Lamellar ciystals

Figure 17. Schematic lepiesentation of a lamellar polyethylene single ciystal. (a) and (b) show enlaiged features of its stnicture in (o) the zigzag conformation of the macromolecular chain inside the crystal in (b) a hypothetical model for the regular folding, drawn according to the calculation reported in ref. 224a.

During isothermal crystallization the actual ciystallization temperature (for example 122.2°C) was not exactly equal to the set temperature (125°C). This is due to the fact that the lamellar size can have only defined values, which depend not only on the conditions of crystallization, but also on the macromolecule structure. 

The exact location of the plasticizer is frequently evaluated to better understand the structure of plasticized polymers. In poly[(vinylidene fluoride)-co-hexafluoropropene], VDF/HFP plasticized with dibutyl phthalate, SAXS measurements indicate that DBP resides in the amorphous zone outside the lamellar stacks. If ciystallization is slow the inclusion of DBP inside the lamellar stacks is also possible. In another contribution for the same copolymer, plasticizer was also found in amorphous phase close to the interface with crystalline stmctures which was evidenced by almost constant spacing in the range of tricresyl phosphate studied.In the plasticized PVC, the plasticizer molecules were found in the amorphous area but were also present in the interlamellar, intefibrillar, inter-spheralitic regions and in the amorphous fold surfaces. ... 

Although SFM is traditionally an intrinsically slow imaging technique, a wealth of growth rate data has been obtained on the lamellar level. Furthermore, recent developments in the area of high speed SPM (289-291) show that realtime SFM at elevated temperatures possesses the potential to develop into an indispensable microscopic tool for the quantitative investigation of polymer phase transitions and other processes at elevated temperatures and as such may help to develop better theories of, eg, polsrmer ciystallization. 

Tracz, A., Ungar, G., AFM study of lamellar structure of melt-ciystallized n-alkane C390H782, Macromolecules 2005, 38 4962-4965. 

We have recently proposed (1) a simulation model of polymer crystallization through a dynamic Monte Carlo approach in a 2-dimensional lattice. Similar results are obtained in a three dimensional cubic lattice (2). We have found that this method can predict the compaction of polymer chains as folded molecules as they are found in polymer lamellar crystals. The model correctly predicts a gradual thickening of the crystals as the temperature is increased. In this paper we want to test further our model in order to determine if it is able to predict other experimental features of polymer ciystallization such as ... 

The ciystallization curve shown in Figure 2 was obtained in a SAXS/WAXS/DSC experiment from iPP [23] and shows the classic features of primary crystallization. The detailed molewlar structure of the polymer, the specific nature of the nucleation processes and the degree of under-coolteg, determines the magnitude of the lamellar thickness and the degree of crystallinity within the lamellar stadcs. The crystallization kinetics are analyzed using the Avrami model [24], expressed in terms of the equation... 

Optical and electron microscopy, melt ciystallized banded and and lamellar spherulites... 

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