Amphiphilic Polymer Conetworks Based on End Group Cross-Linked

Article pubs.acs.org/Macromolecules

Amphiphilic Polymer Conetworks Based on End Group Cross-Linked Poly(2-oxazoline) Homo- and Triblock Copolymers Christian Krumm, Stefan Konieczny, Georg J. Dropalla, Marc Milbradt, and Joerg C. Tiller* Biomaterials and Polymer Science, Department of Biochemical and Chemical Engineering, TU Dortmund, Emil-Figge-Straße 66, 44227 Dortmund, Germany S Supporting Information *

ABSTRACT: Novel amphiphilic polymer conetworks (APCNs) were prepared via end group cross-linking. To this end, poly(2methyloxazoline) (PMOx), poly(2-butyloxazoline) (PBuOx), and the triblock copolymers PMOx-b-PBuOx-b-PMOx were synthesized by cationic ring-opening polymerization in varying block lengths and telechelically modified with N,N-bis(2-aminoethyl)ethylendiamine (TREN). First the cross-linking with 1,4-dibromo-2-butene (DBB) was established for the homopolymers. The swelling of those matches the theoretical value for full cross-linking, indicating that in this way “near perfect” networks could be obtained. Mixtures of the homopolymers and the triblock copolymers were cross-linked with DBB to give APCNs with similar polymer segments but different network topology. AFM showed that all formed APCNs are nanophase separated with slight structural differences in the nanostructures when comparing conetworks with similar composition but different cross-linking strategies. The more drastic difference between APCNs of different topologies was found in their swelling characteristics, which clearly proves the influence of conetwork structure on their properties.

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INTRODUCTION

However, the potential of these conetworks is not fully developed, since they usually contain at least one broadly distributed phase, either from the macromer/monomer approach,21−27 by free radical cross-linking of telechelic block copolymers28,29 or random cross-linking of linear homopolymers30−33 and star-shaped homopolymers34 or dendrimers.35 Only a few examples of APCNs with only narrowly distributed segments, so-called “near perfect” or “ideal” APCNs are known, being PEG/PDMS,36,37 the APCNs of Patrickios, who crosslinks acrylate-based multiblock copolymers by group transfer polymerization17,38−40 and star-shaped PIB and PEG based APCNs of Erdodi and Iván with perfect chain end coupling,34 or by a combination of both strategies.41 According to the various structural analyses, it seems that the different systems show very similar nanostructures independent of the preparation. However, the influence of the cross-linking strategy on the properties of one and the same polymer system has not been explored. We believe this to be a very important detail to better understand the unique properties of APCNs. In this report, we prepared “near perfect” APCNs by controlled end group cross-linking of narrowly distributed homopolymer mixtures and also of well-defined ABA block copolymers. Both of these novel poly(2-oxazoline) conetworks, were compared with respect to their nanostructure and the

Defined, controllable, and switchable nanostructures are the key to modern high performance materials. Among them, polymer networks with narrowly size-distributed polymer segments are of great current interest, because they form such regular nanostructures. Generally, such networks are obtained by crosslinking polymers derived by living or at least controlled polymerization strategies.1−3 Often the narrowly size-distributed network segments afford much more distinguished properties than the respective networks derived by noncontrolled polymerization techniques. Examples are the thermo-response of LCST polymers,4 the selectivity of imprinted conetworks,5 and the porosity of membranes.6 The regular nanostructure is particularly important for amphiphilic polymer conetworks (APCNs), which contain polymer segments with orthogonal properties, e.g. hydrophilic and hydrophobic behavior.7 The regular nanostructures allow addressing one property selectively in one polymer phase while independently addressing another in the second phase. Besides commercial soft contact lenses many other examples of the high performance of APCNs are described in the literature, ranging from tubular networks for insulin delivery,8 their use as activating carriers for the enhancement of enzyme activity in supercritical CO2,9 their use in perfluorinated solvents,10 optical biochemical sensors for peroxide detection,11,12 biomimetic material for synthesis of biological membranes,13 for release of antimicrobials,14 for pH-sensitive drug delivery,15,16 as chemically cleavable conetworks17,18 and for chiral separation.19,20 © XXXX American Chemical Society

Received: March 5, 2013 Revised: April 8, 2013

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dx.doi.org/10.1021/ma4004665 | Macromolecules XXXX, XXX, XXX−XXX

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Table 1. Summary of the Used Amount of DBX and Volumes of Used BuOx, MOx, and TREN as Well as the Polymerization and Termination Conditions for the Preparation of TREN-Functionalized PMOx, PBuOx, and PMOx-b-PBuOx-b-PMOx Polymersa

a

polymer

mDBX [mg]

VBuOx [mL]

TReac [°C]

tReac [h]

VMOx [mL]

TReac [°C]

tReac [h]

VTREN [mL]

TTerm [°C]

tTerm [h]

PMOx19 PMOx42 PBuOx24 PBuOx39 PMOx4-b-PBuOx34-b-PMOx4 PMOx10-b-PBuOx20-b-PMOx10 PMOx12-b-PBuOx12-b-PMOx12 PMOx13-b-PBuOx5-b-PMOx13

232.6 465.2 312.4 155.7 213.9 310.7 336.4 725.7

− − 3.2 3.2 3.2 3.2 2.1 2.1

− − 110 110 110 110 110 110

− − 3.75 7.5 5.5 3.75 2.3 1.2

3 3 − − 0.75 2 3 8

100 100 − − 100 100 100 100

2.5 5 − − 1.25 2.5 3.5 4.3

2.6 5.3 2.5 1.76 3.6 5.3 5.7 12.3

40 40 70 70 70 70 70 70

24 24 48 48 48 48 48 48

The degrees of polymerization given as indices are calculated from the results of 1H NMR spectroscopy discussed in detail in the results.

resulting properties to learn more about the influence of the cross-linking strategy on the properties of “near perfect” APCNs.

