Thermoresponsive Poly(N-isopropylacrylamide) Copolymers

A detailed review of PNIPAM has been carried out by Schild.1 ...... The Journal of Physical Chemistry C 0 (proofing), ... Journal of Polymeric Materia...
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Langmuir 2004, 20, 10138-10145

Thermoresponsive Poly(N-isopropylacrylamide) Copolymers: Contact Angles and Surface Energies of Polymer Films Vincent P. Gilcreest,† William M. Carroll,‡ Yuri A. Rochev,‡,§ Irena Blute,| Kenneth A. Dawson,† and Alexander V. Gorelov*,†,§ Chemistry Department, University College Dublin, Belfield, Dublin 4, Ireland, National Centre for Biomedical Engineering Science, National University of Ireland, Galway, Ireland, Institute of Theoretical and Experimental Biophysics of Russian Academy of Science, Pushchino, Russia, and Institute for Surface Chemistry (YKI), Box 5607, SE-114 86 Stockholm, Sweden Received May 14, 2004. In Final Form: August 27, 2004 Surface properties of poly(N-isopropylacrylamide) (PNIPAM) copolymer films were studied by contact angle measurements and optical and atomic force microscopy. We prepared a series of copolymers of N-isopropylacrylamide with N-tert-butylacrylamide (NtBA) in order of increasing hydrophobicity. The measurements of the advancing contact angle of water at 37 °C were hampered by the observation of a distinct stick/slip pattern on all polymers in the series with the exception of poly(NtBA) (PNtBA). We attributed this behavior to the film deformation by the vertical component of liquid surface tension leading to the pinning of the moving contact line. This was confirmed by the observation of a ridge formed at the pinned contact line by optical microscopy. However, meaningful contact (without the stick/slip pattern and with a time-independent advancing contact angle) angles for this thermoresponsive polymer series could be obtained with carefully selected organic liquids. We used the Li and Neumann equation of state to calculate the surface energy and contact angles of water for all polymers in the series of copolymers and van Oss, Chaudhury, and Good (vOCG) acid-base theory for PNtBA. The surface energies of the thermoresponsive polymers were in the range of 38.9 mJ/m2 (PNIPAM) to 31 mJ/m2 (PNtBA) from the equation of state approach. The surface energy of PNtBA calculated using vOCG theory was 29.0 mJ/m2. The calculated contact angle for PNIPAM (74.5 ( 0.2°) is compared with previously reported contact angles obtained for PNIPAM-modified surfaces.

Introduction The interfacial properties of thermoresponsive polymers have attracted a great deal of interest recently due to their ability to modulate surface properties of materials by a change in temperature or other external stimuli such as pH or light. Poly(N-isopropylacrylamide) (PNIPAM) exhibits a phase transition at ∼32 °C in water which is known as the lower critical solution temperature (LCST). The proximity of the LCST to room temperature and physiological temperature has created a lot of interest in PNIPAM for biological applications. A detailed review of PNIPAM has been carried out by Schild.1 The possibility to create a thermoresponsive surface by modification with PNIPAM has received a lot of attention recently. Many different methods for creating thermoresponsive surfaces have been tested.2-14 They will be briefly * To whom correspondence should be addressed. E-mail: [email protected]. † University College Dublin. ‡ National University of Ireland. § Institute of Theoretical and Experimental Biophysics of Russian Academy of Science. | Institute for Surface Chemistry (YKI). (1) Schild, H. Prog. Polym. Sci. 1992, 17, 163. (2) Schmitt, F. J.; Park, C.; Simon, J.; Ringsdorf, H.; Israelachvili, J. Langmuir 1998, 14, 2838. (3) Ista, L.; Mendez, S.; Perez-Luna, V.; Lo´pez, G. Langmuir 2001, 17, 2552. (4) Balamurugan, S.; Mendez, S.; Balamurugan, S.; O’Brien, M., II; Lo´pez, G. Langmuir 2003, 19, 2545. (5) Liang, L.; Feng, X.; Liu, J.; Rieke, P.; Fryxell, G. Macromolecules 1998, 31, 7845. (6) Liang, L.; Rieke, P.; Liu, J.; Fryxell, G.; Young, J.; Engelhard, M.; Alford, K. Langmuir 2000, 16, 8016.

