**3.2. In situ observation of the deformation kinetics in homopolymers undergoing photocuring and relation to physical aging of the photocured polymer**

Poly(ethyl acrylate) (PEA, *M*w = 1.6 × 10<sup>5</sup> , *M*w/*M*<sup>n</sup> = 2.2) was prepared by conventional free radical polymerization. To be able to cure PEA with UV irradiation, the PEA was chemically labeled with anthracene which served as a cross‐linker of the PEA chains as illustrated in **Figure 5**. Upon irradiation with 365 nm UV light, the anthracene moieties labeled on PEA undergoing photo‐ dimerization, generating PEA networks in the sample. As a consequence, the sample gradually approaches the glassy state and exhibits shrinkage due to the liquid→solid transition. However, this shrinkage in this particular case is fairly small and could not be observed via monitoring the change in the sample thickness by laser‐scanning confocal microscopy as in the case of photo‐ polymerization [15]. As the cross‐link density in the sample reaches a critical value, PEA enters the glassy state. Depending on the rate at which PEA enters the glassy state, physical aging [16] could occur. This feature can be observed via the irradiation intensity dependence of the shrink‐ age associated with irradiation time as shown in **Figure 6**. Here, the deformation *ε* which was calculated from the OPLD data given in Eq. (13) for the case of negligible change in refractive index is plotted versus irradiation time and elapse time. It is worth noting that the physical aging phenomena are evidenced by the continuation of shrinkage after stopping irradiation. These results suggest that the sample with the cross‐link density *γ* ~ 2 already enters the glassy state during irradiation, exhibiting the physical aging phenomena. Compared to the result obtained at low light intensity, it was found that the physical aging of photo‐cross‐linked PEA sample emerges at the cross‐link density *γ* ≥ 2 junctions /chain. This aging process becomes stronger under irradiation with higher light intensity. From the plot of normalized shrinkage (ε/*ε*max ) vs. non‐dimensionalized elapse time defined as (*t* <sup>e</sup> . *k*<sup>a</sup> ) where *t* <sup>e</sup> is the elapse time and *k*<sup>a</sup> is the char‐ acteristic time of the aging process, it was found that all the aging data obtained with a constant irradiation intensity can be expressed by a master curve [17].
