Glass preparation by computer simulation

Glasses are usually formed from the melt either by progressively decreasing the temperature or increasing the pressme. Cooling has been used to pro- 

An alternative and more radical approach is to use the preparation techniques described in Section 5.3 to prepare polymer samples directly in the glassy state. This method has been used for modeling polypropylene and polyethylene. In the latter case careful control over the preparation procedure was used to construct amorphous samples of the same polymer in different configurational states in order to examine the relation between mechanical properties and polymer structure. Such control is only possible, of course, for nonequilibrium states where the structure ean be immediately frozen in. The transformation to equilibrium liquid behavior from sueh samples can be observed by heating.  

Some results of cooling experiments on the PE I model with N = 1000 are shown in Fig. 5.3. Here the density is plotted as a function of decreasing temperature with the pressure controlled at 1 atm. The set marked PE III is for N = 500 (see Section 5.3) and these data show little quantitative relation to polyethylene. For the remainder of this discussion we shall consider only the results for the PE I model. 

Five independent samples were generated and allowed to relax for 500 ps at 500 K at an applied isotropic pressure of 1 bar. Samples at different temperatures were then obtained by cooling or heating at a rate of 1 K ps to the desired temperature under isotropic controlled pressure conditions (1 bar) followed by subsequent periods of relaxation of order 1 ns. Using this procedure additional samples were generated at 600, 400, 300, 200, 100, and 10 K. 

As the temperature is lowered there is a gradual decrease in the thermal expansivity of the polymer (obtained from the slope of the plot) towards values typical of amorphous sohds. The data in the liquid regime show a lower expansivity than the laboratory material but this may be due to shortcomings of this simplified model potential. 

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