Low-temperature direct bonding of poly-methylmethacrylate (PMMA) plates was achieved by pre-treatment with vacuum ultraviolet irradiation in the presence of oxygen gas (VUV/O3), with vacuum ultraviolet irradiation in a nitrogen atmosphere (VUV), or with oxygen plasma. Based on surface analysis by attenuated total reflection Fourier-transform infrared spectrometry (ATR-FT-IR), x-ray photoelectron spectroscopy and near edge x-ray absorption fine structure spectroscopy, chemical changes of the PMMA surface after the pre-treatment were investigated. Changes in morphology and softening point were also investigated by nano-thermal analysis and atomic force microscopy. From the results, the bonding mechanisms proposed are described as follows: during the bonding process, the increased or generated polar groups cause an increase in the dipolar interactions (such as hydrogen bonding) between two pre-treated PMMA surfaces; in the case of VUV/O3 or VUV pre-treatment, a low-T g layer is generated on the surface and this layer acts as an adhesive for the direct bonding of layers by diffusion.
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