Polymorphism and Phase Transitions of Precisely Halogen

Article pubs.acs.org/Macromolecules

Polymorphism and Phase Transitions of Precisely HalogenSubstituted Polyethylene. (1) Crystal Structures of Various Crystalline Modifications of Bromine-Substituted Polyethylene on Every 21st Backbone Carbon Masafumi Tasaki,† Hiroko Yamamoto,† Makoto Hanesaka,† Kohji Tashiro,*,† Emine Boz,‡ Kenneth B. Wagener,‡ Carolina Ruiz-Orta,§ and Rufina G. Alamo*,§ †

Department of Future Industry-Oriented Basic Science and Materials, Graduate School of Engineering, Toyota Technological Institute, Tempaku, Nagoya 468-8511, Japan ‡ The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States § Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, 2525 Pottsdamer Street, Tallahassee, Florida 32310-6046, United States ABSTRACT: Detailed crystal structure analysis has been carried out for four crystalline forms (I, I′, II, and high-temperature phase, HT) of uniaxially oriented specimens from a novel polyethylene-like polymer, −[(CH2)20−CHBr]n− on the basis of the 2-dimensional Xray diffraction patterns and polarized FTIR spectral data. This polymer has Br atoms placed regularly on each and every 21st backbone carbon. The precise Br placement along the polyethylene backbone allows drastically different chain conformation and chain packing modes between the group of forms I and I′ and the group of form II and HT phase. In forms I and I′, the molecule is fully extended adopting a planar all-trans zigzag conformation with layers of Br atoms normal to the chain axis. Conformational disorder and mismatch in relative height of Br atom between the neighboring chains distinguish form I from form I′. In forms II and HT phase, the molecular chains bend at the Br substitutional point and take a large zigzag form consisting of long methylene segmental arms. The molecular bends are caused by the generation of nonplanar gauche conformers at the C−C bonds adjacent to the CHBr groups, while the CH2 segments maintain the all-trans conformation. The major difference between form II and HT is conformational disorder within the methylene runs. Heating at T < 65 °C under unrestrained condition causes an irreversible transition from form I′ to form I, and form I transforms irreversibly to form II in a narrow temperature range of 65−66 °C. The higher temperature heating induces the reversible and apparently continuous transition of form II to the HT phase. On the other hand, the tensile stretching at room temperature causes the irreversible transition of forms I and II to the form I′.

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polyethylenes have a general repeating unit, −[(CH2)m‑1− CHX]n−, where m is 9−21 and X is either a halogen, a methyl group, an ethyl group, or groups of various types.11−13 These polymers are named hereafter PEmX (for example, PE21Br for m = 21 and X = Br). The physical properties of these polyethylenes with pendant groups placed at a precise distance in the backbone are affected in a different manner depending on the type of the side group X and the length of methylene sequence (m − 1). Alamo et al. reported X-ray diffraction and Raman spectra of a series of melt-crystallized unoriented samples of PEmCl with m = 9−21.14 The crystalline properties of the samples cooled from the melt at ∼2 °C/min were compared with those of analogues with a random distribution

INTRODUCTION

Polyethylene [PE, −(CH2CH2)n−], is one of the most widely produced and studied polymers due to its versatility in a very wide range of applications. The properties of polyethylenes can be modified by random copolymerization with 1-alkene or with polar comonomers with drastic effects on physical properties even at relatively low level of comonomer incorporation.1−8 Coupled with the inherent molar mass distribution, the synthesis of random copolymers leads to the products with a bivariate comonomer content-molar mass distribution, that is often difficult to control even for copolymers synthesized with single-site metallocene-based catalysts. Accordingly, the properties of copolymers are also difficult to reproduce at a high level of consistency.9,10 In recent years, Wagener et al. have synthesized new types of PE derivatives, which contain pendant groups, equivalent to those of comonomer units, placed at a constant distance along the PE skeletal chain. The novel © 2014 American Chemical Society

Received: May 17, 2014 Revised: June 18, 2014 Published: June 30, 2014 4738

dx.doi.org/10.1021/ma5009622 | Macromolecules 2014, 47, 4738−4749

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the bromine atoms have much stronger X-ray scattering power compared with the methylene units, giving characteristic 2-D Xray diffraction patterns that are inherently different from those of linear PE. Another advantage of choosing this polymer is that it possesses long methylene segments −(CH2)20− separated by bulk CHBr groups placed at a precise constant backbone length. The long methylene segments are useful for a detailed analysis of the infrared spectra with focus on characteristic progression modes. The progression modes of PE21Br can be contrasted with well-studied vibrational modes of n-alkanes to extract their chain conformations.22−24 Here, we report the crystal structures of four polymorphs of PE21Br extracted via the quantitative analyses of 2-D X-ray diffraction patterns and infrared spectral data. In subsequent papers, more details will be provided on the crystal phase transitions for unoriented and oriented specimens on this and other members of the precision series on the bases of the present work.

of chlorine. The X-ray powder diffraction profiles differed depending on the number m as well as the chlorine distribution, indicating the formation of different crystal modifications for the series. In our more recent work, the isothermal crystallization was studied in a wide range of undercooling for one member of the precision series, PE15Cl.15 Two polymorphic phases were found within a small change in undercooling on the basis of the X-ray powder diffraction and FTIR data analyses. The relatively fast cooling of the melt induces the crystallization to form I. The structural model proposed is the packing structure of the all-trans chains. Conversely, the crystals formed at higher temperatures were proposed to take the packing structure of the nonplanar herringbone-like chains with gauche conformers of the type TGGT···TGG ̅ ̅ T around the halogen substitution, while conserving the all-trans packing of the methylene sequence (form II).15 However, it must be noticed that only for the experimental data collected for the unoriented PE15Cl sample was it difficult to derive the concrete structural models proposed in the previous paper, but the basic knowledge obtained from the structure analyses of the oriented PE21Br samples was helpful, as will be reported in the present paper. In other words, the models proposed in the previous paper are still ambiguous from the crystallographic point of view, because only the X-ray powder diffraction data of the unoriented sample were used in the discussion: for example, the relation between the chain axis and the crystallographic c axis is not clear in the structure model of form I, and the details of the herringbonetype structure of form II are also not clarified concretely. In this way, these structural models proposed in the previous paper15 need to be checked and refined by more quantitative analysis of the 2-D X-ray diffraction data collected for the oriented sample specimens. Besides, the usage of oriented samples helped us to find out more complex phase transitions than initially anticipated from the unoriented sample data. A similar situation can be detected also in the structural studies about the other types of PEmX with methyl, ethyl, carboxylic acid, etc as side group X.16−21 These polymers were found to show the various crystalline phases depending on the side group X as clarified by the X-ray diffraction, electron diffraction, and so on. But, the details of the chain conformation and chain packing mode were not analyzed quantitatively and concretely at all. The aim of the present paper is to describe the results of the crystal structure analyses on the basis of X-ray diffraction and polarized FTIR spectral data analysis which are powerful and complementary for a complete understanding of crystal structures. The knowledge of crystal structure of the various crystalline forms is indispensable for the deep interpretation of the complicated phase transition behaviors of these crystal modifications from the molecular level. In the present paper, PE21Br is chosen here as a challengeable candidate. Although a series of PEmX samples is available, PE21Br sample was able to be highly drawn and to produce the oriented specimens that give the 2-D X-ray diffraction patterns indispensable for the detailed structural analysis. We have found that PE21Br can adopt four types of crystalline modifications (forms I, I′, and II and a hightemperature phase, HT). Given the homopolymer-like crystallization of PEmBr12 and the large size of the Br atom, the Br atoms incorporated in the crystal are expected to affect the torsional angles of PE backbone bonds close to the halogen as well as the self-assembly of chains of this nature. In addition,

