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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.

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We describe the gelation of two-component immiscible blends consisting of a set of low molecular weight organogelators and a polymer. We studied the blends of poly(3-caprolactone) (PCL) and biscarbamates (model compounds for polyurethanes) with two hydrogen bonding motifs separated by a (CH 2 ) 6 spacer and symmetrically attached alkyl side chains. Neither PCL nor the biscarbamates form gels with chloroform in their neat forms, although the latter form gels with several other solvents. It is known that PCL does not form a gel by itself in any solvent. Being part of a copolymer or an inclusion complex is necessary for a PCL based gel. In the current work, mixtures of biscarbamates and PCL in chloroform led to phase separation and gelation upon cooling the solution. This gelation behavior depends on the alkyl side chain length of the biscarbamates. Those with alky side chain length below C 11 remained soluble and beyond C 13 precipitated within the concentration range studied here. Morphological studies showed immiscibility between the biscarbamate and PCL, similar to the case of polyurethanes with PCL as the soft segment. When biscarbamates are minor components in the solvent cast blend films, they form aggregate crystals in the PCL matrix. Upon melting and recrystallizing, PCL forms droplets in the matrix of biscarbamates, although it is the major component. X-ray diffraction studies of the xerogels confirmed the immiscibility of the PCL and biscarbamates and ATR-FTIR spectra showed no change in the hydrogen bonding of these self-assembling biscarbamate molecules. Microscopic studies revealed fibrous morphology of the composite gels, which consist of biscarbamate fibers dispersed in the matrix of PCL fibers. The biscarbamate fibers impregnated in the polymer matrix show the same eavestrough and hollow tube morphology as seen in our previous work (Khanna et al., Langmuir, 2009, 25, 13183).

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We present studies on two‐component organo‐gels of small molecule analogues of polyurethanes. Whereas the two‐component organo‐ or polymer gels reported in the literature so far involve two types of polymers or polymer/ small molecules, this paper deals with blends of homologous pairs of biscarbamates, which differ only in the number of CH2 groups in the side chain. The mixture of any two biscarbamates self‐sort when crystallized from the melt or solution. We also showed before that these molecules, with a side chain shorter than (CH2)6 do not form gels by themselves as a single component. However, in this work, it was found that a blend of biscarbamates with side chains (CH2)3 and (CH2)15 (denoted as C3/ C15 henceforth) formed a homogeneous gel with benzonitrile as the solvent. A small ratio of the C15 component is sufficient to depress the precipitation of C3 and form a gel, leading to co‐assembly in these gels. Such an effect was also seen in the gelation of blends of C8 and C9, which differ by just one CH2 group in the side chains (Δx=1). With the gels of C7/C9 blends (Δx=2), an ageing effect was seen in that the as‐prepared gel did not show any self‐sorting, but the aged gel did. Self‐sorting was observed in the two‐component gels of biscarbamates with Δx > 2. We believe that this is the first case of two‐component organo‐gels of homologous pairs in which co‐assembly and self‐sorting phenomena were observed.

Hydrogen bond-mediated self-assembling biscarbamates have been studied by our group as model compounds of polyurethanes with respect to the crystallization and gelation behaviour in both solid state and solution phase respectively. The aim of my thesis is to investigate the effects on the blending and the gelation of biscarbamates with different alkyl side chain lengths. Melt blending of these molecules shows molecular selectivity and self-sorting behaviour leading to immiscibility. We revealed that the difference in the growth rates of the individual species is responsible for the self-sorting behaviour in these non-chiral synthetic compounds. We discussed methods to bridge the self and non-self that would lead to a more homogeneous system. The gelation properties of biscarbamates with an odd number of carbon atoms in the alkyl side chains, such as the critical gelation concentration, gelation time, gelation temperature, and morphology of the gel fibres were examined. Biscarbamates show odd-even effects in their thermal and gelation behaviours as a function of carbon atom parity in the alkyl side chains in a similar manner to their crystallization behaviours. The blending of the biscarbamate gels shows three different types of blending behaviours. The C8/C9 blend and any gel blends composed of a biscarbamate smaller than C6 display the sergeant-soldier behaviour. The majority rules principle was observed for odd-odd gels blends with an intermediate difference in side chain length. The odd-odd gels blends with a small difference in side chain length self-sort.Organogels with a series of biscarbamates as gelators were prepared using microwave (MW) heating source as well as conventional heating. Biscarbamates with alkyl side chain lengths varying from C5 to C18 were used, with six solvents having dipole moments ranging from 0.07 to 4.3D. The minimum gelation concentration and the amount of heating for ii dissolution were significantly reduced with MW heating with benzonitrile, compared to the conventional heating for all the side chain lengths of the biscarbamates. MW heating is found to be effective with solvents possessing large dipole moments. Although the gels consist of fibers using both methods, an inherent orientation of these fibers was seen with MW heating versus conventional heating methods.iii ACKNOWLEDGEMENTS