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Polyacrylonitrile (PAN) fiber modification can promote the stabilization efficiency and mechanical properties of the subsequent carbon fibers; however, it could cause damage to the fibers to some extent. In this article, the effects of NaIO4 treatment on the chemical and physical properties of PAN fibers were investigated. After NaIO4 modification, the cyclization reaction has partially occurred, owing to the FTIR revealed that the C≡N peak was weakened, but the C=N stretching vibration peak, the –N = C=O stretching vibration peak, the C=O stretching vibration absorption peak, and the short sequence conjugate C=N characteristic peak were enhanced simultaneously, indicating a successful cyclization and dehydrogenation reaction. Moreover, owing to more oxygen-containing functional groups successfully attached to the PAN fibers and the cyclization reaction taking place, the initial temperature, peak temperature, and heat release of cyclization decreased, which means a more efficient and safer stabilization process. X-ray diffraction (XRD) showed that the (100) crystal plane microcrystal size was the smallest at 40 °C of the modification temperature, whereas the (002) crystal plane microcrystal size was the largest at 60 °C. The modified stabilized PAN fibers presented lower crystallinity and a more aromatic structure compared with untreated fibers. Raman spectra revealed a significant increase in the intensity of the G peak after NaIO4 modification; the R-value decreased from 1.65 to 1.43, demonstrating a transformation of the sp3 hybrid carbon structure into a more graphite-like sp2 hybrid C=C structure. Meanwhile, scanning electron microscopy (SEM) images disclosed additional grooves and cracks on the fiber surface after modification. [ABSTRACT FROM AUTHOR]

This study aims to develop highly hygroscopic bio-based co-polyamides (CPs) by melt co-polycondensation of polyamide (PA) 56 salt and PA66 salt with varying molar fractions. The functional groups and the chemical structure of the prepared samples were determined by Fourier transform infrared (FTIR) spectroscopy and proton nuclear magnetic resonance (¹H-NMR) spectroscopy. The relative viscosity was determined with an Ubbelohde viscometer. The melting behavior and the thermal stability of CPs were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Furthermore, the water absorption behavior of CP hot-pressed film was studied. The results reveal that the melting point, the crystallization temperature and the crystallinity of CPs firstly decrease and then increase with the molar fraction of PA66 in CPs. The copolymerization of PA56 with PA66 leads to an obvious increase in water absorption. The CPs with PA66 molar fraction of 50% possess a high saturated water absorption rate of 17.6%, compared to 11.6% for pure PA56 and 7. 8% for pure PA66. [ABSTRACT FROM AUTHOR]

The effect of PBAT (poly(butylene adipate-co-terephthalate)) content and crystallization temperature on the crystallization and heat resistance of PLLA/PDLA blends are studied. The PBAT/PLLA/PDLA blends with different PBAT content were prepared by melt blending. Then, differential scanning calorimetry (DSC) and Vicat softening temperature (VST) were used to investigate the semicrystallization time (t1/2), crystallinity and heat resistant of the blends. The results show that the incorporation of PBAT is benefit to the rate of crystallization and t1/2 of the blends can be reduced from 1.0 min to 0.8 min at 100 °C. After annealing at 220 °C, the degree of stereo-complex (fsc), stereocomplex crystallinity (Xc,S) and VST of PLLA/PDLA blends reaches up to 100.0%, 31.1% and 204 °C, respectively. The values of the former parameters reduce to 96.0%, 15.4% and °C 194 °C after incorporating 12% PBAT. It concludes that the crystallization under high temperature contributes to the improvement of Xc,S and heat resistance. The incorporation of PBAT into PLLA/PDLA promotes the crystallization rate, while it reduces the heat resistance of the blends [ABSTRACT FROM AUTHOR]

The effects of KBrO3 modification on polyacrylonitrile (PAN) precursor fibers at different modification temperatures were studied. The mechanical properties, chemical structure, aggregation structure, morphological structure, and thermal behavior of PAN precursor fibers were analyzed. The mechanical properties of PAN precursor fibers are essentially unaltered after KBrO3 modification, although oxygen-containing functional groups are introduced. KBrO3 can ionize bromate ion in aqueous solution, which has good nucleophilicity and can attack the positively charged carbon atom in C≡N, so that the cyclization proceeds according to the ionic mechanism, allowing the transformation of C≡N to C = N in the PAN precursor fibers generated a cyclic trapezoidal structure containing primary aromatic amine, which reduced the peak temperature of the cyclization reaction from 284.5 °C to 275.1 °C and decreased the heat release. The sp3-hybridized carbon structure changes into a sp2 hybridized C = C structure with increasing aromatization, according to XRD and Raman data, and the KBrO3 alteration accelerates the degradation of the original microcrystalline structure and transforms into a new polycyclic aromatic structure. The degree of conjugation and aromatization of the system rapidly increased with the rise in modification temperature. The stabilized fibers were discovered to have the lowest R-value, the highest aromatization, the smoothest surface, and the fewest flaws at impregnation temperature of 60 °C. [ABSTRACT FROM AUTHOR]