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Title: Watching polymer deformation and fracture with atomic force microscopy
Author: Jackson, Stephen
ISNI:       0000 0004 7428 1546
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2017
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The deformation and fracture of the semi-crystalline polymers, polyethylene and polypropylene is investigated using AFM in order to observe the processes in situ, on the lamellae scale, for the first time. Samples were stretched on a custom built manual stretching stage under the AFM. Consecutive images were collected of the same area so that the deformation and fracture processes could be watched, from unstretched films to high strains. For polyethylene, there was observation of many things that have been previously inferred, and some new things. Crazes are seen to start primarily at existing large gaps between edge on lamellae. Their growth appears to be inhibited by branch points of lamellae and they are seen to not all form at once. Meshes of points are fitted to the images in order to help follow the processes and understand why they occur where they do. This analysis revealed the heterogeneous nature of the strain across the sample, and that local shear plays an important part in governing the deformation of individual lamellae. SAXS is also used to compare to what is happening over a larger area of the sample. For polypropylene, the behaviour is even more localised with the strain occurring almost entirely in crazes; there is virtually no deformation in crystalline areas until they break up into individual crystallites. At high strains, crazes where short sections of fibril are arranged in geometric patterns at angles to each other and to the main parallel fibrils are seen, which we refer to as geometric crazes. The effect of a large scale crack tip in polyethylene is also investigated: A distribution of strain relative to the crack tip is seen, with very high strains close to the edge, and significant shear seen. Similar geometric crazes are also seen here. An attempt was made to use torsional resonance AFM (TRAFM) to monitor the impact of strain on molecular chain orientation. The sensitivity of TRAFM to local organisation of water on the sample surface was confirmed, although no clear impact of surface hydrophobicity was observed. Unfortunately, it was found that any effect of chain orientation on image contrast was over-shadowed by the effect of scan angle.
Supervisor: Hobbs, Jamie Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available