Direct Observation of Conformational Relaxation of Polymer Chains at

17 Sep 2018 - While the relaxation of surface nonequilibrium chains was induced by the enhanced surface mobility, the whole chain motion such as repta...
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Letter Cite This: ACS Macro Lett. 2018, 7, 1198−1202

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Direct Observation of Conformational Relaxation of Polymer Chains at Surfaces Hung K. Nguyen,† Manabu Inutsuka,† Daisuke Kawaguchi,‡ and Keiji Tanaka*,†,§ †

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, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan

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S Supporting Information *

ABSTRACT: Sum-frequency generation spectroscopy was employed to follow the conformation evolution of polystyrene chains at the surface of a spin-coated film in a temperature-ramping mode as well as under isothermal annealing. The conformation of surface chains in an as-cast film was observed to be in a nonequilibrium state, in accordance with reported results for polymer chains in thin spin-coated films. While the relaxation of surface nonequilibrium chains was induced by the enhanced surface mobility, the whole chain motion such as reptation might be a key factor in determining the time scale for equilibrating the surface chain conformation. hin and ultrathin polymer films have become of great importance in various modern nanoscale technologies ranging from organic thin-film transistors to functional biomedical devices.1−3 The chain conformation at the film surface is a key factor in determining their various physical properties, such as adhesiveness, friction, charge transport, and biocompatibility, and thus plays a vital role in controlling the performance of devices.1,2,4−11 These films are generally fabricated by a spin-coating method, resulting in a stretched nonequilibrium conformation of polymer chains in the as-cast films.1,12−15 Thus, as stated above, the physical and chemical properties as well as the device performance of a spin-coated film, in which chains take a nonrelaxed conformation, are supposed to depend on the preparation conditions in addition to the intrinsic dynamics of polymer chains.13,14,16−22 Usually, it takes quite a long time to fully relax chains with nonequilibrium conformations in an as-cast film. For example, the time scale to fully relax the nonequilibrium chain conformation in ∼100 nm thick polystyrene (PS) films has been reported to be several orders of magnitude longer than the reptation time (τ rep ) and is much longer than technologically practical times.15−17,23−25 Because of a relatively large free volume near the surface region with respect to the bulk one,1,8−11 it can be expected that the chain conformation at the free surface of spin-coated films is more easily equilibrated under the thermal treatment.26−29 Unfortunately, so far there still lacks a systematic characterization of the mechanism and time scale, over which the surface nonequilibrium chain conformation can be relaxed. Consequently, although considerable efforts have been made to characterize the chain conformation at the surface of spincoated polymer films in the past, it remains unclear whether the observed conformation represents the one in a nonequilibrium or quasi-equilibrium state.7,26−30

In this letter, we present the first observation of the conformational transition of chains existing at the free surface of spin-coated PS films, from a non- to a quasi-equilibrium, with increasing temperature from below to above the bulk glass transition temperature (Tg), using sum-frequency generation (SFG) spectroscopy. Our SFG results clearly showed that the chain conformation at the surface in an as-cast PS film was in a nonequilibrium state induced by centrifugal force during the spin-coating process. The onset of the relaxation for such a nonequilibrium conformation was related to the segmental motion of the surface chains. However, the characteristic relaxation time for fully relaxing nonequilibrium chains was found to be in the order of the τrep for the corresponding bulk chains. Such an unexpectedly long equilibrating time for surface chains implies a hierarchical polymer dynamics near the surface,23,31 at which chains should be partially entangled with ones existing in the subsurface and/or bulk regions. Monodisperse PS with a number-average molecular weight (Mn) of 183k and deuterated PS (dPS) with an Mn of 178k, purchased from Polymer Source, Inc. (Montreal, QC, Canada), were used. Their characteristics including polydispersity index, bulk Tg, and calculated τrep are shown in Table S1, and their chemical structures are presented in Figure S1 (Supporting Information, SI). Thin films of PS and dPS were prepared by a spin-coating method from solutions in deuterated toluene onto quartz prisms for SFG measurements. Similar films supported on Si/SiOx substrates were used for measuring film thickness with an atomic force microscopy in tapping mode. Films with a thickness of ∼120 nm were used to

T

© XXXX American Chemical Society

Received: May 27, 2018 Accepted: September 13, 2018

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DOI: 10.1021/acsmacrolett.8b00411 ACS Macro Lett. 2018, 7, 1198−1202

