Thermally Responsive Hyperbranched Poly(ionic ... - ACS Publications

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

Thermally Responsive Hyperbranched Poly(ionic liquid)s: Assembly and Phase Transformations Volodymyr F. Korolovych,† Andrew Erwin,† Alexandr Stryutsky,‡ Hansol Lee,† William T. Heller,§ Valery V. Shevchenko,‡ Leonid A. Bulavin,∥ and Vladimir V. Tsukruk*,† †

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Institute of Macromolecular Chemistry, National Academy of Sciences of Ukraine, Kharkivske Shosse 48, Kyiv 02160, Ukraine § Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Taras Shevchenko National University of Kyiv, Volodymyrska Str. 64, 01601 Kyiv, Ukraine

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‡

S Supporting Information *

ABSTRACT: A library of linear and branched amphiphilic poly(ionic liquid)s based on hydrophobic cores and peripheral thermally sensitive shells was synthesized and studied with regard to their ability to form stimuli-responsive, organized assemblies in aqueous media. The thermally responsive derivatives of poly(ionic liquid)s were synthesized by neutralizing 32 terminal carboxyl groups of functionalized polyester cores by amine-terminated poly(N-isopropylacrylamide)s (PNIPAM) (50% and 100%). We observed that these hyperbranched poly(ionic liquid)s possessed a narrow low critical solution transition (LCST) window with LCST for hyperbranched compounds being consistently lower than that for linear PNIPAM containing counterparts. We found that the poly(ionic liquid)s form spherical micellar assemblies with diverse morphologies, such as micelles and their aggregates, depending on the terminal compositions with reduced sizes for hyperbranched poly(ionic liquid)s. Increasing temperature above LCST promoted formation of network-like aggregates, large vesicles, and spherical micelles. Moreover, all PNIPAM-terminated compounds exhibited distinct unimolecular prolate nanodomain morphology in contrast to common spherical domains of initial cores. We proposed a multilength scale organized morphology to describe the thermoresponsive poly(ionic liquid)s micellar assemblies and discussed their morphological transformations during phase transitions associated with changes in hydrophobic−hydrophilic balance of poly(ionic liquid)s with distinct hydrophobic cores and variable peripheral shells.

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poly(methacrylic acid) (PAAM) with bromide (Br− ) anions.6−9 Anion exchange in PAM or PAAM poly(ionic liquid)s from Br− to Tf2N− leads to the formation of micellar aggregates due to the increasing hydrophobicity of the compounds. Linear poly(ionic liquid)s based on N-imidazole-3-propylmethacrylamide ionic monomers and methyl and ethyl alkyl chains form onion-like or cylindrical vesicles with sizes ranging from 60 to 100 nm, depending on the alkyl chain length.10 Well-defined cubosomes with complex internal structures have been constructed from 1-vinyl-1,2,4-triazolium-based linear poly(ionic liquid)s.11,12 Linear poly(3-alkyl1-vinylimidazolium bromide) with variation of alkyl chains forms spherical or worm assemblies (20−40 nm) with ordered internal structures.2,13 Poly(ionic liquid)s based on hyperbranched polyesters (HBP-ILs) with different types of peripheral ionic liquid groups show high thermal stability and low viscosity even for

INTRODUCTION Poly(ionic liquid)s or polymerized ionic liquids are a novel and intriguing subclass of classical ionic liquids.1,2 These polymeric compounds combine the unique properties of ionic liquids, especially high ionic conductivity, with the advantages of assembly and long-chain nature of polymers. Poly(ionic liquid)s possess high chemical, thermal, and electrochemical stability and high ionic conductivity while providing the opportunity to tailor mechanical properties, conductivity, viscosity, and polarity.3,4 Nonflammable electrolytes or binders based on poly(ionic liquid)s demonstrate the potential of combining these properties.5 Moreover, poly(ionic liquid)s form various organized morphologies in semisolid materials (e.g., gels) which are not accessible in fluidic states of low molar mass ionic liquids.1−5 Known morphologies in aqueous media of linear poly(ionic liquid)s include core−corona micelles, micelle aggregates, vesicles, cubosomes, and worm micelles. For example, core− corona micelles are formed by a linear imidazolium bromide derivative of poly(methacrylic acid) with poly(N-isopropylacrylamide) blocks or linear polyacrylamide (PAM) and © XXXX American Chemical Society

Received: April 23, 2018 Revised: June 7, 2018

A

DOI: 10.1021/acs.macromol.8b00845 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. Linear and Hyperbranched Amphiphilic Carboxylate Poly(ionic liquid)s with Thermally Responsive PNIPAM Macrocations Studied in This Work

that are critical in the development ion-conductive media is practically absent. Here, we report on the synthesis and assembly of novel protic hyperbranched poly(ionic liquid)s and their linear analogues with various neutralization degrees of the peripheral carboxyl groups by thermoresponsive amine-terminated PNIPAM macrocations (Scheme 1). These branched poly(ionic liquid)s, the first example of protic HBP-ILs, show unique thermally triggered assembly behavior with morphologies that are controlled by a combination of chemical and thermal stimuli. Their transition temperatures, assembly behavior, and phase transformations can be tailored by the variation of chemical architecture and composition of peripheral groups. Moreover, we suggest that micellar assemblies of these poly(ionic liquid)s possess multilength scale hierarchical ordering which depends on the temperature controlled hydrophobic−hydrophilic balance.

high molecular weight of HBP-ILs which can be synthesized in one-step reaction.4,14−17 The majority of these researches considers aprotic amphiphilic HBP-ILs, and few existing reports describe either synthesis or properties of cationic HBP-ILs.1−9,18,19 The assembly of HBP-ILs in aqueous media and the formation of functional nanomaterials based on HBPILs are a complex process that depends on the poly(ionic liquid)s architecture, chemical composition, and a balance of intramolecular and intermolecular interactions.20−23 Moreover, the assembly behavior of branched poly(ionic liquid)s is rarely analyzed especially in variable environmental conditions.24−26 Some examples include the synthesis of HBP-ILs based on a hyperbranched poly(3-ethyl-3-hydroxymethyloxetane) core with an inner imidazolium cation shell and outer nonpolar nalkyl shell,27 hyperbranched poly(glycidol) terminated with Nmethylimidazole,28 or amine-terminated poly(amido-amine) dendrimers.29 We recently reported the assembly of amphiphilic protic HBP-ILs based on aliphatic hyperbranched polyester with hydrophilic carboxylate or sulfonate ionic liquid groups and Nmethylimidazole (Im) or 1,2,4-1H-triazole (Tr) counterions, which were balanced by peripheral hydrophobic n-octadecylurethene tails.25 Core−corona micelles with dimensions of 12−17 nm were formed at pH ∼ 11 or ionic strengths of 0.1 M.25 Decreasing pH down to 2.6 or increasing ionic strength (up to 0.6 M) tunes the hydrophilicity of HBP-ILs due to ionization of terminal ionic groups, allowing the formation of micelle aggregates with size up to ∼200 nm. Similarly, by varying the number of peripheral hydrophobic tails from 0% (0 arms) to 75% (24 arms) in these HBP-ILs, the size of the micellar aggregates could be increased up to ∼140% (from 100 to 240 nm). This variation was attributed to the change of the amphiphilic properties of HBP-ILs and the effect arising from the hydrogen bonding of urethane groups, which cause the stabilization of the micellar structures. In another study, HBPILs based on a polyether core with peripheral methy(l)imidazolium cations and methyl orange anions were assembled into pH-responsive vesicles (400 nm).18 Namely, decreasing pH from 5 to 2 shrunk the vesicles from 400 to 100 nm.18 There is only one known example of aprotic amphoteric HBPILs with the competing balance of ammonium and carboxylate ionic liquid groups.30 To date, a variety of fundamental questions remain unaddressed, such as the role that poly(ionic liquid)s architecture (linear or branched) and ionic outer shells play in the organization of poly(ionic liquid)s in aqueous media. Moreover, any information regarding phase transformations in branched polyionic liquids under external stimuli

