the hydrogen bond
The hydrogen bond is the strongest of the weak secondary bonds — a special, extra-sticky version of van der Waals attraction that appears whenever a hydrogen atom is covalently bonded to a small, greedy atom like oxygen, nitrogen, or fluorine. It is the bond that makes water behave so strangely, holds the two strands of DNA together, and gives many polymers their strength. Though still far weaker than a primary bond, it is strong enough to shape the everyday world.
Here is why it is special. When hydrogen bonds to, say, oxygen, the greedy oxygen pulls the shared electrons strongly toward itself, leaving the hydrogen end noticeably positive — and because hydrogen has no inner electrons to shield it, that positive end is a bare, exposed proton. That naked positive charge reaches out and grabs the lone electrons of a neighbouring oxygen or nitrogen, forming a bridge. The result is a directed dipole bond with an energy up to about 50 kJ/mol, several times stronger than an ordinary van der Waals force but still ten to twenty times weaker than a covalent bond.
Hydrogen bonding punches above its weight in materials. It is why water melts and boils far higher than its tiny size would suggest (the H-bonds must be broken to melt ice or boil water), and why ice is less dense than liquid water (H-bonds hold the molecules in an open lattice, so ice floats). In polymers such as nylon and cellulose, hydrogen bonds between chains act like extra rivets, raising strength, stiffness, and melting temperature well above what plain van der Waals bonding could give.
Nylon owes much of its toughness to hydrogen bonds between neighbouring chains. Nylon absorbs water partly because water molecules muscle in and form their own hydrogen bonds with the chains, which is why nylon parts swell and soften in humid conditions — a direct, practical consequence of these weak bonds.
Hydrogen bonds between chains: extra rivets that raise strength and Tg.
A hydrogen bond is not a bond to just any hydrogen — it needs hydrogen attached to one of the very electronegative small atoms (O, N, F). A carbon-hydrogen bond, as in a simple hydrocarbon, is not polar enough to form real hydrogen bonds, which is why oils and waxes lack water's special properties.