Wavenumber Dependence of FT-IR Image of Molecular Orientation in

Feb 7, 2013 - The chemical and the molecular chain orientation image of a banded spherulite of crystalline polymers, poly(l-lactic acid) (PLLA) and po...
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Wavenumber Dependence of FT-IR Image of Molecular Orientation in Banded Spherulites of Poly(3-hydroxybutyrate) and Poly(L‑lactic acid) Yuta Hikima, Junko Morikawa,* and Toshimasa Hashimoto Tokyo Institute Technology, 2-12-1, S8-29, Ookayama, Meguro-ku Tokyo 152-8550, Japan ABSTRACT: The chemical and the molecular chain orientation image of a banded spherulite of crystalline polymers, poly(L-lactic acid) (PLLA) and poly(3-hydroxybutyrate) (PHB), are visualized using FT-IR imaging with a newly proposed multipolarization calculation method. The vector representation of FT-IR image at different absorption spectral peaks visualizes the magnitude and direction of molecular chain orientation directly in comparison with the retardation and the slow-axis azimuthal images obtained by birefringence imaging. We found for the first time that the wavenumber dependence of in-plane orientation function in the band structure of a PHB spherulite, which is known as a doubleband structure originated from a biaxial refractive index ellipsoid. The 15 absorption peaks in the IR spectrum of PHB can be classified into three groups by a different appearance of a band structure in view of the magnitude of molecular chain orientation. The azimuthal image of birefringence also shows the double-band structure; however, the FT-IR azimuthal images of orientation axis indicate the completely different distribution in which there is no band structure at any IR absorption peaks. By contrast, a good correspondence between the azimuthal image of the slow axis of birefringence and the orientation axis of FT-IR is observed in the spherulite of PLLA, which has a uniaxial refractive index ellipsoid. The advanced FT-IR imaging method clarifies the wavenumber-dependent two-types single-band structures of PHB, indicating the local molecular chain orientation along the crystallographic axes in a unit cell.

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near-field scanning optical microscope (NSOM),6 electronic microscope (EM), 9 , 1 1 , 1 4 , 2 0 , 3 2 , 3 3 and X-ray diffraction.9,12−15,17,20,24,25,27−31 Polymer physical properties indicate the spatial distribution due to the periodic change of polymer structure along the band structure in a banded spherulite. For example, Orie et al. recently reported that the thermal diffusivity along thickness direction at extinction rings in a PLLA banded spherulite indicate the twice at high-birefringence banded areas.19 PLLA is known as a biodegradable crystalline polymer and has been investigated by many groups. Studies of thermal behaviors during the crystallization process36−38 and crystallographic structure39−41 of PLLA were reported. PLLA forms a negative banded spherulite under the melt crystallization.17,19 PHB is also known as a biodegradable crystalline polymer, and various groups has been reported the investigations about PHB, PHB-based copolymers, and blends. Sato and Ozaki et al. investigated the crystalline structure and crystallization behavior of PHB with infrared spectroscopy (IR), differential scanning calorimetry, and X-ray diffraction and revealed the existence of weak hydrogen bonds along a-axis in PHB crystal.42−50

INTRODUCTION The periodic ring-band pattern of optical anisotropy in a polymeric spherulite is generally attributed to a concerted twisting of crystallographic orientation about the lamella growth direction.1−3 The banded spherulite originating from the rhythmic crystal growth was also reported.4 Ring-banded spherulites are observed in some semicrystalline polymers, such as poly(ethylene) (PE),5−7 poly(ε-caprolactone) (PCL),8 poly(trimethylene terephthalate) (PTT),9−13 poly(butylene adipate) (PBA),14,15 poly(ethylene adipate) (PEA),16 poly(Llactic acid) (PLLA),17−19 poly(3-hydroxybutyrate) (PHB),20−26 and poly(3-hydroxyvalerate) (PHV).23,27 Some polymer blends and copolymers also formulate the banded spherulites.5,15,17,28−32 The crystallization temperature affects the appearance and pattern of band structures of some polymeric spherulites.9,11,17−20,27,32−34 Band spacing in polymeric spherulites has been reported as various values, from less than 1 to almost 100 μm, and tends to become larger under higher crystallization temperature.5,20,33 Toda et al. were experimentally examined the effect of gradient field of temperature on the structural evolution of polymer crystallization and revealed that the temperature gradient enlarge the band spacing due to wider lamellar width.35 The morphology and structure of banded spherulite were investigated with various measurements, for example, atomic force microscope (AFM),7,8,10,14,16,17,21,27,34,35 © XXXX American Chemical Society

