Superfast and Reversible Thermoresponse of Poly(N

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Superfast and Reversible Thermo-Response of Poly(N-isopropylacrylamide) Hydrogels Grafted on Macroporous Polyvinyl Alcohol Formaldehyde Sponges Yanxiong Pan, Bingrui Li, Zhi Liu, Zhongyu Yang, Xu Yang, Kai Shi, Wei Li, Chao Peng, Weicai Wang, and Xiangling Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12395 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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ACS Applied Materials & Interfaces

Superfast and Reversible Thermo-Response of Poly(N-isopropylacrylamide) Hydrogels Grafted on Macroporous Polyvinyl Alcohol Formaldehyde Sponges

Yanxiong Pan†, Bingrui Li†, Zhi Liu†, Zhongyu Yang§, Xu Yang†, Kai Shi†, Wei Li†, Chao Peng†, Weicai Wang†, Xiangling Ji†,* †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Changchun 130022, P. R. of China § Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108, United States of America

ABSTRACT Poly(N-Isopropylacrylamide) (PNIPAAm), a typical thermo-responsive polymer, exhibits potential application in smart materials. However, bulk PNIPAAm hydrogel monoliths undergo slow volume phase transition at least in tens of minutes or hours, as determined by the shape and size of polymers due to the formation of skin layer. In this regard, novel macroporous sponges with rapid thermo-response are prepared via grafting polymerization of N-isopropylacrylamide (NIPAAm) onto the macroporous polyvinyl alcohol formaldehyde (PVF) network, as confirmed by attenuated total reflection-infrared (ATR IR) and 1H-NMR spectra. As-prepared PVF-g-PNIPAAm sponges display interconnected open-cell structures, and their average pore sizes and porosities are approximately 90 µm and higher than 85%, respectively. The equilibrium swelling ratio of PVF-g-PNIPAAm sponges varies from 11 to 50 with temperature. The volume phase transition temperature is at 30-34°C, as detected in the DSC curves of swollen samples. These features indicate that the existence of original PVF network exerts almost no influence on the PNIPAAm temperature responsibility. As-prepared samples can reach the swelling equilibrium in less than 80 s, and their rapid swelling kinetics can be fitted using the pseudo-first-order rate kinetic equation. 1

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Notably, the samples also display rapid deswelling rate in less than 40 s at relative high temperature (48°C), thereby indicating a superfast responsive behavior to temperature change. The PVF-g-PNIPAAm sponges exhibit rapid and reversible thermo-response in repeatable swelling–deswelling cycles, which can satisfy the need of special smart materials. Particularly, combined with iodine solution (i.e. PVF-g-PNIPAAm/I2), these sponges can serve as a novel temperature indicator and exhibit excellent antibacterial performances. KEYWORDS:

thermo-response,

poly(N-Isopropylacrylamide),

macroporous,

polyvinyl alcohol formaldehyde sponge, grafting polymerization, temperature indicator, antibacterial materials

2


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1. INTRODUCTION Poly(N-isopropylacrylamide) (PNIPAAm), a thermo-sensitive polymer, has been investigated extensively because of its volume phase transition temperature (VPTT) in water is approximately 32°C, which is close to the physiological temperature.1 Thermo-sensitive PNIPAAm hydrogels exhibit potential applications in drug release, smart coating, shape memory material, thermo-sensor, and structure engineering.2,3 The outmost chain conformation of PNIPAAm hydrogel could reversibly change from the flexible coil into the compact globular state with the increase of temperature4 and eventually form the “dense skin layer” on the surface of traditional PNIPAAm hydrogels once the environmental temperature is higher than the lower critical solution temperature (LCST).5-6 The presence of such “dense skin layer” would prevent the out-diffusion of free water and cargos from the hydrogel matrix and lead to responsive hysteresis; for example, the time to reach swelling and deswelling equilibrium ranges from a few hours to a few days, which is one of the most disadvantages for its practical applications.7-8 To date, the strategies of preparation of PNIPAAm hydrogels with rapid thermo-sensitivity can be divided into chemical and physical processes. Chemical processes, such as incorporation of hydrophilic segment,9 microgel,10 micelle structure,11 and interpenetrating polymer network,12 have been investigated extensively.

Furthermore,

physical

processes,

including

preparation

of

phase-separated heterogeneous13-15 or porous structures using phase separation,16-18 porosigen,19-20 ice template,21 and emulsion template,22 have also been widely applied 3



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in the preparation of rapidly thermo-sensitive hydrogels. Zhang and coworkers16,19 reported the PNIPAAm hydrogels with interconnected porous structure using water and tetrahydrofuran as a mixed solvent or PEG as pore-forming agent during polymerization and crosslinking procedure; additionally, the hydrogels can reach the equilibrium swelling in 20-260 min. David et al.17 studied the rapid deswelling kinetics in 40-130 min of ternary PNIPAAm/PROZO/PHEMA copolymer hydrogels due to incorporation of hydrophilic moieties, improved hydrophobic/hydrophile balance, and long open channels. Zheng et al. reported that incorporation of other hydrophilic segments such as poly(ethylene oxide),13 poly(vinyl pyrrolidone)14 or poly(sodium

p-styrenesulfonate)15

are

effective

strategies

to

increase

the

thermal-response behavior of PNIPAAm hydrogel. Chu’s group21 synthesized PNIPAAm-based nanostructured smart hydrogels with rapid response (600-1800 s) and high elasticity (elongation at approximately 1,710%). Comb-type grafted P(NIPAAm-co-AA) hydrogels show dual thermo- and pH-sensitive response, and deswelling rate is longer than 60 min.23 The PNIPAM-silica nanocomposite hydrogels were prepared and they exhibited fast swelling/deswelling thermo-responsive behavior (less than 90 s) and high force response due to controlled pore wall thickness.24

Maeda

et

al.25 prepared

PNIPAAm-based

hydrogels

through

electrospinning method; these fiber hydrogels displayed a rapid thermo-response without a two-step shrinking process similar to a conventional PNIPAAm hydrogels. Gancheva used PCL as a template and prepared the PNIPAAm hydrogels with tunable pore size of 20-300 µm; these hydrogels exhibited rapid swelling and deswelling 4


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responses in a few minutes.26 PNIPAAm-clay nanocomposite hydrogels displayed excellent responsive bending and elastic properties during temperature modulation.27 PNIPAAm-cellulose nanofiber composite hydrogels showed short deswelling rate (approximately 10-60 min) and high compression properties.28 Grafting polymerization is an effective and versatile approach to modify the surface chemical composition and functional groups of original materials and obtain the materials with desired properties. Variable monomers into poly(vinyl alcohol) (PVA) were prepared using different techniques, such as esterification reaction,29,

30

ring-opening polymerization,31 and redox polymerization by ceric (IV) ions due to the existence of a considerable number of active sites in PVA molecules. The ceric (IV) initiator can produce the radicals at the PVA backbone via single-electron-transfer process, and the monomer can be effectively grafted onto the PVA chains. Traditional polyvinyl alcohol formaldehyde (PVF) sponges with macroporous structure exhibit advantages, such as open-cell structure, good mechanical property at low apparent density, excellent biocompatibility, and super softness at wet state, which make these hydrophilic materials applicable in daily cleaning and medical health areas. Given the large number of hydroxyl groups in the traditional PVF network, the novel macroporous PVF sponges with hydrophobic and hydrophilic surface properties are easily realized through modification.32-34 In the present study, the macroporous PVF-g-PNIPAAm sponges with rapid thermo-sensitivity were prepared via redox polymerization of NIPAAm onto the PVF networks. The chemical composition, pore structure, and rapidly swelling–deswelling as well as antibacterial 5



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behavior of sponges were systematically investigated. To the best of our knowledge, it is the first time the macroporous PVF-based thermosensitive PVF-g-PNIPAAm sponges are synthesized via grafting polymerization. 2

EXPERIMENTAL SECTION 2.1. Materials. Polyvinyl alcohol powder with a degree of polymerization 2000

and saponification degree of 99% was purchased from Shanxi Sanwei Group Co. Ltd. (China). Triton X-100, ceric ammonium nitrate (CAN), N,N′-Methylenebis (acrylamide)

(MBAA),

N,N,N′,N′-Tetramethylethylenediamine

(TEMED),

Ammonium persulfate (APS) and N-isopropylacrylamide (NIPAAm) were obtained from Aladdin (China). Formaldehyde and other reagents, such as ethanol, concentrated sulfuric acid, and disodium hydrogen phosphate, were purchased from Beijing Chemical Works and used as received. Gram-negative E. coli (ATCC 25922) and gram-positive S. aureus (ATCC 6538) were purchased from HuanKai Microbial Company (Guangzhou, China). 2.2. Preparation of Pristine PVF Sponges. A typical preparation of PVF sponge was reported in literature.35 Typically, a certain amount of PVA was dissolved in 450 g of hot water by vigorous stirring with a magnetic stir bar at 95oC until complete dissolution. Subsequently, calculated formaldehyde and Triton X-100 were poured into 60 g of hot PVA solution under vigorous stirring. The liquid froths were obtained after 5 min. Afterward, 30 mL of 50 wt% H2SO4 was poured into the above froth at room temperature. After reaching a maximum volume, the froths were poured into a mold and cured in an oven at 60oC for 5 h. Raw samples were washed with water at least five times to remove unreacted reactants. Finally, the sponge (apparent density,

6


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0.073 g.cm−3), named as PVF sponge, was obtained after drying at 60°C to a constant weight. The basic characterization of PVF can be found in our previous report.30 The degree of acetalization of the original PVF, calculated using H1-NMR spectra, is 74.6, while the average of pore size and porosity is ~ 60 µm and 95.0%, respectively. 2.3.

