Polymer crystallinity has a huge impact on the physical properties including melting point, density, permeability and storage modulus. Therefore, it is important to understand the extent of crystallinity for a polymer.
Although it is possible to measure most of the manifestations of crystallinity, a direct measure of the extent of crystallinity provides a fundamental property from which other physical properties can be predicted.
Differential scanning calorimetry (DSC) is a method of measuring the flow of heat into or out of a material with respect to time or temperature. Polymer crystallinity can be measured with DSC by quantifying the heat associated with melting (fusion) of the polymer. The heat can be evaluated in terms of % crystallinity by ratioing against a polymer of known crystallinity to obtain relative values, or by ratioing against the heat of fusion for a 100% crystalline polymer sample.
Experimental Method
The DSC consists of a thermoelectric disk with raised platforms over which the reference (usually an empty pan) and a metal pan carrying a sample are placed. As heat is transferred through the disk, the differential heat flow to the sample and reference is monitored by area thermocouples. The sample temperature can be directly monitored by a sample thermocouple. The presence of preheated purge gas further provides additional baseline stability in addition to the desired sample - atmosphere interaction.
This study describes analysis of polyethylene samples over the temperature range ambient to 180°C. The programmed heating rate was 5°C per minute. The environment around the sample consists of nitrogen. As the previous polymer thermal history affects the measured extent of crystallinity, the samples were tested “as received” and after a " thermal treatment" to provide equivalent thermal history to all three samples. The thermal treatment involves heating the samples at 10°C/minute to 180°C, and then controlled cooling at 5°C/minute to ambient.
Results
The melting endotherm for one of the polyethylene samples after the initial "as received" heating is shown in Figure 1. The % crystallinity was calculated using the DSC Standard Data Analysis software based on 290J/g for a 100% crystalline material. The table below includes the summary of results of three samples.
| Sample |
Melt Onset Temperature(°C) |
Melt Peak Temperature(°C) |
Enthalpy (J/g) |
Crystallinity(%) |
| 1 |
121.9 |
132.9 |
195.9 |
67.6 |
| 2 |
121.3 |
132.6 |
194.5 |
67.1 |
| 3 |
122.3 |
131.6 |
180.1 |
62.1 |
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Figure 1. Melting endotherm for one of the polyethylene samples after heating
After "thermal treatment", the three polymers exhibit different crystallinities than initially obtained. The results are shown below.
The results show that the crystallinity and melt profile of samples 1 and 2 are similar, indicating that both the polymers were subjected to similar processing conditions. However, sample 3 exhibits a sharper melt and lower crystallinity denoting different end-use properties and different processing conditions.
The crystallinities of the three polymers were different, post thermal treatment when compared to their initial crystallinities. The results of three polymers were shown below.
| Sample |
Melt Onset Temperature(°C) |
Melt Peak Temperature(°C) |
Enthalpy (J/g) |
Crystallinity(%) |
| 1 |
119.9 |
132.7 |
187.2 |
64.6 |
| 2 |
119.5 |
132.5 |
187.7 |
64.7 |
| 3 |
119.2 |
132.6 |
188.1 |
64.9 |
These results show the elimination of earlier processing thermal history effects. It can be assumed that the polymers would have identical final properties. The methods of optimizing processing conditions can be further explored by allowing polymer samples to different "thermal treatments" in the DSC before the crystallinity determination.
This information has been sourced, reviewed and adapted from materials provided by TA Instruments.
For more information on this source, please visit TA Instruments.