Simultaneous Determination of Critical Micelle Temperature and

May 17, 2013 - Mary J. Burroughs , Dane Christie , Laura A. G. Gray , Mithun Chowdhury , Rodney D. ... Emily K. Leitsch , William H. Heath , John M. T...
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Simultaneous Determination of Critical Micelle Temperature and Micelle Core Glass Transition Temperature of Block Copolymer− Solvent Systems via Pyrene-Label Fluorescence Christopher M. Evans,† Kevin J. Henderson,‡ Jonathan D. Saathoff,† Kenneth R. Shull,‡ and John M. Torkelson*,†,‡ †

Department of Chemical and Biological Engineering and ‡Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: The critical micelle temperature (CMT) and micelle core glass transition temperature (Tg) for a poly(methyl methacrylate) (PMMA)-poly(tert-butyl methacrylate) (PtBMA) diblock copolymer system were measured by fluorescence via single temperature (T) ramps. Synthesis yielded identical block lengths in unlabeled and pyrenelabeled diblocks, the latter with dye at the PMMA block terminus. Studies were conducted at 5−18 wt % diblock in 2-ethylhexanol (2EH) with a trace of labeled diblock (0.2 wt % of total copolymer). The T dependence of pyrene-label fluorescence intensity yielded the CMT and micelle core Tg in systems where the PMMA-block and the 2EH within the cores constituted 1.9−7.8% of sample mass. While the CMT can be measured by many methods, this is the first direct measurement of micelle core Tg at low core content (e.g., 1.9 wt %) in a block copolymer/solvent system. Differential scanning calorimetry was done on diblock samples, showing severe limitations for sensing and characterizing core Tg. Fluorescence from trace levels of labeled diblock was used with 5−20 wt % PMMA−poly(n-butyl acrylate)−PMMA triblocks in 2EH. The micelle core Tg is important in triblock systems that form thermoreversible gels because it fundamentally underlies the viscoelastic to elastic gel transition. Fluorescence results demonstrated the dependence of the CMT and the near invariance of the micelle core Tg on core-block molecular weight in these diblock and triblock systems for PMMA blocks with Mn = 15−25 kg/mol and solvent in the micelle core.

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INTRODUCTION

Of the three transition temperatures, two have been quantitatively characterized in many diblock and triblock copolymer/solvent systems by a variety of techniques. In the case of the CMT, dynamic light scattering,11−17 small-angle neutron scattering (SANS),18,19 and small-angle X-ray scattering (SAXS)9,20 provide sensitive measurements and also yield characterization of micelle dimensions; in addition, SANS and SAXS can yield the aggregation number (number of blocks in the micelle core).9,18−20 As shown in Pluronic (poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide)) systems in water, differential scanning calorimetry (DSC) can characterize the CMT when there is a sufficiently large change in the heat capacity with micelle formation.21,22 Like DLS, fluorescence spectroscopy provides excellent, general characterization of the CMT in block copolymer/solvent systems.23−33 Sensitivity to the CMT can occur via intrinsic fluorescence of chromophores naturally present on the copolymer23,24 or by extrinsic fluorescence of dye labels covalently attached to the polymer25−27 or free dye probes added to the system at trace levels that partition to the core upon micelle formation.28−32 In

At sufficiently high concentration and appropriate temperature (T), ABA triblock copolymers in a midblock-selective solvent can form gels.1 These thermoreversible gels are of technological interest, including use as bioerodible, biocompatible scaffolds for drug delivery2 and wound healing,3 pressure-sensitive adhesives,4 vibration dampeners,5 microfluidic substrates,6 and high-performance dielectric elastomers.7 Three transition temperatures are important in defining the behavior of such triblock copolymer/solvent systems: the critical micelle temperature (CMT) where incipient end block aggregation occurs upon cooling;8 the gel temperature (Tgel) where the relaxation time is comparable to the experimental time scale;9 and the micelle core glass transition temperature (Tg) where the gel changes from viscoelastic to elastic.10 In a strongly midblock-selective solvent, micelle cores consist of (nearly) neat end blocks and are often glassy at all use Ts, resulting in an elastic gel response. However, when the solvent is less strongly selective for the midblock, the micelle cores contain solvent and their Tg often differs significantly from Tgel. Such gels exhibit viscoelastic response between Tgel and the micelle core Tg. A schematic illustration of these various transition temperatures can be found in ref 9. © XXXX American Chemical Society