reviewed in the discussion section of this paper. By modifying a surface with PNIPAM, the surface can be switched from hydrophilic to hydrophobic by increasing the temperature above the LCST. The ability to change the hydrophobicity of a surface by adjusting temperature has found many potential applications, including chromatography,15-17 temperature-sensitive membranes,18 bacterial biofilm,13 and mammalian cell release surfaces.19,20 (7) Liang, L.; Rieke, P.; Fryxell, G.; Liu, J.; Engehard, M.; Alford, K. J. Phys. Chem. B 2000, 104, 11667. (8) Geuskens, G.; Etoc, A.; Di Michele, P. Eur. Polym. J. 2000, 36, 265. (9) Della Volpe, C.; Cassinelli, C.; Morra, M. Langmuir 1998, 14, 4650. (10) Takei, Y.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. Macromolecules 1994, 27, 6163. (11) Yakushiji, T.; Sakai, K.; Kikuchi, A.; Aoyagi, T.; Sakurai, Y.; Okano, T. Langmuir 1998, 14, 4657. (12) Kidoaki, S.; Ohya, S.; Nakayama, Y.; Matsuda, T. Langmuir 2001, 17, 2402. (13) Cunliffe, D.; Alarcon, C.; Peters, V.; Smith, J.; Alexander, C. Langmuir 2003, 19, 2888. (14) Yim, H.; Kent, M. S.; Huber, D. L.; Satija, S.; Majewski, J.; Smith, G. S. Macromolecules 2003, 36, 5244. (15) Kanazawa, H.; Kashiwase, Y.; Yamamoto, K.; Matsushima, Y.; Kikuchi, A.; Sakurai, Y.; Okano, T. Anal. Chem. 1997, 69, 823. (16) Annaka, M.; Matsuura, T.; Kasai, M.; Nakahira, T.; Hara, Y.; Okano, T. Biomacromolecules 2003, 4, 395. (17) Hosoya, K.; Sawada, E.; Kimata, K.; Araki, T.; Tanaka, N.; Frechet, J. M. J. Macromolecules 1994, 27, 3973. (18) Park, Y. S.; Ito, Y.; Imanishi, Y. Langmuir 1998, 14, 910. (19) Rochev, Y.; Golubeva, T.; Gorelov, A.; Allen, L.; Gallagher, W. M.; Selezneva, I.; Gavrilyuk, B.; Dawson, K. Prog. Colloid Polym. Sci. 2001, 118, 153. (20) Ito, Y.; Chen, G.; Guan, Y.; Imanishi, Y. Langmuir 1997, 13, 2756.

10.1021/la0487996 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/12/2004

Surface Energies of NIPAM Copolymers

Our interest in these polymers stems primarily from ongoing efforts to exploit the properties of thermoresponsive polymers in the biomedical field. By adhering cells to a thermoresponsive surface, it is possible to detach the cells from the surface by a change in temperature (switching the surface from hydrophilic to hydrophobic) without the use of chemicals such as EDTA or proteolytic enzymes that have been found in some instances to damage cells. In studies of cell adhesion and growth on PNIPAM and tissue culture polystyrene (TCP) surfaces, it has been found that TCP was notably superior.19 Cell attachment and growth have been found to be dependent on the hydrophobicity of the surface on which they are grown.21 On this basis, the poor ability of PNIPAM surfaces to support cell growth could be attributed to their relative hydrophilicity. By increasing the hydrophobicity of the surface by addition of N-tert-butylacrylamide (NtBA), the cell adhesion and growth properties are improved, and it has been reported that NIPAM/NtBA copolymer films can support cell adhesion and growth in a manner dependent on the composition of copolymers.19 As will be discussed later, there is a wide spread of contact angles reported for these surfaces. This is perhaps due to the different manners in which the surfaces were prepared, each resulting in different degrees of surface coverage. To elucidate difficulties arising due to low polymer surface coverage, we prepare our films by casting an ethanol solution of the polymer onto a glass slide. This produces a polymer film with a thickness of ∼3-4 µm, which removes any dependence of the substrate upon the contact angle obtained. In this work, we study PNIPAM and copolymers of PNIPAM with NtBA surfaces. We attempted to obtain values for surface energies and water contact angles for the series of polymer surfaces above the LCST. We also investigated the effect of increasing the composition of the hydrophobic monomer NtBA on the contact angle and surface energy of the polymer surfaces. Experimental Section Materials. N-Isopropylacrylamide (NIPAM) (97%, Aldrich) and NtBA (purum, Fluka Chemie, Switzerland) were recrystallized from n-hexane and acetone, respectively. 2,2′-Azobis(2-methylpropionitrile) (AIBN) (Phase Separation Ltd., Queensferry, Clwyd, U.K.) was recrystallized from methanol. Liquids that were used for contact angle measurements include benzyl benzoate (γlv ) 45.95 mJ/m2), 1-iodonaphthalene (γlv ) 42.92 mJ/m2), dibenzylamine (γlv ) 40.8 mJ/m2), 1,3diiodopropane (γlv ) 46.51 mJ/m2), bromonaphthalene (γlv ) 44.31 mJ/m2), ethanolamine (γlv ) 48.32 mJ/m2), glycerol (γlv ) 65.02 mJ/m2), and 1,2-dibromoethane (γlv ) 39.6 mJ/m2). Water (γlv ) 72.7 mJ/m2) was purified with a Milli-Q purification system (Millipore). All values of γlv excluding 1,2-dibromoethane were obtained from Kwok et al.22 The value of γlv for 1,2-dibromoethane was obtained from the Handbook of Chemistry and Physics.23 For van Oss, Chaudhury, and Good (vOCG) calculations, the following liquid components were used: water (γLW L ) 21.80 mJ/ LW 2 2 m2, γ+ L ) 25.50 mJ/m , γL ) 25.50 mJ/m ), glycerol (γL ) 34.0 + 2 2 2 mJ/m , γL ) 3.92 mJ/m , γL ) 57.40 mJ/m ), 1-iodonaphthalene 2 2 ) 42.9 mJ/m2, γ+ (γLW L L ) 0.00 mJ/m , γL ) 0.00 mJ/m ). The component values were obtained from van Oss et al.24 No data are available for the components of 1-iodonaphthalene, and it (21) Schakenrad, J. M.; Busscher, H. J.; Wildevuur, C. R. H.; Arends, J. J. Biomed. Mater. Res. 1986, 20, 773. (22) Kwok, D. Y.; Neumann, A. W. Adv. Colloid Interface Sci. 1999, 81, 167. (23) Handbook of Chemistry and Physics: Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1995; Section 6, pp 151-154. (24) Wu, W.; Giese, R. F., Jr.; van Oss, C. J. Langmuir 1995, 11, 379.

