Prof. Steve Armes

Dept. of Chemistry, U. Sheffield

National Physical Laboratory

29.11.2016

Disc Centrifuge Photosedimentometry: A

High Resolution Particle Sizing Technique

for Characterisation of Polymer Colloids

Armes Group Research Interests

SyntheticPolymer

Chemistry

ColloidScience

PolymerParticles

Latexes

Microgels

Nanocomposites

Nanoparticles

Armes Group Techniques

TEM SEM DLS SLS Aqueous electrophoresis

XPS SAXS Rheology Stopped-flow kinetics LV1

NMR FT-IR UV-visible Fluorescence spectroscopy

Laser diffraction Disc centrifuge LUMiSizer AccuSizer

GPC BET (N2) Thermogravimetry Helium pycnometry

Fractionation of particles occurs within a disc centrifuge during measurement

t = [K..ln(Rd/Ri)]/[2dw2]

Particles thrown out radially to the disc periphery and detected by change in light intensity

Detection time, t, is given by: K and ln(Rd/Ri) are known constants

is the solution viscosity

is the centrifugation rate, typically 500-15,000 rpm

is the density difference between the particles

and the spin fluid

dw is the weight-average particle diameter

How does a DCP instrument work?

i.e. t .dw

2

1

Time

At injection time = 0

Fractionation time = t

Disc

Spin fluid

Dispersion

at injection

Larger

particles

detected

first

Pros and Cons of Disc Centrifuge Photosedimentometry

Advantages

Disadvantages

1. Wide dynamic range:100 nm

10 nm60 mm

2. Short analysis times (10 - 45 minutes)

3. Excellent resolution compared to DLS (because of fractionation during measurement)

4. Gives weight-average particle diameter (dw) 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

Disc Centrifuge PhotosedimentometryA 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

DLS cannot resolve trimodal PSD

DLS has lower resolution than DCP

Light scattering biased toward

larger particles (since Iscat ~ R6)

1 mm

620 nm

1 mm

1020 nm

DLS diameter = 700 nm

1 mm

370 nm

D. Dupin et al.Langmuir,

2006, 22, 3381

370 nm 620 nm

1020 nm

1

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Weight-average Diameter (µm)

Sig

nal

DCO

DCPtraces

Effect of coating a sterically-stabilised micrometer-sized polystyrene latex with a ‘sticky’ overlayer

Ultrathin ‘sticky’ polypyrrole overlayer causes incipient flocculation of polystyrene latex

DCP is sensitive to weak aggregation: confirms presence of doublets, triplets, floccs etc.

Singlet

doublet

Higher floccs

Weakly Flocculated

PPy-coated PS Latex

Colloidally Stable

1.0 µm PS Latex

singlet

Particle Diameter (mm)D

iffe

ren

tial

vo

lum

e d

istr

ibu

tio

n

triplet

S. F. Lascelles and S. P. Armes, J. Mater. Chem., 1997, 7, 1339

Sterically-stabilised 1.0 µm polystyrene latex

PVP-stabilized

PS latex

Polypyrrole-coated 1.0 µm polystyrene latex

PPy-coated,

PVP-stabilized

PS latex

pyrrole FeCl3, water, 20oC

J. R. Lovett et al. Adv. Funct. Mater. 2014, 24, 1290; Meteroritics Planetary Sci., 2014, 49, 1929

MPS

1 µm

1 µm

Bare 1 µm silica particles

1 µm

18.5 nm PPy thickness (7.5 %)

Effect of coating micrometer-sized silica particles with a ‘sticky’ polypyrrole overlayer

PPy-silica particles are flocculatedas aqueous dispersions owing to

high Hamaker constant of PPy

B. Vincent et al. Colloids. Surf. 1990, 51, 239

DCP

0.0 0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0

Y A

xis

Title

X Axis Title

Inte

ns

ity

Particle Diameter (μm)

0.0 0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0

Y A

xis

Title

X Axis Title

Inte

ns

ity

(a)

(b)

0.0 0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0

Y A

xis

Title

X Axis Title

Inte

ns

ity

Particle Diameter (μm)

0.0 0.5 1.0 1.5 2.00.0

0.2

0.4

0.6

0.8

1.0

Y A

xis

Title

X Axis Title

Inte

ns

ity

(a)

(b)

+

Particle diameter / µm

J. A. Balmer et al., Langmuir, 2010, 26, 13662

DCP Studies of Silica ExchangeRedistribution of silica between latex particles

Addition of bare latex particles to latex/silica nanocomposite

particles results in redistribution of silica nanoparticles

+ +P2VP

latex

P2VP-silica

(full silica

coverage)

P2VP-silica

(partial

silica

coverage)

P2VP-silica

(partial

silica

coverage)

J. A. Balmer et al., JACS, 2010, 132, 2166 20 nm silicananoparticles

J. A. Balmer et al., JACS, 2011, 133, 826 TR-SAXS: silica redistribution is very fast!

250 nm250 nm 250 nm

+ =

250 nm

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:

Non-trivial problem: Took more than a year to solve. Involved small-angle x-ray

scattering, analytical ultracentrifugation and the solution of a quintic (x5) equation…

Low-density

polymer core

High-density

silica shell

What is the effect of a density distribution

superimposed on a DCP particle size distribution?

