Improved mechanical properties of composites using ultrafine electrospun fibers

Aromatic heterocyclic PBI(poly(2,2{dollar}spprime{dollar}(m-phenylene)-5,5{dollar}sp prime{dollar}bibenzimidazole)) polymer obtained from Hoechst Celanese Corporation was electrospun from a 20 wt% polymer solution in dimethyl acetamide (DMAc) with lithium chloride (LiCl). These ultrafine electrospun fibers were used to make fiber-reinforced composites.; The objectives of this dissertation were (1) to produce ultrafine fibers, from polymer solution using electrospinning process, with diameters a few hundred nanometers, and (2) to improve mechanical properties of the composites using these electrospun nanofibers.; A jet, formed when electrical forces overcome surface tension, was directed onto an aluminum sheath on a rotating cylinder. The PBI fibers, formed by electrical forces, were attracted to the take-up reel. The fibers collected in sheets about 8 centimeters wide and 50 centimeters long, with a compressed thickness of a few tens of microns. The mass per unit area of the sheets was 15 to 50 g/m{dollar}sp2.{dollar} The sheets were treated with sulfuric acid to prevent shrinkage of the fibers. The average diameter of the electrospun fibers was around 300 nanometers. The electrospun PBI fibers were birefringent. The morphological features of these fibers were observed with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM).; The nanofiber-reinforced matrix materials were shown to improve mechanical properties such as fracture toughness (k{dollar}rmsb{lcub}Ic{rcub}){dollar} and Young's modulus (E). The ultrafine fibers provide a very high ratio of surface area to volume. The fine fibers toughened the brittle epoxy resin. The fracture toughness and the modulus of the fiber (15wt%)-reinforced epoxy composite were both higher than of in an epoxy composite with PBI fibrids (17wt%) which are whisker-like particles of PBI. In an elastomeric matrix, the Young's modulus and tear strength of the chopped nanofibers-reinforced SBR was higher than those of the unfilled SBR. Photomicrographs of the fracture surfaces were obtained by scanning electron microscopy (SEM).