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Polymer Science and Engineering

Elena StachewCase Western Reserve University

Advisors: Dr. Myunghwan Byun, Prof. Ryan C. Hayward

Shape control of thermally responsive non-Euclidean

sheets with patterned swelling

2011 REU SYMPOSIUMBackground: Why is this important?

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Overall Goal: To formulate design rules for preparation of 3D shapes by 2D metrics on elastic sheets

Constrained/patterned swelling provides access to tailored actuating structures: sensors, microfluidics, active biomaterials, artificial muscles, soft robotics

Understand morphogenetic mechanisms of biological materials

2011 REU SYMPOSIUMShaping Mechanism: Buckling

Balance of stretching and bending energies dependent upon thickness of ruler

Sharon, et al., Soft Matter 6 5693 (2010)

Stretching energy:

Bending energy:

2011 REU SYMPOSIUMUtilizing Mathematical Tools to Create 3D

Shapes Spatially varying growth (i.e.

distinct patterned swelling) defines a target Gaussian curvature

The shape of this configuration is characterized by the two principle radii of curvature at each point, R1 and R2

• Mean curvature: ½ ()• Gaussian curvature:

Target metric of disk: Target Gaussian curvature of disk:

Sharon, et al., Soft Matter 6 5693 (2010)

http:/en.wikipedia.org/wiki/Gaussian curvature

2011 REU SYMPOSIUMSwelling Behavior of Poly(NIPAm-co-BPAm-co-

AAc) Hydrogel Monomers:

• NIPAm: temperature-responsive, hydrophilic• BPAm: photo cross-linker, hydrophobic• AAc: hydrophilic• RhBMA: fluorescent

Temperature responsive behavior • Contribution of hydrophilic domains greater at lower

temperatures • Contribution of hydrophobic domains greater at higher

temperatures

Kim et. al. In preparation. 2011.

2011 REU SYMPOSIUM

Previous Work: Bi-strip Objectives: Multi-strips In-plane stresses that

result from difference in swelling result in out-of-plane buckling

If length of each strip is a:• a ~ w >> h2/3w1/3 >> h• w > a > h2/3w1/3>> h• w >> h2/3w1/3 > a > h• w >> h2/3w1/3 >> h > a

Outline

R ~ Δ ~ h2/3w1/3


a a a

w

2011 REU SYMPOSIUMGrayscale Gel Lithography, Development

and Swelling A-Prepare substrate and spin-

coat sacrificial layer B-Pattern hydrogel with

specific UV dose and photomask pattern

C-Each irradiation step provides fluorescence contrast for alignment of subsequent photomasks

D-Developed by washing with a marginal solvent

E-Immersion in aqueous medium Kim et. al. In preparation. 2011.

2011 REU SYMPOSIUMAreal Swelling of Poly(NIPAm-co-BPAm-co-AAc)

Copolymer Disc as a Function of UV Exposure TimeΩ = 4.1

Ω = 3.04 Ω = 2.84 Ω = 2.51

60 sec

90 sec 120 sec 180 sec

Ω = 5.31 Ω = 4.78

Ω = 2.04

480 sec

30 sec 45 sec

Ω = 2.45

240 sec Scale bar = 400 µm

Mask

0 100 200 300 400 500

2

3

4

5

6

7

8

9

Are

al S

wel

ling

(A/A

0),

UV Exposure Time (sec.)

10 100 10002

3

4

5

6

7

8

9

Area

l Sw

ellin

g (A

/A0)

,

UV Exposure Time (sec.)

UV dose = UV intensity (73.2 mW/cm2) X sec

2011 REU SYMPOSIUMResults and Analysis of Structures with Swelling

Conditions Ωhigh = 4.1 and Ωlow = 2.04

R = 180 microns R = 150 microns R = 110 microns



2011 REU SYMPOSIUMResults and Analysis of Structures with Swelling Conditions Ωhigh = 8.2 and Ωlow =

2.04


R = 135 microns R = 120 microns

2011 REU SYMPOSIUMHow is the radius affected by the number of

transitions? n strips

n-1 transitions

R ~

1 10

0.2

0.3

0.4

0.5

0.6

Low swelling contrast High swelling contrast

App

roxi

mat

e A

vera

ge R

adiu

s

Number of Transition Regions

-1/3

2011 REU SYMPOSIUMDemonstration of Temperature

Dependent Behavior

Swelled state at approx. RT De-swelled state at approx. 50⁰C

Increasing temp.

2011 REU SYMPOSIUMConclusions and Future Work

Briefly examined relationships:• a ~ w >> h2/3w1/3 >> h• w > a > h2/3w1/3>> h• w >> h2/3w1/3 > a > h

R ~ Examine relationship: w >> h2/3w1/3 >> h > a Solid qualitative analysis of radius Change contrast in swelling ratio Change thickness and width of hydrogel films What sets Δ and R?

2011 REU SYMPOSIUM

Acknowledgments

This work was funded by the NSF MRSEC on Polymers (DMR-0820506).

Thank you to Professor Hayward, Dr. Myunghwan Byun and the rest of the Hayward group.

Thank you to Jennifer Green and MRSEC staff for helping with arrangements for the summer program.

2011 REU SYMPOSIUMPreparation of Substrate

Key objective: Ensure silicon wafer surface is free of particles, so as to avoid poor adhesion, de-wetting

Process:• Sonication with acetone and

ethanol• UVO exposure

Key objective: Deposit thin, uniform layer of water-soluble polymer

Process: Spin coat 2.5-3wt% PAA-Na+(aq)

solution Thermally anneal at 150⁰C for 3 hours Ionically crosslink the PAA chains

using CaCl2 solutionKim et. al. In preparation. 2011.

2011 REU SYMPOSIUMEvaporation of Solvent

Key Objective: Prevent skin formation from solvent evaporation, so no residual stress acts within hydrogel

Process:• Heat drop-casted silicon wafer in enclosed glass

chamber at 60⁰C overnight

Reasoning:• High vapor pressure of chloroform, absence of free

surface and increased mobility of polymer chains


2011 REU SYMPOSIUMMask size = 400 × 400 µm2

Swelled sheet ≈ 623 × 887 µm2

WMask = 33.3 µm LMask = 400 µmWhigh Ω ≈ 93 µm and Lhigh Ω ≈ 662 µmWlow Ω ≈ 54 µm and Llow Ω ≈ 623 µm

Linear swelling ratio along the X axis High Ω area = 1.7 Low Ω area = 1.6Linear swelling ratio along the Y axis High Ω area ≈ 2.9Low Ω area ≈ 1.6

X

Y

400 µm

X

Y

Mask size = 400 × 400 µm2

Swelled sheet = 623 × 887 µm2


Linear swelling ratio along the X axis High Ω area ≈ 1.7 Low Ω area ≈ 1.6Linear swelling ratio along the Y axis High Ω area ≈ 2.7Low Ω area ≈ 1.7 400 µm

2011 REU SYMPOSIUM

X

Y

400 µm

X

Y

400 µm




Linear swelling ratio along the X axis High Ω area ≈ 1.7 Low Ω area ≈ 1.6Linear swelling ratio along the Y axis High Ω area ≈ 3.1Low Ω area ≈ 1.2




Linear swelling ratio along the X axis High Ω area = 1.7Low Ω area = 1.6Linear swelling ratio along the Y axis High Ω area ≈ 3.0Low Ω area ≈ 1.2

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