Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Anisotropic Dynamics of Benzonitrile Confined in δ and ε Clathrate Phases of Syndiotactic Polystyrene Hideo Kobayashi, Sho Akazawa, Osamu Urakawa,*,† Fumitoshi Kaneko,† and Tadashi Inoue Department of Macromolecular Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
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S Supporting Information *
ABSTRACT: Anisotropic dynamics of benzonitrile (BzCN) confined in the cocrystalline region of syndiotactic polystyrene (sPS) was studied by means of dielectric spectroscopy. Especially examined was the effect of orientation of the crystalline phase for two types of cocrystals, δ and ε clathrates, on the dielectric relaxation behavior. The relaxation time of the BzCN in the δ cocrystal, regardless of its orientation, was found to be longer than that in the ε cocrystal, indicating that the δ structure has higher degree of constraint on the guest motion than the ε structure. With respect to dielectric intensity Δε due to the guest BzCN relaxation, uniaxial stretching treatment caused different effects in the δ and ε cocrystals: Δε increased for the δ cocrystal and decreased for the ε crystal by stretching. This observation was attributable to the difference in directionality of anisotropic motion of the guest molecule inside the cavities of these cocrystals. The preferable orientation of the molecular axis of BzCN corresponding to the vector direction of the dipole moment was perpendicular to the sPS chain axis (c-axis) for the δ cocrystal, whereas it was parallel to that for the ε cocrystal. Since the stretching of the sPS/BzCN film made the c-axis align parallel to the film surface, the normal direction of the rotation plane of BzCN was set perpendicular to the electric field imposed on the sample films (to the thickness direction) for the δ cocrystal and parallel to it for the ε cocrystal. Such arrangements of the rotation axis to the electric field resulted in the increase and the decrease in Δε for the stretched δ and ε cocrystals, respectively. The experimental findings indicated the possibility that the dielectric properties of sPS cocrystal films could be changed by controlling the structure and crystallite orientation of sPS cocrystals.
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INTRODUCTION Syndiotactic polystyrene (sPS) is known to form cocrystals with a variety of low-mass guest molecules. Its cocrystalline structure can be classified into four major groups, monoclinic δ clathrate,1−3 triclinic δ clathrate,4 monoclinic δ intercalate,5,6 and orthorhombic ε clathrate. 7−12 Among them, the monoclinic δ clathrate belonging to the P21/a space group has two isolated cavities in a unit cell, and each cavity usually accommodates one guest molecule.1 The sPS chains in this cocrystal take TTGG helical conformation extending along the c axis. Meanwhile, the ε clathrate belonging to the Pbcn space group does not have isolated cavities but has channel like cavities parallel to the c-axis surrounded by the TTGG sPS helicies.9 Since Trezza and Grassi13 unveiled that benzene molecules imprisoned in the sPS δ cocrystal rotate around the C6 axis of the benzene ring by means of 2H NMR method, it has been recognized that guest molecules in the crystalline region are mobile and behave as a kind of molecular rotor, which is similar to the molecules in plastic crystals.14 Recently, we found that the dielectric relaxation of several polar guest molecules, 2-butanone (MEK), benzonitrile (BzCN), and two methyl-benzonitrile isomers (o-MBzCN and p-MBzCN) in the δ clathrate phase, appeared and their relaxation times exhibited © XXXX American Chemical Society
the Arrhenius-type temperature dependence with different activation energies ranging from 34.8 (MEK) to 99.3 kJ mol−1 (p-MBzCN).15−17 It has also been suggested by 2H NMR measurements13,18 and MD simulation19 that the rotational motion of monosubstituted benzenes in the δ clathrate is highly restricted and decoupled into two types of thermal motions: an in-plane rotation and a flip motion of the phenyl ring. The in-plane rotational axis of substituted benzenes (the normal axis of the phenyl plane) is known to align parallel to the sPS helix backbone of the δ phase,4,13,18 which suggests the possibility to control the direction of the rotational axis of the guest molecule by adjusting orientation of the cocrystalline region, which leads to the control of anisotropic physical properties, such as dielectric permeability,20 light absorption,21,22 fluorescence,23 nonlinear optical properties,24 magnetic permeability,25 guest transportation,26 thermal conductivity, and so forth. However, except for the 2H NMR,13,18 and the MD simulation,19 the anisotropic dynamics of guest molecules in sPS cocrystals has not been fully examined. In order to develop advanced materials based on the sPS/guest Received: April 14, 2018 Revised: October 1, 2018
