Influence of surface morphology and chemistry on the enzyme catalyzed biodegradation of polycarbonate-urethanes

Polycarbonate based polyurethanes were synthesized with varying hard segment content as well as hard segment chemistry based on three different diisocyanates,1,6-hexane diisocyanate (HDI), 4.4'-methylene bisphenyl diisocyanate (MDI) and 4,4-methylene biscyclohexyl diisocyanate (HMDI). The surface chemistry and morphology were characterized using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The polymers were incubated with cholesterol esterase (CE) in a phosphate buffer solution at 37 degrees C over 10 weeks. XPS results showed that the surface chemistry changed as the size and chemistry of the hard segment varied within the materials. AFM images exhibited distinctive surface morphologies for all polymers, and this was particularly apparent with changes in the hard segment chemistry. The results showed that the surface of HDI polymers consisted of relatively stiff rod-like structures, which corresponded to the soft segment domains. Polymers with a higher HDI content exhibited a dense top layer containing a relatively higher hard segment component, covering the sub-surface matrix of rod like structures. The MDI based polyurethane had large aggregates on its top surface, which corresponded to the aggregation of harder components. The HMDI based polycarbonate-urethane presented a relatively homogeneous surface where no phase separation could be detected. The relative differences in hard and soft segment content in their surface structure was supported by XPS findings. The analysis of the biodegradation results, concluded that enzyme catalyzed biodegradation within these materials was initiated in amorphous soft segment regions located in the region of the interface between hard and soft segments. A higher hard segment content at the surface contributed significantly to an increase in biostability. The findings provided an enhanced understanding for the role of surface molecular structure in the enzyme catalyzed biodegradation of polyurethanes.