Crystal structures and polymorphism of polymers: Influence of defects and disorder

The crystal structures and the polymorphism of polymers are described on the basis of the main principles that define the conformation of polymer chains in the crystalline state and the modes of packing of macromolecules. We show that the presence of defects and disorder in the crystals influences the polymorphic behavior. The cases of polymorphism of isotactic poly(butene) (iPB) and syndiotactic poly(styrene) (sPS) are ilustrated as examples. In the case of iPB, the effect of the presence of defects of stereoregularity and of comonomeric units on the crystallization of form I and form II is described as an example of alteration of the crystallization behavior because of the modification of both thermodynamic stability and crystallization kinetics from the melt of the polymorphic forms. The case of sPS is taken as an example of a very complex polymorphic behavior arising from the presence and development of structural disorder

Nanometal Skin of Plasmonic Heterostructures for Highly Efficient Near-Field Scattering Probes

In this work, atomic force microscopy probes are functionalized by virtue of self-assembling monolayers of block copolymer (BCP) micelles loaded either with clusters of silver nanoparticles or bimetallic heterostructures consisting of mixed species of silver and gold nanoparticles. The resulting self-organized patterns allow coating the tips with a sort of nanometal skin made of geometrically confined nanoislands. This approach favors the reproducible engineering and tuning of the plasmonic properties of the resulting structured tip by varying the nanometal loading of the micelles. The newly conceived tips are applied for experiments of tip-enhanced Raman scattering (TERS) spectroscopy and scattering-type scanning near-field optical microscopy (s-SNOM). TERS and s-SNOM probe characterizations on several standard Raman analytes and patterned nanostructures demonstrate excellent enhancement factor with the possibility of fast scanning and spatial resolution <12 nm. In fact, each metal nanoisland consists of a multiscale heterostructure that favors large scattering and near-field amplification. Then, we verify the tips to allow challenging nongap-TER spectroscopy on thick biosamples. Our approach introduces a synergistic chemical functionalization of the tips for versatile inclusion and delivery of plasmonic nanoparticles at the tip apex, which may promote the tuning of the plasmonic properties, a large enhancement, and the possibility of adding new degrees of freedom for tip functionalization

Propylene–Butene Copolymers: Tailoring Mechanical Properties from Isotactic Polypropylene to Polybutene

Isotactic propylene-butene copolymers [i(P-co-B)] with precise and controlled molecular structures were synthesized with various organometallic catalysts having different stereoselectivities. Stereoregular and stereodefective samples of i(P-co-B) with 1-butene (B) content variable in the whole range of composition were synthesized. All samples crystallize regardless of composition, indicating cocrystallization of propene and 1-butene units, which are incorporated in the unit cells of polymorphic forms of isotactic poly(propylene) (iPP) and isotactic poly(1-butene) (iPB). The copolymers show a continuum change of crystal morphology with the composition, transforming from big spherulites to bundle-like and needle-like crystals, to granular crystals. The cocrystallization allows maintaining high crystallinity of copolymers for any composition and provides an opportunity to develop outstanding mechanical properties that can be tailored by changing the isotacticity and composition. This allows, ideally, combining in the same material the different properties of stiffness of iPP and flexibility of iPB. These copolymers show, indeed, mechanical properties intermediate between iPP and iPB, ranging from stiffness/brittleness and ductility/flexibility depending on the composition and isotacticity, with high strength and Young's modulus that may be regulated by the stereoregularity of the iPP and iPB sequences, which is, in turn, dictated by the catalyst structure

Morphology of Isotactic Polypropylene–Polyethylene Block Copolymers Driven by Controlled Crystallization

A study of the morphology of diblock copolymers composed of two crystalline blocks of isotactic polypropylene (iPP) and polyethylene (PE) is shown. The samples form phase-separated structures in the melt because of the incompatibility between iPP and PE blocks. Cylindrical PE microdomains are visible at room temperature in the sample with a PE volume fraction of 26%, rapidly quenched from the melt in liquid nitrogen. In the quenched sample, PE crystallizes inside the PE cylindrical microdomains, whereas crystals of iPP are not visible in the iPP domains because the quenching prevents crystallization of the lamellar α form. Less rapid cooling of the melt produces, instead, breakout crystallization, where the phase-separated structure of the melt is destroyed by the slow crystallization of the α form of iPP and of PE. The succession of crystallization of iPP and PE and the resulting final morphology have been analyzed by inducing selective and different orientations of iPP and PE crystals through epitaxial crystallization onto the benzoic acid (BA) crystal substrate. Epitaxy produces oriented crystallization of iPP and PE, with a unique alignment of PE lamellar crystals and a double orientation of iPP crystals on to the (001) exposed face of BA. Epitaxy destroys the phase-separated structure of the melt and induces the formation of ordered lamellar nanostructures with alternated layers of iPP and PE, whose orientation is defined by the alignment of PE or iPP crystals, which, in turn, is determined by epitaxy. The results indicate that crystalline block copolymers offer the opportunity to create nanoscale patterns on thin films and improve the possibility of controlling the microstructure of block copolymers and the alignment of microdomains by controlling the crystallization process

