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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Specific Supramolecular Interaction Regulated Entropically Favorable Assembly of Amphiphilic Macromolecules Pradip Dey,†,§ Priya Rajdev,‡,§ Prithankar Pramanik,†,§ and Suhrit Ghosh*,†,‡ †

Polymer Science Unit and ‡Technical Research Center, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Kolkata, India 700032

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S Supporting Information *

ABSTRACT: This article reports molecular interaction driven aqueous assembly of supramolecularly engineered amphiphilic macromolecules to cylindrical structure. Each polymer contains a single hydrophobic trialkoxybenzamide-linked naphthalene diimide (NDI) chromophore at the chain terminal as the supramolecular structure directing unit (SSDU). Irrespective of the structure of the appended hydrophilic polymer, H-bonding promoted J-aggregation among the NDI chromophore leads to the formation of thermally stable spherical micelle (critical aggregation concentration: 0.01−0.03 mM) which reorganizes to cylindrical micelle after a few hours. The reorganization time can be regulated by pH in the case of the anionic polymer as it affects the dynamics. Isothermal titration calorimetric (ITC) studies reveal positive ΔS values for assembly of all the polymers, reflecting the self-assembly process is favored by the entropy factor similar to the elegant examples in the biological domain. In contrast, a small molecule analogue of these polymers, having a short hydrophilic wedge (instead of a water-soluble polymer), shows a reverse trend, typically expected in a process of supramolecular organization. This can be attributed to the tightly packed J-aggregation of the NDI chromophore of the SSDU that compels a close packing of the hydrophilic polymer chains in the corona, leading to the release of the surrounding water molecules and causing entropy enhancement.

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block copolymers.15−18 On the other hand, directional interactions like H-bonding, π−π stacking, charge-transfer interaction, or dipole−dipole interaction have been successfully utilized in the design of wide-ranging functional supramolecular systems with defined internal order,19−26 which has also been demonstrated in aqueous medium27−30 including polymeric systems31 endowing access to exciting supramolecular biomaterials.30 We have recently introduced32 supramolecularly engineered amphiphilic macromolecules (Scheme 1) which assemble to a predictable structure, primarily regulated by H-bonding. Consequently, P0 and P1 (Scheme 1), which have the same hydrophobic/hydrophilic balance, but merely differ by the single H-bonding functional group (hydrazide and amide, respectively), produced polymersome and cylindrical micellar structure, respectively.32 In fact, we showed that in the P0 series of polymers polymersome structure was formed for all three cases with n = 25, 50, or 100, and similarly for P1 series (P1-a, P1-b, P1-c), cylindrical

INTRODUCTION The field of amphiphilic polymers has grown strength to strength in the recent past owing to their broad range of applications in drug delivery, imaging, sensing, catalysis, and other related areas.1−10 Aggregation of amphiphilic block copolymers leads to various structures including spherical or rod-like micelle, lamellar, and polymersome in a block selective solvent. The nature of the nanostructure is generally governed by the packing parameter (p) (expressed by eq 1) in which v, a, and l are the volume of the hydrophobic chains, optimal area of the headgroup, and length of the hydrophobic tail, respectively. v p= (1) al Spherical micelle, cylindrical micelle, or polymersome structures are formed when p ≤ 1/3, 1/3 < p ≤ 1/2, and 1/ 2 < p ≤ 1, respectively.11−14 Primarily these aggregates are formed by repulsive forces between the solvent and a particular block, and therefore it is difficult to regulate the nanostructure with molecular scale precision. In contrast to such immiscibility driven aggregation, recently, crystallization of the hydrophobic block was used as the primary driving force to produce well-defined nanostructures from specific examples of © XXXX American Chemical Society

Received: May 14, 2018 Revised: June 12, 2018

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DOI: 10.1021/acs.macromol.8b01025 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Structure of Various Polymers (n ∼ 50 in P2, P3, and P4) and Control Moleculesa

a

The supramolecular structure directing unit (SSDU) is shown inside a box in the structure of NDI-CTA. It remains the same for all the polymers studied in this paper.

