Broer, Dirk Jan

English

The scope of this thesis concerns a new method to produce oriented polymers by the in-situ photopolymerization of oriented liquid-crystalline acrylates. The desired macroscopic molecular order is already accomplished in the monomeric state prior to the polymerization. ... Zie: Summar

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   University of GroningenIn-situ photopolymerization of oriented liquid-crystalline acrylatesBroer, Dirk JanIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1990Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Broer, D. J. (1990). In-situ photopolymerization of oriented liquid-crystalline acrylates. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 13-02-2023l 5 lSummaryOriented polymers are of a long-standing theoretical and practical in-terest because of their highly anisotropic properties. Well-known featuresare (i) an anisotropic mechanical behaviour with a high modulus and ahigh strength in the direction of the molecular orientation, which is alreadywidely applied in polymeric fibers and filaments, (ii) an anisotropic opticalbehaviour with a large difference in refractive index or optical absorptionbetween the principal directions, which is usefull for the manufacturing ofoptical components, and (iii) an anisotropic thermal behaviour with a lowthermal expansion coefficient in the direction of the molecular orientation,which is desirable for coating and encapsulation of inorganic substrates inorder to avoid thermal stresses.The scope of this thesis concerns a nelv method to produce orientedpolymers by the in-situ photopolymerization of oriented liquid-crystallineacrylates. The desired macroscopic molecular order is already accom-plished in the monomeric state prior to the polymerization. Because of thecombination of low-viscosity and anisotropic physical properties of thet52 Sttnmturtmesogenic monomers, the molecules orient themselves instantaneously andwith a low concentration of orientional defects under the action of an ex-ternal field, e.g. an electric or a magnetic field, or by the interaction witha treated substrate, e.g. a thin rubbed polyimide coating. A subsequentshort UV exposure generates free-radicals from a dissolved photoinitiator,initiating the chain-polymerization. During the bulk polymerization theliquid-crystalline ordering of the monomer persists and is frozen in as soonas the reacting medium passes a glass transition or crystallization temper-ature. The photopolymerization of diacrylates rather than monoacrylatesleads to densely crosslinked oriented polymers in which the mesogenicmoieties are anchored at both ends to the polymer network. yielding apermanent and temperature-insensitive molecular orientation.The application of photo-initiated polymerization oÍ'fers a number ofadvantages in comparison with for instance thermally-initiatedpolymerization. Firstly, it can be carried out in a broad temperature rangeand the reaction does not start before the exposure. This is very importantsince a particular liquid-crystalline ordering is often observed only in arelatively narrow temperature range. Consequently, one is able to succes-sively heat the monomer to the desired temperature, shape it into the de-sired form and orient the monomer molecules without the occurrence ofearly polymerization which would fix a wrong type of ordering, e.g. an-other mesomorphic phase during heating to the appropriate temperature,or orientational defects. e.g. imposed during flow of the monomers duringprocessing. Secondly, photopolymerization proceeds very fast which pre-vents phase segregation in case the polymer formed does not dissolve in itsmonomer. The latter is observed for instance when a smectic polymer isformed from a nematic monomer at low polymerization rates. Thirdly,by the use of photopolymerization one has the possibil i ty to structurize theoriented polymer by pattern-wise irradiation, which might be usefull espe-cially in opto-electronic applications.In Chapter 2 of this thesis the formation of liquid-crystalline side-chainpolymers by photopolymerization of liquid-crystalline monoacrylates isdiscussed. ln Chapter 2.1 it is shown that the macroscopic orientation ofthe monomer is preserved during polymerization, although both the orderparameter and the type of mesogenic ordering of the polymer formed maybe different from those of the monomer. During polymerization theliquid-crystalline transition temperatures shift to higher values. This meansthat for the monomer studied, an initially nematic monomeric orderingbecomes smectic-A for the polymer-monomer mixture during the reaction,which can be observed by a step-wise increase of the birefringence. In thecase of a polymerization below the melting temperature of the