Crystal Structure of Isotactic Propylene−Hexene Copolymers:  The Trigonal Form of Isotactic Polypropylene

Abstract

A study of the structure of isotactic propylene-hexene random copolymers, prepared with singlecenter metallocene catalysts, is reported. For low concentrations of hexene comonomeric units, up to nearly 10 mol %, copolymers crystallize in the R form of isotactic polypropylene. Copolymers with hexene contents higher than 10 mol % crystallize in a new polymorphic form that melts at nearly 50 °C. The hexene units are partially included in both crystals of R form and of the new form. The crystallization of the new crystalline form allows incorporation of a high amount of hexene units. The melting temperature and the crystallinity decrease rapidly with increasing hexene content. A slower decrease of melting temperature and crystallinity is observed at high hexene concentrations because of the crystallization of the new form. The crystal structure of the new form has been studied by analysis of X-ray powder and fiber diffraction patterns of samples containing hexene concentration higher than 10 mol %. Chains in 3-fold helical conformation of propylene-hexene copolymers are packed in a trigonal unit cell according to the space group R3c or R3 hc. The values of a and b axes of the unit cell depend on hexene concentration, and values of a ) b ) 17.5 Å and c ) 6.5 Å have been found for the sample with 26 mol % of hexene. The structure contains high degree of disorder due to the constitutional disorder of the random copolymer chains that produces disorder in the positioning of the lateral groups in the unit cell. Moreover, statistical disorder in the up-down positioning of the helical chains and slight disorder in the orientation of chains around the 3-fold axes are present. The inclusion of hexene units in the crystals induces a suitable increase of density that allows crystallization of 3-fold helical chains in the trigonal form, where the helical symmetry of the chains is maintained in the crystal lattice. The structure is similar to that of form I of isotactic polybutene. This form does not crystallize, and has never been observed so far for the polypropylene homopolymer because, in the absence of bulky side groups, it would have a too low density. This structure represents an example of entropydriven phase formation.

Comonomer Length Influence on the Structure and Mechanical Response of Metallocenic Polypropylenic Materials

“…Density values in these CiPH correlate rather well with those reported in the literature for analogous copolymers. [19] On the other hand, Figure 1 shows the WAXD profiles at room temperature for the different CiPOD copolymers and that found in the iPP homopolymer for comparison reasons. It can be observed that the profiles of the neat iPP and CiPOD1.0 and CiPOD2.3 copolymers exhibit the five main diffractions [(110), (040), (130), (111), and (130, 041) planes] characteristic of the a monoclinic iPP crystalline modification.…”

“…[14][15][16] An interesting finding is that a crystal structure neither resembles those of the well-known a, b, and g phases of isotactic poly(propylene) (iPP) nor that of isotactic poly-1-hexene has been uncovered in copolymers of propylene-1-hexene (CiPH) with relatively high molar contents in 1-hexene (>10%), crystallized at moderate rates and annealed at room temperature. [17][18][19] On the other hand, specimens of low molecular weight CiPH copolymers with relatively low 1-hexene molar fractions (up to a 5 mol-%), prepared by a fast cooling from the molten state, [20] show a much easier formation of mesomorphic structures similar to those observed in either effective-quenched or stretched iPP. [21,22] Moreover, the effect of comonomer size on the final properties is expected to be more dramatic in the case of propylene copolymers, although contradictory results have been reported in the open literature.…”

Application of a multi-phase multi-morphology crystallization model to isotactic polypropylenes with different molecular weight distributions

“…Elevated cooling rates (>100°C/s) inhibit the stable monoclinic modification in favour of a mesomorphic form [1,12]. Two other modification (δ [13] and ∊ [14]) have been reported in literature, but they are outside the scope of this work.…”

Influence of structure on the properties of polypropylene copolymers and terpolymers

“…The corresponding values are shown in Table 3. 7 The crystal size in the (110) direction, lc (110), has been also determined from the width at half height of the corresponding diffraction, calculated through Scherrer equation. [43] These values are also indicated in Table 3.…”

“…In addition, a new trigonal form has been described in the case of copolymers of iPP with high contents of 1-hexene or 1-pentene as comonomers. [5][6][7][8][9][10] Moreover, a kind of mesomorphic form has been found to appear not only in homopolymer but also in different random copolymers, with comonomers such as ethylene, 1-butene, 1-pentene and other -olefin copolymers. [11][12][13][14] Among the different paths for modifying the ability to crystallize the iPP and the type of polymorph desired, this paper focuses on the introduction of comonomer units.…”