Various Phase Behaviors of Weakly Interacting Binary Block

May 23, 2013 - Various Phase Behaviors of Weakly Interacting Binary Block. Copolymer Blends. Hyungju Ahn,. †,§. Yonghoon Lee,. †,§. Hoyeon Lee,...
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Various Phase Behaviors of Weakly Interacting Binary Block Copolymer Blends Hyungju Ahn,†,§ Yonghoon Lee,†,§ Hoyeon Lee,† Young Soo Han,‡ Baek Seok Seong,‡ and Du Yeol Ryu*,† †

Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 120-749, Korea Neutron Science Division, Research Reactor Utilization Department, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea

‡

ABSTRACT: We investigated the phase behaviors of block copolymer (BCP) blends composed of the weakly interacting (with no specific interaction) polystyrene-b-poly(n-butyl methacrylate) (PS-b-PnBMA) and polystyrene-b-poly(n-hexyl methacrylate) (PS-b-PnHMA), using small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and depolarized light scattering (DPLS). A miscible phase between the PnBMA and PnHMA blocks in the BCP blends allows the various composition-dependent phase behaviors, which display a lower disorder-to-order transition (LDOT) to an order-to-disorder transition (ODT) on heating as the blend composition varies from PS-b-PnBMA to PS-bPnHMA. In between these two extremes, intriguingly, the miscible PS-b-PnBMA/ PS-b-PnHMA blends display a closed-loop phase transition consisting of a LDOT and an upper order-to-disorder transition (UODT).

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INTRODUCTION The phase behavior of multicomponent polymeric materials has been the subject of many experimental and theoretical studies, since it is essential to the compatibility or self-assembly in most of practical applications involving polymer blends and block copolymers (BCPs).1−5 Diblock copolymers with the two dissimilar polymer blocks undergo an order-to-disorder transition (ODT) upon heating, when unfavorable segmental interaction (χ) between the two blocks weakens and the phases mix together.4−6 This is analogous to the upper critical solution transition (UCST) in the binary polymer blends.2,4,5 Meanwhile, in the weakly interacting (with no specific interaction) BCPs and polymer blends, a dominant effect of thermal compressibility (or thermal expansion) difference between the two components causes the lower disorder-to-order transition (LDOT) on heating and the lower critical solution transition (LCST), respectively.3,7−11 Especially in the weakly interacting BCP homologues of polystyrene-b-poly(alkyl methacrylate) (PS-b-PAMA) copolymers, a variety of phase behaviors have been verified depending on alkyl chain length (n) in the methacrylate unit, as described in Figure 1. Polystyrene-b-poly(methyl methacrylate) (PS-bPMMA) for n = 1 displays a typical ODT type,12−14 but polystyrene-b-poly(ethyl methacrylate) (PS-b-PEMA), polystyrene-b-poly(n-propyl methacrylate) (PS-b-PnPMA), and polystyrene-b-poly(n-butyl methacrylate) (PS-b-PnBMA) for n = 2, 3, and 4, respectively, display a combined ODT and LDOT type.10,15−17 The entropically driven LDOT occurs at higher temperatures due to the growing repulsive interactions arising from thermal expansion difference between the two compo© XXXX American Chemical Society

Figure 1. Schematic phase diagrams for the BCP homologues of polystyrene-b-poly(alkyl methacrylate) (PS-b-PAMA) copolymers. The n indicates the alkyl chain length in the methacrylate unit in the BCPs.

nents, although the reduction in the enthalpic repulsive interaction leads to an ODT at lower temperatures.6,18−22 Moreover, polystyrene-b-poly(n-pentyl methacrylate) (PS-bPnPeMA) for n = 5 and its homopolymer blend exhibit a combined ODT and closed-loop type, in which the closed loop involves a LDOT and an upper order-to-disorder transition (UODT).23−29 The phase diagram, namely closed loop above the ODT, involves each phase behavior observed in the weakly interacting BCP homologues of PS-b-PnAMA copolymers.17 It should be noted that the compressible UODT is distinct from Received: December 7, 2012 Revised: May 13, 2013

