Crystallization natural rubber

Further progress in understanding natural rubber had to await the discovery of more microscopic probes of matter. Jean Perrin (1870-1945) established that ordinary matter is composed of atoms of the currently understood size. What probe is also of this size X-rays or electrons Soon after the experimental field of X-ray scattering had been developed, natural rubber was examined by Johan R. Katz (1880-1938) of Amsterdam [16]. Rubber at rest yielded only an amorphous halo, characteristic of a liquid. When the sample was stretched until it crystallized, sharp features appeared. This confirmed the notion of strain induced crystallization. But, the shock was that the unit cell for crystallized natural rubber was quite small. If... 

Many authors studying the formation of ECC from melts and solutions suggested that preliminary unfolding and extension of macromolecules occurs. Keller and Maehin25 have shown that in all known cases (including such extreme variants as the crystallization of natural rubber under extension and a polyethylene melt under flow) the same initial process of linear nucleation occurs and fibrillar structures is formed by the macromolecu-lar chains oriented parallel to the fibrillar axes27. ... 

Very frequent are the cases of stress-induced crystallizations. A typical case is that of slightly vulcanized natural rubber (1,4-m-polyisoprene) which, under tension producing a sufficient chain orientation, is able to crystallize, while it reverts to its original amorphous phase by relaxation [75],... 

A similar comparison can be made with cis-poly(isoprene), natural rubber, by taking advantage of the fact that the polymer is very slow to crystallize [164], Consequently, the comparison can be made between the supercooled, noncrystalline polymers at 0°C and the semi-crystalline polymer (31% crystalline) at the same temperature. The Tlc values for each of the five carbons involved were again found to be the same for the completely disordered polymer and the semicrystalline one, so that a similar conclusion can be made with regard to their chain structure. 

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. 

Their crystallization behavior compares with natural rubber, as follows (1) their rate of crystallization is more rapid and (2) their amount of crystallinity is temperature dependent, but considerably less strain dependent. These experimental rubbers have excellent green strength and building tack. 

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 stretching of amorphous but crystallizable materials can greatly increase the rate of crystallization in some cases. Natural rubber and polyethylene terephthalate are examples. The stretching of the polymer initially causes the crystallites to grow so that the chains in the crystallites are oriented parallel to the applied stress. Thus the growth of the crystallites... 

C. J. Carman Earlier in your talk you showed the carbon Ti data and NOEF for partially crystalline and amorphous poly-isoprenes. Was this a natural rubber which had been allowed to crystallize to different degrees or was this a synthetic rubber ... 

The most relevant property of stereoregular polymers is their ability to crystallize. This fact became evident through the work of Natta and his school, as the result of the simultaneous development of new synthetic methods and of extensive stractural investigations. Previously, the presence of crystalline order had been ascertained only in a few natural polymers (cellulose, natural rubber, bal-ata, etc.) and in synthetic polymers devoid of stereogenic centers (polyethylene, polytetrafluoroethylene, polyamids, polyesters, etc.). After the pioneering work of Meyer and Mark (70), important theoretical and experimental contributions to the study of crystalline polymers were made by Bunn (159-161), who predicted the most probable chain conformation of linear polymers and determined the crystalline structure of several macromolecular compounds. 

Chain flexibility also effects the ability of a polymer to crystallize. Excessive flexibility in a polymer chain as in polysiloxanes and natural rubber leads to an inability of the chains to pack. The chain conformations required for packing cannot be maintained because of the high flexibility of the chains. The flexibility in the cases of the polysiloxanes and natural rubber is due to the bulky Si—O and rxv-olelin groups, respectively. Such polymers remain as almost completely amorphous materials, which, however, show the important property of elastic behavior. 

Insoluble Sulfur. In natural rubber compounds, insoluble sulfur is used for adhesion to brass-coated wire, a necessary component in steel-belted radial tires. The adhesion of rubber to the brass-plated steel cord during vulcanization improves with high sulfur levels ( 3.5%). Ordinary rhombic sulfur blooms at this dose level. Crystals of sulfur on the surface to be bonded destroy building tack and lead to premature failure of the tire. Rubber mixtures containing insoluble sulfur must be kept cool (<100°C) or the amorphous polymeric form converts to rhombic crystals. 

Epoxidized natural rubber is still a strain crystallizing mbber and therefore retains the high tensile strength of natural rubber. However, as can be seen from Table 5, in other respects they have very little in common. The epoxidation renders a much higher damping mbber, a much-improved resistance to oil swelling (insofar as a 50 mol % modified natural mbber has similar oil resistance to a 34% nitrile mbber), and much-reduced air permeability. This latest form of modified natural mbber therefore widens the applications base of the natural material and enables it to seek markets hitherto the sole province of some specialty synthetic mbbers. 

Fig. 12. Crystal structure of natural rubber. According to Nyburg, molecules described by coordinates (x, y, 2) (A, B, C) may be statistically replaced by the isoclined molecules having coordinates (x, Vs— y, ) (A, B, C, D )...

Natural rubber exhibits unique physical and chemical properties. Rubbers stress-strain behavior exhibits the Mullins effect and the Payne effect. It strain crystallizes. Under repeated tensile strain, many filler reinforced rubbers exhibit a reduction in stress after the initial extension, and this is the so-called Mullins Effect which is technically understood as stress decay or relaxation. The phenomenon is named after the British rubber scientist Leonard Mullins, working at MBL Group in Leyland, and can be applied for many purposes as an instantaneous and irreversible softening of the stress-strain curve that occurs whenever the load increases beyond... 

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. 

Early on, before the existence of macromolecules had been recognized, the presence of highly crystalline structures had been suspected. Such structures were discovered when undercooling or when stretching cellulose and natural rubber. Later, it was found that a crystalline order also existed in synthetic macromolecular materials such as polyamides, polyethylenes, and polyvinyls. Because of the polymolecularity of macromolecular materials, a 100% degree of crystallization cannot be achieved. Hence, these polymers are referred to as semi-crystalline. It is common to assume that the semi-crystalline structures are formed by small regions of alignment or crystallites connected by random or amorphous polymer molecules. 

By analogy with the works which dealt with cellulose micro crystal-reinforced nanocomposite materials, microcrystals of starch [95] or chitin [96, 97] were used as a reinforcing phase in a polymer matrix. Poly(styrene-co-butyl acrylate) [95,96], poly(e-caprolactone) [96], and natural rubber [97] were reinforced, and again the formation of aggregates or clustering of the fillers within the matrices was considered to account for the improvement in the mechanical properties and thermal stability of the respective composites processed from suspensions in water or suitable organic solvents. 

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