Impact of synthetic technique on PLA-PEO-PLA physical hydrogel properties

Previous work has shown that the stiffness of poly(lactide)-b-poly(ethylene oxide)-b-poly(lactide) [PLA-PEO-PLA] hydrogels can be influenced by crystallinity. Those hydrogels with crystalline PLLA end blocks had a higher storage modulus (up to 1 order of magnitude) than the amorphous equivalents. All the previous work was done with polymers synthesized in the bulk. This paper reports the difference in mechanical properties when two different synthetic techniques are used - bulk and solution synthesis. Solution-synthesized polymers consistently formed stiffer hydrogels than bulk-synthesized polymers. Further investigation determined the following: crystalline polymers from solution synthesis still form stiffer gels than the amorphous analogues, but not to the extent previously reported; the solution synthesized polymers have narrower distributions, but this alone does not account for the mechanical differences. However, the presence of asymmetric triblock copolymers, which act like an effective diblock copolymer, within the bulk-synthesized materials appears to lower the overall stiffness of the gel. The impact on modulus is much larger for amorphous PLA than for crystalline PLLA end block materials. These findings suggest bulk-synthesized polymers likely have more asymmetric triblock copolymers, that decrease the relaxation time of the system, possibly by lowering the junction lifetime, or lead to dangling ends in the network, which cause a loss in mechanical properties when compared to solution-synthesized polymers.