Nekane Guarrotxena

Influence of tacticity on the thermal degradation of PVC: 8. A comprehensive study of the local isotactic GTTG− conformation dependence of the mechanism of initiation

Some new attempts to demonstrate the superiority of the GTTG− conformation of isotactic triads relative to other labile structures containing allylic or tertiary chlorine in inducing the thermal degradation of poly(vinyl chloride) (PVC) are presented. They concern: (i) the degradation of polymers after nucleophilic substitution in two types of solvent involving the removal of the GTTG− arrangement in a quite different way; (ii) the degradation of PVC blends with a few additives which, as proved by Fourier transform infrared (FTi.r.) spectroscopy, are able to interact specifically with the mmr structure adopting GTTG−TT conformation at the end of isotactic sequences; (iii) the degradation of PVC after the disappearance of GTTG− conformations as a result of the GTTG−TT x → GTGTTT irreversible conformational change provoked by stretching of films at temperatures near the glass transition; and (iv) the degradation of PVC after FTi.r. checked changes in content of GTTG− conformation, induced by annealing at temperatures between 70 and 120°C. The correlations so obtained give evidence of the PVC thermal-instability-determining role of the GTTG− isotactic triad conformation.

Antitags: Nanostructured Tools for Developing SERS-Based ELISA Analogs

Immunology laboratories widely rely on the enzyme linked immunosorbent assay (ELISA) for protein detection and identifi cation. [ 1 , 2 ] A typical 96-well plate sandwich ELISA product provides inside each well an adsorbed primary antibody that binds its specifi c antigen in the fi rst step of the assay. Subsequently, a biotinylated antibody is added and binds to the antigen via a second recognition site. Streptavidin-linked horseradish peroxidase is then introduced, binds to the biotin moiety and is used to catalyze the oxidation of a substrate, thereby chemically amplifying the colorimetric or fl uorometric output signal of the reporter molecule. Raman spectroscopy allows one to distinguish a specifi c analyte based on the multiple narrow peaks characteristic of vibrational modes. However, due to its low cross-sections, it does not allow highly sensitive detection. Surface enhanced Raman spectroscopy (SERS) refers to the increase in signals observed in the vicinity of certain metallic nanostructures. [ 3 , 4 ] Single-molecule detection has been observed in some cases. [ 5 ] The increased sensitivity, coupled with the spatial resolution and vibronic information, has led to a variety of schemes relevant to biodetection assays and in vivo incorporation. [ 6 , 7 ] A suitable environment resides between closely spaced nanoparticles (NPs) and can thus be readily obtained in suspensions of NP dimers or small aggregates. [ 8 , 9 , 10 ] Such solution-based systems offer the advantage of incorporating tags whose Raman enhancement is independent from surface-specifi c interactions. In this contribution we disclose the design, preparation and function of responsive multicomponent nanostructures we have

Local chain configuration dependence of the mechanisms of analogous reactions of PVC. I. A conclusive study of the microstructure evolution in SN2 nucleophilic substitution

The nucleophilic substitution in poly(vinyl chloride) (PVC) with sodium benzenethiolate has been studied in two kinds of solvent differing in the molecular structure in the vicinity of the carbonyl group. From the evolution of the content of isotactic (mm), heterotactic (mr), and syndiotactic (rr) triads; and of mmmm, mmmr and rmmr isotactic pentads, in the unmodified parts of the polymer, as followed by 13C-NMR, it is unambiguously inferred that any chlorine but the central one of either the isotactic triad at mmr tetrads or the heterotactic triad at rmrr pentads is unreactive. Only a small fraction of mmr tetrads reacts occasionally by the central chlorine of its mr triad instead of the mm. Of those structures, the mmr, especially when located at the end of long isotactic sequences, proves to be highly reactive compared to the rmrr structure. By comparing quantitatively the microstructural changes with degree of substitution and taking into account that the reaction is of SN2 type, the mechanisms of substitution through the three foregoing reactive chlorines have been stated. They are found to be independent of the type of solvent and to account for all the changes in triad and pentad content as experimentally found. Instead, the solvent dependence of the ratio between the mmr- and rmrr-based processes of substitution is such that the depletion of mmr compared to that of rmrr structure may be controlled. The conformational sensitivity of this behavior is discussed on the basis of side work in our laboratory. As a whole, the results of the present work provide some original concepts as to the role of the tacticity dependent microstructure and the related local conformations in the chemical reactions of PVC.

