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polymers Article

Temperature-, pH- and CO2-Sensitive Poly(N-isopropylacryl amide-co-acrylic acid) Copolymers with High Glass Transition Temperatures Yeong-Tarng Shieh 1, *, Pei-Yi Lin 1,2 , Tao Chen 3 and Shiao-Wei Kuo 2, * 1 2 3

*

Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 81148, Taiwan; [email protected] Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan Ningbo Institute of Material Technology and Engineering, Key Laboratory of Graphene Technologies and Applications of Zhejiang Province, Chinese Academy of Science, Zhongguan West Road 1219, Ningbo 315201, China; [email protected] Correspondence: [email protected] (Y.-T.S.); [email protected] (S.-W.K.); Tel.: +886-7-591975 (Y.-T.S.); +886-7-5252000 (ext. 4079) (S.-W.K.)

Academic Editor: Jinlian Hu Received: 14 November 2016; Accepted: 7 December 2016; Published: 14 December 2016

Abstract: A series of poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAAm-co-PAA) random copolymers were synthesized through free radical copolymerization in MeOH. The incorporation of the acrylic acid units into PNIPAAm tended to enhance the glass transition temperature (Tg ), due to strong intermolecular hydrogen bonding between the amide groups of PNIPAAm and the carboxyl groups of PAA, as observed using 1 H nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopic analyses. The lower critical solution temperature (LCST) increased upon increasing the pH of the aqueous solution containing PNIPAAm-co-PAA because the COOH groups of the PAA segment dissociated into COO− groups, enhancing the solubility of the copolymer. In addition, high-pressure differential scanning calorimetry revealed that the LCSTs of all the aqueous solutions of the copolymers decreased upon increasing the pressure of CO2 , suggesting that CO2 molecules had displaced H2 O molecules around the polar CONH and COOH groups in PNIPAAm-co-PAA, thereby promoting the hydrophobicity of the copolymers in the aqueous solution. In addition, the values of Tg of a film sample increased upon treatment with supercritical CO2 , implying that intermolecular interactions in the copolymer had been enhanced after such treatment. Keywords: temperature-sensitive; pH-sensitive; CO2 -sensitive; hydrogen bonding; copolymers

1. Introduction Increasing the glass transition temperatures (Tg ) of polymeric materials is interesting in polymer science due to the strong economic incentives arising from their potential industrial applications [1–4]. In general, values of Tg are strongly dependent on the chemical and physical nature of polymeric materials, including their molecular weights, numbers of bulky groups, degrees of branching, degrees of crosslinking, and strength of intermolecular interactions [5–9]. In previous studies [10–13], we found that copolymerization of two monomers possessing strong hydrogen bonding donor or acceptor units can lead to significant enhancements in glass transition temperatures, due to so-called “compositional heterogeneity” [14]. Smart materials exhibit changes in their properties that are dependent on their environment. For example, they could shrink or swell in response small variations in pH, temperature, or ionic strength [15,16]. Poly(N-isopropylacrylamide) (PNIPAAm) is a well-known smart material that Polymers 2016, 8, 434; doi:10.3390/polym8120434

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undergoes a sharp coil-to-globule transition in aqueous solution at approximately 32 ◦ C (its lower critical solution temperature (LCST)), from a hydrophilic (hydrated) state below this temperature to a hydrophobic (dehydrated) state above it [17,18]. Poly(acrylic acid) (PAA) also undergoes phase transitions in aqueous solution that are dependent on pH, allowing it to be used in site-specific drug delivery [19]. Combinations of PNIPAAm and PAA in random copolymers not only result in dual stimuli-responsive behavior (temperature and pH) in aqueous solution [20–22] but also significant increases in values of Tg in response to strong intermolecular hydrogen bonding in the bulk state. In addition, the carbonyl (C = O) groups in PNIPAAm and PAA polymers are capable of interacting with CO2 molecules through the dipole–dipole interactions [23,24]. The CO2 molecules can around the C = O groups in PNIAAm and PAA in aqueous solution, thereby displacing H2 O molecules from around the polar CONH and COOH groups of PNIPAAm and PAA, respectively. This phenomenon may lead to a decrease in the LCST of PNIAAm-co-PAA copolymers in aqueous solution, due to increased hydrophobicity. In this study, we prepared a series of PNIAAm-co-PAA random copolymers through free radical copolymerization in MeOH solution. Although PNIAAm-co-PAA random copolymers have been studied previously as superhydrophobic surfaces, hydrogels, nanoparticles, and microgels for drug delivery [25–28], to the best of our knowledge their thermal properties, hydrogen bonding, and LCST behavior under CO2 atmosphere or after CO2 treatment have not been investigated. Therefore, the copolymer compositions, hydrogen bonding interactions, thermal properties, and LCST behavior of PNIAAm-co-PAA random copolymers were characterized in this study using 1 H nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), and high-pressure differential scanning calorimetry (HP-DSC) analyses. 2. Experimental 2.1. Materials N-Isopropylacrylamide (NIPAAm) was purchased from Tokyo Kasei (Tokyo, Japan) and recrystallized from n-hexanes. Acrylic acid (AA) was obtained from the Alfa Aesa (Heysham, Lancashire, UK). All solvents were obtained from Sigma-Aldrich (St. Louis, MO, USA) and used as received. PNIPAAm-co-PAA copolymers were synthesized from different monomer molar ratios through free radical copolymerization using azobis(isobutyronitrile) as the initiator in MeOH under a N2 atmosphere (Scheme 1). The 2 M monomer mixture containing 0.05 M AIBN was stirred at 55 ◦ C for 4 h in a circulated thermostat water bath. For purification of the pure PNIPAAm homopolymer, the product was dissolved in MeOH at room temperature and then added into deionized water at 45 ◦ C, causing precipitation of the PNIPAAm homopolymer [29]. This dissolution/precipitation procedure was repeated three times and then the product was dried in a vacuum oven at 60 ◦ C for 1 day. For purification of the pure PAA homopolymer, the reaction mixture was concentrated and then was ether added to precipitate the PAA. The purified PAA homopolymer was obtained following vacuum drying for 1 day. To purify the PNIPAAm-co-PAA copolymers, the products were dissolved in MeOH and then ether was added to precipitate the copolymers. The settled products were dried in an oven; the dried precipitates were dissolved in 0.15 M aqueous NaOH and heated at 60 ◦ C to precipitate PNIPAAm, which was filtered off. The filtrates were treated with a few drops of concentrated HCl and then heated at 60 ◦ C to precipitate the PNIPAAm-co-PAA copolymers. The purified PNIPAAm-co-PAA copolymers were washed with ether until neutral and then dried in a vacuum oven for 1 day.

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Scheme 1. Synthesis of temperature-, pH- and CO2 -responsive PNIPAAm-co-PAA copolymers. Scheme 1. Synthesis of temperature-, pH- and CO2-responsive PNIPAAm-co-PAA copolymers. Chemical structures of the: (a) N-isoproylacrylamide monomer; (b) acrylic acid monomer; and Chemical structures of the: (a) N-isoproylacrylamide monomer; (b) acrylic acid monomer; and (c) (c) PNIPAAm-co-PAA copolymers. PNIPAAm-co-PAA copolymers.