Received: April 2, 2013 Revised: May 7, 2013

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the case of Tgel, values are commonly obtained via oscillatory shear rheology measurements,9,15,34−36 although other methods sensitive to increases in specific viscosity upon gelation have also been used including capillary viscometry.37 In contrast to the many studies that have reported CMT and Tgel values in diblock and triblock copolymer/solvent systems, few studies have characterized the micelle core Tg. Most previous investigations of micelle core Tg have used DSC; however, the sensitivity to Tg is limited by the volume of glass former in the sample.10,38−40 For example, micelle core Tgs by conventional second-heat DSC heat flow measurements have been reported only down to limits of ∼30 wt % micelle core (PS block forming the core) by Lai et al.38 and 50 vol % diblock copolymer (with both blocks contributing to the observed Tg) by Virgili et al.39 Using 15−35 vol % triblock copolymer gels, Kim et al.40 and Drzal and Shull10 reported that conventional Tg characterization via second-heat DSC heat flow curves yielded null results. However, Kim et al.40 reported the presence of glass transitions from physical aging studies where a DSC enthalpy peak was observed on first heat. Monitoring physical aging over 130 days, Drzal and Shull10 estimated a micelle core Tg by extrapolating the location of the physical aging enthalpy peak to zero aging time. Mok and Lodge41 also found that second-heat DSC heat flow curves could not resolve micelle core Tg values in diblock copolymer/ionic solvent systems. However, using first derivatives of second-heat DSC heat flow curves, a method previously applied to polymer blends42−44 and gradient copolymers,45 they resolved very broad T ranges over which the cores underwent a glass- to liquid-state transition. They supported their DSC measurements of glass transition breadth with fluorescence studies of dye mobility within the cores. Rheology has been used to infer the Tg of micelle cores with PS blocks; the shift factors from shear modulus measurements followed the bulk PS T dependence but shifted to lower T.46,47 Additionally, micelle core Tgs have been reported using SAXS which can measure the thermal expansion of a micelle core using a synchrotron source.20 Clearly, the few methods that have been used to characterize micelle core Tg in block copolymer/solvent systems have major limitations. In contrast, fluorescence spectroscopy is well suited to studies of micelle cores, as evidenced by past characterization of CMT and critical micelle concentration,23−33,48−50 and can easily report Tgs in polymeric systems. The use of fluorescence to measure bulk polymer Tg was first reported in the 1970s and 1980s.51,52 Within the past decade, intrinsic and extrinsic dye fluorescence have been used to measure Tg as a function of confinement in neat polymer,53−56 diblock copolymer,57 and polymer containing small levels of solvent,54,56 sometimes in films or layers with thickness approaching 10 nm.53−57 These studies most often characterized Tg by measuring the T dependence of fluorescence intensity which changes at the glass transition. Additionally, fluorescence has been applied to measure the Tg of blend components present at 0.01−0.1 wt % in single-phase polymer blends.44,58,59 Here, we have adapted past experience with fluorescence intensity characterization of Tg in order to provide the first direct measurements of micelle core Tg in diblock and triblock copolymer/solvent systems under conditions of relatively low micelle core volume or weight fraction, for example down to 1.9 wt % core where DSC and rheology yield null results. Additionally, we demonstrate that a single temperature sweep, rather than one ramp to measure Tg and a second ramp to

measure CMT on a separate piece of equipment, is sufficient to characterize both the CMT and Tg by integrated fluorescence intensity measurements in these systems. This is also significant because past fluorescence characterization of the CMT in block copolymer/solvent systems has been done most typically by measuring intensity ratios or spectral shifts associated with solvatochromatic effects28−32 or fluorescence resonance energy transfer25−27 rather than integrated intensities. In particular, we have used 1-pyrenylbutyl methacrylate (BPy) as a fluorescence label and synthesized BPy-labeled methacrylate-based diblock copolymer, with BPy at the terminus of the core-forming block. We have added the labeled diblock at trace levels to 5−20 wt % unlabeled diblock or triblock copolymer systems in 2ethylhexanol (2EH). Besides measuring the T dependence of BPy-label fluorescence intensity in order to characterize the CMT and micelle core Tg, we also compare fluorescence results to characterization of the CMT by oscillatory shear rheology and the micelle core Tg by DSC.

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EXPERIMENTAL SECTION

A poly(methyl methacrylate) (PMMA)−poly(tert-butyl methacrylate) (PtBMA) diblock copolymer was synthesized by anionic polymerization in tetrahydrofuran at −78 °C by sequential addition of tertbutyl methacrylate followed by methyl methacrylate. (The initiator was formed by reacting sec-butyllithium with diphenylethylene.) After reaction, a small portion of the synthesis mixture was terminated with degassed methanol. The remainder was reacted with 1-pyrenebutyl methacrylate (BPy) to yield diblock copolymer with a pyrenyl dye at the PMMA block terminus. The BPy was synthesized by esterification of 1-pyrenylbutanol (Aldrich) and methacryloyl chloride (Aldrich) following procedures described for a related monomer.60 Thus, the pyrene-labeled diblock has an identical unlabeled diblock analogue made from the same reaction mixture. The molecular weight of the PtBMA starting block (obtained by aliquot removal from the reaction mix prior to the MMA addition) was determined by gel permeation chromatography (GPC; Waters Breeze) using polystyrene standards in tetrahydrofuran at 30 °C and appropriate Mark−Houwink parameters from the literature for universal calibration. The Mn of the PtBMA block (15 000 g/mol) from GPC was used in combination with 1H NMR measurements to determine the diblock copolymer composition and thus the Mn of the PMMA block (40 000 g/mol). The diblock copolymer molecular weight characterization is summarized in Table 1. Using toluene solutions, the pyrene-label content in the labeled diblock was determined by UV absorbance spectroscopy (PerkinElmer Lambda 35) following a method described previously.44 At 345 nm, pyrene has a molar extinction coefficient of 38 700 M−1 cm−1 while PtBMA and PMMA have zero absorbance. Assuming negligible effect

Table 1. Molecular Weights and Dispersities for the Diblock and Triblock Copolymers diblocks BPy-PMMA15− PtBMA40 PMMA15−PtBMA40 unlabeled triblocks PMMA23−PnBA31− PMMA23 PMMA25−PnBA116− PMMA25

