Molecular Relaxation of the Side Groups in Poly(vinyl alcohol) Films in GHz Frequency Range
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Molecular Relaxation of the Side Groups in Poly(vinyl alcohol) Films in GHz Frequency Range Siva Kumar-Krishnan, Olimpia L. Arias de Fuentes, Evgen Prokhorov, Araceli Mauricio-Sanchez, Moisés Oviedo Mendoza and Gabriel Luna-Barcenas. Cinvestav del IPN, Unidad Querétaro, Libramiento Norponiente 2000, Querétaro, QRO 76230, MEXICO. ABSTRACT Poly(vinyl alcohol) (PVA) is a synthetic polymer which has been used in a wide variety of applications. This polymer has been extensively investigated by a large number of techniques to shed light about its physical and chemical properties. In this work, for fist time, high frequency (1x109-3x109 Hz) relaxation process has been observed in the PVA films in the temperature range of -100C to +1200C. This relaxation exhibits negative activation energy below glass transition temperature Tg and at higher temperature positive activation energy with subsequent saturation. Upon cooling the activation energy was negative again. This high frequency relaxation process and its temperature dependence can be attributed to the interaction of the bounded water and the changes of energy and freedom of movement of OH side molecular chains groups. This conclusion has been supported by in situ FTIR measurements. A possible scenario of this relaxation and dynamics of molecular motion has been proposed. INTRODUCTION Poly(vinyl alcohol) (PVA) is a typical synthetic water-soluble polymer possessing one hydroxyl (OH) group per monomer. For many years, PVA has been extensively investigated by a large number of techniques to gain a better understanding of its physical and chemical properties. PVA has been used in a wide variety of applications, which include the biomedical field, sensors, the food industry, and fuel cells, to name a few [1, 2]. PVA exhibits relatively high water absorption. Controlling the water transport in PVA is of great importance in biomedical applications of the polymer and especially in controlled release devices [3, 4]. The water molecules can interact with PVA by means of hydrogen bonding, resulting in moisture absorption. Water molecules can constitute a problem due to their strong plasticizer effects in films by modifying the physical properties of the material [5, 6]. Additionally, it was shown [5, 7] that small amounts of water affects the glass transition temperature Tg. The glass transition temperature play important role in polymer science because it indicates the change from the glassy state into a rubbery state. Additionally, the physicochemical properties such as dissolution, bioavailability, processing, and electrical properties of a polymer can be related to the material’s Tg [8]. Dielectric spectroscopy is one of most commonly used techniques for investigation of glass transition phenomena which is related to α-relaxation [9]. For polymers, α-relaxation corresponds to the segmental motion of chains [10]. Additionally, inter- and intra-molecular interactions can give rise to the appearance a new relaxation process in polymer materials which can be detected using dielectri
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