Page 341 - Engineering Plastics Handbook

Polyarylethersulfones (PAES) 299

It also results in dimensional stability over the service life of a finished part
that may be used in a variety of thermal environments. This excellent
dimensional stability makes polysulfones an excellent choice for many
structural, engineering, and particularly electronic applications.
One illustration of the variation of properties with backbone structure
is that of moisture absorption. Because of the sulfone moiety, polysulfones
are slightly hygroscopic. PES contains roughly twice the content of sul-
fone groups in its backbone relative to PSF and PPSF. Additionally,
because PSF is synthesized from bisphenol A, it contains the hydropho-
bic isopropylidene unit. As a result, PES is the most hygroscopic of the
three polysulfones while PSF is the least hygroscopic, as can be seen in
Table 13.3. Minimizing the moisture absorption in a plastic material is
generally desirable. One reason is that absorbed water expands the
linear dimensions of polysulfones by about 0.010 to 0.012% for every
0.1 wt% of moisture absorbed. This change is quite small, but could be
important in application designs where extremely tight tolerances on
dimensions are required. Absorbed moisture can also slightly plasticize
the resin, which can cause some lowering of the stiffness and strength
if the plastic is in a hot and wet environment. This effect is also small
and can be compensated for by part design considerations.

Mechanical properties
Polysulfones are rigid and tough with excellent strength and stiffness
properties without reinforcement. Their strength and stiffness are quite
high relative to those of amorphous plastics that contain aliphatic back-
bone structures. In addition, polysulfones are ductile and yield in a duc-
tile fashion even at high deformation rates.
As with any other polymer family, the deformation mechanisms and
resulting mechanical properties are closely tied to the subtle differences
in backbone structure [13]. Polysulfones contain aryl groups that are
connected in a linear (unbranched) and para-linked fashion. This
arrangement imparts good strength and toughness as well as favorable
processing attributes. Although all three polysulfones offer good tough-
ness, PPSF offers superior toughness and impact resistance relative to
PES and PSF. The ductility of polysulfones is tied to a second-order (β)
transition that is observed in polysulfones under dynamic mechanical
thermal analysis. The β transition occurs at around −100°C and is
believed to be due to two mechanisms. The first involves 180° flips of
aromatic units around the ether bond [14]. It is hypothesized that the
second is a concerted motion of the sulfone group with complexed
absorbed water [15]. The presence of water increases the magnitude of
the β transition peak although the peak still exists in the absence of
water. The effect of polysulfone backbone structure on the sub-T relax-
g
ations or second-order transitions has been discussed in detail [16].