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Langmuir 1998, 14, 2238-2244
Articles A Thermodynamic Study on the Thermoreversible Poly(vinylidene fluoride) Gels in Acetophenone, Ethyl Benzoate, and Glyceryl Tributyrate Sukumar Mal and Arun K. Nandi* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Calcutta-700 032, India Received August 13, 1997. In Final Form: January 8, 1998 The gel melting temperatures of poly(vinylidene fluoride) (PVF2) gels in acetophenone (ACTP), ethyl benzoate (EB), and glyceryl tributyrate (GTB) are measured by differential scanning calorimetry (DSC) at high PVF2 concentration (WPVF2 g 0.1) and visually for very low PVF2 concentration (WPVF2 e 0.1). The gelation temperatures are also measured in DSC by dynamic cooling method. The enthalpy of gelation and enthalpy of gel fusion vs weight fraction of PVF2 (WPVF2) plots exhibit different nature in the three solvents. It is linear for the PVF2/ACTP gels but the PVF2 gels in the other two solvents exhibit positive deviation from linearity. A thermodynamic analysis of the enthalpy values in PVF2/EB and PVF2/GTB systems indicates the polymer-solvent complex formation in both the systems. The stoichiometry of the PVF2-EB compound is 1:1 molar ratio and that of PVF2-GTB compound is 3:1 molar ratio with respect to PVF2 monomeric unit and the solvent molecule, respectively. The phase diagram of the PVF2/ACTP system is linear, that of PVF2/EB system exhibits compound formation with incongruent melting point, and that of PVF2/GTB system exhibits compound formation with singular point. The spheroidal morphology of the PVF2/ACTP gels has been attributed to the chain folding process, whereas the fibrillar morphology of PVF2/GTB gels is due to polymer-solvent complex formation. The reason for the mixed morphology of PVF2/EB gels is not clear and is probably due to the comparable rates of the polymer-solvent compound formation and the chain-folding processes.
Introduction Thermoreversible gelation is a cross-linking process occurring through the physical forces. The nature of physical forces is different for different systems.1 However, from a detailed study of the poly(vinylidene fluoride) (PVF2) gels in the three solvents, acetophenone (ACTP), ethyl benzoate (EB), and glyceryl tributyrate (GTB) it has been shown earlier that the crystallization force is responsible for the gelation in each case.2-4 The morphology of the PVF2 gel is very much dependent on the nature of the solvent.4 The PVF2/ACTP gels have spheroidal morphology, PVF2/EB gels have a mixture of spheroidal and fibrillar morphology, and PVF2/GTB gels have only the fibrillar morphology. Since crystallization is the governing force for gelation of PVF2 in these solvents, different modes of crystallization (chain folding or fibrillar crystallization) produce different morphologies of the gels.5 Therefore, it is an important question why the mode of crystallization is different in different solvents. To shed light on this problem, a thermodynamic study of the PVF2 gels in the three solvents has been done and is reported here. Thermodynamic study of the gels helps to understand the interactions in the polymer-solvent systems giving (1) Berghmans, H. In Integration of Fundamental Polymer Science and Technology; Lemstra P. J., Kleintjens, I. A., Eds.; Elsevier Applied Science: London, 1988; Vol. 2, p 296. (2) Mal, S.; Maiti, P.; Nandi, A. K. Macromolecules 1995, 28, 2371. (3) Mal, S.; Nandi, A. K. Macromol. Symp. 1997, 114, 251. (4) Mal, S.; Nandi, A. K. Polymer, in press. (5) Keller, A. In Structure-Properties Relationship of Polymeric Solids; Hiltner, A., Ed.; Plenum Press: New York, 1983; p 25.
