Structure‐property relationships in polycaprolactone‐polyurethanes

Abstract

The structure‐property relationships of polycaprolactone‐based segmented polyurethanes were studied using differential scanning calorimetry (DSC), small‐angle x‐ray scattering (SAXS), wide‐angle x‐ray diffraction (WAXD), dynamic mechanical, and stress‐strain testing. The materials studied varied in hard‐segment type [4,4′‐diphenylmethane diisocyanate/butanediol (MDI/BD) or 4,4′‐dicyclohexyl methane diisocyanate/butanediol (H 12 MDI/BD)], soft‐segment molecular weight (830 or 2000 MW polycaprolactone), hard‐segment content (23–77% by weight), and thermal history. The materials with aromatic (MDI/BD) hard segments had semicrystalline hard‐segment domains, while the materials with aliphatic (H 12 MDI/BD) hard segment had mostly amorphous domains. Materials with the shorter polycaprolactone soft segment (830 MW) exhibited thermal and mechanical behavior which indicated a considerable degree of hard‐ and soft‐segment compatibility. The materials which contained a 2000‐MW polycaprolactone soft segment exhibited better‐defined microphase separation. SAXS was used to characterize the microphase structure of each system. The effects of hard‐segment content and soft‐segment molecular weight were similar for the aromatic (MDI) and aliphatic (H 12 MDI) hard‐segment‐based block copolymers. Changing the hard segment from aromatic to aliphatic gave materials with larger interfacial area and slightly higher tensile strength. A range of morphologies between isolated hard domains in a rubbery matrix and isolated rubbery domains in a hard matrix was observed.