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On the differences in micro-deformation mechanism between isotactic polypropylene and [beta]-nucleated isotactic polypropylene as revealed by the confocal laser scanning microscopy.

INTRODUCTION

Isotactic polypropylene (iPP) is a commodity semi-crystalline polymer (1) of great commercial importance used in packaging, carpeting, automotive industry, home appliances, electronics, sporting goods, and medical disposables (2). Crystalline structure of PP controls its mechanical and other physical properties directly (3), (4). Changes in molecular structure introduced via modification of catalyst and/or polymerization technologies are always translated into performance of PP via changes of its crystalline morphology. In majority of applications, high fracture resistance at low temperatures and/or at high strain rates is desired. In addition to rubber toughening or toughening via copolymerization with ethylene, it has been shown, that improvement of fracture resistance of iPP can be obtained by enhancing the extent of [beta]-crystalline phase using [beta]-nucleating agents.

Unsatisfactory fracture resistance at low temperatures or high strain rates is one of the main shortcomings of PP (1), (5-8). It has been demonstrated that the crystalline modification, spherulite size, and number of intra- and inter-spherulitic tie molecules are directly related to fracture toughness of neat PP (1). In an excellent article on the deformation behavior of iPP, Huy et al. (9) have used Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) techniques to elucidate structural differences between a- and [beta]-PP on the micro- and nano-scale and to relate them to the differences in deformation response. Under standard polymerization and processing conditions, monoclinic a-modification dominates the iPP morphology, consisting of two populations of lenticular lamellae arranged with their axes roughly perpendicularly (10). On the other hand, the hexagonal [3-modification exhibits broad lamellae structure with the lamellae forming coplanar stacks. It has been shown, that improvement of fracture resistance at high strain rates can be obtained by enhancing extent of [beta]-crystalline phase in common PP using [3-nucleating agents (9), (11-20). It has been demonstrated that, during fracture, the /i-spherulites allow for better dissipation of the strain energy compared with the a-PP (9), (18).

In this article, an attempt has been made to visualize the difference between the micro-deformation mechanisms in neat a-crystalline iPP and [beta]-nucleated iPP employing easy to use confocal laser scanning microscopy (CLSM) allowing direct in-situ observation of the micro-deformation mechanism without the need for laborious sample preparation required by the more sophisticated techniques.

EXPERIMENTAL

Commercial grade iPP, Mosten GB 005 (Chemopetrol, Czech Republic) was used as received and [beta]-nucleated using 0.03 wt% of N,N'-Dicyclohexyl-2,6-naphthalenedi-carboxamide (Sigma Aldrich, USA). Both neat and [beta]nucleated iPP were compounded using laboratory twin-screw extruder APV 19 (Baker, USA) at 220[degrees]C and 300 rpm, L/D = 36. Pellets obtained were compression molded into 1-mm thick sheets at 210[degrees]C and 100 kN clamping force using the heated plate press TP400 (Fon-tijne, Netherlands). To minimize the effect of the steel mold surface on the crystalline morphology of PP induced during compression molding, the molded sheets were re-crystallized by heating them to 180[degrees]C and, letting them to cool down in air at room temperature (cooling rate approximately 30[degrees]C/min.) with free PP-air surface. Then, 5 mm wide strips were cut out from the re-crystallized sheets and 1-mm deep notch has been cut into side surface of each strip in the direction perpendicular to the loading direction with razor blade. The notched specimens were loaded in tension using the Zwick Z010 tensile tester (Zwick/Roell, Germany) at cross-head speed of 1 mm/min at room temperature. The test was interrupted at 20% relative deformation. Strips were then mounted on the observation table of the confocal laser scanning microscope CLSM LEXT OLS 3000 (Olympus, Japan) for observation.

RESULTS AND DISCUSSION

In Figs. 1 and 2, surface morphology of the neat iPP (a-form) and [3-nucleated iPP is depicted, revealing their undeformed spherulitic structure. The neat iPP surface (see Fig. 1) exhibits more or less uniform morphology formed by the a-spherulites, with diameter ranging from 10 to 30 fim9 in an intimate contact with relatively few voids left after crystallization from melt. On the other hand, the surface of the [beta]-nucleated PP (see Fig. 2) was clearly composed of two types of spherulites. There are relatively large irregularly shaped [beta]-spherulites embedding well defined a-spherulites. The different the shape of a-spherulite and [beta]-spherulite is a result of the different lamellae stacking during the spherulite growth, schematically shown in Fig. 3. The presence of both a- and [beta]-modification in the [beta]-nucleated PP has been confirmed using X-ray diffraction of 10-jum thick film (see Fig. 4).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

