Article pubs.acs.org/Langmuir
Tailoring of Morphology and Surface Properties of Syndiotactic Polystyrene Aerogels Xiao Wang and Sadhan C. Jana* Department of Polymer Engineering, The University of Akron, Akron, Ohio 44325, United States S Supporting Information *
ABSTRACT: This study evaluates a method for rendering syndiotactic polystyrene (sPS) aerogels hydrophilic using polyethylene oxide (PEO) of different molecular weights. The highly porous sPS aerogels are inherently hydrophobic although applications involving absorption of moisture and removal of particulate solids may benefit from the high surface area of sPS aerogels provided some degree of hydrophilicity is induced in these materials. In this work, sPS gels are prepared by thermo-reversible gelation in tetrahydrofuran in the presence of PEO. The gels are dried under supercritical conditions to obtain aerogels. The aerogels are characterized by scanning electron microscopy, nitrogen-adsorption porosimetry, helium pycnometry, and contact angle measurements. The data reveal that the pore structures and surface energy can be controlled by varying the concentration and molecular weight of PEO and using different cooling rates during thermo-reversible gelation. In the first case, sPS aerogels, aerogels containing PEO of a low molecular weight or low concentration show superhydrophobic surface presenting the “lotus effect”. In the second case, PEO at a higher concentration or with higher molecular weight forms phase-separated domains yielding new hydrophilic macropores (>10 μm) in the aerogel structures. These macropores contribute to the superhydrophobic surface with the “petal effect”. The cooling rate during gelation shows a strong influence on these two cases.
1. INTRODUCTION
achieved. The impact of roughness on contact angle is given by Wenzel’s equation,14
Nature provides the best examples of nanostructured surfaces. Water droplets can roll off the lotus leaf spontaneously with a slight tremble; dust particles are removed from the surfaces in the same manner. This unusual self-cleaning character is termed the “lotus effect”.1,2 Another interesting phenomenon is the “petal effect”,3 whereby the nearly spherical water droplets stick to the rose petals even when they are tilted upside down. The surfaces exhibiting lotus and petal effects show a water contact angle (CA) greater than 150°,4−6 but surfaces with petal effects offer higher contact angle hysteresis (CAH).3 CAH arises from concomitant force of retention, which causes droplets to stick to the surfaces and to resist motion.7 Surfaces exhibiting the “lotus effect” and “petal effect” find important applications, such as artificial nonwettable superhydrophobic coatings on medical devices, textiles, or electronics to achieve self-cleaning and long-lasting protection.8,9 Sticky superhydrophobic surfaces, on the other hand, have been proposed for applications such as transportation of water microdroplets and wall-climbing robots.10,11 The hierarchical structure composed of micro- and nanoscale roughness and regulated by the surface energy of the bulk material are responsible for superhydrophobic nature of the surfaces.12 So far, the maximum contact angle value reported for a nontextured surface is only approximately 120° for smooth CF3-terminated surfaces.13 Thus, surface roughness becomes a necessary requirement if superhydrophobicity characterized by water contact angle larger than 150° is to be © XXXX American Chemical Society
cos θ′ = γ cos θ
(1)
where θ′ is the apparent contact angle on a rough surface, θ is the intrinsic contact angle on a flat, smooth surface, and γ is the roughness factor. Wenzel’s equation predicts that surface roughness reduces the value of contact angle for a droplet on a hydrophilic surface and increases the contact angle of a droplet on a hydrophobic surface. However, the Wenzel model is not sufficient for dealing with heterogeneous surfaces, such as those composed of two or more materials with different surface energy values. Cassie−Baxter model15 can describe a nonhomogeneous or composite regime with a three phase solid− water−air interface, in which the air pockets are trapped between the solid surface and water. The contact angle is then given by an average of the cosine values of the contact angles of the solid materials and air, as in eq 2: cos θ′ = f cos θ − (1 − f )
