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[Koichiro Uto](https://orcid.org/0000-0001-7091-0585), Yihua Liu, Mingwei Mu, Rie Yamamoto, Chinami Azechi, [Mizuki Tenjimbayashi](https://orcid.org/0000-0002-8107-8285), Adrien Kaeser, Marie‐Adeline Marliac, Mohammad Mydul Alam, Jun Sasai, [Mitsuhiro Ebara](https://orcid.org/0000-0002-7906-0350)

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[Humidity‐Responsive Polyvinyl Alcohol/Microcrystalline Cellulose Composites with Shape Memory Features for Hair‐Styling Applications](https://mdr.nims.go.jp/datasets/b0f4eed3-7a4a-47e5-80f5-e5b59b740793)

## Fulltext

Humidity‐Responsive Polyvinyl Alcohol/Microcrystalline Cellulose Composites with Shape Memory Features for Hair‐Styling ApplicationsRESEARCH ARTICLEwww.advmatinterfaces.deHumidity-Responsive Polyvinyl Alcohol/MicrocrystallineCellulose Composites with Shape Memory Features forHair-Styling ApplicationsKoichiro Uto,* Yihua Liu, Mingwei Mu, Rie Yamamoto, Chinami Azechi,Mizuki Tenjimbayashi, Adrien Kaeser, Marie-Adeline Marliac, Mohammad Mydul Alam,Jun Sasai, and Mitsuhiro Ebara*This study investigates the maintenance of humidity-responsive shapememory polymer composites comprising polyvinyl alcohol (PVA) and cellulosemicrocrystal for maintaining hair curls in humid environments. Thecomposite films are prepared by a simple blending method using aqueoussolutions of different polymer ratios and characterized for thermal properties,chemical structure, and surface morphology. The hydroxyl groups in the PVAand cellulose provided hydrogen-bonding interactions and improved themiscibility of the composites. Increasing the cellulose content in thecomposites enhanced mechanical properties and curl-shape recoveryperformance, with 20–25 wt.% cellulose achieving maximum recovery. Whenthe PVA/cellulose solution is applied to natural hair, it effectively maintainsthe shape of curled hair bundles for at least 6 h under 80% humidity andpromotes the hair shape recovery rate to 8–10%. Overall, The proposedhumidity-responsive PVA/cellulose composites present promising applicationpotential in the field of hairstyling to withstand humid weather.1. IntroductionFrizzy hair is a benign problem in the everyday life of modernpeople. It stems from the penetration of air moisture in the haircuticle, which results in hair swelling. In particular, in muggyand humid weather, moisture penetrates the hair very actively,K. Uto, Y. Liu, M. Mu, R. Yamamoto, C. Azechi, M. EbaraResearch Center for Macromolecules and BiomaterialsNational Institute for Materials Science (NIMS)Tsukuba, Ibaraki 305–0044, JapanE-mail: UTO.Koichiro@nims.go.jp; EBARA.Mitsuhiro@nims.go.jpM. TenjimbayashiResearch Center for Materials Nanoarchitectonics (MANA)National Institute for Materials Science (NIMS)Tsukuba, Ibaraki 305–0044, JapanThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/admi.202300274© 2023 The Authors. Advanced Materials Interfaces published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution License, which permits use, distributionand reproduction in any medium, provided the original work is properlycited.DOI: 10.1002/admi.202300274leading to a fluffy and frizzy appearance.Modifying the hairstyle using flat iron-ing to regulate this phenomenon is a verypopular method. Nowadays, people pre-fer to try different hairstyles regularly, in-stead of adhering to a permanent style.Temporary one-day curls achieved by hotironing are used on a daily basis, partic-ularly from people with straight hair. Inhumid environments, however, hair curlsare easily compromised; therefore, effec-tive hair curl maintenance products arerequired to prevent the hairstyle from be-coming frizzy and spoiled.Polyvinyl alcohol (PVA) is a well-known, human skin-compatible, andenvironment-friendly polymer that iswater-soluble, easy to form, non-toxic,and biodegradable.[1,2] To achieve ecolog-ical sustainability, PVA has been widelystudied for compositing with naturalfillers, such as chitosan,[3–5] cellulose,[6–8] and starch.[9–11] Fur-thermore, PVA composites have been successfully used fordrug delivery, contact lenses, filtration materials, and mem-brane formation.[4,12,13] In addition, the safety assessment of PVAas a binder, thickener, and film-former in cosmetic products,e.g., makeup and skincare, has been verified.[14,15] The hydroxylA. Kaeser, M. M. Alam, J. SasaiL’Oréal Research and InnovationKawasaki, Kanagawa 213–0012, JapanM.-A. MarliacL’Oréal Research and InnovationCentre Charles Zviak, Saint-Ouen 93400, FranceM. EbaraGraduate School of Pure and Applied SciencesUniversity of TsukubaTsukuba, Ibaraki 305–8577, JapanM. EbaraGraduate School of Industrial Science and TechnologyTokyo University of ScienceKatsushika, Tokyo 125–8585, JapanAdv. Mater. Interfaces 2024, 11, 2300274 2300274 (1 of 10) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbHhttp://crossmark.crossref.org/dialog/?doi=10.1002%2Fadmi.202300274&domain=pdf&date_stamp=2023-11-01www.advancedsciencenews.com www.advmatinterfaces.deFigure 1. A) Schematic illustration of humidity-responsive shape memory mechanism of PVA/cellulose composites and its application in hair styling.B) Schematic representation of hydrogen bonding interaction among PVA, cellulose, and water molecules.groups in PVA strongly affect its solubility in water and responseto humidity, resulting in water retention and swelling properties.Although these properties provide many benefits to countlessusers, another attractive property of PVA is its shape memoryability, which has not been fully exploited in the cosmetic prod-ucts field.Cellulose is a polysaccharide that constitutes the cell wallof plants and is the most abundant, biodegradable, renewable,and inexpensive polymer found in nature.