Mechanical behaviour of photopolymerized materials

The photopolymerization process used for the production of additively manufactured polymers employed in advanced applications, enables to obtain objects spanning a large dimensional scale thanks to the molecular size achievable by the solidification process. In fact, the photopolymerization is based on the physical-chemical network cross-linking mechanism taking place at the nanoscale. Since the starting raw material is a liquid resin that progressively becomes solid upon the irradiation by a suitable light source, the mechanical properties – and so the corresponding mechanical response of the final printed structural material – heavily depend on the degree and distribution of the polymerization induced in the material itself. In the present study, starting from the governing equations of the light-induced polymerization process, we determine the chain density formed within the solid domain. Then, the mechanical response of photopolymerized elements obtained with different photopolymerization parameters is investigated. Moreover, the microstructure optimization of polymeric elements in relation to the achievement of the desired mechanical response with the least energy spent in the polymer’s formation, is studied. Finally, some interesting considerations related to the modelling of the photopolymerization process are outlined

Smart actuation of liquid crystal elastomer elements: cross-link density-controlled response

Liquid crystalline elastomers (LCEs) exhibit some remarkable physical properties, such as the reversible large mechanical deformation induced by proper environmental stimuli of different nature, such as the thermal stimulus, allowing their use as soft actuators. The unique features displayed by LCE are originated from their anisotropic microstructure characterized by the preferential orientation of the mesogen molecules embedded in the polymer network. An open issue in the design of LCEs is how to control their actuation effectiveness: the amount of mesogens molecules, how they are linked to the network, the order degree, the cross-link density are some controllable parameters whose spatial distribution, however, in general cannot be tuned except the last one. In this paper, we develop a theoretical micromechanical-based framework to model and explore the effect of the network cross-link density on the mechanical actuation of elements made of liquid crystalline elastomer. In this context, the light-induced polymerization (photopolymerization) for obtaining the elastomers’ cross-linked network is of particular interest, being suitable for precisely tuning the cross-link density distribution within the material; this technology enables to obtain a molecular-scale architected LCEs, allowing the optimal design of the obtainable actuation. The possibility to properly set the cross-link density arrangement within the smart structural element (LCE microstructure design and optimization), represents an intriguing way to create molecular-scale engineered LCE elements having material microstructure encoded desired actuation capabilities

a phase field approach for crack modelling of elastomers

Abstract The description of a problem related to an evolving interface or a strong discontinuity requires to solve partial differential equations on a moving domain, whose evolution is unknown. Standard computational methods tackle this class of problems by adapting the discretized domain to the evolving interface, and that creates severe difficulties especially when the interface undergoes topological changes. The problem becomes even more awkward when the involved domain changes such as in mechanical problems characterized by large deformations. In this context, the phase-field approach allows us to easily reformulate the problem through the use of a continuous field variable, identifying the evolving interface (i.e. the crack in fracture problems), without the need to update the domain discretization. According to the variational theory of fracture, the crack grows by following a path that ensures that the total energy of the system is always minimized. In the present paper, we take advantage of such an approach for the description of fracture in highly deformable materials, such as the so-called elastomers. Starting from a statistical physics-based micromechanical model which employs the distribution function of the polymer's chains, we develop herein a phase-field approach to study the fracture occurring in this class of materials undergoing large deformations. Such a phase-field approach is finally applied to the solution of crack problems in elastomers

Fractiles Based Sampling Procedure: a new probabilistic approach to evaluate the design resistance of a structural element