(2)
In eq 2 f is the fraction of the solid material in contact with the droplet and (1 − f) corresponds to the fraction of air, indicating that the fraction of trapped air plays a vital role in determining superhydrophobicity of a surface, as is encountered in aerogels. Received: February 5, 2013 Revised: April 5, 2013
A
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aerogels are expected to provide the following attributesthe aerogel framework is derived from hydrophobic sPS and PEO contributes hydrophilic character. On exposure, PEO absorbs water and other polar liquids, while the sPS framework retains the shape and integrity of the aerogel. The degree of hydrophilicity and the size of PEO-rich domains are expected to be a function of PEO concentration, PEO molecular weight, and cooling temperature. A recent study showed that sPS easily formed gels and sPS strands locked carbon nanotubes in the meso- and macropores when a suspension of carbon nanotubes in sPS solution in chlorobenzene was cooled to room temperature.48 The carbon nanotube networks reinforced the aerogel and exhibited strain-dependent electrical conductivity. Another interesting paper presented the formation of mixed aerogel with sPS and a polyether, i.e., poly(2,6-dimethyl-1,4phenylene)oxide, exhibiting nanoporous−crystalline phases of both polymers.49
Aerogels are highly porous materials characterized by extremely large surface area and very small pore size, with unique applications as thermal/sound insulator,16,17 catalyst support,18 molecular storage,19 molecular separator,20 etc. Manipulation of pore structures and surface properties of aerogels still remains an interesting area of research. For example, silica aerogels offer high porosity, but their surfaces do not show lotus and petal effects due to inherent hygroscopic nature of the silica backbone. These materials absorb moisture from humid surroundings and disintegrate due to capillary stress. Also, immediate collapse occurs in contact with liquid water.21 A variety of methods have been reported for surface modification of silica aerogels to render them hydrophobic, namely, using alkyl alkoxy coprecursors,22−24 converting Si− OH functionalities into Si−CH3 groups,25,26 applying polymer to bridge the neck between neighboring secondary silica particles,27−30 grafting polyhedral oligomeric silsesquioxane molecules with multiple Si−OH functional groups,31 to name a few. Nevertheless, to the best of our knowledge, possible lotus and petal effects have not been reported for silica aerogel surfaces. This work focuses on physically bonded aerogels derived from a crystalline polymer, syndiotactic polystyrene (sPS), and evaluates an experimental method for creation of “lotus effect” and “petal effect” surfaces by introducing a hydrophilic polymer as a means for alteration of both morphology and surface energy. Syndiotactic polystyrene aerogels contain polymer networks originating from three-dimensional connectivity at crystalline junctions of sPS chains and pore structures consisting of micropores of diameter (Φ) < 2 nm within the crystalline junctions and macropores (Φ > 50 nm) formed by the fiber-like polymer strands of diameters in the range of 30− 200 nm.32 The physically bonded crystalline networks of sPS fibrils emerge as sPS solutions are cooled. It is reported that sPS easily forms thermo-reversible gels with many lowmolecular-mass guest molecules, such as tetrahydrofuran,33 benzene, 34,35 toluene, 35−37 chloroform, 37−40 chlorobenzene,41,42 o-xylene,41 naphthalene,43,44 