Article pubs.acs.org/Macromolecules
“Grafting Through”: Mechanistic Aspects of Radical Polymerization Reactions with Surface-Attached Monomers Michael Henze, Daniel Mad̈ ge, Oswald Prucker, and Jürgen Rühe* Laboratory for Chemistry and Physics of Interfaces, Department for Microsystems Engineering (IMTEK), University of Freiburg, G.-Köhler-Allee 103, D-79110 Freiburg, Germany S Supporting Information *
ABSTRACT: In this paper, we investigate the influence of selected reaction parameters on the formation of surfaceattached polymer monolayers. The process is based on the use of self-assembled monolayers containing a polymerizable group and the performance of a bulk free radical polymerization reaction (“grafting through polymerization”). To this, methacryl moieties were immobilized on silica gel surfaces via a silane linker. During the polymerization reaction in a conventional way, free polymer is formed in solution. However, every now and then during chain growth also surface-attached monomers become integrated in the polymer chains, leading instantaneously to covalent linking of the growing polymer molecules to the surfaces. As more and more polymer chains become attached, this leads to the formation of a surface-attached polymer layer on the silica surface. Various sets of polymerization reactions were performed and the influence of a variation of temperature, reaction time and concentration of monomer, initiator, and immobilized monomer onto the layer formation are investigated. We propose a model of the layer formation process and the grafting-through process is compared to grafting-to and grafting-from techniques.
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INTRODUCTION
The surface properties of materials often need to be tailored to fit to the desired applications. Surface modification techniques which yield well-defined polymer coatings play an outstanding role in this context and are important, for example in biomedical applications, as protective coatings or for the improvement of adhesion between different materials.1−9 The attachment of polymers to surfaces can be carried out by physical methods or chemical binding of the molecules to the surfaces (chemisorption). While deposition of polymeric materials by physical techniques is usually a simple and generally applicable method, it suffers often from rather weak interactions between the surface and the coating material. Therefore, chemical attachment of polymers to the surfaces of materials to be modified, frequently called chemical grafting reactions, play an important role in the modification of solid surfaces. This is due to the fact that in the course of the surface modification process covalent chemical bonds between surface and coating are established, which are in many cases stable even under quite adverse conditions.10−12 A plethora of different methods for the covalent attachment of thin polymer films have been developed. They are often subdivided into “grafting to” and “grafting from” reactions. “Grafting to” (Figure 1a) refers to a situation where the covalent attachment is achieved by using preformed polymer chains containing functional groups, which allow a reaction with appropriate surface groups.13−16 These groups can be located either at the end or on the backbone of the polymer chain or be part of side chains. “Grafting from” (Figure 1b), frequently also called “surface© 2014 American Chemical Society
Figure 1. Schematic depiction of different techniques of chemical grafting of polymers to solid surfaces: (a) “grafting to”, (b) “grafting from”, and (c) “grafting through” consisting of the attachment step and further chain growth.
initiated polymerization” describes the growth of polymer chains from the surface using surface-attached/self-assembled initiator moieties.17−21 A third possibility which is also used frequently is based on the use of surface-attached monomer groups, or in other word, surfaces carrying a self-assembled monolayer containing polymerizable groups.9,22 For a polymer layer generation through this method, the growth of polymer chains is initiated in solution. During propagation, occasionally a surface-bound monomer unit can be integrated into the growing chains, which directly results in a permanent anchoring of the polymer chains. After that, chain growth can proceed and further free or surfaceattached units can be added to the growing chain. So in essence Received: December 20, 2013 Revised: April 7, 2014 Published: April 28, 2014 2929
dx.doi.org/10.1021/ma402607d | Macromolecules 2014, 47, 2929−2937
Macromolecules
Article
wavenumber range of 4000 to 400 cm−1 and a resolution of 4 cm−1. For measurements on modified silica the device was equipped with a DRIFT compartment and purged with nitrogen to reduce carbon dioxide and water bands from the gas phase. The incidence angle was 45° and dry potassium bromide was used for sample preparation. The spectra evaluation was done with the “Win-IR Pro” (v2.95) software. For GPC measurements an Agilent 1100 system equipped with the “WinGPC scientific (v6.20)″ software from PSS (Polymer Standards Service, Mainz, Germany) was used. In the case of polystyrene samples the SDV oligo column system was run with tetrahydrofuran (THF) as eluent and calibrated with polystyrene standard samples with a narrow molecular weight distribution. The separations were carried out at a flow rate of 1 mL·min−1. All calibration