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EXPERIMENTAL SECTION

Materials. Poly(N-isopropylacrylamide)amine-terminated (PNIPAM, Mn = 2500 g/mol, Aldrich) was used as received. Poly(di(ethelene glycol) adipate) (PDA, Mn = 800 g/mol, Aldrich) was dried under vacuum using a rotary pump (1−3 mmHg vacuum pressure) at 85 ± 5 °C for 4 h before use. Hyperbranched aliphatic oligoether polyol Boltorn H30 (HBP-OH. Perstorp, Sweden) with Mw of 3500 g/mol (comprising 32 terminal OH groups in outer shell) was purified by precipitation of dimethylformamide (DMF) solution in diethyl ether followed by vacuum drying at 25−30 °C for 6 h (an equivalent MW measured by hydroxyl groups via acetylation technique is 117 g/equivalent). Phthalic anhydride was purified by sublimation. DMF, ethanol, diethyl ether, acetone, and acetonitrile were dried and distilled before use. The ultrapure water was used in all experiments (Milli-Q Plus 185, resistivity ≥18.2 MΩ·cm). The dispersion in aqueous medium was performed by the solvent-addition method.24,25 Specifically, 0.05 g of the poly(ionic liquid) compounds dissolved in 1 mL of THF at room was added to water dropwise with the rate of 1 mL/min under stirring. Finally, THF was evaporated with stirring (24 h, room temperature). The final concentration of poly(ionic liquid)s was 5 mg/mL. Characterization. Fourier transform infrared (FTIR) spectra were recorded using a Bruker Vertex 70 FTIR spectrophotometer operating in the 600−4500 cm−1 range. Proton nuclear magnetic resonance (1H NMR) spectra were recorded with a Varian VXR-400 MHz spectrometer using DMSO-d6 (Cambridge Isotope Laboratories, Inc.) as solvent. Thermal properties were characterized by differential scanning calorimetry (DSC) using a TA Instruments (Discovery DSC model) under a nitrogen atmosphere (50 mL/min) with the temperature range from −50 to 200 °C at the heating/ cooling rate of 10 °C/min twice. Morphology, dimensions, and zeta (ζ)-potential of poly(ionic liquid)s were determined by dynamic (DLS), electrophoretic (ELS) B


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Macromolecules

Scheme 2. Synthesis of the Hyperbranched HBP-CA-50% PNIPAM, HBP-CA-100% PNIPAM (a) and Linear L-CA-50% PNIPAM, L-CA-100% PNIPAM (b) Poly(ionic liquid)s with 50% and 100% Neutralization Degree of Terminal Carboxylic Groups by PNIPAM Macrocations (16 and 32)

rates were between 0.1 and 0.5 Hz, and the resolution was 512 × 512 pixels. Silicon wafers for AFM and SEM experiments were pretreated by “piranha solution” (caution: strong oxidizer!) according to the common procedure.25 For the sample preparation, a drop of poly(ionic liquid)s solution was placed onto a silicon wafer and dried. The LCST behavior of poly(ionic liquid) solutions was determined by the temperature dependence of turbidity at 500 nm (Chirascanplus spectrometer, Applied Photophysics) in the temperature range from 23 ± 0.2 to 50 ± 0.2 °C. The slow (0.15 °C min−1) heating and cooling rates were used to reduce kinetic effects. Small-Angle Neutron Scattering (SANS) Experiments. SANS measurements were conducted using the time-of-flight EQ-SANS instrument at the Spallation Neutron Source of Oak Ridge National Laboratory.33 Data were collected with three instrument configurations: 8 m sample-to-detector distance with the minimum wavelength set to 10 Å; 4 m sample-to-detector distance with the minimum wavelength set to 10 Å; and 1.3 m sample-to-detector distance with the minimum wavelength set to 2.5 Å. In all cases, the wavelength-defining choppers operated at 60 Hz. The beam was collimated using a 25 mm diameter source aperture, except for the 8

light scattering, transmission (TEM) and scanning (SEM) electron microscopies, and atomic force microscopy (AFM). The average hydrodynamic size, DDLS, and size distribution of poly(ionic liquid) assemblies in aqueous media were determined from DLS using a Zetasizer Nano ZS (Malvern) with Non-Invasive Back-Scatter (NIBS) technology (HeNe gas laser operating at a wavelength of 633 nm, scattering angle is 173°). The autocorrelation functions were calculated by the Zetasizer software.25 The ζ-potential was obtained by averaging three independent measurements of 35 runs each. Molecular models were built with Chem3D Ultra 15.1 (MM2 force field).31 TEM images of poly(ionic liquid)s assemblies were collected with a Hitachi HT7700 electron microscope at 100 kV by drop casting on carbon-Formvar-coated copper grids (Ted Pella). SEM images were collected with a Hitachi H3400 electron microscope at accelerating voltage between 5 and 10 kV. Before SEM experiments, a DESK IV cold sputter was used to apply a 1.5 nm thick gold coating to all samples. AFM data were collected by ICON microscope (Bruker) in the soft tapping mode with AFM tips (HQ:XSC11/Al BS, MicroMasch) according to the established procedure.25,32 The scan C


