Behavior of the Interphase Region of an Amphiphilic Polymer

Article pubs.acs.org/Macromolecules

Behavior of the Interphase Region of an Amphiphilic Polymer Conetwork Swollen in Polar and Nonpolar Solvent Attila Domján,*,† Péter Mezey,‡ and Jenő Varga§ †

Institute of Structural Chemistry, ‡Institute of Materials and Environmental Chemistry, §Institute of Biomolecular Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1025 Budapest, Pusztaszeri ut 59-67, Hungary S Supporting Information *

ABSTRACT: The most attractive property of amphiphilic polymer conetworks (APCNs) is their ability to swell in both polar and nonpolar solvents. Depending on the composition, their structure is phase separated on the nanometer scale possessing highly different morphologies. This special nanophase-separated structure provides numerous possible applications for heterogeneous chemical and biological processes. Although the interphase region can fundamentally influence the material transport between the different polarity phases, there has been no specific information regarding its nature. Recent work demonstrates that by selective labeling of the crosslinking molecules by deuterium, information on the mobility of the interphase region can be obtained by solid-state NMR techniques. The first results show that this interphase region behaves differently in dry state as well as when swollen in polar or nonpolar solvents. Although the cross-linker is a polar molecule, its mobility hardly changes upon swelling in water; however, its mobility increases drastically by swelling in heptane. Additionally, the amount of nonreacted, thus non-cross-linked, chain ends could be quantified by solid-state NMR methodologies.

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INTRODUCTION Since the first successful synthesis of amphiphilic polymer conetworks (APCNs),1,2 a broad interest has risen from both synthetic and application points of view. Generally, APCNs are composed of covalently connected hydrophilic and hydrophobic chains with phase-separated structure. The size of the separated domains lies on the nanometer scale with diverse morphology. Because of their special chemical composition and morphology, they are able to swell in both polar and nonpolar solvents, keeping their superior mechanical properties as opposed to homopolymer gels. APCNs have been synthesized with various chemical and molecular structure in the past.3−6 Extensive research has been carried out to describe the structure and morphology of these materials by AFM,7,8 TEM,7,9 SEM,10 SAXS,7,11 SANS,9 and solid-state NMR11 methods. All these studies have confirmed the phase-separated structure, but only the 1H spin-diffusion study11 suggested that an interphase region exists between the phases, with a size not exceeding 1 nm. Because of of its small size, neither the macroscopic properties can be observed nor its existence can be proved by microscopic or scattering methods. The special properties of APCNs in combination with their large chemical and structural variety have inspired an array of different application-oriented studies in chemical, biological, and medical fields, such as nanoreactors,7,12 catalysis,13 solid-phase extractions,14 pervaporation matrices,15 promoted release hosts,16 medical implants and immonoisolators,17 cell culture18 and antifoulding surfaces,19 drug delivery matrices,20 contact lenses,15 tissue engineering scaffolds,18,19b enzymatic catalysis supports,12 sensors, etc. To directly observe and study the interphase region and to gain information on its properties, poly[2-(N,N-dimethylamino)ethyl © 2012 American Chemical Society

methacrylate]-l-polyisobutylene (PDMAEMA-l-PIB) samples of different compositions (31, 49, 65 wt % of PIB) were synthesized. The conetwork samples were prepared by the macromonomer method, using deuterium-labeled telechelic α,ωdimethacrylate(d5)-polyisobutylene (Scheme 1). Static deuterium Scheme 1. Chemical Structure of the Cross-Link Point of the Poly[2-(N,N-dimethylamino)ethyl methacrylate]-lpolyisobutylene Amphiphilic Polymer Conetworka

a

The cross-linker molecules are labeled by deuterium.

NMR spectra were recorded, deconvoluted, and analyzed in dry state as well as swollen in polar (water) and nonpolar (n-heptane) solvent.

