Disc Centrifuge Photosedimentometry: A High Resolution Particle Sizing Technique for Characterisation of Polymer Colloids

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1 Disc Centrifuge Photosedimentometry: A High Resolution Particle Sizing Technique for Characterisation of Polymer Colloids Prof. Steve Armes Dept. of Chemistry, U. Sheffield National Physical Laboratory

2 Armes Group Research Interests Latexes Synthetic Polymer Chemistry Nanoparticles Polymer Particles Nanocomposites Microgels Colloid Science

3 Armes Group Techniques TEM SEM DLS SLS Aqueous electrophoresis NMR FT-IR UV-visible Fluorescence spectroscopy Laser diffraction Disc centrifuge LUMiSizer AccuSizer GPC BET (N 2 ) Thermogravimetry Helium pycnometry XPS SAXS Rheology Stopped-flow kinetics LV1

4 How does a DCP instrument work? Particles thrown out radially to the disc periphery and detected by change in light intensity Disc Dispersion at injection Time At injection time = 0 Spin fluid Fractionation time = t Larger particles detected first Fractionation of particles occurs within a disc centrifuge during measurement Detection time, t, is given by: t = [K..ln(R d /R i )]/[ 2 d w2 ] i.e. t 1.d w 2 K and ln(r d /R i ) are known constants is the solution viscosity is the centrifugation rate, typically ,000 rpm is the density difference between the particles and the spin fluid d w is the weight-average particle diameter

5 Pros and Cons of Disc Centrifuge Photosedimentometry Advantages 1. Wide dynamic range: Disadvantages 100 nm 10 nm 2. Short analysis times (10-45 minutes) 60 mm 3. Excellent resolution compared to DLS (because of fractionation during measurement) 4. Gives weight-average particle diameter (d w ) directly 5. Works well for hard spheres (non-solvated particles): gives good results for silica sols, PS latex, iron oxides, nanocomposites, ceramics (+ emulsions?) 6. Can easily assess the degree of dispersion / flocculation of dilute dispersions 1. Requires accurate particle density (need helium pycnometry) 2. Less good for solvated particles (e.g. sterically-stabilized latexes, microgels) because particle density is not known precisely 3. Assumes a spherical morphology (not true for clay platelets, nanorods etc.) 4. Reduced dynamic range if small Δρ between particles, solvent (e.g. PS latex in water) Overall: Quick, reliable, convenient preferred sizing technique for hard spheres in water

6 Signal Disc Centrifuge Photosedimentometry A High Resolution Particle Sizing Technique Consider a 1:1:1 ternary mixture of three near-monodisperse poly(2-vinylpyridine) latexes: 370, 620 and 1020 nm 370 nm 620 nm 1020 nm D. Dupin et al. Langmuir, 2006, 22, mm 1 mm 1 mm 1 DC O DCP traces 370 nm 620 nm 1020 nm Weight-average Diameter (µm) DLS diameter = 700 nm DLS cannot resolve trimodal PSD DLS has lower resolution than DCP Light scattering biased toward larger particles (since I scat ~ R 6 )

7 Differential volume distribution Effect of coating a sterically-stabilised micrometer-sized polystyrene latex with a sticky overlayer S. F. Lascelles and S. P. Armes, J. Mater. Chem., 1997, 7, 1339 Polypyrrole-coated 1.0 µm polystyrene latex PPy-coated, PVP-stabilized PS latex singlet doublet triplet Weakly Flocculated PPy-coated Singlet PS Latex Higher floccs pyrrole FeCl 3, water, 20 o C PVP-stabilized PS latex Colloidally Stable 1.0 µm PS Latex Sterically-stabilised 1.0 µm polystyrene latex Particle Diameter (mm) Ultrathin sticky polypyrrole overlayer causes incipient flocculation of polystyrene latex DCP is sensitive to weak aggregation: confirms presence of doublets, triplets, floccs etc.

