Imaging of Two-Dimensional Distribution of Molecular Orientation in

Oct 1, 2012 - Yuta Hikima , Junko Morikawa , and Toshimasa Hashimoto ... Xuewen Wang , Jitraporn Vongsvivut , Yuta Hikima , Jingliang Li , Mark J. Tob...
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Imaging of Two-Dimensional Distribution of Molecular Orientation in Poly(ethylene oxide) Spherulite Using IR Spectrum and Birefringence Yuta Hikima, Junko Morikawa,* and Toshimasa Hashimoto Tokyo Institute Technology, 2-12-1, S8-29, Ookayama, Meguro-ku Tokyo 152-8550, Japan ABSTRACT: The two-dimensional quantitative imaging of molecular chain orientation is presented together with a chemical FT-IR imaging, in comparison with the optical birefringence imaging. The molecular chain orientation of the spherulite of poly(ethylene oxide) (PEO) is visualized using the four polarizations method of FT-IR imaging. The magnitude of orientation function and the azimuth angle of orientation axis are calculated with the FT-IR absorbance measured with a linear polarizer settled at four different angles. Retardation and the slow axis in a refractive index ellipsoid are calculated using transmission intensity measured with an electrical-controlled variable elliptical analyzer and a circular polarizer. In spite of the 6 times difference of the spatial resolution, the azimuth angle image determined from FT-IR imaging detects the change of molecular chain orientation, giving the similar line profiles of azimuth angle to that of slow axis determined from birefringence on the spherulite interface. A good correspondence in the direction of orientation axis of the molecular chain with the slow axis of a refractive index ellipsoid at each point is confirmed visually from the comparison of vector representation. The FT-IR imaging with four polarizations method enables to visualize the difference of molecular chain orientation of each component of the polymer blend spherulite.

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INTRODUCTION The imaging method is suited to analyze the morphology in polymer spherulite, having various kinds of lamellae orientations with different chemical structures and crystallization conditions. The Fourier transform infrared spectroscopic (FTIR) imaging method has achieved the two-dimensional chemical imaging that has been applied to various systems.1−24 Recently, the Hermans orientation function25,26 has been visualized using FT-IR imaging that gives the degree of molecular chain orientation and chemical information at once.27−33 Hikima et al.34 proposed the method to visualize the magnitude of molecular chain orientation and the azimuth angle of the orientation axis independent of the polarizer angle. The FT-IR imaging method has been applied to the analysis of polymer spherulite with chemical imaging;30−33,35−37 however, the quantitative analysis of molecular chain orientation is still to be discussed. Polarized optical microscopy is one of the most common techniques for the observation of the morphology in polymeric spherulite. Mei and Oldenbourg38 developed the birefringence imaging system based on the polarized optical microscope. This system visualizes the spatial distributions of retardation and the slow-axis direction at once, using two liquid crystal variable retarders. Some groups applied this system to the local anisotropy analysis, the structure modified glass,39 and sapphire40 by the femtosecond laser pulse and the liquid crystal.41,42 In terms of the biomedical application, the visualization of the living cell is allowed by this system.43−45 Ye et al.46 reported the application of birefringence imaging method to the banded polymer spherulite. © 2012 American Chemical Society

In this paper, FT-IR imaging and birefringence imaging are compared by the vector representation of the magnitude and the direction of molecular chain orientation in the PEO spherulite. The vector representation provides the intuitive and quantitative visualization of orientation, especially in the center and in the interface of the spherulite. Furthermore, the advantage of vector representation is clearly shown in the polymer blend, poly(L-lactic acid) (PLLA)/PEO, where the distribution of azimuth angle of orientation assigned to each chemical absorbance is visualized combined with the chemical imaging.

