Letter pubs.acs.org/macroletters
Effects of Tailored Dispersity on the Self-Assembly of Dimethylsiloxane−Methyl Methacrylate Block Co-Oligomers Bernd Oschmann,† Jimmy Lawrence,† Morgan W. Schulze,† Jing M. Ren,†,∥ Athina Anastasaki,† Yingdong Luo,# Mitchell D. Nothling,†,∥ Christian W. Pester,† Kris T. Delaney,† Luke A. Connal,∥ Alaina J. McGrath,† Paul G. Clark,⊥ Christopher M. Bates,*,‡,§ and Craig J. Hawker*,†,‡ †
Materials Research Laboratory, ‡Department of Materials, and §Department of Chemical Engineering, University of California, Santa Barbara, California 93106, United States # Center for Nanophase Materials Sciences, Oak Ridge National Laboratories, Oak Ridge, Tennessee 37831, United States ∥ Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, Australia ⊥ The Dow Chemical Company, Midland, Michigan 48667, United States S Supporting Information *
ABSTRACT: The effect of dispersity on block polymer selfassembly was studied in the monodisperse limit using a combination of synthetic chemistry, matrix-assisted laser desorption ionization spectroscopy, and small-angle X-ray scattering. Oligo(methyl methacrylate) (oligoMMA) and oligo(dimethylsiloxane) (oligoDMS) homopolymers were synthesized by conventional polymerization techniques and purified to generate an array of discrete, semidiscrete, and disperse building blocks. Coupling reactions afforded oligo(DMS−MMA) block polymers with precisely tailored molar mass distributions spanning single molecular systems (Đ = 1.0) to low-dispersity mixtures (Đ ≈ 1.05). Discrete materials exhibit a pronounced decrease in domain spacing and sharper scattering reflections relative to disperse analogues. The order−disorder transition temperature (TODT) also decreases with increasing dispersity, suggesting stabilization of the disordered phase, presumably due to the strengthening of composition fluctuations at the low molar masses investigated.
T
of truly discrete oligomers to study the influence of dispersity on AB diblock copolymer self-assembly. A complementary study on discrete diblocks using alternative block chemistries from the Meijer group accompanies this work.13 One synthetic route toward discrete block polymers is an iterative pathway that involves successive monomer coupling and protecting group removal steps. In a pioneering work, Meijer and colleagues recently demonstrated that this strategy is indeed tractable and for the first time reported AB diblock copolymers with Đ = 1.0.14 To complement this approach, we have recently reported the facile purification of macromolecules derived from controlled polymerizations (e.g., anionic, ATRP, RAFT, and NMP),15−19 isolating discrete oligomers in high yields with automated flash chromatography.20 Here, we couple these oligomers to form block co-oligomers (BCOs) with tailored molar mass and composition dispersity. Three types of distributions with the same Mn but different breadths were prepared: discrete (Đ = 1), semidiscrete (Đ ≈ 1.001), and disperse (Đ ≈ 1.03). (Note that our present definition of “disperse” would by virtually all other accounts be considered
raditional synthetic polymerizations generate materials with molar mass and composition dispersity. Despite this historically unavoidable lack of precision, polymers with controlled molecular structure (molar mass, dispersity, architecture) have been successfully implemented in a myriad of advanced technologies. A quintessential example is AB diblock copolymers, which typify a versatile class of selfassembling molecules comprising chemically distinct sequences of monomers. Over five decades of theoretical developments have culminated in remarkably successful statistical treatments that can account for the geometry, dimensions, and most ordered-state symmetries observed with block polymers using only three universal parameters: block−block incompatibility (interaction parameter(s) χij), average overall degree of polymerization (N), and relative volume fractions ( f i).1 Notably, the theories underpinning block polymer selfassembly are often predicated on the assumption of perfectly monodisperse molar mass distributions (Đ = 1) and molecular composition, in contrast to real systems. While a number of experimental2−7 and theoretical8−12 studies have together probed the effect of realistic dispersity (Đ > 1) on attendant properties, the behavior as Đ → 1.0 remains empirically unexplored due to synthetic challenges. Herein, we leverage recent advances in polymer chemistry that enable the isolation © XXXX American Chemical Society
Received: April 8, 2017 Accepted: June 8, 2017
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DOI: 10.1021/acsmacrolett.7b00262 ACS Macro Lett. 2017, 6, 668−673
Letter
ACS Macro Letters
Figure 1. (a) Synthesis of block co-oligomers via click chemistry using α-alkynyl-oligoMMA (M) and ω-azido-oligoDMS (D) as building blocks. (b) Left: MALDI-MS analysis of discrete (no asterisk) and semidiscrete (*) oligomers obtained after chromatographic separation. Right: corresponding MALDI-MS data of the resulting block co-oligomers.
