Crystalline properties and morphological changes in plastically deformed isotatic polypropylene evaluated by X-ray diffraction and transmission electron microscopy

Abstract

The isotatic polypropylene (i-PP) plastically deformed by uniaxial plane strain compression was investigated using X-ray diffraction and transmission electron microscopy. The apparent crystallinity was evaluated by means of X-ray diffraction with a profile matching using a FULLPROF code. Two crystalline phases, α and β, were identified in the non-deformed polymer as well as with an amorphous halo. The deformation with 3, 10 and 100 MPa, induces an increase of the amorphous halo. The microstructural orientation of the i-PP, before and after deformation was evaluated using the “Quantikov” image analysis of transmission electron microscopy (TEM) data. The non-deformed material presents spherulitic structure, without any preferential orientation. The high resolution, in the nanometer scale, shows two preferential orientation axes. The deformation at 3, 10 and 100 MPa, affects the spherulitic, as well as, the lamellae structure, introducing fiber orientation, a break up of the lamellae in to small blocks and consequently amorphization of the system.

Introduction

There is a great technological interest to a better understanding of the morphology of isotatic polypropylene (i-PP). Although in the last 30 years many studies have been done in this field, there are many questions that are still open from the point of view of basic science.

One of the major interests is to understand and control the processes that induce the morphological changes due to a plastic deformation of semi-crystalline polymer during stress application. The plastic deformation in polymers is a complex process, and in the case of semi-crystalline polymers the high applied pressures can transform completely the initial orientation of the microspherulitic structure into fiber orientation. Several modes of deformation above glass transition have been identified in these materials. The first reports concerning the morphological changes in semi-crystalline polymer, after the stress application were made by Flory and Yoon [1] and Peterlin [2]. Flory proposed that plastic deformation occurs by fusion and re-crystallization of the crystalline phase, while Peterlin added three stages of plastic deformation during the application of stress on the sample. In the first stage the pre-necking of the microspherulitic structure occurs, that is, the plastic deformation of the original spherulitic structure. In the second one, the discontinuous transformation of the spherulitic into fiber structure by micronecking takes place, and in the third one the plastic deformation of the fiber structure occurs. Initially, the deformation occurs by strain of the interlamellar amorphous regions with a combination of interlamellar separation, stack rotation, interlamellar shear deformation and cavitation [2], [3], [4], [5]. Recent studies [6] reported that the initial spherulitic morphology is destroyed and transformed into stacks of crystalline lamellae with their normal rotating towards the load direction, while the chain axis tends towards the flow direction. It was possible to show [7], [8], [9], [10], [11] that several crystallographic mechanisms supported by deformation of amorphous component, like interlamellar shear, are sufficient for a continuous transformation of a semi-crystalline polymer from an initial spherulitic morphology to a final highly textured material.

The structural orientation of a polymer is an important parameter regarding the production of polycrystalline and amorphous materials with specific physical properties. The characterization of the distribution of microstructural orientation of polymeric materials is important for the prediction of the properties of oriented materials and the better understanding of the influence of different parameters in the manufacturing process [12], [13].

Recently, several studies on morphological and density changes [14], [15], [16], [17], [18], [19], [20], [21] using deformation dimensional analysis, densitometry, X-ray diffraction, DSC, SAXS and optical polarizing microscopy techniques, showed that the plastic deformation process of semi-crystalline polymers at temperatures between T_g and T_m produces a system characterized by a non-equilibrium thermodynamical state.

Lima et al. studied i-PP subjected to large deformation using DSC and WAXD. They concluded that the WAXD method provides more accurate crystallinity values for deformed material, than those obtained from DSC method, which does not reproduce the real crystallinity of the deformed material. This happens due to the inherent heating process of the method, which causes a relaxation and a significant change in the crystallinity of the deformed material, providing values nearer to the intrinsic equilibrium state [16].

The polypropylene often presents three different crystalline phases, namely α, β, and γ [22], [23], [49], which have a monoclinic, trigonal and triclinic unitary cells, respectively. The γ-phase is preferentially formed under pressures higher than 200 MPa, when the polymer is totally crystallized. The α species are characterized by the presence of transverse crystallites in addition to the main structure of radial lamellae so-called “cross-hatched” lamellar morphology. The β-phase presents only radial lamellae with two alternatives: a straight lamellae or a sheaf-like structure. This phase was firstly mentioned by Padden and Keith [24], [25], and it is only obtained under specific conditions, as shown in the literature [26], [27]. The β-phase can be formed by adding nucleation agents or due to the cooling rate in the crystallization region or even by orientation in the shear zone [26], [27], [28], [29], [30]. Usually it is found in samples that were submitted to mechanical deformation in extruded or injection molded products. These factors enhance the local crystallization and consequently the nucleation of β-crystallites [26], [27]. In the present work, the β-phase was formed exclusively following the injection parameters; therefore, texture effects are introduced in the sample preparation since the results are obtained without erasing the thermomechanical memory of the material. Thermodynamically the most stable of the three phases is the monoclinic α, as it is documented in the literature [25].

The aim of the present work is the investigation of morphological properties before and after deformation by uniaxial compression using wide angle X-ray diffraction, WAXD, and transmission electronic microscopy, TEM. The orientation of microstructural elements in isotatic polypropylene is also evaluated from a computer analysis of the TEM images before and after deformation using the computer code “Quantikov” [31], [32], [33], [34], [35], [36] developed for microstructural characterization [12], [13], [14], [31], [36], [37], [38]. The relative crystalline variation of each phase (α and β) was determined by “FULLPROF” [39], computer code using a profile matching refinement with multiple phases.