Epoxy-amine networks

Figure 2. Theoretical curves for superimposed reduced moduli Gd /RT(mol/ cm 3) of epoxy-amine networks versus the gel fraction ws. Fraction of epoxy groups in monoepoxide is O, 0 , 0.2 A, 0.33 V, 0.5. Key O, , A, V, dry networks , A, , swollen networks value of front factor A indicated.

As can be seen from Table 4, self-association of the hydroxyl groups is a predominant type of association at room temperature. The fraction of the self-associates of the hydroxyl groups in the epoxy-amine networks of stoichiometric composition amounts to 85-90 %. An abundant literature on the self-association of the hydroxyl compounds (alcohols, phenols, acids) 38,47,48) shows that a number of different n-mer linear and cyclic self-associates exists in equilibrium. The cyclic associates usually existing in the form of the trimers... 

Now, the question arises to what extent the thermodynamic information on the low-molecular-weight model compounds can be applied to real epoxy-amine network polymers. A clear answer to this question is given by a direct comparison of the... 

The computer-assisted modelling of formation of the epoxy-amine networks indicates that the density fluctuations of the crosslinks does not exceed the value predicted by the statistical theory, which is much lower than the size of the globules in... 

A good example of an A4 +B2 stepwise polymerization is the formation of an epoxy - amine network, starting from a difunctional epoxy molecule (B2)... 

In addition, from thermal and thermomechanical measurements, it is found that typical epoxy-amine networks exhibit one glass transition temperature, Tg, and one sharp well-defined relaxation peak. The same techniques were used for crosslinked polyurethanes based on triol and diisocyanate or diol and triisocyanate (Andrady and Sefcik, 1983). Similar conclusions to those found for epoxy-amine networks were attained. 

It can be stated that networks based on a simple formulation (one monomer reacting with a comonomer), obtained from the step-polymerization process will exhibit a homogeneous structure. This is the case for epoxy-amine networks (the most studied) and polyurethane networks that have been used very often as ideal networks for structure-property correlations. 

The calculated equilibrium network moduli of elasticity and experimental moduli measured in the rubbery state10,34) of different epoxy-amine networks show quantitative agreement. This means that the real structural nonuniformity of cross-linked epoxy-aromatic amine polymers exists only at the level of statistical deviations of network structure from the ideal one. A comparison of Ecalc and E p is shown in Fig. 4. 

Boyer 48) has introduced into polymer science the ACpTg quantity and has found its correlations with some properties of amorphous polymers. It is interesting to compare the data for epoxy-amine networks with those given in Boyer s paper. The... 

Weakly crosslinked epoxy-amine networks above their Tg exhibit rubbery behaviour like vulcanized rubbers and the theory of rubber elasticity can be applied to their mechanical behaviour. The equilibrium stress-strain data can be correlated with the concentration of elastically active network chains (EANC) and other statistical characteristics of the gel. This correlation is important not only for verification of the theory but also for application of crosslinked epoxies above their Tg. 

St John, N. A. (1993) Spectroscopic Studies of the Cure and Structure of an Epoxy Amine Network, Thesis, Department of Chemistry. St Lucia, University of Queensland. 

A polymer is more likely to fail by brittle fracture under uniaxial tension than under uniaxial compression. Lesser and Kody [164] showed that the yielding of epoxy-amine networks subjected to multiaxial stress states can be described with the modified van Mises criterion. It was found to be possible to measure a compressive yield stress (Gcy) for all of their networks, while the networks with the smallest Mc values failed by brittle fracture and did not provide measured values for the tensile yield stress (Gty) [23,164-166]. Crawford and Lesser [165] showed that Gcy and Gty at a given temperature and strain rate were related by Equation 11.43. 

Plangsangmas et al [172] illustrated the measurement of the (Tf and KIc of epoxy-amine networks by using quantitative fractography. They showed that networks with lower Mc values (higher crosslink densities) have lower fracture toughness values. 

Pierre et al [173] considered the effects of crosslink density on dynamic mechanical properties and plastic deformation of epoxy-amine networks, varying chain stiffness by using... 

Based on studies of an homologous, endlihked, epoxy/amine network series, the simple theory of rubber elasticity has proved effective for determining reasonable cross-link densities from equilibria modulus measurements in the rubbery state. 

Figures 10 and 11 illustrate the level of reproducibility in the CR spectra. The former was obtained in the temperature range of sub-Tg ( 3 and y) relaxations, for two samples of the model epoxy-amine network taken from two separate batches. Figure 11 shows the spectra for semi-crystalline poly(oxymethylene) (POM) with an extremely complicated crystalline structure [22]. Simultaneously, the contours of their DMA spectra are also given. These materials and their spectra are discussed in Sects. 3.2 and 3.6.1, respectively.

Fig. 10 Reproducibility of the CR spectra obtained at sub-Jg temperatures and compressive stress of 20 MPa for two samples of the model epoxy-amine network (DGEBA-DDM/60BAN, see Sect. 3.2) coming from two separate batches [20], The dotted line corresponds to its contour of DMA spectrum (1 Hz)...

Creep rate spectroscopy has been used for the discrete dynamic analysis of polymer networks such as PU network [19] PU-butyl methacrylate/dimethacrylate triethylene glycol copolymer interpenetrating networks (PU/BMA copolymer IPNs) [132, 133] commercial epoxy networks [134,135] the model epoxy-amine networks varying in crosslink density and rigidity [20] and the cross-linked hyperbranched polyimides [136],... 

Finally, the CRS technique was used for studying molecular motion in the glassy state (sub-Fg relaxations) of a series of model epoxy-amine networks [20], The experiments were carried out over the temperature range 100-350 K, which covers the regions where the secondary relaxations are basically expected to occur. 

The interpretation of P-relaxation in these epoxy-amine networks with the extraordinarily complicated chemical structure still remains disputable despite many experimental (NMR [137-139] and other) results obtained. Different authors have attributed this relaxation to motion of the hydroxypropylether (HPE) units, the DGEBA ring flips, or their combined motions, as well as to motions in the long enough flexible aliphatic sequences. The y-relaxation is unambiguously related to localized motion in the aliphatic sequences. 

For epoxy-amine networks, at room temperature and e 10 pi 0.2 (Crawford and Lesser, 1999). 

The proposed dendro-aminosilane hardeners give the possibility to introduce the siloxane fragments into aromatic stmcture of diphenylolpropane based epoxy-amine network polymers. Additional hydrolysis of aminosilane oligomer creates the secondary nano-stmctured network polymer that improves the service properties of the compound. Branched (dendro) polyamine hardeners are novel direction in epoxy and cyclocarbonate and acryl resins chemistry. 

Figure 2.115. Reaction-induced phase separation (RIPS) occurs when a homogeneous mixture of epoxy (bifunctional sticks ), amine (tetrafunctional sticks ), and polymeric modifier (dashed line) is heated and the epoxy-amine network formation induces phase separation. The position of Tcure to the glass transition of the modifier Tgniod determines whether vitrification of a phase rich in this component will occur during cure.

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