Mechanical and management improvements in composite materials require the use of new ways to produce them. The actual tendencies and global developments of novel materials include the introduction of natural, synthetic, and recycled polymers into the composite materials. Recently, composite materials have been used for certain applications, which depend on several conditions as materials nature, concentrations, methodology for their elaboration, among others. Nevertheless, certain properties are not reaching with traditional components; thus, different materials are needed as fillers for reinforcing and improving such properties. A worldwide tendency is to use recycled or natural materials and to evaluate their influence after adding into the composite materials. In this sense, polymers are used in a very large variety of composite materials, mainly as fillers or reinforcements. Their chemical structure and physical morphology have been modified by using different kinds of energies, including thermal, chemical, and radiation energy. The perspectives are to obtain new materials with improved properties at low cost by using easy technologies, and chemically sustainable for the environment. The knowledge and understanding of such modified composite materials are therefore of great importance for our living. In this issue, we invite researchers to contribute with original research articles as well as review articles that encourage our understanding of natural, synthetic, and recycled polymers in composite materials. The topics include, but are not limited to:

• Natural polymers as fillers for improvements of composites materials
• Natural polymers used for encapsulation of polymer composites
• Synthetic polymers in polymer-polymer composite for solvent detection
• Polymer fibers as reinforcement of polymer composites
• Impregnated-polymer nanoparticles used in polymer composites
• Recycled polymers for improvement of polymer and cement concrete
• Chemical and thermal-modified polymers to be used in polymer composites

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