Melt Rheology of Tadpole-Shaped Polystyrenes - ACS Publications

Nov 17, 2015 - which were longer than the entanglement molecular weight. (Me = 18.0 kg/mol ..... star relax much faster than the other long arm and di...
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Melt Rheology of Tadpole-Shaped Polystyrenes Yuya Doi,† Atsushi Takano,*,† Yoshiaki Takahashi,‡ and Yushu Matsushita*,† †

Department of Applied Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Institute for Materials Chemistry and Engineering, Kyushu University, 6-1, Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan

‡

S Supporting Information *

ABSTRACT: A series of highly purified tadpole-shaped polystyrene (PS) samples were prepared by anionic polymerizations and multistep HPLC fractionations, and their linear melt rheology was investigated. Three tadpole samples were composed of a common ring chain (R-60; Mw = 59.8 kg/mol) and three different lengths of linear chains (L-30, L-70, and L120; Mw = 27.1, 68.9, and 122 kg/mol, respectively), all of which were longer than the entanglement molecular weight (Me = 18.0 kg/mol for PS). All tadpole samples revealed remarkably slower terminal relaxation than the constituent ring or linear chains and also than the ring/linear blends. These results are evidently owing to the newly generated characteristic entanglements such as the intermolecular ring−linear penetrations. Two rheological parameters, i.e., the zero-shear viscosity, η0, and the steady-state recoverable compliance, Je, were estimated, and their molecular weight dependence was discussed. Tadpoles used in this study exhibited a drastic viscosity enhancement with the increase of the length of the linear tail compared with the simple linear chains. Moreover, the molecular weight dependence of their η0 is similar to that of star polymers when plotted against the molecular weight of a tail chain (Mw,tail) for tadpoles and that of one arm (Mw,arm) for stars. The molecular weight dependence of Je for tadpoles was also similar to that of stars rather than the linear chains. These results strongly suggest that the relaxation mechanism of tadpole chains adopted in this study is similar to that of star polymers, being well understood by contour length fluctuations like the arm retraction model. These characteristic rheological properties of tadpoles must originate from their unique architecture where a ring and a linear chain are introduced into one molecule.

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INTRODUCTION Polymer chain architectures greatly affect the physical properties of polymers. Ring polymers are one of the fascinating model polymers, especially from the aspect of the dynamics, because they have no chain ends.1 Over the past several decades, a large number of studies on the dynamics of ring polymer melts have been pursued in aspects of theories,2−8 simulations,9−23 and experiments.24−42 Recently, the striking rheological behavior of ring polymer melts was observed: truly pure rings do not exhibit a rubbery plateau, even though they have much larger molecular weights than the entanglement molecular weight, Me.34,39,42 In addition, their viscosity increases proportionally with the molecular weight up to about 5 times of Me as Rouse-like motion7,8,43 while large rings with more than 5Me show clearly higher viscosities than the Rouse chains due to certain intermolecular interactions. These demonstrate that the relaxation mechanism of rings is completely different from that of linear chains, which is well understood in the framework of the reptation model.44,45 Ring/linear blends are another interesting topic of the rheology. Kapnistos et al. reported experimentally the effect of the linear contamination in the ring samples on their rheology.34 They mentioned that a tiny amount (surprisingly only 0.07%) of long linear chains, with the molecular weight of about 10Me in the ring samples with the same molecular © XXXX American Chemical Society

weight, sensitively caused a change in the rheological response and that the sample including 1% of the linear chains exhibited a plateau. Halverson et al. reported on the systematic change in the rheological properties of ring/linear blends depending on their composition, where both ring and linear chains have the same molecular weight of 10Me, by molecular dynamics (MD) simulations.20 They also revealed the evident increase of the viscosity due to linear contaminations. Moreover, the blend viscosity is about twice as large as that of the pure linear chain for 50/50 linear/ring blend. From these results, the rheological properties of rings were found to be significantly affected by linear contaminations because there generated multiple ring− linear penetrations. Tadpole-shaped polymers have a unique chain architecture, where one or more linear chains are connected at a certain point on a ring chain. They must be an intriguing model polymer from the rheological viewpoint because they have both ring and linear units in their architecture. In particular, they are expected to show some characteristic rheological properties originating from the intermolecular ring−linear penetrations like the ring/linear blend systems. High-performance liquid Received: August 31, 2015 Revised: November 12, 2015

