CO2-responsive polyacrylamide microspheres with interpenetrating networks

Accepted Manuscript CO2-responsive polyacrylamide microspheres with interpenetrating networks Meng Mu, Hongyao Yin, Yujun Feng PII: DOI: Reference:

S0021-9797(17)30249-7 http://dx.doi.org/10.1016/j.jcis.2017.03.012 YJCIS 22109

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

12 December 2016 28 February 2017 1 March 2017

Please cite this article as: M. Mu, H. Yin, Y. Feng, CO2-responsive polyacrylamide microspheres with interpenetrating networks, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis. 2017.03.012

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CO2-responsive polyacrylamide microspheres with interpenetrating networks

Meng Mua,c, Hongyao Yin*a,b, Yujun Feng* a,b

a

Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, P. R. China b

Polymer Research Institute, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China. c

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

*Corresponding author. Tel. & Fax: +86 (0)28 8540 8037. E-mail address: [email protected]

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Abstract Hypothesis CO2-responsive microspheres fabricated via co-polymerization protocol are attractive due to their promising applications. However, the inevitable particles-agglomeration restrained their further utilizations. Towards this challenge, interpenetrating network (IPN) protocol would be a potential choice to construct the “intelligent” microspheres, which presents superiority in comparison with co-polymerization mode. Experiments A novel CO2-responsive microspheres with polyacrylamide (PAM)/poly(dimethyl aminopropyl methacrylamide) (PDMAPMA) IPN-structure were fabricated, via inverse seed suspension polymerization. A systematic study was performed via adjusting DMAPMA concentration and crosslinking-degree of seeds. The resultant particles and responsiveness were examined using Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), Optical microscopy (OM) and Laser particle size analyzer (LS), respectively. Findings The interior-structure and fracture-morphology of IPN-particles could be intuitively observed by SEM, showing homogeneous and compact structure without phase separation, offering the direct proof for the formation of IPN-microstructures; the particle morphology altered from IPN to IPN-membrane when gradually increasing DMAPMA content inside. Upon alternating treatment with CO2 and N2, these particles experience reversible volume expansion and collapse. Besides, the non-agglomerated responsive particles with varying composition can be prepared via varying the crosslinking-degree of seeds, from which maximum responsiveness, relative swelling volume (RSV), could reach 11.6 when PDMAPMA loading is at 87%.

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Keywords: Microspheres; Interpenetrating network; Polyacrylamide; Inverse-suspension polymerization; CO2-responsiveness.

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1. Introduction Since the first example of microspheres reported by Staudinger and Husemann 80 years ago,1 particles in microscale as well as nanoscale have emerged as one of the most promising research fields. In recent years, particles responsive to an external stimulus including temperature,2,3 pH,4−6 light7,8 and electric field,9 are of great interest due to their wide range of applications in numerous fields, such as controlled drug delivery,10,11 water treatment,12 and pickering emulsions,5,6 etc. Nevertheless, the aforementioned traditional stimulation modes usually bring about high energy consumption and spatial restrictions.13 Furthermore, the inevitable build-up of background salt that contaminate the surrounding may possibly lead to a weakened sensing by the externally provided stimuli hence compromising the efficiency.5,6,14 Therefore, it is highly desirable to develop novel smart-particles based on residue-free and environmentally-benign triggers. As a readily available and water-soluble gas, CO2 is just such a mild trigger due to its weak acidity (the pH of saturated carbonic acid, ≈4).15 Moreover, easy removal and free of contamination endows CO2 system a better “smart” reversibility.16,17 Consequently, CO2 has been employed to construct reversible surfactants,16 solvents,18 polymer assemblies,19,20 and ionic liquids.21 Particularly, the pioneering work of Zhao’s group22,23 on CO2-responsive hydrogel made the door widely open for CO2-switchable polymers.22 Soon later, Armes and co-works6 reported the CO2-responsive micro-gel; Ma’s group24 prepared homo-polymer particles with high sensitiveness to CO2. To broaden the application of CO2-sensitive particles, random25,26 and graft27,28 copolymerizations protocols were introduced in this smart microspheres. For instance, Miura’s group29,30 prepared intelligent particles via pseudo-precipitation copolymerization of Nisopropyl acrylamide (NIPAM) and dimethyl aminopropyl methacrylamide (DMAPMA). Nevertheless, the particles appeared abnormal change in size from 387 to 1972 nm (with PDI