Langmuir, Vol. 20, No. 23, 2004 10139 was assumed that it has only a dispersive component (LW), as is the case for 1-bromonaphthalene.24 Acetone was shaken with anhydrous CaSO4 (Drierite) for 2 h. The decanted acetone was then distilled from fresh Drierite. Ethanol was dried by reaction with magnesium ethoxide by addition of magnesium and iodine. The dried ethanol was then distilled off. Hexane was distilled from sodium metal. Copolymer Synthesis. PNIPAM, poly(N-tert-butylacrylamide) (PNtBA), poly(NIPAM(80)-co-NtBA(20)) (PN80), and poly(NIPAM(60)-co-NtBA(40)) (PN60) (where the number in parentheses is the molar percentage for the respective monomers) were prepared by free radical polymerization, using AIBN (0.5 mol % of AIBN per monomer) as an initiator in benzene under argon. After polymerization at 60 °C for 24 h, the mixture was precipitated in n-hexane. Precipitation was repeated three times using acetone as a solvent and n-hexane as a nonsolvent, and the product was dried at room temperature in a vacuum. Film Preparation. An 80 µL aliquot of a 2% solution of the polymer in dry ethanol was spread on a glass cover slide (22 mm × 22 mm). The glass slides were cleaned with toluene and ethanol before casting of films. The films remained within an enclosed polystyrene Petri dish, which was left overnight in a desiccator to ensure slow drying of the film. The films were then dried for 2 h at 60 °C under a vacuum. Film thickness was estimated based on a film surface density of 0.35 mg/cm2. Assuming a dry polymer density of ∼1 g/cm3, this corresponds to a thickness of approximately 3-4 µm. The films were inspected under a phase contrast microscope (Leitz DMIRB/E, Leica, Oberkochen) before measurements to ensure that they were free of any physical inhomogeneities and surface corrugations on the scale 1-500 µm. Polymer Characterization. The molecular weight (Mw) and polydispersity (PD) of the polymer series were determined by gel permeation chromatography (GPC) using a glucose bound polydivinylbenzene packed column (Jordi Assoc.) with DMF (0.25 M LiBr) as the solvent. A series of monodisperse polystyrene polymers were used for calibration. The LCSTs of the polymers were measured by the cloud point technique by simply observing an increase in turbidity by eye. Measurements were repeated three times. Atomic Force Microscopy (AFM). A Dimension 3100 AFM (Digital Instruments, Santa Barbara, CA) was used to observe the topography of polymer films. AFM was run in the tapping mode using silicon tips (TESP tips from Veeco with a nominal spring constant of 20-100 N/m and a nominal tip radius of 5-10 nm). Images (5 × 5 µm2) were acquired in air, at ambient conditions. Three different films were measured for each polymer composition. The roughness of the films was reported as rootmean-square (rms) roughness values, where rms denotes the standard deviation of the Z-values along the reference line. Contact Angle Measurements. Advancing contact angle measurements were performed using the advancing drop method on a home-built goniometer. The goniometer was assembled on an optical rail from Newport Optics with opto-mechanical components from Newport Optics and Edmund Optics. DROPimage software marketed by Rame Hart and developed by F. K. Hansen was applied for determining contact angles. Two methods are available to calculate contact angle in DROPimage software. The first method utilizes the fitting of the experimental drop profile to the theoretical drop profile generated by the numerical integration of the Young-Laplace equation. This method is similar to axisymmetric drop shape analysis (ASDA). The use of ASDA requires that the whole drop must be visible and that the surface must be undisturbed (no needle can be present). In the second method, a filtered drop profile is obtained and a travelling secant method (with linear extrapolation to the contact point) is used to obtain the contact angle. For our measurements, we choose the second method, as the first one is dependent on perfectly symmetrical drops. For some of our measurements, we did not obtain such symmetrical droplets due to the unusual interactions of some of the liquids with our surfaces. Also the first method requires an undisturbed drop profile to be visible, which makes the measurements of advancing contact angle cumbersome. We compared the two methods by the measurement of water contact angles of symmetrical drops created on a polystyrene surface. A contact angle of 83.6 ( 0.3° was obtained using ASDA, and a

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Gilcreest et al.

Table 1. Characterization Data for Polymers and Films polymer

Mw (×106)

PD

LCST (°C)

rms (nm)

PNIPAM PN80 PN60 PNtBA

2.2 1.9 1.4 0.57

1.96 2.05 2.09 1.77

32 24 14 nonea

3.8 ( 2.2 2.2 ( 0.8 5.8 ( 1.3 2.2 ( 0.2

a

PNtBA is insoluble in water at all temperatures.

contact angle of 83.0 ( 0.4° was obtained using the specified method that we adopted. Polymer samples were placed in a temperature-controlled environmental chamber mounted on a tilt stage. The temperature on the surface was monitored using a thermocouple attached to the slide. In a typical experiment, a drop was deposited on the surface with an initial radius of about 3 mm. For the advancing contact angle experiment, a thin stainless steel needle (gauge 22) was inserted in the center of the drop from above. The volume of a drop was increased by pumping liquid into the drop using a syringe pump. The pumping speed was adjusted to maintain the rates of advancing below 0.5 mm/min. Drop images were acquired every 3 s. Some of the films were inspected after the contact angle experiments using a phase contrast microscope.

