Structure of Poly(vinyl alcohol) Cryo-Hydrogels as Studied by Proton Low-Field NMR Spectroscopy

Abstract

The network structure of poly(vinyl alcohol) (PVA) hydrogels obtained by freezing−thawing cycles was investigated by solid-state 1H low-field NMR spectroscopy. By the application of multiple-quantum NMR experiments, we obtain information about the segmental order parameter, which is directly related to the restrictions on chain motion (cross-links) formed upon gelation. These measurements indicate that the network mesh size as well as the relative amount of nonelastic defects (i.e., non-cross-linked chains, dangling chains, loops) decrease with the number of freezing−thawing cycles but are independent of the polymer concentration. The formation of the PVA network is accompanied by an increasing fraction of polymer with fast magnetization decay (∼20 μs). The quantitative study of this rigid phase with a specific refocusing pulse sequence shows that it is composed of a primary crystalline polymer phase (∼5%), which constitutes the main support of the network structure and determines the mesh size, and a secondary population of more imperfect crystallites, which increase the number of elastic chain segments in the polymer gel but do not affect the average network mesh size appreciably. Correspondingly, progressive melting of the secondary crystallites with increasing temperature does not affect the network mesh size but only the amount of network defects, and melting of the main PVA crystallites at ∼80 °C leads to the destruction of the network gel and the formation of an isotropic PVA solution.

Epoxy resin curing reaction studied by proton multiple-quantum NMR

“…Nevertheless, due to its selectivity, double-quantum (DQ) or more generally multiple-quantum (MQ) NMR is nowadays considered as the most precise and effective experimental approach for the quantification of weak RDCs of soft polymers, 8 and thus one of the most powerful and successful tools for the quantitative characterization of dynamics in polymer melts, [17][18][19] the gel point, 20 vulcanization kinetics, 21 and the complete and quantitative characterization of polymer network structures. [22][23][24][25] In those studies, MQ experiments are based upon the lengthy but quite robust Baum-Pines (B-P) sequence 8,26,27 (shown in Fig. 1), which shows ideal performance for studying soft (weaker dipolar-coupled) polymers systems such as elastomers [23][24][25]28,29 and gels.…”

“…1), which shows ideal performance for studying soft (weaker dipolar-coupled) polymers systems such as elastomers [23][24][25]28,29 and gels. 22,30 Nevertheless, the use of B-P sequence is limited for characterizing high dipolar coupled polymers (epoxy resins would belong to this type of samples), as it was recently demonstrated in the study of ionic elastomers. 31 The main problem in these systems is the fast decay of the associated NMR signal, which completely vanishes during a single cycle of the B-P sequence ($0.09 ms).…”

Significance of epoxy network properties for the toughening effect of flaky and fullerene‐like WS2 nanoparticles

“…In the recent years, it has been successfully applied to characterize the network structure of elastomers, 17,18 hydrogels, 19 and ionomers, 20 however it has not been applied to thermosets like epoxy until recently. 21 This method allows measuring a material's residual dipolar coupling D res , which corresponds to the overall rigidity of the network chains after delineating it from the FIGURE 1 Transmission electron microscopy images of NP agglomerates.…”

A Brief History of Polymeric Cryogels

“…Features of the microstructure of PVA cryogels were ascertained using various microscopy techniques [330,331,333,[338][339][340][341][342][343][344][345][346] and other physicochemical analysis methods [237,326,[347][348][349][350][351]. In these studies, the following five principal facts were established:…”