Gladys Ronca

Applications of Successive Self-Nucleation and Annealing (SSA) to Polymer Characterization

A new technique to thermally fractionate polymers using DSC has been recently developed in our laboratory. The applications of the novel successive self-nucleation and annealing (SSA) technique to characterize polyolefins with very dissimilar molecular structures are presented as well as the optimum conditions to thermally fractionate any suitable polymer sample with SSA. For ethylene/α-olefin copolymers, the SSA technique can give information on the distribution of short chain branching and lamellar thickness. In the case of functionalized polyolefins, detailed examinations of SSA results can help to establish possible insertion sites of grafted molecules. The application of the technique to characterize crosslinked polyethylene and crystallizable blocks within ABC triblock copolymers is also presented.

Heterogeneous nucleation and self-nucleation of poly(p-dioxanone)

The changes in nucleation behaviour upon addition of Boron Nitride (BN), Talc and Hydroxyapatite (HA) to poly(p-dioxanone) (PPDX) were monitored by DSC and Polarised Optical Microscopy (PM). Self-nucleation DSC studies evidenced the existence of the usual three self-nucleation domains depending on the self-nucleation temperature (Ts) employed. By far the best nucleation agents for PPDX were its own self-nuclei and this result was independent of the presence or absence of any of the other nucleating agents employed; once Domain II was reached, self-nucleation dominated the nucleation process. BN and Talc were able to nucleate PPDX, thereby increasing its nucleation density, its dynamic crystallisation temperature upon cooling from the melt (Tc) and its enthalpy of crystallisation (ΔHc). BN was a better nucleating agent than talc. HA on the other hand caused an “antinucleation” effect on PPDX characterised by a decrease in its nucleation density, a decrease in its Tc and in ΔHc. Isothermally crystallised PPDX exhibited large banded spherulites whose morphology changed as a function of crystallisation temperature from single banded structures with a very clear Maltese cross to double banded spherulites. PPDX also shows a change in growth regime upon increasing crystallisation temperature (from Regime III to Regime II) according to the kinetic interpretation of growth rate data. BN did not cause any significant modification of the spherulitic growth kinetics (in Regime II) except for a small decrease in surface free energy of PPDX crystals (σe). On the other hand HA was found to increase the spherulitic growth rate and the overall crystallisation rate of PPDX, this increase was caused by a degradation process experienced by the polymer during the treatments involved in isothermal crystallisation that was only present in the samples with HA. It is postulated that the interaction between the phosphate groups on the surface of HA and the ester groups of PPDX are responsible for both the antinucleation effect and the catalysis of the hydrolytic degradation of PPDX.

Crystallization of the polyethylene block in polystyrene-b-polyethylene-b-polycaprolactone triblock copolymers, 1. Self-nucleation behavior

The self-nucleation of branched polyethylene chains of different degrees of chain mobility was studied. The polyethylene block (PE block) within poly(styrene-b-ethylene-b-caprolactone) triblock copolymers (SEC) of varying compositions was studied. Differential scanning caloriometry was used to determine the self-nucleation domains as a function of the self-nucleation temperature (T s ). The self-nucleation behavior of PE chains within SEC block copolymers was found to be anomalous in comparison to the classical self-nucleation behavior exhibited by homopolymers. When the degree of chain constraint is high, as in the the case where the SEC copolymer only contains 15% of PE, domain II (only self-nucleation domain) completely disappears and annealing can take place before self-nucleation occurs. This means that chain constraint complicates the self-nucleation process and this situation persists until, upon decreasing the self-nucleation temperature (T s ), annealing has generated crystals tha are big enough to act a self-nuclei for the less restricted portions of the chain. If the PE content in the copolymer is very low (15%), two crystal populations can be distinguished. This may reflect the differences in diffusion of the PE chain segments close to the interfaces with the other two blocks and those segments that are close to the middle of the PE block. the influence of chain constraint on determining the difficulty of the chains to self-nucleate was further explored using a crosslinked low-density polyethylene (XLDPE). In this case, crosslinking junctions instead of covalent links with other blocks restrict chain mobility. Nevertheless, a similar difficulty in self-nucleation was found as in the case of the PE block within SEC triblock copolymers in contrast to neat LDPE, a polymer that exhibited the classical self-nucleation behavior with the usual three domains.

Application of the SSA calorimetric technique to characterise an XLPE insulator aged under multiple stresses

In this work the thermal behaviour of crosslinked low density polyethylene (XLPE) used as an insulator for commercial underground high tension (15 kV) cables was studied. Three types of materials were selected: an uncrosslinked low density polyethylene (NXLPE) used as a control sample, an XLPE and an aged XLPE sample. The ageing conditions involved the application of multiple stresses: temperature, voltage and voltage impulses during 60 d under time and temperature cycles that are the most representative load of daily operation in Caracas, Venezuela. The effect of morphology segregation or thermal fractionation under multiple stresses conditions was analysed by measuring the percentage of crosslinking before and after the ageing tests were performed, and by investigating the thermal response of the material by conventional DSC and by the application of the successive self-nucleation and annealing (SSA) thermal fractionation technique. The degree of crosslinking was found to vary in the material depending on the distance from the conductor because a thermal gradient is generated radially during the curing reaction. Such differences did not significantly affect the usual DSC heating scans of the samples. However, when SSA was applied, a difference in the distribution of thermal fractions was detected as a function of the distance towards the conductor that could be correlated to the variations in the cross-linking degree. After the accelerated ageing the thermal response of XLPE changes as evidenced by the presence of multiple melting peaks in subsequent DSC heating scans. This multiple melting was interpreted, as a first approximation, as arising from thermal fractionation during ageing (ignoring the possible effects of the other stresses applied) and SSA was able to simulate a similar fractionation that was very accurate in the prediction of the exact temperatures of the melting peaks produced.

TR-XLPE medium voltage cables subject to accelerated aging cycle under multiple stresses

In this article we report the most important results about the evaluation of TR-XLPE cables after an accelerated aging process under multiple stresses: temperature, AC voltage, impulse voltages and humidity. This work was developed at the Simon Bolivar University High Voltage Laboratory, Venezuela. 15 kV cables were subjected to thermal cycles between 55 - 130/spl deg/C, 26 kV AC voltage and (+/-) 88 kV impulse voltages applied periodically. The cables were aged in a duct filled with water. Dissipation factor and permittivity were evaluated periodically. Differential scanning calorimetry (DSC), crosslinked percentage, crystallinity and tree formations were evaluated periodically in a radial direction across the thickness of the samples and longitudinally along the cable. The most important observation was the increment in the dissipation factor after impulse applications and their corresponding reduction after thermal cycles. Important changes in DSC and considerable loss of crosslinked reticulation percentage, mainly in the region near the conductor and the termination zone, were detected. This research makes evident the nonuniformity of the properties according to the radial and longitudinal position. This is a result of different thermal-electric stress across the thickness and in the termination area. In consequence, at these points multiple stresses lead to premature degradation.