Structure and mechanical anisotropy of cold-rolled ultrahigh molecular weight polypropylene.
It has been generally known that crystal orientation is often accompanied with crystal transformation and/or twinning during cold-rolling and biaxial stretching of highly crystalline polymers. Frank and co-workers (1), investigated the deformation mechanisms in a cold-rolled polyethylene in terms of crystal plasticity. The structure development during the drawing of polyethylene single crystals was examined by Geil et al. (2, 3) by analyzing the twinning of the orthorhombic crystal and a phase transformation to a monoclinic unit cell. In a previous paper (4), we observed the crystal orientation and twinning in simultaneous biaxial stretching of gelation crystallized ultrahigh molecular weight polyethylene (UHMWPE) films. The X-ray pole figures indicated that there is a significant contribution of crystal twinning to the plastic deformation. The phenomenon of crystal orientation and twinning has been extensively explored for polyethylenes, but limited studies on the deformation mechanisms of biaxially stretched or cold-rolled polypropylene (PP) can be found in the literature (5-8).
The studies on ultrahigh molecular weight polypropylene (UHMWPP) have increased over the years; however, most studies were directed to uniaxial drawing of UHMWPP (9, 10) or its blends with UHMWPE (11). To the best of our knowledge, the relationship between processing, structure, and properties of biaxially oriented UHMWPP has not been systematically explored due to the limited availability of biaxial specimens and lower tensile strength and modulus relative to UHMWPE. The melt viscosity of such UHMWPP is very high, therefore the conventional extrusion or injection molding is not practical, and thus cold-rolling or calendering is a preferred method of processing. In this study, the structure development in cold-rolled UHMWPPs has been examined in relation to the mechanical anisotropy of the films.
Ultrahigh molecular weight polypropylene (UHMWPP) powders were supplied by Himont Co. whose viscosity average molecular weight (Mv) was 3.9 x [10.sup.6]. The conventional polypropylene (PP) was obtained from Mitsuitoatsu Chemical Co. Weight average molecular weight (Mw) and number average molecular weight (Mn) of conventional PP were determined to be 3.3 x [10.sup.5] and 4.8 x [10.sup.4], respectively. UHMWPP powders were melt-pressed in a compression molder (Wabash Metal Product, Inc.) at 463 K and 0.02 kg/[m.sup.2] pressure. A rectangular shape spacer was used during compression to adjust sample thickness. Conventional PP pellets were compression molded in a melt-press (Gonnon-hydrostatic Co.) at 473 K under a pressure of 0.02 kg/[m.sup.2]. UHMWPP films were rolled at about 408 K using a two-roll mill (Premixing mill, Brabender Instrument Co.). For conventional PP, rolling was carried out between the third and fourth rolls of a calender machine. The rolling temperature was 373 K.
Wide angle X-ray diffraction (WAXD) photographs were obtained on an X-ray generator (JDX-5P, Nihondenshi Co.) with a plate camera (XDC-CB). WAXD pole figures were acquired on a 2 kW Rigaku X-ray diffractometer with a pole figure attachment (RINT-1200). SAXS experiments were carried out at the Oak Ridge National Laboratory (ORNL) using the 10 m SAXS camera with a 200 by 200 mm two-dimensional position sensitive detector. The sample to detector distance was 5.126 m with a scattering wavenumber range of 0.08 to 1.0 n[m.sup.-1]. The 12 kW Rigaku X-ray generator was operated at 40 kV and 80 mA. Refractive index measurements were undertaken on an Abbe refractometer (Shimazu) at room temperature.
Dynamic moduli of UHMWPP and PP were determined using Rheo-Vibron (DDV-III, Orientec Co.) at 293 K and a frequency of 110 Hz. The sample dimensions were 64 mm in length, 3 mm in width, and 200 [[micro]meter] in thickness. The gauge length was 40 mm. For tensile measurements, a dumbbell shape specimen (JIS-PVC Film) was stretched using a tensile tester (Tensilon UTM 4LH, 5T, Orientec Co.) at a cross-head speed of 3 mm/min at room temperature. The true stress was calculated by estimating the cross-sectional area from the photographs of deformed samples, assuming constant volume.
