Viscoelastic and mechanical properties of vinyl ester VE)/multifunctional polyhedral oligomeric silsesquioxane POSS) nanocomposites and multifunctional POSS±styrene copolymers G.Z. Li a , L. Wang a , H. Toghiani b , T.L. Daulton c , C.U. Pittman Jr. a, * a Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA b Department of Chemical Engineering, Mississippi State University, Mississippi State, MS 39762, USA c Naval Research Laboratory, Marine Geosciences Division, Stennis Space Center, MS 39529-5004, USA Received 20 December 2001; received in revised form 15 March 2002; accepted 21 March 2002 Abstract Vinyl ester VE) composites containing chemically bonded, multifunctional polyhedral oligomeric silsesquioxane POSS), POSS-1 C 6 H 5 CHCHO) 4 Si 8 O 12 )CHyCHC 6 H 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 composi- tions. 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 magni®cation. 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 in¯uence on T g or on the width of the tan d 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 tetrahydro- furan 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 b-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 d 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 0 , 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 ¯exural modulus of the vinyl ester resin is raised by incorporation of POSS-1 while the ¯exural 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. q 2002 Published by Elsevier Science Ltd. Keywords: Viscoelastic property; Mechanical property; Multifunctional polyhedral oligomeric silsesquioxane/vinyl ester nanocomposite 1. Introduction The development of polymer±inorganic nanocomposites with improved properties has attracted much research Polymer 43 2002) 4167±4176 0032-3861/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S0032-386102)00232-X www.elsevier.com/locate/polymer * Corresponding author. Tel.: 11-601-325-7616; fax: 11-601-325-7611. E-mail address: cpittman@ra.msstate.edu C.U. Pittman Jr.).