Methods for making improved quantum dot resin formulations

Abstract

The present invention provides methods for making polymerizable monomer compositions comprising purifying a (b) monomer mixture of (i) one or more monomers having at least two polymerizable vinyl groups and (ii) one or more monomers having a single polymerizable vinyl group as part of a (meth)acrylate ester group by any one or more of treating the monomer mixture in an activated porous alumina or silica column, sieve drying the monomer mixture in a vacuum followed by drying over dried molecular sieves having average pore sizes of from 2 to 20 Angstroms, freeze-pump-thaw (FPT) treating by freezing the monomer mixture in a vessel or container to a temperature below 75 C., degassing the monomer mixture by application of vacuum in the range of 10.sup.2 to 10.sup.2 Pa, sealing the vessel or container under vacuum, and thawing the composition to room temperature; and, combining in an inert gas atmosphere the resulting monomer mixture (b) with a composition (a) of quantum dots in dry form or organic solvent solution.

Claims

1. A method for making polymerizable monomer compositions comprising: Purifying a (b) monomer mixture of (i) one or more monomers having at least two polymerizable vinyl groups as part of a (meth)acrylate ester group or attached directly to an aromatic ring or a cycloaliphatic group and (ii) one or more monomers having a single polymerizable vinyl group as part of a (meth)acrylate ester group in which the ester contains a cycloaliphatic group or a C.sub.6 to C.sub.24 alkyl group by any one or more of treating the monomer mixture in an activated porous alumina or silica column having, for example, a surface area of from 100 to 300 m.sup.2/g; sieve drying the monomer mixture in a vacuum followed by drying over dried molecular sieves having average pore sizes of from 2 to 20 Angstroms; freeze-pump-thaw (FPT) treating by freezing the monomer mixture in a vessel or container to a temperature below 75 C., degassing the monomer mixture by application of vacuum in the range of 10.sup.2 to 10.sup.2 Pa), sealing the vessel or container under vacuum, and thawing the composition to room temperature storing the purified monomer mixture (b) in a dry and inert gas; and, combining in an inert atmosphere the resulting monomer mixture (b) with a composition (a) of quantum dots in dry form or in organic solvent solution to make a polymerizable monomer composition that comprises 150 ppm or less of H.sub.2O, 100 ppm or less of total polymerization inhibitor compounds and 150 ppm or less of organic radically active molecules.

2. The method as claimed in claim 1 in which the purifying of the monomer mixture (b) comprises freeze-pump-thaw (FPT) treating, followed by drying the resulting treated monomer mixture over a dried molecular sieve.

3. The method as claimed in claim 1 in which the purifying of the monomer mixture (b) comprises degassing the monomer mixture under a vacuum of from 10.sup.2 to 10.sup.2 Pa and then treating it in an activated alumina column packed into a disposable polypropylene column.

4. The method as claimed in claim 1 in which the purifying of the monomer mixture (b) comprises degassing the monomer mixture under vacuum of from 10.sup.2 to 10.sup.2 Pa and then drying it by exposure to or by mixing with molecular sieves which have been dehydrated at a temperature of from 75 to 200 C. for a period of 4 to 24 hours and kept in an inert atmosphere.

5. The method as claimed in claim 1 in which in the combining of the monomer mixture (b) and the composition (a) of quantum dots, the (a) quantum dots are dried under vacuum to remove any organic solvent before the combining or the polymerizable monomer composition together with the (a) quantum dots is dried under vacuum to remove any organic solvent.

6. The method as claimed in claim 1 in which in the combining of the monomer mixture (b) and the composition (a) of quantum dots, the quantum dots comprise cadmium-free quantum dots.

7. The method as claimed in claim 6 in which the composition (a) of quantum dots comprises core shell cadmium-free quantum dots that have a core shell structure wherein the shell material has a wider band gap than and a small lattice mismatch to the core material.

8. The method as claimed in claim 1 in which the monomer mixture (b) comprises (i) divinyl benzene, tricyclodecane dimethanol diacrylate, isobornyl dimethacrylate, or mixtures thereof and (b)(ii) one or more monomers having a single polymerizable vinyl group as part of a (meth)acrylate ester group in which the ester contains a cycloaliphatic group or a C.sub.6 to C.sub.24 alkyl group.

