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Materials Chemistry and Physics
j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / m a t c h e m p h y sSynthesis of poly(4-vinylphenol) (PVPh) and polyhedral oligomeric
silsesquioxanes-poly(4-vinylphenol) (POSS-PVPh) with low surface energy and
their surface properties
Chun-Hsiung Liao
a, Fu-Ming Chien
a, Chen-Ming Chen
a, Lung-Chang Liu
a, Ming-Hua Chung
a,∗,
Feng-Chih Chang
baMaterial and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 30011, Taiwan, ROC bDepartment of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan, ROC
a r t i c l e i n f o
Article history: Received 11 June 2011
Received in revised form 6 September 2011 Accepted 8 September 2011 Keywords: Polymers Thin films Chemical synthesis Surface properties
a b s t r a c t
In this paper, we have synthesized poly(4-vinylphenol) (PVPh) and polyhedral oligomeric silsesquioxanes-poly(4-vinylphenol) (POSS-PVPh) with low surface energies and investigated their sur-face properties. Experimental results reveal that the sursur-face properties of PVPh and POSS-PVPh can be manipulated with the length of PVPh segment and POSS contents, respectively, resulting in the variation of intermolecular hydrogen bonding interactions. In addition, the surface energies of PVPh after ther-mal treatment of 180◦C for 24 h can be less than 20 mJ m−2and those of POSS-PVPh without thermal treatment can be less than 25 mJ m−2while those of poly(tetrafluoroethylene) (PTFE) are 22 mJ m−2.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Performances of polymeric materials can be modulated by their surface properties (e.g. wettability, friction, adhe-sion, etc.). In recent years, hydrophobicity and oleophobicity have attracted tremendous interest due to their miscellaneous applications [1–4]. Both poly(dimethylsiloxane) (PDMS) and poly(tetrafluoroethylene) (PTFE) are two well-known polymeric materials with low surface energies [5–8]. PTFE, with a sur-face energy of approximately 22 mJ m−2, may be regarded as the benchmark for polymeric materials with low surface energies and exhibits water repellency[9]as well as other excellent physical properties[10]. Low surface energies of PTFE come from weak inter-molecular forces of fluorinated polymer chains because the fluorine atom has small size, high electronegativity, low polarizability, and strong repulsion[11]. However, PTFE and many fluorinated poly-mers possess some application limitations such as high cost, poor processibility, and the potential to be carcinogenic[12–14]. There-fore, research of an alternative material for PTFE with low cost, easy processibility, and good film-forming characteristics has become an important issue.
Surface properties of polymers highly depend on the hydrogen bonding interactions. Generally, amorphous comblike polymers
∗ Corresponding author. Tel.: +886 3 5732475; fax: +886 3 5732347. E-mail address:[email protected](M.-H. Chung).
with a flexible linear backbone on the side chain exhibit weak intermolecular interaction and low surface energy[15]. In poly-benzoxazine system, the intermolecular hydrogen bonds between the hydroxyl groups increase their surface energies[12]. More-over, Jiang et al.[16]have found that the compact and collapsed conformation of poly(N-isopropylacrylamide) (PNIPAAm), which is induced by intramolecular hydrogen bonds between C O and N–H groups of main chains, causes a low surface energy and a high contact angle for water at the temperatures above its lower crit-ical solution temperature (LCST). When the temperature is below LCST, however, intermolecular hydrogen bonds between PNIPAAm main chains and water molecules predominate, leading to a higher surface energy and a lower water contact angle. Similarly, Chung and Co-worker[17]have also reported that the amide groups in a fluorinated-main-chain liquid-crystalline polymer system induce strong intermolecular hydrogen bonds, resulting in higher surface energies and higher degrees of hydrophilicity. The nature of pen-dent chain has a significant effect on the determination of surface energy. Thus, polymeric materials with low surface energies can be obtained by the reduction of intermolecular interactions between the comblike polymers with flexible linear backbones[18].
