Epoxy Networks Modified by Multifunctional Polyhedral Oligomeric Silsesquioxanes (POSS) with Bulky Organic Branches

R. J. J. WILLIAMS; I. E. DELL'ERBA

East Lansing

Conferencia; The 35th Annual Conference on Thermal Analysis and Applications; 2007

North American Thermal Analysis Society (NATAS)

Polyhedral oligomeric silsesquioxanes (POSS), of generic formula (RSiO1.5)n (n = 6, 8, 10?) or Tn, are nanosized cage structures that can be incorporated into linear or thermosetting polymers to improve thermal and mechanical properties. The definition may be extended to include imperfect polyhedra, Tn(OH) (n = 7, 9, 11?), containing one free SiOH group in the structure. Depending on the number of organic groups bearing reactive functionalities, POSS can be classified as non-functional, monofunctional or multifunctional. The aim of this presentation is to show the variation of thermal and mechanical properties of epoxy networks modified by the incorporation of multifunctional POSS bearing bulky organic branches. Two different types of POSS consisting in narrow distributions of perfect and imperfect polyhedra: T7(OH), T8, T9(OH), T10, and T11(OH), are used. One of them, OH-POSS, contains 3 secondary hydroxyl groups per organic branch; the other one, COOH-POSS has 2 (β-hydroxyester) groups per organic branch. Both were soluble in the epoxy monomer based on diglycidylether of bisphenol A (DGEBA). In order to produce covalent bonds of these POSS, DGEBA was polymerized in the presence of a tertiary amine (benzyldimethylamine, BDMA, or 4-(dimethylamino)pyridine, DMAP). In this reaction, C-OH groups are covalently bonded to the network structure by chain transfer reactions: a propagating polyether chain with an alkoxide end group is terminated by abstraction of a proton from the C-OH group, leaving an alkoxide anion that initiates a new chain (1). On the other hand, carboxyl acid reacts with epoxy groups in the presence of a tertiary amine, with the formation of a hydroxyester group. However, transesterification reactions take place at a fast rate and the generated C-OH groups can also participate as chain transfer agents in the homopolymerization of the epoxy excess (2-4).The incorporation of OH-POSS or COOH-POSS produced a significant decrease of the rubbery modulus by chain transfer reactions. The use of DMAP as initiator produced a 100 % increase in the rubbery modulus when compared with BDMA. The reason is the increase in the average length of primary chains, as was recently proved by a model reaction based on the homopolymerization of phenylglycidylether (5). The glass transition temperature of both types of networks decreased when increasing the amount of POSS in the formulation. This may be explained by two concurrent factors: (i) the decrease in crosslink density produced by increasing the amount of POSS, (ii) the flexibility of the organic branches present in POSS cages (effect of the chemical structure). The glass transition temperature of the neat epoxy network initiated by BDMA was 100 ºC while the corresponding value for the network initiated by DMAP was close to 160 ºC. This confirms the higher crosslink density obtained when using DMAP as initiator of the epoxy homopolimerization.On the other hand, the addition of POSS increased both the glassy modulus and the yield stress of epoxy networks modified by OH-POSS (Table 2). For COOH-POSS the glassy modulus increased to a maximum value but then decreased with the POSS amount