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SANS Study of Ring Topology Effects on the Miscibility of Polymer Blends Yuki Kobayashi,†,§ Yuya Doi,† Siti Sarah Abdul Rahman,†,∥ Eunhye Kim,‡,⊥ Tae-Hwan Kim,‡,⊥ Atsushi Takano,*,† and Yushu Matsushita*,† †

Department of Molecular and Macromolecular Chemistry, Nagoya University, Nagoya, Aichi 464-8603, Japan HANARO Research Reactor Utilization Development, Korea Atomic Energy Research Institute (KAERI), Daejeon 305-353, Korea

‡

S Supporting Information *

ABSTRACT: Highly purified hydrogenous ring poly(4-trimethylsilylstyrene) (hPT) and deuterated ring polyisoprene (d-PI) samples as well as their linear counterparts were prepared, and the miscibility of three kinds of polymer blends, i.e., linear−linear, ring−linear, and ring−ring, denoted as L−L, R−L, and R−R, respectively, with all 50/50 vol % was evaluated by small angle neutron scattering (SANS) measurements. PT and PI are known as a miscible polymer pair with lower critical solution temperature (LCST) type phase diagram. At low-q regime of the scattering profiles, R−R blend exhibits much higher scattering intensity than L−L and R−L, while the latter two show similar profiles. Moreover, from Zimm’s analysis, the spinodal temperature of R−R was estimated to be about 100 °C lower than the other two blends, L−L and R−L. These results suggest that the miscibility of R−R is considerably lower than the other two blends, which is a clear manifestation of the topological effect on the phase behavior of the present blend.

1. INTRODUCTION Understanding and controlling the miscibility of polymer blends is one of the fundamental subjects in polymer physics as well as industrial applications.1,2 In general, most of the binary polymer blends composed of different polymer species are immiscible and thus exhibit phase separation due to little gain in entropy of mixing. However, some miscible polymer pairs have been found up to date.3−7 The miscibility of polymer blends is governed by the Flory−Huggins segmental interaction parameter, χ,8,9 between the components in blends, and their phase behavior can be generally classified into two types of phase diagrams depending on the temperature dependence of χ: one is the upper critical solution temperature (UCST) type where phase separation occurs on cooling, while the other is the lower critical solution temperature (LCST) type where phase separation occurs on heating. The architectures of component polymer chains are conceived to affect the miscibility of polymer blends. To examine the effects of branched architecture on the miscibility, experimental10−16 and theoretical17,18 studies have been conducted by using model branched polymers with defined structures such as star-, comb-, and pom-pom-shaped polymers. For example, Faust et al. examined the miscibility of the 22-arm star polystyrene (PS)/linear poly(vinyl methyl ether) (PVME) blend by cloud point measurements with light scattering10 and found that the star−linear blend exhibits considerably lower miscibility (i.e., a higher χ value) than the corresponding linear−linear blend with the same molecular weights. Here, in principle, χ should be a purely enthalpic term related to the excess free energy,8,9 and hence this parameter can be © XXXX American Chemical Society

determined only by the chemical structures of two polymer species of the blend without depending on chain architecture. However, in those studies, χ was regarded to possess some entropic contribution, and the effects of chain architecture on the miscibility have to be put into the effective interaction parameters, χeff.10−18 Along this line, ring polymer is another candidate to vary the miscibility of polymer blends. Due to the recent development in chromatography techniques,19,20 which enables getting highly purified ring polymer samples, it has been found out that pure ring polymers exhibit obviously different static21−23,63 and dynamic24−27 properties from linear ones. For example, under θ conditions (i.e., the second virial coefficient A2 is zero) for dilute solutions of linear polymers, solutions of rings exhibit positive A2 values.21,23,28 These results indicate that the internal osmotic pressure generates within the ring chains due to the topological constraints, where the inter/intramolecular chain crossing is not allowed, and thus the rings tend to swell by incorporating solvent molecules even under θ conditions. It also turned out that ring chains in bulk possess more compact dimension than the Gaussian rings from both experimental26,29,63 and theoretical/simulation30−36 studies. These results also reflect the topology of ring chains, and hence the rings tend to stand rather isolated states in bulk. Moreover, when linear chains are added to rings in bulk, the linear ones Received: November 4, 2017 Revised: February 7, 2018

