Epoxy diamine networks

It has been shown that the majority of the polar moitles in epoxy-diamine networks, such as hydroxyl groups generated by the cross-linking reaction and residual amino hydrogens, take part in inter- or intramolecular hydrogen bonds (6, 7 ). Indeed, at ambient temperatures, the infrared hydroxyl stretch frequency of epoxy-diamine systems are characteristically shifted from its free value of 3600 cm-1 to a hydrogen bonded value of 3440 cm 1 (8). 

In this contribution, we report equilibrium modulus and sol fraction measurements on diepoxidet-monoepoxide-diamine networks and polyoxypropylene triol-diisocyanate networks and a comparison with calculated values. A practically zero (epoxides) or low (polyurethanes) Mooney-Rivlin constant C and a low and accounted for wastage of bonds in elastically inactive cycles are the advantages of the systems. Plots of reduced modulus against the gel fraction have been used, because they have been found to minimize the effect of EIC, incompleteness of the reaction, or possible errors in analytical characteristics (16-20). A full account of the work on epoxy and polyurethane networks including the statistical derivation of various structural parameters will be published separately elsewhere. 

As explained in Sec. 7.1, epoxy networks have been and are still the subject of controversy. This is mainly based on the particular interpretation of results obtained using microscopy techniques. On the contrary, results obtained with small-angle neutron scattering (SANS) proved that typical diepoxy-diamine networks were homogeneous (Wu and Bauer, 1985). 

The structure of precursors, the number of functional groups per precursor molecule, and the reaction path leading to the final network all play important roles in the final structure of the polymer network. Some thermosets can be considered homogeneous ideal networks relative to a reference state. It is usually the case when networks are prepared by step copolymerization of two monomers (epoxy-diamine or triol-diisocyanate reactions) at the stoichiometric ratio and at full conversion. 

For the amine system under non-isothermal cure at 0.2°C min goo of 245 °C causes devitrification to occur at a temperature more than 150°C above vitrification. The importance of this extended mobility-restricted cure on the final material s properties should be emphasised. For this tetrafunctional epoxy-diamine system, an increase in Tg of ca. 170°C, corresponding with a residual cure of ca. 44% and a reaction enthalpy of more than 230 J g is caused by diffusion-controlled reactions and drastically influences the flnal network structure (crosslink density). 

DDS (/I4 = 2870) Epoxy modei networks from a diepoxy prepolymer, DGEBA, and three different diamines or mixtures of a monoamine and a diamine. 373.9 20.50 50.0 374.2 [156] ar,s of the softening dispersion from creep measurement. 

Cure is illustrated schematically in Figure 1 for a material with co-reactive monomers such as an epoxy-diamine system. Reaction in the early stages of ciu-e (a to b in Fig. 1) produces larger and branched molecules and reduces the total number of molecules. Macroscopically, the thermoset can be characterized by an increase in its viscosity r] (see Fig. 2 below). As the reaction proceeds (lb to Ic in Fig. 1), the increase in molecular weight accelerates and all the chains become linked together at the gel point into a network of infinite molecular weight. The gel point coincides with the first appearance of an equilibrium (or time-independent) modulus as shown in Figure 2. Reaction continues beyond the gel point (Ic to Id in Fig. 1) to complete the network formation. Macroscopically, physical properties such as modulus build to levels characteristic of a fully developed network. 

In a first testing series, the fracture behavior of the neat, fully crosslinked epoxy network was studied. A fully unstable crack propagation behavior was observed and the critical stress intensity factor, Kj (0.82 MPaxm ), and the critical energy release rate, Gj (0.28 kj/m ), were determined [87]. These are typical values for highly crosslinked epoxy networks prepared with DGEBPA and aromatic or cycloaliphatic diamines. 

The control resin network used in this study was a diglycidyl ether-based epoxy resin crosslinked with a cycloaliphatic diamine. Cooligomeric modifiers were prepared having varying percentages of TFP and DP siloxane and aminoethylpiperazine end groups. Both siloxane and ATBN and CTBN elastomers were used as epoxy modifiers, the latter two having been included to facilitate direct comparisons between modifiers in similarly prepared networks. 

