The Conformation of a 

Hydrophilic Di-block 

Copolymer on Silica

Krista Balto

University of Delaware, Department of Chemistry & Biochemistry 

Mentors: John Riley and Michael Zhang
Background (De)stabilize Colloidal Dispersions

Targeted drug delivery 

applications Waste Water Treatment

Diblock

Copolymer DNA/RNA 

Alvarez-Lorenzo, C.; Barreiro-Iglesias, R.; Concheiro, A.; Iourtchenko, L.; Alakhov, V.; Bromberg, L.; 

Temchenko, M.; Deshmukh, S.; Hatton, T. A. Langmuir 2005, 21 (11), 5142–5148. 

Interactions of diblock copolymers with 

surfaces and oppositely charged materials is 

of practical interest

2

Paint

https://www.wateronline.com/doc/understanding-separation-essentials-for-wastewater-treatment-0001

http://paintpad.com.au/wp-content/uploads/2015/11/paint.jpg
Di-Block Copolymer Adsorption on Silica

Poly-ethylene oxide-poly(2-

(dimethylamino) ethyl methacrylate) 

(PEO(9500)-b-PDMA(9000))

Water Soluble

Silica 

Nanoparticles 

Silicon Wafer

pH, salt concentration and 

concentration of silica and PEOb-PDMA were varied

3

Surface charge varies with pH 

https://ec.europa.eu/jrc/en/science-update/first-certified-mixture-silica-nanoparticles (accessed Jul 23, 2018). 

https://www.polymersource.ca/index.php?route=product/category&path=2_2190_17_131_953&subtract=1&categorystart=A-2.2.6.16&serachproduct= 

https://www.indiamart.com/proddetail/ludox-16213896088.html

https://www.2spi.com/catalog/addimages/4176GSW-AB-2.jpg
Hypothesis

Acidic

(more Si-OH) 

Basic

(more SiO-

)

PDMA

(interacts electrostatically with Si-O-)

PEO

(interacts with Si-OH through hydrogen 

bonding)

OH

SiO2 O

OH

O

O

HO

OH

O

PEO-b-PDMA 

competition?

++++

4

https://www.cmu.edu/maty/materials/Synthesis_of_well_defined_macromolecules/block-copolymers.html 

https://patentimages.storage.googleapis.com/WO2008051616A2/imgf000019_0002.png
Experimental Techniques 

Dynamic Light Scattering (DLS) Neutron Reflectometry

Size and Charge of Nanoparticles

Conformation of PEO-b-PDMA on Silica Wafer

NG7 Reflectometer

5

https://lsinstruments.ch/index.php?--/frontend/handler/document.php=test&id=222

https://ncnr.nist.gov/instruments/ng7refl/
Dynamic Light Scattering

Technique used to obtain size and charge of a particle 

when suspended in solution

Particle size is related to diffusion coefficient through 

Stokes Einstein Equation

RRh =

kkBBTT

6ππηηssDD

Charge of particle is related to electrophoretic mobility 

through modified Debye-Smoluchowski equation

μμee =

2εεrrεε0ζζ

3ηss

ff(κκκκ)

Szilagyi, I.; Trefalt, G.; Tiraferri, A.; Maroni, P.; Borkovec, M. Soft 

Matter 2014, 10 (15), 2479. 

Hassan, P. A.; Rana, S.; Verma, G. Langmuir 2014, 31 (1), 

3–12. 

6
pH Series of Bare Silica (.25 w/w%) at 

10mM NaCl using DLS

Bare silica nanoparticles are stable between pH 4 

and pH 10

Provides baseline measurements for adsorbed 

polymer data 

7

SiO2 OH

OH

OH

OOOH

Acidic Conditions 

SiO2 OH

OOH

O-

- O

OBasic Conditions

2 4 6 8 10

32

34

36

38

40

42

44

46

Diameter

Zeta Potential

pH

Diameter (nm)

-40

-30

-20

-10

0

Zeta Potential (mV)
Adsorption Isotherm for HPEO-b-HPDMA 

at 1mM NaCl

Data indicates a strong charge reversal at 

lower pH and a weak charge reversal at 

high pH.

Saturation point of silica particles 

changes with pH.

8 Szilagyi, I.; Trefalt, G.; Tiraferri, A.; Maroni, P.; Borkovec, M. Soft 

Matter 2014, 10 (15), 2479. 