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χ≈

Vs is molar volume of the solvent in this case methanol, R is the gas constant, T is the temperature (298 K), and δP is the solubility parameter of poly(2-methyloxazoline), which was calculated by the an incremental method of FEDORS (27.6 J1/2/cm3/2).43 The solubility parameter δs of methanol was derived by the heat of vaporization ΔHLV (37600 J/(molK)) to be 29.42 J1/2/cm3/2.

EXPERIMENTAL SECTION

Instruments. 1H NMR spectra were recorded in CDCl3 using a Bruker Avance DRX-400 spectrometer with a 5 mm sample head operating at 400.13 MHz. Size exclusion chromatography (SEC) was performed on a Viscotek GPCMax equipped with an refractive index (RI) detector (tempered to 55 °C) using a Tosoh TSKgel GMHHR-M (5.0 μm pores, 2× + 1× precolumn) column set. As eluent, saline N,N-dimethylformamide (DMF + LiBr, 20 mmol) was used at 60 °C at a flow rate of 0.70 mL·min−1. Calibration was performed with poly(styrene) standards (from Viscotek). AFM images were recorded with a Veeco Dimension Icon Scanning Probe Microscope (Veeco Instruments) equipped with a Nanoscope V Controller and an AVH-1000 Workstation. All measurements of the cross sections were performed in tapping mode using commercial tapping mode etched silicon probe (RTESP) cantilevers of various frequencies from 300 to 400 kHz. Phase images were recorded at 5% below the fundamental resonance frequency of the cantilever, with a typical scan speed of 1 Hz and a resolution of 512 samples per line for a 1000 nm scan size. The microwave-assisted polymerizations were carried out in CEM Discover synthesis microwaves; reaction temperature was constantly monitored with a vertically focused IR temperature sensor. Swelling Measurements and Determination of Mc. The swelling behavior of the polymer films was detected by volumetric measurements. Therefore, small film pieces of ca. 2 mm2 were observed under a stereomicroscope (MZ 95, Leica, Germany) and reflected light (KL Electronics 1500, 3000 K), in dry state (V0) and swollen equilibrium state (Veq). The degree of swelling results from increasing edge sizes in swelling medium, after swelling for 48 h in the respective media. S=

Veq V0

=

δS =

2

ΔH LV − RT VS

With these assumptions and with regard to the high degrees of swelling the molecular weight-average between two cross-links can be calculated by a modified Flory−Rehner equation:42 MC ≅

2ρP VS ϕ5/3(1 − χ )

Materials. Purification of α,α′-Dibromo-p-xylene, trans-1,4Dibromo-2-butene, 2-Methyl-2-oxazoline, and 2-Phenyl-2-oxazoline. The initiator α,α′-dibromo-p-xylene (DBX) and the cross-linking agent trans-1,4-dibromo-2-butene (DBB) were purchased from Acros Organics. DBX was recrystallized from dry chloroform (CHCl3) twice. DBB was recrystallized twice from n-hexane. Both reagents were dried under reduced pressure at 25 °C and stored in an argon atmosphere at −20 °C. The monomers 2-methyl-2-oxazoline (MOx) and 2-phenyl-2oxazoline (PhOx) were purchased from Acros Organics and SigmaAldrich. They were distilled twice from CaH2 under reduced pressure and argon atmosphere. All these chemicals were stored in an argon atmosphere and over molecular sieves (4 Å) at −20 °C. All other chemicals were of analytical grade or purer and used without further purification if not noted otherwise. Synthesis of 2-Butyl-2-oxazoline. The synthesis was performed according to a modified protocol of Schubert et al.44 To this end, a mixture of 85 mL (0.813 mol) of valeronitrile, 3.57 g zinc acetate dihydrate (0.02 equiv) and 58.87 mL ethanolamine (1.2 equiv) was heated to 140 °C in a microwave reactor for 16 h. After cooling to room temperature, the reaction mixture was dissolved in 200 mL of cyclohexane. Then the organic phase was washed five times with water (250 mL) until it turned slightly yellow. At last the organic phase was washed with brine and dried over MgSO4. After filtration and evaporation of the solvent, the crude product was purified by distillation from CaH2 under reduced pressure and argon atmosphere (isolated yield: 60%). 1 H NMR (400 MHz; CDCl3): δ (ppm) = 4.04−3.99 (t, 2H, −NCH2CH2O); 3.64−3.39 (t, 2H, −NCH2CH2O); 2.09−2.05 (t, 3H, C−CH2CH2CH2CH3); 1.44−1.40 (m, 2H, C−CH2CH2 CH2CH3); 1.21−1.15 (m, 2H, C−CH2CH2CH2CH3); 0.75−0.71 (t, 3H, C− CH2CH2 CH2CH3). Synthesis of N,N-Bis(2-aminoethyl)ethylendiamine-Functionalized Poly(2-methyloxazoline)s (PMOx42 and PMOx19), Poly(2butyl-2-oxazoline)s (PBuOx39 and PBuOx24) Homopolymers and