Received: December 13, 2012 Revised: January 30, 2013

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Measurements. FT-IR imaging measurement was performed with Spectrum Spotlight 300 (PerkinElmer Inc.). The infrared absorption spectrum was measured with transmission mode, from 4000 to 650 cm−1 spectral region, 4 cm−1 spectral resolution, and 16 scans. The imaging pixel size is 6.25 μm × 6.25 μm. A gold wire-grid polarizer was employed for the polarization measurement. At the polarization measurement of PLLA sphrulite, the polarizer was settled at 0°, 45°, 90°, and 135° for the four polarizations method. In the case of PHB spherulite, the 13 polarizer angles from 0° to 180°, at the intervals of 15°, were employed for the calculation of the multipolarizations method. The raw data of FT-IR imaging consist thousands of infrared spectra with a spatial resolution. Before the calculation of the each FTIR images for the analysis, all absorption peaks were corrected by the subtraction of baseline at each pixel. The calculation of the in-plane orientation function and orientation azimuth angle was performed at each pixel. The orientation function was calculated under an assumption that α, which is the angle between the chain axis and the transition dipole moment of the used absorption peak, is 0° at the parallel dichroism and 90° at the perpendicular dichroism. The birefringence imaging was performed with Abrio-IM (CRi Inc.). The Abrio system in an optical microscope (ECLIPSE LV-100POL, Nikon Instruments Inc.) contains a light source, a 546 nm interference filter, a variable elliptical polarizer, a sample stage, an objective lens (20×, 50×), a circular analyzer, and a digital camera connected via a computer. The variable elliptical polarizer is composed of a pair of electrical-controlled liquid-crystal retarder cells with 45° between their principal axes and a linear polarizer.77,78 Taking several polarized optical images with controlling the variable elliptical polarizer, the calculation of retardation and slow-axis azimuth images were automatically performed by the Abrio-IM software. Introduction of the Multipolarization Method. An infrared absorption spectrum of an anisotropic polymer specimen varies with the polarization direction when a linear polarized radiation is employed for the incident light. This variation is attributed to the molecular chain orientation. The dependence of absorbance at each absorption peak, which is called as the dichroism, is determined by the orientation distribution of the transition dipole moments about the molecular chain axis. In order to evaluate the orientation distribution of molecular chain about an orientation axis quantitatively by infrared absorption spectroscopy, we usually measure two absorption spectra under a pair of orthogonal polarizations, parallel and perpendicular with the orientation axis of the molecular chain, respectively. The ratio of the absorbance is called as a dichroic ratio D and related to the orientation distribution function (ODF). Hermans orientation function f, which corresponds to the second moment of ODF, can be calculated from the equation72

Moreover, the role of hydrogen bonding in PHB−PHV copolymer,51 PHB/poly(4-vinylphenol) (PVPh),52,53 and PHB/cellulose acetate butyrate (CAB) blends54,55 were also investigated. Recently, the effect of composition on the crystallization process of the blend film of typical PHB and ultrahigh-molecular weight PHB was reported. 56 Band structures can be formed under various crystallization temperatures in PHB spherulites.20 There is an obvious difference of band structure between PLLA and PHB spherulites. The band structure in a POM image of PLLA spherulite contains an extinction ring and a high-birefringence banded area in a period. On the other hand, in PHB banded spherulite, the band structure contains an extinction ring, a higher-birefringence band, another extinction ring, and a lower-birefringence band in a period. This structure in PHB spherulites is called a “doubleband structure”. These birefringence band structures are attributed to the molecular chain orientation in polymeric spherulites. However, they do not always reflect the actual structures. As mentioned above, microscopic imaging measurement is suited to analyze the morphology in polymer spherulite. The Fourier transform infrared spectroscopic (FT-IR) imaging method is one of the two-dimensional chemical imaging measurements and has been applied to various systems.57−64 Moreover, the combination of the polarization measurement and FT-IR imaging enables to visualize the chemical information and the molecular chain orientation simultaneously 65−71 with the Hermans orientation function.72,73 Recently, the application of the FT-IR imaging method to polymer spherulite has increased.26,69−71,74 However, the quantitative analysis of band structure in polymeric spherulite using FT-IR imaging is seldom reported. We proposed the “four polarizations method” to visualize the magnitude of molecular chain orientation and the azimuth angle of the orientation axis independent of the polarizer angle using the FT-IR imaging method.75 Moreover, FT-IR imaging with the four polarizations method was applied to the analysis of poly(ethylene oxide) spherulite with chemical imaging.76 In this paper, the FT-IR imaging method with polarization measurement is applied to banded spherulite of two crystalline polymers, PLLA and PHB. The chemical image and orientation function image of a PLLA banded spherulite are visualized and compared at a crystalline peak and an amorphous peak. As for a PHB banded spherulite, the wavenumber dependence of band structure in the in-plane orientation function image is investigated using the newly proposed multipolarization method. The comparison of band structure in the spherulite between FT-IR images and optical birefringence images provide a new classification of infrared absorption peaks in PHB.

