Abstract:
Provided are filled polyimides that can be used in films and articles comprising the films. The films are useful in coverlay applications and have advantageous optical properties. Also provided are blends of cellulosic polymer and polyimide precursor which can be used to obtain the filled polyimides.

Description:
FIELD OF THE INVENTION 
       [0001]    The present disclosure relates generally to filled polyimides that can be used in films and articles comprising the films. The films are useful in coverlay applications and have advantageous optical properties. 
       BACKGROUND 
       [0002]    A coverlay is used as a protective cover film for flexible printed circuit boards, electronic components and the leadframe of an integrated circuit package. The coverlay can prevent damage from scratches, oxidation and contamination. 
         [0003]    Coverlays desirably have certain electrical properties (e.g., dielectric strength), as well as acceptable structural and optical properties to provide visual appeal and security against unwanted visual inspection and tampering of the electronic components protected by the coverlay. 
         [0004]    Typically, a coverlay comprises a single layer or multilayered polyimide film approximately 0.5 mil (12.5 microns) thick, with an adhesive coated on one side. For some applications, it is especially desirable to have a black, opaque coverlay. 
         [0005]    Black, opaque polyimide films can be made by incorporating additives such as dyes, pigments, and/or carbon black into polyimide precursor solutions prior to imidization of the polyimide. Although this can provide films with the desired optical properties, other critical properties (e.g., film strength, dielectric properties) can be adversely affected by the additives. 
         [0006]    There remains a need for an improved black polyimide film with good mechanical strength and dielectric properties for use in coverlay applications. 
       SUMMARY 
       [0007]    One aspect of the present invention is a composition comprising a polymer blend comprising:
   a cellulosic polymer and a polyimide precursor;   
 
         [0009]    wherein:
       the polyimide precursor forms a continuous phase in the blend and is derived from:
           i) at least 50 mole percent of an aromatic dianhydride, based upon a total dianhydride content of the polyimide precursor, and   ii) at least 50 mole percent of an aromatic diamine based upon a total diamine content of the polyimide precursor; and   
           the cellulosic polymer forms domains in a discontinuous phase in the blend, has a loading of from about 1 weight percent to about 40 weight percent, based on the weight of polyimide obtainable from the polyimide precursor, and has an average molecular weight in the range of about 500 to about 300,000 dalton.       
 
         [0014]    In some embodiments, the aromatic dianhydride is selected from the group consisting of: pyromellitic dianhydride, 3,3′,4,4′-biphenyl tetracarboxylic dianhydride, 3,3′,4,4′-benzophenone tetracarboxylic dianhydride; 4,4′-oxydiphthalic anhydride, 3,3′,4,4′-diphenyl sulfone tetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane, Bisphenol A dianhydride, and mixtures thereof. 
         [0015]    In some embodiments, the aromatic diamine is selected from the group consisting of: 3,4′-oxydianiline, 1,3-bis-(4-aminophenoxy)benzene, 4,4′-oxydianiline, 1,4-diaminobenzene, 1,3-diaminobenzene, 2,2′-bis(trifluoromethyl)benzidene, 4,4′-diaminobiphenyl, 4,4′-diaminodiphenyl sulfide, 9,9′-bis(4-amino)fluorine, and mixtures thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0016]      FIG. 1A  shows a scanning electron microscopy (SEM) cross-section of an uncured film prepared from a blend of cellulose acetate (5 wt % loading) and polyimide precursor as disclosed in Example 1. 
           [0017]      FIG. 1B  shows an SEM cross-section of a cured polyimide film prepared as disclosed in Example 1. 
           [0018]      FIG. 1C  shows a transmission electron microscopy (TEM) cross-section of a cured polyimide film prepared as disclosed in Example 1. 
           [0019]      FIG. 2A  shows an SEM cross-section of an uncured film prepared from a blend of cellulose acetate (10 wt % loading) and polyimide precursor as disclosed in Example 2. 
           [0020]      FIG. 2B  shows an SEM cross-section of a cured polyimide film prepared as disclosed in Example 2. 
           [0021]      FIG. 3A  shows an SEM cross-section of an uncured film prepared from a blend of cellulose acetate (20 wt % loading) and polyimide precursor as disclosed in Example 3. 
           [0022]      FIGS. 3B and 3C  show SEM cross-sections of a cured polyimide film prepared as disclosed in Example 3. 
           [0023]      FIG. 4A  shows an SEM cross-section of an uncured film prepared from a blend of cellulose acetate (40 wt % loading) and polyimide precursor as disclosed in Example 4. 
           [0024]      FIG. 4B  shows an SEM cross-section of a cured polyimide film prepared as disclosed in Example 4. 
           [0025]      FIG. 5A  shows an SEM cross-section of an uncured film prepared from a blend of cellulose acetate butyrate (10 wt % loading) and polyimide precursor as disclosed in Example 5. 
           [0026]      FIG. 5B  shows an SEM cross-section of a cured polyimide film prepared as disclosed in Example 5. 
           [0027]      FIG. 6A  shows an SEM cross-section of an uncured film prepared from a blend of cellulose acetate butyrate (10 wt % loading) and polyimide precursor as disclosed in Example 6. 
           [0028]      FIG. 6B  shows an SEM cross-section of a cured polyimide film prepared as disclosed in Example 6. 
           [0029]      FIG. 6C  shows a TEM cross-section of a cured polyimide film prepared as disclosed in Example 6. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    The following discussion is directed to the preferred embodiments of the present invention only, and nothing within the following disclosure is intended to limit the overall scope of this invention. The scope of the present invention is to be defined solely by the claims, as presented at the end of this specification. 
       Definitions 
       [0031]    The methods disclosed herein are disclosed with reference to the following terms. 
         [0032]    As used herein, where the indefinite article “a” or “an” is used with respect to a statement or description of the presence of a step in a process disclosed herein, unless the statement or description explicitly provides to the contrary, the use of such indefinite article is not intended to limit the presence of the step in the process to one in number. 
         [0033]    As used herein, when an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range. 
         [0034]    As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having,” “contains” or “containing,” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a composition, a mixture, process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, mixture, process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
         [0035]    As used herein, the term “about” modifying the quantity of an ingredient or reactant employed refers to variation in the numerical quantity that can occur, for example, through typical measuring and liquid handling procedures used for making concentrates or use solutions in the real world; through inadvertent error in these procedures; through differences in the manufacture, source, or purity of the ingredients employed to make the compositions or carry out the methods; and the like. The term “about” also encompasses amounts that differ due to different equilibrium conditions for a composition resulting from a particular initial mixture. Whether or not modified by the term “about”, the claims include equivalents to the quantities. The term “about” may mean within 10% of the reported numerical value, preferably within 5% of the reported numerical value. 
         [0036]    “Dianhydride” as used herein is intended to include dianhydrides, precursors or derivatives thereof, which may not technically be a dianhydride but would nevertheless react with a diamine to form a polyamic acid which could in turn be converted into a polyimide. 
         [0037]    “Diamine” as used herein is intended to include diamines, precursors or derivatives thereof, which may not technically be a diamine but would nevertheless react with a dianhydride to form a polyamic acid which could in turn be converted into a polyimide. 
         [0038]    “Precursor” and “polyamic acid” may be used interchangeably and as used herein each is intended to mean a relatively low molecular weight polyamic acid solution which is prepared by using a stoichiometric excess of diamine in order to give a solution viscosity of approximately 40-100 Poise. 
         [0039]    “Continuous phase” as used herein refers to the major component of a polymer blend, a filled polyimide polymer, or a filled polyimide film which extends without interruption. “Discontinuous phase” as used herein refers to a component of a polymer blend, a filled polyimide polymer, or a filled polyimide film which is dispersed within the continuous phase. 
         [0040]    In describing certain polymers it should be understood that sometimes applicants are referring to the polymers by the monomers used to make them or the amounts of the monomers used to make them. While such a description may not include the specific nomenclature used to describe the final polymer or may not contain product-by-process terminology, any such reference to monomers and amounts should be interpreted to mean that the polymer is made from those monomers, unless the context indicates or implies otherwise. 
         [0041]    “Void” as used herein refers to an interstice in a polyimide phase which is free of cellulosic polymer and free of polyimide, and which may contain substantially amorphous carbon. “Closed void” as used herein refers to a void which is isolated from other voids by intervening polyimide and thus is not part of an interconnected network of polymer-free space. 
         [0042]    “Substantially amorphous carbon” as used herein refers to carbon of which at least 90% on a weight basis does not have any clear shape, form, or crystalline structure. 
         [0043]    As used herein, the phrase “polyimide obtainable from the polyimide precursor” refers to the theoretical amount of polyimide polymer which can be formed from the polyimide precursor, accounting for the stoichiometry of the monomers employed and discounting the water formed during imidization. 
       Polyamic Acid Solution 
       [0044]    A polyamic acid solution is formed from a diamine component and a dianhydride component forming a polyimide precursor in a suitable solvent. Therefore, the polyamic acid solution comprises a polyimide precursor and a solvent. In some embodiments, the polyimide precursor is derived from at least 50 mole percent of an aromatic dianhydride, based upon a total dianhydride content of the polyimide precursor, and at least 50 mole percent of an aromatic diamine based upon a total diamine content of the polyimide precursor. In some embodiments, the aromatic dianhydride is selected from the group consisting of:
       pyromellitic dianhydride (PMDA),   3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA),   3,3′4,4′-benzophenone tetracarboxylic dianhydride (BTDA),   4,4′-oxydiphthalic anhydride,   3,3′,4,4′-diphenyl sulfone tetracarboxylic dianhydride,   2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane,   Bisphenol A dianhydride, and   mixtures thereof.       
 
