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Surface Force and Friction: effects of adsorbed layers and surface topography
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Interfacial features of polymers are a complex, fascinating topic, and industrially very important. There is clearly a need to understand interactions between polymer layers as they can be used for controlling surface properties, colloidal stability and lubrication. The aim of my Ph.D study was to investigate fundamental phenomena of polymers at interfaces, covering adsorption, interactions between polymer layers and surfactants, surface forces and friction between adsorbed layers.

A branched brush layer with high water content was formed on silica surfaces by a diblock copolymer, (METAC)m-b-(PEO45MEMA)n, via physisorption. The adsorption properties were determined using several complementary methods. Interactions between pre-adsorbed branched brush layers and the anionic surfactant SDS were investigated as well. Surface forces and friction between polymer layers in aqueous media were investigated by employing the Atomic Force Microscopy (AFM) colloidal probe technique. Friction forces between the surfaces coated by (METAC)m-b-(PEO45MEMA)n in water are characterized by a low friction coefficient. Further, the layers remain intact under high load and shear, and no destruction of the layer was noted even under the highest pressure employed, about 50 MPa.

Interactions between polymer layers formed by a temperature responsive diblock copolymer, PIPOZ60-b-PAMPTMA17 (phase transition temperature of 46.1 °C), was investigated in the temperature interval 25-50 °C by using the AFM colloidal probe technique. Friction between the layers increases with increasing temperature (25-45 °C), while at 50 °C friction was found to be slightly lower than that at 45 °C. We suggest that this is due to decreased energy dissipation caused by PIPOZ chains crystallizing in water above the phase transition temperature.

The structure of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers was determined by X-ray reflectometry. Surface forces and friction between DPPC bilayer-coated silica surfaces were measured utilizing the AFM colloidal probe technique. Our study showed that DPPC bilayers are able to provide low friction forces both in the gel (below ≈ 41°C) and in the liquid crystalline state (above ≈ 41°C). However, the load bearing capacity is lower in the gel state. This is attributed to a higher rigidity and lower self-healing capacity of the DPPC bilayer in the gel state.

Friction forces in single asperity contact acting between a micro-patterned silicon surface and an AFM tip was measured in air. We found that both nanoscale surface heterogeneities and the µm-sized depressions affect friction forces, and considerable reproducible variations were found along a particular scan line. Nevertheless, Amontons’ first rule described average friction forces reasonably well. Amontons’ third rule and Euler’s rule were found to be less applicable to our system.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , xiv, 78 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2014:57
Keyword [en]
Friction, nanotribology, surface forces, diblock copolymer, lipid, QCM-D, AFM, optical reflectometry, neutron reflectivity, X-ray reflectivity, micro-patterned surface
National Category
Chemical Engineering
Research subject
Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-157321ISBN: 978-91-7595-362-5 (print)OAI: oai:DiVA.org:kth-157321DiVA: diva2:769603
Public defence
2014-12-18, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20141209

Available from: 2014-12-09 Created: 2014-12-08 Last updated: 2015-08-27
List of papers
1. Electrostatically anchored branched brush layers
Open this publication in new window or tab >>Electrostatically anchored branched brush layers
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2012 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 28, no 44, 15537-15547 p.Article in journal (Refereed) Published
Abstract [en]

A novel type of block copolymer has been synthesized. It consists of a linear cationic block and an uncharged bottle-brush block. The nonionic bottle-brush block contains 45 units long poly(ethylene oxide) side chains. This polymer was synthesized with the intention of creating branched brush layers firmly physisorbed to negatively charged surfaces via the cationic block, mimicking the architecture (but not the chemistry) of bottle-brush molecules suggested to be present on the cartilage surface, and contributing to the efficient lubrication of synovial joints. The adsorption properties of the diblock copolymer as well as of the two blocks separately were studied on silica surfaces using quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry. The adsorption kinetics data highlight that the diblock copolymers initially adsorb preferentially parallel to the surface with both the cationic block and the uncharged bottle-brush block in contact with the surface. However, as the adsorption proceeds, a structural change occurs within the layer, and the PEO bottle-brush block extends toward solution, forming a surface-anchored branched brush layer. As the adsorption plateau is reached, the diblock copolymer layer is 46-48 nm thick, and the water content in the layer is above 90 wt %. The combination of strong electrostatic anchoring and highly hydrated branched brush structures provide strong steric repulsion, low friction forces, and high load bearing capacity. The strong electrostatic anchoring also provides high stability of preadsorbed layers under different ionic strength conditions.

Keyword
Adsorption, Block copolymers, Brushes, Electrostatics, Friction, Ionic strength, Polyethylene oxides, Silica, Synthesis (chemical)
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-107330 (URN)10.1021/la3028989 (DOI)000310664300015 ()2-s2.0-84868538507 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20121212

Available from: 2012-12-12 Created: 2012-12-10 Last updated: 2017-12-07Bibliographically approved
2. Association of anionic surfactant and physisorbed branched brush layers probed by neutron and optical reflectometry
Open this publication in new window or tab >>Association of anionic surfactant and physisorbed branched brush layers probed by neutron and optical reflectometry
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2015 (English)In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 440, 245-252 p.Article in journal (Refereed) Published
Abstract [en]

Pre-adsorbed branched brush layers were formed on silica surfaces by adsorption of a diblock copolymer consisting of a linear cationic block and an uncharged bottle-brush block. The charge of the silica surface was found to affect the adsorption, with lower amounts of the cationic polyelectrolyte depositing on less charged silica. Cleaning under basic conditions rendered surfaces more negatively charged (more negative zeta-potential) than acid cleaning and was therefore used to increase polyelectrolyte adsorption. The structure of adsorbed layers of the diblock copolymer was as determined by neutron reflectometry found to be about 70 nm thick and very water rich (97%). Interactions between the anionic surfactant sodium dodecylsulfate (SDS) and such pre-adsorbed diblock polymer layers were studied by neutron reflectometry and by optical reflectometry. Optical reflectometry was also used for deducing interactions between the individual blocks of the diblock copolymer and SDS at the silica/aqueous interface. We find that SDS is readily incorporated in the diblock copolymer layer at low SDS concentrations, and preferentially co-localized with the cationic block of the polymer next to the silica surface. At higher SDS concentrations some desorption of polyelectrolyte/surfactant complexes takes place.

