Toughened poly(methyl methacrylate) nanocomposites by incorporating polyhedral oligomeric silsesquioxanes
Edward T. Kopeskya, Gareth H. McKinleyb and Robert E. Cohena, 
, 
aDepartment of
Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 2139,
USA
bDepartment of Mechanical Engineering,
Massachusetts Institute of Technology, Cambridge, MA 2139, USA
Received
23 September 2005; revised 28 October 2005; accepted 31 October 2005.
Available online 21 November 2005.
We have investigated the toughenability of poly(methyl methacrylate) (PMMA) using polyhedral oligomeric silsesquioxane (POSS) nanocages at loadings between 0 and 15 wt%. Three distinct POSS species were used: a crystallizable type that did not disperse on a molecular scale within the PMMA matrix (cyclohexyl-POSS), and two types of POSS that formed homogeneous mixtures over the loadings we have investigated (methacryl-POSS and trisilanol-phenyl-POSS). Each of the three types of POSS was able to toughen PMMA in slow-speed tension tests at loadings ≤5 wt%; however, the reproducibility was poor due to the high flaw sensitivity of these binary blends. Ternary blends containing both cyclohexyl-POSS and methacryl-POSS showed the greatest increase in tensile toughness and also excellent reproducibility of toughening. A blend containing 2.5 wt% of both cyclohexyl-POSS and methacryl-POSS maintained the same modulus as the unfilled PMMA while increasing the toughness by a factor of 4. Electron micrographs showed extensive particle–matrix debonding of the PMMA from the cyclohexyl-POSS crystallites and some evidence of plastic deformation on the fracture surface. In high rate (1000 s−1) split-Hopkinson pressure bar (SHPB) tests, binary blends of POSS and PMMA were able to improve the impact toughness of PMMA; however, once again the combined addition of both cyclohexyl-POSS and methacryl-POSS led to the greatest reproducibility of toughening. Comparison with previous results suggests that in order to toughen PMMA with rigid fillers, weakly-adhering particles with sizes on the order of 100 nm are required.
Keywords: Nanocomposies; Mechanical properies; POSS