Poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) crystallize into a stereocomplex with a melting point 50°C higher than the crystals of the enantiomers. The racemic crystal is formed by packing -form 31-helices of opposite absolute configuration alternatingly side by side. Single crystals of the stereocomplex exhibit triangular shape. The drastic difference of the powder patterns evidences the different packing of the -form in the stereocomplex and in crystals of the pure lactides. By force field simulation of the stereocomplex and the PLLA unit cells and of their powder patterns, the reasons for the different packing could be clarified. Between the -helices in the stereocomplex, van der Waals forces cause a specific energetic interaction-driven packing and, consequently, higher melting point. Helices of identical absolute configuration pack different from pairs of enantiomer -helices. Packing favors R-type helication. A well-defined 103-helix has not been found. Good agreement with the experimental powder patterns proves the correctness of the simulations. On the basis of morphology, packing calculations, and atomic force microscopy, we propose a model of stereocomplex crystal growth, which explains the triangular shape of single crystals. Thus, for polymer components beyond chain folding length, the stereocomplex formation by simultaneous folding of the two types of chains is plausible. The triangular type of crystallizing offers favorable position for the polymer loops during the crystal growth. Our study of the PLA complexation mechanism may offer a chance to predict other polymeric stereocomplexes and their properties.

Blends of two polymers, poly(ethene-co-styrene) (PES) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), were examined with tapping mode atomic force microscopy (AFM) using various values of the driving amplitude A 0 and set-point amplitude ratio r sp = A sp/A 0, where A sp is the set-point amplitude. In height and phase images of PPO/PES blend samples, the relative contrast of chemically different regions depends sensitively on the r sp and A 0 values. As the tip−sample force is increased from small to large, both phase and height images of PPO/PES blend samples can undergo a contrast reversal twice. This makes it difficult to assign the features of height and phase images to different chemical components without performing additional experiments. Phase and height images were interpreted by analyzing several factors that affect the dependence of phase shift and amplitude damping on r sp and A 0.

We have analyzed the influence of different amounts w bc of two diblock copolymers, poly(styrene-b-methyl methacrylate) (sm blend series) and poly(cyclohexyl methacrylate-b-methyl methacrylate) (cm blend series), on the morphological and rheological characteristics of a blend containing w = 7.5 wt % polystyrene in poly(methyl methacrylate) matrix. The morphological analysis is based on the sphere size distribution function, which was determined from the image analysis of the transmission electron micrographs. Using this function and assuming that all block copolymers are located at the interface, the interfacial area per copolymer joint, Σ, was calculated. From its hyperbolic dependence on w bc the value at the critical micelle concentration, Σcmc, was found to be about 10 nm2 for both systems. The rheological analysis reveals that in addition to the form relaxation process, well-known for polymer blends, a new relaxation process is observed for these systems. Its relaxation time, τβ, has been studied in dependence on the amount of added block copolymers. The observed phenomena for each blend series, i.e. constant blend viscosity, slight shift of the form relaxation times τ1, and systematic shift of the interface governed relaxation time τβ (τβ > τ1), have been interpreted quantitatively. In contrast to τ1, τβ is less influenced by the interfacial tension but is mainly governed by an additional contribution, the interfacial shear modulus. Formulas were derived from an expanded version of the Palierne emulsion model which allows the determination of the proposed interfacial properties from rheological measurements. In general, the interfacial tension decreases with increasing amount of block copolymer, and the decrease is more pronounced for the cm blend series. The interfacial shear modulus increases during compatibilization from 0 to amounts which are in the range of 20−30% of the interfacial tension. The decrease of interfacial tension is in good agreement with predictions from Leibler's brush model extended by Dai et al. In conclusion, it was found that the Palierne model with an nonisotropical interfacial stress state is quantitatively correct to describe the observed phenomena for those blends.

Since the publication of reports of scanning tunneling microscopy (STM) images of liquid crystals and normal alkanes on graphite,"] the generation and interpretation of STM images of organic insulating molecules adsorbed on conducting substrates have received much attention.r2] In general, the contrast variations of these images correspond to the geometrical features of the adsorbed molecules.All STM studies of normal alkanes adsorbed on graphite show that the alkane chains are aligned parallel to the graphite surface, in agreement with the conclusions drawn from earlier calorimetric studies.[31 However. it is still a matter of debate whether the orientation of the all-truns carbon skeleton of the adsorbed molecule is parallel[4. 51 or perpendicularL6] to the graphite surface. So far the question as to why insulating molecules adsorbed on a conducting surface are imaged by STM has not satisfactorily answered.In view of the increased use of STM in studying the twodimensional (2D) ordering and dynamics of the adsorbed molecules on surfaces,I6, ' I it is crucial to know the electronic origin of the STM imaging of insulating molecules adsorbed on graphite.Recently, it has been shown['] that the STM images of layered transition-metal compounds can be rationally interpreted on the basis of their partial electron density plots, p ( r o , ef),[91 calculated by the extended Huckel tight binding (EHTB) electron band structure This approach has also been successful for the interpretation of the STM images of several 2: 1 salts of the organic donor molecule bis(et1iylenedi thi0)tetrathiafulvalene (BEDT -TTF ), i .e., (BEDT-TTF),X.["] These salts possess a layered structure in which layers of donor cations BEDT-TTFo.5Q alternate with layers of the Xo anions, so that the flat surfaces of the crystal samples can be either the anion or the cation layer. For metallic salts of BEDT-TTF, the anion layer is insulating, and the cation layer is metallic. ['21 By analogy with the case of a monolayer of organic molecules (insulating) adsorbed on a semimetallic graphite sur-

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