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Hydrophobic Monomer Type and Hydrophilic Monomer Ionization Modulate Lyotropic Phase Stability of Diblock Co-oligomer Amphiphiles Minh Thu Lam, Paul A FitzGerald, and Gregory G. Warr Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03133 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017

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Hydrophobic Monomer Type and Hydrophilic Monomer Ionization Modulate Lyotropic Phase Stability of Diblock Co-oligomer Amphiphiles Minh T. Lam, Paul A. FitzGerald and Gregory G. Warr* School of Chemistry, The University of Sydney, NSW, 2006, Australia

ABSTRACT The phase behavior and self-assembly structures of a series of amphiphilic diblock cooligomers comprising an ionisable hydrophilic block (5 to 10 units of acrylic acid) and a hydrophobic block (5 to 20 units of n-butyl acrylate, t-butyl acrylate or ethyl acrylate), synthesized by RAFT polymerization, has been examined by polarizing optical microscopy and small angle x-ray scattering (SAXS). Self-assembled structure and lyotropic phase stability in these systems is highly-responsive to the degree of ionization of the acrylic acid hydrophilic block (i.e. pH), concentration and the nature of the hydrophobic block. Increasing head group ionization switched the amphiphiles from behaving like soluble to insoluble surfactants. Liquid isotropic (micellar), hexagonal, lamellar and discrete cubic phases were found under different solution conditions. The surfactant packing parameter was adapted to understand the self-assembly structures in these diblock co-oligomers. Hydrophobic chain structure and length were shown to strongly affect relative stabilities of these phases, allowing self-assembly structure to be varied at will.

Key words: co-oligomer, acrylic acid, butyl acrylate, ethyl acrylate, phase diagram, RAFT

*

Corresponding Author: [email protected]

1 Environment ACS Paragon Plus

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INTRODUCTION Amphiphilic diblock

copolymers are molecules composed of covalently linked

hydrophobic and hydrophilic blocks that can self-assemble spontaneously in melt or in solutions in which the solvent preferentially dissolves one block.1 Amphiphilic diblock copolymers are widely used in numerous applications, e.g., as stabilizers, emulsifiers, dispersants, compatibilizers, thickeners, foamers and drug delivery agents.2,3 The self-assembly of various amphiphilic block copolymers has been investigated in a variety of solvents over many decades.4,5 Gao et al. observed reverse micellization of poly(styrene)-b-poly(butyl acrylate) in toluene.6 Tang et al. obtained sphere, vesicle, rod-like aggregates and cauliflower-like aggregates of self-assembling poly(ethylene oxide)-b-poly(pnitrophenyl methacrylates) in different solvent mixtures, dimethyl sulfoxide - methanol or ethanol or water.7 He et al. found spherical, worm-like micelles and bilayered vesicles from a system of poly(1,2-butadiene)-b-poly(ethylene oxide) in an ionic liquid.8 Many studies of self-assembly have focused on the phase behavior of several amphiphilic diblock copolymers in aqueous solution. For instance, poly(styrene)-b-poly(acrylic acid),9,10 poly(dimethylsiloxane)-b-(poly(2-dimethylamino)ethyl acrylate),11 poly(n-butyl acrylate)-bpoly(acrylic acid),12,13 and poly(methylene)-b-poly(acrylic acid).14 Poly(ethylene oxide) and poly(propylene oxide) block copolymers are among the most widely investigated, and are known to form micelles and lyotropic liquid crystals whose structure and stability depend on the ethylene oxide/propylene oxide ratio.15 It is difficult to systematically vary hydrophobic and hydrophilic block lengths to understand design rule of self-assembly structure in water. With the notable exception of poly(ethylene oxide) or other ethoxylated moieties, most hydrophilic blocks are charged, e.g., anionic acrylate or methacrylate, or cationic, protonated aminoethylacrylates. Conversely, hydrophobic blocks become water-insoluble at quite modest degrees of polymerization, leading to kinetically-trapped, non-equilibrium structures. Most of the amphiphilic diblock copolymers studied were comprised of long chain polymers or polymers with high average molecular weights.


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Amphiphilic diblock co-oligomers are similar to amphiphilic diblock copolymers but with shorter block chain length and lower average molecular weight. The precise definition of the term oligomer is somewhat inconsistent. The polymer handbook defines oligomers as polymers with molecular weights up to 2000 g/mol,16 while an oligomer is defined by IUPAC as a compound composed between 3 and 10 repeating monomer units.17 In the literature, an oligomer describes short chain polymers with an average molecular weight up to 5000 or 6000 g/mol.18,19,20 Many conventional surfactants, such as sodium dodecyl sulfate, are excellent at reducing interfacial tension but they have high critical micelle concentrations (cmc), which requires large quantities of surfactant in commercial products to obtain the desired properties. In contrast, amphiphilic diblock copolymers with high molecular weights have small cmcs, but their self-assembly requires very long equilibration times.12,21,22 The self-assembly of amphiphilic diblock co-oligomers is an attractive option to avoid both of these problems. Short amphiphilic diblock copolymers have better solubility of hydrophobic block than long amphiphilic diblock copolymers and surface activity similar to conventional surfactants. Siaw et al. studied phase behavior of two short amphiphilic diblock copolymers in aqueous solution, poly(ethylene glycol)-b-poly(styrene) (mPEG16-b-PS2,4,6) and poly(acrylic acid)-bpoly(styrene) (PAA14-b-PS16,33).18 Both systems formed micelles in water at a concentration of 6 mg/mL. The diameter of the micelles increased with an increase in the length of the hydrophobic block. Heinen et al. focused on the phase behavior of amphiphilic diblock cooligomers n-butyl acrylate-b-2-hydroxyethyl acrylate (BA5-b-HEA5,6,7,8), styrene-b-acrylic acid (S5,9,12-b-AA5) and n-butyl acrylate-b-acrylic acid (BA5,9,12-b-AA5) in aqueous solution.23 Systems of BA5-b-HEA5,6,7,8 are nonionic amphiphilic diblock co-oligomers, whereas both S5,9,12-b-AA5 and BA5,9,12-b-AA5 are ionic. Spherical micelles were found in BA5-b-HEA5,6,7,8, while both spheres and lamellar structures were formed in BA5-b-HEA6,7,8. In ionic cooligomer systems, S5,9,12-b-AA5 only assembled micelles, while BA5,9,12-b-AA5 aggregated into spherical and lamellar mesophases as the concentration of the co-oligomer was increased. Self-assembly of short amphiphilic diblock copolymer, poly(butyl acrylate)-b-


