The process of ageing in rubbers requires monitoring over long periods (days
to years). To do so in non-conducting rubbers, small amounts of carbon-black
particles were dispersed in a fractal network through the rubber matrix, to
make the rubber conducting without modifying its properties. Continuous
monitoring of the resistance reveals the structural changes around the glass
and melting transitions and especially details about the hysteresis and ageing
processes. We illustrate the method for the semicrystalline polymer
poly(dimethylsiloxane) (PDMS).Comment: 4 pages, 4 figure

Brom, H. B.

Magis, J. G.

Michels, M. A. J.

Romijn, I. G.

van der Vleuten, M.

English

arXiv

The aging effects around the glass and melting transitions in poly(dimethylsiloxane) were investigated. Small amounts of carbon-black (CB) particles were dispersed in a fractal network in nonconducting rubbers for monitoring the structural changes on small length scales due to aging. Long time constants in the relaxation were observed, even above the melting temperature, which indicated that the sample was not in thermal equilibrium. Continuous monitoring of the resistance revealed the structural changes around the glass and melting transitions and details about the hysteresis and aging processes

Brom, H.B.

Romijn, I.G.

Magis, J.G.

Michels, M.A.J.

NARCIS 

Aging effects around the glass and melting transitions in poly(dimethylsiloxane) visualized by resistance measurements [erratum in Applied Physics Letters 85 (16) 3638-3638]

H. B. Brom

I. G. Romijn

J. G. Magis

M. van der Vleuten

M. A. J. Michels

Crossref

Aging effects around the glass and melting transitions inpoly(dimethylsiloxane) visualized by resistance measurements

