1. Volume-Phase Transitions in
Surface-Tethered Networks and
Implications for Swelling
Instabilities
Ryan Toomey
Department of Chemical & Biomedical Engineering
University of South Florida
Tampa, FL 33620
1-D Fibers 2-D Coatings 3-D Structures
5 μm 5 μm
5. 1E-3 0.01 0.1 1
1
10
100
DegreeofVolumetricSwelling
1/Nc
1E-4 1E-3 0.01
1
2
3
4
5
6
7
8
9
LinearSwellRatio(L/L0
)
1/Nc
Unconstrained
Surface-Attached
Unconstrained
Surface-Attached
Unconstrained and
Surface-Attached
Poly(dimethylacrylamide)
networks
Volumetric Swelling
Linear Swelling
The surface-attached networks
experience a higher degree of
linear deformation than the
unconstrained networks
5/3
c
o
s
N
V
V
Vs
Vo
µ Nc
1/3
Toomey et al., Macromolecules (2004)
6. Can we use the phase diagram
of linear polymers to predict
the volume-phase transition
behavior in confined
geometries?
13. UV Cross-Linkable Copolymer Films
Benzophenone based monomers form
cross-links with free aliphatic groups
365 nm
Statistical copolymers comprising UV-sensitive benzophenone moieties are
deposited and photo-cross-linked: Strategy permits multilayer build-up of
several polymer types
14. • Activated at 365 nm (non-damaging)
• Provides both cross-linking and surface-attachment
• Cross-links in the presence of oxygen and water
O
R
H
C
H
R
C
R H
C O
R
C O
H
R
C R
C O
H
R
365 nm
h
Photo-Crosslinking Benzophenone
BENZOPHENONE
15. Reflected
Beam
Neutron Reflection
Heated Jacket
Quartz
D2O
Poly(NIPAAm)
Incident
Beam
Heated Jacket
q q
Neutrons are incident at low angles
Reflection arises due to mismatch between averaged scattering length
densities of atomic nuclei in the direction normal to the interface
Measurement of Reflectivity versus provides “fingerprint” of
density profile with Angstrom scale resolution
Material SLD (x 106)Å2
Quartz
Poly(NIPAAm)
D2O
4.17
0.96
6.33
q sin4zq
16. • Plot Intensity vs.
• Fringe spacing length
scale of sample
• Solution to the reflectivity
profile is non-unique
• Modeling profile determines
real space interpretation
zq
Neutron Reflection
Poly(NIPAAm-co-MaBP (3%))
Dry Layer Thickness = 320 Å
0.00 0.05 0.10 0.15 0.20 0.25 0.30
-7
-6
-5
-4
-3
-2
-1
0
0 100 200 300 400
0
1
2
3
4
SLD(Å-2
)x10
6
z (Å)
Quartz Substrate
Log(R)
qz
(Å
-1
)
Against Dry Air
at 40
o
C
17. 0.00 0.05 0.10 0.15 0.20 0.25 0.30
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 100 200 300 400
0
1
2
3
4
SLD(Å-2
)x10
6
z (Å)
Quartz Substrate
Against D2
O Vapor at 40
o
C
Log(R)
qz
(Å
-1
)
Against Dry Air
at 40
o
C
Neutron reflection of dry film
Neutron Reflection Profiles of Poly(NIPAAm-co-MaBP (3%))
Dry Layer Thickness = 320 Å
Scattering length density ~20% lower than expected. Suggests 2-3 water
molecules associated with each polymer segment
18. 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08
10
-14
10
-13
10
-12
10
-11
10
-10
10
-9
10
-8
33
o
C
29
o
C
27
o
C
Reflectivity*q
4
z
Momentum Vector (Å
-1
)
15
o
C
Best fit NR profiles for temperature range 15oC-33oC
Neutron reflection of wet films
Neutron Reflection Profiles of Poly(NIPAAm-co-MaBP (3%))
Dry Layer Thickness = 270 Å
19. • Approximately 2-3 D2 O molecules per polymer segment in the collapsed state
• Extended interface between swollen network and D2O
0 500 1000 1500 2000 2500
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
15
o
C
22
o
C
25
o
C
29
o
C
31
o
C
33
o
C
42
o
C
VolumeFraction
z (Angstroms)
Segment Density Profiles
Extended profile
due to surface buckling
20. 15 20 25 30 35 40 45
0
200
400
600
800
1000
1200
<Z>(Å)
Temperature (o
C)
