Friday, March 18, 2011

Selçuk TEVRÜZ, 030070128, 6th week

Maxwell Model (March 18, 2011 - 09:14)

The simplest series combination of mechanical models is the Maxwell model. A given stress σ applied to the model produces a deformation ϵ1 on the spring and a deformation ϵ2 on the dashpot. The stress-strain relation in the spring is

σ = MUϵ1

where MU is the elasticity constant of the spring. The subindex U denotes “unrelaxed”. Its meaning will become clear in the following discussion. The stress-strain relation in the dashpot is

σ = ηθtϵ2 η > 0,

where η is the viscosity. Assuming that the total elongation of the system is ϵ = ϵ1 + ϵ2, the stress-strain relation of the Maxwell element is

θtσ/MU + σ/η = θtϵ

(Carcione J.M., Wave fields in real media: wave propagation in anisotropic, anelastic, porous and electromagnetic media, second edition, pg.68)

Glass Transition Temperature (March 18, 2011 - 09:24)

The glass transition temperature (Tg) is a key parameter in thermosetting polymers, not only from the product performance point of view, but also from the processing point of view, since it may strongly affect the reaction kinetics. The glass transition temperature marks the boundry bbetween the glassy, rigid state of a polymer and the soft, flexible (or fluid) state of the polymer. Below the glass tranition temperature, the available energy is insufficient to allow the molecules coordinated mobility (although there may be some localized motion), so the material is rigid; above the glass transition, the molecules can flow past each other above the glass transition temperature - the polymer is a "melt". In the case of thermoset polymers above the glass transition temperature, the chemical crosslinks prevent the molecules from flowing, but there is enough mobility for molecules to cooperatively relax, and the polymer becomes flexible and "rubbery".

(Cheng S.Z.D., Handbook of thermal analysis and calorimetry: applications to polymers and plastics, 2002, pg.315,316)

Crystalline Polymers (March 18, 2011 - 09:36)

The plastic deformation of crystalline polymers, in a particular polyethylene, has been studied intensively from the viewpoint of changes in morphology. It is now evident that very drastic reorganization occurs at the morphological level, with the structure changing from a spherulitic to a fibrillar type as the degree of plastic deformation increases. The molecular reorientation processes are very far from being affine or pseudo-affine and can also involve mechanical twinning in the crystallites. It is surprising that some of the continuum ideas for mechanical anisotropy are nevertheless still relevent, although they must be appropriately modified.

(Ward I.M., Sweeney J., An introduction to the mechanical properties of solid polymers, second edition, pg.270)

Plasticizer (March 18, 2011 - 09:42)

A plasticizer is a substance, which is incorporated into a plastic or a coating to increase flexibility, workability or distensibility. This is achieved by loosening the strength of intermolecular forces resulting in a higher flexibility of macromolecules or segments of macromolecules (Brownian motion). A plasticizer thus may reduce the melt viscosity and lower the temperature of the second-order transition or glass transition temperature, Tg, of the product. Plasticizers are inert, organic substances with low vapour pressures, predominantly esters.

(Eyerer P., Weller M., Hübner C., Agnelli J.A., The handbook of environmental chemistry: polymers - opportunities and risks II: sustainability, product design and processing, 2010, pg.120)

No comments:

Post a Comment