Polyethylene solution phase diagram

In situ polymerisation does not however guarantee homogeneous blends as two phase regions can exist within the polymer/polymer/monomer three component phase diagram. In the case of vinyl chloride polymerisation with solution chlorinated polyethylene, the vinyl chloride has limited solubility in both poly(vinyl chloride) and chlorinated polyethylene. The phase diagram has the form shown in Fig. 3 The limit of swelling of vinyl chloride in the chlorinated polyethylene is A and the highest concentration of PVC prepared by a one-shot polymerisation is B. 

Diphasic liquid systems used in CCC may have a wide variety of polarities. The most polar systems are the ATPS made by two aqueous-liquid phases, one containing a polymer, for example, polyethylene glycol (PEG), the other one being a salt solution, for example, sodium hydrogen phosphate. The less polar systems do not contain water there can be two-solvent systems, such as heptane/acetonitrile or dimethylsulfoxide/hexane systems or mixtures of three or more solvents. Intermediate polarity systems are countless since any proportion of three or more solvents can be mixed. Ternary phase diagrams are used when three solvents are mixed together. 

The phase diagram (see Figure 1) shows that there are two solution processes a low-temperature process (below 100 °C) for the production of amorphous copolymers like ethylene/propylene elastomers (EPR or EPM) [2], and a high-tempera-ture process (far beyond 100 °C) for the production of semicrystalline homo- and copolymers like high-density polyethylenes (PE-HD), linear low-density poly-ethylenes (PE-LLD) and ethylene waxes [1, 3]. Polypropylenes (PP) cannot be made in high-temperature solution processes, except for propylene waxes. 

There is another type of phase diagram enconntered in some polymer solutions, where the LCST lies below the UCST. This is called closed-loop and appears for polymer solutions where hydrogen bonding effects are dominant snch as polyethylene gly-col/water and polyvinyl alcohol/water (Figure 16.4). 

At higher concentrations block copolymers form lyotropic liquid-crystalline phases. Their range of stability can depend strongly on temperature. In aqueous solutions polyethylene oxide (PEO) is usually the soluble block. An increase of temperature reduces the solubility of the PEO block which can result in phase transitions into different phases. Most of the present knowledge on lyotropic phase behavior of block copolymers was obtained from studies of Pluronics,i.e., poly(ethyleneoxide-h-propyleneoxide-h-ethyleneoxide) (PEO-PPO-PEO) [31]. Phase diagrams of block copolymers with shorter chains resemble those of low-molecular surfactants. 

Fig. 6. Phase diagrams of solutions of A HPC/Ethylene glycol B) HPC/diethylene glycol. C) HPC/Polyethylene glycol = 300 D) HPC/dimethylacetamide.

Figure II. Model predictions of the phase diagram for 10 mass % solutions of polyethylene (M= 108,000) in binary solvent pentane + carbon dioxide with 20, 30 and 40 % carbon dioxide content at a system pressure of 15, 35 and 65 MPa. With increasing pressure, the systems shift from one showing an hour-glass shaped region of immiscibility to one showing both LCST and UCST, and finally to one showing only UCST. [Refs.32 and 33].

A further striking example can be given of the relation between molecular characteristics and phases occurring in the solution. In non-ionic surfactants, the length of the polyethylene hydrophilic segment can be varied in an almost continuous manner. Phase diagrams then show steady evolution in the extent of the various phases. 

With nonionic surfactants of the ethoxylate type an increase in temperature for a solution at a given concentration causes dehydration of the polyethylene oxide (PEO) chains and at a critical temperature the solution become cloudy. This is illustrated in Fig. 3.3 which shows the phase diagram of Below the cloud point (CP) curve one can identify different liquid crystalline phases hexagonal-cubic-lamellar which are schematically shown in Fig. 3.4. 

Phase diagram for the formation of aqueous two-phase systems in polyethylene glycol-Na2S04 solutions. Phase formation occurs... 

CPC and spinodal curve touch each other and share a common tangent as required by thermodynamics. Only for mixtures of exactly two chemical species, e.g. pure solvent plus completely homodisperse polyethylene, is the critical point situated at a common extremum of both the spinodal and binodal curve. In the case of UCST behaviour the extremum is a maximum if it is a minimum, it is called a lower critical solution temperature (LCST). In polydisperse polymer solutions, even for polymers characterized by a narrow MWD, the critical point moves appreciably away from the extremum of the CPC towards the polymer rich side of the phase diagram as is shown in Figure 1. 

Figure 4.14 Solid solution of polyethylene fractions in comparison with the experimental phase diagram of (a) two polyethylene fractions [76] and (b) PCL/trioxane blends [77], showing the liquidus-solidus loop and eutectic phase behavior, respectively.

Both, natural and synthetic polymers with associative properties arising from hydrophobic interactions give aqueous solutions with LCST. Among the most known systems having LCST behaviour should be mentioned polyethylene glycol-water and aqueous solutions of methyl cellulose. Also, in poly(methacrylic) acid, LCST phase diagrams were determined from the change in shear modulus and turbidity. For alkali chitin, the main key role played by hydrophobic interactions in LCST is evident from the decrease in the fluorescence ratio observed in Fig. 3b. 

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