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EXPERIMENTAL SECTION

Static solid-state NMR spectra of the samples were recorded on a Varian NMR System operating at a 1H frequency of 600 MHz (92.1 MHz for 2H) Received: October 26, 2011 Revised: December 15, 2011 Published: January 9, 2012 1037

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quadrupolar coupling constant of ∼δ/3. Appearance of a Pake doublet shows that both nanophases are in a rubbery state, the interphase region; i.e., the cross-linker molecules are only capable of hindered motions. The CD3 methyl groups can rotate freely, and the CD2 methylene groups make crankshaft motion.24 For the relatively rigid chains revealing a kink motion was not detectable. This phenomenon shows that for the dynamics of the cross-link points both phases have crucial role. At room temperature the PDMAEMA homopolymer is a solid, while the PIB is a viscous liquid. The mobility of the crosslinking points is much closer to the hydrophobic than to the hydrophilic component. Swelling of the conetwork sample in water increases the mobility of the hydrophilic PDMAEMA chains, but astonishingly the deuterium spectra do not change dramatically (Figure 1b). On the contrary, swelling in n-heptane results in drastic changes (Figure 1c). The molecular motions are becoming very fast, and the quadrupolar interactions are averaging out almost completely. Two signals can be observed in the spectrum with approximately the same shape. The lower chemical shift signal at 1 ppm can be assigned to CD3, while the higher shift at 3.5 ppm can be attributed to the CD2 groups. Detailed analysis of the deuterium spectra provides more information. Therefore, the spectra of dry samples and samples swollen in water were deconvoluted into a Pake doublet and two or three Gaussian peaks by the DMFIT25 software. Because of the complexity of the motions and hereby the ways of averaging of the quadrupolar tensor, no physical meaning was attached to the Gaussian signals. The determined quadrupolar coupling constants were assigned to crankshaft motion and/or to methyl rotation. In samples swollen in water an additional narrow signal emerges around 3.5 ppm, which can be attributed to the CD2 group of non-cross-linked PIB-MA chain ends. The CD3 signal of these non-cross-linked chain ends cannot be differentiated from cross-linked CD3 signals because their shape is mostly determined by the fast rotation. No signal belonging to double bonds can be detected; thus, all the PIBMA chain ends were reacted with DMAEMAE, but part of them are not connected to the network structure. The intensity of this additional peak is about 1−2% relative to the central line; thus, the amount of non-cross-linked chain ends can be estimated to be 2−3%. This observation opens up the possibility of quantification and direct determination of the ratio of “free” chain ends not only in the given conetwork but for APCN samples in general. As shown in Figure 2, the quadrupolar coupling constant decreases in the dry samples and increases in the water swollen samples with increasing PIB content. When interpreting this information, it is important to take into account that with increasing PIB content the main chain length of the hydrophilic PDMAEMA chains decreases, whereas the macromonomer MA-PIB-MA chain length remains constant with a very narrow distribution. This feature arises from the basic properties of the macromonomer method we used for the synthesis of the investigated APCN samples. A smaller coupling constant indicates that the mobility of the cross-link points, that is, the rate of the crankshaft motion, increases with decreasing of the PDMAEMA chain length. The faster cross-link motion suggests that at least a portion of the DMAEMA units in the vicinity of the cross-link points are more mobile. This finding is in a good agreement with the observation that the glass transition temperature of the PDMAEMA

with a Chemagnetics 6.0 mm narrow-bore triple-resonance T3 probe in double-resonance mode. Spectra were recorded by single pulse technique with 12 μs delay between the π/2 pulse (8 μs) and acquisition (20 ms). The relaxation delay was 2 s, and 40 000 transients were collected for each spectrum. The samples were kept in vacuum oven on 50 °C for 3 days before measurement. The swollen samples were measured after 2 days of soaking in water and n-heptane. The equilibrium swelling degree (Qeq = mswollen/mdry) of the samples was determined gravimetrically (Table 1).

