ARTICLE pubs.acs.org/Langmuir
Surface Characteristics of a Self-Polymerized Dopamine Coating Deposited on Hydrophobic Polymer Films Jinhong Jiang, Liping Zhu,* Lijing Zhu, Baoku Zhu, and Youyi Xu MOE Key Laboratory of Macromolecule Synthesis and Functionalization, Department of Polymer Science and Engineering, and The Engineering Research Center of Membrane and Water Treatment Technology, Ministry of Education, Zhejiang University, Hangzhou 310027, PR China
bS Supporting Information ABSTRACT:
This study aims to explore the fundamental surface characteristics of polydopamine (pDA)-coated hydrophobic polymer films. A poly(vinylidene fluoride) (PVDF) film was surface modified by dip coating in an aqueous solution of dopamine on the basis of its self-polymerization and strong adhesion feature. The self-polymerization and deposition rates of dopamine on film surfaces increased with increasing temperature as evaluated by both spectroscopic ellipsometry and scanning electronic microscopy (SEM). Changes in the surface morphologies of pDA-coated films as well as the size and shape of pDA particles in the solution were also investigated by SEM, atomic force microscopy (AFM), and transmission electron microscopy (TEM). The surface roughness and surface free energy of pDA-modified films were mainly affected by the reaction temperature and showed only a slight dependence on the reaction time and concentration of the dopamine solution. Additionally, three other typical hydrophobic polymer films of polytetrafluoroethylene (PTFE), poly(ethylene terephthalate) (PET), and polyimide (PI) were also modified by the same procedure. The lyophilicity (liquid affinity) and surface free energy of these polymer films were enhanced significantly after being coated with pDA, as were those of PVDF films. It is indicated that the deposition behavior of pDA is not strongly dependent on the nature of the substrates. This information provides us with not only a better understanding of biologically inspired surface chemistry for pDA coatings but also effective strategies for exploiting the properties of dopamine to create novel functional polymer materials.
1. INTRODUCTION Surface chemical characteristics are of prime importance to polymer materials in various applications. Polymer materials, as is well known, always exhibit distinctive physical and chemical properties such as excellent mechanical strength, impact resistance, insulation performance, and corrosion resistance as well as low cost. Recently, polymer materials have been used more and more in packaging and printing, membrane separation, magnetic materials, optical functional materials, composites, biomedical fields, and so forth. However, most polymer materials are inherently inert and have low surface energy, which is usually undesirable for use in industrial and academic areas, as previously reported. Therefore, surface modification is a pressing need in overcoming these shortcomings and obtaining surface properties as expected. Surface modification methods for polymers include surface grafting,1 layer-by-layer deposition,2 self-assembled monolayers,3 plasma treatment,4 and so forth. Nevertheless, these methods are somewhat complex and strict under the reaction conditions in some cases and lack a general applicability to diverse polymer materials. Fortunately, a facile and versatile method for the surface modification of solid materials by dip coating in dopamine solution was proposed by Lee et al.5 on the basis of basic research on bionics in recent years. Dopamine is a kind of biological neurotransmitter that widely exists in living organisms. Dopamine and its catecholic derivatives are able to undergo oxidative polymerization r 2011 American Chemical Society
in the presence of oxygen as an oxidant under alkaline conditions. During the polymerization of dopamine, a tightly adherent polydopamine (pDA) layer is created on the surface of a substrate that is immersed in the dopamine solution for a certain period of time. The interactions between the pDA layer and the substrate include covalent and noncovalent interactions such as the hydrogen bonding interaction, ππ interaction, and electrostatic interaction.6 Actually, the pDA coating on polymer surfaces has good stability and durability in various environments, except in strongly alkaline solution (pH >13).5,7,8 Such novel means for the surface modification of polymers has been widely applied in hydrophilic modifications of polymer membranes,7,9 the modification of nonwetting surfaces for cell adhesion,10 second treatments, and surface functionalization by using adherent pDA as an intermediate layer.5,11,12 In our previous work, hydrophobic polymer membranes including polyethylene (PE), PVDF, and PTFE were surface modified by dopamine in aqueous solution with success.7 The active groups in the adherent pDA layer on the membrane surface allow the covalent immobilization of biomolecules such as heparin and bovine serum albumin (BSA), thus improving the hydrophilicity Received: July 25, 2011 Revised: October 13, 2011 Published: October 19, 2011 14180
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and biocompatibility of the resultant membranes.13,14 However, the surface characteristics of the pDA coating on membranes have not been investigated in detail. This basic information is important in tailoring a polymer surface with desirable properties. In this work, four typical polymer films were used as hydrophobic samples for surface modification to avoid the possible disturbance of the pore structure with respect to the experimental results of the surface characteristics. Four polymer films—PVDF, PTFE, PET, and PI— were modified by the pDA surface-modification technique mentioned above, and changes in the surface free energy of pDA-coated polymer films were studied. Surface energy, an important parameter of surface properties, is often used to estimate the surface adhesivity, wettability, paintability, printability, biocompatibility, and so forth. Determinations of the surface energy by contact angle measurements are able to provide a better understanding of the material compatibility. Moreover, the polymerization behavior of dopamine in solution and in the deposition process on various substrates under different reaction conditions was investigated as well.
