CO2-Switchable Multi-Stimuli-Responsive Polymer Nanoparticle

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CO2-Switchable Multi-Stimuli-Responsive Polymer Nanoparticle Dispersion Yeong-Tarng Shieh, Fang-Zi Hu, and Chih-Chia Cheng ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00237 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Nano Materials

CO2-Switchable Multi-Stimuli-Responsive Polymer Nanoparticle Dispersion Yeong-Tarng Shieha*, Fang-Zi Hua, Chih-Chia Chengb* a. Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 81148, Taiwan. E-mail: [email protected] b. Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 10607, Taiwan. E-mail: [email protected]

KEYWORDS. Nanoparticles; Carbon dioxide switchable emulsifiers, Emulsification/demulsification, Multiple stimuli-responsive polymeric materials; Reversible phase transitions.

ABSTRACT. CO2-switchable multiple stimuli-responsive polymer nanoparticles (MSPNs), a combination of protonated poly(2-dimethylamino-ethylmethacrylate) (PDMAEMA) and hydrophobic poly(methyl methacrylate) (PMMA), were prepared by carbon dioxide (CO2)assisted emulsion polymerization. These MSPNs exhibited excellent thermal properties and unique temperature/pH-responsive and CO2/nitrogen (N2)-switchable abilities, making them highly attractive multifunctional polymer nanoparticles with potential for many applications. Importantly, dispersion experiments and morphological studies clearly confirmed these newly-

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developed

nanoparticles

not

only

possess

efficient,

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reversible

CO2/N2-switchable

aggregation/redispersion ability, but can also remain in stable colloidal dispersion at low pH and promote rapid precipitation of nanoparticles at high pH. With simple preparation and high reproducibility, this approach provides a potentially novel pathway for the development of nextgeneration multifunctional MSPNs.

INTRODUCTION

In order to address the complex requirements of environmental systems, multiple stimuliresponsive polymeric materials (MSPMs) have recently received a great deal of attention in multifunctional

nanoparticle

research.

MSPMs

allow

efficient

manipulation

of

hydrophilic/hydrophobic transitions in aqueous solution, as they respond to changes of external environment, such as changes in temperature, pH, light, gas, and magnetic and electric fields.1-5 Compared to general single-stimulus responsive polymers, MSPMs possess more intriguing physical properties as the desired functions can be achieved by tuning environmental parameters, which results in well-controlled modulation of the MSPMs.6-7 Poly(2-dimethylaminoethylmethacrylate) (PDMAEMA) is one of the frequently used existing cationic MSPMs and has been widely applied for development of effective drug delivery systems and surface-modified biomaterials.8-11 PDMAEMA aqueous solution undergoes phase separation when heated above its lower critical solution temperature (LCST) or cloud point (CP), which is around 35 °C under basic conditions (pH 10) and around 47 °C at neutral (pH 7), though PDMAEMA exhibits no CP behavior under acidic conditions (pH 4).12 In addition to temperature- and pH-responsiveness, PDMAEMA is also carbon dioxide (CO2)-responsive.13 In the last decade, CO2 has been reported


2

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to function as a ‘green’ stimulus that can transform long-chain alkyl amidine compounds in water into charged surfactants for styrene-in-water emulsion polymerization.14 Subsequent bubbling of nitrogen (N2), argon (Ar) or air through amidinium bicarbonate solutions removes the CO2,15 reversing the reaction and thus breaking the emulsion. PDMAEMA containing a tertiary amine group, which is less basic than the amidine group in water, can also be charged by protonation via the carbonic acid (H2CO3) formed in solution during CO2 bubbling. Therefore, the water solubility of PDMAEMA increases, resulting in an increase in its CP.13 Bubbling N2 or Ar can remove CO2 and deprotonate PDMAEMA, reducing its solubility and CP in water.16 However, at environmental temperatures below its CP, PDMAEMA in aqueous solution tends to strongly interact with water molecules, resulting in formation of extended random coil conformations.17,18 Therefore, identification of a suitable synthetic route to promote the formation of uniform PDMAEMA nanoparticles in aqueous solution at temperatures below its CP remains a major challenge. As a possible solution to this challenge, PDMAEMA can be easily converted into cationic polymers due to its ability to rapidly respond with hydrochloride (HCl) or by protonation of the tertiary amine side groups in PDMAEMA during bubbling CO2,19 which leads to formation of hydrophilic side groups on the hydrophobic polymer backbone; this endows surface activity, enabling PDMAEMA to serve as a surfactant (or stabilizer) in emulsion polymerizations. The latexes in the emulsion polymerization aggregate on either adding base to neutralize the protonated tertiary amine or bubbling N2 to remove CO2 to deprotonate the amine, and can be redispersed by either adding acid or bubbling CO2.20 To demonstrate how CO2-switchable surfactants work, Cunningham's group prepared colloidal latexes of polystyrene and poly(methyl methacrylate) (PMMA) by emulsion polymerization using cationic amidine-based switchable


