Effects of Solvents on the Assembly of 

Nanoparticles from Poly(styrene-bethylene glycol) 

Ethan Gasper, Kelsi Rehmann, Katie Weigandt
Forcing Polymers 

to Self-Assemble 

into 

Nanoparticles

• Polymers

 Long repeating chains of identical molecules 

 Common uses: Hand sanitizer, rubber, and plastics

• Nanoparticles

 Nanometer sized particles

 Using polymer at different molar masses and 

using different synthesis processes to study the 

structure of nanoparticles

• Scattering Methods 

 Shot a beam of radiation (light/x-ray/neutrons) at 

the sample particles, parts of the radiation will 

scatter off, which is measured by a detector

 Use the scattering data to determine the 

structure of the nanoparticles

2
Using different synthesis conditions to change the 

structures of nanoparticles

• Study how different solvents, mixing methods of copolymers, and purification 

processes affect the synthesis of nanoparticles 

• Understanding the structures and self-assembly of nanoparticles produced from 

blended polymers

• Studying the reproducibility of the process to make nanoparticles 

• Improved targeted drug delivery 

 Drug delivery nanoparticles: nanoparticles that act as a transportation system for 

drugs to circulate through our bodies

• Our samples will act as a model system to more easily control the final structure, and 

study a variety of structures

Kelley, E. G., Smart, T. P., Jackson, A. J., Sullivan, M. O., & Epps, T. H. (2011). Structural changes in block copolymer micelles induced by cosolvent mixtures. 

Soft Matter, 7(15), 7094. https://doi.org/10.1039/c1sm05506b

3
Dissolving Polymers with Solvents

Polymer: Copolymer: Block Copolymer: 

Long repeating chains of 

monomers/molecules

monomer

Repeating chain of 2 or 

more different polymers 

(can be random or 

alternating)

Uninterrupted chain of 1 

polymer connected to 

another uninterrupted 

chain of a second polymer

polymer A polymer B polymer A

polymer B

Solvents: swell and dissolve polymers

• Due to polymers’ long chains, they can adopt different conformations based on 

the degree of solubility (how much a solvent will associate with a dissolved 

polymer)

Hiemenz, P. C., & Lodge, T. (2007). Polymer chemistry (2nd ed). CRC Press. 4
Polymer Solubility 

• Poly(styrene-b-ethylene glycol) (PS-b-PEG) will be the block copolymer used to 

synthesize the nanoparticles we want to study

n

b m

O

Poly(styrene)

Poly(ethylene glycol)

• Changing molar mass affects the solubility 

of polymers and other physical properties 

(polymer’s ability to stabilize nanoparticles)

• 1.6/3.8-b-5: molar mass of PS-b-PEG 

(M [=] kDa) 

• The 2 different copolymer ends have 

different solubility 

• If both ends are soluble, the whole 

polymer is soluble

• If only one end is soluble, then 

polymers can assemble into different 

structures

Hiemenz, P. C., & Lodge, T. (2007). Polymer chemistry (2nd ed). CRC Press.

5
How Polymers Form Nanoparticles

• The nanoparticles we are working with have a core-shell structure

Hydrophilic shell: Poly(ethylene glycol)

(hydrophilic: attracted to water)

Hydrophobic core: Poly(styrene) 

(hydrophobic: repels water)

• Forcing copolymers to assemble into nanoparticles by changing the solvent

 Solvent good for both polymer ends Solvent (water) only good for 1 polymer end

• Block copolymers can form many different structures

 Spheres and cylinders are primary examples, but others exist (Vesicles, Y-junctions, and more)

 We are trying to make only spheres and learn when the nanoparticles stop assembling into spheres

Jain, S., & Bates, F. S. (2004). Consequences of Nonergodicity in Aqueous Binary PEO−PB Micellar Dispersions. Macromolecules, 37(4), 1511–1523. 

https://doi.org/10.1021/ma035467j

6
Why Scattering Methods are Used

• A beam of radiation (light/x-ray/neutrons) is shot at the sample nanoparticles, parts of 

the radiation will scatter off, then be measured by a detector

• The detector measures the amount of scattered radiation at multiple angles, which is 

analyzed to determine particle structure and size

7

Different structures

Radiation

(Light, x-ray, neutrons)

Sample

Detector
Scattering Methods and What it Measures

• Dynamic light scattering (DLS):

 Measures the radius of the whole structure 

of the nanoparticles (hydrodynamic radius)

• Small angle x-ray scattering (SAXS):

 Depends on the polymers and solvent being 

used and their x-ray contrast

 For PS-b-PEG in water, should be measuring 

the radius of the core and some PEG that is 

close to the surface

• Small angle neutron scattering (SANS): 

