Lorentz - 2008

Atomistic Modelling of Deformed Polymer Glasses

Alexey Lyulin

Group Polymer Physics, Eindhoven Polymer Laboratories and Dutch Polymer

Institute, Technische Universiteit Eindhoven, The Netherlands

Lorentz - 2008

Participants

TU Eindhoven TU Athens MPI-P Mainz

Thijs Michels Doros Theodorou Nico v.d. Vegt B. Vorselaars C. Tzoumanekas V. Harmandaris T. Mulder L. Peristeras E. de Caluwe H.E.H. Meijer L. Govaert

IMPB RAN, PuschinoICP RAN, Moscow Tver University N.K. Balabaev M.A. Mazo A.S. Pavlov E.F. Olejnik I. Neratova

Lorentz - 2008

Motivation

Brittle

Polystyrene

Polycarbonate

vs.vs. ToughTough

Lorentz - 2008

Another puzzle

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Stress-strain behaviour

Intrinsic microscopic response vs chemical structure unclear(e.g. H.E.H. Meijer et al., TU/e and DPI)

PCPS

extension

PC

PS

compression

Lorentz - 2008

Thermal and mechanical rejuvenation

Thermal: heating up above Tg, then quenching

H.G.H. van Melick, PhD thesis, Eindhoven, 2002

Lorentz - 2008

Thermal and mechanical rejuvenation

H.G.H. van Melick, PhD thesis, Eindhoven, 2002

Mechanical: deformation above the yield point, then compression

Lorentz - 2008

Thermal and mechanical rejuvenation

Thermal: heating up above Tg, then quenching

Mechanical: deformation above the yield point, then compression

Bulk mechanics similar

Microscopically the same ????

No !

Lorentz - 2008

PS vs PC as model amorphous polymers

PS fails brittle, PC tough PS shows more post-yield stress drop, large strain softening

What is the relation with molecular structure and chain dynamics ?

Lorentz - 2008

T ~ Tg P = 1 atm

Equilibration

PS PC

Lorentz - 2008

Characteristic ratio

PS PC8.2C 1.7C

SANS: Boothroyd et al., 8.7-9.6simulations: Han and Boyd, 10.2

Sun and Faller, 6.5

SANS: Gawrisch, Brereton, Fischer, 1.9-2simulations: Hutnik, Argon, Suter, 1.6

1/ 2(1 )nC C n

(Wittmer, Meyer, Baschnagel, Johner, Obukhov, Mattioni, Müller, Semenov, PRL, 2004)

Lorentz - 2008

Cooling down below Tg

Cooling time, c 10 ps (quenched)

25 ns (annealed)

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Orientational mobility

22( ) 3/ 2( (0) ( )) 1/ 2P t tb bb

polystyrene polycarbonate

Lorentz - 2008

Equilibrated films, T =540 K

8x80, 38 Å

16x80, 65 Å

32x80, 112 Å

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Orientational mobility

film

bulk

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P2 relaxation-time distribution (CONTIN analysis)

( ) (ln )exp( / ) lnACF t F t d

AVL, M.A.J. Michels, J. Non-Cryst. Solids 2006

-process

-process

PC

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Temperature dependence of P2 relaxation times

polystyrene polycarbonate

~ 50 ps ~ 500 ps<<T = 300K

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Uniaxial extension

PS: 4 chains x N=160, 8 chains x N=80 PC: 64 chains x N=10, 8 chains x N=80

L=0 L=65% L=110%

AVL, N.K. Balabaev, M.A. Mazo, M.A.J. Michels, Macromolecules 2004

1000y ps 0.005 Å/ps

Lorentz - 2008

PC:

T << Tg

PS:

Lorentz - 2008

Simulation vs. experiment

PC

PS

H.G.H. van Melick et al., Polymer 2003AVL, B. Vorselaars, M. Mazo, N. Balabaev, M.A.J. Michels, Europhys. Lett. 2005

Lorentz - 2008

Simulation vs. experiment

quenched

annealed

polystyrene

AVL, M.A.J. Michels, Phys. Rev. Lett., 2007H.G.H. van Melick, PhD thesis, Eindhoven, 2002

polystyrene

Lorentz - 2008

Three time scales

cooling: c ~ 10 ps (quenched) << 25000 ps (annealed)

deformation: y ~ 1000 ps

- relaxation: ~ 50 ps (PS) << 500 ps (PC)

(PS) c (quenched) (PC) >> c (quenched)

c (annealed) >> ,y

for both polymers

~

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Stretching - compression loop: quenched samples

mechanical overaging because of the process

- faster for PS, slower for PC- effect is larger for PC

Lorentz - 2008

Stretching - compression loop: annealed samples

Lorentz - 2008

Energy partitioning

AVL, M.A.J. Michels, Phys. Rev. Lett., 2007

En

erg

y d

istrib

utio

n

mechanically rejuvenated glass is different from thermally rejuvenated glass

Lorentz - 2008

Summary, questions

Tg, overaging and rejuvenation for typical polymer glasses have been simulated;

Key factors are ratios between three time scales: - relaxation; - cooling time; - deformation time; Fast relaxation for PS, slow for PC;

Thermal and mechanical rejuvenation are microscopically different

Direct measurement of segmental mobility under mechanical deformation