Polymers are made by a lot of little molecules together to make one long molecule, like a string of beads. The little molecules are called monomers and the long molecules are called polymers. According to the American Federal Trade Commission "polyester fibers are chemically composed of at least 85% by weight of an ester of a substituted aromatic carboxylic acid, including but not restricted to substituted terephthalate units and Para substituted hydroxyl benzoate units". Polyester is made by reacting ethylene glycol and terephthalic acid. Polyester fabric is highly amorphous in the molecular structure, and hydrophobic in nature, which resist water on the surface of the fabric.
Variation of comforts is depending on:
Thermal comfort is depends upon the heat release from human body & moisture release is one of the way of heat loss, a man can release about 1200 ml perspiration/hr. in heavy activity with his Apocrine sweat glands & Eccrine sweat glands. We know that polyester fiber moisture regain is only about 0.4% whereas cotton fiber moisture regain is 7.0-8.5% at 65% RH. If we observe the cross section of the fiber of cotton we can see convolution form of fiber where more than 70% portions are crystalline and rest of it is amorphous portions. The cotton fiber is very absorbent, owing to the countless polar -OH groups in its polymers; these attracts water molecules, which are also polar can able to absorb water6. And the major portion of polyester is hydrophobic in nature which hardly allows moisture through the fabric. We feel discomfort whenever our perspirations are on in summer through the body & if the sweat drops are begins to deposit on our skin in order to failure of moisture regain of the garments. Cotton can absorb our perspiration & polyester hardly can do it, therefore polyester garments wearer perceives discomfort.
Moisture Vapor Transmission:
The rate at which water vapor moves through a fabric plays an important role in determining the comfort as it influences the human perception and the cool/warmth feeling.9This process is called moisture vapor transmission. Cotton is a hygroscopic fiber that is it's a hydrophilic fiber whereas polyester is hydrophobic in nature which resists water in its surface. Moisture vapor transmission is relevant to the nature of the fiber. Due to the hydrophilic nature of cotton it attract moist vapor in its surface and then begins to transfer the vapor though the garments. Though it is possible to transfer vapor through the pore of fabric but polyester doesn't allow moisture on its surface because of its hydrophobic nature therefore it cannot pass vapor easily through the pore of the fabric. When a fabric allows the transport of water vapor at a faster rate, it is said to be a breathable fabric. In other words, the faster a fabric breath, the better is its comfort. So we can say that cotton is more breathable fabric than polyester.