Subscriber access provided by University of South Dakota
Article
“Grafting-from” Polymerization of PMMA from Stainless Steel Surfaces by a RAFT-mediated Polymerization Process Nico Zammarelli, Michael Luksin, Hannes Raschke, Roland Hergenroder, and Ralf Weberskirch Langmuir, Just Accepted Manuscript • DOI: 10.1021/la402870p • Publication Date (Web): 20 Sep 2013 Downloaded from http://pubs.acs.org on September 24, 2013
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 35
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
“Grafting-from” Polymerization of PMMA from Stainless Steel Surfaces by a RAFT-mediated Polymerization Process Nico Zammarelli,† Michael Luksin,† Hannes Raschke, § Roland Hergenröder, § Ralf Weberskirch*,† AUTHOR ADDRESS †Department of Chemistry and Chemical Biology, Otto-Hahn Str. 6; TU Dortmund, D-44227 Dortmund, §Leibniz-Institut für Analytische Wissenschaften - ISAS - e.V., Bunsen-Kirchhoff Str.11, D-44139 Dortmund KEYWORDS RAFT-polymerization, stainless steel surface, grafting-from polymerization.
* Corresponding author. Phone: +49 203 755 3863. E-mail:
[email protected].
†
Department for Chemistry and Chemical Biology, Otto-Hahn Str. 6; TU Dortmund, D-44227
Dortmund
§
Leibniz-Institut für Analytische Wissenschaften - ISAS - e.V., Bunsen-Kirchhoff Str.11, D-
44139 Dortmund
ACS Paragon Plus Environment
1
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 35
ABSTRACT: The synthesis of grafted PMMA homopolymer films is reported using a surface initiated reversible addition-fragmentation chain transfer (SI-RAFT) polymerization from a RAFT-agent immobilized on a silanized stainless steel surface. Therefore, stainless steel surfaces were hydroxylated with piranha solution followed by silanization with 3-aminopropylsilane (APS). The pendant primary amino groups of the crosslinked polysiloxane layer were reacted with 4-cyano-4-[(dodecylsulfanylthiocarbonyl)-sulfanyl] pentanoic acid N-hydroxysuccinimid ester to produce a surface with covalently immobilized RAFT agents. PMMA homopolymers of different molecular weights between 13.060 and 45.000 g/mol were then prepared by a surface initiated RAFT polymerization. Molecular weight (MW) and polydispersity index (PDI) were determined from sacrificial polymerization in solution. The different steps of stainless steel surface modification and the ultrathin films were investigated using atomic force microscopy (AFM), static, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance-infrared spectroscopy (ATR-IR) and ellipsometry.
ACS Paragon Plus Environment
2
Page 3 of 35
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Introduction. Stainless steel (FeCr-based alloys) describes an extremely versatile family of engineering materials due to its excellent mechanical and physical properties. Aside from many traditional areas of application today about 1 % of stainless steel is used in medical devices such as orthopedic implants1 and vascular stents2 due to their remarkable mechanical durability. A major disadvantage of such materials, however, is their limited ability to interact in a defined manner with their biological environment in vivo and e.g. support cell adhesion, proliferation or healing processes.3 Moreover, it is well known today that stainless steel is not corrosion resistant and can only be used as a temporary implant.4 A particularly attractive way to modify stainless steel surfaces is based on the use of organosilanes that allow tailoring of surface properties by their specific molecular structure and functional groups.5 The stability of the substrate-metal bond depends largely on the surface pretreatment methods and the deposition conditions forming either thick and dense films that are required for corrosion protection or thin polysiloxane networks with high adhesion strength to the surface.6 Hydrolytic stability of such polysiloxanes on inorganic surfaces remains a critical issue due to the polar nature of the Si-O-metal bond.7-9 In an attempt to prepare more stable surfaces A.Y. Fadeev et al. studied the hydrolytic stability of self-assembled monolayers based on long chain C18H37Si(CH3)2Cl and C18H37SiH3 compounds on Ti and Zr-surfaces at different pH-values ranging from 1 to 10 for one week at 65°C. The results indicated, however, no stability of the monofunctional silane and limited stability for the trifunctional silane reagents.10 Best results were found for long chain phosphonic acids (C18H37PO(OH)2) that interact strongly with metal oxides and form a densely packed selfassembled monolayer presenting a well ordered -CH3-surface. While such a surface showed excellent hydrolytic stability further functionalization remains difficult.11