Scheme 2. Schematic Showing Formation of Spherical/Cylindrical Micellar Structure by Directional Molecular Interaction in SEAMs

is similar to P3 in terms of the pendant group, the backbone is different. In this article we elucidate the comparative selfassembly studies of these SEAMs with specific highlight on the thermodynamic aspects that reveal the vital role of entropy on the self-assembly process similar to examples in the biological macromolecules.

micellar structure was noticed irrespective of the degree of polymerization within the tested window. Therefore, it was evident that irrespective of the packing parameter, the single H-bonding functional group, when was hydrazide, could direct polymersome assembly while the amide group produced cylindrical structure. However, in the previous report, such a phenomenon was demonstrated with one particular polymer. But it still remained to be seen whether such strategy would be able to act as a generalized structure directing guideline for structurally diverse polymers with different hydrophilicity, backbone functionality, charge, etc. To test these possibilities, we have now studied a series of polymers in which the supramolecular structure directing unit (SSDU) remains the same (Scheme 1) but the polymer structure varies. We are interested particularly about this SSDU containing the amide functionality as in the case of P1, it produced relatively less abundant (compared to spherical nanostructures) cylindrical morphology,32 which appears to be attractive for many applications33−37 in nanotechnology and biomedicine including flow insensitive drug delivery, enhancing the toughness of epoxy resins, nanoscopic etch resists, enhanced cellular uptake, mineralization of hydroxyapatite, and so on. P1 contains a nonionic hydrophilic polymer with a methacrylate backbone. In P2, it is not only a methacrylamide backbone but also the pendant group that is ionizable depending on the pH. P3, on the other hand, has a methacrylamide backbone but nonionic structure. While P4

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RESULTS AND DISCUSSION Synthesis and Characterization of the Polymers. Synthesis of P1-a−P1-c has been reported elsewhere.32 They were made by RAFT polymerization of triethylene glycol monomethyl ether attached methacrylate monomer using the NDI-CTA. The synthetic scheme for the other polymers is shown in Scheme 3. P2 and P3 were synthesized by postpolymerization modifications of a parent amine-reactive polymer (P-NHS).38 P-NHS was also synthesized39 by RAFT polymerization of the N-hydroxysuccinimide methacrylate (M1) monomer using a functional chain-transfer agent (CTA) containing the amide functionalized SSDU and the hydrophobic wedge. NDI-CTA was prepared in a few steps following our previously reported procedure (1H NMR, Figure S1).32 P-NHS was isolated as white solid in 90% yield and structurally characterized by 1H NMR (Figure S2) and UV/vis spectroscopy (Figure S4). Size exclusion chromatography trace (Figure S3) showed unimodal peak with Mn = 13 kDa (PDI = 1.26), indicating the degree of polymerization (DP) to be approximately 50 as estimated from the monomer/CTA ratio. B


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Macromolecules Scheme 3. Synthetic Scheme of Polymers P2, P3, and P4a

a Reagents and conditions: (a) anisole, 75 °C, 6 h, 90%; (b) DMF, Et3N, 12 h, rt, quantitative; (c) DMF, 12 h, rt, quantitative; (d) 1,4-dioxane− water (5:1), 75 °C, 6 h, 80%.

Figure 1. (a) Normalized UV/vis spectra of P2 (in water and pH 10) and NDI-CTA in THF. (b) Variable temperature UV/vis spectra of an aqueous solution P3. c = 1.0 mg/mL and l = 1 cm. Blue = 25 °C; red = 85 °C.

Substitution of the NHS esters by γ-aminobutanoic acid (GABA) and serinol lead to the formation of P2 and P3, respectively.40 The complete substitution of the NHS ester by GABA and serinol was confirmed by the disappearance of the characteristic peak at δ = 2.83 ppm in 1HNMR spectra (Figure S5 and S6). FTIR spectra further confirmed the complete substitution by the disappearance of the characteristic peaks at 1810, 1781, and 1735 cm−1 corresponding to the NHS ester group (Figure S7). P4 was synthesized41 by polymerization of the monomer M2 using the same NDI-CTA in 1,4-dioxane and water solvent mixture (4:1) at 75 °C using AIBN as initiator with the ratio of monomer/initiator/CTA = 50:0.3:1. P4 was isolated as a waxy solid in 80% yield and characterized by 1H NMR (Figure S8). The SEC trace (Figure S9) revealed Mn = 10 kDa (PDI = 1.32), indicating a close match between the estimated and observed values.