Table 1. Equilibrium Swelling Degree of Samples in Water and n-Heptane

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wt % of PIB

Qeq in water

Qeq in n-heptane

31 49 65

1.75 1.51 1.24

1.95 2.55 3.19

RESULTS AND DISCUSSION

The shape of deuterium solid-state NMR spectra shows a large variety depending on the rate and symmetry of the molecular motions.22 Changes in the molecular motions as well as in the thermomechanic behavior of polymers (glass transition, melting, and crystallization, etc.) cause clearly detectable changes in the spectrum. Using the single pulse method instead of quadrupole echo causes overestimating of contribution of the narrow signals in the center of spectra. On the other hand, if motions are present in the intermediate regime, the echo line shape and intensity depend strongly on the delay time.23 Because there are not any previous investigations on the molecular motions of these kinds of materials, in the present study a single pulse technique was used. In our experiments, spectra were recorded above the glass transition temperature (Tg) of both components, i.e., 90 °C above the Tg of PIB and 10 °C above the Tg of the PDMAEMA homopolymer. According to the deuterium spectra of dry APCN (Figure 1a), it consists of two different

Figure 1. Static solid-state deuterium (2H) spectra of (a) dry, (b) swollen in water, and (c) swollen in n-heptane PDMAEMA-l-PIB amphiphilic polymer conetworks containing 49 wt % of PIB. The middle regions are enlarged on the right side of the spectra.

types of components. There is a narrow signal in the middle of the spectra which can be attributed to fast molecular motions as well as a signal reminiscent of a Pake doublet with a 1038

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the nonuniform swelling of the hydrophilic nanophases plays a significant role. In conclusion, the behavior of cross-link points in the investigated amphiphilic polymer conetwork samples differs significantly from both hydrophilic and hydrophobic nanophases. The interphase region of amphiphilic polymer conetwork plays an important role in heterogeneous processes. Because of the small size of the phases, there is an incredibly large interface region between the hydrophilic and the hydrophobic phases. The molecular motion of the interphase region is similar to the less mobile hydrophobic nanophases, and its mobility increases only a little by swelling in a polar solvent. On the contrary, its mobility increases dramatically if the conetwork is swollen in a nonpolar solvent, although the cross-linker is a polar molecule. This peculiar behavior has to be taken into account not only at the interpretation of heterogeneous processes but also at the thermodynamic description of APCNs. Additionally, we have shown that the amount of non-cross-linked chain ends can be quantified by solid-state NMR methodology.

Figure 2. Quadrupolar coupling constants of dry and in water swollen APCN samples as a function of the composition. Coupling constants were determined by deconvolution of the recorded static deuterium NMR spectra.

component decreases with decreasing its chain length in the conetwork. Swelling of the conetwork sample in water has only a small influence on the basic characteristics of the deuterium spectra, but as a result of the higher mobility of the swollen PDMAEMA nanophases the quadrupolar constant decreases. This decreasing effect is larger for longer chains. It appears that PIB chains influence stronger the mobility of the cross-link points. Swelling in n-hexane supports this observation because no Pake-like doublet can be perceived on the resulting spectra. The methyl rotation and the crankshaft motion are becoming very fast, and probably their axes can wriggle, too. The broadening of the quadropular doublet depends on the composition in both dry and water swollen samples (Figure 3).

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ASSOCIATED CONTENT

S Supporting Information *

Description of the synthesis procedure of the deuterium-labeled APCN samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Hungarian project GVOP-3.2.1.-2004-04-0210/3.0 for the NMR equipment. We thank Prof. Béla Iván for the helpful discussions on APCNs.

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Figure 3. Lorentzian broadening of the quadrupolar doublet in dry and in water swollen APCN samples.

Increase in the broadening arises from different modes of motion of the cross-link points. Since the PIB chains are of narrow molecular weight distribution, the different modes may originate from the distribution of the PDMAEMA chain length. This suggests that less PDMAEMA content, thus shorter PDMAEMA chain length, results in a broader chain length distribution. Similar to the coupling constants, the effect on the broadening is different in samples swollen in water. In the water swollen samples the increase of the broadening is much larger at high PDMAEMA content. This broadening cannot be explained exclusively by differences in chain length distribution. Most likely 1039

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