Covalently crosslinked SMPs

Determination of Crosslink Density in Covalently Crosslinked SMPs... 

Fig. 10 a-e diagram for a cyclic, thermomechanical experiment. Cyclic, thermomechanical experiment of a covalently crosslinked SMP prepared from crystaUizable oligo(e-caprolactone) dimethacrylate segments and /i—butyl acrylate as comonomer ivith Tiiigh — 70 C, 7 0w — 0 = C, and = 50% (a = stress). The graph shows data obtained from five cycles. Taken from ref [77], Copyright 2005. Reprinted with permission of John Wiley Sons, Inc. 

Fig. 20 Thermomechanical model for covalently crosslinked SMPs. (a) Schematic diagram of the micromechanics foundation of the 3-D SMP constitutive model (1). Existence of two extreme phases in the polymer is assumed. The diagram represents a polymer in the glass tiansition state with a predominant active phase (b) In the 1-D model, the frozen fraction (pf = Lf (T) /L(T) is defined as a physical internal state variable that is related to the extent of the glass transition, (c) Frozen fraction, (j>f (T), as a function of temperature, derived from curve fitting of the modified recovery strain curve divided by the predeformation strain, (d) Prediction of the free strain recovery responses during heating for polymers predeformed at different levels. Fig. (a) and (b) reprinted with permission from ref. [92], Copyright 2005, Materials Research Society, Warrendale, PA. Fig. (c) and (d) reprinted from [71], Copyright 2006, with permission from Elsevier.

The additive reaction of oxiranes was also utilized for the preparation of SMP networks with covalent crosslinks. The crosslinking of 3-amino-1,2,4-triazole with epoxidized natural rubber catalyzed by bisphenol-A resulted in polymer networks having shape-memory capability with Ti ms = Tg [82]. The Tg could be controlled by the 3-amino-1,2,4-triazole content in the range between 29 and 64 °C. Recently, the crosslinking reaction of oxiranes and amines was systematically investigated in two polymer systems based on the reaction of the diglycidyl ether of bisphenol A epoxy monomer cured with the bifunctional poly(propylene glycol)bis(2-aminopropyl)... 

Incorporation of light-sensitive groups as molecular switches in the polymer networks enabled the development of light-induced SMPs. In this way, SME could be induced independently from any temperature effect [36]. Instead of increasing the sample s temperature, light of different wavelength ranges was used for the fixation of the temporary and the recovery of the permanent shape. CA or cinnamyliden acetic acid (CAA) have been used as photosensitive molecular switches on the molecular level as they are able to form covalent crosslinks with each other in a... 

There are four main classifications for shape memory polymers with different sources of their transition temperature 1) covalently crosslinked glassy polymers where the glass transition (Tg) is the temporary network transition temperature (To) 2) covalently-crosslinked, semicrystalline elastomers where Tm is T 3) physically-crosslinked, semicrystalline thermoplastics where Tc = Tg and 4) physically-crosslinked, semicrystalline block copolymers where T = T of the soft segment. Each of these types of SMPs requires considerable polymer syntheses if one wishes to vary T(, and/or the modulus of the SMP. 

---