Thermomechanical Properties of Poly(methyl methacrylate)s Containing Tethered and Untethered Polyhedral Oligomeric Silsesquioxanes

Abstract

Poly(methyl methacrylate)s (PMMA) containing both tethered and untethered polyhedral oligomeric silsesquioxanes (POSS) were examined through the use of wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and rheological characterization. The presence of tethered-POSS in entangled copolymers leads to a decrease in the plateau modulus (G N 0 ) when compared with PMMA homopolymer. Two untethered-POSS fillers, cyclohexyl-POSS and isobutyl-POSS, were blended with PMMA homopolymer. Both DSC and rheological results suggest a regime at low untethered-POSS loadings (φ e 0.05) in PMMA in which much of the POSS filler resides in the matrix in a nanoscopically dispersed state. This well-dispersed POSS decreases the zero-shear-rate viscosity (η 0). Above this regime, an apparent solubility limit is reached, and beyond this point additional untethered-POSS aggregates into crystallites in the PMMA matrix. These crystallites cause both the viscosity and the plateau modulus to increase in a way consistent with classical predictions for hard-sphere-filled suspensions. The principles of timetemperature superposition are followed by these nanocomposites; however, fits to the WLF equation show no strong trend with increasing POSS loading. Isobutyl-POSS was also blended with a POSS-PMMA copolymer containing 25 wt % tethered isobutyl-POSS distributed randomly along the chain. Blends of untethered-POSS with copolymer show a significant increase in η 0 for all loadings, greater than that expected for traditional hard-sphere fillers. This is a result of associations between untethered-POSS and tethered-POSS cages in the blend, which retard chain relaxation processes in a way not observed in either the homopolymer blends or the unfilled copolymers. Time-temperature superposition also holds for the filled copolymer system, and these blends show a strong increase in the WLF coefficients, suggesting that both free volume and viscosity increase with filler loading.

Miscibility and viscoelastic properties of acrylic polyhedral oligomeric silsesquioxane–poly(methyl methacrylate) blends

“…These fractional free volume values are plotted in Figure 9. The free volume data in Figure 9 help clarify our previous results for PMMA filled with crystallizable-POSS species [13]. This earlier study reported that the POSS had a strong tendency to phase-separate into crystallites, even at loadings of φ = 0.01, and we could not find a clear trend in free volume with increasing POSS content.…”

“…In both cases, the POSS filler tended to phase separate into microcrystallites, even at loadings as small as φ = 0.01. Despite this phase separation, we observed a slight decrease in the viscosity for loadings φ < 0.05 [13]. This decrease was attributed to a small amount of molecularly-dispersed POSS particles that plasticized the matrix in the melt state at small loadings; however we did not observe a decrease in either T g or an increase in the fractional free volume f 0 , which would be expected if plasticization were occurring.…”

“…In light of the recent work on polymer−nanoparticle systems, the hybrid structure of POSS particles, with a silica core and a variable organic shell, offers a precise way to vary the polymer−nanoparticle interaction and thereby achieve either plasticization or reinforcement, depending on the application. A wide variety of studies have been carried out on POSS-containing copolymers and POSS-homopolymer blends [9], probing their thermal [10][11][12][13][14][15][16][17], morphological [10,13,14,[17][18][19][20][21][22][23][24][25], mechanical [21,23,24,26,27], and selfassembly [10,28] properties. The rheological behavior of POSS-filled homopolymers has been studied by us [13] and by others [29].…”

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

“…In our initial study of POSS-poly(methyl methacrylate)(PMMA) blends (Kopesky et al 2004), we incorporated two monodisperse, crystallizable T 8 POSS components into PMMA: cyclohexyl-POSS and isobutyl-POSS. Each was blended separately with PMMA and we observed a strong thermodynamic driving force toward phase separation of the POSS into crystallites with observed diameters in the range 50 nm ≤ d ≤ 5 µm.…”

Dye‐Doped POSS Solutions: Random Nanomaterials for Laser Emission

“…Thus, POSS nanoparticles can be dispersed at a molecular level (1-3 nm) [4] and, because of their synthetically well-controlled functionalization, can be incorporated into polymers by different polymerization techniques with minimal processing disruption. [5][6][7] This excellent dispersion at the molecular scale prevents phase separation and assures the macroscopic homogeneity of the materials. [8] In this way, new optical hybrid materials based on POSS nanoparticles as the inorganic part overcome some of the most important limitations intrinsic to sol-gel hybrid composites while maintaining the physical, chemical, and mechanical combined advantages of organic-inorganic systems.…”