INTERACTION POLYMER CHROMATOGRAPHY

Yefim Brun*, Chris Rasmussen, B. McCauley

DuPont Central Research & Development yefim.brun@dupont.com

GPC/SEC and Related Techniques, Frankfurt, 2014

2 Liquid chromatography for polymer characterization

Dissolution Separation Detection Data Reduction

Interaction Polymer Chromatography IPC

separation by everything but size

Size Exclusion Chromatography SEC

separation by molecular size

Separation media • porous particles (HPLC, UPLC) • non-porous particles (HDC) • monolithic columns • capillary

2D-Chromatography IPC/SEC, IPC/IPC, SEC/IPC

3

Biomolecules, nanoparticles, micelles, vesicles

Molecular properties of polymers & other nanostructures

Molecular heterogeneities (distributions): strong effect on end-use properties

Synthesis Molecular Structure Property

End-groups Topology Molar mass, size and shape

Homopolymers

statistical alternating block gradient

telechelic comb star linear

Copolymers

Chemical Composition and Microstructure

Size, shape, interaction, surface chemistry

4

Open literature

1. SEC is by far the most popular LC technique for polymer characterization

2. IPC: mostly isocratic separations (LCCC+ barrier techniques) 3. HPLC practitioner: do not use isocratic mode in separation of big molecules!

DuPont

1. IPC/SEC: 50/50(%)

2. IPC: gradient approaches (including TGIC) >90%

3. IPC: practically all separations problems are more effectively solved in gradient mode (vs. isocratic)

DuPont vs. polymer characterization community

Perception: more difficult to control the mechanism of separation in gradient mode

5 How to control separation • Order of elution (selectivity of separation) is determined by mechanism of retention

• Retention is result of interaction between molecules and solid surface of porous particles

• Right balance between different types of interaction is the key to separation mechanism Steric Interaction

(SEC)

Adsorption (HPLC)

IPC

Phase Transition / Partition (Precipitation LC)

Critical point of adsorption (LCCC)

Entropy loss Enthalpy gain

Mutual compensation effect : MW-independent elution

LCCC: LC at critical conditions, or LC at critical point of adsorption (CPA)

6

Elution of Narrow Polystyrenes on Symmetry® C18

Elution time tR, min (flow rate 0.5ml/min)

2.6 3.0 3.4 3.8 4.2 4.6 5.0

474

5,570

THF suppresses non-polar interaction

SEC mode at Φ =100%THF : big elutes first

710,000

Mobile phase THF

4

Non-polar interaction prevails

Adsorption mode at Φ = 45%THF : big elutes last (or does not elute at all)

6 8 10 12

474

9,100 18,100

THF- ACN (45/55)

5,570

tliq ti

Different modes of isocratic separation

7

Elution of Narrow Polystyrenes at Symmetry® C18

tR, min (flow rate 0.25ml/min)

3 4 5 6 7 8 9 10 11 12 13

474

37,900

186,000

THF-ACN (48/52)

Transition mode: MW-independent elution at Φ = Φcr (48%THF)

Critical Point of Adsorption (CPA)

tliq

Log

M

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Elution volume VR (mL)

0 1 2 3 4 5 6

Vliq V0

100

80

70

50

ΦCR= 48% 45

40

Elution at different compos. Φ (THF,%) SEC LAC LCCC

Different modes of isocratic separation

8

Applications

•Separation by functional groups

•Separation of block-copolymers by length of individual blocks

•Separation of homopolymer blends

• Not applicable to statistical copolymers

Limitations

•Often limited to oligomers/low-M polymer separations in wide-pore columns

•Very sensitive to experimental conditions

•Temperature/solvent composition fluctuations resulting in peak splitting, limited mass recovery

LCCC

Let’s discuss after Falkenhagen’s and Hiller’s talks Minutes 0 5 10 15 20 25 30

Flow rate (ml/min):

0.5 0.25 0.125

Nova-Pak C18 Column (60A, 4 µkm, 3.9 x 300 mm) THF-ACN (49/51)

