Smart StarchâPoly(N-isopropylacrylamide) Hybrid Microgels

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Smart Starch-Poly(N-isopropylacrylamide) Hybrid Microgels: Synthesis, Structure, and Swelling Behavior. Daiani Canabarro Leite, Sergej A. Kakorin, Yvonne HertleHannappel, Thomas Hellweg, and Nadya P. da Silveira Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00706 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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Smart Starch-Poly(N-isopropylacrylamide) Hybrid Microgels: Synthesis, Structure, and Swelling Behavior

Daiani C. Leite,† Sergej Kakorin,‡ Yvonne Hertle,‡ Thomas Hellweg,‡ and Nádya P. da Silveira*†

†

Institute of Chemistry, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves

9500, 90650-001 Porto Alegre, Brazil ‡

Faculty of Chemistry, Universität Bielefeld, Universitätstrasse 25, 33615 Bielefeld,

Germany

*Corresponding Author: [email protected] (Nádya P. da Silveira) Phone: +555133086265

1 ACS Paragon Plus Environment

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ABSTRACT. In this study, we present hybrid microgels made of starch nanoparticles (SNPs) and poly(N-isopropylacrylamide) [p(NIPAM)]. SNPs were formed through nanoprecipitation. Hybrid microgels were prepared by surfactant-free precipitation polymerization (SFPP) or in the presence of surfactant (PP) at different NIPAM:SNP ratios. Dynamic light scattering results of hybrid microgels synthesized by SFPP revealed changes in volume phase transition temperature according to SNP amount, where the increase in hydrophilic content caused small shifts in the lower critical solution temperature (LCST), reaching nearly 35 °C. Colloidal stability was improved with SNP content, leading to increased stability due to the hydroxyl groups. Small-angle X-ray scattering indicates a core–shell structure above the LCST, where SNPs chains cover a p(NIPAM) core. Swelling curves experimentally obtained were analyzed using the Flory-Rehner model, where the interaction parameter (χ) has been modeled either by a series expansion on the swelling ratio or by a Hill-like equation for a cooperative thermotropic transition.

KEYWORDS. starch nanoparticles, precipitation polymerization, starch-co-p(NIPAM), core-shell, Flory-Rehner model, cooperative thermotropic transition


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INTRODUCTION Starch is a well-known renewable and inexpensive carbohydrate, which has earned special attention due to its high biocompatibility and hydrophilicity.1,2 The use of starch as a biomaterial for synthesizing starch-based nanoparticles (SNPs) has received growing attention3-7 and various applications of SNPs can be found in the literature, such as drug delivery systems8-10 and biocomposites.11-13 The different properties, such as greater surface area,14,15 are the main reasons for the use of nanoparticles instead of bulk material. Several techniques, like nanoprecipitation,4,6,16 pressure application,17 acid hydrolysis,12,18 and ultrasound19,20 are used in SNP formation. Among these, nanoprecipitation has some advantages, given it is an easy and quick procedure. Additionally, particles with a narrow size distribution can be obtained.6,16,21 Biopolymers, due to their properties, are gaining increasing interest as components of stimuli-responsive systems.1,22-24 Among those "smart" materials, microgels are versatile particles with an ability to rapidly and reversibly swell and deswell, in response to various chemical and external physical stimuli, for instance, light,25 temperature,26,27 electrical fields,28 and pH.29 Synthetic/biomacromolecule hybrid microgels are expected to result in highly particular materials.30,31 One of the most studied stimuli-responsive polymers is poly(N-isopropylacrylamide) p(NIPAM), which has a lower critical solution temperature (LCST) of approximately 32 °C.32,33 The volume phase transition temperature (VPTT) and size of p(NIPAM)-based microgels can change and be adjusted, according to hydrophilichydrophobic interactions and the ratio between the polymers involved.34,35 Zhang and Zhuo36 synthesized native gelatinized starch-co-p(NIPAM) hydrogels with different copolymerization ratio, leading to improved surface properties when compared with pure p(NIPAM) hydrogels. Based on the experiment, more hydrogen bonds 3 ACS Paragon Plus Environment

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(due to the hydroxyl groups of starch) were formed in the hydrogel synthesized in the presence than absence of starch, creating a more stable hydration structure around the hydrophobic groups. Ten years later, Tan and coworkers37 synthesized well-shaped p(NIPAM) hydrogels with a high mechanical strength, by photopolymerization using starch with allyl groups as the crosslinker. To the best of our knowledge, starch nanoparticles-cop(NIPAM) hybrid microgels made by (surfactant-free) precipitation polymerization (SFPP and PP, respectively) are not yet reported in the literature. With the use of SFPP or PP, spherical particles with low polydispersity can be achieved.38 In this work, we report the synthesis of crosslinked three-dimensional SNP-cop(NIPAM) microgels by PP and SFPP in aqueous solution, at different NIPAM:SNP ratios. The use of SNPs instead of native starch polymers is expected to be more suitable for incorporation in a microgel network. At first, SNP formation and characterization will be discussed, followed by an analysis of their influence on the size, VPTT, and colloidal stability of the hybrid microgels synthesized. A small-angle X-ray scattering (SAXS) study will also be presented, aiming at the elucidation of SNPs/p(NIPAM) interaction and organization inside microgel particles. Moreover, the swelling behavior of the network will be described in terms of the Flory-Rehner theory.39-41 In the literature already a number of articles can be found which used Flory-Rehner theory to describe the volume phase transition of microgels.42-46 In this context the work by Lopez and Richtering is very interesting because it summarizes all available experimental data related to this topic However, the simple forms of the interaction parameter (χ), usually used in models for microgels, do not adequately describe our data on the SNP-co-p(NIPAM) hybrid microgels swelling. In the present work, the interaction parameter (χ) of the Flory-Rehner equation