Self-Assembly and Morphology. Supramolecular-assembly of these polymers was studied by UV/vis spectroscopy (Figure 1 and Figure S10) in aqueous solution. As none of these polymers were soluble in tested common organic solvents (DMF, CHCl3, THF, or CH2Cl2) in which usually NDI remains as monomer, NDI-CTA was used as the reference compound to represent the spectral features of the monomeric dye. NDI-CTA in THF exhibits sharp absorption bands in the region of 340−400 nm, which is typical of monomeric NDI.42,43 However, for P2, in either water or basic pH, a bathochromic shift of ∼12 nm, together with reversal of intensities of the two vibronic peaks, clearly indicate offset πstacking, which is typical of NDI chromophore.44 Similar spectral features were also noticed (Figure S10) for the other polymers indicating that irrespective of the nature of the hydrophilic polymer; in each case, there is a self-assembly. However, the extent of the bathochromic shift is maximum for C


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Figure 2. (a, b) TEM (negative staining with uranyl acetate) and (c, d) AFM height images of P2 in water for (a, c) freshly prepared and (b, d) aged (72 h) samples. (e) TEM and (f) AFM height images of freshly prepared samples of P2 in aqueous NaOH (pH 10). Concentration of the polymer solutions = 1.0 mg/mL in all experiments: Dark spots in the TEM images possibly represent the crystals of uranyl acetate.

(Figure 2d) revealing identical cylindrical morphology with width of approximately 25 nm. In general, width measured by AFM is not as accurate as high-resolution TEM images, which may be the reason for the minor discrepancy in the diameter of the cylinders measured by these two different techniques. Furthermore, the morphology of P2 was examined when dissolved in aqueous NaOH (pH 10). In this case cylindrical morphology was noticed even for the fresh sample by either TEM or AFM (Figures 2e and 2f) with width in the range of 15−20 nm. Thus, it is evident that the ability of the SSDU to regulate mesoscopic structure is not only limited to nonionic polymers like P1 but also applicable for charged polymer like P2. However, for P2 at pH 10, the intermediate spherical structure could not be captured possibly because the charge−charge repulsion made the system highly dynamic and consequently the transition from spherical to cylindrical micelle was too fast to capture. P3, having the same methacrylamide backbone like P2, but nonionic in nature, also showed spherical aggregates in freshly prepared solution as evident from the AFM image (Figure S14). However, unlike P2, in this case the morphology remained almost similar after aging and no cylindrical micelle

P2 at pH 10 (anionic) which may be attributed to the maximum lateral shift of the stacked chromophores due to charge−charge repulsion among the carboxylate groups of the attached polymer. Figure 1b shows the effect of temperature on the UV/vis spectra of aggregated P3 in water. Surprisingly even at 90 °C, no signature of disassembly was noticed (Figure 1b). Likewise, there was no change in aggregation behavior for the other polymers (P1, P2, and P4) on heating (Figure S11), suggesting remarkable thermal stability in all cases. Morphology of the self-assembled polymers in water was studied by transmission electron microscopy (TEM) and atomic force microscopy (AFM) (Figure 2). TEM images (Figure 2a) show spherical objects for P2 in freshly prepared aqueous solution with an average diameter of about 30−40 nm, indicating spherical micelle formation which corroborates with the DLS results (Figure S12) and is consistent with the results obtained for P1 in our earlier report.32 The AFM image also confirms the spontaneous formation of spherical micelle in water (Figure 2b). In contrast, the TEM image of P2 in aged solution reveals cylindrical micellar morphology45,46 with diameter of approximately 20 nm (Figure 2b) and the length extending over a few micrometers, which corroborates with AFM image D