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f=

D−1 2 D + 2 3 cos2 α − 1

(1)

where D is the dichroic ratio and α is the angle between the chain axis and the transition dipole moment of the used absorption peak. The range of Hermans orientation function f is allowed from −0.5 to 1. If molecular chains show the random orientation, f will be equal to 0. However, if the direction of used polarizations forms 45° with the orientation axis, f will also exhibit 0. The Hermans orientation function reflects both of the magnitude of molecular orientation and the difference of direction between the used polarization and the orientation axis. In order to avoid this problem, we introduced the four polarizations method.75 From the infrared spectra measured under four linear polarizations, in-plane orientation function fΨ and orientation azimuth angle Ψ are calculated. In-plane orientation function corresponds to the magnitude of molecular chain orientation in the sample plane. The orientation azimuth angle corresponds to the direction of orientation axis in the sample plane and independent of the magnitude of molecular chain orientation. Here, we expand four polarizations method to multipolarization method for calculation of fΨ and Ψ with higher precision.

EXPERIMENTAL SECTION

Sample. The film of poly(L-lactic acid) (PLLA; Aldrich, Mw = 100 000−150 000) and poly(3-hydroxybutyrate) (PHB; Aldrich) was prepared by casting from the chloroform solution on a microscope slide glass. Each film was dried at room temperature in the vacuum for 3 h, peeled off the slide glass, and cut up into a 5 mm × 5 mm section. The thickness of both PLLA and PHB films was ∼11 μm, and it was put on a potassium bromide (KBr) disk (Edmund Optics; diameter 13 mm, thickness 1 mm). The PLLA and PHB films on a KBr disk were placed on the hot stage and melted at 190 °C. PLLA sample was transferred on another hot stage and crystallized under isothermal condition at 130 °C. PHB sample was also transferred and crystallized under isothermal conditions at 90 °C. After the spherulite growth, the samples were quickly cooled down to room temperature. B

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When the uniaxial oriented sample is measured under a linear polarized infrared radiation, the absorbance at an absorption peak varies according to the equation A γ = A max cos2(γ − ψ ) + A min sin 2(γ − ψ )

(2)

where γ is the polarizer angle, Amax and Amin are the maximum and minimum absorbance, respectively, and ψ is the polarizer angle when A = Amax. The in-plane orientation function fΨ can be calculated from the ratio of Amax and Amin. The orientation azimuth angle Ψ can be obtained from ψ. In order to calculate fΨ and Ψ, four particular polarizer angles γ = 0°, +45°, −45°, and 90° are substituted into eq 2 in the four polarizations method. Equation 2 can be transformed into A γ = A 0 sin 2γ + A1 cos 2γ + A 2

(3)

In eq 3, new parameters A0, A1, and A2 are introduced instead of three parameters Amax, Amin, and ψ in eq 2. The relationship among six parameters is expressed as the following equations:

(A 0 2 + A12 ) =

A max − A min 2

⎛A ⎞ 0.5 tan−1⎜ 0 ⎟ = ψ ⎝ A1 ⎠ A2 =

A max + A min 2

Figure 1. Images of nonpolarized infrared absorbance obtained at (a) 872 cm−1 (C−COO stretching, crystalline peak) and (b) 956 cm−1 (CH3 rocking, amorphous peak) in the banded spherulite of poly(Llactic acid) measured with the FT-IR imaging method. The value of absorbance corresponds to the false color shown as right-side color bar. (c) Optical microscopic image of the same area. (d) Profiles of absorbance at (black) 872 cm−1 and (red) 956 cm−1 on the dotted lines in upper images.

