Morphology of Neutralized Low Molecular Weight Maleated Ethylene−Propylene Copolymers (MAn-g-EPM) As Investigated by Small-Angle X-ray Scattering

Abstract

The thermoreversible nature of the ionic associations in aggregates makes ionomers with a low glass transition temperature interesting candidates for thermoplastic elastomers. In this paper, a low-T g ionomer based on low molecular weight ethylene−propylene copolymers modified by maleic anhydride (MAn-g-EPM) is introduced, and the morphology of a series of ionomers, neutralized with Cs+, Na+, K+, Li+, Zn2+, Ba2+, and Mg2+, was investigated with small-angle X-ray scattering (SAXS). To determine the size and composition of the ionic aggregates, the observed SAXS peak was interpreted with the help of the Yarusso−Cooper model, which describes the ionomer morphology by spherical aggregates of the ionic species with a high electron density surrounded by a layer with a restricted mobility. The results from the ionomer precursors suggest that there is a critical concentration above which aggregation of the polar groups in the apolar matrix occurs. Upon increasing degree of neutralization, the average dimension of the aggregate remains almost constant, while the restricted mobility layer increases. The size of the aggregates is much larger in comparison to other ionomer systems, and as a consequence, the number of acid groups within an aggregate is much larger. For low degrees of neutralization, it was shown that the aggregates contain a large fraction of EPM fragments of at least 60 vol %. For the ionomers neutralized with divalent cations, it was observed that the morphology changes drastically beyond degrees of neutralization of 50%; the number of the aggregates decreases, and the size increases. This was explained with the help of the coordination mechanism.

Review and critical analysis of the morphology of random ionomers across many length scales

“…In the case of MAH grafting, two carboxylic acid groups are located near one another; while in the case of copolymerization with methacrylic or acrylic acid, ionic groups are approximately randomly distributed along the polymer backbone. In ethylene-propylene copolymers (5.6 mol% acid groups), the acid form shows definite evidence of phase separation in SAXS experiments unlike acid forms of any carboxylic acid copolymerized materials [79,80]. Further, the characteristics of phase separation as probed by SAXS, other than the electron density, do not change greatly upon neutralization for a wide variety of cations indicating that acid groups are almost certainly found in aggregates.…”

“…Form-factor scattering is scattering due to the shape of a scattering object, structure-factor scattering is due to the arrangement of scattering objects in space. The form [84] SPU (Na, Ca, Ni, Cd Zn, Cs, Eu) Effect of polyol type and molecular weight [127] Polyisoprene telechelic carboxylate (Ca, Sr, Ni, Zn, Cd) Effect of cation type [126] SPU (Na) Effect of polyol type and molecular weight, various extension of Yarusso-Cooper model tested [128] SPU (Na) Using deformation to evaluate models [81] EMAA (Na, Zn, Cu, Fe), SMAA (Na) Effect of ion content, neutralization level, and ion type [129] EMAA (Na), SMAA (Na) Effect of temperature and humidity [79] MAH ionomers (Zn, Cs, Li, K, Na, Ba, Mg) Cation type [80] MAH ionomers (Zn) Neutralization level [57,130] SMAA (Cu) Compare Yarusso-Cooper and STEM [124] SMAA (Cu), 3-methylstyrene-MAA (Cu) Sample preparation method [131] Sulfonated poly(vinylidene fluoride) (Na) Check to see if Yarusso-Cooper model fit pattern factor of monodisperse small spheres can change the location of the peak maximum substantially; however, a reasonable polydisperse size distribution does not. Hence, it is probably perfectly acceptable to calculate an average spacing between scattering objects from the position of the SAXS peak.…”

Effect of Rubber Polarity on Cluster Formation in Rubbers Cross-Linked with Diels–Alder Chemistry

“…For EPM- g -MA and EPM- g -furan the scattering peak is observed at a scattering vector values ( q ) of 0.057 Å –1 , which is in good agreement with data reported before. 9 , 17 The broad scattering peak confirms the microphase separation of the grafted anhydride and furan groups into MA- and furan-graft rich domains, which is driven by the large polarity difference between the polar MA and furan grafts and the apolar EPM polymer backbone (the solubility parameters of EPM and EVM are 16 and 22 MPa 0.5 , respectively, while those of MA, furan, and BM are 28, 27, and 24 MPa 0.5 , respectively). 17 For EVM- g -MA and EVM- g -furan such a scattering peak is not observed, which confirms that the polar vinyl acetate in EVM impedes the formation of MA-graft rich clusters.…”

“…The related SAXS peak is known to shift to lower q values upon such an increase in cluster size. 9 , 36 , 37 …”

Characterization of maleic anhydride-grafted ethylene–propylene–diene terpolymer (MAH-g-EPDM) based thermoplastic elastomers by formation of zinc ionomer

“…Ionomers have been characterized by rheological properties, dynamic mechanical properties, infrared spectroscopy (IR), smallangle X-ray scattering (SAXS), nuclear magnetic resonance spectroscopy (NMR), and X-ray absorption spectroscopy (XAS) [29][30][31][32][33]. Datta et al [29] analyzed maleated EPDM using IR and reported that neutralization of maleated EPDM by zinc oxide resulted in an ionic elastomer.…”