shape-memory behavior of thermally stimulated polyurethane for medical applications

Shape-Memory Behavior of Thermally StimulatedPolyurethane for Medical Applications

G. Baer,1 T. S. Wilson,2 D. L. Matthews,2 D. J. Maitland2

1Department of Biomedical Engineering, University of California Davis, Davis, California 956162Division of Medical Physics and Biophysics, Lawrence Livermore National Laboratory, Livermore, California 94551

Received 24 February 2006; accepted 27 September 2006DOI 10.1002/app.25567Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Shape memory polymers (SMPs) have beenof great interest because of their ability to be thermally actu-ated to recover a predetermined shape. Medical applica-tions in clot extracting devices and stents are especiallypromising. We investigated the thermomechanical proper-ties of a series of Mitsubishi SMPs for potential applicationas medical devices. Glass transition temperatures and mod-uli were measured by differential scanning calorimetry anddynamic mechanical analysis. Tensile tests were performedwith 20 and 100% maximum strains, at 37 and 808C, whichare respectively, body temperature and actuation tempera-ture. Glass transitions are in a favorable range for use in thebody (35–758C), with high glassy and rubbery shear moduliin the range of 800 and 2 MPa respectively. Constrained

stress–strain recovery cycles showed very low hysteresis af-ter three cycles, which is important to know for precondi-tioning of the material to ensure identical properties duringapplications. Isothermal free recovery tests showed shaperecoveries above 94% for MP5510 thermoset SMP cured atdifferent temperatures. One material exhibited a shape fix-ity of 99% and a shape recovery of 85% at 808C over onethermomechanical cycle. These polyurethanes appear par-ticularly well suited for medical applications in deploymentdevices such as stents or clot extractors. � 2006 Wiley Period-icals, Inc. J Appl Polym Sci 103: 3882–3892, 2007

Key words: polyurethanes; mechanical properties; thermalproperties; glass transition

INTRODUCTION

Shape memory polymers (SMPs) represent a very in-teresting class of polymers, because of their ability torecover a predetermined shape upon controlled heatactivation.1 They are increasingly being investigatedfor use as smart materials in a variety of applicationsranging from textiles to ergonomic utensils.2 In themedical field, new SMPs with adjustable moduli, highrecovery ratios, sharp and adjustable transition temper-atures, and biocompatible drug-eluting surfaces haveenabled their use in a variety of new applications. Spe-cifically, SMPs are currently being researched for appli-cations in aneurysms embolization,3 as clot extractiondevices,4–6 and for vascular stents.7,8 Because of theirdifferent mechanical properties above and below theglass transition temperature, the properties of heat

activated SMPs can be optimized to offer both the flex-ibility, the resistance in compression, and the highshape recovery needed for proper insertion, position-ing, deployment, and functionality of these devices.They can be deployed by a variety of heat sources:light illumination,9,10 electric current,11 or simple hotsaline solution. In addition, their surface can readily bemodified for biocompatibility and drug elution.12,13

Thermomechanical behavior and shape memoryproperties have been reported for SMP materialsmade from polyurethanes,1,2,14–17 ethylene-vinyl ace-tate copolymers,18 oligo(e-caprolactones),19 poly(ethyl-ene glycol), and poly(ethylene terephthalates).20 Theshape memory effect of thermal SMPs resides in thestructure of the polymer, which is often described asbeing a two phase structure comprised of a hard, orfixing phase, and a soft, or reversible phase. The hardphase consists of a dispersed glassy phase, crystallin-ity, or/and chemical crosslinking. The soft phase canbe either amorphous or semicrystalline. Varying theamount of hard and soft phase results in significantdifferences in shape memory behavior, as shown byLin and Chen’s early studies with polyurethanes.21,22

Thermomechanical properties in thin films of Mitsu-bishi Heavy Industry polyurethane materials havepreviously been investigated.2,23,24 Their shape mem-ory behavior was also studied when replacing andvarying the amount of hard segment content.25 Thesestudies have shown that such materials have potentialfor biomedical applications, since they exhibit high

Correspondence to: T. S. Wilson ([email protected]).Contract grant sponsor: U.S. Department of Energy by

Lawrence Livermore National Laboratory (LLNL); contractgrant number: W-7405-ENG-48.Contract grant sponsor: National Institutes of Health/

National Institute of Biomedical Imaging and Bioengineer-ing; contract grant number: R01EB000462.Contract grant sponsor: LLNL Directed Research and

Development; contract grant number: 04-ERD-093.Contract grant sponsor: National Science Foundation

Center for Biophotonics (CBST).

