title: Crystallization and hardening of poly(ethylene-co-vinyl acetate) mouthguards during routine use.
abstract: Mouthguards (MGs) made from poly(ethylene-co-vinyl acetate) (EVA) are widely used in contact sports to prevent injuries such as breaking teeth and lip lacerations and to reduce brain concussion. However, the changes in morphology and the molecular mobility of EVA, which can affect its physical properties during practical usage, have not been precisely examined. Therefore, we attempted to determine the main factors which lead to changes in MG performance after one season of practical use by high school rugby players. Solid-state nuclear magnetic resonance (NMR) and pulse NMR measurements showed the hardening of MGs, which was associated with an increased crystallinity of the EVA resulting from prolonged usage. Furthermore, our data indicated that the increase in the relative amount of the crystalline phase may be primarily attributed to temperature fluctuations and repeated changes in pressure, which could cause the hardening of EVA and eventually diminish the protective ability of MGs.
authors.affiliation: Division of Molecular Science, Graduate School of Science and Technology, Gunma University, Kiryu, Gunma, Japan.
authors.forename: Ryoko
authors.initials: R
authors.lastname: Kuwahara
authors.affiliation: Division of Molecular Science, Graduate School of Science and Technology, Gunma University, Kiryu, Gunma, Japan.
authors.forename: Ryotaro
authors.initials: R
authors.lastname: Tomita
authors.affiliation: Division of Molecular Science, Graduate School of Science and Technology, Gunma University, Kiryu, Gunma, Japan.
authors.forename: Natsumi
authors.initials: N
authors.lastname: Ogawa
authors.affiliation: Department of Oral Health and Clinical Science, Division of Sports Dentistry, Tokyo Dental College, Chiyoda-ku, Tokyo, Japan.
authors.forename: Kazunori
authors.initials: K
authors.lastname: Nakajima
authors.affiliation: Department of Oral Health and Clinical Science, Division of Sports Dentistry, Tokyo Dental College, Chiyoda-ku, Tokyo, Japan.
authors.forename: Tomotaka
authors.initials: T
authors.lastname: Takeda
authors.affiliation: Division of Molecular Science, Graduate School of Science and Technology, Gunma University, Kiryu, Gunma, Japan.
authors.forename: Hiroki
authors.initials: H
authors.lastname: Uehara
authors.affiliation: Division of Molecular Science, Graduate School of Science and Technology, Gunma University, Kiryu, Gunma, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Yamanobe