         [0053]    In some embodiments, the aromatic diamine is selected from the group consisting of:
       3,4′-oxydianiline (3,4′-ODA),   1,3-bis-(4-aminophenoxy)benzene (RODA),   4,4′-oxydianiline (4,4′-ODA),   1,4-diaminobenzene (PPD),   1,3-diaminobenzene (MPD),   2,2′-bis(trifluoromethyl)benzidene,   4,4′-diaminobiphenyl,   4,4′-diaminodiphenyl sulfide,   9,9′-bis(4-amino)fluorine and   mixtures thereof.       
 
         [0064]    In another embodiment, the diamine is 1,4-diaminobenzene and the dianhydride is 3,3′,4,4′-biphenyl tetracarboxylic dianhydride. In another embodiment, the diamine is 4,4′-oxydianiline and the dianhydride is pyromellitic dianhydride. In yet another embodiment, the diamine is a mixture of 1,4-diaminobenzene and 1,3-diaminobenzene, and the dianhydride is 3,3′,4,4′-biphenyl tetracarboxylic dianhydride. 
         [0065]    In some embodiments, the polyimide precursor is derived from: 10 to 90 mole % of biphenyl tetracarboxylic dianhydride; 90 to 10 mole % of pyromellitic dianhydride; 10 to 90 mole % of 1,4-diaminobenzene; and 90 to 10 mole % of 4,4′-oxydianiline. 
         [0066]    In some embodiments, the diamine component is selected from 1,4-diaminobenzene and 4,4′-oxydianiline. The dianhydride component is selected from pyromellitic dianhydride and 3,3′,4,4′-biphenyl tetracarboxylic dianhydride. 
         [0067]    In another embodiment, the diamine is a mixture of 1,4-diaminobenzene (PPD) and 4,4′-oxydianiline (ODA) and the dianhydride is a mixture of pyromellitic dianhydride (PMDA) and 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA). In one embodiment, the polyimide is derived from: 10 to 90 mole %, or 30 to 50 mole %, of biphenyl tetracarboxylic dianhydride; 90 to 10 mole %, or 70 to 50 mol %, of pyromellitic dianhydride; 10 to 90 mole %, or 60 to 80 mole %, of 1,4-diaminobenzene; and 90 to 10 mole %, or 40 to 20 mole %, of 4,4′-oxydianiline. 
       Solvent 
       [0068]    Suitable solvents for forming the polyamic acid solution are those capable of dissolving one or both of the polymerizing reactants and the polyamic acid polymerization product. The solvent should be substantially unreactive with all of the polymerizing reactants and with the polyamic acid polymerization product. Suitable solvents include sulfoxide solvents (e.g., dimethyl sulfoxide and diethyl sulfoxide), formamide solvents (e.g., N,N-dimethylformamide and N,N-diethylformamide), acetamide solvents (e.g., N,N-dimethylacetamide and N,N-diethylacetamide), pyrrolidone solvents (e.g., N-methyl-2-pyrrolidone and N-vinyl-2-pyrrolidone), phenol solvents (e.g., phenol, o-, m- or p-cresol, xylenol, halogenated phenols, and catechol), hexamethylphosphoramide, tetrametyl urea, dimethylsulfone and gamma-butyrolactone. These solvents can also be used in combination with aromatic hydrocarbons such as xylene and toluene, or ether containing solvents such as diglyme, propylene glycol methyl ether, propylene glycol, methyl ether acetate, and tetrahydrofuran. 
       Polyamic Acid Solution—Formation 
       [0069]    The polyamic acid solutions are generally made by dissolving the diamine in a dry solvent and slowly adding the dianhydride under conditions of agitation and controlled temperature in an inert atmosphere. In one embodiment the diamine is present as a 5 to 15 weight percent solution in the solvent and the diamine and dianhydride are usually used in about equimolar amounts. 
         [0070]    Numerous embodiments of formation are possible, such as: (a) a method wherein the diamine components and dianhydride components are preliminarily mixed together and then the mixture is added in portions to a solvent while stirring, (b) a method wherein a solvent is added to a stirring mixture of diamine and dianhydride components, (c) a method wherein diamines are exclusively dissolved in a solvent and then dianhydrides are added thereto, (d) a method wherein the dianhydride components are exclusively dissolved in a solvent and then amine components are added thereto, (e) a method wherein the diamine components and the dianhydride components are separately dissolved in solvents and then these solutions are mixed in a reactor, (f) a method wherein the polyamic acid with excessive amine component and another polyamic acid with excessive dianhydride component are preliminarily formed and then reacted with each other in a reactor, particularly in such a way as to create a non-random or block copolymer, and (g) a method wherein a specific portion of the amine components and the dianhydride components are first reacted and then the residual diamine components are reacted, or vice versa, (h) a method wherein the components are added in part or in whole in any order to either part or whole of the solvent, also where part or all of any component can be added as a solution in part or all of the solvent, and (i) a method of first reacting one of the dianhydride components with one of the diamine components giving a first polyamic acid, then reacting the other dianhydride component with the other amine component to give a second polyamic acid, and then combining the polyamic acids in any one of a number of ways prior to film or fiber formation. 
         [0071]    The dianhydride and diamine components are typically combined in a molar ratio of aromatic dianhydride component to aromatic diamine component of from 0.90 to 1.10. Molecular weight can be adjusted by adjusting the molar ratio of the dianhydride and diamine components. 
         [0072]    In one embodiment, the polyamic acid solution is dissolved in an organic solvent at a concentration from about 5, 10 or 12% to about 12, 15, 20, 25, 27, 30% by weight. 
         [0073]    If the filled polyimide is to be used as a film, the polyamic acid solution may be combined with conversion chemicals, including: (i) one or more dehydrating agents, such as, aliphatic acid anhydrides (e.g., acetic anhydride) and aromatic acid anhydrides; and (ii) one or more catalysts, such as aliphatic tertiary amines (e.g., triethylamine), aromatic tertiary amines (e.g., dimethylaniline) and heterocyclic tertiary amines (e.g., pyridine, picoline, and isoquinoline). The anhydride dehydrating material is often used in a molar excess of the amount of amide acid groups in the copolyamic acid. The amount of acetic anhydride used is typically about 2.0-3.0 moles per equivalent of copolyamic acid. Generally, a comparable amount of tertiary amine catalyst is used. 
       Cellulosic Polymer 
       [0074]    Cellulosic polymer useful in the processes disclosed herein can comprise microcrystalline cellulose, a cellulose ester, a cellulose ether, or a combination of two or more thereof. In one embodiment, the cellulosic polymer may comprise microcrystalline cellulose. In one embodiment, the cellulosic polymer may comprise a cellulose ester such as, for example, cellulose acetate, cellulose triacetate, cellulose acetate propionate, or cellulose acetate butyrate. In one embodiment, the cellulose ester may comprise cellulose acetate, cellulose acetate butyrate, or a combination of two or more thereof. In one embodiment, the cellulosic polymer may comprise a cellulose ether such as, for example, hydroxypropyl cellulose, methyl ethyl cellulose, or carboxymethyl cellulose. In one embodiment, the cellulosic polymer may have an average molecular weight in the range of about 500 to about 300,000 dalton. In another embodiment, the cellulosic polymer may have an average molecular weight in the range of about 5000 to about 100,000 dalton. In another embodiment, the cellulosic polymer may have an average molecular weight in the range of about 10,000 to about 70,000 dalton. In one embodiment, a mixture of at least two cellulosic polymers, each having a different average molecular weight, is used. Typically, cellulosic polymers disperse well in the polyimide precursor matrix. Cellulosic polymers are typically commercially available in either powder or pelletized form, and either form can be used. 