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altered from 0.11 to 0.31), when the feed-ratio of DMAPMA increased from 20 to 30 mol%. Besides, the final products presented severe agglomeration while the feed ratio of DMAPMA increased up to 100%. Furthermore, the adsorption-ratio to CO2 decreased from 1.1, 1.0 to 0.2, accordingly. That is, during the co-polymerization process, the increase of DMAPMA would lead to particles-coalescence and even the adverse influence on CO2-adsorption. Therefore, there exists a need for novel procedure to develop spherical particles with high PDMAPMA loading but little particles-coalescence. Apart from the copolymerization mode, fabricating interpenetrating-network (IPN) is another strategy to broaden the applications of smart polymers,31 which has got increasing attention due to the distinctive structure.32−34 IPN-structure is usually composed of two polymers, which is obtained when at least one polymer network is prepared or cross-linked independently in the immediate presence of the other component.35 Different from the simple blend or copolymerization, the two different parts in IPN usually interact with each other through weak interactions such as hydrogen bonds and electrostatic interactions. In comparison with copolymerization route, IPN-fabricating procedure usually endows particles comparable or even superior properties.36−38 Thus the IPN-fabricating protocol could be a better choice for constructing CO2-sensitive particles. Nevertheless, all the previously reported procedure for fabricating CO2-responsive particles were confined to homo-polymerization and copolymerization mode.25−30 Herein, we propose the first example of CO2-sensitive microspheres with IPN-structure (composed of PAM and PDMAPMA), which is synthesized via inverse-suspension polymerization (Scheme 1). For one thing, the massive -NH2 groups endow the PAM with capability of forming hydrogen-bond with other molecule or polymer;39−43 hence the PAM could be introduced as the skeleton for constructing IPN polymer.32,38,44 For another thing, the prominent property of hydrolysis-resistance,45 CO2-responsiveness29,30 and hydrogen-bond

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interaction enable DMAPMA as the good candidate to polymerize into the second network. Towards the resultant IPN-microspheres, we discussed the influence of seeds’ crosslinkingdegree and monomer concentration on the formation of the IPN structure, as well as the responsive property. The current work could enrich the protocol for fabricating IPN-particles, and provide a promising route to meet the challenges of particles with well CO2responsiveness simultaneously without particles-agglomeration.

Scheme 1. The route for fabricating IPN-microspheres, and the responsiveness of IPNparticles upon alternatively purging with CO2 and N2. (a) Dried PAM microspheres used as seeds. (b) DMAPMA monomers penetrate into seed’s interior and surround on the surface, the hydrogen-bonding between PAM and DMAPMA is represented by the dotted lines, indicated in the enlarged chemical structure (right). (c) PAM/PDMAPMA IPN-particles; (d)

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The swelled IPN-particles induced by CO2 bubbling, the red lines represent protonated PDMAPMA⋅H+, indicated as the enlarged chemical structure (red one).

2. Materials and methods 2.1 Materials Acrylamide (AM) (AR), N, N-methylenebisacrylamide (MBA) (AR), sodium hydroxide (NaOH) (AR), mono-(9Z)-9-octadecenoate (Span-80) and sorbitan monooctadecanoate (Span-60) (AR), were purchased from Chengdu Kelong Chemical Factory Co., Ltd. (China). N-[3-(dimethylamino) propyl] meth acrylamide (DMAPMA, ≥99.0%), 4,4-azobis(4cyanovaleric

acid)

(ACVA,

≥

98.0%),

2,2'-azobis[2-(2-imidazolin-2-yl)propane]

dihydrochloride (AIBI, ≥98.0%) were supplied by Sigma-Aldrich. Cyclohexane and ethanol with AR grade were bought from Guanghua Sci-Tech Co., Ltd. (Guangdong, China) without further treatment. Deionized water (conductivity, κ=7.9 µS·cm–1) was produced by the ultrapure water purification system (CDUPT-Ш, Chengdu Ultrapure Technology Co., Ltd., China). The CO2 and N2 gas with a purity above 99.99% were provided by the Jinnengda Gas Co. (Chengdu, China).

2.2 Synthesis 2.2.1 Preparation of PAM seed microsphere 105.00 g cyclohexane and 0.50 g stabilizer (Span 60) were added into a 250 mL fournecked round-bottomed flask, followed by vigorously stirring. A mixture of 12.30 g AM, 0.12 g initiator AIBI, a designed amount of cross-linker (MBA) and water was mixed, followed by purging with N2 for 30 min to remove the dissolved oxygen, and then added dropwise to the flask in half an hour. After that, the reaction was allowed to proceed for 6 h at 60 °C. 7

A series of PAM microspheres with varied cross-linking degree but similar size when swelled in water, were prepared by altering the amount of MBA and stirrer rate during the synthesis process, and the experimental details is displayed in Table 1. After reaction, the microspheres were separated from cyclohexane and washed with ethanol, then dried overnight at 45 °C under vacuum to afford white powder product. Table 1 Recipe for the PAM seed microspheres with different cross-linking degree

a

Da (µm)

Seed

AM (g)

MBA (g)

water (g)

PAM-1

12.30

0.025

28.80

273

270±2.5

PAM-2

12.30

0.12

28.80

200

272±3.0

PAM-3

12.30

0.61

28.80

250

269±2.0

Agitating speed (rpm)

Refers to the diameter of seeds dispersed in distilled water, which was measured by Malvern Mastersizer-2000.

