Nandika Anne D'Souza

Exfoliated and intercalated polyamide-imide nanocomposites with montmorillonite

Abstract Polyamide-imide (PAI) is a high performance condensation polymer, which has high heat resistance and high radiation resistance. Solvent suspensions of PAI are widely used in magnetic wire coatings. Montmorillonite (MMT) nanocomposites were investigated for the concentration effects on dispersion, glass transition, degradation, and mechanical properties. Samples were prepared using a controlled torque stirrer and slow solvent extraction was followed for the cast samples. Optical microscopy shows that the surface of the cast sample has increased edge–edge clay platelet attraction. Transmission electron microscopy of the through thickness sample indicated platelet edges, increased face–face coagulated states and some edge–edge flocculated states of tactoid formation. X-ray diffraction indicated that for 1% sample a highly exfoliated structure was obtained while between 1.5 and 3% intercalated and exfoliated dispersions were obtained. The glass transition was not significantly affected by clay presence but a drop in specific heat change was observed for all samples showing a 001 clay peak presence. The first heating scan showed PAI solvent and MMT organic emission but these emissions did not affect the PAI chemically. Degradation was altered by the level of matrix shielding by the clay. Hardness values were increased with clay presence but unaffected by concentration.

Epoxy+liquid crystalline epoxy coreacted networks: I. Synthesis and curing kinetics

Abstract In situ copolymerization of diglycidyl ether of 4,4′-dihydroxybiphenol (DGE-DHBP) with diglycidyl ether of bisphenol F (DGEBP-F) networks using an anhydride curing agent has been investigated. DGEBP-F is a commercial epoxy while cured DGE-DHBP shows liquid crystal transitions. Curing kinetics are determined using differential scanning calorimetry (DSC). The data were fitted using an autocatalytic curing model for both pure and mixed components. Isothermal and non-isothermal methods were compared. The glass transition ( T g ) was evaluated as a function of composition using DSC. The results show that the DGE-DHBP constituent affects the curing kinetics of the epoxy resin and that the network exhibits one T g .

Electrosynthesis of nanocrystalline cerium oxide/layered silicate powders

Cerium oxide/montmorillonite layered silicate powder nanocomposites were synthesized electrochemically. The produced nanocomposites were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. X-Ray diffraction shows the nanocomposites retain the face-centered cubic structure of cerium oxide while incorporating delaminated platelets of the layered silicate into the matrix. By a negative shift in the OH stretching band position, infrared spectroscopy measurements indicate the layered silicates are delaminated. Photoluminescence background in the Raman spectra also shows the incorporation of the layered silicates within the cerium oxide matrix. The thermal properties of the nanocomposites were investigated with sintering and differential scanning calorimetry. The results indicate that low concentration of layered silicate mixed into the cerium oxide increases crystal growth rate whereas high concentration retards the increase of crystallite size during the densification process.

Epoxy + liquid crystalline epoxy coreacted networks: II. Mechanical properties

Abstract The effect of coreacting diglycidyl ether of bisphenol F (DGEBP-F) with diglycidyl ether of 4,4′-dihydroxybiphenol (DGE-DHBP) on mechanical properties is investigated. DGE-DHBP shows a liquid crystal (LC) transition upon curing. Tensile, impact and fracture toughness test results are evaluated. Dynamic mechanical analysis is conducted to determine the effect of the DGE-DHBP component. Scanning electron microscopy of fracture surfaces shows changes in failure mechanisms compared to the pure components. The results indicate that the mechanical properties of these blended samples are significantly improved at 10–20% by weight of DGE-DHBP. This is a consequence of the rigidity of the LC component which thus provides the reinforcement.

Entropy and fragility in vitreous polymers

Abstract The study of the sub T g relaxation in a semi-rigid polymer family for which the size of the lateral chain varies from one carbon to three carbon atoms have been performed. We used an entropic model proposed by Hutchinson et al. [Macromolecules 33 (2000) 5252] introducing a new parameter x s called the entropic non-linearity parameter. For our samples we found x s =0.45±0.05. On comparison with the data obtained on other linear polymers, we have shown that the rigidity of the main chain involves an increase in the entropic non-linearity parameter x s . Furthermore, we have shown, by using the fragility concept, that this new entropic model is conceptually equivalent to the Random Walk Model proposed by Arkhipov et al. [J. Non-Cryst. Solids 172 (1994) 396; J. Phys. Chem. 98 (1994) 662].

Property enhancement of soy protein isolate-based films by introducing POSS.

To enhance the mechanical and water-resistant properties of soy protein isolate (SPI) based films, hydrophobic TriSilanolPhenyl polyhedral oligomeric silsesquioxanes (POSS) was incorporated to modify the SPI films. POSS has three SiOH groups in a molecular, which is employed to cross-link SPI with the help of 3-glycidoxypropyltrimethoxysilane (GPTMS). POSS is a structure of eight phenol groups, playing a critical role in improving the physical and mechanical properties. The X-ray diffraction (XRD) and attenuated total reflectance (ATR) Fourier transform infrared spectroscopy (FT-IR) were used to characterize the films. The degree of reaction of SiOH groups in the POSS was estimated to be 53.0% according with the absorbance of ATR FT-IR spectra. Although the elongation at break was reduced by 52.6%, the tensile modulus, tensile strength and 10% offset yield strength were significantly increased by 86.6%, 34.0% and 56.8%, respectively, due to the cross-linking reactions among SPI, GPTMS and POSS. The results of water-resistant tests showed that the 24-hour water absorption was dramatically reduced by 54.7%.

