Terahertz time-domain spectroscopy as a tool to monitor the glass transition in polymers

Abstract

We demonstrate the suitability of terahertz time-domain spectroscopy as a non-destructive, contact-free tool to monitor the glass transition in polymers--a core feature of the amorphous phase. Below the glass transition temperature T(g), segmental motions along the polymer chain are frozen due to the lack of free volume between neighboring macromolecules. We show that this transition also reflects in the temperature dependence of the refractive index at terahertz frequencies. Two domains can be identified, which differ in their sensitivity to temperature changes. To verify the proposed approach, we determine the glass transition temperature T(g) of semi-crystalline poly(oxymethylene) (POM) with terahertz time-domain spectroscopy and validate the results by destructive differential scanning calorimetry (DSC) measurements.

Terahertz lens made out of natural stone

“…Both the amplitude and phase information of a sample can be obtained simultaneously through THz-TDS, without resorting to a complex analysis [2]. Previous studies demonstrated the powerful spectroscopic capability of THz-TDS in characterizing many materials, including polymers [3], explosive materials [4], solidstate materials [5,6], chemical compounds in liquids [7], ions [8], biomaterials [9], and even the material phase transitions [10].…”

Non-destructive evaluation by terahertz spectroscopy for penetration of acid solutions into epoxy resin

“…The main advantages of THz-TDS for NDT studies are the high penetration of THz electromagnetic waves into polymers [13][14][15][16][17][18], high sensitivity to the inter/intra molecular vibrations (especially for water) [18][19][20][21][22][23][24][25][26], and ability to estimate the materials dielectric properties without direct contact [27][28][29]. The THz spectra of various thermoplastics [13][14][15] and thermosets [16,17] measured by THz-TDS were reported.…”

“…The THz spectra of various thermoplastics [13][14][15] and thermosets [16,17] measured by THz-TDS were reported. In contrast with FT-IR spectra, the THz spectra shows a few clear features to distinguish polymer molecular structures.…”

“…In contrast with FT-IR spectra, the THz spectra shows a few clear features to distinguish polymer molecular structures. However, THz spectroscopy can be useful for monitoring the glass transition of the polymer because the polymer chain motion above and below the glass transition temperature affects the THz refractive index [14]. In addition, owing to the significant difference of dielectric properties between polymers and other materials, spectral features of inclusions such as inorganic fillers [15] and carbon nanotubes [17] in polymer materials were also recognized in the THz spectra.…”

“…Although free and bound water was experimentally studied by FT-IR [10,30,31], NMR [32], thermomechanical analysis [31,32] etc., it is still a highly controversial subject. Since the interactions of the bound water and free water show slightly different dielectric properties [14,20,[22][23][24][25]29], THz-TDS could have an advantage to distinguish these two states of water. In fact, the THz dielectric spectra of bulk water is composed of several spectral peaks related to the different relaxation times and intermolecular stretching vibration of water [23,24].…”