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EXPERIMENTAL SECTION

Samples. PE21Br was used as a bromine precisely substituted sample, which belongs to the special families recently synthesized via acyclic diene methatesis polymerization (ADMET).13 A characteristic of ADMET is the inability to control tacticity of the −CHBr during the polycondensation reaction. The result is an atactic crystalline homopolymer with the repeating unit −[(CH2)20−CHBr]n−. The molecular weight was determined by GPC using a Waters GPCV 2K instrument and Waters Styragel 1 HR 5E column at 40 °C with HPLC grade THF and a calibration with polystyrene standards (Mw = 94 100 g/mol, Mw/Mn = 2.2). As mentioned in the introduction, this polymer exhibits four crystalline forms depending on the sample preparation conditions. Details of the preparation of each crystalline form will be described in a later section. Measurements. 2-D X-ray diffraction patterns of uniaxially oriented samples were measured using Rigaku X-ray diffractometers R-axis VII. An imaging plate (IP) detector of flat IP detector was used for R-axis VII at a camera-sample distance of 150.0 mm. The incident X-ray beam was a graphite-monochromatized Cu Kα line with a wavelength of 1.5418 Å. A homemade temperature controlled heating stage was used to collect, at ∼68 °C, the 2-D X-ray diffraction pattern of the high-temperature phase. Polarized FTIR spectra were collected at room temperature for the oriented films of about 20 μm thickness using a Varian FTS 7000 Fourier-transform infrared spectrophotometer equipped with a wiregrid polarizer at a resolution power of 2 cm−1. The temperature dependence of FTIR spectra was measured using a Linkam FTIR600 hot stage with KBr windows. The oriented film was placed between a pair of KBr plates and positioned at the Linkam aperture.

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RESULTS AND DISCUSSION 1. Formation of Four Crystalline Modifications. In the interest of clarity, and to facilitate discussion of differences between four polymorphs that we have identified for PE21Br, we start with a brief summary of the drawing and thermal conditions that lead to each of the crystal forms. The schematic diagram for the transition of forms I, I′, and II and the HT phase is given in Figure 1. Analogous to the case of PE15Cl,15 a rapid quenching from the melt gives unoriented form I at room temperature, while slow cooling at 1 °C/min leads to form II. Stretching form I at room temperature gives the oriented form I′. Heating from room temperature, the unrestrained form I′ transforms to oriented form I at about 60 °C and on further heating to form II in a narrow temperature range from 65−66 °C. Finally, form II transforms continuously to a high temperature phase (HT) at T > 66 °C before melting at 70− 4739


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Figure 1. Schematic profile of uniaxial deformation and temperature paths, describing the formation of four crystal modifications of PE21Br.

75 °C. (More strictly speaking, the form II can be obtained by relatively rapidly heating the oriented form I′ (and form I) sample in a narrow temperature region of 65−66 °C under tension free condition, whereas the slow heating in the same temperature region gives the HT phase or the mixture of form II and HT phase, as shown in Figure 1.) The form I′ heated under constrained condition did not transform to form II and HT phase, but melted at a high temperature of 70−75 °C. The form II that appears in a narrow temperature region during continuous heating can be frozen by cooling to room temperature. The HT phase observed at T > 66 °C transforms to the oriented form II on slow cooling to room temperature. When rapidly cooled, the HT phase can be kept even at room temperature. Under uniaxial deformation at room temperature, forms I and II (and the HT phase) transform irreversibly to the oriented form I′. In summary, forms I and I′ transform irreversibly to form II and to the HT phase on heating under tension free condition, while the transformation from form II to HT phase occurs reversibly and continuously, and the uniaxial stretching causes the irreversible transition of forms II and I to form I′. A comparative summary of 2-D X-ray diffraction patterns for the four crystalline modifications of PE21Br is given in Figure 2 in conjunction with the pattern of an oriented linear polyethylene (PE) sample. The chain conformation and chain packing mode of these crystalline modifications are extracted from these 2-D X-ray diffraction patterns, as described in the next sections. 2. Structure of Crystalline Form I′. The 2-D X-ray diffraction pattern of form I′ shown in Figure 2a suggests a triclinic structure. The reflections observed along the offmeridional line are typical for the triclinic cell and can be assigned to 00l (l = 1, 2, 3, ...), indicating that the c* axis of the reciprocal lattice is tilted from the draw direction or from the chain axis (c axis). From the interlayer spacings between the equatorial line and the layer lines, the repeating period along the chain axis was evaluated to be c = 53.2 Å based on the socalled Polanyi’s equation.25,26 This value is in good agreement with the repeating period calculated for planar-zigzag chain conformation, 53.6 Å with C−C bond length 1.54 Å and CCC bond angle 110.0°. Hence the molecular chain of form I′ is considered to take essentially a planar-zigzag conformation. The X-ray data analysis was performed following procedures described in the previous papers.27,28 The (x, y) coordinates of the observed reflections were read from the 2-D X-ray

Figure 2. 2-D X-ray diffraction patterns observed for oriented PE21Br samples. (a) Form I′, (b) form I with a small amount of form I′, (c) form II, (d) the HT phase, and (e) linear polyethylene. The contrast of the central area of each diffraction pattern was decreased for easier identification of diffraction spots in this area.