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poly(acrylic acid) (BA15-b-AA5, BA13-b-AA9) was considered by Lamprou et al.24 The morphology was spherical vesicles. According to this study, self-assembly of such a short BA-b-AA amphiphilic diblock copolymer is dynamic in water, which is different from kinetically frozen aggregation reported in the literature when the polymerization number of BA is greater than 20.12,13,21,22 Also, the degree of polymerization for the butyl acrylate block studied by Heinen is smaller than the co-oligomers in Lamprou’s work, e.g., BA5,9,12-b-AA5 versus BA15-b-AA5, BA13-b-AA9. The assembled morphology is an essential factor in designing the desired properties of surfactants.25 Various morphologies are formed during the self-assembly of amphiphilic block copolymer in aqueous solution, including spheres, cylinders, cubic phases, bilayers, etc. For example, the morphology of drug carrier is specifically addressed when considering applications of therapeutic agents. Many drug carriers have been designed using spherical micelles for delivering hydrophobic drugs,26,27,28,29 while others have used hexagonal micelles due to its higher stability in mammalian cells, wider range of cell growth media and biodegradation.30,31,32 Cubosomes33,34,35 or vesicles36,37,38 have been employed as drug carriers containing both hydrophobic and hydrophilic pharmaceutical ingredients. In other applications, self-assembly of cylindrical micelles of poly(ferrocenyldimethylsilane)-b-poly(2vinylpyridine) block copolymer using as a templates for synthesis polyaniline nanofibers is potentially applied for nanodevice’s components.39 Mesoporous silica thin films have been used in sensors, membranes, chromatography, catalysis and electrochemical cell based applications. Morphology of nanoporous of mesoporous silica thin films prepared on a solid electrode surface affected the permeability of the films, e.g., mass transport rates decreased when the nanopore structure changed from cubic to 3D hexagonal and then to collapsed 2D hexagonal system.40 Therefore, determining the conditions for a specific morphology is necessary in order to form stabilized structures for targeted applications. In this work, the phase behavior and self-assembly structure of three different series of amphiphilic diblock co-oligomer, n-butyl acrylate-b-acrylic acid (BA5AA5), tert-butyl acrylateb-acrylic acid (tBAxAA5, x = 5, 7 or 10) and ethyl acrylate-b-acrylic acid (EAyAA5, y = 5, 10 or 4 Environment ACS Paragon Plus

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20) was examined in aqueous solution by a combination of polarizing optical microscopy and small angle X-ray scattering (SAXS). Each co-oligomer amphiphile was synthesized by reversible addition fragmentation chain transfer polymerization (RAFT), yielding copolymers with well-defined architectures, controlled molecular weights and low polydispersity.41,42 Poly(acrylic acid) is a well-known weak polyelectrolyte, which can self-organize into different structures depending on the degree of ionization and the pH of the surrounding solution.23 Due to this property of the acrylic acid hydrophilic block, the effect of ionization of the acrylic acid groups on the phase behavior of these amphiphilic diblock co-oligomers in water was studied to understand conditions under which micelles, hexagonal, cubic and lamellar mesophases are formed. The generic structure of the co-oligomers examined is shown in Figure 1 and abbreviations used summarized in Table 1.

Figure 1: Representative structure of co-oligomers examined: m denotes the average degree

of oligomerization of the hydrophobic monomer. In all cases the average degree of oligomerization of the acrylic acid block was 5 (see text). Table 1: Structures of R groups of hydrophobic monomers shown in Figure 1, together with

degrees of oligomerization (m) and abbreviations used for each co-oligomer. R group structure

Abbreviation

m

n-butyl

BAmAA5

5

t-butyl

tBAmAA5

5, 7 or 10

ethyl

EAmAA5

5, 10 or 20

MATERIALS AND METHODS Acrylic acid (AA) monomer and 1,4-dioxane (both from Sigma-Aldrich) were purified by distillation under reduced pressure. Other monomers (Sigma-Aldrich), n-butyl acrylate (BA),


