Effect of Topological Constraint and Confined Motions on the

Jan 11, 2017 - Reference QuickView. Top of Page ...... Plazek , D. J. 1995 Bingham Medal Address: Oh, Thermorheological Simplicity, Wherefore Art Thou...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Effect of Topological Constraint and Confined Motions on the Viscoelasticity of Polyrotaxane Glass with Different Interactions between Rings Kazuaki Kato,* Tomoki Mizusawa, Hideaki Yokoyama, and Kohzo Ito* Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan S Supporting Information *

ABSTRACT: A series of polyrotaxane glasses composed of poly(ethylene glycol) backbones and threaded α-cyclodextrins (α-CDs) with different substituents were found to exhibit unique viscoelasticities attributed to their interlocked structures, beyond a mere miscible blend of the different components. Differences in the interactions between CDs, which occupied 80 wt % or more of the materials, brought about wide variations in the glass transition temperature and also influenced the cooperativity of segmental motions of the polyrotaxanes near Tg. These polyrotaxane melts typically showed negligible cooperativity compared to conventional polymers, behaving as so-called “strong” glass formers. The cooperativity gradually emerged with increasing interaction strength between CDs, and elimination of the threading polymer resulted in “fragile” glass formers. These results suggest that the cooperative motion can be shielded by the topological constraint of the threading polymer, whereas the increasing dominance of conventional interactions between CDs develops unavoidable cooperativity. Strong secondary relaxation was observed for all of the polyrotaxane glass samples, and they exhibited considerably higher activation energies than glass made of CD alone. The common secondary relaxation dynamics insensitive to the substituents on the CDs indicate large-scale motion of minor backbone polymers under confinement by similar frozen CD frameworks in the glass state.

■

chains though the cross-links.8 This material design was successful because the sliding ability of polyrotaxane in solution was maintained through the cross-linking for gelation. On the other hand, however, the mechanical properties of solid polyrotaxanes reported until recently have rarely been different from those of conventional materials. Solid polyrotaxanes can be divided into two categories based on different mechanical properties. One is polyrotaxanes with crown ethers (CEs) as their cyclic components. Many CE-based polyrotaxanes are homogeneous thermoplastic materials with a single glass transition temperature (Tg), indicating that the materials can be regarded as miscible blends of CEs and base polymers.9,10 Thus, threaded or unthreaded, the CEs basically behave as a conventional plasticizer, although a small but complex change in the mechanics was reported for an inclusion complex between a CE and a polymer.11 The other type is polyrotaxanes with cyclodextrins (CDs). CD-based polyrotaxanes generally show a phase-separated structure, where the CDs aggregate to form crystalline domains and naked backbone polymers are separated to form their domains.12−14 Such CD domains are formed by strong hydrogen bonds between

INTRODUCTION Polyrotaxanes, necklacelike mechanically interlocked polymers consisting of a backbone polymer and threaded cyclic molecules, have the ability to exhibit isolated molecular motion of different components such as sliding and rotation of the cyclic components within the constraint of the backbone. These characteristic motions, particularly sliding, are enhanced when the backbones are sparsely covered with cyclic components, and thus, such sparse polyrotaxanes have been utilized as precursor polymers for the design of various functional polymers and soft materials.1−4 The attachment of functional moieties to the cyclic components is a mainstream molecular design for enhancing the mobilities of the moieties. A substrateattached polyrotaxane achieved efficient binding to immobilized enzymes on a solid surface because the distribution of the substrates in the polyrotaxane was adjustable to the positions of the enzymes.5 Polyrotaxanes with cationic moieties on the cyclic components were found to work as efficient gene carriers because of the flexibility of the moieties,6 and those with mesogenic moieties were designed for fast-response liquidcrystal polymers.7 To express microscopic sliding as a macroscopic property, the cyclic components of polyrotaxanes were cross-linked to form polymer gels; various macroscopic peculiarities such as solvent permeability and mechanical properties were observed because of the sliding of the polymer © 2017 American Chemical Society

Received: November 11, 2016 Revised: December 23, 2016 Published: January 11, 2017 1861

