A Study on the Miscibility of Selected Blends of Methyl Methacrylate

Dec 6, 1999 - The blend compatibility of methyl methacrylate (MMA)−benzyl methacrylate (BzMA) copolymers with different monomer compositions has bee...
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Ind. Eng. Chem. Res. 1999, 38, 4675-4681

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A Study on the Miscibility of Selected Blends of Methyl Methacrylate-Benzyl Methacrylate Copolymers Joohyeon Park,† Kookheon Char,† and C.-W. Park*,‡ Division of Chemical Engineering, Seoul National University, Seoul, Korea 151-742, and Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611

The blend compatibility of methyl methacrylate (MMA)-benzyl methacrylate (BzMA) copolymers with different monomer compositions has been studied to investigate their feasibility as plastic optical fibers. Copolymers with variable comonomer composition were first synthesized, and the blend compatibility between them was investigated by solution blending and melt blending. The results of 1H NMR, ellipsometry, and differential scanning calorimetry indicate that the copolymers synthesized in this study are random copolymers, with their refractive index and the glass transition temperature varying linearly with their monomer content. While the solutionblended samples indicated immiscibility between copolymers with large differences in their monomer composition, melt-blended samples indicated miscibility showing single glass transition temperature and homogeneity in scanning electron micrographs. The difference between the results of solution blends and melt blends may be attributed to the slow kinetics of phase separation and the intensive shear energy input during melt blending. These results suggest that MMA-BzMA random copolymers are appropriate materials applicable to the coextrusion method of Park and Walker (U.S. Patent 89 929 161, 1997) for the fabrication of graded-index plastic optical fibers. 1. Introduction With the fast advancement of microelectronics technology, graded-index plastic optical fibers (GI-POFs) have drawn much interest recently as new media for high-speed data transmission for local area networks (LANs). While high bandwidth communication can be achieved by single-mode glass optical fibers (GOFs), the GOFs are as thin as 5-10 µm in diameter. Consequently, they are extremely fragile and difficult to make connections with, making them practically unusable in LANs where cables must be installed along very curved paths and where frequent connections must be made. GI-POFs, on the other hand, are flexible and durable, allowing the fibers to be large in diameter (in the order of 1 mm) and making them a viable substitute for singlemode GOFs. GI-POF refers to a fiber with a continuously varying refractive index profile in the radial direction in contrast to a step-index (SI) fiber, which has a core-cladding structure with a step change in the refractive index profile. The refractive index profile of GI-POFs, which is nearly parabolic, minimizes the modal dispersion of input signals, thereby making high bandwidth data transmission possible.1 A continuously varying refractive index profile of a GI-POF can only be created by changing the material composition in the direction of the refractive index change (i.e., in the radial direction). To date, three different types of methods have been suggested for the fabrication of GI-POFs. The first method is known as the interfacial-gel polymerization technique, which utilizes a mixture of two monomers with different reactivity or diffusivity or a mixture of a monomer and a nonreactive additive (or dopant) to create a continu* To whom all correspondence should be addressed. † Seoul National University. ‡ University of Florida.

ously varying material composition.2-8 This method is a batch process in which optical fibers are made by preform manufacturing followed by thermal drawing of the preform. The second is an extrusion method utilizing diffusional characteristics of low molecular weight materials.9,10 In these methods, the refractive index profile of the GI-POFs is solely determined by the diffusional characteristics of the materials. Thus, there exists a difficulty in controlling the refractive index profile which is one of the most important properties of GI-POFs to achieve a high data transmission rate (or high bandwidth). Recently, another method has been introduced by Park and Walker.11 Their method is a coextrusion process in which a continuously varying material composition is created through blending of two polymers in an innovative die. Because the refractive index profile is due to the variation of the blend composition of two polymers, the refractive index profile is stable. In addition, this method is capable of controlling the refractive index profile by mechanical means unlike other methods.11,12 This method, however, requires two polymers with different refractive indices which are miscible at any blend composition. While miscibilities between different polymers are poor in general, the required difference in refractive indices for GI-POFs is rather small. Thus, copolymers with slightly different monomer composition, which may be totally miscible, can be used for this application. We have studied the blend compatibility of methyl methacrylate (MMA)-benzyl methacrylate (BzMA) random copolymers with different monomer compositions to investigate their feasibility as the materials applicable to the processing method of Park and Walker for GI-POFs, and the results are reported in this paper. MMA and BzMA monomers have been chosen because they can be readily polymerized by the same mechanism

