Crystallization of Polyethylene Brushes and Its Effect on Interactions with Water

The crystallization process of brushes of polyethylene chains is investigated using all-atom molecular dynamics simulations at degrees of polymerization of 40 and 120. Through temperature reduction in nanosecond time scales, spontaneous crystallization is observed for polymer chains that are grafted on gold crystals and are in contact with vacuum on the other side. Crystalline nuclei form in the middle region of the brushes separated by nanometers from the gold and vacuum interfaces. The crystallization actually begins when the density of carbon atoms exceeds a threshold value. Increases in the grafting density and/or the degree of polymerization result in an increase in the density of carbon atoms in the middle region of the brush and thus leads to an elevation in the crystallization temperature. The free energy of inserting a water molecule into the brush is also analyzed. It is found to be more unfavorable in the crystalline brush than in the fluid state. This is due to the high density of the crystalline state, and in locally disordered regions in the crystalline brush, the insertion free energy is comparable to that in the fluid.