Results and Discussion
4.2 The Explanation of Superhydrophobic State by Wenzel，s Model and Cassie- Baxter Model
4.4.1 The effect of hydrogen bonding and sequence distribution in PVPh/PMMA system
Formulations and thermal properties of these synthesized copolymers are summarized in Table 4-1. It is well-known that a high temperature above Tg tends to partially disrupt hydrogen bond formation, and this is why we chose 180°C as the thermal treatment temperature and 180°C are far lower than the decomposition temperature (Table 4-1), thus the thermal treatment should not damage the polymer structure.
Table4-2 lists the surface roughness, advancing contact angles, and surface free energies of all specimens, before and after thermal treatment. The surface roughnesses of all
specimens are lower than 5 nm; therefore, the influence of topography on the surface free energy is negligible. The advancing contact angle is relatively less sensitive to surface roughness and heterogeneity than the receding angle; thus, the advancing contact angle data are commonly used to calculate the components of surface and interfacial tension.[6,7] In our previous study, we have found that the surface free energy of PVPh homopolymer decreases substantially after thermal treatment, resulting in a significant decrease in surface free energy (from 41.8 to 15.7 mJ/m2) and the sequence distribution of the vinylphenol group in PVPh-co-PS copolymers plays an important role in dictating the final surface energy after thermal treatment. In this paper, we change the immiscible PVPh-co-PS copolymer to miscible PVPh-co-PMMA copolymer to investigate the effect of hydrogen bonding between PVPh and PMMA on the surface free energy. Before we discussed the effect of hydrogen bonding between PVPh and PMMA on the surface free energy we studied the preparing process effect of hydrogen bonding first.
From Figure 4.11, we find pure PVPh homopolymer possesses quiet different FTIR spectrum after different preparing process. Different from solvent casting process, the PVPh homopolymer prepared from spin coating process possessed higher content of the free hydroxyl group and the hydroxyl groups involved in intramolecular hydrogen bonding. According to previous studies, solvent-cast films from volatile solutions such as chloroform, toluene or tetrahydrofuran may not be thermodynamically equilibrated due to rapid solvent evaporation during the spin-casting process, and the resulting surface could primarily be the result of solvent effects.[8,9] Thus, the PVPh homopolymer prepared from spin coating process possessed higher content of the free hydroxyl group and the hydroxyl groups involved in intramolecular hydrogen bonding than it prepared from solvent casting. In our previous report,[7] it is more favorable to re-form hydrogen bonds from neighboring hydroxyl groups or those in the vicinity (most likely from the same
chain, defined as an intramolecular hydrogen bond) in PVPh system. This is probably the reason for PVPh homopolymer prepared from 180°C thermal treatment possessed higher content of the free hydroxyl group and the hydroxyl groups involved in intramolecular hydrogen bonding than it prepared from spin coating process.
For PVPh-co-PMMA copolymer system, we have reported that the hydrogen-bonding strength of poly (vinylphenol-co-methyl methacrylate) copolymers depended on sequence distribution and polydispersity index due to its intramolecular screening and functional group accessibility effects.[2]The FTIR spectra shown in Figure 4.12 a and 4.12 b are good evidences for intramolecular screening and functional group accessibility effects. However, the FTIR spectra shown in Figure 4.12 c and 4.12 d are quiet different from Figure 4.12 a and 4.12 b. The carbonyl stretching band for PMMA appears at 1730 cm-1 and the peak at 1705 cm-1 corresponding to the hydrogen-bonded carbonyls and they can be fitted well to the Gaussian function (Table 4-3). From Figure 3c, we can find that the hydrogen-bonded carbonyls are few in PVPh/PMMA blends and PVPh-PMMA block copolymer except in PVPh-PMMA random copolymer. Furthermore, we find the similar phenomenon in the FTIR spectra of all polymer films (Figure 4.13) which prepared by spin coating process. From mention above, the reason for this phenomenon is due to rapid solvent evaporation during the spin-casting process.[8,9] We deduced that it is more favorable to form hydrogen bonds from neighboring hydroxyl groups and carbonyls during the rapid evaporation of tetrahydrofuran. As a result, there is more hydrogen-bonded carbonyl can be found in PVPh-PMMA random copolymer.