available ω-hydroxy-polydimethylsiloxane (Mn = 1000 g/mol) using a reversed-phase C18 column cartridge and methanol/ hexane eluent as described in the Supporting Information (SI). Unlike the aforementioned synthesis of oligo(MMA) derivatives, the anionic, ring-opening polymerization of hexamethylcyclotrisiloxane produces a mixture in which each species is separated by three siloxane repeat units, facilitating separations of pure unimolecular materials at substantially larger N as confirmed by MALDI-MS. After separation and isolation of discrete ω-hydroxy-polydimethylsiloxane, the hydroxyl chain end was converted to a terminal azide through a mesylated intermediate (Figure S3a,b) to produce a discrete series of 10-, 13-, 16-, and 19-mer segments for block co-oligomer synthesis. The discrete ω-azido-oligoDMS building blocks were purified by automated chromatography (see SI), resulting in high purity as confirmed by MALDI-MS analysis (Figure 1b) and 1H NMR (Figure S3). The versatility of this approach allows tailored mixtures to be prepared simply by combining discrete oligoDMS species. For example, a semidiscrete oligoDMS material with an average N = 16 and Đ = 1.001 was obtained by mixing six different discrete oligoDMS species (Figure S4). Ligation of oligoMMA and oligoDMS via Cu-mediated “click” chemistry results in a triazole linker between the blocks. Successful coupling was verified by the concomitant appearance of a triazole proton resonance (7.70 ppm) and disappearance of the oligoMMA alkyne resonance (2.45 ppm) using 1H NMR spectroscopy (Figure 2a and Figure S5). MALDI-MS spectra exhibit a clear shift toward larger molecular ions (Figure 1b) in agreement with conventional SEC analysis (Figure 2b), where the BCOs emerge at smaller elution times than either constituent block individually. Although SEC lacks sufficient resolution to accurately determine the dispersity of BCOs, peak maxima of semidiscrete and disperse samples with the same number-average degree of polymerization consistently overlay; progressively larger breadths are attributable to band broadening along the size exclusion column during analysis (Figure S6). Coupling well-defined oligomers thus enables precise
“monodisperse”.) Dimethylsiloxane (DMS) and methyl methacrylate (MMA) were selected as building blocks since previous reports have established that these two constituents are highly incompatible, permitting the formation of microphase-separated structures at low N.21 Small-angle X-ray scattering (SAXS) reveals subtle but consistent differences in the selfassembly of discrete and disperse materials. We find that dispersity influences ordered state stability by suppressing TODT, presumably due to composition fluctuations enhanced at small N. BCOs were synthesized via “click” couplings of discrete, semidiscrete, and disperse oligomers bearing azide and alkyne end groups. An alkyne-containing α-bromoester atom-transfer radical polymerization (ATRP) initiator was used in the synthesis of oligoMMA to quantitatively introduce the terminal functionality. This controlled polymerization procedure generated disperse oligoMMA (Đ = 1.2) with number-average N = 19 (abbreviated M*19*, where the ** indicates disperse; Mn ≈ 2000 g/mol) as determined by size exclusion chromatography (SEC) and 1H nuclear magnetic resonance spectroscopy (NMR), respectively (Figure S1). Discrete 6-mer and 7-mer oligoMMAs (M6 and M7, where no asterisk designates a discrete material) were isolated from the parent M*19* with an optimized acetonitrile/toluene solvent gradient. Separations targeting higher degrees of polymerization (i.e., N > 14) produced mixtures containing a small but measurable distribution of molar masses denoted “semi-discrete” with a single asterisk (e.g., M*14).20 Molar masses extracted from MALDI-MS are in agreement with those calculated by 1H NMR end group analysis, and from the relative peak intensities, exceptionally narrow dispersities were achieved with these semidiscrete materials (ĐMALDI < 1.006) (Figure 1). Interestingly, the discrete oligoMMA derivatives showed well-resolved 1 H NMR backbone resonances (Figure S2), in stark comparison to the broad overlapping peaks observed for the disperse starting material. Discrete siloxane-based oligomers, oligoDMS, were similarly obtained through chromatographic separation of commercially 669
DOI: 10.1021/acsmacrolett.7b00262 ACS Macro Lett. 2017, 6, 668−673
Letter
ACS Macro Letters
oligoMMA derivatives with larger discrete oligoDMS blocks produced samples of asymmetric composition (D19M6 and D10M7 with f D ≥ 0.57) that are disordered in the melt due to low overall N values. In contrast, all compositionally symmetric BCOs (0.44 ≤ f D ≤ 0.56) form an ordered morphology regardless of dispersity; however, significant differences between semidiscrete and disperse samples are apparent. Discreteness results in sharper first-order scattering maxima with decreased full widths at half-maximum (Figures 3 and S7
Figure 3. SAXS profiles for a series of semidiscrete and disperse homologues.
Figure 2. (a) 1H NMR verifies the conversion of alkyne and azide end * -alkyne groups to the triazole ring. (b) SEC of D16-azide (red), M14 * (black). (blue), and D16M14
and Table S7), indicating either enhanced long-range order or sharper block−block interfaces. Samples generally exhibited SAXS reflections at (q/q*) = 1, 2, and 3, consistent with a lamellar morphology, with the exception of the samples D16M14 * , D16 * M14 * , D16M14 **, and D16 * M14 **, in which a structure factor extinction at 2q* occurs as expected for f D ≈ 0.5. Interestingly, for this series, the (semi)discrete samples (D16M14 * , D16 * M14 * ) produce a higher-order 3q* reflection
control over block polymer composition and chain length, a characteristic unattainable with conventional synthetic routes. The self-assembly of each oligo(DMS−MMA) BCO listed in Table 1 was investigated using small-angle X-ray scattering (SAXS) to identify the equilibrium morphology and characteristic domain spacing (d = 2π/q*). At smaller oligomer sizes, BCOs formed from coupling discrete hexamer and heptamer Table 1. Summary of Oligo(DMS−MMA) Samples entry
samplea
Mn,NMRb
ĐD,MSc
1 2 3 4 5
D16 * M14 ** D16M14 ** D16M*19* D19M*14* D19M19 **
3.0 3.0 3.5 3.2 3.7
1.031
