Entanglement networks of 1,2-polybutadiene crosslinked in states of strain. VII. Stress-birefringence relations |
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Authors: | Rick L Carpenter John D Ferry |
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Abstract: | Crosslinks are introduced by γ irradiation into 1,2-polybutadiene while strained in uniaxial extension near Tg with stretch ratio λ0, thereby trapping a proportion of the entanglements originally present. The stress at any subsequent strain λ is accurately given by the sum σN + σx, where σN is the stress contributed by a trapped entanglement network with λ = 1 as reference and a Mooney–Rivlin stress-strain relation, and σx is that contributed by a crosslink network with λ = λ0 as reference and neo-Hookean stress-strain relation. The birefringence is accurately given as δn = ?NσN + ?xσx, where the ?'s are the respective stress-optical coefficients. From measurements at λ = λ0 where σx = 0, ?N can be determined separately. For polymer with 88% 1,2 microstructure, ?N and ?x are nearly equal and independent of irradiation dose, though strongly dependent on temperature. For polymer with (95–96)% 1,2, ?N and ?x are different (even opposite in sign) and dependent on dose. This behavior is associated with a side reaction of cyclization by the γ irradiation, which is inhibited by the 1,4 moiety in the polymer with lesser 1,2 content. It is responsible for residual birefringence in the state of ease (λ = λs) where σN = –σx and the stress is zero. |
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