Materials Science

Welded Together

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Science  15 Mar 2013:
Vol. 339, Issue 6125, pp. 1253
DOI: 10.1126/science.339.6125.1253-d

Welding depends on a strong bond forming between two pieces that have been heated to the point where they partially melt at the area that is being fused together. In joining two polymer pieces together, it was thought that the welded area would attain the same strength as the bulk material when the polymer chains had diffused by a distance close to their radius of gyration, but experimentally, bulk strength has been obtained in much shorter times. Ge et al. use molecular dynamics simulations to probe the weld region between two homopolymer segments in order to determine the correlation between individual chain motion and the shear strength of the welded pieces. At short weld times, the dominant failure mode is caused by the pullout of individual chains at the interface. Then there is a transition, once the interface chains are sufficiently embedded and entangled into the opposite region, where chain pullout decreases and bond breaking occurs. At first, this chain scission occurs primarily at the interface region, but then evolves so that bonds break uniformly across the specimen, showing that the welded region has achieved the bulk strength. This corresponds to when the areal density of entanglements matches the bulk value, and confirms that this is the key parameter to determining the time needed to achieve a strong weld.

Phys. Rev. Lett. 110, 098301 (2013).

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