Editorial

Poly(methyl methacrylate) (PMMA)/hydroxyapatite (HA) composite has potential application in denture base materials. The denture base materials should exhibit good mechanical properties and dimensional stability in moist environment. Silane coupling agent [3-methacryloxypropyltrimethoxy silane (γ-MPS)] was used to treat the HA fillers in order to enhance the interfacial interaction between the PMMA and HA. In this research, the kinetics and effects of Simulated Body Fluid (SBF) and water absorption on the flexural properties of PMMA/HA composites were studied for an immersion duration of 2 months. The mathematical treatment used in analyzing the data was the single free phase model of diffusion, which assumed Fickian diffusion and utilized Fick’s second law of diffusion. The kinetics of water absorption of the PMMA/HA composites conformed to Fickian law behavior, whereby the initial moisture absorption follows a linear relationship between the percentage gain at any time t and t^1/2, followed by saturation. It was found that the equilibrium moisture content and the diffusion coefficient are depending on the concentration of γ-MPS in PMMA/5HA composites. The reduction of equilibrium moisture content of PMMA/5HA is due to the hydrophobic behavior of γ-MPS and compatibility of PMMA with HA. The retention ability in flexural modulus and strength of PMMA/HA composites upon subjected towater absorption are considerably good. The reduction of flexural strength of the PMMA/HA composites after water absorption and SBF absorption could be attributed to the plasticizing effect of water molecules.

Zinc disorbate (ZDS) was in situ formed by the reaction between sorbic acid (SA) and zinc oxide (ZnO) in nitrile rubber (NBR). The effects of SA amount on the curing characteristics, crosslink density and mechanical properties of peroxide- cured NBR were studied. The results showed that ZDS was generated mainly during the rubber vulcanization, rather than the open mill compounding phase. The results from the crosslink density determinations showed that the formation of ZDS significantly increased the ionic bond content in the vulcanizates. In addition, the formation of ZDS greatly enhanced the mechanical properties of NBR vulcanizates. The modulus, tensile strength, tear strength and hardness were found to be increased with the loading of ZDS. Preheating the compounds before compression moulding was beneficial to the formation of ZDS, and consequently the increases in mechanical properties. At 40 parts per hundred rubber (phr) of SA and 16 phr ZnO, five to six folds of tensile strength and tear strength of the neat NBR vulcanizate were achieved. Transmission electron microscopy (TEM) results confirmed the nano-dispersion structure of the polymerized ZDS in the NBR matrix.

A new UV-reactive monomer piperazine-N,N′-bis(acryloxyethylaryl-phosphoramidate) (N-PBAAP) containing phosphorus and nitrogen was synthesized and used as flame retardant for an ether-type UV-cured polyurethane acrylate (PUA) coating. The thermal properties of the PUA films were investigated by thermogravimetric analysis (TGA) in air and nitrogen atmosphere. The TGA results showed that the incorporation of N-PBAAP can obviously enhance the char residue of the PUA coatings. From the TGA and real time Fourier transformed infrared spectroscopy (RT-FTIR) results, different degradation behaviors were observed in the PUA coatings with different N-PBAAP content. The combustibility of the PUA coatings was evaluated by microscale combustion calorimeter (MCC). The MCC results revealed that the addition of NPBAAP in the coatings can significantly reduce the peak Heat Release Rate (pHRR), Heat Release Capacity (HRC) and the Total Heat Release (THR) of the samples. Furthermore, dynamical mechanical thermal analysis (DMA) was employed to examine the viscoelastic properties of the PUA films. It was found that the incorporation of N-PBAAP in the formulation can bring in more functional groups to the coatings, which results in an increase of the glass transition temperature (T_g) and cross linking density (XLD) of the films.

Wettability has been recognized as one of the most important properties of fibrous materials for both fundamental and practical applications. In this study, the plasma induced grafting of acrylic acid (AAc) was applied to improve the wettability of the electrospun poly(vinylidene fluoride) (PVDF) nanofiber membranes. The diameter and chemical structure of the modified PVDF nanofibers were characterized by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR). Nitrogen adsorption based on BET (Brunauer, Emmett and Teller) principle was employed to measure the specific surface areas and porosities of the modified nanofiber membrances. The contact angles of the modified membrane were evaluated by drop shape analysis (DSA) and the modified Washburn method. The dependence of contact angles on specific surface area and porosity was also discussed in this paper. Water adsorptions were used to evaluate the dynamic wetting behavior of the grafted membranes by a dynamic adsorption apparatus (CDCA100-F). The experimental results revealed that the wettablity of the modified PVDF membrane was significantly affected by both surface and porous contact angles.

Poly(vinyl alcohol) (PVA)-montmorillonite (MMT) clay nanocomposite films up to 10 wt% clay concentration were synthesized by aqueous solution grown technique. The relative complex dielectric function, alternating current electrical conductivity, electric modulus and impedance properties of these organic-inorganic nanocomposites were investigated in the frequency range from 20 Hz to 1 MHz at ambient temperature. The PVA-MMT clay nanocomposite films show a large decrease in the real part and loss of relative dielectric function at 1 wt% MMT clay loading compared to pure PVA film and anomalous behaviour is observed with further increase of clay concentration in PVA matrix. Two dielectric relaxation processes were observed in these nanocomposites; a high frequency relaxation associated with PVA segmental motion and a low frequency relaxation resulting from PVA/dispersed MMT clay interfacial polarization (Maxwell-Wagner relaxation). Dielectric relaxation times corresponding to these processes were determined by fitting dc conductivity corrected complex dielectric function data to the Havriliak-Negami expression. A correlation between change in dielectric relaxation strength, relaxation time and hindrance to the PVA-chain dynamics with the degree of exfoliated structures of dispersed nanoscale MMT clay filler in the PVA matrix were explored. Results confirm the application of dielectric relaxation spectroscopy as a potential tool in the confirmation of nanocomposite formation and their structure characterization.

Nanocomposites of PMMA+MMT Brazilian clays were developed by mechanical mixing in co-rotational twinscrew extrusion and injection molding with varying weight fraction of MMT Brazilian clays. The clays were purchased in crude form and then washed and purified to extract the organic materials and contaminants. Dynamic friction and wear rate of these composites were studied as a function of concentration of the Brazilian clay. With an increase in the amount of MMT Brazilian clay, the dynamic friction of the nanocomposites increases, a clear but not large effect. It can be explained by sticky nature of clay; clay in the composite is also on the surface and sticks to the partner surface. The wear rate as a function of the clay concentration passes through a minimum at 1 wt% MMT; at this concentration the clay provides a reinforcement against abrasion. At higher clay concentrations we see a dramatic increase in wear – a consequence of clay agglomeration and increased brittleness. The conclusions are confirmed by microscopy results.

Interface related mechanical properties of unidirectional continuous glass fibre (GF)/polypropylene (PP) composites made of commingled yarns have been systematically studied according to a three level, three-factor factorial design. The three systematically varied factors comprised different silane coupling agent and film former contents in GF sizings as well as a varying GF diameter. Besides the statistical evaluation of those main effects on the transverse tensile and compression shear strengths of the composites, interfacial shear strength measurements on model composites have been performed. The latter ones as well as the results of the dynamic mechanical thermal analysis support the statistical significance of sizing components, the sizing content on the GF, and GF diameter for the mechanical properties of the composites. This highlights the interplay of proper sizing formulation and reproducible GF-spinning conditions, as both affect the interfacial bonding of continuously reinforced GF/PP composites.