Preparation

of

Macroporous

PVF-g-NIPAAm

Sponges.

Grafting

polymerization of NIPAAm on the macroporous PVF was carried out using CAN as initiator under nitrogen atmosphere at room temperature.33-34,36 Typically, 2 g (10 mmol) of PVF was swollen in 100 mL of nitric acid solution (0.01 M) and kept for 0.5 h under nitrogen stream. Subsequently, 20 mL of diluted nitric acid solution (0.01 M) was dissolved with 0.548 g (1 mmol) of CAN and added to the flask. Afterward, 11.316 g (100 mmol) of NIPAAm was added to the above mixture under stirring at 25oC for 24 h. As-prepared samples were washed completely with deionized water/ethanol mixture to remove unreacted reactants and dried in a vacuum oven at 60oC until constant weight was attained; these samples were named as PVF-g-PNIPAAm-t, where t is the reaction time. The grafting percentage (GP) and grafting efficiency (GE) of PVF-g-PNIPAAm-t sponge were calculated using gravimetric method through the following equations: GP = (W2-W0)/W0× 100,

(1)

GE = (W2-W0)/W1× 100,

(2)

where W0, W1, and W2 were the weights (g) of pristine PVF, NIPAAm, and PVF-g-PNIPAAm after the homopolymer was removed, respectively. To confirm the accuracy of GP and GE values obtained using the above equations, the GP and GE were also calculated through the resonance in 1H-NMR spectra. 2.4. Preparation of macroporous PNIPAAm (macro-PNIPAAm) gel. To prepare the macro-PNIPAAm hydrogel, 1 g (8.8 mmol) NIPAAm and 0.15 g (0.1 7



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mmol) MBAA were dissolved in 20 mL DD water and degassed with high-purity nitrogen at 0ºC for 30 mins. Then, 0.02 g (0.09 mmol) APS and 20 µL (0.13 mmol) TEMED were added into the system under vigorously stirring for 30 seconds. The well-mixed solution was moved into a 10 mL syringe and kept at -12oC for 12 hr. The sample was thawed at ambient temperature, washed with DD water for at least five times, and further vacuum dried at 40 ºC for 24 hr. 2.5. Measurement of Swelling Ratio. The gravimetric method was utilized to monitor the swelling behavior of PVF-g-PNIPAAm at 20oC in deionized water. At certain time intervals, the samples were removed from the water, and the weights of wet sponges were recorded after being drained for 0.5 min on stainless steel mesh (pore size: 0.15 mm). The absorption amount (Qt) at time t was defined as follows: Qt = (Wt – W3)/W3

(3)

where W3 and Wt were the weights of dried and wet samples at time t, respectively. The state of equilibrium swelling could be obtained after immersion in water for 10 min. The saturated absorption capacity (Qs) could be obtained as follows: Qs = (Ws - W3)/W3

(4)

where W3 and Ws were the weights of dried and saturated samples, respectively. The Qt and Qs can also represent the swelling ratio at time t and equilibrium state, respectively. The kinetics of deswelling ratio of samples were also measured using gravimetric method through immersing samples in hot water at 48oC.37 Specifically, A sample saturated in cold water was immersed into hot water at 48ºC for the needed interval, ca. 5 s, and subsequently placed onto the stainless-steel mesh. After equilibrating for 30 s, the sample was weighed to obtain the deswelling data point at 5 s. The weighed sample was then placed back to cold water for 2 mins to reach the swelling 8


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equilibrium. This saturated sample was then placed to hot water (48ºC) for the next time point, ca. 15 s, followed by equilibrating on the mesh and weighing. Similarly, data points at 30, 45, and 60 s were obtained. The swelling ratio was Qt and calculated using Equation (3). The equilibrium swelling ratio was absorption capacity Qs and calculated using Equation (4). All of the representative data points of swelling and deswelling performances result from three specimens of the same sample. The standard deviation of three specimens was used as the error bar for each data point. 2.6. Rapid Swelling–Deswelling Behavior. PVF-g-PNIPAAm sponges were also evaluated by temperature cycle. The swollen sponges in deionized water at 20°C were transferred into hot water at 48°C. Their weights at 30, 60, and 120 s were measured, respectively. Once the sample weight reached the absorption equilibrium, the samples were placed into the water at 20°C. Weight changes at 30, 60, and 120 s were monitored, respectively. The iodine-PVF complex was prepared via immersing the dried PVF-g-PNIPAAm with size of 15 mm × 25 mm × 5 mm into water at ambient temperature, then five droplets of I2 aqueous solutions (I2 concentration was 1 wt%) were added, after drying, the samples were used for thermo-responsive tests. Specifically, the PVF based-iodine sample with blue color was placed into the water medium with temperature controller, the color change with temperature was detected. In order to investigate the influence of temperature on volume and color change of iodine-PVF, which was prepared and transferred into one end terminated glass tube and observed its volume and color change in cold water (27oC) and hot water (48oC), respectively. 2.7. Antibacterial Performance. The antibacterial activity of PVF based sponges before and after adsorbed the iodine was evaluated by GB/T 20944.3-2008, specifically, Gram-negative E.coli (ATCC 25922) at concentrations of about 2.7 × 104 CFU/sample and Gram-positive S.aureus (ATCC 6538) at concentrations of about 3 × 9



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104 CFU/sample were used for the tests. In brief, a colony from the LB agar plate was taken with a sterilized tip and placed into 10 mL of LB broth medium for incubating at 37ºC for 12 hr with shaking at 200 rpm. Then this bacterial suspension was placed on

LB

plates for evaluating

the antibacterial behavior

of

PVF/I2

and

PVF-g-PNIPAAm/I2. Then the incubated bacterial suspension was diluted using a sterilized phosphate buffer (pH = 7.0, 0.03 M) to the desired concentration (104 CFU/mL) and 0.75 g different sponges (particularly, PVF/I2 and PVF-g-PNIPAAm/I2 samples were prepared through adding five droplets of I2 aqueous solution into PVF or PVF-g-PNIPAAm sponges swollen in 80mL water, after drying, the samples were available) were added into the suspensions. After shaking at 37°C with 200 rpm for 18 h, the solution was diluted to 50-fold and 50 µL diluted solution was plated on trypticase soy agar (TSA) plates. The plates were incubated at 37°C for 24 h. The total colony forming units (CFU) was counted using the viable cell count method.38 2.8. Instruments and Characterization. Fourier transform infrared spectrum was obtained on Bruker Vertex 70 spectrometer with attenuated total reflection (ATR) attachment.

1

H-NMR spectrum was determined on a Bruker AV 400 NMR

spectrometer in deuterium oxide. For scanning electron microscopy (SEM) tests, the dried samples were cut into sheets and immersed into liquid N2 for 5 min. Then these samples were quickly broken to obtain a random brittle-fractured surface, which was then coated with a layer of gold for SEM observation. SEM was conducted through a field-emission environmental SEM (Micro FEI Philips XL-30-ESEM-FEG) operating at 15 and 20 kV. Pore size and porosity were obtained by an automatic intrusion 10


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porosimeter (Autopore IV 9500, Micromeritics, USA) using mercury as the intrusion reagent, which could immerse into the open-cell of sample under a certain pressure. All the samples were run under lower pressures to fill the penetrometer with mercury, and subsequently, under higher pressure to complete the analysis. To prepare the DSC samples and check the thermo-responsive behavior of sponges, ~ 0.5 mg dried sponges were immersed into DI water for 10 mins to form the water-swollen sponges, which were subsequently placed into the hermetic aluminum sample pans and covered with a lid for DCS test. The VPTTs of samples were analyzed using Perkin-Elmer DSC 7 with a heating/cooling rate of 10oC·min−1 under nitrogen flow. Heating and cooling scans were performed in the temperature range of 10 to 50°C.

3

RESULTS AND DISCUSSION 3.1. Characterization of the Chemical Structure of As-prepared PVF-based

Sponges. Ceric (IV) ion is an efficient initiator to create active sites in the PVA network; this initiator is used for grafting polymerization of water-soluble monomer in the water medium39-40 due to the easy formation of the redox initiation system with the hydroxyl groups in PVA network.41 The successful grafting of PNIPAAm into the PVF sponges is confirmed by ATR-IR spectra, as shown in Figure 1. The broad absorption peaks at 3200–3600, 2843–2943, 2778, and 1012 cm−1 are attributed to the O–H stretching vibration, C–H strength vibration of the alkyl chain, C–H strength vibration of acetal, and C–O–C strength vibration, respectively. For PVF-g-PNIPAAm sponges, the new peaks at 1640, 1538, 1386 and 1366 cm−1 are the isopropyl group, N–H stretching vibration, and C=O stretching vibration of PNIPAAm, respectively. These peaks confirm the successful grafting of PNIPAAm onto the macroporous PVF 11



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sponge. The existence of characteristic peaks of PNIPAAm, including at 3200–3600, 1625, 1546, 1390 and 1371 cm-1 (Fig. S2), respectively, demonstrates the formation PNIPAAm hydrogel. The 1H-NMR spectra in Figure 2 are also used to further confirm the chemical structure

and

composition

of

PVF-g-PNIPAAm

samples.

