Density and Calorimetric Methods

The most obvious difference between amorphous and crystalline regions is the density of chain packing. Accurate measurements of the density, with a precision of about 0.2 kg m are carried out in a density gradient column or alternatively by a floating method, and then very accurate measurement of the mixture density. The mass crystallinity can be obtained from the following equation  

The ready availability of differential scanning calorimetry (DSC) and in recent years moduluated DSC have significantly increased the use of the enthalpy of fusion of crystals as a method for crystallinity determination. An illustration of this method was shown for plastic crystals in Chapter 4. The enthalpy of the solid is a combination of components from the amorphous and crystalline regions. The enthalpy at any given temperature His therefore 

The difference in enthalpy of the sample between temperatures Tj and T2 is given by 

In order to determine AH21 — AHaii, it is necessary to extrapolate the postmelting scanning baseline down to lower temperatures. Often the extrapolated baseline interseets the premelting scanning baseline and this helps selection of T. The area under the eurve starts at and ends once melting has been completed. More details on the methods ean be obtained elsewhere.  

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