Anomalous Swelling Behavior of Poly(N-vinylimidazole)-l-Poly

Article pubs.acs.org/Macromolecules

Anomalous Swelling Behavior of Poly(N‑vinylimidazole)‑l‑Poly(tetrahydrofuran) Amphiphilic Conetwork in Water Studied by Solid-State NMR and Positron Annihilation Lifetime Spectroscopy Attila Domján,*,† Csaba Fodor,‡ Szabolcs Kovács,§ Tamás Marek,*,∥ Béla Iván,‡ and Károly Süvegh§ †

NMR Spectroscopy Laboratory and ‡Department of Polymer Chemistry, Institute of Organic Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Pusztaszeri út 59-67, P.O. Box 17, H-1525 Budapest, Hungary § Department of Nuclear Chemistry, Eötvös Loránd Science University, P.O. Box 32, H-1518 Budapest, Hungary ∥ Department of Surface Modifications and Nanostructures, Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Pusztaszeri u. 59-67, P.O. Box 17, H-1525 Budapest, Hungary S Supporting Information *

ABSTRACT: Poly(N-vinylimidazole) homopolymer (PVIm) and poly(N-vinylimidazole)-l-poly(tetrahydrofuran) (PVIm-lPTHF), a novel amphiphilic polymer conetwork (APCN), were synthesized to compare their solid state structure and investigate the swelling behavior of this unique conetwork in water. A short-range ordered structure stabilized by second-order interactions between the imidazole pendant groups was found in the PVIm homopolymer and in the PVIm phase of the dry conetwork as revealed by solid-state NMR 13C cross-polarization magic angle spinning (CP MAS) and two-dimensional 1H−13C frequencyswitched Lee−Goldburg (FSLG) HETCOR measurements. With increasing swelling ratio, structural and conformational changes were recognized in the hydrophilic PVIm phase of the APCN. In the kinetic swelling study, unexpectedly, four different swelling ranges were identified by gravimetric measurements, solid-state NMR methods, and positron annihilation lifetime (PAL) spectroscopy before the APCN reached its equilibrium swelling state. In the first period, which takes place in several minutes, the ordered structure disintegrates in the PVIm phase and the water uptake is relatively slow. This structural realignment is followed by the main course of water uptake governed by Fickian diffusion in the second stage. Close to the equilibrium swelling ratio, the swelling curve becomes nonmonotonic caused by a realignment of main chains of the hydrophilic phase in the third stage of swelling. Thus, by the unique combination of conventional swelling kinetics and solid-state NMR as well as PAL spectroscopies for investigating the aqueous swelling of the PVIm-l-PTHF amphiphilic polymeric conetwork, it was revealed that unexpected noncontinuous swelling occurs which is due to structural changes of the PVIm component in the conetwork in the course of this process.

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(SANS),8 and solid-state NMR structural studies.7,9 In addition to their unique nanophasic morphologies, APCNs belong to a special class of hydrogels with special properties of rapidly increasing interest not only in polymer and material sciences but also in the medical sciences as well.10 One of the most interesting properties of these materials is their ability to swell

INTRODUCTION Amphiphilic conetworks (APCNs)1−10 are multicomponent, three-dimensional macromolecular structures composed of covalently bonded, otherwise immiscible, hydrophilic and hydrophobic polymer chains. The nonmiscible hydrophilic and hydrophobic components form separated nanodomains in these two- or multicomponent systems, as it has been shown by different methods, such as transmission electron microscopy (TEM),4,5 atomic force microscopy (AFM),3a,4,6 small-angle Xray scattering (SAXS),4a,6b,c,7 small-angle neutron scattering © 2012 American Chemical Society

Received: July 23, 2012 Revised: September 3, 2012 Published: September 17, 2012 7557

dx.doi.org/10.1021/ma301533g | Macromolecules 2012, 45, 7557−7565

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free volumes between polymeric chains. According to the Tao− Eldrup equation,23 the lifetime of o-Ps is a monotonic function of the size (radius) of the free volume within a polymeric sample. As the pick-off annihilation is also governed by the electron density at the site of annihilation, the lifetime of o-Ps is a function of sterical and chemical properties of the studied polymeric system. The structure of an ACPN is very complex, and positrons scan all parts of it before their annihilation. Thus, the measured o-Ps lifetimes reflect every change within both the hydrophilic and the hydrophobic substructure. Results, while giving unique information, cannot be treated without further consideration. Previously, it was reported by us22 that changes in the free volume and in the pore structure of an APCN caused by the swelling process can be revealed by PAL spectroscopy in detail. Although numerous investigations have been dedicated to studying the swelling behavior of hydrogels, the realignments in the structure of the polymer systems have not been observed without doubts until now. However, a better understanding of structural changes during the swelling process is essential for many purposes, such as drug and substance release devices or for hydrogel containing hybrid materials. The aim of the present work was to investigate the structural realignments of polymer chains and their conformational changes during the swelling process of a conetwork, PVIm-l-PTHF, in water by combining solid-state NMR and positron annihilation lifetime spectroscopy. According to our best knowledge, such a combined study has not been reported so far on the swelling process of polymer gels.

