Photonic and Optoelectronic Polymers 9780841235199, 9780841216365, 0-8412-3519-8

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Photonic and Optoelectronic Polymers

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

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

Downloaded by 217.66.152.141 on October 7, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.fw001

ACS

SYMPOSIUM

SERIES

672

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Photonic and Optoelectronic Polymers

Samson A. Jenekhe, EDITOR University of Rochester

Kenneth J. Wynne, EDITOR Office of Naval Research

Developedfromasymposiumsponsored by the Pacific Polymer Federation at the Pacific Polymer Conference

American Chemical Society, Washington, DC

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

Library of Congress Cataloging-in-Publication Data

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Symposium on Polymers for Advanced Optical Applications (1995: Kauai, Hawaii) Photonic and optoelectronic polymers: developed from a symposium sponsored by the Pacific Polymer Federation at the Pacific Polymer Conference, Kauai, Hawaii, December 12-16, 1995/ Samson A. Jenekhe, editor, Kenneth J. Wynne, editor. p.

cm.—(ACS symposium series, ISSN 0097-6156; 672)

Includes bibliographical references and indexes. ISBN 0-8412-3519-8 1. Photonics—Materials—Congresses. 2. Polymers—Optical properties— Congresses. 3. Optoelectronic devices—Materials—Congresses. 4. Polymers— Electric properties—Congresses. I. Jenekhe, Samson Α. II. Wynne, Kenneth J., 1940- . ΙII. Pacific Polymer Federation. IV. Pacific Polymer Conference (4th: 1995: Kauai, Hawaii) V. Title. VI. Series. TA1505.S986 621.36—dc21

1995 97-22245 CIP

This book is printed on acid-free, recycled paper. Copyright © 1997 American Chemical Society All Rights Reserved. Reprographic copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Act is allowed for internal use only, provided that a per-chapter fee of $17.00 plus $0.25 per page is paid to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, M A 01923, USA. Republication or reproduction for sale of pages in this book is permitted only under licensefromACS. Direct these and other permissions requests to ACS Copyright Office, Publications Division, 1155 16th Street, N.W., Washington, DC 20036. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN THE UNITED STATES OF AMERICA

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

Advisory Board

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ACS Symposium Series

Mary E. Castellion

Omkaram Nalamasu

ChemEdit Company

AT&T Bell Laboratories

Arthur B. Ellis

Kinam Park

University of Wisconsin at Madison

Purdue University

Jeffrey S. Gaffney

Katherine R. Porter

Argonne National Laboratory

Gunda I. Georg University of Kansas

Lawrence P. Klemann Nabisco Foods Group

Richard N . Loeppky University of Missouri

Cynthia A. Maryanoff R. W. Johnson Pharmaceutical Research Institute

Duke University

Douglas A. Smith The DAS Group, Inc.

Martin R. Tant Eastman Chemical Co.

Michael D. Taylor Parke-Davis Pharmaceutical Research

Leroy Β. Townsend University of Michigan

Roger A. Minear University of Illinois at Urbana-Champaign

William C. Walker DuPont Company

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

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Foreword 1HE ACS SYMPOSIUM SERIES was first published in 1974 to provide a mechanism for publishing symposia quickly in book form. The purpose of this series is to publish comprehensive books developed from symposia, which are usually "snapshots in time" of the current research being done on a topic, plus some review material on the topic. For this reason, it is necessary that the papers be published as quickly as possible. Before a symposium-based book is put under contract, the proposed table of contents is reviewed for appropriateness to the topic and for comprehensiveness of the collection. Some papers are excluded at this point, and others are added to round out the scope of the volume. In addition, a draft of each paper is peer-reviewed prior to final acceptance or rejection. This anonymous review process is supervised by the organizer(s) of the symposium, who become the editor(s) of the book. The authors then revise their papers according to the recommendations of both the reviewers and the editors, prepare camera-ready copy, and submit the final papers to the editors, who check that all necessary revisions have been made. As a rule, only original research papers and original review papers are included in the volumes. Verbatim reproductions of previously published papers are not accepted.