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tert butyl acrylate (tBA) and ethyl acrylate (EA), were purified by passing through a basic alumina (Sigma-Aldrich) column to remove inhibitor. Chain transfer agent (CTA) 2(((butylthio)carbonothioyl)thio) propanoic acid (C4RAFT, Dulux, Australia), 4,4’-azobis(4cyanovaleic acid) (V501, Fluka), deuterated dimethyl sulfoxide (DMSO-d6, Cambridge Isotope Laboratories), methanol (MeOH, Merck) and sodium hydroxide pellets (NaOH, Ajax Finechem) were used as received. Water used in this work was purified by a Millipore-Q plus water purification system (Milli-Q water, resistivity 18.2 MΩ cm). 1

H NMR spectra were recorded using a Bruker Ultra Shield Avance 300 MHz

spectrometer at 25°C. DMSO-d6 was used as the solvent for characterization. Solvent residual signals were utilized as internal references. All chemical shifts are reported in ppm (δ). The degree of polymerization of the first block C4RAFT-AA5 was identified by electrospray ionization using a Finnigan LCQ spectrometer with XCalibur 2.0.7 control software for data processing. Crude C4RAFT-AA5 was diluted in MeOH, and 20 µL of the sample was injected with a 0.3 mL/minute MeOH flow into the ESI-MS in negative ion mode. Mass spectrum of the first block was recorded from 500 to 1000 m/z. The phase behavior of amphiphilic diblock co-oligomers in water was determined using a Leica DM2500P optical microscope equipped with a camera and crossed polarizers, including samples of solvent penetration experiments and equilibrated samples in water. Anisotropic liquid crystal mesophases (hexagonal or lamellar) were identified based on characteristic textures observed between crossed polarizers. In a solvent penetration experiment, a small amount of co-oligomer was introduced onto a glass slide, covered by cover-slip, and then pressed to obtain a thin film. After, water was dropped at the edge of the cover-slip, and allowed to slowly penetrate into the co-oligomer, and mesophase structures subsequently formed were observed under the microscope. Small angle x-ray scattering was performed using an Anton Paar SAXess using a sealed X-ray tube and generator with two cameras, one with point collimation and the other with line collimation. The X-ray scattering intensities of equilibrated diblock co-oligomers in water at


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various concentrations were collected and analysed. The scattering intensity were recorded on image films, which were then read and reduced to intensity profiles by Optiquant and SAXSquant 2D softwares, respectively. The relative locations of the intensity peaks were used to identify liquid crystal structures. Each amphiphilic diblock co-oligomer sample at a specific neutralized percentage was mixed with the required amount of Milli-Q water to obtain the desired concentration. The cooligomer and water were continuously mixed by a roller-mixer at 40 rpm, for five days at room temperature in order to reach equilibrium. Samples were typically found to equilibrate after 24 h, and selected monophasic samples subjected to heating/cooling cycles recovered their originally-equilibrated structures over a similar time period. Multi-phase samples in some cases took longer to recover, so 5 days was used as a conservative baseline. The cooligomer sample was then characterized by SAXS and polarizing optical microscopy. Diblock co-oligomers were synthesized by reversible addition fragmentation chain transfer polymerization (RAFT). The first monomer AA (4.58 g, 0.0635 mol), chain transfer agent C4RAFT (2.98 g, 0.0125 mol) and V501 initiator (0.175 g, 6.25 x 10-4 mol) were dissolved in 1, 4-dioxane (22.98 g). The reaction mixture was purged with nitrogen for 15 min, and then stirred at 70°C in an oil bath for 3 hr. After, the reaction mixture was cooled to room temperature. The degree of polymerization of the C4RAFT-AA5 agent was characterized using 1H NMR and ESI-MS. The conversion was 94%, which was determined by 1H NMR. 1H NMR (DMSO d6, 300 MHz) δ (ppm) 0.85-0.92 (t, 3H, CH3-(CH2)3-S), 0.991.18 (d, 3H, -CH(CH3)COOH), 1.2-2.47 (m, CH3-CH2-(CH2)2-S-, CH3-CH2-CH2-CH2-S-C(S)-, AA5 backbone -CH2, AA5 backbone -CH, -CH(CH3)(COOH)), 4.6-4.8 (m, 1H, -C(S)-S(CH(COOH)(CH2))5-CH(COOH)(CH3)), 12.2-12.5 (s, 5H, COOH). For the synthesis of the second block, an appropriate amount of the second monomer, dioxane and initiator (0.0126 g, 4.5 x 10-5 mol) were added into 2.2 g solution of C4RAFTAA5 agent (0.538 g, 9.0 x 10-4 mol). The reaction mixture was degassed by nitrogen for 15 min, and subsequently stirred at 70°C in an oil bath for 3 hr. Reaction details of each type of second monomer and the conversion of the corresponding reaction are listed in Table 2.


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Diblock co-oligomers were characterized using 1H NMR. The dioxane solvent was then removed using a rotary evaporator under vacuum pressure to obtain the dry co-oligomer. Table 2: Reacted quantity from synthesis of each type of second block and conversion yield. Co-oligomer

Monomer

Amount (g)

Dioxane (g)

Conversion (%)