10.1021/ie9903083 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/06/1999

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Table 1. Copolymers Synthesized in This Study polymer

fMMAa

FMMAb

MWc × 103

PId

PMMA PBzMA copolymer 2 copolymer 3 copolymer 4 copolymer 5 copolymer 6

1.0 0.0 0.85 0.70 0.50 0.30 0.15

1.0 0.0 0.854 0.748 0.548 0.354 0.232

240 112 210 305 338 384 151

1.92 3.28 2.90 1.83 2.96 2.63 2.22

a MMA mole fraction in the feed. b MMA mole fraction in the copolymer. c Number-average molecular weight. d Polydispersity index.

such as free-radical polymerization and their homopolymers (i.e., PMMA and PBzMA) have a large difference in refractive index (1.492 vs 1.562). In addition, the copolymers of MMA and BzMA are known to be appropriate for optical fiber applications.2-4,9,13,14 In this study, random copolymers of MMA and BzMA with variable comonomer compositions were made, and a mixing study was conducted with the samples prepared by both solution mixing and mechanical mixing in the molten state to examine the blend compatibility between the copolymers. 2. Experiments and Results The present study involved syntheses of MMA-BzMA copolymers with variable comonomer composition and characterization of each copolymer for their comonomer composition by 1H NMR, the refractive indices by an ellipsometer (Rudolph Research, model Auto EL-II), and the glass transition temperature by a differential scanning calorimeter. Blend compatibility between the copolymers at various mixing ratios was then investigated by measuring the cloud points of films cast from solution blends using light scattering technique and by examining the scanning electron micrographs and the glass transition temperatures of melt-blended samples. 2.1. Copolymer Preparation and Characterization. PMMA and PBzMA homopolymers and MMABzMA copolymers with different comonomer compositions were made by bulk polymerization in a test tube without agitation at 65 °C using 2,2′-azobis(isobutyronitrile) (AIBN) as an initiator. Each polymerization reaction was terminated at about 10% conversion to avoid composition drift, which may result from the difference in the relative reactivity of the comonomers. The homopolymers and copolymers synthesized in this study are listed in Table 1. The mole fraction of the comonomers in each copolymer was measured by 1H NMR to calculate monomer reactivity ratios and the χ value of the copolymer. χ is a parameter which indicates the randomness of the copolymer. χ ) 0 and χ ) 1 represent a block copolymer and a purely random copolymer, respectively, whereas χ ) 2 represents a completely alternating copolymer. As will be shown, the χ value can be calculated once the reactivity ratios of the monomers are known. Figure 1a shows the mole fraction of MMA in each copolymer as a function of comonomer feed composition (i.e., FMMA vs fMMA). The synthesized copolymers apparently have a slightly higher mole fraction of MMA than that in the feed (i.e., monomer mixture). This indicates that the relative reactivity of MMA is slightly larger than that of BzMA. If the copolymerization reaction is driven to a much higher conversion, the difference in the MMA mole fraction between the copolymer and the feed may

Figure 1. (a) Dependence of the instantaneous copolymer composition (FMMA) on the initial comonomer feed composition (fMMA). (b) Determination of reactivity ratios of MMA and BzMA using the Fineman-Ross method.

be minimized in an average sense although compositional nonuniformity between copolymer molecules may exist. Using the values of fMMA and FMMA, the reactivity ratios of MMA and BzMA can be estimated by the Fineman-Ross method:15,16

()

x(1 - X) x2 ) rBMA - rMMA X X

(1)

Here X and x are the molar ratios of MMA to BzMA in the copolymer and in the feed, respectively (i.e., x ) fMMA/(1 - fMMA) and X ) FMMA/(1 - FMMA)). rMMA and rBzMA are the reactivity ratios of MMA and BzMA, respectively. Using the values of fMMA and FMMA given in Table 1, X and x for each copolymer were calculated and, subsequently, the x(1 - X)/X vs x2/X plot was prepared as given in Figure 1b. This figure indicates a linear relationship between x(1 - X)/X and x2/X, and the slope and the intercept of the straight line yield the reactivity ratios of each monomer. The estimated reactivity ratios were 0.968 and 0.546 for MMA and BzMA, respectively, and the coefficient of correlation (R2) was 0.996. These values are somewhat different from those reported by Koike and Nihei, which are 0.93 and 1.05 for MMA and BzMA, respectively.3 This disagreement can be due to the differences in the polymerization conditions which result in different microstructures of the resulting copolymers in general. In our study, the copolymerization was carried out up to a 10% conversion

Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4677 Table 2. Microstructure of the Copolymers polymer

χa

〈lMMA〉b

〈lBzMA〉c

copolymer 2 copolymer 3 copolymer 4 copolymer 5 copolymer 6

1.066 1.119 1.155 1.147 1.099

6.484 3.205 1.968 1.415 1.171

1.096 1.239 1.546 2.273 4.091

a Randomness of copolymers (χ ) 1, random copolymer; χ ) 2, alternating copolymer). b,c Number-average sequence length of MMA and BzMA, respectively.

at 65 °C, whereas it was driven to a complete conversion of near 100% at 70 °C in Koike and Nihei.3 As was pointed out previously, if the copolymerization reaction is driven to a complete conversion, the difference in the MMA mole fraction between the copolymer and the feed may be minimized in an average sense because compositional nonuniformity between copolymer molecules can be created. Once the reactivity ratios rMMA and rBzMA are known, the number-average sequence length (〈lMMA〉 and 〈lBzMA〉) of each monomer and the randomness parameter χ of MMA-BzMA copolymers can be calculated using the first-order Markov statistics as follows:15,17

〈lMMA〉 ) 1 + rMMAx

and

〈lBMA〉 ) 1 + rBMA/x (2a,b)

χ)

rMMAx + 2 + rBMA/x rMMAx + 1 + rMMArBMA + rBMA/x

(3)

Using the values for rMMA and rBzMA determined from eq 1 along with x in Table 1, the number-average sequence length for each copolymer (〈lMMA〉 and 〈lBzMA〉) and the χ values were determined and the results are listed in Table 2. As was pointed out previously, the χ values of 0, 1, and 2 indicate a block copolymer, a purely random copolymer, and a completely alternating copolymer, respectively. As the χ values in Table 2 indicate, all copolymers in the present study are nearly random copolymers, which is desirable for the purpose of GI-POF application. It is also noted that the numberaverage sequence length of MMA, 〈lMMA〉, decreases and 〈lBzMA〉 increases at the same time as the BzMA content in the copolymer is increased. In Figure 2, the refractive index and the glass transition temperature of the copolymers are plotted as a function of the BzMA content in the copolymer. When the BzMA fraction in the copolymers is increased, the refractive index of the copolymer increases whereas the glass transition temperature decreases with increasing BzMA. We may note that their dependence on the BzMA content is almost linear in both cases because a simple linear relationship (solid line) provides a good fit to the data. The molar refraction is known to be proportional to the induced dipole moment and, consequently, the phenyl group has higher molar refraction than other groups in the MMA-BzMA copolymers. Thus, the copolymer with the higher BzMA fraction has the higher refractive index.18,19 The relationship between the refractive index and the BzMA content in the copolymer is important information for POF applications because the refractive index profile determines the numerical aperture of either SIPOFs or GI-POFs, which is an indication of lightgathering ability, and the bandwidth (or data transmission rate) of GI-POFs for communications. The

Figure 2. Effect of the BzMA content on (a) the refractive index and on (b) the glass transition temperature.