 Depends on the polymers and solvent being 

used and their neutron contrast

 There is more contrast between H2O and 

D2O, and by mixing these solvents we can 

differentiate between the PS, PEG, and the 

whole particle

SAXS

DLS

SANS

8
Overview of 

the Project

1. Synthesis of nanoparticles at 

different conditions

2. Thin film rehydration

(removing the solvent 

before water is added)

1a. Changing molar mass 

of copolymer 

2a. Using dichloromethane 

(DCM) solvent

3a. Using tetrahydrofuran 

(THF) solvent 

4. Use scattering methods to 

compare differences between 

the processes

3. Solvent switch 

(removing the solvent while 

water is in solution)

1b. Test solubility for 

different molar masses

Wright, D. B., Patterson, J. P., Gianneschi, N. C., Chassenieux, C., Colombani, O., & O’Reilly, R. K. (2016). 

Blending block copolymer micelles in solution; obstacles of blending. Polymer Chemistry, 7(8), 1577–1583. 

https://doi.org/10.1039/C5PY02006A 9
Changing Molar Mass of Polymers 

10

• 1.6-b-5 molar mass

 Soluble in water and produces a clear 

solution of nanoparticles with a 

hydrodynamic diameter ~ 40 nm

• 3.8-b-5 molar mass

 Slightly soluble in water

 It was also found that blending small 

amounts of the 3.8 with a majority (> 70%) 

of 1.6-b-5 results in nanoparticles

• We want to further study this finding and understand how mixing molar mass of polymer affects 

the structure of nanoparticles

 Is the 3.8 polymer always insoluble in water, or is there a concentration where it forms 

nanoparticles in solution? 

 How does molar mass affect polymer solubility?

 How do different processes affect the size of synthesized nanoparticles? 
3.8 Polymer Solubility Test

• Is the 3.8-b-5 polymer soluble is water, and at what concentration?

• What is the structure if it is soluble?

Kelley, E. G., Smart, T. P., Jackson, A. J., Sullivan, M. O., & Epps, T. H. (2011). Structural changes in block copolymer micelles induced by cosolvent mixtures. 

Soft Matter, 7(15), 7094. https://doi.org/10.1039/c1sm05506b (ESI)

• Hard to measure solubility of both copolymer blocks

 Literature: Using UV-Vis to measure absorbance and 

changing the solvent mixture to increase solubility

 When abs reaches 0, polymer is soluble

 Instead of changing the solvent mixture, we are 

changing polymer concertation to affect solubility 

11

*Absorbance at 600nm wavelength

Absorbance of PB homopolymer in 

H2O/THF cosolvent mixtures

Absorbance (abs)

THF content (vol%)

• Procedure

 Create a concentration series of PS-b-PEG (3.8-b-5) in 

water

 Use UV-Vis to determine the changes in absorbance 

and turbidity (cloudiness) of the sample
Thin Film Rehydration Procedure 

DCM 

Solution

Evaporate DCM 

from vial

Water Solution

Add water into vial

Polymer in 

vial

12

• Mix 2 PS-b-PEG 

copolymers that have 

different molar masses 

of poly(styrene) in DCM 

solvent

• Evaporate DCM from the 

sample, add water to the 

thin film of polymer

• Add solution to a mixing 

plate to allow samples to 

reach a final structure

*Procedure completed by Kelsi Rehmann
Solvent Switch Using an Ultrafiltration Stir Cell

THF Solution

Slowly add water 

into solution

THF + H2O Solution

Add to ultrafiltration 

stir cell

Pressure 

input

Stir bar

H2O Solution

20 nm pores

Excess THF 

and H2O

• Mix 2 PS-b-PEG 

copolymers that have 

different molar masses 

of poly(styrene) in THF 

solvent

• Slowly add water into solution 

to initiate self assembly of 

nanoparticles

• Allows the particles to be 

closer to reach a final 

structure

• Add solution and extra water 

(H2O) to ultrafiltration stir cell to 

remove the THF from the solution 

• Silica membrane with 20 nm 

pore diameter used to 

remove THF from solution

13
How DLS Works Differently from Small Angle Scattering

• Measures the movement of particles in solution over time, which can provide 

information about the particles size (assumes particle is a sphere)

• DLS will also pick up the movement of the solution surrounding the nanoparticles; 

therefore, hydrodynamic radius is larger than the actual radius

Stetefeld, J., McKenna, S. A., & Patel, T. R. (2016). Dynamic light scattering: A practical guide and applications in biomedical sciences. Biophysical 

Reviews, 8(4), 409–427. https://doi.org/10.1007/s12551-016-0218-6

14

Blue: Measured values

Purple: Regularization fit

Radius (nm)

Intensity

Intensity AutoCorrelation

Time (μs)

• Standardized 50nm PS nanoparticles (~0.09 wt% sample)
Intesity (

c

m

−

1

)

Q (A

−1

)

Small Angle Scattering Data 

• Detector graphs data based on Intensity of 

radiation vs Q(angle of scattering) 

• SAXS/SANS data is fitted to highly researched 

structural models

 Spheres, core-shell spheres, cylinders, etc. 