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surfactants. Destabilization of the latexes for aggregation required only air (or N2) and heat to remove CO2 from the system and switch the active amidinium bicarbonate surfactant to a surface inactive amidine compound.21 Aggregated polystyrene latexes can be redispersed using CO2 and sonication.22 Aryl amidine and tertiary amines, which are more basic than alkyl amidine, also performed well as CO2-switchable surfactants. Destabilization of the latexes occurred much more rapidly in the presence of more basic aryl amidine- and tertiary amine-based surfactants than alkyl amidine surfactants.23 Based on these findings, PDMAEMA can be suggested to be used as a stabilizer for CO2-assisted emulsion polymerization in aqueous solution that can directly react with hydrophobic monomers to form a variety of latex polymers. Bubbling CO2 or N2 into the latex solutions may manipulate the hydrophobic and hydrophilic properties of the resulting nanoparticles in aqueous environments. Therefore, the emulsion copolymerization of DMAEMA and hydrophobic monomers can result in the formation of PDMAEMA-based copolymeric nanoparticles at a temperature below the CP. Recently, colloidal nanoparticles have been produced using amphiphilic polymers as emulsifying agents during aqueous emulsion polymerization. Many studies have explored copolymers with hydrophilic polymer grafts, and have demonstrated the stability of the oil-inwater emulsion increases with the proportion of amphiphilic polymer and its molecular weight.2427

Amphiphilic polymers can prepared by block copolymerization of a hydrophobic monomer

(e.g., styrene or methyl methacrylate [MMA]) with a tertiary amine-containing methacrylate monomer [for instance, DEAEMA] that becomes hydrophilic after protonation by CO2 bubbling. This block copolymer can also function as a CO2-switchable surfactant. For instance, Zhu and coworkers protonated PDMAEMA-b-PMMA with HCl to confer amphiphilic behavior for a use as a surfactant in PMMA emulsion polymerizations. The PMMA latexes obtained readily


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coagulated on addition of trace amounts of caustic soda and could be redispersed in water to form stable latexes via CO2 bubbling with sonication. The recovered latexes could then be coagulated by N2 bubbling with gentle heating.28 Cunningham and coworkers blockcopolymerized a small amount of DEAEMA with styrene or MMA via sequential reversible addition-fragmentation chain transfer (RAFT) copolymerization in water under a CO2 atmosphere.29 The initially formed block (PDEAEMA) was CO2-responsive and could be used in situ as a stabilizer in subsequent emulsion polymerization of styrene in water under a CO2 atmosphere. The obtained latexes could be coagulated by N2 bubbling and subsequently redispersed by CO2 bubbling.29 The aggregation and redispersion behavior of the CO2-switchable latexes could be tuned via altering the proportion of PDMAEMA blocks as stabilizing moieties.30-34 These findings confirm PDMAEMA-based amphiphilic nanoparticles exert a strong

influence

on

CO2/N2-triggered

emulsification/demulsification

and

reversible

hydrophilic/hydrophobic behavior and–based on the external environmental conditions–the ability to generate rapid, stable, programmable MSPMs. We confidently hypothesized that a hydrophobic polymer could be physically incorporated within protonated PDMAEMA via aqueous emulsion polymerization processes to enable the formation of uniformly structured MSPNs, which would drastically impact the self-assembly behavior and amphiphilic features of the resulting nanoparticles in aqueous environments. In this study, we used PDMAEMA as a CO2-responsive surfactant for emulsion polymerization of MMA in water in the presence of CO2 at 50 °C using hydrophobic azobisisobutyronitrile (AIBN) as an initiator. We prepared three different molecular weights of PDMAEMA with narrow distributions via atom transfer radical polymerization (ATRP). The presence of CO2 in water during emulsion polymerization of MMA made PDMAEMA water-soluble and served as an


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emulsifier for hydrophobic droplets of MMA during stirring. Furthermore, we investigated the effects of the molecular weight of PDMAEMA on emulsion polymerizations of MMA and the CO2-switchable behavior of PMMA particles. To the best of our knowledge, this is the first report of well-defined protonated PDMAEMA, of the desired molecular weights with narrow dispersity, covering PMMA nanoparticles in aqueous solution exhibiting behavior of reversible CO2/N2-switchable aggregation/redispersion. Other studies used either block copolymers where one of the blocks was PDMAEMA, or a small amount (~1%) of a CO2-switchable surfactants2834

and did not investigate the effects of the molecular weights and dispersity of PDMAEMA on

the CO2 switchability of the synthesized particles. Therefore, this newly-developed synthetic system offers a highly efficient route for the fabrication of highly effective MSPNs that enable more precise control over the composition, physical properties and environmentally responsive performance of nanoparticles in aqueous solution.

