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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. DESIGN AND FABRICATION OF ROLL-TO-ROLL PROCESSING LINE FOR FIELD ASSISTED ASSEMBLY
  5. VALIDATION EXPERIMENTS
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. REFERENCES

Many functional films including heat spreaders, ultrahigh density information storage systems, capacitors, batteries, and fuel cell membranes require enhanced through thickness properties. In this paper, we describe a design and demonstration of a multipurpose novel film formation process to orient and align functional nanoparticles and polymer phases using external electric, magnetic, and thermal gradient fields. This roll-to-roll processing line uses a casting system that deposits liquid film of a monomer and/or polymer solution on to a flexible substrate. Substrate is carried by belt through an electric field zone that can apply DC, AC, or a biased AC to orient the phases and particles in the vertical direction while subjecting it to UV through its built in transparent conductive carrier. To orient magnetic particles, an electromagnet located along the machine may be used. The final tool that is built on this machine is the thermal alignment zone which is designed to apply a “line of heat” oriented transverse to the machine direction at nine different zones. Using this processing line, we are able to reduce the cost of manufacturing by limiting the amount of functional fillers through directional alignment while enhancing the thickness properties. POLYM. ENG. SCI., 55:34–46, 2015. © 2014 Society of Plastics Engineers


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. DESIGN AND FABRICATION OF ROLL-TO-ROLL PROCESSING LINE FOR FIELD ASSISTED ASSEMBLY
  5. VALIDATION EXPERIMENTS
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. REFERENCES

In the past decade, a great deal of research has been devoted to field assisted assembly using electric [1-4], magnetic [5-8], and thermal gradients [9-12] for drastically improving the directional properties of polymer blends, block copolymers, liquid crystals, and polymer nanocomposites. Though field assisted assembly of nanostructured phases/particles have been extensively demonstrated in small scale for a range of applications including flexible electronics [13-16], membranes [17], supercapacitors [18, 19], fuel cells [20, 21], photovoltaics [10, 22, 23], etc., a large scale manufacturing platform was not developed for commercial viability of these concepts. In the past, preferred orientation and organization of phases/nanoparticles commonly were controlled through self-assembly [24, 25], patterned substrates [26-28], and other techniques [29, 30]. The use of external fields to obtain their organization and preferred orientation can be more effective [31-33], creating beneficial directional anisotropy in properties particularly through the thickness direction that is not commonly achievable in traditional polymer film processing operations. Oriented nanopathways can be created through directional alignment to enhance the selectivity and flux of polymer films for various applications such as ion exchange, water filtration, and fuel cell membranes [17, 20].

In order to use the external fields to organize and orient phase/particle matrix pairs and sensitization of these particles and phases through the external aid of chemical modifications [34]. However, large improvements in directional physical properties can be achieved through this orientation the particles/phases while keeping their concentration low which also leads to easier processing.

The use of external fields to manipulate the properties of matter has its origin in electro (ER) and magneto (MR) rheological fluids [35]. ER and MR fluids are suspensions of micrometer size particles in solvents or oil. When an ER fluid is subjected to electric field particulate additives reversibly form fibrous structure within the non-conducting fluid. Similar morphological evidence has been observed in MR fluids where instead of electric dipoles, the particle–particle interaction arises from magnetic dipoles. The interaction of opposite dipoles leads to a long range chaining of particles in both applied electric and magnetic fields.

Electric field has been also been used to orient colloidal systems and suspensions using both dielectric and conducting particles [36, 37]. In these systems, the surface charge of the particles plays a critical role orienting easily at low electric fields as they exhibit low viscosities. Nanoparticles dispersed in aqueous medium were shown to assemble into microwires, nanowires, chains, dendrites, and 1D, 2D crystals by dielectrophoresis [38]. For field assisted particle alignment in polymer solutions or monomers or phase alignment in case of polymer blends or block copolymers a much higher electrical field is required to overcome the viscous drag forces.

The basic principle of alignment is based on the difference between the dielectric properties of the matrix and the particles. When an electric field is applied to a suspension between two electrodes, it induces molecular dipoles at the particle surfaces, resulting in attraction between oppositely charged particle surfaces, causing a pearl chain formation. However, as the particles become smaller, the thermal randomization forces compete with the polarization forces.

The ratio of polarization to thermal forces is given by λ [35]:

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In these equations, ε0 is the vacuum permittivity, εm is the relative dielectric constant of the matrix, R is the radius of the particles, βdiel is the Clausius–Mossotti relation describing particle dipole coefficient, KB is the Boltzmann constant, T is the temperature, and λ is the ratio of polarization to thermal forces. Hence when λ » 1 particles will polarize and tend to form a pearl chain morphology parallel to electric field, when λ « 1 thermal randomization overcomes the electric forces leading to no alignment.

