A Tetragonal Phase Self-Organized from ... - ACS Publications

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A Tetragonal Phase Self-Organized from Unimolecular Spheres Assembled from a Substituted Poly(2-oxazoline) Marian N. Holerca,† Dipankar Sahoo,† Mihai Peterca,†,‡ Benjamin E. Partridge,† Paul A. Heiney,‡ and Virgil Percec*,† †

Roy & Diana Vagelos Laboratories, Department of Chemistry, and ‡Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States S Supporting Information *

ABSTRACT: Synthesis and living cationic ring-opening polymerization of a 2-oxazoline containing self-assembling minidendrons with hexadecyl groups are reported. Structural analysis using X-ray diffraction, molecular modeling, and reconstructed electron density maps revealed multiple selforganized periodic arrays, including columnar hexagonal P6mm, cubic Pm3̅n, and tetragonal P42/mnm phases, which have not been observed before in a single homopolymer. The degree of polymerization (DP) of the poly(2-oxazoline) programs the sequence of these periodic arrays. The cubic Pm3̅n phase was observed for DP = 5 whereas the tetragonal P42/mnm for 10 ≤ DP ≤ 50. For DP ≥ 75, only a columnar hexagonal P6mm phase was accessible because the polymer chains are too long to form a single supramolecular sphere. This sequence was rationalized by the columnar character of supramolecular spheres to provide the richest diversity of structures assembled from a single polymer and an unprecedented example of a tetragonal phase organized from macromolecular spheres comprising a single polymer chain.

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INTRODUCTION Quasi-equivalent building blocks are critical to the self-assembly of functional structures in biology.1−3 A classic example is the self-assembly of icosahedral viruses, in which a nucleic acid is coated with quasi-equivalent protein units.1,2 Polymers functionalized with self-assembling dendrons have been elaborated as mimics of biological quasi-equivalent assembly.3−5 Second-generation monodendrons with peripheral dodecyl alkyl chains appended to a poly(methacrylate) or poly(styrene) backbone adopted a conical conformation at low degrees of polymerization (DP), forming spheres which organized into a cubic Pm3̅n lattice.3,5 Polymers with higher DP adopted a hexagonal packing of columns with the polymer backbone at their cores jacketed by monodendrons with a flat-tapered conformation.3,5 Poly(2-oxazoline)s, also known as N-acyl-substituted poly(ethylenimine),6 were employed as models to generate 2D and 3D periodic arrays in the 1990s7−12 and have undergone a recent revival in interest due to their potential biomedical applications, as smart materials and for hierarchical selfassembly. 13−16 Vesicles assembled from AB and ABA amphiphilic block copolymers containing the water-soluble poly(2-methyloxazoline) in the hydrophilic segment(s) undergo directed protein insertion and act as functional mimics of biological membranes controlled by the composition of the block copolymers.17−23 Supramolecular structures can also be controlled via the length of the peripheral alkyl chain of the monodendrons, defined by n, the number of methylene units in © XXXX American Chemical Society

the chain. Dendronized poly(2-oxazoline)s with n = 13 generated columnar hexagonal arrays at all values of DP,24 whereas spheres were generated from polymers with n = 14 and 15 at DP ≤ 75 and DP ≤ 50, respectively.25 These spheres selforganized into cubic Pm3̅n lattices like those observed in dendronized poly(methacrylate)s and poly(styrene)s.3,5 Spherical assemblies of a poly(2-oxazoline) with n = 12 have also been reported to organize into a body-centered cubic (BCC) 26,27 Im3m ̅ structure. Structural and retrostructural analysis28,29 has discovered other 3D phases generated from spheres in self-assembling dendrons,30 including tetragonal P42/mnm,31 and 12-fold liquid quasicrystalline (LQC).32 These phases have also been transplanted to other forms of organized soft matter33,34 including block copolymers35−40 and lyotropic liquid crystals self-organized from surfactants.41−43 Whereas cubic Pm3̅n lattices organized from spheres comprising a single polymer chain have been observed in poly(methacrylate)s,3,5 poly(styrene)s,3,5 and poly(oxazoline)s,24,25 and tetragonal P42/ mnm phases from poly(oxazoline)s have been mentioned,31 no examples of these spheres organizing into a tetragonal P42/ mnm phase have been reported. Here we report a poly(oxazoline) functionalized with monodendrons with n = 16 that self-organizes into a tetragonal Received: October 22, 2016 Revised: November 30, 2016