[16] As such, it hasbeen extensively used in a wide range of fields because of itsbiocompatibility, hydrophilicity, and thermal stability.[17] Nano-and micro-sized celluloses are employed as well-designed fillersin the medical, cosmetic, filtration, and food packaging fieldsbecause their intramolecular hydrogen bonds provide stiffnessto the polymer chain and improve the mechanical properties ofcomposite materials.[18–21] The presence of hydroxyl groups incellulose facilitates the formation of hydrogen bonds with PVA;therefore, good composite properties and satisfactory perfor-mance, such as improved mechanical and barrier properties,[22,23]high moisture resistance and thermal stability,[24,25] and supe-rior UV absorption,[26,27] have been achieved through theinteraction between cellulose and PVA. Although excel-lent cellulose-based materials have been successfully ap-plied, more approaches still need to be explored for practicalapplications.Despite the existing knowledge on the shape memory per-formance of PVA, few attempts have been made to improve orenhance its shape memory ability. In fact, the long-term shapememory ability of PVA in humid environments still presentslimitations. In this study, we investigate PVA/cellulose com-posites for application in hair curl maintenance products, withthe purpose of improving their shape memory performancein response to humidity (Figure 1). The design criteria of ourPVA/cellulose composites were to precisely tailor the interac-tion between PVA and the cellulose filler to enhance the shapememory ability, without compromising the humidity response.Composite films were prepared by mixing PVA and cellulosein different weight ratios (PVA/cellulose = 4/1, 3/1, and 2/1).The thermal properties, chemical structures, and surface mor-phologies of the PVA/cellulose films were evaluated using dif-ferential scanning calorimetry (DSC), Fourier-transform infrared(FTIR) spectroscopy, and scanning electron microscopy (SEM).Furthermore, the moisture absorption and mechanical proper-ties of the PVA/cellulose composite films were evaluated, withthe results indicating an excellent shape memory performanceeven under high humidity conditions. In addition, preliminaryfindings on hair curl maintenance were obtained by applyinga PVA/cellulose solution to a natural hair sample and expos-ing it to an 80% humidity chamber at ≈30 °C for 6 h. Overall,our proposed PVA/cellulose composite material demonstratesAdv. Mater. Interfaces 2024, 11, 2300274 2300274 (2 of 10) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2024, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300274 by National Institute For, Wiley Online Library on [24/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensewww.advancedsciencenews.com www.advmatinterfaces.deFigure 2. DSC curves of PVA, cellulose, and PVA/cellulose compositeswith different compositions.noteworthy potential for a wide range of applications in the cos-metics field.2. Results and Discussion2.1. Characterization of PVA/Cellulose Films2.1.1. Thermal Properties of PVA/Cellulose Composite FilmsHomogenization of PVA/cellulose suspensions adjusted to aPVA to cellulose weight ratio of 4/1, 3/1, and 2/1 by stirringand sonication dramatically suppressed the sedimentation ofcellulose microcrystals used as filler in all compositions, re-sulting in uniform composite suspensions (Figure S1, Support-ing Information). When films were prepared from the obtainedPVA/cellulose suspensions by the solvent casting method, trans-parent films were obtained with PVA, whereas opaque compositefilms were obtained with PVA/cellulose (Figure S2, SupportingInformation). Regardless of the composition of PVA/cellulose,no macroscopic aggregation of cellulose microcrystals was ob-served in the composite films, and a relatively uniform film wasformed. Thermal characterization is a convenient technique toanalyze physicochemical characteristics and changes, such asphase transitions, glass transition temperature (Tg), and melt-ing temperature (Tm). Figure 2 shows the differential scanningcalorimetry (DSC) curves of the as-prepared pure PVA, cellulose,and PVA/cellulose composite films in the temperature range of25—250 °C at a heating rate of 10 °C min−1. Broad endother-mic peaks at ≈80 and 125 °C were observed for the pure cellu-lose and PVA films, respectively, which were attributed to theevaporation of residual water in the films. The higher evapora-tion temperature of bulk water in the PVA films, compared tothat in their cellulose analogs, suggested that the hydrogen bondsbetween PVA and water molecules were stronger than those be-tween cellulose and water molecules. Ergo, the water evaporationtemperature of the PVA/cellulose composite films strongly de-pended on their composition, shifting toward lower temperatures(from 110 to 93 °C) as the cellulose content increased.PVA forms crystals through inter- and intramolecular hydro-gen bonding and, as expected, shows another endothermic peakon the higher temperature range, with a Tm of 182 °C. Interest-ingly, the compositing of PVA with cellulose, which does not ex-hibit a Tm, resulted in an increase of the Tm of PVA to ≈190 °C.PVA-polyethylene glycol (PEG) composite materials, using PEGinstead of cellulose, are known to possess lower Tm values owingto PEG addition.