Self-healing of Structural Composites Containing Dendrimers as Healing Agent

Dendrimers exhibit healing functionalities on polymer level. In the present chapter, the effect of hydrogen-bonded supramolecular polymers (SP) into high performance aerospace carbon fiber reinforced plastics (CFRPs) is assessed. More precisely, the interlaminar fracture toughness of unidirectional (UD) SP-modified composites (containing SP interleaves) and their healing capability were measured under mode I and mode II fracture loading conditions. During testing, these modified samples exhibited extended bridging between the interlaminar crack flanks, which considerably enhanced their interlaminar fracture toughness. Furthermore, SP pre-impregnated fiber layers (prepregs) were fabricated to facilitate the introduction of the self-healing agent (SHA) into the composite laminated structure. SP prepregs were used to modify quasi-isotropic CFRPs in a symmetric fashion, and the damage tolerance of the modified composites was investigated. To that direction, the SP-modified laminates were tested under low velocity impact (LVI) conditions, and compression after impact (CAI) tests were conducted prior and after the activation of the healing. Finally, examination of the morphology of fracture surface led to qualitative conclusions regarding the involved failure and healing mechanisms

Chapter Self-healing of Structural Composites Containing Dendrimers as Healing Agent

Dendrimers exhibit healing functionalities on polymer level. In the present chapter, the effect of hydrogen-bonded supramolecular polymers (SP) into high performance aerospace carbon fiber reinforced plastics (CFRPs) is assessed. More precisely, the interlaminar fracture toughness of unidirectional (UD) SP-modified composites (containing SP interleaves) and their healing capability were measured under mode I and mode II fracture loading conditions. During testing, these modified samples exhibited extended bridging between the interlaminar crack flanks, which considerably enhanced their interlaminar fracture toughness. Furthermore, SP pre-impregnated fiber layers (prepregs) were fabricated to facilitate the introduction of the self-healing agent (SHA) into the composite laminated structure. SP prepregs were used to modify quasi-isotropic CFRPs in a symmetric fashion, and the damage tolerance of the modified composites was investigated. To that direction, the SP-modified laminates were tested under low velocity impact (LVI) conditions, and compression after impact (CAI) tests were conducted prior and after the activation of the healing. Finally, examination of the morphology of fracture surface led to qualitative conclusions regarding the involved failure and healing mechanisms

Mode II Fatigue Delamination Growth and Healing of Bis-Maleimide Modified CFRPs by Using the Melt Electro-Writing Process Technique

In the current study, the interlaminar fracture toughness behavior of high-performance carbon fiber-reinforced plastics (CFRPs) modified with Bis-maleimide (BMI) resin was investigated under Mode II quasi-static and fatigue remote loading conditions. Specifically, CFRPs were locally integrated with BMI resin, either nano-modified with graphene nano-platelets (GNPs) or unmodified, using the melt electro-writing process (MEP) technique. Following the modification, two types of CFRPs were manufactured: (a) CFRPs with pure BMI resin and (b) CFRPs with GNP-modified resin. Quasi-static tests demonstrated that the interlaminar fracture toughness properties of both modified CFRPs were significantly improved compared to the unmodified/reference CFRPs. Conversely, fatigue tests were conducted under displacement control, with crack length measurement performed using a traveling microscope. Delamination length and load quantities were measured at specific cycle intervals. The results indicated that both modified CFRPs exhibited enhanced resistance to delamination under Mode II fatigue loading, with earlier crack arrest, compared against the reference CFRPs. Additionally, the CFRPs displayed low healing efficiency (H.E.) after the healing cycle was activated. Overall, this approach shows promise in improving the delamination resistance of CFRPs under Mode II

