molecules

Review Recent Advances in The Polymer Dispersed Liquid Crystal Composite and Its Applications

1, 2, 1 1 2 Mohsin Hassan Saeed †, Shuaifeng Zhang †, 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. †


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 to UV modification techniques related to the process and material to enhance the E-O properties of PDLCs polymerization, modification techniques related to the process and material to enhance the E-O (Figure2) are cited. However, some papers were exceptionally important and are cited. Furthermore, properties of PDLCs (Figure 2) are cited. However, some papers were exceptionally important and previous reviews related to these materials [32–38] can be referred to gain a full understanding of are cited. Furthermore, previous reviews related to these materials [32–38] can be referred to gain a the topic. full understanding of the topic.

FigureFigure 2.2. AnAn illustrationillustration ofof thethe preparation,preparation, electro-opticalelectro-optical propertiesproperties optimizationoptimization andand applicationsapplications ofof PDLC.PDLC.

2.2. PreparationPreparation ofof ConventionalConventional PDLCPDLC CompositeComposite FilmFilm (Meth)acrylate(Meth)acrylate monomers monomers are are generally generally utilized utilized to prepareto prepare PDLC PDLC films films by UV-polymerization by UV-polymerization [39]. The[39]. modulation The modulation of the of morphologythe morphology and and E-O E-O properties properties are are limited limited in in the the flexible flexible (meth)acrylate(meth)acrylate monomers.monomers. Therefore, some unique structures such as hydroxy, epoxy, branchedbranched methylenemethylene/methyl,/methyl, cycliccyclic methylene,methylene, phenylphenyl andand bisphenolbisphenol werewere incorporatedincorporated into thethe conventionalconventional acrylates [[40–44].40–44]. OnceOnce thesethese structures structures were were incorporated incorporated into theinto polymer the polymer matrix, matrix, the E-O propertiesthe E-O properties of PDLC composite of PDLC filmscomposite could befilms optimized could bybe varyingoptimized the hydrogen-bondby varying the interaction,hydrogen-bond the viscosity interaction, of acrylate the viscosity monomers, of theacrylate refractive monomers, index the of polymerrefractive matrix index of and polyme polymerr matrix network and polymer morphologies network [45 morphologies]. Among them, [45]. theAmong presence them, of the the presence hydroxyl of groupthe hydroxyl promoted group the pr formationomoted the of formation a thin polymer of a thin layer polymer at the surfacelayer at ofthe PDLC surface films of PDLC even atfilms low even LC loadings, at low LC attributed loadings, toattributed improving to im theproving E-O properties the E-O properties [46]. Recently, [46]. aRecently, low-voltage a low-voltage driven PDLC driven system PDLC was system achieved was throughachieved an through elegantly an simpleelegantly and simple uniquely and designeduniquely acrylatedesigned monomer acrylate monomer (A3DA) featuring (A3DA) featuring a benzene a moietybenzene with moiety a dodecyl with a dodecyl terminal terminal chain [47 chain]. In [47]. our recentIn our study,recent we study, focused we focused on the comparison on the comparis of methylon of and methyl acrylate and monomersacrylate monomers as well as as terminal well as structuresterminal structures [48]. We found [48]. We that found the asymmetrical that the asymme substitutiontrical substitution of the methyl of the group methyl on the group quaternary on the carbonquaternary in the carbon main in chain the main increased chain increased the steric the hindrance steric hindrance which reducedwhich reduced the polymerization the polymerization rate. Therate. contrast The contrast ratio (CR)ratio was (CR) improved was improved many-fold many by-fold changing by changing the monomer the monomer structure structure from flexible from toflexible rigid. to Similarly, rigid. Similarly, when the when monomer the monomer with siloxane with siloxane was incorporated was incorporated into the into polymer the polymer matrix, matrix, the mechanical properties were enhanced without sacrificing the E-O properties [49]. Table 1 lists some of the (meth)acrylate monomers employed for the preparation of PDLC composites.