in solvents of different polarity (both aqueous and nonaqueous environment) without dissolving. Recently, poly(N-vinylimidazole)-l-poly(tetrahydrofuran) (PVIm-l-PTHF) (“l” stands for “linked by”) APCNs have been successfully synthesized in our laboratories, and investigations have been started to reveal the fundamental structural features, properties, and possible applications of these new materials.1 Imidazole is a well-known important structural unit in essential biomacromolecules, such as DNA, RNA, and proteins, and also in pharmaceutical compounds. PVIm is a neutral polymer which can become a polyelectrolyte, since the imidazole side groups are weak bases and thus can be protonated in acidic solutions. Swelling studies of homopolymers PVIm11 and PVIm based gels12 have already been reported. It was shown that polymer chains undergo conformation change during swelling in polar media as a function of pH originating from the hydrogen bonds between protonated and nonprotonated forms of imidazole rings as well as from the charge repulsion of the protonated polymer side groups.11,12d,e The hydrophobic, biocompatible PTHF cross-linker in the PVIm-l-PTHF APCN possesses good flexibility, hydrolysis resistance, and water stability. These properties make PTHF adapted for “soft” segments in thermoplastic elastomers13 and biomaterials.14 Comprehensive structural and conformational studies during swelling in pure water have not been reported on cross-linked structures with amphiphilic character, namely hydrophilic PVIm and hydrophobic PTHF based (PVIm-lPTHF) conetworks, yet. Solid-state NMR spectroscopy is an effective technique which is widely used to characterize macromolecular systems,15 especially amorphous and mixed systems that cannot be characterized by scattering methods. One-dimensional solidstate 13C cross-polarization magic angle spinning (CP MAS) and direct excitation (DE MAS) measurements can provide valuable pieces of information on the changes of chain conformation and mobility of main and side chains of polymers. The two-dimensional frequency-switched Lee−Goldburg (FSLG) HETCOR 1H−13C technique provides more information than the one-dimensional method by increasing the resolution and, additionally, providing spatial proximities at the atomic scale. In the past years, this method has been successfully used not only to small molecules but also to various macromolecular systems as well.16−20 By combination of the mentioned solid-state NMR techniques, small changes in the conformation of polymers can be recognized in order to elucidate the mechanism of the swelling process. Positron annihilation lifetime (PAL) spectroscopy has also become a useful technique to characterize polymer systems in the past decades.21,22 The positron, being the antiparticle of electron, annihilates with electrons of the sample. The rate of the annihilation, the reciprocal of the so-called lifetime, depends on the electron density of the sample. For macromolecular materials, the most reliable information on the structure of the sample can be obtained by the aid of the orthopositronium (o-Ps) atom. This species is the hydrogenlike bound state of a positron and an electron. It has the same size as a hydrogen atom and its intrinsic lifetime is about 0.1 μs. This lifetime is reduced in polymers by surrounding electrons through the so-called pick-off annihilation. In this process, the positron of the o-Ps annihilates with an electron of the sample instead of its “self-electron”. Fortunately, the lifetime of o-Ps remains long enough (nanoseconds) to scan intermolecular

