Study On The Relationship Between Polymer Molecular Chain Response And Crystallization Under Flow Field

For almost all kinds of polymer processing methods,polymer melts undergo complex flow field which dramatically influences the crystallization behavior such as crystallinity and crystalline morphology,thus the physical properties of final products.Therefore,flow-induced crystallization(FIC)of semicrystalline polymers is of vital importance for industry and always a fascinating subject in the realm of polymer physics for decades.Although a lot of mechanisms have been proposed to describe the experimental phenomena and predict the experimental results of FIC,most of them are obtained based on the assumption that polymer melts can flow homogeneously during FIC experiments.In fact,under common processing conditions,polymer melts always show nonlinear rheological behavior which would lead to the inhomogeneous response of molecular chains to the flow.Even for the homogeneous flow field,the chain deformation cannot be consistent perfectly with the macroscopic flow field.Therefore,attentions must be payed to the response of molecular chains to the macroscopic flow field in the research of FIC.In this thesis the correlation between the nuclei morphologies and the real chain deformation under different draw ratio has been studied by using extension rheometer and small angle neutron scattering technique.Meanwhile,the influence of the shear inhomogeneity on the rheological and crystalline behavior has also been studied based on a new device named fast cooling shearing device.The results are summarized as follows:(1)By combining extensional rheological and in situ small-angle neutron and synchrotron X-ray scattering(SANS,SR-SAXS)techniques,the correlation between chain deformation and morphology of nucleus in a lightly cross-linked deuterated PE(D-PE)/hydrogenated PE(H-PE)blend has been studied at different draw ratios.The deformation of molecular chain obtained by SANS shows that shish forms at a rather small chain deformation of about 1.3,which does not support either coil-stretch transition or the simple stretch network model.Combining these data with the SAXS results,we speculate that the conformation ordering couples with density,instead of single chain conformation,is the key factor for shish formation.Meanwhile,due to the different alignment of the center of mass of the stretched network in samples with different draw ratios,a periodic concentration modulation of D-PE appears after crystallization of the samples stretched to the hardening zone,which,however,does not occur under the small draw ratio.(2)In order to simultaneously detect the rheology data and the crystalline data,a fast cooling shearing device associated with the synchrotron radiation X-ray scattering technique has been developed.The response of molecular chains to the flow field can be reflected by the rheological behavior while the structure evolution during the crystallization stage can be characterized by the X-ray scattering method.By combining these data,the influence of shear inhomogeneity on the rheological and crystalline behavior can be studied.(3)The influence of shear homogeneity on rheology and FIC behavior of polymer is investigated by contrasting the sudden startup shear with the slow shear rate ramp-up procedure.Results obtained sufficiently illustrate that the FIC is dependent on the rate ramp-up time.It is not the slower the rate ramp-up procedure,the better the flow-induced crystallization.Actually,there exists an optimal rate ramp-up time for a fixed shear rate.In addition,the optimal rate ramp-up time is not the same for different shear rate.These results suggest that rate ramp-up time should be considered as a critical parameter for FIC.(4)The rheological and crystallization behavior of sample sheared under various shear rate and ramp-up time have been characterized by using the fast cooling shear device combining with the synchrotron radiation X-ray wide angle scattering(SR-WAXS).The results show that different ramp-up time would lead to the different disentanglement mode of the molecular chain network under shear flow which will cause the dramatic changes in crystallization behavior.