ACS Paragon Plus Environment
3
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 35
An alternative approach to produce polysiloxane layers on the metal surface with the possibility of further functionalization may be achieved by grafting polymers on the metal surface. Graftingfrom polymerization has become a powerful method to modify inorganic and organic surfaces.1215
While first reports were based on the use of free-radical16 and anionic/cationic17
polymerization techniques, more recently controlled polymerization techniques such as ATRP,18 NMRP19 and RAFT20 have been intensively investigated. In the past years several papers dealt with the grafting-from polymerization from stainless steel surfaces by using ATRP by using different initiator and techniques to immobilize them to the surface.21-23 Although ATRP has many advantages the use of Cu-based catalysts remains a challenge especially when medical application may be envisaged. A potential solution is the grafting polymerization by using the RAFT process. First RAFT-mediated polymerization from surface-immobilized initiators was reported by T. Fukuda et al. where they investigated in detail the mechanism and kinetics of RAFT-mediated styrene polymerization on a silica surface.20 Fukuda could show that the enhanced recombination is specific to the RAFT-mediated graft polymerization and is due to the effective migration of radical on the surface by sequential degenerative (exchange) chain transfer. The presence of a sufficient concentration of the RAFT species in the bulk phase however helps effectively to maintain the concentration of dormant graft chains by the exchange reaction of a graft radical with a dormant free chain, thus keeping the graft polymerization under control even at high conversions. In the past years, RAFT polymerization has been successfully used to prepare polymer brushes using various monomers and various solid substrates such as silicon wafer24, silica particles25-26, Au nanoparticles27, CdSe nanoparticles28, and cellulose fiber.29-30 Two strategies were used to immobilize a RAFT-agent to a surface, either by the Z-group or the R-group. In the
ACS Paragon Plus Environment
4
Page 5 of 35
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
Z group approach, polymerization takes place it solution and in order to undergo the RAFT reaction during the course of polymerization the polymer needs to get close to the solid surface across the barrier of polymer brush layer, typically leading to a lowered graft density similar to a grafting-onto approach.31-33 On the other hand, the R group approach is essentially the same as the so-called grafting-from technique, wherein the RAFT process takes place near the free surface of brush layer. This is a clear advantage over the Z group approach leading to polymer brushes with a high grafting density.34-38 Two different approaches can be used to immobilize the RAFT agent via the R-group on the solid surface either by using a silane coupling agent bearing RAFT group to directly introduce initiating sites on solid surfaces or by silanization and RAFTagent immobilization in two consecutive steps. By using the latter approach we describe herein the first RAFT-mediated polymerization process of methyl methacrylate (MMA) from silane-CTA functionalized stainless steel surfaces. The synthetic procedure is shown in Scheme 1.
ACS Paragon Plus Environment
5
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
OH
MeO Si O Si O Si OMe O O O
MeO Si OMe OMe
OH
stainless steel
CN
stainless steel
Condensation
Piranha treated
CH3 CH2 11 S S S
H2N H2N H2N
H2N
OH
Page 6 of 35
O O O N
NHS coupling CH3 CH2 11 S S S O
+ free polymer
O
n
O
CH3 CH2 11 S S S O O
n
CN
CN O
O O HN H2N HN
CH3 CH2 11 S S S
CH3 CH2 11 S S S
CN
CN
O O HN H2N HN
O O MeO Si O Si O Si OMe O O O
C12H25
S
S S
OH CN
MeO Si O Si O Si OMe O O O stainless steel
stainless steel
AIBN, toluene, T = 70°C
Scheme 1. General procedure for the preparation of CTA-modified stainless steel surfaces and subsequent formation of PMMA-brushes by reversible addition-fragmentation chain transfer polymerization.