EXPERIMENTAL SECTION

Chemicals, instrumentation, and some characterization procedures are described in detail in the Supporting Information section. ATRP of DMAEMA For preparation of high molecular weight PDMAEMA, a mixture of DMAEMA (14 g, 0.09 mol), CuBr (0.13 g, 9 × 10-4 mol) and PMDETA (0.31 g, 1.8 × 10-3 mol) in 22 mL of EtOH/H2O (1/1 v/v) was degassed via three freeze–pump–thaw nitrogen cycles. MBIB (0.08 g) was then injected into the mixture using a syringe to initiate ATRP at room temperature. The molar ratio of DMAEMA/CuBr/PMDETA/MBIB was 100/1/2/0.5. To prepare medium and low molecular


6

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weight PDMAEMA, 0.16 and 0.32 g of MBIB were injected, corresponding to molar ratios of DMAEMA/CuBr/PMDETA/MBIB of 100/1/2/1 and 100/1/2/2, respectively. Each reaction mixture was magnetically stirred at room temperature for 115, 92, and 68 h for the high, medium, and low molecular weight preparations of PDMAEMA, denoted h-PDMAEMA, mPDMAEMA, and l-PDMAEMA, respectively. For purification, the mixture was filtered twice through aluminum oxide to remove CuBr/PMDETA, the filtrate was concentrated using a rotary evaporator, 150 mL n-hexane was added, and the clear solution in which unreacted DMAEMA was dissolved was decanted. To completely remove residual unreacted DMAEMA, 150 mL fresh n-hexane was added and the clear solution was decanted. The settled product was dried in a vacuum oven for 6 h to obtain PDMAEMA. The yields of h-PDMAEMA, m-PDMAEMA, and lPDMAEMA were 73.5, 75, and 80 wt%, the weight average molecular weights (Mw) were 56,500, 29,500, and 14,000 g/mol, and dispersity values were 1.3, 1.3, and 1.2, respectively, as determined by gel permeation chromatography (GPC).

CO2-assisted emulsion polymerization of MMA Typical emulsion polymerization of PDMAEMA/PMMA nanoparticles is shown in Scheme 1. Emulsion polymerization of MMA was conducted in 500 mL in a 4-neck flask equipped with mechanical stirring and a condenser. PDMAEMA (1.28 or 2.4 g) was added to 160 g of deionized water in the flask and the aqueous solutions (0.8 or 1.5 wt% PDMAEMA) were heated to 50 °C under CO2 bubbling to complete dissolution. AIBN (0.77 g, 4.69 × 10−3mol) dissolved in MMA (18.8 g, 0.19 mol) was injected through the septum into the flask reactor to initiate emulsion polymerization of MMA at 50 °C with stirring at 200 rpm for 3 h. The protonation of


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PDMAEMA in water under CO2 to maintain dissolution of PDMAEMA at 50 °C and CO2assisted emulsion polymerization of MMA initiated by AIBN are illustrated in Scheme 1.

Scheme 1. Schematic diagram illustrating polymerization of monodispersed PDMAEMA-coated PMMA nanoparticles.

RESULTS AND DISCUSSION

Synthesis and phase-transition behavior of PDMAEMA. The main synthetic procedures and polymeric structures for the high, medium, and low molecular weight preparations of PDMAEMA (denoted h-PDMAEMA, m-PDMAEMA, and l-PDMAEMA, respectively) are presented in Scheme 1. These polymers were successfully synthesized by copper-mediated ATRP, as described in detail in the Experimental Section. The characterizations of these polymers were described in Supporting Information. Figure S1 (see Supporting Information)


8

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presents the GPC curves for all synthesized PDMAEMAs obtained via ATRP. The numberaverage molecular weight (Mn), mass-average molecular weight (Mw) and dispersity values determined from Figure S1 are listed in Table 1. The dispersity ranges for each polymer produced by ATRP exhibited narrow distributions between 1.2 and 1.3. The Mn and Mw of the three PDMAEMA preparations followed the simple kinetics of living radical polymerization, as the molecular weight of polymer was inversely proportional to the amount of initiator. The molar ratio of initiator in h-PDMAEMA, m-PDMAEMA, and l-PDMAEMA was 1, 2, and 4, respectively, almost exactly inversely proportional to the Mn and Mw ratios for the three PDMAEMA preparations. These observations indicate that the ATRP of DMAEMA exhibited living polymerization characteristics and well-defined water-soluble PDMAEMA was successfully synthesized. Table 1. Mn, Mw and dispersity obtained from GPC for various PDMAEMA and PDMAEMA/PMMA preparations. Sample

Mn

Mw

Dispersity

l-PDMAEMA

11,000

14,000

1.2

m-PDMAEMA

21,200

29,500

1.3

h-PDMAEMA

41,000

56,500

1.3

Peak 1

439,300

632,300

1.43

Peak 2

11,700

14,100

1.2

Peak 1

480,600

668,200

1.39

Peak 2

22,000

29,000

1.31

Peak 1

425,300

562,800

1.32

Peak 2

29,000

32,200

1.1

l-PDMAEMA/PMMA

m-PDMAEMA/PMMA

h-PDMAEMA/PMMA


9


(a)

(b)

(c)

(d)

Page 10 of 35

Figure 1. Visible transmittance at 550 nm at different pH values as function of temperature (from high to low) for aqueous solutions of (a) h-PDMAEMA, (b) m-PDMAEMA and (c) lPDMAEMA. The CP assigned from the inflection point on each curve in (a, b, c) was plotted as a function of pH in (d) for the aqueous solutions.