From Table 3, we find all polymer specimens possess lower surface free energy after thermal treatment. In our previous report, we deduced that the decrease of surface energy is due to the decrease of the intermolecular hydrogen-bonding interaction for PVPh/PMMA system. For clarity, the spectra display the hydroxyl stretching region
between 2800 and 3800 cm-1 and the carbonyl stretching region between 1660 and 1800 cm-1 are shown in Figure 4.14, 4.15 and 4.16. According to a recent study,[10] the -OH band can be fitted by three Gaussian functions: a narrower shoulder band at 3525 cm-1 represents the free hydroxyl group, the peak at ν ≒ 3280 cm-1 corresponds to the hydroxyl groups involved in intermolecular hydrogen bonding, and the peak at ν ≒3420 cm-1 corresponds to the hydroxyl groups involved in intramolecular hydrogen bonding.
Besides, the peaks at 1730 and 1705 cm-1 corresponding to the free carbonyl and hydrogen-bonded carbonyls and the band at 3440 cm-1 represents the hydroxyl groups interacting with carbonyl groups. [2] We find all polymer films possess fewer fractions of the hydroxyl groups involved in intermolecular hydrogen bonding resulting in lower surface free energy after thermal treatment in Figure 4.15. Most of intermolecular hydrogen bondings of hydroxyl groups convert into free hydroxyl groups, intramolecular hydrogen bonding and hydrogen bonding between hydroxyl groups and carbonyl groups after thermal treatment.
In our previous work,[11] we have studied the effect of an inert diluent segment on the immiscibility behavior of PVPh-r-PS copolymers and found that the incorporation of a styrene moiety into the PVPh polymer chain can dilute and decrease the strong self-association in the PVPh component. The spacing of these vinylphenol groups tends to decrease the average hydroxyl-hydroxyl distance and increase the fraction of free hydroxyl in PVPh/PS random copolymers and provides a positive effect to lower the surface energy of the polymer. However, the contact angles and resulting γ of PVPh/PS blends show no significant change before or after 180 °C thermal treatment in PVPh/PS systems.[1] We find miscible polymers, PVPh-co-PMMA, with different sequence distribution present different surface properties from immiscible PVPh-co-PS copolymers because of its hydrogen bonding between hydroxyl groups and carbonyl groups. It is
interesting to note that the surface free energy of PVPh/PMMA blends increase with the increasing of PMMA content (Figure 4.17). The PVPh-r-PMMA and PVPh-b-PMMA copolymers possess the most drastic reduction in surface energy after the thermal treatment in comparison with corresponding blends under comparable compositions. In the PVPh/PS systems, the interference of the styrene segment tends to prevent the vinylphenol segment from migrating to the surface, which can be regarded as a negative effect, i.e., an increase in the surface energy of the material. Thus, the PVPh-r-PS copolymers possess the most drastic reduction in surface energy after the thermal treatment in comparison with corresponding block copolymers and blends under comparable compositions. From the second-run DSC data of both PVPh-co-PMMA copolymers and PVPh/PMMA blends, revealing that essentially all PVPh/PMMA specimens possess only one glass transition temperature. Single glass transition temperature strongly suggests that these systems are fully miscible and possess a homogeneous amorphous phase. Besides, it have been reported that hydrogen bonding interaction would reduce surface enrichment.[12] As a result, there is no surface enrichment occurs in PVPh/PMMA blends and block copolymers.
