The Study Of PE/MMT Nanocomposites Prepared By In Situ Polymerization

In recent years polyolefin/layered silicate (PLS) nanocomposites have attracted great interest both in industry and in academia. Polyethylene is one of the fastest growing classes of thermoplastics due to their good balance of physical and chemical properties, low cost, light-weight, facile processing and recycle characteristics. Nevertheless, it also possesses some shortcomings such as poor gas barriers properties and thermal and dimension stability, which limit their further applications to some extent. Polyethylene/clay nanocomposites are attracting significant research interest since the synergistic effects of silicate layers and resin molecules alter the morphology and crystallinity of the matrix polymer, leading to improved process ability in addition to the benefits to barrier, strength and stability. Therefore, the low cost polyolefin/clay nanocomposites may replace metal, high performance engineering thermoplastics, and some environment unfriendly plastics.Two strategies, melt intercalative compounding and in situ polymerization, have been used to prepare PE/MMT nanocomposites. Most recent studies mainly focus on melt intercalative compounding process. Good dispersion of MMT in polyethylene by this method requires sufficiently favorable enthalpic contributions to overcome entropic barriers. For most polar or polarizable polymers, an alkylammonium surfactant modified MMT is adequate to offer sufficient excess enthalpy and promotes the nanocomposite formation. However, in the case of none polar polyolefins, there is no favorable excess enthalpy to promote polyolefin /alkylammonium/MMT dispersion. A third component, such as maleic anhydride grafted polyolefin, which is utilized to overcome the incompatibility between polyolefin and MMT, has to be added. The steps of modifying the silicate to render highly hydrophilic montmorillonite organophilic by ion exchange with various orgnanic cation molecules, polarizing the polyethylene or its oligamers and the chemical or physical deteriation of the virgin components during meltprocess may offset the benefits of the silicate to some extent. Thus, melting blend method usually suffers from a number of disadvantages.Another method of making PE/MMT nanocomposites is in situ polymerization, which involves a silicate that is intercalated by catalyst or any other coordination catalyst and upon introduction of a monomer. The system is driven to disperse the layered silicate in the resin by the driving force of the polymerization reaction. This method allows formation of nanocomposites based on the quite apolar polymers such as polyethylene, while the less-polar monomers, which undergo a small entropic loss while diffusing in the interlayer spacing, can intercalate more easily than their respective polymers.PE/MMT nanocomposites studied in this dissertation were prepared by in situ polymerization using new supported MMT/MgCl2/TiCl4 catalyst, where MMT was not modified. The purpose of this dissertation is to investigate the relationship between the microstructure and macro-properties. The study of PE/MMT nanocomposites prepared by in situ polymerization is thus of vital importance, especially in the aspect of morphology, crystallization kinetics, and rheology properties.The content of MMT is 0.1~1.2wt% in the nanocomposites. Transmission electron microscope measurements were conducted to investigate the dispersion state of clay in PE/MMT nanocomposites. The dispersion morphology of MMT has four forms: exfoliation, intercalation, conglomeration, and flocculation, where the content of flocculation (exceeds 50%) is much larger than that of other three forms. The polymer particles obtained by in-situ polymerization show various phase morphologies. Template effect makes PE layered accumulation, and then this platelet further accumulates, and finally forms the flower-like irregular sphere, which is attributed to the edge effect of supported MMT/MgCl2/TiCl4 catalyst. The most spherical diameter is 40~60μm. The less small size PE sphere also exists, but the proportion is little. The multilevel phase morphology and various dispersion of MMT are attributed to the various phase morphology of supported MMT/MgCl2/TiCl4 catalyst, which is correlated with MMT catalyst content and structure in the ethylene environment. This structure is a pristine reason why PE/MMT nanocomposite differs significantly with a common macrocomposite prepared by melt blending, although having the same components.The dynamic rheological test of PE/MMT nanocomposites (PEIs) were conducted using Sweden Rheologica. The storage modulus of PEIs increases due to the good dispersion of MMT, and also due to the large interface area and very strong interaction between PE and MMT, which obviously confines the motion of polymer chains. Further, the rheological characteristics of the nanocomposites and the steady shear response of PEIs were investigated. It is found that the zero shear viscosity of PEIs is larger than that of pure PE due to the strong interaction between PE and MMT. The flow activation energy of PEIs also increases.The melting temperature and the crystallization temperature of PEIs are higher than those of pure PE, since the nucleus effect of MMT and the crystal ability of PE become stronger. In addition, the equilibrium melting temperature of PEIs is higher than that of pure PE. This is because the heterogenous nucleation effect of MMT makes the lamellar thicker, and thus the crystallite of PEIs is more perfect. Avrami equation is applicable for the isothermal crystallization of the PE/MMT nanocomposites. The effect of MMT on crystalline structure is obvious in light of the XRD patterns. A substantial intensity increase is detected for the 110 reflection, which implies the crystalline degree increase of PEIs. The Avrami exponent n of PEIs is lower than that of PE due to the decrease of growth dimension. The addition of MMT makes PE crystallization rate increase, and half time of crystallization decrease. Based on the crystallization dynamic theory, the spherulite growth rate is investigated. It is found that both the nucleation constant Kg and lateral surface free energyσincrease, which is attributed to the heterogenous nucleation effect of MMT.The dynamic properties of PEMs prepared by melt blending, PEIs prepared by in situ polymerization, and pure PE were also studied. The storage modulus of PEIs is larger than that of PEMs and PE, which is due to the good dispersion of MMT and strong interaction between MMT and PE. However, the storage modulus of PEMs is slightly larger than that of PE. The reason is that MMT is common filler in the PEMs while PEMs is macrocomposite. The interaction between PE and PE matrix in PEMs is weak. The peak position ofαtransition is higher about 10oC than that of PE, thus the confinement of polymer chain in the PEIs is evident. It has a strong effect on the crystal region. The crystal morphology of PE and PEI1, PEI3 were observed at 120°C by POM. With the increase of MMT content, the nucleation density increases. This leads to the decrease of ring spherulite dimension.