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Surface Structure of Isotactic Polypropylene by X-Ray Diffraction.

KATSUHIKO NAKAMAE [*]

With the glazing angle incidence of an X-ray beam, the surface structure of compression molded isotactic polypropylene film was investigated by X-ray diffraction. A comparison of the crystallinities between the surface and the bulk was made for films annealed in air at different temperatures and times. The crystallinity was lower for the surface compared with the bulk irrespective of annealing conditions. A drastic change in crystallinity occurred up to depths of several micrometers from the surface, and crystallinity decreased near the surface. The apparent crystallite sizes were smaller for the surface. This showed that crystal growth was restricted near the surface because of localized defects (including chain ends), and high chain mobility. Annealing at higher temperatures induced surface oxidation, which also decreased the surface crystallinity. The residual stress near the surface, attributable to quenching from the melt, was also detected by this method.

INTRODUCTION

Polymer surfaces play an important role not only for phenomena such as adsorption, crystal growth, abrasion, and skin-core effect, but also for industrial applications such as adhesives, biomaterials, and composites. Surfaces are well known to show different microstructures and properties from the bulk. Recently, numerous techniques have been developed for surface analysis. Most common for polymers are contact angle measurement, attenuated total reflection of infrared spectroscopy (ATR FT-IR), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) [1]. Recently, Rutherford back scattering [2, 3] and a near edge X-ray adsorption spectroscopy (NEXAFS) [4-6] have also been applied for polymer surface analysis. Further, in addition to optical and electron microscopes, scanning probe microscopes give us an atomic image, the mechanical properties, and the chemical species of the surface [7]. The knowledge of polymer surfaces thus accumulated mainly concerns the morphology, chemical conten t, chemical bonding, and some properties of the surface.

Normal X-ray diffraction gives information on the averaged structure of the bulk, because X-rays are insensitive to the surface structure. On the contrary, with the glazing angle incidence of X-ray beams onto polymer surfaces, most of the X-rays are scattered near the surface [8]. When the geometry of the equipment is suitably fixed, the scattered X-ray intensity becomes very sensitive to the surface structures. This technique is called thin film X-ray diffraction or glazing incidence X-ray diffraction (GIXD) depending on the incident angle. GIXD was first applied to polymers by Buckley and Taylor using the X-ray film method [9];, after that, Factor et al. reported that spin coated polyimide film on Si wafer possessed a more ordered structure near the surface by using synchrotron radiation [10]. Horiuchi and Matsushige also investigated organic thin films, and vacuum-deposited polymer films by using white X-ray beams and an energy dispersive detector [11].

In this study, surface structure of compression molded isotactic polypropylene (it. PP) film was measured by X-ray diffraction, and the structural difference between the bulk and the surface was investigated.

EXPERIMENTAL SECTION

Sample

it.PP used was commercial grade sample (Noblen MA6) from Mitsubishi Kasei Corporation. it.PP was reprecipitated from 5%w/w solution of tetralin to methanol several times, followed by soxhlet extracting with n-heptane for 8 h under nitrogen atmosphere to remove the additives. The absorbance ratio at 997[cm.sup.-1] and 974[cm.sup.-1] (D997[cm.sup.-1]/D974[cm.sup.-1]), measured by FTIR spectrophotometer (Shimadzu, FTIR-4000), increased from 0.88 to 0.92. which indicates that the isotacticity of PP increased with extraction [12]. The extract was compression molded between two pieces of optical glass at 230[degrees]C, followed by quenching and annealing at the desired conditions as shown below. The film was isotropic, judging from the X-ray diffraction photographs taken from through, edge, and end directions. We tried to make films their surfaces as smooth as possible. Surface roughness was observed using an atomic force microscope (Topometrix, TMX-2 100, Explorer) with [Si.sub.3][N.sub.4] tips (a spring constant of 0.032 N/m). The maximum roughness was about 10 A, which corresponds to that of the glass substrate. The thickness of the film was 0.5 mm. To avoid contamination, releasing agent was not used throughout this experiment.

X-ray Diffraction

Figure 1 is a schematic illustration for the geometry of X-ray diffraction. When the angle [alpha] of the incident X-ray beams decreases, so that the reflective index is less than unity, total reflection occurs below the critical angle [[alpha].sub.c] of the X-rays, and diffracted and scattered signals at the angle 2[theta] can arise mainly from the limited depth from the surface. [[alpha].sub.c] depends on the X-ray wavelength ([lambda] = 1.5418 A), and the refractive index of material (13), and can be expressed with the following equation (14).

