Mesoscopic structure of polymer mediated microemulsion networks

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Mesoscopic structure of polymer mediated microemulsion networks

D. Vollmer, U. Hofmeier, H.-F. Eicke

To cite this version:

D. Vollmer, U. Hofmeier, H.-F. Eicke. Mesoscopic structure of polymer mediated microemulsion networks. Journal de Physique II, EDP Sciences, 1992, 2 (9), pp.1677-1681. �10.1051/jp2:1992227�.

�jpa-00247759�

J. Phys. II France 2 (1992) 1677-1681 SEPTEMBER 1992, PAGE1677

Oassification Physics Abstracts

82.70K 61.25H 61.16D 61.42

Short Communication

Mesoscopic structure of polymer mediated microemulsion networks

D. Vollmer, U. Hofmeier and H.-F. Eicke

Universit£t Basel, Klingelbergstr. 80, 4056 Basel, Switzerland

(Received 6 July 1992, accepted in final form 15 July1992)

Abstract. Water in oil microemulsions containing block-copoly(oxyethylenelisoprene/oxy- ethylene)

investigated by freeze fracture electron microscopy. Contrary to pure microemul-

sions, where the droplets

irregularly arranged, they

closed packed in these systems ^and show medium _range order. This feature makes polymer containing microemulsions

an

interesting system to study disorder to order transitions,

it combines typical features of colloidal particle and block-copolymer systems.

1. Introduction.

Recently the study of order-disorder transitions _{as a} function of interaction _range and strength

has attracted much attention, _as liquids containing supramolecular objects form specifically

useful systems to investigate these transitions experimentally [ii. In particular spherical col- loidal particles _{[2, 3]} and block copolymer systems [4, 5] are frequently studied in this context.

Spherical colloidal particles (e. _g. ^latex particles) in _a solvent form excellent approximations

to hardcore systems [6, ii. They allow straightforwardly to test theoretical and numerical predictions for these systems. ^In particular _one observes

fluid to crystal transition _as function of particle density, _pressure and temperature [2, 7, _8]. This order to disorder transition

be

influenced by suitable modifications of particle surface properties, thereby manipulating the interaction potential _[9].

Block copolymers are composed of _sequences of chemically distinct, homogeneous polymer

chains. Their microstructure is essentially determined by the overall degree ofpolymerisation

N and by the Flory-Huggins interaction parameter _x ^of the _monomers building _up the blocks [4, 10]. Transitions between ordered and disordered states

be obtained by varying the

product XN. In the strong segregation limit (large xN) _seven ordered phases _were identified.

Two of them show lattice like symmetries [4, iii.

1678 JOURNAL DE PHYSIQUE II NOg

In this _paper

we report on preliminary results of ordering in microemulsion mediated polymer

networks [12] ^obtained by freeze fracture electron microscopy. These systems combine typical

features of both, colloidal particle and block-copolymer systems. In water-I-oil microemulsions [13] ^water droplets of well defined size [14, 15] are dispersed in _an oil medium. These systems are

frequently viewed _as macrofluids, with droplets interacting via hard _core potentials [16]. The

droplets _are irregularly arranged with _a tendency to agglomeration [18]. Microemulsions form

a selective solvent for both components ^of block-copoly(oxyethylenelisoprene/oxyethylene),

I-e- _a triblock copolymer build _up by a hydrophilic(POE)-hydrophobic(PI)-hydrophilic(POE)

sequence. At high enough copolymer concentration this system ^forms viscoelastic networks [19] ^{that exhibit medium} range [21] ^close packed order.

2. Experimental procedure.

2, I PREPARATION _OF BLOCKCOPOLYMER-MICROEMULSION DISPERSIONS. ROT and ISOOc-

tone are of hi ghest grade commercially available from Fluka. Water is purified with the Alpha-Q Reagent Grade system, Millipore. The triblock copolymer consists of two POE blocks with number average molecular weight Mn

11000 g mol~~ each, _a PI block with Mn

39000 _g mol~~ _and

a

POE/PI (weight/weight) ratio of 36/64. 4.6 _x 10~~ _g ml~~ copolymer _are added to _a microemulsion (H20/ACT (mol/mol)

=

61.8, [ACT]

0.9 mol dm~~), yielding _on the average 16.5 copolymers _per droplet (R

=

16.5). ^The dispersion is stirred (4 5 days) ^with

magnetic stirrer at 313 K until it becomes homogeneously transparent. At

temperature the sample forms _a single phase.

2.2 FREEZE-FRACTURE REPLICATION TRANSMISSION ELECTRON MICROSCOPY. A 400 mesh

copper TEM-grid is immersed into sample material (294 + 0.5 K) and then positioned between two gold plates of about 4 _mm diameter, I.e. the liquid specimen is squeezed between the

plates (sandwich-method) _[18]. The sandwiches

hold together by stainless steel tweezers and quenched by hand plunging into

mixture of liquid 2-methylbutan and _propan (volume

fractions: 1/3 2-methylbutan, 2/3 propan) at 81 K. Typical artifacts of hand plunging (phase separation, crystallization, _aggregate segregation) [22] do not _occur with these systems, as an

enhanced viscosity reduces the mobility of the droplets.