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or twin-tail, and the following numbers represent the molecular weights in the unit of kg/mol for ring/linear chains. Linear dynamic viscoelasticities were measured by the ARES-G2 rheometer (TA Instruments) with 8 mm diameter and 0.1 rad angle cone plates or 4 mm diameter parallel plates. Temperatures for measurements were varied in the range of 120−200 °C depending on the samples under the nitrogen atmosphere to prevent the chain degradations. The frequency range was 0.1−100 s−1 in a linear strain region. Samples for rheological measurements were prepared in the same manner as previously reported.42 Master curves of the dynamic storage and loss moduli, G′(ω) and G″(ω), were constructed by applying the time−temperature superposition (TTS) principle49 with the reference temperature Tref of 160 °C. Specifically, the experimental data obtained at each temperature were vertically shifted by bT = ρ(Tref)Tref/ρ(T)T, where ρ(T) is the density of PS estimated by the relationship ρ(T) = 1.2503−6.05 × 10−4T.50 Subsequently, the data were horizontally shifted by aT, which is so-called temperature dependent shift factor, to attain the best fittings. For linear PSs, the following relationship of aT was obtained: log aT = −c1(T − Tref)/(c2 + T − Tref) with Tref = 433 K, c1 = 6.3, and c2 = 110 K.42,49 After the rheological measurements, all tadpole samples were tested by SEC and IC to check chain degradations.

chromatography (HPLC) such as liquid chromatography at the critical condition (LCCC) or interaction chromatography (IC) is an essential technique to evaluate the purity of ring polymers because of the efficient separation of rings from linear chains.46,47 Indeed, the recent development in these techniques enables us to prepare truly pure ring samples. Recently, we reported on the successful synthesis of tadpole-shaped polystyrene samples with high purity.48 Both ring and linear chains of the tadpoles have much larger molecular weights than Me, and their purities were confirmed to be over 99% by IC measurements. Therefore, our tadpole samples are sufficiently suitable for the accurate rheological measurements. In this article, melt rheological properties for a series of highly purified tadpole-shaped polystyrenes were reported. All samples used in this study were precisely synthesized by anionic polymerizations and purified by multistep HPLC fractionations. Tadpole samples were designed to have a common ring chain and three different lengths of linear tails, all of which have the molecular weight higher than Me. To elucidate the molecular motion of the tadpole chains, their dynamic moduli were compared with those for the component ring and linear chains and also the ring/linear blends. Moreover, the molecular weight dependence of the zero-shear viscosities, η0, and the steadystate recoverable compliances, Je, of the tadpoles was discussed.

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RESULTS AND DISCUSSION Molecular characteristics of tadpole-shaped polystyrene (PS) samples are summarized in Table 1. All samples were confirmed to have molecular features as designed because their molecular weights exactly match the sum of those for the component ring and linear chains. The purity of the tadpole samples was evaluated by IC measurements as shown in Figure 1 for the tadpole-60/70 series as an example. In this figure, the peaks for tadpoles (eluted at around 16−24 min) are completely separated from that for the component linear chains (12−14 min). From the peak area ratio of the tadpoles and linear chains, the purities of both tadpoles are estimated to be 99.6%. Likewise, IC chromatograms for tadpole-60/30 and 60/120

