Y. Mao
Cavendish Laboratory, Madingley Road,
Cambridge, CB3 OHE, UK.
We present a calculation of the rubber
elasticity (with finite chain extensibility) of an
isotropic crosslinked network (of freely
jointed chains) under affine deformation.
The result is derived in its full tensorial
notation, and its predictions for general biaxial
deformations are presented. The negative value
of 
, observed experimentally, is explained.