polymer, thesample crystallizes during the reaction. When a polymerization temper-ature is selected above the polymeric melting temperature, crystallizationtakes place after PolYmettemperature to room temA kinetic studY of tmesomorPhic Phase, aspolymerization ProceedsThe reaction Proceedscrvstallization of the reacu io* *onomer diffusiomesomorPhic monomer jl iqu id crYsta l l ine dur ingmains wi th mutual lY d i l fand, therefore, a decreaslieht intensitY decreasesln ChaPter 3 of this tby photoPolYmerizatron-ono-"t sYnthesized (Crange between 88 and Itemperatures. The ordetnear the smectic Phase tPolymerization in the nepolymerization Processmonomer dePletion andThe order Parameter ofmonomer in case of a hiorder Parameter it increThe attachment of aliquid-crYstalline diacrYtion temPeratttres andequal reduced temPeratorientation in the monrsity of the mesogenshinderance due to theparameters in the PoiYworks hardlY change b'A longer flexible alland the acrYlate motetature and the nemat ic-stabil izes the formattotstate the smectic-C Plwhich Persist during Pdegree of  or ientat ion iat equal reduced temPnetworks the shorter irameters of both the t1 5 3takes place after polymerization during cooling from the polymerizationtemperature to room temperature.A kinetic study of the polymerization of the monoacrylate in itsmesomorphic phase, as described in Chapter 2.2, revealed that thepolymerization proceeds fast both in the nematic and in the smectic state.The reaction proceeds untill depletion of monomer. In the case ofcrystallization of the reaction mixture, complete conversion is hindered bya low monomer diffusion in the crystalline state. Polymerization of themesomorphic monomer in an initially isotropic state gives a transition toliquid crystalline during the reaction. In that case the formation of do-mains with mutually different orientation directions causes light scatteringand, therefore, a decrease of the overall polymerization rate since the UVlight intensity decreases in the deeper sections of the polymerizing flrlm.In Chapter 3 of this thesis the formation of oriented polymer networksby photopolymerization of liquid-crystalline diacrylates is discussed. Themonomer synthesized (Chapter 3.1) exhibits a broad nematic temperaturerange between 88 and 155'C and a monotropic smectic-C phase at lowertemperatures. The order parameter in the nematic phase varies from 0.7near the smectic phase transition to 0.4 near the isotropic phase transition.Polymerization in the nematic phase proceeds fast in the initial stage of thepolymerization process and more slowly at higher conversions due tomonomer depletion and a reduced mobility of the remaining vinyl groups.The order parameter of the polymer network formed is equal to that of themonomer in case of a high initial order parameter, e.g.0.7. At lower initialorder parameter it increases during polymerization.The attachment of a lateral methyl group to the central moiety of theliquid-crystalline diacrylate (Chapter 3.2) is found to decrease the transi-tion temperatures and to suppress the formation of smectic phases. Atequal reduced temperature the methyl group does not affect the degree oforientation in the monomeric state. However, the increased packing den-sity of the mesogens after polymerization leads to an enhanced sierichinderance due to the side-group and, as a consequence, to lower orderparameters in the polymeric state. The mechanical properties of the net-works hardly change by the substitution of the methyl group.A longer flexible alkylene spacer between the mesogenic central groupand the acrylate moiety (Chapter 3.3) decreases both the melting temper-ature and the nematic-isotropic transition of the diacrylate monomers andstabilizes the formation of a smectic-C phase. In the macroscopic orientedstate the smectic-C phase is characterized by local director fluctuationswhich persist during polymerization. In the monomeric nematic state thedegree of orientation is not influenced by the spacer length when studiedat equal reduced temperatures. However, upon formation of the orientednetworks the shorter alkylene spacers lead to somewhat lower order pa-rameters of both the mesogenic unit and the spacer itself. It is proposed154 Sununartthat at short spacers the steric hindrance between the densely packed cen-tral units, as imposed by the periodicity in the acrylate main chains, leadsto conformational changes of the mesogenic moieties, e.g. to a smallerangle between the para-axes of the benzene rings by a rotation of thesemoieties over the connecting ester groups.From the studies presented in Chapters 3.1,3.2 and 3.3 we concludethat the anisotropic optical properties of the polymer networks can bevaried over a wide range by the variation of the spacer