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THEORY

FT-IR Imaging Method with the Four Polarizations Method. In order to obtain the degree of molecular chain orientation by infrared spectroscopy, we need two absorption spectra measured by a pair of orthogonal polarizations, parallel and perpendicular with the orientation axis, respectively. The Hermans orientation function, which corresponds to the second moment of orientation distribution function, can be calculated from the ratio of these absorbance. However, it is difficult to calculate the quantitative distribution of the Hermans orientation function only from the two images measured by a pair of orthogonal polarizations, when the spatial Received: May 21, 2012 Revised: August 26, 2012 Published: October 1, 2012 8356

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where αM is the angle between the chain axis and the transition dipole moment vector. In-plane orientation function fΨ is calculated from the ratio of absorbances, so that fΨ is not in proportion to the sample thickness. Birefringence Imaging Method. Mei and Oldenbourg38 developed the birefringence imaging system based on the polarized optical microscope with the variable elliptical analyzer. According to ref 47, one of the measurement systems contains a left-circular polarizer in the illumination part and a variable elliptical analyzer in the imaging part in a microscope. The variable elliptical analyzer is composed of a pair of liquidcrystal (LC) retarder cells with 45° between their principal axes and a linear polarizer.38 The retardation of a LC retarder is α and that of another is β. An elliptical polarization P0 which can pass through the variable elliptical analyzer without intensity loss is determined by the set of α and β. An elliptical polarization state P0 can be written with Stokes parameter as [S1, S2, S3] = [cos α, sin α sin β, sin α cos β]. On the other hand, an elliptical polarization P is obtained when the leftcircular polarization passes through the birefringent specimen with retardation Δ and slow-axis azimuth angle φ of an index ellipsoid. P can be written as [S1, S2, S3] = [sin Δ sin 2φ, −sin Δ cos 2φ, −cos Δ]. When the elliptical polarization P is incident on the variable elliptical analyzer, the transmission light intensity I is described by a function of the central angle θ between P and P0 on the Poincaré sphere (shown in Figure 1b):

distribution exists in the direction of orientation axis in the sample. We proposed the four polarizations method in order to obtain the quantitative orientation function image by FT-IR imaging method.34 The absorbance Aγ at the polarizer angle settled as γ indicates a value between the maximum and minimum absorbance depending on the difference of direction of the linear polarization and the sample anisotropy. This relationship is expressed by the equation A γ (γ , ψ , A max , A min ) A max + A min A − A min + max cos 2(γ − ψ ) (1) 2 2 where γ is the settled polarizer angle, Amax and Amin are the maximum and minimum absorbance, respectively, and ψ is the polarizer angle when A = Amax. The relationship expressed by eq 1 is shown in Figure 1a. Four absorbances measured with =

Figure 1. (a) Schematic depiction of IR absorbance dependent on the angle of a linear polarizer, γ, in the uniaxial oriented sample. ψ, Amax, and Amin are determined by the direction and magnitude of molecular chain orientation of the sample and the dichroism of the used absorption band. (b) Schematic depiction of two polarization states, P and P0, and the central angle θ between them in the three-dimensional space of the three of Stokes parameters, S1, S2, and S3. The polarization states P and P0 move on the Poincaré sphere according to the birefringence of sample (Δ and φ) and the set of retardation (α and β) of variable LC retarders, respectively.

I (θ ) =

Dmax − 1 2 2 Dmax + 2 3 cos αM − 1

(4)

where Imax and Imin are the maximum and minimum intensity, respectively. This equation is under an assumption of the perfect transmittance of the analyzer. The cos θ is equal to the inner product between the position vectors of P and P0. An elliptical polarization state P0 can be moved on the Poincaré sphere by controlling the set of retardation, α and β of variable retarders. Thus, the elliptical polarization state P, which is described with Δ and φ, can be calculated by the intensities measured under four or five specific sets of retardation, α and β. The details of a LC retarder and algorithms of birefringence imaging method are described in refs 38 and 47.

four different polarizer anglesγ = 0, +45, −45, and 90° follow eq 1, and Amax, Amin, and ψ can be calculated from these absorbances. The Hermans orientation function calculated from Amax and Amin, which is called as “in-plane orientation function ( fΨ)”, corresponds to the magnitude of molecular chain orientation within the sample plane. Moreover, the azimuth of orientation axis of the molecular chain within the sample plane, which is called as “orientation azimuth angle (Ψ)”, can be calculated from ψ. In the case of the parallel band, in-plane orientation function, fΨ, is calculated from Dmax (= Amax/Amin): fΨ =

