Melting behavior of inclusion complex formed between polyethylene glycol oligomer and urea

Inclusion complexes formed from host small molecules and guest polymers have provided a novel platform to study both the physical properties of isolated polymer chains and crystallization behavior of host molecules. In this report, we have studied the crystalline structure, melting behavior and infrared information of inclusion complexes formed between polyethylene glycol oligomers and urea molecules. The effect of molecular weight and terminal group of guest PEG oligomer on the melting behavior of inclusion complex is presented. FTIR spectra reveal that terminal -OH of PEG is hydrogen-bonded to urea in inclusion complex, while the terminal -OCH3 is free. The melting behaviors of complexes are interpreted using either Gibbs-Thomson equation for lamellar crystals with definite channel length or Flory equation for crystals with infinite channel length and included defects. The results show that the interaction between terminal -OH in PEG chain and urea induces larger interfacial free energy or stronger defect effect, compared to the terminal -OCH3 in the inclusion complexes, which results in the particular order of melting point: Tm(DPEG500UIC) > Tm(MPEG550UIC) > Tm(HPEG600UIC).