Magnetic field application in orienting the particles or phases is analogous to that of electric field discussed above. Both magnetic and electric fields show similar effect on alignment of particles forming chains and orienting parallel to the magnetic field; however, in case of electric field relative dielectric constant between matrix and dispersant determines the degree of alignment, whereas in case of magnetic field relative magnetic susceptibilities determine orientation. However, electric field is limited in applicability by the amount of potential difference that can be applied between two parallel plates, the limit being electric breakdown of air, whereas magnetic field has no such breakdown value. But there is a technological limitation on the strength of the magnetic field that can be generated [39]. It is very difficult to achieve magnetic fields of more than a few teslas with an electromagnet. Hence, this limits us to the use of highly susceptible particles including magnetic nanoparticles or phases such as liquid crystals.

Thermal gradient also known as zone solidification, zone melting was first developed by William Gardner Pfann [40] at Bell Labs in the early nineteenth century to prepare high purity materials for manufacturing transistors. In this process, the material travels under a narrow heater slowly. When it encounters a thermal gradient the crystal is molten and subsequently it recrystallizes when it encounters the other cooling side of this gradient leaving impurities behind in the molten zone. Lovinger [9, 41] later applied this to semicrystalline polymers and obtained directional crystallization. Zone melting has also been used to recrystallize small organic molecules obtain single crystals of millimeter size thin films, due to the simultaneous growth and purification of organic single-crystal thin films it led to a large improvement of the optoelectronic properties [10, 42]. Thermal gradient has also been applied on block copolymers, Hashimoto and co-workers [43, 44] for the first time applied this method to a lamella-forming diblock copolymer in bulk, to reduce the number of defects and to form a single-crystal structure in the block copolymer films. A polystyrene-block–polyisoprene diblock copolymer was exposed to moving temperature gradient (ΔT = 70°C/mm) and obtained a single crystal of lamella microdomain structure in the bulk film by imposing the moving gradient at a rate of 25 nm/s.

Generating reconfigurable orientation of micrometer (or sub-micrometer) particles requires complex tooling with reconfigurable systems. In this paper, a novel design for roll-to-roll processing of polymeric film with oriented particles and phases is described. The process consists of three fundamental field assisted assembly techniques: electric, magnetic, and thermal gradients. We will show the utility of each field with an illustrated example below.

DESIGN AND FABRICATION OF ROLL-TO-ROLL PROCESSING LINE FOR FIELD ASSISTED ASSEMBLY

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. DESIGN AND FABRICATION OF ROLL-TO-ROLL PROCESSING LINE FOR FIELD ASSISTED ASSEMBLY
  5. VALIDATION EXPERIMENTS
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. REFERENCES

An outline of electromagnetic processing line is shown in Fig. 1, and a photograph of the actual equipment is shown in Fig. 2. As shown in the figures, the processing line is designed to produce 6″ wide functional films on the 70 ft long roll-to-roll machine. The roll-to-roll processing line consists of seven different zones: (i) unwinding/casting zone, (ii) electric field zone, (iii) magnetic field zone, (iv) UV curing movable station, (v) thermal gradient zone, (vi) high temperature oven, and (vii) rewind station. As shown in Fig. 1, the equipment consists of block-type structures where each block or zone can be used as a standalone or simultaneously depending on the task requirements.

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Figure 1. Schematic of electromagnetic processing line. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Figure 2. Electromagnetic processing line. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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All the zones discussed above lie on the frame created for the endless 10″ wide pre-tensioned stainless steel belt that rides on two drum rollers. This nonmagnetic stainless steel belt carries the substrate and the cast film through different zones for field assisted assembly.

Zone I—Unwind/Casting Zone

A polymer solution is applied to a carrier via a casting blade. Solution casting is chosen over other techniques to make films in this processing line because the polymer molecules and particles in the solution cast films are mobile until the solvent evaporates. This provides a low viscosity system in which it is easier to orient particles in the case of electric and magnetic fields.

The first zone in the processing line is unwind and casting zone, in this zone the web/flexible substrate is unwound at specific tensions which keeps the film under a constant pull force. The film edge is monitored using a ball screw drive to keep the film in place. The unwind module has a second spindle which allows for a secondary roll of web. The substrate is unwound at a constant feed rate and tension is drawn under the casting head located on the infeed ultra-precision granite plate as can be seen in Fig. 3. The feed rate can be varied from 0.0025 cm/s to 400 cm/s using a computerized control. Once the coating has been applied, the film passes under the laser micrometer which measures the wet coating thickness before the film moves to other zones. We employ two different types of solution casting methods depending on the requirement of user. The two methods include doctor blade casting and flow coating.

