Kinetics of isothermal crystallization in isotactic polypropylene monitored with rheology and Fourier-transform rheology

Linear and Fourier-transform rheology were used to study the influence of the oscillatory shear amplitude, γ 0 , 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, I 3 ( 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 ) ∼ t n , was observed at t ∼ 4 h. For t < 4 h, n was between 3 and 4. This change in growth kinetics was also accompanied by a maximum in I 3 ( t ). Below γ 0 = 0.05, these results were independent of the applied strain amplitude. Above γ 0 = 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 I 3 ( t ). Additionally, the presence of even harmonics in the spectrum was observed after failure. Notably, a plateau of I 3 ( t ) at least 1 h before actual failure indicating a greater sensitivity to its onset than that of G ′( t ).