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Controlled Polymer Grafting from Nanoparticles for the Design of Dielectric Nanocomposites
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology. (Ympgruppen)ORCID iD: 0000-0002-3310-9964
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The interest for polymeric nanocomposites has rapidly grown during the last decades, fuelled by the great potential and superior properties of nanoparticles (NPs). The production volumes of commercial NPs have increased exponentially during the last ten years, and the quality has been significantly improved. The aim of this study was to design polymer grafted commercially available metal-oxide NPs, and graphene oxide (GO), to develop isotropic (homogeneous) and anisotropic (heterogeneous) polymer nanocomposites for dielectric applications. The motivation was to formulate functional insulation materials for compact components in future power-grid systems using high-voltage direct-current (HVDC) or high-voltage alternating-current (HVAC), and to fabricate responsive sensor materials for monitoring e.g. temperature and voltage fluctuations in so called “Smart Grids”.

Aluminium oxide (Al2O3), zinc oxide (ZnO) and reduced GO (rGO) NPs were modified with sparse polymer grafts via a controlled “covalent route” and were mixed with silicone (PDMS) or polyethylene matrices (EBA and LDPE) commonly used in HV-cable systems. The graft length and the graft-to-matrix compatibility were tailored to obtain nanocomposites with various self-assembled NP-morphologies, including well-dispersed, connected and phase-separated structures. The graft length was used to adjust the inter-particle distance of nanocomposites with continuous morphologies or connected (percolated) NPs. It was found that nanocomposites with percolated NPs and short inter-particle distances exhibited 10-100 times higher conductivity than the unfilled (neat) polymer, or displayed a rapid non-linear increase in conductivity (~1 million times) with increasingelectric field, while well-dispersed NPs with long inter-particle distances exhibited 10-100 times lower conductivity (i.e. higher resistivity) as an effect of their trapping of charge carriers. These tunable and functional properties are desirable for HV-insulation, field-grading applications, and flexible electronics.

In addition it was shown that GO modified with dense polymer grafts via a “physisorption route” formed suspensions with liquid crystals, or matrix-free GO-composites with well-dispersed GO in isotropic or nematic states. These materials were reinforced by the GO, and exhibited elevated glass transition temperatures and a rapid thermo-responsive shape-memory effect, and are thus proposed to have a great potential as sensor materials and responsive separation membranes.

Abstract [sv]

Intresset för polymera nanokompositer har snabbt ökat under de senaste decennierna, drivet av den stora potentialen och de överlägsna egenskaperna hos nanopartiklar (NPs). Produktionsvolymerna för kommersiella NP har ökat exponentiellt under de senaste tio åren, och kvaliteten har förbättrats avsevärt. Syftet med denna studie var att polymer-ympa kommersiellt tillgängliga metalloxid-NPs, och grafenoxid (GO), för att designa isotropa (homogena) och anisotropa (heterogena) polymera nanokompositer för dielektriska tillämpningar. Motiveringen var att formulera funktionella isoleringsmaterial för kompakta komponenter i framtida kraftnätssystem som använder högspänd likström (HVDC) eller högspänd växelström (HVAC), samt att tillverka responsiva sensormaterial för övervakning av t.ex. temperatur- and spänningsvariationer i så kallade "Smart Grids".

Aluminiumoxid (Al2O3), zinkoxid (ZnO) och reducerad GO (rGO) NPs modifierades med glesa polymerympar via en kontrollerad "kovalent väg" och blandades med silikon (PDMS) eller polyeten matriser (EBA och LDPE) som är vanliga i HV-kabelsystem. Ymplängden och ymp-till-matrix kompatibiliteten skräddarsyddes för att erhålla nanokompositer med olika självordnande NP-morfologier, inklusive väldispergerade, länkade och fasseparerade strukturer. Ymplängden användes för att justera partikelavståndet i nanokompositer med förbundna morfologier eller länkade NPs. Man fann att nanokompositer med länkade NPs och korta interpartikelavstånd uppvisade 10-100 gånger högre konduktivitet än den ofyllda (rena) polymeren, eller erhöll en snabb icke-linjär ökning i konduktivitet (~1 miljon gånger) med ökande elektriskt fält, medan väldispergerade NPs med långa interpartikelavstånd uppvisade 10-100 gånger lägre ledningsförmåga (dvs. högre resistivitet) som en effekt av deras infångande av laddningsbärare. Dessa inställbara och funktionella egenskaper är önskvärda för HV-isolering, fältstyrande applikationer och flexibel elektronik.