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has been successfully modeled, either by a series expansion of the swelling ratio (ϕ) or a more sophisticated Hill-like equation which describes the microgel collapse as a cooperative thermotropic transition. EXPERIMENTAL SECTION Materials. Corn starch (Amisol 3408) was a gift from Ingredion (Jundiaí, SP, Brazil) and it was dried at 60 °C for 48 h before use. N-isopropylacrylamide (NIPAM; purity 97%) (recrystallized with hexane), N,N'-methylenebisacrylamide (BIS; purity 99%), ammonium persulfate (APS; purity 98%), and sodium dodecyl sulfate (SDS; purity 99%) were obtained from Sigma-Aldrich, Germany. Dimethylsulfoxide (DMSO) (Vetec, Brazil), absolute ethanol (Nuclear, Brazil) and all other chemicals were of analytical or reagent grade and were used without purification. Methods. SNP Preparation. Based on a starch solubilization method47 (with some modifications) and a nanoprecipitation procedure,4,16 SNPs were formed as follows: 2% (w/v) starch solution was prepared by dissolution of 2.0 g corn starch in 100 mL DMSO/H2O (9:1 ratio) solution, under magnetic stirring at 40 °C for 2 h, until complete dissolution. After cooling to room temperature, the viscous and slightly turbid solution was sonicated (20 kHz; Branson Digital Sonifier model 250 and 450, Branson Ultrasonics, Danbury, USA) for 1 min at 100% amplitude, to decrease the molecular weight.47 Then, 1 mL of sonicated starch solution was added dropwise to 20 mL of absolute ethanol at 900 rpm. After 1 h stirring, the suspension was purified by three successive centrifugations (3000 rpm, 15 min) and re-suspension steps using absolute ethanol, and then dried at 40 °C for 24 h. The nanoprecipitation procedure was repeated several times until the necessary SNP weight for the microgel synthesis was achieved.


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SNP-co-p(NIPAM) Microgel Synthesis. Certain amounts of SNPs and NIPAM were placed in a 100-mL three-necked-flask with degassed Milli-Q water at 80 °C, until complete dissolution of NIPAM and dispersion of the SNPs. After temperature equilibrium, SDS and BIS were added, and dried nitrogen was bubbled through the solution for at least 30 min before polymerization. Then, the initiator (APS) was added to the solution. The mixture became turbid and the reaction proceeded at 80 °C for 4 h. After this time the mixture was cooled to room temperature and stirred overnight. Next, the copolymer was ultracentrifuged and re-dispersed with Milli-Q water five successive times.32 The corresponding amounts of synthesis components are listed in Table 1.

Table 1. Feeding ratios used for the microgel synthesis NIPAM:SNP

NIPAM

BIS

SDS

APS

ratio (w/w)

(mol/L)

(mol/L)

(mol/L)

(mol/L)

NIPAM(1):SNP(0.5)SDS

1:0.5

0.053

0.0041

0.0011

0.0027

NIPAM(1):SNP(0.5)

1:0.5

0.053

0.0041

----

0.0027

NIPAM(1):SNP(1)SDS

1:1

0.053

0.0041

0.0011

0.0027

NIPAM(1):SNP(1)

1:1

0.053

0.0041

----

0.0027

NIPAM(1):SNP(2)SDS

1:2

0.053

0.0041

0.0011

0.0027

NIPAM(1):SNP(2)

1:2

0.053

0.0041

----

0.0027

Sample ID

Characterization. Nuclear Magnetic Resonance (1H NMR). NMR spectra were obtained in D2O for hybrid microgels (freeze-dried for 3 days and then re-dispersed in D2O) on a Bruker Avance 500 MHz spectrometer (USA). The chemical shifts were reported as δ values (ppm). SNP-co-p(NIPAM), sample NIPAM(1):SNP(1)SDS (Figure S1): 1H NMR


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(D2O, 500 MHz): δ 1.04 (6H, br s), 1.20 – 1.70 (1H, br t), 1.71 – 2.09 (2H, br d), 3.30 – 3.60 (5H, br s), 3.60 – 3.95 (1H, br m), 5.19 – 5.37 (1H, s). Dynamic Light Scattering (DLS). DLS measurements for SNPs in ethanol were performed at 25 °C using a Brookhaven Instruments (USA) standard setup (BI200M goniometer, BI9000AT digital correlator) with a vertically polarized coherent He–Ne laser (λ = 632.8 nm), using a fixed scattering angle of θ = 90°. The scattering volume was minimized using a 400 µm aperture and an interference filter in front of the entrance of the photomultiplier. Polarized homodyne intensity autocorrelation functions g2(t) were obtained using a multi-τ mode correlator with 224 channels. The presented data were an average of three individual results. The swelling curves of microgels were measured at a fixed scattering angle of θ = 60°, using a solid-state laser (Toptica Photonics AG, Germany) at a wavelength of λ = 661.8 nm and a fast correlator (ALV-6010, ALV-GmbH, Langen, Germany) with a thermostatic bath (Haake Phoenix II, Thermo Scientific, Germany). The experiments were carried out from 10 °C to 50 °C, with five individual measurements at each temperature. From the intensity correlation function, we computed the field correlation function using the Siegert relation. For nearly monodisperse particles, the decay of the field correlation functions is a single exponential function given by g1(τ)= exp (-Гt). The relaxation rate (Г) is connected to the diffusion coefficient (D) and can be related to the hydrodynamic radius (RH) via the Stokes-Einstein relation.48 The polydispersity index (PDI) was estimated from a second order cumulant fit of the moment.49 In this method the intensity autocorrelation function is described by ‫ ܩ‬ሺଶሻ ሺ߬ሻ = ‫ ܤ‬+ ߚ݁‫݌ݔ‬ሺ−2߁‫ݐ‬ሻ ቀ1 +

ఓమ ଶ!

߬−

ఓయ ଷ!

ଶ

߬ + ⋯ ቁ and the term


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ቀ1 +

ఓమ

߬− ଶ!

