Volume Phase Transition Mechanism of Poly[di(ethylene glycol)ethyl

Subscriber access provided by Service des bibliothèques | Université de Sherbrooke

Article

Volume phase transition mechanism of poly[di(ethylene glycol)ethyl ether acrylate] based microgels involving thermosensitive poly(ionic liquid) Lan Ma, Hui Tang, and Peiyi Wu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02884 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

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

Langmuir

Volume phase transition mechanism of poly[di(ethylene glycol)ethyl ether acrylate] based microgels involving thermosensitive poly(ionic liquid)

Lan Ma, Hui Tang and Peiyi Wu* State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science and Laboratory for Advanced Materials, Fudan University, Shanghai 200433, China.

1

ACS Paragon Plus Environment

Langmuir

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

Page 2 of 32

ABSTRACT

Microdynamic volume phase transition mechanism of poly[di(ethylene glycol)ethyl ether acrylate] (PDEGA) based microgels with newly developed thermo-responsive poly

(ionic

liquid)

(PIL)

(poly(tetrabutylphosphonium

styrenesulfonate),

P[P4,4,4,4][SS]) moieties was studied by applying temperature-variable Fourier transform infrared (FTIR) spectroscopy in combination with two-dimensional correlation spectroscopy (2Dcos) and perturbation correlation moving window (PCMW) technique. It can be found that the content of hydrophilic PIL moieties plays a significant role in the thermally induced phase transition behavior of microgel system, namely, the microgels containing less PIL moieties present a sharp transition behavior and a gel-like state (10%, w/v) in water while the microgels with more PIL moieties exhibit a slightly broad phase transition process and a flowable solution state. Herein, the C=O···D2O-PIL hydrogen bonds as the interaction between PDEGA and P[P4,4,4,4][SS] moieties result in a complete dehydration process for the microgels with less PIL moieties and the dehydrated behavior of SO3- groups acts as the driving force during the phase transition. As for the microgels with more PIL moieties, the whole transition process is dominated by the hydrophobic interaction of C-H groups. Even though the intermolecular hydrogen bonds (C=O···D2O-PIL) appear as well, the more remarkable effect of coulombic repulsive force of PIL restrain the water molecules from breaking away, thus causes a gradual and incomplete dehydration process during heating.

2


Page 3 of 32

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

Langmuir

1.

INTRODUCTION

As a congener of oligo(ethylene glycol) methacrylate (OEGMA), poly[di(ethylene glycol)ethyl ether acrylate] (PDEGA), first reported by Lutz et al, can be demonstrated to have good biocompatibility,1-3 outstanding antifouling properties,4-6 and controlled stimuli-responsive properties including tunable lower critical solution temperature (LCST) and reversible phase transition.7,8 Specially, copolymers with accurately controlled phase transition temperature approaching to physiological temperature region were achieved by introducing more hydrophilic comonomers to the PDEGA system considering that the LCST of PDEGA was much lower than that of poly(N-isopropylacrylamide) (PNIPAM).9 For instance, the cloud points of random copolymers were tuned in the full thermal range of water by altering the content of hydrophilic poly[oligo(ethylene glycol) acrylate] moieties of PDEGA-included copolymerization system via atom transfer radical polymerization method.10 Based on reversible addition-fragmentation chain transfer (RAFT) polymerization process, Sumerlin et al. obtained copolymers comprising DEGA and N,N-dimethylacrylamide (DMA) with various feed ratios.11 The resulting cloud points varied linearly with DMA mole fraction over 10-90 oC range. Besides thermo-sensitive linear copolymer systems, the study associating with microgels (nanogels) comprising PDEGA analogue polymers has attracted a great deal of attention with regard to the widespread application areas including drug delivery,12-14 nanoreactor,15 and molecule carrier.16,17 By using free radical 3


Langmuir

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

Page 4 of 32

polymerization, Hu et al. prepared poly(ethylene glycol) (PEG) analogue-based microgels with narrow size distribution.18 An et al. reported the synthesis of thermo-sensitive core-shell nanogels containing linear PEG or nonlinear polymers with oligo(ethylene glycol) side chains as the shell and the copolymers of POEGMA and poly[di(ethylene glycol) methyl ether methacrylate] (PMEO2MA) as the core via RAFT method.19 The nanogels could be well-dispersed in the biologically relevant solutions such as bovine serum albumin and fetal bovine serum solutions, and possessed good monodispersities as well as tunable thermo-sensitivities and sizes. Moreover, the CTAs with reinforced chemical stability rendered the good biocompatibility to the nanogels. In addition, Du et al. synthesized a kind of novel thermosensitive

microgels,

poly[di(ethylene

glycol)

methyl

ether

methacrylate-co-2-methoxyethyl acrylate] (poly(DEGMMA-co-MEA)) microgels, which exhibited interfacial interaction with several kinds of charged proteins by electrostatic interaction.20 The PDEGA-analogue moiety of poly(DEGMMA-co-MEA) microgels, namely PMEA, had a capability of impairing the interaction between microgels and proteins. The unique biocompatibility, thermo-responsive property together with monodisperse nature endowed oligo(ethylene glycol) acrylate-based microgels to be utilized in various scientific application fields.2 Among various studies of thermo-responsive microgels which have been focused on qualified materials, microgels involving ionic liquids (ILs) moieties had been discussed due to the inherent features of ILs including thermal and chemical stability21,22,

excellent

electrical

conductivity23-25,

good

4


solubility26-28

and

Page 5 of 32

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

Langmuir

inappreciable vapor pressure

24,29, 30

, etc. By means of concentrated emulsion

polymerization of 1-vinyl-3-ethylimidazolium bromide, PIL microgels were prepared in the presence of N,N-dimethylenebisacrylamide (BIS) crosslinker by Mecerreyes et al.31 The resultant microgels could be swollen in broad range of solvents after modification with different anions and possessed good thermal stability which could be valuable for the research work on the biocatalysis and biosensors. Moreover, ionic microgels synthesized by surfactant-free emulsion polymerization (SFEP) of NIPAM and 1-vinylimidazole (VIM) with dibromide as quaternized cross-linker were reported by Du et al..32 The thermo-sensitive microgels with hydrophilic PIL cross-linking moieties manifested higher transition point and pH tunable degradation ability. Recently,

Wu

et

al.

