Confined Crystallization Behavior Of Supramolecular Polyethylene Glycol Bonded By Hydrogen Bonds

Supramolecular polymers are a new type of materials formed by the association of noncovalent bonds.Hydrogen bonding is a typical noncovalent bond,which can direct self-assembly through precise complementation.It has been extensively used in the preparation of supramolecular polymers,due to its high selectivity,directionality and temperature sensitivity.Thymine(Thy)and diaminotriazine(DAT)are two common hydrogen bond units,which can form the self-complementary double hydrogen bonds by dimerization,and the blending of the two can form triple hydrogen bonds.When the Thy or DAT units are covalently attached to the terminals or side chains of the polymers,supramolecular polymers can be formed via the hydrogen bonds interactions.Compared with traditional covalently bonded polymers,the crystallization behavior of supramolecular polymers undergoes a "confined" manner.Among the numerous synthetic polymers,Polyethylene glycol(PEG)is a commercially available semicrystalline polymer,which has attracted much attention due to its good water-solubility and biocompatibility.In recent years,the research on the confined crystallization of the PEG focuses on the spatial confinement,and there are few reports on the chain confinement.The incorporation of supramolecular units and interactions of hydrogen bond units are the key factors to affecting the crystallization behavior of polymer segments.Therefore,the incorporation of hydrogen bond units at the end of the PEG chains will help to further understand the mechanism of the confined crystallization behavior of PEG.In this thesis,telechelic PEG with various molecular weights were functionalized by Thy firstly.The viscosity test results in solution or melt state showed that Thy-functionalized PEGs can form supramolecular polymers through the interaction of double hydrogen bonding.The crystallization kinetics and melting behaviors of Thy-functionalized PEGs were systemically investigated by comparison with hydroxyl-terminated PEGs.Crystallinity and melting temperature of linked PEG chains were decreased by the presence of Thy groups and the depression effects were strengthened by increase of Thy content.Crystallization rate and temperatures at which crystallization occurred were more complicated.Lower crystallization rate was obtained for Thy-functionalized PEGs with highest Thy content,a signature for confined crystallization by covalent links to Thy groups.However,in the case of moderate Thy content,the trend of crystallization rate was shifted to lower temperature regions,where the maximum and minimum of crystallization rates were comparable with free polymer chains.For Thy-functionalized PEGs with low Thy content,crystallization rates were similar with that of non-functionalized PEGs.The differences in supercoolings might give explanation on differences in crystallization behaviors among three conditions.Secondly,we investigated the effects of different hydrogen bonding strengths on the crystallization kinetics and melting behaviors of PEG.DAT-terminated PEG was prepared by nucleophilic aromatic substitution reaction,and Thy functionalized PEG was synthesized by amide reaction.The two components were mixed in solution at equimolar ratio to prepare supramolecular PEG blend.The viscosity-temperature dependence of the prepared blend in the melt state was stronger than that of DAT or Thy functionalized PEG,which proved that the triplet hydrogen bonding interactions was greater than the double hydrogen bonding.Compared with DAT functionalized PEG,Thy functionalized PEG showed a lower crystallization rate.This was due to the PEG chains covalently linked to Thy units underwent a pre-arrangement process driven by the amide hydrogen bonds,which leaded to poor aggregation of the Thy units and thus hindered the diffusion and accumulation of the PEG chains.However,the crystallization rate of supramolecular PEG blends prepared by equimolar blending was improved.This was due to the triple hydrogen bonds interaction effectively inhibited the pre-arrangement process.As the degree of supercoolings decreased,the interaction of triple hydrogen bonds finally achieved a dominant position,making the crystallization rate of supramolecular PEG blends appeared to be the minimum.In conclusion,the competitive relationship between the interaction of the triple hydrogen bonds and the pre-arrangement structure was the determining factor to explain the difference in the crystallization behavior of supramolecular PEG bonded by different hydrogen bonding strengths.