EFFECT OF BENZOXAZINE RESIN ON NETWORK FORMATION OF ALIPHATIC EPOXY FOR SHAPE MEMORY POLYMERS

Abstract

Shape memory polymers (SMPs) are polymers that can fix the temporary shape and recover to the permanent shape by external stimuli such as heat. Epoxy resins are an important family of SMPs due to their excellent stability, thermomechanical endurance and the high values of shape fixity. However, epoxide ring opening polymerization of epoxy requires a curing agent which typically possesses some disadvantages such as high cost, toxic problem to the user, corrosion to the processing apparatus, as well as no direct benefit to the shape memory performance. In this research, novel series of SMPs were prepared from aliphatic epoxy as reversible switching transition and cured by benzoxazine resin (BA-a) which also acted as stable polymer networks. The composition of BA-a in aliphatic epoxy/benzoxazine SMPs was controlled to be in the range of 30 to 50 mol%. Thermomechanical properties and flexural properties as well as essential shape memory properties of the BA-a/aliphatic-epoxy SMPs were characterized by dynamic mechanical analysis (DMA), flexural test and shape memory test. The results revealed that storage modulus at room temperature of the obtained SMPs was increased from 2.8 GPa at BA-a content of 30 mol% to the value of 4.5 GPa at 50 mol% of BA-a. The glass transition temperature (Tg) was substantially increased with an increase in BA-a content and reported to be in the range of 51 to 140oC. At benzoxazine content 50 mol%, the flexural modulus and flexural strength of the SMP samples were found to be as high as 3.97 GPa and 132 MPa at room temperature, at rubbery state 10.39 and 2.3 MPa respectively. From the shape memory test, all SMP samples exhibited a high value of shape fixity close to 100% and needed only a few minutes to fully recover to their permanent shape. Furthermore, the presence of BA-a content in aliphatic epoxy/benzoxazine SMPs imparted a greater recovery stress ranging from 0.25 to 1.59 MPa or about 6 times improvement. All of the SMP samples fully recovered to their permanent shape at the end of 100 cycles with minimum change on their flexural strength. These SMPs based on aliphatic epoxy/benzoxazine are highly attractive for shape memory materials to be used in a broader range of applications at elevated temperature such as hinge or deployable structure.