         [0075]    For use, the cellulosic polymer is dissolved in a solvent, for example dimethylformamide (DMF), N,N-dimethylacetamide (DMAC) or N-methylpyrrolidone (NMP). Mixtures of such solvents may also be used. A solution of cellulosic polymer can be formed by heating the cellulosic polymer with stirring in the selected solvent. The rate of dissolution of the cellulosic polymer can be increased by using higher temperatures. Solutions of about 1 wt % to about 40 wt % cellulosic polymer are useful for forming dispersions of cellulosic polymer in the polyimide precursor, for example solutions of about 1 wt % to about 5 wt %, or about 1 wt % to about 10 wt %, or about 1 wt % to about 20 wt %, or about 1 wt % to about 30 wt %, or about 10 wt % to about 40 wt %, or about 20 wt % to about 40 wt % cellulosic polymer. The solvent contained in the solution comprising the cellulosic polymer may be the same or different from the solvent contained in the solution comprising the polyimide precursor. 
         [0076]    As the polyimide precursor is converted to polyimide in the curing step, the cellulosic polymer is pyrolyzed and closed voids containing amorphous carbon are formed. 
       Blend Formation 
       [0077]    One embodiment is a composition comprising a polymer blend comprising a cellulosic polymer and a polyimide precursor in which the polyimide precursor forms a continuous phase and the cellulosic polymer forms a discontinuous phase consisting of cellulosic polymer domains. The domains of cellulosic polymer are dispersed in the continuous phase of the polyimide precursor. The polyimide precursor phase may contain solvents in addition to the precursor. The domains of cellulosic polymer may contain solvents in addition to the polymer. The polymer blend is referred to herein as a cellulosic polymer/polyimide precursor blend. 
         [0078]    The blend is formed by contacting at least one solution comprising a cellulosic polymer and at least one solution comprising a polyimide precursor. The contacting is typically performed by mixing. For best results, paddle blades are used, for example, with a high torque mixer. The blend comprises about 1 to about 40 weight percent cellulosic polymer, based on the weight of polyimide obtainable from the polyimide precursor. The cellulosic polymer solution and the polyimide precursor solution should be well dispersed in the blend in such a manner that their dispersion is not mass transfer limited. 
         [0079]    The composition comprising a polymer blend comprising a cellulosic polymer and a polyimide precursor can be used to form a film, which may optionally be dried to remove solvent, for example in air between about 80° C. and 200° C. Such a film is referred to as uncured because the polyimide precursor has not been converted to polyimide. Typically, the domains of the cellulosic polymer phase in an uncured, dried film comprising the composition have an average longest dimension of about 0.1 microns to about 20 microns, for example from 0.1 microns to 8 microns, or from 0.1 microns to 6 microns, or from 0.1 microns to 4 microns, as determined by SEM. The cellulosic polymer phase domains can be spherical or elongated in shape, or of other shapes. When the film is cured, the polyimide precursor is converted to a continuous polyimide phase and the cellulosic polymer domains are converted to dispersed, closed voids containing substantially amorphous carbon. The cured film is referred to herein as a filled polyimide film. 
       Formation of a Polymer Blend Film, a Filled Polyimide Polymer, and a Filled Polyimide Film 
       [0080]    The composition comprising the polymer blend can be cast or applied onto a support, such as a glass, metal or polymer substrate or an endless belt or rotating drum, to give a film. Next, the solvent-containing film can be converted into a self-supporting film by heating in air or nitrogen at 80° C. to 200° C. In some embodiments, the film is then separated from the support, oriented such as by tentering, with continued heating (curing) in nitrogen at 300-500° C. to provide a filled polyimide film in which the polyimide precursor has been converted to a polyimide and the cellulosic polymer has been converted to a substantial number of closed voids containing amorphous carbon. In some embodiments, a cure temperature of about 400° C. to about 450° C. is used. In other embodiments, the film remains on the support through the curing process. The cure time depends in part on the cure temperature and can range from several minutes to an hour or longer. 
         [0081]    Typically, the closed voids in the polyimide phase have an average longest dimension of about 0.1 microns to about 10 microns, for example from 0.1 microns to 8 microns, or from 0.1 microns to 6 microns, or from 0.1 microns to 4 microns. The size of the voids may be determined, for example, by SEM. The closed voids can be spherical or elongated in shape, or of other shapes. 
         [0082]    In one embodiment, at least 25 percent of the voids have an average longest dimension of less than 1 micron and the balance of the voids have an average longest dimension of about 2 microns to about 10 microns. In one embodiment, at least 50 percent of the voids have an average longest dimension of less than 1 micron, and the balance of the voids have an average longest dimension in the range of about 2 microns to about 10 microns. In one embodiment, at least 75 percent of the voids have an average longest dimension of less than 1 micron and the balance of the voids have an average longest dimension in the range of about 2 microns to about 10 microns. The average longest dimension of the voids can be determined, for example, by the use of SEM. 
         [0083]    The substantially amorphous carbon contained within the voids is typically in the form of a particle, a coating, or both. Other forms are also possible. In one embodiment, a particle may fill a void partially or nearly entirely. In one embodiment, a coating may be present on at least a portion of the surface of the polyimide phase in contact with a void (i.e. on the walls of the void), including on the entirety of the surface of the polyimide phase surrounding the void. 
         [0084]    In one embodiment, a filled polyimide polymer is obtained by a method comprising the steps of:
       a) contacting at least one solution comprising a cellulosic polymer having an average molecular weight in the range of about 500 to about 300,000 dalton and at least one solution comprising a polyimide precursor to form a cellulosic polymer/polyimide precursor blend in which the polyimide precursor forms a continuous phase and the cellulosic polymer forms a discontinuous phase consisting of cellulosic polymer domains;       
 