EXPERIMENTAL SECTION

Synthesis, purification, and characterization of a series of tadpoleshaped polystyrene (PS) samples were carried out in the same way as reported previously as follows.48 First, a highly purified ring PS (R-60; Mw = 59.8 kg/mol) with bifunctional 1,1-diphenylethylene (DPE) type vinyl groups at the coupling point was prepared by utilizing the end-to-end coupling reactions of a telechelic linear PS followed by multistep HPLC fractionation treatments, i.e., size-exclusion chromatography (SEC) and interaction chromatography (IC). Subsequently, three living linear PSs, L-30, L-70, and L-120 (Mw = 27.1, 68.9, and 122 kg/mol, respectively), were anionically polymerized with secbutyllithium, and each living linear PS was reacted with R-60 for more than 24 h. In this study, all three linear chains are designed to have larger molecular weights than the entanglement molecular weight (Me = 18.0 kg/mol for PS). By this synthetic pathway, two kinds of tadpole polymers which possess one or two linear tail chains in one molecule, single-tail and twin-tail, respectively, were produced together. They were isolated from coupling products separately by multistep HPLC fractionations. The weight-average molecular weights, Mw, the molecular weight distribution, Mw/Mn, and the purity of the samples were determined from SEC-MALS, SEC, and IC measurements, respectively. Note that the term “purity” in this study actually implies how much the tadpole samples contain the linear chains. The details of these analyses were described in a previous paper.48 The molecular characteristics of all the tadpole samples obtained are summarized in Table 1. As for the sample codes, the initials S or T denote single-tail

Table 1. Molecular Characteristics of a Series of TadpoleShaped Polystyrene Samples sample

10−3Mwa

Mw/Mnb

10−3Mw,taila

Mw/Mn,tailb

purityc

R-60 S-60/30 T-60/30 S-60/70 T-60/70 S-60/120 T-60/120

59.8 86.8 114 130 192 183 300

1.02 1.02 1.02 1.01 1.01 1.02 1.02

− 27.1 − 68.9 − 122 −

− 1.02 − 1.02 − 1.03 −

99.9 99.3 99.5 99.6 99.6 99.1 99.7

Figure 1. IC chromatograms of tadpole-60/70 series: (a) L-70, (b) R60, (c) S-60/70, and (d) T-60/70. Solid and dotted arrows indicate the peak position of linear chains and the solvent, respectively.

a

Estimated from SEC-MALS measurements. bEstimated from SEC measurements. cEstimated fromIC measurements. B

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Figure 2. Master curves of G′ and G″ for tadpole PSs compared with those for the component ring and linear PSs at Tref = 160 °C: (a, b) tadpole60/30; (c, d) 60/70; (e, f) 60/120 series.

series are shown in Figures S1 and S2 of the Supporting Information. All samples used in this study were confirmed to possess high purities over 99%. Figure 2 shows the angular frequency ω dependence of the storage and loss moduli, G′(ω) and G″(ω), for tadpole-60/30 (a, b), -60/70 (c, d), and -60/120 (e, f) series, while Figure 3 shows the temperature dependence of the horizontal shift factors aT for tadpole PSs. SEC and IC chromatograms for tadpole-60/120 series before and after rheological measurements are compared as an example in Figures S3 and S4, respectively. The peaks for the tadpoles in both SEC and IC were almost unchanged after rheological measurements, suggesting that the samples are not broken through the measurements. First of all, in Figures 2 and 3 the time−temperature superposition can hold for all tadpole samples in this study, and their horizontal shift factors aT have almost the similar dependence to those for linear PSs with high molecular weights. Moreover, in high ω regime in Figure 2, the dynamic moduli G* for all tadpoles are in accordance with those for linear PSs, suggesting that the tadpoles have essentially the same viscoelastic segment size as the linear chains. In Figure 2, the simple ring R-60 exhibits no rubbery plateau even though its molecular weight is larger than the critical

Figure 3. Temperature dependence of the horizontal shift factors aT for tadpole PSs at Tref = 160 °C. The solid curve indicates the dependence of aT for linear PSs estimated from the WLF equations.

entanglement molecular weight Mc (= 2Me = 36.0 kg/mol) for linear PS, which have been already reported in our previous work.42 In contrast, all tadpoles in this study show a rubbery C

