Abstract:
Disclosed is an optical multilayer comprising a polymeric substrate having a non-zero out-of plane birefringence and an amorphous polymeric overlayer that comprises an amorphous polymer having a Tg value above 160° C. and having the sign of its out-of-plane birefringence opposite to that of the polymeric substrate so as to provide a total out-of-plane phase retardation of said optical multilayer of between −30 nm and 30 nm for wavelengths of light between 400 and 700 nm.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an optical multilayer comprising a polymeric substrate having a non-zero out-of plane birefringence and an amorphous polymeric overlayer having an out-of-plane birefringence of opposite sign to the substrate. The multilayer has an overall low out-of-plane phase retardation.  
       BACKGROUND OF THE INVENTION  
       [0002]     Due to the low material cost and ease of processing, polymeric materials are widely used in opto-electronic components. An ongoing objective is to replace inorganic glasses that are known to be “fragile”, “heavy” and “hard for machining”. Polymeric materials, however, have optical characteristics that are process dependent, especially birefringence. All optical grade polymers are transparent and amorphous. When amorphous polymers are processed into a desirable shape, they are not optically isotropic, unlike the inorganic glasses. That is, the three indices of refraction, nx, ny and nz, are not equal. This is due to polymer-chain orientation that is unique to polymers. Thus, given a process condition, the observed optical anisotropy depends on the degree of polymer chain alignment. Polymer molecules have intrinsic birefringence Δn int  that is determined by factors, such as the polarizabilities of functional groups and their bond angles with respect to the polymer chain. The polymer products have extrinsic birefringence (in-plane or out-of-plane) that is different from the intrinsic birefringence and that is strongly process dependent. Depending on the application, the birefringence has to be controlled to meet the application requirement. In many cases, it is desirable to have substantially low birefringence or phase retardation in both the in-plane and out-of-plane directions.  
         [0003]     In the optical disk application such as Compact Disk (CD) and Digital Video Disk (DVD), the substrate materials must satisfy conditions such as, 1) high transmission, 2) low humidity permeation, 3) dimensional stability and 4) low birefringence. Typically, the reading of optical disks involves the detection of slight changes n the polarization state or a change in the intensity of the reflected light from a disk surface. Thus, the birefringence in the disk substrate will have detrimental effects on the readout, such as read-error or noise. Optical disk substrates are manufactured by injection molding of polymers. Polycarbonate (PC) has been widely used for substrates for CD and DVD. It has high transmission, high dimensional stability against heat and humidity, and high mechanical strength. PC, however, has relatively high intrinsic birefringence Δn int . The process of injection molding generates alignment of polymer chains. Thus, a polymer with high intrinsic birefringence, such as PC, is prone to generate unacceptable levels of in-plane retardation R in  and out-of-plane retardation R th . In order to prevent this problem, one typically adjusts the molding conditions, such as temperature and flow-rate. This optimization of process conditions has been successfully applied to significantly reduce the R in  through the reduction of Δn in . In some cases, the in-plane birefringence Δn in  for normally incident light can be made as low as 1˜3×10 −5 . On the other hand, the out-of-plane birefringence Δn th  is typically negative and with the optimized molding process the value is −6˜−5×10 −4 . Even though the value of Δn th  is small, the corresponding phase retardation for obliquely incident light is not negligible due to the substantial thickness of substrate, ˜1 mm. Thus, the light incident on the substrate at an oblique angle φ (measured from the substrate normal direction) will suffer a phase retardation that scales as φ 2  for small φ. In some cases, the total phase retardation, taking into account reflection, at φ=30° can reach as much as −150 nm.  
         [0004]     In typical Liquid Crystal Displays (LCDs), a liquid crystal cell is situated between a pair of polarizers. Incident light polarized by the polarizer passes through a liquid crystal cell and is affected by the molecular orientation of the liquid crystal, which can be altered by the application of a voltage across the cell. The altered light goes into the second polarizer. Typical polarizers used widely for liquid crystal displays (LCDs) have a structure such that absorptive polarizing layer (e.g., iodine dye absorbed Polyvinyl Alcohol (PVA) layer) is sandwiched between the triacetylcellulose (TAC) substrate. TAC is widely used for polarizer manufacturing partly because of its low Δn int . For a typical un-stretched TAC, the Δn in  is in the order of 5×10 −5 . Thus TAC with 100 μm thickness has R in ˜5 nm. This amount of phase retardation is not significant and light linearly polarized by the PVA layer essentially remains linearly polarized going through the TAC layer., However, this is true only when light is normally incident to the plane of the polarizer. Most of the TAC substrates are known to have negative Δn th  of the order ˜−5×10 −4 . That would give R th ˜−50 nm. This out-of-plane phase retardation R th  is responsible for the change in the state of polarization for obliquely incident light. It is favorable to have finite negative Δn th  in TAC substrates for some modes of LCDs. This is because of the fact that the negative R th  can compensate positive R th  of the liquid crystal molecules that are aligned perpendicular to the liquid crystal cell plane. However, negative Δn th  of TAC has a detrimental effect in the LCD mode where the liquid crystal remains essentially parallel to the plane of the cell. This is the case for In-Plane-Switching LCDs, in which liquid crystal molecules rotate while remaining substantially parallel to the plane of the cell.  
