Article pubs.acs.org/Macromolecules
Chain Exchange Kinetics in Diblock Copolymer Micelles in Ionic Liquids: The Role of χ Yuanchi Ma† and Timothy P. Lodge*,†,‡ †
Department of Chemistry and ‡Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *
ABSTRACT: Chain exchange kinetics of diblock copolymer micelles with lower critical micellization temperature (LCMT) phase behavior were investigated using time-resolved smallangle neutron scattering (TR-SANS). Three nearly identical isotopically substituted pairs of poly(methyl methacrylate)block-poly(n-butyl methacrylate) (PMMA-b-PnBMA) diblocks were used in mixtures of the room temperature ionic liquids 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. In this case, the h-PnBMA and d9-PnBMA blocks form the micellar cores. The results are consistent with previous measurements in other systems, in that the barrier to chain extraction scales linearly with the core block length. By varying the ratio of the two homologous solvents in the mixture, the value of χ between the core block and the solvent is varied systematically. The results show that the solvent selectivity has a remarkable effect on the chain exchange rate, as anticipated by a previous theory. However, in contrast to an assumption in previous studies, we find that the barrier to chain exchange is not simply proportional to χ. Accordingly, we propose a more elaborate function of χ for the energy barrier, which is rationalized by a calculation in the spirit of Flory−Huggins theory. This modification can account for the chain exchange behavior when χ is relatively modest, i.e., in the vicinity of the critical micelle temperature.
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INTRODUCTION Block copolymers can self-assemble into various structures in selective solvents, such as spherical micelles, cylindrical micelles, and vesicles, of which the thermodynamic properties have been extensively studied.1−6 Despite this, relatively few studies have been conducted on the kinetics of molecular exchange among these micelles. Compared with their small molecular weight counterparts, diblock copolymer micelles have much slower chain exchange dynamics for two reasons: (i) the high incompatibility between the core block and the solvent, embodied in the interaction parameter χ; (ii) large numbers of repeating units in the core block, Ncore. Both factors contribute to the high activation barrier for the chain expulsion from the micelle and add to the difficulty of the experiments. Chain exchange kinetics between micelles have been studied using T-jump light scattering,7−11 nonradiative energy transfer spectroscopy and fluorescence quenching spectroscopy,12−18 and transmission electron microscopy;19−21 however, a general, quantitative picture has not emerged. Richter and co-workers22−28 used small-angle neutron scattering (SANS) to investigate chain exchange for poly(ethylene oxide)-blockpoly(ethylene-alt-propylene) (PEO-b-PEP) in water/N,Ndimethylformamide (DMF) mixtures and later reported a quasi-logarithmic time dependence for the overall chain exchange rate.23−25 Lodge, Bates, and co-workers systematically studied chain exchange of a series of poly(ethylene-alt© XXXX American Chemical Society
propylene)-block-poly(styrene) (PEP-b-PS) diblocks in squalane and elucidated the effect of Ncore29,30 and its dispersity (Đ),31 temperature,29−33 concentration,32and polymer architecture (using PS-b-PEP-b-PS and PEP-b-PS-b-PEP)33 on the micelle chain exchange kinetics. A numerical model29 was established that quantitatively explained the remarkably strong dependence on core block length and the apparent logarithmic decay, which was also consistent with previous analyses of chain diffusion in bulk sphere-forming block copolymers.34−37 Both experiment and simulation38,39 results on chain exchange kinetics in block copolymer micelles are still scarce and mainly focus on the two model systems mentioned above. In particular, the dependence of the barrier to chain extraction on χ has not been fully elucidated. Here, we report a TR-SANS study on a lower critical micellization temperature (LCMT) system: poly(methyl methacrylate)-block-poly(n-butyl methacrylate) (PMMA-b-PnBMA) in mixtures of the ionic liquids 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI)] and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]). PMMA is well soluble in both ILs, while PnBMA has lower critical solution temperature (LCST) behavior, where the critical Received: October 10, 2016 Revised: November 23, 2016
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DOI: 10.1021/acs.macromol.6b02212 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules Table 1. Characterization of PMMA-b-PnBMA Diblock Copolymers PMMA block Mn (kg/mol)a
(d)PnBMA block Mn (kg/mol)b
NPMMAc
N(d)PnBMAd
Đ
25 25 25 25 25 25
24 25 35 38 53 54
250 250 250 250 250 250
169 166 246 252 373 354
1.05 1.05 1.05 1.05 1.08 1.09
PMMA-b-PnBMA (25−24) PMMA-b-dPnBMA (25−25) PMMA-b-PnBMA (25−35) PMMA-b-dPnBMA (25−38) PMMA-b-PnBMA (25−53) PMMA-b-dPnBMA (25−54)
a Number-average molecular weight of the PMMA block was determined by light scattering detection during SEC, with 0.084 mL/g used as the dn/ dc of PMMA.51 bNumber-average molecular weight of (d)PnBMA block determined by 1H NMR spectroscopy. cDegree of polymerization of PMMA block. dDegree of polymerization of (d)PnBMA block.