Dessutom visades att GO, som modifierats med täta polymerympar via en "fysisorptionsväg", bildade suspensioner med flytande kristaller, eller matrisfria GO-kompositer med väldispergerad GO i isotropa eller nematiska tillstånd. Dessa material armerades av GO och uppvisade förhöjda glastransitionstemperaturer och en snabb värmeresponsiv form-minneseffekt, och föreslås därigenom ha en stor potential som sensor-material och responsiva separationsmembran.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , 96 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2017:16
National Category
Polymer Chemistry
Research subject
Fibre and Polymer Science
Identifiers
URN: urn:nbn:se:kth:diva-204036OAI: oai:DiVA.org:kth-204036DiVA: diva2:1083969
Public defence
2017-04-28, Kollegiesalen, Brinellvägen 8, KTH-huset, KTH Campus, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Note

QC 20170323

Available from: 2017-03-23 Created: 2017-03-23 Last updated: 2017-08-10Bibliographically approved
List of papers
1. Polymer-grafted Al2O3-nanoparticles for controlled dispersion in poly(ethylene-co-butyl acrylate) nanocomposites
Open this publication in new window or tab >>Polymer-grafted Al2O3-nanoparticles for controlled dispersion in poly(ethylene-co-butyl acrylate) nanocomposites
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2014 (English)In: Polymer, ISSN 0032-3861, E-ISSN 1873-2291, Vol. 55, no 9, 2125-2138 p.Article in journal (Refereed) Published
Abstract [en]

We report a model system to control the dispersion and inter-particle distance of polymer-grafted Al2O3-nanoparticles in high molecular weight poly(ethylene-co-butyl acrylate). The proposed methods make it possible to extend the use of surface initiated atom transfer radical polymerization (SI-ATRP) in combination with more commercial grades of silanes and particles, showing the versatility of this polymerization process. The nanoparticles were surface-modified by an amine-terminated silane, forming multilayered silane coatings to which moieties capable of initiating ATRP were attached. Subsequently, "short" (DP: 117) and "long" (DP: 265) chains of poly(n-butyl acrylate) were grafted from the particles via SI-ATRP. The graft density was found to be in accordance with the density of the accessible amine groups and could therefore be assessed directly after the initial silanization step using UV-Vis spectrometry. From AFM micrographs, the grafted nanoparticles were found to be well-dispersed in the matrix. This observation was corroborated by a novel simulation method capable of transforming the inter-particle distances from 2D to 3D, for the closest and more distant neighbors. Further, we calculated the deviation ratios and concluded that the dispersions were homogeneous and that the inter-particle distances were related to the graft length. The homogeneous dispersions were explained by dominating enthalpic contributions of the polymer grafts to the nanocomposites in combination with shielding of the nanoparticle core-core attraction by the silane multilayer (similar to bimodal systems).

Keyword
Surface-initiated atom transfer radical, polymerization (SI-ATRP) of aluminum, oxide nanoparticles, Nanocomposites of poly(ethylene-co-butyl acrylate), Simulation of particle dispersion
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-145825 (URN)10.1016/j.polymer.2014.03.005 (DOI)000335205200001 ()2-s2.0-84898859798 (Scopus ID)
Funder
Swedish Research Council, IFA 2007-5095
Note

QC 20140604

Available from: 2014-06-04 Created: 2014-06-02 Last updated: 2017-03-23Bibliographically approved
2. Novel Nanocomposites of Poly(lauryl methacrylate)-Grafted Al2O3 Nanoparticles in LDPE
Open this publication in new window or tab >>Novel Nanocomposites of Poly(lauryl methacrylate)-Grafted Al2O3 Nanoparticles in LDPE
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2015 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 7, no 46, 25669-25678 p.Article in journal (Refereed) Published
Abstract [en]

