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The Importance of Thermodynamic Interactions on the Dynamics of Multicomponent Polymer Systems Revealed by Examination of the Dynamics of Copolymer/Homopolymer Blends

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journal contribution
posted on 13.05.2008, 00:00 authored by Sudesh Y. Kamath, Michael J. Arlen, William A. Hamilton, Mark D. Dadmun
The effect of copolymer composition on their dynamics in a homopolymer matrix has been studied using specular neutron reflectivity (NR). We have monitored the segregation process of random copolymers, containing styrene (S) and methyl methacrylate (MMA), to the d-PS/d-PMMA interface from a polymer matrix. Four random copolymers containing 50, 54, 67, and 80% MMA were studied at 10 wt % loading in d-PMMA, where the interfacial excess, Z*, growth scaled as t1/2 as predicted by theory. These results are correlated to the diffusion-limited growth of a copolymer wetting layer at the d-PS/d-PMMA interface. The mutual and tracer diffusion coefficients and the effective friction coefficients for these copolymers were then determined. The results demonstrate that the copolymer composition has a significant impact on its dynamics. Copolymer dynamics are significantly faster than those for a diblock copolymer at the same composition, which indicates that the impact of the change in composition is more than that due to an increase in the MMA content in the copolymer. Analysis of the friction factor using the Lodge−McLeish model indicates that the local composition around a copolymer is richer in styrene than the model predicts. We attribute this to the fact that the model uses only chain connectivity to calculate the self-concentration and does not include contributions due to thermodynamic interactions between the two blend components. The observation that the local environment around a copolymer is richer in styrene is in agreement with our simulation results and indicates that the styrene monomers in the copolymer aggregate together to minimize contact with the PMMA matrix. These results exemplify the importance of thermodynamic interactions on the dynamics of multicomponent polymer systems, particularly miscible homopolymer/copolymer blends.