1 ϕ

Determination of Mc was performed by using the Flory−Rehnerequation:42

Mc = − 1

VS (δP − δS)2 RT

ρP VSϕ1/3 2

χϕ + ln(1 − ϕ) + ϕ

In this equation, Mc is the average molecular weight between two cross-links, ρp is the specific density of the polymer (for PMOx is 1.14 g/cm3), Vs is the molar volume of the solvent in this case methanol (40.56 cm3/mol), Φ is the volumetric fraction of the polymer in the swollen state, which is the same as 1/S, and χ is the Flory −Huggins parameter for methanol and poly(2-methyloxazoline). They were calculated by using the following equation. B


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ABA PMOx-b-PBuOx-b-PMOx Triblock Copolymers PMOx-b-PBuOxb-PMOx. Typical procedure for the example of the triblock copolymer PMOx10-b-PBuOx20-b-PMOx10 is given. Under argon atmosphere, a mixture of 310.7 mg (1.2 mmol) [mDBX] of DBX and 3.2 mL (24 mmol) of BuOx [VBuOx] were dissolved in 15.0 mL of dry CHCl3. The mixture was heated in closed vessels at 110 °C [Treac] for 3.5 h [treac] in a microwave reactor with magnetic stirring and without active cooling using the maximum available power. Immediately after reaching 110 °C, the power was reduced to a basis level maintaining the target temperature. To avoid short-term temperature peaks, the vessels were counter-cooled by compressed air shocks within the next 3−5 min. After 3.5 h and cooling to 50 °C 2.0 mL (24 mmol) of MOx were added, and then the mixture was heated at 100 °C for 2.5 h. The living polymer solution was terminated by addition of 5.3 mL [VTREN] of N,N-bis(2-aminoethyl)ethylendiamin and heated at 70 °C [TTerm] for 48 h [tTerm]. PMOx42, PMOx19, PbuOx39 and PbuOx24 were functionalized using 20 equiv of TREN. The resulting mixture was added to a large amount of diethyl ether (Et2O) to precipitate a polymeric material, which was reprecipitated in CHCl3/Et2O three times (with exception of PBuOx39 and PBuOx24 homopolymers, in these cases the organic solvent was removed and the polymers were dialyzed without further reprecipitation). The resulting polymer was dialyzed several times in methanol for 24 h using benzoylated cellulose membranes (ZelluTrans, Roth, 1000 g·mol−1 molecular weight cutoff). After methanol removal in vacuum, 3.59 g (63% yield) of polymer was obtained. For the other PMOx-b-PBuOx-b-PMOx polymers, the amount of MOx and the time of polymerization were adjusted according to the block-ratios adjusted. A summary of the experimental data for all synthesized polymers is given in Table 1. 1 H NMR [TREN-functionalized PMOx42] (400 MHz; CDCl3): δ (ppm) = 7.13−7.04 (b, 4H, −CH2−C6H4−CH2−, Ar H); 4.52−4.46 (b, 4H, −CH2−C6H4−CH2−); 3.59−3.15 (b, n × 4H, N−CH2− CH2−); 2.75−2.43 (b, 24H,−N+H2−CH2−CH2N−(CH2−CH2− NH2)2); 2.14−1.90 (b, n × 3H, −NCOCH3). 1 H NMR [TREN-functionalized PBuOx24] (400 MHz; CDCl3): δ (ppm) = 7.13−7.04 (b, 4H, −CH2−C6H4−CH2−, Ar H); 4.57−4.47 (b, 4H, −CH2−C6H4−CH2−); 3.63−3.23 (b, n × 4H, N−CH2− CH2−); 2.79−2.48 (b, 24H,−N+H2−CH2−CH2N−(CH2−CH2− NH2)2); 2.36−2.12 (b, n × 2H −NCO−CH2−CH2−CH2−CH3)− 1.64−1.45 (b, n × 2H −NCO−CH2−CH2−CH2−CH3); 1.37−1.20 (b, n × 2H −NCO−CH2−CH2−CH2−CH3); 0.96−0.81 (b, n × 3H, −NCOCH3). 1 H NMR [TREN-functionalized PMOx10-b-PBuOx20-b-PMOx10] (400 MHz; CDCl3): δ (ppm) = 7.13−7.04 (b, 4H, −CH2−C6H4− CH2−, Ar H); 4.57−4.47 (b, 4H, −CH2−C6H4−CH2−); 3.63−3.23 (b, n × 4H, N−CH2−CH2−); 2.79−2.48 (b, 24H,−N+H2−CH2− CH2N−(CH2−CH2−NH2)2); 2.36−2.12 (b, n × 2H −NCO−CH2− CH2−CH2−CH3); 2.09−1.93 (b, n × 3H, NCO−CH3); 1.64−1.45 (b, n × 2H −NCO−CH2−CH2−CH2−CH3); 1.37−1.20 (b, n × 2H −NCO−CH 2 −CH 2 −CH 2 −CH 3 ); 0.96−0.81 (b, n × 3H, −NCOCH3). More detailed information on the 1H NMR spectra of the synthesized polymers is given in the Supporting Information. Preparation of N,N-Bis(2-aminoethyl)ethylendiamine-Functionalized Poly(2-methyloxazoline)-block-poly(2-phenyloxazoline)block-poly(2-methyloxazoline) PMOx-b-PPhOx-b-PMOx ABA Triblock Copolymers. A typical procedure was described for PMOx-bPPhOx-b-PMOx10/20/10. Under argon atmosphere, a mixture of 158.4 mg (0.6 mmol) of DBX and 1.58 mL (12 mmol) of PhOx were dissolved in 12.0 mL of dry CHCl3. The mixture was heated in closed vessels at 160 °C for 16 h in a microwave reactor with magnetic stirring and without active cooling using the maximum available power. Reaction time and temperature were kept constant for all inner blocks of the PMOx-b-PPhOx-b-PMOx. Immediately after reaching 160 °C, the power was reduced to a basis level maintaining the target temperature. To avoid short-term temperature peaks, the vessels were counter-cooled by compressed air shocks within the next 3−5 min. After cooling to 50 °C 1.02 mL (12 mmol) of MOx were added, and then the mixture was heated at 100 °C for 2.5 h. In the cases of the other PMOx-b-PPhOx-b-PMOx triblock copolymers the amount of

