Poly(2

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Biomacromolecules 2008, 9, 1678–1683

Rapid Deswelling Response of Poly(N-isopropylacrylamide)/ Poly(2-alkyl-2-oxazoline)/Poly(2-hydroxyethyl methacrylate) Hydrogels Geta David,*,† Bogdan C. Simionescu,†,‡ and Ann-Christine Albertsson§ Department of Natural and Synthetic Polymers, “Gh. Asachi” Technical University, Iasi, 700050, Romania, “Petru Poni” Institute of Macromolecular Chemistry, Iasi, 700487, Romania, and Royal Institute of Technology, Stockholm, 10044, Sweden Received February 26, 2008; Revised Manuscript Received March 28, 2008

Ternary poly(N-isopropylacrylamide)/poly(2-alkyl-2-oxazoline)/poly(2-hydroxyethyl methacrylate) (PNIPAAm/ PROZO/PHEMA) hydrogels were prepared by the free-radical copolymerization of N-isopropylacrylamide (NIPAAm), 2-hydroxyethyl methacrylate (HEMA), and poly(2-alkyl-2-oxazoline) (PROZO) multifunctional macromonomers. The resulting polymeric materials were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), as well as by equilibrium swelling experiments. All synthesized hydrogels display temperature sensitivity in the 28-38 °C range. A high rate of response was registered as compared to that of materials based only on PNIPAAm. The swelling-deswelling peculiar behavior was related to the chemical composition (hydrophile/hydrophobe balance), the length of the inserted PROZO sequence, and inner morphology, an aspect which points on its possible control by synthesis. It was evidenced that the architecture of the resulting porous materials has a high order degree, emerging from the self-assembling of the microgel particles, which provided numerous, nearly uniform, large water release channels.

Introduction Hydrogels are attractive soft materials with applications in both chemical and technological fields. Due to the high water content, these materials are very similar to natural tissue and often exhibit good biocompatibility, allowing their use in drug delivery systems, biosensors, contact lenses, super absorbents, and artificial tissue (cell culture carriers, tissue scaffolds). Hydrogels are also good candidates for membrane applications in which water allows solute diffusion through the gel. Last years are characterized by an increasing interest for “intelligent” hydrogels, materials able to respond to external stimuli (pH, temperature, light, pressure, electric impulse, or the presence of ions/chemicals) in a controlled and reproducible manner.1–3 In this respect, the thermoresponsive hydrogels based on PNIPAAm are probably the most studied ones.2,3 This polymer presents a lower critical solution temperature (LCST) of 31 °C, when the PNIPAAm chain, even included in hydrogels, shows a phase-transition, as a result of the change from a coiled state to a globular one.4,5 Thus, PNIPAAm molecules in aqueous solution exhibit a rapid and reversible hydration-dehydration process in response to small temperature changes around LCST, which is close to body temperature of homeothermic animals. Accordingly, biorelated applications (i.e., basic component for artificial muscles, “on-off” switching materials, use in bioseparation processes, component for size-selective separation processes, or controlled drug delivery systems) may be promoted.2 Typically, certain molecules physically deposited in such thermosensitive hydrogels can be released during the deswelling in proportion to changes in the hydrogel volume. The main limitations of PNIPAAm materials are the biocom* To whom correspondence should be addressed. Phone: (+40) 232 278683. Fax: (+40) 232 211299. E-mail: [email protected]. † “Gh. Asachi” Technical University. ‡ “Petru Poni” Institute of Macromolecular Chemistry. § Royal Institute of Technology.

