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|Title:||Dielectric and Ultrasonic Studies of Macromolecular Growth During Polymerization|
|Authors:||Parthun, Giles Matthew|
|Keywords:||Materials Science and Engineering;Materials Science and Engineering|
|Abstract:||<p>Measurements by dielectric spectroscopy, ultrasonics and calorimetry ofseveral low viscosity monomeric liquids undergoing spontaneous chemical reaction, to form three new, linear chain polymers under isothermal conditions, have been used to determine how the number ofcovalent bonds formed during the growth of a linear chain affects the dielectric and ultrasonic properties, their respective relaxation times, and their spectral shape. The dielectric properties changed in the following manner. During this reaction, the static permittivity decreased and the relaxation time increased towards limiting values. As the number of covalent bonds increased towards the Avogadro number, the change in the complex permittivity as measured for a fixed frequency was phenomenologically similar to that observed on varying the frequency, although the exact formalisms in both cases differed. In both cases the relaxation function could be well described by a stretched exponential or sum ofexponentials, characterized by a temperature and system dependent exponent that decreased as the state of the system changed from a monomeric liquid to a fully reacted polymer. At later stages of chemical reaction a second relaxation process at higher frequencies is revealed. The dielectric manifestation of the irreversible process of covalent bond formation is remarkably similar to that observed on supercooling a molecular or polymeric liquid.</p> <p>Longitudinal velocity and attenuation of ultrasonic waves travelling through the three molecular liquids at different temperatures have been measured as its molecules combine irreversibly to form large entities and thereby decrease the diffusivity and increase the configurational restrictions to their dynamics. From these data, the longitudinal modulus and compliance are calculated, and the molecular relaxation time and related properties are deduced and interpreted in terms ofthe number of covalent bonds formed, by a formalism that connects the size ofthe molecules in the liquid with its elastic behaviour. This relaxation time increases monotonically with increase in the molecule's size, tending to infinity as the number ofcovalent bonds formed approaches Avogadro's number. The complex plane plots ofthe modulus and compliance have a shape which is described by a skewed arc function, with a temperature dependent exponent ϒ, that ranges in values from 0.33- 0.31 for modulus and 0.39-0.45 for compliance. Departure from this shape is shown to be due to contributions from non-zero shear viscosity for relatively small size of molecules, and contributions from a faster, or sub Tg-relaxation process when the molecular size is large, which is similar to the behaviour for the dielectric properties. Simulation of the data suggests that this sub Tg-relaxation process, which is progressively more separated from the main relaxation process as the molecular size increases, contributes significantly to the high frequency elastic properties. The measured longitudinal modulus has been deconvoluted to show that the increase in the bulk modulus, and not the shear modulus, dominates the elastic properties when the molecular size increases. Comparison ofthe calculated relaxation times for the longitudinal modulus and compliance with the dielectric relaxation time show that the compliance and dielectric data change in a remarkably similar manner with increasing time of chemical reaction, which is unexpected owing to their different mechanisms.</p> <p>In the last part of this work, the dipolar diffusion in the glassy and supercooled liquid states of 9 additional molecular liquids and oftheir linear chain or network polymerized states formed by condensation-polymerization at different temperatures and times have been studied by measuring the dielectric properties for a fixed ac frequency of 1 kHz. The study showed that as the extent of polymerization increased with increasing isothermal temperature of polymerization, the sub-Tg relaxation peak due to localized molecular motions in the molecular state became gradually extinct, and a corresponding peak at a higher temperature evolved and reached its maximum height. The temperature of the sub-Tg relaxation peak in the polymerized state differed from that of the α-relaxation peak of the supercooled molecular liquid by as much as 70K, but, in several cases, the two temperatures were similar. Reasons for the latter occurrence are given in phenomenogical terms. It is concluded that the localized relaxation modes of the polar segments of the macromolecule are not related to the modes of molecular diffusion in the monomeric liquid state above its Tg. The localized relaxation characteristic of the glassy molecular state persists in the incompletely polymerized state, where it is seen as a ϒ-relaxation.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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