Sanoria, Abhishek (2016)
Developing Raman microscopy as a routine spectroscopic technique for morphology and microstructure characterization of plastics.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication
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Item Type: | Ph.D. Thesis | ||||
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Type of entry: | Primary publication | ||||
Title: | Developing Raman microscopy as a routine spectroscopic technique for morphology and microstructure characterization of plastics | ||||
Language: | English | ||||
Referees: | Rehahn, Prof. Dr. Matthias ; Busch, Prof. Dr. Markus | ||||
Date: | 12 December 2016 | ||||
Place of Publication: | Darmstadt | ||||
Date of oral examination: | 12 December 2016 | ||||
Abstract: | Raman microscopy holds potential advantages over spectroscopic techniques like nuclear magnetic resonance and infrared spectroscopy. These include minimum sample preparation and fast measurement times. When used as an imaging technique (Raman microscopy) the spatial resolution is an additional advantage over infrared microscopy. The aspects of sample preparation and measurement times have been exploited to quantify the comonomer content in ethylene/1-olefin copolymers. In the thesis the aspect of spatial resolution has been explored for crosslinked ethylene/vinyl acetate (EVA) laminates and welds between polypropylene (PP). The results are summarized in the following: Quantifying the comonomer content in ethylene-1-olefin copolymers is important to develop structure property↔relationships and unambiguously identify samples. Nuclear magnetic resonance spectroscopy has been widely used for this purpose, but quantitative measurements require significant amounts of sample, therefore limiting its scope for cases, where measurements are limited by the amount of sample available, such as liquid chromatography or forensics. In this study, experimental protocols for Raman spectroscopy have been developed to quantify the comonomer content in copolymers of ethylene and even numbered 1-olefins, ranging from 1-hexene to 1-octadecene. With increasing comonomer content the spectra reflect the combined effect of the decrease in ethylene content, the associated changes in the phase composition of polyethylene and the scattering from the comonomer sequences. The band ensemble characteristic for the comonomer itself cannot be used for quantification alone since the intensity of the spectra is affected by the sample focus and the spectral acquisition time. Therefore an internal standard for normalizing the intensity of the band ensemble is required, and the intensity of the band at 1295 cm-1 in the C-C twisting region was found to be appropriate for this purpose. This method has also been extended for determining the comonomer content of a cyclic olefin copolymer (ethylene/norbornene) having low crystallinity where the bands arising due to scattering from the comonomer sequences start dominating. Hence, the Raman spectra of copolymers of ethylene with 1-olefins ranging from 1-butene to 1-octadecene and the cyclic norbornene have been analyzed and a method has been developed to quantify the comonomer content. Due to the minimal sample preparation, fast spectral acquisition time and its non-destructive nature, the Raman spectroscopic approach holds potential for quality control. A strong need for such a technique also exists in liquid chromatography where the amounts of sample acquired are regularly very small. EVA resins are widely used as encapsulant for photovoltaic modules and the degree of crosslinking plays a critical role in the cell efficiency. A crucial question when developing processproperty relationships for the process of encapsulation is, how the degree of crosslinking, XC, is spatially distributed i.e., identifying inhomogeneities in the latter. Techniques such as DSC, FT-IR spectroscopy and Soxhlet extraction have been previously used for this purpose. Yet, all these approaches lack spatial resolution per se, as they are limited to bulk differences in XC, and local inhomogeneities are averaged out. In the investigations it could be shown that the sensitivity of FT-IR and DSC analysis, which is based on analyzing the amount of residual crosslinker, limits their applicability. In the present study a method based on Raman microscopy has been developed to study local variations in crosslinking of EVA laminates using model sheets crosslinked between Teflon sheets. The intensity of the respective νCH2 vibration at 2934 cm-1 and the νCH3 vibration at 2885 cm-1 reflect the transformation of the methyl groups of the VA comonomer into methylene bridges as a consequence of crosslinking, and thus the conversion. Yet, this information is of relative nature, and to translate it into absolute values, a calibration was carried out using a non-crosslinked EVA sample and the results from Soxhlet extraction for a highly crosslinked sample as reference points. The developed method has then been applied to study crosslinking inhomogeneities in EVA/glass laminates. In this case the quantification of crosslinking yielded significantly higher values for XCR compared to the model EVA laminates. This can be explained by differences in the cooling pattern of the glass laminate, leading to the formation of multiple crosslinks, which are then recorded by Raman spectroscopy. This demonstrates the limitations for transferring results from a model system to the real process. PP is widely used to produce pipes for civil engineering and construction. However processing of PP regularly leads to anisotropies induced in the melt due to shearing which in the solidified product embody themselves in the form of variations in the degree of crystallinity (Xc), the chain orientation and the polymorphic composition. The spatial variations induced in the polymer morphology have been identified and analyzed through the Raman spectrum of the α- and β-polymorph of PP. Changes in the form of band shifts distinct for the specific polymorph (β polymorph at 809 and 841 cm-1) have been seen and a new method has been developed which can selectively ascertain the presence of the α polymorph in PP plates. A method for qualitatively determining the differences in the crystallinity across the spherulites has been developed using the shift in the position of the band at 2954cm-1. This method has been used to compare the crystallinity profiles for both polymorphs. PP welds show a large impact of the processing conditions on the morphology and thereby act as weak spots susceptible to rupture under application of load. Measurements using techniques such as DSC and FT-IR microscopy are limited to the high step width in measurements and intricate changes across the weld seams are averaged out. These were investigated with Raman microscopy and a gradient of ~18 % in the degree of crystallinity was seen to be present in narrow channels across the weld seams. The effect of the welding on the morphology was also investigated and the cooling time in the weld core region favored the formation of the β-polymorph, which was identified and profiled using spectral band shifts. A binate weld system of a PP homopolymer (PP-H) and a propylene copolymer with ethylene (PP-R) was investigated to study these morphological variations occurring across the welds comprising of two different materials. The crystallinity in the PP-R region of the weld core was affected more in this case and no formation of the β-polymorph was observed in the weld core region from PP-H. The weld core belonging to PP-H, however, showed a similar profile as observed for the previous sample. Using Raman microscopy it was also possible to map the spatial distribution of the two polymers and a clear boundary separating them was observed, indicating that no intercalation of the two components takes place upon welding. The above work shows the prospects of utilizing Raman microscopy for characterization of plastic materials. The experimental protocols can be implemented in quality control of ethylene/1-olefins copolymers with regard to comonomer content. The homogeneity of crosslinking in EVA modules impacting the solar cell behavior can for the first time be determined at a μm level. The impact of the processing parameters on the morphology and crystallinity of PP welded materials have been analyzed and analytical information which could not be retrieved previously can now be accessed easily. |
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URN: | urn:nbn:de:tuda-tuprints-58642 | ||||
Classification DDC: | 500 Science and mathematics > 540 Chemistry | ||||
Divisions: | 07 Department of Chemistry > Ernst-Berl-Institut > Fachgebiet Makromolekulare Chemie | ||||
Date Deposited: | 15 Dec 2016 08:29 | ||||
Last Modified: | 09 Jul 2020 01:29 | ||||
URI: | https://tuprints.ulb.tu-darmstadt.de/id/eprint/5864 | ||||
PPN: | 396678556 | ||||
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