Polymers reinforced with well-defined inorganic fillers with nano-sizes (i.e. organic/inorganic hybrid nanocomposites) have attracted much attention because of their potential applica-tions. Among these systems, polyhedral oligomeric silsesquioxanes (POSS) compounds, which possess unique cage-like structures and nano-scale dimensions, are of particular interest for utilization
0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.09.053
Fig. 1. Procedure for syntheses of POSS-PVPh copolymers.
as hybrid materials. POSS compounds exhibit inorganic/organic hybrid architecture since they contain not only an inner inorganic framework composed of (SiO1.5)xbut also organic groups. Because POSS moieties can be readily incorporated into polymer matrices through copolymerization, many types of polymer/POSS nanocom-posites have been synthesized [19–28]. In this paper, we have synthesized poly(4-vinylphenol) (PVPh) homopolymers and then introduced POSS into PVPh to prepare POSS-PVPh copolymers, fur-ther heightening their fur-thermal stabilities and chemical resistances. Moreover, their surface properties have also been investigated. Experimental results manifest that the surface energies of PVPh homopolymers after thermal treatment increase with the length of PVPh segment and those of POSS-PVPh copolymers decrease with the silicon contents owing to the variation of hydrogen bonds.
2. Experimental details
2.1. Materials and instruments
All the starting materials utilized in this study were purchased from Aldrich Co. and used without further purification.1H NMR spectra were recorded on a NMR spectrometer (Varian Unity Inova 500 FT) operated at 500 MHz. Thermal analyses were performed with a differential scanning calorimeter (DuPont DSC-9000) oper-ated at a scan rate of 20◦C min−1within a temperature range from 30 to 250◦C. The sample was quenched to 20◦C from the melt state
for the first scan and then rescanned between 20 and 250◦C at 20◦C min−1. Glass transition temperature (Tg) was obtained at the inflection point of jump heat capacity. Fourier-transform infrared (FTIR) spectroscopic measurements were conducted on a Nicolet Avatar 320 FTIR spectrophotometer and 32 scans were collected with a spectral resolution of 1 cm−1. All the sample preparations for FTIR spectroscopic measurements were under continuous nitro-gen flow to ensure minimal sample oxidation or degradation. For the measurements of contact angles, deionized water (H2O) and diiodomethane (DIM) were chosen as testing liquids because sig-nificant amounts of data were available for these liquids. The advancing measurements of contact angle for a polymer sample were determined at 25◦C after injection of a liquid drop (5L) onto the surface and a goniometer (Krüss GH-100) interfaced to image-capture software was employed to perform the measure-ment. A two-liquid geometric method was employed to determine the surface energy[29].
2.2. Syntheses of PVPh homopolymers
PVPh homopolymers used in this study was synthesized by liv-ing anionic polymerization of 4-tert-butoxystyrene followed by selective removal of the tert-butoxy protective group through a subsequent hydrolysis reaction. The detailed synthesizing pro-cedures of PVPh homopolymers have been reported previously
Table 1
Physical and surface properties of PVPh homopolymers.
Polymer Length of PVPh segmenta M
w/Mn Tg(◦C) Before thermal treatment After thermal treatment
Contact angle (◦) s(mJ m−2) Contact angle (◦) s(mJ m−2)
H2O DIM H2O DIM PVPh28 28 1.09 138 77 57 34 105 88 14 PVPh42 42 1.05 178 79 56 33 104 87 15 PVPh50 50 1.15 183 78 55 34 104 87 15 PVPh80 80 1.04 184 77 53 35 101 86 16 PVPh95 95 1.15 188 79 53 35 98 86 16 PVPh160 160 1.06 189 76 53 36 96 84 18 PVPh165 165 1.07 183 77 53 35 94 83 19 PVPh254 254 1.12 184 77 52 36 92 81 20 PVPh346 346 1.15 188 76 54 35 90 79 21 PVPh1250 1250 1.03 189 75 50 37 77 64 31
aThe repeat unit of PVPh homopolymer is obtained from GPC.
[30,31]. Physical and surface properties of PVPh homopolymers are summarized inTable 1.
2.3. Syntheses of POSS-PVPh copolymers
With corner-capping reaction, as shown in Fig. 1, POSS-Cl was firstly prepared by reacting trichloro[4-(chloromethyl)phenyl]silane (1.00 mL) with POSS (4.05 g) in the presence of triethylamine (2.20 mL) and dry THF (30 mL) at room temperature for 7.5 h under argon. After filtration to remove the HNEt3-Cl byproduct and precipitation with acetonitrile, POSS-Cl was obtained (yield: 95%). Then POSS-PAS was prepared with POSS-Cl (0.33 g) and 4-acetoxystyrene (3.00 g) in the presence of CuBr (0.15 g) and bipyridine (0.55 g) by the atom transfer radical polymerization at 100◦C for 7.5 h (yield: 76%). Finally, POSS-PVPh was synthesized by the hydrolysis of POSS-PAS (1.27 g) in the presence of hydrazine (3.00 mL) and 1,4-dioxane (27.00 mL) under nitrogen at room temperature for 10 h. The product was eventually concentrated by evaporation of toluene, washed with deionized water, and dried in a vacuum oven at room temperature for 2 days (yield: 93%).
