Effect of Partial Hydrogenation on the Phase Behavior of Poly

Oct 3, 2012 - We studied the effect of selective hydrogenation of the polyisoprene block in poly(isoprene-b-styrene-b-methyl methacrylate) (ISM) tribl...
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Effect of Partial Hydrogenation on the Phase Behavior of Poly(isoprene‑b‑styrene‑b‑methyl methacrylate) Triblock Copolymers Maev̈ a S. Tureau and Thomas H. Epps, III* Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: We studied the effect of selective hydrogenation of the polyisoprene block in poly(isoprene-bstyrene-b-methyl methacrylate) (ISM) triblock copolymers on nanoscale network phase formation. The morphologies of the resulting poly((ethylene-alt-propylene)-b-styrene-b-methyl methacrylate) (EPSM) triblock copolymers and several EPSM copolymer/homopolymer blends were investigated using a combination of small-angle X-ray scattering and transmission electron microscopy, where well-ordered HEX, SA, and Q230 (network) nanostructures were identified. Variations in the nanoscale morphologies and phase boundaries between EPSM copolymers and their corresponding ISM precursors can be attributed to the differences in conformational asymmetry and block interactions. Of particular interest, the growth in the gyroid network region in the EPSM relative to the ISM is highlighted as the expansion of this region could further enable the creation of network-forming nanoporous membranes made from materials that are expected to demonstrate improved resistance against thermal and oxidative degradation.

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INTRODUCTION Block copolymers (BPCs) are versatile macromolecules that demonstrate unique properties based on their distinctive molecular architecture.1 The chemical connectivity between each block, the composition and length of the copolymer chains, as well as the degree of incompatibility between the blocks, drive block copolymers to self-assemble on nanometer length scales.2 As a result, BCP materials can exhibit welldefined and periodic nanostructures with tailored chemical and physical properties that can be applied to advanced technologies such as nanotemplating (e.g., electronics,3−5 lithographic masks,6−8 and solar cells9) and nanoseparation devices (e.g., nanofiltration membranes).10−13 In AB diblock copolymers, the mechanism of nanoscale phase separation of two covalently-bound homopolymer blocks, A and B, is governed by two major parameters: the component volume fraction (fA, where f B = 1 − fA) and the segregation strength (χABN), which is the product of the Flory− Huggins segment−segment interaction parameter (χAB) and the degree of polymerization (N).2 Upon the addition of a third block, ABC triblock copolymers give rise to increased morphological complexity due to additional tunable parameters, including three χ-parameters (χAB, χBC, and χAC), two independent volume fractions (fA and f B, where f C = 1 − fA − f B), and the overall degree of polymerization (N).2 Over the past few decades, numerous theoretical and experimental studies dedicated to the morphological characterization and compositional mapping of the complex morphologies exhibited © 2012 American Chemical Society

by ABC triblock melts have been reported. The major morphologies include two-domain and three-domain LAM morphologies,14−16 core−shell versions of cylinders (HEX) and spheres (S),2,15,17−20 three equilibrium network phases (core− shell gyroid Q230, alternating gyroid Q214, and orthorhombic O70),14,16,18,20−29 and other complex nanostructures.2,21,30−36 Among these nanoscale morphologies, ABC network phases are interesting, as they possess multidimensional cocontinuous domains with high degrees of internal interfacial area per specimen volume. This feature provides materials with enhanced mechanical properties (e.g., enhanced structural stability due to higher mechanical strength and toughness in comparison to their two-dimensional [lamellae or cylindrical] counterparts).24,37 Unfortunately, many of these ABC materials are polydienecontaining copolymers. Polydiene-containing BCPs are known to exhibit poor thermal stability as well as degradation in the presence of oxygen and UV irradiation, making them unsuitable for many applications.38,39 To overcome these limitations, a postpolymerization modification such as hydrogenation of carbon−carbon double bonds of the polydienes has been employed to enhance the polymers’ resistance to thermal, oxidative, and radiation-induced degradation and to improve Received: August 17, 2012 Revised: September 22, 2012 Published: October 3, 2012 8347

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homopolymer blends between the ISM triblock precursors and EPSM materials is discussed based on the differences in interaction parameters (χij) and statistical segment length ratios (bi/bj) between the ISM and EPSM systems, which results in a noticeable network phase extension and slight shifts in the phase boundaries.

