title: Large negative thermal expansion of a polymer driven by a submolecular conformational change.
abstract: Mechanoresponsive polymers hold great technological potential in drug delivery, 'smart' optical systems and microelectromechanical systems. However, hysteresis and fatigue (associated with large-scale polymer chain rearrangement) are often problematic. Here, we describe a polyarylamide film that contains s-dibenzocyclooctadiene (DBCOD), which can generate unconventional and completely reversible thermal contraction under low-energy stimulation. The films exhibit a giant negative thermal expansion coefficient of approximately -1,200 ppm K(-1) at ambient or near-ambient temperatures, much higher than any known negative-thermal-expansion materials under similar operating conditions. Mechanical characterization, calorimetry, spectroscopic analysis and density-functional theory calculations all point to the conformational change of the DBCOD moiety, from the thermodynamic global energy minimum (twist-boat) to a local minimum (chair), as the origin of this abnormal thermal shrinkage. This newly identified, low-energy-driven, thermally agile molecular subunit opens a new pathway to creating near-infrared-based macromolecular switches and motors, and for ambient thermal energy storage and conversion.
authors.affiliation: 1] Materials Science and Engineering, School of Engineering, University of California at Merced, 5200 North Lake Road, Merced, California 95343, USA [2] State Key Laboratory of Molecular Engineering of Polymers and Department of Macromolecular Science, Laboratory of Advanced Materials, Fudan University, 220 Han Dan Road, Shanghai 200433, China.
authors.forename: Xingyuan
authors.initials: X
authors.lastname: Shen
authors.forename: Christopher
authors.initials: C
authors.lastname: Viney
authors.forename: Erin R
authors.initials: ER
authors.lastname: Johnson
authors.forename: Changchun
authors.initials: C
authors.lastname: Wang
authors.forename: Jennifer Q
authors.initials: JQ
authors.lastname: Lu