Elsevier

Polymer

Volume 47, Issue 1, 3 January 2006, Pages 489-497
Polymer

Application of an interpenetrating network model to the necking in the microcrystalline region in four annealed isotactic polypropylene films subjected to uniaxial stretching at room temperature

https://doi.org/10.1016/j.polymer.2005.05.159Get rights and content

Abstract

The microscopic infrared dichroism, mesoscale deformation and macroscopic stress measurements are made on the microcrystalline region in four annealed isotactic polypropylene (iPP) thin films subjected to uniaxial stretching at room temperature. Results reveal that volume dilatation might occur during stretching and the necking causes the anisotropic shrinkage in the thickness and the width directions. The average orientation function fav and the true stress as a function of local draw ratio in the samples showing volume dilatation can be respectively overlapped onto those of the sample undergoing constant volume deformation. The pseudo-affine deformation is applicable for molecular orientation at fav<0.50 and the true stress–strain relationship on the mesoscale can be well described in the same region by the interpenetrating network model previously proposed for necking in the quenched iPP film. This model becomes invalid for deformations above fav=0.50 due to that plastic deformations in the crystalline phase, depending on the annealing time, start to play a major role.

Introduction

Semicrystalline polymers being composed of crystalline lamellae and entangled amorphous chains show a complicated behavior under tensile strain. They exhibit large plasticity when being deformed above the glass transition temperature Tg of the amorphous phase [1]. The structure–mechanical property relations have been investigated on levels of crystal-lattice and lamella as well as large-scale superstructures such as spherulites [2]. However, the inhomogeneous structural nature on both microscopic and macroscopic scales makes it quite difficult to understand molecular orientation mechanisms underlying the tensile behavior on the basis of direct observation of the localized plastic deformations. While microstructural deformations in spherulites and microcrystalline region are well documented [3], their contributions to macroscopic behaviors are still ambiguous [4]. Therefore, it is difficult to establish a straightforward relationship between the microstructural deformations and the applied macroscopic stress. On the other hand, Strobl and coworkers have performed extensive investigations on the deformation mechanisms utilizing true stress–strain measurements at constant strain rates [5] and argued that stretching of the amorphous network dominates the deformation at strains beyond the yield point where a critical stress is attained for destructing crystallites.

Using an equipment for simultaneous kinetic measurements of microscopic infrared (MicIR) dichroism from a predetermined mesoscale sampling area and of macroscopic stress of a polymer film subjected to uniaxial stretching at a constant elongation rate [6], we studied the molecular orientation of a quenched isotactic polypropylene (iPP) film during necking at room temperature [7]. The deformation is truly inhomogeneous on a macroscopic scale. However, drastic deformation and molecular orientation mainly occur in the neck shoulder [8]. We disclosed a pseudo-affine deformation up to a local draw ratio λL∼4.5 determined on a mesoscale of 200 μm, i.e. the film shrinks anisotropically in the width and the thickness directions whereas the molecular orientation function in the amorphous phase, fam, as a function of λL could be described by the affine deformation model. Since the orientation function of the crystalline phase, fc, has been found to be in proportion to fam in the same λL region [8], the average orientation function of molecules in both phases, fav, as a function of λL can also be described by the affine model. By measuring the molecular orientation along the draw axis of a quenched film stretched to different macroscopic strains, we showed that fam≈0.45 is the upper limit of the pseudo-affine deformation occurring in the narrow neck shoulder region [9]. This fam value agrees with the theoretical prediction on the highest value attainable for the ideal rubber network and partly explains why the neck shoulder appears to propagate smoothly.

