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 σ²σ , where σ 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 σ³ instead of σ ² σ the value for σ is found to be 10.57 ergs/cm² which is in reasonable agreement with the value 9.6 ergs/cm² 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, σ²σ 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 σ²σ levels off and the average value of σ²σ for samples 23.0 to 249 K was found to be 19,000 ergs³/cm⁶. The experimental value of the absolute nucleation frequency I ₀, was found to differ from the theoretical value by approximately 1 × 10¹². If one assumes that the surface free energies are temperature dependent [i.e., $$ and $$ where $$ and $$ ] the average value of σ²σ > changes only slightly (to 19,800 ergs³/cm⁶) due to the compensating effects in the signs of the temperature corrections and I ₀ is close to the theoretical value, 1 × 10³⁴ nuclei/cm³/s.