Emerging ultra high-pressure/high-temperature applications in downhole service have created the need for the development of new sealing materials. To address this emerging need, a series of modified polymeric materials were developed. The new material displayed strong mechanical properties below glass transition temperature (Tg). Above Tg, the material displayed stable elastomeric properties up to 400° C. The thermal rating of the material is beyond the available commercial thermoplastics and elastomeric materials. Furthermore, the Tg of the material can be adjusted from 97° C to 215° C. The material was aged in a crude oil at high temperature. Dynamic Mechanical Analysis (DMA) measurements were performed on the material before and after aging. The material displayed excellent resistance to the crude oil. In addition, API extrusion-resistance measurements were performed on the new material at various temperatures and various pressures. The extrusion resistance of the new material was superior to the available commercial elastomeric materials, including perfluoroelastomers. Thus, the material has great potential for use in emerging high-temperature sealing applications and other oil and gas applications. This paper describes the work involved in conducting this development work and test results.