Bismaleimide Based Shape Memory Polymers:

Correlation Between Chemical Composition and

Mechanical Properties

Amber J.W. McClung

AFRL/RXBC, Materials and Manufacturing Dir., Wright-Patterson AFB, OH 45433

National Research Council, USA

Joseph A. Shumaker

AFRL/RXBC, Materials and Manufacturing Dir., Wright-Patterson AFB, OH 45433

University of Dayton Research Ins., 300 College Park, Dayton, OH 45469

Jeffery W. Baur

AFRL/RXBC, Materials and Manufacturing Dir., Wright-Patterson AFB, OH 45433

Stephanie D. Reed

AFRL/RXBC, Materials and Manufacturing Dir., Wright-Patterson AFB, OH 45433

Universal Technology Corporation, 1270 North Fairfield Road, Dayton, OH 45432

Shawna A. Matthys

AFRL/RXBC, Materials and Manufacturing Dir., Wright-Patterson AFB, OH 45433

Shape memory polymers have attracted great interest for application in adaptive or

morphing structures. In order to understand the origins of the mechanical performance

and durability of these polymers, the chemical structure must be understood, which is of-

ten hindered by the proprietary nature of commercially available shape memory polymers

and their additives. In the present study, a series of novel linear polyaspartimide-polyurea

based polymers utilizing 4-4-bismaleimidodiphenylmethane and an aliphatic Jeffamine di-

amine are thermomechanically characterized as a function of chemical structure. A di-

isocyanate resin is incorporated to create a thermosetting material with both improved

toughness and variable cross-link densities. The thermal and mechanical properties are

determined utilizing thermogravimetric analysis and dynamic mechanical analysis. In ad-

dition, the shape memory cycle with free recovery is conducted on these polymers. The

thermomechanical and shape memory properties are analyzed to elucidate the correlation

between the chemical composition and changes in the mechanical performance. With a

thorough understanding of structure-property relationships, the chemical composition of

these novel bismaleimide based shape memory polymers can be tailored to meet the desired

mechanical performance requirements of aerospace applications.

I. Introduction

Shape memory polymers (SMPs) are materials which show great promise in reconfigurable structures.This class of material exhibits the ability to withstand large deformations and then to recover to the originalun-deformed state.1 The recovery process is triggered by an external stimulus such as an electric current,heat, or light. Beloshenko et al.2 provided a review of heat triggered SMPs and listed several advantagesof these materials over shape memory alloys such as low density, high strain-recovery ability, processability,and relative low cost. These polymer materials show great promise in composite skin materials for morphing

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AIAA 2011-2112

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vehicles and for enabling active joints in reconfigurable Micro Air Vehicles (MAVs). However, before suchapplications can be attempted, the mechanical behavior of the SMPs must be thoroughly understood. Byunderstanding critical mechanical performance parameters (such as temperature dependent storage modulus,shape fixity, shape recovery, and response time), the next generation of adaptive vehicles may exploit thememory capabilities of the SMPs for morphing aircraft, deployable space structures, and micro air vehicles.3

SMPs can change their shape in a predefined way from a locked-in (deformed) shape to their originalshape when exposed to an appropriate stimulus, as illustrated with heat as the stimulus in Figure 1. Thematerial begins at state A1 with a relatively high “glassy” modulus. Heat is applied to the sample whichcauses the modulus to drop by several orders of magnitude to its “rubbery” modulus. While in this hightemperature state (B in the figure), the sample is deformed into its new shape (in the present study it isgiven a 90 ◦ bend as signified in C). The deformed shape is held in place while the sample is cooled back toits “glassy” modulus. Once it is cooled, the sample is in the locked-in state D. When heat is reapplied tothe locked-in sample, the material reheats to its “rubbery” modulus and returns to its original “memorized”shape. The unconstrained sample returns to state E. The sample is then cooled to state A2 which is closeto state A1. The better the “memory” properties of the SMP, the closer state A2 is to state A1.

