Melt processing and Mechanical Properties of Polyolefin Block Copolymers Alhad Phatak Adviser: Frank Bates
Combine properties of different polymers Poly(lactic acid)-poly(ethylene) blend (Wang et al. J. Polym. Sci. 2001) Polymer-polymer blend Poly(styrene)-Poly(isoprene) BCP (Khandpur et al. Macromolecules 1995) Block copolymer
Block Copolymer Morphologies PS-PI diblock copolymer χn ~ 1/T χ = Flory-Huggins interaction parameter N = Overall degree of polymerization T ODT Khandpur et al. Macromolecules 1995, 28, 8796.
Commercial Applications Polystyrene based BCPs: Thermoplastic Elastomers (Kraton); PS-PI-PS, PS-PB-PS High impact thermoplastics (K-resins, Chevron Philips); (PS-PB) n Adhesives Footwear Asphalt additives Packaging www.kraton.com
Poly(cyclohexylethylene) - PCHE ( ) n ( ) n + H 2, Pt/Re/SiO 2 170 ºC, 500 psi PS PCHE Higher T g (147 ºC vs. 105 ºC) Better thermal, oxidative, and UV stability High entanglement molecular weight (~ 40 kg/mol) BRITTLE Solution: Make block copolymer with polyethylene Hucul and Hahn, Adv. Mater., 2000; Bates, Fredrickson, Hucul, and Hahn, AIChE Journal, 2001
Glassy Semicrystalline Block Copolymers C ( ) n E [( ) ( ) 0.08m]ran 0.92m Poly(cyclohexylethylene) H 2 -ed PS T g 145 ºC M e 40 kg/mol Polyethylene H 2 -ed 1,4-PB T m 100 ºC T g -100 ºC M e 1 kg/mol Hard block Soft block
Outline Lamellae-forming C/E Block Copolymers Mechanical Properties Melt Processing Polymer Nanofibers by Melt Blowing
Outline Lamellae-forming C/E Block Copolymers Mechanical Properties Melt Processing Polymer Nanofibers by Melt Blowing
CE diblock copolymer - Brittle (Failure strain 1%) Loose chain ends Chain pullout Lim et al, Macromolecules 2004 ECE Brittle (Failure strain 1%) CEC - Ductile (Failure strain 300%) 0% PE chains anchored at C/E interfaces 100% PE chains anchored at C/E interfaces Soft block must be anchored at lamellar interfaces
PCHE PE Polymer Molecular weight, kg/mol w C T ODT, ºC (Rheology) d *, nm (SAXS) CEC* 34 0.56 262 18 ECEC 34 0.48 253 20 ECECE 48 0.65 205 18 Control degree of anchoring of PE block * Made by Dow Chemical Company
Measurement of Tensile Properties anneal above T ODT Sample: 10mm x 5mm x 1mm Extension rate = 10mm/min Room temperature Stress (σ) versus strain (ε) measurement Failure strain (ε f ) Measure of toughness
Common Parameter - ψ E ψ E = Weight fraction of PE anchored at C/E interfaces 0, CE and ECE ψ E = 0.33, ECECE 0.5, ECEC 1, CEC and CECEC ψ = E n M + n M CEC E,CEC ECECE E,ECECE n M + 3n M CEC E,CEC ECECE E,ECECE ECECE/CEC blends ψ = E n M + n M CEC E,CEC ECEC E,ECEC n M + 2n M CEC E,CEC ECEC E,ECEC ECEC/CEC blends
Common Parameter - ψ E Limited by PCHE Weak PE Pure ECECE Pure ECEC Toughening of PE A. Phatak, L. S. Lim, C. K. Reaves, F. S. Bates Macromolecules (2006)
Summary Sensitivity of mechanical properties to molecular design Tying down soft block is critical ψ E - Design parameter for making tough BCPs Manipulate molecular architecture Control mechanical toughness
Outline Lamellae-forming C/E Block Copolymers Mechanical Properties Melt Processing Polymer Nanofibers by Melt Blowing
CEC CECEC Polymer M w, kg/mol w C T ODT, C CEC* 34 0.56 262 CECEC* 52 0.56 231 * Made by Dow Chemical Company
Extrusion Capillary rheometer (Goettfert Rheo-Tester 1500) Constant velocity mode γ = ap 6Q WH 2 σ aw = H P H 2L 1 + W Extrusion flow curve - γ ap vs. σ aw
Flow Curves - CEC T < T ODT v σ > σ sh v σ < σ sh Scale bar = 50 µm
Flow Curves - CECEC T < T ODT v σ > σ sh v σ < σ sh Scale bar = 50 µm
Surface Profiles CEC and CECEC σ > σ sh CEC σ > σ sh CECEC σ < σ sh Scale bar = 50 µm σ < σ sh
Average surface roughness CEC and CECEC A. Phatak, C. W. Macosko, F. S. Bates, S. F. Hahn; J. Rheol. (2005)
CEC/CECEC blends σ aw > σ sh T = 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 CEC CECEC ECECE ECEC DESIGN BLOCK COPOLYMER MOLECULES MECHANICAL TOUGHNESS MELT PROCESSIBILITY
Outline Lamellae-forming C/E Block Copolymers Mechanical Properties Melt Processing Polymer Nanofibers by Melt Blowing
