Glassy polymer melts

Dynamics Simulation of a Glassy Polymer Melt Incoherent Scattering Function. 

Hoffman supposed that ED was not a constant, but that the diffusive transport in a melt could be described by a WLF function, in the same way as visco-elastic deformations in a glassy polymer melt near Tg may be described by it. 

Monte Carlo simulation results for the non-equilibrium and equilibrium d3oiamics of a glassy polymer melt are presented. When the melt is rapidly quenched into the supercooled state, it freezes on the time scale of the simulation in a non-equilibrium structure that ages physically in a fashion similar to experiments during subsequent relaxation. At moderately low temperatures these non-equilibrium effects can be removed completely. The structural relaxation of the resulting equilibrated supercooled melt is strongly stretched on all (polymeric) length scales and provides evidence for the time-temperature superposition property. 

Coarse-Grained Lattice Simulations for Glassy Polymer Melts Bond-Fluctuation Model and Monte Carlo Approach... 

Guo HY, Bourret G, Corbierre MK, Rucareanu S, Lennox RB, Laaziri K, Piche L, Sutton M, Harden JL, Leheny RL (2009) Nanoparticle motion within glassy polymer melts. Phys Rev Lett 102(7) 075702. doi 10.1103/PhysRevLett.l02.075702... 

Fig. 22.4. The random walk of o chain in a polymer melt, or in a solid, glassy polymer means that, on average, one end of the molecule is -yJn)A away from the other end. Very large strains (=4) are needed to straighten the molecule out.

This was originally a very slow batch process, because of the need to dissolve gaseous CO2 in solid glassy polymers such as polycarbonate and polystyrene. The low diffusivity meant that the time taken was many hours. The foam was formed when there was a phase change from the glassy to the melt state. In recent developments of the process, supercritical CO2 (for temperatures >31 °C, and pressures >7.2 MPa) is... 

Mixtures of poly(vinylidene fluoride) with poly (methyl methacrylate) and with poly (ethyl methacrylate) form compatible blends. As evidence of compatibility, single glass transition temperatures are observed for the mixtures, and transparency is observed over a broad range of composition. These criteria, in combination, are acceptable evidence for true molecular intermixing (1, 19). These systems are particularly interesting in view of Bohns (1) review, in which he concludes that a compatible mixture of one crystalline polymer with any other polymer is unlikely except in the remotely possible case of mixed crystal formation. In the present case, the crystalline PVdF is effectively dissolved into the amorphous methacrylate polymer melt, and the dissolved, now amorphous, PVdF behaves as a plasticizer for the glassy methacrylate polymers. 

As discussed earlier, solid polymers can be distinguished into amorphous and the semicrystalline categories. Amorphous solid polymers are either in the glassy state, or - with chain cross linking - in the rubbery state. The usual model of the macromolecule in the amorphous state is the "random coil". Also in polymer melts the "random coil" is the usual model. The fact, however, that melts of semi-crystalline molecules, although very viscous, show rapid crystallisation when cooled, might be an indication that the conformation of a polymer molecule in such a melt is more nearly an irregularly folded molecule than it is a completely random coil. 

Simha et al. (1973) showed that the Tait relation is also valid for polymers in the glassy state. In this case the value of bx is about the same as for polymer melts, but b2 is smaller ( 2 3 x 10 3). Even nowadays frequently use is made of the Tait equation. 

Different from the dissolution of amorphous polymers is that of semi-crystalline ones. Dissolution of these polymers is much more difficult than that in the glassy state, as the enthalpy of melting has to be supplied by the solvent. Many solvents, which are able to dissolve tactic but glassy polymers, are unable to dissolve the same polymer in the crystalline state. Asmussen et al. (1965) have found that the velocity of dissolution of crystalline polymers as a function of temperature closely resembles the velocity of crystallisation versus temperature curves. Polymers formed at the highest rate of growth also dissolve at the highest rate. 

In materials that are often used in construction-wood, concrete and steel—there is still an enormous variation in modulus, by a factor of 15 or so. You can also see that most polymer materials, at least in their usual melt processed form, are not very stiff at all, polyethylene having a modulus of about 150 MPa, while even a glassy polymer like atactic polystyrene has a modulus of only about 3000 MPa (about l/20th that of window glass). 

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