| PPTA fiber or aramid fiber 1414 in China,is an important high-performance materials which is characterized by high strength,high modulus and excellent thermal stability.It is also the only organic fiber in the industrial production of high-performance fibres.The preparation process of aramid fiber has many influence factors,such as the molecule weight of the polymer,draw ratio,spinning solution concentration,spinning temperature,and so on.But the key parameters determining the strength of the aramid fiber are the molecule weight and the quality of the polymer.Therefore,preparation of PPTA resin with high molecule weight,narrow distribution and no crosslinked and branched structures is an important prerequisite for the preparation of high quality aramid fiber.In addition,the polycondensation of the PPTA have the characteristic of high reactivity,quickly reaction speed and high reaction heat.It is difficult to achieve the in-situ observation the polycondensation process of PPTA.And the obtained PPTA is easily separated out from the polymerization system because of its rigid-rod structure and the interaction between those molecular chains,resulting in a low molecule weight of the polymer and a wide molecule weight distribution.Therefore,it is of great important to study the molecular structure and the condensed state in the polymerization of PPTA,in order to obtain high molecular weight polymer.Also,lots of work have been done in the study of the dynamic and the parameters in the condensation polymerization of PPTA.The polymerization mechanism is still not clear.Furthermore,the polycondensation process of PPTA is featured with multiple reaction conditions,hard to precise control,taking a long time to screening and cost a lot.A new method is quite essential to be built to further study the phase transition and reaction kinetics of polycondensation to make up the scarcity of the research.Besides the theory study and experiment research,computer simulation that has been newly named as ‘molecular simulation',has become the third important research method to study the structure and behavior of the real system.And the Monte Carlo simulation method is widely used because its random characteristics is similar to the movement behavior of polymer chains.By using this new method,it will be easily to control the polymerization,monitor the reaction process at any time,and detect the physical and chemical phenomenon simultaneously.We adopt this new technique simulation studies on the polycondesation process,especially on the polycondensation process of PPTA.Firstly,a lattice model was established for studying monomer,chain-like molecules and the kinetics of polymerization process.The dynamic Monte Carlo simulation method was introduced systematically,such as molecular models,fundamental principles,sampling methods,energy parameters,etc.Our simulations were performed in lattice space,and simulated motions of single chain and chain-like molecules.Relaxation of polymers was accomplished by combining single site jumping and successive sliding diffusion.By introducing parallel interaction between neighbor bonds(Ep),polymer could crystallize spontaneously with polymerization.For each bond–bond connection along the chain,the energetic cost of all noncollinear bonds are defined as Ec and used to describe the flexible of polymer chains.The demixing parameter(B)was introduced to represent the driving force of phase separation.Sampling method was Metropolis sampling in canonical ensemble,that could realize the evolution process from the initial state to the equilibrium state.Evolution time in the dynamic Monte Carlo simulation corresponds to the number of iterative simulation circles noted as Monte Carlo cycles(MCC).The reaction probability(kMC)was introduced to describe the polymerization rate,that makes it possible to study the dynamic competition between the polymerication process and other physical phenomena.Secondly,a concise expression of reaction kinetics in the later stage of polymerization or fast polymerization process was proposed based on the scaling analysis;afterwards,the crossover from the reaction controlled to the diffusion controlled was studied theoretically and further verified by simulations.Thirdly,the crystallization process in polymerization was first studied on a molecular level and the complex apparent dynamic behavior and the acceleration caused by crystallization-induced polymerization in heterogeneous systems were interpreted.Finally,the relationship between the reaction conditions and sequence structure of molecular chains and the congregated structure of polymer was studied.A new method that used to control the polymerization process by copolymerization and to adjust the structure of the condensed state was proposed.Several aspects content are included in this paper as following:1.A homopolycondensation model for studying the kinetics of polycondensation was established by the Monte Carlo simulation described above for the first time.It was compared with the classical theory of Flory's about the stepwise polymerization.The simulation results fitted well with the second-order reaction kinetics of Flory's and is well reproduced of the polycondensation process.The diffusion effect in the last stage of polymerization or fast reaction process was investigated by increasing of the reaction rate and concentration.A concise expression of reaction kinetics in diffusion controlled regime and the transition of kinetics from the classical reaction-controlled to diffusion-controlled regime was predicted and examined via simulation.This method studied the polycondensation of PPTA at