Synthesis and Surface Morphology of High-Density Poly(ethylene

Oct 31, 2003 - Robust, Solvent-Free, Catalyst-Free Click Chemistry for the Generation of Highly Stable Densely Grafted Poly(ethylene glycol) Polymer B...
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Langmuir 2003, 19, 10179-10187

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Synthesis and Surface Morphology of High-Density Poly(ethylene glycol) Grafted Layers Bogdan Zdyrko, Viktor Klep, and Igor Luzinov* School of Materials Science and Engineering, Clemson University, Clemson, South Carolina 29634 Received June 3, 2003. In Final Form: September 15, 2003 The present study was focused on permanent grafting of poly(ethylene glycol) (PEG) layers (brushes) from the melt to adsorbed reactive macromolecules. A poly(glycidyl methacrylate) (PGMA) was used to form the reactive primary adsorbed layer. It was found that a thin (1.5 nm) PGMA layer had a high grafting efficiency, and elevated temperatures considerably accelerated the grafting process. The grafting extent was eventually limited by concentration and accessibility of functional reactive groups in the primary adsorbed layer. Several distinct types of brush morphologies were observed depending on the grafted layer height. Isolated crystals were detected for brushes possessing lower grafting density. At higher levels of PEG attachment, the crystalline formations uniformly covered the entire surface of the substrate. The development of highly extended crystalline formations on the surface and marked expansion of the PEG brushes in water indicated that an extremely high grafting density was attained. The developed grafting protocol was utilized to modify various substrates with PEG grafted layers.

Introduction Poly(ethylene glycol) (PEG) ultrathin coatings with controlled properties are of great interest for medical and bioengineering applications because they can prevent nonspecific adsorption of proteins.1,2 There are several probable factors involved in the protein-resistant character of PEG surfaces in aqueous solutions.1 First, the interfacial free energy of PEG films with water is minimal, and as the energy approaches 0, the driving force for protein adsorption decreases. Second, PEG segments nicely fit into the structure of water without significant distortion of water lattices and minimize the tendency for hydrophobic interactions that cause the nonspecific protein binding. The third reason is the steric barrier effect offered when the relatively dense layer of PEG attached by one end to a surface is created. In this case, the anchored chains have high exclusion volume due to high conformational entropy and, therefore, repel (bio)polymers, including proteins.3,4 These factors taken together suggest unique properties of PEG thin films deposited on arbitrary surfaces. Namely, the PEG layers in water with rapidly moving hydrated PEG chains and a large excluded volume tend to repel protein molecules that approach the surface.1 PEG layers anchored to a substrate can be prepared using various methods, including covalent grafting of PEG molecules possessing reactive end groups,5,6 graft (co)polymerization of PEG (macro)monomer onto the surface,7,8 and physical adsorption of PEG onto substrates in the form of a surfactant or a block copolymer.9,10 * To whom correspondence should be addressed. E-mail: [email protected]. (1) Lee, J. H.; Lee, H. B.; Andrade, J. D. Prog. Polym. Sci. 1995, 20, 1043-1079. (2) Amiji, M.; Park, K. ACS Symp. Ser. 1994, 540, 135-146. (3) Sato, T.; Ruch, R. Stabilization of Colloid Dispersions by Polymer Adsorption; Dekker: New York, 1980. (4) Hermans, J. J. Chem. Phys. 1982, 77, 2193-2203. (5) Jo, S.; Park, K. Biomaterials 2000, 21, 605-616. (6) Holmberg, K.; Tiberg, F.; Malmsten, M.; Brink, C. Colloids Surf., A 1997, 123, 297-306. (7) Sun, Y. H.; Hoffman, A S.; Gombotz, W. R. Polym. Prepr. 1987, 28, 282. (8) Chinn, J. A.; Horbett, T. A.; Ratner, B. D.; Schway, M. B.; Haque, Y.; Hauschka, S. D. J. Colloid Interface Sci. 1989, 127, 67-68.

However, the latter simple and widely used technique has a main drawback because the immobilized polymers do not permanently remain on the surface. Thus, the covalent grafting of PEG or PEG derivatives is the most effective way of creating permanent PEG surfaces.1 In addition to the covalent character of the PEG attachment, an important parameter controlling the performance of the protein-repelling PEG films is the grafting density. It has been shown theoretically11-13 and experimentally14 that attainment of sufficient grafting density is extremely important in rejecting proteins. Dense attachment of the randomly coiling polymeric chains results in formation of a polymer brush, when chain spacing is close to the radius of gyration of the grafted macromolecules. When the brush regime is attained, the grafted chains begin to stretch away from the surface and the steric barrier effect is maximized.15 A grafted PEG brush can be readily built from polymers with a functional end group that can react with the substrate surface and, thus, be used as an anchor. Consequently, the coupling of PEG to the substrate requires complementary functional groups on the surface and in the polymer chain. There are two common approaches for PEG chain attachment. The first one includes the formation of a monolayer consisting of functional groups with an affinity for terminally functionalized (e.g., epoxide, amine, anhydride, or hydroxide) PEG.16 A different approach relies on the reactions between endfunctionalized PEG chains and native functional groups originally present on the substrate surface.14,17 Silane, thiol, and epoxy chemistries have proven to be suitable (9) Cheng, Y. L.; Darst, S. A.; Robertson, C. R. J. Colloid Interface Sci. 1987, 118 (1), 212-223. (10) Lee, J. H.; Kopeckova, P.; Kopecek, J.; Andrade, J. D. Biomaterials 1990, 11, 455-464. (11) Jeon, S. I.; Lee, J. H.; Andrade, J. D.; de Gennes, P. G. J. Colloid Interface Sci. 1991, 142, 149-158. (12) Szleifer, I. Biophys. J. 1997, 72, 595-612. (13) Fang, F.; Szleifer, I. Langmuir 2002, 18 (14), 5497-5510. (14) Sofia, S. J.; Premnath, V.; Merrill, E. W. Macromolecules 1998, 31, 5059-5070. (15) Alexander, S. J. Phys. (Paris) 1977, 38, 983-987. (16) Zhu, X.-Y.; Staarup, D. R.; Major, R. C.; Danielson, S.; Boiadjiev, V.; Gladfelter, W. L.; Bunker, B. C.; Guo, A. Langmuir 2001, 17, 77987803.