ఓయ

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ଶ

߬ + ⋯ ቁ is cut off at its second segment. It introduces a polydispersity index ଷ!

(PDI) as an indicator of the size distribution of the particles: PDI =

ఓమ ௰మ

Scanning Electron Microscopy (SEM). SNPs were observed using an SEM instrument (EVO MA10, Zeiss, Germany). Diluted particle suspensions were dropped on glass slides, dried and sputtered with an Au layer. Particle size analyses of SEM images were performed using the ImageJ software.50 Zeta Potential (ζ) Analysis. The zeta potentials were measured in a Zetasizer Nano instrument (Malvern, USA) at λ= 632.8 nm, using a He-He laser. Diluted samples were measured at 20 °C and 45 °C (below and above the LCST). The electrophoretic mobility of each sample was measured three times, and at least 10 runs were performed per measurement. The zeta potential values were calculated from electrophoretic mobility, according to Henry's equation and Smoluchowski approximation.51 Small-angle X-ray scattering (SAXS). The SAXS measurements of microgels in solution were performed on the D01B/SAXS1 beamline at the Brazilian Synchrotron Light Laboratory (LNLS). The sample-to-detector distance was 3000 mm, covering a scattering vector q ranging from 0.04–1.5 nm-1. Experiments were conducted at 20 °C and 45 °C (below and above the LCST), using a controlled thermobath. Water was measured as a background and subtracted from sample intensities, and it was also used for absolute normalization of the intensity. We adopted the strategy of initiating the SAXS measurements with very diluted suspensions, increasing the concentration of particles to the point where the intensity signal was enough to obtain the SAXS curves. Hence, the interparticle structure factor had only a negligible effect on the curve fitting procedure. The data were fitted by a core-shell form factor and a hard sphere structure factor with 8 ACS Paragon Plus Environment

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monodisperse approximation model using the SASfit package (PSI, Switzerland). Size distribution was fitted using the Schulz-Zimm model, by applying the same software (Eq. 1). The SASfit package describes a core-shell structure with an inner radius R2 and outer radius R1 (Eq. 2 – 4):

N SZ (R, N , Ra , k ) = Ra

 R   Ra

  

k −1

k k exp − kR  Ra   , Γ(k )

(1)

where N is the particle number density and R is the radius of the sphere. The parameter k of the size distribution is related to the variance σ by k = 1/σ2. Ra is the scaling parameter and defines the maximum of the size distribution for large values of k. I (q, R1 , R2 , ∆η , µ ) = [K (q, R1 , ∆η ) − K (q, R2 , ∆η (1 − µ ))] , 2

(2)

where K is given by

4 sin qR − qR cos qR K (q, R, ∆η ) = πR 3 ∆η 3 3 (qR )3

(3)

and lim q =0

[

]

2

4  I (q , R1 , R2 , ∆η , µ ) =  π∆η R13 − R23 (1 − µ )  3  .

(4)

In Eq. 4 the scattering contrast relative to the matrix of the core is µ∆η and the one of the shell is ∆η. Statistical evaluation of the quality of the fit was made through the chi-square test. The values of radius of gyration (Rg) were estimated from the slope of the curves at the lowest q values, using the Guinier approximation. RESULTS AND DISCUSSION SNPs Preparation. DMSO/H2O solvent system has been widely used as a solubilizing agent for starch granules, allowing the solubilization at mild conditions without 9 ACS Paragon Plus Environment

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addition of any other chemicals.52 Viscosity and turbidity of those solutions are usually high and depend on starch concentration.53 During the solubilization, the granular structure is destroyed, releasing the corresponding amylose and amylopectin polymers, with high molecular weight. The use of ultrasound techniques in size and molecular weight reduction, as well as viscosity, has been widely explored.12,19,47 After the ultrasound, a transparent starch polymer solution was observed, due to the molecular weight reduction. The non-solvent (absolute ethanol) associated with the high regular stirring rate allowed the formation of spherical and individual nanoparticles with a relatively narrow size distribution, as seen by SEM (Figure 1-B). Figure 1-A shows the DLS measurement of SNPs, which resulted in an RH = 75.2 nm and PDI = 0. 268, estimated from a second order cumulant fit of the moment.

Figure 1. In (A), DLS temporal intensity correlation function (full black square), fit function (blue line) and relaxation time distribution (full black circle) of SNP. In (B), a typical SEM image of SNPs is shown (scale bar = 2 µm). Inset, particle size analysis using ImageJ.


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To better characterize the internal structure of SNPs, wide-angle X-ray scattering (WAXS) analysis was performed and a crystalline degree was calculated. The crystalline content was determined to be higher for SNPs (55.3%) if compared to native rice starch (36.5%). Whereas native starch presents a crystallinity type A, SNPs show crystallinity type B+V. In the SNPs, intermolecular forces seem to be strengthened by more hydrogen bonds. This situation suggests that more energy has to be transferred to SNPs (for instance, by heating) in comparison to native starch, to have their internal crystalline structure affected. To better understand the stability of SNPs under SNP-co-p(NIPAM) preparation conditions (stirring at 80 ºC), we have performed dynamic light scattering experiments with SNPs after stirring at 80 ºC. The measurements were carried out at 80 ºC and the RH of SNPs under these conditions was determined to be 97.7 nm, with a PDI of 0.264. We concluded that the primary structure of the SNPs becomes swollen due to temperature and stirring, resulting in a SNP 30% bigger than the original under these harsh conditions. Microgel synthesis and characterization. SNP-co-p(NIPAM) surfactant-free microgels were prepared by SFPP of NIPAM and SNPs in the presence of APS as initiator and BIS as crosslinker agent. Also, microgels with surfactant (SDS) were prepared under the same conditions, by PP. In this study, a direct initiation of polysaccharide was made: the initiator abstracts hydrogen radicals from the hydroxyl groups of SNPs to form the initiating radicals on the polysaccharide chains. The appearance of the emulsion remains unchanged during polymerization, evidencing the maintenance of the colloidal structures, which conferred the same milky aspect to the suspension (NIPAM is highly soluble in water under the reactions conditions applied). During SNP-co-p(NIPAM) polymerization, initiation occurs directly in OH groups present in SNPs. Due to the SNPs structure, there is the immediate availability of the 11 ACS Paragon Plus Environment

