Linear rheology and structure of molecular bottlebrushes with short side chains

We investigate the microstructure and linear viscoelasticity of model molecular bottlebrushes (BBs) using rheological and small-angle X-ray and neutron scattering measurements. Our polymers have short atactic polypropylene (aPP) side chains of molecular weight ranging from 119 g/mol to 259 g/mol and narrow molecular weight distribution (M-w/M-n 1.02-1.05). The side chain molecular weights are a small fraction of the entanglement molecular weight of the corresponding linear polymer (M-e,M-aPP = 7.05 kg/mol), and as such, they are unentangled. The morphology of the aPP BBs is characterized as semiflexible thick chains with small side chain interdigitation. Their dynamic master curves, obtained by time-temperature superposition, reveal two sequential relaxation processes corresponding to the segmental relaxation and the relaxation of the BB backbone. Due to the short length of the side chains, their fast relaxation could not be distinguished from the glassy relaxation. The fractional free volume is an increasing function of the side chain length (N-SC). Therefore, the glassy behavior of these polymers as well as their molecular friction and dynamic properties are influenced by their N-SC values. The apparent flow activation energies are a decreasing function of N-SC, and their values explain the differences in zero-shear viscosity measured at different temperatures. (C) 2015 The Society of Rheology.