10.1021/la034974r CCC: $25.00 © 2003 American Chemical Society Published on Web 10/31/2003

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Figure 1. (a) Schematic representation of reactive polymer attached to the substrate; (b and c) scheme of chemical bonding of the PGMA layer to the surface.

for the grafting in this case. Polymer grafting can be done either from a solution or from a melt.18-20 However, the attachment from the melt offers potential advantages over grafting from solution as a result of screening of the excluded volume interactions21 and, thus, the possibility of obtaining high grafting density. For instance, Piehler et al.17 and Zhu et al.16 successfully grafted PEG chains from the melt and synthesized brushes possessing relatively high grafting density. Usually, the coupling methods are relatively complex and specific for certain substrate/polymer combinations. An alternative method for synthesis of a PEG modified surface involves primary polymer (mono)layer with activity toward both surface- and end-functionalized macromolecules.22-24 The polymer is used for the initial surface modification as well as the generation of the highly reactive primary layer (Figure 1a). When deposited on a substrate, the primary layer first reacts with the surface through formation of covalent bonds. The reactive units located in the “loops” and “tails” sections of the attached macromolecules are not connected to the surface.25 These free groups offer a synthetic potential for the further chemical modification reactions and serve as reactive sites for the subsequent attachment of the end-functionalized PEG. If the polymer used for building the primary layer contains functional groups highly active in various chemical reactions, the primary layer approach becomes virtually universal toward both surface- and end-functionalized macromolecules being used for the brush formation. (17) Piehler, J.; Brecht, A.; Vialokas, R.; Liedberg, B.; Gauglitz, G. Biosens. Bioelectron. 2000, 15, 473-481. (18) Luzinov, I.; Julthongpiput, D.; Malz, H.; Pionteck, J.; Tsukruk, V. Macromolecules 2000, 33, 1043-1048. (19) Karim, A.; Tsukruk, V. V.; Douglas, J. F.; Satija, S. K.; Fetters, L. J.; Reneker, D. H.; Foster, M. D. J. Phys. II (France) 1995, 5, 14411456. (20) Norton, L. J.; Smigolova, V.; Pralle, M. U.; Hubenko, A.; Dai, K. H.; Kramer, E. J.; Hahn, S.; Begrlund, C.; DeKoven, B. Macromolecules 1995, 28, 1999-2008. (21) Jones, R. A. L.; Lehnert, R. J.; Schonerr, H.; Vancso, J. Polymer 1999, 40, 525-530. (22) Kothe, M.; Muller, M.; Simon, F.; Komber, H.; Jacobasch, H.-J.; Adler, H.-J. Colloid Surf., A 1999, 154, 75-85. (23) Zdyrko, B.; Klep, V.; Luzinov, I. Polym. Prepr. 2002, 43 (1), 586. Iyer, K. S.; Klep, V.; Luzinov, I. Polym. Prepr. 2002, 43 (1), 455. (24) Shibanova, O. B.; Medvedevskikh, Y. G.; Voronov, S. A.; Tokarev, V. S.; Stamm, M.; Antipov, E. M. Polym. Sci. Ser., A 2002, 44, 258-266. (25) Fleer, G. J.; Stuart, C. M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: New York, 1993.