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ACS Macro Letters avoid a complex effect from the substrate interface.1,16 The prepared films were dried in vacuum at room temperature for 24 h, which are hereafter referred to as as-cast films. They were then subjected to SFG measurements in temperature-ramping mode from 298 to 400 K with a heating rate of 1 K·min−1. Some of them were annealed in vacuum at 393 K for different times to obtain the characteristic relaxation time. For SFG measurements, a visible beam with a wavelength of 532 nm and a tunable infrared (IR) beam with a wavenumber ranging from 2800 to 3120 cm−1 were introduced from the quartz prism side with incident angles of 70° and 50° relative to the substrate normal, respectively (Figure S2 in the SI).32 These angles provide the total internal reflection for both incident beams at the PS/air surface and thereby maximize the SFG signal from the PS/air surface with respect to that from the quartz/PS interface.26 The experimental and theoretically calculated SFG intensities at the PS/air surface were much larger than those at the quartz/PS interface by a factor of ∼50 and ∼70, respectively (Table S2 and Figure S3 in the SI). Details regarding the sample geometry and experimental apparatus are provided in the SI. The measurements were carried out using ssp (SFG/s; visible/s; and IR/p) polarization combination. Figure 1 shows the SFG spectra with the ssp polarization combination for an as-cast PS film at several temperatures from

For PS chains, both theoretical and experimental studies revealed that the preferred orientation of the C2v symmetric axis of a phenyl ring is along the direction normal to the plane defined by the adjoining skeletal C−C bonds.34,35 The orientation of phenyl rings along the direction normal to the surface therefore indicates a preferential alignment of polymer chains in the plane of the as-cast film, in accordance with previous conclusions from different methods.12,33,36 In fact, a similar orientation for PS phenyl rings in contact with a quartz surface was only observed in spin-coated, but not in solventcast, films.33 It is therefore reasonable to claim that the SFG spectra for the PS surface in the as-cast film, as observed here and generally reported in the literature,26−29 represent the chain conformation in a nonequilibrium state. With increasing temperature, the intensity of the peaks from phenyl rings gradually decreased together with an increase in the intensity of the peaks from aliphatic groups (Figure 1). The onset temperature for such a change was observed at ∼340 K for the ν2 mode of phenyl rings and ∼355 K for the CH2as mode as shown in Figure 2. As discussed below, the latter one

Figure 2. Temperature dependence of SFG intensity at 2936 cm−1 for the CH2as mode and at 3070 cm−1 for the ν2 mode. The heating rate was 1 K·min−1.

is related to the segregation of chain ends to the PS surface, which probably occurs more slowly than the rotation process of phenyl rings. Nevertheless, these temperatures are well below the bulk Tg, but they are comparable to the Tg values reported for the PS surface region.1,8−11 This indicates that the segmental motion of PS chains at the surface might stimulate the relaxation of the nonequilibrium chain conformation in the spin-coated film. This scenario is indeed supported by our recent results on different polymers in contact with a substrate interface.37,38 In contrast with the enhanced segmental motion of the polymer surface, the segmental motion of interfacial polymers at the substrate interface was observed to occur at temperatures more than 100 K above the bulk Tg.37,38 Although the peak intensity of the ν2 mode gradually decreased with increasing temperature, it was still dominant at 400 K (Figure 1), 22 K above the bulk Tg. This means that the conformation exhibiting the intense ν2 and weak ν20b SFG peaks is in an equilibrium state as generally accepted in the literature26−29 or that the conformation is still moving toward an equilibrium state and thus is altered with a further annealing. To examine the possibility of relaxing the surface chain conformation, the as-cast PS film was annealed in vacuum for different times at 393 K. Representative examples of SFG spectra measured at room temperature for this film after 12 and 32 h of annealing are shown in Figure 3a. Interestingly, once the annealing treatment for 12 and 32 h was applied, SFG peaks arisen from aliphatic C−H groups became more intense than those from phenyl rings. That is, after the

Figure 1. SFG spectra with the ssp polarization combination for an ascast PS film measured at different temperatures.

