Chapter 2 Synthesis of Siloxane-Imide-Containing Benzoxazines
2.2 Synthesis of Siloxane-Imide-Containing Benzoxazine (BZ-A6)
2.2.4 Synthesis of siloxane-imide-containing benzoxazine (BZ-A6)
Aniline (3.8 g, 0.04 mol) was added dropwisely into a mixture of A6-OH (18.95 g, 0.02 mole), paraformaldehyde (2.4 g, 0.08 mole), and 1,4-dioxane (120 ml) in a 250 ml round-bottom flask equipped with a magnetic stirrer bar (Scheme 2-4). The mixture was then heated under reflux at 115 °C for 20 hrs, gradually becoming homogeneous and turning dark brown. The resulting mixture was filtered and the solvent was evaporated under vacuum. The residue was dissolved in ethyl acetate and washed five times sequentially with 0.5 N aqueous NaOH and distilled water. Evaporation of the
solvent and vacuum drying in an oven provided BZ-A6 as a viscous dark brown liquid product (yield: 87.7%). 1H-NMR (CDCl3) (Figure 2-5) δ: 6.70~7.30 ppm (aromatic protons), 5.35 ppm (OCH2N), 4.65 ppm (Ar–CH2–N). FT-IR (KBr) (Figure 2-6): 1256 cm-1 (C–O–C, stretching), 1178 cm-1 (C–N–C, stretching), 1307 cm-1 (CH2, wagging of oxazine), 1502 cm-1 (trisubstituted benzene ring).
8 7 6 5 4 3 2 1 0
Figure 2-5.1H-NMR spectrum of the siloxane-imide–containing benzoxazine BZ-A6.
4000 3500 3000 2500 2000 1500 1000 500
Si CH3
CH3 O Si
CH3
CH3 O Si CH3
CH3 N N
O
O
O
O O O
N N
n
1178cm-1 1502cm-1
v (cm-1)
T (%)
1256cm-1 1307cm-1
Figure 2-6. FT-IR spectrum of the siloxane-imide–containing benzoxazine BZ-A6.
O
Scheme 2-3. Syntheses of compounds A6 and A6-OH
Si
Scheme 2-4. Preparation of compound BZ-A6
References
[1] Holly, F. W.; Cope, A. C. J. Am. Chem. Soc. 1944, 66, 1875.
[2] Ghosh, N. N.; Kiskan, B.; Yagci, Y. Porg. Polym. Sci. 2007, 32, 1344.
[3] Li, H.T; Chang H.R.; Wang, M. W; and Lin, M. S. Polym Int 2005, 54, 1416.
[4] Eddy, V. J; Hallgren, J. E. and Robert, E. J Polym Sci Part A: Polym Chem 1990, 28, 2417.
Chapter 3
Curing Behavior of Siloxane-Imide-Containing Benzoxazines
To understand the polymerization reaction of benzoxazines, an understanding of the chemical structure of its oxazine ring is very important. The ring opening of the benzoxazine was first discussed by Burke et al. [1] In the reaction of 1,3-dihydrobenzoxazine with a phenol, having both ortho and para position free, it was found that aminoalkylation occurred preferentially at the free ortho position to form a Mannich base bridge structure, along with small amount reaction at para position. A cross-linked network structured polybenzoxazines, with higher Tg and degradation temperature, can be obtained when benzoxazines undergo polymerization.
It has been observed that during synthesis of a difunctional benzoxazine (from bisphenol A, formaldehyde and aniline) form by the subsequent reactions between the rings and ortho position of bisphenol A hydroxyl groups. These free phenolic hydroxyl structure containing dimmers and oligomers trigger the monomer to be self-initiated towards polymerization and crosslinking reactions. [2] The curing behavior of siloxane-imide-containing benzoxazines, BZ-A1 and BZ-A6, are discussed in this section.
3.1 Curing behavior of the siloxane-imide–containing benzoxazine BZ-A1
Typically, benzoxazines undergo exothermic ring opening reactions at ca.
