Mengjin Jiang

Hydrogen bond and crystalline structure of the junction network in polyvinyl alcohol/dimethysulfoxide gels

The hydrogen bond and aggregation structures of polyvinyl alcohol (PVA) gels prepared with dimethylsulfoxide (DMSO) were investigated as were the the rheological properties. The variable temperature Fourier Transform Infrared (FTIR) and X-ray results of PVA-DMSO gels show that the hydrogen bond and crystalline structure are destroyed as temperature increases. In the Tanaka model, the value of ζ (the number of sequential units per chain in the junction) is 1.45 and s (the number of polymer chains in a single junction), which reveals the ability of solution to form gel, varied as a function of gel melting temperature Tgm. The results of dynamic rheology show that the elastic modulus G′ is proportional to the gel concentration and inversely proportional to the temperature. It was found that gels formed from solutions with same viscosities showed different properties. This is because a gel with a higher polymerization degree has a lower value of s.

Effects of the reaction degree of melamine-formaldehyde resin on the structures and properties of melamine-formaldehyde/polyvinyl alcohol composite fiber

Composite fibers made of polyvinyl alcohol (PVA) and melamine-formaldehyde (MF) resins with different reaction degrees were prepared by wet spinning. The phase structures of MF/PVA spinning dopes and composite fibers were observed by using optical microscope (OM) and scanning electron microscope with energy-dispersive X-ray spetroscopy (SEM-EDS). Crystal structures of composite fibers were studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The loss of MF resins in the spinning process was calculated by using Kjeldahl. The mechanical properties, the flame retardant property, the water resistant property, and the thermal stability of composite fibers were also tested. Results show that with an increase in the reaction degree of MF resin, the phase separation degrees of spinning dopes and composite fibers rise up, the size of MF microphase grows larger, and the loss of MF resin diminishes; consequently, the hot water resistance and the flame retardancy of the fiber ameliorate while the tensile strength and the thermal stability perform a tendency of dropping after rising.

A novel method to prepare a flame-retardant polyvinyl alcohol fiber with modified acrylonitrile coatings

To improve the flame resistance of polyvinyl alcohol (PVA) fibers, the well-designed permanent flame-retardant coatings were introduced on the surface of PVA fibers. Acrylonitrile (AN) was firstly grafted on the PVA fiber surface and then the grafted fibers (PVA-g-AN fibers) were reacted with hydrazine hydrate and copper sulfate solution to form the coatings with flame retardant performance. The structure of the fibers was characterized by Fourier transform infrared spectroscopy and scanning electron microscopy. The flame-retardant performance of the fibers was evaluated by limiting oxygen index (LOI) and micro calorimeter (MCC) tests. It was found that the coatings were effective in improving the flame resistance of PVA fibers. SEM photos of char residues and the results of the TG-IR technique revealed that flame retardance is mainly provided through the barrier action of the coatings with the partial effect of gaseous phase.

Preparation of Syndiotacticity-Rich High Molecular Weight Polyvinyl Alcohol by Low Temperature Emulsifier-Free Emulsion Copolymerization of Vinyl Acetate and Vinyl Pivalate

Low temperature emulsifier-free emulsion copolymerization of vinyl acetate (VAc) and vinyl pivalate (VPi) was carried out using a redox initiation system to prepare syndiotacticity-rich high-molecular weight (HMW) polyvinyl alcohol (PVA) with high yield. The effects of the polymerization conditions on the conversion, the molecular weight, the structure of poly (vinyl acetate-co-vinyl pivalate) (P (VAc-VPi)) and syndiotacticity of PVA were investigated. With the increases of the initiator concentration, the VPi molar ratio in monomer and polymerization temperature, the degree of polymerization (DP) of PVA decreases distinctly, but VPi contents in copolymer and syndiotactic diad (S-diad) contents of PVA increase obviously. PVA with the maximum DP of 11030 can be prepared by complete saponification of P(VAc-VPi) obtained under condition that the molar ratio of KPS to monomer is 1/2000, the molar ratio of VAc to VPi is 7:3 and reaction temperature is 12°C. The highest S-diad content in PVA can be up to 59.31% while the DP of PVA decreases to 5180 under the condition that the feed ratio of VPi is 70 mol%, which means the syndiotacticity-rich HMW PVA can be prepared successfully. And the conversions of all reactions are above 80%. Syndiotacticity-rich HMW PVA is effectively prepared in this study, which is useful for the preparation of high-strength and high-modulus PVA fiber.

Study of synthesizing energy storage microcapsules in PVA spinning solution and thermal regulating fibers prepared by this solution

Thermal regulating fiber has been a research hotspot worldwide recently. In this paper, the energy storage microcapsules composed of silicon dioxide (SiO2) as shell and paraffin as core were synthesized in the spinning solution of polyvinyl alcohol (PVA). This solution was used to prepare thermal regulating PVA fibers by wet spinning directly. Orthogonal experiment was conducted to optimize the synthetic conditions of the microcapsules. Chemical structure and morphology of the fibers were characterized by Fourier transform infrared-attenuated total reflectance (FTIR-ATR) and scanning electron microscope (SEM) respectively. The thermal properties of the fibers were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Results present that energy storage microcapsules are successfully synthesized in PVA spinning solution with a mean particle size of 1.39 µm. The fibers containing such microcapsules show a high latent heat storage density of 45.39 J g−1, which also achieve a relatively better thermal stability.

Study of the thermal decomposition and flame-retardant mechanism of sulfonated polyoxadiazole fibers

Various proportions of 4,4′-oxybisbenzoic acid (OBBA) were incorporated to obtain the spinning solution of sulfonated polyoxadiazole (SPOD) copolymer through copolymerization in oleum. And the sulfonated fibers were prepared from the solution by using wet spinning approaches. The chemical structures of the fibers were characterized using Fourier transform infrared spectroscopy (FTIR) and elemental analyzer. The results indicated that sulfo groups were successfully introduced into the macromolecular chain. And the flame retardancy of the SPOD fibers was evaluated by limiting oxygen index, UL-94 and microscale combustion calorimeter tests, showing that flame-retardant properties could be significantly enhanced. And the thermal decomposition and flame-retardant mechanism were studied through various analytical methods. And thermogravimetric (TG) analyzer and scanning electron microscope results showed that the incorporation of OBBA would help fibers form more compact carbon residue. X-ray photoelectron spectroscopy and TG-FTIR results revealed that sulfur element increased carbon residue rate in the condensed phase and some converted into SO2 in the gas phase. Py-GC/MS results showed that thermal decomposition happened at oxadiazole rings and the introduction of OBBA did not alter the decomposition mechanism of POD essentially.