Study on the Graft Reaction of Poly(propylene) Fiber with Acrylic Acid Wei Wang, Li Wang,* Xu Chen, Qiang Yang, Tianxu Sun, Junfeng Zhou State Key Laboratory of Polymer Reaction Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China Fax: (þ86) 571 87951612; E-mail: opl_wl@dial.zju.edu.cn Received: July 8, 2005; Revised: October 19, 2005; Accepted: December 9, 2005; DOI: 10.1002/mame.200500248 Keywords: fibers; graft; modification; poly(propylene) (PP) Introduction Because of its low density, high tensile strength, low cost of production, and relative high resistance to chemicals, poly(propylene) (PP) fiber is popular for reinforcing con- crete matrices. However, its hydrophobic character results in a relatively poor combination with concrete matrices. [1] Hence, the surface modification of PP fiber with hydrophilic groups seems very significant. One of the simplest methods is to treat the fiber with a surfactant or impregnate it with a polymer solution and then evaporate the solvent. However, it is difficult to sustain the surfactant for a long time because the surfactants are only physically absorbed and do not bind chemically to the surface of PP fiber. [2] Therefore, chemical modification technologies have been used to modify or improve the surface properties of PP fiber. Among them, surface graft modification through free radical polymerization techni- que is one of the most commonly used methods, [3–7] and methacrylic acid (MAA), methacrylamide (MAAm), acrylamide (AAm), and 2-hydroxyethyl methacrylate (HEMA) are taken as grafting monomer for modification of PP. [8–25] However, one major limitation for this method is that the grafting degree is rather low because a large quantity of homopolymers come into being as side products in the reaction. As we know, the initiator not only initiates the PP fiber to introduce active sites on the PP backbone but also initiates the homopolymerization of monomers as a side reaction in the grafting polymerization. Therefore, if the excess initiators are squeezed out of the reaction system after the PP fiber having been initiated, the possibility of the homopolymerization of monomers can be greatly reduced and in turn facilitate the grafting process. To our knowledge, a method [9] has been reported. However, the microstructure of the grafted products, namely the number of the active sites formed on the PP backbone and the length of the grafted poly(acrylic acid) chain on each active sites, which are the two main aspects having an important influence on the dispersion ability of PP fibers in the concrete matrices, are still unknown. In the present paper, the number of the active sites formed on the PP backbone, the length of the grafted poly(acrylic acid) chain on each active site and the effects of different reactive conditions on them were studied in detail. Also, the method to determine the grafting degree and the reaction mechanism during the grafting reaction were discussed. Summary: In this paper, the graft of poly(propylene) fiber with acrylic acid is investigated. The effects of grafting temperature, monomer concentration, and grafting time on the grafting degree of acrylic acid onto poly(propylene) fiber are discussed. In contrast to the conventional method of determining the grafting degree gravimetrically, the acid- base titration method used in this paper was more efficient, even at low grafting degree. High-performance liquid chromatography (HPLC) was used to estimate the averaged length of the grafted poly(acrylic acid) chains on each grafted site of poly(propylene) backbone. And also a mechanism for the grafting polymerization is proposed. Possible microstructures of two PP-g-AA samples at the same grafting degree. Macromol. Mater. Eng. 2006, 291, 173–180 ß 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Full Paper DOI: 10.1002/mame.200500248 173