Huawei Zou

Simultaneously enhanced tensile strength and fracture toughness of epoxy resins by a poly(ethylene oxide)-block-carboxyl terminated butadiene-acrylonitrile rubber dilock copolymer

A PEG-b-CTBN diblock copolymer was synthesized via the esterification reaction of hydroxy terminated methoxypolyethylene glycols (PEG) in the presence of a carboxyl-terminated butadiene-co-acrylonitrile copolymer (CTBN) with 4-dimethylaminopyridine (DMAP) as the catalyst. Epoxy thermosets were modified via the formation of a nanostructure. The result of transmission electronic microscopy (TEM) shows spherical domains were homogeneously dispersed into the continuous epoxy matrix and the size is nano-scale. It was noted that the glass transition temperatures decreased with an increase in the content of PEG-b-CTBN, which was evidenced by Dynamic Mechanical Analysis (DMA) and a Differential Scanning Calorimeter (DSC). Through mechanical tests, we found the tensile strength and fracture toughness of epoxy resins were simultaneously enhanced by nanostructured PEG-b-CTBN.

Morphology and properties of porous polyimide films prepared through thermally induced phase separation

Porous polyimide (PI) synthesized from 4,4′-oxydiphthalic anhydride (ODPA) and 4,4′-diaminodiphenyl ether (ODA) monomers is a promising material with an ultralow-dielectric constant. We report here a strategy toward engineering PI films of various porous textures using a small molecular phase dispersion agent, dibutyl phthalate (DBP), as porogen. In the presence of DBP, the ODPA–ODA poly(amic acid) solution, the precursor to PI, undergoes phase separation as N,N-dimethylacetamide (DMAc) solvent is slowly evaporated, forming spherical domains of DBP phase uniformly dispersed in the polyamidic acid matrix. Upon thermal imidization, a polyimide film with high porosity is attained after acetone extraction of DBP. It is demonstrated in this study that the porous texture of PI films can be readily engineered by tailoring the initial DBP content. The average pore size increases with increasing concentration of DBP, but was no larger than 6 μm. The PI film achieves a dielectric constant of 1.7 at an optimal porosity of 72%, This study examined the pore formation mechanism, the imidization chemistry, the surface morphology, the density, the thermal stability and mechanical properties of the formed porous PI films. Thermo-gravimetric analysis indicated that porous films retain the inherent exceptional thermal stability of polyimides, with thermal decomposition onset above 500 °C in nitrogen atmosphere.

The effect of epoxy–silicone copolymer content on the thermal and mechanical properties of cured epoxy resin modified with siloxane

In this research, a bisphenol-A type epoxy resin (DGEBA) was modified with epoxy-block-silicone copolymers and hydroxyl-terminated silicone oligomers by physical blending and a chemical reaction, respectively. The chemical structure of the siloxane-bridged epoxy resin terminated by –OHs was characterized by Fourier transform infrared spectroscopy (FTIR), 1H-NMR and an epoxy equivalent weight (EEW) test. Both the samples showed better elongation at break and impact strength than neat resin. The TGA-FTIR results revealed that the residue of the modified epoxy resin at 600 °C increased with the increase in siloxane content, but the thermal stability was slightly reduced compared with that of the neat epoxy resin. Morphology studies indicate that the increase in izod notched impact strength is due to the suitable diameter of silicone phases because of the silicone toughening effect. Therefore, it is believed that the modified epoxy resin, with good toughness and high thermal residual weight, will have potential applications in anti-corrosion coatings and structure bonding materials.

Preparation and characterization of amphoteric polycarboxylate and the hydration mechanism study used in portland cement

A series of comb-like polymer, amphoteric polycarboxylic (APC), was designed as a dispersant for portland cement. APC was synthesized in aqueous solution by macromonomer of butenyl alkylene polyoxyethylene-polyoxypropylene ether (BAPP), sodium methacrylate (SMA), acrylic acid (AA) and [2-(methacryloyloxy)ethyl]trimethylammonium chloride solution (MAEAC). The molecule structure of amphoteric polycarboxylic dispersant was characterized by Fourier transformer infrared (FTIR), and the molecular weight of the APC was determined by gel permeation chromatography (GPC). The dispersion capacity of the APC in cement was also measured. The result shows that the amphoteric polycarboxylic dispersion agent is suitable for portland cement. The APC dispersant with both anionic group and cationic group will provide the better anchoring action with the cement via the adsorption of ions. The hydration mechanism of cement with a certain amount of APC is discussed. We found that the APC could affect the hydration process, which was performed through retarding the generation of ettringite and affecting the crystal enthalpy and its thermal stability in the hydrated product.

Enhanced thermal conductivity of polyamide 6/polypropylene (PA6/PP) immiscible blends with high loadings of graphite

In this study, flake graphite was dispersed in immiscible polyamide 6/polypropylene (PA6/PP) blends to improve their thermal conductivity. Blends of PA6/PP were prepared in different relative fractions and thermal conductivity of the graphite-containing blends showed enhanced values for immiscible blends when compared with those of mono-PA6-based composites at the same graphite loading fractions. Moreover, a PA6/PP mass ratio of 8/2 showed better dispersion of graphite and less interfacial defects, resulting in better heat conduction ability when compared with other immiscible blends. At graphite loading of 40 wt%, thermal conductivity of PA6/PP/graphite composite reached 2.896 W/mK, which is nearly 40% higher than that of mono-PA6/graphite composite. Morphology and melt rheology measurements confirmed that flake graphite began to form thermally conductive network at 30 wt% in immiscible blends which is crucial to the improvement of thermal conductivity.

