Nanoscale Structures of Poly(oligo ethylene glycol methyl ether methacrylate) Hydrogels Revealed by Small-Angle Neutron Scattering

Significance 

Water-swollen hydrogels exhibit remarkable physical, mechanical and biocompatibility properties. In addition, they have a unique response to external stimuli, allowing the control of their volume, properties and functionality. Among the existing hydrogels, temperature-responsive hydrogels have been extensively studied. Such gels have drawn significant research attention as potential candidates for biomedical applications, including drug delivery, sensing and molecular separation.

Poly(oligo-ethylene glycol methyl ether methacrylate) (POEGMA) polymers are novel temperature-responsive polymers consisting of hydrophilic ethylene oxide side chains and hydrophobic methacrylate main chains. Copolymerization of OEGMA and diethylene glycol methacrylate is commonly used to prepare POEGMA-based chains. During polymerization, the copolymerization ratio can be adjusted to effectively control the volume transition temperature of the produced gels.

POEGMA-based materials have been extensively researched for potential advanced biomedical applications, which requires a thorough understanding of their physical and structural properties. However, the structures and physical properties, as well as the temperature-responsive behavior of cross-linked POEGMA polymers, remain unclear. Additionally, POEGMA-based gels have been confirmed to have a unique temperature-dependent microphase separation mechanism, although their nanostructures are largely underexplored. This can be partly attributed to the limitations of the current methods used to study POEGMA gels like dynamic light scattering measurements.

Herein, Professor Takuma Kureha, Mr. Masashi Ohira, Ms. Yuki Takahashi, Professor Xiang Li, Dr. Elliot Gilbert and Professor Mitsuhiro Shibayama applied elegant experimental studies using small-angle neutron scattering (SANS) to investigate the nanoscale structures of temperature responsive and biocompatible POEGMA hydrogels. The gels were copolymerized with two different monomers with different ethylene glycol side chain lengths: long side chain OEGMA and short side chain diethylene glycol methacrylate. Gels with different compositions were evaluated and the relationship between the obtained nanostructures and the swelling behaviors, rheological measurements and other obtained physical properties was discussed. The work is currently published in the research journal, Macromolecules.

The researchers showed that the temperature-responsive behavior of the gels was attributed to the nanoscale structures and was highly influenced by the copolymerization ratio. The SANS profiles of OEGMA-rich gels in the swollen state displayed a characteristic peak as illustrated by the TS model applicable to the bicontinuous structure. The peaks indicated a strong correlation with the hydrophobic main chain domains within the hydrophilic matrix characterized by phase separation. The OEGMA long hydrophilic side chains provided the much-needed cushioning between the domains, allowing domain distribution at desirable periodicity.

On the other hand, similar peaks were absent in the SANS profiles of the diethylene glycol methacrylate-rich gels, suggesting a random domain distribution in the gels attributed to the effects of hydrophobic segments with low LCST of about 20 °C. An increase in the temperature-induced hydrophobic interactions between the hydrated polymers resulting the disappearance of the domain periodicity in the SANS profiles. This phenomenon also led to an increase in the low-q intensities and a subsequent decrease in the correlations. The domain structure growth was further quantitatively verified through SANS intensity invariant.

In summary, the authors are the first to investigate the nanoscale structures of POEGMA hydrogels using SANS. Compared with other measurement methods, SANS measurement allows easy manipulation of the scattering contrast in the sample by producing hydrogel in deuterated solvents, thereby minimizing the incoherent scattering associated with H-containing materials. SANS is thus an effective method for investigating hydrogels. In a joint statement to Advances in Engineering, the authors explained that their findings provided new insights into the nanostructures and properties of hydrogels and would provide a general guideline for preparing gels suitable for advanced biomedical applications.

About the author

Takuma Kureha received his PhD (2018) from Shinshu University through a JSPS Research Fellowship (DC1, 2015-2018) on the synthesis of functional nanogels conducted under the supervision of Prof. Daisuke Suzuki. He then worked as a JSPS Postdoctoral Fellow (PD) at the Institute for Solid State Physics, University of Tokyo under the supervision of Prof. Mitsuhiro Shibayama, and since 2020, he has been leading an independent research life as an Assistant Professor at Hirosaki University. His current research interests include characterization of hydrogels and nanogels by scattering methods to control their functions.

About the author

Mitsuhiro Shibayama graduated from Kyoto University in 1982 with Dr. of Engineering in 1983. He served as Assistant Professor (1984), Associate Professor (1988) and Professor at Kyoto Institute of Technology (1997). Then, he moved to Univ. Tokyo in 2000 as a Professor , Institute of Solid State Physics (ISSP), The University of Tokyo. He became Director of Neutron Science Laboratory, ISSP in 2009. From 2020, he is an emeritus professor at The University of Tokyo, and Director of the Comprehensive Research Organization for Science and Society (CROSS). His recent awards include the Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology (2019, Japan), and Colloid and Polymer Science Lecture Award (2017, Germany).

Reference

Kureha, T., Ohira, M., Takahashi, Y., Li, X., Gilbert, E., & Shibayama, M. (2022). Nanoscale Structures of Poly(oligo ethylene glycol methyl ether methacrylate) Hydrogels Revealed by Small-Angle Neutron ScatteringMacromolecules, 55(5), 1844-1854.

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