The

PVF

and

PVF-g-PNIPAAm sponges exhibit similar chemical shifts in the range of 3.0–5.0, 3.6–4.2, 4.2–4.5, and 4.6–4.9 ppm, which correspond to the methane Hb and Hc, hydroxyl group He, and methylene Ha, respectively. The degree of acetalization of the original PVF, calculated from the corresponding signal, is 74.6%. After grafting PNIPAAm onto the PVF, a new peak at 7.2 ppm is attributed to the imine Hf, indicating a successful grafting polymerization of PNIPAAm segment onto the PVF networks. 3.2. Grafting Percentage and Efficiency. Table 1 provides a list of the GP and GE of PVF-g-PNIPAAm prepared at different reaction time points. According to calculated results based on gravimetrical method, the GPs of PVF-g-PNIPAAm sponges increase from 257% at 12 hr to 297% at 72 hr, and correspondingly the GEs change from 43% to 50%, respectively. The GPs and GEs calculated using 1H NMR display similar trend, namely, the GPs changes from 212% to 353% while the GEs increase from 30% to 58%, respectively. These values from NMR are comparable with those calculated from the gravimetrical methods, demonstrating that PNIPAAm segments were efficiently grafted onto the PVF network. The high grafting efficiency of cerium salt-PVF redox initiator system is attributed to the formation of active sites via complex reaction between the Ce(IV) ions and hydroxyl groups on PVA even at low reaction temperature,42 the excellent accessibility reactant due to open-cell structure of PVF, and the swelling of polymer networks upon grafting the PNIPAAm 12


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segment. Considering the possible weight loss of PVF-g-PNIPAAm during the polymerization, purification, and subsequent handlings, the results obtained 1H NMR are more accurate. 3.3. Pore Size and Morphology. Figure 3 shows the pore size distributions of PVF and PVF-g-PNIPAAm sponges. The original PVF sponge exhibits a unimodal size distribution with a peak at 60 µm and a high porosity of 95.0%. The average pore size of all PVF-g-PNIPAAm sponges is concentrated at 90 µm with a broad distribution. The porosities of samples prepared at 12, 24, 48, and 72 hr are 89.8%, 91.1%, 89.4%, and 85.9%, respectively. These results indicate that the PNIPAAm-grafted samples possess large average pore size and higher porosity, which is probably attributed to the changes in the chain interaction of PVF-g-PNIPAAm after the introduction of PNIPAAm segments on PVF networks. Specifically, both PVF and PVF-g-PNIPAAm were prepared in water but have to be characterized in dry states. Without PNIPAAm, the amphiphilic PVF can be dried directly in an oven. Such drying, however, generates a large extent of pore shrinking, which yields the observed, decreased pore sizes. For PVF-g-PNIPAAm, due to the high hydrophilicity of PNIPAAm at lower temperatures ( < 32ºC) and undergo conformation change under higher temperature, direct drying (typically 60ºC) causes hydrophobic aggregation of the PNIPAAm segments, which further induced structural collapse of PVF-g-PNIPAAm sponges and seals the pores due to the conformation change at relative higher temperature. Therefore, we have to use anhydrous EtOH to replace the water in sample’s network and then dry the sample. EtOH is not a good solvent for PNIPAAm43,44 and PVF and both materials become stiffer and retain the original pore size. The drying, therefore, does not cause major shrinking of the pore size. Taken together, the pore sizes of the PVF-g-PNIPAAm are larger than those of PVF. As shown in Fig. S3, the average pore 13



size of macro-PNIPAAm is also located at ~ 90 µm with a relative lower peak as compared to that of PVF based sponges, while the porosity is only 54.1%. Those features indicate the PVF and PVF-g-PNIPAAm sponges hold higher porosity. The pore morphology is also observed using SEM in Figure 4 and Fig. S6. All samples exhibit open-cell structure, i.e., PVF-g-PNIPAAm sponges maintain the open-cell structure well and macroporous rough surface, which facilitates the wetting of good solvents into PVF network. The statistical pore size of PVF-g-PNIPAAm sponges prepared at 12, 24, 48, and 72 hr are ~91.7, ~101.7, ~81.4, and ~86.9 µm, respectively. Those results are close to 90 µm as obtained from mercury porosimetry, confirming that introduction of the PNIPAAm segments increases the average pore size of PVF-g-PNIPAAm sponges. 3.4. Swelling Behavior and Fitting of Kinetic Procedure of PVF-g-PNIPAAm at 20ºC. Generally, the incorporation of interconnected pore structures is an effective strategy to enhance the swelling rate of hydrogels in the water medium.45-47 In this work we use the hydrogel term “swelling ratio” to indicate the amount of absorbed water in our macroporous hydrogels because our materials are essentially similar to the traditional hydrogels in terms of water adsorption. The only difference is our free water contains two components, free water trapped in the crosslinked polymer networks and pores. The same term has been used in the literature as well.48 The swelling behavior of as-prepared original PVF and PVF-g-PNIPAAm sponges is shown in Figure 5 and Fig. S8. Original PVF can reach swelling equilibrium within 2 min, and it exhibits a saturated water absorption capacity of ~20.0 g.g−1. After grafting polymerization of NIPAAm into the PVF network, all of the samples prepared at different time points also exhibit a rapid swelling behavior similar to that of original PVF, but they display relatively higher absorption capacity. For example, the dried 14


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PVF-g-PNIPAAm-12 hr almost reaches swelling equilibrium within 80 s, which is slightly quicker than that of samples prepared at 24, 48, and 72 hr that need 2–3 mins to attain saturated adsorption. In comparison to PVF-g-PNIPAAm, macro-PNIPAAm needs 300 mins to reach the swelling equilibrium with the maximum absorption capacity ~ 7.8 g.g−1 (Fig. S4a). This fact highlights the excellent water adsorption performance of PVF-g-PNIPAAm. The swelling procedure of samples in water at 20°C is fitted using the pseudo-first-order kinetic models, which is expressed as follows: ln /( − ) = k1t,

(5)

where the Qs and Qt are the swelling ratios at equilibrium and time t, respectively, and k1 is the swelling constant of pseudo-first-order rate kinetic equation. The fitting curves are plotted in Figure 5a. As listed in Table 1, the fitted Qss increase from 20.2 g.g−1 for PVF to 54.0 g.g−1 for PVF-g-PNIPAAm prepared at 72 hr, and the corresponding experimental values are from 20.1 to 52.7 g.g−1, respectively. Remarkably, the correlation coefficients (R2) are in the range of 0.953–0.982. Moreover, the k1 values of the original PVF and PVF-g-PNIPAAm-12h are 0.049 and 0.027 min−1, respectively, which are higher than those of other samples prepared at 36, 48, and 72 hr with k1 in the range of 0.018–0.019 min−1. For macro-PNIPAAm, the adsorption profile shown in Fig. S4a was fitted using the pseudo-first-order kinetic equation. The obtained Qs, k1, and R2 are 8.1 g.g−1, 0.011 min-1 and 0.987, respectively. The pseudo-second-order rate kinetic equation is also used to investigate the swelling kinetics, as given in the following equation:

=

+

(6)

where the Qs and Qt are the swelling ratios at equilibrium state and time t, 15



respectively, and k2 is the swelling constant of pseudo-second-order kinetic equation. The fitted curves are shown in Figure 5b. Table 1 shows that the fitted Qss are 20.4, 36.5, 52.1, 55.4, and 55.7 g.g−1 for PVF and PVF-g-PNIPAAm samples prepared at 12, 24, 48, and 72 hr, respectively. These results are similar to the experimental ones. The k2 values increase from 0.018 g g−1 min−1 to 0.148 g g−1 min−1. The adsorption profile of macro-PNIPAAm was also analyzed using the pseudo-second-order kinetic equation and linear fitting (Fig. S4b and Table 1). The Qs, k2, and R2 are 10.0 g.g−1, 0.125 g g−1 min−1 and 0.982, respectively. The relative smaller difference of Qs calculated from pseudo-first-order kinetic models as compared with that from pseudo-second-order rate kinetic model indicates that the former one would be better to evaluate the water absorption behavior. This reduction can also be confirmed from the relatively higher R2 of the former model. Qss from fittings based on the pseudo-first-order rate kinetic model is closer to the experimental Qss as compared to those from the pseudo-second-order rate kinetic model. We, therefore, propose that the swelling process of PVF-g-PNIPAAm agrees with the pseudo-first-order rate kinetic model, which can be further confirmed by the excellent reversible swelling/deswelling behavior. Indeed, both equations were used in the literature to explain the swelling

process.49-50 The pseudo-first-order equation, also known as the Berens-Hopfenberg differential equation, is based on a physical model wherein the relaxation of the polymer chain dominates the swelling process while assuming the diffusion of solvents remain the same during the swelling process.51-54 The pseudo-second-order equation, on the other hand, is based on a physical model wherein the swelling degree is linear related to the contact time between polymers and solvents, or the diffusion of solvents, in the entire swelling process. Herein the swelling process is directly related 16


