Search

In this paper, we report the temperature effects on the luminescence of polymer optical fibers doped with Lumogen dyes (red and orange). Specifically, we have analyzed the intensity and the average wavelength of the spectra emitted by both fibers when they are heated from 30 °C to 90 °C. An almost full self–recovery of the signal in the fiber doped with Lumogen red has been observed when the fiber is subjected to thermal cycles with oscillating temperature variations between 30 °C and 90 °C. The emitted spectra are red/blue shifted when the fiber sample is heated/cooled with a final temperature sensitivity around 0.07 nm/°C when a stationary situation is reached., Ministerio de Ciencia, Innovación y Universidades (MAT2014- 57429-R, PGC2018-095364-B-I00, PID2021-122505OB-C31, TED2021- 129959B-C21); Gobierno Vasco/Eusko Jaurlaritza (IT1452-22); ELKARTEK (KK‑2021/00082, KK‑2021/00092)

In this work, we detail two types of fabrication processes of four polymer optical fibers doped with lumogen dyes. The fiber preforms have been manufactured with two different methods: extrusion and casting. We have compared the performance of the two types of fibers as luminescent solar concentrators by calculating their optical efficiencies and concentration factors. The obtained results show better performance for those fibers manufactured by the casting process. We have also studied the photostability of the two types of fibers doped with the dye lumogen red under solar light radiation. A high thermal stability of the doped fibers has been observed., This work was funded in part by the Fondo Europeo de Desarrollo Regional (FEDER), in partby the Ministerio de Ciencia, Innovación y Universidades under projects RTC2019-007194-4, RTI2018-094669-B-C31 and MAT2014-5742-R and in part by the GobiernoVasco/Eusko Jaurlaritza IT933-16,ELKARTEK KK-2019/00101 (μ4Indust), and ELKARTEK KK-2019/00051 (SMARTRESNAK

This work presents a simple one-pot protocol to achieve core-doped shell nanohybrids comprising silver nanoparticles, curcumin and thermoresponsive polymeric shell taking advantage of the reducing properties of phenolic curcumin substance and its ability to decorate metallic surfaces. Silver nanoparticles were synthesized, via sodium citrate and silver nitrate addition into a boiling aqueous solution of curcumin, monomers and surfactant. Curcumin and sodium citrate promoted silver nucleation, acting as reducing and stabilizing agents. These curcumin-capped AgNPs enabled, after adding the radical polymerization initiator, the assembling of the growing polymer chains around the hydrophobic AgNP surface. The resultant core-doped shell nanohybrids exhibit plasmonic, luminescent and volume thermoresponsive properties, with improved possibilities to be used as successful therapeutic platforms. In fact, the possibility to nanoconfine the synergistic antioxidant, antiviral, antibacterial features of silver and curcumin in one bioavailable hybrid paves the way to promising applications in the biomedical field., The authors gratefully acknowledge the financial support provided by the Spanish Ministerio de Ciencia, Innovación y Universidades (Grant MAT2014–57429-R and PGC2018–095364-B-100). A.S.-Q. acknowledges CONACYT for the grant EPE 2017–291168. The authors would like to thank David Gómez Vargas assistance with the SEM measurements.

This work focuses on achieving the modulation of optical and swelling properties of nanomaterials based on the self-assembly of thermostimuli polymers around metallic nanoparticles (NPs) for the obtention of functional plasmonic-polymer core/shell nanohybrids. Moreover, an important effort is made for understanding the role of the polymer assembly, molecule interactions, and contributing effect of each nanohybrid’s single component (i.e., polymer and NP) in the final temperature-tune optical properties. In this regard, a two-step route for building-up uniform core/shell hybrid nanogels with tunable thermoresponsive and plasmonic properties is considered as the key vehicle to get a better knowledge of the processes governing the synthesis, behavior, and properties of these materials. These plasmonic nanohybrids are made of gold NPs coated with poly(2-(2-methoxyethoxy)ethyl methacrylate) P(MEO2MA) nanogels with tunable shell thickness by surfactant (SDS) participation. Thereby, this controlled methodology w...