[28] In this case, it was inferred that the hydrogenbonds formed between PVA and PEG disrupted the regular struc-ture and movement of the PVA chains, resulting in a lower Tmfor PVA. Conversely, the PVA/cellulose composite films in thisstudy exhibited an increased Tm by ≈8 °C regardless of composi-tion, suggesting that the hydrogen bonds formed between PVAand cellulose enhanced the regular structure and mobility of thePVA chains.The evaporation temperature of the PVA/cellulose compos-ite films was observed as a single peak, regardless of the PVAcomposition, similar to the Tg observed in the PVA/cellulosesystem.[29,30] Furthermore, rather than demonstrating a simpledecrease in enthalpy, the Tm of PVA shifted to a higher range ow-ing to its compositing with cellulose. These results suggest thatthe composites formed a miscible homogeneous phase, high-lighting the capability of cellulose as a filler to establish stronghydrogen bonds with PVA during film formation. As discussedbelow, the compositing of PVA with cellulose has a marked effecton its hydrogen bonding performance by altering its crystallinity,in turn enhancing its mechanical properties and humidity re-sponse.2.1.2. Chemical Structure and Intermolecular Interactions ofPVA/Cellulose Composite FilmsThe as-prepared PVA/cellulose composite films were then char-acterized using Fourier-transform infrared (FTIR) spectroscopyto identify their chemical structure and interaction between PVAand cellulose (Figure 3). The FTIR spectra of the pure PVA filmshowed characteristic peaks at 3302, 2943, 1419, and 1087 cm−1,assigned to the O–H stretching, C–H stretching, C–H bending,and –C–O– stretching vibrations, respectively.[6,31] Another char-acteristic peak was observed at 1735 cm−1, attributed to the C=Ostretching vibration, which originated from the unhydrolyzedacetate group on PVA. The FTIR spectra of the PVA/cellulosecomposite films exhibited additional absorption bands at 1159and 1051 cm−1, corresponding to C–O–C glycosidic ring stretch-ing, C–O/C–C asymmetric stretching, and O-C-H bending vi-brations of cellulose, respectively, suggesting the presence ofcellulose in the composite films.[32,33] On the other hand, boththe pure PVA and PVA/cellulose films exhibited a wide O–Hstretching vibration band in the range of 3000–3700 cm−1. Withinthis region, the O–H stretching vibration bands correspondingto the free alcohols in the amorphous phase and the boundalcohols in the crystalline phase appeared at 3600–3650 and3200–3570 cm−1, respectively.[28] The band at 3302 cm−1 in thePVA film, originating from the bonded hydroxyl groups in thecrystalline phase, shifted to a lower wavenumber upon com-positing with cellulose, and the level of the shift became largeras the cellulose content increased (3286, 3287, and 3283 cm−1at PVA/cellulose ratios of 4/1, 3/1, and 2/1, respectively). PVAAdv. Mater. Interfaces 2024, 11, 2300274 2300274 (3 of 10) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2024, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300274 by National Institute For, Wiley Online Library on [24/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensewww.advancedsciencenews.com www.advmatinterfaces.deFigure 3. FTIR spectra of PVA and PVA/cellulose composite films with dif-ferent compositions. The figure below shows the chemical formulas of PVAand cellulose used in this study, with the functional groups circled in colorscorresponding to the FTIR spectral assignments.forms intra- and intermolecular hydrogen bonds via side-chainhydroxyl groups, while PVA/cellulose composite films can formadditional hydrogen bonds between PVA chains and cellulose.The shift of the bonded hydroxyl groups in the crystalline phase ofPVA toward a lower wavenumber indicated that the newly formedhydrogen bonds between PVA and cellulose were more robustthan those in pure PVA, which was consistent with the DSC re-sults showing that the Tm of PVA was improved by compositingwith cellulose. Therefore, it was inferred that the presence of cel-lulose in the PVA matrix acted as a filler that enhanced hydrogenbonding in the PVA/cellulose composite films.2.1.3. Surface MorphologyIn addition to the appearance of the films, the DSC and FTIRresults above indicate that when cellulose is used as a filler inPVA/cellulose composite films, it is homogeneously dispersedand forms hydrogen bonds that affect the thermal properties ofthe film. To investigate the compatibility of the PVA and cellu-lose components within the film structure, the surface morpholo-gies of the resultant films were observed using scanning electronmicroscopy (SEM) (Figure 4). The cellulose used as filler in thisstudy had a plate-like microcrystal morphology with a long axislength of 10–70 μm, and its cast film surface was uneven andrugged. On the other hand, the PVA/cellulose composite filmsexhibited a more homogeneous and smooth surface morphol-ogy, although unevenness derived from the cellulose microcrys-tals could still be observed. As the PVA content of the compositesincreased, the surface structure became smoother. Cellulose mi-crocrystals comprise strong intra- and intermolecular hydrogenbonds and are simultaneously insoluble and highly dispersibleFigure 4. SEM images of cellulose microcrystals and PVA/cellulose com-posite films with different compositions (scale bar = 100 μm).in water, because of the hydroxyl groups present on their sur-face. In PVA/cellulose composites, interfacial interactions suchas hydrogen bonding between the PVA chains and cellulose mi-crocrystals largely improve the dispersibility and miscibility ofcellulose in PVA matrices, resulting in a more uniform and ho-mogeneous surface morphology.[34] The homogeneity and dis-persibility of cellulose microcrystals in the composites stronglydepend on the PVA content, with higher PVA ratios having amore positive effect on physical homogeneity. Overall, the re-sults of controlling the interfacial interactions between the matrixcomponents and the cellulose used as a filler to enhance disper-sion in the composite were consistent with previously reportedresults.