Flow-induced crystallization of a multiblock copolymer under large amplitude oscillatory shear: Experiments and modeling

Following a previous work investigating the flow-induced crystallization (FIC) of polybutylene terephthalate/polytetrahydrofuran (PBT/PTHF) multiblock copolymers under steady shear, we propose here to deal with the case of large amplitude oscillatory shear (LAOS). For this purpose, we focus on a single copolymer ( M w ¯ = 50 kg mo l − 1) made, in average, of a sequence of nine soft and eight hard segments. We show unambiguously that LAOS accelerates the polymer crystallization when increasing (i) the frequency from 0.5 up to 50 rad s−1 (at a constant strain amplitude of 100%) or (ii) the strain amplitude from 10 to 300% (at a constant frequency of 2.5 rad s−1). Based on this data, we demonstrate that high oscillatory shear rates have similar effects as the steady shear rate regarding the gelation time, i.e., that frequency- and strain amplitude-related effects are secondary. We carefully analyze the stress response through Fourier-transform decomposition that emphasizes the rich rheological behavior of our material during its liquid-to-solid phase transition. With the help of x-ray scattering experiments (ex situ SAXS and WAXS), we then propose a global scenario rationalizing the whole set of rheological observations based on the copolymer structure. In parallel, we propose to use a recent model that we developed to simulate the stress response in the case of steady shear-promoted FIC and adapt it to the case of LAOS. Remarkably, our model, which is based on modified Doi–Edwards equations only, provides good qualitative agreement with the data when varying the strain amplitude or the frequency. Furthermore, it is found to predict quantitatively the gelation time of the system.