The fabrication of functional surfaces based on arrangements of metallic nanoparticles (MNPs) is challenging for biomedical, optoelectronic, and catalytic applications. Herein, the fabrication of 3D nanostructured microfibers is reported by the controlled cooperative precipitation of MNPs on electrospun polymer fiber mats, at the critical solution temperature of a thermoresponsive copolymer coating the MNPs surface. To obtain the metallic decorated surfaces, several factors must be taken into consideration, such as the previous functionalization of the metallic nanoparticle surface and the selection of a suitable polymer concentration, pH, and temperature. The results show that the plasmonic properties of the prepared surfaces depend on the solution temperature. Among potential applications, the high specific area of the resulting Au@polymer@fiber nanocomposites is ideal to be exploited for catalytic applications. Thus, the reduction of 4¿nitrophenol has been studied as a model., The authors gratefully acknowledge the financial support provided by Ministerio de Economía y Competitividad of Spain (Grants MAT2014- 57429-R and MAT2015-65184-C2-2-R). The authors thank David Gómez Vargas assistance with the SEM measurements

The combination of multifunctionality and synergestic effect displayed by hybrid nanoparticles (NPs) has been revealed as an effective stratagem in the development of advanced nanostructures with unique biotechnology and optoelectronic applications. Although important work has been devoted, the demand of facile, versatile and efficient synthetic approach remains still challenging. Herein, we report a feasible and innovative way for polymer-shell assembling onto gold nanoparticles in competitive conditions of hydrophobic/hydrophilic feature and interfacial energy of components to generate core-shell nanohybrids with singular morphologies. The fine control of reaction parameters allows a modulated transformation from concentric to eccentric nanostructure-geometries. In this regard, a rational selection of the components and solvent ratio guarantee the reproducibility and efficiency on hybrid-nanoassembly. Furthermore, the simplicity of the synthetic approach offers the possibility to obtain asymmetric Janus NPs and new morphologies (quizzical-aspheric polymer-shell, named Emoji-N-hybrids) with adjustable surface-coating, leading to new properties and applications that are unavailable to their symmetrical or single components., This work was supported by the Ministerio de Economía y Competitividad (MINECO), Project MAT2014-57429-R.

Four different dye-doped polymer optical fibers (POFs) have been fabricated following a two-step fabrication process of preform extrusion and fiber drawing, using poly-(methyl methacrylate) (PMMA) as host material and dye derivatives from perylene and naphtalimide as active dopants. The side illumination technique (SIT) has been employed in order to determine some optical properties of the fabricated fibers, such as the side illumination coupling efficiency, optical loss coefficients, and their performance under solar simulator excitation. The aim of this work is to investigate the performance of the manufactured fibers for fluorescent lighting applications, specially targeting on fluorescent fiber based solar concentrators., This work has been funded in part by the Fondo Europeo de Desarrollo Regional (FEDER); by the Ministerio de Economía y Competitividad under project TEC2015-638263-C03-1-R; by the Gobierno Vasco/Eusko Jaurlaritza under projects IT933-16 and ELKARTEK (KK-2016/0030 and KK-2016/0059); and by Ministerio de Economía y Competitividad under project MAT2014-57429-R. The work carried out by Itxaso Parola has been funded by a research grant given by the Departamento de Educación, Política Lingüística y Cultura del Gobierno Vasco/Eusko Jaurlaritza for her PhD thesis

Gold nanoparticles (AuNPs) are functionalized with a thermoresponsive polymer shell of a cross-linked poly(2-(2-methoxyethoxy)ethyl methacrylate) (P(MEO 2 MA)). To provide a covalent attachment of the polymer to the NP surface, AuNPs are fi rst modifi ed using butanoic acid to develop the encapsulation with the biocompatible thermoresponsive polymer formed by free-radical precipitation polymerization. Both the MEO 2 MA concentration and the shell cross-linking density can be varied and, in turn, the thickness and the shells' free volume can be fi ne-tuned. By downscaling the size of the polymeric shell, the lower critical solution temperature (LCST) is decreased. The LCST in the nanohybrids changes from 19.1 to 25.6 °C when increasing the MEO 2 MA content; it reaches almost 26 °C for P(MEO 2 MA) (bulk). The maximum decrease in the volume of the nanohybrids is around 40%, resulting in a modifi cation of the light scattering properties of the system and causing a change in the turbidity of the gel network. The sizes of the nanohybrids are characterized using both transmission electron microscopy and dynamic light scattering measurements. Optical properties of the colloidal systems are determined using the derived count rate measurements as an alternative to absorbance or transmittance measurements, confi rming the colloidal stability of the nanohybrid systems.

