Pudupadi R. Sundararajan

Influence of double hydrogen bonds and alkyl chains on the gelation of nonchiral polyurethane model compounds: sheets, eaves trough, tubes and oriented fibers.

We describe the gelation, upon self-assembly of a series of nonchiral molecules, resulting in tubular morphology of the fibers. These are biscarbamates, which are model compounds for polyurethanes with two hydrogen bonding groups separated by a (CH(2))(6) spacer and symmetrically substituted with alkyl chains on either side varying in length from C(6) to C(18). Upon gelation, these molecules form a sheet initially. The sheets then tend to wrap, leading to tubules. Those with partial wrapping resemble eaves troughs. With the predominant growth along the hydrogen bonding direction, the energy of interaction between the molecules along this direction would be more dominant than just the van der Waals interaction in the other two, leading to asymmetry of interaction in the sheet. We rationalize such tube formation in this case on the basis of the theories [Schnur, J. M.; Ratna, B. R.; Selinger, J. V.; Singh, A.; Jyothi, G.; Easwaran, K. R. K. Science, 1994, 264, 945. Schnur, J. M. Science, 1993, 262, 166. McKierman, R. L.; Heintz, A. M.; Hsu, S. L.; Gido, S. P.; Penelle, J. Polym. Mater. Sci. Eng. 2001, 84, 416.] which were developed for amphiphilic mono and bilayers, which state that these bilayers are not at their minimal energy when they are flat, and prefer a bent state. The gelation behavior of this series with double hydrogen bond is compared with the monocarbamates with a single hydrogen bonding motif [Moniruzzaman, M.; Sundararajan, P. R. Langmuir 2005, 21, 3802.]. We attribute the tendency toward both gelation and crystallization in some cases to the competing contributions of the hydrogen bond and van der Waals interactions between the long alkyl side chains. Oriented fibers for X-ray diffraction were obtained by simply using a magnetic stir bar during gelation. It is also found that the solvents that gel with diureas [van Esch, J.; Kellogg, R. M.; Feringa, B. L. Tetrahedron Lett. 1997, 38, 281] also form gels with biscarbamates. While the biscarbamates with even number of CH(2) groups in the side chain form short fibers, one with (CH(2))(9) side group formed long fibers characteristic of organo gels. In addition, when this odd side chain biscarbamate was mixed with one having a (CH(2))(8) side group, the gelation led to long fibers, following the sergeant and soldier principle.

Solvent mixture induced self assembly of a terthiophene based rod-coil block co-oligomer.

We describe the synthesis and self-assembly of a terthiophene-oligostyrene based block co-oligomer Ter-OSa. While the self-assembly of block copolymers is normally achieved using block-selective solvents, tetrahydrofuran (THF) is a good solvent for both terthiophene and oligostyrene blocks. The self-assembly in this case arises with the nonsolvent (H(2)O) in THF/H(2)O mixtures. The absorption spectra showed a blue shift and fluorescence spectra showed quenching of fluorescence with increasing water content indicating that the Ter-OSa self-assembled to form H-aggregates. The morphology was studied using scanning electron microscopy. Depending upon the amount of water the Ter-OSa undergoes morphological transition from hollow to rigid spheres. However at higher water content the Ter-OSa formed large spheres from the fusion of smaller spheres. A mechanism of morphological transition from hollow spheres to rigid spheres is proposed.

Effects of Carbon Atom Parity and Alkyl Side Chain Length on the Crystallization and Morphology of Biscarbamates, A Set of Model Compounds for Polyurethanes