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Figure 3. Schematic of the unwind/casting zone. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Doctor Blade Coating

When the coating thickness ranging from micrometers to millimeters is required, the doctor blade coating is employed. The doctor blade assembly is 17″ long and 7″ wide which is designed to fit on 1ft wide granite, the doctor blade is comprised of three blades for multilayer casting each blade being 6″ wide. The three doctor blades can be adjusted separately for different thicknesses using micrometers or feeler gauges. The polymer dope solution is then poured in the doctor blades and the substrate is allowed to move using a computerized control making a thin coating of polymer on the top of the substrate. Multilayer casting can be used with different polymer solutions or similar solutions. Schematic of multilayer casting doctor blade is shown in Fig. 4a and picture of the doctor blade is shown in Fig. 4b.

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Figure 4. (a) Schematic and (b) picture of multilayer casting doctor blade system. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Flow Coating

Flow coating is an ideal process to generate thin films ranging from nanometer scale to micrometer scale. The foremost importance of flow coating in comparison to spin coating [45-47], dip coating [48, 49], solvent casting [50, 51], etc. is the ability to use the flow coater as a cost effective roll-to-roll process of generating thin films. Flow coating device is based on the design by Stafford et al. which was developed at NIST [52]. Flow coating is based on a blade kept at a gap of few microns from the substrate at an angle. The polymer solution is injected between the blade and the substrate which is held between the blade and the substrate by capillary forces. The substrate or the blade is then set into motion, due to the relative motion between the substrate and blade, two forces act simultaneously on the polymer solution: (i) capillary forces holding the polymer solution between the blade and the substrate; and (ii) the frictional drag forces of the substrate on the polymer solution which is proportional to relative velocity. As can be anticipated, when the drag forces/velocity increases more liquid is dragged from the blade hence a thicker film is coated on the substrate, this is due to the competing action between the capillary and drag forces. Hence by changing the velocity of the substrate, we can gain access to different coating thicknesses.

The flow coater consists of an optical breadboard on which everything is mounted, two goniometers, y–z stage, x-stage, blade holder, and a blade. The optical breadboard sits on the top of the ultra-precision granite with spacers and the mylar substrate which moves on the top of the granite, the setup is depicted in Fig. 5. The y–z stage is used for the motion of the blade in y- and z-directions, the movement in the y direction guides the coating on the top of the substrate and the z-direction movement controls the distance between the substrate and the blade. The x-tilt goniometer helps in adjusting the blade tilt angle with respect to the substrate whereas the y-tilt goniometer helps in precisely controlling the perpendicularism of the blade with respect to the substrate. The x-axis stage is used if a translation motion is required of up to 1 cm. The blade holder is designed to fit glass blades which are 3 in. wide; however, the holder can easily be modified for blades up to 6 in. wide. The syringe pump is used for delivering the polymer solution continuously in between the glass blade and the substrate. This continuous delivery of solution helps in making indefinitely long films. The syringe pump is calibrated with the speed of the substrate to determine the amount of solution to be delivered per min. The syringe pump we use is a Harvard PHD 2000 with infusing and withdrawing capability.

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Figure 5. (a) Schematic and (b) photo of the flow coating setup. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Zone 2—Electric Field Zone

The most commonly used setup for electric field alignment is limited in approach where two electrodes are placed on the same plane [53, 54] generally glass or other dielectric material separated by a dielectric spacer as shown in Fig. 6a. The composite mixture is then poured in between the two electrodes hence achieving the orientation in the planar direction. To achieve a vertical alignment [3, 55], i.e., through the thickness, two conducting metal plates on the top of each other separated by spacers, as shown in Fig. 6b, are generally used. The solution is poured between the electrodes and then frozen after the alignment either through UV curing as shown in Fig. 6, or through thermal curing.

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Figure 6. (a) Setup for in plane alignment and (b) out of plane alignment used in the laboratory batch process. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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These approaches, however simple and easy to apply, are not practical as they cannot be used to make large films on an industrial scale. In case of planar electrode assembly, the dimensions of the films are limited to the maximum voltage that can be applied, as with increasing dimensions a larger voltage is required. However, in case of vertical alignment, the dimensions of the two electrodes are limited.

A continuous roll-to-roll process to align/orient nanoparticle filled polymer systems through electric field in the out of plane direction was developed to create anisotropy through the thickness of the flexible polymer film. We discuss various strategies to align particles in a continuous process with different polymer matrix systems which include photocurable resin, polymer solution, and polymer melts.

