Materials Letters

Volume 265, 15 April 2020, 127434

📷

X-ray analysis and Rietveld refinement of polyvinyl alcohol

Author links open overlay panel

https://doi.org/10.1016/j.matlet.2020.127434Get rights and content

Highlights

• •Rietveld refinement of the polyvinyl alcohol semicrystalline structure.
• •New structural model of the polyvinyl alcohol.
• •X-ray diffraction analysis of polymeric polyvinyl alcohol.
• •Experimental X-ray diffraction of polyvinyl alcohol.

Abstract

The Rietveld method has been used to refine the polyvinyl alcohol (PVA) structure. The PVA is a polymeric material exhibiting a semicrystalline “head-to-tail” arrangement of repeating units. Experimental X-ray pattern of PVA was adjusted considering a model based on monoclinic symmetry, but using the P21/m and P21/c spatial groups. The lattice parameters and atomic positions were adjusted in each case. The P21/c based model showed a better Rietveld adjust than the P21/m. Then, the P21/c model was used to analyze the structural behavior of the PVA subjected to different pH environments. From refinement results, the PVA subjected to acid environment remained unchanged and suffered a regalement of its crystalline structure when it is subjected to a basic pH, causing a loss of its permanence and crystallinity. The results provide a powerful model to evaluate the crystallinity degree of the PVA and its structural variations using the Rietveld refinement method.

Introduction

The polyvinyl alcohol (PVA) is a biocompatible and water-soluble synthetic polymer having unique properties used in many applications, going from coatings, paints, consumer and hygienic items, biodegradable plastics, and containers for drug release [1]. PVA is current an important material for design nanocomposites wherein their physical properties are strongly dependent on the fillers [2], [3]. The incorporation of nanofillers can induce structural variations on the PVA capable of modifying the physicochemical behavior, and the electric, magnetic, optical, thermal, or mechanical properties of the nanocomposite [4], [5]. However, PVA can change its crystallinity degree and suffer a loss of permanence when it is exposed to acid/basic environments [6]. Then, understanding the structural behavior of PVA becomes important for determining its influence on the change of the functional properties in composites.The PVA structure is composed of a carbon spine with hydroxyl groups attached to methane carbons. This polymer has a high level of crystallinity resulting from strong intermolecular interaction between their chains through intermolecular hydrogen bonding [7]. Mooney proposed a structural model for the PVA based on analytical methods. He assumed a monoclinic angle close to 90° due to an equal probability of the chains of locating in either direction regarding the chain axis [8]. Then, Bunn inferred that the high degree of crystallinity is from a regular molecular structure, and established the structural parameters by analyzing the X-ray intensities [9]. Bunn considered a monoclinic P21/m system having a repeat distance of 2.52 Å, lattice parameters a = 7.81 Å, b = 2.52 Å, c = 5.51 Å, and β = 91°42′, and atomic positions C1 (CH2) at 0.190, 0.250, 0.806; C2 (CH) at 0.277, 0.750, 0.708; O1 at 0.451, 0.750, 0.782, and O2 at 0.267, 0.750, 0.450 [9]. The hydroxyl groups randomly situated at both directions are enough to account for the experimental X-ray pattern, although no structure embodying regular molecules could be properly adjusted analytically. In 1974 Colvin determined that both monoclinic and orthorhombic-type structures could coexist in PVA [10]. Then, Bath showed a reduction of the crystallinity degree in PVA films subjected to γ-radiation [11]. Several works studied the physical properties of the PVA combined with organic and inorganic components. However, there is a mutual interaction that changes the PVA crystalline properties [6], [12], [13]. At present, polymeric matrices play a critical role in determining the physical properties of many composites, so the structural features of the PVA result in a key factor for designing new multifunctional composites allowing to satisfy emerging applications.In 1967, Hugo Rietveld proposed a method for the characterization of crystalline compounds by neutron diffraction considering the whole pattern. The method was rapidly expanded to the X-ray powder diffraction, and for the analysis of semi-crystalline compounds [14]. Two different ways were proposed to refine a semicrystalline pattern, the first evaluates the amorphous fraction regarding a standard, and the second adjusts the amorphous scattering in a theoretically sound fashion and compatible with the Rietveld rules [15]. Le-Bail considered a microstrained crystalline model to prove that the structure and atomic coordinates of amorphous silica can be refined using the Rietveld method [16]. Then, Lutteroti analyzed amorphous structures using the nanocrystalline concept and the Rietquan (MAUD) program [17]. These works concluded that it is impossible to distinguish between a nanocrystalline strained model from an amorphous structure. The main difference is that the broad of peaks is computed from the crystallite size instead of the classical Caglioti law FWHM = (U tan2θ + V tanθ + W)1/2. This approach proved to be accurate to refine the X-ray patterns of amorphous and semi-crystalline compounds [15]. In this work, the semicrystalline structure of the PVA was refined using the Rietveld method and the MAUD program. The P21/m and the P21/c space groups were used as initial models. The best analytical and graphical results were obtained from the P21/c model. Besides, the crystallinity degree of the PVA subjected to acid and basic environments was evaluated by using the crystallite size and microstrain obtained from the Rietveld adjust.