patibility, mechanical properties, and the slow rate of response. The last characteristic is attributed to the presence of a surfacecondensed polymer layer, generated during the initial deswelling stage from temperatures situated above LCST, acting as a skintype barrier, which suppresses the release of internal water molecules to the outside of the hydrogels. However, these environment sensitive materials are promising in different applications, particularly in areas that undergo only minute changes in temperature. In this context, the enhancement of the deswelling rate is of significant interest, and accordingly, a lot of research was developed on this subject. Some alternative solutions were proposed: thinner and smaller hydrogels (microgels),3 gels with macropore structure, micropores formation,6–8 grafting (PNIPAAm grafts with freely mobile ends),9 inclusion of hydrophilic or hydrophobic moieties,10 interpenetrating polymer network forming,11 and so on. The goal of this study is the development of a hydrogel with a fast deswelling rate and potential use for biomedical applications by the copolymerization of NIPAAm with hydrophile poly(2-alkyl-2-oxazoline) (PROZO, R ) methyl or ethyl) bior multifunctional macromonomers,12–14 therefore, based mainly on the chemical modification and porous gel forming route. The structural factors controlling the temperature response rate were investigated and the use of the resulted hydrogels for the release of bovine serum albumin (BSA) was evaluated.

Experimental Section Materials. 2-Hydroxyethyl methacrylate (HEMA, Aldrich) was purified by vacuum distillation. N-Isopropylacrylamide (NIPAAm, Aldrich) was recrystallized from benzene/n-hexane mixture. R,R′Azoisobutyronitrile (AIBN, Fluka), used as initiator, was purified by recrystallization from methanol just before use. Ethanol (EtOH), diethylether, acetone, all of analytical grade, were used as received. The PROZO macromonomers (multifunctional, with unsaturated polyester structuresPE, or bifunctional, with cinnamoyl end-groupssBC, re-

10.1021/bm800215d CCC: $40.75  2008 American Chemical Society Published on Web 05/16/2008

Deswelling of PNIPAAm/PROZO/PHEMA Hydrogels spectively) were synthesized according to literature data by end-capping growing PROZO living chains with maleic or cinnamic acid as nucleophile reagent.12,13 Bovine serum albumin (BSA, Fraction V, Merck) was used as a protein model in physical sorption experiments, performed in sodium phosphate buffer (PBS, pH 7.4). Hydrogel Synthesis. Hydrogels were obtained as previously reported,14 by simple free-radical copolymerization by precipitation technique of HEMA, NIPAAm, and PROZO macromonomer (crosslinker) with AIBN as an initiator (2 wt % relative to total monomers) in ethanol at 60 °C for 20 h under an inert atmosphere (Ar). Typically, a 1:1:1 w/w/w ratio was used (i.e., 0.5 mL HEMA/0.5 g NIPAAm/0.5 g PROZO macromonomer, in 3 mL ethanol p.a.). The crude solid product was washed with diethyl-ether and acetone, centrifuged, stored in vacuum for 3 h, and then washed repeatedly with water and acetone to remove unpolymerized monomer, macromonomer, or water-soluble fractions. After the separation by centrifugation, the wet samples were packed in aluminum foil and stored in an exsiccator, at 32-35 °C, on double-glass plates until constant weight was attained. The total yield of the reaction was in the range of 70-75%. Dried thick polymer films were then separated from the double-glass plates, and disk-shaped samples of ∼0.5 g were carefully cut and subjected to swelling investigation. Swelling-Deswelling Measurements. Equilibrium swelling of the synthesized materials was investigated by monitoring the weight gain of the sample immersed in an excess amount of buffered water (pH 7.4), at an established temperature. The mass of the wet sample was obtained after the removal of the solvent by carefully wiping the sample surface. Three measurements were performed for each specimen. The equilibrium swelling ratio (R ) WH2O/Wd) was defined as the averaged weight of the absorbed water (WH2O) for the three measurements per weight of dried gel (Wd). Equilibrium swelling weights for the gels in water at various temperatures were measured gravimetrically. At specific time intervals the gels were taken out of buffered medium and weighed after removing excess water from the gel surface. Swelling and deswelling kinetics were defined as temporal weight changes for the gels. The deswelling behavior was analyzed by rapid changes in the temperature of the aqueous media from 19 °C (below LCST) to 55 °C (above LCST). The hydrogels were taken out of PBS at regular intervals and weighed after removal of excess water from the surface. Water retention was defined as the weight changes between equilibrium swollen (100%) and equilibrium shrunken (0%) states. The swellingdeswelling cycle was repeatable with reasonable reproducibility. Below 10 °C, the samples are very soft and adhesive. Their shape can be easily modified. At this temperature, after repeated cycles the sample begins to partially disrupt in microparticles. Characterization. The characteristics of the PROZO macromonomers (polymerization degree, functionalization efficiency) and of the resulted copolymers were obtained from 1H NMR spectra, performed with a Bruker AC250/Avance DRX 400 apparatus. The used solvents were CDCl3 and DMSO-d6, respectively. The FT-IR spectra were obtained with a Specord M 90 instrument, at ambient temperature. The thermal behavior of the samples (DSC and TGA) was evaluated with a Mettler Toledo Star System (Mettler DSC 820 and TGA/SDTA 851e instrument, respectively). Before testing, the samples were heated at 110 °C to eliminate the water and absorbed moisture. The registration was carried out under nitrogen flow (10 and 50 L/min, respectively) by using a heating rate of 10 °C/min. The initially synthesized microgel particles were characterized by transmission electron microscopy (TEM) with a TESLA-BS-513 apparatus. Specimens for TEM were deposited from diluted dispersions on carbon-coated grids. The particle size and size distribution were calculated with the formula

