Optimised use of dielectric spectroscopy at microwave frequencies for direct online monitoring of polymerisation reactions

This study reports the first use of dielectric spectroscopy over at a wide frequency range to monitor, in real time, the progress of a ring opening polymerisation. An open-ended coaxial line sensor was placed directly into the reaction medium of a polymerisation of ε-caprolactone and used to characterise the dielectric properties of the polymerisation mixture both ‘in-situ’ and with time at microwave frequencies. In addition to measurements obtained by the sensor, samples of the medium were extracted at various time points for off-line analysis, to confirm the level of conversion and polymer molecular weights that had been achieved. The results demonstrated that the dielectric properties values exhibited by the reaction medium with time could be correlated directly to the progress of the reaction. Thus, the experimental data allowed the construction of a calibration curve which could be used to predict the conversion of monomer to polymer at any given point of the reaction. The dielectric data permitted also the identification of key reaction parameters, such as the optimum point of termination for the reaction. Furthermore, the analysis of the dielectric data over a wide frequency spectrum enabled the identification of the most suitable frequencies for the practical operation of the sensor, in terms of linearity and sensitivity. This will enable the development of suitable instrumentation and an improved strategy for the online monitoring and control of a broad range of polymerisation reactions

An Oxalate Bridged Binuclear Iron(III) Ionic Liquid for the Highly Efficient Glycolysis of Polyethylene Terephthalate under Microwave Irradiation

An oxalate-bridged binuclear iron(III) ionic liquid combined with an imidazolium based cation, (dimim)2[Fe2Cl4(µ-ox)], was synthesized and characterized by a wide range of techniques. This halometallate ionic liquid was active in catalyzing the depolymerization of polyethylene terephthalate (PET) by glycolysis, under conventional and microwave-assisted heating conditions. Both methodologies were very selective towards the production of bis(2-hydroxyethyl)terephthalate (BHET). The employment of microwave heating proved beneficial in terms of time and energy saving when compared to the use of thermal heating. Indeed, dielectric spectroscopy studies revealed that the binuclear iron-containing ionic liquid exhibits an excellent heating response under an electromagnetic field. The catalyst provided quantitative conversions to BHET in the glycolysis of post-consumer PET bottles in only 3 h through microwave heating, as compared to 80 % conversion after 24 h under conventional heating

Selective molecular annealing:in situ small angle X-ray scattering study of microwave-assisted annealing of block copolymers

Microwave annealing has emerged as an alternative to traditional thermal annealing approaches for optimising block copolymer self-assembly. A novel sample environment enabling small angle X-ray scattering to be performed in situ during microwave annealing is demonstrated, which has enabled, for the first time, the direct study of the effects of microwave annealing upon the self-assembly behavior of a model, commercial triblock copolymer system [polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene]. Results show that the block copolymer is a poor microwave absorber, resulting in no change in the block copolymer morphology upon application of microwave energy. The block copolymer species may only indirectly interact with the microwave energy when a small molecule microwave-interactive species [diethylene glycol dibenzoate (DEGDB)] is incorporated directly into the polymer matrix. Then significant morphological development is observed at DEGDB loadings ≥6 wt%. Through spatial localisation of the microwave-interactive species, we demonstrate targeted annealing of specific regions of a multi-component system, opening routes for the development of "smart" manufacturing methodologies

Direct online monitoring and control of chemical reactions using dielectric spectroscopy

