Barbara, The (monomer?) chain length of PDMS is variable. It is possible to purchase PDMS of many different monomer lengths via the viscosity of the solution. The relationship between viscosity and molecular weight is available in the literature: Molecular weight dependence of low shear rate viscosity for poly(dimethylsiloxanes). UTC =3D Silicon Compounds: Register and Review, 5th ed., United Chemical Technologies, Pennsylvania (1993); Lee et al =3D C.L. Lee, K.E. Polmanteer, E.G. King, =93Flow Behavior of Narrow-Distribution Polydimethylsiloxane,=94 J. Polym. Sci., A-2, 8, p1909-1916 (1970); Barry =3D A.J. Barry, =93Viscometric Investigation of Dimethylsiloxane Polymers,=94 J. Appl. Phys., 17, p1020-1024 (1946); Mills =3D N.J. Mills, =93The Rheological Properties and Molecular Weight Distribution of Polydimethylsiloxane,=94 Eur. Polym. J., 5, p675-695 (1969); Rahalkar et al =3D R.R. Rahalkar, J. Lamb, G. Harrison, A.J. Barlow, =93Viscoelastic Studies of Reptational Motion of Linear Polydimethylsiloxanes,=94 Faraday Symp. Chem. Soc., 18, p103-114 (1983); El Kissi =3D N. El Kissi, J.M. Piau, P. Attan=E9, G. Turrel, =93Shear Rheometry of Polydimethylsiloxanes. Master Curves and Testing of Gleissle and Yamamoto Relations,=94 Rheol. Acta, 32, p293-310 (1993); Dow Corning =3D Dow Corning fluids product information.