I sistemi ingegneristici, intesi come strutture o più in generale componenti meccanici o dispositivi, non sono perfetti. Una progettazione perfetta richiede che il sistema rimanga operativo e persegua tutte le sue funzioni, durante una predefinita vita utile, senza ammettere possibilità di danneggiamento o collasso. Questo approccio di progettazione è certamente qualcosa di ideale, impraticabile ed economicamente difficile da perseguire. Anche se la conoscenza tecnica non è più un fattore limitante nella progettazione, produzione, costruzione e gestione del sistema finale, il costo di sviluppo dei test, dei materiali e dell'analisi ingegneristica può superare di gran lunga le prospettive economiche di tali sistemi. Pertanto, limitazioni pratiche ed economiche delineano il perseguimento di progettazioni imperfette. Nonostante questo aspetto imprescindibile, i progettisti hanno il dovere di minimizzare la probabilità di crisi di tali sistemi. A tal proposito, specie per strutture d’elevata importanza o esistenti, analisi strutturali sempre più complesse vengono svolte[Report RTD:1016-1:2017, 2017, “Guidelines for Nonlinear Finite Element Analysis of Concrete Structures”]. La crisi di un sistema ingegneristico, in questo caso di un elemento strutturale, è legata alla presenza di incertezze che nell’analisi e nella progettazione sono da sempre presenti. Tali incertezze, possono riguardare tutte le parti di un sistema, come le caratteristiche intrinseche (geometria, armatura..), i carichi agenti, i fattori ambientali o incertezze legate ai parametri meccanici dei materiali. A questo proposito, l’approccio tradizionale (Partial Safety Factor, PSF) semplifica la progettazione assumendo che le incertezze siano di tipo deterministico, mediante l’introduzione di coefficienti di sicurezza. Tale approccio può rivelarsi in generale troppo conservativo, o in alcuni casi non affidabile; in particolare esso non fornisce informazioni su quale sia il grado di sicurezza raggiunto. Le normative forniscono poi altri approcci di tipo stocastico (Probabilistic Safety Formats). Tali approcci, nella loro forma esatta (Fully Probabilistic, FP) richiedono un elevatissimo sforzo computazionale. All’interno dei metodi stocastici le normative forniscono anche metodi probabilistici di tipo semplificato (Estimation Of Coefficient Of Variation, ECOV) che però spesso forniscono risultati non cautelativi. In questo lavoro di tesi, è stato proposto un metodo probabilistico semplificato (Fractiles Based Sampling Procedure, FBSP), alternativo a quanto proposto dalle norme, sviluppato nell’ambito della collaborazione tra l’Università di Parma e la Boku University di Vienna. Il metodo è basato sui risultati ottenuti dal Latin Hyperbolic Sampling (incardinato nel FP). Esso consente di raggiungere una resistenza di progetto, riducendo di gran lunga il numero d’analisi non lineari da condurre. In particolare, dagli N set di parametri meccanici generati dal software statistico (FReET), che andrebbero utilizzati per condurre N analisi non-lineari nell’ambito di un approccio FP, viene estratto un sotto-campione formato da soli sette set di parametri meccanici, scelti sulla base dei valori che un parametro specifico (leading parameter) assume in corrispondenza di sette frattili. Il leading parameter può essere uno solo di tutte le proprietà meccaniche che descrivono il sistema ingegneristico, ad esempio la resistenza a compressione. Attraverso lo studio di una trave a T precompressa, soggetta a taglio, sono stati proposti alcuni metodi volti alla determinazione del leading parameter. Una volta determinato tale parametro, è stata valutata la resistenza di tale trave e confrontata con le resistenze ottenute dai sopra menzionati approcci (FB, PSF e ECOV). Come verrà mostrato mediante un’analisi parametrica condotta su pannelli con differenti percentuali meccaniche d’armatura ed orientamento, la scelta di questo parametro è fortemente dipendente dal meccanismo di rottura dell’elemento strutturale analizzato, dal quale il progettista non può dunque prescindere

SWELLING MECHANISM IN SMART POLYMERS RESPONSIVE TO MECHANO-CHEMICAL STIMULI

Smart materials and smart structural systems are of paramount importance in the development of modern technological applications such as in bioengineering (tissue engineering, drug delivery, microscale surgery, …), packaging, data storage, molecular machines, smart sensors and actuators, etc. Smart polymers, often termed as responsive polymers, show physical or chemical changes in response to environmental stimuli of physical (ionic, temperature, radiation, light, mechanical stress, …), chemical (specific ions, pH, chemical agents, …), or biochemical (enzyme, substrates, ligands, …) nature; sometimes they could also respond to a combination of two or more triggering actions at the same time. In the present study the responsiveness of smart polymers to a chemical and mechanical stimuli at the same time is considered. In particular, being the chemical action typically carried by a solvent medium, the interplay of the fluid absorption mechanism and the chemical triggering one is accounted for. Moreover, in our study we focus also on the responsiveness capability of providing a detectable mechanical response at the mesoscopic scale, induced by a geometric change (typically a volume expansion) at the molecular scale. The mechanics of polymers undergoing swelling and showing responsiveness is investigated through a micromechanical model, subsequently implemented in a computational framework, for the prediction of the mechanical behavior of this class of active materials

