Polymer International
Polym Int 55:808–815 (2006)
Controlled growth of poly (2-(diethylamino)ethyl methacrylate) brushes via atom transfer radical polymerisation on planar silicon surfaces Paul D Topham,1 Jonathan R Howse,2 Colin J Crook,1 Andrew J Parnell,2 Mark Geoghegan,2 Richard AL Jones2 and Anthony J Ryan1∗ 1 Department 2 Department
of Chemistry, University of Sheffield, Sheffield S3 7HF, UK of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
Abstract: Progress in making pH-responsive polyelectrolyte brushes with a range of different grafting densities is reported. Polymer brushes of poly(2-(diethylamino)ethyl methacrylate) were synthesised via atom transfer radical polymerisation on silicon wafers using a ‘grafted from’ approach. The [11-(2-bromo-2-methyl) propionyloxy]undecyl trichlorosilane initiator was covalently attached to the silicon via silylation, from which the brushes were grown using a catalytic system of copper(I) chloride and pentamethyldiethylenetriamine in tetrahydrofuran at 80 ◦ C. X-ray reflectivity was used to assess the initiator surfaces and an upper limit on the grafting density of the polymer was determined. The quality of the brushes produced was analysed using ellipsometry and atomic force microscopy, which is also discussed. 2006 Society of Chemical Industry
Keywords: poly(2-(diethylamino)ethyl methacrylate); polymer brushes; ATRP; grafted from; X-ray reflectivity; ellipsometry
INTRODUCTION The potential for responsive polymers that can change their conformation according to external conditions is hugely important in soft nanotechnology. Changes in their environment translate directly into changes in the thickness of the polymer layer.1,2 One way in which these thickness changes have been exploited is to line nanoscale pores in a membrane with a pH-responsive brush, resulting in a membrane whose permeability alters with the acidity of the environment.3 Another recent application exploits the fact that a collapsed polymer brush offers a much more favourable surface for protein adsorption than an expanded one, to create a programmable surface to hold and release proteins on demand.4 The synthesis of polymer brushes has developed substantially over the last few years; however, many are made by conventional radical polymerisation.3 This yields polymers with large polydispersities (nonuniformity), as little control can be exerted over the final molecular weight of the polymer, and, in many cases, produces branched or cross-linked structures. The more recent polymer brush syntheses are an interesting collection of well-defined polymer chains attached by one end to a surface or interface.5 – 8 In order to produce well-defined brushes there are
two strategies that can be utilised. The ‘grafting to’ strategy involves the synthesis of the desired polymer in dilute solution by one of a number of living polymerisation techniques.9 A functional initiator is used, or a functionalising termination step, in order to introduce a reactive group at the very end of the chain. This group is then used to react with the desired surface, tethering the chain. This technique has the advantage of a more facile synthesis, by virtue of being a solution process and so easier to handle and keep homogeneous; and the resultant polymer is also easier to characterise for the same reasons. However, the approach has a significant drawback, in that once a critical number of chains have been tethered to the surface, they form a steric barrier which prevents further reaction. It is therefore only possible to produce relatively dilute brushes using this technique. The ‘grafting from’ strategy overcomes the problem of low brush density.10 A functionalised initiator is employed for the polymerisation, which is bound to the desired surface prior to the polymerisation, to produce monolayer coverage. The initiator is chemically bonded to the substrate using a variety of chemical approaches, such as silane or thiol linkages.8 Attachment of the chains close to one another causes
