Patent Publication Number: US-2017368858-A1

Title: Apparatus and method of microcontact printing for improving uniformity

Description:
TECHNICAL FIELD 
     The present disclosure relates to apparatus and methods of micro-contact printing for improving uniformity. 
     BACKGROUND 
     Micro-contact printing can produce fine patterns on a flexible web in a roll-to-roll (R2R) process. One of the unique characteristics of the articles created by micro-contact printing are the small scale features the process is capable of generating suitable for use in the electronics industry. For example, patterns constructed of lines with line widths smaller than 10 microns having high optical transmission and relatively high electrical conductivity can be prepared over a large area. This small line width size, along with a low density of the lines, is enabled by very fine patterning of a micro-contact printing stamp to produce materials suitable for use as, for example, a touch screen. 
     SUMMARY 
     There is a desire to improve uniformity in articles produced by R2R micro-contact printing. Some R2R micro-contact printing processes are related to the transfer of ink from one or more printing stamps mounted on a roll to a functional layer on a web where the roll can be driven by the web with a contacting pressure therebetween. 
     In one aspect, the present disclosure describes an apparatus of applying a pattern onto a web. The apparatus includes a roll that is configured to be rotatable about an axis thereof. One or more micro-contact printing stamps are disposed on an outer surface of the roll and are arranged in a down-web direction on the circumference of the roll with a seam between each adjacent transverse edge of the one or more micro-contact printing stamps. The web is guided along a web path such that the web contacts the outer surface of the roll along a transverse contacting line and rotates the roll. When the transverse contacting line runs across the seam, the transverse contacting line also runs across at least a portion of the one or more micro-contact printing stamps such that the web always contacts the one or more micro-contact printing stamps. 
     In another aspect, the present disclosure describes an apparatus for applying a pattern onto a web. The apparatus includes a cylindrical core having an outer surface, a first end with a first basal edge, and an axis thereof defining an axial direction. The cylindrical core being configured to rotatable about the axis. One or more micro-contact printing stamps are disposed at the outer surface of the cylindrical core and each include transverse edges that extend along the axial direction with a seam between each adjacent transverse edge. The respective projections of the adjacent transverse edges on the first basal edge span the projection of the seam onto the first basal edge. In some embodiments, a web is guided along a web path such that the web contacts the outer surface of the cylindrical core along a transverse contacting line and rotates the cylindrical core. When the transverse contacting line runs across the seam, the transverse contacting line also runs across at least a portion of the one or more micro-contact printing stamps such that the web always contacts the one or more micro-contact printing stamps. 
     In yet another aspect, the present disclosure describes a method of applying a pattern onto a web. The method includes providing a roll that is configured to be rotatable about an axis thereof. One or more micro-contact printing stamps are provided on an outer surface of the roll and are arranged in a down-web direction on the circumference of the roll with a seam between each adjacent transverse edge of the one or more micro-contact printing stamps. The method further includes guiding a web along a web path such that the web contacts the outer surface of the roll along a transverse contacting line and rotates the roll. When the transverse contacting line runs across the seam, the transverse contacting line also runs across at least a portion of the one or more micro-contact printing stamps such that the web always contacts the one or more micro-contact printing stamps. 
     Various unexpected results and advantages are obtained in exemplary embodiments of the disclosure. One such advantage of exemplary embodiments of the present disclosure is that the web always contacts micro-contact printing stamps mounted on an outer surface of a roll, and a consistent contacting pressure can be obtained between the web and the micro-contact printing stamps to improve uniformity in printing patterns on the web. 
     Listing of Exemplary Embodiments 
     Exemplary embodiments are listed below. It is to be understood that any of embodiments A-N, O-T, and U to Z can be combined. 
     Embodiment A is an apparatus comprising: 
     a roll being configured to be rotatable about an axis thereof; 
     one or more micro-contact printing stamps on an outer surface of the roll, the one or more micro-contact printing stamps being arranged in a down-web direction on the circumference of the roll with a seam between each adjacent transverse edge of the one or more micro-contact printing stamps; and 
     a web path along which a web is guided such that the web contacts the outer surface of the roll along a transverse contacting line and rotates the roll, 
     wherein when the transverse contacting line runs across the seam, the transverse contacting line also runs across at least a portion of the one or more micro-contact printing stamps such that the web always contacts the one or more micro-contact printing stamps. 
     Embodiment B is the apparatus of embodiment A, wherein the adjacent transverse edges of the one or more micro-contact printing stamps have curved shapes that are complementary with each other.
 
Embodiment C is the apparatus of embodiment B, wherein the adjacent transverse edges have a chevron shape.
 
