Eetive volume fration and rystallization

Crystal

Crystal

Glass 0.494 0.545

0.58 0.64

0.74

f F f

M

f _G

f _eff

Figure2.26: Phase diagramof a hard spheres dispersion [116118℄.

degrees of rosslinking. The nuleation and growthof the rystalliteshas been visualized

by polarizedlightmirosopyand theinuene ontherheologial properties isdisussed.

2.3.2 Experimental

The dierent ore-shell laties presented in the hapter 2.1 were used in this setion.

Following the systems the rosslinking dened by the amount of BIS in the shell was

adjusted to 1.25

mol.%

^{(KS1), 2.5}

mol.%

^{BIS (KS2) and 5}

mol.%

^{(KS3) (see setion}

2.1.2).

The ow urves and the dynami measurements performed onthe KS2 suspensions have

been investigated usingastrain-ontrolledrotationalrheometerRFSIIfromRheometris

Sienti, equipped with a Couette system (up diameter: 34

mm

^{, bob diameter: 32}

mm

^{, bob length: 33}

mm

^). Measurements have been performed on 12

mL

^{solution and}

the temperaturewasset with an aurayof 0.05

o

C. A stress ontrolled rheometer MCR

301 (Physia) has been used for the experiment performed onthe KS3.

Polarized mirosopy has been performed with aLeia DMRXE. Sample were lled into

a0.1

mm

^{thik apillary}thermostatedwith anaurayof 0.05

o

C. In orderto followthe

rystallizationproessathermostatedellwasdesignedfor0.1and0.5

mm

^thikapillary

(see g. 2.27). The ellisthermostatedand the temperaturewithinthe ellis ontrolled

by a thermoouple with a preision of 0.1

o C

^{. The large surfae of ontat between the}

ell and the apillary allows a fast quenhing of the sample and makes it ideal for the

diret observation of the rystallization proess. Images of the samples were taken in a

darkroomwithoutlter.

2.3.3 Eetive volume fration and rystallization

Theestablishedliquid-rystaloexistenedomainforhardspheres laysbetween the

freez-ing volume fration

φ F

^at

φ ef f

^{= 0.494 and the melting volume fration}

φ M

^at

φ ef f

⁼

0.545asobtained fromomputer simulation[153℄. An experimentalphasediagramould

be ahieved by determining the rystal fration of the samples from the position of the

Capillaries Apertures

Temperature sensor

Cooling

3cm

Figure2.27: Thermostated ell.

oexistene liquid-rystal boundaries after sedimentation. This an be linearly

extrapo-latetodeterminethe beginningandthe endofthe oexistenedomain[117,154℄. Tothis

purpose solutions with weight onentrations rangingbetween 6 and 14

wt.%

^{have been}

prepared. The samples KS1 and KS3 were shaken after preparation to destroy residual

rystallitesandstoredformorethanone monthatroomtemperature20.5

o C ±

^0.5

^o C

^{. In}

the aseoftheKS2 thesuspensionshavebeenheatedto30

o

Cinordertodestroypossible

rystalsthatmayhaveformedatroomtemperature. Thesesuspensionsaresubsequently

ooled down quikly to 21

o C

^{and kept at this} temperature for a time of typially two months.

Aftersreeningofthe eletrostatiinterationsbyadding5.10

−2 mol.L ⁻¹

^{,allthe samples}

rystallizeatdened onentrations. Thishintstothe lowpolydispersityofthe partiles.

Thehydrodynamiradiusofthemirogel

R H

^{anbeusedtoalulatetheeetivevolume}

fration

φ ef f

^fortemperatures below25

o C

^by

φ ef f = φ c

R H

R c

3

(2.33)

where

R c

^{is the ore radius alulated from the ryo-TEM and}

φ c

^{is the volume fration}

of the ores in the system. The latter quantity an be approximated from the weight

onentration ofthe partiles inthe system and the mass ratiobetween the ore and the

shellofthepartiles. Toavoidpossibleerrorsduetothesmallexperimentalunertaintyof

R H

^{, averagedvalues have been taken fromthis graphby} approximating

R H

^{by astraight}

linein this region of temperatures.