A

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were used for cyclized h-PT and also cyclized d-PI products. Under this condition, the critical point was confirmed to be 31 °C for h-PT, referring to 40 °C for d-PI. After the fractionation process, highly purified ring samples, h-R-PT and d-R-PI, were obtained. Details of the sample preparation were described in the Supporting Information. Weight-averaged molecular weight, Mw, and molecular weight distribution, Mw/Mn, of the samples were determined by SEC-MALS and SEC measurements, respectively, and the measurement conditions were described elsewhere.49 The purity of the ring samples was estimated by IC measurements in the same conditions as applied in the preparative IC fractionations. Molecular characteristics of the samples obtained in this study are summarized in Table 1. The numbers of the sample codes in Table 1

initiatively thread into the rings, and consequently they swell.37−40 From the above thermodynamic basis, we may expect the tendency of the miscibility; ring−ring blends favor rather isolated states, and hence the miscibility must be lower than linear−linear blends, while ring−linear blends tend to be more miscible states in molecular scale. Up to date, there are several theoretical and simulation studies on the miscibility of ring polymers.41−43 Recently, Sakaue et al.43 theoretically predicted the miscibility of ring polymer blends based on the mean-field theory incorporating the idea of the topological volume.35,36 They expected the large enhancement in the miscibility for ring−linear polymer blends, whereas the opposite trend toward demixing was conceived for ring−ring blends. Poly(4-trimethylsilylstyrene) (PT) and polyisoprene (PI) are one of the miscible polymer pairs with the LCST-type phase diagram, and Harada et al. carefully examined its miscibility.44 One distinctive feature of this polymer system is that both component polymers, PT and PI, can be synthesized by anionic polymerizations under control, which give polymer samples with narrow molecular weight distribution. In this study, highly purified ring PT and ring PI samples as well as the corresponding linear ones were prepared by anionic polymerizations followed by high-resolved purification with liquid chromatography, and the miscibility of three kinds of blends, i.e., linear−linear, ring−linear, and ring−ring blends, denoted as L−L, R−L, and R−R, respectively, was evaluated by small-angle neutron scattering (SANS) measurements. SANS is a powerful technique to evaluate and quantify the miscibility of polymer blends. One of the most distinctive features of SANS is that high scattering contrasts can be given by the isotope labeling, where some hydrogen atoms are substituted for deuterium atoms. Thus, in this study, fully deuterated PI and hydrogenous PT samples were utilized. The SANS data obtained were analyzed by the Zimm method,45 and the spinodal temperature of three blends was quantitatively evaluated.

Table 1. Molecular Characteristics of h-PT and d-PI Samples

a

samples

10−3Mwa

N

Mw/Mnb

h-L-PT-42 h-R-PT-42 d-L-PI-28 d-R-PI-28

42.0 41.7 28.0 27.8

239 237 368 366

1.06 1.07 1.05 1.07

purityc ∼99% ∼99%

Estimated by SEC-MALS. bEstimated by SEC cEstimated by IC.

