Spinodal Decomposition of Polydispersed ABA' Triblock Copolymers Determined from the Random Phase Approximation

Triblock copolymers produced by free radical polymerization are typically characterized by each of the blocks having a broad distribution of molecular weight. To investigate the effect of this polydispersity on the stability of the homogenous phase of a triblock copolymer, the spinodal decomposition of an ABA' copolymer consisting of ideal Gaussian chains was determined using the random phase approximation (RPA), with each block length characterized by a Zimm-Schulz chain length distribution. The spinodal stability and scattering behavior resulting from microphase separation were determined for volume fractions 0.1 $\le \quad \phi _{B} \quad \le $ 0.9 and polydispersity indices 1.67 $\le $ PI $\le $ 100. Consistent with the reported behavior of fully dispersed multiblock copolymers and diblock copolymers having a single polydispersed block, polydispersity in symmetric ABA' triblocks decreases the stability of the homogenous phase and lowers the value of the Flory-Huggins mixing parameter $\chi _{HF}$ required for microphase separation at the Lifshitz point by more than a factor of two.