Dn ) ΣNiDi ⁄ ΣNi,

Dw ) ΣNiDi4 ⁄ ΣNiDi3,

PI ) Dw ⁄ Dn

The surface and fracture of macroscopic hydrogel samples were analyzed with a Jeol JSM-5400 scanning electron microscope (SEM).

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The samples prepared for SEM investigation were used after vacuum drying and platinum sputtering for 30 s. The hydrogel composition was evaluated from the 1H NMR and FT-IR spectra, N% elemental analysis, and hydroxyl group content determinations. The BSA concentration in the desorption medium was evaluated by spectrophotometric measurements with a Specord M42 apparatus. BSA Absorption Experiments. Typically, in BSA absorption experiments performed in a batch fashion, a ∼0.5 g gel sample was first loaded with BSA for 4 h, at 10 °C, in 5 mL solution in buffered water (pH 7.4), with an initial concentration of 1 mg/mL. The gel sample carrying a certain amount of BSA was then separated from the sorption medium, dried, and transferred for 1 h into 4 mL of buffer maintained at 18 °C to remove excess BSA retained at the surface. After separation, the swelled gel was dried and transferred in 4 mL of buffered water (pH 7.4). The BSA content in the desorption medium, after a 4 h release period at 37 °C, was determined by using UV spectrophotometry (λ ) 278 nm).

Results and Discussion Thermosensitive Gel Preparation and Analysis. Gels, either micro- or macro, are temperature-sensitive if most of the polymer in the gel network displays temperature-sensitive volume phase transition in the swelling solvent. Consequently, their behavior is dependent on the content in thermo-sensitive polymer chains and also on the adopted topology and morphology. With this aim in mind, hydrogels were prepared by freeradical polymerization, using precipitation technique, including in their feed recipe monomers like NIPAAm (LCSTPNIPAAm ) 32 °C) or macromonomers based on poly(2-ethyl-2-oxazoline) (Table 1), a polymer known to have a LCST in the physiological domain, that is, situated at 36 °C.15,16 A third component, HEMA, was added to introduce new gel functionality and to improve its mechanical properties. Precipitation polymerization is especially suitable for preparing thermosensitive microgels with a particle size distribution of narrow dispersity in the submicron size range, because one can control the precipitation/ deswelling tendency by changing temperature.17 In our case, the high amount of PROZO cross-linker gave unstable suspensions. The presence of PHEMA, PNIPAAm, and PROZO sequences in the copolymerization product was supported by FT-IR spectra and chemical analysis (N%, content in OH groups). The elemental analysis results are presented in Table 1. The FT-IR spectra (Figure 1) include signals situated at 1660 cm-1, 1630 cm-1, and 1560 cm-1, specific to PROZO and PNIPAAm sequences (CdO stretching in tertiary and secondary amide groups and NH deformation, respectively). The two typical bands of CH vibrations at ∼1387 cm-1 and ∼1367 cm-1 are ascertained to the divided bands of the CH(CH3)2 symmetric group. An intense increase of the bands ascertained to the esteric groups in the polyvinyl sequences, at 1730 cm-1 (CdO stretching, esteric group), 1158 cm-1 and 1080 cm-1 (C-O-C asym and sym stretching vibration, esters) can be also observed. The 1H NMR data (Figure 2) usually indicated a PNIPAAm/ PROZO molar ratio of 1:1. Small amounts of the final reaction mixtures were diluted and spread onto carbon-coated copper grids and then subjected to TEM observation (Figure 3). Microparticles morphology showed irregular spheres and narrow size distribution. As shown in Table 2, the calculated average size and size distribution of the particles were in the range of 30-200 nm and 1.05-1.15, respectively, depending on copolymer structure. Characterization of the Macroscopic Hydrogel. The aggregation of compacted wet colloidal microgels during drying

