effect of crosslinking and long-term storage on the shape-memory behavior of (meth)acrylate-based...
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Cite this: Soft Matter, 2012, 8, 7381
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Effect of crosslinking and long-term storage on the shape-memory behavior of(meth)acrylate-based shape-memory polymers†
Alicia M. Ortega,*a Christopher M. Yakacki,b Sean A. Dixon,c Roxanne Likos,c Alan R. Greenberga
and Ken Gallcde
Received 9th February 2012, Accepted 10th May 2012
DOI: 10.1039/c2sm25298h
This work highlights the free- and fixed-strain shape-memory response of amorphous (meth)acrylate-
based shape-memory polymers as the level of crosslinking is varied from uncrosslinked to highly
crosslinked (corresponding to a decrease in failure strains and overall increase in rubbery moduli and
failure stresses). The effect of long-term storage on the free-strain shape-memory response is also
considered. Tensile deformation levels during the shape-memory cycle are 90% of failure strain values
to determine the full extent of free- and fixed-strain recovery behavior. All materials demonstrate full
shape-recovery under free-strain conditions (material is unconstrained during recovery); however, total
recoverable strains increase with decreasing crosslinking level, with uncrosslinked and lightly
crosslinked materials recovering strains on the order of 3–10� that of moderately and highly
crosslinked materials. In contrast, under fixed-strain conditions (material is fully constrained in the
fixed shape during recovery), the magnitude of recovery stress generation increases with increasing
crosslinking level, with highly crosslinked materials demonstrating recovery stress levels 4–20� that of
lightly crosslinked and uncrosslinked materials. The ability to produce recovery stresses on par with
those reached during deformation also increases with crosslinking level. Stored-shape fixation and free-
strain recovery levels remain stable after long-term storage in the deformed temporary state at 20 �C;however, recovery onset temperatures increase (by up to 9 �C) with storage time spanning �1 year, as
do rates of free-strain recovery (by up to 9�), due to physical aging. Results indicate that aging could
potentially be used as a method for shape-memory response optimization.
aDepartment of Mechanical Engineering, University of Colorado atBoulder, Boulder, CO, USA. E-mail: [email protected]; Fax:+1 303 492 3498; Tel: +1 303 492 7151bDepartment of Mechanical Engineering, University of Colorado Denver,Denver, CO, USA. Fax: +1 303 556 6371; Tel: +1 303 556 8516cSchool of Materials Science and Engineering, Georgia Institute ofTechnology, Atlanta, GA, USA. Fax: +1 404 894 9140; Tel: +1 404 8942888dGeorgeW.Woodruff School of Mechanical Engineering, Georgia Instituteof Technology, Atlanta, GA, USA. Fax: +1 404 894 1658; Tel: +1 404 8943200eResearch and Development, MedShape, Inc., Atlanta, GA, USA. Fax: +1404 249 9158; Tel: +1 404 249 9155
† Electronic supplementary information (ESI) available: Table S1:Shape-memory material compositions used in this study in wt% andmole% crosslinking agent (CA); Fig. S1: Representative scans fromNIR analysis of the four polymer compositions tailored for this study.Double-bond conversion was determined by the disappearance of thepeak at approximately 6165 cm�1; Fig. S2: Swelling ratios, in2-propanol, as a function of swelling time (time in solvent) for thethree crosslinked shape-memory materials used in this study; Fig. S3:Representative (a) storage modulus and (b) tan delta curves asa function of temperature for the four shape-memory materials used inthis study; Fig. S4: Representative tensile stress–strain curves where the‘‘x’’ indicates sample failure (fracture); Table S2: Stored recovery ratios(RRs) for the four materials used in this study after storage (forvarying storage times up to �1 year) and recovery. See DOI:10.1039/c2sm25298h
This journal is ª The Royal Society of Chemistry 2012
Introduction
Shape-memory alloys, polymers, and ceramics are characterized
by their ability to store a temporary shape and recover
a permanent shape with the application of an external stimulus,
lending added functionality to material and device design. An
increased interest in shape-memory polymers (SMPs) in recent
years can be in part attributed to factors such as lower cost/
greater ease of manufacturing/processing, lower density, and
relatively large recoverable strains compared to metal and
ceramic counterparts,1–4 as well as the potential for multifunc-
tional characteristics such as through the incorporation of
biodegradable linkages.5
A general schematic of a typical shape-memory cycle for
thermally activated SMPs is presented in Fig. 1. First, for both
free- and fixed-strain recovery processes, the polymer (in its
original, manufactured state) is heated to an elevated deforma-
tion temperature, which generally corresponds to a transition
temperature, such as a glass transition temperature (Tg) or
a melting temperature. Once at the deformation temperature, the
material is deformed to the desired, temporary shape. With this
temporary shape held in place, the temperature is decreased,
fixing the polymer in this secondary shape. Once the secondary
Soft Matter, 2012, 8, 7381–7392 | 7381
Fig. 1 General schematic of the free- and fixed-strain recovery shape-
memory cycles.
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shape is fixed, the constraints can be removed from the material,
which should stay in the temporary shape until activation is
desired. For recovery, the material can be subjected to a free-
strain recovery process (if full recovery of the stored deformation
is desired), a fixed-strain recovery process (if the generation of an
actuation force/stress is desired), or a combination of both. As
presented in Fig. 1, for the free-strain recovery process, the
temperature of the material, still in the unconstrained, temporary
shape, is increased. This increase in temperature initiates shape-
recovery, and since the material is unconstrained, it recovers to
the original, manufactured shape. For the fixed-strain recovery
process, constraints are applied to the material while in the
temporary fixed shape. Then, the temperature is increased, and
recover is initiated. The material then generates an actuation
force against the constraints as it tries to recover to the initial,
manufactured shape.
A wide range of applications has been proposed for SMPs
including microfluidic devices,2 deployable space structures,6 and
temperature sensors,4 but biomedical applications are one of the
most rapidly growing areas of interest. Proposed biomedical
applications for SMPs include self-tightening sutures,7 stents,8,9
micro-scale neuronal probes,10 soft-tissue fixation devices,11
blood-clot removal devices,12,13 and dialysis needle adapters.14
For highly developed applications and devices such as these, it is
necessary to tune and/or tailor the thermomechanical properties
and shape-memory response of SMPs for a specific application.
Moreover, a fundamental understanding of the relationship
between composition, resulting structure, thermomechanical
characteristics, and shape-memory behavior in SMPs is essential
for the effective use of such materials in device design.