298 to 400 K. Multiple peaks were observed in each spectrum. The assignment of the discernible peaks is also presented in Figure 1.33 A schematic illustration of these vibration modes is provided in Figure S4 (SI). For an as-cast film at 298 K, the ν2 and ν20b mode peaks originating from phenyl rings in the wavenumber region from 3000 to 3100 cm−1 were dominant. The former was much stronger than the latter, qualitatively indicating that phenyl rings were mainly oriented along the direction normal to the surface.28 In fact, the tilt angle ϕ between the C2v symmetric axis of a phenyl ring and the surface normal in this case has been previously quantified to be 15° ± 15°.26,28,29,33 In contrast, the peaks representing aliphatic groups, i.e., C−H symmetric and antisymmetric stretching vibrations of methylene (CH2s and CH2as), methyl (CH3s and CH3as), and methyne (CH), on the backbone and/ or at the chain end portions appeared relatively weak. 1199

DOI: 10.1021/acsmacrolett.8b00411 ACS Macro Lett. 2018, 7, 1198−1202

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ACS Macro Letters

along the direction parallel to the surface.28 The tilt angle ϕ of the phenyl rings in this case was calculated to be 75° ± 15° (see the detailed discussion in the SI). Therefore, a quantitative analysis of the relaxation behavior for the nonequilibrium chain conformation can be performed relying on the change in the orientation of phenyl rings induced by annealing. This change is reflected in a decrease of the amplitude ratio, Aν2/Aν20b, of the peaks corresponding to the ν2 and ν20b modes (see the detailed explanation in the SI). Figure 3c shows the Aν2/Aν20b ratio for the PS film as a function of annealing time at 393 K. This ratio decays markedly after a few hours of annealing and reaches an equilibrium value of 0.35 ± 0.07 after 32 h. By further annealing at 423 K for 24 h, this value was almost unchanged, 0.31 ± 0.06 (Figure S7 in the SI). Thus, it can be claimed that the Aν2/Aν20b value of ∼0.3 shows the quasi-equilibrium conformation of PS at the surface. The characteristic relaxation time for the nonequilibrium conformation can be obtained by fitting Aν2/Aν20b data with an exponential function, (A0−Aoff)·exp(−t/τ) + Aoff, where A0 and Aoff are the initial and offset values of the Aν2/Aν20b ratio, corresponding to the surface non- and quasi-equilibrium conformations for the as-cast and well-annealed films, respectively. The fitting curve is also presented in Figure 3c, providing the relaxation time τ = 7.5 ± 1.2 h. This time scale is in the order of the τrep of ∼3.3 h (Table S1 in the SI), suggesting that the quasi-equilibrium conformation of surface chains could only be attained when they were wholly relaxed. Although this time scale is much shorter than reported ones for relaxing nonequilibrium chain conformations within a spin-cast film or near a substrate interface,15−18,24,25,33 it is longer than the characteristic times observed for most surface phenomena within a depth region of 2−3 nm.1,8−11 A strong connection between surface chains and ones from subsurface and/or bulk regions through, for example, a partial entanglement might be an important factor in determining the relaxation process of nonequilibrium chains at the free surface. Actually, the τ value was strongly dependent on film thickness as well as molecular weight of PS, strongly supporting the hypothesis for the connectivity of surface chains to the subsurface region. A conclusive study on this issue will be reported in the near future. On the basis of our SFG observations and published knowledge of chain conformation, a reasonable mechanism describing the evolution of the surface PS chain conformation can be illustrated in Figure 4. Chains in an as-cast film are elongated along the in-plane direction.37,40 However, in a quasi-equilibrium state after a thermal annealing, the in-plane

Figure 3. (a) SFG spectra for a PS film annealed in vacuum at 393 K for 12 and 32 h. (b) SFG spectra for a 120 nm thick dPS film before and after the annealing for 36 h at 393 K. (c) The amplitude ratio for the ν2 and ν20b mode peaks, Aν2/Aν20b, as a function of annealing time at 393 K. The solid line is the fitting by an exponential function, and the dashed line represents the quasi-equilibrium ratio for a wellannealed film.