In order to determine the amount of PDMAEMA needed to effectively stabilize emulsion polymerizations of MMA, the surface tension of PDMAEMA in water was determined by measurements on pendant drops of the aqueous solutions. The surface tension of the PDMAEMA aqueous solutions decreased as the concentration of PDMAEMA increased up to 0.8 wt%, above which the surface tension leveled off (Figure S2), indicating PDMAEMA confers surface activity in water. The molecular weight of PDMAEMA insignificantly affected


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the concentration-dependent surface tension. The concentration of 0.8 wt% PDMAEMA can be considered the critical micelle concentration (CMC). Thus, we used the three molecular weights of PDMAEMA at the CMC (i.e., 0.8 wt%) and, for comparison, at concentrations above the CMC (1.5 wt%) as stabilizers for emulsion polymerizations of MMA. To further evaluate the effects of different Mw of PDMAEMA on CP behavior, typical turbidity experiments were performed at 1 wt% by monitoring transmittance at a fixed wavelength (550 nm) between 25 °C and 60 °C. Figure 1 shows the CP values of the three different molecular weights of PDMAEMA in water. The CP of PDMAEMA increased as pH decreased up to pH 7.2, at which point the three PDMAEMA solutions did not exhibit CP behavior, indicating that protonation of the tertiary amine groups in PDMAEMA, due to the reduction in pH, which increased the hydrophilicity and thus solubility of the polymer in water. Although we only tested pH values up to pH 11.2, CP can be expected to continue to decrease as pH increases further. Moreover, at each pH, the CP of PDMAEMA decreased as molecular weight increased (Figure 1d). This can be associated with increased entanglement of higher molecular weight polymer chains, which promotes aggregation and thus lowers the CP. In other words, increasing the molecular weight significantly reduced the CP of PDMAEMA in aqueous solution, possibly as a result of decreased hydration interactions between water molecules and the polar groups in PDMAEMA. Table 2. Compositions of the synthesized PDMAEMA-coated PMMA particles. DMAEMA/MMA (mol/mol)

PDMAEMA/PMMA (wt/wt)

h-PDMAEMA/PMMA

10/90

15/85

m-PDMAEMA/PMMA

11/89

17/83

l-PDMAEMA/PMMA

11/89

17/83


11


e CH dBr

O CH3

H3C

g

3

CH2

O

n CH3

CDCl3

CH2

bH C

a

N H 3C

c

O

O

f

CH3

a

c

i

j

TMS

H2O

2

(A)

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b

g k

CH3

CH2

H2O

n O

O

(B)

CH3

f d

e

i

j

k

H2O

k (C)

a

g

j

b c

i,d f

k (D) (E)

8

7

6

5

a

g

b c

a

k g

b c

4

3

j

f f

2

i,d i,d

1

0

Chemical shift (ppm)

Figure 2. 1H NMR spectra of (A) PDMAEMA, (B) PMMA, (C) l-PDMAEMA/PMMA, (D) mPDMAEMA/PMMA, and (E) h-PDMAEMA/PMMA in CDCl3.

CO2-assisted aqueous emulsion polymerization of hydrophobic monomers. We further explored both the surfactant-activating ability and emulsion-polymerization control ability of PDMAEMA using MMA as a hydrophobic monomer. Protonation of PDMAEMA in water


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under CO2 to maintain dissolution of PDMAEMA and subsequent emulsion polymerization of MMA were carried out in the presence of AIBN as an initiator at 50 °C, as illustrated in Scheme 1. After 3 h polymerization, the final latex products were obtained without further purification. To understand the compositional fraction of the PMMA and PDMAEMA moieties, we generated proton nuclear magnetic resonance (1H NMR) spectra for all samples in deuterated chloroform (CDCl3) at 25 °C. From the 1H NMR spectra of pure PMMA, pure PDMAEMA and three PDMAEMA-coated PMMA preparations in CDCl3 in Figure 2, the composition of the hydrophilic PDMAEMA outer segment and hydrophobic PMMA inner segment of the synthesized PMMA particles can be determined by integration of the proton peak for N-(CH3) in PDMAEMA at 2.35 ppm and proton peak for O-CH3 in PMMA at 3.6 ppm. The integrated peak area ratios, 10/90 for h-PDMAEMA/PMMA and 11/89 for both m-PDMAEMA/PMMA and lPDMAEMA/PMMA, represent the molar ratios of DMAEMA/MMA protons (Table 2). Thus, the weight ratios of PDMAEMA/PMMA could be calculated by multiplying the molar ratios by the molecular weights of the constituent monomers (Table 2). To further confirm the

1

H NMR results, gel permeation chromatography (GPC) was

performed to examine the molecular weights of the polymers. Figure 3 shows the GPC curves for the PMMA particles prepared using three different molecular weights of PDMAEMA as stabilizers. All three PDMAEMA/PMMA particle samples contained two GPC peaks: the PMMA peak with a shorter retention time and the PDMAEMA peak with a longer retention time. The peak with a longer retention time was assigned to PDMAEMA, as its retention time is close to the pure PDMAEMA peak. This indicates all three PMMA particle samples all have a spherical structure with PMMA in the inner layer and PDMAEMA in the outer layer. (This spherical structure will be subsequently confirmed by scanning electron microscopy [SEM]). The