2.2. Characterization of PNIPAAm-co-PAA Copolymers AqueousSolutions Solutions and 2.2. Characterization of PNIPAAm-co-PAA Copolymers ininAqueous andFilms Films The molecular weights polydispersityindices indices (PDIs) (PDIs) of PNIPAAm-co-PAA The molecular weights andand polydispersity ofthe thesynthesized synthesized PNIPAAm-co-PAA copolymers were determined using a Waters 510 gel permeation chromatography (GPC) system copolymers were determined using a Waters 510 gel permeation chromatography (GPC) system equipped with a refractive index detector and three Ultrastyragel columns (100, 500 and 1000 Å) equipped with a refractive index detector and three Ultrastyragel columns (100, 500 and 1000 Å) connected in series. N,N-Dimethylformamide (DMF) was the eluent, at a flow rate of 0.8 mL/min, at connected in series. N,N-Dimethylformamide (DMF) was the eluent, at a flow rate of 0.8 mL/min, 30 °C. A Bruker Tensor-27 (Billerica, MA, USA) FTIR spectrometer was used to quantitatively at 30 ◦characterize C. A Bruker MA, USA)inFTIR spectrometer was were usedcast to quantitatively theTensor-27 hydrogen (Billerica, bonding interactions the copolymers, which from THF characterize theonto hydrogen bonding interactions thecollected copolymers, which were solutions solutions KBr crystal plates. The spectra in were at a resolution of 4 cast cm−1from and aTHF sensitivity −1 onto KBr crystal spectra were collected at Inova-500MHz a resolution of 14Hcm a sensitivity of 32 scans of 32 scans plates. at roomThe temperature. A Varian Unity NMRand spectrometer (McKinley 1 Hcharacterize Scientific, Sparta, A NJ,Varian USA) was used to quantitatively the copolymers’ compositions at room temperature. Unity Inova-500MHz NMR spectrometer (McKinley Scientific, interactions when dissolvedthe in copolymers’ DMSO-d6. The LCST behavior the Sparta,and NJ, hydrogen USA) wasbonding used to quantitatively characterize compositions and of hydrogen PNIPAAm-co-PAA copolymers was in measured from plotsLCST of their visible light transmittance at 550 bonding interactions when dissolved DMSO-d . The behavior of the PNIPAAm-co-PAA 6 nm with respect to temperature, using wt %visible copolymer aqueous solutions at at 550 various of pH. to copolymers was measured from plots of5their light transmittance nmvalues with respect The inflection point of the transmittance–temperature curve was assigned as the LCST of the temperature, using 5 wt % copolymer aqueous solutions at various values of pH. The inflection point copolymer aqueous solution. A Q100 DSC apparatus (TA Instrument, New Castle, DE, USA) was of the also transmittance–temperature curve was assigned as the LCST of the copolymer aqueous solution. used to determine the LCSTs of the 5 wt % copolymer aqueous solutions at various values of A Q100 DSC (TA Instrument, New Castle, USA)from was10 also used theatLCSTs pH. Theapparatus DSC system was operated with heating at DE, 2 °C/min to 55 °C to anddetermine then cooling 2 of the °C/min 5 wt %tocopolymer aqueous solutions at various values of pH. The DSC system was 10 °C, followed by heating again at 2 °C/min to 55 °C to record the endothermic peakoperated (i.e., ◦ C/min to 10 ◦ C, followed by heating with heating at 2A◦ C/min from 10 to 55 ◦ C(TA andInstruments) then coolingwas at 2used the LCST). Q20 HP-DSC apparatus to investigate the effect of a CO2 ◦ ◦ atmosphere of 5 wtthe % endothermic aqueous solutions thethe PNIPAAm-co-PAA copolymers at again at 2 C/minon to the 55 LCST C to record peak of (i.e., LCST). A Q20 HP-DSC apparatus various valueswas of pH. The was first CO22 atmosphere for 5 min before under 2 (TA Instruments) used tochamber investigate the purged effect ofwith a CO on heating the LCST of CO 5 wt % from 16 to 45 °C at a rate of 2 °C/min. The peak temperature and area of the endothermic peak in the aqueous solutions of the PNIPAAm-co-PAA copolymers at various values of pH. The chamber was first heating curve of each sample were recorded and taken as the LCST and◦enthalpy, respectively. first purged with CO2 for 5 min before heating under CO2 from 16 to 45 C at a rate of 2 ◦ C/min. The glass transition temperatures (Tg) of the PNIPAAm-co-PAA copolymers of various compositions The peak temperature and area of the endothermic peak in the first heating curve of each sample were determined after casting them from aqueous solutions at various values of pH. They were were recorded and taken as the LCST and enthalpy, respectively. The glass transition temperatures measured through DSC, with an initial heating and cooling cycle at 20 °C/min between 40 and 200 °C (Tg ) ofand thethen PNIPAAm-co-PAA compositions wereofdetermined with heating againcopolymers at 10 °C/minof to various 200 °C. The CO2-dependence the values ofafter Tg of casting the them from aqueous solutions various values of pH. They measured through DSC, with cast films of the variousatcopolymer compositions was were also investigated after treatment in an ◦ C/min between 40 and 200 ◦ C and then with heating again initial supercritical heating andcarbon cooling cyclefluid at 20 dioxide (scCO 2) at 2000 psi and 32 °C for 1 h, with a depressurization time 1 h. to 200 ◦ C. The CO2 -dependence of the values of Tg of the cast films of the various at 10 ◦of C/min copolymer compositions was also investigated after treatment in supercritical carbon dioxide fluid (scCO2 ) at 2000 psi and 32 ◦ C for 1 h, with a depressurization time of 1 h.

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3. Results and Discussion 3. Results and Discussion 3.1. Synthesis of PNIPAAm-co-PAA Random Copolymers 3.1. Synthesis of PNIPAAm-co-PAA Random Copolymers We synthesized a series of PNIPAAm-co-PAA random copolymers through free radical We synthesized a seriessolution of PNIPAAm-co-PAA copolymers through free radical copolymerization in MeOH (Scheme 1). Therandom compositions of these PNIPAAm-co-PAA copolymerization in MeOH solution (Scheme 1). The compositions of these PNIPAAm-co-PAA random 1 random copolymers were determined using H NMR and FTIR spectroscopy. Figure 1A displays the 1 H NMR and FTIR spectroscopy. Figure 1A displays the 1 H NMR copolymers were determined using 1H NMR spectra of PNIPAAm-co-PAA random copolymers recorded as DMSO-d6 solutions. For the spectra of PNIPAAm-co-PAA random copolymers as DMSO-d solutions. For the pure pure PNIPAAm homopolymer, the signals for the recorded CH and CH 2 groups6 on the main chain were PNIPAAm homopolymer, the signals for the CH and CH groups on the main chain were located 2 and CH3 protons for located between 1.46 and 1.96 ppm. The other alkyl CH pure PNIPAAm between and 1.96 ppm. Theand other alkyl CHrespectively; and CH3 protons for pure PNIPAAm appearedthe as appeared1.46 as multiplets at 3.86 1.02 ppm, a singlet at 7.21 ppm represented multiplets at 3.86 and 1.02 ppm, respectively; a singlet at 7.21 ppm represented the proton of the amide proton of the amide group (CONH). Similarly, for the pure PAA homopolymer, the CH and CH2 group Similarly, the pure PAAbetween homopolymer, the2.19 CH ppm, and CH on the 2 groups groups(CONH). on the main chainforwere located 1.46 and with a singlet at main 12.26 chain ppm were located between 1.46 and 2.19 ppm, with a singlet at 12.26 ppm corresponding to the proton corresponding to the proton of the carboxyl group (COOH). The compositions of PAA in of the carboxyl group (COOH).from The compositions in /(A copolymers were determined from the copolymers were determined the peak ratioofofPAA ACOOH COOH + ACONH), based on the signals peak ratio of A /(A + A ), based on the signals 12.26 and 7.21 ppm. copolymers The compositions COOH CONH 12.26 and 7.21COOH ppm. The compositions of these PNIPAAm-co-PAA random were of these PNIPAAm-co-PAA random copolymers were confirmed using FTIR spectroscopy (Figure 1B). confirmed using FTIR spectroscopy (Figure 1B). For pure PNIPAAm, the two peaks at 1644 and 1544 −1 represented the amide I and amide II For pure PNIPAAm, the two peaks at 1644 and 1544 cm cm−1 represented the amide I and amide II stretching vibration modes; for pure PAA, the stretching vibration for pure PAA, the band at 1710 cm−1 represented the −1 represented characteristic band atmodes; 1710 cm the characteristic self-association hydrogen-bonded COOH groups. −1 self-association hydrogen-bonded COOH groups. Clearly, the ratio of the signal area at 1710 cm −1 Clearly, the ratio of the signal area at 1710 cm (COOH) with respect to the amide I (CONH) peak (COOH) to the amide (CONH) increased upon increasing the PAA content increasedwith uponrespect increasing the PAA Icontent in peak the PNIPAAm-co-PAA random copolymers. Tablein1 the PNIPAAm-co-PAA Table 1 summarizes the and feed ratios of the NIPAAm/AA summarizes the feedrandom ratios copolymers. of the NIPAAm/AA monomers the resultant copolymer 1 H NMR spectroscopy. monomers and the resultant copolymer compositions determined based on 1 compositions determined based on H NMR spectroscopy. The reactivity ratios were determined The reactivity ratios determined using and Tudospreviously methodology, as we have 2described using the Kelen and were Tudos methodology, asthe weKelen have described [30,31]. Figure displays previously [30,31]. Figure 2 displays the results graphically; for the PNIPAAm-co-PAA copolymers, we the results graphically; for the PNIPAAm-co-PAA copolymers, we determined values of rPNIPAAm and determined values of r and r of 1.44 and 0.95, respectively, suggesting a tendency for ideal PNIPAAm PAA rPAA of 1.44 and 0.95, respectively, suggesting a tendency for ideal random copolymerization. random copolymerization.

