Heat of Fusion and Crystallinity

The determination of the heat of fusion of the pure crystals of polymers,, involves always a coupling of measurement of the heat of fusion of semicrystalline samples, A/7f, and their weight-fraction crystallinity, Wc, as expressed by  

Either, one uses an independent method for the crystallinity determination, such as dilatometry. X-ray diffraction or infrared spectroscopy for the determination of Wc [4], or one tries to determine the amorphous fraction, Wa, from the measured increase of the heat capacity at the glass transition temperature, ACp, and the same quantity for the fully amorphous sample, ACf 

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The thermal and morphological behaviors of PP/EPDM blends were studied by Da Silva and Coutinho (6) using differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. Crystallization kinetics of PP/ EPDM blends were found similar. Ten to twenty weight percent addition of EPDM resulted in increasing of spherulite size (Fig. 14.3). Heat of fusion and crystallinity degree of PP/EPDM systems decreased when EPDM contents were increased. 

FIGURE 31.2 Plots of crystalline melting point, heat of fusion and percent crystallinity of ethylene-vinyl acetate (EVA) samples versus (a) radiation dose (b) trimethylolpropane trimethacrylate (TMPTMA) level from differential scanning calorimetry (DSC) studies. (From Datta, S.K., Bhowmick, A.K., Chaki, T.K., Majali, A.B., and Deshpande, R.S., Polymer, 37, 45, 1996. With permission.)... 

Quantitative measurements of the crystallinity content of the block copolymers were made from the determination of the heat of fusion and from the density of the polymer. 

The most common applications of DSC are to the melting process which, in principle, contains information on both the quality (temperature) and the quantity (peak area) of crystallinity in a polymer [3]. The property changes at Tm are often far more dramatic than those at Tg, particularly if the polymer is highly crystalline. These changes are characteristic of a thermodynamic first-order transition and include a heat of fusion and discontinuous changes in heat capacity, volume or density, refractive index, birefringence, and transparency [3,8], All of these may be used to determine Tm [8],... 

When a pure substance is liquefied from the solid state or vaporized from the liquid at constant pressure, there is no change in temperature but there is a definite transfer of heat from the surroundings to the substance. These heat effects are commonly called the latent heat of fusion and the latent heat of vaporization. Similarly, there are heats of transition accompanying the change of a substance from one solid state to another for example, the heat absorbed when rhombic crystalline sulfur changes to the monoclinic structure at 95°C and 1 bar is 360 J for each gram-atom. 

One may attempt to derive the ideal shear strength So of the van der Waals solid normal to the chain axis from the value of the lateral surface free energy, a. This value is well known for common polymers such as PE or polystyrene (PS) (Hoffman et al, 1976) or else can be calculated from the Thomas-Stavely (1952) relationship a = /a Ahf)y, where a is the chain cross-section in the crystalline phase, Ahf is the heat of fusion, and y is a constant equal to 0.12. If one now assumes that a displacement between adjacent molecules by Si within the crystal is sufficient for lattice destruction then the ultimate transverse stress per chain will be given by So = cr/31. The values so obtained are shown in Table 2.1 for various polymers. In some cases (nylon, polyoxymethylene, polyoxyethylene (POE)) the agreement with experiment is fair. In the others, deviations are more evident. In order to understand better the discrepancy between the experimentally observed and the theoretically derived compressive strength one has to consider more thoroughly the micromorphology of polymer solids and the phenomena caused by the applied stress before lattice destruction occurs. 

Perhaps the greatest number of applications of DTA and DSC in recent years has been in the area of polymeric materials. These two techniques are routinely used to measure glass transition temperatures, Tg melting points, Tm degree of crystallinity heats of fusion and/or crystallization decomposition temperatures and numerous other parameters. Several commercial... 

In Section 4.3, it is shown with Figure 4.55 that the heat of fusion and its calibration to 100% crystallinity can be best accomplished by standard DSC, but the baseline is best checked or established by MTDSC. A well-established baseline of heat flow rate of the liquid is sufficient if the temperature dependence of the heat capacity is known (see Figures 4.23,4.25 and 4.57). A detailed, simple description of the kinetics of the glass transition of semicrystalline samples is illustrated in the example of PET (Figures 4.58. 60). Both frequency of measurement and the existing crystallinity affect the appearance of the glass transition as can be seen from the data in Table 4.1. 

Experimental values of crystalline heat of fusion for common packaging plastics vary from 8.2 kJ/mol for polyethylene to 43 kJ/mol for nylon 66. ASTM D3417 describes a method for measuring the heat of fusion and crystallization of a polymer by differential scanning calorimetry (DSC). 

The specific heat above the melting point increases slowly with temperature. The area under the melting peak of the Cp, T curve equals the heat of fusion AH, multiplied with the crystalline weight fraction. Both the heat of fusion and the percent crystallinity are dependent on the thermomechanical history of the polymer, as discussed in the previous section. 

Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) are similar techniques. They measure change in the heat capacity of a sample. These techniques can be used to determine various transition temperatures (T , Tg, T , Tp, etc.), specific heat, heat of fusion, percent crystallinity, onset of degradation temperature, induction time, reaction rate, crystallization rate, etc. A DSC instrument operates by compensating electrically for a change in sample heat. The power for heating is controlled in such a way that the temperature of the sample and the reference is the same. The vertical axis of a DSC temperature scan shows the heat flow in cal/s. 

Thermodynamic approaches provide powerfiil tools to characterize the properties in identifying these metastable states to imderstand the effects of phase size, dimensionality, and composition on the materials properties. One well-known example is the density gradient column method to determine densities of semicrystalline polymers. Based on known equilibrium crystalhne and amorphous densities, the crystallinity of a semicrystalline sample can be calculated by using equations 4 and 5. However, it should be noted that the determination of these equilibrium density data is not trivial. Proper extrapolations are necessary to ensure the equilibrium nature of the results. Detailed issues discussed can be foimd in Reference 146. Another commonly used method is to measure the heat of crystallization or fusion by using dsc. By knowing the equilibrium heat of fusion, the crystallinity of a sample can be easily calculated. 

The crystallites are destroyed upon melting and reform upon cooling. Their type and quantity depends on the sample s thermal history. Crystallinity values have been determined [26] for poly(p-biphenyl acrylate) and poly(p-cyclohexylphenyl acrylate) from both heat of fusion and heat capacity measurements by DSC. DSC has also been used to study the degree of crystallinity of Nylon 6 [27, 28] and crosslinked PVOH hydrogels submitted to a dehydration and annealing process [29]. 

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