Investigations of PCM microcapsules with paraffin filling by positron lifetime spectroscopy

1. Investigations of PCM microcapsules with paraffin filling by positron lifetime spectroscopy Bożena Zgardzińska Department of NuclearMethods Institute of Physics, Maria Curie-SklodowskaUniversity, Lublin

2. Outline Samples Selectedphysicalpropertiesof alkanes PALS measurements for alkane, polymer and microcapsules Conclusions

3. Samples PCM – PhaseChangeMaterial chemical preparation Polymer (polysiloxane) Alkane (n-eicosane) Microcapsules The investigationswere carried out on three materials

4. n-alkanes – generalinformation n-alkanes (paraffins)– saturatedhydrocarbon chains in linear form H C liquid rotational phases molecularformula TEMPERATURE, K CnH2n+2 rigidphase Alkanes phase diagram (temperature, carbonchainlength) NOTE:Odd-numbered alkaneswith n≤9 and even-numbered with n≤22 do not show the existence of a rotator phases. C20H42 CHAIN LENGTH (n) R.R. Nelson, W. Webb, J.A. Dixon, J. Chem. Phys. 33, no. 6 (1960). M. Marconcelli, S. P. Qi, H. L. Strauss, R.G. Snyder, J. Am. Chem. Soc. 104, 6237 (1982); M. Marconcelli, H. L. Strauss, R.G. Snyder, J. Chem. Phys. 82, 2811 (1985) F. Guillaume, J. Doucet, C. Sourisseau, A.J.Dianoux, J. Chem. Phys. 91, 2555, (1989) T. Goworek, R. Zaleski, J. Wawryszczuk, Chem. Phys. Letters 394, 90 (2004)

5. n-alkanes – generalinformation Lamellarstructure Rigidphase The number of carbon atoms in the chain          odd even distance of layers molecular orthorombic, monoclinic, triclinic

6. n-alkanes – generalinformation conformers Rotator phases Schematicpresentation of molecules 109,50 all-trans end-gauche double-gauche kink hexagonal, pseudohexagonal liquid

7. Freeannihilation e+ 2=0,35 ns     Alkanes – PALS measurement In the PAL spectra in neatalkaneswe candistinguishusuallythreecomponentscorresponding to: The component related to o-Ps annihilationis most sensitive to the changes in the crystalstructure. Positronium annihilation p-Ps 1=0,125 ns e- e+ Ps pick-off process o-Ps 3=0,710 ns  

8. Freevolumesize To determine the size of freevolume we canuse the Tao-Eldrup model: τ – o-Ps lifetime R – void radius Δ=0,166 nm – empiricalconstant And the size of freevolume in liquid, we use the bubble model.The radius R of a bubble is determined by the minimum of energy: EPs(R) is the energy of Ps in the potential well σ – the surface tension p– the external pressure

9. LED power supply LED PUMP sample and source thermocouple heater AC heater THERMOREGULATOR Cu Rs-232 LN2 COMPUTER Measuringprocedure Temp.,oC „cold finger” +60 roomtemperature -150 0 24 26 …. Time, h

10. C20atlowtemperature Growthof intensity I3-typical for purealkane. Temperature -150oC The growthof intensity I3can be described by an exponential curve: where: I30- the initial value of the intensity; I3tr - maximumincrease in intensity due to Ps formation on trapede- timeconstant C20H42 θC20≈8,3 h

11. C20 – temperaturedependence The value of lifetimeτ3 in rigid and liquidphasetypicalfor alkanes Stepwisechange of τ3 and I3at the melting temperature The area of thermalemptying of the traps C20H42

12. Polysiloxane – PALS results Twodifferentlifetimeτo-Psmeanstwodifferentfreevolumes Only one of themchanges with temperature Lack of a stepwisechange in lifetimes and intensitiesat37,5oC At -150oC the lifetimes and intensitiesarestable in time The intensities of both o-Ps componentsaremoreor less stable

13. Distinguishabilitycomponents Blue – n-eicosane Red, green - polysiloxane At the lowtemperaturerangetwo o-Ps lifetimesτ3_C20and τ3_PSXarealmostidentical Above 38oC the lifetimesτ3_C20and τ4_PSXarequitesimilartoo

