Thermomechanical response of polymer nanocomposites with preparation protocol controlled nanoparticle dispersion

Tato dizertační práce je zaměřená na základní výzkum procesů samouspořádávání nanočástic v polymerních kapalinách a na vlastnosti připravených polymerních nanokompozitů s řízenou disperzí nanočástic. Navzdory současnému pokroku v porozumění polymerních nanokompozitech, stále chybí mnohé fundamentální znalosti relaxačních a mechanických vlastností polymerních nanostruktur, které by mohly poskytnout klíčové informace pro návrh hierarchických funkčních kompozitů zpracovatelných aditivními výrobními technikami. Hlavní důraz byl kladen na výzkum vlivu postupu přípravy nanokompozitu na finální stav disperze nanočástic, přípravu řízených nanostruktur – individuálně dispergované nanočástice, řetězci vázáné klastry a kontaktní agregáty - a určení jejich relaxačních a mechanických vlastností. Navíc byly nanočástice využity jako „sondy“ v polymerní matrici, které ovlivňují segmentální uspořádání a relaxační dynamiku polymerních řetězců a mohou poskytnout o těchto dějích zásadní informace. Tento přístup může pomoci nalezení vztahů mezi segmentální dynamikou na nano škále a mechanickými vlastnostmi polymerních skel na makro škále, což je náročný fundamentální problém s extrémní technologickou důležitostí. Neroubované keramické nanočástice a polymerní skla byly použity, aby se minimalizoval vliv silných interakcí mezi nanočásticemi a řetězci. Podrobný výzkum byl vykonán na modelovém systému PMMA/SiO2 a následně rozšířen na systémy s jinými matricemi (PC a PS) a jinými nanočásticemi (ZnO2 and Fe2O3) za účelem zobecnění obdržených výsledků. Byla určena závislost relaxačních a mechanických vlastností (teplota skelného přechodu, reptační čas, modul kaučukovitého plata, počet zapletenin, napětí na mezi kluzu, pokles napětí po mezi kluzu, elastický modul, modul deformačního zpevnění a odezvy při toku za studeny) na nanostruktuře, objemovém zlomku a složení. Získané výsledky byly interpretovány za použití současných modelů. Stanovené relaxační a mechanické vlastnosti byly propojeny, aby poskytli informace o molekulárních deformačních procesech řídících mechanickou odezvu makroskopických kompozitních těles.This thesis is focused on a fundamental investigation of nanoparticle self-assembly in polymer liquids and on properties of the prepared polymer nanocomposites with controlled nanoparticle dispersion. Despite recent progress in understanding polymer nanocomposites, there are still unfilled gaps in the fundamental knowledge of relaxation phenomena and mechanical properties of various nanostructures that would provide key information for designing hierarchical or multidomain nanocomposites processable by additive manufacturing technologies. The emphasis was put on the investigation of the preparation protocol influence on the final dispersion state, preparation of various nanostructures – individually dispersed NPs, chain bound clusters, and contact aggregates at a constant composition, and determination of their relaxation and mechanical properties. Moreover, nanoparticles were utilized as “probes” in polymer matrix that affect the segmental ordering and the relaxation dynamics of polymer chains. This approach can help to derive the relationship between the nano scale segmental dynamics and macro scale mechanical properties of polymer glasses. It is a challenging fundamental scientific problem with an extreme technological importance. Non-grafted ceramic nanoparticles and polymer glasses were used to avoid the focus to deflect from the study of the nanoparticle–polymer interaction influence towards the influence of the graft–polymer interaction or the altered crystalline structure. A thorough investigation was performed for the PMMA/SiO2 model system and subsequently broadened to systems with different matrices (PC and PS) and nanoparticles (ZnO2 and Fe2O3) in order to generalize the obtained results. Nanostructure, volume fraction, and composition dependences of relaxation – glass transition temperature, reptation time, plateau modulus, number of entanglements, and mechanical properties – yield stress, yield drop, elastic modulus, strain hardening modulus, and creep response were determined. Achieved results were interpreted by means of the recent models. The determined relaxational and mechanical properties were connected to provide information about the molecular processes responsible for the mechanical response of the polymer nanocomposites.

