The Influence of Structure on Autohesion (Self-Tack) and other forms of Diffusion into Polymers

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Abstract
Autohesion is discussed emphasising in particular the influence of polymer chain structure on apparent rates of diffusion. The full range of autohesive levels which exists amongst rubbery polymers is rationalised by proposing a distinction between inter-chain free volume and “intra-chain” free volume, the latter being a collation of free volume regions “contained” in cavities associated with permanent polymer chain structural features which especially apply for certain polymer types. The coincidence of several such cavities plus the intervening inter-chain space causes the formation of holes which may attain sufficient size during normal chain thermal fluctuations to facilitate forward motion of an incoming chain. The concept of free volume has been used to demonstrate the proposed model semi-quantitatively. Comparison with a simple model, involving inter-chain free volume only, is made throughout the calculation. The proposed model, in showing general agreement for several elastomers between free volume considerations and autohesive characteristics, provides an explanation for the large difference in tackiness which exists between natural rubber and ethylene propylene copolymers. The relationship of the proposed model with chain co-operative motion is discussed, and its apparent correlation with the diffusion through polymers of a range of gases and solvents is discussed in some detail.

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