Amorphous reversion

Jeong, H.M., Ahn B.K., Cho S.M. and Kim B.K. (2000), Water vapor permeability of shape memory polyurethane with amorphous reversible phase. Journal Of Polymer Science Part B-Polymer Physics, 38(23) pp. 3009-3017. 

The effect of pressure and temperature on the phase diagram of P4MP1 was studied by Rastogi et al. [104], see Fig. 7. The polymer crystalline under ambient conditions becomes amorphous reversibly on increasing pressure in two widely separated temperature regimes (20°C and 200°C). The transformation occurs via a liquid crystalline state. The lower-... 

Kim B K, Shin Y J, Cho S M and Jeong H M (2000) Shape memory behavior of segmented polyurthanes with an amorphous reversible phase The effect of block length and content, J Appl Polym Sci 38 2652-2657. 

B. K. Kim, Y. J. Shin, S. M. Cho, and H. M. Jeong, "Shape-Memory Behavior of Segmented polyurethanes with and Amorphous Reversible Phase The Effect of Block Length and Content," Journal of Polymer Science Part B Polymer Physics, vol. 38, pp. 2652-2657, 2000. 

No polymer is ever 100% crystalline at best, patches of crystallinity are present in an otherwise amorphous matrix. In some ways, the presence of these domains of crystallinity is equivalent to cross-links, since different chains loop in and out of the same crystal. Although there are similarities in the mechanical behavior of chemically cross-linked and partially crystalline polymers, a significant difference is that the former are irreversibly bonded while the latter are reversible through changes of temperature. Materials in which chemical cross-linking is responsible for the mechanical properties are called thermosetting those in which this kind of physical cross-linking operates, thermoplastic. 

At very low concentrations of water, or in foods held below the free2ing point of water, physical conditions may be such that the available water may not be free to react. Under these conditions, the water may be physically immobi1i2ed as a glassy or plastic material or it may be bound to proteins (qv) and carbohydrates (qv). The water may diffuse with difficulty and thus may inhibit the diffusion of solutes. Changes in the stmcture of carbohydrates and proteins from amorphous to crystalline forms, or the reverse, that result from water migration or diffusion, may take place only very slowly. 

Because of the capacity to tailor select polymer properties by varying the ratio of two or more components, copolymers have found significant commercial appHcation in several product areas. In fiber-spinning, ie, with copolymers such as nylon-6 in nylon-6,6 or the reverse, where the second component is present in low (<10%) concentration, as well as in other comonomers with nylon-6,6 or nylon-6, the copolymers are often used to control the effect of sphemUtes by decreasing their number and probably their size and the rate of crystallization (190). At higher ratios, the semicrystalline polyamides become optically clear, amorphous polymers which find appHcations in packaging and barrier resins markets (191). 

Pentahydrate is reversibly converted to an amorphous dihydrate, at 88°C and 0.26 kPa (2 mm Hg) or by boiling with xylene (73,75). The heat of dehydration for the pentahydrate to tetrahydrate has been calculated to be 53.697 kj (12.834 kcal) per mole of water (74). Thermogravimetric analyses show that 2.75 moles of water are lost on heating to 140°C. Like borax, pentahydrate puffs when heated rapidly to give a product having a bulk density of 0.042 g/mL (79). 

Interleukin-2 (recombinant human) [94218-72-1] Mr-15,000, amorphous. Purified by reverse phase HPLC. [Weir and Sparks Biochem J 245 85 1 987 Robb et al. Proc Natl Acad Sci USA 81 6486 1984.]... 

Block copolymers can contain crystalline or amorphous hard blocks. Examples of crystalline block copolymers are polyurethanes (e.g. B.F. Goodrich s Estane line), polyether esters (e.g. Dupont s Hytrel polymers), polyether amides (e.g. Atofina s Pebax grades). Polyurethanes have enjoyed limited utility due to their relatively low thermal stability use temperatures must be kept below 275°F, due to the reversibility of the urethane linkage. Recently, polyurethanes with stability at 350°F for nearly 100 h have been claimed [2]. Polyether esters and polyether amides have been explored for PSA applications where their heat and plasticizer resistance is a benefit [3]. However, the high price of these materials and their multiblock architecture have limited their use. All of these crystalline block copolymers consist of multiblocks with relatively short, amorphous, polyether or polyester mid-blocks. Consequently they can not be diluted as extensively with tackifiers and diluents as styrenic triblock copolymers. Thereby it is more difficult to obtain strong, yet soft adhesives — the primary goals of adding rubber to hot melts. 

Synthetic rubber (elastomers) are high molecular weight polymers with long flexible chains and weak intermolecular forces. They have low crystallinity (highly amorphous) in the unstressed state, segmental mobility, and high reversible elasticity. Elastomers are usually cross-linked to impart strength. 

We have been able to show that the dearrangements which are indicated below take place, but have not as yet been able to show that the reactions are reversible. Nor have we as yet any light upon the dearrangement in the other possible manner beyond the fact that both of the substances, heated to decomposition, yield ammonia and amorphous ill-defined residues. 

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