Determination of Polymer Crystallinity with DSC Measurements - METTLER TOLEDO

Determination of Polymer Crystallinity with DSC Measurements

Introduction

A large number of polymers are able to form crystalline structures in which parts of the macromolecules are orientated parallel to one another. Unlike low molecular weight substances, the degree of crystallinity that can be achieved by polymers is much lower than 100 percent and depends on the molecular structure. Besides crystallites, amorphous regions can also be formed in these materials.

In practice, one distinguishes between two kinds of amorphous regions in partially crystalline polymers based on the different types of molecular mobility possible. The mobile amorphous regions are between the crystallites. They also determine the step height of the glass transition. At the surface of the crystallites there are rigid amorphous regions that exhibit an amorphous structure but despite this do not take part in the glass transition because of their reduced mobility (see UserCom 11).

The temperature at which crystallization occurs, how rapidly it takes place, and the degree of crystallinity achieved, depend on the molecular structure of the sample. The size of the crystallites formed during crystallization depends on how easily the polymer chains fit into the crystal structure. In general, polymer chains are less mobile at lower temperatures, and only small, less stable crystals are produced. These crystals have a low melting point. The mobility of the molecules is greater at higher temperatures so that larger, more perfect crystals are formed that melt at higher temperatures. The melting curve of a partially crystalline polymer therefore contains information on the size distribution of the crystallites present in the material. If the melting enthalpy of a 100 percent crystalline material (∆Hf 100 %) is known, the crystallinity of a sample can be calculated from the area of the melting peak. Table 1 summarizes the typical values of the melting enthalpies for a number of completely crystalline polymers.



Conclusions

The crystallinity of a sample can be estimated from its melting behavior. The melting enthalpy of a pure fictive 100 percent crystalline reference sample presents a problem. In addition, the result is greatly influenced by the choice of the baseline and the integration limits. To obtain reproducible results, it is best to extrapolate the baseline in the melt to the region immediately after the glass transition.

Determination of polymer crystallinity with DSC measurements | Thermal Analysis Application No. UC 134 | Application published in METTLER TOLEDO Thermal Analysis UserCom 13