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The effect of crystallization on the structure and morphology of polypropylene/clay nanocomposites.

INTRODUCTION

Polymer/clay nanocomposites have received a great deal of attention during the past decade. Polypropylene (PP)/clay nanocomposite (PPCN) is one of the model systems in the field of nanocomposites. Their preparation and properties have already been discussed in several publications (1-6). The properties of nanocomposites strongly depend on the intercalation or exfoliation of silicate layers in the matrix polymer. But the control of intercalated and/or exfoliated structure remains an unsolved problem. Apart from the nature of clay, crystallization might be a good tool for controlling the structure of nanocomposites and thereby the properties (mechanical, heat resistance, gas barrier, etc.). in our previous studies (5) we reported on the crystallization kinetics, clay particle distribution, and the effects of intercalation/exfoliation on the mechanical properties of PPCNs.

In this paper we focus on the effect of crystallization on PPCNs and thereby their crystalline structure and morphology. Emphasis is given to the mechanism of intercalation through crystallization. The lamellar texture is determined by using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM).

EXPERIMENTAL

Materials and preparation. Maleic anhydride grafted polypropylene [PP-MA) from Exxon Chemical was used as the matrix ([M.sub.w] == 1.95 [kg.mol.sup.-1] [M.sub.w]/[M.sub.n] = 2.98 and maleic anhydride content = 0.2 wt%). The PPCNs were prepared through melt extrusion with PP-MA and montmorillonite intercalated with stearyl ammonium ions. Two clay contents were used: 4 and 7.5 wt%. Henceforth these materials will be termed PPCN4 and PPCN7.5, respectively. Compounding was performed using a twin-screw extruder (TEX30[alpha]-45.5BW, Japan Steel Works Ltd.). The details of PPCN preparation were given in previous publications (1, 2).

Crystallization. A sample of 10 X 10 x 0.5 [mm.sup.3] size was cut from the compression molded sheet and melted at 190[degrees]C in a hot plate for 5 min (to remove the thermal history) and quickly transferred to a hot stage (Linkam) set at a predetermined temperature and crystallized until full solidification (confirmed from the kinetics of crystallization using the polarizing optical microscope). The crystallized samples were characterized using wide-angle X-ray diffraction (WAXD), SAXS, TEM, and differential scanning calorimetry (DSC).

WAXD. The X-ray diffraction experiments were performed using a MXlabo diffractometer (MAC Science Co.) with [CuK.sub.[alpha]] radiation and graphite monochromator. The generator was operated at 40 kV and 20 mA. The crystallized samples were placed inside the aluminum sample holder at room temperature and were scanned from 2[theta] = 2[degrees] to 30[degrees] at the rate of 0.5[degrees] [min.sup.-1].

SAXS. The SAXS measurements were conducted to determine the lamellar thickness and the long period of the crystallized samples of PP-MA and PPCNs. The system consisted of a 6 kW rotating anode generator (MAC Science Co.) operated at 50 kV and 24 mA with [CuK.sub.[alpha]] radiation and germanium monochromator. The data were collected for 3 hours. The correction of slit-width smearing was not needed because of the finite cross section of the primary beam (7).

DSC. The crystallized samples were characterized by using temperature-modulated differential scanning calorimeter (TMDSC, TA2920, TA Instruments) at the heating rate of 5[degrees]C [min.sup.-1], to determine the melting temperature ([T.sub.m]) and heat of fusion ([DELTA]H) of PP-MA and PPCNs. The DSC was calibrated with Indium before use.

TEM. The dispersability of the clay particle in the matrix and the lamellar morphology were checked by using transmission electron microscope (H-7100 Hitachi Co.) operated at an accelerating voltage of 100 kV. The samples have been stained with [RuO.sub.4] for sufficient time. A thin layer of about 70 nm was cut from the stained sample at -80[degrees]C using a Reichert ultramicrotome equipped with a diamond knife.

RESULTS AND DISCUSSION

Formation of [gamma]-Form

Figure 1 shows the typical wide-angle X-ray diffraction (WAXD) patterns of PPCN7.5 crystallized at indicated temperatures. The peaks are assigned as reported in the literature (8). The two peaks, shown by the arrows, are corresponding to the (130) planes of a and [gamma]-crystal. Interestingly, with the increase of crystallization temperature ([T.sub.c]) the peak intensity of [alpha]-form decreases, while the intensity corresponding to the [gamma]-form increases gradually. Figure 2 shows the temperature dependence of the fraction of the [gamma]-form ([f.sub.[gamma]]). The value of [f.sub.[gamma]] calculated from the area of the specific peak is compared to the total area of [alpha] and [gamma]-forms. At low [T.sub.c] (< 100[degrees]C), PP-MA does not exhibit [gamma]-crystals, but their content increases with increasing [T.sub.c]. Anyway, the fraction of [gamma]-form consistently increases with clay content in PPCNs, compared with PP-MA, at every [T.sub.c] studied here.

