|dc.description.abstract||The purpose of this study was to use concepts from classical polymer physics to develop a fundamental understanding of the interdependent relationship between structure, properties and processing in NovateinTM Thermoplastic Protein (NTP). NTP is produced from bloodmeal, a by-product of the meat industry which is 95% protein, by extruding with sodium sulphite, urea, sodium dodecyl sulphate, water and tri-ethylene glycol. Dynamic mechanical thermal analysis (DMA) and differential scanning calorimetry (DSC) were used to investigate thermal transitions and chain relaxation, accompanied by synchrotron based Fourier transform infrared spectroscopy (FT-IR) to investigate chain architecture and structural changes. Extrusion, injection moulding and mechanical testing were used to investigate macroscopic properties, such as processability and mechanical properties.
Material pocket DMA enabled detection of glass transition temperatures not only for moulded NTP test pieces, but also for each processing step: bloodmeal, NTP prior to extrusion, extruded NTP, and conditioned NTP. The pockets increased resolution for injection moulded and conditioned samples, revealing multiple transitions which indicated the presence of more than one phase. Spatially resolved FT-IR experiments were used to characterise the relative content and distribution of protein secondary structures in bloodmeal and NTP after each processing step. Increased chain mobility was observed due to the additives used, and also drastic structural rearrangement consistent with consolidation into a thermoplastic material after extrusion. Viscoelastic phenomena such as creep and stress relaxation further confirmed processed NTP was a consolidated thermoplastic, exhibiting time dependent behaviour characteristic of thermoplastic polymers.
The thermal properties of NTP suggested an underlying semi-crystalline structure, with protein secondary structures such as α-helices and β-sheets constraining motion in the amorphous phase, contributing to a broad glass transition region. These structures do not melt at typical temperatures encountered during extrusion processing, but are dispersed more evenly throughout an amorphous matrix. Other than plasticisation, strong hydrogen bonding interactions stabilising secondary structures were the dominant influence on mechanical properties of the processed NTP. When TEG was included as a plasticiser, the degree of crystallinity decreased, and the fraction of randomly coiled protein chains was greater at all stages of processing compared to NTP without TEG. TEG competes with protein side groups for hydrogen bonding sites on the protein backbone, reducing secondary structure formation. β-sheets increased when NTP was heated in the absence of shear, displacing TEG and causing it to migrate out of the particles. The short residence time and presence of shear and mixing during extrusion and injection moulding prevented migration during processing, but an overall increase in β-sheet content was still observed after these processing steps.
The relationship between structure, properties and processing of NTP is therefore characterised by its semi crystalline nature, in which additives overcome protein-protein interactions in the amorphous phase enabling β-sheet dispersion throughout the material during processing. Future attempts at modifying properties should be informed by this understanding.||