DOI: 10.1021/acs.jpcc.6b11362 J. Phys. Chem. C 2017, 121, 1861−1869

Article

The Journal of Physical Chemistry C

Preparation of Polyrotaxane Glass. The same methoxyethylated polyrotaxane as reported previously, with 46% hydroxyl groups,21 was reused in this work (denoted as PR-ME46), and another methoxyethylated polyrotaxane with a higher modification degree was synthesized by the following procedure: First, a crude polyrotaxane that consisted of poly(ethylene glycol) (PEG) with Mn = 20 000 and αcyclodextrin (α-CD) was purified. The polyrotaxane was then dissolved in 1 N NaOH (aq) and stirred at 333 K for 4 h. The solution was poured into deionized water to precipitate the polyrotaxane. The precipitate was washed repeatedly with deionized water and then dried under a vacuum to obtain purified unmodified polyrotaxane (PR) as a white powder. The coverage was 25% according to the results of 1H NMR spectroscopy, and size-exclusion chromatography (SEC) confirmed the isolation of PR without free PEG or α-CD. Then, methoxyethylation of the hydroxyl groups of the cyclic components of the purified PR was performed. PR (2.03 g) was dissolved in anhydrous dimethyl sulfoxide (DMSO) (60 mL), and then potassium tert-butoxide (5.99 g, 53.4 mmol) was added and dissolved. Next, 2-methoxyethyl p-toluenesulfonate (20.5 mL, 108 mmol) was added and stirred for 1 h at room temperature. After neutralization with acetic acid (3.4 mL, 59 mmol), the solution was poured into tetrahydrofuran, and the obtained suspension was filtered. The filtrate was evaporated to obtain a solution in DMSO. Deionized water (300 mL) was added, and then the solution was dialyzed with a cellulose tube [molecular weight (MW) cutoff of 14 000 Da]. The obtained aqueous solution was freeze-dried. The obtained solid was dispersed in dichloromethane and reprecipitated repeatedly in diethyl ether. Finally, the obtained solid was dissolved in water and ultrafiltered with a poly(ether sulfone) (PES) membrane (MW cutoff of 10 000 Da). The obtained aqueous solution was freeze-dried to obtain 1.89 g of polyrotaxane with a methoxyethylation degree of 61% (PR-ME-61) as a white solid. Crude hydroxypropylated polyrotaxanes composed of poly(ethylene glycol) (PEG) with Mn = 32 000 and hydroxypropylated α-CDs with different degrees of modification were refined by the following method: Each polyrotaxane was dissolved in deionized water and ultrafiltered with a PES membrane (MW cutoff of 50 000 Da). The resultant aqueous solutions were freeze-dried to obtain purified polyrotaxanes with hydroxypropylation degrees of 48% and 78% (PR-HP-48 and PR-HP-78, respectively) as white powders. Each glass-forming polyrotaxane was individually thermoformed at a temperature 100 K higher than its glass transition temperature under a vacuum for 12 h and then cooled to room temperature. The molded glass samples were cut into rectangular shapes (30 × 5 mm, 0.5−1.0 mm thick) for viscoelastic measurements. Definition and Calculation of the Coverage of Polyrotaxane. Coverage is a measure of how densely packed with CDs the main-chain polymer is. Close packing corresponds to a coverage of 100%. For polyrotaxanes consisting of PEG and α-CDs, close packing was defined in a molecular modeling study as the case in which the CD/repeat unit ratio was 1:2.25 For instance, in the case of PR-ME-46, the molar ratio obtained by the 1H NMR spectrum was 1:8. Thus, the coverage was calculated to be 25%. Preparation of a Glass-Forming Cyclodextrin and Its Glass. For comparison, previously reported data on glassforming cyclodextrin prepared by the same methoxyethylation method applied to α-CD instead of PR are reproduced in this

adjacent threaded CDs, and thus, the domains are too stable to be dissociated until they reach the thermal decomposition temperature.15 Therefore, once polyrotaxanes are isolated as a solid, they never melt and flow, unlike conventional thermoplastic polymers. In addition, the volume fraction of cyclic components is generally dominant because of the bulkiness of CDs. Thus, the CDs form a rigid framework in solid polyrotaxanes and largely determine their mechanical properties. Although the glass transition of the naked backbone domains and local fluctuations inside the CD cavities can be detected thermally or spectroscopically,16,17 these motions have little effect on the mechanical properties. In this way, the solidstate properties of CD-based polyrotaxanes are very similar to those of porous glasses or zeolites that take up polymers inside their cavities.18−20 Recently, we reported a new type of solid polyrotaxane, a socalled polyrotaxane glass.21 This material was prepared by thermopress melt molding of a thermoplastic CD-based polyrotaxane whose cyclic components were methoxyethylated. The polyrotaxane glass exhibited unique glass transition dynamics and strong secondary relaxation that promises high impact resistance.22 This was the first time that the unique interlocked structure of polyrotaxanes had been found to affect the viscoelasticity significantly in such a hard material (Young’s modulus, E ≈ 109 Pa), even though the properties of much softer CD-based polyrotaxanes have been investigated.23,24 Because weakening of the interactions between CDs is a key requirement for polyrotaxane glass formation, the effect of substituents on the CDs is thought to be critical to the viscoelastic properties. We report here the influence of interactions between CDs on the unique viscoelasticity of the original polyrotaxane glass. Four polyrotaxane glass samples were prepared from CD-based polyrotaxanes with different substituents and different degrees of substituents on their cyclic components, as shown schematically in Figure 1. The glass

Figure 1. Structures of glass-forming polyrotaxanes and the thermomolded polyrotaxane glass. The image at the lower right shows a transparent sheet of glass prepared from a hydroxypropylated polyrotaxane (PR-HP) sample.

transitions and secondary relaxation (subrelaxation) dynamics of the samples were analyzed and compared to elucidate the generality of the unique viscoelasticity and its molecular origin.

■

EXPERIMENTAL SECTION Materials. 2-Methoxyethyl p-toluenesulfonate was purchased from Tokyo Chemical Industry Co., Ltd. All other chemicals and solvents were purchased from Wako Pure Chemical Industries, Ltd. All reagents were used without further purification. Crude polyrotaxanes were purchased from or provided by Advanced Softmaterials, Inc., and these polyrotaxanes were used after purification. 1862

DOI: 10.1021/acs.jpcc.6b11362 J. Phys. Chem. C 2017, 121, 1861−1869

Article

The Journal of Physical Chemistry C article.21 The glass-forming CD and its thermoformed glass are referred to as CD-ME and CD glass, respectively. Measurements. 1H NMR spectra at 400 MHz were recorded on a JEOL JNM-AL400 spectrometer at 298 K in deuterium oxide (D2O). Chemical shifts were calibrated using water (4.65 ppm) as the internal standard. SEC, with DMSO/ LiBr as the eluent, was performed on a Shodex OHpak SB-G and two Shodex OHpak SB-806MHQ columns at 323 K at a rate of 0.4 mL/min using refractive index detection and PEG standards. The LiBr concentration was 10 mM. Powder X-ray diffraction measurements were performed using a Rigaku SmartLab X-ray diffractometer with Cu Kα radiation (λ = 1.542 Å). Heat-flux differential scanning calorimetry (DSC) was performed using a DSC7000X instrument with a liquidnitrogen cooling system (Hitachi High-Tech Science Corporation). Each sample was sealed in an aluminum pan. The scanning rate was 20 K/min. Attenuated total reflectanceFourier transform infrared (ATR-FT-IR) spectra were recorded on a Nicolet 4700 spectrometer (Thermo Electron Co., Ltd.) equipped with a diamond attenuated total reflection (ATR) accessory (DurasamplIR II, SensIR Technologies) in air. Viscoelastic measurements on the thermoformed polyrotaxane and CD glass samples were performed by a straincontrolled oscillatory rheometer (RSAIII, TA Instruments). Frequency sweeps were conducted from 80 to 0.01 Hz, under a 0.05% oscillatory tensile strain amplitude. Time−temperature superposition was applied using a program in TA Orchestrator (TA Instruments) for the data at different temperatures to obtain master curves without vertical shifts. For temperature sweeps, a 0.05% or 0.1% oscillatory tensile strain at 1 Hz was applied.

