Chapter 5. Thermal Analysis of Binary TAG Mixtures

Transcription

1 Chapter 5: Thermal Analysis of Binary TAG Mixtures 95 Chapter 5 Thermal Analysis of Binary TAG Mixtures 5.1 Introduction Fats, multi-component mixtures of TAGs, are main components in many food products. The physical properties of fats firmly influence the processing of fats in food materials, the final characteristics of food products (hardness and texture) and the storage time of these products 1. For instance, the composition of fat mixture is a crucial factor that determines the properties like rheology and crystallization behaviour 2. In order to achieve desirable characteristics of the products during manufacturing, the adequate control of fat solidification is an important step. The crystallization conditions, such as cooling rate and thermal history, have significant effects on the crystallization kinetics and physical properties of the crystallized products 3. To reveal valuable information about crystallization kinetics and the type of polymorphs formed, recent studies deal with modeling of fat crystallization under isothermal 4 and non-isothermal 5 conditions. Besides the chemical composition of fats and the crystallization conditions, one of the most important pieces of information required to control crystallization is the equilibrium phase diagram of the system. These diagrams are quite complex for real fats due to various interactions between different types of TAG molecules 6. Besides, the crystallization kinetics of fats is highly affected by operating conditions. That is reflected in the occurrence of the crystalline phases and phase transformations between them, which do not involve only thermodynamic phase diagram, but also the kinetics of these processes. Most investigations of the mixtures of TAGs refer to binary blends. Studying of their phase behaviour, which is not that complex as for the real fats, provides useful information about the interactions of TAG molecules in the mixtures. Monitoring of the phase transitions of binary mixtures of TAGs is usually performed by means of the DSC and X-ray diffraction (XRD) techniques Some authors use the combination of the mentioned techniques with NMR, which enables studying the organization of the fatty acid chains in the most stable

2 96 Chapter 5: Thermal Analysis of Binary TAG Mixtures polymorphic forms 13, 14. In other studies, the synchrotron radiation XRD (SR XRD) has been introduced as a powerful tool for the construction of binary phase diagrams and distinct characterization of the metastable polymorphs 15, 16. All these investigations reveal the various polymorphic forms occurring in the binary mixtures and give insight in the mixing properties of different solid phases. Regarding the miscibility in the solid phase, the effect of TAG interactions in mixtures may result in the formation of solid solutions (mixed crystals), eutectic mixtures and chemical compounds 8. The primary factors determining the phase behaviour of TAG mixtures are the differences in the chain lengths of the TAGs, the degree of saturation and the position of the fatty acid chains, and polymorphic forms involved. If the difference in the chain lengths of monoacid saturated TAGs is two carbon atoms or more, the components typically exhibit limited miscibility in the most stable polymorph (example PPP-SSS) 1, 13. The bigger the difference in the number of carbon atoms, the regions where the mixture forms solid solutions become smaller. Poor miscibility in the β-form also occurs for the monoacid saturated and cismonoacid unsaturated TAGs 7,8,13, while trans-unsaturated molecules exhibit higher solubility in the saturated ones 11. Furthermore, for sets of TAG mixtures containing mixed chains of saturated and unsaturated acids (SOS-SSO, PPO-POP), the formation of a molecular compound with molar ratio 1:1 is quite common 16. Regarding the less stable forms (α and β ), they often exhibit higher miscibility than the most stable polymorph, forming solid solution phases in all proportions of the components. However, increasing the difference between the properties of two TAGs may result in limited miscibility for all polymorphic forms (example PPP-POP, POP-PPO) 12, 17. In this work, we investigate the phase behaviour of the three binary TAG mixtures, being EEE-SSS, EEE-PPP and PPP-SSS, by DSC and adiabatic calorimetry. The phase diagrams, constructed from the DSC measurements of the β-polymorph, are presented and discussed. To reveal more information about the mixing properties in the solid phase, the DSC results are supplemented by several adiabatic measurements of the given TAG mixtures.