Aluminum oxide nanoparticles (NPs) were surface-modified by poly(lauryl methacrylate) (PLMA) using surface-initiated atom-transfer radical polymerization (SI-ATRP) of lauryl methacrylate. Nanocomposites were obtained by mixing the grafted NPs in a low-density polyethylene (LDPE) matrix in different ratios. First, the NPs were silanized with different aminosilanes, (3-aminopropyl)triethoxysilane, and 3-aminopropyl(diethoxy)methylsilane (APDMS). Subsequently, a-BiB, an initiator for SI-ATRP, was attached to the amino groups, showing higher immobilization ratios for APDMS and confirming that fewer self-condensation reactions between silanes took place. In a third step SI-ATRP of LMA at different times was performed to render PLMA-grafted NPs (NP-PLMAs), showing good control of the polymerization. Reactions were conducted for 20 to 60 min, obtaining a range of molecular weights between 23?000 and 83?000 g/mol, as confirmed by size-exclusion chromoatography of the cleaved grafts. Nanocomposites of NP-PLMAs at low loadings in LDPE were prepared by extrusion. At low loadings, 0.5 wt % of inorganic content, the second yield point, storage, and loss moduli increased significantly, suggesting an improved interphase as an effect of the PLMA grafts. These observations were also confirmed by an increase in transparency of the nanocomposite films. At higher loadings, 1 wt % of inorganics, the increasing amount of PLMA gave rise to the formation of small aggregates, which may explain the loss of mechanical properties. Finally, dielectric measurements were performed, showing a decrease in tan d values for LDPE-NP-PLMAs, as compared to the nanocomposites containing unmodified NP, thus indicating an improved interphase between the NPs and LDPE.

Place, publisher, year, edition, pages
American Chemical Society, 2015
Keyword
SI-ATRP, grafting from, nanocoomposite, LDPE, PLMA grafted nanoparticles, Al2O3
National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-180125 (URN)10.1021/acsami.5b06427 (DOI)000366005600016 ()2-s2.0-84948682543 (Scopus ID)
Note

QC 20160114

Available from: 2016-01-14 Created: 2016-01-07 Last updated: 2017-03-23Bibliographically approved
3. Tailoring Dielectric Properties using Designed Polymer-Grafted ZnO Nanoparticles in Silicone Rubber
Open this publication in new window or tab >>Tailoring Dielectric Properties using Designed Polymer-Grafted ZnO Nanoparticles in Silicone Rubber
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2017 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 5, 14241-14258 p., C6TA11237DArticle in journal (Refereed) Published
Abstract [en]

Polymer grafts were used to tailor the interphases between ZnO nanoparticles (NPs) and silicone matrices. The final electrical properties of the nanocomposites were tuned by the grafted interphases, by controlling the inter-particle distance and the NP-morphology. The nanocomposites can be used in electrical applications where control of the resistivity is desired. Hansen's solubility parameters were used to select a semi-compatible polymer for grafting to obtain anisotropic NP morphologies in silicone, and the grafted NPs self-assembled into various morphologies inside the silicone matrices. The morphologies in the semi-compatible nanocomposites could be tuned by steering the graft length of poly(n-butyl methacrylate) via entropic matrix-graft wetting using surface-initiated atom-transfer radical polymerization. Image analysis models were developed to calculate the radius of primary NPs, the fraction of aggregates, the dispersion, and the face-to-face distance of NPs. The dielectric properties of the nanocomposites were related to the morphology and the face-to-face distance of the NPs. The dielectric losses, above 100 Hz, for nanocomposites with grafted NPs were approximately one decade lower than those of pristine NPs. The isotropic nanocomposites increased the resistivity up to 100 times compared to that of neat silicone rubber, due to the trapping of charge carriers by the interphase of dispersed NPs and nanoclusters. On the other hand, the resistivity of anisotropic nanocomposites decreased 10–100 times when the inter-particle distance in continuous agglomerates was close to the hopping distance of charge carriers. The electrical breakdown strength increased for compatible isotropic nanocomposites, and the temperature dependence of the resistivity and the activation energy were ∼50% lower in the nanocomposites with grafted NPs. These flexible dielectric nanocomposites are promising candidates for low-loss high-voltage transmission cable accessories, mobile electronic devices, wearables and sensors.

Place, publisher, year, edition, pages
RSC Publishing, 2017
Keyword
tailoring nanoparticle arrangement, polymer grafting, tuning of dielectric properties
National Category
Materials Chemistry Polymer Chemistry
Research subject
Fibre and Polymer Science
Identifiers
urn:nbn:se:kth:diva-204038 (URN)10.1039/C6TA11237D (DOI)000405190000036 ()2-s2.0-85023756284 (Scopus ID)
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy StorageSwedish Foundation for Strategic Research , SM14-0034
Note