polymers were bought from PSS. 1 H and 13C nuclear magnetic resonance (NMR) spectra were obtained from a Bruker Avance 250 MHz spectrometer. All substances and polymers were dissolved in deuterated solvents (CDCl3, D2O or (CD3)2SO) and the solutions were measured at room temperature. For the UV/visible absorption measurements a Varian Cary 50 Bio spectrometer was used at a wavelength range from 800 down to 200 nm with variable scan rates. In most cases the “medium” speed program with 1 nm interval at 10 nm per second was used. Immobilization of Trimethoxysilanes onto Silica Gel. Silica gel (6.00 g) was suspended in 150 mL toluene in an atmosphere of dry nitrogen. A solution of MPS (2.08 g, 8.40 mmol) in 50 mL toluene and 10 mL triethyl amine (∼74.0 mmol) was added to the mixture. The reaction vessel was heated until the solution started to reflux (120 °C) and stirred for 3 h. To avoid grinding of the silica beads with magnetic stir bars, the reaction mixture was agitated with a circular shaking device (IKA KS 260 control) at 160 rounds per minute (rpm). The modified silica gels were centrifuged (Sorvall Super T21, 12500 rpm, 16.750g) for 15 min and washed subsequently with toluene, ethanol, ethanol/water (1/1 v/v, acidified with HCl), ethanol/water (1/1 v/v), ethanol and diethyl ether. The remaining colorless solid was dried in vacuo. SiO2−MPS. IR (DRIFT) ν [cm−1]: 3112, 2986, 2961, 2899, 2857, 1724, 1704, 1637, 1250−1000 cm−1. Anal. Found (LC700-ME): C, 7.30; H 1.15. General Procedure Polymerization of Polystyrene on Silica Gel with Surface-Attached Monomers. Silica gel was suspended in toluene under nitrogen atmosphere and the monomer and AIBN were added. The reaction mixture was degassed in vacuum through five freeze and thaw cycles and finally heated in a water thermostat (60.0 ± 0.1 °C). After 25 h of reaction time, the polymerization was stopped and cooled down. Then the polymerization solution was centrifuged at 12500 rpm for 20 min to separate the silica gel. After decanting, the remaining solid were put into a filter inside of a Soxhlet extractor and hot extraction with toluene was carried out overnight (∼18 h). The modified substrate was freeze-dried from benzene. All samples were analyzed by DRIFT and elemental analysis. The free polymers were precipitated from the reaction solution through a slow addition to 10x excess of methanol. The precipitates were filtered off and dried. The molecular weights were determined by gel permeation chromatography (GPC). SiO2−PS. IR (DRIFT) ν [cm−1]: 3080, 3061, 3028, 2920, 2852, 1724, 1602, 1493, 1450, 1200−1000, 762, 697 cm−1. Anal. Found (LC700-ME-PS): C, 32.85; H, 3.45%. Mn (free polymer): 36 000 (g/mol). Influence of Polymerization Time. Polymerization was performed as described in refs 17 and 18, using silica gel (2.00 g), polystyrene (66.0 mL), toluene (133 mL), and AIBN (9 mmol/L). After chosen time periods, samples (20 mL) were withdrawn from the reaction under nitrogen. The silica gel samples were centrifuged at 12500 rpm (15 min) to separate the silica gel. After decanting, the remaining solids were washed with toluene (∼40 mL) resuspended and again centrifuged. These steps were repeated (usually five times) until no precipitate formed when the decanted washing solution was added dropwise to excess methanol. Finally the silica gel with the polystyrene monolayer was filtered off and freeze-dried from benzene (7 wt % silica gel ≈ 100 mg silica gel in 1.63 mL benzene) The supernatant reaction mixture containing the nonattached polymer chains was slowly poured in excess methanol (>10× volume) under
the attachment of a polymer chain through such a process consists from a principle point of view of both “grafting to” and “grafting from” steps. If one stays within the same terminology such a process could be called a "grafting through reaction" (Figure 1). Despite the fact that such processes are widely applied, even in industrial applications, mostly for adhesion promotion (so-called application of a “primer”), surprisingly, the “grafting through” approach is from a scientific point of view not extensively explored and the details of the mechanism are not well understood. Several studies have been carried out so far to investigate the “grafting through” process. Hamann and Laible studied the attachment of polymer layers on oxide surfaces through surface-attached monomers.23 A model describing the overall reaction kinetics of the layer formation was developed by Chaimberg and Cohen,24 which, however, does not discuss the mechanistic details. In a study by Bialk et al., it was shown that integration of more than one surfacebound monomer is not favored.25 However, it is not yet understood what the relative importance of the “grafting to” and the “grafting from” steps are and what the limits of the reaction are. To study the mechanistic details of this “grafting through” reaction, the influence of several reaction parameters on the grafting density and the amount of grafted polymer of the resulting surface-attached polymers is investigated. More specifically, we study how a variation of temperature, concentration of initiator or monomer in solution, polymerization time, and the surface-concentration of surface-attached monomer influences the formation of the surface-attached polymer layer and elucidate what the implications of the obtained results are on the layer formation mechanism (Figure 2).
