Orientation Structures in Injection-Molded Pellets of Polystyrene

Article pubs.acs.org/IECR

Orientation Structures in Injection-Molded Pellets of Polystyrene/ Carbon Nanotube Nanocomposites Cuiping Yuan,†,‡ Guangming Chen,*,† and Jiping Yang*,‡ †

Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ School of Materials Science and Engineering, Beihang University, Beijing 100191, China ABSTRACT: The orientation structures of both carbon nanotubes (CNTs) and polymer molecular chains in the skin−core structure of polystyrene/multiwalled carbon nanotube (PS/MWCNT) nanocomposite melt-injection-molded pellets were examined. Scanning electron microscopic (SEM) and transmission electron microscopic (TEM) images clearly show the MWCNT preferred orientation along the melt shearing direction, while polarized Raman spectroscopy results consolidate the MWCNT preferred orientation in each of the skin, medium, and core layers of the injection-molded pellets. By infrared dichroism measurements, a preferred orientation of the PS side groups (phenyl rings) perpendicular to the melt shearing direction was deduced at all depths of the present injection-molded nanocomposite pellet. Meanwhile, the MWCNTs have little effect on the random orientation of the PS main chain (−CH2− groups) with a dichroic ratio near unity.

1. INTRODUCTION Carbon nanotubes (CNTs) are typical one-dimensional (1D) nanomaterials with unique structures and excellent mechanical, conductive, and thermal properties.1,2 To translate the extraordinary properties of CNTs to macroscale polymer materials, various techniques of CNT dispersion in polymer matrixes have been developed. So far, the studies of polymer/ CNT nanocomposites concentrate on the preparation, structural characterization, and properties.3−11 Both crystallization and orientation belong to condensed structures, which have significant effects on a material’s macroscale properties. In sharp contrast to the extensive studies of polymer crystallization in polymer/CNT nanocomposites,5−7 the research of orientation structures, especially polymer chain orientation, in polymer/CNT nanocomposites is very limited. In the reported studies of orientation behaviors in polymer/ CNT nanocomposites,12−19 most focus on the orientation of CNTs and/or polymer crystallites in polymer/CNT nanocomposite film or fiber specimens. As for melt-injection molding, one of the most important processing techniques for polymeric materials, studies of orientation structure in skin−core regions for melt-injection-molded pellets of polymer/CNT nanocomposites have seldom been reported. So far, to investigate the CNT orientation in polymer/CNT nanocomposite films and fibers, polarized Raman spectroscopy,13−18 scanning electron microscopy (SEM),12−14 and transmission electron microscopy (TEM)14,15,19 have been employed. On the other hand, CNT-induced polymer lamella orientation16 and crystallite alignment17,19 have been observed by small- and wide-angle X-ray scattering (SAXS and WAXS)16 and two-dimensional (2D) X-ray diffraction (XRD)17,19 techniques. It is well-known that polymer chain orientation is very important for polymer material macroscale properties, especially mechanical properties. The addition of nanoscale inorganic fillers always greatly affects the polymer chain orientation due to large interfacial surface areas, strong © 2012 American Chemical Society

interfacial interactions, and limitations to polymer chain motions by adjacent inorganic fillers.20−23 Recently, a theoretical study based on molecular dynamics (MD) simulation suggested that polymer molecules around CNTs aligned parallel to the nanotube axis direction.24 Unfortunately, few experimental studies about polymer chain orientation by infrared dichroism in polymer/CNT nanocomposites have been found. Infrared dichroism is an effective and convenient technique to quantitatively study polymer macromolecular orientation, and it has been successfully employed in the research of polymer/inorganic nanocomposites.21−23 It is well-known that melt injection is a major melt-processing process for polymer materials. However, there are no reports on the orientation investigations about melt-injection pellets of polymer/CNT nanocomposites in our literature survey. In this study, we report the orientation structures of both CNTs and polymer macromolecules in melt-injection-molded polystyrene (PS)/ multiwalled CNT (MWCNT) nanocomposite pellets. The orientation structures in each of the skin, medium, and core layers of the melt-injection-molded pellets were studied in detail.