Journal of Applied Polymer Science, Vol. 103, 3882–3892 (2007)VVC 2006 Wiley Periodicals, Inc.

strain recovery, high glass/rubber modulus ratios,and their glass transition temperatures are in a favor-able range for use in the human body. Additionally,the good biocompatibility of polyurethanes is wellknown (e.g., catheters26 and heart valves27) and hasbeen studied for these SMPs. In vitro and in vivo stud-ies by Metcalfe et al. of the same Mitsubishi polyur-ethane materials used as foams for endovascular inter-ventions have recently shown the material to be non-cytotoxic and poorly thrombogenic.3 In vitro studiesfrom our group (in the process of publication) showthat the material used here has minimal effect on trig-gering an inflammatory response, thrombogenesis,and activation of platelets and neutrophils, reducingthe risk of rejection of the implanted device. SuchSMPs would appear useful for interventional medicaldevices, such as stents.

Other requirements of a SMP stent reside in its ther-momechanical properties. The material needs to havea high shape recovery ratio when deployed at hightemperature so that a small device can be inserted andnavigated through the vasculature and deployed intoa large device that fits the target artery. In addition,when in place at body temperature, a high modulus isdesired for resistance in tension and compression.Finally, thermomechanical properties need to be stableover time. These general requirements would apply toany SMP interventional medical device.

However, results comparing the thermomechanicalproperties of different Mitsubishi materials of variousglass transition temperatures available have not beenreported. This knowledge would be extremely helpfulas a base for choosing a particular material with opti-mal properties for a specific medical application.Additionally, it is well known that two phase systemssuch as segmented polyurethanes have structures thatare affected by process history including shear, stress,and solvents if solution cast1; however, to our knowl-edge, the effect of cure profile on the behavior of SMPshas not been reported. It is of interest to find out towhat extend varying the curing temperature mightconfer different properties and shape memory behav-ior to SMPs.

In this study, we have examined a variety of Mitsu-bishi polyurethanes within the scope of researchingmaterials for stent applications. Several thermoplasticand thermoset polyurethanes with glass transition tem-peratures ranging from 35 to 758C were investigated.In addition, we studied the effect of curing tempera-ture on thermal transitions and thermomechanicalproperties of one of the thermosets, by having a twostep curing protocol and varying the temperature ofthe first step. Thermal transitions were characterizedby differential scanning calorimetry (DSC), dynamicmechanical properties were analyzed by dynamic me-chanical analysis (DMTA), and cyclic tensile and recov-ery properties were examined using an MTS tensile

tester. MTS experiments were performed at two tem-peratures relevant to a medical device functionality:378C (body temperature) and 808C (actuation tempera-ture). Previous studies reported by our group4 haveshown that temperatures of Tg 6 208C are well into theglassy and the rubbery plateau respectively. Elonga-tion to failure, stress recovery cycles, and free recoverycycles were studied at maximum strains of 20 and100%, which are reasonable ranges of deformationexpected in medical device actuation.

EXPERIMENTAL

Materials

SMPs

The SMPs used in this study were obtained fromDiAPLEX Company (a subsidiary of Mitsubishi HeavyIndustries, Tokyo, Japan) either as two part thermosetresins or thermoplastic resin in pellet form. The firsttwo digits following the two letters represent the tem-perature, in degree centigrade, of the first glass transi-tion, indicated by the manufacturer. While the compo-sitions are proprietary, it is known that the material isa segmented polyurethane, which has a microphaseseparated morphology.28 The primary shape is formedat a temperature above the highest glass (Tgh) or crys-talline (Tm) transition and the polymer is either cooled(thermoplastics) or crosslinked (thermosets) to fix theshape. The secondary shape is obtained by heatingthe material above the glass transition temperature ofthe soft phase (Tgs) or the soft phase crystalline melt-ing temperature (Tms), applying a strain to the mate-rial, and cooling it down below the same soft phasetemperature to fix the shape. The deformation isstored elastically as macromolecular chain orientationwithin the soft phase, while hard phase segments arerelatively imperturbed by this stress. Thus, there is anentropic potential for shape recovery. By heating thematerial above Tgs or Tms, soft phase segments springback to their lower entropic state, resulting in recoveryof the primary shape.