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The present study is focused on permanent grafting of PEG layers from the melt onto a surface utilizing the primary polymer layer concept. For the majority of our experiments, we use silicon wafers as a substrate because it is considered a “standard” surface (along with mica and gold) for research in the field of thin polymer layers. A substantial number of polymer brush investigations have been made using wafers as a model surface, and, thus, we can compare our results with the work of others. However, we also report on PEG grafting to other substrates to demonstrate the universality of the approach used. Nevertheless, universality is an important but secondary goal of our research. The primary objective of the present investigation is the synthesis and characterization of PEG brushes possessing an extremely high grafting density. A high grafting density has been achieved owing to the high surface concentration and effectiveness of the active groups offered by the primary polymer layer technique. The grafting from melt also has aided the efficient grafting. To the best of our knowledge, the grafting density of PEG brushes we report in this paper is significantly higher than those previously attained by other researchers.5,6,14,16,17 Poly(glycidyl methacrylate) (PGMA) was used to form a reactive primary polymer layer. A polymer with epoxy functionality was chosen because the reactions of epoxy groups are quite universal and can covalently anchor PGMA to the substarte surface.22 The glycidyl methacrylate units located in the “loops” and “tails” sections of the attached PGMA chain were not connected to the substrate (Figure 1b). These free groups could serve as reactive sites for the subsequent attachment of PEG macromolecules with a complementary functional group. The thickness of the PGMA primary layer was altered to vary the surface concentration of the reactive epoxy groups. The dependence of PEG grafting on the temperature, time, and thickness of the PGMA layer was studied. The morphology and thickness of obtained PEG brushes in the dry and wet states were investigated. Experimental Section Highly polished single-crystal silicon wafers of 〈100〉 orientation (Semiconductor Processing Co.) were used as a substrate. The wafers were first cleaned in an ultrasonic bath for 30 min, placed in a hot “piranha” solution (3:1 concentrated sulfuric acid/30% hydrogen peroxide) for 1 h, and then rinsed several times with high purity water. Metal films were obtained on the silicon wafer surface by vacuum deposition (Edwards Coating System, model E306A). Silicone polymer films were prepared from Dow Corning Silgard 184 resin cured at 90 °C for 24 h. Glycidyl methacrylate from Aldrich was polymerized radically to give PGMA, Mn ) 84 000, PDI ) 3.4 (GPC). The polymerization was carried out in methyl ethyl ketone (MEK) from VWR at 60 °C. Azobisisobutyronitrile from Aldrich was used as an initiator. The polymer obtained was purified by multiple precipitations from the MEK solution in diethyl ether. PEG monomethyl ether, with Mn of about 5000 (Aldrich), was modified by succinyl anhydride (Aldrich) to give the carboxy end group derivative (PEG). Acylation was done by the refluxing of PEG monomethyl ether with a large excess (ca. 20) of succinyl anhydride in tetrahydrofuran (THF) from VWR. PEG was purified by multiple precipitations from THF solution in diethyl ether. Dodecylamine (DA) from Aldrich was used as received. PGMA was dissolved in MEK at different concentrations, and thin films (1-10 nm) were deposited on the substrate by dip coating (Mayer Feintechnik, model D-3400) and dried overnight. The thickness of the deposited PGMA films was controlled via concentration of the PGMA solution. The PEG powder was deposited onto the surface of a clean glass slide and was covered with the silicon wafer modified by the PGMA primary layer. The specimens were placed in a vacuum oven at an elevated

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temperature for different amounts of time to enable the end groups to anchor to the epoxy-modified substrate. At high temperatures, carboxylic groups are able to react with the epoxy groups of the PGMA layer.18 Unbound PEG was removed by multiple washing with toluene at 75 °C, including washing in an ultrasonic bath. To characterize the polymer layers, several parameters have been evaluated.26 The surface coverage (adsorbed amount), Γ (mg/m2), was calculated from the ellipsometry thickness of the layer, h (nm), by the following equation:

Γ ) hF

(1)

where F is density of attached (macro)molecules. The density of PGMA (1.08 g/cm3) was assumed to be the same as that for poly(propyl methacrylate).27 The density data for PEG (1.09 g/cm3) was provided by the supplier.28 The chain density, Σ (chain/nm2), that is, the inverse of the average area per adsorbed chain, was determined by

Σ ) ΓNA10-21/Mn ) (6.023Γ × 100)/Mn

(2)

where NA is Avogadro’s number and Mn (g/mol) is the numberaverage molar mass of the grafted polymer. The distance between grafting sites, D (nm), was calculated using the following equation:

D ) (4/πΣ)1/2

(3)

The radius of gyration for the PEG macromolecules was estimated by the equation

6Rg2 ) L2

(4)

where L ) 5.44 nm is the end-to-end distance of PEG.29 The free energy of mixing (∆GM) for the PEG/PGMA pair was estimated by the Flory-Huggins equation:30

∆GM ν1ν2χ ν1 ln ν1 ν2 ln ν2 ) + + V0KT Vx V1 V2

(5)

where ν1 and ν2 are volume fractions of the two components, V1 and V2 are volumes per polymer molecule, K is Boltzmann’s constant, V0 is the volume occupied by N0 number of cells in the Flory-Huggins theory (taken as 1 cm3), and Vx is given by

1 2 ) 1 - /VR Vx Z

(

)

(6)

where Z is the lattice coordination number ranging from 6 to 12 and VR is the volume occupied by a monomer unit. In our calculations, the geometrical mean of the PEG and PGMA monomer unit volumes and Z ) 10 were used. The interaction parameter, χ, for the PEG/PGMA pair was estimated by the following equation:30

χ)

Vr(δ1 - δ2)2 RT

(7)

where Vr is molar volume of the monomer unit of the polymer, δ1 and δ2 are the solubility parameters of PEG and PGMA, R is the universal gas constant, and T is the temperature in Kelvin. In our estimations, the geometrical mean of Vr for the PEG and PGMA monomer units was used. The solubility parameters were estimated using the atomic increments approach proposed by (26) Henn, G.; Bucknall, D. G.; Stamm, M.; Vanhoorne, P.; Jerome, R. Macromolecules 1996, 29, 4305-4313. (27) Van Krevelen, D. W. Properties of Polymers; Elsevier: Amsterdam, 1997; p 82. (28) Handbook of Fine Chemicals and Laboratory Equipment; Aldrich: Milwaukee, 2003-2004; p 1515. (29) Brandrup, J.; Immergut, E. H.; Grulke, E. A. Polymer Handbook, 4th ed.; John Wiley & Sons: New York, 1999. (30) Sperling, L. H. Polymeric Multicomponent Materials; John Wiley & Sons, Inc.: New York, 1997.