3 Chapter 5: Thermal Analysis of Binary TAG Mixtures Overview of phase diagrams The phase diagram allows the determination of the amount and the composition of the solid phase fraction in a crystallized fat, if equilibrium conditions are attained. However, it is important to point out that fats are usually not in equilibrium, and the phase diagram does not necessarily tell us what will happen in complex food systems. Kinetic constrains may have significant impact on the crystallized product and may lead to the formation of different phases than those predicted by the phase diagram. Nevertheless, the equilibrium phase diagrams of fats provide a useful guideline for proper control of crystallization during processing of food products. However, they are not well known in many cases and are usually difficult to obtain. Here, we give a classification of main types of phase diagram that are commonly observed in binary mixtures of TAGs (Fig. 1), as introduced by Timms 18 : Monotectic continuous solid solutions, which are formed when the TAGs are very similar in melting temperature, molecular volume and polymorphism. Eutectic systems, which are often found in TAG mixtures, tend to occur when the components differ in molecular volume, shape and polymorph, but not too much in melting temperature. Monotectic partial solid solutions form in preference to a eutectic system if the difference in melting temperature of the TAG components is increased. Peritectic systems have been found to occur only in mixed saturated/unsaturated systems where at least one TAG has two unsaturated acids. For the description of the solid-liquid phase equilibrium, beside the pure component properties, the excess Gibbs free energy properties in the coexisting phases must be known. Since the fats exhibit polymorphism, the excess Gibbs energy of all possible solid phases is required. In principal, there is relatively little information about the excess properties of TAG mixtures. Generally, the deviation from ideal mixing of TAGs in the liquid phase becomes noticeable if the difference in chain length exceeds 15 carbon atoms. As for the mixing properties in the solid phase, we refer to the work of Wesdorp 9, who developed a thermodynamic model to describe the phase behaviour of TAG mixtures. In this work, a review of experimental binary phase diagrams of TAGs is provided. These diagrams were

4 98 Chapter 5: Thermal Analysis of Binary TAG Mixtures used to determine the interaction parameters that occur in an excess Gibbs energy model. Using a fitting procedure the interaction parameters were adjusted until the calculated and measured phase diagrams agreed. By extracting binary interaction coefficients between TAGs, it is possible to extrapolate to ternary and more complex mixtures. Based on these results, the phase behaviour of multi-component TAG mixtures in any polymorphic form was obtained. In general, the α-polymorphs behave as ideal mixtures, while β - and β-forms exhibit significantly non-ideal behaviour. Figure 1. Phase diagrams in binary TAG mixtures: (A) Monotectic continuous solid solution; (B) Eutectic; (C) Monotectic partial solid solution; and (D) Peritectic.

5 Chapter 5: Thermal Analysis of Binary TAG Mixtures 99 However, the unreliability of the interaction parameters obtained from the measured phase diagrams is considerable. Even relatively small experimental errors in the position of the liquidus and solidus lines lead to large uncertainty in the interaction parameters. Moreover, it is not possible to measure reliable phase diagrams of unstable polymorphs. Study of the phase behaviour of a TAG mixture in a metastable solid state (α and β ) can only be established under certain conditions, which definitely do not respect the rule of a true thermodynamic equilibrium. Namely, the accuracy of the measured phase diagram is disturbed by several experimental parameters, such as the purity of materials and the stabilization procedure used for producing the most stable phase 9. The usage of the DSC also introduces certain experimental limitations. Sometimes small heat effects are difficult to detect using the DSC technique due to too large sample size or scanning rate. In some cases the melting peaks are not sharp, so that it is not possible to determine exact starting and end temperatures of the transitions. These issues, concerning the usage of the DSC, may result in a set of false liquidus and solidus points. Usually, the measured solidus line is unreliable due to the kinetics effects. Therefore, revealing accurate phase diagrams for fat mixtures remains a challenging task to be done mostly. 5.3 The role of kinetics The strong influence of kinetics on the crystallization of fats is observed in the occurrence of different polymorphs. Preferably, fats crystallize in the thermodynamically least stable α-polymorph. Although the final energy state of the lower-stability polymorph is higher, the energy barrier that has to be overcome for the nucleation is less (see Fig. 2). On heating or during storage, the α-polymorph eventually transforms or re-crystallizes to more stable forms (β - or β). From experiments it turns out that no significant undercooling is needed for the nucleation and crystallization of the α-polymorph. The reason for that is the ordering of molecules into lamellae that takes place in the liquid phase before the crystallization 16. Typically, at the crystallization temperature of the α-polymorph the undercooling for β - and β-polymorphs is very high, but these phases will not crystallize due to a high nucleation barrier and the slow growth kinetics. The β- polymorph tends to

6 1 Chapter 5: Thermal Analysis of Binary TAG Mixtures crystallize from the melt under conditions where little or no undercooling of the less stable form is present. Figure 2. Schematic diagram presenting the Gibbs free energy change during nucleation of different polymorphs. Another influence of the kinetics is reflected in the existence of composition gradients within crystals, so that the crystallized material is not homogeneous. By slow cooling of the melt, the composition of the growing solid that is in contact with the liquid phase might reach the equilibrium. However, the composition of inner regions of the crystal does not change due to the low diffusion rate in the solid phase. Additionally, the effect of the undercooling at the interface is quite strong and it will induce segregation that might deviate significantly from that predicted by the equilibrium phase diagram. In case of TAG mixtures, it is shown that increased undercoolings result in high reduction of segregation 19. Finally, for an adequate description of the crystallization process, the segregation at the given undercooling should be

