It is a weak physical side bond that is easily broken by water or heat.
But the other type of connection that can form between adjacent keratin proteins, a hydrogen bond, is much weaker and temporary, with hydrogen bonds breaking and new ones forming each time your hair gets wet and dries again. If you straighten and dry your hair, it’ll be like the metal spring, completely straightened out into a rod. But if it’s a humid day, and your hair is prone to curling, water molecules will steadily be absorbed and incorporated into hydrogen bonds, inevitably pulling the metal rod back into a coiled shape. Water is made of hydrogen and oxygen—it's called H2O because every molecule of water has two hydrogen atoms (H2) and an oxygen atom (O). When you wet your hair, water molecules sneak in between the proteins of the cortex and join these hydrogen bonds. Your hair swells up, absorbing up to 30 percent of its weight in water. In wet hair, one protein molecule doesn't bond directly to another protein molecule. In a perm, you don't just break hydrogen bonds, you also break the disulfide bonds that hold the proteins together. You add chemicals that break the disulfide bonds (bonds between sulfur atoms). Then you reshape your hair and add chemicals that reconstruct those disulfide bonds, holding your hair in a new shape. An electronegative atom such as fluorine, oxygen, or nitrogen will be the hydrogen bond acceptor, whether or not it is bonded to a hydrogen atom. An example of a hydrogen bond acceptor that does not have a hydrogen atom bonded to it is the oxygen atom in diethyl ether. A hydrogen attached to carbon can also participate in hydrogen bonding when the carbon atom is bound to electronegative atoms, as is the case in chloroform, CHCl 3. An example of a hydrogen bond donor is ethanol, which has a hydrogen bonded to an oxygen. It is the hydrogen bonds found in the hair's cortex which cause hair to bend and become curly. Curling Iron / Straightener works by driving moisture out of the hair and locking the ionic hydrogen bonds in the hair.  This source provided by bobthejoe states that temporarily straightening hair involves the breaking of hydrogen bonds between keratin molecules within hair. One of the reasons hair is curly is because of hydrogen bonds between the proteins (keratin) that make up your hair [...]. Another way to alter hydrogen bonds is with heat; electric straighteners work because of this principle.  Side Bonds of the Cortex. The middle layer of the hair, the cortex, is made up of millions of polypeptide chains cross-linked with each other by three different types of side bonds. The bonds that link up the polypeptide chains of the hair are hydrogen, salt and disulfide bonds. Hydrogen bonds account for one-third of the hair's strength. The hydrogen bond is a weak physical side bond that is easily broken by water or heat. Hydrogen bonds can be reformed by drying or cooling the hair. These bonds are very abundant in the hair. Salt bonds are also weak physical side bonds that can easily broken by weak alkaline or acid solutions and changes in pH. There are various elements found in the hair and they are used to make amino acids, keratin, melanin, and protein. The average compostion of normal hair is composed of 45.2 % carbon, 27.9% oxygen, 6.6% hydrogen, 15.1% nitrogen and 5.2a% sulphur. The keratin found in hair is called hard keratin. Disulphide bonds also give your hair its elasticity and strength. Hydrogen bonds, on the other hand, are easily broken by the application of water and can be temporarily reset with heat until they become wet again (either from washing or humidity).  These hydrogen-bond attractions can occur between molecules (intermolecular) or within different parts of a single molecule (intramolecular). The hydrogen bond (5 to 30 kJ/mole) is stronger than a van der Waals interaction, but weaker than covalent or ionic bonds. A hydrogen attached to carbon can also participate in hydrogen bonding when the carbon atom is bound to electronegative atoms, as is the case in chloroform, CHCl 3. An example of a hydrogen bond donor is ethanol, which has a hydrogen bonded to an oxygen. Humid air causes hydrogen bonds to form between the proteins in your hair, triggering curls and frizz. Image via Flickr user Simon Gotz. If you have long hair, you probably don’t need to look up a weather report to get an idea of how much humidity’s in the air: You can simply grab a fistful of hair and see how it feels. If you straighten and dry your hair, it’ll be like the metal spring, completely straightened out into a rod. But if it’s a humid day, and your hair is prone to curling, water molecules will steadily be absorbed and incorporated into hydrogen bonds, inevitably pulling the metal rod back into a coiled shape.
http://www.smithsonianmag.com/science-nature/why-humidity-makes-your-hair-curl-21127724/ http://www.exploratorium.edu/exploring/hair/hair_4.html https://en.wikipedia.org/wiki/Hydrogen_bond http://haircare-design.com/Curlerwork.php http://biology.stackexchange.com/questions/2180/molecularly-why-can-you-straighten-or-perm-hair http://texascollaborative.org/hildasustaita/module%20files/topic3.htm http://www.philipkingsley.co.uk/hair-guide/hair-science/the-biology-of-your-hair/ https://en.wikipedia.org/wiki/Hydrogen_bond http://www.smithsonianmag.com/science-nature/why-humidity-makes-your-hair-curl-21127724/
what is hydrogen bonds in hair