| 1. |
- Jain, Shubham, 1990-
(författare)
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Engineering 3D degradable pliable scaffolds for adipose tissue regeneration : Advancing cell-material interactions by understanding the influence from thermal, chemical, mechanical properties and scaffold design
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In soft tissue defects that arise due to trauma, tumor resections and complex burns, a significant loss in adipose tissue remains a considerable challenge due to the insufficient regenerative capacity of the tissue. This thesis focuses on assessing cell-material interactions between degradable 3D polymer scaffolds with different designs and adipose tissue-derived stem cells. This knowledge can be used to engineer 3D scaffolds with adequate physio-chemical and mechanical properties along with an appropriate design that augments adipose tissue regeneration.Salt-leaching 3D scaffolds were fabricated from various medical-grade polyesters, and cellular behavior was evaluated by correlating the physical, chemical, and mechanical properties of the scaffolds. The results showed that the glass transition temperature modulated the mechanical properties of the scaffolds, affecting stem cell proliferation and adipogenic differentiation. The same sets of polymers were further used in melt extrusion-based 3D printer and printability was established for the fabrication of customized 3D scaffolds. Based on printability and cell-scaffolds interaction results, poly (L-lactide-co-trimethylene carbonate) was used to print 3D scaffolds in different soft and pliable designs that promoted adipogenic differentiation. To fabricate even softer, and more hydrophilic 3D scaffolds, poly (ɛ-caprolactone-co-p-dioxanone) and a unique scaffold design were utilized within the research group. The copolymer 3D scaffolds were further combined with knitted mesh and electrospun nanofibers to develop scaffolds with multilayer architecture, modular scaffolds. The in vitro results asserted that the modular scaffold enhanced cell-material interactions by almost five times of those observed for the scaffold alone. Therefore, it can be concluded that softness and pliability are crucial and an appropriate scaffold design with adequate mechanical support is required for enhancing cell-material interaction. The in vitro results asserted that the modular scaffold enhanced cell-material interactions by almost five times of those observed for the scaffold alone. Therefore, it can be concluded that softness and pliability are crucial and an appropriate scaffold design with adequate mechanical support is required for enhancing cell-material interaction. The in vitro results asserted that the modular scaffold enhanced cell-material interactions by almost five times of those observed for the scaffold alone. Therefore, it can be concluded that softness and pliability are crucial and an appropriate scaffold design with adequate mechanical support is required for enhancing cell-material interaction.
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| 2. |
- Azwar, Edwin
(författare)
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Agro-Waste Derived Additives for Polylactide and Tapioca Starch
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Development of materials from renewable resources is one of the big challenges facing our world. In this thesis agro-industrial waste derivatives were developed and evaluated as additives for two common renewable polymer matrices, polylactide (PLA) and starch. Two waste products, wood flour (WF) and rice bran (RB) were evaluated in different forms. Milled WF and RB were either used directly to prepare PLA and starch biocomposites or they were liquefied by acid catalyzed hydrolysis to low molecular weight products. The complex polyol mixtures from liquefaction were tested directly as PLA and starch additives or utilized as monomers for synthesis of esters. The synthesized esters were evaluated as plasticizers for PLA and starch. The effect of different additives on tensile properties, miscibility, surface chemistry and morphology were evaluated by Instron, DSC, FTIR, FTIR imaging and SEM. In the case of polylactide films the influence of additives on hydrolytic degradation rate and process was evaluated by following the weight loss, surface changes, compositional changes and/or water-soluble migrants and degradation products by FTIR, SEM, pyrolysis-GC-MS and ESI-MS. The most marked difference in mechanical properties was observed in the case of PLA modified with liquefied wood flour derived ester plasticizer (PWF). Addition of 10 and 30 weight-% plasticizer increased the strain at break from a few percent for pure PLA to over 100 and 300%, respectively. The liquefied rice bran based ester, however, did not form miscible blends with PLA and it did not function as plasticizer. In some cases the impact of additives on the following degradation process was significant. Depending on the used additive, they could either concentrate in the matrix during the hydrolysis of polylactide or they were rapidly released to the surrounding water. In some cases clear hydrolysis catalyzing effects were observed. Liquefied rice bran in combination with mineral fillers and/or traditional plasticizers seemed to have the best potential as starch plasticizer.