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Macromolecules plateau and remarkably slower terminal relaxation than their component ring and linear chains. All three twin-tail tadpoles exhibit slightly slower terminal relaxations than the corresponding single-tail ones, but this difference may not be essential. From the master curves in Figure 2, two rheological parameters, i.e., the zero-shear viscosities, η0, and the steadystate recoverable compliances, Je, for all tadpole samples at Tref = 160 °C were able to be estimated. Both parameters reflect the characteristic relaxation behavior in the terminal region, represented by the equations η0 = lim G″(ω)/ω

(1)

Je = (1/η0 2) lim (G′(ω)/ω 2)

(2)

ω→ 0

ω→ 0

Figure 4. Schematic illustration of the intermolecular entanglement between ring and linear chains of single-tail tadpole molecules.

those of the ring/linear blends. In this study, two blend samples, R-60/L-70 and R-60/L-120, were prepared with the ratio of 50/50 and 33/67 mol %, which are corresponding to the molar ratio for single-tail and twin-tail, respectively. Figure 5 shows the master curves of G′(ω) and G″(ω) for (a, b) R-60/ L-70 and (c, d) R-60/L-120 blends, being compared with the corresponding tadpole samples. In this figure, the rheological spectra for ring/linear blends almost completely overlapped with those for the simple linear chains in a whole ω regime. The rheological parameters, η0, Je, and G0N, for the ring/linear blends were estimated from Figure 5 and are summarized in Table 3. The viscosities η0 and the compliances Je for the blends do not much differ from those for the simple linear chains. The plateau modulus G0N for the blends is slightly smaller than that for the linear chains and is close to that for tadpoles. These results imply that the intermolecular ring−linear penetrations occur also in ring/linear blend systems. The rheological measurements for the ring/linear blends are our ongoing works, and the detailed results will be reported elsewhere. Consequently, the tadpole chains relax much slower than the corresponding ring/linear blends. This demonstrates that the connection between a ring and a linear chain in one molecule induces a drastic change in their rheological properties. The linear chains in ring/linear blends have two free chain ends, and they can basically follow the reptation mode as with the simple linear chains. In contrast, the motion of a linear tail on the tadpole is probably restricted due to the existence of the ring on a chain end of the tail, which drastically delays their chain motion. To elucidate the relaxation mechanism of tadpole chains, we now discuss the molecular weight dependence of two rheological parameters, η0 and Je, at Tref = 160 °C. We treated these parameters in two ways as functions of (i) the total molecular weight (Mw,total) and (ii) the molecular weight of a linear tail of tadpoles (Mw,tail). The latter is probably more significant in considering the universality of their dynamics, since the rheological behavior of tadpoles does not strongly depend on the number of the linear tails but on the molecular weight of a linear tail. The data of η0 and Je for tadpoles were compared with those for linear, ring, and star PSs. The reason for choosing star polymers here is that they have some similarities with the tadpoles; i.e., both star and tadpole polymers have a single branch point in their architecture, and their rheological properties do not sensitively depend on the number of tails or arms.51 Figure 6a shows η0 for tadpole PSs plotted against their total molecular weight in double-logarithmic scales. The data for linear and ring PSs were obtained from the previous study,42 which includes the old data reported by several groups, while those for four-arm and six-arm star PSs are used here from the report by Graessley et al.52 In this figure, both single-tails and twin-tails exhibit a strong Mw dependence, i.e., a drastic viscosity enhancement compared with the simple linear polymers in the same molecular weight range. This implies that the relaxation mechanism of the tadpoles is totally different

The rheologically important two parameters were determined in the same way as the previous report42 and are summarized in Table 2. Table 2. Rheological Parameters, η0 and Je, for the TadpoleShaped Polystyrenes and Their Components at Tref = 160 °C sample R-60 L-30 S-60/30 T-60/30 L-70 S-60/70 T-60/70 L-120 S-60/120 T-60/120

