Conformational Relaxation of Poly(styrene-co-butadiene) Chains at

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Conformational Relaxation of Poly(styrene-co-butadiene) Chains at Substrate Interface in Spin-Coated and Solvent-Cast Films Biao Zuo,† Manabu Inutsuka,‡ Daisuke Kawaguchi,§ Xinping Wang,*,† and Keiji Tanaka*,‡,∥ †

Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China ‡ Department of Applied Chemistry, §Education Center for Global Leaders in Molecular Systems for Devices, and ∥International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: The local conformation of poly(styrene-co-butadiene) rubber (SBR) chains in direct contact with a quartz substrate was examined by interface-sensitive sum-frequency generation (SFG) spectroscopy. SFG signals, which could be obtained from functional groups only oriented at the interface, were clearly observed for SBR in a film at room temperature which was much higher than the bulk glass transition temperature (Tg). When the film was thermally annealed, SBR chains at the quartz interface changed their conformation to one with a lower energy state, accompanied by the randomization of both the main and side chain parts. The characteristic temperature, at which interfacial chains started to lose their orientations, was much higher than the bulk Tg. Also, the extent found to be more remarkable for the spin-coated film than for the solvent-cast one. This implies that the stress accumulated at the interface, which resulted from the centrifugal force during the spin-coating process, accelerates the mobility of chains there. Finally, the kinetics experiment well supports the slower orientation relaxation at the interface.

1. INTRODUCTION When rubber is applied to composite materials for engineering applications, reinforcement with an inorganic filler is indispensable.1−4 Although extensive efforts have been hitherto devoted to studying rubbers containing fillers, a complete understanding of the reinforcement mechanism at a molecular level has not yet been achieved.5,6 Of the hypotheses proposed thus far, the one on the basis of adsorbed chains onto the filler surface, so-called bound rubber, is generally accepted as one of the most reasonable arguments.7−10 The bound rubber could not be generally swollen and/or dissolved in good solvents. The segmental dynamics in the bound rubber were extremely suppressed in comparison with those in the corresponding bulk region.11−14 The bound rubber on the filler surface induces an internal stress in the composite and thus is believed to play a crucial role in the reinforcing effect.15−24 However, the physical properties and the formation mechanism for the bound rubber at the interface with the filler have yet to be elucidated. Meanwhile, the structure and dynamics of typical glassy polymer chains near solid interfaces have been studied recently. A possible aggregation state for chains adsorbed on the interface is a “train-loop” conformation.25,26 This structure, which is driven by an enthalpic gain to cover the filler surface, could be realized, leading to the formation of a high-density layer at the solid interface.27−31 In this layer, the segmental motion is restricted, and the glass transition temperature (Tg) becomes strikingly higher than the corresponding bulk.32−35 © XXXX American Chemical Society

Conversely, however, the local segmental dynamics, and thereby Tg, of chains at an interface, were also found to be unchanged.36,37 Furthermore, even an enhanced molecular mobility38,39 has been reported. These apparent discrepancies imply that the relaxation process and dynamics of chains in the interfacial layer are much more difficult to comprehend than previously thought. To gain a better understanding of polymer chains at solid interfaces, emphasis should be placed on the following three issues. First, the aggregation states and segmental mobility vary on a gradient with increasing distance from the solid surface.33,35 This means that different techniques with various depth resolutions would give rise to different results. Second, the interfacial adsorbed chains have been usually obtained and studied after removing unadsorbed bulk chains using a solvent leaching treatment. This leads to the fact that the interfacial layer so obtained also has a free surface at which the segmental mobility is enhanced.40−45 Third, the extent of the relaxation for interfacial chains is subject to variation depending on the preparation history because polymer chains, especially under a nanoscopic confinement condition, are usually trapped in a state of nonequilibrium.46−50 In addition, of course, it is also strongly dependent on which kind of solid surface is adopted. Received: January 3, 2018 Revised: February 25, 2018

A

DOI: 10.1021/acs.macromol.7b02756 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Taking these issues into account, it is necessary to carry out in situ measurements for chains at the buried interface with the substrate in the film. The objective of this study is to gain a better understanding of typical rubbery chains, poly(styrene-co-butadiene) rubber (SBR), at a solid interface using in situ sum-frequency generational (SFG) spectroscopy which allows direct probing of the buried interface,51−55 and thus the aforementioned difficulties of the first and second issues can be effectively overcome. Therefore, the effects of sample preparation history on the interfacial relaxation are herein examined as the focus of our interest.