linear relationship will make the fine-tuning of the refractive index profile easier. 2.2. Solution Blends. Blend compatibility is an essential requirement for the polymers to be used in the method of Park and Walker11 to produce GI-POFs. Because the refractive index profile of GI-POFs is obtained by the continuous variation of the blend composition in the radial direction of the fiber, two polymers used in the method of Park and Walker must be miscible at any blend composition. We have investigated the blend compatibility of the MMA-BzMA copolymers listed in Table 1. In the case of GI-POFs for high-bandwidth communications, the most appropriate value of the overall difference in the relative refractive index profile (∆n) is about 0.01.4 Here ∆n ) (n1 - n2)/ n1 where n1 and n2 are the refractive indices at the center and at the outer edge of a GI-POF, respectively. We have chosen three different combinations of the copolymers with which the difference in the refractive indices can be matched closely to the ideal value. Those are copolymer 2/4, copolymer 4/5, and copolymer 5/6 pairs. These copolymer pairs have relative refractive index differences (∆n) of 0.015, 0.009, and 0.009, respectively. Solution blends of the three copolymer pairs were made with variable blend compositions by dissolving them in toluene at 10 wt %. Films were made by solvent casting followed by drying at room temperature for 7 days and additional drying for 3 days in a vacuum oven at 60 °C. The cloud point of each blend was determined by monitoring the intensity of scattered light at a 20° angle through the cast film. A He-Ne laser (λ ) 633 nm) was used as the light source. The films were

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Figure 3. Cloud point vs weight fraction of the A component for copolymer2(A)/4(B) and copolymer5(A)/6(B) pairs. (Curves are drawn to indicate the trend only.)

initially annealed in a preheating block at 60 °C prior to the light-scattering experiment followed by gradual heating to 250 °C at a rate of 1 °C/min while examining the light-scattering behavior. The temperature at which the intensity of scattered light started to show a sudden increase was defined as the cloud point of the sample. The phase behaviors of solution-blended mixtures are known to depend on the solvent used and the casting temperature.20,21 We tested several solvents for our system including toluene, acetone, and tetrahydrofuran (THF). While clear films of MMA-BzMA copolymer pairs were obtained upon casting from toluene, the films casted using acetone or THF became turbid. Because transparent cast films indicate that the solution blends of each copolymer pair may be in a homogeneous state, we used toluene as the solvent for this study. As the quiescent phase diagrams shown in Figure 3 indicate, two copolymer pairs (i.e., copolymer 2/4 and copolymer 5/6) showed phase separation virtually at all blend compositions. The blends of each copolymer pair were initially homogeneous as indicated by the transparency of the films at 60 °C. Upon increasing the temperature, however, the films became turbid, indicating phase separation between the two copolymers. The cloud-point curves of these copolymer pairs exhibit the lower critical solution temperature (LCST) behavior. While the binary mixtures of copolymers shows no evidence of phase separation at all compositions below the LCST, phase separation becomes evident above the LCST. It is also noted that the cloud point of the copolymer 2/4 blend is about 30 °C lower than that of the copolymer 5/6 blend. This is believed to be due to the larger difference in the monomer composition of copolymer 2/4 compared to that of copolymer 5/6. This result seems to indicate that the critical temperature of the MMA-BzMA copolymer blend is quite sensitive to the difference in the monomer composition between the two copolymers that are blended presumably because of the inherent incompatibility between MMA and BzMA segments. However, the copolymer 4/5 pair showed no evidence of phase separation at all blend compositions up to 250 °C. This apparent miscibility is inconsistent with above results considering the fact that the difference in the monomer composition of copolymer 4/5 is greater than that of the copolymer 5/6 pair which showed a phase separation. As will be described later, this inconsistency may be attributed to the lower mobility of the blend pair associated with the higher