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Table 1. Copolymerization Dataa macromonomer b

copolymer c

code

type

DP

N (%)

OH groups (mmol/g)

PHEMA/(PHEMA+PROZO+PNIPAAm) (g/g)

E15 E25 E35 E45 M6 M15 M25 M35 M45 BC1 BC2

PEE15 PEE25 PEE35 PEE45 PEM6 PEM15 PEM25 PEM35 PEM45 BCE6 BCE25

18.0 24.0 30.0 47.0 6.5 17.0 26.0 30.0 41.0 7.0 26.0

6.7 6.7 7.0 6.9 6.4 7.0 7.5 7.8 8.0 5.8 6.5

3.8 3.9 3.6 3.7 3.5 3.7 3.5 3.4 3.4 3.7 3.6

0.49 0.49 0.47 0.48 0.45 0.48 0.45 0.44 0.44 0.48 0.47

a Reaction conditions: w/w/w feed ratios of NIPAAm/HEMA/PROZO macromonomer, 1/1/1; solvent, ethanol; AIBN, 2 wt % relative to total monomers, 60 °C, 20 h, Ar. b PEE/Mn: PEOZO/PMOZO multifunctional macromonomer of unsaturated polyester type, with a predetermined polymerization degree n of the PROZO sequence inserted between polymerizable groups. BCEn: PEOZO bifunctional macromonomer with cinnamoyl end groups with a predetermined polymerization degree n. c DP: polymerization degree of the PROZO sequences inserted in the macromonomer chain calculated from 1H NMR data.

Figure 1. FT-IR spectra of (a) macromonomer PEE45 and (b) hydrogel E15. Figure 3. TEM micrograph of microgel particles (sample M25). Table 2. Characteristics of the Microgel Particles Sa

M6

M25

M35

M45

E15

E25

E45

BC2

Dn (nm) PI

27 1.05

158 1.09

195 1.10

111 1.05

140 1.12

175 1.10

200 1.09

180 1.09

a

S: sample code.

Figure 2. Typical 1H NMR spectrum. Sample E15 (DMSO-d6 as a solvent).

gave macroscopic gels. The dried hydrogel macroscopic samples were all yellowish, clear, and transparent (Figure 4), relatively brittle, with a layered structure, suggesting the absence of macroscopic phase separation and a high crystalline phase content. These observations were confirmed by the presence of endothermic peaks, related to an ordering tendency, in the first run DSC plots. They are situated in the 140-205 °C range, being ascertained to the PMOZO, PNIPAAm, and PHEMA crystallizable inserted sequences18,19 (Figure 5). The broadening of the peak in the 170-200 °C range may reflect a microheterogeneity of the phase domains.

Figure 4. Typical SEM micrograph of the macroscopic gel surface (sample BC1).

