Single-Walled Carbon Nanotube Nucleated Solution-Crystallization of

Nov 30, 2007 - Solution crystallization of polyethylene (PE) with single-walled carbon .... Eun Park , Jihun Kim , Soonjong Kwak , Sehyun Kim , Yongso...
0 downloads 0 Views 2MB Size
Download PDF
18950

J. Phys. Chem. C 2007, 111, 18950-18957

Single-Walled Carbon Nanotube Nucleated Solution-Crystallization of Polyethylene Hiroki Uehara,* Kaori Kato, Masaki Kakiage, Takeshi Yamanobe, and Tadashi Komoto Department of Chemistry, Gunma UniVersity, Kiryu, Gunma 376-8515, Japan ReceiVed: May 23, 2007; In Final Form: October 2, 2007

Solution crystallization of polyethylene (PE) with single-walled carbon nanotubes (SWNTs) was investigated by morphological observations and thermal analyses of the resultant films. In this study, long chains of ultrahighmolecular-weight (UHMW) PE were chosen for a possible interaction with SWNTs. The solvent used was dichlorobenzene, which can disperse both SWNTs and UHMW-PE chains in a hot solution. Electron microscopy observation revealed that a unique network morphology composed of PE single-crystalline lamellae connected by SWNT backbones is developed after cooling to room temperature. A comparison of the obtained morphologies for different SWNT concentrations suggested that SWNT surfaces function as nuclei for solutioncrystallization of UHMW-PE. The melting behavior of the films prepared from the aggregates of UHMWPE/SWNT composites indicated that SWNTs prevent the thickness doubling of solution-crystallized PE lamellae during heating, which is usually observed for the stacked single crystals of pure PE.

Introduction Carbon nanotubes have excellent properties, such as high mechanical strength and superior thermal conductivity. Therefore, attempts have been made to disperse carbon nanotubes as a filler to improve various properties of the polymer matrix.1 In particular, a single-walled carbon nanotube (SWNT) has a much larger surface area than a multiwalled carbon nanotube (MWNT) when compared on a unit weight basis. Thus, even a small amount of SWNTs is expected to improve various properties of the polymer matrix. The homogeneous dispersion of SWNTs in the polymer matrix is necessarily required in order to achieve such expected property development. One practical approach for dispersion of SWNTs is melt extrusion,2 which is widely applied for other filler materials including carbon black or glass fibers. However, an as-prepared SWNT sample is composed of assembled bundle structures, and therefore, simple melt extrusion cannot achieve the ideal dispersion of each SWNT. Chemical modification3-5 and polymer coating6-8 on SWNT surfaces are other possible approaches for better distribution of SWNTs in solution. However, SWNTs can be highly dispersed in certain organic solvents, as reported by Bahr et al.9 If the matrix polymer is soluble in the same solvent and this polymer solution is mixed with the above-mentioned SWNT dispersion, SWNTs and matrix polymer molecules will be homogeneously mixed in the solution state. Especially for semicrystalline polymers including polyethylene (PE), the critical step in that processing is the coprecipitation of the polymer and SWNT by reducing the temperature.3,10,11 Subsequently, the solvent is dried off to leave the precipitate. The dried precipitate containing SWNTs and polymer matrix could be melt-extruded or melt-pressed to prepare the bulk materials. The mechanical3 and electrical properties12 of these materials have been compared for different SWNT concentrations in the polymer matrix. In the case of semicrystalline polymers, the nuclei effect of SWNTs on meltcrystallization of matrix polymer is also discussed.10 In contrast, * Corresponding author. E-mail: [email protected].