To further investigate the importance of hydrogen bonding between the hydroxyls and carbonyls, we turn our attention back to the FTIR spectra of the carbonyl stretching region between 1660 and 1800 cm-1 of PVPh/PMMA random and block copolymers and their corresponding blends were shown in Figure 4.16. From Figure 4.16, we clearly know that the fraction of hydrogen bonded carbonyl group increases after 180°C 24h thermal treatment. It indicates that we increase the interaction between PVPh and PMMA after thermal treatment. For PVPh/PMMA random and block copolymers we speculated that it is more favorable to re-form hydrogen bonds from neighboring hydroxyl groups or carbonyl groups in the vicinity (most likely from the same chain, defined as an
intramolecular hydrogen bond) resulting in the most drastic reduction in surface energy after the thermal treatment. Unlike PVPh/PMMA random and block copolymers, the surface free energy of PVPh/PMMA blends increases with the increasing of PMMA content (Figure 4.17). Different from PVPh (after 180°C 24 h thermal treatment), PMMA homopolymer possessed higher surface free energy. In addition, intermolecular hydrogen bonding between hydroxyl groups and carbonyls increased after 180°C 24 h thermal treatment. Thus, the surface free energy of PVPh/PMMA blends increases with the increasing of PMMA content
4.4.2 Conclusions
An easy method was used for the preparation of a TiO2 photocatalyst on the superhydrophobic surface. Small amounts of TiO2 photocatalyst prepared on BA-a led to the purification of the surface by the photocatalytic degradation of the attached pollutants under UV light irradiation, so that it could recover the super-hydrophobic property of surface. The TiO2/BA-a is promising for the applications in waterproof materials because it has water-repellent properties induced by BA-a and the self-cleaning properties induced by TiO2 photocatalyst.
The decrease of the intermolecular hydrogen-bonding fraction between hydroxyl groups of PVPh in PVPh/PMMA systems through a simple thermal treatment procedure tends to decrease the surface energy. The sequence distribution of the vinylphenol group in PVPh-co-PMMA copolymers plays an important role in dictating the final surface energy after thermal treatment. Besides, there is no surface enrichment occurs in PVPh/PMMA systems because of its hydrogen bonding between hydroxyl groups and carbonyl groups. The effects of molecule weight on surface free energy were also investigated carefully in this paper
References
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C. J. Phys. Chem. B 2007, 111, 3404.
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Macromolecules 2005, 38, 6435.
[3]. D. G. LeGrand, G. L. Gaines, Jr., J. Colloid Interface Sci. 1969, 31, 162.
[4]. D. G. LeGrand, G. L. Gaines, Jr., J. Colloid Interface Sci. 1973, 42, 181.
[5]. S. Wu, J. Colloid Interface Sci. 1969, 31, 153.
[6]. Xu, Y.; Graf, J.; Painter, P. C.; Coleman, M. M. Polymer 1991, 32, 3103.
[7]. Wang, L. F.; Pearce, E. M.; Kwei, T. K. J. Polym. Sci., Polym. Phys. Ed. 1991, 29, 619.
[8]. Garbassi, F.; Morra, M.; Occhiello, E. Polymer Surfaces: from Physics to Technology;
John Wiley & Sons: Chichester, 1998; p 291.
[9]. Green, P. F.; Christensen, T. M.; Russell, T. P.; Jerome, R. J. Chem. Phys. 1990, 92, 1478-1482.
[10]. Yuan, F.; Wang, W.; Yang, M.; Zhang, X.; Li, J.; Li, H.; He, B. Minch, B.; Lieser, G.; Wegner, G. Macromolecules 2006, 39, 3982.
[11]. Kuo, S. W.; Chang, F. C. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 1661.
