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Potential importance of purity/non-ether contaminants
Release time:
2023-09-26
During the synthesis of polycarbonate diols from vinyl carbonate at elevated temperatures, the formation of oxyethylene groups may occur: these ether bonds are present in the polycarbonate urethanes as "impurities" and can affect physical properties and thermoplastic processing. When polycarbonate diols with these oxyethylene groups are incorporated into polyurethanes, it is expected that there will be a reduction in hardness, a difference in modulus at the same level of hardness, and possibly a reduction in biostability (oxidative stability).
General Properties of Polymers with a Single Soft Chain Segment Chemistry
General trends exist between the different classes of polyurethane copolymers. The contributions of the hard and soft chain segments can be easily adjusted by compositional changes during synthesis and usually vary monotonically until a transition point is encountered. In polymers with only one soft and one hard chain segment chemistry, composition affects many of the overall properties including modulus, hardness, toughness and even permeability, although there is not much control over selectivity or molecular weight cut-off. At high soft segment concentrations, e.g., 60-80 wt% in TPU and ~70-90 wt% in solution PUU, where the soft segments are in a continuous phase, the polymer ranges from a soft, viscous gel to a rubbery elastomer, to a "lively" elastomer (i.e. tensile modulus > natural rubber). As the ratio of soft chain segments decreases to the point where it is no longer the phase with the highest volume fraction, the slope of the plot of a given property versus composition may change. Polymers with intermediate compositions may have co-continuous phases and may be leathery but still flexible in vivo, especially in structures with thin cross sections. Polyurethanes may resemble impact modified plastics when the soft chain segments are reduced to become a smaller dispersion/discontinuity volume fraction, depending on the composition and molecular weight of the soft chain segments. Reducing the concentration of soft chain segments typically increases biostability, tensile strength, indentation hardness, and compression, tensile, and flexural moduli. It decreases ultimate elongation and permeability, the latter due to the glassy or semi-crystalline nature of the hard segments. In harder grades of TPUs with higher hard segment concentrations and molecular weights, compatibility is improved by lowering the molecular weight of the soft segments.
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