Poly crystal polymorphism

The crystal polymorphism of the chiral but racemic P5MH1 is, to some extent, very reminiscent of that of isotactic polypropylene. It exists in two crystal modifications. One crystal modification is stable at high temperature, and was observed early on by Corradini et al [39]. Its structure has been redefined as a chiral, frustrated one based on a trigonal cell with three threefold helices per cell. We have also discovered a second crystal modification produced from solution. It has an orthorhombic unit cell that contains four chains in - again - three-fold helical conformation, for which one must assume coexistence of two right- and two left-handed helices. Contrary to the a and ft phases of iPP, the frustrated structure of poly( 5-methyl-hexene-1) is the more stable one [40]. 

It was shown already above that cis-1,4-poly butadiene melts in one step with the expected entropy of fusion. In contrast, cis-l,4-poly(2-methylbutadiene) (natural rubber) has a more complicated fusion and crystallization behavior . The reported entropy of fusion of the common monoclinic (P2ja) crystal polymorph is only 14.4 J/(K mol), less than half of the expected value. The crystal structure has been reported statistically disordered, but only relative to packing of chains that are mirror images of each other along the crystallographic a-axis. Such geometric disorder cannot account for a 50% decrease in entropy of fusion. A full study of the thermod5mamic functions as available for the polybutadienes would be of value. 

Cocca, M., Lorenzo, M.L.D., Malinconico, M. and Frezza, V. (2011) Influence of crystal polymorphism on mechanical and barrier properties of poly(L-lactic acid). European Polymer Journal, 47, 1073-1080. 

T , = T. Complications arise when crystal polymorphs are possible, as seen in poly(vinyhdene fluoride). The polymorphs must be identified by X-ray diffraction, as described in Appendix 15. Also, the appearance of the melting peaks closer to the zero-entropy-production condition is characteristic for different polymers and may depend on the heating rate. 

Di Lorenzo, M.L., Rubino, P., Luijkx, R., and Helou, M. (2014) Influence of chain structure on crystal polymorphism of poly(lactic acid). Part 1 effect of optical purity of the monomer. Colloid Polym. Sci., 292, 399 -409. 

Di Lorenzo, M.L., Cocca, M., and Malinconico, M. (2011) Crystal polymorphism of poly(L-lactic acid) and its influence on thermal properties. Thermochim. Acta, 522, 1657 -1677. 

Yang J, Pan P, Hua L, Dong T, Zhu B, Inoue Y, et al. Fractionated crystallization, polymorphic crystalline structure and spherulite morphology of poly(butylenes adipate) in its miscible blend with poly(bntylenes succinate). Polymer 2011 52 3460-8. 

Yu L, Cebe P. Crystal polymorphism in electrospun composite nanofibers of poly(vinylidene fluoride) with nanoclay. Polymer 2009 50(9) 2133 1. 

The ring adopts the chair Ibrm and its dimensions are compared with those of other polymorphs in Table 15.5. Note that cyc/o-S has the smallest bond angle and dihedial angle of all poly-sulfur species for which data are available and this, Uigether with the small hole at the centre of the molecule and the efficient packing within the crystal, lead to the highest density of any known polymorph of sulfur (Table 15.6). 

Also the polymorphic behavior of s-PS can be altered by blending, in particular with poly-2,6-dimethyl-l,4-phenylene oxide (PPO), both for the case of crystallization from the melt [104] and for the case of crystallization from the quenched amorphous phase [105]. 

Data concerning the chain conformations of isotactic polymers are reported in Table 2.1. In all the observed cases the torsion angles do not deviate more than 20° from the staggered (60° and 180°) values and the number of monomeric units per turn MIN ranges between 3 and 4. Chains of 3-substituted polyolefins, like poly(3-methyl-l-butene), assume a 4/1 helical conformation (T G )4,45,46 while 4-substituted polyolefins, like poly(4-methyl-1-pentene), have less distorted helices with 7/2 symmetry (T G )3.5-39 When the substituent on the side group is far from the chain atoms, as in poly(5-methyl-1-hexene), the polymer crystallizes again with a threefold helical conformation (Table 2.1). Models of the chain conformations found for the polymorphic forms of various isotactic polymers are reported in Figure 2.11. 

Most polymers do not form crystals suitable for single crystal X-ray diffraction, so powder or film methods are usually employed. X-ray and LJV data recorded at various temperatures provide the detailed information required to correlate conformational and electronic properties, since the former is sensitive to the inter- and intrachain packing, and the latter is sensitive to the conformation. DSC provides further evidence for any phase transitions. Detailed studies have been performed by Winokur and West,260 261 who reported a comparison of the polymorphism, structure, and chromism in poly(di- -octylsilylene), (Si- -Oct2), 89, and poly(di- -dccylsilylcnc)(Si- -Dcc2) , 90. These investigations will be described in detail for the useful insights into polysilane structures that they afford. 

Information on the crystal to liquid crystal transitions is scarce and is to be treated with caution since partial crystallization is prominent and polymorphism of the smectic phase is frequent. Only the data on poly(acryloyloxybenzoic acid) (entry 2 of Table 5) have been extrapolated to 100% crystallinity. As with the low molecular weight liquid crystals, the total heat of transition is lower than expected for fully ordered crystals. Various combinations of two phase structures as suggested by Fig. 3 could be produced for the poly(acryloyloxybenzoic acid)21>. 

Poly (ethylene terephthalate) can be grown to extended chain crystals similar to the nylons 164). No special high pressure polymorph has been suggested. But there may be, as in the nylons, the possibility that a continuous increase in mobility exists in the crystal phase at higher pressure and temperature. This mobility would have to be based on conformational changes, i.e. a condis crystal phase. 

Molecular complexes, such as the complex formed between poly(N-vinylcaibazole) and 2,4,7-trinitro-9-fluorenone, and dye-polymer aggregates were widely used as generation materials in many early applications. Since these materials are not infrared sensitive, there has been increasing emphasis on pigments. The more widely studied are various azo, phthalocyanine, squaraine, and peiylene diimide derivatives. A common feature of all of these materials is that they are polymorphic and exist in many different crystal forms. The properties are thus very sensitive to the conditions used in their preparation. Further, the sensitivity of these materials is strongly field dependent as well as dependent on the transport material. For a review of generation materials, see Law (1993). 

The simplest and most straightforward application of thermal analysis is concerned with studies of the relative stability of polymorphic forms. For example, DTA thermograms enabled the deduction that one commercially available form of chloroquine diphosphate was phase pure, while another consisted of a mixture of two polymorphs. DTA analysis was used to demonstrate that in spite of the fact that different crystal habits of sulfamethazine could be obtained, these in fact consisted of the same anhydrous poly-morph.f In a study aimed at profiling the dissolution behavior of the three polymorphs and five solvates of spironlactone, DTA analysis was used in conjunction with powder X-ray diffraction to establish the character of the various materials. ... 

A great deal of information about polymorphic changes has been obtained from studying pure triacylglycerols (TAG). However, in commercial fats, especially if they contain several ingredients, it may be more difficult to predict the likelihood of p crystal formation to occur. Factors that influence the stability of the p poly-... 

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