Microgels with Internal Nanophase Separated Structures

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Poly(N-isopropylacrylamide-co-1-vinyl-3-alkylimidazolium bromide) Microgels with Internal Nanophase Separated Structures Qingwen Wu, Chao Lv, Zhijun Zhang, Yuqing Li, Jingjing Nie, Junting Xu, and Binyang Du Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01575 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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Poly(N-isopropylacrylamide-co-1-vinyl-3-alkylimidazolium bromide) Microgels with Internal Nanophase Separated Structures ‡

Qingwen Wu, † Chao Lv, † Zhijun Zhang, † Yuqing Li, † Jingjing Nie, Junting Xu, † and Binyang Du †* †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China ‡

Department of Chemistry, Zhejiang University, Hangzhou 310027, China

*

Corresponding author. E-mail: [email protected]. 1

ACS Paragon Plus Environment

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ABSTRACT Microgels with internal nanophase separated structures were fabricated by surfactant free emulsion copolymerization of N-isopropylacrylamide (NIPAm) and ionic liquid comonomers, namely 1-vinyl-3-alkylimidazolium bromide (VIMnBr) with various lengths n of long alkyl side chain, in aqueous solution at 70 ºC using N,N'-methylenebisacrylamide (BIS) as the crosslinker. Combined techniques of transmission electron microscopy (TEM), dynamic & static light scattering (DLS & SLS), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and polarized optical microscopy (POM) were employed to systematically investigate the sizes, morphologies and properties of the obtained microgels as well as the microstructures and phase transition of nanophases inside the microgels. The obtained P(NIPAm/VIMnBr) microgels are spherical in shape with narrow size distributions, and the nanophases have a radius of about 8-12 nm and are randomly distributed inside the microgels. The cooperative competition of the hydrophilic quaternary vinylimidazole (VIM) moieties and hydrophobic long alkyl side chains determines the thermal sensitive behavior of the P(NIPAm/VIMnBr) microgels. DSC and WAXD results reveal that the nanophases consist of the ordered alkyl side chains with a layered crystalline structure at low temperature, which exhibit a low melting temperature and a broad melting transition. SAXS results further show that the nanophases form a layered liquid crystalline structure at high temperature for the microgel suspensions and freeze-dried microgels.

KEYWORDS: Microgels, N-isopropylacrylamide, ionic liquid monomer, internal nanophase, alkyl side chain, crystalline structure

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INTRODUCTION Environmental responsive microgels, which exhibit both characteristics of hydrogels and colloidal particles and can respond to external stimuli like temperature, pH value and light, have been found to possess potential applications in various technological fields such as controlled release systems, nanoreators, chemical and bio- sensors.1-8 The applications of microgels are tightly determined by their structures and properties. Intensive studies have thus been carried out in order to tune the size, structure and properties of the microgels with a controllable manner by varying the monomer, comonomer, crosslinking agent as well as the preparation procedure.9-19 Scattering techniques like static & dynamic light scattering (SLS &DLS) and small-angle neutron scattering (SANS) are applied to investigate the internal structures of microgels, especially the inhomogeneity of the crosslinking networks.18-29 The structure information of the microgels is in turn applied to guide the design of novel microgels. Most of the efforts focus on improving the homogeneity of microgel’s crosslinking networks, which mainly results from the different reaction rates of monomer and crosslinker. However, the reports of microgels with nanophase separated internal structures are rare.22,30 Microgels consisting of internal nanophases with different functionalities might find applications in the areas where various functionalities are simultaneously required, for example, as hydrophilic and hydrophobic dual-drug carrier materials. Richtering group 22

reported the first example of thermo-sensitive microgels with an internal nanophase separated

structure, as revealed by using SANS. By copolymerizing nearly equimolar amounts of N-isopropylacrylamide (PNIPAm) and N-isopropylmethacrylamide (PNIPMAM) via free radical dispersion polymerization, the resultant copolymer microgels exhibited an internal nanophase separated structure at their volume phase transition temperature (VPTT), which could be described by a “dirty snowball” form factor model.22

Two different nanophases were located inside one

microgel particle at the VPTT, which was attributed to the collapsed PNIPAm domains and swollen PNIPMAM regions.

22

However, below and above the VPTT, where the copolymer microgels

were at swollen and collapsed states, respectively, no nanophase separation was observed and the 3


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structure of copolymer microgels could be described by a form factor model for homogeneous spheres with graded surfaces, which is usually used for microgels obtained via free radical polymerization.22 Recently, Du group et al. reported the thermo-sensitive microgels with hydrophilic quaternization crosslinking structures and hydrophobic crosslinking inner nanodomains, which were prepared via surfactant free emulsion polymerization (SFEP) by using N-isopropylacrylamide (NIPAm) as the main monomer, 1-vinylimidazole (VIM) as the quaternizable comonomer, 2-(cinnamoyloxy) ethyl methacrylate (CEMA) as the cross-linkable comonomer, and 1, 6-dibromohexane as the quaternization crosslinker.30 The authors demonstrated that these microgels with amphipathic crosslinked networks were suitable to load and simultaneously release the hydrophilic drug, diclofenac sodium (DS), and hydrophobic drug, doxorubicin (DOX). 30 In present work, we shall report a series of thermo-sensitive microgels with internal nanophase separated structures.

By incorporating a sort of ionic liquid comonomers with long

alkyl side chains, namely 1-vinyl-3-alkylimidazolium bromide (VIMnBr, n = 10, 12, 14, 16 or 18), into the thermo-sensitive PNIPAm networks, the obtained P(NIPAm/VIMnBr) microgels exhibit internal nanophase separated structures. The nanophases have a radius of about 8-12 nm and are randomly distributed inside the P(NIPAm/VIMnBr) microgels, as clearly revealed by transmission electron microscopy (TEM). The hydrodynamic size and thermo-sensitive behavior of the P(NIPAm/VIMnBr) microgels are determined by the competition between the hydrophilicity of the quaternary vinylimidazole (VIM) moiety and the hydrophobicity of the long alkyl side chain. The structures and phase transition of nanophases were systematically investigated and verified by combined techniques of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and polarized optical microscopy (POM). The experimental results indicate that the nanophases consist of ordered alkyl side chains with a layered crystalline structure at low temperature and a layered liquid crystalline phase above the melting

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temperature. These thermo-sensitive microgels with internal nanophase separated structures might have potential applications as hydrophilic and hydrophobic dual-drug carrier materials.