PMMA Mn (kg/mol)a

PtBMA Mn (kg/mol)a

dispersitya

15

40

1.06

PMMA Mn (kg/mol)b

PnBA Mn (kg/mol)b

dispersityb

23

31

1.18

25

116

1.39

a

Determined by GPC via a universal calibration curve and appropriate Mark−Houwink parameters for THF and 1H NMR. bReported by supplier. B

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copolymer in 2EH. At 10−18 wt % diblock, G″ and |η*| show sensitivity to micelle formation (on cooling) and dissolution (on heating) and allow estimation of CMT values. Next, we describe the limitations of DSC in sensing the micelle core Tg in the diblock copolymer system. We then discuss the T dependence of fluorescence intensity of BPy-PMMA15− PtBMA40 copolymer present at trace levels in its unlabeled PMMA15−PtBMA40 analogue in 2EH. These data yield excellent quantitative determinations of both CMT and micelle core Tg at all concentrations studied. Finally, we show that trace levels of BPy-PMMA15−PtBMA40 yield CMT and micelle core Tg in systems containing 5−20 wt % PMMA−PnBA−PMMA triblock copolymers in 2EH where the PMMA end blocks have higher molecular weight than in the diblock copolymer. These results indicate that a BPy-labeled diblock copolymer can sense micelle formation and core Tg in such copolymer systems of different architecture and composition when two conditions are met: the core block is the same composition (e.g., PMMA) in the diblock and the copolymer under study, and BPy is labeled to the core block. A. Sensitivity of G″ to the CMT in Diblock Copolymer/ Solvent Systems. Previous rheology studies of micelle or aggregate formation in block copolymer/solvent systems have focused predominantly on ABA triblock copolymers where micelle formation at sufficient copolymer concentration is a prerequisite for achieving percolating networks or physically cross-linked gels. The gel point is often characterized as the condition where the shear storage modulus (G′) is equal to G″.9,34−36,61 If the parameter being varied is T, then the gel point is called the gel temperature. At the gel point, the gel relaxation time is comparable to the experimental time scale. Figure 1 shows that a transition from viscous liquid at higher T

of covalent attachment on dye absorbance, there is a 1 to 2 ratio of pyrene end label to diblock copolymer chains. Three PMMA−PnBA−PMMA triblock copolymers (Kuraray Co.) were used as received, where PnBA refers to poly(n-butyl acrylate). Table 1 lists the molecular weight data. In the naming convention in Table 1, the subscript numbers refer to the Mn of each block in kg/ mol. Solutions were prepared for fluorescence studies by blending a given unlabeled block copolymer with BPy-labeled diblock copolymer at trace levels (∼0.2 wt % of total copolymer) and dissolving in 2EH at 100 °C overnight in a sealed glass vials. Diblock solutions used for DSC were taken from the same vial as those used for fluorescence. Rheology samples were prepared identically but in separate vials without BPy-labeled diblock copolymer. Steady-state fluorescence spectra were obtained using a SPEX Fluorolog 2 in front-face emission geometry. Copolymer systems containing BPy-labeled diblock were loaded into a home-built sample holder with both the holder and solution at 100 °C in an oven. The samples were held between a green glass slide (rear face) and a clear quartz slide (front face). The two slides were separated by ∼1 mm using miscroscope slides which were cut to a smaller size with a diamond pen. The assembly was held together by Scotch-Weld DP100 two-part clear epoxy system. Samples were excited at 324 nm and emission monitored at 360−460 nm (or to 600 nm in order to confirm the near-absence of excimer fluorescence) with 2.5 mm emission slit width (4.5 nm bandpass). Temperature was controlled with an INSTEC heater/liquid N2 cooling system and Wintemp software. The diblock copolymer system was measured on cooling. The PMMA25− PnBA116−PMMA25 system was measured on heating to approximate the conditions of Seitz et al.9 where CMT and Tgel were characterized by small-angle X-ray scattering and rheology. The 15 and 20 wt % PMMA23−PnBA31−PMMA23 systems were measured on heating and cooling, respectively, showing no significant dependence on ramp direction. For cooling studies, the system was heated to 100 °C, held for 20 min, and then cooled in 5 °C steps, waiting 5 min for equilibration at each T plus 2 min for collecting a spectrum. For heating studies, after heating to 100 °C and holding for 20 min, each system was cooled to 0 °C, held for 20 min, and then heated in 5 °C steps, waiting 5 min for equilibration at each T plus 2 min for collecting a spectrum. Normalized integrated intensity was plotted as a function of T to determine CMT and Tg by fluorescence. Linear regressions for the glassy and liquid regimes were fit starting at the T extrema and adding data points to the fit until the fit lines no longer passed through the next point in series (R2 > 0.990). The data between the CMT and Tg were fit starting in the middle and adding points in both directions as long as the regression continued to pass through the next data points in series. Small-amplitude oscillatory shear rheology measurements were done using a Paar Physica MCR-300 rheometer in a double-gap Couette geometry (26.66 mm diameter, 0.30 mm gap) on cooling and heating, yielding similar results. For cooling studies, each sample was heated to 95 °C and held for 20 min; measurements were taken on cooling to 0 °C at a rate of 1 °C/min under a 1% strain and 10 rad/s frequency. Heating studies were done after cooling, heating from 0 °C at a rate of 1 °C/min under a strain of 1% and frequency of 10 rad/s. Temperature was controlled by a water-based Peltier system. For DSC (Mettler-Toledo DSC822e) analysis, diblock copolymer/ 2EH solutions prepared at 100 °C were cooled to room temperature (20−22 °C) where they aged for 7 days. Systems were then loaded into aluminum DSC pans, sealed with aluminum lids, and quenched to −50 °C at 10 °C/min. After holding for 10 min at −50 °C, the systems were heated to 100 °C (first heat), cooled to 0 °C, and heated again to 100 °C (second heat), all at 10 °C/min.