200–250 °C, which can be monitored using DSC. DSC was performed using a TA Instrument DSC-Q10 apparatus operated at a heating rate of 10 °C/min under a N2
atmosphere. The gas flow rate was 40ml/ min. Benzoxazine samples of approximately 5 mg were scanned in hermetic aluminum sample pans. The reaction point of the bisphenol A–type benzoxazine Ba is 228.7 °C; the energy of the exothermic ring opening reaction is 296.0 J/g (Figure 3-1). The thermogram of BZ-A1 in Figure 3-1 reveals a ring opening exothermic reaction having an onset temperature at 194.9 °C and a peak point at 232.7 °C. The exothermic energy of BZ-A1 is 173.7 J/g; i.e., it is lower than that of Ba, presumably due to molecular weight effect, molecular weight of BZ-A1 (879 g/mol) is significantly higher than that of Ba (462 g/mol). The PBZs of Ba (PBa) and BZ-A1 (PBZ-A1) were then cured in an oven under the curing conditions listed in Table 3-1.
Exo Up Universal V4.4A TA Instruments
Figure 3-1. DSC thermograms of Ba and BZ-A1.
Table 3-1. Curing conditions for PBZs
Benzoxazine Ba BZ-A1
200 °C/2 hrs + 230 °C/2 hrs 200 °C /2 hrs + 230 °C/4 hrs Curing conditions
200 °C /2 hrs + 230 °C/6 hrs
PBZs usually exhibit good thermal properties after polymerization. [3] The glass transition temperature of PBZ-A1 after cross-linking was 186.1 °C (Figure 3-2), which is substantially higher than that of typical PBZs (PBa: Tg= 150.0 ℃). [4] In general, the longer and flexible of siloxane segments in the matrix structure results in lower of Tg(Tg from tan δ peak of CP-F-Bz/BATMS-Bz-100 is 116 ℃) as discussed by Liu et al. [5] Our PBZ-A1 structure features both siloxane and imide segments in the benzoxazine monomer where the imide segment tends to raise the glass transition temperature.
186.1 deg. C
-0.60 -0.55 -0.50 -0.45 -0.40 -0.35
Heat Flow (W/g)
100 120 140 160 180 200 220
Temperature (°C)
Exo Up Universal V4.4A TA Instruments
Figure 3-2. Glass transition temperature (Tg) of PBZ-A1, determined from the DSC trace.
3.2 Curing behavior of the siloxane-imide–containing benzoxazine BZ-A6
In general, benzoxazines undergo exothermic ring opening at temperatures of ca.
200–250 °C [6-9] which can be monitored using DSC. The thermogram of BZ-A6 in Figure 3-3 reveals a ring opening exothermic reaction having an onset temperature at 153.7 °C and a peak maximum at 214.2 °C with exothermic energy of 57.9 J/g. After curing at 200℃ for 2 hrs, the reaction heat is decreased to be 37.9 J/g from 57.9 J/g.
We performed the polymerization of BZ-A6 using a two-step process; the first step
involved benzoxazine ring opening at 200 °C and the second involved post curing at a 230 °C. PBZs were cured in an oven under the curing conditions listed in Table 3-2.
BZ-A6 monomer
200℃/ 2h
Tp = 214.2 ℃ Delta H = 57.9 J/g
Tp=221.1 ℃ Delta H = 37.9 J/g
200℃/ 2h + 230℃/ 2h
200℃/ 2h + 230℃/ 4h
200℃/ 2h + 230℃/ 6h
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4
Heat Flow (W/g)
75 125 175 225 275
Temperature (°C)
Exo Up Universal V4.4A TA Instruments
Figure 3-3. DSC thermograms of BZ-A6 monomer and polymerized BZ-A6 (after curing).
Table 3-2. Curing conditions for PBZs
Benzoxazine Ba BZ-A1 BZ-A6
200 °C/2 hrs + 230 °C/2 hrs 200 °C/2 hrs+ 230 °C/4 hrs Curing conditions
200 °C/2 hrs+ 230 °C/6 hrs
References
[1] Burke, W. J.; Bishop, J. L.; Glennie, E. L. M.; Bauer, W. N. J. Org. Chem.1965, 30, 3423.
