Eight 6FDA-TFDB polyimide(PI)samples with absolute molecular weights ranging from 1.25 × 10^5 g mol^-1 to 3.11 × 10^5 g mol^-1 are obtained by precipitation fractionation.Rheological experiments are conducted to determine the influence of molecular weight on the associating behavior of PI in A/X-dimethylformamide(DMF)solutions in a broad volume fraction,including abnormal steady shear flow,solution heterogeneity,and scaling behavior.Abnormal flow behaviors,i.e.,multi-region shear thinning and weak shear thickening,are studied,and these behaviors have not been reported in literature.The heterogeneity of PI/DMF solutions is examined by dynamic rheological test.By plotting qsp versus four concentration regions of l-IV can be distinguished for all PI samples with various molecular weights.The scaling results in different concentration regions are in good agreement with the associative polymer theory proposed by Rubinstein and Semenov.The scaling exponents do not show molecular weight dependence in concentration regions I and II.In concentration regions Ⅲ and Ⅳ,the scaling exponents change little when the molecular weight is below 242 k but increase when the molecular weight increases from 242 k to 311 k.This work can help us to understand polyimide solution properties from dilute to semidilute entangled solutions,and will guide the polyimide solution preparation for different processing.
Hong-Xiang ChenEn-Song ZhangMei HongWei LiuXue-Min DaiQuan ChenXue-Peng QiuXiang-Ling Ji
A series of polyimide（PI）/multi-walled carbon nanotube（MWCNT） composite fibers were prepared by copolymerizing a mixture of monomers and carboxylic-functionalized MWCNTs, followed by dry-jet wet spinning, thermal imidization, and hot-drawing process. The content of the carboxylic groups of MWCNTs significantly increased when treated with mixed acid, whereas their length decreased with treatment time. Both the carboxylic content and length of MWCNTs influenced the mechanical properties of the composite fibers. Fiber added with 0.1 wt% MWCNTs treated for 4 h exhibited the best mechanical properties, i.e., 1.4 GPa tensile strength and 14.30% elongation at break, which were 51% and 32% higher than those of pure PI fibers, respectively. These results indicated that a suitable MWCNT content strengthened and toughened the resultant PI composite fibers, simultaneously. Moreover, raising draw ratio resulted in the increase of tensile strength and tensile modulus of the composite fibers.
In this study, polyimide fibers at different stages of imidization were characterized by TGA, DSC, and FTIR. The imidization degree （ID） calculated by TGA was based on the weight loss of each sample, which was caused by the imidization of residual amic acid groups. The results of TGA showed good regularity with the thermal treatment temperature of the PI fibers. For DSC, the ID was calculated based on the area of endothermal peak of each sample. Compared with TGA, DSC showed a relatively higher value because the endothermal peak was reduced by the exothermic re-formation of polyamic acid which may be partially degraded during thermal treatment. The IDs obtained by the FTIR spectra generally showed poorer regularities than those obtained by both TGA and DSC, especially for the results calculated using the 730 cm^-1 band. Based on the 1350 cm^-1 band, the obtained IDs showed better agreement with the TGA or DSC results. The results obtained by these three methods were compared and analyzed. The ID obtained by TGA showed much more reliability among these three methods.