2. EXPERIMENTAL SECTION 2.1. Materials and Film Preparation. As a sample, SBR with a number-average molecular weight of 177K, a polydispersity index of 2.4, and a (polystyrene (PS):1,4-polybutadiene (1,4-PB):1,2-polybutadiene (1,2-PB)) weight ratio of 20:65:15 was supplied by Asahi Kasei Corporation. Figure 1 and Figure S1 show the chemical

Figure 2. SFG spectra collected from (a) spin-coated and (b) solventcast SBR films sandwiched between a quartz prism and substrate. The inset of panel b shows the film geometry for detecting SFG signals from the SBR/quartz interface. where Ivis and IIR are intensities of the input visible and infrared laser beams, respectively, and χ(2) eff is effective second nonlinear susceptibility. Since χ(2) eff can be generated only from interfaces in which the macroscopic centrosymmetry is broken, SFG is an interface-specific technique with submonolayer interfacial sensitivity. The χ(2) eff can be expressed in the following form:

Figure 1. Chemical structure of the poly(styrene-co-butadiene) rubber (SBR).

(2) (2) χeff = χNR +

∑ q

Aq ωIR − ωq + i Γq

(2)

χ(2) NR

is the nonresonant background contribution, and the where second term on the right-hand side represents molecular resonance contribution; ωIR is the frequency of the IR beam. Also, ωq, Aq, and Γq are resonant frequency, strength, and line width of the qth resonant vibrational mode, respectively. Aq and Γq were obtained by performing the Lorentzian fit to an SFG peak using eqs 1 and 2.

structure of SBR and temperature dependence of loss modulus for SBR by dynamic mechanical analysis. SBR exhibited a single αrelaxation peak located around 213 K, and the Tg value obtained by differential scanning calorimetry was 204 K. This number was higher and lower than the Tg values of 1,4-PB (174 K56) and PS (373 K) homopolymers, respectively. This means that the PS and PB segments in this sample were in a miscible state. Films of SBR were prepared from toluene solutions of 8.0 and 1.4 wt % onto quartz substrates by spin-coating at 3000 rpm and solvent-casting, respectively, resulting in films with a thickness range of 800−900 nm. Two films prepared on a quartz prism and a quartz window were then attached together in a face-to-face geometry and annealed at 303 K under vacuum for 24 h so that an SBR film sandwiched between quartz substrates was obtained, as shown in the inset of Figure S2.34,55 To evaluate the conformational relaxation, the films were annealed at elevated temperatures under vacuum for 1 h and quickly cooled, and then the SFG measurements were conducted at room temperature. 2.2. Sum-Frequency Generation Spectroscopy. A customdesigned SFG spectrometer (EKSPLA, Lithuania) was used to detect the local conformation of SBR at the quartz interface. In the measurement, 532 nm visible and tunable infrared (IR) laser beams passed through a prism and overlapped at the polymer/quartz interface to generate sum frequency signals, as shown in the inset of Figure 2b. The incident angles of visible and IR beams were 70° and 50° with respect to the surface normal, respectively. The visible beam was generated by frequency-doubling fundamental output pulses from a picosecond Nd:YAG laser (PL2143, EKSPLA). The IR beam, tunable between 1000 and 4300 cm−1, was obtained from an optical parametric generation/amplification and difference frequency generation (OPG/OPA/DFG) system based on LBO and AgGaS2 crystals. The SF signals were collected by a monochromatic spectrograph with the ssp (SF/s, visible/s, and IR/p) and ppp polarization combinations. The SFG signal intensity (ISFG) was expressed by the following equation:34,55 (2) 2 ISFG ∝ |χeff | I visIIR

