Birefringent properties of cyclic block copolymers and low

Birefringent properties of cyclic block copolymers and low-retardation-filmdevelopment

Weijun Zhou (SID Member)Charles DiehlDan MurrayKurt A. KoppiStephen HahnShin-Tson Wu (SID Fellow)

Abstract — Cyclic block copolymers (CBCs) are a new class of optical polymers made by fully hydro-genating block copolymers of styrene and conjugated diene. This class of materials has excellentoptical transparency, photostability, and good thermal resistance. By changing the copolymer com-position and the resulting block-copolymer morphology, a unique set of birefringence properties canbe achieved. The focus of this work was to study various sources of birefringence in block copolymersusing a series of model CBC materials. One particularly interesting finding relates to the developmentof an ultra-low-phase-retardation CBC film. Unlike the conventional approach of using an additive orblend, a CBC film prepared by melt extrusion can readily achieve near-zero retardation in both thefilm plane and thickness direction. This nearly isotropic CBC film is useful as a polarizer protectionfilm in flat-panel displays. When used as the inner protective layer of a polarizer, CBC film helps toreduce the color shift of IPS-LCDs at oblique angles and offer a wider viewing angle.

Keywords — Optical film, birefringence, low retardation, polarizer, LCD.

DOI # 10.1889/JSID18.1.66

1 IntroductionCyclic block copolymers (CBCs) are a new class of opticalpolymers recently developed at The Dow Chemical Com-pany. They are made by fully hydrogenating styrene-conju-gated diene block copolymers. As high-performanceengineering polymers, CBC materials have many attractiveproperty features such as excellent optical transparency,high-glass-transition temperature, good moisture-barrierproperty, and lightweight.1 They can be considered to belongto the same general class of polymer as cyclic olefin materi-als,2,3 e.g., cyclic olefin polymers (COP) and cyclic olefincopolymers (COC), but differ from the existing cyclic olefinmaterials in that CBC can exhibit distinct block morpholo-gies due to the microphase separation of different blocksthat are covalently bonded together.

Among various unique structure–property behaviorsexhibited by CBC materials,4,5 the birefringence property isparticularly interesting, but not well understood. The mainpurpose of this paper is to investigate the birefringenceproperty of CBC and its commercial implication for displayfilm applications, especially for polarizer protective films.

Low retardation or zero birefringence is a critical fea-ture needed for polarizer protection films. Triacetyl cellu-lose (TAC) film has been a mainstay for the polarizerindustry and is used in virtually all LCDs, but has someinherent drawbacks and shortcomings. First, TAC film isvery sensitive to moisture and can exhibit dimensional fluc-tuations in conditions where the relative humidity is chang-ing. Secondly, conventional TAC film has a small in-plane

birefringence, but has a high out-of-plane birefringencewhich is undesirable for high-performance LCDs. Recentefforts have been made to reduce the out-of-plane birefringenceof TAC using an additive approach.6 Tagaya and co-workershave proposed a zero-birefringence polymer approach by dop-ing negative-birefringent inorganic crystals into an acryliccopolymer.7 There are also reports of making a low-birefrin-gent polymer composition by blending two different poly-mers together or by the random copolymerization of twomonomers with distinctly different photoelastic proper-ties.8,9

Despite a significant body of research focused on thedesign of zero-birefringence polymer, to the best of ourknowledge, very little work has been conducted to under-stand the birefringence property of block copolymers, par-ticularly with respect to achieving low birefringence. In thispaper, a series of CBC materials were synthesized for thepurpose of understanding various sources of birefringenceas well as their relative importance to the total birefringencein these microphase separated systems.

2 Experimental

2.1 Materials and synthesisCyclic block copolymers used in this study were synthesizedat The Dow Chemical Company. A total of eight CBC materi-als with varying comonomer content were prepared for thisstudy (Table 1). All but one CBC polymer (CBC-3) weremade in a lab reactor. The CBC-3 polymer was produced

Expanded revised version of a paper presented at the 2009 SID International Symposium (Display Week 2009) held May 31–June 5, 2009 in SanAntonio, Texas, U.S.A.

W. Zhou, C. Diehl, D. Murray, K. A. Koppi, and S. Hahn, The Dow Chemical Co., Core R&D – New Products, 2301 N. Brazosport Blvd., B-1470,Freeport, TX 77541; telephone 979/238-1347, e-mail: [email protected]

S-T. Wu, College of Optics and Photonics, University of Central Florida, Orlando, FL, U.S.A.

