Comprehensive Characterization of Natural Rubber Samples using Thermal Field-Flow-Fractionation coupled with MALS and Triple Detection

Importance of the Topic

Thermal field-flow fractionation (Thermal FFF) in combination with multi-angle light scattering (MALS) and multiple concentration detectors offers a powerful approach for analyzing high molecular weight polymers and biopolymers. This technique enables separation based on thermal diffusion, size, and chemical composition, addressing key challenges in polymer characterization such as shear degradation, column interactions, and gel filtration artifacts.

Objectives and Study Overview

The primary goal was to showcase the capabilities of the TF2000 Thermal FFF system coupled with MALS and triple (or penta) detection for comprehensive characterization of natural rubber (polyisoprene). The study covered method development, instrument configuration, validation with polystyrene standards, and an application example on natural rubber samples.

Methodology

Separation took place in a ribbon-like FFF channel under a controlled thermal gradient (ΔT up to 120 °C) and laminar flow conditions. Key parameters included:

• Solvent: tetrahydrofuran (THF)
• Flow rate: 0.3 mL/min
• Temperature gradient: from 90 K down to 0 K
• Analysis time: 10–120 min

Detection signals were related to sample properties by:

• LS signal ∝ (dn/dc)² · concentration · M_w
• RI signal ∝ (dn/dc) · concentration
• UV signal ∝ ε · concentration
• Viscometer signal ∝ [η] · concentration

From these signals, weight-average molar mass (M_w), number-average molar mass (M_n), radius of gyration (R_g), intrinsic viscosity ([η]), hydrodynamic radius (R_h), and Mark–Houwink plots were derived.

Used Instrumentation

• TF2000 Thermal FFF separation unit
• PN3621 multi-angle light scattering detector
• PN3310 viscometer detector
• PN3211 UV detector
• PN3150 refractive index detector
• PN3510 evaporative light scattering detector

Main Results and Discussion

Application to natural rubber revealed a bimodal distribution with two distinct fractions:

• First peak (13–48.5 min): w-average molar mass ~4.7×10⁵ g/mol, z-average R_g ~42 nm, fractal dimension ~1.78 (stiff linear chains)
• Second peak (48.5–56 min): w-average molar mass ~3.6×10⁸ g/mol, z-average R_g ~218 nm, fractal dimension ~2.92 (compact, branched or cross-linked structures)

The sample contained ~1.3 % high-molar-mass gel. Overlay plots of molar mass vs. detector signal and R_g vs. detector signal confirmed multimodal separation. Differential and cumulative molar mass distributions further illustrated the broad range of sizes and the presence of nanoscale impurities differentiated by chemical composition.

Benefits and Practical Applications

Key advantages of Thermal FFF with multiple detection include:

• Enhanced resolution for ultra-high molar mass species
• No shear-induced degradation or stationary phase interactions
• Elimination of gel filtration and column clogging
• Two-dimensional separation combining size and chemical composition
• Flexible fractionation power via adjustable thermal gradients

These strengths make the approach well suited for characterizing rubbers, polymers, latices, gels, cross-linked networks, and complex biopolymers.

Future Trends and Possible Applications

Ongoing developments may include advanced two-dimensional FFF coupling, integration with elemental analysis (e.g., FFF-ICP-MS), expanded detector arrays, and automated workflows. Emerging applications span copolymer analysis, quality control of industrial polymers, biopolymer conformational studies, and detailed investigation of cross-linked or branched architectures.

Conclusion

Thermal FFF combined with MALS and multi-detection provides a versatile and high-resolution platform for polymer and biopolymer characterization. By overcoming limitations of traditional SEC/GPC, this technique unlocks new insights into molecular weight, size, structure, and composition, supporting research and quality control in materials science.