2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. 3,4-Dihydroxyphenethylamine (dopamine) was purchased from Sigma-Aldrich and used as received.
Table 1. Surface-Energy Components of Probe Liquidsa surface-energy components (mN/m) σl
σdl
σpl
water
72.1
19.9
52.2
diiodomethane
50.0
47.4
2.6
liquids
Here, σl represents the total surface energy of the liquid, and σdl and σpl represent its disperse and polar parts, respectively. a
Tris(hydroxymethyl)aminomethane (Tris) was supplied by Sinopharm Chemical Reagent Co., Ltd. PVDF resin (solef 1015, Mn = 238 000 g/mol, Mw = 573 000 g/mol) was obtained from Solvey Co., Ltd. Other chemicals were of commercial analytical grade and used without further purification. PVDF films with different surface roughnesses were prepared following a reported procedure.15,16 A 10 wt % PVDF solution in N,N0 -dimethylacetylamide (DMAc) was cast on a glass plate at room temperature. PVDF1, PVDF2, and PVDF3 films were obtained by evaporating the solvent from the casting films at different temperatures and rinsing in deionized water to remove the residual solvent thoroughly. 2.2. Thickness Determination. Monocrystalline silicon chips (pretreated with H2SO4/H2O2) were used for the thickness determination of the deposited pDA layer. A 1 wt % PVDF solution in DMAc was spin coated onto the surface of the pretreated silicon wafer for comparison. The thickness of the spin-coated PVDF film measured by spectroscopic ellipsometry was 12.1 ( 2.4 nm. A 2.0 g/L dopamine solution was prepared in advance by using a Tris-HCl buffer solution (10 mM, pH 8.5) as the solvent. Then the Si and PVDF spin-coated Si substrates mentioned above were immersed vertically in the freshly prepared dopamine solution in an open vessel, in continuous contact with atmospheric oxygen. The thickness of the pDA layer deposited on these two substrates was measured by a spectroscopic ellipsometer (M-2000, U.S.). The ellipsometry measurements were performed at a continuous wavelength ranging from 190 to 1700 nm and angle of incidence of both of 65 and 70°. Δ and Ψ values measured at a wavelength of 6001700 nm were chosen for data analysis. Monocrystalline silicon with thickness of 1 mm was used as the substrate, and the Cauchy model was used to determine the thickness of the deposited pDA layer. The An and Bn parameters of the Cauchy layer were set as 1.45 and 0.01, respectively, as fit parameters. Then the Cauchy parameters and film thickness that best fit the pDA coating could be automatically obtained after computer calculation and fitting analysis. The thickness of the pDA layer deposited on the PVDF-spin-coated Si
Figure 1. (a) Thickness evolution of the pDA layer deposited on Si and the PVDF spin-coated Si substrates. (b) ATR-FTIR spectra for the near surface of the PVDF film and the pDA-coated PVDF film. FTIR spectra of dopamine and pDA deposition. (c) Water contact angles of original and pDA-coated substrates. (C = 2.0 g/L, t = 24 h). 14181
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substrate was calculated by subtracting the thickness of the PVDF coating layer from the measured value.
