Static time-of-flight secondary ion mass spectrometry (ToF-SIMS) was performed for acquiring the high-resolution surface spectra of four types of synthesized imidazolium ionene membranes. These novel membranes have aromatic ether–ketone–ether linkages inspired by poly(ether ether ketone) (PEEK). The PEEK-ionenes synthesized for this study have imidazolium cations placed in the polymeric backbone with bistriflimide [Tf2N] counterions. The attention given to synthetically modified PEEK derivatives, such as PEEK-ionenes, is considerable due to their ability to selectively capture CO2 molecules and other light gases. Therefore, it is important to characterize the surface of these synthesized novel PEEK-ionenes. In this work, characteristic and unique peaks were identified in the positive spectra of each sample. The differences in mass spectra among the samples provide insights for optimizing or fine-tuning the PEEK-ionenes synthesis to achieve a high-performance CO2 separation membrane with enhanced permeability, selectivity, and mechanical stability. The SIMS spectra and identified characteristic peaks of these synthesized ionenes will serve as a reference in the positive mode, complementing the corresponding spectra reported in the negative ion mode (Paper II).

  • Accession#: 01886, 01887, 01888, and 01889

  • Technique: SIMS

  • Specimen: PEEK-ionenes

  • Instrument: ION-TOF TOF-SIMS 5

  • Major Species in Spectra: C, N, O, F, and S

  • Minor Species in Spectra: None

  • Published Spectra: 4

  • Spectral Category: Reference

Poly(ether ether ketone) (PEEK) and related materials are well known for their exceptional chemical, thermal, and mechanical stability, as well as biocompatibility.1,2 Inspired by the ultrahigh-performance (UHP) polymer region of PEEK, a novel approach of producing PEEK-ionene has been developed in Bara’s group for achieving outstanding properties toward high-performance gas separation membrane applications.3 

Considerable efforts were made to design and develop the key monomers of PEEK-ionenes, aiming to enhance the selectivity and permeability of light gases, as well as other properties such as mechanical and thermal stabilities. Four types of PEEK-ionenes analyzed in this work are the ionic polymers where the imidazolium cation resides in the PEEK backbone and with the incorporation of bistriflimide [Tf2N]. This anion was selected as the counterion in that it is thermal and mechanical stability and, thus, suitable for making thin film for study as membranes.4 The monomer of each PEEK-ionene differs in the ether–ketone–ether (EEK) linkage backbone with or without xylene linkage, number of the imidazolium cations incorporated in the PEEK backbone, and arrangement of ether–ether–ketone linkages, as shown in Fig. 1. The variation and mediation of backbone linkages and imidazolium cations may change their corresponding functional roles, which influence the affinity for certain light gases (CO2 and CH4) and organization of the polymer matrix.

The first example of PEEK-ionene was characterized by 1H nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), and matrix-assisted laser desorption/ionization-time of flight (MALDI-ToF) mass spectrometry (MS). Although NMR and FTIR can provide identification and quantification of functional groups, they are not able to focus on sample surface where the polymers interact with gases in the first place. The results from MALDI-ToF MS were not able to distinguish specific ions.3 Besides, MALDI-TOF MS is limited by the sensitivity, affected by the matrix effect, and low in discriminatory power resulting in being unable to differentiate similar chemical species.5 

Static time-of-flight secondary ion mass spectrometry (TOF-SIMS) is an ideal tool for characterizing PEEK-ionene membrane due to its chemical specificity and surface sensitivity. An earlier attempt was made to characterize the surface spectra of PEEK-ionene using ToF-SIMS.6 However, that work did not detect the molecular ions of imidazolium cation or Tf2N anion. This report includes the full spectra of four types of PEEK-ionene membranes acquired in positive ion mode, using ToF-SIMS 5 (IONTOF, Germany). A low analysis current (0.0002 nA) was applied to avoid peak intensity saturation. Samples No. 1 and 4, membranes were cut into ∼8 × 8 mm2 square piece and mounted on a ToF-SIMS sample holder. Samples No. 2 and 3 were in powder form. They were pressed onto indium foils before mounting separately. 50 keV Bi3++ was applied as the analysis beam. Characteristic peaks of each sample that reflecting their respective chemical structures were identified.

The rapid development and growing interest in the preparation of polymetric membranes for gas separation urge us to have better understanding of the customized membrane and the role of its functional groups. This work can provide complementary information to other analytical techniques (NMR) to understand the role of functional groups in each specifically designed PEEK-ionenes and ultimately guide the production and design of high-performance gas separation membranes.

