In-situ SAXS study on the alignment of ordered systems of comb-shaped supramolecules:A shear-induced cylinder-to-cylinder transition

A tooth rheometer, designed to investigate in-situ the influence of large-amplitude oscillatory shear on the macroscopic orientation of complex fluids, is used to study the alignment of two supramolecular systems composed of a polyisoprene-block-poly(2-vinylpyi-idine) block copolymer with octyl gallate (OG) hydrogen bonded to the vinylpyridine block. The molecular ratio x between OG and pyridine groups in these two PI-b-P2VP(OG)(x) systems is 0.50 and 0.75, respectively. In both cases, a hexagonally ordered cylindrical self-assembly was revealed by small-angle X-ray scattering in a broad temperature range. The spacing of the hexagonal structure decreases significantly on heating and reversibly increases on cooling. In in-situ SAXS experiments, performed with the tooth rheometer, a gradual macroscopic alignment of the nanoscale structure is observed on heating for both supramolecular systems. The most striking feature is a shear-induced transition from one hexagonal structure to another, more aligned, hexagonal structure observed for PI-b-P2VP(OG)0.75 in the temperature range 120-140degreesC. The transition is accompanied by an abrupt reduction of the domain spacing and additionally by a decrease of the phase angle measured by the rheometer. In the PI-b-P2V-P(OG)(0.5) system a comparable reduction in the spacing is observed at 90-95degreesC. In this case, it coincides with the most intensive macroscopic alignment of the sample, proceeding in a continuous rather than discontinuous fashion. This behavior is discussed in terms of the breaking of the hydrogen bonds between OG and P2VP being facilitated by shear

Intermediate segregation type chain length dependence of the long period of lamellar microdomain structures of supramolecular comb-coil diblocks

A characteristic intermediate segregation type chain length dependence of the long period D of the lamellar microdomain structure of a class of comb-coil supramolecules is reported. The supramolecular comb-coil diblock copolymers studied consist of a polystyrene (PS) “coil” block and a “comb” block of poly(4-vinylpyridine) (P4VP) either hydrogen bonded to pentadecyl phenol (PDP) (i.e., P4VP(PDP)-b-PS) or first protonated with methanesulfonic acid (MSA) and then hydrogen bonded to PDP (i.e., P4VP(MSA)(PDP)-b-PS). In both cases we find a scaling D ~ Ntotδ, δ ≈ 0.8, where Ntot denotes the total number of monomers of the P4VP-b-PS backbone. In the case of diblock copolymers this would correspond to a characteristic intermediate segregation regime behavior. Pure PS-b-P4VP, on the other hand, shows the expected strong segregation behavior D ~ Ntotδ, δ ≈ 0.7.

Self-Assembly of Supramolecular Triblock Copolymer Complexes

Four different poly(tert-butoxystyrene)-b-polystyrene-b-poly(4-vinylpyridine) (PtBOS-b-PS-b-P4VP) linear triblock copolymers, with the P4VP weight fraction varying from 0.08 to 0.39, were synthesized via sequential anionic polymerization. The values of the unknown interaction parameters between styrene and tert-butoxystyrene and between tert-butoxystyrene and 4-vinylpyridine were determined from random copolymer blend miscibility studies and found to satisfy 0.031<χS,tBOS<0.034 and 0.39<χ4VP,tBOS<0.43, the latter being slightly larger than the known 0.30<χS,4VP≤0.35 value range. All triblock copolymers synthesized adopted a P4VP/PS core/shell cylindrical self-assembled morphology. From these four triblock copolymers supramolecular complexes were prepared by hydrogen bonding a stoichiometric amount of pentadecylphenol (PDP) to the P4VP blocks. Three of these complexes formed a triple lamellar ordered state with additional short length scale ordering inside the P4VP(PDP) layers. The self-assembled state of the supramolecular complex based on the triblock copolymer with the largest fraction of P4VP consisted of alternating layers of PtBOS and P4VP(PDP) layers with PS cylinders inside the latter layers. The difference in morphology between the triblock copolymers and the supramolecular complexes is due to two effects: (i) a change in effective composition and, (ii) a reduction in interfacial tension between the PS and P4VP containing domains. The small angle X-ray scattering patterns of the supramolecules systems are very temperature sensitive. A striking feature is the disappearance of the first order scattering peak of the triple lamellar state in certain temperature intervals, while the higher order peaks (including the third order) remain. This is argued to be due to the thermal sensitivity of the hydrogen bonding and thus directly related to the very nature of these systems.

Temperature modulated calorimetry of glassy polymers and polymer blends

The theoretical modeling of the relaxation behavior of polymers in the glass transition region, advocated by Moynihan and co-workers, has been used to analyze the heat flow and the relaxation of polymer systems during isothermal modulated DSC experiments in the glass transition region. An analytic solution for the frequency dependent fictive temperature is obtained, which takes a particularly simple form in the high-frequency region. The maximal phase lag of the fictive temperature Tf is βπ/2, where the exponent of the stretched-exponential characterizing the enthalpy relaxation, β, is on the order of 0.1-0.7. The corresponding maximal phase lag in the heat flow is much smaller, on the order of 2-5 deg. It is once more iterated that, as observed long ago by Birge and Nagel, the loss heat capacity corresponds to the entropy production due to a redistribution of energy over the heat baths. The possibility of using specific-heat spectroscopy as a tool to determine miscibility in polymer blends whose constituents possess similar glass transition temperatures is discussed. Compared to conventional differential scanning calorimetry, the resolution is enhanced. However, in many cases an unambiguous conclusion still requires additional enthalpy relaxation of the blend induced by physical aging in the glassy state.