Synthesis Of Poly(Benzoxazole-Co-Imide)and Fabrication Of The Fibers

Aromatic polyimide fiber,as one of the most important high performance polymer fiber,has a good application prospect in so many fields,such as fire safety,environmental protection,aerospace,national defense and so on,in view of its heat resistance,good mechanical properties,radiation resistance,chemical corrosion resistance and other excellent comprehensive performance.However,PI fibers are still deficient on comprehensive performance compared with poly-pphenylenebenzobisoxazole(PBO)whose tensile strength and modulus are up to 5.8 GPa and 270 GPa,respectively.Therefore,introducing the phenylenebenzobisoxazole structure,the repetitive unit of PBO,into the PI molecular main chain to improve the order degree of molecular chain and regulate aggregation structure of PI fibers becomes a new idea to enhance its overall performance.In this dissertation,a novel benzoxazole-containing diamine monomer with high purity,2,6-(4,4'-diaminodiphenyl)benzo[1,2-d:5,4-d'] bisoxazole(PBOA),were firstly synthesized.And then,poly(benzoxazole-co-amic acid)/N,N'-dimethylacetamide(DMAc)solution was preoduced via polycondensation of 3,3',4,4'-biphenyltetracarboxylic anhydride(BPDA),4,4'-diaminodiphenyl ether(ODA)and PBOA in DMAc.Poly(benzoxazole-co-imide)films and fibers were fabricated by flow casting method and wet-spinning method,respectively,and structure and performance of films and fibers were characterized and analyzed in detail.The major contents are as follows.(1)In synthesization of PBOA,the influence of reactant ratio,dosage of SnCl2 and recrystallization times on the yield and purity of final product was studied by element analysis and high performance liquid chromatography(HPLC).The test result shows that the purity and yield of PBOA can reach 98.9 % and 60.0 %,respectively after recrystallization for 3 times when the molar ratio of paminobenzoic acid(PABA)and 4,6-diaminoresorcinol dihydrochloride(ODA)is 2.05 : 1 and the dosage of SnCl2 is 2 wt%,which is thought to be the optimum condition.(2)Poly(benzoxazole-co-amic acid)was synthesized from the self-synthesized PBOA and commercial BPDA and ODA in DMAc.The inherent viscosity of poly(benzoxazole-co-amic acid)/DMAc solution tends to be higher when the molar ratio of PBOA and ODA increase,and it can reach 2.33 g/dL when nPBOA? nODA is 5:5.The steady flow behavior of above mentioned co-PAA/DMAc solution demonstrate that poly(benzoxazole-co-amic acid)/DMAc solution is a shear-thinning fluid,and its zero shear viscosity(0)and structure viscosity index(?)decrease with the increasing of temperature.On the contrary,the non-Newtonian index tends to be larger and get close to 1.0,indicating that it shows more Newtonian properties.(3)In order to study the features of poly(benzoxazole-co-imide)molecular structure,the initial research focused on its films.According to the WAXD test result,the microstructure of poly(benzoxazole-co-imide)is mainly amorphous,so the performance depends on its chemical structure rather than crystal structure.The test result from TGA and DMA shows that the heat resistance is improved markedly owing to the incorporation of benzoxazole into the PI molecular main chain.Compared with the PI film from BPDA and ODA,the tensile strength and modulus of poly(benzoxazole-co-imide)film increases 61 % and 76 %,respectively when nPBOA? nODA is 5:5.Highly conjugated structure of benzoxazole is beneficial for the hydrophobicity of poly(benzoxazoleco-imide)films,but weakens its optical transparency.(4)A series of poly(benzoxazole-co-imide)fibers were fabricated via wet-spinning method,thermal imidization and thermal stretching process.The morphology and structure of precursor fibers were characterized by scanning electron microscope(SEM),revealing that the cross section is irregular or porous when the coagulation bath is DMAc:H2O=3:7(v/v),but it turns to be homogeneous and dense fibrous structure when the coagulation bath is DMAc:H2O=5:5(v/v).The aggregation structure of poly(benzoxazole-co-imide)was studied by means of WAXD.It shows that introducing of benzoxazole structure is beneficial for a better arrange of polymer chain and improvement of crystal structure in PI fiber.In the process of thermal stretching,the crystallinity and degree of orientation,as well as crystal size,in PI fibers become larger as a result of high temperature and stress together.It is noteworthy that the increase of crystal size along the radial direction of fiber is larger than that along the fiber axis direction in the thermal stretching process.The rigid rod molecular structure and highly ordered aggregation structure bring enhanced mechanical and heat resistant performance to the PI fibers.When nPBOA? nODA is 5:5,the tensile strength and modulus are 2.38 GPa and 70.03 GPa,respectively,and the glass transition temperature and decomposition temperature are 320 ? and 610 ?,respectively,which is very promising in application.