RESUMO
We present neutron spin echo and structural measurements on a perdeuterated miscible polymer blend: poly(ethylene oxide)[PEO]/poly(methyl methacrylate)[PMMA], characterized by a large difference in component glass transition temperatures and minimal interactions. The measurements cover the q range 0.35 to 1.66 A(-1) and the temperature range Tg -75 to Tg +89 K, where Tg is the blend glass transition. The spectra, obtained directly in the time domain, are very broad with stretching parameters beta approximately 0.30. The relaxation times vary considerably over the spatial range considered however at none of the q values do we see two distinct relaxation times. At small spatial scales relaxations are still detectable at temperatures far below Tg. The temperature dependence of these relaxation times strongly resembles the beta-relaxation process observed in pure PMMA.
RESUMO
The controversial origins of the unusual dynamics of miscible polymer blends are incisively probed through computer simulations. The distribution of mobilities experienced by a probe monomer in a miscible blend of chains with disparate glass transition temperatures is found to be much broader than in the pure polymers, providing clear evidence for local concentration variations in the mixture. These concentration fluctuations yield distinctly different temperature dependences for the dynamics of the two different components, in a manner that closely mimics experiments.
RESUMO
The effect of copolymer composition on the dynamics of random copolymers in a homopolymer matrix is studied using computer simulations within the framework of the bond-fluctuation model on blends containing low concentrations (10%) of A-B copolymers, where A and B are two different types of monomers, dispersed in a homopolymer matrix of chains with only A-type monomers. Four copolymer compositions were studied, phi(A)=0.33, phi(A)=0.5, phi(A)=0.66, and phi(A)=0.82, while maintaining a statistically random sequence distribution. For this study, we have only included intermolecular interactions between A and B monomers. Our results indicate, in agreement with experimental data, that copolymer composition has an impact on system dynamics. Analysis of the structure reveals that copolymers with majority A content are expanded in the homopolymer matrix, have fewer interchain copolymer-copolymer contacts, and are well dispersed in the homopolymer matrix. On the other hand, copolymers with lower A content form a more compact structure, have more interchain contacts, and form aggregates that are short lived. This in turn leads to slower system dynamics. Both the radius of gyration (Rg) and copolymer end-to-end vectors (Re) increase with increasing A content until phi(A)=0.66 and then decrease. Copolymers with lower A content form more compact structures as the repulsive interactions between unlike species are minimized by the copolymers folding back on themselves and forming aggregates of copolymer chains. Thus, these results provide insight into the variation of copolymer dynamics with composition in the system by documenting the correlation between the thermodynamics of this mixture, the conformation of a copolymer chain in a homopolymer matrix, and the dynamics of both components in this blend.