Structure and Mechanical Behavior of the Mesomorphic Form in a Propylene-b-Poly(ethylene-co-propylene) Copolymer and Its Comparison with Other Thermal Treatments

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...

Metallocenic Isotactic Poly(propylene) and its Copolymers with 1‐Hexene and Ethylene

“…[4,28] In fact, only two wide maxima, centered at around 15 and 21 8, are observed in the mesomorphic form. This intermediate-ordered mesomorphic structure has been also found in copolymers of Figure 1.…”

“…Consequently, the use of coupling agents is necessary to attain an optimum blend and really improve the impact resistance. One possibility of overcoming the difficulties of miscibility of the different polymeric phases is the use of reactor-granule technology, which leads to the achievement of heterophasic copolymers [2] with an excellent stiffness/impact balance [3,4] and a wide range in ethylene content (up to 25% by weight) as the rubber phase.…”

“…The viscoelastic response of iPP and the copolymers is influenced by variables that affect their crystalline structure, [14] such as comonomer content, degree of crystallinity and thermal treatment. [4] Isotactic poly(propylene) obtained with Ziegler-Natta catalysts, as well as metallocenic poly(propylene), show three viscoelastic relaxations, [24,33] as observed in Figure 9, designated as g, b…”

Rheological and structural details of biocidal iPP-TiO2 nanocomposites

“…The orthorhombic γ modification, easily identified because of its characteristic (117) diffraction peak at around 20º in 2θ profiles [23], is specially favored in the case of iPP synthesized by metallocenic catalysts due to the presence of errors homogeneously distributed among the different polymer chains [24]. It can be observed in this Figure 4a that these profiles exhibit the main diffractions characteristic of the α monoclinic iPP crystalline modification [24,25]. On the other hand, it is noticeable that there is practically no evidence of the γ modification in the pristine iPP.…”

The role of the compatibilizer on the microindentation hardness of iPP/PC blends

“…We also applied the microindentation hardness (H) test to the study of semicrystalline polymer blends, both, unmodified and compatibilized [18][19][20][21][22][23]. Block copolymers [17,24], PMMA/natural rubber blends [25], ethylene/1-octene copolymers and their blends with HDPE [26,27] and new reversibly crosslinked isotactic PP (iPP)/LDPE blends [28].…”