Fabrication of Polyurethane@Poly(Amic Acid) Spheres and Their Toughening for Epoxy Resin

Aiming to fabricate soft‐core/hard‐shell spheres for toughening epoxy resin, cross‐linked polyurethane (PU) is selected as the core and poly(amic acid) (PAA) as the shell material. Cross‐linked polyurethane spheres are synthesized by the self‐emulsification method. Then, polyurethane@poly(amic acid) (PUAA) spheres are synthesized from PU spheres and poly(amic acid) (PAA) oligomers terminated by anhydride groups. The structure, particle size, and morphology of PUAA spheres are analyzed and characterized by Fourier transform infrared spectra, dynamic light scattering laser particle size meter, and scanning electron microscope. The influence of the mass ratio of PU to PAA and the amount of PUAA spheres on the mechanical performance of toughened epoxy resin are investigated in detail. The results show that the PUAA spheres have a regular round shape and the particle sizes of 200–300 nm. The PUAA spheres can be well dispersed in the epoxy resin without significant aggregation due to the functional groups on PAA. PUAA spheres with different mass ratios of PU to PAA are fabricated, and their toughening effects on epoxy resin are investigated. When the mass ratio of PU to PAA is 1:2 and the amount of PUAA spheres is 1 wt% of the epoxy resin, the impact strength and tensile strength of toughened epoxy resin are 16.4 kJ/m 2 and 53.0 MPa, respectively, which are 400.0% and 111.8% of those of pristine epoxy resin. Results indicate that PUAA spheres can both enhance the tensile and impact strengths of epoxy resin, and the epoxy resin toughened by PUAA spheres maintains good heat resistance.