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compositions of the h-PDMAEMA/PMMA, m-PDMAEMA/PMMA, and l-PDMAEMA/PMMA particles, as determined by 1H NMR, were approximately 15/85, 17/83, and 17/83 wt/wt, respectively (Table 2). The integrated peak area ratios of each sample (Figure 3) were roughly similar to the weight ratios obtained by 1H NMR (Figure 2). As shown in Figure 3C, the pure hPDMAEMA peak had a shorter retention time than the h-PDMAEMA shell of the PMMA particle, although pure m-PDMAEMA (Figure 3B) and l-PDMAEMA (Figure 3A) exhibited almost the same retention times as the corresponding peaks for the shells of the PMMA particles. In other words, h-PDMAEMA in the shell of PMMA particles has a lower molecular weight than pure h-PDMAEMA, whereas m-PDMAEMA and l-PDMAEMA in the shell of the PMMA particles have approximately the same molecular weights as pure m-PDMAEMA and lPDMAEMA. Quantitative comparisons of molecular weights are shown in Table 1. The cause to the slight discrepancy between the molecular weight of pure h-PDMAEMA and the hPDMAEMA segment on the PMMA is not clear. As shown in Table 1, the dispersity of PMMA prepared by emulsion polymerization in this work was low, ranging from 1.32 to 1.43. It was not clear what caused this narrow dispersity. We suspected the inappropriate use of PS standard might cause this narrow dispersity. We compared PDMAEMA (Mw 38,700, dispersity 2.2) prepared by regular radical polymerization as a stabilizer for emulsion polymerization of MMA, but failed to obtain PMMA particles. The PDMAEMA with broad molecular weight distribution (large dispersity) that failed to synthesize PMMA particles might be attributed to the fact that proper micelles could not form during emulsion polymerization of MMA, leading to an absence of PMMA particles.


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(A)

l-PDMAEMA l-PDMAEMA/PMMA

40 35 30

mV

25 20 15 10 5 0 0

1

2

3

4

5

6

7

8

9

10

8

9

10

8

9

10

Retention time (min) (B)

m-PDMAEMA m-PDMAEMA/PMMA

40 35 30

mV

25 20 15 10 5 0 0

1

2

3

4

5

6

7

Retention time (min) (C)

h-PDMAEMA h-PDMAEMA/PMMA

40 35 30 25

mV

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


20 15 10 5 0 0

1

2

3

4

5

6

7

Retention time (min) Figure 3. GPC curves for PMMA particles prepared from (A) l-PDMAEMA, (B) mPDMAEMA and (C) h-PDMAEMA as stabilizers for emulsion polymerization of MMA.


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Figure 4. DSC curves for (a) PMMA, (b) h-PDMAEMA/PMMA, (c) m-PDMAEMA/PMMA, (d) l-PDMAEMA/PMMA, (e) h-PDMAEMA, (f) m-PDMAEMA and (g) l-PDMAEMA. These results encouraged us to further investigate the phase behavior and morphology of PDMAEMA-coated PMMA particles in more detail via differential scanning calorimetry (DSC) and SEM. Figure 4 shows the DSC curves of pure PMMA, pure PDMAEMA at three different molecular weights and three PDMAEMA/PMMA preparations. Clear glass transition temperatures (Tg) were observed for each sample. The Tg of pure PMMA is close to 101 °C compared to 25, 24, and 18 °C for pure h-PDMAEMA, m-PDMAEMA, and l-PDMAEMA, respectively. A higher molecular weight of PDMAEMA was associated with a higher Tg. A single Tg close to 122 °C was observed for the three PDMAEMA/PMMA preparations, which was unexpectedly higher than the constituent polymers. The single Tg suggests PDMAEMA and PMMA are compatible within the PDMAEMA/PMMA particle. A positive deviation in Tg for


16

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the PDMAEMA/PMMA particles was also observed for the 10:90, 50:50, and 80:20 blends of the constituent polymers in m-PDMAEMA:PMMA (Figure S3). The 10:90 blend has a single Tg near 122 °C, indicating a compatible blend. The 50:50 and 80:20 blends have two Tg close to 22 and 120 °C (or 122 °C for 80:20), indicating two separate phases in the blend: a PDMAEMA phase and a mixed phase of PDMAEMA and PMMA. Figure S3 supports the findings in Figure 4, in that the positive deviation in Tg may be due to the strong specific interactions between PDMAEMA and PMMA, leading to an increase in Tg. Morphology analyses of PDMAEMA-coated PMMA particles under different pH conditions were conducted via SEM. Figures 5 and S4-5 are SEM images taken at 25 °C for the three samples of PMMA particles prepared using 0.8 or 1.5 wt% h-PDMAEMA, m-PDMAEMA or lPDMAEMA, respectively, as stabilizers for emulsion polymerization of MMA. The pH of the PMMA particle solutions (pH 7) was adjusted using aqueous HCl or sodium hydroxide (NaOH) to pH 2 or 12. The average pkaH of PDMAEMA is about 7.5 which is slightly varied with the molecular weight of the polymer.35 The reason why we chose pH 2 (< pkaH) was to protonate the tertiary amines in PDMAEMA and pH 12 (> pkaH) was to deprotonate the amines. As shown in Figure 1, the PDMAEMA aqueous solutions at pH 2 and 7 had no CP behavior and were completely water-soluble at 25 °C. At pH 12, the PDMAEMA aqueous solutions had a CP below 35 °C, which depended on the molecular weight of PDMAEMA. As shown in Figures 5 and S45, PMMA particles with PDMAEMA on the surface could stably disperse in aqueous solutions at 25 °C at both pH 2 and 7, but settled in pH 12 solution due to significant deprotonation and low hydrophilicity (this will be demonstrated later). After removal of water, the products that deposited on the glass plates from the pH 2 and 7 solutions were observed by SEM as spherical particles with diameters of about 125 nm, without a clear dependence on the molecular weight of