Molecules 2020, 25, 5510 4 of 22

the mechanical properties were enhanced without sacrificing the E-O properties [49]. Table1 lists some of the (meth)acrylate monomers employed for the preparation of PDLC composites. Molecules 2020, 25, x FOR PEER REVIEW 4 of 23 Molecules 2020, 25, x FOR PEER REVIEW 4 of 23 Molecules 2020Table, 25, x FOR 1. Some PEER (meth)acrylate REVIEW monomers used for the fabrication of various PDLC composites.4 of 23 MoleculesTable 2020, 251., Somex FOR (meth)acrylatePEER REVIEW monomers used for the fabrication of various PDLC composites. 4 of 23 Molecules 2020Table, 25 1., x Some FOR PEER (meth)acrylate REVIEW monomers used for the fabrication of various PDLC composites. 4 of 23 Molecules Table2020,No. 25 1., xSome FOR PEER(meth)acrylate REVIEW Monomers monomers used for the fabrication Chemical Structureof various PDLC composites. Reference 4 of 23 MoleculesNo. 2020Table , 25 1., x SomeFOR PEER (meth)acrylate REVIEWMonomers monomers used for the fabrication Chemical Structure of various PDLC composites.Reference 4 of 23 MoleculesNo.Table 2020 , 25 1., xSome FOR PEER(meth)acrylate REVIEWMonomers monomers used for the fabrication Chemical ofStructure various PDLC composites.Reference 4 of 23 MoleculesNo. Table2020 , 25 1., xSome FOR PEER(meth)acrylate REVIEWMonomers monomers used for the fabrication Chemical Structureof various PDLC composites.Reference 4 of 23 No.Table 1. Some (meth)acrylateMonomers monomers used for the fabrication Chemical of Structure various PDLC composites.Reference No.Table 1. Some (meth)acrylateMonomers monomers used for the fabrication Chemical Structureof various PDLC composites.Reference No.1 Table 1 1. Some (meth)acrylateLaurylMonomers Lauryl acrylate acrylatemonomers used for the fabrication Chemical Structureof various PDLC composites.Reference[48[48]] No.1 LaurylMonomers acrylate Chemical Structure Reference[48] No.1 LaurylMonomers acrylate Chemical Structure Reference[48] No.1 LaurylMonomers acrylate Chemical Structure Reference[48] 1 Lauryl acrylate [48] 1 Lauryl acrylate [48] 1 Lauryl acrylate [48] 1 Lauryl acrylate [48] 21 LaurylLauryl methacrylate acrylate [50][48] 2 2Lauryl Lauryl methacrylate methacrylate [50[50]] 2 Lauryl methacrylate [50] 2 Lauryl methacrylate [50] 2 Lauryl methacrylate [50] 2 Lauryl methacrylate [50] 2 Lauryl methacrylate [50] 32 HydroxypropylLauryl methacrylate methacrylate [49,50][50] 23 HydroxypropylLauryl methacrylate methacrylate [49,50][50] 3 3Hydroxypropyl Hydroxypropyl methacrylate methacrylate [49[49,50],50] 3 Hydroxypropyl methacrylate [49,50] 3 Hydroxypropyl methacrylate [49,50] 3 Hydroxypropyl methacrylate [49,50] 43 3,Hydroxypropyl 5, 5-trimethelhexyl methacrylate acrylate [49,50][51] 43 3,Hydroxypropyl 5, 5-trimethelhexyl methacrylate acrylate [49,50][51] 43 3,Hydroxypropyl 5, 5-trimethelhexyl methacrylate acrylate [49,50][51] 4 4 3, 3,5, 5,5-trimethelhexyl 5-trimethelhexyl acrylate acrylate [51[51]] 4 3, 5, 5-trimethelhexyl acrylate [51] 4 3, 5, 5-trimethelhexyl acrylate [51] 54 3, 5,Glycidyl 5-trimethelhexyl methacrylate acrylate [49][51] 54 3, 5,Glycidyl 5-trimethelhexyl methacrylate acrylate [49][51] 54 3, 5,Glycidyl 5-trimethelhexyl methacrylate acrylate [49][51] 5 Glycidyl methacrylate [49] 5 5Glycidyl Glycidyl methacrylate methacrylate [49[49]] 5 Glycidyl methacrylate [49] 5 Glycidyl methacrylate [49] 65 TetrahydrofurfurylGlycidyl methacrylate acrylate [48][49] 56 TetrahydrofurfurylGlycidyl methacrylate acrylate [49][48] 6 Tetrahydrofurfuryl acrylate [48] 6 Tetrahydrofurfuryl acrylate [48] 6 Tetrahydrofurfuryl acrylate [48] 6 6Tetrahydrofurfuryl Tetrahydrofurfuryl acrylate acrylate [48[48]] 6 Tetrahydrofurfuryl acrylate [48] 76 TetrahydrofurfurylTetrahydrofurfuryl methacrylate acrylate [48,49][48] 67 TetrahydrofurfurylTetrahydrofurfuryl methacrylate acrylate [48,49][48] 7 Tetrahydrofurfuryl methacrylate [48,49] 7 Tetrahydrofurfuryl methacrylate [48,49] 7 Tetrahydrofurfuryl methacrylate [48,49] 7 Tetrahydrofurfuryl methacrylate [48,49] 7 7Tetrahydrofurfuryl Tetrahydrofurfuryl methacrylate methacrylate [48[48,49],49] 7 Tetrahydrofurfuryl methacrylate [48,49] 87 TetrahydrofurfurylBenzyl methacrylate methacrylate [48,49] 8 Benzyl methacrylate [48,49] 8 Benzyl methacrylate [48,49] 8 Benzyl methacrylate [48,49] 8 Benzyl methacrylate [48,49] 8 Benzyl methacrylate [48,49] 8 Benzyl methacrylate [48,49] 8 8Benzyl Benzyl methacrylate methacrylate [48[48,49],49] 98 CyclohexylBenzyl methacrylate methacrylate [48,49] 9 Cyclohexyl methacrylate [48,49] 9 Cyclohexyl methacrylate [48,49] 9 Cyclohexyl methacrylate [48,49] 9 Cyclohexyl methacrylate [48,49] 9 Cyclohexyl methacrylate [48,49] 9 Cyclohexyl methacrylate [48,49] PDLC9 films composedCyclohexyl of methacrylatethiol-ene monomers are usually prepared by UV-polymerization.[48,49] PDLC9 films composedCyclohexyl of methacrylatethiol-ene monomers are usually prepared by UV-polymerization.[48,49] NorlandPDLC optical films9 adhesives composed Cyclohexyl are of good thiol-ene methacrylate examples monomers of these are kinds usually of monomers. prepared byThese UV-polymerization. monomers[48,49] have NorlandPDLC optical films adhesives composed are of good thiol-ene examples monomers of these are kinds usually of monomers. prepared byThese UV-polymerization. monomers have highNorland PDLCconversion optical films efficiency,adhesives composed fastare of goodspeed,thiol-ene examples water monomers resistance, of these are excellentkinds usually of thmonomers. preparedermal insulation, by These UV-polymerization. monomers better inertness have highNorlandPDLC conversion optical films efficiency,adhesivescomposed fastare of speed,goodthiol-ene examples water monomers resistance, of these are excellentkinds usually of thmonomers.preparedermal insulation, by These UV-polymerization. monomersbetter inertness have toNorlandhigh oxidation PDLCconversion optical films and efficiency,adhesives lesscomposed light initiatorfastare of goodspeed,thiol-ene dosage examples water monomers than resistance, of the these acrylates are excellentkinds usually systemof monomers.th preparedermal [52–54]. insulation, byThese The UV-polymerization. thiol-ene monomers better inertness reaction have Norlandtohigh oxidationPDLC conversion optical films and adhesives efficiency,lesscomposed light initiatorare fast of good speed,thiol-ene dosage examples water monomers than resistance, of the these acrylates are kindsexcellent usually systemof monomers. thpreparedermal [52–54]. insulation, byThese The UV-polymerization. thiol-ene monomers better inertnessreaction have usuallytohighNorland oxidation PDLCconversion occurs optical films and inefficiency, adhesives lesscomposed two light ways: initiatorfastare of thegoodspeed,thiol-ene radical-mediateddosage examples water monomers than resistance, of the these acrylates areanti-Markovnikov excellentkinds usually systemof thmonomers. preparedermal [52–54]. additioninsulation, byThese The UV-polymerization. thiol-eneand monomers better the inertness reactionbase have or highusuallyNorlandto oxidation conversion PDLC occursoptical and films inefficiency,adhesives less two composed light ways: fastinitiatorare speed,goodthe of thiol-eneradical-mediateddosage examples water than resistance, monomers of the these acrylatesanti-Markovnikov excellentkinds are usually systemof thmonomers.ermal prepared[52–54]. insulation,addition These The by thiol-eneand UV-polymerization.monomers better the inertness basereaction have or nucleophile-initiatedusuallytoNorlandhigh oxidation conversion occursoptical and in efficiency,adhesivesless two thiol-Michaellight ways: initiatorarefast goodthespeed, addition.radical-mediateddosage examples water than resistance,In ofthe the these former, acrylates anti-Markovnikov excellentkinds the system ofinitiator monomers.thermal [52–54]. absorbs additioninsulation, These The energy thiol-ene andmonomers better fromthe inertness reaction baselight have or tonucleophile-initiatedhighusually oxidationNorland conversion occurs opticaland inefficiency,less two adhesives lightthiol-Michael ways: initiator fast are speed,the good addition.dosageradical-mediated water examples than resistance,In the the of former, acrylates theseanti-Markovnikov excellent kindsthe system initiator ofthermal monomers. [52–54]. absorbs insulation,addition The energy These thiol-ene and better monomers fromthe inertness reaction baselight or haveor heatusuallynucleophile-initiatedhighto oxidation andconversion occurs produces and inefficiency, less freetwo thiol-Michaellight radicalsways: initiatorfast the speed,to initiate addition.radical-mediateddosage water the than resistance,In reaction the the former, acrylates anti-Markovnikov [53]. excellent theHowever, system initiator thermal [52–54]. the absorbs addition insulation,latter The energyemploys thiol-eneand better fromthe a baseinertness reaction baselight or or a usuallyheattonucleophile-initiated oxidationhigh and conversionoccurs produces and in less twofree effi lightthiol-Michaelciency, radicalsways: initiator fastthe to speed,initiate radical-mediatedaddition.dosage water the than In reaction resistance, thethe former, acrylatesanti-Markovnikov [53]. excellent theHowever, system initiator thermal [52–54]. the absorbsaddition latter insulation, The employsenergy thiol-eneand betterfromthe a base reactionbase light inertness or or or a nucleophilenucleophile-initiatedheattousually oxidation and occurs produces whichand in less couldfreetwo thiol-Michaellight radicalsways: weaken initiator the to the initiate addition.dosageradical-mediated bond the energythan In reaction the the offormer, acrylates theanti-Markovnikov [53]. alkene theHowever, system initiator double [52–54]. the absorbsbond addition latter toThe energyemploysstart thiol-eneand the fromthe reactiona base reaction baselight or orin a nucleophile-initiatednucleophileusuallyheatto and oxidation occurs produces which andin couldtwofree less thiol-Michael radicalsways: light weaken initiator the to the addition.initiateradical-mediated bond dosage theenergy In thanreaction the offormer, the theanti-Markovnikov acrylates[53]. alkene the However, initiator double system theabsorbs bond [52addition latter–54 to]. energy employsstart The and thiol-enethe fromthe reactiona base baselight reaction or ororin a mildheatnucleophileusuallynucleophile-initiated andconditions. occurs produces which in In couldfreetwo China, thiol-Michael radicalsways: weaken Wang the to the initiateetaddition.radical-mediated bondal. [55–58] the energy In reaction the from offormer, the anti-Markovnikov Sichuan[53]. alkene theHowever, initiatoruniversity, double the absorbsbond addition latter in totheir energy employsstart andvarious the fromthe areaction base studies,baselight or orin a heatmildnucleophile-initiatednucleophileusually and conditions. produces occurs which In freeincould China, thiol-Michael two radicals weaken ways:Wang to theinitiate theetaddition. al.bond radical-mediated [55–58] the energy In reaction the from offormer, the [53].Sichuan anti-Markovnikovalkene theHowever, initiatoruniversity, double the absorbs bond latter in addition totheir employsenergy start various the and from a reaction base the studies,light baseor orina or employednucleophilemildnucleophile-initiatedheat andconditions. produces the which thiol-Michael In couldfree China, thiol-Michael radicals weaken Wangaddition to the initiateetaddition. bondal.to produce[55–58] the energy In reaction the from dye-doped offormer, the Sichuan[53]. alkene the However, PDLC initiatoruniversity, double films the absorbsbond latterwith in totheir lowenergy employsstart variousdriving the from areaction base studies,voltagelight or orin a nucleophileemployedheatmild and conditions. produces the which thiol-Michael In couldfree China, radicals weaken Wangaddition to the initiateet bond al.to produce[55–58] the energy reaction from dye-dopedof the [53].Sichuan alkene However, PDLC university,double films the bond latterwith in to their lowemploysstart variousdriving the reactiona base voltagestudies, or in a (Vmildemployedheatnucleophiledr). and conditions.In ourproduces the which group thiol-Michael In couldfree ZhongChina, radicals weaken etWang addition al. to the [1initiateet6] bondal. toemployed produce[55–58] the energy reaction from vinyldye-doped of the Sichuan[53]. ether alkene However, PDLCand university, double thiol films the bondto latter withfabricatein totheir low employsstart variousdrivingPDLCs the areaction base studies,voltagevia orthe in a mild(Vnucleophileemployeddr). conditions.In our the which group thiol-Michael In could China,Zhong weaken Wanget addition al. the[1et6] al.bond toemployed [55–58]produce energy from vinyl dye-dopedof the Sichuan ether alkene andPDLC university, double thiol films bondto withinfabricate totheir lowstart various drivingPDLCs the reaction studies, viavoltage the in Markovnikovemployed(Vnucleophilemilddr). conditions.In our the which group reactionthiol-Michael In could ZhongChina, and weaken investigatedet Wangaddition al. the [1et6] bondal. toemployed theproduce[55–58] energy morpholo from vinyldye-doped of the gySichuan ether alkeneand PDLCelectro-opticaland university, double thiol films bondto withfabricatein properties totheir low start variousdrivingPDLCs the of reaction the studies,voltagevia films. the in employedMarkovnikovmild(Vdr). conditions.In our the groupreactionthiol-Michael In China,Zhong and investigated Wangetaddition al. [1et6] al.to employed theproduce[55–58] morpholo from dye-dopedvinyl gySichuan ether and PDLCelectro-opticaland university, thiol films to with infabricate properties their low variousdriving PDLCs of the voltagestudies,via films. the Zhang(VMarkovnikovmildemployeddr). conditions.In et oural. the [59] group reactionthiol-Michael demonstratedIn ZhongChina, and investigatedetWang addition al.that [1et the6] al. toemployed thiol–acry the[55–58]produce morpholo from latevinyldye-doped systems gySichuan ether and PDLCelectro-opticalandcould university, thiol befilms used to withfabricatein toproperties their producelow various drivingPDLCs ofPDLC the studies,voltagevia films.films the (VZhangemployedMarkovnikovdr). In et our al. the [59]group reactionthiol-Michael demonstrated Zhong and etinvestigated addition al. that [1 6]the toemployed thiol–acry theproduce morpholo latevinyldye-doped systemsgy ether and andPDLCcouldelectro-optical thiol befilms usedto withfabricate toproperties producelow drivingPDLCs ofPDLC the viavoltage films.films the withMarkovnikovZhangemployed(Vdr). higherIn et oural. the CR,[59] group reactionthiol-Michael butdemonstrated Zhongthe and driving investigated etaddition al.that voltage [1 the6] toemployed thiol–acry thewasproduce morpholo also latevinyldye-dopedhigh. systemsgy Sunether and et PDLCelectro-opticalandcould al. [60]thiol befilms studied used to withfabricate toproperties the producelow effects drivingPDLCs ofPDLC of the variousvoltagevia films. films the Markovnikovwith(VZhangdr). higherIn et our al. CR, [59] groupreaction butdemonstrated Zhongthe and driving investigatedet al. that voltage [1 6]the employed thiol–acry thewas morpholo also vinyllatehigh. gysystems etherSun and et electro-opticaland couldal. [60]thiol be studied usedto fabricate propertiesto the produce effects PDLCs of PDLC ofthe variousvia films. films the Zhangwith(VMarkovnikovdr). higherIn et oural. [59]CR, group reaction demonstratedbut Zhongthe and driving investigatedet al.that voltage [1 the6] employed thiol–acry thewas morpholo also late vinylhigh. systemsgy etherSun and et andelectro-opticalcould al. [60]thiol be studied usedto fabricate toproperties theproduce effects PDLCs ofPDLC of the variousvia films.films the ZhangMarkovnikovwith higher et al. [59]CR, reaction demonstratedbut the and driving investigated that voltage the thiol–acry thewas morpholo alsolate high. systemsgy Sun and et couldelectro-optical al. [60] be studiedused toproperties producethe effects ofPDLC ofthe various films.films withMarkovnikovZhang higher et al. CR,[59] reaction butdemonstrated the and driving investigated that voltage the thiol–acry thewas morpholo also latehigh. systemsgy Sun and et electro-opticalcould al. [60] be studied used toproperties theproduce effects ofPDLC of the various films. films withZhang higher et al. CR,[59] butdemonstrated the driving that voltage the thiol–acry was also latehigh. systems Sun et couldal. [60] be studied used to the produce effects PDLC of various films Zhangwith higher et al. [59]CR, demonstratedbut the driving that voltage the thiol–acry was alsolate high. systems Sun et could al. [60] be studied used to theproduce effects PDLC of various films with higher CR, but the driving voltage was also high. Sun et al. [60] studied the effects of various with higher CR, but the driving voltage was also high. Sun et al. [60] studied the effects of various

