Mass transport in polymers

The study of mass transport in polymeric membranes has grown in importance due to its potential application in many processes such as separation of gases and vapors, packaging, controlled drug release. The diffusion of a low molecular weight species in a polymer is often accompanied by other phenomena like swelling, reactions, stresses, that have not been investigated in all their aspects yet. Furthermore, novel materials have been developed that include inorganic fillers, reactive functional groups or ions, that make the scenery even more complicated. The present work focused on the experimental study of systems where the diffusion is accompanied by other processes; suitable models were also developed to describe the particular circumstances in order to understand the underlying concepts and be able to design the performances of the material. The effect of solvent-induced deformation in polymeric films during sorption processes was studied since the dilation, especially in constrained membranes, can cause the development of stresses and therefore early failures of the material. The bending beam technique was used to test the effects of the dilation and the stress induced in the polymer by penetrant diffusion. A model based on the laminate theory was developed that accounts for the swelling and is able to predict the stress that raise in the material. The addition of inorganic fillers affects the transport properties of polymeric films. Mixed matrix membranes based on fluorinated, high free volume matrices show attractive performances for separation purposes but there is a need for deeper investigation of the selectivity properties towards gases and vapors. A new procedure based on the NELF model was tested on the experimental data; it allows to predict solubility of every penetrant on the basis of data for one vapor. The method has proved to be useful also for the determination of the diffusion coefficient and for an estimation of the permeability in the composite materials. Oxygen scavenging systems can overcome lack of barrier properties in common polymers that forbids their application in sensitive applications as food packaging. The final goal of obtaining a membrane almost impermeable to oxygen leads to experimental times out of reach. Hence, a simple model was developed in order to describe the transport of oxygen in a membrane with also reactive groups and analyze the experimental data collected on SBS copolymers that show attractive scavenging capacity. Furthermore, a model for predicting the oxygen barrier behavior of a film formed as a blend of OSP in a common packaging material was built, considering particles capable of reactions with oxygen embedded in a non-reactive matrix. Perfluorosulphonic acid ionomers (PFSI) are capturing attention due to a high thermal and chemical resistance coupled with very peculiar transport properties, that make them appropriate to be used in fuel cells. The possible effect of different formation procedure was studied together with the swelling due to water sorption since both water uptake and dilation can dramatically affect the fuel cells performances. The water diffusion and sorption was studied with a FTIR-ATR spectrometer that can give deeper information on the bonds between water molecules and the sulphonic hydrophilic groups and, therefore, on the microstructure of the hydrated ionomer.

Liquid crystal polymers: macromolecular design for enhanced performances

During this work, done mainly in the laboratories of the department of Industrial Chemistry and Materials of the University of Bologna but also in the laboratories of the Carnegie Mellon University in collaboration with prof. K. Matyjaszewski and at the university of Zaragoza in collaboration with prof. J. Barberá, was focused mainly on the synthesis and characterization of new functional polymeric materials. In the past years our group gained a deep knowledge about the photomodulation of azobenzene containing polymers. The aim of this thesis is to push forward the performances of these materials by the synthesis of well defined materials, in which, by a precise control over the macromolecular structures, better or even new functionality can be delivered to the synthesized material. For this purpose, besides the rich photochemistry of azoaromatic polymers that brings to the application, the control offered from the recent techniques of controlled radical polymerization, ATRP over all, gives an enormous range of opportunity for the developing of a new generation of functional materials whose properties are determinate not only by the chemical nature of the functional center (e.g. azoaromatic chromophore) but are tuned and even amplified by a synergy with the whole macromolecular structure. Old materials in new structures. In this contest the work of this thesis was focused mainly on the synthesis and characterization of well defined azoaromatic polymers in order to establish, for the first time, precise structure-properties correlation. In fact a series of well defined different azopolymers, chiral and achiral, with different molecular weight and highly monodisperse were synthesized and their properties were studied, in terms of photoexpansion and photomodulation of chirality. We were then able to study the influence of the macromolecular structure in terms of molecular weight and ramification on the studied properties. The huge amount of possibility offered by the tailoring of the macromolecular structure were exploited for the synthesis of new cholesteric photochromic polymers that can be used as a smart label for the certification of the thermal history of any thermosensitive product. Finally the ATRP synthesis allowed us to synthesize a total new class of material, named molecular brushes: a flat surface covered with an ultra thin layer of polymeric chain covalently bond onto the surface from one end. This new class of materials is of extreme interest as they offer the possibility to tune and manage the interaction of the surface with the environment. In this contest we synthesized both azoaromatic surfaces, growing directly the polymer from the surface, and mixed brushes: surfaces covered with incompatible macromolecules. Both type of surfaces acts as “smart” surfaces: the first it is able to move the orientation of a LC cell by simply photomodulation and, thanks to the robustness of the covalent bond, can be used as a command surface overcoming all the limitation due to the dewetting of the active layer. The second type of surface, functionalized by a grafting-to method, can self assemble the topmost layer responding to changed environmental conditions, exposing different functionality according to different environment.