EXPERIMENTAL SECTION Materials N-isopropylacrylamide

(NIPAm)

(Acros,

99%),

1-vinylimidazole

(VIM)

(TCI,

99%),

1-bromodecane (J&K, 98%), 1-bromododecane (J&K, 98%), 1-bromotetradecane (J&K, 98%), 1-bromohexadecane

(J&K,

98%),

2,2′-azobis(2-methylpropionamidine)

1-bromooctadecane

dihydrochloride

(AIBA)

(J&K,

(Aldrich-sigma,

98%), 99%)

and

N,N'-methylenebisacrylamide (BIS) (J&K, 96%) were used as received without any further purification. Preparation of 1-vinyl-3-alkylimidazolium bromide (VIMnBr, n = 10, 12, 14, 16 and 18) The ionic liquid monomers with various lengths of alkyl chains were synthesized via a general protocol given below: 0.03 mol VIM and 0.03 mol alkyl-bromide (1-bromodecane, 1-bromododecane, 1-bromotetradecane, 1-bromohexadecane, or 1-bromooctadecane) were first dissolved in 10 mL N,N-dimethylformamide (DMF) solution, respectively. After complete dissolution, the reaction mixture was stirred at 80 oC for about 24 hours. The mixture was precipitated in 500 mL diethyl ether. The precipitates were washed by diethyl ether several times to further remove the unreacted reactants, and then dried under vacuum to give the targeted ionic liquid monomer. Five monomers, namely 1-vinyl-3-decylimidazolium bromide (VIM10Br), 1-vinyl-3-dodecylimidazolium bromide (VIM12Br), 1-vinyl-3-tetradecylimidazolium bromide (VIM14Br),

1-vinyl-3-hexadecylimidazolium

bromide

(VIM16Br),

and

1-vinyl-3-octadecylimidazolium bromide (VIM18Br) were obtained. The chemical structures and corresponding 1H-NMR spectra of the five monomers are given in supporting information (Figure S1).

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Preparation of P(NIPAm/VIMnBr) microgels The P(NIPAm/VIMnBr) microgels were prepared using NIPAm as the main monomer, VIMnBr as the comonomer, and BIS as the crosslinker via surfactant-free emulsion polymerization (SFEP), as shown in Scheme 1. Briefly, NIPAm and BIS were first dissolved in 45 mL deionized water in a three-necked flask. The mixture was heated up to 70 oC under stirring and nitrogen was bubbled into the mixture for 30 minutes to eliminate the oxygen dissolved in the solution. 5 mL AIBA aqueous solution (5 mg/mL) was then injected into the mixture to initiate polymerization. After 4 minutes of polymerization, a given amount of VIMnBr dissolved in 2 mL DMF was injected into the reaction mixture. The reaction further proceeded for 6 hours. After polymerization, the as-prepared microgels were dialyzed in DMF for 3 days, then in deionized water for 4 days, during which the dialyzed solution was replaced by fresh DMF or deionized water for several times every day.

Similar

method

has

been

used

to

incorporate

hydrophobic

cross-linker

N,N′-bis(acryloyl)cystamine (BAC) or hydrophobic dibromo-compounds for synthesizing degradable thermo-sensitive microgels in our previous works. 17,31 The feeding ratio of NIPAm and VIMnBr is given in Table 1. The obtained P(NIPAm/VIMnBr) microgels are coded as PNIn-X, where n represents the length of the alkyl side chains in the ionic liquid comonomers, and X represents 10 times of the molar ratio of VIMnBr to NIPAm. For example, PNI10-2 means that the comonomer is VIM10Br with decane side chain and the molar ratio of VIMnBr to NIPAm is 0.2. The yields of P(NIPAm/VIMnBr) microgels are about 70-89% for the total feeding amounts of monomer, comonomer and cross-linker, as shown in Table S1.

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Scheme 1. Synthesis procedure of P(NIPAm/VIMnBr) microgels.

To determine the conversion ratio of VIMnBr, P(NIPAm-co-VIMnBr) linear copolymers were also synthesized by copolymerization of NIPAm and VIMnBr using the identical procedure of P(NIPAm/VIMnBr) microgels and the same feeding amounts of NIPAm and VIMnBr (Table S1). After polymerization, the as-prepared linear copolymers were dialyzed in DMF for 3 days and in deionized water for 4 days. The molar fractions of VIMnBr in the P(NIPAm-co-VIMnBr) linear copolymers were determined by 1H-NMR (Figure S2) and the conversion ratios of VIMnBr in the P(NIPAm/VIMnBr) microgels were then estimated by assuming that the molar fractions of VIMnBr in the microgels were the same as those in the corresponding linear copolymers. The conversion ratios of VIMnBr in the P(NIPAm/VIMnBr) microgels were estimated to be about 33-63%, depending on the sort and feeding amount of the comonomers, as summarized in Table S1. The details for calculation are given in the supporting information. Characterizations 1

H-NMR spectra were recorded on a Bruker spectrometer (400 MHz) with DMSO-d6 as solvent

and tetramethylsilane as the internal standard. Thermogravimetric analysis (TGA) was performed on a TGA Q50 (TA Instruments) under air. The samples were packed in alumina crucibles and heated to 400 oC or 600 oC at 10 °C/min. Differential scanning calorimetry (DSC) measurements of the freeze-dried microgels and VIMnBr (n = 10,12,14,16 and 18) were carried out on a DSC Q20 (TA Instruments) calorimeter with a heating rate of 10 /min under nitrogen atmosphere. For the freeze-dried microgels, the temperature range was set from -50 oC to 200 oC, whereas for the 7


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VIMnBr comonomers, the temperature range was set from -50 oC to the temperature that was lower than their decomposition temperature obtained from its TGA result (Figure S3). The morphologies of microgels were observed by transmission electron microscopy (TEM) (JEOL JEM-1230 electron microscope, 80kV). The optical images of the microgel suspensions were recorded with a polarized optical microscope (POM, Olympus BX51) equipped with a hot stage. The hydrodynamic diameter 〈Dh〉 (or radius 〈Rh〉 = 〈Dh〉/2), size distribution and thermo-responsive behavior of the obtained microgels were measured by dynamic light scattering (DLS) using a 90 Plus particle size analyzer (Brookhaven Instruments Corp.) at scattering angle θ of 90°. The size distributions of microgels were analyzed by using the CONTIN program integrated in the instrumental software. The corresponding gyration radius 〈Rg〉 of microgels was measured by static light scattering (SLS) using an ALV-CGS-3 light scattering instrument in Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. The sample solutions were equilibrated at each measuring temperature for 15 min before measurements. Temperature-variable small-angle X-ray scattering (SAXS) measurements were carried out at BL16B1 beamline in Shanghai Synchrotron Radiation Facility (SSRF), China, with a wavelength at 1.24 Å and a sample-to-detector distance of 2000 mm.

