Session Z47: Self-Assembly and Polymer Composites

Highly piezoelectric Biocompatible and Soft Composite Fibers

We report the fabrication of highly piezoelectric biocompatible soft fibers containing Barium Titanate (BT) ferroelectric ceramic particles dispersed in electrospun poly lactic acid (PLA). These fibers form mats that have two orders of magnitude larger piezoelectric constant per weight than single crystal barium titanate films. We demonstrate that the observed super-piezoelectricity results from the electrospinning induced polar alignment of the ferroelectric particles and the increased surface area compared to single crystal films. Due to the biocompatibility of PLA that encases the ferroelectric particles, these mats can be applied even in biological applications such as bio-sensors, artificial muscles and energy harvesting devices.   

Phase Behavior of Polymer Nanocomposites from Gravity-based Combinatorial Simulations

The phase behavior of polymers mixed with nanoparticles, termed nanocomposites, has been of great current interest. Holding the promise of improved engineering properties, the property improvement is critically dependent on the spatial dispersion of nanoparticles, which is controlled by polymer-nanoparticle phase behavior. Enumerating the appropriate phase diagrams, however, still remain outside the realm of most simulation-based methods. The difficulty arises since inserting a large particle into a dense sea of small particles, critically for establishing phase equilibrium, is prohibitively difficult. Here we extend a gravity-based combinatorial method, first devise by Biben et al., and used primarily in the context of pure fluids to circumvent this insertion issue. Our results are validated against available simulations for colloid mixtures, but provide new insight into the phase behavior of polymers and nanoparticles.   

Diffusive and re-orientation dynamics in lyotropic gels of self-assembled organic nanotubes

It is known that dispersing lithocholic bile acid (LCA) in aqueous solutions of sodium hydroxide or ammonia leads to the formation of organic nanotubes with well-defined diameters on the nanometer scale. The suspensions appear to be tunable from liquid- to solid-like via the LCA concentration. The length of the tubes is decreasing as a function of temperature, favoring the formation of a phase where the tubes form hexagonally ordered bundles for an appropriate LCA concentration and temperature. In the present study, we have used X-ray Photon Correlation Spectroscopy (XCPS) to probe the diffusive dynamics of the nanotubes in these lyotropic gels under various conditions (concentration, counter ion, temperature). The multispeckle analysis of the coherent scattering pattern reveals domains in the sample with differing diffusive dynamics. The re-orientation process of different domains of nanotube bundles into a preferred parallel alignment has been observed.   

Carbon Nanotube Liquid Crystals: Nematic Droplets and Coarsening Dynamics

On a fundamental basis, carbon nanotubes (CNTs) offer a new model molecule to explore the dynamics and phases of rigid rods and test theories. Their large aspect ratio (100 to 100,000) and persistence length ($\sim $ 100 microns) allow exploring the physics of nematic phases with high Frank elastic constant. Moreover, understanding of CNT liquid crystals is key to their rational processing into ordered materials such as fibers. Here we report the formation of elongated nematic droplets of CNTs in chlorosulfonic acid. In nematic droplets, a continuous transition from a homogeneous to bipolar nematic director field is expected theoretically, as a function of droplet volume; yet, experimental determination of such transition has been elusive. We show that CNT nematic droplets display such transition. We study the coarsening dynamics of positive and negative nematic droplets and observe that two or more droplets merge by matching their nematic director. Merging scenarios that lead to defect formation are not observed. Negative tactoids (isotropic phase in liquid crystalline continuum) merge through attractive forces induced by the nematic director distortion with quadrupolar symmetry.   

Free volume in a granular nanocomposite

Free volume plays a determinative role in the unusual molecular sieving and rheological characteristics of nanocomoposites. To help elucidate the nanoparticle influence on polymer-chain packing, we mixed acrylic spheres with aluminum ball-chains and ascertained from measurements of the bulk density the partial molar sphere volume at small but finite sphere volume fractions. This macroscopic mechanical model permits a systematic exploration of how the chain length, sphere size, and mixture composition influence free volume. We show that the nanoparticle radius to the chain loop size is the primary dimensionless parameter that bridges the solvent and continuum scaling regimes. In the intermediate regime, where the inclusion and polymer nanoscales are comparable, the primary sphere-chain interaction increases free volume---up to twice the intrinsic inclusion volume per particle---and the primary sphere-sphere interaction decreases free volume. Comparisons will be made with data gleaned from PALS and the bulk density of molecular nanocomposites reported in the literature.   

