Shape fixing

Bulk polymerization consists of heating the monomer without solvent with initiator in a vessel. The monomer-initiator mixture polymerizes to a solid shape fixed by the shape of the polymerization vessel. The main practical disadvantages of this method are the difficulty in the removal of polymer from a reactor or flask and the dissipation of the heat evolved by the polymerization. 

The shape fixed by vulcanization (e.g., in a compression mold) cannot be changed unless it is subjected to mechanical work. 

Finally, we optimised the coordinates of the slab + metal atoms again keeping the cell volume and shape fixed. 

Traditional manufacture of ceramics is based on the use of fine natural raw materials which are capable of producing easily worked plastic mass (paste) with water. The formed bodies maintain their shape. The products are strengthened and the shape fixed by firing at temperatures which were attainable even in the primitive kilns. Kaolin and clays are the main raw materials which exhibit these required properties. 

TM7 accomodating the phenyl substituent. As indicated for 36 in Fig. 3 A, in the Dj receptor, differing from D2 for Cys-283 instead of Ile-384, and Phe-319 instead of Leu-414, the pocket has a suitably open shape, fixed by hydrogen bond linking of Cys-283 to the asparagine residue corresponding to Asn-418 in D2... 

In addition to the molecular shape fixed by chemical bonding, variations in the overall shape and size of the polymer chain may occur due to rotation about primary valence bonds (conformation). A polymer molecule may assume a large or limited number of conformations depending on ... 

Three-dimensional (3-D) plot of the shape memory cycle for (a) a shape memory polymer (SMP) and (b) vulcanized natural rubber. The star indicates the start of the experiment (initial sample dimensions, temperature, and load). Both the SMP and the rubber were deformed under constant loading rate at constant temperature. The deformation step was then followed by a cooling step under constant load. At low temperature, the load was removed and shape fixing was observed for the SMP, but an instant recovery was seen for natural rubber. Shape recovery of the primary equilibrium shape was obtained by heating the SMP. (Adapted from Liu, C., Qin, H., and Mather, P. T. 2007. Review of progress in shape-memory polymers, journal of Materials Chemistry 17 1543-1558. Copyright Royal Society of Chemistry. Reproduced with permission.)... 

Figure 3.12 Schematic of classical tension programnting (steps 1 to 3), free shape recovery (step 4), fuUy constrained stress recovery (step 5), and schematic of molecular mechanisms for shape fixing and shape recovery...

Linear poly(L-lactide-co-e-caprolactone)s (PLACLs) were produced by Lu s team [27]. The lactide/lactone ratio was varied between 90/10 and 60/40 which was associated with a large change in the (reduction from 54 to 14°C). For shaping, = T + 1Z°C, whereas for shape fixing, T=T — 15°C were... 

Carbon molecular sieves were prepared from activated carbons by coke deposition from the thermal cracking of propylene. The heat treatment was carried out in a smaller diameter U-shaped fixed bed reactor made of an A inch 316 SS tube. A 2 kW vertical electric furnace was used for heating the reactor, while the design of gas flow and temperature control instruments in the activation system permitted also their use in the smaller reactor system. The... 

Figure 3.6 Photographic demonstration of the shape recovery of the composite prepared with 12 wt% cellulose nanowhiskers (a) temporary shape fixed in the dry and cool state (elongation of around 300%) (b) temporary shape after stimulation with heat (c) temporary shape after stimulation with cool water (d) original shape after stimulation with hot water (68°C). From Luo et al. [42], with permission.

SME of shape memory polyurethanes (SMPUs), the shape recoverability and shape fixing ability, ate dependent on the polyurethane network structure and phase transition of the SMPU, respectively. The roles played by the reversible phase and fixed phase are complex. Recovery stress is essential for most of the applications for SMPs since in practical applications the shape recovery can be impeded by external stimuli. 

Abstract With a melting transition as the switch, shape memory polymers can exliibit the shape memory effect (SME). This chapter demonstrates the e q)erimental findings in the study of a series of T jj-SMEUs. The morphological variation with chemical composition and the influences of morphology and thermo-mechanical conditions including deformation strain, shape fixing speed and pre-deformation on the SME are discussed. 

As shown in Fig. 2.8(b), the intensity of the diffraction peaks of the segmented polyirrethanes increase substantially after shape fixing. Even the polyirrethanes PCL4000-B-45 and PCL4000-B-50 show diffraction peaks. Because the positions of the diffraction peaks are identical to those shown in the WAXD tests before exterrsiorr, the peaks are still ascribed to the crystallization of the soft phase. The increase of the X is attributed to the strain-induced crystallization of the soft segments after deformation and shape fixing. The X of PCL4000-B-40,... 

Tracing the structural changes in shape memorization by the WAXD tests (a) before extension (b) after shape fixing (being extended 100% and fixed in a temporary shape) (c) after shape... 

SS) and the hard phase e (HS). The shape fixing and shape recovery are strongly related to (SS) and (HS). The distribution of viscoelastic deformation between soft and hard phases is dependent on the two-phase morphology such as phase separation, phase composition, domain sizes, domain concentration and coimectivity, etc. Thus the shape memory properties ate defined by the two-phase morphological stmcture of the segmented polyurethanes. 

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