Abstract
In this paper, we have addressed the question of the dynamic miscibility in a blend characterized by very different glass-transition temperatures, Tg, for the components: poly(ethylene oxide) and poly(methyl methacrylate) (PEO/PMMA). The combination of quasielastic neutron scattering with isotopic labeling and fully atomistic molecular dynamics simulations has allowed us to selectively investigate the dynamics of the two components in the picosecond-10 nanoseconds scale at temperatures close and above the Tg of the blend. The main focus was on the PEO component, i.e., that of the lowest Tg, but first we have characterized the dynamics of the other component in the blend and of the pure PEO homopolymer as reference. In the region investigated, the dynamics of PMMA in the blend is strongly affected by the alpha-methyl rotation; an additional process detected in the experimental window 65 K above the blend-Tg can be identified as the merged alphabeta process of this component that shows strong deviations from Gaussian behavior. On the other hand, pure PEO displays entropy driven dynamics up to very large momentum transfers. Such kind of motion seems to freeze when the PEO chains are in the blend. There, we have directly observed a very heterogeneous and moreover confined dynamics for the PEO component. The presence of the hardly moving PMMA matrix leads to the creation of little pockets of mobility where PEO can move. The characteristic size of such confined islands of mobility might be estimated to be of approximately 1 nm. These findings are corroborated by the simulation study, which has been an essential support and guide in our data analysis procedure.
Highlights
One of the fundamental questions of all polymer based soft matter systems is the way how local friction arises in crowded environments that are chemically heterogeneous
The main focus was on the PEO component, i.e., that of the lowest Tg, but first we have characterized the dynamics of the other component in the blend and of the pure PEO homopolymer as reference
We address the dynamics of the miscible blend of polyethylene oxideand polymethyl methacrylatePEO/PMMAby quasielastic neutron scatteringNS
Summary
One of the fundamental questions of all polymer based soft matter systems is the way how local friction arises in crowded environments that are chemically heterogeneous. Different magnitudes and temperature dependencies for the dynamics of the different components, different distribution functions and even the observation that one component could exhibit two component relaxation distributions These observations are very often rationalized in terms of thermally driven concentration fluctuations with spatial regions rich in the component A or B giving rise to environments of different compositions which in turn lead to different glass transition behavior. Local heterogeneities arise from the self-concentration which comes about from the fact that within the volume of a Kuhn length the monomers of a given chain have a higher concentration than on average With this model McLeish and Lodge were able to describe semiquantitatively the mean component dynamics of a number of miscible polymer blends systems. The paper is organized as follows: ͑i First, we describe sample preparation, experimental techniques as well as the simulation approach. ͑ii we present selected experimental and simulation results; ͑iiithereafter these results are analyzed and brought together; andivfinally, the outcome of the analysis is discussed and summarized
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