LCCC of 186K at different flow rates

9

Critical point of adsorption (CPA): 50% ACN in H2O on Symmetry® C4

Elution of linear PEG (MW=40K) with different end groups at CPA on Symmetry® C4

mono-brominated PEG PEG standard with 2 OH ends

Minutes 1.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15 2.20 2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.60

Δt = 0.13 KLCCC = 1 + q

tliq tliq + Δt(q)

LCCC: separation by end groups

Φ = Φcr = 50%ACN

10 Barrier Techniques

• Liquid Chromatography under Limiting Conditions (D. Berek, et al) of adsorption (LGA): Φsample > ΦCR, Φeluent < ΦCR of solubility (LCS): Φsample > ΦSOL, Φeluent < ΦSOL of desorption (LCD): Φsample < ΦCR, Φeluent > ΦCR

• SEC-Gradients method (W. Radke)

Let’s discuss after Radke’s talk

• Limitations: local solvent gradient depends on viscosity, column hydrodynamics, chromatographic conditions (flow rate, injection volume, concentration), separation is limited to column volume

11

polystyrene / styrene-acrylonitrile copolymer / styrene-butadiene copolymer Chromatographic characterizations of polymer blend:

Isocratic separation by size in THF at SEC column set

Minutes 14 15 16 17 18 19 20 21 22 23 24 25 26 27

SEC: separation by size

0

10

20

30

40

50

60

70

80

90

100

Gradient separation by composition in ACN-THF gradient at C18 RP column

Minutes 0 2 4 6 8 10 12 14 16 18 20

St-Ac

Polystyrene

SBR Eluent Gradient

Elu

ent C

ompo

sitio

n,Φ

%

Gradient IPC: MW-independent elution

SEC vs. gradient IPC

12

Elution of narrow polystyrenes at Symmetry® C18

ACN – THF gradient (0 -100%) over 10 min

Elution time, min (flow rate 1 ml/min)

30

10

9,100 50

40

20

0

2,890

Eluent gradient

8 6 5 4 3 7

Elu

ent C

ompo

sitio

n Φ

(TH

F,%

) ΦCR

oligomers LAC Gradient Elution (GE)

at CPA

Practical advice: even if you want to use LCCC, run gradient first: gradient elution is the best way to find critical point of adsorption

190,000 355,000 710,000

1,260,000

43,900 ΦCR = 48% THF

6.5 Log M

0

10

20

30

40

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Φg (

at e

lutio

n}

Different modes of gradient separation

Brun, Alden, J. Liq. Chrom 2002

13 Solvent gradient methods

Adsorption Precipitation

Precipitation

Steric

Adsorption

Precipitation-redissolution chromatography (elution at dissolution point) (GPEC) separation by MW and chemistry

redissolution

GE at critical point of adsorption (CPA) separation by chemistry, topology

Steric

Adsorption Precipitation

CPA

Steric

CPA

LAC separation by MW, chemistry, etc.

redissolution

CPA does not exist

14

Eluent gradient

Elution time, min

14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5

20

40

60

80

100

0

MeOH – THF (0 -100%) over 10 min ΦCR does not exist: elution at redissolution

Gradient elution at redissolution point (GPEC)

Eluent gradient

ACN – THF (0 -100%) over 10 min Φ*sol < ΦCR: elution at CPA

100

14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0 18.5 19.0 19.5

20

40

60

80

0

Φ(%

THF)

Gradient elution at CPA

ΦCR

Elution of narrow polystyrenes at Nova-Pak® C8 Different modes of gradient separation

Brun, Alden, J. Liq. Chrom 2002

15

Elution of polystyrenes at Nova-Pak® C8

X: MeOH ( ) - elution at redissolution ACN ( ) –elution at CPAn

Only elution at CPA is MW-independent!

