Preparation Of Single-Chain Polystyrene And Study On Its Glass Transition Behavior

With rapid development of nanotechnology,polymer nanomaterials have attracted extensive attention in the exploration of novel materials.The properties of nanomaterials are determined by the dynamic of polymer in nanoscale.The single-chain polymer logically represents the smallest particles in macromolecular systems and can be viewed as the extreme limit of nanoconfinement.The investigation of the condensed state of single-chain can provide a new perspective on the structure,morphology and mechanical properties of polymer nanomaterials and help us understand the molecular dynamics of polymer on molecular level,which provide theoretical guidance and technical support for the preparation of nano-devices and nanomaterials with excellent performance.However,there only few researches were carried out until now.The difficulty in preparation and characterization of single-chain seriously restricts the development of its experimental and theoretical research.In this paper,we prepared single-chains polystyrene(PS)with various molecular weights via surface-initiated atom transfer radical polymerization(SI-ATRP)on substrate with superlow initiator density.The molecular weight of PS single-chain was estimated by adding free initiator to the reaction solution to obtain free PS molecule.The PeakForce Quantitative mechanical property mapping(PeakForce QNM)mode on the atomic force microscope(AFM)was employed to in-situ monitor the variation of adhesion of PS single-chain with temperature,in which the turn-point is related to glass transition temperature(T_g).The feasibility of the above method was verified by ellipsometry on PS films with various thickness.Moreover,the molecular weight dependence of the T_g of single-chain PS particles was also studied.The main conclusions are as follows:(1)A facile method of preparing polymer single-chains on planar substrates was developed.The Si-O bond between the initiator and the substrate could be cleaved by tetrabutylammonium fluoride(TBAF),which led to the decrease of initiator density with the increasing time treated by TBAF.The PS single-chains with various molecular weights were prepared via SI-ATRP on substrate with superlow initiator density by controlling the reaction time.As the reaction time increased from 3 h to 28 h,the molecular weight increased from 8 to 123 kg/mol,and the diameter of PS single-chain particles increased from~3 to~8 nm.(2)The T_g of the PS film determined by the PeakForce QNM mode through the adhesion turn-point is consistent with the results measured by the ellipsometry method under the same heating procedure.It is confirmed that this is a reliable method to characterize the T_g of single-chain by the PeakForce QNM mode.(3)When the molecular weight is higher than the entanglement molecular weight(M_e),the T_g of PS single-chain is 74?.And its T_g is independent of the molecular weight and size of single-chain particles.When the molecular weight is lower than the M_e(8 kg/mol),the T_g of PS single-chain is 68?.The molecular weight makes much fewer influence on T_g of single-chain particle than that of corresponding bulk.For the bulk with same mass,the content of end-group with high mobility decreases with the increase of the molecular weight,while for PS single-chain there is only one end-group exist in the particle regardless of molecular weight.Thus,the effect of molecular weight on single chain T_g is weakened.(4)When the original height of the single-chain particle(7.8 nm)is close to the thickness of the thin film(8.5 nm),the T_g of single-chain particle(74?)is much higher than that of thin PS film(45?).This may be attributed to the difference of conformation between the single-chain particles and PS films,which lead to the cohesional entanglement density in single-chain particles is higher than thin films.