# Patent

## Title
Inks including segment copolymer grafted pigments via azide chemistry

## Abstract

Pigment based inks are provided. The inks include a non-polar carrier fluid; and a surface-functionalized pigment particle including a nitrogen-inked moiety to the surface of the pigment particle through a nitrogen link at one end of the nitrogen-linked moiety and a segment copolymer having at least two blocks attached at another end, the pigment particle suspended in the non-polar carrier fluid. A combination of an electronic display and an electronic ink employing the pigment and a process for making the pigment-based inks are also provided.

## Background
BACKGROUND
Ultrathin, flexible electronic displays that look like print on paper have many potential applications including wearable computer screens, electronic paper, smart identity cards, store shelf labels, and signage applications. Electrophoretic or electrokinetic displays are an important approach to this type of medium. Electrophoretic actuation relies on particles moving under the influence of an electric field. Accordingly, the desired particles must exhibit good dispersibility and charge properties in non-polar dispersing media. Non-polar dispersing media are desirable because they help minimize the leakage currents in electrophoretic or kinetic devices.
Current commercial products based on electrophoretic display technology are only able to provide color and white states or black and white states. They cannot provide a clear or transparent state, which prevents use of a stacked architecture design. A stacked architecture of layered colorants would allow the use of transparent to colored state transitions in each layer of primary subtractive color resulting in print-like color in one display.



BRIEF DESCRIPTION OF THE DRAWINGS
Features and advantages of examples of the present disclosure will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear.
FIG. 1 depicts a cross-sectional view of one example of a stacked electro-optical display.
FIG. 2 illustrates a cross-sectional view of one example of a lateral electro-optical display.
FIG. 3 is a schematic diagram of an example reaction scheme for forming a tetrafluorophenyl azide useful in the practice of the processes disclosed herein.
FIG. 4 is a block diagram depicting an example process employed in the practice of the present invention.



