**3.3. Local deformation of miscible polymer blends under photocuring and relation to physical aging**

Mach‐Zehnder interferometry was also utilized to detect the local deformation in miscible polymer blends polystyrene derivative (PS) and poly(vinyl methyl ether) (PVME). The curing Applications of Mach-Zehnder Interferometry to Studies on Local Deformation of Polymers Under Photocuring http://dx.doi.org/10.5772/64611 33

Samples for MZI studies were obtained by solvent casting method and were dried under vac‐ uum at least one night. All the samples PS/PVME mixtures with the dimension (20 mm × 20 mm × 10 μm) were annealed under vacuum over 2 h at temperature above the glass transition tem‐

) of the blend to erase the thermal history of the preparation process.

To photo‐cross‐link a polymer with UV irradiation, photosensitive anthracene was chemically labeled on a given polymer by copolymerizing its monomer with a photoreactive monomer by copolymerization. By doing so, photoreactive anthracene moieties were introduced into the polymer component under examination. The labeling content of anthracene can be adjusted by varying the ingredients of the coupling reactions. Upon irradiation with 365 nm UV light, anthracene undergoes photodimerization as illustrated in **Figure 5** for the case of PEA chains.

**3.2. In situ observation of the deformation kinetics in homopolymers undergoing** 

, *M*w/*M*<sup>n</sup>

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.

<sup>e</sup> . *k*<sup>a</sup>

acteristic time of the aging process, it was found that all the aging data obtained with a constant

Mach‐Zehnder interferometry was also utilized to detect the local deformation in miscible polymer blends polystyrene derivative (PS) and poly(vinyl methyl ether) (PVME). The curing

**3.3. Local deformation of miscible polymer blends under photocuring and relation to** 

) where *t*

= 2.2) was prepared by conventional free radical

<sup>e</sup> is the elapse time and *k*<sup>a</sup>

is the char‐

**photocuring and relation to physical aging of the photocured polymer**

Poly(ethyl acrylate) (PEA, *M*w = 1.6 × 10<sup>5</sup>

non‐dimensionalized elapse time defined as (*t*

**physical aging**

irradiation intensity can be expressed by a master curve [17].

**3.1. Photodimerization of anthracene as a photocuring reaction**

perature (*T*<sup>g</sup>

32 Optical Interferometry

**Figure 5.** Photodimerization of anthracene chemically labeled on poly(ethyl acrylate): (a) before photodimerization, (b) after photodimerization with the formation of photodimer between two segments of PEA.

**Figure 6.** Strain relaxation observed for a PEA sample under curing at different irradiation conditions and the evidence of physical aging phenomena.

reaction was performed taking advantages of the photodimerization of anthracene chemi‐ cally labeled on the PS chains. The curing reaction was followed by monitoring the change in the absorbance of the photo‐cross‐linker chemically labeled on PS. It was found that both the curing kinetics and the deformation induced by the curing reaction can be described by the Kohlrausch‐Williams‐Watts (KWW) kinetics for kinetically inhomogeneous systems [18]:

$$\gamma(t) = A \left[ 1 - \exp\left\{-\left.k\_c \, t\right\}^a \right] \tag{17}$$

$$\varepsilon(t) = A \left[ 1 - \exp\left\{-\left.k\_d \ t\right\}^\beta \right] \tag{18}$$

where A and B are constant, *k* c and *k* 0 are, respectively, rate constant of the reaction and shrinkage. The KWW exponent *α* and *β* are less than 1 and in between 0.7 and 0.8.

From the kinetics data expressed by Eqs. (17) and (18), *k* d c can be obtained from MZI data and *k* c can be deduced from the cross‐linking data. It was found that there exists a strong correla‐ tion between the curing and the shrinkage processes as illustrated in **Figure 7** [19].

However, the correlation between the cross‐link process expressed by the reduced cross‐link density *γ*<sup>r</sup> <sup>≡</sup> (*γ*/*γ*max ) and the deformation process indicated by the reduced strain *ε*<sup>r</sup> <sup>≡</sup> (*ε*/*ε*max ) cannot be well expressible by a master curve, particularly at high cross‐link density under high light intensity as shown in **Figure 8**. It is worth noting that *γ*max and *ε*max are the maximum value at which the cross‐link density and the deformation can be achieved at a given light intensity. The increase in the concentration fluctuations in the blends under curing, particu‐ larly under high light intensity, would be responsible for the deviation from the master curve.

From the data obtained by MZI, it was also found that the glass transition temperature also plays an important role in the deformation of the cured PS/PVME blends. From the in situ measurements of deformation by MZI under curing, it was found that the photocured sample

**Figure 7.** The correlation between the curing reaction kinetics expressed by *k* c and the deformation process revealed by *k* d observed for a PS/PVME (30/70) blend.

Applications of Mach-Zehnder Interferometry to Studies on Local Deformation of Polymers Under Photocuring http://dx.doi.org/10.5772/64611 35

the curing kinetics and the deformation induced by the curing reaction can be described by the Kohlrausch‐Williams‐Watts (KWW) kinetics for kinetically inhomogeneous systems [18]:

can be deduced from the cross‐linking data. It was found that there exists a strong correla‐

However, the correlation between the cross‐link process expressed by the reduced cross‐link density *γ*<sup>r</sup> <sup>≡</sup> (*γ*/*γ*max ) and the deformation process indicated by the reduced strain *ε*<sup>r</sup> <sup>≡</sup> (*ε*/*ε*max ) cannot be well expressible by a master curve, particularly at high cross‐link density under high light intensity as shown in **Figure 8**. It is worth noting that *γ*max and *ε*max are the maximum value at which the cross‐link density and the deformation can be achieved at a given light intensity. The increase in the concentration fluctuations in the blends under curing, particu‐ larly under high light intensity, would be responsible for the deviation from the master curve. From the data obtained by MZI, it was also found that the glass transition temperature also plays an important role in the deformation of the cured PS/PVME blends. From the in situ measurements of deformation by MZI under curing, it was found that the photocured sample

**Figure 7.** The correlation between the curing reaction kinetics expressed by *k* c and the deformation process revealed by

tion between the curing and the shrinkage processes as illustrated in **Figure 7** [19].

*α*

} *β*

d

are, respectively, rate constant of the reaction and shrinkage.

] (17)

] (18)

c can be obtained from MZI data and

*γ*(*t*) = *A* [1 − exp {− *k*<sup>c</sup> *t*}

ε(*t*) = *A* [1 − exp {− *k*<sup>d</sup> *t*

c and *k* 0

From the kinetics data expressed by Eqs. (17) and (18), *k*

The KWW exponent *α* and *β* are less than 1 and in between 0.7 and 0.8.

where A and B are constant, *k*

34 Optical Interferometry

*k* d observed for a PS/PVME (30/70) blend.

*k* c

**Figure 8.** Correlation between the reduced strain and the reduced cross‐link density obtained for a PS/PVME (30/70) irradiated with different light intensity ranging from 0.1 to 5.0 mW/cm<sup>2</sup> .

undergoes shrinking during the irradiation process, but the sample also partially recovered by swelling back after stopping irradiation. This process is determined by the difference between the experimental temperature and the resulting glass transition temperature *T*<sup>g</sup> of the cured sample at the time of stopping irradiation. This particular swelling behavior was observed upon raising the experimental temperature [19].
