Glassy polymer specific volume

Figure 1.67 Specific volume as a function of temperature on cooling from the melt for a polymer that tends to crystallize. Region A is liquid, B liquid with elastic response, C supercooled liquid, D glass, E crystallites in a supercooled liquid matrix, F crystallites in a glassy matrix, and G completely crystalline. Paths ABCD, ABEF, and ABG represent fast, intermediate, and very slow cooling rates, respectively. From K. M. Ralls, T. H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Vo Specific volume of the densified glassy polymer (cnrfyg)... 

The density or its reciprocal, the specific volume, is a commonly used property for polymeric materials. The specific volume is often plotted as a function of pressure and temperature in what is known as a pvT diagram. A typical pvT diagram for an unfilled and filled amorphous polymer is shown, using polycarbonate as an example, in Figs. 2.10 and 2.11 The two slopes in the curves represent the specific volume of the melt and of the glassy amorphous polycarbonate, separated by the glass transition temperature. 

Polymers above their Tg are in a state of equilibrium much like simple liquids. However, upon cooling below Tg, polymers are not able to achieve an equilibrium state since the polymer chain segments lack sufficient mobility to reach this state in realizable time scales. Thus, glassy polymers exist in a nonequilibrium state that is a function of the prior history of the sample. It is useful to think of simple volumetric thermal expansion where at equilibrium the specific volume at a given temperature and pressure is Veq(T,p) the specific volume of a rubbery polymer is given by Veq. The... 

There exists an alternative explanation. Egami, Maeda et al. 85) introduced different kinds of defects of n , p and t type to characterize the glassy state of matter. Defects of n and p-type correspond to negative and positive density fluctuations while the defects of x-type are the shear defects which do not change the specific volume of the system. Different defects through which the total free volume of a sample is distributed affect different properties. If curing at different Tcur<. leads to polymers with different defect ratios, the differences in the macroscopic behaviour can be explained. 

FIGURE10-54 Schematic plots of temperature versus volume (top) and specific heat (bottom) for a glassy polymer. 

Figure 3.1. Schematic illustration of temperature dependences of the specific volumes of amorphous materials. This figure also illustrates the effects of the nonequilibrium nature of glass structure, which results from kinetic factors. Glass 1 and Glass 2 are specimens of the same polymer, but subjected to different thermal histories. For example, Glass 1 may have been quenched from the melt very rapidly, while Glass 2 may either have been cooled slowly or subjected to volumetric relaxation via annealing ( physical aging ) in the glassy state.

The glassy polymers such as the aromatic polyamides and polycarbonates have significant hindrances to intramolecular mobility. The data for these materials appear to be correlated fairly well in terms of the "specific free volume" discussed by Lee(52). Structural variations that suppress the ability to pack tend to reduce the quality of the barrier while those that improve the ability to pack produce better barriers. The free volume in this case is defined as the difference between the actual polymer molar volume at the temperature of the system and at 0°K. This latter parameter is determined by group contribution methods. 

In the NET-GP analysis, the glassy polymer-penetrant phases are considered homogeneous, isotropic, and amorphous, and their state is characterized by the classical thermodynamic variables (i.e. composition, temperature, and pressure) with the addition of a single-order parameter, accounting for the departure from equilibrium. The specific volume of the polymer network, or, equivalently, the polymer density Pp, is chosen as the proper order parameter. In other words, the hindered mobility of the glassy polymer chains freezes the material into a non-equilibrium state that can be labeled by the... 

Figure 4.3 Specific volume-temperature curves for a semicrystalline polymer. (A) Liquid region (B) viscous liquid with some elastic response (C) rubbery region (D) glassy region (E) crystallites in a rubbery matrix (F) crystallites in a glassy matrix. 

At low temperature, an amorphous polymer is glassy, hard, and brittle, but as the temperature increases, it becomes rubbery, soft, and elastic. There is a smooth transition in the polymer s properties from the solid to the melt, as discussed above, so no melting temperature is defined. At the glass transition temperature, marking the onset of segmental mobility, properties like specific volume, enthalpy, shear modulus, and permeability show significant changes, as illustrated in Fig. 3.43. 

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