Preparation, Structure And Properties Of Exfoliated Polypropylne/Montmorillonite Nanocomposites

Improving properties of polypropylene (PP) is always great attention of researchers in materials. Compounding of PP and montmorillonite (MMT) is an effective way to modify PP. According to the dispersed state of MMT layers in PP matrix, PP/MMT nanocomposites are divided into three types:macroscopic two-phase composite, intercalated nanocomposites and exfoliated nanocomposites. The PP/MMT nanocomposites with exfoliated structure are especially desirable for the nanoscale silicate layers evenly dispersion in the PP matrix. However, it is difficult to disperse MMT layers uniformly in PP matrix because of the incompatibility of hydrophobic PP and hydrophilic MMT. Solution blending, melt blending and in situ polymerization methods have been used to prepare PP/MMT nanocomposites. In situ polymerization is an effective method to prepare exfoliated PP/MMT nanocomposites compared with melt and solution blending methods.Alkylimidazolium (Im) and alkyltriphenylphosphonium (Ph) surfactants with high thermal stability were used as surfactants for MMT. Im-modified MMT (IMMT) and Ph-modified MMT (PMMT) were prepared via ion-exchange procedures. The wide angle X-ray diffraction (XRD) and TEM results indicate that the d-spacing of IMMT and PMMT increase to2.76nm and3.34nm, respectively. The thermo gravimetric analysis (TGA) results indicate that the IMMT and PMMT can bear higher temperature compared with that of common alkyl-ammonium surfactants-modified MMT (AMMT).Ziegler-Natta/IMMT compound catalyst was prepared by chemical reaction method and exfoliated polypropylene/IMMT nanocomposite was synthesized by in-situ interlayer intercalative polymerization with the above compound catalyst. Effects of mole ratio of Al to Ti, polymerization temperature, time and solvent type on catalyst activity and catalytic stereospecificity were studied. It was found that under optimal conditions:n (Al): n (Ti)100,60℃,1h and heptane as solvent, the activity of compound catalyst is about78.4kg/(mol-h). High isotacticity PP could be produced by the compound catalyst, and solvent used could affect the isotacticity of PP synthesized with compound catalyst. The existence of MMT do not change weight-average relative molecular mass and its relative molecular mass distribution of of PP synthesized. The microstructure of PP/IMMT nanocomposites were investigated by XRD and transmission electron microscopy (TEM). The results indicate that exfoliated PP/IMMT nanocomposites are produced successfully by in-situ polymerization and MMT nanolayers are dispersed uniformly in the PP matrix in the nano scale. The average thickness of the MMT lines in PP matrix is less than10nm. The increase in MMT content enhances the thermal stability of PP materials. The long-time shear inevitably leads to the aggregation of some MMT layers and this aggregation of MMT can be avoided by shortening the shear time. The a phase is the dominate crystal phase in the in-situ synthesized PP/IMMT nanocomposites. The appearance of β phase is related with the annealling condition.PMMT was used to prepare Ziegler-Natta/PMMT compound catalyst and exfoliated i-PP/PMMT nanocomposites were prepared by in situ intercalative polymerization of propylene with Ziegler-Natta/PMMT catalyst. Effect of polymerization conditions and the existence of MMT on catalyst activity and catalytic stereospecificity were studied. The catalytic efficiency of the above catalyst under optimum polymerization condition can reach as high as1300kg/(molTi·h) and the combining of PMMT with Ziegler-Natta catalyst do not change the stereo-regulation catalytic properties of the Ziegler-Natta catalyst. The synthesized PP possess high isotacticity, melting point and molecular weight. XRD and TEM examinations prove the nanocomposites obtained are exfoliated ones. a-Phase crystallite is the main crystallite formed in PP/PMMT nanocomposites.