the physical properties of the resulting saturated materials (e.g., superior tensile strength and elongation at break).38−47 In previous work, the morphological behavior of the poly(isoprene-b-styrene-b-methyl methacrylate) (ISM) triblock copolymer system was studied experimentally, and several network-like nanostructures were reported.25,26,48 However, to reduce the thermal and oxidative degradation of the polyisoprene (PI) block, these ISM polymers could benefit from the hydrogenation of the PI block to create tougher and more environmentally resistant materials. The hydrogenation of unsaturated polymers is typically accomplished through catalytic or noncatalytic methods. Catalytic hydrogenation methods can produce large batches of hydrogenated copolymers43 and are thus, a preferred route on the industrial scale. The hydrogenation can be either heterogeneous44,49,50 or homogeneous30,43,47,51 in nature, and the effectiveness of the catalytic process toward the selective hydrogenation of unsaturated polymers is a function of the catalyst activity and hydrogenation conditions.47 For heterogeneous catalysis, highly effective PI hydrogenation may be achieved, but it usually requires harsh conditions that can induce polymer degradation and branching.43 Additionally, most heterogeneous catalysts can become ineffective over time due to the absorption of the polymer chains onto the catalyst and have low selectivity in many cases.44 On the other hand, homogeneous catalysts, including Wilkinson’s catalyst30,43,47,51 or other soluble catalysts such as Ni/Al,52 are typically effective under milder reaction conditions that minimize side reactions. Such catalysts also have shown to have better selectivity and tolerance toward the presence of functional groups.47,53,54 However, homogeneous catalytic hydrogenations accomplished using the Wilkinson’s catalyst typically require high-pressure autoclaves43,55 and often are time-intensive reaction processes (>3 days).51,55 Unlike catalytic hydrogenation methods that typically require higher pressures,30,43−45,51 a low-pressure noncatalytic approach was utilized in this study.38,39,41,56 Here, fully or partially hydrogenated poly(ethylene-alt-propylene-b-styrene-bmethyl methacrylate) (ESPM) triblock copolymers were produced from the selective PI hydrogenation of ISM precursors using diimide intermediates, generated in situ from the thermal decomposition of p-toluenesulfonyl hydrazide (TSH).38 Upon hydrogenation of the PI block, the EPSM triblock system becomes a promising candidate for the creation of nanoporous separation membranes due to the toughness given by the combined interactions of the glassy polystyrene (PS) and rubbery poly(ethylene-alt-propylene) (PEP) blocks, the mechanical strength provided by the PS block, and the ease of removal of the poly(methyl methacrylate) (PMMA) block. We note that nanoporous membranes made from partially hydrogenated materials contain remaining double bonds in the PI block that can be chemically cross-linked to improve solvent resistance in addition to the thermal and oxidative resistance imparted by the PI hydrogenation. Previous work on the ISM triblock system demonstrated the discovery of three multidimensional and cocontinuous nanoscale networks that are potentially desirable for separation membranes due to their well-defined diffusion pathways and high structural stability.25,26 This article focuses on the nanoscale characterization of selected EPSM materials that were generated with the goal of preserving the network-like nanostructures found in their ISM triblock precursors. A comparison of the morphological behavior of neat and triblock/

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EXPERIMENTAL SECTION

Material Synthesis. The poly(isoprene-b-styrene-b-methyl methacrylate) (ISM) triblock copolymers were synthesized using anionic polymerization techniques. Synthetic protocols are detailed elsewhere.25,26,57,58 ISM Hydrogenation. ISM triblock copolymers were chemically modified by the selective hydrogenation of the polyisoprene (PI) block with diimide in toluene, produced in situ by the thermal decomposition of p-toluenesulfonyl hydrazide (TSH).38,39,46,56 TSH (97%, Sigma-Aldrich) was initially purified by recrystallization from ethanol and dried under vacuum prior to use. Toluene (ACS certified, Fisher Scientific) was degassed by sparging with argon for 30 min and then dried by passage through an alumina column and a Q5 column. The hydrogenation reaction was carried out under a nitrogen blanket in a round-bottom flask equipped with a reflux condenser, nitrogen inlet port, and glass connectors fitted with a needle port capped by a Teflon-coated septum. The connector ports were used to bleed out excess nitrogen built up during the reaction. The reaction mixture was continuously stirred and brought to reflux by slowly heating to ∼80− 90 °C to allow for the complete dissolution of TSH in toluene and then heated to 120 °C for 4 h and 135 °C for 3 h. Upon completion of the reaction, the hydrogenated samples were precipitated into cold methanol and thoroughly washed ∼3 times with methanol/distilled water mixtures (∼75 vol % methanol) to remove any excess TSH and byproducts of the TSH decomposition. Finally, the purified EPSM copolymers were dissolved in toluene, precipitated into methanol, and dried under vacuum. Chemical Characterization. Number-average molecular weight (Mn) and polydispersity index (PDI) for ISM and EPSM triblocks were determined by gel permeation chromatography (GPC), using polystyrene standards with tetrahydrofuran (THF) as the mobile phase. The GPC traces were obtained on a Viscotek VE 2001 equipped with a Viscotek VE 3580 refractive index detector and Waters Styragel HR1 and HR4 columns (7.8 × 300 mm each) in series. No chemical degradation or cross-linking was detected in the powdered hydrogenated materials. Most Mn values, as determined from GPC using PS standards, were found to be slightly higher upon hydrogenation, likely due to the different hydrodynamic volume of the PEP versus the PI block. The component mole fractions of the ISM precursors and EPSM copolymers were calculated from integrated 1H NMR peak intensities using a Bruker AV-400 instrument and were further converted to volume fractions using homopolymer densities at 140 °C (0.83, 0.969, 1.13, and 0.79 g mL−1 for PI, PS, PMMA, and PEP, respectively59). We note that all ISM triblock copolymers showed ∼94% 1,4-PI addition (and ∼6% 3,4-PI addition), as expected under the anionic polymerization conditions.60 The degree of hydrogenation of the EPSM materials was obtained by calculating the mole percentage reduction in the PI units based on the 1H NMR spectra of the ISM triblock precursor and resulting hydrogenated (EPSM) analogue. No significant TSA byproducts were incorporated to the PI double bonds of the hydrogenated EPSM materials, corresponding to the fraction of incorporated tosyl groups (i.e., −CH3 group found at ∼2.3−2.4 ppm in the 1H NMR spectra).38,55 Small-Angle X-ray Scattering (SAXS). Lab source SAXS experiments were conducted at the University of Delaware (UD) on a Rigaku SAXS instrument as described elsewhere.26 These data are referred to as “UD-SAXS data”. SAXS data were acquired at 30 °C and plotted as azimuthally integrated scattered intensity versus scattering vector modulus, q = 4πλ−1 sin(θ/2), where θ is the scattering angle. Details on the thermal annealing conditions for each sample are reported in Table S2 of the Supporting Information. SAXS patterns 8348