Thermoset sample preparation

Generally, the thermoset SMPs MP3510, MP4510, andMP5510 were processed according to the samplepreparation guide provided by DiAPLEX.29 Briefly, Aand B components were vacuum dried at 708C and 1Torr for 1 h, then mixed at the appropriate weight ra-tio, and degassed under vacuum (1 Torr) for 60 s. Themixture was placed into a syringe and immediatelyused to inject specific molds. Aluminum molds werefabricated to cast samples according to ASTM stand-ards. DMTA samples were molded into 4.65 mm di-ameter, 5-cm-long cylinders; tensile test samples weremolded into 1-mm-thick dogbones for failure analysis

SMPs FOR MEDICAL APPLICATIONS 3883

Journal of Applied Polymer Science DOI 10.1002/app

(ASTM standard D 638 type V), and 1 mm thick, 1 cmwide bars (ASTM standard D 882) for other tensiletests. Once the molds were injected, MP5510 sampleswere cured in an oven under nitrogen for 1 h (curestep 1) at one of the following temperatures: 258C (Tg

� 308C), 458C (Tg � 108C), 558C (Tg) or 658C (Tg þ108C) followed by 4 h (cure step 2) at 808C. MP3510and MP4510 samples were cured in one step for 4 h at808C. Samples were then cooled at room temperatureand stored in sealed containers before testing. DSCsamples were cut from molded strips. It should be notedthat the ‘‘thermoset’’ grade resins are actually comprisedof difunctional monomers and when fully cured at rec-ommended conditions were found to be soluble in ap-propriate solvents and melt processible. For example,heating these thermosets above the hard phase Tg

would allow for re-training of the primary shape.

Thermoplastic SMP sample preparation

Test specimens were prepared from the as receivedthermoplastic SMP MM5510, MM4520, MM5520,MM6520, and MM7520 pellets for DSC and DMTAtesting as follows. The pellets were first vacuumdried (40 mTorr, 508C) for 120 h. They were thencompression molded into plaques with dimensions3.2 mm � 100 mm � 100 mm using a Carver hotpress (2008C, 15,000 pounds, 10 min). The pressedplaques were quickly cooled by placement on an alu-minum block, demolded, stored in plastic bags, andcut into 1 cm strips for testing.

Experimental procedures

Thermal and mechanical characterization of SMPswere done using the following methods.

Differential scanning calorimetry

DSC measurements were made using a Perkin–ElmerDiamond DSC equipped with an Intracooler 2, over atemperature range of �20 to 2008C using a heatingrate of 208C/min, with 5 6 1 mg samples, consistentwith ASTM D 3418. Two heating/cooling cycles wereused and the Tg’s from both heating cycles arereported as the average of two repeats.

Dynamic mechanical analysis

DMTA measurements were made with an ARES LS2under dynamic oscillatory mode at a frequency of1 Hz and initial strain of 0.01%. Testing was doneunder dry air, from 25 to 1208C at a linear ramp rateof 18C/min, consistent with ASTM D 4065. For testingthermosets, test specimens were used having a cylin-drical geometry of 4.65 mm diameter and 5 cmlong, while torsion rectangles of 3.2 mm � 12 mm

� 50 mm were used for testing thermoplastics. Theshear storage modulus G0, shear loss modulus G00,and the ratio of loss to storage modulus (tan d) wereplotted as a function of temperature; Tg was deter-mined from the tan d peak, and the glassy and rub-bery moduli were read from the G0 curve. In the caseof the thermoplastics, first heating, first cooling, andsecond heating cycles were performed to capture theeffect of thermal history (two repeats).

Tensile

Stress–strain measurements were performed on anMTS Synergie 1000 tensile tester with a load cell of5 kN, equipped with a custom forced air heating anda cooling system and a constant temperature chamberto which was attached a thermocouple. Tests wereperformed at 37 and 808C after heating the sample ata rate of 108C/min, and maintaining the sample atthe desired temperature for 5 min before performingthe experiment. Uniaxial elongation to failure testswere conducted at an engineering strain rate of0.5 min�1, as specified in ASTM D882. Initial modu-lus, peak load, peak stress, and strain at break wererecorded for various thermosets with three repeatseach.

Constrained stress recovery

Constrained stress recovery cycles at high temperaturewere conducted on the MTS Synergue 1000 at a con-stant strain rate of 5 min�1 with maximum strains of20 and 100% and at constant temperatures of 808C,according to the same heating protocol mentioned ear-lier. The machine was programmed to perform cyclesof elongation to the maximum desired strain andreturn back to zero strain. Percentage recovery strainwas calculated for each cycle. Five cycles were per-formed on various thermosets with three repeats.

