Solvent uptake

The interaction between polymer matrix and filler leads to the formation of a bound polymer in close proximity to the reinforcing filler, which restricts the solvent uptake [13]. The composites containing acetylated cellulose fillers exhibited higher uptake of toluene compared to water in accordance with their hydrophobic nature. 

The overall sorption value tends to decrease with the addition of the nanoclays. The decrease is maximum for the unmodified-clay-fiUed sample. As the ternperamre of swelling increases, the penetrant uptake increases in all the systems (Table 2.5). The rate of increase of solvent uptake is slower for the unmodified-clay-filled sample compared to the modified one. From Table 2.5 it can be seen that the values are higher for THE compared to MEK in every composite system. The higher sorption can be explained from the difference in solubility parameter of solvent and rubber (9 — 99 and polarity. The solubility parameter value of MEK, THE, and the mbber is 19.8, 18.6, and 14.8 MPa, respectively. This difference is lower (3.8 MPa ) in the case of THE than that of MEK (5.0 MPa ). 

Solutions to the Fickian sorption problem for a variety of systems are given by Crank [142], For a flat sheet with an aspect ratio greater than 10, initially free of solvent, the solvent uptake as a function of time is given as... 

Pure PHEMA gel is sufficiently physically cross-linked by entanglements that it swells in water without dissolving, even without covalent cross-links. Its water sorption kinetics are Fickian over a broad temperature range. As the temperature increases, the diffusion coefficient of the sorption process rises from a value of 3.2 X 10 8 cm2/s at 4°C to 5.6 x 10 7 cm2/s at 88°C according to an Arrhenius rate law with an activation energy of 6.1 kcal/mol. At 5°C, the sample becomes completely rubbery at 60% of the equilibrium solvent uptake (q = 1.67). This transition drops steadily as Tg is approached ( 90°C), so that at 88°C the sample becomes entirely rubbery with less than 30% of the equilibrium uptake (q = 1.51) (data cited here are from Ref. 138). 

As the concentration of MeOH increases, the divergent diffusion behavior between the two membrane types is a reflection of fhe difference in MeOH solubility and its concentration dependence within each membrane. This was verified by solvenf upfake measurements. Upon increasing MeOH concentration, Nafion 117 showed a steady increase in mass, while a sharp drop in total solution uptake was observed for BPSH 40. The lower viscosity of MeOH also affecfs fhe fluidity of the solution within the pores. The constant solvent uptake and the increased fluidity of the more concentrated MeOH solutions accounted for fhe slight increase in diffusion coefficienf of Nafion 117. For BPSH 40, increasing the MeOH concentration resulted in a decrease in MeOH diffusion. The solvent uptake measurements showed very similar behavior, indicating that the membrane excludes the solvent upon exposure to higher MeOH concentrations. 

When a polymer is initially in the dry state, solvent must penetrate into the network by diffusion. When the polymer is rubbery, this diffusion process is rate limiting. If the polymer is in the form of a thin slab, then solvent uptake will initially be correlated with the square root of time [30,31]. When the polymer is in an initially glassy state, swelling kinetics become more complicated [30, 32-34]. While solvent diffusion into the polymer still initiates the swelling... 

The elementary osmotic delivery system consists of an osmotic core containing drug and, as necessary, an osmogen surrounded by a semiper-meable membrane with an aperture (Fig. 7.1). A system with constant internal volume delivers a volume of saturated solution equal to the volume of solvent uptake in any given time interval. Excess solids present inside a system ensure a constant delivery rate of solute. The rate of delivery generally follows zero-order kinetics and declines after the solute concentration falls below saturation. The solute delivery rate from the system is controlled by solvent influx through the semiper-meable membrane. 

Figure 18a,b displays SFM images of SV films that have been prepared from chloroform and from toluene solutions, respectively. The mixed pattern of featureless areas and round-shaped stripes in Fig. 18a can be identified as in-plane lamella and perpendicular-oriented lamellae, respectively. The microstructure prepared from toluene solutions (Fig. 18b) is attributed to P2VP micelles surrounded by the PS shell. The micelle morphology is a result of the SV self-assembly in a selective solvent [119], We have made use of this morphological difference to study the microstructure response to solvent uptake by block copolymer films. 