Results Characterization of Polymers and Films. The results of polymer and film characterization are presented in Table 1. All polymers are of sufficiently high molecular weight so that any effect of end groups on the surface composition of the resulting films can be ignored. The LCSTs obtained were compared with previous measurements carried out on polymers of similar composition using differential scanning calorimetry (DSC).19 Though our measurements of the LCSTs of the polymers are approximate, they are in good agreement (less than 1 °C difference) with more accurate measurements carried out using DSC and confirm that the LCSTs for the polymer series are below the measurement temperature. We also note from our measurements that the LCST decreases with increasing NtBA content. This reflects the increasing hydrophobicity of the polymers. On analysis of the rms data for the films, there is no pronounced trend evident and all films have roughness on the nanometer scale. Contact Angles of Water on Polymer Surfaces. The measurement of the water contact angle is an important characterization parameter for surfaces of biomedical devices. All measurements were carried out at 37 °C, which is above the LCST of all the polymers. In the course of our measurements on PNIPAM and the copolymer films, we encountered stick/slip behavior (Figure 1). In the case of PNIPAM, it was necessary to leave the PNIPAM films in a water-saturated environmental chamber at 37 °C for 2 h prior to measurements to obtain reproducible contact angle against time patterns. Stick/slip behavior has been reported before, and its origin is not entirely understood, though it is believed that it can be caused by factors such as liquid absorption into the film or surface roughness.22 As can be seen from Figure 1A, the contact angle increases, but the radius remains the same. However, as the drop volume increases the drop front slips to a new position, leading to a decrease in contact angle with an increase in radius. Stick/slip behavior was observed for advancing and receding contact angles on PN60 (Figure 1B). Receding contact angles were too low to be measured accurately for PNIPAM and PN80. We name the angle just prior to the drop front slipping the “slip angle”. The angle after the drop has jumped to a new position is named the “stick angle”. It is evident that the angle at which the droplet slips is not reproducible, leading to a large spread in the angles

Figure 1. (A) Drop radius (R) (upper curve) and advancing contact angles (θ) (lower curve) of water on PN80 film as a function of time. (B) Stick/slip behavior for advancing (upper curve) and receding (lower curve) contact angles on PN60. Table 2. Slip and Stick Advancing Contact Angles for Polymer Films at 37 °C polymer

slip angle (deg)

stick angle (deg)

PNIPAM PN80 PN60 PNtBA

96-118 96-126 85-91 87.0 ( 0.5

74.0 ( 1.4 78.6 ( 2 82.9 ( 0.8 87.0 ( 0.5

measured (Table 2). The stick angles are quite reproducible, with the value at which the droplet sticks depending on polymer composition. To investigate the possible cause of this stick/slip behavior, we imaged the surfaces after a drop had been removed using a phase contrast microscope. A series of concentric rings was observed on the surface of the polymer film (Figure 2a). Each ring corresponds to the pinning of a triple line. The rings correspond to the region at which the droplet sticks. Figure 2b is a close-up on one of the rings in which a ridge can clearly be seen (highlighted by an arrow). This ridge reflects the deformation of the film along the line where the drop is pinned to the surface. The other features present on the film surface are small water droplets on the film. These are due to the withdrawal of the water drop that was used to carry out the contact angle measurement prior to imaging of the film under the phase contrast microscope. Contact Angles of Organic Liquids. Characterization of surfaces by surface energies has become increasingly popular, and a lot of work has been carried out to develop more accurate methods of doing this. Calculation of the surface free energy usually utilizes the Young

Surface Energies of NIPAM Copolymers

Langmuir, Vol. 20, No. 23, 2004 10141

equations to determine the three unknown solid surface + tension components γLW S , γS , and γS . The correct use of the vOCG approach in determining solid surface tensions has been discussed by Della Volpe et al.27 Della Volpe et al. pointed out that improper selection of liquid triplets led to a strongly ill-conditioned set of equations. “The use of improper triplets, without dispersive liquids or with two liquids prevalently basic or prevalently acidic, strongly increases the ill-conditioning of the system”.27 Another approach is to use an equation of state. The Young equation on its own is not enough to determine the solid surface energy, as only two of the four unknowns are measurable. It is necessary to have another relationship between the unknowns. By modifying the geometric mean combining rule, Li and Neumann28 proposed a further relationship, which they combined with the Young equation. The resulting equation of state is shown in eq 4.

cos θY ) -1 + 2

Figure 2. (A) PN80 film after water contact angle measurements illustrating ring formations. Scale bar ) 100 µm. (B) Magnified region of the film showing ridge formation (arrow pointing to ridge). Scale bar ) 25 µm.

equation that defines the mechanical equilibrium, which exists with a sessile drop under the action of three interfacial tensions:

γlv cos θY ) γsv - γsl

(1)

here γlv, γsv, and γsl are the liquid-vapor, solid-vapor, and solid-liquid interfacial tensions, respectively, and θY is the Young contact angle. The Young equation contains four unknown terms, only two of which can be measured experimentally. There are two semiempirical approaches used to extract the solid surface energy using contact angle measurements with a series of liquids. One approach is to use surface tension components. The most popular of these approaches are Fowkes25 and van Oss, Chaudhury, and Good.26 These methods utilize liquids that have different components contributing to their total surface tension. In vOCG theory, if a solid surface is represented by Lifshitz-van der Waals (LW) and Lewis acid/base (AB) interactions then the total surface tension (γTOT) is the sum of LW and AB interactions: AB ) γLW γTOT S S + γS

