A novel soft segment adipic acid-polyoxypropylene diamine copolymer with Mw = similar to 2000 (AA-PPA 2000) was well obtained through chain extending of bio-based polyoxypropylene diamine D400 using adipic acid (AA). This copolymer was then used to modify the diglycidyl ether of bisphenol A (DGEBA) epoxy/diethyl toluene diamine (DDM) curing system while polyoxypropylene diamine (D2000) was taken as the reference. The variable temperature FTIR (V-FTIR) spectral was used to determine the hydrogen bonds in cured epoxy resins. The crosslinking density of epoxy composites was further checked using the low field nuclear magnetic resonance (NMR) and dynamic mechanical analysis (DMA). Interestingly, SEM images of the fracture surfaces showed that there were many small holes on the fracture surface of D2000 cured composites while rough cracks were observed instead of tiny holes on the fracture surface of AA-PPA2000 modified composites indicating AA-PPA2000 had good compatibility in epoxy composites. All the findings revealed that AA-PPA 2000 was a preferable tougher compared to D2000. Higher crosslinking density was generated in AA-PPA2000 cured epoxy resin due to the formation of intermolecular hydrogen bonds and more complicated networks in the epoxy composites, leading to the synchronous enhancement of strength and fracture toughness.