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to the dried fraction that can swell at time t and internal specific boundary area.55 In our case, water molecules can rapidly penetrate the sponge network upon contact due to the macroporous structure of the gel, making the diffusion experience the “fast” step of the swelling. On the other hand, the wetter polymers in the network need some time to reach the relaxation equilibrium, making this process exhibit the rate-limiting step, which dominates the swelling kinetics. Therefore, we believe the first-order equation is more suitable to fit our swelling data. This also explains why the first-order equation fitting give a better agreement between experimental and fitting Qss. The rapid swelling kinetics of sample in water is attributed to the interconnected macroporous structure and hydrophilic surface of PVF-g-PNIPAAm sponges at lower temperature. For example, at 20°C, the sponge surface is readily wetted by water molecules, which further penetrates into the pore channels via convection control and capillary effect.56-58 Additionally, the high porosity (~ 90%) of sponges effectively reduces the character size (thickness of pore wall) to microscale, which results in the superfast swelling kinetics of gels in suitable solvents.59-60 These features allow the PVF-g-PNIPAAm sponges to reach swelling equilibrium in tens seconds. In contrast, the relative lengthy swelling time (approximately a few hours) of macro-PNIPAAm is attributed to the diffusion control process due to the relatively lower porosity and smaller pore size. Herein water molecules need relative longer times to diffuse in between the channel walls even if the samples contain macroporous pores and hydrophilic surface at lower temperatures. The Qs of original PVF at 20oC is 20.1 g.g−1 and that of the PVF-g-PNIPAAm sponges prepared at 12, 24, 48, and 72 hr is 36.1, 46.7, 49.7, and 52.7 g.g−1, respectively, indicating the swelling ratio increases with the GP because high GP means many NIPAAm segments can be involved in improving the solvation degree of 17



PVF-g-PNIPAAm at low temperature. 3.5. Thermo-responsive Behavior. PNIPAAm, a typical thermo-responsive polymer, exhibits a volume change at a VPTT of ~ 32°C, which is close to the body temperature. VPTT can be regulated by incorporation of hydrophobic or hydrophilic comonomer.61-62 To clarify the thermo-responsive property, the swelling ratios of original PVF and PVF-g-PNIPAAm sponges at different temperatures are investigated, as shown in Figure 6. The original PVF sponge maintains a constant swelling ratio at ~20 g.g−1 in the wide range of 20-48oC. All of PVF-g-PNIPAAm sponges display evident VPTT at 30-34oC. Further increasing temperature would result in the shrinkage of the swollen sample and the decreased water absorption capacity. This result is attributed to the conversion of PNIPAAm chain from the flexible coil to compact globular state at around about 30-34oC. As illustrated in Figure 7, at low temperature, such as 20oC, the C=O and N–H in PNIPAAm segments of PVF-g-PNIPAAm form intermolecular hydrogen bonds with water molecules, and the PNIPAAm is in hydration and swollen state. However, under high temperature, such as 48oC, the formed intermolecular hydrogen bonds by the C=O and N–H of PNIPAAm would change to intramolecular hydrogen bonds, which lead to the collapse of polymer chains. Previous reports demonstrate that the increase of environmental temperature can weaken the hydrogen bonding interactions between chains and water molecules as well as enhance the hydrophobic interactions between PNIPAAn chains. Further increasing the temperature to above the VPTT results in the phase separation, which is a combination of entropy decrease due to the enhanced hydrophobic interactions and entropy increase due to the detached water molecules. In general, this endothermic phase separation can be studied by DSC using the swollen sample,63-64 wherein the onset temperature and the maximum temperature of the 18


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endothermic peaks are denoted as the VPTT.65-66 As shown in Figure 8, the onset temperatures of the DSC thermograms for PVF-g-PNIPAAm sponges prepared at 12, 24, 48, and 72 hr are 32.9, 32.0, 32.4, and 31.8oC, and the corresponding peaks locate at 35.0, 34.5, 34.7, and 34.2oC, respectively. Notably, the onset temperature of macro-PNIPAAm is approximately 30ºC and the endothermic peak is broad (from 34.4 to 36.5ºC) compared to that of PVF-g-PNIPAAm. On the contrary, no VPTT is observed for original PVF. These results demonstrate that the introduction of PNIPAAm chains results in the thermo-responsive behavior of the sponge. Moreover, the existence of original PVF network exerts almost no influence on the VPTT values of PVF-g-PNIPAAm sponges due to the hierarchical pore structure in sponges and amphiphilic PVF network. 3.6.

Deswelling

Procedure

and

Fitting

of

Kinetic

Equations

of

PVF-g-PNIPAAm at 48oC. Most traditional PNIPAAm hydrogels exhibit a slow responsive rate due to the formation of “dense skin layer” on the outer surface of hydrogel during the collapse process of polymer chains at relatively high temperatures; 67-68

this skin layer blocks the outward diffusion of internal water molecules, restricts

the deswelling kinetics of hydrogel, and further limits their applications. Generally, incorporation of interconnected pore structure in the PNIPAAm hydrogel is an effective approach to realize the rapid swelling–deswelling responsive behavior of thermo-sensitive gels.17,28,37,69 In Figure 9, the PVF-g-PNIPAAm sponges can reach deswelling equilibrium at less than 1 min after rapid removal of the saturated samples from cold water (20°C) and placing them into the hot water (48°C) whereas approximately 4 - 5 minutes are needed for the macro-PNIPAAm to reach the deswelling equilibrium. This result demonstrates that incorporation of pore structure could improve the responsive rate 19



Page 20 of 53

considerably.17,19,22 The deswelling kinetics can be explained using the pseudo-first-order rate analysis,70-72 which is given by the following equation: ln () = ln

= −

(7)

where Qs, Qt, and Q∞ represent the swelling ratio at equilibrium, time t, and collapsed states, respectively, and k is the rate constant. The DS is the degree of swelling ratio of sample at time t as compared with the swelling ratio at equilibrium. The linear fitting results of macro-PNIPAAm was shown in Fig. S5 and Table 1, its k value and R2 is 0.88 min-1 and 0.982, respectively, indicating the deswelling process of macro-PNIPAAm can be evaluated with the pseudo-first-order rate, which matches with the previous report.69 However, the superfast deswelling behavior of PVF-g-PNIPAAm in hot water results in that the linear fitting is not as good as that of traditional gels, which display relatively slow deswelling kinetic in hot water; consequently, large amount of data point could be tested, and relative accuracy linear fitting results could be obtained. All samples reach deswelling equilibrium within 1 min, particularly PVF-g-PNIPAAm-72 h, which reaches equilibrium at less than 0.67 min. Thus, the data points during initial 1 min were selected for linear fitting, and the related results are shown in Table 1. Notably, the R2 of PVF-g-PNIPAAm prepared at 12, 24, 48 and 72 hr are 0.938, 0.890, 0.939 and 0.819, respectively, indicating that the pseudo-first-order kinetic analysis is close to fit the deswelling procedure. The corresponding k values of PVF-g-PNIPAAm prepared at 12, 24, 48, and 72 hr are 3.47, 2.91, 3.33, and 3.02 min−1, respectively. The above results demonstrate a rapid shrinkage of PVF-g-PNIPAAm in the hot water. Tanaka et al.59, 73 demonstrated that the characteristic time (τ) can be used to describe the swelling and deswelling kinetics of responsive hydrogels to a sudden 20


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change in external stimuli, such as pH and temperature. In the present study, the deswelling kinetics of PVF-g-PNIPAAm sponges at 48oC is also investigated using τ. τ = l2/ π2D,

(8)

where l and D are the edge length of cubic gels and the diffusion coefficient of the sponges, respectively. For a spherical hydrogel with small radius change, the swelling degree is can be calculated by the following formula.72,74 ~

6 exp − " (9) !

Kokufuta64 demonstrated that the natural logarithm of DS in equation 7 exhibits linear relation versus time for the cylindrical gel after equation transformation, (Part 2, Supporting Information). In here, we use the Qt as swelling ratio and the reciprocal of k in Equation S6 equals to the τ in equation. Therefore, the τ of PVF-g-PNIPAAms could be approximately equal to the reciprocal of k obtained from Equation (7). The evaluated τ values of above four samples are 0.29, 0.34, 0.30, and 0.33 min, respectively. These values exhibit the same order with the detected equilibrium deswelling time (0.67 -1 min), and they are much shorter than that of conventional PNIPAAm-based hydrogels that require a few hours and even a few days to reach a deswelling equilibrium. 3.7. Superfast and Reversible Thermo-responsive Behavior. On the basis of the rapidly swelling–deswelling kinetics of PVF-g-PNIPAAm sponges, repeatable swelling–deswelling procedures from 20 to 48oC are investigated, as shown in Figure 10. After removing the sample from cold water (20oC) and placing into hot water (48°C), the swollen sample quickly shrinks. The corresponding saturated absorption capacity of PVF-g-PNIPAAm-24h for water also rapidly decreases from 42.1 to 13.8 g.g−1 within 1 min. The absorption capacity of collapsed sample still remains at 13.8 g.g−1 even immersing in hot water for more than 2 min. This result demonstrates that 21



the sample reaches the deswelling equilibrium at less than 1 min. Notably, when the collapsed sample is removed from hot water and immersed into the cold water, the saturated absorption capacity also increases from 13.8 g.g−1 to 41.9 g.g−1 within 1 min. Furthermore, the sample displays similar swelling/deswelling behavior even after five cycles, indicating excellent oscillatory swelling–deswelling behavior. This behavior also can be confirmed from other samples with relatively high GP. As a contrast, it takes ~ 5 min for the macro-PNIPAAm gel to reach the deswelling equilibrium and another 5 min to realize the deswelling equilibrium. These features suggest that as-prepared samples overcome the drawbacks of traditional PNIPAAm hydrogels and make them ideal candidates as rapid thermo-sensor. 3.8. PVF-g-PNIPAAm Combined with Iodine Solution as Thermometer. PVA75 and PVF76 exhibit a characteristic blue color like starch in iodine solution due to the existence of hydroxyl groups and the special stereoregularity. Our experimental results demonstrate that PVF-g-PNIPAAm sponge can also immobilize iodine and display a blue color due to the formation of the complex in the system. Aforementioned results prove that as-prepared PVF-g-PNIPAAm sponges possess a large volume change at phase transition temperature (~33oC). So it is feasible to design a thermometer based on PVF-g-PNIPAAm with relatively rapid and accurate performance, as shown in Figure 11. First, the white PVF-g-PNIPAAm sponge (15 mm × 25 mm × 5 mm) is immersed into deionized water in a Petri dish at 26oC to reach swelling equilibrium, and then five droplets of I2 aqueous solution are added into the above solution at 27oC. The sponge exhibits a blue color in a few seconds due to the formation of the clathrate compound. When the temperature is gradually increased to 30oC, the sample almost exhibits no volume and color change. Nevertheless, at 31-32oC the volume of sample shrinks evidently. At 33oC the sponge 22