         [0086]    wherein: 
         [0087]    the polyimide precursor is derived from at least 50 mole percent of an aromatic dianhydride, based upon a total dianhydride content of the polyimide precursor, and at least 50 mole percent of an aromatic diamine based upon a total diamine content of the polyimide precursor; 
         [0088]    the cellulosic polymer has a loading of from about 1 weight percent to about 40 weight percent, based on the weight of the polyimide obtainable from the polyimide precursor; and
       b) heating the cellulosic polymer/polyimide precursor blend to a temperature of about 300° C. to about 500° C. to convert the cellulosic polymer domains to dispersed closed voids having an average longest dimension of about 0.1 microns to about 10 microns and containing substantially amorphous carbon, and to convert the polyimide precursor to polyimide.       
 
         [0090]    In one embodiment, a filled polyimide polymer comprises a continuous polyimide phase containing dispersed closed voids having an average longest dimension of about 0.1 microns to about 10 microns and containing substantially amorphous carbon, wherein the polyimide is derived from a polyimide precursor derived from:
       i) at least 50 mole percent of an aromatic dianhydride, based upon a total dianhydride content of the polyimide, and   ii) at least 50 mole percent of an aromatic diamine based upon a total diamine content of the polyimide.       
 
         [0093]    In one embodiment, at least 25 percent of the voids of the filled polyimide polymer have an average longest dimension of less than 1 micron and the balance of the voids have an average longest dimension of about 2 microns to about 10 microns. 
         [0094]    In one embodiment, a filled polyimide film is obtained by a method comprising the steps of:
       a) contacting at least one solution comprising a cellulosic polymer having an average molecular weight in the range of about 500 to about 300,000 dalton and at least one solution comprising a polyimide precursor to form a cellulosic polymer/polyimide precursor blend in which the polyimide precursor forms a continuous phase and the cellulosic polymer forms a discontinuous phase consisting of cellulosic polymer domains;       
 
         [0096]    wherein: 
         [0097]    the polyimide precursor is derived from at least 50 mole percent of an aromatic dianhydride, based upon a total dianhydride content of the polyimide precursor, and at least 50 mole percent of an aromatic diamine based upon a total diamine content of the polyimide precursor; and 
         [0098]    the cellulosic polymer has a loading of from about 1 weight percent to about 40 weight percent, based on the weight of the polyimide obtainable from the polyimide precursor;
       b) forming a film from the cellulosic polymer/polyimide precursor blend; and   c) heating the cellulosic polymer/polyimide precursor blend film to a temperature of about 300° C. to about 500° C. to convert the cellulosic polymer domains to dispersed closed voids having an average longest dimension of about 0.1 microns to about 10 microns and containing substantially amorphous carbon and to convert the polyimide precursor to polyimide.       
 
         [0101]    In one embodiment, a filled polyimide film comprises a continuous polyimide phase containing dispersed closed voids having an average longest dimension of about 0.1 microns to about 10 microns, wherein the polyimide is derived from:
       i) at least 50 mole percent of an aromatic dianhydride, based upon a total dianhydride content of the polyimide, and   ii) at least 50 mole percent of an aromatic diamine based upon a total diamine content of the polyimide; and the voids contain substantially amorphous carbon.       
 