2.2.2 Preparation of IPN-microspheres In a typical procedure, 90 mL cyclohexane and 1.14 g Span 80 were firstly added into a 250 mL four-necked round bottom flask with mechanical stirrer, nitrogen inlet, reflux condenser and thermometer, then bubbling N2 into the solution for 30 min to remove the dissolved oxygen. 16.4 mg MBA, 2.8 g DMAPMA, 0.04 g AVCA, and 0.04 g NaOH were dissolved in designed amount of water, subsequently. Then 1.05 g PAM seed powder was introduced into the solution to swell for 3 h under mild stirring at 25 °C. Afterwards, the mixed solution was charged into cyclohexane, and the following reaction was allowed to proceed for 12 h at 70 °C. After polymerization, the microspheres were separated from cyclohexane and washed with ethanol, then dried overnight at 45 °C under vacuum. For the purpose of comparison,

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both

random

co-polymerization

of

P(AM-DMAPMA)

microspheres

and

homo-

polymerization of PDMAPMA cross-linked microspheres were also prepared. However, the former method only resulted in bulk hydrogel, while the latter one gave viscous solution. The recipe for the preparation of a series of IPN-structured particles was listed in Table 2. Table 2 Recipe for IPN-microspheres prepared from PAM seed-swelling polymerization

a

IPNMicrosphere a

Seed (g) b

AM (g)

DMAPMA (g)

H2 O (mL)

MBA (mg)

D (µm) c

IPN-4

PAM-2

/

2.80

36

16.40

307±3.0

IPN-5

PAM-2

/

2.79

27

16.70

312±4.5

IPN-6

PAM-2

/

2.82

18

16.30

296±6.0

IPN-7

PAM-1

/

2.79

27

16.40

286±6.0

IPN-8

PAM-3

/

2.79

27

17.40

277±3.5

P(AMDMAPMA

/

1.40

1.40

27

16.30

/

PDMAPMA

/

/

2.81

27

16.30

/

Refers to IPN-microsphere prepared in the presence of the stabilizer Span-80 and the initiator ACVA; b

Denotes PAM microspheres with same mass (1.05 g) of 1.05g, but different crosslinking-degree; c Stands for the diameter of swelled particles in distilled water, measured by LS.

2.3 Characterizations 2.3.1 Fourier transform infrared spectroscopy (FTIR) The infrared spectra were recorded on a Nicolet MX-1E FT-IR spectrophotometer (USA) with KBr pellet method in the transmittance mode operated at a resolution of 4 cm–1 at 25 °C covering the wavelength range of 4000–400 cm–1.

2.3.2 Scanning electron microscopy (SEM)

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Scanning electron microscopy (SEM) observation was conducted on a JSM-5900LV (Japan) electron microscope operated at an acceleration voltage of 20 kV. The microsphere samples were treated under vacuum-drying at 40 °C for 24 h, and some microspheres were grinded into fragment for fracture-morphology observation.

2.3.3 Particles-size analysis The size of the microspheres was determined on a laser particle size analyzer of Malvern 2000 instrument (Malvern, UK) equipped with a small-volume Hydro 2000SM sample dispersion unit (ca. 500 mL). A He-Ne laser operating at 633 nm and a solid-state blue laser operating at 466 nm were used to measure the size of the microspheres suspended in aqueous or cyclohexane media. The stirring rate was fixed at 1000 rpm in order to avoid particles aggregation during analysis. After each measurement, the cell was rinsed twice with corresponding solvent, and the laser was aligned centrally on the detector. Towards the particles displayed in SEM images, the corresponding particles size were calculated by software of “Nano Measurer”.

2.3.4 Optical microscopy (OM) The optical microscopy images were recorded on an XTL-340 microscope (Shanghai Optics Instruments Ltd.) connected to a computer to record images that emerge variation in size by in situ alternatingly bubbling CO2 and N2. A drop of the dilute microspheres aqueous solution was placed on the microscope slide, followed by direct visualization. This technique was also employed to observe the morphology of microspheres upon treatment with CO2 and N2 successively.

2.3.5 pH measurements

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The pH variation was monitored by a Sartorius basic pH meter PB-10 (±0.01) calibrated with standard buffer solutions, and conducted at 25 °C.

3. Results and Discussion 3.1 Characterization of PAM seed and IPN microspheres The component of the resultant microspheres was firstly examined using FTIR by taking PAM-2 (seeds), IPN-5 (the corresponding IPN-particles) and monomer DMAPMA as examples (Fig. 1).