diffraction pattern, and converted to the corresponding values for the cylindrical (ξ, ζ) coordinates of the reciprocal lattice.25,26 The unit cell parameters were estimated by indexing all the observed reflections using the ξ values. The thusdetermined reciprocal unit cell parameters are a* = 0.237 Å−1, b* = 0.237 Å−1, c* = 0.028 Å−1, α* = 137.3°, β* = 111.7° and γ* = 61.9°. The corresponding real unit cell parameters are listed in Table 1. The space group is P1̅. The agreement Table 1. Unit Cell Parameters of the Various Crystalline Forms of PE21Br crystal form

a/Å

b/Å

c/Å

α/deg

β/deg

γ/deg

I I′ II HT

4.90 4.79 5.00 4.40

5.75 6.55 5.65 5.70

52.7 53.2 47.0 41.5

43.2 46.8 77.5 90.0

109.8 87.7 112.0 90.0

107.9 108.4 65.0 65.0

between observed and calculated lattice spacings using the unit cell parameters of Table 1 is quite reasonable, as shown in Table 2. Judging from the a- and b-axial sizes, one all-trans chain is included in the triclinic unit cell. The parameters to be determined were the setting angle of the all-trans chain, the isotropic temperature factors of the C and Br atoms, and the scale factor of the diffraction intensity. These parameters were changed on a trial-and-error basis until the calculated structure factor |Fcalc| closely matched the value of the observed structure 4740


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Table 2. Comparison between the Observed and Calculated Lattice Spacings and Structure Factors of PE21Br Crystal Form I′ a

a

Nonobserved reflection, the structure factor of which was assumed to be 10.

Table 3. Atomic Fractional Coordinates of PE21Br Crystal Form I′ a

a

atom

x

y

z

atom

x

y

z

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12

0.078 −0.083 0.082 −0.080 0.086 −0.076 0.090 −0.072 0.094 −0.120 0.035 −0.167

−0.006 0.037 0.023 0.066 0.052 0.095 0.081 0.124 0.110 0.041 −0.001 −0.076

0.013 0.033 0.058 0.078 0.103 0.123 0.148 0.169 0.193 0.223 0.250 0.281

C13 C14 C15 C16 C17 C18 C19 C20 C21 Br1 Br2

0.005 −0.186 0.022 −0.169 0.039 −0.152 0.056 −0.135 0.073 0.097 0.435

−0.109 −0.155 −0.079 −0.125 −0.049 −0.095 −0.019 −0.065 0.011 −0.370 0.377

0.306 0.335 0.351 0.380 0.396 0.425 0.441 0.470 0.486 0.275 0.223

Isotropic temperature factors of C and Br atoms are 6 and 10 Å2, respectively. The occupancy of Br atom is 0.5.

factors |Fobs| The |Fobs|; values were evaluated from the equation Iobs = KALPm|Fobs|2, where Iobs is the observed diffraction intensity, and L, P, A, and m are the Lorentz factor, the polarization factor, the absorption factor (neglected) and the multiplicity of reflection, respectively.25,26 K is a scale factor. The structural model thus obtained was further refined using a home-written constrained least-squares program. The variables are the torsional angles of the skeletal chain, in particular those in the vicinity of CHBr groups, while the bond lengths and bond angles remain fixed at the standard values.29,30 Values for the observed and calculated structure factors are listed comparatively in Table 2. The reliability factor (R), a measure of agreement between the observed (|Fobs|) and calculated (| Fcalc|) structure factors, was computed as R=

Here the summation spans over all the observed diffraction spots. The R factor after refinement was 15.2% for the 25 observed reflections. The fractional coordinates of all backbone carbons of the repeating unit are listed in Table 3, and the crystal structure of PE21Br form I′ is given in Figure 3. Notice that to account for the atactic nature of the chain, the Br atoms are placed on a methylene carbon at a 0.5 statistical probability. The best values for torsional angles are appended to the PE21Br structure below. Values of torsional angles around the Br substitution deviate from the ideal trans value (180°) due to a steric effect caused by bulky Br atom compared with H atom size.

2.1. Introduction of Structural Disorder. The unit cell and chain packing mode obtained for form I′ (Figure 3) are

∑ ||Fobs| − |Fcalc||/∑ |Fobs| 4741


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Figure 3. Crystal structure of PE21Br form I′ (model A), viewed from two different projections.

acceptable judged by a relatively low R factor. However, the structural model shown in Figure 3 gave the calculated 2-D Xray diffraction pattern given in Figure 4b, which did not very

well reproduce the observed pattern (Figure 4a), where the simulation of the diffraction pattern was carried out using the Cerius2 commercial software by Accelrys, USA. Some of the layer line reflections are stronger than the observed ones, and the characteristic diffuse scattering along the horizontal reflections in the observed X-ray diffraction pattern need to be accounted for. As is well-known, such horizontal diffuse lines may originate from disorder in the relative height of the neighboring chains.25,26,31,32 On this basis, the several possible models are made to explain such a disordered structure. These models are given in Figure 5. Model A is the initially constructed model for form I′, shown in Figure 3 and reproduced in Figure 5a, that did not quite reproduce the observed streak lines (Figure 4). In Model B, the molecular chains are displaced randomly along the chain axis (Figure 5b). In an extreme case, the relative height of the neighboring chains does not have any correlation with each other and the X-ray diffraction pattern is equivalent to that of an isolated chain. The simulated 2-D pattern for this model, Figure 4c, contains multiple streaks along the horizontal lines, while in the observed pattern, the Bragg reflection spots overlap with the streak lines. Consequently, model B was ruled out. Model C (Figure 5c) maximizes the packing of the methylene sequences between bromines by a longitudinal shift of one carbon, instead of a random shift of molecular chains along the c axis, as for model B. The methylene segments form the domain of a regular triclinic subcell structure, as for model A. However, while in model A the Br atoms orient in the same direction along the a′ axis forming a

Figure 4. 2-D X-ray diffraction patterns for oriented form I′: (a) observed pattern, (b) calculated pattern for the structure shown in Figure 3 (model A), (c) calculated pattern for an isolated all-trans chain, basically identical to that of a model constructed by a random shift of neighboring chains (model B), and (d) diffraction pattern calculated for model C (refer to Figure 5). 4742


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pattern for model C reproduces closely the characteristic features of the observed X-ray diffraction pattern. 2.2. Chain Conformation Supported by Infrared Spectra. Infrared spectra provide additional evidence in support of the chain conformations of the various crystalline forms. Figure 6

Figure 6. Polarized infrared spectra of oriented forms I, I′, and II and the HT phase. The solid and broken lines indicate, respectively, the infrared spectral components measured with the incident infrared electric vector perpendicular and parallel to the draw direction. The roman numbers are the k values of the phase angles predicted for the methylene progression bands (refer to the text).