© Copyright 2010 Society for Information Display 1071-0922/10/1801-0066$1.00

66 Journal of the SID 18/1, 2010

with greater than 100-kg quantity to allow a large-scale filmextrusion study. In all cases, the unsaturated precursor poly-mers were synthesized by anionic polymerization of styreneand isoprene through sequential addition of two monomers.The resulting block copolymers were fully hydrogenatedusing a heterogeneous porous silica supported platinumcatalyst (Fig. 1).10–12 Dry polymers were obtained after asubsequent de-volatilization of the hydrogenated polymersolution. The molecular structure of each polymer wasestablished by a combination of 1H nuclear magnetic reso-nance (NMR) and gel permeation chromatography (GPC)measurements. Both characterizations were conducted onthe polymers prior to their hydrogenation.

The molecular composition of each CBC polymer wasdetermined by NMR spectroscopy using a Varian INOVA300 NMR spectrometer. Polymer samples were dissolved inCDCl3 solvent at approximately 40-mg polymer in 1-ml sol-vent for the analysis. A delay time of 10 sec was used toensure complete relaxation of protons for quantitative integra-tions of different chemical shifts. The percent of co-monomerin CBC (i.e., % styrene content) and the percent of 1,4-poly-merization vs. 3,4-polymerization for isoprene monomerincorporation were calculated from the NMR spectrum.The incorporation of isoprene onto the polymer backbonechain during the anionic polymerization stage was found tobe predominantly through 1,4-polymerization with onlyabout 10% in 3,4-polymerization. The degree of hydrogena-tion on CBC polymers (i.e., post hydrogenation) was alsocharacterized by NMR spectroscopy. Better than 99% ofhydrogenation was obtained for both styrene ring andun-saturated double bonds of isoprene, suggesting the CBCpolymers are fully saturated.

Molecular weights of each CBC polymer was deter-mined from the precursor polymer prior to hydrogenation

using an Agilent 1100 series GPC instrument equipped witha refractive index detector. Tetrahydrofuran (THF) wasused as the solvent for dissolving the polymer and the chro-matograph analysis. Calculation of the molecular weightswas based on the mono-dispersed polystyrene standards. Alleight CBC polymers (prior to hydrogenation) were found tobe nearly monodisperse with the ratio of weight-averagemolecular weight (Mw) and number-average molecularweight (Mn) less than 1.1.

The glass-transition temperature (Tg) for the rigidphase (i.e., hydrogenated styrene phase) of each CBC mate-rial was determined by differential scanning calorimetry(Q1000 differential scanning calorimeter, TA Instruments,Inc.). The scan rate was 10°C/min and Tg was determinedfrom the second heat scan curve.

Table 1 shows the molecular composition and basicproperty characteristics of the eight CBC polymers used inthis study. All polymers are completely amorphous with nomeasurable amount of crystallinity. Tg values reported inTable 1 refer to the CBC rigid phase and they range from110 to 120°C.

2.2 Film fabricationSeveral fabrication methods have been employed to studyCBC film birefringence properties. Thin films of approxi-mately 150 µm thickness were compression molded fromCBC materials at 250°C. Under this molding condition, verylittle molecular orientation is expected in the polymer-filmsample. To understand the effect of orientation on the bire-fringence properties of CBC materials, the compressionmolded films were uniaxially stretched by a tensile machineequipped with an environmental chamber. Micro-tensilespecimens were cut out of the compression molded films forthe stretching study. The stretching temperature was heldconstant at 135°C for all film samples. This is about15–20°C above the glass-transition temperature of CBCmaterials as determined by differential scanning cal-orimetry. Prior to stretching, the micro-tensile bars werethermally equilibrated in the environmental chamber for5 minutes at 135°C. The stretching rate was set at 200%strain per sec. Once a desirable strain had been reached(e.g., 50%, 100%, etc.), the environmental chamber wasopened to allow the film sample to cool down to room tem-perature for removal while still under tension. The birefrin-gence of a stretched film was measured at the same spotbefore and after applying the stretching.

Optical-quality CBC film was also prepared by a com-mercial melt extrusion process in a clean-room environ-ment. This fabrication was only carried out on one CBCpolymer (CBC-3) as it was available in large quantities. Themelt extrusion temperature was held at 250°C. The width ofextruded film was about 1.35 m and the target film thicknesswas 60 µm.

FIGURE 1 — Schematics for cyclic-block-copolymer structure andsynthesis. Hydrogenation reactions were conducted at about 600 psi ofhydrogen pressure and 170°C using cyclohexane as the solvent.

TABLE 1 — Molecular composition of CBC materials and basic propertycharacteristics.