Systems in which physical and thermomechanical properties can
be modified by simple changes in comonomer chemistries and
ratios have been suggested for the development of tunable SMP
systems,15–17 that ideally allow application specific property opti-
mization. Amorphous (meth)acrylate-based polymers ((M)ABP)
are one system that has potential for relatively easy property
7382 | Soft Matter, 2012, 8, 7381–7392
tailoring.8,15 Amorphous (M)ABP networks belong to a class of
thermally activated SMPs termed covalently crosslinked glassy
thermoset networks.18 SMPs belonging to this class which have
been described in the literature include epoxies,2,6,16,17,19–21 cross-
linked oil-based polymers,22 crosslinked amorphous poly-
urethanes,23 and crosslinked styrene copolymers,24 along with
amorphous (M)ABP networks.5,8,9,11,15,25–27 Liu et al.18 also grou-
ped high molecular weight (MW), amorphous polymers in this
class of SMPs with entanglements in these materials acting as
physical crosslinks, allowing for shape recovery.28
In such amorphous SMPs, the chemical crosslinks (or entan-
glements for high MW polymers) serve to prevent chain slippage
and ultimately permanent, unrecoverable strains during defor-
mation. In these materials the vitrification process allows for
fixing the deformed shape. Cited benefits of this class of SMPs
include a high degree of recovery due to an absence of chain
slippage, good fixity due to the large difference in modulus above
and below the transition temperature, and a tunable work
capacity (recovery stress and strain) dependent on the extent of
crosslinking.3,18
In such systems, properties that are key to shape-memory
behavior include Tg (which is related to the storage and recovery
onset temperatures), modulus (which is related to the energy
stored in the material upon deformation), and strain-to-failure, 3f(which serves as an upper bound for the maximum possible
recoverable strain). Thermomechanical properties (such as
transition temperatures, modulus, and 3f) that effect the shape-
memory behavior of amorphous polymer networks can be
modified and controlled by simple changes to comonomer
chemistries, concentrations, and functionalities in (M)
ABPs,15,29,30 making them promising candidates for tunable
shape-memory polymer systems.
Initial studies have shown how aspects of the shape-memory
behavior of (M)ABPs are influenced by chemical/structural
modifications in moderately crosslinked networks confirming
this methodology as one route for designing ‘‘tailorable’’ SMP
systems. Yakacki et al. demonstrated how Tg and rubbery
modulus (ER) could essentially be tuned independently through
modifications to both the crosslinking agent concentration and
MW.9,11 This work also showed that a number of factors such as
Tg, deformation/recovery temperature, and crosslink density
affect the rate of deployment (recovery) in moderately cross-
linked, packaged stents.9 Additionally, the fixed-strain stress
generation in moderately crosslinked systems has been shown to
increase with increasing ER values of network materials.11 Voit
et al.27 recently investigated shape recovery in very lightly
crosslinked (M)ABPs and demonstrated the capability of such
materials to recover high strain levels if factors such as cross-
linking chemistry and concentration, photoinitiator type and
concentration, and glass transition temperature are optimized;
however, study of the extent to which shape-memory properties
can be tailored and optimized and trade-offs in recovery
behavior that may occur in such SMP systems through simple
comonomer chemistry and concentration modifications remains
underdeveloped. Specifically, the relationship between the extent
of crosslinking and free- and fixed-strain recovery limits has yet
to be fully studied.
The storage stability, or shelf life, of SMPs is another area of
interest for the proper design and use of these materials in
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real-world applications. Relatively little work has been done to
understand the long-term storage stability and recovery behavior
of SMPs in their temporary, stored state. Studies on SMP epoxy
resin composites6 and polyurethane foam31 found that, for
samples stored under constraint at room temperature for up to
15 days6 or 2 months,31 the level of strain recovery was inde-
pendent of storage time. Another study of SMP polyurethane
foam considered the effects of storage time (at a temperature of
Tg – 60 K) on shape fixity during storage and shape recovery
after unconstrained storage for designated times up to 180
days.32 These materials demonstrated good storage stability and
recovery across this designated time period. A recent study
evaluated the ability of moderately crosslinked packaged stents,
produced from (M)ABP networks, to maintain their packaged,
temporary shape unconstrained throughout a �30 day storage
period.9 Results indicated that the storage stability of such
polymer networks is affected by a number of factors such as Tg,
deformation/recovery temperatures, and crosslink density of the
polymer network, with premature, free-strain recovery occurring
during storage in some of the studied materials. Clearly,
premature recovery occurring during storage of pre-packaged
SMP devices could result in a number of complications with
practical use of such devices. Thus, the effect of the degree of
crosslinking and length of storage time on shape stability and
recovery behavior of such polymers over extended periods of
time could prove pivotal in the design and use of such materials.
Fundamental understanding of the relationships governing
thermomechanical and shape-memory behavior in SMPs is
critical for the proper development and use of such materials in
biomedical applications. The purpose of this paper is to
demonstrate the effect of varying levels of chemical crosslinking
(via varying comonomer concentrations) on essential aspects of
shape-memory behavior such as free-strain recovery, fixed-strain
stress generation, and long-term (up to 1 year) storage stability
and recovery behavior in amorphous (M)ABPs. This work
should aid in the understanding of the trade-offs in shape-
memory response that occur through variations in crosslinking
levels and over long-term (extended) storage times, as well as
how such variations can be used to tailor and optimize the shape-
memory response for specific applications.
Experimental
Materials
Materials were prepared by the copolymerization of tert-butyl
acrylate (tBA) with poly(ethylene glycol) dimethacrylate (Mn ¼550 i.e. PEGDMA550 and Mn ¼ 330 i.e. PEGDMA330). A
mixture of PEGDMA550 and PEGDMA330 monomers was
developed that, when polymerized, evidenced a Tg equal to
that of the homopolymerized tBA (42.9 wt% PEGDMA330, 57.1
wt% PEGDMA550). This PEGDMA monomer mixture is
referred to as the ‘‘crosslinking agent’’ throughout the text.
Different amounts of the crosslinking agent (0, 2, 10, and 40
wt%; see ESI Table S1† for mole% values) were copolymerized
with tBA to produce four different polymer materials with
similar Tg values and four quantitatively different levels of
crosslinking (uncrosslinked, lightly crosslinked, moderately
crosslinked, and highly crosslinked, respectively). The
This journal is ª The Royal Society of Chemistry 2012
photoinitiator 2,2-dimethoxy-2-phenylacetophenone was added
to the comonomer solution at a concentration of 1 wt% of the
total comonomer weight and mixed manually until fully dis-
solved. All materials used in the polymer synthesis were
purchased from Aldrich and used as received.
Polymer synthesis and sample production
Glass slides were coated with a non-reacting release agent (Rain-
X, SOPUS Products). The comonomer–photoinitiator mixture
was injected between two coated glass slides, which were sepa-
rated by 1 mm spacers and secured by binder clips. Polymeri-
zation was accomplished by exposing the entire configuration to
UV light (Blak-Ray, Model B100AP) for 10 minutes (intensity
�8 mW cm�2). All samples were cut from bulk slide samples with
a laser cutter and polished with 500–600 grit sandpaper prior to
testing to remove edge effects caused by laser cutting.
Near-infrared spectroscopy
Near-infrared (NIR) spectroscopy (Nicolet 750 Magna FTIR
Spectrometer) was used to determine the final double-bond
conversion of the four compositions (0, 2, 10, and 40 wt%
crosslinking agent) subjected to the polymerization protocol (n¼2). Spectra were acquired from 16 scans with a 2 wavenumber
resolution. Samples were scanned pre- and post-photo-
polymerization, and conversions were based on the ]C–H
absorption peak located at approximately 6165 cm�1.33 Percent
conversions were calculated by subtracting the ratio of the
absorbance peak area post- to pre-polymerization from one34
and multiplying the final value by one hundred. All compositions
demonstrate conversions of �96 to 97%. Representative NIR
scans are presented in ESI Fig. S1†.