DETAILED DESCRIPTION
Reference is made now in detail to specific examples, which illustrates the best mode presently contemplated by the inventors for practicing the invention. Alternative examples are also briefly described as applicable.
Reference is made now in detail to specific examples, which illustrates the best mode presently contemplated by the inventors for practicing the invention. Alternative examples are also briefly described as applicable.
In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific examples in which the disclosure may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of examples can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other examples may be utilized and structural or logical changes may be made without departing from the scope of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims.
As used herein, the term “grayscale” applies to both black and white images and monochromatic color images. Grayscale refers to an image including different shades of a single color produced by controlling the density of the single color within a given area of a display.
As used herein, the term “over” is not limited to any particular orientation and can include above, below, next to, adjacent to, and/or on. In addition, the term “over” can encompass intervening components between a first component and a second component where the first component is “over” the second component.
As used herein, the term “adjacent” is not limited to any particular orientation and can include above, below, next to, and/or on. In addition, the term “adjacent” can encompass intervening components between a first component and a second component where the first component is “adjacent” to the second component.
As used herein, the term “electronic ink display” is a display that forms visible images using one or more of electrophoresis, electro-convection, electro-osmosis, electrochemical interactions, and/or other electrokinetic phenomena.
As used herein, “about” means a ±10% variance caused by, for example, variations in manufacturing processes.
The article ‘a’ and ‘an’ as used in the claims herein means one or more.
Significant progress has been made towards developing working electronic inks based on the electrokinetic mechanism using conventional stabilization techniques and materials. However, improvements in reliability are still needed for commercially successful applications. These previous electronic inks are based on pigments with additional surfactants and charge directors, in which both charging and stabilization related functionality are not covalently bonded to the pigment surface. In this case, the pigment can lose charge with time under electric field or repeated switching cycles. The adsorbed stabilizing polymer material on the pigment surface is capable of desorbing and the free polymeric species in the solvent are capable of degradation as a result of cell operation. Additional surfactants in the solvent also result in higher background charges which can lead to field screening effects.
Surface modification of TiO2 pigment has been demonstrated, using a “random graft polymerization” method to introduce a polymer onto the TiO2 pigment surface through polymerizable or polymerizationinitiating groups attached to the surface of the particles. The major drawback of this approach is that polymers are formed in the presence of the particles. It is more difficult to obtain well controlled systems where the unintended products can be cleaned up more easily.
Bi-state and/or tri-state electrophoretic display cells (or elements) having a three-dimensional architecture for compacting charged colorant particles within the display cells are described in US Patent Publication 2010/0245981, published Sep. 30, 2010. A bi-state display cell having a dark state and a clear state is provided by an electronic ink with charged colorant particles in an optically transparent fluid. A clear state is achieved when the colorant particles are compacted and a colored state is achieved when the colorant particles are spread. An electronic ink with charged white particles in a colored fluid enables white and spot-color states, with the color of the colored state depending on the color of the fluid. The ink fluid is colored by a dye, nanoparticle colorants, pigments, or other suitable colorants. A white state is achieved when the white particles are spread and held in proximity to the surfaces closest to the viewer, and a colored state is achieved when the white particles are compacted to allow absorption by the colorant fluid and subsequent reflection by a diffuse reflector in the back of the cell, or when the white particles are distributed throughout the colorant fluid to backscatter the light that has not been absorbed by the colorant fluid. By combining the white particles in the colored fluid with a different colored resin on the back of the display cell, a tri-state display cell is provided.
An electrophoretic display cell may include a three-dimensional architecture to provide a clear optical state. In this architecture, the geometrical shape of the display cell has narrowing portions in which electrophoretically/-electrokinetically translated colorant particles collect and compact in response to appropriate bias conditions applied to driving electrodes on opposite sides of the display cell. The three-dimensional structure of the display cell introduces additional control of electrophoretically/electrokinetically moving colorant particles. As a result, desired functionalities can be achieved with a more stable electrophoretic/kinetic ink that resists irreversible agglomeration of the particles, but maintains its ability to both disperse and collect and compact the particles. The driving electrodes are passivated with a dielectric layer, thus eliminating the possibility of electrochemical interactions through the driving electrodes from direct contact with the electrophoretic ink. In other examples, the driving electrodes are not passivated, thus allowing electrochemical interactions with the electrophoretic/kinetic ink.
An example of a stacked device architecture is shown in FIG. 1. This configuration allows stacking of colored layers for electrophoretic/kinetic displays.
FIG. 1 illustrates a cross-sectional view of one example of stacked electro-optical display 100. Electro-optical display 100 includes a first display element 102a, a second display element 102b, and a third display element 102c. Third display element 102c is stacked on second display element 102b, and second display element 102b is stacked on first display element 102a. 
Each display unit includes a first substrate 104, a first electrode 106, a dielectric layer 108 including reservoir or recess regions 110, thin layers 112, a display cell 114, a second electrode 116, and a second substrate 118. Display cell 114 is filled with a carrier fluid 120 with colorant particles 122. In some examples, thin layers 112 may be opaque. In other examples, thin layers 112 may be transparent.
First display element 102a includes thin layers 112a self-aligned within recess regions 110. First display element 102a also includes colorant particles 122a having a first color (e.g., cyan) for a full color electro-optical display.
Second display element 102b includes thin layers 112b self-aligned within recess regions 110. Second display element 102b also includes colorant particles 122b having a second color (e.g., magenta) for a full color electro-optical display.
Third display element 102c includes thin layers 112c self-aligned within recess regions 110. Third display element 102c also includes colorant particles 122c having a third color (e.g., yellow) for a full color electro-optical display. In other examples, colorant particles 122a, 122b, and 122c may include other suitable colors for providing an additive or subtractive full color electro-optical display.
In the example illustrated in FIG. 1, in the electro-optical display 100, first display element 102a, second display element 102b, and third display element 102c are aligned with each other. As such, thin layers 112a, 112b, and 112c are also aligned with each other. In this example, since recess regions 110 and self-aligned thin layers 112a, 112b, and 112c of each display element 102a, 102b, and 102c, respectively, are aligned, the clear aperture for stacked electro-optical display 100 is improved compared to a stacked electro-optical display without such alignment.
In an alternate example (not shown), first display element 102a, second display element 102b, and third display element 102c may be offset from each other. As such, thin layers 112a, 112b, and 112c are also offset from each other. In this example, since recess regions 110 and self-aligned thin layers 112a, 112b, and 112c are just a fraction of the total area of each display element 102a, 102b, and 102c, respectively, the clear aperture for stacked electro-optical display 100 remains high regardless of the alignment between display elements 102a, 102b, and 102c. As such, the process for fabricating stacked electro-optical display 100 is simplified. The self-aligned thin layers 112a, 112b, and 112c prevent tinting of each display element due to colorant particles 122a, 122b, and 122c, respectively, in the clear optical state. Therefore, a stacked full color electro-optical display having a bright, neutral white state and precise color control is provided.
As indicated above, this architecture enables both clear and colored states. However, developing electronic inks that work in this architecture has been challenging. The materials used in presently-available commercial products do not work in this architecture, since they do not provide clear states. Significant progress toward developing working electronic inks for this architecture has been made; see, e.g., PCT/US2009/060971 (“Electronic Inks”); PCT/US2009/060989 (“Dual Color Electronically Addressable Ink”); and PCT/US2009/060975 (“Electronic Inks”), all filed Oct. 16, 2009.
The foregoing discussion is directed primarily to stacked cells in an electro-optical display. However, the functionalized pigments disclosed herein may also be employed in lateral cells in an electro-optical display.
FIG. 2 illustrates a cross-sectional view of one example of lateral electro-optical display 200. Electro-optical display 200 includes a display element 202. Additional display elements may be disposed laterally in the x and y directions, as side-by-side sub-pixels or segments, with each display element containing inks 120+122 having colorant particles 122 of different colors, or having black colorant particles that are collected to reveal patterned color filters or wavelength-selective reflectors below.
Each display element 202 is similar to electro-optical display 100a previously described and illustrated with reference to FIG. 1. Each display element 202 may include circular shaped thin layers 110a self-aligned within recess regions 108. Each display element 202 may also include colorant particles 122 having a color (e.g., cyan, magenta, yellow, black, or white) for a full color electro-optical display. In other examples, colorant particles 122 may include other suitable colors for providing an additive or subtractive full color electro-optical display.
In accordance with the teachings herein, new segment copolymer grafted pigment colorant particles via azide chemistry are provided for stable electronic inks 120+122. This disclosure also discloses methods of grafting particles with segment copolymers via living polymerization techniques such as atom transfer radical polymerization (ATRP). The particles grafted with these novel functionalized segment copolymers can be self-dispersed into non-polar solvents and supply both steric stabilization and particle charging functionality while minimizing the need for additional surfactants. These functionalized segment copolymers are designed to have two portions that are grafted to the particle or subsequent polymer ends in a step-wise fashion. The first portion contains bulky organic groups to help facilitate the solubility of such functionalized polymers in the solvent and provide a steric stabilization to the resulting particle dispersion. The second portion contains either acidic or basic functionalized side groups that facilitate charging of the particle. Such stable and chargable/charged particle dispersions can be used for a variety of applications such as pigments as colorants in electrophoretic displays. This surface modification technology can be applied to both organic and inorganic pigments.
ATRP (Atom Transfer Radical Polymerization) is a surface-initiated living polymerization method, also referred to as living polymerization method, by which polymers can be formed. In this method, polymerization can only occur on an initiator group, and is subsequently transferred to the end of the just added polymer chain. There are publications in the scientific literature about the application of this method to make new polymers, but none of them deals with grafting functional segment copolymers onto a pigment surface.
In polymer chemistry, living polymerization is a form of addition polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is also much larger than the rate of chain propagation. The result is that the polymer chains grow at a more constant rate than seen in traditional chain polymerization and their lengths remain very similar (i.e. they have a very low polydispersity index). Living polymerization is a popular method for synthesizing block copolymers since the polymer can be synthesized in stages, each stage containing a different monomer. Additional advantages are predetermined molar mass and control over end-groups.
In ATRP, the uniform polymer chain growth, which leads to low polydispersity, stems from use of a transition metal-based catalyst. This catalyst provides an equilibrium between active, and therefore propagating, polymer and an inactive form of the polymer; known as the dormant form. Since the dormant state of the polymer is vastly preferred in this equilibrium, side reactions are suppressed. This equilibrium in turn lowers the concentration of propagating radicals, therefore suppressing unintentional termination and controlling molecular weights. ATRP reactions are very robust in that they are tolerant of many functional groups such as allyl, amino, epoxy, hydroxy, and vinyl groups present in either the monomer or the initiator. ATRP methods may also be advantageous due to the ease of preparation, commercially available and inexpensive catalysts (copper complexes), pyridine based ligands and initiators (alkyl halides).
In accordance with the teachings herein, a step-wise method of grafting novel functionalized segment copolymers onto a particle or pigment surface via azide chemistry is provided, along with the formulation of stable electronic inks based on such surface-modified pigments.
The process depicted herein employs a tetrafluorophenyl azide-based initiator. As shown in FIG. 3, the tetrafluorophenyl azide initiator may be prepared by reacting commercially-available methyl pentafluorobenzoate (1) with sodium azide to yield compound (2), in which the azide functionality substitutes in the para position. Hydrolysis of compound (2) with sodium hydroxide yields tetrafluorophenyl azide acid (3). Reaction of tetrafluorophenyl azide acid (3) with thionyl chloride gives tetrafluorophenyl azide acid chloride (4), which in turn reacts with 2-bromo-N-(3-hydroxypropyl)-2-methylpropanamide (5) to give tetrafluorophenyl azide-based initiator (6). The azide-based initiator may be used to form the tri-block (or di-block) copolymers, as described in greater detail below.
Examples of structures of segment copolymers with different segments are shown in Scheme 1 as segment copolymers 1, 2 and 3, each with slightly different segment chains.




