LETTER pubs.acs.org/Langmuir
HostGuest Interactions between a Nonmicellized Amphiphilic Invertible Polymer and Insoluble Cyclohexasilane in Acetonitrile Ananiy Kohut,† Olena Kudina,† Xuliang Dai,‡ Douglas L. Schulz,‡ and Andriy Voronov*,† †
Department of Coatings and Polymeric Materials and ‡Center for Nanoscale Science and Engineering, North Dakota State University, P.O. Box 6050, Fargo, North Dakota 58108-6050, United States
bS Supporting Information ABSTRACT: Hostguest interactions between cyclohexasilane (Si6H12) and amphiphilic invertible macromolecules based on PEG and sebacic acid in acetonitrile (neither a solvent for cyclohexasilane nor a support for the micellization of amphiphilic invertible macromolecules) have been investigated. Despite the extended conformation of the macromolecules and the absence of self-assembled polymeric domains, a macromolecular amphiphilicity itself contributes to localizing Si6H12 by AIP and thus enables Lewis acidbase interactions between Si6H12 and the AIP carbonyl groups. The obtained results demonstrate an interesting phenomenon in that insoluble Si6H12 can be localized by AIP macromolecules in a medium that does not support the formation of polymeric domains.
’ INTRODUCTION Designing artificial scaffolds to bind guest molecules using specific interactions is an important aspect of supramolecular chemistry.1,2 Amphiphilic molecules—those with polar and nonpolar moieties—tend to self-organize and build assemblies because of solvophobic interactions as the primary driving force for binding guest molecules.3,4 These assemblies, such as micelles, vesicles, and helical structures, possess container properties because of their ability to solubilize lipophilic or hydrophilic guest molecules in the micellar core in an aqueous57 or organic811 environment, respectively. Macromolecular block copolymer surfactants are known to form different types of assemblies depending on the block length and composition.1214 Similar to small molecules, the macromolecular assemblies can effectively sequester hydrophilic guest molecules in nonpolar solvents and lipophilic guests in polar media. The driving force for the block copolymer surfactant selfassembly is an incompatibility of macromolecular fragments or better solvation of one of the fragments by solvent. It has recently been shown that the mutual incompatibility of the fragments in amphiphilic homopolymers could be engineered on a smaller scale, when such properties are provided by each monomeric unit in a homopolymer.1517 If the incompatibility of the amphiphilic fragments in a macromolecule is achieved on a small length scale, then it allows greater tunability in self-assembly just by changing the fragment’s length. Such a structural tuning can afford different types of polymeric assemblies for various applications. In early studies, we documented the synthesis of amphiphilic invertible polymers (AIPs) based on poly(ethylene glycol) (PEG) and aliphatic dicarboxylic acids. Amphiphilic invertible r 2011 American Chemical Society
polymers have been shown to form micelles and to self-assemble in micellar aggregates, changing conformation in response to changes in the polymer composition, concentration, and solvent polarity.1822 Controlled self-assembly is targeted in our approach by having a precise number of short hydrophilic and hydrophobic fragments with a well-defined length in the amphiphilic macromolecules.18,19 Additionally, this approach benefits from having the hydrophilic and hydrophobic fragments distributed alternately in the macromolecule. It has been demonstrated that as the polymer concentration increases the amphiphilic macromolecules form micelles that self-assemble into micellar superstructures in both polar and nonpolar solvents. The selfassembled macromolecules undergo inverse changes in conformation when the polarity of the medium changes.21,22 Recently, we utilized self-assembled amphiphilic macromolecules to study hostguest interactions in a nonpolar solvent (benzene) between micellar assemblies and molecules of liquid cyclohexasilane (Si6H12).23 Cyclohexasilane—a potential precursor for electronic materials—undergoes polymerization, transforming the polyhydrosilane (SiH2)n into a Si and then a crystalline silicon material under heat treatment. However, polyhydrosilane is nearly insoluble in organic solvents, which complicates its solution-based processing. Self-assembled polymeric domains could serve as potential nanocontainers to localize cyclohexasilane and host Si6H12 polymerization and further transformations, presumably in both polar and nonpolar solvents. As previously mentioned,23 upon addition of Si6H12 to Received: May 19, 2011 Revised: July 27, 2011 Published: July 28, 2011 10356
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Figure 1. Si6H12 added to (A) acetonitrile-d3 (6% w/v) and to (B) AIP solution in acetonitrile-d3 (6% w/v Si6H12 in 6% w/v AIP).
