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Molecular crystallization provides an experimentally simple way to arrange molecules on the nanoscale. Synthetic polypeptides present the potential to control ultimate crystal structure by design of primary structure and/or secondary structure adjustment in solution during crystallization. In this work, hexagonal single crystals of synthetic poly-l-lysine (PLL) were produced from aqueous solution by the addition of divalent counterions. It was found that PLL having 100−200 amino acids crystallized in an unfolded-chain form with the crystal thickness controlled directly by PLL molecular weight. PLL chains exhibited chain folding in the crystal state when the PLL was greater than 265 residues. PLL with less than 100 residues were not crystallizable under the same crystal growth conditions. X-ray diffraction data revealed that PLL chains had an α-helical secondary structure and hexagonal symmetry in the crystal state. The introduction of divalent counterions is necessary to initiate the crystallization process and stabilize the crystal structure.

Disk and cylindrical micellar assemblies were formed through self-organization of poly(acrylic acid)-b-poly(methyl acrylate)-b-polystyrene (PAA-b-PMA-b-PS) amphiphilic triblock copolymers with organic diamines as counterions in water/ tetrahydrofuran (THF) solvent mixtures. The system was investigated by means of transmission electron microscopy and cryogenic transmission electron microscopy. It was found that the assembled-state morphologies could be modified by alteration of the type and concentration of cationic diamine counterion undergoing interaction with the negatively charged, polyelectrolyte PAA corona block, the relative amount of water in the water/THF mixture, and the hydrophobic block chain length. Multivalency of the organic amine counterion was critical for disk formation. It was further demonstrated that a single block copolymer underwent disc-to-cylindrical micellar transitions reversibly with variation in the relative water/THF ratio. The ability to form disks beginning from either THF-rich or water-rich solutions indicated that the disk morphology was thermodynamically stable and that THF was important in keeping the micellar structure from becoming kinetically frozen. The nanoassemblies were produced having low size dispersities and were stable for at least one month. Intermediate structures between disks and cylinders were also observed, indicating two distinct kinetic pathways between the two micelle structures.

Time-lapsed Fourier transform infrared (FTIR) spectroscopy was employed to probe chain conformational changes during the isothermal crystallization of poly(l-lactic acid), PLLA, and its nanocomposites from the melt. To study the effect of clay exfoliation extent on the crystallization behavior of PLLA, intercalated and exfoliated nanocomposites were fabricated by solution casting two commercially available organoclays with different degrees of matrix miscibility. By solely varying the degree of miscibility between the organic modifier and the polymer matrix, fully intercalated and exfoliated nanocomposites were obtained. This system allowed us to systematically investigate the effect of degree of clay exfoliation on the chain conformational changes during the crystallization process. In the case of neat PLLA, interchain interactions preceded intrachain interactions during the crystallization. Conversely, the exfoliated nanocomposites exhibited an opposite behavior in that 103 helix formation and backbone reorientation were followed by the interchain interactions. These results explain the nonnucleating behavior of highly miscible and exfoliated nanocomposites reported in our previous contribution.

Intramolecular folding events, triggered by the presence of salt, induce the self-assembly of β-hairpin peptides into hydrogel networks at physiological conditions. At pH 7.4 and low ionic strength solution conditions, dilute, homogeneous solutions of peptide (≤2 wt %) exhibit the viscosity of pure water. Circular dichroism spectroscopy shows that, at pH 7.4 in the absence of salt, peptides are unfolded. By raising the ionic strength of the solution, electrostatic interactions between charged amino acids within the peptide are screened, and a β-hairpin conformation is adopted. Folded β-hairpin molecules supramolecularly assemble via hydrophobic collapse and hydrogen bonding into a three-dimensional hydrogel network. FTIR and X-ray scattering data demonstrate that these hydrogels are rich in β-sheet. Dynamic oscillatory rheological measurements demonstrate that the resultant supramolecular structure forms an elastic material whose structure, and thus modulus, can be tuned by salt concentration and temperature. Storage moduli of hydrogels increase with increasing salt concentration. Robust hydrogelation is also observed in cell growth media at physiological conditions. Transmission electron microscopy reveals that the hydrogel elasticity arises from a network nanostructure consisting of semiflexible fibrillar assemblies.

The effect of organically modified montmorillonite clay addition on crystallization of a polymer matrix has been studied. Two types of commercially available organoclays with different extent of miscibility with a polymer matrix were employed, leading to fully exfoliated (high miscibility) and intercalated (low miscibility) nanocomposite morphologies. The nanocomposites were fabricated via the exfoliation−adsorption technique with the biodegradable polyester matrix polymer poly(l-lactic acid) (PLLA). As-cast nanocomposite films were melted and isothermally crystallized at different temperatures. Bulk kinetics studies and radial spherulite growth rates indicate that when a high degree of filler−polymer matrix miscibility is present, nucleation properties of the organoclay are low relative to the less miscible organoclay. Therefore, the overall bulk crystallization rate was increased in the intercalated system and somewhat retarded in the exfoliated system. Surprisingly, spherulite growth rates were significantly increased relative to the bulk in the fully exfoliated nanocomposite. This phenomenon, linked with less effective nucleation properties, resulted in significantly bigger spherulite sizes in the exfoliated nanocomposite. The overall percent of crystallinity and the size of crystalline domains decreased by addition of organoclays and are the lowest in the fully exfoliated case.