reported

the

synthesis

of

thermo-responsive

poly(tetrabutylphosphonium styrenesulfonate) (P[P4,4,4,4][SS]) microgels possessing highly efficient catalytic activity.33 Several research works had been focused on the certain effect of ILs features on the volume phase transition behavior of linear polymers or microgels and proved that the addition of IL may influence the microdynamic phase transition mechanism by regulating the interaction between polymers (or ILs) and water molecules.34,35 Li et al. studied the influence of various ILs on the phase transition behavior of poly(N-vinylcaprolactam) (PVCL) and revealed the hydrogen bonds between unsaturated

C-H

of

hydrophobic

1-ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl) imide ([EMIM][NTf2]) and amide C=O of PVCL despite of the lower concentration of [EMIM][NTf2].35 While the interactions between 5


Langmuir

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

Page 6 of 32

hydrophilic 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and water molecules had indirect effect on the phase behavior of PVCL. Zhou et al. prepared POEGMA-based ionic microgels and discovered that less PIL cross-linking moieties led to sharp phase transition behavior and the hydrogen bond between ester C=O group and PIL-D2O association were developed while broad transition process was presented in highly cross-linked microgels.36 Thus, great significance is exerted on deepening the studies relating to the influence of IL on the transition mechanism of thermosensitive microgels. Herein, thermo-responsive PDEGA-based microgels copolymerizing with IL monomer

(tetrabutylphosphonium

styrenesulfonate,

[P4,4,4,4][SS])

bearing

polymerizable vinyl groups were synthesized via free radical copolymerization. Different from the previous in situ quaternization and the formation of PIL cross linker in microgels, thermo-responsive P[P4,4,4,4][SS] act as comonomer in this system and the volume phase transition mechanism of microgels with different monomer ratios together with the interactions among PDEGA, P[P4,4,4,4][SS] and water molecules were clarified. It is expected that the thermally induced conformational changes of different chemical groups in microgel systems and the variation of hydrated states upon heating can be illuminated by temperature-dependent FTIR spectroscopy together with perturbation correlation moving window (PCMW) technology and two dimensional correlation techniques(2Dcos).

2. EXPERIMENTAL SECTION

6


Page 7 of 32

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

Langmuir

2.1 Materials. Di(ethylene glycol)ethyl ether acrylate (DEGA) were purchased from J&K Scientific and passed through Al2O3 column before used. Potassium peroxodisulfate (KPS) were purchased from Aladdin and recrystallized from deionized water for purification.

Poly(ethylene glycol) diacrylate (PEGDA, Mn =

258 g/mol) were purchased from Sigma-Aldrich. Tetrabutylphosphonium bromide ([P4,4,4,4]Br), sodium-4-vinylbenzenesulfonate (Na[SS]) and other chemicals were used as received.

2.2 Synthesis of PDEGA-co-P[P4,4,4,4][SS] microgels. [P4,4,4,4][SS] monomers were prepared by ionic exchange reaction of [P4,4,4,4]Br and Na[SS] as described in the previous literature.37 Emulsifier free radical precipitation polymerization was applied here to synthesize PDEGA-co-P[P4,4,4,4][SS] microgels with different ratios.38 Specifically, for sample M-1, DEGA (574 mg, 3.05 mmol), [P4,4,4,4][SS] (150 mg, 0.339 mmol), cross linker PEGDA (17.4 mg, 0.067 mmol) and stabilizer SDS (19.5 mg, 0.068 mmol) were dissolved in 15 ml deionized water. The whole solution was degassed with nitrogen for 30 min. Then, 9.3 mg (0.034 mmol) initiator KPS dissolved in 1 ml deionized water was injected into the reaction system at 70 oC and the reaction was carried out under vigorous stirring and continuous nitrogen purging for 4 h. After the process of cooling to room temperature naturally, the target microgel solution was dialyzed in deionized water for a week (cellulose dialysis tube, MWCO 14000). The polymerization results of microgels with different ratios are shown in Table 1. Copolymer ratios of samples were measured by 1H NMR approach. Corresponding 1H NMR spectra can be found in the Supporting Information. 7


Langmuir

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

Table 1 Polymerization results of microgels with different ratios.

a

Measured at 20 oC by DLS measurement.

Scheme 1 Synthesis route of PDEGA-co-P[P4,4,4,4][SS] microgels via free radical polymerization in the presence of KPS as initiator, PEGDA as cross-linker and SDS as stabilizer.

2.3 Instruments and Measurements. Malvern Zetasizer Nano ZS instrument with detection angle of 90o was utilized to conduct dynamic light scattering (DLS) measurements of PDEGA-co-P[P4,4,4,4][SS] microgels in H2O (0.1%, w/v) during heating. Bruker AV (500 MHz) spectrometer with 4 scans and a relaxation delay of 6s was applied to record the 1H NMR spectra of microgel samples.

Nicolet 6700 spectrometer attached by DTGS detector was used to measure the temperature-dependent FTIR spectra of microgels at a resolution of 4 cm-1 and 32 scans. Microgels in D2O (10%, w/v) were arranged at 4 oC for a week to make sure the microgels swell sufficiently. The heating rate was controlled at 0.3 oC min-1 with an increment of 1 oC by electronic cell holder. After baseline correction using

8


Page 8 of 32

Page 9 of 32

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

Langmuir

OMNIC ver. 8.0 software, PCMW and 2Dcos analysis were applied using 2D Shige ver. 1.3 (© Shigeaki Morita, Kwansei Gakuin University, Japan, 2004–2005) software with proper window size (2n + 1 = 11). The contour maps were plotted by Origin ver. 8.0 software with warm colors (red) indicating positive intensities and cool colors (blue) indicating negative ones.

3. RESULTS AND DISCUSSION

3.1 DLS measurements

Figure. 1 DLS measurements of PDEGA-co-P[P4,4,4,4][SS] microgels with different ratios in aqueous solution (0.1%, w/v) (left). The images of (a) M-1 and (b) M-3 solution at room temperature (10%, w/v)) (right).

DLS measurements were performed for the microgels with different ratios in aqueous solution to monitor the changes of hydrodynamic diameters with raising temperature. Detailed copolymer ratios and resultant hydrodynamic sizes of microgels are listed in Table. 1. Specifically, mutual coulomb repulsive force of charged groups inside the microgel expands the volume of microgels and the hydrophilic P[P4,4,4,4][SS] moieties attract more water molecules and induce higher swelling ratio. Along with the 9


Langmuir

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

increasing amount of P[P4,4,4,4][SS] moieties, the microgel size becomes larger as well as the polydispersity index (PDI).39,40 It can be surmised that both the steric hindrance effect from bulky cations and coulomb repulsive force of higher content of P[P4,4,4,4][SS] count against the synthesis of microgels41, resulting in the nonuniformity of microgel with higher proportion of P[P4,4,4,4][SS] moieties. Particularly, when the feed ratio of DEGA and [P4,4,4,4][SS] monomers reaches 6:4, the hydrodynamic diameter of microgel upon heating was too large and inhomogeneous to be captured.