MECHANICS OF MULTI-STIMULI TEMPERATURE-RESPONSIVE HYDROGELS

The temperature-sensitivity of a certain class of elastomeric gels (cross-linked elastomers keen to uptake a fluid into their network, shown for example by poly-N-isopropylacrylamide (pNIPAm) hydrogels), is an interesting property allowing a temperature-controlled swelling. Being the elastomer-fluid affinity sensible to the temperature variation, this implies the possibility to control the amount of fluid uptaken by the material by properly changing the temperature. This capability is particularly interesting in biomedical applications or in the development of sensors whose responsiveness is often required to depend on the environmental temperature. Further, the incorporation of photo-thermal particles into the gel enables the use of light for controlling the material response. In this way, smart untethered multi-stimuli sensors or actuators can be obtained. In the present study, we consider the mechanical behavior of temperature-sensitive hydrogels and, relying on a theoretical multiphysics-based model accounting for light diffusion, heat generation and transfer, fluid absorption, and mechanics, the response of morphing elements is studied. Light-induced morphing due to photo-thermal effect is also considered and mathematically modeled. Validation and parametric simulations of the emerging deformations confirm the soundness of the approach and demonstrate the wide range of morphing functionalities obtainable by harnessing the temperature-dependent sensitivity of pNIPAm

From responsiveness in biological matter to functional materials: analogies and inspiration towards the systematic design and synthesis of new smart materials and systems

Living organisms and, in general, bio-materials respond to external stimuli exhibiting specific functionalities (such as shape-morphing, color change, tissue growth and remodelling, programmed mechanical responses, adaptation of material properties, etc.) required for different needs (camouflage, locomotion, defense, food supply, biological processes, etc.). Functionalities in nature come from biochemo-physical-mechanical responsiveness of the complex architectures in which natural structures are organized across the nano-, micro- and meso-scales. Often inspired by natural structures and bio-functionalities, the development of synthetic responsive materials has attracted a huge interest in recent years, and increasingly still attracts the efforts of scientists to synthesize new smart materials and devices. The paper illustrates the most compelling morphing and functional responses observable in nature - displayed by biological matter, living individuals or by the collective behavior of large groups of organisms - developed for different functional purposes, and discusses the related underlying mechanisms. In parallel, the most relevant functional materials being developed in the last decades are presented with the related mathematical models, and their underlying driving mechanisms are compared with those observable in nature. The study is aimed at providing a broad overview and to explain the strategies used in nature to obtain functionalities; the analogies with those shown in artificial functional materials, with a particular emphasis on polymer-based or polymer-like materials, are investigated. Some multi-physics models, describing the response and enabling the systematic design, optimization and synthesis of functional materials suitable to the development of new advanced applications, are also illustrated. The knowledge of natural cunning can push forward the research in the field, offers new possibilities worth of investigation by materials scientists, physicists and engineers, and opens unexplored scenarios not yet fully considered in the existing literature

MECHANICS OF MATERIALS WITH EMBEDDED UNSTABLE MOLECULES

Active structural systems and materials, typically used to develop sensors or actuators, are those capable of responding to external stimuli with some physical detectable reaction. The triggering stimuli can be of various nature, ranging from temperature, pH, light, mechanical stress, etc. In this context, responsive materials have the capability to react at the molecular level, showing some changes in their microstructure that can be detected and measured at the meso- or macro-scale level. This ability can be obtained in polymers and polymer-like materials through the introduction in their network of switchable molecules, characterized by two geometrically distinct stable states, where the switch from one to the other has can be seen as an instability phenomenon. In this research, we present a continuum mechanical model, developed starting from the micromechanics of the polymer network joined to molecules whose switching instability is induced by mechanical or chemical actions. The theoretical framework is presented by considering the general case of poly-disperse polymers and some final examples are given and discussed; the proposed model can be used as a tool for the development and design of smart responsive systems, applicable across a wide range of length scales

Viscous and Failure Mechanisms in Polymer Networks: A Theoretical Micromechanical Approach

Polymeric materials typically present a complex response to mechanical actions; in fact, their behavior is often characterized by viscous time-dependent phenomena due to the network rearrangement and damage induced by chains’ bond scission, chains sliding, chains uncoiling, etc. A simple yet reliable model—possibly formulated on the basis of few physically-based parameters—accounting for the main micro-scale micromechanisms taking place in such a class of materials is required to properly describe their response. In the present paper, we propose a theoretical micromechanical approach rooted in the network’s chains statistics which allows us to account for the time-dependent response and for the chains failure of polymer networks through a micromechanics formulation. The model is up-scaled to the mesoscale level by integrating the main field quantities over the so-called ‘chains configuration space’. After presenting the relevant theory, its reliability is verified through the analysis of some representative tests, and some final considerations are drawn