On the thermal stability of PS-b-PMMA block and P(S-r-MMA) random copolymers for nanopatterning applications

This study addresses the thermal stability and degradation path of symmetric and asymmetric PS-b-PMMA block copolymers, widely employed to generate templates for nanopatterned materials, and of the associated P(S-r-MMA) random copolymers, through a combination of TGA and TGA-GC-MS techniques. The monomer evolution under isothermal and dynamic conditions was able to delineate the thermal degradation characteristics of all the copolymers. Although the temperature corresponding to the maximum of the main monomer loss is similar for the random and block copolymers, the onset of the main degradation for the random copolymers is observed at a temperature about 50 C lower than the one for the block copolymers. In addition, the degradation profile of the random copolymers is highly asymmetrical in the low temperature side. This is partially due to the presence of TEMPO end groups, which behave as weak links. The successive evolution of the two monomers in the random copolymers occurs in a parallel fashion, indicating the occurrence of depolymerization via an unzipping process. In contrast, in case of the block copolymers, the two chemically different blocks depolymerize at different temperatures due to their distinct thermal stabilities and different end groups nature. Considering the overall system, comprising the block copolymer film on the grafted random copolymer layer, the monomer volatilization which occurs when the random copolymer is heated at temperatures equal or above 280 C, leads to a reduced control of morphology orientation in the microdomains of the block copolymer film with an eventual local detachment of the block copolymer films. These results clearly indicate that the range of accessible temperature in the processing of these self-assembling materials is mainly limited by the instability of the grafted RCP layer.