Gel permeation chromatography
Triple detector (light scattering, viscosity, refractive index) gel
permeation chromatography, GPC (Viscotek), was used to
characterize the homopolymerized tBA (n ¼ 3). Samples were
dissolved in tetrahydrofuran, THF (Pharmco-Aaper, HPLC
grade). The flow rate and column temperature were 1.0 ml min�1
and 35 �C respectively. The molecular weight of the polymer was
determined relative to a 99 K polystyrene standard. The deter-
mined number average molecular weight (Mn), weight average
molecular weight (Mw), and polydispersity index (PDI) for the
tBA homopolymer are 157 600 � 27 488 Da, 371 819 � 64 473
Da, and 2.36 � 0.06, respectively.
Swelling experiments
Swelling experiments (n $ 3) were completed by placing samples
approximately 1 mm � 14 mm � 20 mm in size in �45 ml of
2-propanol (Sigma-Aldrich), a solvent for the tBA homopol-
ymer. Initial sample weights (Wi) were measured prior to
immersion into the solvent. Sample weights were then measured
periodically until equilibrium (swollen) weights (Ws) were
reached. The equilibrium swelling ratio of each material is taken
as the value of the swelling ratio in the plateau region of the
corresponding swelling ratio versus time curve. The equilibrium
weight-swelling ratio (q) was calculated from the relationship:35
Soft Matter, 2012, 8, 7381–7392 | 7383
Fig. 2 Schematic of the two shape-memory cycles performed in this
study. (a) Free-strain recovery shape-memory cycle. Step (1): sample is
deformed under tensile load (at Td, in this case Tg) to 90% of mean 3f;
Step (2): sample is held at constant extension as temperature is reduced to
20 �C (Tl); Step (3): stress is removed from sample; and Step (4): strain of
the unconstrained sample is monitored as temperature is increased. (b)
Fixed-strain stress-generation shape-memory cycle. Steps 1–3 are the
same as those of the free-strain recovery shape-memory cycle; Step 4:
extension is held constant and stress is monitored as temperature is
increased.
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q ¼ Ws
Wi
(1)
Dried weights (Wd) were obtained by removing the samples
from the 2-propanol and placing them in an 80 �C oven to dry.
The samples were weighed periodically until the sample weight
no longer decreased with further drying time. The percent gel
content (GC) was calculated from the relationship:
GC ð%Þ ¼�Wd
Wi
�� 100 (2)
Dynamic mechanical analysis
Samples for dynamic mechanical analysis (DMA) were approxi-
mately 4 mm� 1 mm� 17 mm (n$ 4). Sample ends were wrapped
with aluminum foil to prevent failure at the grips during testing.
Testing was performed under tensile loading with a dynamic strain
of 0.1%, a frequency of 1 Hz, a force track of 150% and a heating
rate of 2 �Cmin�1 (TA Instruments, Q800 DMA). TheTg is defined
in this study as the peak of the tan delta curve obtained fromDMA
testing, where tan delta is the ratio of the loss modulus to the
storagemodulus.36ER values are defined in this study as the value of
the storage modulus at a temperature of Tg + 50 �C.
Tensile testing
Sample geometry for strain-to-failure testing was a flat dog-bone
with a gauge cross-sectional area of 3 mm � 1 mm and a gauge
length of approximately 9 mm. Strain-to-failure and shape-memory
tests were performed using a mechanical tester with a 500 N load
cell (Instron, Model 5567). A thermal chamber (Instron, Model
3119-506-A2B3) with liquid nitrogen cooling was used to control
test temperature. Strain-to-failure testing (n $ 4) was performed at
the overall mean Tg of 56 � 1 �C. Tests were run in displacement
control at an extension rate of 5 mm min�1. Strain was measured
externally by a video extensometer (Instron, Model 2663-821).
Shape-memory testing
Free-strain recovery tests (n$ 2) consisted of four steps (Fig. 2a):
(1) the sample was deformed to 90% of the mean 3f (at the overall
mean Tg ¼ 56 �C) previously determined for the particular
composition at an extension rate of 5 mm min�1; (2) the
temperature of the sample was decreased from Tg to 20 �C while
extension was held constant (plus a 10 min isothermal hold at 20�C); (3) the sample was unloaded; and (4) with constraints
removed, the temperature was increased at a rate of 2 �C min�1
and strain was monitored until the recovery strain reached
a relatively constant value. Fixed-strain recovery tests (n $ 2)
also consisted of four steps (Fig. 2b): (1)–(3) same as that for free-
strain recovery tests; and (4) with the sample re-constrained in its
stored shape, the temperature was increased at a rate of 2 �Cmin�1 and stress was monitored until reaching a relatively
constant value or until sample failure/fracture. The 0 wt%
crosslinking agent composition did not fail (fracture) prior to
reaching the extension limits of the mechanical tester during
strain-to-failure testing, thus in both free-strain and fixed-strain
recovery tests this composition was deformed to the same strain
as the 2 wt% crosslinking agent composition.
7384 | Soft Matter, 2012, 8, 7381–7392
Long-term storage testing
For long-term (extended) storage free-strain recovery tests, steps
1–3 of the free strain recovery procedure were completed. The
samples were then placed unconstrained in a low-temperature
incubator (VWR, Model 2005) held at 20 � 0.1 �C. At pre-
determined mean storage times (1, 2, 7, 14, 27, 60, 90 � 1, 183 �1, 386 � 3 days), samples were removed from the incubator and
were subjected to the recovery portion of the free-strain recovery
procedure (step 4, Fig. 2a). Strains for long-term storage testing
were measured both by physical measurement of gauge marks
placed on the sample (prior to and after elongation, extended
storage, and recovery) and via a video extensometer (during
recovery). Long-term storage recovery data is reported for
a minimum of two samples (n $ 2), with the following excep-
tions: 2 wt% crosslinking agent composition’s NSRs (see
Nomenclature section) after shape-memory cycle, Tonset (see
Nomenclature section), and rate of strain recovery values at 91
days (n ¼ 1), 10 wt% crosslinking agent composition’s Tonset and
rate of strain recovery values at 27 days (n ¼ 1), and 40 wt%
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crosslinking agent composition’s Tonset and rate of strain
recovery values at 1 day and 2 days (n ¼ 1). Rate of strain
recovery values were calculated from the slope of the strain versus
temperature or strain versus time curve in the range of 20–40% of
total strain recovered during the recovery portion of the shape-
memory cycle.
Fig. 3 Representative free-strain recovery profiles of (a) absolute strain
and (b) normalized deformation strain (NSRd, see Nomenclature section)
recovered as a function of temperature. A NSRd value of 0 and 1 indicate
no and full recovery of deformation strain, respectively.
Results and discussion
Results
The equilibrium swelling ratios (q) in 2-propanol and gel content
(GC) of the materials designed for this study are presented in
Table 1. As previously reported,29 the homopolymerized tBA (0
wt% crosslinking agent) dissolves in 2-propanol implying an
uncrosslinked, linear structure; thus, no q or GC values are listed
in Table 1 for this composition. As the concentration of cross-
linking agent increases, q decreases, confirming an increase in
crosslink density. Representative swelling ratios versus time
curves are presented in ESI Fig. S2†. Mean GC values of the
three network materials are $93%, and increase with increasing
crosslinking agent concentration (Table 1).
Dynamic mechanical results of storage modulus and tan delta
as a function of temperature (ESI Fig. S3a and S3b†, respec-
tively) indicate that the transition from the glassy state to the
rubbery state occurs in a similar temperature range for each of
the four materials used in this study. This is reflected in the
resulting nearly identical Tg values of the materials as listed in
Table 1. The calculated overall mean Tg of all the materials is 56
� 1 �C. The uncrosslinked, homopolymerized tBA (0 wt%
crosslinking agent) material loses mechanical integrity at
temperatures just above its glass transition region, thus no ER
value is reported in Table 1. As expected, with the addition of
crosslinking agent, the network materials demonstrate plateaus
in storage modulus in their rubbery region (i.e. ER), the mean
values of which increase as the concentration of crosslinking
agent increases and range from 0.9 to 20.5 MPa (Table 1).