8 Effect of coating micrometer-sized silica particles with a sticky polypyrrole overlayer J. R. Lovett et al. Adv. Funct. Mater. 2014, 24, 1290; Meteroritics Planetary Sci., 2014, 49, 1929 DCP 1 µm MPS Bare 1 µm silica particles 18.5 nm PPy thickness (7.5 %) 1 µm 1 µm PPy-silica particles are flocculated as aqueous dispersions owing to high Hamaker constant of PPy B. Vincent et al. Colloids. Surf. 1990, 51, 239

9 Redistribution of silica between latex particles 20 nm silica nanoparticles P2VP-silica (full silica coverage) J. A. Balmer et al., JACS, 2010, 132, 2166 P2VP-silica P2VP (partial + latex silica + coverage) P2VP-silica (partial silica coverage) Intensity Y Axis Title DCP Studies of Silica Exchange J. A. Balmer et al., Langmuir, 2010, 26, (a) Addition of bare latex particles to latex/silica nanocomposite particles results in redistribution of silica nanoparticles X Axis Title 0.8 (b) + = Intensity Y Axis Title nm 250 nm TR-SAXS: silica redistribution is very fast! 250 nm J. A. Balmer et al., JACS, 2011, 133, X Axis Title Particle Diameter (μm) Particle diameter / µm

10 What is the effect of a density distribution superimposed on a DCP particle size distribution? Part 1 See L. A. Fielding, S. P. Armes, P. W. Fowler et al., Langmuir, 2012, 28, 2536 Core-shell nanocomposite particles of finite polydispersity exhibit artifactual narrowing of their DCP size distribution due to a superimposed density distribution: Low-density polymer core Patrick Fowler High-density silica shell Sasha Mykhaylyk Non-trivial problem: Took more than a year to solve. Involved small-angle x-ray scattering, analytical ultracentrifugation and the solution of a quintic (x 5 ) equation

11 What is the effect of a density distribution superimposed on a DCP particle size distribution? Part 2 B. Akpinar, L. A. Fielding, P. W. Fowler, O. O. Mykhaylyk, S. P. Armes et al., Macromolecules, 2016, 49, 5160 Model sterically-stabilised diblock copolymer nanoparticles of finite polydispersity with high-density cores, low-density (solvated) shells DCP Bernice Akpinar Dr Lee Fielding In this case get an artifactual broadening of the DCP size distribution

12 Synthesis of diblock copolymer vesicles for in situ encapsulation of silica nanoparticles C. J. Mable, S. P. Armes et al. JACS, 2015, 137, e.g. G 58 H 250 diblock copolymer vesicles in presence of 20 % w/w silica nanoparticles 400 nm 400 nm 100 nm Excess non-encapsulated silica nanoparticles removed after six centrifugation cycles

13 Disc centrifuge particle size analysis of silica-loaded diblock copolymer vesicles Assuming empty vesicle ρ = 1.10 g cm -3 Corrected ρ values calculated using constant SAXS vesicle diameter of 291 nm to normalise DCP data Vesicle density increases with [silica] o : can calculate silica loading!

14 DCP analysis: silica-loaded G 58 H 250 diblock copolymer vesicles C. J. Mable, S. P. Armes et al. JACS, 2015, 137, Number of silica nanoparticles per vesicle calculated from DCP analysis Silica loading efficiency is well below theoretical maximum Seems to be a diffusion-limited mass transport problem!

15 Transmission LUMiSizer Analytical Centrifugation g Near IR light source A PC-controlled analytical photocentrifuge Sample Obtain Space- and Time-resolved Extinction Profiles for up to 12 samples simultaneously (high throughput) Time Time colour coded Transmission profiles Near-IR light source illuminates entire sample cell: detect light transmitted through sample cell(s) Transmission converted into extinction: particle size can be calculated given particle density and refractive index Space Radial Position CCD detector Ideally suited for assessing the size and degree of dispersion of colloidal particles Particularly useful for carbon black nanoparticles dispersed in n-alkanes (highly coloured plus disposable cells!) Unlike DCP, the operating temperature range for the LUMiSizer is 4 o C to 60 o C Centrifugal sedimentation

16 Sterically-Stabilised Polystyrene Latexes P. C. Yang and S. P. Armes, Macromol. Rapid Comm., 2014, 35, 242 Well-defined PHEMA macromonomers via ATRP x = M w /M n < 1.20 styrene dispersion polymerisation PS Latex Variation in synthesis parameters enables good particle size control Steric stabiliser thickness is negligible compared to particle diameter So particle size distributions readily assessed using LUMiSizer