9. The method as claimed in claim 1 in which the monomer mixture (b) comprises (i) one or more monomers having at least two polymerizable vinyl groups as part of a (meth)acrylate ester group or attached directly to an aromatic ring or a cycloaliphatic group and (b)(ii) isobornyl acrylate (IBOA).

10. The method as claimed in claim 1 wherein the polymerizable monomer composition comprises 150 ppm or less of H.sub.2O as determined by Karl Fisher Titration, and comprises 75 ppm or less of total dissolved oxygen as determined by a photochemical method as described in the reference (J. Polym. Sci. A: Polym. Chem. 2004, volume 42, pages 1285-1292).

Description

EXAMPLES

(1) The following examples illustrate the present invention. Unless otherwise indicated, all units of temperature are room temperature and all units of pressure are standard pressure or 101 kPa.

(2) The following test methods were used:

(3) Film thicknesses were determined by measurement of the cured films with a micrometer and then subtracting out the thickness of any barrier film thickness.

(4) Photoluminescent Quantum Yield (PLQY)

(5) In handling both liquid and films, photoluminescent Quantum Yield (PLQY), was measured with a Quantaurus-QY Absolute PL quantum yield spectrometer (C.sub.11347-01 model) (Hamamatsu Photonics KK, Hamamatsu City, Japan). For each reported example, a total of three (3) measurements were taken from three (3) randomly selected points in each indicated analyte substrate and the indicated PLQY represents an average of the measurements taken.

(6) Full-width half-max of the emission peak (FWHM) was measured with a Quantaurus-QY Absolute PL quantum yield spectrometer (C11347-01 model) integrating sphere (Hamamatsu Photonics KK).

(7) Film thicknesses were determined by measurement of the cured films with a micrometer and then subtraction of the barrier film thickness.

(8) Peak emission wavelength (PWL) was determined using Quantaurus-QY Absolute PL quantum yield spectrometer (C11347-01 model, Hamamatsu Photonics KK); for green QD in the examples, below, the target wavelength is from 520 to 540 nm; for red QD, the target wavelength is from 620 to 640 nm.

(9) Karl-Fischer Titration:

(10) Water content of monomers was analyzed by a Metrohm model 831 coulometric KARL FISCHER titrator (Metrohm Ltd, Herisau, Switzerland), calibrated as set forth in the manufacturer's literature and 703 Ti stand (Metrohm).

(11) Monomer Purification Methods:

(12) The following methods were used, as indicated, in the Examples that follow:

(13) Freeze Pump Thaw (FPT):

(14) The indicated monomer mixture was loaded into 20 mL scintillation vials. If sieves were used to dry the monomers, the sieves were added to the vial, and then the vial was fitted with a septum. The vials were submerged into liquid N.sub.2 until the monomer had frozen (freeze). Once frozen, vacuum (1-0.1 Pa) was pulled to evacuate the headspace using a needle fitting on a Schlenk line (pump). The monomers were allowed to thaw under static vacuum until completely melted. Freeze-pump-thaw cycles were performed 3 times or until bubbles were no longer visible on the thaw cycles. Freeze-pump-thaw (FPT) purified monomer vials were backfilled with N.sub.2, and then brought into an N.sub.2 filled glovebox for subsequent handling. FPT monomers were used within 48 hours of purification.

(15) Column Purification:

(16) Activated Aluminum oxide (pH 4.5-9.5 in water, Aldrich, St. Louis, Mo.) was dried at 80 C. in vacuum for 4 h, and then packed into a disposable polypropylene column. The indicated monomer(s) was/were degassed in a vacuum oven (0.1-1 Pa) for 1 hour and then passed through the column slowly. Columns were run in inert atmosphere and purified monomers were stored in a nitrogen purge box. Purified monomers were used within 48 hours of purification.

(17) Sieve Drying:

(18) In the indicated Example, zeolite molecular sieves (4 average particle size) were used to dry monomers. Sieves were dehydrated in a 110 C. oven overnight and loaded into a N.sub.2 filled glovebox while hot. Monomers were dried. Sieves were added to the monomers in the N.sub.2 filled glovebox, were gently shaken, and then allowed to sit at room temperature for several hours prior to use. Sieve dried monomers were used within 48 hours of purification.

