## 3 Results and Discussion
### Matrix Toughening by Polymerization-Induced Phase Separation
For thermally-cured materials (Figure 12a), the gradual increase of PES in the blend leads to the development of a relaxation peak at 205-210 \({}^{\circ}\)C which corresponds to the Tg of pure PES (220 \({}^{\circ}\)C), the position of the epoxy network transition being almost unchanged, while the transition associated to PES at higher temperature increases in intensity almost proportionally with the PES content and is shifted towards higher temperatures by 10 \({}^{\circ}\)C. The system appears as composed of two coexisting phases and with minor cross-solubility. On the contrary, in EB-cured samples (Figure 12b), the transition associated with the epoxy network has its maximum shifted to higher temperatures while the distribution of relaxation temperatures is widened, suggesting the incorporation of a significant amount of PES in the network, with possible formation of a mixed interphase. The peak corresponding to the PES phase appears at the same temperature (200 \({}^{\circ}\)C), yet with an enhanced intensity as the amount of PES increases. Scanning electron microscopy provides complementary information consistent with the observations made by DMA. The micrographs selected for Figure 13 were obtained from the cryo-fractured surfaces of bars prepared from the same two families of blends cured by the two modes of activation. Figure 13a,b shows the fracture profile of the untoughened DGEBA network prepared by EB curing. At low magnification, shear banding is clearly observed together with fragments of ripped material which also appear on the expanded micrograph [47]. In EB-cured materials containing rather low amounts of PES (Figure 13c,d), one observes small PES particles or their imprint in the broken matrix. The particles are evenly distributed with a characteristic size between 100 and 300 nm. In the thermally-cured samples, with 7.5 (Figure 13e,f) or 19 wt-% of PES (Figure 13i,j), one observes larger  
Figure 11: Influence of PES 5003P toughening agent and of silica filler on the kinetic profiles of epoxy group polymerization of DGEBA–PGE blends (83:17 \(w/w\)) containing TAS-SbF\({}_{6}\) as the photo-initiator exposed to 385 nm UV-LED radiation for 0 to 230 min with an irradiance of 150 mW cm\({}^{-2}\). spheroidal phase-separated domains (0.5 to 10 \(\upmu\)m in size) with multiphase inclusions observable at larger magnification. With a PES content of 19 wt-%, the spheroidal clusters are more dispersed in size than those in the material containing 7.5 wt-% of PES. EB-cured samples present a different morphology which can be assigned to a co-continuous phase-separated matrix [48]. Such a structure was obtained by a thermal post-treatment of EB-cured samples containing low amounts of PES, as exemplified by the micrographs of Figure 13c,d for specimens before thermal post-treatment, to be compared to the pictures of Figure 13g,h obtained from post-cured specimen. Samples with higher contents in PES were obtained directly with the morphology assigned to a co-continuous structure after EB curing (Figure 13k,l) [42]. Figure 12: Evolution of the storage modulus E\({}^{\prime}\) and of the loss factor tan \(\delta\) recorded by DMA as a function of the content in PES (0 to 19 wt-% in samples P0 to P6) and on curing conditions, thermal curing (**a**) and EB curing (**b**) of DGEBA/polyTHF-based blends with the appropriate initiator system. Figure 13: SEM micrographs (**a**-**l**) revealing the changes in morphology for DGEBA/poly/Thr-based matrices depending on the amount of PES introduced as a topughening agent and curving conditions, either by thermal or EB process. Scale bars appear on each micrograph. Indication of magnification factors used during image recording is provided as a qualitative guide for facilitating the comparison between images. The overall observations can be rationalized by the following explanations. Cured materials obtained from formulations containing various amounts of PES (from 7.5 to 19 wt-%) exhibit a biphasic morphology contrasting with the one observed with the PES-free epoxy matrix.