Modification of cement mortar with recycled ABS

Abstract Cement mortar was modified using recycled acrylonitrile butadiene styrene (ABS) in powder form. Mixtures with polymer–cement ratios of 8, 15, and 25 wt.% were investigated for changes in compressive properties and adhesion to steel rebar. Compressive tests indicated an increase in Youngs modulus for samples with 8% and 15% ABS. Adhesion strength to the steel rebar decreased on adding the ABS. However, when the ABS was treated with maleic anhydride, an increase in adhesion strength was obtained. The decrease in adhesion of the untreated ABS-modified cement to steel was attributed to the disruption of the interface between the cement mortar and steel rebar. Scanning electron microscopy (SEM) indicated changes in the cured cement with addition of ABS. Gas adsorption measurements of pore size distribution indicated an increase in pore volume of the 8 and 15 wt.%-containing cement mortar.

Hybrid boron nitride-natural fiber composites for enhanced thermal conductivity

Thermal conductivity was dramatically increased after adding natural fiber into hexagonal boron nitride (hBN)/epoxy composites. Although natural fiber does not show high-thermal conductivity itself, this study found that the synergy of natural fiber with hBN could significantly improve thermal conductivity, compared with that solely using hBN. A design of mixtures approach using constant fibers with increasing volume fractions of hBN was examined and compared. The thermal conductivity of the composite containing 43.6% hBN, 26.3% kenaf fiber and 30.1% epoxy reached 6.418 W m−1 K−1, which was 72.3% higher than that (3.600 W m−1 K−1) of the 69.0% hBN and 31.0% epoxy composite. Using the scanning electron microscope (SEM) and micro computed tomography (micro-CT), it was observed that the hBN powders were well distributed and ordered on the fiber surfaces enhancing the ceramic filler’s interconnection, which may be the reason for the increase in thermal conductivity. Additionally, the results from mechanical and dynamic mechanical tests showed that performances dramatically improved after adding kenaf fibers into the hBN/epoxy composite, potentially benefiting the composite’s use as an engineered material.

Dynamic mechanical analysis, surface chemistry and morphology of alkali and enzymatic retted kenaf fibers

Bast fibers grow in the bark layer of many plants and have been used for textiles and cordage for over 6000 years. Bast fibers are expanding into new markets of non-woven fabrics and composite materials, and a comparative assessment of surface reactive groups and mechanical properties after different retting procedures is of value. Here, bast fiber of kenaf (Hibiscus cannabinus L., Malvaceae) were prepared by (1) alkali retting with 2% NaOH and (2) enzymatic retting with pectinase, and compared with commercially-available fiber retted by the natural microbe population in ocean water. Fiber structure was analyzed by fluorescence and electron microscopy; fiber chemistry was assessed by Raman and X-ray photoelectron spectroscopy, and by carbohydrate analysis; and mechanical properties were determined by dynamic mechanical analysis. Collectively, these show that enzymatic and microbial retting preserve the natural fiber structure and result in superior mechanical properties compared with alkali retting, which disrupts structure and degrades quality. The impacts of the retting procedure on fiber chemistry, morphology and mechanical properties are discussed.

Osteoconductive bio-based meshes based on poly(hydroxybutyrate-co-hydroxyvalerate) and poly(butylene adipate-co-terephthalate) blends.

Poly(butylene adipate-co-terephthalate) (PBAT) and Poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) are biopolymers that have the potential to be used in applications of bone healing. In this study, it is hypothesized that the polymer blend has the combined strength and osteoconductivity to support osteoblast collagen formation. PBAT (PBAT 100), and a blend with 20% PHBV (PBAT 80) were extruded in the form of fibers and then knitted in the form of mesh. These were tested in the warp as well as weft direction for the tensile properties; these showed that the weft direction had higher performance than the warp. The individual fibers were kept in phosphate buffered saline (PBS) over the period of 8 weeks and were tested for the storage and loss modulus using a dynamic mechanical analyser (DMA). The results indicated that mechanical relaxation strength showed a decrease and then an increase. In vitro osteoconductivity studies were done by using differentiating osteoblasts (MC3T3-E1 subclone 4 cells). Environmental Scanning Electron Microscopy (ESEM) showed that pre-soaking the samples in α-MEM for two weeks resulted in cell attachment and growth. X-ray diffraction (XRD) was used to determine the change in structure of polymers due to in vitro degradation for two weeks. Raman spectroscopy showed that all scaffolds supported the formation of a collagenous network over the scaffold surfaces. For a combination of knittable manufacturing, mechanical performance and osteoconductivity, blends offer an effective route.