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Scheme 1. Reactions Occurring During the Hydrogenation of the PI Units in the ISM Triblock Copolymer Using Diimidea

Reactions: (1) diimide generation from the thermal decomposition of TSH at a decomposition temperature Td ∼ 107 °C; (2) hydrogenation of ISM triblocks with diimide, leading to EPSM triblock materials; (3) disproportion reaction of diimide, generating hydrazine (N2H4) and nitrogen (N2). a

Table 1. Characterization Data for ISM Precursor Triblocks, EPSM Triblocks, and Associated Blends volume compositiona polymer

f EP/I

fS

fM

Mn (g mol−1)

PDI

0.00/0.13 0.00/0.24 0.00/0.25 0.00/0.29

0.43 0.59 0.57 0.66

0.44 0.17 0.18 0.05

28 100 31 200 20 100 31 100

1.07 1.11 1.09 1.04

T1 T2 T3 T4

0.13/0.01 0.25/0.00 0.23/0.03 0.29/0.02

0.43 0.58 0.56 0.64

0.43 0.17 0.18 0.05

27 400 31 500 20 700 30 500

1.09 1.15 1.09 1.07

HT1/PS HT1/PMMA1 HT3/PS/PEP HT4/PEP HT4/PEP HT4/PMMA2

0.10/0.01 0.10/0.01 0.28/0.03 0.31/0.02 0.37/0.02 0.27/0.01

0.57 0.34 0.55 0.62 0.57 0.58

0.32 0.55 0.14 0.05 0.04 0.14

precursor

ISM triblocks T1 T2 T3 T4 EPSM triblocks HT1 HT2 HT3 HT4 EPSM blendsc bHT1_1 bHT1_2 bHT3 bHT4_1 bHT4_2 bHT4_3

% hydrogenation (molar basis)

phase LAM HEX → Q230b DIS HEX

90 100 86 95

Q230 SA HEX HEX Q230 HEX HEX HEX HEX HEX

a

Volume fractions of hydrogenated materials are calculated based on their respective mole percent hydrogenation and volume fractions of ISM triblock precursors. bHEX-to-Q230 transition on cooling. cEPSM blends refer to blends of EPSM triblocks with constituent homopolymers. Homopolymer Mn’s for PS, PMMA1, PMMA2, and PEP are 13.7, 11.3, 1.1, and 6.0 kg mol−1, respectively, with corresponding PDIs of 1.06, 1.30, 1.07, and 1.12. Blend bHT1_1 is a mixture of triblock HT1 with 26 vol % PS content, while blend bHT1_2 is a mixture of HT1 with 21 vol % PMMA1. bHT3 is a blend of triblock HT3 with 10 vol % PS and 9 vol % PEP. bHT4_1, bHT4_2, and bHT4_3 are blends of triblock HT4 with 3 vol % PEP, 11 vol % PEP, and 9 vol % PMMA2, respectively. Transmission Electron Microscopy (TEM). TEM micrographs were obtained at the W.M. Keck Electron Microscopy Facility at UD using a JEOL JEM-2000FX transmission electron microscope operating at 200 kV. The annealed SAXS samples were cut on a Leica Reichart Ultracut cryo-microtome at temperatures ranging from −60 to −75 °C (depending on the PI or PEP copolymer content) using a Diatome diamond knife. The ISM sample preparation is described elsewhere.25,26 The microtomed slices (70−100 nm in thickness), cut from hydrogenated (PEP-containing) materials, were placed on 400 mesh copper grids and vapor-stained using a 0.5% aqueous solution of RuO4 for 1.5 min to enhance the electron density contrast.