Figure 2. (a) 1H NMR spectra (400 MHz, D2O, 298 K) and (b) SEC traces (eluent, DMSO/LiBr; detector, refractive index) of glassforming polyrotaxanes.

Table 1. Characteristics of Glass-Forming Polyrotaxanes and the Cyclodextrin

■

RESULTS AND DISCUSSION Structure of Polyrotaxane Glasses. Isolation of polyrotaxanes from impurities such as free (unthreaded) CDs and free polymers is necessary for understanding the material properties of polyrotaxane glass because miscible impurities can behave as plasticizers and affect the glass transition behavior. Figure 2b shows the SEC traces for the four polyrotaxanes in this work. In each chromatogram, a single broad peak without low-molecular-weight impurities such as free PEG and CDs, whose retention times are 41 and 46 min, respectively, and with a considerably higher apparent molecular weight than PEG was detected. The formation of mechanically interlocked architectures is suggested by both the substantial increase of the apparent molecular weights of the samples from their components and the stability of their complexes in DMSO, which dissociates the inclusion complexes without end-capping groups. The 1H NMR spectra indicate signals from both PEG and the cyclic components in each sample, and all those signals were assigned as shown in Figure 2a. From the ratios of the integral values between each of the two components, we determined the molar ratios between the CDs and the repeat unit of PEG. As mentioned in a previous section, polyrotaxane is considered to be densely covered if it has a 1:2 ratio between the CDs and the repeat unit of PEG.25 The obtained information on the chemical structures for the four polyrotaxanes, including modification degree, coverage, and calculated weight fractions of CDs, wCD, are summarized in Table 1. These refined polyrotaxanes were thermopress melt-molded at temperatures that were 100 K higher than their respective Tg values (see the next section about their Tg values) under a

substituent group PR-ME-46 PR-ME-61 CD-ME PR-HP-48 PR-HP-78

CH2CH2OCH3 CH2CH2OCH3 CH2CH2OCH3 CH2CH(CH3) OHa CH2CH(CH3) OHa

modification degree (%)

coverage (%)

wCD (%)

46 61 64 48b

28 25 − 25

82 82 − 80

78b

20

80

Oligomeric substituents − [CH2CH(CH3)O]n−H (n ≥ 2) coexist. Oligomeric substitutions were also counted to calculate average numbers of the substituent group per glucose unit.

a b

vacuum for 12 h and then cooled to room temperature to obtain polyrotaxane glasses as transparent plates. X-ray diffraction revealed that these polyrotaxanes were amorphous, as shown in Figure 3, whereas the precursor PR had clear Bragg peaks of a common channel-type crystal formed by its unmodified CDs. It is noteworthy that all of the polyrotaxanes exhibited a common amorphous halo at 2θ = 7°, which corresponds to the short-range order between their cyclic components of 1.3 nm. The relatively sharp signal of PR-ME-61 might indicate more ordered packing of the CDs, possibly because the CDs could have more defined structures; the OH groups at C2 and C6 were almost entirely methoxyethylated. Another common halo at 2θ = 21°, corresponding to a correlation length of 0.42 nm, might have arisen from the distance between the facing walls in the single α-CD, as similarly observed in other amorphous α-CD derivatives, although no discussion was provided in that report.26 Thus, these four thermoplastic polyrotaxanes formed glasses, although 1863

DOI: 10.1021/acs.jpcc.6b11362 J. Phys. Chem. C 2017, 121, 1861−1869

Article

The Journal of Physical Chemistry C

native CDs were interrupted and hence the modified groups formed no strong interactions. The increase in the modification degree actually resulted in a decrease in Tg: PR-ME-61 exhibited a lower Tg value, at −6 °C, than PR-ME-46, although, for these samples, Tg is ambiguous because of their broad DSC profiles. Because the modification degree of PRME-61 was similar to that of CD-ME (64%), the polyrotaxane glass can be compared to a blend of CD-ME and the PEG backbone. If the blend is miscible with equal contributions of the two components, then the Tg value of the polyrotaxane glass can be described according to the Fox equation w w 1 = PEG + CD Tg Tg,PEG Tg,CD (1) where wPEG is the weight fraction of PEG and Tg,PEG and Tg,CD are the Tg values of the respective components. From the weight contents (wring = 0.82, wchain = 0.18) and measured values of Tg,PEG (−57 °C) and Tg,CD (20 °C), the Tg value of the miscible blend was calculated from eq 1 to be 2 °C. This rough agreement with the measured Tg value, defined as the onset temperature extrapolated from the broad curve, suggests that the two components were distributed randomly, without phase separation. Although physical blends between CDs and PEG could not be prepared for technical reasons, such blends would be phase-separated without the topological constraint. Another series of polyrotaxane glass samples were obtained by hydroxypropylation of the cyclic components of PR. The Tg value of PR-HP-48 was 85 K higher than that of PR-ME-46, despite their similar modification degrees. This indicates that hydroxypropylation is less effective in terms of weakening the interactions between the CDs of PR than methoxyethylation, even though their general formulas, C3H7O, are the same. Figure 5 shows partial Fourier transform infrared (FT-IR)

Figure 3. XRD profiles (Cu Kα) of four polyrotaxane glasses, a cyclodextrin glass (CD-ME), and PR. The Miller indices were assigned for PR based on a typical hexagonal columnar crystal formed by its cyclic components, α-CDs.

their unique properties as interlocked polymers have not been identified at this point. Glass Transition Temperature. Each polyrotaxane exhibited a single glass transition, as shown in Figure 4. As we

Figure 4. DSC profiles of polyrotaxane glasses and a CD glass made of CD-ME upon a second heating after being cooled to −150 °C at 10 K/min. The exothermal peaks at approximately 55 °C are due to the negligible crystalline parts formed by their respective PEG backbones during the first cooling [amounts of heat are much smaller than that of bulk PEG (144 J/g)].

