Adhesion between glassy polymers

Mangipudi V S ef a/1996 Measurement of interfacial adhesion between glassy polymers using the JKR method Macromoi. Symp. 102 131-43... 

Schnell, R., Stamm, M. and Creton, C., Direct correlation between interfacial width and adhesion in glassy polymers. Macromolecules, 31, 2284-2292 (1998). 

The SFA, originally developed by Tabor and Winterton [56], and later modified by Israelachvili and coworkers [57,58], is ideally suited for measuring molecular level adhesion and deformations. The SFA, shown schematically in Fig. 8i,ii, has been used extensively to measure forces between a variety of surfaces. The SFA combines a Hookian mechanism for measuring force with an interferometer to measure the distance between surfaces. The experimental surfaces are in the form of thin transparent films, and are mounted on cylindrical glass lenses in the SFA using an appropriate adhesive. SFA has been traditionally employed to measure forces between modified mica surfaces. (For a summary of these measurements, see refs. [59,60].) In recent years, several researchers have developed techniques to measure forces between glassy and semicrystalline polymer films, [61-63] silica [64], and silver surfaees [65,66]. The details on the SFA experimental procedure, and the summary of the SFA measurements may be obtained elsewhere (see refs. [57,58], for example.). 

Johnson and coworkers [6], in their original paper on the JKR theory, reported the measurements of surface energies and interfacial adhesion of soft elastomeric materials. Israelachvili and coworkers [68,69], and Tirrell and coworkers [62, 63,70,88-90] used the SFA to measure the surface energies of self-assembled monolayers and polymer films, respectively. Chaudhury and coworkers [47-50] adapted the JKR technique to measure the surface energies and interfacial adhesion between self-assembled monolayers. More recently, Mangipudi and coworkers [55] modified the JKR technique to measure the surface energies of glassy polymers. All these measurements are reviewed in this section. 

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

Thermoplastic polymers, such as poly(styrene) may be filled with soft elastomeric particles in order to improve their impact resistance. The elastomer of choice is usually butadiene-styrene, and the presence of common chemical groups in the matrix and the filler leads to improved adhesion between them. In a typical filled system, the presence of elastomeric particles at a level of 50% by volume improves the impact strength of a brittle glassy polymer by a factor of between 5 and 10. 

Requirements for rubber toughening of glassy polymers include (I) good adhesion between the elastomer and matrix, (2) cross-linking of the elastomer, and (3) proper size of the rubber inclusions. These topics are reviewed briefly in the order listed ... 

For electrostatic and steric stabilization, the particles can be viewed effectively as colloids consisting of a soft and deformable corona surrounding a rigid core. Colloidal particles with bulk elastomeric properties are also available. These particles, which are generally of submicron size, are developed and used as reinforcement additives to improve the Impact resistance of various polymer matrices [28-30]. The rubber of choice is often a styrene/butadiene copolymer. The presence of chemical groups at the matrix-filler interface leads to improved adhesion between them. Typically, the addition of about 30% by volume of these elastomeric particles increases the impact strength of a brittle glassy polymer like polystyrene by up to a factor of 10. For some applications, particles with more complex architecture have been... 

As the two smooth spherical surfaces approached each other, within a few micrometers of contact, the familiar Newton s ring pattern could be seen in the narrow gap between the smooth surfaces. Then, as the rubber lenses were moved still nearer, a sudden jump of the rubber was observed and the black contact spot grew rapidly to a large size as the rubber deformed and spread under the influence of molecular adhesion (Fig. 3.12(c)). The appearance of this was very similar to the liquid drop spreading over a glassy polymer surface. To get the rubber lenses apart, a tensile force had to be applied to overcome this molecular adhesion. It is this cracking apart of adhering surfaces which we consider next. 

Many of the entries in this book dealing with the science of a(fliesion and Theories of adhesion are relevant to composition materials in general. There are a number of articles that discuss specific aspects of composite materials that come within the scope of this book. Toughened adhesives, particnlarly Epoxide adhesives and Toughened acryiics, consist of polymers with a rubbery phase dispersed as small spheres within a more glassy matrix. Appropriate adhesion between the phases is crucial for effective toughening. 

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