Leishan Shao

Fabrication of high strength PVA/rGO composite fibers by gel spinning

High strength composite fibers were prepared from poly(vinyl alcohol) (PVA) (degree of polymerization: 6100) reinforced by reduced graphene oxide (rGO). The macroscopically homogeneous PVA/rGO dispersion was obtained through solvothermal reduction of graphene oxide (GO) in PVA/dimethyl sulfoxide (DMSO)/H2O solution, and then extruded into composite fibers by gel spinning followed by hot drawing. It was found that the mechanical properties of PVA fibers were greatly improved by incorporating rGO. At 0.1 wt% rGO loading, tensile strength increased from 1.8 GPa for the pure PVA fiber to 2.2 GPa for the PVA/rGO composite fiber. The results of mechanical properties and FTIR spectra for PVA/rGO composite fibers suggest the relatively strong interfacial interactions between rGO nanosheets and PVA that improve the load transfer from the polymer matrix to the reinforcing phase. Meanwhile, the thermal stability of the composite fibers was also enhanced by rGO addition.

The effect of the reduction extent on the performance of graphene/poly(vinyl alcohol) composites

The reduction extent of graphene oxide (GO) was tailored by adjusting the solvothermal reduction time. The effects of the reduction extent of GO on the structure and properties of graphene/poly(vinyl alcohol) (PVA) composites were investigated. Results show that the mechanical properties of polymer composites are sensitive to the reduction extent of GO, and reduced graphene oxide (rGO) with an optimum reduction extent (C/O ratio = 3.94) could more efficiently improve the mechanical properties of PVA than low/high reduction extent rGO. Low reduction extent rGO could enhance the interfacial interactions through the formation of hydrogen bonds with PVA chains, but its mechanical strength is smaller. High reduction extent rGO has stronger mechanical strength while the deep deoxidation of GO significantly weakens interfacial interactions between rGO nanosheets and PVA chains. Achieving optimum reinforcing effects requires balancing and optimizing interfacial interactions and mechanical strength of rGO.

Enhancement of water and organic solvent resistances of a waterborne polyurethane film by incorporating liquid polysulfide

Environmentally friendly, waterborne polysulfide-based polyurethane (WSPU) films have been successfully synthesized from liquid polysulfide (PSF), polytetramethylene ether glycol (PTMG), isophorone diisocyanate (IPDI) and dimethylolpropionic acid (DMPA). The chemical structures, mechanical and thermal properties of the chemically modified films have been carefully investigated using ATR-IR, gel permeation chromatography (GPC), tensile tests, dynamic mechanical analysis (DMA) and a thermogravimetric analyzer (TGA). As the PSF content increases from 0 to 20%, after an immersion in deionized water for 96 h, the water absorption decreases from 6.2% to 3.4% and the tensile strength retention increases from 74.3% to 91.7%. The chemical resistance to organic solvents is also improved significantly. The enhanced water and organic solvent resistances of waterborne polyurethanes (WPUs) are primarily attributed to the addition of a PSF containing sulfur group. However, the tensile strength and thermal performance are weakened to some extent. This study produces a new composite derived from polysulfide, and its high water and solvent resistance performance could contribute to surface material research.

Effect of the functional diamine structure on the properties of a polyimide liquid crystal alignment film

A novel functional diamine containing triphenylamine moiety and biphenyl as well as a long alkyl chain, 4-dodecyloxy-biphenyl-4′,4′′-diaminotriphenylamine (DBDTA), was synthesized and characterized. A series of polyimides (PIs) were copolymerized from DBDTA, 3,3′-dimethyl-4,4′-methylenedianiline (DMMDA) and 4,4′-oxydiphthalic anhydride (ODPA) via a one-step method. The chemical structures of the diamine and PIs were characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (1H NMR). Properties such as the solubility, rubbing resistance, thermal stability and pre-tilt angle of the PIs were investigated. Furthermore, the results were compared with the PIs-BZA derived from 4-dodecyloxy-biphenyl-3′,5′-diaminobenzoate (DBPDA). The PIs-TPA derived from DBDTA exhibited better transparency and thermal stability than the PIs-BZA from DBPDA. When the content of functional diamines was only 10%, the PI1-TPA derived from DBDTA displayed better solubility than the PI1-BZA from DBPDA. In addition, all PIs could induce uniform vertical alignment of the liquid crystals (LCs) before and after the rubbing process, but PI1-BZA only induced parallel alignment of the LCs after rubbing process. It is suggested that the PI1-TPA film is more resistant to the rubbing process than PI1-BZA.