X – THF (0 -100%) over 10 min

3

13

23

33

43

53

63

2.0 4.0 6.0 8.0

Φg

(%TH

F)

ΦCR

Log M

Elution at CPA

LAC

Elution at redissolution

Different modes of gradient separation

16 Gradient elution at redissolution point (GPEC)

Liquid-liquid phase transition in the presence of solid surface: ∆F ∼ M (Φ – Φ*sol)

Usually, Φ*sol > Φsol

Threshold depends on polymer molecular weight and concentration

(Armstrong et.al., 1984)

3 5 7

0.4

0.6

0.8

Log M

*solΦ Gradient elution at CPA can occur

only if Φ*sol < ΦCR Otherwise, ΦCR does not exist and elution is MW-dependent

Φinj < Φ*sol

17 How to control the transition from LAC to GE at CPA

Log Q

Q < 1 - retention depends on molar mass M (LAC)

Q > 1 (CPA)

Φ0

ΦCR

0 –1 1

Transition point: Q ~ 1

′ΦΦ

= XX

2

2

dd

DaR

Q g ε

gR - radius of gyration D - pore diameter

ε - segment interaction energy Φ′ - gradient rate

Transition to CPA occurs faster (at lower M) for: narrow pores (low D) steep gradient (high Φ′) high selectivity (high dε / dΦ)

18

Log M

Gradient time: 5 min 10 min 15 min 20 min 25 min 30 min

10

15

20

25

30

35

40

45

3.5 4.0 4.5 5.0 5.5 6.0

Φg (

%TH

F)

3

13

23

33

43

53

63

Effect of operating conditions Elution of narrow polystyrenes at NovaPak® Silica

Hexane – THF gradient (0 -100%) over 10 min

Effect of gradient rate: shallower gradient – later transition to CPA

Theory vs. Experiment ΦCR

19 Temperature gradient interaction chromatography (TGIC) • IPC at adsorption conditions (LAC): separation by M, not size, but for oligomers only

• ΦCR depends on T, but dε /dΤ << dε/dΦ

• Temperature gradient should be much more effective in separation by MW

Adsorption Precipitation

Steric TGIC

CPA

T > TCR

Φisocr = Φ(TCR) T0

TCR

20 DuPont Confidential October 8, 2014 20

Temperature vs. solvent gradients

0

2

4

6

8

10

12

14

16

18

20

3 4 4 5 5 6 6

retn

etio

n tim

e [m

in]

log M

T0 = 40°C

T0 = 60°C

Solvent gradient, T=60C

• Able to resolve high M species • Solvent gradient is still useful to identify critical conditions • Intrinsic relation between solvent & temperature gradient • TGIC could be coupled to SEC in 2D setting: SEC-IPC for branching analyses

T’ = 2°C/min, flow rate 1 ml/min, Φisocr = 44% THF, TCR = 60C

Start temp.:

TGIC of narrow polystyrenes at Symmetry® C18

21 Gradient elution at CPA Applications

• Separation of polymer blends

• Separation of telechelic polymers by functional groups

• Separation of copolymers by chemical composition and microstructure, e.g. Blockiness

• Copolymer purification by flesh chromatography based on IPC

• Separation of nanoparticles by surface chemistry

Benefits • Easiest way to find CPA

• Broad range of eluents (including non-solvents)

• Practically no limitation on MW

• Superb resolution, especially at narrow pore columns

• No peak splitting or some other chromatographic artefacts

Potential problems • Break-through effect

• Multiple interaction mechanisms for complex polymers

• Competition with dissolution mechanism of retention for non-solvents

22

Separation of poly(alkyl methacrylate) blend (MW ~ 300K) at Symmetry® C18

ACN – THF gradient (0 – 100%) over 30 minutes

4

8

12

16

20

24

0

25

PMMA

PLMA

PnHMA

PnBMA PEMA

5 10 15 20 Elution time, min

Selectivity of gradient elution at CPA

Brun, Alden, J. Liq. Chrom 2002

23

ACN – THF (0 -100%) over 10 min

Separation of ultra-high M ( > 106) polyacrylates at Nova-Pak® C18

Efficiency and resolution on narrow pores (D ~ 6 nm)

PiBMA PEA

Elution time, min 14 15 16 17 18 19 20 21 22 23 24

PMMA PnBA PnBMA

PEHA

20

40

60

80

100

0

Elue

nt c

ompo

sitio

n (T

HF,

%)

Acrylate copolymers (core-shell) blend

D << gR ~50 nm

Only “flower” conformations contributes to retention Brun, Alden, J. Liq. Chrom 2002

