All issues / Volume 5 (2011) / Issue 1 (January)
This is an editorial article. It has no abstract.
A series of hybrids composed of styrene crosslinkable vinyl ester (VE) and acrylated epoxidized soybean oil (AESO) were produced via free radical-induced crosslinking. The VE/AESO ratio was changed between 75/25 and 25/75 wt%. Moreover, to support phase grafting the VE/AESO = 50/50 wt% hybrid was modified with phthalic anhydride in various amounts (1, 5 and 10 wt%). The structure of the hybrid systems was investigated by dynamic mechanical thermal analysis (DMTA), differential scanning calorimetry (DSC), and atomic force microscopy (AFM). The properties of the systems were assessed by static flexural and fracture mechanical tests. The resistance to thermal degradation was inspected by thermogravimetric analysis (TGA). The results suggested that the hybrids have an interpenetrating network (IPN) structure. With increasing AESO content the stiffness (modulus), strength and glass transition temperature (Tg) of the hybrids decreased, whereas their ductility increased. Phthalic anhydride caused an adverse trend. Both the fracture toughness and fracture energy increased with increasing AESO content. They were less affected by adding phthalic anhydride phase couplant. Interestingly, the hybrids outperformed the parent VE and AESO in respect to resistance to thermal degradation.
Poly(3-hydroxybutyrate) is a biodegradable polyester produced by microorganisms under nutrient limitation conditions. We obtained a biodegradable poly(3-hydroxybutyrate) composite having 8 to 55% of chemically in situ polymerized hydrochloric acid-doped polyaniline nanofibers (70-100 nm in diameter). Fourier transform infrared spectroscopy and X-rays diffractometry data did not show evidence of significant interaction between the two components of the nanocomposite, and polyaniline semiconductivity was preserved in all studied compositions. Gamma-irradiation at 25 kGy absorbed dose on the semiconductive composite presenting 28% of doped polyaniline increased its conductivity from 4.6*10-2 to 1.1 S/m, while slightly decreasing its biodegradability. PANI-HCl biodegradation is negligible when compared to PHB biodegradability in an 80 day timeframe. Thus, this unprecedented all-polymer nanocomposite presents, at the same time, semiconductivity and biodegradability and was proven to maintain these properties after gamma irradiation. This new material has many potential applications in biological science, engineering, and medicine.
Linear low density polyethylene (LLDPE) was melt compounded with various amounts of a cycloolefin copolymer (COC). Scanning and transmission electron microscopy evidenced, at qualitative level, some interfacial adhesion between LLDPE and COC. Another indication of interactions between the components was the increase of crystallinity degree with rising COC content and the enhancement of COC glass transition temperature with the LLDPE fraction. In order to explain this behaviour, the incorporation of ethylene segments of COC into the LLDPE crystalline phase, leading to an increased number of norbornene units in the remaining COC component undergoing the glass transition, was hypothesized. The thermo-oxidative degradation stability of LLDPE was substantially enhanced by COC introduction for filler contents higher than 20 wt%, especially when an oxidative atmosphere was considered. An increasing fraction of COC in the blends was responsible for an enhancement of the elastic modulus and of a decrease in the strain at break, while tensile strength passed through a minimum, in agreement with the model predictions based on the equivalent box model and equations provided by the percolation theory. The introduction of a rising COC amount in the blends increased the maximum load sustained by the samples in impact tests, but decreased the blend ductility. Concurrently, a significant reduction of the creep compliance of LLDPE was observed for COC fractions higher than 20 wt%.
In this paper, core-shell TiO2/polystyrene (TiO2/PS) microspheres with superhydrophobic properties were prepared via a facile method. Our method needs neither special apparatus nor complicated chemical treatment. The whole process includes two steps: firstly, coupling agent was used to modify TiO2 by sol-gel method; secondly, fabrication of TiO2/PS dispersions was carried out via in-situ free-radical polymerization strategy. The component and structure of the TiO2/PS particles were characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), field emission scanning electron microscope (FE-SEM) and transmission electron microscopy (TEM). The TiO2 gel particles with average diameter of 1 μm exhibited irregular spherical shape and obvious aggregation. Compared with the TiO2 particles, the resulting TiO2/PS particulates showed regular spherical shape, better dispersion and bigger size. By directly depositing the resulted TiO2/PS dispersion on a Cu foil, the coating showed superhydrophobic property which was reflected by the contact angle (CA) of water on the surface with high water adhesion. The apparent CA of water is 153.5±1.5°, suggesting that this composite possesses well superhydrophobicity.
