Article pubs.acs.org/Langmuir
Stick−Slip Phenomenon in Measurements of Dynamic Contact Angles and Surface Viscoelasticity of Poly(styrene-b-isoprene-bstyrene) Triblock Copolymers Biao Zuo, Fan Fan Zheng, Yu Rong Zhao, TianYu Chen, Zhuo Hua Yan, Huagang Ni, and Xinping Wang* Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of the Education Ministry, Zhejiang Sci-Tech University, Hangzhou 310018, China ABSTRACT: In this paper, a series of poly(styrene-bisoprene-b-styrene) triblock copolymers (SIS), with different chemical components, was synthesized by anionic polymerization. The relationships between surface structures of these block copolymers and their stick−slip phenomena were investigated. There is a transition from stick−slip to a closely smooth motion for the SIS films with increasing PS content; the patterns almost vanish and the three-phase line appears to move overall smoothly on the film surface. The results show that the observed stick−slip pattern is strongly dependent on surface viscoelasticity. The jumping angle Δθ, which is defined as θ1 − θ2 (when a higher limit to θ1 is obtained, the triple line “jumps” from θ1 to θ2 with increases in drop volume), was employed to scale the stick−slip behavior on various SIS film surfaces. Scanning force microscopy/atomic force microscopy (AFM) and sum frequency generation methods were used to investigate the surface structures of the films and the contributions of various possible factors to the observed stick−slip behavior. It was found that there is a linear relationship between jumping angle Δθ and the slope of the approach curve obtained from AFM force measurement. This means that the stick−slip behavior may be attributed mainly to surface viscoelasticity for SIS block copolymers. The measurement of jumping angle Δθ may be a valuable method for studying surface structure relaxation of polymer films. contact angle, θa, which is the contact angle found at the advancing edge of a liquid drop. The lower limit is the receding contact angle, θr, which is the contact angle found at the receding edge. The difference between the advancing and receding contact angles is known as the contact angle hysteresis, θhyst:
1. INTRODUCTION Surface tension is an important thermodynamic parameter that plays a dominant role in numerous industrial and biological processes. Because of difficulties encountered in making a direct measurement of surface tension involving a solid phase, contact angle measurements are used as an indirect approach for this purpose.1−7 Over the past several decades, numerous techniques4−10 have been used to measure contact angles, which were inspired by application of the equation first derived by Thomas Young in 1805. Young’s equation governs the equilibrium of the three interfacial tensions and the Young contact angle θY of a liquid drop on a solid γlv cos θ Y = γsv − γsl (1)
θ hyst = θa − θr
Nearly all solid surfaces exhibit contact angle hysteresis, and because of this hysteresis, the contact angle interpretation in terms of Young’s equation is contentious. Not all of the experimentally measured or observed contact angles are reliable and appropriate. Although contact angle hysteresis has been studied extensively over the past several decades, the underlying causes and its origins are not completely understood. Studies have attributed contact angle hysteresis to surface roughness11−13 and heterogeneity,14−18 as well as metastable surface energetic states.15,17,19 Some researchers found that the hysteresis decreases with increasing molecular volume of the
where γlv is the liquid−vapor surface tension, γsv the solid− vapor surface tension, γsl the solid−liquid surface tension, and θY is the Young contact angle. The derivation of Young’s equation assumes that the solid surface is smooth, homogeneous, and rigid; it should also be chemically and physically inert with respect to the liquids to be employed. Ideally, according to Young’s equation, a unique contact angle is expected for a given system (e.g., a liquid drop on a solid surface). In a real system, however, a range of contact angles is usually obtained. The upper limit of the range is the advancing © 2012 American Chemical Society
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Received: January 9, 2012 Revised: February 10, 2012 Published: February 13, 2012 4283
dx.doi.org/10.1021/la300119n | Langmuir 2012, 28, 4283−4292
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relatively infrequent: the process may be predominantly “stick”, with little “slip”. To date, neither the mechanisms responsible for the stick−slip of the three-phase line nor the thermodynamic significance of the corresponding contact angles are well understood. In this paper, we present the first systematic study of advancing contact angles of water on various poly(styrene-bisoprene-b-styrene) triblock copolymers (SIS) film surfaces using the ADSA-P method. A series of SIS triblock copolymers with different chemical components was synthesized by anionic polymerization, and the relationship between the surface structure of these block copolymers and their stick−slip phenomena was investigated. The goal of this study is to obtain new insight into the mechanisms responsible for the stick−slip of the three-phase line using contact angle measurements, complemented by film surface analysis using atomic force microscopy (AFM), scanning force microscopy (SFM), and sum frequency generation (SFG) spectroscopy.
