Two component gels of immiscible blends of biscarbamates (polyurethane model compounds) and poly(ε-caprolactone)

Abstract

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(ε-caprolactone) (PCL) and biscarbamates (model compounds for polyurethanes) with two hydrogen bonding motifs separated by a (CH₂)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 C₁₁ remained soluble and beyond C₁₃ 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).