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EXPERIMENTAL SECTION

Materials. N-Vinylimidazole (VIm, Aldrich) was vacuum-distilled from CaH2 (95%, Aldrich) at 72 °C and kept under nitrogen until used. Poly(tetrahydrofuran) dimethacrylate (PTHFDMA) was prepared by end-functionalization of hydroxyl-ended poly(tetrahydrofuran) (Terathane 1500 polyether glycol, PTHF, Mn ∼ 1500, Fluka). 2,2′-Azobis(2-methylpropionitrile) (Aldrich) was recrystallized from methanol before use. Freshly distilled absolute ethanol and benzene (Spektrum 3D) were used as solvents for the copolymerization and homopolymerization, respectively. Distilled water was used in experiments with water. Preparation of Poly(N-vinylimidazole) (PVIm) Homopolymer. The PVIm homopolymer was synthesized by radical polymerization of N-vinylimidazole (VIm) in benzene with AIBN as initiator. The desired amount of monomer (1.92 mL, 21.2 mmol) was dissolved in benzene, and then the initiator stock solution (18.5 mg, 0.11 mmol) was added to the reaction mixture. Oxygen was removed by a freeze− thaw process. The reaction mixture was kept in a glass reactor tube in an oil bath at 70 °C under nitrogen using constant stirring for a period of 48 h. Then the polymer was dissolved in methanol (30 mL) and precipitated in acetone. The precipitated polymer was filtered and dried first in air and then in a vacuum oven at 60 °C. The yield was 67%. The purity and structure of the polymer were analyzed by 1H solution-state NMR spectroscopy. Synthesis of PVIm-l-PTHF Conetwork Sample. The end-group modification of hydroxyl-ended poly(tetrahydrofuran) macromonomer was carried out by reacting PTHF with methacryloyl chloride to result in poly(tetrahydrofuran) dimethacrylate (PTHFDMA). The PTHFDMA was characterized by GPC (Mn = 1660, Mw/Mn = 1.84) and by 1H solution-state NMR for end-group functionality (Fn = 2.0). For the synthesis of the APCN, the desired amounts of PTHFDMA cross-linker (0.657 mmol) and comonomer (10.6 mmol VIm), initiator stock solution (0.022 mmol AIBN), and common solvent, EtOH, were measured into glass vials. The total volume of solutions was 6 mL. The reaction mixture was homogenized, and the oxygen was removed by nitrogen purging. The solution was poured into a Teflon 7558


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mold in an AtmosBag (Sigma-Aldrich) under a nitrogen atmosphere. Then the mold was closed under nitrogen and kept in an oven at 65 °C for 3 days. Subsequently, the mold was cooled to room temperature, and the solvent was evaporated followed by drying under vacuum overnight. Then the resulting gel was extracted with water, water/ethanol mixture, and ethanol to remove any extractable impurities, 1 week for each solvent, giving 3.4 wt % extractables. Finally, the extracted conetwork was dried to constant weight under vacuum at 50 °C. The composition was determined by elemental analysis, giving 54 wt % PVIm and 46 wt % PTHF content. Swelling Experiments. After the extractables were removed from the cross-linked polymer sample, the inner part of the sample was cut into small pieces then dried. These small samples were used for further measurements. Two independent measurements were performed for studying swelling as follows: the dried conetwork sample was immersed into a deionized water bath at room temperature and was left to swell. The samples were taken out from the water bath at given time intervals, wiped by using a filter paper, weighed and placed again in the water bath. The degree of swelling (Q) as a function of time was calculated from the weight of the water imbibed sample per unit weight of the dry sample:

Qt =

mt − m 0 m0

Since the time to record a PAL spectrum was much longer than the swelling times applied at the first stage of the experiment, it was crucial to test the reliability of obtained data. To fulfill this requirement, we have chosen three stages of the swelling process where three consecutive spectra were recorded. These stages were after 20, 180 and ∼6500 min of swelling time. No significant change occurred in any of these measurements, so we can state that the sample was stable during PALS measurements.

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RESULTS AND DISCUSSION Synthesis and Swelling Experiments. The poly(Nvinylimidazole)-l-poly(tetrahydrofuran) (PVIm-l-PTHF) conetwork was prepared by radical copolymerization of VIm and methacrylate−telechelic PTHF (PTHFDMA) macromonomer.1,2 The copolymerization reaction and the schematic representation of the resulting conetwork are displayed in Scheme 1. This simple process yields a transparent polymer Scheme 1. Formation of Poly(N-vinylimidazole)-lPoly(tetrahydrofuran) (PVIm-l-PTHF) Conetwork by Radical Copolymerization of N-Vinylimidazole (VIm) with Telechelic Poly(tetrahydrofuran) Dimethacrylate (PTHFDMA) Macromonomer

(1)