Embodiment D is the apparatus of embodiment B or C, wherein the chevron shape has a depth D in the down-web direction, the seam has a width W in the down-web direction, and the ratio of D/W is no less than 1.
 
Embodiment E is the apparatus of embodiment A or B, wherein a first of the adjacent transverse edges includes a protrusion in the down-web direction at each end of the first transverse edge.
 
Embodiment F is the apparatus of embodiment A, B or E, wherein a second of the adjacent transverse edges includes a dent or chamfer at each end of the second transverse edge that is configured to receive the protrusion of the first transverse edge.
 
Embodiment G is the apparatus of embodiment A or B, wherein the adjacent transverse edges have a smooth-curve shape.
 
Embodiment H is the apparatus of any one of the preceding embodiments, wherein the transverse contacting line has first one or more sections in contact with the micro-contact printing stamps, and second one or more sections covers the seam, and the length ratio of the first sections and the second sections is no less than 5.
 
Embodiment I is the apparatus of any one of the preceding embodiments, wherein the roll further comprises a sleeve on the outer surface.
 
Embodiment J is the apparatus of embodiment I, wherein the roll further includes a core, and the sleeve is positioned over the core with the sleeve supported for rotation by a layer of air between the sleeve and core.
 
Embodiment K is the apparatus of embodiment I or J, wherein the sleeve is from 5 mils (0.127 mm) to 30 mils (0.762 mm) thick.
 
Embodiment L is the apparatus of any one of the preceding embodiments, wherein the web path comprises an entry roller and a take-off roller, and a free span of the web between the entry roller and the take-off roller contacts the micro-printing stamp.
 
Embodiment M is the apparatus of embodiment L, wherein the free span is in contact with the outer surface of the roll over less than 25% of the circumference thereof.
 
Embodiment N is the apparatus of any one of the preceding embodiments, wherein the web path comprises a positioning roller, such that the web contacts the micro-contact stamp at a nip formed between the positioning roller and the sleeve.
 
Embodiment O is a method comprising:
 
     providing a roll that is configured to be rotatable about an axis thereof; 
     providing one or more micro-contact printing stamps on an outer surface of the roll, the one or more micro-contact printing stamps being arranged in a down-web direction on the circumference of the roll with a seam between each adjacent transverse edge of the one or more micro-contact printing stamps; and 
     guiding a web along a web path such that the web contacts the outer surface of the roll along a transverse contacting line and rotates the roll, 
     wherein when the transverse contacting line runs across the seam, the transverse contacting line also runs across at least a portion of the one or more micro-contact printing stamps such that the web always contacts the one or more micro-contact printing stamps. 
     Embodiment P is the method of embodiment O, wherein the adjacent transverse edges of the one or more micro-contact printing stamps have curved shapes that are complementary with each other.
 
Embodiment Q is the method of embodiment O or P, wherein the adjacent transverse edges have a chevron shape.
 
Embodiment R is the method of embodiment Q, wherein the chevron shape has a depth D in the down-web direction, the seam has a width W in the down-web direction, and the ratio of D/W is no less than 1.
 
Embodiment S is the method of any one of embodiments O-R, wherein the transverse contacting line has first one or more sections in contact with the micro-contact printing stamps, and second one or more sections covers the seam, and the length ratio of the first sections and the second sections is no less than 5.
 
Embodiment T is the method of any one of embodiments O-R, wherein the web path comprises an entry roller and a take-off roller, and a free span of the web between the entry roller and the take-off roller is in contact with the roll over less than 25% of the circumference thereof.
 
Embodiment U is an apparatus comprising:
 
     a cylindrical core having an outer surface, a first end with a first basal edge, and an axis thereof defining an axial direction, the cylindrical core being configured to rotatable about the axis; and 
     one or more micro-contact printing stamps being disposed at the outer surface of the cylindrical core, the one or more micro-contact printing stamps each including transverse edges that extend along the axial direction with a seam between each adjacent transverse edges, 
     wherein the respective projections of the adjacent transverse edges on the first basal edge span the projection of the seam onto the first basal edge. 
     Embodiment V is the apparatus of embodiment U, further comprising a web path along which a web is guided such that the web contacts the outer surface of the cylindrical core along a transverse contacting line and rotates the cylindrical core, wherein when the transverse contacting line runs across the seam, the transverse contacting line also runs across at least a portion of the one or more micro-contact printing stamps such that the web always contacts the one or more micro-contact printing stamps.
 
Embodiment W is the apparatus of embodiment U or V, wherein the adjacent transverse edges of the one or more micro-contact printing stamps have curved shapes that are complementary with each other.
 
