Unusual Phase Transition Behavior of Poly(

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Unusual Phase Transition Behavior of Poly(N-isopropylacrylamide)co-Poly(tetrabutylphosphonium styrenesulfonate) in Water: Mild and Linear Changes in Poly(N-isopropylacrylamide) Part Ge Wang, and Peiyi Wu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00392 • Publication Date (Web): 29 Mar 2016 Downloaded from http://pubs.acs.org on March 31, 2016

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Unusual Phase Transition Behavior of Poly(N-isopropylacrylamide)-co-Poly(tetrabut ylphosphonium styrenesulfonate) in Water: Mild and Linear Changes in Poly(N-isopropylacrylamide) Part

Ge Wang, Peiyi Wu* State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Polymers and Polymer Composite Materials, Department of Macromolecular Science and Laboratory for Advanced Materials, Fudan University, Shanghai 200433, China

1

ACS Paragon Plus Environment

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ABSTRACT

In this paper, one LCST-type thermoresponsive poly(ionic liquid) (PIL), poly(tetrabutylphosphonium styrenesulfonate) (P[P4,4,4,4][SS]) was introduced to poly(N-isopropylacrylamide) (PNIPAM) by two different ways, mixing and copolymerization, respectively. Interestingly, they show distinct thermoresponsive phase transition behaviors, evidenced by temperature-variable 1H NMR and FT-IR in combination with perturbation correlation moving window (PCMW). The PNIPAM/P[P4,4,4,4][SS] mixture exhibits a sharp and drastic phase transition, similar to pure PNIPAM. While in the statistical copolymer, PNIPAM-co-P[P4,4,4,4][SS], the thermosensitivity of P[P4,4,4,4][SS] is largely suppressed resulting in a linear, mild and incomplete phase transition, which has never been reported before. This abnormal phenomenon is shown to arise from the the outstanding hydration ability of P[P4,4,4,4][SS]. Our findings should be conducive to deepen our understanding of the interaction between LCST-type polymers with distinct structures and provide a new perspective to prepare thermoresponsive materials with linear phase transition behavior.

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1. Introduction

Thermoresponsive polymers have attracted a great deal of interests owing to their thermal sensitivity and potential applications as “smart” materials in sensors1, biomedical materials2-4, separation5, 6 fields, to name but a few. According to their critical miscibility behaviors, traditional thermoresponsive polymers with lower critical solution temperature (LCST) can be divided into three types.7,

8

Poly(N-vinylcaprolactam) (PVCL)9, 10 is a typical example of Type I, which follows “classical” Flory-Huggins miscibility rule, in other words, the critical point shifts toward lower concentration with the chain length of the polymer increasing. The critical points of Type II are independent of the polymer chain length. The well-studied poly(N-isopropylacrylamide) (PNIPAM)11, 12 with an LCST of 32 oC is the typical Type II LCST polymer. Type III polymers are characterized by two off-zero limiting critical concentrations and a zero one, such as poly(vinyl methyl ether) (PVME)13, 14. However, these traditional LCST polymers are usually non-ionic or weakly-charged when dissolved in water. Recently, a new class of LCST polymers, thermoresponsive poly(ionic liquid)s (PILs), gain more and more interests.15 PILs are a kind of wholly-charged polymers composed of cations and anions. By adjusting the balance of hydrophilicity and hydrophobicity between cations and anions, LCST-type PILs can be obtained. As a milestone, poly(tetrabutylphosphonium styrenesulfonate) (P[P4,4,4,4][SS]) is the first LCST-type PIL reported by Ohno’s group in 2012.16 Almost at the same time, another anionic LCST-type PIL, poly(tributylhexylphosphonium 3-sulfopropylmethacrylate) (P[P4,4,4,6][MC3S]), was synthesized.17 Later on, Yuan’s 3


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group reported a cationic LCST-type PIL, poly(tributyl-4-vinylbenzylphosphonium pentanesulfonate) (TVBP-C5S).18 Our group also studied the LCST behaviors of both thermoresponsive IL monomers and PILs to elucidate their different dynamic phase transition mechanisms. Generally, PILs exhibit a weaker dehydration than their corresponding ILs.19,

20

Nevertheless, compared with numerous researches on

traditional LCST polymers, the study of LCST-type PILs is still lacking, especially the distinction between wholly-charged LCST polymers and the traditional ones is still ambiguous so far except their structural difference. Hence, it is necessary to make a comparison between traditional LCST polymers and LCST-type PILs to discern their different phase transition behaviors.

