The effects of free surfaces and molecular confinement on relaxation processes in thin polymer films

Yang, Haidong (2009) The effects of free surfaces and molecular confinement on relaxation processes in thin polymer films. PhD thesis, University of Nottingham.

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Abstract

Glass transition, physical aging and dielectric relaxation in ultra-thin polymer films (< 100 nm) were investigated using complementary techniques (ellipsometry and dielectric spectroscopy).

PtBMA films of different thicknesses were prepared and the thickness dependence of the glass transition temperature (Tg(h)) was investigated with ellipsometry. The uncapped films thinner than 40nm showed signicant depression of Tg and this was explained using the enhanced molecular mobility near the free surfaces. Several sets of PtBMA film samples were then prepared and capped by Al layers; the coating procedure is different for each set. The Tg(h) in PtBMA films with evaporated Al capping layers was essentially the same as that of the uncapped PtBMA films, and this suggests evaporated Al capping layers can not remove the free surface effect. Another set of samples was capped with Al layers using a novel '2(h=2)' sample preparation procedure, and was expected to have no free surface effect. These samples exhibit no apparent thickness dependence. These results suggest that the effect of free surfaces are responsible for the altered dynamics in thin polymer films. However great care has to be taken in attempt to remove the free surface effects caused by solid capping layers.

The frequency dependence of the thickness dependence of α relaxation temperature (Tα(h)) in ultra-thin PVAc films was measured using dielectric spectroscopy. Films thinner than 80nm exhibit smaller Tα's than the bulk values at a measurement frequency lower than 10Hz, but there is no signicant thickness dependence of Tα at measurement frequencies higher than 10Hz. The results demonstrated that the Tα(h) has an intrinsic dependence on measurement frequency. We also measured the cooling rate dependence of the Tg(h) using the same samples, and compared Tg(h) with Tα(h) for different cooling rates/measurement frequencies. The results help to address the existing controversial reports about the apparent discrepancies observed between measurements that are performed using different techniques.

Dielectric spectroscopy in the temperature domain ε"(T) was measured on thin(<486nm) PVAc films. The data were plotted against ln τ and the parameters describing the widening and distortion of the α relaxation peaks are constant for all measurement frequencies. The observed symmetry of the α relaxation peaks in ε"((T)were related to the constant shape parameters of the asymmetric α relaxation peaks in ε"(ln τ). An explanation is given to the observed widening of the α relaxation peaks with increasing measurement frequency in ε"(T). At a lower measurement frequency (<1Hz), the data obtained from thinner PVAc films showed wider α relaxation peaks (in ε"(T)) than the data obtained from thicker films, and at a higher frequency (>1Hz) this thickness dependence doesn't exist. This was explained using the existence of the liquid-like surface layer with enhanced molecular mobility, and using the measurement frequency effects.

PS films of different thickness were prepared and physical aging process in these samples was investigated using ellipsometry. On each sample the measurements were performed at different aging temperatures, and a trend of a decreasing aging rate with decreasing aging temperature is found for all samples. In ultra-thin PS films (<100nm thick) a maximum aging rate at ~17K below Tg film is observed, and these observations were explained using the competition of the two factors that affect the aging, that is, the distance to the equilibrium and the fraction of the free volume. A comparison between the thickness dependence of aging rate and the thickness dependence of Tg suggests that the aging rate serves as a more sensitive probe in the study of polymer molecular dynamics than Tg does.

Item Type:Thesis (PhD)
Supervisors:Sharp, J.S.
Faculties/Schools:UK Campuses > Faculty of Science > School of Physics and Astronomy
ID Code:780
Deposited By:Dr. Haidong Yang
Deposited On:09 Dec 2009 13:49
Last Modified:09 Dec 2009 13:49

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