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Samples Composition ‘Used for Each Crosslinking Reaction Performed at 80°C

Table 1 Samples Composition ‘Used for Each Crosslinking Reaction Performed at 80°C

Related Figures (12)

Scheme 1 Possible crosslinking reaction mechanism of citric acid with cellulose.
Scheme 1 Possible crosslinking reaction mechanism of citric acid with cellulose.
Figure 2. DSC curing thermograms of CMCNa (a); HEC (b); and CMCNa/HEC (3/1) hydrogel (3.75% of CA) (c). DSC analysis of neat CMCNa and HEC powders are reported in Figure 2. The peak in the first scan indicates that some water is still absorbed in the polymers. Above 100°C, a possible degradation peak of CMCNa is detected. Both CMCNa and HEC show a thermal stability below 100°C.
Figure 2. DSC curing thermograms of CMCNa (a); HEC (b); and CMCNa/HEC (3/1) hydrogel (3.75% of CA) (c). DSC analysis of neat CMCNa and HEC powders are reported in Figure 2. The peak in the first scan indicates that some water is still absorbed in the polymers. Above 100°C, a possible degradation peak of CMCNa is detected. Both CMCNa and HEC show a thermal stability below 100°C.
Figure 1 DSC curing thermogram of citric acid.
Figure 1 DSC curing thermogram of citric acid.
Figure 3 FTIR spectra of citric acid (CA) (a); A10 reaction mixture before heating (b); A10 reaction mixture after 5 h of heating (c); B10 reaction mixture before heating (d); and B10 reaction mixture after 6.5 h of heating (e).
Figure 3 FTIR spectra of citric acid (CA) (a); A10 reaction mixture before heating (b); A10 reaction mixture after 5 h of heating (c); B10 reaction mixture before heating (d); and B10 reaction mixture after 6.5 h of heating (e).
Figure 6 Carbonyl band area ratio (1738 cm !/1590 cm’) of reactions B10 (@) and B20 (M) at different reac- tion times. Figure 6 shows peak area ratio A;/A2 of HEC polymer when the reaction is performed at 80°C with 20% of CA or 10% of CA. The anhydride band intensity increase from 0 to 0.098 when the reaction time increase from 0 h to 6.5 h, but drop to 0 when the reaction time reaches 24 h. The B20 reaction (Fig. 6, Ml) follows exactly the same trend providing a maximum of 0.079 value at 5 h. Assuming that the crosslinking mechanism is the same described for CMCNa, the anhydrification and esterification reac- tions appear superimposed in this case. Therefore, HEC polymer shows a single peak in Figure 6. This latter result was in accordance with conclusion of Xie and Liu.”* They studied the degree of substitu- tion, as evaluation of crosslinking esterification, on starch thermally reacted with CA at different reac- tion time and found a maximum after few hours.
Figure 6 Carbonyl band area ratio (1738 cm !/1590 cm’) of reactions B10 (@) and B20 (M) at different reac- tion times. Figure 6 shows peak area ratio A;/A2 of HEC polymer when the reaction is performed at 80°C with 20% of CA or 10% of CA. The anhydride band intensity increase from 0 to 0.098 when the reaction time increase from 0 h to 6.5 h, but drop to 0 when the reaction time reaches 24 h. The B20 reaction (Fig. 6, Ml) follows exactly the same trend providing a maximum of 0.079 value at 5 h. Assuming that the crosslinking mechanism is the same described for CMCNa, the anhydrification and esterification reac- tions appear superimposed in this case. Therefore, HEC polymer shows a single peak in Figure 6. This latter result was in accordance with conclusion of Xie and Liu.”* They studied the degree of substitu- tion, as evaluation of crosslinking esterification, on starch thermally reacted with CA at different reac- tion time and found a maximum after few hours.
Figure 5 Carbonyl band area ratio (1738 cm '/1590 cm’) of reaction A10 at different reaction times.
Figure 5 Carbonyl band area ratio (1738 cm '/1590 cm’) of reaction A10 at different reaction times.
Figure 4 Carbonyl band area ratio (1738 cm '/1590 cm~’) of reaction A20 at different reaction times.
Figure 4 Carbonyl band area ratio (1738 cm '/1590 cm~’) of reaction A20 at different reaction times.