         [0104]    After curing, the filled polyimide film is highly colored, with the colors ranging from brown to black. More intense colors are achieved using higher ratios of cellulosic polymer to polyimide precursor and/or by more intense heating during the curing step (i.e., higher temperatures and/or longer times). Thus the color intensity can be fine tuned by adjusting the weight percent loading of cellulosic polymer, the cure temperature, and/or the cure time. 
         [0105]    Typically, the cured film has a matte finish as compared to the finish of a cured polyimide film prepared similarly but without the addition of a cellulosic polymer. In one embodiment, the gloss of the cured polyimide film is less than 90 as measured at 60 degrees. In one embodiment, the gloss of the cured polyimide film is less than 40. If desired, the matte finish can be enhanced by adding matte agents at any stage of the process prior to casting or by treating the surface of the cured film. Typical matte agents include amorphous silica, such as precipitated silica, fumed silica, diatomaceous silica, and silica gels. Other matte agents include inorganic particles, metal stearates, and nanoparticles. 
         [0106]    In some embodiments, the optical density (opacity) desirable (e.g., to hide the conductor traces in the flex circuits from view) is greater than or equal to 2. An optical density of 2 is intended to mean 1×10 −2  or 1% of the light is transmitted through the film. In some embodiments, the desired optical density is greater than or equal to 1. In some embodiments, the optical density of the polyimide film is greater than 1 for a 1 mil (about 25 microns) film and the gloss is less than 90 as measured at 60 degrees. In some embodiments, the optical density of the polyimide film is greater than 2 for a 1 mil film and the gloss is less than 40 as measured at 60 degrees. 
         [0107]    Because the blend contains solvent that is removed during the drying and converting steps, the cast film generally is restrained during drying to avoid undesired shrinkage. In continuous production, the film can be held at the edges, such as in a tenter frame, using tenter clips or pins for restraint. Alternatively, the film can be stretched by as much as 200 percent from its initial dimension. In film manufacture, stretching can be in either the longitudinal direction or the transverse direction or both. If desired, restraint can also be provided to permit some limited degree of shrinkage. 
         [0108]    High temperatures can be used for short times to dry the film and induce imidization to convert the polyimide precursor to a polyimide in the same step. Generally, less heat and time are required for thin films than for thicker films. 
         [0109]    The thickness of the film may be adjusted depending on the intended purpose of the film or final application specifications. It is generally preferred that the thickness of the film ranges from 2, 3, 5, 7, 8, 10, 12, 15, 20, or 25 microns to about 25, 30, 35, 40, 45, 50, 60, 80, 100, 125, 150, 175, 200, 300, 400 or 500 microns. Preferably, the thickness is from about 8 to about 125 microns. 
         [0110]    A uniform dispersion of closed voids containing substantially amorphous carbon not only does not increase the electrical conductivity, but additionally tends to produce uniform color intensity of the polyimide. In contrast, adding fillers like carbon to polyimide would typically result in the formation of conducting networks because of the percolation of the filler in the polyimide, with a resulting increase in the electrical conductivity. In some embodiments, the mean particle size of the cellulosic polymer-derived carbon contained in the voids is between (and optionally including) any two of the following sizes: 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, and 10.0 microns. The thickness of the film can be tailored to the specific application. 
         [0000]    Formation of a Filled Polyimide Fiber from the Polymer Blend Composition 
         [0111]    Filled polyimide fibers can also be made from the composition comprising a polymer blend comprising a cellulosic polymer and a polyimide precursor. Fiber can be spun from the composition and then heated to convert the polyimide precursor into a continuous polyimide phase and the cellulosic polymer into closed voids containing substantially amorphous carbon. The high temperature imidization/carbonization step can be carried out on the fiber immediately after spinning. Alternatively, the yarn, fabric or article made from the fiber can be heated to the appropriate temperature. 
         [0112]    Filled polyimide polymer of shapes other than films or fibers can also be made from the composition comprising a polymer blend comprising a cellulosic polymer and a polyimide precursor using methods known in the art. 
       Coverlay 
       [0113]    The filled polyimide film disclosed herein can be used as a coverlay film for flexible printed circuit boards, electronic components or the leadframe of an integrated circuit package. 
       Adhesives 
       [0114]    In one embodiment, the coverlay comprises a filled polyimide film and an adhesive layer for maintaining the placement of the coverlay film once applied. The adhesive may be coated on at least one side of the filled polyimide film. Examples of adhesives useful in forming the adhesive layer include thermoplastic polyimide resins, epoxy resins, phenolic resins, melamine resins, acrylic resins, cyanate resins, and combinations of two or more thereof. In some embodiments, the adhesive is a polyimide resin that can flow and bond at temperatures below the polyimide decomposition temperature. In one embodiment, the adhesive is a polyimide thermoplastic resin, optionally further comprising a thermosetting adhesive, such as, epoxy resin and/or phenolic resin. For adhesives having both thermoplastic and thermosetting components, the content of the thermosetting resin in the adhesive layer generally ranges from 5 to 400 parts by weight, preferably from 50 to 200 parts by weight, per 100 parts by weight of resin components other than the thermosetting resin. 
         [0115]    In one embodiment, the adhesive consists of an epoxy resin and hardener, and, optionally, further contains additional components, such as an elastomer reinforcing agent, curing accelerator, filler, and flame retardant. 
         [0116]    In some embodiments, the adhesive is an epoxy resin selected from the group consisting of: bisphenol A epoxy resins; bisphenol F epoxy resins; bisphenol S epoxy resins; phenol novolac epoxy resins; cresol novolac epoxy resins; biphenyl epoxy resins; biphenyl aralkyl epoxy resins; aralkyl epoxy resins; dicyclopentadiene epoxy resins; multifunctional epoxy resins; naphthalene epoxy resins; phosphorus containing epoxy resins; rubber modified epoxy resins, and mixtures of two or more thereof. 
         [0117]    In some embodiments, the epoxy adhesive contains a hardener. Suitable hardeners include phenolic compounds selected from the group consisting of: novolac phenol resins; aralkyl phenol resins; biphenyl aralkyl phenol resins; multifunctional phenol resins; nitrogen-containing phenol resins; dicyclopentadiene phenol resins; and phosphorus-containing phenol resins. 
         [0118]    In another embodiment, the hardener is an aromatic diamine compound selected from the group consisting of: diaminobiphenyl compounds, e.g., 4,4′-diaminobiphenyl and 4,4′-diamino-2,2′-dimethylbiphenyl; diaminodiphenylalkane compounds, e.g., 4,4′-diaminodiphenylmethane and 4,4′-diaminodiphenylethane; diaminodiphenyl ether compounds, e.g., 4,4′-diaminodiphenylether and di(4-amino-3-ethylphenyl)ether; diaminodiphenyl thioether compounds, e.g., 4,4′-diaminodiphenyl thioether and di(4-amino-3-propylphenyl)thioether; diaminodiphenyl sulfone compounds, e.g., 4,4′-diaminodiphenyl sulfone and di(4-amino-3-isopropylphenyl)sulfone; and phenylenediamines. In one embodiment, the hardener is an amine compound selected from the group consisting of: guanidines, e.g., dicyandiamide (DICY); and aliphatic diamines, e.g., ethylenediamine and diethylenediamine. 
       EXAMPLES 
       [0119]    The methods disclosed herein are illustrated in the following examples. From the above discussion and these examples, one skilled in the art can ascertain the various embodiments of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the methods and compositions disclosed herein to adapt it to various uses and conditions. 
         [0120]    The following materials were used in the examples. All commercial reagents were used as received unless otherwise noted. 
         [0121]    Polyamic acid (PAA) was supplied by DuPont Chip Packaging Materials, Circleville Ohio. Cellulose acetate (CAc) having 39.7 wt % acetyl content and an average molecular weight of 50,000 dalton was purchased from Sigma Aldrich (St Louis, Mo.). Cellulose acetate butyrate (CAcB) having an average molecular weight of 12,000 dalton with 16-19 wt % acetyl and 30-35 wt % butyryl content was purchased from Sigma Aldrich, as was cellulose acetate butyrate having an average molecular weight of 70,000 dalton with 12-15 wt % acetyl, 35-39 wt % butyryl, and 1.2-2.2 wt % hydroxyl content . Pyromellitic dianhydride (PMDA) was purchased from Sigma Aldrich and dimethylacetamide (DMAc) was purchased from EMD (Gibbstown, N.J.). The high torque mixer (model RW20.n) was from IKA (Wilmington, N.C.) and the viscometer (model DV-I Prime) was from Brookfield (Middleboro, Mass.). 
         [0122]    Shear viscosity was measured at room temperature using spindle number 5 and the speed of the spindle was adjusted so the torque was at least 50%. The viscometer drives the spindle immersed in the fluid through a calibrated spring and the viscous drag of the fluid against the spindle is measured by the spring deflection, which is measured with a rotary transducer. 
         [0123]    Tear strength-Initial was measured according to ASTM D-1004-90 on 3 to 4 different parts of a cured film sample along the machine direction (MD). Typically 3-4 samples were tested per film made and the results averaged for reporting. 
         [0124]    Optical density (OD) was measured at 3-4 different locations on the film with a 301 Densitometer (X-Rite Inc, Grand Rapids Mich.) and the OD values were normalized to 1 mil thickness. An optical density of 2 is intended to mean 1×10 −2  or 1% of light was transmitted through the film. 
         [0125]    Gloss was measured at 60 degree angle with a Micro-Tri-Gloss meter (model GB-4825) from Tricor Systems (Elgin, Ill.). Gloss measurements are based on the principle that light reflection from a glossy surface is relatively higher than from a matte surface due increased light scattering on the matte surface. Therefore, light reflected from rough surfaces tends to be diffusely scattered in all direction and the image forming capabilities are diminished (blurry image). 
         [0126]    Dielectric constant (Dk) and dissipation factor (Df) were measured with an Hewlett Packard (Santa Clara, Calif.) multi-frequency meters (4272A and 4275A model) at 1 KHz frequency on film samples coated with aluminum (Al) electrodes at least 100 nm thick on both sides. Al was sputtered on both sides of 4 inch×4 inch (about 10.1 cm×10.1 cm) cured film samples through a 1 inch (2.54 cm) diameter mask to provide electrical contact. The Al coated films were held between test tweezers (model 16034B) from Agilent (Santa Clara, Calif.). The multi-frequency meter was calibrated with and without an air gap prior to the measurement. 
         [0127]    Film cross section samples were prepared for SEM imaging as follows. Two sections of film approximately 5 mm by 10 mm were cut from each submitted sample. The film was cut across the 5 mm width in the transverse direction, that is, perpendicular to the machine direction as marked on the sample as received, unless otherwise requested. One end of the 5 mm×10 mm sample was wrapped in masking tape. 
         [0128]    The 5 mm×10 mm film was submerged in liquid nitrogen for at least 20 seconds. One of the following methods was then used to obtain a cross sectioned film.
       1. Brittle samples fractured readily when touched to the bottom of the dewar containing the liquid nitrogen.   2. If a sample did not fracture when touched to the bottom of the dewar, a second attempt was made by folding the sample in a gentle loop before immersion in liquid nitrogen. After at least 20 seconds of immersion, the sample was pinched with a pair of needle-nosed pliers while still submerged in the liquid nitrogen.   3. If neither of the above two techniques was successful, a fresh single-edged blade was immersed in liquid nitrogen concurrently with the sample. After at least 20 seconds of immersion, the sample and the blade were withdrawn and the blade was chopped down on the sample on top of fresh glassine paper.       
 