7 Chapter 5: Thermal Analysis of Binary TAG Mixtures 11 coupled with mass and heat transport limitations, as discussed in Chapter 3. It was shown that during the crystallization of TAG mixtures, the heat transport limitation could significantly enlarge the segregation compensating (partly) the effect of undercooling 19. Rapid crystallization of TAG mixtures may result in poorly packed crystals, which thermal properties may deviate significantly from those of well-packed ones. For example, the melting temperature of such imperfect crystals is typically lower than the melting point predicted by thermodynamics. These imperfections in the crystals may persist for years if the liquid phase is not present 2. However, by re-crystallization the badly packed crystals can easily rearrange into well-packed crystals. Clearly, the kinetics plays an important role in the fat crystallization. For prediction of the amount and composition of the solid phase, the knowledge of equilibrium thermodynamics is indispensable, but not sufficient. Kinetic factors are as important as thermodynamic ones in determining which polymorph will crystallize from the melt and the amount, composition and properties of the crystalline phase. 5.4 Equilibrium phase diagrams of β-polymorph In this section, the phase diagrams of three TAG binary mixtures measured by DSC are presented. Generally, the diagrams are constructed by using the melting temperatures derived from the heating scans of the measured mixtures. The diagrams presented here also contain the corresponding crystallization temperatures obtained from the cooling scans. When the mixture forms a solid solution upon crystallization, the DSC heating scan contains one endothermic effect and the temperature of this endotherm lies between the melting temperatures of the pure components. In case that a mixture exhibits limited miscibility in the solid phase, in principle two endothermic effects are present on the DSC heating scan. However, these two endothermic peaks usually overlap on the experimental scans, so that it is difficult to determine their melting onsets 21. Therefore, we constructed the phase diagrams by using the values of melting peak temperatures. The exothermic effects on cooling DSC scans, registered at low cooling rates, do not show the overlap. Thus, we were able to derive the onset temperatures of the exothermic peaks, which were plotted as a function of composition in the presented phase diagrams.

8 12 Chapter 5: Thermal Analysis of Binary TAG Mixtures Beside the DSC results, we present the adiabatic measurements of the binary TAG mixtures. As it will be discussed further in the text, the diluted TAG mixtures might exhibit different phase behaviour when measured in the adiabatic calorimeter as compared to the DSC results. EEE-SSS mixture Several mixtures of EEE-SSS were measured in the DSC. Their melts were cooled to 273 K at a rate of.2 K min -1 and the obtained solids were melted with the rate of 1 K min -1. No exothermic re-crystallization effects were observed on the heating scans, meaning that the solid phases crystallized in the most stable polymorphic form. In Figure 3, the heating and cooling scans are shown for the mixture x SSS =.558. Both on cooling and heating scans two peaks are clearly distinguished, implying that the mixture exhibits phase separation in the solid phase. 5 heat flow / mw Figure 3. Cooling and heating DSC scans of the EEE-SSS mixture of composition x SSS =.558, registered at the cooling rate of.2 K min -1 and the heating rate of 1 K min -1.

9 Chapter 5: Thermal Analysis of Binary TAG Mixtures 13 On continuous cooling of the melt, the SSS-rich crystalline phase solidifies first, followed by the crystallization of another solid phase, rich in EEE. According to our measurements, the mixture of EEE-SSS shows the described behaviour in most of composition range. 4 3 x SSS =.5 2 heat flow / mw x SSS =.98 1 heat flow / mw Figure 4. Cooling (bold lines) and heating (thin lines) DSC scans of two diluted EEE-SSS mixtures of compositions x SSS =.5 and x SSS =.98, registered at the cooling rate of.2 K min -1 and the heating rate of 1 K min -1.