Analysis of a-SiOC:H films by selected spectroscopic techniques

Táto bakalárska práca sa zaoberá prípravou tenkých vrstiev plazmových polymérov na báze tetravinylsilanu pomocou metódy plazmochemickej depozície z plynnej fázy a ich charakterizáciou pomocou vybraných spektroskopických techník. Teoretická časť je literárna rešerše z oblasti plazmochémie a spektroskopických metód charakterizácie tenkých vrstiev plazmových polymérov. Praktická časť pozostávala z prípravy dvoch sérii vzoriek a ich charakterizácie. Pripravené tenké vrstvy plazmových polymérov boli charakterizované pomocou vybraných spektroskopických techník. Hrúbka a optické parametre boli stanovené pomocou spektroskopickej elipsometrie. Chemická štruktúra bola charakterizovaná pomocou infračervenej spektroskopie. Výsledky poukazujú na možnosť riadiť fyzikálno-chemické vlastnosti tenkých vrstiev plazmových polymérov na báze tetravinylsilanu pomocou depozičných podmienok a teda na možnosť prípravy materiálov šitých na mieru pre najrôznejšie použitie.This bachelor thesis deals with preparation of thin plasma polymer films on the basis of tetravinylsilane by plasma-enhanced chemical vapour deposition and film characterization by selected spectroscopic techniques. The theoretical part is a background research about plasma chemistry and spectroscopic methods of characterisation of thin plasma polymer films. The practical part consisted of preparation of two series of samples and their characterization. Prepared thin plasma polymer films were characterized by selected spectroscopic techniques. Thickness and optical constants were determined by spectroscopic elipsometry. Chemical structure was characterized by infrared spectroscopy. The results indicate the possibility of managing physico-chemical properties of thin plasma polymer films on the basis of tetravinylsilane by deposition conditions and thus possibility of preparation materials tailored to a variety of applications.

Viscoelasticity of polymer glasses

Táto práca sa zameriava na štúdium relaxačného správania polymérnych skiel. Ako typický zástupca polymérnych skiel bol zvolený polymetylmetakrylát. Relaxačné procesy boli študované pomocou dynamickej mechanickej analýzy a ako doplnková metóda bola použitá diferenciálna skenovacia kalorimetria. V termomechanických spektrách bol pozorovaný relaxačný proces nad Tg a vysoké hodnoty modulu kaučukového pláta. Vysokoteplotný relaxačný prechod bol študovaný z pohľadu tepelnej histórie, bola študovaná frekvenčná závislosť, závislosť na axiálnom napätí a vplyv molekulovej štruktúry. Boli stanovené zdanlivé aktivačné energie skúmaných procesov a závislosť zdanlivých aktivačných energii skúmaného polymetylmetakrylátu na axiálnom napätí. Na základe získaných dát bola vypracovaná hypotéza, ktorá spojuje vysokoteplotný relaxačný proces s molekulovým procesom zodpovedným za deformačné spevnenie.This work focuses on polymer glasses relaxation behavior. Polymethylmethacrylate was chosen as a typical representative of polymer glasses. Relaxation processes were studied by dynamical mechanical spectroscopy and differential scanning calorimetry was used as a supplemental analysis. Relaxation process above Tg and high values of rubberlike plateau modulus were observed in thermomechanical spectra. High temperature relaxation transition was studied from the perspective of thermal history, frequency and axial stress dependence and influence of molecular structure was also investigated. Apparent activation energies of studied processes and their axial stress dependence for polymethylmethacrylate were determined. On the basis of obtained data, a hypothesis was developed which connects high temperature relaxation process with molecular process responsible for strain hardening.

Multifunctional Electrospun Nanofibers Based on Biopolymer Blends and Magnetic Tubular Halloysite for Medical Applications

Tubular halloysite (HNT) is a naturally occurring aluminosilicate clay with a unique combination of natural availability, good biocompatibility, high mechanical strength, and functionality. This study explored the effects of magnetically responsive halloysite (MHNT) on the structure, morphology, chemical composition, and magnetic and mechanical properties of electrospun nanofibers based on polycaprolactone (PCL) and gelatine (Gel) blends. MHNT was prepared via a simple modification of HNT with a perchloric-acid-stabilized magnetic fluid–methanol mixture. PCL/Gel nanofibers containing 6, 9, and 12 wt.% HNT and MHNT were prepared via an electrospinning process, respecting the essential rules for medical applications. The structure and properties of the prepared nanofibers were studied using infrared spectroscopy (ATR-FTIR) and electron microscopy (SEM, STEM) along with energy-dispersive X-ray spectroscopy (EDX), magnetometry, and mechanical analysis. It was found that the incorporation of the studied concentrations of MHNT into PCL/Gel nanofibers led to soft magnetic biocompatible materials with a saturation magnetization of 0.67 emu/g and coercivity of 15 Oe for nanofibers with 12 wt.% MHNT. Moreover, by applying both HNT and MHNT, an improvement of the nanofibers structure was observed, together with strong reinforcing effects. The greatest improvement was observed for nanofibers containing 9 wt.% MHNT when increases in tensile strength reached more than two-fold and the elongation at break reached a five-fold improvement

In-situ self-assembly of silica nanoparticles into microfibers with potential to reinforce polymers

Silica nanosphere with a diameter of 10–15 nm were organized into fibers with a lenght of 15 mm and an aspect ratio of 100 by self-assembly in 1,4-dioxane. Dioxane causes a positive zeta potential on the silica surface thus silica in dioxane may behave as an acceptor (base catalyzer) causing decomposition of dioxane to acetaldehyde and its consequent polymerization into oligomer or polymer (polyoxyethylene) chains that bond the particles together. This process was proved using a thermogravimetric analysis which showed that the amount of polymerized dioxane is in the rang 2–3.5 wt. %. Composition of the polymerized dioxane was elucidated employing FTIR. The formation of fibrillar structures was driven kinetically during solidification. The size of the fibers was controlled by the drying rate. Fast-drying results in longer and thinner fibers. Nanosilica fibers can also be formed in a polymer matrix (e.g., polycarbonate) via the solvent-casting method. Formation of fibers in-situ in a soft rubber polymer matrix in one-step processing can provide a polymer reinforcement at two hierarchical levels – at the nanoscale by immobilizing polymer chains due to the presence of nanoparticles and the microscale by strain transfer to the fibers. Elastic modulus of the fibers was determined by wrinkling technique by compression on the elastic surface and by thermal treatment in the polycarbonate matrix. Both techniques showed modulus 43–46 MPa