E A2 A1

B

DC

Apply heat

Bend while hot

Cool

Apply heat

Sample recovers

Cool≈

Figure 1. Shape memory cycle with free recovery for the heat activated SMP BMI-JA-400-50DW

I.A. Background

Damage and failure of polymeric materials in aggressive environments have a direct impact on operationalcost and fleet readiness of advanced aircraft. Relevant aggressive environments include, but are not limitedto, UV radiation, jet oil, moisture, and heat. Previous research has shown commercially available SMPs ex-hibit damage when exposed to these aggressive environments.4 Due to the proprietary nature of commercialmaterials, the stability issues cannot be traced back to the chemical composition of the materials. In thepresent study, bismaleimides (BMIs) are chosen as the basis of a series of novel SMPs due to the demon-strated durability and the ability to achieve a higher range of glass transition temperatures (Tgs) on currentadvanced aircraft components.5, 6 A series of linear polyaspartimide-polyurea based SMPs, utilizing 4-4-bismaleimidodiphenylmethane and an aliphatic Jeffamine diamine, are thermomechanically characterized.To increase material toughness and vary the cross-link density, a diisocyanate resin is being incorporated tocreate a thermosetting material that can be easily processed in a variety of shapes.

The path-dependent behavior of the candidate materials will be evaluated by means of a shape memorycycle (schematic in Figure 1). Liu et al.7 defined a set of performance parameters such as the shape fixingparameter and the shape recovery parameter from the displacement at various points in the shape memorycycle. For an ideal shape memory material, the original state is exactly achieved. However, in reality, shape

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memory materials achieve a state close to their original shape. The shape recovery parameter is a measureof how closely the material returns to that original shape in the shape memory cycle (A2 comparison to A1

in Figure 1). Ratna and Karger-Docsis8 defined two similar parameters, also measured from similar shapememory cycles, which measure the performance of the material when subjected to multiple cycles. Thesevarious pre-defined parameters are useful in comparing the performance of candidate SMP materials. Thepresent study focuses on the durability performance parameters defined by Tandon et al.4

The current research adapts the shape memory performance parameters defined in axial strain by Tandonet al.4 to a bending angle measurement in degrees used to quantify the deformation in the sample. Let α

be the measured angle in the two portions of the folded sample. The shape fixity Rf can be defined as theratio between the angle immediately after unloading to point D in Figure 1, αD, to the locked-in angle ofdeformation at point C, αC . (An example angle αD measurement is shown in Figure 2.)

Rf =180 − αD

180 − αC

x 100% (1)

The linear shape recovery ratio Rr can be defined as a function of the final angle αA2 (point A2) and theinitial angle αA1 (point A1)before the shape memory cycle.

Rr =

(

2 −

αA2

αA1

)

x 100% (2)

These parameters will be used to quantitatively compare the SMPs in the current research. (The sample isinitially a straight line αA1 = 180 ◦.)

Figure 2. Image from camera with angle measurement of αD shown.

II. Material

A series of novel cross-linked polyaspartimide-polyurea based polymers utilizing 4-4-bismaleimidodiphenyl-methane, an extended chain aliphatic diamine, and a diisocyanate have been synthesized and chemicallycharacterized by the authors.9 The idealized chemical structure during the synthesis of BMI-JA-400 and ofthe BMI-JA-400 cross-linked with the diisocyanate are illustrated in Figure 3. The synthesis involves cou-pling of the bismaleimide with the diamine to create a linear polymer of both rigid and soft segments. Forin-depth detail on the synthesis see Shumaker et al.9 In the present study, three of the final polymers fromFigure 3(b) are chosen for further study of their thermomechanical properties. These materials are signifiedas BMI-JA-400-50DW, BMI-JA-400-100DW, and BMI-JA-400-150DW (idealized chemical structures illus-trated in Figure 4). The 400 signifies that Jeffamine D-400 is used (with a molecular weight of approximately

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430g/mol). The -50, -100, and -150 signify the stoichiometric amount of aliphatic diamine introduced, thehigher the number, the higher the potential cross-link density of the material (and the tighter the networkof polymer chains). The BMI-JA-400-50DW has the potential to react 25% of the cross-link sites, while theBMI-JA-400-100DW has the potential to react 50%, and the BMI-JA-400-150DW has the potential to react75% of the cross-link sites. In general when comparing the expected mechanical performance of polymers,a linear polymer is expected to be viscoelastic while the modulus, Tg, and thermal stability will all increasewith increasing cross-link density. By analog we anticipate that the modulus, Tg, and thermal stability ofthe SMPs may increase as we increase the cross-link density.