10 14 Chapter 5: Thermal Analysis of Binary TAG Mixtures Still, we observed complete solubility of EEE in SSS for the composition range x SSS >.9. In another study of the EEE-SSS mixture 11, a broader region of solubility was reported, occurring for x SSS >.65. To illustrate the solubility in the EEE-SSS mixture, we present the scans recorded for two diluted mixtures, being of compositions x SSS =.5 and x SSS =.98 (see Fig. 4). On both cooling and heating scans of the SSS-rich mixture, one peak was observed, indicating that the mixture formed a solid solution. For the EEE-rich mixture, although one exothermic peak appears on the cooling scan, two endothermic effects can be distinguished on the heating scan. This suggests that there is still some separation of the components in the solid phase. These results support already observed mixing trend in the mixtures of saturated-unsaturated TAGs; the saturated TAGs exhibit less or no solubility in the unsaturated ones 13. Based on the transition temperatures extracted from the heating and cooling scans of the measured mixtures, the diagram shown in Figure 5 is constructed. The line that represents the onset temperatures of the first crystallization lies about 2 K below the liquidus line derived from the corresponding melting temperatures. This temperature difference indicates the undercooling needed to induce crystallization at the cooling rate of.2 K min -1. Accordingly, the space between these two lines represents a metastable zone, where nucleation does not take place. The width of the metastable zone depends on the crystallization conditions, particularly on the cooling rate. When higher cooling rates were applied, being 5 and 1 K min -1, more undercooling was needed to initiate crystallization. The lines showing the crystallization onsets at different high cooling rates are very close to each other (Fig. 5). This suggests that there is a maximal value of undercooling that can be achieved for the given mixture. However, the solid phases obtained at higher cooling rates were not in the most stable polymorphic form. The line presenting the onset temperatures of the second crystallization under the rate of.2 K min -1 lies about 4 K below the line of the corresponding melting temperatures (Fig. 5). Thus, less undercooling is needed for the solidification of the second crystalline phase, since it nucleates on already existing crystals of the first solid phase, which is easier. The values of the crystallization and melting temperatures of the second solid phase as plotted for different compositions of the mixture, form lines at approximately constant temperature. The

11 Chapter 5: Thermal Analysis of Binary TAG Mixtures 15 line showing the melting temperatures of the second solid phase is very close to the melting point of pure EEE x (SSS) Figure 5. The phase diagram of the EEE-SSS mixture. Bold lines with circles present the melting temperatures derived from the heating scans registered at the rate of 1 K min -1 ; thin lines with triangles present the crystallization temperatures upon slow cooling of the melt at the rate of.2 K min -1 ; dashed lines correspond to the crystallization temperatures during cooling of the melt at the rates of 5 and 1 K min -1. In the DSC experiments the mass of the sample is in the order of a few milligrams. For the experiments in the adiabatic calorimeter, about 1 time higher amounts of sample can be used, which might cause different phase behaviour of the mixture. To compare with the DSC results, we performed experiments in the adiabatic calorimeter with an EEE-SSS mixture of composition x SSS =.499. Before the sample was mounted in the calorimeter, the mixture was melted in an oven, then shaken to ensure complete mixing in the liquid phase,

12 16 Chapter 5: Thermal Analysis of Binary TAG Mixtures and immersed into ice. Inside the calorimeter the solid phase was again melted, and the melt was cooled to 25 K at a rate of.1 K min -1. The obtained solid phase was heated to 37 K and no re-crystallization exothermic effects were observed, meaning that the mixture crystallized in the β-polymorph. The enthalpy curves measured during cooling and heating of the mixture are given in Figure 6, where each curve shows two transitions. This illustrates the limited miscibility of the components in the solid phase. It was also noticed that the first crystallization took place at somewhat lower undercooling in the adiabatic calorimeter than in the DSC. The reason for that is the larger amount of sample as used in the adiabatic measurements. 2 H vessel+mix / J heating enthalpy cooling enthalpy Figure 6. The enthalpies of the vessel and the EEE-SSS mixture (x SSS =.499) measured during cooling and heating in the adiabatic calorimeter. Both cooling and heating rates were.1 K min -1.

