The present disclosure relates to antistatic polymers for electronic circuitry.
Beyond kids' play at electrifying balloons and “static cling,” accumulation of static electricity on polymers is a serious technological problem responsible for shocks, explosions, as well as damage of satellites and other electronic equipment measured in billions of dollars per annum. Despite centuries of research, it is not known why certain polymers charge more than others and, most importantly, how to design polymers that would resist static electricity and could potentially act as general-purpose antistatic coatings avoiding the limitations of currently used materials.
Strategies of controlling static electricity and electrostatic discharge are known in the art. In the existing approaches, specialized materials such as ionic conductors, carbon or metal-filled resins, or conducting polymers are used to increase the humidity or conductivity of antistatic coatings resulting in charge dissipation (M. Angelopoulos, IBM J. Res. Dev. 45, 57 (2001); M. Tolinski, Additives for Polyolefins: Getting the Most out of Polypropylene, Polyethylene and TPO (Elsevier, Oxford, 2009), pp 79-91). These “traditional antistatics” all have their limitations (e.g., ionic conductors are moisture sensitive, resins require high levels of loading that alter material properties) which become even more prominent with the ever-decreasing sizes of the circuits. Their resistivities are typically 1019-1012 W/sq., relatively close to 109 W/sq, which is the “classified electrostatic discharge (ESD) level” of resistivity required to prevent the damage of sensitive electronics (M. Tolinski, Additives for Polyolefins: Getting the Most out of Polypropylene, Polyethylene and TPO (Elsevier, Oxford, 2009), pp 79-91).
Electrification of polymers due to physical contact leads to spatially heterogeneous transfer of charge and material, as well as the creation of mechanoradicals upon homolytic bond cleavage. See, for examples, the works of H. T. Baytekin et al. Science 333:308 (2011); H. T. Baytekin et al. Angew. Chem. Int. Ed. 51:4843 (2012); B. Baytekin, H. T. Baytekin, & B. A. Grzybowski, J. Am. Chem. Soc. 134:7223 (2012); W. R. Salaneck & A. Paton, J. Appl. Phys. 47:144 (1976); T. A. L. Burgo et al. Langmuir 28:7407 (2012); and M. Williams, AIP Adv. 2:010701 (2012).
In a first aspect, an anti-static polymer composition is provided. The composition includes a polymer and a free radical scavenger additive.
In a second aspect, an electrostatic sensitive device having reduced propensity to retain static electricity is provided. The device includes the electrostatic sensitive device and an anti-static polymer composition. The anti-static polymer composition includes a polymer and a free radical scavenger additive.
In a third aspect, a method of reducing the propensity of a polymer to retain static electricity upon electrification of the polymer is provided. The method includes the step of doping the polymer with a free radical scavenger additive.
These and other features, objects and advantages of the present invention will become better understood from the description that follows. In the description, reference is made to the accompanying drawings, which form a part hereof and in which there is shown by way of illustration, not limitation, embodiments of the invention.
The features, objects and advantages other than those set forth above will become more readily apparent when consideration is given to the detailed description below. Such detailed description makes reference to the following drawings.
FIG. IF depicts a representative KFM image of surface charges on contact-charged surfaces an acrylate polymer (Scotch™ tape) as in
While the present invention is amenable to various modifications and alternative forms, exemplary embodiments thereof are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description of exemplary embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the embodiments above and the claims below. Reference should therefore be made to the embodiments and claims herein for interpreting the scope of the invention.
The compositions and methods now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all permutations and variations of embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. These embodiments are provided in sufficient written detail to describe and enable one skilled in the art to make and use the invention, along with disclosure of the best mode for practicing the invention, as defined by the claims and equivalents thereof.
Likewise, many modifications and other embodiments of the compositions and methods described herein will come to mind to one of skill in the art to which the invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of skill in the art to which the invention pertains. Although any methods and materials similar to or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein.
Moreover, reference to an element by the indefinite article “a” or “an” does not exclude the possibility that more than one element is present, unless the context clearly requires that there be one and only one element. The indefinite article “a” or “an” thus usually means “at least one.”
As used herein, “about” means within a statistically meaningful range of a value or values such as a stated concentration, length, molecular weight, pH, sequence identity, time frame, temperature or volume. Such a value or range can be within an order of magnitude, typically within 20%, more typically within 10%, and even more typically within 5% of a given value or range. The allowable variation encompassed by “about” will depend upon the particular system under study, and can be readily appreciated by one of skill in the art.
