In contrast to most diblock copolymers which exhibit the classical upper critical ordering transition (UCOT), polystyrene-b-poly n-butyl methacrylatePS-b-PBMAhas been shown to undergo ordering upon heating through a lower critical ordering transition (LCOT). Here we report the phase behavior of a family of diblock copolymers formed from styrene and a homologous series of n-alkyl methacrylates, as determined by combined dynamic rheological testing and small-angle neutron scattering (SANS). It is shown that the shortest side chain methacrylates, with the exception of methyl methacrylate, exhibit the LCOT, while for side chains longer than n-butyl, the copolymers exhibit the classical UCOT behavior. Combined group contribution/lattice fluid model calculations of the solubility parameter and specific volume of the corresponding homopolymers qualitatively support these observations. The same calculations were further employed to molecularly design LCOT behavior into a new diblock material consisting of styrene and a random copolymer of methyl and lauryl methacrylate, denoted PS-b-P(MMA-r-LMA). The success of this approach suggests a simple semiquantitative method for predicting and designing the phase behavior of weakly interacting polymer pairs.
Mayan crude, residuum, and hydrocracked asphaltenes have been separated into two fractions by extended Soxhlet extraction in n-heptane. Although the solubility, composition, and molecular structures differ slightly, the greatest difference between the two asphaltene fractions is the degree to which they associate in solution. The vapor-phase osmometry molecular weight, molecular size by size-exclusion chromatography, and small-angle neutron scattering indicate that approximately 25% of Mayan asphaltene is not highly associated in aromatic solvents and, thus, is noncolloidal. By contrast, the remaining asphaltene forms large, rodlike colloidal particles in solution and has a higher apparent molecular weight. Although laser desorption mass spectrometry indicates that the molecular weight of the individual molecules in maltenes and asphaltenes is not very different, high-resolution mass spectrometry indicates that the size of the aromatic core of asphaltenes is significantly larger than those in maltenes. Furthermore, the tendency of the residuum fractions to form coke during thermal cracking is likely related to the size of the largest polyaromatic rings.
22.5 have been studied by small angle neutron scattering ͑SANS͒, transmission electron microscopy ͑TEM͒, and differential scanning calorimetry ͑DSC͒. SANS data of samples annealed isothermally at 623 K exhibit an interference peak centered at qϭ0.46 nm Ϫ1 after an incubation time of about 100 min. TEM and DSC investigations confirm that the respective periodic variation in the scattering length density is due to the formation of nanocrystals embedded in the amorphous matrix. These observations suggest that during the incubation time a chemical decomposition process occurs and the related periodic composition fluctuations give rise to the observed periodic arrangement of the nanocrystals. © 1996 American Institute of Physics. ͓S0003-6951͑96͒04804-2͔In metallic glasses, crystallization and decomposition have been observed in undercooled liquids during cooling from the melt as well as during reheating experiments. [1][2][3] Phase separation requires diffusivity of the atomic species over several interatomic distances and at least two local minima of the Gibbs free energy versus composition function. In multicomponent alloys a large difference in the heat of mixing of the constitutive binary liquids can act as a thermodynamical driving force for phase separation. 4 In these systems the nucleation of crystalline phases is more difficult than for conventional binary metallic glasses, as expressed by the ''Confusion Principle. '' 5 Recently, a unique family of multicomponent glass forming systems with a high thermal stability and excellent glass forming ability was found. 6,7 The critical cooling rate to form these bulk metallic glasses ͑BMG͒ from the melt drops orders of magnitude compared to conventional metallic glasses. For the particular Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 alloy critical cooling rates as low as 1 K/s were found. 8 These low cooling rates allow one to observe phase separation taking place below a miscibility gap in the undercooled liquid region, so that decomposition in the as-prepared amorphous Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 alloy on a length scale of 50-80 nm was observed earlier. 9,10 This decomposition during the cooling process mainly involves the fast diffusion of Be. 11 In this letter, we will show that primary crystallization of the amorphous Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 alloy reheated into the supercooled liquid involves a decomposition process. Phase separation with respect to slower moving species determines the time scale of the primary crystallization in the supercooled liquid. The formation of nanocrystals during isothermal annealing of the Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 alloy has been investigated by small angle neutron scattering ͑SANS͒, transmission electron microscopy ͑TEM͒, and differential scanning calorimetry ͑DSC͒. The results from these studies suggest a decomposition process on a length scale of 13.7 nm preceding the primary crystallization of this alloy when annealed at 623 K.Amorphous samples were prepared from a mixture of the pure elements by...
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