Synthesis of high temperature polyaspartimide-urea based shape memory polymers

Article abstract of DOI:10.1016/j.polymer.2012.08.021

Thermally activated shape memory polymers (SMPs) have attracted great interest in recent years for application in adaptive shape-changing (morphing) aero structures. However, these components require materials with transition temperatures well above the glass transition temperatures of most widely available SMPs while also maintaining processability and property tailorability. In the present study, a series of novel polyaspartimide-urea based polymers are synthesized and characterized. The glass transition temperature and shape memory properties are varied using a diisocyanate resin creating a urea crosslinking moiety between the polyaspartimide chains. Overall, a family of high temperature SMPs was synthesized and characterized showing high thermal stability (>300 °C), toughness, strong shape memory effects, and tailorable properties.

Full text of DOI:10.1016/j.polymer.2012.08.021

Polymer 53 (2012) 4637e4642
Contents lists available at SciVerse ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of high temperature polyaspartimide-urea based shape memory
polymers
,
,
,
J.A. Shumaker a c, A.J.W. McClung b c, J.W. Baur c
*
a University of Dayton Research Institute, 300 College Park, Dayton, OH 45469-0060, USA
b National Research Council, AFRL, Wright-Patterson AFB, OH 45433, USA
c Air Force Research Laboratory, Materials and Manufacturing Directorate, WPAFB, OH 45433, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Thermally activated shape memory polymers (SMPs) have attracted great interest in recent years for
application in adaptive shape-changing (morphing) aero structures. However, these components require
materials with transition temperatures well above the glass transition temperatures of most widely
available SMPs while also maintaining processability and property tailorability. In the present study,
a series of novel polyaspartimide-urea based polymers are synthesized and characterized. The glass
transition temperature and shape memory properties are varied using a diisocyanate resin creating
a urea crosslinking moiety between the polyaspartimide chains. Overall, a family of high temperature
SMPs was synthesized and characterized showing high thermal stability (>300 ꢀC), toughness, strong
shape memory effects, and tailorable properties.
Received 18 April 2012
Received in revised form
8 August 2012
Accepted 9 August 2012
Available online 16 August 2012
Keywords:
Thermoset
Shape memory polymer
Morphing
Published by Elsevier Ltd.
1. Introduction
properties in the “rubbery” regime that depend on the permanent
covalent crosslink network established. This class of SMPs also has
Shape memory polymers (SMPs) are materials which show great
promise in shape-changing (morphing) adaptive structures. This
class of material must exhibit the ability to withstand large defor-
mations, being “fixed” into a temporary shape, and then recover to
the original un-deformed state when triggered and sufficiently
unconstrained [1]. The recovery process is typically triggered by
a change in temperature, although alternative stimuli including
light, infrared laser, magnetic, electric or electromagnetic field have
also been explored [2]. By understanding the relationship between
the chemical composition, the triggering stimulus and key material
properties, critical performance parameters can be optimized and
applied to the next generation of adaptive structures that could
include morphing aero-component, deployable space structures,
unattended responsive ground sensors and soft robotics [3].
Covalently crosslinked amorphous thermoset networks are one
type of SMP that are highly processable and more easily charac-
terized than semi-crystalline polymers due to the absence of
crystalline domains whose properties are highly path dependent. In
contrast, amorphous thermosets have more consistent properties
above and below the glass transitions temperature (Tg) and tunable
characteristics that include high degrees of shape recovery, tunable
work capacity during recovery, and suppression of creep due to
these strong chemical cross-links. The permanent shape is fixed
through formation of the covalent crosslink network upon initial
processing. These materials typically retain the “memory” of their
original shape despite the large deformations and temporary
shapes to which they may be reconfigured [4].
