In the field of characterization techniques of materials, and in particular of IR and Raman spectroscopy, this work sheds light on the usefulness and reliability achieved by state-of-the-art computational methods for the investigation, at the molecular scale, of the spectroscopic and structural properties of the semi-crystalline polymers. The research and development of new polymeric materials, as shown by the most recent papers appeared in the literature, are still very active. Acting on the physical and mechanical properties of these materials, involves a deep knowledge of the structure and arrangement of the molecular chains and in particular of how they influence the physical-chemistry of the material. For this reason, the need for innovative characterization techniques, which lead to a rational design of the molecular structure of polymeric materials, is taking ground. By means of the current CRYSTAL14 and Gaussian09 implementations, we report a detailed study on the spectroscopic properties of the poly(p-phenylene terephthalamide) (PPTA). The potential of this code applied to polymer materials, are still almost completely unexplored. This work shows how these computational models are able to accurately reproduce the vibrational spectra (IR and Raman) of PPTA, providing an effective method to interpret in details the information obtained by experimental characterization techniques. In addition to the assignment of the main group frequencies of vibrational spectra IR and Raman of the 3D and 1D (infinite isolated chain) crystals by means of CRYSTAL14, we also simulated the effect induced by the mechanical stretching of PPTA fibers on the Raman spectrum, observing a downward frequency shift of the bands related to backbone vibrations. The results obtained turned out to be in direct agreement with data found in the literature. In particular, we also verified how structural changes, due to fibers stretching, play a fundamental role in modifying the vibrational spectra. By means of CRYSTAL14 we determined the elastic modulus of the PPTA crystalline phase, and we simulated the IR spectrum with polarized incident light, obtaining once again results in good agreement with experimental measurements. Furthermore, by taking advantage of Gaussian09 we also carried out calculations of the IR and Raman spectra of different kinds of PPTA oligomers. Making a comparison between these latter oligomers spectra with CRYSTAL14 and experimental results, we were able to deeply understand the role of intra and intermolecular interactions in modifying the spectroscopic properties of the polymer. From the methodological point of view, this work shows the different results obtained by means of different exchange-correlation functionals and basis sets, sheding light on their strengths and weaknesses in simulating real systems and obtaining on this way useful information on the computational set up of the DFT calculation of polymeric systems. All the experimental spectra reported in this work has been carried out in the FunMat laboratory of the chemistry and materials department of the Politecnico di Milano, while the acquisition of the Raman spectrum performed during the PPTA fibers tensile test took place in collaboration with Prof. Claudia Marano of Politecnico di Milano. The proposed approach, emphasizes the importance of computational methods not only for a reliable interpretation of vibrational spectra, but also to understand those molecular phenomena responsible of many physical and chemical properties of polymeric materials. Furthermore, its potentialities are not only limited to the basic study of the material, but have also a significant impact in applied research also in the industrial and technological fields.
Nell’ambito delle tecniche di caratterizzazione dei materiali e in particolare della spettroscopia Raman e infrarossa, questo lavoro mette in luce l’utilità e l’affidabilità raggiunta dai metodi computazionali allo stato dell’arte per lo studio, a livello molecolare, delle proprietà strutturali e spettroscopiche dei polimeri semicristallini. La ricerca e lo sviluppo di nuovi materiali a base polimerica, come dimostrato dalla letteratura più recente, è tuttora molto attiva. Intervenire sulle proprietà fisiche e meccaniche di questi materiali implica una profonda conoscenza della struttura e dell’organizzazione delle catene polimeriche, in particolare di come esse influenzino le proprietà fisico-chimiche del materiale. Per questo motivo, nasce la necessità di tecniche di caratterizzazione sempre più evolute, che permettano un design razionale della struttura molecolare dei materiali polimerici. Attraverso l’attuale implementazione di CRYSTAL14 e Gaussian09 proponiamo uno studio dettagliato sulle proprietà spettroscopiche del poly(p-phenylene terephthalamide) (PPTA). Le potenzialità di questo codice di calcolo applicato a materiali polimerici, sono quasi del tutto ancora inesplorate. Questo lavoro mostra come tali metodi computazionali siano in grado di riprodurre fedelmente gli spettri vibrazionali Raman e IR del PPTA, fornendo un valido metodo per consolidare e meglio interpretare le informazioni fornite dalle tecniche di caratterizzazione sperimentali. Oltre all’aver identificato le principali frequenze di gruppo degli spettri vibrazionali (Raman e IR) del cristallo 3D e 1D (singola catena infinita) attraverso il codice CRYSTAL14, abbiamo anche simulato l’effetto che la trazione meccanica delle fibre di PPTA ha sullo spettro Raman, osservando uno spostamento delle principali bande associate alle vibrazioni dello scheletro delle catene polimeriche verso frequenze più basse, in ottimo accordo con i dati presenti in letteratura. In particolare, abbiamo anche verificato come i cambiamenti strutturali, dovuti allo stretching meccanico, giochino un ruolo fondamentale nel modificare