13 Chapter 5: Thermal Analysis of Binary TAG Mixtures 17 From this analysis, we conclude that the β-polymorph of EEE-SSS mixtures show phase separation in the most of composition range, while it forms solid solution in the SSSrich mixtures for x SSS >.9. Such a type of phase diagram belongs to the group of monotectic partial solid solution phase diagrams and it is typical for TAG mixtures where the difference in the melting temperatures of components is high. In case of EEE and SSS, this difference is quite significant, being about 3 K. However, according to Wesdorp 9, EEE and SSS should form completely miscible β-form for the whole composition range. The shape of the liquidus line could point to such a conclusion, but our results clearly show that phase separation occurs in the solid phase. EEE-PPP mixture Mixtures of EEE-PPP were measured in the DSC under the same experimental conditions as the mixture of EEE and SSS. The derived crystallization and melting temperatures of the β-polymorph are plotted for different compositions of the mixture, resulting in a diagram given in Figure 7. The line presenting the first crystallization temperatures upon cooling of.2 K min -1 lies about 15 K below the corresponding melting temperature line. For the second crystallization much less undercooling is needed, about 2 K. Besides, we plot the crystallization temperatures registered during fast cooling. As for the EEE-SSS mixture, it appears to be a maximal achievable undercooling for increasing cooling rate. When the mixture becomes richer in EEE, the crystallization at the cooling rate of.2 K min -1 starts at temperatures that exceed the undercooling of 15 K. For the mixtures of compositions x PPP =.19 and x PPP =.39, the crystallization onset was 26 K lower than the corresponding liquidus point. In these cases, the crystallization temperatures are quite close to the melting point of the EEE α-polymorph, being 288 K. On the cooling scans of these mixtures, broad crystallization peaks appear, while the heating scans contain the exothermic re-crystallization effects just before the melting (Fig. 8). Clearly, in the case of these EEE-rich mixtures the β-polymorph was not obtained during cooling of the melt at the rate of.2 K min -1. Therefore, only the melting points are presented for these two diluted mixtures in Figure 7. In contrast, EEE-rich samples of the EEE-SSS mixtures solidify in the most stable

14 18 Chapter 5: Thermal Analysis of Binary TAG Mixtures form upon slow cooling. Probably, that is so because SSS molecules are longer than PPP ones and thereby allow for easier formation of the most stable form x (PPP) Figure 7. The phase diagram of the EEE-PPP mixture. Bold lines with circles present the melting temperatures derived from the heating scans registered at the rate of 1 K min -1 ; thin lines with triangles present the crystallization temperatures upon slow cooling of the melt at the rate of.2 K min -1 ; dashed lines correspond to the crystallization temperatures during cooling of the melt at the rates of 5 and 1 K min -1.

15 Chapter 5: Thermal Analysis of Binary TAG Mixtures x PPP =.19 heat flow / mw x PPP =.39 heat flow / mw Figure 8. Cooling and heating DSC scans of two EEE-rich mixtures of EEE and PPP, having the compositions of x PPP =.19 and x PPP =.39. The cooling rate was.2 K min -1 and the heating rate was 1 K min -1.

16 11 Chapter 5: Thermal Analysis of Binary TAG Mixtures On the other hand, the PPP-rich mixtures crystallize in the β-polymorph under slow cooling. The cooling scan of an EEE-PPP mixture of composition x PPP =.95 shows one sharp exothermic peak (Fig. 9). On the heating scan two endothermic effects can be distinguished, suggesting that the components do not mix ideally in the solid phase. Note the difference in shape of the crystallization peaks for the EEE-rich and PPP-rich mixtures. The broad peaks by the crystallization of the EEE-rich mixtures point to the crystallization of more unstable polymorphs, while the PPP-rich mixture solidifies instantly in the β-polymorph. 6 4 x PPP =.95 heat flow / mw Figure 9. Cooling and heating DSC scans of the PPP-rich mixture of EEE and PPP, being of the composition x PPP =.95. The cooling rate was.2 K min -1 and the heating rate was 1 K min -1.

17 Chapter 5: Thermal Analysis of Binary TAG Mixtures 111 To get a better insight into the behaviour of the EEE-rich mixtures, an EEE-PPP mixture of composition x PPP =.17 was measured in the adiabatic calorimeter. The mixture was prepared for adiabatic measurements in the same way as for EEE-SSS mixture. After slow cooling of the melt under the rate of.1 K min -1, no β-polymorph was formed. The heat capacity curve, measured upon slow heating of the obtained solid, contains an exothermic kink just before melting (see Fig. 1). Thus, in both adiabatic calorimeter and DSC we could not obtain the most stable polymorph for an EEE-rich mixture just by slow cooling of the melt. The heat capacity of the same EEE-PPP mixture stabilized at 313 K for 24 hours is given in Figure 1. This heat capacity curve corresponds to the most stable polymorph, which seems to have the property of a solid solution. Apparently, the β-polymorph exhibits enhanced miscibility when re-crystallized from the less stable and basically more miscible α- polymorph. 5 4 c p / J(gK) solid after slow cooling of melt solid stabilized at 313 K Figure 1. The heat capacities of the EEE-PPP mixture of the composition x PPP =.17 measured in the adiabatic calorimeter at the heating rate of.1 K min -1.

