Formation and Properties of Some Two-phase Polymer Systems

Polymers of different natures usually demix. The phases can interdisperse into domains only if their formation is thermodynamically favoured. This can be obtained by grafting or by synthesizing block polymers, but the effective structure depends on the technological process. Examples are given of interdisperse domains of grafted polymers obtained both from solutions and from heterogeneous phase polymerization. Quasi-spherical shapes are obtained by the first method. The domain structure has very strong consequences on the properties. By quantitative stress analysis it is possible to demonstrate that a central domain, in a rigid matrix, when swollen, generates pressures which can easily break the matrix producing crazes. A range of water permeability is achieved in polyacrylic acid—polypropylene grafted films having different structures. The regularly distributed domain structure is the most permeable. If the matrix is rubberlike permeability is very small, in accord with theory. In a stress field the tensile strength of a rigid matrix is lowered by the presence of included domains, but its impact strength is higher when the included domains are rubberlike. The necessary conditions to get high impact structures are reviewed (difference in relaxation times, regular interdispersion of the phases, good adhesion, proper domain sizes). These structures can be obtained both starting by an elastomeric phase and successive grafting of rigid polymers and starting by a rigid phase and grafting of elastomeric polymers. Experimental examples are given. Chemically different polymers separate very easily. Starting from a random mixture of two polymers A and B the tendency to separation is measured by the mixing free energy change AGm with the opposite sign, which for amorphous polymers may be calculated according to the Huggins and Flory theory' (Figure 1). This treatment leads to introduction of the dimensionless interaction coefficient XB,A (MB/pBRT) (B — A) where MB, PB' B respectively are molecular weight, density and solubility parameter of polymer B, considered as the solvent, öA is the solubility parameter of polymer A, R is the gas constant, T is the absolute temperature.