an insight to the gelation mechanism of each system. Some reports on the thermodynamic studies of the thermoreversible and the thermal gelation are found in the literature.6-11 However, no generalization on the thermodynamic behavior of these gels can be made, because it is dependent on the individual polymer-solvent system. In the thermoreversible poly(vinylidene fluoride) gels no thermodynamic study has yet been reported. In this thermodynamic study we draw attention mainly on the enthalpy changes during gelation/gel formation and also on the phase diagrams in the three solvent systems. PVF2 is an important polymer because of its piezoelectric and pyroelectric properties.12 It is not completely isoregic in its chain structure, and apart from the usual head to tail structure, it has also some head to head (H-H) defect structures.12,13 It is also an aim of this work to study how the phase diagrams of the gels depend on the H-H defect structure of PVF2. The influence of molecular weight on the phase digrams of PVF2 gels will also be delineated here. (6) Guenet, J. M.; McKenna, G. B. Macromolecules 1988, 21, 1752. (7) Daniel, Ch.; Deluca, M. D.; Guenet, J. M.; Brulet, A.; Menelle A. Polymer 1996, 37, 1273. (8) Spevacek, J.; Saini, A.; Guenet, J. M. Macromol, Rapid. Commun. 1996, 17, 389. (9) Loyen, K.; Iliopoulos, I.; Audelert, R.; Olsson, U. Langmuir 1995, 11, 1053. (10) Sakai, M.; Sotoh, N.; Tsujii, K.; Zhang, Y. Q.; Tanaka T. Langmuir 1995, 11, 2493. (11) Tipton, D. L.; Russo, P. S. Macromolecules 1996, 29, 7402. (12) Lovinger, A. J. In Developments in Crystalline Ploymer-1; Basset, D. C., Ed.; Applied Science Publishers: London, 1981; p 195. (13) Cais, R. E.; Solane, N. J. A. Polymer 1983, 24, 179.
S0743-7463(97)00915-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/03/1998
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Table 1. Characteristics of the Samples Used
sample
source
KY-201 Pennwalt Corp. Sol-10 Solvey Corp. Sol-12 Solvey Corp.
a
hv× M hw× M hn× M 10-5 10-5 10-5
PDI
H-H defect (mol %)
3.12 2.14 3.01
2.82 2.09 2.57
5.31 4.19 4.06
6.71 3.96 6.27
8.81 4.48 7.74
Experimental Section Samples. Three commercial PVF2 samples are used in the work. The characteristics of the samples are presented in Table 1.2 The samples are recrystallized from dilute solutions of acetophenone, repeatedly washed with methanol, and dried in a vacuum at 80 °C for 3 days. The solvents acetophenone (ACTP) (E. Merck) and ethyl benzoate (EB) (E. Merck) are dried over CaCl2 and are fractionally distilled. The middle fraction of each solvent is used in the work. Glyceryl tributyrate (GTB) (Sigma, USA) is used as received. Thermodynamic Study. The thermodynamic study of the PVF2 gels has been done by two methods: (i) differential scanning calorimeter (DSC) method and (ii) visual method. DSC Method. A Perkin-Elmer differential scanning calorimeter (DSC-7) is used in the work. The instrument is calibrated with indium before each set of experiments. The required amounts of polymer and solvent with varying compositions are taken in the Perkin-Elmer LVC capsules and are tightly sealed with the help of a quick press. These sealed capsules are melted at 180 °C in the DSC for 10 min and are made homogeneous by occasional shaking. These are then quenched at 50 °C in the DSC and kept there for 30 min to complete the gel formation. The gels are then scanned at a heating rate of 40 °C/min from 50 to 180 °C. The higher heating rate is chosen to avoid any melt recrystallization during the heating process.4,14 From the area of the melting endotherm the enthalpy of gel fusion is measured, and from the peak temperature of the endotherm the gel melting temperature is measured. The gel formation enthalpy and the gel formation temperature are measured in DSC by the dynamic cooling method. In this method the samples in the LVC capsules are kept at 180 °C and after the samples become homogeneous they are cooled at the rate of 5 °C/min from that temperature to 50 °C and the exotherms are recorded. From the peak area the enthalpy of gelation is determined, and from the peak temperature the gelation temperature is determined. The representative melting endotherms and crystallization exotherms of the KY PVF2/GTB system are shown in parts a and b of Figure 1, respectively. Visual Method. This method has been applied for dilute PVF2 gels (WPVF2 e 0.1, where WPVF2 ) weight fraction of PVF2). Here the gels are prepared in sealed glass tubes. Appropriate amounts of polymer and solvents are taken in glass tubes, sealed at one end. The tubes are then degassed by repeated freeze-thaw technique and are sealed under vacuum (10-3 mmHg). The sealed tubes are then melted at 180 °C for 10 min in an air oven and after becoming homogeneous are quenched rapidly to room temperature (30 °C) for 30 min. The tubes containing the gels are then kept inverted in a thermostatic bath whose temperature is increased by 1 °C in every 2 h. The temperature where the gel breaks and falls down to the bottom has been called the gel melting temperature. At this temperature, however, opacity or an aggregate-like structure still remains, and further increase of temperature by 2-4 °C helps it to transform into a transparent and homogeneous solution. This transformation temperature has been denoted as the crystal dissolution temperature. Scanning Electron Microscopy (SEM) Study. The PVF2 gels are prepared in sealed glass tubes as mentioned earlier. Then a small portion of these gels are taken out by breaking the seal and dried at 40 °C under vacuum for 3 days. These are then gold coated and SEM pictures are taken from a SEM instrument (Hitachi S-415 A).
