T. N. Lebedeva

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

The full phase diagram of an isotactic polypropylene (i-PP)–dibutyl phthalate (DBP) mixture is for the first time constructed by an optical method and discussed within the concept of semicrystalline polymers as microheterogeneous liquids with a three-dimensional network structure. It is demonstrated that the liquidus in this and other polymer–solvent systems is not thermodynamically equivalent to the liquidus in low molecular weight (MW) mixtures. Qualitatively different thermal behavior of those two types of binary systems in the liquidus vicinity is corroborated by differential scanning calorimetry (DSC) experiments. In the former case, a liquid-solid transition resulting in the formation of polymer crystallites does not lead to separating the mixture into crystalline and amorphous phases. On cooling, the system remains macroscopically single phase until the low MW liquid can be fully dissolved in the amorphous regions of the polymer. The correct location of the corresponding borderline is crucially important for the microporous membrane formation via thermally induced phase separation (TIPS). It is also argued that the topology of a phase diagram polymer–low MW liquid does not depend on whether the polymer is amorphous or crystalline.

The phase diagram of the high-density polyethylene-m-xylene system

A phase diagram of the high-density polyethylene (HDPE)-m-xylene system has been plotted and discussed in the context of the concept according to which semicrystalline polymers are three-dimensionally structured liquids with thermotropic grid nodes in the form of crystallites from the thermodynamic point of view. It has been shown that the specific feature of the semicrystalline-polymer-good-solvent systems is the change in the number of phases in them that is exclusively due to the reversible process of dissolution of the low-molecular-mass component in the amorphous regions of the polymer. It has been proposed that the coordinates of the imaging point that corresponds to the minimum temperature of the complete amorphization of the polymer in the presence of the liquid in the phase diagram of these systems can be used as a characteristic for the level of the thermodynamic affinity of the semicrystalline polymer and the liquid.

Phase Diagrams of Semicrystalline Polymer–Crystalline Substances: Polyolefins–1,2,4,5-Tetrachlorobenzene

The full phase diagrams of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and isotactic polypropylene (i-PP) mixtures with 1,2,4,5-tetrachlorobenzene (TeCB), including the solubility curve of TeCB in a solid polymer, were constructed by an optical method. The diagrams contain a eutectic point that corresponds to the situation when the crystallization of TeCB out of its solution in a polyolefin is accompanied by the crystallization of monomer units of the macromolecules. As a result, the polymer acquires a gel structure with crystallites as crosslinks and amorphous regions saturated with TeCB. It is demonstrated that the eutectic point position on the phase diagram can be used for ranking polymers with respect to their thermodynamic affinity to a solvent. For the studied systems, the affinity to TeCB was decreased in the order i-PP, HDPE, and LDPE. Direct experimental evidence was obtained that TeCB crystals can be dissolved in a solid polymer via a vapor phase mechanism, which leads to the polymer amorphization.

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

A hypothesis proposed for semicrystalline polymers as metastable, microheterogenous, three-dimensionally structured liquids with crystallites as grid sites is developed. The phase diagram of the isotactic polypropylene (PP)-dibutyl phthalate system obtained via this approach is discussed in detail. Additional arguments for the thermodynamic nonequivalence of the liquidus line in this and similar systems, on one hand, and in the systems of low-molecular-mass crystalline-liquids, on the other hand, are given. The appearance of a boundary curve, reflecting the transition of the original two-phase system into a single phase, such as a liquid-solution in a polymer, on the phase diagram of the semicrystalline polymer-liquid system is shown to fundamentally change the technological meaning of the diagram, thereby clarifying the mechanism of microporous-membrane formation from solutions of semicrystalline polymers via thermally induced phase separation.

Phase diagrams of low-density polyethylene–alkylbenzene systems

Complete phase diagrams for mixtures of low-density polyethylene with p- and m-xylene are plotted by optical means in developing the concept of which partially crystalline polymers are microstructured liquids. It is shown that in contrast to the ones presented in the literature, both diagrams contain the solubility boundary curve of the low-molecular weight component in the polymer, above which the polyethylene has the structure of a single-phase gel (crosslinks formed by crystallites and amorphous regions saturated with xylene). At the figurative point on the diagrams, a situation is observed in which the dissolution of all the liquid contained in the initial two-phase system in the polymer is accompanied by its simultaneous complete amorphization. The parameters of the figurative point allow us to estimate the thermodynamic affinity of different alkylbenzenes toward polyethylene.

Phase equilibria and transformations in low-density polyethylene–p-xylene system

The state diagram of the low-density polyethylene–p-xylene system that contains the solubility curve of p-xylene in the polymer is first built using the optical method with the involvement of the DSC data. It has been shown that, at 72°C, in a blend with a polymer mass fraction of 0.28, the full amorphization of the polymer proceeds simultaneously with the dissolution of p-xylene in it. Blends with a lower polymer content are also homogenized at the same temperature, although the duration of this process can take several days. In blends with a high polymer content, homogenization occurs at a higher temperature. This process is preceded by the formation of microheterogeneous single-phase gel with network junctions as polymer crystallites and amorphous regions saturated by p-xylene. Cooling of the homogeneous blends to–20°С is not accompanied by the full separation of low- and high-molecular-mass phases; in this case p-xylene not only forms an individual crystalline phase, but also partially crystallizes in amorphous regions of the polymer.

Phase diagram of the high-density polyethylene-1,2,4,5-tetrachlorobenzene mixture

The full phase diagram of a high-density polyethylene-1,2,4,5-tetrachlorobenzene (TeCB) mixture, which includes the solubility curve of TeCB in the polymer, is constructed for the first time by the optical method. It is shown that TeCB crystals are dissolved in polyethylene via vapor phase mechanism on heating, which leads to the polymer amorphization. The eutectic mixture composition corresponds to the situation when the crystallization of TeCB out of its solution in polyethylene is accompanied by the crystallization of elementary units of macromolecules. As a result, the polymer acquires a gel structure with crystallites as crosslinks and amorphous regions saturated with TeCB.

Interaction of water-DMSO mixtures with cellulose

The character of the interaction of cellulose with water-dimethyl sulfoxide mixtures is investigated. It is shown that, in the concentration range 0 < xD < 0.28, cellulose predominantly sorbs water; in this case, in the presence of small amounts of DMSO (xD ∼ 0.1), the sorption is higher than that for pure water. It is established that the formation of hydrogen bonds between hydroxyl groups of cellulose and oxygen atoms in DMSO molecules is most probable at xD > 0.6, where large clusters dominate over hydrophobic dimers.

Estimating the melting temperature of macromolecular poly-3,3-bis-(azidomethyl)oxetane

A melting temperature of 90.4°C (complete amorphization) for macromolecular poly-3,3-bis(azidomethyl)oxetane is determined via the hydrostatic-weighing method. This value corresponds to the end of the endothermic peak of the DSC thermogram and is ~10°C higher than the maximum value. The amorphization of the polymer is found to proceed mainly owing to thermomechanical destruction of the crystallites, and only a very small part of them is melted purely thermally at the temperature of complete amorphization. The melting temperature is significantly reduced in the presence of the oligomer fraction in the polymer. The density and equilibrium heat of fusion of the crystalline regions, which coincide for various polymer samples, are found with consideration for the data of X-ray scattering.

Enthalpies of interaction of water with microcrystalline cellulose having different degrees of dispersion

Changes in enthalpy during the interaction of different kinds of microcrystalline cellulose and water were determined. The integral and differential enthalpies of polysaccharide interactions with water were calculated. The amount of water retained by the cellulose was determined. The effect of mechanical processing on these characteristics was demonstrated.