Polysiloxane-Based Side Chain Liquid Crystal Polymers: from Synthesis to Structure–Phase Transition Behavior Relationships

Review Polysiloxane-Based Side Chain Liquid Crystal Polymers: From Synthesis to Structure–Phase Transition Behavior Relationships

Lanying Zhang 1,2 ID , Wenhuan Yao 3, Yanzi Gao 1, Cuihong Zhang 1 and Huai Yang 1,2,* 1 Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China; [email protected] (L.Z.); [email protected] (Y.G.); [email protected] (C.Z.) 2 Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China 3 College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China; [email protected] * Correspondence: [email protected]; Tel.: +86-10-6276-6919

Received: 11 June 2018; Accepted: 11 July 2018; Published: 19 July 2018

Abstract: Organosilicon polymer materials play an important role in certain applications due to characteristics of much lower glass transition temperatures (Tg), viscosities, surface energy, as well as good mechanical, thermal stabilities, and insulation performance stemming from the higher bond energy and the larger bond angles of the adjacent silicon-oxygen bond. This critical review highlights developments in the synthesis, structure, and phase transition behaviors of polysiloxane-based side chain liquid crystal polymers (PSCLCPs) of linear and cyclic polysiloxanes containing homopolymers and copolymers. Detailed synthetic strategies are elaborated, and the relationship between molecular structures and liquid crystalline phase transition behaviors is systematically discussed, providing theoretical guidance on the molecular design of the materials.

Keywords: linear and cyclic polysiloxanes; side chain liquid crystal polymers; synthetic strategies; phase structure

1. Introduction Organosilicon polymers, which can be divided into three categories—polyorganosiloxanes, polyorganosilanes, and polyorganosilicons—according to structural differences in the main chain, have received widespread attention as one of the most important inorganic backbone polymers since commercial products were developed in the 1940s [1,2]. Polyorganosiloxanes (also known as silicones or silicone elasomers) have gained special interest as a result of unique properties such as diverse structures, excellent physical and chemical properties, as well as a wide range of applications [3,4]. From the viewpoint of diverse structures, polyorganosiloxanes can be linear, cyclic, polyhedral oligomeric silsesquioxane (POSS), comb-like, ladder-like, fish-bone like, and so on; the diversity offers infinite space for a variety of performances. Side Chain Liquid Crystal Polymers (SCLCPs), which integrate the special anisotropy of liquid crystalline materials with the excellent mechanical performance of polymers, have attracted extensive attention over the past few decades [5–8]. Polysiloxane-based Side Chain Liquid Crystal Polymers (PSCLCPs), distinguishing themselves from traditional polyacrylate-based or polymethacrylate-based SCLCPs with much-lower glass transition temperatures (Tg), viscosities, surface energy, as well as good mechanical, thermal stabilities, and insulation performance stemming from higher bond energy and larger bond angles of the adjacent silicon-oxygen bond [9], have received considerable interest and have been applied in various fields, ranging from liquid crystal display(LCD), information storage,

Polymers 2018, 10, 794; doi:10.3390/polym10070794 www.mdpi.com/journal/polymers Polymers 2018, 10, 794 2 of 37 nonlinear optics, gas separation film, etc. [10–12], especially areas that require LCPs with a lower phase transition temperature. Polymers 2018, 10, x FOR PEER REVIEW 2 of 37 PSCLCPs are constructed by attaching the rigid LC mesogens side-on or end-on fixed to the siloxaneinformation main chain storage, through nonlinear flexible optics, spacers. gas separation The siloxane film, mainetc. [10–12], chain especially can be linear areas or that cyclic, require and the rigidLCPs LC mesogens with a lower can phase be of transition any shape temperature. (such as rod, disk, or ring) or with different polarities; a tiny change inPSCLCPs structure are will constructed bring about by attaching great changes the rigid of materialsLC mesogens in nature. side‐on Comparedor end‐on fixed with to polymers the basedsiloxane on carbon-carbon main chain through main flexible chain, spacers. silicon-oxygen The siloxane main main chain chain is can easier be linear to introduce or cyclic, and various the LC mesogensrigid LC into mesogens the side can chain be of byany post-modification shape (such as rod, method,disk, or ring) making or with them different a promising polarities; functionala tiny change in structure will bring about great changes of materials in nature. Compared with polymers material. Since the first discovery of PSCLCPs by hydrosilylation between hydrogen-containing based on carbon‐carbon main chain, silicon‐oxygen main chain is easier to introduce various LC siliconemesogens oil and into vinyl the liquid side chain crystalline by post monomers‐modification in 1979method, by Finkelmannmaking them [ 13a promising], under the functional guidance of the flexiblematerial. decoupling Since the first strategy discovery [14], aof long PSCLCPs list of PSCLCPsby hydrosilylation have been between prepared hydrogen and characterized.‐containing Accordingly,silicone oil and invinyl this liquid critical crystalline review, monomers the long in list 1979 of by PSCLCPs Finkelmann is [13], summarized under the guidance from synthetic of strategiesthe flexible and the decoupling relationship strategy between [14], a long the molecular list of PSCLCPs structures have been and prepared LC phase and transition characterized. behaviors and structure,Accordingly, in order in tothis provide critical review, theoretical the long guidance list of forPSCLCPs the molecular is summarized design from of thesynthetic materials. Influencingstrategies factors and the such relationship as the structuresbetween the of molecular backbone, structures species and of LC spacers, phase transition etc. are behaviors summarized, particularlyand structure, for the in structure–phase order to provide transition theoretical behavior guidance relationship. for the molecular design of the materials. Influencing factors such as the structures of backbone, species of spacers, etc. are summarized, 2. Syntheticparticularly Methods for the ofstructure–phase Polysiloxane-Based transition Side behavior Chain relationship. Liquid Crystal Polymers (PSCLCPs)

From2. Synthetic the viewpoint Methods of Polysiloxane synthetic methods,‐Based Side the hydrosilylationChain Liquid Crystal reaction Polymers has dominated (PSCLCPs) for a long time, althoughFrom the there viewpoint are several of synthetic other methods, methods, the such hydrosilylation as esterification, reaction etherification, has dominated for etc. a Inlong recent years,time, the although preparation there of are a newseveral functional other methods, polysiloxane such as enabledesterification, the developmentetherification, etc. of more In recent efficient syntheticyears, methods the preparation such as of click a new traction functional and freepolysiloxane radical polymerization, enabled the development among others. of more In efficient this section, we willsynthetic review methods these reactions such as inclick detail. traction and free radical polymerization, among others. In this section, we will review these reactions in detail. 2.1. Hydrosilylation Reaction 2.1. Hydrosilylation Reaction Hydrosilylation reaction between polymethylhydrosiloxane (PMHS) and unsaturated carbon–carbonHydrosilylation bond based reaction monomers between under polymethylhydrosiloxane Pt catalysts was the earliest (PMHS) technique and unsaturated to obtain carbon– PSCLCPs, the successcarbon ofbond which based undoubtedly monomers under created Pt conditionscatalysts was for the the earliest study technique of the structure to obtain and PSCLCPs, performance the of success of which undoubtedly created conditions for the study of the structure and performance of the organosilicon LCs. Since the first publication of PSCLCPs in 1979 by Finkelmann [13], this method the organosilicon LCs. Since the first publication of PSCLCPs in 1979 by Finkelmann [13], this method has been a common and most-used method for the synthesis of organosilicon LCs by researchers; has been a common and most‐used method for the synthesis of organosilicon LCs by researchers; this this cancan be be ascribedascribed to the ready ready availability availability of of raw raw materials, materials, simplicity simplicity of the of themethod, method, as well as wellas easy as easy monitoringmonitoring of the of the reaction. reaction. A typicalA typical synthetic synthetic routeroute is shown shown in in Scheme Scheme 1.1 .

Scheme 1. Typical synthetic route of PSCLCPs by hydrosilylation reaction. Scheme 1. Typical synthetic route of PSCLCPs by hydrosilylation reaction.

However, the traditional hydrosilylation method has some imperfections: (1) A small number However,of unreacted the Si–H traditional bond can hydrosilylation easily crosslink during method post has‐processing, some imperfections: which is not beneficial (1) A small for numberthe of unreactedinvestigation Si–H of bond the LC can phase easily structure; crosslink (2) the during coexistence post-processing, of Markovnikov which and is anti not‐Markovnikov beneﬁcial for the investigationaddition reactions of the LC can phase result structure; in complexity (2) the of coexistence the chemical of Markovnikovstructures and performance and anti-Markovnikov of the

Polymers 2018, 10, 794 3 of 37 addition reactions can result in complexity of the chemical structures and performance of the products, limiting application; (3) difficulty in purification owing to the usage of expensive noble metal platinum catalysts often makes products go grey or black, thus proving unbeneficial for the study of the LC phase structure;Polymers 2018 (4), 10, x because FOR PEER theREVIEW molecular weight and distribution of hydrogen-containing3 of 37 organic silicon polymersproducts, limiting are different, application; the liquid (3) difficulty crystal in performance purification owing described to the byusage different of expensive authors noble may vary greatly,metal restricting platinum precise catalysts referencing. often makes For products the above go grey reasons, or black, researches thus proving are focusing unbeneficial on improvementfor the measuresstudy such of the as developmentLC phase structure; of platinum/rhodium/palladium (4) because the molecular weight and complexes distribution with of hydrogen highly efficient‐ catalyticcontaining activity, organic addition silicon of additives, polymers are and different, selection the of liquid right crystal solvents, performance and are described also seeking by other different authors may vary greatly, restricting precise referencing. For the above reasons, researches effective methods [15–20]. are focusing on improvement measures such as development of platinum/rhodium/palladium complexes with highly efficient catalytic activity, addition of additives, and selection of right 2.2. Chlorination Reaction solvents, and are also seeking other effective methods [15–20]. Chlorination reaction for PSCLCPs, which means direct polycondensation of the mesogenic 2.2. Chlorination Reaction dichlolosilanes, is an improvement of the hydrosilylation reaction between polymethylhydrosiloxane (PMHS) andChlorination unsaturated reaction carbon–carbon for PSCLCPs, bondwhich means based direct monomers. polycondensation As shown of the in mesogenic Scheme 2[ 21], the hydrosilylationdichlolosilanes, is reaction an improvement between of the the hydrosilylation vinyl mesogens reaction between and dichlorosilane polymethylhydrosiloxane was conducted (PMHS) and unsaturated carbon–carbon bond based monomers. As shown in Scheme 2 [21], the at 35–40 ◦C in the presence of hexachloroplatinic acid. The polycondensation of mesogenic hydrosilylation reaction between the vinyl mesogens and dichlorosilane was conducted at 35–40 °C dichlolosilanesin the presence with water of hexachloroplatinic was carried out acid. for The 24 polycondensation h; trimethylchlorosilane of mesogenic was dichlolosilanes then added with to produce trimethylsilyl-terminatedwater was carried out polymers. for 24 h; trimethylchlorosilane The resulting product was is then purified added by to repeatedproduce trimethylsilyl precipitation‐ using the THFterminated and methanol polymers. system, The resulting which product results is in purified a notable by repeated amount precipitation of loss of polymers.using the THF However, and as an improvedmethanol method, system, therewhich areresults several in a notable advantages. amount of It loss overcomes of polymers. the However, incompleteness as an improved of the active method, there are several advantages. It overcomes the incompleteness of the active hydrogen hydrogen addition reaction, limits easy reduction, solves the difficulty to recycle the Pt catalyst, addition reaction, limits easy reduction, solves the difficulty to recycle the Pt catalyst, and restricts and restrictsgeneration generation of crosslinking of crosslinking products stemming products from stemming unreacted from Si‐H unreacted bond. This Si-H method bond. produces This method producesmore more-ordered‐ordered PSCLCPs. PSCLCPs.

Scheme 2. Typical synthetic route of PSCLCPs by chlorination reaction. Scheme 2. Typical synthetic route of PSCLCPs by chlorination reaction. 2.3. Thiol–Ene Click Chemistry 2.3. Thiol–Ene Click Chemistry Thiol–ene click chemistry, namely hydrothiolation of a C=C bond, as a representative click Thiol–enereaction clickwith chemistry,high efficiency, namely strong hydrothiolation stereo‐selectivity of a(almost C=C bond, only anti as a‐Markovnikov representative addition click reaction with highproducts), efﬁciency, simple strong reaction stereo-selectivity conditions with (almostno side products, only anti-Markovnikov etc., is a very effective addition method products), to prepare simple polysiloxane LCs (the typical reaction route is shown in Scheme 3). Although this method was used reaction conditions with no side products, etc., is a very effective method to prepare polysiloxane in recent years to prepare polysiloxane LCs, it has been well known since 1905 [22]. It has aroused LCs (the typical reaction route is shown in Scheme3). Although this method was used in recent years to prepare polysiloxane LCs, it has been well known since 1905 [22]. It has aroused great Polymers 2018, 10, 794 4 of 37

Polymers 2018, 10, x FOR PEER REVIEW 4 of 37 interest in the development of organic synthetic chemistry, polymer synthetic chemistry, biological chemistry,great andinterest materials in the science.development By virtue of organic of the thiol–enesynthetic chemistry, reaction, functional polymer synthetic polymer chemistry, materials with linearbiological [23–25], chemistry, crosslinked and [26 materials–28], or science. networked By virtue [29,30 of] the structures thiol–ene were reaction, designed functional and polymer synthesized. Reviewsmaterials on the with mechanism linear [23–25], of thiol–enecrosslinked click [26–28], chemistry or networked and various[29,30] structures applications were designed in polymers and and materialssynthesized. synthesis Reviews were on also the reasonably mechanism elaborated of thiol–ene [31 click,32]. chemistry In regard and to LCvarious materials, applications this method in has beenpolymers widely and materials used in synthesis the preparation were also of reasonably polymer elaborated dispersed [31,32]. liquid In regard crystals to (PDLCs)LC materials, [33 –35]. H. Yangthis etmethod al. reported has been a serieswidely of used main in chainthe preparation polysiloxane of polymer liquid dispersed crystal polymers liquid crystals [36,37 (PDLCs)], side chain polysiloxane[33–35]. H. liquid Yang crystal et al. reported polymers a series [38–40 of], main and liquidchain polysiloxane crystal elastomers liquid crystal (LCEs) polymers [41,42] by [36,37], thiol–ene side chain polysiloxane liquid crystal polymers [38–40], and liquid crystal elastomers (LCEs) [41,42] click chemistry, in which only anti-Markovnikov addition products were obtained. Although this by thiol–ene click chemistry, in which only anti‐Markovnikov addition products were obtained. methodAlthough overcomes this method most shortcomings overcomes most of the shortcomings hydrosilylation of the reaction, hydrosilylation extensive reaction, relevant extensive research has not beenrelevant carried research out duehas not to thebeen relatively carried out few due types to the of relatively raw materials few types available. of raw materials available.

Scheme 3. Typical synthetic route and mechanism of PSCLCPs by thiol–ene click chemistry. Scheme 3. Typical synthetic route and mechanism of PSCLCPs by thiol–ene click chemistry. 2.4. Atom Transfer Radical Polymerization (ATRP) Technique 2.4. Atom Transfer Radical Polymerization (ATRP) Technique Atom transfer radical polymerization (ATRP), which represents a simple, inexpensive, and more Atomgeneral transfer method radical for controlled/living polymerization radical (ATRP), polymerization which represents (CRP), a simple, was first inexpensive, independently and more generaldiscovered method by for Mitsuo controlled/living Sawamoto [43] radical and polymerizationby Jin‐Shan Wang (CRP), and Krzysztof was first Matyjaszewski independently in discovered 1995 by Mitsuo[44,45]. Sawamoto It offers several [43] and advantages: by Jin-Shan tolerance Wang of and several Krzysztof functional Matyjaszewski (such as allyl, in amino, 1995 [epoxy,44,45]. and It offers severalhydroxyl) advantages: and vinyl tolerance groups of severalpresent functionalin monomers (such or initiators; as allyl, amino, ease of epoxy, preparation; and hydroxyl) commercially and vinyl available and inexpensive catalysts (copper complexes); and pyridine‐based ligands and initiators groups present in monomers or initiators; ease of preparation; commercially available and inexpensive (alkyl halides) [46,47]. During the polymerization process, the dormant species is activated by the catalysts (copper complexes); and pyridine-based ligands and initiators (alkyl halides) [46,47]. During transition metal complex to generate radicals via the one electron transfer process. Simultaneously the polymerizationthe transition metal process, is oxidized the dormant to a specieshigher state. is activated This reversible by the transition process metalrapidly complex establishes to generate an radicalsequilibrium via the onethat electronis predominately transfer shifted process. to Simultaneouslythe side with very the low transition radical concentrations. metal is oxidized The to a highernumber state. of This polymer reversible chains process is determined rapidly by establishes the number an of equilibrium initiators. Each that growing is predominately chain has the shifted to thesame side probability with very to lowpropagate radical with concentrations. monomers to form The living/dormant number of polymer chains chains (R–Pn–X). is determined As by thea result, number well of‐defined initiators. polymers Each with growing controlled chain molecular has the weights same and probability narrow molecular to propagate weight with monomersdistribution to form can living/dormant be prepared. Accordingly, polymer chains the control (R–Pn–X). over Asmolecular a result, weight well-defined and functionality polymers with controlledobtained molecular by ATRP weights allows for and the narrow synthesis molecular of numerous weight materials distribution with novel can topologies, be prepared. such Accordingly, as graft copolymers [48–51], star polymers [52–55], and hyperbranched polymers [56–59], among others. the control over molecular weight and functionality obtained by ATRP allows for the synthesis of Take advantages of ATRP, H. D. Tang et al. first successfully prepared a graft side‐chain numerous materials with novel topologies, such as graft copolymers [48–51], star polymers [52–55], crystalline polymer PI with polysiloxane as the backbone and a liquid crystal polymer, poly{6‐(40‐ and hyperbranchedoctyloxyphenyl‐400 polymers‐benzoyl)hexyl [56–59 ],acrylate}, among others.via atom transfer radical polymerization (ATRP) Taketechnique advantages (as shown of in ATRP,Scheme H. 4) [60]. D. Tang Differing et al. from first conventional successfully polysiloxane prepared LCs, a the graft prepared side-chain crystallinegraft LC polymer polysiloxane PI contains with polysiloxane no Si‐H residues, as which the backbonerestricts a subsequent and a liquid crosslinking crystal reaction polymer, poly{6-(40-octyloxyphenyl-400-benzoyl)hexylby the residual Si–H. Additionally, this method acrylate}, also effectuates via atom the high transfer density radical of mesogenic polymerization units, (ATRP) technique (as shown in Scheme4)[ 60]. Differing from conventional polysiloxane LCs, the prepared graft LC polysiloxane contains no Si-H residues, which restricts a subsequent crosslinking Polymers 2018, 10, 794 5 of 37 reactionPolymers by the2018, residual 10, x FOR PEER Si–H. REVIEW Additionally, this method also effectuates the high density of mesogenic5 of 37 units,which which have have a pronounced a pronounced effect effect on the on temperature the temperature window window of the mesophase of the mesophase and the order and degree the order degreein inthe the mesophase mesophase [61,62]. [61,62 A]. large A large number number of novel of novel polysiloxane polysiloxane LC LCmaterials materials with with complex complex topologiestopologies can can be developed be developed using using the the ATRP ATRP method. method.

