Macromolecular Systems I -- Introduction

Last revised:  Sunday, August 22, 2010


Cheap substitutes or gateway to nanometer-scale technology?   

    Polymers are often considered cheap substitutes for natural materials, therefore boring.  Who prefers the fake wood trim in a Taurus to the real wood in a Jaguar?  Yet polymers are actually much more than cheap substitutes.   Strong fibers such as Kevlar provide unprecedented strength for light-weight materials.  Other polymers modify the characteristics of motor oil to help provide good viscosity characteristics at both high and low temperature.  Dendrimers represent precision macromolecules--beautiful molecular flowers with untold potential for drug delivery, new electrochemistry, coatings and structural materials.  Many students nevertheless abhor the production and widespread use of polymers; a styrofoam cup thrown carelessly away will last a very long time, whereas the paper cups we used 40 years ago would decay naturally.  Even if you are one of those students, stay in this class because the paper cup is also made of structural biopolymers.  Other biopolymers perform daily miracles by catalyzing reactions that let us burn fuel without burning up.  

    Polymer means "many parts".  At LSU, we usually prefer to speak of macromolecules--large molecules.  The historical reason for this is that our strengths were traditionally in biopolymers, and the biological crowd did not wish to be tainted with those cheap substitutes made from oil.  In truth, synthetic and naturally occurring macromolecules require you to learn many of the same concepts.   After taking MS-I and MS-II, you will know the terms below (and others).  All people who work in macromolecules need these terms, be they chemists, biochemists, or engineers.  For this reason, some people think Macromolecules is a separate discipline.  At LSU we don't go that far.  The goal is to become an expert chemist (chemical engineer, biochemist, whatever) who is also familiar at how to work with those from other disciplines.  

Macromolecular Jargon What it Means
   
Random coil  
Persistence length  
Polydispersity index  
Radius of gyration  
Hydrodynamic radius  
Free-draining  
Lattice theory  
Partial specific volume  
Binodal  
Spinodal  
Kramers-Kronig relationship  
Reaction injection molding  

    Most chemists know a molecule when they see it, but it's a hard thing to define.  Molecules have definite composition, but so do compounds.  You almost never encounter a single "instance" of NaCl--one sodium and one chlorine atom.  In some tiny sample, you might have something like Na5333333888Cl5333333888.   In molecular substances, you really can get little chunks of matter with small integer numbers of atoms, like C2H4.  The six atoms in this molecule (ethylene) hang together, essentially forever.  The ethylene molecules can join together to make polyethylene, one of the common polymers most responsible for the impression that polymers are cheap--so cheap you use them to wrap garbage.    

    m(C2H4)   [C2H4 ]m 

When is a molecule big enough to be considered a macromolecule?   There is no specific answer, but let's say things get interesting at masses in excess of 10,000 or so.  For the polyethylene above, this means m ~ 360.   Macromolecules can be much more macro than that!  The deoxyribonucleic acid (DNA) of the lung fish has a mass of ~69,000,000,000,000 g/mol.  Let's convert that to tons/mol: 

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Why do we (and nature) need big molecules?  Because "hanging together" is very useful.  Houses are not made of sand, but of bricks which are made of sand.  If we had to assemble every grain of sand, construction would be greatly slowed.  Also, the house would tend to fall down or be blown away easily by wolves.  

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A fast process to make bricks is therefore essential to construction.  Similarly, if we want a water-impervious barrier, we might combine some amino acids (or ethylenes) into large molecules before trying to assemble it. 

 

How much of polymeric materials do we use?  Download the spreadsheet to see! 

Homework Assignment.  Fill in the blank yellow column in tons/person on the spreadsheet.  Do this efficiently in Excel.  If you don't know how to use Excel or where to get it, see me or the TA. 


A sordid history

   Macromolecules are the Rodney "I get no respect" Dangerfield of chemistry.  Your book by Elias has a nice, short and cleverly written history that I won't even try to improve.  Briefly, consider the following weird discoveries about polymers that suggest they are not molecules at all, but perhaps loose assemblies of small and probably impure stuff.

 

Enter Staudinger (Nobel Prize, 1953).  Staudinger could not afford one of the spiffy new analytical ultracentrifuges that the Swede, Theo Svedberg, had used to measure true molecular weights, especially of biopolymers, with a solid thermodynamic basis and without some of the difficulties of the other methods.  He relied on a very simple hypothesis:

If secondary, physical forces hold  polymers together, they should eventually go away if we continually dilute the polymer solution. 

Moreover, Staudinger's poor finances were good luck.  He elected a rapid, highly precise technique to test for "polymerness".  This was viscosity, which can be measured quickly with fabulous precision.  Only recently have any macromolecular characterization methods begun to approach the precision of viscosity--and then only for small molecules.  (I refer to MALDI-TOF mass spectroscopy). 

Hermann Staudinger, 1881-1965                             1953 Nobel Prize in Chemistry

 

Caveat:  this Staudinger-centric view of history is challenged by some notables, such as Charles Tanford, who gives more credit to protein physical biochemists (such as Svedberg) for establishing the existence of very large molecules.