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Abstract

This systematic literature review aims to study the thermal properties (TP) of poly(acrylic acid)-based hydrogels (PAA) by differential scanning calorimetry. The data sources were obtained from Google Scholar via Publish or Perish software from 2018 to 2020. The synthesis results show that most of the analysis of TP in PAA-based hydrogel consists of glass transition temperature ( $T_{g}$ ), melting temperature ( $T_{m}$ ), and enthalpy ( $Δ H_{m}$ ). The subsequent thermal properties analysis is important for a new development and depth study in the future application.

Keywords

Poly(acrylic acid)hydrogels differential scanning calorimetry thermal properties systematic literature review

Introduction

Wichterle and Lím introduced hydrogel in 1960. A network is formed from cross-linked hydrophilic polymers. They can take up water extensively without ripping their structure.^1–3 The potential application of hydrogel materials is not only seen from their ability to absorb moisture. However, the hydrogel performance can be tested when absorbing aqueous solutions such as urea solution, metal salt solution, or metal ion solution to determine their potential application as diapers,⁴ fertilizer medium,⁵ water treatment,⁶ wound dressing-healing,⁷ contact lenses,⁸ and drug delivery.⁹

Over the years, hydrogels from synthetic polymers have become more interesting with their superior properties. They have an excellent water absorption capacity, long life duration, and enhanced mechanical properties functionalized with their structure.^2,10 One of the synthetic polymers often used for hydrogel synthesis is poly(acrylic acid) (PAA).^11–18 PAA is made from acrylic acid monomer (Figure 1.) and cross-linked to produce hydrogels. It is an interesting class because it can produce more water adsorption by swelling.^19,20 Moreover, PAA-based hydrogels have good properties such as biodegradability, excellent mechanical and adhesion strength but have no good thermal properties (TPs). Thus, various materials need to build hydrogel composites that blend for better TP.²¹ It can be applied in a wide range of application areas.^22–27

Figure 1.

Chemical structure of acrylic acid (source: https://pubchem.ncbi.nlm.nih.gov/compound/Acrylic-acid).

To date, TP is the main indicator widely used in the characterization of polymeric materials.²⁸ It is usually analyzed by differential scanning calorimetry (DSC) to determine the TP, namely, $T_{g}$ , $T_{m}$ , and $Δ H_{m}$ .^28–31 However, in some previous reports, PAA-based hydrogels with various copolymers also changed their TP. Consequently, we have to conduct a specific review to identify the TP of PAA-based hydrogels and realize their transformation. The type of review used is a systematic literature review (SLR).

SLR is a good recommendation for a specific review in a scientific investigation. It uses systematic and explicit methods to identify, select, extract, and synthesize the available scientific information from the studies.^32–34 This method successfully leads to evidence-based conclusions that are more objective and organized in many fields, such as medical, business, and energy.^35–39 In this work, the SLR is conducted to study the TP from several parameters such as $T_{g}$ , $T_{m}$ , and $Δ H_{m}$ based on eligibility criteria. Based on the search and knowledge of the author, the study of the TP of PAA-based hydrogels using SLR has never been done before. In addition, the Boolean algorithm has been used in database search using Publish or Perish (PoP) software as a new strategy for SLR studies⁴⁰, which gives very good results when using Google Scholar as the data source because it has a maximum limit of 1000 meta-databases. Therefore, this research can provide consideration for researchers among academics or practitioners in the synthesis of materials, especially poly(acrylic acid)–based hydrogels and their copolymerized forms.

Methodology

Protocol

The present systematic review was conducted based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) method. It consists of well-defined review stages, developed and described eligibility criteria of an information source, literature search strategy, literature selection process, and data synthesis based on selected literature.^32,37 We used these steps as a review strategy in finding the significance of TP in PAA-based hydrogels. Therefore, this systematic approach includes identified and discussed TP from PAA-based hydrogels through their parameters referring to DSC results.

Eligibility criteria

The eligibility criteria consist of inclusion and exclusion criteria. In this present SLR, the inclusion criteria were: (1) articles that reported TP of PAA-based hydrogels; (2) published articles in 2018 until 2020; (3) articles that have comprehensive related information about TP. Besides, the exclusion criteria were: (1) articles without citation; (2) review articles types; (3) books and book chapters; (4) patents; (5) articles published in a language other than English; and (6) articles about PAA-based hydrogels without thermal analysis.