Molecular packing in bone collagen fibrils prior to mineralization

The three-dimensional packing of collagen molecules in bone collagen fibrils has been largely unknown because even in moderately mineralized bone tissues, the organic matrix structure is severely perturbed by the deposition of mineral crystals. During the past decades, the structure of tendon collagen (e.g. rat tail) --- a tissue that cannot mineralize in vivo, has been assumed to be representative for bone collagen fibrils. Small-angle X-ray diffraction analysis of the native, uncalcified intramuscular fish bone has revealed a new molecular packing scheme, significantly different from the quasi-hexagonal arrangement often found in tendons. The deduced structure in bone collagen fibrils indicates the presence of spatially discrete microfibrils, and an arrangement of intrafibrillar space to form ``channels'', which could accommodate crystals with dimensions typically found in bone apatite.   

Understanding the self-assembly of meso-tetra(4-sulfonatophenyl)porphyrin (H$_{2}$TPPS$^{4-})$ in aqueous solutions

The aggregation of \textit{meso}-tetra(4-sulfonatophenyl)porphyrin (H2TPPS4-) in phosphate solutions was investigated as a function of pH, concentration, time, ionic strength, and solution preparation (either from dilution of a freshly prepared 2 mM stock or by direct preparation of $\mu $M solution concentrations) using a combination of complementary analytical techniques. UV-Vis and fluorescence spectroscopy indicated the formation of staggered, side-by-side (J-type) assemblies. Their size and self-associative behavior was determined using analytical ultracentrifugation and small angle X-ray scattering. Our results indicate that in neutral and basic solutions of H$_{2}$TPPS$^{4-}$, porphyrin dimers and trimers are formed at $\mu $M concentrations and in the absence of NaCl to screen any ionic interactions. At these low concentrations and pH 4, the protonated H$_{4}$TPPS$^{2-}$ species self-assembles leading to the formation of particularly stable aggregates bearing 25 $\pm $ 3 macrocycles. At higher concentrations, these structures further organize or re-organize into tubular, rod-like shapes of various lengths which were imaged by cryogenic and freeze-fracture transmission electron microscopy. Micron-scale fibrillar aggregates were obtained even at $\mu $M concentrations at pH 4 when prepared from dilution of a 2 mM stock solution and/or upon addition of NaCl.   

Self-Assembly of Polystyrene Tethered Hydrophilic POSS/C60 Nanoparticles

Self-assembly of nanobuilding block provides unique opportunities to arrange nanoparticles into larger, functional ensembles. Anisotropy or symmetry breaking will lead to the formation of specific hierarchical structures with novel properties. Polymer tethered particle is one of the approaches to break the symmetry of nanobuilding blocks, which are promise to become the elementary building blocks for self-assembled materials. Shape and interactions are two important factors to determine the self-assembly of these molecules. Although a few examples have been reported during the last decade, little is known about ordered self-assembly structures from these anisotropic building blocks in bulk or solution by experiments. To solve this problem, hydrophilic POSS and C60 have been designed and synthesized, to which hydrophobic polystyrene tail(s) was tethered with controlled topology. The designed molecules could form micelles and colloidal particles in solutions. The effects of PS tail length, solvent properties, and molecular topology on self-assembly morphologies have been studied. The designed molecules could form ordered nanophase separation structures in bulk state with the feature sizes of sub-22 nm, which has great potential applications in advanced lithography technologies.   

Self-Assembly of Narrowly Dispersed Brush Diblock Copolymers with Domain Spacing more than 100 nm

Self-assembled structures of high molecular weight (MW), narrow molecular weight distribution brush block copolymers containing polylactic acid (PLA) and polystyrene (PS) side chains with similar MWs were studied in both the melt and thin films. The polynorbornene-backbone-based brush diblock copolymers containing approximately equal volume fractions of each block self-assembled into highly ordered lamellae with domain spacing over 100 nm, as revealed by SAXS, GISAXS and AFM. The domain size increased approximately linearly with backbone length, which indicated an extended conformation of the backbone in the ordered state. The length of side chains also played a significant role in terms of controlling the domain size. As the degree of polymerization (DP) increased, the symmetric brush diblock copolymers with longer side chains tended to form larger lamellar microdomains in comparison to those that have the same DP but shorter side chains.   

