Viscoelastic and mechanical properties of vinyl ester (VE)/multifunctional polyhedral oligomeric silsesquioxane (POSS) nanocomposites and multifunctional POSS–styrene copolymers

Vinyl ester (VE) composites containing chemically bonded, multifunctional polyhedral oligomeric silsesquioxane (POSS), POSS-1 ((C6H5CHCHO)4(Si8O12)(CHCHC6H5)4), nanoparticles were prepared with VE/POSS-1 95/5 and 90/10 w/w ratios. The mole percents of POSS-1 in these two composites are low (<0.5 and <1%, respectively) due to the high mass of POSS-1 (mwt=1305). VE composites of two non-functional POSS-3 (octaisobutyl POSS) and POSS-4 (dodecaphenyl POSS) derivatives were also prepared with 95/5 w/w compositions. Additionally, POSS-1 was also incorporated into styrene copolymers at levels of 5 wt% (0.42 mol%) and 10 wt% (0.88 mol%) of POSS-1. The composites and copolymers were characterized by dynamic mechanical thermal analysis and mechanical testing. The POSS-1 units incorporated into the vinyl ester network were well dispersed. No phase-separation in the VE/POSS-1 90/10 composite could be detected by TEM from low to 8×105 magnification. In VE composites containing 10 wt% POSS-1, silicon-rich phases were observed ranging in size from a few nm to ∼75 nm by electron energy loss spectroscopy (EELS). TEM, EDXS, EELS and extraction studies suggest that some POSS-1-rich nanoparticles in the VE/POSS-1 90/10 composite are present and also a fraction of the POSS-1 is molecularly dispersed within the VE resin. The POSS-1-rich dispersed phase portion is cross-linked, insoluble and contains some VE. VE/POSS-3 and VE/POSS-4 composites exhibited larger-sized POSS phases which do not contain VE. Incorporating low mole percentages of POSS-1 into the VE network by chemical bonds or blending non-functional POSS-3 or 4 into VE resin have almost no influence on Tg or on the width of the tan δ peak in the glass transition range. POSS-1–styrene copolymers exhibit good miscibility at 5 wt% POSS-1 but serious phase-separation occurs in the copolymer with 10 wt% POSS-1 content. POSS-1–styrene copolymers swelled but did not dissolve in tetrahydrofuran (THF) demonstrating they had been cross-linked by POSS-1. No POSS-1 was extracted into the THF. The POSS-1–styrene copolymers have higher Tg values versus pure polystyrene (PS) prepared at the same conditions. The Tg elevation could be due to the cross-linking resulting from the four β-substituted styryl functions in POSS-1 and due to the effect of high molecular weight POSS units retarding segmental motion of a portion of the chain segments. The Tg of the 10 wt% POSS-1 copolymer is almost the same as that of the 5 wt% POSS-1 copolymer because the continuous phase in the 10 wt% POSS-1 copolymer might have a cross-linking density similar to that of the 5 wt% POSS-1 copolymer. The low POSS-1 mole percentage means that many all-styrene segments exist that can undergo segmental motion without being retarded by POSS. The tan δ peak for 10 wt% POSS-1 copolymer is much broader and less intense than that for PS or 5 wt% POSS-1 copolymer. A higher average cross-linking density and much less segmental motion in the dispersed POSS-1-rich phase account for this behavior in the 10 wt% copolymer. The bending storage modulus, E′, values of the VE/POSS-1 composites and the POSS-1–styrene copolymers are higher than those of either the neat vinyl ester resin or pure PS, respectively, over entire temperature range, especially at the low POSS-1 content (5 wt%). The incorporation of multifunctional POSS-1 into vinyl ester or PS by chemical bonding improves the thermal dimensional stabilities. The flexural modulus of the vinyl ester resin is raised by incorporation of POSS-1 while the flexural strengths are lowered. VE resin and VE/POSS-1 composites gave negligible weight gains after 50 days in toluene. The VE and composite samples cracked and fragmented after submersion in THF.