Li et al.12-15 reported that the solution-crystallization with SWNTs yields a unique morphology composed of SWNT shish and kebab lamellae of matrix polymer. However, the effect of SWNT concentration on solution-crystallization has not been investigated yet. Such lack of interest in the composite structure of SWNTs with solution-crystallized lamellae is due to undesired formation of many voids within the film prepared by castingand-evaporation or filtering-and-drying procedures. Since the objective of most investigations of SWNT/polymer composites is to produce the high-performance materials, such voids are usually removed by compression molding3 or extrusion5,10,11 above the melting temperature (Tm) of the matrix polymer. Unfortunately, such melt-processing transforms the prior singlecrystal structure of the solution-crystallized lamellae of the matrix polymer into the usual spherulite structure of meltcrystallized lamellae. This means that the above-mentioned characteristic structure of solution-crystallized composites of SWNT/polymer is destroyed. This study compares solution-crystallized composites prepared under different SWNT concentrations. As a matrix polymer, we selected the most popular semicrystalline polymer, PE, with the simplest molecular architecture. Here, the theoretical calculations16,17 indicate that PE chains can wind around SWNTs, causing higher interaction between the SWNT and matrix PE. In such a case, the longer chain length of PE has an advantage for ease of winding around the SWNT. Therefore, we chose ultrahigh-molecular-weight (UHMW) PE, having a higher MW of over 106, as a matrix polymer. The obtained solution-crystallized UHMW-PE/SWNT composite was filtered onto the film, whose structures were also investigated. Indeed, UHMW-PE has excellent mechanical properties, and preparations of UHMW-PE composite with not only SWNTs and but also MWNTs have been widely examined.18-21 Experimental Section Materials. Purified HiPco SWNTs were purchased from Carbon Nanotechnologies Inc. The matrix UHMW-PE was Hifax 1900 supplied from Montel. Its molecular characteristics were analyzed by gel permeation chromatography in a solvent of 1,2,4-trichlorobenzene at Tosoh (Yokkaichi, Japan) using

10.1021/jp074005v CCC: $37.00 © 2007 American Chemical Society Published on Web 11/30/2007

Solution-Crystallization of Polyethylene on SWNTs

J. Phys. Chem. C, Vol. 111, No. 51, 2007 18951

Figure 1. TEM images of the deposited structure of SWNTs dispersed in decalin (a) and DCB (b) under ultrasonic treatment for 24 h. Scale bar, 500 nm.

Figure 2. TEM images of the deposited structure of SWNT/PE blend solution prepared by adding SWNT dispersion to hot PE solution at the boiling point of DCB with a SWNT/PE content of 4 wt %. Obtained mixtures were cooled at room temperature. Scale bar, 400 nm in (a). Enlarged image is presented in (b). Scale bar, 200 nm.

standard methods. The obtained weight and number average MWs were 2.6 × 106 and 5.6 × 105, respectively. Preparation of SWNT Dispersion. Two solvents, decalin and o-dichlorobenzene (DCB), were chosen for preparing the SWNT dispersion. After the as-received SWNTs were added to these solvents, ultrasonic treatment was applied for 24 h to obtain a homogeneous distribution of SWNTs. It has been reported that the maximum concentration for a homogeneous distribution is 95 mg/L for DCB.9 A slightly lower concentration of 80 mg/L was thus preferred for preparing an ideal SWNT dispersion in this study. Solution Blending and Film Preparation. A solution of UHMW-PE was also prepared at the boiling point. The polymer concentration based on solvent was always 0.2 wt %, which is lower than the overlapping concentration calculated from sample MW and critical entanglement MW of 3.8 × 104 for PE.22 Subsequently, the above SWNT dispersion was added to the polymer solution while maintaining the solution at the boiling point. The SWNT concentration based on the matrix UHMWPE was controlled by the relative amounts of the two solutions. This hot blend of dispersed SWNTs and dissolved UHMW-PE was slowly cooled to room temperature. A series of solution-crystallized composite films was prepared from these blended solutions having different SWNT/UHMW-

