Controlling the nano-deformation of polymer by a reversible photo-cross-linking reaction

The irradiation-time dependence of the optical absorbance OD(t_irr) was observed for a PEA-A film at 365 nm during the modulated irradiation process. Figure 1(a) shows the change in the normalized optical absorbance OD_N(t_irr) versus experimental time under three irradiation intensities I  =  1, 2 and 3 mW cm⁻². Here, the experimental time is defined as the sum of ON and OFF-time, i.e. the total elapse time of the experiment. It was found that the photodimerization kinetics of anthracene can be well fitted to the modified Kohlrausch–Williams–Watts (KWW) stretched exponential function [22] during the irradiation time interval:

$$\begin{eqnarray}&&\text{O}{{\text{D}}_{\text{N}}}\left({{t}_{\text{irr}}}\right)=(1-D)\exp \left[-{{\left({{k}_{c}}{{t}_{\text{irr}}}\right)}^{\beta}}\right]+D.\end{eqnarray} \tag{ 3 }$$

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Here OD_N(t_irr) is the normalized absorbance defined as OD(t_irr)/OD₀ where OD₀ is the initial absorbance of the sample. It should be noted that OD₀ is less than unity for all the samples used in this experiment. D is the baseline of the decay, expressing the limitation of the crosslink reaction under the experimental conditions using 365 nm light. k_c and β are respectively the average reaction rate and the inhomogeneity index of the cross-link process.

On the other hand, it is worth noting that the optical absorbance did not change during the OFF time intervals. The crosslink density generated during the photopolymerization was calculated by using the following equation:

$$\begin{eqnarray}&&\gamma \left({{t}_{\text{irr}}}\right)=\frac{\alpha}{2}\frac{\text{O}{{\text{D}}_{0}}-\text{OD}\left({{t}_{\text{irr}}}\right)}{\text{O}{{\text{D}}_{0}}}\centerdot \end{eqnarray} \tag{ 4 }$$

In the above equation, the numerical factor 1/2 on the right hand side arises from the fact that two anthracenes moieties form one crosslink function by the photodimerization. During the irradiation time interval, the time-evolution of crosslink density γ(t_irr) was found to be also well fitted to the modified KWW stretched exponential function [22]. Furthermore, the crosslink density was unchanged during the OFF-time interval.

On the other hand, as shown in figure 1(b), the crosslink density $\gamma$ increases during the irradiation-time intervals and remains constant during the OFF time, indicating that the PEA-A sample is apparently under equilibrium when the light is turned off. Since the rise of cross-linking density γ with irradiation time is limited by the ON–OFF irradiation time, the full rising process cannot be analyzed by fitting to a model function. As a consequence, the initial slope defined as ${{k}_{\text{c}\left(\text{ON}\right)}}\equiv \text{d}\gamma \left({{t}_{\text{irr}}}\right)/\text{d}{{t}_{\text{irr}}}$ at t_irr  →  0 was used as a measure for the cross-link rate of each irradiation cycle.

In addition, figure 2 shows that the crosslink rates obtained for two successive irradiation cycles increases with increasing the irradiation intensity. The results are consistent with the data obtained for a PEA-A under continuous irradiation with three intensities 1, 2 and 3 mW cm⁻² as illustrated in figure 3. For further understanding, the cross-linking kinetics of a PEA-A under ON–OFF modulated irradiation was compared with those monitored under continuous irradiation. It was found that the OFF-time interval does not modify the absorbance and the crosslink density of the sample. The normalized absorbance $\text{O}{{\text{D}}_{\text{N}}}$ and the cross-link density $\gamma$ are respectively replotted versus the irradiation time t_irr in figure 3. Obviously, both the reaction yield and the cross-link density increases with increasing the irradiation intensity.

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For comparison, figure 4 shows the change in the normalized absorbance of anthracene OD_N (figure 4(a)) and in the crosslink density γ (figure 4(b)) under continuous and ON–OFF modulation versus irradiation time. It was found that the behavior of the reaction kinetics corresponding the ON–OFF irradiation was similar to that of the continuous irradiation. In addition, the dependence of the crosslink density on irradiation time could also be fitted to the modified KWW stretched exponential function [22]. It can be seen from figure 4 that the normalized optical density OD_N and the crosslink density γ obtained by the ON–OFF modulation was slightly deviated from those obtained under continuous irradiation. Probably for such stimuli (reaction) and response (deformation) systems, the difference in the magnitude of the deviation observed for these two cases (continuous versus modulated) could be observed due to the deformation of the samples occurring during the cross-linking reaction. The details will be discussed in the following section.

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For our curing conditions, it was found from the experiments with prism coupler that the change in refractive-index associated with photodimerization of anthracene in PEA matrix is negligibly small (Δn  ⩽  5  ×  10⁻⁴). As an approximation, the strain ε is directly proportional to the change in the optical path length difference (OPLD) [19]

$$\begin{eqnarray}&&\varepsilon =\frac{ \Delta d}{{{d}_{0}}}=\frac{\text{OPLD}}{(n-1){{d}_{0}}}\centerdot \end{eqnarray} \tag{ 5 }$$

From the negligible change in refractive index before and after the photo-crosslinking reaction, the optical path length difference can be approximately expressed by the following equation:

$$\begin{eqnarray}&&\text{OPLD}\cong \Delta d(n-1),\end{eqnarray} \tag{ 6 }$$

where

$$\begin{eqnarray} &&\Delta d=d-{{d}_{0}}.\end{eqnarray} \tag{ 7 }$$

Here, d₀ and d are respectively the thickness of the blend before and after the photo-crosslink reaction. On the other hand, n and 1 are respectively the refractive index of the sample and of the air.