BA5AA5

n-BA

0.68

1.20

95

tBA5AA5

t-BA

0.66

1.20

94

tBA7AA5

t-BA

0.81

1.26

97

tBA10AA5

t-BA

1.21

1.82

98

EA5AA5

EA

0.51

1.10

94

EA10AA5

EA

1.03

1.45

96

EA20AA5

EA

1.82

2.35

97

BA5AA5 1H NMR (DMSO d6, 300 MHz) δ (ppm) 0.8-0.95 (t, 18H, CH3-(CH2)3-S, -CH3 of BA5), 1.0-1.2 (d, 3H, -CH(CH3)COOH), 1.2-2.45 (m, CH3-CH2-(CH2)2-S-, CH3-CH2-CH2-CH2S-C(S)-, AA5 backbone -CH2, AA5 backbone -CH, BA5 backbone -COO-CH2-, CH(CH3)(COOH)), 3.75-3.85 (t, 2H, CH3-(CH2)2-CH2-S-C(S)-), 4.6-4.8 (m, 1H, -C(S)-S(CH(COOH)(CH2))5-CH(COOH)(CH3)), 12.2-12.4 (s, 5H, COOH) tBA5AA5 1H NMR (DMSO d6, 300 MHz) δ (ppm) 0.8-0.95 (t, 3H, CH3-(CH2)3-S), 1.0-1.2 (d, 3H, -CH(CH3)COOH), 1.2-2.45 (m, CH3-CH2-(CH2)2-S-, CH3-CH2-CH2-CH2-S-C(S)-, AA5 backbone -CH2, AA5 backbone -CH, tBA5 backbone -COO-C-(CH3)3, -CH(CH3)(COOH)), 4.6-4.8 (m, 1H, -C(S)-S-(CH(COOH)(CH2))5-CH(COOH)(CH3)), 12.2-12.4 (s, 5H, COOH) 1

H NMR of tBA7AA5 and tBA10AA5 are similar to tBA5AA5 1H NMR, but the numbers of

protons were different from 1.2 to 2.45 ppm. EA5AA5 1H NMR (DMSO d6, 300 MHz) δ (ppm) 0.8-0.95 (t, 3H, CH3-(CH2)3-S), 1.0-1.2 (d, 3H, -CH(CH3)COOH), 1.2-1.4 (t, EA5 backbone -COO-CH2-CH3), 1.4-2.45 (m, CH3-CH2(CH2)2-S-, CH3-CH2-CH2-CH2-S-C(S)-, AA5 backbone -CH2, AA5 backbone -CH, EA5 backbone -COO-CH2-, -CH(CH3)(COOH)), 3.75-3.85 (t, 2H, CH3-(CH2)2-CH2-S-C(S)-), 4.64.8 (m, 1H, -C(S)-S-(CH(COOH)(CH2))5-CH(COOH)(CH3)), 12.2-12.4 (s, 5H, COOH)


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1

H NMR of EA10AA5 and EA20AA5 are similar to EA5AA5 1H NMR, but the numbers of

protons were different from 1.2 to 2.45 ppm. Each type of dry co-oligomer (0.005 g) diluted in 10 mL Milli-Q water was titrated with 1.0 M NaOH using a Metrohm 744 pH meter to achieve the equivalence point of reaction between carboxyl groups in the corresponding co-oligomer and NaOH. The amount of NaOH at the equivalence point is equal to the required amount of NaOH to fully neutralize carboxyl groups in the co-oligomer. Based on the value of the equivalence point, we calculated the required amount of 1.0 M NaOH solution for 25, 50, 75 and 100 % neutralization of each type of co-oligomer. After neutralization, each co-oligomer sample was dried to constant weight under vacuum. Results and Discussion Characterization of synthesized C4RAFT-AA5 and amphiphilic diblock co-oligomers The C4RAFT-AA5 homopolymer synthesized by the RAFT technique has a well-defined structure with the 1H NMR spectrum (see Materials and Methods) of C4RAFT-AA5 showing an average of five protons from the AA groups observed between 12.2 and 12.4 ppm. Figure 2 shows the distribution of AA units for the first block (from ESI-MS) about a mode of five AA units. Note that both the mass spectra and NMR results indicate a slightly longer average degree of polymerization for the AA block than in our previous study,23 consistent with the polymerisation being carried out for longer (3 h vs 2 h) and at higher temperature (70 °C vs 60 °C) in the present work.

Figure 2: ESI-MS of C4RAFT-AA5 first block


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The structure of synthesized co-oligomers after adding the second block was analysed by 1

H NMR, yielding conversions between 94% and 98% (Table 2). From the 1H NMR spectra,

each type of co-oligomer contained five carboxylic groups from five AA units (see Materials and Methods) and the correspondingly desired second block. Self-assembly of tBAx AA5 (x = 5, 7 or 10) The isothermal phase diagrams of tBA5AA5, tBA7AA5 and tBA10AA5 in water are shown in Figure 3. These show the effect of ionization of the hydrophilic acrylic acid blocks as a pseudo-ternary system comprising protonated co-oligomer (e.g., tBA5AA5) and its salt (e.g., tBA5AA5-Na+ - including the ionisable RAFT agent end-group) in aqueous solution. No ordered structures are formed in mixtures of the acidic (hydrogen) form of tBA5AA5 and water. The mixtures included a swollen solid and water, which are denoted SSW in the phase diagrams (see captions). The swollen solid was caused by water absorption by the hydrophilic acrylic acid block.43 In contrast, the fully deprotonated (100% ionized) acrylate species are much more soluble in water, and these form extended liquid isotropic (micellar) phases up to approximately 25 wt %. At higher concentrations, one or more lyotropic mesophases are present along the binary H2O - tBA5AA5-Na+ axis, and at lower degrees of ionization.


