Cross-linking in resins helps increasing:

The resin matrix in most dental composites is based on a blend of aromatic and/or aliphatic dimethacrylate monomers such as bis-GMA and urethane dimethacrylate (UDMA), to form highly cross-linked, strong, rigid, and durable polymer structures. Anusavice, Shen, Rawls. Phillip’s Science of Dental Materials. Edition 12. Page:​295