e

(A)

d c

amide

(B)

b a (a)

COOH

(b) (c) (d) (e) (f) f 14

(g)

12

10

8

6

4

2

Chemical Shift (ppm)

0 4000

3000

2000

1000

Wavenumber (cm-1)

Figure 1. (A) 11 H NMR; and (B) FTIR spectra of: (a) pure PNIPAAm; (b) PNIPAAm97-co-PAA3; Figure 1. (A) H NMR; and (B) FTIR spectra of: (a) pure PNIPAAm; (b) PNIPAAm97-co-PAA3; (c) PNIPAAm87-co-PAA13; (d) PNIPAAm80-co-PAA20; (e) PNIPAAm57-co-PAA43; (f) PNIPAAm30-co(c) PNIPAAm87-co-PAA13; (d) PNIPAAm80-co-PAA20; (e) PNIPAAm57-co-PAA43; (f) PNIPAAm30PAA70; and (g) pure PAA. co-PAA70; and (g) pure PAA.

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Table 1 Characteristicsofofthe thePNIPAAm-co-PAA PNIPAAm-co-PAA copolymers used in this study. Table 1. Characteristics copolymers used in this study. Monomer feed Polymer composition Tg Polymer composition (mol (mol %) %)%) AbbreviationMonomer feed (mol ◦ C) (mol %) (°C) T ( g Abbreviation NIPAAm AA NIPAAm AA NIPAAm AA NIPAAm AA PNIPAAm 100 0 100 0 140.6 PNIPAAm 100 PNIPAAm97-co-PAA3 95 0 5 100 96.9 0 3.1 140.6 142.3 PNIPAAm97-co-PAA3 95 PNIPAAm87-co-PAA13 85 5 15 96.9 86.4 3.1 13.6 142.3 149.5 PNIPAAm87-co-PAA13 85 15 86.4 13.6 149.5 PNIPAAm80-co-PAA20 75 25 79.6 20.4 154.5 PNIPAAm80-co-PAA20 75 25 79.6 20.4 154.5 PNIPAAm57-co-PAA43 5050 50 57.3 57.3 42.7 42.7 159.1 159.1 PNIPAAm57-co-PAA43 50 PNIPAAm30-co-PAA70 25 75 29.2 70.8 140.0 PNIPAAm30-co-PAA70 25 75 29.2 70.8 140.0 PAA 0100 100 0 0 100 100 88.888.8 PAA 0

Mn Mn (g/mol)

(g/mol)

3.1 × 105

5 3.1 4.2××10 106 6 4.2 × 10 2.1 × 106 2.1 × 1066 6.8 × 10 6.8 × 1066 5.0 × 10 5.0 × 106 8.9××10 1066 8.9 3.1××10 1055 3.1

PDI

PDI

1.64 1.701.64 2.241.70 2.24 1.31 1.31 1.561.56 1.591.59 1.641.64

1.5

Experimental Data y = 1.95x-0.51 rPNIPAm = 1.44, rPAA=0.95

1.2

0.9

η

0.6

0.3

0.0

-0.3 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

ζ Figure 2. Kelen–Tudos random copolymers. Figure 2. Kelen–Tudosplots plotsof ofPNIPAAm-co-PAA PNIPAAm-co-PAA random copolymers.

3.2. Thermal Properties of PNIPAAm-co-PAARandom RandomCopolymers Copolymers 3.2. Thermal Properties of PNIPAAm-co-PAA Figure 3A displays DSC thermograms of pure PNIPAAm, pure PAA, and various

Figure 3A displays DSC thermograms of pure PNIPAAm, pure PAA, and various PNIPAAm-co-PAA random copolymers, recorded at temperatures ranging from 60 to 180 °C. The PNIPAAm-co-PAA random copolymers, recorded at temperatures ranging from 60 to 180 ◦ C. The pure pure PNIPAAm and pure PAA provided values of Tg of approximately 140 and 88 °C, respectively; PNIPAAm pure PAA provided values of Tg of approximately 140 andof88Tg◦ C, respectively; each ofand the PNIPAAm-co-PAA random copolymers provided a single value in the range 140– each ◦ of the159 PNIPAAm-co-PAA random a single value of positive Tg in the range 140–159 °C, significantly higher thancopolymers those of theprovided homopolymers. This large deviation in the C, significantly of the temperatures homopolymers. This large positive of deviation in the behavior of behaviorhigher of thethan glassthose transition suggests the presence strong intermolecular interactions between the two polymer segments. the glass transition temperatures suggests the presence of strong intermolecular interactions between The Kwei equation is commonly adopted to explain the behavior of the values of Tg of system the two polymer segments. displaying strong intermolecular interactions The Kwei equation is commonly adopted[7]: to explain the behavior of the values of Tg of system displaying strong intermolecular interactions [7]: W1Tg1 + kW2Tg 2

Tg =

+ qW1W2

(1)

W1++kW kW2 T W1 Tg1 2 g2 Tg = + qW1 W2 (1) W1 + kW2 where Tgi and Wi represent the glass transition temperature and weight fraction of each component i; and k and q are both fitting constants describing the strength of the intermolecular interactions.

whereUsing Tgi and the in glass transition temperature and fraction of each component i represent this W equation (red line Figure 3B), we calculated values of weight k and q of 1 and 160, respectively. i; andThis k and q are both fitting constants describing the strength of the intermolecular interactions. positive value of q (higher than the linear rule, green line) suggests that intermolecular Usinghydrogen this equation (red line in Figure we calculated valuessegments of k and q (Scheme of 1 and 160, bonding between the 3B), PNIPAAm and PAA 2c) respectively. in the PNIPAAm-co-PAA was stronger thanline) the self-association hydrogen bonding of This positive value of q random (higher copolymers than the linear rule, green suggests that intermolecular hydrogen either the PNIPAAm segments (Scheme 2b) or the PAA segments (Scheme 2a). bonding between the PNIPAAm and PAA segments (Scheme 2c) in the PNIPAAm-co-PAA random copolymers was stronger than the self-association hydrogen bonding of either the PNIPAAm segments (Scheme 2b) or the PAA segments (Scheme 2a).

