Role of the interactions between carbonate groups on the phase separation and properties of waterborne polyurethane dispersions prepared with copolymers of polycarbonate diol

Abstract

Different aliphatic waterborne polyurethane dispersions (PUDs) were synthesized by using different polyols (Mw: 1000 Da) of randomly copolymerized polycarbonate diols with hexamethylene and pentamethylene (C6–C5), tetramethylene (C6–C4) and trimethylene (C6–C3); these copolymers differed in the length of the methylene groups and the structural regularity due to the combination of even and odd units. Brookfield viscosity, extent of particle crowding and broadening of the particle size distribution of the PUD synthesized with C6–C4 polyol followed a different trend than for the other because of the even number of methylene units in the polyol. The PUDs showed monomodal particle size distribution which was narrower in C6–C4 (i.e. the dispersion with higher structural regularity) and the mean particle size decreased by decreasing the length of the methylene unit of the copolymer. The properties of the polyurethanes were affected by the phase separation between the hard and soft segments, the more regular packing of even methylene units in the copolymer and the crystallized polar segments due to carbonate groups. Thus, the glass transition values of the soft segments in the polyurethanes were similar because of the more regular packing of even methylene units in C6–C4 polyol and the crystallized segments produced by interactions of carbonate groups. PU(C6–C5) and PU(C6–C4) showed similar degree of phase separation, the higher degree of phase separation corresponded to PU(C6–C3). Furthermore, the crystallinity of the polyurethanes increased with decreasing the number of methylene units in the polyol, but PU(C6–C4) was the most crystalline because of the more regularly packed even methylene groups in the polyol chain. The thermal stability of the polyurethanes increased from PU(C6–C5) to PU(C6–C3) because the more net interactions between the carbonate groups in the soft segments. The lower was the number of methylene groups between carbonate units in the copolymer, the higher was the elastic modulus of the polyurethanes. The tensile strength and elongation-at-break values of the polyurethanes increased by increasing the number of methylene groups between carbonate units in the copolymer. Finally, the peel strength was maximal in the joint made with PU(C6–C5) and the shear strength was the highest in the joint made with PU(C6–C3), in agreement with the variation of the viscoelastic and mechanical properties of the polyurethanes.