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superficial OH groups, which undergo the attack of the initiator transforming into free radicals. These react immediately with NIPAM clusters (primary particles). Considering that the polymerization kinetics of NIPAM is faster, it is expected that there is a higher reactivity between NIPAM groups, which is greater than the rate of radical formation in the inner structure of the SNPs. This fact induces the formation of a crosslinked nuclei of pNIPAM (primary particles) to which the SNPs bind. In this way, microgels are being formed in the presence of pNIPAM particles in the collapsed state. The impact of SNP on the size and VPTT of the hybrid microgels synthesized via PP and SFPP was evaluated through DLS measurements. The VPTT was obtained by determination of the curve inflection point (using a second derivative). From DLS measurements (Figure 2-A), two distinct behaviors can be observed in microgel particles made by SFPP. Depending on the ratio, the SNP content leads to higher transition temperatures of the microgels and also influences the microgel size below the LCST.

Figure 2. RH vs. temperature for microgels prepared by SFPP (A): NIPAM(1):SNP(0.5) (■), NIPAM(1):SNP(1)

(●),

NIPAM(1):SNP(2)

(▲);

and

PP

(B):


(■),


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NIPAM(1):SNP(1)SDS (●), NIPAM(1):SNP(2)SDS (▲). Arrows show the volume phase transition temperatures of the microgels.

The SNPs are hydrophilic and interact with the NIPAM chains at the surface of the hybrid

particles,

leading

to

higher

VPTTs.

The

microgel

particles

named

NIPAM(1):SNP(1) and NIPAM(1):SNP(2) show a higher VPTT (34.5 and 34.9 °C, respectively) than NIPAM(1):SNP(0.5) (VPTT of 32.9 °C), which contains a low amount of SNPs (Figure 2-A). The VPTT of pure p(NIPAM)32 is ~32 °C. The major effect of the starch is a slight shift of the LCST to higher values. With hydrophobic comonomers, the opposite is found.44 The SNPs/NIPAM hydrophilic interaction is also responsible for size differences: when the SNPs amount is increased, the microgels become smaller (below the LCST). It is known that good polymer-solvent interactions cause expansion of the chains, which increases the hydrodynamic radii. In the presence of increasing amounts of SNPs, the primary particles of p(NIPAM) tend to achieve colloidal stability with a smaller size due to steric barriers during polymerization resulting in smaller microgels, the larger the amounts of SNPs used in the synthesis recipe. Beyond the LCST, the microgels have nearly the same size. A different behavior was observed when the microgel was synthesized using a surfactant (Figure 2-B): SDS adsorbs onto dispersed p(NIPAM) particles increasing their colloidal stability.54 The same occurs for SNPs, which are susceptible to the SDS action as well. We conclude that SDS suppresses the influence of the SNPs during synthesis, leading to microgels which size is independent of SNPs feed in the synthesis. P(NIPAM) temperature sensitivity arises because the polymer has a considerable hydrophobic 13 ACS Paragon Plus Environment

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character and this characteristic evokes strong interactions with surfactants used during the synthesis in the reaction medium. Usually, the surfactant affects the determined average hydrodynamic radius of the microgel. The RH (Figure 2-B, Table 2) values are smaller than those of the microgels synthesized without the use of surfactant, but the SNPs content seems to make no difference in the size and VPTT of the particles. As noted elsewhere,44 the particle size and particle properties are strongly influenced by the interfacial tension during the nucleation phase of the reaction.55 The effect of SNPs on colloidal stability was also studied. Figure 3 reveals that all hybrid microgel particles present negative zeta potential values, which can be attributed to the presence of hydroxyl groups of SNPs macromolecules entangled on the surface of the microgels, as reported by other authors.56,57

Figure 3. Zeta potential of microgel particles. Sample identification corresponds to those described in Table 1.

Comparing the microgels synthesized via PP vs. SFPP, a decrease in zeta potential value at 20 °C is observed, probably due to the anionic character of SDS. Furthermore, with 14 ACS Paragon Plus Environment

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the increase in SNPs content a decrease in zeta potential values at 20 °C has been observed, caused by the higher concentration of OH- groups of SNPs in the microgels. p(NIPAM)-based microgels can increase and decrease their electrostatic surface potential during heating and cooling cycles, and this behavior is observed for measurements at 45 °C. All samples measured at 45 °C exhibit zeta potential values < -25 mV. This behavior can be attributed to the shrinkage of the microgel, which induces enhancement in the surface charge density. The decrease in zeta potential, when compared with the other samples, is less pronounced when a high amount of SNPs was used during synthesis, and could be related to the higher SNPs content and its enhanced hydrophilicity and steric hindrance with SDS. As interpreted from the zeta potential results, SAXS measurements have shown that SNPs are probably entangled on the surface of microgels and, above the LCST, the hybrid materials exhibit a core–shell structure (see Figure 4-B). The SAXS curves for samples NIPAM(1):SNP(0.5) and NIPAM(1):SNP(0.5)SDS below the LCST could not be well fitted because of the poor signal to noise ratio (Figure 4-A). The SAXS curves presented in Figure 4-A show patterns which are significantly different from the curves above the LCST of the same samples (Figure 4-B). Such differences are expected, given that below the LCST the microgel structure is swollen, given a poor signal/noise ratio, as discussed below. However, for the samples below the LCST (Figure 4-A), a certain level of organization is noticed due to slight minima in the regions of q around 0.05 and 0.08 nm-1.


