Upper critical solution temperature polymer blends

Vlassopoulos, D., Koumoutsakos, A., Anastasiadis, S.H., Hatzikiriakos, S.G., and Englezos, P. (1997) Rheology and phase separation in a model upper critical solution temperature polymer blend. 

Polymer blends may be characterized in terms of the temperature dependence of the Flury-Huggins interaction parameter (j)- In the case of an upper critical solution temperature (UCST) blend, / decreases with temperature, and the blend remains miscible. For phase separation to occur in a UCST blend, the temperature must be lower than the critical solution temperature. In the case of a lower critical solution temperature (LCST) blend, x increases with temperature, and thus phase separation occurs above the critical solution temperature. The ability of CO2 to mimic heat means that miscibility is enhanced in the case of UCST blends, and for the case of LCST blends the miscibihty is depressed. Ramachandrarao et al. [132] explained this phenomenon by postulating a dilation disparity occurring at higher CO2 concentration as a result of the preferential affinity of CO2 to one of the components of the blend, inducing free-volume and packing disparity. 

Where the solubility parameter rule is in error is for natural rubber and polybutadiene. The differential in solubility parameters is around 0.6 but the two polymers are immiscible. Polybutadiene grade IISRP 1207 and an oil extended polymer such as IISRP 1712 have a differential of less than 0.1 and in this case the two elastomers are nearly fully miscible between the lower and upper critical solution temperatures. The blended elastomers mechanical properties then become a function of the filler type, distribution, vulcanization system, and any processing aids present. 

Thin polymer films composed of two layers with different composition have been used for almost two decades to determine the diffusion coefficient [12,78-82] on the basis of observed broadening of their initial profiles ( >(z). When the two layers are built of two fully miscible phases (T>TC regime for blends with upper critical solution temperature UCST), a free interdiffusion takes place with the interface growing with time t as w1/2°=t1/2. This process proceeds without limits and results in a single homogeneous phase. 

This figure clearly shows the temperature and composition windows where it is either a two-phase system or a single-phase system. The characteristic features of an upper critical solution temperature (UCST) and a lower critical solution temperature (LCST) corresponding to the phase transition are identified. For a particular composition of two immiscible polymers, if the temperature is increased, the UCST is the highest temperature at which two phases may co-exist in the blend. There is then a window of miscibility as the temperature is increased further, followed by phase separation again at the LCST. This type of diagram is often seen for polymer solutions, e.g. polystyrene in cyclohexane. Often polymer blends show... 

In general, the miscibility of a pair of polymers depends on temperature and composition. Figure 10.1 schematically shows three typical phase diagrams. The ordinate and the abscissa axes represent temperature and composition, respectively. The solid line in Fig. 10.1(a), below which the blend becomes immiscible (two-phase), is referred to as an upper critical solution temperature (UCST). However, Fig. 10.1(b) shows a lower critical solution temperature (LCST) behavior. Some polymer pairs display both UCST and LCST as shown in Fig. 10.1(c). As will be shown in the following, UCST is rarely observed for a polymer blend. 

Fig. 10.1. Schematic illustration of a phase diagram for a polymer blend, showing (a) upper critical solution temperature (UCST) (b) lower critical solution temperature (LCST) and (c) UCST + LCST. Ordinate and abscissa show temperature and composition, respectively.

The formulation of Scott (44) does not present the range of phenomena occurring in polymer blends. Various binary blends exhibit lower critical solution temperatures (LCST) where phase separations occur at lower temperature. Other blends exhibit upper critical solution temperatures (UCST) where miscible blends exhibit phase separations at higher temperatures (45). It was shown by McMaster (46) that volume changes occurred in mixing. 

The polyethylenes with higher functionality were soluble in epoxy resin and required lower temperamre and time for forming homogeneous blend systems. The miscibility of the polymers was dependent on the type of epoxy resin also. Cycloaliphatic epoxy resin showed more miscibility with the polymers compared to DGEBA resin and phase separation occurred in these blend systems as a result of crystallization of PE. Upper critical solution temperature (UCST) behavior was... 

When the temperature of the system increases, incompatibility decreases and the system becomes a single-phase system at temperature Tao- The temperature of this transition may vary depending on the system composition and it reaches its maximum at some point Tj ,. Above Tjj, called the upper critical solution temperature, UCST, both components are compatible in any proportion. The diagrams of polymer-plasticizer blends differ from dia-... 