Before measurement, the solid samples of

freeze-dried microgels were heated to 100 oC for 2 hours and then cooled down to room temperature.

The liquid samples were microgel suspensions with a concentration at 0.5~1 wt%,

depending on the type and feeding amount of the comonomer. The samples were heated to the pre-set temperature at a rate of 10 oC/min and held for 3 min to reach equilibrium before acquisition of SAXS data. A Linkam hot-stage was used to regulate the temperature of the samples. For each scan, the exposure time was about 5 s for the freeze-dried microgels (solid samples) and 30 or 300 s for the microgel suspensions (liquid samples). Wide-angle X-ray diffraction (WAXD) patterns were recorded on a Rigaku Ultima IV X-ray diffractometer equipped with a Cu Kα (λ = 1.5405 Å) X-ray source at about 12-14 oC.

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Table 1. The preparation conditions and characterization of P(NIPAm/VIMnBr) microgels. DLSa

Comonomer Sample codes

NIPAm (mmol)

TEM

V10Br

V12Br

V14Br

V16Br

V18Br

(mmol)

(mmol)

(mmol)

(mmol)

(mmol)

(mmol)

Rh (nm)

PDI

Rg (nm)

Swelling

R

Ratio

(nm)

(V25/V60)

Rg/Rh

PNI10-1

0.2

564±10

0.13

248±3

250±21

17±2

0.44±0.01

PNI10-2

0.4

584±6

0.09

206±3

285±28

21±1

0.35±0.01

PNI10-3

0.6

734±5

0.21

194±6

283±18

30±5

0.26±0.01

PNI12-1

0.2

740±8

0.12

228±2

328±37

109±8

0.31±0.01

PNI12-2

0.4

326±5

0.01

187±3

253±24

6±1

0.57±0.02

PNI12-3

0.6

295±3

0.06

194±2

303±17

8±1

0.66±0.01

363±6

0.11

201±2

149±12

11±1

0.55±0.01

312±3

0.13

201±2

168±16

16±1

0.64±0.01

PNI14-1 PNI14-2

0.2 2

PNI14-3

a

SLSa

BIS

0.2

0.4

288±3

0.12

200±2

268±19

9±1

0.70±0.01

PNI16-1

0.6 0.2

312±3

0.12

206±3

286±31

12±1

0.66±0.01

PNI16-2

0.4

257±4

0.06

181±3

256±25

12±1

0.70±0.02

PNI16-3

0.6

229±5

0.02

175±2

228±21

6±1

0.76±0.03

PNI18-1

0.2

255±2

0.02

191±2

197±17

11±1

0.75±0.01

PNI18-2

0.4

238±2

0.01

196±3

215±14

8±1

0.82±0.02

PNI18-3

0.6

221±3

0.08

191±2

268±21

7±1

0.86±0.02

o

measured at 25 C.

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RESULTS AND DISCUSSION Morphology, size and size distribution of P(NIPAm/VIMnBr) microgels Five monomers of 1-vinyl-3-alkylimidazolium bromide (VIMnBr, n =10, 12, 14, 16 and 18) were synthesized by quaternization of 1-vinylimidazole (VIM) and bromoalkane with various lengths n of alkyl chain as described in the experimental section. A series of P(NIPAm/VIMnBr) microgels were successfully obtained via SFEP with N-isopropylacrylamide (NIPAm) as the main monomer, VIMnBr as the comonomer, and N,N'-methylenebisacrylamide (BIS) as the crosslinker at 70 oC, as shown in Scheme 1. The feeding amounts of NIPAm and BIS were fixed to be 2 mmol and 0.2 mmol, respectively, whereas the amounts of VIMnBr varied, as listed in Table 1.

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Figure 1. Representative TEM images and the statistic size distributions measured by TEM (insets) for P(NIPAm/VIMnBr) microgels: (A) PNI10-3, (B) PNI12-3, (C) PNI14-3, (D) PNI16-3, (E) PNI18-3.

Figure 1 shows the typical TEM images and size distributions of P(NIPAm/VIMnBr) microgels with sample codes of PNI10-3, PNI12-3, PNI14-3, PNI16-3, and PNI18-3. The TEM images indicate that all the P(NIPAm/VIMnBr) microgels are spherical in shape with relatively narrow size distributions. The average radii of the PNI10-3, PNI12-3, PNI14-3, PNI16-3, PNI18-3 microgels calculated from the TEM images are about 283±18 nm, 303±17 nm, 268±19 nm, 228±21 nm, and 268±21nm, respectively. Interestingly, dark nanodomains are observed inside the PNI12-3, PNI14-3, PNI16-3, PNI18-3 microgels. These nanodomains are randomly distributed inside the microgels with a radius of about 8-12 nm. These nanodomains appear darker than the microgel matrixes, indicating the higher densities of the nanodomains. Furthermore, no clear dependence of the size and size distribution of nanodomains on the length of alkyl chain of VIMnBr can be observed. 〈Dh〉s and size distributions of the obtained P(NIPAm/VIMnBr) microgels in aqueous solutions were investigated by DLS at 25 oC. Figure 2A shows 〈Dh〉s of P(NIPAm/VIMnBr) microgels as a function of length n of the alkyl side chain of the comonomer VIMnBr. It can be seen that 〈Dh〉s of P(NIPAm/VIMnBr) microgels decrease with increasing the length n of the alkyl side chains. Furthermore, for PNI12, PNI14, PNI16, PNI18 series of microgels, 〈Dh〉s of the microgels also decrease with increasing the comonomer content from 10% to 30%.