We report on a study of the overall surface-enhanced Raman scattering (SERS) response from several thiol-terminated molecules located at interstitial sites between Ag nanoparticles. Multiplexing of the SERS signal was demonstrated along with its dependence of the molecular length, regiochemistry, and the number of thiol groups. The data collected establish pathways for the rational design of the SERS reporters and multiplexed sensory applications.

Three films of poly(methyl methacrylate) (PMMA) (samples X, Y, and Z) were prepared by compression molding of purified polymers as polymers having different tacticities. They were exhaustively characterized by 1H NMR spectroscopy so as to be able to evaluate the differences in content of the repeating stereosequences:…mmrm… (termini of isotactic sequences); ….rrrmr (breaking off syndiotactic sequences; pure heterotactic …mrmr… moistures; and atactic parts. By applying the so-called thermal step method, the behavior in space charge showed a priority charge injection from both electrodes and some polarization at the cathode. Some strong differences between the samples were evidenced and they accurately agree with the variations in the above repeating stereosequences for the samples. The results allow to define every stereosequence as specific trap of the distinct sorts of space charge, namely injection from the anode or the cathode and polarization. These attributions are common with those proposed for poly(vinyl chloride) (PVC) and poly(propylene) (PP) in earlier work. Conversely, whether the priority charge is of injection or polarization type, depends markedly on the type of polymer. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 633–639, 2009

Three samples of poly(methyl methacrylate) (PMMA) were characterized after careful purification. Their composition in terms of some repeating stereosequences, especially mmr termini of isotactic sequences, was determined. Their space charge behaviour was analysed using the thermal step (TS) method. A correlation between the electrical current intensity, as given by the TS method, and the stereochemical microstructure content in each sample was evidenced. The results are explained in the light of previous data on the specific microstructures present in the samples at different contents. The physical ageing of the samples was studied, quantitatively, through differential scanning calorimetry measurements prior to and after equally performed thermal treatments of the samples. The corresponding values of the loss of enthalpy agree with the content of mmr tetrad termini of isotactic sequences. The results are analysed on the basis of the differences in stereochemical composition of the samples. The results give further evidence of the role of some definite stereosequences in physical properties as proposed in earlier works on poly(vinyl chloride) and polypropylene. Copyright © 2006 Society of Chemical Industry

Poly(vinyl chloride) (PVC) samples prepared at various temperatures have been submitted to a substitution reaction for purposes of selectively removing the GTTG-TT conformations of the mmr termini of long isotactic sequences. The physical ageing of all samples has been studied by means of differential scanning calorimetry techniques. Surprisingly, a straight relation between the loss of enthalpy and the above GTTG-TT content, which pass through a minimum, has been found. Such a novel relationship should further lead to better understanding of the reasons why polymeric materials experience nonequilibrium states which are known to cause physical ageing.

A poly(vinylchloride) (PVC) obtained by bulk polymerization at 70 °C was modified through substitution reaction with sodium benzenethiolate (NaBT) in cyclohexanone solution at 25 °C, at various conversions up to 15 mol %. According to earlier work, this allows to change, in a controlled way, the stereochemical microstructure in terms of mainly the content of mmr (meso, meso, racemic) tetrad termini of isotactic sequences of at least one heptad in length, and of its likely chain conformations. The samples were all studied by Dielectric Relaxation Spectroscopy, which allows to measure the real and the imaginary parts of the complex dielectrical constant in a frequency range between 1 and 106 Hz, and a temperature range between 100 and 600 K. Most of the changes in the physical quantities that can be analyzed by this technique were found to parallel the changes in stereochemical microstructure with substitution extent. The temperature of the maximum of the dipolar losses, which relates to the temperature range where α relaxation occurs, appeared to increase somewhat up to 0.7% molar conversion. Then the tendency is towards stabilization. The relaxation time τ(T) as deduced from the Havriliak‐Negami (NH) function, tends to decrease between 0 and 0.7 substitution percent, and to increase afterwards. The evolution of the Ngai's β parameter with substitution degree is also highly illustrative. It clearly decreases up to 0.7% and then it increases rapidly up to 7% and slowly afterwards. These changes are discussed by taking into account that 0.7 and 7% substitution extents agree with the removal of mmr under GTTG−TT conformation and of the same structure under GTGTTT conformation, respectively. From the results, some original correlations between stereochemical microstructure and molecular dynamic behaviors are proposed., JJ.dV. is grateful to the Ministerio de Ciencia y Tecnología (projects CICYT MAT92‐0355 and MAT2001‐0070) for partial finantial support. A.E. thanks the Eusko Jaurlaritza for a grant.