Ανάπτυξη τεχνικών αυτοΐασης ινωδών συνθέτων υλικών

During the last decades, fiber reinforced composites (FRPs) are year by year replacing metals due to their high stiffness and high strength in combination with low specific weight and corrosion resistance. However, during their service life, these composites due to their laminated structure (no fibers are present in transverse direction) appear matrix cracking and delaminations between the reinforcing plies. A primary limitation of these composites is the poor interlaminar toughness and strength. The mismatch of anisotropic mechanical and thermal properties in between plies of deferent principal directions promotes out of plane stresses at the edges of the structures as well as in the case of stringer run out, thickness variation, holes and structural stiffeners joined to composite skin, and are only some of the candidate areas for delamination under in plane and out of plane loadings. Delaminations are among the most frequent modes of failure encountered in laminated composites and are resulted either from fatigue loadings or low velocity impact events.Conventional repair techniques of composites have a lot of drawbacks; are expensive, require extensive human work and cannot repair defects deep inside the material. Self-healing polymers is an approach which has not yet been incorporated to commercial composites but promises to face some principal weak points. This smart technology aims to in-situ repair matrix cracks and matrix/reinforcement debonding and thus to extend the effective life-span of the composites, to reduce the maintenance needs and costs and to improve the damage tolerance and reliability of composite structures. Self-healing composites have previously been developed by embedding healing agents into the matrix using microcapsules or vascular networks, that will release the healing agent upon crack damage. A different approach towards self-healing composites is matrices that comprise reversible polymers that are able to proceed with multiply healing cycles at the same damaged site.In the present investigation, the utilization of three different reversible polymeric systems (based on their chemistry) as healing agent into CFRPs was studied. More precisely, common thermoplastics such as PET and Polyamides (Nylon-66) based on reversible covalent bonds, Bis-maleimide polymers (pure and blends) based on Diels Alder (DA) and Retro-DA reactions through special covalent bonding and finally Supramolecular polymers based on hydrogen bonds were integrated into aerospace-grade CFRPs. A variety of methodologies (i.e. blending, interleaving, sieving and pre-preging) was utilized for the modification. The assessment of potential knock down effects and the healing capability of the resulting composites were investigated under mode I and mode II fracture tests, low velocity impact (LVI), compression after impact (CAI) and three-point bending (3PB) tests. Optical microscopy, SEM examinations and acoustic emission activity (AE) of the samples was monitored and led to qualitative conclusions regarding the involved failure and healing mechanisms.According to all these experimental campaign, it was shown that by the incorporation of all these SHAs to the composites the mode I and II fracture toughness characteristics were significantly increased with samples containing supramolecular interleaves to exhibit dramatically increased fracture toughness characteristics (e.g., GIC increased with more than one order of magnitude at approximately 1550%). These modified composites exhibited healing efficiency values from 60% to 100% after the application of the first healing cycle. In addition the effect of the curing regime on the toughening and