Intuitively, the volume phase transition temperature of PDEGA-co-P[P4,4,4,4][SS] microgels increases with the increasing amount of P[P4,4,4,4][SS] moieties. This can be readily comprehensible since increasing hydrophilic component in LCST-type system tends to strengthen the original interactions between polymer chains and water molecules.38,42 As well, the content of P[P4,4,4,4][SS] moieties may affect the dehydrated efficiency of complex microgels to some extent since M-1 experiences sharp phase transition with relatively narrow transition temperature region while the volume phase transition of M-3 appears slightly gradual.

Different physical states of the microgel aqueous solution at room temperature are identified as shown in Figure. 1. Light blue gel-like state is shown for M-1 sample with relatively high concentration in D2O (10%, w/v). Li et al. reported the thermally induced gelation of doubly thermos-responsive PDEGA-PDMA-P(NIPAM-co-BIS) nanogels. Due to the lower transition temperature of PDEGA, gelation of nanogels

10


Page 10 of 32

Page 11 of 32

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

Langmuir

occurred at temperatures higher than LCST of PDEGA caused by hydrophobic association of PDEGA segments.43 Similar phenomenon was displayed in PEO-PPO-PEO triblock copolymer aqueous solution at high concentrations due to the arrangement of micelles, which is usually called the gelation.44,45 Due to the hydrophobic association of PDEGA moieties, M-1 microgel particles stack tightly and present the gel-like state macroscopically. As for M-3 with relatively high concentration in D2O, visually transparent solution is discerned, which can be illustrated by the surfactant binding of P[P4,4,4,4][SS] onto microgel surface and stabilization effect from electrostatic repulsion among particles46.

3.2 Conventional FT-IR Analysis

Temperature-dependent FT-IR spectroscopy was utilized here to explore the conformational variations of different chemical groups during phase transitions. Based

on

this

method,

detailed

information

of

dehydrated

process

in

thermos-responsive homopolymer47 together with copolymer bearing unique topological structure (linear copolymers48, microgels49, hydrogels50, etc) had been acquired. In this study, D2O

was chosen as the solvent instead of H2O to eliminate

the overlap of δ(O-H) peak with the υ(C=O) at about 1640 cm-1, besides the broad δ(O-H) peak with υ(C-H) around 3300 cm-1. Temperature-dependent FTIR spectra of PDEGA-co-P[P4,4,4,4][SS] microgels with different ratios in D2O (10%, w/v) during heating are shown in Figure. 2. Three spectra regions including C-H stretching region (3010-2840 cm-1), C=O stretching region (1760-1680 cm-1), and O=S=O stretching

11


Langmuir

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

region (1140-1115 cm-1) are discussed in details.

Figure. 2 Temperature-variable FT-IR spectra of (a) M-1 (10-30 oC) and (b) M-3 (30-50 oC) in D2O.

Figure. 3 Temperature-dependent frequency shifts of (a) υas(CH3), (b) υas(CH2), (c) υs(CH2), (d) υ(C=O) and (e) υ(S=O) of M-1 and M-3 in D2O during heating process.

In terms of C-H stretching region, three discernible bands corresponding to asymmetric stretching band of CH3 (υas(CH3)), asymmetric stretching band of CH2 (υas(CH2)) and symmetric stretching band of CH2 (υs(CH2)) are identified. It can be observed that all the C-H groups shift to lower wavenumbers during heating, indicating the variations of interaction between hydrophobic C-H groups and water molecules. Given the fact that more water molecules surrounding the C-H groups of soluble polymer will lead to higher vibrational frequency, the red shift of C-H groups 12


Page 12 of 32

Page 13 of 32

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

Langmuir

indicates the occurrence of dehydration process with temperature rising. As for the spectral changes of ester C=O groups which tend to form hydrogen bonds with water molecules in aqueous solution, blue shift which points out the fracture of hydrogen bonds between oxygen atoms of PDEGA and water is exhibited. Owing to the hydrophilic nature, SO3- groups are also prone to develop hydrogen bonds with surrounding water.41 The red shift of SO3- groups demonstrates that they undergo the changes of hydrated state as well.

To obtain detailed information of hydrated states variation of chemical groups in microgels during the volume phase transition process, quantitative analysis of IR spectra is presented according to the temperature-dependent frequency shifts of υ(C-H), υ(C=O) and υ(S=O) as shown in Figure. 3. For M-1, both C-H and SO3absorptions shift to lower wavenumbers and the variation tendencies exhibit an anti-sigmoid shape. Opposite variation is observed in the ester C=O absorptions. It can be explained that the hydrated groups experience the breakage of hydrated interactions and hydrogen bonds upon heating.47,48 As far as M-3 is concerned, the wavenumbers of characteristic absorptions are linearly dependent on temperature, indicating a continuous and mild dehydration process in the microgel. Similar phenomenon has been reported in PNIPAM-co-P[P4,4,4,4][SS] copolymer.51 The strong hydration nature of P[P4,4,4,4][SS] restrains the sharp phase transition process and lead to the linear and gradual variations of PDEGA during dehydration.

Moreover, further observation of initial frequencies of mentioned-above groups

13


Langmuir

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

displays that the initial wavenumbers of C-H, C=O and SO3- bands in M-1 are higher than that in M-3, except υas(CH2) band which may be affected much by the methylene on the cation of P[P4,4,4,4][SS] in M-3. It proves that the hydrated interactions of C-H and anionic moieties in M-1 are more prominent than that in M-3. Due to the higher content of P[P4,4,4,4][SS] moieties with stronger hydrophilic nature in M-3, the ester C=O of PDEGA in M-3 are surrounded with more water molecules before phase transition compared with M-1.

