âGrafting-fromâ Method - American Chemical Society

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A simple preparation of photoactive glass surfaces allowing coatings via the “grafting from” method Marco Sangermano, Monica Periolatto, Micaela Castellino, Jieping Wang, Kurt Dietliker, Joëlle Levalois Grützmacher, and Hansjörg Grützmacher ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05822 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 17, 2016

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ACS Applied Materials & Interfaces

A simple preparation of photoactive glass surfaces allowing coatings via the “grafting from” method Marco Sangermano,[a*] Monica Periolatto,[a] Micaela Castellino,[b] Jieping Wang,[c] Kurt Dietliker,[c] Joëlle Levalois Grützmacher, [c][d] and Hansjörg Grützmacher[c][ e]. [a] Politecnico di Torino, Dipartimento di Scienza Applicata e Tecnologia, C.so Duca degli Abruzzi 24, 10129 Torino, Italy. [b] Center for Sustainable Futures - CSF@Polito, Istituto Italiano di Tecnologia, Corso Trento 21, 10129 Torino,Italy [c] Department of Chemistry and Biosciences, ETH Zürich, 8093 Zürich, Switzerland.[d] Université Antilles Guyane, BP 250, 97157 Pointe à Pitre Cédex, Guadeloupe, France.[e] Lehn Institute of Functional materials (LIFM), Sun Yat-Sen University, 510275 Guangzhou, China.

KEYWORDS. surface grafting, photopolymerization, glass functionalization ABSTRACT: A simple and straight forward synthesis was developed to prepare the siloxy-substituted bis(acylphosphane)oxide 4-(trimethoxysilyl)butyl-3-[bis(2,4,6-trimethylbenzoyl)phosphinoyl]-2-methyl-propionate TMESI2-BAPO. This new photoinitiator was successfully fixed to glass surfaces. Subsequent irradiation with UV light in the presence of either a partially fluorinated acrylate or a specifically synthesized polysiloxane containing polymerizable acrylate functions allowed to generate polymer chains which grew from the surface in an efficient radical polymerization process (“grafting-from” procedure). Durable hydrophobic surfaces were prepared which have contact angles between 93° and 95°. The silanization process with the photoinitiator and the grafting process were followed and analyzed with various techniques including high-resolution X-ray photoelectron spectroscopy (XPS).

Introduction Materials that have excellent bulk physical and chemical properties do often not possess the surface characteristics required for a specific application. For this reason, surface modification that can transform these materials into valuable finished products has become an important topic both in academic research and for industrial applications.[1] [2] [3] [4] These modifications are aimed at controlling substrate-environment or substrate-organism interactions by designing structures that are functionally appropriate, efficient to apply end environmentally compatible. Target functionalities include a wide range of different features, such as for example controlled wettability, improved laminate strengths in composite materials, repulsive properties for antifouling surfaces, binding and resorption capabilities for drug delivery, or minimal protein absorption for medical and microfluidic analytical devices. Surface modification can be achieved by two different approaches: Physical modification or chemical modification. While physical adsorption of a polymer on a substrate is the simplest way of surface modification, the surfaces thus obtained are often thermally unstable, withstand only low shear forces and may be easily displaced by chemicals or proteins and cells. Chemical modification, achieved by the grafting of a suitable surface layer onto

the bulk material, is more efficient in providing stable systems due to the formation of covalent bonds between the two materials. This is thus the preferred approach, if the coated object has to withstand extended use under practical conditions. There are two different concepts for grafting a thin polymeric film onto a surface. Preformed polymers possessing suitable functional groups can be chemically linked to the surface by the reaction of these groups with reactive sites on the surface (grafting-to method). Alternatively, grafted polymer chains can be produced in situ by a polymerization reaction starting from an active site on the surface (grafting-from method, also known as surface-initiated polymerization). Because of the absence of suitable functional groups on most surfaces, both surface grafting concepts commonly include two reaction steps: (i) Surface activation leading to the generation of active sites on the surface that can be used in the grafting process, and (ii) the actual grafting process. Depending on the type of the bulk material, different surface activation processes are possible, such as the generation of reactive groups by chemical reactions,[5] [6] UV-irradiation, high-energy electron, γ-ray or plasma treatment, corona discharge or ozone exposure. From a practical point of view, the selected method should be simple to apply and not damage the bulk material.