2.3. Characterization of the Product in Dopamine Solution. A freshly prepared 2.0 g/L dopamine solution was stirred in the reaction temperature range from 20 to 60 °C for 24 h in an open vessel and was in continuous contact with atmospheric oxygen. The products from the solution were collected by centrifugation (5000 rpm) for 5 min. The shapes and particle sizes of pDA particles and their aggregates in the supernatant were detected by a transmission electron microscopy (TEM, JEM-1230EX, Japan). The morphologies of the deposition were observed by field-emitting scanning electron microscopy (FESEM, Hitachi S-4800, Japan). The chemical composition of the deposition was analyzed by Fourier transform infrared spectra (FT-IR, VECTOR 22, Germany). 2.4. Surface Modification of Hydrophobic Films. Hydrophobic polymer films of PTFE, PET, PI, and PVDF were ultrasonically cleaned in methanol, acetone, and deionized water for 15 min in sequence prior to use. The aforementioned polymer films were immersed in a freshly prepared 2.0 g/L dopamine solution in an open vessel and were stirred and in contact with air continuously. After reacting for a period of time (t), the resultant films were removed and washed with ethanol and deionized water alternately to remove nonfirmly adsorbed pDA particles. Then the films were dried to constant weight in a vacuum oven at 40 °C and used for characterization. 2.5. Film Characterization. The changes in the chemical composition in the near surfaces of hydrophobic polymer films after modification were analyzed by attenuated total reflectance Fourier transform infrared spectra (ATR-FTIR, Nicolet 6700, U.S.). The surface morphologies of films were observed by a field-emitting scanning electron microscope (FESEM, Hitachi S-4800, Japan). The surface topographies of the modified films were analyzed by an atomic force microscope (AFM, SPI-3800N, Japan). The AFM images were obtained in tapping mode. The root mean square (rms) was used to evaluate the surface roughness of the polymer films on the basis of a 1.0 μm 1.0 μm scan area. The reported rms value was an average of five measurements. The surface hydrophilicity and surface energy (SE) of polymer films were characterized by water contact angle measurements (CA, Dataphysics OCA20, Germany). Five measurements were performed for each film at 25 °C and 70% relative humidity, and the mean value was taken as the reported result. The SE value was determined according to the OWRK method by using two kinds of test liquids (Table 1). Polar and disperse contributions to the surface energy are combined by forming the sum of both parts. The calculative process of SE and its components is described in detail in the Supporting Information.
3. RESULTS AND DISCUSSION 3.1. Thickness and Chemical Compositions of Deposited Polydopamine. The thickness evolution for the deposited poly-
dopamine (pDA) layer is shown in Figure 1A. An increase in the thickness of the deposited layer with increasing time and temperature was observed. The growth of deposited pDA was nearly linear with reaction time during the initial 10 h, and then the growth rate decreased after 15 h and the thickness of the deposited layer gradually reached a constant value. Deposited thicknesses of 23.930.4 nm for the Si substrate and 48.966.8 nm for the PVDF spin-coated Si (Si/PVDF) substrate were obtained in the temperature range of 2045 °C after 24 h of reaction. It was revealed that an elevated reaction temperature resulted in a higher reaction rate for the dopamine monomer, thus leading to a thicker deposited layer attached to the substrate. Moreover, the different surface chemistries between Si and Si/PVDF substrates made an obvious difference in the deposition behavior under the
Figure 2. Morphologies of the products in dopamine solution. (a) Photographs of dopamine solution before and after centrifugation. (b) TEM imagines of pDA particles in the supernatant. (c) SEM images of pDA deposition. (C = 2.0 g/L, t = 24 h).
Table 2. Surface Energy and Its Components in the Original Polymer Filma contact angle (deg)
surface-energy components (mN/m)
water
diiodomethane
σs0
σds0
σps0
PTFE PI
124.2 ( 0.8 82.0 ( 0.7
98.4 ( 0.7 38.9 ( 1.3
9.3 39.8
9.2 35.9
0.1 3.9
PET
85.8 ( 1.5
35.2 ( 1.6
41.3
39.0
2.2
PVDF1
91.6 ( 2.6
51.3 ( 1.5
33.0
31.0
2.0
PVDF2
94.0 ( 2.4
55.5 ( 1.3
30.7
28.9
1.7
PVDF3
96.8 ( 1.4
60.1 ( 1.9
28.1
26.6
1.5
samples
Here, σs0 represents the total surface energy of the original polymer film, and σds0 and σps0 stand for its disperse and polar parts, respectively. a
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Figure 3. (a) AFM topographies and (b) surface SEM images of original PVDF films.
