| For polymeric materials,pressure is not only the fundamental parameter to describe the thermodynamic state,but also the key factor in practical processing,such as injection molding,compression molding,extrusion and high pressure foaming processes.For semicrystalline polymers,pressurization could lead to an increase in equilibrium melting temperature and thus obtain the extra undercooling for the crystallization.Recently,studies have shown that rapid pressurization can induce fast solidification of polymer melts and form metastable structures,which seem to be a new way to prepare materials.However,so far,there have been few reports on the quantitative study toward the effect of controllable pressurization on the crystallization behavior of polymers,which will address basic science issues,as will as providing new methods for material structure design.Isotactic polypropylene(iPP)is a semicrystalline polymer,the equilibrium melting temperature of iPP increases with increasing of the pressure.According to the correspondence relationship between pressure and temperature,pressurization can also induce solidification and crystallization of iPP melt.Therefore,in this paper,we studied the effect of pressure and pressurization rate on iPP crystallization.Furthermore,the iPP mesophase was prepared,and the pressure-pressurization rate-phase structure diagram of iPP was established for the first time.The main contents are as following:1.Effect of controllable pressurization on solidification behavior of iPP melts(1)The effect of both pressure(0.75-2 GPa)and pressurization rate on the crystallization of iPP melts was investigated at 200 oC.It was found that when the pressure is less than or equal to 0.75 GPa,?-iPP is obtained regardless of the pressurization rate.When the pressure is 0.75-1.25 GPa,the mesophase and ? phase coexist with larger pressurization rate,and when the pressure is higher than or equal to 1.25 GPa,the mesophase is formed when the pressurization rate is greater than a certain critical value.Based on the influence of pressure and pressurization rate on iPP melt crystallization,the pressure-pressurization rate kinetic phase diagrams of mesophase and ? phase were plotted.(2)The structural analysis of the iPP mesophase obtained by pressure-induction was carried out.In addition,compared with the quenched iPP mesophase,the WAXD results showed a negligible difference of 32% between the two phases.The SAXS results showed that the pressure-induced mesophase was also in "nodular" morphology(7.66 nm),which has a more regular size of smaller than quenched(8.37 nm).(3)The pressure induced mesophase was annealed under different pressures at 200 oC,and it was found that the phase transition from mesophase to the g phase occurred.Such results illustrated that the mesophase is a kinetically stable phase,and during the annealing process,it can change to a thermodynamically stable phase(? phase).2.Effect of controllable pressurization on solidification behavior of CNTs/iPP nanocompositesThe CNTs is an excellent rigid filler,which can promote polymer crystallization as an effective nucleating agent.In this paper,we studied the effect of pressure and pressurization rate on nanocomposites crystallization.It was found that compared with pure iPP,the CNTs/iPP nanocomposites mesophase required higher pressures and pressurization rates to prepare.The SAXS was also used to quantitatively analyze the lamellar structure,and results indicated that under the same pressurizing pressure,the long period of pure iPP was decrease with the decrease of the pressurization rate,while the trend was reversed for CNTs/iPP nanocomposites. |
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