Chem 421: Introduction to Polymer Chemistry
Polymer Properties and MW
Polymer Properties
- Glass Transition Temperature (Tg)
- The temperature (actually a broad range of temperatures) at which a glassy
polymer softens into a viscous liquid or rubbery phase. On the molecular level,
it is the temperature at which chains in amorphous (i.e., disordered) regions
of the polymer gain enough thermal energy to begin sliding past one another at
a noticable rate. For an amorphous polymer, the
Tg reports the minimum processing temperature. The Tg is
strongly dependant on polymer structure (including stereochemistry), and ranges
from far below 0 °C for very flexible chains, and above 400 °C for
very stiff chains.
The transition is detectable by a variety of methods, including Differential
Scanning Calorimetry (DSC), and Thermal-Mechanical Analysis (TMA). Note that
some authors report the Tg as the onset temperature (i.e., the
beginning of the range), while others report the midpoint of the range.
- Melting Temperature (Tm)
- The temperature (actually a narrow range of temperatures) at which the
ordered regions of a crystalline polymer melt, similar
to a small molecule. Crystallization is essential for many high-performance
polymers because it greatly increases the strength of the material.
Like the Tg, the Tm is detectable by DSC, TMA, and other
techniques.
- Decomposition Temperature
- The temperature above which chemical degradation occurs.
This temperature is conveniently measured by Thermogravimetric Analysis (TGA),
a technique in which one simply weights the sample continuously while heating
it. Once decomposition begins, small molecular fragments are released which
distill away, and the sample loses weight.
- Modulus
- The proportionality constant between stress and strain, and therefore can
be thought of as stiffness.
Molecular Weight
Because virtually all polymers are mixtures of many large molecules, one
must resort to averages to describe molecular weight. Among many possible ways
of reporting averages, three are commonly used: the number average,
weight average, and z-average molecular weights. The weight
average is probably the most useful of the three, because it fairly accounts
for the contributions of different sized chains to the overall behavior of the
polymer, and correlates best with most of the physical properties of interest.
The ratio of Mw to Mn is known as the
polydispersity index (PDI), and provides a rough indication of the
breadth of the distribution. The PDI approaches 1.0 (the lower limit) for
special polymers with very narrow MW distributions, but, for typical commercial
polymers, is typically greater than 2 (occasionally much greater). Here is a
typical MW distribution curve, measured by Size Exclusion Chromatography (SEC):
Dependance of Polymer Properties on MW
Many polymer properties of interest (Tg, modulus, tensile
strength, etc.) follow a peculiar pattern with increasing MW. Small molecules
have small values, then there is a sharp rise in properties as the chains grow
to intermediate size (oligomers), and then the properties level off as
the chains become long enough to be true polymers.
However, a few properties important for polymer processing, like melt
viscosity and solution viscosity, increase monotonically with MW. This means
that the goal of polymer synthesis is not to make the largest possible
molecules, but rather, to make molecules large enough to get onto the plateau
region. Increasing the MW beyond this does not improve the physical properties
much, but makes processing more difficult.
A few properties are dictated by the repeat units alone, and therefore these
are not changed much by MW. Examples: color, dielectric constant, and
refractive index.
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