Thedensityofthepartilesandtheirsizeisnothighenoughtoensurerapidsedimentation

of the rystalline phase. Only after two months, rystals whih an be seen by eye by

means of the Bragg-reetions,have sedimented. Fig. 2.28 exhibits the liquid-rystalline

region of the dierent samples and the orresponding phase diagram. The experimental

phase diagram was taken from the hange in the position of the oexistene boundary

indiated by the dashed lines. As expeted the experimental points desribe a linear

dependeneinthebiphasiregion. The datahavebeenresaled to

φ F = 0.494

^inorderto

omparethe dierentexperimentalphasediagrams. FortheKS1,the oexistenedomain

Ks2

Ks3

0 0.2 0.4 0.6 0.8 1.0

0.40 0.45 0.50 0.55 0.60 0.65

0.556

0.494

f _eff

C ry st a lF ra ct io n

Ks1

Ks3

1cm

Figure2.28: Dierent ore-shell suspensions in the biphasi region with 1.25

mol.%

^{(KS1), 2.5}

mol.%

^{(KS2) and 5}

mol.%

^(KS3) rosslinking after two months and their orre-sponding experimental phase diagram. The rystal frations determined from the

heightof theoexistene boundariesindiatedby thedashed lines onthe photographs

were tted by a linear regression (solid line) for the KS2 (hollow triangles) and

the KS3 (hollow irles). The results obtained for the KS1 are indiated by hollow

squares. Thedata have beenresaled to

φ _F =

^{0.494. The oexistene domain forthe}

1mm

Figure2.29: Crystallization of a 9.48

wt.%

^{solution at dierent} temperatures orresponding to dierent eetive volume frations

manifested by the presene of distint rystals has been observed for eetive volume

between 0.494 and 0.535,whih issmalleraswhat isexpeted forhard spheres. This an

berelatedtothe softness ofthe systemasalready observed for PNIPAM mirogel[7℄. At

higher degrees of rosslinking (KS2 and KS3) the resaled oexistene domain has been

found between 0.494 and 0.556. This is inaord with the theoretial values

φ M = 0.545

[153℄.

Fig. 2.28 shows also that the rystallization study of the thermosensitive partiles is

partially hampered by the strong turbidity of the system. This motivates the use of thin

apillary for a diret observation of the rystallites. In this ase polarized mirosopy

an beusedtoinvestigatethe rystallizationkinetis[135℄. Theseexperimentshave been

performed on the KS2, for a onentration of 9.48

wt.%

^{(see g. 2.29) and 8.22}

wt.%

(see g. 2.30) at dierent temperatures. The same experiment wasrepeated for the KS3

at a onentration of 13.01

wt%

^{at 20}

^o C

^{and will be disussed in the next setion. The}

samples were rst maintained at about 30

o

C in the 0.1

mm

^{thik apillary and then}

quikly ooled-down to thetemperatureof investigationinthe thermostatedell(see g.

2.27).

Below the melting temperature the rystallization proess of olloidal partiles an be

interpreted in the framework of nuleation and growth. The rst rystals have been

observed around

Φ ef f = 0.50

^. Considering the time of observation of one hour, this value is in good agreement with the phase diagram. The onset of the rystallization

1mm

Figure2.30: Crystallization of a 8.22

wt.%

^{solution at dierent} temperatures orresponding to dierent eetive volume frations

0.1 1 10

0 1000 2000 3000 4000

Time [s]

G ', G '' [P a ]

Figure2.31: Visoelasti behavior of rystallizing suspensions vs. glassy systems: The storage

modulus

G ^′

^{(lled symbols) and the loss modulus}

G ^′′

^{(open symbols) are measured}

as funtion of time in the linear visoelasti regime at 1

Hz

^{and 1}

%

^{after 5 min of}

shearingat100

s ⁻¹

^forarystallizingsystem(irles)andaglassysystem(squares).

Thetriangles refertotheliquidstate(

φ _{ef f} =

^{0.49),The irlestoa volumefration}

of 0.52 (two-phase regime) whereas the squares give the results for the glassy state

(

φ _{ef f} =

^0.65).

is manifested by the apparition of large rystals growing on the walls of the apillary.

At lower temperatures orresponding to higher

Φ ef f

^{most rystals are formed in bulk,}

the nuleation inreases, whereas the size dereases. Above

Φ ef f = 0.542

^{no rystal}

was observed in the 9.48

wt.%

^{solution. This is in good agreement with} observations performed on olloidal hard spheres assimilated suspension by light sattering [123℄. It

was demonstrated that when the melting onentration is exeeded, nuleation events

beome orrelated and high nuleation rate densities suppress rystal growth. At higher

eetive volume frations the rystals are indeed strongly ompressed impeding their

growth. Ontheontrarysomerystals anstillbeobserved until

Φ ef f = 0.57

^forthe8.22

wt.%

^{solution,whihanbeattributedtoaslightvariationofthesoftnessofthe partiles}

for the lower temperatures.

Im Dokument Structure, Dynamics and Association of Thermosensitive Core-Shell Particles (Seite 49-54)