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DOI: 10.1021/acs.macromol.6b02298 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Synthesis of (3,4)16G1-Oxz. 3,4-Bis(n-hexadecane-1-yloxy)benzoic acid (1) was synthesized by standard techniques used in our laboratories.47 3,4-Bis(n-hexadecane-1-yloxy)benzoyl Chloride (2). Compound 1 (21 g, 34 mmol) was suspended in CH2Cl2 (200 mL), and a catalytic amount of DMF was added. SOCl2 (40 mL, excess) was added dropwise, and the mixture was heated to reflux for 0.5 h. After cooling to room temperature, the solvent and the excess SOCl2 were distilled to yield 21.5 g (99.9%) of a white powder which was used without further purification. 1H NMR (CDCl3, δ ppm, TMS): 0.89 (overlapped t, 6H, CH3), 1.25−1.65 (m, 52H, CH3(CH2)13), 1.79 (m, 4H, CH2CH2OAr), 4.04 (overlapped t, 4H, CH2OAr), 6.92 (d, 1H, J = 9.2 Hz, meta to COCl), 7.56 (d, 1H, J = 1.8 Hz, ortho to COCl), 7.71 (dd, 1H, J = 9.2 Hz, J = 1.8 Hz, ortho to COCl). N-[3,4-Bis(n-hexadecane-1-yloxy)benzoyl]-2-aminoethanol (3). Compound 2 (20.8 g, 33 mmol) was dissolved in CH2Cl2 (125 mL) and slowly added to ice-cooled ethanolamine (25 mL, excess) under vigorous stirring. The mixture was stirred at 0 °C for 1 h and was then heated to 40 °C whereupon it was stirred for another 5 h. The solution was poured into a separatory funnel and washed three times with H2O, dried over MgSO4, and filtered. The solvent was removed by rotary evaporation, and the crude product was recrystallized twice from acetone at 0 °C to yield 17.8 g (82%) of a white powder. Purity (HPLC): 99+%. TLC (1:1 hexanes:EtOAc): Rf = 0.21. 1H NMR (CDCl3, δ ppm, TMS): 0.89 (t, 6H, CH3, J = 6.3 Hz), 1.25−1.69 (overlapped peaks, 52H, CH3(CH2)13), 1.86 (m, 4H, CH2CH2OAr), 3.2 (bs, 1H, OH), 3.62 (q, 2H, CH2OH, J = 5.0 Hz), 3.83 (t, 2H, NHCH2, J = 5.1 Hz), 4.02 (overlapped t, 4H, CH2OAr), 6.60 (bs, 1H, NH), 6.88 (d, 1H, meta to CONH, J = 8.2 Hz), 7.28 (dd, 1H, ortho to CONH, J = 8.2 Hz, J = 2.4 Hz), 7.40 (d, 1H, ortho to CONH, J = 2.4 Hz). 13C NMR (CDCl3, δ ppm): 13.8 (CH3), 22.4 (CH3CH2), 26.0 (CH2CH2CH2O), 29.0, 29.1, 29.2, 29.3, 29.5 (CH3CH2CH2(CH2)10), 30.2 (CH2CH2O), 31.8 (CH3CH2CH2), 42.7 (NHCH2), 62.4 (CH2OH), 69.1, 69.5 (CH2OAr), 112.5, 112.7 (meta to CONH, ortho to CONH and O), 119.7 (ortho to CONH), 126.3 (ipso to CONH), 148.6, 152.0 (meta to CONH, ipso to O and para to CONH), 168.2 (CONH). 2-[3,4-Bis(n-hexadecane-1-yloxy)phenyl]-2-oxazoline, (3,4)16G1Oxz. Compound 3 (20 g, 31 mmol) was dissolved in CH2Cl2 (750 mL), and SOCl2 (6.56 mL, 0.09 mol) was added dropwise at 23 °C. The mixture was stirred for 15 min, after which the 1H NMR indicated complete conversion of the starting material. The reaction was neutralized by addition of NaHCO3 solution (sat. aq, 750 mL) and vigorous stirring for 1 h. The aqueous layer was discarded, and the organic layer was washed three times with water (300 mL), then dried over MgSO4, and filtered. The solvent was distilled, and the product was recrystallized twice from hexanes, purified on column chromatography (SiO2, hexanes:EtOAc 5:1) and recrystallized again in acetone to yield 15.54 g (80%) of a white solid. Purity (HPLC): 99+%. TLC (CHCl3): Rf = 0.50; mp 65−67 °C. 1H NMR (CDCl3, δ ppm, TMS): 0.88 (t, 6H, CH3, J = 6.4 Hz), 1.25−1.70 (m, 44H, CH3(CH2)13), 1.78 (m, 4H, CH 2 CH 2 OAr), 4.02 (overlapped t, 6H, CH 2 OAr, OCH2CH2N), 4.42 (t, 2H, OCH2CH2N, J = 9.2 Hz), 6.87 (d, 1H, meta to CON, J = 9.2 Hz), 7.48 (d, 1H, ortho to CON, J = 2.6 Hz) 7.48 (dd, 1H, ortho to CON, J = 9.2 Hz, J = 2.6 Hz). 13C NMR (CDCl3, δ ppm): 14.2 (CH3), 22.3 (CH3CH2), 26.2 (CH2CH2CH2O), 29.2, 29.4, 29.7 (CH3CH2CH2(CH2)10), 30.2 (CH2CH2O), 31.9 (CH 3 CH 2 CH 2 ), 54.9 (NCH 2 ), 67.8 (OCNCH 2 ), 69.3, 69.5 (CH2OAr), 112.6, 113.1 (meta to OCN, ortho to OCN), 119.8 (ipso to CON) 121.7 (ortho to OCN), 148.9 (meta and para to CON), 164.8 (OC = N). Anal. Calcd for C41H73O3N: C 78.41, H 11.72. Found: C 78.55, H 11.61. Polymerization of (3,4)16G1-Oxz. A Schlenk tube equipped with a magnetic stirrer was dried at 200 °C overnight and charged with monomer (3,4)16G1-Oxz (0.313 g, 0.5 mmol), which was dissolved in a minimum amount of dry benzene and freeze-dried in the septumsealed Schlenk tube. After freeze-drying, the Schlenk tube was flushed with Ar. The initiator, MeOTf, was added via syringe according to the desired theoretical DP, and the tube was brought on a preheated oil bath at 160 °C. The melt was stirred until 1H NMR indicated the

P42/mnm array. At DP < 50, multiple polymer chains assemble into supramolecular spheres that organize into cubic Pm3̅n and tetragonal P42/mnm structures. Polymers with DP = 50 generate a tetragonal P42/mnm array formed from unimolecular supramolecular spheres. At higher DP (DP ≥ 75), only columnar structures are accessible.