18 112 Chapter 5: Thermal Analysis of Binary TAG Mixtures The above-presented results suggest that the components EEE and PPP are highly immiscible in the β-polymorph. We found no evidence of the solid solution formation during slow cooling of the melt. Such behaviour originates from significant differences between their molecules; the chain lengths differ in two carbon atoms and EEE is trans-unsaturated. However, accurate determination of mixing properties in EEE-rich mixtures remains difficult, since these mixtures do not tend to crystallize in the β-polymorph upon slow cooling. PPP- SSS mixture The phase diagram of the PPP-SSS mixture, as measured by DSC 22, is shown in Figure 11. The experimental procedure consisted of cooling the melt at a rate of.1 K min -1 and afterwards heating of the obtained solid phase at a rate of 1 K min -1. The diagram of the β-polymorph is constructed by using the corresponding melting temperatures registered during the experiments. Additionally, we present the crystallization temperatures of the α- polymorph, derived from rapid cooling of the melts. The phase diagram of the β-form of the PPP-SSS mixture clearly shows eutectic character. The components exhibit limited immiscibility in most of the composition range, having the regions of complete mixing close to the pure component sides. According to the experimental data, the mixed crystals are expected to form in the following composition ranges: at PPP side for x SSS <.46 and at SSS side for x SSS >.88. The phase diagram presented here is in agreement with that determined in Ref. [9]. To illustrate the phase behaviour of the diluted PPP-SSS mixtures, we present cooling and heating scans of the SSS-rich mixture (x SSS =.95) as registered during the described DSC experiments (Fig. 12). At a cooling rate of.1 K min -1 the mixture crystallized at K, which is close to the melting point of the α-form of SSS, being K. According to the heating scan, which contains one exothermic re-crystallization effect at K, we conclude that the mixture did not crystallize completely in the most stable polymorph. However, when the mixture of the same composition was cooled under the same rate in the adiabatic calorimeter, the β-polymorph crystallized at K, 4 K above the crystallization temperature in the DSC. This discrepancy illustrates the effect of the difference in mass of the adiabatic and the DSC samples. Surprisingly, the β-polymorph obtained in the adiabatic

19 Chapter 5: Thermal Analysis of Binary TAG Mixtures 113 calorimeter shows limited miscibility of the components. This is in contrast with the phase diagram measured by DSC (Fig. 11), which suggests the formation of mixed crystals at the given composition of the PPP-SSS mixture. The DSC results point to higher miscibility in the solid phase, since the β-polymorphs of the diluted PPP-SSS mixtures were formed by the recrystallization of the less stable α-polymorph. In conclusion, regarding the mixing status in the most stable β-polymorph, one should consider the manner in which it was formed x (SSS) Figure 11. The phase diagram of the PPP-SSS mixture. Bold lines with circles present the melting temperatures derived from the heating scans at the rate of 1 K min -1 and dashed line with squares corresponds to the crystallization temperatures during rapid cooling of the melt at the rate of 1 K min -1.

20 114 Chapter 5: Thermal Analysis of Binary TAG Mixtures heat flow / mw Figure 12. Cooling and heating DSC scans of the SSS-rich mixture of PPP and SSS, being of the composition x SSS =.95. The cooling rate was.1 K min -1 and the heating rate was 1 K min -1. To investigate the mixing properties of SSS-rich mixtures, we measured a mixture of the composition x SSS =.97 in the adiabatic calorimeter under the same conditions as the previous mixture. The β-polymorph formed during slow cooling of the melt shows phase separation, as can be seen from the measured heat capacity curve (Fig. 13). When the melt was cooled rapidly, the α-polymorph solidified. Subsequently, on heating it re-crystallized at 332 K (Fig. 13). In the next experiment, the thermodynamically unstable α-form was heated to 332 K and left at that temperature to stabilize under adiabatic conditions for 24 hours. Afterwards, the sample was cooled slowly to 25 K and the heat capacity of the obtained solid phase was measured (Fig. 13). The β-polymorph obtained by re-crystallization is apparently in the state of a solid solution. Thus, the β-polymorph tends to higher miscibility when formed via re-crystallization of the less stable polymorph. Using the measured enthalpies and heat capacities of two types of the β-polymorphs, one being in solid solution and other in eutectic form, we calculated their relative Gibbs free energies (see Fig. 14). From thermodynamic point of view, the solid phase with the lowest value of the Gibbs energy should preferably

21 Chapter 5: Thermal Analysis of Binary TAG Mixtures 115 crystallize from the melt. Since the given Gibbs energy curves are very close to each other, the difference in driving force for the crystallization of the β-polymorph into separate phases and a solid solution is very small. This suggests that the solid solution may occur due to kinetics. 5 4 c p / J(molK) Figure 13. The heat capacities of PPP-SSS mixture x SSS =.97 measured in the adiabatic calorimeter during heating at the rate of.1 K min -1 ; α-form (dashed line), β-form crystallized during slow cooling of the melt at the rate of.1 K min -1 (solid line with circles), β-form obtained by re-crystallization at 332 K (bold line).

