Study of solution properties of block copolymers Final report

Solution properties of polybutadiene and polystyrene block copolymer

Piezoelectric copolymer hydrophones for ultrasonic field characterization

Hydrophones to be used in the characterization of medical ultrasonic transducers have been fabricated using a new polyvinylidene fluoride/trifluoroethylene (VF2/VF3) copolymer. The copolymer has an advantage over VF2 in that it does not require prestretching before poling. Thin copolymer films can be cast from solution and then poled using the corona discharge method. As there is a need for small‐diameter hydrophones to provide good spatial resolution in measuring highly focused ultrasonic beams, hydrophones with diameter as small as 0.1 mm have been made. Both needle‐type and line hydrophones have been tested and their performance reported. In the case of line hydrophones, the output signal is proportional to the line integral of the acoustic pressure and a computer tomographic technique has been used to reconstruct the beam profiles

Encapsulation of DNA by cationic diblock copolymer vesicles

Encapsulation of dsDNA fragments (contour length 54 nm) by the cationic diblock copolymer poly(butadiene-b-N-methyl 4-vinyl pyridinium) [PBd-b-P4VPQ] has been studied with phase contrast, polarized light, and fluorescence microscopy, as well as scanning electron microscopy. Encapsulation was achieved with a single emulsion technique. For this purpose, an aqueous DNA solution is emulsified in an organic solvent (toluene) and stabilized by the amphiphilic diblock copolymer. The PBd block forms an interfacial brush, whereas the cationic P4VPQ block complexes with DNA. A subsequent change of the quality of the organic solvent results in a collapse of the PBd brush and the formation of a capsule. Inside the capsules, the DNA is compacted as shown by the appearance of birefringent textures under crossed polarizers and the increase in fluorescence intensity of labeled DNA. The capsules can also be dispersed in aqueous medium to form vesicles, provided they are stabilized with an osmotic agent (polyethylene glycol) in the external phase. It is shown that the DNA is released from the vesicles once the osmotic pressure drops below 105 N/m2 or if the ionic strength of the supporting medium exceeds 0.1 M. The method has also proven to be efficient to encapsulate pUC18 plasmid in sub-micron sized vesicles and the general applicability of the method has been demonstrated by the preparation of the charge inverse system: cationic poly(ethylene imine) encapsulated by the anionic diblock poly(styrene-b-acrylic acid).Comment: 35 pages, 11 figures, accepted for publication in Langmui

Synthesis and properties of a new AB-cross-linked copolymer membrane system

The alcohol permeability and permselectivity properties as well as the morphology of membranes made of a newly developed AB-cross-linked copolymer system composed of elastomeric and glassy components were investigated. The copolymer was synthesized by a hydrosilylation reaction between poly(styrene-stat-isoprenes) (Mn from 40,000 to 100,000 g/mol) with high content in unsaturated side groups (≈ 60% of entire isoprene content) and polyhydrogen polysiloxanes with varying SiH content (0.75 10.7 mol %) and molecular mass, Mn, from 2,500 to 36,000 g/mol. A two-track approach was taken to determine the morphology of the copolymer system. The first employed the usual polymer characterization methods such as electron microscopy, DSC, IR spectroscopy, the density gradient method, and mechanical measurements. For the second approach, different copolymer permeability models were tested so as to give an insight into the copolymer morphology. As a final step, the permeability and permselectivity properties were correlated with the morphological structure of the copolymer system. It was observed that the respective continuous microphase dominated the copolymer's physical properties, as, e.g., permeability, permselectivity, and mechanical properties. The microphase inversion in the copolymer system was proved by the permeability/permselectivity as well as by the mechanical measurements

Palladium nanoparticles by electrospinning from poly(acrylonitrile-co-acrylic acid)-PdCl2 solutions. Relations between preparation conditions, particle size, and catalytic activity

