Viscoelastic properties of silicone polymers with liquid crystalline behaviour

Braatz, K.; Finkelmann, Heino; Gleim, Wolfgang; Kock, H. J.; Rehage, G.

doi:10.1007/bf01534308

Cited by 23 publications

(4 citation statements)

References 4 publications

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“…While there exists a lot of theoretical ,, and experimental − investigations of their static properties, their dynamical properties are less well-known. − Rheological experiments made over a wide frequency range have begun in the past few years. Gallani et al .…”

Section: Introductionmentioning

confidence: 99%

“…5 While there exists a lot of theoretical 3,6,7 and experimental [8][9][10][11][12][13] investigations of their static properties, their dynamical properties are less well-known. [14][15][16][17][18] Rheological experiments made over a wide frequency range have begun in the past few years. Gallani et al 19 studied the behavior of various polydomain elastomers exhibiting nematic and smectic-A phases, and Weilepp et al 20,21 studied that of polydomain and monodomain samples which only have a smectic-A phase.…”

Section: Introductionmentioning

confidence: 99%

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Influence of Molecular Parameters on the Elastic and Viscoelastic Properties of Side-Chain Liquid Crystalline Elastomers

et al. 2002

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We investigated the influence of molecular parameters (amount of mesogen, cross-linking density, cross-linker length) on the complex shear modulus G* = G‘ + i G‘ ‘ of polydomain smectic-A side-chain liquid crystalline elastomers composed of side-chain polysiloxane cross-linked by aliphatic chains. G‘ presents two components in the isotropic phase. One is independent of the frequency and reflects the presence of the permanent network; the other is frequency-dependent and is characterized by a scaling law behavior G‘ = G‘ ‘ ∼ f 0.5 of the Rouse type, where f is the frequency. We show that this scaling law is independent of the molecular parameters except when the amount of mesogen becomes very small. In contrast, the crossover frequency between the elastic and the Rouse regime is a function of the cross-linking density and the cross-linker length but is independent of the amount of mesogen, except when the latter is very small. The elastic plateau follows the same behavior as the crossover frequency. It is no longer visible in the smectic-A phase, and the dynamics of this phase are essentially governed by a transient network characterized by a f 0.3 scaling law which is independent of the cross-linking density.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Influence of Molecular Parameters on the Elastic and Viscoelastic Properties of Side-Chain Liquid Crystalline Elastomers

et al. 2002

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“…Consequently, rheological experiments in these systems open the possibility to study directly the influence of the liquid crystalline side-chains on the local flow properties. After the first viscoelastic experiments done on SCLCEs were reported by Oppermann et al in 1982, work on this interesting topic was recently resumed by Gallani et al , In their experiments; , the emphasis is laid on the rheological properties of a polydomain sample close to the isotropic−nematic and the nematic−smectic A transitions. It is pointed out there that the rheological behavior of the SCLCEs does not change when crossing an isotropic−nematic transition, whereas there is a crucial change at the nematic−smectic A transition.…”

Section: Introductionmentioning

confidence: 99%

Rheology of Liquid Crystalline Elastomers in Their Isotropic and Smectic A State

et al. 1999

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The dynamics of two different side-chain liquid crystalline elastomers (SCLCE) exhibiting a smectic A phase is investigated by low-frequency shear and compression experiments. We find that both the isotropic and the smectic A phase have their own characteristic viscoelastic behavior, which seems to be independent of the compound under investigation and thus universal. The relaxation in the isotropic phase shows a distribution of relaxation times, which gives rise to a scaling law of the elastic moduli with an exponent of 0.5. The viscoelastic response in the smectic A phase, however, exhibits a much broader spectrum of relaxation times containing very long-lived modes. It appears that there exists a low-frequency scaling law with an exponent of 0.3 characterizing the smectic A phase. We propose that this result stems from a transient smectic network with a very long lifetime that was set up by the smectic domains. At the phase transition, this transient smectic network disappears, which leads to a sharp decrease of the dynamic shear as well as the compression modulus.

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“…One of the first viscoelastic studies on PDMS-LC copolymers (of loosely crosslinked SC-LCP) was done by Finkelmann and co-workers [53]. Some persistent shear-induced anisotropy was observed, as well as 'liquid-crystalline elasticity' upon isotropic → nematic transition.…”

Section: Rheological Properties Of Physically Crosslinked Pdmsmentioning

confidence: 99%

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

et al. 2020

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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.

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Viscoelastic properties of silicone polymers with liquid crystalline behaviour

Cited by 23 publications

References 4 publications

Influence of Molecular Parameters on the Elastic and Viscoelastic Properties of Side-Chain Liquid Crystalline Elastomers

Influence of Molecular Parameters on the Elastic and Viscoelastic Properties of Side-Chain Liquid Crystalline Elastomers

Rheology of Liquid Crystalline Elastomers in Their Isotropic and Smectic A State

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

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