3. RESULTS AND DISCUSSION 3.1. Local Conformation at Interface. Panels a and b of Figure 2 show the SFG spectra from the SBR/quartz interfaces of the spin-coated and solvent-cast films, respectively. An intense peak appearing at 2840 cm−1 was attributed to the symmetric C−H stretching vibration of methylene groups (CH2s), which were included both in PS and PB units. A peak at 2900 cm−1 could be originated from the antisymmetric C−H stretching vibration of methylene groups (CH2as) in PB units57 and/or the C−H stretching vibration of methyne groups (CH) in PS units.34,58 A broad peak observed in the wavenumber range from 2950 to 3015 cm−1 might be composed of two contributions from the C−H stretching of methyne groups in CHCH (2979 cm−1) from 1,4-PB units and symmetric C−H stretching of vinyl methylene groups (CH2s) (2999 cm−1) in 1,2-PB units.59 In addition, a peak observed at 3059 cm−1 was assignable to v2 vibration mode of phenyl groups.60−63 The appearance of these clear peaks in the SFG spectra indicates that the local conformation of SBR chains was ordered at the quartz interfaces both for spin-coated and solvent-cast films. To discuss the local conformation of polymer chains at the interface, the orientation of side chain groups (i.e., phenyl groups in PS units and vinyl methylene groups in 1,2-PB units (=CH2)) were focused on herein. In the case of the spin-coated film, only symmetric A1 vibrational mode61 such as ν2 (3059 cm−1) was observed. That is, no B1 symmetric peaks were discerned. Such a spectral feature indicates that phenyl rings were oriented at the interface.61,63 On the other hand, no

(1) B

DOI: 10.1021/acs.macromol.7b02756 Macromolecules XXXX, XXX, XXX−XXX

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S4. The intensity of all SFG peaks gradually decreased with increasing temperature, indicating the loss of the conformational ordering for SBR chains at the quartz interface. A similar trend was also found for the solvent-cast film, as shown in panel b of Figure 3. These results imply that SBR chains at the quartz interface were thermally relaxed accompanied by the loss of the ordering. That is the relaxation behavior likely corresponds to the stabilizing process of chains at the interface. Taking into account that chains at the interface significantly lost their conformational entropy owing to the ordering structure, it seems reasonable that they changed the local conformation to be more random once the temperature went beyond the critical temperature (Tc), as shown in Scheme 1. Such can be realized by the arching up of some segments, which were weakly attached to the substrate surface.28,30,64 In addition, others might get in closer touch with the substrate surface, resulting in an enthalpic gain.28,30,64 These factors make it possible that short loops with high-density anchoring points (i.e., a matured flattened layer27,28,30,64,65) would be formed to minimize the total free energy. That is these adsorbed chains at the interface should be extremely stable in terms of thermodynamics. A more conclusive study coupled with molecular dynamics simulation is underway to verify the proposed mechanism for the adsorption of rubber chains onto the substrate surface. The above-mentioned interfacial relaxation induced by the thermal annealing has been previously observed for polyisoprene (PI) with a quartz substrate65 but not for PS34 and poly(methyl methacrylate) (PMMA).66 This difference can be explained in terms of Tg. Even if the interfacial Tg is much higher than the corresponding bulk value, the relaxation for a polymer having a lower Tg such as PI and SBR is detectable within the measurable temperature range and the experimentally accessible time scale. In contrast, the interfacial relaxation for a glassy polymer such as PS and PMMA cannot be observed due to the experimental temperature limit. The peak intensity was detected as a function of temperature upon the continuous heating/cooling process. Figure 4a shows the temperature dependence of the intensity for the peak at 2900 cm−1, which is composed of CH2as in PB units and/or CH in PS units, for the spin-coated film. The intensity decreased upon the heating, and the slope of the plot was changed at 340 ± 2 K, which was herein defined as Tc,onset. It

signals were detected in the wavenumber region of 3020−3100 cm−1 for the solvent-cast SBR film, meaning that phenyl rings in the solvent-cast film were randomized at the quartz interface. The quantitative analysis of Figure 2 revealed that the tilt angles of CH2= at the interface were estimated to be about 24° and 51° with respect to the surface normal for the spin-coated and solvent-cast films, respectively, as shown in Figure S3. In short, the local conformation of SBR chains at the quartz interface is schematically summarized in Scheme 1. Overall, Scheme 1. Plausible Local Conformations of SBR Chains at the Quartz Interface for (a) Spin-Coated and (b) SolventCast Films, Respectively, and (c) the Corresponding Conformational Relaxation Induced by Thermal Annealing