QC 20170323

Available from: 2017-03-23 Created: 2017-03-23 Last updated: 2017-08-15Bibliographically approved
4. Reduced and Surface-Modified Graphene Oxide with Nonlinear Resistivity
Open this publication in new window or tab >>Reduced and Surface-Modified Graphene Oxide with Nonlinear Resistivity
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2017 (English)In: Macromolecular rapid communications, ISSN 1022-1336, E-ISSN 1521-3927Article in journal (Refereed) Published
Abstract [en]

Field-grading materials (FGMs) are used to reduce the probability for electrical breakdowns in critical regions of electrical components and are therefore of great importance. Usually, FGMs are heavily filled (40 vol.%) with semi-conducting or conducting particles. Here, polymer-grafted reduced graphene oxide (rGO) is used as a filler to accomplish percolated networks at very low filling ratios (<2 vol.%) in a semi-crystalline polymer matrix: poly(ethylene-co-butyl acrylate) (EBA). Various simulation models are used to predict the percolation threshold and the flake-to-flake distances, to complement the experimental results. A substantial increase in thermal stability of rGO is observed after surface modification, either by silanization or subsequent polymerizations. The non-linear DC resistivity of neat and silanized rGO and its trapping of charge-carriers in semi-crystalline EBA are demonstrated for the first time. It is shown that the polymer-grafted rGO improve the dispersibility in the EBA-matrix and that the graft length controls the inter-flake distances (i.e. charge-carrier hopping distances). By the appropriate selection of graft lengths, both highly resistive materials at 10 kV mm-1 and FGMs with a large and distinct drop in resistivity (six decades) are obtained, followed by saturation. The nonlinear drop in resistivity is attributed to narrow inter-flake distance distributions of grafted rGO.

Keyword
tuning of properties, non-linear resistivity, polymer-grafting, field-grading, rGO
National Category
Polymer Chemistry
Research subject
Fibre and Polymer Science
Identifiers
urn:nbn:se:kth:diva-204040 (URN)10.1002/marc.201700291 (DOI)000407849000013 ()2-s2.0-85022346084 (Scopus ID)
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy Storage
Note

QC 20170323

Available from: 2017-03-23 Created: 2017-03-23 Last updated: 2017-09-12Bibliographically approved
5. Hydrophobic matrix-free graphene-oxide composites with isotropic and nematic states
Open this publication in new window or tab >>Hydrophobic matrix-free graphene-oxide composites with isotropic and nematic states
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2016 (English)In: Nanoscale, ISSN 2040-3364, E-ISSN 2040-3372, Vol. 8, 14730-14745 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate a novel route to synthesise hydrophobic matrix-free composites of polymer-grafted graphene oxide (GO) showing isotropic or nematic alignment and shape-memory effects. For the first time, a cationic macroinitiator (MI) has been immobilised on anionic GO and subsequently grafted with hydrophobic polymer grafts. Dense grafts of PBA, PBMA and PMMA with a wide range of average graft lengths (MW: 1–440 kDa) were polymerised by surface-initiated controlled radical precipitation polymerisation from the statistical MI. The surface modification is designed similarly to bimodal graft systems, where the cationic MI generates nanoparticle repulsion, similar to dense short grafts, while the long grafts offer miscibility in non-polar environments and cohesion. The state-of-the-art dispersions of grafted GO were in the isotropic state. Transparent and translucent matrix-free GO-composites could be melt-processed directly using only grafted GO. After processing, birefringence due to nematic alignment of grafted GO was observed as a single giant Maltese cross, 3.4 cm across. Permeability models for composites containing aligned 2D-fillers were developed, which were compared with the experimental oxygen permeability data and found to be consistent with isotropic or nematic states. The storage modulus of the matrix-free GO-composites increased with GO content (50% increase at 0.67 wt%), while the significant increases in the thermal stability (up to 130 °C) and the glass transition temperature (up to 17 °C) were dependent on graft length. The tuneable matrix-free GO-composites with rapid thermo-responsive shape-memory effects are promising candidates for a vast range of applications, especially selective membranes and sensors.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2016
Keyword
graphene oxide, macro-initiator, ARGET ATRP, shape memory, isotropic, nematic, well-dispersed, orientation, matrix-free
National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-193102 (URN)10.1039/C6NR01502F (DOI)000381442500006 ()2-s2.0-84981501478 (Scopus ID)
Funder
SweGRIDS - Swedish Centre for Smart Grids and Energy StorageEU, European Research Council
Note

QC 20160929

Available from: 2016-09-28 Created: 2016-09-28 Last updated: 2017-05-30Bibliographically approved

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