Askadskii.31 Calculated values of the solubility parameter were 22.01 and 20.49 (J/cm3)1/2 for PEG and PGMA, respectively. The interphase thickness was approximated by the following equation:32

Sth ) 2a/

[x ( ( 6 χ-

)

)]

1 1 + 2 ln 2 N1 N2

(8)

where a is the statistical segment length and N1 and N2 are the degrees of polymerization of the two polymers. The statistical segment length for PGMA was assumed to be the same as that for poly(methyl methacrylate) (0.6 nm).30 The segment length for PEG (0.29 nm) was calculated from the known end-to-end distance, L: a ) (L2/3N)1/2.33 In our calculations by eq 8, the geometrical mean of the PEG and PGMA segment lengths was used. Static contact angle measurements were made using a contact angle goniometer (Kruss, model DSA10). Calculation of the contact angle was made using the tangent method. Contact angle measurements were made with water (pH 7.0) and a static time of 60 s before the angle measurement. Ellipsometry was performed with a COMPEL automatic ellipsometer (InOmTech, Inc.) at an angle of incidence of 70°. Original silicon wafers from the same batch and silicon wafers with a PGMA layer were tested independently and used as reference samples for the analysis of grafted polymer layers. Scanning probe microscopy (SPM) studies were performed on a Dimension 3100 (Digital Instruments, Inc.) microscope. We used the tapping and contact modes to study the surface morphology of the PEG films in ambient air and under water. Silicon tips with spring constants of 50 N/m (tapping mode) and 0.25 N/m (contact mode) were used. Imaging was done at scan rates in the range 1-2 Hz. The root-mean-square (rms) roughness of our samples was evaluated from the SPM images recorded.34

Results and Discussion PGMA Primary Layers. The silicon wafers covered with the adsorbed PGMA films were vigorously rinsed by a series of highly polar solvents, including dimethyl sulfoxide and THF. It was found that the layers could not be removed from the wafer after the solvent treatment, suggesting that PGMA was chemically bonded to the surface (Figure 1c).22 SPM studies of PGMA layers revealed that the films were smooth and homogeneous. Figure 2a demonstrates that the PGMA layer uniformly covered the substrate surface on the microlevel. The morphology of the primary polymer layer on the nanolevel is shown in Figure 2b. The layer was molecularly flat with a rms roughness less than 0.3 nm. During the grafting of PEG to the PGMA layer at a high temperature, two competitive processes may occur: (a) reaction between the carboxy groups of PEG and epoxy functionalities of PGMA and (b) self-cross-linking of the PGMA layer owing to the high concentration of the epoxy groups in the layer. The latter can reduce the surface concentration of the epoxy moieties available for the PEG macromolecules being attached and consequently decrease the efficiency of the grafting. To study the extent of this deactivation, we grafted DA to the PGMA films with thicknesses of 1.5-2.5 nm. The samples were preliminarily annealed at 120 °C in a vacuum for different amounts of time to provoke the PGMA self-cross-linking. We used this low-molecular-weight substance as a probe for the presence of the accessible epoxy groups. The amine attachment was carried out in warm (40 °C) toluene (31) Askadskii, A. A. Physical Properties of Polymers: Prediction and Control; Gordon and Breach Publishers: Amsterdam, 1996. (32) Broseta, D.; Fredrickson, G. H.; Helfand, E.; Leibler, L. Macromolecules 1990, 23, 132. (33) Ligoure, C.; Leibler, L. J. Phys. (Paris) 1990, 51, 1313-1328. (34) Scanning Probe Microscopy: Training Notebook; Digital Instruments, Veeco Metrology Group: Santa Barbara, 2000; p 40.

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Figure 2. SPM topography images of the PGMA primary monolayer deposited on the silicon wafer: (a) 10 × 10 µm; (b) 1 × 1 µm. Vertical scale: (a) 10 nm; (b) 2 nm. rms roughness: (a) 0.286 nm; (b) 0.233 nm.

Figure 3. Activity of PGMA layers toward the DA grafting versus annealing/aging time. (0) Samples aged at ambient conditions; (∇) samples annealed at 120 °C.