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Figure 4. SAXS measurements of hybrid microgels NIPAM(1)SNP(0.5)SDS (□) and NIPAM(1)SNP(0.5) (○), below (20 °C) (A) and above the volume transition temperature (45 °C) (B). The other SAXS curves are given in SI (Figures S2 and S3).

The two experimental curves in Figure 4-B present some differences between them. They exhibit very well-defined minima and could be fitted with the same form factor model. The curve related to the microgel synthesized in the absence of SDS showed a higher organization, whereas, in the absence of SDS, the SNPs were able to interact with p(NIPAM) chains to a greater extent, which reflects in a larger number of minima in the respective spectrum. The SAXS measurements allowed the confirmation of the presence of SNPs in the microgel structure. Even at small concentrations and in the presence of SDS, SNPs can be noticed in the microgel structure. The core–shell form factor can be understood based on the slightly higher electron density of the polysaccharide compared to p(NIPAM) assuming assembly of the SNPs on p(NIPAM) microgel cores. The scattering length densities of the core (p(NIPAM)) and the shell (SNPs) were calculated as being equal to 9.36x1010 cm-2 and 11.52x1010 cm-2, respectively, by using the average chemical composition and density of p(NIPAM) ((C6H11NO)n, density 1.1 g/cm3) and starch nanoparticles ((C6H10O5)n, density 1.275 g/cm3)7. The calculated scattering length of water 16 ACS Paragon Plus Environment

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(9.46x1010 cm-2) is incidentally close to the one calculated for the core. Consequently, contrast changes (or changes in the electron density of the core) are better seen above the VPTT of p(NIPAM). Besides the low contrast between the two microgel constituent polymers and water, a high level of organization is observed inside the microgel structure above the LCST, leading to a prediction of how the two polymers interact and are disposed inside the particle. In our model we do not take into account a contribution of starch-based nanoparticles (SNPs) to the free energy of core-shell microgel particles because there is no an experimental evidence for this contribution. The fit of the conventional Flory-Rehner equation to the swelling curves yields very small chi-quadrat values, i.e. a very good fit. From our point of view, that should be an evidence that the contribution of SNPs to the microgel swelling-behavior is negligible. The starch nanoparticles loosely decorate the outer surface of swollen microgel particles. In the collapsed state, the microgels are covered by the SNPs - compact monolayer, without interactions between SNPs. That point is clearly pictured in the schematic of Figure 5:


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Figure 5. Schematic representation of the SNP-co-p(NIPAM) hybrid microgel. The starch nanoparticles (blue) loosely decorate the outer surface of swollen microgel particles (black). In the collapsed state, the microgels are covered by the SNPs - compact layer, without interactions between SNPs.

SNPs are not able to enter inside the p(NIPAM) network during its formation, and their interaction with the hydrophilic regions of p(NIPAM) occurs on the microgel surface. The disposal of SNPs around the p(NIPAM) structure is expected to provide stability to the entire structure, as observed by Zhang and Zhuo36 in analogous hydrogels. The Rg values are shown in the Table 2 together with the structure sensitive parameter (ρ=Rg/RH) for all the samples and both temperatures (20 ºC and 45 ºC).


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Table 2. Zeta potential (ζ ), radius of gyration (Rg), hydrodynamic radius (RH) and structure sensitive parameter (ρ) of hybrid microgels in swollen (20 °C) and collapsed (45 °C) state. Sample

T (°C)

ζ (mV)

Rg (nm)

RH (nm)

PDI*

ρ

20

-13.1 ± 0.8

70.4 ± 2.9

219.3 ± 1.1

0.029

0.32

45

-24.7 ± 0.4

90.7 ± 2.1

109.1 ± 0.4

0.017

0.83

20

-16.4 ± 0.4

86.2 ± 1.8

139.0 ± 0.6

0.032

0.62

45

-32.6 ± 0.7

56.8 ± 0.8

74.9 ± 0.3

0.041

0.76

20

-20.2 ± 0.4

51.0 ± 2.9

193.2 ± 2.4

0.041

0.26

45

-27.4 ± 0.7

67.6 ± 5.0

110.7 ± 0.6

0.032

0.61

20

-25.8 ± 0.8

62.4 ± 3.1

140.9 ± 2.2

0.042

0.44

45

-33.5 ± 1.3

51.4 ± 2.4

72.6 ± 0.4

0.025

0.71

20

-30.3 ± 1.6

46.1 ± 4.8

176.4 ± 1.9

0.100

0.26

45

-31.4 ± 1.0

59.0 ± 0.3

105.5 ± 0.7

0.126

0.56

20

-34.8 ± 1.1

63.0 ± 4.2

140.6 ± 0.4

0.029

0.45

45

-33.1 ± 0.4

54.9 ± 0.4

80.8 ± 0.1

0.064

0.68

NIPAM(1):SNP(0.5)


NIPAM(1):SNP(1)

NIPAM(1):SNP(1)SDS

NIPAM(1):SNP(2)

NIPAM(1):SNP(2)SDS

* Polydispersity index determined for RH was calculated by means of the cumulant method

Rg/RH is a characteristic structure sensitive parameter indicating the conformation of a polymer chain in solution and can be used to further elucidate the microgels structure. The structure sensitive parameter at 20º C falls approximately in the range typical for a