Results The Enthalpy of Gelation/Gel Fusion. Thermoreversible gelation is usually accompanied by enthalpy (14) Prest, W. M., Jr.; Luca, D. J. J. Appl. Phys. 1975, 46, 4136.
b
Figure 1. (a) DSC thermograms of the gel fusion of KY-201 PVF2/GTB gels prepered at 50 °C (heating rate 40 °C/min) for indicated weight fractions of PVF2. (b) Gelation exotherms of KY-201 PVF2/GTB gels obtained from dynamic cooling (cooling rate 5 °C/min) for indicated weight fractions of PVF2.
changes (first-order transition)15 and the variation of enthalpy with composition yields some important information regarding the molecular mechanism of gelation.6-8 In Figures 2-4 the enthalpy of gelation and enthalpy of gel fusion have been plotted with weight fraction of KY201 PVF2 (WPVF2) for the three solvents. In the KY-201 PVF2/ACTP gels the data fit very well in a straight line for both the enthalpy of gelation and gel melting (Figure 2). However, the same plots for KY-201 PVF2/EB gels and KY-201 PVF2/GTB gels show positive deviations from linearity. The enthalpy of gel formation/gel melting may be considered as consisting of four contributions:16 (15) Daniel, C.; Dammer C.; Guenet, J. M. Polymer 1994, 35, 4243. (16) Here ∆H has been expressed per gram basis. By replacing the weight fraction (wi) of eq 1 by mole fraction, it will be per mole basis.
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Figure 2. Plots of enthalpy of gel fusion and enthalpy of gel formation (∆H) per gram of gel vs weight fraction of PVF2 for KY-201 PVF2/ACTP gels. (b) gel fusion and (O) gel formation.
Figure 4. Plots of enthalpy of gel fusion and enthalpy of gel formation (∆H) per gram of gel vs weight fraction of PVF2 for KY-201 PVF2/GTB gels: (b) gel fusion and (O) gel formation.
(Figure 3 and Figure 4) corresponds to ∆Hm + ∆Hc. The enthalpy of random mixing of the polymer-diluent system is usually expressed as:17
∆Hm ) RTχ1Φ22
Figure 3. Plots of enthalpy of gel fusion and enthalpy of gel formation (∆H) per gram of gel vs weight fraction of PVF2 for KY-201 PVF2/EB gels: (b) gel fusion and (O) gel formation.