However, an opposite effect 11


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is observed for PNI10 series of P(NIPAm/VIM10Br) microgels, of which 〈Dh〉s increase with increasing the VIM10Br content. It is known that the sizes of microgels in equilibrium swollen state in aqueous solutions are highly influenced by the relative hydrophilicity and hydrophobicity of their components, namely main monomer, comonomer and crosslinker.

For P(NIPAm/VIMnBr)

microgels with fixed feeding amounts of NIPAm and crosslinker BIS, the comonomer VIMnBr plays a key role for the size of microgels in aqueous solutions. VIMnBr monomers are quaternary ammonium salts with different long alkyl side chains. There is a competition between the hydrophilicity of the quaternary VIM moiety and the hydrophobicity of the long alkyl side chain. Consequently, as the length n of alkyl side chain increases, the hydrophobicity of VIMnBr might increase. The results of Figure 2A also suggest that the comonomer VIMnBr changes from relatively hydrophilic to relatively hydrophobic when the side chain length n increases from 10 to 12. In another word, the VIM10Br is relatively hydrophilic so that 〈Dh〉s of P(NIPAm/VIM10Br) microgels increase with increasing the amount of VIM10Br. For PNI14, PNI16, PNI18 series of microgels, the corresponding comonomer exhibits a relatively hydrophobic property, leading to the decrease in 〈Dh〉s with the increase of comonomer content.

For example, 〈Rh〉s of PNI14-2,

PNI16-2, and PNI18-2 microgels are about 312±3 nm, 257±4 nm, and 238±2 nm, respectively. When the other conditions were fixed, the increase of hydrophobicity of comonomer would finally result in the decrease of hydrodynamic diameters of the swollen microgels in aqueous solutions. Hence, as the hydrophobicity of comonomers rises from PNI12 to PNI18 series microgels, 〈Dh〉s of corresponding P(NIPAm/VIMnBr) microgels decrease (Figure 2A).

Figure 2B shows the size

distributions of P(NIPAm/VIMnBr) microgels in aqueous solutions at 25 oC, indicating the narrow size distributions of P(NIPAm/VIMnBr) microgels, which is consistent with the TEM results. The 〈Dh〉s and polydispersity indexes (PDI) of P(NIPAm/VIMnBr) microgels obtained by DLS are summarized in Table 1.

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B

1600

100 10% 20% 30%

1400

PNI18-2

PNI16-2

PNI14-2

PNI12-2

PNI10-2

80

1200

Intensity(a.u.)

A Hydrodynamic diameter, Dh (nm)

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

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1000 800

60

40

20

600

0

400 10

12

14

16

18

420 450

Length n of alkyl side chain

510 540

600 630

600 630 1080 1200

Hydrodynamic diameters,Dh(nm)

Figure 2. (A) The hydrodynamic diameters Dh of the P(NIPAm/VIMnBr) microgels as a function of length n of alkyl side chain of the comonomer as measured by DLS at 25 oC. (B) Representative size distributions of P(NIPAm/VIMnBr) microgels with the VIMnBr/NIPAm feeding ratio of 0.2 in aqueous solutions as measured by DLS at 25 oC.

The radii of gyration 〈Rg〉s of the obtained P(NIPAm/VIMnBr) microgels were also measured by static light scattering (SLS) at 25 oC (Figure S4). The 〈Rg〉s and corresponding ratios of 〈Rg〉/〈Rh〉 are summarized in Table 1. 〈Rh〉 is the hydrodynamic radius of the obtained P(NIPAm/VIMnBr) microgels, i.e. 〈Rh〉 = 〈Dh〉/2. The value of 〈Rg〉/〈Rh〉 could be used to reflect the crosslinking density distribution of the microgels.

32

For example, 〈Rg〉/〈Rh〉 is 0.778 for uniform hard spheres and

〈Rg〉/〈Rh〉 is about 0.55~0.6 for PNIPAm microgels with inhomogenous crosslinking network structures.26,27 However, for the PNIPAm-related microgels with more homogeneous cross-linked structures made by post-crosslinking, self-crosslinking, or in-situ quaternization crosslinking, 〈Rg〉/〈Rh〉 values of 0.74-0.96 were observed.18,19,21,23,29 For PNI10 series of microgels, 〈Rg〉/〈Rh〉 ratio decreases with increasing the content of VIM10Br. 〈Rg〉/〈Rh〉 values of PNI10-1, PNI10-2 and PNI10-3 are 0.44, 0.35 and 0.26, respectively, indicating that P(NIPAm/VIM10Br) microgels possess inhomogeneous crosslinking network structures. However, for the PNI12, PNI14, PNI16, PNI18 series of microgels, an opposite tendency was observed. In these cases, the 〈Rg〉/〈Rh〉 values of microgels increase with increasing the comonomer content.

Furthermore, the 〈Rg〉/〈Rh〉 values

of the P(NIPAm/VIMnBr) microgels also increase with increasing the length n of the alkyl side 13


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chain of the comonomer.

Page 14 of 31

Especially, for PNI18 series of microgels with alkyl side chain length of

18, the 〈Rg〉/〈Rh〉 values of PNI18-1, PNI18-2 and PNI18-3 microgels are about 0.75, 0.82, and 0.86, respectively, which are close to the 〈Rg〉/〈Rh〉 value of 0.778 for uniform hard spheres. It suggests that PNI18 series of microgels might have more homogeneous network structures. The above results indicate that the length n of alkyl side chain and amount of the comonomer strongly affect the size and network structure of the resultant P(NIPAm/VIMnBr) microgels.

Thermo-responsive behavior of P(NIPAm/VIMnBr) microgels The thermo-responsive behaviors of the obtained P(NIPAm/VIMnBr) microgels were further investigated by DLS, as shown in Figures 3, 4 and S5. For all the P(NIPAm/VIMnBr) microgels in aqueous solutions, the 〈Dh〉s decrease with increasing temperature, which is expectable and attributed to the characteristic of poly(N-isopropylacrylamide) (PNIPAm) network chains.