(2)

Here γAB ) 2(γ+γ-)1/2; γ+ is the electron acceptor and γis the electron donor component of the surface tension. Van Oss et al.26 expanded the Young-Good-GirifalcoFowkes equation to formulate the following relationship: LW 1/2 - 1/2 + 2(γ+ + γlv(1 + cos θY) ) 2(γLW L γS ) L γS )

2(γL

1/2 γ+ S)

(3)

By selecting three liquids and determining the contact angles for the series, it is possible to obtain the set of

x

γsv -β(γlv-γsv)2 e γlv

(4)

To obtain accurate values of surface energy by either of these approaches, we need a series of liquids from which we may obtain contact angles. In choosing our liquids, we were limited in that we had to use liquids which have a surface tension greater than the surface energy of the surfaces; otherwise one would get complete wetting of the surface and no contact angle could be obtained. PNIPAM is a polar surface and would be expected to have a high surface energy. When liquids that have a low surface tension such as n-hexane (γlv ) 18.43 mJ/m2) were placed on the surface, complete wetting occurred. Due to the unusual properties of PNIPAM and the copolymer series that are soluble in water (below LCST) as well as in a broad range of polar liquids, we were further limited in which liquids we could use. These restrictions eliminated many easily available organic liquids from our selection. In making this selection, we used the Handbook of Chemistry and Physics23 and identified all available liquids with a surface tension greater than 30 mJ/m2. In some instances, stick/slip behavior was observed, whereas in other cases a contact angle that decreased with time was observed. Data were only used from liquids that produced a constant contact angle with time. We were unable to come up with liquids that fulfilled the criteria set down by Della Volpe to apply the vOCG approach for the whole series of polymers, though we were able to obtain a surface energy value for PNtBA using water, glycerol, and 1-iodonaphthalene as liquids. We obtained a surface energy of 29.0 ( 0.54 mJ/m2. A greater selection of liquids for the equation of state approach is available due to the fact we were not limited to liquids with specific surface tension components. The contact angle measurements for each surface were fitted to eq 4 using β ) 0.000 124 7. This value of β was determined by Neumann et al.29 and was obtained from an averaging process carried out on different surfaces. The data and fits are presented in Figure 3. It was only possible to obtain three data points for PN80 and two data points for PN60. This was due to difficulties in obtaining liquids for these surfaces that gave meaningful contact angles. These copolymer surfaces share some of the properties of PNtBA (25) Fowkes, F. M. Ind. Eng. Chem. 1964, 12, 40. (26) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. Rev. 1988, 88, 927. (27) Della Volpe, C.; Maniglio, D.; Brugnara, M.; Siboni, S.; Morra, M. J. Colloid Interface Sci. 2004, 271, 434. (28) Li, D.; Neumann, A. W. J. Colloid Interface Sci. 1990, 137, 304. (29) Li, D.; Neumann, A. W. J. Colloid Interface Sci. 1992, 148, 190.

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Gilcreest et al. Table 3. Comparison of Advancing Stick Angle and Calculated Water Contact Angle polymer

θwaterb (deg)

stick angle (deg)

PNIPAM PN80 PN60 PNtBAa

74.5 ( 0.2 78.8 ( 1.0 81.8 ( 0.7 87.8 ( 0.7

74.0 ( 1.4 78.6 ( 2.0 82.9 ( 0.8 87.0 ( 0.5

a PNtBA does not display stick/slip behavior. b Calculated from eq 4.

Figure 3. γlv cos θY vs γlv for PN (9), PN80 (O), PN60 (1), and PNtBA (b).

Figure 4. Effect of NtBA on the surface energy of polymer films.

and PNIPAM making it more difficult to obtain suitable liquids for the measurements. The resulting surface energies that we obtained for the polymer film series were in the range of 38.9 mJ/m2 for PNIPAM to 31.0 mJ/m2 for PNtBA. The lowering of surface energy with increasing NtBA composition can be seen clearly in Figure 4. Discussion. The series of polymer films were found to have surface energies in the range of 38.9-31.0 mJ/m2. It was possible to determine the surface energy of PNtBA using the equation of state (31.0 ( 0.4 mJ/m2) and the vOCG (29.0 ( 0.5 mJ/m2) approaches. The values are quite similar considering the two approaches are based on very different theoretical assumptions. Della Volpe et al.27 have compared the two approaches and found that they give similar surface energies when the liquid triplets used for vOCG calculations produced equation sets that were not ill-conditioned. As mentioned previously, we were able to carry out vOCG calculations only for PNtBA as we could not obtain suitable liquid triplets for the other polymers in the series. The increasing composition of NtBA in the surfaces has the effect of lowering the surface energy. In comparing the surface energies that we obtained with the surface energies of other materials, we can see that PNtBA films have a surface energy comparable to that of polystyrene (29.5 mJ/m2).22 On the other hand, PNIPAM films have a surface energy of 38.9 mJ/m2, which is comparable to that of poly(methyl methacrylate) (PMMA) (38.2 mJ/m2).22 The surface energy is a significant parameter in determining surface properties, but these thermoresponsive polymers generally find their applications in water-based systems, in which the most important characterization is

