Thermal Analysis UserCom 36

Many materials exhibit temperature-dependent magnetic properties. These properties can be measured by thermogravimetry (TGA) and differential scanning calorimetry (DSC). The materials are ferromagnetic at low temperatures, but lose their macroscopic magnetic behavior above the Curie point or Curie temperature.

Introduction

Magnetism is an important property of a material and has many practical uses in everyday life. The applications range from refrigerator magnets, compasses, hard disks to sorting machines to separate valuable metals and waste material.

The general term called magnetism is in fact the macroscopic effect of a microscopic property known as ferromagnetism. Ferromagnetism is a physical phenomenon in which the individual electron spins align parallel to one another in the same direction within a microscopic region (domain). This phenomenon is shown schematically in Figure 1.

Besides ferromagnetism, there are also several other related magnetic properties, for example ferrimagnetism, paramagnetism and diamagnetism.

In ferrimagnetism, some spins are arranged antiparallel and others parallel. The opposing magnetic moments do not however cancel each other out because the magnetic moment is stronger in one direction. This gives rise to a weak magnetic field.

In paramagnetic materials, the spins are randomly oriented so that no macroscopic magnetism results. Diamagnetic materials react to an external magnetic field by creating an opposing magnetic field that weakens it.

The topic of this article is however ferromagnetism and in particular the ferromagnetic-paramagnetic transition temperature or Curie point and its determination.

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The mechanical properties of a polyurethane (PUR) film were investigated using DMA in the tension mode. Besides α- and β-relaxation, a further process was found above the glass transition. Additional DSC measurements and DMA multi-frequency scans help to explain the effects observed in the first and second heating runs.

Introduction

This article shows how the DMA 1 can be used to measure a relatively soft PUR sample in the tension mode. Besides the glass transition and secondary relaxation, other effects were also found. These were interpreted as crystallization and melting using DSC measurements. This was confirmed by performing DMA multi-frequency scans in the tension mode. In contrast to relaxation processes, no frequency dependence is expected for crystallization and melting.

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The Flash DSC 1 expands thermal analysis to scanning rates (heating and cooling rates) of several 10 000 K/s (ten thousand Kelvin per second). At such rates, the formation of structure in materials can be investigated in order to gain a better understanding of the behavior of materials in technical processes. A number of practical questions however arise when using the Flash DSC 1.

Possible questions

Examples:

• How do you determine the mass of a sample?
  
• How do you measure the blank curve needed especially for normal and medium scanning rates?
  
• How do you avoid artifacts that occur at low scanning rates with some materials?
  
• How do you obtain measurement curves that you can evaluate in the first heating run?
  

This article proposes various solutions to answer these questions.

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In this article, we show how SEM and TGA measurements were used to characterize novel osteochondral scaffolding material that can be used as implants for repairing injuries to articular cartilage or for producing biomimetic biological tissue (tissue engineering).

Introduction

We rely on different types of joints to allow coordinated movement of our limbs and body. In many cases, joints are points at which the ends of two bones are connected; the rounded end of one bone fits into the socket of another bone. The articular cartilage together with the synovial fluid distributes the pressure on the bones uniformly over the entire joint and prevents friction between the bones.

In contrast to skin injuries, injuries to articular cartilage take much longer to heal because articular cartilage exhibits poor healing and self-repair capacity [1].

For this reason, different possibilities are currently being investigated to accelerate the healing process of injuries to articular cartilage through surgical operations. This involves transplanting articular cartilage together with pieces of bone.

It would be much easier if suitable implants were available. Such an implant would have to consist of the bone material and a layer that mimicks articular cartilage whereby both layers would have to correspond as closely as possible to the biomechanical properties and structures of the bone and articular cartilage [2]. In recent years, we have developed a process by means of with which we can prepare very homogeneous layers [3].

In this article, we investigate the chemical-physical properties of such layers using scanning electron microscopy (SEM) and thermogravimetry (TGA) and compare our material with material prepared using current standard procedures [4, 5].

The morphological and structural properties of the materials are of special interest.

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References

[1] Hunter, W.; Of the structure and disease of articulating cartilages. Clin. Ortop. Retat. Res. (1995), 174 3:3 - 6
[2] Sherwood, J. K.; Riley S. L.; Palazzolo, R.; Brown, S. C.; Monkhouse, D. C.; Coates, M.; Griffith, L. G.; Landeen, L. K.; Ratcliffe, A.; A three-dimensional osteochondral composite scaffold for articular cartilage repair. Biomaterials (2002), 23:4739 –4751.
[3] Pressato D., Dolcini L., Nicoletti A., Fiorini M. WO 2011/064724 A1 (patent application) Biomimetic composite materials, preparation process thereof and use thereof to produce mono-, bi- or multi-layer structures for the regeneration of bone, cartilaginous and osteocartilaginous tissue.
[4] Wang X., Li X., Lu J., Zhao H., Zhang X., Gu Z. Investigation of a collagen-chitosan-hydroxyapatite system for novel bone substitutes. Key Engineering Materials (2007), 3 3 0 – 3 32:415 - 418.
[5] D. Shi, D. Cai, C. Zhou, L. Rong, K. Wang, Y. Xu; Development and potential of a biomimetic chitosan/ type II collagen scaffold for cartilage tissue engineering. Chin. Med. J. (2005), 118:1436–1443.

In UserCom 34 we announced the acquisition of Triton Technology Ltd. (www.triton-technology.co.uk). This acquisition has allowed us to expand our range of instruments for dynamic mechanical analysis (DMA). The TT DMA and DMA 1 are excellent choices both for standard DMA applications and for quality control. In addition, they offer two new options. First, there is the possibility of performing measurements under conditions of controlled relative humidity by equipping the instruments with a humidity chamber and coupling it to a humidity generator. Second, there is the fluid bath option, which allows you to measure a sample immersed in a liquid.

Introduction

When a material is immersed in a liquid, it can soften, react (harden) or dissolve. The liquid can affect a material to such an extent that it breaks, deforms or can no longer be used for a particular application. Figure 1 shows the TT DMA with the fluid bath option in the open and closed position.

The analytical head is facing downward. The bath is filled with the liquid of interest and its temperature is controlled by an external cryostat.

Figure 2 shows the effect of water on a polyamide sewing thread measured by DMA in the tension mode. The thread was 10 mm long and had a diameter of 0.15 mm. The measurement mode used was “Ratio tension” with a value of 1.5, which corresponds to 150%, and a displacement of 0.20 mm.

The elastic modulus (E') was first measured in air. The thread was then immersed in water at 20 °C at a time marked by the vertical dashed blue line in Figure 2.

The experiment was repeated at 40 °C using a new thread. The curves show that the thread softens in water and that the water temperature influences softening. The elastic modulus decreased from 4.5 GPa to less than 1.3 GPa.

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