∗ Correspondence to: Anthony J Ryan, Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK E-mail:
[email protected] Contract/grant sponsor: Industrial Chemicals Industry (ICI) Contract/grant sponsor: Engineering and Physical Science Research Council (EPSRC); contract/grant number: GR/R77544; GR/S47496 (Received 25 July 2005; revised version received 17 October 2005; accepted 14 November 2005) Published online 16 May 2006; DOI: 10.1002/pi.2061
2006 Society of Chemical Industry. Polym Int 0959–8103/2006/$30.00
Growth of (2-(diethylamino)ethyl methacrylate) brushes on silicon surfaces
them to stretch away from the surface to avoid overlapping. This is only true if the distance between neighbouring attachment sites is sufficiently small to force the chains to adopt a stretched conformation.11 The polymerisation then takes place with the chains growing from the surface out into the monomercontaining solution. This provides a way to tailor the surface properties and interactions, which have many novel applications, such as colloid stabilisation,12 lubrication13 and adhesion.14 The constraint of this technique reduces the number of polymerisation techniques available, but one of the best is ‘living’ radical polymerisation, and in particular atom transfer radical polymerisation (ATRP).8,15,16 ATRP is a technique tolerant of a wide range of vinyl monomers, including some ionic monomers. During polymerisation the growing ends spend most of their time in a dormant state, occasionally transferring a halide to a metal salt to leave a radical, resulting in a burst of propagation, before returning to a dormant state. The result of this is that the concentration of active radicals at any one moment is tiny, resulting in minimal interaction between chain ends with the risk of termination. This is particularly important for the synthesis of dense brushes where the growing polymer chains are so close to each other. In this study poly(2-(diethylamino)ethyl methacrylate) (PDEA) brushes are synthesised from an initiatorcoated silicon surface to yield polymer brushes, which will become cationic or neutral depending on the pH. As well as control over the molecular weight, and hence brush thickness, control is also demonstrated
over the amount of initiator attached to the silicon, and so yielding brushes with varying grafting densities.
EXPERIMENTAL Materials The substrates used were single-crystal silicon in the form of wafers (Mitsubishi Research) or blocks (Crystran UK) polished to the (100) face, having a ˚ [11-(2native oxide thickness of approximately 15 A. Bromo-2-methyl) propionyloxy]undecyl trichlorosilane was synthesised according to the method used by Matyjaszewski et al.,17 as shown in Scheme 1. 2-(diethylamino)ethyl methacrylate (DEA; Aldrich, 99%) was dried over calcium hydride (ACROS, 93%) overnight and was distilled using a high-vacuum, short-path distillation set-up. Copper(I) chloride (Aldrich, >98%) was purified using repeated washings with glacial acetic acid over nitrogen, followed by subsequent washings with petroleum ether and ethanol, and was then stored over nitrogen before use. Triethylamine (Aldrich, 99%) was filtered through a 0.45 µm PTFE Acrodisc CR filter (PALL Life Sciences) immediately prior to use. N,N,N ,N ,N pentamethyldiethylenetriamine (PMDETA; Aldrich, 99%) and methyl-2-bromopropionate (Aldrich, 98%) were used as received. Initiator surface preparation The silicon surface was first rendered hydrophilic by cleaning in a bath of hydrogen peroxide (35 wt%), ammonia (30 wt%), and distilled water in a volume
Scheme 1. Synthesis and surface binding of the ATRP initiating species [11-(2-bromo-2-methyl) propionyloxy]undecyl trichlorosilane.
Polym Int 55:808–815 (2006) DOI: 10.1002/pi
809
PD Topham et al.
