Study On Crystallization, Melting Behaviors, And Mechanical Properties Of Isotactic Polypropylene Nucleated By A Liquid Crystalline Polymer

Isotactic polypropylene?iPP?has a variety of crystal forms,of which ?-form crystal has better impact resistance and higher heat distortion temperature.The most commonly used and efficient method to prepare ?-iPP is adding ?-nucleating agents.However,the common inorganic and low molecular weight organic ? nucleating agents have the disadvantages of poor compatibility,low nucleation efficiency,color,low thermal stability,and toxicity.Based on this,a main-chain liquid crystalline polyester?PBDPS?with side phenyl group was synthesized,and the iPP/PBDPS composites were prepared by melt blending with iPP.The chemical structure of PBDPS was characterized by infrared spectroscopy?FT-IR?,nuclear magnetic resonance?¹HNMR?.Differential scanning calorimetry?DSC?,wide angle X-ray diffraction?WAXD?,polarized optical microscopy?POM?were used to study the influence of PBDPS on the crystallization,melting behaviors,and crystallization kinetics of iPP.The variable temperature WAXD,2D WAXS,rotational rheology and scanning electron microscope?SEM?were used to investigate the nucleation mechanism of PBDPS.The influence of PBDPS on the physico-mechanical properties was investigated by using universal testing machine and thermal deformation tester.The results are as follows:1.PBDPS is a highly efficient ? nucleating agents of iPP.The relative content of ?-crystal of iPP?k_??increases with the increase of PBDPS content.When adding 4 wt% PBDPS,the value of k_? can reach a maximum of 96.6% under isothermal crystallization at 135?.The side phenyl groups of PBDPS stack like a zipper and form strong ?-? interactions.The excellent nucleating effect was mainly ascribed to the lattice matching between the unit cell parameter in the c-direction of ?-iPP and a corresponding distance of stacked phenyl rings of PBDPS.The introduction of PBDPS simultaneously increased the toughness and stiffness of iPP.When adding 4 wt% PBDPS,the elongation at the break and cantilever impact strength increased from 183 to 1443%,and 2.6 to 4.8 kJ/m²,respectively.In addition,the heat distortion temperature increased from 99.1 to 116.3?.2.Moreover,molecular weight dependence of liquid crystal phase transitions,rheological and ?-nucleation behaviors of PBDPS have been studied in details.The phase transition and melt rheological behaviors of PBDPS exhibited a strong molecular-weight dependence.As the molecular weight Mn increases from 5.2k to 51.9k,the PBDPS shows a phase transition from smectic B to smectic A.Meanwhile,There is a power law relationship(?₀? M_w^4.32±0.17)between the zero-shear viscosity??₀?and molecular weight of PBDPS.The nucleation efficiency and selectivity of PBDPS also exhibited a strong molecular-weight dependence.PBDPS with low molecular weight tended to induce ?-iPP.However,it had a tendency to induce ?-iPP while incorporating high molecular weight PBDPS.Moreover,the relative content of ?-iPP crystals?k_??increased first and then decreased with the increase of molecular weight of PBDPS.And,it reached a maximum k_??86.1%?while incorporating 2 wt% PBDPS21.2k.Furthermore,the stiffness,toughness and heat distortion temperature of iPP were simultaneously improved by incorporating PBDPS.The elongation at the break and cantilever impact strength increased from 645 to 1422%,and 3.0 to 5.0 kJ/m² respectively.Besides,the heat distortion temperature increased from 99.3 to 113.5?.3.The isothermal crystallization kinetics and isothermal crystallization activation energy??E?of iPP/PBDPS composites were analyzed by Avrami equation and Arrhenius equation respectively.PBDPS showed an efficient heterogeneous ?-nucleation effect,the size of iPP spherulites dramatically decreased as increased PBDPS.The crystallization activation energy and halftime of crystallization(t_1/2)significantly reduced while introducing PBDPS.Meanwhile,the results of isothermal crystallization kinetics indicated that crystal growth pattern of iPP changed from two-dimensional to two-dimensional coexisting with three-dimensional.When the content of PBDPS is 4 wt% and crystallization temperature is greater than 130?,the value of k_? approaches 100%.The nonisothermal crystallization kinetics of iPP/PBDPS can be described very well by Jeziorny and Mo methods.Meanwhile,PBDPS can induce both ? and ?-crystals for iPP.However,it exhibited a strong selective nucleation affinity for the ?-crystal of iPP.The value of k_? increased with the increasing concentration of PBDPS and the decreasing cooling rate,reaching a maximum value 97.1% with an addition of 4 wt% PBDPS and a cooling rate of 2.5?/min.