Figure 5. Partial ATR-FT-IR spectra of polyrotaxane glass, PR, and CD-ME samples, focusing on their O−H stretching vibrations.

spectra focusing on the O−H stretching vibrations. Because of the strongly hydrogen-bonded CDs, PR exhibits a peak at 3330 cm−1. All modifications in this work shifted the peak toward higher wavenumbers, indicating that the hydrogen bonds were interrupted. Thus, the more moderate shifts in the hydroxypropylated samples suggest that they retained stronger hydrogen bonds than the methoxyethylated samples. This is because hydroxypropylation retains hydroxyl groups, whereas methoxyethylation loses hydroxyl groups and provides no better hydrogen-bonding acceptors. On the other hand, further hydroxypropylation resulted in a dramatic decrease in Tg (ΔTg

reported previously, the crystalline PR, which exhibited neither melting nor a glass transition, turned out to become a polyrotaxane glass through the methoxyethylation of its cyclic components (the glass is denoted as PR-ME-46 in this article).21 This suggests that methoxyethylation effectively weakened the interactions between the cyclic components, probably because the strong hydrogen bonds between the 1864

DOI: 10.1021/acs.jpcc.6b11362 J. Phys. Chem. C 2017, 121, 1861−1869

Article

The Journal of Physical Chemistry C

glass transition, master curves were created from the results of frequency sweeps at different temperatures based on the time− temperature superposition principle, and the results are shown in Figure 7. This successful superposition of the data indicates thermorheological simplicity, which is rarely present in conventional polymers.28 The segmental motion of conventional polymer melts is gradually restricted by the surrounding chains as the temperature approaches Tg, and at the same time, the apparent activation energy increases dramatically.29,30 Their complex intermolecular cooperative motions (caused by interactions between neighboring, nonbonded segments) invalidate the time−temperature superposition principle.28,31,32 Thus, the exceptional dynamics in polyrotaxanes suggest a significantly low intermolecular cooperativity, similar to that of polyisobutylene (PIB), which is an exceptional polymer with thermorheological simplicity.33 Note that all of these polyrotaxanes behaved as typical polymer melts, in considerably low-frequency regimes, following the Rouse model, with E′(ω) ≈ E″(ω) ∼ ω0.5. On the other hand, CD-ME did not follow the Rouse mode (Figure S1), but rather, it flowed immediately after Tg. This behavior confirms that the CDs in polyrotaxane melts do not diffuse randomly but instead coordinate with neighboring CDs connected through a common backbone polymer; this is the segmental motion of polyrotaxanes. It is noteworthy that these polyrotaxanes, except for PR-HP-48, reached their glass states without an increase according to a power law, whereas conventional polymers generally exhibit a higher-power-law frequency dependence of E′(ω) near Tg.34 The cooperativity of segmental motions near Tg was analyzed using the shift factor aT, which is the ratio of the relaxation time at T to that at a reference temperature T0 and is obtained through the creation of master curves. Figure 8 shows Arrhenius plots of aT versus temperature normalized by Tg for these polyrotaxanes. The slight increase in their apparent activation energies, which is defined as the slope of the plot, toward Tg indicates that these polyrotaxanes show very small cooperativity in their segmental motions. Particularly in the methoxyethylated PRs, the apparent activation energies were almost constant through the transition. Methoxyethylated PRs are so-called “strong” glass formers.35 The plots were well fitted by the Williams−Landel−Ferry (WLF) equation36 at each reference temperature, where tan δ is 0.1, which is close to the Tg value determined by the broad profile of DSC. Then, the obtained fitting parameters were used to obtain the strength parameter, D, from the Vogel−Fulcher−Tammann equation, which is equivalent to the WLF equation37,38

= 83 K), as in the case of PR-HP-78; nevertheless, the hydrogen bonds were not affected significantly. It is notable that hydroxypropylation occurred not only on the hydroxyl groups of the native CDs but also on the hydroxyl groups of the attached hydroxypropyl groups to form oligo(propylene glycol).27 This unavoidable oligomerization became pronounced when the steric hindrance around the hydroxyl groups of the native CDs increased with increasing modification degree. Therefore, PR-HP-78 should have longer side chains than PR-HP-48, and such longer chains could disturb the interaction between the cyclic components of the polyrotaxanes. In this way, the Tg values of polyrotaxane glasses are widely variable and can be adjusted by changing the substituents on their cyclic components and their modification degrees. Glass Transition Dynamics. Although the Tg values of polyrotaxane glasses can be explained by an analogy to miscible blends of CDs and PEG, their interlocking geometries make a clear difference in the glass transition dynamics. Figure 6 shows

⎛ TD ⎞ τ = τ∞ exp⎜ V ⎟ ⎝ T − TV ⎠

(2)

where TV is a reference temperature called the Vogel temperature. The D values of PR-ME-46 and PR-ME-61 were calculated to be 103 and 46, respectively. These values indicate that these glass-forming polyrotaxanes are stronger than the strongest glass-forming polymer, PIB, whose D value is 20.33 Although the fragilities of PR-ME-46 and PR-ME-61, where fragility is defined as the degree of deviation from Arrhenius dynamics and increases with smaller D values, are not very low as compared to the fragilities of the ME-PRs, the D values of HP-PRs were comparable to those of the strongest conventional polymer (24 and 17 for HP-PR-48 and HP-PR-78, respectively).