and its derivatives.45 A widely accepted mechanism for gel network formation in sPS involves the following stepsfirst, polymer chains change from random coil to helix as solvent molecules are inserted between the adjacent phenyl groups to stabilize the helix in solution. Second, the helices agglomerate to form the crystalline domains, which in turn physically interlock many chains and yield gels.34,46 The main goal of this work is to manipulate the meso- and macropore structures and surface properties of sPS aerogels so as to obtain surfaces with “lotus effect” and “petal effect”. For this purpose, polyethylene oxide (PEO) is dissolved along with sPS in the solvent and sPS is allowed to undergo thermoreversible gelation such that a PEO-rich solution forms separate domains inside the gel. The size of the PEO-rich domain is anticipated to be a function of the concentration and molecular weight of PEO. The rationale for such material selection is as follows. First, sPS and PEO are respectively hydrophobic and hydrophilic materials and their combinations should provide a handle on adjustment of hydrophilicity of the resultant aerogels. Second, as PEO is thermodynamically incompatible with sPS, various phase structures can be obtained in the gels based on thermodynamic and kinetic conditions.47 Third, PEO is insoluble in liquid carbon dioxide and therefore, remains in the aerogel structure after supercritical drying. These sPS/PEO
2. EXPERIMENTAL SECTION 2.1. Materials. Syndiotactic polystyrene (Mw = 300 000 g/g mol, density = 1.05 g/mL) in the form of pellets was purchased from Scientific Polymer Products Inc. (Ontario, NY). The sPS pellets were ground into powder for easy dissolution in solvents. Polyethylene oxide of molecular weight (Mw) 300 000, 100 000, and 20 000 g/g mol was purchased from Sigma-Aldrich. Tetrahydrofuran (THF) was used as the solvent of sPS and PEO. 2.2. Preparation of sPS/PEO Aerogels. The sPS/PEO aerogels were prepared as follows. First, solutions of sPS and PEO in THF were cooled such that sPS formed thermo-reversible gel. Second, the solvent THF in the gel was exchanged with liquid carbon dioxide (CO2). Third, the gel was subjected to supercritical drying. In typical experiments, sPS and PEO powder were mixed with THF and heated to 110 °C in a sealed vial until complete dissolution of the solids occurred. The solution was transferred to a cylindrical mold and allowed to stand and gel for 24 h at room temperature. A series of sPS/PEO gels were prepared with PEO of different molecular weight and several weight ratios of sPS and PEO. A native sPS gel was prepared using the same procedure and used as control. The gels were kept in a closed chamber filled with liquid CO2 for 2 h to exchange THF with liquid CO2. The solvent was drained off and the chamber was refilled with fresh liquid CO2. The soaking and washing process was repeated five times. The vessel was heated to 45 °C and 11 MPa above the supercritical point of CO2 (31 °C, 7.4 MPa).50 After 1 h, the vessel was depressurized at 45 °C for venting of CO2 under supercritical condition. Any residual solvent was removed by keeping the specimens overnight in a vacuum oven. Table 1 lists the compositions of gel specimens along with aerogel sample codes. The specimen “sPS-PEO20000 0.04−0.01” denotes aerogel prepared from a solution containing 0.04 g/mL sPS and 0.01 g/mL PEO with PEO molecular weight of 20 000 g/g mol.
Table 1. Composition of Gels content
B
sample identifier
sPS (g)
PEO (g)
THF (mL)
sPS-4 sPS-PEO20000 4−1 sPS-PEO20000 4−2 sPS-PEO20000 4−3 sPS-PEO100000 4−1 sPS-PEO100000 4−2 sPS-PEO100000 4−3 sPS-PEO300000 4−1 sPS-PEO300000 4−2 sPS-PEO300000 4−3
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
0 0.1 0.2 0.3 0.1 0.2 0.3 0.1 0.2 0.3
10 10 10 10 10 10 10 10 10 10
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Table 2. Shrinkage, Bulk Density, Skeletal Density, and Porosity of Aerogel Specimens sample identifier
