Polymer Dispersed Liquid Crystal Smart Film Technologies: Overview

Abstract

Liquid crystal (LC)-based research and its technological output vary from daily-use personal electronics and flat panels to switchable optical devices such as sensors. Optical and dielectric anisotropy is a key attribute of LCs, imparting functionality and broadening the scope of smart film systems to such products. Among LC smart films, the polymer dispersed liquid crystal (PDLC) smart film depicts an electro-optical (EO) composite sandwiched by transparent conductive oxide electrode-coated polyethylene terephthalate (PET) films. LC orientation and optical transparency in the composite are readily tuned by altering the electric field. The competitiveness of such PDLC devices reflects its favorable response time, energy conservation potential, and manufacturing convenience, all attributes that are readily endorsed by smart home appliances and areas of architecture and the automotive industry. In response to unrelenting market demands, sustainable, energy-efficient, and “greener” PDLC variants have appeared. Particularly worthy of mention are systems featuring transparency at zero field (reverse-mode). Others boast very high energy efficiencies (%80). In this chapter, the science and technology of PDLC, reverse-mode PDLC, and related LC smart films will be reviewed with a highlight on fabrication methods and operating principles. Market potential and research prospects compared to non-LC smart film technologies will also be touched upon.

1. Introduction

Since the discovery of liquid crystallinity in 1888 by Friedrich Reinitzer [1], potential applications and developments in science and technology have led to a thorough understanding of this remarkable phenomenon and its specific properties [2]. In contrast to the classic traits of soft condensed matter, LC behavior arises from mesophases existing between the solid crystalline form and isotropic liquid phase, which exhibit optical and dielectric anisotropy as a function of position and orientation. A plethora of registered examples [3] show such phase properties, including tobacco, many proteins, and even various soap solutions. Among these, only a selected few LCs specifically used in PDLC manufacturing and upcoming PDLC technologies and potential technologies will form the subject of this chapter.

Modern PDLC designs may be regarded as having evolved in response to economic growth, consumer demands, and improved LC manufacturing technologies. The first LC light valve device was patented in 1936 [4]. LC displays (LCD), flat screens, sensors, and many other LC-based technological products followed in the late 1960s [4, 5]. Among those LC-based smart films entering the market, the technology underlying PDLC-based films in particular was patented four decades ago [6, 7, 8]. With essentially no fundamental revisions thenceforth in terms of operating principle, modern PDLC designs can be physically described as a nematic LC droplet encapsulated in a polymeric matrix. The resultant film is sandwiched by plastic substrates coated along their inner face with transparent conductive oxides (TCO). Indium tin oxide (ITO) is the most common TCO coating used to contact the film, enabling potential use of the film as an electro-optically active device. A typical electro-optically active composite formed accordingly generally works under 60 V_rms AC bias at 50–60 Hz, hence bypassing infrastructural energy management issues and facilitating convenient deployment and integration into standard voltage platforms. Improvements to PDLC design constraints are still being shaped by novel interdisciplinary LC manufacturing techniques [5, 9], thin film deposition methods, transparent conductive oxide coating techniques [10, 11, 12], modern platform constraints, and customer needs.

After passing a period of relative inactivity due to patent disputes [13, 14, 15], PDLC manufacturing has experienced a substantial jump in worldwide interest. In particular, an increased smart glass market demand for esthetic, visually “comforting” products was encouraged by the strong performance of various manufacturers. Global climate/energy expenditure mitigation policies as well as the parallel development of complementary technologies such as electrochromic [16, 17, 18, 19] or suspended particle devices [20, 21, 22] also fueled activities and growth. Indeed, the inevitable continual advancement of LC smart film technologies, both academic and industrially, has proven attractive and lucrative.

In keeping with global urbanization, %67 of world population will move to cities by 2050 (approximately 6.2 billion people). To meet housing demands, the current building count will likely double [23, 24, 25]. Already >30% of the global energy consumption and > 35% of carbon dioxide emissions are attributable to the routine operation of residential and nonresidential buildings (i.e., heating, ventilation, air-conditioning (HVAC), and artificial lighting). These values may exceed 50% by 2050 [26, 27, 28, 29, 30].

Given the obvious need to mitigate environmental as well as urban growth sustainability issues, the stringent implementation of environmental and energy policies, actions, and sanctions [26, 31, 32, 33] is clearly foreseen on a global scale. In addressing both environmental (e.g., climate change) and urban energy use concerns (i.e., HVAC energy consumption [34, 35]), it follows that old habits must die, making room for new construction concepts, innovative building designs, and mitigated material solutions, particularly along building exteriors [36, 37]. Indeed, the routine, if not legislatively mandatory practice of constructing energy-efficient buildings using recyclable construction materials and utilizing renewable energy, seems just on the horizon. A substantial part of this overall global strategy will be to incorporate active and passive smart glass solutions [38, 39] to selectively control the entry of daylight. For instance, when configured for cold weather, sunlight striking the smart glass surface will serve not only as a lighting source but also as a source of infrared heating. Conversely, when solar heating is unwanted, smart glass systems may be adjusted to block out the infrared part of the light spectrum from entering windows. In this way, dynamic light control technologies will permit not only the fine-tuning of transmitted light for visual and working comfort but also the selective passage or blockage of solar heating, thereby reducing energy consumed for HVAC [40, 41] purposes.

Windows clearly represent the weakest construction element of an exterior wall in terms of thermal insulation traits. Indeed, the heat flux ratio of an insulated wall compared to a single-pane window is on the order of 1:100 [42]. In spite of such compelling data encouraging the limitation of windows along external walls (and rooftops), the reality is that exterior window utilization nonetheless continues to proliferate both in terms of number and relative surface area. From an architectural standpoint, windows are esthetically pleasing and permit the exploitation of day illumination and reduction of artificial lighting, the latter consuming 14% of the total energy of EU countries and 19% of countries worldwide [43]. Moreover, sunlight, and an ability to view the outdoors, yields a strong psychological benefit on human behavior, health, and job performance [43, 44, 45]. Hence, window elements will continue to partake in building exteriors as a major design feature. Already certification and regulations such as the Green Building Rating system has led to the utilization of smart window techniques. It follows to suppose that future smart window systems will target the optimization of heat transfer, among other issues.

Among all commercially available, energy-efficient, and environmentally green smart glass technologies, the PDLC-based window can be manufactured and installed most cost-effectively into building exteriors. Indeed, modern variants of PDLC technologies fall into the scope of the European Union Net Zero Emission Building (NZEB) Directive as well as various International Energy Agency policies [46, 47]. Thanks to significant scientific and technical advances in smart and/or specialty nanomaterials, current PDLC systems feature improved dynamic light and heat flux control traits. Complementary smart glass systems such as electrochromic (EC) and suspended particle device (SPD) technologies also provide visual comfort and energy regulation [48, 49] features in buildings (see Table 1). Both are intrinsically limited, however, in that neither can be manufactured as cheaply or simply as PDLC systems.

Attribute	Polymer dispersed liquid crystal (PDLC)	Electrochromic devices (EC)	Suspended particle devices (SPD)
Compatibility to Green Building Standards (LEED, BREEAM, etc.)	Yes – in progress	Yes	Yes
Color	Clear	Blue/Green	Dark Blue
Switching speed	5–15 ms	2–15 min	1–5 s
Privacy in “Off” state	Yes	No	Yes
Energy need for “On” state	Yes	No	Yes
Operating temperature	$-20-70°C$	$-20-70°C$	- 20 - 80 °C
Operating voltage	48–65 V AC	12 V DC	∼100 V AC
Power consumption for switching	5 W/m^2	2.5 W/m^2	5–10 W/m^2
Power consumption for clear state	5 W/m^2	∼0.2–0.4 W/m^2	5–10 W/m^2
Price range (USD)	250–600	1200–2400	1800–2500

Table 1.

A comparison of active smart glass systems sandwiched in glass panes.

