Effects of molecular weight distribution on the flow-enhanced crystallization of poly(1-butene)

Stefano Acierno1, Salvatore Coppola2, Nino Grizzuti3

1Dipartimento di Ingegneria, Università del Sannio di Benevento2Centro Ricerche Elastomeri, Polimeri Europa S.p.A.

3Dip. di Ingegneria Chimica, Università di Napoli Federico II

J. BRAUN, H. WIPPEL, G. EDER, and H. JANESCHITZ-KRIEGL, Polym. Eng. Sci., 43, 188-203 (2003)

“Depending on the shear rates and shearing times, either spherulitic or shish-kebab crystallization takes place. In the mechanical work done on the sample, the number of spot-like nuclei increases tremendously…”

“In duct flow, high shear rates lead to highly oriented surface layers, consisting of a kind of shish-kebab…”

“Shear-induced crystallization is apparently caused by a change in the structure of the polymer melt…”

CRYSTALLIZATION UNDER ROCESSING CONDITIONS

CRYSTALLIZATION UNDER ROCESSING CONDITIONS

•Flow induces changes to crystallizationFlow induces changes to crystallization•Crystallization induces changes to rheologyCrystallization induces changes to rheology

PolymerPolymerprocessingprocessing

ThermaThermall

historyhistory

FlowFlowhistoryhistory

FinalFinalPropertiesProperties

CRYSTALLINITYCRYSTALLINITY

Outline

Crystallization under shear flow

Concluding remarks

Rheological behaviour of the molten phase

Motivation

Materials: HMW – LMW iPB blends

Model comparison

RHEOLOGY OF THE MOLTEN PHASE

Crystallization implies a reorganization of the molten phaseA good micro-rheological model is highly desirable

Doi-Edwards model

THE STEP-STRAIN EXPERIMENT

Characteristic time

Shear rate

Dis Rouse

Chain neither oriented nor stretched

1/ Dis Chain oriented

but not stretched

1/ Dis Chain oriented and stretched

1/ Rouse

ORIENTATION VS. STRETCHING

MICRO-RHEOLOGICAL MODELING

No flow

0 1

m

L S qGT

TG G HG

Flow

q fG G G

1 N

f f

fn

q q

nn

q q

1 1exp 1

1 1

G T G GG

K

G

exp exp

a nnG

G

E KN CkT

kT TIsothermal nucleation rate*:

* Lauritzen and Hoffman, 1960 and Ziabicki, 1996

FLOW-INDUCED FREE ENERGY

• Reptation is considered as the only relaxation mechanism (no constraint release)

• Chain segments are considered as non-interacting rigid rods (Independent Alignment Approximation, IAA)

For shear deformation*:

3 , ' , ' '

tA

fe

NG kT t t A t t dt

ME

* Marrucci & Grizzuti, 1983

2 2 4 4 2 2 21

0

1 4 2 11ln

2 2

x x x

A dx

Memory function

2

2 2

8 1, , expSR d

p odd d

p t tt t

p

For simple reptation* the memory function is given by:

*Doi & Edwards, 1986 **des Cloizeaux,1990

Simple reptation does not account for any constraint release coming from reptation of the surrounding chains.

2, , , , , , ,DR dH dL H SR dH L SR dLt t t t t t

For this reason we choose the double reptationdouble reptation** approach:

CRYSTALLIZATION + MICRO-RHEOLOGY

0

3

e

f DeG kT z A z dzM

N0 1

m

L S qGT

TG G HG

Kn, H0, Tm, Me, d (in De)ARE NOT ADJUSTABLE

PARAMETERS!(only at one single temperature is

fitted)

f f

fn

q q

nn

q q

1 1exp 1

1 1

G T G GG

K

G

Materials & methods

Mn

[kg/mole]

Mw

[kg/mole]

Mw/Mn Tf

[C]

0@140°

[Pa s]

PB800 (L) 37 115 3.1 130.4 757

BR200 (H) 125 851 6.8 144.3 288,500

Blends of two isotactic iPB’s

System A: “diluted”, i.e. H-Molecular weight component up to 10 wt%

System B: “concentrated”, i.e. H-Molecular weight component form 30 to 90 wt%

*3

5 wt%

wH

g A

Mw a

R N

Quiescent crystallization

Temperature [°C]

78 80 82 84 86 88

t 0.5

[s]

102

103

B0B50B91B100

Kn = 2.6 1010 K J/m3 and n = 1

System A: Linear viscoelasticity

frequency [rad/s]

10-3 10-2 10-1 100 101 102 103

[

Pa

s]

101

102

103

104

105

106

A0A1.25 A5A10A100

weight % of BR200

0.1 1 10 100

zero

-she

ar v

isco

sity

[Pa

s]

102

103

104

105

106

system B system A pure Hpure L

time [s]

0 2000 4000 6000

[°

C]

100

120

140

160

[Pa*s]

0

2e+6

4e+6

6e+6

8e+6

1e+7

Rheology during crystallization

10 min annealing at 160°C to erase any crystalline memory

Rapid cooling to the crystallization temperature of 95°C

A constant shear rate is applied and the polymer viscosity is monitored

The crystallization time scale is characterize by an induction time (time needed for the viscosity jump)

System A: crystallization under flow

time [s]

0 1000 2000 3000 4000 5000

/ ss

0

1

2

3

4

0.005 s-1

0.01 s-1

0.03 s-1

0.1 s-1

0.25 s-1

time [s]

100 1000 10000

/ ss

0

1

2

3

4

A0A1.25A5A10A100

Sample A0 Shear rate 0.01 s-1

System A: crystallization under flow

shear rate [1/s]

10-4 10-3 10-2 10-1 100 101

t i [s]

102

103

104

A0A1.25A5A10A100

System B: Linear viscoelasticity

frequency [rad/s]

10-3 10-2 10-1 100 101 102 103

[

Pa

s]

101

102

103

104

105

106

B0B30B50B91B100

weight % of BR200

0.1 1 10 100

zero

-she

ar v

isco

sity

[Pa

s]

102

103

104

105

106

system B system A pure Hpure L

System B: crystallization under flow

time [s]

0 1000 2000 3000

/ ss

0

1

2

3

4

0.0001 s-1

0.00032 s-1

0.001 s-1

0.0032 s-1

0.01 s-1

0.032 s-1

0.1 s-1

Sample B91

time [s]

0 1000 2000 3000 4000 5000 6000 7000

/ ss

0

1

2

3

4

B0B30B50B91B100

Shear rate 0.01 s-1

System B: crystallization under flow

shear rate [1/s]

10-4 10-3 10-2 10-1 100 101

t i [s]

101

102

103

104

B0B30B50B91B100

ConclusionsShear flow accelerates crystallization kinetics and higher molecular weights are more sensitive to flow intensity (i.e., the shear rate).

The addition of a small amount of high MW-polymer (< 6 wt%) to a low MW sample does not produce any appreciable effect upon the crystallization kinetics under both quiescent and shear flow conditions.

Greater elevated amounts of high MW-polymer produce evident effects upon (both quiescent and flow-enhanced) crystallization. Nevertheless the effect is not dramatic.

This behavior can be attributed to constraint release of high MW chains due to the relaxation of the shorter chains. Such a physical phenomenon is successfully described by the double reptation theory, which can be used to predict the flow-induced enhancement in crystallization rate under steady flow conditions. In the case of steady shear flow the agreement between calculations and experimental results is good.