molecules Review Recent Advances in The Polymer Dispersed Liquid Crystal Composite and Its Applications 1, 2, 1 1 2 Mohsin Hassan Saeed y, Shuaifeng Zhang y, Yaping Cao , Le Zhou , Junmei Hu , Imran Muhammad 1, Jiumei Xiao 3, Lanying Zhang 1,* and Huai Yang 1,* 1 Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China; [email protected] (M.H.S.); [email protected] (Y.C.); [email protected] (L.Z.); [email protected] (I.M.) 2 Beijing Advanced Innovation Center for Materials Genome Engineering, Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (S.Z.); [email protected] (J.H.) 3 Department of Applied Mechanics, University of Sciences and Technology Beijing, Beijing 100083, China; [email protected] * Correspondence: [email protected] (L.Z.); [email protected] (H.Y.) These authors contributed equally to this work. y Received: 27 October 2020; Accepted: 13 November 2020; Published: 25 November 2020 Abstract: Polymer dispersed liquid crystals (PDLCs) have kindled a spark of interest because of their unique characteristic of electrically controlled switching. However, some issues including high operating voltage, low contrast ratio and poor mechanical properties are hindering their practical applications. To overcome these drawbacks, some measures were taken such as molecular structure optimization of the monomers and liquid crystals, modification of PDLC and doping of nanoparticles and dyes. This review aims at detailing the recent advances in the process, preparations and applications of PDLCs over the past six years. Keywords:polymer-dispersedliquidcrystals; polymerizationinducedphaseseparation; doping;applications 1. Introduction Polymer dispersed liquid crystals (PDLC) were identified as electrically switchable materials four decades ago and have since then remained a topic of intense scientific curiosity. PDLC composite films consist of micrometer or nanometer size liquid crystal (LC) droplets embedded in a polymer matrix. In general, these films exhibit a milky white scattering state due to the random orientation of LC droplets in the polymer matrix [1–3]. Once an electric field of sufficient strength is applied, the film becomes transparent and this phenomenon is attributed to the alignment of LC droplets along the direction of the electric field (Figure1), if the ordinary refractive index n o of LC matches with the refractive index np of the polymer matrix [4]. Spurred by the employment of PDLC in display technologies, research on these materials has rapidly grown in recent decades and is now extending into areas beyond smart windows [5] and displays [6], including diffuse film [7], antipeeping film, quantum dots (QDs) film [8] and components of organic light emitting diode (OLEDs) [9], field effect transistors (FETs) [10], energy storage [11] and solar-energy harvesting [12]. Molecules 2020, 25, 5510; doi:10.3390/molecules25235510 www.mdpi.com/journal/molecules Molecules 2020, 25, 5510 2 of 22 Molecules 2020, 25, x FOR PEER REVIEW 2 of 23 (a) (b) Figure 1.1. OperatingOperating principleprinciple of of a commona common polymer polymer dispersed dispersed liquid liquid crystal crystal (PDLC) (PDLC) device. device. (a) o ff(a-state,) off- (state,b) on-state. (b) on-state. PDLCs cancan bebe preparedprepared byby emulsion emulsion or or phase phase separation separation methods methods [13 [13–15–15].]. However, However, the the latter latter is widelyis widely reported reported in the in literature the literature as it allows as it great allows control great of thecontrol morphology, of the morphology, and thus the characteristics,and thus the ofcharacteristics, the final film. of This the methodfinal film. can This be induced method bycan thermal be induced action, by solvent thermal evaporation action, solvent and/ orevaporation monomer polymerization.and/or monomer The polymerization. formed LC droplets The formed exhibit LC great drop variationlets exhibit depending great variation on the depending method of on phase the separationmethod of phase employed. separation For thermally employed. induced For thermall phasey separation induced phase (TIPS), separation LC is mixed (TIPS), with LC a polymeris mixed atwith high a polymer temperature, at high then temperature, the mixture then is allowed the mixture to cool is at allowed a specific to ratecool toat causea specific phase rate separation to cause and,phase as separation the polymer and, hardens, as the polymer the LC domains hardens, appear. the LC domains For the solvent-induced appear. For the phasesolvent-induced separation phase (SIPS) process,separation both (SIPS) theLC process, and polymer both the are LC dissolved and