22 116 Chapter 5: Thermal Analysis of Binary TAG Mixtures 5 G rel / Jmol -1 (Thousands) solid solution limited miscibility Figure 14. Relative Gibbs free energies of the PPP-SSS mixture (x SSS =.97), for solid solution formed by re-crystallization at 332 K and for the solid phase with limited miscibility that crystallizes during slow cooling of the melt at the rate of.1 K min The crystallization of TAG mixtures at high cooling rates EEE-SSS mixture Several mixtures of EEE and SSS were melted in the DSC. After keeping the melt at 373 K for 3 minutes the mixtures were cooled at high cooling rates, being 5, 1 and 2 K min -1. After each cooling to 273 K, the heating scans of the obtained solids were taken at a rate of 5 K min -1. It was expected that the mixtures would crystallize in the least stable polymorphic form at the mentioned high cooling rates. However, this was not always the case, since the mixture of composition x SSS =.229 crystallized in the β-form under all three cooling rates. Each cooling scan basically contains two exothermic peaks (Fig. 15), where the onset of

23 Chapter 5: Thermal Analysis of Binary TAG Mixtures 117 the first crystallization is in the vicinity of 314 K. During cooling of the mixture at a rate of 2 K min -1, one additional exothermic peak appears. On the heating scan of this solid phase, a small exothermic effect occurs at 288 K. This temperature corresponds to the melting temperature of EEE α-polymorph, suggesting that part of EEE component solidified in the least stable form. Anyway, the rest of the mixture crystallized in the β-form characterized by limited miscibility of the components. The appearance of more exothermic peaks on the cooling scans suggests that the components EEE and SSS do not mix easily in the solid phase even under fast cooling. -1 heat flow / mw K min -1-1 K min -1-2 K min -1 Figure 15. Cooling scans of the EEE-SSS mixture of composition x SSS =.229. On the other hand, an EEE-SSS mixture of composition x SSS =.883 crystallized around K for all three cooling rates, giving one crystallization peak. On each of the heating scans one exothermic peak appears at K (Fig. 16). This temperature coincides with the melting temperature of SSS α-polymorph, suggesting that part of the sample, rich in SSS, solidified in the α-form upon fast cooling. In contrast to the previously discussed EEE-

24 118 Chapter 5: Thermal Analysis of Binary TAG Mixtures SSS mixture, which crystallized in the most stable form at high cooling rates, this mixture obviously solidified in more polymorphs. Thus, as the mixture becomes richer in SSS, the probability of the formation of the α-polymorph, apparently rich in SSS, increases. 2 heat flow / mw Figure 16. Heating of the EEE-SSS mixture of composition x SSS =.883. The solid phase is obtained using a cooling rate of 1 K min -1. EEE-PPP mixture The phase behaviour of the EEE-PPP mixture at fast cooling was investigated in the same experimental conditions as described for the EEE-SSS mixture. The cooling scans of the mixture x PPP =.657 show two exothermic peaks in case of all three cooling rates. The onset of the first peaks is always in the vicinity of 311 K, while the position of the second crystallization peak shifts toward a lower temperature for increasing cooling rate (Fig. 17 a, b, c). Only during the cooling with the rate of 5 K min -1, both solid phases are in the most stable form (fig. 17a).

25 Chapter 5: Thermal Analysis of Binary TAG Mixtures a) heat flow / mw b) heat flow / mw c) heat flow / mw Figure 17. Cooling and heating scans of the EEE-PPP mixture of composition x PPP =.657, for different cooling rates: a) 5 K min -1, b) 1 K min -1 and c) 2 K min -1.

