Free Surfaces Cause Reductions in the Glass Transition Temperature of Thin Polystyrene Films

Abstract

The effect of free surfaces on the glass transition temperature (T(g)) of thin polystyrene films was studied. Measurements were performed on films (8 nm<h<290 nm) with one free surface and on films capped with a 5 nm thick metal layer (no free surfaces). Potential problems with evaporative deposition were eliminated by studying samples made of two supported films placed with their free surfaces in contact and annealed. Uncapped films displayed reduced T(g) values for h less, similar 40 nm while all "properly" capped films exhibited a T(g) value the same as that of the bulk polymer (370+/-1 K). When the free surface was restored, the measured T(g) values the same as those of uncapped films of the same thickness. These results show that free surfaces are crucial for observing T(g) reductions in thin polymer films and address the role of the sample preparation history.

Calorimetric glass transition temperature and absolute heat capacity of polystyrene ultrathin films

“…They are consistent, though, with the earlier work of Forrest et al 26 on supported films, in which no significant differences between PS thin films on SiO x with one free surface and those on SiO x capped with an SiO x layer were found, although the artifacts potentially introduced by vapor deposition were recently discussed by Sharp and Forrest. 19 We also note that the T g and DC p depressions that we observe with decreasing film thickness by calorimetry differ from the results of recent nanocalorimetric measurements [43][44][45] in which no changes were observed (although in ref. 45, a change in C p was found, and the issue of DC p is difficult to address because of the increasing breadth of the transition).…”

“…The seemingly small effects of the free surface on T g in our results seem to be at odds with experiments in which films are stacked and annealed, such as those in the work of Ellison and Torkelson, 16 in which T g gradients were investigated, and in the work of Sharp and Forrest, 19 in which the T g s of supported thin films having their free surfaces capped with Al or Au were the same as those of the bulk. They are consistent, though, with the earlier work of Forrest et al 26 on supported films, in which no significant differences between PS thin films on SiO x with one free surface and those on SiO x capped with an SiO x layer were found, although the artifacts potentially introduced by vapor deposition were recently discussed by Sharp and Forrest.…”

“…13 On the other hand, freely standing films appear to have a strong dependence on the molecular weight above approximately 500,000 g/mol, and T g decreases linearly with the film thickness for thicknesses below a molecular-weight-dependent critical value. 9,10 It is clear that free surface and interface effects are important factors in determining the magnitude and direction of changes in T g in polymer thin films; [5][6][7][8][9][10][13][14][15][16][17][18][19]40,41 however, the details of these effects are often contradictory or disputed, and a full understanding remains elusive.…”

A coarse-grained molecular dynamics study of segmental structure and mobility in capped crosslinked copolymer films

“…The LJ parameters ε and σ had the same values with their non-bonded counterparts. This particular choice of bonded and non-bonded interactions prevents chain crossings 16 and full crystallization 43,17,18 and allows the system to undergo a transition to a supercooled state, allowing thus the study of the glass-transition temperature in a generalized context (i.e., disregarding material-specific properties). Further, bead-spring polymer chains do not become stiffer with decreasing temperature.…”

“…[14][15][16][17][18][19][20][21][22] The consensus is that the dynamic response of a polymer film is highly dependent on the degree of confinement, the presence of free interfaces, and the polymer-wall interactions. The results, though, seem rather influenced by the employed experimental technique and the preparation procedure of the samples, as contradictory results have often been reported.…”