Tensile stress–strain responses (to failure i.e. fracture) of the
three network materials at the overall mean Tg (56�C) demon-
strate a transition from an elastomeric response to a brittle, linear
response (ESI Fig. S4†), corresponding with an overall decrease
in 3f and increase in stress-at-failure (sf) as crosslinking agent
concentration increases from 2 wt% to 40 wt% (Table 1). Initial 3fresults have been previously reported.37 Ultimate tensile tough-
ness is highest in the lightly crosslinked material (2 wt% cross-
linking agent) reflecting the dominating influence of the large 3f(Table 1). Ultimate properties of the tBA homopolymer are not
Table 1 Mean property values: q¼ equilibrium swelling ratio, GC¼ gel contTg + 50 �C, 3f ¼ strain-to-failure, sf ¼ stress-at-failure, toughness ¼ area undeof all compositions is 56 � 1 �C (sample size (n) ¼ 17). CA ¼ crosslinking ag
Wt% CA q (g g�1) GC (%) Tg (�C) ER (
0 — — 56 � 1 (n ¼ 5) —2 3.30 � 0.04 (n ¼ 4) 93.0 � 0.4 (n ¼ 4) 57 � 2 (n ¼ 4) 0.9 �10 1.85 � 0.01 (n ¼ 3) 97.2 � 0.2 (n ¼ 3) 56 � 1 (n ¼ 4) 3.2�40 1.26 � 0.00 (n ¼ 3) 98.9 � 0.3 (n ¼ 3) 55 � 1 (n ¼ 4) 20.5
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listed in Table 1 since no failure occurred before reaching the
extension limits of the mechanical tester.
Fig. 3 presents representative instantaneous (i.e. no extended
storage) free-strain shape-recovery of the four materials used in
this study, showing (a) absolute (engineering) tensile strain and
(b) strain normalized to the maximum strain reached during the
deformation stage of the shape-memory cycle (NSRd), as
a function of temperature (similar initial results have been pre-
sented previously37). Each material composition is strained to
approximately 90% of 3f, resulting in the uncrosslinked and 2
wt% crosslinking agent compositions storing (and ultimately
recovering) strains 3–10� that of the 10 and 40 wt% crosslinking
agent compositions (Fig. 3a). For all compositions, as the
ent, Tg ¼ glass transition temperature, ER¼ rubbery (storage) modulus atr tensile engineering stress–strain curve up to failure. The overall mean Tg
ent
MPa) 3f (%) sf (MPa)Toughness(MJ m�3)
— — —0.2 (n ¼ 4) 316 � 12 (n ¼ 6) 2.9 � 0.5 (n ¼ 4) 2.1 � 0.3 (n ¼ 4)
0.6 (n ¼ 4) 104 � 14 (n ¼ 7) 2.2 � 0.6 (n ¼ 5) 1.0 � 0.3 (n ¼ 5)� 3.3 (n ¼ 4) 37 � 9 (n ¼ 6) 5.1 � 1.3 (n ¼ 4) 1.0 � 0.4 (n ¼ 4)
Soft Matter, 2012, 8, 7381–7392 | 7385
Fig. 4 Representative fixed-strain stress generation recovery profiles of
(a) absolute stress generation and (b) normalized stress generation (NSG)
as a function of temperature. A NSG value of 0 and 1 indicate no stress
generation and stress generation equal to that reached during deforma-
tion, respectively.
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temperature is increased from the 20 �C storage temperature,
strains decrease from the stored strain value to a value
approaching 0% strain, indicating deformation recovery. In
Fig. 3b a value of 0 indicates no recovery and a value of 1 indi-
cates complete recovery. The 0, 2, and 10 wt% crosslinking agent
compositions demonstrate initial stored NSRd values close to 0,
while the 40 wt% crosslinking agent composition demonstrates
a stored NSRd value of �0.1, indicating partial recovery during
the unloading step (step 3) of the shape-memory cycle. This
behavior is also reflected in the slightly lower mean fixity ratios of
the 40 wt% crosslinking agent composition (Table 2). Tonset
decreases with increasing crosslinking agent concentration
(Fig. 3b). All material compositions demonstrate full recovery as
temperature is increased, with all NSRd values approaching
a value of 1 (Fig. 3b) and mean recovery ratios (with respect to
deformation and storage strains, RRd and RRs respectively) of
$98% (Table 2).38
Fig. 4 presents representative instantaneous (i.e. no extended
storage) fixed-strain stress developed (as a function of temper-
ature) during recovery of the four materials used in this study,
after deformation to and storage of strains of approximately
90% 3f. Fig. 4a presents recovery profiles of the materials in
terms of absolute (engineering) stress. The stress levels gener-
ated in these materials increase with crosslinking agent
concentration, with the highly crosslinked material (40 wt%
crosslinking agent) demonstrating stress generation levels of 4–
20� that of the lightly crosslinked (2 wt% crosslinking agent)
and uncrosslinked materials. Aside from the uncrosslinked tBA
homopolymer, all compositions demonstrate sample failure
(fracture) during fixed-strain shape-memory tests as the
temperature is increased to or beyond Tg, as indicated by the
abrupt decrease in stress at the end of the curves. The uncros-
slinked material demonstrates the lowest level of stress genera-
tion, which gradually decreases with an increase in temperature
after reaching a maximum value at around 45–50 �C. As with
free-strain recovery, the onset temperature of the recovery (in
this case stress generation) decreases as the crosslinking agent
concentration increases. Fig. 4b shows the stress generation
profiles of these materials in terms of stress normalized to the
stress each material reaches during the deformation step of the
shape-memory cycle (NSG), where a value of 0 indicates no
stress and a value of 1 indicates a stress level equal to that
reached during deformation. The ability to generate recovery
stress levels on par with deformation stress levels decreases with
decreasing crosslinking. This is also demonstrated via calculated
stress generation ratios (SGR) as shown in Table 2.39
Representative normalized (to storage strains, NSRs) free-
strain recovery profiles after various storage times at 20 �C (in the
deformed state) are presented in Fig. 5. A NSRs value of
Table 2 Deformation fixity ratios (FRd), deformation recovery ratios (RRsamples with storage time ¼ 0 days. CA ¼ crosslinking agent
Wt% CA FRd (%) RRd (%)
0 95 � 1 (n ¼ 3) 98 � 1 (n ¼2 97 � 2 (n ¼ 3) 99 � 1 (n ¼10 97 � 1 (n ¼ 2) 101 � 2 (n ¼40 88 � 9 (n ¼ 3) 99 � 5 (n ¼
7386 | Soft Matter, 2012, 8, 7381–7392
0 indicates no recovery from the stored state and a value of 1
indicates complete recovery of the stored strain. All materials
demonstrate recovery of the stored strain such that with an
increase in temperature, NSRs values approach 1. Additionally,
even after �386 days of storage, all compositions demonstrate
starting NSRs values of �0, indicating minimal recovery during
storage, with the exception of the 183 and 386 day samples for
the 40 wt% composition, which demonstrate NSRs values larger
than 0. Moreover, the Tonset temperatures increase with storage
time for all compositions tested. Finally, the slope of the recovery
curve (i.e. rate of strain recovery) increases with storage time,
resulting in sharper recovery profiles.
