科研之友
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摘要
In the classical swelling theory a polymer gel is a very (infinitely) large cross-linked polymer molecule that encages the solvent molecules. Here, a new swelling model is developed. Firstly, this model permits many cross-linked polymer molecules with finite molar mass instead of a single one. The gel formation is described by a huge number of consecutive swelling equilibria incorporating a solvent molecule by solvent molecule. Thus, the gel is a polydisperse mixture of cross-linked polymer molecules encaging different amounts of solvent. The polydispersity is expressed by a gel-distribution function, the width of which decreases with increasing molar mass of the cross-linked polymer. In the limit of infinite molar mass the new swelling theory is equivalent to the classical one. Secondly, different to the classical treatment, the polymer network is considered to be finitely extensible. Both theories are applied to hydrogels of poly(N-isopropylactylamide). The mixing contribution of the Helmholtz energy is described by a modified Flory-Huggins expression, the parameters of which are fitted to cloud-point data of a solution of a linear poly(N-isopropylacrylamide). For the elastic contribution the phantom network theory is applied. The new model calculates the swelling degree as a function of temperature nearly perfect in comparison with the experimental data. The classical theory fails in the range of high swelling degrees. Furthermore, the classical treatment predicts a first-order transition between a swollen and a shrunken gel. The new model predicts also a sharp but continuous transition.
- 出版日期2014-10
