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Aqueous Solutions of poly(ethylene oxide)- poly(N-isopropylacrylamide): Thermo-sensitive Behavior and Distinct Multiple Assembly Processes Qiuwen Wang, Hui Tang, and Peiyi Wu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00878 • Publication Date (Web): 26 May 2015 Downloaded from http://pubs.acs.org on June 3, 2015

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Aqueous

Solutions

of

poly(ethylene

poly(N-isopropylacrylamide):

oxide)-

Thermo-sensitive

Behavior and Distinct Multiple Assembly Processes Qiuwen Wang, Hui Tang,* Peiyi Wu*

State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Polymers and

Polymer Composite Materials, Department of Macromolecular Science and Laboratory for Advanced Materials,

Fudan University, Shanghai 200433, China

ABSTRACT: Detailed phase transition and conformational changes taking place as the function of temperature in poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) semi-diluted aqueous solutions are elucidated in the present study. By the use of elaborate vibrational

spectroscopy techniques

in

combination

with

two-dimensional correlation

spectroscopy (2Dcos), three transition regions including respective rich domains (37oC) are depicted. Specially, subtle variations of hydrogen bonds are detected even under the lower critical solution temperature (LCST) and the respective rich domain regime is marked with strong participation from hydrogen bonding at different concentrations and compositions. Both the formation of intermolecular hydrogen bonds and less hydration degrees of PNIPAM segments compared with PNIPAM homopolymer at elevated temperatures can verified the evolution of PNIPAM from their own domains to loose aggregations with PEO shells. Dense micelles are formed beyond the LCST of PNIPAM while the outmost PEO play as buffer layers and postpone the shrinkage of PNIPAM chains. Due to the existence buffer layer, higher phase transition temperatures compared with

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PNIPAM homopolymer are observed.

INTRODUCTION Polymers which are sensitive to environmental variations have attracted a great deal of attention owning to their unique characters and potential applications.1-4 As one of the most extensively investigated thermo-sensitive polymers, poly(N-isopropylacrylamide) (PNIPAM) is water-soluble below its LCST and undergoes reversible coil-to-globule transition near the physiological temperature.5,6 The thermal transition behavior may be attributed to the hydration interactions between hydrophobic groups and water together with variations of hydrogen bonds among hydrophilic groups.7,8,9 Meanwhile, by copolymerizing hydrophobic/hydrophilic comonomers, the LCST can be tuned and the self-assembly behavior of block copolymers have been investigated for the fabrication of stimuli-responsive nanoparticles.10-13 Recently, there has been increasing interest in double-hydrophilic block copolymers (DHBCs) which comprise two water-soluble blocks of different chemical nature.14,15,16 Molecularly dissolved block copolymers self-assemble in situ when one block becomes hydrophobic due to the external stimulus such as pH17 or temperature variations18,19 and then micro-phase separation is conducted.20 Among all these DHBCs, poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) has attracted much attention.21-27 Their phase transition behavior had been investigated in terms of copolymer compositions, concentrations, architecture, heating rate, and additives.24,28-33 In addition, the association-dissociation processes of PEO-b-PNIPAM involved intrachain transition and interchain aggregation had also been discussed.24,25,26 Five regions in the phase diagram were distinguished by Annaka et al. with dynamic light scattering, small-angle neutron scattering and fluorescence spectra measurement. Spectroscopic investigation indicated 2