The commercial smart glass systems mentioned in Table 1 fundamentally differ in their mode of light control. While PDLC enables tint/haze control via light scattering, electrochromism (EC) and suspended particle devices (SPD) permit light control by light absorption and/or reflection. U-value and solar heat gain coefficients (SHGC) are two parameters describing energy transfer at window panes. Both coefficients are strongly related to the light control mechanism underlying SPD, EC, and PDLC. U-value, meaning thermal transmittance, describes the rate of nonsolar heat transfer through a material. It carries the units of W/m²K. In contrast, SHGC describes the fraction of total solar energy transmitted through a window into a building. A low SHGC value arising from limited solar heat transmittance reflects strong shading traits. Such window attributes would be desired in hot climates in order to mitigate air conditioning costs during the day. In contrast, high SHGC values, reflecting weak shading traits, would be configured in devices used to facilitate the heating of building interiors in cold climates [50]. Table 2 highlights the experimental and simulated thermal data of various PDLC window configurations used throughout the world. As shown, PDLC systems with differing characteristics permit the adjustment of near infrared (NIR) transmission, yielding different SHGC and u-values. The results also support the substantial capacity of PDLC windows to reduce energy consumption and CO₂ emissions compared to conventional window systems. In addition to mitigating energy consumption, another major benefit of PDLC is the economic advantage of downsizing the air-conditioning infrastructure and reducing the frequency of periodic maintenance.

SHGC		u-value (W/m^2-K)		Configuration	Summary	Ref.
ON	OFF	ON	OFF
0.252	0.22	1.666	1.666	8 mm PDLC - 12 mm air −6 mm low-e	PDLC double glazing reduced building energy consumption by 25.1% and raised daylight performance by 149.1% compared to a low-e double glazed building.	[51]
0.541	0.536	1.758	1.758	6 mm clear glass - 14 mm air - 6 mm clear glass - 23.1 mm air −13 mm PDLC laminated glass	PDLC overlaid along a conventional glass window showed a 22.4% energy reduction.	[52]
0.68	0.63	2.79	2.44	4 mm glass PDLC film −4 mm glass	The studies showed that the PDLC glazing yields low NIR transmission (44%) in the translucent state.	[53]
0.25	0.23	N/A	N/A	6 mm window glass-0.4 mm PDLC film	PDLC film glass accounts for 101.76 USD/m^2 annual air-conditioning cost mitigation.	[54]
0.63	0.42	N/A	N/A	Double glazed PDLC window	PDLC double glazing reduced indoor temperatures 1.2–3$°C$ compared to conventional double glazing	[55]
0.68	0.63	2.79	2.44	4 mm glass-PDLC film-4 mm glass	PDLC showed potential to reduce HVAC energy loads (4.8–12.8%) and enhanced daylight performance	[56]
0.562	0.363	2.41	2.57	Double glazed PDLC window	NIR transmission varied from 42 to 24% depending on the choice of transparent and opaque phases. CO2 emissions were also reduced (32.5–89 kg CO2/m^2).	[57]
0.53/0.64	0.39/0.49	5.1	5.0	Double glazed PDLC window	PDLC window systems reduced HVAC energy usage %22.35. CO2 emissions fell.	[58]

Table 2.

Experimental and simulated dataset of PDLC glazing systems optimized for operation in different climate zones.

ON: Transparent state of PDLC (electrically active); OFF: Opaque state of PDLC (electrically inactive); Ref: References; N/A: Not available.

Interestingly, the economic gains, merit, and sustainability of smart window systems have rarely been analyzed. In fact, this subject generally lacks sufficient recognition and understanding in the public eye. One study claimed that the high price and luxury perception of PDLC technologies compared to those of conventional glass systems are among the reasons why smart glass systems are not more widespread in the market [59]. Ironically, the costs of nonrenewable energy and environmental remediation will likely continue to increase for as long as fossil fuel combustion continues. Hence, the energy savings potential and short amortization period of PDLC is such that smart glass technologies should be viewed by consumers and builders as a necessity as opposed to a luxury.

Studies pitting the monetary and environmental costs of nonrenewable energy against the return on investment (ROI) period and life-cycle costs of PDLC [54] and EC [60, 61, 62] are limited. This lack of dissemination is unfortunate, as such studies would be required to inform and emphasize the importance and necessity of PDLC. It is hoped that this review may contribute to a better visibility and appreciation of PDLC, hence expediting its deployment and scope and prompting the routine and worldwide use of PDLC devices as an element of exterior building walls.

Compared to PDLC, EC fabrication plants with a typical capacity of 4 million m² cost over 100 million USD to construct and carry along high operational expenses (i.e., energy, maintenance, and qualified workforce). The use of energy-intensive high technology manufacturing equipment such as large physical vapor deposition, high-temperature annealing, and top-tier characterization/quality-control (QC) systems is another discouraging trait of EC, which markedly contrasts against the rapid setup capability and cheap and technically simple nature of PDLC manufacturing. In fact, PDLC manufacturing plants of the same output capacity cost less than 1 million USD. The low operational costs of solution-processed roll-to-roll manufacturing, which lies at the heart of PDLC, is equally compelling. The sales price of SPD technologies is even more than EC, but the root cause of this high expense is likely related to the very low number of available suppliers utilizing propriety knowledge as opposed to the costs of establishing and running a manufacturing plant. Shown in Table 1, the current retail sales price of PDLC is 1/3 to 1/5 of the price of competing technologies. Given that PDLC production methods and raw materials are cheaply and relatively accessible, given that PDLC manufacturers are firmly and broadly established compared to EC and SPD manufacturers, and given that PDLC products can be easily installed without necessitating special training or equipment, it follows to reason that future PDLC technologies will contribute substantially to the energy efficiency of buildings. In fact, price trend comparisons strongly suggest that the market competitiveness of PDLC compared to the aforementioned other two commercial dynamic light and heat influx modulating systems will continue to increase even more over the course of the short and medium term. Factors underlying the favorable future prospects of PDLC include but are not limited to the perception of PDLC as a sustainable element of green growth and its potential for even better performance through continued R&D. Even now, PDLC performance has been further enhanced by the contribution of auxiliary constituents such as dyes, inorganic and organic particle substitutes, adhesive layers, and specific mesogens in the overall formulation. These more recent efforts have opened a door to specialized PDLC devices, which typically fall under one of the categories of polymer stabilized liquid crystal (PSLC) devices [63, 64], reverse-mode LC devices [65, 66, 67, 68], dye and/or particle-doped LC devices [69, 70, 71, 72], or guest-host systems [73, 74].

In the following passages, the fabrication of conventional and modern variants of PDLC systems as well as their chemical and EO operating principles will be reviewed. Additionally, specialized PDLC systems and their constituents will be overviewed and implications relating to performance will be assessed. It follows that a better visibility and appreciation of the science underlying the function of PDLC may expedite the deployment and scope of PDLC devices in energy conscious, “green building” projects.

2. Structure and characteristics of LCs as well as other constituents of PDLC technologies

2.1 General remarks

The LC journey sparked by Friedrich Reinitzer and Otto Lehman in 1888 was revitalized by fresh ideas rife with potential applications at a conference in 1965. The chemical structure, synthesis, application, and research of materials aimed at yielding LC products have undergone many revisions since then [75]. Among these, the chemical structure of LC has been the primary focus of research ever since the early years of investigation. LC materials have been subjected to a variety of methods to better understand aspects of their synthesis, characterization, modification, and performance. Many chemical compounds can exist in one or more LC phases. While each compound is chemically distinct in its own right, all LC materials share some common physico-chemical-optical traits. For instance, in spite of structural differences, the original and nearly most cited LC compound cholesteryl benzoate [76] responds and performs similarly to the currently most studied compound N-(4-methoxybenzylidene)-4-butylaniline (MBBA) (Figure 1) [77].

Figure 1.

Structures of N-(4-methoxybenzylidene)-4-butylaniline (MBBA) and cholesteryl benzoate.

This section outlines and groups the various chemical compounds being used in PDLC smart glass production and comments on their function and interrelationship.

2.2 Definition and distinguishing general traits of an LC

The LC is a thermodynamically stable 4th state of matter residing within a moderately broad temperature range. As might be surmised by the name, the LC state of matter relates to those special compounds, which can intrinsically display traits intermediate to that of ordered solids and isotropic liquids. On the lower end of this temperature continuum exists a phase transformation from LC to solid crystal while at the higher end is a second phase transformation from LC to isotropic liquid. Understandably, the LC state of matter has appropriately been dubbed “mesophase” where meso denotes “middle”. Not every compound can behave like an LC, as most directly transform from solid to liquid. Compounds proceeding indirectly from the solid to the liquid phases via a mesophase are said to be mesophasic or to display mesophasic traits. These are dubbed “mesogens”. The LC state is distinguishable in that the molecules (i.e., mesogens) tend to become aligned along a common axis. This axis, termed the “director” (vide infra for further discussion), shows the preferred direction of molecular orientation in mesophases.