The solution cast films are obtained using one of the two processes described above first encounter the electric field zone. The electric field zone is designed so as to obtain a preferential alignment of particles and phases along the z-axis or vertical orientation and to create undulations on the surface of the films. The electric field zone consists of two electrode system, where one electrode is kept at high voltage and the other is grounded; in this case, the top electrode is charged and the bottom electrode is grounded stainless steel belt. The charged electrode is connected to an HV amplifier (Matsuda, model 20B20), which can be used to generate a DC voltage; this amplifier is combined with an H–P function generator hence generating three types of fields AC, DC, and biased AC fields. The HV amplifier can generate voltages from −20 kV to +20 kV, this voltage is controlled using a one turn potentiometer located on the console panel; however, the output current is limited to less than 1 mA. If the signal is derived from the H–P function generator different AC waveforms such as sine, square, sawtooth, pulse, and burst can be generated and these AC voltages can be combined with DC voltage to create a biased AC. These waveforms and voltages can be visualized using the oscilloscope located on the console. The potential difference between the two plates along with the change in voltage can also be changed by increasing or decreasing the gap in between the two electrodes. The charged electrode is connected to a servo motor for upward and downward motion. The gap between the charged electrode and the belt hence can be changed from 10 mm to 55 cm.

The electric field zone consists of two interchangeable assemblies; the first setup consists of a charged plate separated by an air gap from the grounded substrate which is used when surface undulations are required, and the second setup uses a top moving electrode which touches the cast film; schematic of these setups is shown in Figs. 7 and 8.

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Figure 7. Electric field zone with a high voltage plate. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Figure 8. Electric field setup where two electrodes touch the wet cast film. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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The basic setup shown in Fig. 8 consists of two PVC rollers which are connected to a back plate which in turn sits on two rails connected to pneumatic motor used to jog the whole assembly up and down, hence controlling the distance between the rollers and the steel belt or any conducting substrate which acts as a ground. A conducting film sits on the top of these rollers; the film is tensioned using the spring loaded assembly on one of the rollers and acts as the top electrode. The rollers are also connected to a motor allowing a forward and backward motion of the conducting film at different speeds. In most cases, the speed of the conducting film on the top of the rollers is matched with the speed of the stainless steel belt below. This is done to prevent any shearing effect in the cast film. After adjusting the speed, the whole assembly is lowered to match the distance between the conducting film and the substrate to that of the thickness of the cast films. A copper roller is placed on the top of the conducting film as shown in Fig. 8. This copper roller is connected to a high voltage power supply. The copper roller is spring loaded and hence always is in contact with the conducting film creating a potential difference between the conducting film on the top and the substrate.

Scenario I: Alignment of Particles/Phases in a Photocurable Matrix

We employ a few changes in the general setup described above which includes using a transparent conducting indium tin oxide (ITO)-coated polyethylene terephthalate (PET) film (any transparent conducting film can be used) and an UV wand that radiates at the wavelength suitable for curing the resin as shown in Fig. 9. To start the process, particles are first dispersed in the photocurable resin using sonication, magnetic stirrer, high shear mixer, or other dispersion techniques; the solution is then cast using a doctor blade assembly. The blade gap is set using the micrometers or feeler gauges to the desired thickness of the films. The gap between the top electrode film (sitting on the rollers) and the conducting substrate is kept the same as that of the thickness of the cast film. The speed of the substrate and the top conducting film is then matched. As the film approaches the electric field setup, it touches the transparent conducting film on the top; the top film is maintained at a high voltage using the copper roller. After the sample orients due to the potential difference between the two electrodes, it is necessary to freeze the structure before the film reaches the end of the setup. This is done by irradiating UV light uniformly through the width of the sample using a UV wand as shown in Fig. 9; therefore, the cast film is cured before it reaches the end of the setup. Surface energy of the substrate is always kept higher than that of the transparent top electrode; this helps the film to always peel off from the top electrode making the film stick to the bottom substrate. During the rewinding of the films, these oriented films can first be peeled off and rewound separately or they can be wound with the substrate.

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Figure 9. Electric field alignment setup when using photocurable resin as the matrix. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Scenario II: Alignment of Particles/Phases in a Polymer Melt or Thermally Curable Polymers

In this case, we use a metal belt as the top electrode and we use a heating and cooling system as shown in Fig. 10. The particles are first dispersed in the thermally curable polymers, polymer solutions, or melt films using film extrusion. The polymer solutions are cast into films and are stripped of the solvent completely before entering the electric field setup. The distance between the electrodes, i.e., the top and bottom metal belts, is adjusted to be the same as that of the thickness of dried film or the wet cast thickness in the case of thermally curable polymers. The speed of the substrate and the top metal belt is maintained the same as discussed in the previous case. The heating zone is kept above the melting point of the polymer to reduce the viscous forces hence allowing faster alignment, whereas the cooling zone is kept below the Tg to prevent the relaxation of the particles after alignment and hence freezing the morphology.