Figure 5. Disordered chain packing models for PE21Br form I′. (a) Model A (see Figure 3), (b) model B constructed by a random shift of neighboring chains, and (c) model C, in which the arbitrarily chosen molecular chains are rotated by 180° around the c axis followed by a 1.25 Å (one CH2 unit) shift along the c axis (refer to the scheme given to the right of model C).

shows the polarized spectra of oriented forms I′, I, II, and HT phases in the two frequency regions. Following vibrational spectroscopic analysis of model alkyl bromide compounds in the C−Br bond stretching mode region, the Br atom bonded to a trans CCC sequence displays a strong IR absorption peak at 540−535 cm−1, while if the CCC sequence is in a gauche conformation, the absorption occurs in the 620−605 cm−1 region.37,38 As seen in the spectrum of form I′, only the C−Br stretching band at 535 cm−1 is observed, supporting the alltrans conformation around the C−Br moiety, as earlier revealed by the X-ray structure analysis. The infrared bands observed for form I′ in the frequency region of 730−1350 cm−1 are associated with progression modes of the all-trans methylene segments of finite length.22−24 The conclusion from the X-ray analysis that the sequence −(CH2)20− in form I′ exists in all-trans conformation can be corroborated by the equivalence of progression modes to the nalkane cases. In the coupled oscillators model,22−24,39 1dimensionally arrayed all-trans methylene units are assumed to form a finite chain of oscillators, where each oscillator is

symmetric layer between the methylene segment domains, in model C the Br atoms orient randomly right and left along the a′ axis, as illustrated in Figure 5c. This model can be constructed from model A by rotating randomly chosen chains 180° around the c axis. A simple rotation would cause close face-to-face contact between Br atoms. This is avoided by a shift of one CH2 unit along the chain axis, as shown schematically in the lower right drawings of Figure 5. The result is a normal all-trans packing of the methylene units similarly to the triclinic phase of n-alkane crystals.33−36 The 2-D X-ray diffraction pattern was calculated for model C, with 196 all-trans chains included in a large cell and the Br atomic positions were displaced following the above-mentioned arrangement pattern. Figure 4d shows the 2-D pattern calculated with Cerius2, using a lattice size of 150 × 150 × 150 Å3, a lattice strain 0.5 × 0.5 × 1.0 % and a degree of misalignment of the c-axis 2°. The calculated X-ray diffraction 4743


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in Figure 7a contains some amount of form I′, but distinct Xray reflections of form I can be easily distinguished. The repeating period along the chain axis is 52.7 Å, a little shorter than that of form I′ (53.2 Å). The heat treatment (T < 65 °C) or thermal relaxation induces a transition from the highly tensioned form I′ to form I, consistent with a slight difference in the fiber period between these two crystal forms. Since the total number of observed X-ray diffraction spots was small (Figure 7a), it was difficult to determine the unit cell parameters uniquely. Hence, an indirect approach was used to elucidate the crystal structure. In calculating the 2-D pattern of form I, the structure of form I′ was modified so as to closely reproduce the observed X-ray diffraction pattern of form I. The thus estimated unit cell parameters are listed in Table 1. Similar to form I′, planar all-trans chains are packed in a triclinic unit cell. The X-ray diffraction diagrams calculated for forms I and I′ and their combined 50/50 diagram (Figure 7b) indicate a relatively good correspondence to the observed pattern of form I (Figure 7a). As shown in Figure 8, the Br atoms are packed in

represented by one methylene unit, for example. These oscillators are coupled due to covalent bonding of the CH2 units. The observed infrared progression bands can be identified in reference to the vibrational frequency-phase angle dispersion curves of n-alkane crystals.22−24,40,41 The spectroscopically active vibrational modes are limited to the positions on the dispersion curves at phase angles δk between neighboring methylene units, given as δk = kπ/(m + 1). k is the mode order (1, 2, 3, 4, ...), and m the number of contiguous methylene units that are effective as coupled oscillators. In other words, the observed frequencies can be assigned to vibrational modes (k = 1, 2, 3, ...) by identifying the proper number of methylene units (m) that are effective as oscillators in the all-trans-zigzag sequence. In the analysis of IR progression bands of a polymer chain with long methylene sequence, it must be noted that number of methylene units to form effectively coupled oscillator sequences is not necessarily equal to p given in the chemical formula −[(CH2)p−X]n−. For example, in the interpretation of a series of progression bands observed for aliphatic nylon mn, −[NH(CH2)mNHCO(CH2)n‑2CO]n−, the effective all-trans methylene segments were found to be (CH2)m‑2 and (CH2)n‑4, indicating that the methylenes adjacent to the amide and to the carboxylic group do not couple with the inner methylene segments.42−44 Similarly, for PE21Br form I′, the observed progression bands can be reasonably assigned assuming m = 18, as shown with the corresponding k values in Figure 6.22−24,39 The effective number of methylene oscillators in PE21Br is 18, indicating that the vibration of one methylene unit adjacent to the halogen substitution is not coupled with the effective inner methylene oscillators because of some structural disorder around the methine environment. A similar situation was found also in the case of PE15Cl, where m = 12 was effective and the methylene units of both ends are not included in the coupled oscillations of the long methylene segment.15 3. Crystal Structure of Form I. Figures 2b and 7a show the observed X-ray diffraction diagrams of oriented form I. The preparation of highly oriented and pure form I was quite difficult because this form transforms easily to form I′ under uniaxial deformation. The 2-D X-ray diffraction pattern shown

Figure 8. (a) Crystal structure model of PE21Br form I crystal obtained by energy minimization, which gives the X-ray diffraction pattern shown in Figure 7b, and (b) the crystal structure of form I′. The c-axial translational disorder found for form I′ may be existent also for form I as judged from the streak lines in the 2-D X-ray diffraction pattern (Figure 2b). However, these structural disorders are not taken into consideration in this figure in order to show the difference in the characteristic chain packing mode between these two crystalline forms clearly.

layers, similarly to the case of form I′, but the relative height of Br atoms between the neighboring chains is different between these two forms. It must be noticed here that the crystal structure of form I′ has the statistical c-axial translational disorder of one methylene unit to avoid the repulsive force between the Br atoms of the neighboring chains, as already mentioned in section 2.1. The similar disorder may be existent

Figure 7. (a) Observed 2-D X-ray diffraction pattern of oriented form I crystal, which contains a small amount of form I′ crystal (refer to Figure 8). (b) Calculated X-ray diffraction patterns of forms I and I′ and their 50/50 combined pattern. 4744