         [0005]     In a typical backlight LCD, the backlighting assembly contains several optical films that improve the light distribution and polarization before reaching the liquid crystal cell. This backlighting assembly  201  is illustrated in  FIG. 2 . Light exiting the backlight,  203 , first encounters optical films that improve light distribution in the display, such as, diffusing films,  205  and brightness enhancement films,  207 . Light is then incident on a reflective polarizer  209  that contains a substrate,  211 , and a polarizing layer,  213 , which transmits one polarization state and reflects the other polarization state. The next component in the optical path is the absorptive polarizer  215 , which contains a bottom substrate,  217 , an absorbing polarizing layer,  219 , and a top substrate,  221 . The transmission axis of the absorptive polarizer and that of the reflective polarizer are parallel. Ideally, the polarization state that is transmitted by the reflective polarizer  209  is the same polarization state transmitted by the absorbing polarizer  215 . The optical stack between the backlight  203  and the reflective polarizer  209  recycles the polarization state that is reflected. The polarized light incident on the absorption polarizer  215  must be substantially linearly polarized so that light is effectively transmitted and not absorbed. As stated earlier, typical absorption polarizers contain TAC as a substrate  217 ,  221  on either side of the absorbing polarizing layer  219 . Negative out-of-plane birefringence of TAC used as the bottom substrate  217  converts the linearly polarized light, incident on the absorption polarizer  215 , to elliptically polarized light. The polarizing layer  219  will then absorb a portion of the elliptically polarized light. Thus, decreasing the light through put of the display. To have the most light through put, the bottom substrate  217  between the reflective polarizer  209  and the absorptive polarizing layer  219  must have small Δn th  and R th .  
         [0006]     As mentioned before, careful adjustment of the process can significantly reduce the Δn in , thus the R in  of the polymeric substrate. It is conceivable that additional optimization of the processing condition would further decrease the residual negative Δn th . However, it increases the manufacturing cost. Alternative method is to form a multilayer. That is to dispose an overlayer with positive R th  on the polymeric substrate having negative R th . This process provides an optical multilayer that has low R th  (−30 nm&lt;R th &lt;30 nm) for wavelength λ in the range 400 nm&lt;λ&lt;700 nm.  
         [0007]     Several methods of generating a layer with non-zero Δn th  thus R th  have been known.  
         [0008]     As is well known to those who are skilled in the art, liquid crystals that is uniformly aligned perpendicular to the substrate generate positive Δn th  if Δn int  of liquid crystal is positive. Polymerizable liquid crystal, such as the one disclosed in U.S. Pat. No. 6,261,649 gives perpendicular alignment. However, liquid crystal compounds generally have a high cost and creating a uniform alignment of liquid crystals in large manufacturing scale is complicated and not trivial. In some cases, it requires photo-polymerization process in order to freeze the perpendicular alignment, adding extra process and cost.  
         [0009]     Li et al. (Polymer, Vol. 37, Page  5321 - 5325 ,  1996 ) describe the process of generating the non-zero R th  by spin-coating polyamides on a transparent substrate. The random orientation of polyimide polymer chain is generated. The disclosed process is simple coating of polymers. However, the resulting Δn th  and R th  are negative. Therefore, the method only enhances the negativity of the Δn th  of the polymer substrates described above.  
         [0010]     With process optimizations, it is difficult to obtain a polymer substrate with sufficiently small R th . Also, the prior art fails to provide a simple method to generate a polymer layer with positive Δn th , thus making the manufacturing process for the polymeric multilayer with low R th  difficult. Therefore, it is a problem to be solved to provide a polymeric multilayer and a simple method of making it where the multilayer includes a polymer layer with positive Δn th  that can be disposed on polymeric substrate with negative R th  to form a multilayer having low R th .  
       SUMMARY OF THE INVENTION  
       [0011]     The invention provides an optical multilayer comprising a polymeric substrate having a non-zero out-of plane birefringence and an amorphous polymeric overlayer that comprises an amorphous polymer having a Tg value above 160° C. and having the sign of its out-of-plane birefringence opposite to that of said polymeric substrate so as to provide a total out-of-plane phase retardation of said optical multilayer of between −30 nm and 30 nm for wavelengths of light between 400 and 700 nm.  
         [0012]     The invention thus provides a polymeric multilayer and a simple method of making it where the multilayer includes a polymer layer with positive Δn th  that can be disposed on polymeric substrate with negative R th  to form a multilayer having low R th . 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a view of a layer with thickness d and x-y-z coordinate system attached to the layer;  
         [0014]      FIG. 2  is an elevation schematic of the typical LCD backlighting unit;  
         [0015]      FIG. 3A ,  FIG. 3B  and  FIG. 3C  are elevation schematics of the optical multilayer;  
         [0016]      FIG. 4A  and  FIG. 4B  are schematic views of perpendicular alignment of liquid crystals, and random in-plane orientation of amorphous polymer chain, respectively;  
         [0017]      FIG. 5A  and  FIG. 5B  are elevation schematics of polarizer with optical multilayer;  
         [0018]      FIG. 6  is an elevation schematic of the optical recording medium;  
         [0019]      FIG. 7  is a graph showing the wavelength λ dependence of the out-of-plane phase retardation R th  of the exemplary multilayer according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     The following definitions apply to the description herein:  
         [0021]     Order parameter, S refers to the degree of alignment of the polymer with respect to the reference direction. It is given by  
         S   =       3   ⁢     〈       cos   ⁢           ⁢     θ   2       -   1     〉       2       ,       
 
