Article
Biodegradable Shape Memory Polymeric Material
from Epoxidized Soybean Oil and Polycaprolactone
Takashi Tsujimoto *, Takeshi Takayama and Hiroshi Uyama
Received: 28 September 2015 ; Accepted: 21 October 2015 ; Published: 27 October 2015
Academic Editor: Wei Min Huang
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita,
Osaka 565-0871, Japan; [email protected] (T.T.); [email protected] (H.U.)
* Correspondence: [email protected]; Tel.: +81-6-6879-7365; Fax: +81-6-6879-7367
Abstract: This article deals with the synthesis of plant oil-based shape memory materials from
epoxidized soybean oil (ESO) and polycaprolactone (PCL). PolyESO/PCLs were synthesized by
an acid-catalyzed curing in the presence of PCL. During the reaction, PCL scarcely reacted with
ESO and the crystallinity of the PCL component decreased to form a semi-interpenetrating network
structure. The incorporation of the PCL components improved the maximum stress and strain at
break of ESO-based network polymer. The polyESO/PCL was gradually degraded by Pseudomonas
cepasia lipase. Furthermore, the polyESO/PCLs exhibited excellent shape memory properties, and
the strain fixity depended on the feed ratio of ESO and PCL. The shape memory-recovery behaviors
were repeatedly practicable. The resulting materials are expected to contribute to the development
of biodegradable intelligent materials.
Keywords: renewable resource; biodegradable; epoxidized soybean oil; shape memory polymer
1. Introduction
In recent years, a growing interest in the exploitation of renewable resources in the area of energy
and materials has been one of the major scientific and industrial issues, because of the depletion of
petroleum resources and economic considerations. Bio-based polymers and composites derived from
renewable agricultural feedstock are a demonstrative case of a relevant dimension for sustainable
development [1–3]. They have a large potential to substitute traditional petroleum-based products in
the industry. Moreover, the advantages of these bio-based materials sometimes include composting
and biodegradability after their use.
Among renewable resources, plant oils such as soybean oil, palm oil, linseed oil, and sunflower
oil are expected as an ideal alternative chemical feedstock, since plant oils are found in abundance
in the world [4]. Triglycerides are the major components in all plant oils, and these contain
both saturated and unsaturated fatty acids. Inexpensive triglyceride plant oils have been utilized
extensively for coatings, inks, plasticizers, lubricants, resins and agrochemicals, in addition to their
applications in food industry. However, direct radical or cationic polymerization of most plant
oils is structurally difficult due to their non-conjugated and internal double bonds, resulting in the
formation of viscous liquid polymers with low molecular weight [5]. Due to these properties, when
triglycerides have been a minor component in polymeric materials, they have been used solely as a
modifier to improve their physical properties.
The most common modification of plant oils is the introduction of oxirane groups that replace
double bonds in the unsaturated fatty acids. Epoxidized soybean oil (ESO), which is one of the most
inexpensive vegetable oils in the world, is commercialized in large volumes at a reasonable cost.
ESO is primarily used as a plasticizer for poly(vinyl chloride) to improve stability and flexibility.
Polymers 2015, 7, 2165–2174; doi:10.3390/polym7101506
www.mdpi.com/journal/polymers
Polymers 2015, 7, 2165–2174
The polyols and acrylates, which are converted from epoxidized plant oils, have been developed as
starting materials for functional bio-based polymers [6–10]. Furthermore, the cationic polymerization
of epoxidized plant oils has been investigated using photoinitiators, latent catalysts, and acid
catalysts to produce bio-based epoxy resins. Although these epoxy compounds from renewable
resources possess a high potential as a starting material for bio-based thermosetting plastics, the
triglycerides are made up of aliphatic chains, and consequently, the plant oil-based materials are
incapable of the rigidity and strength required for some applications by themselves [11–15]. ESO was
cured in the presence of inorganic chemicals to produce organic-inorganic hybrid materials [16–19].
Plant oil-based green composites were developed using biofibers such as kenaf, flax, hemp, and rosin
derivatives as renewable compounds to improve their poor mechanical properties [20–23].
Shape memory materials are one of the early-developing and emerging smart materials.
Shape memory materials change their shapes reversibly from temporary to permanent by mechanical
loading and external stimulus, which may be temperature, light, pH, humidity, chemical, and
electricity [24–30]. Because of their good processability, low cost, and high recovery ability, shape
memory polymers have received much attention and have been used in various fields, with
applications such as sensors, transducers, actuators, and medical implants. The thermally-triggered
shape memory polymer is first processed to receive a permanent shape. The temporary shape is
set by deformation above a certain transition temperature (switching temperature) and subsequent
cooling under loading. This process typically relies on vitrification, crystallization, and other
physical interactions such as hydrogen bonding and ionic bonding. When reheated above the
switching temperature without loading, the oriented polymer chains are released, resulting
in a macroscopic recovery of the original shape. Up to now, numerous polymers, including
polyurethane, polynorbornene, trans-polyisoprene, and styrene-butadiene copolymer, have been
found to have attractive shape memory effects [31–38]. Bio-based shape memory polymers have
also been developed from glycerol, glycolide, and lactide [39–44]. Several researchers exploited
polycaprolactone (PCL)-based shape memory polymers, and these materials showed sharper and
faster shape memory-recovery behaviors by using melting/recrystallization of the PCL component
as a switching transition [45–47]. However, most of them are block copolymers that were prepared
by means of a two-stage synthesis or coupling method.