Figure 1. Temperature dependence of G′ (open squares) and G″ (filled squares) taken upon heating at 1 °C/min with 1% strain and a frequency of 10 s−1 taken from ref 9. The shaded gray area corresponds to the transition region determined from SAXS data upon heating in ref 9. This range corresponds to the onset of a decrease in SAXS intensity at 50 °C which continues up to a temperature of 70 °C followed by a plateau in intensity. Reprinted with permission from ref 9.

to an elastic or viscoelastic solid at lower T as measured by rheology occurs over a T range that can approximately coincide with that observed for end block aggregate or micelle formation. Figure 1 is taken from Seitz et al.,9 who studied a triblock copolymer system investigated here (see section D). From Figure 1, Tgel = ∼63 °C based on the G′ = G″ criterion. However, the T range over which G′ and G″ evolve dramatically is comparable to the T range for aggregate or micelle formation. The shaded region in Figure 1 shows the T range over which SAXS contrast is lost upon heating because of

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RESULTS AND DISCUSSION This section is divided into several parts. First, we describe the T dependence of the shear loss modulus (G″) and complex viscosity (|η*|) of 5−18 wt % PMMA15−PtBMA40 diblock C

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be taken as the onset of micelle formation, i.e., the CMT. For the 10 wt % diblock in Figure 2a, G″ values are lower than for the 18 wt % system. However, G″ shows sufficient T sensitivity to yield CMT = 65 ± 3 °C. A similar CMT is obtained from the T dependence of G″ in the 15 wt % system. However, no CMT can be estimated at 5 wt % copolymer (data not shown) because G″ values are so low and the data so noisy that no pronounced increase is observed in G″. We note that it may be possible to determine CMT in a 5 wt % system in certain newer, high sensitivity rheometers. The diblock system CMT values from G″ are given in Table 2. Figure 2b shows complex

micelle dissolution or gained upon cooling because of micelle formation. A factor that may account for some of the T breadth for micelle formation on cooling or dissolution on heating is the 1.39 copolymer dispersity. With this understanding, for the triblock copolymer system in Figure 1, the CMT = ∼70 °C based on SAXS data and 74−80 °C based on the rise of G″ with decreasing T. Figure 2a,b shows semilogarithmic plots of the T dependence of G″ and |η*| for 18, 15, and 10 wt % PMMA15−PtBMA40 in

Table 2. Tg and CMT Values for BPy-Labeled Diblock in 2EH by DSC, Rheology, and Fluorescence wt % diblock BPy−PMMA15− PtBMA40 5 10 15 18

Tg (°C) (±2)

CMT (°C) (±3)

DSC

fluorescence

rheology

fluorescence

22−38 16−36 16−38

21 24 25 23

65 65 67

64 67 65 70

viscosity as a function of temperature for the same solutions and the CMT is the same within error as determined by G″ data. Within error, the CMT is invariant with concentration. As measured here, neither |η*| nor G″ provides sensitivity to the micelle core Tg. B. Limited Sensitivity of DSC to the Micelle Core Tg in Diblock Copolymer Solvent Systems. Drzal and Shull10 demonstrated the inability of conventional second-heat DSC heat flow curves to sense to the micelle core Tg in a 15 vol % PMMA−PtBA−PMMA triblock copolymer in 2EH. (The copolymer had an overall Mn = 139 kg/mol and was 34 wt % PMMA.10) However, the presence of a core glass transition was observed by physical aging experiments. (Physical aging occurs only for T < Tg.) They aged samples up to 130 days at room temperature and measured enthalpy recovery peaks in first-heat DSC heat flow curves; the peak shifted to lower T with increased aging. By extrapolating the peak location to zero aging time, they estimated a PMMA micelle core Tg of ∼36 °C.10,62 A possible interpretation of the resulting Tg value is that it may relate to the Tg end set (measured upon heating) in a DSC heat flow curve. Figure 3a shows DSC results for a 15 wt % PMMA15− PtBMA40 system aged at room T (∼20 °C) for 7 days. Both first-heat (after physical aging) and second-heat (with thermal history erased) curves are given along with a DSC heat flow curve for neat 2EH, the latter as a control showing little noise and none of the curve shifts or bumps present in the copolymer data. In agreement with the triblock study by Drzal and Shull,10 the second-heat DSC heat flow data yield no sensitivity to Tg of the diblock system at 15 wt % copolymer or any other concentration (data not shown). However, the first-heat curve exhibits a very small enthalpy peak with a maximum near 50 °C. Figure 3b shows first heat curves for 10, 15, and 18 wt % diblock systems aged 7 days; each curve has a small enthalpy peak maximum near 50 °C which increases slightly in size with copolymer concentration. (The 5 wt % sample showed no discernible enthalpy peak.) Figure 3c gives an enlarged view of the first-heat DSC heat flow curve for the 15 wt % system and shows a broad T range

Figure 2. (a) Loss moduli and (b) complex viscosity for 10, 15, and 18 wt % PMMA15−PtBMA40 in 2EH. The intersection of lines drawn to G″ or complex viscosity cooling curves for the high-temperature baseline and transition region denote the rheology CMT. The value does not change within error for CMTs determined from heating G″ curves.

