Probing the Role of Side-Chain Interconnecting Groups in the

Article pubs.acs.org/Langmuir

Probing the Role of Side-Chain Interconnecting Groups in the Structural Hydrophobicity of Comblike Fluorinated Polystyrene by Solid-State NMR Spectroscopy Su-Yeol Ryu,† Jae Woo Chung,*,‡ and Seung-Yeop Kwak*,† †

Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, Korea Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 156-743, Korea

‡

Downloaded by UNIV OF NEBRASKA-LINCOLN on August 25, 2015 | http://pubs.acs.org Publication Date (Web): August 18, 2015 | doi: 10.1021/acs.langmuir.5b02192

S Supporting Information *

ABSTRACT: In order to probe the role of side-chain interconnecting groups (−O−, −S−, and −SO2− linkages between the polystyrene (PST) main chain and fluorooctyl side chain) in the hydrophobicity of the comblike fluorinated polystyrenes, the molecular motion and structure of polymers are explored using the spin−lattice relaxation times (T1 and T1ρ) by solid-state 1H and 19F nuclear magnetic resonance spectroscopy. The chain-end motions of the polystyrene main chain and the fluorooctyl side chain are homogeneous, regardless of the interconnecting groups, which means that the chain-end motions of the main chain and the side chain maintain consistency, and these are irrelevant to each other. However, the local dynamic of the main chain shows the structural heterogeneity composed of the mobile and rigid regions, attributed to the rigidity of the side chain. The mobile dynamic portions of the main chain for PST−O and PST−S increase, and their rigid dynamic portions decrease as the temperature increases, whereas the ratio of structural heterogeneity for PST−SO2 is maintained despite increasing temperature. The activation energies (Ea) corresponding to the local motion of fluorooctyl side chains for PST−O and PST−S are drastically increased on the fast motion side compared to the slow motion side, suggesting the motional transformation of side chains for PST−O and PST−S from the small local motion into the large-scale movements related to a cooperative segmental motion when heated. Also, the local motion of the fluorooctyl side chain for PST−SO2 has similar Ea values on both sides, indicating that the relaxation time of PST−SO2 does not change with temperature. Therefore, PST−SO2 is structurally more stable than PST−O or PST−S, which can be attributed to the densely packed fluorooctyl side chain structure caused by the large dipole moment of the sulfone interconnecting group.

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fluorinated polystyrenes (PST−O and PST−S) due to the more densely packed fluorinated alkyl side chain structure with short interchain distances at the film surface. These results clearly reveal that the physical performance of fluorinated polymers was affected by the molecular structure of the fluorinated polymer, or in other words, the properties of the fluorinated polymer can be controlled by manipulating their molecular structure. From this point of view, it is important to explore the structure of the fluorinated polymer to understand the physical properties. Structures of fluorinated polymers have been widely investigated by a variety of analysis techniques, including wide-angle X-ray diffraction (WXRD), small-angle X-ray scattering (SAXS), grazing-incidence small-angle X-ray scattering (GISAXS), near-edge X-ray absorption fine structure (NEXAFS), and nuclear magnetic resonance (NMR) spectroscopies.9,12−17 Among them, NMR spectroscopy, especially

INTRODUCTION Over the past few decades fluorinated polymers have attracted significant attention in varied scientific and industrial fields due to their outstanding properties including low dielectric constant, low refractive index, good transparency, low surface energy, low friction coefficient, low wettability, low adhesion, high chemical resistance, and high thermal resistance.1−7 In particular, surface properties such as the surface energy, surface stability, surface tension, and hydrophobicity can be improved by controlling the orientation and length of the fluorinated side chains,8−11 and such unique characteristics render the fluorinated polymer useful as a repellent against water, oil, and dust. Previously, as a model comblike fluorinated polymer, we synthesized polystyrene with a fluorooctyl side chain bearing a variety of interconnecting groups including ether (−O−), thioether (−S−), or sulfone (−SO2−).12 We investigated the effect of the interconnecting chemical structures on the hydrophobicity of a comblike fluorinated polymer, and we showed that the PST−SO2 film had significantly better longterm hydrophobic stability than did the other comblike © XXXX American Chemical Society