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all-trans methylene segments with gauche-type linkages around the CHBr group moiety. A plausible conformational model is given in Figure 10a, which can be represented as

also in the crystal structure of form I as known from the horizontal streak lines in the 2-D X-ray diffraction pattern (Figure 2b). These structural disorders are not taken into consideration in Figure 8 in order to show the difference in the characteristic chain packing mode between these two crystalline forms clearly. The infrared spectrum of form I is analogous to the spectrum of form I′, as seen in Figure 6, but with some difference in the relative intensity of some of the progression bands. Only progression bands with odd k values were detected in the infrared spectra of form I′ crystal as expected, while in the case of form I the progression bands of even and odd k values are detected. This difference suggests that the long methylene segments are packed with a relatively lower symmetry in form I crystal compared with form I′. For example, a slightly contracted conformation due to a slight deviation from the regular all-trans form would explain the appearance of forbidden (even) modes in form I,45 this also in agreement with a slightly contracted chain conformation as derived from the X-ray diffraction analysis. 4. Crystal Structure of Form II. As mentioned in the early section, the oriented crystalline form II is obtained by heating the oriented form I (and I′) in the narrow temperature region of 65−66 °C. Annealing at a little higher temperature induces an easy transition to the HT phase. Since the temperature region where form II is enabled is quite narrow in this way, the preparation of pure form II necessary for structural analysis, was performed by monitoring the X-ray diffraction patterns measured stepwise during heating of the oriented form I′. When the X-ray diffraction pattern of form II was detected, the sample was quickly quenched to room temperature. The X-ray diffraction pattern of form II taken at room temperature is shown in Figures 2c and 9a. The repeating period along the

Figure 10. Crystal structures of (a) PE21Br crystal form II and (b) the HT phase. The methylene runs have conformational disorder in the HT phase cell.

The repeating period calculated for this model is ca. 46 Å, close to the observed value of 47.0 Å, and the unit cell parameters of the triclinic lattice of form II are listed in Table 1. The lattice energy was minimized for this model using Cerius2 with COMPASS force field as the potential functions.46 Among the candidates extracted in these procedures, the best structural model at the present stage is shown in Figure 10a. The calculated X-ray diffraction diagram is shown in Figure 9b. Compared with the observed diffractogram given in Figure 9a, the agreement is fairly reasonable, although further refinement may be needed, in particular the refinement of the torsional angles of the skeletal chain. The atomic fractional coordinates are listed in Table 4. 5. Crystal Structure of High-Temperature Phase. As already mentioned, oriented form I and form I′ transform to oriented form II on heating in a narrow temperature window of 65−66 °C. With a further increase in temperature, the pattern of form II changes easily to the HT phase. The whole X-ray diffraction pattern of HT is similar to that of form II, but the fiber repeating period is shorter, about 41.5 Å, compared with 47.0 Å of form II. The most remarkable difference in diffraction pattern between these two forms is seen in the 00l reflections (see for example, Figure 2, parts c and d). In form II, these reflections are located on lines tilted about 30° from the meridional line, but they are along the meridional line in the HT phase (Figure 11a). This characteristic X-ray diffraction pattern of the HT phase can be reproduced reasonably by changing the angles α and β in the unit cell of form II to 90°, i.e., by the translational shift of neighboring molecular chains along the chain axis. The X-ray diffraction pattern calculated for the HT phase model, after energy minimization, is shown in Figure 10b, which is in relatively good correspondence to the observed one. The atomic fractional coordinates are shown in

Figure 9. (a) Observed and (b) calculated X-ray diffraction patterns of oriented PE21Br form II.

chain axis is 47.0 Å, a value appreciably shorter than those of forms I and I′, suggesting that the molecular chain must contain gauche CC skeletal bonds. The polarized infrared spectra shown in Figure 6 support the nonplanar characteristics of the form II chain conformation. The C−Br stretching band is detected at 613 cm−1, indicating the existence of gauche CC bonds in the vicinity of CHBr groups.15,37,38 The methylene progression bands observed in Figure 6 are located at frequencies close to those predicted for all-trans methylene segments of forms I and I′. In fact, the observed methylene progression bands can be also assigned reasonably using the effective trans sequence of −(CH2)18−. (A deviation of the predicted frequency for the higher mode orders of forms I, I′ and II is explained by the slight change in torsional angles as pointed out earlier.) We hence conclude that the chain conformation of form II consists of a combination of long 4745


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Table 4. Atomic Fractional Coordinates of PE21Br Crystal Form IIa

Table 5. Atomic Fractional Coordinates of PE21Br HT Phasea

atom

x

y

z

atom

x

y

z

atom

x

y

z

atom

x

y

z

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 Br1 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21

0.637 0.366 0.509 0.248 0.407 0.158 0.333 0.096 0.288 0.079 0.334 0.079 0.250 0.044 0.289 0.105 0.338 0.152 0.398 0.221 0.482 0.311

0.470 0.526 0.432 0.461 0.340 0.352 0.197 0.174 −0.023 −0.099 −0.330 −0.683 −0.473 −0.300 −0.327 −0.110 −0.181 0.046 −0.005 0.231 0.188 0.425

0.009 0.020 0.058 0.070 0.109 0.121 0.160 0.172 0.211 0.224 0.261 0.266 0.285 0.299 0.333 0.344 0.380 0.390 0.426 0.435 0.468 0.478

C22 C23 C24 C25 C26 C27 C28 C29 C30 C31 C32 C33 Br2 C34 C35 C36 C37 C38 C39 C40 C41 C42

0.583 0.420 0.693 0.522 0.783 0.605 0.851 0.665 0.898 0.714 0.960 0.754 0.925 0.670 0.925 0.716 0.908 0.671 0.846 0.597 0.755 0.495

0.380 0.617 0.572 0.808 0.765 1.001 0.950 1.177 1.106 1.323 1.296 1.470 1.680 1.326 1.095 1.020 0.823 0.799 0.644 0.656 0.536 0.564

0.510 0.519 0.551 0.561 0.594 0.603 0.639 0.649 0.685 0.696 0.730 0.744 0.762 0.768 0.805 0.818 0.856 0.869 0.908 0.920 0.959 0.971

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 Br1 Br2 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21

0.025 −0.271 −0.340 −0.673 −0.687 −1.030 −1.033 −1.369 −1.373 −1.718 −1.815 −1.869 −2.273 −1.593 −1.777 −1.204 −1.564 −1.022 −0.926 −0.662 −0.492 −0.218 −0.045

−0.380 0.372 0.513 0.552 0.636 0.835 0.898 1.104 1.132 1.323 1.391 1.708 1.443 1.266 1.252 0.978 1.180 0.962 0.717 0.700 0.444 0.444 0.195

0.005 0.013 0.046 0.061 0.096 0.107 0.143 0.155 0.191 0.203 0.240 0.242 0.246 0.270 0.302 0.330 0.332 0.362 0.381 0.406 0.424 0.447 0.466

C22 C23 C24 C25 C26 C27 C28 C29 C30 C31 Br3 C32 Br4 C33 C34 C35 C36 C37 C38 C39 C40 C41 C42

0.216 0.387 0.596 0.772 0.946 1.095 1.290 1.421 1.649 1.520 2.070 1.706 1.389 1.836 1.593 1.678 1.345 1.362 1.054 0.915 0.641 0.471 0.202

0.206 −0.040 −0.015 −0.271 −0.260 −0.524 −0.534 −0.797 −0.821 −0.914 −1.359 −1.080 −1.198 −0.987 −0.897 −0.755 −0.594 −0.445 −0.383 −0.125 −0.110 0.143 0.136

0.489 0.509 0.537 0.555 0.586 0.602 0.633 0.649 0.678 0.708 0.717 0.737 0.753 0.767 0.796 0.823 0.841 0.871 0.892 0.910 0.933 0.952 0.975

a

Occupancies and isotropic temperature factors of all the atoms (C and Br) are 1.0 and 5.0 Å2, respectively.

a

Occupancies of C and Br atoms are 1.0 and 0.5, respectively. The isotropic temperature factors of all the atoms are 10 Å2.

