Potentials and Limitations of Vibrational Spectroscopy for the Characterisation of Polymers

Katharina Bruckmoser

Research output: ThesisDoctoral Thesis

Abstract

In this thesis the potentials and limitations of vibrational spectroscopy for the characterisation of polymers are evaluated. The focus is laid on the evaluation of the capabilities of Raman and IR spectroscopy to detect ageing, crystallinity and interdiffusion mechanisms in polymers. Specific differences in the sensitivity of Raman and IR spectroscopy to detect different ageing mechanisms in thermoplastic polyurethanes (TPU) were demonstrated. Thereby, TPU were exposed to application relevant influences such as ultraviolet radiation (UV radiation) or elevated temperatures for different times. A high sensitivity of IR spectroscopy compared to Raman spectroscopy was found for the detection of UV-aged TPU. In contrast, a high sensitivity of Raman spectroscopy compared to IR spectroscopy was detected for heat aged TPU. The specific strength of IR and Raman spectroscopy to detect different ageing mechanisms in TPU was explained by different sensitivities on the formed non-polar and polar molecular groups. Based on these results the heat ageing program as well as the evaluation procedure of the Raman spectra were extended. TPU were exposed to different temperatures and different atmospheres. The elevated temperatures provoked the formation of degradation products detectable by Raman spectroscopy. The formation of new Raman bands and changes in certain Raman bands (e.g. intensity and shape) were detectable. By performing peak fitting of the most affected Raman bands and calculation of intensity ratios a tool for the quantitative description of the state of ageing in heat aged TPU was developed. Due to the specific strength of Raman and IR spectroscopy on both, molecular and supramolecular level, their potentials and limitations to detect ageing in poly(lactic acid) (PLA) as well as crystallinity in PLA staple fibres were elicited. The investigations of aged PLA included an analysis of the effect of morphology on ageing mechanisms induced by exposure to different consumer goods. Distinct limitations of Raman and IR spectroscopy were found in order to detect hydrolysis and changes in crystallinity in aged PLA. This was explained by minor ageing of PLA under the investigated conditions. Therefore, the ageing mechanisms were not detectable for both spectroscopic techniques. The potentials and limitations of Raman spectroscopy to detect crystallinity in PLA staple fibres were also evaluated within this thesis. A significant influence of the fibre alignment in relation to the polarisation direction of the incident laser beam in order to detect crystallinity was demonstrated. For a non-optimal alignment of the fibres in relation to the polarisation direction of the laser no insight into crystallinity of the fibre is provided. Though, a proper alignment of the fibres allowed insights into the crystalline fraction. Nevertheless, the Raman bands indicating crystallinity in PLA did not allow a differentiation of fine changes in the degree of crystallinity in PLA. The potentials and limitations of Raman spectroscopy to detect interdiffusion are also outlined in the thesis. By performing line scans across the interfaces in two-component injection moulded parts the interdiffusion lengths were determined as a function of material type and different mass temperatures of the second component. A good sample preparation, optimized measuring parameters and characteristic Raman bands which allow a differentiation of the materials were identified as necessary requirements for the application of Raman spectroscopy. An evaluation based on the intensities of characteristic Raman bands was performed to detect interdiffusion lengths in polymer interfaces ranging above and below the spatial and quantitative resolution limit of Raman spectroscopy.
Translated title of the contributionMöglichkeiten und Grenzen von Schwingungsspektroskopie zur Charakterisierung von Polymeren
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Hild, Sabine, Assessor B (external), External person
  • Pinter, Gerald, Assessor A (internal)
Publication statusPublished - 2015

Bibliographical note

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Keywords

  • vibrational spectroscopy
  • Raman spectroscopy
  • Infrared spectroscopy
  • aeging
  • cristallinity
  • interdiffusion

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