Pure OAI Repository

 Aging effects around the glass and melting transitions inpoly(dimethylsiloxane) visualized by resistance measurements[erratum in Applied Physics Letters 85 (16) 3638-3638]Citation for published version (APA):Brom, H. B., Romijn, I. G., Magis, J. G., van der Vleuten, M., & Michels, M. A. J. (2004). Aging effects around theglass and melting transitions in poly(dimethylsiloxane) visualized by resistance measurements [erratum inApplied Physics Letters 85 (16) 3638-3638]. Applied Physics Letters, 85(4), 570-572.https://doi.org/10.1063/1.1776334DOI:10.1063/1.1776334Document status and date:Published: 01/01/2004Document 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. 2023Aging effects around the glass and melting transitions inpoly (dimethylsiloxane ) visualized by resistance measurementsH. B. Brom,a) I. G. Romijn, and J. G. MagisKamerlingh Onnes Laboratory, Leiden University, P. O. Box 9504, 2300 RA Leiden, The NetherlandsM. van der Vleutenb) and M. A. J. Michelsc)Group Polymer Physics, Technische Universiteit Eindhoven, P.O. Box 513, 5600 MB Eindhoven,The Netherlands(Received 26 January 2004; accepted 2 June 2004; publisher error corrected 13 September 2004)The process of aging in rubbers requires monitoring over long periods(days to years). To do so innonconducting rubbers, small amounts of carbon-black particles were dispersed in a fractal networkthrough the rubber matrix, to make the rubber conducting without modifying its properties.Continuous monitoring of the resistance reveals the structural changes around the glass and meltingtransitions and especially details about the hysteresis and aging processes. We illustrate the methodfor the semicrystalline polymer poly(dimethylsiloxane). © 2004 American Institute of Physics.[DOI: 10.1063/1.1776334]The properties of amorphous polymers(AP) oftenchange in time below the rubber-glass transition temperatureTg.1 For T,Tg the material can be regarded as a solidifiedsupercooled liquid of which the volume, enthalpy, and en-tropy are larger than in the equilibrium state.2,3 The gradualapproach to equilibrium affects various properties of the sys-tem (not only mechanical, like elasticity, creep-, and stress-relaxation rates, but also dielectric and optical4), and is calledphysical aging to distinguish it from chemical influences. Formany plastics or rubbers the aging range includes the tem-peratures for practical application. The same processes areplaying a role in semicrystalline polymers(SCP), like poly-(dimethyl siloxane) (PDMS), but in addition the presence ofvery thin crystal lamellae makes them inhomogeneous, giv-ing a strong resemblance to composites.3 But unlike theamorphous rubbers, semicrystalline polymers turn out to ageat temperatures above theirTg. Usually these changes aremonitored via mechanical probes(elasticity measurements),or in case of polar constituents also spectroscopically. Fornonconducting nonpolar crystalline and amorphous polymersthe addition of carbon-black(CB) particles has been used tomake these material conducting, and the conductivity andnoise data were shown to have marked relaxation effects.5Here we revisit this approach and demonstrate that properlydispersed small amounts of conducting CB particles form afractal network through the polymer, of which the conduc-tivity properties can be well understood.6 The CB particlesdo not influence the rubber and glass transition in an essen-tial way and hence the time evolution of the conductivity isrepresentative for changes in the polymer itself. We illustratethe method for PDMS. The data turn out to be not only avery sensitive detector for subtle changes due to the rubberand glass transitions as found previously,5 but also demon-strate that continuous monitoring of the resistance gives in-sight in the long time constants of the aging process.PDMS has aTg of the amorphous phase around 150 Kand a melting transition temperatureTm of the crystallinephase at 222 K1,7. This difference of 70 K makes the mate-rial very convenient to study the influence of both transitionson the polymer structure below room temperature. ThePDMS chains have a typical length of hundred repeat units,i.e., about 100 nm, and are crosslinked at the end points only.As conducting filler material CB is attractive as it is knownto form fractal distributions8 with percolation thresholds ofthe order of a few tenths of a percent or lower in volumefraction, when dispersed under the proper circumstances.6The CB (Printex XE-2 from Degussa, Probst) in the presentstudy is quite similar to the Ketjenblacks used previously.6The primary particles are hollow half spheres with a typicaldiameter of 30 nm, which in the gas phase have formed ag-gregates of 100 nm. The sample quality was checked by con-focal scanning and optical microscopy.As shown by Struik,3 in most of its aspects aging in APand SCP can be explained in a straightforward way fromfree-volume conceptsVfd, see Fig. 1. The segmental mobil-ity of the polymer chains in a dense system is primarilydetermined by the free volume, and above a critical degree ofpacking steeply falls to zero. Changes inVf are broughtabout by a redistribution of empty spaces, the rate of whichis determined by the segmental mobility. So,Vf determinesthe mobility, while the mobility determinesdVf /dt. Sincethis is a closed loop, it implies that the volume relaxationprocess is basically nonlinear. Below the glass temperature,the AP and SCP will behave similarly, but as mentioned al-ready, unlike AP the SCP turn out to age above theirTg, forthe following reason. Let us consider a SCP aboveTg. Sincethe amorphous polymer chains are connected to the crystal-a)Electronic mail: brom@physics.leidenuniv.nlb)Also project student at: Océ Technologies, Venlo, The Netherlands.c)Also at: the Dutch Polymer Institute Eindhoven, The Netherlands.FIG. 1. Relation betweenTg andVf for an AP. During aging, which typicallyoccurs belowTg, Vf is reduced.APPLIED PHYSICS LETTERS VOLUME 85, NUMBER 4 26 JULY 20040003-6951/2004/85(4)/570/3/$20.00 © 2004 American Institute of Physics570Downloaded 08 Nov 2007 to 131.155.108.71. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsplites, the segmental mobility near the particle surface will bereduced. Only far away from the crystallites, the mobilityand other properties of the rubber matrix will be equal tothose of the amorphous rubber. The glass transition of a SCPwill be broadened on the high temperature side. By aging,which will now occur in a wide range of temperatures aboveTg, the amorphous regions will become smaller(a clear shiftin Tg will only be seen if the amorphous phase is completelydisturbed). It means that there are three regions to discrimi-nate: aboveTm the polymer is completely amorphous andbelow Tg it has become quite rigid. In between these twotemperatures the rubber has the possibility of(re)crystalliza-tion and especially the crystallization will crucially dependon the thermal history.The electrical conduction in systems with a low concen-tration of conducting particles dispersed in a nonconductingmatrix, is well documented and occurs via Mott variable-range hopping. TheT-dependent conductivity can be writtenas s=s0 expf−sT0/Tdgg, where the value ofg depends ondetails of the filler distribution, the filler density of states,and the importance of Coulomb interactions.6,8We performed two kinds of measurements: temperaturescans(dependence of the resistanceR on T and cooling/warming ratedT/dt) and time scans(time evolution ofR atconstantT). In the latter we compared results of a “down-quench” Troom↓T with those of a “down-up-quench”Troom↓10 K↑T, where we discriminate between three re-gimes for the final temperature:T,Tg, Tg,T,Tm andTm,T. The (four-points) data were also taken with pulsedfields with different field strengths to check for ohmic behav-ior while avoiding possible heating. Data taken at room tem-perature as function of filler volume fractions are well rep-resented by sspd=s0sp−pcdt with t=2.3±0.2 and pc=0.0033±0.0004. In Figs. 2 and 3(a) we show the resistivityas function of temperatureT.R as function of T. When we compare cooling-down andwarming-up curves, the data only merge below 80 K andabove 320 K, and show strong hysteresis in between. Appar-ently the region between 320 and 80 K is affected by themelting and rubber–glass transitions, which from differentialscanning calorimetry data for all studied rubbers were foundto be around 220 and 160 K, respectively, in agreement withthe literature.1 Dynamical mechanical analysis(DMA ) of thesame systems revealed the rubber–glass transition as a peakin tan d at 154 K and the melting temperature around 220 Kby a loss in viscosity. Especially because the resistance datashow the same pattern independent of CB filler fractions, weconclude that at these small concentrations the CB particlesdo not change the polymer properties in an essential way andthe aging effects ins, which are in general due to subtlechanges in the particle network, are caused by the polymermatrix. As in previous CB samples,6 sufficiently far belowthe glass transition the conductivity is well described bys=s0 expf−sT0/Tdgg. From fits tos between 20 and 80 K forthe 1.5% sample we findg=0.68 andT0=50±7 K. For theinterpretation we refer to Ref. 6, where these values(whichcan be seen as a measure for the quality of the dispersion)were argued to be characteristic for percolation on a fractalcarbon network.R as function of time t. For Tm,T,300 K, the resis-tance after a down-quench remained constant(measured upto 105 s, not shown). The same is true forT,Tg. Strong timedependencies were seen forTg,T,Tm, for which a typicalresult is shown in Fig. 3(b). Down-up-quench results with afinal temperature belowTg do not show noticeable changes,which is also the case for the down-quench data. ForT.Tmthe similarity between the two kinds of measurements is ab-sent and long-time tails become visible after heating, see Fig.4. In the regionTg,T,Tm the time constants are againquite long, see Fig. 3(b).How can we explain the overall features of the resistivityversus temperature and what is their connection with themelting and glass transition?FIG. 2. TheT dependence of the resistivityr for various filler concentra-tions p. The value ofp is seen to affect only the absolute resistance values.The data while cooling down(a) or heating up(b) are clearly different, seealso Fig. 3.FIG. 3. (a) Hysteresis inR vs T for p=0.15. Cooling-down(↓) andwarming-up(↑) rates were a few degrees Kelvin/min. Data taken with twoand four-contacts(separate current and voltage contacts) were identical(dif-ferences at 200 K in the↓ data are due to differences in cooling rate).Around 220 K the maximal↑ and ↓ resistances differ by more than twoorders of magnitude.