Duigong Xu’s research while affiliated with China Academy of Engineering Physics and other places

The growth and perfection of the crystalline region during the annealing of amorphous poly(ethylene terephthalate) (PET) was studied systematically using a homemade temperature gradient (T‐gradient) ranging from 50 to 250°C. Double melting peaks were observed from the differential scanning calorimetry (DSC) thermograms when PET film was annealed at the temperatures (Ta) above the PET crystallization temperature (Tc ≈ 120°C) with a constant primary melting peak at around 256°C and an increased secondary melting peak on the range of 120 and 250°C. The profound broadening of the wide angle X‐ray diffraction (WAXD) peaks in both transmission and reflection modes indicated that there were a lot of tiny crystals when PET films were annealed between 120 and 200°C, and the shift of the WAXD peaks revealed the presence of the paracrystals in the PET films. The growth and perfection of crystalline region of PET in condition of amorphous and annealed (Ta = 128°C) PET samples were investigated on the basis of Fourier transform infrared spectra and simultaneous WAXD‐DSC measurement. It was found that the conformational transition of the soft ethylene glycol moiety and rigid phenylene moiety in amorphous PET began at around 120 and 200°C, respectively. The microcrystal sizes in the PET film annealed at 128°C exhibited no change at the time that the secondary melting peak showed up, which meant the secondary melting peak might be attributed to the melting of the paracrystals inside or on the surface of the microcrystals. PET paracrystals are distributed on the surface and inside of the PET microcrystal, which is an interpretation of the double melting phenomenon of the annealed PET sample.

The banded to sheaf‐like morphology transition of linear low‐density polyethylene spherulites in the micron‐thick film was investigated systematically for the first time. It was confirmed that this transition predominantly resulted from the thickening long period and the weakening surface stress of the multilamellar crystals as the crystallization temperature was increased. The mechanism might be different from the confinement effect of the ultrathin film. Moreover, based on the high‐selectivity of combinatorial method, a series of unique spherulitic crystals, such as double‐nested, multiple‐nested and hollow spherulites, were successfully constructed, which might provide a potential application in model substrate and surface modification studies.

Epoxy polymer with damage indicating ability was very usable for ships and bridges to detect the cracks at an early stage and to prevent corrosion. 2′, 7′-dichlorofluorescein (DCF), as a damage indicator, was used to report the mechanical damage of epoxy-amine polymer by a strong color change from a light yellow to bright red due to the molecular structure transition from the acid molecular form to the base ion form. The effect of water on damage indicator and damaged epoxy-amine polymer film was evaluated by an immersion test and the properties were characterized by ultraviolet-visible spectrophotometry (UV-Vis), scanning electronic microscopy (SEM), energy dispersive X-ray spectrometer (EDS), zeta potential and thermal gravimetric analysis (TGA). The results showed that DCF was an easy, stable and permanent indicator for epoxy-amine polymer and the water only had a slight influence on the indication stability of damaged epoxy polymer.

Autonomic indication of mechanical damage and self-healing epoxy materials was conducted using 2′,7′-dichlorofluorescein (DCF) and glycidyl methacrylate (GMA) solution. When mechanical damage occured, the released DCF reacted with the amine group in the crack plane to sense the damage colorimetrically, and the GMA rebound the cracks in the epoxy matrix by chemical or physical interaction.