longer annealing, aliphatic C−H groups on the polymer backbone and/or at chain end portions became more oriented at the free surface. Methylene groups on the PS backbone are adjacent to phenyl rings through methyne groups. Thus, it could be hardly imagined that methylene and methyne groups on the backbone would have a higher degree of orientation than phenyl rings on the side chain portion.34,35 In other words, the chain end portions are probably segregated and oriented at the film surface with annealing. To address this issue, SFG measurement was applied to a corresponding dPS film, which possessed C−H bonds only in the sec-butyl group, a fragment of the initiator at one end (Figure S1b in the SI). Thus, the contribution from the chain end portion to the SFG signal in the C−H region can be selectively extracted. SFG spectra for a dPS film in as-cast and well-annealed, at 393 K for 36 h, conditions are shown in Figure 3b. The intensity of the peaks corresponding to the C−H stretching vibrations of chain ends became substantially stronger with annealing. The analytical depth of SFG, at a subnanometer scale,39 is much smaller than the chain dimension. In this spatial scale, the number density of phenyl rings should be constant independent of chain conformation, while that of chain ends depends on the chain conformation. This result makes it clear that the number density of chain ends at the surface increases with the thermal treatment. The intensity of the ν2 mode peak gradually decreased and eventually became much weaker than that of the ν20b mode peak (Figure 3a), indicating that phenyl rings became oriented

Figure 4. Schematic describing the effect of the thermal treatment on the surface chain conformation of the as-cast (a) and well-annealed (b) PS films. 1200

DOI: 10.1021/acsmacrolett.8b00411 ACS Macro Lett. 2018, 7, 1198−1202

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ACS Macro Letters chain dimension becomes comparable to that in the bulk.41 Taking these into account, it seems reasonable to surmise that chains shrink along the in-plane direction but stretch along the depth direction with the annealing process. This means that the population of repeating units oriented along the in-plane direction decreased with annealing time. If that is the case, the orientation of phenyl rings, which directly attached to the main chain part of PS, should be randomized with annealing. That is, as-cast polymer chains are preferentially aligned in plane with the film surface, in which the phenyl rings are almost oriented in the direction perpendicular to the surface. Well-annealed polymer chains are relaxed with a random alignment, in which the number of chain ends markedly increases at the free surface.42 In summary, we here comprehensively provided the first evidence of a mechanism for the conformational relaxation of nonequilibrium chains at the spin-coated PS surface with temperature. Our result clearly demonstrated the relaxation process of the surface nonequilibrium conformation over a time scale much shorter than those reported for equilibrating the chains in bulk and interface regions. This finding can, therefore, provide important insights into understanding the mechanism behind the relaxation process of the highly stretched chain conformation in spin-coated films. In addition, a relationship between the surface chain conformation and the annealing condition observed here is critical for designing functional polymer surfaces with different chain ends and/or side chains.