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EXPERIMENTAL SECTION

Materials. THF (Fisher, ACS reagent) was refluxed over sodium ketyl and freshly distilled before use. CH2Cl2 (Fisher, ACS reagent) was refluxed over CaH2 and freshly distilled before use. Benzene (both Fisher, ACS reagents) was shaken with concentrated H2SO4, washed twice with water, dried over MgSO4, and finally distilled over sodium ketyl. H2SO4, dimethylformamide (DMF), KOH, and NaHCO3 (all Fisher, ACS reagents) and SOCl2 (97%) and ethanolamine (99%) (all Lancaster) were used as received. Methyl trifluoromethanesulfonate (MeOTf) (Fluka, 97%) was vacuum-distilled. Methods. The purity and the structural identity of the intermediary and final products were assessed by a combination of techniques that includes thin-layer chromatography (TLC), high pressure liquid chromatography (HPLC), 1H and 13C NMR, and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. TLC was carried out on precoated glass plates (silica gel with F254 indicator; layer thickness, 200 μm; particle size, 2−25 μm; pore size, 60 Å, from Sigma-Aldrich). HPLC was carried out using PerkinElmer Series 10 high pressure liquid chromatography with a LC100 column oven, Nelson Analytical 900 series integrator data station, and two PerkinElmer PL gel columns, 5 × 102 Å and 1 × 104 Å. THF was used as solvent at an oven temperature of 40 °C. UV absorbance at 254 nm was used as detector. Relative weight-average (Mw) and number-average (Mn) molecular weights were determined with the same instrument from a calibration plot constructed from polystyrene standards. 1H NMR (500 MHz) and 13C NMR (126 MHz) spectra were recorded on a Bruker DRX 500 instrument using the solvent indicated. Differential Scanning Calorimetry (DSC). Thermal transitions were measured on PerkinElmer DSC-7 differential scanning calorimeter (DSC). The heating and cooling rates were 10 °C/min. The transition temperatures were measured as the maxima and minima of their endothermic and exothermic peaks. Glass transition temperatures (Tg) were read at the middle of the change in heat capacity. Indium and zinc were used as calibration standards. An Olympus BX-40 optical polarized microscope (100× magnification) equipped with a Metler Toledo FP82HT hot stage and a Metler Toledo FP80 central processor was used to verify thermal transitions and to characterize anisotropic textures. X-ray Diffraction (XRD). X-ray diffraction (XRD) measurements were performed using Cu Kα1 radiation (λ = 1.542 Å) from a BrukerNonius FR-591 rotating anode X-ray source equipped with a 0.2 × 0.2 mm2 filament and operated at 3.4 kW. Osmic Max-Flux optics and triple pinhole collimation were used to obtain a highly collimated beam with a 0.3 × 0.3 mm2 spot on a Bruker-AXS Hi-Star multiwire area detector. To minimize attenuation and background scattering, an integral vacuum was maintained along the length of the flight tube and within the sample chamber. Samples were held in glass capillaries (1.0 mm in diameter), mounted in a temperature-controlled oven (temperature precision: ±0.1 °C, temperature range from −10 to 210 °C). Aligned samples for fiber XRD experiments were prepared using a custom-made extrusion device.44 The powdered sample (∼10 mg) was heated inside the extrusion device. After slow cooling, the fiber was extruded in the liquid crystal phase and cooled to 23 °C. Typically, the aligned samples have a thickness of 0.3−0.7 mm and a length of 3−7 mm. All XRD measurements were done with the aligned sample axis perpendicular to the beam direction. Primary XRD analysis was performed using Datasqueeze (version 3.0.7).45 Molecular Modeling and Simulation. Molecular modeling and simulation experiments were performed using Material Studio (version 5.0) software from Accelrys. Details were reported previously.46 B


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Macromolecules Scheme 1. Synthesis of (3,4)16G1-Oxza

Reagents and conditions: (i) SOCl2, cat. DMF, CH2Cl2, reflux, 0.5 h; (ii) HN(CH2)2OH, CH2Cl2, 0 °C, 1 h then 40 °C, 5 h; (iii) SOCl2, CH2Cl2, 23 °C, 15 min; (iv) sat. aq NaHCO3, 23 °C, 1 h.

a

complete conversion of the monomer. The melt was allowed to cool to RT, whereupon it was dissolved in THF (2 mL) and end-capped by the addition of KOH solution (25% aq w/w, 2 mL) followed by stirring for 1 h. The solid polymer was isolated by filtration.

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RESULTS AND DISCUSSION Synthesis of Poly[(3,4)16G1-Oxz]. Poly[(3,4)16G1-Oxz] was prepared via living cationic ring-opening polymerization of a monodendritic 2-oxazoline monomer (Scheme 1) synthesized according to reported procedures (Scheme 2).48 PolymerScheme 2. Polymerization of Monodendritic 2-Oxazolines (3,4)16G1-Oxz into DPma

a

m = 5, 10, 20, 30, 40, 50, 75, 100, and 200 Figure 1. DSC traces of poly[(3,4)16G1-Oxz] recorded upon (a) first heating and (b) first cooling at a rate of 10 °C/min. Phases determined by XRD are indicated. Phase notation: k = undetermined crystalline phase; g = glassy phase; Φh = columnar hexagonal P6mm phase; Cub = cubic Pm3̅n phase; Tet = tetragonal P42/mnm phase; i = isotropic liquid.