where mt is the weight of the swollen hydrogel at time t and m0 is the weight of the dry sample at time t = 0. The normalized swelling degree (Qn) was defined as the ratio between the swelling degree at time t (Qt) and the swelling degree at the equilibrium stage (Qeq). Solid-State NMR Measurements. Solid-state magic angle spinning (MAS) spectra of samples were recorded on a Varian NMR system operating at a 1H frequency of 600 MHz with a Chemagnetics 3.2 mm narrow-bore triple resonance T3 probe in double resonance mode. The spinning rate of the rotor was 10 kHz in all cases. For the one-dimensional 13C CP MAS (cross-polarization magic angle) and DE (direct excitation) spectra, 2000 transients were recorded with SPINAL-64 decoupling with a strength of 83 kHz.24 The dry samples were measured with CP MAS (with 2 ms of contact time) and the swollen samples with the DE technique. A recycle delay of 5 s was used for all experiments, which is 5 times larger than T1H. 13 C spectra were deconvoluted with the DMFIT software.25 Twodimensional FSLG HETCOR 1H−13C spectra26 were recorded with contact time of 300 μs to detect only the close connectives. The FSLG scaling factor was 0.53. FSLG HETCOR spectra were recorded with 128 transients and 384 increments in the t1 dimension with 2 s of recycle delay. The temperature of all the measurements was 25 °C. Adamantane was used as external chemical shift reference (38.55 and 29.50 ppm). The 90° pulse lengths were 3 μs for both the proton and the carbon channels for all the NMR experiments. Positron Annihilation Lifetime Spectroscopy. Positron annihilation lifetime (PAL) spectra were performed by a conventional fast−fast coincidence system. BaF2 scintillation crystals were mounted on Philips XP2020Q PMTs, and their signals were processed by standard Ortec units. Spectra were recorded in 4096 channels of a Nucleus PCAII card. The positron source was made of carrier free 22 NaCl of the activity of about 105 Bq sealed between Kapton foils. The spectra were analyzed by the RESOLUTION27 code. Three lifetime components were resolved in each spectrum. The time resolution of the system was about 225 ps. The swelling experiments were carried out in a climatized laboratory; the temperature was kept at 17 °C with 45% relative humidity. Two identical pieces of the APCN sample were left to swell in water for a given time and then taken out. The excess water was lightly wiped off, and the sample−source sandwich was prepared. The recording of a spectrum took 2 h during which the sample/source sandwich was tightly sealed in aluminum foil. After the recording of the PAL spectrum, the sample−source sandwich was taken apart and the swelling was resumed. The swelling time was defined as the total time the samples were allowed to soak; i.e., the measurement time was totally ignored.

conetwork in which the hydrophobic PTHF is a macromolecular cross-linker of hydrophilic PVIm chains. Low amounts, 3.4 wt %, of extractables were obtained, which indicate highly efficient conetwork formation. The dry, optically clear APCN with a composition of 46 wt % PTHF content was cut into sections, and these pieces were used for all the different measurements. The swelling experiments of the PVIm-l-PTHF APCN sample were carried out in water to investigate the swelling process of this new material in details. The investigated conetwork sample, as many other APCNs, has a unique nanophase-separated structure indicated by two glass transition temperatures (Tg): one in the region of the Tg of PTHF and another one at around the Tg of PVIm (DSC thermogram and analysis data of the PVIm-l-PTHF APCN are given in the Supporting Information). The phase-separated morphology of the polymer conetworks influences their fundamental physical properties and thus their swelling behavior as well. In APCNs, the hydrophilic and hydrophobic phases are separated on the nanometer scale.4−9 The separated nanophases of components are not independent of each other because they are connected together by chemical bonds. They are able to swell in a wide variety of polar and nonpolar solvents;4b that is, APCNs have amphiphilic character. The selective swelling of a given phase is possible in an appropriate solvent, and the equilibrium swelling ratio depends on the composition.1a,4a−c Most of the APCNs investigated so far possess special cocontinuous structure for both phases in the medium composition ranges.4a−c,7 First, the swelling process of the PVIm-l-PTHF conetwork was studied by conventional gravimetric swelling degree measurements at 25 °C. The swelling degrees as a function 7559


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of the first 60% of water uptake of hydrogels. For a Fickian diffusion, n varies around 0.5 (from 0.43 to 0.51) and depends on the geometry of the sample.29a The extensive swelling model tries to describe the full swelling range of hydrogels. This theory of extensive swelling of hydrogels considers the diffusion of the solvent and the relaxation of polymer chains in the network structure, and it can be described by the Robinson−Shott second-order kinetic equation:30

of time in Figure 1 indicate that the aqueous swelling of this conetwork is not a uniform process. Similar swelling curves

dQ t dt

= ks(Q eq − Q t )2

t t 1 = A + Bt = + Qt Q eq ksQ eq 2

have been obtained previously for other conetworks as well10l,m,28 (examples with PVIm-l-PTHF conetworks are shown in Figure S2). Various interpretations exist to describe the swelling kinetics of polymer hydrogels. A simple approach is based on water diffusion into the polymer gel neglecting the changes in the gel structure.29 The initial section of the swelling can be approximated by an exponential relation in this approach: Q eq