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core-shell structure (0.3-0.6).58 According to the literature low values of the structure sensitive parameter are observed essentially when the segment density of the particles decreases radially resulting in the presence of a density gradient between the inner and outer segments of spherical particles. In such cases, the inner core has a higher density than the outer shell. Typically, the values of Rg/RH for hard-sphere micelles, random coil, and rod-like structures are reported as 0.774, 1.78, and ~ 2, respectively.59,60 In Table 2 the values of Rg/RH as a function of temperature remains lower than the hard sphere model value of 0.774 (except only for the value of 0.831 observed for the sample NIPAM(1):SNP(0.5) at 45 ºC). The deviation of Rg/RH from the hard sphere value indicates that the microgels are spherical in shape and consist of a dense core covered with SNP chains as the microgel corona. Modeling the swelling behavior. The swelling of microgels is often treated in the framework of the Flory-Rehner model.41,43-46 In the present case we assume that the SNPs do not contribute to the volume phase transition (VPT). This is similar to the work by Ballauff et al. on microgels with a non-swelling polystyrene core.43 In our point of view this assumption is valid since the SNPs are crystalline to a large extent and starch is not known to exhibit a significant LCST type behavior in the studied T range. Moreover, the hybrid materials are made in the collapsed state of p(NIPAM) and the SNPs cover the microgel surface in this state. Hence, no additional constraints for the shrinking and swelling are expected due to repulsive interactions between the SNPs. In microgel particles, interactions between polymer and solvent are analytically described by the dimensionless parameter χ (i.e., the Flory-Huggins polymer-solvent interaction energy parameter). Empirically, it has been found that χ can be modeled by different functions (e.g.,


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χ = A + B / T or χ = A + B / T + C / T 2 , where T is the thermodynamic temperature, and A, B, and C are the virial coefficients).45,61 However, such functions do not adequately describe the swelling data of the SNP-co-p(NIPAM) hybrid microgels. Alternatively, in line with Nigro et al.,62 the Flory-Huggins χ parameter can be modeled by the series expansion up to ϕ3 term:

χ(T, ϕ) =

1  θ − A 1 −  + χ 2ϕ + χ3ϕ2 + χ 4ϕ3 2  T

(5)

where A is the dimensionless parameter, θ is the characteristic temperature of the volume transition (θ-temperature), χଶ,ଷ,ସ are the dimensionless expansion coefficients, and ϕ is the polymer volume fraction within the microgel particle, described by the classical FloryRehner equation:

ϕ ln(1 − ϕ) + ϕ + χϕ + 0 N Gel 2

 ϕ 1/ 3 1 ϕ    −  = 0. 2 ϕ0   ϕ0  

(6)

In Eq. 6, ϕ0 is the polymer volume fraction of the collapsed reference state and NGel is the average number of segments per chain between two crosslinking points. If the volumes of a molecule of solvent and a molecule of monomer are identical, the number of segments per chain NGel and be identified with the number of monomers per chain.63 Note that the average degree of polymerization between crosslinks NGel and the so-called number of segments per chain NSeg are described by the same relationship NGel = NSeg = ϕ0.NA /(νs.Nc),


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where NA is the Avogadro`s constant, νs the molar volume of solvent and Nc the number of chains in network. For the isotropic swelling, ϕ is related to the hydrodynamic radius RH of particles as:

ϕ  R H,0  =  ϕ0  R H 

3

(7)

where RH,0 is the hydrodynamic radius at the collapsed reference state. Results of the parameters fitted using the Flory-Rehner model (Eq. 6) with the series expansion of χ (Eq. 5) to the data on swelling of hybrid microgels, can be found in Table 3, while the respective fits can be found in Figure 6. Statistical evaluation of the quality of the fits was made through the chi-square test.

Table 3. Parameters used for fits shown in Figure 6 and chi-square values. Parameter χ was modeled by series expansion up to ϕ3 term, according to Eq. 5. Chiϕ0

A

NGel

χ2

χ3

χ4


0.88

-9.2

314

35

0.647

0.096

0.273

8.8

NIPAM(1):SNP(0.5)

0.90

-14.7

310

39

0.576

0.385

0.050

7.5

NIPAM(1):SNP(1)SDS

0.88

-10.8

316

36

1.598

-2.114

1.866

5.8

NIPAM(1):SNP(1)

0.80

-7.6

316

34

0.709

8.43.10-4

0.272

8.6

NIPAM(1):SNP(2)SDS

0.88

-8.0

316

36

0.912

-0.511

0.697

5.7

NIPAM(1):SNP(2)

0.80

-5.5

318

33

0.658

0.023

0.397

5.9

0.88

-9.3

315

36

0.85

-0.35

0.59

(±0.03)

(±2.3)

(±2)

(±2)

(±0.27)

(±0.64)

(±0.46)

Sample ID

θ/K

Average

square/10-3

7.0


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Figure 6. Polymer volume fraction ϕ vs. temperature t at the three NIPAM:SNP feeding ratios: (A, D) 1:0.5, (B, E) 1:1, (C, F) 1:2, for microgels without SDS (A, B, C), and with SDS (0.0011 M) (D, E, F). The function ϕ(T) has been calculated using Eq. 6 with the χ-series expansion, Eq. 5. The fitting has been performed with the software Mathcad Prime 3.0, using the MinErr optimization function, for which a very good fit is achieved.

The average volume fraction of the microgel in the collapsed state ϕ0,avg= 0.88(±0.03) is in agreement with results of most studies, where the value ϕ0 = 0.8 is often obtained from the fits.45 However, Lopez and Richtering have calculated ϕ0 on the basis of literature data for molar mass and radius of a lot of different microgels.44 These calculations yield ϕ0 = 0.44. This is by a factor of different compared to our best fits. Nevertheless, we have tried to use forced fit with 0.44 for ϕ0. However, this was unsuccessful. Determination of molar masses in the range of 108 g/mol is not an easy task and a possible explanation for the found discrepancy might be that the determination of the molar mass of microgels is