Thickness versus Temperature
• Gradual transition between 15-29 oC followed by strong collapse
“Dry” layer thickness
21. Prediction of Discontinuity?
0.0 0.2 0.4 0.6 0.8 1.0
285
290
295
300
305
310
315
320
1-Phase
Volume Fraction
Binodal (prediction)
Linear poly(NIPAAm) (experimental)
3% cross-linked network
Temperature(K)
Critical Point
2-Phase
Avidyasagar et al. Macromolecules (2008)
22. 20 25 30 35
1.0
1.5
2.0
2.5
3.0
3.5
4.0
<Z>/Zdry
Temperature (o
C)
Effect of cross-link density
MaBP (10%)
MaBP (3%)
Poly(NIPAAm-co-MaBP (x%))
23. 0.0 0.2 0.4 0.6 0.8 1.0
285
290
295
300
305
310
315
320
2-Phase
3% cross-link denisty
10% cross-link density
1-Phase
Volume Fraction
Temperature(K)
Critical Point
Effect of cross-linking on phase behavior
Layers prepared to the left of the critical point can enter the 2-phase region, whereas
layers prepared to the right of the critical point remain in the 1-phase region
24. Effect of cross-linking on phase behavior
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
285
290
295
300
305
310
315
320
10% MaBP5% MaBP3% MaBP
1% MaBP
Volume Fraction
Temperature(K)
Cloud Curve
2-Phase
Repeat of previous experiments: photo-crosslinked samples above demixing
temperature to drive out water
Avidysagar et al. Soft Matter (2009)
26. O
NH
Release of ordered water from hydrophobic shells overcomes
entropy loss with collapse of network
Hydrophilic hydration
shells stay intact?
At higher temperatures hydrophobic forces dominate and drive collapse
High Temperatures Favor Collapse
27. 4000 3000 2000 1000
0
2
4
6
N-D
Amide II
N-H
Amide II
D2
O
102
O
C (Saturated Vapor)
50
O
C (Liquid)
Absorbance
Wavenumber(CM
-1
)
H2
O
Is water left in the collapse?
Isotopic substitution of N-H with D2O readily observed in collapsed state
28. 0 10 20 30 40 50 60 70 80
1.0
1.5
2.0
2.5
3.0
3.5
4.0
SwellingRatio(Hw
/Hdry
)
Temperature(
o
C)
nPAAm
DEAAm
PNIPAAm
cPAAm
NVIBAAm
Effect of monomer structure
NH
O
NH
O
NH
O
NH
O
29. 10 20 30 40 50 60 70
1.0
1.5
2.0
2.5
3.0
3.5
4.0
poly(NVIBm)
poly(NVIBm)
poly(CPAAm)
poly(CPAAm)
H/Hdry
Temperature (
o
C)
Cross-linked Films of
Poly(cPAAm) and poly(NVIBm)
Poly(NVIBAm) has a discontinuous transition with hysteresis. Poly(CPAAm) has a
continuous transition with no hysteresis. Both cross-linked with 3 % mole MnBP.
Soft Matter, 2013
30. Cloud Point Curves
0 5 10 15 20 25 30 35 40 45 50
10
15
20
25
30
35
40
45
50
Poly(CPAAm)
Poly(NVIBm)
Temperature(o
C)
Polymer Weight %
Poly(NVIBm) has an off-zero critical point, allowing a discontinuous transition.
Poly(cPAAm) has a critical point near zero concentration, preventing a discontinuous
transition.
31. 1700 1650 1600 1550 1500
0.0
0.5
1.0
1.5
2.0
Intensity
Wavenumber (cm
-1
)
25
o
C
40
o
C
70
o
C
2nd Order Derivarive of 70
o
C
1700 1650 1600 1550 1500
0
1
2
Intensity
Wavenumber (cm
-1
)
FTIR of poly(PVIBm) Coating
NH
O
O2H
H2O
(Amide II)
(Amide I)
(Amide I)
(Amide II)
Subbands
1650 1625
1540
Two distinct hydrogen bonding populations are observed at Amide I. The populations
change through the phase transition
20 30 40 50 60 70
0.30
0.32
0.34
0.36
0.38
0.40
0.42
0.44
0.46
0.48
A1650
/Atotal
Temperature
o
C
Change in area of
1650 sub-band
to total Amide I
32. 1700 1650 1600 1550 1500
0.0
0.5
1.0
1.5
2.0
Intensity
Wavenumber (cm
-1
)
25
o
C
40
o
C
70
o
C
2nd Order Derivative 70
o
C
FTIR of poly(CPAAm) coating
NH
OO2H
O2H (Amide II)
(Amide I)
(Amide I)
(Amide II)
1630
1540
In contrast to poly(NVIBm), only a single distinct sub-band found for the Amide I.