liquid on monolayers.20,21 In more recent studies, contact angle hysteresis was found to be related to molecular mobility and packing of the surface,22,23 liquid penetration, and surface swelling.24 Contact angle measurement has been a major experimental approach to many problems concerning solid−liquid interfaces. There are three main techniques for measuring the contact angle on flat solid surfaces. These are the sessile drop, the Wilhelmy plate, and the inclined plane methods. Axisymmetric drop shape analysis profile (ADSA-P), one of the sessile drop methods, is a novel technique for determining liquid−fluid interfacial tensions and contact angles from the shape of axisymmetric menisci (i.e., from sessile as well as pendant drops).25 Its basic principle is to fit the experimental drop profile to a theoretical one given by the Laplace equation of capillarity, and the surface tension is generated as a fitting parameter. Other parameters, such as contact angle, drop volume, surface area, and three-phase contact radius, can also be obtained. An automated ADSA-P has been shown to be a very powerful method for determining the advancing and receding contact angles.26 From these experimental results, four patterns of advancing contact angle were observed: (1) time-dependent advancing contact angle, (2) constant advancing contact angle, (3) stick−slip, and (4) no advancing contact angle. A phenomenon which was termed “stick−slip” may occasionally be observed during the evolution of a drop, in which the wetting front remains static for most of the time, but from time to time moves quite abruptly. The same type of behavior can sometimes be seen when liquid evaporates from a sessile drop or a sliding drop slowly moving triple line.26−30 Since stick−slip phenomena exist in some systems during dynamic contact angle measurement, a resulting contact angle higher than θa or less than θr may be obtained. Several studies on the mechanisms responsible for the stick− slip behavior during contact angle measurement were performed by Neumann.27,31,32 He proposed earlier that such stick−slip behavior could be due to noninertness of the surface.33 Phenomenologically, an energy barrier for the drop front exists, resulting in sticking, which causes the contact angle to increase at constant contact radius r. However, as more liquid is supplied into the sessile drop, the drop front possesses enough energy to overcome the energy barrier, resulting in slipping, which causes the contact angle to suddenly decrease. Another reason for stick−slip behavior was attributed to swelling processes or partial dissolution of the surface of noninert polymer materials.31,32 Contact angles of a series of nalkanes (n-heptane to n-hexadecane) were studied by Neumann27 on two functionalized maleimide copolymers [poly(ethene-alt-N-(4-(perfluoroheptylcarbonyl)aminobutyl)maleimide) (ETMF) and poly(octadececene-alt-N-(4(perfluoroheptylcarbonyl)aminobutyl)maleimide) (ODMF)]. The observed stick−slip was attributed to cooperative nonuniform vapor adsorption on the heterogeneous domains on the surface. Shanahan30 investigated the mechanisms responsible for the stick−slip of the three-phase line of a drop during evaporation. He proposed that the jump distance δr will depend on U1/2, where U is the potential energy barrier, when sufficient energy is available to overcome the hysteresis barrier effect, and the triple line jumps to its next equilibrium position. Thus, for an ideal solid, δr→0 and a smoothly receding triple line would be expected. Conversely, for a system presenting an important hysteretic energy barrier, the jumps may be large and
2. EXPERIMENTAL SECTION 2.1. Materials. Chemicals, including styrene and isoprene, were purchased from Shanghai Reagent Co. Premium cover glass (24 mm × 30 mm × 0.1 mm) was purchased from Fisher Scientific Co. Solvents were purified in the usual manner, dried by refluxing over sodium, and distilled prior to use. 2.2. Polymer Synthesis and Characterization: Poly(styreneb-isoprene-b-styrene) Triblock Copolymer (SIS). Triblock copolymers SIS-1 to SIS-8 with 1,2- and 3,4-PI (shown in Scheme
Scheme 1. Schematic Representation of SIS Triblock Copolymers with Various Polystyrene Contents