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While a pre-formed polymer is used for grafting-to process, a wide variety of polymerization processes[7] are available for the grafting-from approach, including for example ring-opening polymerization (ROP), ring opening metathesis polymerization (ROMP)[8] or radical polymerization. The latter may be further tuned by using one of the well-established techniques for controlled polymerization, such as atom transfer radical polymerization (ATRP),[9] nitroxide mediated radical polymerization (NMP)[10] [11] or reversible addition-fragmentation chain transfer (RAFT).[12] While the polymerization can be initiated by a thermal initiator or directly by radicals formed by high energy treatment of the surface, light induced triggering of the grafting reaction has recently found increasing interest. This technology is well suited for a grafting process, since the layer thickness of the grafted material is usually small and light penetration down to the interface with the bulk material easy. The method is energy efficient since heat is not required, which would warm up not only the surface layer, but also the bulk material. Furthermore, using irradiation sources such as LEDs that do not emit heat, also heat-sensitive bulk materials can easily be grafted without damage. The most outstanding feature of light-induced grafting processes is, however, the fully spatio-temporal control of the grafting process, allowing an easy access to structured surfaces. Glass is a very useful material with widespread uses both in bio-medical and industrial applications. Several approaches to modify the surface of glass to meet individual requirements have been reported in the literature. Different approaches are for example aimed at preventing loss of sensitivity in molecular diagnostic or immunological assays by nonspecific binding of the analyte to the surface of glass vials, or producing antifouling surfaces for drug delivery vehicles. Glass surfaces have been modified by grafting-to approaches, e.g. by attaching poly(ethylengylcol) polymers,[13] dendritic polyglycerol,[14] branched poly(ethylenimine) modified with trimethoxysilyl anchoring groups, or chitosan.[15] The grafting-from concept has been realized using different thermally induced polymerization processes. Grafting of different functionalized monomers has been used to achieve protein and cell-resistant surfaces with non-biofouling properties.[15] [16] [17] [18] Poly(ethyleneimine) layers containing polycationic moieties have been grown onto glass surfaces in order to obtain antimicrobial surfaces.[19] For engineering applications, the surface of glass fibers has been modified in order to improve the compatibility with the organic matrix in glass-fiber reinforced construction materials, thereby reducing delamination failures during use. Both grafting-from polymerization with different monomers,[20] and the grafting-to attachment of functionalized hyperbranched polymers has been used.[21] Most grafting processes on glass reported so far are based on thermal polymerization or condensation reactions, and only few light-triggered applications are reported in

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the literature. A mask-less photolithographic process has been used to produce protein chips on glass, but the photolithographic pattering step was applied on the already grafted material containing a spacer with photolabile caged amino group that is deblocked upon irradiation.[15] A photo-triggered approach using the photochemically driven quasi-living iniferter (initiator-transfertermination) technology, has been reported.[22] While this proceeding allows for the design of sophisticated spatioresolved three-dimensional molecular architectures on glass, this process is far from being easy to apply. Thus, there is a need for a simple light-induced grafting process applicable on glass. Bis(acyl)phosphane oxides (BAPO) are a well-known class of highly efficient photoinitiators [23] [24] that have found widespread use in industrial applications including pigmented and clear coatings, UV-curable printing inks, adhesives, electronic devices and dental restoring material. Surfaces decorated with covalently bound bis(acyl)phosphane oxide derivatives are thus ideally suited for photoinduced grafting from processes, as it was demonstrated in earlier work for grafting of fluorinated acrylates onto cotton fabrics using a tri(methoxy)silyl substituted BAPO derivative.[25] Broad use of this concept was, however, hampered by the tedious synthetic access to the functionalized BAPO compounds. Recently, a novel synthesis of functionalized bis(acyl)phosphane oxides has been developed that allows an easy access to a wide variety of functionalized bis(acyl)phosphane derivatives via a stable bis(mesitoyl)phosphane intermediate.[26] This concept has been used to attach BAPO moieties onto cellulose nanoparticles, which were used to produce nanoparticles with a high compatibility with an organic matrix by the light triggered grafting of poly(methyl methacrylate) chains onto the surface. Here we report the synthesis of novel BAPO derivatives that can be linked to oxide-based inorganic surfaces and demonstrate its use for the facile coating of glass surface with organic polymers. Experimental Part Materials Chemicals: 1H,1H,2H,2H-perfluorooctyl acrylate, tertbutyl hydroperoxide (5.5 M in decane) and dimethoxydimethylsilane were purchased from Aldrich and used as received. 3-(Trimethoxysilyl)propyl methacrylate (1) and 1,1,3,3-tetramethylguanidine were obtained from Acros and ABCR, respectively. DME, n-hexane, and toluene were degassed and purified using an Innovative Technologies PureSolv system. Deionized water was used in all experiments.

