Competitive Growth of - -Crystals in -Nucleated Isotactic Polypropylene under Shear Flow

Journal Article · Fri Jan 01 04:00:00 EST 2010 · Macromolecules

DOI:https://doi.org/10.1021/ma101006e· OSTI ID:1019926

Chen, Y; Mao, Y; Li, Z; Hsiao, B

It has been well established that, although both shear flow and {beta}-nucleating agent could separately induce {beta}-crystals in isotactic polypropylene (iPP) in an efficient manner, their combination in fact depressed the content of {beta}-crystals when compared with quiescently crystallized {beta}-nucleated iPP. In the current study, in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) measurements were performed to investigate this behavior. The WAXD data obtained were quantitatively analyzed to determine the independent contributions of applied shear flow and added {beta}-nucleating agent in terms of nucleation stage and subsequent {alpha}- and {beta}-crystal growth stage. In the nucleation stage, the addition of {beta}-nucleating agent increased the amount of {beta}-nuclei, while the application of shear flow and the interactions between shear and {beta}-nucleating agent enhanced the amount of {alpha}-nuclei (the amounts of {alpha}- and {beta}-nuclei were in the same order of magnitude). As a result, in the initial crystallization, {alpha}- and {beta}-crystals grew competitively, causing simultaneously increments of {alpha}- and {beta}-crystals. However, in the growth stage, the growth rate of {beta}-crystals was faster than that of {alpha}-crystals where the epitaxial growth of {beta}-crystals on {alpha}-crystals also occurred (due to more favorable isothermal crystallization temperature for {beta}-crystal growth). Consequently, the content of {beta}-crystals became dominant in the limited growth space; however, it was still less than that formed from the quiescent isothermal crystallization of {beta}-nucleated iPP. As the shear rate increased, more shear-induced {alpha}-nuclei were formed, further decreasing the amount of {beta}-crystals. Nevertheless, when shear and {beta}-nucleating agent coexisted, {beta}-crystals emerged earlier than {alpha}-crystals. The SAXS results indicated that the combination of shear and {beta}-nucleating agent changed the stacking manner of molecular chains, so that the long period of sheared, {beta}-nucleated iPP was comparable to that of quiescently crystallized iPP.

• Research Organization: Brookhaven National Laboratory (BNL) National Synchrotron Light Source
• Sponsoring Organization: DOE - OFFICE OF SCIENCE
• DOE Contract Number: AC02-98CH10886
• OSTI ID: 1019926
• Report Number(s): BNL--95772-2011-JA
• Journal Information: Macromolecules, Journal Name: Macromolecules Journal Issue: 16 Vol. 43; ISSN 0024-9297
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CHAINS
CRYSTALLIZATION
NUCLEATION
POLYPROPYLENE
SCATTERING
SHEAR
SYNCHROTRONS
X-RAY DIFFRACTION
national synchrotron light source