Apostolos Avgeropoulos

Non-covalent functionalization of carbon nanotubes with polymers

Carbon nanotubes have emerged as very promising materials in various research fields spanning from biotechnology to energy storage and transformation. Their poor solubility in aqueous and organic solvents and limited compatibility with polymer matrices are major drawbacks, rendering these materials incapable of achieving their full potential. Covalent or non-covalent functionalization with polymers is considered a major key in circumventing this issue. In this feature article, the non-covalent functionalization through various types of interactions between polymers and carbon nanotubes is highlighted and their potential applications are discussed.

Robust Block Copolymer Mask for Nanopatterning Polymer Films

The formation of well-oriented cylinders with perpendicular morphology for polystyrene-b-polydimethylsiloxane (PS-PDMS) thin films was achieved by spin coating. The self-assembled PS-PDMS nanostructured thin films were used as templates for nanopatterning; the PDMS blocks can be oxidized as silicon oxy carbide microdomains, whereas the PS blocks were degenerated by a simple oxygen plasma treatment for one-step oxidization. As a result, freestanding silicon oxy carbide thin films with hexagonally packed nanochannels were directly fabricated and used as masks for pattern transfer to underlying polymeric materials by oxygen reaction ion etching (RIE) to generate topographic nanopatterns. By taking advantage of robust property and high etching selectivity of the SiOC thin films under oxygen RIE, this nanoporous thin film can be used as an etch-resistant and reusable mask for pattern transfer to various polymeric materials. This approach demonstrates a simple, convenient, and cost-effective nanofabrication technique to create the topographic nanopatterns of polymeric materials.

Synthesis of model nonlinear block copolymers of A(BA)2, A(BA)3, and (AB)3A(BA)3 type

of THF was always used. Methyltrichlorosilane and tetINTRODUCTION rachlorosilane were fractionally distilled on the vacuum line and were subdivided into ampules. sec-ButyliDue to the academic and industrial significance of block thium, prepared from sec-butyl chloride and a lithium copolymers, the design and synthesis of new such matedispersion, was the initiator and benzene the solvent rials is of great interest. For this reason nonlinear block for the polymerizations leading to v-monofunctional copolymers have recently received much attention. A living polymers. Sodium napthalenide prepared from family of nonlinear block copolymers are the so-called napthalene (purified by triple sublimation) and sodium miktoarm (mikto comes from the Greek word mikto4 was the initiator and a mixture of tetrahydrofuran and meaning mixed) star copolymers, which are starbenzene the solvent for the synthesis of a, v-difuncshaped molecules having chemically different arms. tional living polymers. In this study, anionic polymerization and chlorosilane Fractionation was carried out by adding methanol chemistry were used to prepare new model miktoarm to the polymer solution (ca. Ç 0.5% w/v) in toluene stars of the A(AB)2 and A(AB)3 type, as well as, model at room temperature with stirring until turbidity was bridged miktoarm stars of the (AB)3A(BA)3 (or Super established. The mixture was then heated gently with H-shaped) type (see Scheme 1). In all cases A is polystirring until clear, transferred to a warm separatory styrene (PS) and B polyisoprene (PI). The intermedifunnel and allowed to equilibrate at room temperature ate and final products were characterized by size excluovernight. This procedure was repeated until no precursion chromatography, low-angle laser light scattering, sors or undesirable products were shown to be present laser differential refractometry, membrane osmometry, by size exclusion chromatography (SEC). H-NMR, and UV spectroscopy. SEC experiments were carried out at 307C using a Waters model 510 pump, Waters Model 410 differential refractometer and Waters model 486 tunable abEXPERIMENTAL sorbance detector. Three Phenomenex (type: phenogel 5 linear, pore size: 50–1006 Å) columns were used. THF distilled over CaH2 and sodium was the carrier solvent The purification of styrene (Merck), isoprene (Fluka), at a flow rate of 1 mL/min. and benzene (Merck) to the standard required for anThe weight-average molecular weight (Mw ) of the ionic polymerization has been described elsewhere. final polymers was determined with a Chromatix KMXTetrahydrofuran (THF, Merck) was stirred overnight 6 low-angle laser photometer (LALLS). This instruover CaH2, distilled on the vacuum line to a sodium ment, equiped with a helium–neon laser was operating mirror and was left for 24 h to react. This procedure at a wavelength of 633 nm. THF, purified over CaH2 was repeated until no degradation of the sodium mirror and sodium-distilled prior to use, was the solvent at was observed. Then it was distilled into a flask con257C. The refractive index increments, dn /dc in THF taining a sodium mirror and traces of styrene. The apat 257C, were measured with a Chromatix KMX-16 repearance of red color due to the formation of PSNa is fractometer, operating at 633 nm and calibrated with an indication of the THF high purity. A middle fraction NaCl solutions. The number-average molecular weight (Mn ) was deCorrespondence to: N. Hadjichristidis termined with a Wescan Model 230 membrane osmoJ Polym Sci A: Polym Chem 35: 813–816, 1997 meter at 357C. Toluene, distilled over CaH2, was the solvent. q 1997 John Wiley & Sons, Inc. CCC 0887-624X/97/040813-04