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(A) (A)

180 180

(a)

160 160

(b) (c) (c) (d) (d)

(f) (g) (g)

60 60

80 80

100 120 140 160 100 120 140 160 o

180 180 160 160

140 140

140 140

120 120

120 120

100 100

100 100

80 80 0.0 0.0

o

(e) (e) (f)

Experimental Tg Predicted By Kwei Equation Experimental k =1, q= 165 Tg Predicted Equation PredictedBy ByKwei Linear Equation k =1, q= 165 Predicted By Linear Equation

o Tg T ( C) ( C) g

Heat Flow (Endo, Down) Heat Flow (Endo, Down)

(a) (b)

(B) (B)

80 1.080 1.0

0.2 0.4 0.6 0.8 0.2 Weigth 0.4 0.6 0.8 PAA Fraction

Temperature ( C) PAA Weigth Fraction Temperature (oC) Figure 3. (A) DSC thermograms of PNIPAAm-co-PAA random copolymers: (a) pure PNIPAAm; Figure 3. (A) DSC thermograms of PNIPAAm-co-PAA random copolymers: (a) pure PNIPAAm; Figure 3. (A) DSC thermograms of PNIPAAm-co-PAA copolymers: (a) pure PNIPAAm; (b) PNIPAAm97-co-PAA3; (c) PNIPAAm87-co-PAA13; (d)random PNIPAAm80-co-PAA20; (e) PNIPAAm57(b) PNIPAAm97-co-PAA3; (c)(c) PNIPAAm87-co-PAA13; (d)PNIPAAm80-co-PAA20; PNIPAAm80-co-PAA20; (e) PNIPAAm57(b) PNIPAAm97-co-PAA3; PNIPAAm87-co-PAA13; (d) (e) PNIPAAm57co-PAA43; (f) PNIPAAm30-co-PAA70; and (g) pure PAA. (B) Experimental values of Tg, with lines co-PAA43; (f) PNIPAAm30-co-PAA70; and PAA. (B) (B)Experimental Experimental values Tg , with g, with lines lines co-PAA43; (f) the PNIPAAm30-co-PAA70; and(g) (g) pure pure PAA. values of Tof predicted by Kwei and linear equations. predicted by the andand linear equations. predicted by Kwei the Kwei linear equations. (a) Self-Association of PAA (a) Self-Association of PAA

(b) Self-Association of PNIPAAm (c) Inter-Association of PNIPAAm-PAA (b) Self-Association of PNIPAAm (c) Inter-Association of PNIPAAm-PAA

O

O

O

N

O

N

O

O

O H

N

O

N

H

H

H

H

H O O

H

H

H O

H O

H N

O

H O

O

O

N

O

O

Scheme 2. Noncovalent interactions of PNIPAm-co-PAA copolymers: (a) self-association of PAA Scheme 2.self-association Noncovalent interactions ofunits; PNIPAm-co-PAA copolymers: self-association of ofPAA PAAunits; Scheme 2. (b) Noncovalent interactions of PNIPAm-co-PAA copolymers: (a)(a) self-association units; of PNIPAAm and (c) inter-association of PNIPAAm-co-PAA. units; (b) self-association of PNIPAAm units; and (c) inter-association of PNIPAAm-co-PAA. (b) self-association of PNIPAAm units; and (c) inter-association of PNIPAAm-co-PAA. NMR and FTIR spectroscopy are two highly effective methods for characterizing NMR and interactions. FTIR spectroscopy two 1highly methods characterizing intermolecular Figure 4Aare presents H NMReffective spectra of various for PNIPAAm-co-PAA 1H NMRmethods NMR and FTIR spectroscopy are two highly effective for characterizing intermolecular intermolecular interactions. Figure 4A presents spectra of various PNIPAAm-co-PAA random copolymers in DMSO-d6 solution. The proton of the COOH groups from the PAA segments 1 interactions. Figure 4A presents H6 solution. NMR spectra of various random copolymers in DMSO-d The value proton thePNIPAAm-co-PAA COOH groups the segments underwent a high-field shift from an initial ofof12.26 ppm for purefrom PAArandom to PAA 11.97copolymers ppm for in underwent a high-field shift from an initial value of 12.26 ppm for pure PAA to 11.97 ppm for DMSO-d solution. The proton of the COOH groups from the PAA segments underwent a high-field 6 PNIPAAm87-co-PAA13, consistent with intermolecular hydrogen bonding between the CONH and PNIPAAm87-co-PAA13, consistent with intermolecular hydrogen bonding between the CONH and shift COOH from an initial of 12.26 ppm for pure PAA to 11.97 2c) ppm PNIPAAm87-co-PAA13, groups in value PNIPAAm-co-PAA random copolymers (Scheme [32].for Figure 4B illustrates the COOH groups in PNIPAAm-co-PAA random (Scheme [32]. Figure 4BCOOH illustrates the FTIR spectral region of the C=O groups ofcopolymers the PNIPAAm-co-PAA random The consistent with intermolecular hydrogen bonding between the2c)CONH andcopolymers. groups in FTIR spectral region of the C=O groups of the PNIPAAm-co-PAA random copolymers. The −1 spectrum of purerandom PNIPAAm displays two peaks at 2c) 1644[32]. and 1544 cm 4B , corresponding to the amide I PNIPAAm-co-PAA copolymers (Scheme Figure illustrates the FTIR spectral spectrum ofIIpure PNIPAAm displays twofor peaks atPAA, 1644 and 1544 cm−1, corresponding toatthe amide −1I, and stretching modes; pure a characteristic band appeared 1710 cm region ofamide the C=O groupsvibration of the PNIPAAm-co-PAA random copolymers. The spectrum of pure −1, and amide II stretching vibration modes; for pure PAA, a characteristic band appeared at 1710 cm corresponding to the self-association hydrogen-bonded COOH groups. Upon increasing the PAA −1 , corresponding PNIPAAm displays two peaks at 1644 and 1544 cm to the amide I and amide II corresponding thePNIPAAm-co-PAA self-association hydrogen-bonded COOH the PAA composition intothe random copolymers, thegroups. signal Upon for theincreasing amide groups of −1 ,I corresponding stretching vibration modes; for pure PAA, a characteristic band appeared at 1710 cm composition in the random copolymers, signal for theatamide I groups of −1 and the PNIPAAm split intoPNIPAAm-co-PAA two bands, which we assigned to the freethe amide I groups 1644 cm −1 and thein the to thePNIPAAm self-association hydrogen-bonded COOH groups. Upon increasing the PAA composition split into two bands, which we assigned to the free amide I groups at 1644 cm −1 intermolecular hydrogen bonded amide groups at 1615 cm (Scheme 2c). These two absorption PNIPAAm-co-PAA random using copolymers, thegroups signal for1615 the amide PNIPAAm split into two intermolecular hydrogen bonded at cm−1 (Scheme 2c).ofThese absorption bands were resolvable theamide Gaussian function (Figure 5);I groups the fractions oftwo intermolecular − 1 bands were resolvable using the Gaussian function (Figure 5); the fractions of intermolecular bands, which we assigned togroup the free I groups at 1644 and the intermolecular hydrogen hydrogen-bonded amide areamide summarized in Figure 4Ccm (insets to Figure 4B). The fraction of −1 summarized hydrogen-bonded group in increasing Figure (insets to composition Figurebands 4B). The fraction of bonded amide groupsamide at 1615 cmare (Schemeupon 2c). These4C two absorption resolvable hydrogen-bonded amide I groups increased the PAA inwere the random amide groups upon increasing the PAA composition inenhanced the random copolymers. These strongI intermolecular hydrogen bonding interactions presumably the usinghydrogen-bonded the Gaussian function (Figureincreased 5); the fractions of intermolecular hydrogen-bonded amide copolymers. These strong intermolecular hydrogen bonding interactions presumably enhanced the glass transition temperatures of the PNIPAAm-co-PAA random copolymers, as determined from the group are summarized in Figure 4C (insets to Figure 4B). The fraction of hydrogen-bonded amide I glass transition temperatures of the PNIPAAm-co-PAA random copolymers, as determined from the DSC analyses. groups increased upon increasing the PAA composition in the random copolymers. These strong DSC analyses.

intermolecular hydrogen bonding interactions presumably enhanced the glass transition temperatures of the PNIPAAm-co-PAA random copolymers, as determined from the DSC analyses.

