Glassy Polymers and the Role of Cross-links

Highly drawn crystalline fibres produce abundant radicals at strains exceeding about 8% and rising to the breaking strain of, say, 15 to 20%. Such fibres of course have already undergone draw ratios of 500% or more from the isotropic condition. 

No detectable signals can be obtained in the brittle fracture of isotropic polymers (even crystalline ones), but once plastic flow is induced (eg by small amounts of orientation above Tg before testing below Tg), radicals are readQy obtained during deformation. 

The radical concentration is strongly dependent on the presence of molecular anchorages, notably cross-links. Mead studied cross-linked polybutadienes deformed plastically below Tg. Pre-strains of different magnitudes were applied above 

Tg before testing, but comment here is limited to pre-strains below 250% where no strain induced crystallization occurs. AWiou weak signals ( 0 to 8 x lO spins/g) were recorded in uncross linked material beyond yield, this radical concentration was two orders of magnitude smaller than that obtained on even lightly crosslinked specimens. F re 17 shows the effect of cross4ink density for specimens pre-extended by 200% on the radical concentration after a further 200% test strain and at various temperatures of test. The effects of temperature and strain will be discussed later. 

At lower temperatures (ie. more than about 40 °C below Tg) a further upturn in radical formation is observed as the cross-link density rises above 10 moles/cm. This may reflect the higher stresses required to induce flow in the glassy material at 

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