visiovis technical progresses problems and suggestions...

Antonella ESPOSITO 248

Chapter V

VISIOVIS TECHNICAL PROGRESSES

Problems and suggestions for further amelioration

In the previous chapter we presented Visiovis – a visualization tool thoroughly

designed, assembled and developed in our laboratories to monitor, model and optimize

polymer melt processing in screw/barrel systems. Lately, we intended adapt such a tool

to monitor mixing during compounding of molten polymers with inorganic fillers –

namely, to monitor polymer-clay nanocomposite processing. We have amply stressed

how real-time monitoring techniques would facilitate the work of materials engineers –

since in situ characterizations are generally less time-consumptive, less labor-intensive

and more cost-efficient than ex situ characterizations. Nonetheless, the intensive work

required to conceive, develop, test and then validate any new characterization technique

shouldn’t be underestimated. Sometimes, the way which leads to the set up of a new

VISIOVIS TECHNICAL PROGRESSES Problems and suggestions for further amelioration


characterization method isn’t exactly a highway, but does rather look like a tortuous

path which requires the collaboration of several experts from different domains and,

manifestly, more than a few years of hard and meticulous working. From time to time, it

is required and may be useful to make a point about the progresses accomplished

(ameliorations or just simple evolutions) – which doesn’t exempt from recognizing any

possible mistake or identifying the limitations of the system in its actual configuration.

The main objective of this chapter is explicitly to make a point on the technical

progresses made during the last three years on Visiovis – giving an instant picture of the

actual situation but also some suggestions for further amelioration, whenever a problem

has been encountered.

V-1 MATERIALS

As previously underlined, real-time process monitoring often constrains to use

pilot equipments and model materials – in particular, when the objective is to visualize

the flows in geometrically-complex equipments, process engineers frequently have to

choose model fluids. As our purpose was to monitor polymer-clay melt processing, we

had to found both a model fluid and some model fillers having specific and suitable

optical properties (optical inertness for the fluid and optical activity for the fillers).

Model fluid. The choice of poly dimethylsiloxane (PDMS) as the model fluid

has been largely vindicated in Chapter IV. Indeed, the first reflex was of course to make

profit of the choices previously made by Moguedet and coworkers [1] and eventually

adapt them by taking into account the new requirements. This is the fundamental reason

why we chose PDMS: the same kind of fluid had been previously and successfully used

for Visiovis experiments. However, silicone oil with lower viscosity (10 Pa·s rather than

100 Pa·s) would have been easier to handle (that is to say, it could make it easier to fill

up and empty the screw/barrel system).

Nevertheless, whilst PDMS represented an acceptable compromise for Moguedet

and coworkers, it didn’t really fulfill all the additional requirements we needed to obtain

polymer-clay nanocomposite morphologies by melt compounding. We have previously

Chapter V

PhD INSA de Lyon (2008) 250

called attention to some problems encountered during the preparation of masterbatches

(PDMS + photo-active lamellar fillers) for the calibration of Visiovis detection systems

(§ IV-2): the photo-active lamellar filler CNa+ 0.25CEC RhP couldn’t be visualized by

the CCD cameras, and the aspect of the mixture prepared with such filler was different

in comparison with the other mixtures (Figure IV-F18). We then evocated two possible

reasons for these evidences: (1) the chemical composition of the commercial clays used

to prepare the photo-functional complexes, and (2) the interactions of the photo-active

lamellar fillers with the silicone oil. As the first explication didn’t really persuade us,

we got more and more convinced that the problem is essentially related to the chemical

composition and molecular arrangement of the chosen PDMS fluid (Figure V-F1). To

confirm our suspicions, we carried out some rheological measurements, whose results

will be shown soon after (§ V-4.1). Meanwhile, we searched through the literature to

find out any similar observation reported about the interactions of PDMS and lamellar

mineral fillers.

Figure V-F1 Chemical formula of poly dimethylsiloxane (PDMS), the silicone oil we used as

the transparent, viscous model fluid for Visiovis experiments. The macromolecular backbone

consists of alternating Si and O atoms (contrarily to the majority of organic polymers, based on

C atoms) and the only accessible groups are methyl groups (–CH3).

