Sub-Tg̳, solid-state, plasticity-induced bonding of polymeric films and continuous forming

Abstract

If two pieces of a glassy polymer are brought into intimate contact (within molecular proximity) at temperatures well below their glass transition temperature (Tg) negligible adhesion, due to lack of inter-diffusion of macromolecules, will be noted. Because polymer chains are kinetically trapped well below the Tg, the time-scales for relaxations in the glassy state are extremely large, and the system is effectively frozen with respect to any long range diffusive motion. This thesis shows the discovery of a new phenomenon of solid-state, plasticity-induced bonding at temperatures well below Tg which take on the order of a second, by subjecting amorphous polymers to active plastic deformation. In spite of the glassy regime, the bulk plastic deformation triggers requisite molecular mobility of polymer chains to cause inter-penetration across the interface. Quantitative levels of adhesion and morphology of the fractured interfaces validate the sub-Tg, plasticity-induced, molecular mobilization causing bonding. The absence of bonding during compression of films in a near hydrostatic setting (which inhibits plastic flow), and between an 'elastic' and a 'plastic' film, further establishes the explicit role of plastic deformation in this newly reported sub-Tg solid-state bonding. Other components of this thesis comprise the design and fabrication of machines to perform continuous forming and sub-Tg, solid-state bonding of polymer films. A peel-test fixture has also been designed and developed to improve the T-peel test for determination of mode-I fracture toughness of thin and flexible adhered laminates. This research has been conducted as a part of an ongoing activity at the Novartis-MIT Center for Continuous Manufacturing at MIT.