Received: June 15, 2015 Revised: August 10, 2015

A

DOI: 10.1021/acs.langmuir.5b02192 Langmuir XXXX, XXX, XXX−XXX

Article


Langmuir solid-state NMR, is the most effective and useful tool in elucidating the dynamics and structure of the fluorinated polymer because it can ascertain the structural features of the fluorinated polymer from the molecular dynamic (or motional) heterogeneity caused by the structural difference of the fluorinated polymer.18−22 As NMR parameters for investigating molecular motion, the spin−lattice relaxation time in the laboratory frame (T1) and the spin−lattice relaxation time in the rotating frame (T1ρ) provide accurate insight into the molecular motion of the polymer. T1 presents the overall motion of the polymer chain on the time scale of the resonance frequency (MHz order), and the local motion of the polymer that occurs in the kHz motional regime can be identified from the decay behavior of the T1ρ magnetization relaxation. The T1 or T1ρ relaxation is used to derive the correlation times (τc) that are defined as the average time required for motional events.23−25 Furthermore, the activation energy (Ea) of the molecular motion, which is evaluated from the τc, is used efficiently as a measure of the molecular motion of the polymer chains. Here, we employed solid-state 1H and 19F nuclear magnetic resonance (NMR) spectroscopy to probe the role of side-chain interconnecting groups (ether (−O−), thioether (−S−), or sulfone (−SO2−)) in the hydrophobicity of the comblike fluorinated polystyrenes. We show the strong influence of the interconnecting groups on the structure of comblike fluorinated polystyrenes through solid-state 1H and 19F NMR. The overall motion of the main chain was homogeneous, while the local main chain motion was heterogeneous. In particular, PST−O and PST−S were confirmed to have a mobile structure by the observation of magnetization decay, correlation time, and activation energy with temperature for the local motion of the main chain and side chain, whereas PST−SO2 showed a stable packing structure. On the basis of these results, we ascertained that the interconnecting groups affected not only the side-chain stability but also the local motion and stability of main chain. These structural characteristics of main chain and side chain derived by the interconnecting groups lead to the enhancement of surface stability and hydrophobicity of the comblike fluorinated polystyrenes.

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Figure 1. Chemical structures of the comblike fluorinated polystyrenes (PST−O, PST−S, and PST−SO2) and the controls. time of approximately 12 μs and a magnet temperature of 40 °C. The powder samples (2.5 g) with a particle size of less than 100 μm were packed and pressed into specially designed 10-mm-diameter NMR glass tubes so as to obtain a higher density. The sample measurements were performed over a temperature range of 200−340 K using a Bruker BVT-3000 temperature control unit. The sample temperature was regulated by modulating the steady flows of both dry air at room temperature and cold nitrogen from a liquid-nitrogen dewar below room temperature. During the measurements, the monitored temperature typically remained within ±0.1 K of the target temperature. To prevent complications associated with supercooling or hysteresis, all measurements were performed by increasing the temperature from a low initial temperature. The 1H NMR free induction decay (FID) signal accumulation measurements were collected using a 90° pulse width of 2.04 μs and a 180° pulse width of 4.26 μs, whereas the 19F NMR FID signal accumulation measurements were collected using a 90° pulse width of 3.52 μs and a 180° pulse width of 7.04 μs. T1 and T1ρ relaxation times were determined by analyzing the magnetization decay after the inversion−recovery (i.e., 180°−τ−90°) pulse sequence and the spin-lock-delay τ (i.e., 90°−τ) pulse sequence, respectively, with a receiver gain of 61−107 dB and a recycle delay of 5 s over 128 scans to reduce noise. FIDs were integrated to characterize the individual decay curves. Finally, the relaxation times were obtained from the slopes of the semilogarithmic plots of the magnetization intensity versus τ. Powder samples deposited on a substrate were investigated with in situ XRD measurement. The examinations were performed in air with a Bruker D8 Advance X-ray diffractometer using a LynxEye high-speed strip detector and Cu Kα radiation (λ = 1.5406 Å) at a tube voltage of 40 kV and a tube current of 40 mA. XRD measurements were within the range of 300−340 K (10 K interval). Samples were heated at 2 °C/ min to the desired temperature and equilibrated for 30 min at each temperature before testing. The parameters of tests were 2θ = 3−25°, 0.02° step size, and 2°/min for the scanning speed.

EXPERIMENTAL SECTION

Materials. Poly[p-[[(perfluorooctylethylene)oxy]methyl]styrene] (PST−O) (average Mn = 15 400 and PDI = 1.74), poly[p[[(perfluorooctylethylene)thio]methyl]styrene] (PST−S) (average Mn = 20 300 and PDI = 1.24), and poly[p-[[(perfluorooctylethylene)sulfonyl]methyl]styrene] (PST−SO2) (average Mn = 21 400) were synthesized as described in the literature12 and were used after purification via precipitation using a 3,5-bis(trifluoromethyl)phenol/ methanol cosolvent system. Control samples of polystyrene (average Mw = 35 000) and poly(4-methylstyrene) (average Mw = 72 000) were purchased from Sigma-Aldrich and were used after purification via precipitation using a tetrahydrofuran (THF)/methanol cosolvent system. The chemical structures of the samples are shown in Figure 1. Characterization. Solid-state NMR measurements were performed at least 1 month after sample preparation. During the month between preparation and the measurements, the samples were allowed to reach structural equilibrium at room temperature.26,27 The 1H and 19 F spin−lattice relaxation times in the laboratory frame (T1) and in the rotating frame (T1ρ) were measured for the PST−O, PST−S, and PST−SO2 powder samples. Measurements were carried out using a Minispec mq20 (Bruker Analytik GmbH, Rheinstetten, Germany) spectrometer at 0.47 T with a permanent magnet. The instrument was operated at a proton resonance frequency (ν0H) of 19.95 MHz and a fluorine resonance frequency (ν0F) of 18.77 MHz with a probe dead