Zhou et al. / Birefringent properties of cyclic block copolymers 67

2.3 Characterization of optical propertyLight transmission measurements were carried out on a 3-mm-thick plaque prepared by injection molding. A PerkinElmer Lambda 950 UV-Vis double-beam spectrometer wasused for the study. Measurements were conducted in thewavelength range of 200–800 nm with a slit width of 2 nmper step.

The refractive indices of CBC polymers, n0, weremeasured on a 20-mm-long × 8-mm-wide CBC film speci-men using a multi-wavelength Abbe refractometer (AtagoDR-M2). The bottom surface of the film samples was pol-ished so that it could make a complete contact with the mainprism surface of the Abbe refractometer. A small drop ofcontact liquid, mono-bromonaphthalene, was used betweenthe sample and the main prism during measurement. Carewas taken to ensure the thin contact liquid was spread evenlybetween the sample and the main prism with no bubbles.

Optical retardation and birefringence of film sampleswere measured on an EXICOR™ 150ATS (Hinds Instru-ment, OR) birefringence measurement system at threewavelengths: 436, 546, and 633 nm. The in-plane retardation(R0) and thickness direction retardation (Rth) are defined asfollows:

(1)

(2)

where d is the film thickness and nx, ny, and nz represent therefractive index along three principal axes x, y, and z, respec-tively. The directions x and y define two mutuallyorthogonal axes in the film plane, and z is along the filmthickness direction. The direction along which the refractiveindex is higher in the film plane is denoted as the slow axis.In this paper, the x-direction is assigned to be the slow axis.

R0 was measured with incident light normal to the filmsurface. Determination of the Rth values was based uponretardation measurements made with incident light normalto the film surface (i.e., R0) and at an oblique incidenceangle of 40° (R40). Measurement of R40 was made by tiltingthe film around the slow axis at 40°. By solving the followingfour equations, nx, ny, and nz can be determined and substi-tuted into Eq. (2) for calculating Rth values

(3)

(4)

(5)

(6)

where n0 is the nominal refractive index of the polymer andθ is the direction of light propagation within the film whenit is tilted at 40°.

2.4 Surface characterizationThe surface quality of CBC films prepared by melt extrusionwas examined by atomic force microscopy (AFM). Film sur-face topography was captured at ambient temperature usinga Digital Instruments Multi-Mode AFM equipped with aNanoScope IV controller. Nano-sensor probes with a springconstant of 55 N/m and a resonant frequency in the vicinityof 167 kHz were used for phase imaging. The samples wereimaged at a frequency of 0.5–2 Hz and a set point ratio of~0.8. Film-surface roughness (i.e., z-direction height) wascalculated on the sample size of 40 × 40 µm2. The geometricmean deviation of all points on the surface from the mean,commonly denoted as root-mean-square (RMS) roughness,is reported in this study.

3 Results and discussion

3.1 General optical properties of CBC polymerCBC polymers are fully saturated olefin block copolymerswith excellent light transmission. The overall transmissioncharacteristics are similar to those of cyclic olefin polymers(COP).2 As shown in Fig. 2, a 3-mm-thick injection moldedCBC plaque shows better than 92% light transmittanceacross the entire visible wavelength range from 380 to 780nm. This level of optical transmittance is a few percenthigher than those of optical polymers such as polycarbonate(PC) and polyethylene terephthalate (PET) and is close tobeing best in class among all optical polymers.

CBC polymers also have very low haze. For an optical-quality CBC film of 80 µm thickness or thinner, haze wasmeasured to be lower than 0.2. In comparison, the haze ofTAC and PC optical film is at about 0.3, whereas the haze of

R n n dx y0 = - ¥( ) ,

Rn n

n dthx y

z=+

-FHG

IKJ ¥

2,

n n n nx y z+ + = 3 0,

( ) ,n n d Rx y- ¥ = 0

nn n

n n

dRx

y z

z y

-+

F

HGG

I

KJJ ¥ =

2 2 2 2 40cos sin cos

,q q q

sinsin

,q = 40

0

o

n

FIGURE 2 — Light-transmission spectrum of a 3-mm-thick injectionmolded CBC plaque.


PET film is considerably higher and reaches nearly 1.8 atabout the same film thickness.13

The color of CBC polymers and fabricated articles isalso superior. No inherent color bodies exist in the CBCmolecular structure that could otherwise lead to yellowingor other light stability problems commonly encounteredduring the long term use of aromatic polymers (e.g., PC).The yellowness index (YI) of a CBC film at 80 µm thicknessor thinner is typically smaller than 0.3. In contract, a con-ventional TAC film has a YI value about 1.0 and is visiblymore yellow than a CBC film. These excellent optics prop-erties demonstrate a good suitability of CBC polymers foruse in display film applications.

