Shear rheometry characterization of fracture-healing behavior displayed by a physically associating gel

Abhishek Bawiskar and Prof. Kendra A. ErkSchool of Materials Engineering, Purdue University, West Lafayette, IN USA

Jamming in concentrated comb-polymer-adsorbed MgO suspensions, Lisa Murray The flow-induced jamming behavior of concentrated suspensions of MgO microparticles containing adsorbed PAA-PEO comb-polymer (ADVA) was studied with shear rheometry. Increased comb-polymer concentrations resulted in larger overshoot peaks which may indicate particle network formation, while multiple overshoots in the wide particle size distribution suspensions may be caused by a hydrocluster aggregates.

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She

ar S

tress

(Pa)

Time (s)

0.0 mg/g

0.06 mg/g

0.6 mg/g

6.0 mg/g

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She

ar S

tress

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Time (s)

0.001 s-1

0.01 s-10.1 s-1

1.0 s-1

0.1 1 10 1000

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She

ar S

tress

(Pa)

Time (s)

0.001 s-1

0.1 s-1

1.0 s-1

0.01 s-1

0.1 1 10 1000

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0.0 mg/gShe

ar S

tress

(Pa)

Time (s)

6.0 mg/g

Narrow MgO Distribution

(0.5-40 microns)

Wide MgODistribution

(0.5-400 microns)

Shear Start-Up = 0.001, 0.01, 0.1, 1.0 s-1

Shear Start-Up = 1.0 s-1 , comb-polymer concentrations

Networked Particles

Hydrocluster Aggregates

Polymers with temperature-responsive gelation are of particular significance as biomedical materials. For certain applications, it is important that the gels have the ability to heal if their structure becomes damaged. Here, we have devised a method to quantify healing kinetics using a shear rheometer.

T > 34C

Hold fractured sample for

specified aging time

Const. shear rate:

1 s-1

Cool sample

Const. temp.

T = target temperature

PMMA: Poly(methyl methacrylate) (9 kg/mol)PnBA: Poly(n-butyl acrylate)(53 kg/mol)

Dissolved in 2-Ethylhexanol at high temp; forms a physically associating network at T < 34°C..

Model triblock copolymer:

34°C

Endblock aggregates r ~5 nm

Midblock bridges

Mesh size, ξ ~ 40 nm

0 2 4 6 8 10

0

500

1000

1500

2000

2500

3000

60 min aging time

Stre

ss (P

a)Strain

First fracture curve

30 min aging time

5 min aging time

5 10 15 20 30 45 600.0

0.2

0.4

0.6

0.8

1.0

Rat

io

Healing time (min)

20C 25C 28C

• To fracture, applied shear is fast (relative to relaxation time of the gel).• Peak stress just prior to fracture is measure of the gel’s elastic strength.• Healing is assessed by the aging time required for the gel to regain

total elastic strength upon subsequent re-fracture; quantified by peak stress ratio.

• Higher temperature results in faster healing: e.g., almost 80% strength restored after 5 min. age at 28°C while it takes over 30 min. at 25°C.

• Lower temperature results in slower healing kinetics as well as lower strain to fracture, indication of brittle behavior.

0.00332 0.00336 0.00340

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-8

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0

Aging time

ln(1

/tim

e (s

))

1/T (1/K)

Relaxation time

slope = Ea/ RT

Relaxation time (determined from step-strain experiments) can be approximated to an Arrhenius plot:

Healing time (aging time required for 100% recovery) also fits the same plot with different constant for parallel shift:

Ea corresponds to activation energy associated with polymer chain pull out

COO-

COO-Ca2+

COO-

COO-Al3+COO-Na+

COO- Na+

Na+COO-

dry SAP

swollen acrylic-acid-based SAP

Superabsorbent polymer (SAP) hydrogels are used as internal curing agents to mitigate shrinkage/cracking in high-performance concrete. During the hydration process of concrete, multivalent ions such as Ca2+ are released into the system. Our results show that when these ions are present, the overall swelling capacity of the SAPs decrease, swelling kinetics change, and stiff-shell/soft-core structures can form.

wt% AA-AM 17% 33% 67% 83%

H2O solution 48.7 54.5 66.7 72.5

Na+ solution 26.9 37.1 44.9 44.0Ca2+ solution 9.5 7.2 3.3 2.3Al3+ solution 4.0 2.3 4.9 5.2

Total swelling of poly(acrylic acid-acrylamide) SAP with 2% xlinker

wt% AA-AM 17% 33% 67% 83%

1.0% xlinking 12.1 7.4 3.4 2.2

1.5% xlinking 10.5 6.9 2.4 2.0

2.0% xlinking 9.5 7.2 3.3 2.3

Ionic sensitivity of hydrogels used for internal curing of cement, Qian Zhu

0 50 100 150 200 2500

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Ca2+ solutionAl3+ solution

Na+ solution

Sw

ellin

g R

atio

, Q (g

/g)

Time (min)

H2O solution

Stiff-shell / soft-core structure forms in Al3+ solutions

Effect of covalent xlinking on swelling in Ca2+

Main Conclusions• SAP hydrogels are sensitive to the presence of ions,

especially Ca2+ and Al3+ which can form ionic crosslinks within the gel, leading to reduced swelling capacity and altered swelling kinetics.

• Gels containing more acrylic acid have more COO- groups available for ionic xlinking and thus, are more sensitive to the presence of ions.

6.0 mg/g

6.0 mg/g

Const. shear rate:

1 s-1

T < 34C, gel formation

Fracture! Re-Fracture!