Imax + Imin I − Imin cos θ + max 2 2

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EXPERIMENTAL SECTION

Sample. The powder of poly(ethylene oxide) (PEO; Aldrich, Mv = 100 000) and the pellet of poly(L-lactic acid) (PLLA; Aldrich, Mw = 100 000−150 000) were used. The powder and pellet were dissolved in the chloroform, and a film was prepared by casting from this solution on a microscope slide glass. For the evaporation of the chloroform, the film was kept at room temperature in the vacuum for 3 h. The film was peeled off the slide glass and cut up into a 5 mm × 5 mm section. The thickness of PEO film was ∼8 μm, and it was put on a potassium bromide (KBr) plate, which is transparent to the infrared light. The PLLA/PEO blends film was sandwiched between two KBr plates. The PEO film on a KBr plate was placed on the hot stage, melted at 80 °C, and was crystallized under nonisothermal condition during cooling down to the room temperature. It is known that PEO crystal has the stable monoclinic system (a = 8.05 Å, b = 13.04 Å, c = 19.48 Å, β = 125.4°) in a lamella.48,49 The PLLA/PEO blends film was placed on the hot stage, melted at 190 °C for 2 min, and cooled down to 130 °C with a cooling rate of 30 K/min. PLLA and PEO are reported to be miscible in the melt and amorphous,50 and PEO does not cocrystallize with PLLA.51 The PLLA was crystallized under isothermal conditions at 130 °C, and the PEO was crystallized under

(2)

Ψ=ψ

In the case of the perpendicular band, in-plane orientation function, fΨ, is calculated from Dmin (= Amin/Amax): D −1 2 fΨ = min Dmin + 2 3 cos2 αM − 1 (3) Ψ = ψ + 90° 8357

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nonisothermal conditions during cooling down from 130 °C to room temperature. Measurements. The FT-IR imaging method was performed with Spectrum Spotlight 300 (PerkinElmer Inc.). Infrared absorption spectra were 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. All absorption peaks were corrected by the subtraction of baseline before the calculation of the molecular chain orientation. The orientation function was calculated under an assumption that αM in eqs 2 and 3 is 0° at parallel bands and 90° at perpendicular bands. The birefringence imaging was performed with Abrio (CRi Inc.). The Abrio system in an optical microscope (BX-51, Olympus inc.) contains a light source, a 546 nm interference filter, a left circular polarizer, a condenser, a sample stage, an objective lens (10×), a variable elliptical analyzer, and a digital camera connected via a computer. The variable elliptical analyzer is composed of a pair of electrical-controlled liquid-crystal retarder cells with 45° between their principal axes and a linear polarizer.38,47 Taking several polarized optical images with controlling the variable elliptical analyzer, the calculation of retardation and slow-axis azimuth images were automatically performed by the Abrio software. The spatial resolution is approximately equal to 1 μm when a 10× objective lens is used.

Results of FT-IR Imaging Method. Figure 3a,b shows the comparison of the image of in-plane orientation function fΨ of

Figure 3. Images of in-plane orientation function fΨ calculated at (a) 962 cm−1 (CH2 asymmetric rocking, parallel band) and (b) 842 cm−1 (CH2 asymmetric rocking, perpendicular band) in the spherulite of poly(ethylene oxide) measured with the FT-IR imaging method with four different polarizer angles. The value of fΨ corresponds to the false color shown as the right-side color bar. (c) Optical microscopic image of the same area without a polarizer.