(19) Abbreviations used in Examples:

(20) IBOA is isobornyl acrylate; SR833 is tricyclo[5.2.1.0.sup.2,6]decane dimethanol diacrylate; I-819 and I-651 are IRGACURE photoactive polymerization initiators (BASF AG, Leverkusen, DE); Finex zinc oxide particles (Sakai Chemical Industry co., LTD., Japan); and CFQD stands for cadmium free quantum dots. Green CFQD comprise core-shell QDs having Indium containing cores and exhibit an 73.9% QY, 44.1 nm FWHM, and a 534.4 nm PWL (at absorbance=0.3); Red CFQD comprise core-shell QDs that have indium containing cores and exhibit an 85% QY, 52.8 nm FWHM, and a 630 nm PWL (at absorbance=0.35).

(21) Unless otherwise indicated, in the following Examples, the formulations were prepared, as follows:

(22) All QD and monomer mixture formulations were prepared in an inert environment. After all indicated components except quantum dots or quantum dot solutions were loaded to a crimp vial, the vial was degassed and mixed for 3 to 5 minutes using a dual axis planetary mixer (THINKY ARE-310, Thinky CA). Quantum dots were transferred from toluene to monomer by removing the toluene from the QDs by nitrogen purge for 10 min, and then the dried QD powder was dispersed in the indicated monomer mixture. The quantum dots were pre-dispersed in isobornyl acrylate (IBOA) or the indicated monomer mixture, then mixed with the other components followed by mixing using a dual axis planetary mixer for 1 min in N.sub.2 atmosphere. Monomer mixture components were combined with the dried QD powder after treating in the indicated fashion, which includes a short degas to introduce them to an inert atmosphere (Comparative only) or were purified by running over activated alumina column (column purified), drying with molecular sieves for >2 h (sieve dried), 3 cycles of freeze-pump thaw with or without sieves (FPT w/sieve or FPT only), or degassed at room temperature by application of vacuum for 48-72 h (RT degas).

(23) Film Preparation:

(24) Films of the formulations were prepared by lamination of the resin formulations between two i-Component PET barrier films. Approximately 2 mL of resin was dispensed on the bottom film and the top has applied with a gap coating bar with gap setup based on desired film thickness. Samples were cured in a Fusion UV F300S or a FUSION UV SYSTEMS, INC (DRS-10/12 QNH, Fusion UV Systems, Inc., Gaithersburg, Md.) curing system with UVA 400 mJ/cm.sup.2.

(25) Impact of Monomer Purification on PLQY:

(26) Solution PLQY was used to determine the compatibility between QDs and monomers using approximately 1 gram of each solution (0.025 wt. % QD solids, based on the total weight of the composition) in Table 1, below, to a 1 mL vial and measuring PLQY at 450 nm excitation.

(27) TABLE-US-00001 TABLE 1 Solution PLQY measurement Peak Exam- Composition with Absor- QY Wave- Peak ple QDs QD.sup.1 bance (%) length FWHM 1* Green in Toluene 0.300 73.9 534.4 44.1 (anhydrous with molecular sieves) 2* Green degassed IBOA 0.315 59.7 3 Green Column purified 0.374 71.5 degassed IBOA 4* Green Column purified 0.272 68.8 degassed IBOA + 400 ppm MEHQ 5* Green Degassed 0.356 55.8 IBOA:SR833 (3:2 by weight) 6 Green Column purified 0.264 71.3 degassed IBOA:SR833 (3:2 by weight) 7 Green IBOA FPT 0.26 72.3 w/sieve 8 Green IBOA Molecular 0.29 67.6 Sieve dried 9 Green IBOA FPT only 0.26 67.9 10* Red in Toluene 0.35 85.0 630 52.8 (anhydrous with molecular sieves) 11* Red degassed IBOA 0.34 74.0 12 Red Column purified 0.40 86.0 degassed IBOA 13* Red Column purified 0.28 79.0 IBOA + 1500 ppm water *Denotes Comparative Example; .sup.1Degassed materials treated 60 min under vacuum (0.1-1 Pa).

(28) As shown in Table 1, above, quantum dots exhibit a decent quantum yield (QY) in solvent (see Comparative Examples 1 and 10) but lose much of that QY upon transfer to a monomer mixture or monomer formulation (see Comparative Examples 2 and 11). However, column purification, FPT or the combination of FPT with sieve drying retains much of the QY of the original QY of the quantum dot solution after transfer into a monomer mixture regardless of the monomers used. See Examples 3, 6, 7 and 12. Merely sieve drying retains nearly as much of the QY of the quantum dot solution as does FPT. See Examples 8 and 9.