The refractive index of the CBC-3 polymer wasmeasured to be approximately 1.507 at room temperatureand 589 nm (D-line). This is slightly higher than bothPMMA and TAC materials that have a refractive index ofabout 1.49. Variation of the comonomer ratio (i.e., styrenevs. isoprene) does not significantly affect the refractiveindex value of CBCs. This is due to the close match of therefractive indices between the hydrogenated styrenephase (n = 1.51) and the hydrogenated isoprene phase (n= 1.486).

Figure 3 shows the effect of wavelength on the refrac-tive index properties of CBC-3 polymer. The solid curverepresents the fit of refractive index data to the two-coeffi-cient Cauchy equation:14

(7)

There is a reasonably good fit between the experi-mentally measured refractive-index values and the Cauchyequation. For the particular polymer shown in Fig. 3, val-ues of A and B were determined to be 1.4915 and 0.0042(µm2), respectively. This set of dispersion values will beused for the color-shift calculation of CBC polarizer film inSec. 3.5.

3.2 Birefringence properties of CBCsThe birefringence properties of a polymer can be quitecomplex. Both molecular structure and processing condi-tions affect the measured birefringence in a fabricated poly-mer film or article. Figure 4 shows the birefringence ofCBC films prepared by compression molding as a functionof rigid co-monomer content (i.e., styrene). In all cases, theamount of birefringence is very low (ca. 10–6). A slightdecrease in birefringence is observed with an increase ofstyrene content. A low birefringence is expected for thinfilms of CBC materials made by compression moldingbecause such a fabrication process should not lead to anyappreciable amount of molecular orientation.

Two possible root causes may explain the effect ofco-monomer content on birefringence shown in Fig. 4: (1)block-copolymer microphase separated structure and (2)individual chain polarizability. Polymer films prepared bycompression molding are often not completely isotropic andthere tends to be a preferential (albeit minor) alignment ofblock-copolymer domains. Such alignment is more likely inmaterials with a high degree of microphase separation. CBCmaterials with relatively low styrene content (e.g., CBC-1and CBC-2) are more prone to microphase separation becauseof their more symmetrical blocky structure. Microphaseseparated chains are not in random coil configurations, andif these extended chains are arranged in a macroscopicallyanisotropic matrix (i.e., preferential domain alignment), theresulting material can exhibit birefringence even though thebirefringence of an individual chain is low (or nearly zero).

The second possibility is that CBC polymers with rela-tively high rigid co-monomer content are slightly more iso-tropic. In another words, the difference between thepolarizability along the chain axis (b1) and perpendicular tothe chain axis (b2), ∆b = b1 – b2, becomes smaller whenincreasing the rigid comonomer content in CBC materials.

n AB

( ) .ll

= + 2

FIGURE 3 — Refractive index of CBC-3 polymer as a function of wave-length.

FIGURE 4 — Birefringence of CBC polymers at pseudo isotropic state asa function of styrene monomer content. The measurements wereconducted on thin films of approximately 150 µm thickness prepared bycompression molding.


According to a statistical segmental model analysis,15,16 ∆bfor the chain segment consisting of hydrogenated styreneunits is calculated to be negative at –2.1 × 10–12, whereas forthe chain segment consisting of hydrogenated isopreneunits, ∆b is positive at 19.6 × 10–12. It is noted here that thenegative ∆b for the rigid block is primarily due to the pen-dant cyclohexane ring structure. If ignoring the volumetricdifference between two monomer units, a simple mixing-rule analysis suggests that ∆b approaches zero at about90 wt.% of rigid comonomer content, representing the mostisotropic state a CBC chain can possibly have. When therigid monomer content is less than 90 wt.%, a CBC polymerwould be expected to display a positive birefringence. Onthe other hand, if the rigid monomer content is greater than90 wt.%, there is a tendency for CBC polymer to exhibitnegative birefringent behavior. Indeed, by varying theco-monomer content, CBC polymer exhibits a crossoverfrom positively birefringent to negatively birefringent behav-ior. However, a CBC polymer with greater than 90 wt.% ofrigid co-monomer content tends to be very brittle and is notsuitable for use in display film applications.

The influence of fabrication and, in particular, the ori-entation effects on the birefringence behavior of CBCmaterials were also investigated. This knowledge is impor-tant for designing a block copolymer that is suitable formanufacturing an optical film by melt extrusion, a processthat would inevitably lead to some degree of molecular ori-entation and possibly block-copolymer morphological align-ment in the film. Various films that were uniaxially stretchedat an elevated temperature by a tensile-stretching machinewere used for this study. Prior to stretching, films were pre-pared by compression molding with a target thickness of150 µm and then cut into micro-tensile bars. The stretchingtemperature was kept constant at 135°C for all the CBCfilms. The elongation relative to the original film length (i.e.,stretching ratio) varied from 30% to 200%. After thestretching operation, the film specimen was taken out of thetensile stretching machine and left at the room temperaturefor 48 hours before optical measurements were performed.The birefringence at the middle point of the specimen wasmeasured as a function of the stretching ratio.