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RESULTS AND DISCUSSION Chain-End Mobility of Main Chain and Side Chain in the Comblike Fluorinated Polystyrenes. The spin−lattice relaxation time in the laboratory frame, T1, with a time scale of the resonance frequency in the range of MHz, is adequate to analyze the rapid motion of end groups of polymers.28 In order to estimate the chain-end motion of the polystyrene main chain and the fluorooctyl side chain in the fluorinated polystyrenes, respectively, T1 values of the comblike fluorinated polystyrene B


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side chain observed by the solid-state 19F NMR measurement was also homogeneous on the time scale of T1, as shown in Figure 2b. The T1 values of the main chain and the side chain gradually increased with temperature, as shown in Figure 3, which

powder samples (PST−O, PST−S, and PST−SO2) were measured using the solid-state 1H and 19F NMR in the temperature range of 200 to 320 K below their melting endothermic transitions (PST−O = 346 K, PST−S = 373 K, and PST−SO2 = 460 K),12 and these measurements were reported for powder average values. When applying the 180°−τ−90° inversion−recovery pulse sequence, a single T1 relaxation complies with the following condition ln

Me − M τ τ =− 2Me T1

(1)


where Me is the intensity of the resonance signal at equilibrium (τ ≥ 5T1) and Mτ is the intensity of the resonance at the delay time (τ). As shown in Figure 2a, solid-state 1H NMR results

Figure 3. (a) 1H T1 vs the inverse temperature (1000/T) and (b) 19F T1 vs the inverse temperature (1000/T) of PST−O (■), PST−S (●), and PST−SO2 (▲).

revealed that both chains had the high-frequency-side (fastside) molecular motion (ω0τc ≪ 1) in the measuring temperature range.29,30 In order to obtain quantitative information on the chain-end motion of the main chain and side chain at the different interconnecting groups, we applied the T1 value to the Bloembergen−Purcell−Pound (BPP) theory.31−35 The theory provides the relationships among the relaxation rates, internuclear distance, resonance frequencies, and spectral density function of the molecular motions (i.e., an observation of the relative amount of motion). The spin−lattice relaxation behavior is given by 1 3 = γ 4ℏ2I(I + 1)[J (1)(ω0) + J (2)(2ω0)] T1 2 (2)

Figure 2. (a) Solid-state 1H NMR. Logarithmic plots of the resonance intensity vs the delay time, τ, for PST−O (■), PST−S (●), and PST− SO2 (▲). The slope of the line yields the 1H spin−lattice relaxation time in the laboratory frame, T1, at 280 K. (b) Solid-state 19F NMR. Logarithmic plots of the resonance intensity vs the delay time, τ, for PST−O (■), PST−S (●), and PST−SO2 (▲). The slope of the line yields the 19F spin−lattice relaxation time in the laboratory frame, T1, at 280 K.

showed that all of the ln[(Me − Mτ)/(2Me)] plots of the fluorinated polystyrenes as a function of τ yielded a straight line like a typical polystyrene and poly(4-methylstyrene) (Figure S1 in the Supporting Information), indicating a single T1 event. The observation of the single T1 suggests that the chain-end motion of main chain was dynamically homogeneous. Similar to the solid-state 1H NMR results, the chain-end motion of the

where γ is the magnetogyric ratio of the observed species, ℏ is the reduced Planck’s constant or Dirac constant (ℏ = h/2π), I is the spin quantum number (I = 1/2 for 1H and 19F), ω0 is the angular frequency (ω0 = 2πν0), and ν0 is the Larmor frequency (resonance frequency). J(q)(ω0(1)) with q = 0, 1, 2 denotes the Lorentzian-shaped spectral density functions at a particular frequency and is given by C


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Langmuir J (q)(ω) = G(q)(t = 0)

τc 1 + (ωτc)α

⎛ τ ⎞ Mτ = M 0 exp⎜⎜ − ⎟⎟ ⎝ T1ρ ⎠

with α = 2 (original BPP) (q)


G (t )

