Isotactic polypropylene trigonal form

These crystal modifications differ in their molecular and crystal structures as well as in their physical properties. Many types of crystalline modifications are reported, including a stable orthorhombic phase and metastable monoclinic phase for PE a, and y forms for isotactic polypropylene (/-PP) trigonal and orthorhombic phases for polyoxymethylene a and y forms for Nylon 6 and others. Poly(vinylidene fluoride) (PVF), for example, appears in at least four types of crystalline modification (Lovinger, 1985 Dunn Carr, 1989). 

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. 

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