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Macromolecules formation could be noticed. In contrast, P4, which has the same pendant group like P3, but the backbone is methacrylate (like P1), showed similar behavior like P1 or P2. Spherical micelle was formed in freshly prepared solution which changed to cylindrical morphology after aging (Figure S14). Thus, it is believed that in P3 the amide backbone freezes the initially formed spherical micellar structure (possibly by interchain Hbonding), preventing it from further reorganization to the cylindrical structure. On the other hand, in P2, although the polymer backbone is the same, the presence of charge makes the system dynamic which appears to be playing critical role in the morphology transition. This proposition is further supported by the fact that P2 at pH 3 showed no transformation from spherical to cylindrical micelle (Figure S13) even after 10 days. Thermodynamics Studies. The present polymers contain merely 3−4 wt % of hydrophobic segment but still exhibit remarkably stable self-assembly features in water, as evident from the lack of disassembly at elevated temperature in the variable temperature UV/vis experiments. Intrigued by this fact, we sought to examine the thermodynamics of selfassembly of these polymers by isothermal titration calorimetry (ITC) studies. Concentration of the aqueous aggregated solution of a particular SEAM was adjusted in such a way so that when it was injected into pure water, the concentration reached lower than the critical aggregation concentration (CAC), and thus the heat change would indicate heat of dissociation. Figure 3 shows such an ITC dilution experiment of P1-b at 30 °C revealing an exothermic heat flow which eventually saturates beyond a certain concentration. This was attributed to CAC, as above this concentration, there is no more disassembly of the micelles, but only the heat of dilution persists. From the ITC data, CAC and ΔH (enthalpy of micellization) (Table 1) were determined according to the method reported in the literature.47 It is evident from the exothermic nature of heat flow during disassembly that the assembly in this case is an enthalpically disfavored process, which corroborates with our previous finding, revealing that the system did not disassemble at higher temperature. From the ITC data, the free energy (ΔG) and the entropy (ΔS) of micellization could be estimated using eqs 2 and 3, in which CAC, T, and R are the critical aggregation concentration, absolute temperature, and universal gas constant, respectively. Then, ΔS can be expressed by using the Gibbs−Helmholtz equation (eq 3). ΔG = RT ln CAC

(2)

ΔG = ΔH − T ΔS

(3)

Figure 3. (a) ITC dilution experiment of freshly prepared aqueous solution of P1-b. Top: heat release per injection of P1-b (c = 2.5 mg/ mL) into pure water at 303 K. Bottom: corresponding enthalpogram. (b) Determination of CAC from ΔH versus concentration plot.

Table 1. Thermodynamic Data Obtained from ITCa polymer

CAC (mM)

ΔH (kcal/mol)

ΔG (kcal/mol)

ΔS (cal/mol/K)

P1-a P1-b P1-c P2 P2 (pH 10) P3 P4

0.03 0.013 0.01 0.015−0.025 0.02 0.015−0.02 0.02

3 4 4.5 1.5 1.0 2 1.1

−6.27 −6.75 −6.93 −6.5 −6.51 −6.55 −6.5

30.6 35.5 37.7 26.4 24.8 28.2 25.1

a

Data represent the micellization process for all the polymers at 303 K.

The positive ΔS value for P1-b implies that the aggregation though is enthalpically disfavored but is entropically favorable and therefore eventually becomes a spontaneous process. This observation holds true irrespective of the molecular weight variation, as the nature of the respective ITC enthalpogram for P1-a (lower molecular weight) and P1-c (higher molecular weight) (Figures S15 and S16) and the corresponding thermodynamic parameters (Table 1) were found to be almost similar. We have further extended this study with the other polymers, and in all cases the ITC dilution enthalpogram revealed a similar trend (Figures S17−20). From the ITC data, using eqs 2 and 3, thermodynamic parameters were obtained for all the polymers (Table 1) which revealed a few interesting features: (i) CAC was comparable and very low (in the range

0.01−0.03 mM) for all the polymers, indicating although the hydrophobic part was only 3−4 wt % in these polymers in contrast to 30−50% in common amphiphilic block copolymers, it did not have any adverse effect on the self-assembly. (ii) Positive ΔH as well as ΔS in all examples indicating the self-assembly is entropically favored but enthalpically disfavored. (iii) Although the marginally, but noticeable, fact is the increase in ΔS values with increase in the degree of polymerization in the P1 series. How does one rationalize these experimental observations? It is imperative to understand that the self-assembly of the present series of polymers is triggered by the directional interaction among the SSDU as the E


