Modifying the pom-pom model for extensional viscosity overshoots

(eng) We have developed a variant of the pom- pom model t hat qual itatively d escr ib es two s urpr is ing f ea -tur es r ecently obser ved in filament str etchi ng rheometer e xper iment s o f uniaxial e xt en sional flow of industri al b ranched polymer r esi ns: (i) Overshoots o f t he tr an si en t s tr es s duri ng s teady flow and (i i) strongly a ccelerated str ess relaxation u po n c essa tion o f the fl ow b eyond the over shoot. W i thin the con-text of our model , t hese overshoots o ri ginat e fr om ent anglement s trippi ng (ES ) dur ing the p rocesses o f normal c hain retraction and branch point withdr aw al. W e d emonstrate that, for a si ng le mode , the pre-dictions of our over shoot model a r e qual itatively consistent wi th experimental data. To p rovide a quan-tit at iv e fit, we r epr esent a n industr ia l m elt b y a s uperpositi on of sever al i ndi vidual m odes. We show that a mi ni mal ver sion o f our model, in whi ch ES due to nor ma l chain retraction is o mitted, can p r o-vide a reasonable, but not per fect, fi t to t he data. W i th regard the str ess r elaxation a ft er (ki nemat ic all y) steady fl ow, we demonstr at e that the dif ferential v er si on of t ube or ie ntation dynamics in the o rigi na l pom- pom model p erf orms anomal ou sly. We discuss the reasons for this and suggest a s uitable alternative.