Chain length effects on the thermodynamic properties of n-alkane crystals

Molecular simulations are used to elucidate the effects of chain length on the unit cell dimensions, thermal expansion coefficients, heat capacities, and melting temperatures of n-alkane crystals. As the chain length increases, chain ends become less significant because they represent a smaller fraction of the chain, and the properties of the n-alkane crystals approach the properties of perfect polyelhylene crystals. Energetic effects, associated with poorer packing at the chain ends, lead to a decrease in the density of shorter chain crystals. These energetic effects are compounded by entropic effects, associated with increased vibrational motion at chain ends, which lead to increased thermal expansion of shorter chain crystals. The heat capacity at constant volume is found to increase (on a per carbon basis) as chain length decreases, because shorter chains have more interchain modes in comparison to longer chains. Shorter chain crystals are also found to undergo instabilities related to melting at lower temperatures. The results presented here are expected to be qualitatively similar to the effects of lamellar thickness in semicrystalline polymers.