Elsevier

Polymer

Volume 42, Issue 21, October 2001, Pages 9031-9043
Polymer

Rheology and structure of isotactic polypropylene near the gel point: quiescent and shear-induced crystallization

https://doi.org/10.1016/S0032-3861(01)00402-5Get rights and content

Abstract

Crystallizing isotactic polypropylene (iPP) develops large-scale spherulites and thick threads, large enough for observation by optical microscopy, and undergoes a liquid-to-solid transition as an expression of increased connectivity of the structure. In order to relate the time scales of structural and rheological changes, we measured time-resolved small-angle light scattering (SALS) and transmittance properties in a single experimental run, which then was repeated in an optical microscope for direct observation of growth of large-scale structures, and in a rheometer for mechanical spectroscopy. The results for quiescent and shear-enhanced melt crystallization of a high molar mass iPP are presented. In quiescent crystallization, iPP nuclei are only formed in the beginning (and not in later stages) and grow simultaneously at the same rate, which leads to spherulites of equal size. The critical gel point is reached close to the instant of the maximum of density fluctuations, but before spherulites impinge. Crystallinity estimates from Hv SALS (estimation method of Stein) are much higher than values from DSC. The discrepancy may be caused (in addition to the simplifying assumptions in the estimate) by the enhanced crystallization in the rheo-optical cell due to surface and sample loading effects. Shear flow induces anisotropic molecular conformations, preferably in the high molecular weight component of the iPP sample. The resulting orientation fluctuations (of highly oriented long chains and less oriented short chains) cause (1) an increase in nucleation rate, (2) possibly an increase in crystallization rate and (3) formation of highly elongated structures (threads) which are visible in the optical microscope and in anisotropic SALS patterns. The threads thicken until, at later stages, additional spherulites start to grow, presumably from the shorter chains and nucleated on by the threads.

Introduction

The study of polymer melt crystallization stimulated by flow has drawn much interest because it implies the possibility of controlling the final morphologies and properties of semi-crystalline polymers. By tuning the processing conditions (temperature, strain) and molecular composition, the wide range of possible molecular morphologies, such as spherulitic, shish–kebab, or row-nucleated structures, can be altered in desired ways. Flow-induced orientation and structure formation in elongational flows were thoroughly studied by Keller and co-workers [1]. Even for quiescent crystallization, the mechanism of crystal growth from the melt has become a subject of increased attention [2], [3], [4]. More experiments are needed to support further development of theory.

Isotactic polypropylene (iPP) is a useful polymer for these studies because of its large-scale, semi-crystalline structure, which not only causes a connectivity transition as measured in rheology [5], [6] but also can be seen with optical experiments such as small-angle light scattering (SALS) [7], optical microscopy [8], [9], [10], [11], and transmission intensity [12], [13], [14], [15], [16], [17], [18], [19], [20]. An attempt needs to be made to compare the time scales of these observations in order to merge them into a unified framework. For this purpose, we built a rheo-optical device which, in combination with an optical microscope, allows measurement of the above functions during early stages of crystallization.

Various rheo-optical methods have been used to study shear-induced orientation phenomena and structure changes in polymeric solids [21] and fluids [22], [23]. Hashimoto et al. [24], Läuger and Gronski [25] and Schmidt [26] integrated SALS into a rotational rheometer. Their rheo-SALS systems are suited for simultaneous fast measurements of 2D SALS and rheological properties of polymer melts. Higgins and co-workers [27] built a rheo-optical apparatus for investigating the structure of polymeric fluids under Poiseuille flow using small-angle neutron and light scattering. Important features of the design include low sample volumes and high shear-rates at the wall. Pioneering studies of shear-enhanced crystallization of polymer melts at Linz, headed by Janeschitz-Kriegl [12], [13], [14], [15], [16], used inhomogeneous capillary flow with parabolic velocity profile and high shear rates at the wall. Crystallization kinetics and structure development were followed by monitoring the optical retardation and turbidity. The growth of birefringence during and after short-term shear was explained by the formation of linear precursors. The shear-induced molecular orientation was assumed to be the driving force for accelerated crystallization. Due to strong shear rate and temperature gradients (shear and cooling rates were much higher in the wall layer as compared to central layer), the structure distribution in crystallized iPP reveals three distinct regions: skin-oriented layer, intermediate fine-grained layer, and spherulitic core in the center. Kornfield and co-workers [17], [18], [19], [20] extended these experiments (short-term shearing, slit flow, high level of wall shear stress) by combining polarimetry with synchrotron X-ray scattering. They monitored the optical response of supercooled iPP melts at different temperatures upon start-up of flow and found that the time of upturn of the birefringence correlates with the rheological shift factor of the polymer melt.

The main goal of this paper is to compare the time scales of the optical, rheological and morphological changes during quiescent and shear-induced crystallization of iPP starting from the melt. In addition to dynamic mechanical spectroscopy, we report direct microscopical observation of growing thread-like structure in conjunction with time-resolved simultaneous measurements of scattered and transmitted light. A new rheo-optical device was built for this purpose and is described in this paper (Fig. 1).

Section snippets

Materials

iPP homopolymer was chosen for this study since, below its nominal melting temperature of Tm=163°C, it forms large, semi-crystalline structures which are visible in the optical microscope. Quiescent crystallization was performed on a Ziegler–Natta iPP of Fina (Mw=351×103, Mw/Mn=4.0) which had been studied previously [6], [7], and shear-induced crystallization was studied with a very similar iPP (Mw=369×103, Mw/Mn=4.0) supplied by ExxonMobil. Polymer films (thickness 0.4 mm, diameter 20 mm) were

Quiescent crystallization and morphology development of iPP

Fig. 3a–c shows the evolution of storage modulus, loss modulus and tan δ as a function of aTω at increasing crystallization times at Tx=148°C, starting from the melt. The curve with open circles belongs to the mastercurve of the melt, shifted to Tref=148°C. Within the accessible frequency range, G′(aTω) and G″(aTω) in Fig. 3a and b do not reach the typical low frequency asymptotes of slopes 2 and 1. This broad crossover to the terminal zone is the expression of a broad molecular weight

Conclusions

Quiescent crystallization of iPP occurs in the typical spherulitic morphology. All spherulites are nucleated at about the same time and grow at equal rate, which results in a narrow size distribution. The gel point is reached before the spherulites actually touch each other which means that interaction between spherulites is effective through the amorphous phase. The crystallinity (from DSC) at the gel point is still low, however, it is large enough to give a substantial increase in the Qδ

Acknowledgements

We are thankful to Richard Stein and Benjamin Hsiao for stimulating discussions. The support under grant from ExxonMobil (Baytown, TX) and under the MRSEC program at the University of Massachusetts, Amherst, (NSF DMR 9809365) is gratefully acknowledged. HHW thanks the Von Humboldt Foundation for support.

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