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that the hydrophobicity started to increase and the contraction of the PNIPAM block was observed under LCST.25 Alternately, three distinct stages “association-loose aggregation-micelles” with two critical temperature points in dilute aqueous solution were illustrated by Liang et al. via laser light scattering measurement. The abnormal aggregates below LCST were attributed to the incompatibility between PNIPAM and PEO based on the analysis of Rg/Rh ratio. Polymer chains associated together to form individual rich domains which built fast equilibrium with single chains under the LCST and then loose aggregations together with micelles were observed successively.26 The most important factors influencing the morphologies of the aggregates are copolymer compositions, concentrations, the nature of solvents, etc.34 Shi et al. revealed that the critical aggregation temperature of PEO110-b-PNIPAM44 was increased with decreasing the concentration from 2.0 to 0.2 mg/mL. The higher copolymer concentration preferred to form narrowly distributed, small and dense micelles (2 mg/mL), while larger and loose micelles were formed at lower concentration (0.2 mg/mL).35 Tenhu et al. reported that at low solution concentration or PNIPAM/PEO molar fraction, the aggregations were spherical. Upon dilution or decreasing PNIPAM/PEO, the phase separation between the PNIPAM and PEO blocks was enhanced.24 When molar fraction of PNIPAM/PEO was changed from 2 to 10, the large distribution below LCST could

still

be

presented

at

the

concentration

from

1

to

0.2

mg/mL.26,36,37

PNIPAMx-PEO20-PPO70-PEO20-PNIPAMx pentablock behavior with various PNIPAM lengths in diluted and concentrated solutions was reported by Du et al.. They found the microstructures in concentrated solutions like 40 wt % (at least 110 mg/mL for PNIPAM segment) were strongly dependent on the environmental temperatures and relative lengths of each block.38 Zhang et al. studied the concentration effect on association and dissolution of liner PNIPAM chains in dilute 3


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and semi-dilute aqueous solutions. They demonstrated that the solution with a concentration higher than 10.0 mg/mL could be considered as semi-dilute, and both the phase transition temperature and enthalpy decreased when the solution became more dilute while adverse change happened in the semi-dilute regime.39 Based on above studies, not only the morphologies and sizes of aggregations, but also the self-assembly behavior and mechanism of copolymers may be concentration dependence.40 However, as far as we are concerned, investigation of the aggregation behavior of PEO-b-PNIPAM in higher concentrations, as generally performed in PNIPAM homopolymer, has seldom been reported. Herein, a series of narrowly distributed PEO-b-PNIPAM block copolymers especially with relative higher molar fraction of hydrophilic PEO blocks is synthesized via reversible addition-fragmentation chain transfer polymerization (RAFT). Different from previous researches focused on static and dynamic laser light scattering, and fluorescence measurements which required highly diluted solutions, block copolymer solutions in higher concentration are applied here to analyze the microdynamics phase separation mechanism. Temperature-resolved FTIR in combination with 2Dcos and perturbation correlation moving window (PCMW) technique which had been performed on the phase transition process of PNIPAM and its complex systems is employed here to elucidate the dynamic mechanism of copolymers in aqueous solutions.41-47 More detailed information about the molecular motion process including subtle changes of hydrogen bonds and sequential order of different chemical groups was explained. EXPERIMENTAL SECTION Synthesis. Synthesis process, materials and instrument measurements are provided in the supporting information. According to the integration of 3.64 ppm (CH2 protons in PEO main 4


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chains) and 1.09 ppm (CH3 protons in PNIPAM side chains) in 1H NMR spectra, the molar ratio of PEO and PNIPAM repeating units in the blocks can be calculated, as shown in Figure S1 and Table 1. Investigation Methods. A Nicolet Nexus 6700 spectrometer equipped with DTGS detector by signal-averaging 32 scans at a resolution of 2 cm-1 was used to measure the time-resolved FTIR spectra at different temperatures. The solutions in D2O with diverse concentrations were prepared by sealing them between two ZnS tablets. Temperatures were controlled with electronic cell holder at a rate of 0.3 oC/min with an increment of 1 oC. FTIR spectra collected during the heating (24-52 oC) and cooling (52-25 oC) process were used to PCMW analysis and 2D correlation analysis. The introduction of 2D correlation analysis and PCMW is presented in supporting information. RESULTS AND DISCUSSION Dynamic light scattering measurements. Self-assembly behaviors of the block copolymers together with information about the hydrated sizes are investigated by DLS, as shown in Figure S2. At lower temperatures, the vast majority of copolymers tend to exist as single chains in aqueous solutions while a fraction of abnormal larger aggregations is observed (the distribution diagrams of PEO-b-PNIPAM19 are shown in Figure S3). The hydrated radiuses increase sharply and the unimers disappear gradually with temperature increment, indicating that the volume phase transitions occur. Further dehydration of PNIPAM has be concluded from the abrupt drop of slope since the hydrophobic segments compact inner parts with increment of intermolecular interactions and decrement of water molecules inside. Micelles with stable dimensions are formed above 52 oC. Kjøniksen et al. reported that the cloud points were shifted toward higher temperatures with 5


