Amylose triacetate

Fig. 38.—Circular Dichroism of Amylose Triacetate in 1,1,1-Trifluorethanol at Various Temperatures. (Redrawn from Ref. 88.)...

Acetates can be used instead of the free polysaccharide for preparing polysaccharide adducts. For example,17 at concentrations of 0.1 M alkali metal (lithium, sodium, and potassium) hydroxide in 25% aqueous ethanol, amylose triacetate is deacetylated to form adducts containing approximately one molecule of alkali metal hydroxide per three D-glucose residues. Partially crystalline amylose adducts can be obtained by immersing stretched filaments of amylose triacetate in the deacetylating solution. 

The earliest attempts at model analysis of polysaccharides -typified by the x-ray crystal structure analysis of amylose triacetate - were usually conducted in three steps ( L). In the first step, a model of the chain was established which was in agreement with the fiber repeat and the lattice geometry, as obtained from diffraction data. In the second step, the invariant chain model was packed into the unit cell, subject to constraints imposed by nonbonded contacts. This was followed, in the third step, by efforts to reconcile calculated and observed structure factor amplitudes. It was quickly realized that helical models of polysaccharide chains could be easily generated and varied using the virtual bond method. Figure 1 illustrates the generation of a two-fold helical model of a (l- U)-linked polysaccharide chain. 

Figure 2. X-ray fiber diffraction patterns for ("top, left to right) V -amylose VrnlSo-amylose KOH-amylose (bottom, left to right) B-amylose, amylose triacetate I, triethylamylose I-nitromethane complex...

The double-helical structures of native A- and B-amyloses are found in the fourth group. It is interesting that in both h as well as the d and dyg spacings, they are comparable with the structure of amylose triacetate I (ATAI). In part, this may arise because the packing of the bulky acetate substituents in ATAI is similar to the close-packing of two amylose chains into a double helix. In the latter, one chain may act as the "substituent" for the other chain. At any rate, all three structures contain similar, cylindrical-shaped helices. Somewhat unexpectedly, the distances cL and d-yo are very close for the two native polymorphs, even though their unit cells and packing are... 

A series of alkali-amylose complexes can be obtained during the solid-state deacetylation of amylose triacetate, as first described by Senti and Witnauer (32). The unit cells of the individual members of the series of LiOH-, K0H-, NH3OH-, CsOH and guanidinium hydroxide-adducts of amylose appeared to fit an iso-morphous series based on the space group P212 2i> and a tentative crystal structure was proposed (32). The detailed structure of the KOH-amylose complex has now been determined (ll) and the overall structure is similar to that proposed earlier. It is, therefore, likely that all members of the series are isomorphous. 

Figure 12. A section of the amylose triacetate helix showing acetate positions...

SolubUity differences of the triacetates of the purified starch fractions have not yet been investigated in detail. It might be expected, however, that amylose triacetates, which are composed of linear molecules capable of close packing, would be more insoluble than the somewhat loosely packed molecules of amylopectin triacetate. Furthermore, various amylose triacetates, because of their uniform composition, might be expected to show similar solubilities, whereas different amylopectin triacetates, due to the variation of their phosphoric acid contents, might show dissimilar solubilities. 

Meyer and coworkers have investigated the viscosities of amylose and amylopectin fractions obtained by hot water extraction. Such fractions probably are not pure (see page 261). They have shown that the reduced viscosity, of their corn amylose triacetate in chloroform or in tetrachloroethane varies as a linear function of the concentration, C, whereas the reduced viscosity of corn amylopectin triacetate does not. Amylose triacetate exhibits a considerably higher viscosity than amylopectin triacetate of equal molecular weight. This is probably E. O. Kraemer, Ind, Eng. Chem., 40, 1200 (1038). 

Typical stress-strain curves for this unplasticized amylose triacetate film and for film containing 20% dibutyl phthalate plasticizer are shown... 

Fig. 1. Stress-atrain relationship of amylose triacetate film containing...

In general, the properties of these amylose triacetate films are very similar to those of cellulose triacetate films. A comparison of some of their properties is given in Table II. 

Amylose triacetate prepared from butanol-precipitated starch may be plasticized with almost any of the common plasticizers which are applicable to the plasticization of cellulose triacetate. For example, some suitable plasticizers are dimethyl or diethyl tartrate, tributyl citrate, tributyl phosphate, tricresyl phosphate, polyethylene glycol, and pentaerythritol tetraacetate. Addition of only 10-20% plasticizer is sufficient to give amylose triacetate films a useful and lasting degree of... 

Paralleling their film-forming properties, these amylose triacetates can be molded into strong, tough plastics, but the amylopectin triacetates yield only very brittle products. Similarly, Mullen and Pacsu have shown that whole starch acetate produces clear, transparent, molded pieces which, however, are brittle. 

Since amylopectin acetate produces only brittle films and plastics, its molecules probably have a non-linear structure which may be branched or coiled. A further striking difference between amylose triacetate and amylopectin triacetate is that the former can be obtained in the form of a highly fibrous mass, whereas the latter occurs only as a fine powder. 

Fig. 6.—X-ray Diffraotion Diagram of Amylose Triacetate. (Note the x-shaped...

The acetate of potato amylose is more flexible than cellulose acetate, but the persistent length of the latter is significantly higher.2026 Starch triacetate, amylose triacetate, and amylopectin triacetate are reported to be suitable for... 

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