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Figure 10. Electric field alignment setup when using polymer melt/thermally curable resin as matrix. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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The cast film on the substrate moves closer to the electric field setup, the heating zone is employed to melt the dried polymer film, and the melt then touches the top electrode kept at the required voltage. After the alignment takes place, the cooling zone is employed to freeze the polymer film encapsulating the resulting morphology after the alignment. Similar to the above scenario, the surface energy of the substrate is kept higher than that of the top electrode, so the film adheres to the substrate. However in the case when thermally curable polymers are used as a matrix, the heating zone helps in curing the film during electrical field application and hence freezing the morphology.

Scenario III: Alignment of Polymer Particles/Phases in Polymer Solutions

Instead of metal belt like in the last scheme, we use a perforated metal belt with small holes and a heating system beneath the substrate. The particles are dispersed in the polymer solution using sonication, magnetic stirrer, high shear mixer, or other dispersion techniques; the solution is then cast using a doctor blade assembly. The blade gap is set using the micrometers or feeler gauges to the desired thickness of the films. A sample test is carried out to gauge the final thickness of the dried film, i.e., after the solvent evaporation. The corroborated dry thickness is then used to set the gap between the top perforated metal belt and the conducting substrate. The speed of the substrate and the top electrode is then matched.

The solution is then poured in the reservoir of the doctor blade and allowed to cast a wet film on the conducting substrate. This film then approaches the electric field setup where it touches the top perforated metal electrode sitting on the rollers, the top electrode is maintained at high voltage using the copper roller. After the sample orients due to the potential difference between the two electrodes, it is necessary to freeze the structure before the film reaches the end of the setup. This is done using the continuous heating below the substrate as shown in Fig. 11. As the solvent starts to evaporate under the electric field, the morphology of the aligned particles is frozen due to film drying. Since the gap between the two electrodes was corroborated with the dry thickness of the polymer film, the two electrodes are always touching the matrix.

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Figure 11. Electric field alignment setup when using polymer solution as the matrix. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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In all the three scenarios, the speeds of the substrate and the top electrode are determined experimentally. In the case of photocurable resins and thermally curable resins, the speed should be maintained such that the samples cure before reaching the end of the setup. Similarly in the case of polymer solution, the heating rate and speed are adjusted so that the film dries before it goes out of the electric field zone. For polymer melts, speed is maintained so that the film melts before reaching the electrical field zone and cools down before reaching the end of the zone.

Zone 3—Magnetic Field Zone

The magnetic field can be used in conjugation with the electric field or as a standalone system to obtain a preferential alignment of particles susceptible to magnetic field along the z-axis or with a vertical orientation. The electromagnet is oriented in such a way that the stainless steel belt carrying the substrate along with the solution cast film lies in between the two poles of the magnet as shown in Fig. 12. The two poles are kept very close to each other to get the magnetic field line preferentially parallel to each other in the center.

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Figure 12. Depiction of the magnetic field zone on the processing line. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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The electromagnet is a GMW (Model 3474-1 40 250mm) magnet, the maximum magnetic field that can be achieved is 2.2 T with an 80 mm gap between the two poles. The gap between the two poles can be increased or decreased to change the field strength. The electromagnet has 6″ diameter poles and an SENIS 3-axis magnetic probe mounted below the stainless steel belt. The magnetic field can be controlled by changing current (%) to the magnet. A calibration curve of the magnetic field in Tesla and the current setting required to obtain required field is shown in Fig. 13. Hence depending on the magnetic susceptibilities of the particles in the film, the magnetic field can be adjusted along with the gap between the two poles of the magnet. Figure 13 shows the magnetic field distribution along the machine direction and transverse direction.

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Figure 13. 3D contour plot of magnetic field as experienced by the substrate and calibration curve to determine the magnetic field at required current settings. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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The entire magnetic assembly is mounted on a 35 mm linear rail, hence making the whole assembly moveable up to 10 ft horizontally; the UV assembly is also mounted on the same rail as shown in Fig. 14. This is done so as to use the magnetic field in conjugation with the electric field by moving it very close to electric field; the UV zone can also be moved just after the magnetic or electric field depending on where the curing of the matrix is required. The magnetic and UV assembly can also be set in motion in tandem to move back and forth along the rail.