One can observe (Table 3) that typically one single glass temperature (Tg) was registered for the extracted samples in the third run, in accordance with a good compatibility between

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Table 4. LCST Values for the Synthesized Hydrogels sample code

LCST (°C)

M6 M15 M25 M45 E15 E25 E35 E45 BC1 BC2

27.5 32.0 33.0 38.0 28.5 30.0 32.0 31.5 28.5 27.6

Figure 5. DSC melting endotherms of samples (a) BC2 and (b) M45. Table 3. Glass Transition Temperatures of the Hydrogels sample code

Tg (°C)

M6 M15 M25 M45 E15 E25 E35 E45 C1 C2

95 97 100 97 109 104 108 112 120 80

components, favored by the presence of PROZO chains, polymer compatible with most common polymers,16,18 and the neighboring glass transition domains of the inserted sequences (PROZO 50-70 °C,18 PNIPAAm 85-130 °C,19 PHEMA 84.6-94.6 °C, depending on polymer chain length and water content19). Another explanation is the facile dehydration reaction of the OH groups in the PHEMA sequences at high temperatures. The thermal stability was investigated using TGA. The synthesized hydrogels decompose gradually at temperatures higher than 250 °C. Swelling/Deswelling Behavior. Measurements of equilibrium swelling ratios for the prepared gels, shown in Figure 6, demonstrated that all hydrogels exhibit volume phase transition from swelling states to deswelling states in the 28-38 °C temperature range (Table 4). By increasing the temperature, the gels changed their appearances from transparent to opaque. Higher values of the transition temperature can be observed for the copolymers containing PMOZO sequences as compared to those with PEOZO ones, due to the higher hydrophilic character of the formers. As known, the use of hydrophilic comonomers

Figure 6. Equilibrium swelling ratio of some synthesized gels as a function of temperature in PBS (pH 7.4). Samples: (O) BC25, (9) E25, (×) M25.

shifts the LCST to higher temperatures as compared to that of the PNIPAAm homopolymer gel, by stabilizing the conformation of the macromolecular chains in their hydrated state.20,21 For the same reason, the swelling rate is higher for the samples of the M series comparative with the samples of E or BC type, as shown in Figure 7. By contrast, further analysis of deswelling kinetics by rapid changes in the temperature of the aqueous media from 20 to 55 °C, across the volume phase transition temperature (Figure 8), revealed that deswelling is faster for the samples, which include PEOZO sequences, due to facilitated hydrophobic aggregation and increased content in thermo-sensitive polymer chains (PEOZO and PNIPAAm). Table 5 summarizes the rate constants, calculated from the corresponding semilogarithmic plots, that is, a first order rate analysis to the time dependence of the deswelling, according to the relation

ln (R - R∞ ⁄ R0 - R∞) ) -kt where, R0 and R∞ are the equilibrium swelling ratios at the initial and final temperature, k is a constant, and t is time. It is obvious that the deswelling of BC type gels is faster than those of E type and much faster than those of M type. The hydrophobic/hydrophilic balance in the gel network is better in the hydrogels emerging from the PEOZO macromonomers. For the E type gels, the rate constants are increasing with the decrease in the cross-linking density, in accordance with the polymer mesh size modification. Longer thermosensitive PEOZO chains allow a faster dehydration by a strong hydrophobic aggregation of the whole gel. By contrast, longer hydrophilic PMOZO sequences in the M type gels, that is, a decreased number of hydrophobic points at lower cross-linking density, give rise to a slower release of the water and lower deswelling rate. In all cases, the deswelling rate was higher as compared to that of conventional hydrogels based only on NIPAAm. The deswelling response on raising the temperature of the gels studied here above their phase transition temperature takes place

Figure 7. Typical swelling kinetics of the synthesized hydrogels at room temperature (20 °C): (0) sample BC25; (b) sample E25; (O) sample M25.

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Figure 8. Typical deswelling kinetics of the synthesized hydrogels at 55 °C from the equilibrium swelling condition at 19 °C, in buffered water (pH 7.4). Samples: (b) BC2; (×) E25; (O) M25. Table 5. Deswelling Rate Constants for Some of the Synthesized Hydrogels sample code M6 M25 M45 E15 E25 E35 E45 BC2

-1

k (min

)

0.330 0.110 0.107 0.250 0.260 0.270 0.350 0.290

within 10-30 min, whereas a conventionally cross-linked PNIPAAm gel usually requires one month for deswelling. Porous Gel Morphology. Scanning electron microscopy observation revealed a porous structure of the hydrogel, with nearly uniform, large channels (of 5-10 µm) disposed in successive sheets (Figure 9). The channel walls present themselves a porous structure (Figure 9c). Enlarged details evidenced that they are formed by aggregated microparticles (i.e., diameters of about 180-200 nm for sample E35, Figure 9a). It was assumed that this architecture is the result of self-assembling of the originating microgels via colloidal crystals, by capillary forces,22 during their slowly drying procedure. The peculiar self-