Figure 11. Comparison between (a) observed and (b) calculated 2-D X-ray diffraction pattern of the HT phase.

Table 5. Different from the relatively regular conformation of form II, the HT phase adopts conformational disorder more or less, and while maintaining a nonplanar conformation, the HT unit cell conforms with a hexagonal packing. In fact, the infrared spectrum of the HT phase is as a whole similar to that of form II (Figure 6), but the relative intensities are much weaker in the HT phase, also suggesting conformational disorder of the all-trans methylene sequences prior to melting. The shorter repeating period along the chain axis is consistent with the infrared data. 6. Summary of Crystal Phases and Polymorphic Transitions. Figure 12 summarizes schematically the crystal structures of the 4 crystalline forms of PE21Br revealed in the present study. Form I′ adopts a triclinic unit cell consisting of planar all-trans chains with slight disorder around the CHBr moiety. The crystal has a structural disorder in the Br atom layers. Annealing at T ≤ 65 °C an unrestrained oriented form I′ transforms to form I by a translational shift of parallel chains along the c axis, such that the layer structure of Br atoms becomes tilted with respect to the chain axis. The planar all-

Figure 12. Schematic illustration of phase transitions of the various crystalline phases of PE21Br. As seen in Figure 1, the transition from form I (I′) to form II and to the HT phase is dependent on the slight change in heating rate under tension free condition. In this figure the relatively rapid heating case is shown as a typical case.

trans backbone conformation of I′ is maintained in form I although the degree of structural disorder is different between them. Upon heating at T ≥ 65 °C, forms I and I′ transform to form II and further to the HT phase. More strictly, the rapid heating causes the transition from forms I and I′ to pure form II, while relatively slow heating gives the HT phase or the mixture of form II and HT phase depending on the heating rate. In form II crystallites, the molecular chains adopt a highly contracted conformation of long-arm zigzag form in stark contrast to the usual extended zigzag chain. The chain contraction is induced by gauche-type conformations in the backbone bonds adjacent to the CHBr moiety, while the conformation of the main 4746


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methylene segments remains in the all-trans form. This remarkable conformational change from form I (or form I′) to form II might evolve via melting of form I followed by a fast recrystallization to form II. Although the data is not shown here, the DSC thermogram measured in the heating process indicates this possibility: a first endotherm at ∼63 °C associated with melting of form I is followed by a recrystallization exotherm (∼65 °C) and this is immediately followed by a second endotherm at ∼72 °C corresponding to melting of form II. With increasing heating rate, the exotherm and high temperature endotherm are diminished as recrystallization is impeded. The similar situation may occur also for the oriented sample case. Since the melt-recrystallization upon heating of forms I and I′ seems to be a very fast process, the chain orientation is preserved through the transition to form II, as seen in the 2-D X-ray diffraction patterns and the polarized infrared spectra (Figures 2 and 6). There are other examples of phase transitions in which the chain orientation is also maintained during melt and recrystallization on heating: the typical cases are about poly(vinylidene fluoride) (PVDF) crystal form I and 1,4-trans-polyisoprene.47−50 For example, the oriented PVDF form I was reported to show a solid−solid transformation to form III (or V) 49,50 upon heating immediately below the melting point. But, this conclusion was not settled by additional experimental data at that time, and a possibility of melt-recrystallization was suggested also.51,52 Recently, using a combination of simultaneously measured Xray diffraction and infrared spectral data during isothermal heating process, the melting of PVDF form I was evident for the first time by the appearance of the amorphous phase followed by recrystallization in form III.47 The similar isothermal experiment might support the concept of the melt of form I followed by the recrystallization of form II in the case of PE21Br also. Form II crystal of PE21Br adopts a large zigzag-type structure with long trans methylene runs, which transforms continuously to the HT phase at T > 66 °C while keeping the nonplanar conformation and allowing some disorder in the methylene runs between bromines. The disordered HT phase transforms to the regular form II on slow cooling. The transformation between form II and HT phases is reversible. Uniaxial stretching of oriented form II at room temperature causes irreversible transformation to form I′, and hence, into an all-trans form through a gauche-to-trans conformational exchange. This type of transition between fully extended conformation and appreciably contracted zigzag-type conformation through the T-G exchange is known also for the oriented poly(ethylene oxybenzoate) α and β forms.53,54 As seen in the crystal structures extracted from the 2-D X-ray diffraction patterns and FTIR analysis of the four crystal modifications of PE21Br in Figure 12, we notice that the methylene runs form the layer structures and the bromines are located at the layer boundaries. In all forms the interchain packing of methylene runs is equivalent to the packing of nalkane crystals.34−36 Moreover, molecularly, the n-alkane chains display different aggregation modes. As illustrated in Figure 13, the all-trans chains are packed with their chain axes either parallel to the c axis or heavily tilted from the c axis. In the latter case, the so-called polytypism is observed, where layers of tilted chains are stacked in the same direction or they alternate between opposite directions due to interlayer interactions between the methyl end groups.55−57 In the case of crystal forms I and I′ of PE21Br, the all-trans chains are oriented along

Figure 13. Comparison of layered structural models between n-alkanes (a−c) and PE21Br (d and e). In the case of n-alkanes, model a is an orthorhombic type, and models b and c are shown in reference to polytypism of layer structures. Open circles are methyl end groups. In the case of PE21Br, model d is for forms I and I′ and model e is for form II and HT phase, where the CHBr group is represented as an open circle.

the c axis, the layer boundary including the Br atoms is normal to the chain axis or slightly tilted from the chain axis, respectively. These structures are similar to the case (a) of nalkanes. (It must be noted here that the structural model of PE15Cl form I proposed in the previous paper15 looks similar to the structure (b) of n-alkane but it should be compared with the structure (a) of n-alkanes, just likely the case of PE21Br form I and form I′.) On the other hand, in form II and HT phase, layers of methylene segments zigzag in and out of a plane normal to the layer boundary formed by the bromines, similar to the case (c) of n-alkanes. The gauche conformations of carbons in the vicinity of CHBr groups generate the heavy tilt between methylene layers. In the n-alkane case, however, the interactions between the methyl end groups governs the tilting of alkane chains.57 It must be pointed out in the end of discussion that the transition behaviors are more or less different depending on the side group X and methylene segmental length m in PEmX polymer as reported in the literatures.16−21 The transition scheme given in Figure 1 is not necessarily universal to all the PEmX samples reported so far, but it is specialized for PE21Br sample. However, the structural information revealed in the present paper should be a basic and quite important information for the structural studies of the variously different types of PEmX polymers. 4747