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showed a deep purple color, indicating an oxygen- and moisture-free solvent. Degassed monomers with CaH2 (Aldrich) were vacuum distilled over the second purifiers of dried dibutylmagnesium for styrene and trioctylaluminum for n-alkyl methacrylates, until a characteristic color was persisted. The polymer solution terminated by purified 2-propanol was precipitated in excess methanol. The volume fraction (ϕ) of these BCPs was determined by 1H nuclear magnetic resonance (1H NMR) measurement, based on the mass densities of the four components (1.05, 1.13, 1.05, and 1.01 g/ cm3 for the PS, dPS, PnBMA, and PnHMA, respectively).17,33 The number- and weight-average molecular weights, Mn and Mw, respectively, were measured using multiangle laser light scattering (MALLS) combined with size exclusion chromatography (SEC). The dispersities (Mw/Mn) of the polymer samples were less than 1.06. The binary (d)PS-b-PnBMA/(d)PS-b-PnHMA blends were prepared from polymer solutions using a freeze-drying method. For instance, a predetermined amount of the BCPs was dissolved in benzene (∼5 wt % in solution) as a cosolvent, and the quenched solution at liquid nitrogen was sublimated under vacuum for 24 h to remove the solvent at room temperature. The samples were sequentially annealed at a target temperature (120 °C) for 24 h for equilibrium; this condition turned out to be enough to allow a thermally equilibrated state of the BCPs above the glass transition temperatures (Tg) of the (d)PS block (∼100 °C). Small-angle neutron scattering (SANS) experiments were performed in the 18 m SANS beamline at the HANARO, Korea. The wavelength (λ) of the neutron beam was 8.28 Å with a Δλ/λ = 0.12. The scattered intensities for 10 min exposure time were collected on a 2-D area detector and then azimuthally averaged. The experimental conditions were set up in a circular beam with a diameter of 5 mm, a sample thickness of 1 mm, and a sample-to-detector distance of 3 m. Synchrotron small-angle X-ray scattering (SAXS) experiments were also carried out in the 3C and 4C beamlines at the Pohang Light Source (PLS), Korea. The wavelength (λ) of the X-ray beam was 1.175 Å, and the energy resolution (ΔE/E) was 2 × 10−4. A 2-D Mar CCD camera (Rayonix LLC., Marccd-165) was used to collect the scattered intensities. The experimental conditions were set up in a typical beam size of 0.8 × 0.8 mm2, a sample thickness of 1.5 mm, a sample-todetector distance of 3 m, and the exposure times of 60−150 s. Depolarized light scattering (DPLS) experiments, using a polarized beam from a He−Ne laser source at a wavelength of 632.8 nm, were used to probe the transition temperatures for the nondeuterated BCP blends. A sample thickness was set to 1.0 mm in a small bronze template with a 5 mm diameter hole under an ambient condition. The intensity detected at the photodiode was recorded (through A/D converter) as a function of temperature at a heating rate of 0.7 °C/min from 120 to 250 °C under nitrogen flow. All samples for thermal experiments were prepared using compression-molding at lower than 120 °C immediately after the samples were thermally annealed at 120 °C. The heating processes were controlled automatically with a PID temperature controller at a constant heating rate of 0.7 °C/min; the experiments were performed under nitrogen flow to avoid thermal degradation of the polymer samples.

