tools to probe nanoscale surface phenomena in cellulose ...ojrojas/pdf/2009_3.pdf · uncorrected...

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4Tools to Probe Nanoscale Surface

Phenomena in Cellulose Thin Films:Applications in the Area

of Adsorption and Friction

Junlong Song, Yan Li, Juan P. Hinestroza and Orlando J. Rojas

4.1 Introduction

Surfaces and interfaces play important roles in defining material interactions. Severaldevelopments in science and technology highlight the importance of interfaces in appli-cations involving material functionalization, coatings, colloids stability, etc. (Karim andKumar 2000). In many cases, the interfacial properties are more relevant than the natureand composition of the bulk phases and ultimately define the molecular behavior of thesystem.

The ‘thickness’ of a boundary between two phases, if possible to define, is expected tobe extremely narrow. The interface between (bio)polymers or that for a polymer-coatedsubstrate and the surrounding medium typically entails a ‘soft’ layer with molecular ornanoscale dimensions. The use of adsorbed polymers and surfactants to modify solidsurfaces offers unique possibilities to alter or regulate their properties, including surfaceenergy, molecular assembly and composition, among others. In order to effectivelyor permanently modify the interfacial properties the adsorbing material (or adsorbate)has to bind to some degree or extent to the respective surface. Therefore, adsorption isfundamental in many important applications, particularly in the general fields of adhesion,colloidal stabilization, friction, and heterogeneous reactions.

The Nanoscience and Technology of Renewable Biomaterials. Edited by Lucian A. Lucia and Orlando J. Rojasc© 2009 Blackwell Publishing, Ltd

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92 The Nanoscience and Technology of Renewable Biomaterials

D

Figure 4.1 Schematic illustration of polymers adsorbing from solution onto a surface. D issome average thickness of the adsorbed polymer layer, the value of which depends on themethod use to measure it.

Adsorption results as a consequence of the balance between surface energy and thenature of the adsorbing species. While the conformation of a polymer in solution dependson solvency and polymer chain composition and architecture, at an interface the polymercan be perturbed by the interaction of its segments with the surface (see Figure 4.1).When this interaction involve attractive chemical or physical forces the resulting adsorp-tion is classified as chemisorption or physisorption, respectively (Eisenriegler 1993).

Macromolecules possess a broad diversity of properties that are often related to theirdissociation ability in aqueous solution. As such they are classified into ionic (also knownas polyelectrolytes) and nonionic polymers. Ionic polymers are also classified into simplepolyelectrolytes, with either positive or negative charged groups, and polyampholytes,which contain both positive and negative charged groups.

Polymer adsorption has been extensively studied from theoretical and experimentalperspectives. In this chapter, we will first describe the adsorption of a relevant type ofcharged polymer onto cellulose surfaces. We will then review aspects related to bound-ary lubrication in the case of adsorbed nonionic polymer (finish) on the same substrates.Finally, we will present a brief account on the techniques used to study polymer adsorp-tion and lubrication. Specifically, we will discuss two tools to determine the extent anddynamics of polymer and surfactant adsorption: The quartz crystal microbalance QCMand the surface plasmon resonance technique, SPR. We will also discuss the use of lat-eral force microscopy LFM as a useful approach to investigate friction phenomena. Thisinformation presented in this chapter will be helpful to appreciate other chapters in thisbook that cover specific aspects of surface modification (including hemicellulose adsorp-tion and polymer multilayers). Complementary tools for nanoscale characterization ofbiomass will be discussed in other chapters of this book.

4.2 Polyampholytes Applications in Fiber Modification

Hydrosoluble polymers are commonly used in industry. Among these, amphotericmacromolecules or polyampholytes have been employed in papermaking to modify cellu-losic fibers thereby enhancing inter-fiber bonding. Generally speaking, a polyampholyteis defined as charged macromolecule carrying both acidic and basic groups (Dobrynin,

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Tools to Probe Nanoscale Surface Phenomena in Cellulose Thin Films 93

Colby et al. 2004). These polymers find application in several other fields includingcolloid stabilization, wetting, lubrication and adhesion (Mazur, Silberberg et al. 1959;

• Q1

Bratko and Chakraborty 1996; Jeon and Dobrynin 2005; Sezaki, Hubbe et al. 2006a,2006b; Song, Wang et al. 2006; Wang, Hubbe et al. 2006; Hubbe, Rojas et al. 2007a,2007b; Wang, Hubbe et al. 2007).

Under appropriate conditions the acidic and basic groups in polyampholytes disso-ciate in aqueous solution producing ionic groups and their respective counterions. Ifthe ionic groups on the polymer chain are weak acids or bases, the net charge of thepolyampholytes can be changed by varying the pH of the aqueous medium. At theisoelectric point (IEP), the number of positive and negative charges on the polyion isthe same, giving a net charge of zero. In the vicinity of the isoelectric pH, the polymersare nearly charge-balanced and exhibit the unusual properties of amphoteric molecules.At conditions of high charge asymmetry (far above or below the isoelectric pH),these polymers exhibit a simple polyelectrolyte-like behavior (Gutin and Shakhnovich1994; Kantor and Kardar 1995; Ertas and Kantor 1996; Hwang and Damodaran1996; Long, Dobrynin et al. 1998; Lee and Thirumalai 2000; Yamakov, Milchevet al. 2000; Dobrynin, Colby et al. 2004; Jeon and Dobrynin 2005; Lord, Stenzelet al. 2006).

As fiber recycling increases more interesting and new polymer molecular architectureshave been proposed as means to improve product strength from loses (especially intensile and burst strengths) due to reuse (Nazhad and Paszner 1994; Nazhad 2005).After extensive fibers recycling fiber may not longer be useful without the addition ofchemical additives.

While several polymer chemistries are used in the applications explained above,polyampholyte treatments may be less common. To our knowledge, the first reporton the application of polyampholytes to enhance strength of paper was published in1977 by Carr, Hofreiter et al. (Carr, Hofreiter et al. 1977). In this seminal report,starch-based polyampholytes were prepared using xanthating cationic cornstarch deriva-tives, which had either tertiary amino [−CH2CH2N(C2H5)2] or quaternary ammonium[−CH2CHOHCH2N+(CH3)3] groups attached to the macromolecule. Anionic xanthategroups were introduced into the cationic starch amines. The substitution degree ofthe obtained derivatives ranged from 0.023 to 0.33 for the amine cation and 0.005 to0.165 for the xanthate anion. This work demonstrated that wet-end additions of a starchpolyampholyte was effective in improving both wet and dry strengths, exceeding thosegiven by either cationic or anionic starch polyelectrolytes. For a given amine degree ofsubstitution (DS), there was a charge ratio of A (amine, positive)/X (xanthate, negative)at which point each polyampholyte gave a well-defined maximum value for wet strength.This A/X ratio was about 1 for tertiary amine with a low DS (DS of 0.023, 0.035, and0.06) but was about 2 to 3 for tertiary amines with a high DS of 0.33 (see Figure 4.2).The authors also found that polyampholytes with quaternary amines substitution wereslightly more effective than those with tertiary amines.

