Thermal Analysis UserCom 35

Fossilized resins originating from conifers are well known as “amber”. The frequent inclusion of plants and insects in amber is of interest for scientific studies and jewelry manufacture. Because amber is expensive, fake amber and amber imitations made from plastic or artificially aged tree resin are frequently found on the market. The determination of age is therefore a very important criterion for assessing whether an object is true amber or not.

Introduction

Differential scanning calorimetry has proven to be a simple technique for classifying amber and other resins according to age. The method uses the glass transition temperature (T_g) measured in the first heating run. Experiments showed that old fossilized resins exhibit a high glass transition temperature due to the slow crosslinking or curing reaction that takes place over millions of years and through the reduction in oil content.

The glass transition temperature of fresh tree resin occurs at about room temperature and that of 40-million-year-old amber at 150 °C. Experimentally, enthalpy relaxation and a postcuring reaction overlap and mask the glass transition in a conventional DSC measurement. The glass transition temperature can nevertheless be reproducibly determined using TOPEM^® , a temperature-modulated DSC method.

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The glass transition, dehydration, crosslinking and degradation of isolated lignin with different water contents was measured by TOPEM^®. The separation of the total heat flow into reversing and non-reversing components proved to be essential for identifying different thermal events. The apparent activation energy of the glass transition of dry and wet lignin was determined from the frequency dependence of the glass transition by means of multifrequency analysis using TOPEM^®. This allows conclusions to be drawn about the influence of water on molecular interactions in lignin.

Introduction

Lignin together with cellulose is one of the main constituents of wood. It is an integral part of the cell walls of plants and is responsible for its mechanical properties through the process of hardening (lignification). Lignins are complex natural polymers that form randomized three-dimensional networks. The thermal softening of lignin is largely responsible for the viscoelastic properties of wood [1, 2]. The kinetics of the glass transition of lignin has been studied on wood samples using differential thermal analysis (DTA) [6] or dynamic mechanical analysis (DMA) [3 –5].

However, the kinetics of the glass transition of isolated lignin has not been determined up until now. This could be due to its powdery consistency, which makes it more difficult to perform DMA measurements.

In this study, we describe measurements of the glass transition of isolated lignin using temperature-modulated DSC ( TOPEM^®) to investigate the influence of water and thermal aging on behavior at the glass transition.

The frequency dependence of the glass transition was measured by multifrequency analysis using TOPEM^®. This allowed us to determine the apparent activation energy of dehydrated lignin and lignin with sorbed water and to draw conclusions about changes in molecular interactions. The chemical reactions in lignin are also discussed.

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The DMA/SDTA861^e has a thermocouple that is used to measure the sample temperature near the sample. The measured sample temperature can of course be calibrated and adjusted. In this article, we show how this is done for the large or small tension sample holders.

Introduction

In the DMA/SDTA861^e, the sample temperature is measured by a thermocouple. This sample thermocouple (STC) can be separately calibrated and adjusted for each sample holder. This is particularly easy to do with the shear sample holder because the STC is in direct contact with the sample holder and is always reproducibly in the same position relative to the position of the sample.

Besides this, the thermal mass of the sample holder is large and the sample and sample holder are in close thermal contact. This guarantees that the measured sample temperature agrees very well with the actual sample temperature in shear experiments. Furthermore, the calibration substances (indium, zinc, water) normally employed in the thermal analysis can be used.

The situation with bending, tension and compression is somewhat different. Here the STC is not in direct contact with the sample holder but is positioned as near as possible. The measured sample temperature includes an uncertainty contribution that is mainly determined by the reproducibility involved in positioning the STC.

In addition, it is not quite so easy to use the above-mentioned calibration substances. In this article, we will describe how samples for calibration and adjustment of the STC can be prepared for tension experiments and what the uncertainty of the measured sample temperature is. The methodology can be used for both the large and small tension sample holders (tension clamps).

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Conventional DSC, temperature-modulated DSC (TMDSC) and DMA measurements were used to study the phase transitions of crystalline phases in melt-spun PVDF fibers. The results are compared with those obtained from wide-angle X-ray diffraction. The information about the phase transitions can be used to adjust the crystalline β-phase content in fibers and optimize piezoelectric properties.

Introduction

Polyvinylidene fluoride (PVDF) is a fluoropolymer with excellent chemical stability. The polymer is semicrystalline and can form different crystalline phases (so-called α-, β-, γ- and δ-phases). The molecules of this polymer have a large dipole moment perpendicular to the polymer chain. Depending on the crystal structure, this gives rise to different electrical properties that are utilized in many technical applications such as for actuators and sensors [1].

PVDF films are manufactured by melt blowing and PVDF fibers in melt spinning processes. Depending on the production process, up to four different crystal phases occur. The α-, γ- and δ-phases are non-polar. The β-phase exhibits piezoelectric properties. This phase is formed under mechanical tensile stress or in strong electrical fields and is responsible for the ferroelectric and piezoelectric properties of the materials.

The phases and their transitions can be characterized by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and wide-angle X-ray diffraction (WAXD). These methods are employed to investigate the formation of different phases and in particular the β-phase. The results can be used to optimize production and finishing processes.

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References

[1] Walter, S. et al.: Characterization of piezoelectric PVDF monofilaments, Materials Technology (2011), 1.