Fig. 1 The comparison of FTIR spectra for seed microspheres (PAM-2), IPN microspheres (IPN-5, fabricated from PAM-2), and monomer DMAPMA.

In the FTIR spectra of IPN-particles, the absorption peak of 2780 cm–1 was assigned to the stretching vibrations of methyl group attached to tertiary amine; the absorption peak of 1207 cm–1 was assigned to the stretching vibrations of C-N in tertiary amine group, respectively. This clearly indicates the existence of PDMAPMA portions in IPNmicrospheres. Besides, the absorption peak of 3423 cm–1 was assigned to characteristic peaks

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of -NH2 group in PAM chains that confirms the existence of PAM portions in the IPNmicrospheres.

Fig. 2 Size distribution of particles in different condition. The concentration of PAM and IPN-microspheres dispersions was 2×10–4 g⋅mL–1.

To further examine the formation of IPN-structure in particles, the size of seeds and IPNmicrospheres both in dry and swelled states were measured, with cyclohexane and distilled water used as dispersion-medium, respectively. As shown in Fig. 2, the seeds presented diameter of 120 and 272 µm in dried and swollen state; while the IPN-microspheres appeared 190 and 312 µm under the same condition. Thus it was clear that the size of IPN microspheres was 70 µm larger than that of seed under dried condition (increased by 58%), which was in accordance with the SEM results (Fig. 3a, 3b); but it only increased by 14.7% under swollen state, which was in agreement with the OM results (Fig. 5a, 5b). What caused this consequence was that: firstly, the monomer DMAPMA definitely polymerized and took up space within the seed-interior, leading to the significant increase in size under dried state.

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Addditiionnallyy, whe w en sw welleed in waaterr, the t waaterr-abbsorrptiion was maainlly attr a ibuutedd too th he hyddrophiilic PA AM mooietty but b nott thhe hydr h rophobbic PD DMA APMA A coom mponnen nt, thuss it ressulteed in thee sim milaar m miccroggel--size. 3 8 A rep As portted by Wu W aand Peeppaas,36,38 th he IP PN-strructturee off paarticles was w sim mplly conj c jecttureed

andd veri v ifiedd by b sizze-vvariiatioon or reespoonsiive peerfoorm mance, buut theere stilll be b a lacck of o intuuitiivelly iden i ntiffyinng to t the t intterior stru uctuure off thhe IPN I N-paarticcless.46 Taakinng thee chhallleng ge intoo acco a ounnt, w wheen it com c mess too thhe P PDM MA APM MA A/PA AM M IP PN--parrticcles wiith miicroo-sccalee, we w prooceeedeed frac fr turee-m morpphoologgy obs o servvation to the t parrticcles-intteriior wit w th th he hhelpp off SE EM M. A dissplaayedd inn Fig. As F 3bb annd 3c, thhe IPN N-m microosppherres apppeaaredd sm moo oth suurfaace witthout “shhell”-sttruccturre. Beesiddes,, thhe fraactuure-surrfacce of o IPN N par p rticlles preeseenteed hom h moggenneou us witthouut meesooporres orr ph hase-seepaarattion n (F Fig.. 3d). Thhat reveaals thaat the t intterppennetraatin ng nettwoork disttribbutees uunifform mlyy annd ccom mpacctlyy withi w in the t resultaant parrticcle und u der drie d ed stat s te.

Figg. 3 SE EM M im magges forr micr m rosppheeress affter vaacuuum--dryying g at a 40 4 °C ° ffor 244 h.. (aa) Seed S ds of o PA AM--2; (b) IPN-55; (c) ( tthe ennlarggem mennt of o IP PN--5 indi i icatted byy thee arrrow w in n im magge (b); ( ; (dd) th he

fraccture surf s facee off IP PN--5 prep p pareed by b grin ndiing.. Sccalee baars are 1000 µm µ forr (a) annd (b), ( , annd 10 1 µm m foor (cc) and a (d)), reespeectiivelly.

C 2-indducced swelliing//de--sw welliing behavviorr off IP PN m miccrosspherees 3.22 CO A preevioussly repporrtedd,6,255−28 thhe terti As t iaryy amin a ne--invvolv ved paarticcles synt s thessizeed viaa co opollym meriizattionn mode m e coouldd exxpeerieencee siize exppannsion and a colllappse upoon cycclicc puurgiing with CO O2/N N2.. Coonssideerinng thhe PD DMA APM M-iinv volvved miicroosphherees fab f ricaatedd viia IIPN N-coonsstrucctin ng prootoccol in this work w k, we w woondder if tthey y arre still gaas-ssen nsitiive,, thhus alteernnatinng purrginng of o CO O2/N N2 waas inttrodduceed intto the t m rosppherres diispeersiionss too see micr s wh hat woul w ld happpen. Exhhibbitedd inn Fiig. 44a w wass thhe cchanngee in sizze of o IP PN--parrticcless (IP PN--5)..

