Comportamento meccanico dei polimeri

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Marco Marini
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1 modulo: Proprietà viscoelastiche e proprietà meccaniche dei polimeri Comportamento meccanico dei polimeri R. Pantani

2 Comportamento dei polimeri a trazione

4 Comportamento dei polimeri a trazione

5 Comportamento dei polimeri a trazione

6 Comportamento dei polimeri a trazione

7 Comportamento dei polimeri a trazione

8 Comportamento dei polimeri a trazione

9 meccanismo di deformazione polimeri vetrosi

10 meccanismo di deformazione polimeri vetrosi

11 meccanismo di deformazione 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 T 1 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 T 2 failure will take place by yielding for the lower strain-rate and by brittle fracture for the higher strain-rate. At the temperature T 3 failure will take place by yielding for both strain-rates.

12 effetto della temperatura polimeri vetrosi 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.

14 effetto della temperatura polimeri vetrosi

15 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

16 effetto della temperatura polimeri termoindurenti (a T<Tg)

17 effetto della temperatura polimeri termoindurenti (a T<Tg)

18 polimeri semicristallini

19 polimeri semicristallini

20 polimeri semicristallini

21 polimeri semicristallini

22 Effetto della temperatura La prova a trazione polimeri semicristallini

23 Effetto della dimensione delle strutture cristalline (sferuliti) La prova a trazione polimeri semicristallini

24 polimeri semicristallini Effetto della dimensione delle strutture cristalline (sferuliti)

25 polimeri semicristallini

26 polimeri semicristallini

27 polimeri semicristallini

29 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.

30 effetto della velocità di deformazione

31 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

32 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

33 effetto della velocità di deformazione The energy to break drops rapidly with increasing extensional speed.

34 effetto della velocità di deformazione 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 nearisothermal. 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.

35 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 La prova a trazione effetto della pressione 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

36 effetto dell orientazione

37 effetto dell orientazione

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

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