The contact electrification of polymer interfaces opens the possibility to harvest energy from triboelectric (nano)generators (TEG). The electron transfer between contacted-separated surfaces has been considered as the main electrification mechanism for polymers in TEG. The electron transfer mechanism requires to contact chemically different materials, as well as to increase specific contact area, which has been accomplished via nano-structuring. Here we show that contact electrification can be controlled by intramolecular forces in polymer bulk and adhesive forces at contact interface. Results also confirm covalent bond breaking as a mechanism for contact electrification of polymer insulators. By taking in account findings mentioned above, we produced a TEG device using soft thermoplastic styrene-ethylene-co-butylene-styrene (SEBS) block copolymer, which had shown the highest surface charging. We modified the surface of SEBS to make it more adhesive by treating the surface with hydrogen peroxide solution. As a result, we observed increase in separation stress by 38% from 2.97 N/cm2 to 4.11 N/cm2. The increased adhesion of peroxide treated SEBS surfaces can be attributed to formation of quasi-free ends of macromolecular chains. As expected, for modified SEBS surface charge increased 3.14 times from 0.92 nC/cm2 to 2.89 nC/cm2.