Article pubs.acs.org/Macromolecules
Binding of Functionalized Polymers to Surface-Attached Polymer Networks Containing Reactive Groups Mónica Pérez-Perrino,† Rodrigo Navarro,† Oswald Prucker,‡ and Jürgen Rühe*,‡ †
Instituto de Ciencia yTecnología de Polímeros (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain Laboratory for Chemistry & Physics of Interfaces, Department of Microsystems Engineering (IMTEK), University of Freiburg, Georges-Köhler-Allee 103, 79085 Freiburg, Germany
‡
S Supporting Information *
ABSTRACT: To study diffusion and binding of polymers into surface-attached networks containing reactive groups, surface-attached polymer networks bound to oxidized silicon surfaces are generated, which contain succinimide ester groups. The surface-attached polymer layers are brought into contact with poly(ethylene glycol)s (PEG), which carry terminal amine end groups and which have systematically varied molecular weights. The coupling reaction between the active ester groups in the polymer networks and the amine groups in the incoming chains are studied by ellipsometry, surface plasmon spectroscopy, AFM, and Fourier transform infrared spectroscopy (FTIR). The degree of functionalization of the reactive layers by the PEG-NH2 depends strongly on the crosslink density of the network, the active ester content, and the molecular weight of the amine-terminated polymer. A model for the attachment reaction is proposed which suggests that the incoming polymer chains bind only at the outer periphery of the network in a narrow penetration zone. According to this model, when the incoming polymers are rather short, penetration into the layer and binding are prohibited by the high segment density and the anisotropic stretching of the surface-attached networks (“entropic shielding”). For incoming chains with a higher molecular weight and/or networks with a small mesh sizes, size exclusion effects determine diffusion and binding.
1. INTRODUCTION The attachment of thin polymer coatings to a solid support is a versatile and simple route to adjust and control the interactions of the material with its environment. Important examples of properties, which can be strongly changed by the deposition of polymer coatings, are friction, adhesion, or wetting.1−6 For this reason the modification of surfaces with polymeric coatings has gained much interest in many different research fields, such as microelectronics, biomedicine, (bio)sensors, and optical devices.7 When the polymers in such films are covalently attached to the surfaces of the substrates, good stability against a broad spectrum of solvents and other environmental influences is obtained. In addition, surface-attached coatings are versatile polymeric scaffolds because they can host a variety of different functional entities, such as proteins, enzymes, or inorganic nanoparticles.8−11 Commonly, the functionalization of such films is carried out through specific binding of incoming molecules to reactive groups contained in the polymer coating. Several different techniques have been reported in the literature in order to achieve an efficient modification. The immobilization of biomacromolecules has been realized based on maleimide groups and disulfide or thiol units, which can add to double bonds. Further common strategies are based on the © 2014 American Chemical Society
incorporation of succinimidyl esters (NHS), azlactones, aldehyde, epoxy, or carbodiimide moieties, which can easily react with amine groups of incoming molecules.12−17 A key parameter for such surface reactions is the yield of the functionalization reactions.12−14,16,17 It has been previously demonstrated that reactive polymer brush layers with active ester (NHS) units can be functionalized in a quantitative way by low molecular weight molecules. However, the yield of the modification reaction decreases for high molecular weight modifiers.18,19 One of the reasons for this observation is that the diffusion of large molecules into surface attached layers becomes increasingly difficult with increasing graft density of the attached chains. The penetration by free chains into polymer networks has been extensively studied for more than three decades. For example, the diffusion of linear deuterated polystyrene into its cross-linked counterpart was extensively studied using neutron reflectometry and forward recoil scattering.20,21 The free energy of the mixing process depends on the strength of enthalpic interactions between the two types of chains and on changes of Received: February 6, 2014 Revised: March 13, 2014 Published: April 7, 2014 2695
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weight of penetrating macromolecules and cross-linker, or active ester content, is also extensively studied.