3D printing and post-curing optimization of photopolymerized structures: Basic concepts and effective tools for improved thermomechanical properties

The final thermo-mechanical properties of structural parts fabricated by masked stereolithography (MSLA) are highly determined not only by the processing parameters, but also by the post-processing methods. Improper implementation of post-treatment often leads to underperforming printouts. A novel tool for complex characterization of 3D printed bodies was developed and systematically demonstrated on a commercial free-radical photopolymerization (FRP) resin. The method relies on superimposed static and oscillatory mechanical test combining the heat deflection temperature (HDT) measurement together with the dynamic mechanical analysis (DMA) in a single test for fast and reliable characterization of parameters determining the curing behaviour of the photopolymer. The influence of post-curing time was addressed with a special focus on network density. Furthermore, the print orientation, having a high impact on mechanical properties, is discussed with a particular regard on the residual stress mitigation in future applications, such as 3D-printed cellular bodies

PLA toughening via bamboo-inspired 3D printed structural design

Bioinspired structures can attain mechanical properties unseen in conventional artificial materials. Specifically, the introduction of a cellular structure with a precisely designed distribution of cells, cell sizes, and cell walls is expected to enhance the mechanical response. Polylactic acid (PLA) is a biodegradable polymer produced from renewable resources with very interesting properties and good three-dimensional (3D) printing processability. However, its embrittlement during ageing at room temperature after a very short period of time (a few hours) significantly reduces its usability for advanced applications. Intense effort has been invested in improving its toughness via composition modification. However, this approach can worsen some other properties, make processing more difficult, and increase the carbon footprint. Therefore, fused deposition modelling (FDM) 3D printing was used to manufacture porous bamboo-inspired structures of unmodified PLA. The toughening of PLA solely by the pore gradient, which controlled the energy dissipation mechanism, was introduced for the first time. Improvement of the ductility and work at break was observed especially for notched specimens. Prevention of catastrophic failure could enable the use of gradient porous materials in structural components. The fundamental relationships and practical hints resulting from the work provide a foundation for the future design of toughened 3D printed structures

Biaxial porosity gradient and cell size adjustment improve energy absorption in rigid and flexible 3D-printed reentrant honeycomb auxetic structures

This paper compares different uniaxial and biaxial graded designs of auxetic reentrant honeycomb structures to enhance their mechanical properties, especially the specific energy absorption under compressive load. The lattice structures were 3D printed using the vat photopolymerization masked-stereolithography technique from two different materials – tough (OR) and flexible (FR). The results were evaluated from a material and structural point of view, investigating the effect of porosity, cell number, size, graded design, and fracture mode. The universally best energy-absorbing performance was found in a biaxially graded structure with a center-wise location of the highest local porosity. Depending on the used resin, its energy absorption capacity was up to 2–3 times enhanced compared to a reference uniform-porosity auxetic design. The presented data constitutes a fundamental understanding of auxetic structures and identifies practical approaches for tuning the auxetic structures’ performance regarding their mechanical response. Finally, this study demonstrates the potential of shape versatility offered by 3D printing and other additive manufacturing techniques

Antibacterial Electrospun Polycaprolactone Nanofibers Reinforced by Halloysite Nanotubes for Tissue Engineering

Due to its slow degradation rate, polycaprolactone (PCL) is frequently used in biomedical applications. This study deals with the development of antibacterial nanofibers based on PCL and halloysite nanotubes (HNTs). Thanks to a combination with HNTs, the prepared nanofibers can be used as low-cost nanocontainers for the encapsulation of a wide variety of substances, including drugs, enzymes, and DNA. In our work, HNTs were used as a nanocarrier for erythromycin (ERY) as a model antibacterial active compound with a wide range of antibacterial activity. Nanofibers based on PCL and HNT/ERY were prepared by electrospinning. The antibacterial activity was evaluated as a sterile zone of inhibition around the PCL nanofibers containing 7.0 wt.% HNT/ERY. The morphology was observed with SEM and TEM. The efficiency of HNT/ERY loading was evaluated with thermogravimetric analysis. It was found that the nanofibers exhibited outstanding antibacterial properties and inhibited both Gram- (Escherichia coli) and Gram+ (Staphylococcus aureus) bacteria. Moreover, a significant enhancement of mechanical properties was achieved. The potential uses of antibacterial, environmentally friendly, nontoxic, biodegradable PCL/HNT/ERY nanofiber materials are mainly in tissue engineering, wound healing, the prevention of bacterial infections, and other biomedical applications