Effects of chain structure on the dynamics and rheological properties of macromolecules

Abstract

In order to describe the effects of large-scale molecular structure on the relaxation behavior and viscoelastic properties, the tube model theory for monodispersed polymers initially proposed by de Gennes is extended to the inhomogeneous binary blends composed of different chain structures but chemically identical species. In binary blends, the shorter chain is assumed to release with the same relaxation time as its monodispersed state. After its characteristic relaxation time, it behaves as a pure solvent-like molecule, and the longer relaxation-time chain experiences the tube reorganization causing tube enlargement. The degree of tube expansion is determined by the effective blend ratio and the weight fraction. After the tube renewal is completed, the higher chain reptates again in the expanded tube with the shorter relaxation time in the same way as its monodispersed state. The proposed relaxation mechanism in binary blends is then extended to polydisperse blends, where a model chain experiences the constraint release by shorter chains sequentially. The shear stress relaxation modulus of mixtures is also obtained on the basis of the relaxation behavior considered here. So as to test the proposal, dynamic mechanical measurements are made on binary blends containing well characterized narrow molecular weight distribution linear and three-arm star polystyrenes. The effect of structural diversity is examined with linear-linear, star-linear, and linear-llinear-linear blends. Dynamic moduli allow us to obtain an average relaxation time of the longest chain (component) which is influenced by volume fractions and molecular weights of respective components. The proposed model turns out to describe well the relaxation behavior. In linear-linear binary blends, the zero-shear viscosity is proportional to $W_2M_2$ at low $W_2$, and proportional to $W_2^{1+2d}M_2^{3.4}$ at high $W_2$. The recoverable shear compliance shows the similar concentration dependence with the vis...