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Figure 14. Magnetic and UV on a movable rail, both zones can move in tandem or independently. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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The solution cast film which contains magnetically susceptible particles or crystalline phases which are required to be aligned in the matrix are cast using either the doctor blade system for thick films or flow coating system for thin films. While using a doctor blade system, we obtain a wet film, hence rearrangement of particles and crystal phases is possible and the alignment can be done by directly passing the film under the magnetic field for z-orientation alignment. The speed of the belt can be adjusted to employ freezing of the solution cast wet films; however, if a photocurable resin is used as a matrix, the structure can be frozen using a UV zone, which is movable and can be placed just after the magnetic field.

However, while using wet films with the magnetically susceptible particles in magnetic field, there can be problem of aggregation of particles as depicted in Fig. 15 in the zone where the magnetic field is the strongest as can be seen in Fig. 13. Previously, researchers [56-58] have only employed a standalone magnet with uniform magnetic field; but during the continuous process, this problem causes a hindrance to our objective of creating continuous oriented films. To prevent this aggregation of particles, dried solid films can be used which can be heated in a narrow zone right in the center of the magnet, the temperature can be adjusted so as to get the matrix above Tg or Tm. The narrow heating zone is followed by a cooling zone to freeze the structure of aligned film. A schematic of the heating and cooling zones in the magnetic field is shown in Fig. 15.

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Figure 15. Schematic of (a) problem observed during continuous alignment through magnetic field and (b) heating and cooling zones inside the magnetic field zone. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Zone 4—UV Platform

The UV zone shown in Fig. 14 is used when photocurable resin matrix is employed during electric and magnetic field application. The particles in the photocurable resin after orienting through these fields can be immediately cured using UV. The UV sits on a movable frame and can be tilted back depending on use after electric field or magnetic field. It consists of two UV sources from Nordson Inc., a mercury lamp with IR filtering, and a lead lamp for wide spectral output. Since the UV zone is movable along with the magnet, their motions can be controlled using the console, hence a tandem motion can applied in which both magnet and UV move simultaneously with respect to each other.

Zone 5—Thermal Gradient Zone

The annealing zone is where we generate a thermal gradient which can be used to make directional crystals of semicrystalline polymers, block copolymers, or it can be used to align anisotropic particles by constraining the volume using gradient heating and cooling. According to theoretical considerations, it is necessary to increase the gradients to as large as possible thereby decreasing the amount of defects in the oriented single crystals or block copolymer phases; in the past, this has been achieved by using low thermal conductive substrates such as kapton, quartz, glass, etc. However, the approach has been limited to the use of conductive heating and cooling hence limiting us to a few substrates. The cast films, however, can also be heated from the top therefore increasing the gradient on the cast film. In this section, we will discuss both conductive and radiation heating on the cast films to create sinusoidal sharp gradients.

The zone annealing setup follows the electrical, magnetic, and UV zones. This is useful in the sense that the films must be solid before utilizing this zone. Hence solution casting films before reaching the thermal gradient zone can be subjected to electrical, magnetic, and UV zones. The thermal gradient zone consists of two setups: one is a conduction based setup and the other is radiation based; a schematic is shown in Fig. 16a. The intent is to create instantaneous temperature changes of up to 350°C in the hot zone and −15°C in the cold zone in a space of about 0.5″. These zones are repeated 10 times to create sinusoidal gradients. The conduction setup consists of rod heaters encapsulated by copper bars of 0.5″ wide and 10″ long; and 1″ wide and 10″ long copper bars are used as cold zone by circulating water–ethanol mixture which is cooled using a chiller. The hot and cold zone copper bars are placed one after the other with ceramic spacer between them to prevent any direct conduction of heat and to create a larger gradient. Each hot zone consists of thermocouples and these can be read and set to a required temperature at the console, each of the nine zones can be used separately using different temperatures or simultaneously. The temperature in the cold zone is controlled by a chiller, hence maintaining a same temperature in all the cold zones.

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Figure 16. (a) Schematic and pictures of (b) radiation and (c) conduction based thermal alignment zone. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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The second assembly of the thermal gradient setup which works on radiation of heat lies just above the conduction setup separated by a distance of 0.5 cm. The top assembly which works on radiation heating consists of 10 IR lamps which focus the light on hot copper bars in the bottom zone. The IR lamps are placed one after the other so as to face the hot copper bars at the bottom, but they are also separated by cooled metal plates, hence creating the required gradient. The substrate containing the solution cast dried film goes on top of the conduction setup and is separated by the radiation setup by a distance of 0.5 cm. This zone consists of pyrometers placed strategically, so as to monitor the temperatures on the surface of the cast film; each hot and cold zone consists of three pyrometers measuring temperature uniformity across the width of the film. Figure 16 shows the image of the both radiation and thermal gradient zones employed.

