Finite strain response, microstructural evolution and β→α phase transformation of crystalline isotactic polypropylene

Abstract

An experimental study of the finite strain response of annealed α and β crystalline isotactic polypropylene (iPP) was conducted over a range of temperatures (25, 75, 110 and 135 °C) using uniaxial compression tests. Uniaxial compression results indicate nearly identical macroscopic stress vs. strain behavior for α-iPP and for β-iPP to true strains in excess of −1.1 at room temperature despite the different initial morphologies. At larger compressive strains (>1.2), β-iPP shows more rapid strain hardening. The orientation of crystalline planes during straining differs at room temperature from that at high temperature, indicating a change of slip mechanisms as temperature increases. In addition, strain-induced crystallization occurred at the highest temperature examined in α-iPP. A continuous transformation of β crystals to α crystals with inelastic deformation at room temperature was observed and it was facilitated at higher deformation temperatures. Scanning electron microscopy (SEM) observations of deformed β-iPP provide strong evidence that the transformation is achieved via a solid-to-solid mechanism despite the different helical hands in α and β crystal structures. Molecular simulations were used to investigate a conformational defect in the 3₁ helical chains of β-iPP, characterized by a 120° helical jump. The propagation of this conformational defect along molecular chains provides the reversal of helical hand required by the solid-to-solid transformation. The β→α phase transformation in iPP is proposed to be accomplished via a solid transformation that includes slip along β(110) and β(120) planes during shear of the crystal lattice.

Introduction

Isotactic polypropylene (iPP) is a semi-crystalline polymer widely used in industrial and commercial applications. Many aspects of the morphology and mechanical properties of iPP and their relationships have been determined. iPP is polymorphic with three known crystalline phases, monoclinic α, trigonal β and triclinic γ. The iPP chains in the lattices of these three crystalline forms are 3₁ helices with either a right or left helical hand. The setting angles of the helical hand relative to the a and b crystallographic axes (the azimuthal orientation) also vary for different crystal morphologies. The detailed morphological differences between α and β crystal structures are of critical concern in studies of the deformation mechanisms and the β→α phase transformation. For this reason these two morphologies are reviewed.

The well known α phase is the most stable iPP crystal structure with a density of 0.936 g/cm³ and melting point of 165 °C [1]. The crystalline structure of α-iPP was first established by Natta and Corridini [2] as monoclinic with lattice constants a=6.65 Å, b=20.56 Å, c=6.5 Å and β=99.6°. The α crystal structure is characterized by a change in the helical hand and azimuthal orientations of the molecular chains on successive (040) planes. The β phase was first observed by Keith et al. [3]. It is less prevalent and its crystalline structure had been an enigma for more than 35 years partly due to the instability of the β phase when drawn, thus rendering it very difficult to obtain β phase fibers for crystalline structural investigation [4]. This structure was determined to have a trigonal unit cell with lattice constants a=b=11.01 Å, c=6.5 Å and α=90°, β=90°, γ=60°, independently by Meille et al. [5] and Lotz et al. [6] in 1994.¹ In contrast to the alternating helical hands in α crystals, all of the chains in a β crystal lattice have the same helical hand, either all left or all right, while the azimuthal orientation of the corner chains differs by ∼180° from that of the center chains. The β phase has a density of 0.921 g/cm³ and a melting point of 155 °C [1]. A significant difference between the α and β crystalline structures is the existence of lamellar branches in α spherulites. The branching lamellae manifest themselves under all crystallization conditions [6], [7], which becomes an intrinsic property of the α iPP melt-crystallized form. Being thermodynamically less stable than the α phase, the β phase can only be formed under specific conditions [8], [9], [10], [11], [12], [13], [14]. At all temperature ranges, the α phase is nucleated more profusely than β phase while the β phase can be crystallized in the presence of β nucleation agents that can yield β phase percentages as high as 90% [15], [16].

The mechanical response, deformation mechanisms and microstructural evolution of α-iPP and β-iPP during inelastic deformation have been subjects of recent studies. Hirsch and Wang [17] investigated the texture evolution during compression of α-iPP with 62% initial crystallinity at high temperatures (135 and 155 °C) and three strain rates (0.1, 1 and 10 s⁻¹) using X-ray diffraction analysis. They found that at both temperatures, the crystallographic planes are oriented by the alignment of the c axis normal to the compression axis. This orientation can be achieved by a deformation mechanism in which glide occurs on planes normal to the b-axis. They also found strong dependence of texturing on temperature, which indicates that at higher temperature a more selective (planar) glide mechanism exists with a preferred glide plane normal to the crystallographic b-axis. Hirsch and Wang thus determined that glide on planes normal to the b-axis occurs preferentially at 155 °C but not at 135 °C. Pluta et al. [18] studied the morphology and the development of texture in α-iPP subjected to plane-strain compression. DSC and density measurements both indicate a drop in crystallinity as compression ratio increases. The texture orientation of β iPP has not been observed in the literature because it is not stable during inelastic deformation. Transformation to the α phase or the mesomorphic phase may happen during inelastic deformation of β-iPP [19].

