The relaxation of electric field-induced polar orientation in a side-chain-bearing liquid-crystalline polysiloxane was measured
by means of thermally stimulated depolarization currents. Different relaxation mechanisms were identified and characterized:
the glass transition cooperative relaxation exhibits compensation behaviour. On the other hand, lowerTg and upperTg discharges were observed and their molecular nature is discussed.
Summary The thermal behavior of poly(L-lactic acid) (PLLA) was studied with differential scanning calorimetry (DSC) and polarized optical microscopy. For amorphous PLLA samples, double cold crystallisation peaks were observed in the DSC traces during heating process, being strongly dependent on heating rates. The observation was discussed based on the assumption that the quenched PLLA sample presented some remaining metastable or a precrystalline phase. A small exothermal peak was observed before the main melting peak at low heating rates. The probable reason was discussed through melt-recrystallisation mechanism. Influence of thermal history on the cold crystallisation and melting behavior was also performed on heating process for PLLA samples.
Thermally stimulated recovery, TSR, like as thermally stimulated depolarisation currents, is a suitable technique that allows
for the study of conformational mobility in polymeric systems. Due to its relatively low equivalent frequency and transient
nature, the viscoelastic data obtained from this technique are complementary to conventional dynamic mechanical analysis (DMA).
In this work TSR-like experiments, including TSR, thermally stimulated creep and thermal sampling (TS) experiments were carried
out in the same commercial DMA equipment, allowing for the direct comparison of the data. Some advises for running TSR experiments
are presented, such as the need of performing blank experiments and temperature calibrations. The analysis of the data to
obtain the thermokinetic parameters of TS experiments is revised. In particular, from the direct fitting of the data, it is
reported a tendency for a linear relationship between the pairs of values of (Ea, log τ0) that best adjust any TS single experiment. It is concluded that the usual equation for describing TS experiments possesses
an intrinsic compensation between these two thermokinetic parameters.
Authors:J. Mano, S. Lanceros-Méndez, A. Nunes, and M. Dionísio
Dielectric experiments are often performed in non-isothermal conditions. Thus, there is a difference between the temperature of the sample and the sensor temperature. In this work we propose and compare three temperature calibration methods based on the detection of transitions or relaxations: i) the melting of high-purity metallic standards (indium and tin), ii) the 2nd order phase transition of a ferroelectric crystal (TGS); iii) the -relaxation of an amorphous polymer (poly(carbonate)). The results obtained from the three different methods were used to construct a calibration curve for a given heating rate.
Authors:Ana Brás, María Viciosa, Madalena Dionísio, and J. Mano
A detailed dielectric characterization
of the relaxation modes found in a poly(L-lactic
acid), PLLA, film containing 0.4 mass% of water is provided. The sub-glass
relaxation process is a superposition of two processes, one highly influenced
by water with activation energy of 50 kJ mol–1,
and another one, with longer relaxation times and lower intensity having activation
energy of 38 kJ mol–1. Dried PLLA exhibits
an abnormally broad secondary β-relaxation that probably corresponds to
the superposition of multiple processes. Upon water sorption the strength
of the more mobile process significantly increases being shifted to lower
temperatures which allows the detection of the underlying process. The glass
transition relaxation process is deviated to higher frequencies almost one
decade due to the water plasticizing effect. The reported results show that
small quantities of water may have a profound impact in the relaxational features
in PLLA, which should be taken in account when considering the properties
and performance of this system.
Authors:J. Pálinkás, I. A. Szabó, L. Harasztosi, F. R. Soha, S. Manó, and Z. Csernátony
The beneficial effects of hippotherapy lie in the transmission of the horse's movements to the patient. The aim of our work was to create a measurement method for objective evaluation that can be used in natural settings without interfering with therapy.
Our measurement system consists of three treble axis accelerometers connected to a data logger. Software was developed for data analysis and post processing. A commercial DVD recording camera was used to document the hippotherapy session. In this paper we present the results obtained in pilot measurements on ten children with cerebral palsy. For reference, a skilled rider was measured on the same horse using a passive following seat.
The acceleration diagrams show common characteristics useful to the interpretation of the movement transfer during hippotherapy as well as individual patterns indicating alterations in the movement reactions of different patients.