Embodiment X is the apparatus of embodiment W, wherein the adjacent transverse edges have a chevron shape.
 
     Embodiment Y is the apparatus of embodiment X, wherein the chevron shape has a depth D in the down-web direction, the seam has a width W in the down-web direction, and the ratio of D/W is no less than 1. 
     Embodiment Z is the apparatus of embodiment W, wherein the adjacent transverse edges have a smooth-curve shape, and the seam therebetween lacks an interior angle feature. 
     Various aspects and advantages of exemplary embodiments of the disclosure have been summarized. The above Summary is not intended to describe each illustrated embodiment or every implementation of the present certain exemplary embodiments of the present disclosure. The Drawings and the Detailed Description that follow more particularly exemplify certain preferred embodiments using the principles disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be more completely understood in consideration of the following detailed description of various embodiments of the disclosure in connection with the accompanying figures, in which: 
         FIG. 1A  is a perspective side view of an apparatus, according to one embodiment. 
         FIG. 1B  illustrates projections of a portion of the apparatus of  FIG. 1A . 
         FIG. 2A  is a perspective side view of a comparative apparatus. 
         FIG. 2B  illustrates projections of a portion of the comparative apparatus of  FIG. 2A . 
         FIG. 3  is a schematic view of an apparatus for carrying out a micro-contact printing method, according to one embodiment. 
         FIG. 4  is a schematic view of an alternate embodiment of an apparatus for carrying out a micro-contact printing method, according to another embodiment. 
         FIG. 5  is a schematic view of a micro-contact printing stamp, according to one embodiment. 
         FIG. 6  is a schematic view of the micro-contact printing stamp of  FIG. 5  mounted on a roll, according to one embodiment. 
         FIG. 7  is a schematic view of a micro-contact printing stamp, according to anther embodiment. 
         FIG. 8  is a schematic view of a micro-contact printing stamp, according to anther embodiment. 
         FIG. 9  is a schematic view of a micro-contact printing stamp, according to anther embodiment. 
         FIG. 10  is a schematic view of a micro-contact printing stamp, according to anther embodiment. 
         FIG. 11  is a schematic view of one or more micro-contact printing stamps mounted on a roll, according to another embodiment. 
         FIG. 12  is a schematic view of one or more micro-contact printing stamps mounted on a roll, according to another embodiment. 
         FIG. 13  is a schematic view of one or more micro-contact printing stamps mounted on a roll, according to another embodiment. 
     
    
    
