Nucleation of Polypropylene Crystallization by Single-Walled Carbon Nanotubes Brian P. Grady,* Francisco Pompeo, Robert L. Shambaugh, and Daniel E. Resasco Department of Chemical Engineering and Materials Science, The UniVersity of Oklahoma, Norman, Oklahoma 73019 ReceiVed: December 20, 2001; In Final Form: March 28, 2002 Nonisothermal and isothermal crystallization experiments were performed on polypropylene mixed with carbon nanotubes produced by disproportionation of CO on Co-Mo catalysts. Functionalization of the nanotubes with octadecylamine made the tubes hydrophobic and allowed the tubes to be solubilized in an organic solvent. Mixing of the nanotubes with the polymer was accomplished by adding the nanotubes to a Decalin solution that contained dissolved polypropylene, followed by evaporation of the solvent. Dynamic mechanical analysis indicated very little difference in the small-strain mechanical properties between filled and unfilled polymers at the very low solid levels that were tested. By contrast, the crystallization behavior of the filled and unfilled polymer was quite different. Nanotubes promoted growth of the less-preferred beta form of crystalline polypropylene at the expense of the alpha form. In nonisothermal crystallization, the total amount of crystalline material in the sample was the same for the filled and unfilled materials. However, for isothermal crystallization experiments, the percent crystallinity in the filled materials was slightly higher. Most importantly, the rate of crystallization was substantially higher in the filled system. The results presented in this paper clearly show that carbon nanotubes nucleate crystallinity in polypropylene. 1. Introduction The unusually high Young’s modulus and tensile strength coupled with the low density of single-walled nanotubes (SWNT) have prompted investigations of these materials as reinforcement in polymer composites. 1 Of course, the effective utilization of SWNT as reinforcement materials is not only related to their intrinsic properties but also to characteristics such as dispersion, alignment, and interfacial properties. 2 In our laboratory, we have been particularly interested in carbon nanotubes as reinforcing fillers for thermoplastics. One unique aspect of nanotubes is that they can retain a high aspect ratio even when processed as melt-spun fibers, unlike typical micron diameter fillers. The resultant morphologies in this situation, especially in the case where the filler can nucleate crystallinity, might be totally unique. Carbon fibers, which are closely related to carbon nanotubes, can nucleate crystallinity in isotactic polypropylene. 3-9 This nucleating ability leads to transcrystallization, which is a region of highly oriented crystalline material with the c-axis parallel to the fiber surface at regions very close to the fiber surface. 10 This orientation is a result of very dense nucleation on the fiber, which consequently restricts crystal growth in the direction normal to the fiber. However, several studies have shown that the crystalline growth rate, that is, the rate of the crystal front movement independent of nucleation, is no different for a transcrystalline layer versus a normal spherulite. 5,9 The fundamental morphological characteristics of the carbon fiber that lead to the formation of this transcrystalline layer have not been established. Thomason and Van Rooyen 9 showed that high modulus (HM) carbon fibers tend to form transcrystalline layers with isotactic polypropylene, while high-strength (HS) fibers tend to not form transcrystalline layers. A similar distinction was found for syndiotactic polystyrene transcrystal- linity. 11 HM fibers can be produced from either polyacrylonitrile (PAN) or pitch, while the HS fibers are typically only produced from PAN. However, there is not enough evidence to conclude that only pitch-based fibers can cause transcrystallinity. In a second paper by Thomason and Van Rooyen, 12 the authors suggested that transcrystallinity is due to stress between the fiber and the matrix, and hence large differences in axial thermal expansion coefficients between the fiber and the polymer favor transcrystallinity. A similar explanation with some quantification was proposed by another set of authors. 13 However, both sets of authors note that fiber-melt interaction does seem to be important. In the particular case of carbon fibers, the fact that some carbon fibers form transcrystalline layers and others do not would suggest that the interaction between the polymer and the fiber surface is almost certainly important. Sizings (propri- etary coatings often applied to fibers to improve processing characteristics or fiber-matrix compatibility) may also play a role. For glass fibers, different sizings have been shown to affect the ability of the fiber to nucleate crystallinity. 14,15 Clearly, the characteristics required to induce transcrystallinity have not been established. Even if a priori such a prediction could be made for carbon fibers, the specific relevance to carbon nanotubes would not be clear. Hence, the only way to determine whether nanotubes will nucleate crystallinity is through experi- mental measurement. The purpose of this study is to determine whether carbon nanotubes nucleate crystallization in polypro- pylene. 2. Experimental Section 2.1 Materials. The SWNT used in this work have been obtained by a catalytic decomposition method based on the disproportionation of CO over Co-Mo/SiO 2 catalysts at a total pressure of 5 atm and 1123 K. In a previous study, 16 we fully characterized the structure and chemical state of these catalysts by EXAFS, XANES, UV/vis-DRS, H 2 TPR, XPS, and DRIFTS * To whom correspondence should be addressed. 5852 J. Phys. Chem. B 2002, 106, 5852-5858 10.1021/jp014622y CCC: $22.00 © 2002 American Chemical Society Published on Web 05/21/2002