It is well-known that block copolymers can form large core-shell micelles in a selective solvent. The ultrafiltration of such polymeric micelles made of polystyrene (PS) and polyisoprene (PI) block copolymers in n-hexane through small pores (20 nm) is possible only when the flow-rate-dependent hydrodynamic force in the range 10(-15) to 10(-12) N (i.e., 1 fN-1 pN) is sufficiently strong to pull individual copolymer chains out of the core and disintegrate each micelle. Therefore, we are able to find how strong insoluble PS blocks in the core interact with each other from such a critical flow rate. Our results reveal that the micelle retention gradually decreases as the flow rate increases, different from a sharp first-order coil-to-stretch transition of a flexible linear homopolymer chain under the same elongation flow field. As expected, the interaction strength increases as the PS block becomes longer. Each flow-rate dependence of the micelle retention can be converted to a hydrodynamic force distribution f(F-h). For PS-b-PI diblock copolymers, f(F-h) has a single peak in the range 1-200 fN, whereas for PI-b-PS-b-PI triblock copolymers, there are two separated peaks in f(F-h), respectively, in the ranges 3-20 and 30-500 fN, attributing to two kinds of packing of the PS blocks inside the core; namely, the packing of unentangled and entangled insoluble PS blocks.