Lotz et al. (9) reported that [gamma] crystals are nucleated on the lateral (010) faces of the [alpha] crystal and appear to be favored by, or linked to, the absence of chain folding. The mobility of the PP-MA matrix is significantly reduced in the presence of maleic anhydride grafting in the main chain, which causes a lowering of chain folding, especially at high [T.sub.c]. Again, in the presence of clay particles in PPCNs, the movement of polymer chains inside the clay particles is restricted. The correlation length of the clay particles is roughly the same as that of the radius of gyration of the matrix (3). Thus, the formation of the [gamma]-phase is enhanced in the presence of clay particles.

d-Spacing

It is apparent from Fig. 1 that the major diffraction peaks from the (110) and (040) planes slightly shifted toward the lower angle when crystallized at higher [T.sub.c]. Figure 3 shows the d-spacing of PP-MA and PPCNs as a function of [T.sub.c]. From the Figure it is clear that [d.sub.110] and [d.sub.040] are almost constant or slightly increasing with [T.sub.c], but they increase with clay content. As evidenced from the WAXD pattern, the unit cell structure does not change (all monoclinic), but its dimensions change, which indicates that some defects are entering into the lattice with increasing [T.sub.c] and clay content.

Intercalation and Mechanism

The peaks corresponding to 2[theta] around 3.5[degrees] in Fig. 1 are due to the interlayer spacing, which is the diffraction peak from the (001) planes ([d.sub.001]), for organophilic clay in PPCNs, and the dotted line represents peak position for the solid clay. It is apparent that with increasing [T.sub.c], the peak position gradually shifted to a lower angle, i.e., the interlayer spacing of the clay gallery increases with [T.sub.c]. Figure 4 shows the interlayer spacing as a function of [T.sub.c]; higher interlayer spacing occurs at the high-temperature region ([T.sub.c] > 70[degrees]C) for both PPCN4 and PPCN7.5. The extent of intercalation in PPCN4 is significantly higher than that of PPCN7.5. and the phenomenon is explained by the effective tethering to silicate layers. At lower clay content, most of the polymer chains are tethered to the smaller amount of clay, giving rise to higher intercalated spacing. Again, the extent of intercalation increases with [T.sub.c] IN both cases, i.e., the extent of in tercalation depends on time of exposure to the molten matrix polymer. At high [T.sub.c], the crystallization rate is slow enough and polymer chains have sufficient time to intercalate inside silicate galleries. To confirm this we conducted crystallization at low [T.sub.c] (< 60[degrees]C), where the crystallization rate is slow again, owing to the proximity of [T.sub.g], and we found higher intercalation. No intercalation occurs after full solidification/crystallization of the matrix outside the silicate gallery, as evident from the open circle (crystallized at 70[degrees]C for 17 hrs). Here it is to be noted that 20 mm is enough for full solidification of PP-MA at [T.sub.c] = 70[degrees]C. If PPCN4 is annealed just above the melting point at 150[degrees]C ([T.sub.m] [approximately equal to] 142[degrees]C) from melt for 5 hours and then subsequently quenched to 70[degrees]C, we found enhanced intercalation (indicated by the small arrow). These strongly suggest that the extent of intercalation strongly depends on the duration of the matrix polymer in the molten state and the amount of clay present in the system.

Crystalline Morphology From SAXS

The characteristic parameters for crystalline morphology obtained from SAXS were calculated from the measured data using the method of Strobl (10). The correlation function K(z) in Fig. 5 is given by (11)

Where s is the reciprocal-space coordinate, s = 2 sin[theta] / [gamma], 2[theta] being the scattering angle, [gamma] is the wavelength of X-ray radiation and j(s) is the scattering intensity in electron units per unit volume. In Fig. 5, the arrows indicate the lamellar thickness, l and long spacing, L of PP-MA and PPCN4, crystallized at 70[degrees]C. Figure 6 shows the characteristic parameters (l and L) along with [T.sub.m], obtained from DSC, as a function of [T.sub.c]. Both l and L increase with the clay content because some of the silicate layers may reside in the inter-lamellar region, especially at low [T.sub.c]. But at high temperatures, the long spacing becomes the same, so it is believed that most of the clay particles move to the interfibrilar region, and, owing to the segregation phenomena, especially at high [T.sub.c]. some of the silicate layers diffuse out to the interspherulitic region. In our previous study we showed TEM evidence of this phenomenon (5). [T.sub.m]'S of PPCNs are slightly higher than that of PP-MA, which is a result of thicker and more perfect lamella of the PPCNs. The measured crystallinities of PP-MA, PPCN4 and PPCN7.5 were 43.7%, 47.2% and 48.7%, respectively; details about the measurement procedure are described elsewhere (3).