solution for 12-16 h. The extent of the DA anchoring was measured by ellipsometry. Grafting activity was determined as the ratio of the DA amount attached to the annealed PGMA layer to the amount anchored to the layer with the same characteristics before the annealing. The experiment demonstrated that approximately 40% of the epoxy groups were still available for the DA attachment after 4 h of annealing. The drop in initial activity of the adsorbed PGMA toward the DA grafting occurred almost immediately after the sample was heated (Figure 3). The observed self-cross-linking of the PGMA chains brought up an additional question: how stable are the PGMA layers and how long can they be stored without significant deactivation before use? To clarify this question, DA was grafted to PGMA layers aged for different amounts of time at ambient conditions. The amine attachment was carried out in warm (40 °C) toluene solution for 48 h. Figure 3 presents the results of the experiment. Virtually no decrease in PGMA activity toward grafting was observed after 14 h of aging at ambient conditions. Grafting Capacity of PGMA Layers. The ellipsometric thicknesses of two PGMA primary layers used in our grafting experiments were 1.5 ( 0.2 and 2.5 ( 0.25 nm. Therefore, the adsorbed amount, Γ, of PGMA constituting the layers was 1.65 mg/m2 and 2.75 mg/m2. These amounts corresponded to 0.012 and 0.02 PGMA chains/ nm2 or 7 and 11.7 epoxy groups/nm2, respectively. Certainly, the fraction of these groups (responsible for the PGMA attachment to the surface) was situated in the train sections of the adsorbed chain. These glycidyl methacrylate units, as well as the units involved in the PGMA self-cross-linking during the grafting at elevated

temperatures, were not available for the grafting reactions. According to Fleer et al.,25 the train fraction for relatively high-molecular-weight polymer adsorbed on surface is about 0.15-0.25. Additionally, a maximum of 60% of the epoxy groups in the loops and tails may be lost as a result of self-cross-linking (Figure 3). Therefore, in the worst case scenario the thinnest PGMA layer used in this work had a surface concentration of active epoxy groups offered for grafting no less than 2.1 groups/nm2. This value approximated the surface concentration previously reported for an epoxysilane monolayer (≈2 epoxy groups/ nm2).18 For the thicker PGMA layer, the estimations gave 3.5 groups/nm2. Indeed, utilization of the primary polymer layer technique for the surface functionalization allowed a sufficient increase in the concentration of the active to the grafting functional groups if compared to that of the self-assembled monolayers. When end-functionalized polymer is grafted from the melt to a surface modified with the primary reactive polymer, two boundary cases are to be considered (Figure 4). These situations are related to interdiffusion phenomena that always have to be taken into account when interfaces between two polymers are present.35 The polymer interface, involving two different polymers (PGMA and PEG), may remain intact if the polymers are immiscible. Typically, interpenetration at the interface may range from a depth of several angstroms to several nanometers, depending on the statistical segment length, the degree of polymerization, and the interaction parameter χ. If the interpenetration is minute (Figure 4b), PEG chains being grafted can access only glycidyl methacrylate units located at the surface of the PGMA films. Opposite and more favorable for the grafting situation is when the PEG completely penetrates into the PGMA film (Figure 4c). Then, virtually all epoxy groups are available for the reaction. The thermodynamical miscibility for the PGMA/PEG pair at the grafting temperatures used in the present study (70-110 °C) was estimated by eq 5. Because the PGMA sample had a broad molecular weight distribution, the calculations were done for different PGMA fractions present in the sample. The calculations revealed that ∆GM > 0 in our experimental conditions, and, consequently, there is no thermodynamical miscibility for the PGMA/ PEG pair. Thus, when PGMA and PEG are in contact, the interdiffusion zone has to be formed. The extent of interpenetration at the interface (or width of PEG/PGMA interphase) was approximated by eq 8 for different PGMA (35) Sperling, L. H. Introduction to Physical Polymer Science; John Wiley & Sons: New York, 2001.

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Figure 4. Two boundary situations during PEG grafting to the PGMA layer (a). (b) No penetration of PEG into the PGMA layer; (c) formation of the PEG/PGMA interpenetration zone.

fractions. According to the estimations, PEG has to penetrate to some extent (1.5-1.7 nm) inside the PGMA layer. The random diffusion of the PEG molecules into the PGMA layer leads to the formation of a complex surface with fractal characteristics.36 This process increases the dimensionality of the PGMA film (to d > 2) compared to that of a functional self-assembled monolayer deposited on the silicon wafer. In this scenario, the epoxy functional groups located inside the layer are available for the grafting. Obviously, the interphase calculations developed for immiscible polymer pairs in contact could give only rough estimations for the penetration in the adsorbed polymer layer. However, we expected that the difference in grafting capacity between the PGMA films possessing different thicknesses ought to be diminished because PEG might not fully penetrate inside the thicker (2.5 nm) PGMA layer. PEG Grafting to the Primary Polymer Layer. Figure 5 shows how the thickness of the grafted PEG layers varies with the time and temperature for the primary PGMA films possessing different thicknesses and, thus, different amounts of the epoxy groups. Generally, the thickness of the PEG layers increased with the reaction time for both PGMA layers. The rate of PEG attachment was dependent on the temperature. Elevated temperatures considerably accelerated the grafting process. The grafting leveled off after 200-500 min of the reaction at higher temperatures. When the PEG attachment was conducted at lower temperatures, the thickness of the grafted layer reached maximum values after more than 700 min. Processes of polymer grafting to the interface can be divided into two main regimes.33 During the first stage, construction of the brush is limited by the classical (36) Wool, R. P. Polymer Interfaces: Structure and Strength; Hunser Publishers: Munich, 1995; p 102.

