| Inertial confinement fusion(ICF),as one of the most promising methods to achieve controllable nuclear fusion,has always been the focus of researchers around the world.Its development is inseparable from the continuous optimization of the ignition targets,among which one of the core factors comes from the degradation of mandrel materials.After long-term explorations,poly-?-methylstyrene(PAMS)has been confirmed as the most ideal mandrel material at present,and the degradable mandrel technique developed based on it is a key part of the fabrication ability of targets.Whether PAMS mandrel can achieve effective degradation directly affects the physical properties of the final targets.Correspondingly,it brings about the exploration of basic problems about the degradation of the mandrel material under the guidance of national demand.Up to now,although many related processes have been reported,there are still two key problems in the fabrication of targets,that is,how to avoid degradation residues and reduce the thermal degradation temperature of PAMS.Considering that the general nature of degradation corresponds to the breaking of chemical bonds,it is urgent to grasp the physical laws of the complex degradation process of PAMS at the atomic level.Therefore,this thesis carries out the research work on five aspects from exploring degradation mechanism to developing regulation methods:(1)The establishment of physical images for PAMS degradation;(2)The effect of hydrogen atom transfer on PAMS degradation;(3)The effect of inter-monomer arrangements and the saturation state of the end groups on PAMS degradation;(4)The regulation of PAMS degradation related to spatial position of side-chain functional groups;(5)The regulation of PAMS degradation related to the type of side-chain functional groups.Firstly,we study the various possible degradation reaction pathways of PAMS byusing density functional theory calculations and partly combined with the observational data from thermogravimetric experiments.Theoretical calculations show that the degradation at both ends of the main chain involves two kinds of processes:depolymerization and hydrogen-transfer-chain scission.The energy barriers(0.60?0.82 e V)that need to be overcome for depolymerization of chain ends and the hydrogen-transfer-chain scission at the unsaturated tail end are smaller than those for other hydrogen-transfer-chain scission reactions(1.39?4.23 e V).Importantly,the reaction rates of the former are faster than those of the latter,which can result in a difference of 5?31 orders of magnitude at 550 K.Moreover,thermogravimetric experiments show that the activation energy of 2.53 e V for PAMS degradation is between the energy barriers of fast and slow processes,corresponding to the theoretical average of multiple slow processes.Thus,the degradation model combining fast and slow processes is established at the atomic level.The establishment of this model highlights the important role of slow processes in PAMS degradation,thus providing a direction for the effective regulation of degradation.Secondly,based on the establishment of the degradation model,we focuse on theeffect of an uncertain factor about hydrogen atom transfer(HAT)on PAMS degradation.Through the study of the over-barrier thermal process and through-barrier tunneling process,it is found that the temperature range above 300 K,which is usually empirically considered to be dominated by thermal effects,exhibits a strong tunneling-effect-dominated HAT behavior.Results suggest that among various possible HAT pathways,the lower energy barrier and stronger tunneling effect make the HAT associated with the active end of PAMS more likely to occur.In particular,the difference in energy barriers of this reaction and depolymerization reaction are only in the order of 10-2 e V,but the tunneling probability of the former is 14-32 orders of magnitude greater than that of the latter at the provided energy of 0.40 e V.In addition,chain scission after HAT will lead to a variety of products other than monomer.These findings highlight that quantum tunneling may be an important source of uncertainty in PAMS degradation,which will provide a reference for the further development of key technologies in farbrication of ICF target and even for solving the problem of plastic pollution.Thirdly,considering that PAMS is a complex hydrocarbon molecular system polymerized by monomer,the arrangements between monomers and the saturation state of the end groups may be important factors affecting degradation.Thus,we investigate the influence of normal(head-to-tail)and abnormal(head-to-head,tail-to-tail)arrangements under conditions that the chain ends are saturated and unsaturated on degradation.Results show that,the energy barrier of the depolymerization for head-to-head structure with saturated chain ends is 0.78 e V lower than that of the head-to-tail structure,while that of the tail-to-tail structure is 0.58 e V higher.This indicates that the head-to-head structure is more prone to degradation,while the tail-to-tail structure is more difficult to degradation.In addition,we also found that the absence of hydrogen atoms at the chain end leads to a decrease of more than 3.0 e V in the energy barrier required for PAMS degradation compared to the case with hydrogen atoms saturated.Above results indicate that abnormal tail-to-tail arrangement and chain end saturation will inhibit degradation.Morover,calculated synthesis rates show that tail-to-tail structures are easy to generate.These findings provide an important reference for the source of incomplete mandrel degradation,and for the difficult degradation of polymer materials such as plastics.Forthly,considering that effective regulation of PAMS degradation has great significance for the preparation of high-quality target,we explore the possible influence of the spatial position change of side-chain functional groups on degradation.Specifically,the degradation behavior under different spatial arrangements(one isotactic,one syndiotactic and six atactic arrangements)were studied.Calculated results show that the degradation reaction of PAMS under isotactic and most of atactic arrangements to generate monomer are more likely to occur than under syndiotactic arrangement.Therefore,we propose a new method to regulate degradation by realizing spatial isomerization of PAMS.Not only that,further calculations of the synthesis rate ratio show that the atactic arrangements have the largest synthesis reaction rate at temperatures above 80 K,indicating that the atactic structures can be easily synthesized experimentally.These findings will provide valuable references for achieving easier degradation of PAMS and related experimental researches.Fifthly,in order to further develop the regulation methods of the degradation process,we explore the effect of different side-chain functional groups on degradation based on the understanding about the effect of spatial positions of the side-chain functional group on the degradation.The degradation processes of a series of CH cyclic functional groups(including cyclopentane,cyclopentadiene,naphthalene and azulene)replacing the benzene ring on the side chain of PAMS to form chain-like structures are studied.Calculated results show that these side chains cause the degradation energy barrier of different structures to be distributed in the range of 0.52–0.76 e V rather than that of0.72 e V of PAMS,and even the endothermic and exothermic properties of degradation reactions also be changed.Thermodynamics,energy decomposition and frontier orbital analyses indicate that one of the isomers for naphthalene-substituted structure has qualitatively consistent electronic structure characteristics with PAMS,but the required degradation condition is lower,showing the prospect of being a candidate for mandrel material.Thus,we propose a method to regulate the degradation by changing the functional groups of side chains.Further synthesis rate calculations show that this naphthalene-substituted structure has a larger synthesis rate than PAMS at the temperature required for PAMS synthesis,indicating that it is also promising to be experimentally synthesized.In summary,this thesis carried out the basic scientific exploration of mandrel degradation under the guidance of national demand.Through the study of degradation properties for PAMS as mandrel material,this thesis established the degradation model combining fast and slow processes,revealed that the quantum tunneling effect in HAT may be the source of uncertainty in PAMS degradation,clarified the effect of inter-monomer arrangements and the saturation state of the end groups on PAMS degradation,and developed two methods for regulating degradation related to spatial position and type of side-chain functional groups.Based on the above work,we realized the research idea from exploring the degradation mechanism to further developing the regulation methods about degradation process.These findings can provide reference for the further development of target manufacturing technology in ICF,and have reference value for the solution of the widespread plastic pollution problem. |
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