Molecular Mechanism Of Thermosensitive Self-assembly Evolution Of Amphiphilic Block Copolymers

A class of polymers whose physical properties change when the ambient temperature changes are called thermoresponsive polymers.Thermosensitive block copolymers can be used as smart sensors,tissue and cell culture agents,smart surfaces,bioseparation and drug sustained-release materials due to their easy adjustment of thermal response temperature,strong designability of segment structures,and diverse functions.It has a wide range of applications in many fields.Exploring the molecular mechanism of thermal response behavior of thermosensitive block copolymers has important theoretical significance for the structural design and practical application of such materials.In this dissertation,the molecular mechanism of the temperaturesensitive transition process of the reversible sol-gel transition of amphiphilic block copolymers is studied.The main research contents are as follows:1.The AB-type amphiphilic block copolymer PEG-b-PHPA and the ABA-type amphiphilic block copolymer PDMAA were synthesized using the RAFT chain transfer agents DDAAT and BDAAT containing single and double leaving groups,respectively.-b-PDAAM-b-PMAA with PDMAA-bPHPA-b-PDMAA.The synthesized block copolymers were characterized by analytical methods such as NMR,GPC and FT-IR,and the consistency of the copolymer segment structure with the designed structure was verified.The synthesized aqueous dispersions of the three amphiphilic block copolymers all exhibited temperature-sensitive properties of gelation or turbidity with temperature.When the solid content of PDMAA-b-PDAAM-b-PDMAA was20 wt%,Its gel transition temperature is 50 °C.PDMAA-b-PHPA-b-PDMAA,which is easier to prepare and store,was selected to explore the turbidity phase transition process.When the solid content was 10 wt%,the turbidity phase transition temperature was 46 ℃.Molecular mechanism study of thermosensitive transition using these two thermosensitive block copolymers.2.During the heating process,the self-assembled morphology of PDMAA-b-PDAAM-b-PDMAA block copolymer will change from spherical micelles to worm-like micelles,and finally lead to the gelation transition of the material,showing a temperature-sensitive behavior.Using variabletemperature mid-infrared and variable-temperature near-infrared spectroscopy,combined with chemometric methods such as genetic peak splitting algorithm,binary correlation analysis and principal component analysis,in situ analysis of the evolution of the hydrogen bond association structure of polymer segments and water molecules with temperature process,the gel transition can be divided into three distinct stages depending on the temperature.In stage I(low temperature zone)and stage III(high temperature zone),the dehydration rate is slower.However,in the second stage(medium temperature range),dehydration is accelerated during gelation,which is due to the decrease of the contact area between the hydrophilic block PDMAA and water with the morphological change.Furthermore,the dehydration of the PDMAA block was caused by the disruption of the interaction between the S2 water molecule and the bridging copolymer.The driving force for the whole gelation process mainly originates from the dissociation of hydrogen bonds between the NH and C=O groups within the PDAAM block,which causes the relative volume of the hydrophobic phase PDAAM to increase with temperature,resulting in an increase in the filling parameter P,which in turn increases from Spherical micelles transform into worm-like micelles.3.The PDMAA-b-PHPA-b-PDMAA block copolymer exhibits a thermally responsive turbidity phase transition process with temperature,and phase separation occurs below the lowest critical temperature(LCST),making the clear solution opaque.The liquid-liquid separation process of the solution with a solid content of 10 wt% was observed in situ with a microscope equipped with a hot stage.At the same time,dynamic light scattering showed that the phase transition process was a fast response process,and the phase transition point was 46 ℃.Temperature-variable mid-infrared spectroscopy combined with two-dimensional correlation spectroscopy and PCMW technology to in situ analyze the evolution of the phase transition of the block copolymer during the heating process: the interaction between the OH group at the PHPA terminal and water was first weakened,which led to the formation of the HPA terminal methyl group.The hydration layer of the base is destroyed,and the weakening of the interaction between the OH group and water is the main driving force for the haze phase transition of the block copolymer.Then the C=O group on the side chain is dehydrated,the side chain of the block copolymer collapses before the main chain,and finally the main chain is dehydrated.Chain conformational changes lead to polymer aggregation to form phase separation,with a small amount of water present in the polymer-rich phase acting as a bridge for interactions between polymer chains.By synthesizing two different types of temperature-sensitive block copolymer systems and comparing their phase transition mechanisms,the gap in the molecular mechanism of temperature-sensitive block copolymers in the evolution process with temperature is supplemented,which is a better solution for this kind of temperature-sensitive materials.application provides a theoretical basis.