Additional method to measure and confirm viscosity at various PEG wt%

Measure Brownian motion by tracking probe particles within a solution. Movies are analyzed using

MPT algorithms to determine rheological properties of gelation

Measured mean-squared displacement (MSD) determines extent of gelation

PEG cross-linked gels varied above (right) and below

(left) the overlap concentration

.

Cross-Linked Gels Concentrated Over the

Overlap ConcentrationMichelle Mazzeo, Mark Schumm and Professor Kelly Schultz

Department of Chemical and Biomolecular Engineering

David and Lorraine Freed Undergraduate Research Symposium, Lehigh University

Matt Wehrman (Graduate Researcher)

Francisco Esobar IV (ME ’16)

Cross-linked hydrogels are significant within a variety of chemical industries, playing an important role in

fields ranging from the petroleum industry to applications within biomaterials. The overall objective is to

gain a better understanding of these gels at higher concentrations and study their functionality within

different growth mechanisms. The polymer used for these experiments was poly(ethylene glycol) (PEG),

and was applied to photo-polymerization reactions involving click chemistry. While characteristics of PEG

have been previously studied at lower concentrations, this project aims to understand the material

properties of the resulting gel when it is concentrated above its overlap concentration, c*. Since the

overlap concentration reflects the solution’s viscosity, initial experiments increased the percentage of PEG

in solutions to determine dilute, semi-dilute and entangled regions and their corresponding concentrations.

Using this as a foundation, experiments used PEG precursor solutions from each of these viscous regions

to study the effects on gelation and final material properties during polymerization by both chain and step

growth mechanisms. These experiments applied bulk and microrheology to track particles and measure

characteristics within the gelation reaction. Through multiple particle tracking microrheology (MPT),

material properties of the gel were optically tracked and analyzed. Bulk rheology was then used to confirm

these results and provide further material details. These experiments will provide important results

regarding polymer interactions and long term stability that will improve the understanding of concentrated

solution properties. By determining how different polymerization mechanisms effect cellular behavior and

interact within solutions, concentrated gels can provide effective solutions and innovative improvements to

processes within the chemical industry.

Determine PEG overlap concentration using MPT and bulk rheology

Bulk rheology measured constant viscosity for the

duration that the shear rate was applied

Abstract

4-Arm Poly(ethylene Glycol) Molecule

Poly(ethylene Glycol) Gels

Three PEG Overlap Regions

Dilute

c < c*

Semi-dilute

c*<c < c**

Entangled

c >c**

Cross-Linked Gels Molecular Interactions

Overlap Concentration Equations

Particle Tracking Images

Multiple Particle Tracking (MPT) Microrheology Techniques

Particle Movement Progression

Algorithms identify brightness-weighted centroid for each frame

3 μm

20 μm

Liquid Viscoelastic Fluid Gel

Mean-squared displacement graph with particle trajectories

Liquid

Viscoelastic

Fluid

Gel

Bulk Rheology

Bulk Rheometer Apparatus

Concentration Viscosity Measurements

Viscosity vs. concentration graph

shows a change in slope at c*.

Determining c* developed the

necessary operating

concentrations for future studies

above and below the overlap

concentration

Viscosity vs. Concentration

Gelation Reactions

PEG Chain

Growth

Mechanism:

Gelation reactions at concentrations below (3 wt%), at (16 wt%), and above (25 wt%) the

overlap concentration c* were measured using MPT to study the effects on gelation during a

chain growth (acrylate: thiol) photo-polymerization reaction.

MSD of 3 wt% PEG Exposure vs. Slope 3 wt% PEG

Results showed nearly

instantaneous gelation.

Experiments proceeded by

delaying the reaction. This

was completed using neutral

density filters to limit UV

exposure.

Experimentation continued by varying the neutral density filter strength and height of the UV

light, at each PEG concentration, as listed in the table below. The results were then compared

to determine the optimal filter needed to accurately determine the gelation time where the

material transitions from a liquid to a solid.

MSD PEG Solution Comparison Exposure vs. Slope Comparison

Intensity of

Filter

Height (cm)

0 n/a

1 2.8

1 5.6

2 5.6

2 8.4

3 7.3

4 7.3

5 7.3

d Number of dimensions

D Diffusivity

D Diffusivity

T Temperature

a Particle Radius

• Overlap concentration is determined using microrheology and bulk rheology and is 14.31

3.62 wt% and 16.04 6.97 wt%, respectively.

• Filtering out UV light successfully limited UV intensity to accurately measure gelation times

for a chain growth reaction.

• Future work will include characterizing the effects on gelation by further limiting UV intensity,

experimenting with step growth reactions at the overlap concentration and determining the

gel transition using time cure superposition.

Conclusions

Acknowledgement(s):

b Statistical segment length

1°

+ UV LightAcrylate

Thiol Hydrogel

SH SH

texp

25mm

----------------------------------------0.5

54462-DNI7

----------------------------α= 0.5

Liquid

Gel