(4)

In order to calculate fΨ and Ψ with higher precision, the measurement of infrared absorption spectra with a linear polarizer settled at a smaller interval of the angle is required. The parameters A0, A1, and A2 can be obtained from the polarized infrared absorbances by the least-squares method of eq 3. From the parameters A0, A1, and A2, maximum and minimum dichroic ratios, Dmax and Dmin, can be calculated as

Dmax =

2A 2 + 2 (A 0 2 + A12 ) A max = A min 2A 2 − 2 (A 0 2 + A12 )

Dmin =

2A 2 − 2 (A 0 2 + A12 ) A min = A max 2A 2 + 2 (A 0 2 + A12 )

spherulite for the amorphous peak in Figure 1b. Figure 1d shows the profiles of absorbance on the dotted line depicted in Figure 1a,b. The profile of the crystalline peak (black line) indicates that the absorbance at the inside of the spherulite is almost twice the value at the outside. By contrast, the profile of the amorphous peak (red line) shows larger absorbance at the outside than at the inside of the spherulite. Therefore, the PLLA spherulite can be distinguished clearly from the amorphous regions by chemical images. The in-plane orientation function images of the PLLA banded spherulite at 872 cm−1 (crystalline peak) and 956 cm−1 (amorphous peak) are presented in Figures 2a and 2b, respectively. Values of in-plane orientation function are expressed by the false color depicted in each color bar. In spite of the complementary absorbance images shown in Figure 1a,b, the in-plane orientation function images at both wavenumbers indicate a similar concentric ring-band pattern. The difference of amorphous peak images between absorbance (Figure 1b) and in-plane orientation function (Figure 2b) suggests the spatial distribution of the condition of amorphous chain between the inside and the outside of spherulite. There are many isotropic amorphous molecular chains in the nonspherulite region. On the other hand, a small amount of the oriented amorphous molecular chains exists together with the crystalline chain in the spherulite. In our previous work,76 the retardation, which is the product between the magnitude value of linear birefringence of visible light and the thickness of specimens, corresponded well to the in-plane orientation function calculated from the infrared absorption spectra in poly(ethylene oxide) spherulite. The retardation image around the PLLA banded spherulite obtained by birefringence imaging method is shown in Figure 2c and presents the good correspondence to the in-plane orientation function images at the crystalline peak (Figure 2a). The profiles of the value of inplane orientation function at the crystalline peak (black line),

(5)

The in-plane orientation function fΨ and orientation azimuth angle Ψ can be calculated from Dmax or Dmin obtained using eq 5 and ψ from eq 4. When the used absorption peak has a parallel dichroism, fΨ and Ψ are calculated as fΨ =

Dmax − 1 2 Dmax + 2 3 cos2 α − 1 Ψ=ψ

(6a)

When the used absorption peak has a perpendicular dichroism, fΨ and Ψ are calculated as fΨ =

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Dmin − 1 2 2 Dmin + 2 3 cos α − 1 Ψ = ψ + 90°

(6b)

RESULTS AND DISCUSSION PLLA Banded Spherulite. Figures 1a and 1b show the nonpolarized absorbance images of PLLA banded spherulite at 872 cm−1 (C−COO stretching, crystalline peak) and 956 cm−1 (CH3 rocking, amorphous peak),79,80 respectively, of which the optical image is presented in Figure 1c. In Figure 1a, the infrared absorbance of the crystalline peak indicates the higher values at the inside of the spherulite. On the other hand, the higher values are observed at the outside of the PLLA C

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Figure 2. Images of in-plane orientation function calculated at (a) 872 cm−1 (C−COO stretching, crystalline peak) and (b) 956 cm−1 (CH3 rocking, amorphous peak) in the banded spherulite of PLLA measured with FT-IR imaging method with the four polarizations method. The value corresponds to the false color shown as right-side color bar. (c) Retardation image of the same area obtained from birefringence imaging method. (d) Profiles of in-plane orientation function at (black) 872 cm−1, (red) 956 cm−1, and (blue) retardation on the white dotted lines in (a−c).