⎛ |t | ⎞ = G (t = 0) exp⎜ − ⎟ ⎝ τc ⎠

The magnetization intensity (Mτ) decayed exponentially with a time constant of T1ρ, and M0 is the initial intensity of the free induction decay. The local relaxation behaviors of the main chain in typical polystyrene and poly(4-methylstyrene) displayed a single T1ρ phase, indicating that their main chain had dynamically homogeneous local motion (Figure S3 in the Supporting Information). However, the local proton magnetization behavior of the main chain in the comblike fluorinated polystyrenes (PST−O, PST−S, and PST−SO2) was not fit to a single-exponential decay model (eq 6). Instead, as shown in Figure 4, their proton magnetization decays were well fit to double-exponential models with T1ρ relaxation times that depended on two components, T1ρ,A and T1ρ,B:

(q)

(4)

The original BPP model (α = 2) was applied to investigate the T1 relaxation behaviors of the main chain and side chain for all powder samples. The precise values of the correlation time (τc), an average time required for motional events, for the main chain and side chain were determined by applying nonlinear curve-fitting techniques to the T1 values based on eqs 2 and 3. Figure S2 in the Supporting Information shows plots of τc of the polystyrene main chain and fluorooctyl side chains in the comblike fluorinated polystyrenes as a function of the inverse temperature (1/T), respectively. The value of ln τc depended linearly on the inverse temperature, and this means that the temperature dependence of τc was assumed to follow Arrhenius behavior. Thus, the activation energy, Ea, for the chain-end motion of the main chain and side chain was determined according to the equation36 ⎛E ⎞ τc = τ0 exp⎜ a ⎟ ⎝ RT ⎠

⎛ ⎛ τ ⎞ τ ⎞ ⎟⎟ + M 0,B exp⎜⎜ − ⎟⎟ Mτ = M 0,A exp⎜⎜ − ⎝ T1ρ ,A ⎠ ⎝ T1ρ ,B ⎠

(5)

Table 1. Activation Energies of Chain-End Motion for the Main Chains and Side Chains in Comblike Fluorinated Polystyrenes Obtained from the 1H and 19F Spin−Lattice Relaxation Times in the Laboratory Frame (T1) and the Original Bloembergen−Purcell−Pound (BPP) Theory main chain (1H NMR) PST−O PST−S PST−SO2

Ea (kJ/mol) 4.03 ± 0.16 3.83 ± 0.15 4.89 ± 0.18

side chain (19F NMR)

τc,0 (s) −10

2.92 × 10 2.04 × 10−10 1.27 × 10−10

Ea (kJ/mol)

τc,0 (s)

7.28 ± 0.26 7.50 ± 0.09 8.13 ± 0.05

4.75 × 10−11 3.14 × 10−11 2.15 × 10−11

(7)

This suggests the presence of dynamically heterogeneous local phases (phases A and B) of the main chain in the comblike fluorinated polystyrenes. These trends were observed in T1ρ plots as a function of inverse temperature in the range of 205 to 350 K (Figure 5). The shorter T1ρ value (phase A) derived from the fast decay (quick relaxation) represents characteristic behavior corresponding to the mobile region of the main chain. The longer T1ρ value (phase B) was derived from the slow decay, indicating the rigid region of the main chain. Considering that the main chain of the neat polystyrene and poly(4-methylstyrene) showed the homogeneous local motion (Figure S3), we attribute the multiphase of the fluorinated polystyrene main chain to the fluorooctyl side chain and the interconnecting groups. In other words, the fluorooctyl side chain and the interconnecting groups critically affected the local motion heterogeneity of the main chain. Thus, we evaluated the relative fraction between the mobile phase (phase A) and the rigid phase (phase B) of the fluorinated polystyrene main chain with a variation of temperature from 220 to 340 K with 20 K intervals (Figure S4 in the Supporting Information).37 As listed in Table 2, the relative rigid phase fractions of all samples were 71.5−77.0% at 220 K. As the temperature increased, the relative rigid fraction of the PST−O main chain was dramatically decreased, and eventually the relative fraction of the rigid phase was reduced to 34.7% at 340 K. The fraction of the rigid phase for the PST−S main chain also gradually decreased as the temperature increased. However, the fraction of the rigid phase in the PST−SO2 main chain remained, regardless of temperature change. In the case of the mobile phase fraction, PST−SO2 showed almost constant mobile fraction values, despite the temperature increase, while both PST−O and PST−S showed increases in the mobile fraction value. These results clearly indicate that PST−SO2 had a more rigid and stable molecular structure than PST−O or PST−S. In situ XRD also showed the same results to local structural stability for the main chain observed from solid-state 1H NMR (Figure S5 and Table S1 in the Supporting Information). For PST−O and PST−S, the several diffraction peaks below 2θ = 10° corresponding to the locally ordered main-chain structure decreased or disappeared with increasing temperature, indicating that the ordered main-chain structure of PST−O

where τ0 is the correlation time at infinite temperature and indicates the reciprocal frequency with which the observed species attempt to reach a more mobile state. Ea indicates the energy barrier, which has to be overcome for chain-end motion of the main chain and side chain to occur. Contrary to our expectations, samples showed similar values of Ea and τ0, and this implied that the chain-end mobility of all of the samples was not distinguished with the type of interconnecting groups, as listed in Table 1. Therefore, in order to more clearly

sample

(6)