Search strategy and selection process

The database searches were carried out using PoP software with Google Scholar data sources to obtain information in articles published in 2018–2020. The search technique of PoP used the Boolean algorithm as keywords format to search the meta-data. The keywords format is “poly acrylic acid” AND “differential scanning calorimetry” AND “thermal properties” AND hydrogel. We obtained n = 719 meta-data and collected with the Mendeley Version 1.19.4 software to management and selection process as explained in Figure 2 based on eligibility criteria. Finally, 26 articles were selected and presented in Table 1.

Figure 2.

Flow diagram of study selection.

Table 1.

Specific characteristics of articles selected by preferred reporting items for systematic reviews and meta-analyses method.

Authors	Titles	Materials	$T_{g}$	$T_{m}$	$Δ H_{m}$
Micutz et al.⁴¹	Hydrogels obtained via $γ$ -irradiation based on poly(acrylic acid) and its copolymers with 2-hydroxyethyl methacrylate	P(AA-co-HEMA)	✓	✓	—
Deepa et al.⁴²	Synthesis and electrochemical properties of blend membranes of polysulfone and poly(acrylic acid-co-2-(2-(piperazin-1-yl) ethylamino)-2-hydroxyethyl methacrylate) for proton exchange membrane fuel cell	PSF/PIP	✓	—	—
Ghaffari-Bohlouli et al.⁴³	Performance evaluation of poly(L-lactide-co-D, L-lactide)/poly(acrylic acid) blends and their nanofibers for tissue engineering applications	PLDLLA/PAAc	✓	—	—
De la Garza et al.⁴⁴	Fabrication and characterization of centrifugally spun poly(acrylic acid) nanofiber	PAA	✓	—	—
Bialik-Wąs et al.⁴⁵	Bio-hybrid acrylic hydrogels containing metronidazole-loaded poly(acrylic acid-co-methyl methacrylate) nanoparticles and Aloe vera as natural healing agent	PAM-MET	✓	—	—
Ramli et al.⁴⁶ (2019)	Synthesis, characterization, and morphology study of coco peat-grafted-poly(acrylic acid)/NPK slow release fertilizer hydrogel	CP-g-P (AAc)/NPK	✓	—	—
Vijayavenkataraman et al.⁴⁷	Electrohydrodynamic jet 3D-printed PCL/PAA conductive scaffolds with tunable biodegradability as nerve guide conduits (NGCs) for peripheral nerve injury repair	PCL/PAA	—	✓	—
Nath et al.⁴⁸	Synthesis of carboxymethyl cellulose-g-poly(acrylic acid)/LDH hydrogel for in vitro controlled release of vitamin B12	CMC-g-PAAc/LDH	✓	✓	—
Tang et al.⁴⁹	Preparation and absorption studies of poly(acrylic acid-co-2-acrylamide-2-methyl-1-propane sulfonic acid)/graphene oxide superabsorbent composite	P(AA-co-AMPS)/GO	—	✓	—
Taherkhani et al.⁵⁰	Proton conducting porous membranes based on poly(benzimidazole) and poly(acrylic acid) blends for high temperature proton exchange membranes	PBI:PAA	✓	—	—
Purchel et al.⁵¹	Aggregated solution morphology of poly(acrylic acid)-poly(styrene) block copolymers improves drug supersaturation maintenance and Caco-2 cell membrane permeation	PS-b-PAA	✓	✓	✓
Khalid et al.¹⁹	Cross-linked sodium alginate-g-poly(acrylic acid) structure: A potential hydrogel network for controlled delivery of loxoprofen sodium	SA-g-pAA	—	✓	—
Djamaaa et al.²⁰	Poly(acrylic acid-co-styrene)/clay nanocomposites: Efficient adsorbent for methylene blue dye pollutant	P (AA-co-St)/OMMT	✓	—	—
Basturk & Kahraman⁵²	Preparation and performances of UV-cured methacrylated polyacrylic acid-based core–shell hybrid phase change materials	PAA-g-GMA	—	✓	✓
Abdollahi et al.²¹	Thermal and mechanical properties of graphene oxide nanocomposite hydrogel based on poly(acrylic acid) grafted onto amylose	PAA-g-amylose	—	✓	✓
Mohsin et al.⁵³	Glucose-mediated insulin release carrier	PAA-co-PMAAPBA	✓	—	✓
Guin et al.⁵⁴	Radiation grafting: A voyage from bio-waste corn husk to an efficient thermostable adsorbent	PAAc-g-husk	✓	✓	✓
Zheng et al.⁵⁵	Nanostructured polyelectrolyte-surfactant complex pervaporation membranes for ethanol recovery: The relationship between the membrane structure and separation performance	PAA-CTA, PAA-DHDA PAA-HDP, PAA-BDHA	—	✓	—
Son et al.⁵⁶	Acrylic random copolymer and network binders for silicon anodes in lithium-ion batteries	P(AA-co-nBA)	✓	—	—
Nanoyama et al.⁵⁷	Instant thermal switching from soft hydrogel to rigid plastics inspired by thermophile proteins	PAAc/CaAc	✓	—	✓
Baştürk et al.⁵⁸	Form-stable electrospun nanofibrous mats as a potential phase change material	PVB-PAA	—	—	✓
Velasco-Barraza et al.⁵⁹	Study of nanofiber scaffolds of PAA, PAA/CS and PAA/ALG for its potential use in biotechnological applications	PAA/CS, PAA/ALG	✓	—	—
Sithole⁶⁰	Development of a novel polymeric nanocomposite complex for drugs with low bioavailability	HA-PAA-HP-β-CD	—	✓	—
De Lima et al.⁶¹	The production of a novel poly(vinyl alcohol) hydrogel cryogenic spheres for immediate release using a droplet system	PVA-PAA-HAp	✓	✓	—
Xu et al.⁶²	Development of visible-light responsive and mechanically enhanced “smart” UCST interpenetrating network hydrogels	PAAm-PAAc	✓	—	—
Sternik et al.⁶³	Thermal degradation of peat-based activated carbons covered with mixed adsorption layers of PAA polymer and SDS surfactant	AC-PAA	—	✓	✓