Figure 3. Image of orientation azimuth angle calculated at (a) 872 cm−1 (C−COO stretching, crystalline peak) in the banded spherulite of PLLA measured with FT-IR imaging method with four polarizations method. (b) Image of slow-axis azimuth angle obtained from the birefringence imaging method. The false color corresponds to each azimuthal value shown as right-side color circle, and the brightness corresponds to the magnitude of in-plane orientation function or retardation. (c) Vector representation of (white) orientation azimuth angle at 872 cm−1 and (blue) slow-axis azimuth angle in the rectangular areas in upper images. The length of vector is in proportion to the magnitude of in-plane orientation function or retardation. (d) The schematic diagram of a lamella twisting and the molecular chain orientation at the corresponding position in (c). The schematics of the in-plane orientation of molecular chain at each point in a lamella are shown in the upper part of (d).

amorphous peak (red line), and retardation (blue line) are compared in Figure 2d. Every line profile indicates ridges and valleys at the same areas except the center of spherulite. The width of each ring-band with high values is almost 50 μm. The orientation azimuth angle, which is the direction of orientation axis of the molecular chain in the sample plane and independent from the magnitude of orientation function, can be calculated using FT-IR imaging with the four polarizations method.75,76 Figure 3a shows the orientation azimuthal image of PLLA banded spherulite calculated at 872 cm−1 (crystalline peak). The direction is expressed by the false color, depicted in the color circle, and the brightness corresponds to the magnitude of in-plane orientation function. For example, the orientation axis of the molecular chain is parallel to x-axis at the red area (Ψ ≅ 0° or 180°) and parallel to y-axis at the light-blue area (Ψ ≅ 90°). The direction of molecular chain orientation is parallel to the tangential direction of spherulite in Figure 3a. On the other hand, Figure 3b shows the azimuthal image of slow axis of a refractive index ellipsoid of the PLLA spherulite obtained by the birefringence imaging method. The direction of slow axis is also expressed by the false color, depicted in the color circle, and the brightness corresponds to the magnitude of retardation. The slow-axis azimuthal image in Figure 3b shows the similar pattern to Figure 3a. The orientation of the slow axis to the tangential direction of spherulite is the typical characteristic of the negative spherulite. In order to compare the orientation azimuth angle and slow-axis azimuth angle intuitively, the vector representation is introduced. In Figure 3c, the white vectors obtained from FT-IR imaging is overlaid with the blue vectors obtained from birefringence imaging. The vector length is in proportion to the magnitude value of orientation function or retardation. At near the center of spherulite and on the extinction ring, the vector length becomes

very short. The directions of long-length vectors show good correspondence between two methods. The schematic diagram of a twisting lamellar crystal is illustrated in Figure 3d at the corresponding position in Figure 3c. The molecular chain orientation at the edge- and the flat-on lamellar crystal are illustrated in the upper part of Figure 3d. At the edge-on lamella, the molecular chain is oriented perpendicular to the lamella growth direction in the sample plane. On the other hand, at the flat-on lamella, the molecular chain is perpendicular to the sample plane. The spatial change of vector length along the radial direction of spherulite in Figure 3c reflects well the lamella twisting illustrated in Figure 3d. In summary of the above results, the FT-IR images of magnitude and direction of molecular chain orientation of the crystalline and the amorphous PLLA correspond to the birefringence images. This is originated to the correspondence of the molecular chain axis of crystal and the slow axis of the uniaxial refractive index ellipsoid of PLLA. PHB Banded Spherulite/Birefringence Image. Birefringence images of PHB banded spherulite have the different characteristics from a PLLA banded spherulite. Figure 4 shows D

