Characterization of anisotropic structure in poly(phenylene vinylene) films.
It is well known that the properties of conventional polymers are strongly influenced not only by the structure of the material but also by the magnitude of molecular orientation (1). This is no less true for electrically conducting polymers where chain alignment improves the strength and conductivity (2, 3). A knowledge of the anisotropic structure is a principal ingredient for predicting behavior and is also the factor that couples the fabrication process to the physical properties.
Poly(phenylene vinylene)(PPV) has received considerable interest in recent years because of its remarkable electrical and mechanical properties and its tractability through precursor routes. The indirect two-stage synthesis involves a water-soluble precursor intermediate, which can be fabricated into the desired form prior to conversion. A number of precursor salts have been tried and these can be classified into two categories: those containing dialkyl sulfonium pendant groups such as dimethyl sulfonium and those containing cyclic sulfonium groups such as tetrahydrothiophenium (Fig. 1). It was found that PPV obtained through the precursor salt containing cyclic sulfonium side groups is more crystalline and has higher average conjugation length (4).
A significant advantage of the precursor route is that the morphology of PPV can be controlled by varying the conditions of the precursor conversion step. Oriented films are usually prepared by stretching the precursor film while increasing the stretching temperature so that orientation and conversion occur simultaneously. By varying the load and temperature scheme, oriented PPV films with final draw ratios of up to 20 have been obtained (5, 6).
The ability to stretch-align the precursor polymer offers the opportunity to study the anisotropic properties of PPV and its intrinsic structural parameters. Bradley (7, 8) gave detailed infrared assignments and some transition moment angles of PPV converted from dialkyl sulfonium precursor. Granier et al. (5, 9) conducted a detailed analysis of the crystalline structure of PPV converted from dialkyl sulfonium precursor using electron diffraction on thin sections of oriented film. They found the neutral PPV, while highly crystalline, exhibits significant paracrystalline disorder of the second kind. Shah et al. (10) studied the structural anisotropy of unstretched and uniaxially stretched PPV prepared from the cyclic sulfonium precursor using X-ray diffraction. A strong preferred orientation was found for the unstretched cast film with PPV chains lying in the plane of the film. Gymer et al. (11) measured the refractive index of the PPV films converted from cyclic precursor and also indicated a highly anisotrop ic structure of the unstretched PPV film.
In this study, the anisotropic structure of unstretched and stretched PPV films synthesized through the cyclic sulfonium precursor was studied. The precursor films were stretched at a low temperature (60[degrees]C) before converting to PPV at 200[degrees]C; the effect of this procedure on the orientation of the PPV films is discussed.
This study was a collaborative effort between Clemson University and Georgia Institute of Technology. The synthesis and drawn film processing were done at Clemson University while film structure characterization using the waveguide, FTIR and X-ray was done at Georgia Institute of Technology.
Synthesis and Film Preparation
PPV was synthesized via the tetrahydrothiophenium salt precursor route (12). The polymerization was carried out by the aqueous reaction of the sulfonium salt monomer with equal molar amount of sodium hydroxide at 0[degrees]C for 1 hour under nitrogen atmosphere. The reaction was quenched by neutralization with HC1 solution to a final pH of 4-6. The polyelectrolyte precursor polymer was dialyzed against deionized water for three days to remove any residual monomers and oligomers.
The PPV precursor film was obtained by casting the aqueous solution on a glass plate. The films were first drawn at ~60[degrees]C and then heated at fixed length at 200[degrees]C under vacuum for 2 hours to convert to PPV.
The three-dimensional refractive indices of the freestanding PPV films were characterized with a modified prism waveguide coupler (Metricon PC-2010) at 1550 nm. The technique was previously developed and tested on a range of different freestanding and spin coated polymer films (13). In this paper, Nz is the refractive index along the optical symmetry axis, Ny is the refractive index perpendicular to the symmetry axis in the film plane, while Nx is the refractive index normal to the film plane.
