Results and Disussion

Core partiles

T EM analysis

A monodisperse ore solution used forthe synthesis of the ore-shell system desribed in

thesetion2.1.2hasbeenrstputundersrutiny. Thepartileswereobtainedbyemulsion

opolymerizationof styrene and NIPAM (about 5

wt.%

^{). The partilesthus onsiston a}

polystyrene ore of onstant density with a thin layer of PNIPAM [17℄. The dispersion

has been rst investigated by transmission eletron mirosopy. Fig. 2.16 presents the

TEM mirographs obtained from this analysis and the normalized distribution of the

radius obtained on a population of more than 200 partiles. All the mirographs were

taken as lose as possible to the fous with the same dose onditions. The partiles

appear spherial and monodisperse. The average radius was found equal to 51.3

±

^2.6

nm

^{. The} distribution an be desribed by a Gaussian onsidering an average radius of 51.5

nm

^{and a standard deviationof 2}

nm

^{. This results are ingoodagreement with the}

polydispersity of 5

%

^{determined from the SAXS analysis. The normalized gray values}

has been alulated for more than 100 partiles as desribed in the preeding setion in

order to hek the theory. The average gray values are depited on the g. 2.17. The

variationbetweenthemeasurementrepresentedbythesizeoftheerrorbarsisrathersmall,

whihattestsonthe reproduibilityofthemeasurementfromonepituretoanother. The

experimental result has been diretly ompared to the theory onsidering the spheriity

of the partiles, anaverage radiusof 51.5

nm

^{determined fromthe Gaussian} distribution and rst the ontrast of pure polystyrene partiles. The theory desribed relatively well

theexperimentalresults,nevertheless theexperimental

G(r)/G 0

^{valuesarelowerbetween}

30and45

nm

^{andhigherbelow30}

nm

^{. Thiswasattributedtothe adsorptionanddrying}

of the partiles on the grid. The gray value an then simplybe onverted in height

t

^as

onsidering the equation:

t = − ln G

G ₀

/ ̺ p

x _k,p

^(2.31)

0

Figure2.17: Radial relativegray values

G(r)/G ₀

^{of theore partiles analyzed byTEM (irles).}

The full line refers to the theoretial alulation onsidering a ontrast

5.8.10 ⁻³ nm ⁻¹

^{(seetable2.7)andanaverageradiusof51.5}

nm

^{determined fromthestatisti}

(see g. 2.16). The dotted line is the alulation for a ore-shell system with 49.5

nm

polystyrene ore and a dense 2

nm

^{thin PNIPAM shell. The dashed refers}

to polydisperse polystyrene partiles onsidering the distribution of the g. 2.16.

The inset presents a omparison of the prole of the partiles determined from this

analysis with a spherial prole. The small deviation an be attributed to a small

deformation of the partiles following the adsorption and the drying on the arbon

grid.

Theinsetofg. 2.17displaystheaveragethiknessofthepartilesderivingfromequation

2.31 onsidering a pure polystyrene ore. This prole was then diretly ompared to the

prole obtained for asphere.

The average prole of the dryed partiles thus present a maximum deviation of 7

nm

ⁱⁿ

the enter of the partiles respet to aperfet sphereof 103

nm

^{diameter. As mentioned}

before in the setion dediated to the SAXS analysis, a thin layer of PNIPAM of about

2

nm

^{is adsorbed on the the partiles. The relative gray values}

G(r)/G ₀

^{of the ore}

partiles has been ompared to the theory alulated for a ore-shell system with a

polystyrene orewitharadiusof49.5

nm

^{andaPNIPAMlayerof2}

nm

^{. Asanbeseenin}

the g. 2.17 nosigniant deviationan be observed from the TEM. The polydispersity

has been introdued onsidering the gaussian distribution determined previously and

pure polystyrene partiles. This partially explained the deviation observed for

r > 51.5 nm

^{, on the other hand the results for}

r < 51.5

^{are not signiantly aeted by the}

polydispersity. The remaining disrepany an be mainly explained by the unertainty

on the determination of the enter of the partiles during the rotational average and

on the deviation from the spheriity. Nevertheless this approah desribed the TEM

experiments within the experimental error.

As an appliation, this kind of analysis an by diretly applied on the TEM images to

obtain a three dimensional representations of the partiles absorbed on the grid. This

simplyrequires aonstantbakground withan average gray value

G ₀

^{and the knowledge}

0nm 110nm

A) B) B)

Figure2.18: (A) TEM mirographs of the ore partiles. (B) Transformation onsidering the

equation 2.31 to aess to the height of the partiles adsorbed on the grid (see text

for further details). (C) Tomographi representation of the grid. The olor bar is a

linear sale of the height between 0 and 110

nm

^.