Physical mixing and copolymerization are two feasible strategies to compare the difference between two thermoresponsive systems. A few researches focused on the phase transition of physically mixed aqueous solution of two thermoresponsive homopolymers. For example, Spevacek’s group investigated a range of LCST-type mixture systems, including PVME/poly(N-isopropylmethacrylamide) (PNIPMAM), PNIPAM/PNIPMAM and PVCL/PNIPMAM, by temperature-variable

1

H NMR

spectroscopy. In PVME/PNIPMAM and PNIPAM/PNIPMAM systems, the phase transition of the lower-LCST component is not influenced by the phase separation of the higher-LCST one, while the phase transition of the higher-LCST component is affected by the lower-LCST component in the mixtures.21,

22

However, in

PVCL/PNIPMAM mixture, the transitions of PVCL and PNIPMAM both shift toward higher temperatures in comparison with neat polymers, indicating the existence of 4


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direct or indirect interactions between this two segments.23 Chen and co-workers investigated the aggregation process of PVME/poly(2-ethyl-2-oxazoline) (PEOZ) mixture in water by elastic light scattering spectroscopy and a model was proposed to describe the separate phase transitions of PVME and PEOZ.24 Our group also reported the

remarkable

distinctions

in

the

phase

transition

behaviors

of

poly(2-isopropyl-2-oxazoline) (PIPOZ)/PNIPAM and PIPOZ/PVCL mixures.25 Meanwhile,

studies

on

the

random

copolymers

composed

of

different

thermoresponsive units have been reported as well. Crespy et al synthesized a series of VCL and di(ethylene glycol methyl ether methacrylate) (MEO2MA) copolymers, which show fine-tuned cloud points ranged from 26 to 35 oC.26 Besides, several reports focused on the comparison of the phase separation behaviors of copolymers and physical mixtures. Djokpe and Vogt found the statistical copolymers of PNIPAM and PNIPMAM show a linear-changed cloud point depending on the copolymer content, while the physical mixtures of homopolymers exhibit two cloud points.27 Richtering’s group reported the thermosensitive properties of a diblock copolymer, poly(N,N-diethylacrylamide)-b-poly(N-isopropylacrylamide) (PDEAAM-b-PNIPAM), and compared it with their homopolymers, mixtures thereof, and the statistical copolymer. The properties of diblock copolymer resemble more the averaged properties of the homopolymers, while the mixture and the random copolymers exhibit non-ideal behavior leading to a synergistic depression of the cloud points.28 The comparison of LCST-transitions of homopolymer mixture, diblock and statistical copolymers of PNIPAM and PVCL was made by our group, and only one transition 5


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was observed in the investigated polymer systems.29 However, current researches are all limited to traditional LCST polymers and no relevant work to the wholly-charged LCST-type PILs has been reported yet. By physical mixing or copolymerization of LCST-type PILs and traditional LCST polymers, it would be very attractive to trace the phase transition process of this new system and clarify the difference between them.

Therefore, in this work, we attempt to investigate the detailed phase transition behaviors of PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] to reveal the different thermosensitive properties of a traditional LCST polymer and an LCST-type PIL for the first time. Between them, PNIPAM is the most familiar representative of traditional LCST polymers, whose phase separation undergoes a sharp coil–globule transition,11,

12

and P[P4,4,4,4][SS] is one of the mostly studied

anionic thermoresponsive PILs.16, 30-32 Moreover, the phase separation mechanism of P[P4,4,4,4][SS] has been clarified recently, which provides the convenience for studying the hybrid system involving LCST-type PILs.19 Through DSC, optical microscopy, temperature-variable 1H NMR and FT-IR in combination with perturbation correlation moving window (PCMW) technique, the detailed phase transition behaviors of PNIPAM-co-P[P4,4,4,4][SS] and PNIPAM/P[P4,4,4,4][SS] mixture can be elucidated, which is conducive to deepen our understanding of the thermosensitivity of LCST-type polymers with complex compositions.

2. Experimental Section

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2.1

Materials.

Tetrabutylphosphonium

bromide

([P4,4,4,4][Br]),

sodium-4-vinylbenzenesulfonate ([Na][SS]), N-isopropylacylamide (NIPAM) and 2,2’-azobis(2-isobutyronitrile) (AIBN) were purchased from Aladdin Reagent Co. Deuterium oxide (D2O, D-99.9%) was purchased from Cambridge Isotope Laboratories

Inc.