(4)

where coefficients A and B are calculated from the slope and the intercept of the plot by eq 4, respectively. At long swelling times, the value of B is the reciprocal of the equilibrium swelling degree, the value of A is the reciprocal of the initial swelling rate of the network, and ks is the rate constant of swelling. As it is obvious at the first glance in Figure 1, the Fickian approach (eq 2) is not valid for the initial range of the swelling process for our APCN. The solvent uptake is much slower than that expected by Fickian diffusion of water below 30% of swelling (n ≈ 0.34), but it follows a nearly Fickian behavior (n ≈ 0.43) between 30 and 80% of relative swelling. By the analysis of the swelling curve, the water uptake process can be divided into four stages, including the equilibrium state. In the first stage, that is during the first 10 min of swelling, the water transport is hindered, and it is slower than the Fickian model predicts. This can be related to structural rearrangement of polymer chains. In the second stage, the water uptake follows the Fickian model (eq 2) (n ≈ 0.43) and water molecules can diffuse slowly into the sample. The third stage of swelling is close to the equilibrium (Qn > 80%), where an unanticipated effect occurs. The swelling curve becomes nonmonotonic. This unexpected change in the swelling curve suggests changes in the structure of the polymer chains and/or in the morphology of the nanophases. This phenomenon is reproducible, as the data of the repeated experiment show in Figure 1. As the spectroscopic measurements, presented later in this study, also indicate, an unexpected behavior of the APCN sample occurs in this range of the swelling process. The last stage is the equilibrium state of the swelling (Qeq) that occurs after 8 h of swelling.

Figure 1. Swelling isotherms of PVIm-l-PTHF APCN sample (46 wt % PTHF content) in distilled water at room temperature (a) and enlargement of the anomaly of the dynamic swelling isotherms (b). Open and filled symbols represent independent gravimetric swelling experiments.

Qt

(3)

= kt n (2)

where k is a characteristic constant and n is the diffusion exponent. Equation 2 is commonly used for the interpretation

Figure 2. Second-order plot of the swelling data of PVIm-l-PTHF APCN (46 wt % PTHF content) according to eq 3 for the full (a) and the initial (b) swelling time ranges. Open and filled symbols represent two independent measurements. 7560


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are CH carbons and their relaxation behavior is similar. The integration of the aromatic region indicates that the signals at 122 and 117 ppm belong to the C5 carbon atom. To clarify the origin of this signal duality, two-dimensional 1 H− 13 C frequency-switched Lee−Goldburg (FSLG) HETCOR spectra were recorded with a contact time of 300 μs. This time is appropriate to identify magnetization transfer paths that do not belong to covalently bonded CH pairs. In the resulting correlation spectra, one cross-peak pair can be recognized that does not originate from bonded CH, as Figure 4 shows.

A more general description of the swelling of a hydrogel is the so-called extensive swelling kinetic model.30 The t/Qt versus swelling time plot according to eq 4 is shown in Figure 2. The resulting curve differs from the Robinson−Shott secondorder kinetic equation in the initial stages of swelling (Figure 2b), indicating that the swelling process cannot be fully described by either the classical linear or quadratic models commonly used for evaluating the swelling process of homopolymer hydrogels (the data obtained for the initial swelling stages by the two models are summarized in Table S1). The systematic deviation from the second-order kinetic model is most obvious in the first two sections of swelling, as is shown in Figure 2b. The data in this figure differ significantly from the expected straight line, even if the fitting proved to be suitable for longer swelling times. The special structure of APCNs; i.e., all the hydrophilic chains are connected to hydrophobic chains, and changes in chain−chain interactions and/or chain conformation can result in such a deviations from common models. In order to investigate these observed anomalies in the swelling behavior of the PVIm-l-PTHF conetwork, solid-state NMR and positron annihiliation lifetime spectroscopies were applied to reveal the potential structural changes occurring during the swelling process. Solid-State NMR. Cross-polarization magic angle spinning solid-state NMR spectra of poly(N-vinylimidazole) has been already reported in the literature.31 The spectrum consists of broad signals according to the amorphous structure of the glassy polymer, as displayed in Figure 3a. Because of the weak

Figure 4. 1H−13C FSLG HETCOR spectrum of the dry PVIm-lPTHF conetwork sample. The contact time was 300 μs, and 10 kHz of spinning speed was used.

The presence of these cross-peaks shows that polarization transfer is possible from H6 to C2 and from H2 to C6. These untrivial paths are possible only if the rotation of imidazole rings, or at least a significant part of them, is hindered strongly. At the investigated temperature, the PVIm is in a glassy state; however, the rigidity of the main polymer chain cannot explain the hindered rotation and the signal duality of C5 carbon atom. If an interaction between the imidazole rings stabilized by second-order forces is considered, then both observations can be explained. The mentioned second-order interactions might change the shielding tensor and the shielding effect of the aromatic ring, and thus the aromatic ring current can cause the signal duality of the C5 carbon atom. This interaction can explain the ∼80 °C difference between the glass transition temperatures of PVIm and polystyrene. The second-order interaction between the imidazole rings can hinder the segment motions of the polymer chains, which results in higher Tg than that of polystyrene with pendant aromatic groups consisiting of only carbon and hydrogen atoms. Water-swollen PVIm-l-PTHF samples were measured with the direct excitation technique because the increased mobility decreases drastically the efficiency of cross-polarization. The recorded spectra were analyzed by deconvolution of 13C resonances. The increased mobility causes line narrowing, as Figure 5 shows. This line narrowing is most obvious at higher

