Pilar Tiemblo

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.

Surface Modification of Sepiolite in Aqueous Gels by Using Methoxysilanes and Its Impact on the Nanofiber Dispersion Ability

Surface modification reactions on needle-like sepiolite using alkyl and functional silanes have been carried out in the form of aqueous gels. In contrast with modifications in organic solvents, reactions in water make it possible to modify the surface of almost-individual sepiolite fibers and produce either a continuous coating or a nanotexturization of the sepiolite fiber surface, depending on the reaction conditions. This clean procedure substitutes advantageously organic solvent surface modifications and allows the tuning of surface properties such as specific surface area, wetting behavior, and chemical functionalization. A consequence of such tuning is, for example, the excellent dispersion of modified sepiolite nanofibers in a great variety of polymers by routine compounding and processing techniques.

Effect of physical aging on the gas transport properties of PVC and PVC modified with pyridine groups

Abstract The gas transport coefficients of polyvinyl chloride (PVC) and PVC modified with pyridine groups have been studied. It has been observed that there is a strong time dependence of the permeability and diffusivity of oxygen, nitrogen, carbon dioxide and methane in membranes prepared by solvent casting of PVC and pyridine modified PVC. For PVC there is a two-fold reduction of the diffusion coefficients during the first two days, and about one order of magnitude, a month after the membranes are prepared, and no stabilisation of the trend is seen after a month. Membranes prepared from modified PVC show a short-term diffusion rate reduction which is similar to that found in PVC, while at longer times the diffusion rate decrease levels off quickly, attaining constant values after about ten days. The time dependence of the transport coefficients is attributed to the samples’ physical aging and an attempt is made to fit the experimental data by considering a stretched exponential time dependence of the volume contraction on aging.

PVC modification with new functional groups. influence of hydrogen bonds on reactivity, stiffness and specific volume

Abstract The chemical modification of PVC with new bifunctional thiol compounds is reported. Aliphatic as well as aromatic reactives were tested and the influence of protic and non-protic functionalities on reactivity was studied. The chemical structure of the polymers was analyzed using 1 H NMR spectroscopy. The structural changes in the modified samples were monitored by means of density and T g determinations, which are a measure of interchain spacing and chain stiffness. While protic functionalities lead to polymers with strongly enhanced T g values indicating a considerable stiffening of the system due to physical interaction by hydrogen bonds, the softening-point temperature of PVC modified with non-protic substituents does not change very much. The interchain spacing of the polymer is not significantly altered by the presence of mobile hydrogen in the polymer.

The development of electrical treeing in LDPE and its nanocomposites with spherical silica and fibrous and laminar silicates

Electrical treeing in LDPE and three LDPE nanocomposites, with spherical silica and fibrous and laminar phyllosilicates, has been studied. Electrical tests were performed at a 50 Hz frequency and voltages between 8 and 29 kV, and the time to inception of the first electrical partial discharges (TTI) of the electrical trees and the time to breakdown (TBD), related to the electrical stability of the insulator, were determined. Above 15 kV all the nanocomposites show longer inception times and shorter tree growth times than LDPE. It is proposed that both observations are caused by the modification of the polymer crystalline morphology induced by the presence of the fillers and by the development of a large number of interfacial structures, both organo–inorganic and amorphous–crystalline. Below 15 kV the TBD is increased in the nanocomposites with the laminar silicate because of tortuosity and the TTI is increased in the fibrous silicate containing a nanocomposite because of the LDPE crystalline morphology in the presence of the silicate. The nanosilica particles decrease the electrical stability in the whole voltage range by decreasing both TTI and TBD.

Photo-oxidation of thick isotactic polypropylene films I. Characterisation of the heterogeneous degradation kinetics

Photo-oxidation kinetics of thick isotactic polypropylene films have been compared to thermal-oxidation kinetics of thin films, and noticeable differences have been found. The non-classical kinetic trend of the former can be described as a three step evolution: a typical induction/autoacceleration POOH build-up at the beginning, an intermediate slower POOH content increase and, finally, a gentler POOH increase, which can be better described by quadratic functions of the oxidation time than by a linear dependence. In addition, a series of oscillations appearing from the beginning of the photo-oxidation are found. This kinetic heterogeneity is suggested to be related to the progression of the oxidation to the inside of the strips. However, the FTIR analysis of the evolution of the POOH band position in both photo and thermal oxidation, enables the observed shape changes to be associated to kinetic stages.

Wetchemical surface modification of plasticized PVC. Characterization by FTIR-ATR and Raman microscopy

Abstract The chemical modification of plasticized PVC films with amino thiophenol in mixtures of DMF/H 2 0 has been studied. The degree of modification and the distribution of modified groups within the films were determined using Raman spectroscopy and FTIR-ATR. The amounts of plasticizer (bis-2-ethylhexyl phtalate, DOP) bleached out during the reaction and the amount of dimethylformamide (DMF) penetrated into the film were analysed in control reactions without the reactant using 1 H NMR spectroscopy and Raman spectroscopy. Reaction kinetics and surface selectivity of the modification reaction were compared with the corresponding reactions of PVC films without plasticizer.

Physicochemical processes along the early stages of the thermal degradation of isotactic polypropylene I. Evolution of the γ relaxation under oxidative conditions

The evolution of the γ relaxation along the early stages of the thermo-oxidation of two isotactic polypropylenes (i-PP) has been studied. It has been found that regardless of their different thermal stability, they show a similar disappearing of the γ relaxation as soon as the induction period ends. The features of that disappearance and its correlation with some chemical and structural changes on oxidising are discussed.

On a novel interpretation of PVC antiplasticization based on some local chain conformations

SummaryThe β transition of a poly(vinyl chloride) after either nucleophilic substitution to various extents or plasticization with distinct amounts of diocytl phthalate has been measured by Dynamical Mechanical Thermal Analysis. Both sets of experiments are shown to result in similar loss of β transition. The results are argued in the light of prior work on the substitution mechanism determining role of the mmr chain segments at the end of isotactic sequences. They strongly suggest the mmr structures to interact specifically with plasticizer thereby becoming stiffer (antiplasticization).

The effect of physical parameters of isotactic polypropylene on its oxidisability measured by chemiluminescence method. Contribution to the spreading phenomenon

Abstract The physical parameters of the polypropylene powdered samples like average molar mass, the degree of isotacticity and the resulting crystallinity and melting temperature have quite an important effect on the oxidisability of polypropylene expressed in induction time of oxidation or in maximum chemiluminescence intensity. It was shown that it is the increase of both the average molar mass and the degree of isotacticity which predominantly lead to the longer induction times of oxidation while the opposite is true for maximum chemiluminescence intensity for molar mass 40,000–180,000 and temperature interval 80–130°C. The combined model of the homogeneous increase of concentration of hydroperoxides in the oxidation micro-zones and the spreading of the oxidation from these micro-zones has been proposed which fits well the experimental runs of chemiluminescence intensity–time.