Fig. 6 presents (a) normalized stored strain recovery values
(NSRs) for before and after the shape-memory cycle, (b) Tonset
values, and (c) rates of strain recovery as a function of storage
d), stored recovery ratios (RRs), and stress generation ratios (SGR) for
RRs (%) SGR (%)
3) 98 � 1 (n ¼ 3) 70 � 8 (n ¼ 2)3) 99 � 1 (n ¼ 3) 79 � 7 (n ¼ 2)2) 101 � 2 (n ¼ 2) 92 � 6 (n ¼ 3)3) 100 � 5 (n ¼ 3) 114 � 6 (n ¼ 2)
This journal is ª The Royal Society of Chemistry 2012
Fig. 5 Representative normalized (to stored strain) free-strain recovery profiles for the (a) 0 wt%, (b) 2 wt%, (c) 10 wt%, and (d) 40 wt% crosslinking
agent compositions as a function of temperature after selected storage times. A NSRs value of 0 and 1 indicate no and full recovery of stored strains,
respectively.
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time. The NSRs values of the materials at various storage times,
ranging from 1 to �386 days, prior to a shape-memory recovery
cycle, fluctuate slightly around 0 (Fig. 6a). This indicates that the
stored deformation in these materials does not recover during
storage. The NSRs values for the 40 wt% crosslinking agent
composition are more variable than those for the other
compositions and occasionally demonstrate values larger than
0 (most apparent for the 183 and 386 day samples). The NSRs
values of the materials after the shape-memory cycle all remain
�1 as a function of storage time, for all storage times presented
here. This indicates that all compositions still demonstrate good
recovery (mean recovery ratios with respect to storage strain,
RRs, $93%, see ESI Table S2†) after being stored at 20 �C for
up to �386 days. The Tonset values of the materials increase with
storage time (Fig. 6b). The rate of increase of Tonset with storage
time increases with crosslinking agent concentration. The
regression lines in Fig. 6b (R2 > 0.86) represent the overall
increase in Tonset with time (i.e. Tonset increases logarithmically
with storage time). The rate of strain recovery for each given
composition increases with storage time (Fig. 6c). The mean
rates of strain recovery for the 0, 2, 10, and 40 wt% crosslinking
agent compositions increase by magnitudes of 3, 4, 9, and 3,
respectively. The (power) regression lines in Fig. 6c (R2 > 0.77)
represent the overall increase in rates of strain recovery with
time.
This journal is ª The Royal Society of Chemistry 2012
Discussion
The purpose of this work is to systematically determine the effect
of crosslinking on fundamental shape-memory behavior of
amorphous, (M)ABP materials, across a range of crosslinking
densities from uncrosslinked to highly crosslinked. The effect of
crosslinking on free-strain, fixed-strain, and extended storage
shape-memory behavior was evaluated. A tailored set of four
compositions was created that have equivalent Tg values (Table
1) and four levels of crosslinking to eliminate the effect of varying
transition temperatures and to isolate the effect of crosslinking.
Moreover, after determining ultimate tensile deformation prop-
erties, materials were deformed to approximately 90% 3f (for
each given composition) for subsequent shape-memory testing.
This approach was utilized to determine the full extent of free-
strain recovery and fixed-strain stress generation of these
materials.
NIR analysis and swelling tests with GC determination reveal
near complete double bond conversion (96–97%) of these mate-
rials through photopolymerization and good incorporation of
starting materials into the resulting polymer networks (GC $
93%). As shown in previous work on (M)ABP systems,29 the
overall crosslink density is successfully varied through simple
modifications to the crosslinking agent concentration. Crosslink
density variation is confirmed through the corresponding
Soft Matter, 2012, 8, 7381–7392 | 7387
Fig. 6 (a) Normalized stored strain recovery (NSRs) during storage at
20 �C (bottom portion of plot) and after the free-strain recovery shape-
memory (SM) cycle (top portion of plot) as a function of storage time
where a NSRs value of 0 and 1 indicate no and full recovery of stored
strain, respectively; (b) free-strain recovery onset temperature (Tonset) as
a function of storage time; (c) rate of strain recovery during the recovery
portion of the shape-memory cycle, as a function of storage time. B
0 wt%, 2wt%, 10wt%,and 40wt%crosslinkingagent concentration.
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decrease in q values and increase in ER values as crosslinking
agent concentration is increased (Table 1). Moreover, 3f values of
these materials decrease quickly with increasing crosslinking
agent concentration (Table 1). The sf values demonstrate an
initial decrease from the 2 wt% to 10 wt% crosslinking agent
compositions, and subsequent increase with increasing cross-
linking agent concentration from the 10 wt% to 40 wt%
7388 | Soft Matter, 2012, 8, 7381–7392
crosslinking agent compositions. Such trends in ultimate tensile
properties of (M)ABP materials are consistent with those pre-
sented in previous work29 and serve to fix upper limits for
possible recovery strains and actuation stresses.
Previous studies on the shape-memory behavior of (M)ABP
networks demonstrate full free-strain recovery,8,11 an advanta-
geous characteristic of using chemically crosslinked SMPs.18
Previous work has also demonstrated the ability of this group of
SMPs to demonstrate full recovery of deformations approaching
3f in a moderately crosslinked material8 as well as in optimized,
lightly crosslinked materials.27 However, the capacity for such
behavior in (M)ABP materials ranging from uncrosslinked to
highly crosslinked has not been systematically studied. In the
current work, free-strain recovery tests of all materials demon-
strate recovery ratios $98% after being deformed to strains
�90% 3f thus confirming the ability of this class of polymers to
demonstrate ideal recovery of large deformations approaching
that of 3f. This means that the uncrosslinked and lightly cross-
linked compositions are capable of recovering strains on the
order of 3–10� that of the moderately and highly crosslinked
compositions due to larger 3f values (Fig. 3).
While the full recovery of large strains demonstrated by the
uncrosslinked material seems counter-intuitive, good shape-
memory recovery in glassy, uncrosslinked polymers has been
previously reported18,28 and is in contrast to permanent strain in
semi-crystalline polyurethane thermoplastic shape-memory
polymers.27 Good recovery in glassy, uncrosslinked polymers has
been attributed to high (or ultra high) MWs and a correspond-
ingly large number of entanglements, which act as physical
crosslinks.18 Given the somewhat high MW of the homopoly-
merized tBA as determined from GPC (Mn ¼ 157 600 � 27 488
Da) it is possible that a large number of entanglements exist in
the tBA homopolymer and act as physical crosslinks in the
polymer structure, aiding in the recovery of the stored defor-
mation. Additionally, the characteristic ratio of tBAwas recently
calculated and was lower than other mono-(meth)acrylate
monomers, indicating that polymer chains of the homopoly-
merized tBA have a greater ability to coil.30 This propensity to
form coiled polymer chains could enable the tBA homopolymer
to undergo large strain deformations without chain slippage
(permanent deformation), thus contributing to the observed
large recoverable strain levels. It would be of interest to test other
linear, (meth)acrylate based homopolymers with varying char-
acteristic ratios (i.e. chemical structures) to see how this might
relate to shape-recoverability. The presence of a very small
amount of chemical crosslinking (small enough to be undetected
via swelling test) could potentially be another factor contributing
to the good recovery behavior of the homopolymerized tBA.
