A Hexagonal Precursor

1 We note a minor error by these authors who claim that Eq. 1 applies as written also to the orthorhombic/hexagonal transition. It does not but requires to be modified, as in Eq. 11, on account of the different densities of the two phases. 

There are two issues central to this proposal, namely is there an inversion of phase stability at atmospheric pressure and does the hexagonal phase then crystallize before the orthorhombic As will become clear, the available data do not allow a definite answer to the first but probably not, to the second the answer is certainly no. We consider these matters in turn, first testing the inequality 5 against measured parameters. 

the values of T 0 and T h will differ by at most a few K so their ratio may safely be taken as unity in relation to the other terms involved, hence Eq. 5 effectively reduces to 

Because of the relative magnitudes of the specific enthalpies (see below) the implication is that 

2 Keller et al. [4] mistakenly stated that the slope of the Tm vs 1 /A, plot from equation Eq. 2 is (- 2 7e/A/zv) omitting the factor and erroneously deduced the condition ( je/A/zv)h (cre/AhY)0 although this mistake has no significant effect on calculations. 

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Polymers are unique in the extent of the detail of their history which they retain in their morphology, essentially because of the restricted mobility of long molecules once added to a lamella. Indeed polymer morphology has driven almost all advances in understanding the fundamental nature of polymeric self-organization, not least chainfolding. In the present case it demonstrates clearly that polyethylene lamellae crystallized at atmospheric pressure did not have a hexagonal precursor. 

The single crystalline nature of orthorhombic polyethylene lamellae shows simply and clearly that they did not have a hexagonal precursor. Had they done so they would have been threefold twins. 

The variety of finely divided silica known as hexagonal mesoporous silica (HMS) [6,34] is generally prepared by the copolymerization of a silica precursor such as tetraethylorthosilicate (TEOS), in presence of a suitable template. This acid or base catalyzed reaction follows the sol-gel route and it has been found [28] that during silica preparation by this technique, it is also possible to add an organotriethoxysilane [RSi(OEt)3] that would... 

Ethanol when added directly to the PTES/TEOS mixture is thus highly favoring the formation of a hexagonal phase with respect to the cubic phase. However, even if ethanol is not directly added to the precursor mixture, it is produced during the synthetic procedure via hydrolysis and condensation reactions of PTES and TEOS. One can now wonder which results will be obtained if methoxysilanes rather than ethoxysilanes are used. 

Pinnavaia et al. [69,72] prepared a hexagonally packed alumina through the neutral templating approach. In the presence of a polymer surfactant, (PEO)i3(PPO)3o(PEO)i3, an alumina with a d spacing of 63 A and a surface area of 420 m /g was obtained [72]. It was also mentioned that non-layered alumina can be synthesized using octyl or dodecyl amine as template and a neutral aluminum alkoxide precursor. 

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