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Figure 1. 1H NMR spectrum of L-CA-50% PNIPAM (a) and HBP-CA-50% PNIPAM (b) poly(ionic liquid)s. in neutron scattering length density of these domains, A1 and A2 are scaling factors (proportional to the cluster surface areas and ellipsoid domain volume fractions, respectively), and bkg is the background intensity arising from incoherent scattering. In this analysis, F2 is determined by the averaging of the elongated structures about all possible orientations:

m configuration that used a 10 mm source aperture and a 10 mm diameter sample aperture. The poly(ionic liquid) solutions were loaded into 1 mm cylindrical quartz cells (Hellma USA) at a concentration of 5 mg/mL and mounted into the sample tumbler of the instrument, which provides a constant rotation around the axis of the beam to prevent samples from settling out of solution. Data were collected at temperatures of 23 and 38 °C. Data reduction followed standard procedures implemented in the MANTID software,34 which corrects for detector noise (i.e., “dark current”), detector sensitivity, wavelength-dependent flux and transmission, and sample background before performing an azimuthal average to produce I(Q) vs Q, where Q = (4π/λ) sin θ, where 2θ is the scattering angle. The data were converted into absolute units of cm−1 by means of a calibrated porous silica standard.35 The Q-range covered by the configurations employed spanned from just under 0.002 Å−1 to over 1.0 Å−1. SANS data were fit using a generalized power law for aggregate surfaces combined with the form factor for domains with spherical or ellipsoid shapes at large Q:1 I(Q ) =

A1 A + 2 (Δρ)2 F 2 + bkg Qm V

F2 =

∫0

1

F[QR b(1 + x 2(ν 2 − 1))1/2 ] dx

F(z) = 3V

sin z − z cos z z3

(2)

(3)

where Ra and Rb are the polar and equatorial radii, respectively. Their ratio, ν = Ra/Rb, describes the dimensionality and elongation. By using a value of Δρ computed from the chemical composition of the materials in the samples, this expression takes six fitting parameters. First, the low Q regime were used to find the clustering characteristics. Subsequently, the parameters for the ellipsoidal form factor were fit (see Supporting Information). For comparison, other particle form factors were used to fit the data. The models were ranked by the reduced χ2 parameters obtained from the data fitting with the result that χ2sphere > χ2disk > χ2rod > χ2cylinder> χ2ellipsoid, further evidencing the appropriateness of the prolate ellipsoid model (Figure S4). The use of

(1)

where m is an exponent related to the surface fractal dimension by m = 6 − Ds, V is the volume of the ellipsoid domains, Δρ is the contrast D


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Macromolecules eq 2 assumes the lack of interactions between the domains; these interference effects were determined to be relatively small.

protons of amide fragment (o) of PNIPAM macrocations (Figure 1). It is worth pointing out that the spectrum of L-CA50% PNIPAM compound also contains the signal originating from the protons of the ammonium groups (Figure 1a, signal i), arising as a result of the reaction of neutralization of carboxylate groups of synthesized oligomeric acids with PNIPAM primary amine groups. Integration of the 1H NMR spectrum confirmed 50% (for LCA-50% PNIPAM, HBP-CA-50% PNIPAM samples) and full 100% (for L-CA-100% PNIPAM and HBP-CA-100% PNIPAM) neutralization of carboxyl groups by the primary amino groups of PNIPAM macrocations (Figure 1 and Supporting Information). For example, the signals areas ratio i (from acidic protons of ammonium groups caused by neutralization):signals b, c, e (from methylene groups of initial oligomeric acids):sum of signals q, n (from terminal methyl groups) is 1:11:12:4:135, respectively, for L-CA-50% PNIPAM (Figure 1a). For ideal L-CA-50% PNIPAM macromolecule, the ratio should be 1:12:12:4:135 according to their composition. The analysis of 1H NMR spectra of the HBP-CA-50% PNIPAM compound in terms of the ratios of the signals is complicated by conformation varieties and constraints coming from the branched architecture (Figure 1b). The ratio of the total number of protons of ammonium groups and aromatic rings (signals f−h, i, o) to the total number of protons of methyl groups (used as reference) in the hyperbranched core and PNIPAM macrocations for one poly(ionic liquid) branch is 134:561, while for an ideal structure it is 124:561 for the 50%PNIPAM composition (Figure 1b). FTIR spectra of L-CA and HBP-CA poly(carboxylic acid)s demonstrate stretching vibration modes of ether and ester (C− O−C, CO) bonds, alkylene (C−H) fragments, methylene groups (C−H bonds), aromatic rings (C−C, C−H), and carboxylic (COO−H) groups (Figure 2 and Table 1). Hyperbranched HBP-CA acid has a much higher absorption intensity in the vicinity of the stretching vibrations of C−C bonds of benzene rings (1431−1645 cm−1) and a broadened, higher intensity signal corresponding to the stretching vibrations of COO−H bonds (3125−3713 cm−1) in carboxyl

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RESULTS AND DISCUSSION Synthesis of Linear and Hyperbranched Poly(ionic liquid)s. The hyperbranched poly(ionic liquid)s (named HBP-CA-50% PNIPAM and HBP-CA-100% PNIPAM) and their linear analogues (L-CA-50% PNIPAM and L-CA-100% PNIPAM) with PNIPAM chains were synthesized. Specimens were synthesized by neutralization of peripheral carboxyl groups of corresponding initial oligomeric acids (HBP-CA and L-CA) with terminal primary amine groups of PNIPAM up to 50% or 16 tails (samples L-CA-50% PNIPAM, HBP-CA-50% PNIPAM) and 100% or 32 tails (samples L-CA100% PNIPAM, HBP-CA-100% PNIPAM) (Schemes 1 and 2). As a result of the reaction, the stable ammonium carboxylate groups were formed which is typical for protic ionic liquids.14 The hydrophobic parts of linear and hyperbranched compounds consist of the polyester cores, while the hydrophilic/temperature-sensitive shells include the ammonium carboxylate ionic groups and PNIPAM macrocations (Schemes 1 and 2).36 The degree of neutralization of core carboxyl groups was controlled by the ratio of the corresponding oligomeric acids to PNIPAM in combination with the high rate of the neutralization reaction.37 The neutralization degree of the carboxyl groups by PNIPAM was confirmed by FTIR and 1H NMR spectroscopies (see ratios of signal areas of protons corresponding to characteristic groups and from the yields close to the theoretical values in section SI1). The pKa value of carboxylic groups is 3.32 ± 0.10, which is close to both pKa value of benzoic acid (4.2) and phthalic acid (3.0 and pKa2 = 5.28).25 The pKa value of conjugated acid of PNIPAM at the protonation of the terminal amino group is 8.84 ± 0.10 (calculated with the ACD/Laboratories program).14 Finally, pKa values of conjugated acids of PNIPAM n-ethylamine and n-propylamine are 10.6 and 10.5, respectively.38 As starting compounds for synthesis of poly(ionic liquid)s, poly(carboxylic acid)s with linear (sample L-CA) and hyperbranched (sample HBP-CA) architectures were used (Scheme 2). L-CA acids were produced by the common acylation reaction of PDA with phthalic anhydride.25 Hyperbranched HBP-CA acids were produced by the same acylation reaction of hyperbranched polyester precursors (Boltorn H30, fifth generation).25 Thus, we received starting compounds, LCA and HBP-CA acids, with same chemical composition. Detailed information about synthesis of poly(ionic liquid)s is presented in the Supporting Information. 1 H NMR spectra of L-CA and HBP-CA acids contain signals from protons in methylene groups (3.3−3.7, 4.0−4.3 ppm) as well as from protons in benzene rings (7.2−8.2 ppm) (Figures S1−S3). Signals of protons from methylene groups caused by fragments of adipic acid (2.0−2.4 ppm) in spectrum of L-CA as well as additional signals from methyl groups (1.1−1.2 ppm) in the spectrum of HBP-CA are the distinguishing features of the spectra of these compounds. Partial or full neutralization of acids with PNIPAM causes the appearance of intense signals related to PNIPAM, which are partially superimposed with the signals from source acids. 1 H NMR spectra for poly(ionic liquid)s with the neutralization degree of 50%, namely for L-CA-50% PNIPAM and HBP-CA-50% PNIPAM, show signals from protons of methyl (q, n) and methylene (j, k, l, m, p) groups and from