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increasing length of the hydrophilic chains of PEOn-b-PNIPAM71 copolymer from n = 0 to 114.48 In the present DLS study, the transition temperature increased from 32 to 35 oC with shortening the repeating units of PNIPAM from 207 to 19. It was concluded that in the diluted solution range, the longer hydrophobic PNIPAM segments, the lower transition temperature and larger assembled structures were obtained above LCST. Although the association and aggregation mechanism of PEO-b-PNIPAM under aqueous solution had been proposed by several groups, investigation of the aggregation behavior of copolymer in higher concentration solutions has seldom been reported. Considering that the transition temperature or transition process might be dependent on concentration, semi-dilute solutions (3~20 wt % copolymer concentrations) were utilized here and the self-assembly structures together with volume phase transition processes of PNIPAM segments during the heating and successive cooling process were discussed. Table 1. Characterization results, phase transition temperature (Tp), hydrodynamic radius (Rh) and concentration information of the copolymers copolymer

Mna

Mw/Mnb

NNIPAM/PEOc

(g/mol)

a

Tp, d

Rhe

copolymer

PNIPAM

(oC)

(nm)

concentration f

concentration g

PEO-b-PNIPAM207

28000

1.42

1.83

31.4

225

3.6 wt%

8.2 wt %

PEO-b-PNIPAM58

11600

1.26

0.51

32.6

91

5.3 wt %

5.7 wt %

PEO-b-PNIPAM19

7150

1.21

0.17

36.3

64

10.0 wt %

3.0 wt %

Calculated by 1H NMR data; b Measured by GPC in DMF (PMMA as calibration); c Ratio of repeating units;

d

Determined by DSC under 10 wt % copolymer concentrations; e Measured by DLS under 0.05 wt % copolymer concentrations; f Copolymer overall concentration in FTIR analysis when PNIPAM segment concentration is 3.0 wt %; g PNIPAM segment concentration in FTIR analysis when overall copolymer concentration is 10.0 wt %.

Calorimetric measurements. The transition behaviors of block copolymers are elucidated with calorimetric measurements, as shown in Figure S4. Hysteresis between the heating and successive 6


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cooling process is observed, and it can be explained by the formation of intermolecular hydrogen bonds which act as the cross-linking structures and make the chain aggregate like a gel.9,49 Incorporation of hydrophilic PEO segments increases the LCST, which is owning to the stabilization effect of PEO around PNIPAM as corona.24,28,30 In Figure S4a, the transition points of copolymers with different compositions under 10 wt % solutions shown in Figure S4a shift to higher temperatures with decreasing the NIPAM/PEO ratios. Meanwhile, the curve shifts from 32 to 40 oC when the concentration of PEO-b-PNIPAM19 decreases from 20 to 5 wt %, as shown in Figure S4b.

Figure 1. Temperature-dependent FTIR spectra of PEO-b-PNIPAM in D2O (3 wt % PNIPAM concentration) during heating between 24 and 52 oC with an interval of 1 oC in the regions v(CH) (3020-2840 cm-1), v(C=O) (1680-1580 cm-1) and v(C-O-C) (1127-1050 cm-1). Conventional FTIR Analysis. The transition behaviors of the copolymers are studied by temperature-resolved FTIR spectra with 2Dcos, which have been widely used to analyze spectra variations under various external perturbations.41,50 D2O (deuterium oxide) is utilized as the solvent instead of H2O to eliminate the overlap of δ(O-H) band with v(C=O) as well as the overlap 7