Table 3. Effect of Reaction Temperature on the Surface Energy of a pDA-Deposited PVDF Film (C = 2.0 g/L, t = 24 h)a contact angle (deg)
surface-energy components (mN/m)
sample
temperature (°C)
water
diiodomethane
σs
σds
σps
(σs σs0)/σs0
PVDF1/pDA
20
67.5 ( 1.8
42.5 ( 3.5
42.4
30.1
12.3
0.28
30 45
64.7 ( 2.4 54.8 ( 3.4
38.8 ( 1.3 31.4 ( 4.3
44.8 51.5
31.4 32.7
13.4 18.8
0.36 0.56
60
50.4 ( 1.1
26.2 ( 4.0
54.8
33.8
21.0
0.66
Here, σs represents the total surface energy of a pDA-coated polymer film, and σds and σps stand for its disperse and polar parts, respectively. σs σs0/σs0 is defined to evaluate the changes in σs of a pDA-coated film in comparison to that of the original ones. A high value of σs σs0/σs0 corresponds to a large increase in the total surface energy of the polymer film after the coating with pDA. a
same reaction conditions. More dopamine oligomers and polymers adhered to the surface of the Si/PVDF substrate probably because of the increased surface roughness after the coating with PVDF and the stronger interactions between dopamine and the PVDF coating layer.17,18 Much research has suggested that the properties of pDA are compatible with those of eumelanins, including the chemical composition and physical morphology,5,19,20 and it was also confirmed by our experimental results discussed below. As seen in Figure 1B, there was a remarkable change in dopamine in the FTIR spectra after oxidation and self-polymerization. Compared with dopamine, the absorption peaks of pDA broadened and became unseparated from each other, which was almost identical to the reported FTIR spectrum of eumelanin.19,21 For the ATRFTIR spectra of PVDF films in Figure 1B, several new absorption signals appeared after surface modification by dopamine solution. A broad absorbance between 3600 and 3100 cm1 was ascribed to NH/OH stretching vibrations. The peaks at 1600 and 1510 cm1 were attributed to the overlap of the CdC resonance vibrations in the aromatic ring and the NH bending vibrations, respectively. These results proved the incorporation of the pDA composite layer on the surface of the PVDF film after dopamine modification. The water contact angle (CA) measurement is a common method of characterizing the surface relative hydrophilicity and wetting properties. The CA of pDA-coated substrates is shown in
Figure 1C. Despite the obviously different wetting behaviors of the original substrates, all CA values of the pDA-coated substrates were centralized around ∼4565°, which approached the theoretical value of the pDA film as reported by others.5,22 The CA values of the modified substrates decreased with increasing reaction temperature, probably in relation to the thicker pDA layer over the substrates. As a matter of fact, an elevated temperature could accelerate the deposition of pDA on the substrates, creating a rougher pDA layer than that formed at lower temperature. Therefore, a more hydrophilic surface was obtained by elevating the reaction temperature from 20 to 45 °C. 3.2. Reaction Products in Dopamine Solution. A series of experiments were performed at 20, 30, 45, and 60 °C to investigate the effect of temperature on the polymerization behavior of dopamine. During the self-polymerization and cross-linking reaction of dopamine, the color of the solution gradually changed from colorless to dark brown and then to black, finally returning to light yellowish brown on standing. A mass of macroscopic black particles was formed and was mostly deposited at the bottom of the vessel (Figure 2A). The color of the supernatant lightened with increasing temperature and became almost colorless when the reaction temperature reached 60 °C. The changes in the color (Figure 2A) and TEM images (Figure 2B) of the supernatant suggested that the pDA particle sizes and number declined as the temperature increased. Large pDA particles and their aggregates were unstable in water and 14183
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