polymer in the same are solvent.dissolved The in evaporation the same solvent. of solvent The at aevaporation specific rate of inducessolvent phaseat a specific separation. rate induce For thes phase polymerization-induced separation. For the polymerization-induced phase separation (PIPS) method,phase separation the LC is(PIPS) mixed method, with a the monomer LC is mixed/prepolymer with a solutionmonomer/prepolymer to form a homogenous solution to solution. form a Ashomogenous the polymerization solution. As continues the polymerization the polymer continues and LC separatethe polymer from and each LC otherseparate by liquid-liquidfrom each other or liquid–gelby liquid-liquid phase or [16 liquid–gel,17]. The LCphase molecules [16,17]. comeThe LC out molecules of the solution come out and of form the dropletssolution thatand growform untildroplets polymerization that grow until is finished, polymerization i.e., when is the finished, polymer i.e., matrix when becomes the polymer solid enoughmatrix becomes [18]. The solid PIPS methodenough could[18]. The be furtherPIPS method divided could into thermal-initiatedbe further divided polymerization into thermal-initiated and ultraviolet polymerization (UV)-initiated and polymerization.ultraviolet (UV)-initiated Although polymerization. the former method Although offers good the processability,former method less offers contamination good processability, and strong bondingless contamination energy, it requires and strong high temperaturesbonding energy, and longit requires duration high [19, 20temperatures]. The latter, however,and long isduration prompt, solvent-free[19,20]. The latter, and ecofriendly however, is for prompt, the production solvent-free of PDLC and ecofriendly films [21]. for the production of PDLC films [21]. Despite the stability and durability of the PDLC films prepared by UV- polymerization, nuances in UV-irradiation,Despite the stability molecular and durability weight and of the solubility PDLC films of the prepared constituting by UV- LCs polymerization, and polymers nuances could leadin UV-irradiation, to distinct morphologies molecular weight [22]. Thereby, and solubility degree of of the phase constituting separation LCs and and rate polymers of polymerization could lead areto distinct influences morphologies which control [22]. theThereby, morphology degree ofof LCphase droplets. separation Inside and the rate LC of droplet, polymerization the director are configurationinfluences which is mainly control determined the morphology by boundary ofconditions LC droplets. (surface Inside anchoring), the LCLC droplet, material the parameters director andconfiguration droplet shape is mainly and size. determined External factors by boundary (electric or conditions magnetic fields,(surface temperature, anchoring), stress LC and material so on) changeparameters the orientationaland droplet structureshape and which size. a ffExternalects the factors light-scattering (electric byor LCmagnetic droplets fields, and, consequently,temperature, thestress light and transmission so on) change of thethe PDLCorientational film [23 structure–30]. In addition, which affects a slight the di light-scatteringfference in the refractiveby LC droplets index (RI)and, of consequently, both the phases the (LCslight andtransmission polymer) of has the a PDLC significant film e[23–30].ffect on In the addition, light scattering a slight propertiesdifference ofin the PDLCrefractive films. index Moreover, (RI) of the both film the thickness phases also(LCs crucially and polymer) impacts has the a morphology significant andeffect electro-optical on the light (E-O)scattering properties properties of the of PDLC the PDLC films [films.31]. Studies Moreov oner, these the film issues thickness can be useful also crucially in developing impacts PDLC the filmsmorphology for typical and applications. electro-optical (E-O) properties of the PDLC films [31]. Studies on these issues can be useful in developing PDLC films for typical applications. Here, we review the recent developments in preparation, experimental investigation and application of PDLCs, and focus specifically on UV-polymerization. Instead of going into deep Molecules 2020, 25, 5510 3 of 22 Here, we review the recent developments in preparation, experimental investigation and Molecules 2020, 25, x FOR PEER REVIEW 3 of 23 application of PDLCs, and focus specifically on UV-polymerization. Instead of going into deep details, only specific recent papers between 2014–2020 which are particularly related to UV polymerization, details, only specific recent papers between 2014–2020 which are particularly related