Polydimethylsiloxane systems, diffusion

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. 

Single Component System. In a single component system, the drug is encapsulated in its pure form and release rates are essentially zero order (72). Polydimethylsiloxane and polyethylene are the materials most often chosen for encapsulation. Table HI shows some typical release rates reported by Kind, et al. (14), for various steroids through silicone rubber. Clearly, when a solution-diffusion mechanism controls the drug release rate, drug permeabilities can be expected to vary widely. 

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. 

MI-PTR-MS The MI-PTR-MS platform was first explored by Alexander and coworkers [190]. A membrane of polydimethylsiloxane (PDMS) was employed in the MI-PTR-MS as an interface that can penetrate some organic chemicals easily while others may show little or no permeation. With the MI-PTR-MS, the fundamental properties like diffusion and partition coefficients have been determined for different chemical compounds both in nonpolar and polar species [190,191]. If the analyte-membrane interaction is stronger, then the smaller diffusion coefficient and larger partition coefficient will be obtained. The effect of the temperature in the range of 25-65°C on the diffusion and partition coefficient has also been investigated. The detection limit of the MI-PTR-MS system was in the order of tens of ppt, and its response was linear for more than four orders of magnitude. This study demonstrated that MI-PTR-MS has the ability to measure VOCs dissolved in aqueous solutions [191]. 

---