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the PDMAEMA stabilizer, thus further confirming the pH-responsive behavior of the PDMAEMA/PMMA nanoparticles due to the existence of the reversibly pH-sensitive PDMAEMA segment. Figures 5 and S4 demonstrate that addition of a higher amount of high molecular weight hPDMAEMA (1.5 wt%) seemed to make the particles stick to each other more at pH 7 than at pH 2. This finding can be associated with the fact PDMAEMA shells dissolved from different polymer chains became entangled with each other in solution. The pH 2 solution contains positively charged PDMAEMA on the particle surface, and more expulsive forces exist between the positively charged particles at pH 2 than at pH 7, so the settled particles from the pH 2 solution seemed more separated. The PDMAEMA-coated PMMA particles could settle in pH 12 solution, and the settled h-PDMAEMA-coated PMMA particles from the pH 12 solution appeared tightly glued to each other (Figure 5). The formation of textured surfaces might be

caused by the drying procedure of the nanoparticle sample settled from aqueous dispersion. The textured structure did not affect the study of the pH-responsive aggregation/dispersion behavior of the nanoparticles. The sample with a higher amount (1.5 wt%) of h-PDMAEMA exhibited fused particles that appeared difficult to separate from each other. Compared to h-PDMAEMA, lower molecular weight m-PDMAEMA and l-PDMAEMA resulted in a lesser extent of gluing between particles in pH 12 solution (Figures S4 and S5). Furthermore, these results further confirmed that incorporation of the hydrophobic PMMA polymer into the nanoparticle interior did not affect the pH-responsive behavior of PDMAEMA. Therefore, these pH-responsive PDMAEMA-coated PMMA nanoparticles appear to have significant potential as multifunctional dispersion nanoparticles to meet varied specific requirements in many fields of application.


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0.8 wt%

1.5 wt%

pH 2

pH 7

pH 12

Figure 5. SEM images of h-PDMAEMA/PMMA particles prepared using 0.8 or 1.5 wt% hPDMAEMA as a stabilizer during emulsion polymerization of MMA. The resulting PMMA particle solutions had pH values of 7 and were adjusted to pH 2 or pH 12 using aqueous HCl or NaOH.


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To validate the SEM findings, dynamic light scattering (DLS) and zeta potential measurements were performed to evaluate the particle size distribution and surface potential of the nanoparticles. Figure 6 presents the particle size analysis of pH 7 aqueous solutions of hPDMAEMA/PMMA, m-PDMAEMA/PMMA and l-PDMAEMA/PMMA at 25 °C. The average particle sizes of these aqueous samples ranged from 130~140 nm in diameter, 5~15 nm larger than the values determined from the SEM images. This discrepancy is due to the differences between analytical techniques: the particle size analyses were based on particles in aqueous solution whereas the SEM images were of dried particles. The particles may swell in aqueous solution, whereas dried particles could shrink. The particle size distribution was broader for the higher molecular weight PDMAEMA used as a stabilizer for emulsion polymerization of MMA. This finding may be associated with the presence of more tangled chains in the PDMAEMA with a higher molecular weight. Further, the DLS observations also implied that the presence of different molecular weights of PDMAEMA enables control of the particle size and particle size distribution of PDMAEMA/PMMA nanoparticles.


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Figure 6. Particle size distribution of (a) h-PDMAEMA/PMMA, (b) m-PDMAEMA/PMMA and (c) l-PDMAEMA/PMMA in pH 7 solution at 25 °C.


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Next, we determined the zeta potential of PDMAEMA/PMMA nanoparticles using DLS. Table 3 shows the zeta potentials for the three molecular weights of pure PDMAEMA in aqueous solution of pH 8.3 at 25 °C and three synthesized PMMA (i.e., h-PDMAEMA/PMMA,

m-PDMAEMA/PMMA, l-PDMAEMA/PMMA) in aqueous solutions of pH 7 at 25 °C. Due to the basic nature of tertiary amine groups, the three pure PDMAEMA aqueous solutions exhibited negative zeta potentials (Table 3). The synthesized PMMA particles in aqueous solution (pH 7) had more negative zeta potentials than their corresponding pure PDMAEMA in aqueous solution (pH 8.3) as emulsion polymerization of MMA was conducted in the presence of CO2 bubbling, which resulted in protonation of the PDMAEMA stabilizer and a decrease in pH of the aqueous solution. Formation of bicarbonate anions upon CO2 bubbling in water may have resulted in the more negative zeta potentials (in the range of -74.4~-85.6 mV) of the synthesized PMMA particle solutions (Table 3). This phenomenon indicates that the highly negative zeta potentials enabled the synthesized PMMA particles to stably disperse in water, as will be demonstrated later.