Variations in the hydrated states of different groups during phase transition were carried out quantitatively by calculating the specific frequency shift values. Obviously, the shift values of υas(CH3), υas(CH2), υs(CH2), υ(C=O) and υ(S=O) for M-1 are much higher than those for M-3, indicating a more drastic dehydrated process of M-1. Due to the more remarkable electrostatic repulsion and hydrophilicity nature of P[P4,4,4,4][SS],

the dehydration behavior of M-3 cross-linking network is weakened

with increasing the amount of P[P4,4,4,4][SS]. Meanwhile, the frequency shifts of υ(S=O) in two microgels are smaller than that of pure P[P4,4,4,4][SS], which may be attributed to the influence of hydrophilic property of PDEGA on water environment of P[P4,4,4,4][SS] anions. For the P[P4,4,4,4][SS] moieties of two microgels, the wavenumber of SO3- groups in M-1 is slightly smaller than that in M-3 after the phase transition, manifesting that there are less water molecules surrounding the PIL moieties in M-1 and the anionic moieties of P[P4,4,4,4][SS] are wrapped inside the aggregates after stripping water molecules out of the microgels.

14


Page 14 of 32

Page 15 of 32

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

Langmuir

3.3 Perturbation Correlation Moving Window (PCMW)

In this, perturbation correlation moving window (PCMW) method developed by Morita et al. was applied to monitor the sophisticated information of accurate transition temperature and transition temperature range of characteristic groups in FTIR spectra responding to temperature stimuli.52,53 By means of PCMW, the conventional FTIR spectra can offer a pair of synchronous and asynchronous correlations between the spectral variable (e.g., wavenumber) axis and the perturbation variable (e.g., temperature) axis. The positive synchronous correlation represents an increase in the spectral intensity, whereas the negative one indicates a decrease. With respect to the asynchronous correlation, the positive one denotes a convex spectral intensity variation, while the negative one means a concave intensity variation. The temperature, which the strongest spectral intensity at the specific wavenumber in the synchronous and asynchronous spectra corresponds to, can be utilized to determine the transition temperature and transition temperature region of the certain chemical group.

15


Langmuir

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

Figure. 4 PCMW synchronous and asynchronous spectra of M-1 and M-3 microgels in D2O during heating.

Figure. 5 Phase transition temperature and temperature range of (a) M-1 and (b) M-3 in D2O from PCMW spectra.

PCMW synchronous and asynchronous spectra of M-1 and M-3 aqueous solution during the phase transition process are presented in Figure. 4 and the results are collected in Figure. 5 for legible observation. For M-1, -SO3- groups respond to temperature at 16 oC, followed by C-H and C=O groups at 17 oC, which implies that PIL moieties are likely to be covered by the subsequently dehydrated PDEGA moieties due to the outer conformational collapse. As for M-3, all groups experience a synchronous phase transition (around 38 oC) and a broader temperature range (7 oC), suggesting cooperative response behavior of various chemical groups to temperature.

16


Page 16 of 32

Page 17 of 32

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

Langmuir

Zhou et al. examined the volume phase transition behavior of poly[oligo(ethylene glycol)methacrylate] microgels with PIL cross-linkers and revealed that the content of PIL moieties may impact the sensitivity of chemical groups to temperature and higher PIL content endowed the microgel with broader transition region.36 Similar phenomenon is also observed in this system. Additionally, two bands located at 1726 and 1701 cm-1 in the ester C=O stretching mode are identified in M-1. As for M-3, single band at 1720 cm-1 is found. It is inferred that the introduction of hydrophilic PIL moieties may influence the interactions between cross-linked network and water molecules due to the unique charged and hydrophilic feature of P[P4,4,4,4][SS].

3.4 Two-dimensional correlation spectroscopy (2Dcos) Analysis

To explore the spectral fluctuations under external temperature perturbation, 2Dcos analysis was applied in the present study.54,55 By expanding the original spectra along the planar direction, complex information hidden behind the conventional 1D spectra can be revealed. Besides, the detailed sequence order of chemical groups under certain external perturbation can be deduced as well. Several kinds of thermo-responsive systems with three-dimensional network structure including POEGA-based and PNIPAM-based nanogels49, poly(2-isopropyl-2-oxazoline)-based (PIPOZ) hydrogel50 and etc. had been investigated with above-mentioned method, which deepened the theoretical understanding of thermo-sensitive polymer network systems.

The procreant synchronous and asynchronous spectra of two microgel systems are 17


Langmuir

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

presented in Figure. 6. The synchronous spectra reveal the simultaneous variations between two peaks while the asynchronous spectra are used to improve the resolution of original FTIR spectral data. The tentative assignments of characteristic peaks of M-1 and M-3 systems read from asynchronous spectra are listed in Table. 2.

According to Noda’s rule, peak υ1 responds earlier than peak υ2 when the cross-peaks (Φ(υ1, υ2), assuming υ1 > υ2) reflect the same signs in both synchronous and asynchronous spectra, and vice versa. Based on the 2Dcos spectra, the sequence order of chemical groups in M-1 during heating process are listed as follows (“>’’means prior to or earlier than): 1130 > 2884 > 2872 > 1714 > 1701 > 1123 > 2974 > 1733(1743) > 2958 > 2987 > 2932 cm-1 or vs(SO3-···mD2O in anion) > vs(hydrated CH2) > vs(dehydrated CH2) > v(hydrated C=O) > v(C=O···D2O-PIL) > vs(SO3-···(m-n)D2O in anion) > vas(dehydrated CH3) > v(dehydrated C=O) > vas(hydrated CH2) > vas(hydrated CH3) > vas(dehydrated CH2).

18


Page 18 of 32

Page 19 of 32

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

Langmuir

Figure. 6 2D synchronous and asynchronous spectra of M-1 and M-3 microgels in D2O during heating.

Table 2 Tentative assignments of M-1 and M-3 microgels obtained from 2D synchronous and asynchronous spectra during heating.

a

The wavenumbers corresponding to the chemical groups of M-3 microgels.

Without regard to the differences in stretching modes of chemical groups, the sequence order can be simplified as SO3- > C-H > C=O. It presents that SO3- groups of P[P4,4,4,4][SS] respond initially to the temperature perturbation, followed by the dehydration of C-H and C=O groups in PDEGA moieties. It indicates that the driving force of thermally induced phase separation of M-1 is the dehydration of SO3- groups

19


Langmuir

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

Page 20 of 32

of P[P4,4,4,4][SS] moieties which may be wrapped inside the aggregates by subsequently responsive PDEGA moieties during the phase transition process.

