Synthesis and characterization of electrolyte substrate materials based on hyperbranched polyurethane elastomers for anodic bonding

A series of hyperbranched polyurethane elastomers (PEO-HBPUEs) as polymer electrolyte substrate materials was developed for anodic bonding with aluminum (Al) foil in micro-electro-mechanical system (MEMS) devices. The PEO-HBPUEs were prepared by pre-polymerization method with toluene-2,4-diisocyanate(TDI), polypropylene glycol (PPG), 1,4-butanediol(BDO), trimethylolpropane(TMP), lithium bis(trifluoromethanesulphonyl)imide (LiTFSI), and polyethylene oxide (PEO)-based electrolyte in varying proportions via solution casting technique at room temperature. All prepared PEO-HBPUEs exhibited low glass transition temperatures, good thermal stabilities, and suitable mechanical properties. The XRD results showed that PEO-HBPUEs are amorphous, and LiTFSI was well dissolved in the polymer matrix. The component of PEO-based electrolyte in PEO-HBPUEs contributed to increase the ionic conductivity, of which the highest value reached 1.23 x 10(-3) S/cm at 75 degrees C for PEO-HBPUE4. The anodic bonding of PEO-HBPUE substrate with Al foil was conducted by the coupling action of electric field, temperature field, and pressure field. A clear intermediate bonding layer between the substrate and Al foil was observed and the elements diffusion around bonding layer can be detected by SEM, indicating PEO-HBPUEs and Al foil have been jointed together successfully. The highest tensile strength of the bonding interface of PEO-HBPUE4/Al reached 1.88 MPa. All results demonstrated that the prepared PEO-HBPUEs materials would be promising substrates for flexible MEMS device that can be applied to flexible packaging by anodic bonding technology.