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(A) (A) (c) (c)

(B) (B)

(C) (C)

0.4 0.2

fb

HB

fb

0.6 0.4

0.2 0.0 0.0

11.98 11.98

(d) (d)

0.0 0.0

13 13

0.3

0.4

0.5

0.6

0.7

0.8

PAA Molar Fraction 0.3

0.4

0.5

0.6

0.7

0.8

(e) (e) (f) (f) (g) (g)

12.26 12.26

(g) (g)

0.2 0.2

(a) (a) (b) (b) (c) (c) (d) (d)

12.11 12.11

(f) (f)

0.1 0.1

PAA Molar Fraction

12.03 12.03

(e) (e)

14 14

0.8 0.8 0.6

HB

11.97 11.97

12 12

11 11

Chemical Shift (ppm) Chemical Shift (ppm)

10 1800 10 1800

1700 1700

1600 1600

-1

Wavenumber (cm-1 ) Wavenumber (cm )

1500 1500

1H NMR Figure 4. (A)4.1(A) H NMR spectra, presented 14–10ppm, ppm,displaying displaying signal of proton the proton Figure spectra, presentedininthe the range range 14–10 thethe signal of the 1H NMR spectra, presented in the range 14–10 ppm, displaying − 1 displaying −1 4. (A) the signal of the proton of theFigure acrylic acid units. (B) FTIR spectra in the range 1800–1500 cm the COOH of the acrylic acid units. (B) FTIR spectra in the range 1800–1500 cm displaying the COOH and and of the acrylic acidof:units. (B)pure FTIR spectra in(b) thePNIPAAm97-co-PAA3; range 1800–1500 cm−1 (c) displaying the COOH and CONH absorptions of: PNIPAAm; (b) PNIPAAm97-co-PAA3; PNIPAAm87-co-PAA13, CONH absorptions (a)(a) pure PNIPAAm; (c) PNIPAAm87-co-PAA13, CONH absorptions of: (a) pure PNIPAAm; (b) PNIPAAm97-co-PAA3; (c) PNIPAAm87-co-PAA13, (d) PNIPAAm80-co-PAA20; (e) PNIPAAm57-co-PAA43; (f) PNIPAAm30-co-PAA70; and(g) (g)pure purePAA. (d) PNIPAAm80-co-PAA20; (e) PNIPAAm57-co-PAA43; (f) PNIPAAm30-co-PAA70; and (d) PNIPAAm80-co-PAA20; (e) PNIPAAm57-co-PAA43; (f) PNIPAAm30-co-PAA70; and segments (g) pure PAA. (C) Fractions of inter-associated hydrogen bonds between the PNIPAAm and PAA (C) Fractions of inter-associated hydrogen bonds between the PNIPAAm and PAA segments for various PAA. (C) Fractions of inter-associated hydrogen bonds between the PNIPAAm and PAA segments for various PAA molar fractions. PAA molar fractions.

for various PAA molar fractions.

Absorbance (a.u.) Absorbance (a.u.)

(a) (a) (b) (b) (c) (c) (d) (d) (e) (e) (f) (f) 1800 1800

1750 1750

1700 1700

1650 1650

1600 1550 1600-1 1550

1500 1500

Wavenumber (cm ) Wavenumber (cm-1) Figure 5. Curving fitting of data from FTIR spectra (dash lines are curve fitting results) of: (a) pure Figure 5. Curving fitting of data from FTIR PNIPAAm87-co-PAA13; spectra (dash lines are curve fitting results) of: (a) pure PNIPAAm; (b) PNIPAAm97-co-PAA3; (d) PNIPAAm80-co-PAA20; (e) of: Figure 5. Curving fitting of data from(c)FTIR spectra (dash lines are curve fitting results) PNIPAAm; (b) PNIPAAm97-co-PAA3; (c) PNIPAAm87-co-PAA13; (d) PNIPAAm80-co-PAA20; (e) PNIPAAm57-co-PAA43; and (f) PNIPAAm30-co-PAA70. (a) pure PNIPAAm; (b) PNIPAAm97-co-PAA3; (c) PNIPAAm87-co-PAA13; (d) PNIPAAm80-co-PAA20; PNIPAAm57-co-PAA43; and (f) PNIPAAm30-co-PAA70.

(e) PNIPAAm57-co-PAA43; and (f) PNIPAAm30-co-PAA70.

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3.3. LCST Behaviors of PNIPAAm-co-PAA PNIPAAm-co-PAARandom RandomCopolymers Copolymers 3.3. general, the random random coils of PNIPAAm PNIPAAm chains chains in in aqueous aqueous solution solution collapse collapse to to form form dense dense In general, globule chains at approximately 32 ◦°C. C. Our PNIPAAm-co-PAA random copolymers, however, displayed dual temperature- and pH-responsive pH-responsive behavior behavior in aqueous aqueous solution, with the the transparent transparent values of of aqueous solutions becoming opaque at temperatures above a specific temperature and at values pH above above aa specific specific pH, pH, as as revealed revealed in in Figure Figure 66 from from UV–Vis UV–Visspectroscopic spectroscopicanalyses. analyses. The The opaque opaque pH solution reverted into a transparent transparent solution solutionagain againwhen whenthe thetemperature temperaturedecreased, decreased,characterizing characterizinga our areversible reversiblephase phasetransition. transition.InInaddition, addition,the theLCSTs LCSTsincreased increasedupon uponincreasing increasing the the pH pH for for all of our PNIPAAm-co-PAArandom randomcopolymers copolymers (Figure (Figure 6A–C), 6A–C), presumably presumably because because of of their their better solubility PNIPAAm-co-PAA aqueous solutions solutions at higher higher values values of of pH; pH; Figure Figure 6D 6D summarizes summarizes the the results. results. We also used used the the in aqueous heating scan to determine determine the LCST behavior for all of our PNIPAAm-co-PAA PNIPAAm-co-PAA random random DSC first heating copolymers in in aqueous aqueous solutions solutions at at various various values values of of pH pH (Figure (Figure 7). 7). The obvious endothermic peaks copolymers in all allofofthe theheating heating curves corresponded to coil-to-globule transitions, from dissolved to in curves corresponded to coil-to-globule phasephase transitions, from dissolved to turbid turbid of states, of the polymer chains in the aqueous solutions We observed in the states, the polymer chains in the aqueous solutions [33,34]. [33,34]. We observed a trendaintrend the LCSTs LCSTs similar to that determined spectroscopically: theincreased values increased upon increasing for similar to that determined spectroscopically: the values upon increasing the pH forthe allpH of our all of our PNIPAAm-co-PAA random copolymers (Figure 7A–C); Figure 7D summarizes PNIPAAm-co-PAA random copolymers (Figure 7A–C); Figure 7D summarizes the results.the Weresults. found, We found, however, thatdetermined the LCSTs using determined using were higher than those however, that the LCSTs DSC were all DSC higher thanallthose obtained usingobtained UV–Vis ◦ C, at the same using UV–Visby spectroscopy, by2–5 approximately 2–5PAA °C, at the same PAA in the random spectroscopy, approximately composition in thecomposition random copolymer and at copolymer and at apparent the samediscrepancy pH. This apparent discrepancy the and LCSTs the DSC analyses and UV the same pH. This in the LCSTs from theinDSC UV from spectroscopic spectroscopic analyses can be explained by experimental considering the differentwe experimental conditions: we can be explained by considering the different conditions: would expect the dynamic would expect the dynamic from heating scans higher in DSCLCSTs analyses provide measurements from heatingmeasurements scans in DSC analyses to provide thantowould thehigher static LCSTs than would static scans measurements heating scans of UV–Vis spectroscopic analyses. measurements fromthe heating of UV–Visfrom spectroscopic analyses. 100

(A)

80 60

40

20

20

29

30

31

32

33

(C)

34

35

pH value 3.0 3.5 4.0 4.5

80 60

pH value 3.0 3.5 4.0 4.5

60

40

0 28 100

(B)