Surface-directed spinodal decomposition was first observed in an isotopic polymer blend (Jones et al. 1991) thin films of a mixture of poly(ethylene-propylene) and its deuterated analogue were annealed below the upper critical solution temperature and the depth profiles measured using forward recoil spectrometry, to reveal oscillatory profiles similar to those sketched in figure 5.30. Similar results have now been obtained for a number of other polymer blends, including polystyrene with partially brominated polyst)u-ene (Bruder and Brenn 1992), polystyrene with poly(a-methyl styrene) (Geoghegan et al. 1995) and polystyrene with tetramethylbisphenol-A polycarbonate (Kim et al. 1994), suggesting that the phenomenon is rather general. 

The X parameters of a large number of polymer blends exhibit this kind of temperature dependence. An example of this is the SPB(88)/JSPB(78) blend [system 27a], and the temperature dependence of x is shown in Fig. 19.1(a). Increasing temperature in such blends leads to increased miscibility. This behavior is often referred to as upper critical solution temperature (UCST) behavior. A typical phase diagram obtained from such systems is shown in Fig. 19.1(b). The spinodal and binodal curves were calculated for a SPB(88)/JSPB(78) blend with N = 2,000. A 50/ 50 mixture of these polymers is predicted to be two phase at room temperature but single phase at temperatures above 105 °C. The qualitative features of the phase diagrams obtained from all type I blends will be similar to Fig. 19.1(b). Of course the locations of the phase boundaries will depend on A, B, and N. 

Low molecular weight PS was mixed vdth poly(methyl phenyl siloxane), PMPS, to form an immiscible blend with an upper critical solution temperature (UCST) [199]. The viscoelastic properties were studied by dynamic and steady-state shearing the neat polymers showed Newtonian behavior. Within the miscible region the blend viscosity followed the Mertsch and Wolf equation, Eq. (2.35), but with the parameter /fit = calculated from Bondi s tables. The phase separation created a rheolog-ically complex behavior. 

When a homogeneous mixture solution is cooled, phase separation is induced at a certain temperature. This critical phase separation temperature is termed the upper critical solution temperature (UCST). It is a convex upward curve in the plot of composition versus temperature (C-T plot) and its maximum point shifts to a higher temperature with increasing relative molecular mass of the polymer. However, for many polymer-solvent and polymer-polymer blend systems, a decrease in mutual solubility is also observed as the temperature increases. The critical phase separation temperature is called the lower critical solution temperature (LCST). It is a convex downward curve in the C-T plot and the minimum point shifts to a lower temperature with increasing relative molecular mass of the blend components. LCST occurs at a higher temperature than UCST. 

Once the binary interaction parameters for the blend system are known, EOS theory can be used to predict phase separation behavior. Lower critical solution temperature (LCST) is the temperature above which a miscible system becomes immiscible. Upper critical solution temperature (UCST) is the temperature above which an immiscible polymer blend system becomes miscible. Some polymer-polymer systems exhibit either LCST or UCST or both or neither. Another set of phase separation can be obtained as shown in the copolymer-homopolymer example in Section 3.2 by varying the blend volume fraction. The Gibbs free energy of mixing per unit volume for a binary system of two polymers can be written as... 

Matyjaszewski et al. [2] patented a novel and flexible method for the preparation of CNTs with predetermined morphology. Phase-separated copolymers/stabilized blends of polymers can be pyrolyzed to form the carbon tubular morphology. These materials are referred to as precursor materials. One of the comonomers that form the copolymers can be acrylonitrile, for example. Another material added along with the precursor material is called the sacrificial material. The sacrificial material is used to control the morphology, self-assembly, and distribution of the precursor phase. The primary source of carbon in the product is the precursor. The polymer blocks in the copolymers are immiscible at the micro scale. Free energy and entropic considerations can be used to derive the conditions for phase separation. Lower critical solution temperatures and upper critical solution temperatures (LCST and UCST) are also important considerations in the phase separation of polymers. But the polymers are covalently attached, thus preventing separation at the macro scale. Phase separation is limited to the nanoscale. The nanoscale dimensions typical of these structures range from 5-100 nm. The precursor phase pyrolyzes to form carbon nanostructures. The sacrificial phase is removed after pyrolysis. 

When polymers undergo phase separation in thin films, the kinetic and thermodynamic effects are expected to be pronounced. The phase separation process can be controlled to effect desired morphologies. Under suitable conditions a film deposition process can lead to pattern replication. Demixing of polymer blends can lead to structure formation. The phase separation process can be characterized by the binodal and spinodal curves. UCST is the upper critical solution temperature, which is the temperature above which the blend constituents are completely miscible in each other in all proportions. LUST behavior is not found as often in systems other than among polymers. LUST is the lower critical solution temperature. This is the... 

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