Volume 15, Issue 5 - Special Issue: Polymer and Composite Materials (PCM 2015) / Guest Editor: Esteban Broitman

In this paper, several cross-linked polymer materials with high light-transmitting properties are presented. First, mercaptopropyl polyhedral oligomeric silsesquioxane (POSS-SH) was synthesized via the hydrolytic condensation of 3-mercaptopropyl triethoxysilane. Then, glycidyl methacrylate (GMA) and allylmethacrylate were allowed to polymerize with POSS-SH, yielding a hybridized composite. The volumetric shrinkage, thermal, and light-transmitting properties of the materials were studied and investigated using a pycnometer, differential scanning calorimeter, and ultraviolet visible spectrophotometer. The results indicated that POSS-SH decreased the volumetric shrinkage of the GMA-based materials. Thermal analysis indicated that the glass transition temperature ( T g ) of the hybrid composite decreased as the POSS-SH content increased, while the T c and T m of the hybrid composite increased slightly. The light transmittance of the hybrid composite increased as the POSS-SH content increased. When the POSS-SH content increased to 42.9%, the hybrid composite exhibited average light transmittance values of 96% at wavelengths ranging from 200 to 400 nm and 97% at wavelengths ranging from 400 to 800 nm.

Polymer membrane structures with voids can be fractured due to larger deformation and stress concentration with the effect of an externally applied load, which can lead to failure localization and crack propagation. With the modified constitutive model from Gao, finite deformation analysis of a polymer membrane containing a void has been researched. By deducing the basic governing equation for solving the problem, the paper figures out the stress distribution of different constitutive parameters and discusses the effects on membrane deformation by different parameters and the reasons for the failure of the membrane. The results show that the constitutive parameter n has a major impact on the mechanical properties of a polymer membrane with a void, which provides a reference for the design of polymer membranes.

An investigation of the molecular interaction within a hydrogel system was conducted using molecular dynamics simulation, and the interaction mechanism of a polyacrylamide/polyvinyl alcohol (PAM/PVA) hydrogel system was examined specifically at the molecular level. Several characteristics of the PAM/PVA composite hydrogel system that are largely dependent on water content and temperature were studied in this paper, such as cohesive energy density, binding energy, mechanical properties and pair correlation function. The cohesive energy density and binding energy of the hydrogel system increased with higher water content. Results also showed that increased temperatures led to a decrease in the cohesive energy density of the system, while binding energy remained unchanged. The mechanical properties of the system were evaluated by analyzing the static mechanic performance. Results showed that elastic coefficients, engineering modulus and ductility decreased with increasing water content and temperature. In addition, analysis of the pair correlation function revealed mainly hydrogen bonding interactions between H 2 O molecules and surrounding atoms or functional groups. Results also indicated that the strength of these hydrogen bonds was O water >O PVA >O PAM >N PAM , confirming both the potential and the difficulty of hydrogen bond formation. The aforementioned findings help in understanding the interaction mechanisms between the components of a hydrogel system and in demonstrating the effects of water content and temperature on the PAM/PVA hydrogel system, which provides useful information on the possible operating windows of a biomedical hydrogel-making process.

This paper reports the investigation about the usage of an epoxy (EP)-coated spinneret for the preparation of medicated electrospun nanofibers. Cellulose acetate (CA) and acetaminophen (APAP) were used as the polymeric carrier and model drug, respectively. The electrospinning was undertaken using both EP-coated spinneret and traditional stainless steel capillary as spinnerets. According to the images from scanning electron microscopy, it is obvious that the nanofibers produced using the EP-coated spinneret had a finer diameter and a narrower size distribution (450±90 nm) than nanofibers fabricated using stainless steel equivalent (660±180 nm). In vitro dissolution tests revealed that the sustained-release profiles of nanofibers from the EP-coated spinneret were superior to those of their stainless steel equivalents, although APAP existed in a similar amorphous state in both nanofibers. Because the EP-coated material can exploit the electrical forces more effectively than its steel analogue, it can enhance the electrospinning technique for producing polymeric functional nanofibers.

By UVA and UVB irradiating, the performance of poly-p-phenylene benzobisoxazole (PBO) fiber is changed with an increase in irradiation time, and the surface morphology, chemical structure, mechanical performance and thermal properties of the fiber are poorer. After 320-h irradiation, the properties of the PBO fiber irradiated by UVB are stronger than those irradiated by UVA. The strength of PBO fiber irradiated by UVB and UVA declined by 40% and 20%, respectively, than that of non-irradiated fiber. The roughness of fiber surface improved, and there was corrosion on the surface of PBO fiber irradiated by UVB. After the UV irradiation, the degree of macromolecule crystallinity and orientation changed, the macromolecule chain is broken, new groups (-NO 2 ) are produced in the PBO fiber, and the temperature (660°C) of the thermolysis is decreased by 100°C from that of fibril.