 where θ is an angle between the reference direction and the individual segment in the polymer chain. Brackets                     indicate the statistical average. S can take value from −0.5 to 1.0. 
 
         [0022]     In-plane phase retardation, R in , of a layer  101  shown in  FIG. 1  is a quantity defined by (nx−ny)d, where nx and ny are indices of refraction in the direction of x and y. x is taken as a direction of maximum index of refraction in the x-y plane and y direction is perpendicular to it. x-y plane is parallel to the plane  103  of the layer. d is a thickness of the layer in z-direction. The quantity (nx−ny) is referred as in-plane birefringence, Δn in . The value of Δn in  is given at wavelength λ=550 nm.  
         [0023]     Out of-plane phase retardation, R th , of a layer  101  shown in  FIG. 1  is a quantity defined by [nz−(nx+ny)/2]d. nz is the index of refraction in z-direction.  
         [0024]     The quantity [nz−(nx+ny)/2] is referred as out-of-plane birefringence, Δn th . If nz&gt;(nx+ny)/2, Δn th  is positive, thus the corresponding R th  is also positive. If nz&lt;(nx+ny)/2, Δn th  is negative and R th  is also negative. The value of Δn th  is given at λ=550 nm.  
         [0025]     Intrinsic Birefringence Δn int  of polymer refers to the quantity defined by (ne-no), where ne, and no are extraordinary and ordinary index of the polymer, respectively. The actual birefringence (in-plane Δn in  or out-of-plane Δn th ) of polymer layer depends on the process of forming it, thus the order parameter, and the Δn int .  
         [0026]     Amorphous means a lack of long-range order. Thus an amorphous polymer does not show long-range order as measured by techniques such as X-ray diffraction.  
         [0027]     Transmission is a quantity to measure the optical transmissivity. It is given by the percentile ratio of out coming light intensity I out  to input light intensity I in  as I out /I in ×100.  
         [0028]     Chromophore herein is defined as an atom or group of atoms that serve as a unit in light adsorption. ( Modern Molecular Photochemistry  Nicholas J. Turro Editor, Benjamin/Cummings Publishing Co., Menlo Park, Calif. (1978) Pg 77). A non-visible chromophore is one that has an absorption maximum outside the range of 400-700 nm.  
         [0029]     Reference will now be made to the drawings in which the various elements of the present invention will be given numerical designations and in which the invention will be discussed so as to enable one skilled in the art to make and use the invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
         [0030]      FIG. 3A  shows the structure of an optical multilayer  301  according to the invention.  303  is a polymeric substrate and  305  is an amorphous polymeric overlayer. The amorphous polymeric overlayer  305  can be disposed on both sides of the polymeric substrate  303  as shown in  FIG. 3B . Two polymeric substrates  303  can be-disposed on both side of the amorphous polymeric-overlayer,  FIG. 3C . The Δn th  of the polymeric substrate  303  is negative and that of amorphous polymeric overlayer  305  is positive. Generally, the value of Δn th  of the substrate  303  is extremely small (−1×10 −4 ˜−3×10 −5 ). However, if the thickness of the substrate  303  is significant (e.g. ˜1 mm), the R th  is not negligible and would be in the range of −100 nm˜−30 nm. On the other hand, the Δn th  of the overlayer  305  is more positive than 5×10 −3 (0.005). Thus, thickness of the overlayer  305  is much smaller than that of the substrate for an optical multilayer  301  with −30 nm&lt;R th &lt;30 nm for 400 nm&lt;λ&lt;700 nm. For example, in order to balance the R th =−50 nm from the substrate  303  (e.g., thickness 1 mm and Δn th =−5×10 −5 ), the amorphous polymer overlayer  305  would only be 5 μm, if Δn th  of the overlayer  305  is 0.01. To keep overall thickness of multilayer  301  within the reasonable range, the thickness of polymer overlayer  305  is preferably between 1 to 50 μm or more preferably 5 to 20 μm. Transmission of the overlayer  305  should be high enough so that the overall transmission of the optical multilayer  301  remains high. The transmission of amorphous polymer overlayer  305  is preferably higher than 80% or more preferably higher than 90% for 400 nm≦λ≦700 nm.  
         [0031]     As is well known to those who are skilled in the art, the birefringence of amorphous polymer Δn p  is given by Δn p =SΔn int . In the prior art, a perpendicular alignment (in z direction in  FIG. 4A ) of liquid crystals  401  is used to generate positive Δn th . In this case, S is in the range 0≦S≦1 and Δn int  is positive. If the polymer chain  403  is randomly oriented in the plane of the polymer layer as shown in  FIG. 4B , the Δn th  is generated while Δn in  is zero. For such an orientation, the order parameter S of the polymer chain is in the range −0.5&lt;S&lt;0. Thus, in order to obtain positive Δn th  for amorphous polymeric overlayer on the polymeric substrate, polymers with negative Δn int  can be used. Examples of such a polymers would include materials that have non-visible chromophores off of the polymer backbone. Such non-visible chromophores would include: vinyl, carbonyl, amide, imide, ester, carbonate, sulfone, azo, and aromatic heterocyclic and carbocyclic groups (e.g. phenyl, naphthyl, biphenyl, terphenyl, phenol, bisphenol A, and thiophene). In addition, combinations of these non-visible chromophores could be desirable (i.e. copolymers). Examples of such polymers and their structures are shown below.  
       EXAMPLE I  
     poly(4 vinylbiphenyl)  
       [0032]    
       