[[alpha].sub.c] = [([[lambda].sup.2] [r.sub.e]N/[pi]).sup.1/2] (1)

where [r.sub.e] is the classical electron radius (2.82 X [10.sup.-13] cm); N is the electron density per unit volume of the material. [[alpha].sub.c] is calculated to be 0.14[degrees] for it.PP and Cu K[alpha] radiation.

The penetration depth L is here defined as the depth at which the incident X-ray intensity decreases to 1/e. L can be expressed with two equations depending on whether [alpha] is larger or smaller than [[alpha].sub.c] (15).

L = [lambda]/4 [pi] n [([[[alpha].sup.2].sub.c] - [[alpha].sup.2]).sup.1/2] (when [alpha] [less than] [[alpha].sub.c]) (2)

L = sin [alpha]/[micro] (when [alpha] [greater than] [[alpha].sub.c]) (3)

where [micro] is a linear absorption coefficient.

Figure 2 shows the relationship between the calculated penetration depth of X-rays from the surface and a for it.PP. The penetration depth changes drastically below and above [[alpha].sub.c] At a = 0.05[degrees], we can obtain structural information up to 5 nm from the surface. At [alpha] = 0.2[degrees], the penetration depth is on the order of 10 [micro]m. We employed a of 0.05[degrees] ([less than][[alpha].sub.c]), if not otherwise indicated.

Figure 3 is a schematic illustration of the apparatus for X-ray diffraction In order to satisfy the conditions of the total reflection of the X-ray beams, surface smoothness of the film, the setting of the film on an X-ray diffractometer are important. Further, parallel X-ray beams and high wave length resolution are also required. A rotating anode type X-ray generator (Mac Science, Ltd., Sra-Ml8X) was combined with a Ge [111] incident monochromator and a goniometer with a thin-film X-ray diffraction attachment. X-ray beams from a Cu target, operated at 40kv, 200mA, were used. The divergence angle of the monochromated beam was evaluated to be on the order of [10.sup-3] degree in this system. The wave length resolution [delta] [lambda]/[lambda] was evaluated to be 3.77 X [10.sup.-4]. These are considered not to bring a serious error in evaluating the critical angle and the penetration depth. it.PP film was rotated during the measurements in order to avoid the effect of crystallite orientation. The data were d etected by a scintillation counter through a pulse height analyzer with the fixed time method, and were stored and processed with a work station (SUN SP/5).

The X-ray crystallinity was evaluated by the method proposed by Weidinger and Hermans [16]. The diffraction profile was corrected for geometrical broadening, followed by resolved curve, and the X-ray crystallinity was measured.

The crystallinity [X.sub.c] was also measured from sample density and FTIR measurements. The [X.sub.c] value from the sample density d was measured by using the following equation.

1/d= ([X.sub.c]/[d.sub.c]) + (1 - [X.sub.c])/[d.sub.a] (4)

where [d.sub.c], [d.sub.a] are the crystal (0.936 g/[cm.sup.3]) and amorphous (0.85 g/[cm.sup.3]) densities [17]. The crystallinity from FTIR spectra was evaluated by the absorbance ratio at 997 [cm.sup.-1] and 917 [cm.sup.-1] [18]. Attenuated total reflection (ATR) of FTIR was also measured using KRS-5 (face cut angle [gamma] of 45[degrees]K) to obtaine information near the surface. The measurement was carried out with 4 [cm.sup.-1] resolution and 50 scans. With respect to ATR spectroscopy, the penetration depth [L.sub.FTIR] of IR beam has been estimated as a function of a wave number v and a refractive index n of an ATR crystal ([n.sub.1] = 2.4) and a sample ([n.sub.2] = 1.503) as shown below [19].

[([L.sub.FTIR]).sup.-1] = v * [2 [pi] [n.sub.1][{[sin.sup.2] [gamma] - [([n.sub.2]/[n.sub.1]).sup.2]}.sup.1/2]] (5)

The [L.sub.FTIR] was calculated to be [tilde] 2 [micro]m from the surface.

In order to determine the apparent crystallite size, D, in the direction perpendicular to the chain axis, the observed integral width of the equatorial reflections were corrected for both the CuK [alpha] doublet broadening (Jones's method) and the instrumental broadening [20]. After that, D was obtained by using Scherrer's equation [21].

D = [lambda]/[beta] * cos [theta] (6)

where, [beta] is a corrected integral width, and [theta] is a Bragg angle for the (hkO) plane.