the entropy occurring during the process. When the enthalphic interactions are strong, as this is the case when oppositely charged polyelectrolytes are brought into contact with each other, deep penetration will occur (provided that the reaction or complex formation does not yield insoluble products, which would of course strongly limit the extent of the reaction). However, when the enthalphy of the process is rather small or zero, as this is the case when chemically identical polymers penetrate into the layer, the extent of diffusion of free polymer into the surface-attached layer depends on the difference between the entropy of mixing and changes in the conformational entropy of the system. The latter depends strongly on the conformation of the attached chains. When the chains are attached to the surface with a high graft density, they stretch away from the surface to yield a brushlike conformation. However, it has been shown that also the break of the symmetry caused by surface attachment of polymer networks induces anisotropy into the system caused by stretching of the surface-attached chains.22 In a recent publication the attachment of end-functionalized polymers to surface-attached polymer brushes was studied.19 As an example, poly(methyl methacrylate) (PMMA)-based brushes containing succinimid groups were generated at a solid surface and reacted with amine group containing poly(ethylene glycol)s. It was found that the attachment process was very fast and depended strongly on the molecular weight of the incoming chains. The findings were explained by model that describes the diffusion and functionalization of this kind of reactive polymer scaffolds with free polymer molecules. According to this model, penetration of the layer occurs only in a rather thin penetration layer located at the outer periphery of the brush, where the local segment density is low in comparison to the rest of the surface-attached polymer layer. The model relates the amount of bound polymer to the shape of the segment density profile of the brush and the size of the incoming chains. The authors claimed that only at the outer periphery of the brush layer partial penetration is possible and thus sets strong limits on the subsequent grafting reaction.19 It was hypothesized that surface-attached polymer networks, which swell essentially in only one direction22 and which possess in many cases in the swollen state a rather block-like segment density profile, should exhibit a very narrow penetration zone as a consequence of this very sharp segment density profile. It was predicted that surface-bound networks should be even less susceptible to surface attachment of incoming polymer chains than polymer brushes.19 We decided to put this somewhat counterintuitive prediction to a test and study in the present paper the diffusion and subsequent attachment of chemically nonidentical free polymer chains into surface-attached polymer networks. This is carried out introducing into the polymeric scaffolds active esters moieties (succinimide ester, NHS) and react them with ω-monoaminepoly(ethylene glycol)s. After the partial penetration of the polymer networks by free PEG-NH2 chains, the aminolysis reaction takes place. The grafted PEG chains are irreversibly bound, so the position of this situation becomes “frozen in” by covalent binding. Moreover the yield of the aminolysis reaction and the grafted amount of PEG can be monitored through measuring the variation of the polymeric thicknesses as a function of the reaction conditions using ellipsometry. The influence on the grafting reaction of different parameters, such as the concentration of the free amine solution, the molecular
2. EXPERIMENTAL SECTION Materials. Methyl methacrylate (MMA), α,α′-azoisobutyronitrile (AIBN), O-(2-aminoethyl)-O′-methylpoly(ethylene oxide) (monoamino-PEG) 20000, 10000, 5000, 2000, and 750 were purchased from Fluka. Toluene was distilled from molten sodium using benzophenone as indicator. Triethylamine was distilled from calcium hydride. N,N-Dimethylformamide (DMF) was distilled from activated molecular sieves (4 Å, 300 °C for 16 h) after drying with calcium hydride. All other chemicals and solvents (HPLC grade) were used as received. The substrates for transmission FTIR spectroscopy were silicon wafers polished on both sides (Aurel) with a natural SiO2 layer of approximately 2.5 nm. Characterization. 1H NMR spectra were recorded on a Bruker DPX 250 spectrometer using CDCl3 as solvent. Transmission FTIR spectra of the attached polymer layers were recorded using a Nicolet 730 FTIR spectrometer. The layer thickness of the grafted polymers were obtained by SPR using a setup in the Kretschmann configuration with a He−Ne laser (λ = 632.8 nm), 90° prism (BK7, nD = 1.5151, Spindler & Hoyer, and LaSF9N, nD = 1.8449, Berliner Glas), and index match liquid (nD = 1.5160 and 1.700, Cargill). The dry thickness of surface-attached polymer monolayers was measured by ellipsometry (Riss EL-X1, Germany, incidence angle: 40°−80°). Contact angles (CAs) were measured using an OCA20 measurement system from Dataphysics GmbH, Germany. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PerkinElmer PHI 5600 spectrometer using Mg Kα radiation. The atomic force microscopy (AFM) measurements were carried out on a Topometrix 2000 AFM in the constant force mode. Preparation of the Polymers. MMA-MAC2AE-MaBP Synthesis. A series of MMA-MAC2AE-MaBP copolymers were synthesized by statistical free-radical copolymerization of methyl methacrylate, Nmethacryloyl-β-alanine succinimide ester (MAC2AE), and 4-methacryloyl oxybenzophenone (MaBP). The MaBP monomer and MAC2AE were prepared by literature procedures.18,22 The polymerization was initiated using 0.1% α,α′-azoisobutyronitrile (AIBN), and the reactions were carried out at 60 °C for 15 h in DMF under nitrogen after degassing the reaction mixtures by three freeze and thaw cycles. The reactions were stopped by precipitating the mixture in cold diethyl ether. The terpolymers were purified using methylene chloride/diethyl ether (CH2Cl2/Et2O) as a solvent−precipitant system, typically yielding approximately 75% of a white powder. For characterization, the 1H NMR spectra were recorded on a Bruker MSL 300 spectrometer (300 MHz). The molecular weight Mw and the molecular weight distribution (polydispersity Mw/Mn) of the copolymers were determined by gel-permeation chromatography with an Agilent instrument equipped with an RI detector and software from PSS (Polymer Standards Service). For molecular weight calibrations narrow polydispersity PMMA standards (without active ester groups) from PSS were chosen. A systematic abbreviation is used in the following for the prepared P(MMA-MAC2AE-MABP) copolymers indicating first the content of the “active ester” (A) and then the amount of benzophenone (B). For instance, a copolymer labeled 12A-1B contains 12 mol % of active ester moieties and 1 mol % of photo-cross-linker. Preparation of the Polymeric Networks. For the generation of the surface-attached polymer networks first the silicon wafers were covered with a self-assembled monolayer generated from 3(chlorodimethylsilyl)propyloxybenzophenone (ClSi-BP) following the procedure described by Prucker et al.23 Thick overcoats (≈100 nm) of the terpolymers were prepared by spin-casting solutions of the polymers at a typical spin speed of 3000 rpm. The solvent was dichloromethane, and polymer concentration varied between 10 and 80 mg mL−1. The samples were dried in air and used directly for illumination experiments. By treating the layers with UV light (365 nm, 90 min), cross-linking and surface attachment occur in one step. The thicknesses of the resulting polymer layers were determined by 2696
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ellipsometry. To ensure the removal of all nonbound polymer, an extraction in a good solvent was performed after illumination. Aminolysis of the Surface-Attached Polymer Networks. The active ester polymer networks were placed in a reactor with the desired monoamino PEG (33 mg or as indicated in the text), triethylamine (50 μL), and DMF (1 mL) and kept at 40 °C for 16 h. The substrates were then rinsed extensively with DMF and ethanol and finally dried in vacuum. The layers were characterized by FT-IR, XPS, and ellipsometry.
deposited onto substrates by spin-casting. The layer thickness was controlled by adjusting the polymer concentration. The cross-linking and surface attachment of these polymers were carried out by photoactivation of the benzophenone groups situated on the polymer and at the surface. Upon irradiation with UV-light (λ = 365 nm), a biradicaloid triplet state is generated, which abstracts a hydrogen atom from an adjacent C−H bond, followed by recombination of the two resulting radicals.23 In this way the polymer chains become connected to each other, and the forming network becomes attached to the surface. Typically rather thick films of d > 100 nm were studied to avoid any influence of the underlying substrate. After illumination, the active ester networks were extensively extracted with DMF for at least 6 h to remove nonattached polymer chains. As the obtained networks were quite thick, it was possible to characterize the structures of the reactive scaffolds directly by transmission FTIR spectroscopy. Figure S1 (Supporting Information) shows the typical signals from the active ester groups (N-oxysuccinimide) found at 1815 and 1785 cm−1. The predominant band at 1729 cm−1 corresponds to the CO stretching vibration mode of MMA. Additionally, XP spectra on the networks were recorded that confirmed the proposed structure of the scaffolds. In Figure 2, the C 1s peak in the XP spectrum can be deconvoluted into three different peaks that correspond to the different chemical environments of the carbon in the system. The positions of these deconvoluted peaks were close to the expected binding energies of 288.8, 286.4, and 284.7 eV, which were assigned to carboxyl (COOR), ether (C−O), and alkane moieties (C−C) of MMA, respectively. Deconvolution of the O 1s peak of the spectrum gave two main peaks at 532.7 and 534.3 eV, which are indicative of C−O and CO bonds of methoxy and carboxyl groups in