Sintesi, modifica e caratterizzazione di polimeri da fonti rinnovabili

Gli argomenti trattati all’interno di questa tesi di dottorato riguardano la sintesi e la modifica di polimeri ottenuti a partire da fonti rinnovabili. L’acido polilattico (PLA) è stato modificato per ottenere film estensibili per uso alimentare. La scelta del materiale si è basata sull’analisi del suo ciclo di vita e perché è riconosciuto come sicuro per l’utilizzo nel campo alimentare. Le formulazioni testate, a base di PLA, sono state preparate con l’aggiunta di una serie di additivi utilizzati per migliorare le proprietà meccaniche del materiale. La lavorazione è stata eseguita mediante estrusione, ottenendo dei pellet a composizione omogenea successivamente lavorati nell’estrusore a bolla, modalità industriale di produzione dei film estensibili. È stata poi effettuata la sintesi diretta di nuovi poliesteri insaturi a base di dimetil succinato e 1,6-esandiolo. L’insaturazione della catena è stata ottenuta mediante l’uso, durante la sintesi, di derivati dell’olio di ricino, l’acido ricinoleico e il suo derivato insaturo metil undecenoato. Un’altra molecola insatura utilizzata è stata il citronellolo, scelto tra i terpeni. I polimeri così ottenuti sono stati modificati tramite reazioni radicaliche indotte con radiazioni UV utilizzando sostanze antibatteriche (sale 3-esadecil-1-vinilimidazolo bromuro) al fine di ottenere materiali con attività biocida a lungo termine e senza rilascio. Si è proceduto inoltre alla polimerizzazione reversibile di monomeri furanici con oli vegetali utilizzando una strategia di tipo double click. Si è partiti dalla sintesi di monomeri derivanti da oli vegetali contenenti eterocicli furanici attaccati mediante addizione tiol-enica (prima reazione click chemistry) e si è proseguito con la loro successiva polimerizzazione attraverso una reazione di tipo Diels-Alder con molecole con gruppi maleimmidici (seconda reazione click chemistry). I polimeri così ottenuti sono materiali potenzialmente auto-riparanti, grazie alla possibilità di spostare l’equilibrio verso i prodotti o i reagenti semplicemente variando le condizioni di temperatura.

Charge transport properties of organic conjugated polymers for photovoltaic applications

Charge transport in conjugated polymers as well as in bulk-heterojunction (BHJ) solar cells made of blends between conjugated polymers, as electron-donors (D), and fullerenes, as electron-acceptors (A), has been investigated. It is shown how charge carrier mobility of a series of anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene)s (AnE-PVs) is highly dependent on the lateral chain of the polymers, on a moderate variation of the macromolecular parameters (molecular weight and polydispersity), and on the processing conditions of the films. For the first time, the good ambipolar transport properties of this relevant class of conjugated polymers have been demonstrated, consistent with the high delocalization of both the frontier molecular orbitals. Charge transport is one of the key parameters in the operation of BHJ solar cells and depends both on charge carrier mobility in pristine materials and on the nanoscale morphology of the D/A blend, as proved by the results here reported. A straight correlation between hole mobility in pristine AnE-PVs and the fill factor of the related solar cells has been found. The great impact of charge transport for the performance of BHJ solar cells is clearly demonstrated by the results obtained on BHJ solar cells made of neat-C70, instead of the common soluble fullerene derivatives (PCBM or PC70BM). The investigation of neat-C70 solar cells was motivated by the extremely low cost of non-functionalized fullerenes, compared with that of their soluble derivatives (about one-tenth). For these cells, an improper morphology of the blend leads to a deterioration of charge carrier mobility, which, in turn, increases charge carrier recombination. Thanks to the appropriate choice of the donor component, solar cells made of neat-C70 exhibiting an efficiency of 4.22% have been realized, with an efficiency loss of just 12% with respect to the counterpart made with costly PC70BM.