Table 2. Scattering Length Densities and Volume Fractions for Contrast Matching material PMMA PnBMA dPnBMA 50/50 h-/d-PnBMA [EMIM][TFSI] d3-[EMIM][TFSI] [BMIM][TFSI] d3-[BMIM][TFSI]
mol wt of repeat unit (g/mol)
densitya (g/cm3)
coherent scattering length (10−12 cm)
scattering length density (1010 cm−2)
100 142 151
1.18 1.07 1.14
1.491 1.244 10.61
391 394 419 422
1.52 1.53 1.44 1.45
10.29 13.42 10.13 13.25
1.06 0.564 4.82 2.69 2.41 3.14 2.10 2.74
volume fractionb 0.5 0.5 0.62 0.38 0.08 0.92
a
Density of deuterated material was calculated assuming that the molar volume is identical to that of the protonated material. bVolume fraction under which the contrast-matching condition is achieved. Synthesis of d9-n-Butyl Methacrylate. Predistilled methacryloyl chloride (1.405 g, 13.4 mmol) and d10-n-butanol (1.147 g, 13.7 mmol, Cambridge Isotope Laboratories) were combined with 100 mL of methylene chloride in a 250 mL round-bottom flask. Following the addition of triethylamine (2.02 g, 20.0 mmol) and 4-(dimethylamino)pyridine (0.33 g, 2.7 mmol), the reaction was run for 12 h at room temperature. The resulting crude product was washed with 1 mol/L hydrochloric acid and 10% sodium bicarbonate aqueous solution and then collected twice via extraction with methylene chloride. The crude product was then purified by flash column chromatography to give the pure product (eluent: hexanes/ethyl acetate = 10/1). Synthesis of d3-Ionic Liquids. The protonated ionic liquids [EMIM][TFSI] and [BMIM][TFSI] were synthesized using a procedure adapted from the literature.49 The three hydrogens on the imidazole ring of these two ionic liquids were isotopically exchanged to give d3-[EMIM][TFSI] and d3-[BMIM][TFSI], respectively, according to an established method.50 All other reagents were purchased from Sigma-Aldrich and used as received unless otherwise specified. Characterization. The polymers were characterized by size exclusion chromatography (SEC) with a multiangle laser light scattering detector (Wyatt DAWN) and by 1H nuclear magnetic resonance spectroscopy (1H NMR, Varian Inova 500). The numberaverage molecular weight (Mn) and dispersity (Đ) of PMMA, as well as the dispersity of the diblocks, were determined by SEC, in which 0.084 and 0.068 mL/g were used as the dn/dc for PMMA and PnBMA/dPnBMA in THF, respectively. 1H NMR spectroscopy was used to determine Mn of the PnBMA blocks for the normal PMMA-bPnBMA copolymers, based on the integration ratio between the −OCH3 peak (δ = 3.60 ppm) of PMMA and −OCH2− peak (δ = 3.95 ppm) of PnBMA (Figure S1 in the Supporting Information). For the deuterated copolymers, as the −OCH2− peak is no longer present, the peak at 1.6−2.1 ppm representing the −CH2− on the backbone of both PMMA and PnBMA is used for the calculation of the dPnBMA molecular weight (Figure S1). The characterization results of the three pairs of h- and d-PMMA-b-PnBMA copolymers are shown in Table 1. The numbers in parentheses refer to the molecular weight of each block in kg/mol. The SEC traces are given in Figures S2−S4. Sample Preparation. An isotopic solvent mixture of protonated ionic liquid and d3-ionic liquid was used in the contrast-matching