Scheme 4. Typical synthetic route of PSCLCPs by atom transfer radical polymerization [60]. Scheme 4. Typical synthetic route of PSCLCPs by atom transfer radical polymerization [60]. 2.5. Esterification Reaction 2.5. Esterification Reaction Esterification reaction, which can be divided into Fischer Esterification, Steglich Esterification, EsterificationYamaguchi esterification, reaction, whichetc., is one can of be most divided commonly into used Fischer methods Esterification, in organic synthesis Steglich chemistry Esterification, Yamaguchiand has esterification, been widely used etc., in is the one manufacture of most commonly of dyes and used medical methods materials. in organic However, synthesis because chemistry the and hasreaction been is widely reversible, used it does in the not manufacture complete thoroughly. of dyes and medical materials. However, because the reaction isUtilizing reversible, the itSteglich does not esterification complete reaction, thoroughly. J. H. Wendoff et al. synthesized copolysiloxanes Utilizingwith laterally the fixed Steglich trinitrofluorenones esterification (TNF) reaction, units J. as H. side Wendoff groups et(as al. shown synthesized in Scheme copolysiloxanes 5) [63]. The number‐average molar masses of the obtained copolymers are in the range of 3500 g/mol. Due to the with laterally fixed trinitrofluorenones (TNF) units as side groups (as shown in Scheme5)[ 63]. limitations of the esterification reaction, which is especially not suitable for the preparation of The number-average molar masses of the obtained copolymers are in the range of 3500 g/mol. Due polymers, this synthetic method is not widely adopted in the synthesis of PSCLCPs. to the limitationsAcyl chloresterification of the esterification is a mild reaction, and efficient which method is especially for the synthesis not suitable of ester for thecompounds; preparation it of polymers,can be this classified synthetic as Steglich method Esterification. is not widely Accordingly, adopted this in the reaction synthesis can also of PSCLCPs. be used in the synthesis Acylof polysiloxane chloresterification LCs, which is can a mild easily and control efficient content method and introduce for the synthesis LC mesogenic of ester units. compounds; H. D. Tang it can be classifiedet al. first as Steglichsynthesized Esterification. polysiloxanes Accordingly, with carboxyphenyl this reaction group can in also the be side used chain; in the then synthesis via of polysiloxanemacromolecule LCs, which esterification, can easily LC control polysiloxanes content andwith introduce side chain LC of mesogenic chromophore units. groups H. D. Tangwere et al. first synthesizedsuccessfully obtained polysiloxanes with a high with yield, carboxyphenyl the structures group of which in the were side confirmed chain; then by 1H via‐NMR, macromolecule IR and esterification,elemental LCanalysis polysiloxanes (as shown in with Scheme side 6) chain [64]. of chromophore groups were successfully obtained with a high yield, the structures of which were confirmed by 1H-NMR, IR and elemental analysis (as shown in Scheme6)[64].

Polymers 2018, 10, 794 6 of 37 Polymers 2018, 10, x FOR PEER REVIEW 6 of 37 Polymers 2018, 10, x FOR PEER REVIEW 6 of 37

Scheme 5. Typical synthetic route of PSCLCPs by Steglich esterification reaction [63]. Scheme 5. Typical synthetic route of PSCLCPs by Steglich esteriﬁcation reaction [63]. Scheme 5. Typical synthetic route of PSCLCPs by Steglich esterification reaction [63].

Scheme 6. Typical synthetic route of PSCLCPs by acyl chloresterification. SchemeScheme 6. Typical6. Typical synthetic synthetic route route ofof PSCLCPs by by acyl acyl chloresterification. chloresterification. 2.6. Williamson Nucleophilic Substitution 2.6. Williamson Nucleophilic Substitution 2.6. WilliamsonWilliamson Nucleophilic nucleophilic Substitution substitution is a common method to prepare ether compounds, and is of greatWilliamson significance nucleophilic in organic substitution synthesis. isUsing a common this reaction, method Zhang to prepare X. L. ether et al. compounds, synthesized andnovel is Williamsonofcyclotetrasiloxane great significance nucleophilic materials in organic substitution with synthesis. mesogens is Using aor common cyclotetrasiloxane this reaction, method Zhang toLCs prepare basedX. L. et on ether al. halogen synthesized compounds,‐substituted novel and is of greatcyclotetrasiloxane significance in materialsunder organic a phase with synthesis. mesogenstransfer Usingcatalyst or cyclotetrasiloxane this (as shown reaction, in Scheme Zhang LCs based 7) X. [65,66]. L. on et halogen al. However, synthesized‐substituted due to novel cyclotetrasiloxanecyclotetrasiloxanethe influence of materials the under substrate a with phase reactivity, mesogens transfer catalystmolecular or cyclotetrasiloxane (as structuresshown in Schemeand LCs other based7) [65,66]. factors, on However, halogen-substituted the Williamson due to the influence of the substrate reactivity, molecular structures and other factors, the Williamson cyclotetrasiloxane under a phase transfer catalyst (as shown in Scheme7)[ 65,66]. However, due to the influence of the substrate reactivity, molecular structures and other factors, the Williamson nucleophilic Polymers 2018, 10, 794 7 of 37 Polymers 2018, 10, x FOR PEER REVIEW 7 of 37 Polymers 2018, 10, x FOR PEER REVIEW 7 of 37 nucleophilic substitution reaction has been found to be suitable for the synthesis of polysiloxane nucleophilicsubstitution reactionsubstitution has beenreaction found has to been be suitablefound to for be the suitable synthesis for ofthe polysiloxane synthesis of liquidpolysiloxane crystal liquid crystal materials with a relative low degree of polymerization. liquidmaterials crystal with materials a relative with low a degree relative of low polymerization. degree of polymerization.

Scheme 7. Typical synthetic route of PSCLCPs by Wiliamson nucleophilic substitution. SchemeScheme 7. 7. TypicalTypical synthetic route of PSCLCPs by Wiliamson nucleophilic substitution. 2.7. Supramolecular Interaction 2.7. Supramolecular Interaction Supramolecular chemistry is the study of entities of greater complexity than individual Supramolecular chemistry is the study of entities of greater complexity than individual molecules—assembliesSupramolecular chemistry of molecules is that the bond study and of organize entities ofthrough greater intermolecular complexity interactions. than individual The molecules—assemblies of molecules that bond and organize through intermolecular interactions. The designmolecules—assemblies and synthesis of moleculessupramolecular that bond systems and organize invokes through interactions intermolecular beyond the interactions. conventional The design and synthesis of supramolecular systems invokes interactions beyond the conventional chemicaldesign and bond, synthesis using, of for supramolecular example, hydrogen systems bonding, invokes interactions Van der Waals beyond forces, the conventional dipole‐dipole, chemical and π chemical bond, using, for example, hydrogen bonding, Van der Waals forces, dipole‐dipole, and π interactionsbond, using, to for bring example, discrete hydrogen building bonding, blocks together. Van der Waals It is of forces, great dipole-dipole,significance in andthe fieldπ interactions of liquid interactions to bring discrete building blocks together. It is of great significance in the field of liquid crystalto bring science discrete buildingto construct blocks liquid together. crystal It is ofand great study significance its properties in the field and of liquidapplications crystal scienceusing crystal science to construct liquid crystal and study its properties and applications using supramolecularto construct liquid interactions. crystal and study its properties and applications using supramolecular interactions. supramolecular interactions. HydrogenHydrogen-bonded‐bonded liquid crystalline polymers with well well-defined‐defined structures, structures, such such as as main main-chain,‐chain, Hydrogen‐bonded liquid crystalline polymers with well‐defined structures, such as main‐chain, sideside-chain,‐chain, combined, combined, and and network network structures, structures, have havebeen prepared been prepared by the self by‐assembly the self-assembly of polymers of side‐chain, combined, and network structures, have been prepared by the self‐assembly of polymers andpolymers small andmolecules. small molecules.The introduction The introductionof non‐covalent of non-covalentbonding as a bonding key for asthe a formation key for the of and small molecules. The introduction of non‐covalent bonding as a key for the formation of molecularlyformation of assembled molecularly structures assembled yields structures a new type yields of dynamically a new type functional of dynamically materials. functionalKato et al. molecularly assembled structures yields a new type of dynamically functional materials. Kato et al. hypothesizedmaterials. Kato proper et al. control hypothesized of hydrogen proper bonding, control greatly of hydrogen inducing bonding, the molecular greatly inducing ordering the of hypothesized proper control of hydrogen bonding, greatly inducing the molecular ordering of thermotropicmolecular ordering LCs to obtain of thermotropic a new class LCsof side to‐chain obtain liquid a new crystalline class of polymers side-chain [67,68]. liquid Based crystalline on the thermotropic LCs to obtain a new class of side‐chain liquid crystalline polymers [67,68]. Based on the abovepolymers idea [67 ,and68]. Basedstrategy, on theKato above et ideaal. synthesized and strategy, poly(methylsi1oxanes)Kato et al. synthesized poly(methylsi1oxanes)and poly(methy1‐co‐ above idea and strategy, Kato et al. synthesized poly(methylsi1oxanes) and poly(methy1‐co‐ dimethylsiloxanes)and poly(methy1-co-dimethylsiloxanes) with side chains containing with side 4 chains‐alkoxybenzoic containing acid 4-alkoxybenzoic pendant groups acid attached pendant dimethylsiloxanes) with side chains containing 4‐alkoxybenzoic acid pendant groups attached throughgroups attached aliphatic through spacers aliphatic for use spacers as H for‐bond use as donor H-bond polymers, donor polymers, and stilbazole and stilbazole derivatives derivatives as through aliphatic spacers for use as H‐bond donor polymers, and stilbazole derivatives as representativeas representative mesogenic mesogenic or or nonmeqogenic nonmeqogenic H H-bond‐bond acceptors, acceptors, respectively. respectively. Formation Formation of of the the liquid representative mesogenic or nonmeqogenic H‐bond acceptors, respectively. Formation of the liquid crystalline polymeric complexes occurs occurs by by self self-assembly‐assembly between between the the carboxylic acid acid group of the crystalline polymeric complexes occurs by self‐assembly between the carboxylic acid group of the polysiloxanes with the stilbazoles through H H-bonding‐bonding as confirmed confirmed by spectroscopic techniques (as polysiloxanes with the stilbazoles through H‐bonding as confirmed by spectroscopic techniques (as shown in Scheme 88))[ [69–71].69–71]. shown in Scheme 8) [69–71].

Scheme 8. Structure of self‐assembled hydrogen‐bonded supramolecular PSCLCPs [69–71]. Scheme 8. StructureStructure of self self-assembled‐assembled hydrogen hydrogen-bonded‐bonded supramolecular PSCLCPs [[69–71].69–71]. Additionally, ion‐counterion interactions may be of importance to liquid crystal ionic polymers Additionally, ion ion-counterion‐counterion interactions interactions may may be of be importance of importance to liquid to crystal liquid ionic crystal polymers ionic [72]. A. Blumstein et al. synthesized a homopolysiloxane with sulfonic ionic site internal and adjacent polymers[72]. A. Blumstein [72]. A. et Blumstein al. synthesized et al. synthesized a homopolysiloxane a homopolysiloxane with sulfonic with ionic sulfonic site internal ionic and site adjacent internal to the flexible spacer; the interaction of sulfonic ion with pyridine moiety of the small molecule LC toand the adjacent flexible to spacer; the ﬂexible the interaction spacer; the of interaction sulfonic ion of with sulfonic pyridine ion with moiety pyridine of the moietysmall molecule of the small LC resulted in liquid crystalline ionic polysiloxane with quaternized ammonium groups incorporated resultedmolecule in LC liquid resulted crystalline in liquid ionic crystalline polysiloxane ionic polysiloxanewith quaternized with ammonium quaternized groups ammonium incorporated groups [73]. B. Y. Zhang et al. also reported the synthesis and characterization of PSLCP ionomers with the [73].incorporated B. Y. Zhang [73 ].et al. B. Y.also Zhang reported et al. the also synthesis reported and the characterization synthesis and of characterization PSLCP ionomers of with PSLCP the sulfonic acid group [74,75]; they were, however, obtained by hydrosilylation reaction of sulfonicionomers acid with group the sulfonic [74,75]; acid they group were, [74 ,75however,]; they were, obtained however, by hydrosilylation obtained by hydrosilylation reaction of polymethylhydrosiloxane and monomers containing sulfonic acid pendant, which is out of scope of polymethylhydrosiloxanereaction of polymethylhydrosiloxane and monomers and monomers containing containing sulfonic acid sulfonic pendant, acid pendant,which is out which of scope is out of this article. thisscope article. of this article.

Polymers 2018, 10, 794 8 of 37

3. The Relationship between the Molecular Structures and LC Phase Transitions & Structures of Polysiloxane-Based Side Chain Liquid Crystal Polymers (PSCLCPs) Traditional SCLCPs consist of three fundamental parts: the backbone, mesogenic units, and flexible spacers; which determine the generation ability of the LC phase, the LC phase structure type, and phase stability, etc. In this section, we will systematically review the relationship between the molecular structures, and phase transition behaviors and phase structures. To offer more clarity, this section is divided into two parts: The first part discusses the dependence of the type and nature of the main chain, the relative polymerization degree (DP) of the linear polysiloxanes, the ring size of the cyclic polysiloxanes, as well as the shape of the main chain on phase structures and phase behaviors. In the second part, we examine the dependence of the three main spacers (oligooxyethylene, oligomethylene, and thioether) on phase structures and phase behaviors. Special attention is paid to the effect of the heteroatom (such as O, S) in the spacers and their properties. In regard to the dependence of mesogenic units on phase structures and phase behaviors, in addition to the influence of diverse rigid structures [76–78]; the lateral substituents of the mesogenic side groups [79–81], the aliphatic tail length of the mesogenic groups [82–85], the chirality [86–89] and polarity [90] of the mesogenic groups, and the functionality of the mesogenic groups (such as photo-sensitive property [91–94], amphiphilic property [95,96], etc.) exerted a strong effect on mesomorphic properties. Due to the structural diversity and complexity of mesogenic units, the dependence of mesogenic units on phase behaviors and phase structures of the PSCLCPs is not discussed.

3.1. Dependence of the Main Chain on Phase Transition Behaviors & Phase Structures of the PSCLCPs

3.1.1. Dependence of the Type and Nature of the Polysiloxanes Main Chain Common polymer backbones of SCLCPs are categorized as poly(meth)acrylate, polyvinyl ether, polystyrene, polyacrylamide, polysiloxanes, etc. They can be divided into polyhydrocarbon backbone and polysiloxane backbone, the flexibility and physical properties of which differ. Accordingly, there will be significant differences in the mesophase behaviors. G. W. Gray FRS, Dr. D. Lacey, and Dr. P. A. Gemmell at Hull University engaged the hydrosilylation reaction to prepare a series of polysiloxanes with 4-cyano-2-methylphenyl benzoate mesogenic unit and different spacer lengths; the chemical structures and their phase transitions data by DSC determination are presented in Figure1 and Table1. R. Simon et al. then investigated the dielectric relaxation property of the polymers (discussed in Table1[ 97]). This is evident from a gradual reduction of the activation energy on increasing the temperature for all three polymers in the loss process. The activation energies estimated at a constant reduced temperature (T = 0.98Tcl) decrease from 172 and 190 kJ mol−1 for nematic polymers with short polymethylene spacers to a value of 95 kJ mol−1 for smectic polymer with a longer spacer. In comparison with polyacrylates based on similar structure and polarity of cyanobiphenyl side groups reported by Parneix et al. [98], activation energies ranged from 241 kJ mol−l in a nematic material to 126 kJ mol−1 for a smectic polymer; the higher values in evaluation with our results reflect on the fact that the polyacrylate backbone is less flexible than the polysiloxane one. Furthermore, the added relative rigidity of the polyacrylate backbone also generated higher glass transition temperature. V. Percec synthesized polymethacrylates (40-n-PMA) and polyacrylates (40-n-PAC) containing 4-hydroxy-40-methoxy-a-methylstilbene side groups attached to flexible spacers containing 11, 8, 6, 3, and 2 methylenic units and of polysiloxanes (40-n-PS) containing the same mesogenic group connected through flexible spacers containing 11, 8, 6, and 3 methylenic units, respectively [99]. The chemical structures of the polymers and systematic comparison data are depicted in Figure2 and Table2. Results revealed that the nature of the polymer backbone determines the thermal stability of the mesophase and the ease of the side-chain crystallization. That is, for the same spacer length and similar polymer molecular weight, the most-flexible polymer backbone leads to the highest isotropization temperature Polymers 2018, 10, 794 9 of 37 and the highest ease of side-chain crystallization. This may lead to transformation of enantiotropic mesophases into monotropic mesophases. Polymers 2018, 10, x FOR PEER REVIEW 9 of 37 Table 1. The phase transition and activation energies of polysiloxanes and polyacrylate polymer [97,98]. Table 1. The phase transition and activation energies of polysiloxanes and polyacrylate polymer Activation Energies (kJ mol−1) [97,98]. Spacer Polymer Phase Transitions (◦C) Length At Low Temperatures Isotropic Activation EnergiesAt (kJ 0.98 molTcl −1) Spacer in LC Phase Phase Polymer Phase Transitions (°C) At Low Temperatures Isotropic polysiloxaneLength 5 G 19 N 65 I 202At 0.98 172Tcl 142 polysiloxane 6 G 7 N 48 Iin LC 238 Phase 190Phase 144 polysiloxanepolysiloxane 85 G G 19 9 N S 102 65 I I 202 146 172 95 142 —- polysiloxane 6 G 7 N 48 I 238 190 − 144 Spacer Activation Energies (kJ mol 1) Polymerpolysiloxane 8 PhaseG Transitions 9 S 102 I (◦C) 146 95 –– Length N S NI Spacer ReActivation EnergiesAd (kJ mol−1) Polymer Phase Transitions (°C) polyacrylateLength 6 G 32 NRe 80 SAd 124.5 N 132 I N241Re S 126Ad N 193 I 83