(3)

investigate the structural features of comblike fluorinated polystyrenes, we evaluated the spin−lattice relaxation time in the rotating frame, T1ρ, using the solid-state 1H NMR. Local Molecular Structure of the Main Chain in the Comblike Fluorinated Polystyrenes. T1ρ, which is the relaxation of nuclear magnetization in the presence of the constant magnetic field (B0) and a time-dependent magnetic field (B1), is most useful in observing slowly cooperative motions (kHz motional regime) of the polymeric systems in the solid state. When the solid-state polymer had a dynamically homogeneous local phase, the 90° − τ (spin−lock−delay τ) pulse sequence yielded a single T1ρ according to the following condition: D


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Figure 5. 1H spin−lattice relaxation times in the rotating frame, T1ρ, vs the inverse temperature (1000/T) of PST−O (■), PST−S (●), and PST−SO2 (▲): separation of phase A (inset, mobile region) and phase B (rigid region).

Table 2. Mobile and Rigid-Phase Fractions obtained from the 1H Spin−Lattice Relaxation Time in the Rotating Frame (T1ρ) mobile phase (phase A)

Figure 4. Solid-state 1H NMR measurements. Logarithmic plot of the magnetization intensity vs the delay time (a) for PST−O (■) at 225 K, (b) PST−S (●) at 220 K, and (c) PST−SO2 (▲) at 260 K. The dashed lines indicate rapid decay, and the first 10 points were fit using eq 6. The solid lines indicate the fits of all data points to eq 7 and yield the slow-decay time constants for the 1H spin−lattice relaxation times in the rotating frame, T1ρ..

PST−O (K)

T1ρ,A (ms)

220 240 260 280 300 320 340 PST−S (K)

0.53 0.67 0.26 0.55 0.24 0.84 0.75 T1ρ,A (ms)

220 240 260 280 300 320 340 PST−SO2 (K)

0.76 0.52 0.89 0.63 0.37 0.63 0.35 T1ρ,A (ms)

220 240 260 280 300 320 340

0.98 1.00 0.34 0.51 0.82 0.74 0.70

M0,Aa 15.0 13.2 17.3 25.2 31.9 31.9 36.4 M0,Aa 23.9 18.2 17.9 22.5 16.7 25.9 26.1

rigid phase (phase B)

fraction (%)

T1ρ,B (ms)

23.0 21.1 28.0 42.7 56.5 57.7 65.3 fraction (%)

5.61 4.66 2.22 2.42 2.48 2.57 2.61 T1ρ,B (ms)

M0,Ba 50.2 49.5 44.7 33.8 24.6 23.4 19.4 M0,Ba 59.9 60.5 58.1 32.3 33.3 22.4 22.5

fraction (%) 77.0 78.9 72.0 57.3 43.5 42.3 34.7 fraction (%)

M0,Aa

28.5 23.2 23.5 41.0 33.4 53.6 53.7 fraction (%)

7.66 5.36 3.14 3.14 2.43 2.29 1.95 T1ρ,B (ms)

M0,Ba

71.5 76.8 76.5 59.0 66.6 46.4 46.3 fraction (%)

19.4 19.1 19.7 19.1 20.7 20.3 17.8

25.8 27.0 29.6 31.5 35.7 37.0 33.0

7.64 7.36 6.01 5.45 5.29 5.54 4.83

55.6 51.6 46.8 41.5 37.4 34.7 36.1

74.2 73.0 70.4 68.5 64.3 63.0 67.0

a

M0,A and M0,B are the initial intensities of the free induction decay for phases A and B.

and PST−S was broken down as the temperature increased. In contrast, the ordered main-chain structure of PST−SO2 was well maintained with a d spacing of 21.2 Å, even though the temperature increased to 340 K. Therefore, the combined results of solid-state 1H NMR and in situ XRD reveal that the main chain of the comblike fluorinated polystyrenes possessed a structural heterogeneity attributed to the fluorooctyl side chains, and its local motion and stability depended on the type of interconnecting groups, which were in good agreement with the previous endothermic enthalpy change results (ΔH, kJ/ mol): 6.4 (PST−O), 9.6 (PST−S), and 37.9 (PST−SO2).12

