Page 168 - Biodegradable Polyesters

146 6 Shape Memory Systems with Biodegradable Polyesters

and chemically coupled. Li et al. [69] prepared conetwork-structured thermosets
by the peroxide-induced copolymerization of methyl methacrylate and PCL
dimethacrylate. The poly(methyl methacrylate)/PCL ratio was varied between
80/20 and 20/80. The related systems exhibited a single T that changed over
g
∘
a wide range (T = 110 to − 20 C) as a function of composition. The broad
g
T relaxation may be used to memorize multiple shapes via careful program-
g
ming. The authors showed that the related conetwork may show quadruple
SM – “remembering” up to three temporary shapes. The energy stored in the
whole T range can be partitioned into several parts for multi-shape program-
g
ming as long as the partitioned energy during cooling is sufficient for shape
fixing. Erden and Jana [70] modified SM PU with polybenzoxazine. The precured
benzoxazine with its phenolic hydroxyl groups can react with the polyisocyanate
whereby forming a conetwork with the PU. Polybenzoxazine appeared in the
PU matrix in phase-segregated nanoscale domains. They can be treated as net
points of a second fixing phase in addition to the hard segments of the PU. Recall
that the latter, being net points, are responsible for the permanent shape of PU.
Co-cross-linking with benzoxazine shifted the T of the corresponding systems
g
toward higher temperatures. At the same time, the recovery stress was doubled
compared to that of the reference PU. Enhancing the recovery stress is a research
direction that should meet the demand of sensors and actuators.
Epoxy (EP)-PCL conetworks were produced by the group of Hartwig [71, 72].
Crystalline PCL domains, overtaking the role of “switch” phase, were covalently
integrated into the cationically polymerized EP network. The EP/PCL ratio was
∘
varied between 85/15 and 60/40. After deformation at T trans = 70 C and fixing
∘
at T = 20 C, R of 100% was measured. In the companion paper, the authors
f
addressed the nucleation of the PCL phase [72].
6.2.5.2 Semi-Interpenetrating Network
Unlike conetworks, semi-IPNs are composed of two continuous phases from
which one is of thermoplastic nature. It is intuitive that the related entangled
structure should contribute to the onset of SM properties. This was confirmed
recently [73]. It is worth noting that entanglement in semi- and full-IPN struc-
tures is usually not on molecular level although the latter system features one
single T [74]. “Full” IPN denotes that both constituent continuous phases are
g
cross-linked polymers. Semi-IPNs may not only have SM but also self healing
properties. Although the term shape memory-assisted self-healing was coined by
the Mather group [75], the concept should be credited to Karger-Kocsis [76].
The group of Mather [75] produced semi-IPNs containing high MW PCL as
thermoplastic and tetrathiol cross-linked low-MW PCL diacrylate as thermoset
phase. The linear/network PCL ratio was studied between 0/100 and 80/20. At
200% tensile deformation, R increased slightly, whereas R decreased sharply with
f r
increasing amount of the thermoplastic PCL. Self-healing was demonstrated on
the partially broken double-edge, notched, tensile, loaded specimen. Healing hap-
pened by wetting, diffusion, and randomization of the linear PCL component at
∘
T = 80 C (i.e., above its T ).
m