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Figure 5. Thickness of PEG brushes versus grafting time. PGMA thickness: (a) 2.5 nm; (b) 1.5 nm. (4) 110 °C; (O) 90 °C; (]) 84 °C; (0) 70 °C.

diffusion of polymer chains to the interface. As long as the surface coverage is lower than some critical value, the polymer chains are not overlapped and are not stretched. This process is relatively fast, leading to formation of a polymer layer in which chains start to overlap, creating a barrier to further adsorption. The second, slower regime is characterized by a potential barrier that macromolecules being grafted have to overcome to reach the surface and be anchored. If the grafting is carried out in the melt, many polymer chains are not required to diffuse to the surface and overcome the potential barrier. The macromolecules need only to reorient themselves within the first monolayer to expose the terminal groups to the surface functionalities. This phenomenon extends the duration of the first regime of grafting, leading to more efficient chain anchoring. For the reactions from the melt, the second and slower stage starts when practically all the macromolecules that initially have physical contact with the surface are grafted. Namely, the (dry) thickness of the grafted layer virtually surpasses 2Rg before the second regime is in effect. For PEG macromolecules with Rg ≈ 2.2 nm, the thickness of the monolayer is 2Rg ) 4.4 nm. Because PEG can penetrate into the PGMA primary layer, this value can be increased by 1-1.5 nm. Thus, the thickness of the grafted PEG layer that can be attained in the first regime is approximately 5-6 nm. In fact, for the high-temperature anchoring we observed that the rate of PEG grafting became lower when the thickness of the grafted layer reached a value of 5-6 nm (Figure 5). When PEG was attached at lower temperatures, the increase in the layer thickness with time was virtually linear (Figure 5). It revealed that at the low-temperature conditions the grafting was not limited by the diffusion. Apparently, the grafting was controlled by the rate of reaction between the epoxy functionality of PGMA and the carboxy group of PEG. When the second, slower stage of the grafting is considered, the energy gain during the tethering of the

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Figure 6. Grafting density (Σ) of PEG brushes versus grafting time. PGMA thickness: (a) 2.5 nm; (b) 1.5 nm. (4) 110 °C; (O) 90 °C; (]) 84 °C; (0) 70 °C.

terminal group has to prevail over the barrier of the densely grafted layer formed in the first regime.33 The excess of the interaction energy over the excluded volume interactions determines the maximum equilibrium thickness that can be achieved in the second regime. Ligoure and Leibler33 suggested a theoretical approach to predict the equilibrium grafting density for grafting end-functionalized polymer. The solution of the system of eqs 9 and 10 yields the equilibrium grafting density of polymer brush:

N(υ/a3)φ0R ) ∆ + ln(φ0/σ)

(9)

Nσ ) (h/a)φ0(1 + 2/3R)

(10)

where B ) π2/(8N2a2); R ) Bh2a3/φ0υ; ∆ is the energy gain during the surface adsorption of the terminal group (in KT units); h is the brush thickness; υ is the excluded volume parameter; φ0 is the volume fraction of monomer units in solution (equal to unity for grafting from the melt); and N is the degree of polymerization of the polymer being grafted. For the energy gain 13.7 kJ/mol estimated from standard formation enthalpies37 (or 4.3 in KT units) and the excluded volume parameter υ ) 0.8 nm (for a first approximation it was taken as the same as that in ref 33), the maximum (equilibrium) dimensionless grafting density found by solving the system of the equations graphically for the system studied was

σ ) a2Σ ≈ 1.05

(11)

The equilibrium grafting density calculated (σ )1.05) (37) Lide, D. R. Handbook of Chemistry and Physics, 80th ed. CRC Press: London, 1999-2000; pp 5-30.

Figure 7. Distance between grafting sites (D) versus time of grafting. PGMA thickness: (a) 2.5 nm; (b) 1.5 nm. (4) 110 °C; (O) 90 °C; (]) 84 °C; (0) 70 °C.

corresponds to an inaccessible in practice Σ ) 12.5 chains/ nm2. The value is 1 order of magnitude times larger than that experimentally obtained in this work (Figure 6). Therefore, the estimations show that the PEG grafting from melt is not limited thermodynamically. The grafting density is eventually limited by the concentration and accessibility of functional reactive groups on the surface. At the lower grafting temperatures, nearly the same maximum amount of PEG (8-9 nm) was attached to the primary polymer layer independently of the PGMA film thickness. Thus, the accessible amount of the glycidyl methacrylate units available for the grafting reaction was close for both PGMA layers. This result was predicted by the calculation of the PGMA/PEG interphase thickness. The interphase was expected to be the same for the PGMA films possessing different thicknesses. However, for the highest temperature (110 °C) much more PEG was grafted to the thicker PGMA layer. This result appeared to be in contradiction with the interphase estimations. It indicated that application of the interphase calculations developed for immiscible polymer pairs in contact had certain limitations when it was applied to the adsorbed polymer layer. In fact, the adsorbed polymer chain consists of the trains, loops, and tails of a particular length. And the number of monomeric units in the loops and tails pinned to the surface by trains is much lower than the degree of polymerization of the adsorbed macromolecule. Thus, it is necessary to consider contact between the loops and tails of PGMA with penetrating PEG molecules and not the contact of PEG with the entire PGMA macromolecule.

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Figure 8. Water contact angle of PEG brushes versus the brush thickness. PGMA thickness: (a) 2.5 nm; (b) 1.5 nm. (4) 110 °C; (O) 90 °C; (]) 84 °C.