the molecular level,and make up the deficiency of the inability to observe on line due to the fast reaction rate.It provides a new method for studying the kinetics of polycondensation of PPTA.And these kinetics expressions could be applied to the reaction design and control.2.A dynamic Monte Carlo simulation model was established to study the kinetics and crystallization process of heterogeneous polycondensation process.By comparing the polymerization process with and without crystallization,the effect of crystallization on polymerization was investigated in different reaction rate.The simulation results represented that crystal could decrease distances between all chain end groups and increase the reaction probability when the polymerization processes in low reaction rate.By the following four steps,polymerization in solution,polymerization in the formation of crystal nucleus,polymerization in crystal growth and polymerization after absolute crystal,the polymerization would be accelerated and the thinner and more regular crystal could be obtained.However,when the polymerization processes in high reaction rate,the long molecular chains could be obtained in the early stage of polymerization.The crystallization will accordance with the crystallization theory of polymer.The accelerated phenomenon would disappear.Therefore,by strictly control of the temperature and rate of heterogeneous polymerization,or decreasing the reactivity of monomer can effectively delay the crystallization and slow down the crystal growth.The high molecular weight polymer would be obtained directly in solution polymerization.According to this result,the PPTA resin with higher inherent viscosity(?inh=9.2dl/g)was obtained.A good performance PPTA fibers was prepared with two different inherent viscosity of PPTA.3.A cocondensation polymerization model for obtaining linear polymers was established by the Monte Carlo simulation method.Firstly,the relationship between the ratio of monomer activity,feed ratios and the molecular chain sequence was studied.Then,the relationship between the sequential structure and of the copolymer and their aggregation behavior in cocondensation polymerization was investigated.The simulation results showed that when the reactivity of monomers was same,the sequence of copolymer was consistent with the theoretical model and didn't affect by the concentration ratios.It could be concluded that the theory based on the assumption of invariable reactivity center could be applied in the system with similar reactivity of the two monomers or the concentration of chain segment kept constant,such as the system with only small amount of comonomers,whereas total failure with big different reactivity of monomers or high concentration of comonomers in the system.In the system with small amount of low reactivity comonomer,the average sequence length of the two blocks and their ratio were independent with the reactivity of comonomers,but were controlled by the concentration ratios.But in the system with small amount of high reactivity comonomer,the average sequence length of the two blocks increased with the increasing of the reactivity of comonomers.Moreover,the average sequence length of the two blocks in the system with large amount of comonomers would increase with the increasing of the reactivity of comonomers and hadno concern with the reactivity of comonomers.It could be seen that the polymer chains were end-capped by the monomer with low reactivity.The reactivity of comonomer would significant impact on the polymer chain.The alternating copolymer could be mainly obtained by adding low reactivity comonomers,while the block copolymer could be mainly obtained by the higher one.When the two kinds of monomers had same reactivity,the polymer was random copolymer.4.The model was also extended to study the crystallization and crystal morphology of the obtained cocondesation polymers.When the content of the copolymer was low,the length of the amorphous commoner unit would be so short that cannot restrict the growth of the crystal,resulting in large number of irrelevant crystal.Increasing the content of the copolymer,the amorphous copolymer enriched at the surface of the crystal would restrict the growth of the crystal and decrease both temperature of crystallization and the degree of crystallinity.The amorphous copolymer would also make the connect of different crystal structure that resulting in a physical gel network structure.While in the equal molar content,the sequence length increased due to the increasing of the reactivity of the comonomer.The morphology changed from alternating copolymer into block copolymer.The restrict effect of the amorphous on the growth of the crystal structure would be more remarkable.5.The influence of comonomer on the aggregation behavior in cocondensation polymerization was studied by by the Monte Carlo simulation method.The simulation methods showed that the comonomer would significantly delay the nucleation and crystallization.The thinner crystal was obtained with thicker amorphous layer on the crystal surface and lower degree of polymerization.With the comonomer content increasing,a more thinner crystal would be obtained with lots of loose loops and large surface energy.When the concentration of comonomer was larger,the comonomer could obviously prevent the nucleation and the growth of crystal.These crystalline grains that disperse in solution were full of loose loops and had good solubility.That is the reason why the aramide fiber could be obtained by wet spinning when the PPTA copolymerized with 3,4-ODA. |
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