The stretched CBC films were found to have a bire-fringence on the order of 10–4. This is about two orders ofmagnitude higher than the unstretched state, indicating astrong effect of molecular orientation on the measuredoptical anisotropy. When the stretching ratio is at about130%, the increase in birefringence begins to level off. Fur-ther stretching of the film at higher stretching ratio eitherdoes not result in any substantial increase in the measuredbirefringence or the film-stretching operation becomes toounstable to allow for reproducible measurements. For thisreason, the birefringence at the stretching ratio of 150% willbe compared for CBCs with different co-monomer content.

Figure 5 shows the birefringence as a function of rigidco-monomer content for various stretched CBC films. Arather interesting and complex relationship is observed.

Most evidently, a local maximum occurs for the CBC-4 poly-mer having 75 wt.% of rigid comonomer content. If this par-ticular data point is neglected, there is an overall trend ofthe birefringence decreasing with the increase of rigidcomonomer content in the compositional space investi-gated. This is similar to what is observed on un-stretchedfilms due to the polarizability difference (Fig. 4). Thus, adifferent mechanism would have to be involved to explainthe considerably higher birefringence measured on thestretched CBC-4 film.

One possible cause is the contribution of form bire-fringence (∆nf) for the total birefringence of CBC polymers.Form birefringence arises as a result of multiple phases inthe material in which each phase has a different refractiveindex. Additionally, at least one phase needs to be geometri-cally anisotropic (e.g., cylinder or plate).17,18 Thus, a homo-polymer or random copolymer would not exhibit any formbirefringence, but a block copolymer like CBC can exhibitdistinct microphase separation between two blocks thatleads to a considerable amount of form birefringence. Ofthe various CBC polymers studied here, it is believed thatCBC-4 is the only CBC polymer included in this investiga-tion that tends to form a cylindrical morphology with theminor phase (i.e., hydrogenated isoprene blocks) formingdispersed cylinders in the continuous matrix consisting ofhydrogenated styrene blocks.19 The cylinder morphology isknown to produce form birefringence, and its effect will beenhanced when cylinders are uniaxially oriented (e.g., by astretching orientation process). On the contrary, CBC mate-rials of other composition shown in Table 1 are less prone toexhibit form birefringence effect because of their non-cylin-drical morphology.

The magnitude of form birefringence for CBC-4 poly-mer can be calculated by the following equation:

FIGURE 5 — Birefringence of CBC polymers at oriented state as afunction of styrene monomer content. The measurements wereconducted on thin films that were subjected to a uniaxially stretchingorientation at the stretching temperature of 135°C. CBC tends to formdistinctly different morphology at four shaded regions.


where n|| and n⊥ are the apparent refractive index of poly-mers in the direction parallel and perpendicular to the cyl-inder axes, respectively; v1 and n1 are the volume fractionand refractive index of the rigid (matrix) phase; and v2 andn2 are the volume fraction and refractive index of the soft(cylinder) phase. The form birefringence is calculated to be∆nf ~ 1.1 × 10–4. This value is apparently non-negligible, butstill relatively small compared to the total birefringence ofCBC-4 at 6.7 × 10–4. Thus, the form birefringence alone cannot account for the considerably higher birefringence ofCBC-4 relative to other compositions (Fig. 5).

It is now clear that the molecular orientation is prob-ably still the dominant mechanism for the birefringence inall CBC polymers. Furthermore, since the intrinsic birefrin-gence (referring to the maximum birefringence of a polymerchain under perfectly oriented condition) should follow alinear function of comonomer content, the non-monotonicbehavior of the measured birefringence shown in Fig. 5 sug-gests the stretched films possess different degrees of orien-tation after the stretching. This is likely when consideringthe less than desirable stretching protocol used in this study.Equally important, the variable rate of polymer-chainrelaxation during the stretching at a fixed stretching tem-perature of 135°C can also affect the degree of orientationthat can be retained in the solid film sample. It is also con-ceivable that the competition between stretching orienta-tion and thermodynamically driven microphase separationmay have resulted in a different degree of optical anisotropyfor CBC materials possessing different morphology.