Figure 9. SEM micrographs of fractured hydrogels samples: (a) ternary hydrogel E35; (b) ternary hydrogel BC2; (c) ternary hydrogel E35, surface detail; (d) PEOZO/PNIPAAm binary hydrogel based on BCE6 macromonomer (1:1 w/w ratio PEOZO/PNIPAAm).

Figure 10. SEM micrograph of PMOZO/PHEMA (w/w, 1/1) binary hydrogel:14 (a) surface; (b) inner structure. Table 6. BSA Absorbtion/Release Experiments Dataa sample

BC2

BC1

E25

M6

M25

M45

BSA released (mg/g)

7.9

4.6

6.5

0.8

2.6

3.1

a

Conditions: absorption at the initial aqueous solution content of 10 mg BSA/g sample, 10 °C, 4 h; release in 4 h at 37 °C, pH 7.4.

assembling process is mainly based on hydrogen bonds development between the tertiary amide groups of PROZO and the secondary amide groups in PNIPAAm. This assumption is based on the observation of similar structures in hydrogels based on PROZO macromonomers and NIPAAm (Figure 9d), but not in hydrogels prepared with PROZO macromonomers and HEMA (Figure 10).14 Self-assembling of core-shell PNIPAAm/ PHEMA microparticles emerging in colloidal crystals formation was reported in the literature.23 The possible involvement of the aggregation of colloidal microgel in the synthesis of porous macroscopic temperature-sensitive gels was also suggested earlier by T. Gotoh and co-workers.24 BSA Uptake and Release Experiments. Applications of these materials may envisage the controlled drug or macromolecular active agents release and storing. An advantage consists in the lower toxicity of the PEOZO macromonomers as compared to NIPAAm monomer. With this aim, the gels were loaded via physical entrapment method. Absorption/release experiments were performed with BSA as model protein (1 mg/ mL aqueous solutions) at a lower/higher temperature than LCST, respectively. We took in consideration that the mass transport of solute through the hydrogel is facilitated below the volume phase transition temperature (VPTT), due to the high water content of the gel, and the solute release will be permitted above VPTT, the solute-NIPAAm interactions being replaced by NIPAAm-NIPAAm interactions.3 The release of BSA was investigated with respect to variation in the cross-linking density of the hydrogel and PROZO sequences type. The data from Table 6 show a differentiated release, which confirms the role of the thermosensitive chains content and of the implied hydrophobicity of the component sequences, as well as the importance of the inner structure (cross-linking density, porosity) in effecting a higher BSA release. The released (absorbed) BSA amount is decreasing in the same order as the response rate, that is, BC2 > E25 > M25. Increasing the cross-linking density reduced the swelling/deswelling and consequently the release abilities. As can be observed from Table 6 data, the BSA released amount is decreasing in the order M45 > M25 > M6. The observed behavior is in accordance with before mentioned increased loading of BSA into macroporous hydrogels of large pore size and a high pore number.25 For other applications, one may take into account the optical properties for electro-optical devices. The PROZO sequences

Deswelling of PNIPAAm/PROZO/PHEMA Hydrogels

may also be easily modified to polyethylenimine, yielding in pH sensitivity.