The POSS-PVPh copolymers were characterized by FTIR (Fig. 2), 1H NMR (Fig. 3) and gel permeation chromatography (GPC). Physical and surface properties of POSS-PVPh copolymers are summarized inTable 2.
Fig. 2. FTIR spectra of (a) POSS, (b)POSS-Cl, (c)POSS-PAS, and (d) POSS-PVPh.
2.3.1. FTIR and1H NMR analyses of POSS
FTIR (KBr): 3350 cm−1(hydrogen bonded OH), 3000–2850 cm−1 (Si–CH2 rocking), 2957 cm−1 (CH2 stretching), 1200–1000 cm−1 (Si–O–Si asymmetric stretching), 500–450 cm−1(Si–O–Si bending).
1H NMR (500 MHz): ı = 0.91 ppm (6H, SiCH
2CH(CH3)2), ı = 0.55 ppm (2H, SiCH2CH(CH3)2).
2.3.2. FTIR and1H NMR analyses of POSS-Cl
FTIR (KBr):3000–2850 cm−1(Si–CH2rocking), 2957 cm−1(CH2 stretching), 1200–1000 cm−1 (Si–O–Si asymmetric stretching), 500–450 cm−1(Si–O–Si bending).
1H NMR (500 MHz): ı = 0.91 ppm (6H, SiCH
2 CH(CH3)2), ı = 8.0–7.0 ppm (4H, aromatic CH), ı = 0.55 ppm (2H, SiCH2CH(CH3)2).
2.3.3. FTIR and1H NMR analyses of POSS-PAS
FTIR (KBr): 3000–2850 cm−1(Si–CH2rocking), 2957 cm−1(CH2 stretching), 1765 cm−1 (C O stretching), 1603 cm−1 (in-plane aromatic C–C stretching), 1200–1000 cm−1 (Si–O–Si asymmetric stretching), 500–450 cm−1(Si–O–Si bending).
1H NMR (500 MHz):ı = 7.0–6.0 ppm (4H, aromatic CH), ı = 2.41 ppm (3H, OCOCH3), ı = 0.91 ppm (6H, SiCH2CH(CH3)2), ı = 0.55 ppm (2H, SiCH2CH(CH3)2).
2.3.4. FTIR and1H NMR analyses of POSS-PVPh
FTIR (KBr): 3525 cm−1 (free OH), 3350 cm−1 (hydrogen bonded OH), 3000–2850 cm−1 (Si–CH2 rocking), 2957 cm−1 (CH2
Table 2
Physical and surface properties of POSS-PVPh copolymers.
Copolymer Length of PVPh segmenta M
w/Mn Tg(◦C) Contact angle (◦) s(mJ m−2) Silicon content (mol %)
H2O DIM
POSS-PVPh9 9 1.19 210 113 71 23 14.54
POSS-PVPh35 35 1.41 209 110 69 24 11.05
POSS-PVPh120 120 1.38 205 109 69 24 8.79
POSS-PVPh264 264 1.33 203 108 68 25 7.92
aThe repeat unit of PVPh segment of POSS-PVPh is obtained from1H NMR.
stretching), 1765 cm−1 (C O stretching), 1603 cm−1 (in-plane aromatic C–C stretching), 1200–1000 cm−1 (Si–O–Si asymmetric stretching), 500–450 cm−1(Si–O–Si bending).
1H NMR (500 MHz): ı = 7.37 ppm (H, –OH), ı = 7.0–6.0 ppm (4H, aromatic CH),ı = 2.41 ppm (3H, OCOCH3),ı = 0.91 ppm (6H, SiCH2CH(CH3)2),ı = 0.55 ppm (2H, SiCH2CH(CH3)2).
2.4. Preparation of PVPh and POSS-PVPh thin films
PVPh or POSS-PVPh (5 wt.%) was dissolved in the THF solution. Then the polymeric solution was stirred for 6–8 h and cast onto a wafer. After solvent evaporation at 60◦C for 1 day and vacuum drying at room temperature for 2 days, PVPh or POSS-PVPh thin films were obtained. Both PVPh and POSS-PVPh thin films were transparent. Thermal treatment was carried out by placing the pre-pared polymer films in a vacuum oven at 180◦C for 24 h and then quenching to ambient temperature.
