Thermodynamic and molecular properties of amorphous high polymers
| Autor: | A. T. DiBenedetto, Donald R Paul |
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| Rok vydání: | 2007 |
| Předmět: | |
| Zdroj: | Journal of Polymer Science Part C: Polymer Symposia. 16:1269-1288 |
| ISSN: | 1935-3065 0449-2994 |
| DOI: | 10.1002/polc.5070160306 |
| Popis: | A potential energy function and an equation of state for amorphous polymers are developed by using a mathematical, quasi-lattice model to represent the molecular structure. The potential energy function, obtained by dividing a polymer chain into a linked series of interaction centers, is described by three molecular parameters: e*, a force constant which measures the strength of interaction between two polymer segments, p*, a range parameter which measures the minimum separation between point centers on adjacent segments, and l, a structural parameter which indicates the size of the molecular units. A theoretical equation of state is then developed by defining a cell partition function for a polymer segment in terms of the potential energy function and one additional parameter, 3Cm, which is a measure of the number of external degrees of freedom possessed by an interaction center. It is shown that the theoretical equation for the thermal expansion at constant pressure simplifies to: Similar expressions are developed for compressibility, internal pressure, and cohesive energy density. Experimental data for 13 polymers are then correlated using the theoretical equations. Densities, thermal expansion coefficients, and internal pressure measurements are used to evaluate the molecular parameters. Internal pressures over a range of temperatures are correlated to within 1% (in the best case). The cohesive energy density and the glass transition temperature are also interpreted in terms of the model. The molecular force parameters are also evaluated by a modified Slater-Kirkwood calculation, and the results are compared to the experimentally determined values. A physical interpretation is then discussed in light of these calculations. The tentative conclusions are that the thermodynamic properties of amorphous polymers can be correlated with a model containing four molecular parameters. Two of these parameters, e* and p*, are associated with the nature of the interaction potential between structural units along the chain and can perhaps be estimated theoretically. The parameter l, a measure of the size of a structural unit on a polymer chain, has a strong effect on the repulsion between polymer segments and must be evaluated from experimental data. The parameter 3Cm is a measure of the mobility and freedom of a polymer chain and must be evaluated from experimental data. The change of a given polymer from a rubbery to a glassy state probably can be related to large changes in this latter parameter. |
| Databáze: | OpenAIRE |
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