As a point of reference, the isotropic liquid is a fluid displaying short-range order of its constituent molecules and a statistically random spatial distribution at longer ranges, whereas the solid crystal displays 3D long-range order with respect to the spatial position and orientation of its constituent molecules. Indeed, in true solids, molecules are highly ordered and display minimal translational freedom. As might be anticipated for matter lying intermediate to these states, the mesophase displays imperfect positional and/or orientational long-range ordering of its constituent molecules. The tendency of LC molecules to orient themselves contrasts with molecules in the liquid phase, which have no intrinsic order beyond the short range. As the orientational ordering of the LC state lies midway (i.e., 1D or 2D ordering) between the traditional solid and liquid phases, this state is appropriately described as “mesogenic” along the same logic underlying the origin of the term mesophase. Mesogenic and LC states are synonymous.

Compounds with an inherent ability to exist as a mesophase can be made to behave fluid-like, LC-like, and solid crystal-like by varying appropriate parameters, particularly temperature and pressure; depending on the conditions, the observable traits of an LC compound can vary substantially. That being said, most commercial applications deliberately select LC-active compounds, which retain their mesophasic traits throughout the operating temperature range of the device. It follows that in selecting LC compounds to meet a particular purpose, the thermal transition temperature is just one feature to consider, among many other constraints. The unusual traits of mesophasic compounds and their exploitation in LC devices stem not only from their ability to simultaneously display anisotropic crystal and liquid/fluid properties but also from their ability to be modulated by electric fields. It follows to speculate that the continued advancement of the PDLC industry will depend, in part, on developing and selecting intrinsically advantageous LC-active compounds, which microscopically prompt advantageous macroscopically observable traits in a particular glaze formulation.

2.3 Physical properties of the LC

The physical, or put more technically, the stereo-electronic properties of the LC, and to a substantial extent the non-LC environment surrounding the LC molecule (e.g., additives, residual solvent, etc.), all contribute in determining the energetics, behavior, and hence performance of a particular LC system. Since LC systems typically utilize high concentrations of LC-active species, the principle influence on performance generally reflects the intrinsic physical properties of the LC molecule itself. Given the ability of LC compounds to readily undergo various liquid crystalline phase transitions, a key criterion underlying the design of LC applications is the order parameter, S, which defines the degree of order across the boundaries of a phase transition system. Put more simply, S quantifies the arrangement or orientational order of the system, meaning the extent to which molecules deviate from the overall weight-averaged orientation of the system. S values typically range from 0.3 to 0.9, where S = 1 is perfect order. As might be anticipated, S values are highly temperature-sensitive. Expressed mathematically (Eq. (1)), the order parameter S is a second-rank symmetric traceless tensor usually ranging from 0 in one phase to nonzero in another. Eq. (1) requires the measureable parameters $\widehat{n}$ and θ as input.

$$S=\frac{1}{2}<3{\mathit{cos}}^2\theta -1>$$E1

Exemplified in Figure 2, the angle θ between the principle axis of an individual LC molecule and the local director vector, $\widehat{n,}$ provides a simple means to quantify the orientational order of the LC phase. Again, in a hypothetically perfectly ordered phase, there would be no deviations from the local director and so S = 1. Conversely, S = 0 for a perfectly disordered situation, that is, isotropic phase. The phase order of nematic LCs will typically lie within the range of S = 0.5–0.7.

Figure 2.

Typical representation of an LC director vector and its angular deviation, θ, compared to the local director, $\widehat{n,}$ of the collection.

2.4 Anisotropic behavior of LC systems

A typical rod-like LC possesses uniaxial symmetry around the director, which produces shape anisotropy. When coupled to surrounding effects (e.g., an externally applied electric field), shape anisotropy indirectly permits the adjustment of many intrinsically anisotropic LC physical properties such as refractive index (n), dielectric permittivity ($\overline{\epsilon }$), magnetic susceptibility (χ), conductivity, and viscosity.

2.4.1 Optical anisotropy

LCs are inherently optically anisotropy. Consequently, LCs possess optical birefringence; this is to say that LCs yield one refractive index for light polarized along the director and a second refractive index for light polarized perpendicularly to the director. One is termed the ordinary refractive index ($n_0$), whereas the second is the extraordinary refractive index ($n_e$). In the case of the ordinary refractive index, light travels with a constant velocity regardless of its polarization direction. In the case of the extraordinary refractive index, light travels at different velocities depending on its polarization orientation within the material. The overall effect is due to the anisotropic nature of the crystal structure, which results in two different refractive indexes for different polarizations. The difference between the ordinary and extraordinary refractive indexes allows researchers to understand the unique optical properties of anisotropic materials. By knowing these parameters, one can predict how light will interact with the material; for example, in the case of LC, one may surmise how light will be bent or polarized upon entering or passing through the material. This information is crucial in designing optical components like LC-based waveguides and polarizers, which exploit the birefringence trait of LCs to manipulate light in specific ways. Both inputs are needed to assess the optical birefringence, Δn, of an LC system (vide infra for further discussion). These inputs may also be used to define the average refractive index ($n_{\mathit{av}}$) of an LC (Eq. (2)):

$$n_{\mathit{av}}=\sqrt{\frac{1}{3}\left(n_e^2+2n_0^2\right)}$$E2

As defined in Eq. (3), the birefringence, Δn, is the difference between $n_0$ and $n_e$. This difference may be positive or negative, so the sign of Δn physically has meaning. In particular, a negative or positive value reports on the LC orientation with respect to the optical axis. As might be surmised, the birefringence criterion is important in many aspects, and it is used to classify LCs into different categories. For instance nematic (e.g., rod-like) LCs yield positive $∆n$ values (i.e., $n_e<n_0)$. These typically lie between Δn = 0.02 and 0.4. In contrast, chiral nematic/columnar and discotic LCs yield negative $∆n$ values (i.e., $n_0<n_e)$.

$$∆n=n_0-n_e$$E3

Shown in Figure 3, two geometric constructions, termed optical indicatrix or index ellipsoid, schematically represent opposite scenarios in which $∆n$ takes on positive or negative values. Similar to a uniaxial crystal, the indicatrix herein is an ellipsoid in which the rotational and optical axes are also superimposed.

Figure 3.

Indicatrix depiction of optically (a) positive and (b) negative birefringence.

2.4.2 Dielectric anisotropy

The dielectric properties of LCs relate to the response of the LC molecule toward externally applied electric fields. As with many compounds, the permittivity of an LC is a material property, which determines the strength of interaction between a dielectric medium, here the LC, and an externally applied electric field. This extent of interaction is determined by the inherent capability of the material to polarize upon exposure to an electric field. The observed polarization of the LC will thus reflect the vector sum of the external electric field and an opposing induced electric field of lesser magnitude inside the material. For LC materials comprised of nonpolar groups, electronic (notable at optical frequencies) and ionic polarization generally exists. For LC materials comprising polar groups, orientational polarization is also observed. Dielectric anisotropy is arguably the most important physical property of liquid crystalline compounds because it essentially determines the lower threshold voltages of LC displays.

Two principle permittivity coefficients can be defied by arbitrarily orienting the LC director, N, along the z axis of a macroscopic three-dimensional Cartesian system. The first principal permittivity obtained is ${ϵ}_{ǀ}={ϵ}_{\mathit{zz}}$, which is parallel to the director. The second director is ${ϵ}_{\perp }=1/2\left({ϵ}_{\mathit{xx}}+{ϵ}_{\mathit{yy}}\right)$, and it is perpendicular to N. The difference between the first and second principal permittivity coefficients gives the anisotropic permittivity value, $∆ϵ$ (Eq. (4)):

$$∆ϵ={ϵ}_{\mathrm{ǀ}}-{ϵ}_{\perp }$$E4

When the dielectric anisotropy value is positive, the LC is oriented along the electric field, whereas when the value is negative, the orientation is perpendicular to the electric field (Figure 4). Quite simply, LC dielectric anisotropy is what led to the creation of PDLC smart windows, as LC molecules in the glaze could be reoriented in a controlled manner using electric fields. Indeed, given an appropriately chosen LC with positive/negative birefringence and suitable dielectric anisotropy, the alignment of the optical axis can be predicted and controlled via external stimulus.

Figure 4.

Orientation of LC displaying (a) positive and (b) negative dielectric anisotropy in an externally applied electric field.

