It was previously shown that polyhedral oligomeric silsesquioxane (POSS)-based molecular fillers can enhance the mechanical properties of polymer matrices as a viscoelastic material. However, POSS fillers hardly affected the initial pyrolysis step. Therefore, it was still challenging to reinforce thermal durability of polymers. This manuscript demonstrates development of molecular fillers for simultaneously enhancing decomposition temperature and both storage (E') and loss (E'') moduli. The series of molecular fillers based on POSS having various functional groups to make an interaction with poly(methyl methacrylate) (PMMA) matrices were designed and synthesized. Initially, the single and multiple reactive units as methacrylate ester to form covalent bonds with PMMA were introduced into octaisobutyl POSS (iBu-POSS). It was proposed that cross-linking reactions with polymer chains proceeded after annealing at 65 degrees C. Then, significant enhancements of both E' and E'' were observed. To obtain further enhancements especially for thermal stability, the POSS fillers of second generation having amine and 1,2-diol groups, which were expected to make strong interaction with the side chains in PMMA, were designed and loaded into polymer matrices. From the series of measurements with the homogeneous film samples containing POSS fillers, it was presented that all modified POSS derivatives can greatly improve thermal stability and mechanical properties as we expected. (C) 2017 Elsevier B.V. All rights reserved.