         [0132]    The cross sectioned film was mounted on a standard aluminum SEM stub using Duco cement. Once the cement had dried, the sample was coated with 2 nm Os metal coating using a OPC-80T osmium coater (SPI, Supplies). Cross sectioned film samples were imaged in a Hitachi S4000 FE (field emission) SEM at 5 keV accelerating voltage at 10 mm working distance. 
         [0133]    Film cross section samples were prepared for TEM imaging as follows. Pieces of film were embedded in a liquid two-part epoxy mixture (Buehler EpoxiCure) and cured overnight at 60° C. in a 4 mm deep multicavity embedding mold. The hardened blocks were trimmed of excess epoxy to prepare a block face for sectioning. 
         [0134]    To prepare ultrathin specimens, a diamond knife was used to cut sections by room temperature ultramicrotomy. The knife boat used to accumulate sections was filled with a mixture of 90/10 water/acetone. The prepared epoxy block was secured in the flat jaws of the cryoultramicrotome sample holder and sectioned to nominal thickness of 90 nm. The sections floating on the water/acetone mixture were retrieved on slim bar copper mesh grids and blotted on filter paper. 
         [0135]    Unstained sections were examined in a JEOL Transmission Electron Microscope (TEM) operated at 200 KV accelerating voltage. Images of magnification 1000-95,000× were recorded on a digital camera. Sampling should include both the film surfaces in contact with embedding epoxy and interior portions of the film. 
         [0136]    The following additional abbreviations are used: “C” is Celsius, “cm” is centimeter, “mm” is millimeter, “μm” is micrometer, “μl” is microliter, “mL” is milliliter, “min” is minute(s), “g” is gram(s), “wt” is weight, “wt %” means weight percent, “Temp” is temperature, “Comp. Ex.” is Comparative Example, “gf” means gram force, “MW” means average molecular weight. 
         [0137]    The weight of cellulose acetate needed to prepare a desired wt % loading of cellulose acetate in the cellulose acetate/polyimide precursor blend was determined by first calculating the weight of polyimide obtainable from the polyimide precursor solution, and then determining the weight of cellulose acetate to use based on the desired loading. The desired loading is calculated with respect to the weight of the polyimide obtainable from the polyimide precursor. 
         [0138]    For example, in Example 1 the calculation of the amount of cellulose acetate solution required to provide a loading of 5 wt % cellulose acetate in the blend composition was determined in the following manner: 
         [0000]      Weight of polyamic acid=90 g (polymer+DMAc) 
         [0139]    Since the wt % of polyamic acid was 19.5%, the weight of the obtainable polyimide polymer was estimated to be 90 g×0.195=17.55 g. 
         [0140]    Assuming that 5% of the polymer weight was water which evaporates after curing at 400° C., the weight of obtainable polyimide was calculated to be 17.55 g×0.95=16.67 g. 
         [0141]    Thus the weight of CAc needed to prepare 5 wt % CAc was calculated as 16.67 g×0.05=0.83 g of CAc. 
         [0142]    The weight of a 14% CAc solution needed for mixing with 90 g polyamic acid solution was then calculated as 0.83 g/0.14=5.92 g of CAc solution. 
       Example 1 
     5 wt % Loading of Cellulose Acetate in Cellulosic Polymer/Polyimide Precursor Blend 
       [0143]    In this Example, 90 g of a 19.5 wt % solution of polyamic acid (PAA) in dimethylacetamide (DMAc) was mixed with 5.9 g of a 14 wt % solution of cellulose acetate (CAc) dissolved in DMAc for 15 min using a high torque mixer equipped with paddle blades. Then 2.88 g of a 6 wt % fresh solution of pyromellitic dianhydride (PMDA) in DMAc was added to the mixture of PAA and CAc. The resulting mixture, the cellulose acetate/polyimide precursor blend, was mixed for 30 min with the high torque mixer, during which time the viscosity increased to 1110 Poise at room temperature. A small amount (˜10-30 g) of the blend was then poured onto a Melinex® ST504 sheet (10×8 inches) from DuPont Teijin Films and spread with a 10 mil (about 250 micron) film applicator from Gardco (Pompano Beach, Fla.). The cast film was dried in air on a hot plate for 60 min between 70° C. and 90° C., stored under vacuum at room temperature overnight then cured under nitrogen at 400° C. for 10 min. The temperature of the oven was ramped at 4° C./min. The cured film was light brown in color. Table 1 presents the viscosity of the cellulosic polymer/polyimide precursor blend before film casting, as well as the optical density, gloss, dielectric constant, dissipation factor, and initial tear resistance of the cured film. 
         [0144]    The cross sections of both dried, uncured ( FIG. 1A ) and cured films ( FIG. 1B ) were analyzed by scanning electron microscopy (SEM), which revealed that in the uncured film cellulose acetate had formed domains in the polyimide precursor phase due to phase separation, while in the cured film, closed (isolated) voids were dispersed throughout the polyimide phase. The domains of the cellulose acetate phase were observed to have an average longest dimension of about 0.1 microns to about 2.5 microns. The voids were observed to have an average longest dimension of about 0.3 microns to about 4.0 microns. 
       Example 2 
     10 wt % Loading of Cellulose Acetate in Cellulosic Polymer/Polyimide Precursor Blend 
       [0145]    In this Example, 85 g of a 19.5 wt % solution of PAA in DMAc was mixed with 11.2 g of a 14 wt % solution of CAc dissolved in DMAc. Then 2.80 g of a 6 wt % fresh solution of PMDA in DMAc was added to the mixture of PAA and CAc. The methods and procedures of Example 1 were followed except that the film was stored under vacuum at room temperature for 3 days before curing. The cured film was brown in color. Table 1 presents the viscosity of the cellulosic polymer/polyimide precursor blend before film casting, as well as the optical density, gloss, dielectric constant, dissipation factor, and initial tear resistance of the cured film. 
         [0146]    The cross sections of both dried, uncured ( FIG. 2A ) and cured films ( FIG. 2B ) were analyzed by SEM, which revealed that in the uncured film cellulose acetate had formed domains in the polyimide precursor phase due to phase separation, while in the cured film, closed voids were dispersed throughout the polyimide phase. The domains of the cellulose acetate phase were observed to have an average longest dimension of about 1 microns to about 4 microns. The voids were observed to have an average longest dimension of about 0.3 microns to about 6.0 microns. 
       Example 3 
     20 wt % Loading of Cellulose Acetate in Cellulosic Polymer/Polyimide Precursor Blend 
       [0147]    In this Example, 75 g of a 19.5 wt % solution of PAA in DMAc was mixed with 19.8 g of a 14 wt % solution of CAc dissolved in DMAc. Then 2.85 g of a 6 wt % fresh solution of PMDA in DMAc was added to the mixture of PAA and CAc. The methods and procedures of Example 1 were followed except that the film was stored at room temperature under vacuum for 3 days before curing. The cured film was dark brown in color. Table 1 presents the viscosity of the cellulosic polymer/polyimide precursor blend before film casting, as well as the optical density, gloss, dielectric constant, dissipation factor, and initial tear resistance of the cured film. 
         [0148]    The cross sections of both dried, uncured ( FIG. 3A ) and cured films ( FIG. 3B  and  FIG. 3C ) were analyzed by SEM, which revealed the existence of cellulose acetate domains within the polyimide precursor phase of the uncured film, and voids containing carbon in the polyimide phase of the cured film. The domains of the cellulose acetate phase were observed to have an average longest dimension of about 0.2 microns to about 4.0 microns. The voids were observed to have an average longest dimension of about 0.2 microns to about 3.5 microns. Substantially amorphous carbon could be observed in the voids as a coating layer at the interface of the polyimide matrix and the void, and as carbon particles within the void. 
         [0149]    For the films of Example 3, an increase by at least 25% in the number of cellulose acetate domains having an average longest dimension of less than 1 micron was noted compared to the number of cellulose acetate domains in the uncured films produced from 5 and 10 wt % cellulose acetate loadings (Examples 1 and 2, respectively). The voids in the cured film obtained from a cellulose acetate/polyimide precursor blend having 20 wt % cellulose acetate appeared to have more carbon deposits than their counterparts derived from 5 and 10 wt % cellulose acetate, consistent with the increase in optical density for the cured film of Example 3 relative to that of Examples 1 and 2 (see Table 1). Gloss of the cured film of Example 3 decreased by about 50% compared to the gloss of the cured films of Examples 1 and 2. 
       Example 4 
     40 wt % Loading of Cellulose Acetate in Cellulosic Polymer/Polyimide Precursor Blend 