Figure 2. Schematic depiction of the overall reaction scheme for the generation of surface-attached polymer monolayer through a “grafting through” process. The layers are generated using a broad spectrum of parameters: monomer concentration, initiator concentration, reaction time, reaction temperature, and concentration of surface-attached monomer.
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EXPERIMENTAL SECTION
3-Methacryloylpropyl trimethoxysilane (MPS, Fluka) was used as received. Styrene (Fluka) was chromatographically purified over basic aluminum oxide, distilled under reduced pressure from copper(I) chloride (60 °C, 50 mbar) and stored under dry nitrogen at −20 °C until used. Toluene (Fluka) was dried and distilled using molten sodium and benzophenone as an indicator. After distillation it was stored over molecular sieve under dry nitrogen. Triethyl amine (Fluka) was dried and distilled over calcium hydride prior to use. All elemental analysis measurements were carried out on a Vario EL (Elementaranalysensysteme GmbH, Germany) at the Institute for Inorganic and Analytical Chemistry, University of Freiburg. Fourier transform infrared (FT-IR) spectra were recorded on a BioRad Excalibur FTS 3000 spectrometer collecting 128 scans within a 2930
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rapid stirring. The white precipitate was filtered off and dried in vacuum overnight. Influence of Monomer Concentration. In each of the five experiments with silica gel the total volume of solution was 40 mL with 400 mg MPS-modified LiChrospher and 20 mg (3 mM) of AIBN. The input volumes of monomer and solvent (styrene/toluene, v/v) were 1/0, 1/1, 1/2, 1/4, and 1/8. All mixtures were heated at 60 ± 1 °C for 38.5 h. Three experiments with styrene concentration variation and glass beads were carried out. The total volume of solution was 20 mL with 200 mg of MPS-modified GB80 and 10 mg (3 mM) of AIBN. The input volumes of styrene and solvent (v/v) were 1/0, 1/1, and 1/ 2. All mixtures were heated at 60 ± 1 °C for 24 h. Separation, isolation, and analysis follow the general procedure described above. Influence of Initiator Concentration. The polymerizations were carried out in 40 mL styrene/toluene mixtures (1/2 v/v) with 400 mg MPS-modified silica gel each. Five different initiator (AIBN) concentrations were delivered to the solutions: 10−4 mol·L−1 (0.7 mg), 10−3 mol·L−1 (6.6 mg), 3 × 10−3 mol·L−1 (19.7 mg), 10−2 mol· L−1 (65.7 mg), and 10−1 mol·L−1 (656.9 mg). Then all batches were heated at 60 °C for a duration of 26 h. To the solutions five different initiator (AIBN) concentrations were added, and after degassing all batches were heated to 60 °C and agitated via shaking for a duration of at least 24 h. The PS-modified silica gels were then isolated as described in the general procedure. Influence of Temperature. In each reaction vessel, 300 mg of MPS-modified silica gel was suspended in 20 mL of toluene, 10 mL of styrene and 43 mg (0.26 mmol) of AIBN under dry nitrogen. The mixtures were heated to the chosen temperatures in a range from 60 to 90 °C. The heating bath temperature was in all experiments controlled by a contact sensor inside the Schlenk tubes. For one set of polymerization reactions at different temperatures the polymerization time was varied so that it amounted to exactly one-half-life time of the initiator. For another set of experiments, the polymerizations were performed for a constant period of 21.5 h. For details, see Table 1.