[2] Ning, X.; Ishida, H. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1121.
[3] Ghosh, N. N.; Kiskan, B. and Yagci, Y. Prog Polym Sci 2007, 32, 1344.
[4] Ishida, H. and Allen, D. J. J Polym Sci Part B: Polym Phys 1996, 34, 1019.
[5] Liu, Y. L.; Hsu, C. W. and Chou, C. I. J Polym Sci Part A: Polym Chem 2007, 45, 1007.
[6] Chen, K. C.; Li, H. T.; Chen, W. B.; Liao, C. H.; Sun, K. W. and Chang, F. C.
Polym Int in press
[7] Takeichi, T.; Kano, T and Agag, T. Polymer 2005, 46, 12172.
[8] Agag, T. and Takeichi, T. Macromolecules 2003, 36, 6010.
[9] Takeichi, T.; Agag, T. and Zeidam, R. J Polym Sci Part A: Polym Chem 2001, 39, 2633.
Chapter 4
Thermal/ Mechanical Properties of Siloxane-Imide-Containing Polybenzoxazines
The physical, mechanical and thermal properties of polybenzoxazines are primarily decided by the nature of the diphenol and diamine. The properties of polybenzoxazines are shown to compare very favorably with those of conventional phenolic and epoxy resins. DMA reveals that these candidate resins for composite applications possess high modulus and glass transition temperatures. Long-term immersion studies indicate that they have low water absorption and loe saturation compact. Impact, tensile and flexural properties are also good. [1] BZs are cured usually in the temperature window of 160-220℃. The polymer exhibit Tg in the range 160-340℃ depending on the structure, and have higher stability. The high TGA decomposition onset temperature and char yield are attributed to the very strong intramolecular H-bonding between phenolic OH and the Mannich bridge. [2]
In this section, we discussed the thermal and mechanical properties of polymerized siloxane-imide-containing PBZ-A1 and PBZ-A6.
4.1 Thermal stability of the poly-siloxane-imide–containing benzoxazine PBZ-A1
4.1.1 Materials and Characterization
The bifunctional bisphenol A–type benzoxazine (Ba, Figure 4-1) was purchased from Shikoku Chemicals (Japan). The siloxane-imide-containing benzoxazine, BZ-A1, was synthesized from the according method in chapter 2, the structure is shown in
Figure 4-2. DSC was performed using a TA Instrument DSC-Q10 apparatus operated at a heating rate of 10 °C/min under a N2 atmosphere. The gas flow rate was 40ml/ min.
Benzoxazine samples of approximately 5 mg were scanned in hermetic aluminum sample pans. TGA was performed using a TA Instrument TGA-Q500 apparatus operated at a heating rate of 20 °C/min under an atmosphere of N2 or air, respectively.
An energy dispersive system (EDS) was used for element test, which was recorded
Figure 4-1. Structure of the bifunctional bisphenol A–type benzoxazine Ba.
O N
Figure 4-2. Structure of the BZ-A1.
4.1.2 TGA of the poly-siloxane-imide–containing benzoxazine PBZ-A1
PBZs usually exhibit good thermal properties after polymerization. [3] The glass transition temperature of PBZ-A1 after cross-linking was 186.1 °C (Figure 4-3), which is substantially higher than that of typical PBZs (PBa: Tg= 150.0 ℃). [4] In general, the longer and flexible of siloxane segments in the matrix structure results in lower of Tg(Tg from tan δ peak of CP-F-Bz/BATMS-Bz-100 is 116 ℃) as discussed by Liu et al. [5] Our PBZ-A1 structure features both siloxane and imide segments in the
benzoxazine monomer where the imide segment tends to raise the glass transition temperature.
Bisphenol-A is one of the phenolic compounds often used as the starting material for the synthesized of polybenzoxazines. PBa shows high decomposed temperature (T5% c.a. 300-330 ℃) and high char yield (c.a. 30-42 %) from TGA. [3, 6-9] Liu et al.
