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Name: Maria
Status: educator
Grade: n/a
Country: El Salvador
Date: March 2009


Question:
How do you relate the macroscopic properties of a substance (plastic) to its microscopic structure?



Replies:
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 stringy flow.

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.

Vince Calder


Maria,

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 microscopic structure.

Greg (Roberto Gregorius)



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