No abstract

Transitions in polymers have been studied with the use of a differential expansion apparatus modified from a design described by K. L. Floyd. Materials ranging from uncured, soft rubbers to hard, waxy solids have been studied from above room temperature down to liquid air temperatures. Both the first‐order transition temperature, Tm and the second‐order transition temperature, Tg, can be identified. Multiple changes in slope of the expansion curves indicate the possibility of additional second‐order transitions. Expansion curves for several diene rubbers are presented, illustrating typical transition phenomena. Expansion curves for several polyolefin materials are presented which illustrate changes in Tg and in the low temperature expansion coefficient with changes in the structure of the polymer.

A system for the study of polymer structure by differential thermal analysis (DTA) techniques is presented. The need for an inert reference body has been eliminated by the use of a thermal delay within the sample itself. Typical data showing both first‐ and second‐order transition behavior for appropriate polymers are presented. The applicability of the technique to liquids is also shown. Three first‐order transitions have been found in trans‐1,4‐polybutadiene. The insoluble nature of high polymers in one another is again confirmed, even in those systems which disperse well. The latent heat of crystallization of cis‐1,4‐polybutadiene is about 2.1 kcal./mole.

Apparatus of the falling weight type previously developed for tensile testing of rubbers at strain rates up to 14.5 m/sec has been made stiffer and more sensitive to accommodate plastics. Loads up to 225 kg and resolution to about 50 !-,sec have been achieved using a piezoelectric transducer. A modified ring specimen, the "racetrack," is also required to attain high resolution and reduce errors associated with conventional slip in the grip. Transducer voltages are displayed on a 'scope and photographed using well-known procedures. These curves, when calibrated, are stress-strain curves, showing brittle and ductile failures. Elapsed times range from about 0.3 to 2 msec. Tensile fractures usually occur in the middle of the stressed legs, often in both legs simultaneously. Reproducible curves have been obtained from commercial plastics such as polyethylene, rigid vinyls, and polystyrene, from metallic alloys of aluminum and copper, and from elastomers such as tread rubbers.

When any pure material goes through a change in state, its physical properties change greatly. In each phase the physical properties are relatively constant or change slowly enough with temperature that the rate of change of a property such as volume is a constant. This rate of change of volume is the thermal expansion coefficient, (∂V/V)/∂T. The thermal expansion coefficient is almost constant, experimentally, as long as the temperature range over which measurements are made does not include a phase transition. At the transition temperature, abrupt changes in volume are found as illustrated in Figure 1. Polymeric materials often show changes in physical properties not necessarily accompanied by abrupt changes in volume, even though the expansion coefficient does change. Since the expansion coefficient changes, some change in internal structure is suspected, and the name second-order transition (Tg) has been adopted. This kind of change is roughly diagrammed in Figure 2. This latter change at the second-order transition temperature can be found in every known polymer, even though many polymers possess clear, first-order, crystalline transitions as well. Hevea rubber, for example, has a crystalline melting point of 28° C, compared to its Tg about −70°. These data are shown, copied from Boyer and Spencer, as Figure 3.