Browsing by Author "Ruokolainen, Janne, Prof., Aalto University, Finland"

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Nano- and mesoscale structure control in block copolymers and dendritic supramolecules
(2013) Soininen, Antti
School of Science | Doctoral dissertation (article-based)
The topics of this thesis are self-organized templates, hierarchical structure-within-structure morphologies and induced chirality in solid state assemblies of dendritic molecules. They all contribute to our understanding of the mechanisms of structure formation in the nano- and mesoscale which is key to new materials with unparalleled properties and adaptive functionalities. In Publications I and II, the microphase separation of diblock copolymers is utilized to template a novolac-type resin to obtain porous carbonaceous material or flakes depending on the initial amount of resin in the mixture. The porous material readily exhibits mesoscale pores which enhance mass transportation while the amount of micropores and thus the total surface area can be controlled by pyrolyzation time. The flakes have high aspect ratio and may have either a solid surface or be covered by polymer brushes ("hairy" flakes) depending on the preparation method. In Publications III and IV, a second level of structural order is achieved by introducing side-chains to a diblock copolymer. These structures are studied in fixed lamellar morphology on the block copolymer level in bulk and in submicrometer particles. With low side-chain content, side-chains form a single layer sandwiched in the middle of the side-chain block lamellae. This structure changes gradually to liquid crystalline smectic layers ordered perpendicularly to the block domain interfaces as the side-chain content is increased. In the particles, the lamellae form an onion-like morphology except for the high side-chain content block copolymer where the onion-like structure breaks down due to increased splay deformation energy. Publication V explores the induction of chirality in solid state. Addition of chiral side-chains to three generations of three different types of dendritic molecules leads to nine homologous supramolecules. Structural and optical studies on these cases reveal that the final structures are indeed chiral and that the "strength" of the chirality depends on the quality of the structures. The low generation supramolecules with simpler structures result in better ordered and optically higher-quality morphologies than the higher generation and more complex molecules.
Thermally sensitive block copolymer hydrogels in bulk and with decreased dimensions
(2013) Nykänen, Antti
School of Science | Doctoral dissertation (article-based)
The research on stimuli-responsive polymers has increased rapidly during the last two decades. Poly(N-isopropylacrylamide) (PNIPAM) is one of the most studied thermally responsive polymer because its lower critical solution temperature (LCST) 32 ℃ is close to the ambient conditions. Below 32 ℃, PNIPAM is water soluble, but at temperatures above 32 ℃ the polymer phase separates from water. In this thesis, amphiphilic triblock copolymer polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene (PS-b-PNIPAM-b-PS) was used to prepare thermally-responsive hydrogels. We studied self-assembly and phase behavior of PNIPAM rich compositions of PSb-PNIPAM-b-PS in bulk as well as selected blends with low molecular weight PNIPAM homopolymers. In aqueous solutions, the glassy PS domains act as physical cross-links, and therefore hydrogels are formed. We observed, that the bulk block copolymer morphology has a strong effect on the degree of swelling in aqueous solutions below the LCST. Bulk compositions swelled in water up to 58 times by weight, whereas all the samples shrunk close to their dry state, after heating above 32 ℃ as a result of their thermally sensitive character. Hydrogel swelling and shrinking are diffusion based processes. The response time is directly proportional to the square of the size of the hydrogel, which makes bulk hydrogels unattractive for applications, which require fast response. To address this problem, we prepared thin films, submicron sized fibres, and spherical particles, whose morphology and behavior was characterized in water. It was found out that in thin films, the PS domains undergo extensive plastic deformation upon the gel swelling process, although temperature is always maintained well below the bulk PS glass transition temperature. In electrospun fibres and aerosol particles we also noticed, that the molecular architecture and molecular weight can cause stability issues, and the limits of the concept of physical cross-linking were reached. Finally, the applicability of these thermally sensitive gels was demonstrated by preparing thermally switchable molecular filters, by studying thermo-sensitive wetting transition and by evaluating thermally controlled release of an organic dye.