1 Polymer Chains of Large Persistence Length in a Polymer Brush Require Lower Force to be Compressed than Chains with Short Persistence Length José Saúl Hernández-Fragoso 1 , Salomón de Jesús Alas 2,3* , and Armando Gama Goicochea 4* 1 Posgrado en Ciencias Naturales e Ingeniería, Universidad Autónoma Metropolitana Unidad Cuajimalpa, Ciudad de México 05300, Mexico 2 Departamento de Ciencias Naturales, Universidad Autónoma Metropolitana Unidad Cuajimalpa, Ciudad de México 05300, Mexico 3 Departamento de Química, Universidad Autónoma Metropolitana Unidad Iztapalapa, Av. San Rafael Atlixco 186, Ciudad de México 09340, Mexico 4 División de Ingeniería Química y Bioquímica, Tecnológico de Estudios Superiores de Ecatepec, Av. Tecnológico s/n, Ecatepec, Estado de México 55210, Mexico ABSTRACT The force on a polydisperse polymer brush with chains of increasing persistence length,   , is obtained by numerical simulations. Unexpectedly, the force needed to compress a stiff brush is smaller than for a soft brush. This is due to the ordering of the largest polymer chains in bundles, which buckle when   grows releasing pressure, thus reducing the force. Stiffer brushes lose translational entropy, raising the entropy of the solvent. Our predictions agree with optical tweezer force spectroscopy and ellipsometry measurements on polymer brushes. Clues for the interpretation of experiments on human cervical epithelial cells that are covered with polymer – like brushes are offered, which find that cancer cells are softer than normal ones. KEYWORDS: Polymer brushes; polymer persistence length, atomic force microscopy, dissipative particle dynamics, force – indentation profiles.