Figure 2. FTIR spectra of linear (L-CA) and hyperbranched (HBPCA) poly(carboxylic acid)s and amphiphilic linear (L-CA-50% PNIPAM, L-CA-100% PNIPAM) as well as hyperbranched (HBPCA-50% PNIPAM, HBP-CA-100% PNIPAM) poly(ionic liquid)s. E


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Macromolecules Table 1. Selected Absorption Bands of Initial Poly(carboxylic acid)s and Poly(ionic liquid)s peripheral groupsa (%) samples L-CA L-CA-50% PNIPAM L-CA-100% PNIPAM HBP-CA HBP-CA-50% PNIPAM HBP-CA-100% PNIPAM

Mn,theor (g/mol)

− COOH PNIPAM

ν C−O−C (cm−1)

ν CO (cm−1)

amide I (cm−1)

amide II (cm−1)

ν C(O)N−H (cm−1)

1096 3596

100 50

0 50

988−1333 996−1327

1732 1736

1651

1543

3296

6096

3

97

1001−1318

1736

1649

1543

3300

8480 48480

100 51

0 49

1090−1348 1090−1348

1728 1736

1651

1545

3304

88480

3

97

1091−1348

1727

1651

1547

3300

ν al C−H (cm−1) 2878, 2955 2876, 2936, 2974 2878, 2934, 2972 1931 2878, 2936, 2974 2878. 2936, 2974

Tgb (°C) 54 77 34 94 99

a

Based on 1H NMR data. bFrom DSC data (Figure S4).

Figure 3. Size distribution of micellar assemblies from linear L-CA (a), L-CA-50% PNIPAM (b), L-CA-100% PNIPAM (c) and hyperbranched HBP-CA (d), HBP-CA-50% PNIPAM (e), and HBP-CA-100% PNIPAM (f) compounds in aqueous media at 23 ± 0.2 °C (blue) and at 38 ± 0.2 °C (red) and corresponding TEM images (scale bars are 500 nm).

amide II (1543−1547 cm−1), stretching vibrations of C−H bonds of methyl groups (2972, 2974 cm−1), and N−H bonds of amide fragments (3296−3304 cm−1) as well as the simultaneous decrease in intensities of the absorption bands corresponding to the stretching vibrations of CO bonds of carboxylic groups of the initial acids (1732 and 1728 cm−1 for oligomeric acids of linear and hyperbranched compounds) (Table 1 and Figure 2). The glass transition temperature (Tg) is above 0 °C for LCA and HBP-CA poly(carboxylic acid)s and poly(ionic liquid)s, and thermograms indicate amorphous structure

groups than linear L-CA acid due to higher content of peripheral −COOH groups (Figure 2 and Scheme 2). In addition, FTIR spectra of poly(ionic liquid)s contain amide I (1649−1651 cm−1) and amide II (1543−1547 cm−1) signals inherent to polyamides, the stretching vibration of C− H bonds of the methyl groups, and N−H bonds of amide groups from the PNIPAM fragments, together with the signals of stretching vibrations of CO bonds of ester groups form oligoester (Figure 2). Increasing the content of PNIPAM macrocations manifests in the increasing intensity of the related adsorption bands, such as amide I (1649−1651 cm−1), F


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Macromolecules Table 2. Characteristics of Poly(carboxylic acid)s and Poly(ionic liquid)s Assemblies at 23 °C samples

LCST (°C)

DDLSa (nm)

L-CA L-CA-50% PNIPAM L-CA-100% PNIPAM

33.3 ± 0.2 34.0 ± 0.2

448 ± 55 10 ± 2, 380 ± 90 332 ± 74

HBP-CA HBP-CA-50% PNIPAM HBP-CA-100% PNIPAM

34.7 ± 0.2 34.3 ± 0.2

226 ± 109 253 ± 104 222 ± 60

at 38 °C

DTEMb (nm)

ζ-pot. (mV)

linear compounds 375 ± 40 −9.4 ± 4.6 235 ± 60 −1.2 ± 0.2 170 ± 30 −2.6 ± 3.0 hyperbranched compounds 215 ± 60 −31.6 ± 1.6 172 ± 57 −10.8 ± 2.7 70 ± 10 −10.2 ± 3.2

at 50 °C

DDLS (nm)

ζ-pot. (mV)

DSEMc (μm)

520 ± 59 2905 ± 210 3299 ± 354

−15.0 ± 1.0 −2.5 ± 0.2 +1.4 ± 0.5

4.2 ± 1.2 3.1 ± 0.8

−22.1 ± 0.5 +8.9 ± 0.7 +12.5 ± 1.0

220 ± 101 2411 ± 306 507 ± 119

−35.9 ± 2.1 −7.8 ± 1.9 −10.8 ± 1.7

7.2 ± 3.3 1.3 ± 0.3

−32.7 ± 1.4 +1.9 ± 0.3 −1.1 ± 0.3

ζ-pot. (mV)

a

Average size from DLS data. bAverage size from TEM image. cAverage size from SEM image.

and −10.8 mV (branched) at just 50% neutralization due to the addition and screening with shells composed of macrocations with positive surface potential (Table 2).26,45,50 TEM study shows the formation of spherical assemblies with dimensions that are in general agreement with hydrodynamic dimensions (Table 2 and Figure 3). The average size of spherical assemblies from linear compounds is larger than that from hyperbranched counterparts (Figures 3g−l and Table 2). In addition, TEM data indicate the decreased size of assemblies with increasing peripheral PNIPAM macrocation content (Figures 3g−i and Table 2).25,36 Furthermore, AFM imaging confirms the coexistence of large spherical micelle aggregates (average diameter is 300 ± 80 nm and average height is ∼60 nm) and small flattened dried micelles (diameter of 50 nm and height up to 6 nm) for L-CA50% PNIPAM poly(ionic liquid) assemblies (Figure 4). The flattened shape of dried micelles and their aggregates resembles the microcapsule-like morphology with collapsed inner volume after drying.24,25 However, AFM cannot resolve the internal morphology of these micelles in both swollen and dried conditions.