In this study, we synthesized a biodegradable polymeric material with semi-interpenetrating
network structure from epoxidized soybean oil and polycaprolactone, and measured the thermal
and mechanical properties. Furthermore, the shape memory properties of the resulting material
were investigated.
2. Experimental Section
2.1. Materials
ESO and a thermally-latent cationic catalyst (benzylsulfonium hexafluoroantimonate derivative,
Sun-Aid SI-100L) were gifts from Adeka Co. (Tokyo, Japan) and Sanshin Chemical Industry Co.
(Yamaguchi, Japan), respectively. Polycaprolactone (PCL) (Mn = 8.0 ˆ 104 ) was purchased from
Sigma-Aldrich Co. (St. Louis, MO, USA). Other reagents and solvents were commercially available
and were used as received.
2.2. Synthesis of PolyESO/PCL
The synthesis of polyESO/PCL was carried out as follows. ESO (0.5 g) and PCL (0.5 g) were
dissolved in 2.5 mL of chloroform, and a thermally-latent cationic catalyst was added to the solution.
The solution was poured into a Teflon mold (17 mm ˆ 40 mm ˆ 1 mm), and then, the solvent was
allowed to evaporate at room temperature. The residual mixture was kept at 150 ˝ C for 2 h to produce
polyESO/PCL (50/50 wt %).
2166
Polymers 2015, 7, 2165–2174
The polyESO/PCLs with different feed ratios of ESO and PCL were prepared by a
similar procedure.
2.3. Enzymatic Degradation of PolyESO/PCL
The polyESO/PCL (50/50 wt %) with dimensions of 10 mm ˆ 10 mm was placed in a vial
containing 20 mL of phosphate buffer (pH 7.2) and 10 mg of lipase from Pseudomonas cepacia, and the
vial was shaken gently. At the predetermined degradation time, the sample was removed from the
buffer, washed with deionized water, and dried in vacuo for 24 h. The degradation test was performed
three times. The weight of the residue was measured to evaluate the degree of degradation.
2.4. Evaluation of Shape Memory-Recovery Properties of PolyESO/PCL
Shape memory-recovery properties were evaluated by the end-to-end length of the sample to
determine the strain fixity and the shape recovery. The polyESO/PCL with a rectangular bar (sample
length: L) was prepared as a permanent shape. The sample was heated at 80 ˝ C and loaded to
the predetermined strain, followed by cooling and unloading at 20 ˝ C. Upon unloading, a part of
the strain was instantaneously recovered, leaving an unloading strain (εu ). Then, the sample was
reheated at 80 ˝ C to recover the strain, leaving a recovery strain (εr ). The strain fixity and the shape
recovery are defined as follows.
Strain fixity p%q “ pL ´ ε u q{L ˆ 100
(1)
Recovery p%q “ ε r {L ˆ 100
(2)
2.5. Measurements
1H
nuclear magnetic resonance (1 H NMR) spectrum was measured by a DPX-400 instrument
(Bruker Biospin Co., Billerica, MA, USA). Fourier-transform infrared spectroscopy (FT-IR) was
recorded on a Spectrum One (Perkin-Elmer Inc., Waltham, MA, USA). The thermal properties of
the samples were investigated under nitrogen atmosphere by using a DSC6020 differential scanning
calorimeter (DSC) (Hitachi High-Tech Science Co., Tokyo, Japan). The sample was heated from
´100 to 180 ˝ C at a heating rate of 10 ˝ C¨ min´1 , and the temperature was maintained for a
duration of 3 min. Then, the sample was cooled to ´100 ˝ C at a cooling rate of 10 ˝ C¨ min´1 .
The temperature was maintained for a duration of 3 min, and the sample was reheated to
200 ˝ C at a heating rate of 10 ˝ C¨ min´1 . Tensile properties were measured by using a EZ Graph
(Shimadzu Co., Kyoto, Japan) with a cross-head speed of 5 mm¨ min´1 . The sample was cut into a
plate shape of 5 mm ˆ 20 mm ˆ1 mm. Wide-angle X-ray scattering (WAXS) analysis was performed
using RINT2500 instrument (Rigaku Co., Tokyo, Japan) with CuKα radiation at 50 kV/300 mA.
The diffractogram was scanned in a 2θ range of 1.5˝ –40˝ at a rate of 2˝ ¨ min´1 . Dynamic viscoelasticity
analysis (DMA) was performed by using a DMS6100 (Hitachi High-Tech Science Co., Tokyo, Japan)
with a frequency of 1 Hz at a heating rate of 3 ˝ C¨ min´1 .
3. Results and Discussion
3.1. Synthesis of PolyESO/PCL
In this study, epoxidized soybean oil (ESO) was used as an epoxide monomer. The oxirane group
number of ESO, determined by 1 H NMR spectroscopy, was 3.7 per molecule. The polyESO/PCLs
were synthesized by the curing of ESO using a thermally-latent cationic catalyst in the presence of
PCL at 150 ˝ C to produce flexible pale brown materials (Figure 1). In the FT-IR spectrum of the
polyESO/PCL, a peak at ca. 820 cm´1 ascribed to C–C anti-symmetric stretching of the oxirane groups
of ESO disappeared. On the other hand, the peaks ascribed to C=O stretching of carbonyl groups
of the ESO and PCL hardly changed (Figure S1, Supplementary Materials). The polyESO/PCLs
2167
Polymers 2015, 7, 2165–2174
were immersed in chloroform at room temperature for 24 h, and the residue was washed with
chloroform. The PCL components were dissolved in chloroform, and the residue weight was close to
the feed weight of ESO. These results indicate that the PCL component is scarcely reacted during the
crosslinking
of ESO to form a semi-interpenetrating polymer network structure.