10−3η0 (Pa·s) 1.19 1.00 6.45 9.33 12.5 149 260 75.0 1820 2830

± ± ± ± ± ± ± ± ± ±

0.01 0.02 0.16 0.17 0.1 3.1 7.0 3.2 63 93

105Je (Pa−1)

10−5G0N (Pa)

± ± ± ± ± ± ± ± ± ±

1.38 1.34 2.06 1.36 1.80 2.01 1.78 1.90

0.69 0.30 1.02 1.01 0.71 1.65 1.52 0.96 2.21 2.38

0.08 0.02 0.01 0.05 0.03 0.03 0.01 0.09 0.04 0.11

All tadpoles have larger η0 and Je values than the corresponding ring and linear components. Moreover, η0 of each twin tail is only about 1.5 times larger than that of the corresponding single tail, while their Je values are similar. To elucidate the relaxation mechanism of the tadpole chains, the molecular weight dependence of these parameters will be discussed in detail later. The plateau moduli, G0N, of the tadpoles were also determined from the G′ values where the loss tangent (tan δ) has a minimum, and they are also summarized in Table 2. The G0N values for two linear tails, L-70 and L-120, are approximately 2.0 × 105 Pa, while G0N for tadpoles are relatively smaller than that for the linear ones and reveal weak molecular weight dependence. For tadpoles with short tails, their G0N are approximately 70% of those for linear PS, which may be originated from the insufficient formation of entangled networks due to the short tail. In contrast, the tadpoles with longer tails have almost similar G0N values to the simple linear chains, suggesting that they form well-entangled networks and their entanglement density is not much different from that of linear chains. From these results, we can consider that the ring part as well as the linear part of tadpoles contributes to the formation of the entanglement networks. In other words, tadpole chains spontaneously form the intermolecular ring−linear penetrations as schematically illustrated in Figure 4. This is because if the ring part of the tadpoles is not incorporated in the entanglements but just acts like diluents, the plateau modulus of tadpoles should considerably decrease. To examine the effect of the tadpole-shaped architecture on their dynamics, we compared their rheological properties with D

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Figure 5. Comparison of the dynamic moduli, G′ and G″, for the tadpole PSs and the ring/linear PS blends at Tref = 160 °C: (a, b) tadpole-60/70 and R-60/L-70 blends; (c, d) tadpole-60/120 and R-60/L-120 blends.

Table 3. Rheological Parameters, η0 and Je, for the Ring/ Linear Polystyrene Blends at Tref = 160 °C sample L-70 R-60/L-70 (50/50) R-60/L-70 (33/67) L-120 R-60/L-120 (50/50) R-60/L-120 (33/67)

10−3η0 (Pa·s) 12.5 11.7 14.6 75.0 73.2 77.5

± ± ± ± ± ±

0.1 0.1 0.5 3.2 1.1 2.3

105Je (Pa−1)

10−5G0N (Pa)

