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INTERACTION POLYMER CHROMATOGRAPHY
Yefim Brun*, Chris Rasmussen, B. McCauley
DuPont Central Research & Development [email protected]
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