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we estimate the refractive index ellipsoid at each banded area. The schematic diagram of refractive index ellipsoid estimated from retardation and slow-axis azimuth angle at each banded area is depicted in Figure 4e. The orientation of the crystallographic axes was investigated in a PHB banded spherulite with X-ray diffraction analysis.20,24,25 In these literatures, the a-axis is oriented to the radial direction of the spherulite, and the other two axes rotate about the a-axis. Furthermore, the b- and c-axes are oriented to the tangential direction of the spherulite at the high- and low-retardation area, respectively. The schematic diagram of the orientation of crystallographic axes at each banded area is depicted at the lowest part of Figure 4e. From the comparison between the refractive index ellipsoid estimated from the results of birefringence imaging and the orientation of crystallographic axes obtained from X-ray diffraction analysis,20 we estimate that the magnitude correlation between the refractive indices along each crystallographic axis is expressed as nc > na > nb. PHB Banded Spherulite/FT-IR Imaging Method. From comparison Figures 2 and 4, the band structure of PHB spherulite is fine and more complex than that of PLLA spherulite. Thus, the multipolarization method is employed for the first time in order to visualize the band structure of PHB spherulite precisely with FT-IR imaging. Figure 5 shows the inplane orientation function images of the PHB banded spherulite crystallized at 90 °C calculated at (a) 1134 cm−1 (CH3 rocking) and (b) 1230 cm−1 (C−O−C stretching)45 obtained with the multipolarization FT-IR imaging method. The value of the in-plane orientation function is expressed by the false color depicted in each color bar. The retardation image obtained using birefringence imaging method at the same area is shown in Figure 5c. Moreover, the enlarged images of the black square area are placed at the right side of each image. In Figures 5a and 5b, both of in-plane orientation function images indicate the periodic band structure. The spatial periods of band structure in two images are in good correspondence; however, the depicted colors are opposite. As depicted with the gray dotted lines in the three enlarged images, the high-value areas at 1134 cm−1 (in Figure 5a) and the low-value areas at 1230 cm−1 (in Figure 5b) in each in-plane orientation function image correspond to the high-retardation areas in retardation image (in Figure 5c). By contrast, the low-value areas at 1134 cm−1 (Figure 5a) and high-value areas at 1230 cm−1 (Figure 5b) correspond to the low-retardation areas (Figure 5c), as depicted with the black dotted lines. The profiles of white lines in the three enlarged images in Figure 5 are presented in Figure 6a. In Figure 6a, the profiles of in-plane orientation function change like a sine curve, and a black line (at 1134 cm−1) changes in the opposite direction to that of a red line (1230 cm−1); the top of the black line corresponds to the bottom of the red line, and vice versa. On the other hand, the retardation profile (a blue line in Figure 6a) indicates a double-peak top, where a higher peak corresponds to the top of the black line and a small one corresponds to the top of the red line. The valley of a blue line has a discontinuous differential coefficient that does not match the peak or bottom of the black and red lines. Thus, the profile of in-plane orientation function show the wavenumber dependence, indicating two opposite single band structures, which are different from the double-band structure in the retardation profile. This relationship between the profiles of the orientation function from IR spectrum and the optical retardation is completely different from that of

Figure 4. Images of (a) retardation and (b) slow-axis azimuth angle of a poly(3-hydroxybutyrate) banded spherulite measured with birefringence imaging method. The false color corresponds to the right-side color bar (retardation) or circle (azimuth of slow-axis). (c) Optical microscopic image of the same area. (d) Profiles of (black) retardation and (blue) azimuth of slow-axis on the lines in upper images. (e) Schematic diagrams of refractive index ellipsoids estimated from retardation and slow-axis azimuth angle and the direction of crystallographic axes in the sample plane measured by X-ray diffraction analysis20 at each banded area.

(a) a retardation image and (b) a slow-axis azimuth image of a PHB banded spherulite crystallized at 90 °C measured by birefringence imaging method with a 20× objective lens. The retardation image in Figure 4a indicates a double-band structure composed of high-retardation ring-banded (red or yellow) areas, low-retardation ring-banded (light-blue) areas, and narrow extinction rings (dark-blue areas). In Figure 4b, the slow-axis of a refractive index ellipsoid is oriented to the radial and tangential direction of the spherulite at high-retardation and low-retardation ring-banded areas, respectively. In the optical image (Figure 4c), there are periodic cracks along the radial direction of PHB spherulite at the high retardation areas in Figure 4a. The line profiles of retardation (black) and slowaxis azimuth angle (blue) on the white lines in Figures 4a and 4b are depicted in Figure 4d, respectively. The retardation presents the periodic profiles such as a ridge with lower value of about 70 nm, a valley below 20 nm, a ridge with higher value of about 140 nm and another valley. The width of high- and lowretardation band is almost 30 μm. The azimuthal profile indicates also periodic change of the direction of the slow axis such as a tangential direction, a drastic jump, a radial direction, and another drastic jump. The drastic jump of slow-axis azimuth angle is observed at the valley in retardation profile. From these results obtained by birefringence imaging method, E