15

50nm PS nanoparticles SAXS data: sphere model fit

Radius: 21.5 nm

50nm PS nanoparticles SANS data: sphere model fit

Radius: 22.1 nm

Intesity (

c

m

−

1

)

Q (A

−1

)

Orange: model fit line 

Blue: collected data from detector 

Orange: model fit line 

Blue: collected data 

from detector 

*SANS data collection: Kelsi Rehmann

*SAXS data collection: Caitlyn Wolfe

o USAXS measurements

o Low-q upturn is an artifact 

o Noise at high-q
*Absorbance at 

600nm wavelength

Absorbance of PB homopolymer in H2O/THF cosolvent mixtures

Absorbance of 3.8 Polymer in Water at 600nm Wavelength

16

0

0.5

1

1.5

2

2.5

3

3.5

0 0.2 0.4 0.6 0.8 1 1.2

Absorbance (abs)

wt% of polymer in sample

Peak height at 600nm (abs)

Kelley, E. G., Smart, T. P., Jackson, A. J., Sullivan, M. O., & Epps, T. H. (2011). Structural changes in block copolymer micelles induced by cosolvent mixtures. 

Soft Matter, 7(15), 7094. https://doi.org/10.1039/c1sm05506b (ESI) Absorbance (abs)

THF content (vol%)
Structure of 3.8 Molar Mass Particles at Low Concentration 

17

Sample wt%

(clear)

Radius peak 1 

(%Intensity)

Radius peak 2 (% 

Intensity) 

2

nd lowest conc. 126nm (65) 516nm (34)

Lowest conc. 93nm (47) 341.2 341nm (52)

*Radius is not correlated to size

Take away from solubility test

• 3.8 is soluble and forms 

particles at low concentration

Hypothesis: 

• Multiple structures in 

solution, resulting in multiple 

peaks 

 Lowest Concentration sample

Intensity

Radius (nm)

Intensity AutoCorrelation

Time (μs)
Intensity

Radius (nm)

Intensity

Radius (nm)

Structure for Mixed Solvent Switch Samples

• 10% 3.8-b-5 (DLS 28 days after filtration)

18

• 15% 3.8-b-5 (DLS 3 days after filtration)

THF Soln THF + Water 

Soln

H2O Soln

DLS

*Has more THF 

remaining in soln

Sample type Radius (nm) % Polydispersity % Intensity

10% 86 18 91

15% 93 26 100 Intensity AutoCorrelation Intensity AutoCorrelation

Time (μs)

Time (μs)
19

Structure for Mixed Thin Film Rehydration Samples

*Data collection: Kelsi Rehmann

Solvent: D2O

DCM SANS data

Sample type Core radius 

(nm)

Shell radius 

(nm)

Total radius 

(nm)

100% 1.6-b-5 3 5.2 8.1

10% 3.8-b-5 3 4.9 8.0

15% 3.8-b-5 4.4 1.3 5.7 Intesity (

c

m

−

1

)

Q (A

−1

)

Green: 15% 3.8-b-5

Orange: 10% 3.8-b-5

Blue: 100% 1.6-b-5

Intesity (

c

m

−

1

)

Q (A

−1

)

Orange: model fit line

Blue: 100% 1.6-b-5

• SANS data for all samples • SANS data: core-shell sphere model

DCM Soln Polymer Water Soln

in vial
20

DCM Soln Polymer Water Soln

in vial

Comparison Between the Two Processes

DLS radius (nm) DCM SANS data DCM SAXS data

Sample type Solvent 

Switch 

Thin Film 

Rehydration 

Core 

radius 

(nm)

Shell radius 

(nm)

Total radius 

(nm)

Core 

radius 

(nm)

Shell 

radius 

(nm)

Total radius 

(nm)

100% 1.6-b-5 21 3 5.2 8.2 0.7 12 12.7

10% 3.8-b-5 86 27 3 4.9 7.9 8.1 6.4 14.5

15% 3.8-b-5 93 73 4.4 1.3 5.7 7.7 7.1 14.8

THF Soln THF + Water Soln H2O Soln
21

DCM Soln Polymer Water Soln

in vial

Comparison Between the Two Processes

DLS radius (nm) DCM SANS data DCM SAXS data

Sample type Solvent 

Switch 

Thin Film 

Rehydration 

Core 

radius 

(nm)

Shell radius 

(nm)