Molecules 2020, 25, 5510 5 of 22 nucleophile-initiated thiol-Michael addition. In the former, the initiator absorbs energy from light or heat and produces free radicals to initiate the reaction [53]. However, the latter employs a base or a nucleophile which could weaken the bond energy of the alkene double bond to start the reaction in mild conditions. In China, Wang et al. [55–58] from Sichuan university, in their various studies, employed the thiol-Michael addition to produce dye-doped PDLC films with low driving voltage (Vdr). In our group Zhong et al. [16] employed vinyl ether and thiol to fabricate PDLCs via the Markovnikov reaction and investigated the morphology and electro-optical properties of the films. Zhang et al. [59] demonstrated that the thiol–acrylate systems could be used to produce PDLC films with higher CR, but the driving voltage was also high. Sun et al. [60] studied the effects of various acrylate monomers with thiols and optimized the microstructure which improved the CR and driving voltage of the PDLC films. Unlike the previous works focusing on the structures of the monomers, Zhang et al. [61–63] from University of Science and Technology, Beijing, China, systematically investigated the effects of LC molecule structures (fluorinated LC molecules, alkene-terminated LC molecules, and cyano-terminated tolane LC molecules) on the morphology and E-O properties of PDLC composite films. The LC component was obtained by doping fluorinated LC molecules into commercial LC (E8) and fixed 50.0 wt% in PDLC composite films. The doping contents and the terminal chain lengths of the fluorinated LC molecules influenced the physical properties (such as dielectric anisotropy, refractive index and viscosity) of LC and polymer network morphology and the E-O properties of PDLC composite films. The results showed that the optimal 8.0 wt% doping fluorine LC molecules enabled them to realize the significantly low driving voltage of the composite films. According to the above study methods, the optimal E-O properties of PDLC composite films with low driving voltage, high CR and fast response time were obtained by employing alkene-terminated LC molecules or cyano-terminated tolane LC molecules.

