Precursor of shish–kebab in isotactic polystyrene under shear flow

Abstract

We performed polarized optical microscope (POM), depolarized light scattering (DPLS) and small- and wide-angle X-ray scattering measurements on the structure formation process or the crystallization process of isotactic polystyrene (iPS) under shear flow below and above the nominal melting temperature T_m. It was found that an anisotropic oriented structure termed here as a string-like object was formed in μm scale even above the nominal melting temperature and stable for more than 24 h, but melted at around 270 °C far above the nominal melting temperature. The string-like object acts as a nucleation agent for the folded chain lamella crystal (or the kebab), and was assigned to a precursor of the shish–kebab. We also examined the shear rate dependence of the structure formation to find a critical shear rate for the formation of the string-like object, suggesting the relaxation of the chains plays an important role in the formation of the structure. Based on the results we have discussed the inner structure of the string-like object.

Introduction

Extensive studies were carried out on polymer crystallization under flows because polymers crystallize under various kinds of flows during the processes and the properties are dominated by the crystal structure and the final morphology formed during the crystallization process [1], [2], [3]. It is well known that the so-called shish–kebabs are formed during crystallization under flows such as shear, elongation and mixed flows, which are made of an extended chain crystal (shish) in the central core and periodically grown folded chain lamella crystals (kebabs) on the shish in the simplest case [4], [5], [6], [7], [8]. Recent researches have revealed more complex structure as well as formation mechanisms of the shish–kebab [9], [10], [11], [12]. It is believed that the shish–kebab structure is a molecular basis of ultra-high modulus and ultra-high strength fiber [4], [5], [6], [7], [8]. This also promoted extensive studies on polymer crystallization under various kinds of flows. However, the formation mechanism of the shish–kebab is not well understood regardless the considerable efforts. Recent development of advanced characterization techniques such as synchrotron radiation (SR) X-ray scattering, neutron scattering and light scattering has shed light on the substantial nature of the polymer crystallization under flow. Some of these works have focused on the structural formation in the early stage of the crystallization under flow using “short term shearing” technique [13] because it often governs or at least affects the final structure deeply. One of the most important issues in these studies is a precursor of the shish–kebab. In-situ small-angle and wide-angle X-ray scattering (SAXS and WAXS) studies on isotactic polypropylene (iPP) and polyethylene (PE) after pulse shear [14], [15] have shown that a scaffold or network of oriented structures is formed prior to the full-crystallization. In-situ birefringence measurements on iPP after short term shearing by Kornfield et al. [16], [17], [18] suggested formation of a precursor of the shish–kebab structure. The same group [19] also have studied effects of the high molecular weight (HMW) component using model blends of HMW isotactic polypropylene (M_w = 923,000) and low molecular weight (LMW) one (M_w = 186,000), both of which have rather narrow molecular weight distributions, and found that the role of the HMW component in shear-induced crystallization is cooperative, enhanced by the entanglements among the long chains. These studies as well as other ones [20], [21], [22], [23] have demonstrated that a precursor of the shish is formed in the very early stage during the crystallization process after the shear. Furthermore, other works reported [13], [23], [24], [25], [26] that similar precursors were observed even above the nominal melting temperatures. Somani et al. have shown in SAXS measurements on iPP [14] that shear-induced oriented structures or aggregates of polymer molecules were developed even above the nominal melting temperature. Alfonso et al. [24], [25] produced a precursor of the shish of isotactic polystyrene (iPS) in the melt at 260 °C above the nominal melting temperature by pulling a single fiber through a thin layer melt. Fiber pulling experiments were also done on poly(1-butene) melt to estimate the lifetime of the precursor [26]. Similar precursors above the nominal melting temperature were also reported for a blend of ultra-high molecular weight polyethylene (PE) and low molecular weight PE and for some polymers even near the equilibrium melting temperature [27], [28].

Regardless the considerable efforts we have not had a final picture of the precursor. In some works the precursor was observed as streak-like scattering normal to the flow direction by SAXS [9], [14], showing that the precursor is in 10 nm scale. On the other hand, in other works [16], [17], [18], [22] the precursor was observed by LS and POM, suggesting that the precursor has μm scale structure. In our recent SANS measurements [12] we found that a very large oriented structure in μm scale including a small shish (extended chain crystal) with ∼10 nm diameter. This result implies that the oriented structure in μm scale works as a precursor of shish in nm scale. It is evident that the structure study in a wide length scale of nm to μm is absolutely important to elucidate the precursor. Another unsolved problem is if the precursor includes crystals or not, or in which experimental condition the precursor includes crystal and in which not. As mentioned above, in the experiment by Somani et al. [14] non-crystallized shish-like structure has been observed in iPP above the nominal melting temperature by SAXS. However, a recent work by Balzano et al. [29] have shown by SAXS and WAXS measurements that PE blend of low and high molecular weight components formed needle-like precursors in 10 nm scale with limited crystallinity even above the equilibrium melting temperature. It is a good experiment to see the invisible oriented structure (or precursor) above T_m by quenching below T_m to crystallize [24], [25], [26], but it gives no direct conclusions on the problem whether the precursor above T_m includes crystals or not. WAXS and SAXS measurements at high temperatures above T_m (not after quenching) are necessary to solve the problem.

In a previous paper [30], we studied structure formation process of isotactic polystyrene (iPS) after applying a pulse shear above the nominal melting temperature T_m using depolarized light scattering (DPLS) and polarized optical microscope (POM) and found that oriented objects were formed in μm scale during an annealing process above T_m, which could survive more than several hours. This was assigned to a precursor of the shish–kebab. The number of precursors was very few in the previous shear conditions, depending on the annealing temperature, and hence often no oriented objects were observed in the illuminating volume in the DPLS measurements. In addition, the POM measurements revealed that the oriented objects were very widely distributed in the shape and size. In such non-frequent events with the wide distributions POM observation is more appropriate than DPLS one because the former has an advantage to see individual structure while the latter sees the average one. In order to see the characteristics of the oriented objects in more details we therefore performed mainly POM measurements on the structure formation process of iPS after applying a pulse shear for various temperature and shear conditions. In addition, wide- and small-angle X-ray scattering (WAXS and SAXS) measurements were done to see the inner structure of the oriented objects above the nominal melting temperature T_m.