Section snippets

Experimental The samples were fabricated using 2.0 g of PVA (99%) with molecular weight 89000–98000, (Sigma-Aldrich), mixed in 25 mL of deionized water, and stirred at 60 °C for 4 h. The PVA was poured over a glass surface and dried at 40 °C. After, PVA was detached from the glass with enough thickness to allow its X-ray diffraction. A PVA sample was analyzed without further treatment. Other samples were immersed in a nitric acid solution with pH = 4 (PVA-pH4), and in a basic solution with pH = 11

Results and discussion The experimental pattern obtained for the PVA shows an asymmetric main band centered at 23.1° with a shoulder at 26.0° and a minor contribution at 49.5°. An initial model for the refinement was based on the one proposed by Bunn and Mooney using the P21/m space group. Fig. 1(a) shows that this model fitted the main band but fails in adjusting the contribution at 49.5°. Various strategies for refinement were carried out to fit the entire diffractogram. However, the whole pattern cannot be

Conclusions The Rietveld method using the MAUD program was applied to refine the semicrystalline structure of PVA. The Bunn model, P21/m, was used as an initial model for refinement. However, that structure was unable to adjust the whole experimental pattern, particularly the contribution at 49.5°. Better results were obtained with a model based on the P21/c space group, instead of the original P21/m. The Rietveld refinement profile showed a good agreement with the experimental pattern. Also, the P21/c

Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Credit author statement The entire work from its conception, preparation of the samples, experimental techniques, analysis of the results, and manuscript writing was done by the author A. Lobo Guerrero.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References (17)

• B. Sharma et al.Mater. Today Comm. (2018)
• M.C. PopescuInt. J. Biol. Macromol. (2017)
• R. Shi et al.Carbohyd. Polym. (2008)
• M. Abdelaziz et al.Physica B (2007)
• N.V. Bhat et al.Nucl. Instrum. Methods Phys. Res. B (2005)
• H.M. Zidan et al.Int. J. Electrochem. Sci. (2016)
• S. Rashidi et al.Mater. Res. Bull. (2016)
• C. Cannas et al.J. Solid State Chem. (2005)

There are more references available in the full text version of this article.

Cited by (11) Revealing structural and electrical details of PVA/LiClO4 thermoplastic membranes 2024, Materials Science and Engineering: B