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Figure 6. (a) Magnitude profiles of in-plane orientation function at (black) 1134 cm−1 (CH3 rocking, group 2), (red) 1230 cm−1 (C−O− C stretching, group 1), and (blue) retardation on the white lines shown in enlarged images in Figure 5. (b) Azimuthal profiles of orientation azimuth angle at (black) 1134 cm−1, (red) 1230 cm−1, and (blue) slow-axis azimuth angle on the same lines. Figure 5. Images of in-plane orientation function calculated at (a) 1134 cm−1 (CH3 rocking, group 2) and (b) 1230 cm−1 (C−O−C stretching, group 1) in the banded spherulite of PHB measured with the FT-IR imaging method with the multipolarization method. (c) Image of retardation at the same area measured with birefringence imaging method with a 20× objective lens. Enlarged images of each image are placed at right side. Enlarged image of retardation was measured with a 50× objective lens.

obtained from the birefringence imaging. This suggests that the PHB molecular chain axis is oriented to the tangential direction of spherulite at each point. It is the same result of PLLA banded spherulite as shown in Figure 3a. In order to understand the difference of azimuth angles between FT-IR and birefringence image, the vector representation is introduced in Figure 7d, where the vector images in the black rectangular areas in Figures 7a−c are depicted. Black and red vectors represent the direction of orientation axis calculated at 1134 and 1230 cm−1, respectively. The direction of the slow axis obtained from birefringence imaging is presented by blue vectors. The upper part of every vector image is overlaid on the false-colored retardation image. The length of these vectors is fixed and independent of the magnitude value such as orientation function or retardation. The vector image can be divided into two kinds of areas. The area where the black and red vectors (IR) are perpendicular to the blue vector (birefringence) corresponds to the high-retardation banded area. On the other hand, in the low-retardation banded areas, the IR orientation azimuth vectors and slow-axis azimuth vectors are parallel to the tangential direction of the spherulite. In summary of the results shown in Figures 5 and 7, FT-IR images of molecular chain orientation indicate two single band structures with wavenumber dependence and are different from the double-band structure in birefringence images. At the highretardation band in the double-band structure, the in-plane orientation function calculated at 1134 and 1230 cm−1 indicates high and low value, respectively. Moreover, the orientation axis of FT-IR imaging is perpendicular to the slow axis of refractive index ellipsoid. At the low-retardation band, the in-plane

PLLA banded spherulite and PEO spherulite.76 The images of FT-IR and birefringence of PLLA and PEO are in good correspondence, and the wavenumber dependencies are not observed. Figure 6b shows the comparison of line profiles of azimuth angle between FT-IR and birefringence images at the same area of Figure 6a. The profiles of orientation azimuth angle at 1134 (black) and 1230 cm−1 (red) are completely different from the slow-axis azimuth angle (blue). Figure 7 shows the orientation azimuthal images of the PHB banded spherulite crystallized at 90 °C calculated at (a) 1134 cm−1 and (b) 1230 cm−1 calculated with the multipolarization FT-IR imaging method. The slow-axis azimuthal image obtained from birefringence imaging method is shown in Figure 7c for comparison. The direction is expressed by the false color, depicted in the color circle. In contrast with slowaxis azimuthal image (Figure 7c), the orientation azimuthal images at 1134 cm−1 (Figure 7a) and 1230 cm−1 (Figure 7b) show the same distribution without the ring-band structure. In Figure 6b, the profiles of orientation azimuth angle at 1134 cm−1 (black line) and 1230 cm−1 (red line) indicate almost constant angles near 80°, which corresponds to the tangential direction of spherulite, in comparison with the profile reflecting the band structure of slow-axis azimuth angle (blue line) F

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Figure 7. Image of orientation azimuth angle calculated (a) 1134 cm−1 (CH3 rocking, group 2) and (b) 1230 cm−1 (C−O−C stretching, group 1) in the banded spherulite of PHB measured with the FT-IR imaging method with the multipolarizations method. (c) Image of slow-axis azimuth angle at the same area obtained from birefringence imaging method. The false color corresponds to each azimuthal value shown as right-side color circle. (d) Vector representation of orientation azimuth angle at (black) 1134 and (red) 1230 cm−1 and (blue) slow-axis azimuth angle in the rectangular areas in upper images is overlaid on the retardation image with the false color. The length of vector is fixed and independent from the magnitude of inplane orientation function or retardation.