0 0.05 0.10 0.15 0.20 0.25

30 40 50 60 70

R [nm]

N /N to t

A) B)

Figure2.19: (A) CryoTEM mirographs of the ore partiles. (B) Distribution in size obtained

from the CryoTEM analysis, the population an be desribed by a Gaussian

distri-bution (

h R i = 52 nm

^,

σ = 2 nm

^{)(solid line).}

of the ontrast of the partiles absorbed on the grid

̺ p

x k,p

.

An example is given in the g. 2.18, whih presents the treatment performed on a

TEM mirographs of our ore partiles (assimilated to polystyrene partiles) to aess

to the tomography of the sample. First the initial piture (g. 2.18 A) is transformed

following the equation 2.31 (g. 2.18 B)). In this sense the gray value orrespond to

the height of the partiles absorbed on the lm. Fig. 2.18 C) presents a 3 dimensional

representation of the tomography of the grid thus obtained. Due to its simpliity

this kind of analysis presents an elegant way to aess to the third dimension without

requiring omplex tomographi methods and an omplement other analysis suh as

sanning fore mirosopy performedon the dried state.

CryoT EM

Cryogenieletronmirosopywasthenperformedonthesamesystem. Fig. 2.19displays

the mirographs obtained and the resulting normalized distribution of the radius of the

250nm 450nm

A) B)

G ₀

C)

Figure2.20: (A) CryoTEM mirographs of the ore partiles. (B) Mirographs of a hole

per-formed in the lm by eletroni irradiation in the viinity of the aption A. The

piture istakenunder thesameonditionsasin theaption A,andtheaverage gray

values in the hole are dened by

G ₀

^{. C) 3D} representation of the thikness of the HGW lm(only the pointsoutside of the partiles an be onsidered) deriving from

G 0

^{and equation 2.32 The olor bar is a linear sale of the height between 250 and}

450

nm

^.

partiles. The same feature as in the TEM analysis an be observed. The partiles

appears asspheres witha narrowsize distribution. Theaverage radiusfromthis analysis

alsodeterminedovermorethan 200 partilesisequalto51.4

±

^3.2

nm

^{. The} distribution an be desribed by a Gaussian entered on 52

nm

^{with a standard deviation of 2}

nm

^,

whih is ingoodagreement with the TEM, with the SAXS analysis of the ore (50

nm

⁾

and the dynami light sattering (55.0

nm

^{). The ontrast between the partiles and}

the bakground is less pronouned than in the TEM as expeted from the theoretial

alulation. Indeed the ontrast is this time determined by the dierene between the

ontrast of the polystyrene and water

(̺ _p /x _k,p − ̺ _w /x _k,w )

^{, whih is} approximatelyunder ourexperimentalonditionssixtimessmallerthantheoneofthepurepolystyrene

̺ p /x k,p

(seeTable2.7). Moreoverthebakgroundisnotonstantonthewholemirographs,whih

is diretly related to the variation of the thikness of the lm. This parameter is ruial

for the rest of the analysis. Indeed the lm has to be suiently thik to embedded the

whole partiles.

A simple method has been applied to estimate the thikness of the vitried water lm

(see g. 2.20). An idential approahis desribed in the ref. [80℄. First the mirographs

were aptured aslose aspossibletothe fous(g. 2.20A)).Theninanarea losetothe

partiles a hole was done in the lmfollowing an exessive irradiation. A piture of the

hole was taken in the same onditions as the partiles before, the gray value inside the

hole thendene our

G ₀

^{(see g. 2.20B)).} Consideringthe ontrastof the HGWlm

̺ w

x k,w

it is possible to determined its thikness in all the points outside of the partiles in the

rst piture following the same approah as desribed for the TEM analysis. This time

we an use the relation:

Fig. 2.20 C) is a olour representation of the lm thikness following the equation 2.32.

Only the values out of the partiles have to be taken into aount. A strong variation

0.88 0.92 0.96 1.00

0 20 40 60

r [nm]

G (r )/ G 0

50µm

50µm

A)

B)

C)

Figure2.21: Cryo-TEM mirographs of one ore partile with (A) and without (B) lter of the

inelastisattered eletrons.

