Inorganic nanoparticles doped with optically active rare-earth ions and coated with organic ligands were synthesized in order to create fluorescent polymethyl methacrylate (PMMA) nanocomposites. Two different aromatic ligands (acetylsalicylic acid, ASA and 2-picolinic acid, PA) were utilized in order to functionalize the surface of Tb3+ : LaF3 nanocrystals. The selected aromatic ligand systems were characterized using infrared spectroscopy, thermal analysis, rheological measurements, and optical spectroscopy. Nanoparticles produced
Rare-earth (RE) ions doped into inorganic matrices have been utilized as luminescent additives in applications such as light-emitting devices, lasers, optical amplifiers, and biological fluorescence labeling [
With respect to many inorganic materials, polymer matrices have the potential to offer improved production rates, lower cost, and improved processability. However, polymers exhibit inherently high vibrational energies which tend to quench many of the transitions of rare-earth ions thus limiting their application as optical materials [
In this research, the method utilized to overcome high vibrational energy observed in PMMA is to incorporate optically active RE ions into low vibrational energy inorganic nanoparticles which then are dispersed into a polymer matrix by the use of selective organic ligands. The inorganic component is rare-earth ion phosphor, (terbium III), doped lanthanum trifluoride (LaF3) nanocrystals. Terbium (III) (Tb3+) emits green fluorescence as a result of a strong electronic transition, 5D4 → 7F5, occurring near 543 nm. Tb3+ exhibits absorption levels at shorter wavelengths in the ultraviolet (UV) to visible color region (390–780 nm) [
Tb3+ is incorporated into the inorganic crystal structure of lanthanum trifluoride, LaF3 to create the doped nanocrystal (Tb3+ : LaF3). LaF3, was chosen as the host matrix for terbium due to the low phonon energy it exhibits which minimizes the quenching potential of Tb3+ ion emissions [
Aromatic acids have been used as ligands to sensitize and enhance lanthanide fluorescence by reducing the probability of radiationless (heat) energy transfer from the RE ion to the solvent [
ASA molecule (a), ASA anion (b), and ASA chelation of cation (c).
PA molecule (a), PA anion (b), and PA chelation of cation (c).
The incorporation and distribution of nanoparticles has been shown to affect the viscoelastic properties of polymers [
The PMMA (Mw
The nanoparticles were produced by the scheme shown in Figure
Flow chart of nanoparticle synthesis.
Dried precipitated polymer and precipitated polymer nanocomposite samples were analyzed by attenuated total reflectance—Fourier transform infrared (ATR-FTIR) spectroscopy. The spectra were acquired on a Theromo-Fisher Nicolet Magna 550 FTIR spectrometer equipped with a Thermo-SpectraTech Foundation Series Diamond ATR accessory, Nic-Plan microscope, and Omnic software. The spectral resolution was set at 8 cm−1 and 160 scans were conducted at room temperature.
Glass transition temperature (Tg) was determined by a Universal TA Instruments—2920 MDSC V2.6A differential scanning calorimeter (DSC). Approximately 5 mg of sample was placed in a hermetic sample pan for measurements. The instrument was equilibrated at a temperature of 25
Percent nanocrystal loading was determined by a Universal TA Instruments—TGA Q5000 V3.5 Build 252 thermogravimetric analysis instrument (TGA). Samples weighing approximately 5 mg were loaded into a platinum pan and then placed in the instrument under nitrogen. The temperature ramp rate was 10
Size and size distribution of particles were measured on a Malvern Dynamic Light Scattering (DLS) Zetasizer Nano Series Nano ZS at room temperature. Teflon spheres were used to ball-mill the dried samples into a fine powder before a resuspension in a neat solvent. The average of three samples was used to determine the size and size distributions.
Viscosity measurements were performed using a TA Instruments—ARES LS/M 0012701 Rheometer equipped with the TA Orchestrator version 7.1.2.3 software package. Dynamic frequency sweep tests were performed under a nitrogen environment in the frequency range of 0.1 to 500 rad/s at 220
A Perkin Elmer Lambda 900 UV-Vis-NIR Spectrometer with UV Winlab Version 3.00.03 software was utilized to gather optical absorption data. Scans were performed with a 1 nm slit size in the UV and visible range of 260–380 nm. Samples were prepared by dispersing the ligand into water and measurements were done at room temperature.
Photoluminescence measurements were performed with a Jobin-Yvon Fluorolog Tau 3 Fluorometer with 4 nm emission bandpass. The data were collected at 1 nm intervals with 50 ms integration time. All measurements were performed at room temperature. Excitation spectra were fit with Lorentzian curves correcting for a constant background in Igor Pro 6.1 (Wavemetrics, Portland, Oregon) for full width at half maximum (FWHM) measurements.