It should be pointed out that the C-H groups involved in the microgels mainly belong to the PDEGA moieties instead of PIL moieties. According to previous literatures,36,56 the direction of symmetric stretching vibration is perpendicular to the polymer chain axis while that of asymmetric stretching vibration is parallel to the polymer chain. Since the symmetric stretching vibration of C-H groups responds prior to the asymmetric stretching vibration regardless of alkyl type (CH2 or CH3), it is concluded that the side chains of PDEGA moieties carry out the conformational collapse primarily during the dehydration of PDEGA chain and the hydrophilic groups transfer to the external water-rich surroundings while hydrophobic alkyl groups moved along the polymer chain axis gradually. Particularly, three kinds of C=O groups located at 1733, 1714 and 1701 cm-1 are observed in the 2D asynchronous spectra which can’t be identified by conventional IR spectra and PCMW method. Generally, the characteristic bands at 1733 and 1714 cm-1 are assigned to dehydrated C=O and hydrated C=O, respectively.57 It indicates the breakage of hydrogen bonds between C=O and water molecules together with the transformation of C=O groups from water-rich state to water-poor state. Herein, the existence of band at 1701 cm-1 is attributed to unique hydrogen bond with PIL-D2O clathrate

(C=O···D2O-PIL).

1-butyl-3-methylimidazolium

Wang

et

al.

tetrafluoroborate

studied

the

influence

([Bmim][BF4]),

20


on

the

of

IL, phase

Page 21 of 32

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

Langmuir

transition behavior of PNIPAM aqueous solution and proposed C=O···D2O-IL hydrogen bond by which stable interaction network were constructed during the volume phase transition of PNIPAM.34 Lately, similar structure was descripted in POEGMA-co-PIL microgels which contributed to the aggregation of POEGMA chains and promoted the sharp phase transition of microgels.36 In this, PIL-D2O associations are developed since the hydrophilic P[P4,4,4,4][SS] moieties are surrounded by bulky water molecules. Due to the coulomb repulsive force impose less restriction on the contraction of microgels with lower content of charged P[P4,4,4,4][SS], the microgels collapse which decreases the distance between ester C=O and neighboring PIL-D2O association and C=O···D2O-PIL hydrogen bond may be preferred with temperature increment. Given the sequence order v(hydrated C=O) > v(C=O···D2O-PIL) > v(dehydrated C=O), it indicates that the ester C=O groups of PDEGA moieties undergo two kinds of conformational variations during the phase transition process. One is hydrated state to dehydrated state, the other is C=O hydrogen bonded with D2O to C=O hydrogen bonded with PIL-D2O clathrates. Meanwhile, the PIL involved hydrogen bonds can be considered as physical cross-linking points which facilitate squeezing water molecules from the microgels.

Intuitional expression of phase transition process in M-1 is shown in Figure. 7. At low temperatures, the microgels keep swelling state and the polymer chains stretch in the aqueous environment. On account of the hydrophobic association of PDEGA moieties with larger proportion, the microgel particles stack with each other tightly and gel-like state of M-1 aqueous solution (10%) at the macroscopic level is observed. 21


Langmuir

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

Typically, P[P4,4,4,4][SS] moieties respond initially to the temperature perturbation and the dehydration of SO3- groups trigger the whole dehydration process of microgels with temperature increment. Afterwards, the C-H groups on the PDEGA side chains collapse toward main polymer chains and expel water molecules out, inducing the destruction of hydrated environment around polymer backbones. Notably, although partial hydrated C=O groups on PDEGA side chains dehydrate during the heating process, C=O···D2O-PIL hydrogen bonds which are considered as the physical cross-linking points beneficial to interchain aggregation and drastic dehydration of chemical groups in M-1 system are preferred due to the existence of PIL-D2O associations. As a consequence, P[P4,4,4,4][SS] moieties are wrapped in the core of aggregated micelle while the PDEGA moieties distribute on the relatively outer layer.

The sequence order of chemical groups in M-3 during heating process were analyzed via 2Dcos process as M-1 illustrated above and deduced as follows: 2883 > 2873 > 1132 > 1730 > 2962 > 2933 > 2943 > 2981 > 1124 > 1693 > 1720(1709) cm-1 or vs(hydrated CH2) > vs(dehydrated CH2) > vs(SO3-…mD2O in anion) > v(dehydrated C=O) > vas(dehydrated CH3) > vas(dehydrated CH2) > vas(hydrated CH2) > vas(hydrated CH3) > vs(SO3-…(m-n)D2O in anion) > v(C=O···D2O-PIL) > v(hydrated C=O).

Without regard to the differences in the stretching modes of the chemical

groups, the sequence order can be simplified as C-H > SO3- > C=O. Although P[P4,4,4,4][SS] may also be involved in the dehydration process in view of the considerable ratio in the copolymer, distinguishable bands corresponding to C-H stretching vibration of P[P4,4,4,4][SS] are absent in the asynchronous spectra. It can be 22


Page 22 of 32

Page 23 of 32

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

Langmuir

seen that the microgels with higher content of P[P4,4,4,4][SS] moieties have different phase transition mechanism compared with microgels with less P[P4,4,4,4][SS] component. To be specific, the volume phase transition process of M-3 is driven by the hydrophobic interactions of C-H groups for the first response of C-H. Zhou et al. investigated PIL-involved thermo-responsive microgels and revealed that the temperature sensitivity of C-H groups appeared weakened with higher content of PIL which may be attributed to the highly cross-linking structures caused by the larger amount of PIL cross-linker.36 Compared with C=O and SO3- which are either in the hydrated state or hydrogen bonded with each other, the hydrophobic C-H groups are in the less confined circumstance for M-3 and respond prior to the other groups.

Especially, the sequence order of ester C=O groups in M-3 is v(dehydrated C=O) > v(C=O···D2O-PIL) > v(hydrated C=O) and the phenomenon that dehydrated C=O respond earlier than hydrated ones had seldom been reported in ester-included LCST-type systems.48,57 Owing to the steric-hindrance effect of microgel networks, some cations and anions in PIL would not appear in pairs,41 which lead to the enhancement of coulomb repulsive force in the microgels with higher PIL moieties content. The hindering effect of coulombic repulsion inside suppresses the accelerating effect of C=O···D2O-PIL hydrogen bonds on the dehydration behavior of microgels and a gradual dehydration process is exhibited. It is concluded that the M-3 tends to swell due to the internal coulomb repulsive force and hydrated C=O groups or C=O···D2O-PIL association may be preferred to form afterwards. Additionally, according to the temperature-dependent frequency shift graph as shown in Figure. 3 23


Langmuir

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

and the DLS measurement that the final size of M-3 microgels is close to that of M-1 microgels after phase transition, charged SO3- groups are inclined to move to the outer hydrated environment during heating process and result in the weakened coulomb repulsive force inside M-3 microgels.

