Nonuniformity in Natural Rubber As Revealed by Small-Angle Neutron Scattering, Small-Angle X-ray Scattering, and Atomic Force Microscopy

Abstract

The microscopic structures of natural rubber (NR) and deproteinized NR (DPNR) were investigated by means of small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM). They were compared to those of isoprene rubber (IR), which is a synthetic analogue of NR in terms of chemical structure without any non-rubber components like proteins. Comparisons of the structure and mechanical properties of NR, DPNR, and IR lead to the following conclusions. (i) The well-known facts, for example, the outstanding green strength of NR and strain-induced crystallization, are due not much to the presence of proteins but to other components such as the presence of phospholipids and/or the higher stereoregularity of NR. It also became clear the naturally residing proteins accelerate the upturn of stress at low strain. The protein phases work as cross-linking sites and reinforcing fillers in the rubbery matrix. (ii) The microscopic structures of NR were successfully reproduced by SANS intensity functions consisting of squared-Lorentz and Lorentz functions, indicating the presence of inhomogeneities in bulk and thermal concentration fluctuations in swollen state, respectively. On the other hand, IR rubbers were homogeneous in bulk. (iii) The inhomogeneities in NR are assigned to protein aggregates of the order of 200 A or larger. Although these aggregates are larger in size as well as in volume fraction than those of cross-link inhomogeneities introduced by cross-linking, they are removed by deproteinization. (iv) Swelling of both NR and IR networks introduces gel-like concentration fluctuations whose mesh size is of the order of 20 A.

Kinetics of strain-induced crystallization in natural rubber: A diffusion-controlled rate law

“…This suggests that the natural network in NR, formed by proteins and lipids being attached to functionalized chain ends, supports the formation of crystallites [39,40,41]. Lipids are thought to be the main factor rather than proteins [42,43]. The majority of the lipids, however, are not chemically bonded to the polyisoprene chains [43].…”

Nonuniformity in Cross-Linked Natural Rubber as Revealed by Contrast-Variation Small-Angle Neutron Scattering

“…This is due to the fact that among the nonrubber components, protein gives rise to strong scattering due to its aggregated structure. 16 The scattering length densities (SLD), F, of each component in natural rubber are given in Table 2. It should be noted that the typical average SLD of phospholipids is 0.32 Â 10 10 cm -2 and is almost the same as that of polyisoprene (=0.27 Â 10 10 cm -2 ).…”

“…In spite of these works on the roles of nonrubber components such as the phospholipids and proteins in NR, the question has been still mysterious mainly because even deproteinized NR (DPNR) is known to show excellent properties as crude NR. 15,16 Cross-linking is the most important process in the rubber manufacturing process. There are typically two types of crosslinking, i.e., sulfur and peroxide ones.…”

“…Karino et al carried out SANS study for NR in deuterated toluene (D-toluene) as a solvent and showed that there exist large inhomogeneities originating from protein aggregates, in addition to the blob scattering visualized by this method. 16 They also studied NR samples with different peroxide-cross-link densities. However, the cross-linking density dependence of the polyisoprene network structure was left unsolved.…”

Analysis of structures of rubber-filler systems with combined scattering methods

“…I have not mentioned the analysis of the network structure of rubber by scattering methods, but this is an important subject in rubber-filler systems. [78][79][80] In particular, the inhomogeneity of the network at the micron scale should be clarified.…”