indicate the molecular weights of the samples in a unit of kg/mol. Both ring h-R-PT and ring d-R-PI samples were confirmed to possess Mw equivalent to their corresponding linear counterparts, narrow Mw/Mn less than 1.1 and sufficiently high purity. The microstructures of PI are known to reflect its polymerization conditions. Since the linear d-L-PI sample was anionically polymerized in the polar THF solvent in this study, this polymer is rich in 1,2- and 3,4-microstructures. Judging from the previous report,50 the d-L-PI sample prepared in this study is conceived to contain about 10% of 1,4-linkage. Moreover, since the ring d-R-PI sample was obtained by connecting the chain ends of the telechelic d-L-PI, this ring essentially maintains the same microstructure as linear PI. 2-2. SANS Measurements. To evaluate the miscibility of h-PT/dPI blends based on the difference in chain topology, small-angle neutron scattering (SANS) measurements were performed. In this study, three kinds of blends, i.e., (i) h-L-PT/d-L-PI (indicated as L− L), (ii) h-R-PT/d-L-PI (R−L), and (iii) h-R-PT/d-R-PI (R−R), having the same volume fraction of 50/50% were prepared as follows. First, 10% toluene solutions of h-PT/d-PI with 50/50 composition within an error of ±0.3% were cast, then the films obtained were annealed in a quartz cell for SANS measurements with 1.5 mm thickness at 150 °C under vacuum for 24 h, and finally the cell was sealed under high vacuum. In this study, we chose the 50/50% blends because the topology effects are expected to be the most remarkable at this composition, especially for the R−L blend. SANS measurements were conducted on the 40 m SANS at Highflux Advanced Neutron Application Reactor (HANARO) at Korea Atomic Energy Research Institute (KAERI), Korea. The neutron wavelength, λ, chosen was 7 Å, the sample-to-detector distance was 3 m, and a pinhole with 3 mm in diameter was set just upstream of the samples. This equipment condition covered the q range of 0.01 ≤ q/ Å−1 ≤ 0.2. Here, q is the scattering vector given by q = (4π/λ)sin(θ/ 2), where θ is the scattering angle. The SANS intensities were counted on the two-dimensional position-sensitive detector, and they were circularly averaged. The measurements for each blend sample were conducted for 30 min each at various temperatures from 130 to 180 °C with an interval of 10 °C. The treatment of background signals, i.e., the incoherent scattering intensities arising mainly from hydrogen molecules, is a delicate issue for data reduction.51 In this study, we assumed the incoherent background scattering intensity Iincoh as a q-independent constant, and it was estimated by using raw data of the L/L blend sample as follows. In bulk, linear polymer chains are known to follow the Gaussian distribution statistics due to the screening of the excluded-volume effect, and their SANS intensities obey the q−2 power law dependence at high q regime.52,53 Hence, in this study, we carefully determined

2. EXPERIMENTAL SECTION 2-1. Materials. All polymers used in this study were prepared by anionic polymerizations in sealed glass apparatuses with break-seals under high-vacuum (∼1 × 10−3 Pa). Hydrogenous 4-trimethylsilylstyrene (Hokko Chemical Industry Co.) monomer was treated and purified as reported previously.46 Fully deuterated (d8-) isoprene (Polymer Source) monomer was dried over calcium hydride under reduced pressure and purified by distillation with 1,1-diphenylhexyl lithium. Tetrahydrofuran (THF), methanol, potassium naphthalenide (N a p h- K ), 1 ,1- d ip hen yl e th yl ene ( DPE) , a nd 1- [3- ( 3chrolopropyldimethylsilyl)phenyl-1-phenylethylene] (DPE-Cl) were treated and purified as reported previously.47,48 Telechelic hydrogenous linear poly(4-trimethylsilylstyrene) (h-L-PT) with DPE-type vinyl groups on both ends was synthesized by using Naph-K as an initiator in THF at −78 °C, followed by two-step end-capping reactions using an excess molar amount of DPE and DPE-Cl, following the manner as reported previously.47,48 Telechelic deuterated linear polyisoprene (d-L-PI) was also synthesized in the same manner as adopted in h-L-PT synthesis. These telechelic samples were used for the linear chain blends for SANS measurements. Both h-L-PT and d-L-PI were cyclized by using Naph-K in dilute THF solutions (ca. 0.1%) following the manner reported previously,47,48 and the products were terminated by methanol. The cyclization products were purified by size exclusion chromatography (SEC) and interaction chromatography (IC) fractionations, by using preparative chromatography apparatuses. For IC fractionations, octadecylsilyl (ODS) silica gel columns (C18 M 10E; Shodex) and a mixture eluent of dichloromethane/acetonitrile (80/20 in volume) B

