Homogeneous Nucleation in Polyethylene: Molecular Weight Dependence.

The droplet technique was used to obtain estimates of the isothermal rate of homogeneous crystal nucleation in highly supercooled melts of 8 characterized fractions of linear polyethylene (weight average molecular weights from 3,100 to 249,000). The data obtained from these experiments were analyzed in accord with current theories of homogeneous nucleation of chain folded crystals. Values for the quantity σσ , where and σ are the lateral and end-surface free energies of the crystal, were estimated as a function of molecular weight. Sample 3.2 K was found to be anomalous in its nucleation behavior. When we assume that this sample crystallizes in the extended chain form and calculate σ instead of the value for σ is found to be 10.57 ergs/cm which is in reasonable agreement with the value 9.6 ergs/cm found by other investigators for linear hydrocarbons. However, there remains the question as to whether sample 3.2 K ever underwent homogeneous nucleation. For samples 9.70, 11.74, and 23.0 K, σσ was found to increase rapidly due to a decrease in the number of cilia per chain fold as the molecular weight increases. For higher molecular weights the value for σσ levels off and the average value of σσ for samples 23.0 to 249 K was found to be 19,000 ergs/cm. The experimental value of the absolute nucleation frequency , was found to differ from the theoretical value by approximately 1 × 10. If one assumes that the surface free energies are temperature dependent [i.e., and where and ] the average value of σσ > changes only slightly (to 19,800 ergs/cm) due to the compensating effects in the signs of the temperature corrections and is close to the theoretical value, 1 × 10 nuclei/cm/s.