Crystallinity, Supermolecular Structure, and Thermodynamic Properties of Linear Polyethylene Fractions

Abstract

The supermolecular structures or morphologies of crystallized molecular weight fractions of linear polyethylene, covering the range M = 2.53 X lo4 to 8.0 X lo6, were studied by small-angle light scattering complemented by polarized light microscopy. The different forms that can be developed were established as a function of molecular weight and crystallization temperatures. The best defined, or most perfectly developed spherulites, are formed after rapid crystallization (quenching) for M I 8.5 X lo5. This organized structure deteriorates as the molecular weight is increased under these crystallization conditions. The range M = 1-2 X lo6 represents a transitional region where the scattering pattern is very sensitive to molecular weight distribution and details of the quenching process. For higher molecular weights, however, an azimuthally independent light-scattering pattern is observed which can be attributed to randomly oriented crystalline lamella. The lowest molecular weight samples yield thin rod-like structures at all isothermal crystallization temperatures. For intermediate molecular weights, as the crystallization temperature is raised, there is a transition from poorly defined spherulites to rods whose lengths are comparable to their breadth. At the higher molecular weights, randomly oriented crystallites are found at all crystallization temperatures. It is possible to develop samples having the same degree of crystallinity, on a density basis, but which have quite different supermolecular structures. These differences are reflected in other thermodynamic quantities, such as the enthalpy of fusion, which are not only dependent on the degree of crystallinity but on the morphology as well. The possible influence of the supermolecular structure (at the same level of crystallinity) on physical and mechanical properties is portended by these results. Possible mechanistic explanations for the formation of the different morphological forms, which are still in a rudimentary state of development, are briefly discussed.It has been established that virtually all properties of bulk crystallized polymers are extremely dependent on the molecular weight and crystallization temperat~re.l-~ An extremely wide range in properties can be achieved by utilizing molecular weight fractions and varying the crystallization temperature (or undercooling) under which crystallization takes place. These properties correlate well with the levels of crystallinity and the molecular structure of the lamella crystallites and the associated interfacial and amorphous reg i o n~.~-~ The further organization of the lamellar crystallites into higher levels of morphology, or supermolecular structure, are well known. Until recently, a spherulitic morphology was thought to be the universal mode of homopolymer crystallization, although other forms such as ovoids and hedrites had been noted for specific polymers.P6 These were thought to be special or restricted cases.Since there is this very large effect of molecular weight and crystallization temper...

Crystallization of quenched polyethylene. Part III : Mixtures of fractions

“…In P.E. fractions [28,49] and in mixtures of fractions [48] In a subsequent paper, we will show that the correlations between the solid and liquid states found in P.E. exist also in the other polymers.…”

“…Figures 12a and b [28,48,49]. This critical mass decreases somewhat when the molecular weight of the small chains in the mixture decreases, it corresponds to value of the molecular weight above which the crystallinity of quenched mixtures is constant at Z -0.5 ; d) the mass crystallinity and the linear crystallinity present the same variations with the molecular weight.…”

The effect of molecular weight and crystallite structure on yielding in ethylene copolymers

“…The apparatus used quired for 10 repetitive experiments. One is then able to study materials that are in short supply, in this work has been previously described 21 and is very similar to the ones used by Stein 22 and such as materials with narrow molecular weight and composition distributions, such as used here. by Samuels.…”

Contribución al estudio de la degradación ambiental de poliolefinas fotoestabilizadas.

“…Esto indica la presencia de pequeñas lamelas formadas, probablemente, con cadenas moleculares que poseen ramificaciones de distinta longitud y distribuidas al azar. La causa de esta dispersidad se atribuye a la propia naturaleza del LDPE que a su vez depende de las condiciones del proceso de polimerización y del catalizador usado [11,23,61,100,[161][162][163][164][165].…”

“…Los valores medidos son similares a los publicados por otros autores [108,[158][159][160][161][162][163]. En primer lugar destaca que tienen valores superiores al polietileno linear de baja densidad (LDPE).…”