Effect Of Architecture Of Adsorbed Layer On Dynamics Of Polystyrene Films

When the thickness of polymer film was reduced to nanoscale,the molecular dynamic will deviate greatly from those in bulk.But so far,the nature for such deviation is not clear.In addition,most of the polymer films are attached to the substrate in practical applications,and the existence of the substrate has a significant effect on the dynamics of the ultra-thin polymer films.Although a series of fruitful studies on the substrate effect was carried out in recent decades,the mechanism of the substrate effect on the dynamics of the polymer ultra-thin films is still unknown.The adsorbed layer,which refers to an immobilized ultra-thin polymer layer strongly adhered on the solid surface,is worthy of particular attentions,because it was recently revealed to be important in determining the equilibration and non-equilibration dynamic of the confined chains.Our previous work also revealed that t the?T_g(T_g^film-T_g^bulk)?T_g:glass transition temperature?of polystyrene?PS?thin film is well scaled by the ratio of h_ads/R_g(h_ads is the thickness of the adsorbed layer,R_g is the radius of gyration),which indicate that the architecture of the adsorbed layer is significant in governing molecular dynamic of polymer films.Therefore,the architecture of interfacial adsorbed layer must be taken into careful consideration in order to elucidate the mechanism of substrate effect.Herein we used a random copolymer of styrene/p-hydroxyl styrene?P?St-r-Hs??to prepare the adsorbed layer on silicon substrates.We anticipated to adjust the architecture of the adsorbed chain by changing the molar fraction of Hs on the copolymer(f_Hs).The architecture of the adsorbed layer was characterized by atomic force microscope?AFM?and sum-frequency generation spectroscopy?SFG?.The T_g and the propagation depth of substrate effect for PS thin films with different architecture of adsorbed chain were studied.The main conclusions are summarized as follows:?1?The styrene/p-hydroxyl styrene random copolymer P?St-r-Hs?with different f_Hs were synthesized.Frist,the random copolymer of styrene/p-acetoxyl styrene P?St-r-As?with different As content?molar content?were synthetized by radical polymerization.Then,the As units was hydrolyzed by hydrazine hydrate to obtain styrene/p-hydroxyl styrene random copolymer P?St-r-Hs?with different f_Hs.?2?The influences of film-preparation methods,film-formation solvents,annealing temperature and f_Hs values on the architecture of P?St-r-Hs?adsorbed layer were explored.Results showed that the thickness of adsorbed layer increases with the increase of annealing temperature regardless of film-preparation method.If a solvent-casting method was used to prepare the film,the chain in adsorbed layer mainly adopt“brush-like”conformation,results in thicker adsorbed layer.However,when spin coating method was used,the chain in adsorbed layer adopt“loop-like”conformation leading to thinner adsorbed layer.It was found that the thickness of adsorbed layer increases first and then decreases with the increase of f_Hss when spin coating method was used.The result indicated that the thickness of adsorbed layer which is proportional to the height of loop of adsorbed chain can be regulated by altering the f _Hs values.?3?The influence of adsorbed layer architecture on the dynamics of confined polystyrene films was studied.It was found that the T_g of PS film on the adsorbed layer decreases first and then increases with the decrease of film thickness.Such trend was resulted from a competition between free surface and substrate effect.When the film thickness is larger than a critical value?h_L?,free surface effect is the dominant factor and thus T_g decrease with decrease the film thickness.When the film thickness is lower than h_L,substrate effect is the dominant factor and therefore T_g increase with decrease the film thickness.The critical thickness h_L increases linearly with the thickness of the adsorbed layer.The h _L was believed to be the critical point where substrate effect come to leading role and could be a parameter to evaluate substrate effect.The higher the h _L is,the more significant the substrate effect will be.Moreover,we also found that the propagation depth of the substrate effect increase linearly with the thickness of the adsorbed layer,showed the similar trend with h _L.Therefore,we propose that the thickness of adsorbed layer with higher“loop”?i.e.thicker adsorbed layer? is more efficient in delivering the suppressed substrate effect from interface to inner bulk,mostly because of the formation of strong entanglement and wider overlap region in between the adsorbed chain and free chains above,therefore the dynamic of entire film was severely impacted by the present of substrate.Our results pave new way to modulate the substrate effect by adjusting the loop conformation and thickness of adsorbed layer.