molecular weight of the copolymer 4/5 pair than those of the 2/4 and 5/6 pairs. 2.3. Melt Blends. Melt blends were prepared using a Mini-Max Molder at 200 °C and 50 rpm (Custom Scientific Instruments, model CS-183MMX). All materials were annealed in a vacuum at 70 °C for 48 h before mixing in order to remove any volatile components. A dry-blended sample was fed into the preheated mixer and heated for 2 min before mixing started. Two different mixing times (3 and 8 min) were employed for this study. After mixing for the specified duration, the sample was pressed in a hot press. The pressed sample was then fractured in liquid nitrogen, and the fracture surface was examined using scanning electron microscopy (JEOL 840A) at 10 kV of accelerating voltage. The melt blend investigation was specifically performed because it might simulate the actual situation which would occur during the fabrication of GI-POFs by Park and Walker’s coextrusion method. Although the mixing temperature of 200 °C is much higher than the normal processing temperature in extrusion, it was chosen because phase separation was expected to occur at this temperature with the copolymer 2/4 and the copolymer 5/6 pairs according to the results of the solution blend study. The blend morphologies observed after 3 min of mixing are shown in Figure 4. Parts a and b of Figure 4 are for the copolymer 2/4 and the copolymer 5/6 pairs, respectively, at a blend composition of 70/30 wt %. Apparently, the morphologies of these two copolymer pairs are similar to each other, showing homogeneous phase behavior without any evidence of phase separation. Because the copolymer 4/5 blend showed the same homogeneous phase behavior with no evidence of phase separation as in the solution blend study, its micrograph is not shown here. Figure 4c is for the blend of homopolymers (i.e., PMMA and PBzMA blend) at the same blend composition of 70/30 wt %. It is evident that the micrograph of the homopolymer blend shows dispersed domains, indicating immiscibility between PMMA and PBzMA (Figure 4c). Although not shown in the figure, no evidence of phase separation was observed at the blend composition of 30/70 wt % either. Micrographs of the same blend pairs after 8 min of mixing also showed no evidence of phase separation except the homopolymer blend. These results are contrary to those of the solution blend study where phase separation was observed with the copolymer 2/4 and the copolymer 5/6 pairs. This discrepancy may be attributed to the kinetic effect of phase separation, and the shear energy input during the mechanical mixing. In the turbidity measurements with the solution blends, the samples were heated very slowly at a rate of 1 °C/min, whereas in melt blending they were exposed to an abrupt temperature increase and intensive shear. Because the phase separation of the copolymer blends is likely to occur very slowly, it may be suppressed under the processing condition of mechanical mixing in which the samples are exposed to a high temperature with intensive shear for a short period of time. Thus, the blends of all copolymer pairs in the present study are likely to remain homogeneous during the coextrusion process of Park and Walker, indicating that these copolymers are adequate materials to be used for the fabrication of GI-POFs. Besides the scanning electron micrographs, the glass transition temperatures of the melt-blended samples

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Figure 5. DSC thermograms of MMA-BzMA copolymers: (a) PMMA, (b) copolymer 2, (c) copolymer 4, (d) copolymer 5, (e) copolymer 6, (f) PBzMA homopolymer.

Figure 6. DSC thermograms of the melt blends of MMA-BzMA copolymers: (a) copolymer 2/4 at 30/70 wt %, (b) copolymer 2/4 at 70/30 wt %, (c) copolymer 5 and 6 at 30/70 wt %, (d) copolymer 5/6 at 70/30 wt %.

Figure 4. Scanning electron micrographs of MMA-BzMA copolymer blend pairs after 3 min of mixing: (a) copolymer 2/4, (b) copolymer 5/6, (c) blend of PMMA and PBzMA homopolymers. All samples are at the blend composition of 70/30 wt %.

were also investigated using a differential scanning calorimeter (TA DSC-2010). All of the samples were initially heated from -40 to +200 °C at a heating rate of 10 °C/min and then held at 200 °C for 20 min to remove any thermal history of the sample. They were then quenched to -40 °C to fix the phase followed by reheating to 200 °C at the same heating rate of 10 °C/ min. Figure 5 shows the DSC thermograms of the MMA-BzMA copolymers listed in Table 1 while Figure 6 is for the melt blends of the copolymer 2/4 and the copolymer 5/6 pairs. All copolymers show their glass transition temperatures lying between the Tg’s of PMMA and PBzMA homopolymers (Figure 5), and their values are in

proportion to the monomer composition. As Figure 6 indicates, all melt blends show a single glass transition temperature which lies between the Tg values of the individual components (i.e., copolymer 2 and copolymer 4 in the case of copolymer 2/4 blends). Furthermore, the glass transition temperatures of these blends appear to vary monotonically with the blend composition. The glass transition temperatures of the solutionblended samples were also investigated using the same DSC procedure. In Figure 7, the thermograms of the solution blends show that all of the blends have a single glass transition temperature in contrast to the results of cloud-point measurements shown in Figure 3. In general, the rate of phase separation depends on the chain mobility (kinetic factor) of the blend pair and the phase stability (thermodynamic factor).22-24 Hashimoto and co-workers23,24 investigated the extremely slow demixing process of binary mixtures of styrene-butadiene random copolymer (SBR) and polybutadiene in which phase separation was primarily controlled by the mobility rather than the phase stability. Nishimoto et al.22 also investigated the slow phase separation process in PC-PMMA blends. They concluded that slow phase separation of the blends may result in a nonequilibrium homogeneous state rather than a thermodynamic equilibrium state when the blends are prepared by solvent casting. In these blends, the homogeneous phase structure was found to be frozen before phase separation occurred. Thus, the discrepancy between the cloud-point