SBR chains are aligned on the quartz surface for both spincoated and solvent-cast films. However, the extent of such an interfacial ordering was dependent on how the film was prepared. The side-chain groups of SBR (i.e., phenyl groups and vinyl groups for 1,2-PB) were more likely to perpendicularly orient at the substrate interface for the spin-coated film, yet were randomly or less oriented at the interface for the solvent-cast one. The striking interfacial orientation for side chain groups of SBR observed for the spin-coated film can be interpreted as the preferential orientation of the main chain part along the direction parallel to the interface due to the centrifugal force applied to it during the spin-coating process, as evidenced by our previous works.30,34,55 3.2. Conformational Relaxation at Interface. Panel a of Figure 3 shows the SFG spectra with the ssp polarization combination for the spin-coated SBR film annealed at various temperatures. It was preconfirmed that no oxidation of SBR occurred upon the annealing up to 393 K, as shown in Figure

Figure 4. Temperature dependence of SFG intensity at 2900 cm−1 (with ssp) for (a) spin-coated and (b) solvent-cast SBR films upon heating and cooling processes. The heating and cooling rate was 2 K min−1.

Figure 3. SFG ssp spectra for (a) spin-coated and (b) solvent-cast SBR films after the annealing for 1 h at various temperatures under vacuum. C

DOI: 10.1021/acs.macromol.7b02756 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules remained unchanged once the temperature went beyond approximately 374 ± 3 K, Tc,offset, implying that chains at the interface almost lost the ordering. Actually, in this temperature region, the SFG peak was almost indiscernible. Conversely, the peak intensity was not changed upon the cooling process, as shown by blue symbols in Figure 4a. This means that the local conformation at the interface was irreversibly changed to an energetically favorable one by the thermal annealing. Tc should be also determined on the basis of other SFG signals such as the one at 3059 cm−1, which was originated from phenyl groups in PS units. The Tc,onset and Tc,offset so obtained for the spincoated film were very close to those at 2900 cm−1, as shown in Figure 5. The consistency in the Tc,onset and Tc,offset values

Figure 6. (a) Time dependence of the SFG intensity at 2900 cm−1 for the spin-coated SBR film. Solid curves were the best-fit by the KWW equation. (b) Temperature dependence of relaxation time (τ) extracted from panel a. Solid curves were the best fit by the VFT equation.

1 − INorm = A exp( −t /τ )1 − n

where τ, n, and A are characteristic relaxation time, exponent related to the intermolecular cooperativity (1 > n > 0), and a pre-exponential factor, respectively. The data shown could be well reproduced by the KWW equation, as shown in Figure 6a. The n values here estimated were 0.25−0.35 and were definitely lower than the reported bulk values for PB (0.55)68 and PS (0.64).69 According to Ngai’s coupling model,70,71 the intermolecular coupling is believed to be the origin for the slowing down of the local segmental motion, and the n value, or coupling parameter, is a measure of the extent of such a coupling. The reduced n value observed here means that the intermolecular cooperativity, possibly for the segmental level, at the substrate interface is less than that in the bulk. Using broadband dielectric relaxation spectroscopy in conjunction with differential scanning calorimetry, Klonos et al.72 reported similarly reduced cooperativity and slowing down of dynamics for polydimethylsiloxane adsorbed onto a silica surface. A possible explanation for the reduced cooperativity is that the presence of the substrate becomes dominant over the intermolecular interaction because of an interaction between segments and substrate surface. As a result, segments may not easily cooperate, or couple, with each other in the interfacial layer. Figure 6b shows τ, which was extracted on the basis of KWW fitting, for both spin-coated and solvent-cast films as a function of temperature. The τ value was found to be larger for the solvent-cast film than that for the spin-coated one, indicating slower interfacial dynamics for the solvent-cast film. Applying the Vogel−Fulcher−Tammann equation to the data in Figure 6b, the temperatures at τ = 100 s was estimated to be 372 and 389 K for the spin-coated and solvent-cast films, respectively. Interestingly, these temperatures are in good accordance with Tc,offset, rather than Tc,onset, obtained from Figure 4. It has been accepted that the sample-preparation history is related to the residual stress within the film due to the nonequilibrium initial conformation of chains.34,65,73−76 Such an applied internal stress reduces the Tg of the polymer and accelerates the segmental mobility.77−80 In the case of the solvent-cast process, it takes time for chains to be solidified because of the slow evaporation of solvent molecules. Thus, chains are less oriented in the solvent-cast film than in the spincoated one and thereby possess a lesser nonequilibrity. This is more striking at the interface, as shown in the above and our

