Initiation by Mechanical Stresses

In the presence of monomers, of course, graft and block copolymers are formed. The polymerization is initiated by the macro free radicals generated by mechanical stresses (block copolymers) or by free radicals obtained by intermolecular transfer (graft polymers), such as... 

The transformations initiated by mechanical energy do not uniformly and necessarily pass through all three stages. The number and duration of the stages are determined by the nature and intensity of the mechanical stress. 

A basic requirement of the ESR technique is the presence of molecules or atoms containing unpaired electrons. Such species can be generated in polymeric systems by homolytic chemical scission reactions or by polymerization processes involving unsaturated monomers. These reactions can be initiated thermally, photochemically, or with a free-radical initiator, and, in the case of scission, by mechanical stress applied to the system. Therefore, ESR can be used to study free-radical-initiated polymerization processes and the degradation of polymers induced by heat, light, high-energy radiation, or the application of stress. 

The plastification process is initiated the polymer contacts the hot inner wall of the barrel and/or by mechanical stress of the solid particles in the kneading elements. The melting process is continued by the introduction of mechanical energy into the melted product via shear stresses and/or heat conduction from the melt to the as yet un-melted product. The detailed design of the plastification section depends on the product. 

A craze is supposed to initiate by a stress and temperature controlled mechanism and the initiation criterion is motivated by Stemstein s [10] classical work and is taken as... 

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. 

When reviewing the materials of construction consider external corrosion concerns. Chloride stress cracking of stainless steel can be initiated by insulation capturing chlorides or insulation that contains chlorides (stainless steel should be primed). A weather barrier is needed. The principle here is to understand the potential hazards and their mechanisms. 

Oxide films formed at low temperatures are initially continuous and amorphous, but may undergo local crystallisation with the incorporation of the oxide islands , a process that is facilitated by water, heat, high electric fields and mechanical stress... 

Uniform microstractuie is cracial to the superior performance of advanced ceramics. In a cerantic material, atoms are held in place by strong chentical bonds that ate impervious to attack by corrosive materials or heat. At the same time, these bonds are not capable of much "give." When a ceramic material is subjected to mechanical stresses, these stresses concentrate at minute imperfections in the microstmcture, initiating a crack. The stresses at the top of the crack exceed the threshold for breaking the adjacent atomic bonds, and the crack propagates throughout the material causing a catastrophic brittle failure of the ceramic body. The rehability of a ceramic component is directly related to the number and type of imperfections in its microstmcture. 

The mechanical concepts of stress are outlined in Fig. 1, with the axes reversed from that employed by mechanical engineers. The three salient features of a stress-strain response curve are shown in Fig. la. Initial increases in stress cause small strains but beyond a threshold, the yield stress, increasing stress causes ever increasing strains until the ultimate stress, at which point fracture occurs. The concept of the yield stress is more clearly realised when material is subjected to a stress and then relaxed to zero stress (Fig. Ih). In this case a strain is developed but is reversed perfectly - elastically - to zero strain at zero stress. In contrast, when the applied stress exceeds the yield stress (Fig. Ic) and the stress relaxes to zero, the strain does not return to zero. The material has irreversibly -plastically - extended. The extent of this plastic strain defines the residual strain. 

It is often stressed that the technology is environmentally friendly. The stimulation of oil production by in situ bacterial fermentation is thought to be initialized by one or a combination of the following mechanisms ... 

It hag been shown that transition of a backbone carbon from the sp to sp state is promoted by tensile stresses and inhibited by compressive strains (10,44). The acceleration of the process of ozone oxidation of the polymers under load is not associated with the changes in supramolecular structure or segmental mobility of the chain. The probably reason of this effect is a decreasing of the activation energy for hydrogen abstraction (44). The mechanism of initial stages of the reaction of ozone with PP can be represented as ... 

Dining the preparation of 2-azidoethanol from a stirred mixture of 2-bromoethanol (14.6 mol) and sodium azide (15.4 mol) heated on a steam bath, a violent explosion occurred after 100 min. The preparation had been carried out previously without mishap. The need for care in handling azides of low MW is stressed [1], Later detailed studies showed that the most probable cause of the explosion was the extraordinarily high mechanical and thermal sensitivity of the compound, with initiation by vibration from the agitator [2],... 

When dash pot and spring elements are connected in parallel they simulate the simplest mechanical representation of a viscoelastic solid. The element is referred to as a Voigt or Kelvin solid, and it is shown in Fig. 3.10(c). The strain as a function of time for an applied force for this element is shown in Fig. 3.11. After a force (or stress) elongates or compresses a Voigt solid, releasing the force causes a delay in the recovery due to the viscous drag represented by the dash pot. Due to this time-dependent response the Voigt model is often used to model recoverable creep in solid polymers. Creep is a constant stress phenomenon where the strain is monitored as a function of time. The function that is usually calculated is the creep compliance/(f) /(f) is the instantaneous time-dependent strain e(t) divided by the initial and constant stress o. ... 

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