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Table 4-1. Formulations and thermal properties of PVPh-co-PMMA copolymers and PVPh92-r-PMMA8 92 16000 1.67 173.7 362.1 PVPh10-b-PMMA90 10 37000 1.15 143.1 373.3 PVPh30-b-PMMA70 30 16000 1.11 155.0 372.0 PVPh44-b-PMMA56 44 16000 1.15 163.4 370.5 PVPh55-b-PMMA45 55 30000 1.10 164.5 368.0
PVPh75-b-PMMA25 75 22000 1.13 176. 7 365.0
PVPh10/PMMA90 10 124.4 359.9
PVPh30/PMMA70 30 135.5 353.1
PVPh50/PMMA50 50 147.8 351.8
PVPh70/PMMA30 70 159.2 345.9
PVPh90/PMMA10 90 166.8 343.6
PVPh 100 20000 186.8 1.07 372.1
Table 4-2. Root-mean-square surface roughness, advancing contact angle for water and diiodomethane, surface Free energy, and XPS Analysis of PVPh/PMMA Copolymers
Before 180°C thermal treatment After efore 180°C thermal treatment Contact angle (deg) Contact angle (deg)
polymer Roughness (nm) H2O DIM γ (mJ/m2) Roughness (nm) H2O DIM γ (mJ/m2) PVPh 2.1 75.6 47.8 37.9 2.2 107.0 86.6 14.5 PVPh/PMMA=10/90 1.4 103.4 48.4 37.0 1.4 89.5 60.2 28.7 PVPh/PMMA=30/70 2.1 98.7 48.1 36.1 2.3 92.6 66.6 25.4
PVPh/PMMA=50/50 2.0 95.9 48.5 35.4 1.8 96.6 69.3 23.5
PVPh/PMMA=70/30 1.2 92.55 47.5 35.7 1.2 85.8 76.7 23.5 PVPh/PMMA=90/10 1.5 90.2 48.3 35.2 1.5 103.5 80.9 17.3 PVPh10-b-PMMA90 1.8 95.1 52.9 32.7 1.5 100.0 84.9 16.2 PVPh30-b-PMMA70 2.1 91.5 52.3 33.0 1.7 103.0 86.7 15.0 PVPh44-b-PMMA56 1.7 83.4 52.4 33.8 1.7 105.3 87.9 14.2 PVPh55-b-PMMA45 1.3 82.3 53.2 33.7 1.5 105.8 88.2 14.0 PVPh75-b-PMMA25 1.5 80.3 54.6 33.6 1.5 105.9 88.2 13.9
PVPh10-r-PMMA90 2.3 101.3 53.1 33.3 2.1 102.5 82.6 16.6
PVPh30-r-PMMA70 1.1 93.2 53.5 32.3 1.5 104.1 84.8 15.5 PVPh50-r-PMMA50 1.3 91 53.5 32.3 1.3 105.6 88.5 13.9 PVPh76-r-PMMA24 1.4 81.8 53.5 33.7 1.4 106 89 13.7
PVPh92-r-PMMA8 1.2 79.6 53.1 34.4 1.2 106.3 89 13.6
PMMA 2.0 108.4 48.8 38.3
Table 4-3. Results of Curve-Fitting the Data for PVPh-co-PMMA and PVPh/PMMA Blends with different process at Room temperature
copolymer H-bonded C=O Free C=O
υ, cm-1 W1/2,
cm-1 Ab, % υ, cm-1
W1/2, cm-1
Ab,
% fba
Solvent casting
PVPh/PMMA=50/50 1707 25 17.2 1732 19 82.8 12.2
PVPh44-b-PMMA56 1707 24 45.8 1735 19 54.2 36.1
PVPh50-r-PMMA50 1704 24 49.2 1731 19 50.8 39.2
Spin coating
PVPh/PMMA=50/50 1707 25 11.4 1731 19 88.6 7.9
PVPh44-b-PMMA56 1707 24 13.8 1731 19 86.2 9.6
PVPh50-r-PMMA50 1706 25 44.5 1731 19 55.5 34.8
180°C thermal treatment
PVPh/PMMA=50/50 1708 24 34.7 1731 19 65.3 26.2
PVPh44-b-PMMA56 1708 24 38.9 1732 19 61.1 29.8
PVPh50-r-PMMA50 1707 25 42.5 1732 19 57.5 33.0
(a) (b)
Figure 4.1. (a) Shape of a water drop (5 µL ) on the film of BA-a (left). The water contact angleθis 108 °. (b) Shape of a water drop (5 µL) on the surface of Al with nanoparticles (right). The water contact angle θ is 160±3°.
