Frédéric Bonfils

Characterization of natural rubber using size-exclusion chromatography with online multi-angle light scattering: Study of the phenomenon behind the abnormal elution profile

Natural and synthetic poly(cis-1,4-isoprene) were characterized by size-exclusion chromatography coupled with an online multi-angle light scattering detector (SEC-MALS). Unlike synthetic poly(cis-1,4-isoprene) (SR), natural rubber (NR) samples showed anomalous elution profiles. The beginning of elution was very similar to SR but, after a certain elution volume, the molar masses of the eluting macromolecules increased with elution volume instead of continuing to decrease, which resulted in an upturn curve profile. Adding tetrabutylammonium bromide (TBABr) to THF (solvent and mobile phase) removed this phenomenon. In addition, using different concentrations of TBABr showed that TBABr had two simultaneous actions. TBABr reduced the abnormal elution profiles and the quantity of aggregates (insoluble part or gel). These results mean that the main phenomenon involved in abnormal elution was delayed entities adsorbing on the column packing. Their delayed elution was responsible for the artificial increase in molar masses, especially at high elution volumes. The results obtained suggest that these entities are very compact and have a sphere-like structure.

Relations between Wallace plasticity and Mw for natural rubber

Wallace plasticity and the weight-average molar mass (Mw) were measured on natural rubber samples of different origins. A sigmoidal model describes the relations between Wallace plasticity and Mw (0.872 > r2 > 0.992) for given families of samples. The families of samples analyzed differed through their clonal origin, collection method (cup lumps or latex), and type of processing (CV or non-CV). This study showed that two samples of natural rubber can be identical in terms of plasticity, but very different in terms of the average polyisoprene chain length or Mw.

Micro‐organisms in latex and natural rubber coagula of Hevea brasiliensis and their impact on rubber composition, structure and properties

Natural rubber, produced by coagulation of the latex from the tree Hevea brasiliensis, is an important biopolymer used in many applications for its outstanding properties. Besides polyisoprene, latex is rich in many nonisoprene components such as carbohydrates, proteins and lipids and thereby constitutes a favourable medium for the development of micro‐organisms. The fresh rubber coagula obtained by latex coagulation are not immediately processed, allowing the development of various microbial communities. The time period between tree tapping and coagula processing is called maturation, during which an evolution of the properties of the corresponding dry natural rubber occurs. This evolution is partly related to the activity of micro‐organisms and to the modification of the biochemical composition. This review synthesizes the current knowledge on microbial populations in latex and natural rubber coagula of H. brasiliensis and the changes they induce on the biochemistry and technical properties of natural rubber during maturation.

Comparative study of the mesostructure of natural and synthetic polyisoprene by size exclusion chromatography-multi-angle light scattering and asymmetrical flow field flow fractionation-multi-angle light scattering

This paper presents results from the first analyses of the mesostructure of natural rubber (NR) by asymmetrical flow field flow fractionation (AF4). The results are compared with those obtained by size exclusion chromatography (SEC) in terms of average molar masses, radius of gyration and insoluble part (or gel quantity). Comparable results were obtained for the sample not containing gel. Conversely, for samples with gel, significant differences were found due to the presence of microaggregates. Contrary to SEC, AF4 fractionation enables partial fractionation of polyisoprene chains and microaggregates in a single run without preliminary treatment. The results presented here also highlight the special structure (very compact spheres) of microaggregates in NR compared to chemical crosslinked microaggregates in synthetic polyisoprene. The advantages and drawbacks of both techniques for analysing NR samples are also discussed.

DETERMINATION OF THE REFRACTIVE INDEX INCREMENT OF NATURAL AND SYNTHETIC POLY(CIS-1,4-ISOPRENE) SOLUTIONS AND ITS EFFECT ON STRUCTURAL PARAMETERS

The refractive index increment (dn/dc) of a solution is a constant that indicates the variation of the refractive index with the solute concentration. It is used in the multi-angle light scattering technique to determine the concentration and the weight-average molar mass of polymers. It depends on many parameters, including the structure of the polymer, the solvent, the wavelength of the light and the temperature of the solution. To obtain accurate results, it is necessary to determine the dn/dc at the same condition as for the multi-angle light scattering measurement. In this work, the dn/dc at 633 nm of standard synthetic monodispere poly(cis-1,4-isoprene) (PI) and natural rubber (NR) solutions in THF were determined at 25 and 40°C using an Optilab DSP refractometer. The mean value of the dn/dc obtained for NR and PI was 0.13 mL/g. No significant difference between different types of samples and temperatures (25°C and 40°C) were observed. The nanoaggregates in NR solution that were not retained after filtration through 1 μm filter had no effect on the dn/dc. The intercept value of the conformation plot increased in line with the dn/dc, but the Flory exponent remained unchanged.