lo spettro vibrazionale. Per mezzo di CRYSTAL14 abbiamo potuto determinare il modulo elastico della fase cristallina del PPTA e simulare lo spettro IR in luce polarizzata, ottenendo sempre risultati in ottimo accordo con le misure sperimentali. Inoltre, attraverso il codice Gaussian09 abbiamo calcolato gli spettri vibrazionali, Raman e IR, di differenti oligomeri di PPTA. Paragonando quest’ultimi spettri con quelli ottenuti dalla simulazione del cristallo 3D e con gli spettri sperimentali, siamo stati in grado di approfondire il ruolo delle interazioni intra ed intermolecolari (per esempio il legame ad idrogeno) nel modificare le proprietà spettroscopiche del polimero oggetto di studio. Dal punto di vista metodologico, questo lavoro mostra i differenti risultati ottenuti attraverso i differenti funzionali di scambio-correlazione e basis set utilizzati, mettendo in luce i loro punti di forza e debolezze nel simulare un sistema reale, ottenendo in questo modo utili informazioni per il setup computazionale di calcoli DFT su sistemi polimerici. Tutti gli spettri sperimentali proposti in questo lavoro, sia IR che Raman, sono stati registrati da noi nel laboratorio FunMat del dipartimento di chimica e materiali del Politecnico di Milano, mentre l’acquisizione dello spettro Raman effettuato durante la prova a trazione delle fibre di PPTA, si è svolto in collaborazione con la Prof. Claudia Marano del Politecnico di Milano. L’approccio da noi presentato, enfatizza l’importanza di metodi computazionali non solo per un’affidabile interpretazione degli spettri vibrazionali, ma anche per la comprensione di quei fenomeni moleculari responsabili di molte proprietà fisico-chimiche dei materiali polimerici. Inoltre, le sue potenzialità non si limitano solo allo studio di base del materiale ma hanno importanti ricadute anche nella ricerca applicata in campo tecnologico ed industriale.
Quantum chemical simulations and spectroscopic characterization of poly(p-phenylene terephthalamide) fibers
AVANZINI, LORENZO
2015/2016
Abstract
In the field of characterization techniques of materials, and in particular of IR and Raman spectroscopy, this work sheds light on the usefulness and reliability achieved by state-of-the-art computational methods for the investigation, at the molecular scale, of the spectroscopic and structural properties of the semi-crystalline polymers. The research and development of new polymeric materials, as shown by the most recent papers appeared in the literature, are still very active. Acting on the physical and mechanical properties of these materials, involves a deep knowledge of the structure and arrangement of the molecular chains and in particular of how they influence the physical-chemistry of the material. For this reason, the need for innovative characterization techniques, which lead to a rational design of the molecular structure of polymeric materials, is taking ground. By means of the current CRYSTAL14 and Gaussian09 implementations, we report a detailed study on the spectroscopic properties of the poly(p-phenylene terephthalamide) (PPTA). The potential of this code applied to polymer materials, are still almost completely unexplored. This work shows how these computational models are able to accurately reproduce the vibrational spectra (IR and Raman) of PPTA, providing an effective method to interpret in details the information obtained by experimental characterization techniques. In addition to the assignment of the main group frequencies of vibrational spectra IR and Raman of the 3D and 1D (infinite isolated chain) crystals by means of CRYSTAL14, we also simulated the effect induced by the mechanical stretching of PPTA fibers on the Raman spectrum, observing a downward frequency shift of the bands related to backbone vibrations. The results obtained turned out to be in direct agreement with data found in the literature. In particular, we also verified how structural changes, due to fibers stretching, play a fundamental role in modifying the vibrational spectra. By means of CRYSTAL14 we determined the elastic modulus of the PPTA crystalline phase, and we simulated the IR spectrum with polarized incident light, obtaining once again results in good agreement with experimental measurements. Furthermore, by taking advantage of Gaussian09 we also carried out calculations of the IR and Raman spectra of different kinds of PPTA oligomers. Making a comparison between these latter oligomers spectra with CRYSTAL14 and experimental results, we were able to deeply understand the role of intra and intermolecular interactions in modifying the spectroscopic properties of the polymer. From the methodological point of view, this work shows the different results obtained by means of different exchange-correlation functionals and basis sets, sheding light on their strengths and weaknesses in simulating real systems and obtaining on this way useful information on the computational set up of the DFT calculation of polymeric systems. All the experimental spectra reported in this work has been carried out in the FunMat laboratory of the chemistry and materials department of the Politecnico di Milano, while the acquisition of the Raman spectrum performed during the PPTA fibers tensile test took place in collaboration with Prof. Claudia Marano of Politecnico di Milano. The proposed approach, emphasizes the importance of computational methods not only for a reliable interpretation of vibrational spectra, but also to understand those molecular phenomena responsible of many physical and chemical properties of polymeric materials. Furthermore, its potentialities are not only limited to the basic study of the material, but have also a significant impact in applied research also in the industrial and technological fields.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/123914