PAA: poly(acrylic acid); PCL: poly(caprolactone).

Synthesis of result

The synthesis of the results focused on discussing the empirical trends. The data is presented without meta-analysis because it lacks sufficient statistical information. In addition, the search process resulted in some variation data types such as patents, books, and websites. Therefore, it does not support to conducted meta-analysis.

Results and discussion

The 719 reports appropriated the objective or initial inquiry, as shown in Figure 2. This number reduces to the 377 reports after the elimination of the 0 citation works in PoP software. Furthermore, the article elimination via Mendeley software reduced the number of articles to the remaining 31 articles based on exclusion criteria. However, 5 of 31 selected articles are doubtful, and the review team excluded the six works in the author’s discussion. The doubtful works were conducted by Su et al.,⁶⁴ Kertsomboon and chirachanchai,⁶⁵ Yang et al.,⁶⁶ Kozyra et al.,⁶⁷ and Wei et al.⁶⁸ Eventually, 26 articles were included in the present SLR.

$T_{g}$ analysis

Identification of $T_{g}$ in PAA-based hydrogels has so many perspectives with or without the addition of other materials. According to the original DSC output, several reports used the DSC instrument to identify the $T_{g}$ that was performed as heat flow depends on temperature. Theoretically, $T_{g}$ represented the intermolecular interactions between polymer chains which is obtained from equation (1)⁶⁹

\frac{1}{T_{g}} = \frac{w}{T_{g . s c}} + \frac{1 - w}{T_{g . b b}}

(1)

T_g, $T_{g . s c}$ , $T_{g . b b}$ , and w represented glass transition temperature, glass transition temperature for side chain, glass transition for backbone, and molecular weight, respectively. These demonstrated better quantification, as can be seen in Figure 3. In this SLR, we also obtained the analysis of interpreted $T_{g}$ using equation (1) depending on copolymerization. Micutz et al.⁴¹ reported that the two copolymers should exhibit a lower $T_{g}$ than PAA. Specifically, equation (1) explains that $T_{g}$ depends on the different molar ratio between monomers in the initial substrate.⁷⁰

Figure 3.