Isothermal free recovery

Free isothermal recovery tests at high temperaturewere performed on the MTS Synergie 1000 as in con-strained recovery cycles, except that after elongationto a maximum strain, unloading was done by pro-gramming the machine to return to a very low stressvalue close to 0. The sample was allowed to freelyrecover its strain for 15 min before the next cycle. Per-centage strain recovery was then calculated. Variousthermosets were tested with three repeats each.

Thermomechanical cycling

Thermomechanical cycling tests were conducted onthe MTS Synergie 1000 at a constant strain rate of

3884 BAER ET AL.


5 min�1 with maximum strains of 20 and 100% withelongation at a constant temperature of 808C and sub-sequent cooling and recovery at 378C, as described byKim et al.14 Heating was performed according to theprotocol mentioned earlier. The specimen was thenelongated to the desired strain, held, and rapidlycooled down to 258C using nitrogen gas. After hold-ing the sample at this temperature for 5 min, thestress was set to a low value close to 0, and the sam-ple was then allowed to freely recover to its recoverystrain by rapid heating at 808C. Percentage shape fix-ity and Percentage shape recovery were calculated.One cycle was performed with one of the thermosetmaterials, with three repeats.

RESULTS AND DISCUSSION

Differential scanning calorimetry

The glass transition of the soft phase (Tg) was calcu-lated from first and second heating curves generated

by DSC using the half-height technique as describedby Wunderlich30 and as shown in Figure 1. Results forthe thermoset and the thermoplastic series are shownin Table I. Also shown are results for the MP5510 ther-moset series (manufacturer reported Tg of 558C) pre-pared at different precuring temperatures. In general,the glass transition temperatures measured by DSCwere lower than the ones indicated by the manufac-turer, by up to 108C. Since we have no specific infor-mation on the characterization method used by themanufacturer, we can only speculate on the source ofthis difference. The glass transition is a dynamic prop-erty and therefore varies with the cooling and heatingrates of the samples. In addition, DSC and DMTAmeasurements often give different results; e.g., the Tg

measured by DSC is often much lower than that deter-mined by the tan d peak,31 which is discussed later.There was also a significant difference between theglass transition temperature obtained during the firstand second heat, indicative of the importance of previ-ous process history on phase separation and quench-ing of the amorphous soft phase. Surprisingly, curestep 1 temperature did not affect the glass transitiontemperature significantly.

Dynamic mechanical analysis

Results of the DMTA testing are shown in Figures 2through 3. Glass transitions were determined basedon tan d peak and are reported in Table I. Figure 2(a)shows the shear storage modulus G0 and tan d for thedifferent Tg grade thermoset SMPs prepared accord-ing to manufacturer’s recommendations with onestep curing. Figure 2(b) shows G0 and tan d for ther-moplastic SMP MM-5510 during first heating, firstcooling, and second heating. The effect of thermal his-tory is captured here. There is a shift in the curvesbetween first heating and first cooling. The first cool-ing and the second heating display identical curves.Between the first heating and first cooling, there is a

TABLE ISummary of DSC and DMTA Results on DiAPLEX1 (Mitsubishi) SMPs and Comparison with Manufacturer’s Values

SMP typeCure step 1conditions

Cure step 2conditions

Manufacturer’ssoft phaseTg (8C)

Tg by DSCfirst

heat (8C)

Tg by DSCsecondheat (8C)

Tg bytan d(8C)

Glassyplateau

modulus (Pa)

Rubberyplateau

modulus (Pa)

MP3510 – 4 h 808C 35 31.3 25.2 46.1 5.2 E 8 1.6 E 6MP4510 – 4 h 808C 45 42.5 35.9 54.1 7.0 E 8 1.7 E 6MP5510 – 4 h 808C 55 52.4 51.1 65.2 8.1 E 8 1.9 E 6MM5510 – 10 min 2008C 55 51.4 52.0 54.9 8.1 E 8 1.6 E 6MM4520 – 10 min 2008C 45 35.9 47.2 45.9 6.6 E 8 1.3 E 6MM5520 – 10 min 2008C 55 45.9 53.5 57.3 7.7 E 8 1.4 E 6MM6520 – 10 min 2008C 65 64.7 64.9 66.6 8.9 E 8 2.1 E 6MM7520 – 10 min 2008C 75 74.2 74.4 74.6 7.7 E 8 1.8 E 6MP5510 1 h 258C 4 h 808C 55 44.3 52.4 55.9 7.9 E 8 1.5 E 7MP5510 1 h 458C 4 h 808C 55 43.3 51.0 61.3 8.3 E 8 5.9 E 6MP5510 1 h 558C 4 h 808C 55 41.7 55.2 65.4 7.6 E 8 2.5 E 6MP5510 1 h 658C 4 h 808C 55 44.5 52.6 66.3 7.7 E 8 2.1 E 6

Figure 1 General DSC behavior of thermoplastic and ther-moset polyurethane SMPs. Shown are first heating, firstcooling, and second heating curves.



shift to lower Tg. There is also a decrease in tan dpeak height, indicating an increase in the hard phasecontent. In addition, the storage modulus in theglassy phase increases by a factor of 8 between the firstheating and the first cooling. These results can beunderstood as a rearrangement in the phase-separatedstructure of the polymer as a result of thermal historyduring testing. Initially, during preparation, the testspecimens were quickly cooled from the melt and thesegregation of soft phase segments and hard phasesegments into separate phases did not have time toreach equilibrium. The result would be a structuremade up of soft phase containing a relatively highhard-segment content. The results from the first heat-ing cycle are indicative of this structure up to the pointat which the polymer had enough mobility to regainan equilibrium phase structure. On cooling and subse-quent reheating, the polymer has had time to reachthis equilibrated structure in which a higher degree ofphase separation has occurred, with the result that thesoft phase now has a lower hard segment content

(hence lower Tg) and there is a larger amount of hardphase (hence higher modulus). It is likely that morecycling would show identical curves.

Cure temperature effects can also be seen in thethermoset grade SMPs as seen in Figure 3, whichshows G0 obtained with the MP5510 thermoset mate-rials cured in two steps. Glass transitions were deter-mined based on tan d peak. The glassy and rubberymoduli can also be read; they are two elastic moduliof importance, which are approximately equal to theshear storage modulus G0 in the lower temperaturehigher stiffness ‘‘glassy’’ plateau and in the highertemperature lower stiffness ‘‘rubber’’ plateau, respec-tively. For SMPs, the rubbery and the glassy moduliare generally taken as the elastic moduli at Tg 6 258C,respectively. Results are shown in Table I. Overall,glass transition temperatures obtained by tan d peak

Figure 2 Elastic modulus and tan d of SMPs. (a) Thermo-sets cured using suggested temperature profile and (b) firstheating, first cooling, and second heating cycles for theMM5510 thermoplastics.

Figure 3 Dynamic storage modulus (G0) in the first heatfor MP5510 SMP cured at various cure step 1 temperatures.

Figure 4 Glassy (G0 at Tg þ 258C), rubbery (G0 at Tg

� 258C) modulus, and Elastic ratio (G0 at Tg � 258C/G0 at Tg

þ 258C) versus cure step 1 temperature for the thermosetpolymers MP5510.

3886 BAER ET AL.


were higher than the ones obtained by DSC. This isvery common; Tg in DSC is taken at one half of theincrease in heat capacity, while Tg taken at the tan dpeak occurs after the shear loss modulus G00 haspeaked, thus further into the glass transition and at ahigher temperature.30 For the thermoplastic series, Tg

measured by tan d was close to the manufacturer’sTg. For the thermoset series cured in two steps, the Tg

measured by tan d varied between 61.3 and 66.38C.The glassy shear modulus was roughly constantacross all materials, in the order of 800 MPa. The rub-bery shear modulus seemed to be slightly more ele-vated in the thermoset materials when compared tothe thermoplastic materials, and overall varied be-tween 1.3 and 15 MPa, generally increasing with in-creasing Tg. However, for the MP5510 thermosetscured in a two-step process, while the glassy modulusremained essentially unchanged, the rubbery modulussignificantly decreased with increasing precuring tem-perature, reaching a plateau for materials cured above558C during cure step 1, as seen in Figures 3 and 4.