wherein,
L1, L2, and L3 are a covalent bond or chemical structure providing a covalent bond between different segments, such as C—C, C═C, C═N, C≡C, or N≡N, for example.
SG1 and SG2 each independently represent a solublizing oligomer or solubilizing sterically bulky group, which helps to improve the solubility of the polymer and stabilize the colorant particles; they may be any oligomers of olefin, (meth)acrylates, styrenes, oxides, ethers, or esters or sterically bulky group such as alkyl groups, alkoxy groups, branched alkyl groups, branched alkoxy groups, aliphatic esters, branched aliphatic esters, and substituted phenyl groups.
FG represents an oligomer or functional group that provides charging sites/charges to pigment surfaces; it can be an oligomer or monomeric moiety that contains acidic or basic functional groups, wherein examples of acidic functional groups include hydroxyl, carboxylate, a sulfonic acid, a phosphonic acid, a phosphorous acid, and the like, and wherein examples of basic functional groups include primary amine, secondary amine, tertiary amine, pyridine, imidazoline, and the like. Specific examples include oligomers or monomers of (meth)acrylic acid, 2-sulfoethyl methacylate, dimethylamino ethyl(meth)acrylate, and diethylamino ethyl styrene.
The letters x, y and z are each independently an integer between 1 and about 5,000.
The letter n represents an integer between 2 and about 100.
Scheme 2 shows general examples of segment copolymers 4, 5, and 6 that can be grafted onto pigment surfaces, in which different segments are connected with carbon-carbon single bond (L=C—C).




wherein,
SG1 and SG2, FG, x, y, and z, and n are as described for Scheme 1.
Scheme 3 shows a few examples of vinyl-containing small molecule monomers that can be grafted onto pigment surfaces to provide stabilizing functions.




wherein,
R1, R2, R3, R4 and R5 are each independently selected from the group consisting of C1-C30 alkyl, C1-C30 alkenyl, C1-C30 alkynyl, C1-C30 aryl, C1-C30 alkoxy, C1-C30 phenoxy, C1-C30 thioalkyl, C1-C30 thioaryl, C(O)OR6, N(R7)(R8), C(O)N(R9)(R10), F, Cl, Br, NO2, CN, acyl, carboxylate and hydroxy, wherein R6, R7, R8, R9 and R10 are each independently selected from the group consisting of hydrogen, C1-C30 alkyl and C1-C30 aryl, and so forth. R1, R2, R3, R4, R5, R6, R7, R8 and R9 may or may not be identical.
Scheme 3A shows a few more specific examples that can be used for making SG1 or SG2, in which these monomers are oligomeric monomers.




wherein,
m=1 to about 5,000.
The examples shown in Scheme 3A are surfactant molecules, and may be referred to as macromolecule monomers. As used herein, macromolecule monomers refer to oligomers or polymers that have polymerizable groups, such as styrene, acrylate, or methacrylate moieties.
Scheme 4 shows some examples of monomers that can be grafted onto pigment surfaces to provide charges to the pigment surfaces.




wherein,
R1, R2, R3, R4 and R5 are each independently selected from the group consisting of C1-C30 alkyl, C1-C30 alkenyl, C1-C30 alkynyl, C1-C30 aryl, C1-C30 alkoxy, C1-C30 phenoxy, C1-C30 thioalkyl, C1-C30 thioaryl, C(O)OR6, N(R7)(R8), C(O)N(R9)(R10), F, Cl, Br, NO2, CN, acyl, carboxylate and hydroxy, wherein R6, R7, R8, R9 and R10 are each independently selected from the group consisting of hydrogen, C1-C30 alkyl and C1-C30 aryl, and so forth. R1, R2, R3, R4, R5, R6, R7, R8 and R9 may or may not be identical.
Scheme 5 describes a method of grafting such functionalized segment copolymer onto pigment surfaces via azide chemistry. Tetrafluorophenyl azide initiator I initiates the polymerization of the first segment monomer to give the first segment tetrafluorophenyl azide living polymer II. Addition of second monomer to the living polymer II yield two-segment tetrafluorophenyl living polymer III. Addition of third monomer to living polymer III give three-segment tetrafluorophenyl azide polymer IV. By repeating steps 1, 2 and 3 n times, the final segment copolymer V can be formed. It is noteworthy that one can change the order of reaction steps for living polymerization when repeating steps 1, 2 and 3 to form different segment copolymers V. Coupling reaction of inorganic or organic pigments with tetrafluorophenyl azide polymer V upon UV or thermal irradiation gives functionalized segment copolymers grafted pigment VI. Such segment copolymers grafted pigments can be mixed with other dispersants or charge directors to form charged and stable pigment dispersions for electronic ink applications.