Bio-inspired natural polyphenol cross-linking poly(vinyl alcohol) films with strong integrated strength and toughness

Polymer composites exhibit very high strength but this is usually found to be at the expense of toughness, which greatly limits their application in many fields. Inspired by the hierarchical architectures in spider silk, we attempted to combine simultaneous high strength and toughness via a hydrogen bonding network. PVA composites with tannic acid (TA) molecules were fabricated by a green and easy-to-scale-up water casting method. The results showed that the hydrogen bonding crosslinking between PVA and TA has strong effect on the Tg, crystallinity, thermal stability and mechanical properties of PVA. In addition, the PVA composite films with 3 wt% TA exhibited, simultaneously, the best tensile strength and toughness, improving by 46% and 27% compared to the pure PVA, respectively.

Effect of graft polymer prepared by living radical polymerisation on electro-optical properties of polymer dispersed liquid crystal

In this study, a graft polymer matrix prepared by living radical polymerisation had been incorporated into polymer dispersed liquid crystals (PDLCs). The electro-optical properties of the PDLCs were investigated. The results showed that the length and density of graft chain had a great influence on the memory effect of the PDLCs. Low-driving-voltage and weak-memory-effect PDLCs could easily be obtained with a graft polymer matrix.

Effect of macro-RAFT agent on the morphology of polymer dispersed liquid crystals

In this study, macro-(RAFT) reversible additional fragmental chain transfer agent prepared by reversible additional fragmental chain transfer polymerisation has been incorporated into polymer dispersed liquid crystals (PDLCs). The effects of concentration, molecular weight and glass transition temperature of macro-RAFT agent were studied in terms of morphology, polymerisation kinetics, molecular weight of polymer matrix and electro-optical properties of the films. It was found that the key factor influencing morphology was the mobility of macro-RAFT agent chain rather than polymerisation rate and molecular weight of polymer matrix. Furthermore, the decrease in the mobility of macro-RAFT agent chain caused less liquid crystal nematic fraction, smaller liquid crystal domain size and greater driving voltage.

Effect of rGO on polymer-dispersed liquid crystal fabricated by RAFT polymerisation

In this study, we demonstrated that doping polymer matrix with a small amount of reduced graphene oxide (rGO) component (0.05–0.2%) had significant influence on the polymerisation kinetics and electro-optical performances of polymer-dispersed liquid crystal films (PDLCs) fabricated with macro reversible addition-fragmentation chain transfer agents. The effects of rGO content were studied in terms of morphology, compound viscosity, polymer conductivity, polymerisation kinetics and driving voltage of PDLCs. The results exhibited that higher rGO content increased the compound viscosity and the entire process proceeded slowly. Furthermore, the addition of rGO increased the polymer conductivity and local electric field, and reduced the saturation voltage as well as the threshold voltage from 27.3 to 19.5 V and 13.2 to 6.41 V, respectively.

The impact of flexibility of polyimides backbones on the stability of liquid crystal vertical alignment

Two series of polyimides (PIs) which contained identical side chains but different backbones were synthesized from the same functional diamine: RPI-X% and SPI-X% (X = 20, 30, 40, X stands for the molar content of functional diamine). Rigid polyimides (RPIs) are PIs whose backbones are composed of wholly rigid aromatic units and soft polyimides (SPIs) are PIs whose backbones are composed of flexible ether units and aromatic units. The surface morphology, chemical composition and molecular orientation of the polyimide (PI) alignment layer surfaces were investigated by atomic force microscopy, X-ray photo-electron spectroscopy and polarized attenuated total reflection Fourier transformed infrared spectroscopy, respectively. The results showed that both RPI and SPI could induce LC to align vertically without a rubbing process when the molar content of the functional diamine reached 30%. However, the rubbed RPI-30% whose side chains were oriented vertically induced vertical alignment of the liquid crystal (LC), while the rubbed SPI-30% whose side chains were oriented parallel induced parallel alignment of the LC. The cause of this distinction was attributed to the different main chain structures: the wholly rigid aromatic units in RPIs main chains could restrict the movements of molecular chains so the side chains can maintain vertical orientation all the time; flexible ether bands in SPI main chains could endow molecular chains with mobility so the side chains fall over easily after rubbing.