Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 4 – Control of mechanical and degradation properties in composites
In an attempt to improve the quality of composites, in this study we used a dry
preparation method, i.e. the co–rotatory twin–screw extrusion at high temperature.
We chose this process because the co–rotatory screws system would
homogeneously mix dry apatite powder into melted polymer. Further, a fast cooling of
the resulting material would harden the polymer quickly entrapping apatite without
letting it shift. Since extrusion is a high temperature processing method, it may cause
thermal degradation of the polymer. The rotatory mixing in the presence of screws
may also lead to ‘mechanical’ degradation of the polymer due to friction with the
screws and apatite particles. In particular, we expect that increasing the content of
apatite, higher friction will happen enhancing the degradation of the polymer
phase. To test this hypothesis we prepared composites with various apatite contents
and a high molecular weight copolymer. We then evaluated possible changes in the
effect of extrusion on the chemistry, phase content, apatite distribution and polymer
molecular weight of the composite.
Increasing the apatite phase content will enhance the stiffness of composites and
decrease their damping characteristics. However, it would also render the composites
more hydrophilic and therefore allow more fluid uptake. Thus, the higher content of
apatite would absorb more fluids leading to a larger decrease in stiffness and
improving the damping properties. At the same time, composites absorbing more
fluids would trigger quicker hydrolysis of the polymer phase leading to faster
degradation of the composites. To address this, we evaluated the dynamic
mechanical properties of composites in dry and humid conditions and we also studied
their degradation profile at body temperature for three months.
4.2. Materials and Methods
4.2.1. Apatite preparation and characterization
Nano–apatite powder was synthesized by adding (NH4)2HPO4 (Fluka, Steinheim,
Germany) aqueous solution (c=63.1 g L –1 ) to Ca(NO3)2·4H2O (Fluka) aqueous
solution (c=117.5 g L –1 ) at the controlled speed of 12.5 mL min –1 and 80±5ºC, with the
reaction pH kept above 10 by using ammonia (Fluka). After precipitation, the resulting
powder was aged overnight, washed with distilled water to remove ammonia,
dehydrated in acetone (Fluka) and finally dried at 60±1°C. Its chemistry was then
investigated with X–ray diffractometry (XRD, MiniFlex II, Rigaku, Japan) using Cu K
radiation (=1.54056 Å) operating at 30 kV and 15 mA. Spectra were collected at
0.083 deg sec –1 in the 2 range 5–90 deg. Following background subtraction (cubic
spline method) and pattern smoothing (parabolic Savitzky–Golay filter set at 15
points), individual diffraction peaks were fitted (R