∆H ) w1∆H1 + w2∆H2 + ∆Hm + ∆Hc
(1)
where the first term is the contribution of the solvent, and here it is zero since at the temperature of interest the solvent does not crystallize. The second term is that of the PVF2 crystal, the third term is due to the random mixing of the polymer and solvent, and the fourth term contributes the formation of any oriented structure (polymer-solvent compound). So
∆H - w2∆H2 ) ∆Hm + ∆Hc
(2)
Thus the positive deviation (∆) from linearity in the plots
(3)
where χ1 is the polymer-solvent interaction parameter and Φ2 is the volume fraction of the polymer. Therefore, ∆ - RTχ1Φ22 ) ∆Hc and in the PVF2/ACTP system its value is zero at all compositions since both ∆ and ∆Hm have values almost equal to zero (since χ1 in the PVF2 /ACTP system is 0.09).2 The χ1 values of PVF2 in EB and GTB are 0.31 and 0.39, respectively,2,4 and ∆ also have significant values. So, ∆-RTχ1Φ22 values are calculated and are plotted in parts a and b of Figure 5, respectively. It is apparent from the figures that there is a maximum in each plot. By analogy with the ∆Hvs composition plot (Tammns plot) of polymer-solvent systems6-8,18 the stoichiometry of the complex has been calculated from the composition of the maximum. In the PVF2-EB system the composition of the complex formation is WPVF2 ) 0.3, which corrosponds to the 1:1 molar ratio of PVF2 momomeric unit and EB. This indicates that the >CdO group of EB interacts with the >CF2 dipoles of the momomeric unit of PVF219,20 resulting in 1:1 [AB type] complex formation. In the PVF2/GTB gel the stoichiometry of the complex is WPVF2 ) 0.4, which corresponds to the molar ratio of 3:1 with respect to monomeric unit of PVF2 and GTB solvent, respectively. GTB has three >CdO groups in a molecule and these three >CdO groups interact with the >CF2 dipoles of the three monomeric units of PVF2 producing an AB3 type complex. So it can be concluded that AB type and AB3 type molecular compounds are produced in the PVF2/EB and PVF2/GTB systems, respectively, and no such compound formation occurs in the PVF2/ACTP system. (17) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; p 495. (18) Guenet, J. M. Thermochim. Acta 1996, 284, 67. (19) Roerdink, E.; Challa, G. Polymer 1980, 21, 509. (20) Belke, R. E.; Cabasso, I. Polymer 1988, 29, 1831.
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a
b
Figure 5. (a) ∆-RTχ1Φ22 vs weight fraction of PVF2 plot for KY-201 PVF2/EB system. (b) ∆-RTχ1Φ22 vs weight fraction of PVF2 plot for KY-201 PVF2/GTB system.
Now it is required to know the values of enthalpies of complexation which are calculated from the maximum of parts a and b of Figure 5 and have been found to have values equal to 0.2 and 0.14 kcal/mol for the PVF2/GTB system and PVF2/EB system, respectively. The values are much lower than that of chemical changes, so the complexation is very weak and it is easily breakable by application of thermal energy. The Phase Diagrams. (a) From the DSC Method. Parts a-c of Figure 6 represent the phase diagrams of KY-201 PVF2 gels in ACTP, EB, and GTB, respectively. Both the gel melting temperatures and the gelation temperatures are plotted with composition (WPVF2) of the gel and they have the same shape for each system. However, there is a gap of a few degrees between the two plots due to hysteresis. The phase diagrams are, therefore, characteristics of polymer solvent systems formed under equilibrium.18 But, the nature of the plots is not the same in the three solvents. The PVF2-ACTP system exhibits almost linear nature, but in the other two solvents there
Figure 6. (a) Phase diagram of KY-201 PVF2/ACTP gels obtained from the DSC method: (2) gel melting temperature and (4) gelation temperature. (b) Phase diagram of KY-201 PVF2/EB gels obtained from the DSC method: (O) gel melting temperature and (4) gelation temperature. (c) Phase diagram of KY-201 PVF2/GTB gels obtained from the DSC method: (O) gel melting temperature and (4) gelation temperature.
is a considerable deviation from linearity. The phase diagram of the PVF2/EB system (Figure 6b) corresponds to compound formation with incongruent melting point and that of the PVF2/GTB system corresponds to compound formation with a singular point (intermediate between congruent and incongruent melting point).6,21 So the phase diagrams also support that polymer-solvent compound
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Figure 8. Phase diagrams of PVF2/EB gels (WPVF2 e 0.1) obtained by the visual method (closed symbols for Tgm and open symbols for Td): (O) KY-201; (4) Sol-10; (0) Sol-12 PVF2. Figure 7. Phase diagrams of PVF2/ACTP gels (WPVF2 e 0.1) obtained by the visual method (closed symbols for Tgm and open symbols for Td): (O) KY-201; (4) Sol-10; (0) Sol-12 PVF2.
