Monday, August 12, 2019

On Morphology of Thermoplastic Polyester Elastomer (TPE-E) Research Paper

On Morphology of Thermoplastic Polyester Elastomer (TPE-E) - Research Paper Example The amazing versatility and utility of TPE-Es is because of their specialized structures. TPE-Es have a biphasic structure with one phase remaining soft at room temperature and the other remaining hard at room temperature (Holden 2010). The soft segments are amorphous while the hard segments are structured/crystalline, and both the segments are immiscible (Jelinski, Schilling and Bovey 1981; Sarwade & Singh 2003). The hard segment becomes fluidic when heated and imparts a thermoplastic nature to the polymer, while the soft segment imparts an elastomeric nature (Sarwade & Singh 2003; Holden 2010). The hard segments form noncovalent networks that are thermally reversible, relying on intramolecular interactions for their stability (Jelinski, Schilling and Bovey 1981). A simple TPE-E copolymer structure comprises of alternating A-B-A blocks, where A is the hard phase, and B is the soft phase (Holden 2010). The hard and soft phases are randomly joined head-to-tail, yielding the copolymers (Witsiepe 1973). The general structure of TPE-E copolymers is as follows: SOFT HARD In case of TPE-Es, the soft phase comprises of polyethers while the hard phase comprises of polyesters. By varying the relative amounts of each of these two phases, the properties of the copolymer can be modulated (Witsiepe 1973). Electron microscopic studies by Cella have shown that phase separation occurs in these polyether-polyether co-polymers below their melting points (cited in Witsiepe 1973). The morphology of TPE-Es comprises of the soft segment (polyether), which is the continuous amorphous phase along with interspersed segments of polyesters that have not been crystallized because of their small size, high melt viscosity or due to chain entanglement (Witsiepe 1973). While some of the polyesters remain as short segments in the soft continuous phase, the remaining polyesters exist as closely connected fibrillar crystalline lamellae. This crystalline network of the hard phase along with the s oft amorphous continuous phase forms an elastic network. In simple terms, as described by Witsiepe (1973), â€Å"a more or less continuous crystalline network is superimposed on a continuous amorphous network† (p. 50). The chemical structure of hard and soft phases of a thermoplastic elastomer (TPE) affects its mechanical properties. Therefore, by experimenting with different compounds, TPEs with novel properties can be developed for industrial purposes. Examples of industrially prominent TPE-Es include Hytrel, Ecdel, RTP, Pibiflex, Keyflex and Riflex (Fakirov 2005). Hytrel engineering thermoplastic elastomer is a TPE-Es manufactured by E. I. du Pont de Nemours and Co. (Jelinski, Schilling and Bovey 1981; Fakirov 2005). This elastomer is available in varying compositions of m teramethyline terephthalate, which is the hard segment, and n poly (tetramethyleneoxy) terephthalate, which is the soft segment (Jelinski, Schilling and Bovey 1981). The chemical structure of Hytrel is s hown below: Like all TPE-Es, Hytrel has the flexibility and elasticity of rubbers and the strength and rigidity of plastics, is as easily processable as thermoplastics and comes in both standard and high-performance grades with a Shore D hardness ranging from 30 to 82 (Fakirov 2005). C13 NMR studies by Jelinski, Schilling and Bovey (1981) have shown that the widths of the aliphatic carbon chain of the soft segment of Hytrel are a linear function of the average length of the hard

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