Synthesis and Self-Assembly of Amphiphilic Triblock Terpolymers with

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Synthesis and Self-Assembly of Amphiphilic Triblock Terpolymers with Complex Macromolecular Architecture George Polymeropoulos,†,‡ George Zapsas,† Nikos Hadjichristidis,*,‡ and Apostolos Avgeropoulos*,†,‡ †

Department of Materials Science Engineering, University of Ioannina, University Campus-Dourouti, 45110 Ioannina, Greece King Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory, Thuwal, 23955, Saudi Arabia

‡

S Supporting Information *

ABSTRACT: Two star triblock terpolymers (PS-b-P2VP-bPEO) 3 and one dendritic-like terpolymer [PS-b-P2VP-b(PEO)2]3 of PS (polystyrene), P2VP (poly(2-vinylpyridine)), and PEO (poly(ethylene oxide)), never reported before, were synthesized by combining atom transfer radical and anionic polymerizations. The synthesis involves the transformation of the −Br groups of the previously reported Br-terminated 3-arm star diblock copolymers to one or two −OH groups, followed by anionic polymerization of ethylene oxide to afford the star or dendritic structure, respectively. The well-defined structure of the terpolymers was confirmed by static light scattering, size exclusion chromatography, and NMR spectroscopy. The self-assembly in solution and the morphology in bulk of the terpolymers, studied by dynamic light scattering and transmission electron microscopy, respectively, reveal new insights in the phase separation of these materials with complex macromolecular architecture.

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The synthesis by ATRP, as well as the molecular, and morphological characterization of the precursors [3-arm star diblock copolymers of the (PS-b-P2VP)3 type] has been recently reported by Polymeropoulos et al. in detail.10 The successful synthesis of the final 3-arm star triblock terpolymers and the 3-arm star dendritic terpolymer involves two steps. In the first step, the −Br chain-end groups are replaced with amines bearing one or two −OH groups by reaction with ethanolamine or diethanolamine, respectively.11 In the second step, the −OH terminated 3-arm star diblock copolymers were used as precursors for the anionic polymerization of ethylene oxide in the presence of diphenyl methyl potassium (DPMK), under high vacuum conditions in specially designed glass apparatuses. The synthesis of DPMK is described in detail elsewhere.11 In Scheme 1, the general modification reactions with the appropriate amines, as well as the anionic polymerization of ethylene oxide, are displayed. The molecular characterization of all precursor, intermediate, and final products was carried out via size exclusion chromatography (SEC), static light scattering (SLS), and proton/carbon nuclear magnetic resonance spectroscopy (1H NMR and 13C NMR). The self-assembly in mixtures of DMF and H2O was studied by dynamic light scattering (DLS) and in bulk by transmission electron microscopy (TEM).

t is well known that amphiphilic linear diblock copolymers, in selective for one block solvents, form core-shell micelles1, while linear triblock copolymers, in selective for the middle block solvents, form flower-like micelles, where the two identical external chains form the core and the middle chain forms the shell.2−4 PS-b-P2VP-b-PEO, an amphiphilic triblock terpolymer, where PS, P2VP and PEO are polystyrene, poly(2vinylpyridine) and poly(ethylene oxide) respectively, is of significant interest since the addition of water in a dilute solution of the terpolymer in N,N-dimethylformamide (DMF) leads to the formation of core−shell-corona micelles.5 The size of the micelles is strongly dependent on the average molecular weight of each block, the pH of the solution and the molar fraction of DMF and H2O.6 To the best of our knowledge, there are no reports concerning synthesis and self-assembly of complex nonlinear macromolecular architectures of this triblock terpolymer. Therefore, this work is focused on the synthesis, characterization, and study of the influence of the chain-end topology (one or two PEO chains) on the self-assembly in water and in bulk of two 3-arm star triblock terpolymers (PS-b-P2VP-bPEO)3 and the corresponding dentritic terpolymer [PS-bP2VP-b-(PEO)2]3. The synthetic procedure was accomplished by combining atom transfer radical polymerization (ATRP)7 with anionic polymerization using high vacuum techniques8 and appropriate modification reactions.9 © XXXX American Chemical Society

Received: November 7, 2015 Accepted: November 19, 2015

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Scheme 1. Modification Reactions for Converting the −Br to One or Two −OH Groups, Followed by Formation of the PEO Chains to Afford the Final Nonlinear Triblock Terpolymers

