### Session N29: Block Copolymers II

 Wednesday, March 23, 2005 8:00AM - 8:12AM N29.00001: The melt, lyotropic and aqueous phase behavior of poly(ethylene oxide)-poly(butadiene) block copolymers Sumeet Jain , Frank Bates The melt, lyotropic and micellar phase behavior of poly(ethylene oxide)-poly(butadiene) (PEO-PB) block copolymers was studied using small-angle x-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM). A series of sample solutions with varying copolymer concentration were investigated for a set of block copolymers spanning a range of composition and molecular weight. The melt phase behavior was consistent with the self-consistent mean-field calculations for diblock copolymers. A sequence of lyotropic liquid crystalline morphologies was documented upon gradual variation of water in block copolymer solutions. Swelling of the hydrophilic domain with addition of water drives phase transitions from cubic $\to$ hcp $\to$ lamellar $\to$ inverse hcp $\to$ inverse cubic $\to$ micellar. The results establish that the molecular composition and the extent of swelling are the two main parameters governing the self-assembly of lyotropic phases. Despite the thermotropic nature of the PEO-water interactions, the phase behavior of PEO-PB block copolymers was primarily lyotropic. The evolution of micellar phases depends upon the lyotropic morphology of copolymer solutions prior to the complete saturation of hydrophilic block. The findings are discussed in context of various factors that govern the self-assembly behavior in different regimes. Wednesday, March 23, 2005 8:12AM - 8:24AM N29.00002: Visualizing worm micelle dynamics and phase transitions of a charged diblock copolymer in water Dennis Discher , Yan Geng Assemblies of block copolymer amphiphiles are sometimes viewed as glassy, frozen, or static colloids, especially in strongly segregating solutions. Here we visualize by fluorescence microscopy and AFM the dynamics and transitions of single cylindrical micelles and vesicles composed of a charged diblock copolymer in water. In mapping the salt- and pH-dependent phase diagrams of a near-symmetric diblock of polyacrylic acid--polybutadiene, low pH and high salt (NaCl, CaCl)$_{2}$ neutralize and screen the charged corona sufficiently to foster membrane formation and generate vesicles. Decreased salt and neutral pH increases intra-coronal repulsion and drives a transition to multi-branched cylinders and highly stable, but fluid and flexible worm micelles. Ca$^{2+}$ both stiffens cylinders and stabilizes them relative to spheres. Further increase of intra-coronal repulsion generates spherical micelles by fragmentation and pinch-off at the ends of worms. Both transition kinetics and phase diagrams indicate divalent cation is about 5-10 fold more effective than monovalent in stabilizing all non-spherical morphologies. Wednesday, March 23, 2005 8:24AM - 8:36AM N29.00003: Platelet self-assembly of a tetrablock copolymer in pure water Enrique Gomez , Timothy Rappl , Vivek Agarwal , Arijit Bose , Carlos Marques , Nitash Balsara An amphiphilic tetrablock copolymer was synthesized via anionic polymerization, selective hydrogenation and sulfonation to create an A-B-C-A polymer where the hydrophilic ends are poly(styrene sulfonate) and the middle blocks are incompatible poly (methyl butylene) and poly (ethyl ethylene). The aggregation behavior of these polymers in water was studied using dynamic and static light scattering as well as light and electron microscopy. Both scattering and direct imaging experiments are consistent with monodisperse (s = 0.14) monolayer platelets with radii of 147 nm at 45 °C, while at temperatures below 38 °C we find coexistence of the platelets with micelles. Wednesday, March 23, 2005 8:36AM - 8:48AM N29.00004: Effect of the Soluble Block Size on Spherical Diblock Polymer Micelles Isaac Larue , Mireille Adam , Marinos Pitsikalis , Nikos Hadjichristidis , Sergei Sheiko , Ekaterina Zhulina , Michael Rubinstein In order to understand the effect that the soluble block has on the equilibrium size and shape of polymer micelles, we have characterized spherical micelles formed from two series of polystyrene-b-polyisoprene in dilute solutions of the selective solvent, heptane. We show that varying the soluble (PI) block from 10-100K when the insoluble (PS) block is kept constant (20K and 40K) changes the CMC by over an order of magnitude for both series and the aggregation number by an order of magnitude for the 20K series and a factor of 3 for the 40K series. We have also studied the effect that temperature has on the CMC of the two series. The results are found to be in good agreement with recently developed theory. Wednesday, March 23, 2005 8:48AM - 9:00AM N29.00005: Phase Behavior and Local Dynamics of Concentrated Triblock Copolymer Micelles Hasan Yardimci , Brian Chung , James L. Harden , Robert L. Leheny We investigate the phase