1) in PI block chains containing various polystyrene contents were synthesized in tetrahydrofuran at 0 °C with the sodium naphthalene initiating system using Schlenk techniques in a dry nitrogen atmosphere according to the reported method.34 The molecular weights and polydispersities of the resulting polymers were determined by gel permeation chromatography (GPC) using an Agilent 1100 apparatus (with THF as the eluent at a flow rate of 1.0 mL/min). The GPC chromatograms were calibrated against standard polystyrene samples. 1H NMR spectra were recorded on a Bruker Advance AMX500 NMR spectrometer in CDC13 with tetramethylsilane (TMS) as the internal standard. The Fourier Transform Infrared (FTIR) spectra of the copolymers were measured on a Nicolet Avatar 370 FTIR spectrometer. 2.3. Film Formation. Glass plates were purchased from Fisher Scientific Co. The glass plates were washed three times with dichloromethane in an ultrasonic bath for 5 min and then in a sulfochromic solution for 24 h. The substrates were rinsed five or six times in deionized water and then dried in nitrogen flux. Each polymer was dissolved in xylene to make a 4 wt % solution, which was then filtered through a porous PTFE filter (with pores of 0.25 μm in diameter). The films were prepared by spin-coating at 1600 rpm for 10 ± 2 s on cleaned glass plates and then dried in air for 24 h and then in vacuum at 45 °C for another 24 h. The thickness of the resulting film was about 220 nm. 4284
dx.doi.org/10.1021/la300119n | Langmuir 2012, 28, 4283−4292
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Figure 1. 1H NMR (A) and FTIR (B) spectra of SIS-2. The peaks designated a and b in the 1H NMR were assigned as shown in Scheme 1. 2.4. Advancing Contact Angle Measurements. Sessile drop contact angle measurements using ADSA-P were performed as a function of time. An initial drop with a radius larger than 3 mm was deposited onto the sample surface to ensure that the drop was axisymmetric. By use of a motor-driven syringe to pump liquid steadily into the sessile drop, a sequence of images of the growing drop was then captured, and the advancing contact angles were obtained. The advancing rate used in this study was 0.5−5 motor steps/s (one step is equivalent to two micros that the motor piston moves, or 0.083 708 mm3 liquid/s) The static contact angles of water and diiodomethane were used to make estimates of surface free energy for various samples according to the theory of Owens and Wendt.2 In order to ensure that the results were sufficiently credible, the experimental errors in measuring the θ values were evaluated to be less than ±2°. 2.5. Atomic Force Microscopy. The morphology of the top surface of the samples was investigated using XEI-100 scanning probe microscopy (PSIA Co.). SPM measurements were performed in air with an etched silicon probe having a length of 125 μm; the spring constant was varied from 20 to 75 N/m. Scanning was carried out in the tapping mode at a frequency of approximately 300 Hz. Images of residual deformation were obtained by placing a 4 μL sessile water drop on the sample surface for 30 min at room temperature. After the residual drop was completely removed by filter paper, AFM measurement was then performed immediately. Surface elasticity and adhesion forces were readily investigated using the “force−distance analysis” (F/D analysis) mode of the SPM technique.35,36 The slope or shape of the contact part of the force− distance curve can provide information about the elasticity of the sample surface. Commercial V-shaped silicon nitride integrated tip/ cantilevers (NSC36A, Mikromasch, Estonia) with 0.95 N/m of a nominal force constant were utilized for these measurements. The samples were moved toward and away from the tip over a range of 200 nm with a speed of 10 s/cycle. The slope obtained from the F/D curve was the average of at least 25 measurements taken within a 1 × 1 μm area. 2.6. Sum Frequency Generation (SFG) Vibrational Spectroscopy. Sum frequency generation (SFG) vibrational spectra were obtained using a custom-designed EKSPLA SFG spectrometer that has been described in our previous paper.37 Briefly, the visible input beam at 0.532 μm was generated by frequency doubling of part of the fundamental output from an EKSPLA Nd:YAG laser. The IR beam, tunable between 1000 and 4300 cm−1 (with a line width