wherein,
SG1 and SG2 each independently represent a solublizing oligomer or solubilizing sterically bulky group, which helps to improve the solubility of the polymer and stabilize the colorant particles; they may be any oligomers of olefin, (meth)acrylates, styrenes, oxides, ethers, or esters or sterically bulky group such as alkyl groups, alkoxy groups, branched alkyl groups, branched alkoxy groups, aliphatic esters, branched aliphatic esters, and substituted phenyl groups.
FG represents an oligomer or functional group that provides charging sites/charges to pigment surfaces; it can be an oligomer or monomeric moiety that contains acidic or basic functional groups, wherein examples of acidic functional groups include hydroxyl, carboxylate, a sulfonic acid, a phosphonic acid, a phosphorous acid, and the like, and wherein examples of basic functional groups include primary amine, secondary amine, tertiary amine, pyridine, imidazoline, and the like. Specific examples include oligomers or monomers of (meth)acrylic acid, 2-sulfoethyl methacylate, dimethylamino ethyl(meth)acrylate, and diethylamino ethyl styrene.
The letters x, y and z are an integer between 1 and about 5,000.
The letter n represents an integer between 2 and about 100.
The sphere ball represents any possible electrophoretic particles with all possible colors such as RGB or CYMK. It may be a colored pigment, colored polymeric particles with the sizes ranging from 50 nm to 1 μm. It may be organic or inorganic.
Scheme 6 describes a specific example of such segment copolymers grafted pigments that bear negative charges via azide chemistry. This example describes polyacrylic acid and polystyrene based segment copolymers. Tetrafluorophenyl azide initiator I undergoes the first step polymerization with the first segment monomer, for example, substituted styrene, to give the first segment, namely, a polystyrene tetrafluorophenyl azide living polymer II. Living polymer II undergoes a second step polymerization with the second segment monomer, for example, acrylic acid, to give the two-segment living polymer III, namely, a polystyrene and polyacrylic acid tetrafluorophenyl living polymer III. Living polymer III undergoes a third step polymerization with the third segment monomer, for example, substituted styrene, to give the three-segment living polymer IV, namely, a polystyrene, polyacrylic acid and polystyrene tetrafluorophenyl azide polymer IV. By repeating steps 1, 2 and 3 n times, the final segment copolymer V can be formed. It is noteworthy that one can change the order of reaction steps for living polymerization when repeating steps 1, 2 and 3 to form different segment copolymers V. Coupling reaction of inorganic or organic pigments with the three-segment (polystyrene, polyacrylic acid and polystyrene) tetrafluorophenyl azide polymer V upon UV irradiation gives functionalized segment copolymer grafted pigment VI. Such segment copolymers grafted pigments can be mixed with other dispersants or charge directors to form negatively charged and stable pigment dispersions for electronic ink applications.







wherein,
R represents sterically bulky group, which helps to improve the solubility of the polymer and stabilize the nano-composite material. It could be any alkyl groups, alkoxy groups, branched alkyl groups and branched alkoxy groups.
The letters x, y and z are an integer between 1 and about 5,000.
The letter n represents an integer between 2 and about 100.
The sphere ball represents any possible electrophoretic particles with all possible colors such as RGB or CYMK. It can be colored pigments, colored polymeric particles with the sizes ranging from 50 nm to 1 μm.
Scheme 7 describes a specific example of such tri-segment copolymers grafted pigments that should bear positive charges. It describes polyacrylate ammonium salts and polystyrene based tri-segment copolymers. Tetrafluorophenyl azide initiator I undergoes the first step polymerization with the first segment monomer, for example, substituted styrenes, to give the first block, namely, a polystyrene tetrafluorophenyl azide living polymer II. Living polymer II undergoes a second step polymerization with the second segment monomer, for example, acrylate ammonium salts, to give a two-segment living polymer III, namely, a polystyrene and polyacrylate ammonium salts tetrafluorophenyl living polymer III. Living polymer III undergoes a third step polymerization with the third segment monomer, for example, substituted styrene, to give the three-segment living polymer IV, namely, a polystyrene, polyacrylate ammonium salts and polystyrene tetrafluorophenyl azide polymer IV. By repeating steps 1, 2 and 3 n times, the final segment copolymer V can be formed. It is noteworthy that one can change the order of reaction steps for living polymerization when repeating steps 1, 2 and 3 to form different segment copolymers V. Coupling reaction of inorganic or organic pigments with three-segment polystyrene, polyacrylic acid ammonium salts and polystyrene tetrafluorophenyl azide polymer V upon UV irradiation gives functionalized segment copolymers grafted pigment VI. Such segment copolymers grafted pigments can be mixed with other dispersants or charge directors to form charged and stable pigment dispersions for electronic ink applications.







