Plastic deformation, micromechanical properties

The purpose of this paper is to investigate the mechanical properties (plastic deformation, micromechanisms of deformation, fracture) of several amorphous polymers considered in [1], i.e. poly(methyl methacrylate) and its maleimide and glutarimide copolymers, bisphenol A polycarbonate, aryl-aliphatic copolyamides. Then to analyse, in each polymer series, the effect of chemical structure on mechanical properties and, finally, to relate the latter to the motions involved in the secondary transitions identified in [ 1] (in most cases, the p transition). 

The goal of this investigation of the mechanical properties of amorphous polymers (plastic deformation, micromechanisms of deformation, fracture) was to analyse the influence of secondary transition motions on these properties. 

Incorporation of hard particles into the polymer matrix creates stress concentration, which induces local micromechanical deformation processes. Occasionally these might be advantageous for increasing plastic deformation and impact resistance, but usually they cause deterioration in the properties of the composite. Encapsulation of the filler particles by an elastomer layer changes the stress distribution around the particles and modifies the local deformation processes. Encapsulation can take place spontaneously, it can be promoted by the use of functionalized elastomers (see Sect. 6.3) or the filler can be treated in advance. 

When the experimentalist set an ambitious objective to evaluate micromechanical properties quantitatively, he will predictably encounter a few fundamental problems. At first, the continuum description which is usually used in contact mechanics might be not applicable for contact areas as small as 1 -10 nm [116,117]. Secondly, since most of the polymers demonstrate a combination of elastic and viscous behaviour, an appropriate model is required to derive the contact area and the stress field upon indentation a viscoelastic and adhesive sample [116,120]. In this case, the duration of the contact and the scanning rate are not unimportant parameters. Moreover, bending of the cantilever results in a complicated motion of the tip including compression, shear and friction effects [131,132]. Third, plastic or inelastic deformation has to be taken into account in data interpretation. Concerning experimental conditions, the most important is to perform a set of calibrations procedures which includes the (x,y,z) calibration of the piezoelectric transducers, the determination of the spring constants of the cantilever, and the evaluation of the tip shape. The experimentalist has to eliminate surface contamination s and be certain about the chemical composition of the tip and the sample. 

The deformation behavior of amorphous polymers has been studied extensively, partly because the structure is rather simple as compared with semicrystalline polymers thus, the relationship between structure and properties can be established with relative ease. It is well known that two major micromechanisms are involved in the deformation and subsequent fracture of glassy polymers [1,2,13] (see Figs. 18.1 and 18.2). These are crazing and shear yielding, and both involve localized plastic deformation and some energy is dissipated during the deformation. In a craze, polymer chains are stretched along the stress direction and... 

Mina, M. R, Alam, A. K. M. M., Chowdhury, M. N. K., Bhattacharia, S. K., and Balta Calleja, R J. 2005. Morphology, micromechanical, and thermal properties of undeformed and mechanically deformed poly (methyl methacrylate) /rubber blend. Polymer Plastics Technology and Engineering 44(4) 523-537. 

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