Molecular View of Properties of Random Copolymers of Isotactic Polypropylene

The yield behavior during uniaxial drawing of isotactic random copolymers of propene with ethylene (iPPEt), 1-butene (iPPBu), 1-pentene (iPPPe), 1-hexene (iPPHe), and 1-octadecene (iPPOc) is analyzed within the framework of our current understanding of deformation properties of semicrystalline polymers, that is, the intrinsic stability of lamellar crystals and related polymorphism phenomena, along with the ability of entangled amorphous chains to transmit stress. The samples selected for analysis were synthesized using single-site metalorganic catalysts, are highly stereoregular, and contain small amounts of regiodefects caused by secondary 2,1 erythro units. Moreover, the interchain and intrachain distribution of comonomeric units is uniform. In the case of iPPEt copolymers, samples containing ≈3.5 mol% stereodefects were also studied. The yield behavior of these samples depends on the kind and concentration of defects, and is directly related to the level of inclusion in and exclusion from crystals of the comonomeric units. Apart from iPPBu copolymer samples with high butene content, the yield stress of all samples increases with the thickness of lamellar crystals according to a common trend, regardless of comonomer. In the case of iPPBu copolymers containing a high concentration of butene units, the yield stress decreases with increasing lamellar thickness. The increase in yield stress with lamellar thickness is rationalized in terms of the micromechanical model of crystallographic slips, based on thermal activation of screw dislocations. The parameters of the model describing the yield behavior are the critical free energy required to form a screw dislocation and the shear modulus associated with the slip planes of the dislocations. These were set as identical to those deduced for isotactic polypropylene homopolymer samples (iPP) crystallized under different conditions. Study of the yield behavior of these copolymers extends the use of the dislocation model to a set of samples crystallized under similar conditions but characterized by differences in comonomeric unit, degree of crystallinity, lamellar thickness, polymorphism, and intrinsic flexibility of the chain backbone. The results indicate that for a homogeneous class of propene-based copolymers, namely crystallized in the α-form of iPP under similar conditions, lamellar thickness controls the level of plastic resistance provided that the concentration of structural irregularities in the crystals is not too high. iPPBu copolymers with high comonomer concentration do not obey this rule because of the high level of inclusion of comonomers in the crystals, which induces an increase in lamellar thickness but also a decrease in crystal stability.