MOx and the time of polymerization were adjusted, both given in parentheses; PMOx2-b-PPhOx20-b-PMOx2 (0.21 mL, 0.5 h),PMOx3-bPPhOx20-b-PMOx3(0.31 mL, 0.75 h), PMOx5-b-PPhOx20-b-PMOx5 (0.51 mL, 1.25 h), PMOx30-b-PPhOx20-b-PMOx30(3.08 mL, 7.5 h). The living polymer solution was terminated with 2.69 mL (30 equiv) of N,N-bis(2-aminoethyl)ethylendiamine and heated at 70 °C for 48 h. The purifications of these polymers were performed analogous to the previously described PMOx42 and PMOx19. After purification 2.47 g (71% yield) of polymer was obtained. 1 H NMR (400 MHz; CDCl3): δ (ppm) = 7.80−6.80 (b, n × 5H, −NC6H5,Ar H); 4.64−4.51 (b, 4H, −CH2−C6H4−CH2−); 4.05−2.61 (b, n × 4H, N−CH2−CH2−); 2.25−1.92 (b, n × 3H, NCO−CH3). For the other PMOx-b-PPhOx-b-PMOx polymers the amount of MOx and the time of polymerization were adjusted according to the block-ratios. The 1H NMR spectra of all PMOx-b-PPhOx-b-PMOx polymers are given in the Supporting Information. Polymer Network Synthesis. All polymer networks were synthesized as free-standing membranes. Therefore, 120 mg of the respective polymer were dissolved in 180 μL acetonitrile (CH3CN) or 130 μL chloroform (CHCl3). The polymers were dissolved in a Thermomixer (Comfort, Eppendorf) at 35 °C and 650 rpm for 30 min. One equiv of the cross-linking agent trans-1,4-dibromo-2-butene (DBB) per TREN group was dissolved 10 μL CH3CN or CHCl3. The two solutions were pipetted together and thoroughly mixed. Then the resulting solution was poured on a polytetrafluorethylene slide (2 × 8 cm2). The thickness of polymer films was adjusted by coating the ends of a second polytetrafluorethylene slide with additional tape (50 μm per layer) as “spacer” and the first slide was covered with the second one. The mixture was left at 20 °C for 72 h. The obtained network films were subsequently removed with a razor blade. The sol fraction of the network films was extracted with methanol (10 mL, 20 mg film, 20 °C for 48 h). The difference in weight of vacuum-dried films before and after extraction was calculated as sol fraction. The composition of the sol fraction was determined with 1HNMR spectroscopy. The sol contents of the mixed homopolymers conetworks and their resulting compositions are given in Table 2. All other conetwork sol fractions are given in the Supporting Information.

Table 2. Summary of the Mixed Homopolymer Networks Preparation, the Sol Fraction, and the Resulting Network Composition reaction mixture 80 wt % PBuOx24/20 wt % PMOx19 60 wt % PBuOx24/40 wt % PMOx19 40 wt % PBuOx24/60 wt % PMOx19 20 wt % PBuOx24/80 wt % PMOx19

solvent

sol fraction [wt %]

CHCl3

28

CHCl3

38

CHCl3

35

CHCl3

35

network composition after extraction 79.9 wt 66.1 wt 48.9 wt 33.2 wt

wt % PBuOx24/20.1 % PMOx19 wt % PBuOx24/33.9 % PMOx19 wt % PBuOx24/51.1 % PMOx19 wt % PBuOx24/66.8 % PMOx19

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RESULTS AND DISCUSSION In this work, we strived to explore the influence of the crosslinking strategy in near perfect APCNs on their nanostructure and properties. In order to prepare such conetworks, narrowly distributed homopolymers and block copolymers with telechelic cross-linkable end groups are required. We chose the cationic ring-opening polymerization of 2-alkyl-2-oxazolines as suited technique, because the 2-oxazolines are versatile with respect to hydrophilic and hydrophobic character and the resulting polymers can be terminated with a variety of functional end groups.45−48 Wiesbrock et al. and Hoogenboom et al., who thoroughly explored the cationic ring-opening polymerization of 2-R-oxazolines in a microwave reactor C


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Figure 1. Schematic representation of the synthesis of a telechelic poly(2-methyloxazoline), poly(2-butyloxazoline) homopolymers, and ABA triblock copolymers PMOx-b-PBuOx-b-PMOx functionalized by TREN.

