Segmental diffusion pressure

B. J. Punchard and D. B. Adolph. Pressure and temperature dependence of the dilute solution segmental diffusion of anthracene-labeled polyisoprene. Macromolecules, 35 (2002), 3281-3287. 

The activation volumes An are also negative as shown in Section E. Mostly the termination reactions are diffusion-controlled. The reactant molecules undergo translational diffusion at first to form a collision pair. It follows the movement of the reactive sites on these molecules into a position favorable to chemical reaction. This process has been considered to be the rate-determining step. The activation volume involved in this so-ealled segment diffusion is observed. When chain termination is governed by translational diffusion, the activation volume, Au j for viscous flow, is the key parameter for the evaluation of the influence of pressure. 

Unlike solid electrodes, the shape of the ITIES can be varied by application of an external pressure to the pipette. The shape of the meniscus formed at the pipette tip was studied in situ by video microscopy under controlled pressure [19]. When a negative pressure was applied, the ITIES shape was concave. As expected from the theory [25a], the diffusion current to a recessed ITIES was lower than in absence of negative external pressure. When a positive pressure was applied to the pipette, the solution meniscus became convex, and the diffusion current increased. The diffusion-limiting current increased with increasing height of the spherical segment (up to the complete sphere), as the theory predicts [25b]. Importantly, with no external pressure applied to the pipette, the micro-ITIES was found to be essentially flat. This observation was corroborated by numerous experiments performed with different concentrations of dissolved species and different pipette radii [19]. The measured diffusion current to such an interface agrees quantitatively with Eq. (6) if the outer pipette wall is silanized (see next section). The effective radius of a pipette can be calculated from Eq. (6) and compared to the value found microscopically [19]. 

The manufacture of evacuated closed-cell PE foam and potential applications of these foams and of gas-exchanged foams, which were exposed to fluorine in helium after evacuation, are described. The evacuated foams were prepared by rapid evacuation in a chamber to a pressure of 5 to 10mm Hg, during which the foam segments initially expanded and then rapidly relaxed to about their original dimensions as gas diffused out. Thermal insulation is considered to be a potential end-use. 3 refs. 

Rejection of protein adsorption to the outermost grafted surface is attributed to a steric hinderance due to the tethered chains. A grafted surface in contact with an aqueous medium, a good solvent of the chains, has been identified to have a diffuse structure [67]. Reversible deformation of tethered chains due to invasion of mobile protein molecules into the layer would lead to a repulsive force which is governed by the balance of entropic elasticity of the chains and osmotic pressure owing to the rise in the segment concentration. The overlapped repulsive force would prevent the direct contact of protein molecules with the substrate surface. 

It now appears that the dissolution dynamics of glassy polymers is at the very least a complicated process and its kinetics depends on many parameters of the solvent, the polymer and the processing condition. We have also found that one of the dynamical processes that strongly affect the dissolution kinetics of polymers with high Tg is the diffusion rate of the solvent into the polymer matrix which primarily depend on at least 4 factors polymer free volume and segmental mobility, solvent size and the osmotic pressure (chemical potential difference between the polymer solution and pure solvent). We believe that the above finding has important implication in resist technology. Positive... 

If the dnjdt versus t 1/2 plots do not extrapolate to zero at infinite time, as required by a purely diffusion controlled process, this indicates the presence of an activation barrier to the desorption. This behavior was rationalized by MacRitchie [16]. As shown in Fig. 2, most of the segments are located at the interface at low surface pressure. The probability that the transition state configuration could oc-... 

Consider the idealized picture of a mass-transfer process based on the two-film theory as shown in Figure 7.27. The partial pressure profile in the gas film is linear, as called for by steady-state diffusion, but the concentration profile in the liquid film falls below a linear measure as a result of a first-order chemical reaction removing the absorbed gas. A normal mass balance over the differential segment dz (we may assume unit area normal to the direction of diffusion) produces a familiar result ... 

---