**3.4. Local deformation observed by MZI in polymer blends undergoing phase separation in the bulk state**

So far, Mach‐Zehnder interferometry has been utilized to monitor the local deformation in homopolymers and miscible polymer blends under photocuring. For cured polymer mixtures, phase separation took place as the reaction yield exceeds a critical value. The shrinkage induced by curing reaction not only affects the shape of the blend but also influences the phase separa‐ tion process. For the polymer mixtures with a lower critical solution temperature (LCST) like PS/PVME, cross‐linking the PS component will enlarge the unstable region of the mixture and eventually lead to phase separation. The shrinkage of the mixture reveals some unexpected behavior shown in **Figure 9**, as an example, for a PS/PVME (20/80) blend photo‐cross‐linked by irradiation with 365 nm UV light. Here, the peak of the scattering intensity in situ monitored during the curing process appears and gradually moves toward the side of *wide angle* (*large wavenumber q*), suggesting that the length scale of the bi‐continuous structures resulting from the phase separation gradually decreases, instead of increase as in many cases observed for the conventional phase separation process [20]. Taking into account that polymers often undergo shrinkage upon curing, the deformation of a PS/PVME (20/80) blend was in situ monitored by using a Mach‐Zehnder interferometry under irradiation with the same conditions. The period *ξ* of these spinodal structures induced by photocuring was calculated using the Bragg condition *ξ* = 2*π*/*q*max, where *q*max is the wavenumber corresponding to the scattering peak of the intensity

**Figure 9.** Laser scattering profile obtained for a PS/PVME (20/80) blend photo‐cross‐link by irradiation with 365 nm UV light. The data were recorded in situ under irradiation. The number in the figure indicates the curing time [21].

distribution shown in **Figure 9**. The plot of the characteristic length scale *ξ* of the morphology versus the net time of curing clearly shows that *ξ* decreases with increasing curing (irradiation) time. This result is opposite to the conventional phase separation in the absence of shrink‐ age, that is, *ξ* increases as phase separation proceeds. Obviously, as curing time increases, the characteristic length scale gradually decreases and finally reaches a stationary value, indicating the termination of the phase separation process. From the results obtained by Mach‐Zehnder interferometry, the elastic strain defined as (Δ*d*/*d*) was calculated and its dependence on cur‐ ing time was examined. It was found that the elastic strain (Δ*d*/*d*) increases with increasing the irradiation intensity and significantly deviates from these decay curves when phase separation starts. The initial slope of the plot (Δ*d*/*d*) vs. irradiation time, that is (*τ<sup>i</sup>* = (d (Δ*d*/*d* ) )/*dt*) can be used as a measure of the characteristic time of shrinkage process induced by the curing reaction. The rate of shrinkage *k*<sup>i</sup> can be defined as *ki* = (1/τ*<sup>i</sup>* ). **Figure 10** depicts the clear cor‐ relation between the shrinkage obtained from Mach‐Zehnder interferometry and the evolution of morphology detected by light scattering, demonstrating again the effectiveness of the Mach‐ Zehnder interferometry.

A clear correlation was observed between the apparent rate of the phase separation *<sup>k</sup> <sup>ξ</sup>* obtained from light scattering experiment and the apparent rate of shrinkage *<sup>k</sup> <sup>i</sup>* obtained from MZI experiments [20]. The results also reveal the significant roles of Mach‐Zehnder interferometry in the kinetic studies on polymer phase separation.

Applications of Mach-Zehnder Interferometry to Studies on Local Deformation of Polymers Under Photocuring http://dx.doi.org/10.5772/64611 37

**Figure 10.** Correlation between the rate of shrinkage caused by photocuring reaction and the apparent rate of phase separation observed for a PS/PVME (20/80) blend photocured with 365 nm UV at 110°C.

Besides the applications of MZI to the polymer researches described above, the effect of poly‐ mer molecular weight on the deformation of poly(ethyl acrylate) (PEA) was recently inves‐ tigated during the photocuring process. The effects of the entanglement molecular weight of PEA on the shrinkage and swelling process were observed by MZI and the results are discussed in terms of polymer diffusion in entangled polymer networks [21].
