In-situ Study On The Dissolution Behavior Of Poly (Ethylene Oxide) Monolayer Lamellae By AFM

Due to the irregularity of chain sequence structure,the diversity of topological structure and the complexity of chain interaction,macromolecules generally crystallize away from equilibrium state and folded chain lamellae with internal ordered structure differences is formed.The improvement of crystal structure is always accompanied by the use of heat treatment to study the order difference within the lamellae,which limits the further improvement of the resolution.It was found that the dissolution conditions,the type of solvent and the structure of aggregates had significant effects on the dissolution behavior of polymer,and selective dissolution could also be used as a method to study the difference of the degree of order in the lamellae.In this paper,atomic force microscopy(AFM)was used to characterize in-situ the morphology evolution of poly(ethylene oxide)(PEO)lamellae during the dissolution process at different dissolution temperatures.Furthermore,the difference of the dissolution behavior of PEO lamellae with different initial thickness was compared,and the effect of dissolution temperature and end group on the dissolution behavior of PEO lamellae was discussed.The main results are as follows:1.The effects of supercooling degree,molecular weight and end group on PEO crystal morphology were studied by optical microscopy(POM)and AFM.With the increase of crystallization temperature(50-55 ℃),the lamellar thickness increases and the morphology changes from dendrite to regular single crystal.With the increase of molecular weight,the lamellar thickness decreases and the morphology changes from regular single crystal to dendrite with only four main branches.At the same crystallization temperature,the crystal morphology of hydroxy-terminated poly(ethylene oxide)(HPEO)was more regular than that of methoxy-terminated poly(ethylene oxide)(MPEO).2.The morphology evolution of HPEO(Mn = 4.6 and 10.0 kg/mol)lamellae during dissolution was characterized in-situ,and the effects of dissolution temperature on the dissolution behavior were studied.The results show that the lamellar thickness remains unchanged during the dissolution process;the dissolution rates of(100)and(140)crystal faces are distinctly different;the higher the dissolution temperature is,the higher the dissolution rate is,and the dissolution rate decreases exponentially with time at all temperatures;the dissolution characteristic time can be obtained by fitting,indicating a difference in the degree of order in the lamella(the degree of order increases gradually from the front to the center of the lamella);the change of the dissolution characteristic time with the dissolution temperature shows Arrhenius behavior,and the activation energy can be analyzed.3.The effects of initial crystallization temperature and dissolution temperature on the dissolution behavior of MPEO lamellae were studied by in-situ characterization of morphology evolution of lamellae dissolution.The results show that the higher the crystallization temperature is,the lower the dissolution rate is,and the higher the degree of internal order of lamella is;the dissolution rate increases with the increase of the dissolution temperature and decreases exponentially at all temperatures;the dissolution rate of MPEO was lower than that of HPEO at the same molecular weight and lamellar thickness.