       [0150]    In this Example, 60 g of a 19.5 wt % solution of PAA in DMAc was mixed with 31.8 g of a 14 wt % solution of CAc dissolved in DMAc. Then 3.61 g of a 6 wt % fresh solution of PMDA in DMAc was added to the mixture of PAA and CAc. All methods and procedures of Example 1 were followed. The cured film was black in color. Table 1 presents the viscosity of the cellulosic polymer/polyimide precursor blend before film casting, as well as the optical density, gloss, dielectric constant, dissipation factor, and initial tear resistance of the cured film. 
         [0151]    The cross sections of both dried, uncured ( FIG. 4A ) and cured films ( FIG. 4B ) were analyzed by SEM, which revealed the existence of cellulose acetate domains within the polyimide precursor phase of the uncured film, and voids containing carbon in the polyimide phase of the cured film. The domains of the cellulose acetate phase were observed to have an average longest dimension of about 0.5 microns to about 6.0 microns. The voids were observed to have an average longest dimension of about 1.0 microns to about 6.0 microns. Substantially amorphous carbon could be observed in the voids as a coating layer at the interface of the polyimide phase and the void and as carbon particles within the void. 
         [0152]    The SEM analysis also revealed that the average longest dimension of at least 60% of the cellulose acetate domains was equal to or greater than 4 microns, a two-fold increase over the average longest dimension of the cellulose acetate domains obtained for the uncured films of Examples 1 and 2. The voids appeared to have more carbon deposits than their counterparts of Example 3, consistent with an increase in optical density for the cured film of Example 4 (see Table 1). Gloss of the cured film of Example 4 decreased by about 30% relative to that of the cured film of Example 3, due to an increase in surface roughness and consistent with the increase in the diameter of the cellulose acetate domains of the corresponding uncured film. 
         [0153]    The average tear strength of cured films obtained from cellulose acetate/polyimide precursor blends having 5, 10, 20, and 40 wt % CAc was found to vary between 18,900 and 22,200 gf/mm compared to the average tear strength of 26,000 gf/mm for a polyimide film prepared similarly but without the addition of cellulose acetate. The decrease in tear strength was estimated to be between 15 and 30% of the average tear strength of the control and was correlated to the MW of the cellulose acetate/polyimide precursor blend by viscosity (see Table 1). The viscosity of the blends containing 10 and 40 wt % cellulose acetate was about 2-fold lower than with 20 or 5 wt % cellulose acetate. 
         [0154]    The data in Table 1 indicates that both the dielectric constant (Dk) and the dissipation factor (Df) of the cured film of Example 4 were affected by the amount of carbon deposits derived from the high loading of cellulose acetate in the cellulosic polymer/polyimide precursor blend. The result was a 40% increase for Dk and a 200% increase for Df compared to the respective values for the film prepared with 5wt % cellulose acetate (Example 1). 
       Example 5 
     10 wt % Loading of Cellulose Acetate Butyrate (MW 12,000 Dalton) in Cellulosic Polymer/Polyimide Precursor Blend 
       [0155]    In this Example, 75 g of a 19.5 wt % solution of PAA in DMAc was mixed with 7.31 g of a 19 wt % solution of cellulose acetate butyrate (CAcB) dissolved in DMAc. The CAcB had an average molecular weight of 12,000 dalton with 16-19 wt % acetyl and 30-35 wt % butyryl content. Then 2.63 g of a 6 wt % fresh solution of PMDA in DMAc was added to the mixture of PAA and CAcB. All methods and procedures of Example 1 were followed. The cured film was brown in color. Table 1 presents the viscosity of the cellulosic polymer/polyimide precursor blend before film casting, as well as the optical density, gloss, and initial tear resistance of the cured film. 
         [0156]    The cross sections of both dried, uncured ( FIG. 5A ) and cured films ( FIG. 5B ) were analyzed by SEM, which revealed the existence of cellulose acetate butyrate domains within the polyimide precursor phase of the uncured film, and voids containing carbon in the polyimide phase of the cured film. The domains of the cellulose acetate butyrate phase were observed to have an average longest dimension of about 0.2 microns to about 0.8 microns in diameter. The voids were observed to have an average longest dimension of about 0.1 microns to about 1.0 microns. Analysis of a cross section of the cured film by TEM revealed a thin carbon layer coating the inner surface of the voids. 
         [0157]    The SEM analysis also revealed that the average longest dimension of at least 80% of the cellulose acetate butyrate domains in the uncured film, and the voids in the cured film, was less than 1 micron. The number of cellulose acetate butyrate domains per unit area, regardless of their average longest dimension, increased at least by four-fold over the number of domains formed by cellulose acetate of average molecular weight 50,000 dalton and at the same wt % loading in the cellulosic polymer/polyimide precursor blend. The optical density of the cured film of Example 5 is greater than that of the cured film prepared with 10 wt % loading of cellulose acetate having an average molecular weight 50,000 dalton (Example 2) (see Table 1). This result indicates that the number of voids with an average longest dimension of less than 1 micron per unit area controls the optical density of the cured film. 
       Example 6 
     10 wt % Loading of Cellulose Acetate Butyrate (MW 70,000 Dalton) in Cellulosic Polymer/Polyimide Precursor Blend 
       [0158]    In this Example, 80 g of a 19.5 wt % solution of PAA in DMAc was mixed with 12.1 g of a 12.3 wt % solution of CAcB dissolved in DMAc. The CAcB had an average molecular weight of 70,000 dalton with 12-15 wt % acetyl, 35-39 wt % butyryl, and 1.2-2.2 wt % hydroxyl content. Then 2.79 g of a 6 wt % fresh solution of PMDA in DMAc was added to the mixture of PAA and CacB. All methods and procedures of Example 1 were followed. The cured film was dark brown in color. Table 1 presents the viscosity of the cellulosic polymer/polyimide precursor blend before film casting, as well as the optical density, gloss, and initial tear resistance of the cured film. 
         [0159]    The cross sections of both dried, uncured ( FIG. 6A ) and cured films ( FIG. 6B ) were analyzed by SEM, which revealed the existence of cellulose acetate butyrate domains within the polyimide precursor phase of the uncured film, and voids containing carbon in the polyimide phase of the cured film. The domains of the cellulose acetate butyrate phase were observed to have an average longest dimension of about 2 microns to about 20 microns. The voids were observed to have an average longest dimension of about 1.0 microns to about 8.0 microns. Analysis of a cross section of the cured film by TEM ( FIG. 6C ) revealed a thin carbon layer coating the inner surface of the voids. 
         [0160]    The SEM analysis also revealed that the average longest dimension of at least 50% of the cellulose acetate butyrate domains was greater than 1.0 micron. The number of cellulose acetate butyrate domains per unit area, regardless of their average longest dimension, decreased by at least two-fold in comparison to the cellulose acetate butyrate domains in the uncured film prepared in Example 5. The optical density of the cured film is also lower than that of the cured film prepared in Example 5, which is consistent with the difference in the number of domains per unit area noted for the two films. Moreover, the gloss of the film of Example 6 decreased by at least 50% compared to that of the film prepared in Example 5. This result is consistent with the results for Examples 1-4 and indicates that voids with an average longest dimension of greater than 1 micron control the gloss level of the cured film. 
       COMPARATIVE EXAMPLE 
     Polyimide Produced Without Addition of Cellulosic Polymer 
       [0161]    In this Comparative Example, 98 g of a 19.5 wt % solution of PAA in DMAc was mixed with 3.94 g of a 6 wt % fresh solution of PMDA in DMAc for 30 min during which time the viscosity increased to 1500 Poise. The methods and procedures of Example 1 were followed except that the film was cured by ramping the oven at 4° C./min from room temperature to 350° C. and at 5° C./min from 350° C. to 400° C. Table 1 presents the viscosity of the polyimide precursor before film casting, as well as the optical density, gloss, dielectric constant, dissipation factor, and initial tear resistance of the cured film. 
         [0162]    A cross section of the cured film was analyzed by SEM, which revealed a continuous polyimide phase free of voids. Typically, polyimide film of this composition and thickness (1 mil, 25 microns), made as described, would be expected to have Dk and Df values of about 3.4 and 0.0018, respectively. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Loading of Cellulosic Polymer in Blend and Viscosity of Blend, as Well as Optical 
               