In Figs. 5 and 6, the morphology of the deformed PP is shown inside the notch tip process zone before crack initiation. The shape of the process zone has changed from a narrow elongated shape zone for the neat iPP (see Fig. 5) to much larger oval shape plastic zone in the case of [beta]-nucleated iPP (see Fig. 6). This is in agreement with the results showing that the yield strength of the a-phase is greater than that for the [beta]-phase (18), (20) and, at the same time, the amount of mechanical energy dissipated during fracture increases with the amount of the [beta] -phase. The neat iPP a-spherulites deformed under the tensile load very heterogeneously exhibiting series of many relatively narrow highly drawn micro-shear bands. The thickness of these shear bands varied from 3 to 20 firn. Both inter- and intra-spherulitic shear bands were observed. The Intra-Spherulitic bands were significantly thicker than the Inter-Spherulitic ones. Thus, the plastic deformation was highly localized on the Micro-Scale and the notch tip plastic zone was rather narrow (see Fig. 5).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The [beta]-nucleated PP, on the other hand, showed relatively homogeneous deformation of the [beta]-spherulite under tensile load, while the a-phase embedded in the [beta]-nucleated iPP (see Fig. 2), remained either plastically undeformed or narrow, localized, trans-spherulite shear bands were observed, similarly to the neat iPP. Hence, it seems that the [beta]-spherulites were able to dissipate more strain energy, since larger volume of the material underwent plastic deformation and the deformation was distributed more uniformly throughout the notch tip process zone.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

The distinct micro-deformation mechanisms observed for the a- and [beta]-phase PP resulted from their completely different spherulitic structure (20). The monoclinic a-spherulites consisting of cross-hatched lamellae with densely arranged PP helices and higher concentration of tie molecules exhibit stiffer structure compared with the hexagonal [beta]-spherulites composed of thicker lamellae formed under lower undercooling with lower concentration of tie molecules in the inter-lamellar regions (see Fig. 3). Moreover, crystalline lamellae are stacked nearly parallel in the [beta]-phase with morphology very similar to linear polyethylene. Therefore, in analogy to high-density polyethylene (HDPE), the early stage of deformation of the [beta]-phase was accompanied by formation of typical chevron-like structure further followed by fibrillation during the post-yield drawing. Destruction and transition of the crystal skeleton into fibrils is distributed relatively homogeneously within the [beta]-phase (20-25).

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

In Figs. 7 and 8, the 3D micrographs of the sections of the process zone near the crack tip are presented. Whilst the localized deformation bands were observed in the a-spheru-lites (see Fig. 7), the [beta]-spherulitic morphology exhibited diffuse shear banding distributing plastic deformation in a larger volume and leaving the undeformed a-spherulites to form island-like inclusions (see Fig. 8). Both inter- and intra-spherulite shear bands were observed running through the otherwise plastically undeformed a-spherulites in the neat iPP (see Fig. 7). The intra-spherulitic shear bands were significantly larger and run perpendicularly to the loading direction, while the inter-spherulitic sheaf bands were smaller and run around the spherulite boundaries oriented at approximately 45[degrees] to the loading direction. In the [beta]-modified PP (see Fig. 8), only localized trans-spherulite shear bands were observed running through the randomly distributed a-spherulites perpendicularly to the loading direction. No inter-spherulitic shear bands were observed in the [beta]-rnodified PP. These results are in agreement with observations published in literature and correlate with the results from mechanical testing of a- and [beta]-nucleated iPP (20). CLSM can provide alternative route to determine contributions of various deformation modes to the elucidation of deformation and fracture behavior of semicrystalline polymers (10), (26-28).

CONCLUSIONS

The aim of this letter was to show that the CLSM is a simple, yet powerful method for investigating polymer morphology and visualizing micro-deformation processes in polymers. The capability of this method has been demonstrated investigating the differences in micro-mechanisms of plastic deformation between the neat dominantly a-modification iPP and [beta]-nucleated iPP in the process zone at the sharp notch tip. In the neat, a-spherulitic iPP, plastic deformation was localized in narrow deformation bands. Both inter- and intra-spherulitic shear bands were observed. The intra-spherulitic shear bands were significantly larger and run perpendicularly to the loading direction, whereas the inter-spherulitic shear bands were smaller and run around the spherulite boundaries oriented at approximately 45[degrees] to the loading direction. As a result, the shape of the notch tip process zone was relatively thin unable of dissipating large portion of the strain energy. The [beta]-phase PP showed relatively homogeneous plastic deformation delocalized in wide shear bands. The a-phase coexisting within the [beta]-nucleated iPP, remained either plastically undeformed or narrow, localized, trans-spheru-lite deformation bands running perpendicularly to the loading direction were observed, similarly to the neat a-phase. Hence, the [beta]-phase was able to dissipate more strain energy stored in the sample, since larger volume of the material underwent plastic deformation, and the deformation was distributed more uniformly throughout the notch tip process zone. In addition, plastically deformed /J-phase was able to terminate the highly localized shear bands developing in the a-phase.
ABBREVIATIONS

iPP     Isotactic polypropylene
CLSM    Confocal laser scanning microscopy
FTIR    Fourier transform infrared
SEM     Scanning electron microscopy
TEM     Transmission electron microscopy
AFM     Atomic force microscopy
HDPE    High-density polyethylene


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Correspondence to: J. Jancar; e-mail; jancar@fch.vutbr.cz

Author J. Kalfus is currently at Polymer Engineering and Science Center,

University of Massachusetts, USA.

Contract grant sponsor: Czech Grant Agency; contract grant number:

P205/10/2259.

DOI 10.1002/pen.22024

Published online in Wiley Online Library (wileyonlinelibrary.com).

J. Jancar, J. Kalfus, R. Balkova

Institute of Materials Chemistry, Brno University of Technology, Czech Republic
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Author:Jancar, J.; Kalfus, J.; Balkova, R.
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:4EXCZ
Date:Dec 1, 2011
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