24 Separations by end-groups: isocratic vs. gradient

mV

100

200

300

400

500

600

700

Minutes 3 4 5 6 7 8 9

0

1 2 3

4

number of chlorine ends

water/ACN (60.1/39.9, v/v)

Isocratic elution (LCCC)

water – ACN gradient: 20%ACN -100%ACN

mV

50

100

150

200

250

300

350

Minutes 21 22 23 24 25 26 27 28

0

1

2

3

4 number of chlorine ends

Gradient elution

OH

OH

Cl

Cl

4-arm PEGs with OH- and Cl-ends

Water-ACN at XTerra® C18

25 IPC-MS w/LTQ_Orbitrap: 4-arm PEG

30 32 34 36 38 40 42 44 46 48 50 52Time (min)

NL:1.55E8TIC MS 052711_PEG_102_2_in_water_07

0 Cl 1 Cl

2 Cl 3 Cl

4 Cl

Peak3 Peak1 Peak4

Peak2

Peak5

TIC

Theoretical prediction of gradient elution of PEGs with various end-groups

0

4

1 2 3

26 Gradient elution for copolymer characterization

• Critical point of adsorption for statistical copolymers

• Separation by chemical composition (chemical composition distribution)

• Separation by microstructure (blockiness)

27 IPC of copolymers (theory)

• Statistical copolymer Contain a single CPA (like in homopolymer)

CPA of statistical copolymers depends on chemical composition and microstructure (blockiness) of polymer chains

• Block-copolymer Does not contain a single CPA, but each individual block has its own CPA

Retention in gradient elution depends on MW and composition

Could be separated by block-length in gradient mode

• Effect of blockiness on retention in gradient elution Always increases retention

Elution time at same chemical composition: alternating < statistical<block-copolymer

• Practical implication Gradient elution allows for separation by chemical composition and microstructure

Y. Brun, J. Liq. Chrom., 1999 Y. Brun, P. Foster, JSS, 2010

28

Chromatographically, statistical copolymers behave similar to homopolymers: they contain CPA

Isocratic elution of chlorinated polyethylenes (38% Cl)

Minutes

Silasorb 600 Silica, different hexane-CHCl3 compositions

4.6

3.8

4.0

4.2

4.4

4.6

4.8

0 5 10 15 20 25 30 35 40 45

Log

M

ΦCR= 40.5% 36

38

44

Eluent compositions Φ (CHCl3, %)

Y. Brun, J. Liq. Chrom., 1999

29

dwt/d

(logM

)

0.0

1.0

2.0

3.0

Log M 3.4 3.6 3.8 4.0 4.2 4.4 4.6

Sample Mn Stat-1

Stat-2

Stat-3

Stat-4

Stat-5

Stat-6

Stat-7

Stat-8

6450

8694

11585

13240

14540

15289

16065

17107

Mw

7278

9617

12638

14657

15856

16734

17704

18918

PD

1.13

1.11

1.09

1.11

1.09

1.09

1.10

1.11

Conv (1H-NMR)

95

89 73

12

5 3

<1

<1

RAFT polymerization of 2-EHA/n-BA (50/50) statistical copolymers

CH3

OO

CHCH2

CH3CH2CH2CH2

H3CCH2

CHCH2

OOC

CHCH2

1 2

3 4 5

8

6 7

30 Isocratic elution of statistical 2-EHA/n-BA copolymers

C18, different ACN-THF compositions

Log

M

Elution at different compos. Φ (THF,%):

3.8

3.9

4

4.1

4.2

4.3

1.0 2.0 3.0 4.0

Elution volume Ve (mL)

100 70 60 58 56 54

Vliq

Elution time, min (flow rate 0.25ml/min)

ΦCR, stat = 60%THF

Critical Point of Adsorption (CPA)

4 5 6 7 8 9 10 11 12 13 14 15

THF-ACN (60/40) 1 - 8

tliq

0

31 Gradient elution of statistical 2-EHA/n-BA copolymers

0

10

20

30

40

50

60

70

80

90

100

17.0 17.5 18.0 18.5 19.0 19.5 20.0 20.5 21.0 21.5

PBA (33K)

PEHA (42K) 1

2 3

4

5,6,7

8

ΦCR,stat

Elution time, min (flow rate 1 ml/min)