80

0 28 45

30

32

34

36

38 45

(D) PNIPAm97-co-PAA3 PNIPAm87-co-PAA13 PNIPAm80-co-PAA20

40

40

35

35

30

30

o

40 20 0 20

LCST ( C)

Transmittance (%)

100

pH value 3.0 3.5 4.0 4.5

25

30

35

40 o

Temperature ( C)

45

50

25

3.0

3.5

4.0

4.5

25

pH value

Figure (A–C) Transmittance, based on UV–Vis spectra, 550ofnm of aqueous solutions at Figure 6. (A–C) Transmittance, based on UV–Vis spectra, at 550atnm aqueous solutions at various various temperatures andofvalues of (A) pH PNIPAAm97-co-PAA3; for: (A) PNIPAAm97-co-PAA3; (B) PNIPAAm87-co-PAA13; temperatures and values pH for: (B) PNIPAAm87-co-PAA13; and (C) and (C) PNIPAAm80-co-PAA20 copolymers. Corresponding LCSTsofof aqueous PNIPAAm80-co-PAA20 randomrandom copolymers. (D) (D) Corresponding LCSTs aqueous PNIPAAm-co-PAA solutions at various values of pH. PNIPAAm-co-PAA solutions at various values of pH.

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30

35

40

(C)

30

(B)

pH valu 1.96 3.0 3.5 4.0 4.5

45 pH valu 1.70 3.0 3.5 4.0 4.5

35

40

45

Temperature (oC)

50

50

30

35

40

(D)

45 50

PNIPAm97-co-PAA3 PNIPAm87-co-PAA13 PNIPAm80-co-PAA20

45

55

pH valu 1.94 3.0 3.5 4.0 4.5

45

40

40

35

35

30

3.0

3.5

4.0

4.5

LCST (oC)

Heat Flow (Endo, Down)

(A)

30

pH value

Figure 7. 7. (A–C) aqueous solutions at at various values of pH for: for: (A) Figure (A–C)DSC DSCthermograms thermogramsof of aqueous solutions various values of pH PNIPAAm97-co-PAA3; (B) PNIPAAm87-co-PAA13; and (C) PNIPAAm80-co-PAA20 random (A) PNIPAAm97-co-PAA3; (B) PNIPAAm87-co-PAA13; and (C) random copolymers. (D) values of copolymers. (D) Corresponding CorrespondingLCSTs LCSTsof ofaqueous aqueousPNIPAAm-co-PAA PNIPAAm-co-PAAsolutions solutionsatatvarious various values pH. of pH.

Onthe the basis basis of of the the UV–Vis UV–Visspectroscopic spectroscopicand andDSC DSCanalyses, analyses,we we found found that that the the LCSTs LCSTsincreased increased On uponincreasing increasingthethe forPNIPAAm-co-PAA all PNIPAAm-co-PAA copolymers. This behavior can be upon pHpH for all randomrandom copolymers. This behavior can be attributed attributed to the enhanced solubility in aqueous solution arising from dissociation of the COOH to the enhanced solubility in aqueous solution arising from dissociation of the COOH groups of the groups of the PAA3).segments (Scheme 3). At (i.e., lower PAA compositions the (i.e.,LCST for PAA segments (Scheme At lower PAA compositions for PNIPAAm97-co-PAA3), PNIPAAm97-co-PAA3), the slightly LCST offrom the copolymer at pHon 3.0UV–Vis to 32.8 of the copolymer increased 30.8 ◦ C at increased pH 3.0 toslightly 32.8 ◦ Cfrom at pH30.8 4.5,°Cbased °C at pH 4.5, based on UV–Vis spectroscopic analysis, close to that of the PNIPAAm homopolymer. spectroscopic analysis, close to that of the PNIPAAm homopolymer. A further increase in the PAA A further increase in PNIPAAm80-co-PAA20 the PAA composition caused to that the of PNIPAAm80-co-PAA20 caused the26.6 LCST to ◦ C at composition to that of LCST to change significantly, from change significantly, from 26.6 °C at pH 3.0 to 43.5 °C at pH 4.5, implying that the PAA content in the ◦ pH 3.0 to 43.5 C at pH 4.5, implying that the PAA content in the random copolymer did affect the random did affecton the in aLCSTs manner dependent on the pH. Thedecreasing LCSTs decreased LCST in acopolymer manner dependent theLCST pH. The decreased significantly upon the pH significantly upon decreasing the pH for high-PAA-content copolymers, as revealed in Figures 6D for high-PAA-content copolymers, as revealed in Figures 6D and 7D. We attribute this behavior to the and 7D. We attribute this behavior to the higher fraction of intermolecular hydrogen bonds between higher fraction of intermolecular hydrogen bonds between the PNIPAAm and PAA segments in the the PNIPAAm and having PAA segments in the random copolymers having the higher PAA contentsbetween (Figure random copolymers higher PAA contents (Figure 4C), hindering inter-association 4C),amide hindering theof inter-association amide groups of PNIPAAm and the H2O molecules. the groups PNIPAAm andbetween the H2 Othe molecules. As a result, the random copolymer would As a result, the random copolymer would become more hydrophobic in the aqueous solution, become more hydrophobic in the aqueous solution, inducing a lower LCST, as had been proposed inducing a[35]. lower LCST, asthe hadhigher been LCSTs proposed [35]. In the higher LCSTs were previously In contrast, werepreviously found at higher pHcontrast, for random copolymers having found PAA at higher pH(Figures for random copolymers having higher contents (Figures and 7D). As higher contents 6D and 7D). As mentioned above,PAA the COOH groups of the6D PAA segments − units at higher pH, mentioned above, the COOH groups of the PAA segments dissociated into COO − dissociated into COO units at higher pH, thereby increasing the solubility (i.e., the hydrophilicity) in thereby increasing thea solubility theexpect hydrophilicity) in aqueous solutions; as a result, we higher would aqueous solutions; as result, we(i.e., would higher LCSTs for random copolymers having expect higher LCSTs for random copolymers having higher PAA contents. Figure 8 summarizes the PAA contents. Figure 8 summarizes the visual phase changes of 5 wt % of PNIPAAm80-co-PAA20 visual phase changes of 5 wt % of PNIPAAm80-co-PAA20 aqueous solutions at different aqueous solutions at different temperatures and values of pH. The possible chain conformations temperatures and values of phase pH. The possible chain conformations visual corresponding to the visual changes in Figure 8 are presentedcorresponding schematically to in the Scheme 4.phase changes in Figure 8 are presented schematically in Scheme 4.

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Scheme 3. Possible PAA chain PNIPAAm-co-PAA copolymers at values ofof: pH Scheme 3. Possible PAA chainbehavior behavior in copolymers at values of pH (a)of: 3.0;(a) 3.0; in PNIPAAm-co-PAA (b) 3.5–4.0; (b) 3.5–4.0; andand (c) (c) 4.5.4.5.

Figure 8. Photographs PNIPAAm80-co-PAA20 in aqueous solutions at Figure 8. Photographsofofthe therandom random copolymer copolymer PNIPAAm80-co-PAA20 in aqueous solutions at various temperatures and valuesofofpH. pH. various temperatures and values

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Scheme behavior of of PNIPAAm-co-PAA PNIPAAm-co-PAA copolymers Scheme 4. 4. Possible Possible chain chain behavior copolymers in in aqueous aqueous solutions solutions at at various various temperatures and values of pH (red color is PNIPAAm segment and blue color is PAA segment). temperatures and values of pH (red color is PNIPAAm segment and blue color is PAA segment).

