Morphology, reversible phase crystallization, and thermal sensitive shape memory effect of cellulose whisker/SMPU nano-composites
(749–762)Journal of Applied Polymer  Science 123 #2 (2012)
Han et al of Hong Kong Polytechnic University and Sichuan University, China, prepared cellulose whisker/shape memory polyurethane) nano-composite.  The crystallization mechanism of reversible phase in nano-composite gradually evolves to heterogeneous nucleation and crystal growth in two dimensions with the increase of CW content; the excellent nucleation effect of CW for the reversible phase leads to the drop of activity energy from 134.1 kJ/mol (N-0), to 95.1 kJ/mol (N-4).  (RDC 10/12/2011)

Bioinspired Mechanically Adaptive Polymer Nanocomposites with Water-Activated Shape-Memory Effect
(6827–6835) Macromolecules 44 #17 (2011)
Mendez et al , Switzerland, United Kingdom and Ohio, developed biomimetic, stimuli-responsive mechanically adaptive nanocomposites, which change their mechanical properties upon exposure to water and display a water-activated shape-memory effect, were investigated.  These materials were produced by introducing rigid cotton cellulose nanowhiskers (CNWs) into a rubbery polyurethane (PU) matrix.  A series of materials with CNW concentrations of 2–20% v/v was produced by solution blending CNWs and the PU. Films were subsequently prepared by compression molding.  The introduction of CNWs led to an increase of the tensile storage moduli (E′) in the dry nanocomposites. The level of reinforcement scaled with the CNW content and followed the Halpin–Kardos model below and the percolation model above the percolation limit of 7% v/v. Upon exposure to water, the materials with a CNW content above the percolation limit swelled slightly and showed a decrease of E′, for example from 1 GPa to 144 MPa in the case of the material with 20% v/v CNWs.  This effect is the result of competitive hydrogen bonding between water and CNWs, which reduces the hydrogen bonding between the CNWs and weakens the CNW network that drives the reinforcement in the dry state. The mechanically adaptive behavior and a high elasticity of the wet materials are the basis for a shape-memory effect that uses water as the stimulus. Polarized Raman spectroscopy revealed that in the temporary shape, generated by stretching and drying water-swollen nanocomposites, the CNWs display a significant level of uniaxial orientation.  (RDC 9/7/2011)

Comparison of shape memory polyurethanes and polyurethane-ureas having crystalline reversible phase
(314-325) High Performance Polymers 23 #4 (2011)
Abstract
Ji and Hu ofHong Kong Polytechnic University, China, prepared shape memory polyurethanes (SMPUs) and polyurethane-ureas (SMPUUs) with aromatic diamines as chain extenders having crystalline reversible phase were prepared.   When hard-segment content (HSC) was 30% the polyurethane-urea exhibited higher shape recovery than its polyurethane analogue.  With the increase of HSC the shape recoveries of both the SMPUs and the SMPUUs decreased but those of the SMPUUs decreased more apparently.  When HSC was 40%, the shape recovery of the polyurethane-urea was lower than that of the polyurethane.  This is because the great increase of physical cross-linking density of SMPUUs resulted in significant increase of irreversible deformation.  In comparison with those of the SMPUs, the shape recovery curves of SMPUUs shift to lower temperatures. This is because the higher physical cross-link density of the SMPUUs results in more decrease of the melting temperature of soft phase. Moreover, the shape fixities of the SMPUUs were lower than those of the SMPUs with identical HSC also because of their higher physical cross-link density. The recovery stresses of the SMPUs and the SMPUUs increased with the increase of HSC. The recovery stress of a polyurethane-urea was higher than that of its polyurethane analogue by over 100%. Hence it is possible to produce SMPUUs with much larger recovery stress and higher shape recovery without sacrificing much shape fixity by controlling appropriate HSC.  (RDC 7/12/2011)

Studies on the cycling, processing and programming of an industrially applicable shape memory polymer Tecoflex® (or TFX EG 72D)
(300-307) High Performance Polymers 23 #4 (2011)
Abstract
Schmidt et al of theRuhr-Universität Bochum (RUB), Germany, studied the dependence of different stress levels, and the effect of both recovery temperature and recovery time.  As a model material the commercially and industrially applicable amorphous shape memory polymer Tecoflex® was examined and subjected to 50 programming cycles. Tecoflex® is characterized by a glass transition temperature, Tg, of 74 °C, above which it looses all its strength.  During tensile testing at 20 °C (T < Tg), stresses a steady increase to 26 MPa as strains approached the rupture strain of 25%. It is observed that at 60 °C (T < Tg, but near Tg) the material can be strained to more than 2500% before rupture occurs while stresses slowly increase to values less than 0.3 MPa. It turns out that programming, cooling, unloading and heating to trigger the one way effect causes an increase of irreversible strain that is associated with a corresponding decrease of the intensity of the one way effect during the first thermomechanical cycles.  (RDC 7/12/2011)

