NMR and dynamic mechanical studies of drawn polypropylene fibers

Abstract

The influence of drawing on the relaxation processes connected with the glass transition in polypropylene fibers have been studied in terms of the spin—lattice relaxation times T1 and the storage and loss components of the dynamic modulus measured at different temperatures. The measurements were carried out at the frequencies f0 = 30 MHz and fE = 110 Hz, respectively. The experimental results were interpreted using the Williams—Landel—Ferry (WLF) theory. Glass transition tempera-tures Tg and correlation times τg corresponding to this temperature were calculated using conditions for the minimum and maximum of the temperature dependences of the relaxation times T1 and loss modulus E′′. Two sets of the polypropylene fibers drawn within the range of draw ratios 1—5 were investigated. One of them was prepared from polypropylene containing 8 wt% of the polymer modifier, which gives rise to the plastification of polymer, and the other was made from unmodified polypropylene. Under drawing, the former set of fibers was found to give a shift of the Tg towards the lower temperatures whereas the opposite effect appeared in the measurements of the latter set. The values of an isothermal apparent activation energy Ea estimated on the basis of WLF theory lead to the same conclusions as the activation energy values obtained from the Arrhenius relation. Parameters a and b of the modified Fuoss—Kirkwood distribution were computed from the temperature dependences of the spin—lattice relaxation times T1 and loss modulus E′′ using a fitting procedure.