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(a)

(b)

(c)

Figure 3: Ternary phase diagram of (a) tBA5AA5, (b) tBA7AA5 and (c) tBA10AA5 in water with experimental data. The boundaries were approximated to enclose experimental data. L1, H1, Lα, SSW and OS denote micelle, hexagonal, lamellar phases, mixtures of swollen solid and water, and opaque solid. Symbols refer to experimental data points ( - liquid isotropic (micellar) phase (L1), hexagonal phase (H1), - lamellar phase (Lα), - 2 or 3 phases and x - unidentified phase). The dotted lines in (a) show water-dilution paths of co-oligomers at 25, 50 and 75 % ionization of the carboxylate groups.


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In tBA5AA5 at 25% ionization, a lamellar phase (Lα) is first detected in a range of cooligomer concentration between 60 and 70 wt % and remains stable up to at least 50% ionization. At higher ionizations, the lamellar phase is transformed into a direct hexagonal phase (H1), which persists up to 100% ionization, tBA5AA5-Na+, and over a wide concentration range. The same sequence of lyotropic mesophases is observed as the hydrophobic block length is increased to 7 and then 10 tBA units, but two trends are seen: a higher degree of ionization is required before the Lα phase is first observed in the ternary phase diagram, and it remains stable up to 100% ionization. The lamellar phase remains the stable phase at higher co-oligomer concentrations than the hexagonal phase, which only forms near 75% ionization. As the hydrophobic block length increases, Lα is stabilized relative to H1. The sequence of self-assembled structures and the trends with alkyl chain length are consistent with an interpretation based on the surfactant packing parameter model of Israelachvili et al.,44 v/aol, where v is the molecular volume of the hydrophobic tail, ao is the area of the head group at the polar/non-polar interface, and l is the chain length of the hydrophobic tail. The phase behavior of the fully ionized diblock co-oligomers is very similar to that seen for micelle-forming ionic surfactants.45 In dilute solution at low electrolyte concentration (zero added salt), strong electrostatic repulsions between charged polar groups maximizes a0 for each hydrophobic block, whose volume, v, and length, l, are fixed by molecular geometry. This leads to compact, spherical micelles consistent with v/aol < 1/3. As the hydrophobic blocks are quite short and rotation is sterically hindered by the tert-butyl groups in these amphiphiles, they are expected to be better approximated by the “fullyextended alkyl chain” model used for conventional surfactants44 than an elastically-deformed random flight conformation commonly used to describe longer blocks of copolymer amphiphiles.46 This micelle structure is consistent with the literature of a diblock copolymer micelle including a hydrophobic core and a corona of stretched polyelectrolyte chains47,48 (Micelle structure has been examined in detail by small angle neutron scattering, which will


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be detailed in a forthcoming study of the L1 phase). With increasing concentration, the ionic strength increases, screening repulsions between charged head groups and reducing ao and increasing v/aol. When this value exceeds 1/3 the favored aggregate morphology becomes cylinders, corresponding to the hexagonal phase. This transition is also seen upon increasing concentration of non-ionic surfactant solutions, due to decreasing water activity and changes in steric interactions between hydrophilic ethoxy groups.49 Further increasing concentration leads to a lamellar phase in tBA7AA5-Na+ and tBA10AA5Na+,

but

not

tBA5AA5-Na+.

This

trend

is

again

consistent

with

increasing

electrolyte/decreasing water activity further reducing ao until v/aol > 1/2, where planar or bilayer structures become favored over spheres or cylinders. The absence of a lamellar phase in the fully-ionized tBA5AA5-Na+ is a consequence of the short hydrophobic block, which never allows planar structures to be favored before the solubility limit is reached. This matches the trend seen in polyoxyethylene n-alkyl ether (CmEn) non-ionic surfactants, where the Lα phase is also less stable as non-polar, alkyl, chain length m is decreased at a fixed polar, ethoxy, chain length n.49 The effect of ionization of the acrylic acid groups on the phase behavior of tBAxAA5 in water is also consistent with packing parameter considerations. Both the L1 and H1 phases are only stable well above 50% ionization in their respective concentration ranges. Decreasing ionization is expected to decrease ao, and therefore favor less curved aggregates. This is also seen in the behavior of the Lα phase which is not only stable at much lower degrees of ionization than either L1 or H1, but which also becomes stable over a wider composition range as ionization decreases. This reflects the increasing importance of steric rather than electrostatic interactions in stabilising the self-assembly structure. The lower ionization stability limits of the H1 and L1 phases, a little below 75%, correspond to the transition of these co-oligomer amphiphiles from soluble, micelle-forming surfactants into insoluble, bilayer forming (or lipid-like) amphiphiles.50 In other words, the broad two-phase coexistence region of the diluted lamellar phase is a vesicle dispersion in


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excess water, which means that a micelle-vesicle transition can be triggered by a pH change anywhere along the L1 concentration range (0-25 wt% tBAxAA5-Na+). Further decreasing ionization leads to precipitation of solid co-oligomer, usually as an opaque solid or gel with occluded water. The effect of hydrophobic monomer structure on self-assembly structure was examined by comparing the isomeric monomers n-butyl and tert-butyl acrylate in tBA5AA5 and BA5AA5 prepared from the same starting C4RAFT-AA5 oligomer. This is shown in Figure 4. In previous work we compared butyl acrylate with styrene hydrophobic blocks in similar systems, but styrene’s hydrophobicity and high Tg led to phase diagrams dominated by solid phases.23

Figure 4: Ternary phase diagram of BA5AA5 in water with experimental data. The boundaries were approximated to enclose experimental data. L1, H1, Lα, SSW and OS denote micelle, hexagonal, lamellar phases, mixtures of swollen solid and water, and opaque solid. Symbols refer to experimental - hexagonal phase (H1), - lamellar phase data points ( - liquid isotropic (micellar) phase (L1), (Lα), - 2 or 3 phases and x - unidentified phase).