3. Preparation of Modified PDLC Composite Film In the past few years, the modified PIPS technique suggested by Chidichimo et al. [64] of Calabria University, Italy, was further developed as a dual-step polymerization by our research group [65–70]. PDLC composite films could be prepared by UV-UV, UV-thermal, thermal-UV or thermal-thermal polymerization in two steps. Liquid crystalline acrylate, vinyl-ether, or epoxy monomers were frequently used. Among them, radical-initiated liquid crystalline acrylate and cationic-initiated liquid crystalline vinyl-ether were usually used for UV polymerization, while liquid crystalline epoxy initiated by UV or thermal could be used for UV or thermal polymerization. Here, a UV-UV dual step polymerization method was taken as an example to introduce the specific method [66]. Figure3 shows a schematic illustration of the preparation of polymer dispersed and polymer stabilized liquid crystal (PD&PSLC) composite film. The key factor was the lower polymerization rate of the liquid crystalline monomer (LCM) with a rigid structure than that of non-LCM. Thus, the non-LCM radical polymerization was dominant. Further, the microphase separation between LC and the polymer matrix occurred and a porous polymer network structure was formed in the first step, which was similar to PDLC. Subsequently, under sufficient applied electric field and appropriate UV intensity conditions, the oriented LCM was further polymerized to construct the homeotropically aligned polymer network (HAPN) in larger porous structures as shown in Figure4c, similar to PSLC. The resulting composite film was referred to as PD&PSLC. In the said work, the LC with SmA-N* phase transition was used and the resulting composite film could reversibly transverse a transparent and strong light-scattering state under temperature controlled conditions. More importantly, the film combined the advantages of large-area manufacturing and flexibility. Meanwhile, the dual-step polymerization was also carried out for an electrically controllable PD&PSLC composite film. The results showed that the demonstrated PD&PSLC composite film possessed better E-O properties and mechanical properties, besides good film formation. MoleculesMolecules 20202020, 25,, x25 FOR, 5510 PEER REVIEW 6 of 226 of 23

Figure 3. Schematic illustration of the preparation of polymer dispersed and polymer stabilized Figureliquid 3. Schematic crystal (PD&PSLC) illustration composite of the preparation film. (a) Homogeneous of polymer isotropic dispersed mixture and polymer sandwiched stabilized between liquid crystaltwo (PD&PSLC) pieces of plastic composite sheets. (bfilm.) Microphase (a) Homogeneous separation betweenisotropic the mixture liquid crystallinesandwiched mixture between and two piecespolymer of plastic matrix. sheets. (c) Randomly (b) Microphase oriented separation liquid crystalline between mixture the liquid within crystalline a liquid crystal mixture (LC) and droplet. polymer matrix.(d) Perpendicularly(c) Randomly alignedoriented liquid liquid crystalline crystalline mixture. mixtur (e) Homeotropicallye within a liquid aligned crystal polymer (LC) networkdroplet. (d) Perpendicularly(HAPN) formed aligned within liquid an LC droplet.crystalline (f) The mixture. N* phase (e gradually) Homeotropically turns into the aligned SmA phase polymer upon network the (HAPN)consumption formed ofwithin photopolymerizable an LC droplet. monomers. (f) The N* (g phase) Perpendicularly gradually alignedturns into SmA the phase SmA within phase an upon LC the droplet. (h) Focal-conic texture of the heat-induced N* phase within an LC droplet. (i) Photograph of consumption of photopolymerizable monomers. (g) Perpendicularly aligned SmA phase within an the transparent state of the film at a temperature below the phase-transition temperature of the LC. LC droplet. (h) Focal-conic texture of the heat-induced N* phase within an LC droplet. (i) Photograph (j) Photograph of the heat-induced light-scattering state of the film. Reproduced with permission from of theref. transparent [66]. Copyright state 2017, of the ACS film Publication. at a temperature below the phase-transition temperature of the LC. (j) Photograph of the heat-induced light-scattering state of the film. Reproduced with permission from ref. [66]. Copyright 2017, ACS Publication.

Molecules 2020, 25, 5510 7 of 22 Molecules 2020, 25, x FOR PEER REVIEW 7 of 23

Figure 4. Scanning electron microscope (SEM) photographs of the polymer networks of the films Figure 4. Scanning electron microscope (SEM) photographs of the polymer networks of the films observed from a side view of the cells. (a) A porous structure of polymer networks of PDLC composite observed from a side view of the cells. (a) A porous structure of polymer networks of PDLC composite film. (b) The HAPN of PSLC composite film. (c) A coexistent structure of both the porous polymer film. (b) The HAPN of PSLC composite film. (c) A coexistent structure of both the porous polymer networks and the HAPN of PD&PSLC composite film. Reproduced with permission from ref. [66]. networks and the HAPN of PD&PSLC composite film. Reproduced with permission from ref. [66]. Copyright 2017, ACS Publications. Copyright 2017, ACS Publications. 4. Preparation of Nanoparticles Doped PDLC Composite Film 4. Preparation of Nanoparticles Doped PDLC Composite Film In conventional PDLCs, the LC, monomer and initiator are mixed. However, there are certain limitationsIn conventional to an all-organic PDLCs, system. the LC, Doping monomer of nanoparticles and initiator (NPs) are mixed. was attributed However, to there the enhancement are certain limitationsof optical, thermalto an andall-organic mechanical system. stability Doping of the of polymer nanoparticles matrix and (NPs) interaction was attributed with the LC.to NPsthe enhancementmainly influence of optical, the dielectric thermal constant, and mechanical refractive indexstability or anchoringof the polymer forces matrix at the polymerand interaction/LC interface. with theVarious LC. typesNPs mainly of NPs influence have been the probed dielectric to prepare consta PDLCnt, refractive films with index superior or anchoring qualities. forces A few at recent the polymer/LCexamples of interface. the properties Various exhibited types of by NPs NPs-doped have be PDLCen probed are discussedto prepare below. PDLC films with superior qualities.PDLC A compositefew recent materials examples doped of the with proper ITOties NPs exhibited were studied by NPs-doped by various PDLC groups are to discussed optimize below.the films for applications in the field of energy saving due to their thermal insulation properties. In China,PDLC Wu composite et al. [71] ofmaterials Nanjing doped University with ITO doped NPs 1.5were wt% studied ITO powdersby various of groups submicron to optimize size in the the filmsmixture for (E7applications+ NOA65) in and the foundfield of that energy the operatingsaving due voltages to their decreased thermal insulation due to the properties. effects of ITO In China, on the Wumorphology et al. [71] of of the Nanjing PDLC University as the anchoring doped force 1.5 wt% at the ITO polymer powders/LC of boundary submicron was size lowered. in the Recentlymixture (E7Zhang + NOA65) et al. [72 and] of Xijingfound University, that the operating China, also voltag usedes ITOdecreased NPs with due acrylate-based to the effects monomers of ITO on after the morphologysurface treatment of the ofPDLC NP with as the 3-methacryloxypropyltrimethoxysilan. anchoring force at the polymer/LC boundary However, was they lowered. found Recently that the Zhangdriving et voltage al. [72] was of Xijing increased University, and CR China, was decreased also used with ITO the NPs increment with acrylate-based of doping content monomers at 0–20 after wt% surfaceof NPs treatment loading, while of NP the with near 3-methacryloxypropyltrimethoxysilan. infra-red (NIR) absorption property However, of films was they enhanced found that in the the drivingwavelength voltage range was of increased 1300 to 2500 and nm. CR was decreased with the increment of doping content at 0-20 wt% ofSilica NPs NPs loading, hold while significant the near promise infra-red to improve(NIR) absorption the E-O propertiesproperty of of films the PDLCwas enhanced [73]. In in India, the wavelengthJayoti et al. [range74] of of Dr. 1300 B. R. to Ambedkar 2500 nm. National Institute of Technology, Jalandhar, showed the effects of S-NPsSilica on NPs the hold morphology significant and promise E-O properties to improve of PDLC the E-O by dopingproperties S-NPs of the into PDLC the PDLC [73]. basedIn India, on JayotiNOA65 et andal. [74] BL036 of Dr. LC. B. TheyR. Ambedkar reported National a significant Institute decrease of Technology, in the driving Jalandhar, voltage showed by doping the 1.5 effects wt% of S-NPs on the morphology and E-O properties of PDLC by doping S-NPs into the PDLC based on NOA65 and BL036 LC. They reported a significant decrease in the driving voltage by doping 1.5 wt%