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systematically wrong. The average degree of polymerization NGel obtained, which was in the range 33 ≤ NGel ≤ 39, and the averaged parameter Aavg = -9.3(±2.3), both concur with the data of Crassous et al.43 for core–shell NIPAM-covered latexes obtained with the series expansion of χ up to ϕ2. However, the BIS/NIPAM molar-ratio is higher in our study (7.74 mol.%) and, therefore, the average degree of polymerization should be smaller, i.e., NGel < 22. The discrepancy can also be related to our use of the series expansion of χ up to ϕ3. The average θ-temperature of the hybrid microgels θavg = 315 (± 2) K correlates well with several other studies.43 Nonetheless, it is undoubtedly higher than noted in some studies of microgels at similar crosslinker densities.45 The θ-temperature value observed for the samples in this work can be attributed to the amount of SNP incorporated into the microgel structure and its hydrophilic interaction with the amide groups of p(NIPAM), as well as with water molecules. The dependence of χ on temperature has been determined and the obtained curves exhibit the same shape at all NIPAM:SNP ratios (see Figure 7). This also points to the correctness of the assumption that the SNPs do not contribute to the VPT. For particles made without SDS, the curves χ(T) are most similar. For microgel hybrids made in the presence of SDS, the calculated curves χ(T) show a very minor dependence on SNP concentration (see Figure 7-B). All the curves lay approximately at 33.0 (± 0.5) °C at the value χ(T) = 0.5 (Figure 7), showing no dependence of the volume phase transition temperature on SNP or SDS concentrations.


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Langmuir

Figure 7. Interaction parameter χ vs. temperature t at three NIPAM:SNP feeding ratios: 1:0.5, dash-dot black line, 1:1, dash red line and 1:2, solid blue line for (A) microgels without SDS and (B) with 0.0011 M SDS. The function χ(T) has been modeled by Eq. 5.

Analysis of data with Eq. 5 has shown that the χ(T) function makes a characteristic jump between two equally sloped straight lines at T ≈ θ (Figure 7). This behavior is very similar to cooperative transitions in proteins and reminded us of the Hill-like thermotropic cooperative transition between two states.60 Therefore, considering the sudden increase in the χ(T) value looks like a cooperative thermotropic transition, we used the Hill model for cooperative aggregation. The temperature dependence of χ has been modeled by the Hilllike function for a cooperative thermotropic structure-transition with a straight baseline:

ν

 T − T0    T − T0  χ(T) = χ0 + a ( T − T0 ) + b  ref ν  T − T0    +K  Tref − T0 

(8)


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where χ0 is the χ-parameter at T = T0, T0 is the first temperature point of the swelling curve (here t0 = 11.2 °C), Tref is the reference temperature, a is the slope of the basis line, b is the dimensionless amplitude of the Hill transition, K is the half-saturation constant, and ν is the Hill-coefficient (i.e., the stoichiometric coefficient of the reaction: α(H2O)ν ⇄ β + νH2O). The symbol α(H2O)ν denotes the state of hydrophobic-hydrated polymer (hydration of type II) before the VPT, and β denotes the collapsed-state after the transition, when ν water molecules cooperatively left the solvate layer. At the thermotropic transition, the increase in temperature causes an increase in the degree of freedom of the water molecules. Entropically driven, ν water molecules of the solvate layer around a polymer chain become cooperatively “free” particles in bulk water solution. Without the solvate layers, the volume of the microgel particles decreases. Unlike Eq. 5, the Hill-like model does not contain ϕ and has only the variable T. The Hill-model does not contradict to the expansion for the solubility parameter, Eq. 5, because ϕ can be considered as a function of T. Therefore, the two approaches applied here yield very similar χ(T) dependencies. The low-temperature state can be referred to the hydrophobic-hydrated NIPAM chains (hydration of type II). The high-temperature state is characterized by an entropically driven disintegration of the solvate layers around the polymer chains and the collapse of microgel particles. The results of fits obtained by the combination of the Flory-Rehner (Eq. 6) with the Hill-like function χ(T) (Eq. 8) can be found in Table 4, while the fits can be found in Figure 8.


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Langmuir

Table 4. Parameters used for fits shown in Figure 8 and chi-square values. Dependence χ(T) was modeled by the Hill-like (Eq. 8) at Tref = 37.7 °C. In Eq. 6, ϕ0 = 0.79. ChiSample ID

K

v

NGel

a/K

b

χo

square/10-3


0.23

17

60

0.0225

0.44

-0.06

4.4

NIPAM(1):SNP(0.5)

0.015

27

101

0.025

0.45

-0.06

4.8

NIPAM(1):SNP(1)SDS

0.184

17

65

0.0255

0.5

-0.115

4.0

NIPAM(1):SNP(1)

0.46

16

39

0.025

0.5

-0.135

2.4

NIPAM(1):SNP(2)SDS

0.295

14

54

0.0195

0.48

-0.015

2.2

NIPAM(1):SNP(2)

0.34

17

37

0.021

0.49

-0.015

2.5

0.25

18

59

0.023

0.48

-0.010

±0.11

±3

±16

±0.002

±0.02

±0.03

Average

3.4

Figure 8. Polymer volume fraction ϕ vs. temperature t at three NIPAM:SNP feeding ratios: (A, D) 1:0.5, (B, E) 1:1, (C, F) 1:2, for microgels without SDS (A, B, C), and with SDS (0.0011 M) (D, E, F). The function χ(T) in Eq. 6 has been calculated using the Hill-like Eq. 8. The fitting has been performed with the software


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Mathcad Prime 3.0, using the optimization MinErr function. In this case a nearly perfect fit of the experimental data is achieved.

