Strain-Induced Molecular Reorientation and Birefringence Reversion of a Robust, Anisotropic Double-Network Hydrogel

A robust, macroscopically anisotropic double-network (A-DN) hydrogel has been developed by combining the first anisotropic, physically cross-linked network of semirigid poly(2,2'-disulfonyl-4,4'-benzidine terephthalamide) (PBDT) and the second chemically cross-linked network of flexible polyacrylamide (PAAm) through double-network technology. Owing to the cooperative orientation of the self-assembled bundle structure of semirigid PBDTs, the A-DN gel shows a very sensitive and strong birefringence change upon deformation, which is always dominated by PBDT for the wide strain range investigated (epsilon = 0-18), regardless the fact that PAAm is in large excess. When the initial alignment of PBDT is vertical to the elongation direction, a birefringence reversion occurs at epsilon similar to 0.5, indicating that the initial vertical alignment of PBDT is overwhelmed by the strain-induced parallel alignment, even at such a small strain due to the cooperative reorientation of PBDT.