RESULTS AND DISCUSSION
Figures 1a to c depict the SAXS isointensity contour plots of 3.6 x 1.0, 4.0 x 1.0, and 4.5 x 1.0 cold-rolled UHMWPP films. In the through view ([X.sub.1], film normal direction), a two-point pattern is evident, suggestive of oriented lamellae with their long axes being aligned in perpendicular to the rolled direction. The long periods, as estimated from the scattering maxima, are about 20 nm for the above three UHMWPP specimens (Table 1). A four-point SAXS pattern can be discerned in the edge view ([X.sub.2], transverse direction) which may be attributed to lamellar tilting. In Table 1, the estimated long periods and lamellar tilt angles for various rolled specimens are tabulated. The long period and the tilt angle appear more or less the same for the unidirectional rolling. Upon increasing the rolled ratio in the orthogonal direction, i.e., the cross rolling, the two-point maxima appear to broaden and shift to a higher scattering angle [ILLUSTRATION FOR FIGURES 2A AND B OMITTED]. The long period is reduced approximately to 180 [Angstrom]. Concurrently, the four-point pattern transforms to a diffused two-point pattern implying that the tilted lamellae may be fragmented during increased cross rolling.
Figures 3a and b show the comparison of WAXD pictures of cold-rolled conventional PP and UHMWPP from the through ([X.sub.1]), edge ([X.sub.2]), and end ([X.sub.3], machine direction) views. These films were rolled for a ratio of 3.6 in one direction, but the width was kept constant by controlling the rolling conditions. Strictly speaking, the 3.6 x 1.0 specimens may not be uniaxial because the films must be extended in the transverse direction to compensate the possible width shrinkage. However, the circular patterns of the (110), (040), (130), and (111) plane normals suggest the random orientation in the end view. The plane normals of (110), (040), and (130) appear highly oriented along the equatorial direction in the through view than that in the edge view. The long lamellar axis which corresponds to the (040) plane must orient in the transverse direction such that the crystal c-axis aligns in the machine direction and the [a.sup.*] axis in the transverse direction. Since the sample thickness and exposure time were not properly adjusted for the X-ray photographs of the conventional PP and UHMWPP, the quantative comparison of the diffracted intensity from such WAXD pictures may no longer be meaningful. Although the WAXD pictures axe similar for the two polypropylenes, the pole figure studies indicate that crystal orientation in UHMWPP is very complex and different from that of ordinary PP.
Table 1. The Variation of SAXS Long Period and the Tilt Angle as a Function of Rolled Ratio.
Draw Ratio Long Period ([Angstrom]) Tilt Angle ([Chi])
3.6 x 1.0 200 32 [degrees] 4.0 x 1.0 200 31 4.5 x 1.0 204 33 3.0 x 2.0 180 28 4.0 x 2.0 188 22
The tilt angle [Chi] is defined as the angle between the rolled ([X.sub.3]) direction and the vector Joining the SAXS peek and the center (main beam).
As the rolled ratio of UHMWPP increases to 4.0 x 1.0, the orientation of the (110), (040), and (130) plane normals become more accentuated in the edge view [ILLUSTRATION FOR FIGURE 4A OMITTED]. At the 4.5 x 1.0 rolled ratio, the orientation pattern in the through and edge becomes similar, but the intensities are not necessarily the same [ILLUSTRATION FOR FIGURE 4B OMITTED]. It should be noted that there is a significant difference in the SAXS pattern between the through and edge views, suggesting the preferential lamellar orientation and tilting. When the rolled ratio increases in the orthogonal direction to 3.0 x 2.0 and 4.0 x 2.0, the contour lines of the (110), (040), and (130) plane normals broaden due to the increased biaxiality [ILLUSTRATION FOR FIGURES 4C AND D OMITTED].
Figures 5a to c illustrate the observed WAXD pole figures for the 3.6 x 1.0, 4.0 x 1.0, and 4.5 x 1.0 rolled UHMWPP, respectively. The plane normals of (110), (040), and (130) planes axe all concentrated in the thickness direction which is unusual for polypropylene. In the cold-rolled conventional PP, at least one of the plane normals of (040), (110) or (130) was found to be populated in the thickness direction, but the remaining plane normals would align at some angles between the thickness and the transverse directions (8). According to Takahara et al. (7), three types of orientation modes (called [Alpha], [Beta], and [Gamma]) and their combinations may be considered as possible deformation mechanisms. In the case of the [Alpha] type orientation, the plane normal of (040) is concentrated in the thickness direction while that of (110) and (130) are located at about 70 and 35 [degrees] respectively between the thickness and the transverse directions. This mechanism has been attributed to a planar lamellar orientation, i.e., the lamellae are oriented within the film plane. The [Beta] type orientation is associated with crystal twinning at the (110) plane such that the plane normal of (130) orients in the thickness direction, while that of (110) orients at some degree away from the thickness direction. The [Gamma] type orientation occurs upon release of crystal strain such that the (110) plane normal orients preferentially in the thickness direction. The three mechanisms mentioned above would give the preferential c-axis orientation in the machine direction.