wherein,
R1, R2, R3, R4 and R5 are each independently selected from the group consisting of C1-C30 alkyl, C1-C30 alkenyl, C1-C30 alkynyl, C1-C30 aryl, C1-C30 alkoxy, C1-C30 phenoxy, C1-C30 thioalkyl, C1-C30 thioaryl, C(O)OR6, N(R7)(R8), C(O)N(R9)(R10), F, Cl, Br, NO2, CN, acyl, carboxylate and hydroxy, wherein R6, R7, R8, R9 and R10 are each independently selected from the group consisting of hydrogen, C1-C30 alkyl and C1-C30 aryl, and so forth. The residues R1, R2, R3, R4, R5, R6, R7, R8 and R9 may or may not be identical.
The letters x, y and z are an integer between 1 and about 5,000.
The letter n represents an integer between 2 and about 100.
The sphere ball represents any possible electrophoretic particles with all possible colors such as RGB or CYMK. It can be a colored pigment or a colored polymeric particle, with a particle size ranging from about 50 nm to 1 μm. It may be organic or inorganic.
An example process 400 for making the nitrogen-linked surface function-alized pigment particle is illustrated in FIG. 4. The process 400 includes providing 402 an azide. The process 400 further includes causing 404 the azide to initiate polymerization of a first block monomer to give a first block azide living polymer. The process 400 additionally includes adding 406 a second monomer to the first block living polymer to give a two-block azide living copolymer and form a first segment. The process 400 also includes forming 408 at least one additional segment by adding, in any order, a first block monomer and a second block monomer to form a two-block living segment copolymer. The process 400 further includes causing 410 an inorganic or organic pigment particle to undergo a coupling reaction with the azide on the two-block living segment copolymer to form a functionalized di-block segment copolymer coupled to the pigment particle through a nitrogen link.
The example process in the previous paragraph forms a di-block segment copolymer. To form a tri-block segment copolymer, the process may be continued by adding a third monomer to the two-block living azide polymer, which yields a three-block azide living segment copolymer. In the forming step above, at least one additional segment is formed by adding, in any order, a first block monomer, a second block monomer, and a third block monomer to form a tri-block living segment copolymer. Finally, the inorganic or organic pigment particle is caused to undergo a coupling reaction with the azide on the three-block living segment copolymer to form a functionalized tri-block segment copolymer coupled onto the pigment particle through a nitrogen link.
Turning now to electronic inks 120+122 that employ the functionalized pigments 122 discussed above, examples of such electronic inks generally include a non-polar carrier fluid 120 (i.e., a fluid having a low dielectric constant k such as, e.g., less than about 20, or, in some cases, less than about 2). Such fluids tend to reduce leakages of electric current when driving the display, as well as increase the electric field present in the fluid. As used herein, the “carrier fluid” is a fluid or medium that fills up a viewing area defined in an electronic ink display and is generally configured as a vehicle to carry colorant particles therein. In response to a sufficient electric potential or field applied to the colorant particles 122 while driving electrodes of the display, the colorant particles tend to move and/or rotate to various spots within the viewing area in order to produce a desired visible effect in the display cell to display an image. The nonpolar carrier fluid 120 includes, for example, one or more non-polar carrier fluids selected from hydrocarbons, halogenated or partially halogenated hydrocarbons, and/or siloxanes. Some specific examples of non-polar carrier fluids include perchloroethylene, cyclohexane, dodecane, mineral oil, isoparaffinic fluids, cyclopentasiloxane, cyclohexasiloxane, octamethylcyclosiloxane, and combinations thereof.
The colorant particles 122 are dispersed in the carrier fluid. As used herein, the term “colorant particles” refers to particles that produce a color. Some non-limiting examples of suitable colorant particles 122 include the surface-modified pigment particles described above, which may be dispersible in the non-polar carrier fluid 120 due to the presence of the di-block or tri-block segment copolymers attached to the pigment surface. However, the total elimination of dispersants, such as those commonly used in dispersing pigment particles in the non-polar carrier fluid, may not be attained. In that case, the electronic ink 120+122 may include one or more suitable dispersants. Such dispersants include hyperdispersants such as those of the SOLSPERSE® series manufactured by Lubrizol Corp., Wickliffe, Ohio (e.g., SOLSPERSE® 3000, SOLSPERSE® 8000, SOLSPERSE® 9000, SOLSPERSE® 11200, SOLSPERSE® 13840, SOLSPERSE® 16000, SOLSPERSE® 17000, SOLSPERSE® 18000, SOLSPERSE® 19000, SOLSPERSE® 21000, and SOLSPERSE® 27000); various dispersants manufactured by BYK-chemie, Gmbh, Germany, (e.g., DISPERBYK® 110, DISPERBYK® 163, DISPERBYK® 170, and DISPERBYK® 180); various dispersants manufactured by Evonik Goldschmidt GMBH LLC, Germany, (e.g., TEGO® 630, TEGO® 650, TEGO® 651, TEGO® 655, TEGO® 685, and TEGO® 1000); and various dispersants manufactured by Sigma-Aldrich, St. Louis, Mo., (e.g., SPAN® 20, SPAN® 60, SPAN® 80, and SPAN® 85).
In some examples, the concentration of pigment 122 in the electronic ink 120+122 ranges from about 0.01 to 20 percent by weight (wt %). In other examples, the concentration of the pigment ranges from about 1 to 10 wt %. In some examples, the concentration of dispersant in the electronic ink ranges from about 0.5 to 20 percent by weight (wt %). In other examples, the concentration of the dispersant ranges from about 1 to 10 wt %. The carrier fluid 120 makes up the balance of the ink.
There is commonly a charge director employed in electronic inks. As used herein, the term “charge director” refers to a material that, when used, facilitates charging of the colorant particles. In an example, the charge director is basic and reacts with the acid-modified colorant particle to negatively charge the particle. In other words, the charging of the particle is accomplished via an acid-base reaction between the charge director and the acid-modified particle surface. It is to be understood that the charge director may also be used in the electronic ink to prevent undesirable aggregation of the colorant in the carrier fluid. In other cases, the charge director is acidic and reacts with the base-modified colorant particle to positively charge the particle. Again, the charging of the particle is accomplished via an acid-base reaction between the charge director and the base-modified particle surface or adsorption of charged micelles.
The charge director may be selected from small molecules or polymers that are capable of forming reverse micelles in the non-polar carrier fluid. Such charge directors are generally colorless and tend to be dispersible or soluble in the carrier fluid.
In a non-limiting example, the charge director is selected from a neutral and non-dissociable monomer or polymer such as, e.g., a polyisobutylene succinimide amine, which has a molecular structure as follows:




where n is selected from a whole number ranging from 15 to 100.
Another example of the charge director includes an ionizable molecule that is capable of disassociating to form charges. Non-limiting examples of such charge directors include sodium di-2-ethylhexylsulfosuccinate and dioctyl sulfosuccinate. The molecular structure of dioctyl sulfosuccinate is as follows:




Yet another example of the charge director includes a zwitterion charge director such as, e.g., lecithin. The molecular structure of lecithin is as shown as follows:




