Interfacial and topological effects on the glass transition in free-standing polystyrene films

Abstract

United-atom molecular-dynamics computer simulations of atactic polystyrene (PS) were performed for the bulk and free-standing films of 2 nm-20 nm thickness, for both linear and cyclic polymers comprised of 80 monomers. Simulated volumetric glass-transition temperatures (T) show a strong dependence on the film thickness below 10 nm. The glass-transition temperature of linear PS is 13% lower than that of the bulk for 2.5 nm-thick films, as compared to less than 1% lower for 20 nm films. Our studies reveal that the fraction of the chain-end groups is larger in the interfacial layer with its outermost region approximately 1 nm below the surface than it is in the bulk. The enhanced population of the end groups is expected to result in a more mobile interfacial layer and the consequent dependence of T on the film thickness. In addition, the simulations show an enrichment of backbone aliphatic carbons and concomitant deficit of phenyl aromatic carbons in the interfacial film layer. This deficit would weaken the strong phenyl-phenyl aromatic (π-π) interactions and, hence, lead to a lower film-averaged T in thin films, as compared to the bulk sample. To investigate the relative importance of the two possible mechanisms (increased chain ends at the surface or weakened π-π interactions in the interfacial region), the data for linear PS are compared with those for cyclic PS. For the cyclic PS, the reduction of the glass-transition temperature is also significant in thin films, albeit not as much as for linear PS. Moreover, the deficit of phenyl carbons in the film interface is comparable to that observed for linear PS. Therefore, chain-end effects alone cannot explain the observed pronounced T dependence on the thickness of thin PS films; the weakened phenyl-phenyl interactions in the interfacial region seems to be an important cause as well.

Interface characteristics of polystyrene melts in free-standing thin films and on graphite surface from molecular dynamics simulations

“…MD simulations were performed on melts of atactic polystyrene (PS) at 450 K, which is well above its experimental T g of 373 K, following the simulation methods described in detail in the previous studies of polymethylene films [40] and polytetrafluorene (PTFE) films [49]. The simulation box has 20 molecules and each molecule has 15 repeating units, with the stereochemical configuration of each repeat unit chosen randomly as meso or racemic [48,55].…”

“…In doing so, we first performed NPT MD simulations (fixed number of particles, pressure, and temperature) for a cubic simulation box with periodic boundary conditions in all directions [ Figure 1(a)]. After the bulk phase was fully equilibrated by monitoring the density and chain conformations [40,49,55], the simulation box was elongated along the z -direction by about 3 to 4 times without scaling the coordinates of atoms, which subsequently generates free surfaces at both sides ( We used the explicit-atom force field of Smith et al [53] for the intermolecular and intramolecular interactions of PS chains. From the bulk simulations, we obtained the density at 450 K of 0.984 g/cm 3 .…”

“…7 Å thickness. The major significance of this phenyl-depleted interfacial region at the free surface of PS films is discussed separately in a recent united-atom simulation study on the thicknessdependence of T g of PS thin films, which also considers the relative importance of chain-end segregation in decreasing the T g [55]. As compared with the corresponding results from the united-atom simulations on free-standing PS thin film, the extent of phenyl depletion seems to be slightly more pronounced in the explicit-atom simulations; likewise, the chain-end segregation effects (noticeable in Figure 1(c)) are similar [55].…”

“…Therefore, explicit all-atom molecular simulations of PS melts promise to yield potentially valuable insights into the molecular origin of the decreased T g in thin PS films. In this regard, our explicit-atom MD simulations of PS surfaces are also intended to check the general validity of united-atom MD simulations of PS thin films [48], which have recently been employed to investigate the thickness dependence of T g of free-standing PS films [55]. Secondly, PS chains are comprised of a very high fraction (75%) of aromatic carbons and thus provide a unique opportunity to investigate the detailed characteristics and consequences of important aromatic-aromatic interactions in polymer-graphite composites and polymer-graphene nanocomposites [10].…”

Influence of wall heterogeneity on nanoscopically confined polymers

“…Many experimental, theoretical and computational studies have demonstrated that the behavior of polymers in these situations may deviate very significantly from that in the bulk. Examples of such deviations in the dynamical properties include the glass transition [12][13][14] and the diffusivity of polymers in nanocomposites and in narrow slits. 15,16 Important insights have been obtained by studying simple, well-defined model systems, such as polymers sandwiched between smooth, homogeneous surfaces.…”