Article pubs.acs.org/Macromolecules
Flow and Pressure Jointly Induced Ultrahigh Melting Temperature Spherulites with Oriented Thick Lamellae in Isotactic Polypropylene Shu-Gui Yang,† Zhengchi Zhang,† Dong Zhou,† Yan Wang,‡ Jun Lei,*,† Liangbin Li,§ and Zhong-Ming Li*,† †
College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China ‡ School of Textile and Material Engineering, Dalian Polytechnic University, Dalian 116034, China § National Synchrotron Radiation Lab and College of Nuclear Science and Technology, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei 230026, China S Supporting Information *
ABSTRACT: In practical processing, polymer melts generally experience flow and pressure fields simultaneously, but their flow-induced crystallization behavior under pressure was barely investigated. For this reason, we provided an insight into the crystallization behavior and crystalline morphology and structure of isotactic polypropylene (iPP) obtained under the combination of flow (2.5−22.5 s−1) and high pressure (200 MPa) by using self-designed pressurizing and shearing device. Unprecedented iPP spherulites were observed, which are composed of oriented thick lamellae (18 nm) with ultrahigh melting temperature (179.5 °C). Such spherulitic crystals with lamellae aligning perpendicular to flow have never been reported. All samples have a double melting peak behavior including an additional low temperature peak (165−169 °C) apart from the ultrahigh melting peak. The in situ synchrotron X-ray measurements show that the parents of cross-hatched structure are responsible for the ultrahigh melting temperature and the daughters give rise to the low temperature melting peak because the parents crystallized much earlier and had a higher growth rate than the daughters. Moreover, the parents are all in α-form while the daughters consist of α- and γ-forms. Our results afford a new method to obtain thick lamellae in a relatively short time.
1. INTRODUCTION Considerable work has been devoted to flow-induced polymer crystallization in the past half century because processing of semicrystalline polymers always involves varied flow (elongation, shear, or mixed) fields. Some crucial findings behind flowinduced polymer crystallization are uncovered; for instance, external flow fields not only accelerate crystallization kinetics but also strongly affect the morphologies of semicrystalline polymers and thus the resultant physical properties.1−3 However, the available theories are established largely based on the experiments under normal pressure (atmospheric pressure). As a matter of fact, many processing methods, such as the most representative one, injection molding, are invariably coupled with a pressure of tens of MPa apart from flow field during mold-filling and subsequent freezing the melt. Therefore, it is of practical importance to reveal the effect of flow fields on structure and morphology of semicrystalline polymers under pressure (above room pressure). But until now, very limited work has been focused on crystallization of polymers under the coexistence of external flow and pressure, especially moderate and high pressure (e.g., above 150 MPa), due to the grand challenges, such as in situ © XXXX American Chemical Society
detecting structure, implementation of shearing, dynamic sealing, etc. Recently, Peters et al.4−6 employed a customdesigned dilatometer (PVT) to perform distinctive work in which isotactic polypropylene (iPP) was crystallized under pressure (≤120 MPa) after application of strong shear flow. The oriented specimens with typical shish-kebab structure that templates densely branched γ-lamellae were thus created. In another experiment, shear-induced precursors were effectively enhanced by pressure quenching in the polyethylene with a bimodal molecular weight distribution. The results obtained by Peters’ group show the crystallization behavior of iPP under pressure and shear flow is interesting and quite different from either flow-induced crystallization under normal pressure or quiescent crystallization under high pressure. Moreover, the crystallization scenario under pressure and shear flow is close to practical processing. For these reasons, we are inspired to carry out further research on pressure and flow jointly induced Received: May 20, 2015 Revised: July 18, 2015
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DOI: 10.1021/acs.macromol.5b01043 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. Schematic of PSD, radial distribution of shear rate in specimen, and schematic diagram of WAXD and SEM measurements.
crystal are almost aligned in the direction perpendicular to the shear direction. These interesting and significant results were undoubtedly owing to the joint effect of flow and pressure.
crystallization of semicrystalline polymers (iPP in the current work). We deliberately designed and manufactured a set of pressurizing and shearing device (PSD) equipped with welldesigned dynamic sealing structure, which successfully solved the problem of melt leaking (see Figure S1 in Supporting Information). Thus, we are able to carry out a series of experiments to study the mutual effect of pressure (≥150 MPa) and shear flow on the crystalline structure and morphology of semicrystalline polymers. The shear element attached to PSD has a similarity to the torsional disk reported by Mykhaylyks et al.7 It has an advantage of probing different shear rates in a single disk sample with shear rate increasing linearly with radius (see Figure 1). With the aid of PSD, we chose iPP as model polymer to study because iPP is one of the most widely used commercial semicrystalline polymers and exhibits pronounced polymorphisms, at least four crystal forms including monoclinic α-phase, hexagonal β-phase, orthorhombic γ-phase, and smectic phase.8,9 It is well established that pressure is capable of decreasing the specific volume and accelerating the nucleation of iPP. The influence of pressure on the crystallization kinetics is ascribed to the shift of degree of supercooling, which results from the increase of equilibrium melting temperature (T0m) along with pressure, wherein the pressure dependence of equilibrium melting temperature shows about 0.3 °C/MPa for iPP.10 Besides, both the crystalline structure and morphology also rely on the pressure. Increase in crystallization pressure could lead to a transformation of crystal form from α- to γ-iPP. It was reported that the proportion of γ-phase increases from zero at normal pressure to nearly 100% at 200 MPa.11 Compared with crystallization pressure, flow field can induce more complex crystallization behavior for iPP. On the basis of the thermo-optical observation of melt shearing of iPP via fiber pulling, Varga and Karger-Kocsis concluded that the surface of the in situ formed α-row nuclei may induce the nucleation of βphase, resulting in a supermolecular structure of cylindrical symmetry.12 Despite a lot of researches on iPP crystallization under separated pressure or flow field, the flow-induced crystallization process of iPP under pressure still remains poorly understood, especially at elevated pressures. Given this, we investigated the morphology, structure, and melting behavior of iPP crystals induced jointly by shear flow and high pressure (200 MPa) in the present work. Results show that unprecedented iPP spherulites that are composed of oriented thick lamellae (18 nm) with ultrahigh melting temperature (179.5 °C) were obtained. It has never been reported to generate crystallites with such high melting point within so short time (
