Homogeneous Nucleation in Polyethylene: Molecular Weight Dependence

Area of Science:

• Polymer Science
• Materials Science
• Physical Chemistry

Background:

• Understanding polymer crystallization is crucial for material properties.
• Homogeneous nucleation is a fundamental process in polymer solidification.
• Previous studies have explored nucleation in various polymers, but gaps remain for linear polyethylene fractions.

Purpose of the Study:

• To determine the isothermal rate of homogeneous crystal nucleation in supercooled linear polyethylene melts.
• To estimate the lateral and end-surface free energies (σ and σ_e) of polymer crystals as a function of molecular weight.
• To compare experimental nucleation data with current theories and investigate the influence of molecular weight and temperature on nucleation parameters.

Main Methods:

• Utilized the droplet technique to create highly supercooled melts of eight linear polyethylene fractions.
• Analyzed experimental data using established theories of homogeneous nucleation for chain-folded crystals.
• Estimated the quantity σ²σ_e, a product of surface free energies, across a range of molecular weights.

Main Results:

• An anomalous nucleation behavior was observed for sample 3.2 K, suggesting potential crystallization in an extended chain form.
• For samples 9.70 K to 23.0 K, σ²σ_e increased with molecular weight due to decreasing cilia per chain fold.
• At higher molecular weights (23.0 K to 249 K), σ²σ_e stabilized around 19,000 ergs³/cm⁶.
• Experimental nucleation frequencies (I₀) deviated significantly from theoretical values, but this discrepancy reduced when assuming temperature-dependent surface free energies.

Conclusions:

• The study provides valuable estimates of surface free energies in linear polyethylene crystallization.
• Molecular weight significantly impacts nucleation behavior, particularly at lower ranges, due to chain folding dynamics.
• The findings contribute to a refined understanding of homogeneous nucleation theories in polymer melts.
• Temperature dependence of surface free energies plays a role in reconciling experimental and theoretical nucleation rates.