Copolymers are macromolecules composed of linear or non-linear arrangements of chemically different polymeric chain parts. In most cases the different constituting blocks are incompatible, giving rise to a rich variety of well-defined self-assembled structures both in bulk and in selective solvents. These self-assembled structures may form the basis for applications ranging from thermoplastic elastomers to information storage, drug delivery and photonic materials. As a result, there is a continuous investigation of the self-assembly process as well as of the response of these materials to external stimuli. Therefore, it is not surprising that these materials play an important role in contemporary macromolecular science, covering the full spectrum of polymer chemistry, polymer physics and applications. In the present thesis, copolymers of amorphous polystyrene and aliphatic or aromatic polyamide units, having various structures (diblock, multiblock and graft copolymers) were synthesized and their structure-properties relationships were investigated. These copolymers were applied in order to verify a so-called "sticky-blocks" concept, which aims at designing materials with improved processabilty as compared to high molecular weight industrially used polystyrenes. The principle of "sticky-blocks" lies in decreasing the molecular weight of polystyrene (thus improving melt flow) and compensating for the loss of entanglements per individual macromolecule, by creating hydrogen-bonded, semi-crystalline polymeric networks, which would have similar properties as the high molecular weight homo-polystyrenes. Polyamides we chosen to function as "sticky-blocks" due to their relatively low melt viscosity (200-400 Pas) and optimal mechanical and thermal properties at relatively low molecular weights (20,000-30,000 g/mol), inherent in the presence of relatively strong hydrogen-bond interactions. We demonstrated in this thesis that, by using aromatic polyamides (T6T6T) of (reported) high crystallinity and stability of the crystalline phase, segmented multiblock copolymers of polystyrene and T6T6T with molar masses up to around 40,000 g/mol can be relatively easily prepared. However, due to the very high incompatibility of the two phases, the synthesized multiblock copolymers displayed rather weak crystals and premature phase separation (presumed via liquid-liquid demixing), and the desired semi-crystalline network structure could not be obtained. Nevertheless, the thermal stability and moduli improved considerably, not only compared to neat PS of similar molecular mass (50,000 g/mol), but also compared to commercial PS with Mw ˜ 200,000 g/mol. Aliphatic polyamides (polyamide-6) were also used as "sticky blocks" in the preparation of diblock and graft copolymers. The diblock copolymers of polystyrene and polyamide-6 had a maximum molecular weight of 20,000 g/mol and were prepared via anionic polymerization of e-caprolactam, starting from PS end-functionalized macroinitiators. These semi-crystalline materials were presumed to be well-flowing (lack of entanglements), but were too brittle to possess measurable mechanical properties. Therefore, for achieving less brittle copolymers, graft copolymers of higher molecular weight (Mn up to 100,000 g/mol) were made via combined ATRP and reactive processing. The graft copolymers seemed the most promising materials to be used for achieving the goal of this thesis. As revealed by thermo-mechanical analyses (DMTA) and rheology, relatively stable crystalline polymeric networks can be formed. Moreover, the graft copolymers had a lower viscosity in the melt under injection moulding shear conditions, while maintaining and even improving some properties of commercial PS. Summarizing, by using this approach, the goal set for decreasing the molecular weight of the PS (thereby improving its flow properties) and compensating for the loss of entanglements per PS macromolecule by introducing "sticky blocks", can most probably be achieved when some further improvements suggested in this thesis can be realized.
|Qualification||Doctor of Philosophy|
|Award date||30 Nov 2009|
|Place of Publication||Eindhoven|
|Publication status||Published - 2009|