Morphologies of ABC triblock terpolymer melts containing poly(cyclohexadiene): effects of conformational asymmetry.

We have synthesized linear ABC triblock terpolymers containing poly(1,3-cyclohexadiene), PCHD, as an end block and characterized their morphologies in the melt. Specifically, we have studied terpolymers containing polystyrene (PS), polybutadiene (PB), and polyisoprene (PI) as the other blocks. Systematically varying the ratio of 1,2- /1,4-microstructures of poly(1,3-cyclohexadiene), we have studied the effects of conformational asymmetry among the three blocks on the morphologies using transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and self-consistent field theory (SCFT) performed with PolySwift++. Our work reveals that the triblock terpolymer melts containing a high percentage of 1,2-microstructures in the PCHD block are disordered at 110 °C for all the samples, independent of sequence and volume fraction of the blocks. In contrast, the triblock terpolymer melts containing a high percentage of 1,4-microstructure form regular morphologies known from the literature. The accuracy of the SCFT calculations depends on calculating the χ parameters that quantify the repulsive interactions between different monomers. Simulations using χ values obtained from solubility parameters and group contribution methods are unable to reproduce the morphologies as seen in the experiments. However, SCFT calculations accounting for the enhancement of the χ parameter with an increase in the conformational asymmetry lead to an excellent agreement between theory and experiments. These results highlight the importance of conformational asymmetry in tuning the χ parameter and, in turn, morphologies in block copolymers.

Nanostructured block-random copolymers with tunable magnetic properties.

It was recently shown that block copolymers (BCPs) produced room-temperature ferromagnetic materials (RTFMs) due to their nanoscopic ordering and the cylindrical phase yielded the highest coercivity. Here, a series of metal-containing block-random copolymers composed of an alkyl-functionalized homo block (C(16)) and a random block of cobalt complex- (Co) and ferrocene-functionalized (Fe) units was synthesized via ring-opening metathesis polymerization. Taking advantage of the block-random architecture, the influence of dipolar interactions on the magnetic properties of these nanostructured BCP materials was studied by varying the molar ratio of the Co units to the Fe units, while maintaining the cylindrical phase-separated morphology. DC magnetic measurements, including magnetization versus field, zero-field-cooled, and field-cooled, as well as AC susceptibility measurements showed that the magnetic properties of the nanostructured BCP materials could be easily tuned by diluting the cobalt density with Fe units in the cylindrical domains. Decreasing the cobalt density weakened the dipolar interactions of the cobalt nanoparticles, leading to the transition from a room temperature ferromagnetic (RTF) to a superparamagnetic material. These results confirmed that dipolar interactions of the cobalt nanoparticles within the phase-separated domains were responsible for the RTF properties of the nanostructured BCP materials.

Nanoparticle-driven assembly of block copolymers: a simple route to ordered hybrid materials.

The addition of nanoparticles that selectively hydrogen bond with one of the segments of a block copolymer is shown to induce order in otherwise disordered systems. This enables the fabrication of well-ordered hybrid materials with spherical, cylindrical, or lamellar domains at particle loadings of more than 40%, as evidenced by TEM and SAXS. The approach described is simple and applicable to a wide range of nanoparticles and block copolymers, and it lays the groundwork for the design of cooperatively assembled functional devices.

Zwitterionic polymersomes in an ionic liquid: room temperature TEM characterization.

Conventional transmission electron microscopy (TEM) was utilized to characterize vesicles formed by the spontaneous self-assembly of a novel zwitterionic block copolymer in the ionic liquid (2-hydroxyethyl)dimethylammonium methanesulfonate as well as in 0.1 M phosphate buffered saline (PBS). This block copolymer was synthesized via ring-opening metathesis polymerization (ROMP) of a norbornene-based sulfobetaine, followed by its end-functionalization with polystyrene to generate the necessary amphiphilic structure. The ionic liquid enabled the visualization of the vesicles in their swollen state by TEM, demonstrating a new method for improved characterization of polymer vesicles.

