Phase Structures and Morphologies Determined by Competitions Among Self-Organization, Crystallization, and Vitrification in a Disordered Poly(Ethylene Oxide)-B-Polystyrene Diblock Copolymer

A poly(ethylene oxide)-b-polystyrene (PEO-b-PS) diblock copolymer having a number-average molecular weight ((M) over bar(n)) of 11000 g/mol in the PEO blocks and an (M) over bar(n) of 5200 g/mol in the PS blocks has been synthesized (with a volume fraction of the PEO blocks of 0.66 in the molten state). Differential scanning calorimetry results show that this copolymer possesses a single endotherm, which is attributed to the melting of the PEG-block crystals. Based on real-time resolved synchrotron small-angle x-ray scattering (SAXS) observations, the diblock copolymer is in a disordered state above the glass transition temperature of the PS-rich phase (T-g(PS)) which has been determined to be 44.0 degrees C during cooling using dilatometer mode in thermomechanical measurements. The order-disorder transition temperature (T-ODT) for this diblock copolymer is thus experimentally inaccessible. Depending upon different isothermal crystallization temperatures quenched from the disordered state (T(q)s), four cases can be investigated in order to understand the phase relationships among self-organization, crystallization of the PEO blocks, and vitrification of the PS-rich phase: the region where the T-q is above the T-g(PS), the regions where the T-q is near but slightly higher or lower than the T-g(PS) ; and the region where the T-q is below the T-g(PS) . Utilizing simultaneous SPXS and wide angle x-ray-diffraction experiments, it can be seen that lamellar crystals of the PEO blocks in the first case grow with little morphological constraint due to initial disordered phase morphology. As the T-q approaches but is still slightly higher than the T-g(PS) , as in the second case, the PEG-block crystals with a greater long period (L) than that of the disordered state start to grow. The initial disordered phase morphology is gradually destroyed, at least to a major extent. When the T-q is near but slightly lower than the T-g(PS), the crystallization takes place largely within the existing phase morphology. Only a gradual shift of the L towards smaller q values can be found with increasing time, which implies that the initial phase morphology is disturbed by the crystallization of the PEO blocks. In the last case, the PEO blocks crystallize under a total constraint provided by the disordered phase morphology due to rapid vitrification of the PS-rich phase. Substantial decrease of crystallinity can be observed in this case. This study also provides experimental evidence that the PS-rich phase size, which is down to 7-8 nm, can still retain bulky glassy properties. [S0163-1829(99)01138-8]

Helical Single-Lamellar Crystals Thermotropically Formed in a Synthetic Nonracemic Chiral Main-Chain Polyester

Phase structures and transformation mechanisms of nonracemic chiral biological and synthetic polymers are fundamentally important topics in understanding their macroscopic responses in different environments. It has been known for many years that helical structures and morphologies can exist in low-ordered chiral liquid crystalline (LC) phases. However, when the chiral liquid crystals form highly ordered smectic liquid crystal phases, the helical morphology is suppressed due to the crystallization process. A double-twisted morphology has been observed in many liquid crystalline biopolymers such as dinoflaggellate chromosomes (in Prorocentrum micans) in an in vivo arrangement. Helical crystals grown from solution have been reported in the case of Bombyx mori silk fibroin crystals having the beta modification. This study describes a synthetic nonracemic chiral main-chain LC polyester that is able to thermotropically form helical single lamellar crystals. Flat single lamellar crystals can also be observed under the same crystallization condition. Moreover, flat and helical lamellae can coexist in one single lamellar crystal, within which one form can smoothly transform to the other. Both of these crystals possess the same structure, although translational symmetry is broken in the helical crystals. The polymer chain folding direction in both flat and helical lamellar crystals is determined to be identical, and it is always along the long axis of the lamellae. This finding provides an opportunity to study the chirality effect on phase structure, morphology, and transformation in condensed states of chiral materials. [S0163-1829(99)01042-5]