PE concentrations. The solutions were filtered into the films and dried in vacuum. Measurements. The solution-crystallized morphologies were analyzed by transmission electron microscopy (TEM) observations using a JEOL 1200EMX electron microscope operated at 80 kV. The prepared blended solution was deposited on a carbon-coated grid. After the solvent was evaporated, germanium shadowing was applied in the JEOL JEE-220 vacuum chamber. The structures of the composite films were also analyzed using staining and microtoming techniques. The dried films were stained with RuO4 vapor and embedded in epoxy resin. RuO4 vapor selectively stains amorphous regions, thus they appear darker than the crystalline regions in the TEM images. The assembly was cut into ultrathin sections 50 nm thick using a Reichert UltraCut S microtome. The film surface morphologies were also characterized by scanning electron microscopy (SEM) using a Hitachi field emission SEM S-5000 operated at 3 kV. The sample was coated by Pt-Pd with 5 Å thickness by a Hitachi ion sputter E-1030. A Perkin-Elmer Pyris 1 differential scanning calorimeter (DSC) was used to analyze the melting behavior of the samples. The DSC heating scan was performed up to 180 °C at a heating rate of 10 °C/min under a nitrogen gas flow. DSC characteristics,

18952 J. Phys. Chem. C, Vol. 111, No. 51, 2007

Uehara et al.

Figure 3. TEM images of the deposited structure of SWNT/PE ) 0 wt % (a), 0.05 wt % (b), 0.2 wt % (c), 1 wt % (d), 4 wt % (e), and 8 wt % (f) solutions prepared by adding a SWNT dispersion into hot PE solution at the boiling point of DCB. Scale bar, 5 µm (a), 500 nm (b-f).

including melting temperature (Tm) and heat of fusion, were calibrated using indium and tin standards. The sample Tm was evaluated by the peak position (Tm,peak) of the endotherm. Results and Discussion In this study, two solvents, decalin and DCB, were chosen. The decalin is most widely used for preparing a less-chainentangled precursor, yielding the resultant high-performance UHMW-PE materials.20,21 In contrast, DCB exhibits the maximum achievable concentration for SWNT dispersion among various organic solvents.9 Figure 1 depicts the deposited SWNT

structures prepared from decalin (a) and DCB (b) dispersions. The SWNT concentration was 80 mg/L for both solutions. The assembled bundles of SWNTs are observed for decalin dispersion. In contrast, the DCB dispersion gives the better distribution of SWNTs, but the small bundles still exist. However, this is not evidence that similar bundles are formed in the solution state. When the DCB solution containing SWNTs is deposited and evaporated on a TEM grid, the SWNT concentration should increase far from the initial 80 mg/L. Such a concentration increase can nominally provide the bundle structure in the TEM image.