polyacrylateNote: G: glass state;6 N: nematicG 32 NRe phase; 80 SAd 124.5 S: smectic N 132 A I phase; I: isotropic241 phase; Tcl:126 temperature 193 at the83 liquid crystalline-isotropic phase transition; N : re-entrant nematic phase; S : smectic A phase with a double layer. Note: G: glass state; N: nematic phase;Re S: smectic A phase; I: isotropicAd phase; Tcl: temperature at the

liquid crystalline‐isotropic phase transition; NRe: re‐entrant nematic phase; SAd: smectic A phase with 0 aTable double 2. layer.Thermal transition and thermodynamic parameters of polymethacrylates (4 -n-PMA), polyacrylates (40-n-PAC), and polysiloxanes (40-n-PS) [99]. Table 2. Thermal transition and thermodynamic parameters of polymethacrylates (4′‐n‐PMA), GPC Phase Transitions(◦C) and Corresponding Enthalpy Changes (kcal/mru−1) Polymethacrylatespolyacrylates (4′‐n‐PAC), and polysiloxanes (4′‐n‐PS) [99]. 0 −3 (4 -n-PMA) 10 Mn Mw/Mn Heating Cooling −1 Polymethacrylates40-11-PMA 23.4GPC 2.0Phase Transitions(°C) G 12 S 116 (1.25) and ICorresponding Enthalpy Changes I 105 (1.13) (kcal/mru S 4 G ) 0 −3 ഥ ഥ ഥ 4(4-11-PMA′‐n‐PMA) a 10 ࡹ23.4n ࡹw/ࡹn 2.0 G 15 K 55 (0.55) KHeating 83 (0.39) S 119 (0.99) I I 105 (1.13)Cooling S 4 G 44′‐011-8-PMA‐PMA 23.4 24.1 2.0 2.0 G 18 S 55G 12 (0.29) S 116 N (1.25) 112 (0.43) I I I 108 (0.43)I 105 N 53 (1.13) (0.21) S 4 S G 15 G 4′‐4110-6-PMA‐PMA a 23.4 24.7 2.0 2.5G 15 K 55 G (0.55) 32 N K 101 83 (0.39) (0.26) S I 119 (0.99) I I 97I (0.21) 105 (1.13) N 24 S G4 G 4′‐0-3-PMA8‐PMA 24.1 43.6 2.0 2.5 GG 55 18 S S 85 55 (0.05) (0.29) N N 115 112 (0.17)(0.43) I I 110I 108 (0.16) (0.43) N 75N 53 (0.03) (0.21) S 42S 15 G G Polyacrylates4′‐6‐PMA 24.7 2.5 G 32 N 101 (0.26) I I 97 (0.21) N 24 G 10−3M M /M Heating Cooling (44′‐0-n-PAC)3‐PMA 43.6 n 2.5 w n G 55 S 85 (0.05) N 115 (0.17) I I 110 (0.16) N 75 (0.03) S 42 G Polyacrylates 0 10−3ࡹഥ n ࡹഥ w/ࡹഥ n Heating c Cooling (44′‐-11-PACn‐PAC) 15.9 2.0 G 12 K 107 (2.86) S 115 (0.80) I I 107 (0.72) S 37 (1.08) K 3 G G 11 S 66 (0.03) N 83 (0.39) K 99 (0.55) N 44′‐011-8-PAC‐PAC 15.9 12.2 2.0 1.8 G 12 K 107 (2.86) S 115 c (0.80) I I 97I (0.23)107 (0.72) N 63 S 37 (0.03) (1.08) S 6K G 3 G 104 (0.07) c 4′‐8‐PAC 12.2 1.8 G 11 S 66 (0.03) N 83 (0.39) K 99 (0.55) N 104 (0.07) c I 97 (0.23) N 63 (0.03) S 6 G 40-6-PAC 14.0 2.0 G 18 N 101 (0.11) I I 97 (0.10) N 14 G 4′‐6‐PAC 14.0 2.0 G 18 N 101 (0.11) I I 97 (0.10) N 14 G 40-6-PAC b 14.0 2.0 K 55 (1.09) N 103 (0.06) I I 97 (0.10) N 14 G 4′‐6‐PAC b 14.0 2.0 K 55 (1.09) N 103 (0.06) I I 97 (0.10) N 14 G 40-3-PAC 16.3 1.9 G 38 K 59 (0.02) N 81 (0.03) I I 76 (0.10) N 14 G 4′‐3‐PAC 16.3 1.9 G 38 K 59 (0.02) N 81 (0.03) I I 76 (0.10) N 14 G Polysiloxanes −3 0 1010−3ࡹഥ n Mn ࡹഥ w/Mࡹഥwn /Mn HeatingHeating CoolingCooling (4′‐-n-PS)n‐PS) 4′‐0-11-PS11‐PS 13.4 13.4 1.8 1.8 KK 127 127 (3.81) (3.81) S S 138 138 (1.20) (1.20) dd I I 131I 131 (0.76) (0.76) S 90 S 90 (3.75) (3.75) K K 4′‐0-8-PS8‐PS 16.4 16.4 2.0 2.0 KK 128 128 (4.46) (4.46) I I I 117I 117 (0.22) (0.22) N 89N 89 (3. (3. 54) 54) K K 4′‐0-6-PS6‐PS 19.6 19.6 1.9 1.9 GG 10 10 K K 124 124 (3.95) (3.95) II I 103I 103 (0.09) (0.09) N 81N 81 (3.09) (3.09) K K 4′‐0-3-PS3‐PS 14.3 14.3 2.0 2.0 GG 22 22 K K 84 84 (1.43) (1.43) N N 110 110 (0.12)(0.12) I I 107I 107 (0.10) (0.10) N 65N 65 (1.33) (1.33) K K aa Obtained from from ﬁrst first heating heating and and cooling cooling scans; scans;b ﬁrst heating b first andheating cooling and scans; coolingc, d overlapped scans; c, d transitions;overlapped G: transitions;glass state; N: G: nematic glass state; phase; N: S: smecticnematic A phase; phase; I:S: isotropic smectic phase. A phase; I: isotropic phase.

Figure 1. Chemical structures of the polysiloxane and polyacrylate polymers [97]. Figure 1. Chemical structures of the polysiloxane and polyacrylate polymers [97].

Polymers 2018, 10, 794 10 of 37 Polymers 2018, 10, x FOR PEER REVIEW 10 of 37

Polymers 2018, 10, x FOR PEER REVIEW 10 of 37

Figure 2. ChemicalChemical structures structures of ofthe the Polymethacrylates Polymethacrylates (4′‐n (4‐PMA),0-n-PMA), Polyacrylates Polyacrylates (4′‐n‐ (4PAC),0-n-PAC), and andPolysiloxanes Polysiloxanes (4′‐n (4‐PS)0-n-PS) containing containing 4‐hydroxy 4-hydroxy-4‐4′‐methoxya0-methoxya-methylstilbene‐methylstilbene based based mesogens mesogens [99]. [ 99]. Figure 2. Chemical structures of the Polymethacrylates (4′‐n‐PMA), Polyacrylates (4′‐n‐PAC), and AnotherPolysiloxanes work reported (4′‐n‐PS) containing by V. Percec 4‐hydroxy was‐4 ′‐vinylmethoxya ether‐methylstilbene monomers based and mesogens methacrylate [99]. containing Another work reported by V. Percec was vinyl ether monomers and methacrylate containing benzo‐15‐crwon‐5 groups, by hydrosilylation reaction and conventional free radical polymerization, benzo-15-crwon-5 groups, by hydrosilylation reaction and conventional free radical polymerization, respectively;Another the workfirst reportedexamples by V.of PercecSCLCPs was containingvinyl ether monomersmacroheterocyclic and methacrylate ligands containing within their respectively;benzo‐15 the‐crwon first‐5 examples groups, by of hydrosilylation SCLCPs containing reaction macroheterocyclic and conventional ligandsfree radical within polymerization, their mesogenic mesogenic side groups and different backbones were presented (denoted as PSC‐11 and PMC‐11) [100]. siderespectively; groups and the different first examples backbones of SCLCPs were presentedcontaining (denotedmacroheterocyclic as PSC-11 ligands and within PMC-11) their [ 100]. Figure 3 shows the chemical structures of these polymers; the corresponding physical properties are Figuremesogenic3 shows side the groups chemical and different structures backbones of these were polymers; presented the (denoted corresponding as PSC‐11 and physical PMC‐11) properties [100]. summarized in Table 3. Similar to the results mentioned above, PMC‐11 has a polymethacrylate are summarizedFigure 3 shows in the Table chemical3. Similar structures to the of results these polymers; mentioned the above, corresponding PMC-11 physical has a polymethacrylate properties are backbone and undergoes isotropization at 166 °C, while PSC‐11 with a polysiloxane backbone backbonesummarized and undergoes in Table 3. isotropizationSimilar to the results at 166 mentioned◦C, while above, PSC-11 PMC with‐11 has a polysiloxane a polymethacrylate backbone undergoesbackbone isotropization and undergoes at 216isotropization °C. In addition, at 166 PSC°C, while‐11 displays PSC‐11 awith second a polysiloxane highly ordered backbone smectic undergoes isotropization at 216 ◦C. In addition, PSC-11 displays a second highly ordered smectic mesophase.undergoes This isotropization last result agreesat 216 °C.again In addition,with previous PSC‐11 results displays concerning a second highly the influence ordered smecticof polymer mesophase. This last result agrees again with previous results concerning the influence of polymer backbonemesophase. flexibility This onlast mesomorphic result agrees again phase with transitions previous [101–103].results concerning the influence of polymer backbonebackbone flexibility flexibility on mesomorphicon mesomorphic phase phase transitions transitions [101–103]. [101–103].

Figure 3. The chemical structures of PSC‐11 and PMC‐11 [100]. Figure 3. The chemical structures of PSC-11 and PMC-11 [100]. Figure 3. The chemical structures of PSC‐11 and PMC‐11 [100]. Table 3. Phase transition behaviors of PSC‐11 and PMC‐11 [100]. Table 3. Phase transition behaviors of PSC-11 and PMC-11 [100]. TableGPC 3. Phase transitionPhase Transitions(°C)behaviors of and PSC Corresponding‐11 and PMCEnthalpy‐11 Changes [100]. (kcal/mru −1) Compounds ࡹഥ n ࡹഥ w/ࡹഥ n Heating Cooling ◦ −1 PSC‐11 GPC5700GPC 1.3 GPhase 50Phase S1 106 Transitions( Transitions(°C)(0.60) S2 168 (0.80)C) and and N 216 Corresponding (0.18) I I 208 Enthalpy Enthalpy (0.18) N Changes159 Changes (0.86) S2(kcal/mru 88 (kcal/mru (0.64) −S11) ) CompoundsCompounds PMC‐11 ࡹഥ n14,600 ࡹഥ w ࡹഥ2.8n G 50 S 129 (0.54) N 166 (0.37) I I 160 (0.39) N 123 (0.57) S 49 G Mn Mw/M/ n Heating Cooling Cooling PSC‐11 5700 G: glass1.3 state; GN: 50 nematic S1 106 (0.60) phase; S2 168 S: smectic(0.80) N 216phase; (0.18) I: I isotropicI 208 phase.(0.18) N 159 (0.86) S2 88 (0.64) S1 PSC-11PMC‐11 570014,600 1.32.8 G 50 S1G 106 50 (0.60)S 129 (0.54) S2 168 N (0.80) 166 (0.37) N 216 I (0.18) I I 208I 160 (0.18) (0.39) N 159N 123 (0.86) (0.57) S2 S 8849 G (0.64) S1 PMC-11 14,600 2.8 G 50 S 129 (0.54) N 166 (0.37) I I 160 (0.39) N 123 (0.57) S 49 G Chain‐Shu HsuG: glass et al. state; designed N: nematic and synthesized phase; S: smectic side‐chain phase; liquid I: isotropic crystalline phase. polysiloxanes and G: glass state; N: nematic phase; S: smectic phase; I: isotropic phase. polymethacrylates that contained 4‐[(S)‐2‐methyl‐l‐butoxy]phenyl4‐hydroxybiphenyl‐4′‐carboxylate Chain‐Shu Hsu et al. designed and synthesized side‐chain liquid crystalline polysiloxanes and polymethacrylates that contained 4‐[(S)‐2‐methyl‐l‐butoxy]phenyl4‐hydroxybiphenyl‐4′‐carboxylate

Polymers 2018, 10, 794 11 of 37

Polymers 2018, 10, x FOR PEER REVIEW 11 of 37 Chain-Shu Hsu et al. designed and synthesized side-chain liquid crystalline polysiloxanes and 0 moietiespolymethacrylates as mesogenic that units contained and 3 4-[(S)-2-methyl- to 11 methyleneL -butoxy]phenyl4-hydroxybiphenyl-4units as aliphatic spacers (the chemical-carboxylate structures ofmoieties the polymers as mesogenic are unitsshown and in 3 toFigure 11 methylene 4). Differential units as aliphaticscanning spacers calorimetry, (the chemical optical structures polarizing of microscopy,the polymers and are shownX‐ray diffraction in Figure4 ).measurements Differential scanning revealed calorimetry, that flexible optical polymer polarizing backbones microscopy, enhance theand decoupling X-ray diffraction of the measurements motions of the revealed side chain that and ﬂexible main polymer chain and backbones therefore enhance tend to the give decoupling rise to a higherof the motions thermal ofstability the side of chainthe mesophases, and main chain including and therefore the chiral tend smectic to give C phase rise to (as a summarized higher thermal in Tablestability 4) [104]. of the mesophases, including the chiral smectic C phase (as summarized in Table4)[104].

Figure 4. ChemicalChemical structures of the polysiloxanes and polymethacrylates [[104].104].

Table 4. Thermal transitions and phase‐transition enthaplies for polysiloxanes and polymethacrylates Table 4. Thermal transitions and phase-transition enthaplies for polysiloxanes and polymethacrylates [104]. [104].

PolysiloxanesPolysiloxanes Polymethacrylates Polymethacrylates Spacer Length Spacer Length (n) PhasePhase Transitions/°C Transitions/◦ C(Enthalpies/kcal (Enthalpies/kcal mru mru−1)− 1) PhasePhase Transitions/°C Transitions/◦C (Enthalpies/kcal (Enthalpies/kcal mru −1)1 ) (n) Heating/CoolingHeating/Cooling Heating/CoolingHeating/Cooling G 25G 25SA S117117 (1.82) (1.82) I IK K 81 81 (6.65) (6.65) S SB 8686 (0.58) (0.58) S SA 147147 (1.07) (1.07) I I 3 3 A B A I 99I 99(1.80) (1.80) SA S A I I145 145 (1.01) (1.01) S SAA 8585 (0.55) (0.55) S SB B3434 (4.08) (4.08) K K G 18G 18SA S84 84(0.78) (0.78) I I 4 4 A I 80I 80(0.42) (0.42) SA S A G 22 SB 99 (0.89) SA 212 (0.86) I 5 G 22 SB 99 (0.89) SA 212 (0.86) I 5 I 204 (0.68) SA 89 (0.82) SB I 204 (0.68) SA 89 (0.82) SB a G 16 SB120 (1.73) SC* 166 (–) SaA 244 (1.24) I K 101 (1.97) SB 123 (0.58) SA 149 (0.82) I 6 G 16 SB120 (1.73) SC* 166 (–) SA 244 (1.24) I K 101 (1.97) SB 123 (0.58) SA 149 (0.82) I 6 I 236 (0.98) SA 164 (–) a SaC* 109 (1.47) SB I 142 (0.97) SA 119 (0.53) SB 96 (2.00) K I 236 (0.98) SA 164 (–) SC* 109 (1.47) SB I 142 (0.97) SA 119 (0.53) SB 96 (2.00) K G 20 SB109 (0.78) SC* 143 (–) a SA 218 (0.86) I K 88 (1.45) SC* 125 (–) a SA 169 (0.66) I 11 a a G 20 SB109 (0.78) SC* 143a (–) SA 218 (0.86) I K 88 (1.45) SC* 125a (–) SA 169 (0.66) I 11 I 210 (0. 89) SA 142 (–) SaC* 103 (0. 74) SB I 165 (0.61) SA 120 (–) aSC* 81 (0.63) SB 65 (0.07) K I 210 (0. 89) SA 142 (–) SC* 103 (0. 74) SB I 165 (0.61) SA 120 (–) SC* 81 (0.63) SB 65 (0.07) K a a Enthalpy is very small; G: glass state; SA: smectic A phase; SB: smectic B phase; SC*: chiral smectic C Enthalpy is very small; G: glass state; SA: smectic A phase; SB: smectic B phase; SC*: chiral smectic C phase; I: phase;isotropic I: phase. isotropic phase.

Kenneth J. Toyne synthesized a series of chiral, nematic, laterally attached polymers based on Kenneth J. Toyne synthesized a series of chiral, nematic, laterally attached polymers based on polysiloxane, polyacrylate and polymethacrylate backbones, and first showed the comparisons of the polysiloxane, polyacrylate and polymethacrylate backbones, and ﬁrst showed the comparisons of the effect of different natures of the polymer backbones on phase behaviors (as shown in Figure 5) [105]. effect of different natures of the polymer backbones on phase behaviors (as shown in Figure5)[ 105]. Consistent with those reported for polymers with terminally attached side‐chains, a decrease in Consistent with those reported for polymers with terminally attached side-chains, a decrease in mesophase thermal stability and an increase in glass‐transition temperatures were obtained with mesophase thermal stability and an increase in glass-transition temperatures were obtained with increase in rigidity of the polymer backbone. Furthermore, the effects of the different polymer increase in rigidity of the polymer backbone. Furthermore, the effects of the different polymer backbones in the side‐chain systems are more reduced, compared to the related polymers without backbones in the side-chain systems are more reduced, compared to the related polymers without side‐chains (as summarized in Table 5). side-chains (as summarized in Table5).