     In the drawings, like reference numerals indicate like elements. While the above-identified drawing, which may not be drawn to scale, sets forth various embodiments of the present disclosure, other embodiments are also contemplated, as noted in the Detailed Description. In all cases, this disclosure describes the presently disclosed disclosure by way of representation of exemplary embodiments and not by express limitations. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of this disclosure. 
     DETAILED DESCRIPTION 
     There is a desire to improve uniformity in articles produced by micro-contact printing. Some R2R micro-contact printing processes are related to the transfer of ink from one or more printing stamps mounted on a roll to a functional layer on a continuous web where the roll can be driven to rotate about an axis thereof by contacting the web. In the present disclosure, a web is guided along a web path such that the web always contacts one or more of the micro-contact printing stamps along a transverse contacting line and rotates a roll onto which the micro-contact printing stamps are mounted. The one or more micro-contact printing stamps are configured and arranged on the outer surface of the roll such that when the transverse contacting line runs across a seam, the transverse contacting line also runs across at least a portion of the micro-contact printing stamps. In this manner, a consistent contact pressure between the one or more micro-contact printing stamps and the web can be provided and uniform printing patterns can be produced on the web. 
     Referring to  FIGS. 1A and 1B , the present disclosure describes an apparatus  100  for applying a pattern onto a web. The apparatus  100  shown in  FIG. 1A  is a roll that includes a cylindrical core  101  having an outer surface  102 , a first end  103  with a first basal edge  104 , and an axis  105  defining an axial direction  106 . The cylindrical core  101  can be hollow or solid, and is configured to be rotatable about the axis  105 . One or more micro-contact printing stamps  107  are disposed at the outer surface  102 . The one or more micro-contact printing stamps  107  each include transverse edges that extend along the axial direction  106 . A space between a first transverse edge  108  and a second, adjacent transverse edge  109  of the micro-contact printing stamps  107  defines a seam  110 . In the embodiment of  FIG. 1A , the first and second transverse edges  108  and  109  have complementary chevron or “V” shapes.  FIG. 1B  illustrates respective projections of the first and second transverse edges  108  and  109 , and the seam  110  on a projection plane  111  of  FIG. 1A . The projection plane  111  is parallel to the end  103 . The first transverse edge  108  has a first projection  112  onto the first basal edge  104  of the end  103 . The second transverse edge  109  has a second projection  113  onto the first basal edge  104  of the end  103 . The seam  110  has a projection  114  onto the first basal edge  104 . The projections  112  and  113  are on the first basal edge  104 , but each are shown with a displacement with respect to the first basal edge  104  for the purpose of clarity. The first projection  112  of the first transverse edge  108  and the second projection  113  of the second transverse edge  109  span the projection  114  of the seam  110 . That is, the first projection  112  extends from one end of the projection  114 , the second projection  113  extends from the other end of the projection  114 , and the union of the projections  112  and  113  overlaps with the entirety of the projection  114 . 
     Referring to  FIGS. 2A and 2B , a comparative apparatus  200  is now described. In contrast to the apparatus  100  depicted in  FIGS. 1A and 1B , the comparative apparatus  200  includes a cylindrical core  201  having an outer surface  202 , a first end  203  with a first basal edge  204 , and an axis  205  defining an axial direction  206 . The cylindrical core  210  is configured to be rotatable about the axis  205 . One or more micro-contact printing stamps  207  are disposed at the outer surface  202 . The one or more micro-contact printing stamps  207  each include transverse edges that extend along the axial direction  206 . A space between a first transverse edge  208  and a second, adjacent transverse edge  209  of the micro-contact printing stamps  207  defines a seam  210 .  FIG. 2B  illustrates respective projections of the first and second transverse edges  208  and  209 , and the seam  210  on a projection plane  211  of  FIG. 2A . The first transverse edge  208  has a first projection  212  onto the first basal edge  204 . The first transverse edge  208  is a straight line in the axial direction  206  and the first projection  212  is a point. The second transverse edge  209  has a second projection  213  onto the first basal edge  204 . The second transverse edge  209  is a straight line in the axial direction  206  and the second projection  213  is a point. The seam  210  has a projection  214  onto the first basal edge  104 . The projection  214  corresponds to the width of the seam  210 . The first projection  212  of the first transverse edge  208  and the second projection  213  of the second transverse edge  209  do not span the projection  214  of the seam  210 . That is, the projections  212  and  213  merely overlap with the ends or a portion instead of the entirety of the projection  214 . 