MMA, respectively. We have studied the swelling behavior of these networks upon exposure to different solvents by ellipsometry and SPR using acetonitrile as the swelling solvent and by AFM using DMF. The degree of swelling obtained in both solvents was very similar, which suggests that both solvents are of equal quality in this regard. In Figure S2 (Supporting Information) the degree of swelling is represented as a function of the MaBP content. The slope of the obtained straight line is −1/3. This value agrees well with previous work22 where the swelling behavior of surface-attached poly(dimethylacrylamide) polymer networks with different MaBP content was studied by ellipsometry. Reaction of the Active Ester Networks with Monoaminated-PEGs. After the immobilization on the surfaces, these active ester polymer networks were reacted with O-(2aminoethyl)-O′-methylpoly(ethylene glycol) (monoaminoPEG) of different molecular weights (20 000, 10 000, 5000, 2000, and 750 g/mol). Compositional details on the aminoPEGs used are summarized in Table S2 (Supporting Information). After the coupling reaction, the modified polymer networks were extensively washed with DMF to remove nonbound PEG chains and then dried. Finally, the thicknesses of the modified samples were determined by ellipsometry. Using the ratio between the thicknesses before (d1) and after grafting (d2), one can determine the conversion/ degree of functionalization (f) of the reaction with the PEGs carrying the terminal amine group according to
3. RESULTS AND DISCUSSION General Description of the System. To study the binding reaction between reactive groups of the surfaceattached polymer networks and incoming polymer chains, reactive copolymers were synthesized, which were mainly based on MMA and which contained small amounts of succinimide ester (“active ester”) units (molar fraction x = 6−12%) along with benzophenone moieties (molar fraction y = 0.5−5%) as depicted schematically in Figure 1. The AE content and cross-
Figure 1. Schematic illustration of the chemical pathways used to generate the surface attached polymer networks and for subsequent aminolysis of these networks using poly(ethylene glycol)s with a terminal amine group.
linking density of the networks were varied by adjusting the comonomer ratio of MAC2AE and MABP, respectively. Compositional details on the synthesized functional polymers can be found in the Supporting Information (Table S1). As substrates silicon wafers were used, which were functionalized with self-assembled monolayers generated from a benzophenone group containing silane. Onto the thusobtained modified substrates the synthesized copolymers were 2697
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Figure 2. XP detail spectra obtained from surface-attached polymer networks prepared from the active ester polymers used in this study. In both spectra the open symbols represent the measured data in the regions of the C1s (a) and O1s (b) binding energies. The broken lines are simulations for the respective groups as indicated in the plots. The full line represents a superposition of these simulations.
M f + MNetwork (1 − f ) d2 = PEG‐network d1 MNetwork f=
d2 d1
influence of the concentration of the PEG solution on the degree of functionalization. Figure 3 shows that concentration
(1)
−1
MPEG‐network MNetwork
−1
(2)
Here MNetwork and MPEG‑network are the average molecular weight of the repeating units before and after grafting reaction, respectively. Note that MNetwork is calculated according to the chemical composition of the polymer determined by NMR according to Mnetwork = MAEx + MMMA(1 − x), where x is the molar fraction of the AE content of the polymer, MAE is the molecular weight of the active ester repeat unit (226 g/mol), and MMMA is the molecular weight of the MMA repeat units (100 g/mol). The MaBP content of most polymers used in this study was 1% or less and was, hence, neglected in this calculation. The average molecular weight after pegylation MPEG‑network is calculated according to MPEG‑network = [MAE(1 − f) + MPEG‑mono f ]x + MMMA(1 − x), where MPEG‑mono is defined as MPEG‑mono = MAE + MPEG − MAE‑OH. Here, MAE‑OH is the molecular weight of N-hydroxysuccinimide (101 g/mol) and MPEG is the molecular weight of the monoamino-PEG as determined by SEC. It should be noted that even small experimental errors in the determination of the active ester content have a rather large impact on the calculated values for the functionalization degree. The main reason is the active ester content of the different polymers is determined by NMR, and the accuracy of the integration of small NMR signals is limited. Therefore, the obtained values of degree of functionalization should not be taken as absolute but rather as guidance to compare similar experiments within series. The PEG content c after the reaction is given by c=
d 2 − d1 d2
Figure 3. Variation of the degree of functionalization of surfaceattached PMMA networks (9A-1B) containing active ester group as a function of the concentration of amino-PEG 5000 in solution. Conditions: 40 °C, 16 h, Et3N, solvent DMF.
