Schmelzpunkt und kristallitgröße von aus schmelze und lösung kristallisiertem polyäthylen

Abstract

ZUSAMMENFASSUNG:Bei Polyathylen kann die senkrecht zur longitudinalen Lamellenobediiche gemessene Kristalldimension I und damit der Schmelzpunkt der kristallinen Bereiche durch die Wahl der Kristallisationsbedingungen in weiten Grenzen variiert werden. Aus der nach der THoaasoN-Gleichung zu erwartenden und auch gefundenen linearen AbhZingigkeit der mit dem DSC-Kalorimeter gemessenen Schmelzpunkte (lineares Polyiithylen, zwei verzweigte Polyathylene, ein hithylen-Buten-Copolymerisat) von der aus der Langperiode Ll und der Kristallinitiit a ermittelten reziproken Kristalldimension 1-1 wird ein Gleichgewichtsschmelzpunkt T k = 141 f 0,s "C und eine spezifische Gredichenenergie der longitudinalen Grenzschicht oe = 79,s k 2,s erg/cm2 ermittelt.Die Schmelzpunkte von aus Xylollosung in Form von Lamellen kristallisiertem Polyiithylen liegen infolge einer wiihrend des MeBvorganges auch bei hoheren Aufheizgeschwindigkeiten ablaufenden und mit einer Zunahme von 1 verbundenen Rekristallisation vie1 rm hoch. Wird diese Rekristallisation der Lamellenkristalle durch Behandlung mit 100-proz. Salpetersiiure infolge Spaltung der Faltungsschlaufen oder durch Vernetzung der Faltungsschlaufen unterbunden, d a m wird auch fiir Lamellenkristalle die gleiche Abhiingigkeit T, = f(l-1) gefunden wie fiir die aus der Schmelze kristallisierten Proben. Damit wird der Beweis erbracht, daB die longitudinalen Grenzschichten der Lamellenkristalle die gleiche spezifische Grenzflgchenenergie haben wie die aus der Schmelze entstehenden Kristalle. SUMMARY:In polyethylene the crystal dimension 1 measured perpendicular to the longitudinal crystal surface and with it the melting point of the crystalline regions can be varied in wide limits by choosing the conditions of crystallization. According to the THOMSON equation a linear dependence of the melting points (of linear polyethylene, two branched polyethylenes and ethylenelbuten-copolymer) measured with the DSC-calorimeter, on the reciprocal crystal dimension 2-1 (calculated from the long period Ll and the crystallinity a) was to be expected and also found. Based on this linear dependence an equilibrium melting point T& = 141 f 05°C and a specific surface energy of the longitudinal crystal surface oe = 79.5 rt 2.5 erg/cma were found.The melting points of polyethylene crystallized from a solution of xylenc in form of lamellae are much too high, owing to a recrystallization taking place during the measurements even a t higher heating rates and connected with an increase of 1. As soon as the recrystallization of the lamellar crystals is stopped by a scission of the folds following a treatment with 100 yo nitric acid or by a crosslinking of the folds, the same dependence 1 K.-H. ILLERS und H. HENDUS Tm = f (l-l) is to be stated for the lamellar crystals as for the samples crystallized from the melt. Thus a proof is afforded that the longitudinal crystal sarfaces of the lamellar crystals have the same specific surface energy as the crystals originating from the melt.

Differential scanning calorimetry (DSC) and Raman spectroscopy study of poly(dimethylsiloxane)

“…Most likely the effects found are attributable to changes in chain folding. In this respect it is noted that a similar range of melting points, more than 10 K, has been reported for other melt-crystallized polymers and, via small-angle scattering determination of lamellar thickness, assigned to changes in chain folding; see, for example, the compilation of Illers and Hendus 36 for polyethylene.…”

Equilibrium Melting Temperature of Aliphatic Polyesters Using Model Compounds

“…The melting temperatures of C12-1, C12-2, C12-3, and C12-4 were 52.8, 71.6, 80.3, and 85. 4 C, respectively, which were about 20-30 C lower than those of corresponding alkanes with the same carbon number. For both C12-n and C14-n series polyester oligomers, the melting temperatures converged with increasing n, respectively.…”

“…The third utilizes a plot of experimental melting temperatures as a function of the crystallization temperature (Hoffman-Weeks plot method). The first method yields a value of 414.3-419.6 K, 1-3 the second yields a value of 414 K, 4 and the third a value of 416.6-418.9 K.…”

The maximum melting temperatures in strain-crystallized van der waals networks

“…From equation (33) we realize that the partial molar entropy difference of the c-sequences y being in the crystaIIites and in the melt, is continuously diminished with increased intrinsic extension in the non-crystallized regions due to the partial molar entropy term (27) (ag~/an~,) > o.…”

“…The molar enthalpy A h (Tin) per monomer unit corresponds approximately to the data as given by van Krevelen [26]. In correspondence to polyethylene [27], the molar surface enthalpy was computed with the aid of the evident relation oe(Tm) ~-Ah(Tm). The melting temperature T~ is assigned to the value as indicated in table I whereby that value was accepted by which the best fit to the experimental stress-strain curves of natural rubber was obtained.…”