Recently fully synthetic polyampholytes were systematically investigated in our labo-ratories with aims at enhancement of paper strength (Sezaki, Hubbe et al. 2006a, 2006b;Song, Wang et al. 2006; Wang, Hubbe et al. 2006; Hubbe, Rojas et al. 2007a, 2007b;Wang, Hubbe et al. 2007). The employed polyampholytes were prepared by free-radicalpolymerization of cationic monomer N-[3-(N′,N′-dimethylamino)propyl]acrylamide

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0.00 0.04 0.08 0.12 0.16 0.20

400

800

1200

1600

Wet

Bre

akin

g Le

ngth

(m

)

Xanthate DS

DS 0.33

DS 0.06

DS 0.035

DS 0.023

Figure 4.2 Wet strength (wet breaking length) of paper treated with xanthated starch aminehaving various tertiary amine and xanthate degrees of substitution (DS). The paper sampleswere prepared from unbleached kraft furnish treated with 3% XSA, oven dry pulp basis, atpH 7.0. Figure redrawn from Carr, Hofreiter et al. (1977), with permission of TAPPI Press.

(DMAPAA), a tertiary amine, anionic monomer methylene butanedioic acid (knownas itaconic acid, IA), and neutral acrylamide (AM) monomer. Some of the advantagesof synthetic polyampholytes include higher charge densities; simple control of themolecular weight and charge ratio of cationic and anionic groups; uniform molecularweight distribution (lower degree of polydispersity), etc. The superior dry strengthof polyampholytes over simple polyelectrolytes was reported in several publications(Sezaki, Hubbe et al. 2006a, 2006b; Song, Wang et al. 2006; Wang, Hubbe et al. 2006;Hubbe, Rojas et al. 2007a, 2007b; Wang, Hubbe et al. 2007). Under the experimentalconditions used, polyampholytes were applied at 1% addition level on bleachedhardwood kraft fibers. Paper’s breaking length increased 20–50% compared withcontrol experiments (see Figure 4.3). An interesting reported observation phenomenonreported was the fact that the strength of the paper increased as the charge densityincreased reaching a maximum for polyampholytes of intermediate charge density.After reaching a maximum strength value, the strength decreased as highly chargedpolyampholytes were employed. A near neutral pH was found to be optimum conditionto maximize strength performance. This interesting behavior could be explained bythe fact that close to the iso-electric point (IEP) of the polyampholytes, a maximumefficiency for adsorption is achieved and bonding between fibers is promoted.

Despite the fact that a number of theoretical and computational efforts have beenreported (Gutin and Shakhnovich 1994; Kantor, Kardar et al. 1994; Kantor and Kardar1995; Bratko and Chakraborty 1996; Ertas and Kantor 1996; Schiessel and Blumen 1996;Srivastava and Muthukumar 1996; Lee and Thirumalai 2000; Yamakov, Milchev et al.2000), there is still a lack of experimental data regarding the dynamics of adsorption,and interactions at the nanoscale level on polyampholites. Understanding such phe-nomena will lead to new functional formulations and improved performance of fibersafter surface modification. In this chapter we will revisit the issue of polyampholyte

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PA

mp4

PA

mp8

PA

mp1

6

Cat

.

An.

Con

trol

2.0

2.5

3.0

3.5

4.0

4.5

5.0

PA

mp2

pH = 5pH = 8.5

pH = 4

Polymer (1% Treatment Level)

Bre

akin

g Le

ngth

(km

)

Bleached HW Kraft Fibers

Figure 4.3 Effect of macromolecular composition and pH on the tensile strength ofpolymer-treated bleached kraft fibers at 1000 μS/cm conductivity. Polyampholytes denotedas ‘PAmp 2, 4, 8, 16’ correspond to polymers of increased charge density (with the ratio ofanionic-to-cationic groups kept constant) while ‘Cat’ and ‘An’ correspond to the respectivesingle cationic and anionic polyelectrolytes (with same molecular masses). These polymerswere based on cationic DMAPAA (tertiary amine), anionic itaconic acid (IA) and neutralacrylamide (AM) (see text for more details). Reproduced from Song, Wang et al. (2006) withpermission from Pulp and Paper Technical Association of Canada (PAPTAC).

adsorption in the context of adsorbed nanolayers with high viscoelasticity to enhancefiber bonding. This phenomenon can only be explored with some of the tools describedin later sections.

4.3 Cellulose Thin Films

Studies at the nanoscale usually involve substrates that are limited to surrogates of cel-lulose fibers. This is because the intrinsic complexity of natural fibers, which includeschemical and topographical heterogeneities that prevents a detailed study of cause-effectrelationships. A common approach is to use cellulose thin films as model for cellulose.There is an abundance of literature about this topic and the reader is referred to thereview by Konturri et al. for an excellent account on the subject (Kontturi, Tammelinet al. 2006). Here we limit ourselves to spin coated films of cellulose prepared on silicaor gold substrates according to a procedure reported elsewhere (Gunnars, Wagberg et al.2002; Falt, Wagberg et al. 2004) and modified slightly as follows (Song, Liu et al.2008): Cellulose solution was prepared by dissolving microcrystalline Avicel cellulosein 50%wt water/N-methylmorpholine-N-Oxide (NMMO) at 115 ◦C. Dimethyl Sulfox-ide (DMSO) was added to adjust the concentration (0.05%) and the viscosity of thecellulose suspension. Polyvinylamine was used as anchoring polymer of the cellulosefilm. Silica or gold substrates were immersed in PVAm for 20 min followed by wash-ing with water and drying with a gentle nitrogen jet. The cellulose solution was then

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spin-coated (Laurell Technologies model WS-400A-6NPP) by depositing 50–100 μl onthe PVAm-modified substrates at 5000 rpm for 40 seconds. We found these conditionsas optimal for obtaining robust, smooth films. The cellulose-coated substrates wereremoved from the coater and then immersed in water during four hours and placedin an oven for two hours at 80 ◦C. The substrates were then washed thoroughly withwater, dried with a nitrogen jet and stored at room temperature in a clean chamberfor further use. An AFM image of the obtained films as well as its height profileis shown in Figure 4.4. Because of the chemical homogeneity and flat topographysuch thin films of cellulose are useful as platform for nanoscale studies that involveSurface Plasmon Resonance, Quartz Crystal Microbalance as well as Lateral ForceMicroscopy.

1.00

0

1

2

nm

3

4

5

6

7

8

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5μm

00.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5μm

2.0

(a)

(c)

(b)

3.0 4.0 μm

1.0 2.0 3.0 4.0 μm

Figure 4.4 5 × 5 μm non-contact mode AFM height (a), corresponding section analysis (b),and phase (c) images of cellulose thin film on a silica wafer. The film is about 20 nm thick(obtained by ellipsometry) with an RMS roughness of ca. 2 nm.

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4.4 Friction Phenomena in Cellulose Systems

Friction is an important surface phenomenon that is strongly influenced by molecu-lar adsorption. Inter-fiber friction plays an important role in flocculation and networkstrength of paper (Zauscher and Klingenberg 2001). Relevant work related to the mea-surement of friction in cellulose systems can be found in several references (Bogdanovic,Tiberg et al. 2001; Zauscher and Klingenberg 2001; Theander, Pugh et al. 2005;Stiernstedt, Brumer et al. 2006; Stiernstedt, Nordgren et al. 2006).

The science of friction, lubrication and wear, known as tribology, has long beenof both technical and practical interest since the operation of many mechanical systemsdepends on these surface phenomena (Dowson 1998). The field of tribology has receivedincreased attention in response to the inordinate waste of resources that has resultedfrom unwanted high friction and wear. In fact, estimates indicate that proper attention totribology issues could lead to economic savings up to 1.3% to 1.6% of the Gross NationalProduct (GNP) (Jost 1990). Beyond industrial applications tribology is critical in theperformance of body implants, cell adhesion, and interfacial phenomena in compositematerials.

