Letter pubs.acs.org/macroletters
Synthesis and Selective Loading of Polyhydroxyethyl Methacrylate‑l‑Polysulfone Amphiphilic Polymer Conetworks Catarina Nardi Tironi,†,‡ Robert Graf,† Ingo Lieberwirth,† Markus Klapper,*,† and Klaus Müllen*,† †
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Graduate School Material Science in Mainz, University of Mainz, Staudingerweg 9, 55128 Mainz,Germany
‡
S Supporting Information *
ABSTRACT: Polyhydroxyethyl methacrylate-linked by-polysulfone amphiphilic polymer conetworks of two types of segments with Tg above room temperature are presented. The conetworks are prepared by free radical copolymerization of methacryloyl-terminated PSU macromers with 2-ethyl methacrylate, followed by removal of the TMS protecting groups by acidic hydrolysis. Phase separation in the nanometer range due to the immiscibility of the two covalently linked segments is observed using transmission electron and scanning force microscopy. The swelling of the conetworks in water and methanol as polar solvents and chloroform as nonpolar solvent are studied gravimetrically and then in a more detailed fashion by solid-state NMR spectroscopy. Selective swelling and also targeted loading of a small organic model compound specifically to one of the two phases are demonstrated.
A
applications of APCNs as carriers of catalysts18 or biocatalysts19 and as drug delivery systems20 are discussed. Loading and subsequent controlled release of small molecules were reported by the group of Tiller, for example, by the uptake of the biocide cetyltrimethylammonium chloride (CTAC) into 2-hydroxyethyl acrylate/acrylic acid-l-polydimethylsiloxane conetworks and the observation of a slow release of CTAC, thus, rendering the conetworks suitable as antimicrobial coatings.21 Controlled drug release was detected for polymethacrylic acid-l-polyisobutylene conetworks loaded with theophylline.20 Inorganic Ag22 and CdS5 nanoparticles were synthesized in situ inside the hydrophilic phases of poly(N,N dimethylacrylamide)-l-polyisobutylene and hydroxyethyl methacrylate-l-polyisobutylene conetworks, respectively. However, no study on the selectivity of the swelling and loading of small molecules into a particular phase of a conetwork was reported so far. This aspect is of major importance, in particular, when both phases in a bicontinuous material have to be loaded with two different functional materials. Typical examples can be the use of APCNs as templates for spatially separated donors and acceptors in bulk heterojunction solar cells or fuel cell applications of a doped APCN material, where proton conductivity and mechanical stability have to be achieved by separating conductive pathways from a nonpolar scaffold. Herein, we present novel APCNs consisting of the two relatively high Tg polymeric compounds
mphiphilic polymer conetworks (APCNs) are well-known for the formation of nanophases of two immiscible polymeric components, which are covalently linked to each other. Due to the covalent bonding, macrophase separation of polar and nonpolar components is inhibited. 1−3 The morphologies of APCNs have been studied with a variety of techniques, such as transmission electron microscopy,4,5 scanning force microscopy (SFM),5,6 small-angle X-ray scattering (SAXS), 5,7,8 small-angle neutron scattering (SANS),5,9,10 ion conductivity measurements of conetworks containing salt-loaded, ion-conducting PEG domains,11 and solid-state NMR spectroscopy.7 Solid-state NMR 13C crosspolarization magic angle spinning (CP MAS) was also applied in combination with gravimetric swelling experiments and Positron Annihilation Lifetime Spectroscopy to investigate the swelling kinetics of the polar phase in polytetrahydrofuran-lpolyisobutylene conetworks.12 NMR studies with a focus on the interphase region of poly(N,N-dimethylaminoethyl methacrylate)-l-polyisobutylene APCNs swollen in polar and nonpolar solvents were performed with deuterium solid-state NMR on conetworks with selectively deuterium labeled crosslinking molecules. It was shown that the mobility of the crosslink points, despite being polar, hardly changes when the network is swollen in water, but tremendously increases when swollen in n-heptane.13 The specific swelling behavior8,14,15 in both polar and nonpolar solvents is one of the reasons for ongoing research on amphiphilic conetworks since their first description by the groups of Stadler, Weber, and Kennedy in 1988.16,17 Due to the swelling properties and uptake of small molecules, potential © 2015 American Chemical Society
Received: October 6, 2015 Accepted: November 4, 2015 Published: November 6, 2015 1302
DOI: 10.1021/acsmacrolett.5b00714 ACS Macro Lett. 2015, 4, 1302−1306
Letter
ACS Macro Letters Scheme 1. Synthesis of PHEMA-l-PSU Conetworks
Figure 1. Images of PHEMA-l-PSU conetworks (feed composition PSU:PHEMA 50:50 wt %). Left side: TEM image (high annular dark field); samples were stained with phosphotungstic acid; domain size 10−20 nm. Right side: SFM phase contrast image (1 μm × 1 μm); the large dark brown bands through the entire SFM image are crosstalk from steps from the topography. The inset shows a histogram of the width of measured domain sizes (dark areas).