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Figure 1. (a−c) Double-logarithmic plots of the SANS profiles (I(q) vs q) for (a) L−L, (b) R−L, and (c) R−R blends at various temperatures. The black dashed lines indicate the slope of −2 (I(q) ∼ q−2) at high q, while the red solid line in (b) indicates the slope of −1.83 to match the actual experimental data for R−L. (d) Comparison of the SANS profiles for L−L, R−L, and R−R blends at 170 °C. Iincoh by assuming the decay of coherent scattering intensities I(q) (= I(q)all − Iincoh) follows q−2−dependence at high q. The details are shown in the Supporting Information. As a result, we determined Iincoh of the L−L blends as Iincoh = 0.20 cm−1 irrespective of temperature and adapted this value to the other R−L and R−R blend samples, since hydrogen/deuterium ratios are exactly all the same for three blends adopted in this study. Moreover, we confirmed the validity of this assumption for Iincoh by comparing the present method with the other method reported by Shibayama et al.54 The details are shown in the Supporting Information. 2-3. OM Measurements. To complement the SANS results, optical microscopy (OM) measurements were conducted. Here, a hydrogenous PI (h-PI) sample instead of d-PI was utilized for the blend component. The linear h-PI (Mw = 25.0 kg/mol, Mw/Mn = 1.06; coded as h-L-PI-25) was anionically synthesized from isoprene monomer with sec-butyllithium as an initiator in THF. This h-L-PI25 sample has exactly the same degree of polymerization (N = 367) with the d-L-PI-28, and hence it was used for OM measurements by blending with h-L-PT-42 or h-R-PT-42, which are the same samples adopted for SANS measurements, at various compositions. Since the h-L-PI sample possesses no functional groups for cyclization on its chain ends, we cannot prepare the corresponding ring sample from this h-L-PI. Namely, we examined and compared the phase behavior (i.e., miscibility) of two types of blends, h-L-PT/h-L-PI (hL-hL) and hR-PT/h-L-PI (hR-hL), by using OM. The OM measurements were conducted by using OPTIPHOTPOL (Nikon) with a temperature controller FP90/FP84HT (CHINO). Both hL-hL and hR-hL blend films were prepared by casting on cover-glasses from 10 wt % toluene solutions of h-PT and hPI, mixed at various compositions and annealed at 150 °C for 24 h under vacuum. The measurements were performed at a heating rate of 1 °C/min under nitrogen atmosphere. Due to the isotope effect,55 the OM observation exhibits quantitatively different results from SANS measurements, but qualitatively the same miscibility tendency was given. Thus, the OM observation results assist in understanding the SANS results.

3. RESULTS AND DISCUSSION 3-1. Overview of SANS Profiles. Figures 1a, b, and c show the overview of the SANS profiles, i.e., double-logarithmic plots of I(q) vs q, for L−L, R−L, and R−R blends, respectively, at three selected temperatures. As a reference, normal scale plots of I(q) vs q for the blends are shown in Figure S4 of the Supporting Information. As explained in the Experimental Section as well as the Supporting Information, the contribution from incoherent scattering (Iincoh = 0.20 cm−1) was subtracted from the measured scattering intensity data in advance. For all three blends, the scattering intensity, especially in low q regime, increases as the temperature rises, implying that the concentration fluctuation becomes larger with increasing the temperature. This result suggests that all three blends treated in this study exhibit a lower critical solution temperature (LCST) type phase diagram, irrespective of the component polymer chain architecture of the blends. Figure 1d directly compares the SANS profiles for three type of blends at 170 °C. Note that the data at 170 °C were chosen as an example, but the data trend is essentially the same at other temperatures, i.e., 130−180 °C. It is clear in Figure 1d that the intensities of the R−R blend are evidently higher than the other two L−L and R−L blends covering all q range, indicating that the R−R blend exhibits much larger concentration fluctuation than the others. This result is directly pointing to the existing chain topology effects on blend miscibility, since other characters of the blends such as the molecular weights of component polymers and blend composition are exactly the same. Recent experimental and theoretical studies suggest that ring chains possess more compact and isolated conformation, which is associated with a stronger intermolecular repulsive force for R−R, in bulk compared with the Gaussian rings,26,29−36,63 and this feature must be related to the result for the R−R blend in this study, which reveals lower miscibility than L−L and R−L. C

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Figure 2. Kratky plots (I(q)·q2 vs q) for (a) L−L, (b) R−L, and (c) R−R blends at various temperatures. (d) Comparison of the SANS profiles for L−L, R−L, and R−R blends at 170 °C. The symbols are the same in Figure 1d.