NIPAM

was

recrystallized

from

cyclohexane

prior

to

polymerization, while AIBN was recrystallized from ethanol.

2.1 Sample Preparation. The thermoresponsive IL monomer, [P4,4,4,4][SS], was prepared via ion exchange reaction of [Na][SS] and [P4,4,4,4][Br] in distilled water for 12 h.16, 19 The homopolymers (PNIPAM and P[P4,4,4,4][SS]) and statistical copolymer (PNIPAM-co-P[P4,4,4,4][SS]) used in this work were synthesized by free radical polymerization in methanol with AIBN as the initiator. A typical procedure for the preparation of the copolymer was as follows. NIPAM (0.5 g, 4.4 mmol), [P4,4,4,4][SS] (0.5 g, 1.1 mmol) and AIBN (0.003 g, 0.02 mmol) were placed in a Schlenk flask and dissolved in methanol (10 ml) with magnetic stirring. After three freeze–vacuum– thaw cycles, the flask was placed in a 70 oC hot bath for 12h. The product was purified via dialysis against distilled water (MWCO 14 000). The product was finally freeze-dried for 3 days. The GPC data of the synthetic polymers were presented in Figure S1.

The D2O solution (c = 20 wt%) of PNIPAM/P[P4,4,4,4][SS] mixture was prepared with a mass ratio of 1:1. The concentration of PNIPAM-co-P[P4,4,4,4][SS] solution is 20 wt% as well. Before experiments, all samples were placed at 4 oC for at least 2

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days to ensure complete dissolution.

2.2

Instruments

and

Measurements.

Calorimetric

measurements

of

PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] solutions were carried out using a Mettler-Toledo differential scanning calorimetry (DSC) thermal analyzer at a scanning rate of 10 oC min-1. The testing temperature ranged from 10 to 80 oC. Optical microscopic observations were performed on a Leica DM2500P polarizing microscope equipped with a Linkam THMS600 hot stage to control the sample temperature. The sensor accuracy of the hot stage is 0.1 oC and the heating rate is 0.5 o

C min-1. Temperature-variable 1H NMR spectra of PNIPAM/P[P4,4,4,4][SS] mixture

and PNIPAM-co-P[P4,4,4,4][SS] were recorded on a Varian Mercury plus (400 MHz) spectrometer using D2O as a solvent with an increment of 1 oC.

Temperature-resolved FT-IR spectra at different temperatures were recorded on a Nicolet Nexus 6700 spectrometer with a resolution of 4 cm-1, and 32 scans were available for an acceptable signal-to-noise ratio. The PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] solutions in D2O were sealed between two ZnS tablets. Temperature was controlled by an electronic cell holder at a rate of 0.5 oC min-1 with an increment of 1 oC (accuracy is 0.1 oC). The FT-IR spectra were collected from 25 to 50 oC for both PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] solutions. The collected FT-IR spectra were used to perform perturbation correlation moving window (PCMW) analysis. Data processing was performed with the method Morita provided using the software 2D Shige, ver. 1.3 (© Shigeaki Morita,

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Kwansei-Gakuin University, Japan) for the correlation calculation. Finally, the contour maps were plotted using Origin Program ver. 8.0 with red colors indicating positive intensities while blue colors the negative ones.

3. Results and Discussion

3.1 Calorimetric measurements and optical microscopy

The

transition

behaviors

of

PNIPAM/P[P4,4,4,4][SS]

mixture

and

PNIPAM-co-P[P4,4,4,4][SS] solutions were investigated by DSC, as shown in Figure 1. Interestingly, the exothermic peaks of PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] solutions are obviously different. The sharp exothermic peak of PNIPAM/P[P4,4,4,4][SS] mixture (width at half-peak of 3.0 oC) is similar to pure

PNIPAM33,

with

the

peak

point

at

ca.

33

o

C.

In

contrast,

PNIPAM-co-P[P4,4,4,4][SS] solution exhibits a weak asymmetric exothermic peak with a range of more than 25 oC, broader than most thermoresponsive polymers. The start point of this broad exothermic peak is about 32 oC, while the end point is difficult to identify. Hence, it is attractive to explore the two distinct enthalpy changes at the molecular level, which may be the key to understanding the dynamic nature of the continuous phase transition of PNIPAM-co-P[P4,4,4,4][SS] in water. Moreover, only one

phase

transition

temperature

can

be

observed

by

heating

both

PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] solutions. Since pure PNIPAM (ca. 32 oC) and P[P4,4,4,4][SS] (ca. 50 oC) solutions have different phase transition temperatures (in Figure S2 and Figure S3), there are two possibilities 9


