Claudio De Rosa

Microdomain patterns from directional eutectic solidification and epitaxy

Creating a regular surface pattern on the nanometre scale is important for many technological applications, such as the periodic arrays constructed by optical microlithography that are used as separation media in electrophoresis, and island structures used for high-density magnetic recording devices. Block copolymer patterns can also be used for lithography on length scales below 30 nanometres (refs 3,4,5). But for such polymers to prove useful for thin-film technologies, chemically patterned surfaces need to be made substantially defect-free over large areas, and with tailored domain orientation and periodicity. So far, control over domain orientation has been achieved by several routes, using electric fields, temperature gradients, patterned substrates and neutral confining surfaces. Here we describe an extremely fast process that leads the formation of two-dimensional periodic thin films having large area and uniform thickness, and which possess vertically aligned cylindrical domains each containing precisely one crystalline lamella. The process involves rapid solidification of a semicrystalline block copolymer from a crystallizable solvent between glass substrates using directional solidification and epitaxy. The film is both chemically and structurally periodic, thereby providing new opportunities for more selective and versatile nanopatterned surfaces.

Evaluation by Fourier Transform Infrared Spectroscopy of the different crystalline forms in syndiotactic polystyrene samples

Fourier transform infrared (FTIR) spectra of syndiotactic polystyrene (s-PS) semicrystalline samples have been examined by using the spectral subtraction approach. For the crystalline forms including trans-planar chains (trigonal α and orthorhombic β) a number of conformational and structural order effects, not previously described in the literature, have been identified. A method based on the results of the spectral subtraction analysis has been developed for the determination of the crystallinity degree and compared with the standard method based on the wide-angle X-ray diffraction patterns. The spectral subtraction analysis on FTIR spectra allows also an easy evaluation of the amount of α and β crystalline phases (often simultaneously present in melt-crystallized samples) although both contain chains in a same conformation.

Enabling Strategies in Organic Electronics Using Ordered Block Copolymer Nanostructures

Today’s lithographic techniques for carving silicon into circuit patterns are unable to achieve the future target of the semiconductor industry of fabricating ultrahigh density memory devices made of memory cells just few tens of nanometers apart. [ 1 ] The primary metric for gauging progress in the various semiconductor integrated circuit technologies is, indeed, the spacing, or pitch, between the most closely spaced wires within a dynamic random access memory (DRAM) circuit. The circuit components on today’s silicon chips are more than 100 nm across and modern DRAM circuits have 140 nm pitch wires and a memory cell size of 0.0408 μ m 2 . [ 2 ] Improving integrated circuit technology will require that these dimensions decrease over time. However, at present a large fraction of the patterning and materials requirements that we expect to need for the construction of new integrated circuit technologies have no known solution. [ 2 ] Promising ingredients for advances in integrated circuit technology are nanowires, [ 3 ] molecular electronics [ 4 ] and defecttolerant architectures, [ 5 ] as demonstrated by reports of single devices [ 6–8 ] and small circuits. [ 9 , 10 ] Methods of extending these approaches to large-scale, high-density circuitry are largely undeveloped. The need for very high bit density (the number of memory elements per square centimeter) has pushed the research towards the study of new advanced materials that can overcome these limiting scaling diffi culties and of alternative methods for building memory devices from the bottom up using individual molecules. [ 1 ] These methods start with atoms and molecules and climb up to nanostructures through assembly by various mechanisms of molecular recognition. Self-assembly is emerging as an elegant bottom-up method for fabricating nanostructured materials. [ 11–15 ] Particularly attractive is the self-assembly of organic molecules that, when combined with

Conformational and packing energy of the crystalline α modification of syndiotactic polystyrene

Abstract Conformational and packing energy calculations have been performed on the α modification of syndiotactic polystyrene. The conformational energy has been optimized as a function of the internal parameters of the chain. The packing energy has been calculated considering at first the best packing of chains in triplets and then the best packing of triplets in the space groups R 3 and P 3 . The comparison of the results of the energy calculations with the X-ray experimental data corroborates a rhombohedral mode of packing of the triplets, corresponding to the space group R 3 .

On the form IV of syndiotactic polypropylene

The packing of the chains in (T6G2T2G2)n conformation of the form IV of s-PP is revisited on the basis of packing energy and structure factor calculations. According to this analysis, an alternative mode of packing has been suggested. A monoclinic structural model, with the unit cell centered on the C face, is obtained, after small changes of the atomic coordinates in the triclinic structural model as proposed by Chatani et al. The monoclinic model presents a lower packing energy than the triclinic model and a good agreement between the calculated and observed structure factors. The triclinic structural model implies that all the chains are rotated by the same amount around the chain axis with respect to the monoclinic structural model. Since clockwise and counter clockwise rotations are equivalent, the monoclinic structural model may be taken as descriptive of the order in the long range, for the form IV of s-PP, or in other terms, descriptive of an average structure (space group C2, unit cell constants equal to am = 14.17 A, bm = 5.72 A, cm = 11.6 A, and βm = 108.8°). The triclinic structural model for this polymorph, instead (space group P1, unit cell constants equal to at = 5.72 A, bt = 7.64 A, ct = 11.60 A, αt = 73.1°, βt = 88.8°, γt = 112.0°) is probably more properly descriptive of local situation of order (the symmetry, locally, is broken). Analogies between the monoclinic limit ordered structural model for the form IV and the orthorhombic limit ordered structural model for the form II (with chains in the more stable (TTGG)n conformation) of s-PP are also provided.