The elastic ratio, defined as the ratio of the glassyto rubbery modulus (G0(Tg � 258C)/G0(Tg þ 258C)),increased with increasing temperature during curestep 1 of the thermoset series, as shown in Table IIand Figure 4. It has been reported that a high glassymodulus correlates with high shape fixity during si-multaneous cooling and unloading, and elasticityratios above 100 allow for greater resistance to defor-mation and better shape recovery.13 According tothese findings, material with cure step 1 at 258Cshould exhibit less shape recovery than the others,since the elastic ratio is 53 while other elastic ratiosare above 100.

The increase in the elastic ratio, due to a decrease inthe rubbery modulus, could be explained by an in-crease in phase separation of the material, as the tem-perature is increased during cure step 1.14 Visualobservations of the turbidity of the samples cured atthe different temperatures appear to support this idea.It was observed that in general, the degree of turbidityin the samples increased with initial (step 1) cure tem-perature. Samples cured at 25 and 458C in cure step 1showed little turbidity and were almost transparent,while the ones cured at 55 and 658C in cure step 1, and

the ones cured at 808C in one single step, showed in-creasing degrees of turbidity. Higher turbidity seemedto correlate with lower rubbery modulus. Turbidity orcloudiness occurs in multiphase systems because ofthe differences in the refractive indices between phases,in turn relating to different compositions, the presenceof additives or dispersed fillers, or crystallinity. Herewe believe increases in turbidity are related to thedegree of phase separation and/or changes in size ofthe dispersed phase.

MTS mechanical testing

Uniaxial tensile test to failure—MTS (37 and 808C)

Figure 5 shows the stress–strain behavior during uni-axial tensile testing to failure at 37 and 808C. At a giventemperature test, the materials exhibited very similarcurves, with quite different strains at break. The maxi-mum strain at break was 100% at 378C and 270% at808C. The material exhibited necking at 378C, but notat 808C. This is presumably due to cross section con-traction with realignment of the polymer chains alongthe extension direction.14 Table III shows failure test re-sults at 37 and 808C. The strain at break at 378C washighest for the material precured at 258C and de-creased with increasing precuring temperature, sug-gesting a lower degree of phase separation as curing

TABLE IIGlass Transition Temperatures, Modulus, and Elastic Ratio for the MP5510 Thermoset

Series Cured at Various Temperatures in One or Two Steps

Cure step 1temperature(1 h) (8C)

Cure step 2conditions(4 h) (8C)

Tg from tan dpeak (8C)

G0 atTg þ 25 Pa

G0 atTg � 25 Pa

Elasticratio

25 80 55.9 1.5 E þ07 7.9 E þ08 5345 80 61.3 7.2 E þ06 8.7 E þ08 12155 80 65.4 2.7 E þ06 7.6 E þ08 30465 80 66.3 2.6 E þ06 8.2 E þ08 327– 80 65.2 1.9 E þ06 8.5 E þ08 426

Figure 5 MP5510 thermoset series tensile testing to failure.



temperature increases.14 Strain at break at 808C washighest in the material precured at 558C. Peak pres-tresses were similar across materials at a given test tem-perature, around 50 MPa at 378C, and 19 MPa at 808C.

Isothermal constrained stress recovery cycles

Cyclic behavior at 808C and 20% maximum strain. Allisothermal constrained stress recovery cycles showeda hysteresis, which decreased with increasing cyclenumber. Figure 6 shows the stress–strain cyclic behav-ior for the material precured at 258C. As seen in thisfigure, the greatest change in behavior is observedbetween the first and the second cycle. This is presum-ably due to entanglement decoupling of molecularchains, with relaxation of dangling chains and pullingout of hard segments from the hard phase. Cyclicproperties become similar after the third cycle. This isimportant to know in terms of manufacturing and pre-conditioning the material to create a device with uni-form cyclic properties. In addition, as the number ofcycles increases, the initial slope of the loading curvedoes not significantly change, which implies that thestructure is unchanging. These results are similar tothe ones reported by Tobushi et al.2 This stress–strainbehavior was observed in all the precured conditions

studied. Figure 7(a,b) shows the first and the fifthcycle for materials precured at 25, 45, 55, and 658C.The material precured at 258C clearly exhibits higherstresses at 20% strain when compared to the othermaterials, which exhibit identical behavior. The mate-rial precured at 258C also had the largest initial slope,suggesting it has higher resistance to deformationamong these materials. This suggests the existence of atransition precuring temperature between 25 and 458Cbelow which more phase separation occurs duringpreparation of the material.Cyclic behavior at 808C and 100% maximum strain. Figure 8shows the stress–strain cyclic behavior for the material

