Page 367 - Carrahers_Polymer_Chemistry,_Eighth_Edition

330 Carraher’s Polymer Chemistry

TABLE 10.2
Selected Properties as a Function of Silk Worm Species
Approximate
Silk Worm Species Crystallinity (%) Extension at Break Point (%)
Anaphe moloneyi 95 12.5
Bombyx mori 60 24
Antherea mylitta 30 flows then extends to 35

Source: Coates and Carraher, Polymer News, 9(3):77 (1983). Used with permission.

are made of viscid silk that is strong, stretchy, and is covered with droplets of glue. The frame threads
are about as stiff as nylon-6,6 thread and on a weight basis stronger than steel cable. Capture thread is
not stiff but is more elastomeric-like and on a weight basis about one-third as strong as frame thread.
While there are synthetic materials that can match the silks in both stiffness and strength there are
few that come near the silk threads in toughness and their ability to withstand a sudden impact with-
out breaking. Kevlar, which is used in bullet-resistant clothing, has less energy-absorbing capacity
in comparison to either frame or capture threads. In fact, when weight is dropped onto frame silk, it
adsorbs up to ten times more energy than Kevlar. On impact with frame thread, most of the kinetic
energy dissipates as heat, which, according to a hungry spider, is better than transforming it into elas-
tic energy, which might simply act to “bounce” the pray out of the web.
The frame threads are composed of two major components—highly organized microcrystals com-
pose about one-quarter of the mass and the other three quarters are composed of amorphous spaghetti-
like tangles. The amorphous chains connect the stronger crystalline portions. The amorphous tangles
are dry, glassy-like, acting as a material below its T . The amorphous chains are largely oriented along
g
the thread length as are the microcrystals giving the material good longitudinal strength. As the frame
threads are stretched, the tangles straighten out allowing it to stretch without breaking. Because of the
extent of the tangling, there is a lessening in the tendency to form micro-ordered domains as the material
is stretched, though some micro-order domain are formed on stretching. Frame thread can be reversibly
stretched to about 5%. Greater stretching causes permanent creep. Thread rupture does not occur until
greater extension, such as 30%. By comparison, Kevlar fibers break when extended only 3%.

The capture threads are also composed of the same kinds of components but here the microcrys-
tals compose less than 5% of the thread with both the amorphous and microcrystalline portions
arranged in a more random fashion within the thread. Hydrated glue, which coats the thread, acts

as a plasticizer imparting to the chains greater mobility and flexibility. It stretches several times its
length when pulled and is able to withstand numerous shocks and pulls appropriate to contain the
prey as it attempts to escape. Further, most threads are spun as two lines so that the resulting thread
has a kind of build in redundancy. The spinning of each type of thread comes from a different emis-
sion site on the spider, and the spider leaves little to waste, using unwanted and used web parts as
another source of protein.
Cloning of certain spider genes have been included in goats to specify the production of proteins
that call for the production of silk-like fibroin threads that allow the production and subsequent cap-

ture of spider-like threads as part of the goat’s milk.
The beta-keratin structure is also found in the feathers and scales of birds and reptiles.

10.2.3.2 Wool
Wool, while naturally existing in the helical form, forms a pleated skirt sheet-like structure
when stretched. If subjected to tension in the direction of the helix axes, the hydrogen bonds
parallel to the axes are broken and the structure can be irreversibly elongated to an extent of
about 100%.

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