ization in the bulk state at 160 °C and subsequent end-capping of the polymer chain ends with potassium hydroxide afforded nine polymers with degrees of polymerization (DP) ranging from DP = 5 to DP = 200, denoted DPm, where m is the number of monomer repeat units in the polymer chain. Thermal Analysis by DSC and Structural Analysis by XRD. The temperature of thermal phase transitions and their associated enthalpies were analyzed by differential scanning calorimetry (DSC) at 10 °C/min (Figure 1). Phases indicated in Figure 1 were determined by X-ray diffraction (XRD) to be discussed later. The DSC traces of all polymers (DPm, 5 ≤ m ≤ 200) show a large endothermic transition on first heating consistent with the melting of an undetermined crystalline phase, k. Upon further heating, a glass transition, followed by a 2D columnar hexagonal phase, Φh, was observed. The crystallization (Tk) and glass transition (Tg) temperatures are mostly invariant to DP, and in all polymers, Tg > Tk. This sequence of transitions involving crystallization at a lower temperature than the glass transition (Figure 1b) was elaborated by our laboratory49 to decouple the motion of the side groups from that of the backbone in side chain liquid crystalline polymers. Microphase segregation25,49,50 of the polymer backbone from the side groups provides, for example, two glass transition temperatures: one arising from independent motion of the polymer backbone and the other from the

cooperative but independent motion of the side groups. This decoupled motion of the polymer backbone and aliphatic periphery and microphase segregation within the organized arrays have been reported also for poly(2-oxazoline)s.24,25 For DP75, DP100, and DP200, columnar hexagonal Φh is the only ordered array observed above Tg. In contrast, DPm, 5 ≤ m ≤ 50, exhibit an additional 3D phase organized from spheres: DP5 generates a cubic Pm3̅n (Cub) structure while DP10, DP20, DP30, DP40, and DP50 generate a tetragonal P42/mnm (Tet) structure. XRD analysis to be discussed later demonstrates that the Tet array in DP50 is generated from supramolecular spheres comprising a single DP50 molecule. Thermal analysis of poly[(3,4)16G1-Oxz] is summarized in Table 1. The enthalpy changes associated with phase transitions reflect the change of the degree of order of the assemblies of molecules in between the two phases. Hence, large enthalpy changes are observed at the transition from the 3D crystalline k phase to an isotropic liquid or glassy state and also upon melting the 2D Φh phase of DP = 75, 100, and 200 to an isotropic liquid. In contrast, transitions between ordered C


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Macromolecules Table 1. Theoretical and Experimental Molecular Weights Determined by GPC and Thermal Phase Behavior of Poly[(3,4)16G1-Oxz] thermal transitions (°C) and corresponding enthalpy changes (kcal/mol)a,b DP5 DP10 DP20 DP30 DP40 DP50 DP75 DP100 DP200

Mn,th (g/mol)

Mn,GPC (g/mol)

Mw/Mn

3229 6434 12704 18794 25244 31514 47189 62864 136764

4452 7608 10624 14740 18577 19501 25087 23972 23810

1.23 1.20 1.14 1.12 1.09 1.08 1.08 1.08 1.10

heating k k k k k k k k k

45 32 40 33 33 33 43 44 52

(4.23) (6.17) (6.68) (6.18) (5.86) (5.74) (5.67) (5.83) (6.12)

g 63 Φh 80 (0.22) Cub 114 (0.06) i g 66 Φh 80−85b Tet 122 (0.10) i g 72 Φh 85−90 Tet 128 (0.11) i g 68−72 Φh 75−85 Tet 116 (0.02) i Φh 86 (0.05) Tet 105−115 i g 73 Φh 92 (0.14) Tet 100−110 i g 74 Φh 105 (0.15) i g 80 Φh 110 (0.17) i g 84 Φh 119 (0.26) i

cooling i i i i i i i i i

80 (−0.21) Cub * Φh 54 g 33 (−4.40) k 102 (−0.04) Tet * Φh 26 (−5.44) k 95−105 Tet 65−75 Φh 33 (−5.95) k 95−105 Tet 65−75 Φh 25 (−5.99) k 95−105 Tet 65−75 Φh 25 (−5.23) k 95−105 Tet 74 (−0.03) Φh 26 (−5.34) k 81 (−0.12) Φh 32 (−5.27) k 88 (−0.15) Φh 32 (−5.41) k 97 (−0.25) Φh 39 (−5.84) k

Single values are obtained from DSC data from first heating and cooling scans at a rate of 10 °C/min. bRanges of values are estimated transition temperatures obtained from XRD experiments. Exact transition temperature could not be determined by DSC. All phases confirmed by XRD. Phase notation: k = undetermined crystalline phase; g = glassy phase; Φh = columnar hexagonal P6mm phase; Cub = cubic Pm3n̅ phase; Tet = tetragonal P42/mnm phase; i = isotropic liquid. a

Figure 2. (a, b) VWAXS XRD pattern of the Φh phase of (a) DP5 and (b) DP10 collected at 70 °C. The intensity of the top half of the pattern has been reduced to demonstrate the absence of tilt correlation features. (c) Comparison of simulated and experimental WAXS XRD patterns of the Φh phase of DP5. The simulated pattern has been calculated using the model shown in (e−i). Fiber axis and lattice parameters are indicated. (d) Azimuthal plots of the 4.6 Å stacking feature in (a, b) illustrating the absence of tilt correlation features for DP5 and DP10. (e) Monomeric unit of poly[(3,4)16G1-Oxz]. (f−h) Molecular models of DP5 showing (f) full molecules, (g) polymer backbone, and (h) polymer backbone and aromatic rings of the dendrons. (i) Supramolecular column of DP5. Peripheral alkyl chains omitted for clarity. (j) Schematic representation of the Φh phase. (k) Molecular model of DP10. (l−n) Supramolecular column of DP10 showing (l) side view with peripheral alkyl chains omitted for clarity, (m) side view, and (n) top view. (o) Supramolecular columns packed into a hexagonal array. Yellow lines denote unit cells. Color code used in the model: O atoms, red; H atoms, white; N atoms, blue; C atoms in the core, green; C atoms in the phenyl rings, orange; all other C atoms, gray.