Figure 3. 13C cross-polarization magic angle spinning (CP MAS) spectra of PVIm homopolymer (a) and PVIm-l-PTHF conetwork (b) at room temperature (the spinning speed was 10 kHz in both cases; asterisks denote the spinning side bands of the aromatic signals).

resolution, aliphatic signals belonging to different tacticity triads are overlapping with each other. In the aromatic region, four signals can be identified, although imidazole rings contains only three carbon atoms. The spectrum of the dry PVIm-l-PTHF conetwork sample is similar, although, additionally, two sharp signals belonging to PTHF can also be seen in Figure 3b. The similarity of aromatic and aliphatic signals of the PVIm in the two spectra and the narrower PTHF signals suggest a phaseseparated morphology in accordance with the results obtained by DSC measurements (see Figure S1). Four aromatic signals, instead of three, were also recorded by Ruhnau and Veeman,31 but to our best knowledge, not any explanation has been given for this phenomenon. In general, the CP signals cannot be integrated without knowing their cross-polarization build up curves, but in this case, the integral values of aromatic signals are comparable because all of them 7561


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swelling process, except that, at 3 h, both aromatic and aliphatic signal intensities have maxima. At this time, the gravimetric swelling curve becomes nonmonotonic, and solid-state NMR results support the changes of the PVIm chain structure at this stage of the swelling process. The increased chain mobility denotes a further chain rearrangement. Positron Annihilation Lifetime (PAL) Spectroscopy. Positron annihilation lifetime (PAL) spectra of swollen PVIm-lPTHF conetwork were recorded and were analyzed in terms of three discrete components, of which the two shorter did not show significant changes as a function of the swelling time. The lifetime of the shortest component was about 165 ps with a relative intensity of about 40%, and the medium long component was around 410 ps and 47%. On the other hand, the third component, which is associated with o-Ps, indicates an interesting swelling behavior. Figure 7 shows the lifetime and Figure 5. Direct excitation 13C NMR spectra of the water-swollen PVIm-l-PTHF conetwork sample. The signal at 113 ppm (denoted with an asterisk) originates from the Teflon packing of the rotor.

swelling ratios where signals corresponding to different tacticity triads can be observed in the aliphatic region. The most important phenomenon is the change of the structure of aromatic signals. Even at 2 min of swelling, the duality of the C5 signal vanishes; however, the signals are still broad. From this observation, we can conclude that the first step of the swelling process is the decomposition of the interactions between aromatic rings. This can explain the gravimetric swelling results, i.e., the slower water uptake at the first phase of swelling. Simultaneously, the signal intensity of the aromatic and aliphatic resonances increases, as Figure 6 shows. The

Figure 7. o-Ps lifetime as a function of swelling time of PVIm-l-PTHF conetworks. Solid symbols denote data obtained by averaging three independent measurements, and the error bars represent the standard deviation. Open symbols indicate single spectra, and the error bars show the estimated error of the fitted value. Numbers mark the order as the three independent spectra were recorded. Arrows are just guides to the eye.

Figure 6. Relative intensity of aromatic (Iaromatic/IPTHF) and aliphatic (Ialiphatic/IPTHF) signals of PVIm during swelling of PVIm-l-PTHF conetwork in water. The direct excitation 13C spectra were deconvoluted, and the integral values were correlated to the PTHF signals. Arrows are depicted to guide the eye.

Figure 8. Intensity of o-Ps as the function of swelling time of PVIm-lPTHF conetwork. Arrows are just guides to the eye.

intensity values were correlated with the PTHF signals which remained unchanged during the swelling process. Because the repetition time was shorter than 5 times T1 of the carbon atoms, the intensity increase indicates the decrease of T1 relaxation and faster motions. After 2 min of swelling, the intensity of PVIm signals increases and then, for further water uptake, decreases. This first maximum indicates faster molecular motions, which can be connected to the decomposition of secondary bonds between side chains. The intensity of PVIm signals are nearly constant during the subsequent

Figure 8 the intensity of the o-Ps lifetime component as a function of swelling time. In order to illustrate the reliability and reproducibility of these spectral parameters, the data obtained from repeated measurements are plotted separately in several cases, numbers indicating their recording sequence. As there is no correlation between the recording order and the value of the given parameter, one can assume that the composition of the APCN sample did not change throughout 7562


