Morphology assessment of polymer hydrogels using multivariate analysis of viscoelastic and swelling properties Evgeny Karpushkin, AndreyBogomolov 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 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) 8056-8060 1 mm Biomaterials 26 (2005) 1507–1514
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?
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
Preparation of samples glass plates APS diluent (NH4)2S2O8 HEMA TEMED DEGDMA rubber sealing Sample code Variable parameters: • 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
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)
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
Morphology of hydrogels:salting-out diluent at preparation (0.2M NaCl)60/1 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!
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
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?