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Figure 7. DSC thermograms of the solution blends of MMABzMA copolymers (heating rate of 10 °C/min): (a) copolymer 2/4 at 30/70 wt %, (b) copolymer 2/4 at 70/30 wt %, (c) copolymer 5/6 at 30/70 wt %, (d) copolymer 5/6 at 70/30 wt %.

Figure 8. Scattered light intensity as a function of time: (a) copolymer 2/4 at 50/50 wt %, (b) copolymer 5/6 at 50/50 wt %.

and Tg measurements may be attributed to the slow kinetics of the phase separation process of MMA-BzMA copolymer pairs. To examine this kinetic effect in more detail, we conducted an isothermal turbidity measurement for solution blends at 200 °C. Films of the blends prepared by solvent casting were exposed to a constant temperature of 200 °C, and the intensity of scattered light was measured as a function time. Figure 8 shows the scattered light intensity obtained with the copolymer 2/4 and the 5/6 pairs at a blend composition of 50/50 wt %. As the phase separation occurs, the intensity of scattered light rises abruptly at about 50 min for the copolymer 2/4 pair and at 200 min for the copolymer 5/6 pair. The intensity of scattered light of the copolymer 4/5 pair, however, does not increase up to 400 min, indicating a homogeneous phase structure. If the apparent miscibility of the MMA-BzMA copolymer pair is due to the slow kinetics of phase separation, phase separation may occur once the samples are exposed to an elevated temperature for an extended period of time. Figure 8, in fact, shows evidence of slow phase separation, supporting the argument of the kinetic effect. 3. Summary The blend compatibility of MMA-BzMA copolymers has been studied to investigate their feasibility as the materials to be used for GI-POFs. Copolymers with

variable comonomer composition were first synthesized followed by the characterization of each copolymer using 1H NMR, an ellipsometer, and a differential scanning calorimeter. Blend compatibility between the copolymers at various mixing ratios was then investigated by characterizing the blend samples prepared by both solution blending and melt blending. The results of the present study may be summarized as follows: (1) The MMA-BzMA copolymers synthesized under the prescribed conditions are random copolymers, with their refractive indices and the glass transition temperatures varying linearly with their monomer contents. (2) The turbidity measurements conducted on film samples casted from the solution blends of copolymers 2/4 and 5/6 showed the lower critical solution temperature (LCST) behavior whereas the solution blend of copolymer 4/5 did not show such behavior. While the lower cloud point of the copolymer 2/4 blend than the 5/6 blend by 30 °C seems to indicate the sensitivity of the cloud point to the difference in the monomer composition between the two copolymers that are blended, the apparent miscibility of the copolymer 4/5 blend is rather contradictory considering the fact that the difference in the monomer composition of copolymer 4/5 is greater than that of the copolymer 5/6 pair which showed a phase separation. This may be due to the lower mobility of the 4/5 blend pair associated with the higher molecular weight than that of the copolymer 2/4 and 5/6 pairs. (3) The scanning electron micrographs and the DSC thermograms conducted on melt- and solution-blended samples showed no evidence of phase separation for all copolymer pairs. The difference between the results of solution blends and melt blends seems to be due to the slow kinetics of phase separation and the intensive shear energy input during melt blending. The present results suggest that MMA-BzMA random copolymers are appropriate materials applicable to the coextrusion method of Park and Walker for the fabrication of GI-POFs. Acknowledgment This study was conducted in the Division of Chemical Engineering at Seoul National University while C.-W.P. was on leave from the University of Florida. C.-W.P. thanks the faculty of Chemical Engineering Division for providing the work environment for this study. He also acknowledges the support of the Korea Science and Engineering Foundation (KOSEF) and the Korean Federation of Science and Engineering Societies for this study. Literature Cited (1) Halley, P. Fiber Optic Systems; John Wiley & Sons: New York, 1987. (2) Koike, Y. High-bandwidth graded-index polymer optical fibre. Polymer 1991, 32, 1737. (3) Koike, Y.; Nihei, E. Method of Manufacturing a Graded Optical Transmission Medium Made of Synthetic Resin. U.S. Patent 5 253 323, 1993. (4) Koike, Y.; Nihei, E. Method of Manufacturing Optical Transmission Medium from Synthetic Resin. U.S. Patent 5 382 448, 1995. (5) Koike, Y.; Ishigure, T.; Nihei, E. High-bandwidth gradedindex polymer optical fiber. J. Lightwave Technol. 1995, 13, 1475. (6) Ishigure, T.; Nihei, E.; Koike, Y. Graded-index polymer optical fiber for high-speed data communication. Appl. Opt. 1994, 33, 4261.

Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4681 (7) Ishigure, T.; Nihei, E.; Koike, Y. Optimization of the refractive-index distribution of high-bandwidth GI polymer optical fiber based on both modal and material dispersions. Polym. J. 1996, 28, 272. (8) Ishigure, T.; Satoh, M.; Takanashi, O.; Nihei, E.; Nyu, T.; Yamazaki, S.; Koike, Y. Formation of the refractive index profile in the graded index polymer optical fiber for gigabit data transmission. J. Lightwave Technol. 1997, 15, 2095. (9) Ho, B. C.; Chen, J. H.; Chen, W. C.; Chang, Y. H.; Yang, S. Y.; Chen, J. J.; Tseng, T. W. Graded-index polymer fibers prepared by extrusion. Polym. J. 1995, 27, 310. (10) Koike, Y.; Nihei, R. Method of Manufacturing Plastic Optical Transmission Medium. U.S. Patent 5 593 621, 1997. (11) Park, C.-W.; Walker, J. K. A Novel Production Method for Objects with Radially-Varying Properties. U.S. Patent Application Series No. 89 929 161, 1997. (12) Park, C.-W.; Lee, B. S.; Walker, J. K.; Choi, W. Y. A New Processing Method for the Fabrication of Cylindrical Objects with Radially-Varying Properties. Ind. Eng. Chem. Res. 2000, in press. (13) Kaino, T. Preparation of plastic optical fibers for near-IR region transmission. J. Polym. Sci., Part A: Polym. Chem. 1987, 25, 37. (14) Emslie, C. Review polymer optical fibers. J. Mater. Sci. 1998, 23, 2281. (15) Odian, G. Principles of polymerization, 3rd ed.; John Wiley & Sons: New York, 1993. (16) Mohan, D.; Radhakrishnan, G.; Rajadurai, S.; Joseph, K. T. Evaluation of Reactivity Ratio of Acrylate Copolymers by 13C NMR. J. Polym. Sci., Part C: Polym. Lett. 1990, 28, 307.

(17) Koenig, J. L. Chemical microstructure of polymer chains; John Wiley & Sons: New York, 1980. (18) Van Krevelen, D. W. Properties of polymers; Elsevier Science Publisher: Amsterdam, The Netherlands, 1990. (19) Groh, W.; Zimmerman, A. What is the lowest refractive index of an organic polymer? Macromolecules 1991, 24, 6660. (20) Bank, M.; Leffingwell, J.; Thies, C. The influence of solvent upon the compatibility of polystyrene and poly(vinyl methyl ether). Macromolecules 1971, 4, 43. (21) Mandal, T. K.; Woo, E. M. Marginal miscibility and solvent dependent phase behavior in solution blended poly(vinyl methyl ether)/poly(benzyl methacrylate). Macromol. Chem. Phys. 1999, 200, 1143. (22) Nishimoto, M.; Keskkula, H.; Paul, D. R. Role of slow phase separation in assessing the equilibrium phase behavior of PCPMMA blends. Polymer 1991, 32, 1274. (23) Izumitani, T.; Hashimoto, T. Slow spinodal decomposition in binary liquid mixtures of polymers. J. Chem. Phys. 1985, 83, 3694. (24) Takenaka, M.; Izumitani, T.; Hashimoto, T. Slow spinodal decomposition in binary liquid mixtures of polymers; 2. Effect of molecular weight and transport mechanism. Macromolecules 1987, 20, 2257.

Received for review May 3, 1999 Revised manuscript received September 3, 1999 Accepted October 4, 1999 IE9903083