an amphiphilic polymer solution in a nonpolar medium, the 1H NMR study revealed both (i) the interaction between Si6H12 and the oxygen-containing functional groups of the polymer and (ii) an increasing number of additional interactions between cyclohexasilane and hydrophobic polymeric domains formed upon the self-assembly of amphiphilic macromolecules.23 Although we identified hostguest interactions between self-assembled AIP macromolecules and Si6H12 in benzene, which is a good solvent for cyclohexasilane, there is the question of what would happen if a Si6H12AIP mixture is prepared in a polar solvent, where cyclohexasilane is insoluble? Would amphiphilic macromolecules be able to localize Si6H12 and form a stable colloidal solution in that case? And if so, what would be the interaction between cyclohexasilane and inverted macromolecules? To address these questions, we added Si6H12 to the amphiphilic polymer based on PEG (molecular weight 600 g/mol) and sebacic acid solutions in acetonitrile-d3 and conducted 1H NMR spectroscopic studies on mixtures containing similar ratios of amphiphilic polymer and cyclohexasilane, which were used for experiments in benzene.23
Figure 2. 1H NMR spectra of the AIP solutions (0.6% w/v) in acetonitrile-d3 without Si6H12 (lower curves) and with 0.6% w/v Si6H12 (upper curves) for AIP protons a (left) and d (right).
Table 1. Chemical Shifts of the Si6H12 and AIP Protons in 1H NMR Spectra at Different Concentrations of Cycolhexasilane and Polymer in Acetonitrile-d3 chemical shift, ppm concentration, % w/v AIP
’ EXPERIMENTAL SECTION An appropriate amount of the polyester (5, 50, or 160 μg), diethyl sebacate (5 or 50 μg), or poly(ethylene glycol) dimethyl ether (5 or 50 μg) was dissolved in 0.8 mL of acetonitrile-d3 under gentle agitation. The solutions were left for at least 16 h to equilibrate at 22.5 °C. Cyclohexasilane (5 or 50 μg) was added, the mixture was allowed to equilibrate for another 16 h, and then 1H NMR spectra were recorded. Shifts of the signals in the 1H NMR spectra were determined by comparing their spectra with the spectrum of each initial substance alone dissolved in acetonitrile-d3 at the same concentration. 1H NMR spectra were recorded using Joel ECA 400 MHz NMR spectroscopy at 400 MHz and at 22.5 °C. The spectra were referenced to a TMS signal as an internal standard.
’ RESULTS AND DISCUSSION As the first step in defining polymercyclohexasilane interactions in acetonitrile-d3, proton spectra were collected for Si6H12AIP mixtures at polymer concentrations fixed at 0.6, 6.0, and 20% w/v with different amounts of cyclohexasilane added to bring the Si6H12 concentration to 0.6 and 6.0% w/v. Although Si6H12 is insoluble in acetonitrile-d3, forming cloudy mixtures (Figure 1, left), stable, transparent colloidal solutions (Figure 1, right) have been obtained in all instances when 0.6%
Si6H12
0
0.6
0
6
0.6
0
AIP protons a
b
c
d
e
Si6H12 3.362 3.362
2.291 1.567 1.297 4.143 3.549
0.6
0.6
2.291 1.568 1.298 4.143 3.549
3.360
6 6
0 0.6
2.291 1.568 1.297 4.143 3.550 2.291 1.567 1.296 4.143 3.548
3.360
6
6
2.290 1.567 1.296 4.142 3.547
3.371
20
0
2.290 1.566 1.295 4.143 3.549
20
0.6
2.289 1.565 1.294 4.142 3.549
3.361
w/v cyclohexasilane is added to amphiphilic polymer solutions in acetonitrile-d3. The appearance of mixtures indicates that the hosting macromolecules are able to accommodate insoluble guests. When a mixture of 0.6% w/v AIP and a higher concentration of cyclohexasilane (6% w/v) was prepared, no stable colloidal solution was formed. To address the effect of the polymer/cyclohexasilane concentration on stability further, we added 6% w/v Si6H12 to 6% w/v AIP and again observed the formation of a stable, transparent colloidal solution. The recorded 1H NMR spectra indicated the enhanced interaction between cyclohexasilane and the polymer (Figure 1S in Supporting Information). The 1H NMR spectra recorded for the transparent colloidal solutions that formed in acetonitrile-d3 show that the nature of the signals of the neighboring ester group protons (a, d) was changed in polymercyclohexasilane mixtures, indicating the interaction between the polymer ester group and cyclohexasilane molecules (Figure 2). The chemical shift values were not perturbed (Table 1), in contrast to Si6H12/AIP/benzene 1H 10357
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Langmuir Scheme 1. Schematic Illustrating Si6H12AIP Complex Formation
Figure 3. Localization of Si6H12 by AIP macromolecules in acetonitrile-d3.