Elue

nt C

ompo

sitio

n Φ

(TH

F,%

)

Eluent gradient

Log M

20

30

40

50

60

70

3.8 3.9 4.0 4.1 4.2 4.3

ΦCR,stat

Φ at

Elu

tion

Effect of MW on retention ΦCR, PBA = 42% ΦCR,stat = 60% ΦCR,PEHA = 68%

XTerra® C8, ACN – THF (0 -100%) over 10 min

32

Stat. 2-EHA/n-BA copolymers at different C18 columns

ACN – THF (0 -100%) at different gradients

40

45

50

55

60

3.8 3.9 4.0 4.1 4.2 4.3 Log M

Φ(%

TH

F) a

t Elu

tion

55

57

59

61

63

65

67

3.8 4.0 4.2 4.4 Log M

Symmetry ® 300A

XTerra ® 127A Nova-Pak ® 60A

Columns:

Effect of Pore Size

5 min 10 min 15 min 20 min

Gradient time: Column: XTerra ® 127A

Gradient time: 10 min

Effect of Gradient Rate

Φ(%

TH

F) a

t Elu

tion

Gradient elution: theory vs. experiment

33

Ethylene-Methyl Acrylate Copolymers at Nova-Pak® C18 mobile phase gradient: EtAc/MeOH (50/50)-toluene (50 - 100%) over 10 min

mV

20

60

100

140

180

220

260

300

340

Elution time (min) 12 13 14 15 16 17 18 19 20 21

Narrow chemical composition distribution (autoclave)

Broad chemical composition distribution (tubular reactor)

Chemical composition heterogeneity by gradient elution

Y. Brun, M. Pottiger, Macrom. Symp., v. 282, 2009

34 RAFT polymerization of A/B block-copolymers

dwt/d

(logM

)

0.00

1.00

2.00

3.00

Log M

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8

Sample Mn Mw PD % 2-EHA

Block 1 (Homopol A)

Block 2

Block 3

Block 4

Block 5

Block 6

Block 7

Block 8

7117

7548

8482

9256

10169

11293

11801

12127

7786

8569

9660

10629

11811

13666

14585

15473

1.094

1.135

1.139

1.148

1.161

1.210

1.236

1.276

100.00

91

81

73

66

57

53

50

1 2 3 6 7 4 5 8

A (2-EHA)

B (n-BA)

CH3

OO

CHCH2

CH3CH2CH2CH2

H3CCH2

CHCH2

OOC

CHCH2

35 Gradient Elution of Poly(2-EHA-block-n-BA)

Elution time, min 17.6 18.2 18.8 19.4 20.0 20.6 21.0 17.0

0

20

40

60

80

100 El

uent

Com

posi

tion

Φ (T

HF,

%)

Eluent gradient

PBA PEHA 1 2 3 4 5

6

7 8

ΦCR, PBA

ΦCR, PEHA

XTerra® C8, ACN – THF (0 -100%) over 10 min

Composition affects retention stronger than MW

36 Effect of copolymer microstructure on gradient elution

Statistical and block- 2-EHA/n-BA (50:50) copolymers

Nova-Pak® Silica, Hexane – THF (0 -100%) over 10 min

Elution time, min 12.0 17.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5

PEHA PBA

Stat. (Mn 17107)

Block (Mn 12127)

0

20

40

60

80

100

Elu

ent C

ompo

sitio

n (T

HF,

%)

XTerra® C18 ACN-THF (0 -100%) over 10 min

15 0

20

40

60

80

100

16 17 18 19 20 21 22

PEHA PBA

Stat. Block

CPA depends on microstructure: separation by CPA = separation by blockiness Blockiness always increases retention

37 Effect of copolymer microstructure on gradient elution

Statistical and block styrene/MMA (50/50) copolymers

Nova-Pak® Silica, Hexane – THF (0 -100%) over 10 min

Nova-Pak® C18 ACN-THF (0 -100%) over 10 min

Elution time, min

Retention of block-copolymers increases with MW Elu

ent C

ompo

sitio

n (T

HF,

%)

12 13 14 15 16 17 18 19 20

PMMA PS Stat (91K)

Block (93K) Block (407K)