A
DOI: 10.1021/acs.macromol.8b00799 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
without changing the crystalline form was confirmed by FT-IR measurements. BzCN mixed amorphous aPS films (#6 films) were prepared by casting an aPS/chloroform/BzCN ternary solution on a flat glass plate and by vaporizing the chloroform (most volatile component) under vacuum. The complete elimination of chloroform was confirmed by FT-IR measurements. For dielectric relaxation measurements, the sample films prepared as above were cut in a circular shape with a diameter of 20 mm and platinum or gold electrodes were made on both surfaces of all films by means of ion sputtering using a sputtering apparatus (E-1010, Hitachi High-Technologies). For powder X-ray diffraction measurements, all of the sample films were chopped into fine pieces to eliminate the orientation effect. Measurements. Cocrystal structures of prepared samples were confirmed by X-ray diffractometry. Powder X-ray diffraction patterns in reflection mode were measured over a 2θ range of 4−40° with a step-angle of 0.05° at 0.5° min−1 using a Rigaku RINT-2000 diffractometer equipped with a cupper X-ray tube for generating Cu Kα radiation (40 kV and 40 mA) and a graphite monochromator. FT-IR measurements were conducted at a resolution of 2 cm−1 with a spectrometer (DISILAB JAPAN, Excalibur FTS-3000) equipped with an IR polarizer for both the unstretched and stretched sPS/BzCN films, to determine the degree of orientation of the crystalline region. The weight fraction of BzCN in the sPS/BzCN and aPS/BzCN mixtures was determined by thermogravimetric analysis (TGA) using TGA apparatus (Seiko Instruments Inc., TG/DTA6200 thermobalance). Dielectric measurements were carried out on the electrodedeposited films set in dielectric sample chamber purged with nitrogen gas with an LCR meter (QuadTech, 7100 or HP 4284A) at several frequencies in the range of 12 to ∼2.0 × 105 Hz or 20−1.0 × 106 Hz, respectively, in the heating scan at a rate of 0.33 K min−1 from 80 to 420 K.
clathrate system, it is important to clarify the dynamics of various guest molecules by using multiple methods. From this point of view, we have been using dielectric spectroscopy to examine the dynamics of several guest molecules in detail including their temperature dependencies. Through the studies by MD simulations19,27 and several experimental techniques,13,15,16,18,28 understanding of guest dynamics in the δ cocrystal has advanced to some extent in the last 16 years. However, the guest dynamics in other cocrystal systems has not been studied yet to the best of our knowledge. With respect to the guest arrangement, the ε cocrystal has a strong contrast to the δ cocrystal; the guest molecules are imprisoned in channel like cavities in parallel with the sPS chain axis.29 It has not yet clarified how the shape of cavities could change the dynamics of the guest molecules. In this study, dielectric measurements were conducted to investigate the dynamics of benzonitrile (BzCN) stored as a guest in both the δ and ε cocrystals. We have newly found a dielectric relaxation process of the BzCN in the ε cocrystals, corresponding to the flip or rotational motion of the long axis of the guest BzCN in channel like cavities. In the first part of this paper, the crystal structures of the sPS/BzCN film and the degree of orientation are evaluated. In the second part, the dielectric relaxation data of the guest molecules in both the δ and ε cocrystals are presented and discussed. To examine the effect of orientation on the anisotropic guest dynamics, we compare the dielectric intensities between stretched and unstretched samples and discuss the relation of the host polymer chain (c-axis) direction to the direction of the rotational motion of guest BzCN molecules.