Table 3. Characterization of the Homopolymers and ABA Triblock Copolymers from PMOx and PBuOx by 1H NMR and SEC polymer

DPSeta

DPNMRb

DPSECc

Mn,SEC [g/mol]

PDI

Fd

PMOx19 PMOx42 PBuOx24 PBuOx39 PMOx-b-PBuOx-b-PMOx4/34/4 PMOx-b-PBuOx-b-PMOx10/20/10 PMOx-b-PBuOx-b-PMOx12/12/12 PMOx-b-PBuOx-b-PMOx13/5/13

20 40 20 40 5/30/5 10/20/10 14/12/14 17/6/17

19 42 24 39 4/34/4 10/20/10 12/12/12 13/5/13

25 46 24 44 34 38 39 36

2700 4500 3600 6200 4000 4000 3800 3300

1.15 1.26 1.16 1.15 1.11 1.13 1.12 1.12

0.98 0.98 0.97 0.72 0.96 0.78 0.83 0.86

a

Composition expected from the used initiator/monomer ratio. bCalculated from the 1H NMR spectrum by the ratios of the initiating group to the respective POx side group signals presuming symmetric chain growth. cCalculated from the obtained molecular weight Mn (SEC) presuming the block ratios determined by 1H NMR. dCalculated from the ratio of the signals resulting from the initiating and terminating agent in the 1H NMR spectrum.

In order to prepare “near perfect” polyoxazoline APCNs, we chose to synthesize block copolymer networks using poly(2butyloxazoline) (PBuOx) as the hydrophobic component and poly(2-methyloxazoline) (PMOx) as the hydrophilic block (Figure 1). The telechelic functionalization of the polyoxazolines was achieved by starting the polymerization with a bifunctional initiator α, α′-dibromo-p-xylene DBX, according to our previous work.63 The reaction was terminated with a high excess of N,N-Bis(2-aminoethyl)ethylendiamine (TREN) to introduce cross-linkable end groups. The molecular weight and the degree of functionalization of the isolated and dialyzed polymers were determined by 1H NMR spectroscopy (Table 3). Figure 2 reveals the 1H NMR spectrum of the TREN terminated triblock copolymer PMOxb-PBuOx-b-PMOx4/34/4. The α-CH2 groups of DBX show an isolated baseline-separated signal at 4.48 ppm, which was

showed that it is possible to prepare narrowly distributed poly(2-oxazolines) in short reaction times.49,50 Furthermore, the Schubert group investigated and established a library of synthesized amphiphilic triblock copolymers based poly(2oxazoline)s, which shows the possibility to create even complex polymer structures such as block copolymers.51,45 Surprisingly, only a few examples of poly(2-oxazoline)-based networks are known to date.52 Such polymer networks could be obtained by end group functionalization of polyoxazolines with acrylate-groups and subsequent radical cross-linking,53−55 by cross-linking during the polymerization via bisoxazolines,56,57 by complex formation58,59 and by random linking of functional groups in the polymer backbone.57,60,61 So far the only known amphiphilic networks fully composed of polyoxazolines are not APCNs.57,61,62 They are prepared by cross-linking polyoxazolines with randomly distributed functional groups in the polymer backbone. D


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Figure 2. 1H NMR of PMOx-b-PBuOx-b-PMOx4/34/4.

three bifunctional agents react equimolar with respect to the primary amino end groups of the polymers, which sum up to 4 groups per chain. Thus, 1 mol of the cross-linking agent was added to 2 mol of the NH2 functions (equals to 1 TREN end group) at the polymer terminals. The number of end groups and the molecular weight was calculated from the 1H NMR spectra. In all tests PMOx42 was chosen as test polymer and the amount of bifunctional reagent was calculated on the basis of the molecular weight of PMOx42 (Mn,NMR = 4100 g/mol). Unfortunately, the first two reagents were found to be too reactive to process the polymer/cross-linker mixture, i.e. the reaction mixture solidified before the mixing was complete and thus no homogeneous cross-linking could be expected. DBB on the other hand allowed mixing the bifunctional agent with a POx/acetonitrile solution and processing prior to gelation. After several days of reaction time at room temperature, solid films were obtained. These were swelling but not dissolving in different PMOx solvents with varying polarities including N,Ndimethylformamide (DMF), chloroform (CHCl3), N-methyl-2pyrrolidone (NMP), ethanol (EtOH), and methanol (MeOH). This behavior indicates the formation of a polymer network. Figure 3 shows schematically the idealized network formation. When performing the same procedure with a respective PMOx with OH end groups, no network formation occurred, indicating that the network formation is due to the TREN end groups. In order to get further insight in the network formation, PMOx42 was cross-linked with varying molar quantities of DBB and the resulting networks were swollen in methanol for 48 h. It is well-known that the degree of swelling (S) correlates directly to the cross-linking density. As seen in Figure 4, which