behavior and local dynamics of aqueous solutions of the triblock copolymer polyethylene oxide (PEO) - polypropylene oxide (PPO) - polyethylene oxide (Pluronic F108 and F68) by neutron scattering. In solution the polymer self-associates into spherical micelles with PPO cores and corona of solvated PEO. For sufficiently high concentration the Pluronic evolves on heating from fully solvated polymer to a micellar liquid phase then to a micellar crystal phase. The temperature range of the micellar liquid region increases with decreasing chain length, a feature we attribute to fluctuation effects as predicted by recent theory. The local micellar dynamics probed in neutron spin echo display fast and slow modes that depend systematically on concentration and temperature as the liquid-crystal phase boundary is traversed. However, these dynamics are surprisingly insensitive to phase and macroscopic rheology. Contrast matching the PEO corona to the solvent reveals that the slow mode corresponds to translational motion of the core while the fast mode is related to motion of the corona. Wednesday, March 23, 2005 9:00AM - 9:12AM N29.00006: Multicompartment Micelles from ABC Star Terpolymers Zhibo Li , Marc Hillmyer , Timothy Lodge We have synthesized a series of ABC star terpolymers with three mutually immiscible polymeric components: a fluorocarbon, a hydrocarbon, and a hydrophilic segment. We have observed a new class of multicompartment micellar structures in dilute aqueous solutions by cryogenic transmission electron microscopy. The star architecture enforces interfacial contacts among the three components. The incompatible fluorocarbon and hydrocarbon form separate disk-like micellar cores that are protected from the water by the hydrophilic segment. The flat micelle cores are due to the strongly unfavorable interaction between segment pairs. The structures that emerge depend on the relative lengths of the segments and can be tuned from discrete multicompartment micelles to extended wormlike micelles with segmented cores. Wednesday, March 23, 2005 9:12AM - 9:24AM N29.00007: Brownian Dynamics Simulation of Multiblock Copolymers in Selective Solvents Yongsheng Liu , Huifen Nie , Rama Bansil In this talk we will present results of Brownian dynamics simulation on triblock (ABA, BAB), penta-block (ABABA, BABAB) and hepta-blocks (ABABABA or BABABAB) in selective solvents for the A blocks to study the effect of varying the number of blocks, solvent selectivity, concentration and temperature on the association behavior of multi-blocks. Structure factors and percolation analysis of the clusters was used to characterize the resulting structure. At a concentration of 20{\%} we obtained micellar clusters arranged in a BCC lattice, in agreement with scattering experiments. The ratio of number of loops to bridges decreases as the number of blocks in the copolymer increases, as does the polydispersity. The size of the clusters was larger when the outermost block was in the poor solvent condition. Our results imply that as the number of blocks increases it is favorable to form bridges instead of loops. This leads to a larger number of smaller clusters with more bridges. Wednesday, March 23, 2005 9:24AM - 9:36AM N29.00008: Structure and Properties of PBO-PEO Diblock Copolymer Modified Epoxy Junxian Wu , Yonathan Thio , Frank S. Bates Amphiphilic diblock copolymers poly(n-butylene oxide)-b-poly(ethylene oxide) (PBO-PEO) of various compositions were synthesized and studied as modifiers for epoxy resins. In blends of PBO-PEO, epoxy resin, and curing agent, the copolymers formed well-defined microstructures that persisted upon curing of the epoxy. The resulting morphologies were vesicles, wormlike micelles, and spherical micelles (in order of increasing size of PEO block), as well as transitional morphologies. Addition of 5{\%} by weight of these block copolymers improved the fracture toughness of the epoxy remarkably (by as much as nineteen times) with relatively small decrease in the elastic modulus. The highest level of toughness was measured in a system containing branched wormlike micelles. Close examination of the fracture surfaces of these composites suggests that while all the dispersed morphologies played a similar role to inclusions in particle-toughened thermosets, crack deflection toughening contributed to the significantly higher levels of toughness in the wormlike micelle systems. Wednesday, March 23, 2005 9:36AM - 9:48AM N29.00009: Complex Micelle Morphologies Constructed by Charged Block Copolymer Self-assembly Darrin Pochan The combination of electrostatic interactions and block conformational control, when combined with traditional block copolymer parameters such as relative block length, architecture, and amphiphilicity, provides the opportunity to build novel self-assembled structures. Two sets of molecules will be discussed. The first is a synthetic, linear