The complex solution behavior of polymer brushes is key to control their properties, including for biomedical applications and catalysis. The swelling behavior of poly(dimethyl aminoethyl methacrylate) (PDMAEMA) and poly(2-(methacryloyloxy)ethyl trimethylammonium chloride) (PMETAC) in response to changes in pH, solvent, and salt types has been investigated using atomistic molecular dynamics simulations. PDMAEMA and PMETAC have been selected as canonical models for weak and strong polyelectrolytes whose complex conformational behavior is particularly challenging for the development and validation of atomistic models. The GROMOS-derived atomic parameters reproduce the experimental swelling coefficients obtained from ellipsometry measurements for brushes of 5-15 nm thickness. The present atomistic models capture the protonated morphology of PDMAEMA, the swollen and collapsed conformations of PDMAEMA and PMETAC in good and bad solvents, and the salt-selective response of PMETAC. The modular nature of the molecular models allows for the simple extension of atomic parameters to a variety of polymers or copolymers.