Figure 7 FTIR spectra of C10 reaction mixture before heating (a); C10 reaction mixture after 8 h of heating (b); C10 reaction mixture after 13 h of heating (c); C20 reaction mixture before heating (d); C20 reaction mixture after 8 h of heating; (e) and C20 reaction mixture after 13 h of heat- ing (f). Finally, to complete this study, polymer mixtures of CMCNa and HEC were crosslinked. CMCNa has carboxylic groups in its structure that increase the volume variation process in solution. A preliminary attempt to follow the reaction pathway was failed. Probably the reaction systems considered are too complex having many different reaction centers. FTIR spectra of C10 reaction registered before (Curve a), after 8 h (Curve b), and after 13 h (Curve c) of heating are compared in Figure 7. Figure 7 (Curves d-f) shows similar spectra but referred to reaction sample C20. Moreover, it is worth noting that when polymer mixtures are used (C10 and C20) a broad signal appears at about 1715 cm‘ (see spec- tra 7a and 7d), especially when a higher CA concen- tration is used in the reaction. In fact, with 20% of
Figure 7 FTIR spectra of C10 reaction mixture before heating (a); C10 reaction mixture after 8 h of heating (b); C10 reaction mixture after 13 h of heating (c); C20 reaction mixture before heating (d); C20 reaction mixture after 8 h of heating; (e) and C20 reaction mixture after 13 h of heat- ing (f). Finally, to complete this study, polymer mixtures of CMCNa and HEC were crosslinked. CMCNa has carboxylic groups in its structure that increase the volume variation process in solution. A preliminary attempt to follow the reaction pathway was failed. Probably the reaction systems considered are too complex having many different reaction centers. FTIR spectra of C10 reaction registered before (Curve a), after 8 h (Curve b), and after 13 h (Curve c) of heating are compared in Figure 7. Figure 7 (Curves d-f) shows similar spectra but referred to reaction sample C20. Moreover, it is worth noting that when polymer mixtures are used (C10 and C20) a broad signal appears at about 1715 cm‘ (see spec- tra 7a and 7d), especially when a higher CA concen- tration is used in the reaction. In fact, with 20% of
Figure 8 Equilibrium swelling ratio versus reaction time of CMCNa polymer with 10% w/w (M™) or 20% w/w (@) of CA crosslinker concentration (both samples were dried under vacuum at 30°C for 24 h and then reacted at 80°C).
Figure 8 Equilibrium swelling ratio versus reaction time of CMCNa polymer with 10% w/w (M™) or 20% w/w (@) of CA crosslinker concentration (both samples were dried under vacuum at 30°C for 24 h and then reacted at 80°C).
Figure 10 Equilibrium swelling ratio versus reaction time of CMCNa/HEC weight ratio 3/1 mixture with 10% w/w (HM) or 20% w/w (®) of CA crosslinker concentration (bott samples were dried under vacuum at 30°C for 24 h and then reacted at 80°C).
Figure 10 Equilibrium swelling ratio versus reaction time of CMCNa/HEC weight ratio 3/1 mixture with 10% w/w (HM) or 20% w/w (®) of CA crosslinker concentration (bott samples were dried under vacuum at 30°C for 24 h and then reacted at 80°C).
Figure 11 Equilibrium swelling ratio versus reaction time of CMCNa/HEC weight ratio 3/1 mixture with 1.75% w/w (A), 2.75% w/w (Mf) or 3.75% w/w (®) of CA cross- linker concentration (all samples were dried under vacuum at 30°C for 24 h and then reacted at 80°C).
Figure 11 Equilibrium swelling ratio versus reaction time of CMCNa/HEC weight ratio 3/1 mixture with 1.75% w/w (A), 2.75% w/w (Mf) or 3.75% w/w (®) of CA cross- linker concentration (all samples were dried under vacuum at 30°C for 24 h and then reacted at 80°C).
Figure 9 Equilibrium swelling ratio versus reaction time of HEC polymer with 10% w/w (M) or 20% w/w (@) of CA crosslinker concentration (both samples were dried under vacuum at 30°C for 24 h and then reacted at 80°C).
Figure 9 Equilibrium swelling ratio versus reaction time of HEC polymer with 10% w/w (M) or 20% w/w (@) of CA crosslinker concentration (both samples were dried under vacuum at 30°C for 24 h and then reacted at 80°C).
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