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Figure 4. (a) Variable temperature UV/vis spectra of NDI-CTA in methylcyclohexane (MCH). c = 0.2 mM and l = 1 cm. (b) Plot of absorbance (at 386 nm) vs temperature.

rest of the macromolecule is water-soluble. Therefore, to understand the self-assembly of the SSDU alone, a variable temperature UV/vis study was performed with the NDI-CTA (Figure 4) which essentially represents the SSDU. However, lacking the hydrophilic polymer, it was not possible to disperse it in water, and therefore the studies were conducted in MCH (c = 0.2 mM). At rt, the UV/vis spectrum represents the aggregated state of NDI, but with increase in temperature, there is a clear spectral change involving reduction of the intensity of the shoulder band at 386 nm with concomitant increase in the intensity of the bands at 358 and 378 nm and change in the ratio of the intensities of the two vibronic bands, indicating disassembly at elevated temperature. The change in the band intensity at 386 nm with temperature (Figure 4b) indicates complete disassembly at around 50 °C, which is in sharp contrast to the behavior of the same SSDU, when appended with the large water-soluble macromolecule in various SEAMs (Figure 1), as they did not show any signature of disassembly even at 90 °C. Therefore, the assembly of the SSDU is not enthalpically disfavored intrinsically. But of course in the case of the polymer, the solvent is water, and therefore the situation may not be entirely same. Thus, in another control experiment, a NDI-containing bolaamphiphile48 with relatively small hydrophilic wedge was investigated by ITC dilution experiments in water (Figure 5). In this case, an endothermic heat flow was noticed which is typical for an enthalpy-driven process, and thermal disassembly was noticed in the UV/vis spectra at higher temperature. Therefore, it is evident that the observed entropy-driven assembly for the SEAMs is unique and not inherent to either the self-assembly of the SSDU or the hydrophobic effect49−51 intrinsically experienced by this particular π-surface. Entropy-driven assemblies are rather uncommon in amphiphilic block copolymers52 with large hydrophobic blocks and also not common in synthetic supramolecular systems except a few examples.53−58 However, it is more frequently found in biological systems59,60 like collagen fibrils, tobacco mosaic virus, bovine brain tubulin, or β-amyloids. A possible hypothesis to rationalize this behavior is presented in Scheme 4. As evident from the UV/vis studies, the self-assembly involves strong coupling between the stacked NDI chromophores which in that case should be within the π-stacking distance (∼3.4 Å). Therefore, unlike a more familiar immiscibility driven collapse of the hydrophobic block of an amphiphilic block copolymer, in this case the hydrophobic SSDU organizes to a more ordered rigid structure which does not provide adequate space in the corona for accommodating the relatively longer hydrophilic polymer chains. This may lead to the restructuring of the water molecules surrounding the

Figure 5. (a) ITC dilution experiment of freshly prepared aqueous solution of NDI-1 (structure shown in Scheme 1). Top: heat release per injection of NDI-1 (c = 2.5 mg/mL) into pure water at 303 K. Bottom: corresponding enthalpogram. (b) Determination of CAC from ΔH versus concentration plot.

hydrophilic polymer and eventually lead to release of the water molecules to the bulk which is entropically favorable and drives the self-assembly process. However, as these polymer chains contain several functional groups (ether, amide, ester, and carboxylic acid) which can be engaged in H-bonding with the water molecules, the release may be associated with the loss of enthalpy more than it could gain by assembly of the tiny (with respect to the entire polymer chain) single SSDU having only one H-bonding group. F


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Scheme 4. Schematic Showing the Origin of Entropy-Driven Assembly of SEAMs by Release of Water Molecules (Represented by Green Dots)

Therefore, the assembly process becomes an endothermic one, but the release of water molecules from the hydrophilic corona compensates entropically to drive the process to form a highly stable structure. This corroborates with the fact that with increasing degree of polymerization of the hydrophilic block in P1 series of SEAMs, the process becomes more entropically favorable as larger chains would release more number of water molecules upon self-assembly.

structure by changing a single functional group rather than large variation in the polymer structure (degree of polymerization, hydrophobic/hydrophilic balance, etc.), and the remarkable stability and very low CAC further facilitate their functional utility in multiple domain of applied sciences. Such studies are underway together with continuing efforts on further understanding on structural nuances on self-assembly.