Polymers 2015, 7, page–page
Polymers 2015, 7, page–page
Figure 1. Synthesis of polyESO/PCL.
Figure 1. Synthesis of polyESO/PCL.
Figure 1. Synthesis of polyESO/PCL.
3.2. Thermal and Mechanical Properties of polyESO/PCL
3.2. Thermal and Mechanical Properties of polyESO/PCL
In orderand
to investigate
thermalofproperties
of the polyESO/PCL, DSC measurement of the ESO
3.2. Thermal
Mechanical the
Properties
polyESO/PCL
In order to investigate
the thermal
properties
of carried
the polyESO/PCL,
homopolymer,
the polyESO/PCLs,
and neat
PCL was
out (Figure 2).DSC
In themeasurement
DSC curve of of the
In
order
to
investigate
the
thermal
properties
of
the
polyESO/PCL,
DSC
measurement
of the
ESO
ESO homopolymer,
a glass transitionand
wasneat
observed
at −25carried
°C. Onout
the (Figure
other hand,
thethe
curve
of curve
ESOthe
homopolymer,
the polyESO/PCLs,
PCL was
2). In
DSC
homopolymer,
the
polyESO/PCLs,
and
neat
PCL
was
carried
out
(Figure
2).
In
the
DSC
curve
of
˝
neat
PCL
showed a glass
transition
and awas
melting
behavior
at −63
andthe
59 other
°C, respectively.
of the
ESO
homopolymer,
a glass
transition
observed
at ´25
C. On
hand, the curve
the
ESO
homopolymer,
a
glass
transition
was
observed
at
−25
°C.
On
the
other
hand,
the
curve
˝ C, respectively.
In the
curves
of thea polyESO/PCLs,
ESO polymer
and
PCL
not observed, Inof
of neat
PCL
showed
glass transitionglass
andtransitions
a meltingofbehavior
at ´63
and
59were
the
neat
PCL
showed
a
glass
transition
and
a
melting
behavior
at
−63
and
59
°C, respectively.
and the melting temperature (Tm) of the PCL component decreased with an increase
the ESO
curves
of the polyESO/PCLs, glass transitions of ESO polymer and PCL were notinobserved,
and
Incontent.
the curves
of thethe
polyESO/PCLs,
glass
transitions
ofPCL
ESOalso
polymer
andcompared
PCL werewith
not that
observed,
Moreover,
melting enthalpy
(ΔH
m) per 1 g of
decreased
of
theand
melting
temperature
(T
)
of
the
PCL
component
decreased
with
an
increase
in
the
ESO
content.
m
melting
temperature
(Tm) of
the
PCL
component
withwith
an the
increase
in the ESO
neatthe
PCL.
These results
may indicate
that
PCL
components
are decreased
partly miscible
ESO polymer.
Moreover,
the melting
enthalpy
(∆Hm ) and
per
1mg)polyESO/PCLs
of PCL
compared
with higher
thatthat
of neat
content.
Moreover,
enthalpy
(ΔH
per
1 g ofalso
PCLdecreased
also decreased
compared
of
Both curves
of the the
ESOmelting
homopolymer
the
were
almost constant
in thewith
PCL.
These
results
may
indicate
that
PCL
components
are
partly
miscible
with
the
ESO
polymer.
temperature
region,
suggesting
the quantitative
consumptionare
of the
oxirane
groups
of the
ESO.
neat
PCL. These
results
may indicate
that PCL components
partly
miscible
with
ESO polymer.
Both
curves
of of
thethe
ESO
werealmost
almostconstant
constantininthe
thehigher
higher
Both
curves
ESOhomopolymer
homopolymerand
andthe
thepolyESO/PCLs
polyESO/PCLs were
temperature
region,
suggesting
the oxirane
oxiranegroups
groupsofofESO.
ESO.
T g(ESO) consumption
temperature
region,
suggestingthe
thequantitative
quantitative
consumption of
of the
(a)
(b)
endo
T g(ESO)
endo
-100
(c) (a)
(d)
(b)
(e)
T g(PCL)
(c)
(d)
(e)
T g(PCL)
-50
0
50
100
150
Temperature / °C
Figure 2. Differential
Calorimetry
of 100
second heating
-100Scanning-50
0 (DSC) curves
50
150 scan for (a) ESO
homopolymer; (b) polyESO/PCL (75/25 wt %); (c) polyESO/PCL (50/50 wt %); (d) polyESO/PCL
Temperature / °C
(25/75 wt %); and (e) neat PCL.
Figure
Differential Scanning
Scanning Calorimetry
Calorimetry (DSC)
Figure
2. 2.Differential
(DSC) curves
curves of
of second
second heating
heatingscan
scanforfor(a)(a)ESO
ESO
The thermal (b)
andpolyESO/PCL
mechanical (75/25
properties
such
as melting temperature,
melting
enthalpy,
homopolymer;
wt
%);
(c)
polyESO/PCL
(50/50
wt
%);
(d)
homopolymer; (b) polyESO/PCL (75/25 wt %); (c) polyESO/PCL (50/50 wt %); (d)polyESO/PCL
polyESO/PCL
maximum
stress,
and(e)
strain
at
break, are summarized in Table 1. In the tensile test, neat PCL showed
(25/75
%);
and
neatPCL.