Figure 5. (a) Enlarged SFG spectra of Figure 3a. (b) Temperature dependence of SFG ν2 peak intensity at 3059 cm−1 (with ssp) for the spin-coated SBR film upon heating with a rate of 2 K min−1. Filled black circles denote the data in Figure 3.

determined by the SFG signals at two different wavenumbers of 2900 and 3059 cm−1 makes it clear that the conformational relaxation at the interface occurred both with PB and PS units. Thus, it seems reasonable to claim that the interfacial relaxation here observed possesses a relatively large length scale such as the segmental motion. A similar temperature dependence of SFG signals was observed for the solvent-cast film, as shown in Figure 4b. However, the Tc,onset value determined on the basis of the peak at 2900 cm−1 was shifted to 353 ± 2 K for the solvent-cast film, being higher than that of the spin-coated film by 13 K. This indicates that the chain relaxation at the interface is dependent on the film-preparation history via local conformation. 3.3. Relaxation Dynamics at Interface. To discuss the interfacial relaxation dynamics, the time evolution of the SFG peak intensity at 2900 cm−1 at a given temperature was examined. Figure 6a provides the time dependence of normalized intensity (INorm) for the spin-coated film. INorm was simply obtained by the following eq 3: INorm = (I0 − It )/(I0 − I∞)

(4)

(3)

where I0, I∞, and It denote the SFG intensities before and after the annealing for an infinite time and at a given time t, respectively. The INorm decreased with increasing time, corresponding to the relaxation dynamics at the interface. The Kohlrausch−Williams−Watts (KWW) equation,67 which has been often used to describe the segmental relaxation for bulk polymers, was adopted to express the time vs INorm plots: D

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previous publications.30,34,65 Therefore, the spin-coated SBR film with a larger internal stress could make Tc, or Tginter, lower. A more conclusive study taking into account the effect of polymer−substrate interaction will be reported in the near future.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02756. Loss modulus (E″) versus temperature curves of SBR; cartoon for sample preparation; calculation of the orientation of vinyl methylene groups; SFG ssp spectrum in CO region for the SBR films after annealing at 393 K under vacuum (PDF)

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REFERENCES

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4. CONCLUSIONS Using SFG spectroscopy, we studied the conformation relaxation process for SBR chains in spin-coated and solventcast films supported on a quartz surface. SFG signals were clearly detected from the as-prepared films, indicating that chains were aligned at the quartz interface adopting an ordered conformation. Upon the annealing process, interfacial SBR chains relaxed to attain the quasi-equilibrium state, in association with the randomization of their local conformation. The Tc, at which the interfacial relaxation started to accelerate, determined with CH2 groups in PB units was in good accordance with the one with phenyl groups in PS units. This means that the length scale of the interfacial relaxation is perhaps similar to that of the segmental motion. The Tc values were much higher than the bulk Tg of SBR for the spin-coated and solvent-cast films. Also, it can be claimed that the relaxation dynamics at the interface depends on the film preparation process; the interfacial relaxation is faster for the spin-coated film than for the solvent-cast one. This is probably because a larger stress induced by the centrifugal force upon the spincoating process activates the chain mobility at the interface.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.T.). *E-mail: [email protected] (X.W.). ORCID

Biao Zuo: 0000-0002-4921-8823 Daisuke Kawaguchi: 0000-0001-8930-039X Xinping Wang: 0000-0002-9269-3275 Keiji Tanaka: 0000-0003-0314-3843 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partially supported by the JSPS KAKENHI Grant-in-Aid for Scientific Research (A) (No. JP15H02183) and the Natural Science Foundation of China (Grants 21504081 and 21374104). We also appreciate the support from the ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan) and Natural Science Foundation of Zhejiang Province (No. LQ16B040001). E

DOI: 10.1021/acs.macromol.7b02756 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b02756 Macromolecules XXXX, XXX, XXX−XXX