The Advantages of DSC Cooling Measurements for Characterizing Materials

Introduction

Very often, only heating measurements are performed when DSC is used to analyze materials. In many cases, however, heating measurements alone are not sufficient to understand and characterize the properties and behavior of a sample. Cooling measurements are then a simple way to gain valuable additional information.

The data from cooling experiments is extremely useful if you want to:

1. differentiate between materials with different thermal history, for example to distinguish between new and recycled materials;
2. examine differences in molecular structure or composition;
3. rapidly analyze the phase behavior of materials with mesomorphic structures, for example liquid crystals;
4. separate several overlapping thermal effects

This article describes a number of experiments that illustrate the importance of cooling measurements. The experiments were performed using a DSC822e equipped with an IntraCooler and an automatic sample robot.

Differentiation of New and Recycled Polypropylene

The differentiation of new and recycled material is an important aspect of product development and quality assurance. Very often, recycled material and new material differ noticeably in their crystallization behavior.

Figure 1 shows the heating and cooling curves of new and recycled polypropylene (PP) measured at 10 K/min. It can be seen that the heating curves of both samples are practically identical. The cooling curves of the new and recycled PP, however, show significant differences with respect to crystallization behavior. The recycled PP begins to crystallize at a slightly higher temperature. Furthermore, the crystallization peak of the recycled material is broader and not so high as that of new PP. The enthalpies of crystallization (peak areas) are, however, practically the same and are independent of the type of PP.

These results show that the main difference between the two PP samples is to be found in the crystallization behavior. The reason for this is that the content of athermal crystallization nuclei is greater in recycled material.

The cooling curves of several samples of similar weight and prepared in the same way were very reproducible. This means that crystallization behavior can be used as a criterion to distinguish between samples of new and recycled PP.

Characterization of Different Polypropylene Samples

Polymers are often modified structurally or additives are blended with the polymer in order to optimize important properties for a particular application. The following DSC measurements (Fig. 2) show the (second) heating curves and cooling curves of three different polypropylene (PP) samples: a standard PP film, a nucleated PP and a resin-modified PP.

The heating curves do not show any significant differences apart from the small melting peak at 123 °C in the curve of the resin-modified PP. Clearly, it is not possible to distinguish between the three materials in this way. In contrast, the cooling curves show greater differences. As expected, the nucleated PP undergoes crystallization at a higher temperature and also more rapidly than the standard PP. The peak temperature is 116.9 °C, which is about 3 K higher than that of the standard PP. Nucleation does not, however, influence the enthalpy of crystallization of PP. The cooling curve of the resin-modified PP sample shows that the material crystallizes at an even lower temperature and indicates that the resin interferes with and hinders the crystallization process.

In addition, the enthalpy of the crystallization (88.9 J/g) is about 7% less than that of the other two samples.

Characterization of Liquid Crystals

DSC is an excellent method to study the phase transitions of liquid crystalline materials. To rapidly characterize unknown samples (for screening purposes), heating rates of at least 10 K/min are used in order to save time. Under these conditions, the main melting peak (solid-liquid) may overlap some of the liquid-liquid phase transitions, which therefore go unnoticed. In such cases, controlled cooling measurements are a very effective and convenient way to ensure that all the transitions are in fact identified.

^{The Advantages of DSC Cooling Measurements for Characterizing Materials | Thermal Analysis Application No. UC 201 | Application published in METTLER TOLEDO Thermal Analysis UserCom 20}