Rheology modifiers for encapsulating quantum dots

The present invention relates to rheology modifiers useful in a process for preparing a multilayer polymer composite containing quantum dots.

Semiconductor quantum dots (QD) provide optical absorption and emission (photoluminescence PL or electroluminescence EL) behaviors that are significantly different from those of bulk materials. As the particle size decreases, effective energy bandgap (Eg), or available energy levels, increases and creates a blue shifted PL spectrum. This spectrum tunability by the particle size dependent quantum confinement effect within the same material is a critical advantage over conventional bulk semiconductors. Because of their unique optical properties, QD have been of great interest in many display and lighting applications. Most QD have inorganic shells with a larger bandgap material to confine electron and hole pairs within the core region and prevent any surface charge states. The outer shells are then capped by organic ligands to reduce trap states of the shell that can lead to reduced quantum yield (QY). Organic ligands help QD to disperse in organic/aqueous solvents. Typical organic ligands surrounding QD have relatively long alkyl chains which provide high solubility in non-polar solvents or monomers. Unfortunately, QD are very susceptible to photo-oxidation during light absorption/conversion process. Also, moisture can have similar impacts when ligands are not compatible. QD typically are encapsulated in a polymer matrix to protect them from adverse effects of water and oxygen. For example, US2010/0084629 discloses a variety of polymers as encapsulants. However, this reference does not disclose the polymer compositions described herein.

The present invention provides a polymer resin comprising:

(a) quantum dots,

(b) a compound of formula (I)

wherein R 1 is hydrogen or methyl and R 2 is a C 6 -C 20 aliphatic polycyclic substituent, and

(c) a block or graft copolymer having M n from 50,000 to 400,000 and comprising from 10 to 100 wt % polymerized units of styrene and from 0 to 90 wt % of a non-styrene block; wherein the non-styrene block has a van Krevelen solubility parameter from 15.0 to 17.5 (J/cm 3 ) 1/2 .

The present invention further provides a polymer resin comprising:

(a) quantum dots,

(b) a compound of formula (I), and

(c) a homopolymer or random copolymer with M n greater than polymer critical molecular weight, M c and with a van Krevelen solubility parameter from 16.5 to 20.0 (J/cm 3 ) 1/2 .

Percentages are weight percentages (wt %) and temperatures are in ° C., unless specified otherwise. Operations were performed at room temperature (20-25° C.), unless specified otherwise. Boiling points are measured at atmospheric pressure (ca. 101 kPa). “(Meth)acrylate” means acrylate or methacrylate. Quantum dots are well known in the art, see, e.g., US2012/0113672. Number-average molecular weight, M n , is measured by size exclusion chromatography. A hydrocarbon polymer is a polymer having no more than 5 wt % of elements other than hydrogen and carbon, preferably no more than 3 wt %, preferably no more than 1 wt %, percentages based on dry weight of the entire polymer.

The van Krevelen solubility parameter is described by Bicerano (Prediction of Polymer Properties, 3rd edition, Marcel Dekker, New York, 2002, Chapter 5) and tabulated in Table 5.2 (“δ 2 ”) of the same book for a number of commercially relevant polymers. For an arbitrary polymer with known monomer chemical structure, van Krevelen-type solubility parameter can be computed using algorithms outlined in the same reference or using Materials Studio software, Synthia module (http://accelrys.com/products/collaborative-science/biovia-materials-studio/polymers-and-classical-simulation-software.html). For random copolymers, the copolymer solubility parameter can be calculated as weight-average of the solubility parameters of its comonomers.

Critical molecular weight, Mc, is calculated as described by Bicerano (Prediction of Polymer Properties, 3-rd edition, Marcel Dekker, New York, 2002, Chapters 11 and 13). For example, for polystyrene (PS), Mc=30 kg/mol, for poly(methylmethacrylate) (PMMA), Mc=18 kg/mol, etc. For an arbitrary polymer with known monomer chemical structure, critical molecular weight can be computed using algorithms outlined in the same reference or using Materials Studio software, Synthia module (http://accelrys.com/products/collaborative-science/biovia-materials-studio/polymers-and-classical-simulation-software.html).