               
                 Density (OD), Gloss, Dielectric Constant Dk, Dissipation Constant Df and Initial Tear 
               
               
                 Strength of Cured Films Prepared in Examples 1-6 and the Comparative Example. 
               
             
          
           
               
                   
                 Loading of Cellulosic 
                   
                   
               
               
                   
                 Polymer 
                 Viscosity 
                 Cured Film Properties 
               
             
          
           
               
                   
                 Wt % 
                   
                 of Blend* 
                 OD 
                 Gloss 
                 Dk 
                 Df 
                 Tear 
               
               
                 Example 
                 (%) 
                 MW 
                 (Poise) 
                 (25 microns) 
                 (60 deg) 
                 (1 kHz) 
                 (1 kHz) 
                 (gf/mm) 
               
               
                   
               
             
          
           
               
                 1 
                 5 
                 50,000 
                 1110 
                 1.1 
                 85 
                 3.9 
                 0.009 
                 19,700 
               
               
                 2 
                 10 
                 50,000 
                 474 
                 1.2 
                 88 
                 4.2 
                 0.019 
                 19,300 
               
               
                 3 
                 20 
                 50,000 
                 1330 
                 1.3 
                 42 
                 3.6 
                 0.027 
                 22,200 
               
               
                 4 
                 40 
                 50,000 
                 455 
                 3.0 
                 14 
                 5.3 
                 0.030 
                 18,900 
               
               
                 5 
                 10 
                 12,000 
                 1090 
                 2.0 
                 98 
                 NA 
                 NA 
                 17,900 
               
               
                 6 
                 10 
                 70,000 
                 957 
                 1.3 
                 41 
                 NA 
                 NA 
                 19,700 
               
               
                 Comp. Ex. 
                 0 
                 0 
                 1500 
                 0.3 
                 100 
                 NA 
                 NA 
                 24,890 
               
               
                   
               
               
                 *immediately before film casting 
               
               
                 “NA” means “not available” 
               
             
          
         
       
     
         [0163]    The data in Table 1 show that the filled polyimide films of Examples 1 through 6, which comprise a continuous polyimide phase containing dispersed closed voids having an average longest dimension of about 0.1 microns to about 10 microns, have lower gloss and higher optical density than the cured film of the Comparative Example, which does not contain dispersed closed voids.