Schematic diagram of the phase transition behavior of M-3 is exhibited in Figure. 7, which presents distinct dehydration process at molecular level compared with M-1. At low temperatures, transparent aqueous solution is observed at high concentration (10%) due to the strong coulombic force which keeps stretched microgel particles away from each other in the solution. With temperature increment, the C-H groups of PDEGA dehydrate and aggregate together to construct more hydrophobic environment inside the M-3 microgels due to the hydrophobic interactions and the promotion of cationic moieties in P[P4,4,4,4][SS]. Specially, parts of highly hydrophilic SO3- groups shift towards the outer layer of microgel and the remaining internal SO3groups provide coulombic repulsion which supports the hydrated state and prevents water molecules expelling out. Due to the influence of surrounding hydrophilic P[P4,4,4,4][SS] moieties,

the dehydration of C=O groups is hampered. The hindering

effect of coulombic repulsion suppresses the accelerating effect of C=O···D2O-PIL hydrogen bonds on the dehydration behavior of microgels and water molecules connecting with various chemical groups are expelled out continuously, resulting in a gradual dehydration process.

24


Page 24 of 32

Page 25 of 32

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

Langmuir

Figure. 7 Schematic illustration of the volume phase transition mechanisms of M-1 and M-3 microgels during heating. 4. CONCLUSION

PDEGA based microgels with thermo-responsive PIL moieties were synthesized via free radical copolymerisation for exploring the interaction between polymers (or ILs) and water molecules together with the certain effect of ILs features on the phase transition mechanism of microgels. It can be observed that the content of hydrophilic P[P4,4,4,4][SS] moieties has obvious impact on the thermally induced transition mechanism of microgel systems. Based on DLS measurements, the microgels with less P[P4,4,4,4][SS] moieties display a sharp phase transition behaviour while the sample containing more P[P4,4,4,4][SS] moieties present a slightly broad transition behaviour. According to 2Dcos spectra and PCMW analysis, it can be inferred that the existence of the polymer-water-polymer hydrogen bonds (C=O···D2O-PIL) between C=O groups of PDEGA and PIL···D2O association is responsible for the complete dehydration process and the sudden decrease of hydrodynamic size in 25


Langmuir

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

microgels with less PIL. With regard to the charge-rich microgels containing more PIL moieties, the strong coulombic force of PIL moieties counteracts the promotion effect of C=O···D2O-PIL hydrogen bonds on the dehydration behaviour of microgels and limits the flexibility of conformational changes of chemical groups. Thus, coupled with the cooperative responses of different chemical groups, a gradual dehydration process emerges consequently. Moreover, compared with the laggardly responsive order of C-H groups in microgels with less PIL moieties, the C-H groups in microgels with more P[P4,4,4,4][SS] moieties respond firstly to the increasing temperature, indicating that the temperature sensitivity of C-H groups in PDEGA is affected by the content of hydrophilic PIL in the copolymerization system.

ASSOCIATED CONTENT

Supporting Information DLS results, 1H NMR spectra and TEM images of microgels. AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]

ACKNOWLEDGMENTS

We sincerely appreciate the financial support from the National Science Foundation of China (NSFC) (Nos. 21674025, 51473038, 21604024).

26


Page 26 of 32

Page 27 of 32

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

Langmuir

REFERENCES (1) Chua, G. B. H.; Roth, P. J.; Duong, H. T. T.; Davis, T. P.; Lowe, A. B. Synthesis and Thermoresponsive Solution Properties of Poly[oligo(ethylene glycol) (meth)acrylamide]s: Biocompatible PEG Analogues. Macromolecules 2012, 45, 1362-1374. (2) Hu, Z.; Cai, T.; Chi, C. Thermoresponsive Oligo(ethylene glycol)-methacrylatebased Polymers and Microgels. Soft Matter 2010, 6, 2115-2123. (3) Petersen, H.; Fechner, P. M.; Fischer, D.; Kissel, T. Synthesis, Characterization, and Biocompatibility of Polyethylenimine-graft-poly(ethylene glycol) Block Copolymers. Macromolecules 2002, 35, 6867-6874. (4) Bozukova, D.; Pagnoulle, C.; De Pauw-Gillet, M.-C.; Ruth, N.; Jérôme, R.; Jérôme, C. Imparting Antifouling Properties of Poly(2-hydroxyethyl methacrylate) Hydrogels by Grafting Poly(oligoethylene glycol methyl ether acrylate). Langmuir 2008, 24, 6649-6658. (5) Zhao, W.; He, C.; Wang, H.; Su, B.; Sun, S.; Zhao, C. Improved Antifouling Property of Polyethersulfone Hollow Fiber Membranes Using Additive of Poly(ethylene glycol) Methyl Ether-b-Poly(styrene) Copolymers. Ind. Eng. Chem. Res. 2011, 50, 3295-3303. (6) Charnley, M.; Textor, M.; Acikgoz, C. Designed Polymer Structures with Antifouling–antimicrobial Properties. React. Funct. Polym. 2011, 71, 329-334. (7) Vancoillie, G.; Frank, D.; Hoogenboom, R. Thermoresponsive Poly(oligo ethylene glycol acrylates). Prog. Polym. Sci. 2014, 39, 1074-1095. (8) Lutz, J.-F.; Weichenhan, K.; Akdemir, Ö.; Hoth, A. About the Phase Transitions in Aqueous Solutions of Thermoresponsive Copolymers and Hydrogels Based on 2-(2-methoxyethoxy)ethyl Methacrylate and Oligo(ethylene glycol) Methacrylate. Macromolecules 2007, 40, 2503-2508. (9) Vogt, A. P.; Sumerlin, B. S. Temperature and Redox Responsive Hydrogels from ABA Triblock Copolymers Prepared by RAFT Polymerization. Soft Matter 2009, 5, 2347-2351. (10) Skrabania, K.; Kristen, J.; Laschewsky, A.; Akdemir, Ö.; Hoth, A.; Lutz, J.-F. Design, Synthesis, and Aqueous Aggregation Behavior of Nonionic Single and Multiple Thermoresponsive Polymers. Langmuir 2007, 23, 84-93. (11) De, P.; Sumerlin, B. S. Precision Control of Temperature Response by Copolymerization of Di (ethylene glycol) Acrylate and An Acrylamide Comonomer. Macromol. Chem. Phys. 2013, 214, 272-279. (12) Zhou, T.; Wu, W.; Zhou, S. Engineering Oligo(ethylene glycol)-based Thermosensitive Microgels for Drug Delivery Applications. Polymer 2010, 51, 3926-3933. (13) Oh, J. K.; Siegwart, D. J.; Lee, H.-i.; Sherwood, G.; Peteanu, L.; Hollinger, J. O.; Kataoka, K.; Matyjaszewski, K. Biodegradable Nanogels Prepared by Atom Transfer Radical Polymerization as Potential Drug Delivery Carriers: Synthesis, Biodegradation, in Vitro Release, and Bioconjugation. J. Am. Chem. Soc. 2007, 129, 5939-5945. 27


