The Structure And Microphase Separation Of Amphiphilic Graft Copolymers Studied By Monte Carlo Simulation

It is an important topic of polymer science to study the microphase separation of amphiphilic copolymers in selective solvents,which can assemble into various ordered structures like micelle,vesicle.Such ordered structures are widely used in fields such as drug delivery system,tissue engineering,nanomaterials and so forth.Unlike linear copolymers,graft copolymers possess unique topologic structure,and its assembly behavior can be easily tuned by the properties such as the length and the composition of the main/side chains,the grafting density,which has attracted many attentions.Recently,our group have prepared a novel injectable supermolecular hydrogel via the assembly of an amphiphilic graft polymer PPG-g-PEG with cyclodextrin,here PPG is hydrophobic poly(Nphenylglycine)and PEG is hydrophilic polyethylene glycol(PEG).This hydrogel shows a reversible gel-to-sol while exposed to NIR.While the hydrogel is loaded with indocyanine green(IDG),it can not only increase the stability of IDG,but also obtain the combined photothermal therapy(PTT)and photodynamic therapy(PDT)treatment.However,PPG is a rigid hydrophobic polymer,it is difficult to characterize the properties of the graft copolymer in experiment,the underlying microscopic formation mechanism of the hydrogel is still unclear.The studies of the assembly behavior usually focus on copolymer consisting with flexible chain,but seldom with rigid or semiflexible chains.To better understand the formation mechanism of the PPG-g-PEG/cyclodextrin hydrogel,we apply computer simulation to study the structure and assembly behavior of the amphiphilic graft polymer,which has a hydrophobic rigid backbone and hydrophilic flexible side chains.A lattice Monte Carlo(MC)simulation is applied and the effect of chain rigidity,length of the main chain and side chain and grafting density are examined.Tne main contents of this thesis are as follows:Lattice MC simulation is widely applied in the field of polymers due to it high efficacy.Firstly,the background knowledge of MC simulation is introduced in Chapter II,including its basic idea,widely used lattice MC model,Metropolis sampling method,as well as some technical details.It is a primary task to determine the persistence length that characterizes the chain stiffness.In chapter ?,theoretical persistence lengths of polymers with two different bending potentials were analyzed and examined by using various lattice Monte Carlo simulations.We found that the persistence length was approximately consistent with theoretical predictions only in bond fluctuation model with cosine squared angle potential.The reason for this is that the theoretical persistence length is calculated according to a continuous bond angle,which is discrete in lattice simulations.These works have formed the basis for future study.In chapter ?,the collapse transition of a semiflexible polymer from athermal state to low temperature is examined by lattice Monte Carlo simulation.We focused on effect of chain stiffness on the collapse transition.The results proved that flexible polymer changes continuously during the collapse transition,while the semiflexible polymer shows a discontinuous transition.We analyze the conformation and find that a solid globule can be obtained for flexible or not so stiff polymers at low temperature,while the semiflexible chain can collapse into toroid,bundle and other structures.This chapter is helpful to understand the microphase separation and assembly behavior of the graft copolymer.In chapter ?,we focus on the microphase separation of an amphiphilic graft polymer with a rigid main chain and flexible side chains.The influence of chain stiffness,main chain length,graft density and side chain length are examined.The simulation results proved that the temperature of collapse transition decreases with increasing chain stiffness.With the increased main chain length,the graft polymer exhibited collapse state form continuous to discontinuous in studied temperature range.Graft copolymer with shorter main chain will collapse into rod-like structure at low temperature,while the toroid and bundle structure will appear with longer main chain.This study is helpful to study the formation mechanism of the PPG-g-PEG/cyclodextrin hydrogel.