The key challenge in the design of thermal activated SMPs is to
maintain a balance of large reversible strain capability, easy
temporary shape fixing, and full shape recovery at the desired
activation temperature while also maintaining the desired
mechanical properties in the glassy state, the rubbery state, and
during the transition between them. This paper discusses a novel
approach to synthesizing a new polyaspartimide-urea based SMP
with a tailorable chemical structure that is readily processable at
low temperatures. This tailorable chemical structure provides the
ability to control relative covalent crosslink densities and resulting
properties. The aim of the current research is to characterize the
relationship between chemical composition and the materials’
thermomechanical properties in order to enable tailoring of the
high temperature shape memory behavior. Polyurethanes and
polyurea have previously been observed to exhibit shape memory
properties [4], but have largely been investigated at lower transi-
tion temperature than those examined in this study.
* Corresponding author. Survivability & Sensor Materials Division, 3005 Hobson
Way Bldg 651, WPAFB, OH 45433, USA. Tel.: þ1 937 255 9649.
E-mail address: Jeffery.baur@wpafb.af.mil (J.W. Baur).
0032-3861/$ e see front matter Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.polymer.2012.08.021

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J.A. Shumaker et al. / Polymer 53 (2012) 4637e4642
O
O
A polyaspartimide is synthetically produced by reacting a bis-
CH3
CH3
maleimide with a diamine in a Michael addition reaction. Bisma-
leimides, depending on chemical composition and chain lengths,
possess a wide variety of glass transitions and are recognized for
structural rigidity and durability [5,6]. Diamines are readily avail-
able with diverse chemical compositions, which can be used to
tailor the flexibility and relative molecular weight between cross-
links. In this work, the selected bismaleimide and diamine are
reacted to produce a linear polyaspartimide as shown in Fig. 1. The
polymeric backbone offers two secondary amine protons that can
be readily reacted at room temperature with a diisocyanate func-
tionality to form a urea bond and produce a networked polymer
with varying crosslink density. The proposed chemistry produces
amorphous, high temperature SMPs with tailorable Tg, high
thermal stability (>300 ꢀC), and good toughness. Similar studies
that correlate variations in crosslinking and glass transition
temperatures of SMPs were conducted by Xie and Rousseau [7].
However, they focused on an epoxy-based chemical system which
produced lower Tg values.
O
N
CH2
N
+
H2N
NH2
n
n = 6.1
O
O
O
N
O
CH3
CH3
THF, 10 wt% polymer
60oC, 48 hrs.
N
CH2
O
N
N
n
H
H
O
O
m
Fig. 1. Synthesis of BMI-JA-400.
Jeffamine D-400, in solution at a low temperature. Once the linear
polyaspartimide is generated, the polymeric backbone is reacted
further with the addition of 4,40-diisocyanatodicyclohexylmethane,
a diisocyanate, as represented in Fig. 2. The isocyanate functional-
ities react with the secondary amine protons, as mentioned
previously, along the polymer backbone to form urea functional-
ities and crosslink the linear polymer chains. The resulting amor-
phous networked polymer was no longer soluble in common
solvents indicating that crosslinking had taken place. Model
compounds were also generated and studied to confirm the
proposed reaction and structural compositions.
The current paper focuses on the synthesis of amorphous SMPs
with higher glass transition temperatures (110e164 ꢀC) than
previously
examined
commercially
available
SMPs
(typically ꢁ 110 ꢀC) [7,8]. It is desired that the transition temper-
atures of the shape memory polymers be in excess of their oper-
ating temperature to ensure that shape memory polymers will not
be prematurely triggered by the environment. For example, the
operation temperatures of subsonic external aircraft skins can
often reach 121 ꢀC (250 ꢀF), so SMPs and their composites that are
targeted towards these applications require higher transition
temperatures. Shape memory polymers that exhibit glass transi-
tion temperatures greater than 120 ꢀC are uncommon among many
of the current classes of previously examined thermosetting
materials that include epoxies, polyurethanes, and styrene copol-
ymers [7e10]. For example, a study involving a series of poly-
urethane thermosets conducted by Merline et al. recorded Tg
temperatures ranging from 49 to 73 ꢀC, respectively [9]. Styrene
copolymers SMPs have also attracted much attention and typically
have glass transitions temperatures ranging from 35 to 55 ꢀC
depending on crosslink densities [12]. A series of thermosetting
epoxies SMPs were generated that exhibited transition tempera-
tures ranging from roughly 31e94 ꢀC, respectively [7]. This study
included an SMP epoxy containing the Jeffamine D-400, the
diamine examined in this study. However, the epoxy SMP exhibi-
ted a Tg of 44 ꢀC which is substantially lower that the Tg of the BMI
D-400 polymers discussed here [7].
