Crystallinity

Crystallinity

Amorphous vs . crystalline Amorphous - Random orientation of chains - Above Tg : flexible , ductile and soft . Crystalline - Repeatable 3D structure of polymer units - Higher tensile ( and shear ) moduli and strengths - Because chains. are long , when Possibly polymer more brittle Xtzn occurs any polymer chain may be in more than one crystallite . - This inhibits mobility and segments in between -Polymers are thus semi - crystalline ( 20 - 80 %). cannot enter regions crystal make structure amorphous these . polymers -These rather than brittle.

Crystallization 3 - D structure is regular close - packed crystal lattice

Require •stereoregularity ( i . e . high isotacticity , syndiotacticity , cis or trans maller side groups . Bulky groups impede mobility lead wer crystallinity .

•lower branching . ossible internal stabilization - hydrogen bonding. rientation by stress – tensile , drawing , dies , etc .

H - bonding in nylons O O O H

H

H

N

N

N

N N N

H

H

H

O

O

O

O O O H

N

H

N

H

N

PE Crystallization

Lamellae

Folded re - entry model

Spherulites

Bending of lamellae . Include amorphous mater

PP crystallization

Maltese cross forms under polarization

Single crystals

PE crystal grown from solution

Shish - kebab of is dit net ‘ n sosatie?

Shearing during crystallizati

Fringed micelles

roversial . Possible under stress - crystallization e at low crystallinity . Stabilized by VDW forces .

Control of crystallization

f polymer grade . conditions ( temperature or rate ). •Crash cooling leads to amorphous lock - in . s to promote or suppress . •Plasticizers can suppress crystallization ( im •Filler surfaces may promote crystallization . •Nucleating agents may promote nucleation rath rystal growth . Control of transparency .

Crystallization and properties 10000

1000

100

) P '(/M l,G u d ro e h s ic m a n y D

10

1 -160

-120

-80

-40

0

40

Temperature (/ oC)

80

HDPE

120

160

Crystallization and properties

Rate of crystallization Tg Rate of crystallization

Tm Maximum below T m Eight - ninths rule

Driving force ( ∆ Γ ) ι ν χ ρ ε α σ α σ Τ δ ρ ο π σ Β υ τ σ ι µ υ λ τ α ν ε ο υ σ λ ψ Temperature µ ο β ι λ ι τ ψ δ ρ ο π σ .

nal surfaces cool faster and may consequently have l es of crystallinity .

Avrami equation 1 0.9

(

0.8

(t) X

0.6

n=1

0.5

n=2 n=3

0.4 0.3 0.2 0.1 0

Ztn

X(t) = X ∞ 1− e

0.7

Time

or spherulites , n = 3 for discs , n = 2 for rods y 1 for instantaneous nucleation

)

Crystallization and polymer type Polymer

Growth Maximum rate (/ min-1 ) crystallinity

Polyethylene(PE) > 1000 80 Nylon 6,6’ 1000 70 Nylon 6 200 35 isotactic polypropylene (i-PP) 20 63 poly(ethylene terephthalate) (PET) 7 50 isotactic polystyrene (i-PS) 0.30 32 polycarbonate (PC) 0.01 25

Crystallinity determination Density measurements Mass based

Volume based

ρ c ρ sample− ρ am 100 ρ sample ρ c − ρ am

ρ sample− ρ am 100 ρ c − ρ am

Density and volume based methods

ilatometry – volume changes with temperature

y gradient column – 2 miscible solutions straddling range . Use standards .

atation – titration of miscible liquids Density bottle

VS =

M liquid− ( M t − M b − M s ) ρ liquid

Differential scanning calorimetry ( DSC ) sure heat flow as a function of temperature Cold crystallization

Curing , oxidation , crosslinkin

exo

Tg

Degradation Melt , T m

Differential scanning calorimetry ( DSC )

sure heat flow as a function of temperature . ntify enthalpy of melting . sure relative to pure crystalline polymer . ter determined by extrapolation on standards .

∆H f ,s Wc = 100 ∆H f ,c

FTIR absorbance of crystalline and amorphous bands .

Ac Am

Wc = 100 Ac +K Am

lymer specific . Depends on relative extinction coeff talline and amorphous bands .

XRD

∞

Wc =

2 s ∫ I c(s).ds o ∞

2 s ∫ I(s).ds o

2sinθ s= λ