In one preferred embodiment of the invention, a polymer composite made using the resin of this invention is part of a multilayer assembly which also comprises an outer layer on each side of the polymer composite. Preferably, the outer layer is an oxygen bather which also inhibits passage of moisture. Preferably, the outer layer comprises a polymer film, preferably one comprising polyethylene terephthalate (PET), polyaryletherketones, polyimides, polyolefins, polycarbonate, polymethyl methacrylate (PMMA), polystyrene, or a combination thereof. Preferably, the outer layer further comprises oxides or nitrides, preferably silicon oxides, titanium dioxide, aluminum oxide, silicon nitrides or a combination thereof. Preferably the oxides or nitrides are coated on the surface of the polymer film facing the QD layer. Preferably, each outer layer comprises a polymer film having a thickness from 25 to 150 microns (preferably 50 to 100 microns) and an oxide/nitride layer having a thickness from 10 to 100 nm (preferably 30 to 70 nm). In some preferred embodiments of the invention, the outer layer comprises at least two polymer film layers and/or at least two oxide/nitride layers; different layers may be of differing composition. Preferably, the outer layers have a very low oxygen transmission rate (OTR, <10 −1 cc/m 2 /day) and low water vapor transmission rate (WVTR, <10 −2 g/m 2 /day). Preferably, the polymer film in the outer layers has a Tg from 60 to 200° C.; preferably at least 90° C., preferably at least 100° C.

Preferably, the thickness of the polymer composite of this invention is from 10 to 500 microns, preferably at least 20 microns, preferably at least 30 microns, preferably at least 40 microns; preferably no greater than 400 microns, preferably no greater than 300 microns, preferably no greater than 200 microns, preferably no greater than 150 microns. Preferably, the thickness of each outer layer is from 20 to 100 microns, preferably from 25 to 75 microns

Preferably, the polymer composite of this invention is prepared by free radical polymerization of the resin prepared by mixing monomers, QD and other optional additives. Preferably, the resin is coated on a first outer layer prior to curing by typical methods, e.g., spin coating, slot die coating, gravure, ink jet and spray coating. Preferably, curing is initiated by exposing the resin to ultraviolet light or heat, preferably ultraviolet light, preferably in the UVA range.

Preferably, R 2 is a C 7 -C 17 aliphatic polycyclic substituent, preferably R 2 is a C 8 -C 15 aliphatic polycyclic substituent. Preferably, R 2 is a bridged polycyclic substituent; preferably a bicyclic, tricyclic or tetracyclic substituent; preferably a bicyclic or tricyclic substituent. Preferably, R 2 is a saturated aliphatic substituent. Preferred structures for R 2 include, e.g., adamantanes, bicyclo[2,2,1]alkanes, bicyclo[2,2,2]alkanes, bicyclo[2,1,1]alkanes and tricyclodecanes (e.g., tricyclo[5,2,1,0 26 ]decane); these structures may be substituted with alkyl, alkoxy groups, hydroxy groups or (meth)acrylate esters (i.e., the compound of formula (I) may have at least two (meth)acrylate ester substituents; preferably no more than two); preferably alkyl and alkoxy groups have from one to six carbon atoms, preferably one to four. Tricyclodecanes and bicyclo[2,2,1]alkanes are especially preferred, particularly tricyclo[5,2,1,0 26 ]decane, dimethanol dimethacrylate and isobornyl acrylate. More than one compound of formula (I) may be present in the resin. Preferably, the resin comprises a compound of formula (I) having one (meth)acrylate ester substituent and a compound of formula (I) having two (meth)acrylate ester substituents; preferably in a weight ratio from 100:1 to 1:20, respectively; preferably 10:1 to 1:15.

Preferably, the polymer resin comprises from 70 to 95 wt % of the compound(s) of formula (I); preferably at least 73 wt %, preferably at least 76 wt %, preferably at least 79 wt %; preferably no more than 93 wt %, preferably no more than 91 wt %, preferably no more than 89 wt %.

Preferably, the polymer resin of this invention comprises from 0.01 to 5 wt % of quantum dots, preferably at least 0.03 wt %, preferably at least 0.05 wt %; preferably no more than 4 wt %, preferably no more than 3 wt %, preferably no more than 2 wt %. Preferably, quantum dots comprise CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, GaN, GaP, GaAs, InP, InAs or a combination thereof.

Preferably, ligands surrounding the inorganic part of quantum dots have non-polar components. Preferred ligands include, for example, trioctyl phosphine oxide, dodecanethiol and fatty acid salts (e.g., stearate salts, oleic acid salts).

