W. Richard Bowen and Nidal Hilal 4

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226 8. ATOMIC FORCE MICROSCOPy ANd POLyMERS ON SURFACES Ultrathin polymer coatings, in particular, play a crucial role in many processes, ranging from decoration and protection against a variety of degradation agents (corrosion/chemical) [1] to microfabrication methodologies for microelectronics [2] and biomedical devices [3]. In all situations, at the polymer-solid interface, there is a layer of polymer chains that are in direct contact with the solid surface. The coating can be so thin that it actually consists of just one mono-macromolecular layer, i.e. a monolayer of polymer chains that are all in contact with the solid surface. Such monolayers are typically some nanometres thick (the thickness is of the order of the typical size of one polymer chain). In many cases, the solid surface is not fully covered with polymer chains, and we have the formation of a submonolayer, the structure of which can range from a complete layer occasionally disrupted by some isolated holes to a solid surface only partially covered by some isolated polymer islands. Depending on the molecular interactions and the number of chains at the surface (surface coverage), there is a variety of different continuous, semi-continuous and discontinuous nanopatterns and nanostructures that can be formed. Polymer monolayers and submonolayers on solid surfaces play an important role for many technological applications such as nanopatterning and surface modifications used in microelectronics [4], colloidal stability and flocculation [5], polymer reinforcement with nanofillers [6, 7], non-fouling biosurfaces [8], biocompatibility of medical implants [9], separations [10], microfluidics [11], adhesion, lubrication and friction modification [12]. Of particular importance are recent developments in the direction of achieving smart responsive surfaces to various external stimuli using polymer monolayers and nanostructures [13–15]. The atomic force microscope (AFM) [16] provides a spatial and force resolution in the order of angstroms and tens of piconewtons, respectively; therefore, it is ideally suited to study the fine morphology and physico-chemical properties of materials at the nanometre scale, directly in real space. This is of particular importance in the field of polymer- modified material surfaces. The traditional surface techniques lack the lateral resolution at the nanometre scale, which is necessary in order to analyse laterally inhomogeneous monolayers and submonolayers of polymers, macromolecular-size clusters and polymer nanostructures or nanopatterns. Furthermore, the AFM can operate under liquid conditions, providing a direct look of polymers anchored on surfaces in various solvent conditions. In many cases, solution-based processing techniques (e.g. dip-coating, spin-coating, droplet-evaporation, spraying) are employed and consequently, direct measurements within a good solvent (for the polymer) are of particular importance. AFM investigations within liquids can reveal and elucidate the physico-chemical phenomena governing the polymer monolayer behaviour during formation and processing.
8.2 BASIC CONCEPTS 227 Fur-thermore, in many cases, mainly within biotechnology applications, the functional use of the polymer monolayer is under liquid (e.g. aqueous) conditions and in these cases AFM provides the opportunity for direct, real-space and real-time monitoring of the polymer layer during its functional role. In any case, it has to be noted that AFM studies of dry polymer monolayers are more numerous since they are easier to implement. To some extent, they can provide important snapshots of polymeric behaviour in the solution and can elucidate many aspects of the polymer behaviour. In this chapter, first, a concise and simplified version of the fundamental physical ideas of how polymer chains behave when they are in close proximity to surfaces is presented; the following part is dedicated to the main objective of this chapter, which is to demonstrate the potential, versatility and flexibility of the AFM technique to probe in a direct manner the nanoscale structural and physical properties of polymer monolayers and submonolayers. To this end, we use some selected examples from our own AFM studies. We show that the structural regimes depend on many factors such as surface interactions, polymer molecular weight, surface density, solvent conditions, chemical composition and molecular architecture. The structural properties of these ultrathin polymer films have a profound effect on various chemomechanical properties of the modified surfaces. In summary, we show in a direct manner the important role of the AFM as a very appropriate characterisation technique and tool for the investigation of materials surfaces that has been modified and processed by polymer monolayers and submonolayers. 8.2 BASIC ConCEPtS A polymer chain can be attached on a surface by physical (e.g. van der Waals, electrostatic forces) and chemical (e.g. covalent) bonds. Its behaviour depends strongly on the characteristics of the attachment such as the number and position of attachment points (e.g. anchoring by its end or attachment points along the backbone of the chain) and bond strength, with chemical bonds being usually stronger than any physical attractive forces and hydrogen bonds (which are ranked as of moderate strength). One has to note that a large number of weak physical bonds can be a very efficient way of attachment and in this way a polymer chain can take a flat conformation (Figure 8.1). Nevertheless, the adsorption by chemical bonds (chemisorption) is considered irreversible, while adsorption by physical attractive forces (physisorption) is usually reversible under certain processing conditions. A polymer chain in good solvent conditions (e.g. polystyrene in toluene) is swollen since the polymer segments repel each other because of the
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Butterworth-Heinemann is an imprint
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x Preface in their work. Hence, it
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xii Preface them from hostile condi
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xiv About thE Editors Professor Nid
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xvi List of Contributors Dr Daniel
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2 1. BAsIC PRINCIPLEs OF ATOMIC FOR
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32 2. MEASUREMENT OF PARTICLE ANd S
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82 3. QUANTIFICATION OF PARTICLE-BU
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100 3. QUANTIFICATION OF PARTICLE-B
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C H A P T E R 4 Investigating Membr
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4.2 THE RANgE OF POssIbILITIEs FOR
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4.2 THE RANgE OF POssIbILITIEs FOR
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4.3 CORREsPONdENCE bETwEEN sURFACE
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4.3 CORREsPONdENCE bETwEEN sURFACE
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4.4 IMAgINg IN LIqUId ANd THE dETER
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4.4 IMAgINg IN LIqUId ANd THE dETER
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4.5 EFFECTs OF sURFACE ROUgHNEss ON
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4.6 ‘vIsUALIsATION’ OF THE REjE
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4.7 THE UsE OF AFM IN MEMbRANE dEvE
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Dfractional 0.18 0.16 0.14 0.12 0.1
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4.8 CHARACTERIsATION OF METAL sURFA
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At pH 5.5, dissolution began to occ
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REFERENCEs 137 [4] W.R. Bowen, N. H
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C H A P T E R 5 AFM and Development
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5.2 MEAsUREMENT OF ADHEsION OF COLL
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5.2 MEAsUREMENT OF ADHEsION OF COLL
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The antibacterial activity of initi
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5.3 MODIFICATION OF MEMBRANEs 147 t
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5.3 MODIFICATION OF MEMBRANEs 149 B
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Force (nN m -1 ) Loading force (mN
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5.3 MODIFICATION OF MEMBRANEs 153 p
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Adhesion force (mN m -1 ) 10 8 6 4
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Adhesion force (mN m -1 ) 20 15 10
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Adhesion force (mN m -1 ) 10 8 6 4
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5.3 MODIFICATION OF MEMBRANEs 161 F
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5.4 MODIFICATION OF MEMBRANEs wITH
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5.4 MODIFICATION OF MEMBRANEs wITH
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Å 200 100 0 5.4 MODIFICATION OF ME
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AbbrEvIAtIonS And SyMbolS NOM Natur
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REFERENCEs 171 [29] W.R. Bowen, T.A
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174 6. NANOSCALE ANALySIS Of PHARMA
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276 10. FuTuRE PRosPECTs most AFM s
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278 10. FuTuRE PRosPECTs targeted d
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A Actin stress fiber, 197 Adhesion,
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Natural organic matter, 140 Negativ
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W. Richard Bowen and Nidal Hilal 4

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