Crystallization and structure development in polyolefin processing

Abstract EN

We review studies of structure development in polymer processing of several commercial crystalline polyolefins, including polyethylene, isotactic and syndiotactic polypropylenes, isotactic polybutene-1, and isotactic poly(4-methyl pentene-1).

These polymers possess various polymorphic crystal structures.

We described the crystallization, morphology, and orientation development of these polymers in three major polyolefin applications: (i ) melt spinning; (ii ) tubular blown film extrusion; and (iii ) injection molding.

Some polyolefins exhibit polymorphic crystallization behavior during polymer processing.

We find the relationships of the structure with stress level, cooling rate, and material charact ers (e.g.

crystallization rate and stereoregularity).

We also consider structural trans formation processes occurring in some polyolefins, which involve trans formation of a less stable form to a more stable form during ageing or annealing.

Orientation development in polymer processing operations is another major concern of this study.

Through wide-angle X-ray di ffraction and birefringence investigations, we study the orientation of polymer chains and crystals in various processing conditions and find correlations of crystalline orientation and birefringence with stress level at the position of solidification.

The effects of l amellae twisting and lamellae branching are also discussed in studying the orientation of polyethylene and isotactic polypropylene.