1. Asymmetric Flow Field Flow Fractionation – An Alternative to SEC-MALLS
Robert Brüll, Jan-Hendrik Arndt
Fraunhofer-Institute for Structural Durability and System Reliability LBF

2. Agenda Size Exclusion Chromatography (SEC) (also known as GPC)
How does Field Flow Fractionation (FFF) work?
Specifics of Asymmetric Flow FFF (AF4) working principle and analyses
Advantages of Asymmetric Flow Field Flow Fractionation (AF4)
Conclusion: SEC and AF4 in comparison

3. UHMWPE: Getting more out of PE

4. Characterizing molecular heterogenieties in polyolefins
HT – SEC
Dow
HT – 2D LC (HPLC X SEC)
2010, Dow-LBF
MMD
HT – AFFFF
Dow
Solvent Gradient LC
2009 Dow-LBF
Molecular heterogenieties
Interaction
based techniques
TGIC
2010, Dow
Huge amount of research being done in polyolefin characterization. These are just a few of the many important techniques that have come about over the years.
HT-SEC
Crystallization based techniques – TREF, Crstaf and CEF (which will be discussed by Dr. Monrabal who is a pioneer in this field)
TGIC (as discussed in the previous presentation by Dr. Cong)
HT-HPLC : is what I will be focussing on in my presentation
CCD
TREF
1982, Wild
Crystallization
based techniques
CRYSTAF
1994, Dow PolymerChar
CEF
2007, PolymerChar

5. High molar masses – what is the best analytical approach?
processability
mechanical strength
matrix
Smoke, odor
melt strength

6. Size Exclusion Chromatography (SEC)
Detector signal
SEC Separation
Elution volume
Limited in the high molar mass regime by exclusion limit of the column
shear degradation
Long chain branched material retained by additional mechanisms  Abnormal late coelution

7. Field flow fractionation (FFF) – Family of separation techniques
Invented 1966 by Prof. Calvin Giddings, Univ. of Utah (1969)
Coupling of FFF with light scattering, Michel Martin, ENS Paris (1984)
All field flow techniques utilize the same basic separation principle
Different technique - different force field
Smaller particles will elute first due to higher diffusion rates

8. Field Flow Fractionation (FFF) working principle
Force field
Separation
channel,
height 350 µm
Diffusion
Accumulation wall
Separation in a channel not in a column
Force field pushes polymer against one wall (accumulation wall)
Diffusion (Brownian motion) counteracts force field (vertical size separation)

9. Field Flow Fractionation (FFF) working principle
Force field
Separation
channel,
height 350 µm
Diffusion
Accumulation wall
Separation in a channel not in a column
Force field pushes macromolecules against bottom wall (membrane)
Diffusion (Brownian motion) counteracts force field (vertical size separation)
Sample transport by laminar flow (horizontal size separation)
Small molecules (of low molar mass) reach the detector faster

10. Focus flow – start of most experiments
Focus flow inlet
TIP flow
Focus flow
to detector
Cross flow
Focus flow is used during the injection phase
Concentrates the sample at the channel inlet
Reduces peak broadening, enhances separation
Sample can already separate (vertically) before elution starts

11. Injection step During run Top View Side View Cross flow Injection
Detection
Injection step
During run
Cross flow
Top View
Waste
2.4 mL/min0.08 mL/min
0.08 mL/min  0 mL/min
2.4 mL/min
Injection
0.5 mL/min
0.1 mL/min
Detection
0.5 mL/min
Membrane
Waste
2.0 mL/min  0.0 mL/min
2.0 mL/min
Side View

12. HT-AF4 channel construction 1st generation
Membrane is fixed on a steel frit
Operating life time: few days to weeks
Long downtimes
Membrane is highly sensitive to variations in temperature
mechanical shocks etc.
Membrane
Channel cavity

13. HT-AF4 channel construction 2nd generation
Ceramic membrane is sputtered with ZrO2 which tunes porosity
Operating life time: approx. 1 year
Top plate with PTFE spacer
Bottom plate with ceramic membrane

14. Our Setup – HT-SEC and HT-AF4 in one
Both operation modes (SEC/AF4) use the same detectors (Infrared (IR), MALS)
Switch between SEC and AF4 operation by clicking a button (software controlled valves)

15. Recap: AF4 membrane retention capabilities
Very similar results for two different membranes (first membrane four months old, second membrane fresh from manufacturer)
Limit of full retention ~ 100 kg/mol in both cases (!)