Regardless, full free-strain recoverability was demonstrated by
all compositions, ranging from uncrosslinked to highly cross-
linked. However, the effect of crosslinking in the current study
was apparent in the absolute levels of strain recoverability of
these materials, with the uncrosslinked and lightly crosslinked
materials capable of recovering tensile strains significantly larger
than the moderately and highly crosslinked levels. The effect of
crosslinking on the shape-memory response of these (M)ABPs
was also apparent in the fixed-strain recovery behavior.
As previously reported,11 the fixed-strain stress generation
increases with increasing crosslinking; however, the current study
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differs from previous work in that materials are deformed to
levels close to 3f, and thus demonstrate the full stress generation
capacity (for the given test parameters) of each material. Thus,
despite being able to withstand large deformations, the more
lightly crosslinked materials are not capable of producing
recovery stress levels close to that of the more highly crosslinked
material, even when deformed to large strain levels, with the
highly crosslinked material demonstrating stress generation
levels of 4–20� that of the lightly crosslinked and uncrosslinked
materials. For example, due to the somewhat similar sf levels of
the 2 and 10 wt% crosslinking agent materials (Table 1), the
possibility of the more lightly crosslinked material generating
recovery stress levels on par with that of the moderately cross-
linked material was considered but not observed (Fig. 4a). Two
likely causes for this are a steep increase in stress demonstrated
by the 2 wt% crosslinking agent composition approaching 3f (ESI
Fig. S4†) and the decrease in normalized stress generation with
decreasing crosslinking agent concentration (Fig. 4b and Table
2). Due to the large 3f value and steep upturn in stress near 3f of
the 2 wt% crosslinking agent composition (characteristic of
elastomeric materials), when the deformation level is decreased
to 90% 3f for shape-memory testing, a significant decrease in
deformation stress is observed in comparison to the 10 wt%
crosslinking agent material at 90% 3f. This decreases the possible
actuation stress that can be generated by the 2 wt% composition
compared to that of the moderately crosslinked material. Addi-
tionally, the decrease in normalized stress generation (NSG) with
decreasing crosslinking agent concentration should also
contribute to the lower absolute stress levels of the 2 wt%
crosslinking agent compositions compared to the 10 wt% cross-
linking agent compositions.
The cause of this decrease in the normalized stress generation
levels (Fig. 4b) with decreasing crosslinking agent has not yet
been fully explored (i.e. as crosslinking level decreases, so does
the ability of the material to produce actuation stresses on par
with stresses reached during the deformation stage of the shape-
memory cycle). One possible cause is stress relaxation occurring
during the storage step and/or the recovery step of the shape-
memory cycle. It is well established that for network polymers,
the resistance to both creep and stress-relaxation increases as
crosslink density increases.40 Thus as the level of crosslinking
decreases, the resistance to stress relaxation occurring
during both the storage step and recovery step of the shape-
memory cycle decreases with a corresponding reduction in the
recovery stress levels that the materials can generate. This is also
seen in the ability of the moderately and highly crosslinked
materials to maintain a stable level of recovery stress as the
temperature is increased. In contrast, for the uncrosslinked
material, after the initial step increase in stress generation
during recovery, the stress slowly begins to decrease as the
temperature is increased due to stress relaxation. This could
prove problematic in applications in which a stable recovery
stress is required either at elevated temperatures or over
extended periods of time.
To maximize stress generation capabilities of such materials as
the level of crosslinking is decreased, the material must be
deformed to levels as close to 3f as possible to capture (and store)
any upturn in stress exhibited as the material approaches its
point of failure. However, the recovery stress levels of lightly and
This journal is ª The Royal Society of Chemistry 2012
uncrosslinked materials will not be able to reach levels on par
with deformation levels or be maintained at stable levels during
recovery unless stress-relaxation can be prevented. Such behav-
iors should be studied further as they could affect the use and
selection of such materials in applications in which some
constraint is present during recovery.
Except for the uncrosslinked tBA homopolymer, all materials
in this study demonstrated sample fracture during the (tensile)
fixed-strain shape-memory cycle as the recovery temperatures
approached or surpassed Tg (Fig. 4). Studies by Smith41,42
characterizing the failure of elastomers as a function of strain
rate and temperature reveal that on a certain part of a presented
‘‘failure envelope’’, 3f values of elastomeric materials decrease as
temperatures increase and/or strain rates decrease. In a recent
work by Yakacki et al.43 similar behavior was demonstrated for
(M)ABP networks, where tensile 3f values decreased with
increasing temperature at �Tg. Therefore, as materials are held
at a constant deformation during fixed-strain shape-memory
recovery, the effective failure strains could be decreasing as the
temperature is increased. If the effective tensile failure strain (as
a function of temperature) then coincides with the stored
deformation, sample fracture would occur. Thus, use of these
materials requires that this behavior be taken into account so
that deformation levels and working temperatures are appro-
priately chosen.
Much like the significant trade-off between 3f and ER in (M)
ABPs,29,30 there is a trade-off between maximum recovery strains
and actuation stresses in these materials, to some degree limiting
the ‘‘tailorability’’ of these materials. For example, a tailored
material that can recover large strains, such as needed for
minimally invasive surgery applications, could be developed
using the (M)ABP system presented in this study. Similarly,
a tailored SMP that can generate large stress levels upon con-
strained actuation, such as for orthopedic applications, could
also be developed. However a material tailored to demonstrate
both large elongations and actuation stresses would be difficult
to obtain and would possibly require differing monomer chem-
istries to optimize polymer toughness, a potential area of future
study.
In addition to its effect on the free- and fixed-strain recovery
behavior of (M)ABPs, the effect of crosslinking on the long-
term (extended) storage, free-strain recovery behavior was also
investigated along with the effect of storage time, ranging from
1 to 386 days. This can be an important aspect of the ultimate
use of materials for shape-memory devices that are stored for
long periods of time in a deformed, temporary state. All
materials, independent of the level of crosslinking, demon-
strated good recovery of the stored deformation after the shape-
memory cycle (Fig. 6a), even after storage periods of �386 days
(NSRs $ 0.93 or RRs $ 93%, ESI Table S2†). The 0, 2, and 10
wt% crosslinking agent concentrations did not demonstrate
sizeable recovery during storage throughout the length of
386 days (NSRs # 0.04). For the highly crosslinked material (40
wt% crosslinking agent) there was some indication of recovery
during storage at the 183 and 386 day marks (NSRs $ 0.13),
however the overall variability is higher in this composition
compared to that of the other materials (possibly due to a more
heterogeneous network structure or smaller overall deformation
levels resulting in a pronounced effect of small measurement
Soft Matter, 2012, 8, 7381–7392 | 7389
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errors), thus it is not clear if NSRs values vary in any manner
that correlates with time for the highly crosslinked material. In
previous work by Yakacki et al., the recovery (full and partial)
of packaged shape-memory stents was shown to occur during
storage (unconstrained at room temperature) in (M)ABP
networks over a 35 day storage time.9 Additionally, the recovery
rate during storage was shown to increase with increasing
crosslinking agent concentration and a decreasing difference
between Tg and the storage temperature (DT).9 The DT in the
previous work (DT z 27–33 �C (ref. 9)) was slightly smaller
than used in the current study (DT ¼ 36 �C). In addition, the
deformation temperature was significantly lower in the previous
study (37 �C (ref. 9)) than in the current work (56 �C),a decrease in which has been shown to result in a decrease in
recovery temperature.8
Good storage stability, over a 6-month interval, was also
demonstrated for polyurethane-based SMP foams, where �90%
compressive strains were stored in the shape-memory foams after
being deformed at an elevated temperature of Tg + 30 K.32 The
deformed materials were then kept unconstrained at a low
temperature (Tg – 60 K) for various times, ranging from 1 day to
6 months. These materials demonstrated high fixity and recovery
ratios (>98% (ref. 32)) independent of storage time. Thus,
a number of factors such as crosslinking level, the difference
between transition and storage temperatures, and deformation
temperature can affect the long-term storage stability of SMPs.