Ion selective membranes are the active, chemically selective component of many potentiometric ion sensors (7). They have been most successfully used with solution contacts on both sides of the membrane, and have been found to perform less satisfactorily when a solid state contact is made to one face. One approach that has been used to improve the lifetime of solid state devices coated with membranes has been to improve the adhesion of the film on the solid substrate (2-5). However, our results with this approach for plasticized polyvinylchloride (PVC) based membranes suggested it is important to understand the basic phenomena occurring inside these membranes in terms of solvent uptake, ion transport and membrane stress (4,6). We have previously reported on the design of an optical instrument that allows the concentration profiles inside PVC based ion sensitive membranes to be determined (7). In that study it was shown that water uptake occurs in two steps. A more detailed study of water transport has been undertaken since water is believed to play an important role in such membranes, but its exact function is poorly understood, and the quantitative data available on water in PVC membranes is not in good agreement (8-10). One key problem is to develop an understanding of the role of water uptake in polymer swelling and internal stress, since these factors appear to be related to the rapid failure of membranes on solid substrates. 

Conventionally, cross-link density is determined by measurements of the modulus, the glass transition temperature T, and by solvent uptake in swelling experiments. In these procedures, the chemical cross-link density cannot be discriminated from network-... 

Swelling phenomena due to solvent uptake by the films may lead to volume and viscoelastic changes, which limits the information that can be obtained on rigid mass as will be discussed below. 

Scheme 2 Intermolecular bonds are broken and formed in a solvent uptake process (a), while covalent bonds are broken and formed in the reaction of a solid with a volatile substance (b).

In the context of this discussion, it is useful to recall that solvent uptake may be attained by mechanical treatment of unsolvated crystals. Even gentle grinding of a powder product to prepare a sample for powder diffraction may lead to the formation of a hydrated product. 

The examples provided in the previous sections show that one of the means to obtain new pseudo-polymorphs is via solvent uptake. Since the co-crystallizing solvent may be present in the vapour phase (say water), one may regard the process that leads from an unsolvated to a solvated species (and reverse) as a supramolecular reaction whereby a given set of noncovalent bonds (those between molecules in the nonsolvated form, for example) are broken and a new set of noncovalent bonds (those between host and guest molecules) are formed as shown at the beginning of this chapter in Scheme 2. 

The rise in pK pp with a is unexpected because there is no reason to expect the nonidedity characteristics of the a/(l - a) and [Hjg terms in Eq. (25) to change as drastically as this result would appear to imply. As a consequence, the unexpected rise in pK pp has to be attributed to overestimate of Vg in the course of its measurement. Such a possibility has been attributed to the si2able macroporosity of the C-50 gel. Apparently, long-chain polyphosphate ions, the macromolecule used to monitor solvent uptake by the gel [43] through its concentration change in salt solution used to equilibrate with accurately weighed bone dry C-50 samples... 

The polyion domain volume can be computed by use of the acid-dissociation equilibria of weak-acid polyelectrolyte and the multivalent metal ion binding equilibria of strong-acid polyelectrolyte, both in the presence of an excess of Na salt. The volume computed is primarily related to the solvent uptake of tighdy cross-linked polyion gel. In contrast to the polyion gel systems, the boundary between the polyion domain and bulk solution is not directly accessible in the case of water-soluble linear polyelectrolyte systems. Electroneutrality is not achieved in the linear polyion systems. A fraction of the counterions trapped by the electrostatic potential formed in the vicinity of the polymer skeleton escapes at the interface due to thermal motion. The fraction of the counterion release to the bulk solution is equatable to the practical osmotic coefficient, and has been used to account for such loss in the evaluation of the Donnan phase volume in the case of linear polyion systems. 

---