Melt processing and Mechanical Properties of Polyolefin Block Copolymers

Alhad PhatakAdviser: Frank Bates

Poly(styrene)-Poly(isoprene) BCP

Polymer-polymer blend Block copolymer

(Khandpur et al. Macromolecules 1995)Poly(lactic acid)-Poly(ethylene) blend

(Wang et al. J. Polym. Sci. 2001)

Combine properties of different polymers

Block Copolymer Morphologies

χN ~ 1/Tχ = Flory-Huggins interaction parameterN = Overall degree of polymerization

Khandpur et al. Macromolecules 1995, 28, 8796.

PS-PI diblock copolymer

TODT

Commercial ApplicationsPolystyrene based BCPs:• Thermoplastic Elastomers (Kraton); PS-PI-PS, PS-PB-PS• High impact thermoplastics (K-resins, Chevron Philips); (PS-PB)n

www.kraton.com

Adhesives

Asphalt additives

Footwear

Packaging

Poly(cyclohexylethylene) - PCHE

( )n

• Higher Tg (147 ºC vs. 105 ºC)• Better thermal, oxidative, and UV stability

High entanglement molecular weight (~ 40 kg/mol) BRITTLE

( )n

+ H2, Pt/Re/SiO2

170 ºC, 500 psi

Hucul and Hahn, Adv. Mater., 2000; Bates, Fredrickson, Hucul, and Hahn, AIChE Journal, 2001

Solution: Make block copolymer with polyethylene

PS PCHE

Glassy Semicrystalline Block Copolymers

( )n

Poly(cyclohexylethylene)H2-ed PS

Tg ≈ 145 ºC

Me ≈ 40 kg/mol

C

( )0.92m

( )0.08m

[ ]ran

E

PolyethyleneH2-ed 1,4-PB

Tm ≈ 100 ºC

“Hard” block “Soft” block

Tg ≈ -100 ºC

Me ≈ 1 kg/mol

Polymer Nanofibers by Melt Blowing

Melt Processing

Lamellae-formingC/E Block Copolymers

Mechanical Properties

Outline

Outline

Polymer Nanofibers by Melt Blowing

Melt Processing

Lamellae-formingC/E Block Copolymers

Mechanical Properties

CEC - Ductile (Failure strain ≈ 300%)

CE diblock copolymer - Brittle (Failure strain ≈ 1%)Loose chain ends – Chain pullout

Lim et al, Macromolecules 2004

ECE – Brittle (Failure strain ≈ 1%)

“Soft” block must be anchored at lamellar interfaces

0% PE chains anchored at C/E interfaces 100% PE chains anchored at C/E interfaces

202530.4834ECEC

182050.6548ECECE

262

TODT, ºC(Rheology)

18

d*, nm(SAXS)

0.5634CEC*

wCMolecular weight, kg/mol

Polymer

* Made by Dow Chemical Company

PCHE

PE

Control degree of anchoring of PE block

Measurement of Tensile Properties

Stress (σ) versus strain (ε) measurement

Failure strain (εf) – Measure of “toughness”

Sample: 10mm x 5mm x 1mmExtension rate = 10mm/min

Room temperature

anneal above TODT

Common Parameter - ψE

CEC E,CEC ECEC E,ECECE

CEC E,CEC ECEC E,ECEC

n M n Mn M 2n M

+ψ =

+

CEC E,CEC ECECE E,ECECEE

CEC E,CEC ECECE E,ECECE

n M n Mn M 3n M

+ψ =

+ ECECE/CEC blends

ECEC/CEC blends

ψE = Weight fraction of PE anchored at C/E interfaces

ψE =

0, CE and ECE

0.33, ECECE

0.5, ECEC

1, CEC and CECEC

Common Parameter - ψE

Limited by PCHE

Weak PE

Toughening of PE

A. Phatak, L. S. Lim, C. K. Reaves, F. S. Bates Macromolecules (2006)

Pure ECECE

Pure ECEC

Summary

• Sensitivity of mechanical properties to molecular design• Tying down soft block is critical

ψE - Design parameter for making tough BCPs

Control mechanical toughness

Manipulate molecular architecture

Outline

Polymer Nanofibers by Melt Blowing

Melt Processing

Lamellae-formingC/E Block Copolymers

Mechanical Properties

0.56

0.56

wC

23152CECEC*

26234CEC*

TODT, °CMw, kg/molPolymer

CEC

CECEC

* Made by Dow Chemical Company

Extrusion• Capillary rheometer (Goettfert Rheo-Tester 1500) – Constant velocity mode

Extrusion flow curve - vs.