The loops and tails possessing lower molecular weights can form a more extended interphase with PEG and even appear to be miscible with the penetrating chains. The grafting density, Σ, of the PEG grafted layers is plotted versus time of grafting in Figure 6. Σ increases with the reaction time from 0.2 to 1.2-1.5 chain/nm2. For the thinner PGMA layer, the saturation occurred faster than that for the thicker one. From the grafting density data and surface concentrations of the epoxy groups available for the grafting, we can estimate the grafting efficiency of PGMA layers possessing different thicknesses. PEG was grafted to not more than 58 and 43% of the epoxy groups available in the thinner and thicker PGMA layers, respectively; thus, the thinner PGMA layer has a higher grafting efficiency. The difference in the grafting can be connected not only to the interpenetration phenomena but also to competition between the grafting and self-cross-linking processes. It is known that adsorbed macromolecules have mobility reduced by the interaction with the surface. When the adsorbed amount of PGMA increases, the fraction of the segments located in the trains decreases, causing an increase in the length and mobility of the loops/tails.25 We believe that glycidyl methacrylate units located in the longer flexible loops/tails are more susceptible to the self-cross-linking reaction that effectively decrease the concentration of the epoxy groups in the PGMA layer. Thus, the thicker PGMA film becomes less effective in the PEG attachment. Figure 7a,b demonstrates how the distance between grafting sites for PEG layers varies with the time of the grafting. Values of this parameter for obtained layers are between 2.6 and 0.9 nm for the thinnest and the most densely grafted layers, respectively. It is necessary to highlight that the distance between grafting sites for all layers synthesized is close to or lower than Rg for PEG

Figure 9. SPM topography images displaying surface morphologies of PEG brushes. Vertical scale: 20 nm. (a-e) Microstructure, image size, 5 × 5 µm; (f-j) nanostructure, image size, 1 × 1 µm. Grafting amount, mg/m2: (a,f) 1.59; (b,g) 3.64; (c,h) 6.31; (d,i) 9.75; (e,j) 12.6.

(2.2 nm), indicating that all grafted PEG layers are densely grafted and definitely are in the “brush regime”. For the most dense PEG layer having thickness of 11.5 nm, calculated parameters of the grafted layer are as follows: grafted amount of PEG, 12.5 mg/m2; grafting density, Σ ) 1.5 chain/nm2; and distance between grafting sites, D ) 0.9 nm. To the best of our knowledge, the grafting density of the PEG brush obtained in the present work is considerably higher than those reported in the scientific literature.

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Zdyrko et al.

Figure 10. SPM topography images of (a) dry and (b) wet PEG brushes. Grafting density: 0.67 chain/nm2. Vertical scale: 20 nm. (c and d) Scratched PEG brush topography images and topographical cross sections over the scratched region. Grafting density: 1.5 chain/nm2. SPM imaging in air (c) and under water (d). Vertical scale: 50 nm.

Contact Angle Measurements. Contact angle measurements allow estimation of how the surface is screened with the polymer chains deposited on the substrate. The contact angle of the silicon surface covered with the PGMA primary layers was about 60 ( 2°. We found that the contact angle was independent of the thickness of the primary layer because very similar values were determined for the layers possessing different (1.5 and 2.5 nm) thicknesses. The value corresponded to the one measured for the PGMA films possessing a much higher (15-20 nm) thickness. The results indicated that the silicon surface with the initial contact angle (before the PGMA deposition) of 5-10° was completely covered with the PGMA macromolecules. Due to the PEG hydrophilicity, the surface covered with the grafted layer is expected to exhibit low water contact angles. Piehler et al.17 reported values in the range of 27-30° for relatively densely grafted PEG monolayers. Figure 8 shows the variation of the contact angle with the thickness of the PEG brush synthesized in the present study. The wettability of the modified substrate was a

function of the PEG grafted amount. The contact angle decreased from 55° observed for the thinnest grafted layer to 20 ( 4° measured for the thick PEG brush. The value of 30-35° frequently reported in the scientific literature was reached when the thickness of the PEG layer became 5-6 nm. The low values of the contact angle confirmed the presence of a sufficient amount of PEG molecules on the treated grafting surface. SPM Imaging of Grafted PEG Layers. The surface morphology and smoothness of the grafted PEG layers were determined with tapping mode SPM. Figure 9 displays SPM topographical images of PEG brushes attached at different grafting densities to the surface through the PGMA monolayer. The imaging was conducted in ambient air. Generally, SPM showed that the developed grafting process led to complete PEG layers with surface flatness on the nanometric scale. Several distinct types of brush morphologies were observed depending on the grafted layer thickness. Figure 9a,f present the morphologies (on the micro- and nanolevels) of isolated domains observed for a low grafting density.

