Critical thickness in silicone thermosets

Critical thickness effects are utilized to achieve high fracture toughness in brittle polymers. The postulate of critical thickness, which is: "Macroscopically brittle polymers deform in a ductile fashion below a critical dimension" is validated for silicone thermosets and polystyrene using novel "direct" methods by measuring failure strain in thin films. A discussion on polymer intrinsic deformation mechanisms is presented. Using these intrinsic deformation mechanisms as bases, it is argued that all polymers are ductile at the molecular level. Accordingly, it is suggested that polymer properties below a certain length scale (defined as critical thickness) are dominated by intrinsic deformation characteristics. Two analytical models have been developed which predict critical thickness based on the physical properties of the polymer. The first model is based on an energy criterion according to which crack initiation does not take place if the crack driving force is less than the crack resistance. Such a condition for a brittle polymer is achieved at the critical thickness. The second model is based on a mechanics criterion according to which a minimum film thickness (critical thickness) is required for typical fracture features like crazes to exist within it. Further, using these theoretical models as a basis, the effect of network density, temperature and strain rate on critical thickness is discussed. It is also shown that fracture strain is the characteristic material property to measure film toughness. A variety of silicone thermosets are studied to demonstrate engineering applicability of critical thickness.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2005

Author dc:creator

• Deopura, Manish, 1975-

Advisor dc:contributor.advisor

• Frederick J. McGarry.

• Materials Science and Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/30256
• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Identifier URI

http://dspace.mit.edu/handle/1721.1/30256

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/30256