Figg. 4 (a) Siize--disstribbutionn of o part p ticlles (IP PN--5) triigg gereed by CO2 annd N2 bubb b blin ng succcesssiv velyy; thhe meeasuurem mennts weere connduucteed w withh a fix xed gass fllow w raate oof 100 1 0m mL⋅m min n 1. −

(b) Thhe size s e vaariaationn of o th he IPN I N-5 acccom mpaanieed with w h thhe cha c angee of sooluttionn-pH H. Blaack do ots Cl solutioon titra t atioon. Reed dots d s repreesennt N NH3·H2O sollution titrratioon righ r ht afte a er th he reppressentt HC opeerattion n off accid titrratiion.. Bothh (aa) annd (b)) were w e coonductted witth pparrticlles--connceentraatioon of o 0.002% % g⋅mL L–1.

After purging CO2 for 30 min, the particles-size increased from 312 to 432 µm. Simultaneously, the solution-pH decreased from 8.8 to 4.9 induced by the in-situ formulation of carbonic acid, bringing about the protonation degree of PDMAPMA increased from 7.3% to 99.8% (S1, ESI). Subsequently purging with N2 gas for 60 min led to the solution-pH recovered to 8.7, and the particle-size restored to 315 µm. Why we can get such a responsiveness? The tertiary amine group of PDMAPMA reacts with CO2 in aqueous solution to form ammonium bicarbonate29, leading to the electrostatic repulsion and better hydrophilicity. Besides, the subsequent N2-bubbling breaks up these ammonium bicarbonate owing to its weak chemical stability,16 thus releasing CO2 (Scheme 1, c and d). Via successively purging CO2 and N2, the protonation and deprotonation of PDMAPMA component brings up the increase and decrease in hydrophilicity, further leads to the swelling and collapse of the IPN-particles. For comparison, HCl/NH3.H2O titration mode was employed to directly adjust the solution-pH,17,19 as shown in Fig. 4b. During acid-titration process, the particles size appeared initial change at solution-pH of 7.86, which is close to the pKa of 7.7 for PDMAPMA,29 suggesting that the PDMAPM-component with protonation-degree of 40.8% (S1, ESI) is sufficient to induce macro-size transition of IPN-particles. However, accompanied with the solution-pH continuously decreased from 3 to 2, the particles-size appeared abnormal decrease from 440 to 418 µm, which was attributed to the ion-shielding effect of excess H+ to quaternary ammonium ion. Subsequently, NH3·H2O solution was introduced to facilitate the reversion of solution pH. But it was noteworthy that, corresponding to a fixed solution-pH value, the particle-size in alkali titration curve was obviously lower compared with acid titration ones. Such a difference is resulted from the gradual buildup of NH4Cl, which screens the electrostatic repulsion between the cationic PDMAPMA⋅H+ chains, as well as the

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hyddrogen n-boonddingg innterracttionn beetweeenn am midde ggrouup and a d waaterr. Itt is cleear thaat, unli u ike CO O2/N N2 bubbbliingg m modee, the t ineviitabble bacckggrou undd saalt woouldd reestrainn thhe ssizee reecoverry of o IPN Nparrticlles in aciid/aalkaali titra t atioon proc p cesss, whi w ch is in i agre a eem mennt with w h whhat repporrtedd byy Armees teaam.6 I eresstinnglyy, durin Inte d ng gass buubbblinng proc p cesss, the t exppannsioon and a revversion n of o parti p iclees cou c ld be directlly rrecoorded by an optticaal micr m rosccoppe (OM M) (Fig ( g. 5). 5 A Afteer ppurggin ng CO C 2, thhe part p ticlees me moore trannsp pareent tha t an befo b ore (Fig. 5b 5 aandd 5cc), expperiienccedd ann obbvioous expannsioon aandd beecam cauusedd by b thee incrreasse of chhargge den nsitty (qu ( uateernaary am mm moniium m ioon)), w whiich fuurther faccilittated m moree wate w er’s peermeeatiing intto th he par p rticlles.

Figg. 5 OM O im magees for f miicroosphherres swelleed in i ddisttilleed watter.. (aa) PAM P M-22; (b) ( IPN N-55; (c) IPN N-55 treeateed w with CO C 2 fo or 30 3 min m n; (dd) IPN I N-5 alternnatinngly y trreatted wiith CO O2 and a N2, foor 30 3 minn annd 60 minn, resp r pecttiveely. Sccalee baars are a 3000 µm µ ffor all thee im magges.