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SUMMARY We have shown self-assembly of engineered macromolecules by directional interaction along the single SSDU present at the chain terminal of structurally different hydrophilic polymers. Specific H-bonding motif of the amide functionality that was proved to be crucial for cylindrical structure in an earlier communication32 has now been established to be equally effective for providing precise internal order in the assembly of charged polymers and relatively more water-soluble polymers having methacrylamide backbone with as low as 3−4 wt % of the hydrophobic content. While hydrophobically modified polymers have been studied a long time61 and more recently the π-system appended polymers have been shown to be effective for crystallization-driven self-assembly62 or highly stable supramolecular nanogel,63 the present structure guiding parameter, particularly after being successfully extended in this paper with different polymers, could be a much powerful strategy because in this case the resolution of the structure controlling parameters is pinned down to the molecular scale. In a sense these can be compared to peptide−polymer conjugates 64−69 wherein regulation of the mesoscopic structures has been demonstrated by an end-group attached peptide with a specific sequence. The underlying thermodynamic principle has been unfolded, revealing unlike the majority examples of enthalpy driven supramolecular assembly, these systems produced highly stable nanostructure with very low CAC due to the favorable entropy factor. Consequently the self-assembly propensity increased with increasing hydrophilic content in water, which is counterintuitive for aggregation of traditional surfactants. Considering the emerging reports on superiority of cylindrical morphology over spherical structures, the present systems endow a unique opportunity to test the true impact of morphology on important biological events such as cellular uptake, multivalent binding, tissue engineering, or biomineralization, as the standout feature of the SEAMs is the ability to tune the mesoscopic

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Sample Preparation. Polymers were directly taken in distilled water and dissolved by sonication to maintain a required concentration. P2 (0.2 mg, 0.0008 mmol of CO2H) was taken in a vial, and 5 μL of NaOH (concentration 1.0 mg/mL) (0.0008 mmol, 0.034 mg) was added. Then, the volume was madeup to 200 μL to adjust the concentration to 1.0 mg/mL. Then the solution was sonicated until the polymer became fully dispersed. Sample Preparation for Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). For TEM, a 10.0 μL polymer solution (1.0 mg/mL) (fresh or aged for ∼72 h) was drop casted on carbon-coated copper grid (Ted Pella, Inc.), and the excess solution was blotted off carefully with a filter paper. A 10 μL drop of 2 wt % uranyl acetate solution was dropped on the surface for 30 s for negative staining. The excess solution was removed by filter paper, and the grid was allowed to dry in the ambient overnight. For AFM, a 30 μL aqueous solution (1.0 mg/mL) of a polymer (fresh or aged for ∼72 h) was spin-casted on freshly cleaved mica foil at 1000 rpm for 1 min and kept in the desiccator overnight before taking AFM images. Isothermal Titration Calorimetric (ITC) Measurements. The ITC experiments were performed with freshly prepared polymer solutions in a MicroCal iTC200 Malvern instrument. All polymer solutions were degassed before the experiment. The sample cell was loaded with water, and the aqueous polymer solution (c = 2.5 mg/mL, 2 μL titrant volume for each injection) was injected using an injection syringe with continuous stirring at 700 rpm. All the titration was conducted at 303 K. The data were analyzed by MicroCal analysis software and fitted as described elsewhere.47

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01025. Synthesis and characterization of the polymers, materials and methods, and additional spectral data (PDF) G


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.G.). ORCID

Pradip Dey: 0000-0002-1302-0874 Suhrit Ghosh: 0000-0003-4199-4382 Author Contributions §

P.D., P.R., and P.P. equally contributed.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.G. thanks the DST, India, for funding through the SwarnaJayanti Fellowship (Project No: DST/SJF/CSA-01/214-15). P.P. thanks IACS Kolkata, for a research fellowship. P.D. thanks the Science and Engineering Research Board (SERB), India, for funding under National Post-Doctoral Fellowship (Reference no. PDF/2016/001722). P.R. thanks the Technical Research Centre for Molecules and Materials (DST), IACS, for a research fellowship.

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