     Referring to  FIG. 3 , a schematic view of an apparatus  20  according to one embodiment is illustrated. A web  22  of indefinite length is conveyed in a direction D along a web path  24 , which in the depicted embodiment includes an entry roller  26  and an exit roller  28  positioned such that the web  22  touches or wraps at least a portion of a stamp roll assembly  30 . In many convenient embodiments, the entry roller  26  and the exit roller  28  are idle rollers. In other embodiments, one or the other or both of the rollers  26  and  28  can be a driven roller. The web  22  can include any of a variety of materials. In some embodiments, the web  22  may be a polymeric material such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate, and polyimide. The web  22  might conveniently be coated with a thin metal layer such as, for example, silver, gold, copper, nickel, etc. 
     The web path  24  conveys a free span of the web  22  between the entry roller  26  and the exit roller  28  into touching contact with the stamp roll assembly  30 . The stamp roll assembly  30  includes a roll  32  with a cylindrical shape. In the embodiment of  FIG. 3 , the roll  32  includes a sleeve  32 ′ mounted on an air bearing  34 . The air bearing  34  includes a non-rotating steel core  36  having apertures  38  for the egress of an airflow that rotationally supports roll  32 . Heaters or coolers may be placed in or adjacent to the core  36  or the air supply to add or remove heat from the roll  32  if desired to control the temperature of micro-contact printing stamp(s) mounted thereon. 
     While  FIG. 3  illustrates an air bearing to mount the sleeve  32 ′, it is to be understood that the sleeve  32 ′ can be mounted on various bearings including, for example, plain bearings, roll bearings, ball bearings, air bearings, etc. In some embodiment, the roll  32  and the air bearing  34  can be replaced by an conventional dead shaft or live shaft roll supported for rotation by air bearings located on opposing ends of the roll. Such a system would not have a compliant layer of air between the sleeve and the core. A carbon fiber roll could be used to minimize the rotational moment of inertia present. 
     One or more micro-contact printing stamps can be mounted on the sleeve  32 ′. In the embodiment of  FIG. 3 , micro-contact printing stamps  40  and  41  are mounted on the sleeve  32 ′ around its circumference. The micro-contact printing stamps  40  and  41  are arranged in a down-web direction E on the circumference of the sleeve  32 ′ to have respective edges  401  and  411 , and edges  402  and  412  adjacent to each other. Seams or gaps  70  and  71  are formed between adjacent edges  401  and  411 , and adjacent edges  402  and  412 , respectively. The free span of the web  22  contacts one, the other, or both of the micro-contact printing stamps  40  and  41 . When the web  22  is conveyed in the direction D, the sleeve  32 ′ is rotated in a rotation direction (i.e., the down-web direction E) around an axis  320  thereof. The contacting area between the free span of the web  22  and the sleeve  32 ′ is an arc surface which is referred to as a contacting band  220 . The contacting band  220  has a central line thereof extending in a direction generally parallel to the axis  320 . The central line is referred to as a transverse contacting line  220   a  herein. As shown in  FIG. 3 , the contacting band  220  has a length L in the down-web direction E. In some embodiments, the length L of the contacting band  220  may be less than 50%, less than 25%, or even less than 15%, or further even less than 5% of the circumference of the sleeve  32 ′. The length L can be, for example, 2-4 inches (5.1-10.2 cm) of surface arc such as 3 inches (7.6 cm). The wrap angle α as well as the corresponding length L can be determined based on parameters such as web speed, web tension, radius of the roll, etc. Relatively smaller wrap angles can improve print quality, but may not have sufficient contact to drive the sleeve  32 ′. 
     In operation, when the sleeve  32 ′ is rotated by the web  22  in the down-web direction E and the transverse contacting line  220   a  runs across the seam  70  or  71 , the contacting band  220  may cover a non-uniform surface of the sleeve  32 ′ that includes the seam  70  or  71 . The non-uniform contacting surface may result in non-uniform patterns printed on the web  22 . Conventional micro-contact printing stamps have a rectangular shape. When one or more of such rectangular stamps are mounted onto the sleeve  32 ′ such as shown in  FIGS. 2A and 2B , a seam between adjacent stamp edges extends straightly in a direction generally parallel to the axis  320  of the sleeve  32 ′, which may cause the sleeve  32 ′ to deform to be an oblong shape. In addition, when the web  22  touches the surface of the sleeve  32 ′ around the seam, an undesired step change effect due to an abrupt change of contact pressure can be generated, which may cause non-uniformness in printing patterns on the web, for example, resulting in bands of non-uniform line widths in a printed mess pattern on the web. Such non-uniformness in printing patterns are referred to as cross-web “chatter” herein. 
     The embodiments described herein allow the web  22  to always contact one, the other, or both of the micro-contact printing stamps  40  and  41  to mitigate or prevent the cross-web “chatter”. In some embodiments, when the transverse contacting line  220   a  runs across the seam  70  or  71 , the transverse contacting line  220   a  also runs across at least a portion of the micro-contact printing stamps  40  and  41 , which will be discussed further below in  FIGS. 5-10 . 
