Statistical isotactic polypropylene copolymers

Similarly, in statistical isotactic polypropylene copolymers containing ethylene or butene counits, a mixture of a monoclinic and y orthorhombic [52-54] crystal phases was observed, and the proportion of the /form increased with increasing counit concentration [25, 47, 55, 56]. The /form is typically observed in propylene homopolymers when crystallized under high pressure [57,58]. In the case of copolymers, the presence of non-crystallizable counits disrupts the development of polypropylene helices, resulting in an increase in the / phase content [25, 47, 55, 56]. The /phase was also observed in isotactic copolymers of propylene and 1-hexene [59]. Additionally, propylene copolymers that contain 10-25 mol% hexene units were found to crystallize in a crystal form having a trigonal unit cell [60, 61]. This structure is isomorphous to that of poly(l-butene), which. 

Partitioning of branches in both polyethylene and isotactic polypropylene copolymers was also investigated by Hosoda et al. [41, 42]. Ethylene-alkene and propylene-alkene statistical copolymers were degraded by fuming nitric acid to selectively remove their amorphous phase. The degree of branch inclusion in the residual crystalline phase was then determined by solid-... 

Fig. 2.22. Dependence of the elastic modulus E and the mechanical loss factor 6 on temperature for various polymers. Curves 1 elastomer (statistical copolymer of ethylene and propylene) curves 2 isotactic polypropylene (semicrystalline)...

Both isotactic poly(propylene-co-butene) [81, 82] and syndiotactic poly(propylene-co-butene) [83] statistical copolymers exhibit isodimorphic behavior when crystallized. In isotactic poly(propylene-co-butene), the copolymer was found to crystallize over the entire range of butene content. A transition in crystal unit cell structure was observed at about 50 mol% butene, where below this composition, the crystalline phase resembled that for polypropylene and above this composition, the copolymer was essentially polybutene with propylene defect units [81]. The observed isodimorphic behavior can be explained by the similar helical chain structures exhibited by both polypropylene and polybutene in the crystal, which results in a relatively small free energy penalty for co-crystallization [42, 81-83],... 

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