Elongation at break

Incorporating the filler caused a drop of the elongation at break, no matter wether the filler was surface modified or not (figure 8a).

Immersion in Tris buffer caused a drastic loss in elongation at break even when the filler amount was only 10%. Composites with 30% HA filler were extremely brittle with small elongations while composites with 30% PA-HA filler still maintained a pronounced elongation at break of 29%.

Discussion

Covalent bonding of HA with polymer matrix has the potential to significantly improve the interface of HA and polymer matrix, therefore leading to a significant improvement of mechanical properties of composites.

Due to the presence of hydroxyl end groups and carboxylic end groups in Polyactive^TM 70/30, covalent bonding of HA with polymer can be achieved by using HMDI as coupling agent.

The presence of Polyactive^TM 70/30 on the surface of HA has been confirmed by IR spectroscopy and the amount of grafted Polyactive^TM 70/30 could be determined by TGA measurement.

Covalent bonding Polyactive^TM to HA has a distinct effect on the mechanical properties of the composites, especially at higher filler content. The elastic modulus, strength and elongation at break were all higher for PA-HA composites as compared to HA filled composites(figure 5). The most pronounced effect was found between surface grafted and ungrafted HA composites at higher filler content, especially in the case of the elastic modulus.

This agreed well with our previous results [16,17,23] and the results of Jones et al [12], in which it was found that only at filler amounts higher than 6%, significant differences could be found for silane treated versus non-silane treated materials. Apparently the interfacial interactions become more important when the filler amount is higher.

Immersion in water will cause a decrease in mechanical properties for both PA-HA composites and HA composites. Swelling will decrease the mechanical properties of Polyactive^TM, and cause a loose embedding of HA particles in polymer matrix. Thus HA composites at 30% by volume of HA almost completely lost their strength in contrast to the surface grafted HA composites. Apparently the grafted HA particles could maintain a better contact because of the bonding between HA and polymer matrix.

An interesting finding is that 30% grafted filler composites had a higher tensile strength than 10% grafted filler composites, especially in the wet state, allthough both were less strong than unfilled Polyactive^TM. This is different from results with

Chapter 10 138

Error!

nano-apatite fillers in our previous work [23] where an increase of the filler volume was found to only further decrease the strength. In this light the results of the swelling test are helpfull in explaining the difference. If the filler per se is assumed to have zero water uptake, then the the equilibrium swelling degrees of the composites should follow the dashed line of Fig. 4. The actual swelling degree values were found to be lower with the current Merck-powder fillers, but in the case of the nano-apatite fillers, the swelling degree values were found to be higher than the theoretically expected values. In this last case the filler apparently took up water by itself destroying any interaction between it and the matrix.

With the current fillers showing lower than expected swelling degrees it appears that the quality of the interactions between filler and matrix is much higher and even prevents part of the matrix material from swelling in water. Where ungrafted or grafted fillers do not differ in this respect, adsorption of matrix molecules to the filler surface is apparently as effective as covalent bonding.

When mechanically loaded, however, the difference becomes clearer and do grafted filler composites show better mechanical properties than control HA composites, both in dry and in wet state.

Several theoretical models have been proposed to predict the Young's modulus of a composite system from the mechanical properties of the matrix and the filler. Among the models, the Voigt model and the Reuss model are believed to provide the upper limit and lower limit of the composites respectively. In the Voigt model, a homogenous and elastically isotropic composition with continuity of strain across the interface is assumed. The equation of the Voigt model can be written as a simple rule of mixture:

E_c=V_f E_f+V_m E_m

where E is the Young's modulus, V is the volume fraction, c, f, and m denoting the composites system, filler phase and matrix phase respectively.

Reuss model, which assumes that the stress will be identical in each phase, can be written

as follow:

or

Error!

If we take the Young's modulus of filler and matrix as 70 GPa and 37 MPa respectively, according to the Voigt model, the composites with 10%, 20% and 30% volume of filler should

HA-Polyactive composites with Chemical Bonding 139 have Young's moduli 7 GPa, 14 GPa, and 21 GPa. These theoretical values are much higher than

the actually found values. The Reuss model gives lower Young's moduli as compared to the found values of the composite system. Figure 9 gives the theoretical value of Voigt and Reuss models in comparison with the realistic found value of the composites. Therefore, the current composite system behaves somewhere between the model of Reuss and Voigt model, partially as a constant strain system in which the composite modulus is higher than the values predicted by Reuss model, and well below the upper limit predicted by Voigt model. The fact that introducing interfacial bonding of the filler/matrix resulted in higher Young's modulus raises the question whether if more matrix molecules could be chemically bound to the filler surface, moduli much closer to the theoretical values of Voigt model could be achieved. In this experiment, only 4 wt% Polyactive^TM 70/30 was bound to HA filler, which is much lower than the theoretical value. Considering the bound HMDI is 2.6 wt% , which is 0.13 mmol/gHA, in theory, 12 grams Polyactive^TM (Mw = 90,000 Dalton) can be bound to 1 gram HA. By optimizing the grafting procedure, the mechanical properties of the composites might very probably be further improved.

Figure 9. The experimental Young's modulus in comparision with the theoretical values predicted by Voigt and Reuss models.

Conclusion

Chapter 10 140

A copolyether-ester (Polyactive^TM 70/30) was successfully bound to HA filler particles via HMDI, establishing a covalent bonding of HA filler particles to polymer matrix. The mechanical testing results showed that the chemical bonding of HA filler to polymer matrix had a distinct effect on the mechanical properties of the composites. The Young's modulus, tensile strength and elongation at break were significantly improved by grafting, especially at higher filler fractions. A further improvement of the mechanical properties can be expected if higher grafting percentages of Polyactive^TM to HA can be achieved.

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