the incompressible (or less compressible) ODT because the UODT of the closed loop in the PS-b-PnPeMA exhibits a baroplastic property with the strong pressure dependence of transition temperatures (|ΔT/ΔP| ≈ 725 °C/kbar),26,30 as similarly observed in that of compressible LDOT.10,16 A baroplastic property in polymeric materials was highlighted as a desirable nature for industrial applications since it allows ones to save the processing energy in manufacturing or regenerating polymer products.30,31 Particularly for n = 6, the phase behavior of polystyrene-bpoly(n-hexyl methacrylate) (PS-b-PnHMA) was simply introduced to be an weakly compressible ODT type in the literature10 because the PS-b-PnHMA exhibits a weak baroplastic property (|ΔT/ΔP| ≈ 60−115 °C/kbar).16,32 However, it was reported that the binary blend of polystyrene/poly(nhexyl methacrylate) displays a combined UCST and LCST type at ambient conditions, while the corresponding BCP undergoes a single ODT transition on heating, as shown in Figure 1.33 Considering that the phase behavior of a binary blend is identical to that of BCP by the differences in the molecular weights, there might have been a missing point on understanding the phase behavior type for the PS-b-PnHMA especially at higher temperature than thermal degradation. Here, we utilized a miscible phase between poly(n-butyl methacrylate) (PnBMA) and poly(n-hexyl methacrylate) (PnHMA) to trace the phase behaviors of the BCP blends composed of the two compressible PS-b-PnBMA and PS-bPnHMA, using small-angle neutron scattering (SANS), smallangle X-ray scattering (SAXS), and depolarized light scattering (DPLS). The various composition-dependent phase behaviors were generated from a LDOT-type to an ODT-type transitions on heating as the blend composition varies from PS-b-PnBMA to PS-b-PnHMA. Interestingly, a closed-loop type transition consisting of a LDOT and an UODT was found in nearly symmetric composition of the miscible PS-b-PnBMA/PS-bPnHMA blends.

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EXPERIMENTAL SECTION

Table 1 shows three homopolymers, the PS, PnBMA, and PnHMA, and four symmetric nondeuterated and deuterated BCPs, (d)PS-b-

Table 1. Sample Characteristics of Homopolymers and Block Copolymers Used in the Present Study polymers

Mn (g/mol)

Mw/Mn

ϕ(d)PSa

remark

PS PnHMA PnBMA PS-b-PnHMA PS-b-PnBMA dPS-b-PnHMA dPS-b-PnBMA

40 000 32 800 37 700 39 800 66 900 25 300 47 200

1.06 1.06 1.06

1 0 0 0.50 0.50 0.50 0.50

ODT at 226 °C LDOT at 160 °C disordered disordered

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RESULTS AND DISCUSSION

The miscibility between homopolymers was first evaluated by the glass transition temperature (Tg) and optical microscopy (OM) measurements. Figure 2 shows the thermograms measured by differential scanning calorimetry (DSC: PerkinElmer Diamond DSC) at a heating rate of 20 °C/min from −50 to 150 °C. The DSC data were obtained during the second heating run to remove the thermal history of the samples immediately after thermal annealing at 150 °C for 12 h under vacuum. The Tg values for the homopolymersthe PS (40 000 g/mol), PnBMA (32 800 g/mol), and PnHMA (37 700 g/ mol)were identified, as indicated by the arrows in Figure 2, at 105, 42, and −6 °C, respectively. All the molecular weights of homopolymers were controlled to be higher than those of

a

Volume fractions of (d)PS were calculated with mass densities of the components (1.13, 1.05, 1.05, and 1.006 g/cm3 for dPS, PS, PnBMA, and PnHMA, respectively) using 1H and 13C nuclear magnetic resonance (NMR). PnBMA and (d)PS-b-PnHMA copolymers. All the samples were synthesized via living anionic polymerization of each monomer in tetrahydrofuran (THF) at −78 °C under purified argon; these reactions were performed in the presence of LiCl (high purity, Aldrich) using sec-butyllithium (1.3 M, Aldrich) or sec-butyllithium coupled with 1,1-diphenylethylene as an initiator. THF refluxed from CaH2 was repurified with fresh sodium benzophenone complex until it B