Correlation between w and $T_{g}$ for conjugated polymers.⁷⁰

In addition, the increase or decrease of $T_{g}$ values is affected by intermolecular interaction. Djamaa et al.²⁰ obtained the increase of $T_{g}$ values when utilizing the styerene as a copolymer. The increase of $T_{g}$ values was also experienced by Taherkhani et al.⁵⁰ after mixing PAA with poly(benzimidazole). They claimed that the increase of $T_{g}$ is due to the strong hydrogen bonds formation. The hydrogen bonds may restrict the co-operative segmental mobility of the components via electrostatic molecular interaction. Thus, it reduces the mobility of the polymer chain of PAA.^{46,48,56,61,62,71} Nevertheless, a different phenomenon was found by Deepa et al.⁴² They obtained the decrease of $T_{g}$ values on polysulfone-poly(acrylic acid-co-2-(2-(piperazin-1-yl) ethylamino)-2-hydroxyethyl methacrylate). This phenomenon may be caused by the intermolecular interactions from aliphatic chain segments of the copolymer. The intermolecular interaction of aliphatic is only limited to ions mobility.⁷² Therefore, we can conclude that the strong hydrogen bonds from the copolymers are needed to limit the mobility of the polymer chain and increase the $T_{g}$ values.

Interestingly, we obtained another model to demonstrate the dynamics of $T_{g}$ values from this SLR. De la Garza et al.⁴⁴ investigated the TP of PAA bulk powder and nanofibers using DSC at different scanning rates from 1°C to 50°C min⁻¹. They introduced Williams–Landel–Ferry (WLF) model to obtain the $T_{g}$ value, accurately. Time and temperature-dependent phenomena near the $T_{g}$ and its dynamics are well described by the equation (2)^44,73

\ln a_{T} = \frac{C_{1 g} (T - T_{g})}{C_{2 g} + T - T_{g}}

(2)

where

C_{1 g}

and

C_{2 g}

are constants initially assumed to have universal values. The DSC spectra of the heating branch of bulk PAA are shown in Figure 4. They noticed that for all heating rates and the thermogram performs a sigmoidal dependence of heat concerning temperature. The inflection in the middle is noticed as

T_{g}

. Nevertheless, it cannot be accurately estimated; therefore, they used a mathematical transformation to convert those inflection points into extremum points to obtain T_g’s accurate estimated value. Each recorded thermogram, shown in Figure 5, was numerically differentiated, showing the dependence of the derivative of heat flow concerning temperature (

d H / d T

Figure 4.

Differential scanning calorimetryspectra of poly(acrylic acid) bulk at different heating rates.⁴⁴ Permission is granted subject to an appropriate acknowledgement given to David De la Garza, Francisco De Santiago, Luis Materon, et al., Fabrication and characterization of centrifugally spun poly(acrylic acid) nanofibers, Copyright 2011 John Wiley and Sons.

Figure 5.

The derivative of the DSC scan ( $d H / d T$ ) during the third heating run, displaying the $T_{g}$ as the extremum point. The DSC scans were conducted at 10°C min⁻¹ from 40 to 225°C. The experiments were performed under nitrogen atmosphere.⁴⁴ Permission is granted subject to an appropriate acknowledgement given to David De la Garza, Francisco De Santiago, Luis Materon, et al., Fabrication and characterization of centrifugally spun poly(acrylic acid) nanofibers, Copyright 2011 John Wiley and Sons. DSC: Differential scanning calorimetry.

Furthermore, the analysis was continued with equation (2) to improve the accurate $T_{g}$ value. The improvement of $T_{g}$ accuracy is shown in Figure 6. Each thermogram represented the differentiated heat flow versus temperature (at a given constant heating rate), and the extremum point at the peak of each curve was identified as $T_{g}$ .

Figure 6.

Heating rate versus $T_{g}$ graph of bulk PAA and PAA nanofibers spun at 6000 and 8000 rpm.⁴⁴ Permission is granted subject to an appropriate acknowledgement given to David De la Garza, Francisco De Santiago, Luis Materon, et al., Fabrication and characterization of centrifugally spun poly(acrylic acid) nanofibers, Copyright 2011 John Wiley and Sons. PAA: poly(acrylic acid).

Consequently, a conventional value for the $T_{g}$ heating rate is assumed, which mathematically implies that $T_{g}$ is proportional to the heating rate. In Figure 6, the dependence of the heating rate on temperature is represented. The best fit to the experimental data has been obtained for the expression:

HR is the heating rate, $B$ is proportional to $C_{1 g}$ but not identical to $C_{2 g}$ because it was assumed that the T_g’s heating rate is represented by one, A is similar to $C_{2 g}$ , and $T_{g}$ represents the $T_{g}$ . Studying the $T_{g}$ dynamics and thermodynamics in amorphous polymers and glass-forming liquids is complex and beyond this work scope.