[35]2.2. Humidity-Responsive Properties of PVA/CelluloseComposites: Moisture Absorption and Mechanical PropertiesPVA owes its high water absorption properties to the presence ofhydroxyl groups; however, swelling and dissolution in water canseverely hinder the performance of PVA as a hair-coating agent.Compositing PVA with hydrophobic fillers and other materialshas been widely considered to enhance its water resistance.[36,37]Since the moisture absorption behavior under high humidity andits equilibrium state are essential for evaluating the performanceof PVA/cellulose composites, we comparatively investigated themoisture absorption behavior of pure PVA and PVA/cellulosecomposite films under 100% relative humidity conditions bytracking the changes in the film weight (Figure 5A). Both thepure PVA and PVA/cellulose composite films showed an in-crease in weight owing to moisture absorption and swellingwhen exposed to 100% humidity. Specifically, a relatively fastincrease in the swelling rates was observed during the initial3 h, with recorded values of 1.41 for the PVA films and 1.24-1.32 for the PVA/cellulose composite films, indicating that theaddition of cellulose reduced moisture absorption. The reasonfor the increased initial swelling rate in the PVA/cellulose com-posite films, especially with a PVA/cellulose ratio of 4/1, was theincrease in the surface area of the composites, as observed bySEM, or the hydrogen bonding of water molecules to the freeAdv. Mater. Interfaces 2024, 11, 2300274 2300274 (4 of 10) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2024, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300274 by National Institute For, Wiley Online Library on [24/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensewww.advancedsciencenews.com www.advmatinterfaces.deFigure 5. Time course of A) moisture absorption and B) mechanical property changes of PVA and PVA/cellulose composite films with different compo-sitions under 100% relative humidity exposure.hydroxyl groups of cellulose, which promoted the diffusion ofwater molecules to the cellulose microcrystal interface.[32] Inter-estingly, the swelling of the film showed a small peak after 3 h,and the swelling rate tended to decrease slightly after 6 h. Sim-ilar reductions in the swelling rate upon moisture or water ab-sorption have been reported for other PVA-based materials.[38,39]On the other hand, pure PVA absorbed more water, with itsmoisture absorption rate reaching 1.55 after 24 h, in contrast tothe saturated moisture absorption changes of the PVA/cellulosecomposites. In the pure PVA films, moisture absorption wasdriven by the hydroxyl groups of PVA attracting water moleculesthrough hydrogen bonds, until reaching 100% equilibrium at100% humidity. On the other hand, the strong intra- and inter-molecular hydrogen bonds in the composite films blocked wa-ter molecules from accessing the cellulose microcrystal interiors.Thus, increasing the cellulose content in the composite films de-creased the volume fraction of the PVA matrix, which contributedpositively to the swelling and moisture absorption and resultedin a composition-dependent saturated-type moisture absorptionbehavior.The wetting behavior of water on the surface of PVA/cellulosecomposites was evaluated by contact angle measurement (FigureS3, Supporting Information). The contact angle of the dry PVAfilm immediately after the drop of water was 91.5 ± 9.6°. Com-positing PVA with cellulose microcrystals affected the wettabil-ity of the films, with contact angles of 85.0 ± 12.3, 77.5 ± 4.3,and 108.2 ± 10.1° for films with PVA to cellulose ratios of 4/1,3/1, and 2/1, respectively (Figure S3B, Supporting Information).The cellulose content of the composite film at 25 wt.% was foundto be predominantly hydrophilic, while further increase in cellu-lose content was found to be hydrophobic. Changes in surfacechemical structure with time occur on film surfaces in contactwith water, resulting in the contact angle values after 300 s were83.0 ± 7.8, 76.5 ± 13.7, 63.2 ± 4.7, and 82.8 ± 4.2° for PVA andPVA/cellulose composite films with 4/1, 3/1, and 2/1 ratios, re-spectively. Interestingly, while a wetting ridge was observed in thePVA film a short time after the drop of water, no such ridge wasobserved in the PVA/cellulose composite film in either compo-sition (Figure S3A, Supporting Information). The formation of awetting ridge may indicate that PVA swells in contact with waterrather than dissolves,[40,41] suggesting that this swelling behavioris significantly suppressed by compositing PVA with cellulose.These results suggested that the moisture absorption rate of thePVA/cellulose composites can be effectively controlled by the cel-lulose content.The water or moisture absorption in PVA and its compositefilms significantly affects their mechanical properties, which inturn plays an important role in understanding the internal struc-ture of a material. Figure 5B shows the change in the elastic mod-ulus of films exposed to 100% relative humidity for a given time.Here, both the PVA and PVA/cellulose composite films showeda rapid decrease in their elastic modulus after moisture absorp-tion for 3 h. The decrease in the modulus due to moisture ab-sorption was significantly suppressed by the presence of cellu-lose, and the moduli of the PVA/cellulose composites after 24 hwere 6 to 18 times higher than that of the PVA film. The degreeto which the elastic modulus reached equilibrium after moistureabsorption was strongly correlated with the cellulose content inthe composites, clearly indicating that the presence of celluloseresulted in the structural strengthening of the film under wetconditions. In other studies on PVA/cellulose composites, it hasbeen shown that the mechanical properties, such as the elasticmodulus and tensile strength, were improved several-fold when20% to 90% cellulose was introduced, without considering mois-ture absorption.