(a) (b)

Figure 3. Idealized chemical structure (a) during the synthesis of BMI-JA-400 and (b) of the BMI-JA-400 cross-linkedwith the diisocyanate.

Once these SMPs are synthesized, they are tested following a post-cure for 1 h at 145 ◦C for the “-50”,165 ◦C for the “-100”, and 175 ◦C for the “-150”. All chemical structures are based on the relative portionof starting materials and assume full reaction. Elemental analysis was consistent with this assumption.However, quantitative determination of the degree of cross-linking as indicated in Figure 4 was not obtained.Thus the cross-link densities cannot be directly measured. While the respective cross-link densities couldnot be quantified, the expected cross-link density is consistent with the chemical analysis (“-150” sample hasthe highest cross-link density, “-50” sample has the lowest). The current research is devoted to exploringthe thermomechanical shape memory properties of these three materials, and ultimately to correlate theinfluence of the chemical composition (relative cross-link density) on those thermomechanical properties.

III. Experimental Setup

III.A. Thermogravimetric Analysis

Weight loss vs. temperature is measured using a TA Instruments thermogravimetric analysis (TGA) 2950.Post-cured samples are heated from room temperature at 10 ◦C/min to 600 ◦C under flowing nitrogen.Weight losses occur either when volatiles/reaction products are driven off or when degradation of the polymeroccurs (at higher temperatures).

III.B. Dynamic Mechanical Analysis

For dynamic mechanical analysis (DMA), tensile thin film specimens are machined with dimensions 38 mmx 2 mm. The samples have a nominal thickness of 1.0 mm. A TA Instruments RSAIII dynamic mechanicalanalyzer is used to measure the storage and loss modulus of the post-cured resin specimens. The dynamicproperties of the specimens are measured at a frequency of 1 Hz, a strain of 0.1%, and a heating rate of2 ◦C/min from 25 ◦C to the high temperature state of the particular resin.

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(a)

(b)

(c)

Figure 4. Idealized structure illustration of (a) BMI-JA-400-50DW, (b) BMI-JA-400-100, and (c) BMI-JA-400-150

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III.C. Shape Memory Cycles

Shape memory cycles are conducted in order to correlate the shape memory properties and performance ofthe material to the chemical composition. These cycles are conducted on post-cured samples of dimensions38 mm x 2 mm x 1 mm. One shape memory cycle consists of: recording the original shape of the samplewith a digital camera, heating the sample to its high temperature state, folding the sample around a customtooling to give it a 90◦ bend, cooling the sample in the tooling to lock in the bent shape, and finally, freerecovery in an MTS 651 environmental chamber with an optical window for making observations inside thechamber. The chamber is heated at 4 ◦C/min to the temperature where the sample first reaches its lowelastic modulus for free recovery of the sample. For elevated temperature free recovery, type K thermocouplesare attached next to the test specimens to verify the oven calibration during the testing. A digital camerais used to record time lapse pictures of the sample during the free recovery portion of the shape memorycycle. The temperature and time are also recorded automatically to match each photo taken. The video ispost-processed to measure the degree of bend left in the sample vs. temperature (and time) as it unfoldsduring this recovery period.

IV. Results

IV.A. Thermogravimetric Analysis

TGA in nitrogen was used to investigate the thermal stability of post-cured samples over a range of temper-atures. The data for these two materials indicates an expected trend that as the potential cross-link densityis increased, the SMP remains thermally stable to a higher temperature. The resin weight vs. tempera-ture for the “-50”, “-100”, and “-150” samples is shown in Figure 5, the weight loss indicates that as thepotential cross-link density is increased, the SMP remains thermally stable to a higher temperature. Thedecomposition temperature (Td(2%)) is the temperature at which the material has lost 2% of its weight.The Td(2%) of the “-50”, “-100”, and “-150” resins are 267, 291, and 298 ◦C respectively. Note that all threeof these temperatures are well above the previously stated high temperature (and post-cure temperature)indicating that the post-cured SMP can be reasonably cycled to the high temperatures of 145, 165, and175 ◦C respectively without significant thermal degradation.