Figure

3A also shows a tendency that 〈Dh〉s of P(NIPAm/VIMnBr) microgels decrease with increasing the length n of the alkyl side chain at a given temperature. This phenomenon further supports the argument that the hydrophobicity of VIMnBr is enhanced by longer alkyl side chain. Incorporation of a hydrophobic monomer will generally lead to the decrease of size for the resultant PNIPAm-based microgels.

The volume phase transition temperature (VPTT) of microgels can be

determined by differentiating the data curves given in Figure 3A. The corresponding temperature of the lowest derivative is determined as VPTT of the microgels (Figure 3B). The VPTTs of PNI10-2, PNI12-2, PNI14-2, PNI16-2, and PNI18-2 microgels are found to be around 35-37 oC, which are less dependent of the length n of alkyl side chain (Figure 3B). Figure 4A shows that for P(NIPAm-VIM10Br) microgels, increasing the amount of VIM10Br leads to the resultant microgels with a larger particle size and a higher VPTT. The 〈Dh〉s of PNI10-1, PNI10-2, and PNI10-3 microgels are about 1128 nm, 1168 nm, and 1468 nm, respectively. The volume phase transition temperatures (VPTT) of PNI10-1, PNI10-2, and PNI10-3 microgels are determined to be 33, 35 and 37 oC, respectively. Schachschal et al.33 prepared the P(NIPAm-co-VIM) 14


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Langmuir

microgels using BIS as the crosslinker and found that the size of the P(NIPAm-co-VIM) microgels depended on the amount of VIM used. They observed that the increase of the VIM content in PNIPAm microgels led to a larger size of resultant P(NIPAm-co-VIM) microgels, which was attributed to the hydrophilic character of VIM and electrostatic repulsion between partially ionized VIM groups. Hence, the results of Figure 4A suggest that the comonomer VIM10Br is hydrophilic. The incorporation of more hydrophilic VIM10Br then shifts the VPTT of P(NIPAm-VIM10Br) microgels to a higher temperature. However, for VIM18Br with octadecyl side chain (n = 18), the increase of VIM18Br content decreases the size of the resultant P(NIPAm-VIM18Br) microgels when the measuring temperature is below the VPTT of the microgels, as shown in Figure 4B. Note that the VPTT of P(NIPAm-VIM18Br) microgels is about 37 oC and independent of the amount of VIM18Br. For the temperature above the VPTT, the sizes of PNI18-1, PNI18-2, and PNI18-3 microgels are similar and independent of the amount of VIM18Br. Usually, more hydrophobic comonomers incorporated into the microgel network might lead to the decrease of VPTT of the microgels. By contrast, more hydrophilic comonomers incorporated will lead to the increase of VPTT of the microgels, which is the case of PNI10 series of microgels (Figure 4A). Since VIM18Br with octadecyl side chain (n = 18) is relatively hydrophobic, one may expect that the VPTT will shift to a lower temperature if more VIM18Br comonomers were incorporated. However, the VPTT of P(NIPAm-VIM18Br) microgels observed here is independent of the amount of VIM18Br (Figure 4B). It is speculated that the homogeneous network structures of PNI18 series of microgels may account for the unchanged VPTT. It is known that the network structures will affect the VPTT of the microgels. The above SLS&DLS results indicate that the 〈Rg〉/〈Rh〉 values of PNI18 series of microgels are close to that of uniform hard spheres. The decrease rate of hydrodynamic size of PNI18-1, PNI18-2, and PNI18-3 microgels is in the order of PNI18-1 > PNI18-2 > PNI18-3 when the temperature is below the VPTT of the microgels, leading to the similar VPTT of the microgels. When the temperature is above the VPTT, the hydrodynamic sizes of PNI18 series of microgels are similar. Possibly, the hydrophobicity of VIM18Br is similar with that of PNIPAm 15


Langmuir

chains above its LCST so that the amount of VIM18Br does not affect the hydrodynamic sizes of PNI18 series of microgels at the temperature above VPTT. Furthermore, the thermo-sensitive behaviors of the P(NIPAm/VIMnBr) microgels are reversible, as shown in the inset of Figures 4A and 4B. When the microgel suspensions are cooled from 70 °C down to 25 °C, the hydrodynamic diameters of PNI10-2 and PNI18-2 microgels increase again along the similar track of the heating procedure with slight hysteresis.

B

1200 P(NIPAm-VIMnBr) microgels PNI10-2 PNI12-2 PNI14-2 PNI16-2 PNI18-2

1000 800 600

10 0 -10

Derivative

Hydrodynamic diameter, Dh (nm)

A

-20 -30

P(NIPAm-VIMnBr) microgels PNI10-2 PNI12-2 PNI14-2 PNI16-2 PNI18-2

-40

400 -50

200 20

-60

30

40

50

60

20

70

30

40

50

60

70

o

o

Temperature ( C)

Temperature ( C)

Figure 3. (A) The hydrodynamic diameter, Dh of the representative P(NIPAm/VIMnBr) microgels with sample codes of PNI10-2, PNI12-2, PNI14-2, PNI16-2, and PNI18-2 measured by DLS as a function of measuring temperature and (B) the corresponding derivate values of the data shown in (A).

1200 1000

PNI10-2 microgels Heating Cooling

1000 800 600 400 20

30

40

50

60

70

o

Temperature ( C)

800 600 400 20

PNI10-1 PNI10-2 PNI10-3

30

40

50

60

70



1400

1200


B 600

A 1600 Hydrodynamic diameter, Dh (nm)

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 16 of 31

500

400

500 PNI18-2 microgels Heating Cooling

400

300

200 20

30

40

50

60

70

o

Temperature ( C)

300

200 20

PNI18-1 PNI18-2 PNI18-3

30

o

Temperature ( C)

40

50

60

70

o

Temperature ( C)

Figure 4. (A) The hydrodynamic diameters, Dh of PNI10-1, PNI10-2, and PNI10-3 microgels measured by DLS as a function of temperature. (B) The hydrodynamic diameters, Dh of PNI18-1, PNI18-2, and PNI18-3 microgels measured by DLS as a function of temperature. The insets of (A) and (B) are the evolution of hydrodynamic diameter, Dh of PNI10-2 and PNI18-2 microgels during the heating and cooling cycles, respectively.