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PS-rich and PnBMA-rich (or PnHMA-rich) phases, which were confirmed by the turbid OM images in the insets. In contrast, a single Tg value at 15 °C for the PnBMA/PnHMA blend indicates that the PnBMA is miscible with the PnHMA within these molecular weight ranges, as also confirmed by the transparent OM image in the inset. The phase for the PnBMA/ PnHMA blend remained homogeneous up to 250 °C, which is the maximum temperature prior to sample degradation; this exceptionally miscible phase between the two homopolymers is assumed to be a homogeneous phase in the following BCP blends containing each homologous block. In the BCP blends between a LDOT-type PS-b-PnBMA and an ODT-type PS-b-PnHMA, the composition-dependent phase behaviors of the BCP blends are traceable since the PnBMA and PnHMA phases are miscible into a homogeneous phase. Using SANS experiments, we investigated the phase behaviors of the PS-selectively deuterated BCP blends between the two disordered dPS-b-PnBMA (47 200 g/mol) and dPS-b-PnHMA (25 300 g/mol), with a dPS volume fraction (ϕdps) of two samples being 0.50. The transparent samples after thermal annealing at 120 °C for 24 h under vacuum indicated that no macrophase separation occurred because the PnBMA block is miscible with the PnHMA block. Figure 3a shows the SANS intensity profiles for the dPS-b-PnBMA/dPS-b-PnHMA blend with f = 0.5 (weight fraction of dPS-b-PnHMA) as a function of the scattering vector (q), where q = (4π/λ) sin θ, and 2θ and λ are the scattering angle and wavelength of the neutron beam, respectively. The intensity profiles were obtained every 10 °C or at the selected temperatures during heating above 120 °C. Iabs(q) is the absolute intensity in cm−1. A broad maximum at several temperatures during heating varies in the maximum intensity (Imax) with temperature, which is the characteristic correlation hole scattering in a disordered state of the BCP

Figure 2. DSC thermograms for the homopolymers and binary blends prepared at a critical composition: the PS/PnBMA (44/56), PS/ PnHMA (43/57), and PnHMA/PnBMA (49/51) blends. The OM images of thin-layer mixtures on glass slide show a direct turbidity evaluation for the miscibility between the two components.

corresponding blocks in the BCPs. The binary blends were prepared near at a critical composition (ϕc) given by ϕc = √N1/[√N1 + √N2], where Ni is the degree of polymerization for each homopolymer i. For the PS/PnBMA and PS/PnHMA blends, the two distinct Tg values at lower and higher temperatures are attributed to macrophase separation into

Figure 3. (a) SANS intensity profiles for the dPS-b-PnBMA/dPS-b-PnHMA blend with f = 0.5 (weight fraction of dPS-b-PnHMA) as a function of the scattering vector (q). The lines on the symbols are fit curves based on the incompressible RPA. The inset shows the temperature-dependent Imax for the PS-selectively deuterated BCP blend. (b) Effective interaction parameter (χeff) between the dPS and PnBMA/PnHMA phases, where the two plots for the dPS-b-PnBMA and dPS-b-PnHMA were reproduced from ref 17 for comparison. C

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Figure 4. (a) SAXS intensity profiles for the PS-b-PnBMA/PS-b-PnHMA blend with f = 0.2 (weight fraction of PS-b-PnHMA) as a function of the scattering vector (q). All profiles were measured at various temperatures during heating at a heating rate of 0.7 °C/min from 120 to 260 °C, which are well above the glass transition temperatures of PS (100 °C). The intensity profiles were vertically shifted by a factor of 5 to avoid overlapping. (b) Inverse of the maximum intensity (1/I(q*)) and full width at half-maximum (fwhm) as a function of the inverse temperature (1/K).

shows χeff for the dPS-b-PnBMA/dPS-b-PnHMA blends as a function of the inverse temperature (1/T), where the two plots for the dPS-b-PnBMA (f = 0) and dPS-b-PnHMA (f = 1) were reproduced with open symbols for comparison.17 At f = 0 (or dPS-b-PnBMA), an increase in χeff with increasing temperature indicates the LDOT-type phase behavior, but a plateau-like χeff at higher temperatures suggests that the other decrease may occur above the thermal degradation temperature (Td).17 This speculation could be verified in the dPS-b-PnBMA/dPS-bPnHMA blends as the amount of dPS-b-PnHMA increases, as shown for the deuterated BCP blend with f = 0.2 by a linear increase to 200 °C and slight decrease in χeff with increasing temperature. The similar but distinct trend in χeff was observed in the deuterated BCP blend with f = 0.5, corresponding to the closed-loop type phase behavior, as similarly observed in the (d)PS-b-PnPeMA.23,24,30 For the deuterated BCP blend with f = 0.8 and the dPS-b-PnHMA ( f = 1), however, a decrease in χeff with increasing temperature indicates an ODT-like phase behavior, which resembles a mirror (or opposite) image of that in the dPS-b-PnBMA (f = 0). A noteworthy feature is that the χeff curve in the deuterated BCP blend with f = 0.8 exhibits a plateau at lower temperatures more than likely because it passes over the maximum observed in the closed loop, suggesting that the phase behaviors are continuously shifted from a closed-loop type to an ODT type on heating. In addition, it is noticeable that an increasing trend in overall χeff range is seen in Figure 3b, as the blend composition varies from PS-b-PnBMA to PS-bPnHMA. Therefore, the SANS result represents that the various phase behaviors are strongly dependent on composition of the deuterated BCP blends. The real phase transitions were evaluated with the highermolecular-weight BCP blends between the two PS-b-PnBMA (66 900 g/mol) and PS-b-PnHMA (39 800 g/mol), in which the molecular weights were delicately tuned to exhibit transition temperatures. The PS volume fraction (ϕps) of two samples was set to 0.50. Figure 4a shows the SAXS intensity profiles for the PS-b-PnBMA/PS-b-PnHMA blend with f = 0.2 (weight fraction of PS-b-PnHMA) as a function of the scattering vector (q), where q = (4π/λ) sin θ, and 2θ and λ are the scattering angle and wavelength of the incident X-ray

