Vitrimers hold great promise as adaptive materials capable of shape reconfigurability, welding, and self-healing due to dynamic covalent reactions occurring above the vitrimer transition temperature (T v ). Previous literature reports the T v as one value influenced mainly by chemistry; however, literature also reports significant inconsistencies when measuring or identifying T v trends. Herein, we present unique data interpretation methods to analyze stress−relaxation and elongational creep results allowing for excellent agreement between multiple T v measurement methodologies. We also demonstrate that experimental parameters (e.g., heating rate and applied axial force) and catalyst concentration are crucial in dictating the T v range. Varying the catalyst concentration or sample heating rate shifts the T v up to 115 and 43 °C, respectively. Additionally, we present a kinetic model confirming the temperature dependence of the transesterification rate-limiting step, exhibiting excellent agreement with experimental data. Fundamentally understanding the T v will inform future design of vitrimers toward applications ranging from recyclable actuators to structural adhesives.

Cell transplantation by injection of biodegradable hydrogels is a recently developed strategy for the treatment of degenerated tissues. A cell carrier should be cytocompatible, have suitable working time and rheological properties for injection, and harden in situ to attain dimensional stability and the desired mechanical strength. Hydrophilic macromer/cross-linker polymerizing systems, due to the relatively high molecular weight of the macromer and its inability to cross the cell membrane, are very attractive as injectable cell carriers. The objective of this research was to determine the effects of cross-linker, initiator, and accelerator concentrations on the gelation kinetics and ultimate modulus of a biodegradable, in situ cross-linkable poly(lactide-co-ethylene oxide-co-fumarate) (PLEOF) macromer. The in situ polymerizing mixture consisted of PLEOF macromer, methylene bisacrylamide cross-linker, and a neutral redox initiation system of ammonium persulfate initiator and tetramethylethylenediamine accelerator. Measurement of the time evolution of the viscoelastic properties of the network during the sol-gel transition showed the important influence of each component on the gel time and stiffness of the hydrogels. A kinetic model was developed to predict the modulus as a function of composition. Model predictions were consistent with most of the experimental findings. The values of the storage and loss moduli at the gel point were found to be approximately equal for samples with equal PLEOF concentrations, resulting in a simple method to predict the gelation time based on the Winter--Chambon criterion, with the use of the proposed kinetic model. The results of this study can be coupled with component cytocompatibility measurements to predict the effect of composition on the viability of the cells encapsulated in the hydrogel matrix.

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