Synthesis of 3-(trimethoxysilyl)propyl 3-[bis(2,4,6trimethylbenzoyl)phosphinoyl]-2-methyl-propionate (4)

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The synthetic procedure of the new siloxy-substituted BAPO (TMESI2-BAPO, 4) is shown in Scheme 1. A solution of bis(mesitoyl)phosphane 1 (6.86 g, 21 mmol, 1.05 eq.)[27] [28] and 1,1,3,3-tetramethylguanidine TMG (0.25 mL, 2 mmol, 0.1 eq.) in 50 mL dimethoxyethane (dme) was prepared in a 100 mL Schlenk flask. 3(Trimethoxysilyl)propyl methacrylate 2 (4.78 mL, 20 mmol, 1 eq. ) was added. After stirring for 18 h at 60 °C, the solvent was removed under reduced pressure. The yellow oily residue which consists of 3 was dissolved in toluene (40 mL) and tert-butyl hydroperoxide (4 mL, 22 mmol, 1.1 eq., 5.5 M in decane) was added dropwise at 0 °C. After stirring vigorously at r.t. for 4 h, the solvent was removed under reduced pressure. The obtained yellow oil was washed with hexane (3 x 20 mL) and dried under high vacuum for 12 h to yield 10.64 g of the desired product 4 (18.0 mmol, 90%). 1

H-NMR (300.13 MHz, C6D6, 298 K): δ [ppm] = 0.51-0.58 (m, 2 H, SiCH2,), 1.17 (d, 3 H, CHCH3, 3JHH = 6.90 Hz), 1.621.74 (m, 2 H, SiCH2CH2), 1.95 (s, 6 H, p-CH3 Mes), 2.35(s, 6 H, o-CH3 Mes), 2.41 (s, 6 H, o-CH’3 Mes), 2.40-2.46, 2.913.11 (m, 3 H, PCH2CHCO), 3.39 (s, 9 H, Si(OCH3)3), 3.94 (t, 2 H, OCH2, 3JHH = 6.90 Hz), 6.54 (d, 4 H, Har Mes, 4JHH = 2.40Hz). 13

C{1H}-NMR (75.47 MHz, C6D6, 298 K): δ [ppm] = 5.7 (s, SiCH2,), 19.4 (d, 3JPC =8.53 Hz, CHCH3), 20.2 (d, 4JPC =6.41 Hz, o-CH3 Mes), 21.1 (s, p- CH3 Mes), 22.5 (s, SiCH2CH2), 29.7 (d, 1JPC = 54.41 Hz, CH2P), 34.4 (d, 2JPC = 3.55 Hz, CHCO), 50.3 (s, OCH3), 67.1 (s, OCH2), 129.5 (d, 4JPC = 4.98 Hz, C3,5 Mes), 136.4 (d, 3JPC = 28.98 Hz, C2,6 Mes), 136.8 (d, 2 JPC = 41.36 Hz, C1 Mes), 137.1 (d, 2JPC = 40.90 Hz, C1’ Mes),141.1 (d, 5JPC = 9.58 Hz, C4 Mes), 174.8 (d, 3JPC = 6.79 Hz, COCH), 216.7 (d, 1JPC = 54.64 Hz, COMes), 217.0 (d, 1JPC = 53.81 Hz, C’OMes). 29

Si NMR (59.64 MHz, C6D6, 298 K) from 1H - 29Si HMQC: δ [ppm] = -42.3 ppm.