blend. The inset shows the temperature-dependent Imax for the deuterated BCP blend. The Imax increases to 215 °C and then decreases with further increasing temperature. The variation in the value of Imax that is proportional to unfavorable segmental interaction (χ) between the two components reflects the phase behavior types depending on composition in the BCP blends. The effective interaction parameter (χeff) between the dPS and the PnBMA/PnHMA mixture in the dPS-b-PnBMA/dPS-b-PnHMA blends can be obtained by fitting the SANS intensity profiles to the scattering function S(q) using Iabs(q) =

2 ⎛b dΣ(q) b ⎞ = υref ⎜ 1 − 2 ⎟ S(q) dΩ υ2 ⎠ ⎝ υ1

(1)

where bi is the coherent neutron scattering length and υi is the monomeric volume for the ith component. Since the PnBMA block is miscible with the PnHMA block, the reference volume (υref) was assumed with the two-components system according to υref = [υsp ,dPS[M]o,dPS υsp ,mix [M]o,mix ]1/2

(2)

where υsp,i and [M]o,i are the specific volume (cm /g) and the monomer molecular weight of the ith component, respectively, with the weight-average values of the PnBMA/PnHMA mixtures. Based on the random-phase approximation (RPA), S(q) is given by 3

F (x , f ) 1 = − 2χeff S(q) N

(3)

where F is a function of x = (Rgq)2 and the composition. The N and Rg are the overall degree of polymerization and the radius of gyration in the BCP blends, respectively. The N was simply set as a weight-average value depending on composition in the BCP blends, although there was the difference in molecular weights of the two deuterated BCPs. The fit curves matched well with the SANS intensity profiles, as line-plotted in Figure 3a. This step allows ones to extract the effective interaction parameter (χeff) using the S(q) equation within the framework of the incompressible RPA. Figure 3b D

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Figure 5. (a) SAXS intensity profiles for the PS-b-PnBMA/PS-b-PnHMA blend with f = 0.5 as a function of the scattering vector (q). All profiles were measured at various temperatures during heating at a heating rate of 0.7 °C/min from 120 to 260 °C. The intensity profiles were vertically shifted by a factor of 5 to avoid overlapping. (b) Inverse of the maximum intensity (1/I(q*)) and full width at half-maximum (fwhm) as a function of the inverse temperature (1/K). The inset shows TEM image measured after quenching at liquid nitrogen from 200 °C, where the PS block was selectively stained with RuO4.

Figure 6. (a) SAXS intensity profiles for the PS-b-PnBMA/PS-b-PnHMA blend with f = 0.8 as a function of the scattering vector (q). All profiles were measured at various temperatures during heating at a heating rate of 0.7 °C/min from 120 to 260 °C. The intensity profiles were vertically shifted by a factor of 5 to avoid overlapping. (b) Inverse of the maximum intensity (1/I(q*)) and full width at half-maximum (fwhm) as a function of the inverse temperature (1/K).