[5] investigated that siloxane-containg polybenzoxaizne, CP-F-Bz/BATMS-Bz-100, has Td at 369℃ in air. Figure 4-4 displays TGA thermograms recorded in air and results are summarized in Table 4-1. The 5% and 10% weight loss temperatures (T5%
loss and T10% loss, respectively) for PBZ-A1 cured at 200 °C/ 2hrs and 230 °C/ 2hrs were 380.1 °C and 441.1 °C, respectively, which are both higher than those of PBa or siloxane-containing polybenzoxazine. The PBZ-A1 shows higher thermal stability than PBa because of the presence of the siloxane-imide–containing segment. In Liu et al.
siloxane-containing polybenzoxazine TGA study, they found high thermal stability silica layers formation during the thermal oxidation process and the layer structured protect the polybenzoxazine. [10] PBa-PDMS hybrids was investigated that introduced PDMS into PBa results in the improvement of thermal stability of the hybrid. [11] The better thermal stability of PBZ-A1 with higher decomposed temperature and high char amount is come from siloxane and imide group.
In contrast, the presence of siloxane-imide groups improved the thermo-oxidative stability of the benzoxazine by increasing the char yield to 10–12 wt% in air. This char yield is close to the content of inorganic content (Si–O–Si, 8.2%) in the BZ-A1 structure. EDS analysis was employed to analyze the elemental composition of the PBZ-A1 residue after TGA testing in air. Figure 4-5 displays an image of the residue from PBZ-A1 and its EDS data. The silicone content in the residue was significantly higher than those of C and O atom, the residue from PBZ-A1 after TGA testing in air
was primarily inorganic in nature. Thus, the siloxane units of BZ-A1 provide an inorganic content in its structure, therefore, improve its thermo-stability properties after cross-linking.
The same phenomena occurred in the TGA thermograms recorded under a N2
atmosphere (Figure 4-6, Table 4-2). The 5% weight loss temperature of PBa was ca.
328–337 °C under the N2 atmosphere, whereas that of PBZ-A1 was significantly higher (ca. 355–362 °C). The temperatures for 5 and 10 wt% losses of PBZ-A1 were both higher than those for PBa. PBZ-A1 also featured a high weight residue after high temperature decomposition. The char yield of PBZ-A1 after curing at 200 °C for 2 hrs and then 230 °C for 2 hrs was high (48.0 %), i.e., it was improved by the presence of the siloxane-imide groups. It appears that the PBZ-A1 has the potential use as flame-retardant material.
186.05°C
-0.60 -0.55 -0.50 -0.45 -0.40 -0.35
Heat Flow (W/g)
100 120 140 160 180 200 220
Temperature (°C)
Exo Up Universal V4.4A TA Instruments
Figure 4-3. Glass transition temperature (Tg) of PBZ-A1, determined from the DSC trace.
Figure 4-4. TGA thermograms of PBa and PBZ-A1 (in air).
Table 4-1. Thermostabilities of the cured PBZs PBa and PBZ-A1 (in air) Polymer Curing Conditions T5% loss
(°C)
T5% loss: Temperature at which the weight loss was 5%.
T10% loss: Temperature at which the weight loss was 10%.
Td: Decomposition temperature, onset point temperature.
-20
Figure 4-5. Residue and EDS analysis of PBZ-A1 after TGA testing.
Figure 4-6. TGA thermograms of Ba and BZ-A1 (under N2).
0.0 0.5 1.0 1.5 2.0
Table 4-2. Thermostabilities of the cured PBZs PBa and PBZ-A1 (under N2) Polymer Curing Conditions T5% loss
(°C)
T5% loss: Temperature at which the weight loss was 5%.
T10% loss: Temperature at which the weight loss was 10%.
Td: Decomposition temperature, onset point temperature.
