Spreading of Polymer Solution

The dry spreading of polymer solution(66) more complex than that of either the pure liquid or a polymer melt. Assuming the polymer does not adsorb on the surface, the dry spreading of a semidilute solution (from a good solvent) of neutral, flexible chains on a solid surface can be summarized as follows. 

The main polymers used as thickeners are modified celluloses and poly(acrylic acid). Several different modified celluloses are available, including methyl-, hydroxypropyl methyl-, and sodium carboxymethyl-cellulose and their properties vary according to the number and distribution of the substituents and according to relative molar mass of the parent cellulose. Hence a range of materials is available, some of which dissolve more readily than others, and which provide a wide spread of possible solution viscosities. Poly(acrylic acid) is also used as a thickener, and is also available in a range of relative molar masses which give rise to give solutions of different viscosities. 

Corrections for instrumental broadening (also called axial dispersion) are also sometimes applied [15]. This phenomenon arises because of eddy diffusion and molecular diffusion at the leading and trailing edges of the pulse of polymer solution [16]. Hie result is a symmetrical, Gaussian spreading of the GPC... 

SCF technology has spread quickly from molecules such as naphthalene to more complex substances such as polymers, biomolecules, and surfactants. Supercritical fluids can be used to reduce the lower critical solution temperature of polymer solutions in order to remove polymers from liquid solvents(6.26 The technology has been extended to induce crystallization of other substances besides polymers from liquids, and has been named gas recrystallization(4). In other important applications, SCF carbon dioxide has been used to accomplish challenging fractionations of poly(ethylene glycols) selectively based on molecular weight as discussed in this symposium, and of other polymers(. ... 

Ward and co-workers adapted this technique to the preparation of ultrathin slllcone/polycarbonate membranes and advanced it by spreading the dilute solution first on a limited water surface between two plastic blocks and then moving the blocks apart. This method spreads the polymer solution continuously, reducing the thickness of the film. A nitrogen permeability rate of 8.6 x 10 cm (STP)/cm -s-emHg and an 0 /N selectivity of 2.3 are reported for a 1000 X thin film. 

Generally, the polymer solution needs to be filtered before casting and filtration must be done by pressure techniques instead of gravity or vacuum filtration. In a small scale, the polymer film of uniform thickness can be prepared in a laboratory just by spreading the polymer solution over the glass surface and rolled with a glass rod as shown in Figure 1.30. 

Droplets Which Make Fingers (Fig. 1.34). Marangoni instabilities have also been observed in the spreading of a droplet of polymer solution, in the antagonist case where the solvent wets the substrate (S > 0) but the polymer does not S < 0) (Fig. 1.34. Instead of observing a wetting transition at a volume fraction 0w such as S 4> ) = 0, we observe a leak out transition at a composition > w- The solvent is frustrated, because it loves both the... 

Fig. 1.34. Marangoni instability of a dilute drop of polymer solution, (a) The drop spreads too much and too fast. The shape becomes flat, (b) Undulation of the contour is observed coupled to the formation of an unstable rim and digitation, whereas the contact line recedes (dewetting), (c) The figure breaks into droplets and a rim is reformed, which contracts toward the center, (d, e) Interactions of the process are observed, (f) Final stage microdroplets are left on a substrate. [Figure taken from R. Fondeca Macromolecules, Vol. 31, No. 26,1998]...

On the basis of changing values of characteristic viscosity [rj] (which are associated with the rotation and elastic - viscous deformation of macromolecular balls in a stream of solvent) and Hagging s constant one may determine the thermodynamic affinity of solvent to studied polymers or their mixes [2,3]. [rj] is a measure of additional losses of energy during spreading of the solution). Hagging s constant depends on a degree of interaction of polymeric molecules with solvent and is determined by the formula... 

Although the needle did not touch the substrate, a drop of polymer solution clinging to the needle came into contact with the hydrophilic spots. A certain amount of polymer solution was thus transferred and spread onto the circular hydrophilic area. The substrate was then placed on a hot plate to evaporate any remaining solvent. Subsequent UV irradiation led to the formation of covalent bonds between the polymer chains, and a polymer network and resultant microlenses were formed. 

The bubbles of polymer solution will then stay more or less intact unless at that stage the viscosity stays low enough for the solution to spread and wet the surrounding pores, e.g because there is a high solvent concentration in the pores, or the Tg of the polymer is low. 

In the case of polymer solutions, only one component of the binary mixtures suffers firom the restrictions of chain connectivity, namely the macromolecules, whereas the solvent can spread out over the entire volume of the system. With polymer blends this limitations of chain connectivity applies to both components. In other words Polymer A can form isolated coils consisting of one macromolecule A and containing segments of many macromolecules B and vice versa. This means that we need to apply the concept of microphase equilibria twice [27] and require two intramolecular interaction parameters to characterize polymer blends, instead of the one 1 in case of polymer solutions. 

Details of film preparation 10% solution of polymer in 2 parts of CHCI3 and 1 part of CH3OH by weight spreading of the solution on glass plate with the aid of doctor blade film drying details not available. ... 

Asymmetric membranes, which are more important commercially, come in a wide variety of types. I will discuss three types here. The first type of asymmetric membrane is that made by phase inversion. This process, originally developed by Leob and Sourirajan, involves spreading a polymer solution on a moving web. The polymer solution typically contains 20% polymer in a volatile solvent like acetone. Some of the solvent evaporates to form a thin skin of polymer. The moving web then dips into a nonsolvent like water... 

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