diameter shrinkage (%)
sPS-4 sPS-PEO20000 4−1 sPS-PEO20000 4−2 sPS-PEO20000 4−3 sPS-PEO100000 4−1 sPS-PEO100000 4−2 sPS-PEO100000 4−3 sPS-PEO300000 4−1 sPS-PEO300000 4−2 sPS-PEO300000 4−3
8.0 7.0 6.4 5.5 8.2 7.9 7.7 7.5 7.8 5.2
± ± ± ± ± ± ± ± ± ±
0.67 0.92 0.66 0.28 0.77 0.08 0.48 0.80 0.69 0.10
bulk density (g/mL) 0.060 0.071 0.083 0.090 0.072 0.082 0.095 0.070 0.081 0.088
2.3. Characterization. The bulk density ρb of aerogels was obtained from the weight and volume of the cylindrical specimens. The skeletal density ρs of sPS, PEO, and aerogel specimens was measured using Accupyc 1340 Helium Pycnometer (Micromeritics). The skeletal density can be used in conjunction with the bulk density to calculate porosity p. The shrinkage δ in diameter was determined from the difference of diameter of the aerogel and the corresponding gel specimen. The bulk density, porosity, and shrinkage of the aerogels were calculated using the following relationships: ρb =
4m πD 2 h
(4)
D D0
(5)
δ=1−
polarity =
4γLdγSd γLd + γSd
−
94.3 93.4 92.6 91.8 93.4 92.7 91.2 93.5 92.7 92.2
0.0022 0.0036 0.0031 0.0028 0.0027 0.0065 0.0074 0.0006 0.0063 0.0072
γsp γsp + γsd
(7)
3. RESULTS AND DISCUSSION 3.1. Density, Shrinkage, And Porosity. It was expected at the outset that syndiotactic polystyrene would form the gel networks and PEO would form its own domains after phase separation. In view of this, the concentration of sPS was fixed at 0.04 g/mL in all samples, and the concentration and molecular weight of PEO were varied as listed in Table 1. The sPS and PEO solutions were homogeneous at 110 °C for the compositions listed in Table 1. Note that sPS is thermodynamically immiscible with PEO. However, THF plays the role of a cosolvent of sPS and PEO at 110 °C. Translucent and in some cases opaque gels formed after the homogeneous solutions at 110 °C were allowed to cool down naturally to room temperature and allowed to stand for approximately 24 h. The aerogels were recovered from these gels following the procedure described in the Experimental Section. The aerogel specimens exhibit small shrinkage (5−8%) in diameter and show low bulk density values (0.06−0.09 g/mL) as presented in Table 2. It is seen that the bulk density of the aerogels increases with PEO concentration. It is also seen that the bulk density of aerogels with the same PEO content bears weak relationship with molecular weight of PEO. For example, aerogel specimens sPS−PEO100000 4−2 and aerogel sPS− PEO300000 4−2 show similar bulk density, although PEO of
4γLpγSp γLp + γSp
porosity (%)
± ± ± ± ± ± ± ± ± ±
1.056 1.076 1.115 1.093 1.091 1.117 1.076 1.081 1.106 1.125
The values of sliding angle and hysteresis were obtained from the tilting method to illustrate the surface properties of aerogels. For this purpose, the aerogels were carefully fractured and a flat fractured surface was selected for conducting the test. The values of dynamic contact angle were measured as follows: A water droplet of 8 μL volume was placed on the fractured aerogel surface. The stage was then rotated by 90° with a speed of 1°/s and the advancing and receding angles were measured. The sliding angle is the tilting angle of the stage at which the droplet starts sliding downward. The contact angle hysteresis was obtained from the difference of the advancing and receding angles. These measurements were repeated twice by placing water droplets at different locations on the surface of same aerogel specimen. BET surface area and pore size distribution of aerogel specimens were obtained from nitrogen adsorption−desorption isotherms at 77 K, analyzed using Micromeritics Tristar II 3020 Analyzer. The aerogel specimens were sectioned and placed in designated chamber followed by degassing at room temperature for 12 h before collecting the data. The surface area of aerogels was calculated using Brunauer−Emmet− Teller (BET) method and the pore size distribution was obtained using Barrett−Joyner−Halenda (BJH) method.