Langmuir

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

(14) Melle, A.; Balaceanu, A.; Kather, M.; Wu, Y.; Gau, E.; Sun, W.; Huang, X.; Shi, X.; Karperien, M.; Pich, A. Stimuli-responsive Poly(N-vinylcaprolactam-co-2-methoxyethyl acrylate) Core-shell Microgels: Facile Synthesis, Modulation of Surface Properties and Controlled Internalisation into Cells. J. Mater. Chem. B 2016, 4, 5127-5137. (15) Tehrani, S. M.; Lu, Y.; Winnik, M. A. PEGMA-Based Microgels: A Thermoresponsive Support for Enzyme Reactions. Macromolecules 2016, 49, 8711-8721. (16) Boularas, M.; Deniau-Lejeune, E.; Alard, V.; Tranchant, J.-F.; Billon, L.; Save, M. Dual Stimuli-responsive Oligo (ethylene glycol)-based Microgels: Insight into The Role of Internal Structure in Volume Phase Transitions and Loading of Magnetic Nanoparticles to Design Stable Thermoresponsive Hybrid Microgels. Polym. Chem. 2016, 7, 350-363. (17) Dong, H.; Mantha, V.; Matyjaszewski, K. Thermally Responsive PM (EO) 2MA Magnetic Microgels via Activators Generated by Electron Transfer Atom Transfer Radical Polymerization in Miniemulsion. Chem. Mater. 2009, 21, 3965-3972. (18) Cai, T.; Marquez, M.; Hu, Z. Monodisperse Thermoresponsive Microgels of Poly(ethylene glycol) Analogue-Based Biopolymers. Langmuir 2007, 23, 8663-8666. (19) Shen, W.; Chang, Y.; Liu, G.; Wang, H.; Cao, A.; An, Z. Biocompatible, Antifouling, and Thermosensitive Core−Shell Nanogels Synthesized by RAFT Aqueous Dispersion Polymerization. Macromolecules 2011, 44, 2524-2530. (20) Li, Z.; Xiang, Y.; Zhou, X.; Nie, J.; Peng, M.; Du, B. Thermo-sensitive Poly (DEGMMA-co-MEA) Microgels: Synthesis, Characterization and Interfacial Interaction with Adsorbed Protein Layer. Chin. J. Polym. Sci. 2015, 33, 1516-1526. (21) Lu, J.; Yan, F.; Texter, J. Advanced Applications of Ionic Liquids in Polymer Science. Prog. Polym. Sci. 2009, 34, 431-448. (22) Yan, F.; Texter, J. Surfactant Ionic Liquid-Based Microemulsions for Polymerization. Chem. Commun. 2006, 2696-2698. (23) Inoue, T.; Ebina, H.; Dong, B.; Zheng, L. Electrical Conductivity Study on Micelle Formation of Long-Chain Imidazolium Ionic Liquids in Aqueous Solution. J. Colloid Interface Sci. 2007, 314, 236-241. (24) Yoshizawa, M.; Xu, W.; Angell, C. A. Ionic Liquids by Proton Transfer: Vapor Pressure, Conductivity, and the Relevance of ∆pKa from Aqueous Solutions. J. Am. Chem. Soc. 2003, 125, 15411-15419. (25) Baranchugov, V.; Markevich, E.; Pollak, E.; Salitra, G.; Aurbach, D. Amorphous Silicon Thin Films as a High Capacity Anodes for Li-ion Batteries in Ionic Liquid Electrolytes. Electrochem. Commun. 2007, 9, 796-800. (26) Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. Solubilities and Thermodynamic Properties of Gases in the Ionic Liquid 1-n-Butyl-3-methylimidazolium Hexafluorophosphate. J. Phys. Chem. B. 2002, 106, 7315-7320. (27) Shiflett, M. B.; Yokozeki, A. Solubility of CO2 in Room Temperature Ionic Liquid [hmim][Tf2N]. J. Phys. Chem. B. 2007, 111, 2070-2074. (28) Xie, H.; Li, S.; Zhang, S. Ionic Liquids as Novel Solvents for the Dissolution and Blending of Wool Keratin Fibers. Green Chem 2005, 7, 606-608. 28