R ght after N2 buubbllingg, tthe parrticcle revvertted bacck to thee innitiaal ssizee annd apppearredd less Rig trannsppareent, wiithoout anyy deeforrmaatioon inn patte p ern (Fiig. 5d)). The T goood rev r versionn off paarticcless-sizze andd morp m phoologgy sugggeests that the t PA AM M netw workk ffuncctio oninng aas skeeletton couldd sttandd th he subbstaantiial exp e panssionn annd coll c lapsse.

3.3 The influence of monomer concentration on structure and CO2-responsiveness of the IPN particles To better understand the migration of monomer (DMAPMA) from aqueous solution into the seeds’ interior, as well as the formation of IPN-structure, the DMAPMA with variable concentration was introduced into seed-swelling process. For the purposes of evaluating the responsiveness of particles, the relative swelling volume (RSV) is defined as the volume of the swollen particles over that of the collapsed ones, where volume is directly proportional to the cube of diameter. While the DMAPMA-concentration increased from 0.078 to 0.10 g⋅mL–1, the corresponding IPN microspheres (IPN-4, IPN-5) presented good spherical shape with smooth surface (Fig. 6a, 6b). But, when the monomer-concentration increased to 0.15 g⋅mL–1, the corresponding particles (IPN-6) appeared sphericity-membrane morphology (Fig. 6c, 6d). The difference in morphology could be attributed to the polymerization of DMAPMA within the interior of PAM-microgel, or onto the exterior of particles-surface, depicted as Scheme S1 (S4, ESI). Firstly, the PAM microspheres swelled enough to enable the DMAPMA solution to diffuse into seed-interior. Secondly, the hydrogen-bonding interaction between DMAPMA and PAM restrained the diffusion of monomer from particle interior to exterior, resulting in that the monomer concentration within the seed-microgel would be much higher than that of the outside, thus the polymerization occured earlier within the seed-interior. Towards the low monomer-concentration of 0.078 or 0.10 g⋅mL–1, the concentration was too low to yield polymerization outside the particles. Hence, the final IPN particles emerged sphericity with smooth surface (Fig. 6a, 6b), which was in line with Wu’s conjecture36 on the formation of IPN-structure in particles.

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Figg. 6 SE EM M im magges andd CO C 2-resspoonsiivenness foor IPN I N-m micrrosppheres, prrepared from fr m thhe sam s me seeed (PA ( AM--2) buut disti d inctt m monoomeer ((DM MA APM MA) coonccenttrattionn. (aa) IPN I N-4, prrepared base b ed on mono m omeer-cconncenntraatio on of o 0.00788 g⋅mL L–1; (bb) IPN N-5 5, pre p parred baasedd on o moonoomeerconncentraatioon of o 0.10 0 0 gg⋅mL L–1; (cc) IPN I N-6, prepaaredd baased on o mon m nom merr-co onceentrratiion of 0.1 15 – g⋅m mL–1 ; (d) ( Thhe hhighh-reesollutiion im magee fo or IIPN N-6 inddicaatedd byy thhe arroow in im magee (cc); scaale

barrs are a 100 µm µ forr (aa), (b), (cc); andd 10µm m for f (d)), resp r pecttiveely. (e) The T vaariattionn of thhe parrticlle size s e annd solu s utioon-p pH by altternnatin ng treaatm mentt with CO O2 and a d N2. T connceentrratioon of o 2 The

particles solution was 0.02% g⋅mL–1. CO2/N2 alternatingly purged for 30 min and 60 min, with a fixed gas flow rate at 100 mL⋅min−1.

Nevertheless, when the monomer concentration

increased up to 0.15 g⋅mL–1, later

polymerization-reaction would happen on the particle-surface right after IPN-structure’s establishment, resulting in theoretical core-shell structure depicted as Scheme S1 (S4, ESI). But in reality, the morphology presented membranous-material covering onto the particlessurface (Fig. 6d), which was attributed to disability of PDMAPMA-chain to collapse as particle-pattern, in accordance with Miura group’s report.29 Although the difference in microstructure of the aforementioned IPN-particles, it appeared similar size when swelled in distilled water (307, 312, and 296 µm, respectively), corresponding to the solution-pH of about 8.8, as given in Fig. 6e. After bubbling CO2 for 30 min to decrease the solution-pH to 4.9, the size of particles increased to 385, 433, and 485 µm, corresponding to the RSV of 2.0, 2.7 and 4.4, respectively. Right after purging N2 to release CO2, the particles presented good reversion. During this CO2-induced expansion process, the water is inclined to penetrate into the protonated PDMAPMA⋅H+ network to facilitate the final volume expansion; besides, the PAM network is inclined to maintain the original size because of the cross-linking structure. That is, the final equilibrium-swelling could be the outcome of cationic PDMAPMA expansion versus PAM contraction. With respect to the particles (IPN-4, IPN-5, IPN-6) prepared from the same seed (PAM-2), the variation in RSV should be ascribed to the variation of PDMAPMA moieties, which could be positively correlated with the change of monomer-concentration in swelling process.

