RU2310188C2 - Spectroscopic method for determining material porosity - Google Patents

Abstract

FIELD: investigating or analyzing of materials.

SUBSTANCE: method comprises plotting calibrating curve of porosity as a function of the absorption coefficient for reference monolithic specimens with the same chemical nature as the specimen to be analyzed, choosing the intensive absorption band, and determining porosity of the analyzed specimen with regard to its absorption coefficient.

EFFECT: simplified method.

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Description

The invention relates to the field of determining the porosity of porous materials, in particular polymer films.

A known spectroscopic method for determining the porosity of paper (Germany patent No. 19616018, class G01N 15/08, prototype), including

- sample production;

- its placement in front of the radiation source in the visible (with a wavelength of 500-600 nm (nm - nanometer (10 ^-9 m) is the abbreviation adopted in spectroscopy)) or ultraviolet (with a wavelength of about 220 nm) regions of the light spectrum;

- registration of transmission radiation;

- determination of the total pore volume — total porosity — by transmittance extrema.

The disadvantages of this method:

- limited application: thin films with a thickness of not more than 50 microns are investigated;

- insufficient accuracy of determination due to blurring of the extrema of the spectrograms.

The technical result of the present invention lies in the fact that the porosity is determined by intense sharp extremes, which increases the accuracy of the result, and also that the method allows to determine the porosity of bulk films having a thickness of more than 50 microns.

The technical result of the present invention is achieved by the fact that porosity standards are made of a monolithic material having the same chemical nature as the test sample with a known pore diameter, then they are placed in front of the IR radiation source and the porosity is determined by the extrema of their transmission spectrum to obtain the corresponding calibration dependences, then by the extrema of the transmission spectrum of the test sample, using the calibration dependences find its porosity.

A study of the prior art found that methods for determining porosity using IR spectroscopy are not detected.

A known spectroscopic method for determining the porosity of paper (Germany patent No. 19616018, class G01N 15/08, prototype).

However, a comparison of the properties of the totality of the features of the known method and the claimed shows that:

- in the known method, the sample is irradiated either in the visible (wavelength of 500-600 nm), or in the ultraviolet (wavelength of about 220 nm) regions of the electromagnetic range, and in the present method uses infrared radiation (wavelength of more than 760 nm);

- spectrograms of the studied samples obtained in a known manner have blurry extremes, which reduces the accuracy of determining porosity; in the inventive method, the porosity is determined by the intense clear extremes;

- the known method allows the study of thin films with a thickness of not more than 50 microns, and the porosity of both thin films and bulk samples having a thickness of more than 50 microns is determined by the claimed method.

Therefore, the claimed method meets the criterion of "significant differences".

The invention is illustrated in graphic materials (Figure 1 ÷ 4).

Figure 1. Monolithic sample (a) and with pores (b).

Figure 2. The change in the absorption coefficient depending on the porosity of the PNP film (low density polyethylene) 60 mm thick.

Figure 3. Change in the absorption coefficient for PET-based films with total porosity: 11.3%, 15.4%, 22.7%, 31.4%.

The porosity of the material, characterized by voids or cavities, is important in the manufacture of colored synthetic fibers filled with plastics, membranes, filters, superabsorbents. Widespread methods for determining the porosity of polymeric materials are adsorption and mercury porosimetry, density measurements, and visual methods (optical, electronic, and atomic force spectroscopy). Each of them is designed for a specific working range of pore size and has certain limitations. The most significant limitations are the long duration of preparation of samples for analysis, the relatively low processing speed of information and the destructive effect on the sample. In some cases, porosity data refer to a small surface area, as a result of which there is no reliable information on the porosity of the sample as a whole.

The known spectroscopic method for determining the total porosity, based on the irradiation of the test sample with ultraviolet and visible light, gives satisfactory results only in the case of ultraviolet radiation.

The invention consists in the following.

A sample of polymer material may have a porous surface (Figure 1, a), or the pores are distributed throughout its volume (Figure 2, b).

To assess the total porosity (concentration and pore volume) of the sample by the method of IR spectroscopy in the present method, the intensive and least structurally sensitive absorption band is selected, with which the absorption coefficient ε is calculated. IR radiation passing through the sample “sees” the true thickness d _m . The D / d ratio reflects the porosity of the sample in arbitrary units (here D is the optical density of the IR absorption band, d is the thickness of the sample). The greater the porosity, the less absorption. Comparing the absorption coefficients of porous samples ε with the absorption coefficient of a monolithic ε _m , we can calculate the relative porosity of the sample (%) by the formula

Knowing the value of the absorption coefficients of samples obtained under various technological conditions, it is possible to choose the optimal methods for their manufacture.

Porosity standards are made by piercing a monolithic film with a metal needle of a certain diameter, as well as by irradiating it with a beam of heavy xenon or krypton ions accelerated at the cyclotron.

Example 1 of the method

Determination of porosity of LDPE (low density polyethylene).

Porosity standards were made by piercing with metal needles with a diameter of 100 μm and above a monolithic film of LDPE with a thickness of 60 μm.

The IR spectra of the standard of the test sample were recorded on an Equinox 55 Bruker IR Fourier spectrometer and a Carl Zeiss Specord JR 75 grating spectrometer.

To assess the total porosity of both the reference and the sample, an intense extremum corresponding to a frequency of 730 cm ^{−1 was} chosen in the IR spectrum.

The calibration dependence of the change in the absorption coefficient depending on the total porosity for the standards of porosity is presented in Figure 2.

According to the calibration dependence of Figure 2, knowing the absorption coefficient of the test sample, we find the total porosity of the test sample LDPE.

Example 2 of the method.

Determination of the total porosity of PET (polyethylene terephthalate).

Porosity standards, the so-called track membranes, were made at the Joint Institute for Nuclear Research (Dubna) by irradiating monolithic polymer films with a beam of heavy xenon or krypton ions accelerated in the cyclotron. The film thickness is 25 μm, the diameter of the resulting pores will vary from 0.1 to 12.0 μm. To evaluate the pore diameter of the porosity standard, we used an Raster Elektron Microscope Zeiss DSM 962 electron scanning microscope and a Neoplot 30 optical microscope.

To assess the total porosity of both the reference and the sample, an intense extremum corresponding to a frequency of 730 cm ^{−1 was} chosen in the IR spectrum.

The calibration dependence of the porosity of PET on the absorption coefficient is shown in Figure 3. Knowing the absorption coefficient of the sample under study, its porosity was found from the calibration curve.

The inventive method can find application in the production of porous materials, primarily sorbents and semipermeable membranes.

Claims (1)

A spectroscopic method for determining the porosity of porous materials, including the manufacture of a sample of the test material, placing it in front of the radiation source, recording the spectrum, characterized in that first the porosity standards are made of a monolithic material having the same chemical nature as the sample under study, then the porosity standards are placed in front of source of infrared radiation, in the recorded spectrum choose an intense absorption band, determine the absorption coefficient and the calibration dependence st, using which the test sample with the porosity of its absorption coefficient.

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