For comparison, the time-evolution of strain (shrinkage) is plotted versus the experimental time t_exp in figures 5(a) and (b). It is worth noting that t_exp here is defined as the total of the ON- and OFF-time of the experiment. As a PEA-A film was irradiated with the intensity I  =  2 mW cm⁻², two different relaxation behaviors were observed during the OFF-time interval. As illustrated in figure 5(a), a PEA-A film gradually shrinks with irradiation time upon irradiation. After irradiation over 60 min with the intensity 2 mW cm⁻², the sample reveals a crosslink density of γ  =  2.4 (junctions/chain). Subsequently, the excitation light was turned off and the shrinkage of the PEA-A film was continuously monitored during 80 min in the dark. It was found that the deformation of the PEA-A sample was not changed during this dark period as shown in figure 5(a). In the second irradiation cycle under the same conditions, the sample exhibits shrinkage again during 60 min of additional irradiation. However, unlike the first photo-cross-link step, the cross-linked sample continues shrinking during 80 min of OFF-time in the dark.

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After 60 min of irradiation of the second irradiation cycle, the crosslink density was γ  =  3.0 (junctions/chain) was built into the sample. On the other hand, under higher light intensity I  =  3 mW cm⁻², the sample continued shrinking in the dark corresponding to the OFF-time interval of the first cycle. The crosslink density generated after the first and the second irradiation cycles under this light intensity is ca. γ  =  3.0 and γ  =  3.4 (junctions/chain), respectively. Compared to the experimental data obtained under similar conditions for the same PEA-A sample reported previously, the PEA-A sample already enters the glassy state after the first cycle of irradiation under this particular light intensity, revealing the typical behavior of amorphous polymers undergoing physical aging induced by photo-crosslinking reaction [20].

Similar to the temporal modulation experiments using the ON- and OFF-time intervals, the photodimerization process of anthracene, i.e. the photo-cross-linking reaction, driven by two different wavelengths 365 and 298 nm for a PEA-A film was examined by following the irradiation time-dependence of the absorbance of anthracene.

As illustrated in figure 6(a), upon irradiation with 365 nm UV light, the normalized absorption OD_N decreases with irradiation time, indicating that anthracene groups attached on PEA chain undergoes photodimerization, resulting in the PEA-A networks.

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After 60 min of irradiation, the exciting light was switched from 365 nm to 297 nm to induce the photodissociation of the anthracene photodimers. It was found that the normalized OD_N rises up due to the regeneration of anthracene monomers in the sample. As shown in figure 6(a), similar recovery process was observed as the sample was irradiated with several irradiation intensities ranging from 1.0 to 3.0 mW cm⁻². It should be noted that since the same Hg–Xe light source was used for irradiation, the intensity ratio at the two wavelengths 365 nm and 297 nm was determined by the wavelength distribution of the light source spectra. Namely, once the intensity at 365 nm was determined, the intensity at 297 nm is automatically obtained from the intensity sdistribution of the Hg–Xe lamp. As shown in figure 7, upon irradiation with 365 nm, stronger intensity leads to higher crosslink rate, i.e. higher rate of networking. Similarly, the stronger 297 nm intensity results in the higher de-crosslink rate, i.e. higher rate of the network dissociation. It is worth noting that the rate of the reactions was estimated by using the initial slope of the changes in absorbance of anthracene with irradiation time as previously described for the data shown in figure 2. From these experiments, it was found that the rate of crosslink and de-crosslink reactions increases with increasing the irradiation intensity. During the reaction, the residual anthracene monomer decreases with time, leading to a decrease in mobility of the system because of the network formation. As a result, the reaction rate in the second cycle is slower compared to the first one. For example, under irradiation with the intensity I_{365 nm}  =  3.0 mW cm⁻², the crosslink rate in the second cycle is about 15% compared to that in the first one.

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The effects of the reversible photodimerization of anthracene on the deformation process were in situ monitored by Mach–Zehnder interferometry for a PEA-A film at 25 °C. The dependence was observed for various irradiation intensities of the two different wavelengths, 365 nm and 297 nm of UV light. Figure 8 reveals a decrease in the thickness of a PEA-A film caused by the photodimerization of anthracene under two sets of light intensity. In the ON–OFF modulation experiments mentioned above, the sample could continue shrinking after the cross-linking reaction was terminated. This particular behavior is observed when the cross-linked PEA-A enters the glassy state due to the physical aging phenomena. For the two sets of light intensities at two wavelengths 365 nm and 297 nm, the driven shrinkage/swelling behavior is different at the initial stage of irradiation. However, as the cross-link/decross-link cycle is repeated, the repeating shrinkage/swelling behavior tends to approach to each other and merge into one at long experimental time. This particular behavior is determined by the equilibrium of the photodimerization and photodissociation (cross-link and de-cross-link reactions of anthracene). In other words, the regeneration of anthracene monomers in the sample led to the recovery in the thickness of a PEA-A film, i.e. the induced swelling. These experimental results shown here suggest that reversible dimerization of anthracene can be used to reversibly control the swelling and shrinking process of a polymer film in the nanometer scales by using light irradiation.

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