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RESULTS AND DISCUSSION Clathrate Structures of sPS Films Including BzCN. The powder X-ray diffraction patterns for all sPS/BzCN and aPS/ BzCN sample films, which were chopped in powder form, are shown in Figure 1. The sPS/BzCN samples obtained by the guest exchange of sPS/toluene cocrystal (#1 and #2) show two characteristic peaks of the δ cocrystal at 2θ = 7.7° and 9.6−9.7° corresponding to the 010 and 2̅10 diffractions, respectively,16 regardless of whether stretching treatments were conducted or not. On the other hand, for the sample obtained from the unstretched γ phase (#5), two peaks, characteristic of the ε cocrystal, appear at 2θ = 6.8° and 8.1°, which correspond to the 110 and 020 diffractions, respectively. For the samples obtained from the sPS/chloroform cocrystal (#3 and #4), one peak characteristic of the ε form at 2θ = 6.7°, and two peaks characteristic of the δ form at 2θ = 7.7° and 9.7° are observed.31 The appearance of these three peaks indicates the coexistence of the δ and ε cocrystals. These results are consistent with the literature data.12 To further confirm the above cocrystal structure identification, FTIR measurements were conducted on these film samples. The data of unstretched samples are shown in the Supporting Information (SI-1), which shows the consistent results with those of Figure 1. The degree of crystallinity was evaluated from the powder Xray diffraction patterns using the following equation:
EXPERIMENTAL SECTION
Samples. Syndiotactic polystyrene (sPS) was supplied by Idemitsu Petrochemical Co., Ltd. The weight-average molecular weight, Mw, was 1.79 × 105 and molecular weight distribution index, Mw/Mn, was 3.08. Atactic polystyrene (aPS, Toporex GP550−51) of Mw = 2.46 × 105 and Mw/Mn = 1.92 was supplied by Mitsui Chemical Co., Ltd. Benzonitrile (BzCN) was purchased from Wako Pure Chemical Industries, Ltd. We prepared 6 types of sample films: unstretched and stretched sPS/BzCN δ cocrystal films (denoted as #1 and #2, respectively), unstretched and stretched sPS/BzCN δ+ε cocrystal films (mixture of δ and ε cocrystals denoted as #3 and #4, respectively), unstretched sPS/BzCN ε cocrystal films (#5), and aPS/BzCN mixture amorphous films (#6) in the following way. First, amorphous sPS films were prepared by pressing sPS pellets at 300 °C and 20 MPa and successive quenching in iced water. Then the melt quench films were uniaxially stretched at the draw ratio of 3.5 at 100 °C. Both the stretched and unstretched films were exposed to a vapor of toluene or chloroform. Both the toluene and chloroform vapors induced the formation of δ cocrystal.10−12 At this stage, toluene or chloroform molecule was clathrated as a starting guest in the crystalline phase. According to Shaiju and Gowd,11 after extraction of the guest (toluene or chloroform), or after the guest exchange, different structures are developed: for the toluene treated one δ form is preserved (as a result #1 and #2 films were prepared) while the chloroform treated one transforms into the mixed structure of the δ and ε cocrystals (#3 and #4 films could be prepared). For the preparation of ε cocrystal films, we followed the method in the literature.11,12 First the melt quenched amorphous sPS films were dipped in acetone for 3 days in order to obtain γ form sample. After that, the films were soaked in chloroform for 1 week leading to the formation of ε phase (#5 films). In this study, to induce the guest exchange from the initial guest to BzCN, the obtained films were soaked in liquid BzCN, for longer than 3 days at room temperature.30 The completion of the guest exchange process
crystallinity(%) =
Ic × 100 Ic + Ia
(1)
Here, Ic and Ia are integrated intensities of the crystalline and amorphous peaks. In this analysis, the profile of the amorphous halo was assumed to be the same with that of the aPS/BzCN B
DOI: 10.1021/acs.macromol.8b00799 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Here, α is the angle between the chain axis and the transition moment vector of the vibrational mode.32,33 D was obtained as A||/A⊥ with the values of absorbance, A|| and A⊥, measured with polarized incident light parallel and perpendicular to the draw direction, respectively. Since the orientation factor is defined as S = (3 cos2 θ − 1)/2, where θ is the angle between the polymer chain axis and the drawing direction, the perfect or random orientation of the chain axis along the drawing direction corresponds to S = 1 or 0, respectively. The S value of the helical (TTGG) chains of the host sPS in the cocrystalline phase was evaluated by the dichroic ratio of the 571 cm−1 IR band, whose α value is known as ∼0°.34,35 The results for unstretched and stretched sPS/BzCN samples are also summarized in Table 1. For both the δ and ε+δ cocrystals, the uniaxial stretching treatment with the draw ratio of 3.5 increases the degree of orientation to around 0.7−0.8, which is consistent with the literature data.36 The polarized FT-IR spectra in the region of 800−1000 cm−1 are shown in Figure 2a for the stretched δ cocrystal and
Figure 1. Powder X-ray diffraction patterns for finely cut-up sPS/ BzCN and aPS/BzCN films.