Figure 6. Temperature dependence of E′, E″, and tan δ for (a) methoxyethylated PRs, (b) CD-ME, and c) hydroxypropylated PRs.

the temperature dependence of the dynamic complex moduli and loss tangent. As shown in Figure 6b, the storage elastic modulus, E′, of CD-ME decreased dramatically near Tg and underperformed the loss modulus, E″, with increasing temperature. On the other hand, the polyrotaxane glass samples, particularly the methoxyethylated ones, exhibited very broad glass transitions (Figure 6a,c). To analyze the dynamics of the 1865

DOI: 10.1021/acs.jpcc.6b11362 J. Phys. Chem. C 2017, 121, 1861−1869

Article

The Journal of Physical Chemistry C

Figure 7. Master curves of frequency-dependent moduli for polyrotaxane glasses in their glass transition regimes (T0 is the reference temperature).

constraint. Thus, even though the backbone polymer is a minor component, its presence can considerably affect the cooperativity. In conventional polymers, the relationship between fragility and chemical structure has been studied by systematic experiments with various polymers and theoretical models.39,40 Polymers with flexible main chains, nonbulky side chains, and less polarity tend to be strong because of their low intermolecular coupling in local segmental motions. PIB falls into this category, and its low coupling suppresses cooperative motion. As a result of this suppressed cooperativity, PIB exhibits thermorheological simplicity. On the other hand, polyrotaxanes also show thermorheological simplicity and negligible cooperativity in spite of their bulky, polar chains. It is unlikely that the negligible cooperativity in polyrotaxanes can be explained solely by intermolecular interactions. If the CDs of polyrotaxanes were constrained not by intermolecular interactions among the surrounding CDs but rather by the threading backbone, their cooperativity would be low compared to that of conventional polymers or CDs. Thus, the topological constraint could shield the cooperativity of the segmental motions of polyrotaxanes. The effect of shielding seems to be less significant in HP-PRs than in ME-PRs, even though HP-PRs are still strong glass formers. Although both the ME- and HPPRs should have the same level of topological constraint because of their common backbones and α-CDs, the interactions among the CDs are stronger in HP-PRs. Upon reaching the temperature where the CD interactions are removed, the mobility of the backbone polymer could become too high to restrain the CDs from translational diffusion. The observation of the most fragile behavior in HP-PR-78 might be due to its having the weakest topological constraint because its coverage is the lowest. In this way, these polyrotaxanes were strong glass formers, regardless of the substituents on the CDs, and the degrees of cooperativity seemed to be determined by the balance between the interactions of the CDs and the topological constraint. Although this speculative picture of the glass transition is based

Figure 8. Dependence of the shift factor on the temperature normalized by Tg for polyrotaxane glasses. Fitting curves obtained with the WLF equation are also shown.

It is notable that the strong behavior of these polyrotaxanes was not created without the backbone polymer; CD-ME became a “fragile” glass former whose D value was 8.4, as reported previously.21 As discussed above, these polyrotaxane glasses were formed mainly by the interactions between cyclic components that occupy more than 80% of their respective weights. If their backbones were missing, translational diffusion of the single cyclic component would immediately follow the glass transition of the cyclic components. However, release from the interactions would not induce immediate diffusion of the CDs in polyrotaxanes because the constraint imposed by their threading backbones remains. In other words, the diffusion of CDs in polyrotaxanes would no longer be governed above Tg by the interactions of their CDs but by the topological 1866

DOI: 10.1021/acs.jpcc.6b11362 J. Phys. Chem. C 2017, 121, 1861−1869

Article

The Journal of Physical Chemistry C on the independent mobilities of different components and merits further examination, this independence also appears in their secondary relaxations, as discussed in the next section. Simulation studies are expected to reveal the molecular picture of polyrotaxane glasses near Tg, and such findings for these complex glasses will contribute to the full elucidation of the general glass transition mechanism that is a central issue of physics. Secondary Relaxation of Polyrotaxane Glass. In the glass state, fluctuations of the polyrotaxanes are restricted by vitrification because of the interactions between their cyclic components. Although the mobility of their backbone polymers would be considerably affected by the vitrification, it seems that the backbones maintain a relatively high mobility, even in the glass state, because the main chain can rotate and slide within the frozen cyclic components. Through vitrification, the polyrotaxane and CD glass samples commonly exhibited E′ values of ca. 1 GPa, as shown in Figure 6. Notably, with further decreases in temperature, the moduli of the polyrotaxane glasses increased dramatically, by more than 3 times, through secondary relaxation (see also Figure S2; the linear plots of E′, for clarity). Generally, secondary relaxation reflects local molecular motion in the glass state, such as side-chain rotation, and the relaxation strength is related to the impact strength of the polymer glass.22 The observed secondary relaxation strength in polyrotaxane glasses was even higher than that of poly(methyl methacrylate), which is known as a high-impact polymer glass because of the significant rotation of its side chains.41,42 The significance of the secondary relaxation in polyrotaxane glasses cannot be explained in terms of the local motions of their major components because the CD glass exhibited a contrastingly weak relaxation that changed the modulus by only 1.5 times. The original motion of this secondary relaxation of the CD glass was assigned as the rotational motion of the CD side chains on the basis of dielectric relaxation studies.43 Indeed, essentially the same dielectric spectra were observed for the polyrotaxane glasses because polyrotaxane glasses are made up mostly of CDs.21 Therefore, the enhanced secondary relaxation in polyrotaxane glasses should be explained in terms of characteristic motions that cannot be sensitively observed dielectrically but can be observed mechanically. The master curves in their respective secondary relaxation regimes were created from viscoelastic spectra at different temperatures (see Figure S3). Arrhenius plots of the shift factors are shown in Figure 9. All of the polyrotaxane glass samples exhibited very similar secondary relaxation dynamics, and their activation energies were considerably higher than that of the dielectrically observed side-chain rotation of CDs. It is obvious that the enhanced secondary relaxation in polyrotaxane glasses is created by certain major motion, in addition to CD side-chain rotation. This additional motion is probably that of the PEG backbone, which has considerably less electric permittivity than CDs. Although the observed dynamics appear to be non-Arrhenius, they can be separated into two Arrhenius modes with respective activation energies of 110 and 180 kJ/ mol; the inflection point between these two modes is close to the Tg value of the bulk PEG, Tg,PEG. The mobility of the backbone should change significantly near Tg,PEG if the naked parts of the backbone behave similarly to the amorphous parts of bulk PEG. However, such local motion of the minor component is unlikely to create significant secondary relaxation. Amorphous chains in the bulk PEG crystal are bound to the