Fibrous polymeric materials go through different processing stages including pretreat-ment, dyeing, printing and finishing before they are finally assembled into end products(woven and nonwoven webs, composites, etc.). Machinery and equipment are inevitablyinvolved in handling fibers at high rates of deformation. Fibers and related materials arealso subjected to destructive abrasive forces that may result in mutual abrasion betweenfibers and/or between the fibers and equipment surfaces. In order to control frictionand reduce wear between fibers and between fibers and solid surfaces, surface modifica-tion treatments are necessary. Fiber finishes are commonly used during the productionof many different fiber grades (Proffitt and Patterson 1988) and a myriad of differentfinishing formulations exists depending on the intended use of the fibers and the fiberprocessing operation conditions. In general four general classes of boundary lubricantscan be identified:

1. high molecular weight, water dispersible products – significantly reduce abrasiondamage to fibers in aggressive processes and seem to function most effectively indynamic, higher speed situations;

2. waxy materials – traditional boundary lubricants that function in both low speed (fiberto fiber) and high speed (fiber to metal, fiber to ceramic) processing conditions;

3. low molecular weight polymers that have high affinity for the surface of the fiber andtend self-assemble depending on the chemical interactions with the modified substrate;

4. silicone based materials – tend to have high affinity for the surface of many of thefiber forming polymers.

Recent technological developments in fiber processing trend towards higher speed pro-cessing making the dynamics of the adsorption process and the durability of the adsorbedlayer even more relevant. A need to continuously develop high performance finishes forsurface modification is required in order to meet the increasing requirements of modernfiber processing operations.

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4.5 Lubrication

Lubrication phenomena are involved when a finish or lubricant is applied to (moving)objects as means to reduce friction between them. Amonton’s law was proposed in the17th century in order to analytically describe sliding friction at the macroscopic scale(Dowson 1998):

μ = Ff /N (4.1)

where μ is the coefficient of friction, a dimensionless scalar value that describes theratio of the force of friction between two bodies, Ff , the force pressing them togetherand the normal force applied, N . From a macroscopic perspective, μ is a constantrelated to the nature of both contacting objects. The frictional force (Ff ) is independentof the apparent contact surface. The Amonton equation can be applied in many casesat the macroscopic scale and for sliding objects directly in contact. However, simpleexperimental observation has shown that frictional forces do depend on the contact area ,the surface roughness as well as the chemical nature of the sliding substances.

When dealing with fluid lubricants the situation becomes more complicated since thegap between the two moving objects may vary. The friction coefficient may dependon the gap between the sliding surfaces as well as the sliding speeds or shear rates.According to Hamrock (Hamrock, Schmid et al. 2004), four different regimes of fluidfilm lubrication can be defined, i.e. boundary, mixed, elasto-hydrodynamic and hydrody-namic regimes. These regimes depend on a liquid film parameter known as �. A plot offriction coefficient as a function of � is illustrated by the Stribeck curve (Figure 4.5). Thefilm parameter, �, represents the minimum film thickness separating the two surfacesand can be quantified by using Equation (4.2):

� = V × ηb/P (4.2)

Fric

tion

Coe

ffici

ent

Film Parameter, L

Boundary lubrication

Mixed lubrication

Elastohydrodynamic lubrication (EHL)

Hydrodynamic lubrication

a

b

Figure 4.5 Stribeck curve displaying the different regimes of lubrication. Figure redrawnfrom Hamrock, Schmid et al. (2004).

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Lucian A. Lucia c04.tex V1 - 05/04/2009 9:23 P.M. 9


where V is the speed of the moving (sliding) material (for example fiber); ηb is the bulkviscosity of the lubricant and P is the pressure applied between the two sliding surfaces.

In full-film lubrication (aka hydrodynamic lubrication) the surfaces are separated bya thick lubricant film. Ideally there is no wear of the solid surfaces and the frictionis determined by the rheology, surface chemistry, and intermolecular forces of the bulklubricant. During boundary lubrication regime the load is carried by the surface asperitiesand the lubricant film and the friction behavior is determined by the dynamic propertiesof the boundary film. In the intermediate mixed region both the bulk lubricant andthe boundary film do play key roles. Under these conditions the properties of theadsorbed components and the chemistry and dynamics of the interfacial region betweenthe tribosurfaces are of utmost importance.

In the Stribeck curve, the bulk viscosity ηb applies to all the cases considered, fromwide to narrow gaps between the sliding surfaces. However, in reality, the local ormicroscopic effective viscosity ηeff may be quite different from the bulk viscosity ηb

especially in the case of very confined systems of ultra narrow gaps (Cho, Cai et al.1997).

Luengo, Israelachvili and Granick proposed a set of improved Stribeck-type curvesthat are based on experimental data typical in engineering conditions. The correspondinggeneralized map of friction force against sliding velocity in various tribological regimeswere discussed by the same authors (Luengo, Israelachvili et al. 1996). In the boundarylayer film ηeff is noted to be much higher that the bulk value, ηb. As the shear rateincreases a point is reached where the effective viscosity starts to drop with a power-lawdependence on the shear rate. As the shear rate further increases, a second Newtonianplateau is encountered. At higher loads ηeff continues to grow with load and transitionto sliding at high velocity is discontinuous and usually of the stick-slip type. While thischapter covers the general topic of adsorption and lubrication, our emphasis in the nextsections will be the chemistry and adsorbed layer state of polymeric surfactants. Issuesrelated to roughness, asperities and others are not considered here.

4.6 Boundary Layer Lubrication

In the boundary lubrication regime, the load is carried by a lubricant thin film. A typicallubricant film usually has a thickness of 100 nm or lower, i.e., only several to hundreds ofmolecules thick (Guddati, Zhang et al. 2006; Guo, Li et al. 2006; Izumisawa and Jhon2006). Studying the structure of lubricant thin films and how the molecules organizeduring the lubrication process is of utmost importance. In this regime physisorption(as opposed to chemisorption) is a dominant effect since during fiber processing thelubricant film is not always intended to be retained onto the surface (in some cases thelubricant on fiber surfaces could interfere with successive processes or use of the fiber).The robustness or strength of adsorbed layer of lubricants during fiber processing is anissue that has not been addressed systematically.

4.6.1 Thin Films: Property Changes and Transitions

As discussed above, the properties of lubricant thin films change depending on theirdistance from the surface. When the thickness of the adsorbed film is comparable to

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• Viscosity • Elasticity • Relaxation time

Solid Boundary liquid

Bulk liquid

Continuum properties

Å nm μm

Figure 4.6 Schematic diagram of how the effective viscosity, elasticity and relaxation changewith thickness of a lubricant film. Figure redrawn from Cho, Cai et al. (1997) with permissionfrom Elsevier.

the dimensions of the lubricant molecules, the properties of the thin film are quite dif-ferent than those of the bulk medium (Cho, Cai et al. 1997). As shown in Figure 4.6,the effective viscosity, elasticity and relaxation time increase with diminishing thicknessand diverge when the film thickness is sufficiently small. At these dimensions classi-cal continuum considerations, which can be apply to the bulk phase, do not hold forthin films.

The diffusion coefficient of finish molecules in thin films also diverges when comparedwith that in the bulk. Mukhopadhyay et al. (Mukhopadhyay, Zhao et al. 2002) foundthat the molecular diffusion coefficient decreases exponentially from the edges towardsthe center in systems under Hertzian contact. Hertzian contact is an ideal model todescribe deformation and lubrication. In Hertzian contact only small deformation occursin the contact areas as contacting bodies are elastic and therefore only vertical forces needto be considered. Granick et al. (Mukhopadhyay, Bae et al. 2004; Granick and Bae2006) studied the influence of shear behavior on polymer interfacial diffusion. Accordingto their results shear did not substantially modified the Brownian diffusion.