as shown in the bottom of Figure 1. The estimated crystallinities of all sPS cocrystal samples are summarized in Table 1. No obvious difference in the degree of crystallinity is seen between stretched and unstretched samples.
Figure 2. Polarized FT-IR spectra in the range of (a) 1000−800 and (b) 2260−2200 cm−1 for stretched sPS/BzCN films of semicrystalline δ phase (#2) and ε+δ phase (#4).
ε+δ cocrystal films. The 935 and 943 cm−1 absorption peaks due to the ordered (TTGG) sPS chains37 exhibit strong dichroisms. On the other hand, the dichroism of the 840 cm−1 peak, which is mainly due to conformationally disordered sPS chain, is weak. As shown in Figure 2b, the 2228 cm−1 peak due to the C N stretching of the guest BzCN exhibits clear dichroism in the stretched δ cocrystal film. Since the transition moment vector is parallel to the long axis of the BzCN molecule, the larger A⊥ than A|| means that the guest BzCN molecules align perpendicularly to the c-axis of the crystalline phase, which is consistent with the result of the orientation direction of nitrobenzene (NB) and p-nitroaniline (pNA) clathrated in the δ cocrystal film.4,7,20,38 On the other hand, no distinct dichroism was observed for the stretched δ+ε cocrystal film. Taking into account that the guest pNA in the ε clathrate phase aligns parallel to the chain axis,7,10,28 it can be considered that the BzCN molecule in the ε cocrystal region also align parallel to the stretching direction. This means that the long axes of BzCN in the δ and ε clathrate regions of the stretched ε+δ cocrystal film are oriented vertically to each other. As a result, it is considered that no clear dichroisim in the 2228 cm−1 band appears.
Table 1. Crystallinity, Degree of Orientation, and the Guest Contents in sPS/BzCN Films orientation degree
guest contents/wt %
system
crystallinity/%
sPS/BzCN unstretched δ (#1) sPS/BzCN stretched δ (#2) sPS/BzCN unstretched ε+δ (#3) sPS/BzCN stretched ε+δ (#4) sPS/BzCN unstretched ε (#5)
43
0
13.8
46
0.82
15.3
38
0
19.5
38
0.72
12.4
40
0
6.0
For the stretched films, the degree of orientation of the chain axis was evaluated with the dichroic ratio D of polarized IR spectra. The orientation factor S of the crystalline region can be estimated by the following equation: S=
D − 1 2 cot2 α + 2 D + 2 2 cot2 α − 1
(2) C
DOI: 10.1021/acs.macromol.8b00799 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Four major ε″ peaks in the sPS/BzCN δ cocrystal film are seen in Figure 3a. According to our previous study,16 the three peaks designated as a1, a2, and Cδ are assignable as follows. (1) The a1 relaxation is due to the cooperative motion of BzCN with the segmental motion of amorphous sPS because this relaxation appears just above the glass transition temperature of the amorphous region. (2) The a2 relaxation is attributable to the local wiggling motion of BzCN in the glassy matrix, as observed in some aPS/small molecule mixtures.40 The a1 and a2 relaxations also appear at the similar temperatures in the aPS/BzCN system as shown in Figure 3d, supporting the validity of these attributions. (3) The Cδ relaxation can be attributed to the motion of BzCN included as the guest in the δ cocrystal region. In addition to these three relaxation modes, there exists a peak at around 395 K in Figure 3a. This peak has different characteristics from those of other three relaxation peaks: the peak temperature is independent of frequency while the peak height became higher at lower frequencies. (This behavior is clearly seen in the low frequency region (