(b) Growth and decrease of the normalized resistancevs time close toTm during cooling down from 300 K, respectively, warmingup from 10 K. The resistances are both normalized to 0.437 MV, the valuereached after stabilizing the temperature for 2 days at 220 K after warmingup from 10 K.FIG. 4. (a) Increase and decrease ofR vs t after a down-quench from 300 to205 K. (b) Decrease ofR vs t after first cooling down from room tempera-ture to 15 K and thereafter warming up to 326 K.Appl. Phys. Lett., Vol. 85, No. 4, 26 July 2004 Brom et al. 571Downloaded 08 Nov 2007 to 131.155.108.71. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jspThe increase inR with cooling is a general feature forhopping transport, where the available phonon energy playsan important role. The melting point remains almost unno-ticed upon cooling the sample(typical cooling rate of1–3 K/min). This is likely because formation of crystallitesis delayed and hence does not affect the distances in thepercolation path. Things changes around 190 K, where crys-tallization seems to nucleate and starts to change the CBdistances and the segmental mobility. By this process someamorphous regions might even pass through the glass transi-tion (see the following), by which the structural reorganiza-tion process will be slowed down appreciably. TheR versusT curves below 80 K can be explained in the same way as inRef. 6.During heating from about 10 K the warming-up resis-tance curves remain rather smooth above 80 K, deviatingstrongly from the cooling down data. Some reorganizationseems to occur, a process that becomes more important be-tweenTg andTm. Because of the increase in chain mobilityfurther crystallization is possible which leads to more freevolume, etc. We believe that this process is at the heart of theresistance anomaly around the melting temperature of220 K. AboveTm the crystals melt and the whole polymersystem becomes amorphous. Note that, in contrast, in theDMA data mainly the rubber–glass transition shows up dur-ing warming up. This phenomenon might find its explanationin the specific character of the conductance data, which arevery sensitive to changes on small(atomic) length scales,while DMA and other methods that are applied from outsideprobe mean values where longer distances are involved.What more can we learn by looking at the time depen-dence ofR? The fact that after a down-quench toT aboveTmthe resistance remains stable is no surprise as no isothermalvolume relaxation or crystallization is expected: the systemis still in thermodynamic equilibrium. The effect of thedown-quench to 205 K is remarkable[Fig. 4(a)]. First theresistance grows and thereafter it decreases with a muchlonger time constant. It also appears that the closer we go tothe melting temperature the longer the initial time constantbecomes. Most likely the increase in resistance is due to theformation of crystalline regions. This process will redistrib-ute the CB particles and create some longer distances andhence so-called “hard hops” in the percolation path throughthe sample, which will increase the resistance. In the neigh-borhood of the crystalline material the chain mobility isreduced—some amorphous regions are passing through theirglass transition and therefore we are now in the broadenedTgregime, with slow isothermal volume relaxation. The re-moval of free volume will bring the CB particles closer to-gether and lead to a decrease inR.In summary, dispersing small amounts of CB particles ina fractal network in nonconducting rubbers opens a conve-nient way for continuously monitoring the structural changeson small length scales due to aging. Even above the meltingtemperature we observe long time constants in the relaxationand therefore have to conclude that the sample is still not inthermal equilibrium. Only by waiting for several days, thehysteresis loop seems to close again.The authors acknowledge Océ Technologies for provid-ing the samples and hospitality(M.v.V.), and Sander Smitand Iulian Hulea for participation in the measurements. Thework of I.G.R. forms part of the research programme ofFOM-NWO.1G. R. Strobl,The Physics of Polymers, 2nd ed.(Springer, Berlin, 1997).2I. M. Hodge, Science267, 1945(1995).3L. C. E. Struik,Physical Aging in Amorphous Polymers and Other Mate-rials (Elsevier, Amsterdam, 1978); Polymer 28, 1521 (1978); 28, 1534(1978); 29, 1347(1978);4H. Cheun, B. Tanto, W. Chunwaschirasirl, B. Larson, and M. J. Winokur,Appl. Phys. Lett.84, 22 (2004).5C. Klason and J. Kubát, J. Appl. Polym. Sci.19, 831 (1975).6L. J. Adriaanse, J. A. Reedijk, P. A. A. Teunissen, H. B. Brom, M. A. J.Michels, and J. C. M. Brokken-Zijp, Phys. Rev. Lett.78, 1755(1997); D.van der Putten, J. T. Moonen, H. B. Brom, J. C. M. Brokken-Zijp, and M.A. J. Michels,ibid. 69, 494 (1992).7T. Dollase, H. W. Spiess, M. Gottlieb, and R. Yerushalmi-Rozen, Euro-phys. Lett. 60, 390 (2002).8Fractals and Disordered Systems, edited by A. Bunde and S. Havlin(Springer, Berlin, 1991).572 Appl. Phys. Lett., Vol. 85, No. 4, 26 July 2004 Brom et al.Downloaded 08 Nov 2007 to 131.155.108.71. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Brom, HB Hans

Romijn, IG

Magis, JG

Vleuten, M Maarten  van der

Michels, MAJ Thijs

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