Homogeneous Nucleation in Polyethylene: Molecular Weight Dependence.

Abstract
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 e , where σ and σ e 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 σ2σ e 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 e 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 e levels off and the average value of σ2σ e for samples 23.0 to 249 K was found to be 19,000 ergs3/cm6. The experimental value of the absolute nucleation frequency I0, 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., σ = σ 1 ( 1 + x ¯ Δ T ) and σ e = σ e 1 ( 1 + y ¯ Δ T ) where x ¯ = - 0.0073 and y ¯ = 0.014 ] the average value of σ2σ e > changes only slightly (to 19,800 ergs3/cm6) due to the compensating effects in the signs of the temperature corrections and I0 is close to the theoretical value, 1 × 1034 nuclei/cm3/s.

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