2. EXPERIMENTAL SECTION 2.1. Materials. The PS (trademark 666D, Mw ≈ 200 000) was obtained from Yanshan Petrochemical Co., China. The MWCNTs used in this study are commercial Baytubes C150P from Bayer MaterialScience AG, Germany, produced in a highyield catalytic process based on chemical vapor deposition (CVD). The lengths of most of the MWCNTs are between 1 and 10 μm, and their outer diameters mainly range from 10 to Received: Revised: Accepted: Published: 11695

May 2, August August August

2012 12, 2012 15, 2012 15, 2012

dx.doi.org/10.1021/ie301147h | Ind. Eng. Chem. Res. 2012, 51, 11695−11699

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microscope at an acceleration voltage of 15.0 kV. TEM analyses were conducted on a Hitachi H-800 electron microscope operated at 100 kV. A laser scanning confocal microscope, model Olympus (Japan), was used to image the samples which contained a fluorescent dye, Nile blue A. Polarized Raman spectra were recorded with a Renishaw inVia Raman microscope, with an excitation laser at 633 nm. At least three regions were measured, and the average values were adopted. The infrared dichroism measurements were performed on a PerkinElmer System 2000 (Perkin-Elmer Corp.) Fourier transform infrared (FTIR) spectrophotometer with a Perkin-Elmer wire grid polarizer to record the polarized FTIR spectra. The FTIR spectra were recorded at 2 cm−1 nominal resolution with an accumulation of 126 scans. For all transmission FTIR spectra, the film planes of the samples were installed perpendicular to the incident beam direction. Polarization of the beam was done by rotating the polarizer to a desired degree.

80 nm. All other chemicals and organic solvents are of analytical pure reagent (AR) grade. 2.2. Sample Preparation. First, the PS/MWCNT nanocomposites containing of 1.0 wt % MWCNTs were prepared by solution mixing according to our previous study.25 Then, the melt-injection pellets of the nanocomposite were obtained by injection molding performed at 210 ± 10 °C using a CS-183 MMX Mini Max Molder (CSI Custom Scientific Instruments, Inc.). The pellets were about 3.0 mm in thickness, 20−25 mm in length, and 12 mm in width. The film samples for polarized Raman and infrared spectroscopic measurements were sliced at different depths (as shown in Figure 1).

3. RESULTS AND DISCUSSION 3.1. MWCNT Separation and Dispersion. The MWCNT separation and dispersion in PS matrix were characterized by both laser scanning confocal microscopic (LSCM) and SEM images (Figure 2). The LSCM image reveals valuable information on MWCNT dispersion at micrometer scale. The absence of obvious aggregates (Figure 2A) and the low relative noise (2σ/x)̅ of 0.148 (Figure 2C) suggest the MWCNT dramatically separates and homogeneously disperses in the PS/ MWCNT nanocomposite. In addition, since PS is a typical brittle polymer material,26,27 the sectioned surface shown in Figure 2B is very smooth. Interestingly, a lot of individual white

Figure 1. Film sample preparation by slicing at different depths of the melt-injection-molded pellets for polarized Raman spectroscopic or infrared dichroism measurements.

2.3. Characterization. SEM measurements were carried out on a HITACHI S-4300 field-emission scanning electron

Figure 2. (A) LSCM and (B) SEM images and (C) LSCM intensity variation graph scanning along the red line in (A) of the PS/MWCNT nanocomposites. 11696

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curved lines and small points are prevalent, resulting from the pulling out of the well-dispersed MWCNTs. No obvious MWCNT aggregates can be observed. These SEM results also demonstrate that the MWCNTs have been greatly debundled and uniformly dispersed. Moreover, Figure 2B clearly shows that the separated individual MWCNTs were randomly dispersed without any preferred orientation. 3.2. MWCNT Preferred Orientation in Nanocomposite Injection-Molded Pellets. The preferred orientation structure of MWCNTs in the nanocomposite pellets was clearly observed from SEM and TEM images (Figure 3) and

Figure 4. (A) Raman spectra of (a) neat PS, (b) MWCNTs, and (c) PS/MWCNT nanocomposite; (B) polarized Raman spectra of the skin layer; and (C) measured ratios of I0°/I90° in the skin (200 μm), medium (800 μm), and core (1500 μm) layers of the melt-injectionmolded PS/MWCNT nanocomposite pellets. Figure 3. (A) SEM and (B−D) TEM images of PS/MWCNT nanocomposite melt-injection-molded pellets, wherein the arrows point to the melt shearing flow directions in the (B) skin, (C) medium, and (D) core layers sliced at different depths as shown in Figure 1.