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edges become white due to the detachment of I2 molecules from the PVF-g-PNIPAAm network at this temperature. Further increasing of temperature to 35 °C, results in the considerably white color of the sample. At 40°C, almost the whole sample becomes white. Indeed, evident volume shrinkage occurs at 30-35oC. The color change depends on the sample size and concentration of iodine solution, generally ranging from 33-48oC. Remarkably, similar to the rapid volume change of PVF-g-PNIPAAm at relatively high temperature, the color change of sample is also superfast. As shown in Figure 12, the blue color of stained sample (1 mm × 1 mm × 20 mm, I2 solution is added) in glass tube at 27oC water bath fades away within 40 s after it is placed in high temperature bath (48oC). The above white sample can gradually return back to the original blue color within 40 s when immersed at a low temperature bath (27oC). This reversible process can be repeated without any change in the sample structure and thermo-responsive behavior, thereby indicating the excellent repeatability. The superfast color change of PVF-g-PNIPAAm is attributed to its open-cell structure, which endows the relatively free transfer of molecules. It is noted that the destain– stain and deswelling–swelling processes simultaneously occur with increasing– decreasing environmental temperatures. Such feature implies that PVF-g-PNIPAAm sponges can serve as a novel temperature indicator or thermo-sensor with superfast, accurate response and excellent reusability at certain conditions. 3.9 PVF-g-PNIPAAm Combined with Iodine as Antibacterial Materials. Aforementioned results demonstrate that the PVF-g-PNIPAAm holds stronger bonding with I2 molecules at the lower temperature (for example at 20oC) and I2 molecules will escape from sponge at the higher temperature nearby physiological temperature. It is well known that the iodine is an effective antibacterial material. 23



Thus their antibacterial performance of sponges was investigated using the shake-flask method.77-78 From the inhibition zone test shown in Figure 13(a-1, a-2, b-1, b-2), the Gram-negative E. coli colonies from the original PVF and PVF-g-PNIPAAm sponge reach the density of 3.4 × 104 and 1.6 × 105 CFU/mL, respectively, whereas those of Gram-positive S. aureus colonies reach the density of 4.1 × 104 and 4.3 × 105 CFU/mL, respectively, indicating PVF and PVF-g-PNIPAAm sponges have no inhibition effect on the E. coli and S. aureus. Figure 13 (c-1, c-2, d-1, d-2) shows the antibacterial behavior of PVF/I2 and PVF-g-PNIPAAm/I2. Both PVF/I2 and PVF-g-PNIPAAm/I2 plates show no bacterial colonies. Those features indicate that the PNIPAAm segments have almost no influence on the antibacterial performance of PVF-g-PNIPAAm sponges. This was further confirmed by the antibacterial behavior of macro-PNIPAAm hydrogel. As shown in Fig. S7, the density of Gram-negative E. coli and Gram-positive S. aureus colonies are 1.9 × 105 and 1.2 × 105 CFU/mL. Notably, after added iodine into the sponges, the PNIPAAm/I2 gel shows good antibacterial behavior, indicating the existence of PNIPAAm segments do not influence I2 function. Perhaps the PNIPAAm segments in the PVF-g-PNIPAAm gel can help fast release of I2 comparing to PVF only. It means the PVF/I2 and PVF-PNIPAAm/I2 can effectively inhibit the survival and growth of both Gram-negative E. coli and Gram-positive S. aureus due to the effective detachment of the iodine molecules at incubated temperature 37oC to kill these two kinds of bacteria. It also demonstrates that introduction of PNIPAAm chains almost has no influence on the bonding efficiency of polymer network with iodine. Interestingly, for PVF-PNIPAAm/I2, the detachment of I2 and the shrinkage of PVF-PNIPAAm sponges happen simultaneously due to its superfast thermo-responsive behavior. Such result indicates the macroporous PVF-PNIPAAm sponge can serve as rapid release system 24


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for the antibacterial compounds. Further details on antibacterial performance and the controllable release of PVF based-I2 complex is in progress.

4

CONCLUSIONS

Macroporous PVF-g-PNIPAAm sponges are successfully prepared through the grafting polymerization of NIPAAm onto the PVF network effectively. The main results are as follows. (i)

PVF-g-PNIPAAm sponges maintain the interconnected open-cell structure and high porosity of 90%, which similar to the original PVF sample.

(ii)

Both swelling experiment and DSC measurement demonstrate that PVF-g-PNIPAAm hydrogels exhibit evident thermo-responsive behavior with their VPTT at 30-34oC, which is similar to that of pure PNIPAAm. Furthermore, results indicate that PVF network exerts almost no influence on the VPTT of PNIPAAm chains due to macroporous structure.

(iii)

PVF-g-PNIPAAm sponges exhibit a rapid swelling procedure at 20oC in less than 80 s to reach the swelling equilibrium. These hydrogels undergo a rapid deswelling procedure at 48oC less than 40 s. The pseudo-first-order kinetic model is more suitable to fit the swelling procedure meanwhile the first-order kinetic model can fit the deswelling process well for PNIPAAm grafted samples.

(iv)

Swelling–deswelling cycles indicate that the reversible thermo-responsive behavior of PVF-g-PNIPAAm sponges can be realized in tens seconds. These macroporous responsive hydrogels can serve as a kind of novel temperature indicator and antibacterial material.

ASSOCIATED CONTENT 25



Page 26 of 53

Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. GP and GE calculation, τ calculation, basic characterization of macro-PNIPAAm gel including FTIR spectra, pore size and distribution, swelling and deswelling behavior of Macro-PNIPAAm gel, SEM images of PVF-g-PNIPAAm samples, antibacterial performance

of

macro-PNIPAAm

gel

and

water

absorption

kinetics

of

PVF-g-PNIPAAm samples in initial 2 mins.

AUTHOR INFORMATION

Corresponding Author Email: [email protected]. Fax: 86-431-85262075. Phone: 86-431-85262876. ORCID Yanxiong Pan: 0000-0001-7084-8048 Xiangling Ji: 0000-0001-7074-7722 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This research was supported by National Natural Science Foundation of China (General: 51173180) and Department of Science and Technology of Jilin Province (20130206057GX).

REFERENCES

(1) Pelton, R., Temperature-sensitive Aqueous Microgels. Adv. Colloid Interf. Sci. 2000, 85 (1), 1-33. (2) Stuart, M. A. C.; Huck, W. T.; Genzer, J.; Müller, M.; Ober, C.; Stamm, M.; 26


Page 27 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M., Emerging Applications of Stimuli-responsive Polymer Materials. Nature Mater. 2010, 9 (2), 101-113. (3) Gil, E. S.; Hudson, S. M., Stimuli-reponsive Polymers and Their Bioconjugates. Prog. Polym. Sci. 2004, 29 (12), 1173-1222. (4) Halperin, A.; Kröger, M.; Winnik, F. M., Poly (N‐isopropylacrylamide) Phase Diagrams: Fifty Years of Research. Angew. Chem., Int. Ed. 2015, 54 (51), 15342-15367. (5) Kamata, H.; Li, X.; Chung, U. Il; Sakai, T. Design of Hydrogels for Biomedical Applications. Adv. Healthc. Mater. 2015, 4 (16), 2360–2374. (6) Zhang, X.; Wu, D.; Chu, C. C. Synthesis and Characterization of Partially Biodegradable, Temperature and PH Sensitive Dex-MA/PNIPAAm Hydrogels. Biomaterials 2004, 25 (19), 4719–4730. (7) Guan, Y.; Zhang, Y., PNIPAM Microgels for Biomedical Applications: from Dispersed Particles to 3D Assemblies. Soft Matter 2011, 7 (14), 6375-6384. (8) Zha, L.; Banik, B.; Alexis, F., Stimulus Responsive Nanogels for Drug Delivery. Soft Matter 2011, 7 (13), 5908-5916. (9) Yoshida, R.; Uchida, K.; Kaneko, Y.; Sakai, K.; Kikuchi, A.; Sakurai, Y.; Okano, T., Comb-type Grafted Hydrogels with Rapid Deswelling Response to Temperature Changes. Nature 1995, 374 (6519), 240-242. (10) Ngai, T.; Behrens, S. H.; Auweter, H., Novel Emulsions Stabilized by pH and Temperature Sensitive Microgels. Chem. Comm. 2005,

(3), 331-333.