Polarized FTIR Spectroscopy
A Nicolet 6OSX FTIR spectrometer was used as a second characterization technique to study the anisotropy of the same series of stretched PPV films. A double wire grid polarizer was used to measure the polarized infrared spectra along and perpendicular to the stretch direction. The spectra were collected at 2 [cm.sup.-1] resolution with a CsI beamsplitter and TGS detector. To reduce the noise level, 3200 scans were coadded per spectrum.
The Hermans' orientation function, f, was determined through the following equation:
f = 3 < [cos.sup.2][theta] > - 1/2 =[(D - 1/D + 2).sub.v] [([D.sub.0] + 2/[D.sub.0] - 1).sub.v](1)
where [theta] is the average angle between the chain axis and the molecular symmetry axis, D is the dichroic ratio, which is the ratio between the absorbance of a particular band along the symmetry axis and the absorbance perpendicular to the symmetry axis. [D.sub.0] is the intrinsic dichroic ratio and is related to the transition moment angle, al., through Eq 2:
[D.sub.0] = 2 [cot.sup.2][[alpha].sub.v] (2)
Flat Plate X-ray Diffraction
X-ray diffraction patterns were obtained using a Warhus flat plate camera. Rigaku-Geigerflex X-ray tube with rotating copper anode was the X-ray source. The maximum output of the tube was 50 kV-50 mA and the tube was operated at 40 kV and 25 mA. A nickel filter was employed and the wavelength of the [K.sub.[alpha]] radiation was 1.5418 A. X-ray photographs were obtained using a phosphor screen which was exposed for 3 hours. The films were mounted on a sample holder that was Inserted into the sample chamber. The draw direction of the drawn films was parallel to the vertical direction. During the exposure, vacuum was applied to the sample chamber to minimize air scattering. The phosphor screen collected the scattered X-ray signal. The screen was then scanned using a PhosphorImager scanner made by Molecular Dynamics Inc. The image was displayed and analyzed using ImageQuant software provided by Molecular Dynamics Inc.
RESULTS AND DISCUSSION
1. Three-Dimensional Refractive Indices
Figure 2 shows the change in the three-dimensional refractive indices of PPV films during stretching. The first thing to notice is that the undrawn PPV film shows highly planar characteristics. Nz and Ny in the film plane are significantly larger than Nx, the refractive index through the film plane. This means that the polymer chains tend to lie parallel to the surface of the film plane (14). This planar characteristic is similar to that reported by Gymer et al. (11) for PPV films converted from the cyclic sulfonium precursor salt. However, the undrawn film is not completely random in the film plane as Nz is greater than Ny. This is attributed to the film preparation method used in this study. All the precursor films, drawn or undrawn, were held at constant length during conversion to PPV films. The undrawn film, although free from stress in the precursor form, had some stress developed in the film as elimination products from the conversion reaction were removed during heating at 200[degrees]C. As the draw r atio is increased, Nz increases as the molecules align themselves along the draw direction, while Ny decreases. However, there is only a slight decrease in Nx. This again indicates the highly planar characteristics of the PPV films. Because the polymer chains lie mostly flat in the film plane, there is not much room for Nx to decrease further as the draw ratio is increased.
The moloecular anisotropy develops rapidly. At 100% extension, which corresponds to a draw ratio of 2, Nz, reaches the maximum level. Also, Ny and Nx are very close. This means that the refractive indices transverse to the draw direction are symmetric and the film has an optically uniaxial structure. Thus one way stretching of the PPV films converts the film from a planar structure to an optically uniaxial structure. The fact that the sample at a draw ratio of 2.5 has a slightly lower Nz shows some slippage may have occurred for this sample during stretching.
From Fig. 2, we can also see that the average refractive index of the PPV films stays constant during stretching. The average refractive index, which is the average of the three principal refractive indices, is proportional to the density and hence is proportional to the crystallinity. That the average refractive index does not change during stretching implies the crystalline conversion at 200[degrees]C of precursor to PPV is independent of the orientation level of the precursor.