33. Phase diagram of (some) linear polymers may
serve to predict (or approximate) volume-phase
transition behavior in confined systems
In the single-phase region, swelling is determined
by a balance between chain elasticity and mixing
If the system enters the two-phase region, the
swelling jumps discontinuously to the polymer rich
binodal of the phase diagram
Summary … Part I
34. Adjustable Foundations
• Model of biological membranes
– Barrier between bulk phases
• Current status
– Bilayer on solid substrate easy to deposit but poor biological
mimic
– Bilayer on thick hydrated support difficult to deposit
• Solution: Adjustable cushion that provides a viable
surface for both deposition and natural mimicry
39. 3-D Structure at 25 °C
1000~
ScaleLengthplaneofOut
ScaleLengthplaneIn
40. What is happening?
1. How tightly bound is the lipid bilayer to the poly(NIPAAm) layer?
2. Are concentration fluctuations in the bilayer independent of the
underlying poly(NIPAAm) layer?
3. Or are concentration fluctuations in the poly(NIPAAm) layer
transmitted to the bilayer?
Poly(NIPAAM) coatings were imaged with AFM, but no
discernable out-of-plane structure could be measured,
… however poly(NIPAAm) coatings of 100x thickness
present significant creasing effects
10 mWe will deuterate the outer edge of the
poly(NIPAAm) coating to better understand fluctuations
In ultrathin filmsu
43. Effects of Surface Confinement
3 General Forms of Non-Uniform Swelling
Bulk Buckling
Differential Lateral Swelling
Edge Buckling
High Aspect Ratio Low Aspect Ratio
44. Bulk Buckling in pNIPAAm Structures
• Buckling occurs when:
– compressive stresses overcome the energy required to
bend the structure
DuPont et al. Soft Matter (2010)
45. Linear Swelling Model
• Developed by Mora et al. (2006)
• Describes onset and characteristics of bulk buckling
• Linearization of Föppl–von Kármán equations
T. Mora et al. Eur. Phys. J. E (2006)
Buckling Onset: Pc = 0.867 w2/h2
Wavelength: = 3.256h
Model Assumptions
• σ =f(ε) is linear (stress is a linear function of strain)
• kc is small (low magnitude strain)
46. Bulk Buckling in pNIPAAm Structures
Wavelength is only a function of height?
◊ - Dry Height ● - Swollen Height
DuPont et al. Soft Matter (2010)
47. Differential Lateral Swelling
Large Expansion of Structure’s Surface (ε: 0.1 – 3.5)
Low aspect ratio structures likely best for rapid cell release
50μm
48. Engineering Goal
• Culture organized μ-tissues of various shapes/sizes
• Rapid
• Minimize low temperature exposure
Develop Platform for:
Organization, Rapid Release, and Direct Stamping of μ-Tissues
49. Cell Release from pNIPAAm Surfaces
• Thin film pNIPAAm surfaces (<30nm)
• Very little geometric deformation
• Mostly normal to the surface
• Mostly hydration/dehydration of pNIPAAm
chains
• Release is slow (>10 min)
μm
T<32°C
T>32°C
DuPont et al. Soft Matter, 2010.
• 3D Micro-scale Surface Structures
• High geometric deformation
• In all spatial dimensions
• Induces stress in overlying cell sheet
• Lateral swelling of the gel
59. Cell Culture (Summary)
• Alignment of NIH 3T3 fibroblasts on micron-scale pNIPAAm
structures
• Ranging from 1 - 30μm thick, 4 - 100μm wide
• Rapid release of aligned μ-tissues
• Release occurs in the time scale of swelling (~ 3min)
• Temperature reduction minimized (28°C)
• Release of μ -tissues is primarily mechanical in nature
• Does not require metabolic activity
• Cell contractility effects detachment
• Micron-scale pNIPAAm structures as a platform for direct
stamping of μ-tissues
• μ-tissues maintain organization upon transfer
60. Final Remarks…
• Soft “messy” systems with externally cued
responses can show rich range of behavior
• Can we control, understand, and predict
“specificity” in response and behavior?
• How do we control differential swelling in
confined geometries to be able to produce 3D
patterns using standard 2D processing
techniques?
61. Ajay Vidyasagar (Ph.D.) – phase behavior
Leena Patra (Ph.D.) – phase behavior
Samuel DuPont (Ph.D.) – confined structures and cells
Alejandro Castellanos (Ph.D.)
Martiza Muniz (Ph.D.)
Vinicio Carias (Ph.D.)
Ophir Ortiz (Ph.D.)
Gulnur Efe (Ph.D.)
Ryan Cates (M.S.)
Carlos Bello (M.S.)
Prof. Nathan Gallant
USF, Mechanical Engineering
Camille and Henry Dreyfus Foundation
Draper Laboratory
NSF CMMI 107671
NSF CAREER DMR 0645574
NSF EEC 0530444
USF-BITT Center of Excellence
Smart Materials Research Group