PEO spherulite at 962 cm−1 (CH2 asymmetric rocking, parallel band) and 842 cm−1 (CH2 asymmetric rocking, perpendicular band),52 respectively. Figure 3c is an optical microscopic image of PEO spherulite on the same scale. The value is expressed by the false color presented in the side color bar. In-plane orientation function images visualize the shape of spherulite more clearly than the traditional orientation function image shown in Figure 2b. A similar pattern is obtained at both the parallel and perpendicular bands; such as the lower orientation function areas are found in the center and the interface of the spherulite. Figure 4 shows the orientation azimuth angle Ψ image of PEO spherulite calculated at (a) 962 and (b) 842 cm−1. The

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RESULTS AND DISCUSSION Figure 2a shows the traditional polarized light microscopic images of PEO spherulite with a crossed polarizer. The linear

Figure 2. (a) Polarized optical microscopic image of PEO spherulite with a crossed polarizer. The red arrows show the direction of the linear polarizer and analyzer on the measurement optical system. (b) Hermans orientation function image calculated with the dichroic ratio at 962 cm−1 at the same area obtained using FT-IR imaging. The value of Hermans orientation function corresponds to the false color shown as the right-side color bar.

Figure 4. Images of orientation azimuth angle Ψ calculated at (a) 962 cm−1 (CH2 asymmetric rocking, parallel band) and (b) 842 cm−1 (CH2 asymmetric rocking, perpendicular band) in the spherulite of poly(ethylene oxide) measured with the FT-IR imaging method with four different polarizer angles. The value of orientation azimuth angle corresponds to the false color shown as the right-side color circle.

polarizer and analyzer are set at parallel to the red arrows. The high brightness is observed at the high retardation area. However, the extinction is observed at the area where the slowaxis of an index ellipsoid is parallel to the polarization axis of the polarizer or analyzer. Thus, the brightness depends on both of the retardation and the direction of the slow axis. Figure 2b shows the Hermans orientation function images of PEO spherulites calculated from the dichroic ratio at 962 cm−1. A∥ and A⊥ are the absorbances measured under the linear polarizations parallel to the x- and y-axes, respectively. The value of each pixel is expressed as the false-color image with a color bar at the right side. The value of the Hermans orientation function depends on the angle between the orientation axis of molecular chain and the direction of used linear polarization. In order to visualize the magnitude of molecular chain orientation and the azimuth angle of the orientation axis independent of the polarizer angle, the four polarizations method is required.34

orientation azimuth angle shows the direction of orientation axis in the sample plane and independent from the magnitude of orientation function. The direction is expressed by the false color, depicted in the color circle. 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 orientation azimuth angle image calculated at parallel absorption bands almost corresponds to that at perpendicular absorption band. These images suggest that the molecular chain is oriented to the tangential direction of the spherulite at each point. The local drastic change of orientation azimuth angle observed at the most of interfacial areas is due to the difference of the lamella growth direction between the spherulite. This agrees with the fact that the PEO molecular 8358

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image (Figure 3a,b) than the retardation image (Figure 5a). In Figure 6a,b, we compare the profiles of in-plane orientation function (black line) and (gray line) retardation on the lines, containing the interface (line 1) and the center of spherulite (line 2). The width of low in-plane orientation function area is about 6 times of that of low retardation area. This width difference is almost same as the difference of the spatial resolution between two methods. On the other hand, in Figure 7a,b, the profiles of orientation azimuth angles (black line) are close to those of slow-axis azimuth angle (gray line) in spite of the difference of the spatial resolution. In our previous work,34 the orientation azimuth angle visualized the small change of molecular orientation in the early stage of the uniaxial drawing process. Therefore, the orientation azimuth angle is more sensitive to the change of molecular orientation than in-plane orientation function. Comparison with Vector Representation. In order to visualize the magnitude ( fΨ, Δ) and azimuth angle (Ψ, φ) of anisotropy at once, the vector representation is introduced. The magnitude and azimuth angle are expressed as the length and the direction of vector, respectively. Figure 8a shows the vector image of orientation azimuth angle Ψ and in-plane orientation function fΨ at 962 cm−1 determined from FT-IR imaging. Figure 8b shows the vector image of slow-axis azimuth φ and retardation Δ determined from birefringence imaging. These vector images are overlaid on the nonpolarized optical image of PEO spherulite. As it was previously mentioned, the low retardation is not observed at the interface on the line connecting the center points of the adjacent spherulite. The spatial distribution of vectors at the interface near the “centerto-center line” is visualized in the enlarged images of Figure 8a,b. The disturbance of direction with short length vectors is frequently observed at the interface far from the “center-tocenter line”, shown as the upper area in the both enlarged images. The vector representation enables to compare the orientation azimuth angle and slow-axis azimuth angle directly, independent of the difference of measurement coordinate systems between two methods. In Figure 8c, both the redcolored vectors obtained from FT-IR imaging and the bluecolored vectors obtained from birefringence imaging are overlaid on the optical image of PEO spherulite. The redcolored vector obtained from FT-IR imaging method almost corresponds to the blue-colored vectors obtained from the birefringence imaging method. Especially, the correspondence of the direction of orientation axis and slow axis of an index

