Formation and structure of epoxy network interphases at the contact to native metal surfaces

Abstract

Films (20 nm &lt; d EP &lt; 5 μm) and 1-mm adhesive joints of an amine-cured epoxy (DETA-DGEBA) are studied on native surfaces of Au, Al and Cu. For the films , IR spectroscopy reveals the chemistry as a function of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>d</mml:mi> <mml:mrow> <mml:mtext>EP</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . Curing is based on amine-epoxy reactions only. Compared to the bulk, the gross reaction rate slows down for the RT-curing. A metal-specific ‘chemical interphase’ with reduced epoxy group consumption extends over some 100 nm. Post-curing (393 K, 1 h) is not sufficient to consume all epoxy groups. Brillouin spectroscopy and microscopy provide local mechanical stiffness data ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>c</mml:mi> <mml:mrow> <mml:mn>11</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) with better than 10-μm resolution. The RT-cured films possess mechanical heterogeneities on the sub-mm scale. Their glass temperature is well below RT for most films while <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mi>T</mml:mi> <mml:mtext>G</mml:mtext> <mml:mrow> <mml:mtext>bulk</mml:mtext> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> = 326 K. In the adhesive joints , ‘mechanical interphases’ are detected on the metal side with an unexpected width of up to some 100 μm and with a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>c</mml:mi> <mml:mrow> <mml:mn>11</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> -maximum inside the epoxy. Its shape, height and position depend on the metal and the epoxy curing state. It is interpreted as the result of competing influences from epoxy structure and internal stresses. During polymerisation, a ‘morphological interphase’ should form with a 3-dimensional heterogeneous structure that is specific for the metal and the route of compound formation. Hence, the properties of thin films are not generally equivalent to the situation in thick adhesive bonds. .