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Solution for Polyimide (PI) material characterization

To maintain profitability in the specialized materials market, its necessary to develop polymer materials with specialized functions and launch them promptly. For comprehending the performance as a functional polymer material, it is indispensable to analyze with complementary & diverse analytical techniques. Development and improvement of functional polymer materials can be advanced by considering the base polymer design, formulation, physical properties, etc. By employing a characterization approach of chemical testing to grasp aspects at the molecular level and physical testing to grasp the relationship with physical properties of the product itself, development of functional polymer materials can be efficiently improved.

Polyimide (PI) materials

Polyimide, an engineering plastic, is a polymer compound having feature such as high heat resistance, tensile strength, flexural modulus and electrical insulation, so there are products customized for various usage such as forming into a film, flexible OLED display material, flexible printed circuit and binder of Li-ion secondary battery. The general production method is to imidize polyamic acid of precursor by heat treatment. The precursor polyamic acid is soluble in organic solvents, but not the resulting polyimide, hence only technologies that can measure solid samples can be applied after imidization.

There are several techniques that can evaluate the material as exemplified here, such as molecular weight distribution of precursor with GPC(Gel Permeation Chromatography), surface and outer layer analysis of product with direct Mass Spectrometry, structural elucidation of base polymer by pyrolysis, physical property testing with DMA(Dynamic Mechanical Analysis) and DSC(Differential Scanning Calorimetry).

1) Molecular Weight Distribution with APC (Advanced Polymer Chromatography)

It is known that the difference of molecular weight and degree of irregularity (molecular weight distribution) greatly influence the fundamental properties of polymers, and furthermore the properties as materials. In case of polyimide, improvement and control of molecular weight distributions are evaluated by measuring the molecular weight distribution of precursor polyamic acid. The figure below shows the measurement results of polyamic acid with ACQUITY Advanced Polymer Chromatography (ACQUITY APC). For principle and technology of APC, please refer to ”Solution for polymer characterization with APC

Fig. Polyamic acid analysis with Advanced Polymer Chromatography (n=5)

For functional polymer materials, more precise and accurate measurement on molecular weight distribution can be achieved with the adoption of high-resolution size-based separation technology over conventional GPC technology. Moreover, since it is improved from the conventional GPC in terms of throughput and repeatability, it enables daily calibration and supports improvement of material quality by performing accurate evaluation more quickly.

2) Surface analysis with DESI-QTof (Desorption Electro Spray Ionization)

There are technologies for analyzing the material surface directly, such as AEX, XPS, Tof-SIMS and MALDI. However, depending on the type of information we want to obtain, such as elemental information of surface, molecular structure information, concentration and distribution on surface, depth direction information, there are restrictions on the technologies that can be used. DESI is a mass spectrometric imaging technique that extracts, desorbs and ionizes components on material surface by spraying a charged fine droplet of solvent, and introduces the generated ions to high resolution mass spectrometry (HRMS). By leveraging DESI imaging with the following features, it is possible to visualize distribution of polymer additives on surface, comparison of degradation of surface component, adhesion of contaminant, measurement of cleaning effect, chemical change of the surface of material.

A comparison of control sample and sample treated by ultrasonication in NMP is shown in the fig. below. By extracting specific molecular weight with differences in intensity between samples from full scan data, it is possible to visually grasp the differences in abundance and distribution of components on surface. A) is the full spectrum imaging, B) is the result of 10 consecutive measurements on the same spot.

Fig. Polyimide film analysis with DESI-QTof MS

3) Structural elucidation of base polymer with Pyrolyzer-APGC-QTof (Atmospheric Pressure Gas Chromatography)

Structural elucidation and characterization of polymers is the corner stone of the development of new polymer materials with higher performance and new functionality. Pyrolysis-Gas Chromatography Mass Spectrometry(Py-GC/MS) is one the widely used techniques for structural analysis of polymers. The pyrolysis components of polymers are introduced into capillary column, separated, ionized and detected with MS. Although data acquired on Py-GC/MS is used for elucidating element and composition, the Electron Ionization(EI), which is the conventional ionization for GC/MS, generates many fragmented ions rather than molecular related ions, which makes the structural elucidation extremely complicated.

