A Rheological Investigation of Entanglement in Side-Chain Liquid-Crystalline Azobenzene Polymethacrylates

The shear rheological behavior was investigated in a series of high molar mass liquid-crystalline polymers (PMA4 homopolymer and copolymer samples) carrying an azobenzene mesogenic chromophore group in the side chains. The focus was on studying the entanglement effects and testing selected reptation models to ascertain their ability to reproduce the complex shear modulus of the copolymers. We found that ordinary dynamic models worked for the nematic PMA4 copolymers, nicely reproducing the rheological response of the materials. We were able to obtain microscopic information on the materials, such as Rouse time and entanglement molar mass, in a consistent way, as well as to get insight on the macroscopic effects of tube dilatation induced by the nematic order on the master curves of the entangled polymers. Model improvements, accounting for the different nature of the co-units, were also proposed in this work that singled out the friction coefficients of the co-units MMA and MA4. The monomeric friction coefficient ζ0MA4 was found to be constant throughout the series, (5±2)×10–9 kg s–1. Likewise, ζ0MMA had the same value throughout the series, very similar to literature data for PMMA homopolymers, (2.0±0.6)×10−8 kg s−1. Finally, in the framework of the packing length model, constant packing lengths of 3.5 Å and 13 Å throughout the series were found for MMA and MA4 co-units, respectively. Also, it resulted that the viscoelastic behavior of any PMA4 random copolymer could be predicted, provided that the response of the extreme homopolymers of the series has been characterized.