16


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Langmuir

By taking the volume V60 of the microgels at 60 oC as the reference state, the swelling ratio of P(NIPAm/VIMnBr) microgels at 25oC can be estimated by the ratio of V25/V60, where V25 is the volume of the microgels at 25oC (Table 1). Again, since VIM10Br is relatively hydrophilic, increasing the content of hydrophilic comonomer will lead to the increase of swelling ratio for the P(NIPAm/VIM10Br) microgels (cf. Table 1). On the other hand, the increase of comonomer content will also decrease the crosslinking density of the microgels because the amount of crosslinker BIS was fixed. The decrease of crosslinking density usually results in the higher swelling ratio, which is true for P(NIPAm/VIM10Br) microgels.

However, it is not the case for the other comonomers

with longer alkyl side chains. No clear tendency can be observed for the cases with VIM12Br, VIM14Br, and VIM16Br. Both the increase of hydrophobicity and the decrease of crosslinking density with increasing the amount of hydrophobic comonomer have impact on the equilibrium swelling ratio. There might be a balance and cooperative effect, depending on the length n of alkyl side chain. For the longest alkyl side chain with n of 18 used here, the effect of hydrophobic alkyl chain might dominate, leading to the decrease of swelling ratio of P(NIPAm/VIM18Br) microgels with increasing the amount of VIM18Br (cf. Table 1). Most of the swelling ratios shown in Table 1 are in the same magnitude as that observed by Richtering et al.

22

for the copolymer microgel

PNIPMAM-PNIPAm (50/50) with nanophase separated structure. The swelling ratio (V50/V20) of the copolymer microgel PNIPMAM-PNIPAm (50/50) can be estimated from the ratio of RH (50 o o C)/RH(20 C)

to be about 13.5. 22

Structures and phase transition in the nanodomains of microgels The TEM micrographs show that nanodomains may be formed inside some microgels. We speculate that the nanodomains are formed by aggregation of VIMnBr units, since the comonomers VIMnBr (n = 12, 14, 16, 18) have been reported to exhibit crystalline and liquid crystalline phases, depending on the environmental temperature.34 DSC measurements were first conducted to identify the possible thermal transitions of the freeze-dried P(NIPAm/VIMnBr) microgels. Figure 5 shows the DSC cooling and second-run heating scans of freeze-dried P(NIPAm/VIMnBr) microgels with 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

Page 18 of 31

VIMnBr/NIPAm molar ratio of 0.3. No transition is observed for PNI10-3, PNI12-3 and PNI14-3 microgels. However, broad transitions centered around 17 oC and 32 oC are observed for PNI16-3 and PNI18-3 microgels, respectively (Figure 5B). The cooling and second-run heating scans show that the transitions are reproducible. Figure 6 shows the DSC second-run heating curves of VIM16Br and corresponding P(NIPAm/VIM16Br) microgels, VIM18Br and corresponding P(NIPAm/VIM18Br) microgels. The corresponding cooling and first-heating curves are shown in Figures S6 and S7. For P(NIPAm/VIM16Br) microgels, the broad transition is only observed for PNI16-3 microgels with the highest amount of VIM16Br (Figure 6A). Clear transition at about 70 o

C is observed for VIM16Br, which is attributed to the transition from crystalline phase to liquid

crystalline (LC) mesophase and close to the value previously reported. 34 Luo et al. found that the melting temperature of the transition from solid to mesophase is about 67.1 oC for VIM16Br with a scan rate of 2.0 oC/min.34 Note that the scan rate is 10 oC /min in the present DSC measurements. A higher scan rate usually results in a higher transition temperature because the molecular chains have less time to reach conformation equilibrium during the heating scan. The broad transition observed at about 17 oC for PNI16-3 microgels is much smaller than that of 70 oC for the comonomer VIM16Br. Whereas, the broad transition is observed at about 32

o

C for all the three

P(NIPAm/VIM18Br) microgels, i.e. PNI18-1, PNI18-2, and PNI18-3 microgels (Figure 6B). Clear transition at about 56.6 oC with a shoulder peak at about 34 oC is observed for VIM18Br, which is again attributed to the transition from crystalline phase to LC mesophase. The above results indicate that the broad transition observed for the freeze-dried P(NIPAm/VIMnBr) microgels indeed comes from the existence of comonomer VIMnBr. We speculate that the broad transition corresponds to the melting transition of the nanodomains within the microgels, as shown in Figure 1. These nanophases exhibit ordered structures and might be the crystalline alkyl side chains. Gerstl et al. found that the long alkyl side chains of poly(alkylene oxide) (PAOs) could form alkyl nanodomains, which resulted from the nanosegregation of main chains and side alkyl groups.35,36 By combining neutron diffraction, small-angle neutron scattering and molecular dynamics simulation, the authors 18


Page 19 of 31

revealed that the local structures of the alkyl nanodomains were polyethylene (PE)-like crystalline structures. Similarly, the long alkyl side chains of VIMnBr may form alkyl nanodomains in the P(NIPAm/VIMnBr) microgels. Because of the random copolymerization of comonomer VIMnBr and restriction of the main chains, only the adjacent alkyl side chains can aggregate to form the nanophases with ordered structures, resulting in non-uniform size of the nanodomains. The specific surface energy of nanodomains strongly affects the melting transition of polydisperse crystalline nanodomains, leading to the broader transition at evidently lower temperatures, as compared with the melting points of VIMnBr monomers. The existence of crystalline phases in the P(NIPAm/VIMnBr) microgels is further confirmed by WAXD and POM measurements.

B

A

P(NIPAm-VIMnBr) microgels PNI10-3 PNI12-3 PNI14-3 PNI16-3 PNI18-3

Heat flow, Exo up

Heat flow, Exo up

PNI18-3

PNI16-3

PNI14-3

PNI12-3

PNI10-3

-40

-20

0

20

40

60

80

-40

100

-20

0

20

40

60

80

100

o

o

Temperature ( C)

Temperature ( C)

Figure 5. DSC curves of freeze-dried P(NIPAm/VIMnBr) microgels with VIMnBr/NIPAm molar ratio of 0.3. (A) the cooling scans and (B) the second-run heating scans.