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of v(O-H) band with v(C-H) band.51 Three copolymer aqueous solutions under the same 3 wt % PNIPAM segment concentration are presented in Figure 1, while the temperature-dependent FTIR spectra and frequency shifts of vas(CH2) and v(CH) with rising temperature under the same overall copolymer concentration (10 wt %) are presented in Figure S5 and S6, respectively. Spectra of C-H (3020-2840 cm-1), C=O (1680-1580 cm-1) and C-O-C (1127-1050 cm-1) are applied as the most representative wavenumber regions for detailed analysis of the characteristic structures. The band of v(C-H) of PEO-b-PNIPAM at 3020-2840 cm-1 shifts to lower frequency during the heating process, as shown in Figure 1. According to the previous studies, the red shift of C-H stretching band was due to the decrement of interaction between water and hydrophobic moieties of the polymers since water-soluble polymers with plenty of water around them possessed higher vibration frequency in the C-H stretching band.52 Therefore, dehydration process of CH groups in copolymers can be concluded. However, with increment of PNIPAM/PEO ratio, the absorbance ratio of vas(CH3) (2972 cm-1) and vas(CH2) (2938 cm-1) increase. The longer PNIPAM segment length, the more obvious CH3 wavenumber shift is observed. As for CH2 groups, their frequency shifts are less distinct than CH3 which manifest that the peak shifts are only caused by the change of species of the hydrated bonds.41 Similar to PNIPAM in D2O, 41,46 bidirectional spectral intensity changes of C=O bands are observed in the present study. It is clear that the intensity of 1625 cm-1 band v(C=O--D2O) becomes weaker while a new band at 1645 cm-1 v(C=O--DN) appears with the increment of temperature. (DN represents for deuterated amino groups in PNIPAM segments) Hence the binary change of amide I is interpreted as the transformation of hydrogen bonds.45,53 However, the phase transition of PNIPAM homopolymer is much sharper than that of copolymers, which can be explained by the introduction of transitional hydrophilic PEO layers. Diversely, less 8


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distinct shifts are detected in C-O-C region compared with that in C-H and C=O regions. It is roughly considered that the C-O-C region has less responsiveness to temperature. Overall, the shifts of PEO-b-PNIPAM19 in one-dimensional spectra are slight compared with distinct shifts of PEO-b-PNIPAM207 and PEO-b-PNIPAM58. But it should be noted that the copolymer with shortest PNIPAM length can occur temperature transition with increment of temperature. Feijen et al. reported

the

similar

aggregation

phenomenon

and

micelle-forming

behavior

of

PEO113-b-PNIPAM6 and PEO113-b-PNIPAM21 copolymers and revealed that the block copolymers could show thermo-sensitive properties when temperature was above the LCST.54

Figure 2. Integral area of PEO-b-PNIPAM (3 wt % PNIPAM segment concentration) in the regions of (a) hydrated C=O (1625-1580 cm-1) and (b) C=O--DN (1645-1680 cm-1) vs temperature during heating process. Integral area of PEO-b-PNIPAM19 at different copolymer concentrations in the regions (c) hydrated C=O and (d) C=O--DN vs temperature during heating process. Integral areas of v(C=O) in different composition copolymers in the regions of hydrated C=O and C=O--DN vs temperature during heating process are shown in Figure 2a-b. Based on the integral area of carbonyl groups, the transition temperatures increase with decreasing the PNIPAM repeating units from 207 to 19. Specially, the integral area of hydrated C=O in PEO-b-PNIPAM19 copolymers decrease slowly, arrive a short platform with a stable value at about 32-35 oC 9


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temperature range, and then further decrease above 36 oC. For simplicity, two pink dotted lines in Figure 2a mark the platform range. This variation is different from S-shaped change in PNIPAM aqueous solution.41,46 As for the C=O--DN region, drastic increment is detected from 36 to 41 oC. The anti-S-shaped change of C=O--DN region indicates the occurrence of gradual dehydration in the whole temperature process and the existence of distribution gradient of water caused by the outside hydrophilic PEO chains. However, subtle differences between PEO-b-PNIPAM207 and PEO-b-PNIPAM58 are noticed, and gradual decrements of hydrated C=O without platforms are observed.