Page 28 of 32

Page 29 of 32

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

Langmuir

(29) Rebelo, L. P. N.; Canongia Lopes, J. N.; Esperança, J. M. S. S.; Filipe, E. On the Critical Temperature, Normal Boiling Point, and Vapor Pressure of Ionic Liquids. J. Phys. Chem. B. 2005, 109, 6040-6043. (30) Zaitsau, D. H.; Kabo, G. J.; Strechan, A. A.; Paulechka, Y. U.; Tschersich, A.; Verevkin, S. P.; Heintz, A. Experimental Vapor Pressures of 1-Alkyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imides and a Correlation Scheme for Estimation of Vaporization Enthalpies of Ionic Liquids. J. Phys. Chem. A 2006, 110, 7303-7306. (31) Marcilla, R.; Sanchez‐Paniagua, M.; Lopez‐Ruiz, B.; Lopez‐Cabarcos, E.; Ochoteco, E.; Grande, H.; Mecerreyes, D. Synthesis and Characterization of New Polymeric Ionic Liquid Microgels. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 3958-3965. (32) Zhou, X.; Nie, J.; Wang, Q.; Du, B. Thermosensitive Ionic Microgels with pH Tunable Degradation via in Situ Quaternization Cross-Linking. Macromolecules 2015, 48, 3130-3139. (33) Chen, S.; Peng, Y.; Wu, Q.; Chang, A.; Qu, A.; Shen, J.; Xie, J.; Farooqi, Z. H.; Wu, W. Synthesis and Characterization of Responsive Poly(anionic liquid) Microgels. Polym. Chem. 2016, 7, 5463-5473. (34) Wang, Z.; Wu, P. The Influence of Ionic Liquid on Phase Separation of Poly(N-isopropylacrylamide) Aqueous Solution. RSC Adv 2012, 2, 7099-7108. (35) Li, W.; Wu, P. On the Thermodynamic Phase Behavior of Poly(N-vinylcaprolactam) Solution in the Presence of Different Ionic Liquids. Polym. Chem. 2014, 5, 761-770. (36) Zhou, Y.; Tang, H.; Wu, P. Volume Phase Transition Mechanism of Poly[oligo(ethylene glycol)Methacrylate] Based Thermo-responsive Microgels with Poly(ionic liquid) Cross-linkers. Phys. Chem. Chem. Phys. 2015, 17, 25525-25535. (37) Kohno, Y.; Ohno, H. Key Factors to Prepare Polyelectrolytes Showing Temperature-Sensitive Lower Critical Solution Temperature-type Phase Transitions in Water. Aust. J. Chem. 2012, 65, 91-94. (38) Hou, L.; Wu, P. Microgels with Linear Thermosensitivity in a Wide Temperature Range. Macromolecules 2016, 49, 6095-6100. (39) Zhou, X.; Zhou, Y.; Nie, J.; Ji, Z.; Xu, J.; Zhang, X.; Du, B. Thermosensitive Ionic Microgels via Surfactant-Free Emulsion Copolymerization and in Situ Quaternization Cross-Linking. ACS Appl. Mater. Inter 2014, 6, 4498-4513. (40) Schachschal, S.; Adler, H.-J.; Pich, A.; Wetzel, S.; Matura, A.; van Pee, K.-H. Encapsulation of Enzymes in Microgels by Polymerization/Cross-linking in Aqueous Droplets. Colloid. Polym. Sci. 2011, 289, 693-698. (41) Li, W.; Wu, P. Unusual Thermal Phase Transition Behavior of an Ionic Liquid and Poly(ionic liquid) in Water with Significantly Different LCST and Dynamic Mechanism. Polym. Chem. 2014, 5, 5578-5590. (42) Liu, G.; Qiu, Q.; An, Z. Development of Thermosensitive Copolymers of Poly(2-methoxyethyl acrylate-co-Poly(ethylene glycol) methyl ether acrylate) and Their Nanogels Synthesized by RAFT Dispersion Polymerization in Water. Polym. Chem. 2012, 3, 504-513. 29


Langmuir

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

(43) Li, Y.; Ye, Z.; Shen, L.; Xu, Y.; Zhu, A.; Wu, P.; An, Z. Formation of Multidomain Hydrogels via Thermally Induced Assembly of PISA-Generated Triblock Terpolymer Nanogels. Macromolecules 2016, 49, 3038-3048. (44) Hvidt, S.; Joergensen, E. B.; Brown, W.; Schillen, K. Micellization and Gelation of Aqueous Solutions of a Triblock Copolymer Studied by Rheological Techniques and Scanning Calorimetry. J. Phys. Chem. 1994, 98, 12320-12328. (45) Liu, S.; Li, L. Multiple Phase Transition and Scaling Law for Poly(ethylene oxide)–Poly(propylene oxide)–Poly(ethylene oxide) Triblock Copolymer in Aqueous Solution. ACS Appl. Mater. Inter 2015, 7, 2688-2697. (46) Gan, T.; Zhang, Y.; Guan, Y. In Situ Gelation of P(NIPAM-HEMA) Microgel Dispersion and its Applications as Injectable 3D Cell Scaffold. Biomacromolecules 2009, 10, 1410-1415. (47) Sun, S.; Wu, P. Role of Water/Methanol Clustering Dynamics on Thermosensitivity of Poly(N-isopropylacrylamide) from Spectral and Calorimetric Insights. Macromolecules 2010, 43, 9501-9510. (48) Dai, Y.; Wu, P. Exploring the Influence of the Poly(4-vinyl pyridine) Segment on the Solution Properties and Thermal Phase Behaviours of Oligo(ethylene glycol) Methacrylate-Based Block Copolymers: the Different Aggregation Processes with Various Morphologies. Phys. Chem. Chem. Phys. 2016, 18, 21360-21370. (49) Hou, L.; Ma, K.; An, Z.; Wu, P. Exploring the Volume Phase Transition Behavior of POEGA- and PNIPAM-Based Core–Shell Nanogels from Infrared-Spectral Insights. Macromolecules 2014, 47, 1144-1154. (50) Li, T.; Tang, H.; Wu, P. Structural Investigation of Thermo-Responsive Poly (2-isopropyl-2-oxazoline) Hydrogel across the Volume Phase Transition. Soft Matter 2015, 11, 1911-1918. (51) Wang, G.; Wu, P. Unusual Phase Transition Behavior of Poly (N-isopropylacrylamide)-co-Poly (tetrabutylphosphonium styrenesulfonate) in Water: Mild and Linear Changes in the Poly (N-isopropylacrylamide) Part. Langmuir 2016, 32, 3728-3736. (52) Morita, S.; Shinzawa, H.; Noda, I.; Ozaki, Y. Perturbation-Correlation Moving-Window Two-Dimensional Correlation Spectroscopy. Appl. Spectrosc. 2006, 60, 398-406. (53) Watanabe, A.; Morita, S.; Ozaki, Y. Study on Temperature-Dependent Changes in Hydrogen Bonds in Cellulose Iβ by Infrared Spectroscopy with Perturbation-Correlation Moving-Window Two-Dimensional Correlation Spectroscopy. Biomacromolecules 2006, 7, 3164-3170. (54) Noda, I.; Dowrey, A.; Marcott, C.; Story, G.; Ozaki, Y. Generalized Two-Dimensional Correlation Spectroscopy. Appl. Spectrosc. 2000, 54, 236A-248A. (55) Noda, I. Scaling Techniques to Enhance Two-Dimensional Correlation Spectra. J. Mol. Struct. 2008, 883, 216-227. (56) Sun, S.; Tang, H.; Wu, P.; Wan, X. Supramolecular Self-assembly Nature of a Novel Thermotropic Liquid Crystalline Polymer Containing no Conventional Mesogens. Phys. Chem. Chem. Phys. 2009, 11, 9861-9870. (57) Sun, S.; Wu, P. On the Thermally Reversible Dynamic Hydration Behavior of 30


Page 30 of 32

Page 31 of 32

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

Langmuir

Oligo(ethylene glycol) Methacrylate-Based Polymers in Water. Macromolecules 2013, 46, 236-246.

TOC Graphic:

Schematic

illustration

of

the

dynamic

phase

transition

31


mechanism

of

Langmuir

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

PDEGA-co-P[P4,4,4,4][SS] microgels with low content of PIL (M-1) and high content of PIL (M-3) during heating.

32


Page 32 of 32

Volume Phase Transition Mechanism of Poly[di(ethylene glycol)ethyl

Recommend Documents