Influence of the stereoregularity on the crystallization of the trans planar mesomorphic form of syndiotactic polypropylene

The crystallization of the trans-planar mesomorphic form of syndiotactic polypropylene by quenching the melt at 0°C is investigated as a function of the stereoregularity of the samples. The formation of the trans-planar mesomorphic form at 0°C is followed as a function of the permanence times at 0°C by X-ray diffraction and FTIR spectroscopy. Samples kept at 0°C for short time rapidly crystallize into the helical form I at room temperature, whereas longer permanence times at 0°C increase the stability of the trans-planar mesomorphic phase which remains stable and inhibits the normal crystallization of the sample into the helical form at room temperature. The stereoregularity of the polymer sample strongly influences the rate of formation of the mesomorphic form at 0°C. Higher the syndiotacticity, easier the formation of trans-planar mesomorphic form. Very short permanence times (few hours) at 0°C are enough to form and stabilize the mesomorphic form for highly syndiotactic samples, whereas for stereo-irregular samples the amount of mesomorphic form observed when the sample is removed from the 0°C bath and heated to room temperature, remains always very low, even for very long permanence times (months) at 0°C and the crystallization into the normal helical form is not inhibited.

Polymorphism of syndiotactic polypropylene in copolymers of propylene with ethylene and 1-butene

Abstract The structural characterization, by X-ray diffraction and solid-state 13 C n.m.r. CPMAS, of copolymers of propylene with small amounts of ethylene and 1-butene, synthesized with a single centre syndiospecific catalyst, is presented. The effect of the presence of comonomeric units on the polymorphic behaviour of syndiotactic polypropylene is discussed. In the as-prepared samples, the presence of small amounts of ethylene induces the crystallization of conformationally disordered modifications of form II, intermediate between the limit ordered forms II and IV. The conformational disorder is characterized by the presence of long trans-planar sequences, including the ethylene units, and gives rise a structure with kink bands. In copolymers with 1-butene the usual crystallization in form I occurs. In samples of copolymers crystallized from the melt, the usual crystallization of form I is not affected by the presence of any comonomeric units, although disordered modifications of form I are always obtained.

Mechanical Properties and Elastic Behavior of High-Molecular-Weight Poorly Syndiotactic Polypropylene

An analysis of the mechanical properties and elastic behavior of low-stereoregular and nearly amorphous syndiotactic polypropylene (sam-PP), prepared with heterocycle-fused indenyl silyl amido dimethyltitanium complexes, is presented. High-molecular-weight poorly syndiotactic sam-PP samples show good elastic behavior at room temperature in a large range of deformation. While highly stereoregular and crystalline syndiotactic polypropylene shows good elastic properties only for previously stretched oriented fibers, sam-PP samples present good elasticity even for unoriented compression-molded films during the first stretching. Because of the very low crystallinity, these samples experience a negligible irreversible plastic deformation and show a typical behavior of thermoplastic elastomers. The small crystalline domains in the amorphous matrix act as physical knots of the elastomeric lattice, preventing the viscous flow of the amorphous chains. The elastic behavior is associated with a reversible polymorphic...

Polymorphism of syndiotactic poly(p-methylstyrene): oriented samples

Abstract Possible routes for obtaining oriented fibres of the various known crystalline forms of syndiotactic poly( p -methylstyrene) are described. These routes include solvent diffusion and/or annealing on mesomorphic fibres. The X-ray fibre diffraction patterns show that form I, form II and all the clathrate forms are characterized by chains with helical s(2/1)2 conformation and repeating distance of ≈7.8 A, while the mesomorphic form (form IV), form III as well as a new crystalline form (form V), are characterized by chains with trans planar conformation and repeating distance of ≈5.1 A.

Conformational and packing energy calculations on the two crystalline modifications of poly(trans-1,4-butadiene)

Conformational and packing energy calculations have been performed on the two crystalline modifications of poly(trans-1,4-butadiene), and the results have been compared with experimental data. The conformational energy calculations predict the chain axis and conformation of the modification that is stable below 76°C, without any a priori assumption. Packing energy calculations on this modification show that the best space group is P21a and the position of the chains is in good agreement with X-ray data. Conformational energy calculations on the modification that is stable above 76°C predict that the lowest energy conformation is a statistical sequence with a random distribution of the three minimum torsion angles around the single bonds adjacent to the double bond, with a trans conformation around the other single bond.