ABSTRACT:Proton spin-spin relaxation time (T 2 ) has been measured by the pulsed NMR technique for composites of natural rubber and carbon black prepared by Brabender mixing. The amounts and mobilities of tightly bound rubber and loosely bound rubber, which are determined by the free induction decay analysis, change with mixing time. At the first stage of mixing, the amounts of both components increase rapidly and the mobility of the tightly bound rubber decreases. The ratio of the tightly bound rubber to the loosely bound one decreases rapidly with time, reaching a limiting value at the black incorporation time. At the second stage, the amounts continue to increase at a slower rate but the mobilities become independent of the mixing time. At the third stage, no appreciable change occurs in the amount of each component. Thus, the tightly bound rubber appears to be formed very quickly when the rubber segments get in contact with a new active surface of carbon black, followed by the rapid growth of the loosely bound rubber component around the surface of the preformed tightly bound rubber phase.KEY WORDS Natural Rubber I Carbon Black I Mechanical Mixing I Pulsed NMR I Spin-Spin Relaxation Time I Segmental Mobility I Tightly Bound Rubber I Loosely Bound Rubber I Incorporation of surface active carbon black into various kinds of rubber improves the mechanical properties (tensile modulus and strength) as a result of the interactions between carbon black and rubber segments. 1 -3 When the carbon black is incorporated into rubber by mechanical mixing, a significant amount of rubber becomes insoluble due to the strong interactions induced during mixing. 4 This insoluble component is usually called "bound rubber." Pulsed NMR studies 5 -7 have revealed that the bound rubber is composed of two phases having different mobility; i.e., tightly and loosely bound rubber phases. The tightly bound rubber phase is formed around the surface of carbon black particles. The thickness of this phase was estimated to be about 4.5 nm for ISAF grade carbon black, although it changed depending on the nature of carbon black used. 8 Thus, the pulsed NMR studies provided very important information on the structure of the bound rubber phases produced by mechanical mixing. However, no NMR study has been reported on the structural changes during mechanical mixing. In this work, proton spin-spin relaxation time (T2) of the bound rubber was measured as a function of the mixing time by the pulsed NMR technique. On the basis of these results, the mechanism of the bound rubber formation during mixing was discussed in terms of the changes in the segmental mobility and the fractions of the tightly bound rubber (short T 2 ) and loosely bound rubber (long T2) components. The changes were followed by the mixing torque which is one of the most convenient and sensitive measures of the overall structural change during mixing. 149

ABSTRACT:Proton spin-spin relaxation time (T2) was measured by the pulsed NMR technique for bound rubbers in composites prepared from natural rubber and carbon black at different acid group concentrations. The bound rubber consisted of loosely and tightly bound rubbers. By incorporation of the oxidized carbon black with a high concentration of acid groups into natural rubber, the content of the total bound rubber in the composite increased, and the segmental mobility of the loosely and tightly bound rubber constituting the bound rubber was enhanced. The increase in the segmental mobility of the bound rubber and its content resulted from the formation of the bound rubber with fairly long Ioops·on the oxidized carbon black. The change in reactivity of the carbon black brought about nitric acid treatment is also discussed in connection with the results mentioned above.

Proton spin‐spin relaxation times have been measured as a function of temperature for ultradrawn polypropylene with draw ratios λ up to 24. The three relaxation times T2a (the longest), T2i (intermediate), and T2c (the shortest), observed for all the samples, have been ascribed to the relaxations of the amorphous, constrained amorphous, and crystalline components, respectively. T2i and T2a, which reflect the changes in structure and mobility in the noncrystalline regions, decrease with increasing λ; T2i becomes saturated at λ > 9, whereas T2a shows a substantial decrease up to λ = 24. The continued decrease in T2a indicates that the constraint on the amorphous segments keeps increasing up to the highest λ. The associated mass fractions Fa, Fi, and Fc also change with λ. At λ < 9, the increasc in Fi with increasing λ is accompanied by a decrease in Fa, with Fc remaining unchanged. At higher λ, however, Fa is almost constant, and stepwise rises in Fc at about λ = 12 and 24 are accompanied by corresponding drops in Fi. It seems that, in this high draw ratio range, some of the taut molecules are fully extended and are in sufficiently good lateral register to transform into crystalline bridges. This conjecture is supported by the similarity in the λ dependence of Fc and the mass‐fraction crystallinity obtained from the heat of fusion.

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Solid echo NMR measurements were carried out for poly(ethylene sebacate) (2-8), poly(decamethylene 1,16-hexadecanedicarboxylate) (10-16), and chemically degraded 2-8 polyesters. The following results were found: (1) all the samples are composed of three phases: i.e., crystalline, intermediate, and amorphous components; (2) the rand {3relaxations are found in the amorphous component of the 2-8 and 10-16 polyesters, in agreement with earlier conclusions from dielectric measurements; (3) in the {3-relaxation, the chain mobility in the amorphous region and the value of the activation energy increase with increasing ester group concentration in the polyester; but the degree of restriction imposed upon the local mode motion in the amorphous region is greatly influenced not only by the ester group concentration but also by the morphology of the sample; (4) in the chemically degraded 2-8 polyester, the content of the intermediate phase decreases with increasing crystallinity, however, the mobility of this phase is less sensitive to both crystallinity and temperature. KEY WORDS Pulsed NMR I Linear Aliphatic Polyesters I Solid Echo I Poly(ethylene sebacate) I Poly(decamethylene 1,16-hexadecanedicarboxylate) I Chemically Degraded Polyester I Intermediate Component I Chain Mobility I Ester Group Concentration I Morphology I Recently, much attention has been focused on the effects of chemical structure on the morphology of semicrystalline polymers such as polyamides 1 and polyesters. 2 ' 3 It is of interest to investigate the relation between the chemical structure and the relaxation behavior of these crystalline polymers. motion may also be found in the other relaxation mechanisms of the polyesters.