4.2 Thermal stability of the poly-siloxane-imide–containing benzoxazine PBZ-A6
4.2.1 Materials and Characterization
The bifunctional bisphenol A–type benzoxazine (Ba, Figure 4-1) was purchased from Shikoku Chemicals (Japan). The siloxane-imide-containing benzoxazine, BZ-A6, was synthesized from the according method in chapter 2, the structure is shown in Figure 4-7. DSC was performed using a TA Instrument DSC-Q10 apparatus operated at a heating rate of 10 °C/min under a N2 atmosphere. The gas flow rate was 40ml/ min.
Benzoxazine samples of approximately 5 mg were scanned in hermetic aluminum sample pans. TGA was performed using a TA Instrument TGA-Q500 apparatus operated at a heating rate of 20 °C/min under an atmosphere of N2 or air, respectively.
Si
4.2.2 TGA of the poly-siloxane-imide–containing benzoxazine PBZ-A6
Polybenzoxazines usually exhibit good thermal properties. [3] Figure 4-7 displays TGA thermograms recorded under air atmosphere. No residue remained after burning PBa at high temperature, the char was almost zero at 850 °C. The char yield of PBZ-A1 at 850 °C was 10–11 wt% and the elemental analysis confirmed that the residue was inorganic silicon oxide. [12] PBZ-A6 exhibited a higher char yield of 16–17 wt%, presumably due to the longer siloxane chain in the BZ-A6 backbone than that in BZ-A1. Thus, the char yield increased upon increasing the siloxane content in the polymer. The PBa started to decomposed, T5% loss, around 330-350℃ and it was obviously that PBZ-A1 has higher decomposed temperature to 380-395℃ from the result in Figure 4-8. Polybenzoxazine which contained the siloxane-imide segment in the main-chain, PBZ-A1 and PBZ-A6, could improve the thermal stability. The highest T5% loss was observed in the PBZ-A6 curve. The weight residue of PBZ-A6 is 16-18wt% at 850℃ under air atmosphere, which is list in Table 4-3. It was obviously that the char yield is higher than that of shorter siloxane containing PBZ-A1 (10-12 wt%) or the conventional bisphenol A type polybenzoxazine, PBa (almost 0%). It was indicated that longer siloxane chain could make further improvements in the thermal stability since the more siloxane contaning was incorporated into the main chain of PBZ-A6.
PBZ-A6
Figure 4-8. TGA thermograms of PBa, PBZ-A1 and PBZ-A6 (in air).
Table 4-3. Thermostability of cured polybenzoxazine PBa, PBZ-A1 and PBZ-A6 (in air)
2 hrs 337.3 365.5 349.3 0.2%
4 hrs 345.0 376.8 346.4 -0.3%
PBa 2 hrs
6 hrs 349.1 381.5 357.9 -0.1%
2 hrs 380.1 441.1 472.2 10.1%
4 hrs 389.4 444.2 480.7 11.3%
PBZ-A1 2 hrs
6 hrs 392.0 449.3 478.1 11.6%
2 hrs 435.4 497.9 498.3 17.8%
4 hrs 426.7 491.0 498.6 17.4%
PBZ-A6 2 hrs
6 hrs 433.3 482.4 491.5 16.9%
T5% loss: The temperature for which the weight loss is 5%.
T10% loss: The temperature for which the weight loss is 10%.
Td: The decomposed temperature
TGA analysis has revealed that PBa exhibits high decomposition temperature (T5%, ca. 300–330 °C) and high char yield (ca. 30–42%). [3, 6-8] In a previous study, we found that PBZ-A1 exhibited superior thermal properties relative to that of PBa.
[12] Figure 4-9 displays TGA thermograms recorded under N2 atmosphere and Table 4-4 summarizes the results. The 5 and 10% weight loss temperatures (T5% loss and T10%
loss, respectively) for PBZ-A6 (437.1 and 481.3 °C, respectively) were the highest among the polymers investigated in this study. The thermal decomposition temperature of PBZ-A6 was in the range 460–471 °C. PBZ-A1 and PBZ-A6 exhibited higher thermal stability than PBa because of the presence of the siloxane-imide–containing segment. Furthermore, the siloxane content of PBZ-A6 is higher than that of PBZ-A1 and the decomposition temperature of PBZ-A6 was higher accordingly. The siloxane-imide–containing PBZs also featured high weight residues after TGA. The highest char yield was 50.9% from PBZ-A6 due to the presence of longer siloxane-imide group. PBZ-A6 exhibited good thermal stability, the highest decomposition temperature, and the highest char yield. It appears that incorporating siloxane and imide moieties into the benzoxazine main chain can significantly enhance the thermal properties of PBZs, providing the potential to be used as flame-retardant materials.