The reference temperature in Eq. 8 has been adjusted to Tref = 311.9 K (37.7 °C), which is close to the average θ-temperature of the hybrid microgels, θavg = 315(±2) K (41±2 °C), obtained in the later model with the series expansion of χ up to ϕ3. Similar to the fits with the series expansion of χ up to ϕ3, the parameters of the Hill-like function do not show any obvious dependence on the NIPAM:SNP feeding ratio, or on the use of SDS in the synthesis. The average value of NGel,avg = 59 (± 16) from Table 4 is 1.6-fold larger than NGel,avg = 36 (± 2) obtained by the series expansion of χ. The Hill-like description of the χ parameter provides a better quality of the fits than the series expansion of χ, according to the chi-square values obtained from the numerical error minimization (see Tables 3 and 4). The numerical value of the Hill-coefficient ν = 18 suggests that at the VPTT, on average, 18 water molecules cooperatively leave the hydrophobic solvate layer around the polymer chains and enter the bulk solution. For the particles made without SDS, χ(T) curves do not lay as close as seen for the particles prepared in the presence of SDS (refer to Figure 9). Consistent with Figure 9-A, the experimental φ-curves are also different (Figure 8). The curves in Figure 9-B are very similar to the χ(T)-curves in Figure 7-B. As expected, the overall shape of the curves χ(T) is independent of whether Eq. 5 or Eq. 8 has been applied to model χ(T).


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Langmuir

Figure 9. Interaction parameter χ vs. temperature t at three NIPAM:SNP ratios: 1:0.5, dash-dot black line, 1:1, dash red line and 1:2, solid blue line for (A) microgels made without SDS and (B) microgels synthesized with 0.0011 M SDS. The function χ(T) has been modeled by the Hill-like Eq. 8.

The two approaches to model χ(T) use essentially different sets of parameters, which practically cannot be compared. However, the parameters NGel and ϕ0 of the FloryRehner equation obtained either with Eq. 5 or Eq. 8 are quite comparable. Idealtheoretically, NGel and ϕ0 should be independent of the model applied for χ(T). In practice, the average degree of polymerization NGel,avg = 36 (±2) obtained with Eq. 5 is 1.6-fold smaller than NGel,avg = 59±16 of Eq. 8. The average values 36 and 59 are within the region 80 ≥ NGel ≥ 22 at, respectively, 1.25 ≤ n(BIS)/n(NIPAM) [mol%] ≤ 5.00 for NIPAMcovered latexes.43 Though, it is not yet quite clear, why, at the relatively high concentration of BIS (7.74 mol%), the NIPAM:SNP microgels should have such a large degree of polymerization. A possible reason could be the real amount of incorporated SNPs into the microgel, which might change according to the synthetic route used. From Table 4, it can also be observed that the increase in SNP amount decreases the NGel, which reflects the


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SNP presence in the microgel particles. Such a tendency, as well as the differences, could not be observed in the model with the series expansion of χ up to ϕ3 (Table 3).

CONCLUSIONS SNPs were formed using nanoprecipitation, aiming at their use in NIPAM/SNP hybrid microgel formation. Such hybrids were successfully prepared using PP and SFPP. Slight variations in size, VPTT, and colloidal stability of starch/p(NIPAM) hybrid microgels could be observed, mainly for particles synthesized by SFPP that exhibit a certain sensitivity to the SNP content. Besides protection against aggregation, the presence of surfactant during the preparation of hybrid microgels induced a specific retention in the polymeric network, partially suppressing the presence and concentration of the SNPs. This effect may be a result of a partial hampering of incorporation of SNPs during the synthesis. Zeta potential analysis combined with SAXS results provided insight into how the constituents are organized inside the hybrid microgel particles. Above the LCST, a core– shell structure for hybrid microgels was found, where the p(NIPAM) network is the core, while SNPs form the shell. Two different approaches to model the temperature dependence of the χ-parameter of the Flory-Rehner model have been considered in the paper: the series expansion of χ up to ϕ3 term and the novel Hill-like model for a cooperative thermotropic structural transition. Both strategies have been applied to describe the peculiar jump-like temperature behavior of the χ(T) function, where both approaches yield similar χ(T)dependencies. Results of the Hill-like model compare well with those of the conventional series expansion of χ up to ϕ3. The novel Hill-like model suggests that at the VPTT, about 18 water molecules per polymer segment cooperatively leave the solvate layers of each polymer chain in the network. Therefore, the Hill-like thermotropic model offers a new, deeper insight into the underlying processes at the VPTT in microgels compared to the 30 ACS Paragon Plus Environment

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Langmuir

classical models for χ. The fits with both the Hill-like model and the conventional series expansion of χ up to ϕ3 yield very similar average volume fractions of the microgel in the collapsed state, ϕ0 = 0.79 and ϕ0 = 0.88(±0.03), respectively. These results suggest that the value of ϕ0 is practically independent of the way of modelling of the interaction parameter χ. However, the value ϕ0 = 0.79 obtained with the Hill-like model matches better (than ϕ0 = 0.88) with the value ϕ0 = 0.8, often reported in the literature.45 Despite of the advances arising from the Hill-like approach, it remains to be understood why the collapsed values ϕ0 = 0.79 and 0.88 differ so much from the reference volume fraction ϕ(45°C) = 0.44, as expected from the analysis of literature data.45 Note however, that the data basis used for the determination of 0.44 calculated with Mw from the SLS measurements45 shows a rather broad distribution of the ϕ0 values comprising values in range of approx. 0.8. To rationalize why the light scattering measurements by various groups are outside the range of validity of SLS one could propose the three reasons: (1) SLS measurements primary yield the gyration radius Rg and the average molecular weight Mw in form of the ratio Rg2/Mw ~ 1/ISLS. Rg is usually considerably smaller than the hydrodynamic radius Rh of the same microgel particles. Recalculation of Rh (= [3Mw/(4NA ρpπφ)]1/3) using Mw from the SLS measurements45 might be not quite straightforward because Mw could be biased by Rg in the ratio Rg2/Mw. (2) For the SLS method, the two following criteria for the correct measurements should be settled: i) sample concentrations should be chosen so low that multiple scattering is absent. If transmittance values exceed 0.95, multiple scattering can be neglected;64 ii) one should measure within the range of validity of the Rayleigh-GansDebye light scattering formalism defined by 2α.(m - 1)

Smart StarchâPoly(N-isopropylacrylamide) Hybrid Microgels

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