Since the early 40’s, urea inclusion compounds (U-ICs), i.e. non-covalent compounds differing in the identity of the ‘guest’ species, have been intensively studied. As a result, a plethora of literature exists regarding these compounds. For instance, urea has been observed to form ICs with a wide range of small molecules, aliphatic straight-chain hydrocarbons, and some polymers. Other studies have reported the successful utilization of small molecule-U-ICs for solubilization, mixture separations, or as carriers in many applications. During the formation of conventional U-ICs, guest molecules act as the templates, while hydrogen-bonded urea molecules are arranged along the guests, and adopt a helical rotation, either right-handed or left-handed, and form channels where the guest molecules reside. Normally, the diameter of the urea channel formed in the U-IC is approximately 5.5 to 5.8 Å, which, unfortunately, limits the size of the included guest molecules. All in all, an in-depth review of previously reported studies has highlighted that all the polymer U-ICs or complexes show crystalline structures, either hexagonal, tetragonal or layered crystalline lattices.
Recently, North Carolina State University scientists: Dr. Shanshan Li, Dr. Jialong Shen, and Dr. Alan E. Tonelli reported on a complex material formed between poly (methyl methacrylate) (PMMA) and urea (U). Interestingly, they found out that, distinct to ICs or other crystalline complexes formed between urea and other guest polymers, the PMMA/U complexes formed using the acetone-methanol solvent system had non-crystalline structures, with disassociation temperatures higher than the glass transition temperature of PMMA. Their work is currently published in the research journal, Polymer.
The researchers prepared PMMA/U complexes by dropwise addition of methanol urea solutions into PMMA acetone solutions at room temperature. They noted that, for the asr-PMMA used, when forming PMMA/ U complexes with a higher concentration of asr-PMMA (PMMA > 1 wt %), an alkane contaminant was found to be encapsulated by urea molecules in the form of a crystalline U-IC within the PMMA/U complexes. In addition, they observed that the small contaminant molecule encapsulation ability relied on the urea concentration used in the PMMA/U complex preparation. Moreover, it was revealed that for lower urea concentrations, the non-crystalline PMMA/U complexes were formed without a small molecule encapsulation.
In summary, the North Carolina State University research group successfully reported the formation of an amorphous complex material between poly (methyl methacrylate) and urea. Generally, the disassociation temperatures of the PMMA/U complexes were seen to be higher than the glass transition temperature of neat PMMA and lower than the melting temperature of neat urea. Altogether, their study reported a phenomenon with immense potential to provide a viable technique for small molecule encapsulation and incorporation in a PMMA substrate by using urea as an encapsulant.
Shanshan Li, Jialong Shen, Alan E. Tonelli. Self-assembled complexation of urea with poly (methyl methacrylate): A potential method for small molecule encapsulation in PMMA. Polymer, volume 156 (2018) page 95–101.Go To Polymer