Table 1. Raman Band Assignments of PS/MWCNT Nanocomposites

quantitatively studied by polarized Raman spectroscopy (Figure 4). As shown in Figure 3A, individual MWCNTs apparently align almost perpendicularly to the fractured surface of the injection-molded pellet. In other words, melt injection caused the MWCNT preferred orientation to be along the melt shearing flow direction. An alternative direct observation of the oriented MWCNTs in the PS/MWCNT nanocomposite can be seen in the TEM images. Figure 3B−D demonstrates the MWCNT preferred orientation structure in all three layers of the skin, medium, and core, sliced at different depths (as shown in Figure 1). Further quantitative study of the MWCNT orientation was carried out by polarized Raman spectroscopy. The band attributions according to the previous literature28−30 are shown in Table 1. Although the fluorescent background signals are relatively strong at low Raman shifts for PS (Figure 4A(a)), their interference can be neglected in the PS/MWCNT nanocomposites, as shown in Figure 4A(c). Typical polarized Raman spectra of the skin layer in the PS/MWCNT nanocomposite pellets are shown in Figure 4B. The corresponding Raman band assignments are illustrated in Table 1. In this study, the Raman bands of D (1332 cm−1), G (1577 cm−1), and G′ (2662 cm−1) were used to study the MWCNT orientation structure. From Figure 4C, all of the measured intensity ratios of I0°/I90° in the skin (200 μm), medium (800 μm), and core layers (1500 μm) for the D, G, and G′ bands are obviously larger than unity. Therefore, the uniformly dispersed MWCNTs were obviously oriented parallel

freq/cm−1

assignment

1000 1030 1332 1577 1583 1601 2662 2848 2902 3051

in-plane ring deformation + out-of-plane CH deformation in-plane CH deformation MWCNT D band, disorder mode of sidewall defects MWCNT G band, E2g symmetry stretching of graphite sheets in-plane vibration of phenyl rings in-plane vibration of phenyl rings MWCNT G′ band CH stretching aliphatic CH stretching aliphatic CH stretching of phenyl rings

to the melt shearing direction from skin to core layers in the injection-molded pellets. 3.3. Polymer Molecular Orientation Measured by Infrared Dichroism. The infrared dichroism technique was applied to quantitatively study the PS polymer chain orientation in the melt-injection-molded pellets. Five specimens sliced from different depths, as illustrated in Figure 1, were employed for comparison. Since the bands at 2922 and 2850 cm−1 are characteristic of the −CH2− asymmetric and symmetric stretching vibration modes, they were used to characterize the main chain orientation of the PS macromolecules. As for the orientation of the side groups, i.e., phenyl rings, the band at 540 cm−1 was chosen. Figure 5A presents the scheme of the infrared dichroism measurement procedure, wherein the film sample was installed perpendicular to the incident beam direction, and P∥ and P⊥ are the parallel and perpendicular polarized beams, respectively. Figure 5B shows the polarized FTIR spectra of the nano11697

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nanocomposites.33−35 Further detailed studies are under way and will be reported in the future.

4. CONCLUSIONS We show here an orientation study of MWCNTs in the skin− core structure for PS/MWCNT nanocomposite melt-injectionmolded pellets, and a quantitative experimental study of the polymer chain orientation in polymer/CNT nanocomposites by the infrared dichroism technique. The preferred parallel orientation of the MWCNTs to the melt flow direction was clearly observed and quantitatively studied for the meltinjection-molded nanocomposite pellets. The PS macromolecular main chains exhibited random orientation in all layers, sliced at different depths, of the PS/MWCNT nanocomposite pellets. In contrast, the phenyl rings in all of the skin, medium, and core layers of the PS/MWCNT nanocomposite pellets showed obvious preferred orientation behavior perpendicular to the melt flow direction, mainly due to the strong interactions among adjacent phenyl rings of the PS side groups.

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Figure 5. (A) Schematic presentation of the infrared dichroism measurement procedure, wherein transmission modes were adopted. P∥ and P⊥ are the parallel and perpendicular polarized beams, respectively. (B) Polarized infrared spectra of the skin layers of the PS/ MWCNT nanocomposites.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 62561623. Fax: +86 10 62559373. E-mail: [email protected] (G.C.); [email protected] (J.Y.).

composite film samples, sliced from the skin layers of the meltinjection-molded pellets. In Table 2, the measured dichroic

Notes

Table 2. Dichroic Ratios (R) for the Films Sliced at Different Depths of Melt-Injection-Molded PS/MWCNT Nanocomposite Pellets

ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (No. 50873103) for financial support. G.C. acknowledges the support of the K. C. Wong Education Foundation, Hong Kong.

depth [μm]

R2922

R2850

R540

200 500 800 1100 1500

1.04 1.03 1.04 1.04 1.04

1.03 1.03 1.04 1.04 1.03

0.90 0.90 0.87 0.90 0.93

The authors declare no competing financial interest.

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