Effect of hard segments on the thermal and mechanical properties of water blown semi-rigid polyurethane foams

Water blown semi-rigid polyurethane foams with different ethylene glycol (EG) content were prepared. EG is used as chain extender to vary hard segments content of semi-rigid polyurethane foams. The effect of EG on foam molding process, hard segment structure and the relationship between structure and thermal, mechanical properties of foams has been investigated. Results indicate that the addition of EG contributes to preparing flawless foams, and reduces the apparent density of foams due to the increase in cell size. Fourier transform infrared analysis (FTIR) displays the existence of hydrogen bonds among polyurea segments, which is influenced by reactant activity. Dynamic mechanical analysis (DMA) manifests different phase mixing and soft domains Tg of foams with different EG content. Soft domains Tg is dependent on the purity of soft domains controlled by urethane hard segments and dissolved urea groups. Thermo gravimetric analysis (TGA) illustrates that thermal stability of foams is associated with hard segments content in foams. Mechanical tests demonstrate that improvements of mechanical properties of foams can be achieved by modifying foam formulation to increase microphase separation of polyurea segments.

Melt miscibility of HDPE/UHMWPE, LDPE/UHMWPE, and LLDPE/UHMWPE blends detected by dynamic rheometer

The dynamic rheological behavior of high density polyethylene (HDPE)/ultrahigh molecular weight polyethylene (UHMWPE) blends, low density polyethylene (LDPE)/UHMWPE blends and linear low density polyethylene (LLDPE)/ UHMWPE blends was measured in parallel plate rheometer at 200°C. The analysis of log-additivity rule, Cole-Cole plots and Han curves of the three series blends indicated that the LDPE/UHMWPE blends were miscible in the melt, while the HDPE/UHMWPE blends and LLDPE/UHMWPE blends showed phase separation. The DSC results of LLDPE/UHMWPE blends and HDPE/UHMWPE blends were consistent with the rheological properties, while for the thermal properties of LDPE/UHMWPE blends, results revealed three endothermic peaks, which indicated a liquid-solid phase separation in LDPE/UHMWPE blends.

Enhancement in thermal conductivity and mechanical properties via large-scale fabrication of boron nitride nanosheets:

In this study, a facial method of fabricating hexagonal boron nitride nanosheet (BNNS) was proposed. Isopropyl alcohol was employed as the solvent to obtain the BNNS via exfoliation of the pristine hexagonal boron nitride. The yield of the exfoliated BNNS with thickness less than 20 nm was as high as 0.17–0.2 mg mL−1. The BN- and BNNS-filled polyamide 6 (PA6) composites were subsequently prepared by melt blending, and a comparison of thermal conductivity and mechanical properties of the resultant composites were demonstrated. Results indicated that the PA6/BNNS composites showed superior mechanical and thermal conductive properties when compared with that of neat PA6 and PA6/BN composites. At a filler-loading fraction of 40 wt%, thermal conductivity of the PA6/BNNS composite reached 2.496 W mK−1, which was 21.8% higher than that of PA6/BN composites at the same filler-loading concentration. In addition, the tensile strength of PA6/BNNS composites was invariably higher than that of neat PA6, with a 6.23% increment at a filler concentration of 30 wt%. Based on the results of differential scanning calorimetry, a new crystallization peak (TCC, 2) was observed at higher temperature region for the filler-containing composites and the position of the new peak gradually shifted to higher temperatures with an incremental loading concentration of BN and BNNS.

Enhanced mechanical properties of epoxy nanocomposites based on graphite oxide with amine-rich surface

Functional graphite oxide (DGO) with amine-rich surface was synthesized through chemically grafting flexible poly(oxypropylene)diamine, and its epoxy (EP) composites were prepared. Fourier transform infrared spectra (FTIR) etc. confirmed the realization of chemical functionalization of DGO. The covalent functionalization of graphite oxide (GO) with poly(oxypropylene)diamine was favorable to its homogeneous dispersion in epoxy matrix. Meanwhile, the strong covalent interface formed between epoxy and DGO promoted the stress transfer. The addition of 0.3 wt% DGO increased the tensile strength, flexural strength, elongation at break and toughness of the epoxy resins by 20%, 40%, 90% and 145%, respectively. This showed higher improvements than those addition of GO. Therefore, significant improvements both in the strength and toughness of epoxy nanocomposites were achieved by the addition of trace DGO.

Study of different-sized sulfur-free expandable graphite on morphology and properties of water-blown semi-rigid polyurethane foams

In this paper, EG with different nominal particle sizes (70 μm, 430 μm and 960 μm) and different loadings from 0 to 50 pphp were considered to investigate the effect of expandable graphite on the morphology and properties of water-blown semi-rigid polyurethane foams (SPFs). Experimental results showed that a higher content of EG in the SPF was more conducive to the formation of a barrier layer after expansion and exhibited better flame retardancy. Horizontal burning test confirmed the conclusion that the EG-430 μm and EG-960 μm with various loadings could effectively enhance the flame retardant properties of the SPFs. However, EG-70 μm could not improve the fire behavior of the composite due to a weak ability to form an efficient char layer on the surface of the SPF. Thermo gravimetric analysis (TGA) indicated that EG had a positive effect on the thermal stability of SPFs. In addition to the flame retardant behavior, effects of EG on the density, pore structure, thermal properties, mechanical properties and damping properties of the SPFs were also investigated.