presented herein are vertically shifted for clarity, and unidentified, but expected, peak locations are indicated by dashed arrows. Synchrotron SAXS experiments were performed on the DND-CAT beamline at the Argonne National Laboratory (Advanced Photon Source - APS) and on the X27C beamline at the Brookhaven National Laboratory (National Synchrotron Light Source - NSLS). At the APS facility, two setups were used with an incident beam of wavelength λ = 0.729 Å and a Mar CCD detector at a sample-to-detector distance of 4.0 and 6.5 m, while the setup at the NSLS facility used an incident beam of wavelength λ = 1.371 Å and a Mar CCD detector at a sampleto-detector distance of 1.8 m. The synchrotron SAXS data from the APS and NSLS facilities are referred to as “APS-SAXS” and “NSLSSAXS” data, respectively. 8349

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RESULTS

The EPSM triblock materials were generated from the selective PI hydrogenation of neat ISM triblock copolymers with diimide, produced in situ by the thermal decomposition of ptoluenesulfonyl hydrazide (TSH), according to reaction 1 of Scheme 1.39,41 The hydrogenation of the PI block in ISM triblock materials produces an alternating ethylene and propylene unit and leads to the formation of fully or partially hydrogenated EPSM materials, depending on the TSH:PI molar ratio and reaction conditions (see reaction 2 in Scheme 1). Theoretically, an equimolar ratio of TSH:PI repeat unit should lead to complete hydrogenation. However, previous studies have shown this does not occur, as not all of the diimide intermediates formed during the thermal decomposition process react with sites of unsaturation in polydienes.39,41,61 It is known that diimide can simultaneously undergo disproportionation during the hydrogenation process, forming nitrogen (N2) and hydrazine (N2H4), as shown in reaction 3 of Scheme 1.39,61 The rate of disproportionation (k2) was found to be faster than the rate of hydrogenation (k1) of diene repeat units.39,41,46 Conversely, the thermal decomposition of TSH is known to form sulfur-containing byproducts (i.e., mainly ptoluenesulfinic acid and bis(p-tolyl) disulfide), capable of competing with the diimide products by also reacting with CC bonds in the PI block.38,61 Therefore, favoring complete hydrogenation while minimizing byproducts substitutions required a slow thermal decomposition of TSH, which was accomplished by finding a balance between the amount of excess TSH and the reaction temperature (i.e., above the toluene boiling point).41,46 As a result, a nearly complete hydrogenation of the ISM triblock materials was obtained using a solution of 3 wt % polymer in toluene with a 4:1 molar ratio of TSH:PI repeat units. Additional details regarding the PI hydrogenation of ISM materials can be found in the Experimental Section. The phase behavior of the parent ISM materials used in this hydrogenation study was described previously.25,26,48 To facilitate the comparison between ISM precursors and hydrogenated EPSM materials, the characterization data of neat ISM materials and hydrogenated triblock analogues are summarized in Table 1. Additionally, blends between select hydrogenated triblocks and their constituent homopolymers were prepared to probe the differences in phase behavior between the ISM26 and EPSM systems. The characterization data for the blended materials also are reported in Table 1. Extensive Q230 Network Phase Region in the EPSM Triblock Copolymer System. ISM triblock T1 was subjected to two consecutive PI hydrogenations (first hydrogenation yielded an 88 mol % hydrogenated EP/I block) to produce a 90 mol % hydrogenated triblock HT1. UD-SAXS data (Figure 1) for T1 show diffraction peaks at q*, 2q*, 3q*, and 4q*, suggestive of a lamellar morphology (LAM). Although the SAXS data for the ISM triblock (T1) show peaks characteristic of a LAM morphology, the hydrogenated EPSM triblock analogue (HT1) shows a different APS-SAXS signature with distinct reflection peaks located at √6q*, √8q*, √16q*, √20q*, √22q*, √24q*, √26q*, √30q*, √32q*, √34q*, √38q*, √42q*, √46q*, √48q*, and √50q*. Although the peaks at √14q* and √40q* (indicated by dashed arrows) are not identifiable, this peak sequence is consistent with the Ia3̅d space group, indicative of a Q230 network morphology (Figure 1). The corresponding TEM images support the LAM phase

Figure 1. SAXS data acquired at 30 °C for triblock T1, HT1, and associated blends bHT1_1 and bHT1_2, from mixing HT1 with PS and PMMA1, respectively. Triblock T1 exhibits a LAM morphology. HT1 and associated blend bHT1_1 suggest the Ia3̅d space group symmetry, characteristic of a Q230 network nanostructure. Blend bHT1_2 suggests a hexagonal symmetry, indicative of a hexagonally packed cylinder (HEX) morphology.