As used herein, “doping” refers to disposing a substance or composition on a surface by any means known in the art, including dipping, brushing, flow-coating, screen-printing, slot-die coating, gravure coating, powder coating, spraying and spin-coating.
Ranges recited herein include the defined boundary numerical values as well as sub-ranges encompassing any non-recited numerical values within the recited range. For example, a range from about 0.01 mM to about 10.0 mM includes both 0.01 mM and 10.0 mM. Non-recited numerical values within this exemplary recited range also contemplated include, for example, 0.05 mM, 0.10 mM, 0.20 mM, 0.51 mM, 1.0 mM, 1.75 mM, 2.5 mM 5.0 mM, 6.0 mM, 7.5 mM, 8.0 mM, 9.0 mM, 9.9 mM, among others. Exemplary sub-ranges within this exemplary range include from about 0.01 mM to about 5.0 mM; from about 0.1 mM to about 2.5 mM; from about 2.0 mM to about 6.0 mM, among others.
Formulations and coatings are disclosed for doping polymers with small amounts of free-radical scavengers to reduce static development and to prevent static charging. The present inventors discovered that electric charges and the radicals co-localize to the same nanoscopic regions of electrified polymers. The presence of radicals stabilizes the charged species; conversely, the removal of radicals destabilizes charge domains and results in a rapid discharge of the electrified material. The disclosed formulations transform various common polymers into robust antistatic coatings to protect semiconductor devices from failure caused by the build-up of static electricity.
Various types of polymers were electrified either by contact charging them against other materials or by exposing to a corona discharge from an electrostatic gun. Exemplary polymers that were evaluated include acrylate-based adhesive Scotch™ tape, poly(dimethyl siloxane) (PDMS), poly(methyl methacrylate) (PMMA), polycarbonate (PC) and polystyrene (PS). These charged surfaces were examined by several atomic force microscopy (AFM) modalities on a BrukerDimension Icon system. Kelvin Force Microscopy (KFM) was used to visualize surface charge distributions, while Magnetic Force Microscopy (MFM) visualized the distribution of radicals. In addition, conventional AFM was used to visualize surface topography while AFM phase and AFM PeakForce Quantitative Nanomechanical Property Mapping® (PF-QNM) modes provided quantitative elastic property mapping of the surfaces at the nanoscopic level and allowed identification of regions where material was transferred (if electrification involved physical contact).
We considered whether charge-radical co-localization applies to polymers charged not only by contact but also by discharge. Charging by electrostatic/corona discharge from an electrostatic gun does not involve polymer transfer but produces charged species of both polarities as well as radicals. The KFM analysis of the polymers electrified by corona discharge reveals charged regions (of polarity dependent on gun squeezing/releasing, see
The co-localization of surface charges and radicals suggests a possible interplay between these species. A series of charging experiments were performed with native polymers as well as polymers doped with small amounts of chemical substances scavenging the radicals such as (±)-α-tocopherol (vitamin E), bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate (HALS), or 2,2-diphenyl-1-picrylhydrazyl (DPPH) (
Data in
The dopant 2,2-diphenyl-1-picryl hydrazine (“DPPH-H”) differs from the DPPH scavenger by only one hydrogen atom. Despite the close structural similarity, this molecule is not able to scavenge radicals. Importantly, when the PDMS was doped with DPPH-H, its charging propensity was indistinguishable from pure PDMS (
The role of radicals is to stabilize the charges. This is evidenced by experiments illustrated in
Scanning electron microscopy (SEM) charging experiments were performed with pure PDMS and PDMS/DPPH films to investigate charge dissipation kinetics. When pure PDMS is charged under SEM, the region charged by electrons persists for minutes (
To further explore the kinetic relationship of radical species and charged species life-times, AFM, MFM and KFM imaging of surfaces during charge decay was performed. AFM phase detecting changes in material organization on the surface is a more sensitive method of visualizing material transfer during contact-charging than AFM-height (compare FIGS. 5A,B). FIGS. 5C,D illustrate that the radicals visualized by MFM have significantly longer life-times than the charged species imaged by KFM. Although almost all charge decayed after 24 hrs, as measured by KFM (
Thus, discharge kinetics is related to the presence or absence of radicals. The MFM maps accompanying the discharge curves in
The radicals that form as a result of homolytic bond cleavage during electrification are mostly peroxy radicals, ROO. known for their relatively high stability due to resonance stabilization. Without the invention being bound or limited by any particular theory of operation, the charge stabilization can be rationalized, at least, qualitatively, on the basis of molecular orbital theory whereby the singly-occupied molecular orbitals (SOMO) of the radicalic species interact with the empty orbitals of cationic species (
Alternative explanations based on the dopants increasing conductivity of the polymers do not apply. First, surface resistivities for both undoped and doped polymers are on the order of 1016 W/sq well above 1010-1012 W/sq for antistatic materials. Second, there is no increased ability of the doped polymers to condense surface water. The contact angles do not change significantly upon doping, e.g., 106.5°±1.3° for pure PDMS and 107.0°±0.8° for PDMS/0.5×10−2 M DPPH). In this context, the observed trends remain unchanged in a water free-atmosphere, under dry Ar.
