| Polymer crystallization is not only an important theoretical topic of phase tran-sition in polymer physics,but also a practical subject in modern industry,where the crystallization of polymers provides the necessary strength to polymer materials and enables them to be widely used in daily life.Polymers form typical motif structures like small molecules during crystallization.Meanwhile they also show unique features to form lamellar crystals,which results from the basic chain topology.The thermal stability of polymer lamellae decides the performance and usage of polymer materials.Even a slight change of chain structure will result in tremendous variance upon the thermal stability of polymer crystals.The study on the chain structural dependence of the thermal stability of polymer lamellae will improve our understanding of polymer physics and help us to control the properties of polymer materials,refine the procedure in industry and invent new kinds of polymers.The basic topology feature of polymers is their chain-like structure raised from the covalent connection of monomers.Polymer chains perform random coil confor-mations in melt with the size much smaller than the contour length of the fully ex-tended chains.Such random coil simply chooses the kinetic advanced route to make folded-chain lamellar crystals when crystallizes,while the lamellar crystal is in fact a thermodynamic metastable state.The tendency of evolvement towards the thermo-dynamic stable state,the extended-chain crystals,drives the lamellae to change their morphology and properties by chain sliding,especially during annealing or melting upon high temperatures.Modification of the chain structure can be realized either by copolymerization of different monomers,which changes the intramolecular structure,or by cyclization of the whole chain,which changes the intermolecular topology.The former one will make the polymer crystallization interplay with the phase separation of different components;while the latter one will change the chain conformations and result in different thermodynamic conditions for polymer crystallization.The precise definition of polymer structures and the accurate observation of mi-croscopic details of crystals are commanded in order to study the influence of chain structures on polymer crystallization.In this aspect,computer simulations can be an excellent tool.In recent decades,the dynamic Monte Carlo simulation techniques in lattice space have been proved to be an efficient method to study polymer crystalliza-tions and have already had fruitful results.In this thesis,we use this dynamic Monte Carlo method to study some problems about how the chain structure influences the thermal stability of polymer crystals.These problems include the thickening of poly-mer crystals via chain sliding during annealing,the superheating phenomenon during polymer melting,the special memory effect for the crystallization of statistic random copolymers,and the crystallization of cyclic polymers.In Chapter 1,we give a brief introduction on the relationship between the chain structure and polymer crystallization.The basic chain-like structure of polymers can be described by the Gaussian distribution of chain conformations.In copolymer sys-tems,where the intramolecular structure is modified,the chain conformation will be influenced by the phase segregation of multicomponents.In polymers with different outline shape,like cyclic polymers,the conformation can be treated as the Gaussian chain with additional constrains.The process of polymer crystallization is determined by both the thermodynamic and dynamic conditions of polymers.The former one pro-vides the driving force and the latter one influences the route and rate of crystallization separately.We introduce some existing thermodynamic and dynamic models of poly-mer crystallizations.In the last part,we come to the relationship between the polymer structures and polymer crystal structures.Some questions on how the polymer struc-ture influences the polymer crystallizations,which will be discussed in this thesis,are listed up.In Chapter 2,the computer simulation method is introduced.Varieties of comput-er simulation methods have been developed to meet the balance between the computing accuracy and efficiency under different scales.We then introduce the dynamic Monte Carlo model in lattice space of polymer crystallization,which will be used in this the-sis.In this model,the most important energy aspects for polymer crystallizations are considered.The parallel packing energy term for polymer bonds is introduced to drive the polymer crystallization,which is the result of the anisotropic interaction in polymer system.In Chapter 3,we study the thickening of linear polymer lamellar crystals via chain sliding.Three typical models are built.They are the lamellar thickening of long-chain polymers in bulk,the thickening of once-folded-chain lamellae into extended-chain lamellae of short-chain polymers in bulk and the thickening of once-folded-chain lamellae into extended-chain lamellae of short-chain polymers in a ultra-thin film on a substrate.These three cases illustrate the three aspects to determine the polymer thickening processes:the chain mobility in crystals,the possible existing nucleation process and the crystallinity harvest during the thickening process.These three cases give different temperature dependence of thickening rates,and reflect different kinetic mechanisms.In Chapter 4,the superheating phenomenon during polymer melting is studied by a combination of computer simulations and FlashDSC experiments.When heat-ing rate is relatively low,polymer chains can generate thicker crystals during heating via a melting-recrystallization process.The superheating of melting point shows no power law dependence on the heating rate.When the heating rate is medium,polymer crystals can be perfected by chain sliding during heating.The superheating showed a power law dependence on the heating rate with its index sensitive to the chain sliding abilities.When the heating rate is fast,a power law dependence of superheating on the heating rate with a uniform index of 0.37 is found in computer simulations.It implies the melting mechanism is independent with the chain sliding process in such limited situation.The value of index is the result of a combination of roughening melting and nucleation processes on the polymer lateral surface.In Chapter 5,the strong memory effect of homogeneous random copolymer is studied.The comonomers are homogeneously and randomly distributed in this kind of polymers.No macroscopic phase separation or micro-phase separation can happen here.The sequence length of homogeneous random copolymer decides the crystal-lization ability of such sequence.The sequence segregation can happen based on their different crystallization abilities which is trigged by a first-time crystallization.Once the segregation happens,the concentrated sequences change the local environment of system and increases the local melting temperature.The trace of such segregation can only be totally erased by melting under the temperature which is higher than the original equilibrium melting temperature of the system with sequences uniformly dis-tributed.If the trace of segregation was not totally vanished,the crystallization of ran-dom copolymer will show significant memory effect of an accelerated crystallization process.In Chapter 6,we study the crystallization of short cyclic polymer chains.We calculate the free energy change for the crystallizations of single polymer chains and find the cyclic polymer shows higher equilibrium melting temperature compared with the linear polymer.This makes the cyclic polymers get higher supercooling and faster crystallization rates at the same temperature.This phenomenon can be understood by the conformational entropy loss when forming cyclic topology.The entropy change for cyclic polymer crystallization is smaller and a higher equilibrium melting temperature is got.In the last part,a summary of this thesis and a perspective of further study were given. |
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