Motivation Nonwoven products from polymer fibers ($16.4 billion industry*) Properties High specific surface area (~ 1/d) Chemical resistance * Nonwovens Industry Magazine (2004), http://www.inda.org/category/nwn_index.html Grafe et al. International Nonwovens Journal (2003)
Motivation Nanofiber applications (few hundred nm) U.S. patents on nanofibers From Huang et al., Compos. Sci. Tech. (2003)
Motivation Electrospinning Only continuous process to obtain nanofibers 10 nm to 1 µm fibers Slow process Solvent handling
Melt Blowing Fiber formation (draw down) by air Action T g or T c Processing variables Polymer and air temperatures (T p, T a ) Polymer and air flow rates Faster No solvent Limited to microfibers
Melt Blowing Current 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 Air Air Die orifice: d 0 = 0.2 and 0.4 mm
Materials Polymer M n (kg/mol) T c (ºC) T g (ºC) ( ) n PS* 2.1-61 ( ) n PP 15.0 118-2 O ( ) n O PBT MFR = 350 190 35 O O * PS experiments performed by Chris Ellison
Run I.D. T p, T a (ºC) Polymer mass flow rate (g/min) Air volumetric flow rate (SCFM) Γ d av, µm PS-1 180 0.053 8 9 1.61 PS-3 280 0.07 8 6.8 0.38 PP-3 180 0.35 6 0.5 1.23 PP-4 180 0.035 8 13.6 0.45 PP-5 220 0.035 8 13.6 0.30 PBT-4 265 0.35 4.5 0.4 1.22 PBT-5 265 0.035 10 17 0.44 Γ= Air mass flux Polymer mass flux Higher Γ Greater drag force on fibers Finer fibers
Run I.D. T p, T a (ºC) Polymer mass flow rate (g/min) Air volumetric flow rate (SCFM) Γ d av, µm PS-1 180 0.053 8 9 1.61 PS-3 280 0.07 8 6.8 0.38 PP-3 180 0.35 6 0.5 1.23 PP-4 180 0.035 8 13.6 0.45 PP-5 220 0.035 8 13.6 0.30 PBT-4 265 0.35 4.5 0.4 1.22 PBT-5 265 0.035 10 17 0.44 Lower melt viscosity Higher T p Higher draw down temperature window [T p < T < T g (or T c )]
Run I.D. T p, T a (ºC) Polymer mass flow rate (g/min) Air volumetric flow rate (SCFM) Γ d av, µm PS-1 180 0.053 8 9 1.61 PS-3 280 0.07 8 6.8 0.38 PP-3 180 0.35 6 0.5 1.23 PP-4 180 0.035 8 13.6 0.45 PP-5 220 0.035 8 13.6 0.30 PBT-4 265 0.35 4.5 0.4 1.22 PBT-5 265 0.035 10 17 0.44 Sub-micron fibers from all polymers
PBT
PBT PP
PBT PP PS
Fiber Diameter Distribution > 200 fibers in every case Asymmetric Not normal distribution
Log-normal Distribution Data Mean [log(d)] = -0.42 Median [log(d)] = -0.41 St. dev [log(d)] = 0.25 Gaussian Fit Mean, x c = -0.41 St. dev, δ = 0.25 Gaussian Fit: 1 (x - x c ) p(x) = exp - 2 δ 2π 2δ 2 x c : mean δ: standard deviation
Run I.D. d av, µm Mean [log(d)] Med[log(d)] Std. dev [log(d)] Gaussian fit to log(d) x c δ PS- 1 1.61 0.20 0.20 0.07 0.20 0.07 PS- 2 0.62-0.29-0.24 0.28-0.23 0.29 PS- 3 0.38-0.48-0.47 0.24-0.47 0.28 PBT- 1 2.07 0.26 0.23 0.22 0.23 0.20 PBT- 2 2.01 0.23 0.22 0.25 0.21 0.24 PBT- 4 1.22 0.00 0.00 0.26 0.01 0.21 PBT- 5 0.44-0.43-0.46 0.21-0.48 0.17 PP- 1 2.23 0.30 0.29 0.21 0.28 0.21 PP- 2 2.04 0.27 0.26 0.18 0.26 0.17 PP- 3 1.23-0.04-0.03 0.33-0.07 0.37 PP- 4 0.45-0.42-0.41 0.25-0.41 0.25 PP- 5 0.30-0.57-0.58 0.20-0.59 0.20 CEC- 1 9.82 0.83 0.79 0.36 0.79 0.31 CEC- 2 5.05 0.53 0.52 0.38 0.53 0.41 CEC- 3 2.61 0.32 0.30 0.28 0.29 0.28 CEC- 4 2.10 0.23 0.22 0.28 0.21 0.27 CEC- 5 1.69 0.01 0.07 0.28 0.07 0.24 CEC- 6 1.31 0.004-0.04 0.28 0.00 0.24 MH_PBT- 1 0.60-0.28-0.30 0.21-0.31 0.19 MH_PBT- 2 0.44-0.44-0.45 0.28-0.44 0.25
Why distribution of fiber sizes? AIR AIR 1 2 Distribution of drag forces Fiber diameter distribution Fiber formation mechanism
Fiber break up PP High processing temperature and air flow rates Smaller fibers Surface tension driven Average sphere diameter 1 µm; seen in fibers with d av < 0.6 µm Dependent on fiber motion Does this represent an onset of a fundamental limit of melt blowing? PS
Summary 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 required short run time (few hours) Narrow gap between melt blowing and electrospinning Biocompatible polymers Nanoporous fibers (etch out one component from BCP fibers)
Acknowledgements Frank Bates Chris Macosko Lisa Lim, Cletis Reaves C/E mechanical properties Vince Holmberg CEC/CECEC extrusion Chris Ellison, David Giles Jim Stuart, Peter Herman (Cummins Filtration) Melt blowing Polymer group Cummins Filtration, U of M MRSEC Financial support