phases, such as between the 2D Φh and 3D Tet phases in DP = 10, 20, and 30, may be accompanied by very small or even negative enthalpies seen as exothermic transitions. In these instances, transition temperatures could not be accurately determined by DSC. Therefore, in these cases estimated

transition temperatures determined by XRD are provided as ranges of values in Table 1. Retrostructural Analysis of Columnar Hexagonal Phases by XRD and Molecular Modeling. The structures of the periodic arrays generated by poly[(3,4)16G1-Oxz] were D


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Macromolecules

Figure 3. (a, b) VWAXS XRD patterns of the Φh phase of (a) DP75 and (b) DP200 collected at 80 °C. The intensity of the top half of the pattern has been reduced to demonstrate the presence of tilt correlation features. (c) Comparison of simulated and experimental WAXS XRD patterns of the Φh phase of DP200. The simulated pattern has been calculated using the model shown in (e−i). Fiber axis and lattice parameters are indicated. The simulation does not incorporate the distribution of fiber orientations which smears out the (hk1) reflections. (d) Azimuthal plots of the 4.6 Å stacking feature in (a, b) illustrating tilt correlation features of ∼22° for DP75 and DP200. (e−i) Representative models of the supramolecular columns generated from DPm, 20 ≤ m ≤ 200: (e) polymer backbone, side view; (f, g) polymer backbone and aromatic rings of the dendrons, (f) top view and (g) side view; (h, i) full molecules including peripheral alkyl chains, (h) side view and (i) top view. Color code used in the model: O atoms, red; H atoms, white; N atoms, blue; C atoms in the core, green; C atoms in the phenyl rings, orange; all other C atoms, gray.

determined via a combination of XRD measurements of extruded oriented fibers, molecular modeling, and simulation of XRD patterns. Three different sample-to-detector distances were used to probe structural features on different length scales: very wide-angle X-ray scattering (VWAXS) patterns were recorded with a sample-to-detector distance of 7 cm, measuring d-spacings in the range of 2.6 ≤ d ≤ 40 Å; wide-angle X-ray scattering (WAXS) patterns were recorded at 11 cm (3.9 ≤ d ≤ 60 Å); and intermediate angle X-ray scattering (IAXS) patterns were recorded at 54 cm (17 ≤ d ≤ 300 Å). As noted earlier, all samples of poly[(3,4)16G1-Oxz] investigated here (DPm, 5 ≤ m ≤ 200) exhibit a columnar hexagonal Φh phase. Figure 2 shows XRD patterns and molecular models for the columns of the Φh phase of DP5 and DP10. Equatorial features on the VWAXS patterns of DP5 and DP10 (Figures 2a and 3b, respectively) are consistent with a hexagonal packing of columns with diameters (Dcol = a) of 46.5 and 45.9 Å, respectively. There is also a strong meridional feature on both patterns denoting a 4.6 Å stacking distance between repeat units of the supramolecular column. Azimuthal plots (Figure 2d) demonstrate that each of these 4.6 Å features is a single peak without tilt correlation features,51 suggesting that there is no tilt of the stacked unit relative to the column axis. A model consistent with these observations from XRD is shown in Figure 2e−o. The flat-tapered conformation of the dendrons assembles into disks which stack into supramolecular columns (Figure 2e−j). A single molecule generates each disklike column stratum for DP5 (Figure 2f), whereas a molecule of DP10 spans two column strata (Figure 2k). The

columns thus formed (Figure 2m,n) self-organize into a columnar hexagonal Φh array (Figure 2o). XRD analysis of the Φh phase of DPm, 20 ≤ m ≤ 200, indicates that all of these polymers adopt an identical structure in their supramolecular columns. Representative XRD patterns and a general molecular model for the columns of DPm, 20 ≤ m ≤ 200, are shown in Figure 3. The XRD patterns are very similar to those of DP5 and DP10 (Figure 2a,b) and suggest that there is a similar columnar hexagonal structure at all DPs. The diameters of the columns in DPm, 20 ≤ m ≤ 200, range from 42.7 to 45.8 Å, similar to those of DP5 and DP10 (46.5 and 45.9 Å). The longer polymer backbone in DPm, 20 ≤ m ≤ 200, adopts a helical conformation at the center of the supramolecular column (Figure 3e). A 4.6 Å stacking feature observed by XRD (Figure 3a,b) represents the distance between adjacent turns of the supramolecular helix. An azimuthal plot of the VWAXS patterns (Figure 3d) reveals that the 4.6 Å stacking feature is not a single peak, but also includes overlapped tilt correlation features.51 These tilt features indicate that the dendrons are tilted down from the plane of the column stratum by approximately 22°. A model consistent with XRD and experimental density is shown in Figure 3e−i and is very similar to the structure adopted by DP5 and DP10 (Figure 2e−o), with the addition of the ∼22° tilt revealed by XRD. Hence, the columns of the Φh phase of poly[(3,4)16G1-Oxz] with all DPs adopt almost identical structures in which a helical polymer backbone is jacketed by monodendrons with a flat-tapered conformation. Structural Analysis by XRD and Reconstructed Electron Density Maps of Cub (Pm3̅n) and Tet (P42/ mnm) Phases. In addition to columnar hexagonal phases E


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Macromolecules

Figure 4. (a) Intermediate-angle X-ray scattering (IAXS) pattern of the Cub (Pm3n̅ ) phase of DP5 at 90 °C. Fiber axis and lattice parameters indicated. (b) Radial plot of IAXS pattern in (a). (c) Schematic unit cell of Cub. (d, e) Reconstructed electron density maps of the unit cell of the Cub phase: (d) perspective view; (e) z = 0 plane. (f) IAXS pattern of the Tet (P42/mnm) phase of DP20 at 100 °C. Fiber axis and lattice parameters indicated. (g) Radial plot of IAXS pattern in (f). (h) Schematic unit cell of Tet. (i, j) Reconstructed electron density maps of the unit cell of the Tet phase: (i) perspective view; (j) z = 0 plane. In (c, h), spheres connected by continuous lines exhibit columnar character.

