Swelling behavior of hydrophobic polymers in water/ethanol mixtures

Abstract

Swelling behavior of poly(ethylene- co-vinyl alcohol) (EVAL), polyurethane (PU) and poly(ethylene- co-vinyl acetate) (EVAc) in ethanol/water mixtures was investigated. In the case of EVAL, the total swelling curve rises to a maximum that is above the swelling of either pure constituent. On the other hand, in the PU and EVAc systems, the total swelling is intermediate between the swelling of the two pure components. In the case of PU, the total swelling is close to the ideal system as the water-rich region is approached. However, in the case of EVAc, the deviation from ideal is significantly negative. Theoretical swelling values have been derived from Flory–Huggins thermodynamics by using the equilibrium sorption data of the pure component. The theoretical results show good agreement with the swelling behavior of PU and EVAc in ethanol/water mixtures. However, the agreement between theoretical and experimental swelling values for EVAL is reasonably established when the Flory–Huggins equation was modified by incorporating a ternary interaction parameter χ T. This is probably due to the fact that the water/ethanol mixture produces the water–ethanol complex that has a greater affinity for EVAL. In addition, the experimental results indicate that the shape of hydrophobic polymer swelling curve in the water/ethanol mixture is mainly controlled by the interactions between water and the polymer. Therefore, the correlation among the swelling equilibrium data, the polymer hydrophobicity and the structuring of water around the hydrophobic polymers is discussed. It is concluded that the hydrophobic interactions between the polymer and water are a major factor to influence the polymer swelling due to changes in the structuring of water around the hydrophobic polymers. These results might have important implications for the drug delivery and pervaporation, since these processes are influenced by the polymer swelling to a great degree.