Keyword
Polymer brush layer, Diblock copolymer, SDS, Adsorption, Polyelectrolyte-surfactant complex, Optical reflectometry, Neutron reflectivity
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-157337 (URN)10.1016/j.jcis.2014.11.002 (DOI)000346461100031 ()2-s2.0-84911423822 (Scopus ID)
Funder
Swedish Research CouncilVINNOVAEU, FP7, Seventh Framework Programme, 290251
Note

QC 20150220. Updated from manuscript to published article.

Available from: 2014-12-09 Created: 2014-12-09 Last updated: 2017-12-05Bibliographically approved
3. Low friction and high load bearing capacity layers formed by cationic-block-non-ionic bottle-brush copolymers in aqueous media
Open this publication in new window or tab >>Low friction and high load bearing capacity layers formed by cationic-block-non-ionic bottle-brush copolymers in aqueous media
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2013 (English)In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 9, no 22, 5361-5371 p.Article in journal (Refereed) Published
Abstract [en]

Efficient lubricants should be able to build surface layers that result in low friction and high load bearing capacity. In this work we show how this can be achieved in aqueous media by means of adsorption of a diblock copolymer consisting of a cationic anchor block without side chains and an uncharged and hydrophilic bottle-brush block that protrudes into solution. Surface and friction forces were measured between negatively charged silica surfaces coated with adsorbed layers of the cationic diblock copolymer, utilizing the atomic force microscope colloidal probe technique. The interactions between the surfaces coated with this copolymer in water are purely repulsive, due to a combination of steric and electrostatic double-layer forces, and no hysteresis is observed between forces measured on approach and separation. Friction forces between the diblock copolymer layers are characterized by a low friction coefficient, mu approximate to 0.03-0.04. The layers remain intact under high load and shear due to the strong electrostatic anchoring, and no destruction of the layer was noted even under the highest pressure employed (about 50 MPa). Addition of NaCl to a concentration of 155 mM weakens the anchoring of the copolymer to the substrate surface, and as a result the friction force increases.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-124066 (URN)10.1039/c3sm27862j (DOI)000318945100006 ()2-s2.0-84878143627 (Scopus ID)
Funder
Swedish Research CouncilVinnova
Note

QC 20130625

Available from: 2013-06-25 Created: 2013-06-25 Last updated: 2017-12-06Bibliographically approved
4. Tethered Poly(2-Isopropyl-2-Oxazoline) Chai: Temperature Effects of Layer Structure and Interactions Probed by AFM Experiments and Modeling
Open this publication in new window or tab >>Tethered Poly(2-Isopropyl-2-Oxazoline) Chai: Temperature Effects of Layer Structure and Interactions Probed by AFM Experiments and Modeling
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(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-157338 (URN)
Note

QS 2014

Available from: 2014-12-09 Created: 2014-12-09 Last updated: 2014-12-09Bibliographically approved
5. Dipalmitoylphosphatidylcholine (DPPC) Bilayers on Silica Surfaces: The Effect of Temperature on Bilayer Structure and Lubrication Performance
Open this publication in new window or tab >>Dipalmitoylphosphatidylcholine (DPPC) Bilayers on Silica Surfaces: The Effect of Temperature on Bilayer Structure and Lubrication Performance
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(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-157325 (URN)
Note

QS 2014

Available from: 2014-12-08 Created: 2014-12-08 Last updated: 2014-12-09Bibliographically approved
6. Frictional behavior of micro-patterned silicon surface
Open this publication in new window or tab >>Frictional behavior of micro-patterned silicon surface
2015 (English)In: Journal of Colloid and Interface Science, ISSN 0021-9797, E-ISSN 1095-7103, Vol. 456, 76-84 p.Article in journal (Refereed) Published
Abstract [en]

A micro-patterned silicon surface, consisting of depressions with walls having a tilt angle of 30°, was created by photolithography followed by etching. The friction forces in single asperity contact acting between such a surface and an AFM tip was measured in air. This allowed elucidation of the validity of some common friction rules for this particular situation where a small tip traces a surface having roughness features that are significantly larger than the tip itself. The rules that was compared with our data were Amontons' first rule of friction stating that the friction force should be proportional to the load; Amontons' third rule stating that the friction force should be independent of sliding speed, and Euler's rule providing a relation between slope angle and friction coefficient. We found that both nanoscale surface heterogeneities and the mum-sized depressions affect friction forces, and considerable reproducible variations were found along a particular scan line. Nevertheless Amontons' first rule described average friction forces well. Amontons' third rule and Euler's rule were found to be less applicable to our system.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-171092 (URN)10.1016/j.jcis.2015.06.009 (DOI)000358458500010 ()26093236 (PubMedID)2-s2.0-84934895458 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20150720

Available from: 2015-07-20 Created: 2015-07-20 Last updated: 2017-12-04Bibliographically approved

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