chain crystallizes perpendicular to the lamella growth direction.49 Results of Birefringence Imaging Method. The birefringence imaging method visualizes the larger measurement area with higher spatial resolution than with FT-IR imaging method. Figure 5a shows the magnitude image of

Figure 5. Images of (a) retardation Δ and (b) slow-axis azimuth angle φ measured with birefringence imaging method. The false color corresponds to the right-side color bar (retardation) or color circle (azimuth of slow axis). The red rectangular area shows the measurement area of FT-IR imaging method. Black dotted line shows the line connecting the center of a spherulite to that of an adjacent spherulite.

retardation Δ of the PEO spherulite. The value is expressed by the false color, depicted in the color bar. The rectangular area corresponds to the measured area using FT-IR imaging method. In Figure 5a, the retardation value is approximately equal to 70 nm at the light-blue areas in spherulite. At the center and the interface between spherulite, retardation value decreases to ∼20 nm. The black dotted lines in Figure 5a show the line connecting the center points of the adjacent spherulite. On this line, the decrease of retardation at the interface is not observed. The comparison of fΨ and Δ at the interface near the “center-to-center line” is visualized with vector representation at the later part. Figure 5b shows the azimuthal image of slow-axis φ of the same PEO spherulite with false color, depicted in the color circle. The slow-axis is oriented to the tangential direction of the spherulite. This is a typical characteristic of the negative spherulite. The black dotted lines in Figure 5b also show the “center-to-center lines”, on which the slow-axis azimuth angle shows a little difference across the interface. Comparison of Two Methods with Line Profiles. The magnitude images obtained by two methods show the obvious difference. The decrease in the center and the interface are observed at much larger area of in-plane orientation function

Figure 6. Line profiles on (a) line 1 and (b) line 2 of in-plane orientation function image at 962 cm−1 and the retardation image. Black and gray lines show the profiles of in-plane orientation function and retardation, respectively. 8359

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Figure 7. Line profiles on the (a) line 1 and (b) line 2 in the orientation azimuth image at 962 cm−1 and the slow-axis azimuth image. Black and gray lines show the profiles of orientation azimuth angle and slow-axis azimuth angle, respectively.

Figure 8. (a) Vector representation of orientation azimuth angles Ψ calculated at 962 cm−1 is overlaid on the optical microscopic image. The vector length is in proportion to the in-plane orientation function fΨ measured with FT-IR imaging method. (b) Vector representation of slow-axis azimuth angle φ is overlaid on the optical microscopic image. The vector length is in proportion to the retardation Δ measured with birefringence imaging method. (c) Comparison of enlarged image near the interface in (a) and (b) obtained from (red) FT-IR imaging and (blue) birefringence imaging.