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Table 1. Relevant Information Supplied by the Applied Methods on the Swelling Process of PVIm-l-PTHF Conetwork in Water stage I 0−10 min: non-Fickian diffusion (n ≈ 0.34) imidazole ring pairs already opened up at 2 min 0−12 min: τo‑Ps increases, Io‑Ps increases

stage II

stage III

stage IV

swelling experiment 10−120 min: Fickian 120−360 min: nonmonotonic water uptake; saturation-like at diffusion (n ≈ 0.43) first (∼140 min), then step-like behavior Solid-State NMR (hydrophobic substructure remains intact up to 12 h of swelling) 180 min: intensity maximum of aromatic (and aliphatic) C signals, activated movements of main chains PAL 12−300 min: τo‑Ps 300−420 min: τo‑Ps increases, Io‑Ps ≈ constant decreases, Io‑Ps increases

the recording of PAL spectra. Thus, the presented data reflect real structural changes in the APCN sample but not structural changes induced by long measurement times. Intensity parameters of the repeated measurements at 20 min illustrate the drawback of the conventional swelling experiment as well. The sample−source sandwich was prepared independently after every swelling period. As the sizes of the source and the samples were comparable, a slight misplacement of the source could occur easily. In such a case, a varying amount of positrons can avoid the sample and annihilate in the aluminum foil sample holder with a significantly shorter lifetime than that of the o-Ps. The higher relative intensity of these shorter lifetimes decreases the intensity of the o-Ps signal On the other hand, as the lifetime values are far apart, they remain well separated. The overall effect is that, while the recorded o-Ps lifetimes represent real values, the corresponding intensities can carry large errors; thus, intensity data must be treated with precaution. Figure 7 shows the o-Ps lifetimes as the function of swelling time. One can distinguish four distinct stages of the swelling process on the basis of these data. The short first stage ends at about 10 min, then a second stage appears between 10 and 300 min (5 h), a third stage between 5 and 10 h (300 and 600 min), and finally a fourth stage occurs after 10 h. The first and second stages are common in APCNs.22 At first, a quick rearrangement of polymer chains increases the size of free volume holes in the conetwork, which is followed by a significant and continuous water uptake. This latter process decreases the space available for o-Ps atoms, and thus, it also decreases the lifetime. The astonishing result is the third stage of swelling at 300 min. Here, the o-Ps lifetime increases sharply indicating an increase of free volume holes, implying a change in the molecular structure of the swollen conetwork. The fourth stage is just the swelling (or the relaxation) of this new structure. Figure 8 shows the relative intensity of o-Ps formation as the function of swelling time. The four stages observed on the basis of lifetime values are discernible in intensities as well. With a significant increase of o-Ps formation intensity after the first minute of swelling, the o-Ps intensity increases slowly (1 to 12 min) at first and then more markedly (30 to 100 min). Between 300 and 600 min of swelling time the intensity remains constant and, then, starts to decrease to its final value. Swelling Mechanism. PAL results connect conventional swelling data to structural data supplied by NMR spectroscopy. Swelling data were obtained by the very reliable method of mass measurement. Their precisity is high, but they supply only an integrated view of the swelling process. On the other hand, solid-state NMR data represent only the changes and molecular processes within the carbon structure of polymeric chains without any hints for the macroscopic structure. Meanwhile, o-