Phase behaviors of lubricants may change in confined conditions and that is one of themain reasons why properties of thin films differ from those of the bulk. Confinement-induced phase states of lubricant layers could change from liquid-like to an amorphousstate and then to a solid-like state (Yoshizawa, Chen et al. 1993). While low frictionis exhibited by solid-like and liquid-like layers, high friction is exhibited by amor-phous layers. A change of some controlling variables such as temperature and humiditymay shift the phase status from the solid-like towards the amorphous or liquid-likestates. Confinement-induced solidity of lubricant was observed by Denirel and Granick(Demirel and Granick 2001) by placing octamethyl cyclotetrasiloxane (OMCTS) liquidsbetween two rigid mica plates and decreasing their spacing below ca . 10 moleculardimensions of the lubricant. This phenomena was also observed by Israelachvili andcoworkers (Israelachvili, Luengo et al. 1996; Luengo, Schmitt et al. 1997) by shearingpolybutadiene (PBD) of 7000 Daltons. They found that at low shear rates PBD exhibitedbulk-like properties in films thicker than 200 nm while in thinner films (200–220 nm) the

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shear viscosity ηeff and moduli G′ and G′′ became quite different from those of the bulk.On entering the tribology regime (film thickness <30 nm) PBD exhibited highly non-linear behavior and yield points indicative of phase transitions to ‘glassy’ or ‘solid-like’states. Klein et al. (Klein and Kumacheva 1998) discovered that the transition betweenliquid-like behavior and a solid-like phase of the liquids under progressive confinementtake place abruptly at a distance around six molecular layers. The films that are thinnerthan six molecular layers behaved in a solid-like fashion and they required a criticalstress to shear them.

4.6.2 Structure of Lubricant Films

Why can lubricants reduce friction? How do lubricant molecules work and behave undershear? These questions are currently being investigated by several groups. Lubricantmolecules organize themselves under shear as illustrated in Figure 4.7 by Yoshizawa et al.(Yoshizawa, Chen et al. 1993). A critical velocity Vc* exists; if the sliding velocity oftwo surfaces are below Vc* a polymeric lubricant film exhibits amorphous structure andthe polymer chains interplay and entangle with each other. In this case high friction isproduced (static-kinetic sliding). This phenomenon supports experimental observationsin which chain interdigitation was found to be an important molecular mechanism givingrise to ‘boundary’ friction and adhesion hysteresis of monolayer-coated surfaces. If thesliding velocity of two surfaces is above the critical velocity polymer chains will bealigned or ‘combed’ by shear into an ordered conformation and therefore will result invery low friction (superkinetic sliding).

The phenomenon of shear–induced alignment of lubricant molecules has been vali-dated by a number of experiments. For example, Frantz and co-workers (Frantz, Perryet al. 1994) adsorbed polyisoprene onto a single solid surface and found that the back-bone of the polymer oriented in the direction of flow. They also found that the extentof orientation increased with increasing molecular weight. The structure of the lubri-cant, such as chain length (Frantz, Perry et al. 1994), packing densities (Ruths 2003;Ruths, Alcantar et al. 2003), and nature of the polymer (brush-like (Zappone, Ruthset al. 2007) or grafted polymer (Urbakh, Klafter et al. 2004) and chain ends (Chen,Maeda et al. 2005)) have been found to influence molecular alignment of the lubricantunder shear.

Within these investigations, the work of Urbakh et al. (Urbakh, Klafter et al. 2004)is very significant. They used grafted polyelectrolytes, hyaluronan and hylan, to mimic

Static-kinetic sliding Superkinetic sliding

D

V>Vc*V<Vc*

Figure 4.7 Lubricant molecules organized by shear. Figure redrawn from Yoshizawa, Chenet al. (1993) with permission from Elsevier.

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cartilage lubrication. These polysaccharides (outermost cartilage layer) were not expec-ted to be the responsible molecule for the great lubricity of cartilage. However, theauthors found that they may contribute to the loadbearing and wear protection in thesesurfaces. Their study showed that a low coefficient of friction is not a requirement for,or necessarily a measure of, wear protection.

4.7 Techniques to Study Adsorption and Friction Phenomena

It is well known that the function of thin films in boundary lubrication and mixed lubrica-tion regimes is to offer friction reduction and wear protection. A better understanding ofthin film lubrication will improve our knowledge of how lubricants work and this knowl-edge can be used to develop superior lubricant formulations as well as for improvingthe prediction of tribological failures.

In the last few decades, rapid advancements in analytical instrumentation and tech-niques as well as the expansion in computing power have offered an unprecedentedopportunity to unveil the behavior of lubricant polymers under boundary lubricationconditions (at the atomic/molecular or nano levels). For example, Atomic force micro-scope (AFM) with lateral force capabilities can measure the friction between a substrateand sharp tip with contact areas of a few to several hundred atoms. In fact, the lat-eral resolution of LFM can be less than an atomic spacing (Behary, Ghenaim et al.2000; Breakspear, Smith et al. 2003). The surface force apparatus (SFA) can measurethe forces between atomically flat surfaces as their separation is varied with Angstromlevel resolution. The friction and adhesion can be studied as a function of the chem-istry and thickness of the material between the surfaces (Hu and Granick 1998; Sulekand Wasilewski 2006; Drummond, Rodriguez-Hernandez et al. 2007; McGuiggan, Geeet al. 2007; Zappone, Ruths et al. 2007; Zhang, Hsu et al. 2007). Computer simula-tion has also played an important role in interpreting and explaining the findings fromthese experimental methods. Computer simulations and theoretical investigations haveshed much light on the molecular details underlying both structural and dynamic behav-ior of liquids in the highly confined regime (Akagaki and Kato 1988; Kong, Tildesleyet al. 1997).

From a molecular perspective lubricant molecules adsorb on a metal or organic surfaceas ordered or oriented chains. The interactions of solid surfaces and lubricant films couldbe categorized as physical adsorption or chemical reaction (Hsu 2004). As the thickness,the adsorption mass and structure of the adsorbed layer are crucial to the performanceof lubrication (Rabinowi 1967; Grudev and Bondaren 1973; Visscher and Kanters 1990;Gilmour, Paul et al. 2002) in situ techniques that can measure these phenomena areneeded. Surface Plasmon Resonance (SPR) and Quartz Crystal Microbalance (QCM)are well-established noninvasive methods capable of providing a wealth of informationabout interfacial phenomena in situ, in real time and in fluid media (Stockbridge 1966;Nomura, Okuhara et al. 1981; Nomura and Okuhara 1982; Kanazawa and Gordon 1985a,1985b; Johannsmann, Mathauer et al. 1992; Liedberg, Nylander et al. 1995; Rodahl,Hook et al. 1995; Rodahl and Kasemo 1996a, 1996b; Mak and Krim 1997; Homola,Yee et al. 1999; Bailey, Kanazawa et al. 2001; Bruschi and Mistura 2001; Bailey,Kambhampati et al. 2002; Wang, Mousavi et al. 2003; Krim, Abdelmaksoud et al.

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Table 4.1 General comparison between QCM and SPR techniques.