healing behaviour of CFRPs containing bis-maleimide polymers was investigated. It was shown that curing temperatures lower than the melting point of the healing agent slightly decreased the fracture toughness characteristics while increased the healing capabilities of these samples. LVI tests revealed that samples containing supramolecular prepregs or MWCNT doped nylon electrospun veils as interleaves between the primary layers of the composite exhibited higher resistance to delamination and increased CAI characteristics after the application of the healing cycle. Finally, AE recordings showed that by the incorporation of a ductile phase (i.e., healing agent) into the composite the AE activity in terms of hits is typically reduced while both AE characteristics (hits and energy) was reduced after the application of the healing cycles.Τις τελευταίες δεκαετίες σύνθετα υλικά (ΣΥ) υψηλών μηχανικών ιδιοτήτων και προδιαγραφών αντικαθιστούν όλο περισσότερο μεταλλικά μέρη αεροσκαφών και διάφορα άλλα μέρη που σχετίζονται με βιομηχανικές εφαρμογές. Αυτή η τάση έχει τις ρίζες της στη ζήτηση δομικών υλικών με βελτιωμένες μηχανικές ιδιότητες, χαμηλή πυκνότητα και αντίσταση στη διάβρωση. Από την άλλη, τα υλικά αυτά και κυρίως τα πολύστρωτα ΣΥ, κατά τη διάρκεια λειτουργίας τους εμφανίζουν μικρή αντίσταση στη διάδοση διαστρωματικών αποκολλήσεων. Αυτό το μειονέκτημα τα καθιστά ιδιαίτερα ευπαθή σε φορτίσεις εκτός επιπέδου που τυχόν θα αντιμετωπίσουν κατά τη διάρκεια λειτουργίας κατασκευών που τις απαρτίζουν και γι'αυτό τις τελευταίες δεκαετίες έχει γίνει πολύ μελέτη για το πως θα επιτευχθεί η ενίσχυση της διαστρωματικής αντοχής των υλικών αυτών. Οι συμβατικές μέθοδοι επισκευής ως γνωστό παρουσιάζουν πολλές αδυναμίες και μειονεκτήματα (π.χ. είναι χρονοβόρες, κοστοβόρες και όχι πάντοτε αποτελεσματικές). Έτσι η προβληματική κατά το πάχος αντίσταση των υλικών αυτών στη διάδοση διαστρωματικών αποκολλήσεων, σε συνδυασμό με τα αδύναμα σημεία των συμβατικών μεθόδων επισκευής τους, βάζει εμπόδια στην ευρύτερη χρήση τους στις σύγχρονες προηγμένες κατασκευές. Το τελευταίο διάστημα ιδιαίτερο ενδιαφέρον έχει εκδηλωθεί από τους επιστήμονες μηχανικούς για την ανάπτυξη πολύ-λειτουργικών υβριδικών συνθέτων υλικών που θα είναι σε θέση όχι μόνο να φέρουν υψηλά μηχανικά φορτία αλλά και να έχουν τη δυνατότητα της επούλωσης τυχόν βλαβών μέσα στη μήτρα του συνθέτου υλικού. Μια τέτοια δυνατότητα είναι αυτή της "Αυτοΐασης" (self-healing), που με λίγα λόγια σημαίνει ότι εάν το υβριδικό ή τροποποιημένο υλικό παρουσιάσει κάποια εσωτερική βλάβη από κάποιο εξωτερικό αίτιο το ίδιο το υλικό θα είναι σε θέση να αυτό-επισκευασθεί ή αυτό-επουλωθεί. Η ενσωμάτωση υλικών με αυτές τις "ευφυής" ιδιότητες (Smart Materials) στις κατασκευές στην παρούσα φάση βρίσκονται σε επίπεδο εργαστηρίου.Σύμφωνα με τα προαναφερθέντα η ανάπτυξη νέων υβριδικών ή τροποποιημένων πολύστρωτων συνθέτων υλικών με βελτιωμένη διαστρωματική δυσθραυστότητα (interlaminar fracture toughness) τα οποία έχουν τη δυνατότητας της αυτοΐασης (Self-healing functionality) αποτελεί στόχο. Τα τελευταία χρόνια έχουν γίνει προσπάθειες για την αύξηση της διαστρωματικής δυσθραυστότητας των δομικών αυτών συνθέτων υλικών με διάφορες μεθόδους (nano-modification, stitching, z-pinning, interleaving, optimization of the stacking sequence, κ.