     In some embodiments, the web  22  can make a touching contact with one or both of the micro-contact printing stamps  40  and  41  with a relatively low contact pressure, for example, less than 2 psi (13.7 kPa), or even less than 1 psi (6.9 kPa), or further even less than 0.5 psi (3.4 kPa). The web  22  may be in contact with the micro-contact printing stamp for a short time, for example, several milliseconds. Longer contact times may increase the printed width of the pattern on the substrate undesirably. 
     While  FIG. 3  illustrates two micro-contact printing stamps  40  and  41  and two seams  70  and  71  between the respective adjacent stamp edges, it is to be understood that other numbers of stamps (i.e., one, three or more) can be mounted on a sleeve, and other numbers of seams or gaps can be formed between respective adjacent stamp edges. For example,  FIG. 4 , to be discussed further below, includes one stamp  40 ′ which covers almost the entire circumference of the sleeve  32 ′ and has two edges  401 ′ and  402 ′ adjacent to each other and a seam or gap  70 ′ between the adjacent edges  401 ′ and  402 ′. 
     The sleeves described herein such as the sleeve  32 ′ of  FIGS. 3 and 4  can be built out of metals, combination of layers of metals, from composite materials that include PAN carbon fibers, pitch carbon fibers, para-aramid fibers, Kevlar fibers, and glass fibers, or from combination of layers of metals and polymeric materials, for example elastomers like rubbers. These fiber-based materials can be impregnated with polymeric materials that could include epoxies, polyesters, and vinylesters. Examples of metals suitable for building sleeves include nickel, copper, nickel/cobalt, titanium, and aluminum. A thin shell of carbon composite is also believed to be suitable for use as a sleeve. In some convenient embodiments, the sleeve  32 ′ is composed primarily of nickel. The nickel sleeve can have a thickness of, for example, no less than 3 mils (0.076 mm), no less than 4 mils (0.102 mm), or no less than 5 mils (0.127 mm). The thickness can be, for example, no greater than 30 mils (0.762 mm), no greater than 15 mils (0.381 mm), or even no greater than 6 mils (0.152 mm). In some embodiments, the sleeve  32 ′ has a rotational moment of inertia of, for example, less than 150, 100, 50, or 30 lb-in 2  (4300, 2875, 1438, or 860 N-cm 2 ) in various embodiments. As an example, a nickel sleeve with a length of 15 inches (38.1 cm) and thickness of 10 mils and an outer diameter of 8.7 inches (22.1 cm) has a rotational moment of inertia of about 25 lb-in 2  (718 N-cm 2 ). A relatively low rotational moment of inertia for the sleeve  32 ′ can be advantageous in the disclosed micro-contact printing when the sleeve  32 ′ is driven by the web  22 . 
     The micro-contact printing stamps described herein can be made from polydimethylsiloxane (PDMS) such as one described in PCT Publication No. WO 2013003412 (O&#39;Hare et al.). Other suitable micro-contact printing stamps can be made from diverse polymeric materials. Suitable polymeric materials include silicone polymers, epoxy polymers, acrylate polymers, saturated and unsaturated rubbers. Unsaturated rubbers can include: natural polyisoprene, synthetic polyisoprene, polybutadiene, chloroprene rubber, butyl rubber, halogenated butyl rubbers, styrene-butadiene rubber, nitrile rubber, and hydrogenated Nitrile Rubbers. Saturated rubbers can include ethylene propylene rubber, ethylene propylene diene rubber, epichlorohydrin rubber, polyacrylic rubber, silicone rubber, fluorosilicone rubber, fluoroelastomers, perfluoroelastomers, polyether block amides, chlorosulfonated polyethylene, and ethylene-vinyl acetate. 
     Micro-contact printing stamps can be made by a number of methods that include, for example, casting against masters, selective curing by actinic radiation or heat, surface machining, or laser ablation. The micro-contact printing stamp can be made from one material, have multiple layers of different materials, or can have a composite structure. Micro-contact printing stamps can be pre-made and then mounted on a rotatable surface with help of adhesive tapes, magnetic fields, or vacuum. Alternatively, micro-contact printing stamp material can be initially deposited on a rotatable surface, with a curing step following, and with a pattern making step finishing the stamp. 
     The micro-contact printing stamp can include multiple layers of metals, woven and non-woven fibrous materials, rigid polymers, like PET, and foams. Foams are also referred to as expanded or sponge plastics and have at least two phases, a polymer matrix and gaseous phase. Polymeric matrix can have fillers of either inorganic nature, such as glass, ceramic or metal, or of polymeric nature. Foam cell geometry can be open or closed. Suitable foams can have a range of densities from 0.1 lb/ft 3  to 70 lb/ft 3 . Using a layer of foam between the micro-contact printing stamp and the mounting roll can provide additional compliance improving the print quality. 
     The diameter of the sleeve  32 ′ or the roller  32  can vary and is often sized to be a convenient repeat of the micro-contact printing pattern. Smaller diameters may be preferred due to lower inertia and reduced air entrainment, but often pattern geometry and the ultimate size of the printing pattern dictate the diameter of the sleeve  32 ′ or roller  32 . 
     The tension of the web  22  can vary. Higher tensions can be used to generate more driving force for the sleeve or roll and reduce air entrainment, but also can lead to a collapse of the printing features on the micro-contact printing stamp. In some embodiments, suitable tensions can be in the range of 1-2 pound/linear inch (1.75 to 3.5 Newton/linear cm) depending on the wrap angle of the substrate on the roll  32  or sleeve  32 ′. 
