(188k) Thermal Relaxations in Poly (Vinyl Alcohol): Moisture Content Effect On the Alpha-Relaxation

Poly vinyl alcohol (PVA) is used in a wide variety of applications that include the bio-medical field, sensor, food industry and fuel cells due to its good film-forming, excellent mechanical properties, biocompatibility, non-toxicity, flexibility, easy preparation, excellent chemical resistance, and that it allows being crosslinked without any chemical agent. In order to improve or enhance a specific property, this synthetic polymer hasd been blended, chemically modified and combined with a high number of natural polymers and inorganic compounds. Consequently, because of the wide variety of applications and processing of PVA it becomes necessary to fully understand the nature of this polymer, and to gain a better understanding of its physical and chemical properties, poly vinyl alcohol (PVA) has been intensively investigated by a huge number of techniques. In this context, one parameter that belongs and characterizes the nature of pure polymers, its blends and composites, is the glass transition temperature (T_g), this characteristic temperature indicates the change from the glassy state into a liquid or a rubbery state and determines important properties as compatibility or miscibility, or even it can be use as a tool to modify the physical properties of a material such as dissolution, bioavailability, processing and handling qualities. The T_g of PVA has been modified by the inclusion of ions, ceramics, monomers and other polymers, and it has been previously investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and dielectric measurements, however, the effect of moisture has been not taking into account in most of the cases or it has been misunderstood leading to differences in the T_g reported values. This work reports the molecular dynamics analysis of polyvinyl alcohol (PVA) thin films with two different molecular weights, successfully investigated by dielectric spectroscopy and dynamic mechanical analysis in the 20°C-300°C temperature range. Our results show that and appropriate monitoring and elimination of moisture content before discussing the nature of each relaxation process is crucial, since water effect can lead to inaccurate interpretations of the molecular motions nature in this polymer; i.e. the evaporation of water has previously been assigned by other authors as side groups motion or a secondary relaxation. In this work, the dielectric and DMA analysis of PVA relaxation processes in wet and dry films is reported. In the case of wet films, two well-defined Vogel-Fulcher-Tammmann (VFT) non-linear behaviors clearly separated by the evaporation region (between 80°C and 130°C) are shown; and they are labeled as low and high temperature relaxations. The VFT behavior can be related to the glass transition through the Vogel temperature (T₀) obtained by the fitting of the experimental data to this model, and therefore a glass transition temperature (T_g) can be assigned. On the other hand, once water is eliminated by annealing at 130°C, dry films (water content < 0.01%) show that these two VFT behaviors merge into one and as a result, the α-relaxation related to the glass transition phenomenon is unveiled in the whole temperature range until melting. The T_g in PVA is highly dependent on moisture content and water can plasticized PVA, a glass transition temperature can be assigned to PVA depending upon moisture content and molecular weight, since T_g shifts to higher temperatures as moisture content decreases and molecular weight increases.