| The physical properties of the polymer material will change dramatically nearby the glass transition temperature(Tg), which will affect the actual application of the material directly. The glass transition of material in the cooling process will result in that the viscosity increases dramatically and the movement of the molecular chain will be frozen. However, the micro-structure of the polymer can not be observed clearly by experiment. The current issue that what factors cause such a significant change in the physical properties of the material near the glass transition temperature is still a hot topic in the field of polymer physics. In other words, understanding the microscopic mechanism of the glass transition of polymer is benefit to the preparation of a high-performance polymer material which can be used in a wider range of temperatures. But there is no theory which can provide a comprehensive explanation for the microscopic mechanism of the glass transition. Therefore, further exploring the microscopic mechanism of the polymer glass transition to reveal the hidden micro-structure information has important academic and practical value.Compared with experimental work, the method of computer simulation can observe and research the change of motion and micro-structure of polymer in the glass transition directly, in order to analyze the physical root which cause that the glassy molecular chain lose mobility. In this paper, the change of microscopic state in polymer chains during equilibrium process are simulated and calculated through the method of dynamic Monte Carlo simulation and the intrinsic link between the conformational transition and dynamical heterogeneity or fragility are explored as well. Detailed works are as follows:(1) The evolution process of microscopic state in linear polymer chains in the cooling process were simulated and calculated through dynamic Monte Carlo simulation. In addition, the parameters including the motive probability of chain segments(PSM), heat capacity, the motive probability of trans and gauche conformation and the extent of close-packing of conformation were analyzed.According to the calculating results of the mean PSM of system(<PSM>), in the cooling process, the <PSM> curve decline firstly and then kept unchanged afterexperiencing a significant turning point, indicating that chain segments movement was frozen at this time. The evolution trend of <PSM> value in the cooling process was very similar to the change process of chain segments mobility in the glass transition. Therefore, the temperature which was obtained by fitting the <PSM> curve nearby the turning point with line could be considered as Tg(0.25) of linear polymer.The statistical results of heat capacity showed that the heat capacity kept unchanged at first, with the temperature decreased to a critical temperature the curve decreased rapidly and then kept unchanged again in the cooling process. As a whole, the heat capacity curve had a "step" shape at the temperature range of 0.20-0.30, which was the typical characteristic of glass transition, indicating that the Tg could also be acquired by analyzing the change of heat capacity. In addition, the PSM distribution data of all chain segments showed that nearby the Tg(0.30 and 0.20), the PSM value of different chain segments had obvious difference with each other even at the same temperature, suggesting that the dynamical heterogeneity existed among various chain segments. It could be found that the closer packing of trans conformation resulted in the weaker mobility through calculating the <PSM> value and the extent of close-packing of trans and gauche conformation. In summary, the chain model used in our study could reflect the micro-structure and dynamic behavior of the real condensed matter system. The PSM and conformation information of chain segments were analyzed to combine the chain segments mobility with local microscopic conformation, revealing that the structural factor of dynamical heterogeneity in the glass transition process of polymer was that the mobility of trans conformation was different with gauche conformation. Our findings provided the basis to further understand the mechnism of glass transition.(2) The microscopic state change of ring polymer chains in the cooling process were simulated and calculated through dynamic Monte Carlo simulation and the difference between ring and linear were researched. The parameters including <PSM>,heat capacity and local domain of blocked segments(LDBS) were analyzed. In addition, the mean square displacement distribution(MSD) of the linear and ring polymer chain segments in some typical relaxation process were calculated to revealthe reason which caused the difference between the dynamical heterogeneity of ring and linear polymer. The statistical results of <PSM> showed that in the cooling process, the <PSM> of ring polymer decreased as the temperature decreased firstly and then kept unchanged after experienced a turning point, which was similar with the linear polymer. However, the Tg(0.32)of ring polymer obtained by fitting was higher than the Tg(0.25)of linear polymer with same chain length, which was consistent with the experimental results. In addition, the heat capacity curve had "step" shape at the temperature range of 0.25 to 0.35, which implying that the Tg of ring polymer could be obtained by analyzing the change of heat capacity as well. MSD of the two kinds of polymer chain segments were calculated and it could be known that both the MSD of linear polymer and ring polymer deviated from the Gaussian distribution nearby Tg, suggesting the appearance of dynamical heterogeneity. Furthermore, the MSD curve of ring polymer was narrower than linear polymer at same temperature,illustrating that its dynamical heterogeneity was weaker than linear polymer. By calculating the LDBS number of linear and ring polymer in cooling process,respectively, it could be found that the end chain segments with high mobility in linear polymer prevented the number of LDBS to increase directly. That is, the coexist of the end chain segments with high mobility and LDBS led to the stronger dynamical heterogeneity of linear polymer. In conclusion, the presence of the end chain segments in linear polymer resulted in the lower Tg and affected the freezing process of chain segments at low temperature, which caused the difference of dynamical heterogeneity between ring and linear polymer. Our findings described the different glass transition behavior of polymers from the perspective of topology structure and gave the molecular picture describing the influence of the end chain segment on the glass transition, which was benefit to comprehending the micro-mechanism of glass transition.(3) The fragility index of ring and linear polymer were simulated and calculated through dynamic Monte Carlo simulation and the chain segments number of aggregation region of segments with trans-conformation(ARST) in the cooling process was counted to reveal the micro-structural factors which had effect onfragility index. At first, the relaxation time of ring and linear polymer at a series temperature were calculated by auto-correlation function. Through combining the relaxation time with their respective Tg, the fragility curve could be acquired at last.According to the fragility curve, the fragility index of ring and linear polymer were18.35 and 13.26, respectively, indicating that the fragility of linear polymer was higher than ring polymer. The calculating result of chain segments <PSM> in different ARST illustrated that the chain segments <PSM> in larger ARST were lower,suggesting the weaker mobility of chain segments in larger ARST. Thus, the relaxation time of system was decided by the motion of the largest ARST. By calculating the chain segments number of the largest ARST, it could be found that when the temperature decreased near Tg, the chain segments number of largest ARST in linear polymer kept unchanged firstly and then increased suddenly, after experiencing a turning point it kept unchanged again. However, the chain segments number of largest ARST in ring polymer increased at the same rate firstly and kept unchanged at last. Nearby Tg, the faster the largest ARST of linear polymer increase,the faster the relaxation time increase, thus the fragility index of linear polymer was higher according to the fragility formulation. We not only calculated the fragility index of ring and linear polymer successfully but also found that the fragility index of polymer had close contact with the conformational transition in the cooling process.This discovery was in favor of understanding the structural nature of fragility and building the connection between the dynamic information and structural information in the glass transition. |
PDF Full Text Request