The interaction between a single polystyrene particle and a polystyrene substrate has been previously reported by a number of investigators. However, the effects of relative humidity, applied load and contact time on the adhesion of polystyrene surfaces have not been investigated and these effects are poorly understood. It is the primary aim of the current work to characterise the effect of the aforementioned parameters on the adhesion of polystyrene surfaces using atomic force microscopy. The polystyrene used in this study contained 1% of divinyl benzene as a cross-linking agent. The adhesion forces between an individual polystyrene particle, normally 12-14 mum in diameter, and the surface of a compacted tablet of the same material have been measured at various relative humidities using a custom-built instrument and a commercial AFM. The commercial instrument has the capability of scanning the sample surface, and allowing greater control over the relative motion between the sample and probe. One of the achievements of this work is that a technique has been developed whereby an image of the surface of the tablet can be obtained using the attached particle as a probe scanning in non-contact mode. From the work conducted using the custom-built instrument, the dependency of adhesion forces on the relative humidity is greatest at relative humidities above 60% where capillary forces cause a sharp increase in adhesion with increasing relative humidity. Hysteresis was observed in the solid-solid contact gradient of the accompanying force curves, suggesting non-elastic behaviour at the contact area of the surfaces. Using the commercial AFM it was observed that adhesion values are consistently higher than results obtained from the custom built instrument across the range of relative humidity from 2% RH to 50% RH. This is due to the selection of smooth, single particle contact sites in the commercial AFM experiments. The measured adhesion forces from the custom-built instrument were found to be significantly lower than predictions for adhesion from the contact mechanics theories of JKR and DMT. This can be attributed to the effect of surface roughness and multiple contacts, which are not taken into account in either the JKR or DMT contact mechanics theories. At humidites below 60% the results obtained from the commercial AFM are in much better agreement with the predicted pull-off forces from the JKR model, because smooth, single particle contact regions could be selected. Using the custom-built instrument and the AFM Explorer, the effect of applied load on the adhesion force between a polystyrene particle and a polystyrene substrate was studied at low, medium and high relative humidity. Increasing the applied load has little effect on the measured adhesion forces at low and medium relative humidity but at high relative humidity of 60 to 65% RH, a transition was observed at an applied load of 1000 - 1200 nN. Above this transition the adhesion force increased steadily with applied load due to the yield stress of the material in the contact region being exceeded. The experimental values of applied load have been coupled with published values of Young's modulus, Poisson's ratio and hardness to predict the contact area from contact mechanics models of JKR and Maugis-Pollock. When coupled with the published value of yield stress for the material, the applied load for the onset of plastic flow is predicted. The value of 1508 nN predicted by the plasticity-based MP model agrees approximately with the observed transition in behaviour.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:248072 |
| Date | January 2002 |
| Creators | Looi, Lisa |
| Publisher | University of Surrey |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://epubs.surrey.ac.uk/844419/ |
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