Solution-Crystallization of Polyethylene on SWNTs The above-obtained SWNT dispersion was added to the UHMW-PE solution prepared at the boiling point of DCB. The PE molecules are completely dissolved in DCB at the boiling point. Blending these solutions produced a homogeneous distribution of SWNTs and matrix UHMW-PE molecules with the same DCB solution when it is kept at the boiling point. This blended solution was cooled to room temperature, resulting in black suspensions, and deposited on a carbon-coated grid. The obtained TEM images are presented in Figure 2. The preparation concentration of SWNT to PE was 4 wt %. Both single crystals of PE having the typical lozenge shape and dispersed SWNTs are clearly seen in Figure 2a. In the highmagnification image of Figure 2b, the ellipsoid structures are recognized in some places in SWNTs. Such structures were not observed for the SWNT dispersion (Figure 1b). Therefore, this ellipsoid structure is assigned to a precursor of solutioncrystallized PE lamellae. As seen in these TEM images, each SWNT runs through the center of several PE lamellae, and thus the PE lamellar growth is initiated from the above-mentioned precursors on SWNTs and proceeds perpendicular to the longitudinal axis of the SWNT. Corresponding lateral views of the PE lamellae are obtained in the left region of Figure 2a. Usually, the pure PE lamellae attach well to the substrate during solvent evaporation, but the stiff SWNT backbone can support the perpendicular arrangement of the PE lamellae, which prevents the lamellae from adhering to the substrate for our composite structure. Kebab-like crystallization of PE lamellae on the SWNT shish has been reported by Li et al.,12 but the lateral size of the lamellae is much smaller than that obtained in this study. A significant difference lies in the preparation concentration of SWNT to PE (4 wt % in Figure 2 vs 25 wt % in ref 12). The SWNT surfaces function as nuclei for solution crystallization of PE since the SWNT concentration controls the lateral size of PE lamellae, as shown below. In order to confirm the dependence of SWNT concentration on obtained PE lamellar morphology, we prepared a series of blended solutions with different SWNT/UHMW-PE concentrations. Figure 3 compares the TEM images of the depositions prepared from these blended solutions. For comparison, the result for pure UHMW-PE solution is also included in Figure 3a, where the overgrown PE lamellae are aggregated with each other. In contrast, the typical lozenge lamellae are obtained even at the lowest SWNT concentration of 0.05 wt %. Also, ellipsoid precursors are observable on SWNTs. With increasing SWNT concentration, the number of lamellae per unit area gradually increases, but their lateral size decreases. For quantitative analysis, the long-axis lengths of each lozenge shape were measured for over a hundred lamellae. The averaged value was dependent on SWNT concentration, as plotted in Figure 4. In the blended series containing SWNTs, the lateral size gradually decreases with increasing SWNT concentration, indicating the nucleation effect of SWNTs on solution-crystallization of UHMW-PE. In contrast, the value for pure UHMW-PE is much higher than those for the blended series, due to the remarkable overgrowth of lamellae, which nominally increases the lateral size value. At the same SWNT concentration, a very similar size was observed; thus, the effect of the lower-molecular-mass fraction is neglected in this study. However, the typical lozenge shape in Figure 3b implies that the value at the lowest SWNT concentration is ascribed to that of the lamellae composed of the lower-molecular-mass component. In contrast, the characteristic aggregation of overgrown lamellae at the higher SWNT concentrations is similar to that of pure UHMW-PE.

J. Phys. Chem. C, Vol. 111, No. 51, 2007 18953

Figure 4. Averaged long-axis length in lozenge lamellae of PE as a function of SWNT concentration. These lengths were evaluated from over 100 lamellae for each sample.

A continuous arrangement of PE lamellae was observed beyond 1 wt %. This morphology is attributed to the simultaneous growth of lamellae at different positions on the same SWNT. Such a composite structure is further developed into the network structure, where SWNT ties PE lamellae, at a higher concentration of 4 wt % (Figure 2b and Figure 3e). For 8 wt % (Figure 3f), which is the maximum concentration examined in this study, the composite backbone becomes a bundle structure, and the PE lamellae appear to be stacked on each other. These structural features are quite different from the single strand of SWMT obtained at the lower concentration. Here, it should be noted that such an obvious bundle was not observed for prior SWNT dispersions (Figure 1b). Therefore, we can assume that the PE lamellar formation assembles SWNTs into the bundle structures. Considering that the PE single crystals are usually stacked on each other for solution casting, a similar response of solution-crystallized PE lamellae prepared in this study might gather the SWNT backbones during drying after solution deposition. These results indicate that the composite structure is controllable by varying the SWNT amount in the mixture for solution preparation. A series of composite films with different SWNT/UHMWPE ratios was also prepared by casting and subsequent drying of the mixtures examined in Figure 3. Figure 5 compares the surface morphology for the composite films. The network structure is obtained beyond the critical SWNT concentration of 1 wt %, which agrees with the results obtained for the abovementioned deposited morphologies. This means that the composite morphology observed in the solution state is retained in the solid state. Increasing the SWNT concentration also enhances the stacking irregularity of PE lamellae. This is attributed to the prevention of lamellar adhesion due to the SWNT backbone, which was also recognized as the lateral view of PE lamellae for the deposited morphology in Figure 3. The internal morphologies of this series of the solutioncrystallized composite films were observed using TEM. Figure 6 depicts the images obtained for the ultrathin sections of the composite films. The ultrathin sections were microtomed perpendicular to the film surface (i.e., these images were viewed from the film edge). For pure UHMW-PE film, homogeneous stacking of laterally spread lamellae was achieved over the whole viewed area. The lamellar thickness was 9 nm, which is typical for solution-crystallized UHMW-PE.23 With increasing SWNT concentration, the regularity of lamellar stacking gradually decreased, producing the holes viewed as black regions in the TEM images. Such irregular lamellar stacking has been observed for the surface morphologies of the composite films