     In some convenient embodiments, one or more of the entry roller  26 , the exit roller  28 , or the stamp roll assembly  30  may be on adjustable mounts so as to readily adjust the contacting pressure, the wrapping angle, and/or the contacting band  220  between the web  22  and the one or more micro-contact printing stamps mounted on the sleeve  32 ′. 
     Referring to  FIG. 4 , a schematic view of an alternate embodiment of an apparatus  20   a  is illustrated. In this embodiment, an alternate web path  24   a  is employed to convey the web  22  in the direction D into touching contact with stamp roll assembly  30  along a transverse contacting line  220   a ′. The web  22  is maneuvered into touching contact at a nip between nip roller  50  and the stamp roll assembly  30 . In the depicted embodiment, the nip roller  50  is mounted on a pivot arm  52 . The contact force is controlled by a force controller, embodied as a pneumatic cylinder  54  connected to the pivot arm  52 . A position stop  56  is sometimes desirable for providing an absolute limit on the movement of the positioning roller  50  towards the roll  32 . 
     The embodiments described herein are conveniently used for printing onto an indefinite length web of polymeric material such as polyolefin, polyester phthalate, and polyimide films. Metallic surfaces can also be used as printing substrates. The metallic surface can include, for example, elemental metal, metal alloys, intermetallic compounds, metal oxides, metal sulfides, metal carbides, metal nitrides, and combinations thereof. Exemplary metallic surfaces for supporting self-assembled monolayers include gold, silver, palladium, platinum, rhodium, copper, nickel, iron, indium, tin, tantalum, as well as mixtures, alloys, and compounds of these elements. 
     In the embodiments of  FIGS. 3 and 4 , the sleeve  32 ′ or the roll  32  is driven by the web  22  to rotate about its axis  320 . Changes in the contacting pressure between the web  22  and the sleeve  32 ′ may cause undesired frictional forces at the contacting area that lead to printing defects, for example, the above mentioned cross-web “chatter”. The embodiments described herein allow the web  22  to always contact the one or more micro-contact printing stamps mounted on the sleeve  32 ′ so as to mitigate or prevent the cross-web “chatter”. 
     As shown in  FIG. 5 , a micro-contact printing stamp  40   a  has a chevron shape where two opposite edges  60   a  and  60   a ′ having complementary “V” shapes.  FIG. 4  illustrates the arrangement of one or more micro-contact printing stamps  40   a  on the roll  32 . The one or more micro-contact printing stamps  40   a  are arranged in the down-web direction E with a seam or gap  70   a  between the adjacent edges  60   a  and  60   a ′. The “V” shapes of the edges  60   a  and  60   a ′ each have a depth D measured in the down-web direction E. The seam  70  has a width W measured in the down-web direction E. The ratio of D/W can be, for example, no less than 1, no less than 2, no less than 3, no less than 4, or no less than 5. While not wishing to be bound by theory, it is believed that the higher the ratio of D/W, the more consistent contacting pressure between the web  22  and the stamps  40   a  can be achieved so as to more effectively mitigate or prevent the cross-web “chatter” issues. 
     The web  22  of  FIG. 3  can contact the micro-contact printing stamps  40   a  along a transverse contacting line  64  which can be generally perpendicular to the down-web direction E. When the transverse contacting line  64  runs across the seam  70   a,  the transverse contacting line  64  also runs across at least a portion of the micro-contact printing stamps  40   a.  The one or more portions of the transverse contacting line  64  that cover the seam  70   a  have a total length of L1, and the one or more portions of the transverse contacting line  64  that cover the micro-contact printing stamps  40   a  have a total length of L2. In the embodiment of  FIG. 4 , the ratio of L2/L1 is about 9:1. In some embodiments, the length ratio of L2/L1 can be, for example, no less than 2:1, no less than 5:1, or no less than 10:1. While not wishing to be bound by theory, it is believed that the higher the length ratio of L2/L1, the more consistent contacting pressure can be achieved such that the cross-web “chatter” issues can be more effectively reduced. 
       FIG. 7  illustrates a micro-contact printing stamp  40   b,  according to another embodiment. The micro-contact printing stamp  40   b  includes opposite edges  60   b  and  60   b ′. The edge  60   b ′ includes protrusions  605   a  and  605   b  at the ends thereof to define a recess  605 . The protrusions  605   a  and  605   b  each extend in the down-web direction E. The defined recess  605  at the edge  60   b ′ has a complementary shape to an adjacent edge such as the edge  60   b  so as to receive the edge  60   b,  and a seam (not shown) can be formed between the adjacent edges  60   b  and  60   b′.    
       FIG. 8  illustrates a micro-contact printing stamp  40   c,  according to another embodiment. The micro-contact printing stamp  40   c  includes opposite edges  60   c  and  60   c ′. The edge  60   c ′ includes protrusions  606   a  and  606   b  at the ends thereof. The protrusions  605   a  and  605   b  each extend in the down-web direction E. The edge  60   c  includes dents  606   a ′ and  606   b ′ at the ends thereof each having a complementary shape to the respective protrusions  606   a  and  606   b.  The edge  60   c ′ defines a recess  606  to receive an adjacent edge such as the edge  60   c,  and a seam (not shown) can be formed between the adjacent edges  60   c  and  60   c′.    
