X-ray Diffraction Analysis of Poly(vinyl alcohol) Hydrogels, Obtained by Freezing and Thawing Techniques

Abstract

The structure of physical poly(vinyl alcohol) (PVA) hydrogels prepared by subjecting a PVA/ D 2O solution (11% w/w PVA) to freeze (-22 °C)/thaw (+25 °C) cycles has been systematically investigated by X-ray powder diffraction technique as a function of the number of cycles and aging time. The structural analysis has been extended to PVA "dried gels" and PVA hydrogels obtained by rehydrating the dried samples. The results of the present analysis confirm that highly stable PVA hydrogels, with a water uptake higher than 80%, may be obtained upon freeze/thaw cycles. The X-ray diffraction profiles of PVA hydrogels have been interpreted in terms of three components: "free water", crystalline PVA aggregates, and swollen amorphous PVA. The degree of crystallinity and the size of the crystals increase with increasing the number of freeze/thaw cycles and the aging time. Our results support the hypothesis that PVA hydrogels have a porous structure, with pores mainly occupied by water. The porous walls consist of swollen amorphous PVA while the crystalline domains act as knots of the gel network. The presence of crystalline knots ensures a high dimensional stability of the gel and induces elastic properties. Long time aging in sealed vials at room temperature induces large variations in the structure of freeze/thaw PVA hydrogels. The porous structure formed during freeze/thaw cycles in PVA hydrogels, instead, is not greatly altered upon drying and during the successive rehydration step; rehydrated gels, indeed, recover almost completely volume, shape, and physical properties of the as-formed freeze/thaw PVA hydrogels. Thus, the outstanding physical and mechanical properties of freeze/thaw PVA/hydrogels in the as-prepared state, may be preserved even for a long time, drying the samples immediately after the preparation and then restored when needed, upon rehydration of the dried samples.

Fabrication of Bio-Nanocomposite Nanofibers Mimicking the Mineralized Hard Tissues via Electrospinning Process

“…In order to discern any variation in crystalline content as a result of HAp addition to the PVA the XRD measurements on the electrospun fibers PVA with and without HAp were carried out and the resulting XRD patterns were analyzed in more detail (see Figure 5). In the case of the electrospun pure PVA fibers, the diffraction peaks reveal at 2θ = 19.5° (dspacing = 0.454 nm) and 40.5° (d-spacing = 0.223 nm) [49]. The peak corresponds to a typical doublet reflection of (101) and (10-1) planes of the semicrystalline atactic PVA, which contain the extended planar zigzag chain conformation, while the latter belongs to the (220) reflection [50].…”

Characteristics of Mixed Polyvinyl Alcohol and Chitosan Obtained from Shrimp Shells and Squid Pens for Electromagnetic Wave Absorber

“…1, the X-ray diffraction pattern of the as-received PVA revealed a small peak at 2-theta 10.8 and a strong crystalline diffraction at around 2-theta 19.1 with a shoulder at around 2-theta 22.7. These two latter diffraction peaks are characteristic of PVA from (101) and (200) planes with a monoclinic unit cell [22]. Another small peak is found at around 2-theta 40.1.…”

Potential Evaluation of PVA-Based Hydrogels for Biomedical Applications

“…The addition of PEG having higher molecular weight results in an increased biodegradability. The most stable hydrogels were determined as the hydrogels having PEG (M w : 2000) at four cycles of freeze/thawing for all pHs possesing the higher cross-linked hydrogels [36]. 1.…”