Stepwise quasi-equilibrium crystallization of low-density polyethylene

Abstract

Hydrostatic weighing of a polymer in an immersion liquid using low-density polyethylene as an example was used to show that a condition for the lack of recrystallization of partially crystalline polymers upon annealing is the attainment at this temperature of thermomechanical equilibrium that determines the ratio of the amounts of macromolecule elementary units existing in amorphous and crystalline regions during the crystallization of these polymers. A basis was found for the viewpoint that the Gibbs-Thomson equation is applicable only for estimating the dimensions of the last (not bonded to each other by connecting chains) crystallites disappearing at the true melting point.It is commonly thought in discussing crystallization processes of polymers with flexible chains from the melt, their additional crystallization (melting-recrystallization) during annealing, and complete melting [1-8] that these polymers are biphasic (crystalline and amorphous phases) and thermodynamically non-equilibrated (metastable).The experimentally observed manifestations of the metastability of partially crystalline polymers are: -a lower melting point (mp) than the equilibrium value ( 0 m T ) characteristic of ideal crystals; -a dependence of the mp on the heating rate and the thermal history of the sample; -additional spontaneous crystallization with a decrease of the glassification temperature of amorphous regions to the ambient temperature.Additional crystallization was clearly evident during successive stepwise heating of a partially crystalline polymer undergoing hydrostatic weighing in an immersion liquid. Additional crystallization of low-density polyethylene (LDPE) pressed at 160°C and crystallized in air at room temperature (mp = 111.2 ± 0.4°C) was distinctly observed in the range 58.5-110.2°C with such a heating regime [9]. Apparently only amorphization occurred in the range 29.3-58.5°C. Anyway, preliminary storage of the sample in a differential scanning calorimeter cell for 1 h at 40°C was accompanied by a decrease of its heat of fusion (ΔH m ) by 6% [glassification temperature (T g ) of this LDPE sample was about -53°C].Strictly speaking, these results were expected and agreed completely with existing concepts about the thermal behavior of partially crystalline polymers. However, the problem is that if the postulates of Gibbs regarding phase equilibria are strictly adhered to [10], then such polymers should be viewed as monophasic microheterogeneous metastable liquids in which the ratio between the amount of macromolecule elementary units located in the amorphous regions and crystallites is determined by the thermomechanical equilibrium conditions [11].Such treatment of the monophasic state of partially crystalline polymers differs in principle from that proposed earlier [3, p. 326] in that the amorphization process of such polymer liquids is considered to be a first-order phase transition complicated by a three-dimensional strain deformation relative to regions with long-range three-dimensional ordering in the space aro...

Phase equilibrium of a semicrystalline polymer-liquid system: Isotactic polypropylene-dibutylphthalate

“…5 shows the DSC thermo grams of melting of LDPE and isotactic PP obtained at a scanning rate of 10 K/min. The endotherms indi cate the true temperatures of polymer melting, which were found through the method of stepwise isothermal amorphicity in an immersion liquid medium [20].…”

“…Use of the term crystallite destruction in this case emphasizes the fact that the change in the degree of polymer crystallinity with a change in temperature is a reversible thermomechanical process both in the pres ence of liquid [7] and in its absence [19,20]. 1 Only the last crystallites, unbound by passage chains, are destroyed thermally, e.g., melts, [9].…”

Phase Diagrams Semicrystalline Polymer–Liquid Revisited: Isotactic Polypropylene-Dibutyl Phthalate and Other Systems

“…DSC endotherms for LDPE and i-PP recorded at a rate of 10 C/min are shown along with the true melting temperatures T m obtained in Ref. 20 using the method of stepwise isothermal amorphization in the immersion liquid. [19] It is seen that in the case of LDPE, T m is close to the endotherm maximum T max , whereas for i-PP, T m is considerably shifted toward the end of the curve, T f .…”

“…We use the term "destruction of crystallites" to emphasize that changing the polymer degree of crystallinity with temperature is a reversible thermomechanical process independent of the presence [5] or absence [19,20] of a low MW liquid. Its mechanism, based on a gradual pullout of polymer backbones due to excessive stress in the amorphous regions, was first proposed by Alfrey and Mark.…”

Electrochemical Properties and Electropolymerization of Tetrakis(para-aminophenyl)porphyrin in Dichloromethane

“…Анализ полученных результатов позволяет сказать, что пленка является одним из видов высокомолекулярных соединений, которые плавятся в широком интервале температур, а именно, частично кристаллическим медленно кристаллизующимся полимерам. [37][38][39] …”