Natural rubber strain-induced crystallization

Fourier transform infrared spectroscopy (FTIR) has emerged as a valuable tool for rubber analysis. Siesler monitored the onset, progress and decay of strain-induced crystallization of a sulfur-cured NR during a cyclic experiment. The infrared absorbance of NR at 1126 cm assigned to C-CH3 in-plane deformation vibration is a band sensitive to crystallinity. A thickness reference band was taken to be the 1662 cm one assigned to v(C=C) stretching. The ratio of the absorbance bands at 1126 cm and 1662 cm revealed the reversible nature of strain-induced crystallization of NR. 

Toki, S, Sics, 1., Hsiao, B,S Tosaka, M Poompradub, S Ikeda, Y, et al. (2005) Probing the nature of strain-induced crystallization in polyisoprene rubber by combined thermomechanical and in situ X-ray diffraction techniques. Macromolecules, 38, 7064-7073. 

Alternating isoprene-ethylene copolymers (IER) were prepared with the same catalyst. Due to the strictly alternating sequences of diene and olefin units and the absence of chiral carbon atoms IER shows strain-induced crystallization, but at lower temperatures compared to natural rubber. 

The main conclusions of the strain induced crystallization behavior of high trans polybutadiene based rubber and natural rubber are (1) the rate of crystallization is extremely rapid compared to that of NR (2) the amount of strain induced crystallization is small compared to that of NR, especially at room temperature and (3) for the high trans SBR s relative to NR, crystallization is more sensitive to temperature at low extension ratios, and crystallization is less sensitive to strain. 

The curve shown by a soft and strong material like natural rubber is shown next. The small initial slope and modulus show the material to be soft. At higher elongations, however, strain-induced crystallization occurs and this reinforces the elastomer. As a result its ultimate strength is large and it is therefore quite strong. In other words, one has to be strong to pull it apart. 

Figure 6-13. Stress-strain curves of natural rubber at 0 °C and 60 °C. The rise in stress at 0 °C at A > 3 is attributed to strain-induced crystallization. [After K. J. Smith, A. Greene, and A. Ciferri, Kolloid-Z, 194, 49 (1964), by permission of Dr. Dietrich SteinkopfTVerlag.]...

That crystallization increases the elastic stress has already been demonstrated in Figure 6-8, in which the Mooney-Rivlin plot shows a rise at high extension ratios. However, it should be remembered that part of this increase is due to finite extensibility of network chains. In Figure 6-13 we show the stress-strain curves of natural rubber at two temperatures. At 0 °C there is considerable strain-induced crystallization, and we observe a dramatic rise in the elastic stress above X = 3.0. Wide-angle X-ray measurements show the appearance of crystallinity above this strain. At 60 °C there is little or no crystallization, and the stress-strain curve shows a much smaller upturn at high strains. The latter is presumably due only to the finite extensibility of the polymer chains in the network. 

Raw natural rubber shows high green strength by virtue of strain-induced crystallization. Though natural rubber in the raw form is used in a few applications such as adhesives, binders, and sole crepe, it is not truly thermoplastic, primarily due to its high molecular weight [29]. 

The example chosen here to illustrate this type of composite involves a polymeric phase that exhibits rubberlike elasticity. This application is of considerable practical importance since elastomers, particularly those which cannot undergo strain-induced crystallization, are generally compounded with a reinforcing filler. The two most important examples are the addition of carbon black to natural rubber and to some synthetic elastomers and silica to polysiloxane elastomers. The advantages obtained include improved abrasion resistance, tear strength, and tensile strength. Disadvantages include increases in hysteresis (and thus heat buUd-up) and compression set (permanent deformation). 

Kohjiya, S Tosaka, M Fumtani, M Ikeda, Y Toki, S Hsiao, BS. Role of Stearic Acid in the Strain-Induced Crystallization of Cross Linked Natural Rubber and Synthetic cis-l,4-Polyisoprene. Polymer, 2007, vol.48, 3801 -3808. 

Natural mbber crystallizes on elongation—a phenomenon called strain-induced crystallization—what enhances mechanical properties. However, a filler in the form of carbon black is typically added to natural rubber to additionally modify the mechanical properties. Elastomers which cannot undergo strain-induced crystallization contain even more fillers. Carbon black is used in such cases also, but silicone rubbers are filled with silica. 

In this context rheo-optical FTIR spectroscopy has proved a valuable technique to study the phenomenon of strain-induced crystallization on-line to the deformation process of the elastomer under investigation. Whith the aid of an appropriate absorption band which is characteristic of the threedimensional order in the crystalline phase the onset and progress of strain-induced crystallization during elongation and its disappearance upon recovery can be unambigously monitored simultaneously to the mechanical measurements. Representative for several rubber-like materials which have been investigated by this technique in our laboratory the results obtained with sulfur-crosslinked (1.8 % S) natural rubber (100% 1,4-ds-polyisoprene) and a radiation-crosslinked synthetic polyisoprene (93% 1,4-ds-isomer) lall be discussed in some detail here. 

Strain-induced crystallization also has a pronounced reinforcing effect within the network, and this increases the ultimate strength and maximum extensibility (26). The capability of crystallizing with extension while remaining elastomeric at low elongations provides an important self-reinforcement mode for natural rubber. The only other elastomer with these properties is cis-polybutadiene. These materials are the elastomers of choice for many heavy-duty applications. 

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