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leading to this phenomenon: (1) only one thermoresponsive polymer (either PNIPAM or P[P4,4,4,4][SS]) undergoes LCST phase transition and release heat, while the phase transition of the other thermoresponsive polymer is suppressed or its LCST goes beyond 100 oC; (2) both PNIPAM and P[P4,4,4,4][SS] dehydrate together due to some strong interactions between them. In Figure S4, we measured the transition behavior of PNIPAM/P[P4,4,4,4][SS] mixture solutions with different weight ratios, while the concentration

of

P[P4,4,4,4][SS]

is

fixed.

When

the

weight

ratio

of

PNIPAM/P[P4,4,4,4][SS] is 1/10, two phase transition temperatures can be observed during heating. One is ca. 32 oC for PNIPAM, the other is ca. 61 oC for P[P4,4,4,4][SS]. With the weight ratio of PNIPAM to P[P4,4,4,4][SS] increasing, the transition temperatures of P[P4,4,4,4][SS] show an upward trend but the endothermic peak areas of P[P4,4,4,4][SS] tend to decrease. Once the weight ratio of PNIPAM to P[P4,4,4,4][SS] reaches 7/10, the endothermic peak of P[P4,4,4,4][SS] becomes invisible, as well for the weight ratio 10/10. Hence, the addition of PNIPAM would raise the LCST of P[P4,4,4,4][SS] and weaken the phase transition behavior of PIL at the same time. Moreover, according to later analysis, it is the PNIPAM component that undergoes phase transition, while the phase transition behavior of P[P4,4,4,4][SS] is more or less suppressed.

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Figure 1. DSC heating curves of PNIPAM/P[P4,4,4,4][SS] mixture (black, (PNIPAM/D2O

=

10

wt%,

P[P4,4,4,4][SS]/D2O

=

10

wt%))

and

PNIPAM-co-P[P4,4,4,4][SS] (red, c = 20 wt%) solutions at a scanning rate of 10 oC min-1.

To

directly

observe

the

heat-induced

phase

transition

behavior

of

PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] solutions, optical microscopy

was

employed.

The

optical

microscopic

photographs

of

PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] solutions are shown in Figure 2. For PNIPAM/P[P4,4,4,4][SS] mixture solution, the visual field dims significantly at 34 oC, indicating the sharp and drastic transition behavior. What’s more, liquid–liquid phase separation (LLPS) doesn’t occur, which is similar to pure PNIPAM solution. LLPS is a macroscopic phenomenon, which is common for many thermoresponsive polymers (like PVCL10 and PIPOZ34). Because of the complete dehydration of PNIPAM, LLPS has never been reported in PNIPAM aqueous 11


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solutions. As for PNIPAM-co-P[P4,4,4,4][SS] solution, the case is largely different. First, the brightness of visual field is almost invariable during heating; second, LLPS phenomenon occurs in PNIPAM-co-P[P4,4,4,4][SS] solution and the “droplets” formed are small and imperceptible. That is, PNIPAM-co-P[P4,4,4,4][SS] experienced a weak and continuous phase transition process instead of a sharp one and the polymer-rich droplets still retain some water molecules after an incomplete phase separation. The optical microscopic photographs of PNIPAM and P[P4,4,4,4][SS] solutions are shown in Figure S5 as the control data. The detailed phase transition mechanism will be revealed in the following temperature-variable 1H NMR and FT-IR analyses.

Figure 2. Optical micrographs of (a) PNIPAM/P[P4,4,4,4][SS] mixture (PNIPAM/D2O = 10 wt%, P[P4,4,4,4][SS]/D2O = 10 wt%) and (b) PNIPAM-co-P[P4,4,4,4][SS] in D2O (c = 20 wt%) in the heating process from 25 to 50 oC. 3.2 Temperature-variable 1H NMR analysis High-resolution 1H NMR spectra of (a) PNIPAM/P[P4,4,4,4][SS] mixture and (b) PNIPAM-co-P[P4,4,4,4][SS] in D2O are shown in Figure 3 with the assignment of 12


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various protons.16, 19, 22 We chose Hf in PNIPAM and Hd in P[P4,4,4,4][SS] to calculate the copolymer composition. In PNIPAM-co-P[P4,4,4,4][SS] solution, the value of Hf/Hd is 1.8:1 indicating the number ratio of NIPAM/[P4,4,4,4][SS] repeat units is 3.6:1, which is close to feed ratio (4:1). Additionally, We can clearly distinguish the characteristic peaks of P[P4,4,4,4][SS] (Ha in anion and Hb, Hc, Hd in cation) and PNIPAM (He and Hf), which are convenient for us to trace the change of different components of thermoresponsive polymers during the phase separation process.