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CONCLUSIONS In the present paper the crystal structures of PE21Br have been determined quantitatively for form I′ and semiquantitatively for forms I and II and the HT phase on the basis of observed 2-D X-ray diffraction patterns and infrared spectra, as well as by computer simulation method. As long as the unoriented samples were used, a definite answer was difficult to obtain the chain conformation and chain packing mode, although the existence of the two crystalline forms (form I and form II) was proposed on the basis of the detailed analysis of X-ray powder diffraction and infrared spectral data. The utilization of oriented specimens has made it possible for the first time to extract the crystal structures of forms I, I′, and II and HT phase as well as to infer the transitions of these forms under heating and/or stretching. Although four modifications are resolved (forms I, I′, and II and HT) in the present study, the planar or nonplanar chain conformation is a major characteristic among them, and divides the four modifications into two groups. In forms I and I′, the chains assemble in the all-trans packing with layered interchain staggering of Br that presents some longitudinal disorder, as shown in Figure 5. Conversely, in form II and the HT phase, nonplanar gauche conformers are induced around the CHBr moiety, causing a kinked structure. In a first approximation, all the proposed structural models are reasonable in view of inherent difficulties to derive the stereochemically reasonable models from the X-ray diffraction data of quite limited number of observed spots. The precise equidistant placement of Br atoms in the backbone of PE chains leads to multiple crystal modifications, some initially unexpected. The thermal transition between planar or nonplanar forms is reversible. Moreover, the transformation from a nonplanar to a planar form is induced irreversibly by the application of tensile force. A factor that triggers these transformations is the trans−gauche conformational exchange around the skeletal CC bonds, especially in the vicinity of the CHBr group. It is foreseen that placement of other kinds of substituents in the PE backbone in the same precise fashion, such as other halogen types, a methyl, ethyl, or other group, or changing the length of the methylene sequence −(CH2)m−, will affect the stability of these crystalline modifications, especially of the nonplanar one, as well as their phase transitions. In other words, the 3-D aggregation state of precision macromolecules of the type studied here may be affected remarkably by a small change of the chain structure, for example by changing a F for a Br pendant group. The PE21Br case studied here is a nice example that demonstrates the impact of a regular placement on the PE backbone of a group other than CH2 on the aggregation structure and phase transition behavior of PE.

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REFERENCES

(1) Gomez, M. A.; Tonelli, A. E.; Lovinger, A. J.; Schilling, F. C.; Cozine, M. H.; Davis, D. D. Macromolecules 1989, 22, 4441−4451. (2) Alamo, R. G.; Chan, E. K. M.; Mandelkern, L.; Voigt-Martin, L. G. Macromolecules 1992, 25, 6382−6394. (3) Alamo, R. G.; Viers, B. D.; Mandelkern, L. Macromolecules 1993, 25, 5740−5747. (4) Alamo, R. G.; Mandelkern, L. Thermochim. Acta 1994, 238, 155− 201. (5) Stephens, C. H.; Yang, H.; Islam, M.; Chum, S. P.; Rowan, S. J.; Hiltner, A.; Baer, E. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2062− 2070. (6) Chum, P. S.; Swogger, K. W. Prog. Polym. Sci. 2008, 33, 797−819. (7) Reid, B. O.; Vadlamudi, M.; Mamun, A.; Janani, H.; Gao, H.; Hu, W.; Alamo, R. G. Macromolecules 2013, 46, 6485−6497. (8) Chung, T. C. M. Macromolecules 2013, 46, 6671−6698. (9) Vadlamudi, M.; Alamo, R. G.; Fiscus, D. M.; Varma-Nair, M. J. Therm. Anal. Calorim. 2009, 96, 697−704. (10) Vadlamudi, M.; Subramanian, G.; Shanbhag, S.; Alamo, R. G.; Varma-Nair, M.; Fiscus, D. M.; Brown, G. M.; Lu, C.; Ruff, C. J. Macromol. Symp. 2009, 282, 1−13. (11) Wagener, K. B.; Valenti, D.; Hahn, S. F. Macromolecules 1997, 30, 6688−6690. Smith, J. A.; Brzezinska, K. R.; Valenti, D. J.; Wagener, K. B. Macromolecules 2000, 33, 3781−3794. Watson, M. D.; Wagener, K. B. Macromolecules 2000, 33, 8963−8970. Schwendeman, J. E.; Wagener, K. B. Macromolecules 2004, 37, 4031−4037. Sworen, J. C.; Smith, J. A.; Berg, J. M.; Wagener, K. B. J. Am. Chem. Soc. 2004, 126, 11238−11246. Baughman, T. W.; Wagener, K. B. Adv. Polym. Sci. 2005, 176, 1−42. Sworen, J. C.; Wagener, K. B. Macromolecules 2007, 40, 4414−4423. Boz, E.; Nemeth, A. J.; Ghiviriga, I.; Jeon, K.; Alamo, R. G.; Wagener, K. B. Macromolecules 2007, 40, 6545−6551. Baughman, T. W.; Chan, C. D.; Winey, K. I.; Wagener, K. B. Macromolecules 2007, 40, 6564−6571. Berda, E. B.; Lande, R. E.; Wagener, K. B. Macromolecules 2007, 40, 8547−8552. Boz, E.; Wagener, K. B. Polym. Rev. 2007, 41, 511−541. Boz, E.; Ghiviriga, I.; Nemeth, A. J.; Jeon, K.; Alamo, R. G.; Wagener, K. B. Macromolecules 2008, 41, 25−30. Boz, E.; Nemeth, A. J.; Wagener, K. B.; Jeon, K.; Smith, R.; Nazirov, F.; Bockstaller, M. R.; Alamo, R. G. Macromolecules 2008, 41, 1647−1653. Rojas, G.; Berda, E. B.; Wagener, K. B. Polymer 2008, 49, 2985−2995. Zuluaga, F.; Inci, B.; Nozue, Y.; Hosoda, S.; Wagener, K. B. Macromolecules 2009, 42, 4953−4955. Rojas, G.; Inci, B.; Wei, Y.; Wagener, K. B. J. Am. Chem. Soc. 2009, 131, 17376− 17386. Inci, B.; Wagener, K. B. J. Am. Chem. Soc. 2011, 133, 11872− 11875. Inci, B.; Lieberwirth, I.; Steffen, W.; Mezger, M.; Graf, R.; Landfester, K.; Wagener, K. B. Macromolecules 2012, 45, 3367−3376. (12) Boz, E.; Wagener, K. B.; Ghosal, A.; Fu, R.; Alamo, R. G. Macromolecules 2006, 39, 4437−4447. (13) Boz, E.; Nemeth, A. J.; Alamo, R. G.; Wagener, K. B. Adv. Synth. Catal. 2007, 349, 137−141. (14) Alamo, R. G.; Jeon, K.; Smith, R. L.; Boz, E.; Wagener, K. B.; Bockstaller, M. R. Macromolecules 2008, 41, 7141−7151. (15) Kaner, P.; Ruiz-Orta, C.; Boz, E.; Wagener, K. B.; Tasaki, M.; Tashiro, K.; Alamo, R. G. Macromolecules 2014, 47, 236−245. (16) Lieser, G.; Wegner, G.; Smith, J. A.; Wagener, K. B. Colloid Polym. Sci. 2004, 282, 773−781. (17) Qiu, W.; Sworen, S.; Pyda, M.; Nowak-Pyda, E.; Habenschuss, A.; Wagener, K. B.; Wunderlich, B. Macromolecules 2006, 39, 204−217. (18) Hosoda, S.; Nozue, Y.; Kawashima, Y.; Utsumi, S.; Nagamatsu, T.; Wagener, K. B.; Berda, E.; Rojas, G.; Baughman, T.; Leonard, J. Macromol. Symp. 2009, 282, 50−64. (19) Nozue, Y.; Seno, S.; Nagamatsu, T.; Hosoda, S.; Shinohara, Y.; Amemiya, Y.; Berda, E. B.; Rojas, G.; Wagener, K. B. ACS Macro Lett. 2012, 1, 772−775. (20) Matsui, K.; Seno, S.; Nozue, Y.; Shinohara, Y.; Amemiya, Y.; Berda, E. B.; Rojas, G.; Wagener, K. B. Macromolecules 2013, 46, 4438−4446. (21) Buitrago, C. F.; Alam, T. M.; Opper, K. L.; Aitken, B. S.; Wagener, K. B.; Winey, K. I. Macromolecules 2013, 46, 8995−9002. (22) Snyder, R. G. J. Mol. Spectrosc. 1960, 4, 411−434.