Table 1. Molecular Characteristics of the Synthesized 3-Arm Star PS, 3-Arm Star Block Co/Terpolymers and Dendritic Terpolymer samples

(M̅ n)SEC (g/mol)

PDI

(M̅ w)SLS (g/mol)

(PS)3-1 (PS-b-P2VP)3-1 (PS-b-P2VP-b-PEO)3-1 (PS)3-2 (PS-b-P2VP)3-2 (PS-b-P2VP-b-PEO)3-2 [PS-b-P2VP-b-(PEO)2]3

27.000 45.000 52.000 16.000 44.000 102.000 100.000

1.11 1.21 1.47 1.15 1.40 1.33 1.35

40.000 80.000 93.000 30.000 78.000 140.000 138.000

f(PS) [1H NMR; %(w/w)]

f(P2VP) [1H NMR; %(w/w)]

f(PEO) [1H NMR; %(w/w)]

0.55 0.51

0.45 0.34

0.15

0.42 0.20 0.21

0.58 0.32 0.34

0.48 0.45

It should be mentioned that the main reason for synthesizing two different architectures {(PS-b-P2VP-b-PEO)3 and [PS-bP2VP-b-(PEO)2]3} was to investigate the effect of the extra PEO chain on the dimensions/properties of the micelles in water, as well as to study the self-assembly of these star triblock terpolymers in bulk. The successful substitution of −Br groups by the corresponding amines was confirmed by 1H and 13C NMR spectroscopy (Supporting Information, SI, Figures S1 and S2). Three samples were synthesized, two of the (PS-b-P2VP-bPEO)3 type and one of the [PS-b-P2VP-b-(PEO)2]3 structure. Experimental details are given in the SI. Their molecular characteristics are displayed in Table 1. Size exclusion chromatography results are given in the SI for all three samples (Figure S3−S5). The chromatograms of (PS)3 and the corresponding co/terpolymers are also given for comparison purposes. All precursors, intermediates and final materials exhibit monomodal SEC traces with low molecular weight distribution, demonstrating the absence of byproducts (e.g., PEO homopolymer from the final terpolymers). Consequently, the

polymerization of ethylene oxide from the chemically modified 3-arm star diblock copolymers was successful. The 1H NMR spectra of the 3-arm star triblock terpolymers exhibit chemical shifts at 3.5 ppm, which correspond to the protons of the PEO monomeric unit. Although these shifts overlap with those attributed to the protons of the amines (chemical modification), it is evident in the case of the terpolymers that they are much more prominent, leading to the conclusion that the incorporation of the PEO blocks was almost complete. Figure 1 shows the 1H NMR spectra of the final terpolymer (PS-b-P2VP-b-PEO)3-2 (C), the corresponding 3-arm star diblock copolymer modified with ethanolamine (B), as well as the initial 3-arm star diblock copolymer (PS-bP2VP)3-2 (A). Similar 1H NMR spectra are shown in the SI for the dendritic sample [PS-b-P2VP-b-(PEO)2]3 (Figure S6). Structural characterization in solution of the 3-arm star triblock terpolymers and the dendritic analogue was performed by dynamic light scattering (DLS). The existence of micellar assemblies in both cases was confirmed with the dendritic terpolymer {[PS-b-P2VP-b-(PEO)2]3} to possess higher hydrodynamic diameter (DH) due to the two PEO chains per 1393

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Figure 1. 1H NMR spectra of the (A) initial 3-arm star diblock copolymer; (B) intermediate product modified with ethanolamine; and (C) final 3arm star triblock terpolymer (PS-b-P2VP-b-PEO)3.

