ISSN:
0022-3832
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
It is well known that the modulus and equilibrium swellling of long-chain polymers are both related to degree of crosslinking. Modulus theory normally assumes that chain segments are completely free at all points except the junctions of chains (crosslinks). In practice, however, there are trapped entanglements and other steric restrictions on the freedom of chains in highly crosslinked polymers. In addition, real polymers may not conform entirely with the Gaussian distributions assumed by elasticity theory. The present paper discusses these effects, including a steric effect whose contribution to modulus is dependent on chain flexibility and on linkage concentration. Similar but smaller modifications appear necessary in the case of equilibrium swelling in solvent. The molecular weight of the polymer is taken into account by an adaption of Flory's theory, and a simple correction is proposed to take account of entanglements. The relation between modulus and equilibrium swelling is considered for natural rubber when vulcanized by peroxides or by sulfur, and an assessment is made of the relative effects of steric hindrance on modulus and equilibrium swelling. This is converted to an absolute estimate of steric effects by calculations based on Moore and Watson's chemical determinatios of crosslinking. The equations derived have a convenient form in which there is a parameter ω to take account of steric effects. The contribution which this makes to the modlulus is not constant because it is determined by the magnitude of ω relative to the molecular weight Mc between crosslinks. The reciprocal of ω is taken to be a measure of chain flexibility. By measurements of modulus and equilibrium swelling over a wide range of crosslinking a measure of the molecular flexibility can be obtained, together with an estimate of the solubility parameter μ for the polymer and solvent. For peroxide cures it is found thatd that μ is roughly constant when toluene is the solvent, but for sulfor vulcanizates this appears to be true only at low states of cure. The evidence suggests that μ changes from 0.42 to 0.45 when a high proportion of sulfur is combined with the rubber. Therefore peroxide cures should be used for comparing the chain flexibility of polymers. For sulfur vulcanizates these changes in μ make swollen modulus the most reliable methode of measuring crosslinking. It is estimated that eight atoms of sulfur are combined for every crosslink that is formed in the early stages of vulcanization. The work described should assist studies of curative agents and of the difference between polymers.
Additional Material:
8 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/pol.1959.1203412742
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