PCL.
(25/75
wtwt
%);
and
(e) neat
ductile behavior. Following the yield point, neat PCL could be stretched further, followed up by strain
hardening
to an elongation
of 1000% properties
which is thesuch
maximum
extension
limit of our instrument.
On
The thermal
and mechanical
as melting
temperature,
melting enthalpy,
The
thermal
and
mechanical
properties
such
as
melting
temperature,
melting
enthalpy,
the
other
hand,
the
ESO
homopolymer
was
brittle
and
fractured
at
low
strain,
due
to
the
ESO-based
maximum stress, and strain at break, are summarized in Table 1. In the tensile test, neat PCL showed
network
structure.
The
maximum
stress
of
polyESO/PCLs
increased
the
content
increased.
maximum
stress,
and
strain
at yield
break,
aretheneat
summarized
Table
1.as further,
In PCL
the tensile
test,
neat
PCL
ductile
behavior.
Following
the
point,
PCL couldin
be
stretched
followed
up by
strain
Moreover,
the
strain
at
break
of
the
polyESO/PCLs
was
larger
than
that
of
the
ESO
homopolymer.
showed
ductile
behavior.
Following
the
yield
point,
neat
PCL
could
be
stretched
further,
followed
hardening to an elongation of 1000% which is the maximum extension limit of our instrument. On
This
mayhand,
be because
incorporation
PCL and
components
the due
distance
the
other
the ESO the
homopolymer
wasofbrittle
fracturedincreased
at low strain,
to thebetween
ESO-based
crosslinking points of ESO-based network polymer.
network structure. The maximum stress of the polyESO/PCLs
increased as the PCL content increased.
2168
4
Moreover, the strain at break of the polyESO/PCLs was larger than that of the ESO homopolymer.
This may be because the incorporation of PCL components increased the distance between
crosslinking points of ESO-based network polymer.
Polymers 2015, 7, 2165–2174
up by strain hardening to an elongation of 1000% which is the maximum extension limit of our
instrument. On the other hand, the ESO homopolymer was brittle and fractured at low strain, due
to the ESO-based network structure. The maximum stress of the polyESO/PCLs increased as the
PCL content increased. Moreover, the strain at break of the polyESO/PCLs was larger than that of
the ESO homopolymer. This may be because the incorporation of PCL components increased the
distance between crosslinking points of ESO-based network polymer.
Table 1. Thermal and mechanical properties of polyESO/PCL.
Polymers 2015, 7, page–page
T
Sample code
a
∆H
a,b
Maximum stress c
Strain at break c
MPa
%
m
m properties of polyESO/PCL.
Table 1. Thermal
and mechanical
˝C
J¨ g´1
Tm a
ΔHm a,b Maximum stress c Strain at break c
ESO homopolymer
1.0
9
Sample code
–d
– d −1
°C
J·g
MPa
%
PolyESO/PCL (75/25 wt %) 50.6
9.8
2.6
20
d
ESO homopolymer
–d
1.0 8.8
9
PolyESO/PCL
(50/50 wt %) 51.6 –
25.7
63
PolyESO/PCL
(75/25
wt
%)
50.6
9.8
2.6
20
PolyESO/PCL (25/75 wt %) 52.8
41.6
12.3
147
PolyESO/PCL (50/50 wt %)
25.7
8.8 – e
63
Neat PCL
58.6 51.6
62.2
>1,000
PolyESO/PCL (25/75 wt %)
52.8
41.6
12.3
147
a Determined by DSC; b Melting enthalpy of whole sample; c Determined by tensile test; d Not observed;
Neat PCL
58.6
62.2
–e
>1,000
e
Not measured. Melting temperature (Tm ); Melting enthalpy (∆Hm ).
a Determined by DSC; b Melting enthalpy of whole sample; c Determined by tensile test;
observed; e Not measured. Melting temperature (Tm); Melting enthalpy (ΔHm).
d
Not
In order to investigate the microstructure of the PCL component in polyESO/PCL, wide-angle
In order(WAXD)
to investigate
microstructure
of the3).
PCLItcomponent
polyESO/PCL,
wide-angle of the
X-ray diffraction
wasthemeasured
(Figure
is foundinthat
the microstructure
X-ray
diffraction
(WAXD)
was
measured
(Figure
3).
It
is
found
that
the
microstructure
of
polyESO/PCL (50/50 wt %) is characterized by superposition of an amorphous halo and crystalline
the polyESO/PCL (50/50 ˝wt %) ˝is characterized
by superposition of an amorphous halo and
˝
diffraction peaks of 2θ = 21.5 , 22.0 , and 23.7 that correspond to the (010), (111) and (112) planes
crystalline diffraction peaks of 2θ = 21.5°, 22.0°, and 23.7° that correspond to the (010), (111) and
of PCL (112)
crystal
[48].ofThese
suggest
that
thesuggest
microstructure
of the PCL component
in polyESO/PCL
is
planes
PCL crystal
[48].
These
that the microstructure
of the PCL component
in
not changed
compared
neat PCL
excepttofor
a PCL
decrease
polyESO/PCL
is nottochanged
compared
neat
exceptof
forcrystallinity.
a decrease of crystallinity.