⎟⎠⎞

⎜⎝⎛ +

∆=

WH12L

PHσaw

2ap WH6Qγ =

apγ awσ

v

v×∇

Scale bar = 50 µm

Flow Curves - CEC

T < TODT

σ < σsh

σ > σsh

Scale bar = 50 µm

v

v×∇

Flow Curves - CECEC

T < TODT

σ < σsh

σ > σsh

Scale bar = 50 µm

CEC CECEC

σ < σsh σ < σsh

σ > σshσ > σsh

Surface Profiles – CEC and CECEC

Average surface roughness – CEC and CECEC

A. Phatak, C. W. Macosko, F. S. Bates, S. F. Hahn; J. Rheol. (2005)

CEC/CECEC blends

σaw > σshT = 200 ºC

Summary

• CECEC – Sharkskin-like surface fracture at high extrusion rates• CEC – Relatively smooth extrudates, even at high extrusion rates• 20 % CEC – Dramatically reduces surface roughness

Manipulate molecular architecture

Control melt processing behavior

Common theme

ECECE

CEC CECEC

ECEC

DESIGN BLOCK COPOLYMER MOLECULES

MELTPROCESSIBILITY

MECHANICALTOUGHNESS

Outline

Polymer Nanofibers by Melt Blowing

Melt Processing

Lamellae-formingC/E Block Copolymers

Mechanical Properties

Nonwoven products from polymer fibers ($16.4 billion industry*)

* Nonwovens Industry Magazine (2004), http://www.inda.org/category/nwn_index.html

Properties• High specific surface area (~ 1/d)• Chemical resistance

Motivation

Grafe et al. International Nonwovens Journal (2003)

Motivation

Nanofiber applications (few hundred nm)

From Huang et al., Compos. Sci. Tech. (2003)

U.S. patents on nanofibers

Motivation

• Slow process• Solvent handling

Electrospinning – Only continuous process to obtain nanofibers

• 10 nm to 1 µm fibers

Melt Blowing

• Faster• No solvent

Fiber formation (draw down) by air

Tg or Tc

Processing variables• Polymer and air temperatures (Tp, Ta)• Polymer and air flow rates

• Limited to “microfibers”

Action

Melt BlowingCurrent understanding• Models predict fiber diameters up to ~1-2 µm• Correlations between processing conditions and fiber diameter• Supposedly limited to “microfibers”

What is lacking?• What limits fiber attenuation below 1 µm?• Characterization of fiber diameter distributions

* V. A. Wente, Ind. Eng. Chem. 1956

*

Melt Blowing Die

Melt Blowing Die

Die orifice: d0 = 0.2 and 0.4 mm

Air Air

Materials

MFR = 350

15.0

2.1

Mn (kg/mol)

35190PBT

-2118PP

61-PS*

Tg (ºC)Tc (ºC)Polymer

* PS experiments performed by Chris Ellison

( )n

( )n

OO( )n

O O

0.4417100.035265PBT-5

1.220.44.50.35265PBT-4

0.3013.680.035220PP-5

0.4513.680.035180PP-4

1.230.560.35180PP-3

0.386.880.07280PS-3

1.61980.053180PS-1

dav, µmΓAir volumetric flow rate (SCFM)

Polymer mass flow rate (g/min)

Tp, Ta(ºC)

Run I.D.

Air mass fluxPolymer mass flux

Γ =

Higher Γ → Greater drag force on fibers → Finer fibers

0.4417100.035265PBT-5

1.220.44.50.35265PBT-4

0.3013.680.035220PP-5

0.4513.680.035180PP-4

1.230.560.35180PP-3

0.386.880.07280PS-3

1.61980.053180PS-1

dav, µmΓAir volumetric flow rate (SCFM)

Polymer mass flow rate (g/min)

Tp, Ta(ºC)

Run I.D.

