SynopsisA generalized theory for the glass transition temperature of crosslinked and uncrosslinked polymers has been developed, which takes into account the influences of end groups, branching, and crosslinking, and their functionality distribution. DiBenedetto's theory was found to correctly characterize the influence of crosslinks on the glass temperature. Normalized to constant crosslink functionality, the crosslink constant is a universal parameter suggesting that the entropic theory of glasses is applicable to crosslinked systems. Data on linear polymers and networks from the crosslinking of polymer chains, vinyl /divinyl-copolymers and step-growth polymers, such as polyurethanes, amine-cured epoxies, or inorganic glasses, are presented.

A theoretical treatment of the glass temperature of dendritic polymers is presented. The influences of polymer backbone, end group, initiator core, branching unit, composition and functionality are discussed. In dendritic polymers the glass temperature is dependent only on the generation number of dendritic growth and thus only on the molecular weight of a dendron, but not on the molecular weight of the whole molecule. It is governed primarily by the backbone glass temperature and depends little on branching functionality. Only minor differences between linear polymer and dendrite are obtained, since the influences of end groups and branching compensate each other to a large extent. 0 1995 John Wiley & Sons, Inc.

SYNOPSISA theoretical approach to thermoset cure kinetics based on Arrhenius kinetics and mobility was developed by considering the activation of the reacting group and chain mobility as elementary steps for reaction. This extended kinetic equation was successfully applied to the curing of an epoxy by a n amine, the trimerization of a cyanate, and to the polymerization of methyl methacrylate. Full agreement between theory and experimental data was obtained in all cases. The activation energies for chain mobility were exceptionally low (0.3-1 k J / mol for bisphenol-A-based epoxy and cyanate) which indicates that the structural units must undergo only small-angle rotational oscillations to allow a reaction. A theoretical time-temperature-transformation (TTT ) diagram is also presented. 0 1993 John Wiley & Sons, Inc.

Unmodified blends of two thermoplastic polyurethanes (TPU) and six polyolefines were used to study the influence of the component viscosities on the blend morphology and mechanical properties. Blends were produced by melt mixing using a twin screw extruder. Interactions between the blend components could not be detected by DSC, DMA, selective extraction, and SEM micrographs of cryofractures. The variation in tensile strength with blend composition produce a U-shaped curve with the minimum between 40 and 60 wt % of polyolefine. At similar viscosity ratios (h d /h m ), blends with polyether based TPU (TPU-eth) have a finer morphology than blends with polyester based TPU (TPU-est). This is due to the lower surface free energy of the polyether soft segments compared to the polyester soft segments. Different morphologies also lead to changes in mechanical behavior. Blends with TPU-eth show a lower decrease in tensile strength with blend composition than blends with TPU-est. The viscosity ratio between TPU and polyolefines can be directly correlated to the blend morphology obtained under similar blending conditions. TPU/PE blends show a lower dispersity than TPU/PP blends, due to the higher viscosity ratios of TPU/PE blends. This results in a greater reduction in tensile strength with the disperse phase content.

The coalescence behavior of immiscible blends was determined by annealing the melt without shear. The kinetics and the mechanism of coalescence were observed for blends of thermoplastic polyurethane (TPU) and polyolefins. Two different types of measurements were used to observe coalescence in quiescent melt at the processing temperature. On the one hand, the blend granules were annealed in bulk. The resulting particle sizes were determined by light microscopy and by SEM on particles separated from the blend. On the other hand, thin samples were annealed on a hot stage. Coalescence was observed in situ by light microscopy or static laser light scattering. It was found that the higher viscosity and elasticity ratios between polyethylene and TPU lead to a more pronounced coarsening of the morphology of 80/20 blends than in TPU/polypropylene. It has been shown that the process of reshaping coalescence is one mechanism of coalescence that occurs in quiescent melt. Another mechanism that was directly observed is a “domino effect” where one coalescence process causes the next one.