18954 J. Phys. Chem. C, Vol. 111, No. 51, 2007

Uehara et al.

Figure 5. SEM images of SWNT/PE ) 0 wt % (a), 0.05 wt % (b), 0.2 wt % (c), 1 wt % (d), 4 wt % (e), and 8 wt % (f) composite films prepared by filtering, followed by drying at room temperature. Scale bar, 1 µm.

(Figure 5). It is thought that backbone SWNTs prevent the regular stacking of PE lamellae during filtration, which is quite similar to the origin of the edge-viewed lamellar morphology obtained in Figure 3. Beyond a critical SWNT concentration of 1 wt %, the stacking lamellae appear to be divided into the domains of severalhundred-nanometer size. The lamellar orientation is fixed in the same domain, but the domain orientation is mosaic-like. The restricted lateral size of PE lamellae at the higher SWNT concentration, which was observed in TEM images of the deposited structures (Figure 3), would reflect on the domain

size within the dried composite film. However, SWNTs could not be observed in these TEM images even for the higher SWNT concentration. This is ascribed to the ultrathin diameter of asreceived SWNTs, ranging from 0.7 to 1.2 nm. The resultant image contrast of SWNT is much lower than that made by the staining method, and thus clear images of SWNTs could not be obtained for TEM analysis of the film internal structure. The lamellar thickness can be estimated from these TEM images. Over fifty lamellae were selected, and their thicknesses were measured for each composite film. The average value is plotted in Figure 7, as a function of SWNT concentration. A

Solution-Crystallization of Polyethylene on SWNTs

J. Phys. Chem. C, Vol. 111, No. 51, 2007 18955

Figure 6. TEM images of SWNT/PE ) 0 wt % (a), 0.05 wt % (b), 0.2 wt % (c), 1 wt % (d), 4 wt % (e), and 8 wt % (f) composite films prepared by filtering, followed by drying at room temperature. Scale bar, 100 nm.

0.05 wt % SWNT decreases the lamellar thickness to 7.2 nm, compared to the initial 9.2 nm for pure UHMW-PE. Among the SWNT/UHMW-PE composites, the higher SWNT concentration gives the higher value of the averaged lamellar thickness although such thickness differences with SWNT concentration were covered by the error bars in Figure 7. Generally, the lamellar thickness depends on the crystallization temperature for both crystallizations from solution and from melt. Therefore, the greater thickness at the lower SWNT concentration indicates the higher crystallization temperature achieved on solutioncrystallization of the UHMW-PE component. Considering that

the crystallization kinetics are determined by a balance between nucleation and chain diffusion,24 such increase of the crystallization temperature should be related to the larger amount of SWNT nuclei. The nucleation is a rate-determined step at the higher temperature, but a large number of SWNT nuclei for the composite structure emphasize the chain diffusion effect on crystallization. Thus, the solution-crystallization starts at the higher temperature in the cooling process after mixture preparation at the boiling point of DCB. Once the nuclei could be introduced at the higher temperature, the higher chain diffusion rather tends to accelerate the crystallization.