[42–44] In this study, the effect of the cellulose filleron the films’ mechanical properties was more pronounced un-der wet conditions. The presence of free hydroxyl groups in PVAattracted water molecules, resulting in the disruption of hydro-gen bonds in the PVA matrix, which in turn reduced the me-chanical properties. Thus, water molecules acted as plasticizersin this system. As shown by the moisture absorption propertiesin Figure 5A, the PVA/cellulose composites exhibited reducedmoisture absorption and film swelling rates depending on thecellulose content. Because the films were fully plasticized at theequilibrium swelling state after 24 h, the elastic modulus of thecomposite films was considered to be dictated by the crosslinkdensity. The DSC and FTIR results in Figures 2 and 3 further cor-roborate the formation of additional strong hydrogen bonds be-tween the PVA chains and cellulose fillers. In the PVA/cellulosecomposites, hydrogen bonds existed as cross-linking points thatremained stable under wet conditions. In other words, the in-terfacial interactions, primarily hydrogen bonds, between thePVA chains and cellulose fillers remained undisturbed and stablewhen exposed to wet environments. These results suggested thatAdv. Mater. Interfaces 2024, 11, 2300274 2300274 (5 of 10) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2024, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300274 by National Institute For, Wiley Online Library on [24/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensewww.advancedsciencenews.com www.advmatinterfaces.deFigure 6. A) Photographs of humidity-responsive shape fixation and shape recovery process for spiral-shaped PVA and PVA/cellulose composite films.Time curves of B) long-axis length and C) shape recovery rate of temporarily stretched spiral-shaped films under exposure to 100% relative humidity.our PVA/cellulose composite films can greatly mitigate the loss ofmechanical properties associated with moisture absorption andswelling, and can thus be employed in a variety of applications.[45]2.3. Humidity-Responsive Shape Memory Performance ofPVA/Cellulose Composites: Shape Maintenance and RecoveryAbilityTypical PVA hydrogels form many hydrogen bonds between theirmolecular chains and have the potential to become shape mem-ory materials driven by the reversibility of hydrogen bonds andthe decrease in the glass transition temperature in response tomoisture absorption.[46,47] However, pure PVA cannot sufficientlyfunction as a shape memory material in a wet environment, un-less chemical cross-linking is introduced.[48,49] Recently, it wasreported that the shape memory capacity of PVA can be im-proved by introducing additional intermolecular hydrogen bondsvia compositing with specific compounds that promote hydrogenbonding.[50–52] Therefore, the humidity-responsive shape mem-ory behavior of PVA and PVA/cellulose composites was investi-gated.As shown in Figure 6, the PVA/cellulose films exhibited clearshape fixation and shape-recovery abilities even after moistureabsorption. Films with a permanent spiral shape were preparedby heating a sample placed in an indoor humid environment at120 °C for 30 min while it was being deformed (Figure 6A). Theheat treatment of the PVA and PVA/cellulose films at 120 °C al-lowed the creation of a new permanent shape with the removalof residual water and the reformation of hydrogen bonds in thefilms, and the resulting films showed a clear Tg ≈70 °C, indepen-dent of their composition (Figure S4A, Supporting Information).The spiral-shaped film was allowed to stand overnight while a 5 gload was applied, and then changed to a stretched shape (tempo-rary shape) when the load was removed. The change from a spi-ral shape to a temporarily extended shape was attributed to theelongation of the PVA chains owing to the breakage of the rel-atively weak hydrogen bonds under a force load, that preventedthe elongated chains from maintaining their expanded shape.[53]This force-induced stretching of the spiral shape (Figure 6Bshows the length of film at 0 min) was slightly suppressed inthe PVA/cellulose composites, compared to pure PVA. On theother hand, when the temporarily-extended spiral-shaped filmswere exposed to high humidity, a decrease in length was observedin all samples in the first 15 min, implying a shape recovery tothe original spiral shape (Figure 6B). Figure 6C shows the de-gree of shape recovery relative to the original spiral shape. After15 min of exposure to high humidity conditions, the shape recov-ery rates were 42%, 45%, 31%, and 13% for PVA/cellulose ratiosof 4/1, 3/1, and 2/1 and pure PVA, respectively. This clearly showsthat the PVA/cellulose composites exhibited superior shape re-covery compared to the pure PVA films, in addition to a higherresistance to deformation under force loading. Although the purePVA film showed a slight shape recovery after 15 min, it alsolost its original spiral shape after prolonged exposure to a high-humidity environment. In contrast, the PVA/cellulose compos-ites were able to maintain their spiral shape after recovery for alonger period, with a shape recovery rate of 36%, 36%, and 