“Tecoflex® is a family of medical-grade aliphatic polyether polyurethanes. They do not yellow and are available over a wide range of durometers, colors and radiopacifiers, including barium sulfate, bismuth salts and tungsten. The low durometer grades in particular may not be suitable for long-term implant applications because of the potential for stress cracking.” (Microspec Corporation. Tecoflex, 7/13/2011)

Shape Memory Polyurethanes from Renewable Polyols Obtained by ATMET Polymerization of Glyceryl Triundec-10-enoate and 10-Undecenol
(1392–1399)Macromolecular Chemistry and Physics 212 #13 (2011)
Del Rio et al of theUniversitat Rovira i Virgili, Spain and the Institute of Organic Chemistry, Germany, synthesized branched polyols from glyceryl triundec-10-enoate by acyclic triene metathesis polymerization (ATMET).  10-Undecenol is used as a monofunctional comonomer to end-cap polymer chains, functionalize the periphery of the resulting branched materials, and to control the molecular weight.  The thus obtained castor oil derived polyols are reacted with 4,4′-methylenebis(phenylisocyanate) (MDI) to yield a series of semicrystalline polyurethane networks. The investigation of the thermal stability and the thermomechanical and mechanical properties of these thermosets revealed good shape memory properties.  (RDC 6/27/2011

Effect of MDI–BDO hard segment on pyridine-containing shape memory polyurethanes
(5294-5304) Journal of Materials Science 46 #15 (2011)
Chen et al, China, synthesized a series of pyridine-containing supramolecular shape memory polyurethanes (Py-SMPUs) with various diphenylmethane diisocyanate–butanediol (MDI–BDO) contents from the 1,6-hexamethylene diisocyanate (HDI), N,N-bis(2-hydroxylethyl) isonicotinamide (BINA), BDO, and MDI.  The results show that the addition of MDI–BDO segment improves the strength of hydrogen bonding of Py-SMPUs.  It promotes the formation of hard domains, but not influences the phase-separation structure, the intensity, and distribution of physical netpoints.  In addition, the MDI–BDO segment improves the rubber modulus and drops the energy loss of Py-SMPUs.  However, it does not influence the shape memory behaviors which are mainly influenced by the BINA content. If the BINA content is beyond 30 wt%, good shape memory effect can be achieved in the BINA–HDI–BDO–MDI system; and high shape recovery force can be obtained by increasing the MDI–BDO segment.  (RDC 5/24/2011)

Effects of addition of hindered phenol compounds to a segmented polyurethane with shape memory on mechanical yielding
(1264-1271) Journal of Materials Science 46 #5 (2011)

Kuriyagawa et al of Kanazawa University, Japan showed that the yield process is caused by the rotation of hard segment domains within the deformed plastic domains composed of glassy soft segments.  (RDC 1/12/2011)

Thermomechanical characterization of thermoset urethane shape-memory polymer foams 
(3217-3229) Journal of Applied Polymer Science 115 #6 (2010)
Abstract
Domeier et al of Sandia National Laboratories showed that compression near the foam glass transition temperature gave optimum shape-memory performance.  Foams with densities of about 0.5 g/cc and compressed 50% provided a useful balance (time, strain, and load) in the recovery performance. (RDC 11/9/2010)

Relaxation based modeling of tunable shape recovery kinetics observed under isothermal conditions for amorphous shape-memory polymers  
(6212-6218) Polymer 51 #26 (2010)
Heuchel et al of the Institute of Polymer Research, Germany are looking for shape memory polymers, which allow the adjustment of shape-memory properties by variation of physical parameters during programming.  The stress relaxation behaviour of polyurethane (PEU) based shape-memory polymers at temperatures from 0 °C to 80 °C and different strain values from 100% to 250% could be well described by a modified Maxwell–Weichert model of two Maxwell units and a spring. The stress relaxation results in a combination of a slow and a fast decaying process.  (RDC 1/27/2011)