The two setups in the thermal gradient zones can either be used simultaneously or separately. The temperature in each of the hot zone both at the bottom and the top can be controlled separately. The ability to control the temperature in each zone helps us create a programmed gradient or a constant gradient. Figure 17 shows the ability to program required temperature gradient; in this study, a thermocouple was placed on the top of the Kapton substrate to measure the change in temperature as it passes through various zones.

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Figure 17. Examples showing (a) programmed temperature gradient and (b) constant temperature gradient on the thermal alignment zone.

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Hence, the solution cast films which have been subjected to electric or magnetic field or are using just the standalone thermal gradient setup can be used. The substrate chosen for zone annealing should be able to handle the changes in temperatures, and should have a low thermal conductivity to provide a good efficiency of the system. Polyimide substrates are considered good for this system because they can handle high temperatures as well as provide high thermal gradients due to their low thermal conductivity.

Zone 6—High Temperature Oven and Zone 7—Rewinding Zone

The solution cast films after being oriented, can be dried and annealed at high temperatures in a 20 ft long oven, which consists of underbed heaters and counterflowing hot air blower. High temperature oven consists of 44 electric heating units each 10″ long with a rating of 400 W each operating at 480 V providing uniformity throughout the heating section. The ovens are insulated with a ceramic fiber high temperature packing to prevent any heat loses. The underbed heaters are divided into four different zones and temperature of each can be adjusted separately with the maximum temperature of 250°C. Before the dried cast oriented films reach the rewinding zone, the substrate along with the steel belt goes through a release magnet, as there might be some residual magnetic field effects after the high magnetic fields of 2.2 T are applied during the magnetic field zone. After demagnetizing the steel belt, the substrate moves to a chiller table which cools the sample to room temperature as the sample is still at a high temperature after passing through the oven. Finally, the substrate moves to the rewind zone where substrate and the cast film can be collected on a single roll or the dried film can be peeled off and collected separately. The high temperature oven and rewinding zone are shown in Fig. 18.

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Figure 18. High temperature ovens for annealing and drying the solvent cast films and rewind section to collect the dried films. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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VALIDATION EXPERIMENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. DESIGN AND FABRICATION OF ROLL-TO-ROLL PROCESSING LINE FOR FIELD ASSISTED ASSEMBLY
  5. VALIDATION EXPERIMENTS
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. REFERENCES

In order to test the capabilities of the electromagnetic processing line and its applicability as a roll-to-roll process to generate vertically oriented arrays of particles and to create single crystals, three different experiments were carried out.

Electric Field Alignment of Clay Platelets

To create roll-to-roll oriented particles in a polymer film in electric field zone, a mixture of photocurable resin and particles was used and experiments were performed as described above in scenario I of the electric field setup. The experiment used 6 wt% Cloisite 30B organo-modified clay (provided by Southern Clay products) which was dispersed in a photocurable resin Norland 65 (obtained from Norland products). The clay particles were dispersed in resin using sonication and high shear mixing. The solution was cast using the 3″ wide doctor blade assembly keeping a 500-μm blade gap; the same gap was used to set the distance between the top and bottom moving electrodes. Metal plate was used as the bottom substrate and a transparent ITO-coated PET film was used as the top substrate. Both the bottom substrate and the top electrode were kept at a constant speed of 5 cm/min. After casting and reaching, the electric field setup the wet film was allowed to touch the top ITO-coated PET electrode which was kept at 350 V, hence providing a potential difference between the top and bottom electrodes of 700 V/mm. The alignment takes place within a couple of seconds at this voltage in this system, this was measured externally. Hence after the alignment, the morphology of the clay platelets was frozen using the UV wand which was irradiated uniformly throughout the width of the sample. Since a metal substrate has high surface energy compared to ITO-coated PET electrode, the film peels off from the top electrode while still maintaining the conductivity of the ITO-coated PET. The film was later peeled off from the bottom of electrode for characterization.

As can be seen from the X-ray diffraction (XRD) patterns of all the three surfaces as shown in Fig. 19, the X-ray diffraction patterns taken in all the three directions exhibit isotropy in (001) planes indicating random orientation of the clay particles. Application of e-field in ND (z-direction) leads to rapid orientation of the clay particle planes in the thickness (ND) direction. As a result, the WAXS patterns obtained with beam along ND exhibit complete randomness whereas the other two taken in MD and TD exhibit orientation (orientation factor S = −0.32). This is typically what is observed in oriented fibers. Hence, these samples exhibit fiber symmetry or transverse isotropy with the fiber (symmetry) axis in this case oriented in the normal direction.