Figure 9. Arrhenius plots of shift factors in the secondary relaxation regime for polyrotaxane glasses. The solid line represents the activation energy of side-chain rotation in CDs.

crystalline background. On the other hand, the PEG chains in polyrotaxane glasses are only topologically constrained by the porous background formed by frozen CDs. Thus, the PEG chains in the polyrotaxane glasses should have high mobility that the chains in bulk PEG do not have. At T ≫ Tg, similarly to conventional polymers, polyrotaxanes show segmental motion, which is the combined fluctuation between threaded CDs and the backbone PEG, as shown in Figure 10a. When a polyrotaxane is cooled below Tg, the

Figure 10. Schematic freezing process of molecular motion in polyrotaxane. The global motion of the CDs is frozen at Tg, and then the constrained motion of PEG within the framework formed by the CDs is maintained by Tg,PEG.

segment motion is frozen because the diffusion of the CDs is restricted. In other words, the segment motion is frozen by the bulky “side chains”. Interactions between the bulky side chains generally have a decisive influence on the motion of the main chains. The main chains of polyrotaxanes, however, should have a weaker relationship to the threaded side chains. Thus, the main chains do not necessarily lose their mobility beyond the topological constraint imposed by the vitrified CDs, and the chains are allowed to fluctuate through the cavities of the CDs, as shown in Figure 10b. This characteristic translational motion (sliding) of the backbones would continue until their fluctuations outside of the CD cavity are frozen. Near Tg,PEG, the motion of the amorphous parts formed by naked PEG can be frozen, and thus, the sliding is also restricted, as shown in Figure 10c. Only local fluctuations in the cavity can remain without affecting the macroscopic mechanical properties, similarly to polymers confined in porous materials.18−20 It is noteworthy that the secondary relaxation dynamics were hardly affected by the modified groups and their modification degrees. This coincidence in the secondary relaxation dynamics suggests 1867

DOI: 10.1021/acs.jpcc.6b11362 J. Phys. Chem. C 2017, 121, 1861−1869

The Journal of Physical Chemistry C

■

that the outer structure of the CDs does not significantly influence the dynamics of the backbones. Incidentally, the dynamics in PR-HPs were found to be slightly slower than those in PR-MEs, particularly at higher temperatures. A slower subrelaxation is also shown in Figure 6, where the peaks of tan δ are roughly 20 K higher than for PR-MEs. The slight difference between PR-HPs and PR-MEs might rather be because of the difference in the coverage and the length of the backbone. In this way, the main origin of the significant secondary relaxation in polyrotaxane glasses can be attributed to the significant unique motion of the backbone polymers within the glassy frameworks formed by their cyclic components. The significant and unique molecular motions in the glass state would affect other bulk mechanical properties such as impact strength; studies of these effects are now in progress.

AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected] (K.K.). * E-mail: [email protected] (K.I.). ORCID

Kazuaki Kato: 0000-0002-9997-8599 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant JP16H06050 and ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

■

■

CONCLUSIONS We demonstrated in this work that polyrotaxane glasses with different substituents on their cyclic components commonly exhibited unique glass transition dynamics and significant secondary relaxation. In addition to wide variations in their Tg values, the differences in the interactions between the rings that occupied ≥80 wt % of the material affected the cooperativity of their segmental motions near Tg. The negligible cooperativity commonly observed in these polyrotaxanes was attributed to the topological constraint imposed by the threading polymer, and the effects of the constraint seemed to be suppressed slightly by stronger interactions between rings. Although the interactions between rings below Tg prevented the segmental motion of the polyrotaxanes, which was the combined fluctuations between threaded rings and the polymer backbone, the backbone maintained high mobility within the confinement imposed by the frozen rings’ framework. The confined motion appeared to last until the naked part of the backbone vitrified to create significant secondary relaxation. Similar secondary relaxation dynamics in different substituents on the rings support the common original motion of the polymer backbone confined in similar porous frameworks formed by the rings. In this way, the independent mobilities of different components within the topological confinement create the unique viscoelasticity of polyrotaxane glasses. These results suggest that appropriate molecular design of polyrotaxanes through the tuning of the topological constraint and interactions can control macroscopic mechanics, not only the linear viscoelasticity but also high impact resistance and behaviors under large deformations that conventional designs using polymers could never achieve. Studies on the mechanical properties of various types of polyrotaxane glasses with different degrees of topological constraints are now in progress, in combination with detailed observations of the dynamics by using NMR spectroscopy and other techniques.

