Degradation of Films of Block Copolymers: Molecular Dynamics Simulations

R Sayko and ZL Wang and HY Liang and ML Becker and AV Dobrynin, MACROMOLECULES, 53, 1270-1280 (2020).

DOI: 10.1021/acs.macromol.9b02446

Coarse-grained molecular dynamics simulations are used to study the degradation of films of copolymers in a selective solvent for one of the blocks. Simulations were designed to mimic the hydrolytic degradation of glycine, valine, and phenylalanine-based poly(ester urea)s. The analysis of the simulation results shows that the rate of copolymer degradation is a result of the interplay between chain-breaking kinetics, solvent diffusion, and swelling of the solvophilic domains. The evolution of the film structure during the degradation process was monitored by calculating the scattering function S(q) of the copolymer film. In the initial stages of the polymer degradation and swelling, the scattering function has a characteristic peak corresponding to a domain spacing of solvophilic and solvophobic domains. The solvent diffusion into the films results in a monotonic shift of the peak position to smaller q. This shift is accompanied by the increase in the scattering intensity at q << 1 in such a way that the peak completely disappears at the later stages of film degradation with the function S(q) having two characteristic scaling regimes: S(q) similar to 1/q(2) and S(q) similar to 1/q(4). At this stage of film degradation, the film is composed of the interconnected network of solvophobic domains. The number and weight average degree of polymerizations (DPs) of the copolymer fragments monotonically decrease with time. However, the dispersity (D) shows a nonmonotonic dependence. The copolymer degradation kinetics allows for universal description of the time dependence of the copolymer DP and the dispersity in terms of the fraction of the broken bonds.

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