Instrument QCM-D SPR

Principle Piezoelectric/electromechanical OpticalResolution Few ng/cm2 in waterDetection range The detection range varies from

nanometers to micrometers,depending on the viscoelasticity ofthe adsorbed film. In pure water itis approximately 250 nm.

∼300 nm (related to thewavelength of theprobing light)

Informationprovided

– Adsorbed mass– Adsorption kinetics– Dissipation

– Total adsorbed mass– Adsorption kinetics– Reflective index adjacent

to metal surface

2004; Lundgren, Persson et al. 2006). Ellipsometry is another powerful technique thatcan be applied to measure the mass and thickness of adsorbed layers (Fukuzawa, Shimutaet al. 2005). Even though friction cannot directly be measured with these techniques,they can be instrumental in finding a relationship between the extent of adsorption (andviscoelasticity of the adsorbed layer in the case of QCM with dissipation monitoring,QCM-D) and lubrication (as measured by LFM, SFA and others).

The ability to evaluate dynamic behavior is quite similar with both QCM and SPR.Table 4.1 compares these two techniques. QCM-D systems are more sensitive to water-rich and extended layers, while the SPR system is favored for compact and dense layers.The reason for this difference is due to the different physical principles by which thecoupled mass is measured. The mass-uptake estimated from SPR data is based on thedifference in refractive index between the adsorbed materials and water displaced uponadsorption. Therefore water associated with the adsorbed materials, i.e. hydration water,is essentially not included in the mass determination. In contrast, changes in frequencyacquired with QCM-D are affected by the coupled water arising from hydration, the vis-cous drag and/or entrapment in cavities in the adsorbed film. In QCM-D measurementsthe layer is essentially sensed as a ‘hydrogel’ composed of the macromolecules andcoupled water. Changes in the QCM dissipation (D) signals can be related to the shearviscous losses induced by the adsorbed layers. These viscous losses can provide infor-mation to identify structural differences between different adsorbed systems, or structuralchanges in the same type of molecule during the adsorption process. By applying theproper interpretation models one can therefore decouple the effect of coupled solvent.A more detailed account of the principles involved in QCM and SPR is given in thenext section.

4.8 Surface Plasmon Resonance, SPR

A surface plasmon is a electromagnetic wave occurring at the interface between a metaland a dielectric material (Liedberg, Nylander et al. 1995). Surface plasmons are excitedwhen the energy of the photon electrical field is tuned to a specific value at which it can

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UNCORRECTED PROOFS



Light source

Polarized Incident light

I

IIPrism

Flow channel

Inte

nsity

I

II

Detector

Angle

Chip with metal thin film

Reflected light

I II

Cha

nge

in

refr

activ

e in

dex

Time

I

II

Figure 4.8 Schematics of surface plasmon resonance.

interact with free electrons available in the metal surface. This photon energy is thentransferred to a charge density wave and can be observed as a sharp dip in the reflectedlight intensity. The angle at which the sharp dip happens is called ‘SPR angle’. Outsidethe metal surface there is an evanescent electric field which decays exponentially. Thisevanescent field interacts with the close vicinity of the metal. The SPR signal arises underconditions of total internal reflection and depends on the refractive index of solutions incontact with the surface. Molecules in solution exhibit changes in refractive index andthus give rise to a measurable SPR signal if specific interactions occur. A schematicillustration of SPR is shown in Figure 4.8.

The refractive index near the sensor surface changes because of the binding of poly-mers to the surface. As a result, the SPR angle will change according to the amountof bound material. The thickness of the adlayer can be estimated from Equation (4.3)(Bailey, Kanazawa et al. 2001), which assumes that the thickness of the dielectric filmis much smaller than the wavelength of the probing laser:

df = nλ√−εmεs(εs − εm)

2π

εf

(εf − εs)(εf − εm)

(εm + εs

εmεs

)2

�(sin θc) (4.3)

where df is the thickness of adlayer; n is the solvent refractive index; λ is the wavelengthof the incident laser; εf is the dielectric constant of the film; εs is the dielectric constantof the solvent; εm is the real part of the dielectric constant of the metal; and θc is thecritical resonant angle on the plasmon resonance curve. So for a given system withknown solvent and metal, θc is the only variable. Equation (4.3) can be simplified as:

df = k�(sin θc) (4.4)

where k is a factor that can be obtained after a calibration. In most cases, θc is verysmall and there is a linear relationship between the amount of bound material and theshift of the SPR angle (Liedberg, Nylander et al. 1995; Homola, Yee et al. 1999).SPR response values are usually expressed in resonance or refractive index units. Onelimitation of SPR technique is that compounds with molecular weights smaller than

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100–200 Daltons are difficult to detect. Also, due the limited penetration depth of theevanescent wave, adsorbates much larger than this range cannot be measured totally.However, both situations are not relevant in most experimental cases and the linearrelationships hold. The reader is referred to a number of excellent review papers thatdiscuss SPR and its principles of operation (Liedberg, Nylander et al. 1995; Homola,Yee et al. 1999).

4.9 Quartz Crystal Microbalance with Dissipation, QCM

A QCM crystal consists of a thin quartz disc sandwiched between a pair of (gold)electrodes. Due to the piezoelectric properties of quartz, it is possible to excite thecrystal to oscillation by applying an AC voltage across its electrodes.

The resonant frequency (f ) of the Quartz crystal depends on the total oscillating mass,including water coupled to the resonator. When a thin film is attached to the crystalits frequency decreases. If the film is thin and rigid, negligible or minimum energydissipation occurs and the decrease in frequency is proportional to the mass of the film.In this case the Sauerbrey relation can be applied (Sauerbrey 1959):

�m = −ρqtq�f

nf0= −ρqvq�f

2nf 20

= −c�f

n(4.5)

C = typically 17.7 ng Hz-1 cm−2 for a 5 MHz quartz crystal.n = 1,3,5,7 is the overtone number.

Because the change in frequency can be detected very accurately the QCM operates asa very sensitive balance. The quartz crystal microbalance was first used to monitor thinfilm deposition in vacuum or gas atmospheres. Later on, it was shown that QCM maybe used in the liquid phase thus dramatically increasing the number of applications. TheSauerbrey relation was initially developed for adsorption from the gas phase but it isnow extended to liquid media where it holds in most cases. In order to describe softadlayers of polymer adsorbing from liquid media, the dissipation value D was introduced.Rodahl et al. (Rodahl, Hook et al. 1995) extended the use of the QCM technique andintroduced the measurement of the dissipation factor simultaneously with the resonancefrequency by switching on and off the voltage applied onto the quartz. The measuredchange in dissipation is originated by changes in the coupling between the oscillatingsensor and its surroundings and it is influenced by the layer’s viscoelasticity and slipof the adsorbed layer on the surface. The dissipation factor D, is the inverse of theso-called Q factor and it is defined by:

D = 1

Q= Edisspated

2πEstored(4.6)

where Edissipated is the energy dissipated during one period of oscillation and Estored is theenergy stored in the oscillating system. The resonance frequency is measured when theoscillator is on and the amplitude A of the oscillation is monitored when the oscillator

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is turned off. A can be determined in its decay as an exponentially damped sinusoidalfunction:

A(t) = A0e−t/τ sin(ωt + ϕ) + c (4.7)

where τ is the decay time, ω is the angular frequency at resonance, φ is the phase angleand the constant, c, is the offset. The dissipation factor is related to the decay timethrough Equation (4.8).