Table 3. Zeta potentials of the three molecular weights of pure PDMAEMA in aqueous solutions of pH 8.3 and three synthesized PMMA particles in aqueous solutions of pH 7 at 25 °C. Sample l-PDMAEMA l-PDMAEMA/PMMA m-PDMAEMA m-PDMAEMA/PMMA h-PDMAEMA h-PDMAEMA/PMMA

Zeta potential (mV) -10.3 -85.6 -1.8 -81.0 -6.2 -74.4


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(a)

(b)

(c)

Figure 7. CO2-switchable behavior of (a) l-PDMAEMA/PMMA, (b) m-PDMAEMA/PMMA and (c) h-PDMAEMA/PMMA in aqueous solution. The times shown on the caps indicate the times at which the photos were taken after addition of NaOH, CO2 bubbling or N2 bubbling.


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Multifunctional PDMAEMA-coated PMMA nanoparticles with CO2/pH responsive switchable behavior. Based on these results, we reasonably speculated that CO2/pH-responsive PDMAEMA/PMMA nanoparticles could efficiently manipulate the dispersion/aggregation process during CO2/N2 bubbling and adjustment of pH. Dispersion/aggregation of aqueous samples were performed under a variety of test conditions and recorded using a digital camera. The

three

synthesized

PMMA

particle

solutions

(i.e.,

h-PDMAEMA/PMMA,

m-

PDMAEMA/PMMA, l-PDMAEMA/PMMA) had pH values of 7 and stably dispersed in aqueous solution at 25 °C without precipitation. Good dispersion was observed for the watersoluble PDMAEMA on PMMA particles at pH 7 and 25 °C. The PDMAEMA-coated PMMA particles settled when the pH was increased to pH 12, due to deprotonation of the tertiary amine of PDMAEMA, which reduced the hydrophilicity of PDMAEMA; ultimately, the PDMAEMAcoated PMMA particles settled after 8 h (see third vials from left in Figure 7). The settling speed was fastest for the highest molecular weight PDMAEMA (second vials in Figure 7). The phaseseparated solutions could be restored to a stable dispersion by bubbling CO2 (fourth and fifth vials in Figure 7) due to protonation of the tertiary amine of PDMAEMA by the H2CO3 formed during bubbling. Subsequently, bubbling N2 removed CO2 to revert the stable dispersion to phase separation. The PMMA particles synthesized using the highest molecular weight PDMAEMA as a stabilizer required a longer duration of N2 bubbling to remove CO2 and for deprotonation to settle the PMMA particles (compare last two vials for l-PDMAEMA/PMMA solution, last three vials for m-PDMAEMA/PMMA solution, and last four vials for h-PDMAEMA/PMMA solution in Figure 7). As can be seen in Figure 7, a total of 30, 38 and 40 min N2 bubbling were required to

remove

CO2

from

the

l-PDMAEMA/PMMA,

m-PDMAEMA/PMMA,

and

h-

PDMAEMA/PMMA solutions, respectively. This observation is supported by the fact the latex


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particles dispersed and precipitated in response to bubbling CO2 or N2, reflecting changes in the amphiphilic nature of PDMAEMA. In other words, the precipitation occurring after bubbling N2 can be attributed to the structural transition of PDMAEMA from a hydrophilic state to a hydrophobic state. In addition to observing the efficient, reversible CO2/N2-responsive dispersion/aggregation of the amphiphilic PDMAEMA/PMMA system, we further investigated the pH-responsive ability of the synthesized nanoparticles at room temperature as the PDMAEMA segment possesses remarkable pH-responsiveness. Using the h-PDMAEMA/PMMA sample with 1.5 wt% hPDMAEMA as a stabilizer as an example to demonstrate pH-responsiveness (Figure S6), the synthesized PMMA particles exhibited good, stable dispersion in pH 7 aqueous solution, phaseseparation in pH 12 solution and recovery to stable dispersion in pH 2 solution. The phase changes between the pH 7 and 12 aqueous solutions and between pH 12 and 2 aqueous solutions were reversible. SEM indicated the corresponding settled particles (Figure S6) could be produced and had reversible particle sizes and morphologies. The settlement from the pH 12 solution exhibited fused particles that were difficult to separate due to entanglement of the hPDMAEMA chains prior to settlement. The settlement from the pH 2 solution contained separated particles due to the positively charged particles that were expulsive to each other prior to settlement. Figure S7 demonstrates protonation of the tertiary amine groups of PDMAEMA, which resulted in stable dispersion of the PDMAEMA-coated PMMA particles in solution at low pH due to the water-soluble PDMAEMA coating, as shown in Figure S8. After adding NaOH, the PDMAEMA-coated PMMA particles in pH 12 solution settled due to the low hydrophilicity of the PDMAEMA coating at high pH. This suggests the reversibly pH-responsive PDMAEMA/PMMA nanoparticles provide a direct, efficient approach to control hydrophilic-