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Nanomembranes for Directing Cellular Organization toward Flexible Biodevices. Nano Lett. 2013, 13, 3185−3192. (4) Shundo, A.; Hori, K.; Ikeda, T.; Kimizuka, N.; Tanaka, K. Design of a Dynamic Polymer Interface for Chiral Discrimination. J. Am. Chem. Soc. 2013, 135, 10282−10285. (5) Schuettfort, T.; Thomsen, L.; McNeill, C. R. Observation of a Distinct Surface Molecular Orientation in Films of a High Mobility Conjugated Polymer. J. Am. Chem. Soc. 2013, 135, 1092−1101. (6) Hirata, T.; Matsuno, H.; Kawaguchi, D.; Hirai, T.; Yamada, N. L.; Tanaka, M.; Tanaka, K. Effect of Local Chain Dynamics on a Bioinert Interface. Langmuir 2015, 31, 3661−3667. (7) Wang, Y.; Acton, O.; Ting, G.; Weidner, T.; Shamberge, P. J.; Ma, H.; Ohuchi, F. S.; Castner, D. G.; Jen, A. K.-Y. Effect of the Phenyl Ring Orientation in the Polystyrene Buffer Layer on the Performance of Pentacene Thin-Film Transistors. Org. Electron. 2010, 11, 1066−1073. (8) Tanaka, K.; Takahara, A.; Kajiyama, T. Rheological Analysis of Surface Relaxation Process of Monodisperse Polystyrene Films. Macromolecules 2000, 33, 7588−7593. (9) Fakhraai, Z.; Forrest, J. A. Measuring the Surface Dynamics of Glassy Polymers. Science 2008, 319, 600−604. (10) Paeng, K.; Swallen, S. F.; Ediger, M. D. Direct Measurement of Molecular Motion in Freestanding Polystyrene Thin Films. J. Am. Chem. Soc. 2011, 133, 8444−8447. (11) Ediger, M. D.; Forrest, J. A. Dynamics near Free Surfaces and the Glass Transition in Thin Polymer Films: A View to the Future. Macromolecules 2014, 47, 471−478. (12) Frank, C. W.; Rao, V.; Despotopoulou, M. M.; Pease, R. F. W.; Hinsberg, W. D.; Miller, R. D.; Rabolt, J. F. Structure in Thin and Ultrathin Spin-Cast Polymer Films. Science 1996, 273, 912−915. (13) Reiter, G.; de Gennes, P. G. Spin-Cast, Thin, Glassy Polymer Films: Highly Metastable Forms of Matter. Eur. Phys. J. E: Soft Matter Biol. Phys. 2001, 6, 25−28. (14) Reiter, G.; Napolitano, S. Possible Origin of ThicknessDependent Deviations from Bulk Properties of Thin Polymer Films. J. Polym. Sci., Part B: Polym. Phys. 2010, 48, 2544−2547. (15) Chung, J. Y.; Chastek, T. Q.; Fasolka, M. J.; Ro, H. W.; Stafford, C. M. Quantifying Residual Stress in Nanoscale Thin Polymer Films via Surface Wrinkling. ACS Nano 2009, 3, 844−852. (16) Barbero, D. R.; Steiner, U. Nonequilibrium Polymer Rheology in Spin-Cast Films. Phys. Rev. Lett. 2009, 102, 248303. (17) Napolitano, S.; Wü bbenhorst, M. The Lifetime of the Deviations from Bulk Behaviour in Polymers Confined at the Nanoscale. Nat. Commun. 2011, 2, 260. (18) Panagopoulou, A.; Napolitano, S. Irreversible Adsorption Governs the Equilibration of Thin Polymer Films. Phys. Rev. Lett. 2017, 119, 097801. (19) Reiter, G.; Hamieh, M.; Damman, P.; Sclavons, S.; Gabriele, S.; Vilmin, T.; Raphaël, E. Residual Stresses in Thin Polymer Films Cause Rupture and Dominate Early Stages of Dewetting. Nat. Mater. 2005, 4, 754−758. (20) Gin, P.; Jiang, N.; Liang, C.; Taniguchi, T.; Akgun, B.; Satija, S. K.; Endoh, M. K.; Koga, T. Revealed Architectures of Adsorbed Polymer Chains at Solid-Polymer Melt Interfaces. Phys. Rev. Lett. 2012, 109, 265501. (21) Burroughs, M. J.; Napolitano, S.; Cangialosi, D.; Priestley, R. D. Direct Measurement of Glass Transition Temperature in Exposed and Buried Adsorbed Polymer Nanolayers. Macromolecules 2016, 49, 4647−4655. (22) Chen, F.; Takatsuji, K.; Zhao, D.; Yu, X.; Kumar, S. K.; Tsui, O. K. C. Unexpected Thermal Annealing Effects on the Viscosity of Polymer Nanocomposites. Soft Matter 2017, 13, 5341−5354. (23) Liu, Y.; Russell, T. P.; Samant, M. G.; Stöhr, J.; Brown, H. R.; Cossy-Favre, A.; Diaz, J. Surface Relaxation in Polymers. Macromolecules 1997, 30, 7768−7771. (24) Sen, M.; Jiang, N.; Cheung, J.; Endoh, M. K.; Koga, T.; Kawaguchi, D.; Tanaka, K. Flattening Process of Polymer Chains Irreversibly Adsorbed on a Solid. ACS Macro Lett. 2016, 5, 504−508.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00411. Experimental details, Figures S1−S7, and Tables S1 and S2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Daisuke Kawaguchi: 0000-0001-8930-039X 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). We also appreciate the support from the ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

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REFERENCES

(1) Alcoutlabi, M.; McKenna, G. B. Effects of Confinement on Material Behaviour at the Nanometre Size Scale. J. Phys.: Condens. Matter 2005, 17, R461−R524. (2) Kim, C.; Facchetti, A.; Marks, T. J. Polymer Gate Dielectric Surface Viscoelasticity Modulates Pentacene Transistor Performance. Science 2007, 318, 76−80. (3) Fujie, T.; Ahadian, S.; Liu, H.; Chang, H.; Ostrovidov, S.; Wu, H.; Bae, H.; Nakajima, K.; Kaji, H.; Khademhosseini, A. Engineered 1201

