encapsulation technology: principles and applicationsbh.knu.ac.kr/~inwoo/lectureicon/encapsulation...
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Encapsulation technology: Principles and Applications
In Woo Cheong, Ph.D. Associate Professor
Department of
Applied Chemistry, Kyungpook National University
www.imagico.de
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Backgrounds
Small is not only beautiful but also eminently useful - Prof. JH Fendler
www.digital-photography-school.com
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What are capsules ? Nano- or micron-sized containers !!
Core materials: liquid, solid, gas, protein, cell, etc Shell materials: (i) Organics: polymers, lipids, surfactant (gelatin, urea-urethane, melamine resin, block copolymer, etc) (ii) Inorganic ceramics (SiO2, TiO2, Al2O3, etc) (iii) O/I hybrids (R-SiO2, R-TiO2, R-Al2O3, etc)
From “Smart Capsules for Flexible Electronics” by Dr. S.S. Lee at KIST
oil
In-situ polymerization
oil
Interfacial polymerization
oil
Complex coacervation
Emulsion-based encapsulation (o/w system)
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Why do we know about capsules ?
As reaction container
Protection of vulnerable stuff
Mass transport (release)
Nanoparticle formation Polymerization Coupling rxn, etc. Field responsive materials
Bio-active materials Cell & protein encapsulation Fragrant oils, etc.
Drug delivery Anti-corrosive coating Self-healing Redox rxn, etc.
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Back to the principle, ”How to make capsules ?”
Thermodynamic Consideration
Torza S, Mason SG, J Colloid Interface Sci., 33, 6783 (1970)
Spreading Coefficient
Si = γjk - (γij + γik) where, γjk is interfacial tension between j and k phases. Condition for complete engulfing of phase 1 by phase 3 S1 < 0(γ23 < γ12), when S2<0 and S3>0
1: hydrophobic liquid, 2: water, 3: polymer
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Basic Understanding: - Surface Phenomena
Why most of the capsules are spherical ?
The molecules at the surface must have a higher energy than those in bulk, since they are partially freed from bonding with neighboring molecules !
Water
Air
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Basic Understanding: - Surface Phenomena
Therefore, work must be done to take fully interacting molecules from the bulk of the liquid to create any new surface surface tension
How to measure the surface tension ?
Then how about with solid materials ?
A Work
Wc
Wc = 2ⅹsurface energy (2ⅹAⅹγs)
Unfortunately, we can’t define the surface area exactly…
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Basic Understanding: - Surface Phenomena
Measuring contact angle !
θ
γLV
γLS
γSV
θ
liquid
vapor
dl*
dl solid
Top-view
ldlldlldldG SVLVSL γγγ −+= *
θcos* dldl =
θγγγ cosLVSLSV +=Therefore, …Young equation
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Basic Understanding: - Surface Phenomena
Then how to determine γSL and γSV ?
• Measure the contact angle of liquids with various surface energy (γLV) and plot γLV vs. cosθ.
•Extrapolate it with the value of θ becomes 0 (we call this value γc, complete wetting) and then we can obtain (γc =) γSV.
• For specific liquid system, we apply γSV value and get γSL.
20 30 40 50 γLV/mJm-2
cosθ
1.0
γc
γc = γSV
(complete wetting, γSL0)
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Basic Understanding: - Surface Phenomena
• Surface energy of solids is closely related to its cohesive energy (The higher the surface energy, the higher its cohesion)
• Surrounding (water, vacuum, air, etc.) property significantly affect the force required to make a new surface (i.e., crack propagation)
• At the equilibrium, θγγγ cosLVSLSV +=
If we add surfactant, drop will spread, γSV - γSL - γLV > 0
Here we can define a parameter (Spreading coefficient); SLS = γSV - γSL - γLV
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How to make capsules ? Thermodynamic Consideration
Torza S, Mason SG, J Colloid Interface Sci., 33, 6783 (1970)
Spreading Coefficient
Si = γjk - (γij + γik) where, γjk is interfacial tension between j and k phases. Condition for complete engulfing of phase 1 by phase 3 S1 < 0(γ23 < γ12), when S2<0 and S3>0
1: hydrophobic liquid, 2: water, 3: polymer
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Basic Understanding: - Colloidal Phenomena
Grind to submicron size
bulk colloid “true” solution
Fundamental forces operate on fine particles 1. A gravitational force (settling or creaming depends on density difference) 2. Viscous drag force (resistance to motion) 3. Natural kinetic energy of particles and molecules (Brownian motion)
What are Colloids ?