2.5 A closer look at mesogens

As mentioned in the introductory section, a mesogen is an organic molecule, which displays LC properties. For this reason, mesogens may be regarded as disordered solids or ordered liquids, as each mesogen consists of rigid and flexible parts, which determine the order and mobility of the structure [78]. At the macroscopic length scale, a collection of compatible mesogens forms the LC system of a typical device. As might be anticipated from the traits expressed above, mesogens directly determine the properties of the LC device. The rigid parts align mesogen moieties along one direction and prompt distinctive shapes, which may resemble rods or disks. The flexible parts, which are usually alkyl chains, impart flexibility to the mesogen and hinder crystallization to a certain degree. It follows that a balanced combination of appropriately rigid and flexible moieties in the mesogen should result in structural alignment and fluidity between individual LC molecules and a functional device formed by the collection of these LC molecules. On a lighter note, there is a tendency to interchangeably use the terms mesogen and LC, and for the most part, this practice is acceptable. To be absolutely accurate, however, one must bear in mind that the mesogen is truly the smallest molecular building block of an LC system, whereas the true LC is a semi-ordered collection or ensemble of many associated mesogens at a size scale much greater than that of a mesogen. That being said, it is always true that an LC molecule is a mesogen. It should also be noted that mesogens have evolved from their classic role as LC molecules to additional roles as “reactive” mesogens wherein they partake in polymer network formation as chemically reactive monomers.

In the context of PDLC technologies, the most important LC compounds fall into the thermotropic class. The three LC types of the thermotropic class are rod-like [79], discotic [80], and conic (bowlic) mesogens (Figure 5) [81]. The geometry of rod-shaped mesogens is long and anisotropic, allowing for preferential alignment in one spatial direction. Discotic mesogens are disc-like with a flat core and feature adjacent aromatic rings. Conic LC mesogens display cores, which are not flat, but rather rice bowl-like in appearance. Both discotic and conic LCs can engage in two-dimensional columnar ordering. The proper selection of LC candidates requires various considerations, which are typically based on physicochemical structure. For instance, low-temperature mesomorphic behavior, promoted by alkyl terminal groups, is generally sought as it would avoid metastable, monotropic liquid crystalline phases and prove technologically useful under typical operating conditions. In making a selection among the various available mesogens, a low melting point is usually preferable. Aromatic rings are also very much sought, as liquid crystalline behavior is strongly promoted by extended, structurally rigid, highly anisotropic shapes. Hence, many liquid crystalline materials are benzene derivatives.

Figure 5.

Three thermotropic LC classes as exemplified by their characteristic mesogen types, which are demarcated into rigid and flexible moieties.

The assortment of available mesogens and resultant LC systems is virtually endless. Complex compounds with integrated LCs also form a subject of active interest. These are designed and synthesized so that the mesogenic moiety of the molecule is physically distant from the remaining non-mesogenic flexible linker moiety [82]. Such “reactive mesogen” molecules may be grafted, for instance, to the reactive side groups of a polymer chain utilizing the flexible spacer. Due to the limited scope of this chapter, only selected LC examples will be extensively discussed. To begin this discussion, a very commonly used commercial example of the nematic (rod-like) LC type is shown below. Termed E7, this LC formulated by Merck is the most frequently studied and used nematic LC in normal-mode PDLC-based smart windows [83]. E7 is composed of 4 hematogenic compounds depicted in Figure 6. Pure E7 has two transition temperatures, namely, a low-temperature glass transition at T_g = −62°C and a high-temperature nematic–isotropic transition at T_NI = +61°C.

Figure 6.

Components of the Merck E7 LC formulation with empirical formula, chemical structures, and relative weight percentages shown.

2.5.1 The LC mesophase

As the temperature drops, the isotropic phase, which displays neither preferential positional nor orientational order, forms a nematic phase. Within a specific temperature range, the LC assumes a low-viscosity liquid of either a transparent or translucent appearance. In general, the isotropic phase displays short-range order and highest thermal mobility. Lowering the temperature yields long-range positional and orientational order. The LC mesophase lies within these extremes and thus reflects traits intermediate to those of long-range orientational and short-range positional order. Depending on the temperature, orientation, and position of the LCs, several mesophases may potentially form. Nematic, chiral nematic (i.e., cholesteric or twisted nematic), and smectic mesophases are the most commonly studied types. These are also utilized in smart windows manufacturing. The nematic phase is the most common phase among the calamatic or rod-shaped LCs. The orientation and positioning of LCs with respect to the local director ($\overline{n}$) is what distinguishes one mesophase from another. For instance, in the nematic mesophase, the LC molecule is oriented along the local director. In comparison, in any of the possible smectic phases, a different alignment angle (smectic C) and/or positional ordering (smectic A, B) is observed. In the chiral nematic mesophase, the general orientation of an ensemble of LCs mimics the helical structure of a DNA molecule. Figure 7 summarizes these attributes of the various mesophases.

Figure 7.

Summary of orientational and positional ordering of LCs with respect to optical direction ($\overline{n}$). Thermal mobility descreases in proceeding from left to right.

When these mesophases form layers with an inter-layer distance (d), they produce structures with the highest packing density and greatest long-range orientational and positional order.

Chiral dopants are optically active LC materials used to create helical structures in a host nematic LC mixture [35]. In keeping with this theme, cholesteric mesophases can be obtained by doping a nematic LC with the appropriate chiral agents (Figure 8 [84]. The most distinguishing and desirable outcome of obtaining a cholesteric mesophase is the ability to selectively reflect light. Here, the reflected wavelength, λ_max, is determined by n, the average refractive index, and P, the helix pitch (Eq. (5)):

Figure 8.

Illustration of the formation of a chiral nematic mesophase via the addition of chiral dopant to a nematic LC.

$${\lambda }_{\mathit{max}}=\mathit{nP}$$E5

The size of the cholesteric mesophase formed is half of the helix pitch. The helical pitch obtained will depend on the molecular structure and concentration of the chiral agent.

2.6 LC based film compositions

To permit the industry-scale production and processing of LC-based smart device films, a prepolymer component with appropriate physicochemical traits is selected to (i) physically disperse/support the LC component prior to curing; (ii) impart solid state LC immiscibility and liquid state LC miscibility, as well as film compliancy during curing; and (iii) yield acceptable transparency, amorphicity, and mechanical traits in the cured product [83]. The principle role of the resultant polymer matrix is to (i) encapsulate the LC as nanodroplets, (ii) direct the orientation of LC molecules in contact with the alignment layer (generally reverse-mode PDLC (R-PDLC)), and (iii) physically stabilize the LC mixture (i.e., PSLC). In the case of direct-mode PDLC (D-PDLC), the most conventional means of polymer matrix formation is photopolymerization. In this approach, LC-compatible monomers and photo-initiators are used, forming a polymer matrix. The influence of various matrix monomers and photoinitiators on LC device performance has been characterized using the example of commercially available Merck E7, a well-known mixture of four mesogens [85, 86, 87]. Such work on E7 served as a useful reference, encouraging many research groups to conduct similar investigations on industrial or academically significant alternative systems.

In this section, we will focus on the LC response to electric field stimuli. Conventional PDLCs (also known as direct-mode, D-PDLC, or normal-mode, N-PDLC) are the most popular compounds used in smart windows production. Overall, the formulation is a mixture of LC and polymer, the latter typically being greater than >40 wt%. In producing N-PDLC devices, mesogens are initially blended into a polymer matrix. The insolubility of these low molecular weight LCs within the polymer matrix leads to a phase separation, yielding nanosized LC droplets embedded along the matrix. The mixture obtained initially appears milky white, given the mismatch between the refractive indices of LC and polymeric matrix [88]. Said mixture is then sandwiched between two transparent conductive electrodes (with PET films typically used as backing). Further cross-linking (curing) of reactive groups within the mixture is achieved via UV exposure or the application of heat.

In N-PDLC, electric fields applied across the electrodes are used to toggle the device between the opaque (Voltage OFF) and transparent (Voltage ON) states. Typically, AC voltages (50–100 V_rms) at frequencies of less than 100 Hz are employed [89]. When a sufficiently high voltage is applied so as to align the LC director in the direction of electric field, the refractive index difference between LC and polymer matrix minimizes, allowing incident light to enter the system so as to yield an optically transparent mixture. This minimum refractive index value is highly dependent on the chemical structure of the LC and polymer. Work is ongoing to further diminish this value.