Figure 3. (a) Relative areas of C=O--DN hydrogen bonding (1645 cm-1) vs C=O--D2O (1625 cm-1) in PEO-b-PNIPAM19 (3 wt % PNIPAM segment concentration) at different temperatures (from 24 to 52 oC). Temperature dependence of the molar fraction of f(C=O--DN) of the PNIPAM segments in the (b) heating-cooling cycle of PEO-b-PNIPAM19 (3 wt % PNIPAM concentration) and (c) PEO-b-PNIPAM207 and PEO-b-PNIPAM58 (3 wt % PNIPAM concentration). For in-depth analysis of concentration effects in semi-diluted solution range, integral area of v(C=O) in the regions of hydrated C=O and C=O--DN vs temperature at different concentrations in PEO-b-PNIPAM19 copolymer during heating process is shown in Figure 2c-d. With increasing the copolymer concentration from 5 to 20 wt %, the transition points shift from 40 to 32 oC which indicate that the transition temperatures decrease with the increment of concentration. Besides, anti-S-shaped spectra area changes are observed in the wavenumber region of hydrated C=O 10


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during heating as shown in Figure 2c. The integral areas decrease slowly, arrive short platforms at about 32-35 oC, and then decrease continuously in both 5 and 10 wt % solutions, whereas a gradual decrement without a platform is observed in 20 wt % solution. In contrast, in Figure S7c, gradual decrements of PEO-b-PNIPAM207 (and PEO-b-PNIPAM207) without platforms are observed both in 10 and 3.6 wt % (5.3 wt %) without significant differences. It confirms unique and special assembly process of PEO-b-PNIPAM19. Supposed that a 1:1 conversion of the carbonyls,55 relative areas (S) of 1645 cm-1 of the C=O--DN hydrogen bonding in PEO-b-PNIPAM19 (3 wt % PNIPAM segment concentration) at different temperature are plotted against the areas of 1625 cm-1 component (C=O--D2O) as shown in Figure 3a. The slope of the fitted line yields the ratio of the molar absorption coefficient (ɛ1645/ɛ1625) as 0.98. Temperature dependence of the molar fraction of C=O--DN (f(C=O--DN)) defines as S1645/(S1645+S1625*(ɛ1645/ɛ1625)) in the heating-cooling cycle is shown in Figure 3b. By calculating the temperature dependence of the molar fraction of C=O--DN, a discontinuity increment beginning at 36 oC is observed. Notably, comparing with PNIPAM (5220 g/mol) aqueous solution (20 wt %) in previous study which possessed no intermolecular hydrogen bonds below the LCST,41 the f(C=O--DN) of 38% in PEO-b-PNIPAM19 at 25 oC confirms association structures at low temperature. With temperature increment, PNIPAM chains compact and the f(C=O--DN) increases further. Additionally, no apparent hysteresis is noticed in the heating-cooling cycle and better resilience compared with PNIPAM aqueous solution is obtained. For comparison, the temperature dependence of f(C=O--DN) of PEO-b-PNIPAM207 and PEO-b-PNIPAM58 under the same PNIPAM concentration (3 wt %) are provided in Figure 3c. Shortest PNIPAM segment has biggest f(C=O--DN) about 38 % while there are no significant 11


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differences between PEO-b-PNIPAM207 and PEO-b-PNIPAM58 which initial values are about 14~16 %. Diverse f(C=O--DN) indicates that with decrement of PNIPAM/PEO ratio, mutual interference between PEO and PNIPAM becomes weaken and lead to the formation of respective rich domain. Similarly, Tenhu et al. had reported that upon dilution or decreasing PNIPAM/PEO, the phase separation between the PNIPAM and PEO blocks was enhanced due to the competition or interference between PEO and PNIPAM segments.24 In short, intermolecular hydrogen bonds play a critical role in unique multiple-step assembly process. Based on the conventional IR analysis, the whole heating process of PEO-b-PNIPAM19 is divided into two regions as 24-32 oC and 32-52 oC and their assembly mechanism is analyzed in detail. Specially, in order to obtain more detailed spectral variations and inherent nature of volume phase transition temperature, 2Dcos together with PCMW technique which has been widely used to monitor complicated spectral variations along the perturbation direction and discern microscopic variations of complex interactions is performed in the present system.45,56 Subtle bands and their corresponding assignments are presented in Table S1.

