Heterogeneities in the Dynamics of a Molecular Tracer in Mesogenic and Nonmesogenic Azobenzene Copolymers

Abstract

We studied by electron spin resonance (ESR) spectroscopy the reorientational dynamics of the cholestane spin probe dissolved in the matrix of the PMA4 40/60 copolymer containing methyl methacrylate (MMA, 40 mol%) and azobenzene methacrylate (MA4, 60 mol%) counits. Heterogeneities in the probe dynamics at the nanometer length scale were evidenced and attributed to the existence of molecular sites with slow and fast spinning correlation times. Different dynamic regimes were also observed with a crossover temperature T_C that was correlated with the glass transition temperature T_g of the copolymer, T _C  = 1.17T _g . Even though the nematic-to-isotropic transition temperature could not be detected, the probe dynamics experienced the onset of nematic order at the molecular level. The results are compared with those of the closely related nematic PMA4 30/70 copolymer.

Keywords:

• azobenzene
• cholestane probe
• dynamic heterogeneity
• liquid crystal polymer
• nematic

Notes

nd: not resolvable from the glass transition.