Initiation of cavitation upon drawing of pre-oriented polypropylene film: In situ SAXS and WAXD studies

Highlights

• Initiation of cavitation upon drawing of pre-oriented polypropylene was studied.

• Competition relation between cavitation and crystal shear was observed.

• Cavitation initiated at the anchor point of daughter lamellae was proposed.

Abstract

Initiation of cavitation upon drawing of pre-oriented polypropylene films has been investigated via in-situ small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) techniques. The macroscopic strains for the initiation of the cavitation ε_cav and for the initiation of crystal shear ε_cs have been estimated from in-situ SAXS and WAXD tests respectively. Local crystal shear appears before cavitation upon drawing in machine direction (MD) has been revealed via comparison of ε_cs and ε_cav. The initial size of the cavities in MD is larger than the long period of crystalline lamellae while the size in transverse direction is inversely proportional to the relative fraction of daughter lamellae. Macroscopic critical stresses for the initiation of cavitation σ_cav and for the initiation of crystal shear σ_cs have been estimated as well. The critical stress σ_cav is independent on the crystallinity, whereas σ_cs increases with increasing crystallinity. The mechanism for the initiation of cavitation has been proposed, i.e. the cavitation is initiated by the breakage of crystalline lamellae at the anchor point of the daughter lamellae.

Introduction

Semi-crystalline polymers consisting of crystalline lamellae and amorphous phase generally display a complex structural hierarchy from meso-scale to macroscopic level [1], [2]. Generally, plastic deformation of semi-crystalline polymers involves crystal shear and cavitation. It seems that these processes can be activated concomitantly or competitively under tensile loads depending on material structures and experimental conditions [1], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17].

Historically, cavitation during the tensile drawing of semi-crystalline polymers has been treated as a marginal effect, i.e. accompanying plastic deformation of polymer crystals. Butler et al. [3], [4], [5] studied plastic deformation of tensile drawn high density polyethylene and found the cavitation in polyethylene was formed at yield point when martensitic transformation and chain slips begins. Afterwards, Galeski et al. [7] proposed a competition mechanism between the cavitation in inter-lamellar amorphous layers and the crystal shear. This competition relation has been further confirmed in a recent paper [8] via comparing the onset strain of local crystal shear and cavitation during the tensile drawing of polyethylene by in-situ wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) experiments. Alternative understanding has been proposed by Men et al. [18], [19] where investigations on polybutene-1 revealed that cavities initiated first in the crystalline phase as a result of breakage of crystalline skeleton during tensile deformation.

Currently, cavitation in semi-crystalline polymers has been created for many functional applications. Polypropylene (PP) is a semi-crystalline polymer commonly used to produce porous membranes for battery separators [20], [21], [22]. The polypropylene resins are melt-extruded into a uniaxial oriented film which has a crystalline row structure with their long axis perpendicular to the machine direction (MD). Then the film is stretched in MD to create the pore structure at low temperature. So that understanding the ‘open’ mechanism for pre-oriented PP film is important to control the properties of the final products of micro-membranes.

In recent studies, Lin et al. [23] and Lei et al. [24] investigated the structural and processing parameters of pre-oriented film for the micro-pore formation. They found that the orientation degree and the thickness of crystalline and amorphous layers are crucial parameters for the formation of micro-pores. On the other hand, daughter lamellae were found to play important role on the formation and stabilization of pores upon drawing of pre-oriented polypropylene films. Chen and Lei et al. [25] reported that daughter lamellae were destroyed around yield point before parent crystals and then transferred to the connecting bridge between parent lamellae which can stabilize the cavities during deformation. Besides, annealing of the casted films benefits the formation of pores [26], [27], [28], [29]. It was interpreted as that the increase of the crystalline lamellar thickness promotes a better separation of crystalline lamellae. So that, it seems the thickness of the crystalline lamellae is crucial to the pore formation. However, little attention has been paid to the underlying physical mechanism and also the relationship between cavitation and crystal shear upon drawing of pre-oriented polypropylene films.

The initiation of cavitation has been investigated in this work in order to reveal the mechanism of pore formation in pre-oriented PP films. In-situ SAXS was performed to probe the variation of cavities upon tensile drawing. This technique has been largely used to study the strain induced cavitation in semi-crystalline polymers owing to its ability of capturing the very first events of cavitation [6], [19], [30]. It also allows correlating microscopic structure variation to the macroscopic stress and strain. The onset of crystal shear has been studied as well via in-situ WAXD. In-situ WAXD provides the variation of the crystalline structure upon drawing [8], [31]. The initiation of crystal shear and cavitation upon drawing of the pre-oriented PP films has been compared to disclose the relationship between them.