Article pubs.acs.org/Macromolecules
Positron Annihilation Spectroscopy of Polystyrene Filled with Carbon Nanomaterials Somia Awad,† H. M. Chen,† Brian P. Grady,§ Abhijit Paul,‡ Warren T. Ford,‡ L. James Lee,∥ and Y. C. Jean†,* †
Department of Chemistry, University of MissouriKansas City, 5009 Rockhill Rd., Kansas City, Missouri 64110, United States Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078, United States § Carbon Nanotube Technology Center (CaNTeC) and School of Chemical, Biological, and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States ∥ Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, Ohio 43210, United States ‡
ABSTRACT: Positron annihilation lifetime spectroscopy (PALS) was employed to study the free volume properties of polystyrene (PS) containing three different types of carbon nanoparticles: polystyrenegrafted single wall carbon nanotubes (SWCNTs-g-PS), single wall carbon nanotubes (SWCNTs), and carbon nanofibers (CNFs). The glass transition temperature measured via PALS was significantly lower than that from differential scanning calorimetry (DSC), although qualitatively the two methods agreed in that the Tg measured increased as nanotubes were added to the material. There were some specific differences between the two measurements which may have been related to the fact that DSC does not measure Tg of a portion of the material which is immobilized on the surface of the particle, while PALS measures all polymer, whether immobilized or not. PALS was also used to measure the thermal expansion coefficient and distributions of the free volume of the polystyrene.
1. INTRODUCTION Polymer nanocomposites have been investigated extensively due to their advanced mechanical, thermal, and physicalchemical properties.1−5 The interaction between polymers and filler surface is much more important than with conventional fillers, because the interfacial area is much larger for the nanofillers. Glass transition temperature (Tg) is an important parameter and relates to the modulus, brittleness and permeability of polymers. In nanocomposites, the effect of the addition of filler on Tg is expected to depend on the interaction energy between the polymer chains and nanoparticle surfaces. Because of their high aspect ratio and specific surface area,6 high Young’s modulus,7 high tensile strength, electrical conductivity and thermal conductivity,8,9 carbon nanotubes (diameter of 2−80 nm and typical length of 0.5− 10 μm)10−12 and nanofibers (diameter of 70−200 nm and length 50−100 μm)10,13,14 are considered as excellent fillers for the development of advanced multifunctional polymer-based materials. A single-wall carbon nanotube (SWCNT)15−17 can be treated as a conformal mapping of the two-dimensional hexagonal lattice of a single graphene sheet onto the surface of a cylinder. The graphite sheet may be ’rolled’ in different orientations along any two-dimensional lattice vector (m,n) which then maps onto the circumference of the resulting cylinder; the orientation of the graphite lattice relative to the axis defines the chirality or helicity of the nanotube.8 Carbon nanofibers (CNFs)18 are mainly differentiated from nanotubes © 2012 American Chemical Society
by the orientation of the graphene planes: whereas the graphitic layers are parallel to the axis in nanotubes, nanofibers can show a wide range of orientations of the graphitic layers with respect to the fiber axis. CNFs can be visualized as stacked graphitic disks or (truncated) cones, and are intrinsically less perfect as they have graphitic edge terminations on their surface. Because of the poor solubility of CNTs, chemical functionalization of its surface with suitable groups was proposed to enhance the solubility, to induce a better dispersion, and to improve bonding strength between nanotubes and the polymer matrix.19 Functionalizing nanotubes with a compatible polymer with the matrix20 provides one of the best possible interfaces with a host polymer. Carboxylic acid groups created on the CNT surface by oxidation21 provide opportunity to synthesize many different functional groups on SWCNTs22 as well as polymers grafted to SWCNTs. Chemically modified carbon nanotubes are more dispersible than pristine nanotubes in most solvents23 and are more easily incorporated into a polymer matrix. Particle−matrix interactions play an increasingly important role as the filler size drops below 1 μm. It is not the absolute size but rather the specific surface area of the filler, and the resulting interfacial volumes, which significantly influence the properties of the final composite. Interfacial regions can have Received: November 7, 2011 Revised: December 21, 2011 Published: January 6, 2012 933
dx.doi.org/10.1021/ma202458c | Macromolecules 2012, 45, 933−940
Macromolecules
Article
glass transition temperature compared with those from DSC method.
distinctly different properties from the bulk polymer and can represent a substantial volume fraction of the matrix for nanoparticles with surface areas of the order of hundreds of m2/g. The actual interphase volume depends on the dispersion and distribution of the filler particles, as well as their surface area. In traditional fiber composites, the interfacial region is defined as the volume in which the properties deviate from those of the bulk matrix or filler.24 A straightforward calculation25 of the interparticle distance s provides an estimate for the distance between particles: 1/3 ⎤ ⎡⎛ π ⎞ ⎥ ⎟⎟ s = d × ⎢⎢⎜⎜ − 1⎥ 6 φ ⎦ ⎣⎝ v ⎠
2. EXPERIMENTAL SECTION 2.1. SWCNT/PS Material Preparation. SWCNT (HiPCo, Carbon Nanotechnologies Inc., Houston, TX) gently treated with nitric acid21 were used to prepare the SWCNT/PS composites67,68 and the SWCNT-g-PS/PS composites.67,69 SWCNT-g-PS (polystyrene-grafted single walled carbon nanotubes) was prepared by the free radical addition of TEMPO [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl]ended PS of Mn = 15,000 to the SWCNT. The SWCNT-g-PS/PS composites were prepared by coagulation of a PS solution in Nmethylpyrrolidinone containing dispersed SWCNT into water.69 AFM images of the SWCNT and TEM images of the SWCNT-g-PS showed individual SWCNT and bundles of SWCNT 368 K, respectively. The variation from PALS in these two regions is consistent with the reported results obtained by different techniques in similar polystyrene systems.73 We therefore fitted two regions data in two linear regressions and obtained two lines as shown in Figure 3.PALS results do not show hysteresis, which indicates that there is no physical or chemical aging in the temperatures between 300 and 433 K during a period of one month of the PALS experiments. Therefore, the increase of o-Ps lifetime or freevolume size is simply thermal expansion of free volume. In the low T region ( Tg ].80 This large difference comes from the fact that PALS probes only the free volume while the bulk αbulk is contributed from a fraction of free-volume expansion data.62,63 The SWCNT-g-PS/PS and CNF samples show a decrease in thermal expansion coefficient with added nanotubes, while the trend for the SWCNT/PS sample is not clear.
4. CONCLUSIONS We have reported a systematic study of free volumes in a series of polystyrene/carbon nanoparticles composites as a function of temperature between 298 and 433 K using PALS. Overall, nanotubes shift the Tg to higher temperature, which is in agreement with DSC studies. The thermal expansion coefficient drops as expected since the carbon filler has a smaller thermal expansion coefficient than the polymer; however PALS measures just the free volume expansion in the polymer so this drop could not necessarily be predicted.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone: 816-235-2295.
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ACKNOWLEDGMENTS S.A. wishes to thank the financial support of the channel system and mission department of Egypt. This research is supported by the NSF-sponsored Nanoscale Science and Engineering Center for Affordable Nanoengineering of Polymeric Biomedical Devices (NSEC-CANPBD), the National Institute of Standards, and Technology and Army Research Office (W911NF-101-0476), the Oklahoma State Regents for Higher Education (W.T.F.), and the Department of Energy (DE-FGO206ER64239 to B.P.G.). We acknowledge Dr. Xiaohong Gu’s involvement with this project.
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