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Abstract
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We provide a detailed investigation of the
structure and dynamics of stereoregular poly(methyl methacrylate)
(PMMA) chains confined between graphene layers via
atomistic molecular dynamics simulations. The density,
conformation, and interfacial segmental dynamics of low
molecular weight isotactic, syndiotactic, and atactic PMMA
chains are examined at various temperatures, ranging from 490
K up to 580 K. For all stereoisomers, a tendency of chains to
adsorb on graphene via ester-methyl groups is observed.
Compared to other stereoisomers, isotactic chains are stretched
and form better organized layers at the interface. Concerning
dynamical properties, various dynamical modes of PMMA are studied as a function of distance from graphene as well as over
the entire confined system. The interfacial backbone and ester side group motions are restricted, whereas the fast motion of
methyl groups remains unaffected. In the temperature range studied here, the interphase thickness remains almost constant;
however, the ratios of the interfacial correlation times to corresponding bulk values increase with decreasing temperature,
revealing a stronger temperature dependence of the interfacial dynamics than bulk. In addition, apparent (i.e., short time scale
or high cooling rate) dynamic and volumetric glass transition temperatures (Tg) are studied through calculation of the
temperature at which the correlation time of the segmental motion becomes 10 ?s and a simulated dilatometry method with 1 K
ns?1 cooling rate, respectively. A higher apparent Tg relative to bulk was observed close to graphene for all PMMA
stereoisomers. However, isotactic PMMA shows more restricted interfacial motion and a larger shift of the apparent Tg. The
latter is consistent with the better packing of this stereoisomer at the interface.
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