The genetic architecture of the human cerebral cortex

The cerebral cortex underlies our complex cognitive capabilities, yet little is known about the specific genetic loci that influence human cortical structure. To identify genetic variants that affect cortical structure, we conducted a genome-wide association meta-analysis of brain magnetic resonance imaging data from 51,665 individuals. We analyzed the surface area and average thickness of the whole cortex and 34 regions with known functional specializations. We identified 199 significant loci and found significant enrichment for loci influencing total surface area within regulatory elements that are active during prenatal cortical development, supporting the radial unit hypothesis. Loci that affect regional surface area cluster near genes in Wnt signaling pathways, which influence progenitor expansion and areal identity. Variation in cortical structure is genetically correlated with cognitive function, Parkinson's disease, insomnia, depression, neuroticism, and attention deficit hyperactivity disorder

Poly(ethylene terephthalate) Ionomer Based Clay Nanocomposites Produced via Melt Extrusion

Poly(ethylene terephthalate) ionomer (PETI)/organically-modified montmorillonite clay (OMC) nanocomposites were prepared via melt extrusion. Sulfonated PET containing various incorporations of ionic comonomer and clay modifications were investigated. The random incorporation of ionic functionalities along the PET backbone enhances interactions between the matrix polymer and montmorillonite clay resulting in the creation of polymer-clay nanocomposites exhibiting a predominately exfoliated morphology. The morphology is correlated with mechanical properties and crystallization behavior. It is found that incorporation of clay into the random ionomers leads to increased mechanical properties and slower crystallization rates. (c) 2005 Elsevier Ltd. All rights reserved

Crystallization Kinetics as a Probe of the Dynamic Network in Lightly Sulfonated Syndiotactic Polystyrene Ionomers

The effect of alkali metal counterion type on the crystallization kinetics of sulfonated syndiotactic polystyrene with ion contents of 1.4 mol % was investigated. For crystallization temperatures less than 180 °C, the rate of crystallization was independent of counterion type. In contrast, for temperatures greater than 180 °C, the rate of crystallization increased with increasing counterion size. Since the glass transition and melting temperatures were found to be constant for all counterion types, the variations in crystallization behavior were attributed to differences in chain diffusion within the dynamic network of electrostatic cross-links. At low temperatures, the rate of ion-hopping is slow for all of the counterion forms relative to the rate of crystallization, and thus crystal growth occurred in the presence of kinetically stable cross-links. At high temperatures, chain diffusion is controlled by the ion-hopping process, and the kinetics of crystallization become influenced by the strengths of ionic interactions. Between 180 and 215 °C, the rate of crystallization was found to be inversely proportional to the ionic radii of the counterions. As the activation energy for ion-hopping increased with decreasing counterion size, longer periods of time were required to achieve the same degree of crystallinity

Effect of Preferential Plasticization on the Crystallization of Lightly Sulfonated Syndiotactic Polystyrene Ionomers

The incorporation of 2.2 mol % sodium sulfonate groups along the backbone of syndiotactic polystyrene dramatically inhibits crystallization from the melt. However, small amounts of a surfactant, such as sodium dodecylbenzenesulfonate (DBSNa) may be used to selectively plasticize the ionic domains of sulfonated polystyrene ionomers. On the basis of Na-23 SSNMR results and the dynamic mechanical behavior of these ionomers, this preferential plasticization is postulated to perturb the electrostatic interactions within the ionic multiplets (i.e., electrostatic cross-links), destabilize the dynamic network, and thus increase the molecular mobility of the crystallizable chain segments. With enhanced chain mobility, a higher degree of crystallinity and an elevated rate of crystallization are observed. In contrast to the effect of DBSNa, the incorporation of phenyldodecane or sodium benzenesulfonate (i.e., the separate nonpolar and polar components of DBSNa, respectively) into the ionomer does not significantly affect the electrostatic network or enhance crystallization of sulfonated, syndiotactic polystyrene