Table 2. Structural Analysis of Poly[(3,4)16G1-Oxz] by XRD cell parameterb sample DP5 DP10 DP20 DP30 DP40 DP50 DP75 DP100 DP200

T (°C)

phasea

a (Å)

c (Å)

70 90 70 115 80 120 80 100 80 115 80 110 80 80 80

Φh Cub Φh Tet Φh Tet Φh Tet Φh Tet Φh Tet Φh Φh Φh

45.8 87.8 44.6 151.3 45.8 152.2 42.7 151.2 44.8 150.4 43.7 151.3 42.8 43.7 43.0

− − − 82.4 − 81.3 − 82.3 − 81.1 − 82.0 − − −

d10, d11, d20, d21 (Å)c (Φh); d200, d210, d211 (Å)d (Cub); d002, d410, d330, d411, d312 (Å)e (Tet) 40.2, 43.9, 39.1, 41.2, 39.6, 40.6, 37.0, 41.1, 38.8, 40.6, 37.8, 41.0, 37.6, 37.8, 37.7,

22.8, 19.4, 15.1 39.3, 35.9 −, −, 14.2 36.7, 35.7, 33.5, −, −, − 36.9, 35.9, 33.6, −, −, − 36.6, 35.6, 33.5, −, −, − 36.5, 35.7, 33.3, −, −, − 36.7, 35.7, 33.6, 21.8, 18.5, 13.7 −, −, − 21.7, 18.6, 13.8

31.2 31.1 31.2 30.9 31.2

Dcolf (Å) (Φh); Dsphereg (Å) (Cub); Dsphereh (Å) (Tet) 45.8 54.5 44.6 49.3 45.8 49.3 42.7 49.5 44.8 48.9 43.7 49.3 42.8 43.7 43.0

a Phase notation: Φh = columnar hexagonal P6mm phase; Cub = cubic Pm3̅n phase; Tet = tetragonal P42/mnm phase. bCell parameters: a = (2/ √3)(d100 + √3d110 + 2d200)/3 for Φh; a = (2d200 + √5d210 + √6d211)/3 for Cub; a = (√17d410 + 3√2d330)/2 and c = 2d002 for Tet. cd-spacings for Φh calculated using dhkl = 1/(4h2 + k2 + hk)/(3a2) + l2/c2). dd-spacings for Cub calculated using dhkl = a/(h2 + k2 + l2)1/2. ed-spacings for Tet calculated using dhkl = 1/((h2 + k2)/a2 + l2/c2)1/2.53 fFor Φh, Dcol = a. gFor Cub, Dsphere = 2(3a3/32π)1/3. hFor Tet, Dsphere = 2(abc/40π)1/3.47

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Macromolecules Table 3. Structural Analysis of the Cub and Tet Phases of Poly[(3,4)16G1-Oxz] DP5 DP10 DP20 DP30 DP40 DP50

phase

Dspherea (Å)

ρb (g/cm3)

Mwtc (g/mol)

μcelld

μe

μ′f

Cub Tet Tet Tet Tet Tet

54.5 49.3 49.3 49.5 48.9 49.3

1.0 1.0 1.0 1.0 1.0 1.0

3229 6434 12704 18974 25244 31514

126.2 175.2 88.7 59.4 44.6 35.8

15.78 ∼ 16 5.84 ∼ 6 2.96 ∼ 3 1.98 ∼ 2 1.49 ∼ 1.5 1.19 ∼ 1

78.9 58.4 59.2 59.4 59.6 59.5

a Dsphere calculated as in Table 2. bExperimental density at 23 °C. Error ±5%. cTheoretical molecular weight. dNumber of molecules in the unit cell. For Cub μcell = NAρa3/Mwt, and for Tet μcell = NAρabc/Mwt, where NA is Avogadro’s number (6.022 × 1023 mol−1), ρ is the experimental density measured at 23 °C, a, b, c are lattice parameters, and Mwt is the theoretical molecular weight. eNumber of molecules in the supramolecular sphere. For Cub μ = μcell/8, and for Tet μ = μcell/30. fNumber of monodendritic units in the supramolecular sphere, calculated using μ′ = μ × m for DPm.