(11) Noguchi, Y.; Okeyoshi, K.; Yoshida, R., Design of Surfactant ‐ Grafted Hydrogels with Fast Response to Temperature. Macromol. Rapid Comm. 2005, 26 (24), 1913-1917. (12) Gil, E. S.; Hudson, S. M., Effect of Silk Fibroin Interpenetrating Networks on Swelling/deswelling

Kinetics

and

Rheological

Properties

of

Poly

(N-isopropylacrylamide) Hydrogels. Biomacromolecules 2007, 8 (1), 258-264. (13) Zheng, Q.; Zheng, S. From poly(N-Isopropylacrylamide)-Block-Poly(ethylene Oxide)-Block-poly(N-

Isopropylacrylamide)

Triblock

Copolymer

to

poly(N-Isopropylacrylamide)-Block- Poly(ethylene Oxide) Hydrogels: Synthesis and Rapid Deswelling and Reswelling Behavior of Hydrogels. J. Polym. i. Part A 27



Page 28 of 53

Polym. Chem. 2012, 50 (9), 1717–1727. (14) Cong, H. L.; Li, L.; Zheng, S. Poly(N-Isopropylacrylamide)-Block-Poly(vinyl Pyrrolidone) Block Copolymer Networks: Synthesis and Rapid Thermoresponse of Hydrogels. Polymer. 2013, 54 (4), 1370–1380. (15) Li, J.; Cong, H.; Li, L.; Zheng, S. Thermoresponse Improvement of poly(N-Isopropylacrylamide)

Hydrogels

via

Formation

of

Poly(sodium

P-Styrenesulfonate) Nanophases. ACS Appl. Mater. Interfaces 2014, 6 (16), 13677–13687 (16) Zhang, X. Z.; Yang, Y. Y.; Chung, T. S., Effect of Mixed Solvents on Characteristics of Poly(N-isopropylacrylamide) Gels. Langmuir 2002, 18 (7), 2538-2542. (17) David, G.; Simionescu, B. C.; Albertsson, A.-C., Rapid Deswelling Response of Poly(N-isopropylacrylamide)/poly(2-alkyl-2-oxazoline)/poly(2-hydroxyethyl methacrylate) Hydrogels. Biomacromolecules 2008, 9 (6), 1678-1683. (18) Xue, S. W, Allan S. H, and Paul. Y. Synthesis and Characterization of Thermally Reversible Macroporous Poly ( N-Lsopropylacrylamide ) Hydrogels. J. Polym. Sci. Part A Polym. Chem. 1992, 30 (S), 2121–2129 (19) Zhang, X.-Z.; Yang, Y.-Y.; Chung, T.-S.; Ma, K.-X., Preparation and Characterization of Fast Response Macroporous Poly(N-isopropylacrylamide) Hydrogels. Langmuir 2001, 17 (20), 6094-6099. (20) Serizawa, T.; Wakita, K.; Akashi, M. Rapid Deswelling of Porous Poly. Macromolecules 2002, 35, 10–12. (21) Xia, L.-W.; Xie, R.; Ju, X.-J.; Wang, W.; Chen, Q.; Chu, L.-Y., Nano-structured Smart Hydrogels with Rapid Response and High Elasticity. Nature Comm. 2013, 4,2226. (22) Mou, C.-L.; Ju, X.-J.; Zhang, L.; Xie, R.; Wang, W.; Deng, N.-N.; Wei, J.; Chen, Q.; Chu, L.-Y., Monodisperse and Fast-responsive Poly (N-isopropylacrylamide) Microgels with Open-celled Porous Structure. Langmuir 2014, 30 (5), 1455-1464. (23) Zhang, J.; Chu, L.-Y.; Li, Y.-K.; Lee, Y. M., Dual Thermo-and pH-sensitive Poly (N-isopropylacrylamide-co-acrylic

acid)

Hydrogels

28


with

Rapid

Response

Page 29 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Behaviors. Polymer 2007, 48 (6), 1718-1728. (24) Depa,

K.;

Strachota,

A.;

Šlouf,

M.;

Hromádková,

J.,

Fast

Temperature-responsive Nanocomposite PNIPAM Hydrogels with Controlled Pore Wall Thickness: Force and Rate of T-response. European Polym. J. 2012, 48 (12), 1997-2007. (25) Maeda, S.; Kato, T.; Kogure, H.; Hosoya, N., Rapid Response of Thermo-sensitive Hydrogels with Porous Structures. Appl. Phys. Lett. 2015, 106 (17), 171909. (26) Gancheva, T.; Virgilio, N., Enhancing and Tuning the Response of Environmentally Sensitive Hydrogels with Embedded and Interconnected Pore Networks. Macromolecules 2016, 49 (16), 5866-5876. (27) Yao, C.; Liu, Z.; Yang, C.; Wang, W.; Ju, X.-J.; Xie, R.; Chu, L.-Y., Poly(N-isopropylacrylamide)-Clay Nanocomposite Hydrogels with Responsive Bending Property as Temperature-Controlled Manipulators. Adv. Funct. Mater. 2015, 25 (20), 2980-2991. (28) Wei, J.; Chen, Y.; Liu, H.; Du, C.; Yu, H.; Zhou, Z., Thermo-responsive and Compression Properties of TEMPO-oxidized Cellulose Nanofiber-modified PNIPAm Hydrogels. Carbohyd. Polym. 2016, 147, 201-207. (29) Lejardi, A.; Etxeberria, A.; Meaurio, E.; Sarasua, J.-R., Novel Poly(vinyl alcohol)-g-poly(hydroxy acid) Copolymers: Synthesis and Characterization. Polymer 2012, 53 (1), 50-59. (30) Ding, J.; Chen, S.-C.; Wang, X.-L.; Wang, Y.-Z., Synthesis and Properties of Thermoplastic Poly(vinyl Alcohol)-Graft-Lactic Acid Copolymers. Ind. Eng. Chem. Res. 2008, 48 (2), 788-793. (31) Chen, S.-C.; Zhou, Z.-X.; Wang, Y.-Z.; Wang, X.-L.; Yang, K.-K., A Novel Biodegradable Poly(p-dioxanone)-grafted Poly(vinyl alcohol) Copolymer with a Controllable in Vitro Degradation. Polymer 2006, 47 (1), 32-36. (32) Pan, Y.; Wang, W.; Peng, C.; Shi, K.; Luo, Y.; Ji, X. Novel Hydrophobic Polyvinyl Alcohol-Formaldehyde Foams for Organic Solvents Absorption and Effective Separation. RSC Advances 2014, 4 (2), 660–669. 29



Page 30 of 53

(33) Pan, Y.; Shi, K.; Liu, Z.; Wang, W.; Peng, C.; Ji, X., Synthesis of a New Kind of Macroporous Polyvinyl-alcohol Formaldehyde Based Sponge and Its Water Superabsorption Performance. RSC Adv. 2015, 5 (96), 78780-78789. (34) Pan, Y.; Liu, Z.; Wang, W.; Peng, C.; Shi, K.; Ji, X., Highly Efficient Macroporous Adsorbents for Toxic Metal Ions in Water Systems Based on Polyvinyl Alcohol-Formaldehyde Sponges. J. Mater. Chem. A 2016, 4 (7), 2537-2549. (35) H. G. Hammond, US 2668153 1953. (36) Fundueanu, G.; Constantin, M.; Ascenzi, P., Poly (vinyl alcohol) Microspheres with pH-and Thermosensitive Properties as Temperature-controlled Drug Delivery. Acta Biomater. 2010, 6 (10), 3899-3907. (37) Zhang, J. T.; Huang, S. W.; Cheng, S. X.; Zhuo, R. X., Preparation and Properties

of

Poly

(N-isopropylacrylamide)/poly

(N-isopropylacrylamide)

Interpenetrating Polymer Networks for Drug Delivery. J. Polym. Sci. Part A-Polym. Chem. 2004, 42 (5), 1249-1254. (38) Saif, M. J.; Anwar, J.; Munawar, M. A. A Novel Application of Quaternary Ammonium Compounds as Antibacterial Hybrid Coating on Glass Surfaces. Langmuir 2009, 25 (1), 377–379 (39) McDowall, D. J.; Gupta, B. S.; Stannett, V. T., Grafting of Vinyl Monomers to Cellulose by Ceric Ion Initiation. Prog. Polym. Sci. 1984, 10 (1), 1-50. (40) Fundueanu, G.; Constantin, M.; Ascenzi, P., Preparation and Characterization of pH- and Temperature-sensitive Pullulan Microspheres for Controlled Release of Drugs. Biomaterials 2008, 29 (18), 2767-2775. (41) Sarac, A., Redox Polymerization. Prog. Polym. Sci. 1999, 24 (8), 1149-1204. (42) Bischofberger, I.; Calzolari, D. C. E.; De Los Rios, P.; Jelezarov, I.; Trappe, V. Hydrophobic Hydration of Poly-N-Isopropyl Acrylamide: A Matter of the Mean Energetic State of Water. Sci. Rep. 2014, 4, 1–7. (43) Costa, R. O. .; Freitas, R. F. . Phase Behavior of Poly(N-Isopropylacrylamide) in Binary Aqueous Solutions. Polymer (Guildf). 2002, 43 (22), 5879–5885. (44) Gao, B.; Lu, J.; Chen, Z.; Guo, J., Preparation and Recognition Performance of 30


