Abstract
The classical theory of rubber elasticity goes back to the seminal work by Flory 1,2 based on idealization of the polymer chains into Gaussian springs. All interactions between chains are neglected and the response is purely entropic. A number of revised models 3-12 have been proposed dealing with the constraint of the chains on each other (entanglements), but are all entropy based. We report here on an in situ high energy X-ray diffraction study of a prototype rubber Poly-(dimethylsiloxane).(PDMS) under up to 200% elongation, below the threshold for strain-induced crystallization. Due to an unprecedented accuracy we identify novel contributions to the free energy and quantify the associated structural changes. Between chains, at a length scale of 10-50 Å we observe an elastic strain, 3-4 orders of magnitudes smaller than the macroscopic deformation. Remarkably this strain is compressive along the macroscopic tensile direction. Relative changes in the average bond-lengths and –angles are of order 10-4, while the largest energetic contribution to the elasticity of PDMS comes from widening of the associated distributions. Further analysis reveals that the next neighbor SiO bonds are ordered preferentially parallel to the tensile direction and the degree of orientation can be quantified. This is seen as a new route to test models of entanglement.