13 14 15 16 17 18 19 20 21 22 23 24

PS (107K) PMMA (103K)

Stat (91K)

Block (93K) Block (407K)

20

40

60

80

100

0

100

20

40

60

80

0 12

Brun, Foster, JSS, 2010

38 Characterization of fluoro-containing aromatic polyesters

HO OH

m

O

O

O

O

O

OO

CF3CF3F

F F

F H F F

FF

F F

O O

O O

O O

O

O

OO

O

F F

F H F F

F CF3CF3

F F

F F

mn

Tyzorcat

-MeOH, -PDO

+

O O

O O

mO

O

O

O

O

OO

CF3CF3F

F F

F H F F

FF

F F

n

O O

O O

O O

O

O

OO

O

F F

F H F F

F CF3CF3

F F

F F

mn

Tyzorcat

-MeOH, -PDO

+

O

O

O

O+

n

"Monomer first"

"Oligomer first"

How to control microstructure of condensation copolymers?

“Monomer first”: traditional copolycondensation should produce statistical copolymer

PTT

3-GF16-iso

“Oligomer first”: chain extension in melt may produce blocky copolymers if transesterification is suppressed by phase separation

molar mass (g/mol) 1000.0 4 1.0x10 5 1.0x10

diffe

rent

ial w

eigh

t fra

ctio

n (1

/log(

g/m

ol))

0.2

0.4

0.6

0.8

1.0

1.2

PTT 3-GF16-iso

Chain-extended copolymer

N. Drysdale, Y. Brun, E. McCord, F. Nederberg, Macromolecules, 2012

39 Gradient elution of “monomer first” copolymers

0.00

0.04

0.08

0.12

0.16

0.20

10 12 14 16 18 20 22 24 26 28 30 32 34

100

90

50 25

10

AU

Minutes

Chemical composition calibration curve (MW independent)

0

20

40

60

80

100

40 50 60 70 80 90 100

MP composition, HFIP%

gradient 65-100%HFIP

gradient 60-100%HFIP

Polynomial fit

F 16-

iso

in c

opol

ymer

(mol

%)

mol %F16–iso:

UV chromatograms

FluoroFlash F8, water-HFIP

Separation of statistical copolymers by chemical composition (elution at CPA)

40 Chromatographic identification of copolymer microstructure

AU

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

Minutes 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

PTT AU

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Minutes 16.00 18.00 20.00 22.00 24.00 26.00 28.00 30.00

Statistical “monomer first” 50 mol%

Blocky “oligomer first” 50 mol%

41 Purification based on step-wise flash IPC

ELSD Chromatograms in Hexane-THF Gradient on Silica column

Initial solution (PS, PMMA, copolymer)

Intermediate solution (PS removed)

Final solution (PMMA stayed on the column)

Minutes 15 16 17 18 19 20 21

PS removal window

Copolymer removal window

PMMA removal window

1st elution 2nd elution

Purification of St/MMA copolymer from residual homopolymers

42

Nanoparticle

Stationary Phase

Free polymer chains grafted within the monolith pores provide higher degree of interaction between nanoparticle and column

Separating Nanoparticles by Surface Chemistry

• Monolith column produce better recovery for nanoparticles • Rigid surface provides minimal tangential nanoparticle/column interaction • Currently commercially available monoliths from styrene-divinyl crosslinked gel contain relatively short chains. Monoliths with grafted flexible chains are needed • Recently done at the Molecular Foundry at Berkley National lab (Dr. Frantisek Svec)

43 Separation of decorated colloidal silica by degree of coating

Separation of colloidal silica coated with mix of phenyl- and aminophenylsilane

13 14 15 16 17 18 19 20 21 22 23 24 25 26

Si-OH

APhTMS

PhTMS

9:1

3:1

1:3

1:1

PhTMS / APhTMS ratio:

Mobile phase: toluene-DMAc gradient Column: BMA/EDMA monolith grafted with PMMA chains (F. Svec, Berkley )

Elution time, min

• Long and flexible polymer chain functional groups (bonded phase) produce “reversed” critical point of adsorption. • Nanoparticle is “swallowed” by these chains through multiple attachment mechanism

First reported at HPLC 2012

44