Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 7 – Polymer molecular weight and instructive composites
15±1 minutes to stabilize the material prior the measurements. The data measured were
storage modulus and loss tangent.
7.2.8. Statistical analysis
Two tail t–test (for populations with different variance) and post–hoc Tamhane ANOVA
test were used to evaluate differences in the results. The choice between the two tests
relied on the size of the compared data. If the data populations to be compared were two,
t–test was use. If populations were more, ANOVA was used. A p–value lower than 0.05
was considered as significant difference in both statistical tests. The analyses were
performed using Origin software (v8.0773, OriginPro, Northampton, MA, USA).
7.3. Results
7.3.1. Physicochemical characterization of the synthesised apatite
Using XRD we observed that the spectrum of the synthesized powder (Figure 1a) was
comparable to the international JCPDS 9–432 reference for hydroxyapatite. Since the
powder was neither sintered nor calcined, the diffraction peaks were broader than the
international reference, indicating low crystallinity and confirming we obtained a calcium
phosphate apatite powder. Calculations on the diffractometer data led to unit cell
parameters a=9.412±0.047 Å and c=6.880±0.011 Å, which are characteristic for
synthetic apatite. [314] FTIR showed vibrational bands around 604, 632, 962, 1031 and
1095 cm –1 (Figure 1b), which are typical for a calcium phosphate apatite. [351] Acicular–
shaped apatite particles, with dimensions ranging from 200 to 400 nm in length and 20
to 50 nm in width, were observed in TEM images (Figure 1c).
7.3.2. Composites characterization
Two composites (ML and MH respectively) were successfully prepared by extruding low
and high molecular weights poly(D,L–lactide) with calcium phosphate apatite particles.
Measurements with Ubbelohde viscometer showed significant decreases in the weight
average molecular weight for the polymer phase in both composites (Table 1), which
indicates degrading effects of extrusion (i.e. thermal degradation and friction with
apatite) on the polymer phases. In particular, this effect was larger in the case of higher
molecular weight polymer. This fact is in line with observations reported in literature, [336]
and our results in Chapter 4. However, differences in the molecular weight between the
two polymers after extrusion with apatite were still observed and therefore the two
composites contained significantly different molecular weight polymers (Table 1).
Chemical characterization with XRD and FTIR confirmed that the two composites had
similar surface and bulk composition. Diffractometer patterns showed amorphousness of
the pure polymers and the presence of apatite in the composites (Figure 1a). The infra–
red vibrational bands of the two polymers were similar demonstrating chemical
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