From this point of view, it is necessary to ascertain the local molecular motion of the fluorooctyl side chain that was connected to the polystyrene main chain via the varied interconnecting groups to understand the impact of the interconnecting group on the molecular motion of the comblike fluorinated polystyrenes in more detail. Local Molecular Motion of Side Chain in the Comblike Fluorinated Polystyrenes. The magnetization T1ρ relaxation decay of solid-state 19F NMR was used to characterize the local E


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motion of the fluorooctyl side chain and to investigate the effect of the interconnecting groups (ether (PST−O), thioether (PST−S), or sulfone (PST−SO2)) on the local side-chain motion. All data fit a single-exponential decay, as shown in Figure 6a. The single decay suggests that the spin diffusion

able T1ρ values with the type of interconnecting groups (ether (PST−O), thioether (PST−S), or sulfone (PST−SO2)). Interestingly, in the cases of PST−O and PST−S, the relaxation curves exhibit an asymmetric shape around the Tmin, whereas PST−SO2 shows the U-shaped character of a symmetric curve. These asymmetric curves are predicted by several BPP-based relaxation models considering correlation effects such as structural disorder in amorphous and/or defective crystalline materials.34 The asymmetric curves are taken into account in eq 3 when exponent α is allowed to adopt values in the interval 1 < α < 2 (α = 2, the BPP model in the case of symmetric shape).33,34 On the slow side (ω1τc ≫ 1), only a few spin diffusions are sampled in the time interval by ω1. Therefore, side-chain motion occurs on a shorter length scale on the slow side. In contrast, the long-range spin diffusion related to sidechain motion can be probed on the fast side (ω1τc ≪ 1), which reasonably implies Ea,slow < Ea,fast. The correlation time (τc), indicating an average time required for motional events of the side chain, was estimated from the theory, and the local sidechain motion was assessed using the τc. The spin−lattice relaxation behavior is given by 1 3 = γ 4ℏ2I(I + 1)[J (0)(2ω1) + 10J (1)(ω0) + J (2)(2ω0)] T1ρ 8 (8)

where ω0/2π = 18.77 MHz and ω1 = 0.45 Mrad/s in this experiment. PST−O, PST−S, and PST−SO2 showed the lowest T1ρ value at 280, 320, and 250 K, respectively, and the ω1τc value is 0.5 at those temperatures (when B1 ≪ B0).34,40 Finally, the precise values of the correlation time (τc), indicating an average time required for motional events of the side chain, was determined by applying nonlinear curve fitting to the T1ρ value based on eq 8. Figure 7 shows the plots of τc of the fluorooctyl side chains in the comblike fluorinated polystyrenes as a function of the inverse temperature (1/T); we see that τc depended linearly on the inverse temperature. On the basis of the temperature dependence of τc, the Arrhenius activation energy (Ea) of the local motion for the fluorooctyl side chain was determined by

Figure 6. (a) Logarithmic plot of the magnetization intensity vs the delay time obtained by the solid-state 19F NMR of PST−O (■) at 280 K, PST−S (●) at 320 K, and PST−SO2 (▲) at 250 K. The solid lines indicate the fits of all data points to eq 6 and yield the 19F spin−lattice relaxation time in the rotating frame, T1ρ. (b) T1ρ vs the inverse temperature (1000/T) of PST−O (■), PST−S (●), and PST−SO2 (▲).

processes of the fluorines were sufficiently fast to reach equilibrium on the relaxation time scale (i.e., with homogeneous and identical relaxation behavior). It has been reported that the fluoroalkyl chains form a helical structure with a 16.6° helix twist angle.38,39 Thus, the fluorooctyl side chain of the comblike fluorinated polystyrenes can be thought of as a uniformly rigid molecular rod with a strong helical conformation; consequently, the local side-chain motion can exhibit a single relaxation time. Figure 6b displays the plots of T1ρ as a function of the inverse temperature (1000/T) over the range 220−360 K. PST−O, PST−S, and PST−SO2 showed the lowest T1ρ values at 280, 320, and 250 K, respectively (also called Tmin, which means temperature at the minimum T1ρ value) and had the most efficient relaxation of fluorine spins at Tmin. T1ρ located on the fast side (above Tmin, left side of the minimum, ω1τc ≪ 1) implies fast motion, and T1ρ located on the slow side (below Tmin, right side of the minimum, ω1τc ≫ 1) implies slow motion. Despite the homogeneous local motion of the side chain, the fluorinated polystyrenes had distinguish-

Figure 7. Temperature dependence of the correlation times (τc) calculated from the 19F NMR T1ρ data using the BPP equation for PST−O (■), PST−S (●), and PST−SO2 (▲). The slope of the solid line yields the activation energy (Ea). The arrows represent the temperature at the T1ρ minimum, Tmin. The glass-transition temperatures (Tg’s) of PST−O and PST−S were observed at 317 and 332 K, respectively, and the Tg of PST−SO2 was not observed. F