junction point compared with just one in the (PS-b-P2VP-bPEO)3 sample. Solutions of the 3-arm star diblock copolymer (PS-b-P2VP)32, and the corresponding terpolymers (PS-b-P2VP-b-PEO)3-2 and the [PS-b-P2VP-b-(PEO)2]3 samples, were prepared in N,N dimethylformamide (DMF) and the micelles formed by adding milli-Q water were studied. The total number-average molecular weight of the PEO block was approximately constant (∼45.000 g/mol) for (PS-b-P2VP-b-PEO)3-2 and the dendritic terpolymer, therefore, the total monomeric units of PEO remain almost identical. For DLS measurements, literature data were used in order to incorporate known values of viscosity and refractive index for the specific solvent mixture (DMF/H2O) in various ratios.12 The ratio of H2O versus DMF was increased from 5 to 20 wt %, while the polymer concentration in the solutions was approximately equal to 0.5 wt %, in all measurements. It should be mentioned that in order to validate and confirm our results from DLS we repeated all experiments at least three times. The results given for the hydrodynamic radii of the micelles were practically identical with a mean error of ±1.5− 2.0 nm, which is within the margin of error for DLS experiments. Therefore, the micelles dimensions may be considered absolutely reproducible. Table 2 summarizes the hydrodynamic diameter (DH) and polydispersity (I) of the micelles at different H2O/DMF ratios. The 3-arm star diblock copolymer (PS-b-P2VP)3-2, for the first two ratios of DMF/H2O (95/5 and 90/10), does not form micellar structures, and the chains exist as unimers.13 When the H2O concentration is increased, a minor augmentation of the hydrodynamic diameter was observed (SI, Figure S7) due to the tendency of the hydrophobic PS core to avoid contact with water (micelle core), while P2VP is forming the shell. The

Table 2. Hydrodynamic Diameter and Polydispersity of (PSb-P2VP)3-2, (PS-b-P2VP-b-PEO)3-2, and [PS-b-P2VP-b(PEO)2]3 Samples at Concentration Equal to 0.5 wt % in Different Ratios of DMF/H2O Mixtures ratio DMF/H2O 95% 90% 85% 80%

DMF−5%H2O DMF−10% H2O DMF−15% H2O DMF−20% H2O

95% 90% 85% 80%

DMF−5%H2O DMF−10% H2O DMF−15% H2O DMF−20% H2O

95% 90% 85% 80%

DMF−5%H2O DMF−10% H2O DMF−15% H2O DMF−20% H2O

hydrodynamic diameter DH (nm) (PS-b-P2VP)3-2 4.0 7.4 14.2 17.1 (PS-b-P2VP-b-PEO)3-2 8.9 43.9 47.3 56.8 [PS-b-P2VP-b-(PEO)2]3 11.1 43.3 55.9 67.1

I (polydispersity, DLS) 0.317 0.435 0.293 0.278 0.611 0.200 0.186 0.181 0.458 0.133 0.154 0.250

results are different when the PEO blocks are added in the terpolymer samples. Even in high ratio of DMF versus H2O (95/5), the formation of micelles with very small hydrodynamic diameters is evident since increased values were determined when compared to the corresponding copolymer (8.9 and 11.1 nm for the two terpolymers in comparison to 4.0 nm for the copolymer; SI, Figures S8−S9). In the cases where H2O content was the highest toward DMF (80/20), the hydrodynamic diameter was increased considerably, due to the hydrophilicity of the PEO block and the increase of the 1394

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Scheme 2. Schematic Illustration of the Formed Micelles by Addition of Milli-Q Water in DMF Solutions (H2O: 20 wt %) of (PS-b-P2VP)3-2, (PS-b-P2VP-b-PEO)3-2, and [PS-b-P2VP-b-(PEO)2]3 Co/Terpolymers and the Corresponding Hydrodynamic Diameter (nm) Determined by DLS

corresponds to the P2VP blocks (stained with iodine) and the gray/white to the PS/PEO chains. It is evident that thermal annealing led to ordered structure despite the fact that the χ values were decreased.18 Introducing thermal energy to the system results in minimizing thermodynamic constrains (attributed to conformational entropy factors), leading to better-organized microstructures due to the fact that the complex macromolecular architecture is significantly dependent on entropy.19 Without thermal annealing, the chains are highly entangled due to the large number of junction points leading to high disorder, which is evidently improved by thermal annealing. The terpolymer samples are consisting of two amorphous chains (PS and P2VP) and one highly crystalline chain and the self-assembly is highly dependent on the total number-average molecular weight of all blocks. In the specific sample [(PS-b-P2VP-b-PEO)3-1] the PEO chain numberaverage molecular weight is very low (3.000 g/mol) and therefore the PEO domains are not microphase separated from the PS chains, leading eventually to a two-phase system.20 The morphological characterization of the second sample, [(PS-b-P2VP-b-PEO)3-2] revealed a three-phase morphology. The precursor sample (PS-b-P2VP)3-2 at room temperature exhibits alternating lamellae structure as already reported elsewhere,10 but the addition of the PEO block led to hexagonally closed packed core−shell cylinders, consisting of PS chains (dark gray areas), surrounded by the P2VP blocks (dark areas), in the matrix of the PEO domains (light gray areas; Figure 2b). The TEM results could be explained taking into account the higher value of number-average molecular weight of the PEO chains. More specifically, for sample (PS-b-P2VP-b-PEO)3-2, the volume fraction of PEO is high [0.48 compared to only 0.15 for sample (PS-b-P2VP-b-PEO)3-1] and is the matrix in the observed morphology. The exhibited three-phase structure, is in good agreement with theoretical predictions,16 for the corresponding linear terpolymers. A schematic illustration of the morphology is depicted in Figure 2c.