Both tBA5AA5 and BA5AA5 form micelles (L1) in dilute solution at high degrees of ionization, and a hexagonal phase at higher co-oligomer concentrations. Both tBA5AA5 and BA5AA5 also form lamellar phases at higher co-oligomer concentrations, and at lower degree of ionization. However, the difference between tBA5AA5 and BA5AA5 is most pronounced when the lamellar phase of BA5AA5, not only forms at higher minimum degrees of ionization (25% ionization) than tBA5AA5, but also remains stable even at 100% ionization. This is a


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straightforward consequence of tBA monomers being more compact than those of BA. Hence, the hydrophobic block can pack closer together in tBA than BA, leading to a smaller packing parameter at the same degree of ionization. This is consistent with the trends in the phase behavior and morphology of tBA5AA5, tBA7AA5 and tBA10AA5 (Figure 2), which increasingly favors lamellar over hexagonal phase at 100% ionization as the hydrophobic block length increases. The phase behavior of the BA5AA5 - water system shown in Figure 4, and particularly the effect of ionization of the acrylic acid groups, is in a good agreement with our previous work.23 Here, however, we detect a hexagonal phase near 100% ionization of BA5AA5-Na+. This is attributed to the longer reaction time used to prepare the AA block, yielding a longer average block than the previous system. This highlights the sensitivity of self-assembly structure to small changes in the average degree of polymerization of the acrylic acid hydrophilic block. A small change is sufficient to alter the relative stability of the hexagonal and lamellar mesophases, paralleling trends seen in the phase behavior of conventional CnEm non-ionic surfactants with decreasing ethoxy chain length.49 Self-assembly of EAyAA5 (y = 5, 10 or 20) An advantage of block copolymer or co-oligomer amphiphiles afforded by the large library of monomers with varying degrees of hydrophobicity, rather than being constrained to hydroor fluoro-carbon chains of traditional surfactants, has been exploited extensively in poly(ethylene oxide)-poly(propylene oxide) copolymers, for example,51 in which longer hydrophobic blocks lead to larger structures without sacrificing solubility. In this spirit, we have examined the phase behavior of co-oligomer amphiphiles built from ethylacrylate hydrophobic monomers, EA5AA5, EA10AA5 and EA20AA5 in water. Partial phase diagrams are shown in Figure 5.


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(a)

(b)

(c)

Figure 5: Ternary phase diagram of (a) EA5AA5, (b) EA10AA5 and (c) EA20AA5 in water with experimental data. The boundaries were approximated to enclose experimental data. L1, I1, H1, Lα, SSW and OS denote micelle, discrete cubic, hexagonal, lamellar phases, mixtures of swollen solid and water, and opaque solid. Dashed lines are extending phase boundaries. Symbols refer to experimental data points ( - liquid isotropic (micellar) phase (L1), - discrete (I1) cubic phase, hexagonal phase, - lamellar phase (Lα), - 2 or 3 phases and x - unidentified phase).


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In EA5AA5 (Figure 5a), the L1 phase is stable up to almost 50 wt % in water over a much wider range of ionizations than with either the BA or tBA hydrophobic tails. This is taken to be a straightforward consequence of the higher water-solubility of EA monomers, such that EA5 is much less hydrophobic than either BA5 or tBA5. The lamellar phase of EA5AA5 is similarly stable at low degrees of ionization, and like tBA5AA5-Na+, this is attributed to the compactness of the ethyl moiety. At high degrees of ionization, the now-familiar hexagonal phase is observed between the L1 and lamellar phase, which remains stable up to 100% ionization. Increasing EA length to 10 or 20 monomers collapses the L1 phase towards the 100% ionization axis as the block’s overall hydrophobicity becomes comparable with blocks of BA5 isomers. For EA10AA5 the lamellar phase retains its stability from low ionizations, also like the BA5AA5 systems (Figure 4), up to 100% ionization. Here the hexagonal phase region is larger and for the first time a cubic phase is observed between L1 and H1. Indexing of SAXS patterns (√3:√4:√8:√11, see Supporting Information) indicate that this is a discrete (I1) cubic phase comprising micelles on a face-centered cubic lattice.52,53,54 The emergence of an I1 cubic phase with increasing hydrophobic block length is at first sight counterintuitive, as this should favor less-curved (higher packing parameter) structures. However, the I1 phase forms from the L1 region by an ordering transition that does not involve a change in aggregate shape. It is known in ionic surfactant systems that short hydrophobic chains give rise to high cmcs and high ionic strengths that suppress the repulsions needed for I1 phase formation.55 In the EA20AA5 co-oligomer system (Figure 5c), the stability ranges of the hexagonal and lamellar phases are markedly smaller than in EA10AA5. When the chain length of EA is increased to 20 repeating units, the solubility of the co-oligomer in water decreases, so that the concentrations at which hexagonal and lamellar phases formed are lowered. At 75% ionization, and concentrations between 35 and 40 wt%, EA20AA5 forms a discrete (I1) cubic phase.52 The phase is optically isotropic but the Bragg peaks in the SAXS patterns could not be unambiguously indexed to any known cubic phases. This may be due to two (or more) 17 Environment ACS Paragon Plus