RESULTS AND DISCUSSION

Figure 4 shows the X-ray diffraction profiles of the surface ([alpha] = 0.05[degrees] [less than][[alpha].sub.c]), and the bulk of it.PP annealed at 100[degrees]C for 10 min after quenched from the melt. Bulk profile was obtained by symmetrical reflection geometry with 2[theta]-[theta] scan, where X-ray beam could be penetrated into the bulk. It is well known that there exist several crystal modifications for it.PP [22]. All the reflections in Fig. 4 could be indexed with so-called, most popular, [alpha]-form of it.PP It could be clearly observed that the diffraction profile from the surface was more diffuse than, and the peaks were not as sharp as those from the bulk. This suggests that the crystallinity of the quenched film surface was lower than that of the bulk.

Figure 5 shows the effect of annealing temperature (annealing time is 10 min) on (O) the bulk, and (*) the surface crystallinity of it. PP films. With increasing annealing temperature, both crystallinities increased; however, the surface crystallinity was always lower than that of the bulk. This tendency was more pronounced for the lower annealing temperature. This shows that the crystallization of it. PP seems to be restricted near the surface.

Figure 6 shows the relationship between the crystallinity of it. PP annealed at 100[degrees]C, 10 min after quenching and the angle [alpha] of the incident X-ray beams. The apparent penetration depth is also superposed on the upper side of the Figure. The crystallinity was almost constant at 48% in the bulk, but it became low at a low angle of [alpha]. A drastic change in crystallinity occurred up to several [alpha]m depth from the surface, and the crystallinity decreased near the surface. It was rather surprising that a surface effect propagates on the order of microns into the bulk. However, the FTIR crystallinity of the surface of the compression molded it. PP film was about 63%, which was 10% smaller than that of the bulk. Kawamoto et al. [23] also found that a drastic decrease in the crystallinity occurred within a region of about 0.7 [micro]m from the surface by using ATR FTIR with a Ge prism, which coincided with the above infrared observations. Judging from the penetration depths of ATR FTIR and X-ra y diffraction, it is natural to conclude that the surface of compression molded it. PP possesses low crystallinity, which continues up to a few micrometers into the bulk.

Figure 7 shows the relationship between (O) the bulk and (*) the surface crystallinity of it. PP and the annealing time. Annealing was done in air, and the annealing temperature was 100[degrees]C. Crystallinity from the density, which also corresponds to the bulk crystallinity, was also superposed on the Figure with half-filled circles. With increasing annealing time, all the crystallinities increased. The greatest change in crystallinity occurred within 10 min for the bulk, judging from both the X-ray and density crystallinities. Despite the fact that the absolute values were different from each other, the same tendencies were observed for both bulk crystallinities. On the contrary, the crystallization rate was slower for the surface. Thus, slow crystallization is considered to be one reason for the low surface crystallinity, as shown above. The surface crystallinity almost reached the crystallinity of the bulk for it. PP annealed longer than 1 h. However, longer annealing induced the opposite tendency in c rystallinity for the bulk and the surface, that is, the bulk crystallinity increased and the surface crystallinity decreased for it. PP annealed for more than two days. The decrease in the surface crystallinity is due to the surface degradation because of oxidation during annealing in air. This was confirmed With X-ray photoelectron spectroscopy, where the Ols peak increased for it. PP film annealed for two days. The dynamic contact angle of water decreased from 82[degrees] to 68[degrees] after two days of annealing, which also supports the surface oxidation This suggests that structural change is induced not only by temperature, but also by chemical modification, which can be revealed by the X-ray diffraction method.

Figure 8 summarizes the relationship between the bulk crystallinity and the surface crystallinity. The datum of the crystallinity for the film annealed at 100[degrees]C for 48 h was excluded. The broken line indicates the case where the crystallinities coincide with each other. It is clear that the surface crystallinity is always lower than the bulk crystallinity irrespective of annealing conditions. Factor et al. [10] found increased order at the surface compared With the bulk for spin-cast polyimides. This is in contrast with the above mentioned results. However, numerous experimental results support the low crystallinity near the surface. Schonhorn et al. [24] measured the surface tension of molten polymers. The extrapolated surface tension [[gamma].sub.l] to that at 20[degrees]C (in the solid state) was found to be nearly equal to the critical surface tension [[gamma].sub.c] of each polymer measured for solid surfaces at 20[degrees]C. In the case of it. PP, the [[gamma].sub.l] of 28 mJ/[m.sup.2] was almost equal to the [[gamma].sub.c] of 29 mJ/[m.sup.2]. They concluded that the surface layer of melt crystallized polymers is essentially amorphous. Narisawa et al. [25] reported parallelism between the micro-Vickers hardness and the crystallinity of compression molded polyoxymethylene. By utilizing a layer removal method, they found a drastic increase in the crystallinity from the surface to the bulk; in other words, the outer surface was almost totally composed of amorphous components. A large hole size and fraction, that is, lower density near the polymeric surface, was also observed by means of slow positron annihilation lifetime spectroscopy [26]. The low crystallinity on the surface was also revealed by computer simulation [27]. Further, components with lower crystallinity were reported to be localized on the surface for polymer blends [28].