Page 31 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cholic Acid-imprinted Material Prepared with Novel Surface-imprinting Technique. Polymer 2009, 50 (14), 3275-3284. (45) Liu, W.; Zhang, B.; Lu, W. W.; Li, X.; Zhu, D.; De Yao, K.; Wang, Q.; Zhao, C.; Wang,

C.,

A

Rapid

Temperature-responsive

Sol–gel

Reversible

Poly

(N-isopropylacrylamide)-g-methylcellulose Copolymer Hydrogel. Biomaterials 2004, 25 (15), 3005-3012. (46) Yin, L.; Fei, L.; Cui, F.; Tang, C.; Yin, C., Superporous Hydrogels Containing Poly(acrylic acid-co-acrylamide)/O-carboxymethyl Chitosan Interpenetrating Polymer Networks. Biomaterials 2007, 28 (6), 1258-1266. (47) Zhang, X.-Z.; Xu, X.-D.; Cheng, S.-X.; Zhuo, R.-X., Strategies to Improve the Response Rate of Thermosensitive PNIPAAm Hydrogels. Soft Matter 2008, 4 (3), 385-391. (48) Nakaoki, T.; Yamashita, H. Bound States of Water in Poly(Vinyl Alcohol) Hydrogel Prepared by Repeated Freezing and Melting Method. J. Mol. Struct. 2008, 875 (1–3), 282–287. (49) Li, S.; Vatanparast, R.; Lemmetyinen, H. Cross-Linking Kinetics and Swelling Behaviour of Aliphatic Polyurethane. Polymer. 2000, 41 (15), 5571–5576. (50) Lin, H.; Liu, J.; Zhang, K.; Fan, Y.; Zhang, X. Dynamic Mechanical and Swelling Properties of Maleated Hyaluronic Acid Hydrogels. Carbohydr. Polym. 2015, 123, 381–389. (51) Mineart, K. P.; Dickerson, J. D.; Love, D. M.; Lee, B.; Zuo, X.; Spontak, R. J. Hydrothermal

Conditioning

of

Physical

Hydrogels

Prepared

from

a

Midblock-Sulfonated Multiblock Copolymer. Macromol. Rapid Commun. 2017, 38 (5), 1–6. (52) Xia, M.; Wu, W.; Liu, F.; Theato, P.; Zhu, M. Swelling Behavior of Thermosensitive Nanocomposite Hydrogels Composed of Oligo(Ethylene Glycol) Methacrylates and Clay. Eur. Polym. J. 2015, 69, 472–482. (53) Bartil T, Bounekhel M, Cedric C, J. R.; Of. Swelling Behavior and Release Properties Methacrylic, PH-Sensitive Hydrogels Based on Derivatives. Acta Pharm, 2007, 57, 301–314,. 31



Page 32 of 53

(54) Ganji, F.; Vasheghani-Farahani, S.; Vasheghani-Farahani, E. Theoretical Description of Hydrogel Swelling: A Review. Iran. Polym. J. 2010, 19 (5), 375– 398. (55) Schott, H. Swelling Kinetics of Polymers. J. Macromol. Sci. Part B Phys. 1992, 31 (1), 1–9. (56) Loo, S.-L.; Krantz, W. B.; Lim, T.-T.; Fane, A. G.; Hu, X., Design and Synthesis of Ice-Templated PSA Cryogels for Water Purification: Towards Tailored Morphology And Properties. Soft Matter 2013, 9 (1), 224-234. (57) Zhang, J.-T.; Jandt, K. D., A Novel Approach to Prepare Porous Poly(N-Isopropylacrylamide) Hydrogel with Superfast Shrinking Kinetics. Macromol. Rapid Comm. 2008, 29 (7), 593-597. (58) Kim, J. H.; Lee, S. B.; Kim, S. J.; Lee, Y. M., Rapid Temperature/pH Response of Porous Alginate-g-Poly(N-Isopropylacrylamide) Hydrogels. Polymer 2002, 43 (26), 7549-7558. (59) Tanaka, T.; Fillmore, D. J., Kinetics of Swelling of Gels. J. Chem. Phys. 1979, 70 (3), 1214-1218. (60) Chen, J.; Park, H.; Park, K., Synthesis of Superporous Hydrogels: Hydrogels with Fast Swelling and Superabsorbent Properties. J. Biomed. Mater. Res. 1999, 44 (1), 53-62. (61) Alarcon, C. d. l. H.; Pennadam, S.; Alexander, C., Stimuli Responsive Polymers for Biomedical Applications. Chem. Soc. Rev. 2005, 34 (3), 276-285. (62) Imran, A. B.; Seki, T.; Takeoka, Y., Recent Advances in Hydrogels in Terms of Fast Stimuli Responsiveness and Superior Mechanical Performance. Polym. J. 2010, 42 (11), 839-851. (63) Otake, K.; Inomata, H.; Konno, M.; Saito, S., Thermal Analysis of the Volume Phase Transition with N-Isopropylacrylamide Gels. Macromolecules 1990, 23 (1), 283-289. (64) Feil, H.; Bae, Y. H.; Feijen, J.; Kim, S. W., Effect of Comonomer Hydrophilicity and

Ionization

on

the

Lower

Critical

Solution

Temperature

of

N-Isopropylacrylamide Copolymers. Macromolecules 1993, 26 (10), 2496-2500. 32


Page 33 of 53 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(65) Singh, D.; Kuckling, D.; Choudhary, V.; Adler, H.-J.; Koul, V., Synthesis and Characterization of Poly(N-Isopropylacrylamide) Films by Photopolymerization. Polym. Adv. Technol 2006, 17 (3), 186-192. (66) Fei, R.; George, J. T.; Park, J.; Grunlan, M. A., Thermoresponsive Nanocomposite Double Network Hydrogels. Soft Matter 2012, 8 (2), 481-487.

(67) Kamata, H.; Li, X.; Chung, U.-i.; Sakai, T., Design of Hydrogels for Biomedical Applications. Adv. Healthc. Mater. 2015, 4, 2360-2374. (68) Razmjou, A.; Barati, M. R.; Simon, G. P.; Suzuki, K.; Wang, H., Fast Deswelling of Nanocomposite Polymer Hydrogels via Magnetic Field-Induced Heating for Emerging FO Desalination. Environmental Science & Technology 2013, 47 (12), 6297-6305. (69) Kang, X.; Cheng, Z.; Yang, D.; Ma, P. a.; Shang, M.; Peng, C.; Dai, Y.; Lin, J., Design and Synthesis of Multifunctional Drug Carriers Based on Luminescent Rattle-Type Mesoporous Silica Microspheres with a Thermosensitive Hydrogel as a Controlled Switch. Adv. Funct. Mater. 2012, 22 (7), 1470-1481. (70) Zhao, Z. X.; Li, Z.; Xia, Q. B.; Bajalis, E.; Xi, H. X.; Lin, Y. S. Swelling/deswelling Kinetics of PNIPAAm Hydrogels Synthesized by Microwave Irradiation. Chem. Eng. J. 2008, 142 (3), 263–270. (71) Yoon,

J.

A.;

Kowalewski,

Thermoresponsive

Deswelling

T.;

Matyjaszewski,

Kinetics

of

K.

Comparison

Poly(oligo(ethylene

of

Oxide)

Methacrylate)-Based Thermoresponsive Hydrogels Prepared By “graft-from” ATRP. Macromolecules 2011, 44 (7), 2261–2268. (72) Ogawa, K.; Nakajima-Kambe, T.; Nakahara, T.; Kokufuta, E., Coimmobilization of Gluconolactonase with Glucose Oxidase for Improvement in Kinetic Property of Enzymatically Induced Volume Collapse in Ionic Gels. Biomacromolecules 2002, 3 (3), 625-631. (73) Hirokawa, Y.; Tanaka, T., Volume Phase Transition in a Nonionic Gel. J. Chem. Phys. 1984, 81 (12), 6379-6380. (74) Ogawa, Y.; Ogawa, K.; Kokufuta, E., Swelling−Shrinking Behavior of a 33



Polyampholyte Gel Composed of Positively Charged Networks with Immobilized Polyanions. Langmuir 2004, 20 (7), 2546-2552. (75) Imai, K.; Matsumoto, M., The Effect of Stereoregularity on the Polyvinyl Alcohol–Iodine Reaction. J. Polym. Sci. 1961, 55 (161), 335-342. (76) Sakuramachi, H.; Choi, Y.-S.; KIYASAKA, K., Poly (vinyl alcohol)-iodine complex in Poly (Vinyl Alcohol) Films Soaked at High Iodine Concentrations. Polym. J. 1990, 22 (7), 638-642. (77) Gong, J.; Ren, Y.; Fu, R.; Li, Z.; Zhang, J., pH-Mediated Antibacterial Dyeing of Cotton with Prodigiosins Nanomicelles Produced by Microbial Fermentation. Polymers 2017, 9 (10), 468. (78) Zhang, D.; Chen, L.; Zang, C.; Chen, Y.; Lin, H., Antibacterial cotton fabric grafted with silver nanoparticles and its excellent laundering durability. Carbohydrate polymers 2013, 92 (2), 2088-2094.

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Captions for Scheme, Figures and Tables Scheme 1. Figure 1. Figure 2. Figure 3. Figure 4.

