Polymer matrix

As mentioned above, the low thermal conductivity of the polymeric matrix is a drawback for magnetic refrigeration. However, the use of a homogeneous distribution of metallic particles in the polymer and the subsequent increase of the effective heat-transference surface could counterbalance the low conductivity of the polymer. Furthermore, the possibility of building-up devices with high surface to volume ratio may counterbalance the low conductivity of the polymer and make 3D-printed composite structures be even more efficient (from the point of view of heat transfer) than a piece of bulk material with standard shape.

5.2.1.1 Heat-transfer simulation

The suitability of the use of PCL and PLA polymers for the development of heat-exchangers has been preliminary analyzed from numeric calculations. In particular, finite-element simulations (collaborative study with Isaac Royo, Department of Mathematics, UPNa) have been performed to compare the heat transfer process of a standard cubic shape piece of pure active material (MSMA) with that of composite wires containing MSMA particles in various concentrations and different types of polymers as matrix. The volume of active material was kept constant for both geometries, so the length of the wire varies depending upon the diameter and the particle concentration. The transfer rate will depend on the length of the wire, type of material, percentage of particles and quality of cooling/heat transfer to the medium. The simulations describe a situation in which the active material has an initial temperature different from that of the polymer matrix and the environment and calculates the dynamics of the thermal energy transference towards or from the surroundings. As an example, figure 5.1 shows the normalized heat transference as a function of time for a composite wire with PCL matrix and 50% volume fraction of functional particles and for the bulk MSMA piece. The results evidence that wires with MMSMA micro-particles in the PCL matrix transfer more energy per second (higher slope) than the bulk. Consequently, the simulations support the idea that geometries with high specific surface, obtained by 3D printing methods from printable polymer composites can be

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more efficient than a standard metallic piece in terms of heat transference, despite the low thermal conductivity of the polymer matrix. These results encourage the fabrication of printable functional composites for magnetocaloric applications.

Fig. 5.1. Left) Wire geometry (50% in volume of particles), right) Heat transfer as a function of time for Ni₄₅Co₅Mn_36.7In_13.3 bulk (red line) and equivalent volume of particles embedded in PCL matrix (black line).

Simulation condition: 0.6mm wire diameter, 50% volume particle, perfect refrigeration, environment = 20°C and initial temperature = 15°C.

5.2.1.2 Thermal analysis of PCL/PLA polymers

As explain in Chapter 1, the operating range of the polymer is limited by the melting temperature, above which the material loses its mechanical consistency. In order to determine the phase transformation taking place in the polymers to be used, DSC measurements have been analyzed in raw PLA and PCL polymers. Figure 5.2 shows the corresponding thermograms obtained on heating and cooling at the rate of 10K/min. In the case of PCL (figure 5.2a), the endothermic peak around 338K (H^PCLm= 68J/g) corresponds to the transformation from solid to liquid (viscous) phase associated to the melting point of PCL. The exothermic peak around 291K during cooling, is linked to the the crystallization of the polymer (H^PCLc= 51J/g). It is worth noting that intensity of the melting peak is quite lower on the second heating run, whereas the calculated melting enthalpy is (H^PCLm= 48J/g), very close to crystallization enthalpy on cooling.

0 5 10 15 20 25 30 35

0.0 0.2 0.4 0.6 0.8 1.0

PCL+MMSMA 50%

equi. MMSMA bulk 50%

lQ l (J)

time (sec)

h = 1000

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The phase transformations in PLA are shown in figure 5.2b. The inflection in the baseline observed around 330K must correspond to the occurrence of the glass transition in the polymer, which indicates that some fraction of polymer was initially in amorphous state. The endothermic peak observed on further heating (around 440K) corresponds to the melting of the polymer (H^PLAm= 39J/g). Contrary to PCL, PLA does not show any crystallization process on cooling.

When the cooling process is rapid enough to skip the crystallization process, the molecules arrange randomly giving rise to an amorphous state. However, an exothermic peak is indeed observed around 376K on the second heating run, linked to the cold crystallization of PLA. The cold crystallization (H^PLAm= 39J/g) occurs during heating above the glass transition as soon as the molecules gain mobility to rearrange in an ordered manner.

Fig. 5.2. DSC thermograms during heating and cooling ramps for a) PCL and b) PLA raw polymers.

250 300 350 400

-1.0 -0.5 0.0 0.5 1.0

Temperature (K)

H eat flo w (W /g )

exo up melting

a)

300 350 400 450

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

b)

exo up

cold crystallization

Tg

melting

Temperature (K)

Heat flow (W /g)

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Fig. 5.3. DSC thermograms for a 50-50 (wt%) mixture of PCL and PLA.

Different mixtures of PLA and PCL have been used to carry out the fabrication of MSMA/polymer-composites. To evaluate the eventual interaction between the different polymers, the effect of presence of one polymer on the phase transformations taking place in the other one, has been analyzed from DSC measurements. As an example, figure 5.3 shows the thermograms obtained on a 50-50 (wt%) mixture of PCL and PLA. It can be seen that the melting and crystallization peak temperatures for both polymers remain unaffected, which suggests no interaction between the polymers. The same lack of interaction has been observed in all the analyzed mixtures, regardless of the polymer concentrations.