Drying from solutions

Disordered conformation. Figure 5 shows electron micrographs of xanthan D and F obtained from xanthan vacuum-dried from solutions yielding the disordered conformation. The various molecular assemblies are assigned as follow I = single -stranded, II = perfectly matched double stranded, III = branched from double - to single stranded. This assignment will be discussed below. 

Figure 5. Electron micrographs of xanthan sample F (A), and D (B) obtained from replicas of xanthan vacuum dried from solutions containing 0.1 mM ammonium acetate, 50 % glycerol and 3-10 ug /ml polymer. Specie designation I = single-stranded, II = perfectly matched doublestranded, III = branching from double- to single-stranded. Scale bar = 200 nm.

Structural studies of amylose have, in turn, revealed a wide range of crystalline polymorphy, both in chain conformation and in crystalline packing. An example is the group of V-amyloses that exist as complexes with small organic molecules, water, or iodine. The latter complex is particularly interesting because it displays an intense blue color. The V-amyloses can be prepared by precipitation or drying from solution, and they crystallize readily. Consequently, their crystal structures are of interest in connection with any regenerated form of starch material. 

Dried from Solutions of Hydrogen Bonding Capability... 

Figure 6.23. The circular dichroism (in relative unit) spectra of PMLG films dried from solutions of the polymer in (1) dichloroethane (2) dichloroethane/dimethyl formamide, and (3) chloroform/dimethyl formamide. (From Uematsu and Uematsu, 1984.)...

In effect, the physical chemistry of drying from solution is a problem in mechanics. In practice, instead of warping the substrate, a typical lacquer develops stresses (in excess of o as calculated by Equation 12, because some stress relief is brought about by bending) that may exceed a threshold value, whereupon the system ruptures. Inasmuch as the system for all practical purposes is a two-dimensional laminated structure, rupture will occur at or near the interface. Laminar failure does not necessarily signify poor adhesion, for the failure may be cohesive as indicated by ESCA and SEM detection of residual polymer on separated surfaces (66). In some cases, failure is, indeed, adhesive as revealed by the absence of radioactive-tagged molecules (67). 

Drying from Solution. The simplest coatings are those that contain a polymer in a solvent. Organic solvent systems are hardy perennials destined to last for several more decades, until their insult to the environment requires the development of one or more of the new technologies described above. The physical chemistry of drying from solution therefore becomes one s first concern. 

Fusaoka et al. [12] observed a structure composed of fused and deformed polymeric spheres using FE-SEM in a cross section of the membranes, cast and dried from solutions of poly(l-trimethyl-silyl-l-propyne), poly(4-methyl-l-pentene), and some other polymers. 

The vast majority of thermosetting coating powders are prepared by melt muting. Some thermoplastic powders are also produced by this method but most are manufactured by the dry-blend process as shown in Figure 1. Production methods based on spray drying from solution (86) and precipitation from solution (87) have been evaluated but never achieved commercial success because of difficulties in solvent and/or water removal from the powders. Many types of coating powders are still manufactured in small batches, eg, 50-1500 kg, due to differences in color or chemistry, where chemical processes are not economical. 

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