assignment for ISM triblock precursor, T1 (Figure 2a), and reveal two complementary projections of the Q230 network morphology for hydrogenated EPSM triblock, HT1: the “wagon-wheel pattern”, characteristic of the [111] projection (Figure 2b) and an additional orientation resembling the Q230

Figure 2. TEM micrographs obtained from (a) T1 (LAM), (b) HT1 (Q230 network), (c) bHT1_1 (Q230 network), and (d) bHT1_2 (HEX): (a) the PI domains (dark) of sample T1 are stained with OsO4; (b−d) the PS domains (dark gray) and PS/PEP interfaces are stained with RuO4. Scale bars are 100 nm. 8350

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network [125] projection (inset image of Figure 2b). Two blends were generated from mixing the hydrogenated Q230 network-forming material HT1 with either 26 vol % PS content (blend bHT1_1) or 21 vol % PMMA1 content (blend bHT1_2). Blend compositions for bHT1_1 and bHT1_2 were chosen to match composition-equivalent blends from the ISM precursor system (i.e., blends of ISM triblock T1 with the same PS and PMMA homopolymers, respectively).26 Blend bHT1_1 revealed a similar APS-SAXS pattern to that of the hydrogenated material HT1 but showed less apparent ordering than the HT1 precursor material (Figure 1). The corresponding TEM image for blend bHT1_1 (Figure 2c) provides support for the Q230 network assignment. Upon addition of 21 vol % PMMA1 to HT1, blend bHT1_2 shows a different APSSAXS signature with reflections at q*, √3q*, √4q*, √7q*, √9q*, √12q*, √16q*, √19q*, and √21q*, consistent with a hexagonal symmetry (Figure 1). The TEM micrographs confirm a well-defined core−shell HEX morphology, as illustrated in the parallel (Figure 2d) and perpendicular (inset of Figure 2d) orientations, where PEP cylinders are surrounded by a shell of PS in a matrix of PMMA. Additionally, two blends were generated from mixing HT1 with ∼9 vol % PS content (bHT1_3) and ∼18 vol % PS content (bHT1_4), in order to assess the extent of the Q230 network phase region, at intermediate compositions between the Q230 network-forming HT1 and bHT1_1 specimens. The UD-SAXS patterns for blends bHT1_3 and bHT1_4, shown in the Supporting Information (Figure S1), also are indexed according to the Ia3̅d space group, characteristic of the Q230 morphology. Alternating Spheres in the EPSM Triblock Copolymer System. ISM triblock T2, exhibiting a likely HEX-to-Q230 phase transition on cooling, was nearly 100% hydrogenated to produce the EPSM sample HT2. Details on the characterization of the ISM triblock T2 can be found in the Supporting Information (UD-SAXS data in Figures S2 and S3 and TEM micrographs in Figure S4). APS-SAXS data and a TEM micrograph, acquired at 30 °C for sample T2, are presented in Figures 3a and 3b, respectively, in order to allow for a direct comparison with the EPSM triblock analogue HT2. The APSSAXS pattern for sample HT2, provided in Figure 3a, shows Bragg reflection peaks at consecutive integer square roots q*, √2q*, √4q*, √5q*, √6q*, ..., up to √27q*, suggestive of a body-centered cubic spherical symmetry.15,62 However, several higher order peaks were not located at √3q*, √7q*, √15q*, √18q*, √19q*, √22q*, and √23q* (dashed arrows). The corresponding TEM micrograph in Figure 3c further supports an alternating spherical morphology (SA) of PEP (light dots with dark outlines) and PMMA (light dots) spheres in a continuous PS matrix (gray). Inducing Order from a Disordered ISM Copolymer upon Hydrogenation. ISM triblock T3 was characterized as a disordered melt (DIS) using a combination of SAXS and TEM analysis.48 APS-SAXS data for triblock T3 reveal a broad primary peak over the entire range of temperatures explored (Figure 4). This lack of ordering was confirmed in a corresponding TEM micrograph (Figure 5a). Upon 86 mol % hydrogenation of the PI block in triblock T3, the resulting hydrogenated material, HT3, showed a well-defined APS-SAXS pattern with Bragg peaks located at q*, √3q*, √4q*, √7q*, and √9q*, suggestive of a hexagonal symmetry (Figure 4). TEM micrographs support a HEX assignment in HT3 (Figure 5b). We note that, as this material was only partially hydrogenated and had an appreciable PI content, osmium

Figure 3. (a) APS-SAXS data for triblock T2 and HT2 (hydrogenated T2) acquired at 30 °C. Triblock T2 was identified as a possible HEX/ Q230 mixed-morphology resulting from a HEX-to-Q230 phase transition on cooling (see Supporting Information). Sample HT2 is indicative of a possible spherical symmetry. TEM micrographs obtained from (b) T2 (HEX/Q230 mixed-morphology) and (c) HT2 (alternating spheres - SA). (b) Dark regions represent the PI domain (stained with OsO4) and lighter regions correspond to the unstained PS and PMMA domains. (c) Gray region represents the PS matrix (stained by RuO4), the lighter spheres with black outline represent the PEP domain (contours stained with RuO4), and the brighter spheres correspond to the unstained PMMA domains. An illustration of the SA morphology is provided in the inset of (c) for clarity. TEM scale bars are 100 nm.

