No abstract

The development of the mesophase has been studied by either fast scanning calorimetry or conventional DSC in a series of random propylene-co-1octene copolymers. The results show that the mesophase formation rate can be easily tailored in a very wide range of magnitudes, covering almost 4 orders. That decrease is similar to that found previously in propylene-co-1-pentene copolymers at the same wt % of counits. On the contrary, the pertinent unit for the melting point depression (and for the crystallization temperatures) is found to be the mol % comonomer content. Consequently, the "kinetic" parameters in the ordering of iPP copolymers can be tailored somewhat independently of the transition temperatures just by changing the size of the comonomeric units. Additional X-ray diffraction experiments, employing either conventional or synchrotron radiation, were also carried out on these 1-octene copolymers in order to determine the precise nature of the different transitions.

No abstract

ABSTRACT:Several thermal treatments have been imposed along processing to a poly[propylene-b-(ethylene-copropylene)] and their effect on structure, thermal characterization and mechanical response has been explored. Quenching in a dry ice/methanol bath after melting has allowed to obtain a mesomorphic form due to the existence of long isotactic polypropylene (iPP) chains within the block copolymer here studied. An exhaustive analysis of such a mesophase has been performed comparing its morphological details and mechanical properties with the rest of thermal treatments. A microspherulitic superstructure is found in this mesomorphic form which leads to the lowest values in mechanical parameters, such as storage and Young moduli as well as microhardness, compared to those exhibited by the other specimens with distinct thermal histories. However, the ductility of the mesomorphic structure is the highest one, owed to the non-existence of monoclinic crystallites. A phase transition is observed at around 90• C on heating, which suggests the transformation of this mesomorphic form to the monoclinic crystalline structure. KEY WORDS Poly(ethylene-co-propylene) / Mesomorphic Form / Monoclinic Crystallites /The crystallization behavior of isotactic polypropylene (iPP) is very complicated. The morphology and properties after crystallization depend upon thermal history and the detailed microstrucutre of the polymer molecules. The iPP chains adopt a 3 1 helical conformation when crystallized from the molten state. These helical chains can organize into several different spatial arrangements giving rise to three distinct polymorphs: α-monoclinic, β-hexagonal, and γ-orthorhombic forms, depending on the crystallization conditions and catalyst used. [1][2][3][4][5][6][7][8][9][10][11][12][13] Cooling from the melt at low or moderate cooling rates leads usually to the formation of the thermodynamically stable α-monoclinic crystalline lattice, being this α-form the most common one.Moreover, there is also a "mesomorphic" phase, which can be obtained by rapid quenching of molten iPP. The appearance of two broad peaks in the X-ray diffraction pattern is a characteristic of this quenched form. 14 The presence of broad, diffuse peaks in X-ray diffraction tends to suggest that the quenched structure is in a disorder state. These two peaks, however, exhibit an intermediate width between the crystal diffractions and the pattern found for a completely amorphous polypropylene. 15 Natta and Corradini 1 first pointed out that this form had an intermediate degree of ordering between the amorphous phase and the crystalline phase. They categorized it as smectic to indicate the presence of two-dimensional ordering, being better in longitudinal than in transverse chain direction. 16 However, the full evidence for the smectic phase in iPP has not been documented. The description of the mesophase can be quite confusing in the literature. In addition to the smectic phase, 1, 17 other investigators described the mesomorphic phase as paracrystalline, 18...

No abstract

No abstract

No abstract

No abstract