structures of material systems employed. Hence, if the blends in this study form large domain structures with sharp interfaces, it must be originating from phase separation, and I(q) should show the drastic upturn toward low q as expressed by I(q) ∼ q−4. All blend samples adopted in this study, however, did not show such drastic increase in I(q) at all temperature ranges (i.e., 130−180 °C). These results indicate that all three blends are essentially in the miscible state. This fact is helpful for us to analyze the SANS data in order to estimate thermodynamic parameters such as the correlation length ξ and the spinodal temperature TS as will be discussed later. Next, we focus on I(q) curves at high q range in Figure 1. The scattering curves at high q are not directly related to the miscibility of the blends but reflect the local conformation of the chains in the blends. Nevertheless, it may contain some important information to indirectly understand the miscibility of the blends associated with conformations of the component polymer chains. In Figure 1a, L−L blend exhibits I(q) ∼ q−2 in a wide q (0.05 ≤ q/Å−1 ≤ 0.2) range. This is a natural result because we subtracted the incoherent scattering background from the raw scattering intensity to satisfy the relation of I(q) ∼ q−2 for the L−L blend at high q, indicating that both component linear h-PT and d-PI chains follow the Gaussian statistics (see the Supporting Information). Then, the Kratky plots, i.e., I(q)·q2 vs q, are displayed in Figure 2. This q−2 dependence of the L−L blend in a wide q range in Figure 1a is exactly equivalent to the plateau in the Kratky plot at high q range in Figure 2a, although the data at the highest q limit (≥0.18 Å−1) are slightly scattered. I(q) curves of the R−L and R−R blends in Figures 1b and 1c, respectively, exhibit different q dependence from the L−L

Concerning the other two blends, L−L exhibits higher intensities at low q (≤0.03 Å−1), while exhibiting lower intensities at high q (≥0.03 Å−1) than R−L. This crossover in I(q) reflects the difference in the intrinsic component chain architecture as well as the concentration fluctuation. Now, we focus on I(q) at low q where the contribution of concentration fluctuation is dominant, suggesting that the R−L blend exhibits smaller concentration fluctuation, i.e., better miscibility, than the L−L blend. Although this is a simple comparison of I(q) curves at low q, we clearly found out that the miscibility of polymer blends strongly depends on the topology of component polymer chains: the R−R blend shows much lower miscibility than the L−L blend, while the R−L blend exhibits slightly better than the L−L blend. As far as we know, this is the first report on experimental observation as for the difference in the miscibility of polymer blends using ring polymers as either one or two blend components. These results are consistent with the theoretically proposed ones by Sakaue et al.,43 where the enhancement in the miscibility was expected for ring−linear blends than linear−linear ones, whereas the opposite trend toward demixing was predicted for ring−ring blends. To deepen the understanding of miscible behavior of the polymer blends, we discuss the same SANS profiles from some viewpoints as follows. In Figure 1, we focus on the power-law dependence of I(q) on q, i.e., the slope of I(q) curves against q in doublelogarithmic scales, at both low and high q regimes. First, at low q (≤0.02 Å−1), all L−L, R−L, and R−R blends exhibit a weak and similar slope of around −0.3 to −0.5. The scattering intensities at low q regime are known to reflect the global D

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Macromolecules blend at high q. The R−L blend exhibits a little weak q dependence with its slope of −1.83 at 0.08 ≤ q/Å−1 ≤ 0.2, and this corresponds to a constant increase in Kratky plots at the same q range in Figure 2b. In contrast, the R−R blend exhibits a steeper slope than q−2 at intermediate q (0.07 ≤ q/Å−1 ≤ 0.1), while showing q−2 dependence at a higher q range (≥0.1 Å−1). Moreover, in a Kratky representation as shown in Figure 2c, the R−R blend exhibits a well-defined peak, which is known to originate from the ring-shaped architecture of the component chains.56−58 It is probably difficult to extract some conformational information for the component chains in the blends only from the slope of the profiles at high q. This is because the profiles for R−L and R−R blends contain the scattering originating from ring architectures even at a higher q side unlike L−L. 3-2. Zimm Analysis. We applied Zimm’s method45 to our present SANS intensities to quantitatively evaluate the strength of concentration fluctuation and correlation length for L−L, R−L, and R−R blends. Under the assumption of the mean-field approximation, the structure factor S(q) for binary polymer blends can be written as follows S(q) = I(q)NA

