Meijer, H.E.H.

Schaake, R.P.

Vellinga, W.P.

English

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 Effect of molecular weight on friction of PolyStyreneCitation for published version (APA):Schaake, R. P., Vellinga, W. P., & Meijer, H. E. H. (2002). Effect of molecular weight on friction of PolyStyrene.Poster session presented at Mate Poster Award 2002 : 7th Annual Poster Contest.Document status and date:Published: 01/01/2002Document Version:Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)Please check the document version of this publication:• A submitted manuscript is the version of the article upon submission and before peer-review. There can beimportant differences between the submitted version and the official published version of record. Peopleinterested in the research are advised to contact the author for the final version of the publication, or visit theDOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and pagenumbers.Link to publicationGeneral rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            • Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, pleasefollow below link for the End User Agreement:www.tue.nl/taverneTake down policyIf you believe that this document breaches copyright please contact us at:openaccess@tue.nlproviding details and we will investigate your claim.Download date: 04. Oct. 202312/department of mechanical engineering PO Box 513, 5600 MB Eindhoven, the NetherlandsEffect of molecular weight on friction of PolyStyreneR.P. Schaake†‡, W.P.V ellinga†, H.E.H.Meijer†Eindhoven University of Technology, Department of Mechanical Engineering†DutchPolymerInstitute, Eindhoven‡TNOIndustrialTechnology,EindhovenIntroductionSingle asperity friction measurements are an effectivemethod to study the origin and phenomenology of frictionand wear. Using well-defined micron-sized contacts one canprobe friction and wear mechanisms that are relevant formacroscopic contacts [1]. Our aim is to relate microscopicfeatures of polymers to their friction and wear behaviour.In this poster we present measurements on two grades ofmonodisperse PolyStyrene (PS). Mechanical properties of PSare well understood [5] which makes it a good model system.Material and methodsPS samples with a molecular weight of 56 kg/mole and 966kg/mole, and a polydispersity of 1.05 and 1.15 respectively,were used. Pressed samples of these materials were flat-tened by embossing with a silica wafer above Tg. The result-ing surfaces can be considered smooth for our purpose. Forthe single asperity friction measurements the Lateral ForceApparatus described in [4] was used. This apparatus hasrecently been improved and is now capable of spanning 6decades in velocity. A spherical diamond tip with a radiusof 5 µm was mounted on a cantilever with a normal and lat-eral stiffness of 243 and 54 N/m respectively. "Start-stop"experiments were performed, in which the contact was pre-pared by bringing it into a reproducible sliding situation. Tipmotion was then halted at time t=0 for a certain period, afterwhich the motion was resumed. Lateral force and (vertical) z-displacement are measured continuously from t=0 onwards.ResultsAt higher loads both materials showed an increase in frictionin a certain sliding velocity range. The effect of molecularweight on the velocities at which this phenomenon occurredwas consistent with trends in the work by Tanaka et al. [2]who found a shift of a friction peak to lower velocities withincreasing molecular weight.10−210−110010110210300.511.522.5v (µm/s)Ff (mN)FN = 0.7 mNFN = 1 mNFN = 1.7 mNFN = 3 mN10−110010110210300.511.522.5v (µm/s)Ff (mN)FN = 0.01 mNFN = 0.7 mNFN = 1 mNFN = 2.7 mNFigure 1 Average kinetic friction force as a function of velocity onboth the high (right) and low molecular weight material (left).The friction measured on the high molecular weight (MW)material was usually lower than on the low molecular weightmaterial. It is reasonable to assume that a major cause ofthis difference is the size of the contact area.The right graph of figure 2 shows the diamond tip sinks intothe material when standing still. This process is slower forthe high MW material (red) than for the low MW material(blue). After 10 seconds the tip had already sunken 3 µminto the low MW material. The high molecular weight mate-rial sank only 1 µm in the same amount of time at a higherload. From this it could be concluded the contact area wasgrowing faster in the lower MW material.10−210−11001011021031.31.41.51.61.71.8t (s)Ff (mN)10−210−1100101102103−10−505t (s)∆z (µm)Figure 2 Friction and horizontal displacement from t=0 till just be-yond the friction peak. ∆z=0 at t=0. v= 2.5 µm/s. Red line: highMW PS Fn = 2.5 mN. Blue lines: low MW PS, Fn = 2.2 mN.During the period the motion was stopped, the friction forcerelaxed. Upon the subsequent resumption of motion, the tiphad to climb out of the pit it had just created, causing a peakin friction.DiscussionThe lowMW PS has a reduced Tg. This reduction is about 5 K,calculated from a fit through values found in literature. Thethermal mobility of the lowMW chain is therefore higher thanfor the high MW chain, resulting in easier plastic deformationwith decreasing Tg or MW. The easier plastic deformation willlead to a larger contact area.The rise in friction can also be ascribed to the glass transi-tion. The increase in the loss modulus causes an increase infriction. Nanofriction measurements by Wang et al. [3] showthat the friction peak associated with Tg becomes more pro-nounced with increasing load. They explain the presence ofthe peak with the presence of a wedge in front of the tip. Thiscould mean that the appearence of the friction peak in ourexperiments is associated with a transition from a ploughingdeformation without, to one with wedge formation. Charac-terisation of the material used will have to be carried out tobetter understand the mechanisms leading to the observedphenomena.References:[1] YANG, A.C.-M. AND WU, T.W.: J. Pol. Sci. B. 35, 1997, pp. 1295[2] TANAKA, K. ET AL.: Macromolecules30, 1997, pp. 6626[3] WANG, X.P. ET AL.: Langmuir 30, 2002, pp. 7066[4] HENDRIKS, C.P. AND VELLINGA, W.P.: Rev. Sci. Instr. 71, 2000, pp. 2391[5] VAN MELICK, H.E.H.: Deformation of polymer glasses (Technische Uni-versiteit Eindhoven, 2002)

Effect of molecular weight on friction of PolyStyrene

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Effect of molecular weight on friction of PolyStyrene

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