α.), ενώ ταυτόχρονα έχουν αναπτυχθεί διάφοροι τρόποι ενσωμάτωσης της δυνατότητας αυτοΐασης (νάνο ή μικρό-κάψουλες (nano or micro-capsules), μικρό-αγγειακά δίκτυα (vascular networks) και ΘΑΠ (reversible polymers)). Από την έως τώρα βιβλιογραφική επισκόπηση, πολλές προσπάθειες έχουν γίνει μονόπλευρα είτε από την πλευρά της αύξησης της διαστρωματικής δυσθραυστότητας είτε από την πλευρά ενσωμάτωσης της ιδιότητας της αυτοΐασης. Υπάρχει περιορισμένος αριθμός δημοσιεύσεων που αφορούν και τις δυο κατευθύνσεις ταυτόχρονα. Βασική προϋπόθεση κατά την ανάπτυξη του νέου αυτού υβριδικού ή τροποποιημένου συνθέτου υλικού που θα έχει την ικανότητα της αυτοΐασης είναι: α) η προσθήκη του υλικού ή του συστήματος που θα προσδίδει στο σύνθετο υλικό την ιδιότητα της αυτοΐασης να μην προκαλεί μείωση των μηχανικών ιδιοτήτων "knock down effect" (τουλάχιστον σημαντικά), και β) να μην αυξάνει σημαντικά το κόστος παραγωγής και την πολυπλοκότητα παρασκευής των νέων αυτών ΣΥ. Όλα τα ανωτέρω αποτελούν τεράστιες προκλήσεις που πρέπει να απαντηθούν ώστε να προχωρήσει η εφαρμογή αυτής της πολλά υποσχόμενης τεχνολογίας.Το αντικείμενο της παρούσας διατριβής είναι η ανάπτυξη τεχνικών αυτοΐασης σε ινώδη ΣΥ με ενσωμάτωση θερμικά αναστρέψιμων πολυμερών (ΘΑΠ) (reversible polymers) στη μήτρα του συνθέτου υλικού. Πιο συγκεκριμένα η διατριβή αυτή βασίζεται στη στρατηγική των ΘΑΠ γνωστή ως "Reversible Polymers Strategy" η οποία είναι μη αυτόνομη διαδικασία (non-autonomous self-healing) καθώς εξωτερικά αίτια θα επιφέρουν την ίαση (θέρμανση και πίεση για συγκεκριμένο χρονικό διάστημα) στο σημείο όπου εντοπίζεται η βλάβη με χρήση μεθόδων μη καταστροφικών ελέγχου (ΜΚΕ). Η στρατηγική αυτή έχει αρκετά πλεονεκτήματα καθώς δεν απαιτείτε ενσωμάτωση στο υπό επισκευή ΣΥ καψουλών (νάνο ή μικρό) ή μικρο-αγγειακών δικτύων που είναι πολύ πιθανόν να λειτουργήσουν ως σημεία συγκέντρωσης τάσεων, που με τη σειρά τους θα οδηγήσουν στην πρόωρη δημιουργία ρωγμών μέσα στη μήτρα του ΣΥ και κατά συνέπεια τη μελλοντική θράση του. Επίσης η χρήση των θερμικά αναστρέψιμων πολυμερών είναι πολλαπλή καθώς έχουν την δυνατότητα της ίασης για περισσότερες από μια φορές.Στην παρούσα διδακτορική διατριβή έγινε εκτενής μελέτη ενσωμάτωσης τριών τύπων θερμικά αναστρέψιμων πολυμερών βάσει των δεσμών που συνδέουν τις πολυμερικές αλυσίδες μεταξύ τους. Ο κάθε τύπος θερμικά αναστρέψιμου πολυμερούς απασχολεί ένα κεφάλαιο ξεχωριστά στην παρούσα εργασία (Κεφάλαια 3,4 και 5 αντίστοιχα). Αρχικά μελετήθηκαν δυο βασικά υλικά θερμοπλαστικού τύπου (το PET (Polyethylene terephthalate) και το Nylon-66) που κατά κύριο λόγο βασίζονται σε ομοιοπολικούς δεσμούς (covalent bonds). Στη συνέχεια μελετήθηκαν υλικά τύπου Βις-μαλεϊμίδιου (Bis-maleimide (ΒΜΙ)) και μείγματα αυτών με εποξειδικές ρητίνες που βασίζονται σε ειδικούς ομοιοπολικούς δεσμούς (Specialcovalentbonds) και έχουν ως βάση τις Diels-Alder (DA) και Retro Diels Alder (R-DA) αντιδράσεις. Τέλος μελετήθηκαν ΘΑΠ τύπου Supramolecular τα οποία βασίζονται στη Supramolecular χημεία και κατά κύριο λόγο σε δεσμούς υδρογόνου (hydrogenbonds).Τα προαναφερθέντα υλικά τοποθετήθηκαν σε ινώδη ΣΥ από ίνες άνθρακα (CFRPs) είτε με τη μορφή απλής διασποράς μέσα στη μήτρα είτε υπό τη μορφή πολύ λεπτών στρώσεων ή φιλμ (interleaving) μεταξύ των στρώσεων του ΣΥ. Πιο συγκεκριμένα η παρούσα διδακτορική διατριβή επικεντρώθηκε στη μελέτη της διαστρωματικής θραυστομηχανικής συμπεριφοράς τύπου Ι και ΙΙ (interlaminar fracture toughness Ι and II) για ΣΥ που εμπεριέχουν όλων των τύπων των προαναφερθέντων υλικών καθώς και στη μελέτη συμπεριφοράς τους σε πειράματα κρούσης χαμηλής ταχύτητας (low velocity impact tests) και θλίψης μετά από την κρούση (Compression After Impact (CAI)). Ακολούθως εφαρμόστηκε η διαδικασία της αυτοΐασης των βλαβών και τα μηχανικά τεστ επαναλήφθηκαν υπό πανομοιότυπες συνθήκες. Η διαδικασία δοκιμής/επισκευής επαναλήφθηκε αρκετές φορές μετά την αρχική πρόκληση βλάβης στο τροποποιημένο ΣΥ. Ως βοηθητικά εργαλεία στη διεξαγωγή της έρευνας αυτής χρησιμοποιήθηκαν ο ΜΚΕ των CFRP πλακών μέσω C-scan, η παρακολούθηση της ακουστικής δραστηριότητας (Acoustic Emission (AE)) των δοκιμίων κατά τη διάρκεια των πειραμάτων αλλά και η μελέτη της εσωτερικής δομής των ΣΥ χρήσει οπτικού μικροσκοπίου και SEM