The peculiar orientation behavior of UHMWPP during cold-rolling cannot be explained by a single orientation process described above, suggesting that, more than one deformation mechanism must be involved. According to the pole figure analysis (12), it was found that the crystal orientation as well as twinning have taken place concurrently with the crystal slip. The detailed analysis of the deformation mechanisms of the cold-rolled UHMWPP films will be presented in a succeeding paper (12).
Figure 6 represents the anisotropy of refractive indices in the rolled and transverse directions. The refractive index is greater in the rolled direction relative to that in the transverse direction, implying the preferential orientation of the chain axis along the rolled direction. The trend is similar for all unidirectionally rolled films, except that the optical anisotropy of the 3.6 x 1.0 rolled UHMWPP is somewhat larger than that of the conventional PP of comparable rolled ratio.
Tensile measurements were undertaken along the rolled direction (0 [degrees]), the transverse direction (90 [degrees]) and 45 [degrees] of the 3.6 x 1.0 and 4.0 x 1.0 UHMWPP films to compare with the 3.6 x 1.0 conventional PP. As can be seen in Figs. 7a to c, the stress-strain behavior is considerably different for the three drawing directions. In the rolled direction, the Young's modulus as well as the yield stress are appreciably higher than those in the other two directions. However, the specimen fails much earlier in the rolled direction than in the transverse direction. The specimen drawn at 45 [degrees] shows the the stress-strain behavior intermediate between that of 0 and 90 [degrees] specimens. A similar trend was observed in the cold rolled conventional PP. The modulus and yield stress are lower for the conventional PP as compared to the UHMWPP of comparable rolled ratios, but the elongation at break is significantly larger.
As pointed out previously, the lamellae are oriented with their long axes in the transverse direction, i.e., the chain axes are preferentially aligned along the rolled direction. Therefore, the drawing in the rolled and transverse directions would correspond to the drawing perpendicular and parallel to the long lamellar axis, respectively. Hence, the drawing in the rolled direction would be similar to the serially connected lamellar crystal and interlamellar amorphous model. The amorphous chains must be appreciably extended before the lamellar bending and fragmentation could occur. The bending of lamellae naturally create inter-lamellar voids which often leads to a stress-whitening phenomenon. Since these processes would occur over the extensive elongation region, the yielding zone would be broader. As evidenced by the WAXD and refractive index anisotropy, crystal chains are oriented considerably in the rolled direction, therefore a higher modulus can be expected.
On the other hand, the drawing along the transverse direction resembles the drawing along the lamellar axis, i.e., perpendicular to the crystal chain axis, thereby yielding a lower modulus. Moreover, lamellar fragmentation probably occurs which would lead to the yielding at a very low elongation. Once the lamellae are disintegrated, more polymer chains will be pulled out individually (chain unfolding) or collectively as a bundle in a complex fashion as conjectured by various authors (13-15). Unfortunately, no agreed upon opinion has yet emerged on the transformation from folded chain crystals to the extended crystals of micro-fibrils (fiber structure). The molecular chains probably slide by each other during the drawing leading to a broader ductile region and a larger elongation at break. The drawing at 45 [degrees] of the unidirectionally rolled UHMWPP exhibits the stress-strain behavior intermediate between those of 0 and 90 [degrees].
In Fig. 8, the dynamic Young's moduli ([E.sup.*]) are plotted as a function of off-axis angle (16) for different rolled ratios of UHMWPP in comparison with the 3.6 x 1.0 rolled conventional PP. The modulus is the highest in the rolled direction, but it decreases with increasing stretching angle toward transverse direction, i.e., increasing off-axis angle. The modulus of the 3.6 x 1.0 rolled UHMWPP is slightly lower than that of the conventional PP in the rolled direction, but the trend reverses in the transverse direction. The higher rolled ratio UHMWPP specimens show a larger modulus in all directions relative to that of ordinary PP. The mechanical anisotropy increases with increasing rolled ratio which is in good agreement with the optical anisotropy results.
Support of this work by the National Science Foundation, Grant No. MSM 87-13531 is gratefully acknowledged. S. H. thanks the Ministry of Education, Science and Culture for a travel support. T. K. thanks the Oak Ridge Associated Universities for the support of his travel to Oak Ridge National Laboratory. We thank Dr. T. Dziemianowicz of Himont Co. for supplying the UHMWPP specimens. This research was also supported in part by the Division of Material Sciences, US Department of Energy, under contract number DE-AC05-84 OR 21400 with Martin Marietta Energy System Inc.
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|Author:||Hibi, Sadao; Niwa, Takahiro; Wang, Chi; Kyu, Thein; Lin, Jar-Shyong|
|Publication:||Polymer Engineering and Science|
|Date:||Jun 15, 1995|
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