ACS Paragon Plus Environment
6
Page 7 of 35
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
2. Materials and Methods 2.1 Materials. Stainless steel 316l (foil, thickness of 1.5 mm, Fe/Cr-18/Ni-10/Mo-3) were cut into 1 cm 1 cm in size. Methyl methacrylate (MMA) (>99 %) monomer was purchased from Sigma-Aldrich. The monomer was washed three times with an aqueous solution of sodium hydroxide (5 wt %), followed by washing with deionized water until the solution was neutral. The resulting solution was then dried over anhydrous magnesium sulfate, distilled twice at reduced pressure. 3-Aminopropyltrimethoxysilane (97%), N-hydroxysuccinimide, N-(3Dimethylaminopropyl)-N-ethylcarbodiimide
hydrochloride
(EDC)
and
4-cyano-4-
[(dodecylsulfanylthiocarbonyl)-sulfanyl] pentanoic acid were purchased from Sigma-Aldrich and used without further purification. 2.2 SS316L Substrate Preparation and Cleaning. One side of the stainless steel substrates was polished with a P2500 silicon carbide paper by using a Buehler and Struers Planopol-3 machine, followed by 6 µm, 3 µm diamond paste and 50 nm silica pastes. Polished metal surfaces had a mirror-like and highly reflective appearance. In the final step the polished surfaces were sonificated in acetone for 30 min. Surface roughness of the stainless steel samples as determined by ellipsometric and AFM measurements was in the range of 2-3 nm from 40-50 nm before sample polishing.39 The polished substrates were oxidized using a Piranha solution 2:1 (v:v, H2SO4: H2O2) at 60 °C for 10 min.40 All the substrates were sonificated in acetone for 10 min and then blow dried with Ar. Prior to silane deposition, the coupons were cleaned ultrasonically in acetone at RT for 5 min then dipped in acetone at 60 °C for 5 min, rinsed with distilled water and blown dry with nitrogen. Silanization of the stainless steel surface with 3-aminopropyltrimethoxysilane (APS) was carried out according to a recent publication.6 APS solutions (1.2% v/v) were prepared with
ACS Paragon Plus Environment
7
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 35
a 25%/75% v/v mixture of ethanol and deionized water and used at their natural pH = 10.6. The solution was allowed to react for 1 min prior use which is sufficient for the alkoxy groups to start hydrolysis without significant formation of oligomers from condensation reactions. After cleaning and pretreatment, all substrates were subjected to silanization by dipping in the prepared APS solutions for 1 min followed by washing with ethanol. The APS films were blown dry with nitrogen and heated for 15 min at 100 °C in air. After this step, the APS films were rinsed in distilled water for 2 min in order to remove any weakly bound species. 2.3 Synthesis of the RAFT-NHS ester and coupling to the silanized stainless steel surface. RAFT-OSu was prepared by dissolving 4-Cyano-4-[(dodecylsulfanylthio-carbonyl)-sulfanyl] pentanoic acid (1 eq, 0,25 mmol) in DMF under Ar-atmosphere. N-hydroxysuccinimde (2,3 eq, 0,57 mmol) was added in the presence of EDC (1,4 eq, 0,35 mmol). The mixture was stirred for 12 h at room temperature and poured into saturated aqueous NaHCO3 solution, and the product was extracted with ethyl acetate. The organic layer was washed with water and saturated aqueous NaCl, dried (MgSO4), and concentrated under reduced pressure. The crude NHS ester was purified by column chromatography on silica gel using 1:2 ethyl acetate/ cyclohexane as the eluent. Yield 58% 1H NMR (CDCl3 ,500 MHz): δ = 3.34 (t, J=7.5 Hz, 2 H,5), 2.94 (ddd, J=9.6, 6.6, 2.9 Hz, 2 H, 2), 2.86 (s, 4 H, 1), 2.48 - 2.72 (m, 2 H, 3), 1.90 (s, 3 H, 4), 1.71 (dt, J=14.9, 7.5 Hz, 2 H, 6), 1.36 -1.46 (m, 2 H, 7), 1.20 - 1.35 (m, 16 H, 8-15), 0.89 ppm (t, J=7.0 Hz, 3 H, 16) 13
