Modified shape memory cyanate polymers with a wide range of high glass transition temperatures Fang Xie1 , Longnan Huang2, Yanju Liu1, Jinsong Leng3 1 Department of Astronautical Science and Mechanics, Harbin Institute of Technology (HIT),Harbin 150080, People’s Republic of China 2 Department of Materials Science and Engineering, Harbin Institute of Technology at Weihai, Weihai 264209, People’s Republic of China 3 Center for Composite Materials and Structures, Harbin Institute of Technology (HIT), Harbin 150080, People’s Republic of China ＊
ABSTRACT Shape memory cyanate polymers (SMCPs) are a new kind of smart materials, which have huge development potential and a promising future. A series of shape memory cyanate polymers were prepared by cyanate ester and varying content of a linear modifier. The thermal properties of the SMCPs were investigated by Differential Scanning Calorimetry (DSC), Thermal Gravimetric Analysis (TGA) and Dynamic Mechanical Analysis (DMA). The SMCPs we prepared have high glass transition temperature and show good heat resistance. The glass transition temperature Tg can be adjusted from 156.9°C to 259.6°C with the modifier. The initial temperature of thermal decomposition comes up to 300°C, which is enough high for the application in aerospace fields. The shape memory polymer we prepared shows a good shape memory effect, as the shape recovery time is less than 65s and the shape recovery rate reaches 95%. Keywords: Shape memory polymer, cyanate ester, high glass transition temperature
1. INTRODUCTION Shape memory polymer is a kind of smart materials with huge development potential, which have the capability of recovering their original shape upon application of external stimulus. There are many classifications of SMP, including thermo-induced SMP[1], light-induced SMP[2], electro-induced SMP[3], chemical-induced SMP[4] and magnetic-induced SMP[5]. With in-depth research, shape memory polymers have been used in daily life [6], aerospace [7] and medical fields [8]
.
Early researches on shape memory polymers are mostly concentrated on the thermoplastic polymer based SMP and few studies are about thermosetting SMP. However, the thermoplastic SMP creeps seriously in the deformation process, the shape recovery ratio decrease quickly after several shape memory cycles. Furthermore, the application of thermoplastic
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Behavior and Mechanics of Multifunctional Materials and Composites 2012, edited by Nakhiah C. Goulbourne, Zoubeida Ounaies, Proc. of SPIE Vol. 8342, 834210 · © 2012 SPIE CCC code: 0277-786X/12/$18 · doi: 10.1117/12.915128 Proc. of SPIE Vol. 8342 834210-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/09/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx
SMP is greatly limited due to its defects in thermodynamic properties. So there have been more and more studies on thermoset SMP. Rousseau and Xie[9] in GM Development and Research Center have developed a new type of epoxy shape memory polymer. And the composition, structure, properties and shape memory behaviors of epoxy shape memory polymers are in-depth discussion and analysis. In 2009, Leng[10] and his research group using epoxy resin, curing agent and linear epoxy monomers synthesized a series of epoxy SMP whose glass transition temperature is adjustable. Recently McClung[11], who induced bismaleimide (BMI) resin into the ranks of SMP, synthesized BMI based SMPs with the glass transition temperatures 110°C, 137°C, and 144°C. In addition to studying unitary shape memory polymers, researchers are increasingly concerned about the diversity and functionalization of SMP. For example, Tamagawa[12] developed a two-way shape memory polymers. Langer[13] and Xie[14] developed three-way shape memory polymers which can remember two temporary shapes. It is worth noticing that Xie[15] reported multi-shape memory effect of the ion-exchange resin Nafion membranes perfluorinated sulfonic acid (PFSA) in 2010. Leng[16] and his group summarized the shape memory effect, preparation methods, constitutive models, characterization properties, and actuate method of SMP in detail. With the development of aerospace technology shape memory polymer materials must have good adaptability to the space environment, including ultra-high or low temperature environment. Although shape memory polymers reported until now have significant shape memory effect, their application temperature range is generally narrow (about -100°C~100°C[17]), which greatly limits the application in aerospace. Cyanate resin, whose mechanical performance is better than epoxy resin (EP) and bismaleimide resin(BMI), have been used in radar and satellite battery[18]. It is a great matrix material for the SMP using in high temperature in the aerospace. The research of cyanate resin’s shape memory effect is still in its infancy. The CRG company
[19]
is in the forefront of this research and they have synthesized shape
memory cyanate polymer whose Tg is above 150°C. Based on the current development of shape memory polymers, this paper will synthesis a of thermosetting shape memory cyanate polymer which is suitable for high-temperature. Its composition, structure, thermal and mechanical property, especially shape memory performance will be discussed below.