chosen as reference. The CH2 groups of the terminating agent appear at 2.78−2.45 ppm and a degree of functionality of 0.96 was found. The different block lengths were determined from the ratio of the integrals of the CH2-protons of the starter group and the integrals of the respective POx-side group protons. This way the degree of polymerization of PBuOx (DPBuOx) could be determined from the integrals of the signals at 0.85, 1.25, 150 and 2.20−2.27 ppm to be 34. The integral of signal resulting from the CH3 side groups of PMOx at 2.05 ppm gives a DPMox of 4 for each block. This is in good correlation to the set degree of polymerization (DPSet), which was 5/30/5. A reference of poly(2-methyloxazoline) with OH end groups showed no signals at 2.78−2.47 ppm in 1H NMR. The theoretical number of protons, for the positions 8 to 11 in Figure 2, sums up to 24 protons. The fact that the 1H NMR spectrum of sample PMOx-b-PBuOx-b-PMOx4/34/4 showed nearly 24 protons from 2.78 to 2.47 ppm and that this integral is six times greater than the integral of the α-CH2−groups of DBX showed that we could synthesize the aspired telechelic PMOx. Size exclusion chromatography confirmed this result and also showed a narrow molecular weight distribution of 1.11. A summary of the analyses for the different homopolymers and block copolymers is given in Table 3. Next, the cross-linking procedure was explored for plain homopolymer networks composed of PMOx and PBuOx, respectively. The active bifunctional agents, glutaraldehyde, hexamethylene diisocyanate, and trans-1,4-dibromo-2-buten (DBB) were explored regarding their potential to cross-link the telechelic homopolymers. Therefore, we presumed that all E


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Figure 3. Schematical network formation from end group functionalized polyoxazolines via trans-1,4-dibromo-2-butene (DBB). The pictured layer indicates the third dimension by bold printed NH2 (which lead to a layer in front) and gray printed NH2 groups which lead to a layer behind the mapped one.

Figure 4. Obtained degrees of swelling S in methanol for the converted networks obtained by cross-linking telechelic TREN-PMOx42 (left) and TREN-PBuOx24 (right) with different quantities of the cross-linking agent DBB.

depicts S as a function of the added DBB, the lowest S of 9.02 and thus the highest degree of cross-linking is obtained when the TREN end groups react twice. This result indicates that the proposed cross-linked structure is plausible, because 1 mol of DBB seems to react with exactly one TREN end group. It further confirms that the determined molecular weights and the number of end groups are realistic. In further cross-linking reactions, a molar ratio of 1.0 DBB with regard to the TREN end groups was used. The degree of swelling S in methanol decreases to 5.2 when cross-linking PMOx19. Similar results were found for cross-linked telechelic TREN PBuOx24 and PBuOx39, respectively. They show minimal S values in methanol of 4.2 and 8.5, respectively.

Obviously, the cross-linking efficiencies of telechelic TRENterminated PMOx and PBuOx with DBB are similar. According to the determined sol fractions given in the Supporting Information, the efficiency is similar to other end-groups crosslinked systems.17,53 Interestingly, the degrees of swelling of the PMOx networks greatly increase in water to S = 11.2 (PMOx19) and 19.5 (PMOx42). This was also observed for networks derived from PMOx cross-linked by tetrametylenbisoxazoline.56 Even in methanol which was to be found a solvent with less ability to swell the ionic networks derived from PMOx, the degrees of swelling are relatively high. For a closer insight in the efficiency of cross-linking, we calculated Mc the average molecular weight of polymer segments between two crossF


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Figure 5. AFM images of the cross sections of the synthesized networks from the homopolymers PMOx19 and PBuOx24, with different weight fractions varying from 80 wt % (upper left, phase shift 27°), 60 wt % (upper right, phase shift 50°), and 40 wt % (lower left, phase shift 6°) to 20 wt % (lower right, phase shift 18°) PBuOx24. The scale of the high magnification insets is 150 × 150 nm2.

mixture of the homopolymers PMOx and PBuOx and alternatively by cross-linking telechelic TREN-terminated ABA triblock copolymers. PMOx19 and PBuOx24 in different weight fractions varying from 20 to 80 wt % were dissolved in chloroform and subsequently cross-linked with DBB according to the protocol described above. In all cases polymer films were obtained. The sol fraction of these networks between 28 and 38 wt % was in the same range as that of the respective homopolymer networks, indicating a similar cross-linking efficiency as that of homopolymers films. The cross sections of these homopolymer derived APCNs (HB-APCN) were investigated in AFM tapping mode. Figure 5 shows the AFM images of four films with different ratios of PBuOx to PMOx. The amount of the bright PMOx phase rises as expected from 80 to 20 wt % of PBuOx. Additionally, it could be observed that at 80 wt % and at 20 wt % PBuOx24 a relatively fine structure is present and at 60 and 40 wt % of PBuOx24 the separated phases are somewhat larger. Nevertheless, at 80 and 60 wt % PBuOx the hydrophobic PBuOx-phase seems to be interconnected whereas at 40 and 20

links, which should be in the range of the molecular weight of PMOx42. Resulting from 1H NMR measurements a Mn,NMR was calculated to be 4100 g/mol. To correlate S to Mc the theory of Flory−Huggins was used. Therefore, we chose the Flory− Rehner equation42 for high degrees of swelling shown in the experimental part. For the calculation of Mc, the solubility parameter for PMOx was estimated using an incremental calculation method43 and the solubility parameter of methanol was calculated by its enthalpy of vaporization.43 Resulting from these assumptions, a Mc of 3825 g/mol is resulting from a minimal degree of swelling of 9.02 when a molar ratio of 1.0 (DBB:TREN end groups) was used. Compared to the molecular weight Mn,NMR of 4100 g/mol the above value is very close. This indicates that the high values of S resulting from the swelling experiment shown in Figure 4 are not caused by low cross-link efficiency. Further, the networks derived by this cross-linking method are “near perfect” despite of their fairly high sol content. Having established the best cross-linking strategy, amphiphilic polymer conetworks were prepared by cross-linking a G