D = 1

πf τ(4.8)

Combining Equations (4.5) and (4.8) the dissipation changes can be expressed asEquation (4.9). This equation shows that dissipation changes depend not only on theproperties of the adsorbed layer but also the density and viscosity of the solution(Rodahl and Kasemo 1996a):

�D = √n

1

ρqtq

√ηf ρf

2πf(4.9)

Generally, soft adlayers dissipate more energy and thus are of higher dissipation value.From this point of view, the dissipation value is an indicator of the conformation of theadlayer.

A practical QCM-D system records the signals of fundamental frequency (5 MHz)and overtones (e.g. 15, 25 and 35 MHz and even high frequencies for newly developedsystems). Each overtone has its own detection range in thickness. Theoretical work byVoinova and coworkers (Voinova, Rodahl et al. 1999) advanced a general equation todescribe the dynamics of two-layer viscoelastic polymer materials of arbitrary thicknessdeposited on solid (quartz) surfaces in a fluid environment as follows:

�f ≈ − 1

πρ0h0

⎧⎨⎩η3

δ3+

∑j=1,2

[hjρjω − 2hj

(η3

δ3

)2ηjω

2

μ2j + ω2η2

j

]⎫⎬⎭ (4.10)

�D ≈ 1

2πfρ0h0

⎧⎨⎩η3

δ3+

∑j=1,2

[2hj

(η3

δ3

)2μjω

μ2j + ω2η2

j

]⎫⎬⎭ (4.11)

where ρ stands for density; h stands for thickness; η stands for viscosity and δ stands

for the viscous penetration depth (δ =√

2η

ρω). The subscript 0, 1, 2 and 3 denote quartz

crystal, layer 1, layer 2 and bulk solution respectively. From this model, the shift ofthe quartz resonance frequency and the shift of the dissipation factor strongly depend onthe viscous loading of the adsorbed layers and on the shear storage and loss moduli of theoverlayers. These results can readily be applied to quartz crystal acoustical measurementsof polymer viscoelasticity which conserve their shape under the shear deformations anddo not flow as well as layered structures such as protein films adsorbed from solutiononto the surface of self-assembled monolayers. By measuring at multiple frequencies andapplying this model the adhering film can be characterized in detail: viscosity, elasticityand correct thickness may be extracted even for soft films when certain assumptionsare made.

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4.10 Application of SPR and QCM to Probe Adsorbed Films

4.10.1 Monitoring Adsorption and Desorption of Macromolecules

SPR and QCM techniques are useful to determine if a given molecule has affinity ornot with the respective metal/organic/polymeric substrate. They also enable elucida-tion of how strong the affinity is by measuring the actual kinetics of adsorption anddesorption. For example, in a report about the uptake from an organic solution ofoctadecyltrichlorosilane, which is of particular interest for the fabrication of microelec-tromechanical system devices, the authors used quartz crystal microbalance data to fita Langmuir isotherm (Hussain, Krim et al. 2005). In this case the adsorption rate waswritten as follows (Equation 4.12):

φ(t) = β

α[1 − exp(−αt)] (4.12)

where φ is the fraction of free active sites on the surface, α = Cbkaf + kar and β =Cbkaf. Cb is the concentration of adsorbate, while kaf and kar represent the constants ofadsorption and desorption. The parameters α and β can be obtained by fitting frequencydata. Furthermore, from the relation between α and Cb, the values of kar and kaf andthe adsorption equilibrium constant (Keq = kaf/kar) was calculated as well as the freeenergy of adsorption (Equation 4.13):

�G = −RT ln Keq (4.13)

In a typical experiment a baseline is first established prior to injection of the adsorbate(analyte or lubricant, in our case). A sharp change in SPR signal or QCM frequency (anddissipation) will be observed if adsorption occurs. When these monitored signals reachtheir equilibrium values, a large amount of background buffer solution can be injectedas a rinsing step. If the adsorbate is replaced by the solvent (desorption) upon rinsingthe respective signals will tend to go to the original value. This behavior would thenindicate that the molecules in bulk as well as molecules loosely bound on the surfacewere removed by the rinsing step.

Lubricant degradation can also be measured via QCM. In order to monitor the degrad-ing process of lubricants at high temperature, Wang et al. (Wang, Mousavi et al. 2004)used QCM at high temperatures (more than 200 ◦C) to evaluate the thermal stabilityof polyol ester lubricants. Figure 4.9 provides an example that demonstrates how twolubricants showed different sensitivities to temperature. Here the lubricants were heldin a T-controlled chamber. The lubricants degraded gradually when they were heatedto very high temperature leaving solid residues on the tested surfaces. The behavior oftwo commercial-grade pentaerythritol tetrapelargonate based lubricants, represented bythe codes ‘EM’ and ‘AF’ (corresponding to two commercial lubricant compositions), areshown in this figure. During the first nine hours, both EM and AF didn’t change withthe temperature treatment indicating that both lubricants were stable. However, afterexposure to high temperatures for nine hours the frequency of AF decreased rapidlywhile that of EM barely changed. This behavior indicated that EM was much morestable than AF at the tested temperature of 200 ◦C. QCM can thus provide an integralpicture of the thermal stability of lubricants in real-time, and in situ.

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0

Df (

Hz)

−2500

−2000

−1500

−1000

−500

0

2 4 6 8 10 12

Time (hour)

14 16 18

AF

20 22 24

EM

Figure 4.9 Time-dependent frequency change of QCM for EM and AF adsorbed on QCMcrystal at 200 ◦C. From, Mousavi et al. (2004), reproduced with permission from AmericanChemical Society.

4.10.2 Conformation of Adsorbate Layers Revealed by the QCM-D

Indirect information about the conformation of adsorbed layers can also be derivedfrom QCM experimental data. For rigid, ultrathin, and evenly distributed adsorbedlayers, the Sauerbrey equation (Sauerbrey 1959) describes successfully the proportionalrelationship between the adsorbed mass (m) and the shift of the QCM crystals’ resonancefrequency (f ). Under these conditions, the dissipation value is a constant. It doesn’tchange with time or with increasing adsorbed mass. On the other hand, if the adsorbedmaterial exhibits a viscoelastic behavior, such as that exhibited by layers of proteins,substantial deviations from the Sauerbrey equation can occur. Using �D–�f plots onecan eliminate time as an explicit parameter and as concluded in previous studies (Rodahland Kasemo 1996a; Hook, Kasemo et al. 2001; Edvardsson, Rodahl et al. 2005), theabsolute slopes and their gradients provide information about the kinetic regimes andthe conformational changes of the polymer. The magnitude of the slope provides anindication on the conformation of the adsorbed layer: Lower values indicate a softerlayer. If more than one slope exists it can be concluded that more than one conformationstate of the adsorbed layer are present during the adsorption process.

Figure 4.10 shows QCM results (shifts in frequency) for a cellulose-coated sensorafter injection of a high charge density polyampholyte solution using a 1 mM NaClbackground electrolyte solution (130 μl/min flow rate). For comparison, the case of asilica surface is also included.