The foregoing discussion has been directed to the functionalization of TiO2 pigment particles (white color). However, the teachings herein are equally applicable to other pigments, whether inorganic or organic, and of whatever color. Such inorganic and organic pigments are described further below, along with examples of different colors.
The pigment particles are selected from organic or inorganic pigments, and have an average particle size ranging from about 1 nm to about 10 μm. In some examples, the average particle size ranges from about 10 nm to about 1 μm. In other examples, the average particle size ranges from about 30 to 500 nm. In still other examples, the average particle size ranges from about 50 nm to 1 μm. Such organic or inorganic pigment particles may be selected from black pigment particles, yellow pigment particles, magenta pigment particles, red pigment particles, violet pigments, cyan pigment particles, blue pigment particles, green pigment particles, orange pigment particles, brown pigment particles, and white pigment particles. In some instances, the organic or inorganic pigment particles may include spot-color pigment particles, which are formed from a combination of a predefined ratio of two or more primary color pigment particles. To the extent that the generic pigments on the foregoing list can be functionalized as taught herein, such pigments may be used in the practice of the teachings herein. Likewise, to the extent that the following examples of specific pigments can be functionalized as taught herein, such pigments may be used in the practice of the teachings herein.
A non-limiting example of a suitable inorganic black pigment includes carbon black. Examples of carbon black pigments include those manufactured by Mitsubishi Chemical Corporation, Japan (such as, e.g., carbon black No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, and No. 2200B); various carbon black pigments of the RAVEN® series manufactured by Columbian Chemicals Company, Marietta, Ga., (such as, e.g., RAVEN® 5750, RAVEN® 5250, RAVEN® 5000, RAVEN® 3500, RAVEN® 1255, and RAVEN® 700); various carbon black pigments of the REGAL® series, the MOGUL® series, or the MONARCH® series manufactured by Cabot Corporation, Boston, Mass., (such as, e.g., REGAL® 400R, REGAL® 330R, REGAL® 660R, MOGUL® L, MONARCH® 700, MONARCH® 800, MONARCH® 880, MONARCH® 900, MONARCH® 1000, MONARCH® 1100, MONARCH® 1300, and MONARCH® 1400); and various black pigments manufactured by Evonik Degussa Corporation, Parsippany, N.J., (such as, e.g., Color Black FW1, Color Black FW2, Color Black FW2V, Color Black FW18, Color Black FW200, Color Black S150, Color Black S160, Color Black S170, PRINTEX® 35, PRINTEX® U, PRINTEX® V, PRINTEX® 140U, Special Black 5, Special Black 4A, and Special Black 4). A non-limiting example of an organic black pigment includes aniline black, such as C.I. Pigment Black 1.
Other examples of inorganic pigments include metal oxides and ceramics, such as the oxides of iron, zinc, cobalt, manganese, nickel. Non-limiting examples of suitable inorganic pigments include those from the Shepherd Color Company (Cincinnati, Ohio) such as Black 10C909A, Black 10P922, Black 1 G, Black 20F944, Black 30C933, Black 30C940, Black 30C965, Black 376A, Black 40P925, Black 411A, Black 430, Black 444, Blue 10F545, Blue 10G511, Blue 10G551, Blue 10K525, Blue 10K579, Blue 211, Blue 212, Blue 214, Blue 300527, Blue 300588, Blue 300591, Blue 385, Blue 40P585, Blue 424, Brown 100873, Brown 10P835, Brown 10P850, Brown 10P857, Brown 157, Brown 200819, Green 10K637, Green 187 B, Green 223, Green 260, Green 30C612, Green 30C654, Green 30C678, Green 40P601, Green 410, Orange 10P320, StarLight FL 37, StarLight FL105, StarLight FL500, Violet 11, Violet 11C, Violet 92, Yellow 10C112, Yellow 10C242, Yellow 10C272, Yellow 10P110, Yellow 10P225, Yellow 10P270, Yellow 196, Yellow 20P296, Yellow 30C119, Yellow 30C236, Yellow 40P140, Yellow 40P280.
In addition to the foregoing inorganic pigments, the same teachings may be employed with organic pigments. The following is a list of organic pigments that may be treated in accordance with the teachings herein.
Non-limiting examples of suitable yellow pigments include C.I. Pigment Yellow 1, C.I. Pigment Yellow 2, C.I. Pigment Yellow 3, C.I. Pigment Yellow 4, C.I. Pigment Yellow 5, C.I. Pigment Yellow 6, C.I. Pigment Yellow 7, C.I. Pigment Yellow 10, C.I. Pigment Yellow 11, C.I. Pigment Yellow 12, C.I. Pigment Yellow 13, C.I. Pigment Yellow 14, C.I. Pigment Yellow 16, C.I. Pigment Yellow 17, C.I. Pigment Yellow 24, C.I. Pigment Yellow 34, C.I. Pigment Yellow 35, C.I. Pigment Yellow 37, C.I. Pigment Yellow 53, C.I. Pigment Yellow 55, C.I. Pigment Yellow 65, C.I. Pigment Yellow 73, C.I. Pigment Yellow 74, C.I. Pigment Yellow 75, C.I. Pigment Yellow 81, C.I. Pigment Yellow 83, C.I. Pigment Yellow 93, C.I. Pigment Yellow 94, C.I. Pigment Yellow 95, C.I. Pigment Yellow 97, C.I. Pigment Yellow 98, C.I. Pigment Yellow 99, C.I. Pigment Yellow 108, C.I. Pigment Yellow 109, C.I. Pigment Yellow 110, C.I. Pigment Yellow 113, C.I. Pigment Yellow 114, C.I. Pigment Yellow 117, C.I. Pigment Yellow 120, C.I. Pigment Yellow 124, C.I. Pigment Yellow 128, C.I. Pigment Yellow 129, C.I. Pigment Yellow 133, C.I. Pigment Yellow 138, C.I. Pigment Yellow 139, C.I. Pigment Yellow 147, C.I. Pigment Yellow 151, C.I. Pigment Yellow 153, C.I. Pigment Yellow 154, Pigment Yellow 155, C.I. Pigment Yellow 167, C.I. Pigment Yellow 172, and C.I. Pigment Yellow 180.
Non-limiting examples of suitable magenta or red or violet organic pigments include C.I. Pigment Red 1, C.I. Pigment Red 2, C.I. Pigment Red 3, C.I. Pigment Red 4, C.I. Pigment Red 5, C.I. Pigment Red 6, C.I. Pigment Red 7, C.I. Pigment Red 8, C.I. Pigment Red 9, C.I. Pigment Red 10, C.I. Pigment Red 11, C.I. Pigment Red 12, C.I. Pigment Red 14, C.I. Pigment Red 15, C.I. Pigment Red 16, C.I. Pigment Red 17, C.I. Pigment Red 18, C.I. Pigment Red 19, C.I. Pigment Red 21, C.I. Pigment Red 22, C.I. Pigment Red 23, C.I. Pigment Red 30, C.I. Pigment Red 31, C.I. Pigment Red 32, C.I. Pigment Red 37, C.I. Pigment Red 38, C.I. Pigment Red 40, C.I. Pigment Red 41, C.I. Pigment Red 42, C.I. Pigment Red 48(Ca), C.I. Pigment Red 48(Mn), C.I. Pigment Red 57(Ca), C.I. Pigment Red 57:1, C.I. Pigment Red 88, C.I. Pigment Red 112, C.I. Pigment Red 114, C.I. Pigment Red 122, C.I. Pigment Red 123, C.I. Pigment Red 144, C.I. Pigment Red 146, C.I. Pigment Red 149, C.I. Pigment Red 150, C.I. Pigment Red 166, C.I. Pigment Red 168, C.I. Pigment Red 170, C.I. Pigment Red 171, C.I. Pigment Red 175, C.I. Pigment Red 176, C.I. Pigment Red 177, C.I. Pigment Red 178, C.I. Pigment Red 179, C.I. Pigment Red 184, C.I. Pigment Red 185, C.I. Pigment Red 187, C.I. Pigment Red 202, C.I. Pigment Red 209, C.I. Pigment Red 219, C.I. Pigment Red 224, C.I. Pigment Red 245, C.I. Pigment Violet 19, C.I. Pigment Violet 23, C.I. Pigment Violet 32, C.I. Pigment Violet 33, C.I. Pigment Violet 36, C.I. Pigment Violet 38, C.I. Pigment Violet 43, and C.I. Pigment Violet 50.
Non-limiting examples of blue or cyan organic pigments include C.I. Pigment Blue 1, C.I. Pigment Blue 2, C.I. Pigment Blue 3, C.I. Pigment Blue 15, C.I. Pigment Blue 15:3, C.I. Pigment Blue 15:34, C.I. Pigment Blue 15:4, C.I. Pigment Blue 16, C.I. Pigment Blue 18, C.I. Pigment Blue 22, C.I. Pigment Blue 25, C.I. Pigment Blue 60, C.I. Pigment Blue 65, C.I. Pigment Blue 66, C.I. Vat Blue 4, and C.I. Vat Blue 60.
Non-limiting examples of green organic pigments include C.I. Pigment Green 1, C.I. Pigment Green2, C.I. Pigment Green, 4, C.I. Pigment Green 7, C.I. Pigment Green 8, C.I. Pigment Green 10, C.I. Pigment Green 36, and C.I. Pigment Green 45.
Non-limiting examples of brown organic pigments include C.I. Pigment Brown 1, C.I. Pigment Brown 5, C.I. Pigment Brown 22, C.I. Pigment Brown 23, C.I. Pigment Brown 25, and C.I. Pigment Brown, C.I. Pigment Brown 41, and C.I. Pigment Brown 42.
Non-limiting examples of orange organic pigments include C.I. Pigment Orange 1, C.I. Pigment Orange 2, C.I. Pigment Orange 5, C.I. Pigment Orange 7, C.I. Pigment Orange 13, C.I. Pigment Orange 15, C.I. Pigment Orange 16, C.I. Pigment Orange 17, C.I. Pigment Orange 19, C.I. Pigment Orange 24, C.I. Pigment Orange 34, C.I. Pigment Orange 36, C.I. Pigment Orange 38, C.I. Pigment Orange 40, C.I. Pigment Orange 43, and C.I. Pigment Orange 66.
Advantageously, the herein-disclosed methods of grafting particles with novel functionalized segment copolymers and their dispersion in non-polar solvents results in minimal need for additional surfactants or charge directors. The methods use a step-wise process to two or three different chemically-functionalized segment polymers which are consecutively grafted onto the particle/polymer surface. Each block of these functionalized segment copolymers can be designed to optimize its intended function in the system based on the specific particle chemistry, solvent choice, and system requirement. For example, for the tri-block segment copolymer, the inner block can be designed to provide the best compatibility to the particle surface chemistry while exhibiting adequate solubility in the non-polar solvent. The middle block can be designed to achieve the appropriate charge functionality in combination with the particle chemistry and other additives. The outer block can be designed to provide adequate steric stabilization to prevent agglomeration based on system requirements. For example, the outer block could be different for the same particle species in a single species ink compared to a dual species/charge system. Using this new technology, one can also make stable dual color electronic inks based on both positively charged particles and negatively charged particles, since the charges are separated by steric stabilizing groups, which prevents the agglomeration and precipitation of the two particle species due to the attraction of opposite charges.
The electronic inks based on such segment copolymers grafted onto pigment particles are very stable, since the both the charge-producing and steric stabilization groups are covalently bonded to the pigment surface. This approach contributes to the robustness of the particle. Because there is minimal need to add additional surfactants to stabilize the ink, the final electronic ink will have low background charge, and as a result will exhibit less field screening effects which limit the clearing performance of electronic inks. This surface modification technology can be applied to both organic and inorganic pigments.
The foregoing functionalized pigments have been described with specific application to electronic inks. However, the functionalized pigments may find use in other ink technologies that employ non-aqueous inks. An example of such other ink technology is liquid electrophoretic ink (LEP) used in commercial digital printers.