On the relationship between tacticity dependent molecular microstructure and glass transition temperature of poly(vinyl chloride): a novel approach through model polymers with controlled, chemically induced microstructure

Abstract Nucleophilic substitution in poly(vinyl chloride) (PVC) with sodium thiobenzoate (NaTBZ) has been studied in two kinds of solvent. From the evolution of the content of isotactic (mr) and heterotactic (mr) triads in the unmodified parts of the polymer, as investigated by 13C n.m.r., the reaction was found to behave similarly to that previously studied with sodium benzenethiolate (NaBT) in that only the local configurations mmr and rrmr at the end of isotactic and syndiotactic sequences respectively are reactive. The same reaction with both nucleophiles has also been studied in aqueous suspension in the presence of a phase transfer catalyst. In all cases the ratio of mmr content to rrmr content is found to vary according to the experimental conditions, in particular the type of solvent, irrespective of the nucleophile. Accordingly, model polymers having well known tacticity-derived microstructure can be obtained. On the other hand, the evolution of Tg with degree of conversion has been studied by calorimetric measurements. Tg has been found to change linearly with conversion and the correlation between the corresponding rates and the evolution of the ratio between the mmr and rrmr contents proves to be linear too. From these studies it follows that Tg depends on two well defined factors: one, variable in character, is the nucleophile nature; the other is the microstructure in terms of the isotactic and syndiotactic sequences and, especially, the local configurations and conformations at the end of them. Interestingly, this effect is permanent in character and is independent of the substituent. The results that we have obtained provide new concepts on the involvement of some specific molecular microstructural features in the Tg phenomenon.

Local chain configuration dependence of the mechanisms of analogous reactions of PVC—3. An approach to SN2 nucleophilic substitution in condensate state

Abstract The nucleophilic substitution reaction on poly(vinyl chloride) (PVC) with sodium benzenethiolate has been studied in the melt state in the presence of the interacting agents: di-(2-ethylhexyl) phthalate, poly(ϵ-caprolactone), poly(ethylene adipate), poly(ethylene oxide), poly(ethylene glycol) and butyl stearate. The aim was that of viewing whether or not the local chain configuration based mechanism of reaction, as extensively investigated for the same reaction in solution, is a general behaviour and it applies to reaction conditions close to those of the polymer processing. The presence of one interacting agent proves to be necessary for the reaction to exceed 1% conversion. With all the agents the kinetic curves attain a plateau after a rather fast reaction period is elapsed. The reaction yield is found to depend on the agent nature and is not influenced by the amount of agent, which, on the other hand, governs the kinetics. The evolution of isotactic (mm), heterotactic (mr) and syndiotactic (rr) triad contents with degree of substitution for every interacting agent has been followed by 13 C NMR. Comparing the results obtained herein and those previously obtained in solution and in heterogeneous phase, two conclusions may be drawn straightforwardly: one is that the reaction proceeds by the mm or the rm triads at the end of isotactic or syndiotactic sequences respectively, i.e. by the mmr or rrmr local structures, the former being much more reactive than the latter; the other is that this mechanism is independent of the reaction conditions utilized. In contrast the superiority of mmr relative to rrmr in reacting strongly depends on the ability of the interacting agent to make the mmr adopt the highly reactive GTTG-TT conformation. A short overview of this stereospecific mechanism and of its implications is given.

Charge decay properties of poly(propylene) samples (PP) with various stereochemical compositions

Charge decay properties at 90°C have been studied for four poly(propylene) (PP) samples of different content of isotactic (mm), syndiotactic (rr) and heterotactic (mr, rm) triads. Comparing the charge decay properties with the previously measured space charge distribution and taking into account the specific stereochemical sequences which characterize each of the samples, it is apparent that mmrm and rrrm sequences furnish better storage properties. In contrast, mrrr and short heterotactic sequences would both exhibit worse retention power of negative and positive charge. Correlations - although still tentative - justify those specific stereochemical sequences as being well defined factors for determining the electrical properties of PP.

Molecular microstructure dependence of space charges in polymers. Coupling of thermal step and thermally stimulated discharge current methods in PVC after stretching at around 90°C

Abstract A film of poly(vinyl chloride) (PVC) was stretched at 90°C under an inert atmosphere to an elongation ratio, λ, of 1.1, 1.2, 1.3 and 1.5. From the evolution of the dichroic ratio R = A ∥ A ⊥ of υ(CCl) 615 and 637 cm −1 frequencies and of relative absorbance A 615 A 637 in the perpendicular direction, as measured by FT i.r. spectroscopy, it is concluded that stretching causes both the conformational change GTTG − TT ⇒ GTGTTT for mmr isotactic tetrads, and the orientation of chains, to be operative, but the relative proportion of both phenomena varies with λ, the former one being the major effect up to λ = 1.1. The space charge distribution of all the samples has been studied by coupling the ‘thermal step’ (TS) and the ‘thermal stimulated discharge current’ (TSDC) techniques. The results, as analysed in the light of earlier work, show that the nature and distribution of the space charges which appear in the film submitted to high electric field, strongly depend on the GTTG − TT and GTGTTT conformations of mmr, in particular the former.

