All issues / Volume 4 (2010) / Issue 1 (January)
Micrometer and submicrometer diameter of polystyrene (PS) fibers were electrospun from various dimethyl formamide (DMF) solutions at different weight fractions under 35% relative humidity. Increasing polymer fraction in the solution results in a gradual morphological transition from beads-with-incipient to bead-free fibers and also increases the diameter. The formation of uniform glassy skin presumably due to radial capillary flow within the liquid jet was confirmed by scanning electron microscope. The thickness of the skin varies with the weight fraction of PS; therefore, it was normalized with respect to average fiber diameter (AFD). The skin gets thinner as the weight fraction of PS increases. In addition, the fibers exhibit highly porous internal structure and smooth surface along with slight porosity. The development of porosity is attributed to liquid-liquid phase separation of water molecules in atmospheric moisture and DMF.
Starch-g-poly(maleic anhydride-co-vinyl acetate) (SMV) was synthesized via the esterification reaction of starch with the copolymer of maleic anhydride and vinyl acetate. The carboxylic unit percentage (CUP) of SMV was tailored with reaction conditions, and it ranged from 29.8 to 46.9%. The structure and the morphology of the copolymers were characterized with Fourier Transform Infrared spectroscopy and X-ray diffraction analysis. It was found that SMV could form complex with some metal cations such as Ca2+, Pb2+ and Hg2+ or cationic polyelectrolyte chitosan, and precipitate from the solution. The weight of precipitation increases with an increase of the CUP of SMV. In addition, a physically cross-linked hydrogel of SMV/poly(vinyl alcohol) (PVA) was obtained by freeze/thaw technique. Scanning electron microscopy exhibited the hydrogel was uniform. The gel exhibited pH-responsive re-swelling. The maximum swelling-ratio values of SMV/PVA (9:1, wt/wt) gel were 3.29 and 5.34 in HCl (pH 1.0) and phosphate-buffer saline (PBS) (pH 12) respectively.
A series of novel block-type amphiphilic copolymers have been prepared by copolymerizing methacrylate endcapped oligo-urethane and 3-(trimethoxysilyl) propyl methacrylate (TPM) via the sol-gel process. Copolymers with welldefined end groups and narrow polydispersity were prepared through Reversible Addition Fragmentation Chain Transfer (RAFT) polymerization. As-synthesized copolymer was characterized 1H nuclear magnetic resonance (1H NMR), gel permeation chromatography (GPC), Fourier transform infrared (FTIR). The copolymer precursors self-assembled in form of spherical micelles (in selective solvents) have been hydrolyzed and then condensed via sol-gel process in order to generate polyurethane-silica (PU-SiO2) hybrid materials. The hybrid copolymers thus prepared possess excellent thermal stability and mechanical property. The structures and properties of the copolymer precursors and their hybrid copolymers were characterized by thermogravimetric analysis (TGA), tensile test and atomic force microscopy (AFM).
Cellulose nanofibres, 20 nm in diameter and 300 nm long, were prepared by acid hydrolysis of flax yarns. Composite films containing 2.5 and 5.0 wt% flax cellulose (FC) fibres were prepared by solution casting of mixtures of poly(lactic acid) (PLA) solution and cellulose nanofibre suspension in chloroform. The resulting composite films and solution cast pure PLA film, with thickness of around 160 m, showed good transparency. For composites with 2.5 and 5.0 wt% FC, the tensile strength increased by 25 and 59% and tensile modulus by 42 and 47%, respectively, compared to pure PLA film. The composite film with 2.5 wt% FC combined high strength and ductility with tensile strength of 24.3 MPa and 70% elongation at break. Flax cellulose appeared to facilitate nucleation and subsequent crystallisation of PLA more effectively in the amorphous composites than in the crystalline composites.
Aiming to mimic blood vessels, biodegradable thermoplastic elastomer (BTPE) is designed to be elastic, flexible and tough. A series of biodegradable triblock copolymers and poly(ester-urethanes) (PEU) based on ε-caprolactone have been synthesized and studied. The crystallinity of the poly(ε-caprolactone) used as soft segment has been disrupted by incorporating either L-lactide (L-LA) units or trimethylene carbonate (TMC) units. Our studies suggest that soft segment composition does affect the mechanical properties significantly.
The electro-optical performances of polymer dispersed liquid crystal (PDLC) were investigated in the presence of organically modified clays. With the addition and increasing amount of modified clay, driving voltage and memory effect, viz. transparent state of the film after the electricity is off simultaneously increased due most likely to the increased viscosity. Among the two types of modifier, 4-(4-aminophenyl) benzonitrile having greater chemical affinity with LC than hexylamine, gave finer dispersion of clay in liquid crystal, greater viscosity, larger driving voltage and response time, and greater memory effect.
A facile way for alignment of carbon nanotubes in macroscopic polymer matrix was developed by combining electrospinning and in-situ polymerization. The approach is based on the formation of nanofibre scaffolds with wellaligned arrays, which is filled with carbon nanotubes (CNTs). CNTs will be well aligned in macroscopic polymer matrix when the aligned nanofibre scaffold containing CNTs has been incorporated into the poly(methyl methacrylate) (PMMA) matrix by in-situ polymerization. We demonstrate that this scaffold approach is broadly applicable and allows for the fabrication of nanocomposites with accurately aligned nanofillers. The results presented in this report show that the approach is ideal by using polyacrylonitrile (PAN) nanofibres as a scaffold of multiwalled carbon nanotubes (MWNTs), and PMMA as the macroscopic polymer matrix. The tensile strength (7.2 wt% MWNTs/PAN nanofibres loadings) reaches 48.61 MPa, 87% higher than that pure PMMA, and the tensile modulus is increased by 175%.