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Tunable Nanolubrication between Dual-Responsive Polyionic Grafts
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
2009 (English)In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 9, no 8, 2984-2990 p.Article in journal (Refereed) Published
Abstract [en]

This study reports on a direct approach of quantitatively probing the nanotribological response of chemically end-grafted polyions.  A combination of quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM), in the now well established colloidal probe mode was utilized to investigate the stimuli-induced lubrication behavior between poly-2-(dimethylamino)ethyl methacrylate (PDMAEMA) grafts on gold.  Force and friction measurements showed reversible transitions of up to an order of magnitude difference induced by varying the solvent conditions.  The greatly enhanced lubrication observed at low pH was attributed to the formation of a repulsive, highly charged, hydrated cushion.  At high pH the friction was significantly increased.  The system turned attractive above the lower critical solution temperature (LCST) with a small friction reduction interpreted as nanoscopic flattening at the interfacial boundary.

Place, publisher, year, edition, pages
USA: American Chemical Society , 2009. Vol. 9, no 8, 2984-2990 p.
Keyword [en]
AFM, QCM-D, responsive polymer, RAFT, thiol, force, friction, colloidal probe, DMAEMA, LCST, lubrication, nanotribology
National Category
Physical Chemistry
URN: urn:nbn:se:kth:diva-11000DOI: 10.1021/nl901411eISI: 000268797200031ScopusID: 2-s2.0-68949108073OAI: diva2:233866
QC 20100804Available from: 2009-09-03 Created: 2009-09-03 Last updated: 2012-03-22Bibliographically approved
In thesis
1. Interfacial Properties of Biomacromolecular Model Systems: Surface Forces and Nanotribology
Open this publication in new window or tab >>Interfacial Properties of Biomacromolecular Model Systems: Surface Forces and Nanotribology
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The nanotribology, adhesion and related interfacial properties of biomacromolecular systems have been studied.  The aim was to elucidate the role of physisorbed and chemically grafted bio-polymers implicated as mediators for cellulose based processing and material design.  To that end, model surfaces were prepared and characterized by the versatile techniques quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM) in colloidal probe mode.  A prerequisite for the latter in conducting quantifiable force and friction measurements lies in obtaining accurate values for the cantilever spring constants.  An investigation of various recently proposed calibration methods shows that they return essentially the same result, but that the route of Sader et al. is the least strenuous and indeed most reliable.  A novel approach for determining the required frictional detector sensitivity is proposed.

Cellulose model surfaces with varying morphology were evaluated.  For a given surface roughness the adsorption of xyloglucan, a nonionic polysaccharide, leads to a consistent decrease in the friction coefficients for all cellulose substrates, demonstrating that the effects on friction arising from either roughness or chemistry can be decoupled.  Moreover, xyloglucan grafts on gold show biological enzymatic accessibility, and their native affinity towards cellulose is retained, even in an extended brush conformation.

Chitosan, a cationic polysaccharide is also found to mediate lubrication and adhesion of cellulose in a solvent dependent manner, though the mechanisms are unlike those of xyloglucan.  At low pH the most efficient lubrication is achieved due to highly charged chitosan species extending out in the aqueous media, forming a highly hydrated cushion.  Conversely, at high pH the deswelling of the layer results in higher friction.  The same trend is observed between synthetic dual-responsive polyionic grafts on gold where the polymer charge is decisive for the friction properties. Above the lower critical solution temperature where the grafts are fully collapsed and display an attractive force, the friction is slightly reduced due to a nanoscopic flattening at the interface.

Finally, polymer grafts as matrix compatibilizers for bio-degradable cellulose based nano-composite applications were evaluated.  Adhesion is greatly promoted by chain entanglements.  The effect displays a dynamic diffusion based dependence of which the rate is significantly enhanced at a higher temperature close to the melting point of the polymeric layer.  No such adhesion benefit was obtained between ungrafted cellulose and the matrix material.

Abstract [sv]

Nanotribologin, adhesionen samt relaterade gränsskikts egenskaper av biomakromolekylära system har studerats.  Målet var att belysa inverkan av adsorberade samt kemiskt ympade bio-polymerer implicerade som medlare för cellulosa baserad förädling och material design.  För detta ändamål var modell ytor beredda och karakteriserade med de mångsidiga teknikerna kvarts kristall mikro våg med dissipation (QCM-D) och atom krafts mikroskopi (AFM) i kolloidal prob konfiguration.  En förutsättning för den senare i att utföra kvantifierbara kraft och friktionsmätningar ligger i att bestämma noggranna värden för kantilever-fjäderkonstanterna.  En undersökning av olika nyligen föreslagna kalibrerings metoder visar att de ger i huvudsak samma resultat, men att den av Sader et al. är minst krävande och sannerligen den mest tillförlitliga. Ett nytt tillvägagångsstätt för att bestämma den obligatoriska friktions detektor-känsligheten föreslås.

Cellulosa modell ytor med varierande morfologi utvärderades.   För en given ytråhet leder adsorption av xyloglukan, en icke-jonisk polysackarid, till en konsekvent reduktion av friktions-koefficienterna för alla substraten, vilket demonstrerar att effekten på friktion av antingen ytråhet eller kemi kan separeras.  Därtill visar sig xyloglukan ympningar på guld vara biologiskt enzymatiskt tillgängligt, och deras naturliga affinitet för cellulosa är bevarad, även i en utdragen borst-konformation.

Kitosan, en katjonisk polysackarid visar sig också förmedla smörjning och adhesion av cellulosa på ett lösnings beroende sätt, men mekanismerna är olika de för xyloglukan.  Vid lågt pH uppnås den mest effektiva smörjningen p. g. a. de högt laddade kitosankedjorna som sträcker ut sig i vatten lösningen och formar en högt hydratiserad dyna.  Omvänt, vid högt pH resulterar vattenhaltsminskningen av lagret i en högre friktion.  Samma tendens iakttas mellan syntetiskt dubbel-responsiva polyjoniska ympningar på guld där polymerladdningen är avgörande för friktionsegenskaperna.  Ovanför den lägre kritiska lösningstemperaturen där ympningen är helt kollapsad och uppvisar en attraktiv kraft är friktionen något reducerad        p. g. a. nanoskopisk utjämning vid gränsskiktet.

Slutligen, har polymerympningar som matris-kompatibiliterare för bio-nedbrytbara cellulosa baserade nano-komposit applikationer utvärderats.  Adhesionen är mycket gynnad av kedjeintrasslingar.  Effekten påvisar ett dynamiskt diffusionsbaserat beroende, av vilken takten är signifikant förhöjd vid en högre temperatur nära smältpunkten för polymerlagret.  Ingen sådan adhesionsfördel uppnåddes mellan den rena cellulosan och matrismaterialet.

Place, publisher, year, edition, pages
Stockholm: KTH, 2009. vii, 72 p.
Trita-CHE-Report, ISSN 1654-1081 ; 2009:36
afm, surface force, friction, composite material, model surface, quartz crystal microbalance, dissipation, cellulose, xyloglucan, chitosan, colloidal probe, nanotribology, biomimetic, enzymatic, polysaccharide, graft, fiber, thiol, spring constant, calibration, lubrication, polyion, responsive.
National Category
Physical Chemistry
urn:nbn:se:kth:diva-11002 (URN)978-91-7415-394-1 (ISBN)
Public defence
2009-09-25, FD5, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
QC 20100804Available from: 2009-09-08 Created: 2009-09-03 Last updated: 2010-08-04Bibliographically approved

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