José M. Gómez-Elvira

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.

Influence of tacticity on the thermal degradation of PVC. Part 7—Further approaches to the conformational mechanism through a temperature effect study

Abstract The thermal degradation of three different PVC samples has been studied at temperatures ranging from 110 to 200°C. Polymers 1 and 3 were prepared in bulk with 2,2′-azodiisobutyronitrile as initiator at 90 and 20°C, respectively, so as to obtain different isotactic contents and therefore different contents of the GTTG − isotactic triad conformation. Sample 2 resulted from a stereospecific nucleophilic substitution reaction on sample 1 at −15°C. As shown elsewhere, such a treatment involves the disappearance of GTTG − isotactic triads without other significant changes in structure taking place. The content of GTTG − isotactic triads should thus be expected to decrease from sample 1 to sample 3. The Arrhenius plots for the initial degradation rates, make evident the occurrence of two distinct competing degradation processes with activation energies of 7·0 and 19·8 kcal/mol. Their respective contributions to the overall degradation is shown to depend upon the isotactic content of the polymer such that the higher the isotactic content, the higher the polymer fraction involved in the process of lower activation energy. On the other hand, the UV-visible spectra of the polymers after degradation to similar degrees, demonstrate that the main absorption, i.e. the most abundant polyene length, shifts towards shorter wavelengths as the degradation temperature increases. On the basis of previous results which allow a correlation between definite long polyenes with the fraction of isotactic triads that adopt the GTTG − conformation during the polymerization, the above behaviour is shown to prove that the latter conformation is responsible for the degradation process of lower activation energy. Such a conclusion is strongly supported by the behaviour of sample 2, relative to sample 1, in both degradation kinetics and polyene distribution in the degraded polymers. As a whole, the results are fresh support for our prior work proving that the GTTG − isotactic triad conformation is the instability-determining structure in PVC.

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.

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.

Photo-oxidation of thick isotactic polypropylene films II. Evolution of the low temperature relaxations and of the melting endotherm along the kinetic stages

Abstract The photo-oxidation of thick polypropylene films has been studied in this and a preceding work (Castejon ML, Tiemblo P, Gomez-Elvira JM. Photo-oxidation of thick isotactic polypropylene films: I. Characterization of the heterogeneous degradation kinetics. Polym Degrad Stab 2000;70(3):357). In particular, the evolution of thermal and dynamo-mechanical properties and their correlation with POOH build-up and molecular mass decrease have been considered. The observed modifications in the DSC endotherms features and in the γ relaxation have shown that T m and γ relaxation are greatly affected from the very beginning of the oxidation reaction and give an account of the progress of the oxidation during the induction period. On the contrary, the crystallinity content hardly changes during this initial period, while it really does so when the sample is on average at the propagation stage.

Specific polymer-solvent interaction and stereospecificity of nucleophilic substitution reaction of PVC

SummaryThe 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.

Solvent dependence of stereoselective substitution reaction on poly(vinyl chloride). A useful tool to investigate the tacticity effect on Tg

Abstract 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.