|MadSci Network: Chemistry|
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: http://www.madsci.org/posts/archives/nov2000/973635799.Ch.r.html The link on that page has expired, but you can see how the polymer chains stack on Wikipedia: http://en.wikipedia.org/wiki/Kevlar Also, the brand homepage is part of Dupont's website: http://www.dupont.com/kevlar/whatiskevlar.html 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: http://www.bbc.co.uk/dna/h2g2/A791246 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!
Try the links in the MadSci Library for more information on Chemistry.