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Abstract:
Linear and Fourier-transform rheology were used to study the influence of the oscillatory shear amplitude, γ0, on the isothermal crystallization at 140 °C of three commercial isotactic polypropylenes. The development of the crystallization was monitored through the time dependence of the dynamic storage modulus, G'(t), and the normalized intensity of the third harmonic of the stress waveform, I3(t), a quantification of the degree of nonlinearity under oscillatory shear conditions. A change in the exponent, n, of the power law describing growth, according to G'norm(t) similar to tn was observed at t similar to 4 h. For t < 4 h, n was between 3 and 4. This change in growth kinetics was also accompanied by a maximum in I3(t). Below γ0 = 0.05, these results were independent of the applied strain amplitude. Above γ0 = 0.05, failure of the polypropylene in the parallel plate geometry due to stress build-up was often observed in the late stages of crystallization. This was accompanied by a sharp decrease of G'(t), and a simultaneous sharp increase of I3(t). Additionally, the presence of even harmonies in the spectrum was observed after failure. Notably, a plateau of I3(t) at least 1 h before actual failure indicating a greater sensitivity to its onset than that of G'(t).