PBa
Figure 4-9. TGA thermograms of PBa, PBZ-A1 and PBZ-A6 (in N2).
Table 4-4. Thermostability of cured PBZ PBa, PBZ-A1, and PBZ-A6 (in N2) Curing Conditions
2 hrs 334.6 356.8 344.2 34.3%
4 hrs 328.8 360.7 342.7 42.7%
PBa 2 hrs
6 hrs 336.5 369.8 341.6 46.3%
2 hrs 355.7 417.8 452.9 48.0%
4 hrs 361.5 427.2 448.4 48.4%
PBZ-A1 2 hrs
6 hrs 358.5 415.8 446.5 49.3%
2 hrs 437.1 474.2 471.0 45.1%
4 hrs 437.2 481.3 459.9 48.1%
PBZ-A6 2 hrs
6 hrs 430.6 477.4 463.7 50.9%
T5% loss: Temperature at which the weight loss reached 5%.
T10% loss: Temperature at which the weight loss reached 10%.
Td: Decomposition temperature
4.2.3 DMA of the poly-siloxane-imide–containing benzoxazine PBZ-A6
Figure 4-10 displays DMA thermograms of the PBZ-A6 under three curing conditions and results are summarized in Table 4-5. The curing profiles revealed that the storage modulus at room temperature was 600–800 MPa which is much lower than conventional PBa’s. In general, a higher shear storage modulus in the rubbery state indicates a polymer having a high crosslinking density [5]. The storage modulus reached the highest value after longer post curing time at rubbery state which was indicated at 200℃ and 220℃ in Table 4-5. Thus, a longer curing time improved the crosslinking density as would be expected. These results are consistent with the fact that PBZ-A6 exhibited the highest Tg (186.4 °C) of the studied polymers. From a previous study [11] the storage modulus of the brittle PBa was found to be ca. 3.2 GPa at room temperature and Tg(derived from tan δ) of 174 °C.
In generally, it is difficult to obtain free-standing PBZ films without adding plasticizers. Since the siloxane-imide–modified benzoxazine PBZ-A6 exhibits superior flexibility and toughness, PBZ-A6 readily formed a free-standing, bendable film after polymerization at a thickness of ca. 200 μm (Figure 4-11). Notably, the PBZ-A6 film exhibited not only excellent flexibility but also a high value of Tg due to the presence of the rigid imide-norborane rings in the PBZ. DMA results revealed that the presence of siloxane and imide moieties can significantly improve the flexibility and toughness of PBZs without sacrificing high glass transition temperature.
(a)
Figure 4-10. DMA thermograms of PBZ-A6: (1) storage modulus (2) tan δ
Table 4-5. Thermal mechanical analysis data for PBZ-A6 Curing conditions Storage modulus (MPa)
(b) 4hrs 595.8 73.6 46.0 31.4 184.0
(c)
2hrs
6hrs 729.5 111.3 73.7 52.0 186.4
(2) tan δ
(1) storage modulus
Figure 4-11. Photograph of a thin film of PBZ-A6, a siloxane-imide–containing PBZ.
References
[1] Nair, C. P. Reghunadhan Prog. Polym. Sci. 2004, 29, 401.
[2] Shen, S. B. and Ishida, H. Polym. Comp. 1996, 17, 710.
[3] Ghosh, N. N.; Kiskan, B.; Yagci, Y. Porg. Polym. Sci. 2007, 32, 1344.
[4] Ishida, H. and Allen, D. J. J Polym Sci Part B: Polym Phys 1996, 34, 1019.
