1. No category

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
90 MPa
84 MPa
On a macroscale, polystyrene (PS) is brittle and polycarbonate (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 experiments and model fits
(true stress versus compressive strain,λ =draw ratio):
c
70
90
60
70
b
50
40
30
60
50
40
a
0.4
0.6
0.8
1
0
0
1
2
3
4
5
−(λ2−1/λ)
2
2
2
42 MPa
36 MPa
30 MPa
PS is brittle because of high defect sensitivity
PC is tough because of low defect sensitivity
Enhance toughness by minimizing defect sensitivity.
Possible routes:
100
Deformation stages: (a-b) linear elastic; (b-c) nonlinear viscoelastic (c) yield; (c-d) strain softening; (de) strain hardening.
2
48 MPa
PS
10
PC
0.2
−(λ2−1/λ)
2
54 MPa
PC, dilative stress
strain softening: decreasing stress results in
increasing strain → unstable deformation
crazes initiate after yield, triaxial stress level during
craze initiation in PS≈ 40 MPa and PC≈ 90 MPa4,5
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
↓
MPa
43.5 MPa
42
MPa
40.5 MPa
39
MPa
37.5 MPa
36
MPa
34.5 MPa
33
MPa
31.5 MPa
PS, dilative stress
30
−crosslinking
−preorientation
−blending with rubber
1:
−predeformations
50
−addition of plasticizers
−creation of surface (voids)
−addition of heterogeneities
50
100
150
Linear strain [%]
200
3. avoid high triaxial stress states by incorporation of
3. voids or cavitating rubbery particles
PS: more strain softening, less strain hardening
→ Polystyrene shows intrinsically a less stable
→ deformation behaviour than polycarbonate
45
2:
0
0
strain hardening: increase in stress needed for
increase in strain → stable deformation
Polystyrene: at a global strain
of 0.22%, the defect triggers local yielding, resulting in a critical
dilative stresses (> 40MPa)
→ PS crazes
True stress [MPa]
0
0
2
60 MPa
20
10
2
66 MPa
150
30
20
72 MPa
1. reduce yield stress: minimizes (unstable) strain
1. softening and reduces triaxial stresses
2. improve (stabilizing) strain hardening
80
−σzz [MPa]
−σzz [MPa]
e
d
78 MPa
Improving toughness
2,3
80
Polycarbonate:
at a global
strain of 1.1%, the notchtip
causes critical dilative stresses
(> 90MPa) → PC crazes
MPa
Rubber toughening is 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. Reducing 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 polymers. 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.