Crystallization Property of Polybutyrolactam Modified by Deep Eutectic Solvents

Three deep eutectic solvents (DESs), choline chloride-urea (CU), choline chloride-oxalic acid (CO) and choline chloride-glycerol (CG) , were synthesized and utilized to modify polybutyrolactam (PBL) in order to lower its melting point. Differential scanning calorimetry, thermogravimetric analysis, infrared spectroscopy and X-ray diffraction were employed for investigating the effects of DES on the thermal stability, melting point and crystallization of PBL, as well as analyzing the underlying interaction mechanisms. Additionally, tensile testing and melt index measurements were conducted to verify the influence of DES on the mechanical and processing properties of PBL. CU partially disrupted the hydrogen bonds of PBL while forming new ones between NH groups on PBL and C=O on urea. This led to a distortion in the crystal lattice structure and parameters of PBL, resulting in a reduction in crystallinity by 16.2% along with a decrease in melting point from 265 ℃ to 253 ℃. CO distorted the lattice structure by forming new hydrogen bonds between oxalic acid carboxyl groups and free carbonyl groups on PBL, while CG primarily acted as a plasticizer; both had less influence on PBL hydrogen bonds compared to CU. As a result, they reduced crystallinity by only 4.4% and 1.9%, leading to decreases in melting point by 8 ℃ and 6 ℃ respectively. CU, the most effective DES, decreased the tensile strength of PBL by 3.5 MPa, increased the elongation at break by 15.6%, and lowered the melting temperature of PBL to below 32 5 ℃ with further addition of CU.