3.4. LCSTs LCSTs and and Values Values of of TTgg of PNIPAAm-co-PAA PNIPAAm-co-PAAunder underCO CO22and andafter afterscCO scCO22 Treatment Treatment Because CO CO22can caninteract interact with groups of PNIPAAm andthrough PAA through weak with the the C=OC=O groups of PNIPAAm and PAA weak dipole– dipole–dipole interactions, we used the first heating scans of HP-DSC analyses to investigate dipole interactions, we used the first heating scans of HP-DSC analyses to investigate the effectsthe of effects the CO2 pressure on the of LCSTs of aqueous PNIPAAm-co-PAA solutions (Figure 9A–D). the COof 2 pressure on the LCSTs aqueous PNIPAAm-co-PAA solutions (Figure 9A–D). The The endothermic peaks corresponding to LCSTs the LCSTs shifted to lower temperatures upon increasing endothermic peaks corresponding to the shifted to lower temperatures upon increasing the the CO pressure for all three PNIPAAm-co-PAA copolymers, consistent with CO molecules CO2 pressure for all three PNIPAAm-co-PAA copolymers, consistent with CO2 molecules 2 2 displacing displacing H2 O molecules around theCONH polar CONH and COOH groups aqueoussolutions solutions of H2O molecules around the polar and COOH groups in in thetheaqueous 2323. This displacement of H O would enhance the intramolecular hydrogen bonding PNIPAAm-co-PAA PNIPAAm-co-PAA . This displacement of2 H2O would enhance the intramolecular hydrogen of the COOH and CONHand groups (Scheme 2a,b) and the2a,b) intermolecular hydrogen bonding of thebonding CONH bonding of the COOH CONH groups (Scheme and the intermolecular hydrogen and COOH groups 2c) in the PNIPAAm-co-PAA random copolymers, therebycopolymers, enhancing of the CONH and (Scheme COOH groups (Scheme 2c) in the PNIPAAm-co-PAA random the hydrophobicity aqueous solutions and decreasing in the and LCSTs. In addition, the LCSTs. dissolved thereby enhancing in thethe hydrophobicity in the aqueous solutions decreasing in the In CO form carbonic to decrease the pHacid of the decreasing the addition, the dissolved COacid 2 might form carbonic to aqueous decrease solutions, the pH of thereby the aqueous solutions, 2 might LCSTs, discussed above. Schemeas5 summarizes the possible intermolecular interactions of the therebyasdecreasing the LCSTs, discussed above. Scheme 5 summarizes the possible PNIPAAm-co-PAA random copolymers in aqueous solutions under a CO2 atmosphere. 9D intermolecular interactions of the PNIPAAm-co-PAA random copolymers in aqueousFigure solutions summarizes LCSTs of Figure the PNIPAAm80-co-PAA20 random copolymer in aqueous solutions of under a CO2 the atmosphere. 9D summarizes the LCSTs of the PNIPAAm80-co-PAA20 random various values of pH, measured under different CO pressures. In all cases, the LCSTs of the copolymer copolymer in aqueous solutions of various values 2of pH, measured under different CO2 pressures. In in aqueous solutions increasing the CO2 pressures decreasing thethe pH,CO as2 all the cases, the LCSTs of thedecreased copolymerupon in the aqueous solutions decreasedor upon increasing ◦ C, discussed Figurethe 10 summarizes the DSCpreviously. thermograms, recorded in the range 60–180 pressures previously. or decreasing pH, as discussed Figure 10 summarizes the DSC of thin films cast from PNIPAAm-co-PAA solutions at from various values of pH. All of the thermograms, recorded in the range 60–180aqueous °C, of thin films cast PNIPAAm-co-PAA aqueous films derived from aqueous at values of pH greaterPNIPAAm-co-PAA than 3 gave lower solutions at various valuesPNIPAAm-co-PAA of pH. All of thesolutions films derived from aqueous values of Tg. For allofofpH thegreater random copolymers, thevalues value of Tg. Tg increased upon increasing the pH solutions at values than 3 gave lower For all of the random copolymers, of aqueous solution above pH 3. As mentioned in aqueous Scheme 3a, a lower pH pH would the thethe value of Tg increased upon increasing the pH of the solution above 3. Asenhance mentioned intermolecular bonding PNIPAAm and PAA segments, thereby increasing in Scheme 3a, hydrogen a lower pH wouldbetween enhancethe the intermolecular hydrogen bonding between the the value of Tg. As the pH increased, the COOH groups of the PAA segments partially dissociated PNIPAAm and PAA segments, thereby increasing the value of Tg. As the pH increased, the COOH into COO units (Scheme 3b),partially thereby decreasing of intermolecular bonding. groups of–the PAA segments dissociatedthe intostrength COO– units (Scheme 3b), hydrogen thereby decreasing – The negative of charge of the COOhydrogen units would induce repulsive forces andofincrease the– free the strength intermolecular bonding. The negative charge the COO unitsvolume, would leading to a decrease in the value of Tg. Further increasing the pH would cause the COOH groups induce repulsive forces and increase the free volume, leading to a decrease in the value of Tg. of the PAA segments fully dissociate intoCOOH COO– groups units (Scheme 3c). The much to greater repulsive Further increasing thetopH would cause the of the PAA segments fully dissociate – units would result in a rigid structure – units forces of the greater number of negatively charged COOforces into COO (Scheme 3c). The much greater repulsive of the greater number of negatively – units would for the PAA therefore, of Tg would increase. Figure 11 therefore, summarizes DSC charged COOsegments; result the in avalue rigid structure for the PAA segments; the the value of ◦C thermograms of those films11 insummarizes Figure 10 after treatmentofinthose scCOfilms psi and Tg would increase. Figure theadditional DSC thermograms in Figure 1032after 2 at 2000 for 1 h, with a depressurization 1 h. The of the samples prepared at each value additional treatment in scCO2 attime 2000of psi and 32 values °C for 1ofh,Tg with a depressurization time of 1 h. The of pH increased aftersamples the scCO suggesting promotedafter intermolecular hydrogen values of Tg of the prepared at each value ofthat pHitincreased the scCO2 treatment, 2 treatment, suggesting that it promoted intermolecular hydrogen bonding [36]. Kramer et al. reported that the

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bonding [36]. Kramermay et al.increase reportedthe that the hydrostatic pressure may increase the of value of Tg by hydrostatic pressure value of Tg by decreasing the free volume a polymeric decreasing the free volume of a polymeric material when the pressure environment is soluble the material when the pressure environment is soluble in the polymer matrix. In contrast, itinmay polymer Inof contrast, it may value of Tg, a plasticizer effect, when the decrease matrix. the value Tg, through a decrease plasticizerthe effect, when thethrough pressure environment is not soluble pressure environment is not soluble in the polymer matrix [36]. Because CO can interact with the C=O 2 in the polymer matrix [36]. Because CO2 can interact with the C=O groups of the PNIPAAm and groups of the PNIPAAm PAA segments through weak dipole–dipole interactions, and also act as2 PAA segments through and weak dipole–dipole interactions, and also act as a Lewis acid, the scCO – – units atreatment Lewis acid, the scCO would promote thegroups formation of COO COOH groups fromPAA COOsegments, units in 2 treatment would promote the formation of COOH from in the the PAA segments, thereby facilitating intermolecular hydrogen bonding between the PNIPAAm and thereby facilitating intermolecular hydrogen bonding between the PNIPAAm and PAA segments, as PAA segments, as displayed in Scheme 3a, leading to an in Tg,inasFigure summarized Figure 12. displayed in Scheme 3a, leading to an increase in Tg, asincrease summarized 12. Thisinincrease in This increase value of Tg after scCOwas was dependent on content. the PAA For content. For example, 2 treatment the value of in Tgthe after scCO 2 treatment dependent on the PAA example, for the ◦ for the random copolymer PNIPAAm97-co-PAA3, the value of Tg increased whereas for random copolymer PNIPAAm97-co-PAA3, the value of Tg increased byby4 4°C,C, whereas for ◦ PNIPAAm80-co-PAA20 C, after PNIPAAm80-co-PAA20ititincreased increasedby by10 10 °C, afterscCO scCO22 treatment treatmentat atpH pH3. 3.