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Basic Understanding: - Colloidal Phenomena
분자Colloid 입자Colloid Micelle Colloid
Egg, Protein, PVA, etc.
Natural rubber, Latex paint, milk, ice-cream, etc.
Detergent, Shampoo, Liposome, etc.
Type of Colloids
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Basic Understanding: - Colloidal Phenomena
Large surface area :Adsorption property
Light scattering : Tyndall phenomena
Electrically charged : Elecrophoresis Etc.: Brownian motion
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Basic Understanding: - Colloidal Phenomena
• Colloidal particles prepared from natural or synthetic process in nano and micron-sizes.
• Large surface area • Various typical properties (surface property) • Mineral, metals, protein, polymer, etc.
SEM image of heterocoagulated polymer particles
starch latex paint waste water milk treatment
Natural Rubber latex
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Thermodynamic aspect Phase transition accompanies change in standard free energy, ∆Gf = γ ∆A
> 0 < 0
Colloidal stability is poor (Lyophobic) Coagulation Thermodynamically stable (Lyophilic)
∆Gf
∆Gf
∆Gf Bulk Colloids
Basic Understanding: - Colloidal Phenomena
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Thermodynamic aspect Lyophobic colloids, even if they are thermodynamically unstable, can be made “metastable” for long periods of time if an energy barrier of sufficient height can be erected between the bulk and colloidal state. “Kinetically stable”
Hydrophobic tail
Hydrophilic head
Basic Understanding: - Colloidal Phenomena
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Synthesis
www.digital-photography-school.com
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Historical stuffs • Christopher Columbus discovered
natural latex.
• 1839-1844 Charles Goodyear – Vulcanized latex was invented.
• Before World War I, synthetic rubbers from emulsion (exactly not from emulsion, but from suspension).
• 1920s - World War II, “true” emulsion polymerization was conducted.
Natural rubber tree: Hevea Brasilensis
30-40% 100% cis-Polyisoprene 50-60% Serum Etc. Lipids, Proteins, Inorganics
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Historical stuffs Original reasoning: they assumed they could polymerize emulsion droplets ⇒ polymer latex:
free-radical initiator
Poor quality products because of wrong mechanism
water Monomer droplet
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Historical stuffs
Polymer particles ~ 100 nm diameter each containing many polymer chains, stabilized by surfactant
water
monomer
surfactant solution initiator solution
latex (polymer particles 100 nm diameter)
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Heterogeneous Polymerization Generation of tiny particles
From the precept of laborious works on kinetics: Micelles or monomer droplet can be a primary locus of reaction … a state we call “nano- or micro-reactor”
Droplets
∆E
Nano-reactor
Small size Protection Mass and heat transfer
1018~1021 nano-compartments/L
RXN
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Heterogeneous Polymerization Why nanoparticle ?
– Better storage stability
– Better transparency
– Fast film formation rate and permeability
– High reaction rate
Energy of Particle (Etot) = Ei + Es = eiV + γA ei: Energy per unit volume γ: Surface Energy per unit volume Therefore, Etot/unit volume = ei + γ(A/V) Dp (nm) A/V(cm-1) 1 6x107
10 6x106 100 6x105
A Problem: Aggregation or flocculation of nanoparticles
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Heterogeneous Polymerization
Type Typical Particle Radius
Droplet size Initiator Continuous
Phase Discrete phase
(particles)
Emulsion 50 – 300 nm ≈ 1 – 10 µm water or oil soluble Water
Initially absent, monomer-
swollen polymer particles form
Dispersion ≥ 1µm - oil soluble
Organic (poor solvent
for formed polymer)
Initially absent, monomer-
swollen polymer particles form
Suspension ≥ 1 µm ≈ 1 – 10 µm oil soluble Water
Monomer + formes polymer in pre-existing
droplets
Inverse Emulsion 102 – 103 nm ≈ 1 – 10 µm
water or oil soluble
oil Monomer,
cosurfactant + formed polymer
Microemulsion 10 – 30 nm ≈ 10 nm water soluble Water
Monomer cosurfactant +
Formed polymer
Miniemulsion 30 – 100 nm ≈ 30 nm water soluble Water
Monomer, cosurfactant +
formed polymer
The differential types of heterogeneous polymerization systems
www.andrew.cmu.edu/user/kemin/Research.htm
Suspension Emulsion Miniemulsion Microemulsion
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Emulsion Polymerization Free-radical polymerization
• Usually vinylic: CH2= CR1R2 • R1 = H:
– R2 Name – –Ph styrene – –CH=CH2 butadiene – –Cl vinyl chloride – –CO2H acrylic acid – –CO2Me methyl acrylate (butyl, …) – –OCOCH3 vinyl acetate