2. Experimental
2.1. Materials
The 4,4-bismaleimidodiphenylmethane (Matrimid 5292A, BDM)
was purchased from Ciba Specialty Chemicals Corporation and
purified by recrystallization using toluene. The Jeffamine D-400
(Poly(oxy(methyl-1,2-ethanediyl)),
alpha-(2-aminoethylenthyl)
omega-(2-aminomethylethoxy), (JA-400)) was purchased from
Huntsman. Once received, the material was stored/dried over acti-
vated 3A molecular sieves. The 4,40-diisocyanatodicyclohexyl-
methane (Desmodur W, DW) was purchased from Bayer Material
Science and used as received without further purification. The
structures of DBM, JA-400, and DW are depicted in Fig. 3. All other
chemicals were purchased from SigmaeAldrich and used as
received without further purification.
O
O
N
O
CH
N
2
Other studies did investigate higher temperature SMP epoxies
with Tg’s ranging from 69 to 113 ꢀC [11]. An epoxy-based SMP was
also previously available commercially with a Tg of 105 ꢀC [8,10].
Despite a Tg that is slightly lower than desired for subsonic aero
applications, this SMP showed strong promise for select aerospace
applications. However, it is no longer available from the manufac-
turer. Even higher temperature polyimides, such as CP2, have been
considered for their high temperature SMP properties and sharp Tg
at 199 ꢀC [Jacobs 2010]. However, significant processing challenges
remain which may limit their application in fiber-reinforced shape-
changing composites. In short, alternative aerospace-grade SMPs
which are high temperature, processible, and easily tunable are
needed.
The aim of the current investigation is to explore a family
of SMPs that have the potential to meet this need with a high
temperature SMP with desired mechanical, thermal, and process-
ing properties. The reaction schematic illustrated in Fig. 1 repre-
sents the polymer backbone of the materials that have been
produced. The linear system is generated by reacting 4,4-
bismaleimidodiphenylmethane with an aliphatic diamine,
O
N
N
H
6.1
n
O
O
H N
H N
N
O
O
O
N
H
N
O
n
CH
N
6.1
2
O
Fig. 2. BMI-JA-400 crosslinked with 4,40-diisocyanatodicyclohexylmethane.
O

J.A. Shumaker et al. / Polymer 53 (2012) 4637e4642
4639
purified by dissolving in ethyl acetate and passing through a silica
gel column using ethyl acetate as the eluent yielding 5.20 g (97.0%)
of white solid.
a
O
O
Elemental analysis was used to confirm the chemical
composition; calcd (%) for C13H16N2O2 (232.27): C 67.23, H 6.94,
N 12.06; found: C 67.19, H 6.93, N 11.99. FT-IR (KBr), cmꢂ1: 3467
(eCeNe of the succinimide ring), 3288 and 1598 (NeH), 3060
(aromatic CeH), 2966 and 2852 (aliphatic CeH), 1778 and 1709
(C¼O). NMR spectroscopy was used to determine purity and
structural composition of MC-1. 1H NMR (400 MHz, CDCl3, ppm);
N
C H 2
N
O
O
d
-7.39 (t, 1H, 1), 7.49 (t, 2H, 2, 6), 7.27 (t, 2H, 3, 5), 3.97 (t, 1H, 9),
3.10 (dd, 1H, 10a), 2.70 (dd, 1H, 10b), 3.02 (q-5, 1H, 11), 1.15 (t, 6H,
C H 3
C H 3
b
12, 13), 1.82 (s, 1H, 14). 13C NMR (400 MHz, CDCl3, ppm);
-C1d128.7, C2d126.3, C3d129.2, C4d131.6, C5d129.2,
O
n
d
C6d126.3, C7d174.3, C8d177.3, C9d54.7, C10d38.1, C11d48.1,
C12d23.5, C13d22.5. Melting point: 107e108 ꢀC sharp.