The spectra presented here are also companion to the Part II negative ion mode dataset.7 

Specimen: Synthesized PEEK-ionene sample No. 1

CAS Registry #: N/A

Specimen Characteristics: Homogeneous; solid; unknown crystallinity; unknown conductivity; polymer

Chemical Name: PEEK-ionene

Source: Synthesized at University of Alabama

Specimen Composition: N/A

Form: Film

Structure: p([K(EEK)2(2mIm)6(C6)5][Tf2N]6)

History and Significance: PEEK-ionene is developed and optimized for gas separation with improved selectivity and permeability. Polymetric membranes for gas separation are rapidly developing with growing interest.

As Received Condition: As-received film

Analyzed Region: 200 × 200 μm2

Ex Situ Preparation/Mounting: ∼8 × 8 mm2 membrane piece was cut from the membrane film and stabilized on a back-mount holder.

In Situ Preparation: N/A

Charge Control Conditions and Procedures: Low-energy electrons

Temp. During Analysis: 298 K

Pressure During Analysis: 2.1 × 10−5 Pa

Pre-analysis Beam Exposure: N/A

Specimen: Synthesized PEEK-ionene sample No. 2

CAS Registry #: N/A

Specimen Characteristics: Homogeneous; solid; unknown crystallinity; unknown conductivity; polymer; powder

Chemical Name: PEEK-ionene

Source: Synthesized at University of Alabama

Specimen Composition: N/A

Form: Dark brown powder

Structure: p([K(EEK)2(2mIm)4(C6XylC6)][Tf2N]4)

History and Significance: PEEK-ionene is developed and optimized for gas separation with improved selectivity and permeability. Polymetric membranes for gas separation are rapidly developing with growing interest.

As Received Condition: As-received powder

Analyzed Region: 200 × 200 μm2

Ex Situ Preparation/Mounting: ∼100 mg of powder was pressed into an indium foil (5 × 5 mm2), and the latter was stabilized on a top-mount holder.

In Situ Preparation: N/A

Charge Control Conditions and Procedures: Low-energy electrons

Temp. During Analysis: 298 K

Pressure During Analysis: 4.3 × 10−6 Pa

Pre-analysis Beam Exposure: N/A

Specimen: Synthesized PEEK-ionene sample No. 3

CAS Registry #: N/A

Specimen Characteristics: Homogeneous; solid; unknown crystallinity; unknown conductivity; polymer; powder

Chemical Name: PEEK-ionene

Source: Synthesized at University of Alabama

Specimen Composition: N/A

Form: Light brown powder

Structure: p([K(EEK)2(2mIm)4(XylC6Xyl)][Tf2N]4)

History and Significance: PEEK-ionene is developed and optimized for gas separation with improved selectivity and permeability. Polymetric membranes for gas separation are rapidly developing with growing interest.

As Received Condition: As-received powder

Analyzed Region: 200 × 200 μm2

Ex Situ Preparation/Mounting: ∼100 mg of powder was pressed into an indium foil (5 × 5 mm2), and the latter was stabilized on a top-mount holder.

In Situ Preparation: N/A

Charge Control Conditions and Procedures: Low-energy electrons

Temp. During Analysis: 298 K

Pressure During Analysis: 6.3 × 10−6 Pa

Pre-analysis Beam Exposure: N/A

Specimen: Synthesized PEEK-ionene sample No. 4

CAS Registry #: N/A

Specimen Characteristics: Homogeneous; solid; unknown crystallinity; unknown conductivity; polymer

Chemical Name: PEEK-ionene

Source: Synthesized at University of Alabama

Specimen Composition: N/A

Form: Dark brown powder

Structure: p([K(EEK)2(2mIm)4(C6C6C6)[Tf2N]4)

History and Significance: PEEK-ionene is developed and optimized for gas separation with improved selectivity and permeability. Polymetric membranes for gas separation are rapidly developing with growing interest.

As Received Condition: As-received film

Analyzed Region: 200 × 200 μm2

Ex Situ Preparation/Mounting: ∼8 × 8 mm2 membrane piece was cut from the membrane film and stabilized on a back-mount holder.

