Aliphatic polyamide, commonly known as nylon, was the world’s first synthetic fiber and has found its largest application range in tires, carpets, stockings, upholstery, and adhesives. All polyamides have a recurring amide group (–CONH–) present in the molecular structure with hydrogen bonds between these recurring amide groups. In comparison to other polymers such as polyethylenes, polyamides have a high melting temperature. Although polyamides have been extensively studied by many research groups, much is still to be learned and achieved regarding these materials. The first main achievement reached in this thesis concerns a new and improved insight and understanding of the Brill transition seen in many polyamides. The Brill transition is a solid state crystalline transition observed in polyamides on heating. The Brill transition temperature is defined as the temperature at which the characteristic intersheet and interchain reflections observed in wide angle X-ray diffraction (WAXD) merge to a single reflection which is maintained up to the melt. The nature and mechanisms behind the Brill transition has been a matter of debate ever since it was first studied in 1942. The work presented in this thesis creates a better understanding of the mechanisms involved during the Brill transition, and how the Brill transition might be influenced by hydrogen bonding; a major factor influencing many polyamide properties. It appears plausible that the Brill transition would be influenced by hydrogen bonding, or more specifically, by a weakening of hydrogen bonding. By using a unique set of piperazine based copolyamides specially tailored to study the influence of hydrogen bonding on various (physical) properties, we are able to study how the Brill transition relates to hydrogen bonding. We show that the Brill transition is independent of the piperazine content, and therefore also independent of the hydrogen bond density. The Brill transition is caused by conformational changes in the polyamide main chain which cause the methylene units to twist, whilst hydrogen bonding is retained. When the methylene units next to the amide groups are able to twist sufficiently, the Brill transition is observed. The Brill transition is therefore not a classical first or second order transition, but a solid state crystalline transition driven by the crankshaft motions in the polyamide main chain. The work presented on the Brill transition has made a significant contribution towards completely understanding this transition. The use of specially tailored and designed copolyamides together with the use of many high quality analytical techniques proved essential to the successes achieved here. The work presented in this thesis combines the knowledge and expertise from two distinctly different, yet complementary fields in polymer research. The understanding gained from studying the Brill transition and the chain motions present in polyamides provide the possibility for understanding the influence of water, and more specifically the influence of superheated water, which is water above 100¿C, on polyamides in general. The second main achievement described in this thesis involves dissolving polyamide in water. We show that superheated water is a solvent for various (commercial) polyamides, including polyamide 4,6 and polyamide 6,6. The conformational changes in the polyamide during the Brill transition are key in the dissolution process, allowing highly mobile water molecules in the superheated state to penetrate the crystal lattice and break the hydrogen bonds between the amide groups. On crystallization from the water solution, which occurs upon cooling the solution, water molecules associate to the amide group in the crystal lattice, weakening the amide-amide hydrogen bonds. On heating the dried, water crystallized polyamide above the Brill transition, the water molecules are released from the crystal lattice and the hydrogen bonds are restored. The removal of water molecules at the Brill transition is typically observed by an exothermic event in differential scanning calorimetry (DSC) experiments performed on dried sedimented water crystallized polyamide crystals. The influence of water on the crystal lattice is observed very clearly for polyamide 2,14 where the water molecules incorporated in the crystal lattice cause a slip in the hydrogen bonded planes. This slippage results in the coexistence of a triclinic and monoclinic crystal structure observed in WAXD. On heating above the Brill transition temperature, the water molecules exit from the crystal lattice, and the polyamide shows only the triclinic crystal structure. The work presented in this thesis, especially the work related to the use of superheated water as a polyamide solvent, opens the door for an environmentally friendly processing route. A route in which water instead of organic solvents and acids are used to process polyamides. For the use of (superheated) water in processing applications such as film casting and recycling it is essential that the polyamide crystallization from the superheated water is a controlled process. Currently this is not the case; on cooling the polyamide/water solution the polyamide crystallizes from solution rapidly and uncontrolled when sufficient undercooling is obtained. The growth of large single crystals for example is hampered by this fast crystallization. The next step that needs to be taken is to control the crystallization, for example by adding salts to the solution, thereby preventing or manipulating the crystallization,33 even at room temperature. Dependent on the choice of ions and the requirements applicable to the applications under consideration, it would be possible to influence or suppress the crystallization. The possibilities for environmentally friendly polyamide processing using water-based technology are a promising prospect for the future.
|Qualification||Doctor of Philosophy|
|Award date||17 Sep 2008|
|Place of Publication||Eindhoven|
|Publication status||Published - 2008|