Gaetano Guerra

High-sensitivity optical chemosensor based on coated long-period gratings for sub-ppm chemical detection in water

In this work, the use of fiber long-period gratings (LPGs)—coated with nanoscale overlays of Syndiotactic Polystyrene (sPS) in the nanoporous crystalline δ form as specific and highly sensitive chemical sensors for in water monitoring—is proposed. The approach presented here, combines the excellent sorption properties of δ form sPS as a chemosensitive layer with the excellent refractive index sensitivity of LPG-based sensors as ideal transducers. In particular, when overlays with a high refractive index compared with the cladding one are deposited along the grating region, as in this case, the refraction-reflection regime at the cladding-overlay interface occurs. As result of this mechanism, the attenuation bands of coated LPGs would respond to the optical changes induced in the sensitive overlay due to chemical sorption by a significant modification of the peak central wavelength and intensity. The sensitivity depends strongly on the overlay thickness and the grating coupled cladding mode. Here, sensor p...

Vapor sorption in emptied clathrate samples of syndiotactic polystyrene

The analysis of chloroform vapor sorption at 35°C in semicrystalline syndiotactic polystyrene samples shows remarkably different sorption isotherms, depending on the crystalline form of the samples. In particular, “emptied” clathrate (“emptied” δ form) samples are characterized by higher equilibrium sorption levels and the differences are particularly relevant for low vapor activities. Moreover, sorption kinetics detected at a vapor activity equal to 0.5 show that in the case of “emptied” δ form samples the sorption rate is much higher than for the other semicrystalline samples. The larger sorption equilibrium uptakes and sorption rates of the “emptied” δ form samples are essentially due to their ability to absorb chloroform, already for low activities, by clathration in the crystalline phase. The measured equilibrium uptakes and sorption kinetics suggest that “emptied” δ form samples of syndiotactic polystyrene could be suitable for removing polluting chlorinated compounds from vapor and liquid streams.

Coated long-period fiber gratings as high-sensitivity optochemical sensors

In this paper, the numerical and the experimental analyses of coated long-period fiber gratings (LPFGs) as a high-sensitivity optochemical sensor are presented. The proposed structure relies on LPFGs coated with nanoscale high refractive index chemical-sensitive overlays. The deposition of overlays with refractive index higher than the cladding one leads to a modification of the cladding-mode distribution. If the overlay features are properly chosen, a strong field enhancement within the overlay occurs, leading to an excellent sensitivity of the cladding-mode distribution to the coating properties. The effects of overlay thickness and cladding-mode order on sensor performances have been numerically and experimentally investigated. In order to provide a high-sensitivity and species-specific optochemical sensor, this mechanism has been proved with nanoscale overlays of syndiotactic polystyrene (sPS) in the nanoporous crystalline /spl delta/ form. The sensitive material has been chosen in light of its selectivity and high sorption properties towards chlorinated and aromatic compounds. Sensor probes were prepared by using dip-coating technique and an adequate procedure to obtain the /spl delta/-form sPS. Experimental demonstration of the sensor capability to perform subparts-per-million detection of chloroform in water at room temperature is also reported.

Analysis of models for the Ziegler-Natta stereospecific polymerization on the basis of non-bonded interactions at the catalytic site—I. The Cossee model

Abstract The model proposed by Cossee for Ziegler-Natta polymerization of propene is re-examined by computing the non-bonded energies for all sets of possible internal coordinates of the atoms at the catalytic site. For catalytic sites in the bulk of (110) surfaces of TiCl3-α, it is possible to explain the regiospecificity of the reaction but, at variance with Cossee, we see no evidence that the surface conditions the methyl group of the olefin to protrude out of the crystal. The stereospecificity may arise however from the fixed, chiral orientation of the first carbon—carbon chain bond, due to interactions with the surface. Further computations seem to indicate that the polymerization could occur with much less steric hindrances on edges or reliefs of the surfaces, the relative positions of olefin and chain being exchanged in respect to the Cossee model.