16. Recap: General features of AF4 elution profiles
CPMSR13100Y2
Void peak  Low molar mass material, small amounts of unretained material
High molar mass tail (eluting after cross flow is stopped), partially eluted if cross flow is interrupted

17. Achievement of void peak reduction (so far)
NIST SRM 1496 Standard
(broadly distributed HDPE)
The higher the injection flow, the lower the void peak
Amount of unseparated material is decreasing
More accumulation on the membrane in the focussing step

18. AF4 – Optimization of cross flow programs 1
Void peak  small amounts of unseparated material (always present)
Lineares PE Mw: 200 kg/mol Ð: 6,7 (MALS)
Intially used program
Longer linear steps
Goal: Linear separation (analogous to GPC)  Direct impression of MMD
Intially: linear program with two steps
Change of slope clearly visible in elution profile
Longer linear steps considerably more favorable, potential for additional improvements

19. AF4 – Optimization of cross flow programs 2
Exponential flow program
Longer duration
Sudden changes of slope are unfavorable  Switch to exponential flow programs
Further optimization by increasing gradient duration
Result: Significantly more linear separation

20. HT AF4 of LDPE Mw: 2.099 kg/mol; D: 1,06
Higher mass in HT-AF4 detected
Mw: kg/mol; D: 1,06
HT-SEC vs. HT-AF4
Co-elution of large material in HT SEC
NO late co-elution of large material in HT AF4

21. Degradation esp. by stirring Depending on Antioxidants
Analysis of entangled and disentangled UHMWPE
Slow rotation, shaking, stirring
Heating
Entangled
UHMWPE:
Partially entangled
Completely
disentangled
Degradation esp. by stirring
Depending on Antioxidants
Entangled UHMWPE cannot be dissolved completely using only heating
Shaking and stirring are common methods to dissolve UHMWPE
Physical degradation can occur by shear forces
Chemical degradation can occur (Oxidation)
Heating
Completly
disentangled
D-UHMWPE

22. AF4 as a tool to investigate entanglement
Slow rotation
Typical elution profile
Completely
disentangled
Partially entangled
Major differences can be found in the AF4: UHMWPE fraction
Entangled UHMWPE can be found at the end of the peak
Slow rotation
Completly
disentangled
Completly
disentangled
Highest detected molar masses with AF4:
few billion g/mol
No mayor differences should be found in the AF4

23. How to detect LCB?
Polymers in a good solvent (like PE in TCB) typically assume a random coil conformation
Coil size (radius of gyration (Rg)) increases with molar mass
Apparent particle size
Molar mass

24. How to detect LCB?
Relation between molar mass and coil size: Rg = a x Mb
a depends on the type of polymer but b on its topology!
Linear polymer: b ~ 0.58
Polymer with LCB: b < 0.58
b is determined as slope in log-log plot of Rg vs. M (Rg M relation)

25. RgM relation for HDPE
Bimodal slope – multiple active catalytic species
Molar masses > 107 are found

26. AF4-MALS of HMS PP Mw 13040 kg/mol, Mn 144 kg/mol
Slope 0.57
Slope 0.40
Mw kg/mol, Mn 144 kg/mol
Molar masses up to 109 g/mol found

27. How reproducible are these results?
Reproducibilities ~6%
bimodal and unimodal HDPE ~ kg/mol
LDPE, HMS PP
SEC varies ~5% empirically
AF4 results of the same membrane are reproducible compared to SEC
Variation old versus new membrane hasn‘t (obviously) been done yet

28. The separation power of AF4
MMD of PS standards from LS
Well in agreement with name plate values

29. New possibilities: Multiple Injection
Compared to SEC a multiple injection in AF4 is possible
Focussing step will be extended
Polymer can be upconcentrated at the focussing spot
No major deviations in the peak shape
Accessible for fractionation
5 injects
1 inject

30. High Temperature Fractionation
Valve control
Operated by WinGPC software
Self-build fractionator
(can be used for high-temperatures up to 180°C)
Heat control unit
10 port valve
with heating hoses
Fractionation can be carried
out in small vials or bigger flasks

31. Sample Preparation
Due to solubility issues of UHMW polyolefins new preparation methods have to be developed
Stabilizer? Inert gas?
Agitation?
Temperature?
Slow rotation unit

32. Conclusions Void peaks can be eliminated
Gully effect has been minimized
HT-AF4 has been developed towards a robust and stable method
Ultra high molar masses can be analyzed reliably for the first time
Information about LCB becomes accessible
Peculiar co-elution is avoided
New perspectives for preparative fractionation are on the horizon
New methods for sample preparation are now needed
Experimental parameters for AF4 have to be customized for material classes (e.g. LDPE, UHMWPE, BiPE, HMS PP)
Cross flow programs have to be finetuned for in depth information

33. THE REAL HEROES
Dib
Jan
Tibor
Sampat
Subin
Prabhu
Abhishek

34. THE REAL HEROES
Tobias
Nico
Gordian
Guru

35. Acknowledgement