Higher Tp

Lower melt viscosity

Higher ‘draw down temperature window’ [Tp < T < Tg (or Tc)]

0.4417100.035265PBT-5

1.220.44.50.35265PBT-4

0.3013.680.035220PP-5

0.4513.680.035180PP-4

1.230.560.35180PP-3

0.386.880.07280PS-3

1.61980.053180PS-1

dav, µmΓAir volumetric flow rate (SCFM)

Polymer mass flow rate (g/min)

Tp, Ta(ºC)

Run I.D.

Sub-micron fibers from all polymers

PBT

PBT

PP

PBT

PP

PS

Fiber Diameter Distribution

Asymmetric → Not normal distribution

> 200 fibers in every case

Log-normal Distribution

⎡ ⎤⎢ ⎥⎣ ⎦

2c2

1 (x - x )p(x) = exp - 2δδ 2π

xc: meanδ: standard deviationGaussian Fit:

Data• Mean [log(d)] = -0.42• Median [log(d)] = -0.41• St. dev [log(d)] = 0.25

Gaussian Fit• Mean, xc = -0.41• St. dev, δ = 0.25

- 0.45

- 0.30

- 0.04

0.07

0.22

0.30

0.52

0.79

- 0.58

- 0.41

- 0.03

0.26

0.29

- 0.46

0.00

0.22

0.23

- 0.47

- 0.24

0.20

Med[log(d)]

0.28

0.21

0.28

0.28

0.28

0.28

0.38

0.36

0.20

0.25

0.33

0.18

0.21

0.21

0.26

0.25

0.22

0.24

0.28

0.07

Std. dev [log(d)]

0.25- 0.44- 0.440.44MH_PBT- 2

0.19- 0.31- 0.280.60MH_PBT- 1

0.240.000.0041.31CEC- 6

0.240.070.011.69CEC- 5

0.270.210.232.10CEC- 4

0.280.290.322.61CEC- 3

0.410.530.535.05CEC- 2

0.310.790.839.82CEC- 1

0.20- 0.59- 0.570.30PP- 5

0.25- 0.41- 0.420.45PP- 4

0.37- 0.07- 0.041.23PP- 3

0.170.260.272.04PP- 2

0.210.280.302.23PP- 1

0.17- 0.48- 0.430.44PBT- 5

0.210.010.001.22PBT- 4

0.240.210.232.01PBT- 2

0.200.230.262.07PBT- 1

0.28- 0.47- 0.480.38PS- 3

0.29- 0.23- 0.290.62PS- 2

0.070.200.201.61PS- 1

δxc

Gaussian fit to log(d)Mean [log(d)]dav, µmRun I.D.

Why distribution of fiber sizes?

1

2

Distribution of drag forces

Fiber diameter distribution ↔ Fiber formation mechanism

AIR AIR

Fiber break up

PP PS

• High processing temperature and air flow rates → Smaller fibers• Surface tension driven• Average sphere diameter ≈ 1 µm; seen in fibers with dav < 0.6 µm• Dependent on fiber motion

Does this represent an onset of a fundamental limit of melt blowing?

Fundamental aspects• Verified existing correlations between fiber diameter and processing conditions• Melt blowing not limited to 1 µm

• produced few hundred nm fibers with variety of polymers (also with BCP)• Fiber diameter – Log-normal distribution

• characteristic of process (must be related to fiber formation mechanism)• Surface tension driven fiber break up (first time in melt blowing)

Technological aspects• Demonstrated lab scale melt blowing device (single and multi orifice)

small amounts (few grams) of material requiredshort run time (few hours)

• Narrow gap between melt blowing and electrospinning

• Biocompatible polymers• Nanoporous fibers (etch out one component from BCP fibers)••

Summary

Acknowledgements

Frank Bates

Chris Macosko

Lisa Lim, Cletis Reaves – C/E mechanical propertiesVince Holmberg – CEC/CECEC extrusion

Chris Ellison, David Giles Jim Stuart, Peter Herman (Cummins Filtration)

Polymer group

Cummins Filtration, U of M MRSEC – Financial support

Melt blowing