C NMR (CDC3 ,100 MHz): δ = 216.2, 168.5, 166.8, 118.4, 45.7, 36.9, 32.9, 31.7, 29.4, 29.3,
29.2, 29.1, 28.8, 28.7, 27.4, 26.6, 24.9, 25.3, 24.5, 22.4, 13.9 ppm. HR-MS (ESI) calculated C23H36N2O2S3 (M+H)+: 501,1917; found: 501,1910 Coupling of the RAFT-NHS ester to the APS-modified stainless steel surface was achieved by mixing with triethylamine (1 eq) in 3 ml dry DCM and addition to the silanized stainless steel coupons. The reaction mixture was stirred
ACS Paragon Plus Environment
8
Page 9 of 35
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
overnight at room temperature. Finally the coupons were rinsed with toluene, chloroform, acetone to remove uncoupled species. 2.4 Surface-Initiated RAFT Polymerization of MMA from silanized stainless steel surfaces. A
typical
procedure
was
as
follows:
4-Cyano-4-[(dodecylsulfanylthio-carbonyl)-
sulfanyl]pentanoic acid (3 eq, 0.014 g, 0.036 mmol), AIBN (1 eq, 0.002 g, 0.012 mmol, MMA (386 eq, 0.471 g, 4.7 mmol) and toluene (5 mL)were degassed separately for 0.5 h and mixed in a degassed flask and a pretreated RAFT-Si-stainless steel-substrate was added. The mixture was immersed into a preheated oil bath at 70 °C on a shaker with no stirring bar. The polymerization was conducted at 70°C for 20 h. The free polymer was precipitated into ethyl ether, collected by centrifugation and purified one more time by precipitation into ethyl ether. To remove free PMMA
from
the
grafted
stainless
steel
samples
they
were
frist
washed
with
chloroform/toluene/aceton three times followed by extraction residual PMMA with 10 ml THF at 55°C under agitated stirring for 20 h. Afterwards the samples were again washed three times with chloroform/toluene/aceton and placed in oven at 70°C for 30 min. 1H NMR (500 MHz, CDCl3): 4.06 ppm (s, COOCH2), 3.78 3.50 ppm (m, O(CH2)2O), 3.37 ppm (s, OCH3), 2.5 1.5 ppm (backbone CH2), 1.5 0.6 ppm (s, CH3). 2.5 Polymer characterization. 1H NMR spectra were collected on a Bruker DRX 400 MHz spectrometer and Bruker DRX 500 MHz spectrometer. SEC measurements were performed on a self assembled GPC system, equipped with a PSS guard column, a PSS SDV 1000 Å (molecular weight range 100-60000 Dalton) column, a PSS SDV 105 Å (molecular weight range 10001,000,000 Da) column. Detection was performed on a Knauer Smartline RI detector using THF (HPLC grade) as the eluent at a flow rate of 1 mL/min. Analysis of molecular weight and polydispersity index of the polymers was performed against PMMA standards (molecular weight
ACS Paragon Plus Environment
9
Langmuir
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 35
range 102 – 1.190.000 Da). The theoretical Mn;theo of synthesized polymers was determined by equation 1: M n , theo = M RAFT +
x ⋅ [M 0 ] ⋅ M Monomer [RAFT ]0
(1)
with MRAFT as the molar weight of the RAFT-agent, x as the monomer conversion (0