Figure
Absorbance as a function of wavenumber—infrared spectra of undoped precipitated PMMA (red), ASA capped nanocrystals incorporated into precipitated PMMA (blue), and PA capped nanocrystals incorporated into precipitated PMMA (green) in the range of 4000–500 cm−1 (a) and inset—range 1700–1300 cm−1 (b).
The loading of the inorganic components (Tb3+ : LaF3) and Tg obtained from TGA and DSC, respectively, are presented in Table
Percent loadings and glass transition temperature of ligand capped nanocrystals in PMMA.
Ligand | Ligand to nanoparticle ratio | Calculated loading | Experimental loading | Inflection point Tg, |
---|---|---|---|---|
ASA | 2 : 1 | 10% | 9% | 120 |
3 : 1 | 10% | 9% | 121 | |
4 : 1 | 10% | 10% | 121 | |
5 : 1 | 10% | 10% | 121 | |
PA | 2 : 1 | 10% | 7% | 121 |
3 : 1 | 10% | 7% | 122 | |
4 : 1 | 10% | 7% | 120 | |
5 : 1 | 10% | 8% | 121 | |
undoped PMMA precipitated | 0% | 122 |
No significant change in Tg was observed between the undoped PMMA and the nanoparticle-loaded PMMA systems. The glass transition inflection point was used to determine the Tg and the average of two sample runs is stated as the Tg for each sample in Table
The results of the elemental composition of the nanoparticles determined by EDS analysis are summarized in Table
Elemental composition and corresponding atomic percentage of nanoparticles in PMMA at different ligand:nanoparticle ratios.
Atomic percentage | ||||||||
PA | ASA | |||||||
Element | 2 : 1 | 3 : 1 | 4 : 1 | 5 : 1 | 2 : 1 | 3 : 1 | 4 : 1 | 5 : 1 |
La | 61.2 | 46.4 | 49.1 | 30.8 | 43.0 | 30.4 | 32.6 | 30.8 |
Tb | 6.6 | 8.0 | 8.1 | 4.9 | 9.1 | 8.2 | 7.7 | 7.3 |
F | 32.2 | 45.6 | 42.7 | 64.3 | 47.9 | 61.4 | 59.5 | 60.0 |
Suspensions containing different ligand : nanoparticle ratios of PA : Tb3+ : LaF3 and ASA : Tb3+: LaF3 were measured by dynamic light scattering to determine the size of the nanoparticles. Previous work of Ellerbrock using the same nanoparticle synthesis route found that Tb3+ : LaF3 particles without ligand measured
Agglomerate diameters and complex viscosity percent difference of ligand capped nanocrystals in PMMA.
Ligand | Ligand to nanoparticle ratio | Diameter (nm) | Complex viscosity percent difference |
---|---|---|---|
ASA | 2 : 1 | 252 ± 66 | 81% |
3 : 1 | 420 | 66% | |
4 : 1 | 349 ± 85 | 58% | |
5 : 1 | 233 ± 41 | 68% | |
PA | 2 : 1 | 444 | 44% |
3 : 1 | 150 | 82% | |
4 : 1 | 457 ± 176 | 53% | |
5 : 1 | 242 ± 24 | 70% |
STEM image of polymer nanocomposite showing the presence of 50–1000 nm ligand capped Tb3+ : LaF3 agglomerates.
No significant difference was observed between the size of the agglomerates resuspended in water versus those resuspended in THF for either PA or ASA ligand at nanoparticle ratios of 2 : 1, 3 : 1, and 4 : 1. However, a significant particle size difference was observed at the 5 : 1 ratio between the two resuspension methods. The agglomerate sizes in water for ASA and PA systems were measured at 893 ± 418 nm and 1080 ± 192 nm, respectively. The agglomerate sizes in THF/PMMA were measured at 233 ± 41 nm for ASA and 242 ± 24 nm for PA. This diameter variation could be attributed to the mechanical action of the grinding process necessary to achieve the resuspension. Representative histograms of the diameter size distribution for PA : Tb3+ : LaF3 and ASA : Tb3+: LaF3 at the 5 : 1 ligand to nanoparticle ratio with STEM images in the inset are illustrated in Figures
PA : Tb3+ : LaF3 (a) and ASA : Tb3+ : LaF3 (b) agglomerate diameters in water (red) and dispersed in PMMA/THF (blue) at 5 : 1 ligand to nanoparticle ratio. STEM images (inset) show representative agglomerates found in the polymer nanocomposite.