Figure 4.10 illustrates that shifts in frequency upon polyampholyte adsorption onsilica were two times larger than those measured in the case of cellulose films. Also,it is interesting to note that for both substrates, silica sand cellulose, a small changein the measured QCM frequency was observed after replacing the polymer solution

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0

5

10

15

20

25

30

35

40

0 20 40 60 80 100

−Df (

Hz)

Time (min)

Cellulose filmSilica surface

Figure 4.10 Changes in frequency with high-charge density polyampholyte adsorption,before and after rinsing, for silica substrate and cellulose film surfaces. Conditions: pH 4.3;temperature 25 ◦C; and [NaCl] 1 mM. The polyampholyte was injected at time 10 min andafter an incubation time of ca. 55 min rinsing with background electrolyte was performed.

with the buffer solution (rinsing). These observations imply that the interactions forcesbetween the polyampholyte and silica are stronger than for the cellulose film. Also,a faster dynamics of adsorption occurs in the case of silica. These results can beexplained by considering electrostatic interactions as the main driving mechanism foradsorption as both substrates exhibit significantly different surface charge densities. Forlong equilibration times small changes in frequency are evident; this is hypothesizedto be the result of polymer reconformation and exchange at the interface, given thepolydisperse nature of this macromolecule.

The swelling and water-holding ability of adsorbed polyampholyte layers on cellulosefilms as a function of ionic strength was evaluated by using D-f plots. Figure 4.11 showsthe relation between dissipation and frequency change for the same high charge densitypolyampholyte adsorbed on cellulose at different ionic strengths.

Larger variations in energy dissipation imply more viscoelastic layers. Significantchanges in energy dissipation can be seen for intermediate values of salt concentration(e.g., 10 and 100 mM). The viscoelastic character of the polyampholyte layers built upat extreme salt conditions is interpreted as being the result of more rigid structures (e.g.,0.1, 1, and 1000 mM). On the other hand, no major differences are observed on the stateof hydration and extension of the adsorbed layer.

4.10.3 Coupling QCM and SPR Data

While SPR and QCM are often used to monitor adsorption and adsorbed layer dynamics,each technique has its own strengths and weaknesses. Also, as presented before, theyhave assumptions inherent in data collection and analysis (Bailey, Kambhampati et al.

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0

1

2

3

4

5

6

7

8

0 5 10 15 20 25 30 35 40

DD

x 1

06

−Df (Hz)

0.1 mM NaCl1 mM NaCl10 mM NaCl100 mM NaCl1000 mM NaCl

Figure 4.11 �D-Df profiles for polyampholyte adsorption on cellulose surfaces at differentionic strengths. The high charge density polyampholyte consisted of 20% cationic and 16%anionic groups.

0

Nor

mal

ized

Thi

ckne

ss

−0.2

0

0.2

0.4

0.6

0.8

1

1.2

500 15001000

QCM Kinetics

Injection Artifact

SPR Kinetics

Time (s)

2000

Figure 4.12 Comparison of adsorption kinetics of a perfluoropolyether lubricant (FomblinZDOL) deposited on silver surfaces as measured by SPR and QCM techniques. Figure fromBailey, Kambhampati et al. (2002) reproduced with permission from American ChemicalSociety.

2002). However, since the two techniques rely on fundamentally different principlesof physics, namely optical and electromechanical, a more complete perspective of theadsorption phenomena can be achieved by combining them. Figure 4.12 illustrates anexample to demonstrate how QCM and SPR data can be combined to study the kineticsof adsorption of a thin organic film. In this case both curves agree with each othervery well.

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UNCORRECTED PROOFS



Deviations between the signals in QCM and SPR experiments may indicate that thefilm is viscoelastic or that there is some coupled water in the adsorbed layer. Bycarefully considering the nature of each measurement it is possible to decouple theviscoelastic properties and the contributions from coupled water in the film. Belowa more detailed explanation about the role of coupled water is presented. Water canbe used as a boundary lubricant as the fluidity of the hydration layers nanoconfinedbetween two surfaces significantly differs from the behavior of the water in the bulk(Raviv, Laurat et al. 2001; Zhu and Granick 2001; Raviv and Klein 2002; Leng andCummings 2005). The water coupled with lubricant polymers has the same function, i.e.to protect the contact surfaces and minimize abrasion. Measuring of the coupled wateris not an easy task since it is difficult to distinguish the coupled from the bulk water.Below are two alternative ways to decoupled the contribution from water via QCM andSPR measurements.

The first approach is to substitute water solvent with D2O, as reported by Hook andothers (Hook, Kasemo et al. 2001; Craig and Plunkett 2003; Notley, Eriksson et al.2005). D2O substitution increases the density and shear viscosity of the bulk liquid andcoupled water by ∼10% and ∼25%, respectively but presumably it doesn’t change anykinetic and equilibrium state. Therefore, from the slight difference in frequency fromexperiments conducted in normal and heavy water, the coupled water fraction can beobtained through Equation 4.14 (Craig and Plunkett 2003).

Sfraction = �fs − �fd

�fs

(1 − ρd

ρp

)− �fd

(1 − ρs

ρp

) (4.14)

Subscript s, d, p represents solvent, deuterated water and polymer respectively. In somecases where ρp = ρs, Equation (4.14) can be simplified to Equation (4.15):

Sfraction = �fs − �fd

�fs

(1 − ρd

ρp

) (4.15)

Since the difference is very small, only polymers adsorbing in large quantities or carryinglarge amounts of coupled water can be analyzed with this approach.

The second method to decouple hydration from bulk water is by combining QCM andoptical methods, for example SPR or ellipsometry (Hook, Kasemo et al. 2001). Thechange in resonant frequency (f ) of the QCM crystal depends on the total oscillatingmass which includes the coupled water. In the case of SPR or ellipsometry water coupledwith adsorbed molecules doesn’t affect the refractive index hence they are not detectedby these optical techniques. Therefore by subtracting the mass determined from SPR orellipsometry measurements from that obtained from QCM measurements the contributionof coupled water can be revealed. Figure 4.13 is an example used here to demonstrate thecombination of QCM and SPR techniques. The polymer tested was a cationic polyamide(5% cationic groups), with molecular weight ca . 3 million. The surface used in thisexperiment was a negatively charged silica surface. The experimental results indicatethat there was around 25% of water in the adsorbed polymer layer.

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0 500 1000 1500 2000 25000

50

100

150

200

250

Aso

rbed

mas

s (n

g/cm

2 )

Time (s)

SPR

QCM

Rinse with water

Figure 4.13 Decoupling water content through the combination of QCM and SPR mea-surements. The polymer used in this experiment was a cationic polyamide. The calculatedcoupled water determined by this method was found to be 25%.

4.11 Lateral Force Microscopy

Both SPR and QCM allow for the real time in-situ monitoring of adsorption processes.Although relevant to lubrication phenomena these adsorption techniques do not measurefriction behavior in a direct manner. Lateral Force Microscopy (LFM) is a techniquethat can directly measure friction by lateral forces. These direct measurements allow forthe evaluation of lubricants’ performance on specific surfaces with nanoscale resolution.LFM when used with SPR and QCM techniques could unveil a more comprehensiveunderstanding of lubrication phenomena.

LFM is based on scanning probe microscopy and it is one of the few experimentalmethods capable of assessing forces at the single contact or atomic level. LFM andatomic force microscopy (AFM) share the same principles. A typical AFM comprisesthree main components: laser source, cantilever and photo-detector (see Figure 4.14).When an atomic force microscopy (AFM) tip slides on a surface it is deformed both inthe vertical and the horizontal directions (Figure 4.14). The force Fn, which is normalto the surface of the sample, results in vertical bending of the free end of the cantilever.By contrast, the force Fl , which is parallel to the probed surface and is in the oppositedirection to the sliding direction, leads the cantilever into a twisting motion. A typicalAFM measures only the normal force, Fn. What distinguishes LFM form AFM, as thename indicates, is that it measures both Fn and Fl .