In Situ Preparation: N/A

Charge Control Conditions and Procedures: Low-energy electrons

Temp. During Analysis: 298 K

Pressure During Analysis: 1.1 × 10−6 Pa

Pre-analysis Beam Exposure: N/A

Manufacturer and Model: IONTOF TOF-SIMS 5

Analyzer Type: Time-of-flight

Sample Rotation: No

Rotation Rate: N/A

Oxygen Flood Source: N/A

Oxygen Flood Pressure: N/A

Other Flood Source: N/A

Other Flood Pressure: N/A

Unique Instrument Features Used: N/A

Energy Acceptance Window: 20 eV

Post-acceleration Voltage: 10 000 eV

Sample Bias: 0 eV

Specimen Normal-to-analyzer (Θe): 90°

Ion source 1 of Bi3++

Purpose of this Ion Source: Analysis beam

Ion Source Manufacturer: IONTOF (Münster, Germany)

Ion Source Model: Bin+ cluster ion source

Beam Mass Filter: Yes

Beam Species and Charge State: Bi3++

Beam Gating Used: No

Additional Beam Comments: N/A

Beam Voltage: 50 000 eV

Net Beam Voltage (impact voltage): 50 000 eV

Ion Pulse Width: 0.8–1.0 ns

Ion Pulse Rate: 10.0 kHz

DC Beam Current: 14 nA

Pulsed Beam Current: 0.0002 nA

Current Measurement Method: Faraday cup

Beam Diameter: 5 μm

Beam Raster Size: 200 × 200 μm2

Raster Pixel Dimensions: 128 × 128

Beam Incident Angle: 45°

Source-to-Analyzer Angle: 45°

Ion source 2 of Ar cluster

Purpose of this Ion Source: Sputtering beam

Ion Source Manufacturer: IONTOF (Münster, Germany)

Ion Source Model: Ar cluster ion source

Beam Mass Filter: Yes

Beam Species and Charge State: Ar+

Beam Gating Used: No

Additional Beam Comments: For removing potential surface contamination

Beam Voltage: 10 000 eV

Net Beam Voltage (impact voltage): 10 000 eV

Ion Pulse Width: 30 000 ns

Ion Pulse Rate: 10 kHz

DC Beam Current: 12 nA

Pulsed Beam Current: 5.1 nA

Current Measurement Method: Faraday cup

Beam Diameter: 20 μm

Beam Raster Size: 1000 × 1000 μm2

Raster Pixel Dimensions: 128 × 128

Beam Incident Angle: 45°

Source-to-Analyzer Angle: 45°

This work, associated with direct air capture (DAC) of CO2 project, was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Materials Science and Separation Science program, No. FWP 76830.

The authors have no conflicts to disclose.

Lyndi E. Strange: Writing – original draft (equal). David J. Heldebrant: Funding acquisition (equal); Project administration (equal); Supervision (equal); Writing – review & editing (equal). Sudhir Ravula: Investigation (equal); Resources (equal); Writing – review & editing (equal). Ping Chen: Writing – review & editing (equal). Zihua Zhu: Writing – review & editing (equal). Jason E. Bara: Methodology (equal); Resources (equal); Writing – review & editing (equal). Jennifer Yao: Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material.

1.
J. C.
Jansen
and
E.
Drioli
,
Polym. Sci. Ser. A
51
,
1355
(
2009
).
2.
D.
Shukla
,
Y. S.
Negi
,
J. S.
Uppadhyaya
, and
V.
Kumar
,
Polym. Rev.
52
,
189
(
2012
).
3.
K.
O’Harra
,
I.
Kammakakam
,
P.
Shinde
,
C.
Giri
,
Y.
Tuan
,
E. M.
Jackson
, and
J. E.
Bara
,
ACS Appl. Polym. Mater.
4
,
8365
(
2022
).
4.
I.
Kammakakam
,
K. E.
O’Harra
,
J. E.
Bara
, and
E. M.
Jackson
,
Macromolecules
55
,
4790
(
2022
).
5.
W. J.
Perry
,
N. H.
Patterson
,
B. M.
Prentice
,
E. K.
Neumann
,
R. M.
Caprioli
, and
J. M.
Spraggins
,
J. Mass Spectrom.
55
,
e4491
(
2020
).
6.
J.
Gao
,
Y.
Zhang
,
J.
Son
,
J. E.
Bara
,
K. E.
O'Harra
,
M. H.
Engelhard
,
D. J.
Heldebrant
,
R.
Rousseau
, and
X.-Y.
Yu
,
Carbon Capture Sci. Technol.
2
,
100037
(
2022
).
7.
L. E.
Strange
,
Surf. Sci. Spectra
31
,
015002
(
2024
).

Supplementary Material