2EH. Measurements on cooling and heating show substantial similarity, which indicates that micelles are equilibrated or nearly so at experimental conditions. Over the entire T range for all of the diblock concentrations studied, we observed that G″ ≥ G′, indicating liquid-like behavior. Although the change in moduli is smaller than that observed with gelation in triblock copolymer/solvent systems (see Figure 1), the shift in the T dependence over a moderate T range is consistent with a structural change involving micelle formation. With decreasing T the onset of the rising T dependence of G″, which is 67 ± 3 °C for the 18 wt % solution in Figure 2a, may D

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mass. Differentiation of second-heat DSC curves yielded no discernible glass transition regions for copolymer concentrations as high as 18 wt %. The insensitivity of this method to the core Tg in our samples is likely because of the combined effects of the cores constituting a very small fraction of the sample mass and the relatively small heat capacity change that accompanies heating PMMA in 2EH through the glass transition. Thus, based on physical aging studies, DSC provides sensitivity to the presence of a micelle core glass transition in systems containing at least 10 wt % PMMA15−PtBMA40 in 2EH. However, it does not yield a direct measurement of Tg62 and provides no information for the 5 wt % diblock solution. An experimental method is needed that provides strong sensitivity to Tg when the phase of interest is present at low levels. Mok and Lodge have discussed that pyrene fluorescence can provide “direct Tg breadth measurements for block copolymer micelle cores”.41 Recent fluorescence studies have also shown that Tg can be determined for phases present at 0.1 wt % and less in polymeric systems.44,58,59 Section C describes how fluorescence yields direct measurement of both CMT and micelle core Tg in the PMMA15−PtBMA40 in the 2EH system. C. CMT and Micelle Core Tg in Diblock Copolymer/ Solvent Systems by BPy-Label Fluorescence. Figure 4a shows the T dependence of the BPy-label fluorescence spectra for 15 wt % PMMA15−PtBMA40 (with a trace of BPyPMMA15−PtBMA40) in 2EH. Fluorescence intensity decreases with increasing T because the nonradiative decay pathways from the BPy excited state are enhanced in a more mobile environment, which results in a decrease in quantum yield. Although Figure 4a shows spectra over a 360−460 nm range of emission wavelength, measurements were also done at higher wavelength, showing the presence of only very small levels of excimer fluorescence (emission from an excited-state pyrenyl dimer which exhibits a maximum at ∼480 nm and extends beyond 550 nm). The near-absence of excimer fluorescence is anticipated based on considerations of the aggregation number of the micelles and the fraction of chains that contain a BPy label. The study by Seitz et al.9 of related triblock copolymer systems suggests that the aggregation number in our diblock system may be ∼60−80. Taking a value of 60, noting the 1 to 2 ratio of BPy label to “labeled” diblock copolymer chains (and assuming no more than one label per chain), and recalling that systems contain only 0.2 wt % BPy-labeled diblock, there would be an average of 1 BPy label per 17 micelle cores. This would mean that almost no micelles contain two or more BPy-labeled diblocks; thus, intermolecular excimer fluorescence is virtually disallowed in these systems. The near-absence of excimer fluorescence is important because the T dependence of BPylabel fluorescence can be used to characterize CMT and Tg without concern for how the T dependence of substantial excimer formation and emission may affect or contaminate measurements. (As shown in the Supporting Information, there is a very small level of intrapolymer excimer fluorescence in these systems derived from the presence of a small fraction of BPy-labeled diblock having two or more BPy units at the PMMA block terminus. We demonstrate in the Supporting Information that the integrated fluorescence intensity from 360 to 460 nm, excluding the very small amount of excimer fluorescence, yields the same CMT and micelle core Tg values as the integrated fluorescence intensity that includes the small amount of excimer fluorescence.)

Figure 3. (a) DSC heating curves for a 15 wt % PMMA15−PtBMA40 solution upon first heat and second heat along with a pure 2EH scan for comparison. (b) DSC first heat curves (10 °C/min) for 10, 15, and 18 wt % BPy-PMMA15−PtBMA40 in 2EH. Samples were aged at room temperature for 7 days before being run. (c) Enlarged view of the 15% solution demonstrating how a range was selected for Tg determination.