In order to precisely detect the forces between the tip and the surface, a laser beam isreflected off the back of the cantilever onto a quadrant photodiode detector. The outputof the quadrant detector is used to determine the degrees of bending and twisting of thecantilever. The laser beam method is the most commonly used monitoring techniqueas it can achieve a resolution comparable to that of an interferometer while it is alsoinexpensive and easy to use. The availability of lateral force microscopy (LFM) hasmade it possible to explore friction and wear at the molecular level and to examine the

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UNCORRECTED PROOFS



Laser

Surface

Cantilever Fn

F

Photo Detector

Figure 4.14 Schematic of lateral force microscopy and twisting and bending motions actingon the cantilever.

Sample

Sample

LFM Image

different material

LFM Image

Figure 4.15 Lateral deflection of the cantilever from changes in surface friction (top) andfrom changes in slope (bottom) (redrawn from http://mechmat.caltech.edu/∼kaushik/park/1-4-0.htm).

effectiveness of a finishing treatment in modifying a specific behavior of the substrate.LFM has been used extensively to study molecular lubrication phenomena on hard sur-faces, such as mica, silica, and graphite. Studies on polymer surfaces, relevant to fiberapplications, however, have been limited, primarily due to the fact that polymer sur-faces deform easily, which adds complexity to the experiment and to the interpretationof the data.

Lateral force acting on cantilever usually arises from two sources: changes in surfacefriction and changes in slope, as illustrated in Figure 4.15. In the first case, since differentmaterials provide different friction, the cantilever can experience different twisting extent

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−80

−60

−40

−20

0

20

40

60

80

0 3

Surface position (μm)

Late

ral f

orce

(pA

)In air

P65-1

P65-2

P65-3

P65-4

P65-5

1 2 4 5

Figure 4.16 One line scanning profiles for cellulose-coated silica surface while immersedin a nonionic triblock lubricant (E19P29E19) solutions and in air. P65 is used to indicatelubricant E19P29E19, which is a triblock copolymer with 19 E groups at both ends and 29 Pgroups in the middle. P65-1∼P65-5 represent a series of ethanol solutions with the increaseof ethanol concentration.

even though the surface being measured is topographically smooth. In the second case,the cantilever may twist when it encounters a steep slope. In order to eliminate theroughness effect caused by the second case in lubrication, two scans on the same line(back and forth) are performed on the substrate in order to measure the net effect (Behary,Ghenaim et al. 2000).

When a tip in lateral force microscopy is sliding on a surface, lateral force profiles canbe measured both in air (no lubricant applied) and in solution. Figure 4.16 shows lateralforce profiles for a cellulose surface imaged in air and immersed in a solution withnonionic E-P-E triblock polymeric surfactants (commonly used as lubricant finishes).Here E and P represent ethylene oxide and propylene oxide, respectively. During theseexperiments, the lubricant was dissolved in ethanol aqueous solutions at various levelsof ethanol concentration (22, 38, 52, 66, and 87%). It was observed that the frictionforces measured in air were significantly larger than those in the respective solutions,confirming the lubrication attributes of the polymer. However, the force profiles inthe five solutions were undistinguishable, making the effect of ethanol concentrationunimportant.

Studies on copolymer adsorption are usually conducted with hydrophobic surfacesand only a few reports have addressed the case of adsorption on hydrophilic surfaces.The adsorption behavior of E19P29E19 copolymers on hydrophilic cellulose surfaces ishereby briefly discussed. It is expected that the self-assembly mechanism of the blockcopolymer in the case of cellulose will be different from that exhibited by hydrophobicsurfaces such as propylene or polyethylene. Wu et al. (Wu, Liu et al. 2000) carriedout an AFM study involving triblock copolymer chains on hydrophilic silica surfaces.They suggested that in the case of hydrophilic surfaces, the E blocks bind the surface

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UNCORRECTED PROOFS



0 10 20 30 40 50 60 70 800.01

0.1

1

Cellulose

Log,

Fric

tion

Coe

ffici

ent

Normal force (nN)

RP10E13RP13E17E26P40E26E133P50E133AirWater

Figure 4.17 The relationship of friction coefficient (COF) and normal force (Fn) on cellulosefilms in air, water and in the presence of four types of nonionic polymers. E: polyethyleneoxide; P: polypropylene oxide; R: alkyl groups.

because the shared hydrophilicity nature of the E blocks and silica surface (affinitybetween the E blocks and the silica surface). Consequently the P blocks are repelledfrom the surface. A competition between solvency of E segments and the enthalpicE-to-surface attraction is likely to be present in the case of cellulose substrates. Therefore,an anchor-buoy-anchor configuration may be formed on the hydrophilic cellulose surface.Molecular self-assembled structures are formed on the interfaces between sliding surfacesas a result of morphology changes at a nanoscale level. These changes mainly dependon the chemical natures of the surface and the liquid. In boundary lubrication, it isbelieved that surface coatings of organized, molecular liquid films will control frictionand reduce wear in fiber processing.

Figure 4.17 shows an LFM curve for coefficient of friction at different applied loads.Under low normal forces the coefficient of friction decrease as the normal force increases.However, at high normal forces, the value of friction coefficient does increase. Thethreshold for this transition was around 30–40 nN. This behavior can be explained bythe fact that lubricant molecules self assemble onto the surface and form a layer undershear and normal forces. At higher shear rates or normal forces, the polymer alignsbetter and forms a more compact structure with a low coefficient of friction. However,at higher pressures and higher loadings, the polymer film might be distorted (moleculescan be driven out from the interface) and the tip can make direct contact with theunlubricated surface thus measuring a higher coefficient of friction. This phenomenonis especially relevant in the case of sharp LFM tips where even a normal force of only30–40 nN can produce a substantially high pressure.

4.12 Summary

In this chapter we discussed the use of QCM and SPR as tools to monitor the adsorp-tion of molecules on solid surfaces. Some examples were provided with regards to the

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modification of the surface of cellulose thin films via adsorption of polyampholytes andnonionic polymers. These techniques allow the determination of fundamental informa-tion, relevant to lubrication phenomena including (1) affinity of adsorbing molecules tothe substrate, (2) viscoelasticity of adsorbed layers, (3) kinetics of adsorption and des-orption, and (4) thickness of the adsorbed layer as well as the amount of coupled water inadsorbed film. LFM was presented as a useful tool used to directly measure friction onpolymeric surfaces. LFM complements results from the adsorption tests as LFM allowsus to quantify the extent of the adsorption as well as the conformation of adsorbed lay-ers. Based on information provided via LFM, SPR and QCM, a better understanding offriction phenomena on cellulosic systems can be achieved. By correlating the structureand lubricant effect of adsorbates, novel formulations with superior performance can betailored. In return one can significantly improve the efficiency of cellulose fiber andtextile processing and improve the quality of products being manufactured.

Overall, it is concluded that a fundamental understanding of adsorption and frictionbehavior can unveil a more complete understanding about boundary lubrication andnanostructuring phenomena on cellulose systems.

Acknowledgement

Funding supported from the National Textile Center under the Grant number C05-NS09and the National Research Initiative of the USDA Cooperative State Research, Educationand Extension Service, grant number 2007-35504-18290 is gratefully acknowledged.Dr Tom Theyson, from Goulston Corp. is acknowledged for his advice and suggestions.

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Queries in Chapter˜4

Q1. We have shortened the running head since it exceeds the size limit. Please clarifyif it is fine.

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tools to probe nanoscale surface phenomena in cellulose ...ojrojas/pdf/2009_3.pdf · uncorrected...

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