Polysiloxane-Based Side Chain Liquid Crystal Polymers: from Synthesis to Structure–Phase Transition Behavior Relationships
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polymers 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, etc. main [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;