Poly(ethylene glycol) Grafted Layers

We associate the domains with PEG crystals38 being formed by a fraction of the grafted macromolecules. Phase imaging (images not shown) corroborated our conclusions because the domains displayed different mechanical characteristics than the majority of the grafted film. The surface concentration of the crystals increased with the extent of grafting and reached a maximum at a grafting amount of 3-5.5 mg/m2 (Figure 9b,g). At higher levels of PEG attachment, the number of the individual crystalline formations decreased. However, the development of larger crystals covering 15-50% of the sample surface was observed (Figure 9c,h). For thicker (9-13 mg/m2) brushes, the crystalline structure of the grafted layers was the most pronounced. The crystalline formations uniformly covered the entire substrate surface (Figure 9d,e,i,j). The development of highly extended crystalline formations on the surface modified with the grafted PEG brushes once again indicated that, indeed, an extremely high grafting density could be reached utilizing the grafting process designed in the present study. The rms roughness on 1 µm2 of dry PEG brushes was in the range of 0.3-2 nm. The higher values of the roughness were observed for the samples covered with the sufficient fraction of the individual crystalline formations. It is necessary to stress that the crystals practically disappeared when the samples were imaged under water (Figure 10a,b). We determined the SPM thickness of four selected PEG brushes by scanning a box in the contact mode at high loads. The scanning produced a scratched-out area, in which the majority of the grafted PEG layers was removed (Figure 10c). The layer height obtained by SPM was fairly close (within 5-10% error range) to the layer thickness measured by ellipsometry. This result indicated that PEG was densely packed in the grafted layer with a refractive index (and density) very close to the known value for the bulk material. The scratched-out box was also used to estimate the extension of the PEG layers under the water. Because the grafted chains were in the strong brush regime, we expected sufficient elongation of the macromolecules due to the excluded volume effect. The box and surrounding area were imaged in the dry state and under water. Topographical cross sections were measured over the scratched region to estimate the PEG layer thickness (Figure 10c,d). The thickness of the dry brush tested was about 11.5 nm (ellipsometry and SPM measurements). The brush extended more than three times in water (a good solvent) and approached a height of 39 nm. It is necessary to stress that the theoretical length of the fully extended PEG chain with molecular weight of 5000 is approximately 42 nm. Comparison of our experimental data with the theoretical chain length suggests that the PEG chains were grafted at very high densities to the surface through the PGMA monolayer. Versatility of the Reactive Primary Layer Approach. The grafting approach we developed was then used to modify various substrates with PEG grafted layers. We attempted to attach the PEG brushes to gold, silver, titanium, and polymeric films made of silicon resin. Table 1 shows the contact angle values of the substrates covered with the primary PGMA layer. The contact angle of the substrates covered with the PGMA primary layers were between 52 and 60°. Contact angle measurements showed that the surface of substrates was thoroughly screened with the PGMA primary film. We believe that the (38) Sanderson, L. A. W.; Emoto, K.; Van Alstine, J. M.; Weimer, J. J. J. Colloid Interface Sci. 1998, 207, 180-183.

Langmuir, Vol. 19, No. 24, 2003 10187 Table 1. Water Contact Angles of PGMA and PEG Layers on Different Substrates contact angle

a

substrate

PGMA layer

PEG layer

gold silver titanium silicone resina

58.8 ( 0.4 59.4 ( 0.82 51.9 ( 1.26 55.5 ( 1.07

32.2 ( 0.4 35.5 ( 0.9 14.9 ( 0.27 32.8 ( 1.05

Surface was treated with air plasma before PGMA attachment.

deviations in the contact angle values from the expected 60° obtained for the silicon wafer covered with PGMA were connected with the roughness of the substrates used. After the PEG grafting, the wettability of the surfaces was sufficiently increased (Table 1). The contact angle was between 15 and 35°, indicating that PEG was successfully grafted to the various substrates by the designed grafting process. Conclusions PEG attachment utilizing a primary polymer layer approach led to the synthesis of complete and high-density PEG brushes on various substrates. PGMA was used to form the reactive primary adsorbed layer. The dependence of PEG grafting on the temperature, time, and thickness of the PGMA layer was studied. It was found that as the PGMA layer thickness decreased the grafting efficiency increased and that elevated temperatures considerably accelerated the grafting process. At high temperatures, the rate of PEG grafting decreased when the height of the grafted layer exceeded a value of 5-6 nm. If PEG was attached at lower temperatures, the increase in the layer thickness with time was virtually linear. Calculation of maximum possible grafting density showed that the PEG anchoring from the melt was not limited thermodynamically. The grafting extent was eventually limited by the concentration and accessibility of functional reactive groups on the surface. The wettability of the modified substrate was a function of the PEG grafted amount. The contact angle decreased from the 55° observed for the thinnest grafted layer to 20 ( 4° measured for the thick PEG brush. The surface structure and thickness of obtained PEG brushes in the dry and wet states were also investigated by SPM. Several distinct types of brush morphologies were observed depending on the grafted layer thickness. Isolated crystals were observed for the brushes possessing a low grafting density. At higher levels of the PEG attachment, the crystalline formations uniformly covered the entire surface of the substrate. However, the crystals practically disappeared when the samples were imaged under water. The densely grafted brushes expanded more than three times in the vertical direction when exposed to the good solvent. The development of highly extended crystalline formations on the surface and the strong expansion under water indicated that an extremely high grafting density could be reached utilizing the grafting process designed in the present study. Acknowledgment. This work was supported by the Department of Commerce through the National Textile Center, Grant M01-CL03, and in part by the ERC Program of National Science Foundation under Award No. EEC9731680. The authors thank Dr. S. Minko and Dr. M. Ellison for useful discussion and Ms. Kim Ivey for the GPC measurements. LA034974R