Structure-property relations in polymers from renewable resources

The present research project focuses its attention on the study of structure-property relations in polymers from renewable sources (bio-based polymers) such as polymers microbially produced, i.e. polyhydrohyalkanoates (PHAs) or chemically synthesized using monomers from renewable sources, i.e. polyammide 11 (PA11). By means of a broad spectrum of experimental techniques, the influence of different modifications on bio-based polymers such as blending with other components, copolymerization with different co-monomers and introduction of branching to yield complex architectures have been investigated. The present work on PHAs focused on the study of the dependence of polymer properties on both the fermentation process conditions (e.g. bacterial strain and carbon substrate used) and the method adopted to recover PHAs from cells. Furthermore, a solvent-free method using an enzyme and chemicals in an aqueous medium, was developed in order to recover PHAs from cells. Such a method allowed to recover PHA granules in their amorphous state, i.e. in native form useful for specific applications (e.g. paper coating). In addition, a commercial PHA was used as polymeric matrix to develop biodegradable and bio-based composites for food packaging applications. Biodegradable, non-toxic, food contact plasticizers and low cost, widely available lignocellulosic fibers (wheat straw fibers) were incorporated in such a polymeric matrix, in order to decrease PHA brittleness and the polymer cost, respectively. As concerns the study of polyamide 11, both the rheological and the solid-state behavior of PA11 star samples with different arm number and length was studied. Introduction of arms in a polymer molecule allows to modulate melt viscosity behavior which is advantageous for industrial applications. Also, several important solid-state properties, in particular mechanical properties, are affected by the presence of branching. Given the importance of using ‘green’ synthetic strategies in polymer chemistry, novel poly(-amino esters), synthesized via enzymatic-catalyzed polymerization, have also been investigated in this work.

Solubility, diffusivity and permeability of gases in glassy polymers

Gas separation membranes of high CO2 permeability and selectivity have great potential in both natural gas sweetening and carbon dioxide capture. Many modified PIM membranes results permselectivity above Robinson upper bound. The big problem that should be solved for these polymers to be commercialized is their aging through time. In high glassy polymeric membrane such as PIM-1 and its modifications, solubility selectivity has more contribution towards permselectivity than diffusivity selectivity. So in this thesis work pure and mixed gas sorption behavior of carbon dioxide and methane in three PIM-based membranes (PIM-1, TZPIM-1 and AO-PIM-1) and Polynonene membrane is rigorously studied. Sorption experiment is performed at different temperatures and molar fraction. Sorption isotherms found from the experiment shows that there is a decrease of solubility as the temperature of the experiment increases for both gases in all polymers. There is also a decrease of solubility due to the presence of the other gas in the system in the mixed gas experiments due to competitive sorption effect. Variation of solubility is more visible in methane sorption than carbon dioxide, which will make the mixed gas solubility selectivity higher than that of pure gas solubility selectivity. Modeling of the system using NELF and Dual mode sorption model estimates the experimental results correctly Sorption of gases in heat treated and untreated membranes show that the sorption isotherms don’t vary due to the application of heat treatment for both carbon dioxide and methane. But there is decrease in the diffusivity coefficient and permeability of pure gases due to heat treatment. Both diffusivity coefficient and permeability decreases with increasing of heat treatment temperature. Diffusivity coefficient calculated from transient sorption experiment and steady state permeability experiment is also compared in this thesis work. The results reveal that transient diffusivity coefficient is higher than steady state diffusivity selectivity.

New bio-based polymers: from synthesis to characterization

The environmental problems caused by human activity are one of the main themes of debate of the last Century. As regard plastics, the use of non-renewable sources together with the accumulation of waste in natural habitats are causing serious pollution problems. For this reason, a continuously growing interest is recorded around sustainable materials, potential candidate for the replacement of traditional recalcitrant plastics. Promising results have been obtained with biopolymers, in particular with the class of biopolyesters. Their potential biodegradability and biobased nature is particularly interesting mainly for food packaging, where the multilayer systems normally used and the contamination by organic matter create severe recycling limits. In this framework, the present research has been conducted with the aim of synthetizing, modifying and characterizing biopolymers for food packaging application. New bioplastics based on monomers derived from renewable resources were successfully synthetized by two-step melt polycondensation and chain extension reaction following the “Green chemistry” principles. Moreover, well-known biopolyesters have been modified by blending or copolymerization, both resulting effective techniques to ad hoc tune the polymer final characteristics. The materials obtained have been processed and characterized from the chemical, structural, thermal and mechanical point of view; more specific characterizations as compostability tests, surface hydrophilicity film evaluation and barrier property measurements were conducted.