Polymers 2018, 10, 794 12 of 37 Polymers 2018, 10, x FOR PEER REVIEW 12 of 37

FigureFigure 5.5.Chemical Chemical structures structures of the polymersof the polymers based on polysiloxane, based on polyacrylatepolysiloxane, and polyacrylate polymethacrylate and backbonespolymethacrylate [105]. backbones [105].

TableTable 5.5. TransitionTransition temperatures temperatures and and polymer polymer parameters parameters [ 105[105].].

Transition Temperatures/°C Polymers ◦ Molecular Weight (ࡹഥ n) Polydispersity (γ) Transition(Enthalpies/J Temperatures/ g−1) C Molecular Weight Polymers −1 Polydispersity (γ) PS G(Enthalpies/J 22.6 Ch 123.8 (0.27) g ) I 16,300(M n) 1.30 PSPA GG 22.6 30.1 Ch Ch 123.8 110.8 (0.36) (0.27) I I9250 16,300 1.35 1.30 PAPMA GG 30.1 37.8 Ch Ch 110.8 101.5 (0.18) (0.36) I I18,900 9250 1.66 1.35 PMA G 37.8G: glass Ch 101.5 state; (0.18) Ch: cholesteric I phase; 18,900 I: isotropic phase. 1.66 G: glass state; Ch: cholesteric phase; I: isotropic phase. Q. F. Zhou synthesized a series of mesogen‐jacketed liquid crystalline polymers (MJLCPs) with polysiloxane backbones by hydrosilylation of polymethylhydrosiloxane with styrenic derivatives (as Q. F. Zhou synthesized a series of mesogen-jacketed liquid crystalline polymers (MJLCPs) with listed in Table 6) [106]. Compared with structurally similar MJLCPs having a polyethylene backbone polysiloxane backbones by hydrosilylation of polymethylhydrosiloxane with styrenic derivatives [107–110], these MJLCPs based on highly flexible polysiloxane main chain could also self‐assemble (as listed in Table6)[ 106]. Compared with structurally similar MJLCPs having a polyethylene into supramolecular columnar nematic or smectic liquid crystalline phases; however, the stability of backbone [107–110], these MJLCPs based on highly flexible polysiloxane main chain could also the LC phase was greatly decreased. Contributing to the combined effect of the flexible polysiloxane self-assemble into supramolecular columnar nematic or smectic liquid crystalline phases; however, backbone and the less‐rigid side chain structure with the introduction of the ethylene spacer, the glass the stability of the LC phase was greatly decreased. Contributing to the combined effect of the flexible transition temperatures (Tg’s) of the polysiloxane polymers were about 10 °C lower, and their liquid polysiloxane backbone and the less-rigid side chain structure with the introduction of the ethylene crystalline ranges were narrowed significantly (as summarized in Table 7). spacer, the glass transition temperatures (T ’s) of the polysiloxane polymers were about 10 ◦C lower, g and their liquid crystalline ranges were narrowed significantly (as summarized in Table7).

Polymers 2018, 10, 794 13 of 37 Polymers Polymers2018, 10, 2018x FOR, 10 PEER, x FOR REVIEW PEER REVIEW 13 of 37 13 of 37 Polymers Polymers2018, 10, 2018x FOR, 10 PEER, x FOR REVIEW PEER REVIEW 13 of 37 13 of 37 Polymers Polymers2018, 10, 2018x FOR, 10 PEER, x FOR REVIEW PEER REVIEW 13 of 37 13 of 37 Polymers Polymers2018, 10, x2018 FOR, 10 PEER, x FOR REVIEW PEERTable REVIEW 6. Chemical structures of the MJLCPs based on polysiloxane backbones and polyethylene backbone [106]. 13 of 37 13 of 37 Polymers Polymers2018, 10, 2018x FOR, 10 PEER, x FOR REVIEW PEER REVIEW 13 of 37 13 of 37 Table 6.Table Chemical 6. Chemical structures structures of the MJLCPs of the MJLCPs based on based polysiloxane on polysiloxane backbones backbones and polyethylene and polyethylene backbone backbone [106]. [106]. Table 6.Table Chemical 6. Chemical structures structures of the MJLCPs of the MJLCPs based on based polysiloxane on polysiloxane backbones backbones and polyethylene and polyethylene backbone backbone [106]. [106]. Conventional MJLCPs.Table 6.Table Chemical Chemical6. Chemical structures Structures structures of the MJLCPs of the MJLCPs based Mesogen-Jacketedon based polysiloxane on polysiloxane backbones Polyailoxanes backbones and polyethylene and polyethylene backbone backbone Chemical [106]. [106]. Structures ConventionalConventional MJLCPs.Table MJLCPs. 6.Table Chemical 6.Chemical Chemical structuresChemical Structures structures of the Structures MJLCPs of the MJLCPs basedMesogen on based polysiloxaneMesogen‐Jacketed on polysiloxane‐Jacketed Polyailoxanes backbones Polyailoxanes backbones and polyethylene and polyethylene backboneChemical backboneChemical [106]. Structures [106].Structures ConventionalConventional MJLCPs.Table MJLCPs. 6.Table Chemical 6.Chemical Chemical structuresChemical Structures structures of the Structures MJLCPs of the MJLCPs basedMesogen on based polysiloxaneMesogen‐Jacketed on polysiloxane‐ JacketedPolyailoxanes backbones Polyailoxanes backbones and polyethylene and polyethylene backboneChemical backboneChemical [106].Structures [106].Structures ConventionalConventional MJLCPs. MJLCPs. ChemicalChemical Structures Structures MesogenMesogen‐Jacketed‐ JacketedPolyailoxanes Polyailoxanes ChemicalChemical Structures Structures ConventionalConventional MJLCPs. MJLCPs. ChemicalChemical Structures Structures MesogenMesogen‐Jacketed‐ JacketedPolyailoxanes Polyailoxanes ChemicalChemical Structures Structures ConventionalConventional MJLCPs. MJLCPs. ChemicalChemical Structures Structures MesogenMesogen‐Jacketed‐ JacketedPolyailoxanes Polyailoxanes ChemicalChemical Structures Structures PMPCSPMPCS PMPCS Si‐MPCSSi-MPCSSi ‐MPCS PMPCS PMPCS Si‐MPCS Si‐MPCS PMPCS PMPCS Si‐MPCS Si‐MPCS PMPCS PMPCS Si‐MPCS Si‐MPCS PMPCS PMPCS Si‐MPCS Si‐MPCS

PBPCS PBPCS Si‐BPCS Si‐BPCS PBPCSPBPCS PBPCS Si‐BPCSSi-BPCS Si‐BPCS PBPCS PBPCS Si‐BPCS Si‐BPCS PBPCS PBPCS Si‐BPCS Si‐BPCS PBPCS PBPCS Si‐BPCS Si‐BPCS

OXD‐C12pOXD ‐C12p Si‐OXD‐C12pSi‐OXD ‐C12p OXD‐C12pOXD ‐C12p Si‐OXD‐C12pSi‐OXD ‐C12p OXD-C12pOXD‐C12pOXD ‐C12p Si‐OXDSi-OXD-C12p‐C12pSi‐OXD ‐C12p OXD‐C12pOXD ‐C12p Si‐OXD‐C12pSi‐OXD ‐C12p OXD‐C12pOXD ‐C12p Si‐OXD‐C12pSi‐OXD ‐C12p

4C2Vp 4C2Vp Si‐4C2VpSi ‐4C2Vp 4C2Vp 4C2Vp Si‐4C2VpSi ‐4C2Vp 4C2Vp 4C2Vp Si‐4C2VpSi ‐4C2Vp 4C2Vp4C2Vp 4C2Vp Si‐4C2VpSi-4C2VpSi ‐4C2Vp 4C2Vp 4C2Vp Si‐4C2VpSi ‐4C2Vp

12C2Vp 12C2Vp Si‐12C2VpSi ‐12C2Vp 12C2Vp 12C2Vp Si‐12C2VpSi ‐12C2Vp 12C2Vp 12C2Vp Si‐12C2VpSi ‐12C2Vp 12C2Vp 12C2Vp Si‐12C2VpSi ‐12C2Vp 12C2Vp12C2Vp 12C2Vp Si‐12C2VpSi-12C2VpSi ‐12C2Vp

Polymers 2018, 10, 794 14 of 37

TablePolymers 7. 2018Comparison, 10, x FOR PEER of REVIEW the physical data of the MJLCPs based on polysiloxane backbones14 of and 37 polyethylene backbone [106–110]. Table 7. Comparison of the physical data of the MJLCPs based on polysiloxane backbones and ◦ polyethylenePolymers backboneMolecular [106–110]. Weight (Mn) Tg ( C) LC Phase d-Spacing

PMPCS 15,900 118 ΦN 1.59 Polymers Molecular Weight (ࡹഥ n) Tg (°C) LC Phase d‐Spacing PBPCS 12,700 98 ΦHN 1.87 PMPCS 15,900 118 ΦN 1.59 OXD-C12p 142,000 167 SA 3.44 PBPCS 12,700 98 ΦHN 1.87 4C2Vp 83,000 105 SA 3.00 OXD‐C12p 142,000 167 SA 3.44 12C2Vp 80,000 86 SC 4.20 4C2Vp 83,000 105 SA 3.00 Si-MPCS 16,600 111 ΦN 1.58 12C2Vp 80,000 86 SC 4.20 Si-BPCS 19,500 92 ΦHN 1.86 Si‐MPCS 16,600 111 ΦN 1.58 Si-OXD-C12p 29,200 156 S 3.53 Si‐BPCS 19,500 92 ΦHN C 1.86 Si-4C2Vp 22,200 96 No No Si‐OXD‐C12p 29,200 156 SC 3.53 Si-12C2VpSi‐4C2Vp 30,00022,200 7696 No SA No 4.24 ΦN: columnarSi‐ phase;12C2VpΦ HN: hexagonal30,000 columnar phase; S76A: smectic ASA phase; SC:4.24 smectic C phase.

ΦN: columnar phase; ΦHN: hexagonal columnar phase; SA: smectic A phase; SC: smectic C phase. 3.1.2. Dependence of Polymerization Degree of the Linear Polysiloxanes and the Degree of Substitution3.1.2. Dependence of the Mesogenic of Polymerization Structural Degree Unit of the Linear Polysiloxanes and the Degree of Substitution of the Mesogenic Structural Unit Generally, phase transition behaviors, LC textures, and stability of the mesophase in MJLCPs greatly dependGenerally, on phase molecular transition weight behaviors, [107,108 LC,111 textures,]. In polysiloxane and stability of LC the polymers, mesophase the in MJLCPs inﬂuence of moleculargreatly weight, depend composition on molecular of weight mesogenic [107,108,111]. units, and In polysiloxane degree of substitution LC polymers, on the LC influence properties of have molecular weight, composition of mesogenic units, and degree of substitution on LC properties have also been investigated. also been investigated. G. Kossmehl, et al. synthesized a series of LC polysiloxanes with 2,7-disubstituted ﬂuorene as G. Kossmehl, et al. synthesized a series of LC polysiloxanes with 2,7‐disubstituted fluorene as mesogenicmesogenic unit andunit differentand different polymerization polymerization degree degree of the of mainthe main chain chain (as shown (as shown in Figure in Figure6)[ 112 6) ,113]; the phase[112,113]; behaviour the phase investigation behaviour investigation revealed that revealed the longer that polymerthe longer backbone polymer slightlybackbone enhanced slightly the stabilityenhanced of the the LC stability phase, of as the shown LC phase, in Table as shown8. in Table 8.

Figure 6. Chemical structures of the polysixanes with fluorene unit [112,113]. Figure 6. Chemical structures of the polysixanes with ﬂuorene unit [112,113].

Table 8. Thermal transition behaviors of the polysixanes with fluorene unit [112,113]. Table 8. Thermal transition behaviors of the polysixanes with ﬂuorene unit [112,113]. Transition Temperatures/°C Polymers TransitionHeating/Cooling Temperatures/ ◦C Polymers G 130–140 S 170–191 I 1a Heating/Cooling I 180–157 SA 142–138 SE 135–116 G G 130–140 S 170–191 I 1a G 137–143 S 174–194 I 1b I 180–157 SA 142–138 SE 135–116 G I 192–166 SA 139–130 G G 137–143 S 174–194 I 1b G 141–153 SE 153–158 SA 168–186 I 2a I 192–166 SA 139–130 G I 177–155 SA 154–147 SE 143–133 G G G97– 141–153133 SE 152 SE–153–158163 SA 168 SA–194168–186 I I 2a2b I 182I 177–155–163 SA S153A 154–147–141 SE 121 SE –143–13380 G G

G: glass state; S: smectic phase; SA: smecticG 97–133 A phase; S 152–163 SE: smectic S 168–194 E phase; I I: isotropic phase. 2b E A I 182–163 SA 153–141 SE 121–80 G

G: glass state; S: smectic phase; SA: smectic A phase; SE: smectic E phase; I: isotropic phase.

Polymers 2018, 10, 794 15 of 37

ByPolymers the 2018 cationic, 10, x FOR polymerizationPEER REVIEW of 1,3,5,7-tetramethylcyclotetrasiloxane (D40) and15 of 1,3,5,7-37 octamethylcyclotetrasiloxan (D4), Percec synthesized a series of poly(methylsiloxane)s and By the cationic polymerization of 1,3,5,7‐tetramethylcyclotetrasiloxane (D4′) and 1,3,5,7‐ poly(methylsiloxane-co-dimethylsiloxane)soctamethylcyclotetrasiloxan (D4), Percec withsynthesized molecular a series weight of poly(methylsiloxane)s and composition accurately and controllable;poly(methylsiloxane these can‐ beco‐ useddimethylsiloxane)s as model polymers with molecular to study weight the dependence and composition of molecular accurately weight and substitutioncontrollable; these degree can be of used mesogenic as model unitspolymers on to phase study behaviorsthe dependence [102 of,114 molecular]. Polysiloxanes weight and and copolysiloxanessubstitution containingdegree of mesogenic mesogenic side units groups on withphase different behaviors molecular [114,102]. weights Polysiloxanes and compositions and werecopolysiloxanes obtained by hydrosilylationcontaining mesogenic reaction side between groups poly(methylsiloxane) with different molecular and vinylweights monomers and containingcompositions 2-[4-(2(S)-Methyl- were obtainedL-butoxy)phenyl]-5-(11-undecany1)-1,3,2-dioxaborinane by hydrosilylation reaction between poly(methylsiloxane) mesogenic and vinyl unit (as shownmonomers in Figure containing7). Studies 2‐[4 of‐(2(S) liquid‐Methyl crystalline‐1‐butoxy)phenyl] properties‐5 showed‐(11‐undecany1) that all‐1,3,2 the polysiloxanes‐dioxaborinane and copolysiloxanesmesogenic unit exhibited (as shown a smectic in Figure phase, 7). Studies and the of phaseliquid crystalline transition properties temperatures showed increased that all withthe the polysiloxanes and copolysiloxanes exhibited a smectic phase, and the phase transition temperatures polymerization degree (as summarized in Table9) and composition (as summarized in Table 10) of increased with the polymerization degree (as summarized in Table 9) and composition (as mesogenicsummarized unit. X-ray in Table scattering 10) of mesogenic experiments unit. revealedX‐ray scattering that the experiments thickness revealed of the smectic that the layer thickness increased with theof the structural smectic layer units increased containing with mesogenic the structural units, units and containing the ﬂexible mesogenic backbone units, was and squeezed the flexible between smecticbackbone layers was [115 squeezed,116]. between smectic layers [115,116].

Figure 7. Chemical structures of the polysiloxanes and copolysiloxanes containing mesogenic unit [102]. Figure 7. Chemical structures of the polysiloxanes and copolysiloxanes containing mesogenic unit [102].