Figure 4. PCMW synchronous spectra of 10 wt % PEO-b-PNIPAM19 copolymer in D2O between 24 and 32 oC during heating. Warm colors (like red and yellow) are defined as positive intensities and cool colors (like blue) are defined as negative intensities. Analysis of 24-32 oC Perturbation correlation moving window (PCMW). In PCMW analysis, spectral intensity 12


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increment is defined as positive synchronous correlation, whereas negative correlation means the spectral intensities decrement.57 PCMW synchronous spectra of the copolymer during heating from 24 to 32 oC are presented in Figure 4. Typically, in the heating process, the temperature transition point of bands at 2972, 2966, 2946, 2921, 2872, 1102, 1093 and 1085 cm-1 is ca. 27 °C. For 1109 and 1078 cm-1 bands, the transition point is ca. 28 °C, whereas the drastic changing temperature point appears at ca. 29 °C for 2901, 1644, 1616 and 1063 cm-1 bands. It indicates that the hydrophobic CH groups respond earlier than C=O, and the transition in lower temperature region is attributed to the hydrophobic interaction. Additionally, group motional coordination is possessed since all the temperature points are within 3 oC.

Figure 5. 2D synchronous (left) and asynchronous (right) spectra of PEO-b-PNIPAM19 copolymer in D2O (10 wt %) during heating between 24 and 32 oC. Warm colors (red, yellow) are defined as positive intensities, and cool colors (blue) are defined as negative intensities. Two-dimensional correlation spectroscopy (2Dcos). 2Dcos analysis of the block copolymer during heating from 24 to 32 oC is employed to elucidate the contraction of PNIPAM block and weak micro-phase separation in the lower temperature region, as presented in Figure 5. By using the Noda’s judging rule,57 the specific order can be obtained in supporting information. Neglecting the differences in the stretching modes of the chemical groups, the special order can be expressed 13


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as follows: C-O-C (PEO) → -CH2- (PEO) → -CH- (PNIPAM) → -CH2- (PNIPAM)→ -CH3 (PNIPAM) → C=O (PNIPAM). Feijen et al. investigated the micelle-forming behavior with MPEO/MPNIPAM exceeding 3 and revealed aggregation existence below the LCST.54 Based on DLS together with fluorescence measurements, Annaka et al. also depicted the contraction of PNIPAM block and micro-phase separation occurred around 18 °C in water.25 According to the specific order, more hydrophilic C-O-C groups respond earlier than other groups with temperature increment, which is consistent with PCMW analysis. However, the evolution of PNIPAM from their own domains to loose aggregation with PEO shells has seldom been verified according to the previous paper. Comparing with PNIPAM aqueous solution (20 wt %), the pendent group CH3 in homopolymers responded earlier than the main chains while carbonyl groups released hydrogen bonds with water before the formation of intermolecular hydrogen bonds.41 Based on the 2Dcos combined with PCMW analysis, the main chains of PNIPAM respond earlier, indicating that the assembly structure transition occurs with elevated temperatures. Long PEO segments play as continuous phase which are scattered around PNIPAM domains while the movements of pendant groups are constricted by lower temperature assembly structures. PNIPAM segments dehydrate gradually and form loose cores. Besides, the formation of intermolecular hydrogen bonds (C=O--DN) also evidences the transition from respective rich domains to the loose aggregations. PNIPAM aqueous solution (20 wt %) did not have any intermolecular hydrogen bonds, 41 while copolymer presents about 38% hydrogen bonds below 32oC, indicating less hydration degree of PNIPAM segments due to the existence of respective rich domains. The variation of band at 1645 cm-1 which assigns to the hydrogen bonds of C=O--DN declares the transformation from respective rich domains to loose 14


