Study On The Chain Entanglement And Viscoelasticity Of Polymer Brushes

Polymer films have received extensive attention due to its excellent performance and wide applications in the field of nanomaterials.The polymer chain is confined in nano-scale when the thickness of the film reduced to the scale of the polymer chain.By understanding the entanglement behavior of polymer chains under confinement,it can provide a theoretical basis for polymer micro-nano processing technology and viscoelastic behavior of polymer nanometer materials could be predicted.However,the theory of polymer chain entanglement in nano-confined system has not been clarified,and it is one of the great challenges in the field of polymer science polymer field.The entanglement structure and variation of nano-confined polymer chains have not been clearly described.At the same time,since the results obtained by different characterization methods are different,it is necessary to develop new methods for characterizing the entanglement behavior of polymer chains under confinement.in nano-confined.Compared with conventional polymer films,aggregation structure of polymer brushes have been controlled easily and the transition of the brush conformation from random coil to brush can be achieved by controlling the grafting density.By elucidating the relationship between the polymer chain conformation and its chain entanglement behavior,it is helpful to understand the entanglement behavior of polymer chain under confinement.A novel method for measuring the viscosity of supported thin polymer thin films was developed previously.The viscosity can be acquired by probing the growth dynamics of“wetting ridge”,which is a micro-deformation on film surface created by vertical component of surface tension of liquid droplet on film surface.It was found that there existed a plateau area where the height of the wetting ridge remained unchanged with time.According to the viscoelasticity of polymers,such plateau area was caused by the entanglement of polymer chains.Based on the relationship between the height of the wetting ridge plateau area and the plateau modulus,in this paper,the entanglement molecular weights of polymer film and polymer molecular brush are obtained by the height of the wetting ridge plateau area.The main conclusions are as follows:?1?A novel method has been developed to measure the plateau modulus and entanglement molecular weight of polymer film by monitoring the height of the wetting ridge plateau area.The results of the rheological test confirm the feasibility of this method.?2?We prepared poly?n-butyl methacrylate??PBMA?brushes via surface-initiated atom transfer radical polymerization?SI-ATRP?.The density of initiator on the substrate surface was controlled by degrafting method.PBMA brushes of different thickness with grafting densities of 0.60 nm^-2,0.44 nm^-2 and 0.30 nm^-2 were synthesized.Polymerization kinetics showed that the thickness of PBMA brushes increased linearly with the polymerization time,indicating that the polymerization process was controllable.?3?The critical entanglement thickness?h_e?and entanglement molecular weight?M_e?of PBMA brush with different grafting densities were studied by the height of the plateau area.It shows that when?_p=0.60 nm^-2,the critical entanglement thickness of PBMA molecular brushes is 28 nm and the corresponding entanglement molecular weight is 30 kg/mol.When?_p=0.44 nm^-2,the critical entanglement thickness of PBMA molecular brushes is 17 nm and the corresponding entanglement molecular weight is25 kg/mol.When?_p=0.30 nm^-2,the critical entanglement thickness of the PBMA molecular brush is 9.5 nm and the corresponding entanglement molecular weight is 21kg/mol,which is consistent with the entanglement molecular weight of the bulk.The reason is attributed to that the stretch degree of polymer chain increases with the increase of grafting density,which reduces the degree of entanglement between polymer chains,resulting in increasing the molecular weight of entanglement.When?_p=0.30 nm^-2,the chain conformation of PBMA brushes is random coil,so the entangled molecular weight is consistent with the corresponding bulk.?4?The elastic modulus of PBMA films with the same thickness and different graft densities were studied by the height of wetting ridge plateau area.It was found that the elastic modulus of PBMA brush increased with the increase of grafting density.When?_p=0.60 nm^-2,the elastic modulus of the PBMA molecular brush is 1.37 times higher than that of the PBMA film.When?_p=0.44 nm^-2,the elastic modulus of the PBMA molecular brush is 1.18 times higher than that of the PBMA film with the same thickness.When the grafting density of PBMA molecular brush is reduced to?_p=0.30nm^-2,the elastic modulus of PBMA molecular brush is equivalent to that of PBMA thin film.The reason is attributed to that with the increase of grafting density,the stretch degree of polymer chain increases,and the molecular chain has low conformational entropy,leading to the increase of resistance to deformation of polymer brush.