Molecules 2020, 25, 5510 8 of 22

S-NP and also transmittance of the films was enhanced. The NPs located at the LC-polymer interface modified the anchoring energy and thus effected the reorientation of LCs, which was attributed to the voltage drop. The dual size S-NPs was accredited to steric hindrances and resulted in slow phase separation during the photo-polymerization process. In Algeria, Beroguiaa et al. [75] of Université Aboubakr Belkaïd, reported that high concentration of S-NPs caused their aggregation at the LC polymer interface, which increased the anchoring energy and thus the reorientation voltage. Other inorganic NPs such as ZnO, MgO, CuO, BTO, BaTiO3, Fe3O4, TiO2 [76–82] were doped into PDLCs and a decrease in driving voltage was observed. It was found that these particles tended to impact the dielectric constant of the medium or created local field effects which consequently lowered the driving voltage. The doping of ferroelectric NPs created spontaneous polarization which further improved the E-O properties [82,83]. In India, Mishra et.al [76] of Mumbai University, reported the fabrication of PDLC films by the SIPS method and embedded CuO, ZnO and Zn, NPs. A change in the structural and optical properties in the films was observed due to the gradient in the phase transition temperature confirmed by various characterization techniques such as Fabry Perot spectroscopic studies, optical polarized microscopy, differential scanning calorimetry, and Abbe refractometry using DSR Lamda. A change in the refractive index was noted because of the alteration of the orientational ordering properties of LCs and polymer networks due to the temperature increase. In recent years, studies showed that metallic NPs-doped PDLC films exhibited better E-O, dielectric and optical properties. Particularly, Ag and Au-doped films possessed low driving voltages and high CR. This was attributed to surface plasmon excitations at metal-LC interfaces which enhanced the local electric fields [84,85]. A random lasing from dye-doped PDLCs containing Ag NPs was observed due to the cumulative effect of light scattering from nano-sized LC droplets and the local field enhancement around the silver NPs. In Taiwan, Chan et al. [86] of the National Central University, prepared core-shell particles by coating a thin layer of Ag on polystyrene microspheres and reported the effects of doping such particles on the operating voltage of PDLCs. The composite films contained LC (E7) and an acrylate-based polymer. They found that the induced electric field between the particles was enhanced upon the application of the external electric field and resulted in reduction of operating voltage from 77 V to 40 V. Although the NPs have the potential to modulate the morphology of the films due to their intrinsic properties, it is difficult to predict how the NPs-doped PDLCs will perform relative to undoped PDLCs. In addition, the interaction of nanoparticles with the various recipes of the PLDCs is not easy to envisage and the performance comparison of NPs in a graphical or tabulated form to obtain the general sketch is of no use because the PDLC composition varies.

5. Preparation of Dye-Doped PDLC Composite Film In dye-doped PDLCs (D-PDLCs), dye molecules were found to increase light absorption and decrease the power necessary for the optic effect [87]. D-PDLCs have been studied extensively by several groups, and it has been found that a strong microscopic mutual interaction among the dye and LC molecules influenced the LC refractive index, dielectric constant, orientational order and phase transition temperature [88–92]. E-O properties of the films were significantly improved by doping dyes due to their contribution to modifying the morphology of the PDLCs [93–96]. In India, Deshmukh et al. [94] of the Institute of Chemical Technology, Mumbai, reported the growth in the size of LC droplets at a higher concentration of orange dyes. They noticed that during photopolymerization, dye molecules absorbed some of the UV light available for polymerization and slowed down the phase separation process. Sharma et al. [97,98] of Chitkara University, Jhansla, Rajpura, India, also reported the incorporation of orange azo dichroic dye. Their finding revealed that higher dye concentration varied the transition temperature which transpired trans-cis photoisomerization of dye molecules as well as light absorption by the dye molecules to a slight extent. The azo dye molecule exhibited a rod-like shape in the trans form, but was bent and banana-like in the cis form, which weakened the intermolecular ordering interactions and order parameters of the LC. The Molecules 2020, 25, 5510 9 of 22 Molecules 2020, 25, x FOR PEER REVIEW 9 of 23 polymerizationwhich resulted in rate increased was reduced LC droplet since the size vicinities as shown were in occupiedFigure 5. byThe dye studies molecules, showed which that resultedthe LC, polymerin increased and LC dye droplet were chosen size as such shown that in the Figure solubility5. The studiesof dye in showed the polymer that the was LC, low polymer but high and in dye the wereLC. chosen such that the solubility of dye in the polymer was low but high in the LC.

Figure 5. GraphicalGraphical representations representations of of LC LC droplets droplets si sizeze of images observed by POM withwith 5050× × magnificationmagnification takentaken duringduring the the experiment experiment (a )(a bipolar) bipolar and and radial radial configuration configuration of LC of droplets LC droplets and ( band–e) (stableb–e) stable bipolar bipolar configuration configuration with respect with torespect their diverse to their size diverse of LC dropletssize of withLC droplets dye concentration. with dye concentration.Reproduced with Reproduced permission with from permission ref. [97]. Copyright from ref. 2017, [97]. ELSEVIERCopyright Publications.2017, ELSEVIER SEM Publications. images of (f) SEMpure PDLC,images ( gof) PDLC(f) pure+ 1% PDLC, MR, ((hg)) PDLCPDLC+ +3% 1% MR MR, and (h ()i )PDLC PDLC ++ 3%5% MR MR composites.and (i) PDLC Reproduced + 5% MR withcomposites. permission Reproduced from ref. with [87]. permission Copyright 2019,from re ELSEVIERf. [87]. Copyright Publications. 2019, ELSEVIER Publications.

6. Preparation of Carbon Nanofillers-doped PDLC Composite Film 6. Preparation of Carbon Nanofillers-doped PDLC Composite Film Carbon nanofillers nanofillers such as nano nanographite,graphite, fullerenes and carbon nanotubes (CNTs) have been paid more attention due to the advantages of thei theirr high electric, thermal and and mechanical mechanical properties. properties. Specifically,Specifically, the alignment of CNTs isis crucialcrucial forfor anisotropicanisotropic mechanical and electric properties: their properties are extraordinarily large along the tube axis and small along a perpendicular direction. direction. Single wall nanotubes (SWNTs) and multiwall nano nanotubestubes (MWNTs) are two representativerepresentative types of CNTs successfully applied asas dopantsdopants toto PDLCsPDLCs toto improveimprove theirtheir E-OE-O performance.performance. Jain et al. [99] [99] of the InstituteInstitute of Chemical Technology, Mumbai, India, prepared two types of MWCNT-doped PDLCsPDLCs (CPDLC), (CPDLC), C00N C00N and and C36N C36N using using mixtures mixtures of twoof two diff differenterent LCs LCs (LC (LC BL036 BL036 and andLC HPC850100-100), LC HPC850100-100), and NOA65. and NOA65. The LC The droplet LC sizedrop waslet notsize influenced was not byinfluenced the doping by concentration the doping concentrationbecause no UV because light was no absorbed UV lightby was MWCNT. absorbed At by the MWCNT. optimal doping At the concentrationoptimal doping (0.005%) concentration CR and (0.005%)threshold CR voltage and threshold of C36N, voltage CPDLC of composite C36N, CPDLC film was composite 990 and film 1 V, respectively,was 990 and and1 V, thatrespectively, of C00N andCPDLC that composite of C00N CPDLC film was composite 137 and 0.1 film V, respectively. was 137 and However, 0.1 V, respectively. the composite However, film shrank the composite because of filmthe formation shrank because of conductive of the formation channels of between conductive two electrodeschannels between of the composite two electrodes film with of the the composite increment filmof doping with the concentration. increment of For doping the relaxation concentration. behavior, For thethe tworelaxation composite behavior, films both the two showed composite Debye-type films bothbehavior showed having Debye-type a zero value behavior of the distribution having a zero parameter. value of Wuthe etdistribution al. [100] from parameter. the University Wu et of al. Science [100] fromand Technology, the University Beijing, of Science China, and focused Technology, on the improvement Beijing, China of E-O,focused properties on the of improvement PDLC composed of E-O of propertiesacrylate monomers, of PDLC SLC1717, composed and MWCNTof acrylate modified monomers, by a cyanobiphenyl SLC1717, and functional MWCNT group. modified The results by a cyanobiphenylindicated a reduction functional of drivinggroup. The voltage results at anindica optimalted a MWCNTreduction dopingof driving content voltage of 0.01–0.03at an optimal mg, whichMWCNT was doping attributed content to the of enhancement 0.01-0.03 mg, of which the electric was fieldattributed by reducing to the theenhancement resistivity ofof thethe medium electric field by reducing the resistivity of the medium and the increase of the capacitance of the cells induced by the MWCNT. Moreover, the dispersion of MWCNT was further improved through modification,