LDOT-type phase behavior, in which the repulsive interactions (by the entropic driving force) arising from thermal expansion difference between the two components increase with increasing temperature, as similarly observed in PS-bPnBMA.9,16 No other higher-order peak may be attributed to the low contrast in the electron density between the PS and PnBMA/PnHMA phases. Nevertheless, the scattering parameters derived from the SAXS profiles such as the inverse of the maximum intensity (1/I(q*)) and full width at half-maximum (fwhm) allow us to reasonably determine a LDOT at 163 °C by the discontinuous changes from a disordered to an ordered state, as plotted in Figure 4b as a function of the inverse temperature (1/K). As the amount of PS-b-PnHMA increases in the BCP blends, the phase transitions were characterized. The SAXS intensity

beam, respectively. The samples were annealed at a constant temperature of 120 °C to ensure an equilibrium state prior to the measurements, and a homogeneous phase without macrophase separation was visually confirmed by the OM. All profiles were measured at various temperatures during heating at a heating rate of 0.7 °C/min from 120 to 260 °C, well above the glass transition temperatures of the PS block (∼100 °C). At lower temperatures ( 168.2 °C, indicative of microphase separation in the BCP blend. This is the characteristic of the E

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profiles for the PS-b-PnBMA/PS-b-PnHMA blend with f = 0.5 are shown in Figure 5a. A broad maximum near q* = 0.257 nm−1 at lower temperatures ( 235 °C), however, the primary peak significantly weakens and broadens into a broad maximum, indicating a transition from an ordered to a disordered state. These two transitions in the SAXS intensity profiles indicate a LDOT and an UODT in the closed-loop phase behavior, as consistently observed in PS-b-PnPeMA.23,24 The blend morphology by TEM image, measured after quenching at liquid nitrogen from 200 °C, exhibits a lamellar microdomain structure due to the symmetric volume fractions of both BCPs, as evidenced in the inset of Figure 5b. The interlamellar distance (∼24 nm) is in an agreement with d-spacing (24.6 nm) derived from the SAXS data at 200 °C. The scattering parameters of 1/I(q*) and fwhm clearly confirm the closedloop transitions of the PS-b-PnBMA/PS-b-PnHMA blend with f = 0.5, where the BCP blend undergoes disordered−ordered− disordered transitions with increasing temperature. This result consistent with the SANS experiment also suggests that a simple blending between the two miscible PS-b-PnBMA and PS-b-PnHMA provides an easy route to reproducing the closedloop transitions. When the amount of PnHMA further increases in the BCP blends, a completely different phase transition is shown in Figure 6a by the SAXS intensity profiles for the PS-b-PnBMA/ PS-b-PnHMA blend with f = 0.8. A sharp primary peak near q* = 0.269 nm−1 at lower temperatures (T < 217 °C) weakens and broadens into a broad maximum with increasing temperature, indicating a transition from an ordered to a disordered state. The transition temperature was determined at 217 °C by the discontinuous changes in the scattering parameters of 1/I(q*) and fwhm, as plotted in Figure 6b. This ODT-type transition is in contrast to the LDOT observed in the PS-b-PnBMA/PS-bPnHMA blend with f = 0.2. Figure 7 shows the DPLS intensity profiles for the PS-bPnBMA/PS-b-PnHMA blends as a function of temperature (at a heating rate of 0.7 °C/min) from 120 to 250 °C, where the composition of the BCP blends are gradually varied. A discontinuous increase in the DPLS intensity for a LDOTtype PS-b-PnBMA occurs at 159 °C due to a transition arising from the optical isotropy of a disordered state to the optical anisotropy of the lamellar microdomains. These same LDOTs were found in the PS-b-PnBMA/PS-b-PnHMA blend up to f = 0.4. Particularly in the PS-b-PnBMA/PS-b-PnHMA blends with f = 0.5 and 0.6, the discontinuous increase and decrease in the DPLS intensity correspond to a LDOT and an UODT of the closed-loop transitions at lower and higher temperatures, respectively. For the PS-b-PnBMA/PS-b-PnHMA blend with f = 0.7 and the other blends to the PS-b-PnHMA, the DPLS intensity remains higher at lower temperatures and discontinuous decrease with increasing temperature, indicating an ODTtype transition. These transition temperatures, measured by the DPLS intensities in Figure 7, were in good agreement with those characterized by SAXS measurements, within an experimental error range of ±6.0 °C when the transitions were evaluated at the final intensity drop. The values measured using the two methods may have differed either due to the broad transitions or to different measurement mechanisms.