One interesting finding worthy of note is the birefrin-gence of CBC-3 material. This material exhibits a lowerbirefringence than both CBC-2 and CBC-4, but it has anintermediate comonomer content. We suspect CBC-3might be inherently more isotropic due to the possible for-mation of a bi-continuous gyroid phase. Further evidencesupporting such a morphology is the fact that CBC-3 exhib-its an excellent balance of thermal and mechanical proper-ties. However, there is no direct evidence (e.g., microscopyor scattering) confirming the presence of gyroid morphol-ogy in CBC-3.

3.3 Melt extrusion behavior of CBC polymerThe melt extrusion process for making thin films with prop-erties and quality appropriate for use in polarizer applica-tions has presented a series of challenge that was not solveduntil very recently. A viable melt process that preparedusable polarizer film was first demonstrated on COP byZeon Corp.13 Similar to COP, we found CBC materials alsoexhibit very good film forming ability across a wide compo-sitional range. Films with thickness ranging from 20–100µm or higher can be readily cast from a CBC polymer usinga melt-extrusion process.

Figure 6 shows the film-thickness profile of a 1340-nm-wide CBC-3 film. Thickness measurements were madealong the film width direction. The average film thicknesswas measured to be 61.2 µm with a standard deviation ofabout 0.26 µm. No visible die lines or streak-like cosmeticdefects along the machine direction and the transversedirection were observed on the film, suggesting the melt-ex-trusion cast process is inherently capable of manufacturingthin film (i.e., less than 100 µm) with a high degree of uni-formity for display applications.

CBC films prepared by melt casting also exhibit excel-lent surface uniformity at the microscopic level. Figure 7shows the surface morphology of the CBC-3 film measuredby atomic force microscopy (AFM). The film side directlycontacting the cast roller [Fig. 7(a)] was found to be verysmooth. The root-mean-square average roughness (Rq) wascalculated to be only about 1.7 nm. The other side of theCBC film was shown to be slightly rougher [Fig. 7(b)], nev-ertheless, the Rq value is still relatively low at 5.8 nm. Thislevel of surface roughness is better than a standard TAC filmmade by the solution casting process. Rq values measured onthe two sides of a TAC film (e.g., Fujifilm TD80ULM) areat 6.2 and 6.7 nm using a similar AFM surface morphologyanalysis.

In many respects, a melt extrusion process is advanta-geous over the conventional solution casting process formaking a polarizer film. First and foremost, the melt-extru-sion process reduces the total cost of manufacturing anoptical film and it is more environmental friendly. CurrentTAC film manufacturing involves many steps and is inher-ently complex and expensive, partly due to the need to com-ply with environmental regulations of solvent emission andhandling. Secondly, TAC film with thickness less than 40 µmis very difficult to make. As a result, TAC film is becoming aless-desirable choice for use in portable devices such as cellphones, PDAs, etc., where a slim and light display design isnecessary. The excellent film-forming ability of CBC poly-mer can potentially be used in making thinner films that

Dn n nn n

n n nf = - =

-

+ +^||

||

( )

( ),

n n

n n1 2 1

222

1 22

2 122 1

FIGURE 6 — Thickness profile of a CBC-3 film prepared by a commercialmelt-extrusion process.


have not been possible by TAC. Thirdly, the possibility ofmicro-patterning an optical film during the melt-extrusionprocess, an option not feasible in solution cast, opens up anadditional opportunity for cost reduction and size compac-tion of the LCD.

3.4 Results of low-retardation CBC filmLow-retardation CBC film with high optical quality was suc-cessfully made by melt casting CBC-3 material. As we dis-cussed in the earlier sections, CBC-3 provides a goodbalance in terms of melt-extrusion processing, physicalproperties, and low birefringence. Thus, the remaining sec-tions will mainly focus on films made from the CBC-3 material.

Figure 8 shows a comparison of film retardation at dif-ferent light incidence angles for CBC-3 film and a conven-tional TAC film. The retardation was measured at a singlewavelength of 633 nm. Unlike the TAC film which exhibitssignificant dependence on light incidence angle, the opticalretardation of CBC-3 film is essentially constant at all inci-dence angles, suggesting the CBC film is optically isotropic.Moreover, the optical retardation of CBC-3 film is very low

and less than 0.3 nm at all incidence angles. Thus, it shouldbe very suitable for polarizer-film use.

CBC-3 film also exhibits a higher degree of retarda-tion uniformity across a large film area. As shown in Fig. 9,both in-plane retardation (R0) and thickness direction retar-dation (Rth) were found to be nearly zero across the entirefilm width direction. The average R0 is about 0.2 nm with astandard deviation of less than 0.1 nm. The average Rth isapproximately 0.1 nm with a standard deviation of less than0.1 nm. To the best of our knowledge, this level of opticalretardation is the lowest among all display films. It is alsoworth noting here that both R0 and Rth are stable afterexposure of CBC-3 films to moisture because of the excel-lent low moisture absorption properties of CBC materials.