31

P-NMR (121.49 MHz, C6D6, 298 K): δ [ppm] = 24.7 ppm (q, 3JPH = 10.21 Hz). IR (ATR [cm-1]): 2942 (w), 2840 (w), 1732 (m), 1673 (m), 1607 (m), 1456 (m), 1192 (s), 1081 (s), 849 (m), 815 (s), 616 (m), 444 (m). UV/Vis λ[nm] = 232 (sh.), 282, 353, 384. ESI MS: [M + H]+ m/z = 591.2538, meas. 591.2537. Synthesis of acrylated polydimethylsiloxane (7) 3-(Trimethoxysilyl) propyl methacrylate 2 (2.5 mL, 2.60 g, 10.5 mmol, 1 eq.), dimethyldimethoxysilane 6 (13.1 mL, 11.33 g, 94 mmol, 9 eq.), ethanol (60 ml), and aqueous hydrochloric acid (1 M, 15 ml) were added in a 250 ml round bottom flask. The solution was stirred for four days in a closed vessel and eight additional days in an open vessel. The residual solution was dried at 60 °C under high vacuum. The acrylate functionalized silicone 7 was

obtained as co(polymer) in form of a colorless oil as outlined in Scheme 4. 1

H-NMR (300.13 MHz, CDCl3): δ = 6.11 (br., 1H, acrylate), 5.54 (br., 1H, acrylate), 4.12 (t, J = 6.30 Hz, 2H, OCH2), 1.95 (br., 3H, CqMe), 1.79–1.72 (br., 2H, CH2CH2Si), 0.64–0.59 (br., 2H, CH2Si), 0.13–0.09 (br., 6H, SiMe2). 13

C{1H}-NMR 75.47 MHz, CDCl3): δ = 167.4 (CO), 136.5 (Cq), 125.0 (CH2, acrylate), 66.8– 66.5 (br., OCH2), 22.6 (br., CH2CH2Si), 18.3 (CqMe), 10.3–9.7 (br., CH2Si), 1.5–0.9 (br., SiMe).GPC: Mn = 1.85 kDa, Mw = 3.59 kDa, PDI = 1.94. Glass surface functionalization The glass surface functionalization was achieved as following (Scheme 2, step 1): A glass slide was immersed into 100 ml of an ethanol/water solution (1:3 vol/vol). 0.1 gr. of TMESI2-BAPO 4 was gradually added over 15 min and the mixture was stirred under heating (around 60-80 °C) for 4 hours to accomplish hydrolysis and condensation reactions with the silanol groups on the glass surface. The functionalized glass was washed with ethanol and dried at 80 °C overnight. Polymer grafting-from method On the functionalized glass surface, the fluorinated acrylic monomer (1H,1H,2H,2H-perfluorooctyl acrylate) 5 or the acrylate functionalized polydimethylsiloxane 7 were spin-coated to give a film. Subsequently, the surface of the samples were UV-irradiated under nitrogen for 1 minute or for 10 minutes (Scheme 2, step 2) using an Hamamatsu lamp with a light intensity on the surface of the sample of about 50 mW/cm2. After irradiation, the glass was either immediately extensively washed (experiments with 1 min radiation time) or thoroughly washed with acetone after one hour of curing time after the end of the illumination (experiments with 10 min radiation time). This procedure ensures the removal of all non-grafted polymers. Characterization methods Photopolymerization was induced using a medium pressure mercury lamp equipped with an optical guide (Hamamatsu, LC8). The light intensity on the surface of the sample was about 50 mW cm-2 (measured with EIT instrument). TGA analyses were performed in the range between room temperature up to 800 °C in air, with a Mettler TC 10A/TC15 TA controller instrument. Contact angle measurements were performed on a Kruss instrument equipped with a digital camera. A PHI 5000 Versaprobe Scanning X-ray Photoelectron Spectrometer (monochromatic Al Kalpha X-ray source with 1486.6 eV energy), was employed to check the material surface chemistry. High resolution (Pass energy: 23.5 eV) and survey spectra (Pass energy: 187.85eV) have been