Table 9. Thermal transitions and thermodynamic parameters for polysiloxanes with varying degree Table 9. of polymerizationThermal transitions [102]. and thermodynamic parameters for polysiloxanes with varying degree of polymerization [102]. Phase Transitions (°C) and Corresponding Enthalpy Changes (kcal/mru−1) DP (NMR) ࡹഥ w/ࡹഥ n (GPC) Heating Cooling Phase Transitions (◦C) and Corresponding Enthalpy Changes (kcal/mru−1) DP (NMR)7 Mw/M1.17n (GPC ) G ‐5 S 10 (0.12) S 72 (1.90) I I 64 (1.90) S 6 (0.10) S 15 1.14 G 9 S 25 (0.16)Heating S 93 (2.10) I I 85 (2.00) Cooling S 20 (0.15) S 722 1.171.17 G 10 -5 S 1024 (0.15) (0.12) S 98 72 (2.00) (1.90) I I I 87 I (2.00) 64 (1.90) S 16 S (0.13) 6 (0.10) S S 1527 1.141.45 G G 10 9 SS 2521 (0.16)(0.11) S 9395 (2.10)(1.80) I I 85 I 85(1.80) (2.00) S 13 S 20(0.10) (0.15) S S 2243 1.171.60 G 10 S 21 24 (0.10) (0.15) S S 99 98 (1.90) (2.00) I II 90 I 87(1.80) (2.00) S 15 S 16(0.09) (0.13) S S 2754 1.451.63 G 10 S 21 (0.08) (0.11) S S 98 95 (1.60) (1.80) I II 87 I 85(1.50) (1.80) S 16 S 13(0.09) (0.10) S S 43 1.60 G 10 S 21 (0.10) S 99 (1.90) I I 90 (1.80) S 15 (0.09) S 5456 1.631.81 G 10 S 21 (0.13) (0.08) S S 98 98 (1.80) (1.60) I II 90 I 87(1.70) (1.50) S 14 S 16(0.10) (0.09) S S 5692 1.811.72 G G 10 10 S S 24 21 (0.11) (0.13) S S 104 98 (1.80) I I 94 I 90(1.70) (1.70) S 17 S 14(0.12) (0.10) S S 92 1.72G: glass Gstate; 10 S S: 24 smectic (0.11) S 104phase; (1.80) I: Iisotropic phase. I 94 (1.70) S 17 (0.12) S G: glass state; S: smectic phase; I: isotropic phase. Table 10. Thermal transitions and thermodynamic parameters for copolysiloxanes with varying Tablecomposition 10. Thermal [102]. transitions and thermodynamic parameters for copolysiloxanes with varying compositionStructural [102 Units]. Phase Transitions (°C) and Corresponding Enthalpy Changes ۱ሻ۾Containing Side ሺ۵ (kcal/mru−1) StructuralGroups Units Phase Transitions (◦C) and Corresponding Enthalpy Changes a a ഥ (GPCഥ) ഥ Containingmol Side% Groupswt % ࡹw ࡹw/ࡹn Heating (kcal/mru−1) Cooling 23.3 65 26,800 2.95 G1 −98 G2 −8 S −7 (0.16) S 37 (1.30) I I 31 (1.30) S −14 (0.12) S mol % a wt % a M M /M Heating Cooling 38.0 79 21,400w 2.02w n G1 −81 G2 −8 S −6 (0.22) S 47 (1.30) I I 41 (1.20) S −14 (0.21) S 23.356.0 6589 26,80025,900 2.17 2.95 G1 G1 −−6698 G2 G2 5 −S 98 (0.23) S −7 (0.16)S 75 (1.80) S 37 I (1.30) II 67 (1.80) I 31 (1.30) S 2 (0.19) S −14 S (0.12) S − − − − 38.082.0 7997 21,40022,800 1.49 2.02 G1G 881 S G218 (0.24)8 S S 692 (0.22) (2.00) S I 47 (1.30) II 83 (2.00) I 41 (1.20)S 13 (0.26) S 14 S (0.21) S 56.0 89 25,900 2.17 G1 −66 G2 5 S 9 (0.23) S 75 (1.80) I I 67 (1.80) S 2 (0.19) S 82.0100.0 97100 22,80025,500 2.10 1.49 G 8 G S 820 S (0.13) 18 (0.24) S 90 S (1.80) 92 (2.00) I II 81 (1.70) I 83 S (2.00) 13 (0.12) S 13 S (0.26) S 100.0 100a determined 25,500 by NMR; 2.10 G: glass state; G S: 8 Ssmectic 20 (0.13) phase; S 90 (1.80) I: isotropic I phase. I 81 (1.70) S 13 (0.12) S a determined by NMR; G: glass state; S: smectic phase; I: isotropic phase. Polymers 2018, 10, 794 16 of 37

Polymers 2018, 10, x FOR PEER REVIEW 16 of 37 DielePolymers et 2018 al., 10, x explored FOR PEER REVIEW the dependence of the amount of substitution on liquid crystalline16 of 37 structureDiele by means et al. explored of X-ray the diffraction dependence analysis. of the amount They synthesized of substitution several on liquid series crystalline of liquid structure crystalline homopolysiloxanesby meansDiele etof al. X explored‐ andray diffraction copolysiloxanes the dependence analysis. with of Theythe different amount synthesized of distances substitution several between onseries liquid successiveof crystalline liquid crystalline dimesogenicstructure side groupshomopolysiloxanesby means linked of X with‐ray and thediffraction copolysiloxanes polymer analysis. backbone with They (asdifferent shownsynthesized distances in Figure several between8)[ series117 successive,118 of]. liquid Results dimesogenic crystalline showed that all thesidehomopolysiloxanes copolymers groups linked exhibited with and the copolysiloxanes smectic polymer phase backbone with and (as twodifferent shown mesophases distances in Figure were between8) [117,118]. found successive Results for the showeddimesogenic homopolymers. that The phaseallside the groups transitioncopolymers linked temperatures exhibitedwith the polymersmectic of SmA-I phasebackbone and increased (astwo shown mesophases with in Figure the were composition 8) [117,118].found for Resultsthe of thehomopolymers. mesogenicshowed that unit. all the copolymers exhibited smectic phase and two mesophases were found for the homopolymers. Furthermore,The phase X-ray transition diffraction temperatures results of suggestedSmA‐I increased that thewith smectic the composition monolayer of the thickness mesogenic remarkably unit. The phase transition temperatures of SmA‐I increased with the composition of the mesogenic unit. dependedFurthermore, on the X amount‐ray diffraction of substitution, results suggested and due that to the the smectic domaining monolayer interaction thickness of remarkably the mesogenic dependedFurthermore, on theX‐ray amount diffraction of substitution, results suggested and due that to thethe smecticdomaining monolayer interaction thickness of the remarkablymesogenic groups, a ﬂexible polysiloxane backbone was forced to take part in the layer formation (as illustrated groups,depended a flexible on the polysiloxane amount of substitution, backbone was and forced due toto takethe partdomaining in the layer interaction formation of the (as illustratedmesogenic in Figure9). ingroups, Figure a 9). flexible polysiloxane backbone was forced to take part in the layer formation (as illustrated in Figure 9).

Figure 8. The chemical structures of the liquid crystalline homopolysiloxanes and copolysiloxanes Figure 8. The chemical structures of the liquid crystalline homopolysiloxanes and copolysiloxanes withFigure dimesogenic 8. The chemical side groups structures [118]. of the liquid crystalline homopolysiloxanes and copolysiloxanes with dimesogenicwith dimesogenic side side groups groups [118 [118].].

Figure 9. (a) Schematic representation of the DSC and X‐ray investigation of the homopolysiloxanes Figure 9. (a) Schematic representation of the DSC and X‐ray investigation of the homopolysiloxanes Figureand 9. copolysiloxanes(a) Schematic representation(SA: smectic A phase, of the N: DSCnematic and phase, X-ray I: isotropic investigation phase); of (b the) The homopolysiloxanes layer spacings; and copolysiloxanes (SA: smectic A phase, N: nematic phase, I: isotropic phase); (b) The layer spacings; and copolysiloxanes(d) as a function of (S number: smectic of the A phase, inserted N: dimethylsioxane nematic phase, unites I: isotropic (y) [118]. phase); (b) The layer spacings; (d) as a function ofA number of the inserted dimethylsioxane unites (y) [118]. (d) as a function of number of the inserted dimethylsioxane unites (y)[118]. Yonetake et al. designed and synthesized poly[6‐(4‐cyanobiphenyl‐4′‐oxy)‐hexylmetylsiloxanes] (PCS‐Yonetaken, where etn denotedal. designed the andDS, synthesizedas shown in poly[6Figure‐ (410)‐cyanobiphenyl with various ‐degrees4′‐oxy)‐ hexylmetylsiloxanes]of substitution (DS), 0 Yonetakeand(PCS systematically‐n, where et al. n designed denoted investigated the and DS, synthesizedthe as effects shown of in poly[6-(4-cyanobiphenyl-4the Figure degree 10) of with the varioussubstitution degrees-oxy)-hexylmetylsiloxanes] on theof substitutionphase transition, (DS), (PCS-n, and where systematically n denoted investigated the DS, asthe shown effects inof the Figure degree 10 )of with the substitution various degrees on the of phase substitution transition, (DS), and systematically investigated the effects of the degree of the substitution on the phase transition, Polymers 2018, 10, 794 17 of 37 structures, and the liquid crystallization of PCS-n on the basis of thermal analysis, optical microscope, Polymers 2018, 10, x FOR PEER REVIEW 17 of 37 and X-rayPolymers diffraction 2018, 10, x FOR methods PEER REVIEW [119 ,120]. They found that all the PCS-n showed (SA) in the temperature17 of 37 rangestructures, from glass and transition the liquid temperature crystallization (T ofg) PCS to clearing‐n on the temperature basis of thermal (Ti analysis,), in spite optical of various microscope, DS. All the structures, and the liquid crystallization of PCS‐n on the basis of thermal analysis, optical microscope, Tg andandT iXincreased‐ray diffraction linearly methods with DS,[119,120]. and the They temperature found that range all the of PCS the‐ SnA showedphase between(SA) in theTg and and X‐ray diffraction methods [119,120]. They found that all the PCS‐n showed (SA) in the Ti expandedtemperature with range DS. Sincefrom glass the non-mesogenic transition temperature ﬂexible (T backbonesg) to clearing were temperature squeezed (T betweeni), in spite smectic of temperature range from glass transition temperature (Tg) to clearing temperature (Ti), in spite of betweenvarious the DS. SA layer,All the the Tg and layer Ti spacingincreased of linearly the SA withstructure DS, and decreased the temperature with increasing range of the DS. SA In phase addition, various DS. All the Tg and Ti increased linearly with DS, and the temperature range of the SA phase due tobetween the entropy Tg and relaxation Ti expanded of thewith backbones DS. Since the between non‐mesogenic the SA layers, flexible the backbones layer spacing were ofsqueezed the sample between Tg and Ti expanded with DS. Since the non‐mesogenic flexible backbones were squeezed with lowerbetween DS smectic highly between decreased the S withA layer, increasing the layer spacing temperature. of the SA Furthermore, structure decreased the liquid with increasing crystallization between smectic between the SA layer, the layer spacing of the SA structure decreased with increasing of PCS-nDS. In was addition, remarkably due to inﬂuenced the entropy by relaxation DS; PCS-n of the with backbones higher DSbetween exhibited the S fasterA layers, growth the layer rates of DS.spacing In addition, of the sample due to with the lowerentropy DS relaxation highly decreased of the backbones with increasing between temperature. the SA layers, Furthermore, the layer liquid crystalline textures compared with lower DS (as shown in Figure 11). spacingthe liquid of crystallization the sample with of PCS lower‐n was DS remarkablyhighly decreased influenced with byincreasing DS; PCS ‐temperature.n with higher Furthermore, DS exhibited H.thefaster liquid Yang growth crystallization et rates al. of liquid of synthesized PCS crystalline‐n was remarkably textures a series compared influenced of PSLCPs with by lowerDS; PCS copolymers DS‐ n(as with shown higher basedin FigureDS exhibited on 11). poly[3- mercaptopropylmethylsiloxane]fasterH. growth Yang rates et of al.liquid synthesized crystalline (PMMS) texturesa (M.W. series compared 4000–7000) of PSLCPs with with lowercopolymers chiral DS (as cholesteric-based shown based in Figureon poly[3 11). group ‐ and achiralmercaptopropylmethylsiloxane] benzeneH. Yang methyl et al. ether synthesized group (PMMS) via a (M.W. thiol–eneseries 4000–7000) of clickPSLCPs chemistry. with chiralcopolymers cholesteric A battery based‐based of on linear group poly[3 and siloxane‐ tetramersmercaptopropylmethylsiloxane]achiral containing benzene methyl the same ether chiral group (PMMS) and via achiral (M.W. thiol–ene mesogenic4000–7000) click chemistry. with groups chiral were A cholesteric battery also obtained of‐based linear group by siloxane traditional and achiral benzene methyl ether group via thiol–ene click chemistry. A battery of linear siloxane hydrosilylationtetramers containing reaction the (the same chemical chiral and structure achiral mesogenic of the polymers groups were is shown also obtained in Figure by traditional 12)[121, 122]. tetramers containing the same chiral and achiral mesogenic groups were also obtained by traditional Contributinghydrosilylation to the increasedreaction (the length chemical of the structure polysiloxane of the backbone polymers andis shown spacer in as Figure well as12) the [121,122]. cooperative hydrosilylationContributing to reaction the increased (the chemical length structureof the polysiloxane of the polymers backbone is shown and inspacer Figure as 12) well [121,122]. as the interactionContributing between to them,the increased a more length ordered of LCthe phase polysiloxane of smectic backbone E (Sm E)and developed spacer as spontaneously well as the at cooperative interaction between them, a more ordered LC phase of smectic E (SmE) developed low temperaturescooperative interaction for polymers between with them, chiral a cholesteric more ordered unit LC (X cholphase) below of smectic 0.3 and E a(S strongmE) developed dependence spontaneously at low temperatures for polymers with chiral cholesteric unit (Xchol) below 0.3 and a of LC phase structures on the molar content of the chiral group was observed.chol However, for the linear spontaneouslystrong dependence at low of temperaturesLC phase structures for polymers on the with molar chiral content cholesteric of the chiralunit ( Xgroup) below was observed.0.3 and a siloxanestrongHowever, tetramers, dependence for the a lower linearof LC degree phasesiloxane structures of tetramers, polymerization on thea lower molar generated degree content of of only polymerizationthe cholestericchiral group orgenerated was smectic observed. only A phase withHowever, rathercholesteric wide foror temperature smecticthe linear A phase siloxane ranges with (seetetramers,rather Table wide a11 temperature lower). degree ranges of polymerization(see Table 11). generated only cholesteric or smectic A phase with rather wide temperature ranges (see Table 11).

Figure 10. The chemical structures of the poly[6‐(4‐cyanobiphenyl‐4′‐oxy)‐hexylmetylsiloxanes] (PCS‐ Figure 10. The chemical structures of the poly[6-(4-cyanobiphenyl-40-oxy)-hexylmetylsiloxanes] Figuren) [120]. 10. The chemical structures of the poly[6‐(4‐cyanobiphenyl‐4′‐oxy)‐hexylmetylsiloxanes] (PCS‐ (PCS-n)n) [120]. [120 ].

Figure 11. (a) Changes in phase transition temperature (Tg and Ti) and enthalpy change of PCS‐n as a Figure 11. (a) Changes in phase transition temperature (Tg and Ti) and enthalpy change of PCS‐n as a Figurefunction 11. (a) of Changes the degree in phaseof substitution transition (DS); temperature (b) Changes ( Ting layerand ‐Tspacingi) and enthalpyand intermolecular change of spacing PCS-n as a function of the degree of substitution (DS); (b) Changes in layer‐spacing and intermolecular spacing functionof the of PCS the‐ degreen as a function of substitution of the degree (DS); of ( bsubstitution) Changes (DS) in layer-spacing [120]. and intermolecular spacing of of the PCS‐n as a function of the degree of substitution (DS) [120]. the PCS-n as a function of the degree of substitution (DS) [120].

Polymers 2018, 10, 794 18 of 37 Polymers 2018, 10, x FOR PEER REVIEW 18 of 37

Figure 12.12. Chemical structures of the copolymers based on PMMS backbone and linear siloxanesiloxane tetramer backbone [[121,122].121,122].

Table 11.11. Phase behaviors comparison ofof tetramerstetramers [[121,122].121,122]. Phase Transitions a (°C) PMMS‐Based Phase Transitions a (°C) Linear Polysiloxane a ◦ a ◦ Phase TransitionsHeating ( C) Polysiloxane PhaseHeating Transitions ( C) Tetramers PMMS-Based Linear Polysiloxane CoolingHeating CopolymersPolysiloxane Cooling Heating Tetramers Copolymers G55.63SmA234.94ICooling G35.7SmAd139.9SmA Cooling s204.8I PS4‐Xchol‐1.00 PMMS‐Xchol‐1.00 I234.13SmA56.74G I197.9SmAs136.65SmAd34.6G G55.63SmA234.94I G35.7SmAd139.9SmAs204.8I PS4-Xchol-1.00 PMMS-Xchol-1.00 G49.94SmA231.35II234.13SmA56.74G G35.6SmA I197.9SmAd’59.6SmAd152.2SmAs136.65SmAd34.6Gs189.9I PS4‐Xchol‐0.90 PMMS‐Xchol‐0.90 I230.32SmA56.51G I185.5SmAs154.9SmAd59.0SmAd’34.1G G49.94SmA231.35I G35.6SmAd’59.6SmAd152.2SmAs189.9I PS4-Xchol-0.90 PMMS-Xchol-0.90 G45.10SmA206.39II230.32SmA56.51G I185.5SmAG31.9SmAs154.9SmAd59.0SmAd145.1SmAs174.4I ’34.1G PS4‐Xchol‐0.80 PMMS‐Xchol‐0.80 d I206.98SmA43.35G I171.8SmAs144.6SmAd27.8G G45.10SmA206.39I G31.9SmAd145.1SmAs174.4I PS4-Xchol-0.80 PMMS-Xchol-0.80 G50.75SmA187.32Ch213.48II206.98SmA43.35G I171.8SmAG30.3SmAs144.6SmAd164.4I 27.8G PS4‐Xchol‐0.70 PMMS‐Xchol‐0.70 d I211.63Ch187.25SmA45.37G I161.5SmAd28.9G G50.75SmA187.32Ch213.48I G30.3SmA 164.4I PS4-X -0.70 PMMS-X -0.70 d chol G42.62SmA141.65Ch185.21II211.63Ch187.25SmA45.37G chol G22.8SmA I161.5SmAd132.4I28.9G PS4‐Xchol‐0.60 PMMS‐Xchol‐0.60 d I183.69Ch141.39SmA39.57G I129.8SmAd22.6G G42.62SmA141.65Ch185.21I G22.8SmA 132.4I PS4-X -0.60 PMMS-X -0.60 d chol I183.69Ch141.39SmA39.57GG40.51Ch178.89I chol G21.9SmA I129.8SmAd114.7I22.6G PS4‐Xchol‐0.50 PMMS‐Xchol‐0.50 d I177.38Ch39.01G I117.7SmAd20.1G G40.51Ch178.89I G21.9SmA 114.7I PS4-X -0.50 PMMS-X -0.50 d chol G34.48Ch156.72II177.38Ch39.01G chol G20.2SmA I117.7SmAs110.7I20.1G PS4‐Xchol‐0.40 PMMS‐Xchol‐0.40 d I155.20Ch32.59G I108.3SmAs18.6G G34.48Ch156.72I G20.2SmA 110.7I PS4-X -0.40 PMMS-X -0.40 s chol G24.00Ch106.50II155.20Ch32.59G chol G16.0SmE55.9SmA I108.3SmA 18.6Gs102.5I PS4‐Xchol‐0.30 PMMS‐Xchol‐0.30 s I105.40Ch23.00G I99.83SmAs5I.3SmE11.8G G24.00Ch106.50I G16.0SmE55.9SmA 102.5I PS4-X -0.30 PMMS-X -0.30 s chol G18.39Ch82.50II105.40Ch23.00G chol G7.9SmE70.6SmA I99.83SmA 5I.3SmE11.8Gs80.0I PS4‐Xchol‐0.20 PMMS‐Xchol‐0.20 s I81.42Ch15.99G I72.5SmAs63.4SmE9.6G G18.39Ch82.50I G7.9SmE70.6SmAs80.0I PS4-Xchol-0.20 G11.41Ch64.06I PMMS-Xchol-0.20 G11.4SmE’46.8SmE71.9SmAs90.0I PS4‐Xchol‐0.10 I81.42Ch15.99GPMMS‐Xchol‐0.10 I72.5SmAs63.4SmE9.6G I62.53Ch8.98G I76.5SmAs71.9SmE57.4SmE’8.91G G11.41Ch64.06I G11.4SmE’46.8SmE71.9SmAs90.0I PS4-Xchol-0.10 G6.97N32.09I PMMS-Xchol-0.10 G17.1SmE83.4SmAs95.5I PS4‐Xchol‐0.00 I62.53Ch8.98GPMMS‐Xchol‐0.00 I76.5SmAs71.9SmE57.4SmE’8.91G I30.04N4.80G I80.5SmAs68.6SmE15.2G G6.97N32.09I G17.1SmE83.4SmAs95.5I a PS4-Xchol-0.00 PMMS-Xchol-0.00 According to the DSC results;I30.04N4.80G G: glass state; Ch: cholesteric phase; SmA: I80.5SmA smectics68.6SmE15.2G A phase; SmE: aSmecticAccording E phase; to the DSC SmE’: results; Smectic G: glass E phase state; Ch:with cholesteric microphase phase; separation; SmA: smectic SmA A phase;s: smectic SmE: A Smectic phase E with phase; a SmE’:single Smectic layer; ESmA phased: withsmectic microphase A phase separation; with a SmAdoubles: smectic layer; A SmA phased’: with SmA a singled phase layer; with SmA microphased: smectic A phase with a double layer; SmA ’: SmA phase with microphase separation; I: isotropic state. separation; I: isotropic state. d d

