Heat capacity signal

For quantitative MTDSC measurements, it is necessary to calibrate temperature and heat flow as in a conventional DSC. The heat capacity signal is calibrated at a single temperature or as a function of temperature using a reference material. 

Commonly, the heat capacity signal is calibrated at a single temperature. However, the experimental error on the heat capacity can further be reduced by a dynamic calibration over the entire temperature range instead of at a single temperature. The heat capacity calibration constant, Kc., shows a gradual evolution over the entire temperature range, with a total variation of 4% between —50 and 300°C. Below —50°C, the deviation increases. 

It is clear that A Cp react provides information on the reaction mechanism. Instead of normalising the heat capacity signal in terms of a mobility factor (DF = 1 if no mobility restrictions before vitrification), the information available in ACp,react can be exploited for mechanistic modelling. 

The left-hand-side part of this equation (dx/dt)obs is proportional to the non-reversing heat flow according to Eq. (8). For step-growth polymerisation reactions, diffusion control is governed by the vitrification process (section 5.1.1). Therefore, as a first approximation, the normalised heat capacity signal or mobility factor, DF, [Eq. (10)] is proposed to be an in... 

Figure 2.27. Progress of the glass transition in the heat capacity signal during cure of an epoxy-anhydride system. Reference lines are indicated (see text).

Characterisation of Glass Transition Behaviour in Interpenetrating Polymer Networks The multi-phase nature of IPNs results in complicated glass transition behaviour [101]. Figure 3.46 shows that heat capacity changes with temperature for a series 60 40 polyurethane (PU)/ polystyrene (PS) IPNs (see Table 3.5 for the compositional details) [131,132]. It is, however, not possible to obtain much detailed information from these heat capacity signals. 

Before carrying out the actual DSC experiments, both the abscissa and the ordinate have to be calibrated. In a DSC curve, the abscissa displays the temperature (or time for isothermal experiments), while the ordinate displays the heat flow signal (dQ/dt) or heat capacity signal (Cp). 

The initial increase in the heat capacity signal corresponds to the reaction heat capacity or the change in heat capacity from reactants to products (see arrow in Rg. 2.112). A thermodynamic analysis of the epoxy-aromatic amine reaction revealed that the primary amine-epoxy reaction contributes less to the increase in reaction heat capacity than does the secondary epoxy-amine reaction (Swier and van Mele 2003b). Information specific to the different steps in the reaction mechanism can therefore be deduced from the heat capacity signal, in contrast to the global conversion evolution obtained from the total heat flow signal. 

Figure 2.116. Nonreversing heat flow, change in heat capacity, and heat flow phase signal from MTDSC and percent of hght transmittance from optical microscopy (OM) for the reactive blend DGEBA + aniline (r = l)/20 wt% PES cnred at 100/90/80°C the cloud point from OM (A), onset of heat flow phase relaxation corresponding to vitrification of PES-rich phase (O) and epoxy-amine-rich phase ( ) are shown and also indicated in the heat flow and heat capacity signals [reprinted from Swier and Van Mele (2003a) with permission of Elsevier Ltd.].

The glass transition of PETP samples which were subjected to solvent induced crystallisation (SINC) was investigated by modulated differential scanning calorimetry (MDSC) and density measurements. The differential heat capacity signal from MDSC was used to monitor the SINC process. 16 refs. 

A quantitative thermal method, based on the differential of heat capacity signal from modulated temperature differential scanning calorimetry, was described for determining the weight fraction of interface and the extent of phase separation in polymer materials. The interface was modelled as discrete fractions, each with its own characteristic increment of heat capacity. The materials used to demonstrate the range of the method were PS blended with poly(phenylene oxide) (PPO), pure PS, pure PPO, a styrene-isoprene-styrene triblock copolymer (SIS), SIS blended with PPO, PMMA/poly(vinyl acetate) blends and PVC sandwiched with poly(n-butyl acrylate). Two-phase and four-phase systems were used. The calculated results agreed well with experimental results for two- and four-phase systems. 20 refs. 

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