without any signs of crystallization (Table 1 and Figure S4).39 The Tg values for linear poly(ionic liquid)s are lower than those for the hyperbranched analogues (Figure S4). For instance, the Tg values of L-CA-50% PNIPAM and HBP-CA50% PNIPAM compounds are 54 and 94 °C, respectively (Table 1). Difference in Tg of poly(ionic liquid)s can be explained by the difference in nature of glass transition mechanisms. Indeed, glass transition in branched materials relates to translational motion of branched macromolecules, in contrast to segmental motion in linear compounds that requires higher thermal excitation.39 In addition, Tg of linear and hyperbranched poly(ionic liquid)s increases with increasing content of PNIPAM macrocations, as expected due to the higher Tg of the grafted PNIPAM component (within 110−140 °C).40 Assembly of Poly(ionic liquid)s below LCST. In aqueous media, all compounds formed a variety of micellar assemblies (Figure 3 and Table 2). The initial linear and hyperbranched poly(carboxylic acid)s form large micellar aggregates with hydrodynamic diameters of 448 ± 55 and 226 ± 109 nm, respectively (Figure 3a,d). These sizes of the assemblies are common for the micellar aggregates observed for linear polymers dispersed in a similar way41,42 and for carboxyl-terminated hyperbranched polyesters.43,44 Neutralization of carboxylate groups of the L-CA or HBP-CA acids by PNIPAM macrocations leads to the polydisperse assemblies with bimodal size distribution (Figure 3b and Table 2). The bimodal size distribution of L-CA-50% PNIPAM assemblies can be related to different packing of L-CA-50% PNIPAM compounds with various macrocation associations. Full neutralization of carboxylate groups promotes the formation of larger micellar aggregates in linear compounds and more compact assemblies in hyperbranched compounds (Figure 3c and Table 2). Increasing the content of peripheral hydrophilic arms results in decreasing the size of the assemblies in both linear and hyperbranched poly(ionic liquid)s (Table 2). It should be noted that the size of micelles from linear polymers studied here is close to that observed for various PNIPAM-containing polymers45−47 and poly(ionic liquid)s.48,49 Next, for initial polyacids, ζ-potential increases from −9.4 ± 4.6 mV (linear) to −31 ± 1.6 mV (branched) due to the increased density of carboxylic terminal groups in the branched compounds (Table 2). Neutralization of carboxylate groups by grafting PNIPAM macrocations partially screens the negative charge of core carboxylate anions, thereby dramatically decreasing the magnitude of the surface potential of PNIPAM-decorated poly(ionic liquid) to −1.2 mV (linear)

Figure 4. Medium (a) and high (b) resolution AFM images of L-CA50% PNIPAM poly(ionic liquid)s deposited on silicon wafer at 23 ± 0.5 °C and corresponding height profiles along white lines (c, d). Z scale is 30 nm for (a) and 6 nm for (b). G


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Figure 5. SANS from aqueous dispersions of linear (a) and hyperbranched (b) poly(ionic liquid)s with varying degrees of PNIPAM neutralization. The scattered intensities were measured at room temperature (blue squares) and above LCST at 38 °C (red circles). Solid lines denote fits to eq 1.

range between 3 and 4 (Figure 5b). This feature can be attributed to elastic scattering from fractal surfaces, wherein an exponent of 3 indicates rougher or polydisperse interfaces while 4 corresponds to the Porod law for sharp interfaces.53 The feature most likely originates from the partial contributions from the large-scale micelles. However, in our measurements, the Guinier regime is not reached. The maximum length scale probed in these experiments (∼100 nm) is too small to adequately characterize the overall micellar sizes (usually within 100−400 nm, Table 2 and Figure 3) and is further complicated by the contribution from inelastic scattering at the lowest angles.54,55 Consequently, we restrict our analysis to the higher Q range (0.0035 Å−1 < Q < 0.35 Å−1) and rely on other techniques (AFM, TEM, and DLS) to analyze the large-scale morphologies. The higher Q features are known to originate from various sources of intra- and intermolecular inhomogeneities on the molecular and nanoscale scales.53,54,56,57 Such features can be analyzed, for example, with the Debye−Bueche or Debye− Anderson−Brumberger models for randomly distributed systems or the Teubner−Strey model for micellar aggregates.58 However, these approaches failed to account for the pronounced scattering features at intermediate Q. On the other hand, a correlation length model, as modified by Hammouda and co-workers to describe lower-dimensional ellipsoidal aggregates in dendrimer-type compounds, more accurately describes our scattering data from branched molecules.51 Accordingly, we find that the prolate ellipsoidal

Therefore, to elucidate the internal structure of these micelle aggregates under swollen conditions, SANS was measured over a Q range spanning more than 2 orders of magnitude at temperatures below and above the LCST transition (Figure 5, section SI2). To generate contrast, D2O was used in the aqueous solutions to distinguish the hydrogenated poly(ionic liquid) backbones from the surrounding media with the maximum theoretical scattering length density difference of Δρ = 4.4 × 10−6 Å−2. In SANS data, two prominent features are consistently observed: (1) a sharp increase in intensity at very low Q below 0.05 Å−1 corresponding to the larger micelle clusters and (2) a feature at higher Q with a length scale corresponding to the local, molecular-level organization (Figure 5). For poly(ionic liquid)s with PNIPAM macrocations, the latter feature is stretched to higher Q above 0.1 Å−1 with an intensity about an order of magnitude higher than that observed for initial L-CA and HBP-CA acids without PNIPAMs chains (Figure 5). Note that while SANS data of the initial compounds are fully independent of temperature, the PNIPAM functionalized poly(ionic liquid)s feature pronounced scattering changes above the LCST. The emergence of scattering features dominating in two distinct Q ranges is characteristic of multiscale assemblies.54,51,52 Because of the wide separation in length scales of the SANS intensity, no single form factor could be used to adequately describe the data over the measured Q range. The intensity scales as Q−m at low Q range, where m is observed to H


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Figure 6. Elongation of local domains (aspect ratio) as a function of PNIPAM content and temperature as determined by the ratio ν = Ra/Rb from the SANS data (a). Schematic of ellipsoidal domain (inset). (b, c) Molecular models of repeating unit of PNIPAM attached to terminal amino group and molecules projections with labeled atom groups: isopropyl group (1), amide group (2), aliphatic sulfide group (3), primary aliphatic amino group (4), and ethylene group (5). Individual molecule within elongated domain in different projections (b, c).