The number of mmr- and rrm-based structures which occur necessarily whenever an isotactic or a syndiotactic sequence breaks off respectively; the extent to which they are isolated or are extended to atactic sequences, and the fact that the mmr repeating sequence, especially when it takes the GTTG−TT conformation, is shorter and exhibits greater local free volume than rrm, mmm and rrr sequences, are shown to be the stereochemical composition determining structures in poly(vinyl chloride) (PVC) and polypropylene (PP) polymers. These structures, as analyzed by 13C NMR spectroscopy and probability calculations, have been determined as a function of the overall tacticity for one series of samples prepared by bulk polymerization at temperatures varying from −50 °C to 70 °C, and one series of samples prepared by polarity-based fractionation of a commercial polymer. Using this approach, the stereochemical composition of the samples could be identified. The results are of interest to understand the changes in the physical properties as shown in earlier and current work for both series of polymers. © 2003 Society of Chemical Industry

Simultaneous detection of multiple proteins on a single spot can be efficiently achieved by using multiplexed surface-enhanced Raman spectroscopy (SERS)-encoded nanoparticle 'antitags' consisting of poly(ethylene glycol) (PEG)-protected silver dimers (and higher aggregates) and antibody-tagging entities. The effective SERS-based multivariate deconvolution approach guarantees an accurate and successful distinguishable identification of single and multiple proteins in complex samples. Their potential application in multiplexed SERS bioimaging technology can be easily envisaged. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Dynamic mechanical thermal analysis (DMTA) is an accessible and versatileanalytical technique where a sample is submitted to an oscillating stress or strain as a function ofoscillatory frequency and temperature. Through this non-destructive measurement, acomprehensive understanding of the dynamic mechanical properties of polymers may be obtained.Key words: PVC, PMMA, -relaxation, chain conformation, local motion, nucleophilicsubstitution, molecular microstructure.IntroductionThe structure property relationships aregreatly significant industrially and scientifically.Most of abundant research work in this field forthe last years has dealt with approaches to thewhole simulation of the physical behavior and,thence, to a macroscopic view of the relatedphysical properties of polymers. In contrast, noattempts to correlate the physical properties withthe molecular microstructure of polymers havebeen made. The implication is that little informationas to the nature and the mechanisms of themolecular nature involvement in the physicalbehaviour of these materials is available. In fact,the studies to correlate every physical propertywith any specific molecular structure, in particularthe local or sequential chain microstructureregardless of the type of vinyl polymer, are ratherscarce if any. The difficulty lies in the fact thatboth the controlled formation of thosemicrostructures along the polymer chain and theiraccurate determination are far from beingsuccessful.Within this framework a rather abundant workin our laboratory has revealed that there exists astraight relation between some physical andchemical behaviours and the occurrence of a fewrepeating steresequences along the chain,especially the mmmr and the mmmmrx (x = m orr)which occur necessarily whenever an isotacticsequence breaks-off. The prominent role of themin a list of physical properties of poly(vinyl chloride)(PVC) including glass transitions, physical ageing,dielectrical relaxations and electrical space chargenature and distribution, has been extensivelyconveyed [1; 4; 5; 6; 8]. Most of the correlationsfound for PVC have been proved to hold true forpoly(propylene) (PP) [2;7]. They have been alsoextended to dynamic mechanical properties of PVCand PMMA. Actually, this paper deals withapproaches to secondary relaxation knowledge asinferred from found correlations with thestereochemical microstructures mentioned below.In general the stereochemical microstructuresthat have been considered are: 1)the isotactic andsyndiotactic sequences, that relate to the extent towhich a polymer exhibits a non-Bernoullian tacticity