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ACS Macro Letters (25) Nguyen, H. K.; Inutsuka, M.; Kawaguchi, D.; Tanaka, K. Depth-Resolved Local Conformation and Thermal Relaxation of Polystyrene near Substrate Interface. J. Chem. Phys. 2017, 146, 203313. (26) Gautam, K. S.; Schwab, A. D.; Dhinojwala, A.; Zhang, D.; Dougal, S. M.; Yeganeh, M. S. Molecular Structure of Polystyrene at Air/Polymer and Solid/Polymer Interfaces. Phys. Rev. Lett. 2000, 85, 3854−3857. (27) Briggman, K. A.; Stephenson, J. C.; Wallace, W. E.; Richter, L. J. Absolute Molecular Orientational Distribution of the Polystyrene Surface. J. Phys. Chem. B 2001, 105, 2785−2791. (28) Curtis, A. D.; Calchera, A. R.; Asplund, M. C.; Patterson, J. E. Observation of Sub-Surface Phenyl Rings in Polystyrene with Vibrationally Resonant Sum-Frequency Generation. Vib. Spectrosc. 2013, 68, 71−81. (29) Lu, X.; Myers, J. N.; Chen, Z. Molecular Ordering of Phenyl Groups at the Buried Polystyrene/Metal Interface. Langmuir 2014, 30, 9418−9422. (30) Wang, J.; Paszti, Z.; Even, M. A.; Chen, Z. Measuring Polymer Surface Ordering Differences in Air and Water by Sum Frequency Generation Vibrational Spectroscopy. J. Am. Chem. Soc. 2002, 124, 7016−7023. (31) Hanakata, P. Z.; Douglas, J. F.; Starr, F. W. Interfacial Mobility Scale Determines the Scale of Collective Motion and Relaxation Rate in Polymer Films. Nat. Commun. 2014, 5, 4163. (32) A repeating unit of PS has C−C bonds in addition to C−H bonds. Since all C−H bonds are characterized in the current wavenumber region, however, the local conformation of PS can be discussed without having the information of C−C bonds. (33) Tsuruta, H.; Fujii, Y.; Kai, N.; Kataoka, H.; Ishizone, T.; Doi, M.; Morita, H.; Tanaka, K. Local Conformation and Relaxation of Polystyrene at Substrate Interface. Macromolecules 2012, 45, 4643− 4649. (34) Abe, Y.; Tonelli, A. E.; Flory, P. J. Optical Anisotropy of Vinyl Polymer Chains. I. Strain Birefringence of Polypropylene and Polystyrene. Macromolecules 1970, 3, 294−303. (35) Yoon, D. Y.; Sundararajan, P. R.; Flory, P. J. Conformational Characteristics of Polystyrene. Macromolecules 1975, 8, 776−783. (36) Prest, W. M.; Luca, D. J. The Alignment of Polymers during the Solvent-Coating Process. J. Appl. Phys. 1980, 51, 5170−5174. (37) Sugimoto, S.; Inutsuka, M.; Kawaguchi, D.; Tanaka, K. Reorientation Kinetics of Local Conformation of Polyisoprene at Substrate Interface. ACS Macro Lett. 2018, 7, 85−89. (38) Zuo, B.; Inutsuka, M.; Kawaguchi, D.; Wang, X.; Tanaka, K. Conformational Relaxation of Poly(styrene-co-butadiene) Chains at Substrate Interface in Spin-Coated and Solvent-Cast Films. Macromolecules 2018, 51, 2180−2186. (39) Chen, Z.; Shen, Y. R.; Somorjai, G. A. Studies of Polymer Surface by Sum Frequency Generation Vibrational Spectroscopy. Annu. Rev. Phys. Chem. 2002, 53, 437−465. (40) Aoki, H.; Morita, S.; Sekine, R.; Ito, S. Conformation of Single Poly(methyl methacrylate) Chains in an Ultra-Thin Film Studied by Scanning Near-Field Optical Microscopy. Polym. J. 2008, 40, 274− 280. (41) Jones, R. L.; Kumar, S. K.; Ho, D. L.; Briber, R. M.; Russell, T. P. Chain Conformation in Ultrathin Polymer Films Using SmallAngle Neutron Scattering. Macromolecules 2001, 34, 559−567. (42) Kajiyama, T.; Tanaka, K.; Takahara, A. Surface Molecular Motion of the Monodisperse Polystyrene Films. Macromolecules 1997, 30, 280−285.

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DOI: 10.1021/acsmacrolett.8b00411 ACS Macro Lett. 2018, 7, 1198−1202