Transparent Zero-Birefringence Polymers - American Chemical Society

Chapter 2

Transparent Zero-Birefringence Polymers 1,2

1,2

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1,2

Shuichi Iwata , Hisashi Tsukahara , Eisuke Nihei , and Yasuhiro Koike

Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch002

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Department of Material Science, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan Kanagawa Academy of Science and Technology, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213, Japan

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Birefringence is caused by orientation of polymer chains during injection-molding and extrusion and it is a disadvantage in optical systems such as lenses, compact disks, waveguides. We propose new transparent zero-birefringence polymers for any degree of orientation of polymer chains, which are obtained by the following two methods. One is random copolymerization of negatively and positively birefringent monomers. The other is doping of a small amount of large anisotropic molecules into a polymer matrix to compensate the birefringence caused by the orientation of polymer chains. Zero-birefringence polymers synthesized by these two methods have low scattering loss, which is comparable with that of pure poly(methyl methacrylate) (PMMA), and had no observable microscopic heterogeneous structures.

Transparent amorphous polymers such as poly(methyl methacrylate) (PMMA) have been found to be useful materials for polymer optical fibers (POFs) (1,2), waveguides (3), lenses (4), optical disks (5), and other optical components because of their excellent mechanical properties and easy processing. Many recently developed optical applications utilizing polarization techniques need optical polymers for maintaining more accurate polarization. However, applications of optical polymers are limited by birefringence which occurs in the process of device fabrication. 16

© 1997 American Chemical Society

In Photonic and Optoelectronic Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.


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Transparent Zero-Birefringence Polymers

IWATA E T A L .

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Although a polymer may be composed of an anisotropic monomer, birefringence does not occur in it if the polymer structure forms random coils. Birefringence is, however, easily caused by both orientation of the polymer chains and photoelasticity which results from external mechanical stress. Orientational birefringence can be compensated by blending a negative and positive biréfringent homopolymers and such pairs of polymer blends have been reported (6,7). However these polymer blends are inhomogeneous and not transparent, since they phase-separate, resulting in large-domain heterogeneous structures which dramatically increase light scattering. In addition, it is very difficult to blend polymers homogeneously in the extrusion process and injection-molding. In order to circumvent this problem, we propose two methods for compensating the birefringence of polymers. One involves random copolymerization of positive and negative biréfringent monomers and the other involves doping a small amount of large anisotropic molecules into a polymer matrix. Compensation for Birefringence Compensation by Random Copolymerization Zero-birefringence copolymers synthesized from negative and positive biréfringent monomers are more transparent than polymer blends. Perfect random copolymerization is achieved when the monomer reactivity ratios are equal to unity. As a result, the heterogeneous structure of the random copolymer is of the order of less than several monomer units which is much smaller than an optical wavelength, preventing light scattering. In the zero-birefringence copolymer, since the anisotropic polarization of the negative biréfringent monomer units is compensated by the positive biréfringent monomer units on the same polymer chain, the birefringence is zero for any degree of orientation. In the copolymerization reaction between Mi and M2 monomers, the monomer reactivity ratios ri and ri are defined as (8):

where k is the propagation rate constants in the following copolymerization reaction: • • · M[ + Mj —^—*· · · Μ M\ λ

•••M;+M —^ ->·.·Μ Μ ' 2

2

1

2

·Μ' + M 2

••·Μ ' + 2

*

1

?1

>···Μ Μ; 2

ΛΓ — -*-+-M M\ k

2

2

The monomer reactivity ratios r\ and ri between monomers Mi and M2 are estimated by using Equation (2):


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PHOTONIC AND OPTOELECTRONIC POLYMERS

Ί - 7 ρ β χ ρ [ - « ι ( β ι - « 2 ) ]

Here Qi (or Q ) is the reactivity of the monomer Af; (or M ), and e/ (or ei) is the electrostatic interaction of the permanent charges on the substituents in polarizing the vinyl group of monomer Mi (or M ). When r; and r of two monomers are unity, they can be randomly copolymerized perfectly. Table I shows the Q and e values and the monomer reactivity ratios r; and r for the radical copolymerization process. Since the monomer reactivity ratios between negatively biréfringent methyl methacrylate (MMA) and positively biréfringent 2,2,2-trifluoroethyl methacrylate (3FMA) and benzyl methacrylate (BzMA) are nearly equal to unity, these monomers can be randomly copolymerized, resulting in homogeneous and transparent copolymers. 2