For example, elevated deformation temperatures and large DT
values appear to result in good storage stability over long-term
storage times, as shown in the current study and the work by
Tobushi et al.32
In contrast to the overall fixity and recovery stability over time
of the materials used in the current study, Tonset increased with
storage time (Fig. 6b), due to physical aging. Physical aging in
glassy polymers occurs when the material is cooled (10–20 �C)below Tg from an elevated temperature and results in a reduction
of free-volume in the material over time (i.e. densification).44,45
Both aging and shape-memory processing utilize a cooling step
from$Tg to <Tg leaving the material in a state of structural non-
equilibrium, which is lessened over time via small structural
rearrangements,44 explaining the increase in Tonset with storage
time. The overall increase in Tonset over �1 year storage ranged
from �5 to 9 �C, and increased with crosslinking agent concen-
tration. Similarly, polyurethane based SMPs demonstrated an
increase in Tg of >10�C after aging for 72 days at 40 �C and 107
days at room temperature.46 This effect of physical aging could
have a significant impact on the ability to tailor materials and
predict their deployment performance when materials are stored
before actuation. For example, if a material/device is developed
to recover at body temperature for in vivo deployment, but is
stored in the temporary shape for a time, it is possible for the
onset temperature to increase during that storage time such that
proper deployment would not occur. As such, the effect of
physical aging on Tonset of SMPs over extended storage times
needs to be studied and quantified further as well as considered
during material selection and device design when specific and
accurate recovery temperatures are required, as in biomedical
applications.
The rate of free-strain recovery also increased by up to 9�with storage times of up to �1 year for the studied SMPs
7390 | Soft Matter, 2012, 8, 7381–7392
(Fig. 6c). A sharp recovery profile (i.e. decreased profile
breadth) and fast recovery rate is often a desired and sought
after property47 in SMPs, for applications requiring rapid
actuation/deployment. Thus, aging could potentially be used as
a means to optimize shape-recovery behavior and should be
an area of focus in future investigations. Moreover, a more in-
depth study as to how various parameters such as deformation
temperatures, the difference between Tg and storage
temperatures, crosslink density, storage time, storage
conditions, and even applied constraints during storage,
might affect the optimization and successful use of SMPs in
highly developed applications should prove beneficial. For
example, while a reduced storage temperature should aid in
storage stability over long periods of time, a concurrent study
by Choi et al. predicts (via application of a three-dimensional
thermoviscoelastic model) that if the storage temperature is
sufficiently low, the effects of physical aging will be mitigated
due to reduced molecular mobility in (M)ABP networks.48
Thus, further work exploring the long-term storage behavior of
SMPs and how physical aging can be prevented during storage
and/or used as a means to optimize shape-memory behavior
could prove vital for proper material design and
implementation.
Conclusions
This work demonstrates and quantifies trade-offs in free- and
fixed-strain shape-memory behavior, as well as practical issues
such as fixity and recovery behavior over long-term storage, in
(M)ABP materials ranging from uncrosslinked to highly cross-
linked. With all materials in this system of SMPs demonstrating
complete recovery of strains approaching that of failure strains,
total recoverable strains are highly dependent on 3f. Similarly,
actuation stresses were determined to be highly dependent on the
level of stress attained when the material is strained to the desired
deformation, although the ability to reach actuation stress levels
on par with deformation stress levels was shown to decrease as
crosslinking decreased. Thus, there is significant room for ‘‘tai-
lorability’’ of the free- and fixed-strain recovery behavior in this
polymer system for varying applications via changes to cross-
linking level, though limited by the inverse relationship between
recoverable strains and actuation stresses as the level of cross-
linking is varied.
Moreover, physical aging was shown to affect the recovery
behavior of these materials over extended storage times. While
shape-fixity and the level of free-strain recovery remained stable
over long storage times, both Tonset and the rate of strain
recovery increased with storage time due to physical aging. This
change in recovery behavior over extended storage times needs to
be studied further and taken into account for any SMP device
requiring a shelf life to successfully predict and tune the recovery
behavior during use. Moreover, based on the results presented
here, physical aging could potentially be used as a means to
optimize the recovery behavior of SMPs by facilitating sharper
recovery profiles and thus sharper/quicker recovery times in use.
These results should aid in the further development, optimiza-
tion, and ultimate use of SMPs in highly developed applications,
such as those for biomedical-based shape-memory materials and
devices.
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Nomenclature
Td
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Deformation temperature
Tl
Temperature (below Td) to whichmaterial is cooled to fix the temporary
shape
Tonset
Onset temperature of shape recovery3m
Maximum strain during deformation3u
Strain stored in the material afterunloading at Tl
3p
Permanent strain remaining afterrecovery
sm
Maximum stress reached duringdeformation
sp
Plateau stress level reached duringfixed-strain recovery
Normalized
Deformation Strain
Recovery
NSRd ¼ 1� 3
3m(3)
3
Normalized StoredStrain Recovery
NSRs ¼ 1�3u
(4)
Normalized Stress
Generation
NSG ¼ s
sm(5)
�3�
Fixity Ratio
FRd ð%Þ ¼ u3m� 100 (6)
Deformation Recovery
Ratio
RRd ð%Þ ¼�1� 3p
3m
�� 100 (7)
Stored Recovery Ratio
RRs ð%Þ ¼�1� 3p3u
�� 100 (8)
Stress Generation Ratio
SGR ð%Þ ¼�spsm
�� 100 (9)
Acknowledgements
This project was financially supported in part by grant number
F31AR053466 from the National Institute of Arthritis and
Musculoskeletal and Skin Diseases as well as the National
Science Foundation Alliances for Graduate Education and the
Professoriate grant at the University of Colorado (NSF HRD-
0639653). The content is solely the responsibility of the authors
and does not necessarily represent the official views of the
National Institute of Arthritis and Musculoskeletal and Skin
Diseases or the National Institutes of Health. The authors would
like to thank Dr. Scott Kasprzak for performing dynamic
mechanical tests, Professor Chris Bowman for use of NIR
spectroscopy equipment, Dr. Neil Cramer for assistance with
NIR testing, Professor Jeff Stansbury for use of GPC equipment,
and Matthew Barros for performing GPC testing.
Notes and references
1 A. Lendlein, A. M. Schmidt and R. Langer, Proc. Natl. Acad. Sci.U. S. A., 2001, 98, 842–847.
2 K. Gall, P. Kreiner, D. Turner and M. Hulse, J. Microelectromech.Syst., 2004, 13, 472–483.
3 I. A. Rousseau, Polym. Eng. Sci., 2008, 48, 2075–2089.
Society of Chemistry 2012
4 A. M. DiOrio, X. Luo, K. M. Lee and P. T. Mather, Soft Matter,2011, 7, 68–74.
5 N. Choi and A. Lendlein, Soft Matter, 2007, 3, 901–909.6 K. Gall, M. Mikulas, N. A. Munshi, F. Beavers and M. Tupper, J.Intell. Mater. Syst. Struct., 2000, 11, 877–886.
