Crystallizable polymers

Variations of all the above points-all the way to the complete failure of a crystallizable polymer to actually crystallize-are possible, depending on experimental conditions. 

Solomko VP (1980) Filled Crystallizable Polymers Naukova Dumka, Kiev... 

Introduction Fundamental Aspects of Orientational Processes in Crystallizable Polymers... 

For rigid-chain crystallizable polymers, spontaneous transition into the nematic phase is accompanied by crystallization intermolecular interactions should lead to the formation of a three-dimensional ordered crystalline phase. 

At present, it is known that the structures of the ECC type (Figs 3 and 21) can be obtained in principle for all linear crystallizable polymers. However, in practice, ECC does not occur although, as follows from the preceding considerations, the formation of linear single crystals of macroscopic size (100% ECC) is not forbidden for any fundamental thermodynamic or thermokinetic reasons60,65). It should be noted that the attained tenacities of rigid- and flexible-chain polymer fibers are almost identical. The reasons for a relatively low tenacity of fibers from rigid-chain polymers and for the adequacy of the model in Fig. 21 a have been analyzed in detail in Ref. 65. 

Each crystallizable polymer exhibits a characteristic equilibrium melting temperature, at which the crystalline and amorphous states are in equilibrium. Above this temperature crystallites melt. Below this temperature a molten polymer begins to crystallize. 

Crystallization of polymers in chiral crystals, even in the case of achiral polymers, is quite frequent and strictly related to the occurrence of helical conformations of the chains. The crystallizable polymer consists of a regular sequence of a chemical repeating unit which can be chiral if it presents an asymmetric center or achiral. On the contrary, helical conformations assumed by the polymer chains in the crystalline state are intrinsically chiral, even though the chemical repeat is achiral. Three possible cases can be distinguished ... 

The number of isolated MDs should be much larger than the usual number of active heterogeneities present in an equivalent bulk sample of the crystallizable polymer. 

For ideal solutions (x = 0). the melting temperature of the crystallizable polymer from solutions follows as... 

Due to light scattering, crystalline polymers mostly yield turbid films.Their blends with other polymers are always demixed because polymers are not able to form mixed crystals. Consequently, crystallizable polymers only yield homogeneous blends above their melting point. As soon as crystallization sets in, the components will separate. 

Crystallizable polymers tend to form randomly oriented crystallites which are oriented when the polymer is stretched or cold drawn at temperatures below the Tm. Crystallization under pressure may result in a fibrillar structure or extended chain structure. 

The degree of crystallinity may be calculated from the density of the polymer if the density is known for the amorphous and crystalline states. Some crystallizable polymers are polymorphic, i.e., they may exist in more than one crystalline form. An unstable crystalline form may change to a more stable form, and crystalline forms may change under stress. For example, hdpe changes from an orthorhombic crystalline polymer to a monoclinic form when subjected to compressive forces. 

Between the Tg and the Tm temperatures, another transition may be seen. The chains in crystallizable polymers have sufficient mobility so that ordering and crystallization may occur. The temperature at which this occurs is referred... 

Following the procedure used with JeR (Section 5), po data on these and other polymers were correlated in terms of cM/qMc, with results which are shown in Fig. 8.13. The parallel in behavior between the po and JeR master correlations is unmistakable. Even the relative positions of polymers on the master correlations are similar note for example the data on JeR and p0 for polybutadiene. Published data on relatively narrow distribution polyethylene (210,326) have not been included in Fig. 8.13 because departures from tj0 were too small to define P0 with accuracy. However, estimates of po from the data provided suggest that polyethylene may follow a different pattern than other polymers. Departures from rj0 seem to appear at anomalously low shear rates (326). Aside from tj0 values, viscoelastic data on well characterized crystallizable polymers in the melt state are rather scarce. Although not especially anticipated, it is certainly conceivable that erystallizability confers some unusual features to the flow behavior. 

Fig. 4. Specific volume vs temperature for a crystallizable polymer. The dashed line represents amorphous material (6). Numbers refer to individual states.

Some information is available on other acrylates. N,N-disubstituted acrylamides form isotactic polymers with lithium alkyls in hydrocarbons (12). t-Butylacrylate forms crystallizable polymers with lithium-based catalysts in non-polar solvents (65) whereas the methyl, n-butyl, sec-butyl and isobutyl esters do not. Isopropylacrylate also gives isotactic polymer with lithium compounds in non-polar solvents (34). The inability of n-alkylacrylates to form crystallizable polymers may result from a requirement for a branched alkyl group for stereospecific polymerization. On the other hand lack of crystallizability cannot be taken as definite evidence of a lack of stereoregulating influence, as sometimes quite highly regular polymer fails to crystallize. The butyllithium-initiated polymers of methylmethacrylate for instance cannot be crystallized. The presence of a small amount of more random structure appears to inhibit the crystallization process1. 

Oxidative polymerization of 2,6-diphenylphenol yields a crystallizable polymer that is characterized by a very high melting point ( 4fS0°C) and excellent electrical properties. It can be spun into a fiber with excellent thermal, oxidative and hydrolytic stability. It is marketed under the trademark Tenax . 

Let us consider some aspects of thermal behaviour of LC polymers 45> (Fig. 3). In case of crystallizable polymers, which are mainly those containing mesogenic groups in the main chain, the LC state is observed from above the melting temperature (Tm) and up to the clearing temperature (Tcl), the melt displays anisotropy and may flow. The polymer thus behaves alike low molecular liquid crystals (Fig. 3 a), the viscosity of the former being, however, essentially higher. 

---