Nanograin Evolution in Cold Crystallization of Syndiotactic Polystyrene As Illustrated via in-Situ Small/Wide-Angle X-ray Scattering and Differential Scanning Calorimetry

• Published in: Macromolecules 2009-09, Vol.42 (17), p.6656-6664
• Main Authors: Su, C. H, Jeng, U, Chen, S. H, Lin, S. J, Wu, W. R, Chuang, W.-T, Tsai, J. C, Su, A. C
• Format: Article
• Language: English
• Subjects: Applied sciences; Crystallization; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma900537c

Structural evolution of syndiotactic polystyrene (sPS) in a cold crystallization process was quantitatively examined with in-situ small/wide-angle X-ray scattering and differential scanning calorimetry (SAXS/WAXS/DSC). After removal of background scattering from fractal-like matrix structure, SAXS profiles obtained during programmed heating of an amorphous sPS specimen from 30 to 240 at 10 °C/min can be interpreted with a similar sequence of events previously observed in cold crystallization of poly(9,9-di-n-octyl-2,7-fluorene) (PFO). Specifically, the nanograin evolution of sPS involves four stages: (1) the frozen-in stage below 90 °C, (2) the nucleation of oblate-like nanograins with a constant radius of gyration R g ≈ 2.6 nm between 90 and 130 °C, (3) the growth of nanograin size to R g ≈ 3.2 nm concomitant with the emergence and development of WAXS-determined crystallinity (X c,WAXS) from 130 to 180 °C, and finally (4) the coalescence (and thickening) of the nanocrsytals into a greater size of R g ≈ 4.6 nm upon further heating up to 240 °C. Developments in the DSC-determined crystallinity (X c,DSC) coincided with nucleation and growth stages, whereas the SAXS-determined heterogeneity (Q inv) increased steadily throughout nucleation, growth, and coalescence stages. Little changes of morphological features in the nanometer-length scale can be observed with subsequent isothermal annealing at 240 °C up to 1 h; the final size of coalesced nanograins at this temperature is therefore attributed to the balance between the tendency to eliminate lateral surface via coalescence and the opposing strain field (due to locked and tightened entanglements) in the surrounding matrix. Delicate differences in nanograin size and shape during cold crystallization processes of sPS and PFO are discussed in terms of differences in chain rigidity (random coiled vs semirigid) and melt structure (isotropic vs nematic).