tetroxide (OsO4) was used to preferentially stain the remaining unsaturated repeat units on the randomly hydrogenated PI chains. Using this staining agent provided better contrast than staining with RuO4. The resulting TEM images show perpendicular (Figure 5b) and parallel (inset of Figure 5b) orientations of the HEX morphology, in which the stained PEP/PI cylinders appeared dark in a matrix of unstained (and indistinguishable) PS and PMMA domains (light). Lastly, blend bHT3 was obtained by blending the hydrogenated material HT3 with 10 vol % PS and 9 vol % PEP. This sample retained a HEX morphology, as indicated by the NSLS-SAXS data in Figure 4, with well-defined peaks at q*, √3q*, √4q*, √7q*, √9q*, √12q*, √13q*, √16q*, and √19q*. The TEM images for blend bHT3 support this HEX morphology assignment (Figure 5c). Additionally, EPSM triblock material HT4, situated in the vicinity of triblock HT3 on the PMMA-poor side of the EPSM phase diagram, was produced by 95 mol % PI hydrogenation of 8351

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DISCUSSION

In this study, EPSM triblock copolymers were generated by hydrogenating the PI block in ISM triblock precursors. The anionically synthesized ISM precursors provided materials with narrow molecular weight distributions that assembled into distinctive network structures.26 Furthermore, the chemically modified EPSM triblock analogues presented in this study will offer materials with improved chemical and physical properties that will facilitate the removal of the PMMA block, while also preventing the collapse of the nanoscale structure for the creation of tough yet flexible nanoporous membranes. As noted previously,25,26 the ISM copolymers are nonfrustrated with statistical segment length ratios close to unity (bM/bI = 0.95, bI/bS = 1.11, and bS/bM = 0.95)59,63 and a χparameter sequence of χIM64,65 ≫ χIS66 > χSM67 (at 100 °C). Therefore, a direct comparison could be made between the nearly ideal ISM system and a symmetric model ABC phase diagram by Tyler et al.68 Upon hydrogenation, the resulting EPSM triblock system is substantially more asymmetric with appreciably different statistical segment length ratios, in comparison to the ISM precursor system (bM/bEP = 0.85, bEP/bS = 1.25, and bS/bM = 0.95).59,63 Additionally, although the chemical modification of the PI block in the ISM system does not significantly alter the overall degree of polymerization (NEP ≈ NI and hence, NEPSM ≈ NISM = NI (or NEP) + NS + NM) in the hydrogenated materials, it will affect the interaction parameters involving the PEP block (χEPS and χEPM). These changes in effective segregation strengths (χEPSN and χEPMN vs χISN and χIMN, respectively) and segment length ratios (bM/bEP and bEP/bS vs bM/bI and bI/bS, respectively) are expected to affect the phase behavior of the EPSM system with respect to that of the ISM triblock precursors and the Tyler et al. model ABC system.68 The temperature-dependent expressions for χIS and χEPS were reported by Sakurai et al. (χIS = 0.042 and χEPS = 0.069 at 140 °C)69 and as expected, χEPS > χIS (Table S1 in the Supporting Information). To the best of the author’s knowledge, no temperature-dependent equation for χEPM have been reported in the literature, but we believe that χEPM > χIM based on estimations from solubility parameter theory (χIM = 0.1425 and χEPM = 0.2079 at 140 °C; see Table S1 in the Supporting Information). To assess quantitative differences in χ-parameters between the EPSM and ISM systems, the associated χ-parameters were estimated using solubility parameter theory (based on regular solution theory), χ12 = V0(δ2 − δ1)2/RT, where the molar volume of the component pair (V0) is calculated from the geometric mean of the component molar volumes: V0 = (V1V2)1/2, δ is the solubility parameter for each polymer, R is the gas constant, and T is the temperature in kelvin.17 We note that a modified equation to the regular solution theory can be applied, which includes an entropic interaction term (usually a constant ranging between 0.3 and 0.4).70 In this study, however, the entropic contributions are neglected as we are interested in the relative differences between the estimated χ-parameters of the ISM and EPSM triblock systems. Details regarding the calculation of χparameters (see Table S1 in the Supporting Information) indicate that the difference in the neighboring blocks (EP/S vs I/S) χ-parameters is larger in EPSM than in the ISM precursor. However, the EPSM system still follows the same χ-parameter sequence as for the ISM system (i.e., χEPM1,35 > χEPS1,35 ≫ χSM1), which implies that this nonfrustrated and asymmetric

Figure 4. SAXS data for triblock T3, HT3, and bHT3 (a blend of HT3 and PS/PEP homopolymers). All SAXS patterns were acquired at 30 °C. Sample T3 shows a broad primary peak, indicative of a disordered specimen. Sample HT3 reordered upon hydrogenation to a hexagonal symmetry. Blend bHT3 retained the hexagonal symmetry.