the water contact angle. This is reflected throughout the literature in which the water contact angle is the main form of characterization.2-14 Using the surface energies obtained above, we can employ eq 4 to predict a water contact angle for the series of films (Table 3). It is noticeable from the data that the calculated water contact angle increases with rising NtBA content and is a reflection of the increasing hydrophobicity of the surfaces. We were able to obtain experimentally a water contact angle for PNtBA. This is most likely due to its greater hydrophobicity, which prevents water absorbing into the film. From Table 3, we can see that the calculated water contact angle for PNtBA is very close to the experimentally obtained angle. The free surface energy of PNtBA was determined from liquids with a wide range of surface tensions. For the other surfaces, the surface energies were obtained from liquids with a narrower range of surface tensions. To ensure that the surface energy calculations were not sensitive to the range of liquids used, the surface energy was re-evaluated for PNtBA using the first two data points, which gave a range of liquids with surface tensions comparable to those of the other surfaces. A surface energy of 30.8 mJ/m2 was obtained, comparable to the value of 31.0 mJ/m2 that was calculated using all the data. On examination of the slip/stick data, the value at which the droplet starts to stick is close to the calculated water contact angle. The calculated water contact angles were worked out based on the surface energies at 20 °C. The water contact angle measurements were carried out at 37 °C. We carried out water contact angle measurements for PNtBA at 37 and 20 °C and found that the difference between the contact angles obtained at the two temperatures was not significant (less than 1°). The contact angle estimated above for the series of copolymers is a hypothetical contact angle (except PNtBA). This is a contact angle that water would have on the given surface in the absence of water absorption by the polymer film. Polymers based on NIPAM apparently can absorb water even above the LCST. We believe that water absorption leads to a softening of the film. Our microscopy observation connects stick/slip behavior with the deformation of the polymer film along the contact line. This deformation is caused by the vertical component γlv sin θ acting on the polymer film at the triple junction. The resulting deformation creates a ridge with height h on the surface of the polymer and can be estimated as

h≈

γlv sin θ G

(5)

Here G is the shear modulus.30 Usually this deformation is negligible for the substrate with high Young modulus. For example, for a typical polymer with a glass transition temperature above 100 °C the shear modulus will be ∼1 GPa. If γlv sin θ ∼ 0.05 N/m, the height of the ridge created on the surface of the film will be ∼0.05 nm. However, in the case of PNIPAM, water absorbed into the polymer can (30) Shanahan, M. E. R.; Carre, A. Colloids Surf., A 2002, 206, 115.

Surface Energies of NIPAM Copolymers

Figure 5. (a) Movement of a drop in the absence of ridge formation. (b) Unfavorable movement of a drop down a ridge (θµ < 90°). (c) Movement of a drop down a ridge (θµ ) θ ) 90°).

act as a plasticizer reducing its Young modulus dramatically. Mechanical properties of PNIPAM film at 37 °C in equilibrium with water are not available. However, for order of magnitude estimation we can use the Young’s modulus of a weakly cross-linked NIPAM gel in the collapsed state.31 The Young’s modulus of PNIPAM gel with 0.25 mol % of cross-link was 13.9 kPa at 40 °C. The shear modulus can be estimated as ∼5 kPa (taking Poisson’s ratio as 0.4), and consequently h is ∼10 µm. These estimations show that significant deformation is developed in the triple line contact region. It is not surprising that we can see this deformation in the optical microscope. We find that the ridge is formed when the advancing liquid sticks to the surface of copolymers. At present, we can only provide a heuristic explanation for the connection between the ridge formation and the stick/slip phenomenon. We start with the drop that rests on the surface and allow a sufficient time for the formation of the ridge (Figure 5). To simplify the picture, we assume that the equilibrium contact angle (in the absence of the ridge) is θ ) 90° and the drop is small so that its profile is the section of a circle. In the ideal case (without the formation of a ridge), the increase in the volume will result in the movement of the contact line to a new position with the same contact angle (Figure 5a). If the ridge is formed, the movement down the ridge slope is unfavorable because the microscopic contact angle, θµ, on the side of the ridge is less than 90° (Figure 5b). When the contact line is pinned to the ridge, the movement of the drop will be possible at θµ ) θ ) 90° (Figure 5c). The observed slip angle will be equal to the sum of θ and θslope, where θslope is the slope angle of the ridge. Outside the ridge, the actual contact angle will be higher than 90° so the drop will continue to spread until it attains an equilibrium contact angle. The same line of (31) Matzelle, T. R.; Geuskens, G.; Kruse, N. Macromolecules 2003, 36, 2926.