0 50 100 150 200 250 30090

92

94

96

98

100

102

Temperature (°C)

Wei

ght (

%)

BMI−JA−400−50DWBMI−JA−400−100DWBMI−JA−400−150DW

(a)

0 100 200 300 400 500 6000

20

40

60

80

100

120

Temperature (°C)

Wei

ght (

%)

BMI−JA−400−50DWBMI−JA−400−100DWBMI−JA−400−150DW

(b)

Figure 5. BMI-JA-400-50DW, BMI-JA-400-100DW, and BMI-JA-400-150DW percent weight loss vs. temperature for(a) the initial temperature region and (b) the full temperature range measured

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IV.B. Dynamic Mechanical Analysis

The DMA was used to investigate the dynamic properties of the material over a range of temperatures.The tensile storage modulus (E′) and the tan(δ) vs. temperature are shown in Fig. 6 for one DMA test.(The tan(δ) is defined as the relative ratio of the loss modulus to the storage modulus, E′′, it representsthe relative amount of energy being dissipated versus elastically stored in a material.) Three tests wereconducted per material with each showing similar results. For this study, the measured peak of the tan(δ)curve is used for identifying the Tg. In addition a high temperature modulus signified Eh is defined as E

′

measured at the high temperature (low modulus state) of the given material and a low temperature modulussignified El is defined as E′ measured at 25 ◦C. The Tg, Eh, and El from the DMA curves are summarizedin Table 1. The results indicate that both the Tg and Eh increase as the cross-link density increases. Thereis not a discernible correlation in the measured El with the changes in cross-link density.

20 40 60 80 100 120 140 160 18010

5

106

107

108

109

1010

Temperature (°C)

E’ (

Pa)

BMI−JA−400−50DWBMI−JA−400−100DWBMI−JA−400−150DW

(a)

20 40 60 80 100 120 140 160 1800

0.2

0.4

0.6

0.8

1

1.2

Temperature (°C)

tan(

δ)

BMI−JA−400−50DWBMI−JA−400−100DWBMI−JA−400−150DW

(b)

Figure 6. DMA results for the BMI-JA-400-50DW, BMI-JA-400-100, and BMI-JA-400-150DW in tension (a) storagemodulus E′ and (b) tan(δ)

Table 1. Quantitative features of the DMA results given as average for three samples.

50DW 100DW 150DW

Tg (◦C) 110 137 144

High temperature Eh (106Pa) 3.69 5.52 9.04

Low temperature El (109Pa) 1.87 1.09 2.16

IV.C. Shape Memory Cycles

Test samples of each material were marked with 5 lines spaced at a 5 mm interval for measuring the angleof bend throughout the shape memory cycle. Initial photos were taken as the reference state A1 beforebeginning the shape memory cycle. After heating (“-50” to 145 ◦C, “-100” to 165 ◦C, and “-150” to 175 ◦C)and folding the sample around a custom tooling to give it a 90◦ bend and cooling the sample in the toolingto lock in the bent shape, free recovery is conducted on the sample (heated again “-50” to 145 ◦C, “-100”to 165 ◦C, and “-150” to 175 ◦C). Digital pictures of the unfolding are post-processed using Motic ImagesPlus 2.0 software to measure the angles. (A sample image with angle measurement is shown in Figure 2.)The angle during the free recovery portions of the shape memory cycle results are shown vs. temperaturein Figure 7. The samples hold closely to their original angle during the initial heat up, the samples unfold

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toward 180◦ as the temperatures approach the Tg of the sample (110 ◦C for the “-50”, 137 ◦C for the “-100”,and 144 ◦C for the “-150”). Above the Tg, the samples continue to recover the small amount of deformationstill present until nearly all of the deformation is recovered at the designated high temperature for thatparticular material (“-50” at 145 ◦C, “-100” at 165 ◦C, and “-150” at 175 ◦C).