Graphene Nanoplatelet- and Hydroxyapatite-Doped Supramolecular Electrospun Fibers as Potential Materials for Tissue Engineering and Cell Culture

Porous and fibrous artificial extracellular matrices (ECM) called scaffolds are considered to be promising avenues of research in the field of biomedical engineering, including tissue fabrication through cell culture. The current work deals with the fabrication of new matrix-type scaffolds through electrospinning, in order to support future three-dimensional tissue formation. The selected material for the fabrication of these scaffolds was a supramolecular polymer (SP) that is based on ureiodypyrimidone hydrogen bonding units (UPy). More precisely, pure SP and modified electrospun scaffolds with (a) graphene nanoplatelets (GNPs), (b) hydroxyapatite (HA), and (c) a mixture of both were fabricated for the needs of the current study. The aim of this work is to engineer and to characterize SP electrospun scaffolds (with and without fillers) and study whether the introduction of the fillers improve the physical and mechanical properties of them. The obtained results indicate that doping the SP scaffolds with GNPs led to improved apparent mechanical properties while HA seems to slightly deteriorate them. For all cases, doping provided thinner fibers with a more hydrophilic surface. Taking together, these types of SP scaffolds can be further studied as potential candidate for cell culture

Toughening and Healing of CFRPs by Electrospun Diels–Alder Based Polymers Modified with Carbon Nano-Fillers

In the present investigation, thermo-reversible bonds formed between maleimide and furan groups (Diels–Alder (DA)-based bis-maleimides (BMI)) have been generated to enable high-performance unidirectional (UD) carbon fiber-reinforced plastics (CFRPs) with self-healing (SH) functionality. The incorporation of the SH agent (SHA) was performed locally, only in areas of interest, with the solution electrospinning process (SEP) technique. More precisely, reference and modified CFRPs with (a) pure SHA, (b) SHA modified with multi-walled carbon nano-tubes (MWCNTs) and (c) SHA modified with graphene nano-platelets (GNPs) were fabricated and further tested under Mode I loading conditions. According to experimental results, it was shown that the interlaminar fracture toughness properties of modified CFRPs were considerably enhanced, with GNP-modified ones to exhibit the best toughening performance. After the first fracture and the activation of the healing process, C-scan inspections revealed, macroscopically, a healing efficiency (H.E.) of 100%; however, after repeating the tests, a low recovery of mechanical properties was achieved. Finally, optical microscopy (OM) examinations not only showed that the epoxy matrix at the interface was partly infiltrated by the DA resin, but it also revealed the presence of pulled-out fibers at the fractured surfaces, indicating extended fiber bridging between crack flanks due to the presence of the SHA

