Elucidation of the dependence of the thermodynamic energy terms for compaction of carbohydrate polymers upon water vapor sorption and molecular weight.

摘要

The objective of this study was to elucidate the dependence of the thermodynamic energy terms for compaction of carbohydrate polymers upon water vapor sorption and molecular weight. Powder compressions were analyzed in a thermodynamically rigorous manner utilizing a compression calorimeter. A compression calorimeter is capable of simultaneous measurements of force, displacement, and temperature changes within a powder bed undergoing volume reduction. By application of the First Law of Thermodynamics, the irreversible internal energy change (DeltaE IRR) for compaction is calculated from the measured irreversible work (DeltaWIRR) and irreversible heat (DeltaQIRR) terms. Pregelatinized corn starch and maltodextrin polymers of varying molecular weight were employed in this study.;The solid-state properties of these polymers, including particle size and size distribution, true density, crystallinity, and water vapor sorption behavior, were characterized. The deformation behaviors of these polymers were also characterized by Heckel analyses. In addition, compact mechanical properties were studied utilizing a stress-strain analyzer.;Pregelatinized corn starch and maltodextrin polymers exhibited Type II-like sorption isotherms, indicative of both adsorption to the external surface and absorption into the bulk polymeric solid. The sorption behaviors demonstrated a molecular weight dependence and were related to the molar cohesive energies of the bulk polymers. The Heckel analyses indicated a shifting deformation mechanism as a function of changing polymer moisture content and molecular weight. The yield pressures were inversely related to carbohydrate polymer moisture content and molecular weight.;The ductilities of hydrated and high M.W. carbohydrate polymers were also reflected in the thermodynamic energy terms for compaction. At constant applied load, the DeltaWIRR, DeltaQIRR, and DeltaE IRR values for compaction varied systematically with carbohydrate polymer moisture content and molecular weight. In addition, the DeltaEIRR values for compaction were correlated to compact mechanical properties. Thus, compression calorimetry provides a rigorous thermodynamic criterion for tableting success and appears to elucidate the extent of interparticulate bonding in carbohydrate polymer systems. Furthermore, the systematic relationships seen in this work offer the promise for making a priori predictions of carbohydrate polymer compaction behavior, and also for facilitating the design of polymer systems with optimum material properties.