(a)
(b) (c)
Figure 4-2. SEM images of BA-a + nanoparticle films on aluminum substrate (a) flate aluminum substrate without composite (b) sandblasting topographical microstructure on the surface of aluminum, and (c) sandblasting topographical microstructure of aluminum . Films were prepared from solutions having a 100/100 (TiO2 /BA-a) ratio having 5%
BA-a concentration.
Figure 4-3. The change of cosθ with different mass ratio nanoparticles. The sharp change in contact angles occurs at different mass percentages, depending on the TiO2/BA-a ratio concentration of the spin-coated solutions.
(a) (d)
(b) (e)
(c) (f)
Figure 4-4. SEM image of the BA-a + nanoparticle film on rough aluminum surface, obtained from a solution having (a) a 5% BA-a concentration. (b) 20/100 (TiO2 /BA-a) ratio with a 5% BA-a concentration. (c) 40/100 (TiO2 /BA-a) ratio with a 5% BA-a concentration. (d) 60/100 (TiO2 /BA-a) ratio with a 5% BA-a concentration. (e) 80/100 (TiO2 /BA-a) ratio with a 5% BA-a concentration. (f) 100/100 (TiO2 /BA-a) ratio with a 5% BA-a concentration.
(a)
(b) (c)
Figure 4-5. SEM pictures of (a) an area of 60µm 80µm for bare rough aluminum, (b) ╳ the rough substrate containing 100/100 (TiO2 /BA-a) ratio having a 5% BA-a concentration , and (c) magnified view of (b).
Figure 4-6 Advancing and receding of water drops placed on rough aluminum composite as functions of the roughness of the substrate.
Figure 4.7 CAH, △θ＝θa —θr, as a function of microscale roughness of the substrate.
Figure 4.8.Time profiles of the photocatalytic degradation of dirt attached on the surface of BA-a composite obtained by monitoring the contact angle.
Figure 4-9. The mechanism of self-cleaning and recovery properties on superhydrophobic TiO2/BA-a surface.
3 6 0 0 3 2 0 0 2 8 0 0 0 . 0
0 . 2 0 . 4 0 . 6 0 . 8 1 . 0
Absorbance (a.u.)
W a v e n u m b e r (c m- 1) ( a )
( b ) ( c )
Figure 4.10. The FTIR spectra of pure PVPh homopolymer (Mw = 9697) (a) solvent casting (b) spin coating (c) 180°C 24h thermal treatment.
1800 1780 1760 1740 1720 1700 1680 1660
1800 1780 1760 1740 1720 1700 1680 1660
50-r-50 50-r-50
50-r-50
Wavenumber (cm-1) 50/50
3800 3600 3400 3200 3000 2800
Wavenumber (cm-1)
3800 3600 3400 3200 3000 2800
44-b-56
44-b-56
Wavenumber (cm-1)
Figure 4.11. The FTIR spectra of samples having similar PVPh contents preparing by different coating process (a)、(b) solvent casting and (c)、(b) spin coating .
1800 1780 1760 1740 1720 1700 1680 1660
1800 1780 1760 1740 1720 1700 1680 1660 0
1800 1780 1760 1740 1720 1700 1680 1660
10-r-90
3800 3600 3400 3200 3000 2800
010/90
Wavenumber (cm-1)
3800 3600 3400 3200 3000 2800
0
55-b-45
10-b-90
Wavenumber (cm-1)
3800 3600 3400 3200 3000 2800
f
Figure 4.12. FTIR spectra of (a)、(b) PVPh/PMMA blends, (c)、(d) PVPh-b-PMMA copolymers and (e)、(f) PVPh-r-PMMA copolymers at room temperature
1800 1780 1760 1740 1720 1700 1680 1660
1800 1780 1760 1740 1720 1700 1680 1660
f
1800 1780 1760 1740 1720 1700 1680 1660 0
Wavenumber (cm-1)
3800 3600 3400 3200 3000 2800
0
Wavenumber (cm-1)
3800 3600 3400 3200 3000 2800
44-b-56
Wavenumber (cm-1)
3800 3600 3400 3200 3000 2800
92-r-8
Figure 4.13. FTIR spectra of (a)、(b) PVPh/PMMA blends, (c)、(d) PVPh-b-PMMA copolymers and (e)、(f) PVPh-r-PMMA copolymers after the 180 °C thermal treatment procedure.