We proposed an interpenetrating network model for interpreting the true stress–strain relationship of a mesoscale area suffering from necking in the quenched iPP film [9]. In this model, a small portion of crystallites adhered through intercrystalline links [10] forms a rigid crystal (C) network that penetrates through a soft crystallite enhanced amorphous matrix (CEAM) network. The deformation of the C network during the necking is described using Takayanagi-Nitta tie molecule model [11], while that of the CEAM network should obey the pseudo-affine deformation model as far as the test temperature is above the glass transition temperature. For the quenched iPP film, this model is able to account for inhomogeneous deformation accompanied with the localized necking at draw temperatures below 400 K where crystallinity does not change markedly [12]. This model with slight alteration is also valid for uniaxial deformation of films of poly(ether-block-amide) multiblock copolymers [13] and also films of a metallocene polypropylene, and an ethylene-butylene rubber and their 80 wt/20 wt blend at room temperature [14].

It is well known that annealing of iPP film results in various sizes of spherulites with the α-form, which significantly influences the mechanical properties of the final products. Large spherulites often promote brittleness owing to the concentration of structural defects at their boundaries [15]. In investigation of the fcfam relation for the microcrystalline region of annealed iPP films, we noticed that deformation behaviors of a giant spherulite with a diameter of several hundreds of micron are quite different from that of the microcrystalline region, i.e., a giant spherulite could resist the elongational stress by local lamellar rearrangement and shows plastic deformation upon stress concentration [16]. Cracking usually initiates at the spherulite boundary and propagates toward the drawing axis over a pretty long distance. The interpenetrating network model could not describe the inhomogeneous deformation of a film containing both microcrystalline region and spherulites, but would be useful for discussing the true stress–strain relationship during the localized deformation of the microcrystalline region itself among spherulites. Since the cooperativity of the crystalline and the amorphous phases or relative contributions of the two networks in the proposed model might be influenced by fine morphologies formed at various annealing conditions, we in this study further examine the applicability of the interpenetrating network model to the plastic deformation in the microcrystalline region of four iPP films annealed at different conditions. The average size of spherulites in iPP films increases with increasing time of isothermal annealing. However, the influences of annealing condition on the structure in the microcrystalline region and also on the room temperature deformation are not yet clear. We in this study attempt to reveal such aspect executing the MicIR dichroism, mesoscale deformation and macroscopic stress measurements and analyzing the data in the framework of the interpenetrating network model.

Section snippets

Materials and sample preparation

iPP pellets with Mw=3.7×105 g mol−1 were compressed at 210 °C for 5 min to form a film with an average thickness of ca. 30 μm, which was then transferred to an oven and annealed at four different conditions. Fig. 1(a) shows the typical morphologies of the films containing giant spherulites with the α-type of crystal structure. When the film was annealed at 145 °C, the average size of spherulite increased with time and a spherulitic embryo without Maltese cross was observed after annealing of 4 h. The

Nominal stress and molecular orientation

Fig. 2(a) shows the time profiles of nominal stress σn. The films yield at t=3.5–4.9 min. The nominal yielding stress varies with annealing condition. The two samples 145 °C×5 h and 140 °C×2 h show the lowest and the highest nominal yielding stress, respectively. A small sharp stress peak is observed at t=13.4–14.6 min during the strain softening for the films annealed at 145 °C, which is different from the strain softening of the quenched film and also of the film annealed at 140 °C.

As shown in our

Conclusion

The local deformation in the microcrystalline region of four annealed iPP films is studied executing simultaneous measurements of MicIR dichroism, mesoscale strain, and macroscopic stress during unixial stretching at a constant elongation rate and at room temperature. Volume dilatation might be an important characteristic in the deformation of the annealed films except for the sample 145 °C×4 h. The true stress–strain and the favλL curves of the annealed samples showing volume dilatation can be

Acknowledgements

This work has been partially supported by the Grant-in-Aid for the Scientific Research from Ministry of Education, Culture, Sports, Science and Technology, Japan (No. 10305070) and one of the authors, Y. Song, is grateful to JSPS for the Grant-in-Aid for JSPS Fellows relating to JSPS Fellowship for Foreign Researchers (No. 12000317) during his stay in Japan for a postdoctoral research.

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