Over the last decades, the increasing demand for both the production of biobased products and the need for more sustainable material synthesis processes led to the development of novel techniques such as the Nitroxide Mediated Polymerization (NMP), Catalytic Chain Transfer Polymerisation (CCTP), and Ring-Opening Polymerisation (ROP). However, the commercial development of those techniques has been limited, with a key issue being the need to determine how far a particular reaction has progressed in order to continue to the next stage of the process (e.g., addition of other reactants) or terminate the reaction when the target conversion has been reached. Dielectric spectroscopy has been considered a promising technique for ‘in-situ’ monitoring since it is a non-invasive technique which can be applied to most industrial reactors. The aims of this research were to investigate the use of dielectric spectroscopy for the ‘in-situ’ monitoring of chemical reactions at microwave frequencies and relate the dielectric properties with key reaction features such as the molecular weight and the level of conversion that has been achieved. The thesis presents a detailed study of the tin octanoate mediated ROP of ε-caprolactone, and the para toluene sulfonic acid catalysed hydrolysis of sorbitol to sorbitan and isosorbide. Additionally, dielectric spectroscopy was utilised to differentiate the polymer architecture and molecular weight of Styrene–divinylbenzene copolymers synthesised by CCTP and NMP. An open-ended coaxial line sensor was placed directly into the reaction medium and used to measure the dielectric properties of the mixture both “in-situ” and with time. A swept signal (0.5 GHz–20 GHz) was then transmitted from a Vector Network Analyser (VNA), through the open-ended coaxial line, into the sample. Depending on the complex permittivity of the sample, a portion of that signal was reflected to the VNA and the reflection coefficient of the sample was used to calculate the complex permittivity. In addition to the measurements obtained by the sensor, samples of the medium were extracted at various time points for off-line analysis, using Gel Permeation Chromatography and Nuclear Magnetic Resonance spectroscopy to confirm key reaction features, e.g., molecular weight and the level of conversion that had been achieved. The results demonstrated that in case of ε-caprolactone polymerisation and sorbitol dehydration the dielectric property values exhibited by the reaction medium could be correlated to both the progress of the reaction and the structure of the final product. Thus, the experimental data allowed the construction of a calibration curve which could be used to predict the level of conversion that has been achieved. The use of dielectric data also permitted the identification of key reaction parameters, such as the optimum point of termination for the reaction. Furthermore, the analysis of the dielectric data over a wide frequency spectrum enabled the identification of the most suitable frequencies for the practical operation of the sensor, in terms of linearity and sensitivity. This study has demonstrated a method to determine the product properties or conversion during the reaction progress based on the real-time measurement of the dielectric properties during the ε-caprolactone polymerisation and sorbitol dehydration. This work provides a basis for developing process control strategies based on observing the change in the dielectric properties which can be applied to commercial manufacturing. The proposed method could improve product quality (e.g., by terminating the reaction when the target conversion has been reached batch to batch repeatability can be improved) and reduce the production cost of materials, e.g., by optimising the amount of time that the reaction is kept at the required temperature, energy usage and waste generation can be minimised)

Direct online monitoring and control of chemical reactions using dielectric spectroscopy

Over the last decades, the increasing demand for both the production of biobased products and the need for more sustainable material synthesis processes led to the development of novel techniques such as the Nitroxide Mediated Polymerization (NMP), Catalytic Chain Transfer Polymerisation (CCTP), and Ring-Opening Polymerisation (ROP). However, the commercial development of those techniques has been limited, with a key issue being the need to determine how far a particular reaction has progressed in order to continue to the next stage of the process (e.g., addition of other reactants) or terminate the reaction when the target conversion has been reached. Dielectric spectroscopy has been considered a promising technique for ‘in-situ’ monitoring since it is a non-invasive technique which can be applied to most industrial reactors. The aims of this research were to investigate the use of dielectric spectroscopy for the ‘in-situ’ monitoring of chemical reactions at microwave frequencies and relate the dielectric properties with key reaction features such as the molecular weight and the level of conversion that has been achieved. The thesis presents a detailed study of the tin octanoate mediated ROP of ε-caprolactone, and the para toluene sulfonic acid catalysed hydrolysis of sorbitol to sorbitan and isosorbide. Additionally, dielectric spectroscopy was utilised to differentiate the polymer architecture and molecular weight of Styrene–divinylbenzene copolymers synthesised by CCTP and NMP. An open-ended coaxial line sensor was placed directly into the reaction medium and used to measure the dielectric properties of the mixture both “in-situ” and with time. A swept signal (0.5 GHz–20 GHz) was then transmitted from a Vector Network Analyser (VNA), through the open-ended coaxial line, into the sample. Depending on the complex permittivity of the sample, a portion of that signal was reflected to the VNA and the reflection coefficient of the sample was used to calculate the complex permittivity. In addition to the measurements obtained by the sensor, samples of the medium were extracted at various time points for off-line analysis, using Gel Permeation Chromatography and Nuclear Magnetic Resonance spectroscopy to confirm key reaction features, e.g., molecular weight and the level of conversion that had been achieved. The results demonstrated that in case of ε-caprolactone polymerisation and sorbitol dehydration the dielectric property values exhibited by the reaction medium could be correlated to both the progress of the reaction and the structure of the final product. Thus, the experimental data allowed the construction of a calibration curve which could be used to predict the level of conversion that has been achieved. The use of dielectric data also permitted the identification of key reaction parameters, such as the optimum point of termination for the reaction. Furthermore, the analysis of the dielectric data over a wide frequency spectrum enabled the identification of the most suitable frequencies for the practical operation of the sensor, in terms of linearity and sensitivity. This study has demonstrated a method to determine the product properties or conversion during the reaction progress based on the real-time measurement of the dielectric properties during the ε-caprolactone polymerisation and sorbitol dehydration. This work provides a basis for developing process control strategies based on observing the change in the dielectric properties which can be applied to commercial manufacturing. The proposed method could improve product quality (e.g., by terminating the reaction when the target conversion has been reached batch to batch repeatability can be improved) and reduce the production cost of materials, e.g., by optimising the amount of time that the reaction is kept at the required temperature, energy usage and waste generation can be minimised)

Mechanical and Electrical measurements for health monitoring of polymer nanocomposite PVDF