above csolution ≈ 0.75 mmol/mL (concentration of the repeat units) yield identical degree of functionalization. Hence, all experiments described in the following were carried out at a concentration of csolution = 0.75 mmol/mL (33 mg/mL) which represents a compromise between maximizing the concentration of the free polymer to avoid any kinetic effects on the one side and the solubility of the free polymer on the other. The higher molecular weight PEGs were not sufficiently soluble in DMF to allow for generation of solutions with higher concentrations. In a second set of experiments the influence of the initial thickness (d1) of the polymer network on the PEG loading was studied by using reactive surface-attached networks with thicknesses between 80 and 220 nm (Figure 4). With increasing initial layer thickness of the network the degree of functionalization decreases (Figure 4a), while the absolute amount of added PEG is independent from the starting thickness of the network (Figure 4b). Reaction Kinetics. The kinetics of the surface reaction was studied by determining the grafted PEG thickness as a function of the reaction time for two monoamino-functionalized PEGs (PEG-750 and PEG-5000). As shown in Figure 5a,b, the kinetics of the aminolysis reaction are very fast for both PEGs. A plateau is reached after only a few minutes in both cases. The
(3)
This parameter describes how the layer thickness of the network d1 increases after aminolysis due to the incorporation of PEG chains (d2). This parameter is less sensitive to small experimental errors of the chemical composition of the polymers because it directly based on the thickness determination of the polymer layers, which can be measured rather accurately. To better understand the PEG grafting process and to establish standard conditions, we have first analyzed the 2698
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Figure 6. (a) Variation of the PEG content (●) and degree of functionalization (■) of surface-attached 12A-1B networks as a function of the molecular weight of the PEGs used. (b) Degree of functionalization as a function of the radius of gyration (M−0.571) of the polymer chains. Conditions: cPEG = 33 mg/mL, 16 h, 40 °C, Et3N, solvent DMF.
Figure 4. Variation of the degree of functionalization, normalized amount of grafted PEG, and layer increase due to reaction with the PEG (d2 − d1) as a function of the initial thickness (d1) of the polymeric network 9A-1B reacted with PEG 2000. Conditions: cPEG = 33 mg/mL, 16 h, 40 °C, Et3N, solvent DMF.
ability of the PEG chains to diffuse into these layers. The crosslinking density of the surface-attached polymer networks was varied through adjusting of the MaBP content between 0.5% and 5%. The dependence of the resulting PEG content for different networks is shown in Figure 7 as a function of the
Figure 5. (a) Added PEG layer thickness (d2 − d1) and (b) degree of functionalization as a function of reaction time for the aminolysis of the active ester groups in the polymer network 9A-1B with PEG 750 (●) and 5000 (■). Conditions: cPEG = 33 mg/mL, 40 °C, Et3N, solvent DMF. Initial layer thicknesses d1 ≈ 140 nm.
initial speed of functionalization is somewhat faster for the low molecular weight PEG than for the high molecular weight PEG. The reaction kinetics was also monitored by FTIR spectroscopy. In Figure S3 (Supporting Information), the FTIR spectra at different stages of the binding reaction are depicted. All spectra are normalized with respect to the carbonyl band at 1729 cm−1 (attributed to the MMA units) which remains constant during the reaction. After immobilization of PEG-NH2 several new bands from the attached PEG layer can be distinguished. The bands situated at 2949 cm−1 (νas(CH2)) and 2885 cm−1(νs(CH2)) correspond to the C−H stretching vibrations, and the band situated at 1115 cm−1 proceeds from backbone C−O−C vibrations of the grafted polymeric chains. Influence of the Molecular Weight of MonoaminatedPEGs. First results on the influence of the PEG molecular weight onto the grafting reaction are already evidenced from the kinetic data in Figure 5. More results on respective studies are shown in Figure 6. The degree of functionalization decreases strongly with the molecular weight of the penetrating chains and almost vanishes for the highest molecular weight (Figure 6a). However, at higher molecular weight of the incoming chains the exclusion becomes even stronger, and not only the degree of functionalization but also the mass of the attached polymer decreases (Figure 6b). Influence of the Cross-Linker Content. The cross-linking density of the surface-attached polymer networks determines the swelling of the layers. Accordingly, it will also influence the
Figure 7. PEG content of modified networks 6A-xB (x = 0.5%−5%) after aminolysis as a function of the molecular weight of the aminoPEG prepared from polymers containing different amounts of benzophenone moieties. Conditions: cPEG = 33 mg/mL, 16 h, 40 °C, Et3N, solvent DMF.