length, by the ap-plication of a lateral substituent or by influencing the degree of orientationby the polymerization temperature. This, and the fact that the networkprohibits the formation of polymer crystallites as often observed for linearpolymers which would enhance light scattering, makes the materials ulti-mately suited for various optical applications.In the investigations discussed so far, only uniaxially oriented polymernetworks are described. However, by the addition of chiral dopants to theliquid-crystalline monomers it becomes possible to produce polymericnetworks with a helicoidal ordering of the mesogenic crosslinks. Chapter3.4 has been dedicated to this technique which yields polymers with unu-sual optical properties. By the rubbed substrate orientation technique. aplanar anchoring is achieved at both interfaces of the film and conse-quently the molecular helix lies perpendicular to the plane of the film. Thetotal angle of rotation can be adjusted very accurately by the dopant con-centration and the angle between the rubbing directions at both substrates.During the network formation or during temperature cycling afterwards,the pitch of the helix changes rather than the total angle of rotation dueto thickness variations.The anisotropic thermal expansion of both the uniaxially and thehelicoidally ordered polymer networks are described in Chapter 3.5. Incase of a uniaxial orientation, the thermal expansion coefficient in the di-rection of the orientation is low, but still positive, below the glass transi-tion temperature of the networks. As soon as this transition sets in atincreasing temperatures, the network starts to contract in the same direc-tion. This process appeared to be reversible during repeated temperaturecycling. Perpendicular to the molecular orientation, the thermal expansionis much higher and compensates for the anomalous thermal expansionalong the molecular orientation such that the calculated overall volumeexpansion is comparable to that of conventional polyacrylates. The influ-ence of the monomeric structure and the polymerization conditions on theanomalous thermal expansivity is discussed in terms of the small reversibledecrease of the order parameter during temperature cycling. In case of afilm consisting of a helicoidally ordered polymer network with the molec-ular helix perpendicular to the plane of the hlm, a low thermal expansionis measured for the principal directions in the plane of the film. As a result,this type of molecular arrangement is very promising for the coating orencapsulation of Planatavoided in the Plane of tIn the APPendix thephotopolYmerization ofcrystalline diacrYlates muof the temperature rangethe effects of these I-polymerization. Therstudy is carried out usinlcomparative matter forthe polymerization behapolymerization rate incttemperatures the ProPcreasinglY imPortant leiisotropic diacrYlates chithe acrYlate grouP enhrautoacceleration regionthe overall PolYmerizacrystalline ordered statepolymerization rates u1higher temPeratures thdeproPagation.h,1 5 5encapsulation of planar substrates where thermal stresses should beavoided in the plane of the substrate.In the Appendix the influence of relatively high temperatures on thephotopolvmerization of isotropic aitmeitrlacivlatËs is tfi*rrráa. Liquidcrystalline diacrylates must be potymeiized between g0 and 165.c becauseof the temperature tung: .of ttt"ir -"sofhur"r. Ho.,'"ver, littre is known ofthe effects of these high temperat-"rÀ on the kinetics of photo-polymerization. Therefóre, a high-temperature photopolymerizationstudy is carried out u-sing conventionál isoiropic *ooá*"..ïrrïi0". to g"tcomparative matter for estimating the influence of liquid crystailinity onthe porvmerization behaviour. I;is fo;;à ro, u aiÀËirrl"r# that thepolymerization rate increases wrttr temperature up to 160t. At highertemperatures the propa.gation-depropagation "qrritiUriu- Íecomes in-creasingly important reading to lowei oïerall polymerization rates. Forisotropic diacryrates chain tiansfer uà áu.aruction of the a- hydrogen ofthe acrylate group enhances the rate oi-trr. termination reaction in theautoacceleration resion of the polymerization. This leads to a decrease ofthe overall polvmàrization. rate "iià"av"'"uove 90"c. i;;h" hquid-crystalline ordered state chain transfer is'suppressed, which results in highpolymerization rates y? tg rh-" isotropic transition temperature. At stillhigher temperatures this is foilowed iv u-rapio drop of the rate due todepropagation.

In-situ photopolymerization of oriented liquid-crystalline acrylates

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In-situ photopolymerization of oriented liquid-crystalline acrylates

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