All issues / Volume 5 (2011) / Issue 10 (October)
One of the most subtle problem in the characterization of thermoresponsive polymers is the evaluation of the relationship between the lower critical solution temperature (LCST) of the linear polymer and the volume phase transition temperature (VPTT) of the corresponding hydrogel. Here, the LCST and the onset temperature of linear poly(N-isopropylacrylamide-co-N-hydroxymethyl acrylamide) has been determined under pseudo-physiological conditions by cloud point (CP) measurements and by microcalorimetric analysis. The LCSTs, as well as the onset temperatures, determined by the CP method, decrease with increasing the concentration of the polymer solution. On the contrary, microcalorimetric analyses give almost the same values for LCSTs and the onset temperatures regardless of polymer concentration. The VPTT of the hydrogel, determined by the blue dextran method, was found to be closely similar to the LCST of the concentrated polymer solution (10%, w/v), determined by the CP method. In fact, the hydrogel could be considered as a concentrated polymer solution whose concentration could be related to the amount of water retained by the hydrogel. Hydrogel microspheres have been also reported to release diclofenac, a drug model system, in a pulsating way at temperatures slightly below and above the VPTT.
The crystallization kinetics of poly(lactic acid) / talc composites were determined over a range of 0 to 15 wt% of talc. Talc was found to change the crystallization kinetics. The presence of talc increases the crystallization rate and this increase is related to talc concentration and to crystallization temperature. In order to understand the effect of talc and PLA crystallinity on mechanical properties, dynamic mechanical thermal analyses were performed on poly(lactic acid) / talc composites before and after an annealing process. It was demonstrated that the presence of crystals improves thermomechanical properties but in order to achieve good results at high temperatures the reinforcing effect of a filler such as talc is necessary.
It was demonstrated in our previous work that the combined carbon nanofibers (CNFs) and microsized short carbon fibers (SCFs) in epoxy (EP) leads to significant improvements in the mechanical properties of the matrix. In this work, the effect of nano-SiO2 particles, having an extremely different aspect ratio from CNFs, on the tensile property and fracture toughness of SCFs-filled EP was studied. It was revealed that the combined use of SCFs and silica nanoparticles exerts a synergetic effect on the mechanical and fracture properties of EP. Application of SCFs and the nanoparticles is an effective way to greatly enhance the modulus, strength and fracture toughness of the EP simultaneously. The synergetic role of the multiscale fillers was explained by prominent changes in the stress state near the microsized fillers and the plastic zone ahead of the crack tip. The synergetic role of multiscale fillers is expected to open up new opportunities to formulate highperformance EP composites.
Multi-syringe electrospinning has been successfully employed to produce a blended fibermat composed of poly(lactic-glycolic acid) (PLGA) fibers and a composite fiber for bone repair. The composite fiber, siloxane-containing vaterite (SiV)/poly(L-lactic acid) (PLLA), donated as SiPVH has the ability to release soluble silica species and calcium ions at a controlled rate. The SiPVH fibermats have demonstrated excellent bone regeneration ability in vivo at the front midline of the calvaria of rabbits. However, they are brittle and have low tensile strength resulting from the large particulate SiV (60 wt%) content. In this study, co-electrospinning of PLGA with SiPVH was performed in the hope of achieving a blended fibermat with improved mechanical properties. The co-electrospun fibermats showed good homogeneous blending of the PLGA and SiPVH composite fibers that had excellent flexibility. The blended PLGA-SiPVH fibermats had significantly improved mechanical properties compared to the SiPVH fibermats, where more than 20 times higher elongation to failure was achieved on comparison to the SiPVH fibermat. As well as strength, high porosity and large pore size are vital for the migration of cells into the centre of the graft. This was accomplished by heating the PLGA-SiPVH fibermats at 110°C for a fixed time, which induced the softening and flow of PLGA towards the more stable SiPVH fibers. Heating had successfully produced PLGA-SIPVH fibermats with large open pores and inter-fused SiPVH fibers, which also had better tensile mechanical properties than the SiPVH fibermat.