G(r)/G ₀

^{hasbeenalulatedinthetwoases(withlter}

(hollow irles), without lter (hollow squares)) and tted following the equation

2.29 andthe ontrast values of the table2.7 (full anddotted lines) assuming a pure

polystyrene ore of 55

nm

^.

of the thikness from approximately 250 to 450

nm

^{within 1.7}

µm

^{was observed in this}

example. Consideringthe average size of the partilesthis thikness should be suient

inordertoperformaorretanalysis. Partilesobservedinverythinlmpresentastrong

ontrastin theirenter. This eet an be attributedtoa lmthikness whihis smaller

than the diameterof the partiles, or to the deformationof the lm by the partiles. In

this ase the prerequisites of eq. 2.29 are nolonger given. If the lmis suiently thik

and not deformed by the partiles, the thikness gradient does not play a role beause

it will be ompensate by the rotational average of the gray values. In the rest of the

analysis only partiles embedded in a lm with a thikness superior as the diameter of

the partiles have been proessed.

The eet of the fousing has been investigated by taking dierent pitures for dierent

defousing. If taken in fous, the mirographs exhibit a sharp interfae with the

sur-rounding solvent. With inreasing defous, diration phenomena our at the edge of

thepartilesundertheformofFresnelsfringesasexpeted. Consideringthepituretaken

inthefousasreferene,the defousinganberelativelyestimatedfortheotherpitures.

Evidently, these mirographs would need aorretion through the

CT F (α)

^beforedoing

a qualitative evaluation. Fig. 2.19 demonstrates, however, that the ontrast between

the partiles and the vitried water is suient. As mentioned above, no defousing is

needed toenhane theontrast andthe evaluationofthe gray saleproeeds from

miro-graphs taken in fous. The eet of the energy ltering has alsobeen investigated. Fig.

2.21 presents the ryo-TEM mirographsof one singlepartiles taken with (g. 2.21 A))

and without (g. 2.21 B)) energy lter. The normalized gray values have been derived

for the two mirographs and tted following the equation 2.29 onsidering the ontrasts

presented inthe table 2.7onsidering a 55

nm

polystyrene partile (g. 2.21 C)). Equa-tion 2.29 gives a good desription of the radial normalized gray values in the two ases.

Nevertheless the experiments without energy ltering learly lak of ontrast, whih is

more than six times lower than with lter (see table 2.7). Thus any small variations of

0.88 0.92 0.96 1.00

0 20 40 60

r [nm]

G (r )/ G 0

Figure2.22:

G(r)/G ₀

^{of the ore partiles analyzed byryoTEM (irles). The full linerefers to}

the theoretial alulation onsidering the ontrast of pure polystyrenepartiles (see

table 2.7) and a radius of 52

nm

^{determined fromthe statisti (see g. 2.19). The}

dotted lineisthealulation foraore-shell systemwith50

nm

polystyreneore and a swollen 2

nm

^{thin PNIPAM shell (}

φ = 0.5

^{). The dashed refers to} polydisperse pure polystyrene partiles onsidering the distribution of the g. 2.19.

the transmitted eletron intensity will indue a dramati error in the evaluation of the

relative gray values. For this reason we only onsider zero loss images in the rest of the

analysis.

The normalized gray values shown ing. 2.22 have been obtained by averaging over 100

partiles. The symbolsdisplays the mean values while the error bars gives the standard

deviation in eah point. The results has been then diretly ompared to the theoretial

values. The average size was diretly taken equal to 52

nm

^{from the statisti performed}

on the ryo-TEM mirographs and we rst have onsidered the ontrast

( _x ^{̺ p}

k,p − _x ^̺ _k,w ^w )

^of

pure polystyrene in HGW. The obtained values presented by the full line desribed the

experimentalresultverywellonrmingthe spheriityofthe partilesinsolutionandthe

interest of the ryo-TEM respet to normal TEM. The small deviation between the two

resultsanbeattributedtopossibleerrorsinthe determinationofthe absolutedensityof

theHGWandoftheinelastirosssetionofhydrogen. Wealsoinvestigatedtheinuene

ofthethinPNIPAMshellontheabsorbane. ThistimeaswollenPNIPAM shellhasbeen

onsideredasobtained duringtheSAXSanalysis(seesetion2.1.4). Thenormalizedgray

values were alulated for a 50

nm

^dense polystyrene ore, and a 2

nm

^{thin PNIPAM}

shell inthe swollen state (

φ = 0.5)

^{)(dotted lines). The deviation between the tworesults}

israther smallasalready observed by TEM. The methodpresents hereintoevaluate the

mirograph is thus not sensitive enough to reveal this thin layer of PNIPAM in term of

ontrast. Forthis reason weonly onsider pure polystyrene partiles. The polydispersity

obtained fromthe statistiwas alsointrodued,and partially explainedthe deviationfor

r > 52 nm

^{as observed for the TEM analysis.}

As a onlusion a new method for extrating the exess eletron density of olloidal

partiles from TEM and ryo-TEM mirographs has been developed. This method has

been appliedto the orepartileswhihan beassimilatedtopurepolystyrene partiles.

The alulated ontrast as well as the size is in good agreement with the experimental

valuesbothforTEMandryoTEManalysis. Onanotherhand,thenormalizedgrayvalues

an bediretly usedtoaess tothe tomography ofthe partilesinvestigated byTEM as

long asthe ontrast of the partiles isknown. The resultsobtained for the ryoTEM are

even loser to the theory as the partiles are investigated in solution and not absorbed

and driedonasurfae. Goodagreementisfound between the mirosopyand the SAXS,

even if the SAXS was more sensitive to the presene of a thin layer of PNIPAM at the

surfae of the partiles and presents about 2

nm

^{smaller partiles.}

Core-shell partiles

Figure2.23displaysthemirographsofthe ore-shellmirogelsobtainedby ryo-TEM in

purewater. The sampleshave beenkept at23

o

Cpriorryogenization(see setion2.1.4).

The thermosensitive shell islearly visiblein these pitures beause of suientontrast

between the shell and the ore. Moreover, the mirographs show diretly the thermal

utuations and inhomogeneous ross-linkingwhih lead toa further ontributionto the

sattering intensity [3,63, 64℄. This isdiretly obvious fromg. 2.23 A), whih presents

a zoom-inon apartile toevidene the inhomogeneities of the shell.

As disussed in the setion 2.1.4 a feature diretly visible in the ryo-TEM images is

the bukling of the shell (see g. 2.23 and g. 2.1). This nding an be related to the

instabilitiesof swelling or deswelling gels ourring at the surfae of swollen gels axed

to solid substrates [69, 109114℄. This results orroborates reent small-angles neutron

sattering analysis performed on ore-shell PNIPAM/PNIPMAM also synthesized in a

seed emulsion polymerization, whih pointed out the presene of a depletion zone at the

interfae ore-shell [62℄.

As a onsequene, the ore-shell partiles deviate from an ideal spherial symmetry. In

order to demonstrate this, we have evaluated the relative gray sale

G(r)/G 0

^{along the}

linesindiateding. 2.23A).Fig. 2.23B)shows thatthesizealongtheselinesmaydier

appreiably. As already disussed above, this dierene is mainlydue to the bukling of

the shell. Fig. 2.23 C) displays the polymer volume frations that have been evaluated

usingeq. 2.30together withtheontrastsof polystyrene (ore)andPNIPAM(shell). For

speimens embedded in HGW, the alulated ratio of the ontrast between polystyrene

andPNIPAMis0.682(seeTable 2.7). Notethattheratioalulatedwiththe

approxima-tiongivenbyLangmorefortheelastiross-setion(eq. 2.21andeq. 2.22[80℄ wouldgive

aratioof0.650. Fig. 2.23C)demonstratesthatthestrongutuationsoftheshellleadto

strongloalvariations. This fatmust bekept inmindwhen onsideringthe omparison

with SAXS-data disussed further below.

In order to arrive at an average prole that an be ompared to a prole deriving from

SAXS-measurements, the analysis of the partiles has been performed on 45 partiles

taken from dierent mirographs similar to g. 2.23 A). Only isolated partiles were

be analyzed in this way. Prior to taking the gray values, a rotational average has been

performed as shown in Fig. 2.14 B) and E). The average relative gray values resulting

from this analysis are displayed in the gure 2.24.

G(r)/G 0

^{an be deomposed in two}

parts: the ontribution of the ore and the ontribution of the shell. The average result

has been tted onsidering a dense polystyrene ore and a paraboli density prole for

A) B) C)

Figure2.23: Comparison of ryo-TEM and SAXS. A) Average prole

φ(r)

^{evaluated from}

G(r)/G ₀

^{aording to eq. 2.29 and the ontrasts of} polystyrene and PNIPMAM given in Table 2.7. The inset gives the average relative gray sale that has been

used for this alulation. B)Measured SAXS-intensity andthe prole

φ(r)

^deriving

therefrom. C) Comparison of the overall sizeas determined by DLSand ryo-TEM

(solid line) and by SAXS (dashed line). See textfor further explanation.

the shell. This paraboli prole follows the same desription as for the SAXS analysis

and is given by the equation 2.15 (see disussion in setion 2.1.4).

The same proedure was repeated this time after tting eah partile individually. The

average

k(r)φ(r)

^{overthe 45partilesispresented by theopen symbolsintheinset ofthe}

g. 2.24. Thefulllineing. 2.24presentstheorrespondingalulationof

G(r)/G 0

^{. Both}

approahes lead tothesame resultswhihan beattributed tothe lowpolysdispersityof

the system.

Fig. 2.25 A) presents the average density prole obtained from the t of the average

G(r)/G 0

^{shown in the inset. As expeted, this prole exhibits a plateau withinthe ore}

up to

R c =

⁵⁴

nm

^{whih is in good agreement with the 52}

nm

^{of the ore found in the}

previous setion. The average prole an be tted by the eq. 2.15 with

K = 0.23

^,

R hw = 94 nm

^and

σ = 19 nm

^{. Between 55 and 75}

nm

^{the ontrast inreases to reah}

a maximum at 75

nm

^{showing that the shell is not totally attahed to the ore. After}

this, the ontrast dereases paraboliallyuntil

r =

¹¹³

nm

^{is reahed. This value losely}

mathesthe hydrodynamiradiusof thepartilesat23

o

Cequalsto113

nm

^{. Theaverage}

volume fration

φ

^{of PNIPAM in the shell is 0.116 ingood agreement with data derived}

froma ombinationof SANS and SAXS [63℄.

As aomparison the density proleused for the SAXSanalysis (see setion 2.1.4)is also

displayed inFig. 2.25A).The weight perentof theore inthepartilederived fromthis

analysis wasfound equal to53.3

%

^{whihis ingood agreement withthe 53.4}

%

^fromthe

gravitometry and with the 50

%

^{fromthe ryo-TEM.}

The overall size obtained from the SAXS has been ompared from the results obtained

fromtheCryo-TEMmirographs. Fig. 2.25B)displaysthemirographofasinglepartile

together with the overall size determined by ryo-TEM (solid line) as well as by SAXS

(dashed line). The dierenebetween both methodsamounts toa. 13

%

^{. However, this}

0 0.2x10 ^-3 0.4x10 ^-3 0.6x10 ^-3 0.8x10 ^-3 1.0x10 ^-3 1.2x10 ^-3

0 25 50 75 100 125

r [nm]

k ( r) f (r ) [n m -1 ]

0.85 0.90 0.95 1.00

0 25 50 75 100 125

r [nm]

G (r )/ G 0

Figure2.24: Average relative gray values

G(r)/G ₀

^{of the ompositemirogels (irles). The full}

line refers to the t obtained onsidering the funtion

k(r)φ(r)

^{of a ore-shell with}

a solidpolystyrene

54 nm

^{ore (fulllineinthe inset)anda paraboli PNIPAMshell}

(dashed line in the inset) desribed by equation 2.15. The relative gray values have

been tted for eah partiles using equation 2.30 and the average

k(r)φ(r)

^is

repre-sented bythe symbolsin the inset. The orresponding

G(r)/G ₀

^{values are indiated}

by the dashed line. The thin dashed lines of the inset display the hydrodynami

ra-dius from the DLS at 55

nm

^{and 113}

nm

^{obtained for the ore and the ore-shell}

partiles at23

o C

^.

marked disrepany has already observed before when omparing the overall size from

DLS and SAXS/SANS and explainedby single polymer hains protruding fromthe shell

[63℄. Nowthe originofthe disrepany beomesobviousfromlose inspetionofg. 2.25

B):SAXSis onlysensitivetothe averagestruture ofthe partileswhileryo-TEM takes

fully aount the deviations from this average aused by the bukling of the shell. In

this way thepresentanalysis orroboratesthe previousonjeture ofref. [63℄ toaertain

extend.

The bukling of the PNIPAM-shell must be a dynami phenomenon. This is supported

by reent investigations performed on similar system by dynami light sattering (DLS)

and depolarized dynami light sattering (DDLS). The latter method requires a

non-entrosymmetripartile. From astrongDDLS signalthe deviationsfromspherial

sym-metry ould be inferred diretly whih was most pronouned in the swollen state [115℄.

In this investigation a strong oupling of the rotational diusion and the translational

In this investigation a strong oupling of the rotational diusion and the translational

Im Dokument Structure, Dynamics and Association of Thermosensitive Core-Shell Particles (Seite 38-48)