Seeing’ strain in soft materials

Article abstract of DOI:10.3390/molecules24030542

Several technologies can be used for measuring strains of soft materials under high rate impact conditions. These technologies include high speed tensile test, split Hopkinson pressure bar test, digital image correlation and high speed X-ray imaging. Howe

Full text of DOI:10.3390/molecules24030542

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molecules
Article
‘Seeing’ Strain in Soft Materials
Zhiyong Xia 1,*, Vanessa D. Alphonse 1, Doug B. Trigg 1, Tim P. Harrigan 1, Jeff M. Paulson 1,
Quang T. Luong 1, Evan P. Lloyd 1 , Meredith H. Barbee 2 and Stephen L. Craig 2
1
Applied Physics Laboratory, The Johns Hopkins University, Laurel, MD 20723, USA;
vanessa.alphonse@jhuapl.edu (V.D.A.); Doug.trigg@jhuapl.edu (D.B.T.); Tim.harrigan@jhuapl.edu (T.P.H.);
jeff.paulson@jhuapl.edu (J.M.P.); quang.luong@jhuapl.edu (Q.T.L.); Evan.lloyd@jhuapl.edu (E.P.L.)
Department of Chemistry, Duke University, Durham, NC 27708, USA; Meredith.barbee@duke.edu (M.H.B.);
2
Stephen.craig@jhuapl.edu (S.L.C.)
*
Correspondence: Zhiyong.Xia@jhuapl.edu; Tel.: +1-2-402-289-694
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Elisabete C.B.A. Alegria and Maximilian N. Kopylovich
Received: 14 January 2019; Accepted: 30 January 2019; Published: 1 February 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Several technologies can be used for measuring strains of soft materials under high rate
impact conditions. These technologies include high speed tensile test, split Hopkinson pressure
bar test, digital image correlation and high speed X-ray imaging. However, none of these existing
technologies can produce a continuous 3D spatial strain distribution in the test specimen. Here
we report a novel passive strain sensor based on poly(dimethyl siloxane) (PDMS) elastomer with
covalently incorporated spiropyran (SP) mechanophore to measure impact induced strains. We have
shown that the incorporation of SP into PDMS at 0.25 wt% level can adequately measure impact
strains via color change under a high strain rate of 1500 s−1 within a fraction of a millisecond. Further,
the color change is fully reversible and thus can be used repeatedly. This technology has a high
potential to be used for quantifying brain strain for traumatic brain injury applications.
Keywords: spiropyran; impact strain; poly(dimethyl siloxane); mechanophore; strain sensing
1. Introduction
Strains of soft materials at high rates can be characterized using several technologies including
strain gauges and two dimensional digital image correlation (2D-DIC) during high-speed tensile
test [
into the specimen [
high rate testing. For example, the attachment of the strain gauges onto the specimen surface often
alters the local stress field of the soft material resulting in inaccurate strain measurement [ ]. Split
Hopkinson pressure bar test requires the establishment of a uniform stress/strain field through the
entire length of the specimen [ ]. However, due to the low wave speed in soft materials, this is not
always feasible resulting in poor data quality. 2D-DIC uses high speed cameras for capturing the
surface features during the high rate process for strain calculation [ ] but the interpolation function
used for computing the strains can have a large effect on the bias errors of the matching leading to
inaccurate measurement [ ]. When 2D-DIC is used for soft materials, marking inks are often needed
1], split Hopkinson pressure bar test [
2] and high-speed X-ray with lead tracers pre-embedded
3
]. These existing technologies suffer from a number of limitations when used for
1
2
4
5
due to the lack of surface features and the inks need to be compatible with the specimen surface a
condition that is not always possible. Lastly, high-speed X-ray requires very complex instrumentation
and complicated pre-test sample tracer mounting for bulk strain measurement [3]. Most importantly,
none of these existing technologies offers any direct measurements of the three dimensional (3D)
spatial strain distribution inside the specimen during the impact event.
To directly visualize the 3D spatial strain distribution, we turned to the growing field of polymer
mechanochemistry, specifically spiropyran (SP) mechanophore [
6,
7]. Upon mechanical stimulation,
Molecules 2019, 24, 542; doi:10.3390/molecules24030542
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the sp3 spiro carbon-oxygen (C-O) bond in SP breaks leading to the formation of a longer, colored
merocyanine (MC) [ 10]. By introducing ‘pulling points’ on both the chromene and indoline sides of
8–
the S-C-O junction, SP can be covalently bound to the matrix polymer, effectively coupling mechanical
stress in the matrix polymer to the C-O bond. A number of polymer networks and structures have
been demonstrated to exhibit mechanochromism [9–12], here we chose to focus on poly(dimethyl
siloxane) (PDMS) for its tunability in mechanical properties, optical clarity and its wide use as impact
simulants [13]. Alkene groups incorporated on both sides of the spiro junction allow easy covalent
incorporation into commercially available PDMS kits, which form networks using platinum catalyzed
hydrosilylation [11–15]. Since the transition between SP and MC is reversible, the color change in these
polymer systems is fully recoverable and the SP-polymer matrix can thus be used multiple times for
impact sensing.
Here we demonstrate the application of the SP mechanophores for sensing the impact strains
in a PDMS elastomer, referred to as SP-PDMS hereafter. The objective of this work is to use SP
mechanophore as a cost effective, passive strain sensor for measuring 3D spatial strains under impact
conditions. Our ultimate goal is to use SP-PDMS as a brain surrogate for quantifying brain strain for
traumatic brain injury (TBI), an injury that is responsible for 30% of all injury death in the Unites States
alone [16]. The accurate impact strain measurement in brain surrogates can greatly help the design of
impact helmet and further reduce TBI.
2. Experimental
2.1. Quasi-Static Tensile Test and Color Analysis
Quasi-static tensile testing was performed according to ASTM D412 with a die C geometry using
a screw-driven MTS 30G at an extension rate of 0.36 mm/s at room temperature and the strain was
measured via a strain gauge. During the tensile testing, a charge coupled device camera (Logitech
Brio, Newark, CA, USA) was used to observe color change. The color of the quasi-static samples after
stretching was deconvoluted into R, G, B values according to the method developed earlier [14]. Since
the color of the specimen after deformation is primarily blue, we adopted a ‘blueness’ parameter for
quantifying the color of each sample. The ‘blueness’ (B%) was computed by averaging the B value
over the entire R, G, B scale, that is, B% = B/(R + G +B).
2.2. SP Synthesis
30,30-dimethyl-6-nitro-10-(2-(pent-4-enoyloxy)ethyl)spiro[chromene-2,20-indolin]-8-yl pent-4-enoate
or spiropyran was synthesized according to [15] with the following modifications made to
the amounts of reagents and purification methods. To an oven dried round bottom flask,
10-(2-hydroxyethyl)-30,30-dimethyl-6-nitrospiro[chromene-2,20-indolin]-8-ol (3.0 g, 8.14 mmol, 1 equiv)
and 4-dimethylaminopyridine (0.099 g, 0.814 mmol, 0.1 equiv.) were dissolved in dry dichloromethane
(40 mL). The dark green suspension was stirred and 4-pentenoic anhydride (3.20 mL, 17.51 mmol,
2.15 equiv) was added in 3 separate aliquots, with 15 min between each addition. The reaction was
stirred overnight, resulting in a magenta-purple solution. The mixture was extracted with concentrated
sodium bicarbonate solution (1
×
75 mL), 1 N hydrochloric acid (1
×
75 mL), water (2
×
75 mL)
and brine (1 75 mL) before drying over sodium sulfate. The crude product was collected from
×
rotary evaporation as crude purple oil. Boiling petroleum ether (300 mL) was poured into the oil,
then the solution was hot filtered and let stand to develop yellow-green crystalline SP (3.21 g, 74%).
Characterization matched the compound reported in literature [15].
2.3. SP-PDMS Elastomer Block Synthesis
An elastomer block was made with Sylgard®184 from Dow Corning and 0.25 wt% SP
mechanophore [14]. SP was first dissolved in para-xylene at a concentration of 75 mg/mL followed
by incorporating in the Sylgard®184 mixtures with different ratios of base to curing agent by volume