2


2

2

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Preparation of Copolymer Films. In order to eliminate inhibitors and impurities, MMA monomer, 3FMA monomer, and BzMA monomer were distilled at bp 46-47T: / 100 mmHg, bp 56-57*0 / 110 mmHg, and bp 86ΐ) / 0.1 mmHg, respectively. A mixture of MMA and one of the positively biréfringent monomers with a specified amount of initiator, benzoyl peroxide (BPO), chain transfer agent (/i-butyl mercaptan (nBM)), and solvents (ethyl acetate) were placed in a glass tube and heated at KfC. After polymerization, the polymer solution was filtered through a 0.2 μπι membrane filter and precipitated into methanol. The polymer was dried under reduced pressure for about 48 hours. The polymer solution in ethyl acetate was cast onto a glass plate with a uniform film thickness (50-100 μιη) by using a knife-coater. The polymer film was dried under vacuum. Birefringence Measurement. The polymer film was uniaxially heat-drawn at at a rate of ca. 6.6 mm/min. in hot silicone oil. Birefringence of the drawn film was determined by a measuring the optical path difference between the parallel and perpendicular directions to the draw, using a crossed sensitive color plate method (Toshiba Glass Co., SVP-30-II). Zero-Birefringence Copolymers. Figure 1 shows birefringence Δ/ι (n - n ) of poly(MMA-co-3FMA) film sample as a function of the draw ratio. The subscripts "//" and " -L " denote the parallel and perpendicular directions to the oriented polymer chains, respectively. The drawn PMMA has a negative birefringence (Δ/ι0). The value and the sign of birefringence vary with the composition of copolymers. The copolymer of MMA/3FMA in the ratio of 45/55(wt./wt.) had a birefringence ;/


x

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IWATA E T A L .

Transparent Zero-Birefringence Polymers

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of zero at various draw ratios. The zero-birefringence of such a poly(MMA-co3FMA) copolymer was independent of molecular weight from 1.0 X10 to 3.2 X 10 and draw temperature (70-9θΌ). The birefringence, An, of poly(MMA-co-BzMA) film sample as a function of the draw ratio is shown in Figure 2. Poly(benzyl methacrylate) (PBzMA) has a large positive birefringence (Δ/ι>0). However, since the PBzMA film was brittle, we could not show the data of PBzMA in Figure 2. The copolymer of MMA/BzMA in the ratio of 82/18 (wt./wt.) had the orientation birefringence of zero at various draw ratios. Zero-birefringent poly(MMA-coBzMA) was independent of molecular weight from 1.0 Χ10 to 5.5 Χ10 and draw temperature from 70 to 90*0. 5

5


5

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Compensation by Doping a Large Anisotropic Molecule into Polymer Matrix In the random copolymerization method, the monomer reactivity ratios between negative and positive biréfringent monomers must be nearly equal to unity in order to achieve random copolymerization and hence a homogeneous structure. Therefore, combinations of monomers to satisfy that condition are quite limited. Therefore, we propose another method involving doping of large anisotropic molecules into polymers to compensate the birefringence. The advantage of this method is that the birefringence can be compensated with many selections of doping molecules. If the doping molecules are miscible with the matrix of polymer, the molecules are randomly dispersed in the polymer without any aggregation and does not cause excess light scattering loss (9). The dopant molecules are oriented in proportion to the orientation degree of polymer chains. The long axis of the doping molecule tends to point to the stretched direction to minimize the enthalpy. In the case of the rod-like doping molecules such as irans-stilbene and diphenyl acetylene (tolan) used in this paper, they are easily oriented by the orientation of polymer chains when the polymer is heat-drawn or injection-molded. Polarization parallel to the long axis of the doping molecule is much larger than the polarization perpendicular to the axis and hence they have positive birefringence. Therefore, when these doping molecules in the PMMA matrix are oriented according to the orientation of MMA polymer chains, the negative birefringence caused by orientation of polymer chains is compensated by the orientation of these dopant molecules. Birefringence of PMMA with Dopant Molecules. The film sample for investigating birefringence was prepared from a solution of PMMA and the dopant molecules. The methods of processing the films and measuring the birefringence were almost the same as in the random copolymerization method. Since the glass transition temperature (Tg) of the PMMA sample is decreased by the dopant molecules, the film was uniaxially heat-drawn at 70*0. Figures 3 and 4 show the birefringence of PMMA with dopant molecules, stilbene and tolan respectively, as


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PHOTONIC AND OPTOELECTRONIC POLYMERS

Table I. Q and e values of monomers Q value e value Monomer reactivity ratio 0.74 Π = 1.07 0.40 η = 0.83 1.13 0.98 r = 0.86 0.70 0.42 r = 0.94

Monomer MMA 3FMA BzMA

2

2


4 3 Ο

2

s:

Transparent Zero-Birefringence Polymers - American Chemical Society

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