molecular weight of the PEGs. In all four curves a maximum of the amount of bound PEG is observed. This maximum becomes less pronounced and shifts toward lower molecular weight PEGs as the cross-link density of the network increases. The degree of functionalization of the active ester network as a function of the cross-linker contents at a constant molecular weight of the incoming chains is shown for the binding of PEG 2000 (Figure 8). It is observed that the penetration and binding decreased strongly in a nonlinear fashion with the cross-linker contents. While at 0.5% cross-linker contents the degree of functionalization by the amino-functionalized PEG was roughly 60%, at 5% BP contents the degree of functionalization was only 20%.
4. DISCUSSION The most important results of the experiments of the functionalization of NHS-ester group containing swollen polymer networks with poly(ethylene glycol)s carrying a terminal amine group can be summarized as follows: • For a given molecular weight of the free polymer, the maximum degree of functionalization decreases linearly with the initial thickness of the surface-attached networks. Indeed, 2699
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As far as the kinetics of the process is concerned, the incoming free polymer chains must diffuse into the layer against the concentration gradient built up by the high segment density of the polymer network. This phenomenon leads to a very strong slowing down of the process with increased segment density of the surface and is known for all “grafting-to” reactions,1,26,27 so that penetration becomes unfavorable both for kinetic and thermodynamic reasons. The diffusion of penetrating polymer chains into a polymeric network and the subsequent grafting reaction are expected to depend strongly on the shape of the segment density profile of these scaffolds. Close to the surface the segment density of the network is high, as the swelling of surface-attached polymer networks is strongly reduced compared to that of their threedimensional counterparts, i.e., nonattached polymer network.22 At some distance away from the surface the segment concentration drops steeply to that of the polymer solution. A more or less steplike profile is obtained with a rather narrow interphase between the network and the contacting polymer solution. If we view now the local perturbation of the segment density concentration profile (ϕnet(z)) at depth z within the network due to the insertion of a free polymer chain (ϕpoly(z)), diffusion of penetrating chains into the polymer networks will only occur at those locations which are below a certain critical segment concentration value, that means, ϕnet(z) < cpoly. The schematic representation of the concentration profile for a surface attached network is depicted in Figure 9. It should be
Figure 8. Variation of the degree of functionalization of 12A-xB networks by PEG2000 as a function of the benzophenone content. Conditions: cPEG = 33 mg/mL, 16 h, 40 °C, Et3N, solvent DMF.
the number of bound PEG chains remains constant regardless of the thickness of the PMMA carrier layer (Figure 4). • The yield of the binding process, i.e. the degree of functionalization of the NHS groups, shows a strong dependence on the molecular weight of the PEG chains (Figure 6a). The immobilized amount remained quite small, in the case of high molecular weight oligomers. For instance, for PEG 20000 the degree of functionalization is only around 3%. • Despite this monotonic decrease in functionalization, a maximum is found if the immobilized amount of PEG is plotted as a function of the PEG Mw (Figure 4b). This indicates a least two mechanisms that determine the overall reaction behavior. • Finally, the kinetics of the grafting reaction is very fast. The formation of tethered layers is complete after a few minutes, and no further significant variation of the thickness has been detected for longer times (Figure 5). When no specific strong interactions between network and penetrant occur and thus enthalphic contributions can be neglected, the process is determined by the difference ΔS between the entropy of mixing ΔSmix and entropy gains/losses due to conformational changes ΔSconf or in other terms: ΔG = ΔH − TΔS, with ΔH ≈ 0, one obtains ΔG = T(ΔSmix − ΔSconf). Thus, due to the high entropy of mixing, small molecules can easily diffuse into the scaffold generated by the surface-attached network. Therefore, in these cases the degree of functionalization, which is obtained in the grafting reaction, is high, and quantitative conversion can be obtained as shown for surfaceattached polymer brushes.18 However, when the molecular weight of the incoming molecules increases, the entropy of mixing becomes smaller which strongly reduces the driving force of mixing process. On the other hand, it is known that surface-attached polymer networks are anisotropically swollen.22 When now chains penetrate the network, they need to stretch from the coil shape they assume in solution into the stretched conformation. This process is accompanied by a significant loss in entropy. If this energy loss cannot be compensated by an energy gain due to enthalphic interactions, the penetration process becomes unfavorable. In addition, if additional chains become attached to the network, the network (sub)chains have to stretch further, which is accompanied by an additional entropy loss so that the total entropy loss of the system presents a considerable energetic cost.20,21,24,25 Thus, it is not surprising that the degree of functionalization decreases proportional to the size of the molecules/radius of gyration as depicted in Figure 6b.