2 ⎛ b1 b2 ⎞ − ⎟ ⎜ v2 ⎠ ⎝ v1

(1)

where I(q) is the coherent scattering intensity of the blends, NA is Avogadro’s number, bi is the scattering length per monomer, and vi is the volume per monomer of the polymer component i (= 1 or 2). At low q regime, S(q) can be represented by the Ornstein−Zernike (OZ) equation53 S −1(q) =

1 + q 2ξ 2 S(0)

with q2 ·ξ 2 ≤ 1

(2)

where ξ is the correlation length of concentration fluctuation, and S(0) is the structure factor at q = 0. Figures 3a, b and c show S−1(q) plotted against q2 for L−L, R−L, and R−R blends, respectively, at three selected temperatures. Here, we used b = 1.74 × 10−12 cm and v = 0.319 nm3 for h-PT, while b = 8.66 × 10−12 cm and v = 0.137 nm3 were adopted for d-PI.59 Note that b and v values should be independent of the polymer chain architecture. The solid lines in Figure 3 are the least-squares approximate straight lines, and from the slope and intercept of these lines, ξ and S−1(0) for these blends at each temperature were estimated and summarized in Table 2, and they were plotted against the inverse temperature, 1/T, in Figure 4. Both ξ−2 and S−1(0) show good linearity with 1/T in Figure 4. The ξ−2 and S−1(0) are intersected on the 1/T axis for R−L and R− R blends, whereas the crossing points are fairly distant (ΔT ∼ 20 °C) for the L−L blend. This discrepancy is probably due to the limited q range for the L−L blend in the Zimm analysis, and we conceive that the S−1(0) data is more reliable than the ξ−2 for data handling. The inverse of the structure factor at q = 0, S−1(0), can be described as follows60 ⎛1 1⎞ S −1(0) = 2(ΓS − Γ) = 2A⎜ − ⎟ T⎠ ⎝ TS

Figure 3. Zimm plots of structure factors (S−1(q) vs q2) in the range of 1 ≤ q2 × 104/Å−2 ≤ 5, for (a) L−L, (b) R−L, and (c) R−R blends at various temperatures. The solid lines indicate the least-squares approximate fits.

TS, at S−1(0) = 0, and they are 318, 322, and 220 °C for L−L, R−L, and R−R blends, respectively. From these results, we can safely state the following. The TS values extrapolated for all three blends are enough higher than the measurement temperatures of SANS measurements (i.e., 130−180 °C). Namely, the blends examined in this study were at the miscible states during the SANS measurements. Moreover, TS for L−L and R−L are almost the same, and they are ca. 100 °C higher than that for R−R. We note that the extrapolated TS of L−L and R−L are much higher than the SANS measurement temperatures, and hence the accuracy of the extrapolation for those two blends is somewhat lower than that for R−R. Nevertheless, as far as we can see from Figure 4, we can definitely conclude that the miscibility of the R−R blend is much lower than the other L−L and R−L blends. This result is consistent with the recent theoretical study by Sakaue et al.,43 reflecting the effect of ring chain topology on the miscibility of polymer blends. In contrast, if we compare the

(3)

where Γ (= A/T + B) is the Flory−Huggins parameter, ΓS (= A/TS + B) is the parameter at the spinodal point, and the coefficients A and B represent the enthalpic and entropic terms in Γ. From Figure 4, we can evaluate the spinodal temperature, E

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Macromolecules Table 2. Experimental Values of ξ, S−1(0), and TS Estimated from SANS Measurements blends

T (°C)

ξ (nm)

S−1(0) × 10−4 (mol/cm3)

TS (°C)

L−L

130 140 150 160 170 180 130 140 150 160 170 180 130 140 150 160 170 180

2.81 2.97 2.99 3.21 3.36 3.64 2.24 2.34 2.44 2.43 2.67 2.80 2.99 3.14 3.40 3.68 4.06 4.60

