The swelling of cellulose acetate networks obtained by crosslinking in solution

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Abstract
In order to study the applicability of existing network theories for rubberlike polymers to crosslinked celluloseacetate networks, a series of gels was prepared by crosslinking cellulose acetate in dioxane solution with increasing amounts of oxalyl chloride. The swelling of these gels in dioxane, tetrahydrofuran, methyl acetate, and ethyl acetate as well as in cellulose acetate solutions of these solvents was determined by measuring their buoyancy in the swelling agent and in mercury on a slightly modified analytical balance. The solvent activity outside the gel in the case of the cellulose acetate solutions was determined osmometrically. An estimate of the extent of crosslinking was obtained by determinations of the oxalic acid content of the gels. On the basis of these experiments, it is found that Hermans' rather than Flory's theoretical expression for the conformational free energy, stored in the swollen network, is applicable. In the literature most experiments have been restricted to networks obtained by crosslinking in the dry state, and subsequently highly swollen. Under these conditions swelling experiments do not allow a discrimination between the two theories. The description of the swelling of crosslinked polymer networks requires at least three parameters, viz., (1) the molecular weight of the polymer chain between crosslinks Mp; (2) the solvent‐polymer interaction parameter x; and (3) the reference degree of swelling q0, in which state the polymer chains have their unrestrained mean square end‐to‐end distance (〈r20〉). The reference degree of swelling q0 varies with the swelling agent and the extent of crosslinking. From data of q0 as a function of (〈r20〉), which occurs at χ = 1/2, can be estimated. For the quantity (〈r20〉/Mp)1/2 the value 870 × 10−11 is found, which is well in line with values obtained for cellulose tributyrate and caprylate from viscosity data. Another estimate of the flexibility of the cellulose acetate chain yields a statistical chain element of about 10 monomer units. The general equilibrium swelling patterns of polymer networks in terms of the three parameters Mp, χ, and q0 are discussed and compared with the observed behavior of the cellulose acetate gels.