The practical importance of the charge-radical interplay described above is in the ability to engineer antistatic materials by simply doping common insulating polymers with small amounts of radical scavengers.
The ability of the disclosed antistatic polymer composition to effect charge-radical regulation is illustrated in two exemplary electronic circuit experiments (
Referring to
Since similar effects are observed with other types of polymers and scavengers studied including notably, the edible and biocompatible vitamin E. These results provide a general, technically straightforward and environmentally “green” way of protecting electronics of various types from the untoward effects of static electricity.
In view of the foregoing, disclosed herein as a first aspect of the invention is an anti-static polymer composition. The composition includes a polymer and a free radical scavenger additive. In a first respect, the polymer is selected from a group consisting of poly(dimethyl siloxane), poly(methyl methacrylate), polycarbonate, polystyrene, styrene-butadiene-styrene, styrene-ethylene/butylene-styrene, styrene-ethylenepropylene, styrene-isoprene-styrene, polyacrylate, polypropylene, butyl rubber, natural rubber, silicone rubber, ethylene-vinyl acetate, polyvinylether and nitrile. In a second respect, the polymer comprises an acrylate-based polymer. In a third respect, the polymer comprises a composition having the propensity to charge, retain or discharge static electricity. In a fourth respect, the free radical scavenger additive is selected from a group consisting of (±)-α-tocopherol, bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate, 2,2-diphenyl-1-picrylhydrazyl, curcumin, ascorbic acid, and β-carotene. In a fifth respect, the free radical scavenger additive contacts a surface of the polymer. In a sixth respect, the free radical scavenger additive is disposed on a surface of the polymer by dipping, brushing, flow-coating, screen-printing, slot-die coating, gravure coating, powder coating, spraying and spin-coating. In a seventh respect, the free radical scavenger additive is present at a final concentration of at least about 0.01 mM. In an eighth respect, the free radical scavenger additive is present at a final concentration in the range from about 0.01 mM to about 10.0 mM, including any sub-range having numerical values of concentration within this range.
In a second aspect of the invention, an electrostatic sensitive device having reduced propensity to retain static electricity is disclosed. The device includes the electrostatic sensitive device and an anti-static polymer composition. The anti-static polymer composition includes a polymer and a free radical scavenger additive. In a first respect, the anti-static polymer composition contacts a surface of the electrostatic sensitive device. In a second respect, the anti-static polymer composition is disposed on a surface of the electrostatic sensitive device by dipping, brushing, flow-coating, screen-printing, slot-die coating, gravure coating, powder coating, spraying and spin-coating. In a third respect, the polymer is selected from a group consisting of poly(dimethyl siloxane), poly(methyl methacrylate), polycarbonate, polystyrene, styrene-butadiene-styrene, styrene-ethylenebutylene-styrene, styrene-ethylenepropylene, styrene-isoprene-styrene, polyacrylate, polypropylene, butyl rubber, natural rubber, silicone rubber, ethylene-vinyl acetate, polyvinylether and nitrile. In a fourth respect, the free radical scavenger additive is selected from a group consisting of (±)-α-tocopherol, bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate, 2,2-diphenyl-1-picrylhydrazyl, curcumin, ascorbic acid, and β-carotene.
In a third aspect of the invention, a method of reducing the propensity of a polymer to retain static electricity upon electrification of the polymer is disclosed. The method includes the step of doping the polymer with a free radical scavenger additive. In a first respect, doping the polymer with a free radical scavenger additive includes disposing the free radical scavenger additive on a surface of the polymer by dipping, brushing, flow-coating, screen-printing, slot-die coating, gravure coating, powder coating, spraying and spin-coating a solution comprising the free radical scavenger additive on the surface of the polymer. In a further elaboration of the first respect, the additional step of drying the solution comprising the free radical scavenger additive on the surface of the polymer is provided. In a second respect, the polymer is selected from a group consisting of poly(dimethyl siloxane), poly(methyl methacrylate), polycarbonate, polystyrene, styrene-butadiene-styrene, styrene-ethylenebutylene-styrene, styrene-ethylenepropylene, styrene-isoprene-styrene, polyacrylate, polypropylene, butyl rubber, natural rubber, silicone rubber, ethylene-vinyl acetate, polyvinylether and nitrile. In a third respect, the free radical scavenger additive is selected from a group consisting of (±)-α-tocopherol, bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate, 2,2-diphenyl-1-picrylhydrazyl, curcumin, ascorbic acid, and β-carotene. In a fourth respect, the free radical scavenger additive is present at a final concentration in the range from about 0.01 mM to about 10.0 mM.