Patrick Fowler Sasha Mykhaylyk

Part 1

B. Akpinar, L. A. Fielding, P. W. Fowler, O. O. Mykhaylyk, S. P. Armes et al., Macromolecules, 2016, 49, 5160

In this case get an artifactual broadening of the DCP size distribution

Model sterically-stabilised diblock copolymer nanoparticles of

finite polydispersity with high-density cores, low-density (solvated) shells

DCP

What is the effect of a density distribution

superimposed on a DCP particle size distribution?Part 2

Dr Lee FieldingBernice Akpinar

Synthesis of diblock copolymer vesicles for

in situ encapsulation of silica nanoparticles

e.g. G58H250 diblock copolymer vesicles in presence of 20 % w/w silica nanoparticles

100 nm400 nm 400 nm

Excess non-encapsulated silica nanoparticles removed after six centrifugation cycles

C. J. Mable, S. P. Armes et al.JACS, 2015, 137, 16098

Disc centrifuge particle size analysis

of silica-loaded diblock copolymer vesicles

Vesicle density increases with [silica]o: can calculate silica loading!

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

DCP analysis: silica-loaded G58H250 diblock copolymer vesicles

Silica loading efficiency is well below theoretical maximum

Number of silica

nanoparticles per vesicle

calculated from

DCP analysis

Seems to be a diffusion-limited mass transport problem!

C. J. Mable, S. P. Armes et al. JACS, 2015, 137, 16098

LUMiSizer – Analytical Centrifugation

A PC-controlled analytical photocentrifuge

Obtain Space- and Time-resolved Extinction Profiles for up to 12 samples simultaneously (high throughput)

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

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 4oC to 60oC

6 – 2300 g Near IR light source

Sample

Time colour coded

Transmission profiles

CCD detectorSpace

Radial Position

Tran

smis

sio

n

Time

Centrifugal sedimentation

Sterically-Stabilised Polystyrene Latexes

P. C. Yang and S. P. Armes, Macromol. Rapid Comm., 2014, 35, 242

x = 20 - 70Mw/Mn < 1.20

Well-defined PHEMA macromonomers via ATRP

styrenePS

Latexdispersion

polymerisation

Variation in synthesis parameters enables good particle size control

So particle size distributions readily assessed using LUMiSizer

Steric stabiliser thickness is negligible compared to particle diameter

Assessing the degree of dispersion of ‘diablo’ ZnO particles

Y. Ning, S. P. Armes et al., Nanoscale, 2015, 7, 6691

2 µm

A Star Diblock Copolymer: Flocculation vs. Dispersion (BP-funded PhD)

D. J. Growney, O. O. Mykhaylyk et al., Macromolecules, 2015, 48, 3691

1.0

0.8

0.6

0.4

0.2

0

Cu

mu

lati

ve d

istr

ibu

tio

n 0.0005

0.0004

0.0003

0.0002

0.0001

0

Distrib

utio

n d

en

sity (nm

-1)

4000 5000 6000 80007000

Diameter (nm)

LUMiSizer: 6000 ± 1800 nm

OM image for 2.0 % star copolymer

+ carbon black in n-dodecane

OM image for 8.0 % star copolymer

+ carbon black in n-dodecane

Star Diblock Copolymer (6 mol % polystyrene)

Diameter (nm)

1.0

0.8

0.6

0.4

0.2

0

Cu

mu

lati

ve d

istr

ibu

tio

n

0.20

0.15

0.10

0.05

0

80 85 1009590

0.25

Distrib

utio

n d

en

sity (nm

-1)

LUMiSizer: 88 ± 5 nm

10

100

1000

10000

100000

0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10%

4 degrees

20 degrees

60 degrees

copolymer concentration (wt% based on carbon black)

Bridging flocculation Steric stabilization

2 3 987654 10101

0

102

103

104

105

Ap

pa

ren

t vo

lum

e a

vera

ge d

iam

ete

r (n

m)

4oC

20oC

60oC

1

Bridging Flocculation

Steric Stabilisation

Copolymer concentration (wt.%)

Ap

par

ent

dia

met

er /

µm

Disc centrifuge photosedimentometry is a powerful sizing technique

Conclusions

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

Dr. Lee Fielding (now a Lecturer at U. Manchester)

Acknowledgements

Dr. David Growney (now working at Lubrizol – LUMiSizer expert)

Bernice Akpinar (now a PhD student at Imperial)

Prof. Patrick Fowler FRS (Maths) and Dr. Sasha Mykhaylyk (SAXS)

Charlotte Mable, Dr. Joe Lovett (Armes group members)

Thank you for your attention

Dr. Stuart Lascelles, Dr. Damien Dupin, Dr. Jennifer Balmer

Problem of Effective Particle Density (BP-funded PhD)

D. J. Growney, O. O. Mykhaylyk et al., Langmuir, 2015, 31, 8764

Thick steric stabiliser layer leads to lower particle density, incurs LUMiSizer sizing error

WRONG RIGHT

PS

Diameter /nm Diameter / nm

D = 40 ± 5 nm D = 118 ± 15 nm

ρ = 1.89 g cm-3 ρ = 0.91 g cm-3

ρsolvent = 0.75 g cm-3

(n-dodecane)

PEPor PB

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, 10358

Answer depends on the nature of the chosen copolymer dispersant!

Olefinic copolymerPS-PEP diblock copolymer

Carbon blackor diesel soot

Carbon black Carbon black

Diesel soot Diesel soot

Bad mimic

Good mimic

107 ± 2 nm

4.9 ± 2.0 µm

106 ± 4 nm

107 ± 5 nm

Effect of particle density on the lower limit particle diameter measurable by DCP:

Dynamic range depends markedly on particle density due to (see equation)

Colloid Type

PS latex

Silica sol

Magnetite sol

/ g cm-3 / g cm-3 Lower limit dw / nm

1.05 0.05 ~ 100

2.17 1.17 ~ 50

4.36 3.36 10-15

[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.