Linear polyethylene crystallized from the melt or heat-treated at temperature above llO°C. shows in the DTA-thermogram an additional endothermal maximumTm' at a temperature lower by 10 to 2OoC. compared with the normal maximum Tmo. The appearance of T m l depends on the crystallization temperature, time, and temperature of heat-treatment. For a constant crystallization time, the difference between Tmo and Tml decreases with increasing of the crystallization temperature ; at a constant crystallization temperature it increases with the crystallization time. By treating those specimen with two endothermal maxima a t a temperature near that of Tml, Tml splits into two peaks. This effect is interpreted by the assumption, that those crystalline regions which first correspond to the T m l maximum, partly form more stable crystals of greater thickness and higher melting point, and partly melt with subsequent formation of little crystallites of lower melting point during cooling. Under suitable conditions, the three endothermic peaks also can be observed with branched polyethylene.
ZUSAMMENFASSUNG:Bei Temperaturen oberhalh llO°C isotherm aus der Schmelze kristallisiertes oder getempertes lineares Polyathylen zeigt im DTA-Thermogramm ein zusatzliches, endothermes Maximum Tml bei einer um etwa 10 bis 2OoC gegenuber dem normalerweise gefundenen Maximum Tmo niedrigeren Temperatur.Das Auftreten von Tml hangt von Kristallisationstemperatur, Zeit und Tempertemperatur ab. Die Temperaturdifferenz zwischen Tmo und T m l wird, bei gleicher Kristallisationszeit, mit steigender Kristallisationstemperatur geringer und wlchst, bei konstanter Kristallisationstemperatur, mit der Kristallisationszeit an. Wenn man eine Probe, die zwei Maxima zeigt, auf eine Temperatur nahe der von T m l erwiirmt, so spaltet Tml selbst wieder in zwei Maxima auf. Diese Tatsache wird durch die Annahme erklart, daR in dem kristallinen Gefuge gewisse Teile der kristallinen Bereiche, die urspriinglich Tml zuzuordnen waren, durch Dickenwachstum der Lamellen stabilere Kristalle bilden, deren Schmelzmaximum bei hoherer Temperatur liegt, wiihrend andere Anteile aufschmelzen und beim Abschrecken rekristallisieren. Das Auftreten dieser Maxima kann unter geeigneten Bedingungen auch bei verzweigtem Polyathylen beobachtet werden.
Studies of the steric structures of poly(vinyl alcohol), as deduced from studies of the structures of its esters, particularly poly(vinyl formate) are described. Two types of tactic poly(vinyl formate) crystallize with different structures, corresponding to their different molecular structures. Therefore, the type of molecular stereoregularity in poly(vinyl formate) samples can be deduced from an examination of the crystals produced by a given crystallization treatment. From the results of the study of the poly(vinyl formate) crystals, some conclusions can be drawn concerning the steric structure of poly(vinyl alcohol). Crystallizabilities of other tactic poly(vinyl esters) have also been investigated.
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