       FIG. 9  illustrates a micro-contact printing stamp  40   d,  according to another embodiment. The micro-contact printing stamp  40   d  includes opposite edges  60   d  and  60   d ′. The edge  60   d ′ includes protrusions  607   a  and  607   b  at the ends thereof. The protrusions  607   a  and  607   b  each extend in the down-web direction E. The edge  60   d  includes chamfered ends  607   a ′ and  607   b ′ each having a complementary shape to the respective protrusions  607   a  and  607   b.  The edge  60   d ′ defines a recess  607  to receive an adjacent edge such as the edge  60   d,  and a seam (not shown) can be formed between the adjacent edges  60   d  and  60   d′.    
       FIG. 10  illustrates a micro-contact printing stamp  40   e,  according to one embodiment. The micro-contact printing stamp  40   e  includes opposite edges  60   e  and  60   e ′ having complementary shapes of single smooth curve. The edge  60   e  has a convex shape and the edge  60 ′ has a concave shape. When one or more micro-contact printing stamps  40   e  are mounted on the roll in the down-web direction, a seam (not shown) can be formed between the adjacent edges  60   e  and  60   e′.    
     In some embodiments, the one or more micro-contact printing stamps each can include transverse edges that are smooth curves such that a seam between each adjacent transverse edge lacks an interior angle feature. The term “interior angle feature” refers to an angle feature somewhere along the adjacent transverse edges, other than at the termination of the transverse edge.  FIGS. 11-13  illustrate embodiments where each adjacent transverse edges  60   f  and  60   f ′,  60   g  and  60   g ′, and  60   h  and  60   h ′ lack any such interior angle features. 
     In addition to the micro-contact printing stamps illustrated in  FIGS. 5-13 , micro-contact printing stamps described herein can have opposite edges with various shapes including, for example, single curve, chevron shape, dovetail shape, tongue shape, groove shape, combinations thereof. In the present disclosure, the arrangement of one or more of the micro-contact printing stamps allows a web to always contact the stamps. In this manner, a consistent contact pressure between the one or more micro-contact printing stamps and the web can be provided and uniform printing patterns can be produced on the web. 
     Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments” or “an embodiment,” whether or not including the term “exemplary” preceding the term “embodiment,” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the certain exemplary embodiments of the present disclosure. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the certain exemplary embodiments of the present disclosure. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. 
     EXAMPLES 
     Example 1 
     In Example 1, a chevron shaped micro-contact printing stamp was mounted on a sleeve or a roll with a seam between adjacent edges as shown in  FIG. 6 . The chevron shaped micro-contact printing stamp has a depth D in a down-web direction of 0.2 inch (0.51 cm). The seam has a seam width in a direction perpendicular to the edges of about 0.25 inch (0.64 cm). 
     Example 2 
     Example 2 is the same as Example 1 except for the micro-contact printing stamp of Example 2 having a depth D in a down-web direction of 0.35 inch (0.89 cm). 
     Example 3 
     Example 3 is the same as Examples 1 and 2 except for the micro-contact printing stamp of Example 3 having a depth D in a down-web direction of 0.75 inch (1.91 cm). 
     Example 4 
     Example 4 is the same as Examples 1-3 except for the micro-contact printing stamp of Example 4 having a shape as shown in  FIG. 8 . 
     Example 5 
     Example 5 is the same as Examples 1-4 except for the micro-contact printing stamp of Example 5 having a shape as shown in  FIG. 9 . 
     Comparative Example A 
     In Comparative Example A, a rectangular shaped micro-contact printing stamp was mounted on a sleeve or a roll with a seam between adjacent straight stamp edges. The seam extend in a direction generally parallel to the rotation axis of the sleeve and has a seam width of about 0.25 inch (0.64 cm). 
     The above examples are used to apply a pattern onto a web via the apparatus of  FIG. 3  for carrying out a micro-contact printing method. Examples 1-5 show various improvement in “chatter” reduction compared to Comparative Example A. For Examples 1-3, the larger the depth D of the chevron shaped stamp, the more improvement in “chatter” reduction. 
     While the specification has described in detail certain exemplary embodiments, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. Accordingly, it should be understood that this disclosure is not to be unduly limited to the illustrative embodiments set forth hereinabove. In particular, as used herein, the recitation of numerical ranges by endpoints is intended to include all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). In addition, all numbers used herein are assumed to be modified by the term “about.” 
     Furthermore, all publications and patents referenced herein are incorporated by reference in their entirety to the same extent as if each individual publication or patent was specifically and individually indicated to be incorporated by reference. Various exemplary embodiments have been described. These and other embodiments are within the scope of the following claims.