Catalytic palladium (Pd) nanoparticles on electrospun copolymers of acrylonitrile and acrylic acid (PAN-AA) mats were produced via reduction of PdCl2 with hydrazine. Fiber mats were electrospun from homogeneous solutions of PAN-AA and PdCl2 in dimethylformamide (DMF). Pd cations were reduced to Pd metals when fiber mats were treated in an aqueous hydrazine solution at room temperature. Pd atoms nucleate and form small crystallites whose sizes were estimated from the peak broadening of X-ray diffraction peaks. Two to four crystallites adhere together and form agglomerates. Agglomerate sizes and fiber diameters were determined by scanning and transmission electron microscopy. Spherical Pd nanoparticles were dispersed homogeneously on the electrospun nanofibers. The effects of copolymer composition and amount of PdCl2 on particle size were investigated. Pd particle size mainly depends on the amount of acrylic acid functional groups and PdCl2 concentration in the spinning solution. Increasing acrylic acid concentration on polymer chains leads to larger Pd nanoparticles. In addition, Pd particle size becomes larger with increasing PdCl2 concentration in the spinning solution. Hence, it is possible to tune the number density and the size of metal nanoparticles. The catalytic activity of the Pd nanoparticles in electrospun mats was determined by selective hydrogenation of dehydrolinalool (3,7-dimethyloct-6- ene-1-yne-3-ol, DHL) in toluene at 90 °C. Electrospun fibers with Pd particles have 4.5 times higher catalytic activity than the current Pd/Al2O3 catalyst

Self-assembly of triblock copolymers in aqueous solution

Indexación: ScieloThe aggregation of PE4VP-b-PS-b-PE4VP block copolymers was studied in aqueous solution. Triblock copolymers P4VP-b-PS-b-P4VP were synthesized by sequential anionic polymerization of poly(styrene) and poly(4-vinylpirydine) using sodium naphthalene as a bifunctional initiator. Subsequently, the 4-vinylpyridine units were quaternized with ethyl bromide to obtain cationic PE4VP-b-PS-b-PE4VP block copolymers. Both star and crew-cut micelles were formed. The concentrations at which micelles are formed cmc were determined, by steady-state and time-resolved fluorescence probing methods, as a function of quatemization degree. The results indicate that cmc of crew-cut micelles increases with increasing charge density of the PE4VP blocks. For star micelles there is not a clear dependency of cmc with the percentage of quatemization. The lifetime of pyrene fluorescence and the ratio I1/I3 were determined at concentrations of copolymers well above the cmc, and the results show that the location of pyrene into the micelle changes with the charge density of the micelle corona. The micropolarity sensed by pyrene decreases with increasing quatemization degree. The presence of aggregates was confirmed by transmission electronic microscopy.http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0717-97072008000200013&lng=es&nrm=is

Synthesis of Hydrophobically and Electrostatically Modified Polyacrylamides and Their Catalytic Effects on the Unimolecular Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate Anion

A series of hydrophobically and electrostatically modified polyacrylamides (Copol(AM-C12)) has been synthesized by radical-initiated copolymerization of acrylamide with n-dodecylmethyldiallylammonium bromide as the hydrophobe in aqueous solution using ammonium persulfate as the initiator. The formation of hydrophobic microdomains of the copolymers was revealed by large hypsochromic shifts of the longwavelength absorption band of the solvatochromic probe Methyl Orange, noncovalently bound to the macromolecule. It was found that the microdomains formed by these copolymers in aqueous solution are more hydrophobic than those of the cationic polysoaps poly(alkylmethyldiallylammonium halides) containing the same n-dodecyl groups as the side chains as a result of the reduced electrostatic repulsions at the periphery of the microdomains. The reduced cationic character of the copolymers Copol(AM-C12) most likely also accounts for the observation that the anionic dye Methyl Orange does not induce microdomain formation in aqueous solution. The effect of the hydrophobically and electrostatically modified polyacrylamides on the unimolecular decarboxylation of 6-nitrobenzisoxazole-3-carboxylate anion (6-NBIC) has been investigated in aqueous solutions at pH 11.3 and 30 °C. It is suggested that the relatively modest catalytic effects induced by Copol(AM-C12) should be ascribed to hydrogen-bond stabilization of the initial state by NH groups in the macromolecules. The decarboxylation rates of 6-NBIC at binding sites in hydrophobic microdomains increase with increasing n-dodecyl group content in the copolymers.