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3.4 The influence of seed crosslinking-degree on structure and responsiveness of the IPN particles As aforementioned, it presented interesting morphology and high-responsiveness through increasing monomer concentration, thus it was wondered whether the prominent performance could be attained by enhancing seeds capability of absorbing DMAPMA-solution. Keep this in mind, a series of seeds (PAM-1, PAM-2, PAM-3) with distinct cross-linking degree (0.2 wt%, 0.97 wt%, 4.9 wt %) but similar size in water (270, 272, 269 µm, respectively), were prepared as listed in Table 1. For the purpose of evaluating the swelling-property of seeds, the swelling-ratio by absorbing water (SRW) was defined as the size of swelled seeds over that of dried ones. Accordingly, the different SRW (2.73, 2.27, 1.77) of the seeds (PAM-1, PAM-2, PAM-3) could be calculated based on the particles-size (S6, ESI and Fig. 2). Additionally, accompanied with the cross-linking degree decreased from 4.9 wt%, 0.97 wt% to 0.2 wt%, the morphology of dried seeds appeared abnormal change from standard sphericity, to particles with planes embedding onto the surface (inset, Fig. 7c).

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Figg. 7 SE EM M im magees and a d CO O2-responnsivven nesss for m micrrosppheres. (aa) PAM P M-33, w withh crooss-linnkin ng deggreee of o 4.99 w wt% %; (b) IP PN--8, prepaaredd from f m tthe seeed off PAM P M-3 3 and a d DMA D APMA Aconncentraatioon of o 0.1 0 0 g⋅m g mL–11; (cc) P PAM M-1, wit w th ccrosss-linkkingg deegreee of 0.22 wt%; innsett: th he enllarggem mentt off (cc) inndiccateed by b thee arrrow w; (d) ( IPN N-77, prrep pareed ffrom m thhe see s d of o P PAM M-11 an nd moonom merr-coonccenttrattionn off 0.10 g⋅m mL––1; inse i et: the t enlarggem mennt oof (dd) iindiicatted by thee arrrow w. Thee sccalee baar is 100 µm m fo or thhe ima i agess (aa), (b), ( , (c)), (dd); 10 µm m foor innseet in n (cc), and a 500 µm m fo or inset in i (d), ( , reespeectiivelly. (e) Thhe var v iatiion of paarticcless-sizze andd sooluttion n-pH H by b alteernaatin ng

treatment with CO2 and N2. IPN-5 was prepared based on the seed with crosslinking-degree of 0.97 wt%. The concentration of particles solution was 0.02% g⋅mL–1. CO2/N2 alternatingly purged for 30 min and 60 min, with a fixed gas flow rate at 100 mL⋅min−1.

Subsequently, the seeds (PAM-1, PAM-2, PAM-3) were introduced to fabricate IPN particles (the recipes were listed in Table 2), and the resultant IPN-particles presented interesting difference in morphology and the PDMAPMA-loading (S2, ESI). With the crosslinking degree of seeds reduced from 4.9% to 0.97%, the corresponding IPN-microspheres (IPN-8, IPN-5) emerged standard sphericity with smooth surface. However, towards the seed with cross-linking degree of 0.2% (PAM-1), the corresponding IPN-particles (IPN-7) presented “potato”-like pattern and tiny “membrane”-like debris around the particles (Fig. 7d). Simultaneously, with the cross-linking degree of seeds decreased from 4.9%, 0.97% to 0.2%, the difference in size (between seeds and corresponding IPN-particles under dried state) significantly increased from 20, 70 to 96µm, calculated from Fig. S1 (S6, ESI) and Fig. 2. The abovementioned difference in size (between seeds and corresponding IPN-particles under dried state) and morphology could be attributed to the distinct PDMAPMA-loading, as depicted in Scheme S2 (S5, ESI). Right after the seeds (PAM-3, with high cross-linking degree of 4.9%) was introduced into swelling process, merely a small amount of DMAPMA penetrated into seed network due to the low SRW (1.77) of the PAM, subsequently polymerized into the IPN-particle with low PDMAPMA-content of 31%. That is, the PAM moiety still be the dominant one, which function as the skeleton to maintain the IPN-particles’ sphericity. Towards the seeds PAM-2 (with the cross-linking degree of 0.97%), the corresponding PDMAPMA-loading in IPN-5 reached to 74.8%, but the IPN-particles always kept in sphericity pattern (Fig. 3b). Nevertheless, the IPN-particles presented apparent abnormality in morphology when the

