Amorphous polymer-water vapor

Interactions between water vapor and amorphous pharmaceutical solids were evaluated using isothermal microcalorimetry. " The desorption of water from theophylline monohydrate has been investigated using microcalorimetric approaches.The properties of surfactants and surface-active drugs in solution were studied by Attwood et al. " using calorimetry, while titration microcalorimetry has been utilized to elucidate the nature of specific interactions in several pharmaceutical polymer-surfactants systems. " Drug decomposition was evaluated as a function of different... 

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. 

Fourier Transform Raman Spectroscopic Study of the Interaction of Water Vapor with Amorphous Polymers... 

Score values of the first principal component following analysis of the 1550 to 600 cm spectral region plotted against the mole fraction water. Source Reproduced from Taylor, L.S., Langkilde, F.W., and Zografi, G. Fourier transform Raman spectroscopic study of the interaction of water vapor with amorphous polymers, /. Pharm. Set., 90,888-901,2001. With permission of the copyright owner.)... 

Fujita s model is valid for penetrant/polymer systems with diffusion coefficients that exhibit a strong concentration dependence, such as organic vapors in amorphous polymers, (20,22,24-27), but fails to describe the difference between water in poly(vinyl acetate) and in poly(methyl acrylate) (28). This may be due to the hydrogen-bonding nature of water rather than to a failure of the model. Fujita viewed his theory as inappropriate for small penetrant molecules, whose diffusion is largely independent of concentration, because the critical hole size for such penetrants is... 

Transport in semi-crystalline polymers can, thus, be treated on the basis of a simple two phase model, consisting of a dispersed impermeable crystalline phase in a permeable amorphous matrix. Lasoski and Cobbs (12) attempted to correlate the solubility of water vapor in unoriented poly(ethylene terephthalate) and nylon 610 with... 

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. 

Taylor LS, Langkilde FW, Zografi G (2001) Fourier transform Raman spectroscopic study of the interaction of water vapor with amorphous polymers. J Pharm Sd 90 888-901... 

The p for water vapor permeation has been determined as 9.8 kJ/mol for PLA films with 98% L-lactide and —10.1 kJ/ mol for PLA films with 94% L-lactide [ 15]. Ep values for PLA can differ depending on the processing conditions and crystallinity. For example, the Lp values for amorphous and semicrystalline PLA films have been reported as 5 and —0.1 kJ/mol, respectively, at 100% RH [37]. Very few polymers show negative activation energies for water, and usually... 

Jeong, H.M., Ahn B.K., Cho S.M. and Kim B.K. (2000), Water vapor permeability of shape memory polyurethane with amorphous reversible phase. Journal Of Polymer Science Part B-Polymer Physics, 38(23) pp. 3009-3017. 

Duan and Thomas [13] went further to extend the model of the tortuous path effect of crystallinity (obtained via the DSC) on another polymer, polytetraethylene, PET and obtained data as plotted in Figure 12.3 and concluded that the water vapor permeability decreased linearly with the percentage of crystallinity. They found out that the measured values of WVTR decreased linearly with increasing crystallinity of the PLA from 0% to 50% and they readily explained this observation in terms of the effect of crystallinity on the solubility of water vapor in polymers, i.e., that water is insoluble in the crystalline regions and so the solubility coefficient (5) of a semi-crystalline polymer is equal to the solubility coefficient of the amorphous fraction. They therefore concluded that the tortuous path model could also be used to explain the water permeability characteristics of PET. 

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