Table 2. Conditions for the Coimmobilization of MPS and OS to the Surface of Silica Gels as a Function of the Solution Composition
60 62 65 70 80 90 60 62 65 70 80 90
decomposition rate (10−6 s−1)
V(OS) (μL)
m(LiChrospher) (mg)
V(toluene) (mL)
N(Et3N) (mL)
1/0 4/1 1/1 1/4 1/19 0/1
2000 190 120 50 15 −
− 50 130 200 240 300
6000 715 715 715 715 509
200 28 28 28 28 20
10.0 1.0 1.0 1.0 1.0 0.7
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RESULTS To anchor the monomer moieties to the surfaces of a silica gel, which was chosen as a substrate in this study, a trimethoxysilane with a methacrylate moiety was attached to the surface of the silica beads (Figure 3).26,27 When the surface area of the silica gel is assumed to be equivalent to the surface area as determined by BET measurements, the graft density of the surface-attached monomer can be calculated from elemental analysis to ΓSAM = 0.9 ± 0.1 μmol/m2 (reference experiments without surface-attached monomer were performed, in order to validate the results of elemental analysis; in all cases the amount of residual organic material on the silica gel was very low). Influence of Polymerization Time. To investigate the influence of the polymerization time on the formation of the surface-attached polymer layer, a series of reactions were performed under exactly the same conditions with increasing reaction times. The results show that a variation of the polymerization time has a strong influence on the amount of surface-attached polymer only at the beginning of the reaction. After polymerization times of 25−30 h, the increase of the grafted amount levels off (Figure 4a) and the amount of grafted polymer remains almost constant. The molecular weight of free polystyrene formed by the radical polymerization process in the reaction vessel remains constant during the whole course of the reaction (Figure 4) Assuming that the surface-attached polymer has a similar molecular weight as the free polymer, the grafting density of polystyrene in the surface attached layer can be calculated from the mass of surface-attached material. This assumption seems reasonable as the grafting density of the surface-attached polymer chains is in all samples rather low. A strong difference of the molecular weight between the two species (surface-attached and free polymer) can only be expected if the termination reaction is dominated by the reaction of two macroradicals. Such surface-confinement effects can be rather dominant under certain conditions in “grafting from” processes.19 However, as the grafting through reactions are run mostly under conditions, where transfer reactions or termination with low molecular weight radicals occur, reactions of two macroradicals seem to be not dominant and the differences in the molecular weight seem not very likely. It shows also at short reaction times a strong increase, and then levels off, so that a maximum of 20 nmol·m−2 (or in other terms 1 chain per 83 nm2) is reached (Figure 4c). Influence of Monomer Concentration. As the monomer concentration is an important parameter for any free radical polymerization, we investigated the effect of different monomer
polymerization time (h)
Constant Initiator Conversion (50%) 9 12 19 40 150 490 Constant Polymerization Time 9 12 19 40 150 490
V(MPS) (μL)
modified LiChrospher were placed in 30 mL styrene/toluene (1/2 v/ v) with 43 mg (9 mM) AIBN and heated at a temperature of 60 °C for 25 h.
Table 1. Conditions for the Graft Polymerization of MPSModified Silica Gels at Various Temperatures temperature (°C)
batch (MPS/OS) (n/n)
21.50 16.05 10.85 4.00 1.05 0.40 21.50 21.50 21.50 21.50 21.50 21.50
Influence of Surface Concentration of Polymerizable Monomers. MPS and octyl silane (OS) were added to the reaction solution in certain molar ratios (4/1, 1/1, 1/4, 1/19, pure OS). The composition of each batch is given in Table 2. The mixtures, heated to reflux by an oil bath (120 °C), were agitated by circular shaking for 3 h at 160 rpm. After the reaction was completed and between each washing step during work up, the modified silica gels were centrifuged (15 min at 12500 rpm). They were washed with toluene, ethanol, a mixture of ethanol/water (1/1 v/v, acidified with HCl), ethanol/water (1/1 v/v), ethanol and diethyl ether. Remaining colorless solids were dried for 18 h (overnight) at
