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Exploitation of biomaterials derived from renewable resources is an important approach to address environmental and resource problems in the world today. In this paper, novel ionic hydrogels based on xylan-rich hemicelluloses were prepared by free radical graft copolymerization of acrylic acid (AA) and xylan-rich hemicelluloses (XH) by using N,N-methylene-bis(acrylamide) (MBA) as cross-linker and ammonium persulfate/N,N,N',N'-tetramethylethylenediamine (APS/TMEDA) as redox initiator system. The network characteristics of the ionic hydrogels were investigated by Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM), as well as by determination of mechanical properties, swelling, and stimuli responses to pH, salts, and organic solvents. The results showed that an increase in the MBA/XH or AA/XH ratio resulted in higher cross-linking density of the network and thus decreased the swelling ratio. Expansion of the network hydrogels took place at high pH, whereas shrinkage occurred at low pH or in salt solutions as well as in organic solvents. The ionic hydrogels had high water adsorption capacity and showed rapid and multiple responses to pH, ions, and organic solvents, which may allow their use in several areas such as adsorption, separation, and drug release systems.

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Ultimate tensile strength and axial tensile modulus of single high-strength electrospun polyimide [poly(p-phenylene biphenyltetracarboximide), BPDA/PPA] nanofibres have been characterized by introducing a novel micro tensile testing method. The polyimide nanofibres with diameters of around 300 nm were produced by annealing their precursor (polyamic acid) nanofibres that were fabricated by the electrospinning technique. Experimental results of the micro tension tests show that polyimide nanofibres had an average ultimate tensile strength of 1.7 ± 0.12 GPa, axial tensile modulus of 76 ± 12 GPa and ultimate strain of ∼3%. The ultimate tensile strength and axial tensile modulus of the electrospun polyimide nanofibres in this study are among the highest ones reported in the literature to date. The precursor nanofibres with similar diameters and molecular weights had an average ultimate tensile strength of 766 ± 41 MPa, axial tensile modulus of 13 ± 0.4 GPa and ultimate strain of ∼43%. The experimental stress-strain curves obtained in this study indicate that under axial tension, the precursor (polyamic acid) nanofibres behave as linearly strain-hardening ductile material without obvious softening at final failure, while the polyimide nanofibres behave simply as brittle material with very high tensile strength and axial tensile modulus. Furthermore, by using a transmission electron microscope, detailed fractographical analysis was performed to examine the tensile failure mechanisms of the polyimide nanofibres, which include chain scission, pull-out, chain bundle breakage, etc. X-ray diffraction analysis of the highly aligned polyimide nanofibres shows the high chain alignment along the nanofibre axis that was formed in the electrospinning process and responsible for the high tensile strength and axial tensile stiffness.

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