In order to minimize the surface free energy, some impurities, including chain ends, tend to locate in the surface. These would become defects, and are believed to suppress the crystallization of the chain near the surface. Surface enrichment of chain end groups, observed from an SIMS depth profile analysis of chain end deutrated polymers by Takahara, Kajiyama et al. [29], results in a distinct depression of the glass transition temperature of the surface compared with the bulk measured by scanning probe microscopy. A lower [T.sub.g] was also observed by Brillouin light scattering [30], elipsometry [31], and theoretical calculations [32, 33]. The polymer chains near the surface are considered to have higher mobility than those in the bulk because of the two-dimensional array of the molecules, which would also be capable of bringing low crystallinity to the surface. The enhanced surface mobility is a consequence of the decreased surface density, and hence, increased free volume.

Figure 9 shows the relationship between the annealing temperature and the apparent crystallite size D for (O) the bulk, and (*) the surface of it.PP measured for several equatorial reflections. With increasing annealing temperature, D increased in every direction, which shows that crystallites grew isotropically for the hk0 directions. However, D was always smaller for the surface compared with the bulk. This shows that the crystallite growth was restricted near the surface, corresponding to the results in Fig. 5.

Figure 10 shows the relationship between the lattice spacing for the (110) plane of it.PP annealed at various conditions after quenching and the angle [alpha] of the incident X-ray beams. The apparent penetration depth is also superposed on the upper side of the Figure. For it.PP annealed at 100[degrees]C, 10 min, the lattice spacing decreased with decreasing [alpha]. This indicates that the residual stress due to quenching from the melt was not relaxed at this annealing condition. The change in the lattice spacing from the bulk to the surface is equivalent to the lattice strain of -0.3%. By multiplying this strain by the elastic modulus (2.8 GPa) of the crystalline regions in the direction perpendicular to the chain axis [34], a compressive stress of 8.4 MPa was estimated to remain near the surface, On the other hand, the lattice spacing was constant from the surface to the bulk after annealing at 100[degrees]C, 30 min, which means that it took more than 10 min (and 30 min is sufficient) for stress relaxati on. The lattice spacing of it. PP annealed at 150[degrees]C is also constant even at 10 min, which shows that residual stress relaxed faster at higher annealing temperatures. The higher annealing temperature also induced a decrease in the absolute value of the lattice spacing, which is due to the increase in the lattice perfection of it.PP crystals.

CONCLUSIONS

Compression molded it. PP film was found to show lower crystallinity near the surface than in the bulk, irrespective of annealing conditions. An abrupt change in the crystallinity was observed up to a depth of several micrometers from the surface. This observation, together with the smaller crystallite sizes nearby, allows us to conclude that crystal growth was restricted near the surface. High temperature annealing in air was found to induce surface oxidation, which also decreased the surface crystallinity. The residual stress of 8.4 MPa near the surface could be evaluated from the lattice strain of it. PP. This compressive stress was induced by quenching from the melt, and was relaxed by high temperature or longer time annealing. This novel X-ray diffraction method is nondestructive, and the crystalline physical parameters, such as crystallinity and lattice constants, can be quantified from the first few tens of angstroms to the bulk simply by varying the glancing angle. It is a powerful method for examini ng the effect of manufacturing conditions on the surface crystalline features close to the surface of semicrystalline polymers.

ACKNOWLEDGMENT

This work was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Culture, and Sports, Japan.

(*.) To whom correspondence should be addressed. Katsuhiko NAKAMAE, Professor, D. Eng., Department of Chemical Science and Engineering, Faculty of Engineering, Kobe University, Rokko, Nada, Kobe 657-8501. Japan Tel: +81-78-803-6191, Fax: +81-78-803-6205, E-mail: nakamae@cx.kobe-u.ac.jp

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Author:NISHINO, TAKASHI; MATSUMOTO, TAKAYUKI; NAKAMAE, KATSUHIKO
Publication:Polymer Engineering and Science
Article Type:Brief Article
Geographic Code:1USA
Date:Feb 1, 2000
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