In fiber-reinforced polymer pressure-retaining structures, such as pipes and vessels, micro-level failure commonly causes fluid permeation due to matrix cracking. This study explores the effect of nano-reinforcements on matrix cracking in filament-wound basalt fiber/epoxy composite structures. The microstructure and mechanical properties of bulk epoxy nanocomposites and hybrid fiber-reinforced composite pipes modified with acrylic tri-block-copolymer and organophilic layered silicate clay were investigated. In cured epoxy, the tri-block-copolymer phase separated into disordered spherical micelle inclusions; an exfoliated and intercalated structure was observed for the nano-clay. Block-copolymer addition significantly enhanced epoxy fracture toughness by a mechanism of particle cavitation and matrix shear yielding, whereas toughness remained unchanged in nano-clay filled nanocomposites due to the occurrence of lower energy resistance phenomena such as crack deflection and branching.Tensile stiffness increased with nano-clay content, while it decreased slightly for block-copolymer modified epoxy. Composite pipes modified with either the organic and inorganic nanoparticles exhibited moderate improvements in leakage failure strain (i.e. matrix cracking strain); however, reductions in functional and structural failure strength were observed.
Enhancing mechanical properties of thermoplastic polyurethane elastomers with 1,3-trimethylene carbonate, epsilon-caprolactone and L-lactide copolymers via soft segment crystallization
S. S. Liow, V. T. Lipik, L. K. Widjaja, S. S. Venkatraman, M. J. M. Abadie
Vol. 5., No.10., Pages 897-910, 2011
Vol. 5., No.10., Pages 897-910, 2011
Multiblock thermoplastic polyurethane elastomers based on random and triblock copolymers were synthesized and studied. Dihydroxyl-terminated random copolymers were prepared by ring opening copolymerization of ε-caprolactone (CL) and 1,3-trimethylene carbonate (TMC). The triblock copolymers were synthesized by using these random copolymers as macro-initiator for the L-lactide (L-LA) blocks. These random and triblock copolymers were further reacted with 1,6-hexamethylene diisocyanate (HMDI) and chain extended by 1,4-butanediol (BDO). The polymer structure and chemical composition were characterized by 1H NMR 13C NMR and SEC. Their thermal and mechanical properties were studied by using DSC and Instron microtester. Multiblock polyurethanes based on random PCL-co-PTMC copolymers showed strain recovery improvement with increasing PCL content. However, these polyurethanes were unable to sustain deformation at body temperature due to the melting of PCL crystals and low hard segments content. With the presence of crystallizable PLLA blocks, mechanical properties were improved at body temperature without compromising their good strain recovery.
A stable nitroxide radical (2,2,6,6-tetramethylpiperidinyl-1-oxy, TEMPO) was employed to a grafting polymerization of styrene onto the high-cis-1,4 polybutadiene (PB) rubber initiated by 1,1-bis(tert-butylperoxy)cyclohexane (DP275B). The influence of TEMPO/DP275B ratio on the reaction progress, molecular structure, mechanical performance and fracture behavior of the toughened polystyrenes (PS) was systematically characterized. The results showed that a moderate amount of TEMPO used is favorable to the morphology and properties of the as-prepared products, which fracture with a semi-ductile mode. While increasing TEMPO dosage, both rubber grafting and particle size distribution become weaker, and as a result the material also tends to be very brittle and unstable under impact.
In this work, the use of a polar wax, e.g. amphiphilic Tegomer® E 525 (TEG) is investigated with the aim of modifying, and possibly improving, the dispersion of an organically modified nanoclay (OMMT), loaded at 5 wt%, in a polyethylene matrix (PE) at relatively low loading levels. We have indeed found that the incorporation of low loadings, e.g. 0.5 wt%, of TEG, an amphiphilic block co-polymer, into a PE/OMMT sample results in a substantial improvement of the clay dispersion in the nanocomposite and, consequently, of the mechanical and thermomechanical properties of the films. The achieved results are comparable to those obtained for systems containing traditional dispersing agents such as maleated PE (PEgMA) and ethylene-acrylic acid copolymer (EAA), at higher loadings, i.e. at 5 wt%. It has also been found that by increasing the polar wax content, i.e. 1–5 wt%, no useful improvement in the mechanical behaviour and morphology of the PE films was obtained. At high loadings of the polar wax relatively the short chains are arranged into the clay particles galleries and intercalation of the polyethylene chains between the clay platelets may be hindered. Additionally at high TEG loadings, the presence of the new polar groups of the wax also on external surfaces of the clay particles is expected to promote aggregation of the clay particles, with a loss of the beneficial effect of the more dispersed clay particles on the polymer mechanical/thermomechanical properties. The reported results strongly indicate that the amphiphilic TEG dispersing additive, may advantageously be used, at substantially lower loadings, as an alternative to incumbent PEgMA in the formulation of nanocomposites to improve their macroscopic performances.