A

B

-40

-20

0

20

40

60

80

VIM18Br PNI18-3 PNI18-2 PNI18-1

Heat flow, Exo up

VIM16Br PNI16-3 PNI16-2 PNI16-1

Heat flow, Exo up

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

100

-40

-20

o

Temperature ( C)

0

20

40

60

80

100

o

Temperature ( C)

Figure 6. DSC second-run heating curves of VIMnBr and freeze-dried P(NIPAm/VIMnBr) microgels with various amount of VIMnBr. (A) VIM16Br and P(NIPAm/VIM16Br) microgels and (B) VIM18Br and P(NIPAm/VIM18Br) microgels. 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 31

WAXD profiles of freeze-dried PNI16-3 and PNI18-3 microgels as well as the comonomers VIM16Br and VIM18Br are shown in Figure 7. Clear crystalline phases of VIM16Br and VIM18Br are observed. Note that, due to the too large 2θ in WAXD experiments, the first-order peaks of the crystalline phases of VIM16Br and VIM18Br cannot be observed by WAXD. However, the first-order peaks can be observed by SAXS, as shown in Figure S7a. With the d spacing of first-order SAXS peak as the reference, the WAXD diffraction peaks are located at 1/3, 1/4, 1/5, 1/6… for VIM16Br and 1/4, 1/5, 1/6, 1/7… for VIM18Br, respectively. Such a WAXD pattern indicates a layered structure for the crystalline phase of comonomers. It is also noticed that the d spacings of the layered structures in the five comonomers increase linearly with length n of alkyl side chain (Figure S8), which are consistent with the results in literature. 34 Broad weak diffraction peaks are observed for PNI16-3 and PNI18-3 microgels, indicating that most parts of the microgels are amorphous. This agrees with the small melting enthalpies calculated from DSC curves (Figure 6). Nevertheless, three weak diffraction peaks can be identified for the two microgels, of which the d spacings are similar with those observed from WAXD profiles of the corresponding comonomers. These results indicate that the freeze-dried PNI16-3 and PNI18-3 microgels indeed contain crystalline phases, which have similar layered structures with the crystalline phases of the corresponding comonomers. Gerstl et al. found that the structures of alkyl nanodomains formed in PAOs were dependent on the length of alkyl side chains. For n ≤ 4, the alkyl nanodomains in PAOs would present the features of amorphous PE. However, crystallinity signature was observed for longer alkyl side chains, evidenced by the clear intense neutron diffraction peaks. The average characteristic distance Lav of the alkyl nanodomains calculated from the diffraction peak was found to linearly increase with length n of alkyl side chains.35

20


Page 21 of 31

A

B

1.21 nm

VIM18Br PNI18-3

Intensity (a.u.)

Intensity (a.u.)

VIM16Br PNI16-3

0.86 nm

1.31 nm 0.94 nm

1.1 nm

1.64nm

1.51 nm 2.0 nm

5

10

15

20

5

25

10

15

20

25

ο

ο

2θ ( )

2θ ( )

Figure 7. WAXD profiles of (A) freeze-dried PNI16-3 microgels and comonomer VIM16Br and (B) freeze-dried PNI18-3 microgels and comonomer VIM18Br at about 12-14 oC. The wavelength of X-ray is 0.154 nm.

POM observation was also carried out for PNI18-3 microgel suspensions. It is found that the polarized light can indeed pass through the concentrated PNI18-3 microgel suspensions, as shown in Figure S9. However, no crystalline texture is observed due to the small size of the nanodomains (8-12 nm) and the resolution of POM. If there is no ordered structure, the sample is isotropic so that the polarized light could not pass through the polarized filters of 90o cross and the obtained POM image will be completely dark. Nevertheless, the bright POM image obtained here again indicates the existence of ordered phases within the microgels, so that the polarized light can pass through.

A

freeze-dried microgels

B

microgel suspensions

PNI18-3

PNI16-3 PNI14-3

PNI12-3

PNI18-3

Log(Intensity (a.u.))


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

PNI16-3

PNI14-3 PNI12-3

PNI10-3

at 26 C

at 40 C 0.5

PNI10-3 o

o

1.0

1.5

2.0

2.5

0.5

-1

1.0

1.5

2.0

-1

q (nm )

q (nm )

Figure 8. SAXS profiles of (A) freeze-dried P(NIPAm/VIMnBr) microgels with VIMnBr/NIPAm molar ratio of 0.3 at 40 oC, and (B) the corresponding P(NIPAm/VIMnBr) microgel suspensions at 26 oC. The wavelength of X-ray is 0.124 nm. 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

Page 22 of 31

In literature, it is reported that the VIMnBr monomer can form liquid crystalline (LC) mesophase after melting. In order to know whether this characteristic is retained in the microgels after random copolymerization, temperature-variable SAXS measurements were carried out for selected P(NIPAm/VIMnBr) microgels with VIMnBr/NIPAm molar ratio of 0.3 and the corresponding comonomers. Figure 8 shows the SAXS profiles of freeze-dried P(NIPAm/VIMnBr) microgels at 40 o

C and P(NIPAm/VIMnBr) microgel suspensions at 26 oC. With longer alkyl side chain, i.e. n > 10,

scattering peaks are observed not only for the freeze-dried P(NIPAm/VIMnBr) microgels but also for the P(NIPAm/VIMnBr) microgel suspensions, which strongly indicates that there are partially ordered structures within the P(NIPAm/VIMnBr) microgels. Such partially ordered structures have a higher density, leading to the observation of dark nanodomains, as shown in TEM images (cf. Figure 1). Note that the measurement temperature of SAXS experiments is higher than the melting temperatures of nanodomains (Figures 5 and 6), though it is lower than the melting temperatures of the corresponding monomers. For example, the melting temperatures measured by DSC at a heating rate of 2 oC /min are 59.8 oC for VIM14Br, 67.1 oC for VIM16Br, and 70.5 oC for VIM18Br. Therefore, temperature-variable SAXS experiments were performed. Figure 9 shows the SAXS profiles of freeze-dried PNI18-3 microgels and PNI18-3 microgel suspensions at various temperatures. It is observed that, even when the temperature is higher than the melting temperature of VIM18Br, the partially ordered structures are still present in PNI18-3 microgels. As a result, the partially ordered structure at high temperature can be attributed to the liquid crystalline mesophase formed by the copolymerized VIMnBr units, like the VIMnBr monomers. The d spacings of the LC mesophases can be determined from the position of the scattering peak and are summarized in Table 2. There is a tendency that the d spacings increase with increasing the length n of alkyl side chains. The d spacings obtained from the P(NIPAm/VIMnBr) microgel suspensions are slightly larger than those from freeze-dried P(NIPAm/VIMnBr) microgels. The d spacings of LC mesophases are in the range of 2.7-4.1 nm, which are close to those of LC mesophases of VIMnBr comonomers at higher temperature.34 The stacking mode of the alkyl chains 22


