I’m doing a research project which involves rubber toughening - the particular example I'm looking at is the toughening of SAN polymer (styrene-acrylonitrile) with polybutadiene to make ABS. What I'm looking for is a description of why the process makes the material tougher, whereas everything I can find online seems to be research about the effects of different factors on the toughness. If anyone could provide a source that would be great.
I think this Wikipedia article answers your question quite directly:
"This important property [rubber toughening] is entirely due to the small rubbery domains that aid in arresting crack propagation, thereby increasing toughness/energy needed to break the material."
The same article gives an example using the materials sited in your post:
"Well-known rubber toughened polymers include high impact polystyrene (HIPS), rubber-toughened nylon, and acrylonitrile butadiene styrene (ABS). In HIPS, polybutadiene domains are dispersed in a polystyrene matrix; in rubber-toughened nylon, typically cis-polyisoprene is dispersed in a Nylon matrix; in ABS, polybutadiene domains are again used, but for both polystyrene and acrylonitrile."