Polyurethane (PU) damping materials were prepared by mixed-base PU prepolymers and 4,4′-methylene- bis-(2-chloroaniline) (MOCA) as chain extender. The prepolymer was synthesized from poly(hydroxypivalyl hydroxypivalate succinate) (PHPS) or polypropylene glycol 2000 (PPG-2000) with 2,4-toluene diisocyanate (TDI-100), respectively. The structure, morphology, and dynamic and static mechanical properties were characterized by Fourier transform infrared spectroscopy (FT-IR), scanning tunneling microscopy (STM), dynamic mechanical thermal analysis (DMA), and tensile testing machine, respectively. The results indicated that the mass ratios of polyester prepolymer (PESP) and polyether prepolymer (PETP) ( m PESP / m PETP ratio) had significantly affected the dynamic mechanical properties of the PUs. When the m PESP / m PETP ratio was 40/60 and 45/55, and PUs had the temperature range (Δ T ) higher than 60°C for efficient loss factor (tan δ >0.4) in the DMA curve with bimodal. The phase diagrams of STM indicated that more complex combination of hard and soft segments existed in the PUs, and the segments of PESP and PETP formed irregular block structures. Furthermore, the mechanical properties of PUs were of higher value. It is believed that PUs may be used as potential damping materials.

Energy dissipation in polymeric materials was studied using a two-relaxation-time model. A differential form of a constitutive relation was constructed with the viscoelasticity theory. Through the simulation of a cyclic loading and unloading test, the dependence of the dissipated energy on the model parameters and external loading variables was determined and analyzed. In particular, the characteristics of the hysteresis phenomenon of a material with more than one relaxation time were studied in detail.

This study reports the reinforcement and fracture toughening mechanism of pristine multi-walled carbon nanotubes (MWCNTs) on epoxy matrix. The tensile strength and fracture energy ( G IC ) of the epoxy polymer increased simultaneously upon the addition of a small amount of MWCNTs. The fracture surfaces of single-edge-notch three-point bending test specimens were analysed by scanning electron microscopy, and the double-notch four-point bending technique was used to investigate the fracture process by transmission electron microscopy, respectively. MWCNT pull-out and subsequent plastic void growth were found; meanwhile, fracture of MWCNTs was observed along the crack propagation path. The theoretical model of shearing band initiated by the stress concentrations around the MWCNTs is the dominant toughening mechanism. While the crack bridging of MWCNTs and the plastic void growth of epoxy also have a toughening effect.

In recent years, organosilane-containing copolymers have attracted much attention in antifouling coatings, automotive coatings, fiber, composite materials, biomedical materials, and latex and protection coatings against corrosion owing to their excellent weathering and low surface energy. Trimethoxysilane-containing high-solid hydroxyl acrylic resin used for automotive coatings was prepared by in situ free radical polymerization. The surfaces of the resin film were characterized by solid surface energy, atomic force microscopy and X-ray photoelectron spectroscopy (XPS) measurements. With the increase in silicone content from 0 to 2.5 wt.%, the surface energy of trimethoxysilane-containing hydroxyl acrylic copolymer films decreased from 49 to 40 mN/N and the surface roughness (Rms) on the surface of thin films increased from 0.427 to 0.887 Å. The XPS result indicated that the surface accumulation phenomena of silicone occurred in the surface of the thin films. The phenomena can explain the reason why the incorporation of trimethoxysilane (MPS) into acrylic copolymer by in situ polymerization can decrease its surface energy and increase its outdoor weathering.

A series of fire-resistant, temperature-sensitive hydrogels were synthesized via free radical polymerization using N -isopropylacrylamide as a temperature-sensitive monomer, N , N ′-methylenebisacrylamide as a cross-linker, ammonium persulfate (APS) as an initiator, and other raw materials including acrylic acid, 2-acrylamido-2-methyl propane sulfonic acid (AMPS), and attapulgite clay (APT). The structures and properties of superabsorbent hydrogels were studied using different analytical tools such as Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA)/differential scanning calorimetry. The effects of the monomer, cross-linker, and other reactants (AMPS and APT) on the water absorption ability of the hydrogels were analyzed; moreover, the effect of valence of metal ions was also examined. SEM images revealed the surface of the hydrogels as uneven and corrugated. The surface toughness increased with increasing APT. The addition of APT and AMPS increased the thermal resistance of the hydrogels but decreased the temperature sensitivity. Furthermore, the swelling ratio of the hydrogels decreased with the increase in salt solution concentration and valency of metal cation present in the salt solutions.

Authigenic gas foaming hydrogels were synthesized using chitosan (CS), acrylic acid, and attapulgite (APT) as hydrophilic monomers; potassium persulfate as initiator; N,N′-methylenebisacrylamide as cross-linker; and sodium carbonate (Na 2 CO 3 )/acetic acid as foaming agent. Effects of different monomers, amount of foaming agent, and temperature on swelling ratio (SR) of the hydrogels were examined. Morphology, structure, and thermal stability of authigenic gas foaming hydrogels were studied using field emission scanning electron microscope, Fourier-transform infrared spectroscopy, and synchronous thermal analysis. Scanning electron microscope images reveal apparent pores on hydrogels, produced by foaming agent Na 2 CO 3 /acetic acid. Therefore, more foaming agent would bring more pores on hydrogels. Synchronous thermal analysis results suggest that higher amount of CS would result in higher thermostability of hydrogels. However, APT has no substantial effect on thermal stability. SR decreases with increasing amount of CS. Hydrogels prepared at 70°C reaction temperature exhibit maximum swelling. Furthermore, SR decreases with higher covalence of saltion.