Thermodynamic analysis indicates that even small amounts of CO 2 can lead to substantial melting point depression, due to its very low melting temperature and negative deviations to Raoult's law. To whom correspondence should be addressed. View Author Information. Cite this: Ind. Article Views Altmetric -.
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Lopes , Francisco A. Previous studies have shown that, as melting and crystallization occurs in polymers, the intensities of some functional group bands change and can also shift in frequency. We have previously shown that IR can be used to determine the melting point and crystallization temperatures of polycaprolactone PCL with a good correlation to differential scanning calorimetry.
IR studies are used to investigate the melting point depression of PCL caused by the addition of CO 2 and the crystallization behavior upon venting. CP grade CO 2 Diagram illustrating the high-pressure IR system: 1. Variation in the IR spectra of PCL with increasing pressure, at a series of isothermal temperatures, was investigated.
Briefly, a PCL plaque was placed into the preheated cell. Background spectra in the presence of CO 2 were also run and subtracted from the sample spectra. The IR spectra of polymers are sensitive to the local molecular environment and, as a result, can be used to distinguish between crystalline and amorphous morphologies. PCL displays three distinct regions in its spectrum Figure 2.
The major peak is characteristic of a crystalline conformation, whereas the shoulder indicates an amorphous structure. Because the temperature remained constant throughout the experiment, this melting phenomena is a result of the pressure increase alone. The gaseous nature of CO 2 enables it to diffuse into the amorphous regions of the polymer chains.
This reduces the chemical potential of the amorphous regions, shifting the equilibrium towards an amorphous morphology and thereby reducing the crystallinity of PCL. This lowers the chemical potential of the amorphous morphology further leading to PCL displaying melting behavior. Another interesting point to note is that a shift in the absorbance wavenumber is observed in the amorphous conformation as the pressure is increased but not in the crystalline morphology. This shows that CO 2 diffusion is limited to the amorphous regions and does not enter the crystal lattice.
This isothermal experiment was repeated for a range of temperatures to observe how the melting point depression of PCL in the presence of scCO 2 varied. The overlapping of the two carbonyl peaks prevented measurement of the absorbance of the crystalline peak following melting. An increase in temperature results in a concomitant increase in chain mobility, and, therefore, less CO 2 is needed to induce melting. Such a reduction of the polymer melting temperature has important implications, because it allows the viscosity of the polymer to be significantly decreased without additional heating.
This has already enabled therapeutic agents such as proteins to be incorporated into polymers without degrading or denaturing.
Pressure required to melt PCL at a range of isothermal temperatures. As the temperature of the system is increased, the pressure required to melt PCL reduces. It is also interesting to note that, under the conditions adopted in this experiment, the CO 2 was not supercritical over the entire temperature and pressure range Table 1. This is because, at high temperatures, there is greater chain mobility which enables CO 2 to penetrate the amorphous phases of polymers at lower pressures.
The ability of CO 2 to induce crystallization is generally known. The onset of crystallization can therefore be generated by initiating a change in the thermodynamic state of the system by either lowering the temperature below a critical crystallization value or by supersaturation of the system through the venting of CO 2. On venting, PCL originally retained its amorphous nature, but as the pressure reduced further, it recrystallized.
The spectra were recorded for an additional 4 min following venting to ensure the complete removal of CO 2. During this time, PCL retained its crystalline nature. The increased mobility afforded by the chains following plasticization allows the amorphous regions to adopt a more kinetically favorable crystalline form.
Upon venting, the more ordered configuration results in induced crystallization and concomitant changes in the morphology. This behavior follows the classical thermodynamics concept of nucleation described by Gibbs and extended to polymers by Turnbull and Fisher 28 Equation 1 :.
At the melting point, the Gibbs free energy is zero and therefore the system is stable. However, below this temperature a system will spontaneously seek to minimize its free energy by undergoing crystallization.
When CO 2 is vented, fluctuations in the melt viscosity can overcome the Gibbs free energy barrier to nucleation and phase transformation begins when the free energy of crystallization becomes negative.
This initially causes the formation of sub-critical nuclei by way of positive free energy of crystallization. The nuclei then grow spontaneously to the crystal size with an associated free energy, and a crystalline phase ensues.
IR spectroscopy is a powerful tool for detecting local molecular environments and has revealed the conformational changes of PCL in the presence of CO 2 and upon depressurization. This work has highlighted the ability of CO 2 to penetrate into the amorphous regions of semi-crystalline polymers and induce melting below the atmospheric melting temperature.
Scurto and W. Leitner, Chem. To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given.
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Read more about how to correctly acknowledge RSC content. Fetching data from CrossRef. This may take some time to load. An impure solid is typically heterogeneous on the microscopic level, with pure regions of each component distributed through the bulk solid much like granite. When an impure solid is warmed, microscopic melting first occurs in a pure region by the component with the lower melting point compound A in Figure 6.
This microscopic melting is not visible to the eye. The preliminary melting of compound A in Figure 6. As compound B is dissolved into the melt causing it to become more impure , the freezing point of this mixture is depressed. Compound B will continue to dissolve in the melt, until it reaches the eutectic composition point a in Figure 6.
Once the minor component is completely dissolved, further melting continues of the bulk component. This increases the purity of the melt, so the melting temperature increases somewhat. The system follows the melting line in Figure 6. This continues until the entire sample is melted.
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