A very convenient system for performing this investigation consists of epoxy networks formed by reacting aromatic epoxy on aliphatic diamine. Indeed, the different nature (aromatic or aliphatic) of the units allows one to apply 13 C NMR for identifying the groups involved in the transitions and their motions. Furthermore, in the same way as for polyethylene tere-phthalate), specific small molecule additives act as antiplasticisers, which can offer an additional possibility for investigation of the molecular motions and their cooperativity. 

In the case of epoxy networks with a secondary diamine, like DMHMDA, the network architecture is such that flexible aliphatic sequences are present as chain extenders between the crosslink points. In such architectures, the motions of the HPE units can develop towards other HPE sequences (either along the chain or spatially neighbouring) without involving the crosslink points in their cooperativity. Thus, with these systems a different nature of cooperativity exists compared to the other network architectures. The introduction of an antiplasticiser in such a local packing does not affect the cooperativity as much as with the densely crosslinked architecture, for the crosslinks are not so much involved. Once more, it is important to point out that the flexible nature of the aliphatic amines does not matter since the same behaviours are observed for fully aromatic systems with identical architecture [68]. 

However, similar structures were observed with etched surfaces of amorphous linear thermoplastics, such as polystyrene and poly(methyl methacrylate). Moreover, the small-angle X-ray scattering (SAXS) spectra of simple epoxy networks based on diepoxy and diamine monomers were... 

Figure 8.11a TEM photographs of core-shell particles dispersed in an epoxy network, DGEBA-lsophorone diamine, IPD. (A) 10 wt% CS (B) 15 wt% CS. (From LMM Library.)...

Figure 8.12 TEM photographs of triblock copolymers dispersed in a DGEBA-diamine epoxy network. The triblock copolymer is polystyrene-b-polybuta-diene-b-poly(methyl methacrylate), and the epoxy hardener is (a) -methylene bis [3-chloro-2,6 diethylaniline], MCDEA, and (b) 4,4 -diamino diphenyl sulfone, DDS. In the case of the epoxy system based on MCDEA, the PMMA block is miscible up to the end of the epoxy reaction. In the case of the epoxy system based on DDS, the PMMA block phase-separates during reaction. (From LMM Library.)... 

Figure 10.7 Comparison of the damping peaks of tetraglycidyl methylene diamine and diamino diphenyl sulphone (TGMDA-DDS epoxy network) (O) and poly(bismaleimide) (BMI network) ( ).

For many usual, moderately polar networks, such as epoxides of the diglycidyl ether of bisphenol A (DGEBA) diamine type, or vinyl esters, Hs —Hw, so that the equilibrium concentration appears almost temperature-independent. For most of the less polar networks such as polyesters or anhydride-cured epoxies, Coo (or Wro) increases slightly with temperature AW /AT 0.01 0.02% K 1 between 20 and 50°C. 

Lichtenhan et al. [23,24] prepared several monosubstituted POSS epoxides (XIX). The Cs-based chain epoxy (5-9 wt%) was used with 1,4-butanediol diglycidyl ether (EDGE) (XX), diglycidyl ether of Bisphenol A (DGEBA) (XXIII) and polyoxypropylene diamines (XXI) to prepare nano-reinforced epoxy network glasses (Scheme 6) [23]. 

The prepolynier can be reacted further with a wide variety of reagents, and its latent Functionality depends on the particular reaction. The conversion of an epoxy polymer to an interconnected network structure is formally similar to the vulcanization of rubber, but the process is termed curing in the epoxy system. When the epoxy hardener is a primary or secondary amine like m-phenylene diamine the main reaction is... 

An identical mathematical description of the kinetics of curing of reactants different in chemical nature and that obtained on the basis of fundamentally different experimental methods allows us to assume that this apparent selfacceleration course of some rheokinetic parameters is common to the processes of formation of materials with a crosslinked structure. It should be emphasized once more that the self-acceleration" effect must not be identified with the self-catalysis of the reaction of interaction between epoxy monomers and diamines which is studied in detail on model compounds [116, 117]. For each particular curing process the self-acceleration effect is influenced by the mechanism of network formatic, namely, chemical self catalysis [118], the appearance of local inhomogeneities [120], the manifestation of gel eff t [78], parallel course of catalytic and noncatalytic reactions [68]. It is probably true that the phenomena listed above may in one form or another show up in specific processes and make their contribution into self-acceleration of a curing reaction. 

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