3. X-ray diffraction patterns of (a) ESO homopolymer; (b) polyESO/PCL (50/50 wt %); and
Figure Figure
3. X-ray
diffraction patterns of (a) ESO homopolymer; (b) polyESO/PCL (50/50 wt %);
(c) neat PCL.
and (c) neat PCL.
Figure 4 shows the dynamic viscoelasticity (storage modulus and loss factor) as a function of
temperature
for the
homopolymer,
the polyESO/PCLs
andmodulus
neat PCL. and
For allloss
samples,
the storage
Figure 4 shows
theESO
dynamic
viscoelasticity
(storage
factor)
as a function
moduli remained almost constant between −100 and −70 °C, and decreased as temperature increased.
of temperature for the ESO homopolymer, the polyESO/PCLs and neat PCL. For all samples, the
In the ESO homopolymer, the rapid decrease of the storage modulus was observed at −25 °C.
storage moduli remained almost constant between ´100 and ´70 ˝ C, and decreased as temperature
At higher temperatures, the storage modulus of the ESO homopolymer was almost constant,
increased.
In the
homopolymer,
the of
rapid
decrease
modulus
observed at
indicating
theESO
quantitative
consumption
oxirane
groups of
andthe
the storage
formation
of a plantwas
oil-based
´25 ˝ C.network
At higher
temperatures,
storage
of the ESO
homopolymer
wastoalmost
constant,
polymer.
The storagethe
modulus
ofmodulus
neat PCL slightly
decreased
at −70 °C due
the glass
transition
of PCL, and PCL
melted at ca. of
50 °C.
It is found
that the
moduli of polyESO/PCLs
indicating
the quantitative
consumption
oxirane
groups
andstorage
the formation
of a plant oil-based
˝ C duemore
drop
stepwise.The
At lower
temperature,
were glassy
with a storage
network
polymer.
storage
modulusthe
ofpolyESO/PCLs
neat PCL slightly
decreased
at ´70modulus
to the glass
than 1.0 GPa. The storage modulus of the˝ polyESO/PCLs gradually decreased from −60 to 0 °C,
transition of PCL, and PCL melted at ca. 50 C. It is found that the storage moduli of polyESO/PCLs
resulting in the first rubbery state. With an increase in the ESO content, the storage modulus in first
drop stepwise.
At lower temperature, the polyESO/PCLs were glassy with a storage modulus more
rubbery state decreased. Only a broad peak was found in loss factor curves of the polyESO/PCLs,
than 1.0and
GPa.
The
storageto modulus
of the polyESO/PCLs
gradually
decreasedThese
fromindicate
´60 to 0 ˝ C,
the peaks shifted
the higher temperature
region as the ESO
content increased.
resulting
inthe
thePCL
first
rubbery state.
With
an increase
thepolymer
ESO content,
the storage
modulus
that
components
are partly
miscible
with thein
ESO
in polyESO/PCLs.
Upon
further in first
heating, the storage modulus dropped at ca. 50 °C, which was derived from the melting temperature
of PCL, to give the second rubbery state. These results are in good agreement with DSC analysis.
In the second rubbery region, the storage moduli
of polyESO/PCLs were constant due to the plant
2169
oil-based network structure and the polyESO/PCLs with the ESO content of 50 and 75 wt % exhibited
5
Polymers 2015, 7, 2165–2174
rubbery state decreased. Only a broad peak was found in loss factor curves of the polyESO/PCLs,
and the peaks shifted to the higher temperature region as the ESO content increased. These indicate
that the PCL components are partly miscible with the ESO polymer in polyESO/PCLs. Upon further
heating, the storage modulus dropped at ca. 50 ˝ C, which was derived from the melting temperature
of PCL, to give the second rubbery state. These results are in good agreement with DSC analysis.
In the second rubbery region, the storage moduli of polyESO/PCLs were constant due to the
2015, network
7, page–pagestructure and the polyESO/PCLs with the ESO content of 50 and 75 wt %
plantPolymers
oil-based
exhibited a relatively high storage modulus of more than 1.0 MPa. The storage modulus in the second
a relatively
high
storage modulus of more than 1.0 MPa. The storage modulus in the second rubbery
Polymers
2015, 7,depended
page–page on the ESO content. In shape memory polymers, the storage modulus
rubbery
region
region depended on the ESO content. In shape memory polymers, the storage modulus below
below the switching temperature governs the strength of the materials, whereas the modulus above
switching
governs
the than
strength
of the
whereasinthe
above
athe
relatively
hightemperature
storage modulus
of more
1.0 MPa.
Thematerials,
storage modulus
the modulus
second rubbery
the switching
temperature
determines
the
recovery
force.
In this study,
the transition
between
the switching
temperature
determines
theshape
recovery
force.polymers,
In this study,
the transition
region
depended
on the ESO
content. In
memory
the storage
modulusbetween
below
the first
rubbery
state
and
the
second
rubbery
state
was
used
as
a
switching
transition
of
shape
the switching
first rubbery
state and the
second
state
was materials,
used as a whereas
switchingthe
transition
of above
shape
the
temperature
governs
therubbery
strength
of the
modulus
memory-recovery
behaviors.
memory-recovery
behaviors.determines the recovery force. In this study, the transition between
the
switching temperature
the first rubbery state and the second rubbery state was used as a switching transition of shape
(A)
(B)
memory-recovery
0.8
.0E+10
1010 behaviors.