Schmidt [2] and Paquien [3] dealt with filled polysiloxanes during their PhD

research work: the former synthesized, characterized and evaluated the properties of

polysiloxane/layered silicate nanocomposites, whereas the latter investigated mostly the

rheological properties and the filler dispersion of PDMS/silica slurries. Paquien et al.

[4] published soon after the results obtained by dynamic mechanical measurements and

TEM on fumed silica/PDMS suspensions and focused their work on: (1) the modulation

of the interactions between silica particles and PDMS through a controlled silylation of

filler surface; (2) the effect of the procedure used to graft the silanol groups on the silica

surface; (3) the effect of silica volume fraction on dispersion. They finally established a



relation between the silica grafting ratio, the aggregate size and the rheological

properties of the suspensions. Indeed, it is well known that silica particles can establish

favorable interactions with polysiloxane macromolecular networks: Schaer et al. [5]

have even developed a model for the description of silica particles dispersion in silicone

polymers, taking into account the particle structure (porosity and density as a function

of size), the penetration of PDMS into the silica particles, the bound formation between

PDMS and accessible silanol sites, as well as the erosion of silica agglomerates – first in

intermediate fragments, then in aggregates of a few hundreds of nanometers. Some

works are also available about poly dimethylsiloxane reinforced by other fillers, e.g.

barium titanate particles [6] and mica flakes [7]. Recently, lamellar mineral fillers

(clays) have become quite popular also as a reinforcing agent of polysiloxane polymers,

coherently with the general trend registered about other carbon-based polymers (§ I-2).

Burnside and Giannelis [8] reported the first melt-processed layered silicate/poly

dimethylsiloxane nanocomposites synthesized by delamination of the silicate particles

in the PDMS matrix, followed by cross-linking. Afterwards, the number of works about

the synthesis and properties of silicone rubber/clays nanocomposites in the literature

noticeably augmented [9-18]. Wang and coworkers [19] openly proposed organic MMT

as a substitute for aerosilica in liquid silicone rubber systems. However, if compounding

lamellar mineral fillers to polysiloxane networks is nowadays interesting several

research groups, it is worthy highlighting that nobody has ever reported about a PDMS

matrix with the molecular structure of an inert silicone oil (Figure V-F1). Most of the

time clays (whether natural or organically-modified) are added to silanol-terminated

(hydroxyl-terminated) PDMS [8-10][17], vinyl-terminated PDMS [10][13], –NH2 and –

PEO terminated PDMS [17] and some other PDMS matrices containing at least one

reactive site in their repeating unit, for example SiCH3CH=CH2O [18]. The principal

objective is, apparently, to introduce the filler in the liquid precursor, disperse it, then

add the cross-linking agent (e.g. TEOS and/or tin 2-ethylhexanoate or whatever else)

and cure the rubber composite at room [8-10][12] or higher temperatures [13][18][19].

Contrarily to Burnside and Giannelis [8], clay intercalation is typically accomplished in

appropriate solutions [13][14] or with the addition of a dispersing aid, such as small

amounts of chloroform [9] or even distilled water [10] (only for PDMS–OH). Though,

Chapter V


this strategy doesn’t always work as expected1. Moreover, even if probably performed,

the results of rheological measurements on such systems are rarely reported: the authors

rather perform mechanical tests [9][10][12][13][15][18][19] in order to evaluate the

performances of cured samples (as the main purpose is the reinforcement of the polymer

matrix), supported by XRD2, swelling tests, TGA, TEM and sometimes SEM, AFM,

permeability measurements, IR spectroscopy.

The control of the interactions between polymer macromolecules and the surface

of some filler is, in the case of polysiloxanes, more complex than in the case of common

carbon-based polymers. Several authors have faced the difficulty of pointing out which

are the factors influencing the extent of clay exfoliation in polymer composites (whether

the compatibility of clay modification with polymer chemistry, or the processing route

and parameters, or both of them, and in which proportion) [20-26]. Takeuchi and Cohen