Table 1. Assignment and Classification by Band Structure of Spherulite of Absorption Peaks of Infrared Spectrum of PHB26,43−45,48

function at low-retardation areas like as at 1230 cm−1 (group 1). Second, the high values of orientation function are observed at high-retardation areas like at 1134 cm−1 (group 2). Third, no band structure is observed as the in-plane orientation function image (group 3). The assignment,26,43−45,48 the dichroism, and the group of a band structure of spherulite of 15 absorption peaks are summarized in Table 1. It is worth noting that the classification by the kind of band structure does not depend on the corresponding chemical structure or the dichroism of each absorption peak. The FT-IR azimuthal images of orientation axis indicate the molecular chain orientation to the tangential

orientation function indicates opposite tendency to the highretardation band, and the orientation axis is parallel to the slow axis. Therefore, the double-band structure in the birefringence image is transfigured into two single-band structures with the wavenumber dependence by advanced FT-IR imaging of molecular chain orientation. We also calculated at the other 13 peaks in the IR absorption spectra of PHB and investigated the band structure of in-plane orientation function images at those wavenumbers. As a result, 15 absorption peaks can be classified into three groups by the difference of band structure of orientation function images. First, the band structure shows high values of orientation G

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direction of the spherulite without any band structures at all absorption peaks. As shown in Figure 4e, the origins of the complicated periodic distribution called as the double-band structure in birefringence images are estimated as the periodic lamella twisting along the growth direction and the difference of refractive index along each crystallographic axis of PHB crystal. On the other hand, FT-IR images indicate the simple distribution, two-types single-band structures of orientation function reflecting the periodic lamella twisting and the molecular chain orientation perpendicular to lamella growth direction, as observed in a PLLA banded spherulite. The FT-IR image of molecular chain orientation indicates the wavenumber-dependent two-types distributions instead of the double-band structure. Comparing with the birefringence image and X-ray analysis,20 the wavenumber dependence of orientation function image provides a hypothesis that two opposite band structures are originated to the difference of local molecular chain orientation in the crystal structure of PHB. In other words, the orientation function calculated at absorption peaks belonging to groups 1 and 2 presented in Table 1 may correspond to the crystallographic orientation of the c- and baxis, respectively. The wavenumber dependence of band structure in the in-plane orientation function image of PHB is found for the first time while it is not found in PLLA banded spherulite.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Japan Society for the Promotion of Scientists, JSPS, Grant-in-Aid for JSPS Fellows No. 24·9326. REFERENCES

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CONCLUSION The chemical and the molecular chain orientation image of a banded spherulite of crystalline polymers, PLLA and PHB, are visualized using FT-IR imaging with a newly proposed multipolarization calculation method. The vector representation of FT-IR image at different absorption spectral peaks visualizes the spatial distribution of actual polymer structure as the magnitude and direction of local molecular chain orientation in comparison with the retardation and the slowaxis azimuthal images obtained by birefringence imaging. The FT-IR images of magnitude and direction of molecular chain orientation of the crystalline and the amorphous PLLA correspond to the birefringence images, which are originated to the correspondence of the molecular chain axis of crystal and the slow-axis of the uniaxial refractive index ellipsoid of PLLA. On the other hand, we found for the first time that a doubleband structure of PHB that is well-known in the birefringence image is transfigured into a wavenumber-dependent single-band structure in the FT-IR image of molecular chain orientation. Moreover, the FT-IR azimuthal images show the perfectly different distribution from the double-band structure in the birefringence azimuthal image that indicates the orientation of local molecular chain perpendicular to the spherulite growth direction corresponding to the actual direction of lamellar crystal. Fifteen absorption peaks in the IR spectrum of PHB can be classified into three groups by a different appearance of a band structure in view of molecular chain orientation. The wavenumber-dependent two-types single-band structures of PHB indicate the local molecular chain orientation along two crystallographic b- and c-axes in the crystal structure of PHB, which is consistent with the results measured by birefringence and X-ray analyses. The advanced FT-IR imaging method achieves the visualization of band structure in polymeric spherulite indicating the local molecular chain orientation in a crystallographic unit cell. H

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