2. EXPERIMENTAL 2.1.
Preparation of shape memory cyanate polymers (SMCPs)
Materials. The polymer matrix consists of cyanate ester and a linear modifier. The cyanate ester was purchased from Jiangdu Wuqiao Resin Factory, China. The modifier was purchased from Tianjin Guangfu Fine Chemical Research Institute, China. Synthesis of SMCPs. Five mixtures of cyanate ester and the modifier were first degassed at a vacuum oven and then injected into preheated glass molds. The SMCP sheet samples were obtained after undergoing a designed curing prosess. The modifier represents an incremental weight ratio of the amount of cyanate ester corresponding to the samples A1, A2, A3, A4, and A5, respectively (Fig. 1).
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Al
A2
A3
A4
A5
Fig. 1 Samples of SMCPs.
2.2.
Characterization methods
The differential scanning calorimetry (DSC, NETZSCH STA 449 C) measurements were used to characterize the change of thermal properties of the polymers from 25°C to 300°C at a heating/cooling rate of 10°C· min-1. The glass transition temperature (Tg) values were obtained as the inflection temperatures in the DSC curves. The dynamic mechanical analysis (DMA, NETZSCH Q800) experiments were conducted in a tension mode. Samples 3
with dimensions of 30 × 5 × 1mm were used. All runs were performed at 5Hz and 0.05% strain, and the specimens were heated in a hot chamber at a constant rate of 5°C· min-1 from 25°C to 300°C. Thermogravimetric analysis (TGA, NETZSCH STA 449 C) test was carried out from 25°C to 800°C at a constant -1
heating rate of 10°C· min under flowing argon. The shape memory behaviors were examined by a bending test using rectangular strip specimens as the permanent shape. The specimens were heated up to Tg + 40°C in an oven and held for 1 min for full heating. Then the specimens were elastic and were bent into a ‘U’ shape with a restrained mold. They were subsequently cooled to room temperature and then released from the ‘U’ shape mold. The deformed specimens were again heated up to Tg + 40°C and the shape recovery process was recorded by a video recorder.
3. RESULTS AND DISCUSSION 3.1.
Differential Scanning Calorimetry (DSC) Analysis
DSC thermograms of our SMCP samples with marked Tg are shown in Fig. 2. As shown in the Fig. 2, Tg is 212.2°C, 231.9°C, 229.5°C, 209.5°C and 194.7°C for samples A1, A2, A3, A4, A5, respectively. The above figures reveal that Tg first increases and then decreases as the modifier content increases. It is a demonstration that Tg of the polymer can be altered by controlling the amount of modifier content. It can be explained that the level of crosslinking reaction is enhanced when small amount of the modifier was put into the polymer. And at this time the modifier worked as a catalyst, which results in an increase of crosslinking density. However, when it comes to a larger amount of modifier, the
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length of the segments between the crosslinking points is much longer and the crosslinking density of the network drops. The deduction that increasing the modifier content can lead to the decrease of crosslinking density will be demonstrated further by the following result of the DMA test. Thus, increasing the linear modifier, the crosslinking points are far more linked together with higher flexible segment mobility, resulting in a decrease in Tg.
Al
0.6
- - A2
0.4
A3 0.2 212
0.0
''
E
-0.2
194.7°C
-0.4
I
231.9°C
-0.6 -0.8 1.0
25
1111111111
50
75
100 125 150 175 200 225 250 275 300 Temperature(°C)
Fig. 2 DSC thermogram of SMCPs.
3.2. Thermogravimetric analysis (TGA) TGA in argon was used to investigate the thermal stability of SMCP samples over a range of temperatures. The initial degradation temperatures are all above 300°C (Fig. 3), indicating that the SMCPs remains thermally stable to a higher temperature in spite of having decreasing crosslinking densities. Therefore, the SMCPs can respectively be used in the high temperatures above their Tg without significant thermal degradation. As shown in Fig. 3, the residual carbon contents decrease with the increase of linear modifier. It indicates that the thermal stability decreases quickly with the increase of linear modifier when the temperature is over 300°C. It is because that the thermal stability of crosslinking points is much higher than that of the modifier. This should be pay close attention to when using SMCPs in practices.