Molecules 2020, 25, 5510 10 of 22 and the increase of the capacitance of the cells induced by the MWCNT. Moreover, the dispersion of MWCNT was further improved through modification, and the corresponding anchoring effect on the LC molecules was weakened, which improved the transmittance.

7. Applications The efforts dedicated to fabrication methods and various outstanding properties make PDLCs suitable for promising applications in energy saving smart windows, photovoltaics, optical elements, light shutters, switching gratings, sensors, microlenses, lasers, smart food packaging, electrically switchable high-fold-helix spiral phase plates and biocompatible materials, as well as biomedical devices. In this section, we presented the information on the aforementioned applications in detail.

7.1. Traditional Applications Since the beginning, the electrically switchable peculiarity of PDLCs from a light scattering state to a transparent state has led to their excellent performance and extended application prospects in energy-efficient smart windows. Although the fabrication of PDLC-based windows has not changed in recent years, new formulations suitable for this purpose have been investigated thoroughly. Particularly, the fundamental bottleneck was the optical modulation confined within a limited waveband, either visible (400–800 nm) or near-infrared (NIR, 800–2500 nm) region. The shorter wavelength invisible NIR (800 to 1500 nm) carries about 50% of entire solar energy and, therefore, the shielding performance of a smart window in this range is crucial. To develop a highly efficient energy-saving smart window Liang et al. [101] recently demonstrated the broadband optical modulation, the regulation of visible light transmittance [102] and the shielding performance of NIR light from 800 nm to 2500 nm in PDLC films. They combined a poly(vinylpyrrolidone) (PVP) tuning layer between tungsten bronze (CsxWO3) nanorods (NRs) and a polymeric syrup containing liquid crystals with a smectic A (SmA) to chiral nematic (N*) phase transition, followed by forming an elaborately designed polymer structure within the film [103]. In essence, the field of the PDLC for smart windows is still evolving [104–106] and there is still huge room for improvement. Transparent displays based on PDLCs have high visibility and require a black color as well as complete blocking of the background. In Korea, Yoon et al. [107,108] of the National University, Busan, prepared devices which consist of a dye-doped cholesteric liquid crystal (ChLC) layer and a polymer dispersed liquid crystal (PDLC) film (E7 and NOA 65), for simultaneous control of haze and transmittance. In the opaque state, this can not only provide a black color by using the dye-doped ChLCs but also hides the objects behind the display panel by using the PDLC film. The proposed light shutter shows a high haze value of 90.7% with a low specular transmittance of 1.20%.

7.2. HPDLC Holographic PDLCs (HPDLCs) are another class of polymer-LC composite wherein prepolymer/ monomer/oligomer concentrations are quite high compared to PDLC. HPDLC structures (periodic dark and bright fringes) are created in an LC cell filled with a mixture of LC, monomer and photoinitiator (PhI) exposed under the interference pattern generated by two, or multiple, coherent laser beams with proper wavelength and power. The interference pattern formed gives rise to nonuniform photopolymerization and phase separation. High polymerization rate in the bright region (diffusion of the monomer from dark to bright region) and low polymerization rate in dark region (diffusion of LC from bright to dark region) creates a Bragg grating with alternate polymer-rich and LC-rich regions. A periodic refractive index profile is formed in the cell, and thus light can be diffracted. Morphology and diffraction properties of the grating depend upon writing set-up, materials, diffusion rate, curing conditions and the phase separation process Typical morphology, the working principle and electro-optical properties of HPDLCs are thoroughly elaborated in chapter 5 of the recently published book [109]. Promising applications of HPDLCs include diffraction gratings (DGs), polymer slices (POLICRYPS), waveguides, autostereoscopic image splitter, three-dimensional (3D) data storage and photonic crystals [110–117]. Molecules 2020, 25, 5510 11 of 22

Despite several attempts to overcome the need for a strong electric field when the diffraction effects are not required, E-O efficiency of the HPDLCs was compromised. In the United States, TJ. Buning et al. [118] of the Beam Engineering for Advanced Measurements Company, Florida and Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio, demonstrated highly-efficient and fast DGs which were invisible (transparent) in the off state and exhibited strong diffractive properties upon the application of a moderate E-field by employing photocurable LC monomer and nematic LCs using holographic photopolymerization techniques. Similarly, Manda et al. [119] of Chonbuk National University, Korea, obtained DGs which were optically transparent in the visible wavelength regime by tuning the LC droplet size below the visible wavelength. Efficient HPDLCs having low driving voltages and high diffraction efficiency were realized by using a photoinhibitor, acrylamide and doping ZnS nanoparticles [120,121]. Also, a concurrent photoinitiation and inhibition upon green light illumination was disclosed, which significantly improved the diffraction efficiency of HPDLC gratings and afforded their utilization in storage of colored 3D images [122].

7.3. Novel Films Although the displays based on PDLC films are simple and bright, there is huge room for improvement. In this regard, optical diffusers play a critical role in preventing light sources from being seen directly by viewers in the lighting system. Additionally, optical diffusers are a key optical component in light-emitting diodes (LEDs), which can spread the point light source uniformly without high intensity spots. In our group Zhou et al. [7,123–126] designed optical diffusers with very high efficiency. Recently, an optical diffuser was prepared via a thiol ene click reaction comprising of 1:1 of ene and thiol and exhibited high transmission (98.49%) and high haze (91.77%). Similarly, display devices with narrow viewing angle (NVA) and wide viewing angle (WVA) characteristics for privacy protection and information sharing were demonstrated [127]. Han et al. [128] from the University of Science and Technology, Beijing, China, fabricated a controllable antipeeping device with a laminated structure of microlouver and PDLC films which reversibly switched between a wider viewing angle range (VAR) for the WVA mode and a narrower viewing angle range (VAR) for the NVA mode at 0 V and 8 V, respectively. In our recent studies novel QDs films based on a liquid crystals/polymer composite were developed [8,129]. The fluorescence enhancement of QDs-doped TiO2/polymer, LCs/polymer and TiO2/LCs/polymer composite systems were comparatively studied. The photoluminescence enhancement of QDs was induced by the synergistic effect of LCs/polymer composite and TiO2 nanoparticles (150 nm particle size) having 0.1 wt% loading content. The enhancement factor of QDs-doped TiO2/LCs/polymer composite film was approximately 6-fold relative to the QDs-doped polymer. The polymer dispersed vinyl-ether liquid crystals (PDVLC), polymer dispersed crosslinked vinyl-ether liquid crystals (PDCVLC) and fluorine-containing polymer-dispersed crosslinked vinyl-ether liquid crystals (F-PDCVLC) QDs films were fabricated by dual-step polymerization as shown in Figure6. The reported PDVLC film had fluorescence enhancement and tuning, whereas the PDCVLC film not only maintained fluorescence enhancement of the PDVLC film but also improved its water-resistant performance. Similarly, hydrophobic PDLC films for ecofriendly optoelectronic applications were demonstrated by the encapsulation of cholesteryl acetate LC by a chitosan polymer matrix [130]. Molecules 2020, 25, 5510 12 of 22 Molecules 2020, 25, x FOR PEER REVIEW 12 of 23