AUTHOR INFORMATION

Corresponding Authors

*(K.T.) E-mail: [email protected]. *(R.F.G.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported financially by MEXT “Strategic Project to Support the Formation of Research Bases at Private Universities (2010-2014)”. R.G.A. acknowledges support from the US National Science Foundation (DMR1105129). 4748


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(23) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85−116. (24) Schachtschneider, J. H.; Snyder, R. G. Spectrochim. Acta 1963, 19, 117−168. (25) Alexander, L. E. X-ray Diffraction Methods in Polymer Science; Wiley-Interscience: New York, 1969. (26) Tadokoro, H. Structure of Crystalline Polymers; John Wiley & Sons: New York, 1989. (27) Tashiro, K.; Asanaga, H.; Ishino, K.; Tazaki, R.; Kobayashi, M. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 1677−1700. (28) Tashiro, K.; Ishino, K.; Ohta, T. Polymer 1999, 40, 3469−3478. (29) Arnott, S.; Wonacott, A. J. Polymer 1966, 7, 157−166. (30) Takahashi, Y.; Sato, T.; Tadokoro, H.; Tanaka, Y. J. Polym. Sci., Part B: Polym. Phys. 1973, 11, 233−248. (31) Tashiro, K.; Yoshino, J.; Kitagawa, T.; Murase, H.; Yabuki, K. Macromolecules 1998, 31, 5430−5440. (32) Wasanasuk, K.; Tashiro, K. Polymer 2011, 52, 6097−6109. (33) Smith, A. E. J. Chem. Phys. 1953, 21, 2229−2231. (34) Seto, T.; Hara, T.; Tanaka, K. Jpn. J. Appl. Phys. 1968, 7, 31−42. (35) Nyburg, S. C.; Lüth, H. Acta Crystallogr. 1972, B28,, 2992− 2995. (36) Bunn, C. W. Trans. Faraday Soc. 1939, 35, 482−491. (37) Bently, F. F.; McDevitt, N. T.; Rozek, A. L. Spectrochim. Acta 1964, 20, 105−126. (38) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Elsevier: San Diego, CA,1991. (39) Zbinden, R. Infrared Spectroscopy of High Polymers; Academic Press: New York, 1964. (40) Tasumi, M.; Shimanouci, T. J. Mol. Spectrosc. 1962, 9, 261−287. (41) Kobayashi, M. J. Chem. Phys. 1979, 70, 4797−4802. (42) Yoshioka, Y.; Tashiro, K. J. Phys. Chem. B 2003, 107, 11835− 11842. (43) Yoshioka, Y.; Tashiro, K.; Ramesh, C. Polymer 2003, 44, 6407− 6417. (44) Yoshioka, Y.; Tashiro, K. Polymer 2003, 44, 7007−7019. (45) Li, H. W.; Strauss, H. L.; Snyder, R. G. J. Phys. Chem. A 2004, 108, 6629−6642. (46) Sun, H. J. Phys. Chem. B 1998, 102, 7338−7364. (47) Ratri, P. J.; Tashiro, K. Polym. J. 2013, 45, 1107−1114. (48) Ratri, P. J.; Tashiro, K.; Iguchi, M. Polymer 2012, 53, 3548− 3558. (49) Tashiro, K.; Takano, K.; Kobayashi, M.; Chatani, Y.; Tadokoro, H. Polymer 1983, 24, 199−204. (50) Tashiro, K.; Takano, K.; Kobayashi, M.; Chatani, Y.; Tadokoro, H. Polym. Bull. 1983, 10, 464−469. (51) Takahashi, Y.; Miyamoto, N. J. Polym. Sci., Part B: Polym. Phys. 1985, 23, 2505−2515. (52) Lovinger, A. J.; Davis, D. D.; Cais, R. E.; Kometani, J. M. Macromolecules 1986, 19, 1491−1494. (53) Kusanagi, H.; Tadokoro, H.; Chatani, Y.; Suehiro, K. Macromolecules 1977, 10, 405−413. (54) Takahashi, Y.; Kurumizawa, T.; Kusanagi, H.; Tadokoro, H. J. Polym. Sci. Part B: Polym. Phys. 1978, 16, 1989−2003. (55) Amelinckx, S. Acta Crystallogr. 1956, 9, 217−224. (56) Boistelle, R.; Aquilano, D. Acta Crystallogr. 1977, A33, 642−648. (57) Kobayashi, M.; Kobayashi, T.; Itoh, Y.; Chatani, Y.; Tadokoro, H. J. Chem. Phys. 1980, 72, 2024−2031.

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