pubs.acs.org/Langmuir © 2009 American Chemical Society
Surface-Initiated Vapor Deposition Polymerization of Poly(γ-benzyl-L-glutamate): Optimization and Mechanistic Studies Wenwei Zheng† and Curtis W. Frank*,‡ †
‡
Department of Chemistry, Stanford University, 333 Campus Drive, Stanford, California 94305-5080, and Department of Chemical Engineering, Stanford University, Stauffer III, Room 111, 381 North-South Mall, Stanford, California 94305-5025 Received August 31, 2009. Revised Manuscript Received October 31, 2009
Surface-initiated vapor deposition polymerization (SI-VDP) is a very effective approach to synthesize grafted poly(amino acids). In this study, we developed an SI-VDP system with pressure and temperature control and demonstrated highly efficient surface-grafting of poly(γ-benzyl-L-glutamate) (PBLG) on a silicon wafer at pressure 1000 times larger than those in prior reports. More importantly, we developed new methods to quantitatively investigate mechanistic details of the SI-VDP process. First, we monitored the amount of vaporized monomer and developed a VDP reaction profile (VDPRP) method to study the major monomer reservoir processes. Next, we developed a quantitative Fourier transform infrared analysis of both as-deposited PBLG and chemisorbed PBLG films in addition to ellipsometric data to evaluate the major substrate surface processes. We observed two classes of characteristic features (pulses or two peaks) of VDPRPs, which depended upon the monomer temperature, and proposed possible mechanisms. We also found that the two peaks of VDPRPs can selectively track different reservoir processes in real time. For surface processes, we proposed possible mechanisms to obtain the surface-grafted PBLG that are expected to have either high packing density with mostly R-helix segments or low packing density with both random coil and R-helix segments.
1. Introduction Poly(amino acids) are of particular interest as candidates for surface-grafted ultrathin films because of their ability to assemble hierarchically into stable ordered conformations.1,2 Poly(amino acid) chains grafted to a surface can demonstrate strikingly high electromechanical and electrooptical efficiency3,4 as well as reversible response to an external trigger,5,6 and show promise for applications such as biosensors,7,8 biomedical coating,9 biomineralization,10 chiral separation membranes,11 and optical storage and display devices.12 Grafting can be achieved either via one-step chemical bonding between reactive end-groups of preformed poly(amino acids) and complementary reactive groups on a substrate, known as the grafting-to method, or by ring-opening polymerization (ROP) of R-amino acid N-carboxyanhydride (NCA) monomer initiated by primary-amine groups on a substrate, known as the grafting-from method. Because monomer can diffuse to reactive groups more easily than can preformed poly(amino acids),13 the grafting-from method can achieve higher *Telephone: 650-723-4573. Fax: 650-723-9780. E-mail: curt.frank@ stanford.edu. (1) Berg, J. M.; Tymoczko, J. L.; Stryer, L.; Stryer, L. Biochemistry; W. H. Freeman: New York, 2007. (2) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. Rev. 2004, 33, 14–22. (3) Jaworek, T.; Neher, D.; Wegner, G.; Wieringa, R. H.; Schouten, A. J. Science 1998, 279, 57–60. (4) Chang, Y. C.; Frank, C. W.; Forstmann, G. G.; Johannsmann, D. J. Chem. Phys. 1999, 111, 6136–6143. (5) Wang, Y. L.; Chang, Y. C. Macromolecules 2003, 36, 6503–6510. (6) Wang, Y.; Chang, Y. C. Macromolecules 2003, 36, 6511–6518. (7) Deming, T. J. Adv. Mater. 1997, 9, 299–311. (8) Yang, C.; Wang, Y.; Yu, S.; Chang, Y. Biomacromolecules 2009, 10, 58–65. (9) Lahann, J. Polym. Int. 2006, 55, 1361–1370. (10) Wu, J.; Wang, Y.; Chen, C.; Chang, Y. Chem. Mater. 2008, 20, 6148–6156. (11) Lee, N. H.; Frank, C. W. Polymer 2002, 43, 6255–6262. (12) Daly, W. H.; Poche, D.; Negulescu, I. I. Prog. Polym. Sci. (Oxford) 1994, 19, 79–135. (13) Chang, Y.; Frank, C. W. Langmuir 1996, 12, 5824–5829.
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grafting density than the grafting-to method. This approach has been widely used to synthesize grafted poly(γ-benzyl-Lglutamate) (PBLG) chains at the solid-liquid interface.13-15 However, the thickness of the resulting grafted PBLG is generally restricted (e.g., less than 25 nm) because the ROP can be terminated by chemical deactivation of amine end-groups and physical blockage by physisorbed materials.16 As an alternative approach, Chang and Frank developed a vapor phase grafting-from method known as surface-initiated vapor deposition polymerization (SI-VDP).17 In this protocol, vaporized monomer, instead of solvated monomer, go through the same ROP to form surface-grafted poly(amino acids). We refer to this desirable mode of surface polymerization as chemisorbed polymerization. Because the low pressure vapor phase effectively suppresses termination reactions by decreasing moisture and acid impurities, this method can improve grafting efficiency (e.g., resulting in a PBLG film up to 42 nm thick).17 Wang and Chang18 subsequently improved the SI-VDP method by designing their SI-VDP system to use high vacuum, increase vaporized monomer concentration, and add control of substrate temperature. With these modifications, they were able to synthesize a grafted PBLG film up to 187 nm thick. In spite of the significant progress on SI-VDP optimization (e.g., as reflected in the increased thickness of grafted films),17,18 high vacuum was still required to achieve high grafting efficiency. In this study, we developed an SI-VDP system with improved (14) Wieringa, R. H.; Siesling, E. A.; Geurts, P. F. M.; Werkman, P. J.; Vorenkamp, E. J.; Erb, V.; Stamm, M.; Schouten, A. J. Langmuir 2001, 17, 6477–6484. (15) Heise, A.; Menzel, H.; Yim, H.; Foster, M. D.; Wieringa, R. H.; Schouten, A. J.; Erb, V.; Stamm, M. Langmuir 1997, 13, 723–728. (16) Kricheldorf, H. R. [R]-aminoacid-N-carboxy-anhydrides and related heterocycles: syntheses, properties, peptide synthesis, polymerization; Springer-Verlag: Berlin, New York, 1987. (17) Chang, Y.; Frank, C. W. Langmuir 1998, 14, 326–334. (18) Wang, Y.; Chang, Y. Langmuir 2002, 18, 9859–9866.