Figure 6.6. Schematic illustrationillustration of of the the preparation preparation of of crosslinked crosslinked V-LCs V-LCs/polymer/polymer composite composite films. films. (a) The (a) homogeneousThe homogeneous polymeric polymeric syrup syrup containing containing quantum quantum dots sandwiched dots sandwiched between between two conductive two conductive ITO glass substratesITO glass coatingsubstrates with coating PVA. ( bwith) The PVA. PDVLC (b) film The was PDVLC formed film by was the first formed step radicalby the polymerizationfirst step radical of weakpolymerization UV light to of initiate weak microphaseUV light to between initiate V-LCsmicrophase and polymer between matrix. V-LCs (c) and Enlarged polymer V-LCs matrix. domains (c) withEnlarged randomly V-LCs oriented domains liquid with crystal randomly mixture. oriented (d) V-LCs liquid were crystal homeotropically mixture. aligned(d) V-LCs when were the electricalhomeotropically field was aligned applied. when (e) The the second electrical step cationic field was polymerization applied. ( ofe) intenseThe second UV light step was cationic carried outpolymerization to prepare the of intense PDCVLC UV by light the was polymerization carried out to of prepare V-LC monomers. the PDCVLC (f) Enlarged by the polymerization V-LCs domains of withV-LC the monomers. polymerization (f) Enlarged of V-LC V-LCs monomers. domains (g) Thewith F-PDCVLC the polymerization film was developed of V-LC monomers. by introducing (g) The the perfluoroF-PDCVLC acrylate film was monomers developed into by the introducing original syrup the and perfluoro repeating acrylate the above monomers dual-step into polymerization. the original (syruph) Enlarged and repeating polymer matrixthe above with dual-step the perfluoro polymerization. acrylate monomers. (h) Enlarged Reproduced polymer with matrix permission with from the ref.perfluoro [8]. Copyright acrylate monomers. 2019, ELSEVIER Reproduced Publications. with permission from ref. [8]. Copyright 2019, ELSEVIER Publications. 7.4. Device Components 7.4. DevicePDLCs Components have been used as device components for OLED, Micro-LEDs, solar power collector, OFET, and solarPDLCs cells. have In been United used States, as device D.K.Yang components et al. [9 ]for of KentOLED, State Micro-LEDs, University, solar Ohio, power demonstrated collector, PDLCOFET, filmsand insolar which cells. LC dropletsIn United were States, unidirectionally D.K.Yang et aligned al. [9] under of Kent an applied State electricUniversity, field alongOhio, thedemonstrated film normal PDLC direction. films Thisin which PDLC LC film droplets was used we tore enhanceunidirectionally the light aligned outcoupling under e ffianciency applied of OLEDs,electric field because along it could the film selectively normal scatter direction. light This emitted PDLC from film the OLEDwas used into to directions enhance with the largelight incidentoutcoupling angles efficiency but not of light OLEDs, with because small incident it could angles,selectively and scatter thus reduced light emitted light lossfrom due the to OLED total internalinto directions reflection. with Similarly, large incident Wu et angles al. [131 but] of not the light University with small of Central incident Florida, angles, USA, and demonstrated thus reduced volumetriclight loss due light-shaping to total internal PDLC reflection. films for Similarly, mini LED-backlit Wu et al. liquid [131] crystalof the University displays (LCDs). of Central PDLC Florida, films withUSA, ademonstrated thickness 50- volumetricµm were prepared light-shaping without PDLC surface films alignmentfor mini LED-backlit using a mixture liquid crystal containing displays LC (ZLI(LCDs). 1844 PDLC= 49.92%) films and with prepolymer a thickness (NOA 50-μ 60m =were47.11%). prepared With properwithout material surface engineering, alignment using passive a PDLCmixture films containing with angle-selective LC (ZLI 1844= scattering 49.92%) properties and prepolymer were shown (NOA which 60=47.11%). respond With to angles proper rather material than spatialengineering, locations. passive PDLC films with angle-selective scattering properties were shown which respondMonolithic to angles integration rather than of spatial CNT thin-filmlocations. transistor driver circuits with PDLC pixels has been studiedMonolithic [132]. The integration fabricated of PDLC CNT pixelsthin-film possessed transistor good driver contrast circuits and high-performance.with PDLC pixels has In Korea, been Songstudied et al.[132]. [133 The] of thefabricated Kyungpook PDLC National pixels possessed University, good Daegu, contrast demonstrated and high-performance. PDLC integrated-organic In Korea, field-eSong etff ectal. transistors[133] of the (PDLC-i-OFETs). Kyungpook National The PLDCUniversity, sensing Daegu, layers demonstrated were prepared PDLC by embeddingintegrated- 4,4organic0-pentyl-cyanobiphenyl field-effect transistors (5CB) (PDLC-i-OFETs). microdots in the The poly PLDC (methyl sensing methacrylate) layers were (PMMA) prepared matrix, by whichembedding were 4,4 integrated′-pentyl-cyanobiphenyl on the flexible OFETs(5CB) microdots with 200 µ inm-thick the poly poly (methyl (ethylene methacrylate) naphthalate) (PMMA) (PEN) substratesmatrix, which as shown were integrated in Figure7 .on The the PDLC flexible layers OFETs were with found 200 toμm-thick contain well-alignedpoly (ethylene LC naphthalate) micro-dots embedded(PEN) substrates in the PMMAas shown layers. in Figure The fabricated 7. The PDLC PDLC-i-OFET layers were devices found were to able contain to sense well-aligned four different LC micro-dots embedded in the PMMA layers. The fabricated PDLC-i-OFET devices were able to sense four different stimulations such as weak air (gas) flow, strong physical force (touch), light and heat,

Molecules 2020, 25, 5510 13 of 22 Molecules 2020, 25, x FOR PEER REVIEW 13 of 23 asstimulations shown in Figure such as 7b. weak The airconcept (gas) flow,of combining strong physical PDLC and force OFET (touch), could light contribute and heat, to asthe shown further in developmentFigure7b. The of concept multifunctional of combining organic PDLC andsensory OFET coulddevices contribute for various to the applications further development including of humanoidmultifunctional robots, organic artificial sensory skin and devices wearable for various sensors. applications including humanoid robots, artificial skin and wearable sensors.

Figure 7. Structure and basic device performances of flexible PDLC-i-OFETs. (a) Illustration for the device structure and materials used in this work (photographs demonstrate attachment of flexible Figure 7. Structure and basic device performances of flexible PDLC-i-OFETs. (a) Illustration for the PDLC-i-OFET array sensors on human hands), (b) Sensing performance by weak gas flow stimulations device structure and materials used in this work (photographs demonstrate attachment of flexible and mechanism change of 5CB alignment in the channel region of PDLC-i-OFET devices according to PDLC-i-OFET array sensors on human hands), (b) Sensing performance by weak gas flow applied voltages and nitrogen gas stimulations: illustrations for possible orientation of 5CB molecules stimulations and mechanism change of 5CB alignment in the channel region of PDLC-i-OFET devices in the LC micro-dots in the channel layers. Reproduced with permission from ref. [133] Copyright 2017, according to applied voltages and nitrogen gas stimulations: illustrations for possible orientation of Springer Nature Publications. 5CB molecules in the LC micro-dots in the channel layers. Reproduced with permission from ref. [133] CopyrightConventional 2017, PDLCs-basedSpringer Nature windows Publications. require external electricity to operate, and these devices cannot be combined with traditional solar cells for energy savings. To achieve nearly zero energy-consumingConventional PDLCs-based windows, luminescent windows require solar concentrators external electricity (LSCs) to were operate, integrated and these with devices PDLC cannotdevices be [ 134combined–137]. with The schematictraditional illustrationsolar cells for of energy LSC-coupled savings. PDLC To achieve is shown nearly in zero Figure energy-8a,b. consumingAn important windows, aspect of luminescent the LSC device solar is its concentrat capabilityors to harvesting (LSCs) were direct, integrated diffused andwith ground-reflected PDLC devices [134–137]. The schematic illustration of LSC-coupled PDLC is shown in Figure 8a,b. An important aspect of the LSC device is its capability to harvesting direct, diffused and ground-reflected light.

