Thermorheological properties near the glass transition of oligomeric poly(methyl methacrylate) blended with acrylic polyhedral oligomeric silsesquioxane nanocages

Abstract

Two distinct oligomeric species of similar mass and chemical functionality (M w ≈ 2,000 g/mol), one a linear methyl methacrylate oligomer (radius of gyration R g ≈ 1.1 nm) and the other a hybrid organic-inorganic polyhedral silsesquioxane nanocage (methacryl-POSS, r ≈ 1.0 nm), were subjected to thermal and rheological tests to compare the behaviors of these geometrically dissimilar molecules over the entire composition range. The glass transition temperatures of the blends varied monotonically between the glass transition temperatures of the pure oligomer (T g = −47.3°C) and the pure POSS (T g = −61.0°C). Blends containing high POSS contents (with volume fraction φ POSS ≥ 0.90) exhibited enhanced enthalpy relaxation in DSC measurements, and the degree of enthalpy relaxation was used to calculate the kinetic fragility indices m of the oligomeric MMA (m = 59) and the POSS (m = 74). The temperature dependences of the viscosities were fitted by the free volume-based WLF-VFT framework and a dynamic scaling relation. The calculated values of the fragility from the WLF-VFT fits were similar for the POSS (m = 82) and for the oligomer (m = 76), and the dynamic scaling exponent was similar for the oligomeric MMA and the POSS. Within the range of known fragilities for glass-forming liquids, the temperature dependence of the viscosity was found to be similarly fragile for the two species. The difference in shape of the nanocages and oligomer chains is unimportant in controlling the glass-forming properties of the blends at low volume fractions (φ POSS < 0.20); however, at higher volume fractions, adjacent POSS cages begin to crowd each other, leading to an increase in the fractional free volume at the glass transition temperature and the observed enhanced enthalpy relaxation in DSC.

On the Decoupling of Relaxation Modes in a Molecular Liquid Caused by Isothermal Introduction of 2 nm Structural Inhomogeneities

“…The "m fragility" 40 of m-cresol is reported as ~65 (D = 10) 41 , based on viscosity data. The m fragility of pure POSS AM270, according to the variable scan rate differential scanning calorimetry protocol developed by Velikov 42 and Wang 43 , is 76, which is comparable to the value of 74 reported by Kopesky et al for a methacrylate POSS 44 . The conductivities likewise show non-Arrhenius temperature dependences, though we do not consider these dependences here, beyond noting that, in the temperature range of interest, they are the same as for viscosity when no POSS is present.…”

Hybrid Fluorescent Poly(silsesquioxanes) with Amide- and Triazole-Containing Side Groups for Light Harvesting and Cation Sensing

“…Octahedral POSS-amide-Py exhibited not only melting at around T m = 104 °C but also a glass transition (T g = 3 °C) ( Figure S5 ). It may be explained by adjustment of more flexible segments in the crystal structure, analogously to that observed in macromolecular systems [ 52 , 53 , 54 ].…”

Segmental dynamics in hybrid polymer/POSS nanomaterials

“…Homopolymerized octa(methyl-methacrylate) POSS shows a glass transition at 191°C [65], much higher than the unpolymerized moiety as mentioned earlier in this paragraph [55,56] and presumably due to the restriction of dynamics by the bonds. An even higher T g ≈ 320°C has been reported for a a polymerized octabenzoxazine POSS moiety [66].…”

“…Blends of the unmodified acryl-substituted POSS of the previous study with an unentangled oligomeric MMA with M w = 2160 g/mol -comparable to that of the POSSwas the subject of another study of the same group [55]. The T g of the POSS moiety was A follow-up work [115] studied the toughenability of some of the materials studied in refs.…”