The complex shear compliances of five samples of natural rubber cross linked by dicumyl peroxide and three samples cross linked by sulfur, covering a broad range of average crosslink spacing, have been measured over a frequency range of 0.1 to 1000 c.p.s. and a temperature range from -18 to 55". The data were all reduced to 25" by shift factors calculated from an equation of the WLF type. In the transition zone of frequencies, the viscoelastic functions were closely similar ; the loss tangents of the dicumyl peroxide vulcanizates were identical within experimental error, and those of the sulfur vulcanizates were shifted somewhat to the left on the logarithmic frequency scale with increasing degree of vulcanization. In the rubbery zone, the losses persisted to much lower frequencies than could be expected from configurational rearrangements within individual strands on the basis of current molecular theories, reflecting slow relaxation mechanisms whose presence has also been deduced from other measurements. The magnitude of the loss tangent a t a given frequency in this zone (e.g., 1.6 c.P.s.) increases substantially with the average spacing between cross links in both dicumyl peroxide and sulfur vulcanizates. The possible role of trapped coupling entanglements as a source of the slow relaxation mechanisms is discussed.
By measurement of peak areas in the dioxymethylene region of high‐resolution NMR spectra of partly formalized poly(vinyl alcohol), PVA, the relationship between tacticity of PVA, rate of formalization reaction, and stereochemical configuration of poly(vinyl formal) was studied. Formation of cis‐formal at an isotactic part of a PVA chain is faster than that of trans‐formal at a syndiotactic part. When the reaction is allowed to continue for a long period, the migration of formal rings from syndiotactic to isotactic portions is observed. Partition of formal rings in partly formalized PVA into cis and trans types is discussed in connection with the tacticity of original PVA and degree of formalization. The rate‐determining step of acetalization reaction is also discussed.
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