ratio of 1:1:5. This mixture was heated to 80 ◦ C and maintained for 10 min.18 The silicon surface was then rinsed with copious amounts of distilled water and then dried under a stream of nitrogen. Samples were then placed in a vacuum oven at 120 ◦ C for 30 min to remove any last traces of water. After removal from the oven, the dried samples were placed in a PTFE beaker with a tight-fitting lid containing 20 mL of a 1.5 µL mL−1 solution of the [11-(2-bromo-2methyl)propionyloxy]undecyl trichlorosilane initiator in dry toluene. Pre-filtered triethylamine (0.5 mL) was immediately added to the reactants. The triethylamine scavenges any acidic by-products created during the reaction, helping to create smoother surfaces.19 After the required period the silicon block was removed and subjected to two sequential washings with toluene, acetone, and ethanol before finally being dried under nitrogen. The treated surfaces were stored under vacuum until required for brush synthesis. Procedure for grafting PDEA from a flat silicon substrate via ATRP A range of different silicon-based PDEA brushes were synthesised at different polymerisation times and initiator densities. Studies to optimise the conditions were carried out in specifically designed vessels. It has been previously reported that free initiator is required to provide deactivating species, which help to achieve control of surface polymerisation.20 However, we have found that cleaner surfaces, with more control of the living nature of ATRP, can be achieved by the exclusion of free initiator for the synthesis of PDEA brushes. It has been shown that the competition between bound initiator and free initiator disrupts the living nature of the brush growth. This has also been confirmed by gel permeation chromatography (GPC) analysis of the free polymer created during the polymerisation, where bimodal peaks of broad molecular weight distributions have been observed. Described below and depicted in Scheme 2 is the optimised procedure for the brush synthesis on a silicon block of
dimensions 30 mm × 30 mm × 10 mm, suitable for neutron reflectivity experiments. Tetrahydrofuran (24 mL), dried over potassium/sodium benzophenone, was added to the nitrogen-purged vessel, to which copper chloride (240 mg, 2.4 mmol) and PMDETA (501 µL, 2.4 mmol) were subsequently added. The resulting dark green homogeneous solution was stirred using a magnetic follower and the apparatus was thoroughly purged with nitrogen. DEA monomer (48 mL, 240 mmol) was then added via a rubber septum, creating a light blue solution, and the vessel was purged with nitrogen for a further 15 min. Maintaining stirring and a minimum constant flow of nitrogen, the apparatus was lowered into an oil bath at 80 ◦ C. Ensuring that the silicon surfaces did not come into contact with the magnetic follower, the block coated with ATRP initiator and a reference wafer were carefully lowered into the solution. It is important to note that a reference wafer with a saturated initiator surface of dimensions 10 mm × 10 mm × 0.625 mm was also used for each polymerisation batch to assess the consistency of polymer chain growth. In the absence of facile means of measuring the polymer chain molecular weight, this then allows for some comparative assessment of the molecular weight of the polymer chains on the dilute brush under study. In the cases where free initiator was used, methyl-2bromopropionate (89 µL, 0.8 mmol) was then added via a rubber septum. After 5 h polymerisation time, the brushes were lifted from the now dark green solution and were cleaned with tetrahydrofuran, removing any undissolved catalytic material adsorbed onto the surface. On the occasions where free initiator was used, a THF Soxhlet system was required to separate selectively any free PDEA physisorbed to the surface from the grafted polymer. Tetrahydrofuran is a good solvent for PDEA therefore any free polymer present is dissolved, while only swelling the polymer brush, leaving the chains firmly anchored to the silicon surface. For kinetic studies, a specifically designed vessel was made to accommodate eight pieces of silicon at
Scheme 2. Synthesis of tethered PDEA on a flat silicon substrate.
810
Polym Int 55:808–815 (2006) DOI: 10.1002/pi
Growth of (2-(diethylamino)ethyl methacrylate) brushes on silicon surfaces
any one time with each piece in the same chemical environment. The vessel allowed the introduction and removal of each initiator-functionalised substrate individually, therefore providing comparative studies within the same reaction parameters. A schematic representation of the reactor is shown in Fig. 1. Characterisation methods X-ray reflectivity measurements of the initiator surfaces were made using a Bruker D8 Advance X-ray scattering system fitted with a G¨obel mirror on the ˚ (copper Kα) X-ray source emitting radiation at 1.54 A and running at 40 kV and 40 A. The sample size was 20 mm by 60 mm orientated with the longest side parallel to the beam. Reflectivity runs were collected over a 4 h period with step sizes sufficiently small enough to distinguish the features seen. Free polymer was analysed using GPC with one Phenogel 5 µm, 5 cm × 0.78 cm pre-column, two Phenogel 5 µm, 30 cm × 0.78 cm mixed-bed columns, ˚ one Phenogel 5 µm, 30 cm × 0.78 cm, 5 × 104 A single-porosity column and one 5 µm, 5 cm × 0.78 cm, ˚ single-porosity column, using Polymer 5 × 103 A Laboratories ‘‘Caliber’’ detection software against a linear polystyrene standard. All columns were purchased from Phenomenex.