.0E+09
109
(a)
(b)
.0E+08
10
.0E+06
1086 (c)
(a)
(d)
.0E+07
10
.0E+05
1075 (b)
(e)
.0E+06
10
.0E+04
1064 (c)
-100
0
50
100
(d) -50
.0E+05
105
(e)
Temperature / °C
.0E+04
104
-50
0
50
100
tan δ
tan δ
.0E+09
10
.0E+07
1097
-100
(a)
(b)
(c)
(a)
(d)
(b)
(e)
(c)
(d)
(e)
0.6 (B)
0.8
E’ / Pa E’ / Pa
8
.0E+10
10
.0E+08
1010
(A)
150
150
0.4
0.6
0.2
0.4
0
0.2 -100
0
-100
0
100
200
Temperature / °C
0
100
200
Figure 4. Dynamic viscoelasticity of (a) ESO homopolymer, (b) polyESO/PCL (75/25 wt %),
Figure 4. Dynamic viscoelasticity
of (a) ESO homopolymer, (b) polyESO/PCL
(75/25 wt %),
Temperature / °C
/ °Cstorage modulus,
(c) polyESO/PCL (50/50 wt %), (d) polyESO/PCL (25/75 wt %), and (e)Temperature
neat PCL; (A)
(c) polyESO/PCL (50/50 wt %), (d) polyESO/PCL (25/75 wt %), and (e) neat PCL; (A) storage
and (B) 4.
lossDynamic
factor. viscoelasticity of (a) ESO homopolymer, (b) polyESO/PCL (75/25 wt %),
Figure
modulus,
and (B)
loss factor.
(c) polyESO/PCL (50/50 wt %), (d) polyESO/PCL (25/75 wt %), and (e) neat PCL; (A) storage modulus,
PCL(B)
is loss
onefactor.
of the most promising synthetic polymers that can degrade in contact with enzymes
and
Biodegradability
/%
Biodegradability
/%
and microorganisms.
PCLpromising
is also a biocompatible
and nontoxic
polyester,
likeinpoly(lactic
acid)enzymes
and
PCL
is one of the most
synthetic polymers
that can
degrade
contact with
polyglycoside.
Enzymatic
degradation
of
the
polyESO/PCL
(50/50
wt
%)
was
performed
using
lipase
PCL is one of the
most
promising
synthetic polymers
that can polyester,
degrade in like
contact
with enzymes
and microorganisms.
PCL
is also
a biocompatible
and nontoxic
poly(lactic
acid) and
frommicroorganisms.
Pseudomonas
cepasia.
hydrolysis
gradually
took
place,
biodegradability
and
PCL
isThe
alsoenzymatic
a biocompatible
and nontoxic
polyester,
like
poly(lactic
acid)
andlipase
polyglycoside.
Enzymatic
degradation
of the polyESO/PCL
(50/50
wt %)
wasand
performed
using
reached 40% atEnzymatic
5 days (Figure
5). The loss
weight
was close(50/50
to thewt
feed
weight
of PCL. On
thelipase
other
polyglycoside.
degradation
of
the
polyESO/PCL
%)
was
performed
using
from Pseudomonas cepasia. The enzymatic hydrolysis gradually took place, and biodegradability
hand,Pseudomonas
the weight of the The
ESO homopolymer
hardly gradually
changed, took
suggesting and
that biodegradability
the lipase from
from
reached
40% at 5 dayscepasia.
(Figure 5).enzymatic
The losshydrolysis
weight was
close to theplace,
feed weight
of PCL. On the
Pseudomonas
cepasia
degraded
the
PCL
components.
In
our
previous
study,
an
ESO-based
network
reached 40% at 5 days (Figure 5). The loss weight was close to the feed weight of PCL. On the
other
otherpolymer
hand, could
the weight
of
the
ESO
homopolymer
hardly
changed,
suggesting
that
the lipase
be
degraded
in
activated
sludge
[16].
Therefore,
the
resulting
polyESO/PCLs
exhibited
hand, the weight of the ESO homopolymer hardly changed, suggesting that the lipase from
fromPseudomonas
Pseudomonas
cepasia
degradedPCL
the components.
PCL components.
In our previous study, an ESO-based
high
potential
for
biodegradable
cepasia
degraded thematerials.
In our previous study, an ESO-based network
network
polymer
could
be
degraded
in
activated
sludge
[16].
Therefore,
resulting polyESO/PCLs
polymer could be degraded in activated sludge [16]. Therefore, the resultingthe
polyESO/PCLs
exhibited
50
exhibited
high
potential
for
biodegradable
materials.
high potential for biodegradable materials.
40
50
30
40
20
30
10
20
100
0
0
2
4
Time / day
6
8
0
2
4 of polyESO/PCL
6
8
Figure 5. Enzymatic
hydrolysis
behavior
(50/50 wt%).
Time / day
3.3. Shape Memory-Recovery
Behavior of
PolyESO/PCL
Figure 5. Enzymatic
hydrolysis
behavior of polyESO/PCL (50/50 wt%).
Figure 5. Enzymatic hydrolysis behavior of polyESO/PCL (50/50 wt%).
An investigation of shape memory-recovery behaviors of the polyESO/PCLs was performed.
3.3.
Shape
Memory-Recovery
Behavior of PolyESO/PCL
In this
study,
the melting/recrystallization
of the PCL component was used as a switching transition
of shape
memory-recovery
effect.
