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.


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: Full data set: Kinetics of the change in storage (G') and loss (G") modulus (kinetics of physical gelation) upon cooling molten H11-BAFKU 2 from 70°C down to different temperatures ranging between -50°C and +60°C; the course of the temperature of the plates between which the sample was loaded is also depicted.  Fig. 4: Kinetics of the change in storage (G') and loss (G") modulus (kinetics of physical gelation) upon cooling molten H21-BAFKU2 from 70°C to different temperatures ranging between -60°C and 60°C; the course of the temperature of the plates between which the sample was loaded is also depicted.  Fig. 9: Multi-step creep tests of H11-BAFKU2 all data: tests at temperatures between -50 and +20°C; stresses ranging between 200 and 10 000 Pa were applied, followed by recovery steps (at 0 Pa).

H21-BAFKU2
SI- Fig. 10: Multi-step creep tests of H21-BAFKU2 at temperatures between -50 and 0°C; stresses ranging between 200 and 10 000 Pa were applied, followed by recovery steps (at 0 Pa).  Fig. 14: All data: Frequency-stiffening of H11-BAFKU2 observed in frequency sweep tests (1 mHz to 100 Hz) conducted at temperatures between -60 and +60°C; the strain amplitude was different in each frequency decade, ranging from 50% at 1 mHz to 1% at 100 Hz; the effect of strain-induced damage to the network, as well as of its recovery between the frequency decades is clearly visible, especially in case of G' curves at lower temperatures.  Fig. 15: Frequency-stiffening of H21-BAFKU2 observed in frequency sweep tests (1 mHz to 100 Hz) conducted between -60 and +55°C; the strain amplitude was different in each frequency decade, ranging from 50% at 1 mHz to 1% at 100 Hz; the effect of strain-induced damage to the network, as well as of its recovery between the frequency decades is clearly visible, especially in case of G' curves at lower temperatures.

Storage modulus G'
Loss modulus G''  Fig. 16: Frequency-stiffening of H03-BAFKU2 melt observed in frequency sweep tests (1 mHz to 100 Hz) conducted at temperatures between +25 and +70°C; the strain amplitude was different in each frequency decade, ranging from 50% at 1 mHz to 1% at 100 Hz; the effect of strain-induced damage to elastic structures in the melt, as well as their recovery between the frequency decades is clearly visible, especially at lower temperatures.

Explanation of the upward steps in G"
While simple self-healing should lead to decrease in G" (stronger elastomer character due to more crosslinks, shorter elastic chains and hence less friction), the experimentally observed upward steps in G" could be explained by resistance caused by re-assembled larger aggregates (lamellae) of BAFKU, which have time to disconnect at lower frequencies (and high applied strains). Their gradual destruction by shear generates resistance (high G" value), but also leads to a decrease in the number of these secondary aggregates and thus in turn to less than maximum resistance (smaller growth, or in extreme cases even local decrease in G"). During the experimental delay, the smaller BAFKU aggregates ('fragments') re-assemble to larger ones again, and hence can generate considerably increased resistance after the delay (upward step in G"). Temperature T

Storage modulus G' Loss modulus G''
Very small strains also lead to change in measured moduli: Fig. 18: Thixotropy effects as strain-dependence -study of very low strains -of the moduli values and curve course -especially G', less so G", in case of the kinetics of physical gelation of molten H03-BAFKU2 upon abrupt cooling.