95 σ.Στην παρούσα εργασία μελετήθηκε η δυνατότητα χρήσης του νανοσύνθετου πολυβινυλυδενο(δι)φθοριδίου (PVDF) με νανοσωλήνες άνθρακα πολλαπλού τοιχώματος (MWCNTs) ως αισθητήρα επιμήκυνσης σε μονοαξονικό εφελκυσμό. Στόχος της εργασίας είναι η διάγνωση της φθοράς του υλικού αξιοποιώντας την μεταβολή της ηλεκτρικής του αντίστασης λόγω επιβολής μονοαξονικής εφελκυστικής δύναμης. Το κάθε δοκίμιο υποβλήθηκε σε δοκιμή εφελκυσμού μέχρι την θραύση με ταυτόχρονη μέτρηση της μηχανικής τάσης, της διαμήκους παραμόρφωσης και της διαμήκους ηλεκτρικής του αντίστασης σε τρεις περιοχές μέτρησης κατά μήκος του λαιμού του δοκιμίου. Χρησιμοποιήθηκαν δοκίμια με περιεκτικότητα σε νανοσωλήνες άνθρακα από 1,25% έως 8% κ.β., τα οποία παράχθηκαν με την τεχνική της ανάμειξης τήγματος. Για την καλύτερη κατανόηση των ηλεκτρικών και μηχανικών ιδιοτήτων των δοκιμίων καθώς και για την μελέτη της επίδρασης των νανοσωλήνων άνθρακα στο πολυμερές, χρησιμοποιήθηκαν τεχνικές χαρακτηρισμού όπως η Διηλεκτρική Φασματοσκοπία Εναλλασσόμενου Πεδίου και η Δυναμική Μηχανική Ανάλυση. Η προσθήκη των νανοσωλήνων στην πολυμερική μήτρα διαπιστώθηκε ότι προσδίδει αγωγιμότητα στο νανοσύνθετο σε περιεκτικότητες από 1,2% κ.β. και πάνω ενώ δεν επηρεάζει σημαντικά τις μηχανικές ιδιότητες του νανοσύνθετου. Από τα πειραματικά αποτελέσματα προκύπτει ότι υπάρχει συσχέτιση μεταξύ της σχετικής μεταβολής της αντίστασης (ΔR/Ro) του υλικού και της εντατικής του κατάστασης κατά τη διάρκεια της εφαρμογής εφελκυστικής δύναμης στο δοκίμιο μέχρι και τη θραύση του. Εφόσον, δηλαδή, είναι γνωστή η αρχική ηλεκτρική αντίσταση του υλικού Ro, μπορούμε να γνωρίζουμε την κατάσταση της «υγείας» του υλικού (την εντατική του κατάσταση) σε οποιαδήποτε χρονική στιγμή μετρώντας την ηλεκτρική του αντίσταση. Επίσης παρατηρήθηκε μεγαλύτερη αύξηση της σχετικής μεταβολής της αντίστασης στην περιοχή της θραύσης, από ό,τι σε άλλες περιοχές μέτρησης. Αυτό οδηγεί στο συμπέρασμα ότι είναι δυνατόν να υπάρξει έγκαιρη προειδοποίηση της περιοχής όπου αναμένεται η θραύση του δοκιμίου. Μπορούμε να πούμε, δηλαδή, ότι οι νανοσωλήνες άνθρακα, διεσπαρμένοι σε μια πολυμερική μήτρα όπως το PVDF έχουν τη δυνατότητα να λειτουργήσουν ως ένα δίκτυο αισθητήρων για τον έλεγχο της εντατικής κατάστασης του υλικού και την πρόβλεψη της θραύσης του λόγω επιβολής εφελκυστικής δύναμης.In this diploma thesis, conductive PVDF (PolyVinylidene(di)Fluoride)/MWCNT (Multi Wall Carbon Nanotubes) nanocomposites have been used for strain sensing under tensile loading. This was achieved by measuring the relative electrical resistance change (ΔR/Ro) of the sample during longitudinal deformation. Each dog bone shaped sample was subjected to tensile loading until fracture and the longitudinal strain was monitored together with the longitudinal electrical resistance in three regions along the sample’s neck. The samples were prepared by melt-mixing with varying filler content from 1,25wt% to 8wt%. In order to understand the mechanical and electrical properties of the nanocomposite, different material characterization techniques were used, such as Dielectric Relaxation Spectroscopy (DRS) and Dynamic Mechanical Analysis (DMA). The percolation threshold of the nanocomposite was found at 1,2wt% conductive filler content. The mechanical properties of the nanocomposite were found almost equal to the properties of the polymer matrix. The results showed that as strain changes, relative resistance change could be measured on site by suitable equipment to evaluate the strain or damage level and predict the imminent fracture. It is well established that carbon nanotubes dispersed in an insulating polymer matrix, such as PVDF, have the potential to be used as a sensitive network to monitor or predict the damage in polymer/carbon nanotube nanocomposites by measuring the electrical resistance change (ΔR/Ro) of the sample.Αλέξης Ν. Καλαμιώτη