Effect of Initial Polypropylene Structure on Its Deformation Via Crazing

Contents lists available at ScienceDirect EuropeanEuropean Polymer Polymer Journal 100 (2018) Journal 233–240 journal homepage: www.elsevier.com/locate/europolj Effect of initial polypropylene structure on its deformation via crazing mechanism in a liquid medium ⁎ Alena Yu. Yaryshevaa, , Dmitry V. Bagrovb, Artem V. Bakirovc,d, Larisa M. Yaryshevaa, Sergey N. Chvalunc,d, Aleksandr L. Volynskiia T a Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1-3, 119991 Moscow, Russia b Department of Biology, Lomonosov Moscow State University, Leninskie Gory 1-12, 119991 Moscow, Russia c National Research Center Kurchatov Institute, Akademika Kurchatova pl. 1, 123182 Moscow, Russia d Institute of Synthetic Polymer Materials RAS, Profsoyuznaya st. 70, 117393 Moscow, Russia ARTICLE INFO ABSTRACT Keywords: Atomic force microscopy has been employed to study the structure of isotactic polypropylene (PP) deformed in a Semicrystalline polymers physically active liquid medium (PALM) by the crazing mechanism. The investigations have been performed Crazing directly in the liquid, in which deformation is carried out, and under conditions excluding PP contraction. In Atomic force microscopy order to study the effect of the initial PP structure on the crazing mechanism, the polymer structure was varied Polymer structure by annealing. The crazing mechanism, as well as the parameters and morphology of the polymer, has been Polymer deformation investigated at different tensile strain values as depending on the initial PP structure. It has been shown that the Polypropylene development of deformation via the classical or intercrystallite crazing mechanisms is predetermined by the degree of crystallinity and the thickness of lamellae in initial PP. Before annealing, PP is deformed via the classical mechanism, i.e. as a uniform one-phase material, with the formation of crazes and zones of the bulk polymer located between them. Because the lamellae are «weak», the lamellar structure is transformed into a fibrillar structure, with porosity developing in crazes between fibrillar strands. After annealing of PP, the thickness of lamellae, degree of crystallinity, and, accordingly, the drag of crystallites increase, cavitations arise in the amorphous phase, and the fibrillar-porous structure develops in the space between crystallites moving apart from each other. 1. Introduction The theory of the joint action of a medium and stress on the formation of crazes and cracks in polymers had, initially, been developed In view of the diversity and complexity of the structures of crys- for glassy polymers and was, then, used to advantage for crystalline talline polymers, many models and mechanisms of their deformation polymers. In [12,13], a mechanism was proposed that involved stress- have been considered in the literature (reviews [1–7]). The deformation induced swelling and relevant plasticization of polymer amorphous of crystalline polymers is accompanied by various processes, such as the phase. The authors of [14] established the key role of the closeness separation, slip, bending, reorientation, fragmentation, microfibrilla- between the solubility parameters of a polymer and a liquid: the higher tion, local melting, martensitic transformations, stress-induced crys- the affinity of a liquid for a polymer the higher the stress-induced tallization of lamellae, development of cavitations, etc. swelling and plasticization of the polymer. Stretching of polymers in physically active liquid media (PALMs) by Liquids that do not cause swelling of polymers may also efficiently the crazing mechanism is a peculiar type of deformation. It is char- affect their crazing. The influence of such media is associated with the acterized by the development of nanosized porosity in a polymer adsorption-induced decrease in the polymer strength (Rehbinder’s ef- [8–11]. The formed nanopores are filled with the liquid medium, in fect). Crazing is accompanied by the total development of an interfacial which the stretching is performed. Morphologically, such porosity is surface in a polymer due to its dispersion into fibrils – nanosized ag- realized via the nucleation and growth of peculiar zones of a plastically gregates separated by voids. Since the specific surface area of a crazed deformed polymer. These zones (crazes) have a fibrillar-porous struc- polymer amounts to several hundred square meters per gram [10], ture composed of oriented polymer fibrils 5–20 nm in diameter sepa- adsorption of liquid molecules on the surface of such a polymer may rated by microvoids of nearly the same sizes. dramatically reduce the interfacial surface energy. ⁎ Corresponding author. E-mail address: [email protected] (A.Y. Yarysheva). https://doi.org/10.1016/j.eurpolymj.2018.01.040 Received 