18956 J. Phys. Chem. C, Vol. 111, No. 51, 2007

Uehara et al. composite structure. The backbone SWNTs trap the PE lamellae, and thus they hinder the lamellar doubling. Small-angle X-ray scattering (SAXS) measurements may provide the quantitative information for lamellar size distribution corresponding to the melting peak width of DSC thermograms.27 For this analysis, the voids (black region within Figure 6e,f) contained in the sample films should be removed by a compression procedure below the Tm of matrix UHMW-PE lamellae. Therefore, such SAXS analysis is a future study to discuss the detailed correlation between DSC thermograms and unique lamellar morphologies of our SWNT/PE composite films. Conclusions

Figure 7. Averaged lamellar thickness as a function of SWNT concentration in the dried composite films. Thicknesses of over 50 lamellae were measured for corresponding images shown in Figure 5.

Figure 8. Comparison of DSC melting curves for the dried composite films prepared from SWNT/PE ) 0 wt % (a), 0.05 wt % (b), 0.2 wt % (c), 1 wt % (d), 4 wt % (e), and 8 wt % (f) solutions.

Finally, thermal analysis was applied to the series of composite films prepared at different concentrations of SWNT/ PE. Figure 8 compares the DSC heating profiles obtained at a heating rate of 10 °C/min. A single melting endotherm was obtained for all solution-grown composite films prepared in this study. With increasing SWNT concentration, the melting peak temperature (Tm,peak) gradually shifts higher. The Tm of polymeric crystals is parallel to the lamellar thickness, which is quite similar to that of the crystallization temperature. This phenomenon is consistent with the results obtained in Figure 7, where the lamellar thickness increased with increasing SWNT concentration. In contrast, Tm for the pure UHMW-PE film was much higher than the maximum achievable Tm for the composite films prepared at an SWNT concentration of 8 wt %. However, the averaged lamellar thickness of 9.2 nm for the former pure UHMW-PE is less than the 12.7 nm for the latter composite. This discrepancy can be attributed to the reorganization phenomenon during the DSC heating scan for solution-crystallized PE. The single-crystalline lamellae thicken with annealing due to the sliding diffusion of prior regularly folded chain stems.25 Rastogi et al.26 reported that annealing the stacked lamellar morphology induces the doubling phenomenon, where two single-crystalline lamellae are merged. Such lamellar thickening during DSC heating produces a gradual peak shift to higher temperatures. In contrast, the inclinations of Figures 7 and 8 are parallel, thus the reorganization during DSC heating could be eliminated for the composite films containing even very small amounts of SWNTs (the minimum SWNT concentration examined in this study was 0.05 wt %). Such restricted lamellar reorganization for the composite film containing SWNTssuggests a backbone effect of SWNT within the

