Thermoanalytical studies on cross-linked polyurethane networks: effect of polyol molecular weight and structure of cyclodextrins

The applications of polyurethanes (PURs) have increased in recent years. They can have pharmaceutical applications as potential drug delivery carriers, due to their properties which can be adjusted according to their chemical structure. In this paper, the synthesis of PURs and their characterization were examined. Two series of PURs were synthesized by using β-cyclodextrin (βCD) or hydroxypropyl-β-cyclodextrin as cross-linker of alicyclic isophorone diisocyanate and three glycols: polyethylene glycols (PEG2000 and PEG6000) and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (BLOCK 1100). The NCO/OH molar ratio was 3.3, 4.3 and 6.3, and thermal properties of obtained PURs were investigated by thermogravimetry analysis and differential scanning calorimetry. The degree of crystallinity of polyol in soft segment of PURs was examined by using the X-ray diffraction (XRD), the morphology of the PURs was visualized by scanning electron microscopy (SEM), and the structural modifications was analyzed by using FTIR spectroscopy. The XRD patterns revealed that the crystallinity decreased mainly due to the cross-linking process, and FTIR assignments confirmed the formation of urethane linkages. The thermal stabilities of two series of PURs have been correlated with their soft segment molecular weights, structure of cross-linker and NCO/OH molar ratio. The increase in isocyanate ratio increased the cross-linking and thermal stability of the PURs network. The increase in melting point is attributed to an increase in the size of the crystallites or an increase in complete crystalline phase due to the greater phase separation of the hard and soft segments. The results of the SEM micrographs showed that βCD contributes to the microphase separation. By using a various cyclodextrins as a cross-linker, the chain length of the soft segments and NCO/OH molar ratio, the stated properties of PURs have been changed.