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.

Polymeric sensing films absorbing organic guests into a nanoporous host crystalline phase

Abstract Syndiotactic polystyrene (s-PS) semicrystalline films, including the nanoporous crystalline δ phase, have been tested as sensing elements of resonant sensors for vapors of volatile organic compounds (VOCs). In particular, the response to chloroform vapor of quartz crystal microbalance (QCM) sensors, coated with films of semicrystalline δ form of s-PS have been analyzed and compared to analogous systems coated with films of amorphous atactic polystyrene (a-PS). The sensitivity of sensors based on s-PS films was found markedly higher than those based on a-PS, particularly for low chloroform pressures. The higher sensitivity of the semicrystalline s-PS films is associated with a peculiar sorption mechanism: in fact, the organic compound, rather than being dissolved only into the amorphous phase, as it is generally the case for semicrystalline polymers, is mainly absorbed into the nanoporous crystalline phase, each molecule being confined into regularly spaced crystalline nanocavities, thus leading to a clathrate structure.

Normal vibrational analysis of the syndiotactic polystyrene s(2/1)2 helix.

The full vibrational spectra of the gamma, delta, and epsilon crystalline phases of syndiotactic polystyrene (sPS), i.e., phases presenting the s(2/1)2 helical conformation, have been experimentally determined and compared with that calculated at the B3LYP/6-31G(d,p) level of theory for an infinite helix. The assignment of the different modes was highly facilitated and validated by the experimental evaluation of the direction of the transition moment vector of most IR peaks, which was made possible for the first time by measurements on sPS films with different uniplanar orientations of the crystalline phase. The normal vibration analysis of most representative modes of the periodic model allowed for a general description of each one to be obtained, which was further confirmed by the direct inspection of mode animations.

Regeneration of nanoporous crystalline syndiotactic polystyrene by supercritical CO2

Guest desorption procedures for s-PS clathrate samples, leading to formation and regeneration of the nanoporous δ phase, are compared. An extraction procedure, based on supercritical carbon dioxide, allows an easy and fast recovery of the guest molecules operating under relatively mild conditions (90–200 bar, 40°C) and generates a completely empty δ form, also starting from the most stable s-PS clathrate forms. In agreement with a previously proposed crystal structure of the nanoporous δ form, X-ray diffraction patterns of δ form powders obtained by this procedure do not show the peak, which is reduced but still apparent in samples extracted with previous procedures based on boiling solvents.

Optical chemo-sensor based on long period gratings coated with /spl delta/ form syndiotactic polystyrene

In this work, a long period grating (LPG) is coated with a thin film of syndiotactic polystyrene (sPS) in the nanoporous crystalline /spl delta/ form in order to obtain a species-specific chemical sensor. The combination of the intrinsic and high refractive index sensitivity of the LPGs with the high sorption properties toward chlorinated and aromatic compounds of sPS /spl delta/ form allows detecting very low concentrations (in the range of few parts per million) of chloroform in water. Chemical detection is carried out by measuring the wavelength shift and the amplitude changes in the attenuation band of the LPG due to analyte sorption.

Steric control in Ziegler-Natta catalysts: An analysis of nonbonded interactions at model catalytic sites

Abstract The stereospecificity of Ziegler-Natta heterogeneous catalysts is analyzed in terms of non-bonded interactions at the simplest model catalytic sites, which still retain the essential characteristics of chirality. The energetic calculations described show that model catalytic sites, comprising only three titanium atoms, may lead to a reasonable explanation of the stereospecificity in the polymerization of propene. Active sites of this type could be present in all the layered modifications of TiCl3. As well as in the more complex catalytic surfaces examined in previous papers, the main factor in determining the stereospecific behavior of such sites is the fixed, chiral orientation into which the growing polymeric chain appears to be forced. The possible factors favoring the primary insertion of the olefin are also discussed.