Fracture of model polyurethane elastomeric networks
Paul Steeman2011
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Abstract
Fracture properties of model elastomeric networks of polyurethane have been investigated with a double-edge notch geometry. The networks were synthesized from monodisperse end-functionalized polypropylene glycol precursors and a trifunctional isocyanate. All reagents were carefully purified and nearly defect-free ideal networks were prepared at a stoichiometry very close to the theoretical one. Three networks were prepared: an unentangled network of short chains (M n ¼ 4 kg mol À1 ), an entangled network of longer chains (M n ¼ 8 kg mol À1 ) and a bimodal network with 8 kg mol À1 and 1 kg mol À1 chains. The presence of entanglements was found to increase significantly the toughness of the rubber, in particular at room temperature, relative to the bimodal networks and to the short chains network.
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Comparison Between Crosslink Densities Derived from Stress–Strain Data and Theoretically Data Evaluated Through the a-Model Approach for a Polyurethane Network System Based on Hydroxyl Terminated Polybutadiene and Isophorone-Diisocyanate
The crosslink density (CLD) for polyurethane elastomeric networks based on hydroxyl terminated polybutadiene and isophorone-diisocyanate was theoretically calculated with a-model equations the employing the functionality distribution and extent of reaction as input parameters. The theoretical crosslink density (m t ) was compared with the CLD values computed from stress-strain data evaluated at various strain rates. The methods for the calculation of the CLD from stressstrain data were based on the Mooney-Rivlin and Young's modulus approaches. Theoretical stress-strain curves were generated on the basis of m t conforming to both phantom and affine model calculations. The experimental stressstrain plots aligned more closely to the affine model line. The deviation of the experimentally derived stress-strain curves from the theoretical affine curve was probably due to the presence of temporarily trapped physical entanglements. From the stress-strain data, the concentrations of true chemical crosslinks and physical entanglements were estimated individually. V C 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 920-925, 2010
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