Inhibiting the Self-Sorting Behavior in the Blends of a Homologous Set of Polyurethane Model Compounds

Abstract

Self-sorting is the phenomenon in which there is high fidelity recognition and preference only for self and avoidance of nonself (narcissistic self-sorting). It has been observed in a number of biological systems and chiral synthetic molecules. We found that blends of biscarbamates, which are model compounds for polyurethanes, self-sort during crystallization [ J. Phys. Chem. B 2008 , 112 , 4223 - 4232 ], although these are not chiral molecules. Even if the two components in the blend differ only by a couple of CH2 groups in the side chain length, no intercomponent hydrogen bond forms, and the molecules self-sort. They do not show any cocrystallization despite being part of a homologous series. We believe that it is the first reported example such behavior among synthetic nonchiral molecules. This is similar to the behavior of blends of hydrogen-bonding polymers including polyurethanes. We show that the difference in the growth rates of the individual species is responsible for the self-sorting behavior in these nonchiral synthetic compounds. While self-sorting might be advantageous for separation of blends, it poses a challenge for modifying properties such as the melting temperatures, spherulite size, etc., for various applications. We will discuss methods that were attempted to bridge the self and nonself that would lead to a more homogeneous system. We evaluated the miscibility using differential scanning calorimetry (DSC), since the occurrence of a single or multiple endotherms would indicate molecular level miscibility. This is similar to the behavior of glass transition temperatures in the case of polymer blends. Optical microscopy (OM) and X-ray diffraction (XRD) were also used. It is concluded that irrespective of the protocol followed for preparing the mixtures, mutual plasticization occurred in most cases (i.e., mixing of domains of the two species) and not molecular mixing.

Co‐Assembly and Self‐Sorting Effects in Gels of Blends of Polyurethane Model Compounds

“…In contrast to the cases of complimentary mutual self-recognition between segments of molecules mentioned above, [10][11][12][13][14][15][16][17] in the case of biscarbamates, the self-sorting occurs between homologous pairs of molecules that have the same hydrogen-bonding motifs, but differ in the length of the alkyl side chain. [21,22] Similar self-sorting was reported by Sijbesma group, using bis-ureas. [23][24][25] Wü rthner's group reviewed such self-sorting in large aromatic systems.…”

“…As mentioned above, in our previous work, we found that the blends of any two biscarbamates in the homologous series would not mix at the molecular level (i. e., co‐crystallize) but would act as mutual diluents and depress the melting temperatures of each other. We also discussed the influence of the length of the alkyl chain on the rates of crystallization and spherulite growth of these biscarbamates from the melt in a previous publication .…”

“…We showed before that blends of any two biscarbamates self‐sort when crystallized from the melt or solution. Using several approaches to curb the self‐sorting, we showed that these pairs at best form “compatible blends” by mutually depressing each other's the melting temperatures and not homogeneously mix at the molecular level that would lead to co‐crystallization . They would self‐sort irrespective of the way they were crystallized.…”

“…DOI: 10.1002/slct.201601696 when crystallized from the melt or solution. [21,22] Biscarbamates, despite the identical distance between the hydrogen bonds and only a small difference in the number of carbons on the side chain, display the ability to self-sort, and this behavior is more pronounced when the difference between the lengths of the side chains is large. In contrast to the cases of complimentary mutual self-recognition between segments of molecules mentioned above, [10][11][12][13][14][15][16][17] in the case of biscarbamates, the self-sorting occurs between homologous pairs of molecules that have the same hydrogen-bonding motifs, but differ in the length of the alkyl side chain.…”

“…The viscosity and T m can be varied by the choice of the length of the alkyl side chains. It was also observed that blends of two biscarbamates with different side chain lengths, both with the same distance between carboxylic groups, are not miscible when crystallized from the melt or solution . Biscarbamates, despite the identical distance between the hydrogen bonds and only a small difference in the number of carbons on the side chain, display the ability to self‐sort, and this behavior is more pronounced when the difference between the lengths of the side chains is large.…”