Interface Effects on Relaxation Dynamics in Nanostructured Polymer Films Investigated by Local Dielectric Spectroscopy

The motions of a polymer chain can be described on a variety of different length scales, ranging from the size of the individual monomers to the overall size of the macromolecule. By confining polymeric materials to dimensions that are comparable to the different length scales characterizing a polymer chain, their properties, such as viscoelasticity, mechanical and dielectric relaxation, and glass transition temperature, can be significantly altered with respect to the bulk behavior. The understanding of these changes of polymeric materials in confined geometries, for instance ultrathin polymer films with thickness of tens of nanometers or polymer-based nanocomposites, are becoming more important since such confined materials have been used effectively in nanotechnological applications, such as optical coatings, protective coatings, adhesives and dielectric insulators in microelectronic circuits. Though many evidences, including theoretical and experimental studies, show that the interfacial interactions at surfaces or interfaces play a key role in determining the observed dynamics deviations between confined and bulk polymers, the consensus on such point is still far to be obtained as preparing and measuring conditions can greatly affects the results thus altering the observation of true physical phenomenon. In addition, the limited availability of techniques that allow measuring dynamics with the spatial resolution of tens of nanometers, leads to the difficulty in studying the dynamics deviations on interfacial regions of nanostructured polymers, such as nanocomposites or blends. This restriction means, for instance, that the study of physical properties of nanostructured polymers is usually performed by averaging on large portion of the sample and the results are in principle depending on the model used to interpret the data. The goal of this Ph.D. thesis is to study the interfacial effects on dynamic relaxation of confined polymers (two different types of material are considered: ultrathin PVAc films coated on solid substrates and PVAc nanocomposite films with layered silicates and SiOx nanoparticles) at the nanoscale of spatial resolution by means of a technique, so-called local dielectric spectroscopy (LDS), suitably developed for such purpose. By such technique, dielectric measurements can be performed on polymer films having a free upper surface and with spatial resolution at nanometer scale, relying on that many limitations which previous techniques have met can be overcome. The major results in this thesis are listed below: 1. After an annealing procedure under vacuum at 323 K for about 3 days (i.e. above Tg and for a time much longer than any characteristic time scale of polymer mobility), confinement effects on structural relaxation were observed on PVAc350 (Mw = 350 kg/mol) films deposited on gold and aluminum substrates when the film thickness decreases below about 35 nm. However, such effects were not found on Si-supported PVAc350 films as thin as 12 nm. The difference was reasonably related to the interfacial energies between the polymer and substrate surfaces. 2. The influence of polymer molecular weight on the above effects was studied by using Au-supported PVAc films with three different Mw: PVAc15 (Mw = 15 kg/mol), PVAc167 (Mw = 167 kg/mol) and PVAc350. The results indicated that confinement effects on structural relaxation of PVAc films was observed and seemed not to be affected by Mw in the range between 167 and 350 kg. In contrast, there is no observable effect on the dynamics of PVAc15 films when the film thickness decreases from 100 nm down to 9 nm. 3. By further annealing PVAc350 films supported on aluminum substrates at higher temperature (366 K) still under vacuum, the annealing-time dependence of structural relaxation was found on the 23 nm thick Al-supported film. In particular, the bulk dynamics was recovered after six days of annealing. However, no change in dynamics was observed for the 233 nm thick PVAc350 film on aluminum as well as on the 26 nm thick film on silicon when annealed up to 8 days. Our results are consistent with the presence of long living metastable states of the polymer at the interfacial layer close to the substrate. The characteristic time of such states was evidenced to depend on the nature of substrate, and polymer molecular weight as well. The presence of an absorbed polymer layer with annealing time can be also used to explain the thickness independent dynamics observed on PVAc15 supported on gold substrates. 4. The influence of ambient relative humidity of dynamics of PVAc350 films under confinement was studied with increasing RH from 3 % to 75 %. For both the relative thick (128 nm) and thin (21 nm) films supported on gold substrates, relaxation rate of the α-process increases at higher RH. Moreover, with increasing RH up to 40 % the confinement effect on relaxation dynamics is still observed between two films, but it seems to be depressed at the highest RH of 75 %. The impact of supporting substrate on the increase of relaxation rate of the α-process in ultrathin PVAc films was also investigated by using two substrates with a well-different affinity to water, i.e., aluminum and gold. The relaxation rate measured on the Al-supported film is much faster than that measured on the Au-supported one at RH of 40 %, by a factor of about 2, evidencing a strong correlation between the effect of RH on the structural relaxation of ultrathin PVAc films and the supporting substrate on that films deposited. 5. We here for the first time apply LDS method to study the effects of nanoinclusions on the structural relaxation of PVAc at interfacial regions by mapping as well as locally measuring dielectric loss over a range of temperature above the bulk Tg of the polymer. The electric phase shift image on the PVAc-SiOx nanocomposite film evidences the existence of interfacial regions around nanoparticles, the size of such regions can be as large as several times the particle radius. Whereas, dielectric spectra acquired in the boundary region of PVAc and MMT shows a slowing down of structural dynamics of polymer chains due to the presence of the nanoinclusions. Such results directly verify, with local scale measurements, the effect on polymer mobility due to well-dispersed and interacting nanoinclusions, previously only measured on macroscopic scale.