them partially “invisible” for SANS experiments (Figure S7 and Table S3). In this model, the elongated domains are mostly composed of several packed hydrogenated central blocks which are hydrophobic in nature and surrounding PNIPAM chains swollen in D2O (Figure S7). The overall ellipsoid domain volume is similar for branched molecules and estimated to be 29−32 nm3 (Table S3). Surprisingly, these volumes do not increase after dramatic increase in the molecular volume because of branching and added PNIPAM chains and are much smaller than the intrinsic molecular volumes estimated for both densely packed and loosely packed and partially swollen hydrogenated molecules (Table S3). On the other hand, the overall dimensions of ellipsoidal domains match well to the longitudinal molecular dimensions but not transverse dimensions of compounds with either 16 or 32 grafted PNIPAM chains (Figure S6). Moreover, in contrast to expectations, grafting an additional 16 PNIPAM chains during 100% of neutralization does not change the domain volume (Tables S2 and S3). Thus, we suggest that a high content of D2O within terminal PNIPAM chains in good solvent conditions results in dramatically diminished scattering contrast for the outer shells similarly to that for linear compounds. This presence effectively makes PNIPAMs “excluded” from the highly contrasted scattering volume of hydrogenated cores and makes them indistinguishable from the surrounding D2O environment. Analysis of molecular dimensions and conformations under constrained spatial conditions of the individual domains allows us to propose the following molecular models (Figure 6b,c). For all branched molecules, we suggest the extended conformation of hyperbranched cores with branches extending in opposite directions (Figure 6). The longitudinal length of the branched core in this state is about 5 nm, and the thinnest central section dimension is within 0.5−1 nm with the thicker peripheral section of packed carboxylic groups reaching ∼2.9 nm. The overall match is illustrated with the overlapped ellipsoidal shape with characteristic dimensions (the effective length is 11 nm and diameter is about 2 nm) (Figure 6c). The unimolecular composition fits well with this model with assumption that some of PNIPAM chains still participate in the formation of contrasted elongated domains (Figure 6c, see estimation in Table S3). On the other hand, considering that the molecular volume of the just hydrogenated branched cores is somewhat smaller than the domain volume, we can suggest that more than one molecule might constitute an individual domain depending upon the content

form factor best describes the high Q features for the SANS data for PNIPAM-containing compounds (Figure 5 and Table S1).59 For the initial L-CA and HBP-CA acids, for which the decreased scattering intensity at high Q indicates very low level contrast at molecular length scales due to the relatively uniform distribution of solvent within the weakly associated aggregates (Figures 5a,b). Notably, the ellipsoidal form factor describing the low-scattering regions reduces to a simple spherical form factor eq 2, where ν → 1) with very low contrast and a diameter of about 8 nm (Tables S1 and S2). These weakly associated aggregates include few hundreds of individual hydrogenated molecules as can be estimated from direct comparison of molecular volumes for extreme cases of loosely (Vm) and densely (Vρ) packed molecules with volumes of spherical domains (Tables S1−S3).24,25 The domain morphology and effective dimensions change dramatically after grafting PNIPAM chains, as shown by the dramatic rise in scattering intensity in the intermediate range indicating the presence of the highly contrasted hydrogenated central core with large size and addition of a number of tails (Figure 5, Tables S1 and S2). For all linear and branched compounds with PNIPAM chains, the prolate ellipsoidal form factor was found to be the best fitting model (see fitting results in Figure 5 and corresponding parameters in Tables S1−S3). The ellipsoid length 2Ra is within 13−14.0 nm for both linear and branched molecules with PNIPAM chains while the effective width, 2Rb, is much smaller, about 2 nm (Tables S1 and S2). Overall, these ellipsoid models possess relatively high aspect ratios of 6−7 for all PNIPAM-terminated compounds studied here at room temperature (Figure 6, Tables S1 and S2). The experimental domain volume of linear compounds calculated from ellipsoid dimensions (within 27−34 nm3) is much larger than that calculated for densely packed molecules of 5 nm3 (neutralization degree is 50% or one grafter PNIMAM chain) and 9 nm3 (100% or two grafted PNIPAM chains) taking into account molecular weight and the packing density (Table S3). If we suggest that PNIPAM chains are in a random coil conformation below the LCST, the individual ellipsoid domain should contain 3−5 individual molecules with densely packed central cores and tails. On the other hand, these domain volumes are smaller than the molecular volumes estimated from molecular model with randomly packed tails (Figure S6). This discrepancy suggests that the terminal PNIPAM chains are completely dissolved in a good solvent below the LCST, thus reducing effective contrast and making I


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Figure 7. Transmittance of linear L-CA-50% PNIPAM (a), L-CA-100% PNIPAM (b) and hyperbranched HBP-CA-50% PNIPAM (c), HBP-CA100% PNIPAM (d) poly(ionic liquid)s, and PNIPAM (e) aqueous solutions vs temperature temperature (heating and cooling rates are 0.15 °C min−1). LCST (red bars) and ΔT (dark cyan bars) values for poly(ionic liquid)s and PNIPAM (f).

ing compounds (Table 2 and Table S4). This shift can be associated with the increased surface tension at the water/ polymer interface due to destabilization of the hydration shell around of the PNIPAM groups that promote salting-out of the macrocations.63,64 Other changes are insignificant and close to common variability in different runs (Figure S9). Furthermore, the width (ΔT) of transition designated as |ΔT| = |T1 − T2| is very narrow (within 1−2 °C), which is an unusual feature of the PNIPAM-containing polymers (Figure 7). Next, PNIPAM aqueous solution shows temperature hysteresis of about 3.6 ± 0.2 °C, which is in good agreement with literature data.26,76 The temperature hysteresis for PNIPAM-grafted poly(ionic liquid)s is also small, within 2.7−3.2 °C, indicating a fast diffusion of water molecules into poly(ionic liquid)s assemblies during cooling (Figure 7 and Table S5).36,65 Furthermore, the temperature-triggered transformation of PNIPAM containing poly(ionic liquid)s shifts the ζ-potential from about −3 to +13 mV for linear and from −10 to ∼0 mV for hyperbranched compounds (Table 2 and Figure S10). This behavior of poly(ionic liquid)s indicates the partial collapse of PNIPAM chains that leads to the screening the charged groups of core fragments (deprotonated ester and protonated amino groups). Indeed, a similar surface charge behavior has been observed in linear poly(ionic liquid)s with PNIPAM fragments during heating and for PNIPAMs of comparable molecular weight (Figure S10).66 Such charge screening should facilitate

of the solvent inside the domain and H-D2O contrast contribution from PNIPAM tails. Even more surprisingly, the prolated ellipsoids keep their shape above the LCST where PNIPAM tails should collapse and become “more visible”, albeit with somewhat reduced aspect ratio (Figure 6a and Table S3). The aspect ratio of ellipsoid domains reduces dramatically in linear molecules (to 3-fold) while the reduction is modest in branched molecules (by 20−30%). Slightly reduced longitudinal dimensions, volumes, and the aspect ratios can be attributed to the reduced chain stretching. Thus, the similar molecular models with predominantly unimolecular micelles can be suggested for branched compounds at the elevated temperature above LCST. However, we do not observe a significant increase in the hydrogenated molecular volume with increased contrast and exclusion of D2O due to the collapse of PNIPAM chains in bad solvent above the LCST (see more discussion below). LCST Behavior of Poly(ionic liquid)s. Furthermore, the turbidity experiment revealed a sharp (ΔT = 2.0 °C) LCST phase transition for free PNIPAM at 34.8 ± 0.2 °C in an agreement with literature data as well as thermal stability of LCA and HBP-CA acid solutions (Figure 7 and Figure S8).25,36,60−62 After neutralization of terminal carboxyl groups by PNIPAM macrocations, the sharp LCST phase transition is observed (Figure 7 and Figure S9). The LCST values for various thermally responsive poly(ionic liquid)s are shifted by 1−2 °C toward lower temperatures in contrast to correspondJ