Wet-spinning of osmotically stressed silk fibroin.

Based on the phase diagram constructed for water-silk fibroin-LiBr using the osmotic stress method, wet-spinning of osmotically stressed, regenerated Bombyx mori silk fibroin was performed, without the necessity of using expensive or toxic organic solvents. The osmotic stress was applied to prestructure the regenerated silk fibroin molecule from its original random coil state to a more oriented state, manipulating the phase of the silk solution in the phase diagram before the start of spinning. Various starting points for spinning were selected from the phase diagram to evaluate the spinning performance and also physical properties of fibers produced. Monofilament fiber with a diameter of 20 microm was produced. It was found that the fibers whose starting point in the phase diagram were around the phase boundary between silk I and silk II, at very low LiBr concentrations, showed the best spinning process stability and physical properties. This underpins the prediction that the enhanced control over structure and phase behavior using the osmotic stress method helps improve the physical properties of wet-spun regenerated silk fibroin fibers.

Additive-driven assembly of block copolymer-nanoparticle hybrid materials for solution processable floating gate memory.

Floating gate memory devices were fabricated using well-ordered gold nanoparticle/block copolymer hybrid films as the charge trapping layers, SiO(2) as the dielectric layer, and poly(3-hexylthiophene) as the semiconductor layer. The charge trapping layer was prepared via self-assembly. The addition of Au nanoparticles that selectively hydrogen bond with pyridine in a poly(styrene-b-2-vinyl pyridine) block copolymer yields well-ordered hybrid materials at Au nanoparticle loadings up to 40 wt %. The characteristics of the memory window were tuned by simple control of the Au nanoparticle concentration. This approach enables the fabrication of well-ordered charge storage layers by solution processing, which is extendable for the fabrications of large area and high density devices via roll-to-roll processing.

The effect of molecular architecture on the grain growth kinetics of AnBn star block copolymers.

To investigate the effect of molecular architecture on the grain growth kinetics of star block copolymers, a series of AnBn miktoarm star block copolymers with different numbers of arms (n = 1, 2, 4 and 16) was studied. Across this entire series of materials, all the A arms are polystyrene (PS) blocks from the same anionically synthesized batch, and thus all the A arms are identical. Likewise, all the B arms are polyisoprene (PI) blocks from the same anionically synthesized batch, and thus all the B arms are identical. All the stars employed in this study are therefore composed of the same A and B arms liked together in symmetric numbers. The coarsening kinetics of grain growth was monitored in real space by transmission electron microscopy (TEM), followed by subsequent micrograph image analysis. It was found that the molecular architecture influenced the grain growth kinetics of these AnBn star copolymers dramatically. The grain coarsening kinetics was found to follow a scaling law as V approximately t(beta), where V is the characteristic grain volume and t is time. The exponent, beta, was found to be about 0.2 for the diblock copolymer (n = 1) and 0.4 for all three of the star block copolymers (n = 2, 4 and 16) in the series. It is postulated that the difference in grain growth rate between the diblock and the various stars is due to a reduction in molecular entanglements resulting from chain stretching near the junction points in the stars.

Phase behavior and hydration of silk fibroin.

The osmotic stress method was applied to study the thermodynamics of supramolecular self-assembly phenomena in crystallizable segments of Bombyx mori silkworm silk fibroin. By controlling compositions and phases of silk fibroin solution, the method provided a means for the direct investigation of microscopic and thermodynamic details of these intermolecular interactions in aqueous media. It is apparent that as osmotic pressure increases, silk fibroin molecules are crowded together to form silk I structure and then with further increase in osmotic pressure become an antiparallel beta-sheet structure, silk II. A partial ternary phase diagram of water-silk fibroin-LiBr was constructed based on the results. The results provide quantitative evidence that the silk I structure must contain water of hydration. The enhanced control over structure and phase behavior using osmotic stress, as embodied in the phase diagram, could potentially be utilized to design a new route for water-based wet spinning of regenerated silk fibroin.