Miscible polymers molecular weight dependence

Polymer pairs miscible in the amorphous state at room temperature. Molecular weight dependence investigated. These are usually polymer pairs that are miscible when the molecular weights are low and immiscible as when the molecular weights are increased. 

Because polymer-polymer miscibility depends on a balance between AH and AS and because AS , is molecular weight dependent, any statement as to miscibility, partial miscibility or immiscibility should be qualified by the molecular weights of the polymers involved. A system which is partially miscible for one pair of molecular weights may become miscible for mixtures of lower molecular weight species or immiscible if the molecular weight of one or both components is increased. 

The critical value above which the two polymers phase separate can be determined from equation 22 for various mixtures with ri = r2, and xi2 can be compared to the corresponding difference in solubility parameters, as shown in Table 1. The tolerated difference between the solubility parameters for miscibility to be achieved decreases with r and, for high molecular weight polymers to mix, the solubility parameter must be very close. For example, polystyrene ([5 = 18.4 (J/cm )° l and polyla-methyl styrene) [5 = 18.1 (J/cm )° ] are predicted to be miscible up to Mn fi0000 while for miscibility to be achieved between poly(methyl methacrylate) [3 = 19.0 (J/cm )° ] and poly(methyl acrylate) [5 = 19.fi (J/cm )° l Mn needs to be lower than 13000. Miscibility is expected to be strongly molecular/weight-dependent, as illustrated for polystyrene/polyla-methyl styrene) blends in References 11 and 12. 

Table 4. Polymer Pairs Miscible In the Amorphous State at Room Temperature Molecular Weight Dependence Investigated VI-450...

TABLE 4. POLYMER PAIRS MISCIBLE IN THE AMORPHOUS STATE AT ROOM TEMPERATURE, MOLECULAR WEIGHT DEPENDENCE ... 

Most properties of linear polymers are controlled by two different factors. The chemical constitution of tire monomers detennines tire interaction strengtli between tire chains, tire interactions of tire polymer witli host molecules or witli interfaces. The monomer stmcture also detennines tire possible local confonnations of tire polymer chain. This relationship between the molecular stmcture and any interaction witli surrounding molecules is similar to tliat found for low-molecular-weight compounds. The second important parameter tliat controls polymer properties is tire molecular weight. Contrary to tire situation for low-molecular-weight compounds, it plays a fimdamental role in polymer behaviour. It detennines tire slow-mode dynamics and tire viscosity of polymers in solutions and in tire melt. These properties are of utmost importance in polymer rheology and condition tlieir processability. The mechanical properties, solubility and miscibility of different polymers also depend on tlieir molecular weights. 

Fractional precipitation is dependent on the slight change in the solubility with molecular weight. When a small amount of miscible nonsolvent is added to a polymer solution, the product with the highest molecular weight precipitates first. The procedure is repeated after the precipitate is removed. Molecular weights are run for each fraction and a curve developed that is similar to Figure 3.6. 

The precipitation behavior of the polymer is intapreted by a photostimulated change of the critical miscibility temperature T. For polystyrene dissolved in cyclohexane, the polymer precipitates at temperature below T. According Fox and Flory [65], T depends on the molecular weight M as... 

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