Session H19: Focus Session: Long Range Order in Polymeric Structure and Morphology I

Phononic band gap and mechanical anisotropy in spider silk

Spider dragline silk is a semi-crystalline biopolymer exhibiting superior properties compared to synthetic polymers with similar chemical structure, such as polyamides. This is ascribed to the hierarchical nanostructure that is created in the spinning duct. During this process the aqueous solution of the two protein constituents of dragline silk is crystallized, while the macromolecules maintain their high orientation, leading to a high value of the Young's modulus (in the order of 10 GPa) along the fiber. We employed spontaneous Brillouin light scattering to measure the longitudinal modulus ($M_{//}$,\,$M_\bot$) along the two symmetry directions of the native fiber with increased (decreased) pre-strain created by stretching (supercontracting after hydration). A strong mechanical anisotropy is found; at about 18\% strain ${M_{//}}/{M_\bot}\sim 5$. Most important, an unexpected finding is the first observation of a unidirectional hypersonic phononic band gap in biological structures. This relates to the existence of a strain-dependent correlation length of the mechanical modulus in the submicron range along the fiber axis.   

Enhancing commensurability using cylinder-forming block copolymer-homopolymer ternary blends on spotted chemical patterns

Self-assembly of block copolymers can be directed by underlying chemical patterns to create well-defined nanostructures with registration and a high degree of order, and is extremely useful for multiplying feature density from the pattern, if the pattern spacing and surface chemistry are carefully selected. Best hexagonal order of cylinder-forming pure polystyrene-\textit{block}-poly(methyl methacrylate) (bulk inter-cylinder spacing $L_{0})$ on spotted chemical pattern (inter-spot spacing $L_{S})$ is typically achieved within the range of 0.96$L_{0} \quad \le \quad L_{S} \quad \le $ 1.06$L_{0}$ of the pattern and even narrower range for the density multiplication technique. Using ternary blends of block copolymer and low molecular weight homopolymers increase the range of $L_{S}$ for which direct hexagonal cylinders with registration on the spot patterns, up to $\pm $ 10{\%} from $L_{0}$, although the blends have similar $L_{0}$ with pure block copolymer. Additionally, inclusion of homopolymers relaxes the commensurability requirements for multiplying the cylinder density on the spot patterns.   

Supramolecular Structure of inclusion complexes of $\beta $-cyclodextrin with PEO-b-PPO-b-PEO Triblock Copolymer

Inclusion complexes, formed by non-covalent host-guest interactions, can be useful as building blocks for constructing supramolecular structures. Cyclodextrins (CDs), due to their good water-solubility and ability to include a wide range of guest molecules, have been the most intensively studied host molecules. CDs are shaped like a shallow truncated cone, with hydroxyl groups on the outer surface of the molecule. The cavity is hydrophobic and can act as a host for a great variety of molecular guests. A series of host-guest inclusion complexes were prepared with $\beta $-cyclodextrin ($\beta $-CD) and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers of varying molecular weights and compositions. The middle PPO block of the copolymers can be selectively included by $\beta $-CD to form an inclusion complex while the PEO blocks cannot. The inclusion complexes with $\beta $-CD have been studied by two-dimensional rotating frame overhauser effect spectroscopy (2D ROESY) and solid-state NMR experiments. These inclusion complexes can further aggregate into supramolecular structures in aqueous solution. The formation of self-assembled supramolecular structures has been observed by transmission electron microscopy (TEM) and one- and two- dimensional x-ray diffraction (1D and 2D WAXD).   

Periodic Polymers for Control of PhoXons, where X = t + n

1, 2 and 3D periodic nanoscale composites with either dielectric or impedance contrast or both, can serve as photonic and or phononic crystals for electromagnetic and elastic waves. Compared to electromagnetic waves, elastic waves are both less complex (longitudinal modes in fluids) and more complex (longitudinal, transverse in-plane and transverse out-of-plane modes in solids). Engineering of the dispersion relation between wave frequency w and wave vector, k enables the opening of band gaps in the density of phoXon modes and detailed shaping of w(k). Periodic polymeric materials can be made by self assembly of block polymers and by interference lithography. Band gaps can be opened by Bragg scattering, anti-crossing of bands and particle resonances. Current interest is in our group focuses using design - modeling, fabrication and measurement of polymer based phoXonic crystals for applications as tunable optics and control of phonon flow.   

Green Manipulation of Microdomain Structures in Block Copolymer Thin Films using Density Fluctuating Supercritical Carbon Dioxide

Self-assembling block copolymers are now commonly utilized to obtain well-defined arrays of nanoscopic structures in polymer thin films. Here, we introduce the use of supercritical carbon dioxide (scCO$_{2})$ as an environmentally ``green'' alternative to typical external fields, i.e. high temperature annealing, used to enhance the ordering of the nano-arrays. Various scCO$_{2 }$conditions, including the density fluctuation ridge where anomalous swelling behavior and miscibility can be witnessed in homopolymer thin films, were used for poly(styrene-\textit{block}-butadiene-\textit{block}-styrene) (SBS) triblock copolymer thin films. The surface structures of the SBS thin films were then investigated by using GISAXS and AFM. The details of this experiment will be discussed in the talk.   