Characterization of a 4-miktoarm star copolymer of the (PS-b-PI)3 PS type by temperature gradient interaction chromatography

Temperature gradient interaction chromatography (TGIC) was applied for the separation of a complex miktoarm star copolymer which has one polystyrene (PS) arm and three polystyrene-b-polyisoprene (PS-b-PI) diblock copolymer arms. Such miktoarm star polymers are much more difficult to characterize than branched homopolymers since the byproduct, typically polymers with missing arm(s) or coupled products, have not only different molecular weights but also different compositions. TGIC was able to fully separate the byproducts, and the composition of the molecular species corresponding to the different separated elution peaks was determined by two methods, fractionation/NMR and multiple detection (UV and RI). A reasonable agreement between the results of the two methods was obtained. By using the composition found, the corresponding molecular weights were determined by multi-angle light scattering detection. Based on the composition and the molecular weight we were able to identify the structure of the different molecular species.

Continuous Equilibrated Growth of Ordered Block Copolymer Thin Films by Electrospray Deposition

Deposition of block copolymer thin films is most often accomplished in a serial process where material is spin coated onto a substrate and subsequently annealed, either thermally or by solvent vapor, to produce a well-ordered morphology. Here we show that under appropriate conditions, well-ordered block copolymer films may be continuously grown under substrate equilibrated conditions by slow deposition of discrete subattoliter quantities of material using electrospray. We conduct time-resolved observations and investigate the effects of process parameters that underpin film morphology including solvent selectivity, substrate temperature, block-substrate selectivity, and flow rate of the feed solution. For a PEO cylinder-forming poly(styrene-b-ethylene oxide) block copolymer, we uncover a wide temperature window from 90 to 150 °C and an ideal flow rate of 2 μL/min for ordered film deposition from dilute acetone solutions. PEO cylinders aligned with their long axes perpendicular to the film-air interface at optimal spray conditions. Using poly(styrene-b-methyl methacrylate) deposited onto neutrally selective surfaces, we show that the substrate-equilibrated process results in vertically oriented microdomains throughout the film, indicating a preservation of the initial substrate-dictated morphology during the film deposition. Electrospray offers a new and potentially exciting route for controlled, continuous growth of block copolymer thin films and manipulation of their microstructure.

