Heat capacity jump

Measurements of heat capacity jumps at the glass-transition temperatures, Tg, in the matrix material and the composites, carried out from heat-capacity experiments, were intimately related to the extent of the mesophase thickness. Further accurate measurements of the overall longitudinal elastic modulus of the composites and the... 

Fig. 6. The variation of the heat capacity jumps at the respective glass transition temperatures versus inclusion-volume contents of iron-epoxy particulates of different diameters of inclusions. In the same figure is presented the variation of the coefficients X for the same composites and volume contents...

Moreover, the mesophase-volume fractions Oj for the same inclusion-contents were determined from the experimental values of heat-capacity jumps ACp at the respective glass transition temperatures T f by applying Lipatov s theory. Fig. 7 presents the variation of the differences Ars oi the radii of the mesophase and the inclusion (rf), versus the inclusion volume content, uf, for three different diameters of inclusions varying between df = 150 pm and df = 400 pm. 

The definition of the extent of mesophase and the evaluation of its radius r, is again based on the thermodynamic principle, introduced by Lipatov 11), and on measurements of the heat-capacity jumps AC and ACf, of the matrix material (AC ) and the fiber-composites (ACP) with different fiber-volume contents. These jumps appear at the glass-transition temperatures Tgc of the composites and they are intimately related, as it has been explained with particulates, to the volume fraction of the mesophase. 

Here, aL is the root-mean-square displacement, ACp is the heat capacity jump... 

The evolution in calorimetry technology has also led to the development of protocols for quantitative analysis (Buckton and Darcy 1999). Fiebich and Mutz (1999) determined the amorphous content of desferal using both isothermal microcalorimetry and water vapour sorption gravimetry with a level of detection of less than 1 per cent amorphous material. The heat capacity jump associated with the glass transition of amorphous materials MTDSC was used to quantify the amorphous content of a micronised drag substance with a limit of detection of 3 per cent w/w of amorphous... 

Twin models. Figure 2 illustrates the temperature dependence of heat capacity for the two twin models and Table I gives the corresponding numerical data. Figure 2 typifies the Cp(T) curve of conventional glasses with a well defined enthalpy relaxation peak and smooth solid and liquid lines. From the extrapolated solid and liquid lines we can measure the heat capacity jump at Tg, by equation 1. Within our experimental range, the data fit a straight line with slopes (B) as listed in Table n. 

Heat capacity jump, AC/>, at the glass transition temperature, can be related to F in the following way. The configurational entropy of the glass A5, at the melting temperature T is... 

Figure 4.1. Schematic illustration of temperature dependences of specific heat capacities of amorphous polymers. The heat capacity jumps to a much higher value over a narrow temperature range as the polymer goes through the glass transition. It increases more slowly with increasing temperature above Tg than it did below Tg.

Its N value is 1.4 (measured with l p=2.5, 5 and 10 C/min). The second peak at 135 C is the nematic-isotropic transition peak. Its N value is 1.3 (same heating rates). These two N values are typically the ones of first order phase transitions. The heat capacity of PAA is small compared to the height of the peak. Even if an anomalous second order phenomenon occurs, increasing the heat capacity jump under the nematic-isotropic peak by 100% or 200%, it cannot shift N towards two in a detectable way. 

The detected temperature values for the first-order (endothermic peak) and the second-order transitions (heat capacity jump) (Table 13.3) were close to the values of the glass transition temperature and melting points of the pure homopolymers. The slightly smaller values of the PA and polyether melting points probably reflect the low molecular weight of the blocks and the less ordered crystallite structure. They can be therefore attributed... 

With due account of the expression for a heat capacity jump at T, (Equation 1.6-11)... 

Measurement of Heat Capacity Jump at Glass Transition... 

Heat Capacity Jump for Unfilled Polypropylene Resin The corresponding heat capacity jump ACp for unfilled polypropylene resin is shown in Fig. 12.11. The observed glass transition temperature range is about -15° to -5°C. Before and after onset of glass transition, heat capacity versus temperature behavior is linear but with different slopes. The measurement of heat capacity jump ACp is simply the difference of heat capacity values defined by end-points A and B for the two linear regions shown in Fig. 12.11. The resulting value of ACp is 0.105 J/g°C. 

Figure 12.11 Heat capacity jump for polypropylene resin. , fg. (Data given in Ref. 1.)...

Heat Capacity Jump for Given interphase Design... 

At 10 wt% fiber glass content, the heat capacity versus temperature plots for three types of composite material are given in Fig. 12.12. The significant reduction in heat capacity jump value at the glass transition temperature for P7... 

Equation (12.3) was used to compute the interaction parameter A, from known heat capacity jump values for filled and unfilled polypropylene resin. 

Material Heat capacity jump 7g ACp(J/g°C) Interaction parameter X Interfacial thickness Ari( nm) Measured tensile strength (MPa)... 

Middle point of the corresponding heat capacity jump measured at a heating rate of lOC/min on a Perkin-Elmer calorimeter. 

Heat capacity jumps taking place when the number of phases present in the system changes... 

There are some limitations to the use of DSC in phase studies in addition of its inability to identify phases [5], They are that (1) there are difficulties in locating very steep phase boundaries in heterogeneous systems from DSC data alone because heat capacity depends strongly on the slope of phase boundaries and so the heat capacity jumps are small [10] and (2) the determination of phase boundaries in systems with slow nucleation rate, interfacial transport problems, or inherently slow phase changes may not be possible [11], This is why it is hard to discover a liquid miscibility gap by DSC measurements. 

FIG. 2 The heat capacity jump of the dimethyldecylphosphine oxide-water system versus concentration at a constant temperature (Cp in J/g mixture) of 316 K. (From Ref. 6.)... 

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