The invention will be more fully understood upon consideration of the following non-limiting examples, which are offered for purposes of illustration, not limitation.
Materials
Poly(dimethylsiloxane) (PDMS) was prepared by mixing a degassed elastomer base and a crosslinker in a 10:1 w/w ratio (Sylgard 184, Dow Corning). Prepolymer mixture was cast on an atomically flat silicon wafer (Montco Silicon Technologies, Inc.), silanized with 1H,1H,2H,2H-perfluorooctyltrichlorosilane, and cured at 65° C. for 24 h. After curing the prepolymer, the PDMS pieces (ca. 1 cm×1 cm×0.5 cm) were gently peeled off the wafer, washed with 3×1 L of dichloromethane for 24 hrs (to remove catalyst and unreacted monomers) and thoroughly dried prior to the charging experiments.
Instrumentation
All surface images were obtained on a Bruker Dimension Icon AFM microscope. For quantitative measurement of mechanical properties, PeakForce QNM software was used which gave DMT moduli data as direct maps, using PDMS standard of 6.0 MPa. SNL-10 tips (spring constant, k=0.35 Nm) were used in the QNM measurements. For potential (KFM) measurements, conductive SCM-PIT tips (resonance frequency, f=70 kHz) were used, lift height was kept at 100 nm during the potential scan. For magnetic force (MFM) measurements, conductive MESP tips (resonance frequency, f=75 kHz) were used, and lift height was kept at 100 nm during imaging. XPS spectra were recorded on an XPS spectrometer, ESCA Lab 250 that was equipped with EA125 energy analyzer. Photoemission was stimulated by a monochromatic Al κ alpha radiation (1486.6 eV) with the operating power of 300 W. Survey scans and high-resolution scans were collected using pass energies of 70 eV and 26 eV, respectively. Analyzer substrate angle was 45°. Binding energies in the spectra were referenced to the C1s binding energy set at 285 eV. At least three different measurements were performed for each sample. SEM images were taken on a LEO Gemini 1525.
Cross-Correlation Image Analysis.
The mutual localization of charges (visualized by KFM), radicals (by MFM), and transferred material (by AFM-phase) over the same scan domain was quantified by calculating the cross-correlation function defined as:
Where w and f denote the images being compared and displaced by (x,y) with respect to each other,
In some of the contact-charging experiments, a commercial Scotch™ tape from 3M was used. While results were qualitatively similar for all other pairs of polymers we tested, the Scotch™ tape is a convenient model system since the surfaces to be separated are initially in conformal contact (thus eliminating any artifacts related to contact imperfections), and material transfer can be easily quantified. When the tape was peeled off (with typical speed ˜1 cm/sec), it developed surface charge density on the order of 1 nC/cm2 (measured by a house-made Faraday cup & cage system).
PDMS pieces prepared according to the Materials and Methods section above were charged by ten “shots” from an ion gun (Zerostat, Sigma Aldrich) held 1 cm away from the surface of the polymers: pulling the trigger of the gun produced positively charged ions (and positive charges on the polymer's surface) while releasing the trigger generated negative ions and negative charges on the polymer.
A series of contact charging experiments were performed with native polymers as well as polymers doped with chemical substances scavenging the radicals. In particular, in the studies described in
PDMS pieces prepared as described previously were immersed into 0.5×10−2 M and 1.0×10−3M solutions of radical scavengers, e.g., DPPH, in dichloromethane. The swollen polymer pieces were first dried in air and then under high vacuum for 48-96 hrs prior to experiments. To ascertain that no residual solvent had any effect on the charging, control experiments were also performed with (i) pure PDMS, and (ii) pure PDMS soaked in dichloromethane and then dried as described above. No differences in the charging characteristics of these two “types” of pure PDMS were observed.
