Aramids - What is the shape of these molecules?

Question

From a stereochemistry standpoint - what is the shape of an aramid? is there a "general" 3-D shape for the family?

----

Note - i'm not asking for the chemical formula, or even the diagram - i am talking structure, here....3-D structure at the molecular level.

Thanks.

yeah i know what they are and what specific materials are in the group, blah, blah, blah. that's not what i'm asking.

------

i'll just come out and ask what i utimately need to know - is kevlar helical in shape?

if you can't back it up with a source - don't bother.

can you provide a reference for the kevlar-49 helix ? it would seem that the lower strain-to-failure exhibited by 49 vs. 29 would make it the unlikely choice for the helix structure, given a choice between those two.

Answer 1

Aramid is a shortened form of "aromatic polyamide".
They DONOT POSSESS A SPECIFIC 3-D shape.
In fact the aramid family is quite diverse, each having some unique properties.

Although all are SPACE-NETWORKING Polymers.

Sorry i researched it no Helical structure is possible AFTER REFINEMENT.
Although if formed due to certain brige-H bonds they are broken during refining by adding mayerials given below.

Kevlar belongs to the group of polyamides; other compounds that belong in this group are the natural proteins (think of silk with its similar strength/weight properties), and the synthetic Nylon. Another descriptive name for Kevlar is "para-aramid", where para- refers to the linear linking of the two benzene groups in the structure.

The most impressive property of Kevlar is its tensile strength (3.6-4.1 GPa), combined with a low weight. Compared to an equal weight of steel, Kevlar is 5 times stronger. But there are other important features: the material has a high structural rigidity, which means that it does not elongate much by stretching the fiber up to its breaking point. Furthermore, Kevlar has a low electrical conductivity, high chemical resistance. high cut resistance, and it is flame resistant.

The physical properties of Kevlar make it a suitable material for many applications, such as:

body armor: bullet-proof vests and helmets
ropes and cables.
belts and hoses for industrial applications
composites for aircraft body parts, boats, and sporting goods (e.g. skis)
fiber-optic cables for communication, data transmission and ignition.
friction products such as brake pads, clutch linings, gaskets. It is often employed as a replacement for the carcinogenic asbestos
sailing/motorcycle outerwear
adhesives and sealants
The reason for Kevlar's remarkable properties lies in its molecular structure. As mentioned previously, Kevlar has a highly linear structure due to the para-linking on the two benzene groups in the chain. Furthermore, the large benzene groups hinder the twisting of the polymer chain: it is forced into a trans-conformation:

C-R-C C-R-C C-R-C ~
/ \ / \ / \ /
~ N-R-N N-R-N N-R-N

trans-conformation


C-R-C C-R-C C-R-C
/ \ / \ / \
~ N-R-N N-R-N N
/
R
/
N
/
~
cis-conformation
The large benzene (R) groups force the C-N bonds in the trans-conformation, resulting in a linear polymer structure. The cis-conformation, shown at the end of the second chain is sterically hindered; the benzene groups (not to scale) are close to each other, resulting in an energetically unfavorable conformation.

The linear structure of Kevlar results in a highly ordered, or crystalline structure. This ordering makes the Kevlar fibers exceptionally strong; randomly cluttered strands would be easy to pull apart, but lined up they can withstand enormous tensile forces.

Another reason for the tremendous strength of Kevlar is the formation of hydrogen bridge bonds between the O and H groups on the chain. These hydrogen bridges (symbolized by ":") help the structural rigidity of the polymer:


O O O O O O
C-R-C C-R-C C-R-C ~
/ \ / \ / \ /
~ N-R-N N-R-N N-R-N
H H H H H H
: : : : : :
O O O O O O
C-R-C C-R-C C-R-C
/ \ / \ / \
~ N-R-N N-R-N N-R-N-~
H H H H H
The synthesis of Kevlar itself is a relatively simple polymerization. However, because of its tendency to crystallize into rigid structures, it was difficult to process into a useful product. Kevlar does not dissolve easily, and it does not melt (it decomposes at 500 C). Spinning the fibers requires the product to be in a liquid state.

Researchers discovered that the addition of lithium chloride or calcium chloride to the reactant solution prevents the formation of hydrogen bridges. The polymer remains in solution until the product is synthesized by removal of the solvent, and spinning the fibers.

Answer 2

To start with DNA is a double helix, although there is also three stranded DNA. The double helix could be circular (the two ends joined) like bacterial, some viral chromosomes and plasmids, or linear like in eukaryotes. It can be supercoiled-imagine the cord of your telephone as the double helix, with its spiral winding corresponding to the winding of the double helix, and twist it again. If proteins are also involved then you can have even higher organization of the DNA. You can have different types of helical geometries (A,B and Z DNA) with B-DNA being thought to be predominant in cells.