Versatile thiolated thermosensitive polymers synthesized by ATRP of MEO2MA and AcSEMA, a new methacrylic monomer with a protected thiol group

Herein, the synthesis of a new monomer containing a protected thiol group, 2-(acetylthio)ethyl methacrylate (AcSEMA), is introduced. The monomer has been copolymerized via atom transfer radical polymerization (ATRP) with 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) to obtain a series of well-defined hidden-thiol functionalized thermosensitive polymers. The new system exhibits a sharp lower critical solubility temperature (LCST) and after hydrolysis of the acetyl group, the thiolated copolymers exhibit pH responsiveness. Moreover, to show the versatility of AcSEMA, P(MEO2MA-co-AcSEMA) copolymers were in situ hydrolyzed and modified by thiol-ene Michael addition with some acrylate compounds. The click reaction was successfully performed as revealed by NMR and the change in the LCST. We finally demonstrate that the addition of these polymer coatings onto gold nanoparticles (AuNPs) results in the formation of stable, colloidal thermosensitive polymer@AuNP complexes due to bridge formation between the thiol groups of AcSEMA and the metallic NP surfaces. The formation of temperature responsive polymer coated plasmonic nanoparticles shows the promise of P(MEO2MA-co-AcSEMA) copolymers for building multifunctional nanostructures for drug-delivery, diagnosis, tagging, catalysis and organic electronics systems.

Local chain configuration dependence of the mechanisms of analogous reactions of PVC. II. A further evidence through the changes in conformationally sensitive FTIR bands with degree of nucleophilic substitution

SYNOPSIS The evolution of the ν C-Cl bands of the infrared spectrum of a Bernoullian though slightly isotactic poly(vinyl chloride) (PVC), with both the degree of S N 2 substitution reaction with sodium benzenethiolate, as studied earlier, and the increase of the nucleophile infrared bands, has been studied by FTIR spectroscopy. In a parallel way, the changes in the same bands, in particular those at 615 and 637 cm -1 , presumably induced by S N 2 substitution, have been estimated, theoretically, by comparing the sequential order and the number of the distinct conformationally sensitive vibration modes of C-Cl bond, prior and after substitution, for a series of polymer sequences containing the reactive sites, namely the isotactic mmr tetrad and the heterotactic rmrr pentad, according to earlier work. The experimental behaviour of the νC-Cl bands is found to be in close agreement with the theoretical expectations, thereby allowing two main conclusions to be drawn : (i) during the early stage going up to conversions of 10-12%, the reaction proceeds in a nearly exclusive manner, by the mmr and rmrr terminal of long isotactic and syndiotactic sequences, respectively ; and (ii) any reaction event throughout the substitution process proves to be highly dependent upon the local environment in which each of the foregoing reactive structures finds itself. In summary, the local configurational nature of the mechanisms of analogous reactions of polymers is strongly suggested on the grounds of the results given herein.

On the relationship between stereochemical microstructure and the glass transition temperature (Tg) of poly(vinyl chloride) (PVC): some conclusive evidence given by stereospecific modification with sodium 2-mercaptothiobenzoate

Abstract The mechanisms of nucleophilic substitution in poly(vinyl chloride) (PVC) with sodium 2-mercaptothiobenzoate (NaMBT), as studied in recent works, have been outlined. Like substitution with other nucleophiles formerly studied, the reaction proceeds only by the tetrad mmr and the pentad rrmr, the termini of the isotactic and syndiotactic sequences, respectively. In contrast, unlike in those nucleophiles, mmr is found to react exclusively by the mm triad, because the competing reaction by the mr triad proves to be nil with NaMBT. This is demonstrated to be the reason why the evolution of T g with substitution, as studied herein, agrees thoroughly with the ratio of mmr to rrmr, contrary to what happened with the nucleophiles that exhibit a minority reaction by the mr triad of mmr. Such a novel quantitative correlation, compared with the more tentative ones obtained previously, allows one to furnish a valuable insight into the prevailing role of stereochemical microstructure in the process of glass transition in PVC.