Morphology assessment of polymer hydrogels using multivariate analysis

of viscoelastic and swelling properties

Evgeny Karpushkin, Andrey Bogomolov

WSC-9, Tomsk, Feb 2014

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, …

Examples of hydrogel morphology: polyHEMA

Soft Matter 3 (2007) 1176–1184

Reactive & Functional Polymers62 (2005) 1–9

Macromolecules 40 (2007) 8056-8060

Biomaterials 26 (2005) 1507–1514

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

Journal of Controlled Release102 (2005) 3–12

100 mm

100 mm

100 mm

400 mm

1 mm

Materials science: general aim

Preparation conditions

Structure:porosity,topology,

microphases,…

Utilitarian properties

• 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?

Measured propertiesComposition

Remarks on microscopyLight 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

Chamber pressure decreasing

dry

swollen

Preparation of samples

CH2

CH3

O

O

OH

CH2

CH3

OOO

CH2

CH3

O O NNCH3

CH3

CH3

CH3

diluent (NH4)2S2O8glass plates

rubber sealing

• diluent content• diluent nature (water, aqueous NaCl, aqueous Mg(ClO4)2)• cross-linker : monomer ratio• swelling medium: water, dimethylsulfoxide, aqueous NaCl

HEMA

DEGDMA TEMED

APS

(0.1M NaCl) 70/1 (DMSO)

diluent type diluent ratio (wt%)

crosslinker to monomerratio (mol%) swelling medium

Sample codeVariable parameters:

> 100 samples

Observed propertiesMorphology type Equilibrium swelling

Shear deformation

Homogeneous Droplets

Interlocking Fused particles

Mixed

• In water, or DMSO, or aqueous NaCl• Sometimes versus temperature

• Forced oscillatory deformation• Creep (constant shear stress in time)

)()()(

swollenmdrymswollenm

Morphology of hydrogels:diluent at preparation = water

Morphology of hydrogels:salting-in diluent at preparation

70/2

(0.05M Mg(ClO4)2)70/2

(0.2M Mg(ClO4)2)70/2

(0.3M Mg(ClO4)2)70/2

More diluent solvating power = less phase separation

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

Fine tuning – never observed before!Less diluent solvating power = more phase separation

Effect of morphology on swelling

0 1 2 3 4 5

0.4

0.6

0.8 homogeneous intermediate interlocking particulate

IWC = 80 wt%

IWC = 70 wt%

IWC = 60 wt%

IWC = 50 wt%

Equ

ilibr

ium

sw

ellin

g (g

/g)

Crosslinker concentration CC (mol%)

IWC = 40 wt%

0.0 0.2 0.4

0.4

0.6

70/2

60/1

Equ

ilibr

ium

sw

ellin

g, g

/g

Mg(ClO4)2 in preparation diluent (mol/L)

40/1

b)

0.0 0.2 0.4 0.6 0.8

0.4

0.6

70/2

60/1

Equ

ilibr

ium

sw

ellin

g, g

/gNaCl in preparation diluent (mol/L)

40/1

d)

Effect of morphology on swelling

Effect of morphology on equilibrium shear modulus

0 1 2 3 4 5

1

10

100

homogeneous intermediate interlocking particulateIWC = 80 wt%

IWC = 70 wt%

IWC = 60 wt%

IWC = 50 wt%

G' a

t 0.1

Hz

(kP

a)

Crosslinker concentration CC (mol%)

IWC = 40 wt%

0.0 0.2 0.40

40

80

120

160

70/2

60/1

G' a

t 0.1

Hz

(kP

a)

Mg(ClO4)2 in preparation diluent (mol/L)

40/1

a)

0.0 0.2 0.4 0.6 0.8

10

100

70/2

60/1

G' a

t 0.1

Hz

(kP

a)NaCl in preparation diluent (mol/L)

40/1

c)

PCA: 35 water-born samples, 19 variables

-0,4 -0,2 0,0 0,2 0,4

-0,4

-0,2

0,0

0,2

0,4

PEWC

G'-0.1G'-1

G'-10

G''-0.1G''-1G''-10

tg-0.1tg-1

tg-10

J-0.04

J-0.1

J-1J-10J-100

CreepGIWC

CC

p1

p 2

PCA: 35 water-born samples, 19 variables

-8 -6 -4 -2 0 2 4 6

-3

-2

-1

0

1

2

3

4

5

40/0.140/0.540/1

40/2

40/3

40/4

40/5

50/0.1

50/0.550/1

50/2

50/3

50/4

50/5

60/0.1

60/0.560/160/2

60/3

70/0.1

70/0.570/1

60/4

60/5

70/2

70/3

70/4

70/580/0.1

80/0.5

80/180/1.5

80/2

t1 (56%)

t 2 (30%

)

PCA: 35 water-born samples, 19 variables

-0,4 -0,2 0,0 0,2 0,4

-0,4

-0,2

0,0

0,2

0,4

PEWC

G'-0.1G'-1

G'-10

G''-0.1G''-1G''-10

tg-0.1tg-1

tg-10

J-0.04

J-0.1

J-1J-10J-100

CreepGIWC

CC

p1

p 2

PCA: 35 samples, 4 variables

-2 0 2 4

-2

-1

0

1

2

3

4

40/0.140/0.540/140/2

40/3

40/4

40/5

50/0.150/0.550/1

50/2

50/3

50/4

50/5

60/0.160/0.560/160/2

60/3

70/0.170/0.5

70/1

60/4

60/5

70/2

70/3

70/470/5

80/0.1

80/0.580/180/1.580/2 t

1 (51%)

t 2 (42%

)

Equilibrium swelling, low frequency modulus, and pair of loss factorsThese variables are important as such, and therefore they are usually determined

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?