Thermoreversible Crosslinking of Maleic Anhydride- Grafted Ethylene/Propylene Copolymer Using Hydrogen Bonding and Ionic Interactions

C.X.Sun1,2, M.A.J.van der Mee1,3, J.G.P.Goossens*,1,3, M. van Duin4, P.J. Lemstra1

Introduction

Formation of amide-acids and amide-salts

Prevention of imide formation

Conclusions

References

Recycling of materials is becoming more and more important nowadays. This is a major problem for traditional thermoset rubbers, since scrap material and used products cannot be recycled. A solution to overcome this (recycling) problem is to develop elastomers with a thermoplasitc behavior, so-called thermoplastic elastomers (TPEs). Here, the effects of the type and amount of primary amine on the morphology, using SAXS, and the mechanical properties of one kind of TPEs (MAn-g-EPM) are studied, in order to obtain insight in the crosslinking mechanism (s). FTIR spectroscopy is used to study the conversion of MAn-g-EPM with amines and to determine the exact structure of the reaction products.

The amount of alkylamine that is added to the MAn-g-EPM has an important influence on the structure of the functional groups, i.e. it determines whether an amide-acid or an amide-salt structure is formed (Scheme 1).

Three new absorption bands appear upon the addition of 1 eq of hexylamine (C6) to MAn-g-EPM, while the anhydride bands decrease in intensity (Figure 1). Combining the assignment of the characteristic FTIR bands (Table 1), the results indicate that the alkylamide-acids are indeed formed. The intensity of the band around 1710 cm-1 decrease upon addition of 2 eq of hexylamine, indicating the successful formation of the amide-salt structure.

Figure 1. FTIR spectra of MAn-g-EPM modified with different ratios of hexylamine and an excess of NH3.

The spectra of Figure 2 do not change significantly up to 120 ºC, indicating that the amide-acid prevails. A sharp band around 1705 cm-1 appears from 140 ºC onwards, while the bands around 1640 and 1555 cm-1decrease with increasing temperature. This indicates that significant imide formation occurs about 140 ºC for hexylamide-acid. The formation of the imide drastically decreases the polarity of the functional groups compared to the amide-acid, leading to the disappearance of the aggregates for alkylimides (Figure 3) and poorer properties (Table 2).

Figure 3: SAXS profiles of MAn-g-EPM and different imides, viz. NH3-, propyl- (C3) and octadecylimide (C18).

Figure 4 shows representative tensile curves for the alkylamide-acids, while Table 2 shows the average values of the tensile strength (TS), elongation at break (EB) and modulus at 200% strain (σ200) for all materials. All alkylamide-acids have significantly improved tensile properties compared to MAn-g-EPM, due to additional hydrogen bonding (HB). However, the trends in tensile properties, i.e. C18 > C3 ≈ C14 > C10 > C6, are not consistent with the tail length of the primary amines due to distruption of the aggregates by longer apolar alkyl tails and packing of long tails in a crystalline order.

Figure 4: Tensile curves of MAn-g-EPM and different alkylamide-acids, viz. propyl- (C3), hexyl- (C6), decyl- (C10), tetradecyl- (C14) and octadecylamide-acid (C18)

It was shown that imide formation at elevated temperatures leads to a dramatic decrease in properties and the introduction of additional ionic interactions for the alkylamide-salts could not prevent the imide formation (Table 2). The reason for this might be that ionic interactions in the amide-salts are not strong enough. Weiss et al.[1] and Xie et al.[2] show that ionic interactions are significantly stronger for metal counterions than for primary amines. Therefore, potassium hydroxide (KOH) is used for the neutralization of the carboxylic acid group of the hexylamide-acid to prevent imide formation, the results of which can be seen from Figure 5, 6 and Table 2.

• The addition of 1 equivalent of alkylamine results in the formation of amide-acid structure, which is able to form HB, while the addition of an excess leads to the formation of amide-salt structure, for which HB is combined with ionic interactions.• The tensile properties of the amide-acids and amide-salts are significantly improved compared to MAn-g-EPM, due to HB and HB combing with ionic interactions within the aggregates, respectively. • Imide formation, leading to loss of aggregates and poorer mechanical properties, can be prevented by using a different base, i.e. KOH, while improved mechanical properties can be got.

1. Weiss, R.A.; Agarwal, P.K.; Lunderg, R.D. J. Appl. Polym. Sci. 1984, 29, 2719-27342. Xie, H; Xu, J., Angew. Makromol. Chem. 1990, 174, 177-187.

Figure 2. FTIR spectra of hexylamide-acid after CM at different temperatures.

Mechanical properties

Acknowledgments We thank the DUBBLE beamline (BM26) at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France) for the possibility to perform SAXS experiments. Furthermore, we thank the Dutch Polymer Institute (DPI) for financial support under project #346

1Laboratory of Polymer Technology, Department of Chemical Engineering, Eindhoven University of Technology, PO Box 513, 5600MB Eindhoven, The Netherlands

2Department of Macromolecular Science, The Key Laboratory of Molecular Eingineering of Polymers, Fudan University, Shanghai, 200433, People’s Republic of China

3Dutch Polymer Insistute, PO Box 902, 5600 AX Eindhoven, The Netherlands4 DSM Research, PO Box 18, 6160 MD Geleen, The Netherlands

*Corresponding author: E-mail: J.G.P. Goossens@tue.nl, tel:+31 40 2473899, Fax: +31 40 2436999