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collected using a beam size of 100 μm. A combination of an electron gun and an Ar ion gun have been used as neutralizer system to compensate the positive charging effect during the analysis, due to not perfectly conductive surfaces. Fitting procedure and deconvolution analysis have been done using the Multipak 9.6 dedicated software. All core level peak energies were referenced to C1s peak at 284.5 eV (C-C/C-H sp2 bonds). Solution NMR spectra (1H, 13C, 29Si, 31P) were recorded on a Bruker 300 spectrometer operating at 300.13 MHz, 75.47 MHz, 59.64 MHz and 121.49 MHz, respectively. All 1H and 13 C NMR chemical shifts are reported relative to SiMe4 using the 1H (residual) and 13C chemical shifts of the solvent as a secondary standard. Infrared Spectroscopy (FTIR-ATR) of the dried samples were recorded using a Tensor 27 FT-IR spectrometer (Bruker, Switzerland). For each sample, the diamond crystal of an Attenuated Total Reflectance (ATR) accessory was brought into contact with the area to be analyzed. The contact area was a circle of about 1.5 mm in diameter. All spectra were recorded between 4000 and 600 cm−1 with a resolution of 4 cm−1 and 32 scans per sample. Gel permeation chromatography (GPC) measurements were performed on a Viscotek GPC unit. Detection: Triple Detector Array TDA 302 (refractive index, small and wide angle light diffraction and viscosity) and UV detector Viscotek 2500 (λ = 254 nm). The solution of a polymer sample is passed through two columns (Polymer Laboratories, PLgel 5 µm Mixed-C and PLgel 5 µm Mixed-D) to achieve a separation of polymers by size. The temperature was maintained at 35 °C during the measurement. The sample concentration was 1 mg/mL in DMF, the injection volume 100 µL and the flow-rate 1 mL/min. Data analysis: universal calibration with 13 monodisperse PS standards. Results and Discussion Herein, we report a straightforward method for a photoinduced "grafting-from" polymerization on a glass surface achieved by immobilizing bis(acylphosphane)oxides (BAPOs) as photoactive groups on the glass surface. The photoactive groups are subsequently used as initiating species for the photo-induced radical polymerization reaction resulting in a surface modified by organic polymers (see Scheme 2). The (trimethoxy)silyl propyl substituted bis(acyl)phosphane oxide TMESI2-BAPO (4) was synthesized following a straightforward and simple protocol as given in Scheme 1. The Michael addition reaction of the stable bis(mesitoyl)phosphane intermediate 1 onto 3(trimethoxysilyl)propyl methacrylate (2), followed by the oxidation of the phosphane, provides TMESI2-BAPO (4) as yellow oil in high yield.

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Scheme 1: Synthesis of TMESI2-BAPO 4.

Surface activation of the glass substrate was achieved by immersing the glass slide in an ethanol-water solution of 4 at 80 °C for four hours. TMESI2-BAPO 4 was thus grafted onto the glass surface via hydrolyses and condensation reactions taking place between the silanol groups present on the glass surface and the alkoxy silane groups in the TMESI2-BAPO 4 (step 1 in Scheme 2).