exhibited by poly[(3,4)16G1-Oxz] with all DPs, two 3D phases organized from spheres were also observed by XRD: cubic Pm3n̅ (Cub) for DP5 (Figure 4a,b) and tetragonal P42/mnm (Tet) for DPm, m = 10, 20, 30, 40, and 50 (Figure 4f,g). The XRD pattern for DP20 in the Tet phase presented in Figure 4f is representative of the tetragonal phase observed in DPm, m = 10, 20, 30, 40, and 50. The Cub and Tet lattices consist of eight and 30 supramolecular spheres, respectively (Figure 4c,h).31 Spheres connected by continuous lines in Figure 4c,h exhibit columnar character.30,31,52 Electron density maps reconstructed from XRD support the formation of supramolecular spheres by DPm, 5 ≤ m ≤ 50 (Figure 4d,e,i,j). Lattice parameters derived from XRD (Cub of DP5: a = b = c = 87.8 Å and representative Tet of DP20: a = 151.3 Å, b = c = 82.4 Å) indicate that the supramolecular spheres of DP5 in the Cub phase have a diameter of 54.5 Å, whereas those of DP10, 20, 30, 40, and 50 in the Tet phase have diameters of 48.9−49.5 Å. Structural parameters for poly[(3,4)16G1-Oxz] at all DPs are summarized in Table 2. Retrostructural Analysis and Molecular Modeling of Spheres of Poly[(3,4)16G1-Oxz]. Taking the experimental density of poly[(3,4)16G1-Oxz] at 23 °C (ρ = 1.0 g/cm3) with the lattice parameters derived from XRD and molecular weight of the polymer allows calculation of the number of polymer chains in the unit cell, μcell. It is more convenient to consider two related parameters: μ, the number of polymer chains per supramolecular sphere, calculated by dividing μcell by the number of spheres in the unit cell (8 for Cub and 30 for Tet), and μ′, the number of dendrons per supramolecular sphere, calculated by multiplying μ by the number of monomers per polymer chain, m. All three values (μcell, μ, μ′) are tabulated in Table 3. The number of polymer chains per supramolecular sphere, μ, is plotted as a function of DP in Figure 5. As expected from their XRD-derived diameters, the spheres of the Cub and Tet phases contain significantly different numbers of monodendritic units: for Cub, Dsphere = 54.5 Å and μ′ ≈ 80, while for Tet, Dsphere ≈ 49 Å and μ′ ≈ 60. The spheres of DP5, which organize into a Cub array, contain 80 dendrons and hence contain, on average, 16 molecules of DP5 per sphere. The agreement of the lattice parameters and of μ′ for the Tet phases of DP10, DP20, DP30, DP40, and DP50 suggests that the supramolecular spheres in all of these tetragonal phases are essentially identical and contain, on average, 60 monodendrons. This can also be seen from the plot in Figure 5, which shows that a sphere is expected to contain only one molecule (μ = 1) for DP = 60. These spheres can be generated from various sizes of polymer chains provided that the number of dendrons per spheres (μ = μ′ × m) is 60: for example, 6 molecules of DP10,

Figure 5. Plot of the number of polymer chains (μ) per supramolecular sphere as a function of the degree of polymerization (DP). The solid red line represents a reciprocal line of best fit (R2 = 0.99).

3 molecules of DP20, or 2 molecules of DP30. In line with previous studies, the polymer backbones of these molecules are expected to be segregated from the peripheral alkyl chains of the dendrons, although the degree to which this segregation occurs is not determined here. For larger sphere-forming molecules (DP40 and DP50), calculated values of μ would suggest a noninteger number of polymer chains: μ = 1.5 for DP40 and 1.2 for DP50. However, sharing a polymer chain between two (or more) spheres is disfavored, as doing so would disrupt microphase segregation between aliphatic and aromatic regions. Instead, the inherent distribution of molecular weights represented by the polydispersity25 of poly[(3,4)16G1-Oxz] (Table 2) provides a mechanism for the formation of 60-dendron spheres from samples of DP40 and DP50. Two (or more) imperfect molecules within a sample, for example, a DP32 molecule and a DP28 molecule, could self-assemble into a 60-dendron sphere. In addition, μ′ represents an average number of dendron units in a supramolecular sphere, and hence not all spheres within a single sample will necessarily contain exactly 60 dendrons. However, this polydispersity means that a sample of, for example, DP = 50, will contain a substantial number of polymer chains with DP > 60.24,25 Hence, there will be some molecules too long to form a macromolecular sphere, leading to a small proportion of columnar assemblies such as those observed by XRD for DP ≥ 75. A plot of the dependence of transition temperature determined by DSC (Figure 1) or estimated by XRD (Table 1) against DP (Figure 6) exhibits a maximum of the isotropization temperature (Ti) for DP = 20, with a G


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biphasic mixture or no observable Tet phase at all. For DP ≥ 75, the biphasic regime ends, the tetragonal phase is completely eliminated, and only columnar hexagonal arrays are observed. The stability of these columnar hexagonal arrays increases with DP, as indicated by their increasing Ti values. As observed in poly(methacrylate)s and poly(styrene)s,3,5 increasing DP beyond a certain threshold eliminates the generation of 3D phases organized from spheres and instead promotes the formation of columnar structures. This behavior is observed too in poly[(3,4)16G1-Oxz]: for DP ≤ 50, phases generated from spheres are observed, but for DP ≥ 75, only columnar hexagonal arrays are formed. This behavior arises from the geometric constraints of the polymer sphere. As the number of dendrons in a molecule approaches 60, the number of molecules forming the supramolecular sphere tends to one. For molecules with more than 60 dendrons, the molecules cannot pack into a single sphere without creating a hollow sphere, which is disfavored. An alternative assembly, in which the polymer chain forms a 60-dendron sphere and the excess length of the polymer hangs from the sphere like a tail, is also disfavored as it disrupts microphase segregation between aromatic and aliphatic components. Therefore, there is no suitable mechanism via which a molecule larger than DP60 can form a sphere. Instead, the most favorable structure is an elongated column in which the polymer backbone forms a helical core. Hence, only columnar hexagonal phases are observed for poly[(3,4)16G1-Oxz] molecules with DP > 60. Molecular models of the supramolecular spheres formed by DPm, m = 5, 10, 20, 30, 40, and 50, are shown in Figure 7. In all molecules, the dendrons on the polymer backbone adopt a

Figure 6. Dependence of transition temperatures of poly[(3,4)16G1Oxz] as a function of DP. Transition temperatures that could not be accurately determined by DSC and were estimated by XRD (Table 1) are shown with error bars.

