Polydimethylsiloxane self-diffusion

Fig. 2. Self-diffusion of component A in a binary solution of polydimethylsiloxanes (10% A, 90% B) as function of the molecular weight of B. Each curve represents a separate value of the molecular weight of A, labelled, (after Ref. M), with permission).

Fig. 3. Self-diffusion of polydimethylsiloxanes in various solvents as function of concentration. Polymer molecular weights varied from 4,6 x 103 (PDS2, top) to 8 x10s (PDS5, bottom), (after Ref. with permission).

In the present paper, after a rapid presentation of the reptation model in its simplest version, in order to pinpoint the underlying hypothesis, we discuss the interest of complementary self diffusion and viscoelastic measurements, and present the currently available methods for measuring diffusion in entangled polymer systems. Then, results obtained on polydimethylsiloxane (PDMS), a model liquid polymer well above its glass temperature at room temperature will be described, and the consequences on the limits of the entangled regime as seen from diffusion measurements, compared to what is observed in rheometry, will be discussed. 

We present here the results of such a systematic investigation on the dependence of the self-diffusion coefficient of flexible polymer chains as a function of P and N, conducted on polydimethylsiloxane (PDMS). This model polymer is well above its glass temperature at room temperature (Ta = - 120°C), so that one can expect that spurious effects associated with the variation of the free volume and of the local monomer-monomer friction coefficient with the molecular weights of the chains are minimised. 

Walderhaug, H., Structures in a Microemulsion System of an Ethoxylated Polydimethylsiloxane Surfactant, Water, and Oil Studied by NMR Self-Diffusion Measurements. J. Phys. Chem. B 2007, 111, 9821-9827. 

Cosgrove, T., Griffiths, P.C., Hollingshurst, J. et al. (1992) Self-diffusion and spin-spin relaxation in cyclic and linear polydimethylsiloxane melts. Macromolecules, 25,6761-6764. 

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