Modulation On The Structure And Composition Of Layered Double Hydroxides And Their Nucleating Effect On Polypropylene Crystallization

Polypropylene(PP)is an important polymer material with excellent comprehensive performance,which is widely used in food processing,transportation and aerospace fields.However,the semi-crystalline feature of PP normally induces long process duration,low-temperature brittle fracture and poor impact performance.The addition of nucleating agent into PP matrix can significantly increase its nucleation sites,enhance the nucleation rate,and decrease the spherulite size,thereby improving the mechanical properties.In addition,the incorporation of nucleating agent can also induce changes in crystallinity manner and crystal morphology of PP,providing great potential for the regulation of its functionality.As a class of typical two-dimensional(2D)inorganic materials,layered double hydroxides(LDHs)have been widely applied in many polymer materials to improve the thermal stability,flame retardant property and gas barrier property of polymers.In spite of great progress on the development of polymer/LDH nanocomposites,very few reports are focused on LDHs as nucleating agent for PP.There are two main issues with LDHs as PP nucleating agents need to be resolved:(1)How to reveal the nucleation/crystallization mechanism of PP after incorporation with LDHs,which would provide guidance for the design and preparation of nucleating agent;(2)How to design and control LDHs structure to enable PP crystallize into different forms.In order to solve the above problems,a systematic research was performed as follows:(1)Particle-size regulation of MgAl-LDHs and its effect on the crystallization of a-PPTwo kinds of LDHs with nano-sized and micro-sized level were prepared by hydrothermal and homogeneous co-precipitation method respectively,which were used as PP nucleating agents.PP/LDHs composites(composites containing nano-sized and micro-sized LDH were named as 1-PPLx and 2-PPLx respectively,x represents the mass percentage of LDHs)were prepared by using solution blending method.The result reveals that as an inorganic nucleating agent,LDHs has an excellent effect on inducing crystallization of ?-PP.The crystallization temperature was significantly increased(1-PPL5 and 2-PPL5 increased by 8.5 and 12.1 ?,respectively),and the isothermal crystallization rate was significantly enhanced.It should be noted that the micro-sized LDH exhibits stronger nucleation ability towards PP crystallization than the nano-sized LDH,and also among the most efficient inorganic nucleating agents compared with other counterparts reported previously.Avrami kinetic analysis shows that the crystallization of PP follows a heterogeneous nucleation mechanism with the assistance of MgAl-LDHs.It is found that the micro-sized LDHs significantly reduce the surface free energy as well as nucleation free energy,based on Lauritzen-Hoffman theory.In addition,as the LDHs loading increases,the spherulite density of a-PP becomes larger and the spherulite size is significantly reduced.The addition of LDHs markedly improves the thermal stability,gas barrier property and flame retardant property of PP;the obtained PP/LDHs composites are expected to be used in food/drug packaging and engineering plastics.(2)Preparation of PA@CaAl-LDHs hybrid nucleating agent and its effect on inducing(3-PP crystallization.CaAl-LDHs was prepared by co-precipitation method and subsequently modified with pimelic acid(PA)to obtain PA@CaAl-LDHs material,which was used as PP nucleating agent.The effect of PA@CaAl-LDHs on inducing PP crystallization was studied.It is found that PA@CaAl-LDHs not only increases the nucleation density of a-PP,reduces the spherulite size,but also induces a certain proportion of?-crystalline PP.The control experiments show that pure CaAl-LDHs,PA and PA@MgAl-LDHs samples can not induce the crystallization of?-type PP,indicating that both the chemical composition and surface property of LDHs play key roles in the crystallization behavior of PP.Compared with PA@MgAl-LDHs,the PA@CaAl-LDHs exhibits stronger nucleation ability towards ?-PP with a higher crystallization temperature and a faster crystallization rate.The relative content of ?-PP increases along with the increase of PA loading in PA@CaAl-LDHs.Furthermore,the relationship between the structure of the hybrid nucleating agent and the crystallization behavior of PP was revealed:PA has a strong interaction with Ca2+ on the surface of CaAl-LDHs;CaAl-LDHs promote the crystallization of ?-PP while PA induces the crystallization of ?-PP.The synergistic effect of PA and CaAl-LDHs promotes the crystallization of ?-PP and induces crystallization ability of ?-PP.Therefore,this work provides a new strategy for the design and preparation of PP nucleating agent,which can be potentially used in composite materials field.