Oriented crystallization in fiber formation: Inferences from the structure and properties of melt spun syndiotactic polypropylene filaments

Abstract

ABSTRACT:In processes, such as melt spinning, the crystallization behavior of syndiotactic polypropylene (sPP) is found to be substantially different from that of most other linear polymers. The anisotropic stress field in such processes leads invariably to extension as well as alignment (orientation) of the chains in the melt, both of which contribute usually to dramatic enhancement in the rate of crystallization. However, since the primary structure of the sPP chain in its most preferred crystal form is comprised of a "coiled helical," O(T 2 G 2 ) 2 O, sequence, stress-induced chain extension can lead to conformational sequences that are not favorable for crystallization in this form. As a consequence, process conditions that generate higher stress levels can cause a diminution in the rate of crystallization of this polymer. Such conformation-related aspects of oriented crystallization of sPP have been addressed through an analysis of the structure and properties of melt-spun fibers, produced over a range of spinning speeds. The results serve to identify a refinement that is needed in current models of oriented crystallization and also a mechanism to promote the nucleation of crystallization of sPP.

Nucleator-Enhanced Molecular Orientation and Crystallization in Melt-Spun Monofilaments of Syndiotactic Polypropylene

“…[6][7][8][9][10][11][12] As for syndiotactic polypropylene (sPP), however, the aforementioned situation does not occur. [13][14][15][16][17] The extent of crystallization is decreased with increasing flow stress, which is totally different from what is observed in its isotactic analog, iPP. This abnormal crystallization in sPP under flow is related to its complicated crystal modifications and related chain conformations.…”

“…Rather, a flow-induced mesophase with trans-planar conformations is present in the melt-spun sPP fibers. [13,16,17] In commercial processing, on the other hand, incorporation of some amount of inorganic or organic nucleator in sPP is effective to accelerate its crystallization rate. [22,23] In particular, dibenzylidene sorbitol (DBS) and its derivative 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS) show significant heterogeneous nucleation effects towards sPP, manifested by dramatic increasing of the crystallization temperature during cooling from the melt.…”

Morphology and mechanical behavior of isotactic polypropylene (iPP)/syndiotactic polypropylene (sPP) blends and fibers

“…It was found [1,2] that sPP has complex polymorphism, such as form I or II of t 2 g 2 conformation, and form III of planar zigzag conformation. Melt-spun sPP fiber exhibits Form I helical structure at low spinning speeds and Form III trans-planar structure at high spinning speeds [22,23]. The crystal structure of the sPP fiber prepared by the spinning speed of 100 m/min should be the form I.…”

Stress-induced structure transition of syndiotactic polypropylene via melt spinning

“…On the other hand, the increase of nucleation rate will further result in the increase of crystallization rate. For most linear polymers, the structure formation in melt spinning follows the above-mentioned theory, however, recently some researchers argued that syndiotactic polypropylene (sPP) fiber in melt spinning doesn't follow the mechanism of orientation-induced crystallization [4]. It has been found that the crystallinity of sPP fiber decreases with the increase of spinning velocity.…”

“…Along the melt-spinning line, polymers are likely to be influenced by a variety of factors, including temperature, stress, strain, spinning velocity, etc. Although there have been some elementary researches on the unusual crystallization behavior of sPP, and the relationship between structure and performance of related fibers has also been touched upon [4,28,29], the structure transition mechanism during melt spinning still keeps not so unambiguous. It is therefore the main aim of the present work is to investigate the correlations between structural variation and melt-spinning parameters and further to elucidate the abnormal crystallization variation of sPP in the meltspinning process.…”