360−3000 min: equilibrium, no further water uptake

600−10 000 min: τo‑Ps decreases, Io‑Ps decreases

Ps atoms provide information about the intermolecular space, where incorporated water molecules reside. However, one significant aspect of the comparison of the three methods must be addressed here. As PAL measurements were carried out at well below the usual room temperature, i.e., at 17.5 °C instead of 25 °C, the swelling times at which the stages of swelling start or end might differ from temperatures indicated by the other methods. The relevant pieces of information supplied by the three different methods are summarized in Table 1. As the hydrophobic substructure of poly(tetrahydrofuran) remains intact by water up to 12 h of swelling, as shown by the NMR results, the whole swelling process takes place only in the poly(N-vinylimidazole) structure. The changes in the o-Ps lifetime parameters originate from this same substructure. At the first stage of swelling, where the non-Fickian behavior has been observed, both the o-Ps lifetime and its intensity increase (0 to 10−12 min). The solid-state NMR results show that the swelling process starts with the decomposition of imidazole− imidazole interactions. At this stage, the concentration of water molecules is still low in the conetwork; thus, o-Ps reacts only to the increase of the size of free volume holes between polymer chains which is forced by the incorporation of water molecules. The increase of o-Ps lifetime shows that the free volume between the polymer chains increases, indicating that more and more paths become available for water to diffuse in the APCN. In this first stage, the water content is low enough to permit Ps to seek out “water-free” free volumes. The size of these “waterfree” sites increases, as more and more incorporated water molecules are built in between the chains. The origin of the non-Fickian swelling at this stage originates from the continuous change of the size of free volume holes. After about 10 min of swelling, the concentration of water becomes high enough to increase the distance between polymer chains to ensure an almost free diffusion of water molecules into the APCN. As the concentration of water increases, o-Ps has less and less space between the chains, as it is indicated by the decrease of its lifetime. Such a swelling behavior, as mentioned above, has already been observed in the case of poly(N,N-dimethylaminoethyl methacrylate)-l-polyisobutylene conetworks by PAL spectroscopy.22 The significant increase of the o-Ps lifetime at the third stage is unexpected. Since it cannot be attributed to the conventional swelling process, it must be an indication of a structural change in the PVIm-l-PTHF conetwork. While PAL data show this structural change clearly, the other two methods also signal some type of transition at this stage of swelling. A closer examination of swelling data (Figure 1b) suggests that the swelling process reaches a quasi-equilibrium at around 140 min, and then, the water uptake continues with a rate larger than expected. The relative intensities of 13C signals (Figure 6), connected with molecular motions and rearrangements through 7563


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relaxation times, also show a maximum at 180 min. This might be due to a change in molecular movements of main chains at this swelling stage. To explain the findings of the three combined methods within a common model, we propose the following scenario. The first two stages of the swelling process lead to a quasiequilibrium at around 140 min of swelling (∼85% swelling ratio). At this stage, the water fills all the available space within the poly(N-vinylimidazole) structure, and as an effect of the incorporated water, the distance of these main chains increases. Also, the incorporated water acts as a “lubricant”; thus, the increased intermolecular space and the lubricating effect allow the rearrangements of the main chains and the connected imidazole rings. The thermodynamic driving force of such a rearrangement can easily be the formation of a more stable Hbond structure around imidazole rings. This scenario explains the results of the PAL spectroscopy measurements as well. As the stable H-bond structure is formed, the remaining structure becomes less dense. The reforming structure around imidazole rings means a stronger screening of delocalized π-electrons. This leads to an overall decrease of electron density and thus to an increased o-Ps lifetime. The swelling process resumes at the end of this third stage due to the regained flexibility of the backbone of PVIm. The fourth (and final) stage follows the usual swelling characteristics. The slow water uptake near the equilibrium results in a decrease of o-Ps lifetime while it levels off at approximately the same value, as at the end of the second stage.

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CONCLUSION

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.D.); tamas.marek@chemres. hu (T.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the elemental and GPC analyses to Dr. Hedvig Medzihradszky-Schweiger and Dr. Márta Szesztay, respectively. Partial support of this research by the Hungarian Scientific Research Fund (OTKA K81592) and by the Nanomedicine Thematic Program of the Chemical Research Center of the Hungarian Academy of Sciences is also acknowledged. We also gratefully acknowledge the financial support of the Hungarian project GVOP-3.2.1.-2004-04-0210/ 3.0 for the NMR equipment.

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REFERENCES

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By the combination of three methods, conventional swelling kinetics, solid-state NMR, and positron annihilation lifetime (PAL) spectroscopies, to investigate structural changes of polymer chains during swelling, an anomalous swelling behavior an amphiphilic conetwork was observed in water. The swelling of poly(N-vinylimidazole)-l-poly(tetrahydrofuran) (PVIm-l-PTHF) starts with a rapid rearrangement of the pendant groups of the PVIm chains. PAL measurements of the swollen conetwork indicate an increase of the free volume while NMR spectra show the opening up of imidazole ring pairs in the first 10 min of the swelling process. At this 10 min long first stage, the water concentration is very low, as indicated by classical gravimetric measurements. The second stage is the actual swelling between 10 min and 3 h of swelling. The diffusion of water into the conetwork is Fickian, and NMR spectra indicate an increasing movement of main chains at this stage. The anomalous swelling stage occurs around 3 h of swelling. All three methods indicate a structural rearrangement of the PVIm chains at this point. This structural change becomes possible because of the increased mobility of the polymer chains due to the “lubricating” effect of water molecules built in between the chains. After this rearrangement, the swelling process resumes and continues until the equilibrium state. Thus, we can conclude that the aqueous swelling process of PVIm-l-PTHF conetworks, according to the data presented above, is not a continuous but an unprecedented stepwise process.

S Supporting Information *

Figures S1 and S2; Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. 7564


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Anomalous Swelling Behavior of Poly(N-vinylimidazole)-l-Poly

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