ABSTRACT:The isothermal crystallization kinetics of polypropylene/montmorillonite (PP/MMT) nanocomposites synthesized via intercalation polymerization were investigated by using differential scanning calorimeter and polarizing optical microscope (POM). The crystallinity of the nanocomposites decreased with the increase of the montmorillonite content, indicating that the MMT layers dispersed in the PP matrices confined the PP chains and hindered the crystallization of the PP chains. The POM photographs showed that the spherulites of the PP/MMT nanocomposites were greatly decreased in size as MMT was introduced. On the other hand, the crystallization rate increased dramatically with the increasing of MMT content. The interfacial free-energy per unit area perpendicular to PP chains in PP/MMT nanocomposites decreased with increasing MMT content, suggesting that the MMT layers acted as heterogeneous nuclei in the nucleation of crystallization. The nucleus density increased with the increasing of MMT content, leading to a positive effect on the crystallization.
Polypropylene/clay (PP/clay) nanocomposites were synthesized via intercalative polymerization. The nanostructure of the composites was investigated by wideangle X-ray diffractometry (WAXD) and transmission electron microscopy (TEM). The WAXD patterns of the PP/clay nanocomposites indicated that the characteristic diffraction peak of the clay disappeared. The TEM image showed the clay was exfoliated into nanometer size and dispersed uniformly in the PP matrix. The composites exhibited much higher storage modulus compared to that of pure PP. At temperatures higher than T g , the storage modulus of the PP/clay nanocomposites with 8.1 wt % clay content increased three times that of the pure PP. Additionally, the thermal stability of the nanocomposites significantly increased. The maximum decomposition temperature was increased by 44°C with the introduction of about 10 wt % clay. The heat-distortion temperatures (HDTs) of the nanocomposites also increased.
A systematic study of the scope and limitations of B-H chain transfer agents during metallocene-mediated olefin polymerization is discussed in this contribution. The polymerization procedures provide a convenient route to prepare chain-end functionalized polyolefins and polyolefin diblock copolymers containing both polyolefin and functional polymer blocks. With the proper choice of borane chain transfer agents, metallocene catalyst systems, and reaction conditions, the chemistry can be applied to a broad range of polyolefin homo-and copolymers, such as polyethylene, polypropylene, syndiotactic polystyrene, poly(ethylene-co-propylene), poly(ethylene-co-1-octene), and poly(ethylene-costyrene). The molecular weight of the borane-terminated polyolefin is basically inversely proportional to the molar ratio of [borane]/[olefin]. In turn, the terminal borane group is very reactive, which can be quantitatively converted to various functional groups and also can be selectively oxidized to form a stable polymeric radical for living free radical polymerization of functional monomers. This process resembles a transformation reaction from metallocene polymerization to living free radical polymerization via the borane terminal group to produce functional polyolefin diblock copolymers, which are difficult to prepare using conventional initiaors.
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