2. Polarized Infrared
In polarized FTIR, measurements are made when polarized infrared light is aligned along and perpendicular to the stretch direction. The dichroic ratio is defined as the ratio of the absorbances parallel to and perpendicular to the stretch direction. If the dichroic ratio is greater than one, the absorbance band is called a pi band; if the dichroic ratio is smaller than one, it is a sigma band. In Fig. 3, the dichroic ratios of several of the absorbance bands are shown as a function of the draw ratio. The assignments of these bands can be found in Bradley's paper 7. The pi bands are shown in Fig. 3a. These bands have dichroic ratios greater than one. The dichroic ratios increase with draw ratio with some fluctuation at the highest draw ratios. The increase in dichroic ratios of these pi bands indicates increasing molecular orientation along the draw direction. The variation in dichroic ratios at the highest draw ratios is similar to that observed for refractive indices in Fig. 2.
In Fig. 3b the dichroic ratios of the sigma bands are shown as a function of draw ratio. The sigma bands have their transition moments vibrating more or less perpendicular to the polymer chain axis and thus a higher level of orientation leads to a lower value for the dichroic ratio. For all these sigma bands, the dichroic ratios decrease with draw ratio with some fluctuation at the highest draw ratios. The decrease of dichroic ratio of these sigma bands also indicates increasing molecular orientation along the stretch direction.
The Hermans' orientation function of these oriented PPV films estimated using the dichroic function of the 1519 [cm.sup.-1] absorption band is shown in Fig. 4. The transition moment angle of 9[degrees] is adopted from Bradley's paper (7). Since no distinction is made of whether the bands belong to the crystalline or noncrystalline regions, this is assumed to be an average band. As shown in Fig. 4, a high degree of orientation is reached by 100% extension (draw ratio = 2) and then It somewhat levels off. One reason for the rapid increase in orientation function with stretching is the fact that our precursor samples were stretched at a lower temperature (60[degrees]) than is commonly employed. The common practice is to stretch the PPV precursor film while increasing the temperature so that orientation and elimination occur simultaneously in a temperature range of 60[degrees]C-150[degrees]C (8), 60[degrees]C-120[degrees]C (15), 125[degrees]C-180[degrees]C (5). 80[degrees]C-130[degrees]C (16), 100[degrees]C-120 [degrees]C (17), 115[degrees]C-180[degrees]C (18). Although high temperature stretching has the advantage of allowing for high draw ratios, relaxation of the orientated polymer chains can also occur more easily. At a low stretching temperature, relaxation of the polymer chains occurs less readily. Thus the orientation functions for our PPV films stretched at 60[degrees]C are higher than the orientation functions of the stretched PPV films obtained by Gagnon et aL (18) using infrared dichroism at corresponding draw ratios, where the PPV precursor films were stretched in the temperature range of 115[degrees]C-180[degrees]C.
3. X-ray Diffraction
The flat plate X-ray photographs of the oriented PPV films as well as the unstretched film are shown in Fig. 5. The discrete rings and arcs are indicative of a crystalline structure. As the draw ratio is increased, the rings develop into arcs and the arcs become shorter and shorter, showing increasing molecular orientation.
The crystal structure of PPV is found by Granier et aL (5) to have a monoclinic unit cell with the following parameters: a = 0.790 [+ or -] 0.005 nm; [alpha] [congruent to] 123[degrees]; b = 0.605 [+ or -] 0.005 nm; c = 0.658 nm. It does not make a difference whether the polymer is converted from the dimethylsulfonium precursor salt (5) or from the cyclic sulfonium precursor salt (10). The d-spacings of the major reflections in Fig. 5 agree with the reported crystal structure.
These X-ray photographs (Fig. 5) are transmission patterns. As illustrated in Fig. 6a., the photographs were collected with the X-ray beam perpendicular to the film plane (parallel to the x-axis). The normal transmission patterns in Fig. 5 only show the orientation that exists within the film plane.
In order to reveal any anisotropy that may exist through the film plane, X-ray photographs (Fig. 7a and b) were collected with the PPV film rotated 75[degrees] around the z-axis (stretch direction) so that the y-axis is now aligned 15[degrees] from the X-ray beam as shown in Fig. 6b. In this figure, the draw direction (the z-axis) is perpendicular to the paper. The reader is looking down the draw direction at the edge of the film. The angle [theta] of about 15[degrees] was chosen to be just big enough to ensure the X-ray beam hits the polymer film sample.
