Study On Effect Of The Interfacial Chain Conformation On Segmental Dynamic Of Polystyrene Films

With the rapid development of nanotechnology,dynamic of polymer at nanoscale has become an important topic in materials science.When the thickness of the polymer nanofilm is close to the nanoscale,many physical properties of the film will deviate from the bulk.Researches in the past twenty years have revealed that the interfacial chain conformation is an important factor of affecting confined polymer dynamics.But due to limitation in characterization methods,the mechanism of the interfacial chain conformation on the interfacial effect has not been clearly explanation.As the grafting density of polymer brushes decreases,chain conformation will change from the stretched"Brush"conformation to the collapsed"Mushroom"conformation.Such a polymer system is an excellent model for studying the influence of the interfacial chain conformation on the molecular motion of polymer films.In this thesis,polystyrene(PS)brushes are selected as the substrate for thin spin-coating PS film.We have invented the influence depth h~*of the effect PS brushes with different densities on the molecular motion of the PS film,the glass transition temperature(T_g)of the PS film on PS brush substrate,the dewetting behavior of the PS film on the PS brush substrate,and further explored the interpenetrating behavior between PS brush and film.The influencial mechanism of interfacial chain conformation on film molecular motion was discussed.The main conclusions are as follows:(1)The relationship between the T_g of the PS brush-film bilayer system and the density of the PS brush was studied.It was found that the T_g of the PS brush-film bilayer showed a non-monotonic dependence on the grafting densities of PS brush substrate.When the PS brush substrate has a medium density(～0.44 chain/nm~2),there is only minimal decrease in T_g of the brush-film with the thickness of the film decreases.When the PS brush density is higher(>0.44 chain/nm~2)or lower(<0.44 chain/nm~2),the T_g of the PS brush-film significantly decreases with the decrease of the PS film thickness.It shows that as the density of the PS brush chain is decreased,the degree of inhibition of the film molecular segmental dynamic first increases and then decreases.PS brush with a medium density has the strongest inhibitory effect on the PS film molecular segmental dynamic.(2)When PS brushes with different densities used as substrates,the dewetting behavior of PS films and the influence depth h~*of PS brush substrates on the molecular motion of PS films are studied.The broadening degree of the interface between the PS brush and the d-PS film was studied by neutron reflection.It is found that when the grafting density of PS brush at medium density(～0.44 chain/nm~2),the dewetting rate of PS film is the slowest,and the influence depth h~*of PS brush substrates on the molecular motion of PS films is the largest.At this time,the degree of diffusion between PS brush chains and film molecules is the highest,so the degree of broadening of the brush-film interface is the largest.(3)Based on the experimental,the mechanism of interfacial chain conformation on the motion of thin film molecules segmental dynamic is discussed.The reason of the molecules segmental dynamic of the PS film changes with the grafting densities of the PS brush is that the degree of interpenetration between the PS brush and the film.The PS brush with higher graft density(>0.44 chain/nm~2)adopts a vertical and compact"Brush"conformation,which repels the PS film molecules,so the interface is difficult to interpenetrate;PS brushes with lower density(<0.44 chain/nm~2)are easily compatible with the PS film molecules,but because the number of grafted chains is small,the concentration of PS brush chains in the interface area is low,so the degree of interpenetration at the interface is low.Only when the PS brush has medium grafting density(～0.44 chain/nm~2),the compatibility between the brush&film and the brush chain concentration in the interface area reaches a balance,the degree of PS brush and the PS film interpenetration is highest.Therefore,the PS brush substrate can effectively inhibit the dewetting of the PS film,and the interface effect has the largest influence depth h~*of the PS film.At the same time,the high degree of interpenetration at the brush-film interface leads to greater friction with the interface chains,which increases the size and number of microdomains with low mobility at the interface.These micro-domains are connected to each other in polymer system to form a large-scale percolation network with low mobility,which inhibits the motion of the segmental dynamic and increases the T_g of the PS brush-film.