2.6.1 Reverse-mode PDLC (R-PDLC)

Several reverse-mode PDLC techniques have been reported previously. As might be surmised by the term “reverse”, reverse-mode PDLC (R-PDLC) is transparent during the voltage OFF state and opaque during the voltage ON state [90]. The starting materials used to prepare the R-PDLC largely depend on the fabrication method. Various fabrication methods combined with different LCs have yielded satisfactorily performing R-PDLC devices [91]. To date, various strategies have formed the basis of the performance of these devices. Among the list of notable approaches, some of the most successful include the use of large liquid crystals and a polymer matrix formed out of reactive mesogenic network precursors [92], the surface energy modification of polymer droplets [93], the utilization of dual-frequency responsive liquid crystals [65], the technique of nematic emulsion polymerization [94], the use of rough and supportive alignment layers [95], and, finally, the introduction of an anisotropic polymer matrix [96]. From a technical viewpoint, the underlying rationale of these approaches has been to alter the surface energy of positive/negative dielectric anisotropy-type LCs using active monomers, to disperse double frequency-responsive LCs in a polymer matrix, to vertically align LC molecules along rough glass substrates, to disperse LCs into an anisotropic polymer matrix, and to replace passive polymeric matrix elements with low amounts of mesogenic networks (PSLC based systems). Among these approaches, the alignment method [95] is the most conventional and practical strategy to mass produce LC-based smart windows. This method is also conveniently general in that LCs with either positive [92] or negative anisotropic [97] permittivity can be used to produce the R-PDLC devices [90]. PSLC is the only strategy that has shown commercialization potential in the smart glass industry. The performance of some of the other strategies fell short of meeting commercial expectations in terms of haze, contrast ratio, and driver voltage, whereas the remaining ones required complex power electronics or were unsuitable for roll-to-roll film production methods. Given the above, the remainder of this section will focus on PSLC devices, which utilize an alignment layer on transparent conductive electrodes and feature a relatively low weight percentage content of polymer compared to other PDLCs.

2.6.2 Polymer stabilized liquid crystal (PSLC)

In normal-mode PDLC, an unfortunate consequence of utilizing high polymer loadings (>40 wt%) during preparation is that high voltages are required to toggle the resulting device (48–65 V_rms). In this respect, normal as well as reverse-mode PSLC provides a viable and convenient alternative to the requirement and dangers of using high voltages. In PSLC, the prepolymer content is reduced to such an extent (<5 wt%) that the device transitions from transparent (Voltage OFF) to opaque (voltage ON) states at only ∼30 V_rms. In preparing a typical PSLC, a small amount (2–10 wt%) of monomer (typically a reactive mesogen) and photoinitiator are dissolved in a low molecular weight liquid crystal. The resultant precursor mixture is injected between ITO-glass or ITO-PET substrates and cured under UV illumination or via heat. Subsequent cross-linking of monomers affords a myriad of LC ensembles enclosed inside a polymer network. During polymerization, the uncured mixture is made to assume the “voltage ON” alignment between two conductive electrodes using an alignment layer to prompt reorientation. The so-called alignment layer, from which this method was coined, refers to a thin layer of material, typically a commodity plastic, which is pre-coated/pre-melted along the electrode surfaces and subsequently imparted a particular micron-scale surface topology using a brush affixed to a specialized “rubbing” machine. It is this regular micron-scale topology along the plastic layer that associates and enforces appropriate orientational traits to that very thin fraction of liquid crystals directly layered along the electrode surface. Collections of disoriented liquid crystals in the uncured mixture contact this topologically oriented layer and follow suit, orienting themselves in the same direction as the initially aligned body of liquid crystals via what may be loosely described as a templating effect. This effect is transmitted layer-by-layer in a direction normal to the electrode until the entire glaze layer assumes the same alignment. Finally, a small amount of reactive mesogens present within the uncured mixture are chemically cured via UV irradiation or heat, affording a cross-linked polymer network extending from one plastic surface to the other. Like a flexible glue, this network serves to hold all substances in place and provides much needed mechanical support against delamination. As might be surmised by this description of events, the correct initial orientation of liquid crystals and hence the correct performance of the PSLC device critically depend on the quality of the alignment layer [98]. The alignment layer influences device performance directly, via surface interaction with the LCs, and indirectly, via setting up the circumstances by which a proper polymer network comprising cured reactive mesogens is established. The most compatible and generally utilized plastic material for the alignment layer in both industry and academia is polyimide (PI) [99, 100]. Just like normal mesogens, which form the LCs, a most distinguishing aspect of reactive mesogen monomers compared to polyacrylate, polymethacrylate, or epoxy groups used in classical PDLC systems is that reactive mesogens can be oriented or aligned by their interaction with an electric field, other liquid crystals, or alignment layers such as polyimide due to their liquid crystal structure. An oriented polymer network formed from reacted mesogens can be achieved by applying bias to the substrates during UV curing or by incorporating brush-rubbed polyimide or nylon alignment layers along the ITO surfaces. Once oriented to assume a specific morphology, the reactive mesogens can be polymerized, affording a desired PSLC structure and imparting a targeted function. The desired structure is typically a polymer network arranged in parallel or perpendicularly to the substrate surface. Just as in the case of the alignment layer itself, the presence and quality of a mesogenic polymer network on the local alignment of LC molecules also determines the operating mode and performance of the PSLC system.

Compared to conventional N-PDLC, the view angle transparency of PSLC is higher due to the relative scarcity of the polymer network, which greatly limits the influence of any refractive index mismatch existing between the polymer and LC. A lower polymer content also limits the total area delineated by the polymer-LC interface. Since the interfacial area is limited, the influence of stereo-electronic “anchors” or energy terms, which otherwise would resist the reorientation of LCs, is also limited. With a low number of anchors in the collective whole, the device can operate at substantially lower working voltages compared to conventional PDLCs. Monomers used in today’s PSLCs are often based on terminally functionalized acrylate or methacrylate groups. These moieties are chemically attached to both sides of the mesogenic core, typically via a flexible alkyl chain, which provides a measure of distance and isolation. To exemplify the above, the chemical structure of a monomer (and necessary photoinitiator) used in the fabrication of a PSLC device is given in Figure 9 [101].

Figure 9.

Example monomer and photoinitiator used to produce commercial PSLC.

2.7 Doped LC devices

The addition of organic and inorganic substances to a mixture of LC and polymer is of one the well-established routes to modify the device characteristics. The main approaches used to tailor various features of the LC-polymer mixture include dye-doping (guest-host) [102], nanostructured materials-doping [103], and cholesteric (chiral) molecules-doping [84] of liquid crystals.

Guest-host LCs (GH-PDLC) may be viewed as a composite of dye dissolved in a host LC [102]. Generally, a dichroic dye molecule, which is a geometrically anisotropic (rod-like), is used to absorb light of a specific wavelength or range. Being anisotropic, the operative feature of the dye is that it absorbs incident light better from one orientation compared to the others. Given the intimate guest-host-type interaction between dye and LC molecules, it follows that both will reorient together upon the application of and external electric field. Dyes rotating together with their host LC will absorb light so as to favorably modulate the transmission of light. That being said, many traits are to be considered in choosing a dye. The dyes selected should absorb light best along the molecular chain. Potential candidates should also display a high dichroic ratio, a high-order parameter, a high chemical stability, and good solubility in the LC molecule (but not in traces of unreacted monomer or in the polymer matrix). In general, dyes possessing a positive anisotropic permittivity value are preferred. In such dyes, the absorption transition dipole lies along the long molecular axis. For this reason, incident light polarized perpendicularly to the long axis of the dye is weakly absorbed. In contrast, when light polarization lies parallel, light absorption is fastest. As an example, a dichroic azo dye possessing the N=N functional moiety absorbs specific light wavelengths more along one direction than along the others (Figure 10).

Figure 10.

Orientation of Duest dye along the LC long axis and depiction of orientation-dependent absorption.

The modification of LCs using various nanostructured dopants has been vastly cited in scientific literature. For LCs used in smart windows, the general aim of LC doping has been to enhance the EO properties. Nanostructures including nanoparticles [104], nanorods [105], metallic nanoparticles (MNP) [106], Carbon nanotubes (CNT) [107], magnetic nanoparticles [108], and quantum dots (QDs) [70] were all reported to impart a positive effect on the performance of the LC devices.