24%Adv. Mater. Interfaces 2024, 11, 2300274 2300274 (6 of 10) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2024, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300274 by National Institute For, Wiley Online Library on [24/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensewww.advancedsciencenews.com www.advmatinterfaces.deFigure 7. A) Photographs of humidity-responsive shape fixation and shape recovery process of curly hair bundles coated with PVA and PVA/cellulosecomposites. Time curves of B) length and C) shape recovery rate of temporarily stretched curly hair bundles when exposed to 80% relative humidity.for PVA/cellulose ratios of 4/1, 3/1, and 2/1, respectively, after360 min. The main driving force behind the shape recovery wasthe glass/rubber transition associated with moisture absorption,where water molecules disrupted the hydrogen bonds in the filmand simultaneously acted as plasticizers, shifting the Tg of thePVA matrix toward a lower value.[46] Under ambient humidity,the Tg of PVA and PVA/cellulose composites (≈70 °C in dry con-dition) was higher than room temperature (25.5 ± 1.5 °C), allow-ing them to maintain their temporary shape; however, under highhumidity conditions, the Tg decreased below the room temper-ature, causing gravity-defying shape recovery owing to entropyelasticity.These results also indicated that the hydrogen bonds formedbetween the PVA chains and cellulose provided sufficient andstable cross-linking points to maintain the original spiral shape,even after prolonged exposure to high-humidity conditions.Interestingly, in the 20%–33% cellulose content range, therewas no linear correlation between the cellulose content in thePVA/cellulose composites and the shape recovery ability. Al-though the PVA chains initially contributed to shape fixationand recovery significantly, upon moisture absorption for 1 h, theshape-recovery rate dropped to almost 0% because of the col-lapsing hydrogen bonds and the subsequent impairment of themechanical properties. The PVA composite with the highest cel-lulose content of 33% exhibited lower moisture absorption andhigher mechanical strength (Figure 5); however, its shape recov-ery was inferior to those with ratios of 20% and 25%. These re-sults indicated that the quantitative balance between the PVAmatrix responsible for shape memory switching, and the cellu-lose microcrystals providing stronger hydrogen bonds, is criticalfor achieving optimal shape memory properties. Here, the max-imum shape recovery and retention rate of the recovered shapewas achieved at a cellulose content of 20%–25% (PVA/cellulosecomposition of 4/1 to 3/1).2.4. Application of PVA and PVA/Cellulose Composites onNatural HairAnimal hair, including human hair, is composed of 𝛼-keratinfibers, a water-sensitive shape memory polymer, that exhibitsshape-fixing and shape-recovery properties owing to the recombi-nation of intra- and intermolecular hydrogen bonds in the keratinmolecules.[54,55] However, its ability to maintain fixed hair shapesin humid environments is low, creating a need for new productsthat can effectively maintain hairstyles.To investigate whether PVA and PVA/cellulose composites canbe used to maintain hairstyles under high humidity conditions,aqueous solutions with different PVA/cellulose ratios were ap-plied directly to natural hair, and curls were created using ahair iron at 180 °C. Experiments with films confirmed that heattreatment at 180 °C, the ironing temperature, has little effecton the Tg of PVA and PVA/cellulose composites (Figure S4B,Supporting Information). As a control sample, non-polymer-containing water was used to create a curl shape in the same way(Figure 7). Although all curly hair bundles were adjusted tothe same length during hot ironing, the water-treated bundleswere longer than their PVA- and PVA/cellulose composite-treatedequivalents (Figure 7A). This indicated that the inherent shapememory properties of hair can be greatly improved by coatingAdv. Mater. Interfaces 2024, 11, 2300274 2300274 (7 of 10) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2024, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300274 by National Institute For, Wiley Online Library on [24/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensewww.advancedsciencenews.com www.advmatinterfaces.dewith polymer composites. The length of hair swatches whenPVA-coated hair was curled on the hair iron was Lcurl, and theactual length of hair swatches after ironing at 180 °C for 30 sand then removing from the hair iron is L. The shape fixing rate(Rf) of PVA-coated hair can be calculated by Rf (hair) = L / Lcurlx 100, and the shape fixing rate of hair bundle Rf (hair) value was93.7 ± 3.6%. The hair samples coated with PVA were found tostretch 31.3 ± 1.9% under their own weight when hung for eval-uation (e.g., top photo in Figure 7A). When the curled hair bun-dles were then stretched overnight by applying a force load, asin the film shape memory test (Figure 6), there was no signifi-cant difference in their lengths; however, compared to water, thePVA and PVA/cellulose coatings notably suppressed spreadingand frizzing. When the temporarily stretched hair bundles wereplaced in a chamber with 80% relative humidity, their lengthswere shortened by returning to the original curly hair shape. Af-ter 360 min of high-humidity exposure, the water-treated hairbundle showed the poorest curl retention, while the hair bundlescoated with PVA and the composite solution with a PVA/celluloseratio of 4/1 maintained their curl shape better than the other com-positions (Figure 7B). As shown in Figure 6, the PVA/cellulosecomposite films showed a