Ρεολογία τηγμάτων πολυμερικών δακτυλίων : προσομοίωση μοριακής δυναμικής εκτός ισορροπίας και θεωρητική ανάλυση
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Date
2024-06-20
Authors
Παπαργυρίου, Χαράλαμπος
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Abstract
Cyclic polymers are compounds that stand out due to their closed-loop geometry. Unlike their linear counterparts, they do not contain free ends in their stereochemical structure, a feature that gives them unique properties, which in many cases are superior (and particularly desirable by industry) to those of linear polymers. The aspects of everyday life where cyclic macromolecules find broad application are numerous and diverse. The compact and more rigid structures they form are more resistant to degradation from acidic environments, and when used as drug carriers, they exhibit longer circulation times in the blood, resulting in more efficient transport and targeted release of the active substances they encapsulate [1], [2]. Additionally, their use in electronic applications, such as photolithography, is very promising, as in environments with topological constraints (e.g., film arrangements), they exhibit significantly smaller changes in their glass transition temperature even with changes in film thickness by an order of magnitude [3], [4], [5], [6]. They also appear to be more effective molecular switches, as they exhibit faster kinetics in photoisomerization reactions in both UV and visible light [7], [8].
The synthesis of these compounds is primarily achieved through either the ring-closing method [9], [10] or the ring-opening method [11], [12], [13]. Both methods lead to mixtures of cyclic polymers that are more or less admixed with linear polymers of corresponding chemical structure. Their properties differ from those of their linear counterparts in terms of spatial extent, dynamics, configurational, and diffusional behavior. Notably, under conditions of binary mixtures of cyclic-linear polymers, the properties of cyclic polymers show further differentiation, which varies with the volume-by-volume composition of the mixture. Even at a linear polymer composition of 1%v/v, the properties of cyclic polymers are dramatically affected [14]. Identifying the molecular mechanisms governing the change in properties of cyclic polymers is a cornerstone in designing materials with predetermined properties that will more effectively meet societal needs.
The purpose of the forthcoming thesis is to contribute to the study of the structural, dynamic, and diffusional properties of cyclic polymer chains, whether they are in complete purity or in mixtures with minor linear "contamination" . This is an area where systematic research has not been conducted, as studies have mainly focused on mixtures with high linear concentrations . For this study, atomistic molecular dynamics (MD) simulations of poly-ethylene oxide (PEO) systems were conducted at molecular weights greater than the entanglement molecular weight of the corresponding linear polymer (MW = 10,000 – 40,000 g/mol). To this end, the NPT ensemble was chosen, and both the temperature and pressure were kept constant at 413 K and 1 atm, respectively, so that the simulation results could be directly compared with available experimental data under the same conditions.
The trajectories of the atomistic simulations were analyzed in detail to study all the aforementioned properties. Starting from the radius of gyration, which is a measure of the spatial extent of the chains, the analysis indicates a slight swelling of the cyclic macromolecules, which is independent of the concentration of linear "impurities" in the polymer mixture (for small contaminations up to 10%v/v). For pure cyclic melts, the gyration radius shows a power law dependence on the molecular weight of the polymer molecules of the form , consistent with available small-angle neutron scattering (SANS) experimental data. In the study of the rotational relaxation of cyclic PEO molecules, it was found that the increase in linear admixture causes longer relaxation times in cyclic PEO with the same molecular weight. This phenomenon is due to the hindrance of the free movement of the cyclic molecules caused by interpenetration phenomena from their linear counterparts. This topological constraint is quite strong and, in several cases, was found to persist for times much longer than the relaxation time of the cyclic chains. Furthermore, the increase in molecular weight of the polymers results in longer relaxation times, as the polymer chains acquire more degrees of freedom and interactions, making their reorganization and disengagement from these specific topological constraints more difficult. From the analysis of the SANS spectra and Kratky plots, it was found that small linear contaminations do not affect the configuration of segments or the entire cyclic macromolecule. Analysis of the dynamic structure factor (S(q,t)) for pure cyclic melts and comparison with experimental neutron spin echo spectra showed that MD simulations accurately describe the dynamic behavior of both segments and the entire chain. Linear admixtures significantly affect the dynamics across nearly the entire range of segment lengths of the cyclic molecules, except for small segments of a few bonds in length. Finally, data describing the diffusive behavior of polyethylene oxides were analyzed. From the results of the mean square displacement of the center of mass of the cyclic polymers, it was observed that the increase in the concentration of linear homologues, hinders not only the rotational relaxation but also the diffusion of the cyclic chains. This phenomenon is attributed to topological constraints due to the interpenetration phenomena of cyclic molecules by the linear ones. Lastly, the comparison of the diffusion coefficient values from the simulation shows agreement with the experimental data, as the experimental data fall within the uncertainty range of the measurement. Overall, excellent agreement was found between simulation and experiment, confirming that the structure and dynamics of cyclic PEO in both pure melts and mixtures with linear chains can be accurately predicted using atomistic molecular dynamics simulations.
The above conclusions from this thesis aim to serve as the basis for future systematic study of the behavior of cyclic polymers in conditions of pure solution or binary mixtures with minor linear "contamination." The successful understanding of both the effect of the presence of linear polymers and the scaling laws of properties with the variation of the molecular weight of polymer chains may lead to the optimal understanding of the behavior of cyclic polymers in their entirety, improving our understanding of such physical systems.
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Πολυμερή, Προσομοίωση, Δακτύλιοι, Πολυ-αιθυλεν οξείδιο