INTEMA   05428
INSTITUTO DE INVESTIGACIONES EN CIENCIA Y TECNOLOGIA DE MATERIALES
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Epoxy Networks Modified By Multifunctional Polyhedral Oligomeric Silsesquioxanes (POSS) with Bulky Organic Branches
Autor/es:
R. J. J. WILLIAMS; I. E. DELL´ERBA
Lugar:
Lansing, MI, USA
Reunión:
Simposio; 35th NATAS Annual Conference; 2007
Institución organizadora:
NATAS
Resumen:
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 (b-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). Table 1 shows the rubbery modulus of the neat epoxies and of two POSS-modified epoxies of each one of the series.   Table 1 Rubbery modulus (ER) of the neat epoxies and of POSS-modified epoxies.                                                Sample                                                    ER (MPa)                      Epoxy/BDMA without OH-POSS                                      48                      Epoxy/BDMA with 30 wt % OH-POSS                             29                      Epoxy/BDMA with 50 wt % OH-POSS                             19                      Epoxy/DMAP without COOH-POSS                                100                      Epoxy/DMAP with COOH/epoxy = 0.10                           32                      Epoxy/DMAP with COOH/epoxy = 0.15                           22                                  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.   Table 2 Glassy modulus (EG), yield stress (sY), and ratio sY/EG, for epoxy networks modified by OH-POSS                            Sample                                     EG (GPa)                    sY (MPa)                    sY/EG   Epoxy/BDMA without OH-POSS                    2.80                             84                            0.030   Epoxy/BDMA with 10 wt % POSS                  2.97                             88                            0.030   Epoxy/BDMA with 30 wt % POSS                  3.23                             94                            0.029   Epoxy/BDMA with 50 wt % POSS                  3.43                            100                           0.029                                 The main factor affecting EG is the cohesive energy density (CED) of the polymer network that increased with the concentration of H-bond donor groups (6). This explains the increase of EG when increasing the amount of OH-POSS in the formulation. A similar behavior was observed for the variation of the yield stress. The constancy of the ratio sY/EG means that both mechanical properties were affected in the same way by variations in the cohesive energy density.             Therefore, an antiplasticization effect was observed (increase in glassy modulus associated with a decrease in glass transition temperature), due to the increase in cohesive energy density produced by extensive H-bonding associated with a decrease in crosslink density.   REFERENCES 1. J. Berger and F. Lohse, Eur. Polym. J., 1985, 21, 435. 2. L. Matějka, S. Pokorný, and K. Dušek, Polym. Bull., 1982, 7, 123. 3. G. P. Craun, J. Coat. Technol., 1995, 67, 841. 4. C. E. Hoppe, M. J. Galante, P. A. Oyanguren, and R. J. J. Williams, Macromol. Mater. Eng., 2005,     290, 456. 5. I. E. dell’Erba and R. J. J. Williams, Polym. Eng. Sci., 2006, 46, 351. 6. J. P. Pascault, H. Sautereau, J. Verdu, and R. J. J. Williams, Thermosetting Polymers, Marcel Dekker,     New York, 2002.