Why is polystyrene brittle and polycarbonate though and what
can we do about it?
Citation for published version (APA):
Smit, R. J. M., Brekelmans, W. A. M., & Meijer, H. E. H. (1997). Why is polystyrene brittle and polycarbonate though and what can we do about it?. Poster session presented at Mate Poster Award 1997 : 2nd Annual Poster Contest.
Document status and date: Published: 01/01/1997
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Why is polystyrene brittle and polycarbonate
tough and what can we do about it?
R.J.M. Smit, W.A.M. Brekelmans, H.E.H. Meijer
Eindhoven University of Technology, Department of Mechanical Engineering,
P.O. Box 513, NL 5600 MB Eindhoven
Introduction
On a macroscale, polystyrene (PS) is brittle and poly-carbonate (PC) is tough. On a microscale, however, craze craze fibrils (length scale nm) break after 300% strain in PS and 100% in PC1. This contradictory behaviour is elucidated and the toughening by the addition of cavitating rubbery particles is explained.
Intrinsic material behaviour
Uniaxial compression experiments2,3 and model fits (true stress versus compressive strain,λ =draw ratio):
0 0.2 0.4 0.6 0.8 1 0 10 20 30 40 50 60 70 80 PC a b c d e −(λ2−1/λ) − σzz [MP a] 0 1 2 3 4 5 0 10 20 30 40 50 60 70 80 90 PS −(λ2−1/λ) − σzz [MP a]
Deformation stages: (a-b) linear elastic; (b-c) non-linear viscoelastic (c) yield; (c-d) strain softening; (d-e) strain hardening.
2 strain softening: decreasing stress results in
increasing strain→unstable deformation
2 strain hardening: increase in stress needed for
increase in strain→stable deformation
2 PS: more strain softening, less strain hardening
→Polystyrene shows intrinsically a less stable
→deformation behaviour than polycarbonate
2 crazes initiate after yield, triaxial stress level during
craze initiation in PS≈ 40MPa and PC≈ 90MPa4,5
2 model offers accurate description of yield- and
post-yield behaviour in arbitrary 3D stress states3,4
Consequence for toughness
Deformation of a notched bar of PS and PC with a minor defect to model realistic (imperfect) specimen:
defect ↓
Polystyrene: at a global strain of0.22%, thedefecttriggers lo-cal yielding, resulting in a critilo-cal dilative stresses (> 40MPa)
→PS crazes 30 MPa 31.5 MPa 33 MPa 34.5 MPa 36 MPa 37.5 MPa 39 MPa 40.5 MPa 42 MPa 43.5 MPa 45 MPa PS, dilative stress Polycarbonate: at a global strain of 1.1%, the notchtip causes critical dilative stresses (> 90MPa)→PC crazes 30 MPa 36 MPa 42 MPa 48 MPa 54 MPa 60 MPa 66 MPa 72 MPa 78 MPa 84 MPa 90 MPa PC, dilative stress
2 PS is brittle because of high defect sensitivity 2 PC is tough because of low defect sensitivity
Improving toughness
Enhance toughness by minimizing defect sensitivity. Possible routes:
1. reduce yield stress: minimizes (unstable) strain
1.softening and reduces triaxial stresses
2. improve (stabilizing) strain hardening
0 50 100 150 200 0 50 100 150 Linear strain [%]
True stress [MPa]
−crosslinking −preorientation −blending with rubber
2:
−predeformations −addition of plasticizers −creation of surface (voids) −addition of heterogeneities
1:
3. avoid high triaxial stress states by incorporation of 3.voids or cavitating rubbery particles
Rubber tougheningis successful because:
- cavitating rubbery particles reduce triaxial stresses - heterogeneous microstructure eliminates softening6 - rubbery particles improve strain hardening
Conclusion
Brittleness of glassy polymers depends on unstable post-yield behaviour and triaxial crazing stress. Re-ducing softening, improving hardening and avoiding high triaxialities are the keys to enhanced toughness.
References
1. Donald, A.M. and Kramer, E.J. (1982). Deformation zones and entanglements in glassy poly-mers.Polymer, 23, 1183-1188.
2. Hasan, O.A. and Boyce, M.C. (1993). Energy storage during inelastic deformation of glassy polymers.Polymer, 34, 5085-5092
3. Timmermans, P.H.M. (1997),Evaluation of a constitutive model for solid polymeric materials: Model selection and parameter quantification. Ph.D. thesis, Eindhoven University of Technology. 4. Tervoort, T.A. (1996)Constitutive modelling of polymer glasses: Finite, nonlinear viscoelastic behaviour of polycarbonate. Ph.D. thesis, Eindhoven University of Technology.
5. Narisawa, I. and Yee, A.F. (1993), Crazing and Fracture of Polymers. In: Cahn, R.W., Haasen, P., and Kramer, E.J., editors,Materials Science and Technology. A Comprehensive Treatment, Vol. 12:Structure and Properties of Polymers, vol.ed.: E.L. Thomas. page 699. VCH, Weinheim. 6. Smit, R.J.M., Brekelmans, W.A.M., and Meijer, H.E.H., Prediction of the large-strain mechanical response of heterogeneous polymer systems. Part 1.J. Mech. Phys. Solids, submitted.