Al
100
- - A2
90 80 70 60 50 40 30 20 10
0 0
100
200
300
400
500
600
00
800
Tmeperature (°C)
Fig. 3 Thermogravimetric curve of SMCPs.
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3.3. Dynamic mechanical analysis (DMA) For a good shape-memory material, a large and sharp drop in the storage modulus around the glass transition is the most important[100], which can be seen from Fig. 4a) since the modulus systematically decreases with increasing modifier content by almost 2–3 orders of magnitude, whereas the elastic ratio, which is defined as the glass/rubber modulus ratio, is often quoted to estimate the magnitude of modulus change when the polymer undergoes shape recovery. In addition, as shown in Fig. 4a), although the glass and rubbery region extends broader respectively with increasing modifier content, which can be explained by decreasing crosslink density and inhomogeneity of the chemical structures, the storage modulus at the middle of glass transition is still more than 100MPa. It indicates that the modifier works well in adjusting Tg of SMCPs as we can see in the tanδ curve (Fig. 4b)). Tg is 228.8°C, 259.6°C, 243.1°C, 211.9°C and 156.9°C for samples A1, A2, A3, A4, A5, respectively. It also first increases and then decreases as the modifier content increases, which is identical with the DSC results. The above DMA results have shown that the storage modulus of most SMCP samples began to decrease at about 150°C which ensures that the shape memory polymer can be used as structural materials with a higher modulus in high temperature conditions, thus ensuring the structural stability. 1.4
1o4
- -Al
1.2 -
228 8°C
- A2 A3
1.0
0.8 C C,
.259.6°C
/243.1C
0.6
F 211.9°C
0.4
156.9°C
0,2
100 0
50
100
150
200
250
300
Temperature (C)
Fig. 4 DMA curves of SMCPs. a) Storage modulus b) tanδ
3.4. Shape memory behaviors of SMCPs Table 1 is the recovery time for the first five shape memory cycles of SMCPs samples. It can be seen from the table that the recovery time is no more than 65s, mostly in the range of 15s - 30s, indicating that all samples can achieve shape recovery in a short period of time. This is the distinctive and necessary feature for SMCPs using in high temperature condition. In order to avoid serious thermal degradation the SMCP material should not prolonged stay in the high temperature environment.
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Table 1 Recovery time for the first five shape memory cycles of SMCPs samples (s) Shape memory cycles
A1
A2
A3
A4
A5
1
45
23
27
26
44
2
35
20
15
28
24
3
50
22
17
25
28
4
65
20
15
30
21
5
60
25
18
27
18
Average recovery time
51
22
18.4
27.2
27
Visco-elastic is the typical characteristics of SMP and thanks to its viscosity the strain energy will be partially consumed in the form of heat during the shape recovery process. Therefore, the shape recovery ratio is difficult to achieve 100%. The average recovery ratios for the first five cycles of SMCPs are shown in Fig. 5. As shown in the figure, the average recovery ratios are all above 95%, which is 99% for the sample A5. In addition, the recovery ratios were not significantly affected by modifier content, indicating that SMCPs having different Tg could all possess high recovery ratios. This is very beneficial for practical applications. 100
95
90
85
80
Al
A2
A3
A4
A5
Samples
Fig. 5 Average recovery ratios for the first five cycles of SMCPs.
Shape memory behaviors of SMCPs with different temporary shapes were also investigated in this research. As shown in Fig. 6, all temporary shapes recovered their initial ones, showing good shape memory properties.
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a) Illustration of temporary shape
b) Illustration of the recovery shape
Fig. 6 Shape memory behaviors of SMCPs with different temporary shapes.
4. CONCLUSIONS A series of shape memory cyanate polymers were prepared by cyanate ester and varying content of a linear modifier. The thermal property and shape memory behaviors of the shape memory cyanate polymers were investigated in detail. The SMCPs we prepared have a wide range of high glass transition temperatures from 156.9°C to 259.6°C and also show good heat resistance. The initial temperature of thermal decomposition is up to 300°C, which is enough high for the application in aerospace fields. Small amount of modifier could increase the cross-linking density, and enhance the transition temperature of the polymer at the same time. With the increasing modifier, the cross-linking density decreased, resulting in the decrease of transition temperature. The shape memory polymer we prepared shows a good shape memory behavior, as the shape recovery time is less than 65s and the shape recovery ratio reaches 95%. The recovery ratios are not significantly affected by modifier content and SMCPs having different Tg could all possess high recovery ratios. This is very beneficial for practical applications.
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