LECTURE #30 :3.11 MECHANICS OF

MATERIALS F03INSTRUCTOR : Professor Christine OrtizOFFICE : 13-4022 PHONE : 452-3084WWW : http://web.mit.edu/cortiz/www

• PLASTICITY OF POLYMERS II

SUMMARY : LAST LECTUREI. Molecular Origins of Plasticity in Metals• edge and screw dislocations, 2-D and 3-D movement of dislocations, slip systems,

planes, directions, polycrystalline plastic deformationII. Stress versus Strain Curves for Polymers in Uniaxial Tension• structure of semicrystalline polymers: amorphous+cystalline regions, chain folding,

lamellar, spherulites (macroscopically isotropic)

Semicrystalline polymers (T>Tg):Ι. linear elastic deformationII. homogeneous plastic deformation,

strain hardeningIII. NECK forms and grows unstably, i.e.

inhomogeneous plastic deformationIV. neck stabilizes V. COLD DRAWING, neck increases

in length by extracting polymer fromunnecked region of sample

VI. entire sample is drawnVII. begin stretching of completely drawn

sample, strain hardeningVIII. fracture

σ

ε

σ

ε

highly oriented polymer fiber

amorphous, glassy polymer

(T<Tg)

semicrystallinepolymerT>Tg

elastomer (T>Tg)

σ

ε

σmax

Lo

εf

I.

II.

III.

IV. V.VI.

VII.

•

σY

VIII.

MACROSCOPIC MECHANICAL

BEHAVIOR

Polyethylene :used in milk jugs, toys, pens,

ice traysNylon (polyamide)

Plastic Deformation of Semicrystalline Polymers

crystalline lamellae

amorphous region

Stages and Mechanisms of Plastic Deformation1) elongation of amorphous chains (uncoiling along stress axis)2) rotation/tilting of lamellae crystallites toward tensile axis3) separation of crystallites into block segments (partial melting)4) further stretching of crystallites and orientation of amorphous regions along tensile axis, void formation, stress whitening5)2nd strain hardening regim:recrystallization to a fiber like oriented structure, high stiffness

σ, ε

σ, ε

Comparison of Amorphous and Semicrystalline Polymers

Some Applications of Amorphous Polymers

Plastic Deformation of Amorphous, Glassy Polymers or Networks (T < Tg)

σ

ε

Crazing in Amorphous, Glassy Polymers (T<Tg)

σ,εσ,ε

Crazing in Amorphous, Glassy Polymers (T<Tg)

σ,εσ,ε

entanglementoriented polymerchains

fibril

CRAZE NANO

STRUCTURE

main fibrilcross-tie fibril

craze-bulk interface

craze

bulkbulk

polycarbonate

Crazing in Amorphous, Glassy Polymers (T<Tg)

σ,εσ,ε

Shear DZ’s in Amorphous, Glassy Polymers (T<Tg)

σ,εσ,ε

Macroscopic sample

~ necking

Microscopic structure

CRAZING OR SHEAR DZ’s?

σ,ε

σ,ε

σ,εσ,ε

CONSEQUENCES FOR IMPACT RESISTANCE

bisphenol-A polycarbonate

(crazing)

Plastic Deformation and Fracture of Amorphous Components

Plastic Deformation and Fracture of Amorphous Components

TOUGHENING OF AMORPHOUS POLYMERS

TOUGHENING OF AMORPHOUS POLYMERS

Typical uniaxial tensile stress-strain behavior of polystyrene (PS), medium-impact PS (MIPS), high-impact PS (HIPS), and poly(acrylonitrile-co-styrene-graft-butadiene) ABS.