Figure 3. (a) 1H NMR spectra of PNIPAM/P[P4,4,4,4][SS] mixture (black) and PNIPAM-co-P[P4,4,4,4][SS] (red) in D2O (25 oC) and the assignments of various protons.

Temperature-variable

1

H NMR measurement could easily track the phase

transition process of different components and quantitatively describe their phase transition

degrees.

The

temperature-variable

1

H

13


NMR

spectra

of

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PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] were presented in Figure 4 (a) and (b). The integrated intensities of HDO peak (4.79 ppm) have been normalized. All the peaks shift toward a higher chemical shift except the HDO peak during the phase transition. The intensities of some protons decrease during heating, indicating the formation of aggregates with protons being wrapped. However, the degrees of intensity decreasing are much different. In the dotted frames of Figure 4 (a), the He and Hf peaks almost disappear after phase transition (36 oC). While for PNIPAM-co-P[P4,4,4,4][SS], the corresponding peaks are always visible even at 48 oC. This reveals the sharp transition of PNIPAM/P[P4,4,4,4][SS] mixture and the gradual transition of PNIPAM-co-P[P4,4,4,4][SS], in good agreement to the DSC and optical microscopy results. As for P[P4,4,4,4][SS], the changes of peak intensity are very weak. Hence, it is necessary to quantitatively analyze the degree of phase transition for different protons.

The phase separation fraction p was employed to quantitatively estimate the degree of dehydration and phase transition. p is defined as

p = 1 − (I/I0)

(1)

In equation (1), I is the integrated intensity of the selected peak in 1H NMR spectra, while I0 is the integrated intensity of this peak before LCST. In this work, we took the integrated intensity obtained from the initial testing temperature (25 oC) as I0. Ha in the anion of P[P4,4,4,4][SS], Hd in the cation of P[P4,4,4,4][SS] and He, Hf in PNIPAM were chosen for analysis. The temperature dependences of phase separation 14


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fraction

p

were

plotted

for

both

PNIPAM/P[P4,4,4,4][SS]

mixture

and

PNIPAM-co-P[P4,4,4,4][SS] in Figure 4 (c) and (d). In PNIPAM/P[P4,4,4,4][SS] mixture, PNIPAM shows a complete (pmax ~ 0.95) and drastic (less than 5 oC) phase transition, similar to pure PNIPAM. The unchanged p (~ 0) of Hd implies that the cations of P[P4,4,4,4][SS] have no temperature responsiveness in this temperature range. Interestingly, an unusual over-hydration behavior (pmin ~ -0.6) occurs in the anions of P[P4,4,4,4][SS] as a result of the competition of water molecules for hydrogen bonds. This phenomenon has also been reported in a ternary composite system composed of PNIPAM, LCST-type ionic liquid and water.35 Since P[P4,4,4,4][SS] experiences an anion dominated phase transition process19,

20

, the over-hydrated anions cannot

provide enough hydrophobic interaction to induce the phase separation of P[P4,4,4,4][SS]. As for PNIPAM-co-P[P4,4,4,4][SS], the case is much different. It is well-known that PNIPAM usually undergoes a drastic coil–globule transition in homopolymer or copolymers.11,

12, 29, 36

However, the PNIPAM component in

PNIPAM-co-P[P4,4,4,4][SS] exhibits a weak (pmax ~ 0.5), gradual (more than 20 oC) and linear-like transition, which has never been reported before. Without showing the over-hydration phenomenon in the anions due to the covalent bonds, P[P4,4,4,4][SS] doesn’t exhibit thermo-responsiveness during heating as well. In other words, it is PNIPAM that exhibits temperature responsiveness in both PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] solutions, while the phase transition of P[P4,4,4,4][SS] is suppressed. This also explains that only one exothermic peak can be

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found in the DSC curves. The control data of temperature-variable 1H NMR for pure PNIPAM and P[P4,4,4,4][SS] solution can be seen in Figure S6.

Figure

4.