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Table 3. Parameters of the Fluorooctyl Side Chains in the Comblike Fluorinated Polystyrenes Obtained from 19F Spin−Lattice Relaxation Times in the Rotating Frame (T1ρ) and the Original Bloembergen−Purcell−Pound (BPP)- and BPP-Based Theories τc,0 (s)


Ea (kJ/mol) sample

Tmin (K)

α

above Tmin

below Tmin

above Tmin

below Tmin

PST−O PST−S PST−SO2

280 320 240

1.5 1.5 2.0

38.9 ± 3.7 27.8 ± 3.3 9.3 ± 0.4

17.6 ± 2.0 13.1 ± 0.8 8.9 ± 2.1

8.3 × 10−14 4.3 × 10−11 9.5 × 10−9

1.8 × 10−9 1.8 × 10−8 1.4 × 10−8

at 300 K. At an increased temperature of 340 K, the distance in PST−O increased to 5.4 Å, whereas the distance in PST−SO2 was well maintained. In addition, the correlation length of the ordered side chain, which was derived from the full width at half-maximum (fwhm) of the peaks, was evaluated to be 42.3 Å (PST−O) and 61.8 Å (PST−SO2) at 300 K by the Scherrer equation. With increasing temperature of PST−O to 340 K, the correlation length dramatically decreases to 35.6 Å, which implied a diminishment in the arrangement of the side chain and an increase in the unconstrained local motion over a large range, whereas the correlation length of PST−SO2 was firmly maintained. These results suggest that the ordered structure of the side chain for PST−SO2 was less affected by temperature, or in other words, the local motion of side chains for PST−SO2 was less dependent on the temperature, as manifested in the Ea of PST−SO2. The structural stability of the side chain can be explained by the degree of the dipole moment of the interconnecting group (ether (−O−), thioether (−S−), or sulfone (−SO2−)) between the styrene main chain and the fluorooctyl side chain; the dipole moments of dimethyl ether, dimethyl sulfide, and dimethyl sulfone are 1.30, 1.55, and 4.49 D, respectively.41 Thus, the PST−SO2, having the sulfone interconnecting group with a relatively strong dipole−dipole interaction, showed an improved and stabilized arrangement of the side chains. The thermal dependence of hydrophobicity was observed so as to investigate the relationship between the hydrophobicity and the local motion of the fluorooctyl side chain. The water contact angle of as-prepared and heat-treated thin films was measured so as to determine the change in surface hydrophobicity. As-prepared films were heat treated at 350 K for 1 h in an oven (air). As shown in Figure 8, the PST−SO2 film maintained its hydrophobicity, and this result suggests that PST−SO2 possesses a tenacious hydrophobic surface due to the dipole−dipole interactions between the sulfone interconnecting groups which can prevent the reconstruction of fluorooctyl side

eq 5 and is listed in Table 3. Interestingly, PST−O and PST−S showed quite different Ea,fast and Ea,slow values between the fast and slow sides (above and below Tmin) while PST−SO2 showed similar Ea values in both regions. Asymmetrical relaxation behaviors of PST−O and PST−S lead to two sets of different fitting values of τc,0 and Ea on the slow- and fast-side flanks of the T1ρ minimum. The Ea,fast values of PST−O and PST−S were larger than those of PST−SO2 (38.9, 27.8, and 9.3 kJ/mol for PST−O, PST−S, and PST−SO2, respectively) on the fast side (ω1τc ≪ 1), while the values of Ea,slow for all samples were similar (17.6, 13.1, and 8.9 kJ/mol for PST−O, PST−S, and PST−SO2, respectively) on the slow side (ω1τc ≫ 1). In addition, the Ea values follow Ea,slow = (α − 1) × Ea,fast.33,34 These results suggest that the motional event of the fluorooctyl side chain for PST−O and PST−S was transformed on the fast side into motion that required a larger activation energy, while the local motion of the fluorooctyl side chain for PST−SO2 below Tmin remained above Tmin. Moreover, different sets of τc,0 and Ea values represent different motion modes with different fluctuating dipolar fields. From DSC measurements (Figure S6 in the Supporting Information), we confirmed that the glasstransition temperatures (Tg’s) of PST−O and PST−S were observed at 317 and 332 K, respectively. However, the Tg of PST−SO2 was not observed. We note that Tg values of PST−O and PST−S were located on the fast side (above Tmin), which showed an increase in Ea. Generally, it is known that the mainchain segmental motion of a polymer (20−50 carbons moving in cooperation) starts from near Tg when the polymer is heated. To start a cooperative segmental motion in the polymer, a higher Ea is required than the Ea required for a small local motion. Thus, the increase in the Ea of PST−O and PST−S above Tmin could be explained by the transformation of the small local motion, corresponding to β or γ relaxation below Tmin, into the large cooperative segmental motion, such as α relaxation above Tmin. This large cooperative segmental motion of PST−O and PST−S above Tmin would provide their fluorooctyl side chain relatively greater free movement. In other words, the dramatic change in Ea for the fluorooctyl side chain was considered in relation to the occurrence of segmental motion of the main chain with increasing temperature. In contrast, there was no glass-transition behavior of the main chain in PST−SO2, and the motional event of the fluorooctyl side chain was well maintained despite the increase in temperature over Tmin. Thus, it was thought that no large movement, such as a cooperative segmental motion, occurred in PST−SO2, which suggested that PST−SO2 had a confined fluorooctyl side chain capable only of restricted side-chain motion and the small Ea is needed to start the motional event. These different molecular behaviors of the side chains with the type of interconnecting group were also verified by the observation of the structural change in the side chain with temperature variation using in situ XRD as shown in Figure S5 and Table S1. The fluorooctyl side chains of PST−O, PST−S, and PST−SO2 showed a similar side chain distance of ca. 5.0 Å