subsequent decrease in Ti with increasing DP to DP = 50. This suggests the existence of a biphasic regime for DP > 20, in which there is a small but increasing proportion of polymer chains with DP > 60 which disrupt the formation of the Tet phase. XRD data are consistent with the presence of a small proportion of Φh in the Tet phase but cannot provide conclusive evidence due to the overlap of the diffraction features arising from the Φh and Tet phases (compare Figure S6b with S6d). Even for a sample with DP = 60, the small proportion of molecules with exactly 60 monomers in the polymer chain (few % of all chains)24,25 would result in a

Figure 7. Molecular models of the supramolecular spheres of DPm with m = 5, 10, 20, 30, 40, and 50. Top row: single molecule, side view. Second row: single molecule, viewed from the apex of the cone defined by the molecule or, for DP50, viewed from the top. Color code: O atoms, red; H atoms, white; N atoms, blue; C atoms in the core, green; C atoms in the phenyl rings, orange; all other C atoms, gray. Third row: schematic representation of single molecule. Bottom row: schematic representation of molecules packed into a supramolecular sphere. For clarity, the schematic representations depict absolute segregation between the polymer backbone (green) and aliphatic regions (red), which is unrealistic. Color code: green, polymer backbone; red, monodendrons; blue, surface of sphere. H


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Figure 8. Summary of hierarchical self-organization of poly[(3,4)16G1-Oxz] into cubic Pm3n̅ (Cub), tetragonal P42/mnm (Tet), and columnar hexagonal P6mm (Φh) phases as a function of DP.

conical conformation rather than the flat-tapered conformation observed in the columnar hexagonal Φh phase. DP5 forms a molecular cone, 16 of which assemble into the supramolecular spheres of the Cub phase. DP10 and DP20 also form cones which generate spheres in the Tet phase. Larger polymers, such as DP30 and DP40, occupy a hemisphere or larger portion of a sphere, and hence only two molecules assemble into one spherical object. Finally, a single molecule of DP50 generates a sphere. This is the first example of a tetragonal phase selforganized from spheres containing a single polymer chain. Previous dendronized poly(oxazoline)s with n = 14 and 15 also exhibited 3D phases generated from spheres at low DPs and columnar hexagonal arrays at higher DPs.25 However, in both series of molecules, only the Cub (Pm3̅n) phase generated from spheres was observed. Spheres generated from a single macromolecular dendron also organized into a cubic Pm3̅n lattice.54 In contrast, poly[(3,4)16G1-Oxz] exhibits two different 3D phases generated from spheres, which can be programmed via the DP. Previous work has shown that there is a high repulsion between spheres such as those generated by poly[(3,4)16G1-Oxz].31 This repulsion is lowered if alkyl chains can move or deform to accommodate a neighboring sphere.31,55 Indeed, the high proportion of close contact spheres in a Pm3̅n lattice (= 6/8 or 75%) is favored by deformable quasi-equivalent spheres, and hence the most commonly observed 3D phase generated from spheres is Pm3̅n,28,29,55,56 known also as Frank−Kasper A15. Deformation of the sphere requires reorganization of the dense aliphatic corona and coordinated movement of the polymer backbone at the center of the sphere. In a sphere of DP5, there are 16 molecules with short polymer backbones which can move and deform to allow close contact between spheres. However, at higher DP, this becomes more difficult as the backbone becomes longer and movement of the polymer backbones becomes more highly coordinated. Deformation of the sphere is therefore less favorable, and a lattice with fewer close contact

spheres, such as tetragonal P42/mnm (20/30 or 67% close contact spheres), 55 known also as Frank−Kasper σ phase,34,35,39,41 becomes more favored. Hence, increasing DP disfavors a cubic Pm3̅n arrangement and favors tetragonal P42/ mnm. Increasing n at a given DP (e.g., DP50 for poly[(3,4)nG1Oxz]) also seems to exhibit a similar trend (Φh for n = 13;24 Pm3n̅ for n = 14 and 15;25 and P42/mnm for n = 16), but the driving force for this dependence requires further experiments in order to be elucidated. The phases self-organized from poly[(3,4)16G1-Oxz] are schematically summarized in Figure 8.

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CONCLUSIONS

A tetragonal P42/mnm (Tet) phase self-organized from spheres containing a single polymer chain was discovered in a library of poly(2-oxazoline)s functionalized with a self-assembling minidendron. Structural analysis demonstrated the formation of cubic Pm3̅n (Cub), tetragonal P42/mnm (Tet), and columnar hexagonal P6mm (Φh) phases as a function of the degree of polymerization (DP) (Figure 8). This represents the richest diversity of periodic arrays self-organized from a single component polymer known to us. We propose that the transition from cubic to tetragonal phases is mediated by changes in the deformability or quasi-equivalence of the supramolecular spheres. It is anticipated that these results will provide principles for the assembly of additional molecular building blocks into previously unexpected 3D structures. The results reported here will also facilitate the merger of selfassembling dendrons and dendrimers28,29 with block copolymers,35−40,57−64 other self-assembling building blocks,33,34 and surfactants.41−43 I


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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.6b02298. X-ray diffraction data of DP5 (Φh and Cub phases), DP10, DP20, DP30, DP40, and DP50 (Φh and Tet phases), and DP75, DP100, and DP200 (Φh phase) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; tel +1-215-573-5527; fax +1215-573-7888 (V.P.). ORCID

Mihai Peterca: 0000-0002-7247-4008 Benjamin E. Partridge: 0000-0003-2359-1280 Virgil Percec: 0000-0001-5926-0489 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the National Science Foundation (DMR1066116 (V.P.) and DMR-1120901 (V.P. and P.A.H.)), the Humboldt Foundation (V.P.), and the P. Roy Vagelos Chair at Penn (V.P.) is gratefully acknowledged. B.E.P. thanks the Howard Hughes Medical Institute for an International Student Research Fellowship.

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REFERENCES

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