The possibility of making rigid polymer layered nanocomposites from biocompatible/biodegradable matrixes was explored. Three types of commercially available organophilic clay were employed to concurrently study the effects of organic modifier miscibility with the matrix and of the extent of clay modification on overall nanocomposite formation. The nanocomposites were fabricated via the exfoliation−adsorption technique with the matrix polymer poly (l-lactic acid), PLLA, a widely used, biodegradable, synthetic polyester. X-ray diffraction (XRD) data reveal that increasing the miscibility of the surfactant/polymer matrix increases the tendency of the system to exfoliate and randomly distribute the silicate layers. Transmission electron microscopy (TEM) data show that the ordering of silicate platelets is consistent with d spacings obtained from XRD. Because of the nanometer-range dispersion of silicate layers, all the nanocomposites retain their optical clarity. Mechanical properties of the fabricated nanocomposites were probed by dynamic mechanical analysis and show significant improvements in the storage modulus when compared to neat PLLA. The extent of crystallinity was observed to be inversely proportional to the extent of modified clay loading in the final composites.

Neutron reflectivity measurements on deuterated polystyrene (dPS) thin films reveal a strong dependence of the coefficient of thermal expansion (CTE) on both the thickness of the film and the specific confinement boundaries. A direct comparison is made between films on low energy, fluorinated polyimide (fPI) substrates with a free surface (bilayer) and the same films after capping the bilayer with another fPI layer (trilayer). For bilayers, a significant decrease in αr, the CTE above the bulk glass transition temperature, is observed in dPS with thicknesses d < 35 nm. For trilayers, a decrease of αr is observed at larger film thicknesses, d < 60 nm. Replacing the fPI substrate with an SiOx substrate, αr remains bulklike, showing that the length scale for deviations from bulk behavior is sensitive to the specific confining boundaries.

A new equilibrium morphology consisting of randomly oriented wormlike micelles dispersed in a continuous matrix is observed in a neat, strongly segregated I2S simple graft block copolymer. The equilibrium nature of the worm phase is determined via a set of selective solvent casting and prolonged annealing experiments. Transmission electron microscopy (TEM) experiments on quenched samples allow a unique opportunity to directly observe the transition of a kinetically trapped, nonequilibrium folded-layer morphology, formed by casting the sample with a solvent selective for polyisoprene (PI), into the equilibrium, randomly oriented worm phase through an intermediate folded-lace morphology. The folded-lace intermediate is similar to the “mesh” structure previously observed by Hashimoto et al. in starblock/homopolymer blends.¹ The simple graft block copolymer, formed by grafting a single polystyrene (PS) chain onto the center of a polyisoprene backbone, introduces a 2:1 PI/PS arm number asymmetry in the microphase separated state. The 0.81 volume fraction of the PS graft is theoretically predicted² to be the first volume fraction of graft large enough to force the two PI arms per molecule to the concave side of the PI/PS interface in the microphase separated state. This unique volume fraction, coupled with the novel graft architecture, seems to frustrate the system from choosing a lattice during the microphase separation process.

The morphological behavior of a series of well-defined A2B simple graft or “Y” architecture block copolymers is characterized via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). This model architecture is formed by grafting a polystyrene block onto the center of a polyisoprene backbone. The volume fraction windows in which specific strongly segregated microphase-separated morphologies are observed are shifted to higher volume fractions of the PS graft material than in the corresponding linear diblock copolymers. These findings are in good agreement with recently calculated theoretical phase behavior for simple graft, A2B, block copolymers. However, a new morphology, not found in neat linear diblock copolymers, is also observed. This A2B material is microphase separated into wormlike micelles but not ordered on a lattice. This morphology is found at high PS graft volume fraction (φs = 0.81), where the two PI chains per molecule are initially forced to the concave side of the PS/PI interface.

Well-defined graft copolymers with polyisoprene backbones and polystyrene branches, having trifunctional branch points, of the type S2IS2 (H-shaped) and (SI)I(SI) (π-shaped) were synthesized by anionic polymerization high-vacuum techniques. The synthetic strategy involves the preparation of the outer parts of the molecules, carrying one reactive Si−Cl bond, followed by coupling with difunctional living poly(isoprenyllithium) chains. In this way, the number and placement of the branches can be precisely controlled. Molecular characterization of the fractionated samples by size exclusion chromatography with UV and RI detection, membrane osmometry, low-angle laser light scattering, and ¹H-NMR spectroscopy confirmed that the materials exhibit narrow molecular weight distributions and low compositional heterogeneity. The strongly microphase-separated morphologies of these two samples were characterized using TEM and SAXS. The π architecture with a PS volume fraction of 0.21 was found to form body-centered cubic spheres, while the H architecture with a PI volume fraction of 0.64 was found to form a lamellar morphology. The observed morphology for these architectures was rationalized by formally dividing the π and H architectures into component simple (single) graft block copolymers which were mapped onto Milner's morphology diagram for simple graft copolymers.