The dielectric behaviour through DETA system has been studied for six PVC samples of distinct molecular microstructure in terms of content and associated surroundings and conformation of the local configurations mmr termini of isotactic sequences. Both the dielectric studies and the microstructure of the samples, as obtained by stereospecific substitution reaction of an additive-free commercial PVC are to be published in detail separately. For purposes of clarity some of the results are mentioned in Experimental section. The phenomenological correlation between the β parameter as defined by Ngai's theory and the microstructure of the samples, quantities both issued from the above quoted work, is considered in the present paper with the objective of providing some novel ideas on the physical meaning, at molecular level, of the Ngai's coupling parameter, in the light of mobility constraints and local free volume associated with mmr structure at its likely conformations.

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.

The dichroic behaviour of 615 and 637 cm -1 bands of the IR spectrum of poly(vinyl chloride) (PVC) has been analysed through FTIR measurements on PVC films stretched to various elongation ratios. By comparing the evolution both of the dichroic ratios and of the relative absorbance of those bands in the perpendicular direction to that of stretching, the occurrence of local chain orientation related to the conformational change GTTG - TT ⇒ GTGTTT at mmr isotactic tetrads can be differentiated from the normal orientation of syndiotactic sequences, so confirming earlier proposals that the above conformational change takes place when stretching PVC.

By using the thermally stimulated discharge currents (TSDC) technique and coupling it with the thermal step (TS) method, which allows accurate measurement of the space charge distribution (an outstanding electrical property), both the relaxation and the relative contribution of the intrinsic polarization charges and the extrinsic trapped charges have been analysed for a series of poly(vinyl chloride) (PVC) materials after partial hydrogenation as a precursor structure of polyethylene (PE). The α relaxation related to the release of frozen-in dipole orientation, and the ρ relaxation which is due to the trapping of extrinsic charges have been found to vary, both in intensity and in temperature, with the percentage of ethylene units. On the other hand, the TS measurement of the space charge distribution prior to and after destroying the polarization charges according to the TSDC behaviour, allows one to correlate, as a first approximation, those types of space charge with the molecular microstructure, of either PVC or PE parts. Even if further work remains to be done, the results, as discussed in the light of earlier work on tacticity-driven microstructure–PVC property relationships and on space charge distribution of PVC after hydrogenation, studied by the TS method, provide an original although still tentative interpretation of the space charge behaviour of PE.

The nucleophilic substitution reaction on poly(vinyl chloride) (PVC) with sodium 2-mercaptobenzothiazolate has been carried out in solution in cyclohexanone at different temperatures. The kinetic parameters indicate the occurrence of one process having an activation energy of 63 KJ.mol −1 . The evolution of unreacted iso, hetero and syndiotactic triad contents with degree of substitution has been followed by 13 C n.m.r. spectroscopy. By comparing quantitatively the microstructural changes with degree of substitution and taking into account that the reaction is of S N 2 type only the mechanisms of substitution through the mm triad of mmr tetrad and the rm of rrmr pentads are proved to react. On the contrary, owing to steric hindrance the reaction by the mr triad in the mmr tetrad is quite unlikely, thus enhancing the stereoselective nature of the reaction. This conclusion was confirmed on the basis of the FT i.r. results and is more feasible than using other nucleophiles to study the effect of the aforementioned structures on the polymer properties and therefore provides new approaches to better comprehension at the molecular level of the structure–property relationships. These novel contributions are an original approach to the prominent role of the microstructure-dependent local conformation in the mechanisms of analogous reactions of PVC.

The electric space charge behaviour of chlorinated PVC has been studied via thermal step (TS) and thermally stimulated discharge current (TSDC). Chlorination has been found to change strongly the ratio between polarization and injection charges in comparison with unmodified PVC. The results allow one to take the fundamentals of the space charge behaviour of PVC an important step further.

The space charge distribution, an important electrical property, has been studied by means of the thermal step (TS) method for some poly(vinyl chloride) (PVC) samples containing polyethylene (PE) sequences, as the result of hydrogenation reaction of PVC with H4LiAl to conversions ranging from 1 to 12%. Both the preparation and the characterization of the samples by 13C n.m.r. are to be published elsewhere. The aim of the present work is to investigate the effect of the changes in molecular microstructure, whether relevant to the tacticity of PVC or to that occurring when CH2 groups are substituted for CHCl groups, on the nature and distribution of space charges which are formed upon applying an electrical field. The contribution of the PE structure to the overall space charge distribution could be evidenced by comparing the results obtained to those extensively published on tacticity-microstructure/space charge relationships for PVC. Since hydrogenated PVC may be considered as a precursor of PE, the results provide novel information about the space charge behaviour of the latter polymer.