Normalized

temperature-variable

1

H

NMR

spectra

of

(a)

PNIPAM/P[P4,4,4,4][SS] mixture (PNIPAM/D2O = 10 wt%, P[P4,4,4,4][SS]/D2O = 10 wt%), (b) PNIPAM-co-P[P4,4,4,4][SS] in D2O (c = 20 wt%) and their temperature dependence of the phase separated fraction p for different protons in (c) and (d). Moreover, the above 1H NMR data could uncover the distinction between traditional LCST polymers and LCST-type PILs. Unlike traditional LCST polymer showing thermosensitivity only, the LCST-type PIL is the “sweet spot” of two properties. One is the thermosensitivity derived from the balance of hydrophilicity

16


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and hydrophobicity between cations and anions, and the other is the outstanding hydration ability due to their wholly-charged structure. This hypothesis could account for many experimental phenomena. For example, the phase transition degree of LCST-type PIL is usually very low (pmax < 0.5)

19, 20, 35

indicating their incomplete

dehydration, while that of traditional LCST polymer is much higher (pmax > 0.9)10, 21-23, 25, 36

. In this work, after mixing or copolymerizing with PNIPAM (traditional LCST

polymer), P[P4,4,4,4][SS] (LCST-type PIL) shows more its hydration ability instead of thermosensitivity. The over-hydration behavior of PIL in PNIPAM/P[P4,4,4,4][SS] mixture is an evidence of its outstanding hydration ability that PIL captures expelled water from PNIPAM. On the other hand, the hydration ability of PIL in turn weakens the sharp and complete phase transition of PNIPAM, which has been reported before in a random copolymer system consisting of PNIPAM and non-thermosensitive PIL.37 It is speculated that the mild and linear transition of PNIPAM-co-P[P4,4,4,4][SS] is the balance between the sharp transition of PNIPAM and the good hydration ability of P[P4,4,4,4][SS], in the case of copolymer composition. The unusual weak and linear transition of PNIPAM component in PNIPAM-co-P[P4,4,4,4][SS] can be confirmed in the following FT-IR analysis.

3.3 Conventional FT-IR analysis

To deeply understand and compare the dynamic phase transition process of PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS], temperature-variable FTIR spectra have been collected. In order to eliminate the overlap of the δ(O-H)

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band at around 1640 cm-1 with v(C=O) of PNIPAM as well as the broad v(O-H) with the v(C-H) band of the polymer, we used D2O instead of H2O as the solvent. We also collected temperature-variable FTIR spectra of pure PNIPAM and P[P4,4,4,4][SS] solutions for comparison, as shown in Figure S7. Here, two spectral regions v(C-H) (3010-2840 cm-1) and v(C=O) (1670-1580 cm-1) are traced during the phase transition. As shown in Figure 5, it is obvious that the spectral variations of the chemical groups in PNIPAM/P[P4,4,4,4][SS] mixture is much steeper and larger than those in PNIPAM-co-P[P4,4,4,4][SS]. All the frequencies of C-H groups shift to lower wavenumbers with temperature rising. It is reported that hydrophobic moieties of water-soluble polymers are surrounded by well-ordered water clathrates resulting in higher vibrational frequency.38, 39 Thus, the C-H moieties undergo dehydration process, which is common in the phase separation process of thermoresponsive polymers in water. As for C=O, bidirectional spectral intensity changes that the intensity of v(C=O···D2O) (1625 cm-1) decreases while a new intra-/inter-molecular hydrogen bond v(C=O···D-N) (1650 cm-1, D-N represents the deuterated amide groups in PNIPAM after deuteration) can be observed. After careful observation, we can find that there exists an isosbestic point in C=O region of the overlaid spectra. In general, the appearance of an isosbestic point means that one species is quantitatively converted to another single species.40 In the case of PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] solutions, it reveals that the changes of C=O in PNIPAM take place between two single states without intermediate states. 18


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Figure 5. Temperature-dependent FT-IR spectra of (a) PNIPAM/P[P4,4,4,4][SS] mixture (PNIPAM/D2O = 10 wt%, P[P4,4,4,4][SS]/D2O = 10 wt%) and (b) PNIPAM-co-P[P4,4,4,4][SS] in D2O (c = 20 wt%) in the regions of 3010-2840 cm-1 (top) and 1670-1580 cm-1 (bottom) during heating between 25 oC and 50 oC. The temperature increment interval is 1 oC with a heating rate of 0.5 oC min-1.