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(10:1, 20:1 and 30:1). The mixture was then degassed under vacuum for about 30 min until all gas
bubbles were removed. The corresponding tensile and impact samples were then made by curing the
mixers at ambient temperature for 48 h to ensure adequate formation of the network structure.
2.4. High Rate Impact Test and Finite Element Analysis
The high rate impact was performed using an in-house air cannon test system for achieving impact
loading condition [17]. During the test, a domed cylindrical projectile (outer diameter 40 mm, length
52.5 mm, dome radius of curvature 20 mm and mass 29 g) made of 3D printed glass filled nylon was
inserted into the air cannon barrel. The air pressure was then regulated to achieve a projectile impact
speed from 80–110 m/s. Finite element analysis (FEA) used for predicting stress/strain distributions
was performed using LS-Dyna based on parameters obtained from Mooney-Rivlin analysis of the
quasi-static tensile test data. A Lagrangian formulation and an Euler forward time stepping algorithm
were employed, with the time step size automatically chosen for numerical stability.
3. Results
3.1. Quasi-Static Test Results
As discussed, the color change of SP relies on the rupture of the spiro C-O bond and the
transformation of SP into the longer conjugated MC. The latter absorbs strongly at a wavelength
of 550–600 nm and leads to the formation of purple color in SP-PDMS. Since MC is metastable, once the
mechanical stimulus is removed, the spiro C-O bond will be reformed reversing MC into the original
SP. The latter leads to the disappearance of the purple color and thus allows for multiple use. In this
work, alkene functionalized SP [11–
15] was used to couple with a vinyl terminated PDMS-Sylgard®
184. The hydrosilylation reaction between Si-H and alkene groups lead to the formation of covalent
bonding between SP and PDMS, resulting in the transfer of the load and thus color change upon
impact (Figure 1).
(a)
(b)
) Incorporation of SP (spiropyran) into PDMS (poly(dimethyl siloxane)) network structure.
(b) Changing between SP and MC (merocyanine) during uniaxial mechanical deformation.
Figure 1. (a