Total radius 

(nm)

Core 

radius 

(nm)

Shell 

radius 

(nm)

Total radius 

(nm)

100% 1.6-b-5 21 3 5.2 8.2 0.7 12 12.7

10% 3.8-b-5 86 27 3 4.9 7.9 8.1 6.4 14.5

15% 3.8-b-5 93 73 4.4 1.3 5.7 7.7 7.1 14.8

THF Soln THF + Water Soln H2O Soln
Intesity (

c

m

−

1

)

Intesity (

c

m

−

1

)

Q (A

−1

)

22

Comparison Between the Two Processes

DLS radius (nm) DCM SANS data DCM SAXS data

Sample type Solvent 

Switch 

Thin Film 

Rehydration 

Core 

radius 

(nm)

Shell radius 

(nm)

Total radius 

(nm)

Core 

radius 

(nm)

Shell 

radius 

(nm)

Total radius 

(nm)

100% 1.6-b-5 21 3 5.2 8.2 0.7 12 12.7

10% 3.8-b-5 86 27 3 4.9 7.9 8.1 6.4 14.5

15% 3.8-b-5 93 73 4.4 1.3 5.7 7.7 7.1 14.8

Q (A

−1

)

Green: 15% 3.8-b-5

Orange: 10% 3.8-b-5

Blue: 100% 1.6-b-5

Orange: model fit line

Blue: 100% 1.6-b-5
Ongoing Work

23

• SAXS/SANS measurements 

and analysis

• Low yield for Solvent Switch 

Ultrafiltration

• Processes to improve yield

*Data collection: Kelsi Rehmann

Solvent: D

2O 

Intesity (cm−1)

Q (A

−1

)

Orange: model fit 

line

Blue: 100% 1.6-b-5

Orange: model fit 

line

Blue: 15% 3.8-b-5

Intesity (cm−1)

Q (A

−1

)
Ongoing Work

• SAXS/SANS measurements 

and analysis

• Low yield for Solvent Switch 

Ultrafiltration

• Processes to improve yield

• Calibration Curve based on DLS data from standardized 

PS nanoparticles 

 Related DLS counts to concentration through PS 

nanoparticles concentration series to estimate % yield 

of the solvent switch method

y = 0.9289x + 10.447

0

2

4

6

8

10

12

-7 -6 -5 -4 -3 -2 -1 0

normalized intensity: log(counts)

concentration: log(conc) (g/mL)

Calibration Curve of log(conc) vs log(counts)

log(counts)

Filtered samples

Linear (log(counts))

24
Ongoing Work

• SAXS/SANS measurements 

and analysis

• Low yield for Solvent Switch 

Ultrafiltration

• Processes to improve yield

25

THF + H2O

Solution

THF + H2O Solution

THF Solution

Slowly add 

water into 

solution

Add to 

ultrafiltration 

stir cell (70mL 

increments)

Pressure 

input

Stir bar

H2O solution

20 nm pores

Excess THF 

and water

Add to 

200mL of 

water

• Have polymer in more water before filtration

 Less THF concertation in solution, therefore, more likely for 

polymer chains to self-assemble into nanoparticles

• Reuse the same membrane 

 Already has water/polymer trapped in the membrane, won’t 

trap much of the new sample

• Started using 100% 1.6-b-5 to compare data to previous findings
Conclusion

• Solubility test showed that 3.8-b-5 is able to form particles in water at low concentrations

• Ultrafiltration is usable for a solvent switch method, however, the process can still be 

optimized 

• Higher molar masses of the polymer results in a greater particle size and changes in structure

• Possibility that changes in process can affect the nanoparticles size and structure

 Continued work towards this finding

26

SAXS

DLS SANS

DLS radius (nm) DCM SANS data DCM SAXS data

Sample type Solvent 

Switch 

Thin Film 

Rehydration 

Core 

radius 

(nm)

Shell radius 

(nm)

Total radius 

(nm)

Core 

radius 

(nm)

Shell 

radius 

(nm)

Total radius 

(nm)

100% 1.6-b-5 21 3 5.2 8.2 0.7 12 12.7

10% 3.8-b-5 86 27 3 4.9 7.9 8.1 6.4 14.5

15% 3.8-b-5 93 73 4.4 1.3 5.7 7.7 7.1 14.8
Acknowledgements • Kelsi Rehmann – Mentor, DCM procedure, and more

• Katie Weigandt – Co-mentor

• Caitlyn Wolfe – SAXS measurements for PS nanoparticles 

and more 

• Yimin Mao – DCM SAXS measurements 

• Friends of Soft Matter group at NCNR 

• NCNR SURF Directors

 Julie Borchers

 Susana Marujo Teixeira

 Leland Harriger

• NIST SURF Program 

27