Since the tilting angle of 15[degrees] is close to the Bragg angle of the (200) reflection (11.3[degrees], (5)). different patterns of the (200) reflection are expected for different crystal orientations with respect to the film plane. Thus if the (200) crystal planes are randomly distributed, a circle is expected in the X-ray photograph. If all the (200) crystal planes are parallel to the z axis, but uniformly distributed around the z axis, with an equal proportion of planes to the left and to the right of the X-ray beam (Fig. 6c), two spots will show on the X-ray photograph. If all the crystal planes are parallel to the film surface, then only one spot will be shown to the right of the X-ray photograph (Fig. 6d).
Figure 7a is an X-ray photograph of the unstretched PPV film collected with the y-axis tilted 15[degrees] from the X-ray beam. In addition to the (110) circular ring, an obvious short arc of the (200) reflection is shown in the equatorial direction, with the arc on the right much darker than that on the left. According to Shah et al. (10), PPV crystals of the unstretched film exhibit a preferred orientation in the film plane with the a axis perpendicular to the film plane. This means the (200) plane is preferably parallel to the film plane. The predominant (200) planes that are parallel to the film plane contribute to a stronger arc to the right of the X-ray photograph (Fig. 7a). The fewer crystal planes that deviate from the film plane and form the Bragg angle to the right of the x-ray beam will contribute to the relatively weak reflection to the left of the x-ray photograph. Figure 7a confirms the conclusion from Shah et al. (10) that the a axis is preferably perpendicular to the film plane in the unstretch ed cast PPV film.
Figure 7b shows the X-ray photograph of the PPV film stretched at 200% extension (draw ratio = 3.0) collected with the -y-axis tilted 15[degrees] from the X-ray beam. It has an even distribution of the (200) reflections on both sides of the equatorial direction. This means that the (200) crystal planes are no longer predominantly parallel to the film surface; rather, they are uniformly distributed around the stretch direction, In other words, the highly stretched PPV film has fiber symmetry around the stretch direction. This explains why Fig. 7b looks very much like the 200% extension and 100% extension transmission patterns in Fig. 5. Because the drawn film has fiber symmetry, the X-ray patterns obtained will be the same in all directions, so long as the X-ray beam is normal to the z-axis (stretch direction).
An illustration of the change in orientation of PPV film upon stretching is shown in Fig. 8. In this figure, [alpha] is the angle between the b and c axes, since the PPV crystal is monoclinic (5). In the unstretched state, the PPV crystals have a preferred orientation with the a axis perpendicular to the film plane. The b and c axes have a random orientation around the a axis. Since the PPV chains are along the c axis, the chains lie flat in the film plane and have a random orientation in the film plane. When the film is stretched to high orientation, the chains align themselves toward the stretch direction. Now the c axis becomes the symmetry axis and the a and b axes have a random orientation around the c axis. Thus, one-way stretching converts the film from a planar structure to a uniaxial structure. This is consistent with the observations of Shah et al. (10).
Both refractive index measurement and tilted angle X-ray diffraction experiment show unstretched PPV film has a highly planar structure with a tendency for the PPV chains to be lying parallel to the film surface. The a axis of the PPV crystals is perpendicular to the film plane.
Orientation within the PPV film develops rapidly with stretching. One reason may be the stretching of the precursor film at a lower temperature before converting the precursor film.
One-way stretching randomizes the otherwise surface-induced orientation of PPV crystallites and converts PPV film from a planar structure to a uniaxial structure.
The average refractive index of the stretched film is independent of the draw ratio. This implies the amount of crystallinity developed from converting precursor film to PPV film is independent of the orientation state of the precursor film.
The authors wish to thank the National Textile Center for support of this work.
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|Author:||Ou, Runqing; Samuels, Robert; Wang, Xingwu; Gregory, Richard|
|Publication:||Polymer Engineering and Science|
|Date:||Oct 1, 2001|
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