Light-initiated additive manufacturing techniques typically rely on layer-by-layer addition or continuous extraction of polymers formed via nonliving, free radical polymerization methods that render the final materials “dead” toward further monomer insertion; the polymer chains within the materials cannot be reactivated to induce chain extension. An alternative “living additive manufacturing” strategy would involve the use of photocontrolled living radical polymerization to spatiotemporally insert monomers into dormant “parent” materials to generate more complex and diversely functionalized “daughter” materials. Here, we demonstrate a proof-of-concept study of living additive manufacturing using end-linked polymer gels embedded with trithiocarbonate iniferters that can be activated by photoinduced single-electron transfer from an organic photoredox catalyst in solution. This system enables the synthesis of a wide range of chemically and mechanically differentiated daughter gels from a single type of parent gel via light-controlled modification of the parent’s average composition, strand length, and/or cross-linking density. Daughter gels that are softer than their parent, stiffer than their parent, larger but with the same modulus as their parent, thermally responsive, polarity responsive, healable, and weldable are all realized.

Structurally tailored and engineered macromolecular (STEM) gels are polymer networks containing latent initiator sites available for postsynthesis modifications. The STEM gels presented here were synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization, and the network was modified by grafting soft poly­(n-butyl acrylate) (PBA) side chains via atom transfer radical polymerization (ATRP) to create supersoft materials. Modified STEM gels with low Young’s moduli (E = 590–220 kPa) were produced, and the mechanical properties were tunable by varying the grafting density and side chain length. Dissipative particle dynamics (DPD) simulations were used to gain insight into side chain mobility in the network. This approach was also used to made soft elastomers (E = 42 kPa), which could withstand 100% shear strain without permanent deformation. Using spatial control, single-piece materials with hard or soft domains were synthesized.

The efficient formulation of layered gels that encompass both hydrophilic and hydrophobic domains could lead to novel materials with a range of vital functionalities. To facilitate the fabrication of these materials, we perform dissipative particle dynamics (DPD) simulations to model the formation of two-layered stackable gels where the gels are incompatible and their respective solvents are immiscible. The bottom layer of the gel is created first and then a solution of new initiators, monomers and cross-linkers is introduced on top of this first layer. These components then undergo living copolymerization to form the second gel layer. We investigate all possible combinations using free radical polymerization (FRP) and controlled/living atom transfer radical polymerization (ATRP) to form the two

Crystalline solids undergo plastic deformation and subsequently flow when subjected to stresses beyond their elastic limit. In nature most crystalline solids exist in polycrystalline form. Simulating plastic flows in polycrystalline solids has wide ranging applications, from material processing to understanding intermittency of earthquake dynamics. Using phase field crystal (PFC) model we show that in sheared polycrystalline solids the atomic displacement field shows spatio-temporal heterogeneity spanning over several orders of length and time scales, similar to that in amorphous solids. The displacement field also exhibits localized quadrupolar patterns, characteristic of two dislocations of the opposite sign approaching each other. This is a signature of crystallinity at microscopic scale. Polycrystals being halfway between single crystals and amorphous solids, in terms of the degree of structural order, descriptions of solid mechanics at two widely different scales, namely continuum plastic flow and discrete dislocation dynamics turns out to be necessary here.

The structural and mechanical properties of gels can be controlled by promoting the unfolding (and refolding) of loops (stored lengths) embedded within the networks. As a loop unfolds, the released chain length can increase the extensibility and reconfigurability of the gel. Here, we develop a theoretical model that couples the elasticity of the gel to the dynamic transitions occurring in loops that lie between the crosslinks. Using this model, we show that a thermally-induced swelling of the gel generates an internal strain, which unfolds the loops and thereby further increases the degree of gel swelling. We exploit this cooperative behavior to reconfigure the gel by patterning the location of the loops within the sample. Through this approach, we convert flat, two-dimensional layers into three-dimensional forms and introduce architectural features into uniform 3D slabs. At a fixed temperature, an applied force produces analogous structural transformations. The shape-changes are reversible: the systems return to their original structure when the temperature is reset or the force is removed. The findings provide guidelines for creating materials that interconvert thermal, chemical and mechanical energy to perform work. Such systems could be useful for designing soft robotic materials that convert environmental stimuli into useful functionality.