Polyester-polyurethanes having different chemical structure were synthesized and their biodegradabilities were investigated. Average molecular weight of the synthesized polyurethanes ranged from 79,106 to 161,715($M_w$) by gel permeation chromatography. Hydrolytic and enzymatic degradation decreased with the increase of the diol carbon chains in polyol, and increased by substituting aromatic diisocyanate with aliphatic diisocyanate. It is considered that hydrophobicity and hard segment formation seem to resist the hydrolytic and enzymatic degradation of polyurethanes. Synthesized polyurethanes were biodegradable under composting condition to certain extent depending on their chemical structures. As the hard segment content was increased, biodegradation rate decreased. Biodegradation rate of polyurethanes increased with the following order of diisocyanate used : MDI ＜ $H_{12}MDI$ ＜ HDI. Polyurethane composed of aliphatic diisocyanate showed higher biodegradation rate than polyurethane composed of aromatic diisocyanate. These facts indicates that presence and content of hard segment in polyurethane affect the biodegradability under composting condition. As the diol carbon chains of polyol increased, biodegradation rate under composting condition increased. When the polyol used is poly(hexamethylene adipate)diol or poly(caprolactone)diol, polyurethane showed maximum biodegradation rate under composting condition. Surface hydrophobicity, which is related to good adhesion of bacteria on the polymer surface, is considered to be a factor on biodegradation rate under composting condition.
Immersion precipitation method is a general technique in preparation of polyurethane membrane and coated fabric. A thin film of a homogeneous polymer solution is contacted with a nonsolvent. Exchange of solvent and nonsolvent across the interface introduces phase separation of polymer film into polymer-rich phase and polymer-lean phase. The former results in a rigid, structural form of the membranes, while the latter gives a porous substructure. Thus, phase separation process determines the membrane morphology. To investigate the phase separation process, we need to know both thermodynamics and kinetics of phase separation. Firstly, knowledge of thermodynamics such as phase diagrams(binodals, spinodals, and critical compositions) enables one to change the conditions for the preparation of membranes such as compositions of the casting solution and of the coagulation bath to obtain optimal membrane structure. Kinetics of phase separation can be taken into account by solving ternary diffusion equations for the system of polymer/solvent/water. In this work, we measured the interaction parameters and calculated binodal and spinodal curves based on the thermodynamics of polymer solutions. The Flory-Huggins lattice treatment which was extended with a concentration-dependent interaction parameter, was used to describe the thermodynamics of the ternary system. We also formulated ternary diffusion equations relating chemical potential gradients in the polymer film with frictional coefficients between components, and obtained the composition paths in the phase diagrams through calculations of diffusion equations. In the present calculation, instead of equilibrium interfacial boundary conditions, we used a new boundary conditions including mass transfer coefficients. By doing this, we could demonstrate the possible spinodal decomposition mechanism in case of instantaneous demixing.
Based on the basic information about both thermodynamics and kinetics discussed, we investigated the morphology of polyurethane membrane, and proposed the phase separation mechanism involved in morphology formation by immersion precipitation. We observed the macrovoid (finger-like structure) and the cellular structure (sponge-like structure) in polyurethane membranes prepared via phase inversion method. We suggest that macrovoid is resulted from the spinodal decomposition mechanism, and cellular structure is resulted from the secondary phase separation followed by spinodal decomposition.