3. Results and discussion
3.1. Surface properties of PVPh homopolymers
As shown inTable 1, the surface properties of PVPh homopoly-mers before and after thermal treatment have been investigated. In order to avoid polymer degradation, we chose 180◦C as the ther-mal treatment temperature as this is far lower than the therther-mal decomposition temperature (350◦C). Furthermore, a high temper-ature above Tgtends to partially disrupt the formation of hydrogen bond. Before thermal treatment, as shown inTable 1, the surface properties of PVPh homopolymers are independent of the length of PVPh segment. After thermal treatment of 180◦C for 24 h, however, the surface properties of PVPh homopolymers dramatically depend on the length of PVPh segment due to the variations of hydrogen bonds. The experimental results indicated that the contact angles of PVPh homopolymers after thermal treatment decreased (i.e., the surface energy increased) with increasing PVPh segment length due to increased hydrogen bonding interactions. The thermal treatment
Fig. 4. FTIR spectra of PVPh80(a) before thermal treatment; (b) after thermal treat-ment of 180◦C for 24 h.
causes the decrease of hydrogen bonds for PVPh homopolymers because the peaks at wavenumber∼3500 and 3380 cm−1(i.e. free hydroxyl groups) in the FTIR spectra blue-shift to be respectively 3525 and 3420 cm−1 after thermal treatment as shown inFig. 4. Since the thermal treatment effectively reduces the intermolec-ular hydrogen bonding interactions of PVPh homopolymers, the surface energies for all of the samples with thermal treatment are lower than those without thermal treatment. Furthermore, the sur-face energies after thermal treatment increase with the length of PVPh segments as shown inFig. 5owing to the raise of hydrogen bonds while those before thermal treatment are independent of the length of PVPh segments. With thermal treatment, lab-made PVPh homopolymers are potential alternative materials for PTFE because all their surface energies are lower than 22 mJ m−2except PVPh1250.
3.2. Surface properties of POSS-PVPh thin films
Although PVPh homopolymers are good materials with low surface energies, their thermal stabilities as well as chemical resis-tance are dissatisfactory and thermal treatment is necessary for the promotion of surface properties. In addition, thermoplastic PVPh homopolymers are conformation sensitive and unstable as com-pared with thermosetting polybenzoxazine in a previous study by Chang’s group[12]. Therefore, we have tried to introduce POSS into PVPh homopolymers (Fig. 1), generating POSS-PVPh copolymers. As manifested inTable 2, incorporation of POSS into the PVPh raises Tg (>200◦C) and all lab-made POSS-PVPh copolymers exhibit low sur-face energies (25 mJ m−2) without thermal treatment. Moreover, the contact angles of POSS-PVPh copolymers increased (i.e., surface energy decreased) with silicon content (Fig. 6) due to decreased hydrogen bonding interactions. Preliminary experimental results reveal that the surface energies of POSS-PVPh copolymers can
Fig. 5. Dependence of surface energies of PVPh homopolymers on repeat units of
23 23.5 24 24.5 25 15 14 13 12 11 10 9 8 7
Silicon content (mol %)
γ (mJ/m2)
Fig. 6. Dependence of surface energies of POSS-PVPh copolymers on silicon
con-tents.
be modulated with POSS contents and they are good polymeric materials with low surface energies because of their high thermal stabilities, easy processibility, and chemical resistance. Although PVPh homopolymers and POSS-PVPh copolymers are prepared from different methods and exhibit different polydispersities, no dependence of polydispersities on their surface properties can be observed in the present study. Further research for the impact of POSS macromolecules on morphology, the phase segregation,29Si NMR spectra, more IR spectra of POSS and PVPh systems, thermo-gravimetric analysis (TGA) data of PVPh homopolymers as well as PVPh-POSS copolymers, and the surface properties of POSS-PVPh copolymers with thermal treatment will be executed in the near future.
4. Conclusions
PVPh homopolymers and POSS-PVPh copolymers with low surface energies have been successfully synthesized and their sur-face properties highly depend on the intermolecular hydrogen bonds. In case of PVPh homopolymers, the surface energies of PVPh homopolymers with the thermal treatment of 180◦C for 24 h have drastically been heightened owing to the decrease of intermolecular hydrogen bonding interactions. In case of POSS-PVPh copolymers, however, the surface energies of POSS-POSS-PVPh copolymers increase with POSS contents since the intermolecular hydrogen bonding interactions reduce.
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