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argument can be repeated for the receding drop, so it is not surprising that we can see stick/slip behavior in that experiment. Stick/slip behavior is not connected to the kinetics of water absorption into the polymer film. The polymer films we use are in equilibrium with water vapor in advancing drop experiments. In receding drop studies, films are in contact with water prior to experiments. The role of water is to decrease the rigidity of the film, so a significant deformation caused by the vertical component γlv sin θ can develop at the contact line. We speculate that the water absorption capacity decreases with the increase of the more hydrophobic NtBA monomer content in the polymer series. The deformation or the ridge height should also be lower for more hydrophobic polymers, resulting in lower θslope. This, in turn, should decrease the difference between the stick angle, θ, and the slip angle θ + θslope (Figure 5) with the increase in polymer hydrophobicity. Indeed, we observe this tendency in our experiments (Table 2). We can now compare the contact angle on the PNIPAM surface (∼75°) estimated above with published contact angles. There is a wide spread of contact angles published for PNIPAM-modified surfaces. We can speculate that this spread can be explained taking into account the method for the preparation of the polymer surfaces and the experimental technique for the contact angle determination. For example, the measurement of static contact angles should, generally, give a lower value. The contact angle should be also dependent on the surface coverage of the substrate. At lower surface coverage, the influence of the substrate can increase the contact angle of water as well as decrease it, depending on the surface properties of the substrate material. The stick/slip behavior we observed for the PNIPAM and copolymer surfaces can also complicate the measurements and the interpretation of PNIPAM-modified surfaces. For example, the use of the Wilhelmy plate method can give the average value between the stick and slip angles because the slip of the three-phase contact line is unlikely to happen simultaneously at the perimeter of the sample. We attributed the stick/slip behavior to a mechanical deformation of the polymer layer due to the vertical component of surface tension at the three-phase contact line. However, this mechanical deformation will decrease with the thickness of the grafted layer so the stick/slip behavior may not be noticeable on a thin grafted layer. Based on the above considerations, we review briefly the results for the contact angles of water available from the literature. Israelachvili et al.2 carried out water contact angle measurements on PNIPAM surfaces. The PNIPAM contained 5 mol % of positively charged comonomer and was physically absorbed onto the negatively charged mica substrate. The thickness of the layer was found to be in the range of 2-3 nm. They reported a contact angle of 76 ( 2° using the advancing drop method. The contact angle was found to be temperature independent. Lopez and coworkers carried out water contact angle measurements on PNIPAM, which had been modified by two different methods. In the first method, they adopted in situ polymerization of NIPAM onto initiator-derivatized selfassembled monolayers (SAMs) of gold.3 They reported a contact angle of 72.4 ( 4° at 45 °C for a film thickness of 25 nm. For a 15 nm thick film, they noted the increase of the contact angle to 81.2 ( 2° that reflected the hydrophobicity of alkanethiolate SAMs. They also prepared PNIPAM-modified surfaces by atom transfer radical polymerization onto SAMs.4 This method produced dense PNIPAM brush films with a thickness of ∼50 nm. Here

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they report a contact angle of 78° ( 1° using the advancing drop technique. These results support our estimation of the contact angle of water on the PNIPAM surface. They also point out that the thickness of the grafted layer should be in the order of 25-50 nm or higher to avoid the influence of the substrate. Liang and co-workers have carried out contact angle measurements on PNIPAM surfaces, which were prepared in different ways. One method they adopted was to create a cross-linked PNIPAM gel surface by photopolymerization of NIPAM in the presence of N,N′-methylenebisacrylamide onto an initiator-modified glass substrate.5 This produced a grafted layer of ∼320 nm. They reported an advancing contact angle of ∼85° using the Wilhelmy plate method at 40 °C. They also carried out measurements on crosslinked PNIPAM layers that were prepared by thermal free-radical polymerization on a vinyltriethoxysilanemodified silicon substrate.6 They obtained a grafted layer of ∼500 nm and report an advancing contact angle of 92° at 40 °C using the Wilhelmy plate technique. These contact angles are significantly higher than we estimated, as discussed previously.2-4 As noted above, the Wilhelmy plate method can give a high apparent contact angle of water if stick/slip behavior is observed. For a relatively thick gel layer, the formation of a ridge of significant height that will result in a higher contact angle is possible. On the other hand, the same group obtained an advancing contact angle of ∼90° on the surface of linear PNIPAM created by photochemical polymerization on an initiatormodified silicon surface.7 They determined the thickness of the PNIPAM layer to be ∼4 nm. The other possible reason for a higher apparent contact angle obtained with the Wilhelmy plate technique is higher immersion speed (6-10 mm/min) compared to advancing drop experiments (∼0.5 mm/min). Geuskens et al.8 carried out measurements on PNIPAM surfaces, which were prepared by photochemical polymerization of NIPAM on a polyethylene substrate where the initiator was adsorbed on the surface. No data were given on the thickness of the grafted PNIPAM layer though the modification route suggests a relatively low surface coverage. A static contact angle of 80° was obtained at 40 °C using the sessile drop technique. The contact angle of the polyethylene substrate was also 80°. Della Volpe et al.9 carried out measurements using the Wilhelmy plate technique on PNIPAM surfaces that were prepared by Ce(IV)-initiated NIPAM polymerization on air plasma treated polystyrene. No details of the thickness of the PNIPAM grafted layer were given, and no direct evidence existed that polymer was covalently attached to the substrate. They reported an advancing contact angle of 78 ( 5° at 37 °C using the Wilhelmy plate technique. At the same time, the contact angle of the substrate was 74 ( 5 °C. Okano and co-workers carried out measurements on several different surfaces. One method they used was chemically grafting PNIPAM chains, which had a carboxyl end group, to a poly(styrene-co-(aminomethyl)styrene)coated glass substrate.10 They do not report the density of the grafted layer, but this method would most likely give poor coverage due to the steric hindrance encountered in grafting PNIPAM chains. They report an advancing contact angle of ∼75° at 36 °C using the Wilhelmy plate technique. They also grafted a poly(NIPAM-co-acrylic acid) copolymer onto the same surface. This created a surface in which the PNIPAM was multipoint attached to the surface. Here they obtained a higher advancing contact angle of ∼90° at 36 °C using the same technique. In a subsequent publication, they studied graft architectural

Gilcreest et al.

