Differential Scanning Calorimetry (DSC) monitors heat effects associated with phase transitions and chemical reactions as a function of temperature. Differential Scanning Calorimetry (DSC) is used to determine a wide range of physical properties of materials, including the glass-transition temperature Tg, the melting temperature Tm, and solid-solid transitions. In this technique, a sample and a reference material are subject to a controlled temperature program. When a phase transition such as melting occurs in the sample, an input of energy is required keep sample and reference at the same temperature. This difference in energy is recorded as a function of temperature to produce the DSC traces.

A large numbers of polymers are able to form crystalline structures in which parts of the macromolecules are oriented 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 the 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. The temperature at which crystallization occur, 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 (ΔH_{f 100%}) is known, the crystallinity of the 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.

To determine the initial crystallinity the measured melting enthalpy ΔH _f, is compared to the value for 100 percent crystalline sample, ΔH _{f 100%}. The crystallinity, α, is given by the equation:

The melting enthalpy, ΔH_{f 100%}, is the difference between the enthalpy curves of the completely amorphous material and the pure crystalline material. ΔH_{f 100%} cannot be obtained directly in an experiment but is calculated from the structural data of the crystallites that have been determined using X-Ray diffraction. In general, ΔH_{f 100%} is a function of temperature.

Modulated temperature DSC (MTDSC) provides the same qualitative and quantitative information about physical and chemical changes as conventional DSC, and it also provides unique thermochemical data that are unavailable from conventional DSC. The effects of baseline slope and curvature are reduced, increasing the sensitivity of the system. Overlapping events such as molecular relaxation and glass transitions can be separated. Heat capacity can be measured directly with MTDSC in a minimum number of experiments.

Both MTDSC and DSC measure the difference in heat flow to a sample and to an inert reference. The sample and reference cells are identical. However, MTDSC uses a different heating profile. Whereas DSC measures heat flow as a function of a constant rate of change in temperature, MTDSC superimpose a sinusoidal temperature modulation on this rate. The sinusoidal change in temperature permits the measurement of heat-capacity effects simultaneously with the kinetic effect. Typical experimental procedure for an initial MTDSC experiment include a heating rate from isothermal to 5 °C/min and a modulation amplitude from 0.01 to 10 °C. The modulation period can vary from 10 to 100 seconds or, expressed as a frequency, from 10 to 100 MHz.

Please contact Krystyna Brzezinska at kbrzez@mrl.ucsb.edu to schedule training. Before training starts please read the on-line Manual and Presentation.