Effects of He++ ion irradiation on Adhesion of Polymer

Micro-structure Based Dry Adhesives

Paul Day∗a, Mark Cutkoskya, Richard Grecob, and Anastasia McLaughlinb

aDepartment of Mechanical Engineering, Stanford University,Escondido Mall, Stanford, CA 94305, USA

bLos Alamos National Laboratory, Los Alamos, NM, USA

June 21, 2010

Abstract

Irradiation of polymer based gecko-like synthetic ad-hesives (GSAs) using an accelerated beam of He++

ions has been performed. This irradiation simulateslarge α radiation doses that the GSAs may experi-ence if deployed on a robotic platform in some radio-logical environments. After irradiation, the adhesivesamples were tested for adhesion on a three-axis ad-hesion testing stage and were examined via scanningelectron microscope. The GSA samples showed sig-nificant changes in surface morphology at high radia-tion doses. Additionally, radiation doses larger then750kGy resulted in a significant deterioration of theadhesive performance. Eventually, the adhesive sam-ples lost all ability to generate frictional adhesion.Such results allow us to make quantitative statementsabout the applicability of GSAs for robotic applica-tions in nuclear environments.

1 Introduction

Biologically inspired directional dry adhesives havebecome a topic of significant interest in the roboticscommunity. These adhesives have proven to be effec-tive on a growing variety of surfaces, including thosewith surface roughnesses on widely ranging lengthscales. Many adhesives consist of sheets of elas-

∗Author for correspondence.

tomeric polymers with micro-scale surface featuresformed via molding processes [1], [2]. Others haveutilized single- and multi-walled carbon nano-tubesproduced on silicon wafers using additive methodssuch as chemical vapor deposition [3]. The adhesivesdiscussed in this paper consist of a silicone elastomerwith asymmetric micro-features and are considereddirectional adhesives, i.e. they generate maximumadhesion when loaded in a preferred shear direction.Additionally, they can be considered controllable ad-hesives because they produce virtually no adhesionin the absence of a shear load [1].

The Los Alamos National Laboratory has shownrecent interest in utilizing these adhesives in roboticapplications in nuclear environments such as glove-boxes. These airtight stainless steel boxes protectworkers from contamination while allowing them tomanipulate nuclear material by reaching throughports in the box containing sealed rubber gloves.Glove ports allow workers to access most of the glove-box without undue effort. However, some areas areextremely difficult, if not impossible, to reach. Thischaracteristic makes a number of common tasks suchas cleaning and leak testing very difficult. Climbingrobots represent a potential solution to this problem.A small, climbing robotic platform would be able toreach and perform maintenance tasks in areas whereworkers cannot. Robotically assisted maintenancehas a number of advantages over current practicesincluding reduced ergonomic stress on workers due

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25kV x800 38μm

25kV x40.0 750μm

Figure 1: SEM Images of the micro-structure adhesive. Left: A side view of the micro-structure. Right: A largerview of the patterning of the micro-structures on the surface.

to excessive reaching, reduced radiation dose takenby workers, and reduced operational cost.

Ideally, an adhesive for such a platform would haveexcellent resistance to alpha and gamma radiation.Having stable adhesive performance at doses in themany tens or hundreds of kGy would allow for verylong lifetimes given the radiation levels seen in nu-clear facilities. Unfortunately, there is limited infor-mation available on how these adhesives will react inthe presence of radiation doses. Radiation inducedmolecular and mechanical effects [4],[5], and volatileevolution [6], [7], [8], of the bulk silicone have beendiscussed in the literature. While knowledge of thechanges in the chemical and mechanical properties ofthese materials may enable us to make general predic-tions about how the adhesives will perform, furtherexperimentation is necessary in order to determinethe precise functional relationship between adhesionand radiation dose.

In this paper, we focus solely on simulated α ra-diation (in the form of accelerated He++ ions) andits effect on the durability of polydimethlysiloxane(PDMS) elastomer based adhesives. The effect of

varying radiation dose on the adhesive performance,surface morphology, and surface energy are discussed.Of particular interest is the simulated α dose afterwhich the adhesives can no longer generate direc-tional adhesion.

2 Experiments

2.1 Material Preparation

PDMS adhesive pads were prepared using a DowCorning Sylgard 170 elastomer kit and a mold pat-terned via photolithography methods. A detailed dis-cussion of the mold preparation can be found in [1].The Sylgard 170 A and B components were mixedvigorously, vacuum degassed for 1-2 minutes, andpoured under vacuum onto the mold which was thenspun to produce a thin film of the bulk, uncured elas-tomer. The elastomer and mold were then placed inan oven at 85◦C for 15 minutes. The adhesive wasthen manually peeled from the mold and cut into 1cm2 squares for testing. Figure 1 is a SEM of themicro-patterned surface of the adhesive.

