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.
We present the first investigation of plastic weld joints using terahertz waves. Terahertz time-domain spectroscopy clearly reveals contaminations like metal or sand within the weld joint of two high-density polyethylene sheets. Furthermore, areas can be identified where the welding process has failed and the parts to be joined are separated by a small air gap. We show that a three layer structure of polyethylene-air-polyethylene has a characteristic, frequency-dependent transmission behaviour. This allows for a distinction between welded and non-welded material as well as for the calculation of the air layer thickness from the relative transmission spectrum. Consequently, terahertz time-domain spectroscopy provides a promising new non-destructive and even contactless technique, which is desired by the plastics industry for detecting a variety of deviations from the ideal welding process.
Poly(lactic acid) (PLA) is a commercially available biobased material that has become an ideal material in packaging applications because of its low toxicity along with its environmentally friendly characteristics. Unfortunately, PLA is rigid and brittle. These characteristics impede its wide application. The flexibility of PLA can be improved by plasticization. In addition, welding polymer films is essential in the packaging production. Therefore, the weldability by means of ultrasonic welding of the neat and with polyethylene glycol plasticized PLA films was analyzed in this study. Moreover, the study examines the influence of the material composition on the processing window, that is, the range of welding parameters which could be used to weld films efficiently, and on the weld quality. This research showed that all examined films can be welded by ultrasonic welding. Furthermore, it was discovered that the addition of a plasticizer has a strong influence on the processing window and on the weld quality. V C 2014 Wiley Periodicals, Inc. J.Appl. Polym. Sci. 2015, 132, 41351.
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