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

Abstract

Vinyl ester (VE) composites containing chemically bonded, multifunctional polyhedral oligomeric silsesquioxane (POSS), POSS- 1 ((C 6H 5CHCHO) 4(Si 8O 12)(CHCHC 6H 5) 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 8×10 5 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 T g 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 T g values versus pure polystyrene (PS) prepared at the same conditions. The T g 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 T g 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.