Solid state morphology and crystallization behavior of a homologous series of biscarbamate molecules having varying alkyl side chain lengths with different carbon atom parity were investigated. These are model compounds for polyurethanes. We synthesized a set of biscarbamates with double hydrogen bonding motifs separated by a (CH(2))(6) spacer and with alkyl side chains of various lengths ranging from C(3) to C(18) at the ends. Thermal analysis showed an odd-even alternation in their melting temperatures and heats of fusion, with the odd number of carbon atoms in the side chain having higher melting temperatures and heats of fusion than the even numbered ones, in contrast to the case of n-alkanes. The effect of carbon atom parity in the alkyl side chains on the spherulite size, spherulite growth rate, and isothermal crystallization kinetics was studied. Although the spherulite size increases with the alkyl side chain length, the maximum is seen at an intermediate length and not with a short or long alkyl chain for both the odd and even series. Along this series of molecules, a maximum in spherulite size, spherulite growth rate, and rate of crystallization is seen for C(7)C(6) (odd series) and C(8)C(6) (even series) biscarbamates. There is a significant difference in spherulite size with respect to carbon atom parity in the alkyl side chains as well as sample preparation protocol. Hence the length of the alkyl side chain, carbon atom parity in the alkyl side chains, and the sample preparation protocol (i.e., quenching versus slow cooling) play an important role in the morphology of these molecules. We rationalize this behavior with the relative contributions of hydrogen bonding and van der Waals forces as discerned from IR spectroscopy. While the van der Waals interaction increases with the alkyl side chain length in this series, the hydrogen bond contribution remains invariant. The rate of crystallization follows the trend seen with the spherulitic growth. The Avrami exponent is significantly smaller than 3, varying from 1.6 to 2.5 depending on the side chain length, which indicates that the crystallization is not truly three-dimensional. This study on the influence of alkyl side chains shows that while hydrogen bonding enables self-assembly the van der Waals interactions play a significant role in the crystallization and morphology of such self-assembled structures.

Carbon nanotube reinforced porous gels of poly(methyl methacrylate) with nonsolvents as porogens.

We fabricated porous organogels of poly(methyl methacrylate) (PMMA) using nonsolvents as porogens. In contrast to the use of inorganic materials, surfactants, etc., as porogens, we used the nonsolvents for PMMA such as water, methanol, propanol, and cyclohexane. This offers the advantage of not having to extract the porogen after gel formation. The nonsolvent simply evaporates. We find that the pore size can be controlled by matching the solubility parameters of the solvent/nonsolvent mixtures and that of PMMA. Incorporation of CNT itself does not lead to porous morphology in this case, unless a nonsolvent is added. Introducing carbon nanotubes during the addition of nonsolvent for gelation enhances the elongation ratio of the PMMA gel. We show that the CNT forms a network around the pores. Infrared and Raman spectra show no specific interaction between the CNT and PMMA. Hence, the former simply acts as a reinforcing filler.

Thermoreversible physical gels of poly(dimethylsiloxane) without cross-links or functionalization.

The preparation of gels of poly(dimethylsiloxane) (PDMS) reported in the literature so far involves catalysts and chemical cross-links (chemical gels) or functionalization with organogelators. We report that thermoreversible physical gels of PDMS, without cross-links or functionalization, can be made with propylamine or hexylamine as a solvent. The gels consist of spherical domains as small as 20 nm. We show that these spherical domains are part of a network. Differential scanning calorimetry (DSC), optical microscopy, and rheology show that the gel is thermoreversible. With the DSC experiments, we have devised a procedure to achieve thermoreversibility with very similar gel-sol transition endotherms in the first and second heating cycles.

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

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.

Morphological studies on the blends of polyethyleneoxide and cellulose ethers

SummaryThe blends of the water soluble polymers, PEO/HPC and PEO/CMC show crystallization of PEO over a wide composition range. The crystallinity of the PEO fraction decreases drastically beyond an HPC concentration of 70%, whereas it persists throughout the composition range in the case of the blend with CMC. This leads to the conclusion that at low concentrations, PEO is more compatible with HPC than with CMC. The films of the blends exhibit a skin as seen in the cross-sections of the films. Calorimetric studies show that annealing above Tm of PEO or prolonged annealing below Tm causes disorder of the PEO crystalline domains.

Extended and threefold helical conformations in gels of isotactic polystyrene

SummaryGels of isotactic polystyrene (iPS) were prepared, using solvents of different molecular geometries, by rapid quenching of the solutions formed at high temperatures. The extended conformation was observed in gels prepared with bulky, cyclic hydrocarbon solvents, whereas the aromatic solvents promoted the threefold helical structure. The gels containing the threefold helix were more elastic compared to those with the extended conformation. The effect of the solvent geometry is explained qualitatively.