NMR spectroscopic measurements where splitting and shifting were observed.23 The collected data show that although Lewis acidbase interactions between the polymer ester groups and Si6H12 are present, in the 1H NMR spectra (Table 1) no significant changes in the chemical shifts were noted for polymer protons ae (shown in Figure 2). To explain the observed data, we collected 1H NMR spectra on solutions at four different polymer concentrations in acetonitrile-d3 (0.1, 1.0, 10, and 30% w/v). It is clearly seen that the chemical shift values exhibit only slight changes with increasing polymer concentration from 0.1 to 30% (Figure 2S in Supporting Information). This observation indicates that an extended conformation is mostly adopted by macromolecules in acetonitriled3. The dynamic light scattering data recorded for each polymer concentration in acetonitrile-d3 confirms that no micelles are formed. Well-expressed hydrophilic and hydrophobic domains were not formed when the polymer concentration increased in acetonitrile-d3 from 0.1 to 30% w/v. This fact is quite interesting with regard to the formation of stable colloidal mixtures of AIP and Si6H12 upon addition of cyclohexasilane to polymer solutions in acetonitrile-d3. To probe the interactions specific to the amphiphilic polymer functional groups (i.e., esters and ethers) with cyclohexasilane in acetonitrile-d3, we prepared solutions of diethyl sebacate (DES) and poly(ethylene glycol) dimethyl ether (PEGDM) and added Si6H12 to each solution. In this experiment, DES and PEGDM represent polymer building blocks that are not yet combined into the invertible macromolecule. Unlike the amphiphilic polymer, neither DES nor PEGDM improves the Si6H12 solubility in acetonitrile-d3. 1H NMR spectra show that the 1H resonance for Si6H12 in acetonitrile-d3 (3.362 ppm) is nearly unchanged in the presence of DES (3.361 ppm) and PEGDM (3.360 ppm) in solution (data not shown). Also, no significant changes in the collected spectra of DES and PEGDM indicate that there are no specific interactions between the functional groups in both building blocks and Si6H12 in acetonitrile-d3. 1H NMR spectra
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of DES-Si6H12 and PEGDM-Si6H12 mixtures are provided in the Supporting Information (Figure 3S). However, being combined in the macromolecule, the Lewis base carbonyl group undergoes an interaction with Lewis acid Si6H12 (Scheme 1), resulting in the appearance of a multiplet at 2.36 ppm (Figure 2). Similar interactions have been observed in a nonpolar benzene medium.23 Electron donation from the carbonyl oxygen to the cyclohexasilane acceptor enhances the deshielding effect (δ1 < δ10 ) of some of the protons labeled as a in the sebacate moiety (Scheme 1) in the polar acetonitrile-d3 medium indicated by the peak at 2.36 ppm. We attribute the observed new signals in the range of 3.873.95 ppm to dipolar interactions of Hδ+ protons d in the PEG fragment with Hδ atoms of Si6H12 that increase the shielding effect (δ3 > δ30 ) of some of the protons labeled as d (Scheme 1).
’ CONCLUSIONS Despite the extended conformation of the amphiphilic macromolecules and the absence of self-assembled polymeric do mains, the polymer amphiphilicity itself (e.g., the presence of both hydrophilic (PEG) and hydrophobic (sebacate) fragments) contributes to localizing Si6H12 by amphiphilic macromolecules (Figure 3) and thus enables Lewis acidbase interactions between Si6H12 and the carbonyl groups. The obtained results demonstrate an interesting phenomenon in which insoluble Si6H12 can be localized by amphiphilic macromolecules in a medium that does not support the formation of polymeric micelles. ’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental details regarding the synthesis and characterization of the initial substances, general considerations, 1H NMR spectra of amphiphilic invertible polyester in acetonitrile-d3 at different concentrations, 1 H NMR spectra of the mixtures of diethyl sebacate and poly(ethylene glycol) dimethyl ether in acetonitrile-d3 with and without Si6H12, and 1H NMR spectra of the AIP solutions (6% w/v) in acetonitrile-d3 without Si6H12 and with 6% w/v Si6H12. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Fax: +1 701 2318439. Tel: +1 701 2319563. E-mail: andriy.
[email protected].
’ ACKNOWLEDGMENT Financial support from the North Dakota EPSCoR/National Science Foundation (EPS-0447679) and the DoD Defense Microelectronics Activity (DMEA) under agreement number H94003-09-2-0905 is gratefully acknowledged. The U.S. Government is authorized to reproduce and distribute reprints notwithstanding any copyright notation thereon. ’ REFERENCES (1) Lehn, J. M. Angew. Chem., Int. Ed. 1988, 27, 89. (2) Cram, D. J. Angew. Chem., Int. Ed. 1988, 27, 1009. (3) Tanford, C. The Hydrophobic Effect; Wiley-Interscience: New York, 1973. 10358
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