17 Assessing the degree of dispersion of diablo ZnO particles Y. Ning, S. P. Armes et al., Nanoscale, 2015, 7, µm

18 Cumulative distribution Apparent diameter / µm Apparent volume average diameter (nm) Cumulative distribution A Star Diblock Copolymer: Flocculation vs. Dispersion (BP-funded PhD) D. J. Growney, O. O. Mykhaylyk et al., Macromolecules, 2015, 48, 3691 OM image for 2.0 % star copolymer OM image for 8.0 % star copolymer + carbon black in n-dodecane + carbon black in n-dodecane LUMiSizer: 6000 ± 1800 nm Diameter (nm) Distribution density (nm -1 ) Star Diblock Copolymer (6 mol % polystyrene) Bridging Flocculation Bridging flocculation Steric Stabilisation Steric stabilization 44degrees o C degrees o C degrees o C 10 0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10% copolymer concentration (wt% based on carbon black) Copolymer concentration (wt.%) LUMiSizer: 88 ± 5 nm Diameter (nm) Distribution density (nm -1 )

19 Conclusions Disc centrifuge photosedimentometry is a powerful sizing technique Enables assessment of flocculation, as well as particle size analysis Solvated particles, density distributions require a lot of hard work! LUMiSizer, DCP are highly complementary sizing techniques LUMiSizer well-suited to multiple samples in non-aqueous solvents, good for highly coloured pigments, offers temperature control

20 Acknowledgements Dr. Lee Fielding (now a Lecturer at U. Manchester) Dr. David Growney (now working at Lubrizol LUMiSizer expert) Bernice Akpinar (now a PhD student at Imperial) Charlotte Mable, Dr. Joe Lovett (Armes group members) Dr. Stuart Lascelles, Dr. Damien Dupin, Dr. Jennifer Balmer Prof. Patrick Fowler FRS (Maths) and Dr. Sasha Mykhaylyk (SAXS) Thank you for your attention

21 Problem of Effective Particle Density (BP-funded PhD) D. J. Growney, O. O. Mykhaylyk et al., Langmuir, 2015, 31, 8764 WRONG PEP or PB PS RIGHT ρ = 1.89 g cm -3 ρ = 0.91 g cm -3 Diameter /nm D = 40 ± 5 nm ρ solvent = 0.75 g cm -3 (n-dodecane) Diameter / nm D = 118 ± 15 nm Thick steric stabiliser layer leads to lower particle density, incurs LUMiSizer sizing error

22 Is carbon black a good mimic for diesel soot? (BP-funded PhD) D. J. Growney, O. Mykhaylyk, S. P. Armes et al., Langmuir, 2015, 31, PS-PEP diblock copolymer Carbon black Olefinic copolymer Carbon black 106 ± 4 nm Good mimic Bad mimic 107 ± 2 nm Diesel soot 107 ± 5 nm Carbon black or diesel soot Diesel soot 4.9 ± 2.0 µm Answer depends on the nature of the chosen copolymer dispersant!

23 Effect of particle density on the lower limit particle diameter measurable by DCP: Colloid Type PS latex Silica sol Magnetite sol / g cm -3 / g cm -3 Lower limit d w / nm ~ ~ Dynamic range depends markedly on particle density due to (see equation) [Also: DCP is not well suited for particle mixtures with different densities] Can also use an X-ray source/detector (instead of a light source/detector) X-ray disc centrifuge is more expensive, but no assumptions required concerning scattering/absorption characteristics of particles. Not useful for most latexes, since these usually comprise only low Z atoms (C, H, N, O etc.) When analysing larger particles, can extend upper limit by either increasing or decreasing (otherwise particles move to disc periphery too quickly) Two analysis methods: 1. Line-start mode: Differential PSD determined directly so inherently high resolution; standard analysis method. 2. Homogeneous start mode: Needs larger sample volume; measures integrated PSD & calculates differential PSD; better suited for broad PSD s.