To

quantitatively

describe

the

distinct

phase

transition

behavior

of

PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] during heating, the frequency shifts of vas(CH3), vas(CH2), vs(CH2) and the integral area of C=O were plotted in Figure 6 and Figure 7, respectively. The frequencies of vas(CH3), vas(CH2), vs(CH2) shift to lower wavenumbers drastically and uniformly at 33

o

C in

PNIPAM/P[P4,4,4,4][SS] mixture solution. While the wide transition range of

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PNIPAM-co-P[P4,4,4,4][SS] confirm that the dehydration process of C-H is gradual and continuous. What’s more, the peak positions of all the C-H groups in PNIPAM-co-P[P4,4,4,4][SS]

are

higher

than

the

corresponding

peaks

in

PNIPAM/P[P4,4,4,4][SS] mixture, which reconfirms the outstanding hydration ability of PIL. Furthermore, to quantitatively analyze the hydration state variations during the phase transition process, we calculated the shifts of C-H groups. The shifts of vas(CH3), vas(CH2) and vs(CH2) in PNIPAM/P[P4,4,4,4][SS] mixture are 3.6, 1.9 and 2.5 cm-1, respectively, which are larger than those in PNIPAM-co-P[P4,4,4,4][SS] (2.8, 1.1 and 1.7 cm-1). Hence, we come to a conclusion that the PNIPAM/P[P4,4,4,4][SS] mixture experiences a more complete dehydration process than PNIPAM-co-P[P4,4,4,4][SS], which is coincident with the 1H NMR analysis.

Figure 6. Temperature-dependent frequency shifts of (a) vas(CH3), (b) vas(CH2) and (c) vs(CH2) of PNIPAM/P[P4,4,4,4][SS] mixture (black) and PNIPAM-co-P[P4,4,4,4][SS] (red) solutions during heating process.

The integral area of C=O group was presented in Figure 7 in the regions of 1625 to 1580 cm-1 and 1670 to 1650 cm-1, which were assigned to hydrated C=O and C=O···D-N,

respectively.

In

Figure

7

(a),

the

20


S-shaped

curves

of

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PNIPAM/P[P4,4,4,4][SS] mixture exhibit the sharp transition of C=O in PNIPAM as well as an apparent phase transition temperature (Tp) at 33 oC. While in Figure 7 (b), the

normalized

integral

areas

of

hydrated

C=O

and

C=O···D-N

in

PNIPAM-co-P[P4,4,4,4][SS] were linearly dependent on temperature with the linear correlation coefficient R2 more than 0.995 (R2 = 1 signifies completely linear correlation), which has never been reported before. We can find out a turning point at 32 oC in these linear curves, which directly corresponds to the start point of phase transition in DSC analysis. However, it is difficult to figure out the specific Tp of PNIPAM-co-P[P4,4,4,4][SS]. Here, PCMW may show its advantage.

Figure 7. Integral area of (a) PNIPAM/P[P4,4,4,4][SS] mixture (PNIPAM/D2O = 10 wt%, P[P4,4,4,4][SS]/D2O = 10 wt%) and (b) PNIPAM-co-P[P4,4,4,4][SS] (c = 20 wt%)

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solutions in the regions of hydrated C=O (top, 1625-1580 cm-1) and C=O···D-N (1670-1645 cm-1) vs temperature during heating process.

3.4 Perturbation correlation moving window

Perturbation correlation moving window (PCMW) is a new technique, whose basic principles was proposed by Thomas and modified by Richardson41, and then improved by Morita42 through introducing the perturbation variable into the correlation equation. According to this technique, we could monitor the complicated spectral variations along the perturbation direction and determine transition points as well.

Figure

8

presents

PCMW synchronous

and

asynchronous spectra

of

PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] during the phase transition process. PCMW synchronous spectra provide convenience for us to find transition points whereas asynchronous spectra are useful to confirm transition temperature regions by peaks with the strongest intensities. With regard to PNIPAM/P[P4,4,4,4][SS] mixture, the transition point of v(C=O···D-N) is about 33 oC, which is slightly lower than that of v(hydrated C=O) (34 oC). Additionally, the transition temperature ranges of C=O is determined between 30 to 36 oC. In consideration of PNIPAM-co-P[P4,4,4,4][SS], we can clearly capture the Tp of v(C=O···D-N) and v(hydrated C=O) at 39 oC and 40 oC, which is difficult to determine using other characterization methods. The two low-temperature peaks in asynchronous spectra indicate the start point is about 32 oC. It is noted that 22


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high-temperature peaks are weak compared with the low-temperature ones in PNIPAM-co-P[P4,4,4,4][SS], different from PNIPAM/P[P4,4,4,4][SS] mixture. This phenomenon reveals that the dehydration process PNIPAM-co-P[P4,4,4,4][SS] does not finish under our experimental conditions (more than 18 oC). Therefore, the PCMW analysis

can

indirectly

prove

the

mild

and

continuous

synchronous

and

asynchronous

dehydration

of

PNIPAM-co-P[P4,4,4,4][SS].