Time-resolving small angle X-Ray scattering analysis of melt crystallization of mixtures of regular and irregular isotactic polypropylene samples

The melting/crystallization properties of blends obtained by mixing two isotactic polypropylene (iPP) samples synthesized using single-site metallocene catalyst systems and containing a high and low concentration of rr triads as stereo-defects, are studied. The changes occurring at lamellar length scale during a heating/cooling cycle at constant scanning rate are followed in situ by performing time-resolved small angle X-ray scattering (SAXS) measurements. Data analysis demonstrates that the evolution of the SAXS intensity with increase/decrease of the temperature is controlled by the separate melting/crystallization of the two components, the differences in the thermal expansion (contraction) coefficient of the amorphous and crystalline phases and the role of thermal fluctuations in electron density. The two components give rise to different populations of intermixed lamellar stacks in the blends which originate from the good miscibility of the low and high stereoregular samples in the melt.info:eu-repo/semantics/publishedVersio

Nano-in-Nano Approach for Enzyme Immobilization Based on Block Copolymers

We set up a facile approach for fabrication of supports with tailored nanoporosity for immobilization of enzymes. To this aim block copolymers (BCPs) self-assembly has been used to prepare nanostructured thin films with well-defined architecture containing pores of tailorable size delimited by walls with tailorable degree of hydrophilicity. In particular, we employed a mixture of polystyrene-block-poly(l-lactide) (PS-PLLA) and polystyrene-block-poly(ethylene oxide) (PS-PEO) diblock copolymers to generate thin films with a lamellar morphology consisting of PS lamellar domains alternating with mixed PEO/PLLA blocks lamellar domains. Selective basic hydrolysis of the PLLA blocks generates thin films, patterned with nanometric channels containing hydrophilic PEO chains pending from PS walls. The shape and size of the channels and the degree of hydrophilicity of the pores depend on the relative length of the blocks, the molecular mass of the BCPs, and the composition of the mixture. The strength of our approach is demonstrated in the case of physical adsorption of the hemoprotein peroxidase from horseradish (HRP) using 2,2?-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) with H2O2 as substrate. The large surface area, the tailored pore sizes, and the functionalization with hydrophilic PEO blocks make the designed nanostructured materials suitable supports for the nanoconfinement of HRP biomolecules endowed with high catalytic performance, no mass-transfer limitations, and long-term stability

Development of lab-on-chip integrated systems based on block copolymers for advanced sensing

In the present thesis different classes of nanomaterials based on di-block copolymers (BCPs) have been studied, with the purpose to design, characterize and fabricate functional nanostructures to be used as active elements in selective sensing and/or biosensing devices with high sensitivity. The work has been mainly focused on the creation of two classes of BCP based materials: nanocomposite materials, characterized by the selective inclusion of functional nanoparticles (NPs) in specific BCP nanodomanins, and nanoporous materials able to act as ideal support for the physical immobilization of specific biomolecules. Also, an explorative study regarding the use of BCPs as additives in photopolymerizable formulations has been conducted. The spontaneous self-assembly of BCPs has been used to prepare a large number of tailor-made nanostructured materials, that can be used as nanoscopic device components, templates or active layers in biosensors. The obtained results have confirmed that the self-organization of block copolymers offers a wide variety of methods for structuring and functionalizing materials at nanometric length scale. These methods that could overcome several of the intrinsic limitations of current top-down fabrication technologies

G-Qadruplexes from Human Telomeric DNA: How Many Conformations in PEG Containing Solutions?

G-quadruplex structures are an attractive target for the development of anticancer drugs as their formation in human telomere induces a DNA damage response followed by apoptosis in cancer cells. However, the development of new anticancer drugs by means of structural-based drug design, is hampered by a lack of an accurate information on the exact G-quadruplex conformation adopted by the human telomeric DNA under physiological condition. Several groups reported that in a molecular crowded, cell-like environment, simulated by polyethylene glycol (PEG), the human telomeric DNA adopts the parallel G-quadruplex conformation. These studies have suggested that 40% (w/v) PEG concentration induces complete structural conversion from the other known human telomeric G-quadruplex conformations to the parallel G-quadruplex, thus simplifying the high structural polymorphism existing in absence of PEG. In this study, we demonstrate that the structural conversion to the parallel G-quadruplex is not a complete reaction at physiological temperature. We report a complete kinetic and thermodynamic characterization of the conformational transitions involving the (TTAGGG)4TT and (TTAGGG)8TT human telomeric DNA sequences in K+ solution containing PEG. Our data show that the hybrid-type and parallel conformations coexist at equilibrium in presence of PEG at the physiological temperature and the degree of the quadruplex interconversion depends on the PEG molecular weight. Further, we find that telomeric DNA folds in the parallel quadruplex in seconds time scale, a much larger time scale than the one reported for the hybrid quadruplex folding (~ms). The whole of our data allow us to predict the relative amount of each G-quadruplex conformation as function of temperature and time. The effect of other crowding agent like Ficoll 400 and glycerol on the quadruplex interconversion has been also explored