Effects of Humidity on Aggregation Structure at Epoxy / Metal Interface
Department of Chemistry for Materials, Graduate School of Engineering, Mie University,1577 Kurimamachiya, Tsu, Mie 514-8507, Yoshihisa Fujii

1. Introduction
Diffusion of water into the polymer structures can influence the structure and properties ofthe polymers. The absorbed water is believed to degrade the strength and properties of thepolymers and hence it is important to study how it affects the thermal and mechanical properties ofthe polymers. For example, in the field of adhesion which attracts attention as a structural material,
epoxy resins are considered to be hardy polymers capable of withstanding deleterious conditions forconsiderable times. As such, they are much used as adhesives, matrices for composite materials,
encapsulating agents, etc. However, extended exposure to water (or water vapor) can lead todamage, and much work has been devoted to better understanding of the mechanisms involved. The
transport properties of water have been studied on epoxy–amine networks in relation with thepolymer structure. The water diffusion has already been studied in the several literatures
principally considering the specific interactions between the water molecules and the polar groupsof the networks. The chemical structure is then a fundamental parameter, and in that way, thepolar groups concentration and the composition of the reactive system are the main parameters.
However, to guarantee the safety of adhesives, we have to ensure the mechanical strength between
adhesive and substrates. In this situation, concentration of water molecules to adhesive / substrateinterface should be important. Recently, we demonstrated a simple but efficient strategy forcreating ultrathin protective coating for versatile and practically useful structural metal/alloy
substrates. We also revealed the interfacial aggregation structure of ultrathin protective coating byusing neutron reflectivity (NR, SOFIA at BL-16). (RSC Advances 5, 15977 (2015).) Then, in thisstudy, to promote our research to the next stage, we apply one of the most depth sensitivetechniques, NR, to adhesive layer so that we can give a better understanding of relation between
adhesion and interfacial structure. Additionally, to investigate the interfacial structure using bulk
sate is very important in the field of adhesion. Here, we have established a method to clearly
evaluate the structural change of the pseudo bulk state adhesive and the adherend interface so far
thanks to past and present progressive approved proposals by MLF. However, the interfacialconcentration of water molecules could not be sufficiently studied. Therefore, we evaluated theinterfacial aggregation structure between metal adherend and bulk epoxy adhesive by NR.
2. Experiment
In our experiment, we found that the 60-nm metal (Copper) layer is effective to elucidate theinterfacial aggregation structure between adhesive and metal, therefore “Adhesive / Metal(Cu, Al,Au) / Substrate(Si)” geometries are prepared. The existence of this Copper layer enables us to detect
the interfacial information no matter how thick the adhesive layer is. As adherend layers, Al andCu are used. In case of Cu adherend, Cu is directly deposited onto thick silicon blocks with a size of
40 × 40 × 7 mm. On the other hand, in case of Al adherend, Al is deposited onto thick silicon blocks
with Cu surface layer. Figure 1 shows the variety of multi-layer substrates. The thickness of the1metal layer is controlled to be approximately ca. 60 nm
which is appropriate for NR measurements. Then, as(a)adhesives, epoxy films are coated on the top of metal

Cu 60nm

Si reference

Cu 60nm + Al 40nm

Au 60nmlayers mentioned above. To evaluate the intrinsic
interfacial structures of adhesive/metal system, samples
are aged in deuterium oxide (D 2 O) for a given
temperature and time, which is sufficiently enough toinduce contrast change by sorption of liquids. Density
surface in contact with D 2 O are examined by NR.
3. Results
Figure 2 shows the neutron reflectivity curves for
epoxy/Cu/Si

andepoxy resin was thicker than 50 µm, clear fringes wereobserved in the reflectivity curves (blue and redsymbols). Furthermore, since the shape of the fringes
are clearly different from the case of the Cu/Si system, it

epoxy/Cu/Si and D 2 O/epoxy/Cu/Si are compared, theshape of the reflectivity curve also changed. This means
that, by bringing the adhesive layer of the epoxy resin
into contact with deuterated water, water molecules

0.5

q / nm-1

1.0

1.5(c)
6

Cu layer

4Al layer

2Si0

can be said that the data reflects the aggregation
structure of the epoxy resin layer. Even when

10-4
10-6
0

D2O/epoxy/Cu/Si systems,respectively. Despite the fact that the thickness of theexperimental data
fitted curve10-2(b/V)•104 / nm-2

Cu/Si,(b)100Reflectivity

profiles of the films along the direction normal to the020 40 60 80 100 120
Distance from surface / nm

Figure 2. (a) Photo images for metal
covered Si wafers. (b) Neutron reflectivityfor (Al/Cu) bilayer on Si wafer. Open symbols
depict experimental data, and solid line isreflectivity calculated based on the model
scattering length density profiles in panel (c).

diffuse to the vicinity of the interface with the Cu
covered substrate. Moreover, in the system contact with

100

Cu/Si
Epoxy/Cu/Si
D2O/Epoxy/Cu/Si

water, the fringe in the high q region became unclear
thick film, it is considered that water molecules
concentrate near the interface with metal and thedensity profile of the epoxy adhesive gradually changed.

Reflectivity

compared with dry environment. Therefore, despite a

10-2

10-4

4. Conclusion
It is considered that the aggregated structure of
thick adhesive layer near the interface with metal could
be clarified based on the NR measurement. As a result,
it is possible to accurately evaluate the effect of watercontent under actual conditions. We are currently
conducting more precise analysis, and if additional
experiments are needed we are going to apply for next

10-600.5

q / nm-1

1.0

1.5

Figure 2. Neutron reflectivity for Cu layer,
epoxy/Cu on Si wafer. Open symbols depict
experimental data, and blue symbols measure inD2O.

machine time and we are going to publish more details in the near future.
2