Hydrophilic amorphous polymer-water vapor

In this presentation, two examples of the use of vibrational spectroscopy to probe water-solid interactions in materials of interest to the food and pharmaceutical sciences are described. First, the interaction of water vapor with hydrophilic amorphous polymers has been investigated. Second, water accessibility in hydrated crystalline versus amorphous sugars has been probed using deuterium exchange. In both of these studies, Raman spectroscopy was used as the method of choice. Raman spectroscopy is especially useful of these types of studies as it is possible to control the environment of the sample more easily than with infrared spectroscopy. 

Indirect methods of determining the degree of crystallinity start from the fact that a given chemical or physical event proceeds differently in the crystalline phase and in the amorphous phase. Common physical experiments include, for example, the study of water vapor absorption of hydrophilic polymers or the diffusion of a dye into the polymer. Together with a series of chemical reactions (hydrolysis, reaction with HCHO, deuterium exchange), they are used in particular for determining the crystallinity of cellulose. 

An interesting use of GVS, for amorphous dispersions, is detection and quantification of crystallinity. Due to higher hygroscopicity of hydrophilic polymers and amorphous API of ASD, the exposure of a sample to elevated humidity environment triggers the sorption of water molecules by polar/hydrophilic functional groups at the air-solid interface. The a-relaxation time of amorphous material often correlates with its water sorption potential (Bhardwaj and Suryanarayanan 2013). The gravimetric vapor sorption (GVS)-desorption led to the dissimilar structurally reversal of annealed amorphous trehalose when compared to that obtained by heating beyond Tg (Saxena et al. 2013). The sorbed water molecules Tg = -137°C) increases... 

Humidity. Hydrophilic polymers such as polyamides and EVOH ahsorh water from humid air. For example, water sorption isotherms for Nylon have been determined hy Hernandez and Gavara. The presence of water in a hydrophilic polymer affects the permeability of, oxygen, carbon dixide, organic vapor, and flavor and aroma compounds. An increase in moisture content increases oxygen permeability in EVOH (Fig. 11.6). Nylon 6, while for amorphous nylon the permeability tends to decrease (Fig. 11.12). 

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