Chapter 9.5:Natural Rubber

Natural rubber, India rubber and caoutchouc are all names for the solid elastic material isolated, one way or another, from the ‘milk’ or latex of Hevea brasiliensis, and various other tropical plants like Castilloa elastica. Natural rubber and natural rubber products have been known to the ancient ...

Intrinsic Viscosity Characterization of Poly(cis-isoprene) in Hevea Natural Rubber

Intrinsic viscosity [η] characterizes flow, polymerization, or degradation of dilute polymer solutions. Unlike other high polymers, natural rubber hardly completely dissolves in most organic solvents, and eliminating the insoluble phase undermines the quality of the measurement and its interpretation. On using some natural and synthetic polyisoprene samples, the Huggins and Schulz-Blaschke best-fits were most suitable for estimating [η] for very dilute polyisoprene solutions (< 0.06 g/dL in cyclohexane at 30°C). Logarithmic linearization of the Mark-Houwink-Sakurada equation ([η] = K.Ma; M is molar mass) and a curvilinear model on untransformed data gave different Mark-Houwink constants (K and a) for polyisoprene standards, differences that seemed to have no bearing on their viscosity-average molar masses ( ). Plots of against weight-average molar mass enabled differentiation of the polyisoprenes into larger groupings, differences that could be attributed to different extents of branching or of aggregation between macromolecules.

Effect of Rubber Clone on Fatty Acid Composition and Properties of Air Dried Sheet

Properties of air dried sheet (ADS) made from four different clones of Hevea brasiliensis (BPM24, RRIM600, PR255 and PB235) were investigated. BPM24, RRIM600 and PR255 rubbers show a bimodal molar mass distribution (MMD) while PB235 rubber has quasi-unimodal molar mass distribution. It was found that ADS from rubber clone with bimodal MMD (BPM24, RRIM600 and PR255) displayed lower original plasticity (P0) but higher plasticity retention index (PRI). Those rubber samples with bimodal MMD show higher accelerated storage hardening test (ASHT) values than ADS from PB235. Within bimodal rubbers, ADS from PR255 displayed lower PRI and P than the 2 other clones, i.e. RRIM600 and BPM24. ADS made from BPM24 and RRIM600 presented similar fatty acid composition with 3 major fatty acids namely linoleic acid (C18:2), stearic acid (C18:0) and oleic acid (C18:1). In contrast, major fatty acids of ADS made from PR255 were linoleic acid (C18:2) and furan fatty acid. Moreover, ADS from PB235 showed the highest amount of furan fatty acid which confirmed the results of Liengprayoon et al. [1]. Relationship between mesostructure, fatty acids composition and ADS properties are discussed.

Microbial communities in natural rubber coagula during maturation: Impacts on technological properties of dry natural rubber

To characterize microbial communities present in natural rubber (NR) coagula from Hevea brasiliensis latex during maturation and identify microbial taxa (bacteria and fungi) having an impact on dry NR properties.

Mesostructure Evolution during Storage of Rubber Films from Washed Cream Latex and Skim Latex

Fresh field latex (FL) of Hevea brasiliensis was centrifuged to separate the major compartments: (i) the cream, (ii) the skim plus the C-serum, and (iii) the bottom fraction. Cream was dispersed in distilled water to prepare cream washed latex or CL sample. Skim was collected together with C-serum to prepare the skim latex or SKL sample. Then, films were made from FL, CL and SKL. The mesostructure (macromolecular structure and aggregates or gel) of the natural rubber films was studied by size exclusion chromatography coupled to a multiangular light scattering detector (SEC-MALS). The mesostructure was analyzed on fresh films and on films stored in the laboratory for three months (slow structuring). This study showed that FL and CL films had bimodal elution profiles of long and short polyisoprene chains as revealed by the two peaks on the chromatograms at low and high elution volumes, respectively. On the contrary, SKL films exhibited unimodal elution profiles at low elution volume and a highly abnormal molar masses elution profiles. After 3 months storage, the mesostructure of FL evolved dramatically especially the number-average molar masses (Mn), which increased by 60%. The mesostructure of CL films evolved at a lesser extent and, the mesostructure of SKL films exhibited no significant change. These results indicated (i) the skim rubber particles are not prone to slow structuring during storage in presence of serum and (ii) some constituents either in the serum or/and in the bottom fraction could still be present with cream and be involved in the slow structuring of cream particles latex.