formation takes place in the PVF2/EB and in the PVF2/ GTB systems, whereas no such compound formation occurs in PVF2/ACTP gels. In making the phase diagrams, we used the thermograms which usually exhibit single endothermic/exothermic peaks and no separate peak for the compound6,7 is observed for both the cooling and heating procedures applied here. So it may be considered that the melting endotherm is actually composed of two nondistinguishable peaks. The peak temperature of the first endotherm is invarient with concentration and corresponds to
compound f liquid + solid and the second endotherm whose associated temperature is concentration dependent corresponds to
solid f liquid In parts b and c of Figure 6 the nonvarient transition temperatures are indicated by dotted lines. (The dotted line indicates that it is not physically identifiable in this system). (b) From the Visual Method. In Figures 7-9 the phase diagrams obtained by the visual method of PVF2/ ACTP, PVF2/EB, and PVF2/GTB systems are presented for the composition WPVF2 e 0.1. Both the gel melting temperatures (Tgm) and crystal dissolution temperatures (Td) are plotted in each figure. In Figure 7, the phase diagrams of PVF2/ACTP systems are presented for all the KY-201, Sol-10, and Sol-12 PVF2 systems. From the phase diagrams of Sol-10 and Sol-12 PVF2, the influence of molecular weight can be worked out. The phase diagrams of lower molecular weight Sol-10 PVF2 is situated at a somewhat lower temperature region than that of the higher molecular weight Sol-12 PVF2 system, particularly for the low concentration of the polymer. This indicates that the larger molecular weight polymer favors better network formation. However, the crystal dissolution temperature does not exhibit any such dependency. The influence of the H-H defect on the phase diagrams for the gels can be obtained by comparing the gel melting (21) Reisman, A. Phase Equilibria; Academic Press: New York, 1970.
Figure 9. Phase diagrams of PVF2/GTB gels (WPVF2 e 0.1) obtained by the visual method (closed symbols for Tgm and open symbols for Td): (O) KY-201; (4) Sol-10; (0) Sol-12 PVF2.
temperature of Sol-12 and KY-201 systems. Both the gel melting temperature and the crystal dissolution temperature of the later system are much lower than those of the Sol-12 PVF2 gels. This indicates that the gels of higher H-H defect content PVF2 is thermodynamically weaker than that of the lower H-H defect content PVF2 gels. A probable cause is that the H-H defects of PVF2 enter into the crystal lattice as defect structure and the larger the H-H defect is in the chain, the larger is its amount in the crystal lattice.22-24 Since crystallites are forming the junction points in this gelation process,2-4 so the junction points of KY-201/PVF2 gels have a propensity of larger amount of H-H defect than that in the lower defect content Sol-12 sample. These higher defect containing junctions are weaker than those of more regular Sol-12 PVF2 system, and probably this is the cause for lower gel melting temperature of the KY-PVF2 system. The crystal dis(22) Farmer, B. L.; Hopfinger, A. J.; Lando, J. B. J. Appl. Phys. 1972, 43, 4293. (23) Lovinger, A. J.; Davis, D. D.; Cais R. E.; Kometani, J. M. Polymer 1987, 28, 617. (24) Datta, J.; Nandi A. K. Polymer 1994, 35, 4804.
