Dynamics of polymers in ultra-thin films

This thesis presents experiments aimed at understanding more about the dynamics of polymer chains when they are confined in a space close to or smaller than their unperturbed, bulk dimensions. Chapter 2 gives an account of the existing data on polymer thin films emphasizing the controversies existing in the results obtained over the last 15 years on polystyrene (PS) thin films. In chapter 3 the effect of thickness reductions on the glass transition dynamicsin ultra-thin films of polystyrene has been studied by dielectric spectroscopy(DS) and capacitive dilatometry (CD). Upon reduction of the film thickness, a systematic reduction in the dilatometric glass transition temperatures, Tg, was observed via CD, while DS revealed a continuous speed-up and broadening of the α-process, accompanied by only minor reductions in the fragility index. A good agreement between "spectroscopic" and the dilatometric glass transition temperatures was found for films thicker than 20 nm, while for thinner films they become different. A likely explanation for this discrepancy is the presence of another dynamic process showing Arrhenius-behaviour with a pre-exponential factor being indicative for non-cooperative dynamics. Such a new process might be assigned to distinct surface dynamics in polystyrene films as suggested in recent papers. Chapter 4 describes the effect of thickness on the glass transition dynamics in ultra-thin polystyrene films, studied by thin film ac-calorimetry, dielectric spectroscopy and capacitive dilatometry. In all PS-films, a prominent α-process was found in both the ac-calorimetric and dielectric response, indicating the existence of cooperative bulk dynamics even in films as thin as 4 nm. Glass transition temperatures were obtained from ac-calorimetric and from capacitive dilatometry, and reveal a surprising, marginal thickness dependence Tg(L). This result confirms data by Efremov et al., but oppose many previous observations, and is rationalized by differences in film annealing conditions together with the fact that our techniques probe exclusively cooperative dynamics (ac-calorimetry, DS) or allow the effective separation of surface and "bulk"-type mobility. Two other observations, a significant reduction in cp towards lower film thickness and the decrease in the contrast of the dilatometric glass transition, support the idea of a layer-like mobility profile consisting of both cooperative "bulk" dynamics and non-cooperative surface mobility. In chapter 5 the local and cooperative dynamics in atactic (a-PS) and syndiotactic (s-PS) polystyrene were studied by dielectric spectroscopy. Besides the known α-relaxation, two additional relaxation processes β1 and β2 were revealed in various samples of a-PS and s-PS films cast from solvent solutions. These new dynamic processes show Arrhenius behavior, a common activation energy around 80 kJ/mol and cross the α-relaxation region without merging, indicating a molecular origin being phase separated from the amorphous PS fraction. By Fourier transform infrared spectroscopy (FTIR) measurements, a clear link between the existence of T2G2 helix conformation and the occurrence of the β1 and β2 processes was established. Symmetry arguments and systematic differences in the relaxation parameters between a-PS and s-PS favor the assignment of the fast, β1, process to a helix defect mechanism (helix inversion), while the slow mode, β2, likely originates from cooperative helix inversion events that would point to spatially organized aggregates of helices as suggested earlier for PS-gels. The occurrence of such solvent induced structures and their dynamics might have important implication for the interpretation of Tg reductions found in ultra-thin PS films. Chapter 6 gives a different interpretation to the existing controversy regarding the Tg reductions in thin films of PS. In this chapter new evidence from dielectric spectroscopy of the existence of a thin surface layer with distinct dynamics compared with the rest of the film was presented and a new model in which the T2G2 helices existing in the film migrate to the surface after sufficient annealing was proposed. The last chapter of the thesis, chapter 7, presents dielectric spectroscopy measurements on thin films of poly(2-vinylpyridine), a polymer that has a similar molecular structure to polystyrene, but different interaction with the aluminium substrate used for the measurements. The glass transition temperature was found to be almost independent of film thickness showing a slight increase for films thinner than 15 nm. The relaxation strengths of both α and β processes decrease with decreasing film thickness, slightly above a critical thickness and much sharper below this thickness. From the way the glass transition temperature and the relaxation strength depend on film thickness, it was concluded that this behavior is a direct result of the influence that the intermolecular hydrogen bonding with the Al substrate has on the film properties.nrpages: 125status: publishe