Figure 7. Depolarized light scattering (DPLS) intensity profiles for the PS-b-PnBMA/PS-b-PnHMA blends as a function of temperature by varying composition of the BCP blends. A heating rate was set to 0.7 °C/min from 120 to 260 °C. The f denotes the weight fraction of PSb-PnHMA in the BCP blends. The intensity profiles were vertically shifted in an arbitrary factor to avoid overlapping.

On the basis of transition temperatures evaluated by the SAXS and DPLS measurements, a phase transition diagram for the PS-b-PnBMA/PS-b-PnHMA blends is shown in Figure 8a as a function of f (weight fraction PS-b-PnHMA). The transition temperature lines are traceable and dependent on composition in the BCP blends. With increasing f, the LDOTs slightly increase to f = 0.5, rapidly decrease at f = 0.6, and then disappear. In contrast, the UODTs originating from the closedloop transitions at f = 0.5 pass through a minimum at f = 0.7 and then increase to an ODT of PS-b-PnHMA. This experimental result for the miscible BCP blends composed of the PS-b-PnHMA and PS-b-PnBMA reveals two phase boundaries with an ordered region between two cornered disordered regions at lower and higher temperatures. This diagram is a projected 2-dimensional phase transition only at a symmetric PS volume fraction (ϕps = 0.50) of both BCPs. Consequently, a 3-dimensional phase transition diagram for the BCP blends can be proposed with the other variable of PS volume fraction (ϕps) in each BCP, as depicted in Figure 8b, leading to a series of phase behaviors continuously reproduced with increasing f, from a LDOT, to a closed loop with a LDOT and an UODT, to a compressible ODT type of PS-b-PnHMA. For further consideration, we analyzed the d-spacing in an ordered state and thermal expansion coefficient (αd) of dspacing for the PS-b-PnBMA/PS-b-PnHMA blends, as shown in Figure 9a as a function of f. As the amount of PS-b-PnHMA increases (f increases), d-spacing in an ordered state (at 200 °C) decreases linearly by 25.7% from 29.2 nm for the PS-bPnBMA to 21.7 nm for the PS-b-PnHMA. The difference is consistent with an increase by 25.0% in molar volume of the PnBMA/PnHMA from 135.4 cm3/mol (PnBMA) to 169.2 cm3/mol (PnHMA) because the microphase separation occurred between the two phases of the PS block and the averaged PnBMA/PnHMA block in the miscible BCP blends and there was no macrophase separation. Thermal expansion between the two phases of the PS and miscible PnBMA/ PnHMA blocks was described in terms of d-spacing in an ordered range for the PS-b-PnBMA/PS-b-PnHMA blends, F

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Figure 8. (a) Phase transition diagram for the PS-b-PnBMA/PS-b-PnHMA blends as a function of f (weight fraction of PS-b-PnHMA). All data were evaluated by the SAXS and DPLS measurements. (b) 3-dimensional phase transition diagram for the BCP blends.

Figure 9. (a) The d-spacing in an ordered state (at 200 °C) and thermal expansion coefficient (αd) of d-spacing in an ordered range for the PS-bPnBMA/PS-b-PnHMA blends as a function of f (weight fraction of PS-b-PnHMA). (b) The interaction (αd,int) and compressibility (αd,comp) terms of αd.

where d-spacing (or interlamellar distance) is directly correlated to q* position in the SAXS profiles by d = 2π/q*. Accordingly, the slope of d-spacing as a function of temperature (K) is normalized by the initial d-spacing (d0) by αd =

Δd 1 ΔT d0

Assuming that thermal expansion of d-spacing reflects the enthalpic and entropic contributions, the αd can be subdivided into both interaction (αd,int) and compressibility (αd,comp) terms experimentally by ⎡ Δd 1 ⎤ ⎡ Δd 1 ⎤ αd = αd ,int + αd ,comp = ⎢ ⎥ +⎢ ⎥ ⎣ ΔT d0 ⎦int ⎣ ΔT d0 ⎦comp