In eqs 3, 4, and 5, m is the mass of the aerogel specimen, D and h are respectively the diameter and height of the cylindrical aerogel specimen, and D0 is the diameter of the cylindrical gels. The morphology of aerogel specimen was observed using scanning electron microscope (SEM), JEOL JSM5310 with an operating voltage 8 kV. For this purpose, the aerogels were fractured at room temperature and mounted on an aluminum stub using adhesive carbon tape and subsequently sputter-coated by a thin layer of silver particles under argon atmosphere using Sputter Coater, Model ISI 5400. The values of the contact angle were measured using a Rame-Hart Model 500 advanced goniometer equipped with a tilting base and automated dispensing system. In the static sessile drop method, a 5-μL drop of liquid was placed on the sample surface, and the image was captured and analyzed with ImageJ software. Five measurements were taken for each specimen to obtain reproducible data. For sample preparation, the aerogels were carefully fractured and the specimen with the flat fractured surface was selected for the test. In addition, the aerogel samples were compressed under a pressure of 3000 psi for 5 min to remove the pores and to obtain solid discs. These discs were used to obtain information on surface energy and polarity of the polymeric materials from respective contact angle values. The contact angle values of both water and diiodomethane were measured on the fractured specimen surface and on compressed discs. The value of total surface energy (γ), and the polar (γp) and dispersion (γd) components of total energy were calculated using eq 6 based on Wu’s theory:51
γLS = γL + γS −
skeletal density (g/mL)
0.0021 0.0027 0.0031 0.0021 0.0020 0.0005 0.0008 0.0022 0.0014 0.0010
polarity was determined as the ratio of polar component of free energy to surface free energy, and calculated using eq 7:
(3)
⎛ ρ ⎞ p = ⎜⎜1 − b ⎟⎟ × 100 ρs ⎠ ⎝
± ± ± ± ± ± ± ± ± ±
(6)
In eq 6, γLS is the interfacial tension between liquid and solid, γL is the surface tension of the liquid, γS is the surface tension of the solid, and γd and γp are the dispersion (nonpolar) and polar components of surface tension, respectively. The values of surface energy γd = 21.8 dyn/cm and γp = 50.7 dyn/cm for water, γd = 44.1 dyn/cm and γp = 6.7 dyn/cm for diiodomethane were used in calculations.51 Surface C
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Figure 1. SEM images of fractured surfaces of sPS/PEO aerogels. Insets show the images of water droplet sitting on the fractured surface of each specimen.
Figure 2. SEM images of sPS-PEO100000 aerogel specimens. The upper, middle and bottom rows correspond to aerogels with sPS/PEO weight ratio of 4/1, 4/2, and 4/3, respectively. Parts (a), (d), and (g) show the detailed morphology of aerogels with increasing PEO concentration. Parts (b) and (c) show randomly selected microstructure for sPS-PEO100000 4−1. Parts (e) and (h) show the magnified micrograph of the matrix part around the PEO macropores. Parts (f) and (i) show the magnified micrograph of the macropore walls, which show lamellae of PEO crystals.
molecular weight, respectively, 100 000 and 300 000 g/g mol were used. Also, these materials have similar skeletal density. Note that the weight ratio of sPS and PEO and the total polymer content were the same in these materials. However, small increases in skeletal density are observed with the addition of PEO to sPS attributed to higher skeletal density of
PEO (1.25 g/mL) compared to that of sPS (1.06 g/mL). Porosity calculated from the values of bulk and skeletal density shows a small reduction from 94% to 91% with the increase of PEO concentration (Table 2), but its dependence on PEO molecular weight is weak. D
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Figure 3. SEM images of sPS-PEO20000 aerogels with weight ratio of sPS/PEO = 4/1 at (a) low and (b) high magnification, sPS/PEO = 4/2 at (c) low and (d) high magnification, and sPS/PEO = 4/3 at (e) low, and (f),(g) high magnification.
3.2. Pore Structure Regulation. The typical morphology of native sPS aerogel is presented in Figure S1 of the Supporting Information, SI. The fracture surface of the aerogel is relatively smooth as is evident from the low magnification image in Figure S1(a) of the SI. Figure S1(b) of the SI reveals that native sPS aerogel contains fiber-like networks with strand diameters in the range of 50−80 nm. This is quite similar to the morphology observed for polyimide aerogels52−55 exhibiting bundles of polymer fibers tangled together with fiber diameters in the range of 15−50 nm and in polyurea aerogels.56 The SEM image in Figure S1(b) of the SI reveals the macropores (diameter >50 nm) formed by the polymer strands. The micropores (diameter
