Pure Epoxy Networks

The epoxy resins [62,63] are formed from the chemicals indicated in Table 8. The different types of networks that can be obtained are outlined in Fig. 92. 

All the systems contain the digycidyl ether of bisphenol A (DGEBA) and the changes come from the type of amine with which it is reacted  

It should be noticed that linear polymers DGEBA/HA or DGEBA/DMHMDA lead to very brittle samples for this reason lightly crosslinked systems were used instead. 

The code names, compositions and glass transition temperatures of the various networks are reported in Table 9. 

It is worth pointing out that the stoichiometry between epoxy group and NH function has always been controlled furthermore, the curing conditions have been defined in order to have an epoxide-amine reaction as complete as possible 40 °C for 12 h, then at Tg + 30 °C for 24 h. 13C and 15N NMR [64] 

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As for the pure epoxy networks, the interesting measurements deal with t /2 and Tip (13C). 

It is worth noting that in pure epoxy networks, the two groups, HPE and CH2 - N, have the same mobility and behave similarly as a function of temperature. 

Results of ( 1/2)0/( 1/2) determinations as a function of temperature for the protonated aromatic carbons are shown in Fig. 103. The higher content of antiplasticiser (19 wt %) is required to see the occurrence of the onset of mobility of the aromatic carbons at a higher temperature than in pure matrix. Such a behaviour is quite similar to that observed for HPE units, in agreement with the conclusion, reached in pure epoxy networks, of a likely correlation between the motions of the aliphatic units and the ring flips. 

The NMR experiments clearly show a different effect of the antiplasticiser on the mobility of either the crosslink points (CH2 - N) or the hydroxypropyl ether sequence. Indeed, whereas these two groups have similar mobility in pure epoxy networks, the mobility of the crosslink points is hindered by the antiplasticiser, whereas only a slight slowing down occurs for the HPE units. Furthermore, there is no difference in mobility between the HPE sequence in the epoxy network and the one in the antiplasticiser molecule. 

On the other hand, the investigations performed on pure epoxy networks unambiguously assign the p transition to motions of the HPE sequence. 

The dynamic mechanical results and the solid-state 13C NMR measurements lead to a deeper insight of the motions occurring below the glass transition temperature in the considered pure aryl-aliphatic epoxy networks, in particular those involved in the p transition of these systems, and the nature of their cooperativity. 

With just three distances, the antiplasticiser can be located approximately by triangulation relative to the nearest-neighbour network chain, but it is impossible to infer details of the orientation of the antiplasticiser within the network. However, the increase in density, observed for antiplasticised epoxy relative to pure epoxy, suggests, by similarity with the case of bisphenol A polycarbonate, that HPEs tend to align with one another in one direction and isopropylidene moieties in another. 

Dynamic mechanical analysis and the 13C NMR experiments performed on pure aryl-aliphatic epoxy networks, as well as on antiplasticised systems, have led to a deeper insight into the molecular motions involved in the glassy state of these epoxy resins. 

Figure 2.2 Optical micrographs of MWCNT-epoxy composites (a) pure epoxy, (b) 0.001 wt%, (c) 0.0025 wt%, and fd] 0.005 wt%. Scale bar (left) is 1 cm and the sample thickness is about 2.2 mm. The formation of small local NT aggregates at a loading fraction of 0.0025 wt% can be seen, which then leads to the macroscopic network of NTs at higher filler contents. (From Ref. 56. Reprinted with permission of Elsevier)...

Finally, the variation of the glass transition temperature for both systems (DGEBA-IPDA or DGEBA-DETA) versus the stoichiometric ratio (a/e) is reported (e.g., see Fig. 7.6) for either pure or modified materials. Usually, as the functionalities of epoxy and amine monomer were well defined, mixing materials at the stoichiometric ratio of 1 led to the formation of the most crosslinked network having the highest glass transition temperature. From Fig. 7.6 it can be... 

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