A noise attenuation device for a vehicle exhaust system is provided. More specifically, provided are a polyimide-polybenzoxazole copolymer simply prepared through thermal-rearrangement performed by thermally treating a polyimide-poly copolymer as a precursor, a method for preparing the same, and a gas separation membrane comprising the same. The sacrificial layer is heated by microwave energy, and a gas is discharged from the sacrificial layer.