Shape Memory PU

This work involved the synthesis of branched polyols via ATMET polymerisation starting from the same model triglyceride depicted in Scheme 5.13, and its reaction with MDI to produce shape-memory PU [55]. Mn values of those polyols were 1.3-3 kDa, and the PU derived from them exhibited good thermo-mechanical properties. 

Potential applications for shape memory PU exist in almost every area of daily life from self-repairing auto bodies to kitchen utensils, from switches to sensors, from intelligent packing to tools [98]. Other potential applications are drug delivery [99], biosensors, biomedical devices [100,101], microsystem components [102], and smart textiles [103]. Because PU can be made biodegradable, they can be used as shortterm implants so removal by surgery can be avoided. Some important applications are discussed next. 

Shape memory PU and polymers in general have tremendous applications in biology and medicine [104, 105] especially for biomedical devices which may permit new medical procedures. Because of the ability to memorise a permanent shape that can be substantially different from an initial temporary phase, a bulky device could be introduced into the body in a temporary shape (e.g., string) that could go through a small laparoscopic hole and then be expanded on demand into a permanent shape at body temperature. 

Shape memory PU has been proposed as a candidate for aneurysm coils [106]. An intracranial aneurysm can go undetected until the aneurysm ruptures, causing... 

Recently, the concept of cold hibernated elastic memory utilising SMP in open cellular structures was proposed for space-bound structural applications [108]. The concept of cold-hibernated elastic memory can be extended to various new applications such as microfoldable vehicles, shape determination and microtags [109]. Recent studies on shape memory PU-based conductive composites using conducting polymers and carbon nanotubes show considerable promise for application as electroactive and remote sensing actuators [110]. 

The previous discussion indicates the tremendous application of shape memory PU. Extensive work has been carried out for the development of shape memory PU in the last few years, and is reviewed next. 

Table 2.10 Thermal and viscoelastic properties of DiAPLEX (Mitsubishi) thermoset shape memory PU [111] ...

Shape memory PU can be classified into two catergories amorphous and crystalline. The compositions and properties of different types of amorphous PU reported in recent literature are summarised [116-120] in Table 2.11. Wang and Yuen [121] synthesised a series of thermoplastic PU using aromatic chain extenders such as... 

Table 2.12 Structure and properties of crystalline shape memory PU ...

Zhu et al. studied polyurethane foams from soy reinforced with cellulose microfibers. They found an increase on the onset degradation temperature of the thermal degradation of polyurethane with the addition of 2 wt % cellulose fibers. They attributed this fact to the insulator effect of cellulose fibers [52]. Navarro-Baena et al. studied shape memory PU based on PLA-PCL-PLA block copolymer and reinforced with both CNCs and PLA grafted CNCs [72]. Aside the increment on the shape memory behavior of the polyurethane-based nanocomposites, they reported an increase on the thermal stability of the PU matrix in particular, they reported that, although CNCs improved the thermal stability of both PCL and PLEA blocks, in particular the thermal stability of the PCL block was improved in the nanocomposites increasing the maximum degradation temperature of about 40 with respect to the PCL block of the neat PU-matrix [72]. 

Recently, Han et al. used shape memory PU nanofibrous membranes which allowed the material s shape to be retained and recovered through heating. 

They evaluated the shape memory performance and air and water vapor permeability, and all samples showed shape recoveries of at least 99 % and shape retentions of at least 94 %. The stretched and fixed nanowebs showed greater differences of water vapor permeability at 10 °C with 90 % relative humidity (RH) and 15 °C with 90 % RH, than the original nanowebs did, because stretched shapes of the samples could be retained by more than 70 % at 10 °C or 15 °C. The 40 mm shape memory PU nanoweb showed the best performance, maintaining high shape memory and the largest difference in water vapor permeability between when stretched and not stretched at 15 °C with 90 % RH [32],... 

Better dispersion of MWNTs in the polymer matrix caused by the formation of the chemical bonds leads to uniform stress distribution and enhanced shape memory (23). Jana et al. prepared nanocomposites of PU and MWNTs via in-situ polymerization and conventional method (105). PU nanocomposites obtained via an in-situ method with PCL-g-MWNTs showed better shape recovery, compared to conventional nanocomposites. 

Figure 6.6 Chemical pathway of the synthesis of multiblock linear and cross-linked SM PU by incorporating high MW PCL soft segment as switch phase (a) and demonstration of the shape memory effect setting r,j3 5 = 80°C (b). Abbreviations - MDI methylene diphenyl diisocyanate, Pluronic ...

A. M. (2013) Thermal, mechanical and electroactive shape memory properties of polyurethane (PU)/poly(lactic acid)(PLA)/CNT nanocomposites. Eur. Polyrtu /., 49 (11), 3492-3500. 

Figure 2.1 Effect of polyurethane (PU)/polycaprolactone flbn structure on the modulus of 65 layer and blend films in comparison with values predicted from the parallel and series models. From J. Du, S.R. Armstrong, E. Baer, Co-extruded multilayer shape memory materials comparing layered and blend architectures. Polymer (United Kingdom) 54 (20) (2013) 5399-5407.

BD. They reported an improvement in shape memory properties as a result of introduction aromatic structure into the main chain. Yang and co-workers [122] compared the mechanical, dynamic mechanical and shape memory properties of PU block coPolymers with planar shape hard segment (1,6-diphenyl diisocyanate (PDI)) and bent shape hard segment (MDI). The PDl-based PU showed superior properties compared with MDI-based PU (Table 2.11) as a result of better interaction among hard segments due to the planar shape of PDI. 

Table 2.13 Effect of crosslinking on mechanical and shape memory properties of crosslinked PU [92, 93] ...

In the recent study made by us [63], the usual diol chain extender was replaced by a triol (TMP), producing crosslinked PU networks without phase segregation. The aim was to ensure high degrees of strain recoverability, to produce candidate thermally-triggered shape-memory polyurethanes. 

Fig. 19 Mechanical-viscoelastic model of Lin and Chen (1999) with two Maxwell models to describe SME in segmented PUs. (a) General model, (b) Change of the model in the shape-memory cycle, (c) Shape-memory behavior for two PU samples. Solid lines indicate the recoverable ration curves of the model. Taken from ref. [36], Copyright 1999. Reprinted with permission of John WUey Sons, Inc.

Recently Langer andLendlein reported a PU-based shape-memory suture [75]. The smart surgical suture is tied loosely, and after exposure to physiological stress and pH it tightens the knot automatically. These sutures are biodegradable. 

Details are given of the use of the shape memory properties of PU in the design of a fully polymeric vascular endoprosthesis. The possibility of using the stent as a drug delivery system is discussed. 15 refs. 

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