R. Pantani - polymertechnology.it · A temperature basse il polimero ha un comportamento vetroso. A temperature elevate ha un comportamento gommoso A temperature ancora superiori

Comportamento meccanico dei polimeri

modulo: Proprietà viscoelastiche e proprietà meccaniche dei polimeri

R. Pantani

Comportamento dei polimeri a trazione






La prova a trazione meccanismo di deformazione

polimeri vetrosi


polimeri vetrosi


polimeri vetrosi ‘Competition’ between yield and brittle fracture. The curves show schematically the dependences of brittle-fracture stress and yield stress on temperature. The dashed lines correspond to higher strain-rates than do the full lines.

At the temperature T1 the polymer will fail by brittle fracture at each of the strain-rates illustrated and the fracture stress will be higher for the higher strain-rate.

At the temperature T2 failure will take place by yielding for the lower strain-rate and by brittle fracture for the higher strain-rate. At the temperature T3 failure will take place by yielding for both strain-rates.

A temperature basse il polimero ha un comportamento vetroso. A temperature elevate ha un comportamento gommoso A temperature ancora superiori il polimero può subire deformazioni permanenti sotto carico e comportarsi come un liquido altamente viscoso.

La prova a trazione effetto della temperatura

polimeri vetrosi

effetto della temperatura polimeri vetrosi

effetto della massa molecolare polimeri vetrosi

The elastic moduli and other small-strain properties of strain-free glassy polymers such as polystyrene are found not to depend on the molecular weight or molecular weight distribution, except at very low molecular weights. The tensile strength, of polymers having a narrow molecular weight distribution, however, is negligible at low molecular weight, increases with increasing molecular weight, and ultimately reaches an asymptotic value. Results vary with the polydispersity index


polimeri termoindurenti (a T<Tg)


polimeri termoindurenti (a T<Tg)

La prova a trazione polimeri semicristallini





Effetto della temperatura

Effetto della dimensione delle strutture cristalline (sferuliti)


Effetto della dimensione delle strutture cristalline (sferuliti)





Tenacità

Tenacity as a Function of Degree of Polymerization (DP) Curve A: Polyesters, Polyamides, Other Condensation Polymers. Curve B: Polyolefins, Other Hydrocarbon Polymers. Shaded Area: Most Other Polymers.

La prova a trazione effetto della velocità di deformazione


As the strain rate increases, polymers tend to become more brittle with the most apparent result being an increase in yield stress

Strain-Rate-Dependent Tensile Yield Stress at 20°C for A: Amorphous Polyethylene Terephthalate (PET). B: Polyacetal Polyoxynlethylene (POM) Copolymer. C: Unplasticizcd PVC (RPVC). D: Nylon 66 (PA-66). Ambient Humidity. E: Poly-4- Methyl-Pentene


As the strain rate increases, polymers tend to become more brittle with the most apparent result being an increase in yield stress


The energy to break drops rapidly with increasing extensional speed.


At low strain rates, as in standard tensile testing. the heat generated by elongational work is for the most part dissipated to the environment, and so the experiments are near-isothermal. At high strain rates, the energy cannot be dissipated prior to material failure. The result is a localized increase in temperature in the necking region and adiabatic strain response to applied stress. Local temperature increases of 50° to more than 300°C have been measured during high speed elongation. The effective yield stress decreases with increasing temperature and if there is insufficient orientation hardening, cold drawing is prevented.

La prova a trazione effetto della pressione

Both the yield stress and the yield deformation increase with increasing pressure – The initial Young modulus also increases with increasing pressure – The yield depends on the pressure according to such a relationship has been found for many amorphous, glassy polymers: µ, i.e. the coefficient of internal friction, usually shows values between 0.1 and 0.25, depending on the polymer, whereas for semi-crystalline polymers this coefficient is smaller – At very high pressures the polymer fails in a brittle way

La prova a trazione effetto dell’orientazione

La prova a trazione effetto dell’orientazione

Comportamento meccanico dei polimeri Bibliografia: •  D. W. v. Krevelen and K. t. Nijenhuis, Properties of polymers : their correlation with

chemical structure : their numerical estimation and prediction from additive group contributions, Elsevier, Amsterdam, 2009.

•  L. H. Sperling, Introduction to Physical Polymer Science, 3rd Edition, Wiley-Interscience, 2001

•  R. J. Samuels, Structured polymer properties: the identification, interpretation, and application of crystalline polymer structure, Wiley, New York,, 1974.

•  D. I. Bower, An introduction to polymer physics, Cambridge University Press, Cambridge ; New York, 2002.

•  R. C. Progelhof and J. L. Throne, Polymer engineering principles : properties, processes, and tests for design, Hanser Publishers ; Hanser/Gardner, Munich ; New York, Cincinnati, 1993.

R. Pantani - polymertechnology.it · A temperature basse il polimero ha un comportamento vetroso. A temperature elevate ha un comportamento gommoso A temperature ancora superiori

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