■

Article

REFERENCES

(1) Wenz, G.; Han, B. H.; Muller, A. Cyclodextrin Rotaxanes and Polyrotaxanes. Chem. Rev. 2006, 106, 782−817. (2) Harada, A.; Hashidzume, A.; Yamaguchi, H.; Takashima, Y. Polymeric Rotaxanes. Chem. Rev. 2009, 109, 5974−6023. (3) Fang, L.; Olson, M. A.; Benítez, D.; Tkatchouk, E.; Goddard, W. A., III; Stoddart, J. F. Mechanically Bonded Macromolecules. Chem. Soc. Rev. 2010, 39, 17−29. (4) Arunachalam, M.; Gibson, H. W. Recent Developments in Polypseudorotaxanes and Polyrotaxanes. Prog. Polym. Sci. 2014, 39, 1043−1073. (5) Ooya, T.; Eguchi, M.; Yui, N. Supramolecular Design for Multivalent Interaction: Maltose Mobility Along Polyrotaxane Enhanced Binding with Concanavalin A. J. Am. Chem. Soc. 2003, 125, 13016−13017. (6) Yang, C.; Wang, X.; Li, H.; Goh, S. H.; Li, J. Synthesis and Characterization of Polyrotaxanes Consisting of Cationic α-Cyclodextrins Threaded on Poly[(ethylene oxide)-ran-(propylene oxide)] as Gene Carriers. Biomacromolecules 2007, 8, 3365−3374. (7) Kidowaki, M.; Nakajima, T.; Araki, J.; Inomata, A.; Ishibashi, H.; Ito, K. Novel Liquid Crystalline Polyrotaxane with Movable Mesogenic Side Chains. Macromolecules 2007, 40, 6859−6862. (8) Mayumi, K.; Ito, K.; Kato, K. Polyrotaxane and Slide-Ring Materials; Royal Society of Chemistry: Cambridge, U.K., 2016. (9) Gibson, H. W.; Marand, H. Polyrotaxanes: Molecular Composites Derived by Physical Linkage of Cyclic and Linear Species. Adv. Mater. 1993, 5, 11−21. (10) Gong, C.; Ji, Q.; Subramaniam, C.; Gibson, H. W. Main Chain Polyrotaxanes by Threading Crown Ethers onto A Preformed Polyurethane: Preparation and Properties. Macromolecules 1998, 31, 1814−1818. (11) Nagapudi, K.; Hunt, J.; Shepherd, C.; Baker, J.; Beckham, H. W. Synthesis, Dynamic Mechanical, and Solid-State NMR Spectroscopy of Crown-Ether-Based Linear Aliphatic Polyurethane Rotaxanes. Macromol. Chem. Phys. 1999, 200, 2541−2550. (12) Li, J.; Harada, A.; Kamachi, M. Sol−Gel Transition during Inclusion Complex Formation between α-Cyclodextrin and High Molecular Weight Poly(ethylene glycol)s in Aqueous Solution. Polym. J. 1994, 26, 1019−1026. (13) Topchieva, I. N.; Tonelli, A. E.; Panova, I. G.; Matuchina, E. V.; Kalashnikov, F. A.; Gerasimov, V. I.; Rusa, C. C.; Rusa, M.; Hunt, M. A. Two-Phase Channel Structures Based on α-Cyclodextrin−Polyethylene Glycol Inclusion Complexes. Langmuir 2004, 20, 9036−9043. (14) Sakai, Y.; Ueda, K.; Katsuyama, N.; Shimizu, K.; Sato, S.; Kuroiwa, J.; Araki, J.; Teramoto, A.; Abe, K.; Yokoyama, Y.; et al. Fabrication and Structural Analysis of Polyrotaxane Fibers and Films. J. Phys.: Condens. Matter 2011, 23, 284108. (15) Huang, L.; Allen, E.; Tonelli, A. E. Study of the Inclusion Compounds Formed between α-Cyclodextrin and High Molecular Weight Poly(ethylene oxide) and Poly(ϵ-caprolactone). Polymer 1998, 39, 4857−4865.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11362. Master curves of CD glass in the glass transition regime, linear plots of E′ values of polyrotaxane and CD glasses, master curves of polyrotaxane glass in the secondary relaxation regime, and NMR spectra with integrations (PDF) 1868