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cross-linking degree of seed decreased to 0.2 wt%. What cause this consequence was that, on one hand, large amount of DMAPMA permeates into seed network because of the high SRW of 2.73, then the PDMAPMA component turns into the dominant moieties with content up to 87%; resulting in that the preferential seed-skeleton become too weak to maintain the initial spherical-pattern. Besides, the already existing planes (embedding on seeds-surface) would strengthen the deformation of IPN-particles morphology. On the other hand, during seedswelling process, the PAM component become less in comparison with DMAPMA, thus the hydrogen-bonding interaction between PAM and DMAPMA became too weak to restrain monomer-diffusion from seed interior to outside during polymerization. Eventually, it results in that the tiny membrane-like debris adheres on the particle-surface, as depicted in Scheme S2d (S5, ESI). To examine the influence of seeds’ cross-linking degree on the responsiveness of IPNparticles, the aforementioned IPN-particles swelled in water and appeared size of 277 µm (IPN-8), 312 µm (IPN-5), and 286 µm (IPN-7), respectively displayed in Fig. 7e. After purging CO2 for 30 min, the solution-pH decreased to 4.9, accompanied with the particlessize increased to 335, 433 and 648 µm (RSV of 1.8, 2.7 and 11.6, accordingly). Subsequently purging N2 to release CO2, the particles size experienced good reversion, as well as the solution-pH. Towards the different CO2-responsiveness, it could be explicated according to Flory theory47 (S3, SI). First of all, the RSV is positively correlated with the osmotic pressure, which is determined by the difference in ion-concentration between the interior and exterior of the IPN-microgels. Additionally, the RSV is negatively correlated with the elasticity of polymer-network, i.e. the cross-linking degree of PAM portion. That is to say, on one hand, right after treatment with CO2, the PDMAPMA⋅H+ moieties with varied content (within particles of IPN-8, IPN-5 and IPN-7) lead to varied osmotic pressure, which further influence

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the capacity of expansion. On the other hand, the resilience of PAM moieties restricts the expansion of protonated PDMAPMA with distinct binding-force, caused by the different cross-linking degree of seeds. Hence, what could be concluded is that, the high RSV (11.6) of particles (IPN-7) should be ascribed to the synergistic effect of the higher PDMAPMAloading (87%) and the lower cross-linking degree of seed (0.2 wt%).

Conclusions By introducing the isolated seed-swelling procedure and inverse-suspension system into −

the previous approaches for IPN-particles,36 38 this work demonstrated that CO2-responsive microspheres (in micro-scale) with PAM/PDMAPMA IPN-structure could be fabricated through three sequential steps: seed-preparing, seed-swelling and IPN-constructing. FTIR and SEM revealed the composition and spherical-morphology of the IPN-particles. Especially, the fracture-morphology presented the interior of IPN-particle was homogeneous rather than phase separation, offering direct proof for the formation of IPN-structure, which filled the lack of intuitively identifying to the interior-structure of IPN-particles.46 By increasing the DMAPMA-concentration from 0.078 to 0.15 g⋅mL–1, the particles-morphology altered from sphericity to sphericty-membrane clearly observed by SEM, which further verified Wu’s conjecture36 on the formation-process of IPN-particles. In comparison with acid/alkali titration cycles, the IPN particles experienced expansion and better reversion via alternating aeration of CO2/N2, which was in accordance with Armes group’s report.6 Moreover, the CO2-induced volume-transitions was directly observed and recorded for the first time by OM. The particles having responsiveness and composition in different levels could be designed and prepared by varying cross-linking density of seeds. Thus the noncoalescent particles with PDMAPMA-loading of 87% and RSV of 11.6 could be attained by adjusting the crosslinking-degree of seed decreased to 0.2 wt%.

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Compared with the previously reported CO2-sensitive microspheres29,30 synthesized by copolymerization mode, IPN-fabricating mode in this work can meet the demand of highcontent and non-agglomeration for CO2-absorbents. The introduction of isolated seedswelling and inverse-suspension polymerization, made this protocol a better choice for improving responsive-performance or preparing particles with dual responsiveness.

Acknowledgements The authors would like to thank the financial support from the National Natural Science Foundation of China (21273223), the opening fund of State Key Laboratory of Polymer Materials Engineering (sklpme2014-2-06), and the Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province (YQKF201403).

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CO2-rresspoonsivee polyyaccryylam CO mid de micrrosph herees with intterrpeeneetraatin ng net n twoork ks

Meng Muua,c, Hong Me gyaao Yin* Y *a,bb, Yuju Y un F Fenng* a,b

Grrap phiicaal A Absstraactt A nnovvel kin k d oof CO C 2-resspoonsiive miccroosph herees with w h innterrpen netrratinng nettwo ork wer w re dev d velooped d.