Page 330 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Naturally Occurring Polymers—Plants 293

Amylose typically consists of more than 1,000 d-glucopyranoside units. Amylopectin is a
larger molecule containing about 6,000–1,000,000 hexose rings essentially connected with
branching occurring at intervals of 20–30 glucose units. Branches also occur on these branches
giving amylopectin a fan or tree-like structure similar to that of glycogen. Thus, amylopectin is
a highly structurally complex material. Unlike nucleic acids and proteins where specifi city and
being identical are trademarks, most complex polysaccharides can boast of having the “mold
broken” once a particular chain was made so that the chances of finding two exact molecules is

very low.
Commercially, starch is prepared from corn, white potatoes, wheat, rice, barely, millet, cas-
sava, tapioca, and sorghum. The fraction of amylose and amylopectin varies between plant species
and even within the same plant varies depending on location, weather, age, and soil conditions.
Amylose serves as a protective colloid. Mixtures of amylose and amylopectin, found combined in
nature, form suspensions when placed in cold water. Starch granules are insoluble in cold water

but swell in hot water, first reversibly until gelationization occurs at which point the swelling is
irreversible. At this point, the starch loses its birefringence, the granules burst, and some starch
o
material is leached into solution. As the water temperature continues to increase to near 100 C,
a starch dispersion is obtained. Oxygen must be avoided during heating or oxidative degradation
occurs. Both amylose and amylopectin are then water soluble at elevated temperatures. Amylose
chains tend to assume a helical arrangement giving it a compact structure. Each turn contains six
glucose units.
The flexibility of amylose and its ability to take on different conformations are responsible for

the “retrogradation” and gelation of dispersions of starch. Slow cooling allows the chains to align
to take advantage of inter- and intrachain hydrogen bonding squeezing out the water molecules,
leading to precipitation of the starch. This process gives retrograded starch, either in the pres-
ence of amylose alone or combined in native starch, which is more diffi cult to redisperse. Rapid
cooling of starch allows some inter- and intrachain hydrogen bonding, but also allows water
molecules to be captured within the precipitating starch, allowing it to be more easily redispersed
(Figure 9.3).
Most uses of starch make use of the high viscosity of its solutions and its gelling characteristics.
Modification of starch through reaction with the hydroxyl groups lowers the gelation tendencies,

decreasing the tendency for retrogradation. Starch is the major source of corn syrup and corn sugar
(dextrose or glucose). In addition to its use as a food, starch is used as an adhesive for paper and as
a textile-sizing agent.
Oligomeric materials called cyclodextrins are formed when starch is treated with Bacillus mac-
erans. These oligomeric derivatives generally consist of six, seven, eight, and greater numbers of
glucose units joined through 1,4-α linkages to form rings. These rings are doughnut-like with the
hydroxyl groups pointing upward and downward along the rim. Like crown ethers used in phase-
transfer reactions, cyclodextrins can act as “host” to “guest” molecules. In contrast to most phase-
transfer agents, cyclodextrins have a polar exterior and nonpolar interior. The polar exterior allows
guest molecules to be water soluble. The nonpolar interior allows nonpolar molecules to also be

guest molecules. Cyclodextrins are being used as enzyme models since they can first bind a sub-
strate and through substituent groups and act on the guest molecule—similar to the sequence car-
ried out by enzymes.
A major commercial effort is the free radical grafting of various styrenic, vinylic, and acrylic
monomers onto cellulose, starch, dextran, and chitosan. The grafting has been achieved using a
wide variety of approaches, including ionizing and ultraviolet (UV)/visible radiation, charge- transfer
agents, and various redox systems. Much of this effort is aimed at modifying the native proper-
ties such as tensil-related (abrasion resistance and strength) and care-related (crease resistance and

increased soil and stain release) properties, increased flame resistance, and modified water absorp-

tion. One area of emphasis has been the modification of cotton and starch in the production of super-

absorbent material through grafting. These materials are competing with all synthetic cross-linked

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