The bis(methacryloyl)-terminated polysulfone MA-PSU-MA macromer was synthesized via nucleophilic substitution of bisphenol A (BPA) and dichlorodiphenyl sulfone (DCDPS). A slight excess of bisphenol A was used in order to obtain α,ωphenol-terminated polysulfones. The molecular weight was tuned to about 6 × 103 g/mol by the molar ratio of the two monomers. Typical for polycondensations, the polydispersity index (Mw/Mn) was approximately 2 and, as expected,23 traces of low molecular weight cyclics were produced, but these could be extracted after network formation. The α,ω-phenolterminated polysulfone was reacted with methacryloyl chloride,
PHEMA and PSU. PHEMA was chosen due to its hydrophilicity and swelling ability in polar solvents. PSU was selected not only because of its hydrophobicity, but also because of its high temperature stability, which can be of interest in the above applications. The swelling of the conetworks in water and methanol as polar solvents and chloroform as a nonpolar solvent is investigated by solid-state 1H MAS NMR spectroscopy. The incorporation of TMS-cholesterol as a nonpolar organic model compound into the network and the uptake into the nonpolar PSU phase were proven by MAS NMR correlation experiments. 1303
DOI: 10.1021/acsmacrolett.5b00714 ACS Macro Lett. 2015, 4, 1302−1306
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ACS Macro Letters
Figure 2. (a, b) Gravimetrically determined swelling degrees of PHEMA-l-PSU conetworks in water and methanol (left side) and chloroform (right side); (c, e) 1H MAS NMR and (f) 1H DQF MAS NMR spectra (25 kHz) of PHEMA-l-PSU network (PSU/PHEMA 50:50).
yielding α,ω-dimethacryloyl-functionalized polysulfone (MAPSU-MA). The absence of signals of non- or monofunctionalized chains in the MALDI-TOF spectrum (SI, Figure 3) indicated a quantitative reaction. Amphiphilic PHEMA-l-PSU conetworks were synthesized via free radical copolymerization of silylated 2-hydroxyethyl methacrylate (TMS-HEMA) with MA-PSU-MA and subsequent removal of the silyl group by acidic hydrolysis, as illustrated in Scheme 1. The phase separation of a PHEMA-l-PSU conetwork with a feed composition of 50:50 wt % PHEMA and PSU, as depicted in Figure 1, was investigated with TEM and SFM. For TEM measurements, the PHEMA phase was stained with phosphotungstic acid. Both methods show a nanophase-separated structure typical for APCNs. The domain size determined with TEM and SFM was 10−20 nm in both cases. However, as these images are two-dimensional, it is hard to make a statement about the three-dimensional morphology and claim a bicontinuous structure only from the images. Phase continuity will be further discussed in the context of the swelling in polar and nonpolar phases. For selective loading of small compounds into a specific phase, it is important to first study the swelling behavior of the conetworks and to investigate whether the swelling in a
particular solvent is selective for just one of the two phases. For this purpose, swelling ratios were determined gravimetrically and NMR techniques were applied for monitoring the swelling by observing changes in the mobilities of the respective phases. Swelling experiments were done in chloroform as a nonpolar solvent and water and methanol as polar solvents. As expected, the swelling ratio in chloroform (see Figure 2b) increases with a greater weight fraction of the nonpolar PSU component in the network, while the swelling degree in water and methanol increases with increasing amount of polar PHEMA in the conetwork (see Figure 2a). Taking a closer look at the values of the swelling degree, it is obvious that swelling in chloroform is generally much higher than in water. For feed compositions of PSU/PHEMA between 40:60 and 70:30, the swelling degree in chloroform ranges from 315 to 640%, whereas the swelling ratio in water is between 17 and 6% for the same feed compositions. A PHEMA-l-PIB conetwork (64 wt % polyisobutylene) was reported to show a swelling degree of about 8% in an aqueous CdCl2 solution.5 The PHEMA-l-PSU conetwork presented here with 60 wt % PSU has a swelling degree of about 8% as well and is thus comparable. Ranging from 41 to 19 wt % for feed compositions of PSU/ PHEMA between 40:60 and 70:30, the swelling in methanol 1304