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| 3. |
- Claudino, Mauro
(författare)
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Thiol−ene Coupling of Renewable Monomers : at the forefront of bio-based polymeric materials
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Plant derived oils bear intrinsic double-bond functionality that can be utilized directly for the thiol–ene reaction. Although terminal unsaturations are far more reactive than internal ones, studies on the reversible addition of thiyl radicals to 1,2-disubstituted alkenes show that this is an important reaction. To investigate the thiol–ene coupling reaction involving these enes, stoichiometric mixtures of a trifunctional propionate thiol with monounsaturated fatty acid methyl esters (methyl oleate or methyl elaidate) supplemented with 2.0 wt.% Irgacure 184 were subjected to 365-nm UV-irradiation and the chemical changes monitored. Continuous (RT– FTIR) and discontinuous (NMR and FT–Raman) techniques were used to follow the progress of the reaction and reveal details of the products formed. Experimental results supported by numerical kinetic simulations of the system confirm the reaction mechanism showing a very fast cis/trans-isomerization of the alkene monomers (<1.0 min) when compared to the total disappearance of double-bonds, indicating that the rate-limiting step controlling the overall reaction is the hydrogen transfer from the thiol involved in the formation of final product. The loss of total unsaturations equals thiol consumption throughout the entire reaction; although product formation is strongly favoured directly from the trans-ene. This indicates that initial cis/trans-isomer structures affect the kinetics. High thiol–ene conversions could be easily obtained at reasonable rates without major influence of side-reactions demonstrating the suitability of this reaction for network forming purposes from 1,2-disubstituted alkenes. To further illustrate the validity of this concept in the formation of cross-linked thiol–ene films a series of globalide/caprolactone based copolyesters differing in degree of unsaturations along the backbone were photopolymerized in the melt with the same trithiol giving amorphous elastomeric materials with different thermal and viscoelastic properties. High thiol–ene conversions (>80%) were easily attained for all cases at reasonable reaction rates, while maintaining the cure behaviour and independent of functionality. Parallel chain-growth ene homopolymerization was considered negligible when compared with the main coupling route. However, the comonomer feed ratio had impact on the thermoset properties with high ene-density copolymers giving networks with higher glass transition temperature values (Tg) and a narrower distribution of cross-links than films with lower ene composition. The thiol–ene systems evaluated in this study serve as model example for the sustainable use of naturally-occurring 1,2-disubstituted alkenes at making semi-synthetic polymeric materials in high conversions with a range of properties in an environment-friendly way.
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| 4. |
- Dånmark, Staffan
(författare)
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Polyester scaffold: Material design and cell-protein-material interaction
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Tissue engineering has emerged as a valid approach for the regeneration and restoration of bone defects. The concept of bone tissue engineering includes degradable scaffolds, osteogenic cells and osteoinductive growth factors either alone or in any combination of these three. The scaffold bulk material and its design, in particular, are essential for reaching clinically relevant treatments. It is essential that the scaffold is biocompatible and acts as a temporary extra-cellular matrix with a porous 3-dimensional structure, supporting adhesion, proliferation and differentiation of osteogenic cells. Yet another criterion of the scaffold is that is must have sufficient mechanical stability to maintain structural integrity and protect the cells with a gradual transfer of mechanical load to the developing tissue. At the same time, the scaffolds needs to be bioresorbable with a controllable degradation rate depending on its application and the rate of tissue regrowth. In this thesis, aliphatic polyester scaffolds have been modified and shown to be suitable for bone tissue engineering applications. In addition, a new microfluidic device for live imaging of cell behavior within porous 3-dimensional scaffolds has been developed. Highly porous and degradable aliphatic polyester scaffolds with varying pore sizes and interconnected pores were fabricated. The polyesters assayed were random co-polyesters poly(L-lactide-co-ε-caprolactone) [poly(LLA-co-CL)] and poly(L-lactide-co-1,5-dioxepan-2-one) [poly(LLA-co-DXO] and the homopolymer poly(L-lactide) [poly(LLA)]. The inherently different polymers yielded scaffolds with a wide range of properties with respect to surface chemistry, thermal properties, mechanical stability and degradation rate. The polyester scaffolds were shown to support the increased proliferation of bone marrow-derived stromal cells (BMSC) as well as enhanced osteogenic differentiation, with increased levels of osteocalcin gene expression, which emphasized their potential to act as cells carriers in bone tissue engineering. The potential of poly(LLA-co-CL) scaffolds and common biomedical polyesters in bone tissue engineering was further enhanced by surface functionalization. This involved two different methods of immobilization of bone morphogenetic protein-2 (BMP-2), a potent bone-growth-inducing factor, to the assayed polyesters. The first method used BMP-2 immobilized to heparin functionalized polyesters, while the second method covalently bonded BMP-2 to grafted linker groups on polyesters. Both immobilization techniques retain the bioactivity of BMP-2, and growth-factor-modified polyesters showed an increasing expression of osteogenic genes and production of osteocalcin in osteoblasts-like cells as well as increased proliferation in the mouse cell line, C3H10T1/2. The rate of degradation of electron-beam-sterilized polyester scaffolds and the subsequent loss of mechanical stability were strongly dependent on the chemical, physical and macroscopic architecture of the samples. The degradation rate and loss of mechanical integrity were much greater in porous scaffolds with hydrophilic co-monomers. By incorporating hydrophobic co-monomers with a limited ability to crystalize instead of hydrophilic co-monomers, the mechanical stability was retained for a longer time during the degradation process. The polyester supported spreading and flattened the morphology of both BMSC and osteoblast-like cells. The early cell adhesion to synthetic surfaces is mainly governed by the proteins adsorbed from its surrounding fluids. Early adhesion of BMSC to blood-plasma-coated polyesters was limited, despite the ability of the polyesters to adsorb adhesive proteins and expression of appropriate integrins on BMSC. However, adhesion to a purified adhesive matrix protein on the polyesters did occur, suggesting that pretreatment of polyester scaffolds with adhesive proteins or peptides is a feasible way to enhance the efficiency of cell loading into polyester scaffolds. Polyester scaffolds were combined with microfluidics and soft lithography to develop a new method for high-resolution imaging of live cells within porous scaffolds. The microfluidic device was used to frequently follow live cell proliferation and differentiation on the same spatial location within 3-dimansional porous scaffolds over a period of more than four weeks. This device is attractive for the evaluation of cells and materials intended for tissue engineering. We conclude that degradable aliphatic co-polyester scaffolds carefully designed with respect to macroscopic structure, bulk material and surface chemistry are able to meet the specific requirements of various bone tissue engineering applications. In addition, microfluidic devices permit reoccurring high resolution imaging of live cells within porous scaffolds and have a potential as a method of evaluating tissue engineering constructs.
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| 5. |
- Sun, Yang, 1983-
(författare)
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Engineering and Functionalization of Degradable Scaffolds for Medical Implant Applications
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The treatment of bone defects is facing the situation of lacking donations for autotransplantation. As a valid approach, scaffold-based tissue engineering combines the construction of well-defined porous scaffolds with advanced cell culturing technology to guide tissue regeneration. The role for the scaffold is to provide a suitable environment with a sufficient mechanical stiffness, supports for cell attachment, migration, nutrients and metabolite transport and space for cell remodeling and tissue regeneration. The random copolymers poly(L-lactide-co-ɛ-caprolactone) (poly(LLA-co-CL)) and poly(L-lactide-co-1,5-dioxepan-2-one) (poly(LLA-co-DXO)) have been successfully incorporated into 3D porous scaffolds to induce specific interactions with cells and direct osteogenic cell differentiation. In this thesis, these scaffolds have been modified in chemical and physical ways to map and understand requirements for bone regeneration. Scaffold functionalities and properties, such as hydrophilicity, stiffness, size/shape, and reproducibility, were studied. The hydrophilicity was varied by adding 3–20 % (w/w) Tween 80 to poly(LLA-co-CL) and poly(LLA-co-DXO) respectively, which resulted in contact angles from 35° to 15°. With 3 % Tween 80, the resultant mechanical and thermal properties were similar to pristine polymer materials. Tween 80 did not significantly influence cell attachment or proliferation but did stimulate the mRNA expression of osteogenetic markers. The surface functionality and mechanical properties were altered by introducing nanodiamond particles (n-DP) into poly(LLA-co-CL) scaffolds by means of surface physisorption or hybrid blending. Scaffold with n-DP physisorbed showed improved cell attachment, differentiation, and bone reformation. Hybrid n-DP/poly(LLA-co-CL) composites were obtained by direct blending of polylactide modified n-DP (n-DP-PLA) with poly(LLA-coCL). The n-DP-PLA was prepared by sodium hydride-mediated anionic polymerization using n-DP as the initiator. Prepared n-DP-PLA could be dispersed homogenously in organic solvents and blended with poly(LLA-coCL) solution. The n-DP-PLA particles were homogenously distributed in the composite material, which significantly improved mechanical properties. For comparison, the addition of benzoquinone-modified n-DP (n-DP-BQ) did not reinforce poly(LLA-co-CL). This indicated the importance of specific surface grafting, which determined different particle-polymer interactions. For the treatment of critical size defects, a large porous poly(LLA-co-CL) scaffold (12.5 mm diameter × 25 mm thickness) was developed and produced by molding and salt-leaching methods. The large porous scaffolds were evaluated in a scaffold-customized perfusion-based bioreactor system. It was obvious that the scaffold could support improved cell distribution and support the stimulation of human mesenchymal stem cell (hMSC) especially with dynamic flow in a bioreactor. To improve the scaffolding technique, a three-dimensional fiber deposition (3DF) technique was employed to build layer-based scaffolds. Poly(LLA-coCL) scaffolds produced by the 3DF method showed enhanced mechanical properties and a homogeneous distribution of human osteoblasts (hOBs) in the scaffolds. Although poly(LLA-co-CL) was thermally degraded, the degradation did not influence the scaffold mechanical properties. Based on the computerized design, a 3DF scaffold of amorphous copolymer poly(LLAco-CL) provides high-precision control and reproducibility. In summary, the design of porous scaffolds is one of the essential factors in tissue engineering as to mimicking the intrinsic extracellular environment. For bone tissue engineering, an optimized scaffold can maintain a contact angle greater than 35 degrees. Pristine or modified n-DP, introduced as an additive by surface physisorption or direct blending, can improve scaffold mechanical properties and cell response. Various sizes of scaffolds can be easily produced by a mold-mediated salt-leaching method. However, when 100 % reproducibility is required, the 3DF method can be used to create customizable scaffolds.
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| 6. |
- Tagami, Ayumu
(författare)
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Towards molecular weight-dependent uses of kraft lignin
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- There is growing demand for a more efficient use of polymers that originate from renewable feedstocks due to the depleting supply of fossil fuels, based on economic and environmental reasons. As a result, lignin has attracted renewed interest as a resource for various bioproducts. Lignin is a natural biopolymer with a high carbon content and is composed of aromatic moieties, with a high level of polar functionalities. This makes it a unique precursor for certain high-value applications, such as in biofuels, bioplastics, composite materials, carbon fibers and activated carbons and as a source of phenolic monomers and fine chemicals.Industrial lignins are formed as byproducts of pulping processes (such as kraft, sulfite or alkaline pulping) or result from the biorefining process, where carbohydrates are used for sugar production. Lignin’s intrinsic structure is significantly modified during the processing of lignocellulose, resulting in the formation of more diverse, condensed and less reactive raw materials. Since molecular mass and polydispersity are the most important parameters affecting the chemical reactivity and thermal properties of lignin, additional process steps to improve the quality of crude technical lignins, including kraft lignin, are needed. Solvent extraction is a potentially useful technique for further improving the polydispersity of technical lignins.This work summarizes the impact of solvent fractionation on the chemical structure, antioxidant activity, heating value, and thermal and sorption properties of industrial hardwood and softwood kraft lignins. The purpose was to understand the correlation between certain structural features in the lignin fractions and their properties to select the appropriate solvent combinations for specific applications of lignin raw materials.Four common industrial solvents, namely, ethyl acetate, ethanol, methanol and acetone, in various combinations were used to separate both spruce and eucalyptus kraft lignins into fractions with lower polydispersities. Gel-permeation chromatography analysis was used to evaluate the efficiency of the chosen solvent combination for lignin fractionation. The composition and structure of the lignin material were characterized by elemental analysis, analytical pyrolysis (Py-GC/MS/FID) and 31P NMR spectroscopy. The thermal properties of the lignin samples were studied by thermogravimetric analysis. Proximate analysis data (ash, volatile components, organic matter and fixed carbon) were obtained through the direct measurement of weight changes during the analysis, while the high heating values (in MJ/kg) were calculated according to equations suggested in the literature. The sorption properties of fractionated kraft lignins were studied with respect to methylene blue dye.Additionally, lignin fractions with different molecular weights (and therefore various chemical structures) that were isolated from both softwood and hardwood kraft lignins were incorporated into a tunicate cellulose nanofiber (CNF)-starch mixture to prepare 100% bio-based composite films. The aim was to investigate the correlation between lignin diversity and film performance. The transmittance, density and thermal properties of the films were investigated, as were their mechanical properties, including the tensile stress and Young’s modulus.This part of the study addressed the importance of lignin diversity on composite film performance, which could be helpful for tailoring lignin applications in bio-based composite materials based on the material’s specific requirements.
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| 7. |
- Undin, Jenny, 1985-
(författare)
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Functional Degradable Polymers by a Radical Chemistry approach
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One class of polymers that is inherently of great value for many applications is the aliphatic polyesters. Such polymers are very suitable for use as temporary guides, scaffolds, for tissue formation and other biomedical applications, due to their biocompatibility, degradability and appropriate mechanical properties. A prominent way to incorporate sites that allow alterations and modifications of the polymer backbone could be by copolymerization of functional monomers. The focus in this thesis is the development of new monomers and subsequent polymers bestowed with functional groups.Radical ring-opening polymerization (RROP) of cyclic ketene acetals through a free-radical mechanism presents an alternative route to conventional ring-opening polymerization for the synthesis of aliphatic polyesters. By RROP, it is possible to incorporate ester functionality into the backbone of non-degradable polymers by copolymerize cyclic ketene acetals with vinyl monomers.The possibility of creating materials with high degree of functionality is achieved by copolymerization with other and possible functional monomers. Three different copolymerizations including cyclic ketene acetals were performed. First, to increase hydrophilicity of a hydrophobic polymer by copolymerization of two cyclic ketene acetals, 2-methylene-1,3,6-trioxocane (MTC) and 2-methylene-1,3-dioxepane (MDO). Second, to introduce degradability into a non-degradable backbone by copolymerize MDO and vinyl acetal (VAc). Subsequently, the acetate side-group was hydrolyzed into the more hydrophilic alcohol group. Third, to introduce reactive functionalities into the degradable backbone of poly(2-methylene-1,3-dioxepane) (PMDO), by copolymerize MDO and glycidyl methacrylate (GMA). The epoxide side-groups, originating from GMA, were subsequently used in post-polymerization reactions by coupling with the bioactive molecule heparin.The degradability of this class of copolymers was evaluated using the MDO/GMA-based material as model, showing that the materials degrade during 133 days without a rapid release of acidic degradation products or any substantial lowering of the pH. Methylthiazol tetrazolium (MTT) assays were also performed to confirm the innocuousness of the material. The results from the degradation study together with the MTT assays showed that these materials would be interesting for use in biomedical applications.Finally, a combination of controlled radical polymerization with controlled ring-opening polymerization was performed. α-Bromo-γ-butyrolactone (αBrγBL) together with ε-caprolactone (εCL) or L-lactide (LLA) was successfully copolymerized to achieve copolymers with active and available grafting sites for single electron transfer living radical polymerization (SET-LRP). Different acrylates, ranging from the hydrophobic n-butyl acrylate and methyl methacrylate to the hydrophilic 2-hydroxyethyl methacrylate, were subsequently grafted via SET-LRP. All designated acrylate monomers were successfully grafted onto the polymer backbone, thereby emphasizing the versatility and ability of αBrγBL to act as a bridge between SET-LRP and ROP for a wide range of monomers.
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| 8. |
- Wistrand, Anna, 1971-
(författare)
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Mellan närhet och distans : Lärar-elevrelationen som kunskapsinnehåll i grundlärarutbildning
- 2023
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis takes its point of departure in a relational perspective on education, which means that the relationship between teacher and student is seen as fundamental for students’ knowledge- and personal development. The aim is to contribute to a deeper understanding of the teacher-student relationship in teacher education at national and local levels and the different meanings of closeness in this context. The theoretical basis of the study is curriculum theory-based didactics, where questions about knowledge content and teaching as offering meaning are central. The thesis consists of three separate studies. The first investigates closeness in the teacher-student relationship by means of a thematic conceptual analysis of the field of research in educational science on teacher-student relationships. Four different dimensions of closeness were identified: closeness as presence, closeness as care, closeness as feeling and closeness as responsibility. The results show a nuanced way of understanding closeness in the teacher-student relationship and that closeness is challenging for teachers in that it makes their relational work more complex.The central question in curriculum theory - ‘What opportunities do students have to learn about the teacher-student relationship in teacher education?’ - is used as the point of departure in the analysis of educational policy documents at national and local levels. The second study investigates content knowledge at the national level and shows a limited and simplified view of the relationship between teacher and student. By using the dimensions of closeness as analytical tools, a multifaceted teacher-student relationship is constructed as an alternative meaning offering. In the third study, eleven Swedish universities participated in the study of curriculum documents at the local level. The analysis showed three types of knowledge content: relational theory, relational work and relational quality. In this study, the local documents together provided a broad range of knowledge content, but individually a reduced and simplified knowledge content. The study also confirmed the differences between universities in their teacher education programmes and that there are often discrepancies in the knowledge content that is offered.The thesis as a whole contributes to a nuanced understanding of closeness in the teacher-student relationship and provides a set of theoretical tools that can be used in an analysis of different types of research studies. It also indicates that the policy documents relating to teacher education at the national and local levels contain simplified and limited notions about the teacher-student relationship and teachers’ relational work as knowledge content. This restricted knowledge content could limit student teachers’ abilities to create, build and maintain good and close teacher-student relationships and to address the relational challenges in their complex relational work.
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| 9. |
- Fall, Andreas
(författare)
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Cellulose nanofibril materials with controlled structure : the influence of colloidal interactions
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nanoparticles are very interesting components. Due to their very large specific surface area they possess properties in between molecules and macroscopic materials. In addition, a material built up of hierarchically assembled nanoparticles could obtain unique properties, not possessed by the nanoparticles themself. A very interesting group of nanoparticles is the cellulose nanofibrils. The fibrils are found in various renewable resources such as wood, bacteria and tunicates. In this work fibrils extracted from wood is studied. In wood the fibrils are the smallest fibrous component with the approximate dimensions; 4 nm in width and length in the micrometer range, providing a high aspect ratio. In addition, they have a crystallinity above 60% and, hence, a high stiffness. These fibrils are hierarchically ordered in the wood fiber to give it its unique combination of flexibility and strength. The properties of the fibrils make them very suitable to be used as reinforcement elements in composites and, due to their ability to closely pack, to make films with excellent gas barrier properties. The key aspect to design materials, efficiently utilizing the properties of the individual fibrils, is to control the arrangement of the fibrils in the final material. In order to do so, the interactions between fibrils have to be well characterized and controlled. In this thesis the interaction between fibrils in aqueous dispersions is studied, where the main interactions are attractive van der Waals forces and repulsive electrostatic forces. The electrostatic forces arise from carboxyl groups at the fibrils surface, which either are due to hemicelluloses at the fibrils surfaces or chemically introduced to the cellulose chain. This force is sensitive to the chemical environment. It decreases if the pH is reduced or if the salt concentration is increased. If it is strongly reduced the system aggregates. In dilute dispersions aggregation causes formation of multiple clusters, whereas in semi-dilute dispersions (above the overlap concentration) a volume filling network, i.e. a gel, is formed. The tendency of aggregation, i.e. the colloidal stability, can be predicted by using the DLVO theory. In this thesis DLVO predictions are compared to aggregation measurements conducted with dynamic light scattering. Good agreement between experiments and the designed theoretical model was found by including specific interactions between added counter-ions and the carboxyl groups of the fibrils in the model. Thus, the surface charge is both reduced by protonation and by specific interactions. This emphasizes a much larger effect of the counter-ions on the stability then generally thought. Hence, this work significantly improves the understanding of the interfibril interactions in aqueous media. As mentioned above, the fibrils can be physically cross-linked to form a gel. The gelation is an instant process, occurring at pH or salt levels causing the interfibril repulsion to decrease close to zero. If a well dispersed stationary dispersion is gelled, the homogenous and random distribution of the fibrils is preserved in the gel. These gels can be used as templates to produce composites by allowing monomers or polymers to enter the network by diffusion. In an effort to mimic processes occurring in the tree, producing materials with fibrils aligned in a preferred direction, the ability to form gels with controlled fibril orientation were studied. Such networks were successfully produced by applying strain to the system prior or past gelation. Orientation prior gelation was obtained by subjecting the dispersion to elongational flow and freezing the orientation by “turning off” the electrostatic repulsion. Orienting the fibrils after gelation was achieved by applying shear strain. Due to the physical nature of the crosslinks, rotation in the fibril-fibril joints can occur, enabling the fibrils to align in the shear direction. This alignment significantly increased the stiffness of the gels in the shear direction.
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| 10. |
- Guo, Baolin, 1981-
(författare)
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Synthesis, characterization and molecular architecture of electroactive and degradable polymers
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The third-generation biomaterials are designed to stimulate specific cellular responses at the molecular level. Recent studies have shown that electrical signals regulate cellular activities including cell adhesion, migration, proliferation and differentiation. One of the biggest limitations for conductive polymers in tissue engineering applications is their inherent inability to degrade, so the incorporation of conducting polymers into biodegradable polymers to obtain electroactive and biodegradable materials is still a challenge. Architecture plays an important role on the performance of polymers. To achieve the optimal mechanical, degradation, thermal and biological properties for each biomedical application, it is desirable to promote architectural diversity. To combine the electroactivity of conductive polymers and the degradability of aliphatic polyesters, linear, star-branched and hyperbranched copolymers based on Poly(L,L-lactide) (PLLA), Poly(ε-caprolactone) (PCL), and aniline oligomers were synthesized by coupling reactions between the hydroxyl group at the chain end of the PLLAs or PCLs and the carboxyl group of the aniline oligomer, using the N, N’-dicyclohexyl carbodiimide / 4-dimethylaminopyridine (DCC/DMAP) catalytic system. The chemical structures of the polymers obtained were fully characterized by Fourier transform infrared spectroscopy, nuclear magnetic resonance, and size exclusion chromatography. The cyclic voltammetry and ultraviolet spectra of the copolymers demonstrated their good electroactive properties. Differential scanning calorimetry and thermogravimetric analysis studies showed the copolymers were more thermal stable than the corresponding PLLAs and PCLs. The wettability of the copolymer film increased sharply after doping with acid. The copolymers also exhibit much better processibility than conductive polymers because they are soluble in most organic solvents. Macromolecular architecture design as a useful tool to enhance the conductivity of degradable polymers has been presented. The hyperbranched copolymers showed a higher conductivity than that of the linear ones with the same content of conductive segments. It is proposed that the higher conductivity of the hyperbranched copolymers is due to the ordered distribution of peripheral emeraldine state of aniline pentamer (EMAP) segments. Thus, the conductivity of the polymers is controlled by the macromolecular design. In other words, the conductivity of the polymers was increased with the same content of aniline oligomer by macromolecular architecture.The copolymers with different architectures could be used to tailor the thermal properties, degradation properties and surface properties, to give materials that are favorable for the growth of electrically excitable cells in tissue engineering.
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