Figure 8. Thermal dependence of hydrophobicity for PST−O (■), PST−S (●), and PST−SO2 (▲) thin films. G


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Figure 9. Schematic representation of the structure of comblike fluorinated polystyrenes (PST−O, PST−S, and PST−SO2) based on the 1H and 19F spin−lattice relaxation times (T1 and T1ρ), in situ XRD, and hydrophobicity results.

were also consistent with the results of in situ XRD, DSC, and a hydrophobicity test, which evaluated the water contact angle of thin films for PST samples with temperature. Our results clearly show that the interconnecting group has a significant influence on the structure of the comblike fluorinated polystyrenes and affects not only the side-chain stability but also the local motion and stability of the main chain. Furthermore, we show that the motional study using solid-state NMR, a nondestructive and reliable technique, is quite attractive and effective for the identification of the polymer structure and enables academic and industrial researchers to design tailor-made advanced functional polymers and predict their physical performance.

chains on the surface during heat treatment. In contrast, the hydrophobic surface properties of PST−O and PST−S were weakened due to the reconstruction of the side chains during heat treatment. On the basis of the results from the analysis of solid-state NMR, in situ XRD, and the hydrophobicity of the samples, the change in molecular structure for PST−O, PST−S, and PST− SO2 as the temperature increases is schematically illustrated in Figure 9. All fluorinated polystyrenes formed with the ordered main chain and side chain at low temperature, whereas PST−O and PST−S showed unrestrained local structure of the main chain and the freely moving side chain with increased temperature. In the case of PST−SO2, the main chain and side chains maintained a stable local structure regardless of temperature variation. We believe that the interconnecting groups affect not only the side chain stability, which we have reported in our previous work,12 but also the local motion and structural stability of the polystyrene main chain. Furthermore, it could be asserted that the physical properties of the comblike fluorinated polystyrenes (i.e., hydrophobicity and surface stability) are determined by the structural features of main chain and side chain of comblike fluorinated polystyrenes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b02192. 1 H spin−lattice relaxation times in the laboratory frame (T1) and in the rotating frame (T1ρ) of polystyrene and poly(4-methylstyrene). Temperature dependence of the correlation times (τc) calculated from the BPP equation using the 1H NMR T1 data of the polystyrene main chain and the 19F NMR T1 data of the side chains for PST−O, PST−S, and PST−SO2. In situ XRD patterns and related data of PST−O, PST−S, and PST−SO2. DSC curves and thermal properties of the comblike fluorinated polystyrenes. (PDF)

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CONCLUSIONS In this study, we investigated the chain-end motion and local molecular motions of comblike fluorinated polystyrenes with various interconnecting groups (PST−O, PST−S, and PST− SO2) by analyzing the solid-state 1H and 19F NMR spin−lattice relaxation times so as to probe the role of side-chain interconnecting groups in the hydrophobicity of the comblike fluorinated polystyrenes. From analyzing the dynamic heterogeneity of the main chain and the activation energy of the side chains, which were obtained from the spin−lattice relaxation times of the fluorinated polystyrenes, we found that PST−SO2 was structurally more stable than PST−O or PST−S due to the densely packed fluorooctyl side chain structure by the strong dipole moment of the sulfone interconnecting group. These

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AUTHOR INFORMATION

Corresponding Authors

*(J.W.C.) Tel: +82-2-828-7047. Fax: +82-2-817-8346. E-mail: [email protected]. *(S.-Y.K.) Tel: +82-2-880-8365. Fax: +82-2-885-9671. E-mail: [email protected]. Notes

The authors declare no competing financial interest. H


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ACKNOWLEDGMENTS This work was supported by the Research Institute of Advanced Materials, Department of Materials Science and Engineering, Seoul National University.


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