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cubic phases of different symmetry arising at nearby compositions (and refractive indices) in this region. None of the lyotropic phases I1, H1 or Lα could be isolated in binary mixtures of the fully ionized co-oligomer and H2O. Rather, optical micrographs and SAXS patterns consistent with two coexisting phases (see supporting information). From 40 to 60 wt% several broad and overlapping peaks occurred in the SAXS that we attribute to coexisting I1 and H1 phases, and from 65 to 70 wt %, SAXS patterns are consistent with coexisting hexagonal and lamellar phases. This is consistent with I1, H1 and Lα phases remaining stable up to 100% ionization of EA20AA5-Na+, but each only existing alone over a very narrow concentration range. This is indicated in Figure 5c by the dashed phase boundaries extending to the 100% ionization axis. Like the tBAnAA5 and BAnAA5 systems,23 ethyl acrylate blocks also acts as hydrophobes that enable amphiphilic co-oligomer self-assembly when the acrylic acid moieties are at least partially ionized. Also like BA-based co-oligomers, increasing the degree of ionization of the acrylate block increases solubility of longer EA chains. At low ionization the co-oligomers behave like sparingly-soluble surfactants and exhibit a broad two-phase coexistence region (vesicle dispersion) between a lamellar phase and a very dilute aqueous solution. However, increasing the ionization allows micelles to form in the L1 phase, leading to the formation of discrete cubic and hexagonal mesophases. The only departure from this occurs for very short EA5 blocks, which are themselves soluble in water even at low AA5 ionization. Controlling self-assembly Control of self-assembly may be broken into two components: hydrophobicity and structure. Hydrophobicity is determined by the side chains (i.e. ethyl vs t-butyl vs n-butyl), while structure is determined by a combination of tail length, hydrophobic volume, and head group ionization – all of which modify self-assembly through the surfactant packing parameter – although in a modified form for block co-oligomers, as discussed below. Whereas our previous study23 showed that the stability of the L1 phase and solubility limit depended strongly on n-BA or styrene block length, this paper shows that this effect is


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attenuated in tBA, and even more so in EA co-oligomers. This is consistent with their much smaller exposed hydrophobic side-groups, enabling easy decoupling of hydrophobicity from self-assembly structure. We expect that the surfactant packing parameter, v/aolc, can easily be adapted to understand self-assembly structures in these diblock co-oligomers: In conventional surfactants, hydrophobic (alkyl) chain length is a primary control parameter for self-assembly structures, as it determines both v and lc (using Tanford’s formulae56). However, high chainmelting or Krafft temperatures of saturated alkyl chains longer than C16 limits the range of aggregate sizes that can be assembled. The weakly-hydrophobic propylene oxide monomers used to make Pluronic copolymers are one of few widely-utilized counterexamples. These amphiphilic co-oligomers, contain hydrophobic blocks with fully-extended lengths, lc, ranging from 23 Å for 5-mers (comparable to a C16 alkyl hydrophobe) up to 60 Å for EA20. The ethyl-, t-butyl, and n-butyl- side-chains independently alter the hydrophobic packing volume, v, as well as tuning hydrophobicity. Thus, at the same degree of ionization, more highly-curved structures are favored by the more compact ethyl- and t-butylmonomers, at the same block length and degree of ionization. Two factors complicate the adaptation of the packing parameter to diblock co-oligomers, both related to the meaning of lc. The first is well-known in block-copolymer assembly: As block length increases, the probability of adopting an all-trans chain of length lc becomes vanishingly small, so the hydrophobic core needs to be instead considered as an unperturbed chain in its melt. The range of hydrophobic chain lengths examined in these systems represents a transition from C4RAFT+5-mer, with an effective alkyl chain length approximately equivalent to a C16 alkyl tail, and hence most like a small surfactant, up to EA20, which would correspond to about C46. The probability of an all-trans conformer of EA20 is negligible, so chain length in the packing parameter, v/aolc, as applied to conventional surfactants, must be recast in terms of the random coil dimension of the hydrophobic block. Thus, although v increases linearly with degree of polymerization of the hydrophobic block, the hydrophobic core radius of the aggregate should increase more slowly with increasing


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tail length. The packing parameter should therefore increase markedly, favoring less curved aggregates. The second consideration relates to the packing of hydrophobes inside the core of the aggregates formed. Even within the paradigm of the surfactant packing parameter, the alltrans conformation lc, is an upper bound on the core radius and therefore defines the most highly-curved structure that can form. Interpenetration of tails in the core favors more planar structures. In our previous study,23 fitting the SAXS patterns of lamellar phases of BA5-, BA9and S5-AA5 co-oligomers yielded hydrophobic block thicknesses much smaller than two fullyextended hydrophobic chains. This suggested that the hydrophobic blocks were highly intercalated and/or the chains were coiled. Here we have taken a slightly different approach to analyse the lamellar phase SAXS patterns. We partition the co-oligomers into hydrophobic and hydrophilic volume fractions using their known densities, and then use the dilution law for diffraction, φhydrophobe = thydrophobe/D*, where φ denotes volume fraction and D* is the repeat spacing of the primary diffraction peak, to determine the hydrophobic block thickness. In every system examined we find again that the hydrophobic blocks are much thinner than expected, with the overall hydrophobic block thickness close to the fully-extended length of a single chain from 5-mers to 20-mers. Results are summarized in Table 3. We interpret this to be a consequence of the side-chains of the hydrophobic moieties, which do not readily pack into the same orderly arrangement as alkyl chains: They must coil back onto themselves and/or intercalate in order not to leave voids in the hydrophobic core. That is, even the shortest hydrophobic blocks do not pack like the alkyl tails of conventional surfactants, but interpenetrate like a polymer melt. Our results show that BA5 forms slightly thicker bilayers than does tBA5 under the same conditions, and that EA5 bilayers are thinner still. Thicker bilayers are likely to be a consequence of greater steric hindrance to rotation around the backbone, restricting the ability of the chain to adopt more compact conformations.


