Identification of Thermoplastic Polymers: Melting Point Analysis by DSC - METTLER TOLEDO

Identification of Thermoplastic Polymers: Melting Point Analysis by DSC

For many practical applications, it is important to be able to quickly and reliably identify polymers. This article describes how semicrystalline polymers can be identified by measuring their melting points using DSC.

 

Introduction

Thermoplastics consist of macromolecules. In the melt, the molecules are randomly entangled. Amorphous polymers are solid at temperatures below the glass transition. The molecules are arranged similar to in a melt. Besides an amorphous phase, semicrystalline thermoplastics also have a crystalline phase in which the molecular segments are arranged almost parallel to one another.

Melting of the crystallites produces broad melting peaks in the DSC curves. The crystallites are relatively small and surrounded by amorphous regions. The corresponding structure is referred to as a rigid amorphous region due to the restricted mobility of the molecular segments near the surface of the folds of the crystals. The rigid amorphous parts are not directly identified in a DSC measurement curve. In addition to these amorphous regions, there are also mobile amorphous regions that exhibit a glass transition (Figure 1).

On heating, the crystalline regions do not melt at a fixed temperature but rather over a temperature range. The melting point of crystallites depends on their size. Small crystallites melt at lower temperatures than large crystallites. Since crystallites of different size are present, polymers always melt over a temperature range.

This is the reason for the relatively broad melting peak observed in their DSC curves. The peak can be characterized by the peak temperature and the peak width.

 

Experimental details

The measurements were performed using a METTLER TOLEDO DSC 2 equipped with an IntraCooler. The samples were contained in standard 40-μL aluminum crucibles sealed with pierced lids. A flat sample of about 5 to 10 mg was prepared. The thermal history of samples was eliminated by performing a heating-cooling-heating cycle and evaluating the second heating curve. The heating curve was measured at a heating rate of 20 K/min; the cooling rate was 10 K/min. Nitrogen was used as purge gas at a flow rate of 40 mL/min.

Conclusions

We have presented a table with data showing the enthalpies of melting and melting points of a number of well-known polymers. These quantities are important for the identification of semicrystalline polymers by DSC. They are useful for quality control and material analysis. In such analyses, the thermomechanical history of the sample should be eliminated before the actual measurement.

This is done by first melting the sample and then cooling it from the melt at a defined rate. In our case, we chose a cooling rate of 10 K/min and a heating rate of 20 K/min. Due to the possible influence of sample mass on the peak temperature, it is important to use approximately the same sample mass for all measurements.

 

Identification of Thermoplastic Polymers: Melting Point Analysis by DSC | Thermal Analysis Application No. UC 448 | Application published in METTLER TOLEDO Thermal Analysis UserCom 44