Shear-induced crystallization and rheological behavior of syndiotactic polystyrene
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We studied the correlation between shear-induced crystallization and rheological behavior of syndiotactic polystyrene. It was found that after applying a steady shear flow around the nominal melting temperature (Tm 5 270 °C), crystal growth rate was accelerated compared with the quiescent state and a morphology of oriented lamellae (kebabs) was observed. On the other hand, no obvious morphological change was observed when applying a shear flow with relatively slow shear rate. We discussed a possibility that the difference of crystal growth rate and morphology could be attributed to the competition between shear rate and relaxation time such as reptation time. Our rheological results suggested that when the imposed shear rate is close to the reciprocal of reptation time, oriented lamellae (kebabs) are observed but extended-chain crystals (shishs) cannot be formed since the chain segments between adjacent entanglements remain unstretched.
I. INTRODUCTION
Semicrystalline polymers are widely used in polymer industry due to their bi-properties of crystalline and amorphous components. The crystalline component contributes the strength for materials, while the flexibility can be attributed to the amorphous component. Generally, molten polymers have experienced some processes like extrusion or injection and then become plastic products, fibers, and so on. The final morphology and properties of polymer products strongly depend on the processing history. It is well known that when semicrystalline polymers are crystallized under shear or elongational flow, crystallization rate is enhanced and the so-called “shish-kebab” structure might be formed.1–4 As shown in Fig. 1, it is considered that shish-kebabs consist of extended-chain crystals (shish) and fold-chain lamellar crystals (kebab) grow periodically around shish in the simplest case.1–7 It is believed that shish structure is the molecular origin of ultrahigh strength and ultrahigh modulus fibers.1–4 Numerous studies have focused on the formation mechanism of shish-kebabs. Recently, one of the major topics is formation mechanism of shish and shish precursor (“row-nucleated” or “shish-like” precursor) formed during the shearing process, which affects final morphology drastically. For example, Alfonso’s and Kanaya’s groups revealed the existence of precursors of shish-kebabs with a scale of micron size.8–11 Precursors of shish-kebabs in micron size were found to be formed above nominal melting temperatures (Tm) of isotactic polystyrene even close to equilibrium melting temperAddress all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.34
The polymer used in this study is syndiotactic polystyrene (sPS) purchased from Scientific Polymer Products
J. Mater. Res., Vol. 27, No. 10, May 28, 2012
http://journals.cambridge.org
II. EXPERIMENTS A. Materials
a)
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atures (Tm0 ). Rastogi et al. found the existence of precursors of shish-kebabs formed in nanometer scale and elucidated the critical size of precursors.12,13 Van Meerveld et al.14 us
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