26 12 Chapter 5: Thermal Analysis of Binary TAG Mixtures For the other two cooling rates, being 1 and 2 K min -1, the solid phases formed during the second crystallization are clearly in the less stable form, since they re-crystallize just before the melting (Fig. 17 b, c). In these cases, the final solid phase contains more than one polymorph - one stable that crystallizes at 311 K, and the other unstable, of which the properties depend on the intensity of cooling. Accordingly, in the heating scan of the solid phase formed at the cooling rate of 2 K min -1, an additional melting peak occurs at 288 K, followed by a re-crystallization effect (Fig. 17c). This temperature corresponds to the melting temperature of the α-polymorph of EEE, suggesting that a certain portion of EEE crystallized in the less stable polymorph due to the very fast cooling. The PPP-rich mixture, x PPP =.946, exhibits one exothermic effect in case of all applied cooling rates, while the onset of crystallization is at K. On heating the obtained solid phases, one exothermic re-crystallization effect occurs at the temperature close to 318 K (Fig. 18). It seems that the most of the sample crystallizes as a completely mixed stable form, while the remaining part solidifies in the α-polymorph on further fast cooling. The effect of the crystallization of this α-phase is not visible on the cooling scan, probably due to the fact that two polymorphs solidify simultaneously within the temperature interval of the crystallization peak. According to the results presented here, the fast cooling of the EEE-PPP mixtures leads to solid phases that consist of more polymorphic forms. Considering the results for the mixture of composition x PPP =.657, the appearance of two crystallization peaks implies that the components do not mix easily during fast cooling. However, in the case of the PPP-rich mixture, the miscibility of the components is enhanced to a certain extent.

27 Chapter 5: Thermal Analysis of Binary TAG Mixtures heat flow / mw Figure 18. Cooling and heating scans of the EEE-PPP mixture of composition x PPP =.946. The applied cooling rate was 1 K min -1. PPP-SSS mixture The experimental procedure applied for measuring of the previously discussed TAG mixtures in the DSC, is used for the investigation of several PPP-SSS mixtures. For all the measured compositions of the PPP-SSS mixture, one crystallization peak occurs during cooling with the above-mentioned high cooling rates. This suggests that the components mix ideally in the α-polymorph. To illustrate the effect of composition of the PPP-SSS mixture on the DSC trace, we present the cooling and heating scans of two PPP-SSS mixtures, being of compositions x SSS =.522 and x SSS =.879 (Fig. 19 a, b). For both mixtures, the scans refer to the solid phases formed at a cooling rate of 1 K min -1. Both heating scans show first the melting of the α- polymorph, followed by the immediate re-crystallization into the β -polymorph. This polymorph melts on further heating and subsequently re-crystallizes into the most stable β- polymorph. In principle, the phase transitions are the same for both mixtures, but the

28 122 Chapter 5: Thermal Analysis of Binary TAG Mixtures sharpness and the intensity of the transition peaks are clearly different. When the amount of the components is about the same, the peaks on the cooling and the heating scans are somewhat broader than in the case where the component SSS dominates. This is not surprising, in the sense that the purer the sample, the sharper the transition peaks. 1 a) heat flow / mw b) heat flow / mw Figure 19. Cooling and heating scans of the PPP-SSS mixtures: a) x SSS =.552, b) x SSS =.879. In both cases the cooling rate was 1 K min -1.

29 Chapter 5: Thermal Analysis of Binary TAG Mixtures Summary The thermal analysis of three binary mixtures of TAGs, being EEE-SSS, EEE-PPP and PPP-SSS, is performed by means of DSC and adiabatic calorimetry. The phase diagrams of the β-polymorph of the mixtures are constructed using the DSC data obtained from slow cooling of the melts and subsequent melting of the formed solid phases. Regarding the EEE- SSS mixture, the components exhibit limited miscibility in the most of the composition range, while the SSS-rich mixtures form solid solutions. The components EEE and PPP show no mixing in the β-polymorph for the whole composition range. Still, it remains difficult to determine the miscibility in the EEE-rich mixtures, since they do not crystallize in the most stable polymorph during slow cooling. As for the PPP-SSS mixture, the β-polymorph exhibits typical eutectic phase diagram with the regions of solid solutions close to the pure components. However, the β-polymorph of SSS-rich mixtures, formed during slow cooling in the adiabatic calorimeter, shows limited miscibility of the components. That is in contrast to the DSC results, which imply that these SSS-rich mixtures should be in the state of solid solutions. Namely, in the DSC experiments, the β-polymorph of diluted PPP-SSS mixtures is formed by the re-crystallization of a less stable polymorph. In this way, the miscibility of the components in the solid phase is enhanced. These observations point to the impact of kinetics on the mixing properties of the β-polymorph. Finally, the solid states of the mentioned TAG mixtures, formed during fast cooling of the melts in the DSC, are discussed. The results show a remarkable difference between the samples containing the unsaturated EEE component and the PPP-SSS mixture. In the latter sample, the α-polymorph is easily obtained for high cooling rates, whereas for the mixtures containing EEE only part of the sample crystallizes in the α-form. An explanation of this behaviour is that the mixing in the α-polymorph is not ideal for the EEE containing samples, due to the significant structural differences in the molecular shapes of the saturated and the unsaturated TAG component.

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