DOI: 10.1021/acs.jpcc.6b11362 J. Phys. Chem. C 2017, 121, 1861−1869

Article

The Journal of Physical Chemistry C (16) Girardeau, T. E.; Leisen, J.; Beckham, H. W. Chain Dynamics of Poly(oxyethylene) in Nanotubes of α-Cyclodextrin by Solid-State 2H NMR. Macromol. Chem. Phys. 2005, 206, 998−1005. (17) Li, J.; Ni, X.; Zhou, Z.; Leong, K. W. Preparation and Characterization of Polypseudorotaxanes Based on Block-Selected Inclusion Complexation between Poly(propylene oxide)-Poly(ethylene oxide)-Poly(propylene oxide) Triblock Copolymers and αCyclodextrin. J. Am. Chem. Soc. 2003, 125, 1788−1795. (18) Schönhals, A.; Goering, H.; Schick, C.; Frick, B.; Mayorova, M.; Zorn, R. Segmental Dynamics of Poly(methyl phenyl siloxane) Confined to Nanoporous Glasses. Eur. Phys. J.: Spec. Top. 2007, 141, 255−259. (19) Shin, K.; Obukhov, S.; Chen, J. T.; Huh, J.; Hwang, Y.; Mok, S.; Dobriyal, P.; Thiyagarajan, P.; Russell, T. P. Enhanced Mobility of Confined Polymers. Nat. Mater. 2007, 6, 961−965. (20) Yanai, N.; Uemura, T.; Kosaka, W.; Matsuda, R.; Kodani, T.; Koh, M.; Kanemura, T.; Kitagawa, S. Inclusion and Dielectric Properties of a Vinylidene Fluoride Oligomer in Coordination Nanochannels. Dalton Trans. 2012, 41, 4195−4198. (21) Kato, K.; Mizusawa, T.; Yokoyama, H.; Ito, K. Polyrotaxane Glass: Peculiar Mechanics Attributable to the Isolated Dynamics of Different Components. J. Phys. Chem. Lett. 2015, 6, 4043−4048. (22) Wada, Y.; Kasahara, T. Relation Between Impact Strength and Dynamic Mechanical Properties of Plastics. J. Appl. Polym. Sci. 1967, 11, 1661−1665. (23) Inomata, A.; Sakai, Y.; Zhao, C.; Ruslim, C.; Shinohara, Y.; Yokoyama, H.; Amemiya, Y.; Ito, K. Crystallinity and Cooperative Motions of Cyclic Molecules in Partially Threaded Solid-State Polyrotaxanes. Macromolecules 2010, 43, 4660−4666. (24) Sakai, Y.; Gomi, R.; Kato, K.; Yokoyama, H.; Ito, K. Structure and Dynamics of Polyrotaxane-based Sliding Graft Copolymers with Alkyl Side Chains. Soft Matter 2013, 9, 1895−1901. (25) Harada, A.; Li, J.; Kamachi, M. Preparation and Properties of Inclusion Complexes of Poly(ethylene glycol) with α-Cyclodextrin. Macromolecules 1993, 26, 5698−5703. (26) Bettinetti, G.; Sorrenti, M.; Catenacci, L.; Ferrari, F.; Rossi, S. Polymorphism, Pseudopolymorphism, and Amorphism of Peracetylated α-, β-, and γ-Cyclodextrins. J. Pharm. Biomed. Anal. 2006, 41, 1205−1211. (27) Irie, T.; Fukunaga, K.; Yoshida, A.; Uekama, K.; Fales, H. M.; Pitha, J. Amorphous Water-Soluble Cyclodextrin Derivatives: 2Hydroxyethyl, 3-Hydroxypropyl, 2-Hydroxyisobutyl, and Carboxamidomethyl Derivatives of β-Cyclodextrin. Pharm. Res. 1988, 5, 713− 717. (28) Plazek, D. J. Anomalous viscoelastic properties of polymers: Experiments and explanations. J. Non-Cryst. Solids 2007, 353, 3783− 3787. (29) Egelstaff, P. A. An Introduction to the Liquid State; Academic Press: New York, 1967. (30) Kanaya, K.; Kaji, K. Dynamics in the Glassy State and Near the Glass Transition of Amorphous Polymers as Studied by Neutron Scattering. Adv. Polym. Sci. 2001, 154, 87−141. (31) Ngai, K. L.; Plazek, D. J. Identification of Different Modes of Molecular Motion in Polymers That Cause Thermorheological Complexity. Rubber Chem. Technol. 1995, 68, 376−434. (32) Plazek, D. J. 1995 Bingham Medal Address: Oh, Thermorheological Simplicity, Wherefore Art Thou? J. Rheol. 1996, 40, 987−1014. (33) Plazek, D. J.; Zheng, X. D.; Ngai, K. L. Viscoelastic Properties of Amorphous Polymers. I. Different Temperature Dependences of Segmental Relaxation and Terminal Dispersion. Macromolecules 1992, 25, 4920−4924. (34) Rubinstein, M.; Colby, R. H. Polymer Physics; Oxford University Press: Oxford, U.K., 2003. (35) Bohmer, R.; Ngai, K. L.; Angell, C. A.; Plazek, D. J. Nonexponential Relaxations in Strong and Fragile Glass Formers. J. Chem. Phys. 1993, 99, 4201−4209. (36) Williams, M. L.; Landel, R. F.; Ferry, J. D. The Temperature Dependence of Relaxation Mechanisms in Amorphous Polymers and Other Glass-forming Liquids. J. Am. Chem. Soc. 1955, 77, 3701−3707.

(37) Vogel, H. The Temperature Dependence Law of the Viscosity of Fluids. Phys. Z. 1921, 22, 645−646. (38) Fulcher, G. S. Analysis of Recent Measurements of the Viscosity of Glasses. J. Am. Ceram. Soc. 1925, 8, 339. (39) Plazek, D. J.; Ngai, K. L. Correlation of Polymer Segmental Chain Dynamics with Temperature-Dependent Time-Scale Shifts. Macromolecules 1991, 24, 1222−1224. (40) Ngai, K. L.; Roland, C. M. Chemical Structure and Intermolecular Cooperativity: Dielectric Relaxation Results. Macromolecules 1993, 26, 6824−6830. (41) Schmidt-Rohr, K.; Kulik, A. S.; Beckham, H. W.; Ohlemacher, A.; Pawelzik, U.; Boeffel, C.; Spiess, H. W. Molecular Nature of the beta Relaxation in Poly(methyl methacrylate) Investigated by Multidimensional NMR. Macromolecules 1994, 27, 4733−4745. (42) Takiguchi, O.; Inoue, T.; Osaki, K. Studies on Sub-Relaxation of a Series of Methacrylate Polymers by Dynamic Birefringence Measurements. Nihon Reoroji Gakkaishi 1995, 23, 13−19. (43) Kaminski, K.; Adrjanowicz, K.; Kaminska, E.; Grzybowska, K.; Hawelek, L.; Paluch, M.; Tarnacka, M.; Gruszka, I.; Kasprzycka, A. Impact of Water on Molecular Dynamics of Amorphous α-, β-, and γCyclodextrins Studied by Dielectric Spectroscopy. Phys. Rev. E: Stat. Nonlin. Soft Matter Phys. 2012, 86, 031506.

1869

DOI: 10.1021/acs.jpcc.6b11362 J. Phys. Chem. C 2017, 121, 1861−1869