                 
         
             
             
         
       
     
       EXAMPLE II  
     poly(4 vinylphenol)  
       [0033]    
       
                 
         
             
             
         
       
     
       EXAMPLE III  
     poly(N-vinylcarbazole)  
       [0034]    
       
                 
         
             
             
         
       
     
       EXAMPLE IV  
     poly(methylcarboxyphenylmethacrylamide)  
       [0035]    
       
                 
         
             
             
         
       
     
       EXAMPLE V  
     poly[(1-acetylindazol-3-ylcarbonyloxy)ethylene]  
       [0036]    
       
                 
         
             
             
         
       
     
       EXAMPLE VI  
     poly(phthalimidoethylene)  
       [0037]    
       
                 
         
             
             
         
       
     
       EXAMPLE VII  
     poly(4-(1-hydroxy-1-methylpropyl)styrene)  
       [0038]    
       
                 
         
             
             
         
       
     
       EXAMPLE VIII  
     poly(2-hydroxymethylstyrene)  
       [0039]    
       
                 
         
             
             
         
       
     
       EXAMPLE IX  
     poly(2-dimethylaminocarbonylstyrene)  
       [0040]    
       
                 
         
             
             
         
       
     
       EXAMPLE X  
     poly(2-phenylaminocarbonylstyrene)  
       [0041]    
       
                 
         
             
             
         
       
     
       EXAMPLE XI  
     poly(3-(4-biphenylyl)styrene)  
       [0042]    
       
                 
         
             
             
         
       
     
       EXAMPLE XII  
     poly(4-(4-biphenylyl)styrene)  
       [0043]    
       
                 
         
             
             
         
       
