Miscibility and viscoelastic properties of acrylic polyhedral oligomeric silsesquioxane–poly(methyl methacrylate) blends
Edward T. Kopeskya, , Timothy S. Haddadb,
, Gareth H. McKinleyc,
, and Robert E. Cohena,
aDepartment of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA
bERC Inc., Air Force Research Laboratory, Edwards AFB, CA 93524, USA
cDepartment of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Received 14 January 2005; accepted 1 April 2005. Available online 22 April 2005.
We investigate the miscibility of acrylic polyhedral oligomeric silsesquioxanes (POSS) [characteristic size d≈2 nm] and poly(methyl methacrylate)(PMMA) in order to determine the effect of well-dispersed POSS nanoparticles on the thermomechanical properties of PMMA. Two different acrylic POSS species (unmodified and hydrogenated) were blended separately with PMMA at volume fractions up to =0.30. Both POSS species have a plasticizing effect on PMMA by lowering the glass transition temperature Tg and decreasing the melt-state linear viscoelastic moduli measured in small amplitude oscillatory shear flow. The unmodified acrylic-POSS has better miscibility with PMMA than the hydrogenated form, approaching complete miscibility for loadings <0.10. At a loading =0.05, the unmodified acrylic POSS induces a 4.9 °C decrease in the Tg of PMMA, far less than the 17.4 °C decrease in the glass transition temperature observed in a blend of 5 vol% dioctyl phthalate (DOP) in PMMA; however, the decrease in the glass transition temperature per added plasticizer molecule is nearly the same in the unmodified acrylic-POSS–PMMA blend compared with the DOP–PMMA blend. Time-temperature superposition (TTS) was applied successfully to the storage and loss moduli data and the resulting shift factors were correlated with a significant increase in free volume of the blends. The fractional free volume f0=0.046 for PMMA at T0=170 °C while for a blend of 5 vol% unmodified acrylic-POSS in PMMA f0=0.057, which corresponds to an addition of 0.47 nm3 per added POSS molecule at =0.05. The degree of dispersion was characterized using both wide-angle X-ray diffraction (WAXD) and dynamic mechanical analysis (DMA). Diffraction patterns for both blend systems show clear evidence of phase separation at =0.20 and higher, but no significant phase separation is evident at =0.10 and lower. The storage modulus measured in DMA indicates appreciable phase separation for unmodified acrylic POSS loadings ≥0.10, while no evidence of phase separation is present in the =0.05 blend in DMA.
Keywords: POSS; Plasticization; Nanocomposites