The polymer brushes were analysed using ellipsometry. A Gaertner 116B ellipsometer using a He–Ne laser at 628 nm and 70◦ incidence angle was used. The resulting ψ and δ values were fitted using a single-layer model with a refractive index of n = 1.5 on a silicon substrate of n = 3.875, k = 0.018. Multiple measurements were taken over the sample surface and we present here the average values obtained with any associated deviation. AFM images were obtained using a Digital Instruments multimode Nanoscope 4 in tapping mode, using Olympus Oxide sharpened cantilevers with a resonant frequency of typically 300 kHz and a tip radius of 10 nm. The trichlorosilane sample was imaged with Ultrasharp non-contact silicon cantilevers (MikroMasch, Spain). The response of the brushes on silicon blocks to external stimuli (pH and salt concentration) is currently under investigation using neutron reflectivity. Preliminary findings of this investigation can be found elsewhere.21
RESULTS AND DISCUSSION Characterisation of initiator monolayer To assess the dynamics of the formation of the initiator layer on the surface, a wafer aliquot methodology was employed. For this, samples were prepared as described above, but the residence time within the silane solution was varied from 1 min to 1 h. The initiator layer thickness and density were then determined using X-ray reflectivity. For a detailed description of the reflectivity technique and its applications the reader is directed towards the extensive review by Russell.22 From the behaviour of the X-ray reflectivity measured as a function of momentum transfer [Q = (4π/λ) sin θ ], it is possible to extract a layer thickness, roughness, and an X-ray refractive index by optimising a model that best fits the data. In the case of X-rays the refractive index is given by n = 1 − δ + iβ (1) where
Figure 1. Eight-arm reaction vessel used to carry out kinetic studies of up to eight different silicon wafers. Each arm is thoroughly purged with nitrogen before introducing the initiators to the surface of the reaction mixture.
Polym Int 55:808–815 (2006) DOI: 10.1002/pi
δ=
ρi r0 λ2 NA (Zi + f ) 2π A i i
β=
ρi r0 λ2 NA (Zi + f ) 2π A i i
NA is Avogadro’s number, r0 is the classical electron radius (2.82 × 10−15 m), λ is the wavelength of the incident radiation, ρi is the density of element i with atomic weight Ai and atomic number Zi , and f and f are the real (dispersion) and imaginary (absorption) anomalous dispersion factors, respectively. From this it can be seen that there is a direct link between the X-ray refractive index and the electron density of the material. With knowledge of 811
PD Topham et al. Table 1. Results of fitting the X-ray reflectivity data to a single-layer model for varying initiator exposure times and the determined initiator density per unit area
Exposure time (min) 0 1 2 5 10 30 60
0.0 9.0 14.3 20.0 21.3 22.0 23.9
Electron density
(105
0.00 0.17 0.23 0.33 0.33 0.31 0.37
−2 A˚ )
Initiator density (bulk) (106 g m−3 )
Initiator density (surface) (mg m−2 )
Area per 2 molecule (A˚ )
Coveragea (%)
0.00 0.54 0.73 1.06 1.05 0.99 1.16
0.00 0.49 1.04 2.12 2.23 2.17 2.77
– 109 51.1 25.1 23.8 24.5 19.2
0 17.6 38.4 76.5 80.7 78.4 100
Percentage coverage = (Surface initiator density at time t/surface initiator density at time t = 60) ×100.
