Many biological polymers have
remarkable mechanical properties. Pure cellulose and silk, for example, are
stronger than mild steel. In nature, structural biopolymers are not often found
in pure form but are assembled into composite materials in which strong fibres
are embedded in a hydrated, sometimes structurally complex, matrix. Examples
with diverse properties are wood, artery walls, hair and tendon. These
materials have properties that are unfamiliar to structural engineers. They are
commonly less rigid than man-made structures, but this strategy is very
effective and small amounts of biopolymer material can withstand remarkable
loads.
We are just beginning to
understand and model how biological composites function as structural
materials. New methods to see inside them as they deform under stress are
urgently needed. Vibrational spectroscopy is turning out to be useful in a
number of ways, and the aim of this project is to develop and exploit two new
methods based on this principle. At least some of the experiments will be
carried out on wood, the focus of much of the ongoing work in Mike Jarvis’s
lab. If the student wishes they can be extended to mammalian biomaterials.
When a load-bearing covalent
bond is stretched, it becomes weaker and the frequency of its stretching
vibration decreases. This effect can be seen as a small bandshift in the FTIR
or Raman spectrum. Stretching of hydrogen bonds can be detected in a slightly
different way. Hydrogen bonding with an OH or NH group as donor weakens the O-H
or N-H bond and alters the frequency of its vibrational modes. When the
hydrogen bond is stretched, the covalent bond in the donor group shortens and
its stretching vibration moves to higher frequency. These bandshift effects
have not been widely used because they are not large. Often a material will
break before the load is great enough for any bandshifts become readily
measurable. We have recently found a way to process the FTIR spectra that
extracts and disentangles complex patterns of small bandshifts, even when the
parent spectra seem visually identical. One part of the project will consist of
developing this method and finding practical uses for it.
Another kind of elasticity in
biopolymers is associated with their reorientation under mechanical stress.
Polymer orientation in solid materials can be detected by polarised vibrational
spectroscopy. The intensity of a stretching vibrational band is maximal when
the covalent bond responsible (or, strictly, the transition moment of the
vibrational mode which corresponds approximately with the bond axis) is
parallel to the direction of polarisation. However covalent bonds with
convenient stretching vibrations are not usually oriented exactly along the
axis of the polymer chain, which makes the analysis of the data rather
complicated. We have worked out an analysis of this, which has been used
successfully to predict polarisation data for cellulose chains in different
orientations, as part of a project aimed at finding the geometry of hydrogen
bonding within cellulose fibres. This analysis has since been used to measure
the orientation of cellulose in plant biomaterials, but it was not designed for
this purpose and is rather cumbersome to use. The second aim of the PhD project
is to simplify this method and integrate it with the bandshift approach
described above, so that the stretching and the reorientation of polymers in
biocomposite materials can be measured in a single spectroscopic experiment.
Because of the difficulty of
preparing large homogeneous samples of many biomaterials, most of the
experiments will be carried out on an FTIR microscope and will require some
manipulative skill. Mathematical skills would also be useful, but the
mathematical depth in which the experimental methods are developed depends very
much on the student concerned. The project can if desired become quite applied,
and there will be opportunities for close interaction with plant and human
biologists or foresters and wood scientists.