between a glassy heat capacity region at T < ∼16 °C and a liquid heat capacity region at T > ∼55 °C, above the enthalpy peak. We estimated a narrower T range over which a Tg may exist, taking as a lower bound the T where the apparent heat capacity changes from the glassy value (∼16 °C) and as a higher bound the T obtained by extrapolating the high slope portion of the enthalpy peak back to a heat flow equal to that in the glassy state (∼36 °C). The 16−36 °C T range is not an estimate of the core Tg breadth but rather an estimate of the range where the core Tg (onset method) may occur. Using this approach, similar results were obtained at 10 and 18 wt % systems, with T ranges of 22−38 and 16−38 °C, respectively. (The 5 wt % system yielded null results because there was no enthalpy peak.) See Table 2. We considered a second analysis method for characterizing core Tg which uses the first derivative of second-heat DSC heat flow curves.41−45 Mok and Lodge41 used differentiated DSC data to characterize the core Tg breadth in 10−30 wt % polystyrene (PS)−poly(ethylene oxide) (PEO) and PS− PMMA diblock copolymer/ionic liquid systems, where PS blocks form micelle cores without major levels of solvent. With the PS−PEO system, the PS blocks accounted for 6−24 wt % of the total copolymer/solvent system. It must be noted that in comparison with many neat polymers, the DSC heat flow curve for PS undergoes a large change at Tg;63 thus, the size of a glass transition peak in a differentiated DSC curve is larger and more easily discernible for PS than for many other systems. That sensitivity to the PS Tg is the reason that this approach yielded Tg breadths of micelle cores with the cores being only 6−24% of the sample mass. In our PMMA15−PtBMA40 in 2EH system, the PMMA in the micelle cores account for only 1.4−5.5 wt % of the total system. Near Tg at 30 °C, the micelle cores are estimated to contain ∼30 wt % 2EH;10 thus, near Tg the combination of PMMA and 2EH in the cores accounts for only ∼1.9−7.8% of total sample E

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Figure 4. (a) Fluorescence spectra for 15 wt % BPy-PMMA15− PtBMA40 in 2EH measured at 100 (lowest curve), 50 (middle curve), and 0 °C (highest curve). (b) Normalized integrated intensity from 360 to 460 nm as a function of temperature for 5 wt % (circles) and 15 wt % (diamonds) BPy-PMMA15−PtBMA40 in 2EH. Data were normalized to 1.0 at 95 °C and shifted vertically for clarity. The intersection of linear regressions fit to the data denote CMT (higher temperature break) and Tg. The DSC range determined for an aged sample and the CMT determined from G″ data are plotted above the fluorescence data for a 15 wt % solution. For the 5 wt % sample, DSC and rheology provide null results.

Figure 5. Normalized integrated intensity (from Figure 4b) of a 5 wt % BPy-PMMA15−PtBMA40 in 2EH focused on the (a) Tg and (b) CMT regions. Isolating each transition provides a clearer view of both processes.

copolymer/solvent systems. The CMT and Tg values are summarized in Table 2. Within error, the CMT and micelle core Tg values are independent of concentration. In the case of the micelle core Tg, this suggests that at Tg the aggregation number varies insufficiently with concentration to yield differences in Tg. It is noteworthy that the micelle core Tg is far below the Tg of the PMMA diblock in neat diblock copolymer. Measurements of neat diblock copolymer by DSC show an overlapping transition at 100−120 °C because neat PtBMA Tg = ∼100 °C and neat PMMA Tg = ∼120 °C. The vastly lower micelle core Tg can be explained by a substantial level of 2EH in the micelle core, ∼30 vol % at 30 °C near Tg.10 The depression of the PMMA block Tg is consistent with alcohol swelling experiments of PMMA films where Tg was reduced by ∼100 °C in the presence of ∼50 vol % ethanol.64 We note that studies on the effect of nanoscale confinement on polymer Tg also suggest that 3-D confinement in nanospheres, such as micelle cores, may lead to deviations from bulk Tg response.65−67 However, other studies have noted that addition of small-molecule diluent to polymers can suppress or eliminate the Tg-confinement effect.54,56 Future studies should consider the role, if any, of nanoconfinement in modifying the core Tg.

Figure 4b shows the normalized intensity as a function of T for 5 and 15 wt % PMMA15−PtBMA40 systems. The data exhibit a strong T dependence over the 95 °C T range studied, increasing by more than 200%. This T dependence is much larger than that exhibited by BPy-label fluorescence in bulk polymer above and below Tg, where intensity changes by much less than 50% over a similar 80−100 °C breadth of temperature.53,56,57,59 At each concentration, two transitions are apparent: the lower one associated with the micelle core Tg (supported by DSC results in section B and denoted above the fluorescence data) and the higher one with the CMT (supported by G″ results in section A and also shown above the fluorescence data). In Figure 5, both transition regions are isolated to clarify the sharpness of both the Tg and CMT. At 5 wt % copolymer, Tg = 21 ± 2 °C and CMT = 64 ± 3 °C, and at 15 wt % copolymer, Tg = 25 ± 2 °C and CMT = 65 ± 3 °C. Studies were also conducted on 10 and 18 wt % PMMA15− PtBMA40 systems, with similar results. Thus, the T dependence of BPy end-label fluorescence intensity provides robust characterization of the CMT and micelle core Tg in diblock F

