| The performances of crystallized polymer products are strongly dependent on the crystal morphology and structure which are formed during processing.It is of great significance to control the performances of polymer products by studying the variation law of the crystal morphology and structure during processing.It is known that polymer melts,under the influence of oscillatory shear field during the whole crystallization process,exhibit rich ordered structures,different growth processes of crystals and morphology,when compared with the steady sheared melts.In this thesis,the influence of shear conditions on the crystallization of polylactic acid(PLA)with different melt structure is studied.Firstly,we studied the rheological properties of PLA and we got the zero shear viscosity and relaxation time of PLA at different temperature and its non-Newtonian index and the viscous flow activation energy.According to the investigation of the storage modulus variation of PLA melt during and after different large amplitude oscillation shear,We studied the effect of shear conditions(temperature,strain,time,frequency)on the melt state,we found that:(1)the linear viscoelasticity interval of PLA increases with rising of temperature;(2)the storage modulus decreases with increasing of shear time and finally reached the equilibrium state;(3)the storage modulus has a smaller variation at higher temperature and with a larger recovery speed;(4)the variation of storage modulus increases with increasing of shear strain;(5)the variation rate of storage modulus increase with the increase of frequency but with little influence on the final level of storage modulus of PLA melt.We studied the crystallization behavior of PLA melt with different large amplitude oscillation shear by the characterization of the variation of relative storage modulus and we found that:(1)the influence of oscillation shear on crystallization behavior of PLA decrease with increasing of shear temperature;(2)only when the oscillation shear strain is larger than the critical strain,can the oscillation shear have a positive influence on the crystallization behavior of PLA;(3)the influence of oscillation shear on the crystallization behavior of PLA increase with the increase of oscillation shear time;(4)within a certain cycles of oscillation shear,the frequency has a positive effect on the crystallization behavior of PLA;(5)the influence on crystallization behavior of PLA decreases with increasing of relaxation time. |
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