A
typical
example
of shape
of
2170
An investigation of shape memory-recovery
behaviors
of the memory-recovery
polyESO/PCLs wasbehaviors
performed.
the
polyESO/PCL
(50/50
wt
%)
is
shown
in
Figure
6.
During
the
crosslinking
process,
the
primary
In this study, the melting/recrystallization of the PCL component was used as a switching transition
shape
without
stress (permanent
shape)
was setexample
to be a spiral
shape.memory-recovery
Above the meltingbehaviors
temperature
of
shape
memory-recovery
effect.
A typical
of shape
of
the polyESO/PCL (50/50 wt %) is shown in Figure 6. During the crosslinking process, the primary
Polymers 2015, 7, 2165–2174
3.3. Shape Memory-Recovery Behavior of PolyESO/PCL
An investigation of shape memory-recovery behaviors of the polyESO/PCLs was performed.
In this study, the melting/recrystallization of the PCL component was used as a switching transition
of shape memory-recovery effect. A typical example of shape memory-recovery behaviors of the
polyESO/PCL (50/50 wt %) is shown in Figure 6. During the crosslinking process, the primary
shape
without
(permanent shape) was set to be a spiral shape. Above the melting temperature
Polymers
2015, 7,stress
page–page
of the PCL component, the crystal of PCL melted, and the polyESO/PCL (50/50 wt %) was
of the PCL
component,
the crystal
of PCL melted,
and The
the polyESO/PCL
(50/50 wt
was temperature
deformed
deformed
into
a rectangular
bar (temporary
shape).
subsequent cooling
at %)
room
intothe
a rectangular
bar (temporary
shape).above
The subsequent
at room
temperature
fixed
fixed
deformed shape.
Upon reheating
the meltingcooling
temperature
of the
PCL component,
the
deformed
shape.
Upon
reheating
above
the
melting
temperature
of
the
PCL
component,
the temporary shape was restored to its original shape. The recovery time was dependent on the
the temporary
shape and
wasfeed
restored
original
shape.
time
wasofdependent
on
operating
temperature
ratio to
of its
ESO
and PCL,
andThe
the recovery
permanent
shape
polyESO/PCL
the operating
temperature
ratio
of ESO
PCL,deformations
and the permanent
shape of polyESO/PCL
(50/50
wt %) was
recoveredand
at feed
80 ˝ C
after
25 s.and
These
and recovery
processes were
(50/50
wt
%)
was
recovered
at
80
°C
after
25
s.
These
deformations
and
recovery
processes
were of
repeatedly practicable. The plausible shape memory-recovery mechanism is as follows.
The crystal
repeatedly practicable. The plausible shape memory-recovery mechanism is as follows. The crystal
PCL fixes the shape of the sample and the driving force of shape recovery process is the elastic force of
of PCL fixes the shape of the sample and the driving force of shape recovery process is the elastic
the ESO-based network polymer, generated during deformation. Above the switching temperature,
force of the ESO-based network polymer, generated during deformation. Above the switching
PCL components in the polyESO/PCLs melt and the sample is easily deformed. After the subsequent
temperature, PCL components in the polyESO/PCLs melt and the sample is easily deformed. After
cooling, the melt crystallization of PCL components proceeds, and the temporary shape is fixed with
the subsequent cooling, the melt crystallization of PCL components proceeds, and the temporary
theshape
internal
stress
of the
network
reheating
above
thereheating
melting temperature
is fixed
with
theESO-based
internal stress
of the polymer.
ESO-basedUpon
network
polymer.
Upon
above the
of melting
PCL, thetemperature
mobility ofof
polymer
chains
increases,
and
the
shape
of
the
sample
returns
thesample
original
PCL, the mobility of polymer chains increases, and the shape oftothe
shape
by the
entropy
elasticity
ofthe
theentropy
ESO-based
network
polymer.
returns
to the
original
shape by
elasticity
of the
ESO-based network polymer.
Figure
Shapememory-recovery
memory-recoverybehaviors
behaviors of
of polyESO/PCL
polyESO/PCL (50/50
wtwt
%).%).
Figure
6. 6.
Shape
(50/50
To investigate the ability of shape memory-recovery properties, we prepared the polyESO/PCLs
To investigate
thebar
ability
shape memory-recovery
properties,
we prepared
polyESO/PCLs
with
a rectangular
as aofpermanent
shape. The sample
was deformed
to a the
circle
shape as a
with
a
rectangular
bar
as
a
permanent
shape.
The
sample
was
deformed
to
a
circle
shape
as a
temporary shape (Figure S2, Supplementary Materials), and the strain fixity and recovery
were
temporary
shape
(Figure
S2,
Supplementary
Materials),
and
the
strain
fixity
and
recovery
were
evaluated by end-to-end length of the sample (Figure 7). The ESO homopolymer could not be
evaluated
end-to-end
length of the
(Figureshape
7). The
ESOPCL
homopolymer
coulddue
notto be
deformedbybecause
of the brittleness,
and sample
the temporary
of neat
was not prepared
deformed
because
of the brittleness,
andgood
therecovery
temporary
shape
of neat
PCL
waspolyESO/PCL
not prepared
melting. All
of the polyESO/PCLs
showed
ability.
The strain
fixity
of the
(75/25
wt %) decreased
compared
with that of
the polyESO/PCL
(50/50ability.
wt %) The
and the
polyESO/PCL
due
to melting.