20 40 60 80 100 120 140 160 18080

100

120

140

160

180

200

Temperature (°C)

Ang

le (

°)

BMI−JA−400−50DWBMI−JA−400−100DWBMI−JA−400−150DW

Figure 7. BMI-JA-400-50DW, BMI-JA-400-100, and BMI-JA-400-150DW free recovery after fixing at a 90◦ bend.(Note: 180◦ indicates a flat line which represents the original “shape” of the samples.)

The shape fixity and shape recovery values were currently calculated using Equations (1) and (2). InEquation (1), the angle αC is assumed to be equal to 90◦. The resulting shape recovery parameters are shownin Figure 8. All formulations have high fixity (85 to 93 %) and near complete shape recovery (99.8 to 100%).In fact, the highest cross-linked material (“-150”) returns completely to its original shape. Presumably,the greater stiffness Eh at high temperature for a higher cross-linked material is enabling the material togenerate a greater restoring force, thereby returning the material more closely to its original shape. All threeformulations are good SMPs with tailored Tgs.

V. Conclusion

The testing described in the current research has been conducted on the BMI-JA-400-50DW, the BMI-JA-400-100DW, and the BMI-JA-400-150DW with the goal of providing a family of SMPs with good durability,high shape fixity, and good shape recovery with tailored transition temperatures (through modification ofthe chemical structure). As expected, with an increase in the idealized cross-link density, the materialremains thermally stable to a higher temperature and the Tg is increased. In addition, with an increasein the idealized cross-link density, the high temperature stiffness Eh of the material increases. All threeformulations exhibit good shape fixity and shape recovery.

The ongoing research is focused on the further mechanical characterization of this family of durableSMPs with tailored Tgs. This research also aims to elucidate further trends in the thermomechanical shapememory behavior of these materials and correlating those trends to the chemical structure of the materials.Understanding these correlations will insight into what allows SMPs to exhibit shape memory behavior. Inaddition, this family of SMPs serve as aerospace grade designer memory material with tuned mechanicalproperties based on BMIs which have a demonstrated history of good durability in current advanced aircraftcomponents.

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BMI−JA−400−50DW BMI−JA−400−100DW BMI−JA−400−150DW0

20406080

100120

Rf

BMI−JA−400−50DW BMI−JA−400−100DW BMI−JA−400−150DW0

20406080

100120

Rr

Figure 8. Shape fixity Rf and shape recovery Rr parameters

Acknowledgments

This research was performed while the primary author held a National Research Council Research Asso-ciateship Award at the Air Force Research Laboratory.

References

1Lendlein, A., and Kelch, S., 2002. “Shape-memory polymers”. Angew Chem Int Ed, 41, pp. 2035–2057.2Beloshenko, V. A., Varyukhin, V. N., and Voznyak, Y. V., 2005. “The shape memory effect in polymers”. Russ Chem

Rev, 74, pp. 265 – 283.3Vaia, R., and Baur, J., 2008. “Materials Science: Adaptive Composites”. Science, 319(5862), pp. 420–421.4Tandon, G., Goecke, K., Cable, K., and Baur, J., 2009. “Durability assessment of styrene- and epoxy-based shape-memory

polymer resins”. J Int Mater Syst Struct, 20, pp. 2127–2143.5Daniel, I. M., and Ishai, O., 2006. Engineering mechanics of composite materials. Oxford University Press, New York.6Mason, K., 2004. “Composites combat ready in ucavs”. Composites World, High Performance Composites, May.7Liu, C., Qin, H., and Mather, P. T., 2007. “Review of progress in shape-memory polymers”. Journal of Materials

Chemistry, 17, pp. 1543–1558.8Ratna, D., and J.Karger-Kocsis, 2008. “Recent advances in shape memory polymers and composites: A review”. Journal

of Materials Science, 43, pp. 254–269.9Shumaker, J., McClung, A., Mathyss, S., and Baur, J., 2011. “A novel approach to synthesizing polyaspartimide-urea

based shape memory polymers”. Polymer. in progress for submission.

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