Nanofibrous-structured polymers and their engineering applications

Monolithic materials cannot always satisfy the demands of today’s advanced requirements. Only by combining several materials at different length-scales, as nature does, the requested performances can be met. Polymer nanocomposites are intended to overcome the common drawbacks of pristine polymers, with a multidisciplinary collaboration of material science with chemistry, engineering, and nanotechnology. These materials are an active combination of polymers and nanomaterials, where at least one phase lies in the nanometer range. By mimicking nature’s materials is possible to develop new nanocomposites for structural applications demanding combinations of strength and toughness. In this perspective, nanofibers obtained by electrospinning have been increasingly adopted in the last decade to improve the fracture toughness of Fiber Reinforced Plastic (FRP) laminates. Although nanofibers have already found applications in various fields, their widespread introduction in the industrial context is still a long way to go. This thesis aims to develop methodologies and models able to predict the behaviour of nanofibrous-reinforced polymers, paving the way for their practical engineering applications. It consists of two main parts. The first one investigates the mechanisms that act at the nanoscale, systematically evaluating the mechanical properties of both the nanofibrous reinforcement phase (Chapter 1) and hosting polymeric matrix (Chapter 2). The second part deals with the implementation of different types of nanofibers for novel pioneering applications, trying to combine the well-known fracture toughness enhancement in composite laminates with improving other mechanical properties or including novel functionalities. Chapter 3 reports the development of novel adhesive carriers made of nylon 6,6 nanofibrous mats to increase the fracture toughness of epoxy-bonded joints. In Chapter 4, recently developed rubbery nanofibers are used to enhance the damping properties of unidirectional carbon fiber laminates. Lastly, in Chapter 5, a novel self-sensing composite laminate capable of detecting impacts on its surface using PVDF-TrFE piezoelectric nanofibers is presented.

Preparation and characterization of multifunctional bio-based polymers exhibiting enhanced properties

Driven by environmental reasons and the expected depletion of crude oil, bio-based polymers are currently undergoing a renaissance in the attempt to replace fossil-based ones. The present work aims at contributing in the development of the steps that start from biomass and move to new polymeric multifunctional materials. The study focuses on two bio-based building blocks (itaconic and vanillic acids) characterized by exploitable functionalities, i.e. a lateral double bond and a substituted aromatic ring respectively, able to confer interesting properties to the final polymers. The lateral double bond of dimethyl itaconate was functionalized via thia-Michael addition reaction obtaining a thermo-stable building block that can undergo polycondensation under classical conditions of reaction. The addition of a long lateral chain allows the polymer to express antimicrobial activity against Staphylococcus aureus making it attractive for packaging and targeting antimicrobial applications. Moreover, the architecture of the homopolymer was modified by means of copolymerization with dimethyl 2,5-furandicarboxylate thus improving the rigidity and obtaining a thermo-processable material. Potential applications as thermoset or thermoplastic material have been discussed. As concerns vanillic acid, the presence of aromatic rings on the polymer backbone imparts high thermal stability, but brittle behaviour in the homopolymer. Therefore, the architecture of the polyester was successfully tuned by means of copolymerization with a flexible bio-based comonomer, i.e. ω-pentadecalactone, providing processable random copolymers. An in depth investigation of water transport mechanism has been undertaken on the synthesized polyesters. Since the copolymers present a succession of aromatic and aliphatic units, as a consequence of the chemical structure water vapor permeability interposes between polyethylene and poly(ethylene terephthalate) proving that the copolyesters are suitable for packaging applications. Moving towards a sustainable model of development, novel sustainable synthetic pathways for the eco-design of new bio-based polymeric structures with high value functionalities and different potential applications have been successfully developed.

Analytical pyrolysis to study microplastics and other polymers in the environment

Synthetic polymers constitute a wide class of materials which has enhanced the quality of human life, providing comforts and innovations. Anyway, the increasing production and the incorrect waste management, are leading to the occurrence of polymers in the environment, generating concern. To understand the extent of this issue, analytical investigation holds an essential position. Standardised methods have not established yet, and additional studies are required to improve the present knowledge. The main aim of this thesis was to provide comprehensive information about the potential of pyrolysis coupled with gas-chromatography and mass spectrometry (Py-GC-MS) for polymers investigation, from their characterisation to their identification and quantification in complex matrices. Water-soluble (poly(dimethylsiloxanes), PDMS bearing poly(ethylene glycol), PEG, side chains) and water-insoluble polymers (microplastics, MPs, and bioplastics) were studied. The different studies revealed the possibility to identify heterogeneous classes of polymers, fingerprinting the presence of PDMS copolymers and distinguishing chemically different polyurethanes (PURs). The occurrence of secondary reactions in pyrolysis of polymer mixtures was observed as possible drawback. Pyrolysis products indicative of secondary reactions and their reaction mechanisms were identified. Py-GC-MS also revealed its fundamental role for the identification of polymers composing commercial bioplastics items based. The results aided to identify chemicals that have the potential to migrate in sea waters. Investigations of environmental samples demonstrated the capability of Py-GC-MS to provide reliable, reproducible and comparable results about polymers in complex matrices (PEG-PDMS in sewage sludges and PURs and other MPs in road dusts and spider webs). Criticisms were especially found in quantitation, such as the retrieval reference materials, the construction of reliable calibration protocols and the occurrence of bias due to interferences between pyrolysis products. This thesis pursues the greater purpose to develop harmonised and standardised methods for environmental investigations of polymers, that is fundamental to assess the real state of the environment.

