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While ordered bicontinuous double diamond (OBDD) in block copolymers has always been considered as an unstable structure relative to ordered bicontinuous double gyroid (OBDG), here we report the existence of a thermodynamically stable OBDD structure in a diblock copolymer composed of a stereoregular block. A slightly asymmetric syndiotactic polypropylene-block-polystyrene (sPP-b-PS) as cast from xylene was found to display the OBDD morphology. When the OBDD-forming diblock was heated, this structure transformed to the OBDG phase at ca. 155 °C. Interestingly, OBDD was recovered upon cooling even in the temperature range above melting point of sPP, indicating that OBDD was a thermodynamically stable structure for sPP-b-PS melt, which was in contradiction to the conventional view. We propose that the larger free energy cost encountered in OBDD due to the larger packing frustration may be compensated sufficiently by the release of free energy due to local packing of the conformationally ordered segments of sPP blocks, which stabilizes the OBDD structure at the lower temperatures.

The ordered bicontinuous double diamond (OBDD) structure has long been believed to be an unstable ordered network nanostructure, which is relative to the ordered bicontinuous double gyroid (OBDG) structure for diblock copolymers. Using electron tomography, we present the first real-space observation of the thermodynamically stable OBDD structure in a diblock copolymer composed of a stereoregular block, syndiotactic polypropylene-block-polystyrene (sPP-b-PS), in which the sPP tetrapods are interconnected via a bicontinuous network with Pn3̄m symmetry. The OBDD structure underwent a thermally reversible order-order transition (OOT) to OBDG upon heating, and the transition was accompanied with a slight reduction of domain spacing, as demonstrated both experimentally and theoretically. The thermodynamic stability of the OBDD structure was attributed to the ability of the configurationally regular sPP block to form helical segments, even above its melting point, as the reduction of internal energy associated with the helix formation may effectively compensate the greater packing frustration in OBDD relative to that in the tripods of OBDG.

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The crystallization behavior of the blends of two crystalline−amorphous diblock copolymers bearing chemically different crystalline block has been studied. A series of lamellae-forming blends of a polystyrene-block-poly(ethylene oxide) (SEO) with shorter block lengths and a polystyrene-block-poly(l-lactide) (SLLA) with longer block lengths were prepared. Over the composition range investigated, the PS blocks from these two copolymers were found to mix intimately in the PS lamellar microdomains in the melt; moreover, the PEO and PLLA blocks also formed a miscible mixture in their domains. Because of the constraint imposed by the junction points localized at the lamellar interface and the nanoscale confinement effect, the crystallization behavior of PEO and PLLA blocks within the lamellar microdomains was found to deviate from that in the corresponding homopolymer blends. In the SLLA-rich blends, the junction point constraint coupled with the poor chain mobility at low crystallization temperature (≤∼45 °C) hampered the relatively long-range transport of PLLA segments for crystal growth. In this case, a local demixing between a fraction of PEO and PLLA chains took place, yielding the PLLA crystalline domains in which the PLLA crystalline stems were intervened by the PEO chains. This crystalline species gave rise to a relatively broad peak at 2θ = 15.92° in the wide-angle X-ray scattering profile and displayed a much lower melting point of ca. 100 °C compared to that of the typical α-form crystal of PLLA. It was suggested that the inserted PEO chains served as the molecular defects which induced an expansion of the a-axis and b-axis of the α-form PLLA unit cell and lowered the crystal melting point due to introduction of defect free energy.

Changes in particle size distribution play an important role in fluidized bed processes, and these changes are dominated by elutriation and carryover of fines and by attrition or agglomeration. In this study on attrition in gas-fluidized beds, we found that the attrition is a function of the particle size distribution, the jet velocities and the overall superficial gas velocity. Empirical equations have been developed to predict the attrition rate. Les Keywords: attrition, fluidized bed, grid velocity.he particle size distribution in a gas-fluidized bed is con-T tinuously changing as the result of (i) elutriation and carryover of fines, and (ii) attrition or agglomeration. Whereas, the effect of carryover on the bed composition can be limited by recycling the collected fines to the bed, attrition and agglomeration will produce extra amounts of respectively, fine or coarse particles, both resulting in the modification of the bed composition.Agglomeration occurs mostly with sticky particles, in moist air or at high temperature (Ray et al., 1987) and is not studied in this work.Particle attrition in fluid-bed systems was first studied to characterize catalysts for fluid-cracking systems. Forsythe and Hertwig (1 949) illustrate early efforts to select catalysts to resist the effect of high-velocity jets. In spite of subsequent research, as illustrated below, attrition is not fully understood yet. Evaluating attrition is important in many fluid-bed systems for process operation (e.g. bed loss during start-up), control of fines production to minimize downstream deposits or erosion, controlled attrition to maintain material reactivity, low operating cost, or environmental regulations (e.g. minimize fine particle collection cost). Vaw and Keairns (1980) tested several sources, i.e. particle heating, calcination, sulfatation, low and high velocity impacts, and their importance for particle size reduction through wear, fracture, decrepitation, abrasion, splitting, shattering, chipping and desintegration.Attrition of fluidized-bed solids can be caused by several mechanisms, including thermal stress (when particles are being heated or cooled and unequal temperatures in the particles cause uneven expansion, intra-particle stress and decrepitation); chemical stress (when during chemical reaction of a particle its surface will generally react first); static mechanical stress (when a bed particle is stressed at its surface *Author to whom correspondence should be addressed. E-mail address: jan haeyens@cit.kuleuven.ac.be e.g. at the bottom of a bed, or from within e.g. formation of gas by vaporization of hydrated water); and kinetic stress (when slow moving particles suffer surface abrasion upon collision or fast-moving particles shatter completely e.g. in cyclones or by high-velocity (grid) jets). Baeyens et al. (1989) illustrated thermal and chemical stresses during the calcination of limestone where evaporation, hydrate decomposition and/or calcination enhanced production of fines. The magnitude of attrition depends mostly...

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