H 2
N
N H 2
n = 6.1
2.3.2. Synthesis of MC-2
The reaction schematic is illustrated in Fig. 5. A 50 mL single
necked round-bottom flask equipped with a magnetic stirbar was
charged with MC-1 (0.5 g, 0.0022 mol), phenyl isocyanate (0.269 g,
0.0022 mol), and THF (14.62 g, 5 wt%). The transparent solution
was capped and stirred at room temperature for 72 h. The solution
was filtered to remove any particles and the THF vacuum evapo-
rated. The reaction yielded 0.750 g (97.5%) of white solid. The solid
was recrystallized in toluene yielding 0.69 g (90%) of white
crystals.
c
O
C
N
C H 2
N
C
O
Fig. 3. Structures of (a) 4,4-Bismaleimidodiphenylmethane (BDM), (b) Jeffamine D-400
(JA-400), and (c) 4,40-Diisocyanatodicyclohexylmethane (DW).
Elemental analysis was used to confirm the chemical compo-
sition; calcd (%) for C20H27N3O3 (357.43): C 67.21, H 7.61, N 11.76;
found: C 67.18, H 7.64, N 11.70. NMR spectroscopy was used to
2.2. Instrumentation
determine purity and structural composition. FT-IR (KBr), cmꢂ1
:
Elemental Analyses were performed using a CHN analyzer
through combustion processes. Intrinsic viscosities of the linear
polymer in an N-methyl-2-pyrrolidine (NMP) solution were
measured with a Cannon Ubbelohde viscometer at 30 ꢀC at
a concentration of 0.5 g/dL. 1H NMR and 13C NMR spectra were
recorded using a Bruker Avance 400 spectrometer with deuterated
CDCl3 containing 0.1% (v/v) tetramethylsilane (TMS) as a reference.
FT-IR spectra were recorded using a Thermo Nicolet Nexus 470
spectrometer. An electrothermal melting point apparatus was used
to obtain melting point determinations. A TA Instruments 2950
series thermal analysis instrument was used to obtain thermogra-
vimetric analysis (TGA) and collected data at a heating rate of
10 ꢀC/min under flowing nitrogen. For dynamic mechanical analysis
(DMA), tensile thin film specimens are machined (from the cast
plates using an IsoMet 1000 precision saw) with dimensions
38 mm by 2 mm. The samples have a nominal thickness of 1.0 mm.
A TA Instruments RSAIII dynamic mechanical analyzer is used to
measure the storage and loss modulus of the post-cured resin
specimens. The dynamic properties of the specimens were
measured at a frequency of 1 Hz, a strain of 0.1%, and a heating rate
of 2 ꢀC/min from 25 ꢀC to the high temperature (TH) of the partic-
ular resin.
3470 (eCeNe of the succinimide ring), 3415 (broad, urea
formation), 3066 (aromatic CeH), 2932 and 2855 (aliphatic and
cyclic aliphatic CeH), 1782 and 1726 (C¼O), 1621 (NeH). 1H NMR
(400 MHz, CDCl3, ppm); d-7.35 (t, 3H, 1, 2, 6), 7.44 (t, 2H, 3, 5), 4.07
(q-4, 1H, 9), 3.04 (dd, 1H, 10a), 2.97 (dd, 1H, 10b), 3.77 (s-7, 1H, 11),
1.35 (d, 3H, 12), 1.26 (d, 3H, 13), 3.61 (m, 1H, 15), 1.96 (d, 2H, 16a,
20a), 1.70 (dt, 2H, 16b, 20b), 1.37 (m, 2H, 17a, 19b), 1.10 (m, 2H, 17b,
19b), 1.60 (s-6, 2H, 18a, 18b). 13C NMR (400 MHz, CDCl3, ppm);
d
-C1d128.3, C2d126.8, C3d129.2, C4d132.4, C5d129.2,
C6d126.8, C7d174.0, C8d175.5, C9d50.9, C10d36.4, C11d47.7,
C12d22.0, C13d21.3, C14d155.6, C15d49.4, C16d34.1, C17d25.0,
C18d25.6, C19d25.0, C20d33.9. Melting point: 157e158 ꢀC sharp.