1.29 1.16 1.12 1.05 0.94 0.86 1.78 1.61 1.51 1.43 1.26 1.15 0.84 0.72 0.61 0.52 0.42 0.34

318

R−L

R−R

the results were not successful because the blend samples treated in this study are very complicated to do RPA analysis. Specifically, two kinds of components, RT and PI, in this study have different degrees of polymerization, different chain sizes, and different architectures (i.e., linear or ring ones). Moreover, the conformation of chains, especially rings, in the blend is still not clarified well.30−36,61 Hence, it is extremely hard to determine correctly the χ values for the blends in this study. In this regards, further experimental and theoretical/simulation studies on the conformations of ring polymers must be required. 3-3. OM Observation. To complement the SANS results for L−L and R−L blends, OM observations for hL-hL and hRhL blends with various compositions were conducted. As mentioned in Experimental Section, a hydrogenous PI (h-PI) sample instead of a deuterated one (d-PI) was used as the blend component. Figure 5 shows optical images of the hR-hL blend

322

220

Figure 5. Optical images of the hR-hL blend of ΦPT = 0.47 observed at (a) 180 °C and (b) 190 °C. The scale bars in the images indicate 50 μm.

Figure 4. ξ−2 and S−1(0) plotted against 1/T for L−L, R−L, and R−R blends. The solid and dashed lines indicate the least-squares approximate straight lines for ξ−2 and S−1(0), respectively. The arrows point out 1/TS at S−1(0) = 0, where TS denotes the spinodal temperature.

of ΦPT = 0.47 at (a) 180 °C and (b) 190 °C, as an example of the phase transition. Note that the temperature was continuously increased at the rate of 1 °C/min. On the lower T side, the blend exhibits the homogeneous phase as shown in Figure 5a, while on the higher T side, it demonstrates micrometer-ordered domain structures as evidenced in Figure 5b. From the observation of this blend, we confirmed that the phase separation began to occur at 187 °C. In this way, the phase behavior for the hL-hL and hR-hL blends at various compositions was examined by changing temperature, and the results obtained were summarized in Figure 6. Although all plots have a margin of error (at most ±3 °C), the hR-hL blend exhibits evidently higher phase-separation temperatures, i.e.,

results for L−L and R−L blends, there does not exist a big difference in the miscibility. One may consider this result is inconsistent with the theoretical prediction.43 In fact, we studied only one composition of the blends, i.e., 50/50 vol % in the present SANS study; however, L−L and R−L blends may show clearer miscibility difference at different blend compositions. In this regards, we further investigated by optical microscopy (OM) observation in the next section. Finally, we should make two comments on our data analysis. First, in principle, we were able to distinguish the contribution of the enthalpic and entropic terms in Γ (the coefficients A and B, respectively, as described above) in Figure 4, but we did not do it since the data, especially for L−L and R−L, in Figure 4 are slightly scattered. Second, certainly the random phase approximation (RPA) analysis is a powerful method to evaluate directly the Flory−Huggins interaction parameters of the polymer blends.52 In reality, we executed RPA analyses, but

Figure 6. Phase diagram of the hL-hL and hR-hL blends. The curves indicate the guide for eyes. F

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Macromolecules better miscibility, than the hL-hL blend, at all ΦPT ranges examined. This result is qualitatively in good accordance with the recent theoretical prediction.43 Moreover, the difference between two blends is pronounced at lower ΦPT. In Figure 6, we connected the data points by a smooth curve and estimated the critical composition and temperature, Φc,PT and Tc, respectively, as 0.40 and 184 °C for the hL-hL blend and 0.44 and 187 °C for the hR-hL blend. We found out that the Φc value changes depending on the topology of component chains. Based on the Flory−Huggins mean-field theory, Φc of polymer blends where both components are linear chains is derived from the differentiations of the Gibbs free energy of mixing and described by using the degree of polymerization N of components 1 and 2 as53 Φc,1 =

profiles of the blends by Zimm’s method and estimated the spinodal temperatures TS. It has been revealed by this attempt that R−R shows ca. 100 °C lower TS value than the other L−L and R−L blends, indicating the miscibility for R−R is evidently lower than the other two. Furthermore, the OM data are consistent with the SANS results. From these facts, we can assume that the ring chains in bulk tend to possess more compact and isolated conformation, which is a clear manifestation of the topological effect on phase behavior of the present blend. As far as we know, this is the first experimental report on the miscibility of ring polymer blends, and the present results will lead to a further understanding of structures and properties of ring polymers.