(4)

which depends on composition of the BCP blends. The αd of dspacing in the BCP blends decreases gradually from the highest αd (1.433 × 10−3 K−1) for a LDOT-type PS-b-PnBMA, through an intermediate αd at f = 0.5−0.6, exhibiting the closed-loop type phase behavior, to the lowest αd (0.10 × 10−3 K−1) for an ODT-type PS-b-PnHMA. Compared with thermal expansion coefficients (α) for each homopolymer, the PS (0.6 × 10−3 K−1), PnBMA (0.65 × 10−3 K−1), and PnHMA (0.70 × 10−3 K−1),27,34 there was no correlation with the αd for the BCP blends. However, it should be noted that all the positive αd values, even at the lowest αd for the PS-b-PnHMA ( f = 1), are inherently different from the negative αd values for a typical incompressible ODT in the PS-b-PMMA (−0.27 × 10−3 K−1) and a strongly interacting polystyrene-b-polyisoprene (PS-b-PI) (−2.76 × 10−3 K−1), in which the enthalpic repulsive interaction between the two blocks diminishes with increasing temperature.14,35−37

(5)

Furthermore, the αd,int can be expressed with αd,int

⎡⎛ ⎞1/2 ⎤ ⎡ Δd 1 ⎤ χ 1 ⎢ =⎢ ⎥ = ⎜⎜ ⎟⎟ − 1⎥ ⎢ ⎥ ⎣ ΔT d0 ⎦int ⎣⎝ χ0 ⎠ ⎦ ΔT

(6)

since the d-spacing (d) associated with the interaction term is proportional to χ1/2 in the weakly segregation regime.38 Figure 9b shows both interaction (αd,int) and compressibility (αd,comp) terms within temperature range from 120 to 250 °C. With increasing f, the αd,int, calculated from the variations of χeff by the SANS measurements, decreases from the positive to the negative values. However, the extracted αd,comp slightly increases to f = 0.2 and decreases with further increasing f, which displays an asymmetric convexity above the α d,int . This result presumably indicates that in all the miscible PS-b-PnBMA/ PS-b-PnHMA blends the entropic compressibility effect on G

dx.doi.org/10.1021/ma302514s | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

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temperature-dependent d-spacing is dominant in competition with the enthalpic interaction effect, where a compressible character of these BCP blends can be an interesting topic for the future study.

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CONCLUSIONS We investigated the phase behaviors of the binary compressible BCP blends composed of the weakly interacting (with no specific interaction) (d)PS-b-PnBMA and (d)PS-b-PnHMA. Using a miscible phase between the PnBMA and PnHMA blocks in the BCP blends, the blending between a LDOT-type PS-b-PnBMA and an ODT-type PS-b-PnHMA was used as a simple approach to characterizing the phase behavior types of the miscible BCP blends. The various composition-dependent phase behaviors were varied from a LDOT-type to an ODTtype on heating as the blend composition varies from PS-bPnBMA to PS-b-PnHMA. A closed-loop phase transition consisting of a LDOT and an UODT was found in nearly symmetric composition of the miscible PS-b-PnBMA/PS-bPnHMA blends. A series of phase behaviors were proposed in a 3-dimensional phase transition diagram for the BCP blends. On the basis of thermal expansion of d-spacing, we speculated that all the miscible PS-b-PnBMA/PS-b-PnHMA blends have a compressible character by the dominant entropic effect. This study also represents the first experimental evidence that a miscible BCP blend system exhibits a closed-loop transition as well as a LDOT and an ODT transition in the weakly interacting BCP blends.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.Y.R.). Author Contributions §

H.A. and Y.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Nuclear R&D, NRF grants (2011-0022690), Converging Research Center (2010K001430), and APCPI ERC program (R11-2007-05000000), which are funded by the Ministry of Education, Science & Technology (MEST), Korea.

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REFERENCES

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dx.doi.org/10.1021/ma302514s | Macromolecules XXXX, XXX, XXX−XXX