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.
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