16

(B)

0 psi 200 psi 400 psi 600 psi

20

24

28

32

(C)

0 psi 200 psi 400 psi 600 psi

32

20

0 psi 200 psi 400 psi 600 psi

24

28

(D)

32 pH value 2.5 3.0 3.5 4.0

28

32

28

oo

LCST ( C)

Heat Flow (Endo, Down)

(A)

24

16

20

24

28

32 o

Temperature ( oC)

20

24

0

200

400

600

20

CO2 Pressure (psi)

Figure 9.9.(A–C) (A–C) HP-DSC first-heating curves (2 °C/min) of solutions 5 wt %of:solutions of: (A) Figure HP-DSC first-heating curves (2 ◦ C/min) of 5 wt % (A) PNIPAAm97PNIPAAm97-co-PAA3; (B) PNIPAAm87-co-PAA13; and (C) PNIPAAm80-co-PAA20 co-PAA3; (B) PNIPAAm87-co-PAA13; and (C) PNIPAAm80-co-PAA20 random copolymers at random various 2 pressures. (D) Corresponding LCSTs of the PNIPAAm80-co-PAA20 copolymers at various CO CO pressures. (D) Corresponding LCSTs of the PNIPAAm80-co-PAA20 aqueous solutions at various 2 aqueous at various values of pH. values of solutions pH.

Scheme 5. Possible interactions in PNIPAAm-co-PAA random copolymers under a CO2 atmosphere. Scheme 5. Possible interactions in PNIPAAm-co-PAA random copolymers under a CO2 atmosphere.

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(B)

(C)

(B) 1.94

(C) 1.70

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pH value 1.96(A) 1.96

1.94

3.5

1.70

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3.0

Heat Flow (Endo, Down)

Heat Flow (Endo, Down)

pH value

3.0

3.0

3.0

3.0

3.5 3.5

3.5

3.5

4.0

3.5

4.0

4.0

4.0 4.0

4.5

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4.5

4.5

4.5

4.5

60 6090 90120120150 150 180 180 60 60

90 90

120 150 180 180 6060 90 90120120 120 150 150 150 180 180 o

o Temperature C) Temperature ((C)

Figure 10. thermograms of:of:of: (A)(A) PNIPAAm97-co-PAA3; PNIPAAm87-co-PAA13; and Figure 10 thermograms (A) PNIPAAm97-co-PAA3;(B)(B) Figure 10 DSC DSCDSC thermograms PNIPAAm97-co-PAA3; (B)PNIPAAm87-co-PAA13; PNIPAAm87-co-PAA13; (C) PNIPAAm80-co-PAA20 films cast from aqueous solutions at various values of pH. and (C) PNIPAAm80-co-PAA20 films cast from aqueous solutions at various values of pH. and (C) PNIPAAm80-co-PAA20 films cast from aqueous solutions at various values of pH.

(A) (A)

(C)

(B) (B) 1.94

pH value pH value 1.96

(C)

1.94

1.70

3.0

3.0

1.70

Heat Flow (Endo, Down)

Heat Flow (Endo, Down)

1.96

3.0

3.0

3.0

3.0

3.5 3.5

3.5

3.5

4.0

4.0

3.5 4.0

3.5

4.0

4.0

4.0

4.5 4.5

4.5

4.5

4.5 4.5

60

90

120 150 180

60

90

120 150 180

60

90

120 150 180

o

Temperature ( C) 60

90

120 150 180

60

90

120 150 180

60

90

120 150 180

Figure 11. DSC thermograms of: of: (A) (A) PNIPAAm97-co-PAA3; (B)(B) PNIPAAm87-co-PAA13; Figure 11. DSC thermograms PNIPAAm97-co-PAA3; PNIPAAm87-co-PAA13; and o Temperature ( C) atat various (C) PNIPAAm80-co-PAA20 films cast aqueous solutions valuesof of and (C) PNIPAAm80-co-PAA20 films from cast from aqueous solutions various values pHpH andand afterafter ◦C treatment in scCO 22000 at 2000 psi and 32 °C for 1 h. treatment in scCO at psi and 32 for 1 h. 2 Figure 11. DSC thermograms of: (A) PNIPAAm97-co-PAA3; (B) PNIPAAm87-co-PAA13; and (C) PNIPAAm80-co-PAA20 films cast from aqueous solutions at various values of pH and after treatment in scCO2 at 2000 psi and 32 °C for 1 h.

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14 of 16 14 of 16

PNIPAm97-co-PAA3 Without CO2

170

With CO2 PNIPAm87-co-PAA13 Without CO2 With CO2 PNIPAm80-co-PAA20 Without CO2

160

Tg (oC)

With CO2

10 oC

150

6 oC

140 4 oC

130 1.5

2.0

2.5

3.0

3.5

4.0

4.5

pH value of TTgg of PNIPAAm-co-PAA films cast from aqueous aqueous solutions at various values of Figure 12. Values of pH, with or without scCO22 treatment.

4. Conclusions 4. Conclusions We have have synthesized synthesized PNIAAm-co-PAA PNIAAm-co-PAA random random copolymers copolymers of of different different compositions compositions through through We free radical radical copolymerizations. copolymerizations. Significant increases in in T Tgg values free Significant increases values occurred occurred after after incorporation incorporation of of AA AA units into PNIPAAm, due to the formation of strong intermolecular hydrogen bonding between the units into PNIPAAm, due to the formation of strong intermolecular hydrogen bonding between the 1H NMR and FTIR 1 amide groups of PNIPAAm and the carboxyl groups of PAA based on amide groups of PNIPAAm and the carboxyl groups of PAA based on H NMR and FTIR spectroscopic spectroscopic analyses. The LCSTs of PNIPAAm-co-PAA random were increasedthe upon analyses. The LCSTs of PNIPAAm-co-PAA random copolymers werecopolymers increased upon increasing pH − − increasing the pH value because the COOH groups of the PAA segments dissociated into COO value because the COOH groups of the PAA segments dissociated into COO units, thereby enhancing units, thereby enhancing the solubilities of thesolutions. copolymers in aqueous solutions. HP-DSC revealed the solubilities of the copolymers in aqueous HP-DSC revealed that the LCST values of thatcopolymers the LCST values of the copolymers afterimplying applyingthat CO2the pressure, implyingdisplaces that the the decreased after applying decreased CO2 pressure, CO2 molecules CO2 molecules some of thethe H2O molecules the polar CONH andPNIPAAm-co-PAA COOH groups in some of the H2 displaces O molecules around polar CONHaround and COOH groups in the the PNIPAAm-co-PAA copolymers, thereby enhancing hydrophobicity of the copolymers in the copolymers, thereby enhancing the hydrophobicity of thethe copolymers in the aqueous solutions. The Tg aqueous solutions. The T g values of the copolymers increased after treatment with scCO 2 , implying values of the copolymers increased after treatment with scCO2 , implying that intermolecular hydrogen that intermolecular hydrogen bonding had been enhanced. bonding had been enhanced. Acknowledgments: This study was supported financially by the Ministry of Science and Technology, Taiwan, Acknowledgments: This study was supported financially by the Ministry of Science and Technology, Taiwan, under contracts MOST MOST 103-2221-E-110-079-MY3 103-2221-E-110-079-MY3and andMOST MOST104-2221-E-390-024, 104-2221-E-390-024,and andbyby Cooperation fund under contracts thethe Cooperation fund of of the two universities (NSYSU and NUK). the two universities (NSYSU and NUK). Author Contributions: Contributions: Pei-Yi PNIPAm-co-PAA copolymers; copolymers; and Pei-Yi Lin Lin contributed contributed to to the the synthesis synthesis of PNIPAm-co-PAA Yeong-Tarng Shieh, Tao Chen, and Shiao-Wei Kuo coordinated the study, interpreted the results, wrote Yeong-Tarng Shiao-Wei Kuo coordinated the study, interpreted the results, andand wrote the the paper. paper. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest.

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