ACS BOOKS DEPARTMENT

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

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Preface E X P L O S I V E GROWTH OF INFORMATION in the world is fueling the need for efficient, reliable, and low-cost acquisition, storage, processing, transmission, and display technologies. In the last several years, research aimed at addressing this need has demonstrated the great potential of electroactive and photoactive polymers as photonic and optoelectronic materials. Recent discoveries in these areas include high-data-rate and low-loss polymer optical fibers, high-speed polymer-based electro-optic modulators, bright polymer lightemitting diodes, high-diffraction-efficiency photorefractive polymers, highefficiency polymer photodetectors for visible-UV radiation, and organic thinfilm transistors comparable to amorphous silicon devices. Although these recent achievements represent exciting opportunities for major innovations in information technologies, the commercial realization of many segments of polymer-based photonics and optoelectronics awaits further research advances in addressing many scientific and technical challenges. This volume was developed from a symposium presented at the Pacific Polymer Conference (sponsored by the Pacific Polymer Federation), titled "Polymers for Advanced Optical Applications," in Kauai, Hawaii, December 12-16, 1995. We organized the symposium to provide an international forum to discuss recent advances and future prospects in the broad field of photonic and optoelectronic polymers. The symposium attracted more than 80 presentations and included 39 invited leading researchers from several countries. The chapters in this book were developed mostly from the invited presentations at the Kauai conference. In addition, five papers not presented at the symposium were invited for inclusion in the volume for the purpose of broadening the topical coverage of the book. In the spirit of the symposium, this book provides a broad overview of research advances in several areas of photonic and optoelectronic polymers as well as their promising applications. Among the main topics covered are diverse polymers for digital and holographic information storage, including photorefractive polymers; electroluminescent polymers for light sources, high­ speed transmission, high-power amplification, and high-speed modulation of optical signals in polymer waveguides and gradient-index fibers; thermally stable poled polymers for second-order nonlinear optics; self-assembly and nanostructure control as approaches to efficient photoelectronic and photonic properties; and thin-film transistors from organic semiconductors.

xi

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

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An important theme of this volume is the interrelationships among materials chemistry, photonic and optoelectronic properties, and device performance. The design and synthesis of novel polymer compositions and architectures aimed at enhanced properties are emphasized in some chapters. Other contributions feature the development of novel approaches to processing and fabrication of photonic and optoelectronic polymers into thin films, multilayers, fibers, waveguides, gratings, and device structures. These approaches, which emphasize polymer synthesis, processing, and device fabrication, are complementary and synergistic. The interdisciplinary nature of many of the chapters suggests that chemists, chemical engineers, materials scientists, and others interested in the design, synthesis, and processing of diverse photonic and optoelectronic polymers will find this volume useful. This book will also be valuable to physicists, electrical engineers, optical engineers, and others concerned with the design, fabrication, and evaluation of polymer-based electronic, optoelectronic, and photonic devices and components. Acknowledgments The assembly of leading researchers for the symposium and the subsequent editing of this volume for publication would not be possible without the generous financial support of the Office of Naval Research. We also thank X . Linda Chen and Laura Devincentis at the University of Rochester, A. Ervin and M . Talukder at the Office of Naval Research, and Anne Wilson and Vanessa Evans-Johnson at ACS Books for their essential help with the book. SAMSON A. JENEKHE

Departments of Chemical Engineering and Chemistry University of Rochester Rochester, N Y 14627-0166 KENNETH J. W Y N N E

Physical Sciences S&T Division 331 Office of Naval Research U.S. Department of the Navy 800 North Quincy Street Arlington, V A 22217-5660 May 2, 1997

xii

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

Chapter 1

Linear Optical Anisotropy in Aromatic Polyimide Films and Its Applications in Negative Birefringent Compensators of Liquid-Crystal Displays Fuming Li, Edward P. Savitski, Jyh-Chien Chen, Yeocheol Yoon, Frank W. Harris, and Stephen Z. D. Cheng Downloaded by 217.66.152.141 on October 7, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch001