DOI: 10.1021/acsmacrolett.5b00714 ACS Macro Lett. 2015, 4, 1302−1306
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ACS Macro Letters
neous distribution of small molecules in this particular network phase. To evaluate selective loading into the nonpolar PSU phase, TMS-cholesterol was chosen as a small organic model compound because the chemical shift of the TMS is upfield of all others in the system. It was synthesized via a literature procedure.24 A PHEMA-l-PSU conetwork with a feed ratio of 50:50 wt % was immersed in a solution of TMS-cholesterol in chloroform. 13 C{1H} Lee−Goldburg heteronuclear NMR correlation measurements (LG-HETCOR)25 with polarization transfer times of 8 ms reveal that the TMS group of the model compound is spatially located near the methyl groups of the PSU component of the conetwork as indicated by the correlation signal of TMS protons with the methyl carbons of PSU (Figure 3, red circle). The correlation signal of PSU
leads to a higher swelling degree than the swelling in water. This gives rise to the questions whether the swelling in chloroform is selective for just one phase and whether the polar PHEMA phase is completely swollen when the network is immersed in water and methanol. In order to prove the selectivity of the swelling of the respective phases in different solvents, 1H MAS NMR spectroscopy was applied. Swelling of a phase leads to increased mobility of local segments of the respective phase, which is observed as a narrower and better defined signal in the 1H MAS NMR spectrum. Hence, the selectivity of the swelling of one phase in a two-component network can be verified with NMR spectroscopy. The 1H MAS NMR spectrum of the dry PHEMA-l-PSU conetwork shows three broad peaks (see Figure 2c). The first peak in the aromatic region around 7 ppm can be assigned to the aromatic protons of the PSU phase. The middle peak around 4 ppm can be attributed to the OCH2 and OH groups of PHEMA, while the third peak around 1 ppm results from methyl groups of the PSU and the PHEMA backbone. After swelling in water, no change in the aromatic PSU signal at 7 ppm is observed. The peak from PHEMA becomes slightly narrower after swelling, indicating a selective, but incomplete, swelling. This result matches well with the gravimetrically determined, low swelling degree. A significant amount of the PHEMA phase is not swollen. This might be due to some isolated PHEMA domains inside a continuous PSU matrix. However, as discussed above, the degree of swelling in water is determined in the same range as the swelling degree of the PHEMA phase in other APCNs mentioned in the literature.5 The relatively low water uptake might be explained by the rigidity of the PSU phase hindering the swelling of the PHEMA phase. Especially HEMA units near the cross-linking points can be affected. It can be assumed that due to the nonswelling of PSU, this phase acts as a cage and therefore hampers the expansion of the PHEMA phase, which is a prerequisite for the solvent uptake. A simple explanation for this finding is that chloroform is a good solvent for PSU and incompatible for PHEMA, while water is a very poor solvent for PSU and a marginally good solvent for PHEMA. Trying to find a more efficient solvent for the swelling of the polar PHEMA phase of the conetwork, methanol turned out to be a suitable candidate. A significant narrowing of the PHEMA NMR signal around 4 ppm was observed (Figure 2e), while the aromatic signal remained constantly broad. Figure 2f provides double quantum filtered 1H MAS spectra (1H DQF MAS) in which signals of mobile segments are suppressed. With this technique, it is easier to quantify