• Gamma–radiation synthesis and characterization of nanocomposite hydrogels based on poly(vinyl-alcohol)/zirconium NPs/ europium 2024, Radiation Physics and Chemistry
• Morphological, UV blocking, and antimicrobial features of multifunctional cotton fibers coated with ZnO/Cu via sonochemistry 2022, Materials Chemistry and PhysicsCitation Excerpt :However, it requires previous information about the unit cell, space group, and atomic coordinates [70]. Since sonochemical treatment and citric acid concentration can cause modifications over the packaging and molecular organization of cellulosic chains in the crystalline domains, the lattice parameters were determined by the Rietveld refinement (Fig. 4S) to get more information about the effect of these two parameters on the crystalline phases present in CF. The XRD curves calculated present good agreement with the observed x-ray patterns, presenting goodness-of-fit (χ2) between 1 and 2 that is considered an excellent reliability factor for refinement processes involving XRD experimental data of semicrystalline polymer systems [71–73]. The unit cell dimensions of cellulose Iβ are a = 7.84 Å, b = 8.2 Å, c = 10.38 Å, α = β = 90°, and γ = 96.5°, while cellulose II unit cell presents a = 8.1 Å, b = 9.03 Å, c = 10.31 Å, α = β = 90°, and γ = 117.1° [38,55].
• Magnetic, structural, and fluorescence properties of Sr-hexaferrite nanoparticles embedded in a PV-OH film 2022, Physica B: Condensed MatterCitation Excerpt :The hexaferrite phase has hexagonal symmetry, P63/mmc space group, and lattice parameters a = 5.8844 Å, and c = 23.0500 Å (COD-ID: 1006000) [29]. The PV-OH semicrystalline phase was refined using the monoclinic P21/c:b2 space group, and lattice parameters a = 7.768 Å, b = 2.462 Å, c = 5.661 Å, and β = 93.19° [14]. The Rietveld refinement analysis performed on the Sr-M nanoparticles showed a crystallite size of 15.2 (1) nm, cell parameters a = 5.8342 (1) Å, and c = 22.8662 (8) Å.
• New strategy of synthesizing zeolitic imidazolate framework-67 with hierarchical pores for heat storage 2021, Materials LettersCitation Excerpt :PVA was selected as the templating agent due to the characteristics that PVA can dissolve in water at high temperature and then it can form PVA three-dimensional networks while cooling. In the cooling process of PVA solution dissolved with Co2+ ions, PVA molecules could form three-dimensional networks by molecular entanglements, and/or by ionic, H-bonding or hydrophobic forces [11]. Meanwhile, PVA molecules would coordinate with Co2+ ions, which then could react with 2MIM and form ZIF-67.
• Phase transition of Mg/Al-flocs to Mg/Al-layered double hydroxides during flocculation and polystyrene nanoplastics removal 2021, Journal of Hazardous MaterialsCitation Excerpt :The D8 Advance X-ray diffractometer (Bruker, Germany) was used to record the X-Ray Diffraction (XRD) patterns with a copper target Kα source (λ = 0.15418 nm) and Lynx Exe array detector, with a voltage of 10.0 kV, pass energy of 50 eV, and power of 300 W, which has its scanning speed of 17.7 s/step and range from 5° to 70°. For further understanding the composition of crystalline and amorphous phases, Rietveld refinement was used to adjust in a theoretically sound fashion (Lobo-Guerrero, 2020), which is adapted to a multilayered sample and enables a specific analysis of each of individual growth, figuring out the characterization of each crystalline structure and the assessment of each thickness (Ravat et al., 2018). Samples of XRD patterns were Mg/Al flocs in solid state (dried at 60 °C after 4000 rpm centrifugation) at pH 6.0, 7.0, 8.0, 9.0 and 10.0, respectively.

View all citing articles on Scopus

View full text© 2020 Elsevier B.V. All rights reserved.

• Cationic surfactant at high pH in microwave HAp synthesis Materials Letters, Volume 265, 2020, Article 127416D. Sánchez-Campos, …, V. Rodríguez-Lugo
• Hybrid films of reduced graphene oxide modified with gold nanorods and its study as surface-enhanced Raman spectroscopy platform Materials Letters, Volume 265, 2020, Article 127405P.B. Jimenez-Vara, …, S. Sepulveda-Guzman
• Bioactive ophiopogonin release form bioglass-collagen-phosphatidylserine scaffolds to enhance bone repair in vitro Materials Letters, Volume 265, 2020, Article 127436Chunrong Yang, …, Guanghui Li

Citations

• Citation Indexes: 9

Captures

• Readers: 16

View details📷

Thank you so much Sir Suresh Ahuja