حسین اسلامی

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.