repulsive forces between the hydrophobic blocks, which eventually form the insoluble core (PS) and shell (P2VP). A quite interesting observation from DLS measurements is the increased hydrodynamic diameter in high H2O content of the [PS-b-P2VP-b-(PEO)2]3 sample in comparison to the (PSb-P2VP-b-PEO)3-2, despite the fact that both nonlinear terpolymers exhibit almost identical number-average molecular weight for the total PEO blocks (45.000 g/mol). Having two PEO chains per junction point with half number-average molecular weight in the dendritic sample leads to higher hydrodynamic diameter, which may be attributed to overcrowding caused by the increased number of PEO chains. This behavior cannot be correlated to the attractive forces between water and PEO hydrophilic blocks due to the similar PEO total molecular characteristics for both terpolymer samples. An illustration describing the effect of the end block topology vs the hydrodynamic diameter of the formed micelles, is given in Scheme 2. The morphological characterization in bulk of the terpolymers was accomplished by TEM. As in the case of linear triblock terpolymers, the main factors affecting the microphase separation in the nonlinear terpolymer materials are the Flory− Huggins interaction parameters χPS/P2VP,10 χPS/PEO,14 and χP2VP/PEO,15 the degree of polymerization (N), and the volume fraction (φ) of each block.16 In a recent work reported by our group,10 both copolymer samples [(PS-b-P2VP)3-1 and (PS-b-P2VP)3-2], which were used as the precursors for the final terpolymers of this study, adopt the alternating lamellae morphology. The addition of the PEO block eventually leads to different self-assembly behavior and to three-phase systems if the degree of polymerization per block as well as the three different Flory−Huggins interaction parameters are significantly high.17 The TEM results for sample (PS-b-P2VP-b-PEO)3-1, without thermal annealing, led to disordered structures (SI, Figure S10). The results are improved after thermal annealing of the specific sample at 140 °C for 24 h. The observed morphology is alternating lamellae (Figure 2a) where the dark phase 1395

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

G.P. and A.A. would like to acknowledge cofinancing of this research project by the European Union (European Regional Development Fund- ERDF) and Greek national funds through the Operational Program “THESSALY- MAINLAND GREECE AND EPIRUS-2007−2013” of the National Strategic Reference Framework (NSRF 2007−2013). G.P., G.Z., and A.A. would like to acknowledge the Network of Research Supporting Laboratories at the University of Ioannina for using the Electron Microscopy Facility. G.P. and N.H. would like to acknowledge the support by King Abdullah University of Science and Technology.

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Figure 2. (a) TEM image of sample (PS-b-P2VP-b-PEO)3-1 after thermal annealing at 140 °C for 24 h, exhibiting alternating lamellae morphology; (b) TEM image of the (PS-b-P2VP-b-PEO)3-2 sample exhibiting the morphology of hexagonally closed packed core−shell cylinders, consisting of PS (gray), which are surrounded by P2VP (dark), in the matrix of PEO (white); (c) Schematic presentation of the observed hexagonally closed packed core−shell cylindrical morphology.

In summary, we report a rather facile method to synthesize novel amphiphilic terpolymers with complex macromolecular architectures by combining ATR and anionic polymerizations and appropriate chemical modification reactions. Their DLS and TEM characterization led to significant conclusions concerning the influence of the terpolymer structure on the self-assembly in selective solvents and in bulk. These amphiphilic terpolymers with complex macromolecular architectures are potential candidates for integral asymmetric membranes.21

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00795. Experimental details and characterization data (PDF). 1396

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