Effect of an organoclay on the reaction-induced phase-separation in a dynamically asymmetric epoxy/PCL system

The addition of layered silicates can significantly affect the phase behaviour of both immiscible thermoplastic blends and partially miscible thermoset systems that undergo reaction-induced phase separation (RIPS) during curing. This study focuses on the phase behaviour of polycaprolactone (PCL)/epoxy in the presence of organically modified montmorillonite (oMMT). Due to the high dynamic asymmetry caused by the differences in the molecular weights and viscosities of the PCL and the uncured epoxy, the critical point is localised at low PCL concentrations, as indicated by the pseudophase diagram. The addition of oMMT to the system led to the marked shift of the critical point towards higher concentrations of PCL, with an increase in the oMMT content occurring as a consequence of the preferential localisation of the clay in the epoxy phase, making this phase more dynamically slow. Significant changes in morphology, including phase inversion of the PCL/epoxy systems caused by the presence of oMMT, were recorded for PCL concentrations ranging from 10 to 30%

Dipole derivatives and infrared intensities of the ester group. An ab initio and force field study of methyl acetate

Dipole derivatives have been obtained (using both 3-21G and 6-31G basis sets) for methyl acetate. Using a force field refined from experimental data on methyl acetate and its deuterated derivatives, infrared intensities have been calculated. The good frequency and intensity agreement indicate that these force constants and dipole derivatives can serve as satisfactory transferable parameters for the ester group.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27093/1/0000084.pd

Conformational Structure of Ordered Forms of Stereoregular Poly(Methyl Methacrylates)

The results of the studies of the conformational structure of stereoregular PMMA obtained by means of infrared spectroscopy, combined with NMR spectroscopic studies of the kinetics of the aggregation of s-PMMA, are described. It was found that both the aggregation of s-PMMA and PMMA stereocomplex formation lead to the generation of double helices with a large number of monomer units per turn, and that such heli ces also exist in all three crystalline forms of stereoregular PMMA (i-PMMA, s-PMMA, stereocomplex) in the solid state. Interactions leading to the formation of the stereocomplex can enforce this structure even in s-PMMA chains with not too high stereoregularity

C=O stretch mode splitting in the formic acid dimer: Electrostatic models of the intermonomer interaction

The physical origin of the large (74 cm-1) splitting between the symmetric (Ag) and antisymmetric (Bu) components of the C=O stretch mode in the formic acid dimer has previously been attributed to tautomerism effects, transition dipole--dipole coupling, or dynamical charge transfer through the hydrogen bonds. We show that an electrostatic model involving atomic charge--charge interactions can account for a splitting of 56 cm-1, provided the atomic partial charges are allowed to vary in magnitude during vibrational motion. The charges and charge derivatives have been obtained from ab initio Hartree--Fock calculations up to the 6-31G** level. An additional 13 cm-1 of the remaining discrepancy in the splitting of 69 cm-1, compared to the observed value of 74 cm-1.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26690/1/0000237.pd

Organic nanofibers embedding stimuli-responsive threaded molecular components

While most of the studies on molecular machines have been performed in solution, interfacing these supramolecular systems with solid-state nanostructures and materials is very important in view of their utilization in sensing components working by chemical and photonic actuation. Host polymeric materials, and particularly polymer nanofibers, enable the manipulation of the functional molecules constituting molecular machines, and provide a way to induce and control the supramolecular organization. Here, we present electrospun nanocomposites embedding a self-assembling rotaxane-type system that is responsive to both optical (UV-visible light) and chemical (acid/base) stimuli. The system includes a molecular axle comprised of a dibenzylammonium recognition site and two azobenzene end groups, and a dibenzo[24]crown-8 molecular ring. The dethreading and rethreading of the molecular components in nanofibers induced by exposure to base and acid vapors, as well as the photoisomerization of the azobenzene end groups, occur in a similar manner to what observed in solution. Importantly, however, the nanoscale mechanical function following external chemical stimuli induces a measurable variation of the macroscopic mechanical properties of nanofibers aligned in arrays, whose Young's modulus is significantly enhanced upon dethreading of the axles from the rings. These composite nanosystems show therefore great potential for application in chemical sensors, photonic actuators and environmentally responsive materials.Comment: 39 pages, 16 figure

The effect of halloysite modification combined with in situ matrix modifications on the structure and properties of polypropylene/halloysite nanocomposites

The effect of various modifications/intercalations of halloysite and the combination of these modifications with in situ PP matrix modification was investigated with respect to the structure and properties of the polypropylene/halloysite nanocomposites. Hexadecyl-tri-methyl-ammonium-bromide (HEDA), 3-aminopropyltrimethoxysilane and urea were used as the intercalators/modifiers. The best intercalation was found for urea, although an unexpected insignificant impact on the mechanical properties also resulted as a consequence of the urea polarity and the significant decrease in PP crystallinity. However, the simultaneous application of 4,4!-diphenylmethylene dimaleinimide (DBMI) brought about an increase in the mechanical behavior by increasing the halloysite/PP affinity as a result of in situ matrix modification. This effect was further supported by coupling between the PP and halloysite (HNT) in the system containing urea-intercalated HNT. This can be explained by the occurrence of a urea-supported reaction between the imide ring of DBMI and the OH groups of the HNT, which resulted in the best mechanical behaviors achieved in this study