Differential Scanning Calorimetry

As the name suggests, differential scanning calorimetry relies on a differential in heat flow between a sample of interest and a reference sample with known thermal properties. In fact, measuring heat accurately with a heat meter is very difficult. The measurement is further complicated by the fact that the sample is placed within a pan which also absorbs heat and the measurement typically occurs within a larger furnace. A more accurate measurement would involve monitoring the temperature of a sample and calculating what heat flow must have been present in order to produce the temperature change.

Therefore, DSC involves either the simultaneous or sequential measurement of temperatures of both a sample and a reference. To accurately measure heat in and out of the sample while accounting for thermal contributions and losses to the pan and surrounding environment, the measurement of both sample and reference should occur in the exact same environment and heat conditions. Preparations to the pan should also be consistent between reference and sample. These include crimping to seal the pan and poking a hole in the lid, to allow equilibration with the inert atmosphere in the furnace and avoid pressurization in the pan as phase changes occur in the sample.

A schematic of the DSC sample set-up and heat cell are shown in Figure 1. For each scan, the DSC contains an empty reference pan and a sample pan. The DSC reads the difference in power required to keep the reference pan and the sample pan at the set temperature (defined prior to measurement by the user). The sample pan will require more power to heat when the sample absorbs heat (in an endothermic reaction) and more power to cool when the sample gives off heat (in an exothermic reaction).

Figure 1: DSC sample set-up and heat cell schematic.

An empty pan is placed in the reference position for all DSC measurements. For all thermal characterization techniques, a baseline measurement is performed first with an empty pan inside the furnace in the sample position. This measurement accounts for atmospheric changes and is automatically subtracted from the following sample measurement. For a crystallinity measurement, a precisely measured amount of sample material is placed into a separate pan (which is placed in the sample position in the furnace) and run using the same measurement program as the baseline. Percent crystallinity is calculated using values obtained from the sample measurement. The equation used is:

% Crystallinity =  (Equation 1)

A typical DSC results curve is shown in Figure 2. The heat of melting (ΔHm) is obtained by taking the area under the endothermic peak (present during the heating phase of the measurement) and the heat of cold crystallization (ΔHc) is obtained by taking the area under the exothermic peak (present during the cooling phase of the measurement); accompanying software is used to calculate these values from the sample measurement. The known heat of melting of a 100% crystalline form of the sample (ΔHm°) is a material property that must also be known to calculate polymer percent crystallinity.

Figure 2: Schematic of a DSC results curve. Exothermic and endothermic peaks are labeled.

When performing a heat capacity measurement, one additional step is added: prior to running the sample measurement, a measurement identical to the baseline is performed with a precisely measured amount of a standard material. The standard material should be a compound with well-characterized heat capacity, such as sapphire. The sample material is then run using the same measurement program as the baseline and standard. The heat capacity and heat flow in/out of the sample are also calculated by the user in accompanying software. The baseline measurement is subtracted and the heat capacity of the standard material is used to go from temperature to heat flow.

Figure 3 shows the result of a DSC percent crystallinity sample scan on a polybutylene terephthalate (PBT) polymer sample. The result is displayed as a DSC power reading (in milliwatts per milligram of sample) verses time. The power reading, the blue trace in Figure 3, indicates how much additional power was required to change the temperature of the sample pan in comparison to the empty reference pan. The temperature program is also displayed as the dashed red line in Figure 3. The first peak in the blue trace is an endothermic peak; its area gives a value for the heat of melting of the polymer sample. The second peak is an exothermic peak whose area gives a value for the heat of crystallization of the polymer sample.

Figure 4 shows zoomed views of the endothermic and exothermic peaks from the PBT scan (Figure 3). The area of each peak is shown (calculated using the Proteus Analysis software). From these calculated values, the percent crystallinity of this PBT polymer sample is calculated using Equation 1 and a reported value of 142 J/g for ΔHm°:

% Crystallinity =  = 78.6% crystalline

Figure 3: DSC reading vs time for a polybutylene terephthalate polymer sample, run using the DSC 3500. The temperature program used is also shown as the red dashed curve. 

Figure 4: Zoomed view of the endothermic peak (A) and the exothermic peak (B) of PBT polymer DSC scan. Areas under each curve are calculated; these correspond to the heat of melting and heat of cold crystallization of the PBT polymer sample, respectively.

Differential scanning calorimetry is a technique used to determine many thermal properties of materials, such as heat of melting, heat of crystallization, heat capacity, and phase changes. DSC measurements can also be used to calculate additional material properties including glassy transition temperature and polymer percent crystallinity. The DSC requires very small samples that must conform to the size and shape of the pans used in the machine and is based on a differential heat comparison between an empty reference and a sample. Polymer percent crystallinity calculations are relatively simple if the heat of melting of a 100% crystalline form of the polymer being tested is known. Other characterization methods that can determine percent crystallinity include density measurements, which also require a 100% crystalline and a 100% amorphous version of the polymer, and X-ray diffraction, which requires a sample that can be thoroughly mixed with a standard material such as silicon.

Percent crystallinity is an important parameter that significantly contributes to many of the properties of polymer materials used every day. Percent crystallinity plays a role in how brittle (high crystallinity) or how soft and ductile (low crystallinity) a polymer is. Polyethylene is one of the most widely used polymer materials and is a good example of the importance of crystallinity to material properties. HDPE (high density polyethylene) is a more crystalline form and thus is a harder, more brittle plastic used in garbage bins and cutting boards, whereas LDPE (low density polyethylene) has a lower crystallinity and is thus a ductile plastic used in disposable plastic shopping bags. Polymer crystallinity can also affect transparency and color; polymers with higher crystallinity are more difficult to color and are often more opaque. Percent crystallinity plays a large role in how we create and use different plastics and different forms of the same plastic every day, from polymers used in fabrics, to those used in bullet proof vests. Other polymeric characteristics that can affect these properties, and can contribute to percent crystallinity values, include previous heat treatments and degree of crosslinking.