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Figure 8. SEM images of the L-CA-50% PNIPAM (a), L-CA-100% PNIPAM (b), HBP-CA-50% PNIPAM (c), and HBP-CA-100% PNIPAM (d) poly(ionic liquid)s solution drops deposited on silicon wafer at 50 ± 0.5 °C. Scale bars are 20 μm. Size distribution of poly(ionic liquid)s assemblies (insets).

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GENERAL DISCUSSION AND CONCLUSIONS In this study, the library of novel amphiphilic poly(ionic liquid)s with linear or hyperbranched architecture based on hydrophobic polyester cores and peripheral hydrophilic ammonium carboxylate ionic groups and terminal thermally sensitive chains was synthesized and characterized in terms of their solution-based assembly and thermoresponsive behavior. The controlled amphiphilicity of these poly(ionic liquid)s and their temperature-responsive behavior were adjusted by varying the neutralization degree of carboxyl groups with the terminal amine groups of PNIPAM chains (Figures 1 and 2, Figures S1−S3). First, as observed in the literature, the incorporation of PNIPAM chains by covalent bonding to various macromolecular backbones (linear, grafted, branched) typically promotes the significant broadening of LCST transition up to several tens of degrees while shifting the LCST value to higher temperatures. For example, ΔT of linear and hyperbranched poly(ionic liquid)s with PNIPAM chains varies from ∼6 to ∼21 °C (Figure 10).71−75 This behavior indicates efficient suppression of molecular mobility within covalently bound PNIPAMs during transformation (Figure 10).23,39,76,77 In contrast, the opposite trend is observed for all our branched compounds studied here with significant reduction of LCST and narrowing of the LCST transition window (Figure 10, Figure S4, and Table 1). This alternative trend can be attributed to very different nature of PNIPAM grafting in our poly(ionic liquid)s with weakly bond short chain PNIPAM macrocations to well-separated terminal carboxylic groups. The weak core−shell interactions and absence of the restrictive covalent bonding decrease steric hindrance for the polar heads of grafted PNIPAM chains during phase transformation. This higher mobility changes the corresponding dynamics in the conformational freedom driven by the variable hydration state of the associated PNIPAM chains, thus facilitating easier collapse of PNIPAM chains at the lower level of thermal

the poly(ionic liquid)s aggregation trend above the LCST, as discussed below.25 When linear L-CA-50% PNIPAM poly(ionic liquid)s are deposited above the LCST, they form network-like aggregates featuring the average size of 4.2 ± 1.2 μm (Figure 8a,b and Figure S12). Notably, the DLS data also confirm the formation of micrometer-sized assemblies by poly(ionic liquid)s above the LCST (Figure 3 and Figure S11). On the other hand, submicrometer worm-like morphology corresponds to kinetically trapped micellar structures during drying.67 We suggest the network-like aggregates form a similar mechanism to the core−core fusion observed in common worm-like aggregation process.67 At the same time, hyperbranched poly(ionic liquid)s assembled above the LCST form either very large vesicles (giant vesicles68) or highly uniform and much smaller spherical micelles (Figures 8c,d). For example, HBP-CA-50% PNIPAM poly(ionic liquid)s formed vesicles with an average size of 7.20 ± 3.30 μm (Figure 8c), which is similar to those formed by other representatives of hyperbranched polyesters.69,70 Moreover, increasing the content of peripheral PNIPAM tails to 100% promotes the formation of much smaller and highly uniform spherical micelles of 1.35 ± 0.34 μm in diameter which are assembled in chain-like aggregates but without their ultimate merge (Figure 8d). Finally, AFM images of dried micellar aggregates under ambient conditions confirm that L-CA-50% PNIPAM and LCA-100% PNIPAM poly(ionic liquid)s form aggregates with large-scale network-like and smaller-scale spherical micelles (Figures 9a,e,b,f). On the other hand, hyperbranched HBPCA-50% PNIPAM and HBP-CA-100% PNIPAM poly(ionic liquid)s form large vesicles with an average wall thickness of 250 nm and smaller spherical aggregates, which is in agreement with SEM data collected at high-vacuum conditions (Figures 9c,g,d,h). K


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Figure 10. Transition width, ΔT, vs LCST temperature for free PNIPAM chains (green), poly(ionic liquid)s studied here (PILs, red), linear poly(ionic liquid)s (blue), and star or hyperbranched nonionic compounds with covalently bonded PNIPAM fragments from the literature (violet).

Below the LCST, the average hydrodynamic size of micelle aggregates based on hyperbranched HBP-CA acids is half of that found for linear L-CA acids. Morphology of poly(ionic liquid)s assemblies changes dramatically above the LCST with linear poly(ionic liquid)s forming kinetically trapped structures such as network and worm-like aggregates (Scheme 3). At the same time, thermo-induced aggregation of branched HBP-CAPNIPAM compounds results in the formation of large vesicles and highly uniform smaller spherical aggregates above the LCST (Scheme 3).78 At the molecular level, linear and hyperbranched molecules form unique prolate nanodomains with high aspect ratio (Figure 6 and Scheme 3). This is greatly different from common spherical domains of initial poly(carboxylic acid)s (Figure S6). Consideration of the molecular structures shows that dense packing of four hydrophobic polyester branches is impossible within spherical volume. Spherical shape results in incomplete space filling and increase in unfavorable hydrophobic−hydrophilic contacts between polyester cores and surrounding water molecules. The dense packing for the minimization of the interfacial energy can be possible only as a result of the formation of elongated densely packed hydrophobic branches surrounded by the peripheral PNIPAM chains, which are highly swollen in a good solvent below the LCST (Figure 6 and Figure S6). However, such a dense molecular packing of hydrophobic cores results in prolated unimolecular domains filled with hydrogenated cores which are highly contrasted in D2O surrounding and nearly “invisible” PNIPAM tails (Scheme 3). The prolated shape of nanodomains is preserved above the LCST (Figure 6). Expected significant increase in the hydrogenated molecular volume because of collapse of PNIPAM chains above the LCST under poor-solvent conditions and exclusion of D2O has not been observed. We suggest that this behavior can be related to the modest variation of the hydration level of short PNIPAM chains. Indeed, molecular dynamics simulation suggests that above the LCST relatively short PNIPAM chains do not reduce dimensions dramatically but rather shrink modestly by 15%.79 Internal hydrated content does not change significantly as well due to shielding hydrophilic PNIPAM groups by their isopropyl groups in the side chains of PNIPAM (Scheme 1 and Figure 6b). Thus, the scattering contrast in D2O media and therefore the overall effective dimensions of near-unimolecular