3.1.3. Dependence of the Ring Size of the Cyclic Polysiloxanes Main Chain The ringring size size of of the the cyclic cyclic compounds compounds determines determines its molecular its molecular rigidity, whichrigidity, strongly which inﬂuences strongly theinfluences phase behaviors. the phase For behaviors. liquid crystalline For liquid oligo(cyclosiloxanes), crystalline oligo(cyclosiloxanes), the effect is even the more effect pronounced. is even more W. pronounced. W. Staficzyk synthesized a series of well‐defined liquid crystalline

Polymers 2018, 10, 794 19 of 37

Polymers 2018, 10, x FOR PEER REVIEW 19 of 37 Staficzyk synthesized a series of well-defined liquid crystalline oligo[methyl(hydrogen)cyclosiloxanes (theoligo[methyl(hydrogen)cyclosiloxanes molecular structures are shown in Figure (the 13molecular)[123]. Differing structures from are the shown work byin GrayFigure et al.13) [124 [123].], W.Differing Staficzyk from found the a work close by relationship Gray et al. between[124], W. theStaficzyk size of found the ring a close and relationship the crystal-smectic between phase the size transitionof the ring temperature and the crystal (as shown‐smectic in Tablephase 12transition). With increase temperature of the (as ring shown size ofin oligo(cyclosiloxanes),Table 12). With increase ◦ ◦ a decreaseof the ring in thesize temperature of oligo(cyclosiloxanes), from 50 C toa decrease 38–37 C in is the directly temperature correlated from with 50 an °C increase to 38–37 in °C d is valuedirectly from correlated 3.13 nm for with D4 toan 3.27 increase nm for in D8, d value which from may 3.13 probably nm for be D4 due to to 3.27 the morenm for conformational D8, which may freedomprobably of larger be due rings. to the more conformational freedom of larger rings.

FigureFigure 13. 13.Chemical Chemical structures structures of theof the liquid liquid crystalline crystalline oligo[methyl(hydrogen)cyclosiloxanes] oligo[methyl(hydrogen)cyclosiloxanes] [123 [123].].

Table 12. Interlayer distance d (nm) and the phase transition temperatures as a function of the number Table 12. Interlayer distance d (nm) and the phase transition temperatures as a function of the number of siloxane units [123]. of siloxane units [123]. x d (nm) Phase Transitions (°C) ◦ x 4d (nm)3.13 Phase TransitionsK 50 S 118 I ( C) 4 5 3.133.16 KK 50 44 S S 118 116 I I 5 6 3.163.15 KK 44 43 S S 116 124 I I 6 7 3.153.26 KK 43 37 S S 124 119 I I 7 8 3.263.27 KK 37 38 S S 119 118 I I 8 3.27 K 38 S 118 I K: crystal; S: smectic phase; I: isotropic phase. K: crystal; S: smectic phase; I: isotropic phase. By conventional hydrosilylation chemistry, T. J. Bunning et al. synthesized a series of cyclic siloxaneBy conventional LCPs based hydrosilylation on pentamethylhydrosiloxane chemistry, T. J. Bunning and tetramethylhydrosiloxane et al. synthesized a series rings of as cyclic well as siloxanemesogens LCPs with based cholesterol on pentamethylhydrosiloxane and biphenyl [125]. The and chemical tetramethylhydrosiloxane structures are listed in rings Figure as well14; the asvariables mesogens examined with cholesterol included and ring biphenyl size, spacer [125 ].group The length, chemical and structures type and are composition listed in Figure of pendant 14; themesogenic variables examinedgroups. Results included revealed ring size, that spacer the influence group length, of the andring type size andon the composition phase behaviors of pendant has to mesogenicdo with the groups. mesogenic Results unit revealed and spacers that the length. inﬂuence For homopolymers of the ring size and on the copolymers phase behaviors with more has rigid to domesogenic with the mesogenic unit and unitshorter and spacer spacers length, length. greater For homopolymers impact on phase and copolymers behaviors can with be more found. rigid But mesogenicoverall, unitthe andtetramethyl shorter spacer ring length,derivatives greater exhibited impact on higher phase behaviors glass transition can be found. or crystallization But overall, thetemperatures tetramethyl ring than derivatives their pentamethyl exhibited counterparts higher glass due transition to more or rigidity crystallization of the temperaturestetramethyl ring than (As theirillustrated pentamethyl in Table counterparts 13). due to more rigidity of the tetramethyl ring (As illustrated in Table 13).

Polymers 2018, 10, 794 20 of 37 Polymers 2018, 10, x FOR PEER REVIEW 20 of 37

Figure 14. The chemical structures of the cyclic siloxane LCPs [125]. Figure 14. The chemical structures of the cyclic siloxane LCPs [125].

Table 13. Phase behaviors of the homopolymers and copolymers [125]. Table 13. Phase behaviors of the homopolymers and copolymers [125]. 5R 4R Compound a Thermal Transitions (°C) Compound a Thermal Transitions (°C) 5R1 5RG 66 N 135 I 4R1 G 110 Viscous 4R melt Compound a 5R2Thermal TransitionsK1 115 K2 130 (N◦ C)175 I Compound4R2 a K 180Thermal N 200 I Transitions (◦C) K 148 N 175 I K 188 I 5R3 4R3 5R1 G 66I N172 135 N 140 I SC 120 K 4R1I 178 N 148 G 110 K Viscous melt 5R25R4 K1 115 KK2 1130 80 K N2 103 175 SA I172 I 4R3 4R2 K 116 SA 169 K I 180 N 200 I 5R5 K 148 NG 175 76 S IA 247 I 4R5 G 100 SA 232 I K 188 I 5R3 5R6 G 61 SA 228 N* 246 I 4R64R3 G 76 SA 270 N* 277 I I 172 N 140 SC 120 K I 178 N 148 K 5R7 G 50 SA 255 I 4R7 G 58 SA 263 I 5R4 K1 80 K2 103 SA 172 I 4R3 K 116 SA 169 I 5R8 G 49 S*C 100 SA 252 I 4R8 G 55 S*C 100 SA 246 I 5R5 G 76 S 247 I 4R5 G 100 S 232 I 5R1/5 AG 80 N* 204 I 4R1/5 G 75 N* 204 I A 5R65R2/6 G 61 SA 228G N* 50 N* 246 220 I I 4R2/6 4R6 G 57 N* G 237 76 I SA 270 N* 277 I 5R75R3/7 GG 50 41 S SA*C 255130 S IA 198 N* 228 I 4R3/7 4R7 G 35 N* 220 G I 58 SA 263 I * * 5R8 5R4/8G 49 S CG100 26 S SAA 199252 N* I 209 I 4R4/84R8 G 30 SA 204G N* 55 210 S CI 100 SA 246 I a5R1/5 5R and 4R refer to the ring G 80 size, N* 204the last I number corresponds 4R1/5 to mesogen attached G(see 75 Figure N* 204 14). I G: glass5R2/6 state; K: crystal; S GA: 50smectic N* 220 A phase; I S*C: chiral smectic 4R2/6 C phase; N*: chiral G nematic 57 N* 237phase; I I: * isotropic5R3/7 phase.G 41 S C 130 SA 198 N* 228 I 4R3/7 G 35 N* 220 I 5R4/8 G 26 SA 199 N* 209 I 4R4/8 G 30 SA 204 N* 210 I a3.1.4.5R and Dependence 4R refer to the of ringthe Shape size, the of last the number Polysiloxanes corresponds Main to mesogenChain attached (see Figure 14). G: glass state; * K: crystal; SA: smectic A phase; S C: chiral smectic C phase; N*: chiral nematic phase; I: isotropic phase. The shapes of the polymer chain (e.g., linear, cyclic, branched, or networked), which strongly influence the flexibility and tacticity of the polymer, have a pronounced effect on the phase behaviors 3.1.4. Dependence of the Shape of the Polysiloxanes Main Chain of the SCLCPs. Polysiloxanes have diverse backbone structures, such as linear and cyclic polysiloxane The[126,127], shapes tetrahedral of the polymer or octathedral chain silsesquioxane (e.g., linear, cyclic, [128,129], branched, branched or or networked), networked polysiloxane which strongly influence[130,131], the etc., flexibility the influence and of tacticity which on of LC the properties polymer, have have also been a pronounced investigated. effect on the phase behaviorsG. of W. the Gray SCLCPs. et al. synthesized Polysiloxanes a series have of cyclic diverse SCLC backbone polysiloxane structures, and their suchopen chain as linear analogues and cyclic by means of a standard hydrosilylation reaction using both cyclic (the number‐average degree of polysiloxane [126,127], tetrahedral or octathedral silsesquioxane [128,129], branched or networked polymerization were approximately between 4 and 24) and linear (the number‐average degree of polysiloxane [130,131], etc., the influence of which on LC properties have also been investigated. polymerization were approximately between 4 and 13) polymers [124]. The chemical structures of G.the W.polysiloxanes Gray et al.are shown synthesized in Figure a 15. series The results of cyclic reveal SCLC that polysiloxanethe mesophase‐ andisotropic their transition open chain analoguestemperatures by means of the of cyclic a standard polymers hydrosilylation were generally higher reaction than using those both of the cyclic linear (theanalogues number-average (Figure degree16a,b), of polymerization which may be attributed were approximately in part to the absence between of the 4 andtrimethylsilyl 24) and linearend groups (the in number-average the cyclics. degreeSuch of end polymerization groups would were be expected approximately to dilute between the mesophase, 4 and 13)thereby polymers lowering [124 the]. Thetransition chemical structurestemperatures of the polysiloxanes of the linear relative are shown to those in Figureof the corresponding 15. The results cyclic reveal materials. that the mesophase-isotropic transitionH. temperatures Yang et al. designed of the cyclic and polymerssynthesized were cyclic generally and linear higher polysiloxane than those tetramers of the linearwith similar analogues (Figuremesogenic 16a,b), whichgroups may and investigated be attributed the in influence part to the of the absence shape of of the tetramer trimethylsilyl backbone end on the groups phase in the behaviors [122,132]. The chemical structures of the polysiloxanes are shown in Figure 17. The results cyclics. Such end groups would be expected to dilute the mesophase, thereby lowering the transition temperatures of the linear relative to those of the corresponding cyclic materials. H. Yang et al. designed and synthesized cyclic and linear polysiloxane tetramers with similar mesogenic groups and investigated the influence of the shape of tetramer backbone on the phase Polymers 2018, 10, 794 21 of 37 behaviorsPolymers [122 2018,,132 10, x]. FOR The PEER chemical REVIEW structures of the polysiloxanes are shown in Figure 17. The21 of results37 Polymers 2018, 10, x FOR PEER REVIEW 21 of 37 revealPolymers that 2018 owing, 10, x toFOR the PEER increased REVIEW rigidity of the cyclic backbone, abundant LC phase21 structures of 37 reveal that owing to the increased rigidity of the cyclic backbone, abundant LC phase structures with withreveal blue phase that owing (BP) to and the relatively increased highrigidity glass of the transition cyclic backbone, temperatures abundant were LC discoveredphase structures (see with Table 14). blue phase (BP) and relatively high glass transition temperatures were discovered (see Table 14). Theybluereveal pointed phase that out owing(BP) that and to the the relatively increased high rigidity rigidity glass of transition ofthe the cyclic cyclic temperatures backbone, backbone abundant were played discovered LC phase an important structures(see Table role with 14). in the They pointed out that the increased rigidity of the cyclic backbone played an important role in the Theyblue phasepointed (BP) out and that relatively the increased high rigidityglass transition of the cyclic temperatures backbone wereplayed discovered an important (see roleTable in 14).the stabilitystability of the of bluethe blue phase, phase, which which accordingly accordingly improved improved the the helical helical twisting twisting power power (HTP). (HTP). stabilityThey pointed of the outblue that phase, the whichincreased accordingly rigidity of improved the cyclic the backbone helical twisting played anpower important (HTP). role in the stability of the blue phase, which accordingly improved the helical twisting power (HTP).

Figure 15. The chemical structures of the cyclic and linear polysiloxanes [124]. FigureFigure 15. 15.The The chemical chemical structures structures of the cyclic cyclic and and linear linear polysiloxanes polysiloxanes [124]. [124 ]. Figure 15. The chemical structures of the cyclic and linear polysiloxanes [124].

Figure 16. (a) Mesophase‐isotropic transition temperatures for cyclic (•) and linear (▪) polysiloxanes Figure 16. (a) Mesophase‐isotropic transition temperatures for cyclic (•) and linear (▪) polysiloxanes Figurecontaining 16. (a) Mesophase-isotropic the cyanoester mesogenic transition groups; temperatures (b) Mesophase for‐isotropic cyclic ( •transition) and linear temperatures () polysiloxanes for containingFigure 16. (a)the Mesophase cyanoester‐isotropic mesogenic transition groups; temperatures (b) Mesophase for‐ isotropiccyclic (•) transitionand linear temperatures(▪) polysiloxanes for containingcyclic (•) the and cyanoester linear (▪) mesogenic polysiloxanes groups; containing (b) Mesophase-isotropic the cyanobiphenyl mesogenic transition groups temperatures [124]. for cyclic cycliccontaining (•) and the linear cyanoester (▪) polysiloxanes mesogenic containing groups; ( bthe) Mesophase cyanobiphenyl‐isotropic mesogenic transition groups temperatures [124]. for (•) andcyclic linear (•) and () polysiloxanes linear (▪) polysiloxanes containing containing the cyanobiphenyl the cyanobiphenyl mesogenic mesogenic groups groups [124 [124].].

Figure 17. Chemical structures of the cyclic and linear polysiloxane tetramers [122,132]. Figure 17. Chemical structures of the cyclic and linear polysiloxane tetramers [122,132]. Figure 17. Chemical structures of the cyclic and linear polysiloxane tetramers [122,132]. Figure 17. Chemical structures of the cyclic and linear polysiloxane tetramers [122,132].

Polymers 2018, 10, 794 22 of 37

Table 14. Phase behaviors comparison of the tetramers [122,132].

Phase Transitions a (◦C) Phase Transitions a (◦C) Linear Polysiloxane Heating Cyclic Polysiloxane Tetramers Tetramers Heating Cooling G55.63SmA234.94I PS4-X -1.00 PCS4-X -1.00 G65.5SmA233.1Ch248.0BPs266.5I chol I234.13SmA56.74G chol G49.94SmA231.35I PS4-X -0.90 PCS4-X -0.875 G56.4SmA220.2Ch245.4BPs251.0I chol I230.32SmA56.51G chol G45.10SmA206.39I PS4-X -0.80 PCS4-X -0.75 G657.3SmA197.7Ch237.5BPs242.5I chol I206.98SmA43.35G chol G50.75SmA187.32Ch213.48I PS4-X -0.70 PCS4-X -0.625 G55.2Ch230.5BPs234.5I chol I211.63Ch187.25SmA45.37G chol G42.62SmA141.65Ch185.21I PS4-X -0.60 PCS4-X -0.50 G43.4Ch205.2BPs208.0I chol I183.69Ch141.39SmA39.57G chol G40.51Ch178.89I PS4-X -0.50 PCS4-X -0.375 G53.7Ch208.3I chol I177.38Ch39.01G chol G34.48Ch156.72I PS4-X -0.40 PCS4-X -0.25 G51.5Ch192.7I chol I155.20Ch32.59G chol G24.00Ch106.50I PS4-X -0.30 PCS4-X -0.125 G55.5Ch189.2I chol I105.40Ch23.00G chol G18.39Ch82.50I PS4-X -0.20 PCS4-X -0 Cr 45.6Cr 156.9N194.4I chol I81.42Ch15.99G chol 1 2 G11.41Ch64.06I PS4-X -0.10 chol I62.53Ch8.98G G6.97N32.09I PS4-X -0.00 chol I30.04N4.80G a According to DSC results; G: glass state; Ch: cholesteric phase; SmA: smectic A phase; BP: blue phase; Cr: crystal; I: isotropic state.

3.2. Dependence of the Flexible Spacer on Phase Transition Behaviors & Phase Structures of the PSCLCPs The flexible spacers between the mesogenic side group from the polymer backbone, which can usually improve the mobility of the mesogenic groups and facilitate the formation of LC phase structures, plays an important role in determining many physical properties of the resulting liquid-crystal polymers [133–135]. According to the flexible decoupled theory, the spacers can provide an effective decoupling between the polymeric chain and the mesogenic groups. Generally, the longer the spacer is, the more stable the mesophase is. From the perspective of application, elasticity is a very important factor; the longer the spacer, the lower the elasticity constant is. Accordingly, increase in flexibility facilitates response to the electric or magnetic fields. For PSCLCPs, there are mainly three kinds of spacers: polymethylenic, polyoxyethylenic, and thioether. Due to differences in size and nature of carbon, oxygen, and sulfur atoms, the physical properties of the corresponding polymers are obviously different, and interesting applications may be made. For example, polymethylenic flexibility of the spacer is significantly increased by the presence of oxygen and sulfur, which bring about a difference in the sensitivity to electric, optical, magnetic fields.