[10] showed that the reinforcement in PDMS elastomers can be attributed to the

anchoring of the hydroxyl end-group of polymer chains to the silicate surface of the

fillers and that the mechanical properties of the obtained networks cannot be superseded

without further chemical modification of the system. They could enhance the properties

of their PDMS elastomers only if the networks were formed from the hydroxyl-

terminated precursor. LeBaron and Pinnavaia [12] confirmed that organoclay could

readily intercalate linear PDMS molecules terminated by hydroxyl groups, even though

they observed little or no intercalation with analogous molecules terminated by methyl

groups: this would mean that the interactions of terminal silanol groups with the internal

surface of clay galleries represent an essential step of swelling and intercalation

mechanisms of clays. Kaneko and Yoshida [18] recently highlighted that, hitherto, few

researchers have reported that different kinds of clay have different behaviors when

compounded to PDMS matrices. Even if they assure that exfoliation can be achieved in

high-molar-mass PDMS matrices without solvent assistance or high shearing, they also

admit that reinforcing PDMS rubbers by layered silicates is far more complicated than

1 Takeuchi and Cohen [10] observed that attempted network synthesis using just water as a dispersing aid,

whilst efficacious for hydroxyl-terminated PDMS, was unsuccessful for vinyl-terminated PDMS. In this

latter case, they rather added a buffer solution (pH=7) since the addition of water is suspected to prevent

the hydrosilylation cross-linking reaction from occurring. 2 Kaneko et al.[17], for instance, evaluated the morphology of silicone/clay slurries by SAXS and WAXS.



reinforcing them by silica or carbon black, mostly because of their unique morphology

and interactions with the matrix. They observed that the main driving force for polymer

intercalation into clay galleries results from the enthalpic contribution generated by the

establishment of many favorable polymer-filler surface interactions and that, in the case

of PDMS networks, essentially depends on the insertion of terminal segments of PDMS

chains containing specific groups (e.g. Si(CH3)OH groups) into the interlayer spacing.

Such end-groups could effectively interact with the silanol sites present on clay platelet

surfaces (especially at the platelet edges, indeed) by hydrogen bonds. This is the reason

why some authors found that the addition of small amounts of water occasionally helps

clay exfoliation in PDMS–OH matrices [10]. However, in the majority of conventional

organoclays, most of the –OH polar sites aren’t anymore available after cation exchange

process: that’s the reason why Kaneko and Yoshida reported that clay agglomeration

was more evident in PDMS-organoclay composites than in the composites containing

unmodified clay. This observation contradicts somehow our reflections about PDMS

masterbatches containing CNa+ 0.25CEC RhP: at least theoretically, the masterbatches

containing the photo-active filler produced from natural clay should present a better

dispersion than all the other masterbatches prepared from C30B 0.25MC RhP, C10A

0.25MC RhP, C15A 0.25MC RhP. Indeed, rheological measurements (§ V-4.1) showed

no significant differences between the four photo-active lamellar fillers: none of the

masterbatches showed the typical behavior of a properly dispersed polymer-clay

nanocomposite (the principles of morphological characterization by rheology have been

reported in § I-2.2.1). PDMS-clay interactions are maybe more complex than expected.

The evidences discussed so far let presume that, in the case of polysiloxane-clay

composites, a simple compatibilization of the polymer and the filler particles based on

the degree of hydrophilicity/hydrophobicity of the compounded ingredients isn’t enough

to assure the formation of nanocomposite morphologies. This difficulty has been only

recently pointed out and, unluckily, represents the main limitation of our visualization

tool and the associated characterization techniques. The problem is that Visiovis in its

actual configuration (§ IV-1.3) isn’t compatible with reactive fluids, liquid precursors,

solvents used as dispersing aids, etc. because of the material used to fabricate the barrel

(PMMA) and, for the reasons evocated in the previous chapter, it cannot sustain high

Chapter V


temperatures. As a result, a better choice of the model fluid would be possible only if a

PDMS containing different terminal or lateral groups could be synthesized, so that the

polymer chains could favorably interact with the photo-active lamellar filler but remain

inert with respect to the PMMA internal surface of Visiovis barrel. Actually, the quest

for such an ideal transparent fluid could be quite difficult, as Schmidt et al. [16] recently

published a paper about the origins of silicate dispersion in polysiloxane/layered nano-

composites and clearly showed that the factors influencing clay dispersion are numerous

and not always simple to control. They prepared polysiloxane composite samples from a

variety of matrices (with respect to both chemical functionality and molecular weight)

and lamellar mineral fillers (natural and synthetic, hydrophilic and hydrophobic) just to

determine the origins of silicate dispersion in a generic polysiloxane matrix. They found

that, in the case of organoclays, the presence of an appropriate number of long (C12-C18)