Scheme 2: Overal Schematic representation of the grafting-from process initiated by surface boundbis(acyl)phosphane oxides Thermogravimetric analysis (TGA) of the activated glass substrate showed a 5 wt% loss upon heating to 800 °C, which can be interpreted as weight loss due to the decomposition of the organic material that was fixed on the glass surface during the silanization process with TMESI2BAPO 4 (TGA curves not reported here). Contact angles of the untreated and silanized glass were measured in order to evaluate a change in wettability of the surface. The value of the water contact angle increased from 40° for

4


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the untreated glass up to 75° upon grafting with 4, thereby providing good evidence for successful fixation of 4 on the glass surface. For the confirmation of the efficiency of the silanization process, XPS analyses were performed on the treated substrates. From a survey analysis (see Figure 1, black lower line), the presence of C, O, Si and P was confirmed. The relative atomic concentrations. From High Resolution (HR) spectra (Figure 2, column A), the chemical state of each element can be inferred (see Table 2). From the deconvolution procedure applied to the C1s peak, three contributions can be identified: peak I corresponds to the C(C,H) bond, peak II to the C-O bond and peak III to the C=O bond.[29] From the P2p peak, two curves due to the 2p3/2 and 2p1/2 doublet are obtained by deconvolution, separated by 0.8 eV, which can be assigned to P linked to O atoms (132.9 and 133.8 eV respectively).[30] The Si2p peak has also been deconvoluted, resulting in two curves due to the 2p3/2 and 2p1/2 doublet (separated by 0.6 eV), which are attributed to silicon atoms in Si-O groups in trimethoxysilane.[31] These data clearly demonstrate the functionalization of the glass substrate with BAPO moieties. Figure 1: XPS survey spectra for the BAPO functionalized sample (lower, black line) and a sample after grafting with 1H,1H,2H,2H-perfluorooctyl acrylate 5 (BAPO+PFOA, upper, red line)

Table 2: Binding energies of C1s, Si2p3/2, P2p3/2 and F1s core levels lines from the deconvolution procedure and relevant references. Chemical Shift

Binding Energies (eV) C1s

Si2p3/2

P2p3/2

F1s

Ref.

C-(C,H)

284.5

[6]

C-O

286.2

[6]

C=O

288.6

[6]

C-F2

291.3

688.7

[32]

C-F3

293.6

688.7

[32]

Si-O

P-O

[31]

102.2102.5 132.7-132.9

[30]

Figure 2: XPS HR spectra for (A) the BAPO functionalized and (B) the 1H,1H,2H,2H-perfluorooctyl acrylate (5) grafted samples (BAPO+PFOA). C1s, P2p and Si2p peaks have been deconvoluted using Voigt functions and Shirley background removal. Fitting results are shown in Table 2. Scatter points are raw data, continuous line are reconstructed peaks and dot dashed line are fitting curves

Table 1: XPS relative atomic concentraacrylateion values obtained from HR spectra for the BAPO functionalized and the 1H,1H,2H,2H-perfluorooctyl acrylate (5) grafted samples. Atomic Concentration (at.%) C1s

O1s

Si2p

P2p

F1s

BAPO

51.8

33.8

12.2

2.3

-

BAPO+PFOA

51.9

23.2

6.0

2.3

16.6

5


ACS Applied Materials & Interfaces In order to test whether the BAPO coated glass surfaces can be used to initiate polymerization reactions in a “grafting-from” procedure, a thin layer of 1H,1H,2H,2Hperfluorooctyl acrylate (5) was placed on the glass surface by spin coating. Subsequently the glass surface was exposed to UV-irradiation under nitrogen for one minute (see Scheme 3). In this process, light will activate the BAPO moieties and generate radicals on the glass substrates.[33] [34] [35] In a Norrish type I cleavage reaction, the surface bound (O3Si)--P(=O)(COMes)2 groups undergo a stepwise cleavage according to:

step 1

step 2

*

. .