2.4. Polymer synthesis
2.4.1. Synthesis of BMI-JA-400
BMI-JA-400 was synthesized by following the literature method
[13] with some modifications. A 100 mL single necked round-
bottom flask fitted with a nitrogen inlet, a reflux condenser and
magnetic stirbar was charged with BDM (2.00 g, 0.0056 mol),
JA-400 (2.40 g, 0.0056 mol), and tetrahydrofuran (THF) (17.6 g,
20 wt% polymer). The mixture was stirred to a bright yellow colored
solution and was heated at 65 ꢀC for 24 h under a nitrogen atmo-
sphere. The resulting viscous amber colored solution was filtered to
remove any particles and the THF vacuum evaporated at room
2.3. Model compound synthesis
2.3.1. Synthesis of MC-1
The reaction schematic is illustrated in Fig. 4. A 50 mL single
necked round-bottom flask equipped with a magnetic stirbar was
charged with phenylmaleimide (4.00 g, 0.023 mol), isopropylamine
(1.36 g, 0.023 mol), and THF (21.4 g, 20 wt%). The light yellow
colored solution was capped and stirred at room temperature for
6 h. The resulting orange colored solution was filtered to remove
any particles and then the THF was vacuum evaporated. The reac-
tion yielded 5.30 g (98.9%) of pink solid. The solid was further
O
O
N
THF
N
NH2
+
N
O
H
O
Fig. 4. Synthesis of MC-1.

4640
J.A. Shumaker et al. / Polymer 53 (2012) 4637e4642
O
Table 2
O
Compositions of SMP samples.
N
THF
Sample
C (at%)
H (at%)
N (at%)
N
NCO
+
N
N
O
50DW
C42.3H60.6N4O10.1
(918.45)
C53.55H77.85N5.5O11.6
(984.29)
C57.3H83.6N6O12.1
(1050.13)
C64.8H95.1N7O13.1
(1181.81)
Theoretical
Measured
Theoretical
Measured
Theoretical
Measured
Theoretical
Measured
Theoretical
Measured
65.12
64.13
65.35
64.93
65.54
65.51
65.86
65.76
66.11
65.94
7.91
8.01
7.97
7.91
8.02
7.89
8.11
8.12
8.18
8.17
7.63
7.75
7.83
7.84
8.00
7.94
8.30
8.08
8.53
8.38
N
O
O
H
H
75DW
100DW
150DW
200DW
Fig. 5. Synthesis of MC-2.
C72.3H106.6N8O14.1
(1313.50)
temperature overnight, yielding an amber colored polymeric
material in a quantitative yield (4.38 g, 99.6%). NMR spectroscopy
was used to determine the complete removal of the solvent. This
formula is referred to as material “REF” in the subsequent discus-
sions of TGA and DMA following curing overnight at 120 ꢀC.
Elemental analysis was used to confirm the chemical composi-
tion; calcd (%) for C42.3H60.6N4O10.1 (786.77): C 64.58, H 7.76, N 7.12;
found: C 64.52, H 7.74, N 7.04.
Model compound, MC-2, was generated to confirm the
proposed reaction mechanism and formation of the urea bond
linkage resulting from the combination of the secondary amine
proton from MC-1 and the isocyanate functionalities. Supporting
1H and 13C NMR spectroscopy and FT-IR spectra were also taken and
confirmed the chemical structure of the reaction product (not
shown).
2.4.2. Synthesis of BMI-JA-400-xdw shape memory polymers
The detailed formulations of the series of SMPs are summa-
rized in Table 1. A 100 mL single necked round-bottom flask fitted
with a nitrogen inlet, a reflux condenser and magnetic stirbar was
charged with BDM (2.00 g, 0.0056 mol), JA-400 (2.40 g,
0.0056 mol), and THF (17.6 g, 20 wt% polymer). The mixture was
stirred to a bright yellow colored solution and was heated at 65 ꢀC
for 24 h under a nitrogen atmosphere. The resulting viscous
amber colored solution was filtered to remove any particles and
the THF was reduced, increasing the polymer concentration to the
values shown in Table 1. The weighed DW was then added to the
reduced solution, resulting in a 35 polymer wt% solution. The
highly viscous solution was stirred/mixed rapidly for 2 min. The
homogeneous solution was transferred evenly into Teflon casting
dishes (5.0 g solution per dish) and placed into an inert atmo-
sphere for 16 h. The resulting polymeric films yielded an amber
colored amorphous material in a quantitative yield (1.75 g per
film). The generated films were heat treated at the respective high
use temperature (TH in Table 1) for 1 h to ensure reaction
completion. Elemental analysis was used to confirm the chemical
compositions. The theoretical and measured results are summa-
rized in Table 2.