T_m analysis

T_m is one of the key variables in polymer extrusion to determine thermal stability and hence melt quality.⁷⁴ We found some cases which explained $T_{m}$ of PAA-based hydrogels. Zheng et al.⁵⁵ obtained a sharp peak of PAA with various surfactants such as cetyl-trimethyl-ammonium bromide (CTAB), dihexadecyl-dimethyl-ammonium bromide (DHDAB), benzyl-dimethyl-hexadecyl-ammonium chloride (BDHAC), and hexadecyl-pyridinium chloride (HDPC) that was named as PAA-CTA, PAA-DHDA, PAA-BDHA, and PAA-HDP, respectively (as shown at Figure 7). The $T_{m}$ of PAA-CTA, PAA-DHDA, and PAA-BDHA are 12°C, 34°C, and 2°C, respectively. In contrast, PAA-HDP shows endothermic peaks.

Figure 7.

T_m values of polyelectrolyte-surfactant complexes.

Furthermore, a report from Vijayavenkataraman et al.⁴⁷ on poly(acrylic acid) PAA blend poly(caprolactone) (PCL) also performed the same pattern. In this case, a decision about $T_{m}$ from DSC data is limited from the sharp peaks without some depth analysis.

Enthalpy ( $Δ H_{m}$ )

$Δ H_{m}$ is the total integrated zone below the thermogram peak, indicating the sample’s total heat energy uptake after suitable baseline correction influencing the transition.⁷⁵ It can be obtained from endothermic and exothermic peaks, as shown in Figure 8. Guin et al. obtained the thermogram and marked the endothermic and exothermic peaks on their study about PAA-g-corn husk. One endothermic peak and two exothermic peaks in Figure 8(a) represent the water molecules' evaporative loss from corn husk following the decomposition and thermal combustion of degraded some units glycosides, hemicelluloses, pectin, and cellulose.^54,76 At Figure 8(b), two endothermic peaks on the PAA-g-corn husk sample showed evaporation in the lower temperature and greater stability at high temperature.^54,77 Finally, PAA plays a role important in thermal equilibrium.

Figure 8.

Differential scanning calorimetry thermogram of corn husk (a) and PAA-g-husk (b).⁵⁴ Reprinted from Carbohydrate Polymers, Vol 183, Jhimli Paul Guin, Y.K. Bhardwaj, Lalit Varshney, Radiation grafting: A voyage from bio-waste corn husk to an efficient thermostable adsorbent, Copyright (2018), with permission from Elsevier. PAA: poly(acrylic acid).

After that, a similar case has been found by Baştürk et al.⁵⁸ which observed the $Δ H_{m}$ from polyvinyl butyral-PAA based form-stable phase change materials. Their result confirmed that endothermic peaks indicate thermal stability and heat capacity, especially on PAA-based copolymerization.⁵²

Conclusion

PAA-based hydrogels and their copolymerization have some interesting TP parameters to study. Several TP parameters such as $T_{g}$ , T_m, and $Δ H_{m}$ have been identified in this SLR. The Fox equation can determine the $T_{g}$ values from the DSC standard measurement technique depending on the molecular weight of PAA and its copolymers. The dynamics of changes in $T_{g}$ values were also obtained from this SLR study. It requires variations in heating rates on DSC measurement and the WLF equation in determining $T_{g}$ values. Furthermore, $T_{m}$ can be determined by a sharp peak of the thermogram. Meanwhile, the thermogram’s inflection point formed endothermic and exothermic peaks is claimed as $Δ H_{m}$ parameter.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Ahmad Sofyan Sulaeman

Permono Adi Putro

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Thermal studies of hydrogels based on poly(acrylic acid) and its copolymers by differential scanning calorimetry: A systematic literature review

Abstract

Keywords

Introduction

Methodology

Protocol

Eligibility criteria

Search strategy and selection process

Synthesis of result

Results and discussion

T g analysis

Tm analysis

Enthalpy ( Δ H m )

Conclusion

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

References

$T_{g}$ analysis

T_m analysis

Enthalpy ( $Δ H_{m}$ )