Development of a structure-property correlation for castable urethane elastomers

A significant problem in electronic encapsulation is the poor load bearing performance of existing replacements for Adiprene-MOCA urethane elastomer. In response to this problem, this study defines the structural features that control the viscoelastic properties of the following liquid castable elastomers: Adiprene-MOCA, EN-7, and 3121-S. A review of previous investigations on a related class of materials suggests that viscoelastic properties may be more directly related to the physical structure or morphology of these elastomers, rather than their chemical structure. Accordingly, the morphology of the subject elastomers is characterized by means of electron microscopy and x-ray scattering measurements. These measurements reveal that within each elastomer incompatible chain segments cluster into separate domains, or microphases on a scale of 10 +- 1 nm. On this basis, it is concluded that the two major thermomechanical transitions present in each elastomer can be assigned to separate transitions within the two microphases. The above-ambient transition, which determines the upper use temperature of the elastomer, is specifically assigned to the glass transition of an amorphous microphase. The significance of this structure-property correltion for the liquid castable elastomers is twofold: (1) it permits generalization of mechanical property measurements on existing materials in order to predict their performance in unusual applications and (2) it leads to a rational strategy for developing improved elastomers for new, more demanding applications

Morphology and phase composition of an amine cured rubber modified epoxy

The sharpness of the interface between the matrix and the dispersed phase, the volume fraction of the dispersed phase, the distribution of particle sizes, and the concentration of epoxy in the dispersed phase were determined. Scanning transmission electron microscopy coupled with energy dispersive x-ray analysis revealed that the interface width is less than 500 A. Variation of the fraction of mobile hydrogens with temperature determined by nuclear magnetic resonance indicated that a small fraction of segments participated in mixing at the interface. Differential scanning calorimetry and nuclear magnetic resonance indicated that all the rubber precipitates as a discrete phase. The distribution of particles greater than 0.1 ..mu..m in diameter was measured, and the average diameter of these particles was found to be 0.8 ..mu..m. The particles larger than 0.1 ..mu..m accounted for approximately 50% of the total volume of dispersed phase. The epoxy concentration in the dispersed phase was determined using /sup 13/C nuclear magnetic resonance spectroscopy. This concentration was found to be less than that predicted if all the epoxy monomer units attached to the rubber molecules were present in the dispersed phase

A Computational Model for Processing of Semicrystalline Polymers: The Effects of Flow-Induced Crystallization

A computational model for the combined processes of quiescent and flow-induced crystallization of polymers is presented. This modelling should provide the necessary input data, in terms of the structure distribution in a product, for the prediction of mechanical properties and shape- and dimensional-stability. Rather then the shear rate as the driving force, a viscoelastic approach is proposed, where the viscoelastic stress (or the equivalent recoverable strain) with the highest relaxation time, a measure for the molecular orientation and stretch of the high end tail molecules, is the driving force for flow induced crystallization. Thus, the focus is on the polymer that experiences the flow, rather then on the flow itself. Results are presented for shear flow, extensional flow and for injection moulding conditions of an isotactic Polypropylene (iPP)