The aim of this study is to investigate the influence of the tacticity dependent microstructure on the ability of PVC to accumulate space charge. In this connection the Thermal Step Method developed at ‘Laboratoire d'Electrotechnique de Montpellier’ is used to measure space charge distribution in poled samples. Three PVC samples of different tacticity are studied. We show that the space charge increase substantially with isotacticity. An explanation to this phenomenon is also provided.

The space charge distribution, a basic electrical property of insulating materials, has been studied for a poly(vinyl chloride) (PVC) sample before and after nucleophilic substitution reaction up to various degrees with sodium benzenethiolate (NaBT) in cyclohexanone (CH) solution, so as to modify in a controlled way the content of the isotactic tetrad mmr and the heterotactic pentad rrmr located at the end of isotactic and syndiotactic sequences, respectively. We investigated the influence of mmr and rrmr structures on the number and type of electrical charges accumulated in the material when submitting it to an electrical field. To differentiate between the polarization charges and the trapped charges, coupling of the “thermal step” (TS) and the “thermally stimulated discharge current” (TSDC) techniques was applied. Both types of charges were proved to be related chiefly to the conformations GTTG-TT and GTGTT, having higher free volume and local mobility, characteristic of mmr structure.

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.

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. © 1996 John Wiley & Sons, Inc.

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.

The separation of surface-enhanced raman scattering (SERS)-active Ag-multi-nanoparticle (NP) assemblies by low melting point agarose gel electrophoresis was accomplished here by controlling surface charge using NP capping agents, and the pore size of agarose gel matrix. Detailed transmission electron microscopy analysis of excised gel fractions showed dimers and small clusters to have the greatest SERS activity and a mobility in between the monomers and large aggregates. This strategy enables one to: (1) stabilize small multispherical Ag clusters against further aggregation during purification; (2) fractionate and recover spherical assemblies by nuclearity; and (3) analyze SERS-enhancements for each fraction to optimize purification conditions.

The substitution reaction of PVC with sodium benzenethiolate has been performed in two series of solvents. The one series includes solvents bearing CO or O function; the other includes some more basic solvents such as N -methylpyrrolidone and hexamethylenphosphotriamide. 13 C-NMR analysis of the modified polymers shows that reactions in the former type of solvent involve first a fast period in which the mm terminal triad of isotactic sequences of at least one heptad react exclusively. Then, a slower stage involves the mr triads in the atactic parts, besides the mm terminal triads of isotactic sequences shorter than one heptad. The glass transition temperature, as measured by DSC, is shown to decrease linearly with degree of substitution at a definite rate for each series of solvents. The results, as analyzed in the light of earlier work, provide some novel ideas on the role of distinct reactive structures in polymer-solvent and polymer-polymer interactions of PVC.

The aim of this paper is to highlight the determining factor of some normal stereochemical nanostructures, especially the mmmr and mmmmrx, termini of isotactic sequence, on the ability of PVC, hydrogenated PVC (as a precursor of PE) and PP to accumulate, release and transport space charges. The correlations found for these polymers by using TSM, TSDC and IDC methods, provide an original interpretation of the space charge behavior of these insulators and open new prospects in the field of property/nanostructures relationships and performances.

The substitution reaction of PVC with sodium benzenethiolate is studied in a series of solvents in comparison to cyclohexanone as considered in earlier work. As expected from the known influence of solvent on the nucleophile reactivity, the reaction extent is observed to be higher as the solvent polarity increases. In contrast, the stereoselective character of the reaction, as demonstrated for cyclohexanone solution, is found to hold only for solvents containing C=O or-O-function. The others deviate thoroughly from this behaviour. The results are discussed on the grouds of prior findings. They tentatively suggest that the occurrence of the definite conformational changes that are responsible for the stereoselective mechanism, strongly depends on some specific interactions of the solvent with isotactic GTTG and heterotactic GTTT triad conformations.