Characterization of Natural Rubber mesostructure

To study NR mesostructure (macromolecular structure + aggregates or gels) (Figure 1), the NR sample is placed in a good solvent of polyisoprene (cyclohexane, toluene or tetrahydrofuran). In addition to a soluble fraction, there is always an insoluble fraction called macrogel (Figure 1) or gel depending on the authors [1-4]. The gel fraction of NR not only consists of an insoluble fraction or macrogel, but also includes a fair quantity of microaggregates or microgels (sphere-like or core-shell structure) distributed in the soluble fraction with the polyisoprene macromolecules (random coil structure) (Figure 1) [5]. The gel in NR formed because of associations between poly(cis-1,4-isoprene) chains due to interactions with non-isoprene compounds (proteins [6-7] and lipids [8]). The characterization of NR mesostructure by separative methods coupled to a multiangular light scattering (MALS) detector is presented. The separative methods used were size exclusion chromatography (SEC) [5,9] and asymmetrical field field-flow fractionation (AsFlFFF) [10]. These two techniques offer the opportunity to fully characterize NR mesostructure (average molar masses, size, microaggregates, conformation, and gel contents). Two examples of AsFlFFF-MALS use are presented: (i) characterization of NR mesostructure from different Hevea brasiliensis genotypes, (ii) characterization of the dynamic structuring of NR. A focus is on microaggregates with sizes smaller than 1 μm (Gel<1μ). They were purified by SEC and analyzed by TEM [11]. The different changes in the microaggregate structure after mastication (thermal or mechanical) were also studied [12]. Lastly, the relation with the protein content and crosslink density of the macrogel [13] is presented. Figure 1: Different types of aggregates (or gels) present in a solution of natural rubber intetrahydrofuran (THF). References[1] PW Allen, GM Bristow, The gel phase in natural rubber, Rubb Chem Technol, 36 (1963) 1024.[2] T Shiibashi, Gel structure characterization of NR and IR and direct observation of individualpolymer molecules by electron microscopy, Intern Polym Sci Technol, 14 (1987) T/33.[3] Y Tanaka, L Tarachiwin, Recent advances in structural characterization of natural rubber, RubbChem Technol, 82 (2009) 283.[4] F Bonfils, A Doumbia, C Char, J Sainte Beuve, Evolution in the Natural Rubber Native Structureand Plasticity Retention Index from the First Tapping of Clonal Trees, J Appl Polym Sci, 97 (2005) 903.[5] C. Kim, M. Morel, J. Sainte Beuve, A. Collet, S. Guilbert, F. Bonfils, Characterization of naturalrubber using size-exclusion chromatography with online multi-angle light scattering: Study of thephenomenon behind the abnormal elution profile, J Chromato A, 1213 (2008) 181.[6] V Grechanovskii, I Dmitrieva, N Zaitsev, Separation and preliminary characterisation of the proteincomponent from commercial varieties of hevea rubber, Int Polym Sci Technol, 14 (1987) 1.[7] T Karino, Y Ikeda, Y Yasuda, S Kohjiya, M Shibayama, Nonuniformity in natural rubber as revealedby small-angle neutron scattering, small-angle X-ray scattering, and atomic force microscopy,Biomacromolecules, 8 (2007) 693.[8] Y Tanaka, Structural characterization of natural polyisoprenes: solve the mystery of natural rubberbased on structural study, Rubb Chem Technol, 74 (2001) 355.[9] C Kim, J Sainte Beuve, S Guilbert, F Bonfils, Study of chain branching in natural rubber using size-exclusion chromatography coupled with a multi-angle light scattering detector (SEC-MALS), Eur PolymJ, 45 (2009) 2249. [10] S Dubascoux, C Thepchalerm, E Dubreucq, S Wisunthorn, L Vaysse, S Kiatkamjornwong, CNakason, F Bonfils, Comparative study of mesostructure of natural and synthetic polyisoprene by SEC-MALS and AF4-MALS, J Chromatogr A, 1224 (2012) 27.[11] S Rolere, Cl Cazevieille, J Sainte-Beuve, F Bonfils, New insights on Natural Rubber microgelstructure thanks to a new method for microaggregates extraction, Eur Polym J, 80 (2016) 117.[12] S Rolere, F Deme, J Sainte Beuve, F Bonfils, Effect of mastication on the structure of microgelpresent in natural rubber, Rubb Chem Technol, under press (2017).[13] S Rolere, S Bottier, L Vaysse, J Sainte Beuve, F Bonfils, Characterization of macrogel compositionfrom industrial natural rubber samples: Influence of proteins on the macrogel crosslink density,Express Polym Lett, 10 (2016) 408.