Alongside modification methods using doping with nanostructured materials, other alternative methods were reported to effectively impact the EO properties of various LCs. In addressing the vulnerability of LCs in harsh environments, the notion of stabilizing physical traits and/or improving optical traits by tailoring the chemical structure of the parent compound proved promising. For instance, microencapsulation of LCs with a protective layer of compatible polymer to yield a core-shell structure has been widely employed as a protective strategy to preserve the essential traits of LC materials. Also, Reza et al. [109] EO properties of novel side-chain liquid crystalline polymers (SCLCPs) in which LC polymers featuring mesogenic coumarin-based side-groups were fixed along the PMMA backbone via flexible bridging alkyl spacers. A similar strategy utilizing highly compatible Polyurethane (PU) was reported to protect cholesteric LCs and to improve their EO properties, yielding functionality at lower fabrication expenses [110].

3. PDLC displays

3.1 General remarks

The first example of a PDLC system was reported in 1986 by Doane et al. [111]. The classic PDLC structure may be described as a film comprising nematic liquid crystals dispersed in a solid polymer matrix as spherical or elliptical-shaped droplets [112, 113, 114, 115]. Again, an externally applied electric field controls the optical transmittance of the film. The potential difference to create the electric field is typically established by sandwiching the PDLC film between glass/PET substrates coated with a transparent conductive surface such as ITO [116]. It should not be surprising that seminal contributions along these lines prompted the emergence of the smart glass industry [14, 15]. Significant refinements notwithstanding, the conventional PDLC systems still operate by essentially the same principles as those of the original design. To ameliorate haze, for instance, modern designs strive to match the extraordinary refractive indices (n_e) of the LCs and the refractive index (n_m) of the isotropic matrix polymer to high accuracy [112, 113, 114, 115]. In the absence of any electrical potential across the PDLC film, the alignment of the LC directors is set by the LC-matrix interface. This alignment differs for each LC droplet because the droplets and surrounding matrix structure are heterogeneous [117, 118]. To a first approximation, the directors of the droplets are randomly oriented. As such, incident light, traversing from the polymer matrix to the LC, generally experiences scattering at the matrix-LC interface, as the LC in question has a refractive index value between n_o and n_e [116, 119]. The specific value, which is subject to change on the basis of crystal orientation and interfacial integrity, is in all cases marginally different from the refractive index of the matrix [120]. The light scattering mechanism is shown in Figure 11a. Scattering occurs at all visible light wavelengths. Scattering is random due to the spherical shape of the LC droplets, which, again, are randomly dispersed in the heterogeneously structured polymer matrix. As such, the PDLC film appears hazy when switched off [66]. When a potential is applied to the PDLC film, directors having positive dielectric anisotropy in the LC droplets align in the direction of the electric field (Figure 11b). In this well-known EO phenomenon, the refractive indices of the matrix polymer and LC droplets match, so the PDLC film looks transparent [66, 111, 119, 120]. This is to say that light striking the outer surface transmits through the PDLC film without scattering [75].

Figure 11.

Operating principle of a PDLC composite fixed between ITO electrodes.

When a potential is applied, the ideal PDLC film should display high transparency and clarity (i.e., low haze) from a broad range of viewing angles, a millisecond respond time, and a low threshold voltage [15, 66, 121]. These ideals reflect current commercial concerns and demands. Happily, modern PDLC films are not far from these ideals in that commercial PDLC products typically exhibit 80% total optical transmittance and an acceptable base 3.5% haze value when switched on [122, 123, 124, 125]. While haze increases with the viewing angle, this angular dependency is one technical problem, which has slowed the more widespread use of classical PDLC films [126, 127, 128]. Work is ongoing in our laboratories to further ameliorate these technical aspects, and new-generation marketable materials are forecast in the near future. Currently, PDLC films are being used to a limited extent in industrial sectors such as construction, energy, and transport, to name a few [129, 130, 131]. Although the automotive sector is a particularly important market for the smart glass industry, this haze limitation coupled with very strict consumer desires has thus far limited the use of PDLC films to sunroofs [124, 132]. It should be emphasized that the discussion above relates to “normal-mode” PDLC, whereof the device is transparent when powered up and hazy when switched off. However, new-generation reverse-mode PDLC versions are currently being developed in-house to overcome the continuous power consumption issue, and the fruits of these efforts in the auto industry are forecast. Academic reports on reverse-mode PDLC began during the 1990s, and commercial applications as smart glass products began to emerge in 2021 [93, 133, 134].

Even now, basic reverse mode devices have been developed and marketed by a small number of manufacturers. The current industry standard for smart glass, that is, electrochromic devices, is routinely touted for being more energy efficient, and to date, this fact has led to their more widespread adaptation in motor vehicles. Still, projections strongly support the notion that reverse-mode PDLC systems can be refined to such an extent in the near future so as to rehash the shortcomings inherently related to electrochromic devices and thereby negate favorable perceptions of ED technologies in this sector.

Reverse-mode PDLC was primarily developed to address energy concerns in sectors requiring long-term transparency of normal-mode PDLC devices. Indeed, the use of normal-mode PDLC has also been restricted by the high potential and thus high power consumption requirements needed to maintain transparency [48, 70, 131, 135]. For classical (i.e., normal-mode) PDLC films to exhibit maximum optical transmittance, an AC potential of 48 V – 75 V_rms must be applied [135, 136, 137, 138]. AC is the preferred signal, as DC is known to prompt electrolysis of LCs and degradation of ITO coatings. In fact, first-generation PDLC products can show potential requirements as high as 110 V_rms [137, 138, 139, 140]. Understandably, AC signals and high voltage ratings are not very compatible with automotive applications in which 12 V or 24 V DC power supplies have become standard. Moreover, all automotive devices requiring <50 V_rms become subjected to directives demanding the use of complex power electronics, prompting a substantial cost increase. The problem of incompatibility is not as severe in the building sector, as high voltage 50–60 Hz AC power lines can be adopted to standard building needs including architectural glass using simple inexpensive step-down transformers [141]. Still, the architectural glass industry is becoming more energy conscience by the day, so the high voltage requirements of PDLC remains a formidable impasse to rectify [57, 142, 143]. From a commercial viewpoint, normal mode PDLC devices are inherently limited due to the EO nature of the composite films and high power requirements [141, 142]. These EO properties are influenced by the LC-matrix interface, the anchoring effect at this interface, and the size and shape of the LC droplets [144, 145, 146, 147]. The most challenging problem of normal-mode PDLC is the unwanted presence of haze. While haze is minimal in viewing the device straight-on (i.e., normal to the plane of the device), haze becomes increasingly pronounced in viewing the device at increasingly oblique angles compared to the ideal straight-on perspective. The root problem of haze and its angular dependency relates to light scattering. Scattering originates from 2 issues. Firstly, some LC molecules may inadvertently become polymerized with prepolymer molecules. This side reaction slightly alters the refractive index of the isotropic polymer matrix, so haze will occur at every viewing angle due to refractive index mismatches involving no and np. The second issue relates to the isotropic refractive index of the polymer matrix. The refractive index of the polymer matrix is constant and equal in all directions. In an ideal PDLC system, the ordinary refractive index of the LC will match the refractive index of the polymer matrix. However, LCs display birefringence, and more importantly, the ordinary and extraordinary refractive indices differ from one another. As a result, when an electric potential is applied to the PDLC film, the two refractive indices of the LC will only match the matrix refractive index in the direction of the electric field. This results in substantial haze and haze variability at all “off-normal” viewing angles. While strategies exist to increase the miscibility of the LC in the matrix, thus ameliorating the first issue, no strategies founded in theory exist to address the second issue, which originates from material properties. That being said, practical solutions are reported. For example, haze was reduced to an optimal level by adjusting the size of the LC droplets and by using external polarizer filters. Still, these approaches negatively affect the total transmittance and threshold voltage. Optimization has met with partial success; however, a need to perform beyond the intrinsic limitations of normal mode PDLC is what prompted the development of LC-matrix polymer composites, which operate in reverse mode [66, 148, 149]. Such reverse-mode composites naturally consume less power. More importantly, selected variants also exhibit very low haze when transparent, regardless of the viewing angle. Among these, the PSLC device, functioning in reverse mode (R-PSLC), achieved commercialization in the smart glass industry [67, 94, 150, 151, 152]. Indeed, PSLC technology is a candidate, which not only can replace PDLC films in that it displays none of the standard technical shortcomings known to PDLC; moreover, PSLC also exhibits technical advantages, which permit additional applications outside the scope of PDLC and desirable features compared to many other commercial smart glass alternatives [92, 153, 154, 155].