Solvent Annealing Strategies for the Self-Assemblty of Low Molecular Weight Block Copolymers

Templated self-assembly of block copolymer (BCP) films is of intrest for lithographic applications. As the drive for smaller features continues, we explore the extensibility of block copolymer microphase separation towards smaller periods in a series of low molecular weight (5 -15 kg/mol) poly(styrene-b-dimethylsiloxane) (PS-PDMS) diblock copolymers. This material exhibits a large segmental Flory-Huggins interaction parameter (~0.26), enabling microphase separation with a periodicity of <10 nm in 5kg/mol PS- PDMS at room temperature. One of the challenges in low molecular weight systems is to obtain self-assembly while avoiding dewetting. Here we describe the use of solvent annealing and substrate functionalization processes to accomplish microphase separation. We also discuss the use of mixed solvents to selectively swell different blocks of the copolymer resulting in a change in the final morphology of the self-assembled features, e.g. a cylindrical-morphology BCP forming perforated lamellae.   

The Gelation Transition in Confinement: A Field-Theoretic Model and Mean-Field Solution

Reactions among multifunctional monomers yield networks that may grow to macroscopic sizes, thereby resulting in a sol-gel phase transition. The classical Flory-Stockmayer theory of gelation relies on probabilistic arguments to calculate the percolation threshold leading to the formation of an infinite network. Subsequently, Gordon and coworkers reproduced the predictions of Flory-Stockmayer theory by employing statistical mechanical and graph theoretical methods (Gordon and Judd, Nature 1971). However, the description of inhomogeneous polymer networks, such as the formation of microphases during copolymerization, necessitates a field-theoretic model of network formation with account for all isomeric chain conformations. We present a field-theoretic model of reactions among multifunctional monomers, based on the approach of Gordon and coworkers. As an illustration of our model, we quantify the effect of one-dimensional confinement on the gelation transition via the numerical solution of the self-consistent field-theoretic equations.   

Phase equilibria in polymer-blend thin films

To describe equilibrium concentration profiles in thin films of polymer mixtures, we propose a Hamiltonian formulation of the Flory-Huggins-de Gennes theory describing a polymer blend thin film. We first focus on the case of 50:50 polymer blends confined between anti-symmetric walls. The different phases of the system and the transitions between them, including finite size effects, are systematically studied through their relation with the geometry of the Hamiltonian flow in phase space. This method provides an easy and efficient way, with strong graphical insight, to infer the qualitative physical behavior of polymer blend thin films. The addition of a further degree of freedom in the system, namely a solvent, may result in a chaotic behavior of the system, characterized by the existence of solutions with exponential sensitivity to initial conditions. Such solutions and there subsequent contribution to the out-of-equilibrium dynamics of the system are well described in Hamiltonian formalism. A fully consistent treatment of the Flory-Huggins-de Gennes theory of thin film polymer blend solutions, in the spirit of the Hamiltonian approach will be presented. 1. M. Souche and N. Clarke, J. Chem. Phys., submitted.   

Facile Alignment of Amorphous Poly(ethylene oxide) Microdomains in a Liquid Crystalline Block Copolymer Using Magnetic Fields

Large area microdomain alignment in poly(ethylene oxide-b-methacryalte/LC) diblock copolymers was successfully accomplished in the melt state by application of a 6 T magnetic field while cooling from elevated temperatures. Small angle X-ray scattering demonstrated that both lamellar and cylindrical PEO microdomains oriented with their interfaces along the applied field while the smectic layers of the liquid crystalline mesophase are perpendicular to the field. This is in agreement with the positive diamagnetic anisotropy of the cyano-biphenyl mesogen and a homogeneous anchoring condition at the inter-material dividing surface (IMDS) between the two blocks. Addition of homopolymer acrylic acid and LiClO4 suppress PEO crystallinity and produce stronger segregation between the polymer blocks leading to improved order. Our experiments demonstrate the creation of well aligned arrays of amorphous PEO domains over large length scales and offer a route to functional materials, in particular for selective transport applications such as solid ionic electrolytes.   

Ordering of Triblock Copolymer Surfactants by Blending with a Room Temperature Ionic Liquid

Well-ordered block copolymer morphologies were obtained by blending Pluronic{\textregistered} PEO-PPO-PEO triblock copolymer surfactants with the room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate. The selective association of the ionic liquid with the PEO blocks raises the effective interaction parameter between the PEO and PPO blocks. Therefore, the copolymer/ionic liquid blends form well-ordered microdomains in the melt, whereas the neat copolymers are phase mixed in the melt. The ionic liquid was confirmed to interact with the PEO chains by a depression in the melting point of the PEO crystals with increasing ionic liquid concentration. Wide angle x-ray scattering and polarized optical microscopy also indicate disruption of PEO crystallization in the blends. Infrared spectroscopy also indicates a strong interaction between the PEO blocks and ionic liquid. The formation of well-ordered microdomains is demonstrated by small angle x-ray scattering experiments, which show the appearance of higher order peaks with increasing ionic liquid concentration.