Macromolecules Article a) b) 01修 9% 11% EP1-DDM EP1-DDM surface EP2-DDM inside EP2-DDM surace 0 60 400 200 EP2 DDM resulted in the high char yields.The lower under ai attributed to the third sta than that of phosphorus content should lead to of the DDM DGEBA-DDM and DGEBA-DDM of183 d214℃， rely,tha e cr nd th The nt are the sing eq 3. M.="Mp+DMMpDM (3) nd M represent in the As ith R re t d EP. ibited highe Ms than ogen atm osphere.As m vio the phospho densi DD oh- mainly resulted from the higher rigidity of EP1-DDM and EP2- Table 2.Phosphorus and Nitrogen Content of EP-DDM,EP2-DDM,and DGEBA-DDM before and after Burning (wt $6)phospl GEBA-DD. 89 8060h22a2a yield at 700 °C (R700) as high as ∼44.7% under air atmosphere and ∼58% under a nitrogen atmosphere than DEGBA-DDM with R700 of 0% under air atmosphere and 14.0% under a nitrogen atmosphere. As mentioned previously, the phospho￾nate group as a promoter for “char formation” formed the phosphorus-rich layer during the degradation process, which resulted in the high char yields. The lower R700 under air atmosphere than that under a nitrogen atmosphere was attributed to the third stage degradation belonging to the oxidative degradation of residual carbon under air atmosphere, which can be seen in Figure 9. Meanwhile, the phosphorus content of EP2-DDM is higher than that of EP1-DDM, resulting in its higher Td30% and R700 than EP1-DDM. Higher phosphorus content should lead to lower Td5% of the phosphorus-containing systems, while EP2-DDM showed higher Td5% than EP1-DDM. Our previous work proved that increasing the cross-link density can improve the initial degradation temperature.38 EP2-DDM has higher cross-link density relative to EP1-DDM, which corresponds to its higher Td5% than EP1-DDM. Glass Transition Temperature of the Cured Epoxy Resins. Glass transition temperature (Tg) is a major parameter for thermosetting materials. The Tgs of EP1-DDM, EP2-DDM, and DGEBA-DDM were determined by DSC, and the results are summarized in Figure 10. EP1-DDM and EP2-DDM showed higher Tgs of 183 and 214 °C, respectively, than DGEBA-DDM with Tg of 166 °C. The Tgs of the cross-linked polymers have a close tie with their cross-link density and the rigidity of the chain segment structure.49,50 The higher the cross-link density of the thermosets and rigidity of their chain segment are the higher the Tg is. In order to estimate the cross￾link density of the cured epoxy resins, the average molecular weight between cross-link points (Mc) (Table 1) was calculated using eq 3. = + M nM n M n c EP EP DDM DDM DDM (3) where n and M represent the molarity and molar mass of the corresponding component in the epoxy formulations.51 As shown in Table 1, EP1-DDM and EP2-DDM exhibited higher Mcs than DGEBA-DDM, which means that they possessed lower cross-link density than DGEBA-DDM. Thus, the higher Tgs of EP1-DDM and EP2-DDM than that of DGEBA-DDM mainly resulted from the higher rigidity of EP1-DDM and EP2- Figure 7. (a) Digital photos and (b) SEM photographs of the residues after LOI test. Table 2. Phosphorus and Nitrogen Content of EP-DDM, EP2-DDM, and DGEBA-DDM before and after Burning before burning after burning sample phosphorus content (wt %) nitrogen content (wt %) weight residue (wt %) phosphorus content (wt %) phosphorus retention (%) EP1-DDM 6.50 4.40 35.8 6.99 38.5 EP2-DDM 7.18 4.86 41.2 9.32 53.5 DGEBA-DDM 0 2.86 Figure 8. XPS spectra of the surface part and inside part of the char residues of EP1-DDM and EP2-DDM. Macromolecules Article DOI: 10.1021/acs.macromol.7b00097 Macromolecules 2017, 50, 1892−1901 1898