The wavelength dependence of R0 and Rth for CBC-3film and a conventional TAC film are compared in Fig. 10.Neither retardation property shows any measurable wave-length dispersion effect for CBC-3 film; R0 and Rth areessentially unaffected by the change in wavelength. On thecontrary, both R0 and Rth of TAC film exhibit strong wavelength

FIGURE 7 — AFM surface morphology of CBC-3 film: (a) film sidecontacting the extrusion casting roller; (b) opposite side of the film.

FIGURE 8 — Optical retardation of CBC-3 film and a conventional TACfilm measured at different light incidence angles and the wavelength ofλ = 633 nm.

FIGURE 9 — Optical retardation of CBC-3 film across the entire filmwidth direction.


dependence; R0 decreases with the increase of wavelength,whereas Rth increases with the increase of wavelength. Ingeneral, the strong wavelength dependence shown by TACfilm is undesirable for wide-view performance and has to beaddressed during LCD system design.

Additionally, the Rth value of TAC film is rather highand reaches about 50 nm at λ = 550 nm. This is suspectedto be due to the out-of-plane orientation of the cellulosering structure of the TAC molecule. The high Rth values ofthe TAC film, when used as a polarizer protective film, isknown to cause a substantial color shift in IPS-LCD.6 Incontrast, the Rth of the CBC-3 film is roughly 0.1 nm. Theextremely low Rth value of CBC-3 film should be advanta-geous for use as a polarizer protection film in general andparticularly in an IPS-LCD for improved wide view per-formance.

3.5 LCD simulation resultsThe large color shift caused by the non-negligible retarda-tion of TAC protective film in a polarizer assembly has beena well-known problem in IPS-LCD cells. To understand thepotential benefit of using a completely isotropic film likeCBC-3 for polarizer applications, the image-viewing charac-teristics were studied through an optical simulation using a

standard IPS-LCD structure described by Lu et al.20 Figure11 is the simple layout of the IPS-LCD cell used for oursimulation. The active component of the cell used for thesesimulations have the following characteristics. The liquid-crystal mixture MLC-6692 from Merck is homogeneouslyaligned in the LCD cell. The pre-tilt angle of LC director is~2° and the rubbing angle is at 10°with respect to the longi-tudinal direction of the electrodes. The electrode width, theelectrode gap, and the liquid-crystal cell gap are maintainedat 4, 8, and 4 µm, respectively.

Figure 12 shows the calculated CIE 1931 chromaticitydiagram of an IPS-LCD cell using CBC-3 film (R0 = 0.2 nmand Rth = 0.1 nm) as the inner protective film for both thebottom polarizer and the top analyzer. A standard cold-cath-ode fluorescent-lamp (CCFL) source from 380 to 780 nmwas used for the calculation. The incident light was at a 40°angle with respect to the cell normal direction and scannedacross the entire azimuthal range. Results show a relativelyminor color shift. The color shift (∆u′ v′) was calculated tobe about 0.128 across the entire viewing angles.

A similar color-shift calculation was conducted whenusing two conventional TAC films as the inner protectivefilms for both polarizer and analyzer. Results for a TAC film

FIGURE 10 — In plane retardation R0 and thickness direction retardationRth for CBC-3 film and a conventional TAC film at three differentwavelengths: (a) R0 as a function of wavelength; (b) Rth as a function ofwavelength. The measurements were made on a CBC-3 film of 60 µmthickness and a TAC film of 80 µm thickness.

FIGURE 11 — Schematic layout of the IPS-LCD cell for the opticalsimulation.

FIGURE 12 — CIE 1931 chromaticity diagram of an IPS-LCD cell usingCBC-3 film as the inner protective polarizer films at 40° of white-lightincidence angle (z-CBC Film: R0 = 0.2 nm, Rth = 0.1 nm).


having R0 = 5 nm and Rth = 50 nm were shown in Fig. 13.Not surprisingly, a substantially larger degree of color shiftoccurred at large viewing angles than the case of the CBC-3film. The average color shift (∆u′v′) was calculated to beabout 0.305. This is more than twice of the value observedwhen predicting the performance of the low-retardationCBC-3 film.

In addition to substantially reducing the color shiftwhen using CBC-3 film as the inner protective film of thepolarizer, minor improvements on the viewing angle andcontrast ratio was also obtained from the optics calculations.Thus, a completely isotropic film such as CBC-3 should pro-vide some real advantages for improving the overall viewingcharacteristics of an LCD, particularly with respect to IPS-LCD design.