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950 1000 1050 1100 1150 1200 1250−2

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0

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Time (ms)

For

ce (

N)

ShearNormal

Figure 2: Load-Pull Data from a single run of an un-irradiated adhesive sample. Preload is generated by ap-proaching the substrate at 45◦ to a depth of 80µm, andcan be seen as the small positive normal force (dashedline) at approximately 1s. The sample is then loaded inshear (solid line) and the adhesion generated by the sam-ple is represented by the negative normal force.

2.2 Radiation

The irradiation took place in the Ion Beam MaterialsLaboratory at the Los Alamos National Laboratory.The samples were attached to a sample fixture withcarbon tape and placed on a sample goniometer in-side the irradiation chamber which was evacuated toapproximately 10−6 Torr. He++ beam at 2.5MeVwas produced by an NEC Alphatross source on the3MV Tandem accelerator to simulate intermediateenergy alpha particles emitted from a purified pluto-nium source and associated daughter products. Thebeam current on target was 20-30nA and dosage wascalculated by integrating the current on the sampleusing a 1000C Brookhaven Instruments Corporationcurrent integrator. The sample and sample holderwere biased to +150V to allow accurate current inte-gration throughout the irradiations. A homogeneousdose across the entire sample surface was performedby rastering the He++ beam. The irradiation timetook between 1-20 minutes of exposure, dependingon dose.

−10 0 10 20 30 40 50−6

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Shear Pressure (kPa)

Nor

mal

Pre

ssur

e (k

Pa)

Figure 3: Limit curve from a single battery of LP testson an un-irradiated adhesive sample. Maximum adhesivepressure achieved is approximately 3.7kPa. Each pointindicates adhesive failure through slipping or detachmentfrom the surface. Note the adherence to Autumn’s fric-tional adhesion model in which adhesion is generated onlyin the presence of shear loading.

2.3 Adhesion Testing

Data were collected on an experimental setup ca-pable of moving the adhesive samples into contactwith a glass substrate along a specified trajectory andloading the adhesive in normal and tangential direc-tions. The experimental setup consists of a three-axis positioning gantry (Velmex, MAXY4009W2-S4and MA2506B-S2.5) capable of 10µm positioning res-olution in the tangential direction and 1µm position-ing resolution in the normal direction. The gantryis responsible for moving the substrate in and outof contact with the adhesive, which is mounted on astationary, six-axis force/torque transducer (ATI In-dustrial Automation, Gamma Transducer SI-32-2.5).The transducer is mounted on a two-axis (roll andtilt) goniometer (Newport, GON40U/L) to allow theadhesive and substrate to be precisely aligned. Testswere preformed by bringing the adhesive into contactwith the substrate along a 45◦ angled trajectory to acertain pre-load depth, defined as the distance pastwhich the tips of the micro-structure initially make

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ConsistentAdhesion

Failing Adhesion

AdhesionSaturation

Figure 4: The maximum adhesive pressure of sampleswith doses ranging from 0-2500kGy as a fraction of theirun-irradiated performance.

contact with the substrate. Once the sample was atthe appropriate pre-load depth it was loaded in shearand then pulled away at a specified angle. Force datawere recorded and filtered by a low-pass, third-orderButterworth filter with f c = 20Hz. Such tests arereferred to as Load-Pull (LP) tests.

Figure 2 shows the results of a single LP test per-formed on an un-irradiated sample with a pre-loaddepth of 80µm and a pull off angle of 10◦. The nor-mal force (dashed line) can be seen to have a smallpositive peak at approximately 1s. This is the pre-load phase of the trajectory. Subsequently, as shearload (solid line) is applied, the normal force dropsbelow zero, indicating that it is producing adhesion.Note that the sample produces adhesion only in thepresence of shear loading, behavior consistent withthe frictional adhesive model proposed by Autumn,et. al [9]. To obtain the adhesion limit curve [10], abattery of LP tests are performed for pre-load depthsranging from 30-80µm in 10µm increments and pull-off angles ranging from 0-90◦ (with respect to thevertical) in 10◦ increments. This curve, which canbe seen in Figure 3, shows the limit of the adhesive’sperformance for a given shear/adhesion load. If aload lies above the curve in force space, the adhesive

0 500 1000 1500 2000 2500 30000

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Dose (kGy)

Shea

r pre

ssur

e (k

Pa)

{

Frictional Adhesion

Embedded Cone Adhesion{

Figure 5: The shear pressure at which samples producemaximum adhesion pressure for various doses shows thetransition between adhesion models. Data points are av-erage values with error bars denoting the minimum andmaximum measured values. It can be seen that at dosesbelow 1500kGy the samples produce maximum adhesionin the presence of a significant shear load, similar to thethe Autumn frictional adhesion model. At and above adose of 1500kGy, the samples very consistently producetheir maximum adhesion in the presence of virtually noshear loading.

will remain attached. Any load below the curve inforce space will cause the adhesive to detach fromthe surface.

