Cross-Linking of Polypropylene via the Diels–Alder Reaction

Abstract

In this work, the possibility of preparing cross-linked polypropylene (PP) via Diels–Alder (DA) chemistry is explored. The overall strategy involves reaction of maleated polypropylene (the starting material), furfuryl amine (FFA), and bismaleimide (BM) as the cross-linking agent. The occurrence of reversible cross-linking was studied by checking the presence of relevant peaks in FTIR spectra, i.e., CH out-of-plane bending vibrations of the furan ring’s peak (γCH) at an absorption band of 730–734 cm−1, CH=CH of the BM aromatic ring’s stretching vibrations (υCH=CH) at an absorption band of 1510 cm−1, and the DA adduct (C-O-C, δDAring) at an absorption band of 1186 cm−1. In agreement with the spectroscopic characterization, the presence of a cross-linked network is also confirmed by rheology, namely the higher storage modulus (G′) compared with loss modulus (G″) value (G′ >> G″), as obtained via temperature sweep. Both the maleic anhydride (MA) content as well as the annealing temperature (50 °C and 120 °C) favor the DA reaction, while only partial de-cross-linking (retro DA) is observed at the higher temperature range of 150–200 °C. In addition, the products show higher mechanical robustness and thermal stability compared to the starting material.

Cross-Linking of Sago Starch with Furan and Bismaleimide via the Diels-Alder Reaction

“…1) with the spectra of the native sago starch, additional peaks at 1721 and 1579 cm −1 which corresponds to the symmetric stretching from the carbonyl group (υ C=O sym) of the ester product and aromatic furan ring absorption (see Supplementary Materials, Fig. S1, the FTIR spectra of methyl-2 furoate), appears in the IR spectra of the starch furoate product [3,26,27]. The presence of the two additional peaks in the spectra of modified starch suggests that the transesterification of starch with the MF step was successful.…”

Cross-Linking of Polypropylene with Thiophene and Imidazole

“…and also with furan grafted product (26.3% wt.) [ 27 ]. Clearly, API is far more reactive compared with TMA and furfuryl amine (FFA).…”

“…Evidently, IMG1 already shows a relatively higher degree of cross-linking prior to annealing, while this is not the case for TG1 ( Figure 7 a,b) and for FG1 (furan based cross-linked products) as in agreement with the FT-IR and solubility measurements (TG1 and FG1 is still soluble in DCB). Detailed experimental results on cross-linking of PP with furan and bismaleimide were already reported in our recent publication [ 27 ]. The difference in reactivity of IMG1 compared with TG1 and FG1 prior to annealing suggests that the cross-linking between imidazole and bismaleimide may take place via a different reaction route than thiophene or furan with bismaleimide [ 27 ].…”

“…Detailed experimental results on cross-linking of PP with furan and bismaleimide were already reported in our recent publication [ 27 ]. The difference in reactivity of IMG1 compared with TG1 and FG1 prior to annealing suggests that the cross-linking between imidazole and bismaleimide may take place via a different reaction route than thiophene or furan with bismaleimide [ 27 ]. It is reasonable to assume that cross-linking of imidazole and bismaleimide proceeds via Michael addition ( Scheme 1 b) instead of Diels–Alder ( Scheme 1 c) reaction.…”

“…Furthermore, we recently reported the successful application of furan—BM as the cross-linking agent using maleated PP (PPgMA) as the starting material. In the report, we demonstrated a significant influence of maleic anhydride (MA) content and annealing temperature on the degree of cross-linking of the final products [ 27 ]. We also noticed that the furan grafting percentage on the maleated PP is relatively low (% grafted = 26%).…”