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aggregations. Analysis of 32-52 oC Perturbation correlation moving window (PCMW). PCMW synchronous spectra of the copolymer in D2O during heating from 32 to 52 oC are presented in Figure 6. For CH and C=O groups, the phase transition temperature is determined to be 40 oC, which is higher than that of C-O-C groups (37 oC), implying that the C-O-C groups respond with water at lower temperature compared with CH and C=O groups.

Figure 6. PCMW synchronous spectra of PEO-b-PNIPAM19 between 32 and 52 oC. Warm colors (red, yellow) are defined as positive intensities, and cool colors (blue) are defined as negative intensities. Two-dimensional correlation spectroscopy (2Dcos). The volume phase transition and the self-assembly process at 32-52 oC region are investigated via 2Dcos analysis as shown in Figure 7. The final sequence order is described in supporting information. Without considering the differences in stretching modes of the chemical groups, the specific order can be summarized as follows: C-O-C (PEO) →-CH2- (PEO) → -CH2- (PNIPAM) → -CH3 (PNIPAM)→ C=O (PNIPAM). According to the sequence discussed above, the dehydration of hydrophilic ether bands takes place before main chains in the temperature region of 32-52 oC, which is consistent with the PCMW analysis. Considering much longer PEO113 compared with PNIPAM19, PEO shells gather 15


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in the surrounding of PNIPAM to achieve their stabilization effect. Owning to the existence of loose associations, water prefers to get closer to more hydrophilic and longer PEO segments rather than PNIPAM. Thus surrounding PEO chains possess more water and play as buffer layers around cores. Once temperature rises, these buffer layers respond firstly and postpone shrinkage of PNIPAM chains, corresponding with increment of the transition temperature. Then PNIPAM becomes hydrophobic and the volume phase transition occurs. Different from previous study on PNIPAM aqueous solution in which CH3 on the side chains was more sensitive and flexible than the main chains,41 in present study, compaction between PNIPAM intermolecular main chains respond at lower temperature than pendent chains CH3. The dehydration process of main chains instead of variation of hydrogen bonds becomes the driving force of the volume phase transition due to the former assembly process below 32 oC. The pendent chains have been restricted by loose aggregations and their activities are weakened. Particularly, it is found that the formation of C=O--DN occurs earlier than the breakage of C=O--D2O, which is also different from traditional PNIPAM systems.41,45 It indicates that the intermolecular hydrogen bonds have formed and a part of hydrated C=O has released their hydrogen bonds with water far below the LCST. Since the contraction of PNIPAM blocks and micro-phase separation have occurred below the LCST, PNIPAM in loose aggregations may be slightly packed with less hydration. When the copolymers solutions are heated near the phase transition temperature, the PNIPAM in the contraction state responds with higher inter-chain cooperation and a sharp increment of v(C=O--DN) occurs. With further increment of temperature, more releasing of C=O--D2O is observed and main chains collapse with formation of dense micelles. Notably, due to the nature of hydrophilic PEO segments which may hinder the transport of water molecules from cores and play as buffer layers, 16


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Langmuir

higher phase transition temperature is observed.