triblock copolymer with two hydrophobic blocks connected to a charged hydrophilic block. Assembling these molecules in water/organic solvent mixtures containing multivalent organic counterions produces biomimetic micelles such as toroids and disks. These nanostructures can be chosen with the correct choice of counterion and solvent conditions. The second molecule is a linear diblock copolypeptide containing a rigid rod alpha helical hydrophobic block and a random coil, charged hydrophilic block. The self-assembly is defined by the helicity of the hydrophobic block. By controlling the kinetics of assembly one can form interconnected hydrogels, twisted fibrils, or hexagonal plates, all anchored by hydrophobic cores of alpha helical peptide. Transmission electron microscopy, neutron scattering, atomic force microscopy, circular and infrared spectroscopy, and light scattering results will be presented. Wednesday, March 23, 2005 9:48AM - 10:00AM N29.00010: The Effect of Counterion Valency and Solvent Properties on Charged Amphiphilic Triblock Copolymer Assembly into Disks, Cylinders, or Spheres Zhibin Li , Zhiyun Chen , Honggang Cui , Kelly Hales , Kai Qi , Karen Wooley , Darrin Pochan Amphiphilic triblock copolymer with one charged/ion-containing block, polystyrene-b-polymethacrylate-b-polyacrylic acid (PS-PMA-PAA), was studied in a water/THF cosolvent system. By adding an amine-based divalent counterion, polymeric disc micelles were formed. Discs or cylinders can be purposefully formed by choosing the type and amount of the divalent counterions in the system. In comparison, systems without counterion or with monovalent amine counterions will also be discussed. In addition to the counterion effect, the charges and the ionization along the PAA chain can also be tuned by changing the water ratio in the solvent system. This PAA tunability through counterions and solvent content allows one to direclty tune the micellar structure formed during assembly. The system was studied using dynamic light scattering (DLS), transmission electron microscopy (TEM), cryogenic transmission electron microscopy (cryo-TEM), and small angle neutron scattering. Wednesday, March 23, 2005 10:00AM - 10:12AM N29.00011: Separation of PS-PMMA block copolymers from PS precursors via selective adsorption on nanoprous silica Chang Yeol Ryu , Junwon Han We report a simple adsorption-based separation method using nanoporous silica in solution via controlling solvent quality to remove polystyrene (PS) homopolymers from polystyrene-poly(methyl methacrylate) (PS-PMMA) diblock copolymers. In particular, the solvent quality is controlled by employing binary mixed solvents of THF (good solvent) and isooctane (nonsolvent for both PS and PMMA). The aim of this work is to qualitatively study the competitive adsorption between PS and PS-PMMA and to provide a correlative understanding of polymer adsorption in nanopores with interaction chromatography techniques. In addition, the quantitative understanding of polymer adsorption is further employed to develop a simple polymer separation scheme for manipulating polymer adsorption via solvent quality. In particular, concentration changes of PS and PS-PMMA in the supernatant solution have been quantitatively measured for the adsorption studies using solvent gradient interaction chromatography techniques. We found that the PS-PMMA (43k-32k) selectively adsorb over PS (43k) precursors at the THF composition window between 42 {\%} and 55{\%} in THF/IO (v/v) mixed solvents. For THF/IO solvents with composition higher than 60 {\%} THF, polymers did not adsorb to the nanoporous silica due to the good solvent quality. Wednesday, March 23, 2005 10:12AM - 10:24AM N29.00012: Influence of Grain Boundaries on the Deformation Behavior of Block Copolymers: In Situ SAXS Tensile Deformation and Simulation of Bicrystals Panitarn Wanakamol , Theodora Tzianetopoulou , Mary C. Boyce , Edwin L. Thomas The evolution of the microdomain structure of block copolymers (BCP) during tensile deformation have been studied using transmission electron microscopy (TEM) and small angle x-ray scattering (SAXS). Most previous studies have been conducted on isotropic, polygranular materials, where the role of grain boundaries on the deformation is largely unknown. In order to develop a fundamental understanding of boundary defects on the deformation, we have utilized model bicrystals. Such ideal grain boundaries are made by first producing a near-single crystal BCP sample using the roll casting process and then cutting appropriate pieces and adhering these together to yield bicrystal BCP specimens with various types of tilt or twist boundaries. The reciprocal space patterns obtained dynamically using SAXS and step-scanning the small cross-section beam across the boundary region after each increment of applied strain provide detailed insight into the structural evolution of the microdomain across