K(z) = [[integral].sup.[infinity].sub.0] [s.sup.2] j(s) cos(2[pi]zs) ds/[[integral].sup.[infinity].sub.0] [s.sup.2] j(s) ds (1)

Spherulitic and Lamellar Morphology

The spherulitic texture was observed with a polarizing optical microscope. Micrographs of (a) PP-MA (b) PPCN4 at [T.sub.c] = 130[degrees]C are shown in Fig. 7. The negative birefringence (12) and the nucleating effect of clay particles are evident, as the spherulitic size decreases significantly with the addition of clay. In our previous studies (5) we showed the higher crystallization rate of nanocomposite by Rayleigh light scattering and linear growth rate. Figure 8a. b shows the lamellar morphology of (a) PP-MA and (b) PPCN4 crystallized at 130[degrees]C and subsequently stained with [RuO.sub.4] for sufficient time. The matrix (PP-MA) exhibits lamellar orientation having an average thickness of 8 rim, while in PPCN4 a fragmented lamellar structure (thickness about 10 nm) is observed, probably due to the presence of a silicate layer in the crystal growth front. It is clear from Fig. 8b that the clay particles (the deep black region in the micrographs) are randomly distributed inside the spherulite. Interes tingly, the crosshatching phenomenon is categorically absent both in PP-MA and PPCN4, and negative birefringence was observed for both cases, which was also evident from the spherulitic morphology (Fig. 7).

CONCLUSION

Excess [gamma]-phase crystals are formed in PPCNs, owing to the restriction of chain movement between the clay particles. PP d-spacing did not change with crystallization temperature but increased with clay content. With an increase of crystallization temperature and decrease of clay content in PPCNs, more and more polymer chains are tethered onto the silicate galleries, giving rise to higher intercalated species. The extent of intercalation strongly depends on the time the clay is exposed to the molten matrix polymer. The PP-lamellar thickness and long period of PPCNs are higher than those of PP-MA.

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ACKNOWLEDGMENT

This work was partially supported by a Grant-in-Aid for the Academic Frontier Center under the project "Future Data Storage Materials" granted by the Ministry of Education, Science, Sports and Culture (1999-2003).

REFERENCES

(1.) N. Hasegawa, H. Okamoto, M. Kato, A. Tsukigase, and A. Usuki, Macromol. Mater. Eng., 76, 280 (2000).

(2.) N. Hasegawa, H. Okamoto, M. Kato, and A. Usuki, J. Appl. Polym. Sci., 78, 1981 (2000).

(3.) P. H. Nam, P. Maiti, M. Okamoto, T. Kotaka, N. Hasegawa, and A. Usuki, Polymer, 42, 9633 (2001).

(4.) M. Okamoto, P. H. Nam, P. Maiti, T. Kotaka, N. Hasegawa, and A. Usuki, Nano Letters, 1, 295 (2001).

(5.) P. Maiti, P. H. Nam, M. Okamoto, N. Hasegawa, and A. Usuki, Macromolecules, 35, 2042 (2002).

(6.) M. Okamoto, P. H. Nam, P. Maiti, T. Kotaka, T. Nakayama, M. Takada, M. Ohshima, A. Usuki, N. Hasegawa, and H. Okamoto, Nano Letters, 1, 503 (2001).

(7.) O. Glatter and O. Kratky, Small-Angle X-Ray Scattering, Academic Press, London (1982).

(8.) D. R. Marrow and B. A. Newman, J. Appl. Phys., 39, 4944 (1968).

(9.) B. Lotz, S. Graff, and J. C. Wittmann, J. Polym. Sci.: Part B: Polym. Phys., 24, 2017 (1986).

(10.) G. R. Strobl, Acta Crystallogr., 26, 367 (1970).

(11.) C. G. Vonk and G. Kortleve, Kolloid Z. Z. Polym., 19, 220 (1967).

(12.) D. R. Norton and A. Keller, Polymer, 26, 704 (1986).

ABBREVIATIONS AND SYMBOLS

PP-MA: Maleic anhydride grafted polypropylene

PPCN: Polypropylene clay nanocomposite

WAXD: Wide angle X-ray diffraction

SAXS: Small angle X-ray scattering

TEM: Transmission electron microscope

DSC: Differential scanning calorimeter

[RuO.sub.4]: Ruthenium tetroxide

[T.sub.c]: Crystallization temperature ([degrees]C)

[T.sub.m]: Melting temperature ([degrees]C)

l: lamellar thickness (nm)

L: Long period (nm)

K(z): Correlation function

[theta]: Scattering angle

Masami Okamoto *

* Corresponding author. E-mail address: okamoto@toyota-ti.ac.jp
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Article Details
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Author:Maiti, Pralay; Nam, Pham Hoai; Okamoto, Masami; Kotaka, Tadao; Hasegawa, Naoki; Usuki, Arimitsu
Publication:Polymer Engineering and Science
Article Type:Abstract
Geographic Code:1USA
Date:Sep 1, 2002
Words:2315
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