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To understand the effect of PDMS network structure on the color change of SP, two part Sylgard®
184 prepared at three different mixing ratios (10:1, 20:1 and 30:1) were tested under uniaxial tensile
mode. Figure 2 shows the results at a strain rate of 0.014 s−1 for all three mixing ratios. In all three
samples, the SP loading level was controlled at 0.25 wt%. It can be seen that the decrease in blue color
(B%) initiated around the onset of the strain-hardening region. The onset point was determined by
drawing lines tangent to the initial and final portions of the same curve with the interception of the
two lines defined as the onset point. For the 10:1 mixing ratio, the onset true strain for the color change
was found to be around 53%. With the increase in mixing ratio to 20:1 and 30:1, the onset true strain
values for the color change increased to 92% and 102%, respectively. The bottom row of Figure 2 shows
B% values plotted against true stress which demonstrates a close to linear relationship after the onset
of strain hardening.
20:1
10:1
30:1
0.36
0.35
0.34
0.33
0.32
0.31
0.30
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.36
0.35
0.34
0.33
0.32
0.31
0.30
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
0.36
0.35
0.34
0.33
0.32
0.31
0.30
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
20
40
60
80
0
50
100
150
0
20
40
60
80
100
120
True strain (%)
True Strain (%)
True Strain (%)
0.36
0.35
0.34
0.33
0.32
0.31
0.30
0.29
0.36
0.35
0.34
0.33
0.32
0.31
0.30
0.29
0.36
0.35
0.34
0.33
0.32
0.31
0.30
0.29
y = -0.0067x + 0.3596
R² = 0.996
y = -0.0087x + 0.3434
R² = 0.9842
y = -0.0056x + 0.3522
R² = 0.9945
0
2
4
6
8
10
0
1
2
3
4
5
0
0.2
0.4
0.6
0.8
1
1.2
True Stress (MPa)
True Stress (MPa)
True Stress (MPa)
Figure 2. Effects of PDMS (poly(dimethyl siloxane)) mixing ratios on color change quasi-static tensile
test. The top row shows blue color (B%) and true stress plotted against true strain and the dark arrows
show the onset point for blue color change. The bottom row shows the B% plotted against true stress.
To better characterize PDMS network structure, the Mooney-Rivlin analysis was employed
(Equation (1)) [18,19].
σ engr
1
λ
ꢀ
ꢁ
= 2C1 + 2C2
(1)
1
λ −
λ2
where
σ
engr is engineering stress,
λ
is extension ratio, C1 and C2 are materials constants and relate to
the network crosslinking density and the deviation from ideal rubber, respectively. C1 and C2 can be
σ
engr
1
λ
ꢀ
ꢁ
determined by plotting
versus with the slope and intercept being 2C2 and 2C1, respectively.
1
λ−
2
λ
The ratio of 2C2/C1 measures the looseness of the network structure [20]. With C1 and C2 known,
the average molecular weight between crosslinks (
Mc) (kg/mol) and the network crosslinking density
(N) (mol/m3) can be further determined via Equation (2).
ρRT
2C1 + 2C2
ρR
κN
Mc =
=
(2)
In the above equations,
temperature (K) and R is the gas constant of 8.31 J/mol·K.
κ
is the Boltzmann constant of 1.38
×
10−23 J/K,
ρ
is density (g/cm3), T is