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Figure 19. XRD patterns taken with X-ray beam along MD, TD, and ND. Here the applied field is in z (ND) direction and schematic of the aligned clay platelets based on the XRD patterns. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Magnetic Field Alignment of Nickel Nanowires

To validate the z-direction alignment using magnetic field, we use Nickel Nanowires (obtained from PlasamaChem) with average width and length of 200 nm and 200 μm, respectively. Nickel nanowires were dispersed in toluene (obtained from Sigma–Aldrich) through sonication. A 25 wt% polystyrene 685D (obtained from Americas Styrenics) solution in toluene was used as the matrix system and the nickel nanowires were added to the solution to obtain a final concentration of 0.5 wt% of the nanowires with respect to polymer. The solution was then cast using a 3″ doctor blade assembly on the PET substrate. The substrate carrying the wet solution was allowed to move to the magnetic field where a 2T magnetic field was applied, the solution was dried inside the magnetic field zone. This was done so as to prevent any relaxation of the particles while the film is coming out of the magnetic field. The oriented structure of the nickel nanowires in the polystyrene matrix was characterized using SEM and is shown in Fig. 20. As can be seen nickel nanowires align in the direction parallel to magnetic field, which is through the thickness direction. We also observe protruding nanowires out of the cast film towards the magnet. Hence, we can use the magnetic field to create not only oriented films but also patterned structures on the surface of the films by controlling the duration of the applied magnetic flux.

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Figure 20. SEM images of oriented pillars of nanowires on the surface and aligned nanowires in the matrix of the He polymer film under applied magnetic field. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Directional Crystallization using Thermal Gradient

Directional crystals of poly(ethylene oxide) were obtained using thermal gradient or zone solidification. 1 wt% polyethylene oxide (PEO) with Mw of 400,000 (obtained from Scientific Polymer Products Inc.) was first dissolved in dichloromethane (obtained from Sigma–Aldrich). The solution was then cast on glass substrate using flow coater to obtain a thin film with dry thickness of 1 μm; the film was 7 cm wide and 25 cm long. The sample was then subjected to thermal gradient zone; in this example, we only use the conduction heating. Only one zone was utilized instead of all the nine available zones to create gradient. The hot zone was kept at 130°C to obtain 110°C on the film surface and the cold zones were kept at 9°C; the profile of temperature gradient on the top of the glass substrate is shown in Fig. 21a. The hot zone temperature was selected such that it was much higher than the melting point of PEO (Tm = 63°C). The film was then set in motion at a speed of 25 μm/s with a calculated moving gradient of 60°C/cm.

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Figure 21. (a) Temperature gradient observed on top of the substrate and (b) uniaxial crystallization of PEO crystals through directional solidification observed under cross polarizers with lambda plate. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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After directional crystallization, the sample was characterized using polarized optical microscope with lambda plate, and a composite of multiple images is shown in Fig. 21b. We observe elongated crystals with extended sectors of spherulites which start from the nucleation points. The elongated crystals were easily visible by naked eye, and some of the elongated crystals ran through the whole length of the sample.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. DESIGN AND FABRICATION OF ROLL-TO-ROLL PROCESSING LINE FOR FIELD ASSISTED ASSEMBLY
  5. VALIDATION EXPERIMENTS
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. REFERENCES

The novel roll-to-roll process is designed to produce continuous high value added nanostructured products with the aid of several external fields. These include use of electric field, magnetic field, and thermal gradient to promote defect free “z-direction” oriented flexible films, which can be achieved by using one or combination of different capabilities built into this roll-to-roll processing line. Hence using the roll-to-roll capability, we can make flexible, transparent products which can lead to supercapacitors, heat spreaders, gas and water separation membranes, fuel cell membranes, thermal insulators, electrical insulation, fuel cells, etc. as these applications require enhanced through thickness properties which can be achieved by alignment of particles or phases in a polymer matrix.

We also show experimental results validating the use of these external fields in a roll-to-roll process, by orienting clay platelets using electric field, nickel nanowires using magnetic field, and by creating uniaxial oriented crystals of PEO using thermal gradient fields.

ACKNOWLEDGMENT

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. DESIGN AND FABRICATION OF ROLL-TO-ROLL PROCESSING LINE FOR FIELD ASSISTED ASSEMBLY
  5. VALIDATION EXPERIMENTS
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. REFERENCES

The design and construction of this roll-to-roll electromagnetic line was made possible by Ohio Third frontier funding Wright Centers program under CMPND.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. DESIGN AND FABRICATION OF ROLL-TO-ROLL PROCESSING LINE FOR FIELD ASSISTED ASSEMBLY
  5. VALIDATION EXPERIMENTS
  6. CONCLUSIONS
  7. ACKNOWLEDGMENT
  8. REFERENCES