Atactic PVC Research Articles

Stereodynamic insight into the thermal history effects on poly(vinyl chloride) calorimetric sub-glass and glass transitions as a fragile glass model.

The dynamic thermal history impact of poly(vinyl chloride) (PVC) has been explored for a wide range of pre-cooling rates, from 1 to 30 °C min-1. A first macroscopic insight into the dynamic thermal history influence has been highlighted through a decrease in the apparent activation energy (Eapp) in the first stage of the glass transition. The overall glass transition Eapp surface was successfully modeled in a polynomial fashion regarding the pre-cooling range. Raman scattering was used to associate the Eapp variations along the glass transition conversion with the stereochemistry evolution during the polymeric relaxation. Herein, the selection of atactic PVC as the polymer model permits us to monitor the glassy polymer segment stereodynamics during the heating ramp through the C-Cl stretching. The intermolecular H-Cl dipole interactions, as well as intramolecular conformational reorganizations among syndiotactic, isotactic and heterotactic polymer sequences, have been associated with non-cooperative and cooperative motions, i.e. the β- and α-process, respectively. The fruitful comparison of the two extreme values of the pre-cooling rates permits us to propose a thermokinetic scenario that explains the occurrence, intensity, and inter-dependence of β- and α-processes in the glassy state and during the glass transition. This scenario could potentially be generalized to all the other polymeric glass-formers.

Structure and texture analysis of PVC foils by neutron diffraction

Crystalline order of molded and then bi-axially stretched foils prepared from atactic PVC resin is investigated by means of wide-angle neutron diffraction (WAND). The observed high-resolution WAND patterns of all samples are dominated by a sharp maximum corresponding to the inter-planar distance 0.52 nm. Two weaker maxima are also resolved at 0.62 and 0.78 nm. Intensities of the peaks vary with deformation ratios of the samples and their diffraction position. Average size of the coherently scattering domains is estimated as approximately 4-8 nm. Based on the experimental data, a novel model of crystalline order of atactic PVC is proposed.

Cooperative motions in PVC studied by thermally stimulated currents: Comparison with A.C. dielectric derivative analysis

The thermally stimulated current–thermal sampling (TSC–TS) technique was used to study the broadened glass transition in conventional “atactic” poly(vinyl chloride), PVC. The activated parameters obtained from the TSC–TS data, mainly the apparent activation energy (Ea), characterize the breadth of glass transitions in a very sensitive way. These results are compared with those values of Ea obtained from the literature, using a recently proposed method of analyzing a.c. dielectric constants and their derivatives, over the temperature range of −100–130°C. Both techniques detect weak cooperative glass transition-like relaxations well below the main glass transition of ca. 80°C. As is the case with “atactic” PMMA, the data suggest that compositional heterogeneity related to a small fraction of predominantly isotactic sequences contribute to the broad glass transition extending ca. 60°C below the main glass transition in atactic PVC. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 913–918, 1998

Molecular Structures in Poly(vinyl chloride) Thermoreversible Gels: Effect of Tacticity and of Solvent Type

Neutron scattering and neutron diffraction studies have been carried out as a function of the solvent type and of poly(vinyl chloride) (PVC) tacticity. In diesters, while the large scale structure of the gel does not depend significantly upon the nature of the alcohol group, the short-range structure differs markedly. In both cases a PVC−solvent complex is nevertheless formed. Neutron diffraction allows confirmation of the existence of such a complex in PVC/diethyl oxalate gels. By using bromobenzene, it has been observed that gels prepared from a PVC of higher syndiotacticity do not possess the same large scale structure as those produced from usual atactic PVC. Conversely, they are similar to those obtained in diethyl oxalate with the usual atactic PVC. These results highlight again the equivalence between tacticity and solvent type. This equivalence originates, however, in differing short-range molecular arrangements: complex formation in usual PVC/diethyl oxalate gels, “syndiotactic” crystals formation in highly syndiotactic/bromobenzene gels.

Diffraction modelling of poly(vinyl chloride): An approach to the crystallinity enigma

AbstractThis paper demonstrates that changes which are seen to occur in experimental X‐ray diffraction patterns of poly(vinyl chloride) (PVC) with varying sample tacticity are indicative of a crystal structure in which both isotactic and syndiotactic sequences are incorporated within the same lattice. The isotactic sequences are in a conformation first suggested by Juijn et al. which enables them to emulate the shape of the syndiotactic chain. We present a short summary of molecular and diffraction modelling results which show how the intensities of the 200 and 110 peaks change with crystallite tacticity. The results provide a rational explanation for the long standing enigma that the observed crystallinity of atactic PVC is too high to be accounted for in terms of crystallization of syndiotactic sequences only.

Infrared spectral changes with crystallization in poly(vinylchloride): Correlations with X-ray and density data

The influence of crystallinity and stereoregularity on the infrared (IR) spectrum of atactic PVC in the solid state has been studied by many researchers [1-12]. Although the molecules in commercial...

Influence of Tacticity on Some Reactions of PVC

Abstract The influence of the tacticity on PVC reactivity is discussed on the basis of preliminary results obtained in ionic dehydrohalo, genation and chlorination reactions. From the reaction of an atactic PVC and a 70% syndiotactic PVC with LiCl in dimethyl-formamide and hexamethylphosphortriamide as solvents, it follows that both the reaction rate and the polyene sequence distribution depend markedly on the syndiotacticity content. This effect is accounted for by the fact that the isotactic parts are preferred in dimethylformamide and the syndiotactic ones in hexamethylphosphoramide. On the other hand, the chlorination of PVC appears to be easier through the heterotactic parts than through the syndiotactic sequences as shown by 13C-NMR.

Melting of crystallites in polymers with low degree of crystallinity under shear flow conditions I: Poly(vinyl chloride)

AbstractThe aggregates found in dilute solutions of nearly atactic PVC have been shown to consist of 10 to 15 single molecules held together by a crystalline nucleus. Based on the postulate that the last crystallites in a PVC melt have the same structure as found in dilute solution, a relation between the melting temperature in shear (Tdyn), the static melting temperature (Tm) and the shear stress (τ) is found: where Q is a constant. The model predictions are in accordance with data from a Brabender rheometer and a Rheometrics mechanical spectrometer. Q is found to be a constant for samples with weight average molecular weights in the interval between 70,000 and 160,000 corresponding to polymerization temperatures between 40 and 70°C.

Crystallinity in atactic poly (vinyl chloride)

Chain conformations of poly(vinyl chloride) have been investigated in order to explain the relatively high crystalline content of atactic PVC. A study of chain configurations and conformations leads to the conclusion that three different regular straight chain structures exist: the syndiotactic zig-zag chain, the isotactic helix and an isotactic straight chain. This isotactic straight chain conformation has a slightly higher energy than the isotactic helix but it nevertheless occurs in bulk PVC in appreciable amounts.