Structure Evolution of Epoxidized Natural Rubber (ENR) in the Melt State by Time-Resolved Mechanical Spectroscopy

Abstract

In this work, time-resolved mechanical spectroscopy (TRMS) was used to accurately characterize the rheological behavior of an epoxidized natural rubber (ENR) containing 25 mol% of epoxy groups. Conventional rheological tests are not suitable to characterize with accuracy the frequency-dependent linear viscoelastic behavior of materials, such as ENR, in a transient configurational state. For this reason, TRMS was used to determine the true rheological behavior of ENR, as well as to gain some insights into the changes of its macromolecular architecture under the dynamic conditions experienced during the measurements. The constructed master curves for the moduli revealed a gradual transition of the ENR rheological state from liquid-like to solid-like through the formation of an “elastic gel” throughout the bulk of the polymer. Furthermore, the evolution of the stress relaxation modulus revealed a slow relaxation mechanism, resulting from thermally activated reactions in the molten state attributed to the formation of crosslinks. Finally, the crosslink density evolution was estimated from the TRMS data and compared with results derived from equilibrium solvent-swelling measurements. These demonstrated the accuracy of the TRMS data in the prediction of the structural changes that can take place in polymers during processing.

Effect of SiO2 Particles on the Relaxation Dynamics of Epoxidized Natural Rubber (ENR) in the Melt State by Time-Resolved Mechanical Spectroscopy

“…The analysis stems from the approximate power law relationship exhibited by the variation of G’(ω) and G”(ω) with a common relaxation exponent so that the ratio (tan δ) becomes frequency independent and converges and intersects the curves at the gel point, when plotted against time. The multi-frequency plots of tan δ against time for the ENR + SiO 2 system are presented in Figure 10 , from which the intersection point (the gel time) is identified at around 6500 s. For comparison, the measurements performed under the same testing conditions for the ENR sample (matrix component of the ENR + SiO 2 composite of the present study) the gel point appears at around 1500 s [ 31 ]. The estimated increase for the time required to reach the gelation for the composite system can be expected from a possible acceleration of chain scission reactions in ENR the matrix, due to the acidic nature of the SiOH groups on the surface of the silica particles [ 46 , 47 ].…”

“…In this context, it should be noted that we have previously reported the occurrence of thermal-induced gelation reactions (cross-linking) for the same ENR in the pristine state (unfilled) during TRMS measurements performed at the same temperature [ 31 ]. The observed growth trend of the calculated values for static modulus (see Figure 8 B) indicates the occurrence of gelation phenomena with a concurrent increase in the cross-linking degree of the ENR matrix.…”

“…In a previous work, we have explored the potential of a TRMS analysis to evaluate the time-dependent rheological behavior of an epoxidized natural rubber containing 25 mol% of epoxy groups, which experienced structural changes and gelation in the molten state [ 31 ]. The obtained results have indicated an alteration of the polymer relaxation dynamics through the formation of a highly elastic network due to the occurrence of thermal-activated reactions associated with the formation of crosslinks.…”

Rheology, Morphology and Thermal Properties of a PLA/PHB/Clay Blend Nanocomposite: The Influence of Process Parameters

“…6. This representation allows to distinguish any changes in the microstructure of a multi-component polymer-based system through the discrimination of the different relaxation process [56]. The weighted relaxation spectrum (λH(λ)) can be calculated with data coming from small amplitude oscillatory shear measurements, using the method proposed by Honerkamp and Weese [57], which where G′ (ω) and G″(ω) are storage modulus and loss modulus measured through the frequency sweep tests, H(λ) the relaxation time spectrum and λ the relaxation time, respectively.…”

Self-Healable Biocomposites Crosslinked with a Combination of Silica and Quercetin

“…This material is produced by means of epoxidation process, i.e., controlled chemical modification of natural rubber (NR), which is a popular biopolymer derived from the Hevea brasiliensis plant [ 2 ]. However, epoxidized natural rubber has a much more interesting structure as well as improved properties in comparison to its precursor [ 3 , 4 ].…”