Because physical differences between individualsamples as well as minute changes in sample align-ment between batteries may cause variation in ad-hesive performance, an adhesion baseline was estab-lished with each sample prior to irradiation. Eachsample was put through three separate batteries ofLP tests to determine the maximum adhesive pres-sure it was capable of generating. Variation in adhe-sion generation due to misalignment from mountingand un-mounting of the sample over multiple runswas small (<5%) and results from the three batterieswere averaged to produce a reasonable baseline formaximum adhesive pressure. After irradiation, sam-ples are then put through the same adhesion testingprocedure conducted prior to irradiation.

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a b

Shear (kPa) Shear (kPa)

Adh

esio

n (k

Pa)

Adh

esio

n (k

Pa)

0 1 2 3 4 5 6 7−5

0

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Figure 6: Left: The limit curve produced by a flat sample of PDMS. Note the resemblance to the JKR or embeddedcone model. Right: A limit curve produced by a sample of directional adhesive irradiated with a dose of 2500kGy. Atthis dose, all ability to produce directional adhesion has been lost and the sample behaves like a flat piece of PDMS.

3 Results and Discussion

3.1 Adhesion

Post-irradiation testing focused on the ratio of post-to pre-irradiation maximum adhesion production andthe results are summarized in Figure 4. Samples ir-radiated with 50-750kGy maintain the large major-ity (80-95%) of their adhesion generation capability.As radiation doses continue past 750kGy the samplesbegin to lose increasing amounts of adhesion, contin-uing until a dose of 1500kGy. At and above 1500kGyadhesion begins to saturate, i.e. the samples producea constant amount of adhesion, approximately 1kPa,regardless of increased dose. In this region, ratios ofthe post-irradiation adhesion to pre-irradiation adhe-sion become less meaningful due to the fact that thepost-irradiation adhesion values have saturated.

In the saturation region, samples begin to behaveas flat elastomeric adhesives. Their limit curves re-semble the JKR model described in [11] for a roundedelastomeric material contacting a flat surface or thesimpler ”embedded cone” extension of Coulomb fric-tion originally proposed to account for the effects ofadhesion in elastic materials with friction [12, 13].

The transition from frictional adhesion to embed-

ded cone behavior can be seen when examining theshear pressure at which the maximum adhesion isgenerated. Figure 5 shows that the samples followthe frictional adhesion model, i.e. generating maxi-mum adhesion under shear loading, up until a doseof 1250kGy. At 1500kGy and larger doses, maximumadhesion is produced in the presence of close to zeroshear load, a characteristic of the JKR or embeddedcone models.

In these models, adhesive capacity is greatest withzero shear load and decays as shear load is appliedin either direction. A similar limit curve, producedby conducting the same adhesion testing on a flatpiece of PDMS, can be seen in Figure 6a. As dosesincrease past 1250kGy, the limit curves produced bythe samples tend increasingly toward the embeddedcone shape. Figure 6b shows the limit curve from asample irradiated with 2500kGy. The resemblance ofthis curve to the embedded cone model shows the ex-tent of the radiation induced damage as the sampleshave lost all ability to generate directional adhesion,producing curves similar to that of flat PDMS.

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Figure 7: SEM of a sample irradiated with 1.125MGy.No obvious damage is visible in the wedges or the back-ing layer at this dose. Adhesion generation, however, isapproximately 50 percent of the pre-irradiation value.

3.2 Surface Morphology and SurfaceEnergy

Surface properties of the irradiated samples were ex-amined via SEM. The surface morphology of the ad-hesive micro-structures for various doses is shown inFigures 7 and 8. Figure 7 shows the resulting surfacemorphology for doses up to 1250kGy. It can be seenthat there is no obvious surface damage. However,further irradiation begins to produce more noticeablechanges. At doses above 2MGy brittle cracking be-comes evident in the backing layer of the adhesive.As doses increase, the cracking becomes more signifi-cant. Figure 8 is an SEM of a sample irradiated witha dose of 2.5MGy. The fractures are present acrossthe entire sample, some of which are large enough tobe seen with the naked eye. In addition, the embrit-tling effect of high radiation dose is evident given thewidespread brittle cracking of the micro-structure.