Mechanical properties of the double gyroid phase in oriented thermoplastic elastomers

We investigate the mechanical properties of triblock copolymers with oriented double gyroid (DG) morphology in poly(styrene-b-isoprene-b-styrene) (SIS) triblock copolymers by deforming textured samples along both the [111] direction and transverse to this direction. The modulus anisotropy for the two directions of this cubic material is approximately a factor of 5. Deformation along [111] causes the sample to form a distinct neck and draw, while the deformation in the transverse direction proceeds without neck formation. In addition, the mechanical hysteresis of the [111] stretch is 50% higher than that transverse to the [111] direction. Upon unloading and annealing above the polystyrene Tg, the DG structure recovers fully, both macroscopically and microscopically. The mechanical properties of the DG are compared to those of the classical block copolymer morphologies to gain insight into the deformation mechanism.

Silicon oxy carbide nanorings from polystyrene-b-polydimethylsiloxane diblock copolymer thin films

A silicon-containing diblock copolymer, poly(styrene)-b-poly(dimethylsiloxane) (PS-PDMS), with PDMS cylinders in a PS matrix has been synthesized through sequential anionic polymerization. PS-PDMS thin film was prepared by spin coating on silicon wafer. Perpendicular PDMS cylinders in the PS-PDMS thin film can be obtained by solvent annealing. The PS-PDMS thin film was employed as template for the formation of core-shell cylinder thin films through surface reconstruction. By taking advantage of the silicon-containing character, silicon oxy carbide nanopattern can be fabricated due to the crosslinking of PDMS blocks and the degradation of PS blocks through one-step oxygen plasma treatment. As a result, the formation of silicon oxy carbide nanoring arrays can be achieved. This method provides a convenient way to create silicon oxy carbide nanoring arrays from diblock copolymers.

Factors controlling the enhanced mechanical and thermal properties of nanodiamond-reinforced cross-linked high density polyethylene.

A systematic investigation of the factors influencing the notable enhancement of the mechanical and thermal properties of nanodiamonds (NDs)-reinforced cross-linked high density polyethylene (PEX) is presented in this work. The effects of crystal structure and molecular conformation as well as filler dispersion and adhesion with the matrix were found to govern the mechanical properties of the final composites. A considerable increase in the strength, toughness, and elastic modulus of the materials was found for the composites with filler content below 1 wt %. For higher NDs concentrations, the properties degraded. When filler concentration does not exceed 1 wt %, enhanced adhesion with the matrix is achieved, allowing a more successful load transfer between the filler and the matrix, thus enabling an effective reinforcement of the composites. The higher degree of crystallinity along with larger crystal size are also positively influencing the mechanical properties of PEX. Higher filler concentrations, on the other hand, lead to the formation of larger aggregates, which lead to lower adhesion with the matrix, while they also constitute stress concentrators and therefore reduce the positive reinforcement of the matrix. The thermal conductivity of the composites was also found to be significantly increased for low-filler concentrations. This enhancement was less significant for higher NDs concentrations. It is concluded that this reinforcement is due to the heat capacity increase that NDs incorporation causes in PEX. Additionally, a thermal stability enhancement was found for the composite with minimum filler content.

Hydrodynamic properties of A8B8 type miktoarm (Vergina) stars

The dilute solution properties of three (PS)8(PI)8 miktoarm (Vergina) stars were investigated by viscometry and dynamic light scattering in toluene and tetrahydrofuran (THF) (common good solvents), cyclohexane at 34.5°C (theta solvent for PS and good for PI) and dioxane at 34°C (theta solvent for PI and good for PS). Experimental intrinsic viscosity [η] and hydrodynamic radii, Rh, values in all solvents were larger for the miktoarm stars in comparison to the calculated ones using a simple model which describes the size of the copolymers as a weighted average of the sizes of the homopolymer stars with the same total molecular weight and number of arms as the copolymer. This expansion is discussed on the basis of the increased number of heterocontacts, the topological constrains imposed by the common junction point in this highly branched miktoarm architecture and the asymmetry in molecular weights of the different kinds of arms. The conformation adopted in dilute solutions can explain, to some extent, the morphological results obtained on the same materials. The ratios of viscometric to hydrodynamic radii are consistent with previous investigations on linear and star polymers and in accord with the hard sphere model.

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.