EFFECT OF THE MONOMER RATIO ON THE STRENGTHENING OF POLYMER PHASE BOUNDARIES BY RANDOM COPOLYMERS

The fracture toughness G(c) of an interface between the immiscible hom opolymers polystyrene (PS) and poly(2-vinylpyridine) (PVP) reinforced with random copolymers, dPS(f)-r-PVP1-f was measured as a function of the average monomer fraction f and the areal chain density Sigma of th e copolymer. Long symmetric random copolymers (f approximate to 0.48) are shown to be effective in strengthening the interfaces. The effecti veness of the random copolymer at low areal chain densities results fr om each chain establishing multiple covalent connections across the in terface. Whether these connections result from each copolymer chain cr ossing the interface multiple times, entangling with the homopolymer o n either side of the interface, or whether these connections result fr om a ''pairing'' of chains with different monomer fractions f(resultin g from composition drift) is not yet certain. The interfacial G(c) inc reases strongly with increasing Sigma above Sigma approximate to 0.00 4 chains/nm(2) where a transition from chain scission to crazing occur s. At a high areal density (Sigma > Sigma(sat), where Sigma(sat) is th e saturation areal density of the copolymer, above which the random co polymer forms a distinct and continuous layer at the interface), the f racture toughness of the interface reinforced with f = 0.48 random cop olymer becomes a constant. The effectiveness of the copolymer at high Sigma may be due to the presence within the random copolymers of signi ficant fractions of chains with f > 0.48 as well as f < 0.48. Such a s pread in composition is caused by composition drift during the bulk co polymerization. In a thick layer of such a copolymer at the interface, the dPS-rich chains will preferentially segregate to the PS/random co polymer interface while the PVP-rich chains will preferentially segreg ate to the random copolymer/PVP interface, resulting in an overall int erface that is graded in composition and highly entangled. For asymmet ric random copolymers (f = 0.77, 0.60, 0.39, 0.25), the effectiveness decreases markedly as the copolymer becomes less entangled with the ho mopolymer (corresponding to the minor component in the copolymer) on o ne side of the interface. The maximum G(c) for the interface saturated with the random copolymer decreases significantly as f deviates from 0.5.