± ± ± ± ± ±

2.06 1.49 1.78 2.01 1.64 1.57

0.71 0.89 0.82 0.96 1.08 1.14

0.03 0.02 0.05 0.09 0.05 0.08

Figure 7b, Je of tadpoles and stars are plotted as a function of Mw,tail and Mw,arm, respectively. Note that the data points for twin-tails are omitted in this figure because they stand almost the same spots as those for the single-tails. The tadpoles with long linear tails, i.e., tadpole-60/70 and -60/120, reveal similar Je values to the corresponding stars, while tadpoles with short tails, tadpole-60/30, exhibit evidently larger Je than the corresponding star. This is because tadpoles have a ring chain in their architecture, and its contribution to Je is not negligible (the ring chain R-60 itself has Je = 0.69 × 10−5 Pa−1) when the linear tail chain becomes short. To confirm the similarity between the stars and tadpoles further, their dynamic moduli in a whole ω regime were directly compared. Here a four-arm star PS sample, S141A (Mw,total = 521, Mw,arm = 130 kg/mol), was chosen as reference from the previous data reported by Graessley et al.52 to compare its spectra with those of our tadpole sample, S-60/120 (Mw,total = 183 kg/mol, Mw,tail = 122 kg/mol), since the molecular weight of one arm of the star is close to that of a tail of the present tadpole molecule. Figure 8 shows a comparison of the dynamic moduli for S-60/120 and S141A at Tref = 160 °C. The master curves of S141A are almost perfectly overlapped with those for S-60/120 in a whole ω regime adopted. Only a small difference is observed in the terminal region; that is, the terminal relaxation of the star is a little slower than that of the tadpole, which is probably due to the small difference in between Mw,arm and Mw,tail. This result also supports that the relaxation mechanism of tadpole chains in this molecular weight range is in the same manner as that of star polymers. Now there is an open question why the tadpole chains follow the relaxation mechanism like star polymers. We believe that this star-like motion of a linear tail of a tadpole can be realized only when the ring part of the tadpole is well-threaded by linear chains as schematically illustrated in Figure 4, which induces remarkable motion restriction of the ring part as well as one

from that of the simple linear chains. This result must be an intrinsic feature of the present tadpole-type molecules, each of which includes a ring and a tail chain unit in the architecture. In Figure 6b, η0 of tadpole PSs are plotted against the molecular weight of a linear tail chain. In the same way, η0 of 4arm and 6-arm star PSs are plotted against the molecular weight of a linear arm (Mw,arm). Interestingly, the data points of tadpoles stay on the exponential curve of η0 of the stars. This result strongly suggests that the relaxation mechanism of tadpoles in this molecular weight range is similar to that of star polymers, whose dynamical motion being understood by contour length fluctuations like the arm retraction model.53−55 Figures 7a and 7b show Je data plotted against Mw,total and Mw,arm, respectively. The data for linear, ring, and star PSs were transferred from the previous studies.42,52 Regarding the linear PSs with narrow molecular weight distribution, Je increase in proportional to the increase of Mw (Je ∝ Mw1.0) up to 6Me, while they have a constant value (Je ≈ 1.2 × 10−5 Pa−1) at above 6Me. In contrast, Je of star polymers continue to increase with Mw in a wide range of molecular weight as shown in Figure 7a. Correspondingly, Je for tadpoles also keep increasing. In fact, tadpole-60/70 and -60/120 show clearly larger Je values than the linear chains. These results also suggest that the relaxation mechanism of tadpoles is totally different from that of the linear chains but rather similar to that of the stars. In E

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Figure 6. Double-logarithmic plots of η0 versus (a) Mw,total and (b) Mw,tail of tadpole PSs at Tref = 160 °C. The data are compared with linears, rings, and 4-arm and 6-arm star PSs. The green circle is the symbol for R-60. The solid and dashed lines indicate the relations between η0 and Mw for linear and ring chains, respectively, treated in the previous works. The dotted curve indicates an exponential curve for star chains, which is a guide for the eyes.

Figure 7. Double-logarithmic plots of Je versus (a) Mw,total and (b) Mw,tail of tadpole PSs at Tref = 160 °C. The data are compared with linears, rings, and 4-arm and 6-arm star PSs. The green circle is the symbol for R-60. The solid line indicates the relations between η0 and Mw for linear chains, treated in the previous works. The dotted line indicates the approximate straight line with the slope of unity for star chains, which is a guide for the eyes.