All of the
polyESO/PCLs
showed
good recovery
strain
fixity of the
(25/75 wt %), due
to the
of the crystallinity
of thethat
PCLof
component
and the increase
internal
polyESO/PCL
(75/25
wtdecrease
%) decreased
compared with
the polyESO/PCL
(50/50ofwt
%) and
of the ESO-based
polymer
These of
results
indicate that
the PCL
shape
recovery and
fromthe
thestress
polyESO/PCL
(25/75 wt
%), duenetwork.
to the decrease
the crystallinity
of the
component
the
deformed
shape
to
the
original
shape
is
attributed
to
the
relaxation
of
the
inner
stress
of
the
increase of internal stress of the ESO-based polymer network. These results indicate that the shape
ESO-based
polymer
network.
Furthermore,
even
at
the
fifth
cycle,
the
strain
fixity
and
the
recovery
recovery from the deformed shape to the original shape is attributed to the relaxation of the inner
of the
wt %)
and polyESO/PCL
(25/75
wt at
%)the
were
more
than
90%,
suggesting
stress
of polyESO/PCL
the ESO-based(50/50
polymer
network.
Furthermore,
even
fifth
cycle,
the
strain
fixity and
good reusability.
2171
Polymers 2015, 7, 2165–2174
the recovery of the polyESO/PCL (50/50 wt %) and polyESO/PCL (25/75 wt %) were more than
Polymers
2015, 7, page–page
90%,
suggesting
good reusability.
(B)
100
100
80
80
Recovery / %
Strain fixity / %
(A)
60
40
20
60
40
20
0
1
2
3
4
0
5
1
Cycle
2
3
4
5
Cycle
Figure 7. Shape memory-recovery properties of (∆) polyESO/PCL (75/25 wt %), (Ο) polyESO/PCL
Figure 7. Shape memory-recovery properties of (∆) polyESO/PCL (75/25 wt %), () polyESO/PCL
(50/50 wt %), and (◊) polyESO/PCL (25/75 wt %); (A) strain fixity, and (B) recovery.
(50/50 wt %), and (♦) polyESO/PCL (25/75 wt %); (A) strain fixity, and (B) recovery.
4. Conclusions
4. Conclusions
In this study, a plant oil-based shape memory material was synthesized from epoxidized
In this study,
shape memory
material
from epoxidized
soybean
soybean
oil anda plant
PCL. oil-based
An acid-catalyzed
crosslinking
ofwas
ESOsynthesized
in the presence
of PCL produced
oil aand
PCL.material
An acid-catalyzed
crosslinking of network
ESO in the
presence
of resulting
PCL produced
a flexible
flexible
with a semi-interpenetrating
structure.
In the
polyESO/PCLs,
material
with
a
semi-interpenetrating
network
structure.
In
the
resulting
polyESO/PCLs,
PCL
PCL components were partly miscible with the ESO polymer, and the crystallinity of PCL
components
were
partly miscible
with theproperties,
ESO polymer,
the crystallinity
of PCL
components
decreased.
The mechanical
such and
as maximum
stress and
straincomponents
at break,
decreased.
The mechanical
such as maximum
stress
and strain The
at break,
were effectively
were effectively
improvedproperties,
by the incorporation
of the PCL
components.
PCL components
in
improved
by the incorporation
of thehydrolyzed
PCL components.
The
PCLPseudomonas
componentscepasia.
in the Furthermore,
polyESO/PCL
the polyESO/PCL
were gradually
by lipase
from
thegradually
polyESO/PCLs
exhibited
excellent
shape
memory-recovery
Thethe
strain
fixity was
were
hydrolyzed
by lipase
from
Pseudomonas
cepasia. behaviors.
Furthermore,
polyESO/PCLs
dependent
on theshape
feed ratio
of ESO and PCL,
and the The
deformation
and was
recovery
processes
exhibited
excellent
memory-recovery
behaviors.
strain fixity
dependent
on of
the
theratio
polyESO/PCLs
were
repeatedly
These
results
provide
new
strategy for
feed
of ESO and
PCL,
and the practicable.
deformation
and striking
recovery
processes
of athe
polyESO/PCLs
designing
novelpracticable.
biodegradable
smartstriking
materials.
were
repeatedly
These
results provide a new strategy for designing novel
biodegradable
smart
materials.
Supplementary
Materials:
The supplementary materials is available online at: www.mdpi.com/20734360/7/10/xxxx/s1.
Supplementary Materials: The supplementary materials is available online at: www.mdpi.com/2073-4360/
Acknowledgement: This study was supported by a Grant-in-Aid for Young Scientists from Japan Society for
7/10/1506/s1.
the Promotion of Science (JSPS) (No. 268101140).
Acknowledgements: This study was supported by a Grant-in-Aid for Young Scientists from Japan Society for
the Author
Promotion
of Science (JSPS)
268101140).
Contributions:
In this(No.
study,
Takashi Tsujimoto and Hiroshi Uyama designed the experiments and
involved
in
all
data
analysis.
Takashi
Tsujimoto
and Takeshi
Takayama
the experiments
and collected
Author Contributions: In this study, Takashi Tsujimoto
and
Hiroshiperformed
Uyama designed
the experiments
and
data. Takashi
Tsujimoto
wrote
the paper.
involved
in all data
analysis.
Takashi
Tsujimoto and Takeshi Takayama performed the experiments and collected
data. Takashi Tsujimoto wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
Conflicts of Interest: The authors declare no conflict of interest.
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