To reduce the complexity of this structural elucidation process, adoption of Atmospheric Pressure Gas Chromatography (APGC) coupled with high resolution QTof mass spectrometry is more suitable to elucidate polymer structure. Because of soft ionization, less fragmentation is observed when compared with EI, and molecular related ion information can be obtained. As the system can intentionally generate fragment ions inside MS, accurate mass information for molecular related ions and fragment ions associated with precursor ions is obtained in one sample run. In addition, UNIFI software, which supports structural elucidation, makes it easy to estimate the elemental composition and chemical structure of key marker components without using a spectrum-based database for GC analysis. For the principle and technology of APGC, please refer to ”Solution for polymer characterization with novel technology of APGC

The sample and blank sample are measured, the thermal decomposition component (pyrolyzates) peculiar to the sample is extracted by software, and elemental composition is elucidated with accurate mass spectrum of molecular related ion, structure of pyrolyzates is elucidated with assignment of fragment ion. The example of elemental composition and structural elucidation is shown in fig. A), the elucidated polymer structure from the pyrolyzates is shown in fig. B).

Thus, structural elucidation becomes simpler and less complex by using data provided by soft ionization method even GC/MS analysis.

4) Physical properties with high performance Thermal analyzers

To advance the development of materials for specific functions, such as being used under more severe conditions, it is important to grasp the physical properties more precisely and accurately. The knowledge of how the material will behave through this testing can be correlated with why at the molecular level as described above. Polyimide, one of the engineering plastics, is a material that is often evaluated with thermal analysis techniques such as DSC (Differential Scanning Calorimetry) and DMA (Dynamic Mechanical Analysis) because of its high heat resistance and tensile strength. More precise and accurate thermal analysis requires maximizing temperature control and minimizing drift, as well as system design that can handle a wider range of solid samples with different shapes and types.

DSC system from TA instruments can provide not only linear temperature measurement, but also measurement with modulation of the heating rate. In this temperature-modulated DSC, as the total heat flow can be separated into a component that responds to the temperature increase rate (Reversing) and a component that does not respond (Non-reversing), complex heat events can be easily understood by separating Reversing components such as heat capacity and glass transition, and Non-reversing components such as volatilization, crystallization and denaturation.

Fig. Polyimide film analysis with Modulated DSC

Whether the material becomes elastic or viscous as the temperature rises provides a great influence on the physical properties of the material. With DMA measurement, it is possible to measure the viscoelastic behavior in response to temperature changes under stress conditions. The figure below shows the overlay of storage modulus and tan delta versus temperature for two samples (control and extracted sample). As the value of tan delta is less than 1 among the measurement temperature range, elastic factor is dominant in both samples. However, as onset temperature of extracted sample shows lower value, it is inferred that treatment with NMP makes materials more viscous with the onset temperature drop.

By adopting chemical testing method that can obtain information at molecular level such as MW distribution of precursor, structural elucidation of polymer, change of material surface and physical testing method that can obtain physical behavior of material against temperature and stress, development of functional polymer materials can be more efficiently improved.

1) Molecular weight distribution

ACQUITY APC with BEH technology

<ACQUITY APC system>

  • High speed and high resolution
  • Optimally designed low dispersion
  • BEH technology : Hybrid particle column for size exclusion

2) Surface analysis

<DESI – Q Tof>

  • Direct mass spectrometry
  • No matrix required

    SYNAPT G2-Si with DESI

  • Atmospheric pressure ionization
  • Non- destructive analysis

3) Structural elucidation of polymer

<Pyrolyzer-APGC-QTof>

  • Soft ionization with less fragmentation
  • Molecular related ion detection

    Xevo G2-XS Q-Tof with Pyrolyzer-APGC

  • High sensitivity & selectivity
  • Easier elucidation of elements and structure

4) Physical properties

<Thermal analyzers>

  • Modulated temperature profile function
  • Maximize temperature control and minimize drift
  • Wider range of solid analysis techniques

    DSC 2500            TGA 5500          RSA-G2

DESI and APGC are parts of Universal Ion Source, which are compatible with high resolution MS such as SYNAPT G2-Si and Xevo G2-XS Q-Tof, and can be changeable without venting vacuum.

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