Influence of chain architecture on the mechanochemical degradation of macromolecules

We detail here studies into the nature of mechanochemical degradation of macromolecules, effected by means of ultrasonic (US) irradiation. Specifically, we have investigated the effect of long-chain branching (LCB), in the star configuration, on the degradation mechanism of polystyrene dissolved in DMAc/LiCl. The information obtained from size-exclusion chromatography with triple detection (refractometry, viscometry, multi-angle light scattering) shows that the degradation mechanism of stars is radically different from that of linear polymers. Whereas in the latter, from a macromolecular standpoint, it is merely necessary for the molar mass to be greater than some limiting value (M-lim), in the former both molar mass and structural factors affect ultrasonic degradation. We have examined the effects of arm number and of arm molar mass on star degradation, and propose the concept of a spanning molar mass (M-span approximate to 2M(arm)) such that, even in the event that M-arm < M-lim for the stars, mechanochemical degradation may nonetheless proceed if M-span > M-lim. This mechanism has been extended to other types of architecture (e.g., H-branched, dendritic), where it is proposed that a continuous path must exist with M-path > M-lim. for degradation to occur. Examination of the different radii afforded by viscometric and light-scattering detection gives insight into the solution thermodynamics and conformation of the stars with differing arm number and molar mass and of the effects of insonation on macromolecular structure. (C) 2003 Elsevier Science B.V. All rights reserved.