4 ConclusionCyclic block copolymers possess an interesting set of bire-fringence properties. Both molecular orientation and mor-phological effects can contribute to the total birefringence.However, due to a close refractive-index matching betweenthe hydrogenated styrene phase and the hydrogenated iso-prene phase, the form birefringence due to block-copoly-mer morphology is found to be only a small fraction of thetotal birefringence, whereas molecular orientation remainsto be the dominant mechanism for the observed birefrin-gence in CBC films, particularly when the polymer is ori-ented.

By optimizing the comonomer composition, an iso-tropic film was recently developed using cyclic-block-co-polymer technology. This new optical film shows near-zerooptical retardation in both the film plane and thicknessdirection. No measurable wavelength dependence for theretardation property of CBC film was observed. In addition,the CBC film has excellent thermal, mechanical, and barrier

properties. Commercial-scale production of CBC film by amelt-extrusion process has also been demonstrated. The useof melt processes for making a polarizer protective film pro-vides a more cost effective and environmental friendly tech-nology than current commercial TAC film manufacturingprocesses based on solvent casting.

AcknowledgmentsWe would like to thank Dr. Ruibo Lu at the University ofCentral Florida (currently at Pixel Qi) for conducting theoptical simulation of CBC film and TAC film. Assistance onthe optical measurement from Dr. Yi Jin and Vicki Tharp atThe Dow Chemical Company is also acknowledged.

References1 F. S. Bates et al., “PCHE-based pentablock copolymers: Evolution of

a new plastic,” AIChE J. 47, No. 4, 762–765 (2001).2 M. Yamazaki, “Industrialization and application development of cyclo-

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6 H. Nakayama et al., “Development of low-retardation TAC film forprotective films of LCD’s polarizer,” J. Photopolym. Sci. Tech. 19, 169(2006).

7 A. Tagaya et al., “Compensation of the birefringence of a polymer by abirefringent crystal,” Science 301, 812 (2003).

8 R. Hahn and J. H. Wendorff, “Compensation method for zero birefrin-gence in oriented polymers,” Polymer 26, 1619 (1985).

9 S. Iwata et al., “Transparent zero-birefringence copolymer and itsoptical properties,” Appl. Opt. 36, No. 19, 4549 (1997).

10 D. A. Hucul and S. F. Hahn, “Process for hydrogenating aromaticpolymers,” U.S. Patent 5,612,422 (1997).

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FIGURE 13 — CIE 1931 chromaticity diagram of an IPS-LCD cell usinga conventional TAC film as the inner protective polarizer films at 40° ofwhite-light incidence angle (TAC Film: R0 = 5 nm, Rth = 50 nm).


Weijun Zhou received his Ph.D. degree in chemi-cal engineering from California Institute of Tech-nology and his B.S. in materials science andengineering from University of Science and Tech-nology of China. He is currently a Sr. ResearchSpecialist at Core R&D – New Products Group ofThe Dow Chemical Company. His current areasof research interest are optical polymer, opticalfilm, structure–properties relationship, and multi-functional materials for electronics and displayapplications.

Daniel J. Murray is a Senior Research Specialistin Dow’s Core R&D where he is involved in thesynthesis and development of new polymeric sys-tems for a variety of applications. He received hisB.S. degree in chemistry from Saginaw ValleyState University.

Kurt A. Koppi received his B.S. degree in chemi-cal engineering from the University of Illinois in1988 and his Ph.D. degree in chemical engineer-ing from the University of Minnesota in 1993. Heis currently a Sr. Research Specialist in the NewProducts Core R&D Department at The DowChemical Company in Midland, MI. His areas ofexpertise are polymer processing, rheology, andstructure–property relationships.

Shin-Tson Wu is a PREP professor at the Collegeof Optics and Photonics, University of CentralFlorida. Prior to joining UCF in 2001, Dr. Wuworked at Hughes Research Laboratories (Malibu,California) for 18 years. He received his Ph.D.degree from the University of Southern Californiaand his B.S. degree in physics from National Tai-wan University. His studies at UCF concentrate infoveated imaging, bio-photonics, optical commu-nications, liquid-crystal displays, and liquid-crys-

tal materials. Dr. Wu is a Fellow of the IEEE, SID, and OSA. He hasco-authored 2 books: Reflective Liquid Crystal Displays (Wiley, 2001)and Optics and Nonlinear Optics of Liquid Crystals (World Scientific,1993).


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Birefringent properties of cyclic block copolymers and low