Figure 9. Schematic representation of the segment density profile of a surface-attached polymer network. The shaded area represents the interpenetration zone. The dashed line represents the solution concentration of the penetrating polymer.
noted that the width of the penetration zone represented by the shaded area in Figure 9 is intrinsically related to the cross-linker content, the concentration of the polymer in solution, and the quality of the solvent. As a consequence, the extent of functionalization depends (at low concentrations) very strongly on the concentration of polymer in solution as can be seen in the low concentration regime of Figure 3. As the concentration of the polymer in solution is further increased, the penetration zone increases only very little due to the almost steplike nature of the profile, and the amount of grafted polymer becomes independent of the solution concentration (Figure 3, higher concentrations). When the film thickness is increased, the segment density profile remains essentially unchanged. Thus, the size of the penetration zone remains also more or less the same. In agreement with these theoretical considerations, the experimental data in Figure 3b show that the amount of polymer 2700
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PEG-NH2, when the cross-linker content increases as shown in Figure 6. Therefore, in the case of more densely cross-linked networks and high molecular weights of incoming linear polymer, the expected shape of the volume fraction at the interface should be symmetric, implying negligible penetration, and the binding reaction is confined to the outer edge of the interface. This binding reaction at the periphery without significant penetration into deeper regions of the network allows us to explain the rather fast kinetics of the modification reaction. Intuitively, one might expect that the reaction is rather slow as the poylmer chains have to “wiggle” into the network, and the diffusion coefficients for the penetration of polymer chains into nonattached polymer networks are rather small. However,25,29 as the reaction occurs only in the easily accessible outer perimeter of the network, the grafting reaction of surfaceattached networks can be expected to be very fast (Figure 4). On an additional note, any grafted polymer in the outer sections of the network will sharpen the segment density profile further, which will in turn even more strongly prevent the binding of additional chains to inner parts of the networks, which terminates the reaction even faster. It is interesting to compare these findings on surface attached polymer networks with the situation for chemically similar polymer brushes. Previously, we have described the reactions of active ester group containing polymer brushes toward free polymer chains, where the system was more or less chemically identical to the one investigated here.18 Brushes are strongly stretched away from the surface and are thus also quite difficult to be penetrated. However, as the bulk of the surface-attached network is already completely inpenetrable for the free chains, the much stronger stretching of the chains which is present in the brush system cannot change this situation. Less than zero penetration is simply not possible. So the question boils down to the size of the penetration zone of the two systems. The shape of the segment density profile of a polymer brush is characterized by a parabola with a more or less pronounced exponential tail, as a consequence of the polydispersity of the system.30 Thus, the penetration zone of polymer brushes is also small, but slightly larger than that of the corresponding surfaceattached networks, which swell in a rather blocklike fashion. As a consequence, penetration and binding to polymer networks is even lower than that of polymer brushes. In addition, the penetration of surface-attached polymer networks by free chains is different from that of polymer brushes in one additional respect. When the molecular weight of the incoming chains is small compared to the mesh size, the system is dominated by size exclusion effects, so that the mass of attached polymer goes through a maximum, which is quite different from that of polymer brush systems. The influence of this parameter and the location of the maximum is not only related to the molecular weight and shape of the free polymer but also depends strongly on the content of the cross-linker of the network.
bound in the grafting process is independent from the initial layer thickness so that the degree of functionalization decreases with increasing layer thickness. The situation is schematically shown in Figure 10. The ratios of the interpenetration area A to
Figure 10. Schematic representation of density profiles for two polymer networks with different initial thickness but constant interpenetration areas A and B at the layer periphery.
the total area of box A and of the interpenetration area B to the total area of box B linearly decreases when the initial thickness of the polymer increases. When the progress of the tethering process is limited by the diffusion of the penetrating chains, it should be intrinsically related to the radius of gyration of the polymers. The scaling law of this parameter in the investigated molecular region is roughly Rg ∼ √M (or more precisely Rg ∼ M0.571 according to the literature28). Indeed, there is a fairly linear relation between the functionalization degree and the radius of gyration of the incoming polymer chains, as shown in Figure 5c for small Mn (