Thin film of carbon fiber (CF) and poly (L-lactic acid) (PLLA) composite was prepared by a solution casting technique. Influence of crystallization temperature (Tc) on morphology of transcrystalline (TC) on the interface between CF and PLLA was investigated under a temperature-controlled gradient (T-gradient) stage with temperature ranging from 90 °C to 140 °C. Morphology of TC was observed by polarized optical microscope (POM). Typical TC structure isothermally crystallized on the T-gradient stage was obtained at the surface of CF when Tc = 111 ∼ 132 °C. As Tc decreased from 132 to 111 °C, nuclei density on the surface of single fiber increased while the thickness of TC layer decreased. Crystallization kinetics of TC was investigated through isothermal crystalization under POM equipped with a temperature-controlled stage. As Tc decreased, the nucleation rate of TC increased and the growth rate of TC decreased. Furthermore, it revealed that the thickness of TC layer was conducted by the competition between heterogeneous nucleation on the surface of fiber and homogeneous nucleation in the bulk matrix around the fiber. The nucleation rate of TC structure was much higher than homogeneous nucleation rate of spherulites in the bulk matrix when Tc was above 118 °C. In this case, TC had enough space to develop perpendicular to fiber axis. Consequently, thickness of TC layer was larger at higher crystallization temperature. On the contrary, transcrystallization was suppressed by homogeneous crystallization when Tc was below 118 °C due to enhanced nucleation ability of spherulites.

The crystallization behavior of poly(vinylidene fluoride) (PVDF) and transcrystallization in carbon fiber (CF)/PVDF composite were investigated under a temperature gradient. The crystallization temperature (Tc) was controlled in the range of 110–180 °C. For neat PVDF, the results showed that exclusive γ phase formed at Tc above 164 °C, but coexisted with α phase at Tc ranging from 137 to 160 °C. The promotion of γ phase to nucleation of α phase at low Tc was observed for the first time. For CF/PVDF composite, a cylindrical transcrystalline (TC) layer formed on the surface of CF when Tc was between 137 and 172 °C. The TC layer was exclusively composed of γ phase at Tc above 164 °C. The hybrid nucleation was dominated by γ phase though some α phase nuclei emerged on the surface of CF when Tc was in the range of 144–160 °C. As Tc decreased, competition between the hybrid nucleation of α and γ phase became more intense. The γ phase nuclei was soon circumscribed by the rapidly developed α phase when Tc was below 144 °C. Furthermore, some α phase nuclei were induced at the surface of the γ phase TC layer, and developed into α phase TC layer when Tc was in the range of 146–156 °C, which resulted in a doubled TC layer of α and γ phase at the interface of the composite. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43605.

Temperature dependences of spherulite morphology and crystal orientation of poly(vinylidene fluoride) (PVDF) were systematically investigated via a combinatorial method. The method created a temperature gradient ranging from 130 to 200 °C. Results show that the preferential orientation of the crystallites changes with the crystallization temperature. The crystallization at 169 °C gives the most highly developed crystalline state of PVDF crystalline form II (α form), in which the spherulite size is maximal, and the crystallite sizes are also the longest, about 200 nm along the b axes. Besides, the a-axis is almost parallel to the film normal. It indicates that the crystallization rate is the highest in the b-axis direction. The perferential orientation at higher temperatures may be attributed to the confined 2D growth of the PVDF spherulites in the thin film, whereas the spherulites grow in the 3D mode at lower temperatures. The crystallization behavior revealed in the method is consistent with the results of melt isothermal crystallization experiments. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014

Composites of carbon fibers (CF) filled with ultrahigh molecule weight polyethylene/low density polyethylene blend matrix (UHMWPE/LDPE) were prepared by kneading method. The positive temperature coefficient (PTC) effect of electrical resistivity of UHMWPE/LDPE/CF composites was investigated by direct current (DC) and alternating current (AC) measurements over the frequency range of 10(0)-10(6.5) Hz from 30 to 150 degrees C. The onsets of PTC effect were found to be strongly depended upon the CF content even the melting behaviors were almost same for all composites. To interpret this phenomenon, a master curve of temperature frequency resistivity superposition was constructed for composites with different CF contents based on the AC resistivity. The CF content dependence of correlation length was related to the onset of PTC effect. The transitions of conductor insulator were studied quantitatively by complex planes of AC impedance, and the calculated capacitances and resistances showed a similar PTC effect under DC. Based on the analysis of AC capacitance, the average distances between CFs were calculated using a plane capacitance model which varied with CF concentration and temperature, and the tendency was consistent to the vrc effect.