effects on wettability of PNIPAM-modified surfaces.11 In one instance, they end-grafted an active ester terminated PNIPAM chain to the substrate (method A). The substrate was glass modified by (3-aminopropyl)-triethoxysilane. Another method they adopted was to graft poly(NIPAMco-N-acryloxysuccinimide) to the substrate (method B). This led to the formation of multipoint attached PNIPAM loops on the surface. This method produced a significantly higher contact angle of 93° at 40 °C compared to the endgrafted method they used. The final method they adopted was to graft PNIPAM with an amine end-group to the immobilized loops on the substrate (method C). This method also produced a surface with a contact angle of 93° at 40 °C. All measurements were carried out using the advancing Wilhelmy plate technique. No information was given about the contact angle of the substrate used for the modification by PNIPAM. Polymer surface coverage was estimated from the consumption of amine groups on the surface, which was 85% for method A compared to 98% for methods B and C. Surface amine consumption was higher for the surfaces modified by methods B and C, which probably explains the higher contact angles obtained on these surfaces. Matsuda et al.12 carried out measurements on PNIPAM surfaces, which were prepared using an iniferter-based polymerization technique on a dithiocarbamate-derivatized glass substrate. They achieved a dense polymer brush film with a thickness of ∼15 nm. They obtained a contact angle of 64 ( 3° at 40 °C using the advancing drop technique. It is possible that the lower than expected contact angle could be due to the influence of the substrate used, resulting from incomplete coverage. Alexander et al.13 carried out contact angle measurements on PNIPAM and poly(NIPAM (80 mol %)-co-NtBA (20 mol %)) copolymer surfaces which were prepared by grafting of the polymer chains onto amine-functionalized glass. They reported a contact angle of 46° for PNIPAM and 58° (compared to the value of 79° we calculated) for the copolymer at 37 °C using the advancing drop technique. Once again, the low contact angles obtained can be attributed to poor polymer coverage on the substrate. The substrate used was similar to that used by Okano and would be expected to have a low contact angle. Yim et al.14 prepared surfaces using the “grafted-to” method by grafting COOH-terminated PNIPAM to a 40 mol % OH-terminated self-assembled monolayer. The remainder of the monolayer is methyl group terminated. The film thickness was found to be in the range of 1-4 nm. An advancing contact angle of 87.4 ( 8.9° was obtained using the Wilhelmy plate technique. A contact angle of 89.2 ( 2.5° was also reported using the advancing drop technique. From the review of previously published data relating to the contact angle of water on PNIPAM, it is evident that there is support for our findings.2,4,8-11 It is also evident that there are considerable inconsistencies in the contact angles published. We propose a number of reasons for these inconsistencies. Many different methods have been adopted to create PNIPAM-modified surfaces. The variety of methods has created surfaces with differing layer thickness of PNIPAM (∼1-500 nm). In the case of surfaces that have a low coverage, it is conceivable that the substrate, which was modified with the polymer, plays an important role. Due to the variety of substrates that have been used, this could have the effect of increasing5-8,10,11,14 or lowering the reported contact angle.13 For instance, in the case of Alexander et al. the substrate is amine-functionalized glass, which would be expected to have a low contact angle and perhaps explains the

Surface Energies of NIPAM Copolymers

unusually low contact angle which they report (46° for PNIPAM and 58° for the copolymer PN80 (compared to the value of 79° we calculated)). The effect of a more hydrophobic substrate can be seen in the work by Yim et al. where they report a water contact angle of 89.2 ( 2.5°. In this case the substrate, which has not been grafted, is terminated with methyl groups. In some instances, a high contact angle (>85°) is reported on surfaces with a high graft density.5,6,11 These reported contact angles are higher than the contact angle we obtained experimentally for PNtBA (87.8 ( 0.7°). From the surface energies we obtained, the contact angle of PNIPAM should be lower than the contact angle on PNtBA. One possible reason for this discrepancy could be due to the formation of a ridge that could not be detected optically on thinner films (submicron) as on our films. Slip/stick behavior may not be evident using the Wilhelmy plate, which therefore may present a contact angle that is between slip and stick. Conclusions. In our efforts to determine the water contact angle for the thermoresponsive polymer surfaces, we encountered an unusual contact angle pattern known as stick/slip. Stick/slip behavior is not an uncommon phenomenon, though little is understood of its origins or its significance. We attribute this behavior to water absorbing into the film. The water absorption leads to the softening of the film so that the vertical component γlv sin θ of the line tension can create sufficient deformation on the surface of the film. As a result, a ridge is formed on the surface, leading to the pinning of the contact line.

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Using organic liquids and adopting an equation of state approach, we determined the surface energies for the series of films. The surface energies range from 38.9 mJ/m2 for PNIPAM to 31.0 mJ/m2 for PNtBA. We were also able to determine the surface energy for PNtBA (29.0 mJ/m2) using the vOCG approach. From our results, it is evident that incorporating NtBA into the films lowers the surface energy of the films. Using these surface energies, we were able to determine a calculated water contact angle for the entire series by fitting the surface energies of the films into the equation of state. The water contact angles calculated were in the range of 74.5 ( 0.2° for PNIPAM to 87.0 ( 0.5° for PNtBA. In our attempts to understand the stick/slip behavior that we observed, we found that the stick angle was reproducible whereas the slip angle was not. It is also noticeable that the stick angle was very close to the calculated angle that we obtained. The water contact angles clearly reflect the increasing hydrophobicity of the films with increasing NtBA composition. Acknowledgment. This work was carried out with the support of the Health Research Board (Project RT1282002) and Enterprise Ireland (Project SC-02454). K.D. is also grateful to his colleagues in the Physical Chemistry Department of Lund University, Sweden, for helpful discussions on this topic. LA0487996