Influence of molecular weight on the fracture of poly(methyl methacrylate) (PMMA) |
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Authors: | P Prentice |
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Affiliation: | Dept. of Mechanical Engineering, Imperial College of Science and Technology, London SW7 2BX, UK |
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Abstract: | A model is proposed to explain the dependence of fracture parameters on the molecular weight of glassy polymers. The model assumes that the fracture event occurs in two stages; the first involves the orientation of polymer chain segments between entanglement points and the second, the fracture itself. A value has been calculated, (~0.6J m?2), for the fracture surface energy corresponding to the lower critical molecular weight between entanglements, M=Me. Allowing for the simplifying assumptions made in its derivation, this value is in good agreement with that found experimentally. It is proposed that, after the chain segments between entanglements crossing a plane have been fully extended, two possible mechanisms are involved; chain ‘pull-out’ up to a maximum governed by the time scale of the local fracture event, or chain scission. Using the concept of a reptating chain it is proposed that above M ~2 Me there is a relationship between the fracture energy (γ) and the molecular weight of the form γ∝ ∝M2 up to a critical value of M, above which γ is constant. It has been shown that there is some agreement with experimental relationships determined independently. |
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Keywords: | Reptation fracture energy influence of molecular weight entanglements crazing in glassy polymers |
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