Concluding Remarks Thermosensitive hydrogels with a high rate of deswelling were synthesized by batch copolymerization of PROZO macromonomers with NIPAAm and HEMA. Their swelling characteristics, LCST, response rate, and protein absorption/release ability, were proved to be dependent and are controlled by composition (hydrophobe/hydrophile balance, thermosensitive structural units content) and morphological features. The inclusion of PEOZO and PNIPAAm, both with LCST in the proximity of the therapeutic domain (36 and 31 °C, respectively), in gel compositions gave materials with better performances, considering the thermosensitivity range and the rapidity of the response of the resulting materials, as compared to those based on PMOZO. The main reasons for the rapid deswelling response in the studied hydrogels, as compared to PNIPAAm, were considered to be (1) the incorporation of hydrophilic moieties, giving rise to surface skin disruption; (2) the improved hydrophobe/ hydrophile balance and higher thermosensitive chains content in the gels originating in the use of PEOZO macromonomers; and (3) porous gel formation, with long open channels, generated by a self-assembling process of microgel particles. Microgel particles may act as building blocks for complex nanostructured architectures, consisting in successive porous layers with nearly uniform channels. The self-assembling process is facilitated by the H-bonding ability of the components. Applications as advanced biomedical materials, including controlled release of drugs or macromolecular active agents, such as proteins and peptides, may be envisaged for the obtained multifunctional, stimuli responsive porous polymers.

References and Notes (1) Wichterle, O.; Lim, D. Nature 1960, 185, 117. (2) Qiu, Y.; Park, K. AdV. Drug DeliVery ReV. 2001, 53, 321–339.


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(3) Pelton, R. AdV. Colloid Interface Sci. 2000, 85, 1–33. (4) Heskins, M.; Guillet, J. E. J. Macromol. Sci. Chem. 1968, A2, 1441– 1455. (5) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163–249. (6) Caykara, T.; Kiper, S.; Demirel, G. S.; Demirci, G. S.; Cakanyildirim, C. Polym. Int. 2007, 56, 275–282. (7) Kato, N.; Takahashi, F. Bull. Chem. Soc. Jpn. 1997, 70, 1289–1295. (8) Walsh, D.; Hall, S. R.; Moir, A.; Wimbush, S. C.; Palazzo, B. Biomacromolecules 2007, 8, 3800–3805. (9) Kaneko, Y.; Nakamura, S.; Sakai, K.; Aoyagi, T.; Kikuchi, A.; Sakurai, Y.; Okano, T. Macromolecules 1998, 31, 6099–6105. (10) Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W. Macromolecules 1993, 26, 2496–2500. (11) Gil, E. S.; Hudson, S. M. Biomacromolecules 2007, 8, 258–264. (12) David, G.; Ioanid, A. J. Appl. Polym. Sci. 2001, 80, 2191–2199. (13) David, G.; Alupei, V.; Simionescu, B. C. Eur. Polym. J. 2001, 37, 1353–1358. (14) David, G.; Pinteala, M.; Simionescu, B. C. DJNB: Digest Journal of Nanomaterials and Biostructures 2006, 1, 129–138. (15) Kobayashi, S.; Uyama, H. Trends Macromol. Res. 1994, 1, 121 131. (16) Chiu, T. T.; Thill, B. P.; Fairchok, W. J. Water Soluble Polymers. In AdVances in Chemistry Series 213; Glass, J. E., Ed.; American Chemical Society: Washington DC, 1986; p 425-433. (17) Zhou, G.; Elaissari, A.; Delair, Th.; Pichot, C. Colloid Polym. Sci. 1998, 276, 1131–1139. (18) Kobayashi, S. Prog. Polym. Sci. 1990, 15, 751–823. (19) Polymer Handbook, fourth ed.; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; J. Wiley & Sons, Inc.: New York, 1999; Chapter VI, pp 13 and 201. (20) Kaneko, Y.; Nakamura, S.; Sakai, K.; Aoyagi, T.; Kikuchi, A.; Sakurai, Y.; Okano, T. Macromolecules 1998, 31, 6099–6105. (21) Brazel, C. S.; Peppas, N. A. Macromolecules 1995, 28, 8016–8020. (22) Kralchevsky, P. A.; Denkov, N. D.; Paunov, V. N.; Velev, O. D.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. J. Phys.: Condens. Matter 1994, 6, A395–A402. (23) Debord, S. B.; Andrew Lyon, L. J. Phys. Chem. B 2003, 107, 2927– 2932. (24) Gotoh, T.; Nakatani, Y.; Sakahara, S. J. Appl. Polym. Sci. 1998, 69, 895–906. (25) Zhuo, R.-X.; Li, W. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 152–159.

BM800215D

Poly(2

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