Deformation-induced highly oriented and stable mesomorphic phase in quenched isotactic polypropylene

Abstract

Changes of structure and morphology of quenched isotactic polypropylene (i-PP) film during tensile deformation at room temperature have been investigated by applying in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. SAXS patterns show that the isotropic diffuse scattering peak in the initial quenched i-PP film disappears after the film experiences necking and a strong streak, which accompanies obvious film stress-whitening, appears when strain reaches 600% and above. Explanation to the former is the dropping of density contrast between amorphous and mesomorphic phases due to deformation and that to the latter is the formation of microfibrils and/or microvoids in the film. WAXD patterns indicate that i-PP chains in the mesomorphic phase become highly oriented along the drawing direction with the strain value more than 5% and the three-fold helical chain conformation keeps in the deformed mesomorphic phase. The mesomorphic structure in the initial quenched i-PP film does not transform into crystal phase although high orientation appears during tensile deformation and the highly oriented mesomorphic phase is considered to be quite stable. Thermal behaviors of the initial quenched i-PP and deformed i-PP with the strain value of 650% are examined by modulated differential scanning calorimetry (MDSC). MDSC results show that the highly oriented i-PP chains in deformed mesomorphic phase are stable, which brings about restriction to chain mobility and prohibits from phase transformation of mesophase to crystal phase in the temperature range normally measurable for the initial quenched i-PP. A model about tensile deformation of the quenched i-PP is outlined to distinctly illuminate the changes of structure and morphology in both the amorphous and mesomorphic phases.

Introduction

Mesophase of polymers, especially induced in isotactic polypropylene (i-PP), has attracted extensive attention. Mesomorphic phase which has an intermediate order between the amorphous and crystalline phases has been denoted with different names in the literature: smectic [1], [2], paracrystal [6], [7], microcrystal [8], [9], glass [10], condis crystal [11] or mesomorphic phase [12]. Here we would like to use the terms “mesomorphic phase” or “mesophase” in this article. In general, two methods can be used to generate mesophase. Quenching the molten polymer at a drastic cooling rate is an effective way to obtain mesophase which can be considered as a “frozen” intermediate ordering state during crystallization pathway [11]. It is a consequence of severe solidification condition which prematurely hinders molecular motions necessary for crystallization. Up to now, a great deal of work has been dedicated to this historic mesophase, namely “quench-induced mesophase”, in quenched i-PP [1], [2], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], ethylene/vinyl alcohol copolymers [20], and syndiotactic polypropylene (s-PP) [21], [22]. The other kind of mesophase, namely “deformation-induced mesophase”, can be obtained by processing, commonly, cold-drawing. It is well established that uniaxial tensile test [3], [4], [5] is one of the most important experimental methods to characterize the relationship between structure and property, and tensile deformation can generally induce structural evolution of polymers during drawing process [20], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. In some cases, both amorphous and crystalline phases can transform into mesophase during tensile deformation at low temperature. Recently, cold-drawing induced mesophases in amorphous [23], [24] or slush [25] poly(ethylene terephthalate), amorphous poly(ethylene naphthalate) [26], amorphous poly(diethylsiloxane) [27], [28], crystallized ethylene/vinyl alcohol copolymers [20], crystallized i-PP [29], [30], [31], and blends of i-PP and ethylene/propylene copolymer [32] have been found and studied in detail.

i-PP is one of the mostly investigated polymers due to its academic interest and industrial application. i-PP chains can organize into different spatial arrangements giving rise to three basic crystalline polymorphs: α-monoclinic, β-hexagonal, and γ-triclinic phases [2], [33], [34], [35]. The more interesting finding is that i-PP chains are prone to organize into an arrangement of the mesophase [1], [2], [6], [7], which has an intermediate order between amorphous and crystalline phases. Since Natta et al. reported smectic mesophase of i-PP in 1959 [1], the morphology and property of mesomorphic i-PP have been broadly investigated by using X-ray diffraction, infrared and Raman spectroscopy, electron microscopy, electron diffraction and differential scanning calorimetry (DSC). However, the nature of mesophase is not fully understood yet. It has been widely accepted that the partially ordered mesophase of i-PP is made up of bundles of parallel chains [1], [2], which maintain the same three-fold helical conformation just as in the stable α-monoclinic form according to the results of infrared spectra [1], [2], [6], [10], and the long range ordering maintains only along the chain axes, whereas large amounts of disorder in lateral packing are present in small mesomorphic aggregates [1], [2]. Corradini et al. [12] compared several models (corresponding to β-hexagonal and α-monoclinic forms, as well as model showing characters of both forms) with the measured WAXD patterns of quenched mesomorphic i-PP and found that the local correlations of chains in quenched i-PP were close to those in α-monoclinic crystals.

Tensile deformation of i-PP has also been investigated extensively because processing conditions strongly affect the morphology and property of i-PP products. Understanding structural deformation during processing is essential for quantitative prediction of product performance. Ran et al. [29], [30], [32] studied the structure and development of mesophase induced by cold-drawing of crystallized i-PP by synchrotron SAXS and WAXD techniques and found that the mesophase did not show any obvious long range ordering. Stockfleth et al. [36] studied deformation mechanism of oriented crystallized i-PP and PET films under uniaxial stretching by using SAXS, WAXD and birefringence measurements at various temperatures. They interpreted the structural changes by using a lamellar stack model and pointed out the contribution of lamellar separation or lamellar slip depended on strain, temperature and orientation of the stack. AboEIMaaty et al. [37] investigated development of defects in drawn i-PP fibers with an original paracrystalline structure with increasing drawing ratio by using electron microscopy and suggested a simple model to explain the defect formation. Russo and Vittoria [38] determined the intrinsic birefringence of smectic phase during uniaxial stretching and suggested that drawing at room temperature did not allow transformation from smectic phase to crystalline phase. Dasari et al. [39] investigated microstructure evolution of crystallized i-PP from initial surface deformation band to spaced crazes and their inward growth until final fracture as functions of strain and strain rate under uniaxial stretching by using TEM and AFM. Song et al. investigated tensile deformation behaviors of quenched i-PP [40], [41], [42] and annealed i-PP [43] by using microscopic infrared dichroism. They studied molecular orientation, disclosed pseudo-affine deformation behavior for the amorphous phase and proposed an interpenetrating network (IPN) model for interpreting the stress–strain relationship of the mesoscale area suffering from necking. Liu et al. [44] studied the yielding behavior of i-PP by using optical microscopy (OM), scanning electron microscopy (SEM), and DSC. Sakuraia et al. [45] investigated effects of molecular weight, molecular weight distribution and isotacticity on the mechanical deformation behaviors of i-PP during hot-drawing by using synchrotron SAXS and WAXD techniques.

It is evident that a great deal of work has been carried out on subjects of mesophase and deformation behaviors of i-PP. However, limited work has been done to comprehensively investigate tensile deformation behavior of mesomorphic i-PP by using in situ synchrotron SAXS and WAXD techniques. In this study, we have found that the WAXD patterns of deformed mesomorphic i-PP obtained by us and other scientists [13], [18], [46] are quite similar to that of oriented mesomorphic i-PP induced from crystallized i-PP by cold-drawing [29], [30], and the long range ordering shown in the initial quenched i-PP (quench-induced mesophase) disappears during tensile deformation. Moreover, the long range ordering could not be found either in the deformation-induced mesomorphic i-PP [29], [30]. Therefore, it is quite necessary to clarify the differences among the quench-induced mesophase, deformed mesophase, and deformation-induced mesophase of i-PP.