Environmental effects on the mechanical properties of reconstituted collagen hollow fiber membranes

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Abstract
The lateral swelling, changes in length and tensile stress–strain curve of reconstituted collagen hollow fibers, crosslinked with uv irradiation, were measured as a function of the pH and ionic strength of their aqueous environment. From pH 5 to pH 10 the lateral swelling and length are constant; at lower and higher pH the lateral swelling increases rapidly while the fiber shrinks in length. Corresponding to these changes, a pronounced toe develops in the stress–strain curve and extends to a higher strain in more acid (or basic) media. This toe is attributed to the straightening of a microcrimp in the collagen fibrils; the microcrimp is accentuated by the repulsion between the charged fibrils in acid or basic media. Direct small‐angle x‐ray evidence of these microcrimp structure changes at low pH is presented. Increasing the ionic strength of an acid solution by adding NaCl decreases the lateral swelling, increases the length, and decreases the extent of the toe on the stress–strain curve. These changes result from an increased screening of the charge on neighboring fibrils at higher ionic strengths thus decreasing the repulsion between fibrils and the resulting microcrimp. Although the zigzag elastica model of the crimp developed by Diamant et al. [Proc. Roy. Soc., Ser. B, 180, 293 (1972)] provides a good empirical fit to the stress–strain data, it is shown to be inappropriate where the crimp results from repulsive forces between fibrils. A new model taking these forces into account is developed and is shown to be in reasonable agreement with the experimental results.

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