■

N2 N1 +

ASSOCIATED CONTENT

* Supporting Information S

N2

(4)

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02359. Figures S1−S4, preparation of ring h-R-PT and d-R-PI samples, determination of incoherent scattering intensity, SANS profiles (PDF)

Thus, if the hR-hL blend also obeys eq 4 as with the hL-hL blend, their Φc value should be the same. Nevertheless, their Φc values are clearly different. Sakaue et al.43 expected that the Φc value of ring−linear blends changes from that of linear−linear blends, and our experimental results agree with this prediction. Here, we need to make a comment on the difference of the phase separation temperatures determined by OM and SANS measurements, i.e., the temperatures observed by OM are about 100 °C lower than those estimated by SANS. We conceive that this difference mainly originates from the difference in isotopic chemical composition in the component PI, that is, d-PI was used for SANS while h-PI was for OM. This isotope effect on the miscibility was known55 and discussed by considering the difference of the free volumes between hydrogenous and deuterated samples.62 We actually conducted OM measurements for the h-L-PT/d-L-PI sample at 50/50 vol %, which was exactly the same blend sample as that used in the SANS measurements. As a result, no phase separation behavior was observed in the temperature range of 100−200 °C. From the SANS measurements, TS of this sample was estimated to be about 300 °C, which was considerably higher than the temperatures adopted in OM. Thus, we can safely say that the results for h-L-PT/d-L-PI obtained from OM and SANS measurements are consistent. Based on the above experimental facts, exploring the phasebehavior of R−L and R−R blends with different compositions by using SANS is of great interest. The effects of the molecular weight of the blend components on the miscibility are also worth investigating. In fact, Sakaue et al.43 pointed out that the miscibility of ring−linear blends should be more enhanced as the size of rings becomes larger. Thus, if we use much larger rings for ring−linear blends, their miscibility may be enhanced more drastically. Further experimental and theoretical studies are desired to clarify this issue.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (A.T.). *E-mail: [email protected] (Y.M.). ORCID

Yuya Doi: 0000-0001-8029-7649 Atsushi Takano: 0000-0002-5188-5166 Present Addresses §

Toyota Motor Corporation, Toyota, Aichi 471-8571, Japan. Kaneka Corporation, Nakanoshima, Kita-ku, Osaka 530-8288, Japan. ⊥ Department of Quantum System Engineering, Chon Buk National University, Deokjin-gu, Jeonju-si, Jeollabuk-do 561756, Korea. ∥

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Prof. H. Watanabe at the Institute of Chemical Research, Kyoto University for providing deuterated isoprene monomer, and we are grateful to Dr. T. Sakaue, Dr. D. Kawaguchi, and Dr. Y. Takahashi at Kyushu University for their helpful discussions. Travel expenses for the SANS experiment performed at 40m SANS at HANARO (Korea) were supported by General User Program for Neutron Scattering Experiments, Institute for Solid State Physics, The University of Tokyo (proposal No. 13064), at JRR-3, Japan Atomic Energy Agency, Tokai, Japan, and the authors are grateful for the support. This work was partly supported by a Grant-in-Aid for Scientific Research (No. 24350056 for A.T. and No. 25248048 for Y.M.) from the Japan Society for the Promotion of Science.

4. CONCLUSION In this study, we conducted SANS measurements for three kinds of h-PT/d-PI blends at 50/50 vol % with different chain architectures, i.e., L−L, R−L, and R−R, and evaluated their miscibility. We found out from the scattering intensities at low q regime that R−L exhibits slightly better miscibility than L−L, while R−R shows considerably lower miscibility than L−L. These results directly reflect the difference in chain architectures and are consistent with the recent theoretical prediction by Sakaue et al. Moreover, we analyzed the SANS

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