Biomedical Engineering Reference
In-Depth Information
Scheme 6.2
Synthesis of PURs' networks and reaction conducing to CO
2
as blowing agent.
Indeed, understanding the morphology is crucial for the design of materials
with specifi c properties. Molecular organization of PURs has been investigated by
several techniques, including differential scanning calorimetry ( DSC ), wide - angle
X - ray diffraction ( WAXD ), small - angle X - ray scattering ( SAXS ), infrared spectros-
copy (IR), electron microscopy (EM), dynamic mechanical thermal analysis
( DMTA ), and nuclear magnetic resonance ( NMR ) [18] .
DSC experiments show that PURs have several thermal transitions, the inter-
pretation of which is rather complex [19]. Glass transitions of both hard and soft
amorphous microphases can be detected. The
T
g
value of the soft domain, which
appears at the lowest temperature, may be used to evaluate the number of hard
segments in this domain since
T
g
should increase when the degree of mixing is
raised. However, a quantitative analysis is problematic due to the infl uence of
factors like restrictions on the motion of soft segments caused by the presence of
microcrystals. In addition, DSC traces can show multiple endothermic peaks
which may be ascribed to morphological effects and be broadly divided into loss
of long - and short - range order.
Early explanations about these multiple endotherms were based on the disrup-
tion of different kinds of hydrogen-bonding interactions [20, 21]. However, infra-
red thermal analysis led to discarding a clear relationship between endothermic
peaks and these interactions [22]. Hydrogen bonding plays a signifi cant role in the
design of biostable or biodegradable materials as it is a determinant factor of their
hydrolytic stability. Susceptibility to hydrolytic degradation is clearly enhanced
when the carbonyl groups in the hydrolyzable group do not act as hydrogen- bond
acceptors. The knowledge of hydrogen-bond distribution in PUR materials is thus
essential to obtain materials with a specifi c degradation rate.