Figure 5. TEM micrographs obtained from (a) T3 (DIS), (b) HT3 (HEX), and (c) bHT3 (HEX). (a, b) Stained with OsO4. (c) Stained with RuO4. The TEM micrographs of (b) and (c) support the HEX morphology assignment, where both perpendicular and parallel (inset) orientations are noted. Scale bars represent 100 nm.

a HEX-forming ISM triblock precursor (T4). Three blends (bHT4_1, bHT4_2, and bHT4_3) also were prepared by mixing hydrogenated material HT4 with 3 vol % PEP, 11 vol % PEP, and 9 vol % PMMA2, respectively, to probe the EPSM phase space at similar compositions to ISM triblock/ homopolymer blend analogues.26 Based on SAXS and TEM analyses, triblock HT4 and associated blends retained the HEX morphology of the ISM materials. SAXS data (Figure S5) and TEM micrographs (Figure S6) for T4, HT4, and blended materials are reported in the Supporting Information, and these samples will be discussed further in the Discussion section. 8352

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EPSM system also should favor the formation of network structures as the EP and M blocks are unlikely to mix.71 To investigate the differences in phase behavior between the EPSM and ISM systems near network phase regions, judicious blends made from EPSM triblocks and corresponding homopolymers were targeted to match their non-hydrogenated analogue blends (i.e., ISM triblock/homopolymer blends) at similar compositions.26 Using this approach, several morphological differences were revealed between the ISM precursor and the hydrogenated EPSM systems. Three blends were generated by mixing Q230 networkforming HT1 with incremental amounts of PS homopolymer (bHT1_3, bHT1_4, and bHT1_1 with 9, 18, and 26 vol % PS content, respectively) to probe the extent of the Q230 network phase region in the EPSM system. The blend composition for bHT1_1 was chosen to match a composition-equivalent Q230 network-forming ISM blend, reported in a previous publication.26 Although a HEX morphology is expected in the Tyler et al. ABC phase diagram at similar compositions, blend bHT1_1 retained a Q230 network structure. The additional blends bHT1_3 and bHT1_4, generated at intermediate compositions between EPSM triblock HT1 and blend bHT1, appeared to retain a Q230 network morphology. This latter finding provides possible evidence of an extended Q230 network phase in the EPSM system, when compared to the ISM system at similar compositions. The general trend depicted by the shift of the phase boundaries of the EPSM system resembles that of a less idealized poly(isoprene-b-styrene-b-ethylene oxide) (ISO) triblock copolymer system, as seen in the work by Epps et al. and later in the self-consistent field theory (SCFT) theoretical predictions based on the ISO system by Tyler et al.14,68 In contrast to the ISM phase map reported previously,26 conformational asymmetries (bEP/bM = 1.18)59,63 resulted in a shift in the phase boundaries toward the EP-rich corner, along the EP/M edge of the EPSM phase map, which likely contributed to the enlargement of the Q230 network phase region toward the M-rich corner. An illustration of the extended Q230 network phase region in the EPSM system (dashed-green region) as compared to that of the ISM precursor system (bottom green-colored region) is shown in a partial phase map depicting the change in phase boundaries upon hydrogenation (Figure 6). Additional exploration of network phase regions was carried out in the EPSM system at similar compositions to noncubic network-forming ISM/PMMA blends. To this end, a blend, bHT1_2 was made from mixing HT1 with 21% PMMA1. This material was targeted to match blends located in the O70 orthorhombic network region of the ISM system.26 Instead of exhibiting an O70 orthorhombic network structure, blend bHT1_2 exhibited a well-defined HEX morphology (Figure 6). Although this blend lies near the PMMA-rich LAM region of Tyler et al. ABC phase diagram and at close proximity to the Q230 network phase region, a HEX-forming blend at such compositions is not unexpected as the conformational asymmetries of the EPSM system may affect the Q230-to-HEX phase sequence predicted in the Tyler et al. ABC model phase diagram.68 Furthermore, the increase in χ-parameters between the PEP block and the PS and PMMA blocks is reminiscent of the lithium salt doping of the ISO system, which led to a morphological transition from O70 to HEX upon doping.72 Similar results were obtained from simulation efforts;73 however, we note that the change in χ-parameters is not nearly as dramatic in this work.

Figure 6. Partial phase map depicting the estimated ISM phase regions of LAM, HEX, DIS, and three network structures (Q214 alternating gyroid, Q230 core−shell gyroid, and O70 orthorhombic).25,26,48 The phase diagram is overlaid with EPSM triblock copolymers (fully or partially hydrogenated HT1, HT2, HT3, and HT4 materials analogue to the ISM triblock precursors, represented by colored circles with black outline) and associated blends (colored circles). The dashed colored contours highlight the approximate change in phase boundaries in the hydrogenated (EPSM) triblock system based on the samples investigated in this work. We note that the phase boundaries may expand (or contract) as additional samples are examined.