(29) TABLE-US-00002 TABLE 2 Characterization Of Water Content By Karl-Fischer Titration Example.sup.1 ppm Water 1A* - Degassed IBOA 252 16 2A - IBOA (degassed and dried by ND (<50) molecular sieves) 3A - IBOA (degassed and purified ND (<50) by AlO.sub.x column) 4A* - IBOA (degassed by freeze- 303 pump-thaw) 5A - IBOA (degassed by freeze- ND (<50) pump-thaw with drying over molecular sieves) 6A* - IBOA (degassed and purified 785 by AlOx column and then spiked with water) *Denotes Comparative Example; .sup.1Degassed materials treated 60 min under vacuum (0.1-1 Pa).

(30) As shown in Table 2, above, sieve drying and column purification effectively dries a monomer mixture; however, FPT alone, without separately drying does not dry the monomer mixture.

(31) The formulations in Table 3, below, were drawn to make films of a consistent thickness using a bar coater (Paul N. Gardner Co., FL, USA) and PLQY was measured for each film. Each films was prepared by lamination of the indicated formulations between two i-Component PET barrier films. Approximately 2 mL of resin was dispensed on the bottom barrier film and was drawn down with a gap coating bar having a 250 to 300 (10 mil-12 mil) gap to insure the desired film thickness. All formulations were cured using a DRS-10/12 QNH at, 500 mJ/cm.sup.2 UV curing intensity (Fusion UV Systems, Inc., Gaithersburg, Md.). Results are presented in Table 4, below.

(32) TABLE-US-00003 TABLE 3 QD formulation (all parts by weight) Film Formulation 14.sup.1 15* 16.sup.2 17* IBOA 60.036 60.036 58.31 58.31 SR833 36.438 36.438 38.07 38.07 I-819 0.901 0.901 1.00 1.00 Finex 30S LP2 2.125 2.125 2.13 2.13 zinc oxide Green QDs 0.5 0.5 Red QDs 0.50 0.50 Barrier film i-component i-component i-component i-component Target resin 3.1 3.1 3.1 3.1 optical density (OD)/g *Denotes Comparative Example; .sup.1Purified monomers by AlOx column; .sup.2Purified monomers by sieve drying.

(33) TABLE-US-00004 TABLE 4 Characterization of Film PLQY PLQY Peak FWHM Film (%) Absorbance (nm) (nm) 14 44.4 0.200 535.9 44.1 15* 41.5 0.216 534.4 45.3 16 50.0 0.327 640.4 54.6 17* 45.7 0.306 641.9 54.5 *Denotes Comparative Example.

(34) As shown in Table 4, above, the purified compositions of the present invention show a significant improvement in initial quantum yield after column purification.

(35) In the following Examples, formulations of monomer mixtures as indicated in Table 5, below, and QDs were handled, as indicated above with the degassed IBOA purified over an alumina column. Each QD composition was added into the monomer based on the formulation below in an inert atmosphere. 1.0 gram of each formulation was loaded into a glass vial and measured the PLQY using the Quantaurus quantum yield spectrometer. Loading of QD in monomer is 0.0025 wt. %, based on total weight of the formulation. Testing results are shown in Table 6, below.

(36) TABLE-US-00005 TABLE 5 QD toluene Example QD stock solution IBOA quantity 18 CdSe/ZnS core- 5 mg Degassed 1.0 g shell (red) and column purified 19* CdSe/ZnS core- 5 mg Degassed 1.0 g shell (red) 20 InP/ZnS core- 5 mg Degassed 1.0 g shell (red) and column purified 21* InP/ZnS core- 5 mg Degassed 1.0 g shell (red) *Denotes Comparative Example.

(37) TABLE-US-00006 TABLE 6 Formulation Performance Absorption at Peak Red Peak Example PLQY 450 nm Wavelength FWHM 18 0.473 0.212 615.81 28.79 19* 0.448 0.18 616.56 29.91 20 0.321 0.029 624.75 68.3 21* 0.214 0.043 627.73 73.93 *Denotes Comparative Example.

(38) As shown in Table 6, above, the initial quantum yield performance of the inventive formulations is significantly higher than in the comparative formulations, especially in the case of cadmium free InP dots (InP/ZnS core-shell). The inventive formulations also exhibited less red shift in Example 20 than Comparative Example 21.