Reexamination of the equilibrium melting temperature of natural rubber crystals

Abstract The existence of an equilibrium melting temperature, T0m, at 28 ± 1°, for unstretched natural rubber has been established, using dilatometric methods. The lower melting temperatures previously observed are a consequence of the low temperatures of crystallization and the rapid heating rates employed. From melting point studies of mixtures of the polymer with low molecular-weight diluents, the heat of fusion per repeating unit, ΔHu has been evaluated as 15.3 ± 0.5 cal./g. The values of ΔHu and T0m have then been combined with data of other workers to obtain the following information concerning natural rubber: (1) The variation of melting temperature with applied hydrostatic pressure has been calculated from the Clapeyron equation to be 0.0465° C/atm. (2) The degree of erystallinity resulting from maintaining a sample at 0° until the rate of crystallization is negligible has been calculated, by three independent methods, to be in the range 26 to 31 per cent. (3) Analysis of the stress-strain-temperature relationship has indicated that crystallization is the cause of the large internal energy changes that are observed at relatively high elongations.

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Nature of Stark Rubber

Rubber Chemistry and Technology ◽

10.5254/1.3542857 ◽

1955 ◽

Vol 28 (4) ◽

pp. 1007-1020 ◽

Cited By ~ 1

Author(s):

Donald E. Roberts ◽

Leo Mandelkern

Keyword(s):

Natural Rubber ◽

Melting Temperature ◽

Melting Behavior ◽

X Ray Diffraction ◽

Equilibrium Melting Temperature ◽

X Ray ◽

Melting Temperatures ◽

Controlled Conditions ◽

Previous Assignment ◽

Diffraction Patterns

Abstract The melting behavior and x-ray diffraction patterns of four different samples of stark rubber have been investigated. The melting temperatures, 39° to 45.5° C, are substantially higher than that observed for natural rubber crystallized by cooling. The x-ray diffraction patterns indicate that the crystallites in stark rubber are oriented. This observation can explain the higher melting temperatures. Thus, the previous assignment of an equilibrium melting temperature, 28° (±1°) C, to unoriented crystalline natural rubber is shown to be appropriate. Several different methods that have been used successfully in preparing stark rubber under controlled conditions in the laboratory are outlined.

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Effect of Deformation on the Crystallization of Supercooled Cured and Uncured Rubbers

Rubber Chemistry and Technology ◽

10.5254/1.3540332 ◽

1964 ◽

Vol 37 (2) ◽

pp. 404-407 ◽

Cited By ~ 2

Author(s):

M. F. Bukhina

Keyword(s):

General Theory ◽

Natural Rubber ◽

Mechanical Stress ◽

Melting Temperature ◽

Crystallization Kinetics ◽

Crystallization Rate ◽

Approximate Expression ◽

Supercooled Liquids ◽

Half Time ◽

Equilibrium Melting Temperature

Abstract 1. The dependence of the polymer crystallization rates on temperature is considered on the basis of the general theory of crystallization kinetics for supercooled liquids. The coefficients in the equations relating the crystallization half time, τ½, to the degree of supercooling are calculated for natural rubber. 2. An approximate expression is obtained which relates the equilibrium melting temperature of deformed rubber with the mechanical stress applied when crystallization starts. 3. The acceleration of crystallization which is induced by deformation is shown to be basically associated with an increase of the equilibrium melting temperature. 4. The possibility of calculating the crystallization rate at all temperatures and stresses from the results of a small number of experiments is established.

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