Figure

8.

PCMW

spectra

of

(a)

PNIPAM/P[P4,4,4,4][SS] mixture and (b) PNIPAM-co-P[P4,4,4,4][SS] solutions during heating. Herein, warm colors (red and yellow) denoted positive intensities, whereas cool colors (blue) denoted the negative ones.

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3.5 Proposed phase transition dynamic mechanisms of PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] solutions

Based on the above analysis, we can understand the phase transition dynamic mechanisms of PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS] solutions, as well as the distinction between traditional LCST polymers and LCST-type PILs. The schematic illustration is presented in Figure 9.

In general, the thermosensitivity of P[P4,4,4,4][SS] is suppressed due to the existence of PNIPAM. PNIPAM in PNIPAM/P[P4,4,4,4][SS] mixture experiences a sharp and complete

phase transition process, similar to pure PNIPAM, while in

PNIPAM-co-P[P4,4,4,4][SS] the PNIPAM component shows a linear, mild and incomplete phase transition, which has never been reported before. Compared with traditional LCST polymer (PNIPAM as a representative), LCST-type PIL (P[P4,4,4,4][SS] as a representative) not only possesses thermosensitivity but it also has an outstanding hydration ability. For PNIPAM/P[P4,4,4,4][SS] mixture solution, when temperature reaches 33 oC, PNIPAM loses bonded water rapidly to form dense aggregates and P[P4,4,4,4][SS] is forced to over-hydrate at the same time. In the case of PNIPAM-co-P[P4,4,4,4][SS] solution, the excellent hydration ability of P[P4,4,4,4][SS] suppressed the sharp and complete phase transition of PNIPAM resulting in linear variations. Due to the covalent linking of PNIPAM and P[P4,4,4,4][SS] components, P[P4,4,4,4][SS] repeating units could be distributed in the periphery of loose globules formed by partly dehydrated PNIPAM repeating units.

24


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Figure 9. Schematic illustration of the dynamic phase transition mechanism of PNIPAM/P[P4,4,4,4][SS] mixture (top) and PNIPAM-co-P[P4,4,4,4][SS] (bottom) solutions during heating.

4. Conclusion

In summary, in this paper, the thermodynamic phase transition behaviors of PNIPAM/P[P4,4,4,4][SS] mixture and PNIPAM-co-P[P4,4,4,4][SS], as well as the distinction between traditional LCST polymers and LCST-type PILs were investigated.

We found the thermosensitivity of P[P4,4,4,4][SS] is suppressed due to the existence of PNIPAM. PNIPAM in PNIPAM/P[P4,4,4,4][SS] mixture experiences a sharp and complete

phase

transition

process,

similar

to

pure

PNIPAM,

while

in

PNIPAM-co-P[P4,4,4,4][SS] the PNIPAM component shows a linear, mild and 25


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incomplete phase transition, which is unusual for PNIPAM-related systems. Different from traditional LCST polymer, LCST-type PIL locates in the “sweet spot” of thermosensitivity and outstanding hydration ability. This work shall be conducive to deepen our understanding of the interactions between LCST-type polymers with distinct structures and provide a new perspective to prepare thermoresponsive materials with linear phase transition behavior.

Associated Content

Supporting Information Available. [GPC of PNIPAM, P[P4,4,4,4][SS] and PNIPAM-co-P[P4,4,4,4][SS], DSC heating curves of PNIPAM and P[P4,4,4,4][SS] solutions. Control data of DSC thermograms, variable temperature 1H NMR and FTIR, and optical microscopy for pure PNIPAM and P[P4,4,4,4][SS] solutions.]

Author Information

Corresponding Author

*E-mail: [email protected] (P.Wu)

Acknowledgements

We gratefully acknowledge the ﬁnancial support from the National Science Foundation of China (NSFC) (No 21274030 and 51473038) .

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For Table of Contents Entry

Schematic illustration of the dynamic phase transition mechanism of PNIPAM/P[P4,4,4,4][SS] mixture (top) and PNIPAM-co-P[P4,4,4,4][SS] (bottom) solutions during heating.

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Unusual Phase Transition Behavior of Poly(

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