## Claims

1. Pigment based inks including:
a non-polar carrier fluid; and
a surface-functionalized pigment particle including a nitrogen-linked moiety to the surface of the pigment particle through a nitrogen link at one end of the nitrogen-linked moiety and a segment copolymer having at least two blocks attached at another end, wherein the segment copolymer has a formula selected from the group consisting of segment copolymer 1, segment copolymer 2, and segment copolymer 3, as shown below:




wherein,
L1, L2, and L3 are each independently a covalent bond or chemical structure providing a covalent bond between different blocks selected from the group consisting of C—C, C═C, C═N, C≡C, and N≡N;
SG1 and SG2 each independently represent a solubilizing and sterically bulky group, which helps to improve the solubility of the polymer and stabilize the colorant particles, selected from the group consisting of alkyl groups, alkoxy groups, branched alkyl groups, branched alkoxy groups, aliphatic esters, branched aliphatic esters, substituted phenyl groups, and macromolecular monomers;
FG represents a functional group that provides charging sites to pigment surfaces, selected from acidic functional groups and basic groups;
x, y and z are each independently an integer between 1 and about 5,000; and n is an integer between 2 and about 100,
wherein the pigment particle is suspended in the non-polar carrier fluid.

2. The inks of claim 1, wherein the non-polar carrier fluid is a non-polar solvent selected from the group consisting of hydrocarbons, halogenated hydrocarbons, partially halogenated hydrocarbons, and siloxanes.

