| Because of the outstanding physical and chemical properties of polymer materials, the polymer products exist extensively in our life, and they have become our indispensable friends. However, there are some properties and phenomena of polymer materials which are difficult to be explained experimentally. The computer simulation provides us a reliable solution to overcome these problems with the help of theoretical tools. Combined with the experimental methods, it can enrich our understanding on the relation between the polymer materials and the microscopic structures.This paper has two main parts. The first part is to study the adsorption and diffusion of polymer chain on the surfaces at the microscopic scale with the help of computer simulations. The second part is to study the compatibility of PEO/PMMA blends at both microscopic and mesoscopic scales also with the help of computer simulations. In order to study the influence of the external factors on the phase morphologies of PEO/PMMA blends, we add the steady shear, block copolymers and nanoparticles into the blends.The first part: the computer simulation study of the adsorption and diffusion of polymer chain on surfacesIt is common to study the adsorption behavior between the adsorbate and adsorption surface which possess the similar polarity. Among them, the study on the hydrophobic-hydrophobic systems has been carried out by many researchers. The adsorption systems in our researches are different, one is with hydrophilic-hydrophilic type, and the other is with hydrophobic-hydrophilic type. The adsorbates are poly(vinyl alcohol) (PVA) which is hydrophilic and polyethylene (PE) which is hydrophobic, respectively. We adopt the hydroxylatedβ-cristobalite (100) surface for both systems as the adsorption surfaces.(1) Hydrophilic-hydrophilic type:We study the adsorption of a typical hydrophilic chain on a strong hydrophilic surface where hydrogen bonds can form between them. The vacuum and"in solution"conditions are considered separately, and the effects of molecular weight on the adsorption behavior are taken into account. For short chains in vacuum, the adsorbed molecule configuration is ordered and the molecule is oriented along (110). Increasing the molecular weight to a moderate extent, the molecule possesses folded configuration and the whole molecule is still oriented along (110). Therefore we show the possibility of producing ordered single polymer layer and suggest control factors. When the molecular weight is large enough, the adsorbed molecule configuration is frozen with certain height in the Z direction, i.e., not easy to change due to the strong surface-polymer interactions. In solution, there is no orientation preference of the molecules and the configurations are always two dimensional and not ordered, no matter what the molecular weight is. Furthermore, the mobility of the molecules in solution is larger than that in vacuum, which implies that the molecules in solution are easier to be desorbed. These results may relate to the experiments of template-induced self-assembly of molecules.(2) Hydrophobic-hydrophilic type:The PE chain forms ordered folded 2D adsorption configuration on the hydroxylatedβ-cristobalite (100) surface, and the preferential adsorption direction for the whole PE chain is along (110) direction, this is the result from the competition between the intra-chain and chain-surface interactions. The hydroxyl groups with such ordered arrangement arrays on the hydroxylatedβ-cristobalite (100) surface can"direct"the PE chains to form ordered arrangement on the adsorption surface, furthermore, form a PE thin film at last with the increase in the PE concentration.From these results, we can draw a conclusion that no matter the polymer chain is hydrophilic or hydrophobic, the polymer chains all adopt 2D ordered arrangement on the hydroxylatedβ-cristobalite (100) adsorption surface. These results can be applied in the experiments of template-induced self-assembly of molecules.The second part: the computer simulation study of the phase behavior of PEO/PMMA blendsThe study about PEO/PMMA blends is involved both the microscopic and mesoscopic simulation methods: we obtain the representative chain length of the component chains and the Flory-Huggins interaction parameter at the selected temperatures; then we obtain the phase morphologies of the blends via computer simulations from the mesoscopic scale.(1) the calculations at microscopic scaleAt first, we need to calculate the representative chain length of PEO and PMMA, respectively. It involves two steps to do this job: the first step, do minimization and anneal treatment on the PEO/PMMA blends. The second step, do MD simulations with a repeating ensemble cycle from NVT to NPT three times. The aim for these two steps are to reduce the end-end interactions in these oligomer systems, otherwise the calculated data would not be comparable with the polymer bulk properties. From the calculated relation between the solubility parameters of PEO and its chain lengths, we can find that the solubility parameter does not change when N reaches 50. Thus N=50 is the representation chain length for PEO chain. The process of finding the representation chain length of PMMA is the same as that of PEO. Next, we calculate the Flory-Huggins interaction parametersχat 270 K, 298 K and 400 K. Comparing the calculatedχwithχcritical, we find that the PEO/PMMA blends are all miscible. From the change tendency of the calculatedχdata of different PEO/PMMA blends at different simulation temperatures, we can draw a conclusion that the influence of the enthalpy effect performs more intensely than the entropy effect during this blending process. This conclusion is coincident with that of Hopkinson et al.(2) the calculations at mesoscopic scaleWe take 1/4 (PEO volume fraction 0.1021), 3/1 (PEO volume fraction 0.5771) and 8/1 (PEO volume fraction 0.7844) blends as examples for the composition rich in PMMA, roughly equal in PEO and PMMA, and rich in PEO, respectively. We discuss the influence of external factors on these phase morphologies by adding the block copolymers, the steady shear, and the nanoparticles into the PEO/PMMA blends.From the calculations of order parameter of three representative cases (1/4, 3/1 and 8/1 blends) with block copolymer added, we find that when the two ends of copolymer and the dominating composition in plain blends are different components, it can lead to evident local phase separation, especially in PEO-rich blends. The longer the chain, the stronger is the local phase separation. The simulations of testing the shear effect in different composition cases show that the 1/4 blending case suffers the most apparent local phase separation.We now consider the mesoscopic simulations carried out on the plain PEO/PMMA blends, doped with nanoparticles with various size, density and arrangement. The doped nanoparticles can even induce the blends to exhibit phase separation, especially when the blends are rich in PEO or PMMA, such as 6/1 and 1/4 blending cases, their phase morphologies are all sensitive to the doped nanoparticles. From the simulations, we find that the density and the size of doped nanoparticles are the most important factors influencing on the phase morphologies of PEO/PMMA blends. |
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