3.2. Polymer properties
The intrinsic viscosity of BMI-JA-400 was recorded to be
3.18 dL/g and was soluble in a variety of organic solvents (DMAc,
DMSO, THF, and NMP). Upon final reaction with the diisocyanate,
the generated polymeric films became insoluble, but swelled in the
organic solvents.
3.3. Polymer swell testing
Swell tests were conducted on each of the five generated ther-
mosetting SMP materials to gage crosslink densities. A solvent base
was selected covering a broad polarity index, water being the most
polar (9.0), methanol (5.1), THF (4.0), and toluene being the least
polar (2.4). Each sample was exposed for 24 h. From the graph
illustrated in Fig. 6, comparative crosslink densities can be inferred
for the SMPs. As a trend, the greater the amount of diisocyanate
used during synthesis, a greater chemical crosslinking density was
achieved resulting in a decrease in swelling by volume percentage.
The least amount of activity is seen in the tests involving water and
toluene. In water, a very small increase in volume is observed
(w1.0%) across all five materials. This is attributed to the adsorbed
water on the exterior surface due to the high volume of hydrogen
bonding associated throughout the chemical structure. In toluene,
3. Results and discussion
3.1. Model compound characterization
Model compound, MC-1, was generated to confirm the proposed
reaction mechanism and formation of the aspartimide bond linkage
resulting from the combination of the maleimide and primary
amine functionalities. Supporting 1H and 13C NMR spectroscopy
and FT-IR spectra were taken and confirmed the chemical structure
of the reaction product (not shown).
40
H2O
Methanol
35
30
THF
25
Toluene
20
Table 1
Details of SMP synthesis.
15
10
5
Sample
DW to JA-400
Mole ratio (%)
Polymer conc.
in THF (wt%)
TH (ꢀC)
50DW
75DW
100DW
150DW
200DW
25
37.5
50
75
100
31.5
30.1
28.8
26.4
24.4
145
155
165
175
185
0
0
50
100
BMI-JA-400-(X)DW
150
200
Fig. 6. BMI-JA-400-(X)DW swell test results.

J.A. Shumaker et al. / Polymer 53 (2012) 4637e4642
4641
1010
120
100
80
60
40
20
0
a
a
105
20
40
60
80
100 120 140 160 180 200
0
100
200
300
400
500
600
Temperature (°C)
-50DW
-75DW
-100DW
-150DW
-200DW
REF
Temperature (°C)
1.5
b
b
-50DW
-75DW
1.6
1.2
0.8
0.4
0
-100DW
-150DW
-200DW
REF
1
0.5
0
20
40
60
80
100 120 140 160 180 200
Temperature (°C)
0
100
200
300
400
500
600
Temperature (°C)
Fig. 8. DMA (a) storage modulus (E0) and (b) tan(
d) results in tension.
Fig. 7. TGA analysis results of the crosslinked BMI-JA-400-(X)DW systems (a) weight
(%) and (b) derivative of weight (%/ꢀC).
3.5. Dynamic mechanical analysis
The DMA was used to investigate the dynamic properties of
the materials over a range of temperatures. Three tests were
conducted per material with each showing consistent results. The
tensile storage modulus (E0) versus temperature is shown in Fig. 8.