chain end of the linear tail attached on the ring. In other words, if the ring part of tadpoles does not concern any intermolecular interactions, their characteristic star-like relaxation behavior would be suppressed. Figure 8 shows a good evidence to support our hypothesis for the following reason. The tadpole sample S-60/120 may be regarded as an asymmetric three-arm star polymer with two short arms (Mw,short = 59.8/2 = 29.9 kg/ mol) and one long arm (Mw,long = 122 kg/mol), if we assume that a ring part of a tadpole is cut into two chains with the same length and they do not interact with each other. This asymmetric star should show much faster relaxation than the symmetric star with the long arms, which corresponds to S141A in Figure 8, because two short arms of the asymmetric star relax much faster than the other long arm and dilute the remaining entanglements. Such behavior of asymmetric stars was already reported by several groups.56,57 If a ring part of a tadpole S-60/120 does not show any intermolecular interactions like two short arm chains of asymmetric stars, the molecule should relax faster than the symmetric star. However, in fact, S-60/120 exhibits almost the same rheological spectra with S141A as mentioned above. This result supports our hypothesis that the intermolecular ring−linear penetration is evidently generated for tadpole polymers to exhibit their

Figure 8. Comparison of the dynamic moduli, G′ and G″, for the single-tail tadpole PS, S-60/120, and the four-arm PS, S141A at Tref = 160 °C.

characteristic star-like relaxation mechanism. Moreover, we can consider that the relaxation of the ring part threaded by other linear tails occurs only when the ring is released from the threading. In other words, the relaxation of ring parts occurs simultaneously with that of linear tails. F

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Fellowships for Young Scientists (No. 26003393 for Y.D.) and 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. This work was also supported partly by the Collaborative Research Program of Institute for Chemical Research, Kyoto University (Grant No. 2013-45), and A.T. is grateful for the support. This work was also supported by the Program for Leading Graduate Schools at Nagoya University entitled “Integrate Graduate Education and Research Program in Green Natural Sciences”.

Finally, we need to mention that the above characteristic rheological features of tadpole polymers are observed in the limited range of the molecular weights (30 ≤ Mw,tail ≤ 120 kg/ mol) in this study and require further investigation of their dynamics in a wider range of the molecular weight, especially the molecules with much longer linear tails. In addition, the effect of ring size of tadpoles on their dynamics should be also examined, which is closely related to understanding the dynamics of intermolecular ring/linear penetrations.

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CONCLUSION We prepared a series of highly purified tadpole-shaped polystyrenes with three different linear tail lengths, all of which have the molecular weights larger than Me, and investigated their linear melt rheological properties. All tadpole samples used in this study exhibited the slower terminal relaxation than the component ring and linear chains and also than the ring/linear blends. These results suggest that the tadpole chains spontaneously generate characteristic entanglements such as an intermolecular ring−linear penetration. Moreover, they exhibited a characteristic molecular weight dependence of the zero-shear viscosities, η0. In short, they showed a drastic viscosity enhancement compared with the linear chains when plotted η0 against Mw,total, while they revealed similar molecular weight dependence to the star polymers when plotted against Mw,tail and Mw,arm. The molecular weight dependence of the steady-state recoverable compliances Je for tadpoles was also similar to that of stars instead of the linear chains. These results strongly suggest that the relaxation motion of tadpole chains can be taken place in the same manner as that of stars, which can be described by the contour length fluctuations like the arm retraction model, coupled with the intermolecular ring−linear penetrations. Finally, we emphasize again that the tadpole chains revealed a more drastic increase in the melt viscosity than the linear or star chains in the same total molecular weight range, which originates from the unique tadpole-shaped architecture where a ring and a linear chain are introduced into one molecule.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01913. Figures S1−S4 (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*Phone +81-52-789-3211; Fax +81-52-789-3210; e-mail [email protected] (A.T.). *Phone +81-52-789-3211; Fax +81-52-789-3210; e-mail [email protected] (Y.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. J. Roovers for helpful discussions and providing the rheological data for 4-arm and 6-arm star polystyrenes. The authors also thank Prof. Y. Masubuchi of Nagoya University, Prof. T. Inoue of Osaka University, and Dr. D. Kawaguchi of Kyushu University for many fruitful discussions. This work was supported by JSPS Research G

DOI: 10.1021/acs.macromol.5b01913 Macromolecules XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.macromol.5b01913 Macromolecules XXXX, XXX, XXX−XXX