MadSci Network: Chemistry

Re: how large is a single molecule of kevlar?

Date: Wed Nov 16 15:46:41 2005
Posted By: David Akerman, Staff, R&D Scientist, Clear Edge
Area of science: Chemistry
ID: 1126143605.Ch

Hi there, sorry for the delayed reply.

Anyway, to answer help your question I'll refer you to another MadSci page,
which describes the chemical structure of aramids (Kevlar is the brand name
for Dupont's aramid fibres) and the reasons for its high strength:

The link on that page has expired, but you can see how the polymer chains
stack on Wikipedia:

Also, the brand homepage is part of Dupont's website:

As with all polymers, because they are made up of many hundreds or
thousands of monomer units, it isn't usual to think of the size of each
molecule in terms of length. Instead, polymers are usually categorised
according to molecular weight - typically either the average molecular
weight or a range of molecular weights. The molecular weight is the sum of
the weights of all the individual atoms according to the periodic table
i.e. Hydrogen=1 amu (atomic mass unit), carbon=12, nitrogen=14, oxygen-16 etc.

As with most polymers, aramid chains are made from many hundreds of monomer
units giving a chain which maybe 100,000amu or more.

As for actual length, I couldn't find a reported value, but this website
has a list of bond lengths:
So, using these you can work out a rough monomer length of:
2(C-N bonds)+ 2(C-O bonds)+ 2(benzene hexagon width point to point)+
2(angled C-N bonds - approximate this to a single C-N bond)

which all comes to:
(2*147)+(2*142)+(2*280)+147 = 1285pm (pico-metres)

A pico-metre is a millionth of a micron (a hair is fifty microns (or fifty
millions of a metre) wide) so the monomer molecule is obviously tiny to say
the least. However, if you string a thousand of these together, you would
have a polymer strand 1,285,000pm long which is 1.285microns. In theory you
could see something of this size with an electron microscope, but I would
imagine separating a single, unbroken polymer chain from be nigh-on impossible.

Anyway, if you want to see some really large molecules, search for natural
molecules such as haemoglobin or chlorophyll. Natural molecules can be
chains of the order of a million amus, the sequence of which is coded for
by the DNA within cells. Man-made long chain molecules are prehistoric
compared to the complexity of natural ones!

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