Biomedical Engineering Reference
In-Depth Information
D
Figure 4.1
Schematic illustration of polymers adsorbing from solution onto a surface.
Distheaveragethicknessof theadsorbedpolymerlayer,thevalueofwhichdependsonthe
methodusedtomeasureit.
Adsorption results as a consequence of the balance between surface energy and the
nature of the adsorbing species. While the conformation of a polymer in solution depends
on solvency and polymer chain composition and architecture, at an interface the polymer
can be perturbed by the interaction of its segments with the surface (see Figure 4.1).
When this interaction involves 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 their
dissociation ability in aqueous solution. As such they are classified into ionic (also known
as polyelectrolytes) and nonionic polymers. Ionic polymers are also classified into simple
polyelectrolytes, 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 experimental
perspectives. In this chapter, we will first describe the adsorption of a relevant type of
charged polymer onto cellulose surfaces. We will then review aspects related to boundary
lubrication in the case of adsorbed nonionic polymer on the same substrates. Finally,
we will present a brief account on the techniques used to study polymer adsorption
and lubrication. Specifically, we will discuss two tools to determine the extent and
dynamics of polymer and surfactant adsorption: The quartz crystal microbalance QCM
and the surface plasmon resonance SPR techniques. We will also discuss the use of
lateral force microscopy LFM as a useful approach to investigate friction phenomena.
This information presented in this chapter will be helpful to appreciate other chapters
covering specific aspects of surface modification (including hemicellulose adsorption and
polymer multilayers). Complementary tools for nanoscale characterization of biomass
was discussed in chapter 3.
4.2
Polyampholytes Applications in Fiber Modification
Hydrosoluble polymers are commonly used in industry. Among these, amphoteric
macromolecules or polyampholytes have been employed in papermaking to modify cellu-
losic fibers thereby enhancing inter-fiber bonding. Generally speaking, a polyampholyte
is defined as charged macromolecule carrying both acidic and basic groups (Dobrynin,
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