Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST
Article
Rapid and Clean Covalent Attachment of Methylsiloxane Polymers and Oligomers to Silica using B(C6F5)3 Catalysis Daniel H. Flagg, and Thomas J McCarthy Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01751 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on July 29, 2017
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 40
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
Rapid and Clean Covalent Attachment of Methylsiloxane Polymers and Oligomers to Silica using B(C6F5)3 Catalysis Daniel H. Flagg and Thomas J. McCarthy* Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003
ABSTRACT
The rapid, room temperature covalent attachment of alkylhydridosilanes (R3Si-H) to silicon oxide surfaces to form monolayers using tris(pentafluorophenyl)borane (B(C6F5)3, BCF) catalysis has recently been described. This method, unlike alternative routes to monolayers, produces only unreactive H2 gas as a byproduct and reaches completion within minutes. We report the use of this selective reaction between surface silanols and hydridosilanes to prepare surface-grafted poly(dimethylsiloxane)s with various graft architectures that are controlled by the placement of hydridosilane functionality at one end, both ends, or along the chain of poly(dimethylsiloxane) (PDMS) samples of controlled molecular weight. We also report studies of model methylsiloxane monolayers prepared from pentamethyldisiloxane, heptamethyltrisiloxane (two
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 40
isomers), heptamethylcyclotetrasiloxane and tris(trimethylsiloxy)silane. These modified silica surfaces with structurally defined methylsiloxane groups are not accessible by conventional silane surface chemistry and proved useful in exploring the steric limitations of the reaction. Linear mono- and di- hydride-terminated PDMS grafted surfaces exhibit increasing thickness and decreasing contact angle hysteresis with increasing molecular weight up to a particular molecular weight value. Above this value, hysteresis increases with increasing molecular weight of end-grafted polymers. Poly(hydridomethyl-co-dimethylsiloxane)s with varied hydride content (3-100 mol%) exhibit decreasing thickness, decreasing contact angle and increasing contact angle hysteresis with increasing hydride content. These observations illustrate the importance of molecular mobility on 3-phase contact line dynamics on low hysteresis surfaces. To calibrate our preparative procedure against both monolayers prepared by conventional approaches as well as the recent reports, a series of trialkylsilane (mostly n-alkyldimethylsilane) monolayers was prepared to determine the reaction time required for maximum bonding density using dynamic contact angle analysis. Monolayers prepared from hydridosilanes with BCF catalysis have lower bonding densities than those derived from chlorosilanes and the reactions are more sensitive to alkyl group sterics. This lower bonding density renders greater flexibility to the n-alkyl groups in monolayers and can decrease contact angle hysteresis.
Introduction The chemical modification of silica (and other oxide) surfaces through the covalent attachment of alkylsilanes is important in fundamental research as well as in numerous applied technologies.1-5 Typically, covalent attachment is achieved through the reaction of surface
ACS Paragon Plus Environment
2
Page 3 of 40
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
silanols (Si-OH) with reactive chloro-, amino- or alkoxy- silanes either in solution or from the vapor. Molecular monolayers can be prepared through random attachment of functional silanes6,7 or self-assembly of trifunctional n-alkyl silanes.8,9 Polymerized grafted structures can also be formed from di- and tri- functional silanes, but control of graft structure is challenging or impossible due to the sensitivities of the hydrolysis and condensation reactions to the experimental/environmental conditions.7 These silane modifications typically require long reaction times (>24 hours) at elevated temperatures (>70 °C) to form complete monolayers, making this a time and energy intensive process. Moreover, their hydrolysis byproducts may damage substrates, competitively adsorb during reaction - which can negatively affect monolayer structure, catalyze restructuring the monolayer, and need to be removed. It would be advantageous to use hydridosilanes to modify silica surfaces for two major reasons: (1) the byproduct of the condensation of a hydridosilane with a silanol, H2 gas, is non-adsorbing, noncorrosive and disappears (does not need to be removed), (2) hydridosilanes are more stable than chloro-, amino- or alkoxy- silanes, making them easier to handle, and can be purified by column chromatography. The challenge of using hydridosilanes to modify silica is the low reactivity between silanols and hydridosilanes. We note our use of the terminology "hydridosilane" rather than the more conventional "hydrosilane" to emphasizes the electronegativity differences (Pauling values for Si and H are χ = 1.8 and 2.2) and the chemistry of these compounds. Recently, Escorihuela et al.1 reported the rapid modification of oxidized single silicon surfaces, Si(111) wafers, using tris(pentafluorophenyl)borane (B(C6F5)3, BCF) catalysis. In this work, five n-alkyldimethylsilanes (one semifluorinated) were covalently attached using catalyst concentrations of 1 mol% with reaction times of 5-10 minutes at room temperature (equation 1).
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 40
Me
RMe2Si-H OH
CH2Cl2 1 mol% B(C6F6)3 RT, 5 min
O
Si R
(1)
Me
Characterization using x-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), atomic force microscopy (AFM), ellipsometry, static contact angle analysis, as well as hydrolytic and thermal stability studies, suggest the formation of highly stable self-assembled monolayers. Quantum mechanical calculations are consistent with a mechanism that involves the formation of a borane-hydridosilane adduct followed by nucleophilic attack of the electron deficient Si atom by a surface silanol. Subsequent to hydride transfer from silicon to boron, a siloxane bond forms with the surface, molecular hydrogen is formed by acid-base chemistry between the borohydride and protonated surface siloxane and the BCF catalyst is regenerated. This work demonstrates the versatility of the reaction by chemically patterning a surface using microcontact printing and the rapid preparation of superhydrophobic hydridosilane-functionalized silicon nanowires. Prior to this work, Moitra et al.2 reported the modification of amorphous silica with a range hydridosilanes using BCF catalysis. These authors also noted that the reaction proceeds rapidly (