3.2 Production methods of conventional PDLC films and technical facts

In 1984, Fergason et al. patented a microemulsion approach to fabricate an EO film system consisting of LC droplets dispersed in a polyvinyl alcohol (PVA) matrix [6, 156]. Such LC systems predated PDLC and were classified as the nematic curvilinear aligned phase (NCAP) [157, 158]. The NCAP technology lies outside the scope of this review and has been mentioned just as a point of comparison and historical reference. While NCAP films may look and performed similarly to PDLC, the two systems differ from each other in terms of microstructure [159, 160].

The first academic report of a conventional “true” PDLC system appeared 2 years later in 1986 (J. W. Doane et al.) [111]. Many reports followed this seminar contribution in which nematic LCs with positive dielectric anisotropy were dispersed in a polymer matrix either in interconnected pores or as separate droplets [66, 161, 162, 163].

Thermally induced, solvent-induced, or polymerization-induced phase separation techniques are used to produce PDLC films. For practical and economic reasons, commercial PDLC films are preferably manufactured using roll-to-roll (R2R) processes. In R2R processing, polymerization-induced phase separation (PIPS) is understandably advantageous and preferred over solvent or heat-induced phase separation techniques for reasons of simplicity, convenience, and limited processing steps [15, 164, 165]. In this technique, LCs and miscible prepolymer molecules are combined, yielding a PDLC precursor mixture. For nematic type LCs, low molecular weight monomers or oligomers are typically preferred [136, 149, 166, 167]. The precursor mixture obtained is injected between ITO-coated PET substrates, and the sandwiched precursor material is polymerized to yield a flexible film. The LC and prepolymer combination is so chosen that polymerization gradually prompts immiscibility of the two components. As polymerization continues, a gel-like film is obtained [168]. During gelation, the LC undergoes phase separation, precipitating out as droplets dispersed in the polymer matrix [169, 170]. Polymerization is continued until all low molecular weight components are cured and incorporated into the polymer, effectively separating them from the LC phase. The overall polymerization event may thus be arbitrarily divided into an initial gelation stage, in which the film consolidates and phase separation occurs, and a secondary “purification” stage, in which the LCs become purified of any residual reactive monomers [166]. In conventional PDLC systems, the weight ratio of LC to the polymer matrix is a maximum of 80/20 and typically 60/40 [66, 162, 171, 172]. The first PIPS studies utilized epoxide-based curing. In these studies, precursors were prepared by mixing epoxy monomers, a curing agent, and nematic LCs. Polymerization was carried out at temperatures moderately above room temperature [66, 166, 173]. As with the expected temperature dependence of many chemical reactions, the curing temperature was found to affect the size of the LC droplets [174]. Currently, free radical polymerization and UV curing is the most commonly exploited chemical basis of the PIPS approach in producing commercial PDLC films [66, 148, 162, 172]. Compared to epoxide curing, this approach benefits from less shrinkage, zero heat input, and precise control of the UV dose and intensity, which permits fine adjustment of the LC droplet size [117, 175, 176].

3.3 Modern PSLC systems

PSLC composite film systems address and resolve much of the classical PDLC haze problem by deliberately utilizing relatively small amounts of polymer matrix in the composite film. A negligible LC-polymer interfacial surface area is thus obtained. The refractive index mismatch experienced at such a limited interface affords a virtually imperceptible total haze. Early scientific reports on the development of PSLC structures were published in 1991 by R.A.M. Hikmet et al. [153]. In this groundbreaking work, the polymer content of the total film was 10 wt.% or less [150]. Improvements followed such that modern-day PSLC systems utilize approximately 5 wt% of polymer. In spite of the low loading of polymer, the LCs nonetheless still align themselves by interacting with the polymer network [151]. While polyacrylate, polymethacrylate, polyacrylate-thiol, or epoxy systems form standard matrices in PDLC, bifunctional photo-reactive mesogenic monomers are typically used in PSLC to yield the polymeric network [66, 150]. As might be surmised, photo-reactive mesogen monomers are highly advantageous compared to acrylate, methacrylate, or epoxy prepolymers used to construct PDLC systems in the sense that reactive mesogens have an LC structure, and thus, they can be oriented or made to align by their interaction with an electric field, other LCs, and alignment layers such as polyimide (PI) [64, 67]. The reactive mesogens typically utilized are large, rod-shaped molecules terminated at both ends by acrylate or methacrylate groups [177]. As monomeric mesogens exhibit LC properties, it follows to correctly reason that their incorporation into a polymer matrix will also bestow substantial LC traits to the matrix [178, 179, 180].

Normal and reverse mode PSLC structures with different polymer network alignments have been previously reported. Both nematic and cholesteric (i.e., chiral nematic) LC phases can be used to prepare PSLC structures [64, 150, 181, 182]. Those phases based on cholesteric LCs have been subclassified under the category of polymer-stabilized cholesteric texture (PSCT) [150, 183]. Devices operating in normal and reverse modes are available as both phase types. Although many different PSLC-based systems have been reported, three basic structural models have been explained below.

Model 1 - Reverse-mode PSLC: The working principle of a film prepared using a positive nematic LC is shown in Figure 11. A PSLC precursor mixture is prepared by dissolving reactive mesogen monomers in a nematic LC, and a trace of photoinitiator is added to the mixture. A polyimide layer is coated along the conductive surface of the sandwiching substrates, and the polyimide is subsequently rubbed, affording a regularly patterned, unidirectional alignment layer. Now functionally competent, the alignment layer ensures that the LC molecules making up the bulk of the PSLC precursor mixture will align themselves parallel to the substrate surface (Figure 12a) [184]. Interestingly, this alignment of LC molecules also prompts the mesogens, which are dispersed throughout the LC molecules, to similarly align. UV curing follows, and an appropriate film thickness maintains the alignment during this period. Once the film is UV-cured, a polymer network will have formed with an orientation parallel to the substrate surface (Figure 12b) [154]. The LC molecules remain aligned parallel to the surface of substrates without an external electric field (Figure 12a, b) [64, 67]. In this case, the system is transparent because undesired light scattering from the PSLC cell is very low at all angles. The small amount of polymer network in the PSLC cell reaps a refractive index mismatch between polymer and LC exhibiting negligible haze. When a small electric potential is applied to the film, LC molecules in direct contact with the alignment layer and polymer network remain parallel to the substrate surface because adsorptive forces predominate. In contrast, LC molecules within the interior of the film tend to align in the direction of the electric field. The difference in refractive index between perpendicular and parallel LCs imparts a substantial haze to the film, causing it to become opaque (Figure 12c) [185].

Figure 12.

Reverse-mode operation of a PSLC device showing: (a) UV-curable precursor sandwiched between alignment layer-coated electrodes; (b) UV-cured film and LC alignment in the absence of an external electric field; and (c) UV-cured film subjected to an externally applied electric field showing perturbed LC alignment.

Model - 2 Normal-mode PSCT: A PSLC precursor mixture is prepared by dissolving reactive mesogen monomers in a chiral nematic LC, and a trace of photoinitiator is added to the mixture. The precursor mixture is injected between two conductive transparent substrates and cured under UV light (Figure 13a). An alignment layer is not utilized. Instead, the film curing is conducted under AC potential [64]. In this case, the reactive mesogen monomers adopt a perpendicular alignment with respect to the substrate surface. Polymerization yields perpendicular polymer growth, and the resultant polymer network separates the chiral nematic LCs into distinct regions (Figure 13b) [64, 150]. The phase of these regions is sometimes termed a focal conic texture. In the absence of an electric current, chiral nematic LCs are randomly dispersed within the focal conic texture, causing incident light to scatter. The non-energized state of the normal mode will thus be opaque. When a sufficiently strong electric field is applied to the device, the LCs assume a homeotropic phase (Figure 13c). As such, incident light perpendicular to the cell passes through without scattering [186, 187]. Light scattering (i.e., haze) at all other viewing angles is very low, thanks to the negligible optical mismatch between the LC and small amount of polymer network.

Figure 13.

Normal-mode operation of a PSLC device showing: (a) UV curing of a PSCT precursor mixture under the influence of an external electric field as opposed to alignment layers; (b) LC orientation of cured film in the absence of an electric field; and (c) LC orientation of cured film under the influence of an applied potential.