significantly higher shape recoveryrate and subsequent shape maintenance than pure PVA, and thePVA film did not show any notable shape recovery performanceafter exposure to high humidity for an extended period, whereasthe PVA and composite-treated hair bundle recovered ≈10% ofits shape and maintain it thereafter.Hydrogen bonding between the hair and the applied materialis crucial for hair styling. Here, although the application of PVAor the PVA/cellulose composite had a strong enhancing effect onthe shape memory of the hair curls, no significant shape recoverywas observed when applying the film. The interaction betweenhair and the coating material is paramount for realizing a shapememory action. Because the PVA and cellulose used in this studyare hydrophilic and neutral, their interaction with hair was con-sidered to primarily stem from hydrogen bonds. Under insuffi-cient hydrogen bonding between the hair and the coating, theforce generated by the shape-recovery effect of the applied ma-terial cannot be transferred to the hair. Furthermore, it is possi-ble that the cellulose microcrystals used in this study were rel-atively large in size, and thus inefficiently incorporated into thePVA matrix formed on the hair; in fact, in PVA-coated hair bun-dles, the formation of PVA film was observed between hairs bydigital microscopy (Figure S5C, Supporting Information), but itwas clearly thinner than the size of cellulose microcrystals, andthis could have led to the poor performance observed when thePVA/cellulose composite film was directly applied on hair. Nev-ertheless, the presence of the coating effectively inhibited waterpenetration into the hair under high humidity, resulting in re-duced hair bundle spreading and frizz formation. Furthermore,the PVA coated on the hair could be easily washed off with warmwater or shampoo, despite its stability under high humidity con-ditions (Figure S5, Supporting Information). As such, our pre-liminary findings showed that our proposed PVA/cellulose com-posite coating can slightly restore and maintain the curly shapeof stylized hair under heavy-humidity conditions. In the future,it will be important to develop hair-coating agents with ionic andhydrophobic properties that can effectively improve interactionwith the hair even with thin application.3. ConclusionThe chemical structure, thermal and mechanical properties, sur-face morphology, and shape memory retention of PVA/cellulosecomposite films with different compositions in response to hu-midity were investigated. The PVA/cellulose composites showedgood compatibility and miscibility owing to efficient hydrogenbonding, allowing the formation of uniform films. The presenceof hydrogen bonds formed between PVA and cellulose in thecomposite, as revealed by chemical structure analysis, affectedthe film’s thermal, hygroscopic, and humidity-responsive shapememory properties. When PVA films are exposed to high hu-midity conditions, moisture absorption persists for a long pe-riod of time until equilibrium is reached, resulting in a decreasein mechanical strength. The compositing of PVA with cellu-lose markedly inhibits the uptake of water molecules into thefilm and allows the film to maintain low moisture absorptionstates for long periods of time. PVA/cellulose composites can alsomaintain high mechanical properties during moisture absorp-tion, and a positive correlation was observed between the com-position of the composite and its mechanical properties. Evalua-tion of humidity-responsive shape memory using films showedthat composites with cellulose content from 20% to 25% exhib-ited superior shape recovery and maintenance, whereas thosewith cellulose content of 33% adversely reduced their perfor-mance. These results suggest that cellulose is an effective fillerto reinforce PVA against humidity, and that the balance betweenPVA and cellulose content in the composite is critical to achieveboth moderate moisture absorption (water) resistance and shapememory properties. Preliminary experiments with natural hairsamples showed that both pure PVA and PVA/cellulose compos-ite films can effectively maintain the hairstyle under high hu-midity conditions, while preventing spreading and frizzing. Par-ticularly, when exposed to 80% relative humidity, natural haircoated with PVA or PVA/cellulose composites showed an ≈10%shape recovery rate from a temporarily extended state to theoriginal curly shape. To the best of our knowledge, this is thefirst study to report the application of PVA and PVA/cellulosecomposites as humidity-responsive shape memory polymer haircoatings that help style maintenance via moisture-absorptionprevention. When directly applied to hair, the effect of com-positing PVA with cellulose was barely noticeable; however, thecomposite film showed promising results under experimentalconditions. For future practical applications of PVA/cellulosecomposites as moisture-resistant coatings or styling agents onhair, further research will be needed to increase the affin-ity and interaction between the hair and the polymers andfillers.4. Experimental SectionMaterials: PVA (degree of polymerization 1700, saponified 88%), mi-crocrystal cellulose, and Japanese hair swatches were obtained from NI-HON L’ORÉAL K.K (Tokyo, Japan). All other chemicals used in this studywere of laboratory grade.Preparation of PVA/Cellulose Polymer Solutions and Films: 10 wt.