Wettability change of Diamine Polymer Coating via CO2 capture
Waseda University, Takeo Suga
1. Introduction
Superhydrophilic surface based on zwitterion (e.g., sulfobetaine or phosphorylcholine) polymers has been
proposed as anti-fouling or self-cleaning surface. Takahara, et al., investigated superhydrophilic zwitterion
polymer brushes and their detailed hydration status of polymer-water interfaces by neutron reflectivity (Takahara,et al., Polym. J., 2016, 48, 325–331; Polymer, 2016, 98, 464-469). Compared to such intrinsic zwitterion polymers,
we designed diamine containing pre-polymers to accumulate CO2 at water interface to form zwitterion (Scheme1), which induced significant wettability change by just immersion in water under open-air conditions (low CO2conc. of 300 ppm). NMR, IR, and contact angle measurements verified CO2 capture by diamine polymer P1 andP4, but not by P2 and P3. Further understanding and better material design of such environmentally adaptive
coating, it is crucial to track the formation process of zwitterion and swelling behavior at polymer/water interface.
The goal of this research is to estimate superhydrophilic layer thickness formed by CO2 addition. NR was utilized
to analyze deuterated water (D2O) uptake and swelling behavior at interface comparing before and after CO2treatment. Time-slicing measurement of D2O uptake can also monitor CO2 responsive behavior of the diamine
polymer coating.

HOCO2N

CO2Air
WaternO

OmO

HON

r
0.05
OO

O

O

0.95
OOOnH

ONNH 2O

iamine polymer
P1r

HOH

NHOP1 (n = 0.05, 0.10)

R=O

Zwi.erion polymer

NHRONN
HP2orHONOP3P4NH 2Fig. 1 CO2 addition to the diamine polymer coating for superhydrophilic surface formation.
2. ExperimentDiamine polymers P1 (diamine content = 5, 10 mol%) were synthesized and spin-coated from
1-methoxypropanol solution onto quartz substrate (5 x 5 cm). The control samples P2 and P3 were also prepared.
The crosslinked epoxy coating was also prepared using P4 by thermal curing for 3 h. The target thickness of thecoating is 100 nm. Two solvent conditions are employed for NR experiments: a) D2O degassed and treated withargon bubble (CO2 free condition), and b) D2O with CO2 bubbling for 15 min (saturated CO2 condition, 3 x 10-2mol/L). The coating was set in the special liquid cell, and immersed in two solvent conditions. The firstexperiment (A) is to check swelling behavior of the coatings with different composition at equilibrium state. The
sample layer was quite thin (100 nm), so immersion time was set to 1 h for each experiment. To cover the q range
from 0.08 to 2 nm-1, the incident angle of neutrons is set at 0.3, 0.75, and 1.6°, and the reflectivity data wascollected. The data analysis was performed using Motofit, assuming the monolayer model. Change in thickness,surface roughness, depth profile of D2O provided the contribution of zwitterion formation, comparing conditionsa) and b). After the first screening experiment, the optimized composition was determined. The second experiment(B, time-slicing NR measurement) was carried out for the crosslinked polymer coating P4 (20 wt%) using special
liquid cell with auto injector. D2O was set under open air condition for 24 h and saturated with air (CO2 conc. ~1.2 x 10-5 mol/L). The SLD change due to D2O uptake was monitored over time (~3 h), and the profiles wereanalyzed every 10 min. All experiment was performed at 293 K.
1

3. Results
3.1 Swelling Behavior of Diamine Polymer Coating at Equilibrium StateNR profiles of the prepared polymer coating P1-4 were measured and fitted by the monolayer model atdifferent conditions: (a) under air, (b) immersed in D2O saturated with argon, and (c) immersed in D2O saturated
with CO2. Figure 2 shows the additional swelling of P1 (diamine content 10mol%) at condition (c), ascribed to thezwitterion formation by CO2 capture. Compared with P1, the control samples
P2 (with propylene spacer) and P3 (acetate) did not show significant change inswelling behavior, which suggested the slight structural change arounddiamine moieties critically affected the CO2 binding affinity (Table 1). These
results corresponded well with our previous findings on NMR, IR andcross-sectional SIMS analyses.
Table 1 Thickness and SLD profile change of the coating by immersion in D 2O
Thickness Thickness Roughness (nm)Polymer
Condition
SLD(nm)ratio (%) Polymer Glass

P1(10 mol%)

P2

P3

P4

76.4

1002.05.9

D 2 O ratio

70.96-65Ar

79.7

104

8.3

1.7

0.14

CO282.0

107

2.5

1.9

0.18

Air

64.4

100

2.30.94-Ar
CO2Air

69.6
79.7
71.2

108
124
100

1.4
2.6
0.10

6.0

1.9
2.6
0.90

0.18
0.31
-

Ar

74.0

104

1.2

1.0

1.5

0.11

CO2

75.0

105

1.0

1.6

0.13

Air

80.3

100

1.8

Ar

83.1

103

1.0

CO2

83.6

104

3.0

Air
Ar
CO2

68.4
71.6
75.1

100
105
110

2.4
5.6
6.7

0.94

2.00.92

-

1.4

0.087

1.7

0.14

1.2
2.6
2.8

0.28
0.31SLD (10-4 nm-2)

P1(5 mol%)Air

4SiO22040

6080

110

Swelling ratio (%)

min, which clearly revealed this technique can track the dynamic change inswelling behavior at this time scale (Fig. 3). However, the swelling ratio after

108
106
104
102
100

700 min increased to be 108%, and Time-slicing experiments for longer time125

and the swelling trend was not agreed with the data of 120 nm. Thickness

120

dependence and the time scale to reach at equilibrium state will provide kinetics

Swelling ratio (%)

needs to be collected. As for 180 nm film, q range was not suitable to fit the data,

the time-slicing experiments were preliminary, but its applicability could be20Film thickness (nm)Fig. 2 (a) NR profile and fiting
curve of P1 (10mol%) film, and(b) SLD profile at equilibrium
states.

120 nm film, the swelling ratio reached at the constant value of 105% within 30

behavior of CO2-responsive polymer coating. Due to the beam time limitation,

Air0180 nm) were carried out for 2 h using D2O containing low conc. CO2. As forTime-slicing NR technique is a powerful tool to monitor dynamic swelling

Polymer coating

1Time-slicing NR experiments of the crosslinked polymer coating P4 (120 and4. Conclusion

immersed D2O3

3.2 Time-Slicing Experiments for Swelling Behavior

to describe the zwitterion layer formation.

D2OAir
Ar aq
CO2aq

03060
90 120 700
150
Immersed time (min)030
60
90 120 satu.
150
Immersed time (min)115
110105100Fig. 3. Time-dependent swellingverified. Diamine polymer P1 (10 mol%) would be more suitable for further behavior of the crosslinked
investigation because of sufficient change in swelling ratio.
polymer coating P4 (a) 120 nm,
and (b) 180 nm.
2