3.2.1. Dependence of the Shape of the Polysiloxanes Main Chain For all the linking spacers, the polymethylenic spacer is the most commonly used because it is easily carried out and offers good results. The longer the polymethylenic spacer, the more stable the mesophase is. Furthermore, increase of the polymethylenic spacer enables the development of more abundant and ordered LC structures such as SC,SE, etc. [136–139]. However, crystallization often occurs in some early reported PSCLCPs, which is a disadvantage for application. Several research works were carried out to identify ways to decrease or avoid this phenomenon. Percec et al. introduced structural or conformational isomer mesogenic groups in copolymers to remove the crystallization [140–142]. Similarly, Chain S. HSU et al. synthesized polysiloxanes homopolymers Polymers 2018, 10, 794 23 of 37 with cyclohexane based mesogenic units, which exhibited conformational isomerism; the side chain crystallization was efﬁciently avoided, even with usage of very long spacers [143]. In addition, crystallization can also be limited by carrying out block copolymers possessing mesogen structures with non-mesogen ones [144], or by introduction of a voluminous group in spacers [145].

3.2.2. Dependence of the Polyoxyethylenic Spacer on the Phase Transition Behaviors & Phase Structures of the PSCLCPs Oligo(oxyethylene)s groups, which exhibit polarity, flexibility, hydrophilicity, low toxicity, and the ability to form complexation with Li+, have an important position in the field of physical chemistry and other scientific research, and have been used in molecular engineering of rigid rod-like conjugated polymer systems in order to improve solubility in hydrophilic solvents [146], or to obtain “hairy rod” polymers for polymer electrolyte applications [147–149]. Percec et al. designed and synthesized side chain LC homo-polysiloxanes and co-polysiloxanes containing end-on fixed mesogens and oligooxyethylenic spaces (see Figure 18), and systematically investigated the effects of length of oligooxyethylenic spacers and complexation with LiCF3SO3 on their thermotropic mesomorphic behaviors [150]. As shown in Table 15, with the increase of the spacer length, the glass-transition temperatures of the homo-polysiloxanes and co-polysiloxanes increased, while the smectic-isotropic (S-I) transition temperatures decreased. Similarly, (in Table 16), the complexation of the polysiloxane based on the monomer lla-3 (i.e., P-lla-3) with LiCF3SO3 also led to an increase in their glass-transition temperatures and a decrease in their S-I transition temperatures. These complexes are thermotropic liquid-crystalline polyelectrolytes, which can accommodate up to 0.6 mol of salt per mole of ethylene oxide repeat unit (CH2CH2O). Figure 19 presents the possible mechanism of the complexation of LiCF3SO3 by the oligooxyethylene spacer and its effect on the arrangement of the mesogenic units of the polymer in the SA mesophase. At low LiCF3SO3 per mru ratios, the complexation of LiCF3SO3 results in a stiffening of the side chains and consequently in a decrease in segmental mobility; Tg, therefore, increases. On the other hand, the flexibility of the spacer decreases and its conformation changes. Therefore, the SA-I transition temperature decreases. When the amount of LiCF3SO3 becomes high enough so that the majority of the oligooxyethylene of the spacer are participating in complexation, a distortion in the arrangement of the mesogenic side-groups should occur. Consequently, the formation of the SA mesophase cannot be completed.

Table 15. Phase transition behaviors of homo-polysiloxanes and co-polysiloxanes containing end-on ﬁxed mesogenic side groups [150].

◦ −1 a × 3 Thermal Transitions/ C and Corresponding Enthalpy Changes (kcal mru ) in Parentheses Polymers Mn 10 Mw/Mn (GPC) (GPC) Heating Cooling b P-11a-2 2.7 1.51 G18K52(0.98)SA298(0.72) I I268(0.95)SA16G

K51(1.56)SA235(0.41)I P-11a-2/3-2(8/2) 4.9 1.51 I224(0.49)SA10G g 7 SA 231 (0.45) I

G-2K41(0.64)SA158(0.05) I P-11a-2/3-2(6/4) 6.1 1.84 I156(0.33)SA-10G G-6SA159(0.20)I c K40(0.82)SA92(0.02) I c P-11a-2/3-2(4/6) 9.5 1.86 c I92(0.13) SA-21G G-19K31(0.02)SA95(0.01) I K51(2.20)S 265(0.94)I P-11a-3 3.0 1.12 A I257(0.95)S 5G G10SA264(0.86)1 A

K47(1.58)SA204(0.37)I P-11a-3/3-3(8/2) 3.3 1.07 I190(0.26)SA-11G G-5K41(0.03)SA195(0.45)I c K36(0.91)SA105(0.05) I c P-11a-3/3-3(6/4) 3.5 1.10 d c I107(0.14) SA-20G G-16K26(0.08) K34(0.10)SA108(0.05) I

K57(0.46)K121(1.98)SA164(0.18)N212(0.35)I P-11b-2 14.4 1.97 I209(0.20)N157(0.15)SA92(2.03)K68(0.34)K K76(0.39)K123(2.32)SA163(0.15)N213(0.22)I K64(0.40)K107(2.00)N160(0.18)I P-11b-3 10.4 2.00 I155(0.16)N83(1.79)K52 (0.40)K K63(0.38)K107(1.98)N159(0.16)I a K: crystal; G: glass state; SA: smectic A phase; I: isotropic state. Data in the ﬁrst line are from the ﬁrst heating and cooling scans, data in the second line are from the second heating scan; b decomposition; c very broad transition (enthalpy change could not be accurately determined); d crystallization exotherm. Polymers 2018, 10, 794 24 of 37 Polymers 2018, 10, x FOR PEER REVIEW 24 of 37

+ Table 16. Phase transitionTable 16. behaviorsPhase transition of P-11a-3-Li behaviorsCF 3ofSO P3‐11acomplexes‐3‐Li+CF3 [SO1503]. complexes [150].

+ − −1 a + Mole− of Li CFThermal3SO3 Per Transitions/ Thermal◦C and Transitions/°C Corresponding and Enthalpy Corresponding Changes (kcalEnthalpy mru− Changes1) in Parentheses (kcal mrua ) in Parentheses Mole of Li CF3SO3 Per Mole Repeat of P‐11a‐3 Heating Cooling Mole Repeat of P-11a-3 Heating Cooling K51(2.20)SA287 b I 0 b SA5I K51(2.20)SA287 I b 0 b G10SA287 I SA5I G10SA287 I G15SA270 b I 0.3 b SA18I G15SA270 I b 0.3 b G23SA270 I SA18I G23SA270 I K48K55(1.13) c SA237 b I 0.7 c b SA30I K48K55(1.13) SA237 I b 0.7 b G36SA237 I SA30I G36SA237 I K53(0.65)SA234 b I 1.1 b SA29I K53(0.65)SA234 I b 1.1 b G38SA234 I SA29I G38SA234 I K47K58(0.44) c SA231 b I 1.7 c b SA39I K47K58(0.44) SA231 I b 1.7 b G45SA231 I SA39I G45SA231 I K48K57K65(0.83) c SA228 b I 2.0 c b SA40I K48K57K65(0.83) SA228 I b 2.0 b G45SA228 I SA40I G45SA228 I K58(0.16)K66(0.43)SA222 b I 2.5 K58(0.16)K66(0.43)S 222 b I SA41I A b 2.5 b G44SA222 1 SA41I G44SA222 1 K56(0.31)K64(0.34)SA 220 b I 2.7 K56(0.31)K64(0.34)S 220 b I SA38I A b 2.7 b G44SA220 I SA38I G44SA220 I A b K56(0.31)K64(0.21)Sb 221 I 3.0 K56(0.31)K64(0.21)S 221 I SA45I A b 3.0 b G46SA221 I SA45I G46SA221 I b K55(0.35)K64(0.21)Sb A215 I 3.3 K55(0.35)K64(0.21)SA215 I SA44I 3.3 b G47SA215 b I SA44I G47SA215 I c b c K55K64(0.59)b SA 14 I 4.5 K55K64(0.59) S 14 I SA41I A b 4.5 b G43SA214 I SA41I G43SA214 I K: crystal; G: glass state; SA: smectic A phase;a I: isotropic state. a Data in the first line are from the first K: crystal; G: glass state; SA: smectic A phase; I: isotropic state. Data in the ﬁrst line are from the ﬁrst heating and cooling scans,heating data in theand second cooling line scans, are from data the in second the second heating line scan; areb fromDetermined the second by optical heating polarized scan; b Determined by c microscopy; Overlappingoptical polarized transitions microscopy; (enthalpy change c Overlapping is the total transitions of both transitions). (enthalpy change is the total of both transitions).



Figure 18. ChemicalFigure structures18. Chemical of the structures side chain of LCthe homo-polysiloxanesside chain LC homo and‐polysiloxanes co-polysiloxanes and co‐polysiloxanes containing end-oncontaining ﬁxed mesogens end‐on andfixed oligooxyethylenic mesogens and oligooxyethylenic spaces [150]. spaces [150].

Polymers 2018, 10, 794 25 of 37 Polymers 2018,, 10,, x FOR PEER REVIEW 25 of 37

Figure 19. Schematic representation of the mechanism of salt complexation and its effect on the Figure 19. SchematicSchematic representation representation of of the the mechanism of of salt complexation and its effect on the arrangement of the mesogenic side groups of polysiloxanes containing oligooxyethylenic spacers arrangement ofof the the mesogenic mesogenic side side groups groups of polysiloxanesof polysiloxanes containing containing oligooxyethylenic oligooxyethylenic spacers spacers [150]. [150].[150]. Percec et al. also designed and synthesized the ﬁrst examples of side chain LC homo-polysiloxanes Percec et al. also designed and synthesized the first examples of side chain LC homo‐ containing oligooxyethylenic spacers and side-on ﬁxed mesogenic groups (see Figure 20), polysiloxanes containing oligooxyethylenic spacers and side‐on fixed mesogenic groups (see Figure 20), and investigated systematically the effect of oligooxyethylenic spacers on their phase transition and investigatedinvestigated systematically the effect of oligooxyethylenic spacers on their phase transition behaviors [151]. Results revealed that all the polysiloxanes synthesized displayed enantiotropic behaviors [151]. Results revealed that all the polysiloxanes synthesized displayed enantiotropic N Nmesophases, mesophases, and and Tg andTg and Ti decreasedTi decreased obviously obviously with with the length the length of spacers of spacers (see Table (see Table 17). The17). mesophases, and Tg and Ti decreased obviously with the length of spacers (see Table 17). The Thecomplexation complexation of the of thepolysiloxane polysiloxane P‐12 P-12-3‐3 with with LiCF LiCF3SO3SO3 resulted3 resulted in in an an initial initial decrease decrease and and then a complexation of the polysiloxane P‐12‐3 with LiCF3SO3 resulted in an initial decrease and then a subsequent increase of the glass transition of the complexes, and in a decrease of the N-I transition subsequent increaseincrease of the glass transition of the complexes, and inin a decrease of the N‐I transition temperature (as shown in Table 18). These complexes are nematic liquid-crystalline polyelectrolytes up temperature (as shown inin Table 18). These complexes are nematic liquidliquid‐crystalline polyelectrolytes toup a to value a value of 0.4of 0.4 mol mol LiCF LiCF3SO3SO3 per3 per mole mole repeat repeat unit unit of of polymer. polymer. Above Above this this value, value, the the NN mesophasemesophase up to a value of 0.4 mol LiCF3SO3 per mole repeat unit of polymer. Above this value, the N mesophase cannot form owing to its close proximity to the glass-transition temperature. A possible explanation for cannot form owing to itsits close proximity to the glass‐transition temperature. A possible explanation this behavior can arise from the consideration of the mesogen model previously described [152]. for this behavior can arise from the consideration of the mesogen model previously described [152]. According to this model, the helical arrangement of the mesogenic units is responsible for the According to this model, the helical arrangement of the mesogenic units isis responsible for the formation of the N liquid-crystalline phase. A schematic representation of the helical arrangement formation of the N liquidliquid‐crystalline phase. A schematic representation of the helical arrangement of ofthe the mesogenic mesogenic units units of ofthe the uncomplexed uncomplexed polymer polymer in in the the N N mesophase mesophase is is shown shown in in Figure Figure 21.21. the mesogenic units of the uncomplexed polymer in the+ N mesophase is shown in Figure 21. Complexation withwith LiCFLiCF33SO3 results in incorporation of Li+ ionsions within within the the cylindrical cylindrical structure structure and, Complexation with LiCF3SO3 results in incorporation of Li+ ions within the cylindrical structure and, consequently, to to an an increase increase in in its its diameter. diameter. Eventually, Eventually, increased increased LiCF LiCF3SO33SOcomplexation3 complexation destroys destroys the consequently, to an increase in its diameter. Eventually, increased LiCF3SO3 complexation destroys helical arrangement and transforms it into an open structure in which the nematic arrangement of the the helical arrangement and transforms itit intointo an open structure inin which the nematic arrangement mesogens is highly distorted, leading to the disappearance of the liquid-crystalline mesophase. of the mesogens isis highly distorted, leadingleading to the disappearance of the liquidliquid‐crystalline mesophase.

Figure 20.20. Chemical structures of the the side chainchain LCLC homo-polysiloxaneshomo‐‐polysiloxanes containing side-onside‐‐on ﬁxedfixedfixed mesogens andand oligooxyethylenicoligooxyethylenic spacesspaces [[151].[151].151].

Polymers 2018, 10, 794 26 of 37 Polymers 2018, 10, x FOR PEER REVIEW 26 of 37

FigureFigure 21. 21.Schematic Schematic representation representation of of the the mechanism mechanism of salt of saltcomplexation complexation and its and effect its on effect the on the arrangementarrangement of of the the mesogenic mesogenic groups groups of ofa aside side chain chain LC LC homo homo-polysiloxanes‐polysiloxanes based based on on oligooxyethylenic spacers and side‐on fixed mesogenic groups [151]. oligooxyethylenic spacers and side-on ﬁxed mesogenic groups [151].

Table 17. Phase transition behaviors of homo‐polysiloxanes containing side‐on fixed mesogenic side Tablegroups 17. Phase [151]. transition behaviors of homo-polysiloxanes containing side-on fixed mesogenic side groups [151]. −1 a ࡹഥ n × 103 ࡹഥ w/ࡹഥ n Thermal Transitions/°C and Corresponding Enthalpy Changes (kcal mru ) in Parentheses Polymers (GPC) (GPC) Heating Cooling ◦ −1 a M × 103 M /M Thermal Transitions/G41K88(0.29)N122(0.13)IC and Corresponding Enthalpy Changes (kcal mru ) in Parentheses Polymers P‐12‐1 n 7.8 w1.97n I125(0.19)N47G (GPC) (GPC) HeatingG50N129(0.18)I Cooling G41K88(0.29)N122(0.13)IK54(0.45)K90(2.33)N109(0.31)I P‐12‐2 9.7 1.27 I125(0.19)N47GI107(0.20)N35G P-12-1 7.8 1.97 G50N129(0.18)IG39N111(0.19)I K53(0.21)K83(2.99)N104(0.17)I P‐12‐3 12.0 1.84 K54(0.45)K90(2.33)N109(0.31)I I98(0.19)N26G I107(0.20)N35G P-12-2 9.7 1.27 G39N111(0.19)IG31N103(0.23)I a K: crystal; G: glass state; N: nematic phase;K53(0.21)K83(2.99)N104(0.17)I I: isotropic state. Data in the first line are from the first P-12-3 12.0 1.84 I98(0.19)N26G heating and cooling scans, data in the secondG31N103(0.23)I line are from the second heating scan. K: crystal; G: glass state; N: nematic phase; I: isotropic state. a Data in the first line are from the first heating and

cooling scans, dataTable in the second18. Phase line transition are from behaviors the second of heating P‐12‐3/Li scan.+CF3SO3− complexes [151].

−1 a + 3 3− Thermal Transitions/°C and Corresponding Enthalpy Changes (kcal mru ) in Parentheses Mole of Li CF SO Per + − Mole RepeatTable of P 18.‐12‐3Phase transition behaviorsHeating of P-12-3/Li CF3SO3 complexesCooling [151]. K53(0.21)K83(2.99)N104(0.17)I 0 I98(0.19)N26G ◦ G31N103(0.23)I −1 a Mole of Li+CF SO − Per Thermal Transitions/ C and Corresponding Enthalpy Changes (kcal mru ) in Parentheses 3 3 K70(3.68)I Mole Repeat of P-12-30.1 I67(0.06)N16G HeatingG19N71(0.07)I Cooling K53(0.21)K83(2.99)N104(0.17)IK66(3.16)I 0 0.2 I98(0.19)N26GI60(0.07)N20G G31N103(0.23)IG22N 65(0.05) K70(3.68)IK60(2.67)I 0.3 I67(0.06)N16GI55(0.06)N20G 0.1 G19N71(0.07)IG26N61(0.05) K60(1.87)I 0.4 K66(3.16)I I48(0.05)N24G I60(0.07)N20G 0.2 G22N 65(0.05)G28N54(0.06)I K45(0.87)I 0.5 K60(2.67)I I26G G28I I55(0.06)N20G 0.3 G26N61(0.05) K48(0.35)I 0.6 I31G K60(1.87)I G31I 0.4 I48(0.05)N24G G28N54(0.06)I‐ b 0.7 I33G K45(0.87)I G33I 0.5 ‐ b I26G 0.8 G28I I43G G48I K48(0.35)I K: crystal;0.6 G: glass state; N: nematic phase;G31I I: isotropic state. a Data in the first line areI31G from the first heating and cooling scans, data in the -secondb line are from the second heating scan; b No glass 0.7 I33G transition or melting endotherm could beG33I detected. - b 0.8 I43G 3.2.3. Dependence of the Thioether SpacerG48I on the Phase Transition Behaviors & Phase Structures of a K:the crystal; PSCLCPs G: glass state; N: nematic phase; I: isotropic state. Data in the first line are from the first heating and cooling scans, data in the second line are from the second heating scan; b No glass transition or melting endotherm couldIn be the detected. periodic table of chemical elements, sulfur atoms and oxygen atoms belong to the same family. However, the larger atomic radius of sulfur atoms results in a significant difference in polarity 3.2.3.and Dependence compliance of with the oxygen Thioether atoms, Spacer which on may the Phaselead to Transitiona more sensitive Behaviors response & Phase to electro Structures‐optical of the PSCLCPsperformance. In the periodic table of chemical elements, sulfur atoms and oxygen atoms belong to the same family. However, the larger atomic radius of sulfur atoms results in a significant difference in polarity and compliance with oxygen atoms, which may lead to a more sensitive response to electro-optical performance. Polymers 2018, 10, 794 27 of 37

Polymers 2018, 10, x FOR PEER REVIEW 27 of 37 Polymers 2018, 10, x FOR PEER REVIEW 27 of 37 Jean-Claude Milano et al. developed three new cyanobiphenyl-based polysiloxanes containing Jean‐Claude Milano et al. developed three new cyanobiphenyl‐based polysiloxanes containing long thioetherJean‐Claude spacer Milano [153, 154et al.]; developed Figure 22 three presents new cyanobiphenyl the chemical‐based structures. polysiloxanes This typecontaining of spacer long thioether spacer [153,154]; Figure 22 presents the chemical structures. This type of spacer effectivelylong thioether removes spacer the crystallinity [153,154]; Figure phenomenon, 22 presents enables the chemical to obtain structures. a wide range This type of mesophase of spacer and effectively removes the crystallinity phenomenon, enables to obtain a wide range of mesophase and to geteffectively polymers removes presenting the crystallinity an interdigitated phenomenon, smectic enables A (Sm toA obtaind) phase a wide structure range of at mesophase room temperature and to get polymers presenting an interdigitated smectic A (SmAd) phase structure at room temperature to get polymers presenting an interdigitated smectic A (SmAd) phase structure at room temperature (as shown(as shown in Table in Table 19 and19 and Figure Figure 23 23).). Compared Compared with the the polymethylenic polymethylenic spacer, spacer, the theflexibility ﬂexibility of the of the (as shown in Table 19 and Figure 23). Compared with the polymethylenic spacer, the flexibility of the thioetherthioether spacer spacer is increased is increased by by the the presence presence ofof sulfur, which which could could have have interesting interesting consequences consequences on on thioether spacer is increased by the presence of sulfur, which could have interesting consequences on the propertiesthe properties of these of these polymers, polymers, e.g., e.g., the the sensitivity sensitivity to electric, electric, optical, optical, magnetic magnetic fields. ﬁelds. the properties of these polymers, e.g., the sensitivity to electric, optical, magnetic fields.