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Langmuir

in the comonomer units can also be inferred from the d spacings. Generally, the alkyl chains can stack into a head-to-head bilayer structure or an interdigitated monolayer structure.37 The d spacings of LC mesophases calculated from SAXS and the theoretical lengths for the alkyl chains with an all-trans conformation are listed in Table 2. One can see that, the d spacings of freeze-dried microgels are smaller than twice of the all-trans chain length (L) of the corresponding alkyl side chain, which suggests that the alkyl side chains form an interdigitated monolayer structure in the LC mesophases. However, for the microgel suspensions, the d spacings of PNI12-3 and PNI14-3 are comparable to the values of their 2L, but the d spacings of PNI16-3 and PNI18-3 are still smaller than 2L. This shows that the alky chains in PNI12-3 and PNI14-3 stack into a head-to-head bilayer structure, probably due to the pulling effect of swelling in the suspensions. When the LC mesophases become more ordered in PNI16-3 and PNI18-3, the interdigitated monolayer structure is retained in the suspensions. The effect of temperature on the d spacing of the LC mesophase PNI18-3 was also investigated. The d spacings of the LC mesophases of all P(NIPAm/VIMnBr) microgels with VIMnBr/NIPAm molar ratio of 0.3 at various temperatures are summarized in Tables S2 and S3 and shown in Figure S10. Figure 9 shows temperature-variable SAXS profiles of freeze-dried PNI18-3 microgels and PNI18-3 microgel suspensions. One can see from Figure 9 and Table S3 that, for freeze-dried PNI18-3 microgels, the position of the SAXS peak is independent of the temperature up to 80 oC. However, for PNI18-3 microgel suspensions, the scattering peak shifts to lower q value when increasing the temperature from 26 – 70 oC. This shows that the d spacing of the LC mesophase of PNI18-3 microgel suspensions increases with increasing solution temperature. Luo et al. observed that the d spacing of mesophase of VIMnBr decreases steadily with increasing temperature, which is attributed to greater alkyl chain mobility upon increasing the temperature.34 However, for PNI18-3 microgel suspensions, increasing the temperature above its VPTT leads to the strong collapse of the microgels, which in contrast strongly restricts the mobility of alkyl side chains of comonomer VIM18Br, resulting in the increase of d spacing of the ordered structures (cf. Figure 9). 23


Langmuir

Table 2. Interlayer d-spacings of P(NIPAm/VIMnBr) microgels with VIMnBr/NIPAm molar ratio of 0.3 calculated from the SAXS profiles shown in Figure 8.

Microgel Suspensions

Freeze-dried Microgels

Sample Codes

A

d (nm)

q (nm-1)

d (nm)

L (nm)

1.98 1.71 1.75 1.66

3.18 3.67 3.59 3.78

2.29 1.80 1.73 1.70

2.74 3.49 3.63 3.70

1.53 1.79 2.04 2.30

PNI18-3 microgel suspensions

freeze-dried PNI18-3 microgels

1.6

1.7

1.8

1.9

-1

q (nm )

0.5

1.0

1.5

2.0

2.5


o

32 C o 40 C o 50 C o 60 C o 70 C o 80 C 1.5

Length of Alkyl Side Chain

q (nm-1) PNI12-3 PNI14-3 PNI16-3 PNI18-3


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 24 of 31

0.5

1.0

-1

1.5

o

26 C o 33 C o 40 C o 50 C o 60 C o 70 C

2.0

-1

q (nm )

q (nm )

Figure 9. Temperature-variable SAXS profiles of (A) freeze-dried PNI18-3 microgels and (B) PNI18-3 microgel suspensions. The wavelength of X-ray is 0.124 nm.

CONCLUSIONS P(NIPAm/VIMnBr) microgels with interior nano-ordered phases were obtained by surfactant free emulsion copolymerization of N-isopropylacrylamide (NIPAm) and ionic liquid comonomers, VIMnBr with long alkyl side chain, in aqueous solution at 70 ºC. The obtained microgels are spherical in shape with narrow size distributions, and the nanophases have a radius of about 8-12 nm and are randomly distributed inside the microgels. The hydrodynamic diameters of P(NIPAm/VIMnBr) microgels decrease with increasing the length n of alkyl side chain. The thermo-sensitive behavior of the P(NIPAm/VIMnBr) microgels is strongly affected by the cooperative competition of the hydrophilic quaternary vinylimidazole (VIM) moiety and hydrophobic long alkyl side chain. The aggregation of adjacent alkyl side chains of comonomer 24


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Langmuir

VIMnBr leads to the formation of nanophases with ordered structures inside the microgels. A layered crystalline structure with a low melting temperature and broad melting transition is identified for the nanophases. Above the melting temperature, the partially ordered structure of nanophases is attributed to a layered liquid crystalline phase. These results suggest that we can purposely design and synthesize the microgels with internal phase separated nanodomains, which possess different functionalities and order structures, by properly designing and choosing the monomer and comonomer.

Such microgels with different functional nanophases might find

applications in the areas where various functionalities are simultaneously needed.

25


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Page 26 of 31

ASSOCIATED CONTENT Supporting Information 1

H-NMR spectra of comonomers, P(NIPAm-co-VIMnBr) linear copolymers, additional SLS and

DLS data, DSC and SAXS data, POM image of microgels. This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. ORCID Binyang Du: 0000-0002-5693-0325 Junting Xu: 0000-0002-7788-9026 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS The authors thank the National Natural Science Foundation of China (Nos. 21674097 and 21322406), the second level of 2016 Zhejiang Province 151 Talent Project, and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry (201601), Changchun Institute of Applied Chemistry, Chinese Academy of Sciences for financial support. The authors also thank beamline BL16B1 at SSRF for providing the beam time and Prof. Pengju Pan, Zhejiang University, for sharing his beam time.

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Microgels with Internal Nanophase Separated Structures

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