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Figure 6. AFM images the synthesized networks from the PMOx-b-PBuOx-b-PMOx triblock copolymers with different block ratios varying from 4/ 34/4 (upper left, phase shift 21°), 10/20/10 (upper right, phase shift 14°), and 12/12/12 (lower left, phase shift 8°) to 13/5/13 (lower right, phase shift 4°) PBuOx. The scale of the high magnification inlets is 150 × 150 nm2.

seen that in all cases THF increases the swelling with higher PBuOx content, while water does the same with increasing PMOx content. Seemingly, these solvents selectively swell the respective POx phase. Although, the composition for the homopolymer derived APCNs (HB-APCNs) and the triblock copolymer conetworks (TB-APCNs) are equal, there are significant differences in their swelling behavior. Taking a look at the S value of the HBAPCNs in THF (Figure 7a) reveals that the conetworks do not swell in this solvent with 51 wt % PMOx and more. When decreasing the PMOx content to 34 wt % the conetworks swell by more than 200% (S = 3.2). Higher contents of PBuOx do not lead to greatly higher swelling in THF. When swelling the same conetworks in water (Figure 7b), there is no such sharp transition composition, but the conetworks continuously increase the swelling with higher PMOx content. For instance an HB-APCN with 34 wt % PMOx shows equal degrees of swelling in water and THF, while an HB-APCN with 51 wt % PMOx greatly swells in water (S = 5.0) but not in THF (S = 1.0).

wt % PBuOx24 the darker PBuOx phase is interrupted by the hydrophilic PMOx phase. Alternatively, networks of four different triblock copolymers PMOx-b-PBuOx-b-PMOx with varying compositions from 20 to 80 wt % of PMOx were prepared similarly to the networks above. Transparent polymer phases were obtained. The sol content of these block copolymer APCNs ranging between 23% and 37% was similar to that of the above prepared conetworks obtained from homopolymer mixtures. The AFM images of cross sections of the triblock copolymer APCNs (TBAPCN) are presented in Figure 6. Again all APCN show a nanophase separation. Analogous to the copolymer networks consisting of the two homopolymers depicted in Figure 5 the amount of PBuOx rises from PMOx-b-PBuOx-b-PMOx13/5/13 to PMOx-b-PBuOx-b-PMOx4/34/4. We examined the swelling behavior of these networks to investigate the amphiphilic character of the obtained polymer conetworks. Only two solvents were found that selectively swell either the hydrophobic (PBuOx, THF) or the hydrophilic (PMOx, water) phase of the APCNs. The S values in dependence on the composition are shown in Figure 7. It is H


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Figure 7. Swelling behavior of the APCNs derived from mixtures of the homopolymers PMOx19 and PbuOx24 with varying weight fractions in THF (a) and water (b) and of APCNs build up from the triblock copolymers PMOx-b-PbuOx-b-PMOx in water (c) and THF (d).

Table 4. Characterization of the ABA Triblock Copolymers PMOx-b-PPhOx-b-PMOx by 1H NMR and SEC polymera

DPSetb

DPSECc

Mn,SEC [g/mol]

PDI

PPhOxSetd [mol %]

PPhOxe [mol %]

PMOx30-b-PPhOx20-b-PMOx30 PMOx10-b-PPhOx20-b-PMOx10 PMOx5-b-PPhOx20-b-PMOx5 PMOx3-b-PPhOx20-b-PMOx3 PMOx2-b-PPhOx20-b-PMOx2

80 40 30 26 24

69 45 29 29 22

7100 5200 3700 3800 3000

1.25 1.21 1.19 1.23 1.17

25 50 67 77 83

29 52 74 79 93

a

Composition expected from the used initiator/monomer ratio. bTheoretical overall degree of polymerization. cCalculated from the obtained molecular weight Mn (SEC) presuming the block ratios determined by 1H NMR. dPercentage ratio of PPhOx to PMOx calculated from the initial initiator/monomer ratios. eComposition of the PMOx-b-PPhOx-b-PMOx triblock copolymers calculated from ratio of the PMOx to PPhOx signals in 1H NMR spectroscopy.

nanostructure. Although, the AFM images clearly show that the APCN nanostructure changes with the network topology, there is no structural difference recognizable that would explain the different swelling characteristics. In order to broaden the solvent range usable for selectively swelling the APCNs, we changed the PBuOx with the more hydrophobic poly(2-phenyl-2-oxazoline, PPhOx) in order to prepare the respective TB-APCNs. To this end, PMOx-bPPhOx-b-PMOx ABA triblock copolymers in different compositions were prepared and telechelically terminated with TREN. The procedure was performed similar to the PBuOx/PMOx triblock copolymers and the block ratios were determined by 1H NMR. Unfortunately, the TREN specific signals are overlapping with PPhOx specific backbone signals. The characteristics of the five prepared triblock copolymers are given in Table 4. The polymers were cross-linked according to the procedure described above with a molar quantity of 1.0 DBB per TREN end group, presuming a full termination with TREN. In all cases optically clear TB-APCNs were obtained. The sol fraction of the polymer networks was 9−26 wt %, indicating higher cross-linking efficiency than the respective PBuOx based TBAPCNs. The nanostructure of the PPhOx-based TB-APCNs was analyzed by AFM measurements on cross sections. As seen