     
         [0044]     Another important factor is to obtain finite negative value of S. One way to achieve such negative S values is to solvent coat polymers whose glass transition temperature Tg is greater than 160° C. Such polymers will not have sufficient time to relax upon solvent evaporation and will retain a negative S value.  
         [0045]     Examples of polymeric substrate can be made of polycarbonate, TAC, cyclic polyolephin, and other commonly used polymers in opto-electronic device applications. The thickness of polymer substrate should be sufficient to maintain mechanical integrity and handling ease. It is preferably between 10 μm to 5 mm or more preferably between 30 μm to 2 mm.  
         [0046]      FIG. 5A  is the elevation schematic for an absorptive polarizer  501  with an optical multilayer  301 . The multilayer  301  has a structure such as the one shown in  FIGS. 3A, 3B  and  3 C. Polarizing layer  505  is made of, for example, dye absorbed PVA film. The substrate  503  can be the optical multilayer, such as  301  or other single layer polymeric material.  FIG. 5B  is yet another example of polarizer  507 . In this case, polarizing layer  505  is contiguously disposed on the multilayer  301 . This is a typical structure of the reflective polarizer. As is well known to those who are skilled in the art, layer of cholesteric liquid crystal functions as reflective polarizing layer. Also, reflective polarizer based on periodically placed metal thin wire such as the one disclosed in U.S. Pat. No. 6,081,376 can be the polarizing layer  505 .  
         [0047]     Elevation schematic of the optical-recoding medium  601  is shown in  FIG. 6 .  603  is a recording layer. In magneto-optical recording media (MO),  603  is a magneto-optical layer made from, for example, rare-earth-cobalt-iron alloys. Optical multilayer  301  according to the invention is placed on the MO layer  603 . The light  607  to read the recorded signal is incident from multilayer  301  side.  605  is a protective layer.  
         [0048]     The overlayer can easily be disposed on the polymeric substrate by and suitable method such as, for example, solvent casting.  
         [0049]     The present invention is further illustrated by the following non-limiting examples of its practice.  
       EXAMPLE  
       [0050]     Poly (N-vinylcarbazole) (polymer I) was obtained from Acros Organics and found to have a Tg of 161° C. by differential scanning calorimetry (DSC).  
                         
 
         [0051]     Polymer I (15% solids in toluene) was spun cast onto a TAC substrate. R in  and R th  of this sample (and the TAC control) were measured with an ellipsometer (model M2000V, J. A. Woollam Co.) at λ=550 nm. Results are shown in TABLE I.  
         [0052]     The layer of polymer I did not show any sign of a long-range order. Therefore the layer was determined to be comprised of an amorphous polymer. This optical multilayer has a R th  between +30 and −30 nm at a λ between 400 and 700 nm. R th  of TAC and multilayer are shown as functions of λ with dash  701  and solid  703  lines, respectively in  FIG. 7 .  
                                                 TABLE I                                   Polymer I Layer   R in , In-Plane   R th , Out-of-Plane           thickness (μm)   Retardation (nm)   Retardation (nm)                                        0 (control)   3   −63           3   3   −7                      
 
         [0053]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. The entire contents of the patents and other publications referred to in this specification are incorporated herein by reference.  
       Parts List  
       [0000]    
       
           101  film  
           103  plane of the film  
           201  backlight assembly  
           203  backlight  
           205  diffusing film  
           207  brightness enhancement film  
           209  reflective polarizer  
           211  substrate  
           213  polarizing layer  
           215  absorptive polarizer  
           217  bottom substrate  
           219  absorptive polarizing layer  
           221  top substrate  
           301  optical multilayer  
           303  polymeric substrate  
           305  amorphous polymeric overlayer  
           401  liquid crystal  
           403  randomly oriented polymer chain in x-y plane  
           501  absorptive polarizer  
           503  substrate  
           505  polarizing layer  
           507  polarizer  
           601  optical recording medium  
           603  recording layer  
           605  protective layer  
           607  incident light for reading signal  
           701  dash line showing the wavelength dependence of R th  of TAC  
           703  solid line showing the wavelength dependence of R th  of the optical multilayer  
          S order parameter  
          θ an angle between the reference direction and the individual segment of the polymer chain  
          φ angle of incidence of light  
          nx index of refraction in x direction  
          ny index of refraction in y direction  
          nz index of refraction in z direction  
          no ordinary index of refraction  
          ne extraordinary index of refraction  
          Δn th  out-of-plane birefringence  
          Δn in  in-plane birefringence  
          Δn int  intrinsic birefringence of polymer  
          Δn p  birefringence of polymer  
          d thickness of the film  
          R th  out-of-plane phase retardation  
          R in  in-plane phase retardation  
          λ wavelength  
          I out  out coming light intensity  
          I in  input light intensity