It has been observed that a β→α phase transformation occurs during the plastic deformation of β-iPP. Li and Cheung [20] investigated the crystallinity changes of β nucleated iPP (with initial crystallinities of 36% β and 23% α) using WAXS and DSC over the necking region during tensile tests. They observed a steady and significant phase transformation from the β-phase to α-phase after yielding. Also, they recorded a decrease in overall crystal content with inelastic deformation. Deformation in tension is not uniform, however and possible dilatation due to cavitation can obscure the β→α transformation mechanism. Karger-Kocsis [21] attributed the increase of the toughness of β-iPP to the mechanical stress-induced phase transformation from a less dense β phase to a denser α phase crystalline structure. The exothermic characteristic of this transformation also contributes to the increase in toughness since crack tip blunting and subsequent yielding are favored.

The β→α transformation mechanism is not well understood but a melting and recrystallization theory dominates the literature without any clear physical evidence. Stress-induced polymeric phase transformations can be divided into two categories: solid martensitic-like phase transformations and partial melting-recrystallization processes [22]. The martensitic-like mechanism has been used to describe those cases in which the molecular chains of the initial and final crystalline phases have identical conformations, such as the orthorhombic to monoclinic phase transformation of polyethylene (PE), or when the helical hands of molecular chains are preserved, such as the phase II to phase I transformation of isotactic poly(1-butene) (PBu). It has long been suggested that the reversal of helical hand is indeed an impossible molecular event [22]. The β→α phase transformation of iPP has been assumed to involve melting and recrystallization because of the different arrangements of the chain helical hands in α and β crystals. It is believed that since a solid-to-solid transformation has to involve a ‘chain rewinding’ process, it would not be possible [11], [23], [24]. The present work hypothesizes, however that the presence of conformational defects in molecular chains readily reverses the helical hands of iPP chains.

The helical hand reversal accomplished via conformational defects during the phase transformation of polytetrafluoroethylene (PTFE) has been widely studied [25], [26], [27], [28]. PTFE is in Phase II below 19 °C and at atmospheric pressure. The unit cell is triclinic and comprised of 54/25 helical chains with opposite hands. Phase transformation from phase II to phase IV occurs when the temperature rises to between 19 and 30 °C. Phase IV exhibits a hexagonal unit cell which is comprised of 15/7 helical chains with identical helical hand. Partial melting is not possible at this transformation temperature. Helical reversals via the motion of conformational defects (helical reversal points) were proposed and thought to be energetically feasible as indicated by modeling results [25], [29]. Stress-induced α↔β phase transformation of poly(butylene terephthalate) (PBT) was studied by Dobrovolnay-Marand et al. and found to be accomplished via the change of molecular conformation [30]. In α form, the plane of benzene rings is inclined about 19° to the c axis of the unit cell, whereas it is nearly parallel to the c axis in β form. The rotation of aliphatic groups about the benzene–carbonyl bonds was shown to be feasible for accomplishing the phase transformation.

A 120° helical jump conformational defect for iPP has been proposed in the literature [31], [32], [33]. Schaefer et al. [31] have reported 2D solid-state NMR studies on α-iPP and presented evidence of a 120° helical jump, wherein the 3₁ helices appear to execute a 120° rotation about their axes. Rutledge [32] analyzed this 120° helical jump accompanied by a c/3 translation using quasi-static methods of molecular modeling and examined the mechanisms for describing the chain jump process in terms of energetic stability and activation barrier. The energy analysis reveals that the excess conformational energy (above that of a perfect iPP 3₁) is minimal when the length of the conformational defect dispirations is 6 repeat units (monomer units). The role of this defect in accommodating the helical hand changes required of a solid-to-solid β→α phase transformation is developed herein.

SEM has been widely used in the literature to study the spherulitic morphology of iPP. Etched α and β iPP crystals are clearly differentiated using scanning electron microscopy (SEM) because of their different lamellar structures [23], [34]. The α spherulites are characterized by an interlocking structure in which secondary lamellae grow during crystallization. This interlocking structure renders the α spherulites almost impenetrable to etchants used to remove the amorphous parts of the specimens for SEM analysis. The β crystals form sheaf-like lamellae clusters, or immature spherulites [35] without secondary lamellae and are easily attacked by etchants. The etched α crystals have a relatively smooth surface whereas β crystals have a rough surface. This difference results in melt-crystallized α crystals having a dark contrast in SEM secondary electron images when the electron beam is perpendicular to the sample surface [23]. The well etched β-iPP crystals appear bright under these SEM conditions. The present study tests the hypothesis that α crystals formed via the β→α transition arising from a shear transformation rather than melting-recrystallization, are therefore devoid of secondary lamellae, and present a well-etched, bright contrast in SEM.