To gain insight on possible changes in surface en-

Figure 8: SEM of a sample irradiated with 2.5MGy.Widespread brittle cracking of the micro-structure ispresent along with significant cracking in the base layer ofPDMS. Samples irradiated with this dose no longer con-form to the frictional adhesion model and demonstrateembedded cone style adhesion when tested.

ergy induced by the irradiation, water droplet exper-iments were performed. Samples of flat PDMS wereirradiated at the same doses as the micro-structureadhesives. The samples were then tested with a Sur-face Electro Optics Phoenix 300 Contact angle an-alyzer to examine the contact angle of a droplet ofwater with the flat PDMS. To account for any pos-sible localized irregularities, three experiments wereconducted per sample on different areas of the sam-ple. The averaged results for contact angle versusdose are shown in Figure 9.

Contact angles for un-irradiated PDMS samplesbegin at close to 94◦ and gradually increase with in-creasing radiation dose. The rate of increase begins toslow after 750kGy and the contact angle values beginto stabilize between 98◦-99◦. Increasing sample hy-drophobicity indicates that the surface energy of theirradiated PDMS is decreasing. Assuming contact

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−500 0 500 1000 1500 2000 250093

94

95

96

97

98

99

100

Dose (kGy)

Con

tact

ang

le (

deg)

Figure 9: Water droplet contact angles for samples of flatPDMS irradiated with doses up to 2MGy. The increasein contact angle with increasing dose suggest a progres-sive decrease in the surface energy of the PDMS due toirradiation.

angle and surface energy are roughly correlated, thisdata is in agreement with the adhesion experiments.A decrease and subsequent stabilization of surface en-ergy would result in a proportional decrease and sta-bilization of adhesion, much like the trend seen in theexperimental adhesion results.

3.3 Discussion

Evaluating the appropriateness of the adhesive for ra-diological environments requires an understanding ofthe typical doses seen in these environments. A hypo-thetical and quite conservative usage scenario mightinvolve deployment of the adhesives in a radiologi-cal environment until they have absorbed a dose ofapproximately 750kGy, the dose at which they firstbegin to deviate significantly from their un-irradiatedbehavior. The highest radiation environment definedby the Los Alamos National Laboratory and the DOEis a very high radiation area, an environment in whicha dose of over 500 rad an hour would be absorbed.At a conversion rate of 1 rad to 10 mGy this doserate is equivalent to 5Gy per hour. In the given us-age scenario, we will assume a dose of 10Gy per hour

for a more conservative estimate. At this rate, theadhesive would not need to be replaced for greaterthan eight and a half years. This factor, combinedwith the high re-usability of the adhesives shown in[1], make the adhesives an attractive option for radio-logical applications involving high α radiation levels.

4 Conclusions

Polymer based directional dry adhesive were irradi-ated with He++ ions to simulate the effects of highα radiation dose. The adhesives have proved to berobust in the presence of significant amounts of radi-ation, making them a competitive candidate for ap-plications in nuclear and radiological environments.Samples were shown to maintain a large majority oftheir adhesive capabilities at doses as high as 750kGy.Doses within the 750-1500kGy range cause the adhe-sives to begin to lose their ability to generate direc-tional adhesion. Eventually, at a dose of 1500kGy,the adhesives lose all ability to generate directionaladhesion and behave like a pressure sensitive adhe-sive. At this point, adhesion generation ability hassaturated to a value of around 1kPa. SEM exam-ination of the surface morphology of the adhesivesshowed significant embrittlement and cracking at andabove the 2MGy dose. Water droplet contact angletesting suggests that irradiation has altered the sur-face chemistry enough to produce changes in surfaceenergy, a likely cause of adhesive performance degra-dation. Given the robustness shown by the mate-rial and predicted long service life, these adhesivespresent a promising opportunity for new applicationsin high radiation environments.

Additional investigation is necessary to determinethe effects of different types of ionizing radiation onpolymer based adhesives. Future work will involvedetermining the effects of high-energy photons (in theX- and gamma ray spectrum) on adhesion for applica-tions with different types of special nuclear materialas well as space applications.

Acknowledgement 1 The authors would like tothank the staff at the Ion Beam Materials Labora-tory at Los Alamos National Laboratories, New Mex-

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ico, USA, and the Stanford Nanofabrication Facility,California, USA, for the use of their facilities.

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