Aggregation and Self-Assembly of Amphiphilic Block Copolymers in Aqueous Dispersions of Carbon Nanotubes Rina Shvartzman-Cohen, † Marc Florent, †,§ Daniella Goldfarb, § Igal Szleifer, | and Rachel Yerushalmi-Rozen* ,†,‡ Department of Chemical Engineering and the Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion UniVersity of the NegeV, 84105 Beer SheVa, Israel, Department of Chemical Physics, the Weizmann Institute of Science, 76100 RehoVot, Israel, and Department of Biomedical Engineering, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208 ReceiVed December 3, 2007. In Final Form: January 10, 2008 The self-assembly (SA) of amphiphilic block copolymers (poly(ethylene oxide)-poly(propylene oxide)-poly- (ethylene oxide)) was investigated in dispersions of single-walled and multiwalled carbon nanotubes (SWNT and MWNT, respectively) as a function of temperature. Differential scanning calorimetry (DSC) was used for characterization of the thermal behavior of the combined polymers-nanostructures system, and spin-probe electron paramagnetic resonance (EPR) was employed for probing the local dynamic and polarity of the polymer chains in the presence of nanostructures. It was found that SWNT and MWNT modify the temperature, enthalpy, and dynamic behavior of polymer SA. In particular, SWNT were found to increase the cooperativity of aggregating chains and dominate aggregate dynamics. MWNT reduced the cooperativity, while colloidal carbon black additives, studied for comparison, did not show similar effects. The experimental observations are consistent with the suggestion that dimensional matching between the characteristic radius of the solvated polymer chains and the dimensions of additives dominate polymer SA in the hybrid system. Introduction Over the past few years, block copolymers 1 have been utilized for modifying the solution behavior of nanostructures including nanoparticles, 2-4 nanorods, 5 and carbon nanotubes (CNT). 6,7 Block copolymers were shown to disperse nanostructures and colloidal moieties 8,9 in different media and to induce their assembly into mesostructures. 10-12 Recently, it was suggested that block copolymers may be used as a vehicle for directing functional nanostructures (nanoparticles, nanotubes) onto sur- faces. 13 An important prerequisite for utilization of block copolymers for shaping the assembly of nanostructures is the development of an understanding of the interactions that dominate the behavior of the combined systems. Nanostructures may affect the phase behavior, dynamics, and mechanisms of self-association in polymers-nanostructures-solvent systems. Detailed char- acterization of those systems, from the molecular to the macroscopic level, provides a difficult challenge both for experiment and theory. Among the most useful block copolymers are the amphiphilic block copolymers poly(ethylene oxide)-poly(propylene oxide)- poly(ethylene oxide), PEO y PPO x PEO y (Poloxamers (ICI) or Pluronics (BASF)). Pluronic block copolymers are known to self-assemble in water into micelles consisting of a hydrophobic core of PPO and a corona of the solvated PEO. It is accepted that the driving force for micellization in aqueous solutions is entropic. 14 Micellization is initiated (at a fixed temperature) by increasing the concentration to above the critical micellar concentration, CMC, or by increasing the temperature (at a given concentration) to above the critical micellization temperature, CMT. 15,16 An important observation is that the CMC and CMT are highly sensitive to the presence of molecular additives, 17-21 while colloidal particles, such as carbon black (CB), do not affect polymer self-assembly (SA) and do not alter the structure of the formed micelles. 8 What should one expect when nanostructures, such as carbon nanotubes (CNT), are present in solutions of self-assembled polymers? * To whom correspondence should be addressed. Telephone: 972- 8-6461272. Fax: 972-8-6472916. 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