## 3 Results and Discussion
Such improved adhesion characteristics of cemented glasses were also observed after curing and storing as described in detail in Table 1. Figure 5 shows a comparison of the measured adhesive pull strengths (APS) of cemented and differently stored samples before and after plasma treatment. Figure 4: Impact of plasma treatment of 10 s on the contact angle of UV-curable optical cement (error bars represent standard deviation of five measurements). It can be seen that for the samples without any artificial ageing, the APS was significantly increased by a factor of approximately 4.6 (from 0.79 MPa before to 3.63 MPa after plasma treatment). This trend, i.e., an improved adhesion due to the plasma treatment, is also observed for aged samples: After pure tempering as described in Table 1, the APS of the reference samples was 1.47 MPa. This value was increased to 4.34 MPa by the plasma treatment, corresponding to a factor of about 2.9. It thus turns out that again, the adhesion of the cement was improved whereas the initial APS value of the non-treated reference sample is notably higher in comparison to the reference samples without any storing or ageing. This also applies to the reference samples that were tempered and additionally placed in a water bath. This effect can be explained by the fact that the used cement is not only completely cured by the UV-irradiation, but needs some further relaxing time. Here, a certain heating as initially induced by tempering and heating the water bath as performed in the present case is an advantage since an increase in temperature supports the polymerisation process. However, in comparison to the samples without any ageing, the applied long-term tempering procedure results in failure of the cementation. Periodical tempering and cooling contributes to cement delamination by inducing mechanical stress due to different coefficients of thermal expansion of the glasses and the cement [12]. An additional storing in water bath further comes along with diffusion of water and hydrogen into the cement and along the cement-glass interface, leading to decrementing [13]. However, even in this extreme case, which represents an extraordinary aggressive environment, plasma treatment has turned out to improve the stability of cementation. Without plasma treatment, an APS of 1.12 MPa was determined. For plasma treated samples, the APS was 2.31 MPa, ending up in a factor of approximately 2.1. The observed increase in APS in all cases can mainly be attributed to the plasma-induced improvement in wettability and adhesiveness of the cement on the glass surfaces. Such improvement is due to two major effects: First, surface cleaning and second, surface activation by attaching functional groups and an accompanying change in surface energy. It is known from literature that the surface tension of the used liquid cement before curing should be significantly smaller than the surface energy of the substrate, i.e., the glass surface [48]. As an estimation, the difference should at least amount to 10 mN/m. In the present case, a surface tension of the liquid cement of 45.96 mN/m was determined experimentally. The surface tension of the investigated glass was 56.22 mN/m before and 78.81 N mN/m after plasma treatment. Hence, the treatment increased the difference in surface energy from 10.26 mN/m (which is nearly the above-mentioned minimum value) to 32.85 mN/m. This corresponds to an increase in difference by a factor of approximately 3, explaining and confirming the notable plasma-induced enhancement of the cemented bond as described above. Figure 5: Comparison of the particular adhesive pull strengths of non-treated reference and plasma treated (for 10 s) samples after different storing procedures as described in detail in Table 1 (error bars represent standard deviation of five measurements).