n n

hν

n Norrish Type I cleavage

R' = H, CH3

Furthermore, the contact angle with hexadecane showed a value of 85°, again typical of the oleophobicity of fluorinated polymer surfaces. These data give good evidence that the formation of poly(1H,1H,2H,2H-perfluorooctyl acrylate) brushes was achieved by the grafting-from process. These findings were further confirmed by XPS analysis. A survey spectrum (see Figure 1, red top line) confirmed besides C, O, Si, P also the presence of F, in relative atomic concentrations reported in Table 1. From the HR spectra (Figure 2, column B) the chemical state of each element reported in Table 2 can be obtained. From the deconvolution procedure applied to the C1s peak, the same three curves as already shown for the sample coated with BAPO 4 alone were obtained, with the addition of a new component at 291.3 eV (peak IV) which is due to the CF3 bond.[32] Also the F1s peak (not reported here) is detected, which shows the same chemical shift due to CF3 at 688.7 eV. From the deconvolution of the Si2p and P2p HR * peaks the same results as for the only silanized sample were obtained. In order to evaluate the percentage of grafted polymer as a function of irradiation time, samples exposed to fluorinated acrylate 5 were irradiated either for 1 minute or for 10 minutes, respectively. * Thermogravimetric analysis (TGA) for the grafted glass materials, either irradiated for 1 or 10 minutes, are reported in Figure 3.

X = CH2CH2(CF2)6CF3, [(SiR''O)x (SiMe2O)y ] n

. .

n

hν

R'' = OH or polysiloxane

step 4

step 3

*

*

*

*

n

n

n

n

n

Norrish Type I cleavage

Scheme 3: Detailed Schematic representation of the grafting-from process initiated by surface bound bis(acyl)phosphane oxides

Figure 3: TGA curves of the 1H,1H,2H,2H-perfluorooctyl acrylate (5) grafted samples obtained after 1 minute or 10 minute of UV-irradiation.

10 min 1 min Untreated Glass 100

In each of the reaction steps (1) and (3), a phosphoruscentered radical is formed which is chemically linked to the glass surface. Both radicals, which are generated in two subsequent photoinitiated cleavage reactions, efficiently initiate the polymerization of acrylic double bonds, resulting in two polymer chains growing from the surface-fixed phosphorus atom (reaction steps (2) and (4), see also Scheme 2). In the same photoinduced cleavage reactions (1) and (3) two mesitoyl radicals are concomitantly formed, which are not linked to the substrate. Although mesitoyl radicals are known to be less efficient in the initiation of acrylate polymerization than the phosphinoyl radicals, this inevitably leads to the formation of non-bound homopolymers as shown in both reaction steps (2) and (4). [36] [37] [38] These linear photopolymers can, however, easily be removed from the grafted glass surface by careful washing with acetone. Indeed, a further increase of the contact angle with water from 75° for the glass surface silanized with 4 up to 95° after UV radiation in presence of the fluorinated acrylate 5 indicates a successful grafting-from process. The achieved contact angle is typical for a fluorinated surface.

Loss weight %

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80

60

0

300

600

900

Temperature [°C]

When the glass was irradiated only for 1 minute, a very low enhancement of the weight loss compared to the one obtained from a glass substrate coated with (4) alone was observed. A weight loss of about 7% above 250 °C is attributed to the decomposition of the organic material grafted to the surface in the irradiation process. This means that only a very thin polymeric layer was grafted after a short irradiation time. On the other hand by irradiating the glass substrate silanized with (4) for 10 minutes in the presence of monomer 5 leads to coating which gives rise to a weight loss of

6


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40% (see red curve in Figure 3). This is additional and strong evidence that the grafting step has resulted in the formation of polymer brushes of considerable lengths and indicates a remarkable grafting percentage of about 40%. The effect of the irradiation time on the polymer thickness was further demonstrated by microscopy analysis. In Figure 4 the FESEM microscopy analysis of the irradiated glass for 1 minute and 10 minutes are reported. It is evident that 1 minute of irradiation produced a very thin polymer layer, while the glass irradiated for 10 minutes gave rise to the formation of a polymer with a homogeneous thickness of 60 µm. For further grafting studies, the acrylated poly(siloxane) 7 was synthesized as depicted in Scheme 4. In contrast to the perfluorinated monoacrylate 5, compound 7 is a multifunctional oligomer with Mn = 1.85 kDa, Mw = 3.59 kDa, and PDI = 1.94.