• R1 = CH3:
– –CO2Me methyl methacrylate (MMA) (butyl, …) • R1 = Cl:
– –CH=CH2 chlorobutadiene (neoprene)
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Emulsion Polymerization • Initiation:
– e.g. R–N=N–R → 2R• + N2 ; rate coefficient kd R • + M → RM •
• Propagation: (monomer unit M) – –Mn• + M → –Mn+1• rate coefficient kp
• Termination:
– 2R• → dead polymer rate coefficient kt
• Transfer, e.g. to monomer: – –Mn• + M → –Mn + M • rate coefficient ktr – M • then starts another chain
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Emulsion Polymerization
Core/shell Hemisphere Occlusions
Various morphologies: electron microscopic images
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Emulsion Polymerization Various morphologies: electron microscopic images
S Omi et al., J Applied Polym Sci., 66, 7, 1327 (1998)
Snowman-like Porous morphology
Rugby ball-like Raspberry-like
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Emulsion Polymerization
Transmission Electron Micrograph Showing the Cross-Sections of OsO4-Stained Two-Stage (20 PS/80 (S/B)) Latex Particles
100 nm
Polystyrene Core (20 parts)
S/B Copolymer Shell (80 Parts)
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Emulsion Polymerization TEM sample preparation techniques
Shadowing
Pt, Cr particles [RuO4 제조의 예] 2NaIO4 + RuO2 RuO4 + 2NaIO4
Staining
Microtoming
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Microemulsion Polymerization • Microemulsion: transparent liquid system consists of at least
ternary mixtures of oil, water, surfactant. • It exhibits continuous or bicontinuous structure with < 100 nm
scale.
www.baschem.co.uk
W/O
O/W
Oil (O)
Water (W) Surfactant (S)
Bicontinuous
Liquid crystalline
W
O
O
W I
W II
W III
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Microemulsion Polymerization • Surfactants
– SDS : needs co-surfactants, short chain alcohols – Nonionics, some cationics (e.g., CTAB, DTAB), double chain
surfactants (e.g., Aerosol OT) need no co-surfactants
• Features – Thermodynamically stable – Enormous inner surface area – Various morphologies – No steady state reaction rate – Inorganic particle formation – Large amount of surfactant (7-15wt%)
Andrey J. Zarur and Jackie Y. Ying Nature 403, 65-67
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Microemulsion Polymerization Surfactant system: wet template
os lavN /=Packing parameter (shape factor)
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Microemulsion Polymerization
Hsiang Y. W. et al, Chem Mat 2005, 17, 6447
Zhaoping Liu, et al, Langmuir 2004, 20, 214
K. Landfester et al., Macromolecules 2000, 33, 2370
JS Jang et. al., Chem Comm, 2003
Making various morphologies
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Emulsification Techniques • Features
– Post emulsion process – Uncontrollable particle size distribution – Methods:
• Direct emulsification – External surfactant assisted emulsification – Neutralization emulsification
• Other emulsification methods – Emulsification-diffusion emulsification – Nanoprecipitation – Dialysis – Membrane emulsification – Self-assembly technique
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Emulsification Techniques Emulsification-diffusion emulsification
Water + Stabilizer
Emulsification
Adding excess water
Solvent diffusion
PLGA + Solvent
50 nm50 nm50 nm
TEM micrograph of PLGA nanoparticles produced by ED method.
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Emulsification Techniques Nanoprecipitation and dialysis methods
Nanoprecipitation Dialysis
water
emulsifier
polymer solvent drugs
microsyringe pump piezoelectric nozzle
dialysis tube PLGA
hydrophobic probe
TEM micrograph of core-type particles produced by nanoprecipitation.
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Emulsification Techniques Membrane emulsification
Optical micrograph of W/O/W multiple emulsion droplet containing vitamin C by membrane emulsification. O/W W/O/W
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Emulsification Techniques
worms vesicles Starfish vesicle
lamellae large compound vesicles (LCV)
Self-assembly technique by Block Copolymers
Micellization of PS-PAA block copolymers under different conditions (i.e., ionic strength, concentration of polymer, MDF/water ratio, etc.)
Amphiphilic block copolymer
Micelles
vs. Gels
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Block Copolymers A living free-radical polymerization
– No termination or chain transfer – Radical chain remains active when all the monomer is used up – Propagation continues when additional monomer is added
– Block copolymer formation! – Example : atom transfer radical polymerization
. . CH3CHCl + Cu(I)(bpy) CH3CH + Cu(II)(bpy)Cl
φ φ .