Figure 5.

Figure 6.

Synthetic route of PNIPAAm grafted on PVF sponges. FTIR spectra of PVF and PVF-g-PNIPAAm sponges prepared at 12 h, 24 h, 48 h and 72 h, respectively. 1 H NMR spectra of PVF and PVF-g-PNIPAAm-24h in d6-DMSO. Pore size distributions of PVF and PVF-g-PNIPAAm samples. SEM images of PVF-g-PNIPAAm samples prepared at different time, (a) 12 h, (a-2) enlarged view of local (a-1); (b) 24 h, (b-2) enlarged view of local (b-1); (c) 48 h, (c-2) enlarged view of local (c-1); (d) 72 h, (d-2) enlarged view of local (d-1). Swelling procedure of PVF and PVF-g-PNIPAAm samples in water. (a) Experimental (solid symbol) and nonlinear fitting (line) absorption kinetics in water using pseudo-first-order rate kinetic equation, and (b) linear fitting using pseudo-second-order rate kinetic equation. Equilibrium swelling ratio of PVF and PVF-g-PNIPAAm samples from 20oC to 48oC (solid lines and solid symbols) and from 48oC to 20oC (dash lines and open symbols) during a heating-cooling cycle.

Figure 7. Figure 8. Figure 9.

Figure 10.

Figure 11.

Figure 12.

Figure 13.

Thermo-responsive procedure of PVF-g-PNIPAAm at 20oC and 48oC, respectively. DSC thermograms of swollen PVF, PVF-g-PNPAAm and macro-PNIPAAm sponges during heating. (a) Deswelling kinetics of PVF-g-PNIPAAm samples and macro-PNIPAAm at 48oC, (b) linear fitting of deswelling behavior of PVF-g-PNIPAAm samples using pseudo-first-order rate kinetic equation. Variation of swelling ratio of PVF-g-PNIPAAm sponges and macro-PNIPAAm after a sudden temperature change of the water from 20oC to 48oC and vice versa. Photographs taken by digital camera during temperature change. At first, the PVF-g-PNIPAAm-48h sponge was immersed into deionized water in petri dish at 26oC, and then I2 aqueous solution was added into the above solution at 27oC, increasing temperature from 27oC to 40oC gradually. Reversible deswelling-reswelling and destain-stain response of PVF-g- PNIPAAm-48h (I2 aqueous solution was added) in water after a sudden temperature change of the water from 48oC (a-e) to 27oC (f-j) observed in 40 s. Antibacterial performance of PVF based sponges against (1) Gram-negative E. coli (ATCC 25922) and (2) Gram-positive S. aureus. Here, (a-1) and (a-2) are PVF against E. coli and S. aureus; 35



Table 1. Table 2.

(b-1) and b-2) are PVF-g-PNIPAAm against E. coli and S. aureus; (c-1) and (c-2) are PVF /I2 against E. coli and S. aureus, and (d-1) and (d-2) are PVF-g-PNIPAAm/I2 against E. coli and S. aureus, respectively. Related parameters from the different kinetic equations fitting to the experimental results. Swelling and deswelling performance of rapidly thermo-responsive PNIPAAm hydrogels.

TOC

36


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Scheme 1. Synthetic route of PNIPAAm grafted on PVF sponges.

37



Figure 1. FTIR spectra of PVF and PVF-g-PNIPAAm sponges prepared at 12 h, 24 h, 48 h and 72 h, respectively.

38


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Figure 2. 1H NMR spectra of PVF and PVF-g-PNIPAAm-24h in d6-DMSO.

39



Figure 3. Pore size distributions of PVF and PVF-g-PNIPAAm samples.

40


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Figure 4. SEM images of PVF-g-PNIPAAm samples prepared at different time, (a) 12 h, (a-2) enlarged view of local (a-1); (b) 24 h, (b-2) enlarged view of local (b-1); (c) 48 h, (c-2) enlarged view of local (c-1); (d) 72 h, (d-2) enlarged view of local (d-1).

41



a)

b)

Figure 5. Swelling procedure of PVF and PVF-g-PNIPAAm samples in water. (a) Experimental (solid symbol) and nonlinear fitting (line) absorption kinetics in water using pseudo-first-order rate kinetic equation, and (b) linear fitting using pseudo-second-order rate kinetic equation.

42


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Figure 6. Equilibrium swelling ratio of PVF and PVF-g-PNIPAAm samples from 20oC to 48oC (solid lines and solid symbols) and from 48oC to 20oC (dash lines and open symbols) during a heating-cooling cycle.

43



Figure 7. Thermo-responsive procedure of PVF-g-PNIPAAm at 20oC and 48oC, respectively.

44


Page 44 of 53

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Figure

8.

DSC

thermograms

of

swollen

PVF,

macro-PNIPAAm sponges during heating.

45


PVF-g-PNPAAm

and


Page 46 of 53

a)

b)

Figure

9.

(a)

Deswelling

kinetics

of

PVF-g-PNIPAAm

samples

and

macro-PNIPAAm at 48oC, (b) linear fitting of deswelling behavior of PVF-g-PNIPAAm samples using pseudo-first-order rate kinetic equation.

46


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Figure 10. Variation of swelling ratio of PVF-g-PNIPAAm sponges and macro-PNIPAAm after a sudden temperature change of the water from 20oC to 48oC and vice versa.

47



Figure 11. Photographs taken by digital camera during temperature change. At first, the PVF-g-PNIPAAm-48h sponge was immersed into deionized water in petri dish at 26oC, and then I2 aqueous solution was added into the above solution at 27oC, increasing temperature from 27oC to 40oC gradually.

48


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Figure 12. Reversible deswelling-reswelling and destain-stain response of PVF-gPNIPAAm-48h (I2 aqueous solution was added) in water after a sudden temperature change of the water from 48oC (a-e) to 27oC (f-j) observed in 40 s.

49



Figure 13. Antibacterial performance of PVF based sponges against (1) Gram-negative E. coli (ATCC 25922) and (2) Gram-positive S. aureus. Here, (a-1) and (a-2) are PVF against E. coli and S. aureus; (b-1) and (b-2) are PVF-g-PNIPAAm against E. coli and S. aureus; (c-1) and (c-2) are PVF /I2 against E. coli and S. aureus, and (d-1) and (d-2) are PVF-g-PNIPAAm/I2 against E. coli and S. aureus, respectively.

50


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Table 1. Related Parameters from the Different Kinetic Equations Fitting to the Experimental Results swelling process GP

GE

sample

pseudo-first-order kinetic

pseudo-second-order kinetic equation

pseudo-first-order kinetic

results

equation fitting

fitting

equation fitting -1

GP2

GE1

GE2

Qs (g g-1)

Qs (g g-1)

k1(min−1)

R2

Qs (g g-1)

k2 (g g min-1)×102

R2

k (min−1)

τ(min)

R2

PVF

___

___

___

___

20.1

20.2

0.049

0.982

20.4

0.114

0.999

___

___

___

12h

257

212

43

30

36.1

36.3

0.027

0.954

36.5

0.018

0.999

3.47

0.29

0.938

24h

265

286

44

47

46.7

48.0

0.019

0.970

52.1

0.018

0.982

2.91

0.34

0.890

48h

265

324

46

52

49.7

51.3

0.018

0.953

55.4

0.030

0.973

3.33

0.30

0.939

72h

297

353

50

58

52.7

54.0

0.018

0.969

55.7

0.148

0.993

3.02

0.33

0.819

___

___

___

___

7.8

8.1

0.011

0.987

10.0

0.125

0.982

0.88

1.14

0.982

IPAAm 2

experimental

GP1

Macro-PN 1

deswelling process

: Calculated from gravimetric method; : Calculated from 1H-NMR spectra.

51


ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 52 of 53

Table 2. Swelling and Deswelling Performance of Rapidly Thermo-responsive PNIPAAm Hydrogels samples Ternary hydrogels

time to reach time to reach methods swelling equilibrium deswelling equilibrium 90-500 min 40-130 min Phase separation

PNIPAAm hydrogels

20-260 min

Nanostructured PNIPAAm hydrogels PNIPAAm microgels Poly(NIPAAm-co-AA) hydrogels PNIPAAm-silica nanocomposite hydrogels PNIPAAm hydrogels Porous PNIPAAm hydrogels PNIPAAm-clay nanocomposite Hydrogels Cellulose nanofiber-modified PNIPAAm hydrogels PNIPAAm interpenetrating polymer network

20-40 min

Porosigen

360-1200 s

Freeze-drying

250 s

80 -200 s

NA

> 60 min

Polymerization of microfluidic emulsification Copolymerization

≤ 90 s

≤ 90 s

Cryogenic synthesis

NA < 2 min

~ 200 s < 5 min

>10 min

NA

reason for rapid response Open-cell macroporous structure Open-cell macroporous structure Mesh-like structure

references 17 19 21

Microgels with open-cell porous structure Copolymerize with acrylic acid (AA) Interconnected pores

22

Decrease characteristic size Interconnected pores

25 26

~ 10 min

Electrospinning method Template using porous poly(ε-caprolactone) (PCL) Freeze-drying

Uniform porous network

27

> 240 min

10-60 min

Freeze-drying

28

~ 25 h

~ 60 min

Freeze–drying

Three-dimensional porous structure Formation of water releasing channels

52


23 24

37

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TOC

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Superfast and Reversible Thermoresponse of Poly(N

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