Macromolecules ARTICLE Figure 3. (a) PEG trimethylbenzyl thioester. One possible product from radical degradation of PEG-disulﬁde networks where the radical initiator fragment reacts with and cleaves the disulﬁde bridge. (b) 31P NMR spectrum of degraded hydrogel. Peaks are consistent with phosphonothioic acid ester (34 ppm), LAP (13.4 ppm), and hypophosphonate (12.7 ppm) groups. Figure 4. Mechanism of radical degradation and disulﬁde exchange within PEG hydrogels. Hash marks represent connections to the network. (a) Upon cleavage of the acyl phosphinate, a radical initiator fragment (in this case the phosphonate radical is shown) attacks a disulﬁde bond, transferring an electron to one of the two sulfurs generating a thiyl radical and a phosphonothioic acid ester-terminated PEG chain. (b) A thiyl radical transfers to another sulfur by fragmenta- tion and degradative chain transfer. The radical disulﬁde exchange minimizes the free energy of the system, allowing it to accommodate an applied stress on the network. (c) Reversible termination occurs by thiyl radical recombination, re-forming the disulﬁde bond. The degradation of the network by an initiator fragment by chain transfer necessarily results in materials with decreased cross-link density as those sulfurs must no longer participate in disulﬁde bonds. It should be noted, however, that both the benzyl thioester group and phosphonothioic acid ester have been shown to cleave in alkaline and acidic conditions, regenerating a thiol.28 30 Thus, the radial scission and initiator fragment substitution may be considered a temporary protection of thiol groups. For photodegradable materials, the results presented in Figure 2 indicate that the extent of network degradation can be precisely controlled by the concentration of photoinitiator infused into the gel. This level of control can be used to create photopatterned physical features, either by complete degradation of the network in exposed regions or by incomplete degradation of the network, which, when placed in aqueous buﬀer, results in increased swelling of the material in exposed regions due to a decreased cross-linking density. Figure 5a shows the scheme by which this procedure was performed using a transparency-based photo- mask. Noteworthy is that the pattern is transferred to both sides of the hydrogel. Proﬁlometry was used to conﬁrm the presence of the elevated features (broad, exposed lines) and to measure their height, which was ∼100 μm. Here, the increase in feature height results from the partial degradation of the cross-linked polymer structure, reducing the cross-linking density and increasing the local equilibrium degree of swelling. Another way in which topographical features may be created on a disulﬁde cross-linked hydrogel is by taking an impression of a textured surface and employing photorelaxation for negative pattern transfer. For initiator concentrations well below the cross-link density of the network, the primary reaction will be one of adaptation and rearrangement rather than degradation. If a stress is applied to such a network, during exposure the disulﬁdes will rearrange to minimize the free energy and alleviate the stress. If that stress is accompanied by a strain on the materials, some degree of that strain will be preserved in the regions exposed to light where relaxation and even ﬂow are enabled. Figure 6 illustrates this general principle showing the transfer of a pattern with concentric rings when a prefabricated gel is placed on a template mold and exposed to light. Speciﬁcally, Figure 6c is a photograph of a disulﬁde cross-linked hydrogel, immersed in 0.1 wt % LAP initiator solution, that was placed on top of a surface of raised concentric rings. A normal force corresponding to the weight of two glass microscope slides (5 g) was placed on top of the gel (surface area = 0.50 cm2), and the gel was exposed to 10 mW/cm2 light centered at 365 nm. As the disulﬁde rearranged, the gel adopted the negative of the topographical features against which it was pressed. Application of a strain followed by photopatterned exposure can also result in the transfer of topographical features via 2447 dx.doi.org/10.1021/ma200202w |Macromolecules 2011, 44, 2444–2450