Figure 4: FESEM analysis of the coated glass with 1H,1H,2H,2H-perfluorooctyl acrylate (5) after 1 minute (left) or 10 minute (right) of UV-irradiation.

Scheme4: Synthesis of acrylated polydimethylsiloxane 7

In a grafting experiment similar to that with the perfluorinated acrylate, the acrylated siloxane oligomer 7 was used for the coating of a glass surface functionalized with TMESI2-BAPO 4. After irradiation with UV/Vis light under nitrogen and thorough and extensive washing of the

grafted glass substrate in order to remove unreacted monomers or oligomers, the contact angle for water increases from 75° for the silanized glass to 92° for the grafted material. This finding is good evidence that using the acrylate functionalized poly(siloxane) 7, a silicone layer is covalently fixed on the glass surface. This is achieved by the reaction of the photochemically formed surfacebound phosphorus radicals with the acrylate groups. In contrast to the perfluorinated acrylate 5, the oligomer 7 is a multifunctional acrylate, which can undergo crosslinking reactions during the photopolymerization process. Thus, while the grafting from reaction with the monoacrylate 5 produces linear polymer brushes on the glass surface, the same reaction with the multifunctional oligomer 7 is expected to form a crosslinked coating-like layer that is covalently bond to the glass surface. The efficiency of this process is demonstrated by the high hydrophobicity which is maintained after different rinsing processes of the surface.

Conclusions We report a straightforward method for the synthesis of a new siloxy-functionalized bis(acyl)phosphaneoxide TMESI2-BAPO (4). This BAPO molecule is suited to be chemically anchored onto hydroxyl-containing surfaces such as glass. The tethered molecule can be used as a photochemical trigger for inducing a "grafting-from" polymerization of radically polymerizable monomers. Depending on the selection of the monomer, good hydroor lipophobicities can be obtained in a simple two-step process. Due to the strong chemical bonding between the surface layer and the bulk material, an excellent chemical and physical durability of the surface is achieved. In our experiments, glass was selected as a model substrate. A commercially available fluorinated acrylic monomer and a newly synthesized acrylate functionalized poly(siloxane) oligomer were used for the grafting reactions. When the glass sample activated with TMESI2BAPO 4 was irradiated with UV/Vis light in the presence of these acrylates, surfaces with highly hydrophobic properties were obtained, as it is demonstrated by an increase of the water contact angle from 40° for the untreated glass to 93° respectively 95° for the siloxane- and fluoroalkyl-modified surfaces. The high stability of the surface modification was demonstrated by the insignificant loss of these properties during repeated washing processes. The grafting process was further confirmed by XPS analysis, which showed the presence of C, O, Si and P, after silanization, and F atoms after addition of the fluorinated monomer. The density of the grafting on the inorganic surface can be easily controlled by varying the experimental condition of the grafting step, so that the percentage of grafting is dependent on irradiation time, thus allowing for an additional control on the surface properties.

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The grafting method reported here provides an attractive possibility for the simple and efficient modification of inorganic materials possessing hydroxyl groups at the surface, such for example glass, ceramics or similar materials. Since the grafting process is triggered by light, the efficient spatio-temporal control of this medium can in principle be used for image-wise grafting processes which can be used to produce materials with structures surfaces.

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SYNOPSIS TOC: A simple and straight forward synthesis was developed to prepare the siloxy-substituted bis(acylphosphane)oxide. This new photoinitiator was successfully fixed to glass surfaces. Subsequent irradiation with UV light in the presence of partially fluorinated acrylate or a specifically synthesized polysiloxane containing polymerizable acrylate functions allowed to generate polymer chains which grew from the surface in an efficient radical polymerization process (“grafting-from” procedure).

step 1

step 2

n*

. . n

hν

n Norrish Type I cleavage

*

R' = H, CH3 X = CH2CH2(CF2)6CF3, [(SiR''O)x(SiMe2O)y ]n

. .

*

n

hν

R'' = OH or polysiloxane

step 4

step 3

*

*

n

n

*

n

n

n

Norrish Type I cleavage

*

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âGrafting-fromâ Method - American Chemical Society

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