-3 -2 -1 0 1

-50

-40

-30

-20

-10

0

10

20

30

40

pH = 9.5

pH = 7.5

pH = 4

Zeta Potential (mV)

log[PEOPDMA] (mg/mL)
Summary of DLS Results

The zeta potential of the silica particles changes 

based on the amount of PEO-b-PDMA adsorbed. 

The data suggests that the diblock has a different 

conformation at pH 4 and pH 10 because of the 

extent of charge reversal.

9

DLS measurements are useful for detecting 

polymer adsorption onto oppositely charged silica 

nanoparticles.
Neutron Reflectometry 

Fitzsimmons, M. R.; Majkrzak, C. F. Modern Techniques for Characterizing Magnetic 

Materials 107–155. 

Technique that uses specular reflection to 

obtain reflectivity curves 

Fit data to a theoretical model (slab, 

exponential model etc.), generating a 

scattering length density profile

Calculate layer thickness & roughness, 

which should change based on the block 

adsorbed onto the silica

Sensitive to differences in refractive 

indexes across surfaces and interfaces

Rodriguez-Loureiro, I.; Scoppola, E.; Bertinetti, L.; Barbetta, A.; Fragneto, G.; Schneck, 

E. Soft Matter 2017, 13 (34), 5767–5777. 

10

QQzz =

4ππ

λλ sin(θθii)

0.0 0.1 0.2

-8

-6

-4

-2

0

Normalized Reflectivity

Qz (1/Å)
Neutron Reflectometry: Why neutrons?

Probes length scales of interest for adsorbed polymer layers 

(wavelength of neutron is 6 Angstroms) 

11

Highlight components of interest via selective deuteration/ 

contrast matching

HPEO-b-dPDMA

Solvents

HPEO-HPDMA in D20 HPEO-dPDMA in H20 HPEO-dPDMA in D20

http://animatedphysics.com/baryon_decay/xray_neutron_cross_section.png

https://www.polymersource.ca/index.php?route=product/category&path=2_2190_17_131_953&subtract=1&categorystart=A-2.2.6.16&serachproduct= 
Scattering Length Density Profile of 

HPEO-b-HPDMA in D2O

1 = Si Layer

2 = SiO2 Layer

3 = HPEO-HPDMA Layer

4 = D2O Layer

Entire HPEO-HPDMA layer is significantly thicker at pH 10 compared to pH 

4. 

Thickness = 25 Å

Thickness = 225 Å

12

0 100 200 300 400

2

4

6

pH 10

SLD (10^-6 A^2)

z (A)

3 4

2

1

-10 0 10 20 30 40 50

2

4

6

pH 4 HPEO HPDMA

SLD (10^6*A^2)

z (A)

1

2 3

4
Scattering Length Density Profile of 

HPEO-b-dPDMA in D2O

1 = Si Layer

2 = SiO2 Layer

3 = HPEO-dPDMA Layer

4 = D2O Layer

The positioning of the HPEO block at pH 10 is significantly extended away 

from the interface compared to pH 4. 

HPEO is less attracted to surface of the silicon wafer at pH 10.

Thickness=13 Å

Thickness = 53 Å

13

0 50 100

2

4

6

SLD (10^-6*Å^2)

z (Å)

pH 10 HPEOdPDMA

1 2

3 4

-10 0 10 20 30 40

2

4

6

SLD (10^-6*Å^2)

z (Å)

pH 4 HPEOdPDMA

1 2

3

4
Summary of Neutron Reflectometry 

Results

The conformation of the di-block copolymer is different at pH 4 and 

pH 10, which is supported by the varying thickness of the polymer 

layer adsorbed onto the silica wafer. 

pH 4 may not be acidic enough for the PEO block to be the main 

adsorbed block, the polymer may be laying flat. 

Silicon Wafer

- - - - - -

pH 4

14

- - - - - - - - - - - Silicon Wafer

pH 10

+++ +++

+++
Future Work

Further model and improve fits on neutron 

reflectometry data to get a more accurate 

thickness, especially at pH 10. 

Complement reflectometry data with Quartz 

Crystal Microbalance with Dissipation 

(QCM-D) measurements to obtain mass and 

swelling of the adsorbed layer. 

15

https://openqcm.com/wp-content/uploads/2015/02/Quartz-crystal-Microbalance-openQCM-Sensor-1208x440.jpg

Run experiments at pH 2 to see if there is another 

conformational change.

Silicon Wafer

pH 2??
Acknowledgements

Dr. John Riley & Dr. Michael Zhang

Drs. Joseph Dura, Julie Borchers and Brandi Toliver

Center for High Resolution Neutron Scattering

NCNR SURFers 

16
17
Reflectivity Curves 

20