3 November 2017; Received in revised form 16 January 2018; Accepted 30 January 2018 Available online 01 February 2018 0014-3057/ © 2018 Elsevier Ltd. All rights reserved. A.Y. Yarysheva et al. European Polymer Journal 100 (2018) 233–240 Fig. 1. SEM micrographs taken from cleavages of polypropylene deformed by 100% in (a) ethanol via the classical and (b) heptane via the intercrystallite crazing mechanisms [16]. Panel (a): (1) craze boundary, (2) bulk polymer, and (3) craze with fibrils. The stretching axis is denoted by arrow. At present, two types of crazing are considered, which are realized in the medium used for crazing. upon deformation of polymers in PALMs, namely, the classical crazing Isotactic polypropylene (PP) is of special interest for the study and (accompanying stretching of glassy and some crystalline polymers) comparison of the two types of crazing, because this polymer can be [8–10] and the intercrystallite or delocalized crazing (observed only for deformed via both the classical and intercrystallite crazing mechanisms crystalline polymers) [10,15]. The main difference between these two depending on the nature of a liquid medium (Fig. 1). However, there is types of crazing consists in the sizes of local zones of deformation. For one more possibility of the passage from the classical to the inter- polymers deformed by the classical crazing mechanism, the craze width crystallite crazing, namely, the modification of an initial PP structure may be as large as several micron. In this case, crazes develop due to the by annealing followed by deformation in a liquid medium in which the gradual consumption of a nonoriented bulk polymer, which con- polymer is unswellable. For example, before annealing PP is deformed tinuously passes into a highly dispersed oriented state in crazes. The in aliphatic alcohols via the classical crazing mechanisms, and after fibrillar-porous structure is thermodynamically unstable, and, after a annealing PP is deformed in aliphatic alcohols via the intercrystallite medium is removed from the pores and the stress is eliminated, coa- crazing mechanisms. The goal of this work was the visualization of the gulation of fibrils, contraction of a sample, and collapse (healing) of the structures of initial and annealed PP, as well as the structures the porous structure occur. polymer crazed in PALMs before and after annealing. Thus, we in- In the case of classical crazing, the large width of crazes and the vestigated the influence of the initial PP structure on the mechanism of presence of undeformed polymer zones enable one to observe the re- its crazing, i.e., the classical or intercrystallite one. sidual fibrillar-porous structure of crazes with the help of scanning electron microscopy (SEM) even after removal of the medium (Fig. 1a). 2. Materials and methods However, the «native» structure, i.e., the structure formed directly during the classical crazing of a polymer in a liquid medium, has not yet 2.1. Materials been visualized and studied. The case of the intercrystallite crazing is still more complex for Isotactic PP films with a thickness of 90 µm (Mw 250 kDa, Mw/ observation of a polymer structure. Previous attempts to observe in- Mn = 3.5) produced by melt extrusion and the same films annealed dividual crazes with the use of SEM failed because of the collapse of the under laboratory conditions at 140 °C for 180 min were used in the porous structure after stress elimination and removal of a liquid experiments. Ethanol (85%) was applied as a PALM. medium (Fig. 1b) [16]. The majority of the data on the structure of crystalline polyolefins 2.2. Procedure for investigating the structure of PP deformed in the medium deformed in liquid media were previously obtained by the small-angle X-ray scattering (SAXS) and pressure-driven liquid permeability Variations in the structure of polymers being stretched is, as a rule, methods, which made it possible to estimate the effective diameters of studied by AFM using a microscope installed directly on a tensile- pores, the specific surface areas and diameters of fibrils, and the long testing machine. The scanning is performed with a cantilever actually periods for diverse polymers and liquid media [16]. However, the free of limitations on sample sizes and masses. The measurements with calculation of the structural parameters by the aforementioned methods the use this system, which combines an atomic force microscope and a requires the use of model concepts of the fibrillar-porous structure. stretching unit, are usually carried out in the following way: the load on Thus, although the notions of the structure of crazed polymers were a film being stretched is increased stepwise, and an AFM image is taken formulated rather long ago, the visualization of

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