Figure 9. AFM images of the L-CA-50% PNIPAM (a), L-CA-100% PNIPAM (b), HBP-CA-50% PNIPAM (c), and HBP-CA-100% PNIPAM (d) poly(ionic liquid)s deposited at 50 ± 0.5 °C and corresponding height profiles along white lines (e−h). Scale bars are 5 μm for (a−c) and 2 μm for (d). Z scale is 70 nm (a), 800 nm (b), 2.3 μm (c), and 500 nm (d).

mobility and resulting in reduced LCST. This reduction is modest for the compounds we studied here, and further reduction might be promoted by introducing lower grafting density and changing the nature of terminal groups. Overall, the variation of the chemical architecture of poly(ionic liquid)s by the variation of the type of anions or cations can be explored for control the LCST transition as important for various applications.21−23 Second, multilength scale micellar morphologies have been observed for both linear and hyperbranched poly(ionic liquid)s whose characteristic dimensions are in a good agreement with those found in various PNIPAM-containing linear or hyperbranched polymers (Scheme 3).24,25,43−49 In contrast to the initial L-CA and HBP-CA acids which form large, highly charged aggregates, poly(ionic liquid)s with PNIPAM countercations form smaller aggregates with diameter 300 ± 80 nm (Table 2). L


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Scheme 3. Illustration of Assemblies Morphology from Initial L-CA and HBP-CA Acids (a) and Linear, Hyperbranched Poly(ionic liquid)s below (b) and above LCST (c−f); above LCST Poly(ionic liquid)s Form Network-like Aggregates (c), Worms (d), Large Vesicles (e), and Spherical Aggregates (f)

(2) Yuan, J.; Mecerreyes, D.; Antonietti, M. Poly(ionic liquid)s: An Update. Prog. Polym. Sci. 2013, 38, 1009−1036. (3) Qian, W.; Texter, J.; Yan, F. Frontiers in Poly(ionic liquid)s: Syntheses and Applications. Chem. Soc. Rev. 2017, 46, 1124−1159. (4) Xu, W.; Ledin, P. A.; Shevchenko, V. V.; Tsukruk, V. V. Architecture, Assembly, and Emerging Applications of Branched Functional Polyelectrolytes and Poly(ionic liquid)s. ACS Appl. Mater. Interfaces 2015, 7, 12570−12596. (5) Hernández, G.; Işik, M.; Mantione, D.; Pendashteh, A.; Navalpotro, P.; Shanmukaraj, D.; Marcilla, R.; Mecerreyes, D. Redox-active Poly(ionic liquid)s as Active Materials for Energy Storage Applications. J. Mater. Chem. A 2017, 5, 16231−16240. (6) Liang, J.; Wu, W.; Li, J.; Han, C.; Zhang, S.; Guo, J.; Zhou, H. Synthesis and Self-assembly of Temperature and Anion Double Responsive Ionic Liquid Block Copolymers. Front. Mater. Sci. 2015, 9, 254−263. (7) Vijayakrishna, K.; Mecerreyes, D.; Gnanou, Y.; Taton, D. Polymeric Vesicles and Micelles Obtained by Self-Assembly of Ionic Liquid-Based Block Copolymers Triggered by Anion or Solvent Exchange. Macromolecules 2009, 42, 5167−5174. (8) Vijayakrishna, K.; Jewrajka, S. K.; Ruiz, A.; Marcilla, R.; Pomposo, J. A.; Mecerreyes, D.; Taton, D.; Gnanou, Y. Synthesis by RAFT and Ionic Responsiveness of Double Hydrophilic Block Copolymers Based on Ionic Liquid Monomer Units. Macromolecules 2008, 41, 6299−6308. (9) Coupillaud, P.; Taton, D. In Applications of Ionic Liquids in Polymer Science and Technology; Mecerreyes, D., Ed.; Springer: Berlin, 2015; pp 69−102. (10) Manojkumar, K.; Mecerreyes, D.; Taton, D.; Gnanou, Y.; Vijayakrishna, K. Self-assembly of Poly(ionic liquid) (PIL)-based Amphiphilic Homopolymers into Vesicles and Supramolecular Structures with Dyes and Silver Nanoparticles. Polym. Chem. 2017, 8, 3497−3503. (11) Zhang, W.; Kochovski, Z.; Lu, Y.; Schmidt, B. V. K. J.; Antonietti, M.; Yuan, J. Internal Morphology-Controllable SelfAssembly in Poly(Ionic Liquid) Nanoparticles. ACS Nano 2016, 10, 7731−7737. (12) Zhang, W.; Kochovski, Z.; Schmidt, B. V. K. J.; Antonietti, M.; Yuan, J. Crosslinked 1,2,4-Triazolium-type Poly(ionic liquid) Nanoparticles. Polymer 2016, 107, 509−516. (13) Yuan, J.; Soll, S.; Drechsler, M.; Müller, A. H. E.; Antonietti, M. Self-assembly of Poly(ionic liquid)s: Polymerization, Mesostructure Formation, and Directional Alignment in One Step. J. Am. Chem. Soc. 2011, 133, 17556−17559. (14) Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids: Evolving Structure−Property Relationships and Expanding Applications. Chem. Rev. 2015, 115, 11379−11448. (15) Inoue, K. Functional Dendrimers, Hyperbranched and Star Polymers. Prog. Polym. Sci. 2000, 25, 453−571.

domains rebealed with SANS do not change significantly after crossing phase transition. Overall, we demonstrated the complex assembly behavior and phase transformation of novel amphiphilic poly(ionic liquid)s with branched architecture. Relatively weak complexation of terminal macrocationic PNIPAM chains to highly branched acidic cores results in unusual reduction of LCST and the formation of large-scale micellar aggregates with peculiar prolated unimolecular morphology both below and above the phase transition. Such complex multilength scale morphology might result in intriguing ionic transport properties in solution, gel, and solid states, which can be considered for designing of novel polymeric materials with controlled ion transport.

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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/acs.macromol.8b00845. Synthesis of samples, NMR data, molecular models, SEM images, SANS data, ζ-potentials, and transmittance (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; phone 404-894-6081 (V.V.T.). ORCID

William T. Heller: 0000-0001-6456-2975 Vladimir V. Tsukruk: 0000-0001-5489-0967 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the National Science Foundation DMR 1505234, DOE under Contract DE-AC05-00OR22725 and use of the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. We thank Carrie Gao, Emily Mikan, and Luke Pittner for technical assistance.

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REFERENCES

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