Successful retardation or arrest of fatigue crack is observed in self-healing epoxy composite containing dual encapsulated healant, i.e. two types of microcapsules that respectively include epoxy prepolymer and mercaptan/tertiary amine hardener. Fast curing of the released healing agent from the broken capsules leads to rapid development of its bonding strength and fracture toughness at room temperature. It is found that the effects of microcapsules induced-toughening, hydrodynamic pressure crack tip shielding, polymeric wedge and adhesive bonding of the healing agent are responsible for the extension of fatigue life. Depending on the applied stress intensity range, ΔKI, and the competition between polymerization kinetics of the healing agent and crack growth rate, the above mechanisms exert different influences on crack retardation. The results might serve as a reference for further improving the performance of the healant system under fatigue circumstances.
This work reports the functionalization of multi-walled carbon nanotubes (MWCNTs) with crystalline poly(4- vinylpyridine) (P4VP) in CO2-expanded liquids (CXLs). The structure and morphology of MWCNT-induced polymer crystallization are examined, with the focus on molecular weight of P4VP (MW-P4VP), the pressure of CXLs and the concentration of P4VP. First, it is observed that the crystallization morphologies for the P4VP/MWCNTs composite with a low molecular weight P4VP (LMW-P4VP) matrix could be finely controlled in CXLs, and it is surprising to find that the P4VP8700 wrapping patterns undergo a morphological evolution from dot crystals to dotted helical wrappings, and then to dense helical patterns by facile pressure tuning under lower polymer concentration. In other words, the CXLs method enables superior control of the P4VP crystallization patternings on MWCNTs, particularly efficient for LMW-P4VP at lower polymer concentration. Meanwhile, the CXL-assisted P4VP crystal growth mechanism on MWCNT is investigated, and the dominating growth mechanism is attributed to 'normal epitaxy' at lower P4VP concentration rather than 'soft epitaxy' at higher concentration. We believe that this work reports a new crystalline polymer wrapping approach in CXLs to noncovalent engineering of MWCNTs surfaces.
Morphology, grating formation dynamics and electro-optical performance of transflective multiplexing with holographic polymer dispersed liquid crystal (HPDLC) were investigated in the presence of silica nanoparticles (Aerosil R812 (RS) and modified Aerosil 200 (MS)) and silicon monomer (vinyltrimethoxy silane (VTMS)) by using three coherent laser beams. The addition of Si additive significantly augmented the diffraction efficiencies of transmission and reflection gratings due to the enhanced phase separation with large LC channels. The film was driven only with Si additives which are enriched at the polymer-LC interfaces. As the additive content increased, driving voltage decreased to a minimum of 30 V at 2.0 wt% VTMS. It was found that the interface modification and large droplet size are crucial to operate the film. Among the three types of Si additive, VTMS showed the highest electro-optical performance due to its low viscosity and high reactivity.
The effect of a quenching treatment applied on heated cast sheet extruded films of two poly(lactic acid) (PLA) commercial grades, with different optical purities, was studied. The thermal and mechanical properties of the films, as well as their fracture behavior, were assessed by differential scanning calorimetry (DSC), tensile tests, and the essential work of fracture (EWF) approach. The heating-quenching treatment causes a de-aging effect with an increase in the free volume of polymer chains evidenced by a decrease in the glass transition temperature (Tg) and a decrease in the tensile stiffness and yield stress. As a result, there is an abrupt increase in ductility, finding a dramatic change in the fracture behavior, from brittle to ductile. The use of digital image correlation (DIC) of the strain field analysis during fracture testing has allowed relating the decrease on the yield stress promoted by quenching with the crack propagation kinetics. The use of the EWF method to characterize the fracture toughness of PLA has allowed to measure this enhancement on toughness, finding that the specific essential work of fracture (we) and the plastic term (βwp) parameters increased 120% and 1200%, respectively, after the quenching process.