Prior to charging, PDMS pieces were left to discharge for at least 24 hrs under argon. The electroneutrality (i.e., lack of any detectable charge) of these pieces was confirmed in two ways (1) by measurements using a house-made Faraday cup connected to a high precision electrometer (Keithley Instruments, model 6517B). Only pieces with net charge densities below the electrometer's detection limit <±0.005 nC/cm2, were considered to be neutral (vs. densities above 0.1-0.2 nC/cm2 after charging) and used in further experiments; (2) by Kelvin Force Microscopy (KFM) potential imaging; here neutrality was assumed if the highest potential on the scanned surface did not exceed 10 mV (vs. >500 mV for electrified pieces). For images of PDMS prior to electrification, see FIG. IA-C.
Quantification of Charging.
The overall/net charges on the macroscopic pieces of native and doped PDMS pieces developed during charging were measured using a house-made Faraday cup connected to a high precision electrometer (Keithley Instruments, model 6517B).
Surface Resistivity Measurements.
Surface resistivities of PDMS, and DPPH-PDMS were measured using a two-probe method, with w=4.26 mm wide samples, and the distance between electrodes d=100 mm. I-V curves were collected on a Keithley electrometer (6517B) that served as the voltage source and also measured the generated current. Applied voltage was changed from 0 to 100 V in steps of 10 V and also from 0 to −100 V in steps of 10 V, which gave identical results in terms of conductivity. Form the slopes of the I-V curves, the values were calculated for surface resistivity, Rs, according to equation Rs=(V/I)·(w/d). For PDMS, DPPH-PDMS (1 mM and 5 mM) the measured surface resistivities were determined to be ca. 1016 ohm/sq.
0.4 g, 1.0 mmol of DPPH was dissolved in argon-purged THF (100 mL). With stirring, H2O2 (30% w/w, 20 eq.) was introduced into this solution dropwise at room temperature. After further stirring for two days, by which time the dark violet color of the solution turned to red-brown, the solution was washed with 3×100 mL of water, dried over MgSO4, and then evaporated under high vacuum. The remaining solid was purified by column chromatography (using dichloromethane on silica gel). The first band (dark red) was collected to give the final product in 90% yield (0.36 g). 1H NMR (400 MHz, CDCl3) δ0.12 (s, 1H), 9.22 (s, 1H), 8.52 (s, 1H), 7.35 (t, J=8.04 Hz, 4H), 7.21 (t, J=7.75 Hz, Hz, 2H), 7.11 (d, J=7.75 Hz, 4H); ESI-MS (neg) me: [M-H]−1 calcd. for C18H12N5O6− 394.08, found 393.95.
Polystyrene (PS) spheres (1.6 mm) were purchased from Engineering Laboratories Inc. (cat. #BL00625STNA2CC). Polystyrene dishes (35 mm×10 mm) were from VWR (cat. #25382-064). Prior to experiments, both materials were carefully washed with ethanol and dried at 40° C. for several hours. To avoid contamination with dust etc., all subsequent manipulations/procedures were performed in a glove box. The initial charges on the beads (<10 pC) were measured using a house-made Faraday cup connected to a Keithley 6517 electrometer. The beads were placed into a dish, which was affixed to a computer-controlled linear motor (LinMot, P01-23×160). The shaking parameters used in the experiments were 90 sec shaking time, 10 Hz shaking frequency and 25 beads in each dish. After a predetermined shaking time, the beads were carefully removed from the dish one by one, and their charges were measured in a Faraday cup.
To dope with DPPH, the beads and dishes were immersed into a 1 mM solution of DPPH in propylene glycol monomethyl ether acetate and left for 5 min to allow DPPH to soak into the polymer. The polymer pieces were then removed from the solution, carefully dried in air and then under vacuum for 48-96 hours. Lack of any residual solvent effects on charging was ascertained by comparing (i) pure beads; (ii) pure beads soaked in propylene glycol monomethyl ether acetate and then dried.
All of the patents, patent applications, patent application publications and other publications recited herein are hereby incorporated by reference as if set forth in their entirety.
The present invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. However, the invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. Accordingly, one of skill in the art will realize that the invention is intended to encompass all modifications and alternative arrangements within the spirit and scope of the invention as set forth in the appended claims.
This application claims benefit of priority under 35 U.S.C. 119 to U.S. provisional patent application Ser. No. 61/879,607, filed Sep. 18, 2013, and entitled “Control of Surface Charges by Radical Scavengers and Antioxidants as a Principle of Antistatic Polymers Protecting Electronic Circuitry,” the content of which is herein incorporated by reference in its entirety.
This invention was made with government support under DE-SC0000989 awarded by the Department of Energy. The government has certain rights in the invention.
Number | Date | Country | |
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61879607 | Sep 2013 | US |