Non-Classical Ordering of Sphere Forming SISO Tetrablock Terpolymers

ABAC tetrablock terpolymers represent the simplest symmetry breaking multiblock extension of ABC triblocks. The model system poly(styrene-b-isoprene-b-styrene-b-ethylene oxide) (SISO) tetrablock terpolymers were synthesized in relatively monodisperse form and the resulting morphologies were characterized by small-angle X-ray scattering, transmission electron microscopy, differential scanning calorimetry and dynamic mechanical spectroscopy. Two non-classical sphere-based orderedphases have been established in these single component materials with P$_{6}$/mmm (simple hexagonal) and P$_{42}$/mnm (tertragonal sigma phase) symmtery. A third state, tentatively associated with quasicrystalline order, has been identified at temperatures between the hexagonal and sigma phases, which occur at low temperatures, and prior to disordering, respectively. This unusual set of morphologies will be discussed in the context of segregation under the constraints associated with the tetrablock molecular architecture. Self-consistent mean-field theoretical calculations, obtained using the PolySwift++ software package, provide valuable insights into the molecular configurations associated with these morphologies.   

Tear rotation in reinforced natural rubber

We analyze the impact of tear rotation, that is, an abrupt instability in the direction of propagation of a notch, on the tensile strength of natural rubber elastomers reinforced with carbon black or precipitated silica, in single edge notched samples stretched at constant velocity. As a consequence of tear rotation, the energy at break increases by a factor of 6 to 8 in some cases. We show how the tensile strength of a test sample is related to the presence of tear rotations and analyze semi-quantitatively this increase in tensile strength, based on energetic arguments, without entering into a detailed description of the elastic strain field in the vicinity of the tear tip. The proposed interpretation is based on the idea that tear rotations creates a macroscopic tip radius, which relaxes the local strain (or stress) at the tear tip. Materials reinforced with carbon black or precipitated silica aggregates show similar behavior. The relation to strain-induced crystallization is discussed.   

Quantification of Folding in Sheet-like Nanostructures

Two-dimensional nanostructures are of interest to a wide range of scientists from biologists interested in membranes to polymer scientists producing grapheme reinforced nanocomposites. Considering two--dimensional structures as a class of nanomaterials, we could use the sheet thickness and lateral size as structural description. In addition to these parameters, two-dimensional structures are, at times, capable of folding or crumpling, largely depending on the interfacial chemistry and to some extent on thermodynamics. We have studied this crumpling behavior using small-angle neutron and x-ray scattering in a range of nano-materials, specifically, membrane bilayers, graphene oxide, as well as exfoliated nano-sheets of molybdenum sulfide, boron nitride, and tungsten sulfide. A new parameterization of crumpling in these two-dimensional nanostructures will be described with indications of how this quantification can lead to general categories of crumpling behavior that differ in the systems mentioned above. (A helpful discussion with Fyl Pincus assisted with this work.)   

Rheology of cube shaped particles in a suspension

~The rheological properties of suspensions of Brownian cube-shaped particles are interesting because of the greater increase in the translational freedom caused by layering relative to suspensions of Brownian spheres.~Theoretical solutions for a simple shear flow around an isolated cube were obtained using the geometric properties of cubes. Since stress-strain relationship is anisotropic, the suspension viscosity$_{~}$in the limit of zero shear rate can be obtained from the orientational average of the stresslet acting on the cube. The value of intrinsic viscosity was found to be 3.1. Brownian dynamics simulations were carried out with the obtained anisotropic stress relationship to understand hydrodynamic interactions between cubes at low to moderate volume fractions and lubrication hydrodynamic interactions at higher volume fractions. Interesting transitions are observed in rheological properties as the volume fraction of particles in suspension rises above a critical value substantially lower than the minimum close-packing. We have also synthesized model cube-shaped particles with two different chemistries and sizes: Iron oxide, Fe$_{3}$O$_{4}$, nanocubes (20nm and 100nm) and hollow Manganese Carbonate (MnCO$_{3})$ microcubes (1-2microns). MnCO$_{3}$ microspheres were also synthesized to compare their properties with the cubic particle suspensions. At low particle volume fractions, the experimentally determined intrinsic viscosity for the suspension of cubic particles is in excellent accord with expectations from theory. The talk will also compare results of rheological properties of spheres and cubes for both high and low volume fractions over a range of Peclet numbers.