To further probe the phase behavior, sample T2 (assigned to possible HEX/Q230 phase-mixed morphology) was hydrogenated to sample HT2, which exhibited an alternating spherical morphology (SA), as shown in Figure 6. A small SA region was stable along the fA = f C isopleth of the Tyler et al. model ABC phase diagram, and this SA-forming sample HT2 is located in the HEX region of the model phase diagram, but very close to the HEX/SA boundary. This SA finding for HT2 further indicates that the EPSM system, while similar to the ISM system, does not behave as symmetrically with respect to the f EP = f M isopleth as the ABC model system, likely due to the greater difference in the neighboring block χ-parameters (χEPS > χSM), as well as the difference in statistical segment length ratios.1,35,68 Finally, a morphological shift from a DIS state in the ISM triblock precursor T3 to a HEX morphology in triblock HT3 was noted upon hydrogenation. This HEX-forming triblock HT3 is found to be qualitatively consistent with the Tyler et al. ABC phase behavior, as sample HT3 is located in close proximity to the network/HEX phase boundary.68 Blend bHT3, made by mixing HT3 with PS/PEP homopolymers, was targeted to similar compositions to a Q230 network-forming ISM/PS/PI blend analogue;26 however, the hydrogenated blend retained a HEX morphology (Figure 6). Additionally, blends bHT4_1 (3 vol % PEP) and bHT4_3 (9 vol % PMMA2) were targeted to match the compositions of a Q230 8353

dx.doi.org/10.1021/ma301739j | Macromolecules 2012, 45, 8347−8355

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(FA9550-09-1-0706). M. Tureau acknowledges an Air Products and Chemicals, Inc., Fellowship. Use of the DuPont− Northwestern−Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS) was supported by E. I. DuPont de Nemours & Co., the Dow Chemical Company, and the State of Illinois. Use of the APS was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DEAC02-06CH11357. Portions of this work also were performed at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. NSLS is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886. NMR spectra were obtained with instrumentation supported by NSF CRIF:MU, CHE 0840401. We acknowledge the W.M. Keck Electron Microscopy Facility at the University of Delaware for the use of their JEOL JEM-2000FX TEM instrument. We also thank E. Kelley for the synthesis of the PS homopolymer.

network-forming ISM blend and HEX-forming blend analogues, respectively;26 however, these blends also retained the HEX morphology (Figures S5 and S6 in the Supporting Information). The bHT4_3 blend may imply that the HEX-toQ230 phase boundary in the EPSM system occurs at higher PEP contents than in the ISM system (i.e., PI content), as shown in Figure 6. Again, this phenomenon likely arises from the considerable conformational asymmetries in the EPSM system, which results in an expanded HEX phase region as the phase boundaries tend to shift toward compositions richer in PEP as bEP/bS = 1.25 in EPSM.59,63,74

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CONCLUSIONS In summary, four EPSM triblock copolymers were generated by hydrogenation of PI repeat units in ISM triblock copolymer precursors, using diimide as a hydrogenating agent. In view of creating nanoporous templates, the combination of the strong PS and flexible PEP blocks is expected to enhance mechanical integrity as well as environmental resistance, while also facilitating the removal of the PMMA domains. The morphological characterization of the EPSM triblock copolymers and associated EPSM copolymer/homopolymer blends has been investigated using a combination of SAXS and TEM techniques, where well-ordered HEX, SA, and Q230 network morphologies were identified. This hydrogenation study provides experimental understanding for how changing χparameters and statistical segment length ratios impact network formation in an ABC triblock copolymer system. The variations in phase behavior between the EPSM system and the ISM precursor system arise from the significant differences in conformational asymmetry between the two systems (larger statistical segment length ratios bEP/bS > bI/bS and bEP/bM > bI/ bM and segregation strengths χEPSN > χISN and χEPMN > χIMN). Particularly, this study presents an initial assessment on the location of an extended Q230 network window in the PMMArich region of the EPSM system, which gives further insight into the universality of the phase behavior of nonfrustrated ABC triblock copolymer systems.

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ASSOCIATED CONTENT

S Supporting Information *

χ-parameters for ISM and EPSM triblock systems (Table S1); thermal annealing conditions for the reported neat and blended ISM and EPSM materials (Table S2); SAXS patterns for two additional Q230 network-forming blends produced from EPSM triblock (HT1) and PS homopolymer (Figure S1); detailed characterization of triblock T2 (SAXS and TEM in Figures S2, S3, and S4); characterization of HEX-forming ISM triblock (T4), EPSM triblock (HT4), and associated blends of HT4 and homopolymers (SAXS and TEM in Figures S5 and S6, respectively). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF-NER (CBET-0707507), NSF-CAREER (DMR-0645586), and AFOSR-PECASE 8354

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