After quenching the sandwich is transferred into liquid nitrogen and clamped onto _a brass block (Balzer). It is mounted

Balzer freeze edge device (BAF301) at 103 K and subse-

quently the _pressure is reduced _to 5 _x 10~~ _{mbar. After evacuation the} sample is fractured

by separating the gold-plates with _a liquid nitrogen cooled microtome.

To enhance contrast of the surface structure, ^the sample is warmed _{up to} 183 K and edged

for three minutes. During edging the cooled knife of the microtome is positioned close above the sample surface. It acts _{as a} cold trap to prevent recondensation of volatiles _on the fracture

surface. Thereafter the sample is cooled again to 103 K and its surface is shadowed with _a 16 1 _{layer of W-Ta under}

an

angle of 40°. In order to provide sufficient mechanical stability

of the W-Ta film, it is coated with

carbon layer of about 400 1.

After the specimens _are warmed _up to _room temperature and brought to atmospheric _pres-

sure, the replica is washed in chloroform, ^{dried in air and examined} finally with

a HiTachi

H-8000 electron Jnicroscope operating at 100 kV.

N°9 MICROEMULSION NETWORKS 1679

3. Results and discussion.

Figure I shows _a micrograph of _a polymer containing w/o microemulsion _at 294 + 0.5 K in the

one phase region. The figure demonstrates that the droplet structure is preserved and that the

droplets ^show medium range dose packed order at volume fraction id

0.16 Their average radius and the center to center droplet distance _can be measured directly from the micrograph, yielding 11 + 3 _nm and 27 + 5 _nm, respectively. The _errors in the droplet size _are mainly due to _a slight polydispersity, _a restricted visibility of droplets that _are partially ^buried after the fracture and to difficulties _to distinguish droplets from their surroundings. In contrast the variance in the _average center to center distance is mainly due to defects in the ordering of the

sample.

Fig-I- Micrograph of

polymer containing w/o microemulsion at 294 + 0.5 K in the

one

phase region. Close packed arranged water droplets

imbedded in

oil matrix. The length of the bar

corresponds to 200

The influence of the polymers

droplet size and droplet-droplet distance _can be estimated

by comparing ^the values obtained for _a microemulsion with and without polymers. For pure microemulsions such data _are available from both, dynamic light scattering measurements _or

geometric parameters [14]. They _are listed in table I. Measured and calculated droplet radii coincide within the _error margin, whereas the droplet-droplet distance _seems to be slightly

smaller for polymer containing systems. ^This _may ^{be due} to evaporation of isooctane during

the preparation _{process, or} to _an increase in the number of particles die to _a replacement of surfactant molecule by the copolymer. The arrangement ^{of the} droplets changes substantially through addition of the polymers, _{as can} be _seen by comparing figure I with micrographs

obtained for pure microemulsions [18, 22, 23]. Jahn and Strey observe that the arrangement of droplets ⁱⁿ _pure microemulsions is irregular with _a slight tendency to agglomeration. The

droplets _are still agglomerated in clusters of dilserent sizes for nearly twice the droplet volume

fraction (id

0.322) and comparable droplet size (21.2 _nm in the diameter) to

sample.

1680 JOURNAL DE PHYSIQUE II N°9

Table I -Droplet size and droplet-droplet ^distance determined from the microgmph for _a mi- croem«lsion containing polymer (R

16.5, _cw

0.20, _wo

61.8, id

0.16), respectively from dynamic light scattering meas«foments and from ^the geometric _parameters for _p«re microem«I- sion systems (cw

0.20, _wo

61.8, id

0.14).

droplet size droplet-droplet ^distance micrograph dynamic geometric micrograph geometric

light scattering parameters parameters

nm nm nm nm nm

12 + 2 13 + 2 11 + 2 27 + 5 33 + 4

4. Conclusions.

We conclude that the polymers

medium _range ordering of microemulsion droplets into close packed arrangement, ^similar to the microphase behaviour of block-copolymer systems in the strong segregation limit. For _our systems ^the interaction parameter _X ^has to be replaced by _a higher elsective interaction potential between the distinct polymer _sequences, due to the

their preferred solubility in the _water

oil regions.

On the other hand the system _can ^be considered _as dispersion of spherical colloidal parti-

cles. For high polymer concentration the microemulsion droplets

surrounded by

shell of

hydrophobic PI blocks (in _average there _are 16.5 PI blocks per droplet) [24]. This increases the elsective hard _core radius of the droplets [17, 16], ^{while attractive short} term contributions

become much stronger ^for our systems, due to network formation caused by the copolymers.

Acknowledgements.

The authors wish to thank M. H£ner for help with the measurements at the Maurice E.- Miller Institute for High-Resolution Electron Microscopy. One of the authors (D.V.) would like to thank J. Vollmer, W. Sager and F. Stieber for _many useful and stimulating discussions. They

also acknowledge financial support ^{of the Swiss National Science Foundation}

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