Published on Web 12/04/2009
DOI: 10.1021/la9032628
3929
Article
Zheng and Frank
Scheme 1. Process of Surface-Initiated Vapor Deposition Polymerization: (A) Synthesis of NCA Monomer, (B) Vapor Silylation of a Substrate with APTES, and (C) SI-VDP of Vaporized NCA Monomer
pressure and temperature control and have demonstrated highly efficient surface-grafting at pressures 1000 times larger than those in prior reports. More importantly, little quantitative study has been done to understand mechanistic details of an SI-VDP process. The study of the SI-VDP mechanism involves understanding the details of its five major subprocesses: monomer vaporization and reservoir polymerization in the monomer reservoir, which we refer to collectively as reservoir processes; and monomer condensation and physisorbed and chemisorbed polymerization on the substrate surface, which are termed surface processes. Reservoir polymerization and physisorbed polymerization denote monomer polymerization initiated by heating in the reservoir and on the surface, respectively, although both processes are undesirable. However, only ellipsometric data were available in the previous reports,17,18 making it impossible to quantitatively differentiate among these SI-VDP subprocesses. In this study, we developed a real-time tracking method, termed an SI-VDP reaction profile, and a quantitative Fourier transform infrared (FTIR) analysis for application to both as-deposited PBLG and chemisorbed PBLG films. By so doing, we were able to generate sufficient variables to quantitatively investigate the SI-VDP mechanism.
2. Materials and Methods and Chemistry. γ-Benzyl-L-glutamate (BLG) N-carboxyanhydride (NCA) monomer was synthesized by the reaction of BLG with triphosgene (Scheme 1A)19 and purified by rephosgenation and recrystallization.20 All chemicals were purchased from Sigma-Aldrich Chemicals and used as received except (3-aminopropyl)-triethoxysilane (APTES). APTES was purified by vacuum distillation with calcium hydride being used as a drying agent. The wafers were undoped, doublepolished silicon (100) from Montco Silicon Technologies, Inc. Si wafers (1.5 1.8 cm2) were cleaned with a freshly prepared 7/3 (v/v) mixture of concentrated sulfuric acid and 30% hydrogen peroxide within 90-120 °C for 30 min and dehydrated at ca. 120 °C in a vacuum oven for 30 min. They were then placed inside a vacuum desiccator with a Teflon vial inside; added to the vial was 25 μL of APTES. The desiccator was purged with nitrogen for 2 min and evacuated to 0.2-0.3 mbar. It was then heated at 100 °C in an oven for ca. 2 h. Finally, the desiccator was purged seven 2.1. Materials
(19) Daly, W. H.; Poche, D. Tetrahedron Lett. 1988, 29, 5859–5862. (20) Dorman, L. C.; Shiang, W. R.; Meyers, P. A. Synth. Commun. 1992, 22, 3257–3262.
3930 DOI: 10.1021/la9032628
Scheme 2. Surface-Initiated Vapor Deposition System: (A) Vacuum System and (B) Polymerization Reactor
times by nitrogen at ca. 0.03 mbar slightly above 90 °C to remove physisorbed APTES if any. This process is represented as Scheme 1B. Finally, BLG-NCA monomer and an APTES modified substrate were loaded in a vacuum chamber for an SI-VDP process, during which chemisorbed polymerization is desired, as shown in Scheme 1C.
2.2. Automatically Controlled SI-VDP System. 2.2.1. Chamber Configuration. The vacuum system (Scheme 2A)
and polymerization reactor (internal dimension: diameter area = 100 mm 100 mm) (Scheme 2B) were designed to control temperature from 5 to 300 °C and pressure from atmosphere to 10-6 mbar. The experimental setup in the reactor includes a top heating plate for the substrate, a bottom heating plate for the monomer reservoir, and an intervening Al cylinder (inner diameter height = 35 mm 25 mm). The resulting confined space (26 mL volume) ensures a high concentration of vaporized monomer around the surface of the silicon wafer substrate. The gap (2 mm) between the top heating plate and the top of the Al cylinder allows removal of carbon dioxide produced as byproduct during the SI-VDP and eliminates thermal conduction between the top and bottom heating plates. A copper reservoir with monomer (outer diameter height = 32 mm 2.5 mm; inner diameter height = 31 mm 2.0 mm) was placed inside the cylinder. The two heating plates are heated independently by mica thermofoil heaters from Minco Products Inc. to ensure a uniform surface temperature profile (