3500 3000
Absorbance (a.u.)
Wavenumber (cm-1)
RT 180o
(a) C
3500 3000
(b) RT
180o C RT
180o C
Wavenumber (cm-1)
3500 3000
(c)
Wavenumber (cm-1)
Figure 4.14. FTIR spectra of (a) PVPh/PMMA blends, (b) PVPh-b-PMMA copolymers and (c) PVPh-r-PMMA copolymers in 2800cm-1~3800cm-1.
1800 1760 1720 1680
RT 180oC RT
180oC
Absorbance (a.u.)
Wavenumber (cm-1)
RT 180oC
(a)
1800 1760 1720 1680
(b) (c)
Wavenumber (cm-1)
1800 1760 1720 1680
Wavenumber (cm-1)
Figure 4.15. FTIR spectra of (a) PVPh/PMMA blends, (b) PVPh-b-PMMA copolymers and (c) PVPh-r-PMMA copolymers 1660cm-1~1800cm-1.
0 20 40 60 80 100 30
40
Surface energy (mJ/m-2 )
Phenol ratio (%)
0 20 40 60 80 100
0 20 40
Surface energy (mJ/m-2 )
Phenol ratio (%)
Figure 4.16. Surface energy of PVPh/PMMA random copolymers (▲), block copolymers
(●) and their blends (■) (a) before (b) after the thermal treatment process.
Chapter 5 Conclusions
The technology of self-cleaning coatings, superhydrophobic surfaces, have attracted much interest because of potential applications in daily life as well as in many industrial processes. We use a simple method for fabricating a lotus-like micro–nanoscale binary structured surface of aluminum. The aluminum microsheets were generated by sandblasting then cleaned by ultrasonic cleaning ,and the nanoscale was produced by coating a complex polymer of nanoparticles and Polybenzoxazine Thus, a simple drop drying technology has been successfully developed for fabricating a complex lotus-like micro–nanostructure. The coating of polybenzoxazine/nanoparticles on films produced superhydrophobic surfaces with contact angle for water larger than 150° and the rolling angle smaller than 10°
respectively
Self-cleaning coatings have been used in nature for millennia.It is only in the past decade that commercial self-cleaning products have become available to the consumer. The use of a self-cleaning coating is attractive as they are labour saving and effectively improve the appearance of the environment. Two main forms of self-cleaning films have been developed – superhydrophobic films that repel water droplets, and TiO2 films that show attractive photocatalytic properties for the destruction of organic dirt. One interesting new development of the titania-based coatings is their ability to destroy harmful bacteria and viruses by photocatalytic action using sunlight.We introduce the photocatalytic concept to our superhydrophobic system,that can make a smart self-cleaning superhydrophobic
surface. Our results is considered significant importance industrial applications in the future.
The decrease of the intermolecular hydrogen-bonding fraction between hydroxyl groups of PVPh in PVPh/PMMA systems through a simple thermal treatment procedure tends to decrease the surface energy. The sequence distribution of the vinylphenol group in PVPh-co-PMMA copolymers plays an important role in dictating the final surface energy after thermal treatment. Besides, there is no surface enrichment occurs in PVPh/PMMA systems because of its hydrogen bonding between hydroxyl groups and carbonyl groups. The effects of molecule weight on surface free energy were also investigated carefully in this paper.
Introduction to Author
English name: Chang-Yuang Li Chinese name: 李昌謜
Birthday: 1978 May 23
Address: No.148, Wunhua Rd., Erlun Township, Yunlin County 649, Taiwan (R.O.C.)
Education:
1996.09～2000.06 B.S., Department of Chemistry, Chung Yuan Christian University , Taoyuan , Taiwan.
2005.09～2009.06 M.S., Institute of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan.
Experience:
Engineer, Far Eastern Textile Co. Ltd