how much of one phase is swollen. In the case of methanol, the results confirm the assumption that the polar PHEMA phase could be swollen more efficiently than in water as it is a better solvent for PHEMA than water. Since a small, more defined peak for the PSU signal in the aromatic region appears as well, it can be assumed that a small fraction of the PSU phase is mobilized by the methanol. In the 1H MAS NMR spectrum of a network swollen in deuterated chloroform (Figure 2e), the broad signal of the PSU phase splits into different peaks, indicating that the PSU phase is highly mobile due to the swelling. The 1H double quantum filtered MAS NMR spectrum in Figure 2f also proves a complete swelling of the PSU phase while the PHEMA phase is not affected. This leads to the conclusion that the PSU phase is continuous and suitable for a targeted loading and homoge-
Figure 3. 13C{1H} LG-HETCOR measurement of TMS-cholesterol@ PHEMA-l-PSU conetwork.
methyl protons with TMS carbons, however, is not observed due to the significantly shorter T2 relaxation time of the PSU methyl protons. It should be pointed out that the longer T2 relaxation time of the TMS group covalently attached to the model compound is essential to obtain the intermolecular correlation between the TMS groups and 13C sites of the conetwork in close spatial proximity. Due to the low concentration of the TMS model compound, not even a trivial other correlation signal from model compound is observed, whereas the TMS shows the trivial CH correlations with the TMS group as well as intermolecular correlations with the methyl sites of PSU segments but no correlation with any of the very intense PHEMA signals in the system. Hence, a selective uptake of the model compound into the nonpolar PSU phase is demonstrated by the NMR experiments. The high chemical selectivity and the feasibility of through-space intermolecular correlation experiments render condensedphase MAS NMR spectroscopy a suitable method to monitor the swelling and selective loading of the amphiphilic polymer conetworks. In conclusion, the synthesis of novel PHEMA-l-PSU amphiphilic polymer conetworks of two components with Tg above room temperature was presented. The conetworks were prepared by free radical copolymerization of methacryloylterminated PSU macromers with 2-(trimethylsiloxy)ethyl methacrylate, followed by removal of the TMS protecting groups by acidic hydrolysis. Phase separation in the nanometer range was detected by transmission electron and scanning force microscopy. The conetworks showed swelling of the respective 1305
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phases in polar solvents (water, methanol) as well as in a nonpolar solvent (chloroform). The swelling degree in the nonpolar solvent increased with higher feed ratio of the nonpolar PSU component and vice versa in polar solvents. Loading of the organic model compound TMS-cholesterol specifically into the nonpolar PSU phase was proven by solidstate NMR spectroscopy. In the solid-state spectrum of the PHEMA-l-PSU conetwork loaded with TMS-cholesterol, the signal of the TMS group showed a correlation only to the methyl groups of PSU and no correlation with any signal of the polar PHEMA phase. This leads to the conclusion that the loading is directed only to the nonpolar PSU phase.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00714. Experimental details, MALDI-TOF, and NMR spectra (PDF).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2010-2013) under the call ENERGY-2010-10.2-1: Future Emerging Technologies for Energy Applications (FET) under Contract 256821 QuasiDry. C.N.T. is a recipient of a fellowship through the Excellence Initiative (DFG/GSC 266). The Interuniversity Attraction Poles Programme P7/05 (FS2) is acknowledged for funding. We acknowledge Uwe Rietzler and Rüdiger Berger for performing and discussing scanning force microscopy experiments.
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