Contact:
Dr Nikolaus Wellner
T: +44 (0)1603 255012
E: wellner@ifr.ac.uk

Fracture of biopolymer films

Molecular response during mechanical deformation and failure

Hydroxypropylcellulose (HPC)

HPC is a chemically modified cellulose manufactured by reacting alkali cellulose with propylene oxide, which makes it water-soluble. Polarised FTIR was used to image the plastic deformation developed in HPC films under uniaxial stretching. FTIR spectroscopy can be used to study the molecular orientation of polymers and biopolymers because some of the vibrational modes are anisotropic and the corresponding bands exhibit marked dichroism when the molecules are spatially aligned.

Applied in two dimensions, FTIR mapping of the dichroic ratio (parallel divided by perpendicular polarised) pictures the molecular orientation. Dry films showed a brittle break with a sharp edge. With increasing hydration the deformation zone extended further and further. In that zone the film thickness tapered off towards the edge and considerable molecular orientation was observed. The figure below shows FTIR maps of a notched HPC film hydrated at 47 % relative humidity. The panels show successive snapshots of the area around the crack tip as the film was stretched. The maps were coloured according to the height of the dichroic ratio at 1030 cm-1, which is one of the major IR bands from the glucopyranose ring.

       

In the beginning, the film showed very little molecular orientation, but as the crack widened, the dichroic ratio increased strongly on both sides of the crack tip, indicating that a large plastic zone developed around the crack tip in which the HPC molecules became increasingly aligned in the stretching direction.

Cassava starch

Cassava starch is becoming widely used in the food industry because of its ability to perform most of the functions of maize, rice and wheat starches. As with all starches, cassava consists of two polysaccharides. Amylose has long linear chains of (1→4)-linked a-D-glucopyranose residues, some with a few branches. Amylopectin has high molecular weight and highly branched structure consisting of much shorter chains of (1→4)-linked a-D-glucopyranose residues. The mechanical properties of plasticized starch films depend on the amylose/amylopectin ratio. In the case of cassava starch, the low content of amylose resulted in relatively fewer intermolecular interactions compared to high amylose content starches. The amylose, which is mainly responsible for the crystallinity of starch films, did not organize in an ordered structure by forming a crystalline phase. Moreover, this amorphous structure seemed to be stable. Dry cassava starch films are very brittle. Small amounts of water make them quite pliable, but they still fracture easily.

       

Polarised light microscopy of fractured film fragments showed that the bulk of the film was unchanged, but structure change (‘melting’) with some molecular orientation occurred in defined stress regions running parallel along the fracture edge. These areas were thinner than the surrounding areas, and grew with increasing hydration. The cause for this behaviour was most likely the structure of the film, with starch grain remnants (ghosts) and a slightly heterogeneous distribution of amylose and amylopectin.

Gelatin

Gelatin films prepared from aqueous solution are a well-known biopolymer model system. The gelatin conformation varies from amorphous to partially crystalline due to the presence of triple-helical order. The hydration of gelatin films proceeded through three main stages with different water-protein affinities before reaching the glass-rubbery transition at room temperature: At the first stage of hydration (up to 5% w/w water content), and at the third stage (from 14% to 22%) the films show brittle behaviour at fracture. At the second stage of hydration (from 5% to 14%) the films showed slightly improved fracture behaviour. Synchronous and asynchronous FTIR correlation maps for gelatin films indicated a very strong correlation of the protein backbone hydration with the water uptake, but only limited changes in secondary structure, mainly a shift between random coil and helical structures. The behaviour at fracture was predominantly related to the renaturation level of gelatin films, which coincided with the maximum of the triple-helix content. Gelatin films were very brittle at low hydration levels. With hydration they became more elastic, but still failed abruptly. Linear stretching induced consistent transient patterns of dichroic difference bands (parallel / perpendicular) which indicated some reversible changes in the packing of the molecules in stretched films, but there was no clear evidence for molecular orientation. After fracture, no permanent change in molecular structure or orientation was observed away from the fracture edge.

These brittle films fractured very suddenly. In order to visualise the actual cracking it was necessary to use a high-speed camera, generously lent to us by the EPSRC instrument pool. Videos captured at 50000 frames per second show the progression of the crack through a gelatin film..