Relating Macroscopic to Microscopic Properties
Country: El Salvador
Date: March 2009
How do you relate the macroscopic properties of a
substance (plastic) to its microscopic structure?
This is a LARGE question. The simple answer is CHEMISTRY, but I do not mean that
to be condescending. It covers a wide range of studies.
EXAMPLE(1): What substances
dissolve in which solvents? Sounds easy until you find all the "exceptions".
(2): Molecules of certain structures can turn water into a gel or into a long
Before the last century, say 1850 to 1900 chemists had to determine the structure
of a molecule from the way it reacted with other molecules. There were no
sophisticated tools like spectroscopy available to them. I find that one of the
intellectual "giants" of modern science.
Your specific question about polymers is a special case of building the body of
science. Within a body of polymers there are ways to refine the structure and
properties, changing the chemical components, varying the conditions of their
reaction, altering how they are assembled.
And this is only one of any number of ways of altering the microscopic, molecular
composition and structure and macroscopic properties.
The length of my reply only reflects the richness of possibilities, and the lack of
a simple answers to your inquiry.
This is a very big question. There are many factors at the microscopic level that
contribute to a specific macroscopic property and, depending on the macroscopic
property, certain microscopic property will be more important than others - and at
other situations, less important.
For example, in the case of the bullet resistance of Kevlar, the important properties
include, resonance structures and bond strength, hydrogen bonding and crystal
structure. But in the the bullet resistance of Lexan, while the bond strength is
important, resonance is not, while crystal structure plays some role, there is no
hydrogen bonding to speak of, and bond mobility becomes of the utmost importance.
This gets even more complicated when we try to explain the bullet resistance of
Spectra Fiber which has little to do with bond strength, has no resonance, no bond
mobility, no hydrogen bonding, and more to do with crystal alignments.
I think you will find that while certain macroscopic properties can be explained in
general through the microscopic structure of the polymer, to get a better explanation,
one would have to be very specific about the macroscopic property and the related
Greg (Roberto Gregorius)
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Update: June 2012