Amorphous polymers temperature/pressure dependence

Fine crystalline quartz or amorphous substances (gel, quartz glass, etc.) are used as one of initial components for the synthesis [19]. Depending on temperature, pressure, pH of the medium and the presence of salts, silica can exist in solution both as simple ions or molecules, and as more complicated polymer particles. Under normal conditions silica passes into solution in monomer form, as silicic acid Si(OH)4 at large pH, silicate ions SiOj - are formed. Monosilicic acid is a very weak acid however, at increased temperature its dissociation constant increases substantially. The amount of monomer form also increases with temperature. The dissolution of Si02 is due to hydration as well as to depolymerization. 

Each, R., Grellmann, W., Schroeter K., and Donth, E. (1999) Temperature dependence of d5mamic yield stress in amorphous polymers as indicator for the d5mamic glass transition at negative pressures. Polymer 40, 1481... 

Experimental data from our laboratories will be shown for an extensive series of amorphous polymers with glass transitions between Tg = 200 and 500 K. We discuss the temperature dependence of the hole-size distribution characterized by its mean and width and compare these dependencies with the hole fraction calculated from the equation of state of the Simha-Somcynsky lattice-hole theory from pressure-volume-temperature PVT) experiments [Simha and Somcynsky, 1969 Simha and Wilson, 1973 Robertson, 1992 Utracki and Simha, 2001]. The same is done for the pressure dependence of the hole free-volume. The free-volume recovery in densified, and gas-exposed polymers are discussed briefly. It is shown that the holes detected by the o-Ps probe can be considered as multivacancies of the S-S lattice. This gives us a chance to estimate reasonable values for the o-Ps hole density. Reasons for its... 

Next we note that there are two physieally different sources of temperature and pressure dependence of the elastic constants of polymers. One, in common with that exhibited by all inorganic crystals, arises from anharmonic effects in the interatomic or intermolecular interactions. The second is due to the temperature-assisted reversible shear and volumetric relaxations under stress that are particularly prominent in glassy polymers or in the amorphous components of semi-crystalline polymers. The latter are characterized by dynamic relaxation spectra incorporating specific features for different polymers that play a central role in their linear viscoelastic response, which we discuss in more detail in Chapter 5. 

As with amorphous metals and semiconductors, the unit plastic relaxations in glassy polymers are also thermally assisted shear transformations (STs), which control the temperature dependence of the plastic resistance and encompass other phenomena of strain softening and the pressure dependence of the resistance. Moreover, the incremental processes of molecular-segment alignment, resulting... 

The temperature dependence of polymer densities at atmospheric pressure is given in Tables 7.1 and 7.2. Table 7.1 gives densities measured above the glass transition temperature Tg and, for semi-crystalline polymers, above the melting temperature. Table 7.2 lists densities of amorphous polymers below Tg. Volumetric data are presented here in terms of the density p rather than the... 

In addition to the free volume [36,37] and coupling [43] models, the Gibbs-Adams-DiMarzo [39-42], (GAD), entropy model and the Tool-Narayanaswamy-Moynihan [44—47], (TNM), model are used to analyze the history and time-dependent phenomena displayed by glassy supercooled liquids. Havlicek, Ilavsky, and Hrouz have successfully applied the GAD model to fit the concentration dependence of the viscoelastic response of amorphous polymers and the normal depression of Tg by dilution [100]. They have also used the model to describe the compositional variation of the viscoelastic shift factors and Tg of random Copolymers [101]. With Vojta they have calculated the model molecular parameters for 15 different polymers [102]. They furthermore fitted the effect of pressure on kinetic processes with this thermodynamic model [103]. Scherer has also applied the GAD model to the kinetics of structural relaxation of glasses [104], The GAD model is based on the decrease of the crHiformational entropy of polymeric chains with a decrease in temperature. How or why it applies to nonpolymeric systems remains a question. 

According to the experimental observations, the increase of void content is not sensitive to the maximum temperature when the specimen is reheated above Tg (for amorphous polymer matrices) or above the melting temperature, T (for semicrystallized polymer matrices). Also, it was found that the variation of void content is insensitive to the holding time in the reheating process, even when the processing period varied by several minutes. Yet the dependence of void content on the applied external pressure is quite clear. As shown for a GF/PA12 composite laminate, the void content is about 10% at a pressure of 0.1 MPa, and then reduces to about 3% when the pressure increases to 0.5 MPa. No further variations of note are observed when the applied external pressure is over 1.0 MPa. 

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