Viscosity measurements were made by evaluating the relationship between complex viscosity and frequency. The relationship of complex viscosity to frequency is representative of the shear viscosity versus shear rate based on the Cox-Merz rule which states that the magnitudes of complex and shear viscosity data can be compared at equal values of frequency and shear rate [
Complex viscosity as a function of frequency at 220
Overall, the lower viscosity values could be the result of the nanoparticles acting to break up the structure interfering with the local intermolecular hydrogen bonding, and enabling polymer chains to slip past the nanoparticles resulting in less resistance [
Figure
Absoption spectra of PA (blue), ASA (red) ligand, Tb3+ salt (light green), and Tb3+ : LaF3 (dark green) in water.
Figure
Characteristic (a) excitation spectrum (
Neat PMMA and PMMA nanocomposites composed of varying ratios of (a) PA : Tb3+ : LaF3 and (b) ASA : Tb3+ : LaF3 under UV light.
Upon excitation of the
Sharp emission spectra (FWHM < 5 nm) are generally preferred than broad emission spectra (FWHM = 50–200 nm) which organic dyes typically produce for photonic devices [
FWHM values for PMMA nanocomposites composed of varying ratios of PA : Tb3+ : LaF3.
Ligand | Ligand to nanoparticle ratio | FWHM of 490 nm peak—direct ion excitation ( | FWHM of 490 nm peak—ligand excitation ( |
---|---|---|---|
PA | 2 : 1 | 9.64 ± 0.36 nm | 8.51 ± 0.20 nm |
3 : 1 | 9.01 ± 0.19 nm | 8.53 ± 0.18 nm | |
4 : 1 | 9.18 ± 0.34 nm | 8.50 ± 0.20 nm | |
5 : 1 | 9.81 ± 0.39 nm | 8.95 ± 0.20 nm |
Emission of Tb3+ doped LaF3 in PMMA nanocomposites composed of 2 : 1 PA ligand to ion ratio at two different excitation wavelengths (red-
Light-emitting polymer nanocomposites were produced via solution/precipitation chemistry using ligand capped nanocrystals doped with Tb3+ ions loaded into PMMA. The incorporation of ligand (ASA and PA) : Tb3+ : LaF3 nanoparticles within the PMMA matrix was verified by ATR-FTIR spectroscopy (organic) and TGA (inorganic). The absorbance spectrum obtained from ATR-FTIR spectroscopy of the ASA system produced absorption peaks at 1601 cm−1 and 1559 cm−1 which corresponds with benzene ring stretching. The PA : Tb3+ : LaF3 absorbance spectrum exhibited absorption peaks at 1653 cm−1, 1593 cm−1, and 1568 cm−1 which suggest that the pyridine ring of picolinic acid is stretching. The peak located at ~1341 cm−1 could be associated with C-N stretching vibrations of the PA ligand.
Thermogravimetric analysis of PMMA nanocomposites showed that the ASA : Tb3+ : LaF3 contained system ~10 wt% inorganic material, whereas the PA : Tb3+ : LaF3 system contained approximately 7 wt% inorganic material. The EDS analysis confirmed that PA : Tb3+ : LaF3 was on average at a 6 : 1 molar ratio of La3+ to Tb3+ and ASA : Tb3+ : LaF3 was on average at a 4 : 1 molar ratio of La to Tb3+.
The average diameter for PA : Tb3+ : LaF3 and ASA : Tb3+ : LaF3 at varying ligand to nanoparticle ratios produced agglomerates with diameters > 200 nm in PMMA. The resuspension liquid (water or THF) showed no significant difference in agglomerate size for PMMA nanocomposites of PA :Tb3+ : LaF3 and ASA : Tb3+ : LaF3 at ligand : nanoparticle ratios of 2 : 1, 3 : 1, and 4 : 1. However, a significant particle size difference was observed at the 5 : 1 ratio where those resuspended in water were measured at 893 ± 418 nm and 1080 ± 192 nm for ASA and PA systems, respectively. For the same ratio resuspended in THF values were measured at 233 ± 41 nm for ASA and 242 ± 24 nm for PA.
The rheology measurements for all samples produced traditional shear thinning curves. The addition of the agglomerates caused a reduction in the viscosity of all PMMA nanocomposites as compared to neat PMMA. The PA : Tb3+ : LaF3 PMMA nanocomposites exhibited a reduction in viscosity that corresponded with the reduction in the size of the agglomerates. The ASA nanocomposite demonstrated lower viscosity values but did not exhibit the same trend with respect to agglomerate size.
Green light was produced by direct ion and ligand excitation of ASA and PA PMMA nanocomposite. All emission spectra exhibited the corresponding characteristic emission of trivalent terbium.
The authors acknowledge the support and fellowship funding at the time of this work provided by the South East Alliance for Graduate Education and the Professoriate (SEAGEP) on the National Science Foundation Award HRD-0450279, Center of Optical Materials Science and Engineering Technologies (COMSET), and School of Materials Science and Engineering at Clemson University. A note of gratitude is given to Dr. Gregory Von White II, Courtney Kucera, and Kim Ivey for their help with this paper.