The effect of styrene-methyl methacrylate monomeric arrangement on the ultrasonic degradation of copolymers

Ultrasonic degradation is a convenient means of studying the degradation of macromolecules in transient elongational flows. Cavitation results in non-random, near-midchain scission of linear polymers, which reach a limiting molar mass Mlim beyond which degradation is no longer possible. Also, the flow fields produced as a result of ultrasonic bubble collapse are kinetically and fluid-mechanically equivalent to those produced during more experimentally intractable manifestations of transient elongational flow. Here, we describe results from the ultrasonic degradation of dilute solutions of block, random, and alternating copolymers of styrene and methyl methacrylate, along with the degradation of their respective homopolymers. Results from the degradation experiments were monitored using size-exclusion chromatography with on-line triple detection including multi-angle static light scattering, differential viscometry, and differential refractometry. The influence of monomeric arrangement on solution structure was found to have concomitant influence on copolymer degradation, both mechanistically and kinetically. Noteworthy was the correspondence between persistence length and both Mlim and degradation rate k when comparing the alternating to both the block and random copolymers, in qualitative agreement with the previously-postulated modified path theory of polymer degradation in transient elongational flows. Most differences in Mlim and k could be explained by a combination of effects due to molar mass, monomer ratio, monomer arrangement, and resultant chain stiffness in solution, except for differences in Mlim between the random and block copolymers studied, which proved too subtle to measure.