Glass transitions and dynamics in thin polymer films: Dielectric relaxation of thin films of polystyrene

Abstract

The glass transition temperature Tg and the temperature Tα corresponding to the peak in the dielectric loss due to the α-process have been simultaneously determined as functions of film thickness d through dielectric measurements for polystyrene thin films supported on glass substrate. The dielectric loss peaks have also been investigated as functions of frequency for a given temperature. A decrease in Tg was observed with decreasing film thickness, while Tα was found to remain almost constant for d > dc and to decrease drastically with decreasing d for d < dc. Here, dc is a critical thickness dependent on molecular weight. The relaxation time τα of the α-process, which was measured as the frequency at which the dielectric loss realizes its peak value at a given temperature, was found to have a d dependence similar to that of Tα. The relaxation function for the α-process was obtained by using the observed frequency dependence of the peak profile of the dielectric loss. The exponent βKWW, which was obtained from the relaxation functions, decreases as thickness decreases. This suggests that the distribution of relaxation times for the α-process broadens with decreasing thickness. The thickness dependence of Tg is directly related to the distribution of relaxation times for the α-process, not to the relaxation time itself. The value of the thermal expansion coefficient normal to the film surface was found to increase with decreasing film thickness below Tg, but to decrease with decreasing film thickness above Tg. These experimental results are discussed in the context of a three-layer model in which within thin films there are three layers with different mobilities and glass transition temperatures.

Molecular Dynamics Simulation of Size-Dependent Structural and Thermal Properties of Polymer Nanofibers

“…16 A series of polystyrene (PS) films of thicknesses between 10 and 200 nm were prepared on silicon wafers and reductions in T g for films with thicknesses less than 40 nm were measured. Results obtained for PS films on a variety of substrates using numerous experimental techniques such as ellipsometry, 16,17 dielectric spectroscopy, 18,19 X-ray reflectivity, 20 local thermal analysis 21 and probe fluorescence intensity measurements 22,23 show a consistent decrease in T g with decreasing film thickness. However, a modest increase in T g was observed with decreasing film thickness for PMMA films on silicon oxide, demonstrating the importance of polymersubstrate interactions on the measured T g .…”

“…These differences have implications for the properties of such materials confined on the nanometer length scale. For example, the glass transition temperature (T g ) of amorphous polymer thin films has been observed either to increase or decrease with decreasing film thickness, [16][17][18][19][20][21][22][23][24][25][26][27][28] phenomena that have attracted great interest in recent years as part of a larger effort to understand the nature of the glass transition itself. The first systematic study of the dependence of the T g on film thickness in thin polymer films was performed by Keddie et al using ellipsometry.…”

The Relaxation Dynamics of a Supercooled Liquid Confined by Rough Walls

“…Depending on the investigated liquid, the realization of the confinement and even the experimental method, there is a great variety in experimental findings. Some give evidence for an accelerated dynamics in confinement [21,25,26,29,30,36], i.e. the glass transition temperature T g goes down, while others see an increase of T g [24,37,38,39].…”

“…One large class of experiments are done on molecular liquids (such as salol, glycol, ortho-terphenyl) confined to porous material such as Vycor glass, sol-gel glass and zeolithes with pore sizes ranging from several hundred to only a few nanometer. Typical experimental methods are differential scanning calorimetry (DSC) [21,22,23], dielectric spectroscopy [24,25,26,27,28,29], neutron scattering [30,31], solvation dynamics [32,33,34] and many more.…”

Broadening, no broadening and narrowing of glass transition of supported polystyrene ultrathin films emerging under ultraslow temperature variations

“…14 Dielectric investigations of thin films supported on aluminum substrates have shown that the full width at half maximum of dielectric loss increases with decreasing thickness, suggesting that a decrease in T g is directly related to the broadening of the a-relaxation peak. 15,16 Ellipsometric measurement of thermal expansivity in supported polystyrene (PS) thin films has shown that the upper extreme of glass transition, T + , remains at a point close to the bulk T g , whereas the lower extreme, T À , continuously decreases with decreasing film thickness, suggesting a broadening of glass transition in films thinner than 30 nm. 17 However, such a broadening has not been observed in free-standing films.…”