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The T dependence of BPy-label fluorescence intensity has been used previously to probe Tg in homopolymers,53,55 block copolymers,57 and polymer blends.59 In those studies, the T dependence of BPy-label fluorescence was linear both above and below Tg, being stronger above Tg. In contrast, the data between the transition temperatures in Figure 5a,b show stronger, apparently linear dependences of intensity with decreasing T. The different T dependence of fluorescence intensity in the diblock copolymer/solvent system (along with its greater overall T dependence) relative to neat polymer arises from several effects. At high T above the CMT, the copolymer is dissolved in 2EH, and intensity increases with decreasing T because of the reduction in local mobility around the BPy label in the highly mobile solution. When CMT > T > Tg, the BPy labels at the end of the PMMA blocks are located almost exclusively in cores, which also contain 2EH. Thus, the much higher PMMA block concentration in the core than overall results in a greater T dependence of mobility locally near the BPy label in the micelle state. With decreasing T below the CMT, the 2EH level within the core decreases, further enhancing the effect of T on mobility within the cores and thus on BPy-label fluorescence intensity. Below the micelle core Tg, a part of the T dependence of fluorescence intensity derives from how local mobility is affected by T in the glassy state. However, that effect alone would be expected to result in a weaker T dependence below Tg than above Tg as found in studies on homopolymer and block copolymer.53,55,57 A second factor is the T dependence of 2EH concentration within the glassy micelle cores, with 2EH level decreasing with decreasing T. This effect is not kinetically limited in the fluorescence experiments. The micelle core radius is ≤∼10 nm for block copolymer in 2EH systems of the type studied here.9 Small molecules of larger size and more rigid structure than 2EH exhibit diffusion coefficients in polymers near and below Tg that are sufficient to allow for diffusion at short times over length scales exceeding the micelle core radius. For example, the small molecules pyrene and dimethylaminonitrostilbene (DANS) present at trace levels in methacrylatebased polymers at T = Tg have diffusion coefficients of ∼(1−2) × 10−13 cm2/s.68,69 With such diffusion coefficients, over a 10 s interval the average diffusive displacement is ∼30 nm, larger than the core radius. Given the ∼400 s time interval between each fluorescence measurement, 2EH (which will diffuse much faster than the larger, more rigid pyrene and DANS molecules) has more than enough mobility to diffuse in out of micelle cores during the experimental time frame used to measure the T dependence of fluorescence intensity. The CMTs obtained via fluorescence agree well with those from G″ characterization at 10−18 wt % concentration. Fluorescence is superior to G″ for characterizing the CMT at diblock concentrations of 5 wt % and below, where G″ is insensitive to the CMT. Micelle formation has been characterized at concentrations of 10−4−10−5 wt % by endlabel fluorescence in other diblock copolymer/solvent systems,25 meaning that fluorescence can sense the CMT at concentrations many orders of magnitude below the lower limits of most other techniques. We note that the micelle core Tg s determined by fluorescence intensity fall near the low end of the ∼20 °C range estimated by our DSC studies (see section B). Previous studies in bulk homopolymers have shown that the Tg value obtained from fluorescence intensity measurements is typically equal to the onset Tg obtained from second-heat DSC

measurements.44,53−56,58,59 We expect the same correspondence for the micelle core Tgs reported by fluorescence. This means that the Tgs for micelle cores in 5−10 wt % block copolymer systems are the first reported that are comparable to a standard Tg value. D. CMT and Micelle Core Tg in Triblock Copolymer/ Solvent Systems by BPy-Label Fluorescence. Figure 6

Figure 6. Normalized integrated intensity for 20 wt % triblock solutions of PMMA23−PnBA31−PMMA23 (circles) and PMMA25− PnBA116−PMMA25 (squares) in 2EH. Trace amounts (∼0.2 wt % of the total polymer content) of BPy-PMMA15−PtBMA40 were used as the fluorescent probe. Data were normalized to 1.0 at 100 °C and shifted vertically for clarity. Intersections of linear regressions fit to the data denote CMT and Tg at high and low temperature, respectively.

shows the T dependence of fluorescence intensity of trace levels of BPy-PMMA15−PtBMA40 in 20 wt % PMMA25−PnBA116− PMMA25 or PMMA23−PnBA31−PMMA23 triblock copolymer in 2EH. As with the diblock copolymer data in Figure 4, there is a strong T dependence of fluorescence intensity, and both CMT and micelle core Tg are evident for each copolymer system. Studies were also conducted for 5, 10, and 15 wt % PMMA25−PnBA116−PMMA25 and 15 wt % PMMA23− PnBA31−PMMA23, with data similar to those in Figure 5. These results indicate that the BPy-PMMA15−PtBMA40 diblock copolymer added at trace levels to these triblock copolymers serves as an effective sensor of both aggregate or micelle formation on cooling (or dissolution upon heating) and the micelle core Tg. This point is further supported by the CMT values (72−80 °C) and core Tg values (22−29 °C) obtained in these triblock systems, which are summarized in Table 3. If micelles were formed by trace levels of BPy-labeled diblock copolymer independently of micelles formed by these triblock Table 3. Fluorescence Tg and CMT Values for Triblock Systems Doped with Trace BPy-PMMA15−PtBMA40 wt % triblock PMMA23−PnBA31−PMMA23 15 20 PMMA25−PnBA116−PMMA25 5 10 15 20 G

Tg (°C) (±2)

CMT (°C) (±3)

24 28

72 80

22 22 22 29

80 80 77 75

dx.doi.org/10.1021/ma400686j | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

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Given past difficulties with experimental determination of micelle core Tg in block copolymer/solvent systems, no study has focused on the effects of copolymer architecture on micelle core Tg in systems with cores containing solvent at levels comparable to those in this study or in systems in which the cores constitute only 1.9% of sample mass. Here, we have shown that fluorescence intensity measurements of BPy-endlabeled block copolymer can easily yield micelle core Tgs in such systems. Future studies using fluorescence dye-labeled block copolymers are warranted in order to achieve greater understanding of such effects and systems.