Design, synthesis and photovoltaic properties of some thiophene-based conjugated polymers for organic solar cells

The current issue of the resource of energy combined with the tendency to give a green footprint to our lifestyle have prompted the research to focus the attention on alternative sources with great strides in the optimization of polymeric photovoltaic devices. The research work described in this dissertation consists in the study of different semiconducting π-conjugated materials based on polythiophenes (Chapter I). In detail, the GRIM polymerization was deepened defining the synthetic conditions to obtain regioregular poly(3-alkylthiophene) (Chapter II). Since the use of symmetrical monomers functionalized with oxygen atom(s) allows to adopt easy synthesis leading to performing materials, disubstituted poly(3,4-dialkoxythiophene)s were successfully prepared, characterized and tested as photoactive materials in solar cells (Chapter III). A “green” resource of energy should be employed through sustainable devices and, for this purpose, the research work was continued on the synthesis of thiophene derivatives soluble in eco-friendly solvents. To make this possible, the photoactive layer was completely tailored starting from the electron-acceptor material. A fullerene derivative soluble in alcohols was successfully synthetized and adopted for the realization of the new devices (Chapter IV). New water/alcohol soluble electron-donor materials with different functional groups were prepared and their properties were compared (Chapter V). Once found the best ionic functional group, a new double-cable material was synthetized optimizing the surface area between the different materials (Chapter VI). Finally, other water/alcohol soluble materials were synthetized, characterized and used as cathode interlayers in eco-friendly devices (Chapter VII). In this work, all prepared materials were characterized by spectroscopy analyses, gel permeation chromatography and thermal analyses. Cyclic voltammetry, X-ray diffraction, atomic force microscopy and external quantum efficiency were used to investigate some peculiar aspects.

Valorization and recycling of waste biomass and bioplastics into molecules, polymers and catalysts

The enormous amount of goods that world societies consume every day, derives from an immense consumption of energy and raw materials, and leads to an unthinkable amount of wastes. The abuse of fossil-based resources and the mismanaged waste is leading to big environmental pollution and climate change, with consequences on all living beings. To solve this issue and start living in equilibrium with nature, modern societies must stop using fossil fuels massively in favor to clean renewable energies, recycling, and biomass and waste utilization for materials and chemical production. Moreover, bioplastic recycling must be prioritized over their biodegrading and composting. This work is dedicated to the study of new synthetic strategies that fall into these fields.

Enhancing predictive capability of models for solubility and permeability in polymers and composites

The interpretation of phase equilibrium and mass transport phenomena in gas/solvent - polymer system at molten or glassy state is relevant in many industrial applications. Among tools available for the prediction of thermodynamics properties in these systems, at molten/rubbery state, is the group contribution lattice-fluid equation of state (GCLF-EoS), developed by Lee and Danner and ultimately based on Panayiotou and Vera LF theory. On the other side, a thermodynamic approach namely non-equilibrium lattice-fluid (NELF) was proposed by Doghieri and Sarti to consistently extend the description of thermodynamic properties of solute polymer systems obtained through a suitable equilibrium model to the case of non-equilibrium conditions below the glass transition temperature. The first objective of this work is to investigate the phase behaviour in solvent/polymer at glassy state by using NELF model and to develop a predictive tool for gas or vapor solubility that could be applied in several different applications: membrane gas separation, barrier materials for food packaging, polymer-based gas sensors and drug delivery devices. Within the efforts to develop a predictive tool of this kind, a revision of the group contribution method developed by High and Danner for the application of LF model by Panayiotou and Vera is considered, with reference to possible alternatives for the mixing rule for characteristic interaction energy between segments. The work also devotes efforts to the analysis of gas permeability in polymer composite materials as formed by a polymer matrix in which domains are dispersed of a second phase and attention is focused on relation for deviation from Maxwell law as function of arrangement, shape of dispersed domains and loading.