Molecules 2020, 25, 5510 14 of 22 Molecules 2020, 25, x FOR PEER REVIEW 14 of 23

Therefore,light. Therefore, a measurable a measurable amount amountof energy of can energy be generated can be generatedeven in nonideal even in illumination nonideal illumination conditions. Inconditions. Korea, Mateen In Korea, et al. Mateenof Dongguk et al. University, of Dongguk coupled University, an LSC coupled based on an luminophores LSC based on luminophores(MACROLEX Fluorescent(MACROLEX Yellow Fluorescent 10GN and Yellow MACROLEX 10GN and Fluoresc MACROLEXent Red Fluorescent G (Bayer, Germany)) Red G (Bayer, and Germany)) PDLC (E7 and and NOAPDLC 65). (E7 The and PDLC NOA was 65). placed The PDLC on the was backside placed of on the the LSC backside to reduce of thethe LSCbackside to reduce light losses the backside of LSC becauselight losses PDLC of LSCin off because mode acted PDLC as in a obacksideff mode actedscattering as a reflector backside as scattering shown in reflector Figure 8b–d. as shown PDLC in filmFigure not8 b–d.only PDLCreflected film the not untrapped only reflected light into the untrappedthe LSC part light butinto also theredirected LSC part the but light also that redirected was not absorbedthe light that(i.e., wasoutside not absorbedthe absorption (i.e., outsiderange of the the absorption luminophores) range towards of the luminophores) the edges, and towards thus to the the attachededges, and solar thus cells. to the The attached electricity solar produced cells. The by electricity the LSC produced part was by successfully the LSC part employed was successfully for the switchingemployed operation for the switching of the PDLC operation device. of the PDLC device.

Figure 8. (a) Schematic representation and working principle of PDLC-coupled LSC [138](b) Schematic Figure 8. (a) Schematic representation and working principle of PDLC-coupled LSC [138] (b) exploded representations of an active LSC window containing red-NIR phosphorescent octahedral Schematic exploded representations of an active LSC window containing red-NIR phosphorescent molybdenum nanoclusters composed of a PDLC matrix containing the phosphorescent inorganic emitter octahedral molybdenum nanoclusters composed of a PDLC matrix containing the phosphorescent surrounded by PV cells on its edges [138]. Figure8a,b reproduced with permission from Ref. [ 138] inorganic emitter surrounded by PV cells on its edges [138]. Figure 8a,b reproduced with permission Copyright 2017, ACS Publications (c) PDLC backside emissions of an N-CQDs-based waveguide from Ref. [138] Copyright 2017, ACS Publications (c) PDLC backside emissions of an N-CQDs-based coupled with no PDLC and with PDLC. The driving voltage in the ON state was kept constant at 50 V waveguide coupled with no PDLC and with PDLC. The driving voltage in the ON state was kept (d) Backside emission power of PDLC-LSC devices in the OFF and ON mode. The driving voltage constant at 50 V (d) Backside emission power of PDLC-LSC devices in the OFF and ON mode. The in the ON mode was kept constant at 50 V in both devices. (e) Color possibilities of transmitted driving voltage in the ON mode was kept constant at 50 V in both devices. (e) Color possibilities of light through the ON and OFF states of a waveguide-coupled PDLC device plotted on a CIE 1931 XY transmitted light through the ON and OFF states of a waveguide-coupled PDLC device plotted on a chromaticity diagram. Figure8c–e reproduced with permission from Ref. [ 134,136]. Copyright 2019, CIE 1931 XY chromaticity diagram. Figure 8c–e reproduced with permission from Ref. [134,136]. ELSEVIER Publications. Copyright 2019, ELSEVIER Publications. 7.5. Others 7.5. Others The application of PDLC composites in biomedical devices has shown significant promise [139,140]. In Romania,The application Marin et al.of [ 141PDLC–143 composites] from the Institute in biom ofedical Macromolecular devices has Chemistry, shown preparedsignificant ecofriendly promise [139,140].PDLCs for In sensing Romania, applications Marin et by al. embedding [141–143] biocompatible from the Institute LCs in of the Macromolecular polymer matrix ofChemistry, polyvinyl preparedalcohol-boric ecofriendly acid (PVAB). PDLCs The for composites sensing applicat films exhibitedions bywell-defined embedding dropletsbiocompatible of submicrometric LCs in the polymersize (250–650 matrix nm). of polyvinyl It was found alcohol-boric that the composite acid (PVAB). films The possessed composites moderate films wettabilityexhibited well-defined and showed dropletsselective responsivenessof submicrometric for Lsize or D(250–650 sugars, aminonm). It acids was andfound DNA. that the composite films possessed moderatePDLCs wettability were also and shown showed to be selective sensitive responsi to ultrasoundveness for through L or D thesugars, acousto-optic amino acids eff andect. DNA. In the UnitedPDLCs Kingdom, were also Edwards shown et to al. be [144 sensitive] of the to University ultrasound of through Warwick, the Coventry, acousto-optic prepared effect. a In PDLC the Unitedsensor (E7:Kingdom, NOA68 Edwards= 75:25) et andal. [144 studied] of the the University acousto-optic of Warwick, response Coventry, of PDLCs prepared coupled a with PDLC an sensorultrasound (E7: transducerNOA68= 75:25) over aand broad studied frequency the acousto-optic range (0.3–10 response MHz). The of displacementsPDLCs coupled required with an to ultrasoundproduce acousto-optic transducer over clearing a broad of PDLC frequency films wererange between (0.3–10 aroundMHz). The 2 and displacements 50 nm. These required frequency to produce acousto-optic clearing of PDLC films were between around 2 and 50 nm. These frequency dependent displacements could be used for ultrasound visualization in nondestructive testing.

Molecules 2020, 25, 5510 15 of 22 dependent displacements could be used for ultrasound visualization in nondestructive testing. Recently, Firoozi et al. [145] reported a harmonic response in PDLCs to an applied square-wave voltage at low frequencies (4.5–5.0 Hz) enabling their use in interferometers. Another important application of the PDLCs is a polarization-dependent (Micro lenticular lens array) MLA device [146]. This device was used to realize a 3D image when the polarized 2D image arrays encountered the microlens [147].

8. Conclusions As detailed in this review, significant achievements have been made in the preparation, properties and applications of polymer dispersed liquid crystal composite films between 2014 and 2020. The electro-optical properties of the PDLC films were optimized by the structural variation of monomers and liquid crystals, two-step polymerization as well as doping nanoparticles, dyes and CNTs. Specifically, the two-step polymerization paved a new way to improve the electro-optical and mechanical performance of PDLC composite films. Meanwhile, significant progress in applications was also achieved such as diffuser films, antipeeping films, QDs films, device components of OLED, OFET, sensors, photovoltaics, energy harvesting, actuators, biomedical applications, etc. Featuring the advantages of simple preparation, cost-efficiency, large-area manufacturing and flexibility, PDLC composite films promise to promote and expand the development of next-generation smart windows, displays, wearable devices and sensors.

Funding: This work was supported by the National Key R&D Program of China (2019YFC1904702), the National Natural Science Foundation of China (NSFC) (Grant No. 51272026), the joint fund of the ministry of education for equipment pre research (6141A02033238). Conflicts of Interest: The authors declare no conflict of interest.

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