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difference of two melting points, DSC cannot differentiate the two. Slower heating rates in DSC also create a problem in this gel due to the melt recrystallization.4,14 Figures 8 and 9 represent the phase diagrams of PVF2/ EB and PVF2/GTB systems, respectively. The general character of the phase diagrams is the same as in the PVF2/ACTP system with respect to molecular weight and H-H defect of PVF2 sample; however, there is a major difference. The gel melting and the dissolution temperatures are higher by ∼25 and ∼30 °C in the PVF2/EB gel and PVF2/GTB gels, respectively. A probable cause is the solvent power of the three solvents. The polymer-solvent interaction parameter (χ1) values for ACTP, EB, and GTB solvents are 0.09, 0.31, and 0.39,2,4 respectively. Thus both the dissolution temperature and the gel melting temperature are higher for solvents of lower χ1 values. It can, therefore, be concluded that the gel melting temperature depends on the polymer-solvent interaction parameter. Another interesting point is that the difference between gel melting temperature and crystal dissolution temperature is lowest for the PVF2/GTB gel than those of the other two solvents. Though no definite reason is known, it may possibly be due to the different morphology PVF2 gels in GTB than those in the other two solvents as presented in the following section. Discussion In Figure 10 the morphology of PVF2 gels in ACTP, EB, and GTB are shown. It is apparent from the figures that the morphology of PVF2 gels in ACTP is spheroidal and that in GTB is fibrillar. However, in the PVF2/EB system there is a mixed morphology containing both the spheroidal and fibrillar types. Since chain folding is the cause of spheroidal morphology,5 in PVF2/ACTP gels chain folding is occurring. In the PVF2-GTB gel the morphology is fibrillar, and it arises as a result of the inhibition of the chain-folding process. Probably, the compound formation of the PVF2 chain with GTB prevents the chain-folding process, yielding the fibrillar crystals. In the PVF2/EB system compound formation occurs, but yet a mixed morphology of the gel consisting of both spheroidal and fribrilar types is observed. The reason is not clear to us; however, a probable cause may be due to the comparable rates of the chain-folding and the compound-formation processes in this system. The rate of polymer-solvent compound formation in the PVF2-GTB system is possibly much higher than that in the PVF2-EB system because of the higher exponent values in the concentration terms of the rate expression: In GTB:
3>CdO + 3(-CH2-CF2-) f PVF2-GTB complex Figure 10. Scanning electron micrographs of 7% PVF2 gels (w/v) in (a) ACTP, (b) EB, and (c) GTB.
solution temperatures are also much lower for the KY201 PVF2 system than that of the Sol-12 PVF2 system due to the propensity of a large amount of H-H defect in the crystal lattice of the former sample. Now a question may arise regarding the presence of two melting temperatures, e.g., the gel melting temperature and the crystal dissolution temperature in the same system. This is probably due to the fact that at Tgm the disruption of network structure occurs and after Tgm the crystals are present in unassociated form because then the solution does not have the critical gelation concentration which increases with temperature.2 In other words, there are two different types of crystallites present in this gel, one kind involves the network formation that melts at lower temperature and the other kind, which is free, melts at higher temperature. Due to the very low
rate ) k[>CdO]3[-CH2-CF2-]3
(4)
and in EB:
>CdO + (-CH2-CF2-) f PVF2-EB complex rate ) k[>CdO]1[-CH2-CF2-]1
(5)
Thus due to the faster rate of compound formation in GTB, the morphology of this gel is totally fibrillar. The comparatively slower rate of compound formation in the PVF2/EB system than that in the PVF2-GTB system produces a mixed morphology. Conclusion The thermodynamic study indicates that the polymersolvent compounds are formed in case of PVF2/EB and
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PVF2/GTB gels. The enthalpy of compound formation is ∼0.2 kcal/mol, which indicates that the compound is physical in nature. The stoichiometry of the complex is 1:1 for the PVF2/EB gels and 3:1 for the PVF2/GTB gels with respect to the monomeric unit of PVF2 and the solvent, respectively. However, a detailed structure of the compound is not known to us. In the case of PVF2/ACTP gels, no such compound formation takes place. The phase diagrams of the PVF2 gels in the three solvents ACTP, EB, and GTB are different. The PVF2/ ACTP gels show an ideal behavior, whereas in PVF2/EB and PVF2/GTB systems compound formation with incongruent melting point and singular point are observed,
Mal and Nandi
respectively. The phase diagrams of PVF2 gels (WPVF2 e 0.1) are also dependent on the H-H defect structure present in the chain. With higher molecular weight of PVF2, the gel melting temperature and the gelation temperature have somewhat higher values. The spheroidal morpholgy of the PVF2/ACTP gels is due to the chain-folding process, but compound formation inhibits the chain-folding process and produces fibrillar morphology in the PVF2/GTB gels. However, no definite reason can be given for the mixed morphology of PVF2/EB gels from the thermodynamic study alone. LA9709150