Initiation
CH3CH + CH2=CH CH3CHCH2CH φ φ φ Propagation
. Atom transfer CH3CHCH2CH + Cu(II)(bpy)Cl CH3CHCH2CHCl + Cu(I)(bpy)
φ φ φ φ
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Block Copolymers Formation of block copolymers
(a) sequential “controlled/ living “block copolymerization
(sequential addition of monomers)
(b) coupling of linear chains
containing antagonist functions ( X and Y )
(c) switching from one
polymerization method to another
(d) use of a dual (“double-
head”) initiator consisting of two distinct initiating fragment ( I1 and I2 )
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Block Copolymers Crosslinking
Shell crosslinking J Am Chem Soc 2000;122:3642–51.
Core crosslinking Macromolecules 2000;33: 4780–90.
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Block Copolymers Stimuli-responsive nano-assemblies
– Intelligent, smart, environmentally sensitive, etc. – Stimuli : light, temp., solvent, pH, chemicals, etc. – Drug release, encapsulation, intelligent switches
PS-co-P2VP-co-PEO
Core/shell/corona
2-(dimethylamino)-ethyl methacrylate 2-(diethylamino) ethyl methacrylate Poly(DMAEMA/DEAEMA) diblock copolymer
Chem Commun 1997;671–2.
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Applications
www.digital-photography-school.com
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Applications
• Anti-corrosive coatings – Sacrificial means: zinc-rich
coating
– Barrier effect: polymer coatings, inorganic filler (eg. MMT): increases pathway by parallel arrangement, stainless flakes, glass flakes, etc.
– Inhibition: Cr and Pb-based pigments metal phosphate, silicate, titanate or molybdate compounds
• Self-cleaning coatings – Hydrophobic-hydrophilic
effects – Lotus effect – Photo-reactive : TiO2
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Applications Self-cleaning coating with TiO2
– Photo-catalytic titanium dioxide (TiO2): A strong oxidation power & super-hydrophilicity
– TiO2 coating cannot be coated directly onto an organic paint surface as this will attack the paint surface, causing a phenomenon so called paint-chalking.
Substrate
: inorganic linker : organic or polymer
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Applications
52 days 32 days 24 days
Copper plating coating Composite coating 2 days
8 days 12 days 16 days
Swapan K G, Functional Coatings, Wiley-VCH, 2006.
Encapsulation of Ultra-hydrophobes
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Applications
S. R. White et al, Nature 2001, 409, 794
Self-healing Plastics Matrix: Epoxy Microcapsule : Urea-formaldehyde + dicyclopentadiene (DCPD) Catalyst:
U of Illinois at UC
coating without capsule
coating with capsule
0 hr
24 hrs
48 hrs
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Applications pH-induced Micellization
Angew. Chem. Int., Ed 2003;42:1516–9.
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Applications E-paper
Characteristics : • Flexible like news paper • Wide-angular readability • Low energy (No back-light) • Potable (light-weight)
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Applications
+/- charged core-shell particle
~ 200 nm
20 - 50 µm Nature, 394, 16, July 1998.
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Applications
prepolymer migration and crosslinking
in situ polymerization
Urethane prepolymer chain extender
interfacial polymerization
transparency durability flexibility impermeability thermal and chemical resistance
Characteristics of shell materials
E-paper 100μm
From “Smart Capsules for Flexible Electronics” by Dr. S.S. Lee at KIST
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Previous and Current Works on Encapsulation
MF@Fragrant oil J. Microencapsulation, 19(5), 559 (2002)
PAni@PS Synthetic Metals, 151(3), 246 (2005)
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Previous and Current Works on Encapsulation
Phase Change Materials
PCM
Bulk PCM
Microencapsulated PCM
Nanoencapsulated PCM
100 nm TEM image of PCM nanocapsule prepared by using ultramicrotome
Temperature ( ℃)
0 200 400 600 800
Wei
ght (
%)
-20
0
20
40
60
80
100
120 60% PCM PS CapsulePure OctadecanPolystyrene
TGA curve for capsulation efficiency analysis
Octadecane
Polystyrene
Octadecane@PS Korean Patent 10-0612139 (2005)
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Previous and Current Works on Encapsulation
Phase Change Materials
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Previous and Current Works on Encapsulation
Multi-walled Carbon Nanotubes
Macromol. Res., 14(5), 545 (2006) Korean Patent 2006-94071 Korean Patent (출원) 2008-0046401 (2008) Composites Sci. Tech., accepted in 2008
CNT
Amphiphilic Macromolecules
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57 Kyungpook National University