1. Evgeny Karpushkin, Andrey Bogomolov WSC-9, Tomsk, Feb 2014
Morphology assessment of polymer hydrogels using multivariate analysis of viscoelastic and swelling properties
Evgeny Karpushkin, Andrey Bogomolov
WSC-9, Tomsk, Feb 2014

2. What is hydrogel? Large variety of gels, but in this talk
gel = covalently cross-linked 3D (network) polymer swollen in water
Properties:
hydrophilic, swollen, soft materials
biocompatible or biodegradable
responsive to changes of temperature, pH and ionic strength; electrical and magnetic fields
Applications:
artificial implants (contact lens)
supports for enzymes and cells immobilization
sensors, actuators, chemical valves and robots
models of tissues and organs
food industry, cosmetics, …

3. Examples of hydrogel morphology: polyHEMA
100 mm
Reactive & Functional Polymers
62 (2005) 1–9
Pores generation in HEMA gels:
phase separation during polymerization
incorporation of soluble particles
cryo (frozen solvent as porogen)
liquid porogen (emulsion polymerization)
introduction of gas generating substances
100 mm
Journal of Controlled Release
102 (2005) 3–12
400 mm
100 mm
Soft Matter 3 (2007) 1176–1184
Macromolecules 40 (2007)
1 mm
Biomaterials 26 (2005) 1507–1514

4. Materials science: general aim
Preparation conditions
Utilitarian properties
Structure:
porosity,
topology,
microphases, …
Measured properties
Composition
How do preparation conditions influence the material structure?
How does structure of material influence its functional properties?
What material structure corresponds to certain measured properties?
How to prepare a material with desired properties?

5. Remarks on microscopy Light microscope: rapid, good for start,
but resolution not enough
Electron microscope:
for dry samples structure is not always preserved
Electron microscope:
for swollen samples observed structure is highly dependent on conditions
dry
swollen
Chamber pressure decreasing

6. Preparation of samples
glass plates
diluent
APS
(NH4)2S2O8
HEMA
TEMED
DEGDMA
rubber sealing
Variable parameters:
Sample code
diluent content
diluent nature (water, aqueous NaCl, aqueous Mg(ClO4)2)
cross-linker : monomer ratio
swelling medium: water, dimethylsulfoxide, aqueous NaCl
diluent type
diluent ratio (wt%)
(0.1M NaCl) 70/1 (DMSO)
crosslinker to monomer
ratio (mol%)
swelling medium
> 100 samples

7. Observed properties Morphology type Equilibrium swelling
Homogeneous
Droplets
In water, or DMSO, or aqueous NaCl
Sometimes versus temperature
Interlocking
Fused particles
Shear deformation
Mixed
Forced oscillatory deformation
Creep (constant shear stress in time)

8. Morphology of hydrogels: diluent at preparation = water

9. Morphology of hydrogels: salting-in diluent at preparation
70/2
(0.2M Mg(ClO4)2)70/2
(0.05M Mg(ClO4)2)70/2
(0.3M Mg(ClO4)2)70/2
More diluent solvating power = less phase separation

10. Morphology of hydrogels: salting-out diluent at preparation
60/1
(0.2M NaCl)60/1
(0.4M NaCl)60/1
(0.45M NaCl)60/1
(0.475M NaCl)60/1
(0.5M NaCl)60/1
(0.525M NaCl)60/1
(0.6M NaCl)60/1
Less diluent solvating power = more phase separation
Fine tuning – never observed before!

11. Effect of morphology on swelling

12. Effect of morphology on swelling

13. Effect of morphology on equilibrium shear modulus

14. PCA: 35 water-born samples, 19 variables

15. PCA: 35 water-born samples, 19 variables

16. PCA: 35 water-born samples, 19 variables

17. PCA: 35 samples, 4 variables
Equilibrium swelling, low frequency modulus, and pair of loss factors
These variables are important as such, and therefore they are usually determined

18. Conclusions and perspectives
PCA approach is promising for indirect morphology assessment.
Fairly reliable
Fast and cheap as compared with the direct ESEM
Uses experimental variables are important as such
Needs further investigation
Is it possible to exclude swelling data?
Creep curve fitting?
Does the approach work with chemically different materials?