Oriented copolymers with liquid crystalline building blocks

Several series of reversible physical networks based on polydimethylsiloxane (PDMS) chains and liquid crystalline (LC) structural units were synthesized and studied, which belong to three different architecture types: (1) LC-grafted PDMS (with LC quartets at the grafting sites), (2) LC-end-capped linear PDMS, (3) and linear 'infinite' LC-PDMS copolymers (with alternating LC and PDMS segments). PDMS spacers of different lengths of were tested, as well as 6 different polyaromatic mesogens of azo type and azo-free type. Hydrosilylation coupling of vinyl-functional mesogens, obtained as part of an international cooperation, with commercial Si-H-functional PDMS was employed to synthesize all the studied materials. The copolymers were physically crosslinked by the nano-aggregation of the LC units contained in their macromolecules. The thermotropic properties of the LC-nano-aggregates lent interesting physical properties to the whole material, making some of the copolymers attractive as potential smart materials. The PDMS spacer segments were selected for the sake of their extreme flexibility, which should provide elastic properties to the physically crosslinked copolymers, and also because of their (desired) incompatibility with the mesogens. This latter effect was highly helpful for achieving the..

Orientované kopolymery obsahující kapalně krystalické stavební bloky

Shrnutí Bylo syntetizováno a studováno několik sérií reverzibilních fyzikálních sítí založených na polydimetylsiloxanových (PDMS) řetězcích a kapalně krystalických (LC) strukturních jednotkách. Materiály patřily do tří různých typů architektury: (1) PDMS s 'naroubovanými' bočními skupinami LC (čtveřice LC v každé pozici naroubování), (2) PDMS ukončený na obou koncích makromolekul jednotkami LC, (3) a lineární 'nekonečné' kopolymery LC-PDMS s pravidelně se střídajícími jednotkami LC a řetězci PDMS. V rámci syntéz byly testovány řetězce PDMS o různých délkách a 6 různých polyaromatických mezogenů se skupinami azo i bez nich. K syntéze všech studovaných materiálů byla použita hydrosilylační reakce mezi mezogeny s vinylovými funkčními skupinami (získaných v rámci mezinárodní spolupráce) a komerčními Si-H-funkčními polydimetylsiloxany. Kopolymery byly fyzikálně sesíťovány nano-agregací jednotek LC obsažených v jejich makromolekulách. Termotropní vlastnosti nano-agregátů LC propůjčily celému materiálu zajímavé fyzikální vlastnosti, díky nimž jsou některé z kopolymerů atraktivní coby potenciální 'inteligentní' materiály. Řetězce PDMS byly vybrány coby stavební jednotky z důvodu jejich extrémní flexibility, která měla propůjčit elastické vlastnosti fyzikálně sesíťovaným kopolymerům. Dalším důvodem byla také...Several series of reversible physical networks based on polydimethylsiloxane (PDMS) chains and liquid crystalline (LC) structural units were synthesized and studied, which belong to three different architecture types: (1) LC-grafted PDMS (with LC quartets at the grafting sites), (2) LC-end-capped linear PDMS, (3) and linear 'infinite' LC-PDMS copolymers (with alternating LC and PDMS segments). PDMS spacers of different lengths of were tested, as well as 6 different polyaromatic mesogens of azo type and azo-free type. Hydrosilylation coupling of vinyl-functional mesogens, obtained as part of an international cooperation, with commercial Si-H-functional PDMS was employed to synthesize all the studied materials. The copolymers were physically crosslinked by the nano-aggregation of the LC units contained in their macromolecules. The thermotropic properties of the LC-nano-aggregates lent interesting physical properties to the whole material, making some of the copolymers attractive as potential smart materials. The PDMS spacer segments were selected for the sake of their extreme flexibility, which should provide elastic properties to the physically crosslinked copolymers, and also because of their (desired) incompatibility with the mesogens. This latter effect was highly helpful for achieving the...Department of Physical and Macromolecular ChemistryKatedra fyzikální a makromol. chemiePřírodovědecká fakultaFaculty of Scienc

Low-Temperature-Meltable Elastomers Based on Linear Polydimethylsiloxane Chains Alpha, Omega-Terminated with Mesogenic Groups as Physical Crosslinker: A Passive Smart Material with Potential as Viscoelastic Coupling. Part II—Viscoelastic and Rheological Properties

Rheological and viscoelastic properties of physically crosslinked low-temperature elastomers were studied. The supramolecularly assembling copolymers consist of linear polydimethylsiloxane (PDMS) elastic chains terminated on both ends with mesogenic building blocks (LC) of azobenzene type. They are generally and also structurally highly different from the well-studied LC polymer networks or LC elastomers: The LC units make up only a small volume fraction in our materials and act as fairly efficient physical crosslinkers with thermotropic properties. The aggregation (nano-phase separation) of the relatively rare, small and spatially separated terminal LC units generates temperature-switched viscoelasticity in the molten copolymers. Their rheological behavior was found to be controlled by an interplay of nano-phase separation of the LC units (growth and splitting of their aggregates) and of the thermotropic transitions in these aggregates (which change their stiffness). As a consequence, multiple gel points (up to three) are observed in temperature scans of the copolymers. The physical crosslinks also can be reversibly disconnected by large mechanical strain in the ‘warm’ rubbery state, as well as in melt (thixotropy). The kinetics of crosslink formation was found to be fast if induced by temperature and extremely fast in case of internal self-healing after strain damage. Thixotropic loop tests hence display only very small hysteresis in the LC-melt-state, although the melts show very distinct shear thinning. Our study evaluates structure-property relationships in three homologous systems with elastic PDMS segments of different length (8.6, 16.3 and 64.4 repeat units). The studied copolymers might be of interest as passive smart materials, especially as temperature-controlled elastic/viscoelastic mechanical coupling

Low-Temperature Meltable Elastomers Based on Linear Polydimethylsiloxane Chains Alpha, Omega-Terminated with Mesogenic Groups as Physical Crosslinkers: A Passive Smart Material with Potential as Viscoelastic Coupling. Part I: Synthesis and Phase Behavior

Physically crosslinked low-temperature elastomers were prepared based on linear polydimethylsiloxane (PDMS) elastic chains terminated on both ends with mesogenic building blocks (LC) of azobenzene type. They are generally (and also structurally) highly different from the well-studied LC polymer networks (light-sensitive actuators). The LC units also make up only a small volume fraction in our materials and they do not generate elastic energy upon irradiation, but they act as physical crosslinkers with thermotropic properties. Our elastomers lack permanent chemical crosslinks—their structure is fully linear. The aggregation of the relatively rare, small, and spatially separated terminal LC units nevertheless proved to be a considerably strong crosslinking mechanism. The most attractive product displays a rubber plateau extending over 100 °C, melts near 8 °C, and is soluble in organic solvents. The self-assembly (via LC aggregation) of the copolymer molecules leads to a distinctly lamellar structure indicated by X-ray diffraction (XRD). This structure persists also in melt (polarized light microscopy, XRD), where 1–2 thermotropic transitions occur. The interesting effects of the properties of this lamellar structure on viscoelastic and rheological properties in the rubbery and in the melt state are discussed in a follow-up paper (“Part II”). The copolymers might be of interest as passive smart materials, especially as temperature-controlled elastic/viscoelastic mechanical coupling. Our study focuses on the comparison of physical properties and structure–property relationships in three systems with elastic PDMS segments of different length (8.6, 16.3, and 64.4 repeat units)