The swelling of the triblock copolymer derived TB-APCNs show an entirely different behavior. In contrast to the homopolymer derived conetworks, swelling in THF is continuously increasing from S = 1.0 for 22 wt % PBuOx to S = 5.4 for 86 wt % PBuOx (see Figure 7d). The swelling in water (Figure 7c) on the other hand shows a huge jump in swelling when increasing the PMOx content from 40 wt % (S = 6.1) to 57 wt % (S = 31.7). This different swelling behavior indicates that the structures of the TB-APCNs are different from those of the HB-APCNs. This is particularly visible, when comparing conetworks with compositions of 34 and 51 wt % PMOx. In this composition range, the THF swelling of HBAPCNs drops from 200% to 0%, while the TB-APCNs change from 200% to 75%. The opposite happens in water, where the HB-APCN shows nearly no change of swelling for 34 and 51 wt % PMOx, while the water-swelling of TB-APCNs increase more than 5-fold in this composition range. It is worth noting that the swelling in water of S = 31.7 is higher than that of the respective PMOx homopolymer network from PMOx42. We explain this by the fact that the cross-linking efficiency of the TB-APCN was lower than that of the PMOx42 network (S = 19.5), indicated by the higher sol content of the first. We propose that the entirely different swelling characteristics of the HB-APCNs and the TB-APCNs are caused by a different I


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Figure 8. AFM images of three APCNs from PMOx-b-PPhOx-b-PMOx. The block ratios of the three polymers are 3/20/3 (73% PPhOx), 10/20/10 (50% PPhOx) and 30/20/30 (29% PPhOx). The scale of the high magnification inlets is 270 × 270 nm2.

in Figure 8 all conetworks show a very regular nanophasic structure. Interestingly, the APCN with the 73 wt % PPhOx show an interconnected phase of this polymer, while the PMOx phase forms isolated domains. The two APCNs with the highest PMOx content show two interconnected polymer nanophases. As expected from these structures, the hydrophilic PMOx phase can indeed only be swollen when it is interconnected as seen in Figure 9. The same transition of swelling in water was found for the respective PBuOx based TB-APCNs, indicating that the conetwork topology is indeed the reason for this behavior. Unfortunately, we could not find a solvent that selectively swells the PPhOx phase to fully confirm the structure-swelling relations that are suggested by the AFM images.

Figure 9. Measurement of the swelling behavior of the networks resulting from PMOx-b-PPhOx-b-PMOx triblock copolymers in water.

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ymers and compare them to similar APCNs derived by crosslinking the respective ABA triblock copolymers. In order to prepare such networks, narrowly size distributed poly(2oxazoline) homo- and triblock copolymers were prepared and quantitatively telechelically functionalized with N,N-bis(2-

CONCLUSION The goal of this work was to explore the properties of “near perfect” amphiphilic polymer conetworks derived by homopolJ


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aminoethyl)ethylenediamine. The cross-linking with 1,4dibromo-2-butene resulted in nanophasic “near perfect” amphiphilic polymer conetworks. The nanostructure explored with AFM showed significant differences between mixed homopolymers APCNs (HB-APCNs) and networks derived from ABA triblock copolymers (TB-APCNs). While the first showed areas with typical lamellar structures those are not evident in the TB-APCNs of similar compositions. More dramatically is the difference in swelling behavior. Ideally, the nanophase of APCNs, in contrast to common random copolymer networks, should swell independently from the other orthogonal phase in the respective selective solvent. Given the typical nanostructure motives of APCNs this phase should swell only if it is interconnected, which is typically the case around 50% (v/v). The other phase should act vice versa in presence of its selective solvent. Such a typical behavior is rarely found for the numerous published APCNs so far.19,64,65 More typically, the above-described swelling behavior is found for one phase only, while the other phase swells continuously, because it does not form isolated areas below 50% (v/v). We propose that this is controlled by the cross-linking strategy. The comparison of the swelling behavior of HB-APCNs and TBAPCNs clearly supports this thesis. In HB-APCNs the PBuOx phase shows the typical APCN behavior when swollen in THF, while water swells the PMOx phase continuously increasing with greater content. The behavior is fully inverted in TBAPCNs, which follow a different cross-linking strategy of the two poly(2-oxazoline) phases. Thus, we could show that the properties of APCNs with the same polymer phases can be controlled by the cross-linking strategy. This is of utterly importance to the applications of APCNs. Finally, the here presented APCNs are the first examples of this class fully composed of poly(2-oxazoline)s.

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ASSOCIATED CONTENT

S Supporting Information *

Tables showing the degrees of swelling of the polymers and figures showing 1H NMR spectra and composition of the sol fraction. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*(J.C.T.) E-Mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors of this paper would like to thank Thorsten Moll for performing SEC and Dr. W. Hiller for 1H NMR measurements. All polymers were synthesized using CEM Discover microwaves, which were kindly provided by CEM for undergraduate student education.

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REFERENCES

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Amphiphilic Polymer Conetworks Based on End Group Cross-Linked

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