SynopsisDynamic mechanical properties have been used as the basis for some important conclusions with regards to the physical properties of rubber-modified high-impact polymers. This paper attempts to show that conclusions of this type should be limited to fairly narrow groupings of polymers. Since the size, shape, and position on the temperature scale of a damping peak are influenced by composition, morphology, and method of polymer preparation, the significance of the damping peak associated with the rubber phase of the polymer has probably been generalized to too great an extent. Two examples of polymer groupings are given to illustrate the need for caution in attaching significance to the dynamic mechanical properties of polymers. Also given are two fairly narrow polymer groupings to show to what extent dynamic mechanical properties can be used for a correlation with impact strength and rubber concentration.
The mechanical properties of the reinforcing phase of rubber modified polymers are important in determining the performance of the composite material. Of particular interest are the modulus values of the rubber particle phase. Stress concentrations in the rigid phase are determined by the shear modulus of the rubber particles; therefore, the knowledge of these values would be helpful in studying the crazing phenomenon in rubber modified polymers.
For this reason the gel phase of a rubber modified polystyrene was separated by high speed centrifugation and shear modulus determinations made by means of a torsion pendulum over a wide temperature range.
SynopsisDynamic mechanical properties aa determined by a torsion pendulum were used to study the effect on the crystalline structure of polypropylene of annealing a t a temperature very near to its melting point. Crystallinity values aa determined by both x-ray techniques and density measurements were reported for the specimens used in this study. Differential thermal analysis waa used to show the differences in melting b e havior of annealed and unannealed polypropylene. It waa found that, although crystallinity may increase slightly, the major effect of annealing polypropylene is an increase in the size and/or perfection of the resulting crystallitea. A damping peak not normally found for polypropylene waa present a t -50°C. in the damping curves for those specimens annealed in silicone oil without protective wrappings. This peak waa found to be due to the freezing of the absorbed silicone oil, present in the specimen as small droplets.
In the last several years, studies of the dynamic behavior of polymers have shown that many, if not all, polymers exhibit more than one transition region when the investigation covers a sufficiently wide temperature or frequency range. The purpose of this paper is to discuss, for a few selected polymers, those multiple transitions which have been found by using dynamic mechanical methods, with particular emphasis on those exhibited by certain olefinic polymers and copolymers. In addition to work published in the literature, this paper also covers some unpublished results based on measurements made in our laboratory. Although attempts have been made to relate secondary transitions with such types of motion as the rotation and/or oscillation of short side chains, subgroups, and short segments of the main polymer chain, there are few polymers for which secondary transitions have been definitely assigned to a particular motion of a specific group. For the most part, the results to be discussed in this paper are based on measurements made with a torsion pendulum, vibrating reed, or transverse vibration apparatus.
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