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For a given system, the hydrophobic core thickness decreases slightly as the degree of ionization increases. This is consistent with an increase in repulsion between neighboring AA chains within a single bilayer increasing the area per chain; in order to conserve volume the hydrophobic core thins.

Table 3: Hydrophobic block thicknesses (thydrophobe) in amphiphilic block co-oligomer lamellar phases. thydrophobe is calculated from the scattering dilution law (see text), and lc from Tanford’s formula.56 φhydrophobe, D* and lc denote hydrophobic volume fraction, the repeat spacing of the primary diffraction peak and the fully-extended length of a single chain, respectively.

% ionization

wt% cooligomer

D*

φhydrophobe

thydrophobe

lc (Å) 23.0 23.0

BA5 AA5

75

65

(Å) 61.6

0.46

(Å) 28.0

tBA5AA5

50 75

65 65

65.1 59.8

0.47 0.45

30.8 26.6

50 25

65 65

63.4 65.1

0.45 0.46

28.7 30.0

100 75

65 65

62.0 70.1

0.46 0.48

28.7 33.3

50 100

65 65

73.8 73.8

0.48 0.50

35.7 36.8

75 50

65 65

76.5 88.4

0.50 0.51

38.6 45.4

100 75

70 70

51.5 51.8

0.42 0.43

21.5 22.2

50 25

70 70

55.2 56.0

0.44 0.45

24.2 25.1

100 75

70 70

64.7 69.3

0.54 0.55

35.0 38.1

50 25

70 70

71.4 73.0

0.56 0.57

39.9 41.5

100 75

65 65

65.1 71.4

0.51 0.51

33.1 36.8

50 75

65 65

75.3 92.6

0.53 0.57

39.6 52.6

50

65

95.4

0.58

54.9

tBA7AA5

tBA10 AA5

EA5 AA5

EA10AA5

EA20AA5

28.1

35.7

23.0

35.7

35.7

61.0

CONCLUSIONS Controlled radical polymerization techniques such as RAFT, used here and previously by us,23,57 enable the preparation of novel amphiphilic co-oligomers comparable with


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conventional surfactant, and with exquisite control over the lengths and chemistry of the hydrophobic and hydrophilic blocks. These undergo spontaneous self-assembly in aqueous solution into micelles, and subsequent organization into lyotropic phases, which depends on block length and type. For example, here both the monomer hydrophobicity (ethyl-, t-butyl-, and n-butyl- acrylate) and block length (n = 5-10) has been varied, allowing bilayer thickness in lamellar phases to be designed synthetically and then further modulated by changes in ionization of the hydrophilic acrylic acid group. As with conventional surfactants, the stability of different lyotropic phases is determined by packing constraints.49 Somewhat surprisingly, in co-oligomers even the shortest hydrophobic chains examined packed in a coiled and intercalated arrangement, similar to long polymer chains. The range on monomers available for free-radical polymerization promises a vast new library of block-co-oligomer amphiphiles incorporating controllable functionality and responsiveness to environmental variables including pH, electrolyte or specific ions, or temperature,58 both alone and in combination for sensor or triggered release applications.59

ACKNOWLEDGMENTS The work was funded by Australian Research Council Discovery and Infrastructure Grants. We also thank Dr Algi Serelis from Dulux Australia for providing the RAFT agent used in this study, and Prof. Sébastien Perrier for valuable discussions.

Supporting Information SAXS patterns and optical microscopy images of discrete (I1) cubic phase of EA10AA5 100 % ionization (from 30 to 35 wt %), discrete cubic phase of EA20AA5 75 % ionization (from 35 to 40 wt %) and the coexisting hexagonal and lamellar phases of EA20AA5 100 % ionization (from 65 to 70 wt %).


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57. Siauw, M.; FitzGerald, P. A.; Hawkett, B. S.; Perrier, S., Thermoresponsive behavior of amphiphilic diblock co-oligomers of ethylene glycol and styrene in aqueous solution. Soft Matter 2013, 9 (29), 7007-7015. 58. FitzGerald, P. A.; Gupta, S.; Wood, K.; Perrier, S.; Warr, G. G., Temperature- and pHResponsive Micelles with Collapsible Poly(N-isopropylacrylamide) Headgroups. Langmuir 2014, 30 (27), 7986-7992. 59. Deng, H. Z.; Liu, J. J.; Zhao, X. F.; Zhang, Y. M.; Liu, J. F.; Xu, S. X.; Deng, L. D.; Dong, A. J.; Zhang, J. H., PEG-b-PCL Copolymer Micelles with the Ability of pH-Controlled Negative-to-Positive Charge Reversal for Intracellular Delivery of Doxorubicin. Biomacromolecules 2014, 15 (11), 42814292.


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