Shape slow crystallizing polymers

The relatively slow crystallization kinetics of polymers at small supercooling make it possible to control, to some extent, the temperature of crystallization. This enables researchers to study solidification at predetermined isothermal conditions and to link the crystallization and emerging structure with temperature. The temperature at which crystallization occurs influences not only the nucleation and growth of crystals but also the sizes and shapes of the crystals and the overall degree of crystallinity. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Bruk et al. [716] described the low-temperature radiation polymerization of crystalline TFE in detail. It has been established that three solid-phase postpolymerization reactions can take place when irradiated specimens are heated above the melting point low-temperature polymerization (in the interval 77 to IlOK), slow polymerization close to the melting point (in the interval 128 to 138 K), and rapid polymerization during melting of the crystal (142 K). Tabata et al. [717] have found that a significant post-polymerization takes place even in the liquid phase. Kinetic analysis has been made of the in-source and post-polymerizations [718,719]. Post-polymerization is explained by a long lifetime of polymer radicals in the hquid phase at —78 °C due to the slow combination rate of the polymer radicals caused by their rod-like shape. 

In polymers, motions are slowed down compared to when they occur in low molar mass liquid crystals. This often brings processes that are too fast in low molar mass liquid crystals into the kinetic window of line shape analysis or even 2D exchange experiments. Thus motional processes that cannot be studied in low molar mass liquid crystals become accessible in polymers [19]. Examples of NMR applications to liquid-crystalline polymers are given in the following section. 

The discovery of isotactic polystyrene (IPS) gave a new dimension to this material since it now could crystallize and provide a melting point (Tm) of around 250 °C. Although it still has a Tg of 100 °C, the material will maintain its shape and may be used for many applications above this Tg and below the Tm. IPS has been the subject of several intense efforts for commercialization. Ultimately it has been unsuccessful for one primary reason that being the rate at which the polymer will crystallize is too slow under normal forming... 

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