the chemical composition of this layer it is then possible to determine the density of this material. By determining the layer thickness and density of this layer a grafting density in terms of mass per unit area may be determined. This can then be converted to an area per molecule. These calculations have been carried out for the data obtained from a bare silicon substrate and silicon substrates that have had different exposure times to the initiator solution. The results are all shown in Table 1. The data were fitted to a single-layer model adjacent to a substrate of silicon using the Parratt formulae.23 The native oxide layer was neglected from the model due the similarity in refractive index of silicon and silicon oxide. The control piece of silicon was treated by the same procedure (cleaning and washings) but not exposed to the trichlorosilane solution. The reflectivity data for this sample were first fitted to determine the silicon electron density and its surface roughness. The parameters determined for this sample were used for all other samples. For initiator-treated surfaces, a single-layer model was used. Layer thickness, electron density and upper surface roughness were optimised until the best fit was obtained, as exemplified in Fig. 2. As the exposure time is increased there is an increase in the fringe amplitude and the fringe minima moves towards lower Q (scattering vector). This is indicative of an increase in the surface film density and an increase in the film thickness. The trend is borne out by the results of the fitting. This shows a significant increase in film thickness and initiator density between 1 and 5 min. After this, the reaction slows until completion at 60 min, i.e. when smooth monolayer coverage is achieved. All surfaces were further analysed using AFM and were shown to be extremely smooth ˚ with an average surface roughness no greater than 5 A for all the initiator exposure times (Fig. 3). Kinetic study of polyDEA brush growth Polymerisation time The kinetics of the ‘living’ polymerisation used to synthesise the brushes were investigated. Specific fourarmed and eight-armed reaction vessels (Fig. 1) were made to allow silicon wafers to be removed from the reaction mixture at any given time, without affecting the on-going polymerisation. The vertical symmetry 812
1013
Control 1 min. 2 min. 5 min. 10 min. 30 min. 60 min.
1012 1011 1010 109 108 107 106
Reflectivity
a
Initiator ˚ thickness (A)
105 104 103 102 101 100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8
0.0
0.1
0.2
0.3
0.4
0.5
Q/ Å-1 Figure 2. X-ray reflectivities for silicon substrates with exposure times of 0 (control piece) to 60 min. Successive data sets have been multiplied by factors of 100 to aid clarity. The solid line in each case is the fit to the data obtained via the Parratt simulations.
of the cell ensures that all initiator surfaces are in thermal and chemically equivalent environments. This is analogous to taking a homogeneous sample aliquot from an ATRP mixture during its polymerisation to assess its molecular weight progression. The siliconbased brushes were taken away for purification and analysis, whilst maintaining an inert atmosphere at the reactants’ surface, by the constant flow of nitrogen passing through the vessel. Brush thicknesses were measured by ellipsometry once the surfaces had been thoroughly cleaned using tetrahydrofuran and dry nitrogen. Figure 4 shows ellipsometry measurements taken at different polymerisation times for three different systems of varying initiator exposure times. This illustrates that the kinetics of brush thickness Polym Int 55:808–815 (2006) DOI: 10.1002/pi
Growth of (2-(diethylamino)ethyl methacrylate) brushes on silicon surfaces
˚ The Figure 3. Tapping-mode AFM images of the initiator monolayer on a silicon wafer exposed to the solution for 60 min, with a height scale of 9 A. ˚ grafted layer is 24 A˚ thick with an RMS roughness of 5 A.
500 450
Brush Height/ Å
400 350 300 250
Initial Exposure to Initiator:
200
30 mins 1 hour 17 hours
150 100 50 0
100 Polymerisation time/ minutes
1000
Figure 4. Brush height determined by ellipsometry on polyDEA-grafted wafers taken from three different syntheses of different initiator coverage, illustrating the effect of polymerisation time on brush thickness.
are affected only by initiator grafting density, as the growth pathways all follow the same trend. System 1 has an initiator monolayer coverage produced from
30 min exposure time to [11-(2-bromo-2-methyl) propionyloxy]undecyl trichlorosilane. System 2 was given 1 h and system 3 had 17 h of exposure time. The observed trend of brush thickness with polymerisation time is typical for these systems and has been reproduced several times. During the first 5 h of synthesis, the brush height (or thickness) increases, typical of bulk homopolymerisation via ATRP.24 – 26 This increase is relatively rapid at first, but slows down as the polymerisation progresses (indicated by near-linear dependence of the height against the logarithm of the time). As the PDEA chains grow longer, the chain ends become buried within their own coils, inhibiting facile access of an incoming monomer unit or the catalyst sites to the living PDEA chain ends, thus reducing the polymerisation rate.27 All PDEA brushes synthesised with short polymerisations times (