CH3 CH3 CH3 H C SiCHOSi3 CHO3 SiCHCH3 3 35 3 H3C Si OSiO Si CH3 CH3 35 CH3 CH3 CH2 n CH3 CH2 n S S CH2 m CH O 2 m O

Pn,m Pn,m

CN CN Figure 22. Chemical structures of the cyanobiphenyl‐based polysiloxanes containing long thioether FigureFigure 22. Chemical 22. Chemical structures structures of of the the cyanobiphenyl-based cyanobiphenyl‐based polysiloxanes polysiloxanes containing containing long long thioether thioether spacers [154]. spacersspacers [154 ].[154].

Figure 23. Example of an ‘‘interdigitated’’ structure observed by X‐ray analysis: P8, 2 [154]. Figure 23. Example of an ‘‘interdigitated’’ structure observed by X‐ray analysis: P8, 2 [154]. Figure 23. Example of an “interdigitated” structure observed by X-ray analysis: P8, 2 [154]. Table 19. Physical properties of the cyanobiphenyl‐based polysiloxanes containing long thioether Table 19. Physical properties of the cyanobiphenyl‐based polysiloxanes containing long thioether spacers (first heating) [154]. Tablespacers 19. Physical (first heating) properties [154]. of the cyanobiphenyl-based polysiloxanes containing long thioether spacers (ﬁrst heating) [154]. ΔT/°C Polymers (Pn,m) Tg/°C Tk/°C Ti/°C ΔT/°C Mesophase Polymers (Pn,m) Tg/°C Tk/°C Ti/°C (1) (2) Mesophase (1) (2) ◦ P6,2 −15 ‐ 101 116 ‐ ∆TSmA/ C P6,2◦ −15 ‐ ◦ 101 ◦ 116 ‐ SmA Mesophase Polymers (Pn,m) TgP/8,2 −C T20k ‐ / C 118Ti/ C138 ‐ SmA P8,2 −20 ‐ 118 138 ‐ (1)SmA (2) P10,2 −21 ‐ 125 146 ‐ SmA P10,2 −21 ‐ 125 146 ‐ SmA P6,2 −P10,315 −10 ‐ -135 101 145 ‐ 116SmA - SmA P10,3 −10 ‐ 135 145 ‐ SmA P8,2 −P11,220 −22 -45 121 118 ‐ 76 138 SmA - SmA P11,2 −22 45 121 ‐ 76 SmA P10,2 −P11,321 −15 -68 140 125 ‐ 72 146 SmA - SmA P11,3 −15 68 140 ‐ 72 SmA P10,3 −P11,10‐ ‐ -52 181 135 ‐ 129 145 SmA - SmA P11,‐ ‐ 52 181 ‐ 129 SmA SmA:P11,2 smectic A phase;−22 Tg: Glass 45transition temperature; 121 Tk: -Melting 76temperature SmA(from SmA: smectic A phase; Tg: Glass transition temperature; Tk: Melting temperature (from crystallization);P11,3 Ti: Isotropisation−15 temperature; 68 ΔT = Ti − 140Tg (1); ΔT = Ti - − Tk (2). 72 SmA crystallization); Ti: Isotropisation temperature; ΔT = Ti − Tg (1); ΔT = Ti − Tk (2). P11,- - 52 181 - 129 SmA SmA:H. smectic Yang Aet phase;al. developedT : Glass a series transition of polysiloxane temperature;‐basedT : Melting LCPs temperaturewith thioether (from spacer crystallization); via thiol−eneT : H. Yang et al. developedg a series of polysiloxane‐basedk LCPs with thioether spacer via thiol−enei Isotropisationclick chemistry temperature; (as shown∆ Tin= FigureTi − T g24)(1); [39].∆T = OnTi −theT kbasis(2). of the POM, DSC, SAXS, and WAXS results, click chemistry (as shown in Figure 24) [39]. On the basis of the POM, DSC, SAXS, and WAXS results, these polysiloxane‐based LCPs with thioether spacer could readily achieve a wide variety of liquid these polysiloxane‐based LCPs with thioether spacer could readily achieve a wide variety of liquid H.crystalline Yang et phases al. developed of nematic a (N) series phase of polysiloxane-based or smectic (SmX) phase. LCPs Due withto the thioether lack 2D WAXD spacer results, via thiol the− ene crystalline phases of nematic (N) phase or smectic (SmX) phase. Due to the lack 2D WAXD results, the click chemistry (as shown in Figure 24)[39]. On the basis of the POM, DSC, SAXS, and WAXS results, these polysiloxane-based LCPs with thioether spacer could readily achieve a wide variety of Polymers 2018, 10, 794 28 of 37

liquidPolymers crystalline 2018, 10, x FOR phases PEER REVIEW of nematic (N) phase or smectic (SmX) phase. Due to the lack 2D WAXD28 of 37 results, the precise smectic structure of PMMS-g-LC1 and PMMS-g-LC1 was not conﬁrmed. However, accordingprecise smectic to Dreiding structure models of PMMS [155‐g,156‐LC1] and and the PMMS results‐g‐LC1 obtained was not by confirmed. Jean-Claude However, Milano according [153,154], PMMS-g-LC1to Dreiding models and PMMS-g-LC2[155,156] and the might results have obtained SmA and by Jean interdigital‐Claude Milano SmAd phases, [153,154], respectively PMMS‐g‐LC1 (as illustratedand PMMS in‐g‐ TableLC2 might 20). have SmA and interdigital SmAd phases, respectively (as illustrated in Table 20).

Figure 24.24. ChemicalChemical structuresstructures ofof thethe polysiloxane-basedpolysiloxane‐based LCPsLCPs withwith thioetherthioether spacerspacer via via thiol thiol−−ene click chemistry [[39].39].

Table 20. Mesomorphic properties of the polysiloxane‐based LCPs with thioether spacer via thiol−ene Table 20. Mesomorphic properties of the polysiloxane-based LCPs with thioether spacer via thiol−ene click chemistry [39]. click chemistry [39]. Phase Transitions (°C) a Polymers ◦ a PhaseHeating Transitions Cooling ( C) Polymers PMMS‐g‐LC1 HeatingSmX 62 I I 59 SmX Cooling PMMS‐g‐LC2 SmX 82 I I 80 SmX PMMS-g-LC1 SmX 62 I I 59 SmX PMMS-g-LC2PMMS‐g‐LC3 SmXN 8270 I I 68 I 80N SmX PMMS-g-LC3PMMS‐g‐LC4 NN73 70 I I I 71 IN 68 N PMMS-g-LC4PMMS‐g‐LC5 N73N 56 I I I 54 IN 71 N a Detected only by polarizedPMMS-g-LC5 optical microscopy. N 56N I= nematic, SmX I 54 = N undefined smectic phase, I = aisotropic,Detected G only = glass by polarized phase. optical microscopy. N = nematic, SmX = undeﬁned smectic phase, I = isotropic, G = glass phase. By thiol‐ene click chemistry, H. Yang et al. designed and synthesized a series of PMMS‐based PSCLCPs,By thiol-ene the mesogenic click chemistry, units H.of Yangwhich et were al. designed cholesterol and‐terminated synthesized chiral a series unit of PMMS-basedwith achiral PSCLCPs,substituted the4‐methoxyphenyl mesogenic units 4‐(allyloxy) of which benzoate were cholesterol-terminated unit [121], or cholesterol chiral‐terminated unit with chiral achiral unit substitutedwith achiral 4-methoxyphenyl substituted 4‐cyanophenyl 4-(allyloxy) 4 benzoate‐(allyloxy) unit benzoate [121], or unit cholesterol-terminated [90], respectively. The chiral phase unit withbehaviors achiral of substitutedPMMS‐based 4-cyanophenyl polymers based 4-(allyloxy) on cholesterol benzoate‐terminated unit [90], respectively.chiral unit and The achiral phase behaviorssubstituted of PMMS-based4‐methoxyphenyl polymers 4‐(allyloxy) based on cholesterol-terminatedbenzoate unit (abbreviation chiral unit for and PMMS achiral‐OCH substituted3‐based 4-methoxyphenylpolymers) have been 4-(allyloxy) described benzoate in this paper unit (abbreviation above. However, for PMMS-OCH the polymers3-based based polymers) on cholesterol have‐ beenterminated described chiral in this unit paper with above. achiral However, substituted the polymers 4‐cyanophenyl based on cholesterol-terminated 4‐(allyloxy) benzoate chiral unit with(abbreviation achiral substituted for PMMS 4-cyanophenyl‐CN‐based polymers, 4-(allyloxy) the chemical benzoate structure unit (abbreviation of which is for shown PMMS-CN-based in Figure 25) polymers,exhibited a thedrastic chemical change structure in mesogenic of which state although is shown only in Figure cyano substitution25) exhibited was a used drastic instead change of inmethoxy mesogenic substitution. state although Attributing only to cyano the decisive substitution role of the was polarity used instead interaction, of methoxy a strong substitution.dependence Attributingof LC phase to structures the decisive on roleXchol of did the not polarity reappear interaction, in this series a strong of dependencecyano‐terminated of LC phasepolymers. structures With onincreasingXchol did X notchol, reappear polymers in thiswith series cyano of‐terminated cyano-terminated side chains polymers. in this With work increasing developedXchol, polymersa single withmonolayer cyano-terminated interdigitated side SmA chains phase in (as this illustrated work developed in Table 21). a single The results monolayer obtained interdigitated above were Sm allA in accordance with those by Jean‐Claude Milano [153,154].

Polymers 2018, 10, 794 29 of 37 phase (as illustrated in Table 21). The results obtained above were all in accordance with those by PolymersJean-Claude 2018, 10 Milano, x FOR PEER [153 ,REVIEW154]. 29 of 37

Figure 25. ChemicalChemical structures of PMMS PMMS-CN-based‐CN‐based polymers [90]. [90].

TableTable 21. Mesomorphic properties of PMMS-CN-basedPMMS‐CN‐based polymers [[90].90].

Xchol a Mn (g/mol) Phase Transitions (°C) d Polymers a ◦ d In FeedX chol CALC b GPCMn c (g/mol)CALC b HeatingPhase Transitions ( C) Cooling Polymers P1 In0.00 Feed CALC0.000b GPC2200c CALC18,747b G2.4SmAHeatings92.1I I81.3SmA Coolings‐1.4G P2 0.20 0.221 3300 21,412 G10.5SmAs96.3 I I98.5SmAs8.8G P1P3 0.000.40 0.0000.425 22003700 18,74723,872 G2.4SmAG18.7SmAs92.1Is111.7I I113.4SmA I81.3SmAs16.4Gs-1.4G P2P4 0.200.60 0.2210.604 33004000 21,41226,031 G10.5SmAG23.6SmAss96.3176.8I I I175.8SmA I98.5SmAs19.5Gs8.8G P3 0.40 0.425 3700 23,872 G18.7SmA 111.7I I113.4SmA 16.4G P5 0.80 0.819 4400 28,624 G33.1SmAs s189.4I I186.7SmAs29.9Gs P4 0.60 0.604 4000 26,031 G23.6SmA 176.8I I175.8SmA 19.5G P6 1.00 1.000 4800 30,852 G35.7SmAd139.9SmAs204.8Is I197.9SmAs136.6SmAs d34.6G P5 0.80 0.819 4400 28,624 G33.1SmAs189.4I I186.7SmAs29.9G a b 1 c P6Molar ratio; 1.00 According 1.000 to H 4800‐NMR results; 30,852 determined G35.7SmAd139.9SmAs204.8I by GPC in THF I197.9SmA using spolystyrene136.6SmAd34.6G d standards;a Molar ratio; baccordingAccording to 1DSCH-NMR results; results; G:c determinedglass state; by SmA GPCs: in smectic THF using A phase polystyrene with standards;single layer;d according SmAd: smecticto DSC results; A phase G: glasswith state; double SmA layer;s: smectic I: isotropic A phase state. with single layer; SmAd: smectic A phase with double layer; I: isotropic state. 4. Conclusions 4. Conclusions Encouraged by the first publication of PSCLCPs via hydrosilylation reaction between polymethylhydrosiloxaneEncouraged by the (PMHS) first publication and unsaturated of PSCLCPs carbon–carbon via hydrosilylation bond based monomers, reaction a between variety ofpolymethylhydrosiloxane improved synthetic methods (PMHS) and and lots unsaturated of polysiloxane carbon–carbon‐based LC bond materials based with monomers, diverse a structures variety of wereimproved rapidly synthetic developed. methods We attempted and lots of to polysiloxane-based find the factors influencing LC materials phase with transition diverse structures behaviors were and phaserapidly structures developed. from We a attempted molecular to level. find the The factors following influencing trends phasecan be transition concluded: behaviors For the and influence phase factorsstructures of main from achain, molecular (i) flexible level. polysiloxane The following backbones trends can that be concluded:can enhance For the the decoupling influence factorsof the motionsof main chain,of the (i) side flexible chain polysiloxane and main chain backbones tend thatto give can enhancerise to a the higher decoupling thermal of stability the motions of the of mesophases,the side chain and and a maindecrease chain in tendglass to‐transition give rise temperatures. to a higher thermal However, stability for cyclic of the SCLC mesophases, polysiloxane and a anddecrease corresponding in glass-transition open chain temperatures. analogues, However, increased for cyclicrigidity SCLC of the polysiloxane cyclic backbone and corresponding played an importantopen chain role analogues, in the stability increased of the rigidity blue phase, of the cyclicwhich backboneaccordingly played improved an important the helical role twisting in the powerstability (HTP); of the (ii) blue A phase, longer which polysiloxane accordingly backbone improved (namely, the helical a higher twisting degree power of polymerization, (HTP); (ii) A longer DS) canpolysiloxane enhance the backbone stability (namely, of the LC a phase, higher and degree even of more polymerization, ordered phase DS) structures can enhance can thebe developed; stability of (iii)the LCThe phase, phase and transition even more temperatures ordered phase increased structures with can bethe developed; polymerization (iii) The degree phase (DS) transition and temperatures increased with the polymerization degree (DS) and composition of the mesogenic unit. composition of the mesogenic unit. All the Tg and Ti increased linearly with DS, and the temperature All the Tg and T increased linearly with DS, and the temperature range of the LC phase between Tg and range of the LCi phase between Tg and Ti expanded with DS. For the influence factors of spacer, (i) TThei expanded flexible withspacers DS. between For the influence the mesogenic factors ofside spacer, group (i) Thefrom flexible the polymer spacers betweenbackbone the can mesogenic usually improveside group the from mobility the polymer of the mesogenic backbone can groups usually and improve facilitate the the mobility formation of the of mesogenic LC phase groupsstructures. and Generally,facilitate the the formation longer the of spacer LC phase is, the structures. more stable Generally, the mesophase the longer is; the (ii) spacer The increase is, the more in rigidity stable and the polaritymesophase of the is; (ii) spacer The increasecan increase in rigidity the glass and transition polarity of of the the spacer polymers, can increase and widen the glass the range transition of the of mesophase.the polymers, However, and widen too the much range increase of the mesophase. in rigidity However,of the spacer too muchmay also increase destroy in rigidity the ordered of the arrangementspacer may also of the destroy molecules, the ordered and lead arrangement to disappearance of the molecules, of the liquid and lead‐crystalline to disappearance mesophase; of (iii) the The introduction of heteroatoms such as oxygen and sulfur atoms in the spacer can properly improve the flexibility and polarity of molecules, which could have interesting consequences on the properties of these polymers, e.g., the sensitivity to electric, optical, and magnetic fields. The analysis above will help us design molecular structures effectively.

Polymers 2018, 10, 794 30 of 37 liquid-crystalline mesophase; (iii) The introduction of heteroatoms such as oxygen and sulfur atoms in the spacer can properly improve the ﬂexibility and polarity of molecules, which could have interesting consequences on the properties of these polymers, e.g., the sensitivity to electric, optical, and magnetic ﬁelds. The analysis above will help us design molecular structures effectively.

Author Contributions: L.Z. and H.Y. conceived and designed the content of the paper, L.Z. wrote the paper, W.Y. and Y.G. revised the paper, C.Z helps edit the paper. Funding: This work is supported by the National Natural Science Foundation of China (Grants No. 51333001, 51573006, 51561135014, 51573003, 51720105002, 51602007), and the Joint Fund of the Ministry of Education for Equipment Pre Research (Grants No. 6141A020222). Acknowledgments: This work is supported by the National Natural Science Foundation of China (Grants No. 51333001, 51573006, 51561135014, 51573003, 51720105002). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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