Preferably, the block or graft copolymer is a hydrocarbon polymer. The block or graft polymer is added to the polymer resin as a rheology modifier, i.e., a thickener. Preferably, the block or graft polymer comprises at least 15 wt % styrene, preferably at least 20 wt %, preferably at least 25 wt %; preferably no more than 90 wt %, preferably no more than 80 wt %, preferably no more than 70 wt %, preferably no more than 60 wt %, preferably no more than 50 wt %, preferably no more than 45 wt %. Preferably, the block or graft polymer comprises at least 10 wt % of a non-styrene block, preferably at least 20 wt %, preferably at least 30 wt %, preferably at least 40 wt %, preferably at least 50 wt %, preferably at least 55 wt %; preferably no more than 85 wt %, preferably no more than 80 wt %, preferably no more than 75 wt %, preferably no more than 60 wt %, preferably no more than 50 wt %, preferably no more than 45 wt %. Preferably, the non-styrene monomers in the copolymer (non-styrene block) are alkenes, dienes, (meth)acrylates, siloxanes, or combinations thereof; preferably alkenes and/or dienes. Preferably, the non-styrene in the copolymer comprises polymerized units of C 2 -C 8 alkenes and/or dienes, preferably C 2 -C 5 alkenes and/or dienes. Preferably, the block or graft polymer is a block copolymer. Preferably, the C 2 -C 8 alkenes and/or dienes are selected from ethylene, propylene, butylene, isoprene and butadiene. Preferably, M n of the block or graft polymer is at least 60,000, preferably at least 70,000, preferably at least 80,000; preferably no more than 350,000, preferably no more than 300,000, preferably no more than 250,000.

Preferably, the non-styrene block has a van Krevelen solubility parameter less than 17.2 (J/cm 3 ) 1/2 , preferably less than 17, preferably less than 16.5; preferably at least 15.5.

Preferably, a homopolymer or random copolymer with M n greater than polymer critical molecular weight, M c has a van Krevelen solubility parameter no greater than 19.5 (J/cm 3 ) 1/2 ; preferably at least 17, preferably at least 17.5, preferably at least 18.0. Preferably, M n is at least 1.5 times M c , preferably at least twice; preferably no more than 20 times M c , preferably no more than ten times. Preferred polymers having M n greater than M c include, e.g., those comprising polymerized units of styrene, alkenes, dienes, (meth)acrylates, siloxanes, or combinations thereof. M c for polystyrene is 30,000 kDa.

Other additives which may be incorporated into the polymer composite of this invention include UV stabilizers, antioxidants and scattering agents to improve light extraction.

Preferred forms for the polymer composite include, e.g., films, beads, strips, rods, cubes and plates. The polymer composite is useful in many applications, including, e.g., displays, lighting and medical applications. Preferred display applications include public information displays, signage, televisions, monitors, mobile phones, tablets, laptops, automotive dashboards and watches.

Sample Preparation for Examples

A) Liquid Sample Preparation

All QD resin samples were prepared under inert environment. Styrene-based polymers were dissolved in isobornyl acrylate by mixing at 80° C. for ˜30 min using magnetic stirring. After all components except quantum dots were loaded to a crimp vial, the vial is degassed and mixed for 3 to 5 minutes using a dual axis planetary mixer (Thinky ARE-310). Quantum dots were pre-dispersed in isobornyl acrylate, then mixed with the other components followed by rolling for 1 hr.

B) Film Sample Preparation

All samples 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 curing system with UVA ˜400 mJ/cm 2 . The films were then cut into approximately 0.2″ square pieces for Quantum Yield Measurements and 1×1″ square pieces for photo oxidation tests. Freestanding films were also prepared by coating of the resin formulations on glass, followed by curing in a FUSION UV SYSTEMS, INC (DRS-10/12 QNH) with UVA ˜400 mJ/cm 2 . The free-standing films were then delaminated from glass and used for O 2 permeability testing using Mocon ox-tran model 2/21 with 3% O 2 and 97% N 2 at 23° C.

C) Characterization

Viscosity was measured by Brookfield DV-II+ viscometer. Frequency sweep and steady state flow experiments of resins were performed using a AR G2 rheometer at 20° C. Both liquid and film photoluminescent Quantum Yield (PLQY), peak emission wavelength (PWL) and full-width half-max of the emission peak (FWHM) were measured with a Hamamatsu C9920-02G integrating sphere. Film thicknesses were determined by measurement of the cured films with a micrometer and then subtracting out the barrier film thickness. Edge ingress was determined by image analysis of 1″×1″ samples aged on a bare backlight unit. Number average molecular weight (M n ) and polydispersity (PDI) of polymers were determined using a size exclusion chromatography equipped with a mixed A PLgel 20 um×300 mm×7.5 mm (×2+ guard) column with stabilized tetrahydrofuran at 1.0 mL/min @35° C. and a refractive index detector (against polystyrene standards).

D) Chemical Composition Description: