Browsing by Subject "polymeerikemia"

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  • Andersson, Mirja (Helsingin yliopisto, 2014)
    In dilute aqueous solution poly(N-isopropylacrylamide), PNIPAM, chain undergoes a coil-to-globule transition at its lower critical solution temperature, LCST, of ca. 32 °C. PNIPAM is one of the most studied polymers for instance for temperature controlled drug release systems, because its LCST is so close to body temperature. However, applicability of PNIPAM as gel-actuators or as active surfaces depends on the self-assembling microstructures and their physical properties. In this study several types of polymers based on PNIPAM (linear, microgels, macrogels, core-shell particles) were prepared and characterised with several methods. The first part of the research focused in developing a temperature controlled release system for a drug, isobutylmethylxanthine (IBMX), based on a PNIPAM system with tailored properties. It is concluded, that the prepared macroscopic PNIPAM -copolymer gels, with properties adjusted chemically by adding the aromatic esters groups (benzoates and cinnamates) to the structures, exhibited higher IBMX binding capacity than the unmodified PNIPAM gel in pure water above the LCST. The release rates of IBMX from the gels are slowed down by the aromatic moieties in the polymer network. The binding of IBMX to the polymers is concluded to be due to both the specific complex formation between aromatic moieties and IBMX, and to hydrophobic interactions inside the hydrophobically modified PNIPAM. In the second part of the research the structures of PNIPAM microgels synthesised with different concentrations of surfactant (SDS) and crosslinking monomer (MBA). Also PNIPAM microgels as shells on PS particles were studied. With high SDS concentrations during microgel synthesis, the precipitation of PNIPAM is prevented, and consequently tightly packed PNIPAM particle cores are not formed. In other words more homogeneously structured PNIPAM microgels are resulting. The concentration of MBA does not affect to the structure as dramatically as SDS, but the effect is clearly observed. Increasing hydrodynamic radius above the cloud point is observed with the increasing MBA concentration. This owes to the increasing size of the tightly crosslinked and rigid particle core. It is concluded that due to a relatively more rigid structure of the microgel at higher crosslinker concentrations, the volume phase transition broadens, and it is pushed towards higher temperatures. The enthalpy of transition is concluded to decrease with increased crosslinking density. Phase transitions and structural characteristics of microgels were further studied with 1H-NMR spectroscopy including the measurements of the signal intensities as well as the spin-lattice (T1) and spin-spin (T2) relaxation times for the protons of PNIPAM with changing temperature. When analysing the relaxation times, the broad temperature range of study is divided in two parts, to cases above and below the LCST. When the suggested significant structural changes with the MBA concentration, and especially with the SDS concentration are taken into account, the results can be rationalised. In the homogeneous microgel structure also the charges should be more evenly distributed compared to the corresponding heterogeneous microgel structure with highly charged surface and insoluble core. As the zeta potentials are also suggesting, the negatively charged coronal layers (with high local LCST) in the heterogeneous microgels are likely to contribute to the proton signals well above the LCST. According to the relaxation times from NMR-studies, it is concluded that there is more mobile structures of PNIPAM on PS core particles compared to the heterogenous microgel samples (a looser and/or more heterogeneous network structure). The results also show, that PNIPAM microgel shell on PS core inhibited the polymer-cell contact by steric repulsion similarly to PEO grafts, whereas PVCL coated PS was adsorbed on the cells more strongly, especially above the LCST. This result with a PNIPAM shell in the cell interaction study is in correlation with the observed high T2 values referring to mobile components existing in 2-stage particle samples still above LCST, and supports the idea of local high LCSTs of coronal PNIPAM layers in the outermost parts.
  • Hiltunen, Miia (Helsingin yliopisto, 2013)
    Cellulose is the most abundant renewable natural polymer, with many attractive physical and chemical properties including hydrophilicity, biodegradability and biocompatibility. However, its effective use as bio-based material is limited due to its insolubility in water and organic solvents, as well as due to difficult processability. The main objective of this research was to utilize the new controlled/living free radical polymerization (CRP) methods in the modifications of various cellulosic materials, yielding new water-soluble cellulose based graft copolymers (cellulose-g-copolymers) with a uniform molecular structures. The recent developments in CRP methods have enabled the tailoring of macromolecules with sophisticated architectures including block, graft and star structures with predetermined molecular weights, terminal functionalities, and narrow molecular weight distributions. Modification by graft copolymerization using CRP methods provides one of the best ways to combine the advantages of both natural cellulose and synthetic polymers and therefore affect the properties of the cellulose derivatives. The cellulose-g-copolymers have various potential applications over a wide range of areas, such as sensor matrices, recognition devices, selective membranes, organic-inorganic complex materials, and bioactive and biocompatible materials. In this study novel water-soluble cellulose-g-copolymers were successfully synthesized via CRP methods (RAFT and ATRP/SET-LRP). In addition to unmodified cellulose (softwood dissolving pulp), cellulose ethers (carboxymethyl cellulose (CMC) and ethyl hydroxyethyl cellulose (EHEC)) were also used as starting materials i.e. as backbones for the cellulose-g-copolymers. Homogeneous reaction conditions were used to confirm as uniform structure of the graft copolymers as possible. The influence of the side chain length and the grafting density on the aqueous solution and thermal properties of the graft copolymers were studied.
  • Karesoja, Mikko (Helsingin yliopisto, 2015)
    In this study several inorganic-organic hybrids and multiresponsive hybrid polymers were prepared and characterised in detail. Especially the focus has been on stimuli responsive materials but also on nanocomposites based on modified montmorillonite clay. Furthermore thin SiO2-capillaries were modified for electrophoretic separations. In all cases different controlled radical polymerisation techniques have been used. The modification of montmorillonite clay was conducted by surface initiated atom transfer polymerisation. Clay was grafted with random copolymer of butyl acrylate and methyl methacrylate and the modified clay was further mixed with a matrix polymer with the same chemical composition to create nanocomposite films. The relation of the nanocomposite structure to its mechanical properties was in the main focus. The extent of exfoliation of the clay in the composite films clearly affected mechanical properties. Montmorillonite clay was also grafted with pH- and thermoresponsive poly(2-dimethylaminoethyl methacrylate). The thermoresponsive properties of the resulting hybrid materials were compared to similar homopolymer. The inner walls of thin silica capillaries were grafted with a cationic polymer, poly([2-(methacryloyl)oxyethyl]trimethylammonium chloride) (PMOTAC). These capillaries were further used in capillary electrophoresis to separate standard proteins, different β-blockers and low-density as well as high density lipoproteins. The separation of the analytes was not possible with bare SiO2-capillaries but with polymer coated capillaries good separation of the analytes was achieved. Hybrid materials based on mesoporous silica particles grafted with poly(N-vinylcaprolactam-b-polyethylene oxide) (PVCL-b-PEO) were synthesised. The challenging synthesis of these hybrids was performed as a combination of surface initiated atom transfer polymerisation and click reactions. Thermal behaviour and the colloidal stability of these hybrid particles were studied. The role of the PEO block in the colloidal stability of the particles was crucial. Finally, multiresponsive hybrid block copolymers based on N-vinylcaprolactam and 2-dimethylaminoethyl methacrylate was prepared. The thermal properties of these block copolymers can be tuned by varying the chain length of PVCL block. On the other hand the thermal behaviour of PDMAEMA block is highly dependent on the environmental conditions like pH and ionic strength.
  • Pulkkinen, Petri (Helsingin yliopisto, 2014)
    Nanoparticles of gold, copper and copper sulfide with different kinds of protective ligands were successfully synthesized and characterized. Of special interest were particles protected with calixarenes, and their complexation with guest molecules. The copper sulfide nanocrystals were protected with benzyl thiol moieties that were generated by reducing RAFT chain transfer agent in the particle synthesis. The nanocrystals were characterized in detail and their sintering on paper was studied. The as-prepared crystals were in Cu2S (chalcocite) form, but thermal sintering resulted in the consolidation of the crystallites as well as the transformation of the material into a semiconductive Cu1.8S (digenite) film. The metallic copper nanoparticles were obtained by reducing copper salts in the presence of polyethylene imine (PEI) or analogous small organic compounds (tetraethylene pentamine, TEPA). In the case of the polymer, the strong complexation of the copper ions into the polymer matrix retarded the reduction reaction enabling the preparation of crystallites with less than 10 nm diameter. The particles were coated with a surface oxide layer that catalyzed the thermal decomposition of the PEI protecting layer, resulting in low sintering temperatures for the nanoparticles. The sacrificial decomposition of PEI resulted in reduction of the oxide layer back into metallic copper. The particles were sintered on paper and the layers were found to be semiconductive. Calix[4]arene protected gold nanoparticles were prepared and the interactions of pyridinium with the calixarene cavity was examined in great detail. It was found that the calixarenes bound to the gold surface are able to complex pyridinium cations, and that the complexation induced the aggregation of the nanoparticles. Further, the complexing ability of the calixarene could be tuned by preparing mixed monolayers of calixarenes and alkanethiols: key factors were the amount of calixarene in the mixed monolayer, the calixarene type and the length of the calixarene spacer, as well as the length of the alkanethiol chain. Finally, alkanethiol protected gold nanoparticles were prepared, inkjetted and sintered to form an electrode on paper. The results indicated the electrode was of comparable electronic quality to gold electrodes printed onto glass substrates. In this way, gold nanoparticles can be used in inexpensive roll-to-roll printed paper-based electronic platforms.
  • Nuopponen, Markus (Helsingin yliopisto, 2008)
    Controlled radical polymerization techniques, such as RAFT polymerization, are modern alternatives for preparing pre-designed polymers. In RAFT polymerization, chain length, molar mass distribution, microstructure (tacticity and sequencing), composition and functionality can be controlled. This allows the synthesis of a variety of novel polymer architectures, such as block and graft copolymers, stars, hybride materials and bioconjugates. The self-organization of synthetic preformed polymers into controllable nanostuctures is one of the most promising topics in the material science. However, the field of block copolymer self-assembly is still relatively young and current polymeric materials are structurally rather simple compared to biological materials. Thus, novel generations of polymer-based materials offer huge opportunities in material science. In this work, amphiphilic di- and triblock copolymers were synthesized by RAFT polymerization, and their organization into specific structures at nanoscale was studied. In all the block copolymer, one of the blocks was thermoresponsive poly(N-isopropylacrylamide). Thus, polymers and studied materials were temperature sensitive. In addition, control over tacticity in N-isopropylacrylamide polymerization was studied. The self-organization in aqueous solutions was strongly affected by the tacticity and the block sequence. Amphiphilic polymers formed various micellar structures in aqueous solutions. These micellar microcontainers have applications in controlled drug delivery. Amphiphiles have also applications as dispersants in coatings and cosmetics. In bulk, all the stable block copolymer morphologies were observed for triblock copolymers. Hydrogels of triblock copolymers can be used as thermoresponsive membrane materials. Polymers synthesized through the RAFT polymerization can be directly used in the synthesis of polymer grafted nanoparticles. Gold nanoparticles have attracted great interest due to the fact that gold is the most stable and inert noble metal possessing unique surface properties and good conductivity. It was shown that the association and optical properties of the gold nanoparticles grafted with smart polymers can be widely varied by pH and temperature. This type of gold nanoparticles have applications in diagnostics, sensors and cell imaging.
  • Niskanen, Jukka (Helsingin yliopisto, 2013)
    Binding of a flexible polymer chain to a solid surface or air-water interface affects its conformational freedom. A polymer covalently grafted to a surface can adopt three dimensional conformations, limited however by interactions with the surface and the neighboring chains. A dense grafting of polymers forces the polymers to adopt more elongated conformations than what they would take in solutions or amorphous solid state. On the other hand, strong interactions between the polymer and the surface cause the polymer to adsorb to the surface. The air-water interface is a two dimensional space. Also in this space, polymers are more elongated than in solutions. Certain polymers that are insoluble in water can form monolayers at the air-water interface. Water soluble polymers can be anchored to the surface with hydrophobic moieties, so that the polymers do not dissolve into the bulk of the solution during the deformation of the interface. In this work, controlled radical polymerization techniques have been utilized in the syntheses of polymer grafted gold, silver and clay nanoparticles. Gold nanoparticles were grafted with the well-known thermoresponsive poly(N-isopropyl acrylamide), PNIPAM, and poly(N-isopropyl acrylamide-co-N-propyl acrylamides), P(NIPAM-NPAMs). The particles were either dispersible or non-dispersible in water. Monolayers of the polymers and polymer grafted gold nanoparticles formed on an air-water interface were characterized using a Langmuir trough. Silver nanoparticles grafted with soft acrylate copolymers, poly(butyl acrylate-co-methyl methacrylate) were produced to be used in antimicrobial coatings. A block copolymer with an oligomeric acrylic acid block located at the surface of the silver nanoparticles proved to be an optimal choice. The short hydrophilic block promoted the dissolution of silver ions from the coating and also produced the most homogenous particles. Thermoresponsive properties of poly(dimethylaminoethyl methacrylate), PDMAEMA, are strongly affected by the grafting of the polymer to montmorillonite clay nanoparticles. PDMAEMA is a weak polyelectrolyte and thus the charge of the polymer chains can easily be tuned by altering the pH of the solutions. Increasing the charge of the polymer by lowering the pH of the dispersions, or increasing the relative amount of clay in the hybrid material, had significant effects on the thermo responsive properties of PDMAEMA. Both factors change the polymer-polymer and polymer-clay interactions. Increasing the isotacticity of the thermoresponsive polymers PDMAEMA and PNIPAM affects the phase transition at the lower critical solution temperature. In fact, PNIPAM loses its water solubility when the isotacticity is high enough. The effect of increased isotacticity on the phase transition of PDMAEMA was investigated by micro calorimetry and by measuring the zeta potentials of the polymers. The interfacial properties were looked upon by conducting surface tension and interfacial surface rheological measurements on aqueous solutions of both atactic and isotactic-rich PDMAEMA. The behavior of stereoblock polymers of isotactic-atactic PNIPAM was studied at the air-water interface using interfacial surface rheology. The block sequence and thus the different architectures of the polymeric micelles had a great influence on the interfacial properties.
  • Karjalainen, Erno (Helsingin yliopisto, 2015)
    This thesis combines anion responsive polymeric ionic liquids (PILs) with thermoresponsive polymers. The polymers have been synthesized with controlled radical polymerization methods. A water-insoluble PIL was used as a macro-chain transfer agent in synthesis of block copolymers with poly(N-isopropyl acrylamide) (PNIPAm). PNIPAm chains of various lengths were grown to the same PIL-block. These polymers show a lower cirical solution temperature (LCST) type behavior, typical to PNIPAm. The PIL block and the PNIPAm block interact strongly, no phase transition can be observed for the block copolymers with short PNIPAm chains. The block copolymers form complex aggregates in water. The hydrophobic PIL-homopolymer can be used to make stable particles in salt free water. Also triblock copolymers with a long central PNIPAm block and short water soluble PIL blocks were synthesized. These polymers also show interactions between the PIL and PNIPAm blocks. This can be seen for example as reduced enthalpy of phase transition for the triblock copolymers compared to the PNIPAm homopolymer. The triblock copolymers form complex aggregates at elevated temperatures. The LCST-type phase transtiton of weakly cationic poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) can be modified with bis(trifluoromethane)sulfonamide (NTf2) ions. The presence of NTf2 induces also an upper critical solution temperature (UCST) type transition for PDMAEMA, if the polymer is charged enough. NTf2 turns PDMAEMA to a stronger base, presumably due to the effective screening of charges. NTf2 induces an UCST-type transition for strong polycations in the presence of an added electrolyte. The phase behavior of the polycation-NTf2 system can be influenced by addition of sodium chloride. Similar transition can also be induced by trifluoromethanesulfonate (OTf), though OTf is needed in much higher concetrations. This allows the use of OTf as the only salt. The NTf2-polycation interactions influence the phase behavior of copolymers of N-isopropyl acrylamide (NIPAm) and strongly cationic (3-acrylamidopropyl) trimethylammonium chloride (AMPTMA). With low AMPTMA content, the copolymers show LCST-type behavior in the presence of NTf2 and a copolymer with high AMPTMA-content shows UCST-type behavior. If NIPAm and AMPTMA are copolymerized in nearly equal amounts, both transitions may coexist.
  • Strandman, Satu (Helsingin yliopisto, 2008)
    New and emerging technologies based on polymeric materials have increased the demand for more advanced, tailor-made polymers. The synthesis of well-defined polymers and complex polymer architectures has been greatly facilitated due to the recent developments in controlled radical polymerization techniques, which has opened up new possibilities also in the design and preparation of functional nanostructures based on the supramolecular assembly. In nature, numerous structures of varying complexity can be produced upon the self-assembling of individual molecules, such as lipids and proteins, by noncovalent interactions. Such molecules are often amphiphilic, i.e. they consist of both hydrophilic and hydrophobic moieties. Hence, an important class of synthetic polymers possessing similar self-assembling characteristics is amphiphilic block copolymers. Amphiphilic block copolymers are composed of covalently linked hydrophilic and hydrophobic polymer chains, leading to characteristic solution properties. In block selective solvents, these polymers tend to associate to micelle-like aggregates of various morphologies, which can transform from one to another when the solution conditions are changed. Depending on the morphology, the potential applications of the self-assemblies lie in various fields of nanotechnology, for example, in the preparation of nanoparticles of different shapes or in templating of inorganic structures for nanomaterials, as well as in the encapsulation and delivery of compounds like drugs, dyes, anticorrosion agents, flavors, and fragrances. Amphiphilic block copolymers have also been investigated for industrial applications as rheology modifiers, emulsifiers, stabilizing agents of latexes or flocculants. The most commonly utilized amphiphilic block copolymers are linear ones, but recently the research has been directed towards more complex architectures, such as starlike or graft copolymers. Such polymers may exist in aqueous solutions in their self-assembled form but also as single molecules, so called unimolecular micelles having a core-shell structure even at low polymer concentrations, which makes them particularly attractive for solubilizing or binding hydrophobic compounds. The term ‘unimolecular micelle’ could in fact describe the structure of starlike amphiphilic block copolymers, which consist of linear block copolymers tethered to one point. Understanding the association processes is vital for controlling the self-assembling behavior of various polymer architectures. Thus, the current work focuses on investigating the self-assembling characteristics of well-defined amphiphilic star polymers both experimentally and by computer simulations. The overall work can be divided in two sections: the one focusing on the synthesis of star polymers by macrocyclic initiators and the other concentrating on the characterisation of amphiphilic star block copolymers.