Thermal Transitions—Thermoplastics and Elastomers

A key feature of any polymer is how it responds to heating. For example, a polymer that on heating becomes much more flexible, perhaps even fluid, can be shaped or molded into a particular form at high temperatures and then rigidified upon cooling so as to maintain the new shape. Processability issues such as these are crucial in polymer science. As with most topics in this field, we cannot hope to cover all aspects of polymer thermodynamics. Here we simply introduce a few key terms that are commonly used and provide valuable initial insights into a polymer s characteristics. 

Generally, polymers are classified as either amorphous or crystalline. A solid amorphous material that has no significant order on the molecular scale is characterized as a glass. Other polymers are characterized as crystalline, but this does not mean that the entire sample is crystalline, as implied by a crystalline sample of a molecular solid. Rather, in polymer science crystalline implies that some small regions of the material, termed domains, are crystalline and have the ability to diffract x rays. Polymers are rarely fully crystalline 

As an example of how molecular weight and poly-dispersity are apparent from such a method, we show to the right the overlay of four GPC traces from various samples of poly (methylmethacrylate). Here the x axis is the elution volume and the y axis is the response of a refractive index detector. The four samples were formed by continued irradiation of the polymer by y rays. Trace 0 was prior to irradiation, and trace 3 was after the highest dose of irradiation. 

Irradiation of the polymer leads to lower molecular weight species as the retention on the column increases. However, we also see that the width of the chromatography traces becomes increasingly narrow with continued irradiation. This indicates more uniform lengths among the polymer strands upon irradiation, meaning a lower polydispersity. How is this explained  

A crystalline polymer above Tg is a flexible thermoplastic. On raising the temperature further, a second relatively sharp transition occurs at the melting temperature (Tm). It is at this point that the crystalline domains melt, and above T the material is a liquid. By definition, an elastomer is a polymer in the temperature range between Tg and T.  

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