Entropic and Energetic Elasticities of Natural Rubber with a Nanomatrix Structure

Abstract

The entropic and energetic elasticities of natural rubber with a nanomatrix structure are investigated by measuring the viscoelastic properties of deproteinized natural rubber (DPNR)-graf t-polystyrene (PS). A nanomatrix structure is formed by graft copolymerization of styrene onto the surface of natural rubber particles, followed by coagulation. The morphology of the nanomatrix structure is observed with transmission electron microscopy. Natural rubber particles with about 1 μm diameter are well dispersed in a nanomatrix of PS. The horizontal shift factor (a T ) and vertical shift factor (b T ) are determined by superposition to create a master curve according to the time−temperature superposition principle. The positive slope of DPNR appears in the plot of b T versus temperature, suggesting that entropic elasticity occurs. In contrast, the slopes of DPNR-graf t-PS are negative at lower temperatures but positive at higher temperatures. The negative slope, which suggests entropic elasticity, may be attributed to the formation of a nanomatrix structure. Natural rubber with a nanomatrix structure realizes both energetic elasticity and entropic elasticity.

Synthetic Rubber with the Tensile Strength of Natural Rubber

“…For instance, the stress value at 100% strain increases with increase in the number of cross-linking junctions per unit volume (i.e., increase in cross-link density) when the rubber molecules are completely cross-linked. − By contrast, when the rubber molecules are incompletely cross-linked, the value of stress at 100% strain for the partially cross-linked polymer is approximately the same as that for the un-cross-linked polymer used as a source because stress concentration occurs in the un-cross-linked portion . In addition, the value of stress at 100% strain increases substantially when the island-nanomatrix structure is formed. , Thus, we must determine the ratio between cross-linked and un-cross-linked portions because this ratio can be converted into the gel content. The gel content is estimated from the weight of the toluene-insoluble fraction and the weight of the whole sample.…”

“…Measurements of the tensile properties were taken with Toyoseiki Strograph VG10E according to standard JIS K6251 . The as-cast films with a thickness of about 1 mm were cut into type 7 dumbbell shapes.…”

Effect of Cross-Linking Density on Horizontal and Vertical Shift Factors in Linear Viscoelastic Functions of Epoxy Resins

“…It has previously been reported that for cross-linked rubbery polymers, the b T value, which was larger than unity, increased with increasing temperature. 34,37 Such an increment of b T corresponds to an increase in the modulus upon heating and is often explained in terms of entropy elasticity, which becomes more dominant at a higher temperature. 37,60,61 Hence, it is most likely that the b T increase originates from the increase in the modulus due to the entropic response of the epoxy network being in the rubbery state in addition to the density change.…”

“…34,37 Such an increment of b T corresponds to an increase in the modulus upon heating and is often explained in terms of entropy elasticity, which becomes more dominant at a higher temperature. 37,60,61 Hence, it is most likely that the b T increase originates from the increase in the modulus due to the entropic response of the epoxy network being in the rubbery state in addition to the density change. Taking into account that the b T increase is more remarkable in the order of ER12, ER10, ER8, ER6, ER4, and ER2, the contribution of the entropic response to the modulus should increase in this order.…”

“…The TTS principle is based on the assumption that the temperature has the same effect on a relaxation process . So far, great efforts have been made to examine whether the TTS principle was applicable to various types of polymer systems such as polymer melts, − blends, − and composites. − Through these works, the time (frequency) and temperature ranges, in which the TTS principle held (or broke down), have become clear. However, there have been few studies dealing with the verification of TTS reliability for cross-linked polymers including epoxy resins.…”