Study On Molecular Conformations And Dynamics Of Oriented Polymer Film

The oriented polymer chains can significantly improve the physical properties of polymer material,such as mechanical,optical,thermal conduction,and is believed to be a very effective way to improve the performance of polymer materials.Molecular motion,which serves as a link between the microstructure and properties of polymers,is very important for the physical properties of polymer materials.The orientated molecular chain with decreased conformational entropy and the increased free energy due to the deviation in chain conformation from the random-coil ones exhibited much complex dynamics.Thus,it’s necessary to understand the microscopic mechanism of the conformational behavior and relaxation dynamics of the oriented polymer chain in order to achieve a better strategy to optimize the performance of oriented polymer materials.In this work,chain conformation and conformational relaxation dynamics of oriented polymethyl methacrylate（PMMA）films were investigated.The microscopic mechanism of chain orientation in a dense polymer brush was discussed,and as well as relaxation of the oriented PMMA during heating.Instead of using the general method（i.e.,mechanical force）to induce chain orientation,which has many drawbacks such as occurrence of break（or failure）and existence of voids and mechanical stress,we used polymer brushes,in which the chain spontaneously stretched due to the volume exclusion between chains,to prepare the oriented polymer films.The polymer brushes were prepared by surface-initiated polymer method,and then,detached from the substrate surface to obtain an oriented PMMA monolayer film.Microscopic conformational characteristics and conformational relaxation of oriented PMMA were studied by conformation-sensitive infrared spectroscopy,combined with the characterization of film thickness relaxation.The main conclusions obtained in this article are as follows:（1）PMMA brushes on Si Ox/Si substrates surface with different initiator contents were obtained in situ by surface-initiated atom transfer radical polymerization（SI-ATRP）.The Si Ox layer was etched with HF aqueous solution to remove the PMMA brushes from the substrate surface to obtain a uniformly oriented PMMA film.Two PMMA films with different degrees of orientation were prepared in this paper.The grafting densities of brushes were 0.68 and 0.41 nm^-2and the degree of orientation of molecular chains of the two films were 0.38 and 0.23,respectively.（2）The microscopic conformational changes of PMMA brushes with different grafting densities(i.e.,0.68 and 0.41 nm^-2)were studied by IRRAS.It was found that the degree of molecular chain stretching and the number of gauche conformations of the PMMA brushes chains increased with the increasing of grafting density,and concomitantly the side groups changed from trans to cis conformation.This result indicates that the microscopic conformation of the PMMA brushes rearranges with the increase ofσ_P and the extension of the molecular chain.The dense brush chain was stretched through side group shrinkage and main chain buckling.It suggests that the molecular disorder at or below the segmental level increases in the dense brushes.Meanwhile,however,the whole chain in the dense brushes is well stretched.Taken together,this indicate that an increase in local disorder is accompanied with the elongation of polymer brushes chain.Based on this,a microscopic molecular mechanism that induced the extension of the chain of high-density polymer brushes was proposed,that is localized disordering promotes the chain ordering in the dense grafted polymer brushes.The randomization of the local conformation compensates the entropic loss due to global chain orientation,thus stabilizing the conformation of the straightened brush chain and reducing the tension of the molecular chain,promoting the ordering of the entire chain.（3）The dynamics and conformational relaxation of oriented PMMA films were studied using ellipsometry and IRRAS.It was found by ellipsometry that when the temperature was below 352K,the oriented film thickness increased with the increasing temperature;at the temperature range of 355 to 388 K,heat-induced shrinkage behavior occurred.At T>388 K,the film thickness increased against with the increasing temperature with a much lower thermal expansion coefficient(β=2.5×10^-4 K^-1)as compared with that of spin-coating film in the rubber state.In order to reveal the mechanism of the complex thermal expansion behavior,IRRAS was used to characterize the microscopic conformational changes of PMMA during heating.When T<352 K,the ratio of gauche conformation in the backbone increased rapidly with increasing temperature;at 352 K<T<391 K,the increment of the amount of gauche conformer in the main chains slowed down;at T>391 K,the gauche conformation of the backbone turn to decrease with increasing temperature.The number of gauche conformations in the PMMA backbone is related to the increasing degree of molecular chain extension.We then built a connection between the conformational relaxation and film thickness changes during heating by taking into account the trend of changes in film thickness and conformation,and proposed a microscopic mechanism of the thermal expansion behavior of the oriented film.When T<352 K,only the vibration of the side groups can occur.This caused the film thickness slowly increase with increasing temperature,and the backbone gauche conformation increases as well.at 352 K<T<391 K,the movement of the short chain segment（or local segment）caused the chain to shrink obviously.However,since the temperature was still below the T_g of PMMA,the thermal expansion coefficient remains very low.Orientation-induced chain shrinkage and thermally-induced expansion competed with each other to cause the film thickness to decrease with increasing temperature.At T>391 K,PMMA was in a rubber state,the molecular motion was completely activated.The entire chain relaxation causes the large degree of chain contraction,which increase the number of gauche conformations.However,due to the significant increase in the expansion coefficient of the rubbery polymer,the degree of expansion was higher than the chain shrinkage caused by the orientation,resulting in that the film thickness still increases with temperature.However,due to that the chain shrinkage partly offsets the thermally-induced expansion,the apparent expansion coefficient of the oriented PMMA film at high temperature was much smaller than that of the spin-coating film.The results of this study shed new insights into the understanding of the conformational relaxation behavior of the oriented polymer molecular chain,which is significant to improve the thermal performance and structural stability of oriented polymer materials.