solution temperature increases almost linearly with the [BMIM] wt % in the solvent.40−42 To the best of our knowledge, this is the first study that quantifies chain exchange kinetics in ILs, although Meli et al. examined equilibration of micelle size using dynamic light scattering for diblocks in ILs and used TR-SANS to confirm that there was no chain exchange during that process.43,46 Another virtue of this system is that the glass transition temperature (Tg) of the core block, PnBMA, is about 20 °C,44 which avoids the “glassy core” problem that can also hinder chain exchange.45,46 In this paper, we report TR-SANS measurements of chain exchange kinetics for a series of three PMMA-b-PnBMA copolymers with identical PMMA blocks but different PnBMA block lengths. The solvent composition is changed systematically, which gives access to a range of the Flory−Huggins interaction parameter χ between the solvent and the core block; a quantitative expression for χ as a function of solvent composition and temperature was obtained from prior measurements.41,42 The results are compared with the model proposed by Choi et al.,29−31 and particular emphasis is placed on the effect of χ on the overall chain exchange rate. In particular, we show that the previous assumption of a barrier simply proportional to χ is incorrect and offer a more elaborate expression, developed in the spirit of Flory−Huggins theory.
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EXPERIMENTAL SECTION
Materials. Three nearly identical isotopically substituted pairs of PMMA-b-PnBMA diblock copolymers were synthesized by radical addition−fragmentation chain-transfer (RAFT) polymerization. In each pair of these copolymers, one is partially deuterated on the PnBMA block (henceforth denoted as dPnBMA). The normal PMMA-b-PnBMA copolymers were synthesized by sequential RAFT polymerization following a previously documented procedure;47 the deuterated counterparts were synthesized similarly, but using d9-nbutyl methacrylate as the monomer, which was prepared with a modified version of a documented method,48 as briefly described below. B
DOI: 10.1021/acs.macromol.6b02212 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 1. Representative TR-SANS profiles collected at 35 °C over 5 min intervals for (a) PMMA-b-PnBMA (25−24) in [BMIM] wt % = 0%, (b) PMMA-b-PnBMA (25−24) in [BMIM] wt % = 10%, (c) PMMA-b-PnBMA (25−24) in [BMIM] wt % = 20%, (d) PMMA-b-PnBMA (25−35) in [BMIM] wt % = 0%, (e) PMMA-b-PnBMA (25−35) in [BMIM] wt % = 10%, (f) PMMA-b-PnBMA (25−35) in [BMIM] wt % = 20%, and (g) PMMA-b-PnBMA (25−53) in [BMIM] wt % = 30%. to zero at t = ∞. As the coherent scattering intensity of the core, Icoh,core ∼ (ρcore − ρsol)2; therefore, the total scattering intensity also monotonically decreases with time, as the corona scattering remains the same during the whole process. In order to quantitatively reflect the extent of chain exchange, the normalized relaxation function, R(t), is defined as follows:
method, the ratio of which was selected so that the average neutron scattering length density (SLD) of the solvent mixture ρsol is equal to that of a 50/50 PnBMA/dPnBMA micelle core, i.e., ρsol = Φh‑ILρh‑IL + (1 − Φh‑IL)ρd‑IL = (ρPnBMA + ρdPnBMA)/2, where Φh‑IL is the volume fraction of the protonated ionic liquid in the solvent mixture. In this work, the Φh‑IL for [EMIM][TFSI] and [BMIM][TFSI] are calculated to be 0.62 and 0.08, respectively. The calculated SLD results are listed in Table 2. In the preparation of the micelle specimens, methylene chloride was used as a cosolvent to dissolve the PMMA-b-PnBMA and PMMA-bdPnBMA separately. These solutions were then combined with a predetermined ratio of contrast-matched [EMIM][TFSI] and [BMIM][TFSI] mixture at room temperature and then purged with nitrogen overnight to slowly remove the cosolvent. The resulting solutions were dried at 50 °C under vacuum (