TABLE IIITensile Testing Results for the MP5510 Thermoset Series

Specimen

Peakload (N)

Peak stress(MPa)

Strain atbreak (%)

Initialmodulus(MPa)

Cure step 1temperature (8C)

Testtemperature (8C)

25 37 142 6 1 46 6 1 95 6 5 1943 6 1645 37 149 6 1 48 6 1 88 6 8 1962 6 3555 37 158 6 1 51 6 1 71 6 8 2027 6 1865 37 153 6 1 49 6 1 51 6 3 1966 6 1625 80 68 6 4 22 6 2 263 6 12 35 6 245 80 51 6 4 16 6 2 245 6 3 26 6 155 80 60 6 4 19 6 2 271 6 8 23 6 165 80 53 6 4 17 6 2 241 6 8 27 6 1

Figure 6 Constrained isothermal stress recovery cycles at808C for the material cured for 1 h at 558C followed by 4 hat 808C. Maximum strain 20%.

Figure 7 MP5510 thermoset series: constrained isothermalstrain recovery at 808C, (a) cycle 1 and (b) cycle 5.

3888 BAER ET AL.


precured at 558C. As in the curves obtained at 20%maximum strain, the greatest change in behavioroccurs between the first and the second cycle. Unlikein the 20% maximum strain curves, however, a yieldpoint is present but is more prominent in the firstcycle. The initial slope decreases with increasingcycles, suggesting that the resistance to deformationdecreases with the number of cycles. This is presum-ably due to microscopic rearrangement of hard seg-

ments during extension to 100% strain, occurringmostly during the first cycle, as previously describedby Kim et al.14 This rearrangement is also seen as anincrease in the slope after the yield point with increas-ing cycle number. Cyclic properties become similar af-ter the third cycle. This stress–strain behavior wasobserved in all the materials studied. Figure 9(a,b)show the first and the fifth cycle for materials pre-cured at 25, 45, 55, and 658C. The clear difference instresses between the material precured at 258C and theother materials seen in the 20% maximum strain de-formation experiments is not as pronounced here, butthe material precured at 208C exhibits the highest re-sistance to deformation here again.

The percentage strain recovery was calculated asfollows

Rr ¼ ½ðem � epÞ=em�100;

where em is the maximum strain at maximum stress,and ep is the residual strain after recovery, i.e., thestrain at the beginning of the next cycle. Figure 10shows the percentage strain recovery obtained in thedifferent precuring conditions as a function of cyclenumber. As expected, the percentage strain recoveryis higher for a maximum elongation of 20% (88% forthe first cycle) than for a maximum elongation of100% (84% after the first cycle) but they seem to fol-low the same trend and remain parallel to each other,with the largest decrease between the first and thesecond cycle. However, this commonly used calcula-tion does not reflect the fact that the next cycle doesnot start at zero strain, which means that the percent-age recovery actually increases with cycle number. Toappreciate this, the adjusted recovery strain can becalculated as follows for each cycle

Rr adj ¼ ½ðem � epÞ=ðem � e0Þ�100

Figure 8 Constrained isothermal stress recovery cycles at808C for the material cured at 558C for 1 h followed by 4 hat 808C. Maximum strain 100%.

Figure 9 MP5510 thermoset series: constrained isothermalstrain recovery at 808C, (a) cycle 1 and (b) cycle 5.

Figure 10 Percentage recovery strain (constrained) at 808Cas a function of cycle number.



where em is the maximum strain, ep is the residualstrain after recovery, and e0 is the strain at the begin-ning of the cycle. Note that e0 of cycle 1 is 0, and e0 ofcycle N corresponds to ep of cycle N � 1. Figure 11shows the adjusted recovery strain as a function ofcycle number. After the second cycle, 97% strain re-covery is obtained for both 20 and 100% maximumstrain tests. Over 99% recovery is obtained after thefifth cycle. This shows that preconditioning samplesby five cycles of elongation should result in constantdeformation properties. It is important to note thatthese tests were performed in constrained recovery.Thus, the material has even more potential for shaperecovery, if more time is allowed for it to recover, asin free recovery tests.

