Nano Indentation Studies Of Polyacrylamide / Bentonite Nanocomposite For Hip Prosthesis Application

Bentonite clay was used as loading charge in polyacrylamide/bentonite nanocomposites. PAM/BC nanocomposites were synthesized via insitu radicalar polymerization using adiabatic process. The amount of bleachy clay was fixed at 1wt%, 3wt%, 5wt% and the polymerization was performed in aqueous solution. Mechanical properties were carried out using nano indentation technique. The measurements confirm that PAM/BC prepared with one present the best ones. The slope of load curve, storage modulus and nano hardness was presented as function of BC amount clay. The result shows that the structural and mechanical properties of the samples were influenced by the amount of BC. Rising of load tangent with increasing of percentage weight BC in the polymer nanocomposite, the nano-hardness curve decreases for the two starting values of the percentage of weight in BC (1% and 3%), then it comes a little increase from 3% to 5% percent of BC. These variations depend on the structure and surface roughness of samples.

Introduction

Nano-indentation has emerged as a critical technique for the evaluation of the mechanical response of small material volumes and thin films or layers to applied loading. It is generally called as, the depth sensing indentation or the instrumented indentation, this technique allows the measurement of the depth of an indenter with known geometry on the surface materials. It’s meaning that, the profundity of penetration into the specimen surface is measured and combined with the identified geometry form of the indenter to calculate the contact area. The load and unload curves during the indentation practice were described by Ciprari and Van Landingham.

In the present paper, nano indentation studies at different ration of PAM/BC were realized in order to determine the influence of the nanostructure in the mechanical behavior nano- hardness, storage modulus and slope of load curve.

Method and instruments

A Keysight G200 nano-indenter was used to obtain nano-hardness and storage modulus data. Maximum indentation depth was 5 µm, surface approach velocity was 10 nm/s, strain rate target 0,05 s-1, frequency 45-50Hz and temperature 25-26°C.

Bentonite/acrylamide nano-composites were obtained using an adiabatic process of radical polymerization (in-situ) in aqueous solution. The samples were prepared in the ratio of (1-5%:99-95%) in weight of bentonite/acrylamide and ammonium persulfate (APS) was used to propagate polymerization. The diameter of spherical bentonite particles was in the range between the micrometer and the nanometer scale. At first x% of the bentonite suspension was stirred under a nitrogen atmosphere for 30 min, APS (0.2%) was stirred for 5 min, y % of AM dissolved in bi-distilled water was also stirred for 30-35 min after a fast heating. The AM and APS suspensions were added to the bentonite suspension and stirred for 20min. The polymer obtained was dispersed in ethanol, filtered and dried under vacuum for 48h.

Results and discussion

Energy Dispersive X-ray Spectroscopy (EDS) results, providing the elemental percent composition in our polymer nanocomposite prepared with one percent of BC. Table 1, 2 and 3 correspond to the EDS spectrums represent the composition elements in the polymer composite prepared with a ratio of 99 PAM/ 1 BC , 97 PAM / 3 BC, PAM 95/ 5 BC. In the different spectrum, we showed that the important element composition is the carbon, that means that the monomer Acryl amid was polymerized and polymer exists with an important quantity in the final material. EDS results of elements percent in each polymer composite is showed in the next tables.

Curves represented in Figure 1 show the variation of nano-hardness with displacement into the surface for different ratios in weight of PAM/BC and the measurements of the nano-hardness by the nano-indentation technique. From the data represented in fig 1 and (Fig. 4 b), it can be observed that nano-hardness decreases with the percentage of BC in the polymer composite. Increasing of bleachy clay loading in weight induces a fragility of PAM/BC. These results are attributed to the interaction type created between the bentonite sheets and matrix polymer or dispersion of clay particles in PAM matrix. This significantly indicates a formation of Van Der Waals bonds among the constituents of our material. In hence, the polymerization of acrylamid with BC clays already proved the diffusion of the polymer chains into basal space of the bentonite layers which lend to the creation of strong interfacial interactions. This confirms that the data obtained is essentially due to formation of agglomerations of BC sheets. The variation of the nano-hardness as a function of the depth in surface at the surface shows a constant trend for all the measurements carried out on the different samples. Figure 2 represents the modulus variation with the displacement into the surface for different ratios in weight of PAM/BC. In each curve of storage modulus, we observe some fluctuations that can be related to the increase of sample surface roughness under load ≈ 200 mN. Figure 3 shows the evolution of load applied in the surface of PAM/BC for each value of percentage in weight of BC. 11 1 2 3 4 59,610,010,4 modulusmodulus (Gpa)percentage in weight of BC (a) 1 2 3 4 50,300,350,400,45 nano hardness nano hardness (GPa)percentage in weight of BC (%)(b)12 Figure 4: (a) Storage modulus variation with percentage in weight of BC, (b) Nano hardness variation with percentage in weight of BC, (c) Tangent of curve load variation with percentage in weight of BC In figure 4(a) is shown decreasing of storage modulus from 10.4 GPa to 9 GPa with increasing of clay content in polymer composites from 0.5 to 5% wt. Reduction of nano-hardness from 450 MPa to 335 MPa with increasing of BC wt % from 1 to 3% after this value we observed clearly a small gain of nano-hardness from 335 MPa to 400 MPa (figure 4 (b)). Augmentation of tangent load as function of loading clay weight %, which varied respectively, from 10 to 70 N/nm and 0,5 to 5 wt% (figure 4 c).

Result obtained in figure 4 (a) resumed that the storage modulus of our material reduced with augmentation of BC percentage. Storage modulus depends on BC sheet distribution and their organization in the matrix polymer. In fact, correlation between 1 2 3 4 5204060tangent of load curvestangent of load (N/NM)percentage in weight of BC (%)(c)13 the stress fields applied on the sample and orientation of BC sheet in the matrix polymer displays an important factor in this data. The remarkable increase in the tangent of load curve (figure 4 c) as function of loading clay content % depends to the resistance of composite polymer surface to stress and dispersion of BC clay in the matrix polymer. The shape of this curve confirms that resistance against the load applied reduces with growing of weight fraction of BC. The fragility of material is principally due to the links Inter and intra-molecular types.

Conclusion

By taking into account the data discussion the following conclusions can be done: The morphology and the structure obtained with polymer prepared with one percent in weight of BC present the most important mechanical thermal and structural properties. However, the Van Der Waals bonds and the structure of the polymer composite are the dominant factors in decreasing of storage modulus with content of BC% wt. The load tangent increase with %BC in the polymer, the nano-hardness decreases for the two starting values of the percentage of weight in BC (1% and 3%), then it comes a little increase from 3% to 5% percent of BC. These variations depend on the structure and surface roughness of samples. The effect of the level of reticulation PAM on the structure and physical properties of PAM/BC and anti-microbial behavior of system based on PAM/BC nano-composite will be studied in a separate work. 14 The obtained results desire to be investigated and exploited in the fabrication of hip prosthesis based on PAM/BC nano-composite via its biocompatibility, legerity, and good thermal and mechanical behavior.