[5] Liu, Y. L.; Hsu, C. W. and Chou, C. I. J Polym Sci Part A: Polym Chem 2007, 45, 1007.
[6] Hemvichian, K. and Ishida, H. Polymer 2002, 43, 4391.
[7] Agag, T. and Takeichi, T. Macromolecules 2003, 36, 6010.
[8] Takeichi, T.; Kano, T. and Agag, T. Polymer 2005, 46, 12172.
[9] Choi, S. W.; Ohba, S.; Brunovska, Z.; Hemvichian, K. and Ishida, H. Polymer Degradation and Stability 2006, 91, 1166.
[10] Liu, Y. L.; Chiu, Y. C. and Wu, C. S. J Appl Polym Sci 2003, 87, 404.
[11] Ardhyananta, H.; Wahid, M. H.; Sasaki, M.; Agag, T.; Kawauchi, T.; Ismail, H.
and Tachechi, T. Polymer 2008, 49, 4585.
[12] Chen, K. C.; Li, H. T.; Chen, W. B.; Liao, C. H.; Sun, K. W. and Chang, F. C.
Polym Int in press
Chapter 5
Extremely Low Surface Free Energy and UV Stability of Siloxane-Imide-Containing Polybenzoxazines
5.1 Introduction of Surface Free Energy
5.1.1 Interfacial Thermodynamics
The interface (surface) is a region of finite thickness (usually less than 0.1 μm) in which the composition and energy very continuously from one bulk phase to other.
The pressure (force field) in the interfacial zone is therefore non-homogeneous, having a gradient perpendicular to the interfacial boundary. In contrast, the pressure in a bulk phase is homogeneous and isotropic. Consequently, no net energy is expended in reversibly transporting the matter within a bulk phase. However, a net energy is required to create an interface by transporting from the bulk phase to the interfacial zone. The reversible work require to create a unit surface area is the surface free energy, that is
T, P, n
G
A
(5.1)
where is the surface free energy, G the Gibbs free energy of the total system, A the interfacial area, T the temperature, P the pressure, and n the total number of moles of matter in the system.
The work require to separate reversibly the interface between two bulk phases
and form their equilibrium separation to infinity is the work of adhesion.
Wa W (5.2)
Where W is the work of adhesion, a the surface free energy of phase , the surface free energy of phase , and the interfacial energy between phase and (Figure 5-1).
Figure 5-1. Work of adhesion.
This was apparently first purpose by Dupré. [1] When the two phase are identical, the reversible work is the work of cohesion (Figure 5-2),
c jj j j j
W W 0 2 (5.3)
where W is the work of cohesion for phase j.c
Figure 5-2. Work of cohesion.
The work of adhesion is the decrease of Gibbs free energy per unit area when an
W
aW
c jj
interface is formed from two individual surfaces. Thus, the greater the interfacial attraction, the greater the work of adhesion will be. Rearrangement of Eq. (5.1) gives
Wa
(5.4)
indicating that the greater the interfacial attraction, the smaller the interfacial energy will be. The works of adhesion can be related to the cohesion theoretically. Thereby, the interfacial energy can be linked to the properties of the two individual phases.
Thermodynamic discussions of adhesion in solid-liquid systems should be carried out in terms of surface free energy rather than surface tension. Discussions that involve the shape of liquid-gas or liquid-liquid interfaces can be carried out either in terms of surface tension or surface free energy.
5.1.2 Contact Angle Equilibrium: Young Equation
A liquid in contact with a solid will exhibit a contact angle (Figure 5-3). If the system is at rest, a static contact angle is obtained. If the system is in motion, a dynamic contact angle is obtained. Here, static contact angles are discussed. A system at rest may be in stable equilibrium (the lowest energy state), or in meta stable equilibrium (an energy through separated from neighboring states by energy barriers).
Figure 5-3. Contact angle equilibrium on a smooth, homogeneous, planar, and rigid
Figure 5-3. Contact angle equilibrium on a smooth, homogeneous, planar, and rigid