7 A. Lendlein and R. Langer, Science, 2002, 296, 1673–1676.8 K. Gall, C. M. Yakacki, Y. P. Liu, R. Shandas, N. Willett andK. S. Anseth, J. Biomed. Mater. Res., Part A, 2005, 73A, 339–348.
9 C. M. Yakacki, R. Shandas, C. Lanning, B. Rech, A. Eckstein andK. Gall, Biomaterials, 2007, 28, 2255–2263.
10 A. A. Sharp, H. V. Panchawagh, A. Ortega, R. Artale, S. Richardson-Burns, D. S. Finch, K. Gall, R. L. Mahajan and D. Restrepo, J.Neural Eng., 2006, 3, L23–L30.
11 C. M. Yakacki, R. Shandas, D. Safranski, A. M. Ortega,K. Sassaman and K. Gall, Adv. Funct. Mater., 2008, 18, 2428–2435.
12 D. J. Maitland, M. F. Metzger, D. Schumann, A. Lee andT. S. Wilson, Lasers Surg. Med., 2002, 30, 1–11.
13 M. F. Metzger, T. S. Wilson, D. Schumann, D. L. Matthews andD. J. Maitland, Biomed. Microdevices, 2002, 4, 89–96.
14 J. M. Ortega, W. Small IV, T. S. Wilson, W. J. Benett, J. M. Loge andD. J. Maitland, IEEE Trans. Biomed. Eng., 2007, 54, 1722–1724.
15 C. Liu and P. T. Mather, J. Appl. Med. Polym., 2002, 6, 47–52.16 J. Leng, X. Wu and Y. Liu, Smart Mater. Struct., 2009, 18(095031),
1–6.17 T. Xie and I. A. Rousseau, Polymer, 2009, 50, 1852–1856.18 C. Liu, H. Qin and P. T. Mather, J. Mater. Chem., 2007, 17, 1543–
1558.19 Y. Liu, K. Gall, M. L. Dunn and P.McCluskey, SmartMater. Struct.,
2003, 12, 947–954.20 Y. Liu, K. Gall, M. L. Dunn and P. McCluskey,Mech. Mater., 2004,
36, 929–940.21 Y. Liu, K. Gall, M. L. Dunn, A. R. Greenberg and J. Diani, Int. J.
Plast., 2006, 22, 279–313.22 F. Li and R. C. Larock, J. Appl. Polym. Sci., 2002, 84, 1533–
1543.23 J. R. Lin and L. W. Chen, J. Appl. Polym. Sci., 1999, 73, 1305–
1319.24 D. Zhang, X. Lan, Y. Liu and J. Leng, in Behavior and Mechanics of
Multifunctional and Composite Materials, ed. M. J. Dapino, Proc. ofSPIE, Bellingham, WA, 2007, vol. 6526, p. 65262W.
25 T. D. Nguyen, H. J. Qi, F. Castro and K. N. Long, J. Mech. Phys.Solids, 2008, 56, 2792–2814.
26 H. J. Qi, T. D. Nguyen, F. Castro, C. M. Yakacki and R. Shandas, J.Mech. Phys. Solids, 2008, 56, 1730–1751.
27 W. Voit, T. Ware, R. R. Dasari, P. Smith, L. Danz, D. Simon,S. Barlow, S. R. Marder and K. Gall, Adv. Funct. Mater., 2010, 20,162–171.
28 H. G. Jeon, P. T. Mather and T. S. Haddad, Polym. Int., 2000, 49,453–457.
29 A. M. Ortega, S. E. Kasprzak, C. M. Yakacki, J. Diani,A. R. Greenberg and K. Gall, J. Appl. Polym. Sci., 2008, 110,1559–1572.
30 D. L. Safranski and K. Gall, Polymer, 2008, 49, 4446–4455.31 S. J. Tey,W.M. Huang andW.M. Sokolowski, SmartMater. Struct.,
2001, 10, 321–325.32 H. Tobushi, R. Matsui, S. Hayashi and D. Shimada, Smart Mater.
Struct., 2004, 13, 881–887.33 J. W. Stansbury and S. H. Dickens, Dent. Mater., 2001, 17, 71–79.34 L. Rey, J. Galy, H. Sautereau, G. Lachenal, D. Henry and J. Vial,
Appl. Spectrosc., 2000, 54, 39–43.35 N. A. Peppas and B. D. Barr-Howell, Hydrogels in Medicine and
Pharmacy, ed. N. A. Peppas, CRC Press, Inc., Boca Raton, 1986,vol. 1, pp. 27–56.
36 K. P. Menard, Dynamic Mechanical Analysis: A PracticalIntroduction, CRC Press, New York, NY, 1999.
37 A. M. Ortega, C. M. Yakacki, S. A. Dixon, A. R. Greenberg andK. Gall, Active Polymers in Mater. Res. Soc. Symp. Proc., ed. A.Lendlein, V. Prasad Shastri and K. Gall, Warrendale, PA, 2009,vol. 1190, 1190-NN01-02.
38 Note 1: Measured residual or permanent strains of the lightly anduncrosslinked compositions (on average 3 to 5% strain) are slightlylarger than that of the moderately and highly crosslinkedcompositions (on average �1 to 0 % strain).
39 Note 2: One 40 wt% crosslinking agent composition sampledemonstrated a fixed-strain stress level lower than those reported
Soft Matter, 2012, 8, 7381–7392 | 7391
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here (values of which are not included in the mean values presented inTable 2). The cause of this discrepancy in generated stress level is notunderstood at this time.
40 J. M. G. Cowie, Polymers: Chemistry & Physics of Modern Materials,CRC Press, New York, NY, 2nd edn, 1991.
41 T. L. Smith, J. Polym. Sci., Part A: Gen. Pap., 1963, 1, 3597–3615.
42 T. L. Smith, Polym. Eng. Sci., 1977, 17, 129–143.43 C. M. Yakacki, S. Willis, C. Luders and K. Gall, Adv. Eng. Mater.,
2008, 10, 112–119.
7392 | Soft Matter, 2012, 8, 7381–7392
44 P. Hedvig, in Crosslinking and Scission in Polymers, ed. O. Guven,Kluwer Academic Publishers, Boston, MA, 1990, NATO ASI SeriesC, vol. 292, pp. 91–128.
45 S. Wu, Polym. Int., 1992, 29, 229–247.46 V. Lorenzo, A. D�ıaz-Lantada, P. Lafont, H. Lorenzo-Yustos,
C. Fonseca and J. Acosta, Mater. Des., 2009, 30, 2431–2434.47 T. Liu, J. Li, Y. Pan, Z. Zheng, X. Ding and Y. Peng, Soft Matter,
2011, 7, 1641–1643.48 J. Choi, A. M. Ortega, R. Xiao, C. M. Yakacki and T. D. Nguyen,
Polymer, 2012, 53, 2453–2464.
This journal is ª The Royal Society of Chemistry 2012
Addition and correction Note from RSC Publishing This article was originally published with incorrect page numbers. This is the corrected, final version.
The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers.
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