% PVAaqueous solution was prepared using 20 g PVA and 180 g distilled waterin a glass bottle. After stirring for 30 min at room temperature (in thisstudy, the range of 24—27 °C), the glass bottle was continuously stirred atAdv. Mater. Interfaces 2024, 11, 2300274 2300274 (8 of 10) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH 21967350, 2024, 1, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/admi.202300274 by National Institute For, Wiley Online Library on [24/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensewww.advancedsciencenews.com www.advmatinterfaces.de95 °C in an oil bath with a half-sealed cap for 3 h. The PVA aqueous solutionwas then cooled down to room temperature and stirred until producing ahomogeneous solution. For the PVA/cellulose blend solution, an aqueoussolution with 10 wt.% cellulose suspension was prepared first, and thenmixed with a 10 wt.% PVA solution at PVA/cellulose weight ratios of 2/1,3/1, and 4/1. For example, for PVA/cellulose = 2/1, 8 g of 10 wt.% PVAsolution was mixed with 4 g of 10 wt.% cellulose solution. After homoge-nization via stirring for 1 h and ultrasonication, 10 g 10 wt.% of the blendsolution was added to the cast film. Air bubbles were removed from theblended solution through sonication for at least 30 min. The films werecast by pouring 8 g of each blend solution with different PVA/cellulose ra-tios into a plastic dish with a diameter of 10 cm. The dishes were incubatedovernight on a horizontal stand in a 60 °C oven. Finally, the PVA/cellulosefilms were peeled off from the dishes using tweezers after cooling to roomtemperature. The PVA/cellulose films were half-transparent with a thick-ness of ≈0.13 mm.Characterization of PVA/Cellulose Films: The thermal properties of thefilms (5 mg) were evaluated using differential scanning calorimetry (DSC,EXSTAR6000, Seiko Instruments Inc., Japan) at a heating rate of 10 °Cmin−1. Attenuated total reflection-Fourier transform infrared spectroscopy(ATR-FTIR) measurements were performed for film surface analysis us-ing a Thermo Scientific Nicolet 4700 spectrometer (Thermo Scientific,Waltham, MA, USA). The morphology of the films was observed usingscanning electron microscopy (SEM, S-4800, Hitachi High-Technology,Tokyo, Japan). The films were coated with a 30 s gold sputter before ob-servation. The contact angle values for PVA and PVA/cellulose compositefilms were measured using a contact angle meter (Drop Master-SA-Cs 1,Kyowa Interface Science Co., Ltd., Japan). Water was used as the probeliquid, and images of the contact angle were taken every 100 ms for 300 safter 5 μL water was dropped on the film surfaces. The results are reportedas the mean ± standard deviation, n > 5.Moisture Absorption and Mechanical Properties of PVA/Cellulose Films:The films were cut into 0.5 × 3 cm dimensions and dried at 100 °Covernight to remove all the absorbed water. Then, they were placed in achamber with 100% humidity. The films were removed after 15, 30 min,1, 2, 3, 6, and 24 h, respectively. The moisture absorption was calculatedas the ratio of the film weight after absorption (W1) to that before absorp-tion (W0). The elastic moduli of the films were determined using a tensiletest machine (EZ-S, Shimadzu Co., Japan). The results are reported as themean ± standard deviation, n = 3.Shape Memory Properties of PVA/Cellulose Films and Hair Swatches:The shape memory evaluation included three main steps: 1) curl shapemaking, 2) stretching the curls, 3) humidity-responsive curl recovery. Thefilms were cut into 0.5 × 6 cm pieces and enwound on a glass stick, onwhich they were fixed by clipping. The glass stick was incubated in anoven at 120 °C for 30 min to produce a spiral-shaped film. The films werethen removed from the glass stick after cooling to room temperature. A5-gram load was then applied to the spiral-shaped films, left overnight atindoor humidity, and then the load was removed to produce temporarilyextended spiral-shaped films. The films were then placed in a chamber with100% relative humidity at room temperature to evaluate their humidity-responsive shape memory performance. This process was video recordedfor 6 h to produce a time-lapse movie, from which the length of the filmswas analyzed using the ImageJ software (National Institutes of Health,Bethesda, MD). The shape memory properties were evaluated through thefilm length before and after the humidity treatment. The shape recoveryrate was calculated as (L0 − Lmin)/L0 (L0 = film length after overnight ex-tension; Lmin = film length in a specific minute in the humidity chamber).For preliminary application in hair styling, Japanese hair swatches (1 g)were coated with 300 μL of 3 wt.% PVA/cellulose solutions (2/1, 3/1, 4/1),pure PVA, and water. After forming curls via hair ironing at 180 °C for30 s, the curly hair bundles were extended using 13 g weights via the samemethod described above. The hair swatches were then put under 80% hu-midity at ≈30 °C for 6 h to record a time-lapse movie. The hair curl main-tenance and shape recovery rate were analyzed in the same way as withthe films. The results are reported as the mean ± standard deviation, n =3. The surface morphology of clean hair, PVA-coated hair, and PVA-coatedhair that had been washed with warm water or shampoo was obtained bydigital microscopy. Images of pristine hair, PVA-coated curly hair beforeand after warm (42 °C) water or shampoo washing were acquired by thebright field mode of digital microscopy.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThis study was supported by the JSPS KAKENHI Grant-in-Aid for ScientificResearch (B) [JP19H04476, M. E.], Grant-in-Aid for Scientific Research (C)[JP21K12696, K. U.], Grant-in-Aid for Transformative Research Areas (A)[JP20H05877, M. E. and K. U.], and Innovative Science and TechnologyInitiative for Security [JPJ004596, K. 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