Free isothermal recovery tests

Free isothermal recovery tests were performed on theMTS. The specimens were elongated to a maximumstrain of 20% in the chamber at 808C, and thenallowed to freely return to their initial strain for10 min, at the same temperature. For this, the stresswas set to a very small value. Stress and strain wererecorded continuously. The various MP5510 thermo-sets were tested, and recovery strains above 90%were obtained, with no significant difference be-

tween materials, as seen on Table IV. Note that themethod we used for evaluating the free recoverystrain is limited by the resolution of the load cell. Inaddition, the PDI control settings introduce someoscillations, which also introduce errors. These oscil-lations were so prominent during testing at 100%maximum strain that data could not be analyzed. Abetter method would be to measure free recovery byvideotaping.

Thermomechanical cycles

Samples precured at 558C were run through one cycleof shape fixing and recovery experiments, asdescribed in the previous section. The sample waselongated to a maximum strain em at 808C, the shapewas then fixed by rapid cooling at 258C using com-pressed air, and the load was removed. Because ofcontraction of the material during cooling, the newstrain at zero stress eu was lower than em. The mate-rial was then heated at 808C or at 378C and allowedto recover its shape for 10 min back to ep. The per-centage shape recovery and the percentage shape

Figure 11 Adjusted percentage recovery strain (constrained)at 808C as a function of cycle number.

TABLE IVPercentage Strain Recovery of MP5510 Thermosets

Obtained in the Free Recovery Tests Performed at 808Cwith Maximum Elongation of 20%

Cure step 1 temperature (8C) Strain recovery (%)

25 94 6 145 94 6 155 96 6 265 95 6 2

Figure 12 Thermomechanical cycling curves at 808C forMP5510 thermoset material cured at 558C for 1 h, followedby 4 h at 808C. (a) Maximum 20% strain and (b) maximum100% strain.

3890 BAER ET AL.


fixity were calculated as follows

% shape fixity ¼ ðeu=emÞ100

% shape recovery ¼ ½ðem � epÞ=em�100

Figures 12(a,b) show plots of stress versus strain forthe MP5510 thermoset material precured at 558C,elongated to 20% [Fig. 12(a)] or 100% [Fig. 12(b)] maxi-mum strain, cooled at 258C, and heated at a recoverytemperature of 808C. The high shape fixity both at 20and 100% maximum strain is evident from thesecurves. Table V shows shape fixity and shape recoveryobtained at the various maximum strains and recoverytemperatures for this material. The average shape re-covery at a recovery temperature of 808C was 87%,which is less than the values obtained for recoveredsamples in the free recovery experiments. This mostlikely reflects the influence of fixing the shape on therecovery ratio. Recovery at 378C was in the order of10%; this is expected, since this temperature is wellbelow the glass transition temperature of the softphase. Very high percentage shape fixity between 96and 99.5% were obtained.

CONCLUSIONS

We have studied the thermomechanical properties ofcommercial thermoplastic and thermoset polyur-ethanes of various compositions for medical applica-tions. Their glass transition temperatures measuredby DMTA and DSC varied between 45 and 758C. Theaverage glassy modulus was 700 MPa, and the aver-age rubbery modulus was in the order of 1.8 MPa.The glassy modulus was fairly constant across thematerials, but there were significant variations in therubbery modulus between the thermosets cured atdifferent temperatures, and in thermoplastics, as afunction of cooling rate during processing. Sampleturbidity and prior works suggest these effects are dueto differences in microphase structure. This shows thatit is possible to manipulate the elastic ratio by chang-ing curing conditions, to obtain different desired re-covery properties. Stress–strain relationships werestudied at relevant temperatures for medical interven-

tional devices: body temperature of 378C (expandeddevice) and actuation temperature of 808C, at relevantstrains of 20 and 100%. Failure strains of 100 and 250%were achieved at 37 and 808C respectively. Con-strained recovery cycles showed a similar behavior af-ter cycle 5, suggesting that preconditioning is an im-portant consideration for these materials. Finally, ther-mocycles showed shape recovery over 85% and shapefixity up to 99%. Thus, thermomechanical propertiesof these materials can be adjusted and seem to be wellsuited for manufacturing medical interventional devi-ces such as thromboembolic extractors and vascularstents.

CBST, an NSF Science and Technology Center, is managedby the University of California, Davis, under CooperativeAgreement No. PHY 0120999.

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TABLE VShape Fixity and Shape Recovery Obtained after One Thermomechanical Cycle of

MP5510 Thermoset Material Cured at 558C for 1 h, Followed by 4 h at 808C

Recoverytemperature (8C) em ep eu

Percentageshape fixity

Percentageshape recovery

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shape-memory behavior of thermally stimulated polyurethane for medical applications

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