Only one DMA test per SMP material is given in the figure for
clarity. The baseline BMI-JA-400 is also included in the figure as
with the exception of the BMI-JA-400-50DW, each material
remains chemically inert with minimal to no (<0.5%) chemical
interactions. The BMI-JA-400-50DW, the least crosslinked material,
showed w1.3% volume increase in toluene. The materials showed
the greatest interactions with methanol and THF. In both solvents,
the lesser crosslink dense material (BMI-JA-400-50DW) swelled
nearly three times the amount of the materials with the greatest
crosslink densities (BMI-JA-400-200DW). It should be noted
that the percent volume increase appears to plateau between the
BMI-JA-400-150DW and the BMI-JA-400-200DW. This can be
attributed to maximum potential crosslink density being
approached at ꢂ150DW.
“REF” for comparison purposes. The tan(d) is defined as the
relative ratio of the loss modulus (E00), to the storage modulus (E0).
It represents the relative amount of energy being dissipated
versus elastically stored in
a material. For this study, the
measured peak of the tan( ) curve is used for identifying the Tg
d
listed in Table 4. In addition, a high temperature modulus
(signified as EH) is defined as E0 measured at TH which marks the
high temperature of the given material and a low temperature
modulus (signified as EL) is defined as E0 measured at 25 ꢀC. The
TD, EH, and EL from the DMA curves are summarized in Table 4. As
expected, the Tg increases as the crosslink density increases. The
results also indicate that the EH increase as the crosslink density
increases in the SMPs. There is not a discernible correlation in the
measured EL with the changes in crosslink density in the
aspartimide-based SMPs because the permanent covalent cross-
link network is dominated by the physical crosslink network at
low temperatures. The ratio of EL/EH is observed to vary from 500
to 130 as the amount of crosslinking is increased.
3.4. Thermogravimetric analysis
Fig. 7 show the overall comparison of the TGA curves and the
derivative of the weights in nitrogen. The baseline BMI-JA-400 is
also included in the figure as “REF” for comparison purposes. BMI-
JA-400-150DW and BMI-JA-400-200DW show early thermal
instabilities in both curves. This can possibly be attributed to
a transport-limited reaction between secondary amines leading to
species with premature thermal breakdown. Major chemical
decomposition can be seen by examining the derivative of the
weight curves. Each of the materials examined show thermal
stabilities upward of 300 ꢀC with minimal decompositions (<5%)
before major decompositions beginning at w400 ꢀC. The initial
temperatures of decomposition (TD) for the SMP materials are listed
in Table 3.
Table 4
DMA properties of the crosslinked BMI-JA-400 systems (results are given as the
average from three samples for each case).
Polymer
Tg (ꢀC)
TH (ꢀC)
High temperature
Low temperature
EH (MPa)
EL (MPa)
Table 3
50DW
75DW
100DW
150DW
200DW
110
136
137
143
164
145
155
165
175
185
3.69
3.92
5.41
9.39
12.6
1870
1240
1050
2250
1640
Thermal properties of the crosslinked BMI-JA-400 systems.
Polymer
TD (ꢀC, in N2)
50DW
348
75DW
354
100DW
361
150DW
346
200DW
340

4642
J.A. Shumaker et al. / Polymer 53 (2012) 4637e4642
Fig. 9. Shape memory illustration of BMI-JA-400-100DW.
3.6. Shape memory cycles
Acknowledgments
Fig. 9 illustrates the potential shape memory capabilities of BMI-
JA-400-100DW utilizing a 35 mm diameter disc with a thickness of
1 mm. The polymer was easily deformed into a rolled/cylindrical
shape at 150 ꢀC (above its Tg) and fixed into this temporary shape,
when cooled back down to room temperature. The polymer qual-
itatively showed high toughness in both the glassy and rubbery
states. Upon reheating to 150 ꢀC (again above the Tg), the material
returns to the original form within 30 s of thermal exposure. Time
stamps are placed on each photograph in relation to shape memory
recovery time when reheated above the materials Tg. The process
was repeated ten times with no damage observed. In each case the
material appears to recover fully to the initial shape other defor-
mation modes including extension and twist were also observed
with similar results. These phenomena will be quantified in future
research with controlled strain and recovery [14,15].
The authors would like to thank the Materials Integrity Branch
(RXSA), Wright Patterson Air Force Base, Dayton, Ohio for elemental
analysis.
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