Carbohydrate Polymers in Amorphous States: An Integrated Thermodynamic and Nanostructural Investigation
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- 作者：Duncan Kilburn ; Johanna Claude ; Thomas Schweizer ; Ashraf Alam ; and Job Ubbink
- 刊名：Biomacromolecules
- 出版年：2005
- 出版时间：March 2005
- 年：2005
- 卷：6
- 期：2
- 页码：864 - 879
- 全文大小：225K
- 年卷期：v.6,no.2(March 2005)
- ISSN：1526-4602
文摘
The effect of water on the structure and physical properties of amorphous polysaccharide matrices isinvestigated by combining a thermodynamic approach including pressure- and temperature-dependentdilatometry with a nanoscale analysis of the size of intermolecular voids using positron annihilation lifetimespectroscopy. Amorphous polysaccharides are of interest because of a number of unusual properties whichare likely to be related to the extensive hydrogen bonding between the carbohydrate chains. Uptake ofwater by the carbohydrate matrices leads to a strong increase in the size of the holes between the polymerchains in both the glassy and rubbery states while at the same time leading to an increase in matrix freevolume. Thermodynamic clustering theory indicates that, in low-moisture carbohydrate matrices, watermolecules are closely associated with the carbohydrate chains. Based on these observations, we propose anovel model of plasticization of carbohydrate polymers by water in which the water dynamically disruptschains the hydrogen bonding between the carbohydrates, leading to an expansion of the matrix originatingat the nanolevel and increasing the number of degrees of freedom of the carbohydrate chains. Consequently,even in the glassy state, the uptake of water leads to increased rates of matrix relaxation and mobility ofsmall permeants. In contrast, low-molecular weight sugars plasticize the carbohydrate matrix withoutappreciably changing the structure and density of the rubbery state, and their role as plasticizer is mostlikely related to a reduction of the number of molecular entanglements. The improved molecular packing inglassy matrices containing low molecular weight sugars leads to a higher matrix density, explaining, despitethe lower glass transition temperature, the reduced mobility of small permeants in such matrices.