copolymers, then the CMT values reported in Table 3 would be identical or nearly identical to those in Table 2. This is not the case. However, like the results for the diblock system in Table 2, Table 3 shows that the CMT and Tg values are (nearly) invariant with concentration and identical within error for the two copolymers, which can be explained by the nearly identical PMMA end block molecular weights. The ∼10 °C lower CMT in the diblock system than in the triblock system can also be explained by PMMA block molecular weight; the diblock copolymer has a PMMA block with an Mn that is 8−10 kg/mol lower than those in the triblocks, resulting in greater end block solubility in 2EH. These results are in contrast to the Tgel results reported by Seitz et al.9 for the same two triblock systems (e.g., see Figure 1). The Tgel values were concentration dependent and were shown to be identical within error in the two triblock systems when Tgel was plotted as a function of midblock volume fraction. For example, Tgel increased from ∼55 to ∼68 °C as midblock volume fraction increased from 0.03 to 0.18. In contrast, the CMT is not significantly affected by midblock molecular weight or copolymer concentration. A comparison of data from Figure 1 with fluorescence results reveals slight differences in the CMTs interpreted from SAXS, G″, and fluorescence intensity data. The CMT values for a 15 vol % (∼18 wt %) PMMA25−PnBA116−PMMA25 system are ∼70 and 74−80 °C by SAXS and G″, respectively. The fluorescence CMT values in the same system are above the CMTs from SAXS and overlap with those from G″. The higher sensitivity of fluorescence intensity to micelle formation on cooling or dissolution on heating is consistent with fluorescence being sensitive to smaller, incipient changes in block copolymer unimer vs aggregate states28 than SAXS. Like the CMT results, the micelle core Tg exhibits little or no difference between the two triblock systems. This is reasonable given that the triblocks have nearly identical PMMA end block molecular weights (23 and 25 kg/mol), which lead to similar aggregation numbers9 and hence the expectation of similar concentrations of end blocks in the micelles as a function of T. Unlike the CMT, the micelle core Tg in the triblock systems is also in accord with the Tg in the PMMA15−PtBMA40 diblock system, which has a significantly lower PMMA end-block molecular weight and a different composition (PtBMA vs PnBA) for the non-PMMA block. At least a part of this outcome arises from the fact that the micelles contain significant levels of 2EH in the cores, which should decrease the effect of end block molecular weight on core Tg. We also note that the micelle core Tgs from fluorescence are ∼12 °C below the 36 °C Tg estimated by Drzal and Shull10 in their DSC physical aging study of a 15 wt % PMMA−PtBA− PMMA triblock copolymer in the 2EH system, with PMMA end blocks having Mn = 24 kg/mol, nearly the same as the PMMA end blocks in our PMMA−PtBMA−PMMA triblock systems. Given that the Tgs measured by fluorescence intensities are expected to correspond to DSC onset Tgs,44,53−56,58,59 this means that the Tg estimated by Drzal and Shull is not equivalent to a DSC onset Tg. However, given that they extrapolated the location of the maximum of the enthalpy peak to zero aging time, it is possible that their estimated Tg more closely relates to a DSC endset Tg, potentially explaining the difference in values. Further study may resolve this issue.

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CONCLUSIONS We have presented direct measurements of micelle core Tg in diblock and triblock copolymer/solvent systems at low copolymer content (as low as 5 wt %) and micelle core content (as low as 1.9 wt %), where techniques such as differential scanning calorimetry lose their sensitivity to Tg. We measured the T dependence of the integrated fluorescence intensity of pyrene dye labeled at the terminus of the coreforming PMMA block in the diblock copolymer. This labeled diblock was used as a trace sensor both in a model system of unlabeled diblock copolymer of identical composition and molecular weight and in unlabeled triblock copolymers with PMMA end blocks of higher molecular weight and a different midblock composition than in the diblock. Simple breaks in the T dependence of fluorescence intensity yielded determinations of both the CMT and the micelle core Tg in a single T sweep. The micelle core Tgs were reduced by nearly 100 °C relative to that of the neat PMMA blocks due to the presence of 2EH solvent in the cores and were ∼40−60 °C below the CMTs. With the triblock copolymer systems, the CMT measured by fluorescence was ∼10−15 °C above the gel temperature, consistent with the sensitivity of fluorescence to incipient aggregation. Studies are underway using a variety of fluorescence methods to measure Tgs in micelle cores in block copolymer/solvent and block copolymer/homopolymer systems. Specifically, aqueous block polymer systems such as polystyrene−poly(ethylene oxide) or polystyrene−poly(acrylic acid) diblock copolymers are of interest as pyrene would strongly segregate to the hydrophobic polystyrene domains, and thus covalent attachment of a dye may not be required in these systems.

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ASSOCIATED CONTENT

* Supporting Information S

Additional fluorescence data demonstrate that, within experimental error, the inclusion or elimination of the very small level of intramolecular excimer fluorescence from the integrated fluorescence intensities yield identical CMT and micelle core Tg values; temperature dependence of normalized integrated intensity for the BPy-labled diblock is presented in a different solvent, dimethylformamide, where the changes in T dependence corresponding to the CMT and Tg are not discernible. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.M.T.). Notes

The authors declare no competing financial interest. H

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ACKNOWLEDGMENTS We acknowledge the very helpful input of Dr. Michelle Seitz and Prof. Connie Roth regarding this study. We also acknowledge the support of the NSF-MRSEC Program (DMR-0520513), a Northwestern University Terminal Year Fellowship (to C.M.E.), a Ryan Fellowship (to C.M.E.), a 3M Fellowship (to C.M.E.), and an NSF Graduate Research Fellowship (to K.J.H.).

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