1

Department of Polymer Science and Maurice Morton Institute, University of Akron, Akron, OH 44325-3909 Samples of soluble aromatic polyimides of varying chemical structure and molecular weight were synthesized in refluxing m-cresol at elevated temperatures through a one-step polymerization route. Modifications of the dianhydride and diamine monomers were designed to prepare aromatic polyimides having new architectures based on the requirements of liquid crystal display (LCD) applications. The solution– cast films exhibit linear optical anisotropy (LOA), which is called uniaxial negative birefringence (UNB) and is characterized by the presence of a larger refractive index along the in-plane direction than in the out-of-plane direction. It is found that the UNB is critically associated with the backbone linearity and rigidity as well as the intrinsic polarizability of the polyimides. A specific polyimide synthesized from 2,2'-bis(3,4-dicarboxyphenyl)-hexafluoropropane and 2,2'-bis(trifluoro-memyl)-4,4'-diaminobiphenyl was used as an example to study the molecular weight effect on the LOA. For films having a fixed molecular weight, the refractive indices are constant for film thicknesses below 15 μm. They gradually change with further increase of the film thickness. On the other hand, the refractive index along the out-of-plane direction decreases while the in-plane refractive index increases when the polyimide molecular weight increases. The LOA is closely associated with the anisotropy of other second order parameters which are second derivatives with respect to the energy term. These parameters include the coefficient of thermal expansion, modulus, dielectric constant and refractive index. Films with this UNB can be used as negative birefringent compensators in twisted and super-twisted nematic LCDs to improve display viewing angles. It has been recognized since the 1960s that aromatic polyimide films exhibit structural anisotropy in the directions parallel (in-plane) and perpendicular 1

Corresponding author

2

© 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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1. LI ET AL.

3 Linear Optical Anisotropy in Aromatic Polyimide Films

(out-of-plane) to the film surface. This phenomenon has been defined as "in-plane orientation" (1-5). Recently, it has been recognized that such anisotropic structure leads to anisotropic thermal, mechanical, dielectric and optical properties along the in-plane and out-of-plane directions (5). Our interests are particularly focused on the linear optical anisotropy (LOA) in solution-cast polyimide films and their applications. Since aromatic polyimide molecules commonly tend to align parallel to the film surface during the film forming process (1-5), the in-plane refractive index is thus larger than the out-ofplane refractive index. The degree of in-plane orientation and the resultant extent of LOA in the films can be estimated readily using refractive index measurements. Therefore, the LOA can be expressed by the birefringence which is the difference in the refractive indices along the in-plane and out-of-plane directions. In the field of optics, this phenomenon is defined as uniaxial negative birefringence (UNB). One of the applications for the LOA in polyimide films is that they may be utilized to design negative biréfringent compensators for twisted and super-twisted nematic liquid crystal displays (TN- and STN-LCDs). Such films can be used in both active and passive forms to improve LCD viewing angles (6-8). However, it is well known that aromatic polyimides are usually difficult to process since they do not melt flow before decomposition and are insoluble in conventional solvents. The traditional approach is to use a two-step polymerization route to make processing possible through the soluble poly(amic acid) precursors. For example, polyimide films are generally produced by solution-casting or spin-coating and then are either thermally or chemically imidized. The imidization history affects the ultimate structure, morphology and properties of the films (9,10). Conventional aromatic polyimides synthesized via the two-step polymerization often possess strong optical absorption in the low W-visible wavelength region (···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):

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

18

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

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2

2

2

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 ;/

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

x

2.

IWATA E T A L .

Transparent Zero-Birefringence Polymers

19

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

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5

5

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

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

20

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

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4 3 Ο

2

s: