Combined Cononsolvency and Temperature Effects on Adsorbed

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Combined Cononsolvency and Temperature Effects on Adsorbed PNIPAM Microgels Sebastian Backes, Patrick Krause, Weronika Tabaka, Marcus U. Witt, and Regine von Klitzing Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02903 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 22, 2017

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Combined Cononsolvency and Temperature Eects on Adsorbed PNIPAM Microgels Sebastian Backes,

†, ‡

Patrick Krause,

†

Weronika Tabaka,

Regine von Klitzing

†Stranski-Laboratorium

†

Marcus U. Witt,

†, ‡

and

∗,†,‡

für Physikalische und Theoretische Chemie, Technische Universität

Berlin, Straÿe des 17. Juni 124, 10623 Berlin, Germany

‡Institut

für Physik, Technische Universität Darmstadt, Alarich-Weiss-Strasse 10, 64287 Darmstadt, Germany

E-mail: [email protected] Phone: +49 (0)6151 1625647

Abstract The present study adresses the multi-responsive behavior of Poly(N-isopropylacrylamide) (PNIPAM) microgels adsorbed to interfaces. The microgels react to changes in temperature by shrinking in aqueous solution above their volume phase transition temperature (VPTT). Additionally, they shrink in mixtures of water and ethanol, although both individual liquids are good solvents for PNIPAM. The combination of this so-called cononsolvency eect and the temperature response of adsorbed microgels is studied by atomic force microscopy (AFM). Adsorbed microgels are of special interest because they are compressed considerably compared to those in bulk solution. It is shown that the impact of adsorption on swelling depends on the specic surface details, as well as the sample preparation. Thereby the microgels are deposited on two dierent kinds of surfaces: on gold surface and on polycation (PAH) coating which show dierent interactions with the microgels in terms of electrostatic interaction and wettability. In 1

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addition the microgels were deposited from dierent solvent mixtures. This inuences the microgel structure and thereby the swelling properties. Nanorheology studies by dynamic AFM measurements lead to surprising results which are explained by the fact that not only polymer density but a subtle interaction between polymer and solvent might dominate the rheological properties. This work supports the view that preferential adsorption of ethanol at PNIPAM drives cononsolvency, while the shrinking at T>VPTT is caused by general breaking of hydrogen bonds between solvents and PNIPAM.

1 Introduction Poly(N-isopropylacrylamide) (PNIPAM) is a smart polymer that is sensitive to external stimuli, most prominently to temperature, as it exhibits a lower critical solution temperature

◦ 16 (LCST) around 32 C in water. It is soluble below and insoluble above this temperature. This behavior is explained by a balance of hydrogen bond energy (between water and polymer) and entropy. Above the LCST the hydrogen bonds between water and PNIPAM break and the polymer becomes insoluble. PNIPAM-based microgels can be synthesized by adding a cross-linker and thereby building a three-dimensional polymer network. Those microgels

◦ have a volume phase transition temperature (VPTT) of around 32 C, above which they expel water and shrink. Microgels combine the advantages of a fast stimuli response and being colloidally stable even above the VPTT. The responsiveness of PNIPAM oers numerous potential applications, like in medicine and drug delivery

79

or sensors.

10,11

A very interesting phenomenon, the so called cononsolvency eect, can be observed when adding certain organic solvents, such as short-chained alcohols, to an aqueous PNIPAM solution.

1215

Both water and alcohol are good solvents for PNIPAM, but when a small

amount of alcohol is added to water, this causes a shrinking of microgels.

Upon further

addition of alcohol the microgels reswell. There are several approaches to explain the cononsolvency phenomenon, but a generally

2


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accepted theory of the molecular driving forces behind it is still missing.

As the solvent

quality is steadily improving when more and more alcohol is added, classical mean eld theory cannot describe the miscibility gap for intermediate alcohol content. of explanations is focused on the solvent structure.

16

One category

It is assumed that solvent-cosolvent

complexation is preferred over bonds of solvent or cosolvent to the polymer, and that those complexes are a poor solvent for the polymer. by Hao et al.

14,17,18

Another approach in this category

is concentrated on composition uctuations in THF-water solutions, which

were assumed to be the main reason for PNIPAM cononsolvency.

19

A second category of

explanations concerns the microscopic solute-solvent interactions. Tanaka et al. considered competetive adsorption of solvent and cosolvent as the cause for cononsolvency.

2022

In their

theoretical approach cooperative adsorption leads to a higher probability of one solvent molecule to adsorp in the vicinity of another molecule of the same kind along the polymer chain. This nonlinearly amplies the changes in solvent composition. However, they neglect solvent-cosolvent interactions, which contradicts the rst category of explanations, as well as simulations with perturbed chain statistical association uid theory (PC-SAFT) as studied by Arndt et al..

23

Mukherji et al. have found preferential interactions of one solvent with

the polymer as a generic phenomenon to explain cononsolvency. al.

16,24

Rodriguez-Ropero et

however considered the chemistry-specic balance between solvation free energies and

congurational entropy as an explanation for cononsolvency.

25

Bischofberger et al. examined

the water-rich and alcohol-rich phases seperately. They found hydrophobic hydration and the kosmotropic eect of alcohols in the rst case and classical mixing contributions to the thermodynamics of polymer solutions in the second case to explain the cononsolvency phenomenon.

26

Some experimental studies have examined the eect of cononsolvency on the VPTT of PNIPAM microgels.

By addition of up to 30 mol% ( ≈49 vol%) of methanol or 8 mol%

(≈22 vol%) of ethanol to aqueous solutions the VPTT decreases, as has been shown by dynamic light scattering (DLS).

26,27

At higher alcohol concentration, no VPTT is observed,

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as PNIPAM does not show a VPTT in alcohol in the common measuring ranges.

This

can be explained by the stronger hydrogen bonds of PNIPAM with alcohol compared to water.

16

Consequently, the plateau minimum of the hydrodynamic radius at high tempera-

tures increases with increasing alcohol concentration. These characteristics for microgels are generally similar to linear chains.

27

Few studies have been carried out so far concerning the cononsolvency eect on microgels which are adsorbed to solid surfaces. Adsorption of microgels to a surface can lead to a compression by more than one order of magnitude.

28,29

The higher polymer density in

adsorbed microgels can aect their stimuli response in several ways. Therefore, studying the behavior of adsorbed particles is of great importance, especially for potential applications like sensors. One approach was based on microgels adsorbed between two gold layers like an etalon in mixtures of water and methanol.

30

The VPTT was shifted to lower tempera-

tures with increasing methanol concentration, and comparison to microgels in bulk solution did not show any signicant dierence.

An eect of connement was however found in a

combined DLS and atomic force microscopy (AFM) study of PNIPAM and P(NIPAM-coAAA) microgels on silicon wafers coated with the cationic polyelectrolyte poly(allylamine hydrochloride) (PAH).

31

The minimum volume of PNIPAM microgels in bulk was reached

at 10 vol% of ethanol. Conned to the surface, however, the minimum is found at 40 vol% of ethanol. This shift in the minimum volume towards higher organic solvent concentrations is explained by the higher polymer density in conned gels and a preferential water-uptake of PNIPAM-rich phases. This is derived from the larger particle volume in pure water compared to pure organic solvents.

This nding is in contradiction to the results described

above. Additionally, the sizes of microgels spin-coated from dierent water-solvent solutions have been compared under ambient conditions. An increase in ethanol concentration during spin coating leads to higher particles with a reduced width and an overall increased volume. A possible explanation provided for this phenomenon is the entrapment of organic solvent molecules due to their bigger size.

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The contradiction of those ndings from previous studies of adsorbed microgels leads to the present work.

The question is if the degree of hydrophilicity of the surface has an

eect on the solvent composition within the adsorbed microgel. The inuence of adsorption to both PAH and gold surfaces on the cononsolvency eect in water-ethanol mixtures is studied by AFM. Additionally, the temperature eect in cononsolvent systems is analyzed systematically for the rst time on both surfaces, and the inuence of the sample preparation is examined.

Those ndings are compared to the behavior of PNIPAM microgels in bulk

solution, which has been studied by DLS. Additional insights are provided by dynamic force measurements with AFM, which have been performed on microgels for the rst time with varying temperature in dierent solvent compositions. By those measurements the storage and loss modulus can be obtained separately, giving information about elastic and viscous properties.

This allows to draw conclusions about the internal structure of the polymer-

solvent system.

2 Materials and experimental methods 2.1

Materials

N-isopropylacrylamide (NIPAM), N,N'-methylene bisacrylamide (BIS), potassium persulfate (KPS), acrylic acid (AAc), and poly(allylamine hydrochloride) (PAH, Mw ≈58,000) were purchased from Sigma-Aldrich. Silicon wafers (orientation:

The used ethanol was purchased from Chemsolute.

100) were purchased from Siltronic AG (Munich, Germany).

Gold-coated silicon wafers (polycristalline gold with 100 nm thickness on 100 silicon wafer, pre-coated with titanium for better adhesion) were purchased from Georg Albert PVPBeschichtungen (Silz, Germany).

For water purication a three stage Millipore system

(Milli-Q Plus 185) was used.

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2.2

Microgel synthesis

Synthesis of the microgels used in this work was done via surfactant free precipitation polymerization.

32

PNIPAM and P(NIPAM-co-AAc) microgels were synthesized with the nega-

tive initiator KPS. The NIPAM monomers, AAc comonomers if applicable (5 mol%), and crosslinker BIS (5 mol%) were dissolved in Milli-Q water and the solution was degased for 60 min with nitrogen under constant stirring.

The solution was transferred into a reactor

◦ and heated to 70 C. To start the reaction 1 ml of the initiator KPS dissolved in Milli-Q water was transferred into the reactor. The reaction temperature was maintained for 3 h. Then the heat was turned o and the reaction volume was allowed to cool to room temperature over night. At room temperature the microgels were puried by dialysis against Milli-Q water

◦ for 10 days. Afterwards they were freeze-dried at a temperature of -85 C and a pressure of 0.001 mbar.

2.3

Sample Preparation

The gels were deposited on silicon wafers with two kinds of coatings, namely PAH and gold. For the PAH coating, silicon wafers were cleaned in a plasma cleaner and then deposited in a 0.01 M PAH solution (with 0.1 M NaCl) for 30 min. Afterwards, they were deposited in MilliQ water for 1 min to remove residual PAH and then dried in a nitrogen stream. The microgels were then spincoated on the wafers. To do so they were solved in MilliQ water unless noted otherwise. The contact angle of water was measured via the sessile drop method.

◦ PAH is a hydrophilic surface with a contact angle of 44 , while gold is less hydrophilic and ◦ has a contact angle of 87 .

2.4

Scanning force microscopy

The topography of microgels adsorbed to a surface was scanned with an atomic force microscope (AFM). All measurements were done with a Nanowizard II (JPK, Berlin, Germany) us-

6


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ing HQ:NSC18/CR-AU BS tips (Mikromash, Soa, Bulgaria) in intermittent contact mode. The tips have a typical radius of 8 nm and a chromium-gold coating on their backside for better reection. Measurements were performed in a temperature-controlled JPK liquid cell in solutions of water and ethanol. At least three microgels were scanned for each water-ethanol composition at each temperature and the average was taken. The particle analysis was performed with Gwyddion (free software).

The imaging of soft microgels in liquid is rather

challenging and time consuming. Therefore the results presented in the plots of Fig. 2 and 3 could not be achieved within one day. As a consequence the AFM cell with the sample had to be reset another day with another cantilever and the apparatus parameters like setpoint and gain had to be adjusted to get the optimum results. Therefore absolute values for the dimensions of microgels were not always comparable, and only the volume relative to the

◦ minimum value of the collapsed particles at 50 C is shown for the temperature-dependent measurements to ensure comparability. In contrast the data shown in Fig. 4 could be obtained with the same cantilever and the same (or at least very similar) apparatus parameters (setpoint, gain). Therefore absolute values are shown there. The adsorbed microgels ana-

◦ ◦ lyzed in this study possess a contact angle of 20-40 , which is well below 90 . Therefore no major artifacts resulting from the tip geometry are expected, which might arise in systems possessing sharp edges.

2.5

Dynamic force measurements

Dynamic force measurements were done with an MFP-3D AFM (Asylum Reasearch, Oxford Instruments, Santa Barbara, CA) with a CoolerHeater (Asylum Research) for temperature control in liquid, with a drop of the respective solvent being placed upon the sample. HQ:CSC38/NO AL tips (Cantilever C, MikroMasch, Soa, Bulgaria) were used for all measurements. Those tips are uncoated and have a half tip cone angle of 20

◦

and a reference

spring constant of 0.05 N/m. Before the measurements the tips had to be calibrated against a hard surface to obtain the inverse optical lever sensitivity (InvOLS). Then the exact spring

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constant was determined by tting the thermal noise spectrum of the cantilever.

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Those

procedures are built in the Asylum Research software. The dynamic modulus ments.

3336

G∗ = G0 + iG00

can be obtained from dynamic force measure-

First a standard force map was recorded in the area around a microgel particle.

From the height prole the center of the particle can be identied, which is where the tip is then placed for dynamic measurements as shown in Fig. 1a. All dynamic procedures were controlled with the built-in dwell panel of the Asylum Research software. The procedure is exemplarily shown in Fig.1b. The tip is approached to the surface with a velocity of 2

µm/s

until a load force of 2 nN is exerted on the cantilever (area I in Fig. 1b). Then, the tip is kept here to equilibrate for 5 s (area II), until the cantilever starts with a sinusoidal oscillation for 10 s (area III). The amplitude is 10 nm and the frequency is varied between 5, 10, 20, 40, 60, 80, and 100 Hz, which is well below the resonance frequency of the cantilever ( ≈5 kHz). After 10 s of oscillation the cantilever is retracted from the microgel (area IV). Force and indentation signals were used for data analysis. The recorded values depend to some extend on the load force exerted by the tip. Higher forces yield slightly higher values for

G0 ,

which might be referable to the nite size of the

microgels. Measurements with a very low load force however produce an unstable oscillation signal. Therefore, a constant value of 2 nN was chosen for all measurements to ensure reasonable and comparable results.

The indentation was around 100 nm, which is signi-

cantly larger than the tip radius, so a conical indenter shape was assumed. Because of their rather small size, pure PNIPAM microgels are not well suited for indentation measurements with AFM. Therefore, P(NIPAM-co-AAc) microgels were synthesized. Because of the negative charge of the comonomers, a bigger radius is achieved. The cononsolvency behavior of P(NIPAM-co-AAc) is similar to pure PNIPAM.

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Figure 1: a) Height prole from force map on microgel particle. The red dot indicates the location of the tip, where dynamic force measurements are performed.

b) Example force

against time plot during a dynamic force measurement.

Data analysis The Hertzian contact mechanics model

37,38

allows the calculation of the complex modulus

of the sample for a conical indenter by

G∗ = G0 (ω) + iG00 (ω) =

with the Poisson ratio

ν,

1 − ν F (ω) 3δ tan(φ) δ(ω)

(1)

which was assumed to be 0.5 (value for incompressible materials

because of the high water or ethanol content in the gels), the indentation depth half tip cone angle

φ. F (ω)

and

δ(ω)

δ

and the

are the force and indentation signals recorded during

the oscillation of the cantilever, which are given by

F (ω) AF (ω) i(ϕF (ω)−ϕδ (ω)) = e δ(ω) Aδ (ω) with the respective amplitudes

(2)

A and phases ϕ of the force and indentation signals which have

been tted with a sine function. The complex modulus consists of a real part, the storage

0 00 modulus (G ) and an imaginary part, the loss modulus ( G ). Dynamic force measurements allow to separately obtain both parts. the viscous properties of the sample.

G0

represents the elastic properties and

G00

represents

A sample which acts perfectly elastic would give

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zero phase shift between force and indentation, thus giving only a real part for the complex

◦ modulus. On the other hand, a purely viscous sample would give a phase shift of 90 , leaving only an imaginary part for the complex modulus. Real samples like the microgels observed in this study give a phase shift between 0 of elasticity and viscosity. The quotient

◦

and 90

G00 /G0

◦

according to the respective inuence

is called loss tangent and gives information

about whether elastic or viscous behavior is dominating. By tting the approach part of the curves (area I in Fig. 1b), the elastic modulus be calculated additionally to the complex modulus, using the Hertz model

37

E

could

implemented in

the Asylum Reasearch software:

F =

2Eδ 2 tan φ . π(1 − ν 2 )

(3)

Three dierent microgel particles were analyzed for every system and the average was taken. The hydrodynamic drag which acts upon the cantilever was found to be insignicant for the present system, so no systematic hydrodynamic drag correction as has been done in similar studies

2.6

3335,39

was performed.

Dynamic light scattering

The hydrodynamic radius of microgels in bulk solution was measured by dynamic light scattering (DLS). Measurements were performed on an LS spectrometer (LS Instruments, Fribourg, Switzerland) with a HeNe laser at

λ=632.8 nm

with 21 mW. For the correlation

function the LS spectrometer was used. The scattering intensity was measured for 30 s at angles between 30

◦

and 120

◦

◦ in steps of 5 .

In temperature dependent measurements all

◦ ◦ ◦ samples were heated from 14 C to 60 C and then cooled again in steps of 2 C. The data were tted using a self-written script with the cumulant t procedure.

10


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3 Results 3.1

Microgel topography

The topography of adsorbed microgels was scanned with AFM. Fig. 2 shows the relative volumes of PNIPAM microgels on PAH and gold compared to the hydrodynamic radius in

◦ bulk solution. The ethanol concentration is varied at a constant temperature of 20 C. For the surface data the same set of particles was observed throughout the whole composition cycle. On PAH, the ethanol concentration was rst increased from 0 to 100 vol% and then decreased to 0 vol% again. No signicant hysteresis is observed. On gold the microgels are detaching in pure ethanol, so no full composition cycle as on PAH could be performed here. A slight shift of the composition where the minimum is located can be seen for PAH compared to gold and bulk. While the minimum volume in bulk and on gold is observed at 30 vol% ethanol, on PAH 40 vol% ethanol are needed for the maximum collapse. P(NIPAM-co-AAc) microgels show a similar trend, with a plateau minimum on PAH and gold between 30 and 50 vol% ethanol.

Figure 2: Volume relative to the minimum volume of PNIPAM microgels on a) PAH and b) ◦ gold surfaces and c) hydrodynamic radius in bulk in varying water-ethanol solutions at 20 C. The volume of the adsorbed microgels is normalized with respect to the minimum, since not all curves in the paper could be measured with the same tip, leading to slight deviations.

The temperature dependent swelling with varying solvent composition is shown in Fig. 3.

◦ ◦ The VPTT in pure water is always located between 30 C and 35 C. On PAH, 10 vol% ◦ ethanol show no eect on the VPTT, whereas 20 vol% ethanol decrease the VPTT by 5 C.

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No VPTT is observed for 30 vol% ethanol or more. This is in contrast to the bulk behavior,

◦ where 10 vol% ethanol already lead to a decrease of the VPTT by 4 C and 20 vol% ethanol ◦ lead to a decrease by more than 10 C. To investigate the inuence of the type of surface as well as the sample preparation, measurements were also performed on gold wafers and on particles which were spincoated from the same water-ethanol mixture that was used in the swelling experiments afterwards.

In both cases the resulting temperature dependence

◦ resembles the bulk behavior, with a reduction of the VPTT by 5 C for 10 vol% ethanol ◦ and 15 C for 20 vol% ethanol.

Apart from the transition temperature, the swelling ratio

varies depending on the surface and sample preparation conditions as well. Fig. 3 shows that on PAH, microgels only swell by a factor of 2.25, while on gold the swelling factor is 3.75 compared to the collapsed state above the VPTT. Furthermore, microgels spincoated on a PAH surface from an aqueous solution containing 10 vol% ethanol show a higher swelling, with a factor of nearly 3. A reswelling of PNIPAM microgels in bulk solutions of ethanol and water at temperatures

◦ 40 above 50 C has been reported recently. Because the AFM liquid cell used in the present ◦ study is not fully closed, evaporation made measurements above 50 C impossible, so no reswelling at high temperatures could be observed at the surface.

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Figure 3: Volume relative to the minimum volume of PNIPAM microgels on a) PAH surfaces after spincoating from aqueous solution, b) PAH surfaces after spincoating from solutions in the respective water-ethanol mixture, c) gold surfaces, and d) hydrodynamic radius in ◦ bulk during heating in 0, 10 and 20% ethanol. 50 C was the maximum temperature in AFM experiments in order to avoid evaporation.

To better understand the behavior of microgels on surfaces and the inuence dierent surfaces have on the swelling, the gel dimensions are compared on gold and PAH. They are analyzed in ambient (called dry in the following) state after spincoating from pure water, 10 vol% or 20 vol% ethanol, and after swelling in water or ethanol (except on gold, because immersion in ethanol leads to detachment of the microgels here). The results are presented in Fig. 4. Generally, the microgels are always bigger on PAH than on gold, and when spincoated from pure water, they are atter on gold. Their dimensions in both lateral and vertical direction are larger in water than in ethanol.

As reported before, microgels

in dry state are bigger, have a smaller surface area and larger volume when the ethanol concentration in the spincoating solution is increased.

31

The gels are much atter (with

atness being dened as the quotient of the microgel radius on the surface, and the particle

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height) in ambient conditions than in solution, which means that swelling occurs mainly in vertical direction.

Figure 4: Comparison of a) volume, b) projected surface area, c) height, and d) atness (rSurface /height) of PNIPAM microgels on gold and PAH. The microgels were spincoated from aqueous solutions containing 0 vol% (not further indicated), 10 or 20 vol% ethanol and were measured in ambient conditions (dry), in water, or in ethanol. The measurements were ◦ carried out at 20 C.

A good indicator for interactions between microgels and surfaces is the swelling ratio for the surface areas

A,

fA

which can be calculated as

fA = as well as the swelling ratio

fV

Aliquid , Adry

concerning the volumes

fV =

(4)

V,

Vliquid . Vdry

given as

(5)

The swelling ratios given in Tab. 1 clearly show that microgels on gold swell more in the lateral direction than microgels on PAH.

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◦ Table 1: Particle swelling ratio from ambient to liquid conditions at 20 C.

fA,water fA,ethanol fV,water fV,ethanol 3.2

on PAH

on Gold

2.53

3.59

2.47

-

14.18

20.25

11.83

-

Storage and loss moduli of microgels

Dynamic force measurements were carried out with AFM on P(NIPAM-co-AAc) microgels deposited on gold surfaces. AAc monomers add electric charges to the microgels. Therefore, electrostatic forces increase the adhesion to the surface, and those gels stick to gold surfaces even in ethanol, in contrast to pure PNIPAM gels. Fig. 5 shows the cononsolvency behavior

◦ of P(NIPAM-co-AAc) microgels at a constant temperature of 20 C. A minimum volume is observed between 30 and 60 vol% ethanol.

Figure 5: Volume relative to the minimum volume of P(NIPAM-co-AAc) microgels on gold ◦ surfaces at 20 C.

0 Fig. 6a shows the storage modulus ( G ) for dierent water-ethanol mixtures and dierent temperatures.

◦ At 20 C, surprisingly, the highest values for

G0

are not observed at 30%

ethanol, where the volume minimum is located (see Fig. 2), but for 60% and 100% ethanol. No frequency dependence of

G0

◦ is observed at 20 C. The

G0

◦ values obtained at 50 C are

◦ ◦ signicantly higher than at 20 C. In contrast to the values at 20 C, they increase with increasing frequency.

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Figure 6: a)

G0

and b)

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G00 /G0

of P(NIPAM-co-AAc) microgels in dependence of frequency ◦ ◦ for dierent water-ethanol mixtures at 20 C and 50 C. The microgels are spin-coated from aqueous solutions on gold surfaces.

00 0 The loss tangent ( G /G ) is shown in Fig. 6b. Although the than the

G0

G00 data scatter more strongly

data, several trends can be observed. The loss tangent for the microgels in 0%

◦ and 30% ethanol at 20 C is the lowest with values below 0.1, which indicates a mostly elastic behavior. For microgels in a solution with higher ethanol content (60% and 100%), the loss tangent is up to 0.3, which still implies a behavior dominated by elasticity rather than by

◦ viscosity. At 50 C however, the loss tangent both for 0% and 30% ethanol is signicantly higher. This means that, in a highly shrunken state, the role of viscous behaviour is more pronounced. From the force curves obtained during the indentation process of the dynamic AFM measurements, the elastic modulus alongside averaged values for materials 0.5.

41

E

is connected to

The obtained

E/G0

G0

G0

and

E

could be extracted as well.

G00 , are given in Table 2.

by the relation

The obtained values,

For homogenous and isotropic

E = 2G(1 + ν) = 3G

for a poisson ratio

ν

of

values are between 1.27 and 1.57, which is lower than expected.

This would lead to negative values for the apparent Poisson ratio and -0.37.

16


ν=

E 2G0

−1

between -0.07

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0 and the ratio E/G for P(NIPAM-co-AAc) microgels ◦ ◦ for dierent water-ethanol mixtures at 20 C and 50 C.

Table 2: Average values of

G0 , G00 , E ,

◦ Temperature [ C]

Vol% EtOH

G0 [kPa]

G00 [kPa]

E [kPa]

E/G0

20

0

98

6

183

1.87

20

30

156

14

198

1.27

20

60

208

108

320

1.54

20

100

177

62

293

1.66

50

0

544

374

732

1.35

50

30

448

246

680

1.52

4 Discussion A central aspect of this work is the inuence of connement at a surface on the cononsolvency eect for microgels. A slight shift in the concentration at which the particle size minimum occurs has been detected between PAH, with 40 vol%, and gold and bulk, with 30 vol%. This is consistent with literature, as a shift between PAH and bulk has been reported before, while no shift has been found between gold surfaces and bulk.

30

31

This already indicates that

not only connement to a surface, but also the specic properties of the surface play a role in inuencing the cononsolvency eect.

The shift of 10 vol% for PAH which is found

here is however much less signicant than 30 vol% found in Ref.

31

Reason for this deviation

might be the smaller increments in ethanol concentration measured in the present work in comparison to the former work. Temperature dependent measurements and the shift in VPTT provide further insight into the impact of surfaces. The fact that a deviation from bulk behavior can be observed for microgels on PAH surfaces, but not on gold, is consistent with the results from cononsolvency experiments at constant temperature.

Accordingly, adsorption to PAH surfaces

has a repressive impact on the cononsolvency eect. A higher ethanol fraction is needed to achieve a particle shrinking and a decrease of the VPTT, respectively. Thus, there have to be stronger interactions between PNIPAM and PAH than between PNIPAM and gold. This is supported by the swelling ratio in lateral direction

fA ,

which is higher for gold surfaces

than for PAH. Microgels are exibel particles, so they atten upon adsorption, depending

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on internal factors like the cross-linker concentration,

Page 18 of 29

42

and on the respective surface. This

can also be seen in the gel dimensions after spincoating. Microgels on gold are signicanty smaller and atter than the same microgels on PAH. A further hint for the weaker interaction between PNIPAM microgels and gold is the fact that they can be easily removed by exposure to ethanol, while they remain xed at PAH surface.

The attraction of PNIPAM to gold,

which is a good conductor, can be referred to image charges. On PAH however, microgels are adhering to the surface at rst contact, preventing them from spreading out and resulting in larger particles which are less at. Both the opposite charges of PAH (+) and PNIPAM gels (- due to negatively charged initiator), as well as interdigitation between chains of those two polymers might be responsible for this adherence. PNIPAM microgels are consequently not in an equilibrium state after spincoating on a PAH surface. The chain conformation obviously inuences the swelling behavior more strongly than the presence of a surface itself. The theory that simply a higher polymer density is responsible for the shift in microgel volume minimum

31

no longer holds, especially since microgels are even more compressed on gold.

Bonds of PNIPAM to ethanol are preferred over bonds to water. nuclear magnetic resonance (NMR) study

43

16

Furthermore, ndings of a

suggest that the respective methanol or ethanol

concentration conned inside a PNIPAM macrogel is higher than in bulk solution.

This

might indicate that ethanol is even more enriched in PNIPAM microgels of higher PNIPAM density like after adsorption at a surface.

According to our knowledge there is no study

about the water/ethanol ratio as a function of PNIPAM density.

Therefore we can only

speculate about this point. Measurements with azobenzene-containing surfactant have also shown that the microgel is more hydrophobic than the aqueous environment.

44

It is rather

the non-equilibrium microscopic chain arrangement and the reduced exibility that might increase the water ratio inside the gel and hinders the cononsolvency eect on PAH. The solution from which the microgels are spincoated inuences the overall structure and thereby also the internal microscopic chain arrangement.

Microgels spincoated from

mixtures are larger in dry state than microgels spincoated from pure water, so the meshes

18


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of the polymer network might be less compressed.

It is assumed that this is caused by

the larger ethanol molecules. Those larger meshes can take up the respective water-ethanol mixture in the swelling process. If spincoated from pure water however, the meshes are more compressed, and a smaller fraction of ethanol molecules can t in. In the polymer conned to PAH after spincoating from water, the ethanol concentration might therefore be lower than in polymers in bulk solution. This might explain the slight shift of the cononsolvency minimum to higher ethanol concentrations. In summary, the microscopic structure of the gel network, inuenced by the surface coating and the spincoating conditions, terminates the cononsolvency behavior of adsorbed microgels. A second factor which might inuence the shift on PAH compared to bulk and gold is

◦ ◦ the hydrophilicity of the surface. The water contact angle on PAH is 44 , compared to 87 on gold. PAH is more hydrophilic, which might lead to a higher water concentration next to the PAH surface compared to a gold surface. Such small changes in composition can be amplied inside the microgel because of cooperative adsorption.

20

This might explain why

higher ethanol concentrations are needed for PAH to reach a minimum volume, or to achieve a certain VPTT decrease. The RMS roughness for all surfaces is in the sub-nanometer range, so it is not expected to play a role here. Further insights into the swelling mechanisms have been gained by considering the rheological properties and measuring the storage and loss modulus of microgels at dierent temperatures and ethanol contents. It is known from temperature dependent measurements in water that the elastic modulus of PNIPAM microgels increases when particles shrink with increasing temperature.

45,46

Accordingly, the highest storage modulus

G0 is observed at 50◦ C,

both in 0 and 30 vol% ethanol. The breaking of hydrogen bonding between PNIPAM and solvent molecules at T>VPTT leads to a smaller number of solvent molecules remaining within the collapsed structure. Therefore more monomer-monomer contacts occur, and the gel becomes stier. Additionally, the chain conformation changes, so that hydrophobic parts come closer together to avoid exposure to the solvent, and hydrogen bonds between PNIPAM

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Page 20 of 29

◦ monomers form. This also explains the higher loss tangent at 50 C, as PNIPAM chains are rubbing against each other in the absence of solvent molecules. Thereby they are causing a higher friction and a higher loss modulus G. Regarding the cononsolvency eect,

G0

of the microgels at 30 vol% ethanol, where their

◦ volume is minimal at 20 C, is not signicantly decreased compared to

G0

in pure water. This

nding stresses how the shrinking mechanism for the cononsolvency eect diers from the shrinking upon temperature increase. The low storage modulus despite the small size in the case of cononsolvency leads to the conclusion that the remaining number of solvent molecules must eciently lubricate the polymer network. A probable explanation of this is the presence of polymer loops bridged by ethanol molecules. Preferential adsorption of ethanol molecules to PNIPAM monomers compared to the adsorption of water to PNIPAM has been proposed as a reason for the formation of loops in several theoretical studies.

16,24,4749

The microgel

subsequently remains softer in the low-temperature cononsolvency case than in the hightemperature case. Preferential adsorption of one solvent seems however not to be the sole requirement for the appearance of the cononsolvency eect. Scherzinger et al. have analyzed the cononsolvency eect for polymers with secondary amide side chains, i.e. hydrogen bond donor and acceptor functions, and tertiary amide side chains, which only have a hydrogen bond acceptor function.

50

Although all analyzed polymers have a preferentiability towards

methanol compared to water, only those with a secondary amide group show a signicant cononsolvency eect.

An additional requirement therefore seems to be the ability of the

polymer to form intramolecular hydrogen bonds to support the shrinking. The temperature eect however is stronger, and all systems, including those without intramolecular hydrogen bonds, do show a VPTT.

◦ At 20 C the highest moduli were surprisingly observed for 60 vol% and 100 vol% ethanol, i.e. the systems with the highest ethanol content, whereas it was lower for 30 vol% despite the smaller size. This leads to the assumption that the water which is present in the microgel is mainly responsible for its softness, so it might act like a plasticizer for PNIPAM. The

20


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shift in the ethanol concentration where the minimum modulus is observed compared to the minimum volume shows parallels to the shift in transition temperature of PNIPAM microgels in pure water. The transition from low to high modulus takes place at higher temperatures than the volume phase transition.

45,46

Both results show that there is no direct correlation

between size (or swelling ratio) and elastic modulus or shear modulus. The frequency dependence of the storage modulus for high temperatures and its absence for low temperatures is consistent with theory, which predicts a more pronounced frequency dependence of

G0

for materials with a higher loss tangent.

obviously smaller than

51

The obtained

E

values are

3G which would be assumed for homogeneous polymer bulk material.

This is indicative of the nite size and the inhomogenous structure of the microgels, which consist of a highly crosslinked core and a weakly crosslinked shell.

6,52

Additionally, apparent

Poisson ratios are negative, which is only observed for specially structured materials, and very unlikely here. Hence, this is another hint at the invalidity of the relationship known from ideal conditions, and therefore indicates non-ideal conditions. Due to adsorption the material properties within the gel are far form being isotropic and cannot be described with an average number.

5 Conclusion The combination of temperature response and cononsolvency eect of PNIPAM microgels has been compared on dierent surfaces and in bulk solution. It has been shown that the type of surface and the sample preparation process inuence the swelling behavior. A slight shift in the ethanol concentration where the minimum microgel volume occurs can be observed for PAH surfaces compared to gold and bulk phase. Additionally, more ethanol is needed for microgels spincoated from water on PAH to decrease the VPTT. A probable reason for this is a strong interaction of the microgels with the surface which stems from electrostatic attraction and interdigitation of PNIPAM and PAH chains. In this way, the incorporation

21


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Page 22 of 29

of ethanol could be sterically hindered, leading to a higher ethanol concentration necessary to achieve a certain eect. Additionally, the higher hydrophilicity of PAH compared to gold might lead to water enrichment near the PAH surface, which would explain the need for a higher ethanol concentration to shrink microgels. Dynamic force measurements on PNIPAM microgels have revealed a non-trivial relationship between size and elasticity.

◦ Microgels at TVPTT. This is caused by the dierent mechanisms underlying the shrinking. Cononsolvency is explained by preferential ethanol adsorption, leading to ethanol bridging of PNIPAM chains, with many solvent molecules still present.

At high temperatures, however, many PNIPAM-solvent hydrogen

bonds break, and the solvents are being expelled from the microgel. This makes the microgels much stier compared to the cononsolvency case. Subject of further studies in this eld will be microgels with a homogeneous crosslink density, in contrast to standard microgels, where the core is more densely crosslinked than the shell. Because the present study has shown an inuence of the microscopic gel structure on the swelling behavior, homogeneous microgels are likely to behave dierently. This could shed more light on the cononsolvency mechanism.

Acknowledgment:

We thank the German Research Council (DFG) for nancial support

via the project KL1165/12-3.

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Figure 7: For Table of Contents Only.

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Combined Cononsolvency and Temperature Effects on Adsorbed

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