Figure 7. 2D synchronous (left) and asynchronous (right) spectra of PEO-b-PNIPAM19 in D2O (10 wt %) during heating between 32 and 52 oC. Warm colors (red, yellow) are defined as positive intensities, and cool colors (blue) are defined as negative intensities. For comparison, temperature-dependent FTIR spectra of PEO/PNIPAM blend are shown in Figure S8 and its temperature dependence of the molar fraction of f(C=O--DN) is shown in Figure S9a. The f(C=O--DN) of blend at lower temperature is far less than that in copolymers even with the existence of PEO. Besides, the changes of integral areas of C=O, as shown in Figure S9b-9c, are sharper than PEO-b-PNIPAM19 with no short platform in hydrated C=O region. 2Dcos of the blend is presented in Figure S10 and the sequence order is presented in supporting information. For PNIPAM chains, more flexible pendant chains have earlier response than main chains while the variation of hydrogen bonds become the driving force of the volume phase transition instead of the dehydration process of main chains, corresponding with PNIPAM homopolymer.41 For PEO chains, the earliest response of 1103 cm-1 and the latest movement of 1074 cm-1 demonstrate that changes of ether groups occur in the whole heating process and PEO chains do not have distinct shifts during the heating process. It means that PEO and PNIPAM have no detectable mutual influences in the blend system. In contrast, 1104 and 1078 cm-1 bands respond earlier than all 17


Langmuir

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other bands in PEO-b-PNIPAM19, certifying that that PEO buffer layers have direct relationship with PNIPAM and protect inner cores through releasing their own hydrogen bonds with water firstly. Scheme 1. Schematic illustration of the dynamic mechanism of the phase transition of PEO-b-PNIPAM19 copolymer (10 wt %) during the heating process.

Overall, a three-step self-assembly association behavior of thermo-sensitive PEO-b-PNIPAM19 block copolymer in semi-diluted aqueous solution is elucidated with the assistant of 2Dcos and PCMW analyses. A multiple-step consecutive assembly mechanism depicted as “respective rich domains-loose aggregations-dense micelles” is proposed and the changes of the hydration states of PEO-b-PNIPAM19 block copolymers in semi-diluted solution (10 wt %) are illustrated as shown in Scheme 1. When the temperature is under 29

o

C, respective rich domains exist since

intermolecular hydrogen bonds are already presented. The evolution of PNIPAM from their own domains to loose aggregations with PEO shells is verified at elevated temperatures. Long PEO segments play as continuous phase which are scattered around PNIPAM domains while PNIPAM segments dehydrate gradually to form loose cores. With the increment of temperature, PNIPAM becomes hydrophobic and the volume phase transition occurs. Due to the restriction effect from association structures, PNIPAM in the contraction state responds earlier with higher inter-chain cooperation. Notably, higher phase transition temperature compared with the homopolymer is 18


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Langmuir

observed considering the hydrophilic nature of PEO segments which may hinder the transport of water molecules from cores and play as buffer layers as the first responsive groups in 2Dcos. The analysis of cooling process is presented in the supporting information. Generally, the block copolymer has good resilience in temperature cycles and no apparent hysteresis is noticed. Since the hysteresis is usually ascribed to the formation of the intermolecular interaction between chains in the collapsed states, less interchain association with longer hydrophilic PEO block is beneficial for the copolymer to reverse to its original state. CONCLUSIONS PEO-b-PNIPAM block copolymers are synthesized via RAFT method. The effects of concentrations and compositions on the phase transition behavior in semi-diluted solutions are discussed. Higher transition temperature together with smaller structures assembled above LCST is obtained with shorter PNIPAM segments. Meanwhile, with increasing the concentration from 5 to 20 wt %, the transition points of PEO-b-PNIPAM19 shift from 40 to 32 oC which indicate that the transition temperature decreases with the concentration increment. Notably, a distinct and unique multiple-step transition process with short stable platform in hydrated C=O integral area changes distinguish PEO-b-PNIPAM19 from PEO-b-PNIPAM207 and PEO-b-PNIPAM58. 38% f(C=O--DN) of PEO-b-PNIPAM19 confirms association structures at low temperature. With temperature increment, PNIPAM chains compacts and the f(C=O--DN) increases further. To figure out the multiple assembly processes in semi-diluted solutions, vibrational spectroscopy techniques with 2Dcos and PCMW analysis are further applied. Three transition regions of the whole self-assembly process including respective rich domains (37 oC) are depicted. Respective rich domains 19


Langmuir

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are preferred at low temperature (

Aqueous Solutions of Poly(ethylene oxide)-Poly(N ... - ACS Publications

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