Figure 9. (a) Chemical image of absorbance ratio at 842 cm−1 (PEO) to 872 cm−1 (PLLA) of the PLLA/PEO (50/50) blend spherulite sandwiched between two KBr plates. (b) Vector image of PLLA/PEO blend spherulite with red vector calculated at PLLA band (872 cm−1) and light-blue vector calculated at PEO band (842 cm−1). The vector length is in proportion to in-plane orientation function fΨ measured with FT-IR imaging method. The images in (c) and (d) show the enlarged one of the “light-blue area” and “red area” in (b), respectively.

ellipsoid is originated from the chemical structure of PEO, which has no polar group at the side chain. Application to Polymer Blend Spherulite. FT-IR imaging with four polarizations method is applied to the molecular orientation analysis of PLLA/PEO blend spherulite with chemical imaging. Figure 9a shows the chemical image of nonpolarized absorbance ratio at 842 cm−1 assigned to CH2 asymmetric rocking of PEO52 to 872 cm−1 assigned to C− COO stretching of PLLA53 at around a large spherulite in PLLA/PEO (50/50) blends. The value is expressed by the false color presented in the side color bar. This image almost corresponds the distribution of PEO fraction. The low value areas colored in purple, blue, or light blue are observed in the spherulite. On the other hand, much higher value of PEO fraction is shown at the outside area of the spherulite. The PLLA/PEO blend film was crystallized under isothermal conditions at 130 °C, which is above the melting temperature of PEO. Thus, only PLLA was crystallized and the exclusion of PEO component took place during the PLLA spherulite growth at 130 °C. This agrees with the fact that the smaller PEO

fraction (larger PLLA fraction) is observed at the center of spherulite in Figure 9a. The areas colored in blue or light blue in the spherulite shows that few PEO components remains in the PLLA spherulite. This typical chemical image is compared with the quantitative molecular chain orientation image calculated with four polarizations method. In-plane orientation function fΨ and orientation azimuth angle Ψ are calculated at 842 cm−1 (PEO) and 872 cm−1 (PLLA). The vector image of the spherulite of PLLA/PEO blend is shown in Figure 9b. The vector length is in proportion to fΨ and the vector direction corresponds to Ψ. In Figure 9b, the both vectors of PLLA and PEO show the higher value of orientation function in the spherulite, but no orientation at the outside areas. In the spherulite, the redcolored vector distribution calculated at PLLA band is clearly different from the light-blue colored vector distribution calculated at PEO band. The pattern of “light-blue color area” and “red color area” in Figure 9b does not correspond perfectively to the chemical distribution in Figure 9a. However, 8360

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the most of high PEO fraction areas correspond to the “red color areas”. Figures 9c and 9d are the enlarged image of the “light-blue color area” and “red color area” in Figure 9b, respectively. In Figure 9c, at the small PEO fraction area, both the red and light-blue colored vectors are oriented to the tangential direction of the spherulite. On the other hand, in Figure 9c, at the large PEO fraction area, some of the light-blue colored vectors are oriented to the different direction from the red color vector. In consideration of chemical image of Figure 9a, the spherulite growth was disturbed by the existence of PEO molecular chain at the high PEO fraction areas as shown in Figure 9d. The FT-IR imaging method with four polarizations method visualizes not only the typical chemical image but also the difference of spatial distribution of the molecular chain orientation between PLLA and PEO in the spherulite.

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CONCLUSION The molecular chain orientation of the spherulite of poly(ethylene oxide) (PEO) is visualized using four polarizations method of FT-IR imaging in comparison with the optical birefringence imaging. The vector representation enables to compare the distribution of orientation azimuth angle and slowaxis azimuth angle directly, independent of the difference of measurement coordinate systems between two methods. Azimuthal images obtained by both methods show a good correspondence of the orientation axis of the molecular chain in PEO crystal and the slow-axis of a refractive index ellipsoid, and the orientation of those axes is to the tangential direction of the spherulite. In spite of the 6 times difference of the spatial resolution, the azimuth angle determined from FT-IR imaging shows the similar line profiles to that from slow-axis analysis determined by birefringence. The application of the FT-IR imaging method to the molecular orientation analysis of the polymer blend spherulite is examined that enables to visualize the different spatial distribution of molecular chain orientation between PLLA and PEO in the blend spherulite. The advanced FT-IR imaging method is presented with the azimuthal visualization and vector representation for the analysis of molecular chain orientation of polymeric spherulite.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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