Interface and Surface Effects on the Glass Transition in Thin Polystyrene Films

Abstract

Lifetime analysis of positronium annihilating in nanometer voids is used to study the thermal expansion behavior of thin, Si-supported polystyrene films near the glass transition temperature T g . A reduction in void volume expansion is correlated with a reduction in the apparent T g as film thickness decreases. Our results can be fitted using a three-layer model incorporating a 50 Å constrained layer at the Si interface and a 20 Å surface region with reduced T g .[S0031-9007(97)02458-7]

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

“…In the case of polymers, DC p for the microtomed epoxy 20 did not change, although the error in the measurements (65%) was relatively large, whereas a significant decrease in DC p to values less than 20% of the bulk value for 2.5-nm pore sizes was observed for confined oligomeric poly(propylene glycol). 37 Similar changes in the strength of the glass transition at the nanoscale have been observed for polymer ultrathin films with various other methods, including ellipsometry, fluorescent probe intensity, dielectric relaxation, and positron annihilation spectroscopy; 10,13,16,[22][23][24]34,45 similar decreases in DC p were also observed in polymer nanocomposites as the length scale between the particles decreased. 57 Our results clearly show T g depressions in the absence of free surfaces in our stacked PS thinfilm samples for both PS and PIB interfaces.…”

“…A decrease in the relative C p normalized to the film thickness was also observed with decreasing film thickness for PS with ac-nanocalorimetry. 45 In addition, depressed C p s in both the glass and liquid states are supported by analogous depressions of the thermal expansion coefficients from positron annihilation lifetime spectroscopy [22][23][24]34 and neutron reflectometry, 54 although some measurements 10 indicate only changes in the thermal expansion coefficient in the liquid above T g . We note, however, that a depressed C p on the nanoscale is not expected to be universal: water shows an elevated C p when confined to nanometer-size pores because of a density decrease resulting from the disruption of its hydrogen bonding.…”

Glass transition reductions in thin freely-standing polymer films : A scaling analysis of chain confinement effects

“…For polymersubstrate combinations where there is not a specific attractive interaction, the measured glass transition temperature values, T g , are observed to decrease monotonically below the bulk T g for films with thickness less than ~ 50 nm. This same conclusion has been found by different groups using various techniques [2][3][4]. In contrast, for systems where there is a strongly attractive interaction between the polymer and substrate [5] the measured T g values are increased above the bulk value T g bulk .…”

Pore structure and glass transition temperature of nanoporous poly(ether imide)

“…[2][3][4][5][6][7][8][9][10][11] It has been observed that T g shifts occur when the film thickness decreases to ten to several hundred nanometers, and T g depends strongly on both the film/substrate interface and the molecu-lar weight of the polymer. [3][4][5][6][7][8] Attempts have been made to interpret the confinement effects on T g in polymer thin films from different aspects. 2,3,7,[9][10][11] More recently, the glass transition behavior in polymer nanocomposites has been studied.…”