Weiqing Zhou

Extensional rheometer for in situ x-ray scattering study on flow-induced crystallization of polymer

We designed and constructed an extensional rheometer for in situ small and wide angle x-ray study on flow-induced crystallization of polymer. Two rotating drums with an axis distance of 20 mm are employed to impose extensional deformation on the samples. With a constant angular velocity, the two drums generate a constant Henkcy strain rate as sample length for testing keeps constant during deformation. An ionic liquid is used as heating medium to prevent polymer melt from bending downward due to gravity, which is excellent in terms of high thermal stability, low viscosity, and relative low adsorption on x-ray. Flow-induced crystallization experiments are conducted with this apparatus on x-ray scattering station in Shanghai Synchrotron Radiation Facility (SSRF), which allows us to collect rheological and structural data simultaneously and may lead to a better understanding on flow-induced crystallization of polymer.

Relaxation propelled long period change in the extension induced crystallization of polyethylene oxide

The time evolution of the long period in the extension-induced crystallization of polyethylene oxide (PEO) is investigated with a combination of rheological measurement and in situ small angle X-ray scattering (SAXS), aiming to show the dynamic change in the spatial arrangement of nuclei in terms of both the chain stretching and orientation. The main findings are summarized as follows. (i) Strain hardening is observed when the strain is larger than 3.0, indicating a transition from chain orientation to chain stretching. (ii) The time evolution of the long period differs before and after strain hardening, reflecting changes in the evolution of nuclei arrangement. The full strain range is thus divided into two regions, namely the large strain region and the small strain region. In the large strain region the long period exhibits a two-step monotonic decrease. For the small strain region, an increase-plateau-decrease behavior is observed. (iii) A large gap exists between the initial values of the long period in each of the two strain regions. (iv) The crystallization kinetics shows no secondary acceleration after strain hardening. Based on these results, a qualitative model emphasizing the chain relaxation propelled rearrangement of nuclei is proposed to explain the structural evolution.

How flow affects crystallization in a heterogeneous polyethylene oxide melt

Extension-induced crystallization in a heterogeneous polyethylene oxide (PEO) melt is investigated by small angle X-ray scattering (SAXS) and extensional rheology. The crystalline complex of PEO and sodium bromide is the heterogeneous component, which simulates the roles of crystal or strongly interacted additives during flow. It is aimed to demonstrate how the altered chain relaxation by the heterogeneous particle affects the crystallization. The main findings are listed as follows: (i) strain hardening occurs in heterogeneous melts but not in pure PEO. This indicates that the crystalline complex strengthens the entanglement network of free chains, leading to slower chain relaxation. (ii) In morphology, the orientation of lamellae becomes easier with the existence of the crystalline complex. A jump in the long period is also observed with large strain, accompanied by changes in the evolutional trend at the early stage. (iii) The crystallization kinetics converge with increasing strain; even the stress response significantly differs in different samples. Based on these findings, a quasi-network consisting of the crystalline complex and the entangled free chain is anticipated. Nucleation induced by the stretch of the quasi-network is supposed to lead to the change in crystallization observed.

The influence of inertia and elastic retraction on flow-induced crystallization of isotactic polypropylene

The influence of nonquiescent relaxation of isotactic polypropylene on flow-induced crystallization (FIC) is investigated by a combination of particle tracking velocimeter (PTV) with a cone-plate shearing geometry and synchrotron radiation microbeam wide-angle X-ray diffraction (SR-μWAXD), which is aimed to correlate real flow profile and distribution of crystal orientation. With PTV technique, we observed that flow remains homogeneous during shear, while postshear movement and delayed fracture take place after a step strain when large shear rates and strains were imposed. Delayed fracture slices samples into several layers which move either forward or backward after the cessation of shear imposed externally. SR-μWAXD measurements reveal that the layers moving forward keep high crystal orientations while the layers moving backward show low orientations, which gives an inhomogeneous distribution of crystal orientation across the thickness of sheared samples. The correlation between moving direction and cry...