the grain boundaries. A theoretical study using finite element analysis of the deformation of each grain was also performed to compare with the experimental work. Wednesday, March 23, 2005 10:24AM - 10:36AM N29.00013: Grain Growth Kinetics of AnBn Star Block Copolymers in Supercritical Carbon Dioxide Xiaochuan Hu , Samuel Gido , Thomas Russell , Hermis Iatrou , Nikos Hadjichristidis , Ferass Abuzaina , Bruce Garetz Using a series of lamellae-forming AnBn (n = 1, 2, 4, and 16) miktoarm star block copolymers, the effect of the number of arms on the grain growth kinetics has been investigated by annealing in supercritical carbon dioxide (CO2). Across this series all materials have the same A and B block molecular weight and all have the equal number (n) of each type of arm. The grain growth was monitored in real space by transmission electron microscopy, followed by subsequent micrograph image analysis. It was found that supercritical CO2 could be used to promote the grain growth of these AnBn star block copolymers at relatively low temperatures. Also, the molecular architecture was found to have a significant impact on the grain growth kinetics. The grain growth of these AnBn stars annealed in supercritical CO2 was then compared to a previously completed grain growth study of the same materials under simple thermal annealing. It was found that the grain growth kinetics for the AnBn stars with n = 2, 4, and 16 were similar for both supercritical CO2 and thermal annealing. However, the grain growth of the diblock (AnBn with n = 1) was dramatically enhanced in supercritical CO2 relative to thermal annealing. Wednesday, March 23, 2005 10:36AM - 10:48AM N29.00014: A Mesoscopic Archimedean Tiling Having a New Complexity in ABC Star-shaped Block Terpolymers Tomonari Dotera , Atsushi Takano , Wataru Kawashima , Atsushi Noro , Yoshinobu Isono , Nobuo Tanaka , Yushu Matsushita The Archimedean tiling ($3^2.4.3.4$) is a regular but complex polygonal tessellation of equilateral triangles and squares. We have found the tiling in a melt of an ABC star-shaped polymer alloy molecule composed of polyisoprene, polystyrene and poly(2-vinylpyridine). The circumstance of a molecule splits into multiple sites and consequently two microdomains with different sizes and shapes are formed for one component. This complexity is the first observation in polymeric alloy systems and can lead to a new type of mesoscale self-organization. N29.00015: Nanotransforming Assemblies Dennis Discher , Yan Geng Degradable polymeric materials with hydrolysable backbones have attracted much attention because they break down to non-toxic metabolites. They are the key solutions to many environmental problems, and are particularly useful for various biomedical applications. Much work has been focused on degradable polymers and their co-polymers as bulk, or films and monolayers.$^{2}$ Only limited work has explored the degradable amphiphilic copolymer self-assemblies (spherical micelles, worm micelles and vesicles) in solutions, which are quite important for soft-material engineering. Mostly spherical micelles, and in rare cases, vesicles, have been reported made from copolymers with degradable polyester, typically polylactide or polycaprolactone, as the hydrophobic block, connected to biocompatible, stealthy poly (ethylene oxide) as hydrophilic block. Morphological change of such spherical micelles induced by degradation is subtle, and the degradation kinetics and mechanism in assemblies, which can be quite different from that in bulk or film, are not well understood. Here we will describe the phase transformations of worm micelles and vesicles as they degrade and also highlight how these polymeric self-assemblies interact with lipid membranes. N29.00016: Mesophase formation of block copolymer in cylindrical nanopore June Huh , Won Ho Jo , Kyusoon Shin , Hongqi Xiang , Jiun-Tai Chen , Thomas P. Russell We investigate the influence of the confinement on the mesophase formation of diblock copolymer caged in a cylindrical pore in which the surface of the pore preferentially attracts one of the blocks. Using cell dynamics simulation, we construct phase maps as a function of the composition of diblock copolymer ($f)$ and the pore diameter ($D)$ relative to the period at bulk ($L_{o})$. Depending on $f$ and $D$/$L_{o}$, we observe a variety of confinement-induced mesophases ranging from a simple dartboard-like structure to more complicated structures involving various forms of helices or doughnuts. We also find that the creation of a new central domain of the layered structure in the cylindrical pore occurs at a critical value of $D$/$L_{o}$ larger than the critical value for the layered structure under the flat confinement, indicating that the block copolymers under curved confinement afford to be more stretched than that under flat confinement. These results are compared with experimental observations.