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hydrophobic transition behavior, which can be employed to significantly alter the dynamic characteristics of the resulting nanoparticles and endow pH-responsive ability in aqueous solution. There are no previous reports of MSPNs that can undergo rapid, stable and reversible dispersion in this manner. Another interesting observation is that the protonated amines (cations) are ion-paired with the bicarbonate ions (HCO3- anions). This ion pair interacted with the dipole of water molecules resulting in dissolution of PDMAEMA in water. Without protonation by added acid or by bubbling CO2, PDMAEMA is soluble in water at low temperatures (lower than its LCST) due to the intermolecular hydrogen-bonding between PDMAEMA and water but insoluble in water at temperatures above LCST due to weakened hydrogen-bonding at high temperatures. The extent of the dissociation of the ion pairs (i.e., the distance between cation and anion ions), that are formed by protonation of the amines, is dependent on the solvent polarity which is determined by the type of solvent and is affected by temperature. The extent of the dissociation of the ion pairs did not appear to affect the dispersion/aggregation study in this work. Thus, collectively, these findings clearly confirm that this newly-developed PDMAEMA/PMMA nanoparticle not only possesses an effective and reversible CO2/N2-switchable aggregation/redispersion ability, but can also remain stable at low pH conditions (pH 2 and 7) and promote rapid precipitation of nanoparticles at a higher pH (pH 12).

CONCLUSIONS

We successfully developed a simple, highly efficient method for fabrication of MSPNs based on the presence of protonated PDMAEMA and hydrophobic PMMA. Three different molecular


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weights of PDMAEMA with narrow distributions were prepared by ATRP and successfully used as stabilizers for CO2-assisted emulsion polymerization of MMA. GPC and SEM analyses revealed the synthesized spherical PMMA particles were coated with PDMAEMA and had diameters of about 125 nm. The relative compositions of these PMMA/PDMAEMA structured particles were approximately 85/15 wt/wt, as determined by 1H NMR. PDMAEMA with a broad molecular weight distribution prepared by regular radical polymerization failed to produce PMMA particles. Compared to lower Mw PDMAEMA, use of a higher amount of high Mw PDMAEMA as a stabilizer for emulsion polymerization of MMA resulted in more sticky spherical particles when settled from the pH 2 and 7 solutions and more particles fused together from the pH 12 solution. DSC revealed the synthesized PDMAEMA/PMMA particles exhibited a single Tg close to 122 °C, about 21 °C higher than the Tg of PMMA, suggesting that the two polymers were compatible in the mixed-phase state. These observations might be attributed to the specific interactions of PDMAEMA with PMMA in the nanoparticles so that PMMA chain mobility was restricted. The synthesized PDMAEMA/PMMA nanoparticles exhibited excellent pH-responsiveness and CO2-switchable aggregation/dispersion behavior in aqueous solution. The CO2-switchable behavior was associated with the tertiary amine groups of PDMAEMA on the surface of the PMMA particles. Upon bubbling CO2 into the aqueous solution, the tertiary amine groups were protonated by the formed H2CO3, making PDMAEMA hydrophilic and creating water-dispersible PMMA particles. Upon bubbling N2, the CO2 was removed and the PMMA particle dispersion reverted to an aggregation. Compared to lower Mw PDMAEMA, use of the higher Mw PDMAEMA as a stabilizer resulted in faster formation of a stable CO2triggered PMMA particle dispersion and slower CO2 removal on N2 bubbling to aggregate PMMA particles due to the higher amount of tertiary amine groups. Overall, this newly-


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discovered system is the first attempt to prepare multi-stimuli responsive PMMA nanoparticles through the use of the multi-stimuli responsive PDMAEMA as a surfactant and offers a potentially exciting route for the development of highly effective, pH-responsive and CO2/N2switchable nanoparticles for next-generation multifunctional colloidal platforms and smart MSPNs in medical applications.


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ASSOCIATED CONTENT

Supporting Information. Chemicals, instrumentation, and some characterization procedures are described in detail in the

Supporting Information section. This information is available free of charge on the ACS Publications website at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author E-mail: [email protected] and [email protected]

Present Addresses a. 700 Kaohsiung University Rd., Nanzih District, Kaohsiung 81148, Taiwan. b. 43 Keelung Rd., Sec. 4, Da'an Dist., Taipei City 10607, Taiwan.

Author Contributions Y.-T. Shieh and C.-C. Cheng conceived the project, designed the research, and wrote the paper. F.-Z. Hu performed all experiments. All authors discussed the results and commented on the manuscript.

Funding Sources Ministry of Science and Technology, Taiwan (contract no. MOST 104-2221-E-390-024 and MOST 105-2221-E-390-025-MY2).

Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENT This study was supported financially by the Ministry of Science and Technology, Taiwan, under contracts MOST 104-2221-E-390-024 and MOST 105-2221-E-390-025-MY2.

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CO2-Switchable Multi-Stimuli-Responsive Polymer Nanoparticle

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