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.

Mn

Mw

Mz

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):

MW distribution by 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.

MW dependence of properties

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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