3. The inks of claim 1, wherein the pigment particle is selected from the group consisting of black pigment particles, yellow pigment particles, magenta pigment particles, red pigment particles, violet pigment particles, cyan pigment particles, blue pigment particles, green pigment particles, orange pigment particles, brown pigment particles, and white pigment particles.

4. The inks of claim 1, wherein FG is selected from the group consisting of primary amines, secondary amines, tertiary amines, amides, nitriles, isonitriles, cyanates, isocyanates, thiocyanates, isothiocyanates, azides, thiols, thiolates, sulfides, sulfinates, sulfonates, phosphates, hydroxyls, alcoholates, phenolates, carbonyls, carboxylates, phosphines, phosphine oxides, phosphonic acids, phosphoramides, and phosphates.

5. The inks of claim 1, wherein at least one of SG1 and SG2 comprise the macromolecule monomer selected from the group consisting of




wherein m is an integer from 1 to 5,000.

6. The inks of claim 1, wherein one or both of SG1 and SG2 comprises




wherein,
R1, R2, R3, R4 and R5 are each independently selected from the group consisting of C1-C30 alkyl, C1-C30 alkenyl, C1-C30 alkynyl, C1-C30 aryl, C1-C30 alkoxy, C1-C30 phenoxy, C1-C30 thioalkyl, C1-C30 thioaryl, C(O)OR6, N(R7)(R8), C(O)N(R9)(R10), F, Cl, Br, NO2, CN, acyl, carboxylate and hydroxy, wherein R6, R7, R8, R9 and R10 are each independently selected from the group consisting of hydrogen, C1-C30 alkyl and C1-C30 aryl, and wherein R1, R2, R3, R4, R5, R6, R7, R8 and R9 may or may not be identical.

7. The inks of claim 1, further including a charge director, wherein the charge director is a small molecule or polymer that is capable of forming reverse micelles in the non-polar carrier fluid.

8. In combination, an electronic display and an electronic ink, wherein the electronic display includes:
a first electrode;
a second electrode; and
a display cell having a recess defined by a dielectric material, the first electrode, and the second electrode, the display cell containing the electronic ink; and wherein the electronic ink includes:
a non-polar carrier fluid; and
a surface-functionalized pigment particle including a nitrogen-linked moiety to the surface of the pigment particle through a nitrogen link at one end of the nitrogen-linked moiety and a segment copolymer having at least two blocks attached at another end, wherein the segment copolymer has a formula selected from the group consisting of segment copolymer 1, segment copolymer 2, and segment copolymer 3, as shown below:






wherein,
L1, L2, and L3 are each independently a covalent bond or chemical structure providing a covalent bond between different blocks selected from the group consisting of C—C, C═C, C═N, C≡C, and N≡N;
SG1 and SG2 each independently represent a solubilizing and sterically bulky group, which helps to improve the solubility of the polymer and stabilize the colorant particles, selected from the group consisting of alkyl groups, alkoxy groups, branched alkyl groups, branched alkoxy groups, aliphatic esters, branched aliphatic esters, substituted phenyl groups, and macromolecular monomers;
FG represents a functional group that provides charging sites to pigment surfaces, selected from acidic functional groups and basic groups;
x, y and z are each independently an integer between 1 and about 5,000; and n is an integer between 2 and about 100,
wherein the pigment particle is suspended in the non-polar carrier fluid.

9. The combination of claim 8, wherein the electronic display includes a plurality of display cells in a stacked configuration, associated first electrodes and second electrodes, and a plurality of electronic inks of different colors, each display cell containing an electronic ink of a different color.

10. The combination of claim 8, wherein the non-polar carrier fluid is a non-polar solvent selected from the group consisting of hydrocarbons, halogenated hydrocarbons, partially halogenated hydrocarbons, and siloxanes.

11. The combination of claim 8, wherein the colored pigment is a colored particle having a size ranging from 1 nm to 10 μm and is selected from the group consisting of black pigment particles, yellow pigment particles, magenta pigment particles, red pigment particles, violet pigment particles, cyan pigment particles, blue pigment particles, green pigment particles, orange pigment particles, brown pigment particles, and white pigment particles.

12. The combination of claim 8, further including a charge director, wherein the charge director is a small molecule or polymer that is capable of forming reverse micelles in the non-polar carrier fluid.

13. A method for making a nitrogen-linked surface functionalized pigment particle including:
providing an azide;
causing the azide to initiate polymerization of a first block monomer to give a first block azide living polymer;
adding a second monomer to the first block living polymer to give a two-block azide living copolymer and form a first segment;
forming at least one additional segment by adding, in any order, a first block monomer and a second block monomer to form a two-block living segment copolymer; wherein the segment copolymer has a formula selected from the group consisting of segment copolymer 1, segment copolymer 2, and segment copolymer 3, as shown below:





wherein,
L1, L2, and L3 are each independently a covalent bond or chemical structure providing a covalent bond between different blocks selected from the group consisting of C—C, C═C, C═N, C≡C, and N≡N;
SG1 and SG2 each independently represent a solubilizing and sterically bulky group, which helps to improve the solubility of the polymer and stabilize the colorant particles, selected from the group consisting of alkyl groups, alkoxy groups, branched alkyl groups, branched alkoxy groups, aliphatic esters, branched aliphatic esters, substituted phenyl groups, and macromolecular monomers;
FG represents a functional group that provides charging sites to pigment surfaces, selected from acidic functional groups and basic groups;
x, y and z are each independently an integer between 1 and about 5,000; and n is an integer between 2 and about 100, and
causing an inorganic or organic pigment particle to undergo a coupling reaction with the azide on the two-block living segment copolymer to form a functionalized di-block segment copolymer coupled to the pigment particle through a nitrogen link.

14. The method of claim 13, further comprising:
after the adding step, adding a third monomer to the two-block azide living block copolymer, which yields a three-block azide living copolymer,
in the forming step, forming at least one additional segment by adding, in any order, a first block monomer, a second block monomer, and a third block monomer to form a tri-block living segment copolymer; and
causing the inorganic or organic pigment particle to undergo a coupling reaction with the azide on the three-block living segment copolymer to form a functionalized tri-block segment copolymer coupled onto the pigment particle through a nitrogen link.