14. Aboutmicrocapsules properties The shellmaterialdeterminesphysicalproperties (size, strength, toxicity, reactivity, …) The corematerialdeterminesthermalproperties (operatingtemperature, capacity to accumulation and emissionenergy, …) application As phasechange materials (PCM) for thermoregulation, thermalenergystorage and cooling (buildingindustry, textile, …) morphology core-shellmulticorematrix

15. SEM microcapsules image moreor less similar in shape to a sphere grainsize (average) isabout 11 μm W. Fortuniak, Center of Molecular and MacromolecularStudies, PolishAcademy of Science, Łódź

16. Micocapsules – PALS results In microcapsuleswe seetwo o-Ps components Stability in timeat -150oC No increase of intensitymeansthat the electrontrappingpresent in the purealkanedoes not appearwhen the n-ecosaneisenclosedwithin the microcapsule Therecan be tworeasons of this: The impurities in the sample (itsresults in a reductionortotallack of effect) The crystalstructure of n-eicosane in microcapsuleisdifferentthanthat of macroscopic samplesize (the electrontrapsare not created)

17. Micocapsules – PALS results Twodifferent o-Ps components (τ3_PSX/I3_PSXblue and τ4_PSX/I4_PSXyellow) Lifetimes vary smoothly with temperature The intensities change abruptly with temperaturetwice: at 38oC and 42oC Between -30oC and 10oC the I3_PSXincreases and then falls significantly

18. Phasetransition The intensities change abruptly with temperaturetwice: at 38oC and 42oC Between38oC and 42oC the intensity I4isconstant,thus we suppose the appearance of therotator phase. The melting point isshifted by 4.5 K comparedto the pure alkane.

19. Size of freevolume Below38oC the τ3_PSX and τ3_C20arevisible as one component τ3; similarlyabove 42oC τ4_PSXand τ3_C20form ther component τ4. There are two types of free volume. τ4_PSX & τ3_C20 Twodifferent o-Ps components (τ3_PSX/I3_PSXblue and τ4_PSX/I4_PSXyellow) τ3_PSX &τ3_C20 Lifetimes vary smoothly with temperature -150oC-10oC 45oC60oC The intensities change abruptly with temperaturetwice: at 38oC and 42oC Between -30oC and 10oC the I3_PSXincreases and then falls significantly R Ps

20. Mass fraction of substrates Twodifferent o-Ps components (τ3_PSX/I3_PSXblue and τ4_PSX/I4_PSXyellow) Lifetimes vary smoothly with temperature -150oC-10oC 45oC60oC The intensities change abruptly with temperaturetwice: at 38oC and 42oC Between -30oC and 10oC the I3_PSXincreases and then falls significantly

21. Mass fraction of substrates Twodifferent o-Ps components (τ3_PSX/I3_PSXblue and τ4_PSX/I4_PSXyellow) Taking the I3 and I4values for microcapsules and assuming the ratio as found in PALS measurements for purepolymerattemperature we obtain: The intensities change abruptly with temperaturetwice: at 38oC and 42oC Lifetimes vary smoothly with temperature -53oC 52oC Between -30oC and 10oC the I3_PSXincreases and then falls significantly 0,7μm the PSX component is: 4,8μm 38%±2% 42%±2%

22. Mass fraction of substrates Twodifferent o-Ps components (τ3_PSX/I3_PSXblue and τ4_PSX/I4_PSXyellow) Anothermicrocapsule for textileindustry with n-eicosanefilling relativelylargeshellthickness The intensities change abruptly with temperaturetwice: at 38oC and 42oC Lifetimes vary smoothly with temperature Between -30oC and 10oC the I3_PSXincreases and then falls significantly 0,7μm the component of PSX was evaluated: 4,8μm 38%±2% 42%±2%

23. Micocapsules – SEM image A check by SEM microscope:

24. Conclusions PALS technique can be successfully used to study thephase change materials(e.g. microcapsules). Similarity of thePALS results obtained for the microcapsules to the results obtained for materials composingthem (alkane and polymer)isobserved. It should be noted that, in the microcapsules, in contrast to the pure alkane: - no electrontrapping effectatlowtemperature; - meltingpoint is preceded by the rotational phase in the range of 4 K; - meltingpoint is shifted to a higher temperature by4.5 K. Basingon the PALSresults, wecan: - estimate the percentagecontentof components building the microcapsules (here ~40% belongs to the polymer); - notice the morphology of the microcapsulesdifferingfrom that declared by the manufacturer.

25. Thankyou for yourattention