Hugh Brown has shown that interfacial entanglements govern adhesion between two polymers. We demonstrate this for three systems by adding interfacial chains <i>via</i> chemical coupling. The adhesion between polypropylene (PP)/amorphous polyamide (aPA) was reinforced by the coupling reaction of maleic anhydride grafted PP (PP-<i>g</i>-MA) and the primary amine groups on aPA; huge increases in adhesion were observed. A good correlation between critical fracture toughness, <i>G</i><sub><i>c</i></sub>, and PP-<i>g</i>-MA concentration squared follows Brown's crazing mechanism. For a polystyrene (PS)/aPA interface reinforced by the coupling reaction of poly(styrene-<i>r</i>-maleic anhydride) (PS-<i>r</i>-MA)/aPA only modest adhesion increases in <i>G</i><sub><i>c</i></sub> were observed through the whole PS-<i>r</i>-MA concentration range. This different behavior of <i>G</i><sub><i>c</i></sub> <i>vs</i>. functional polymer concentration is believed to be caused by segregation of the formed graft copolymers at the interface. The relationship between <i>G</i><sub><i>c</i></sub> and the extent of coupling was studied quantitatively with a model PS/PMMA system. The interface was reinforced by the coupling reaction of 010% PS-NH<sub>2</sub>/PMMA-anh. <i>G</i><sub><i>c</i></sub> was measured with the asymmetric dual cantilever beam test (ADCB) and the amount of copolymer formed at the interface was determined by a fluorescence labeling technique. <i>G</i><sub><i>c</i></sub> is low and is linear in block copolymer interfacial coverage (&b.Sigma;), indicating a chain scission mechanism. Reasonable agreement was achieved between experiment and theoretical prediction based on the energy to break CC bonds.
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