Embedded Resistance Wire Technique for Epoxy Curing and Self-Healing of PET Thermoplastics

The aim of the present study is to apply an embedded resistance wire technique for curing of thermosetting resins as well as for the self-healing of thermoplastic polymers. The work consists of two parts. In the first part, Kanthal resistance wires embedded in a resin plate acted as heating elements when direct electrical current was flowing through them (Joule heating). During heating, the temperature was continuously monitored using a thermal camera, and accurate temperature times for fixed position diagrams as well as temperature positions for fixed time diagrams were calculated. The effects of curing with this method were evaluated by studying the three-point bending mechanical behavior of the cured resin, comparing it with the corresponding behavior of the same resin when cured using a conventional oven curing method at the same temperature. In the second part of the present work, the possibility of using the same technique for healing existing notches and flaws in a PET thermoplastic is explored. We examined whether providing energy through the resistance wires created the right amount of heat to heal the thermoplastic, or, more specifically, whether it closed the notches and eliminated the abrasions that were artificially created on the specimens. The technique using embedded resistance wires worked equally well, with interesting and promising preliminary results regarding the curing of thermoset resins and the healing of thermoplastics

Toughening and Healing of CFRPs by Diels–Alder-Based Nano-Modified Resin through Melt Electro-Writing Process Technique

In the current study, a novel approach in terms of the incorporation of self-healing agent (SHA) into unidirectional (UD) carbon fiber reinforced plastics (CFRPs) has been demonstrated. More precisely, Diels–Alder (DA) mechanism-based resin (Bis-maleimide type) containing or not four layered graphene nanoplatelets (GNPs) at the amount of 1 wt% was integrated locally in the mid-thickness area of CFRPs by melt electro-writing process (MEP). Based on that, CFRPs containing or not SHA were fabricated and further tested under Mode I interlaminar fracture toughness experiments. According to experimental results, modified CFRPs exhibited a considerable enhancement in the interlaminar fracture toughness properties (peak load (Pmax) and fracture toughness energy I (GIC) values). After Mode I interlaminar fracture toughness testing, the damaged samples followed the healing process and then were tested again under identical experimental conditions. The repeating of the tests revealed moderate healing efficiency (H.E.) since part of the interlaminar fracture toughness properties were restored. Furthermore, three-point bending (3PB) experiments were conducted, with the aim of assessing the effect of the incorporated SHA on the in-plane mechanical properties of the final CFRPs. Finally, optical microscopy (OM) examinations were performed to investigate the activated/involved damage mechanisms

Primary MSCs for Personalized Medicine: Ethical Challenges, Isolation and Biocompatibility Evaluation of 3D Electrospun and Printed Scaffolds

Autologous cell therapy uses patients’ own cells to deliver precise and ideal treatment through a personalized medicine approach. Isolation of patients’ cells from residual tissue extracted during surgery involves specific planning and lab steps. In the present manuscript, a path from isolation to in vitro research with human mesenchymal stem cells (MSCs) obtained from residual bone tissues is described as performed by a medical unit in collaboration with a research center. Ethical issues have been addressed by formulating appropriate harvesting protocols according to European regulations. Samples were collected from 19 patients; 10 of them were viable and after processing resulted in MSCs. MSCs were further differentiated in osteoblasts to investigate the biocompatibility of several 3D scaffolds produced by electrospinning and 3D printing technologies; traditional orthopedic titanium and nanostructured titanium substrates were also tested. 3D printed scaffolds proved superior compared to other substrates, enabling significantly improved response in osteoblast cells, indicating that their biomimetic structure and properties make them suitable for synthetic tissue engineering. The present research is a proof of concept that describes the process of primary stem cells isolation for in vitro research and opens avenues for the development of personalized cell platforms in the case of patients with orthopedic trauma. The demonstration model has promising perspectives in personalized medicine practices

3-Arm star pyrene-functional PMMAs for efficient exfoliation of graphite in chloroform: fabrication of graphene-reinforced fibrous veils

ISSN:2040-3364ISSN:2040-337