Model 3 - Reverse-mode PSCT structure: A PSCT precursor mixture is injected in between conductive transparent substrates, which have been furbished with a competent alignment layer. Again, reactive mesogen monomers directly contacting the alignment layers are aligned in parallel to the substrate. However, this time, the alignment layer also arranges the chiral nematic LC molecules perpendicularly to the substrate surface. In so doing, some to the mesogens intimately contacting the LC molecules follow suit. The dynamics of alignment between alignment layer, mesogens, and LCs is such that when the PSCT mixture is UV-cured, a polymer network oriented parallel and perpendicular to the substrate surfaces is obtained, imparting a multicellular appearance to the film cross section (Figure 14a) [91, 150]. In the absence of an electric potential, the film is transparent because focal conic structural units are aligned in the same direction [64]. When some electrical potential is applied to the device (Figure 14b), focal conical structures directly contacting the alignment layer retain some element of their perpendicular alignment to the surface of the substrates. Focal conical structures not directly contacting the alignment layers align more randomly due to their interaction with the electrically responsive mesogenic polymer network [67]. Hence, the net orientation of LCs becomes a weight-averaged contribution of many energy terms working together. In summary, orientational differences of LCs near the substrate surface and those in more inner regions of the film prompt the complete scattering of transmitted light, creating a strong haze in the film [64, 188].

Figure 14.

Reverse-mode operation of a PSCT film illustrating (a) LC orientation in (a) the absence and (b) the presence of an externally applied electric field.

Selected examples of normal and reverse mode PSLC and PSCT systems as well constraints underlying their preparation are given in Table 3. LCs and reactive mesogens well established in the LCD sector are listed along with their commercial codes and abbreviations [195]. As some reactive mesogens have been identified using more than one commercial code, synonyms are also provided where appropriate. Reactive mesogen content, film thickness, and cure conditions have also been provided. The data listed is typical and provides a useful reference from where to begin, but these starting materials and parameter values are in no way written in stone, and they are certainly subject to change on the basis of need. Given that temperature and cure conditions (i.e., UV intensity and dosing time) alone can markedly affect the microstructure, morphology, and EO performance of a PSLC system [63], the readership is encouraged to experiment further. For instance, the polymerization cure time has been empirically found to relate to the fraction of prepolymer in the curable mixture. The polymerization time of a PSLC film with a 6 wt.% prepolymer loading typically ranges between 30 and 60 min [64]. Other reports have documented that droplet sizes increase at higher UV intensities and diminish when longer cure times are applied at weaker light intensities. In general, PSLC and PSCT films are fabricated thinner than conventional PDLC films in order to extend the stability of the LCs. As a side-benefit to this added longevity, the thinner film and limited interaction between LCs and a small amount of polymer network effectively lowers the threshold voltage requirements [67, 90, 91].

Model	Liquid Crystal (LC)	Reactive mesogen (RM)	RM (%)	Film thickness (μ)	Curing Cond.	Ref.
Reverse mode PSLC	E7 (PN)	C6H (C6/RM1)	3–10	6	UV curing @ 25°C	[92]
Normal- and reverse-mode PSCT	E31 (PN); CB15 (CA)	RM206	2	8	UV curing 4 mW/cm^2 - 1 h @ 25°C	[189]
PSLC	5CB (PN)	R6M (RM82/HCM009/C6M)	10–90	2–7	UV curing @ 25°C	[153]
Normal-mode PSCT	E48 (PN); CB15 (CA)	BMBB6	2	10	UV curing @ 25°C + 100 V BIAS	[150]
Normal-mode PSLC and PSCT	ZLI4389 (PN); E48 (PN); R1011 (CA)	BAB6; BMBB6	4.5–7	15	UV curing @ 25°C + 100 V BIAS	[154]
Normal-mode PSCT	E48 (PN); S6 (SA); ZLI-4572 (CA)	R6M (RM82/HCM009/C6M)	2	14	UV curing - 20 min	[190]
Normal-mode PSCT	SLC-1717 (PN); S811 (CA)	R6M (RM82/HCM009/C6M)		20	UV curing 1.1 mW/cm^2–30 min @ 29°C	[191]
Reverse-mode PSLC	ZLI 4788 (NN)	PAHB; AHBB	14	25	UV curing 0.5 mW/cm^2–10 min @ 45°C	[152]
Reverse-mode PSCT	E48 (PN); CB15 (CA); CPCA-1	RM257 (HCM008)	1–6	5–10	UV curing	[192]
Reverse-mode PSLC	MLC-2136 (PN); MLC-2053 (PN)	ARLM-002	5	5–25	UV curing @ 25°C	[193]
Reverse-mode PSLC	ZLI-4792 (PN); MLC-2136 (PN); E7 (PN); RDP-86483 (PN)	RM257 (HCM008); ARLM002; UCL-017	5	10	UV curing; 20 mW/cm^2–5 min	[194]

Table 3.

Some PSLC and PSCT contributions.

PN: Positive dielectric anisotropy; NN: Negative dielectric anisotropy; SA: Smectic A; CA: Chiral agent; CPCA-1: Chiral photochromic dopant.

Lastly, the nature of PSLC technologies provides flexibility and a diversity of functional tools to researchers striving to further develop smart-glass systems. In the simplest sense, new EO devices can be derived from the strategies utilized in the basic PSLC models listed above. For instance, vertical alignment layers can be used to replace electrical bias during polymerization, or particle-substituted nematic or chiral nematic phase LCs can be selected to prompt an unusual trait or performance attribute [196]. Even dichroic or non-dichroic rod-like dyes have been mixed with nematic or chiral nematic LCs [197, 198, 199]. The dyes become oriented via electrostatic forces created within neighboring LCs molecules and mesogens. Once cured and energized, films containing dyes will display interesting traits because the dyes will remain oriented along the direction of the LCs even when the system is toggled by an external electric field. Indeed, there are several reports of dye-doped PSLC and PSCT cells [199, 200, 201]. Shown in Figure 14, a film containing dye-doped chiral nematic LCs perpendicularly aligned to the substrate depicts a good example of an advanced normal mode PSCT with potentially new applications. In the absence of an electric field (Figure 15a), dye particles, LC molecules, and the alignment layer are parallel. Incident light travels without scattering, but a certain wavelength is nonetheless absorbed by the dye molecules. For this reason, the PSCT film is clear but tinted [202]. When a sufficiently strong electric field is applied, the LCs reorganize into the homeotropic phase, and the rod-like dye particles retain their parallel alignment to the LC molecules. Now, the PSCT cell exhibits marginally higher clarity and not tinted in this state [203].

Figure 15.

A dye-doped PSCT showing (a) the absorption of a particular wavelength of light in the absence of an electric field and (b) the full & free transmission of light under an applied electric field.

This tinting attribute of dye-doped PSLC and PSCT structures is significant and substantial in the sense that it enables LC-based smart glass technologies to compete against SPD and EC approaches in the highly lucrative facade and automotive glass markets. Moreover, future PSLC technological advances can potentially spark the manufacture of even more original smart glass products displaying desirable attributes such as selective transmittance and adjustable IR reflectance [204, 205, 206, 207].

4. Concluding remarks and prospects

Clearly, PDLC appears destined to become a most instrumental and unavoidable construction material of window technologies. This unwavering interest in PDLC is a principle factor driving efforts to continually develop better films. Despite the aforementioned progress and highly encouraging developments, current LC technologies still require subtle yet important improvements to meet client and energy demands. With the basic theory of operation being long established, efforts have focused by and large on resolving more practical problems such as robustness, power consumption, haze, longevity (i.e., stability), and tinting. Still, the basis to prepare impressive next-generation variants nonetheless appears to rest upon better understanding and addressing microscopic factors such as the physicochemical and optoelectronic nature of the various individual components making up the film; the state of the various interfaces separating immiscible components; the surface microstructure and topology of alignment layers; the adsorption, desorption, and reorganization energy terms of LC molecules; and the thermodynamic and kinetic behavior of the system at macroscopic, mesoscopic, and microscopic length scales. Apart from lowering production and operating costs, a most foreseeably general yet crucial technical target might be to optimize the interaction energetics of molecular participants making up the film, thereby facilitating the kinetics of optical change at all ambient temperatures. Such an undertaking will require a precise knowledge of the thermodynamic and kinetic behavior and packing interactions between various LC molecules, matrix polymers, and additives. It follows to reason that many impressive improvements can potentially be realized within the next decade or two by systematically identifying, characterizing, and optimizing all physicochemical and electrooptical contributions underlying a particular problem.