A solution-crystallized composite structure was obtained by cooling hot blends of DCB dispersions of SWNT and UHMWPE. The growth of solution-crystallized lamellae proceeds perpendicular to the longitudinal direction of SWNTs. A unique network morphology composed of SWNT backbones and PE single-crystalline lamellae was developed above a SWNT concentration of 1 wt %. This means that SWNTs function as nuclei for solution-crystallization of the matrix PE. Such nucleation effects of SWNT were also confirmed from quantitative size analyses of the solution-crystallized PE lamellae prepared with different SWNT concentrations. The lateral length of PE lamellae decreased with increasing SWNT concentration for solution preparation, indicating the greater amount of nucleus. In contrast, their thicknesses increased with SWNT concentration, possibly due to the higher crystallization temperature with the SWNT nucleus on cooling. Beyond the abovementioned critical concentration of SWNT/UHMW-PE, the network morphology could be recognized even for the solid composite film prepared by filtering and subsequently drying the blended solutions. DSC measurements of the dried films suggested that usual lamellar thickening during DSC heating is restricted for the SWNT/UHMW-PE composite structure since SWNT backbones prevent the regular stacking of solutioncrystallized PE lamellae. References and Notes (1) Moniruzzaman, M.; Winey, K. I. Macromolecules 2006, 39, 5194. (2) Bhattacharyya, A. R.; Sreekumar, T. V.; Liu, T.; Kumar, S.; Ericson, L. M.; Hauge, R. H.; Smalley, R. E. Polymer 2003, 44, 2373. (3) Zhang, W.; Suhr, J.; Koratkar, N. K. AdV. Mater. 2006, 18, 452. (4) Liu, M.; Yang, Y.; Zhu, T.; Liu, Z. J. Phys. Chem. C 2007, 111, 2379. (5) Moniruzzaman, M.; Chattopadhay, J.; Billups, W. E.; Winey, K. I. Nano Lett. 2007, 7, 1178. (6) Star, A.; Stoddart, J. F.; Steuerman, D.; Diehl, M.; Boukai, A.; Wong, E. W.; Yamg, X.; Chung, S.-W.; Choi, H.; Heath, J. R. Angew. Chem., Int. Ed. 2001, 40, 1721. (7) Star, A.; Liu, Y.; Grant, K.; Ridvan, L.; Stoddart, J. F.; Steuerman, D. W.; Diehl, M. R.; Boukai, A.; Heath, J. R. Macromolecules 2003, 36, 553. (8) Cotiuga, I.; Picchioni, F.; Agarwal, U. S.; Wouters, D.; Loos, J.; Lemstra, P. J. Macromol. Rapid Commun. 2006, 27, 1073. (9) Bahr, J. L.; Mickelson, E. T.; Bronikowski, M. J.; Smally, R. E.; Tour, J. M. Chem. Commun. 2001, 193. (10) Haggenmueller, R.; Fischer, J. E.; Winey, K. I. Macromolecules 2006, 39, 2964. (11) Haggenmueller, R.; Guthy, C.; Lukes, J. R.; Fischer, J. E.; Winey, K. I. Macromolecules 2007, 40, 2417. (12) Li, C. Y.; Li, L.; Cai, W.; Kodjie, S. L.; Tenneti, K. K. AdV. Mater. 2005, 17, 1198. (13) Li, L.; Li, C. Y.; Ni, C. J. Am. Chem. Soc. 2006, 128, 1692. (14) Li, L.; Yang, Y.; Yang, G.; Chen, X.; Hsiao, B. S.; Chu, B.; Spanier, J. E.; Li, C. Y. Nano Lett. 2006, 6, 1007. (15) Kodjie, S. L.; Li, L.; Li, B.; Cai, W.; Li, C. Y.; Keating, M. J. Macromol. Sci., Pt. B 2006, 45, 231. (16) Lordi, V.; Yao, N. J. Mater. Res. 2000, 15, 2770. (17) Wei, C.; Srivastava, D.; Cho, K. Nano Lett. 2002, 2, 647.

Solution-Crystallization of Polyethylene on SWNTs (18) Ruan, S. L.; Gao, P.; Yang, X. G.; Yu, T. X. Polymer 2003, 44, 5643. (19) Zhang, Q.; Lippits, D. R.; Rastogi, S. Macromolecules 2006, 39, 658. (20) Chen, Q.; Bin, Y.; Matsuo, M. Macromolecules 2006, 39, 6528. (21) Ruan, S.; Gao, P.; Yu, T. X. Polymer 2006, 47, 1604. (22) Matsuoka, S. Relaxation Phenomena in Polymers; Hanser: New York, 1992. (23) Uehara, H.; Matsuda, H.; Aoike, T.; Yamanobe, T.; Komoto, T. Polymer 2001, 42, 5893.

J. Phys. Chem. C, Vol. 111, No. 51, 2007 18957 (24) Fatou, J. G.; Marco, C.; Mandelkern, L. Polymer 1990, 31, 1685. (25) Matsuda, H.; Aoike, T.; Uehara, H.; Yamanobe, T.; Komoto, T. Polymer 2001, 42, 5013. (26) Rastogi, S.; Spoelstra, A. B.; Goossens, J. G. P.; Lemstra, P. J. Macromolecules 1997, 30, 7880. (27) Yamanoto, Y.; Inoue, Y.; Onai, T.; Doshu, C.; Takahashi, H.; Uehara, H. Macromolecules 2007, 40, 2745.