Samples of isotactic polypropylene (PP) were zone‐solidified in temperature gradients up to 300°C/cm at growth rates down to 3 μm/min. Oriented α‐type spherulites were obtained only by nucleation. While β nucleation is extremely rare, the β phase is easily initiated by growth transformations along the oriented α front. Since the β phase was found to grow considerably faster than the α phase, the α‐to‐β transformation points diverge across the sample, interrupting growth of the oriented α fibrils. This causes subsequent nucleation to yield teardrop‐shaped α spherulites. Differential scanning calorimetry (DSC) studies of zone‐solidified PP show the β‐phase to be favored by slow growth rates, high temperature gradients, and large degrees of superheat in the melt—all of which tend to suppress nucleation. Differential thermograms of largely β‐PP obtained at a heating rate of 1°C/min show the actual melting and recrystallization of the β spherulites into the α form.

Tensile stress-strain curves for polycrystalline potassium were obtained at liquid-nitrogen temperature (T=0.23Tm) and up to a pressure of 5.15 kbar. Pressure shifts the stress-strain curve upward. The yield stress versus pressure was measured. Theoretical dislocation models for the effect of pressure on the yield stress are reviewed herein. It is shown that the effect of pressure on the yield stress owes its origin to the effect of pressure on the effective elastic constant only. The present experimental results for the yield stress σ0 as a function of pressure P are in excellent agreement with the equation σ0=σ00[1+(Ce′0/C0) P], where σ00 is the yield stress at zero pressure, Ce0 is the value of the elastic constant Ce=G/(1−ν) at zero pressure, and Ce′0 is the value of the pressure derivative of the elastic constant Ce at zero pressure. Here G is the bulk shear modulus and ν is Poisson’s ratio. If Ce′0/C0 is replaced by E′0/E0 (where E is Young’s modulus) or G′0/G0 the agreement is also within experimental error. In this study, the yield stress was found to increase by 42% at 5.15 kbar.

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