ammonium-bound alkyl chains is essential, as well as the presence of sufficient amounts

of polar functional groups. They also concluded that, generally speaking, an otherwise

incompatible polymer can be made compatible with a given filler by the inclusion of the

appropriate number of dispersion-enhancing functional groups either at the chain-ends

or elsewhere in the polymer. It is worthy to report the observation they made that, to the

date of their contribution (i.e. 2006), polymer-clay nanocomposites had generated over

a thousand publications, whilst only a handful dealt with polysiloxane. The publication

by Kaneko et al. [17] came out soon after (2007) and roughly confirmed Schmidt and

coworkers’ results. In particular, Kaneko and coll. reminded that, as previously found

by LeBaron and Pinnavaia [12], the comparison of PDMS–SiOH with PDMS–Si(CH3)3

showed that, with an equivalent molar mass and in the presence of the same organoclay,

the first matrix produced intercalation while the latter did not.

Photo-functional fillers. On the basis of the previous considerations, if a proper

functionalization of PDMS (modification of its terminal groups or synthesis of a novel

repeating unit containing dispersion-enhancing lateral groups) is obtained, as a result of

the experimental activity of the last three years four photo-active lamellar fillers are

nowadays available to perform visualization experiments by Visiovis: CNa+ 0.25CEC

RhP, C30B 0.25MC RhP, C10A 0.25MC RhP, C15A 0.25MC RhP. Any choice about



both the procedure and the parameters used for clay photo-functionalization has been

amply discussed and vindicated in Chapter II, while the results of the characterizations

performed on the photo-functional inorganic-organic complexes have been reported and

interpreted in Chapter III.

If no good alternatives for the choice of the model fluid can be found (since the

requirements imposed to the polymer matrix are numerous and quite strict), another

possibility would be to purposely prepare a new class of photo-functional complexes –

eventually but not necessarily by cation exchange process – in which the photo-activity

would be once again assured by the RhP moieties inserted into clay galleries3, but the

chemical compatibility with the PDMS matrix would be guaranteed by a specifically

designed surfactant. This solution is inspired by the analogous problem encountered by

some researchers who wanted to obtain exfoliated PP-clay nanocomposites by melt

processing without making use of any dispersion-enhancing additives. Wang et al. [27],

for instance, observing that many research efforts had been focused on dispersing MMT

in PP (and that such efforts failed because of the absence of strong interactions between

clays and highly apolar polyolefins), proposed to overcome the hurdle of formulating

complex compounds by a compatibilization method which can be certainly generalized

to other systems: synthesizing ammonium-terminated polymer chains (belonging to the

same family but eventually shorter than those present in the polymer matrix) and using

them to perform cation exchange processing of clay mineral fillers. This idea has been

afterwards proposed by Schmidt [2] to obtain PDMS-modified clays by cation exchange

processing a MMT-Na+ with short ammonium-terminated PDMS chains resulting from

the acidification of a commercial amino-terminated PDMS. In such a situation – that is,

when the surfactant for clay modification is insoluble in most of the solvents capable of

swell clays – some other procedure for cation exchange processing must be used and the

choice of the experimental protocol can be long and labor-intensive: Schmidt developed

a melt exchange technique, but the work of Ma et al. [14] let also guess that some other

method can be found, for instance choosing a different solvent (or a mixture of solvents)

3 This condition must be assured because, if another fluorescent dye is selected, the lighting source should

be consequently changed.

Chapter V


as the exchanging medium4. The same idea has been developed by Li et al. [15]: they

modified clay by siloxane surfactants and dispersed it into polymethylsilsesquioxane

(PMSQ) by solution intercalation. Afterwards, they compared the system with the same

matrix compounded with a commercial organoclay (Cloisite ® 15A). As a confirmation

of what previously reported, they concluded that organoclays modified by carbon-based

surfactants are not suitable to prepare PMSQ-clay nanocomposites.

If no good alternatives for the choice of the fluid or the photo-functionalization

of the lamellar mineral fillers are found, there’s yet one alternative: synthesizing photo-

active silica fillers. The photo-functionalization of silica particles could be envisaged

as: (1) a photo-functionalization ex situ, including a modification of the silica particles

right after their synthesis (and typically introducing the fluorescent cationic dye on their

external surface) or (2) a photo-functionalization in situ, viz. performed at the same time

of the synthesis (which could eventually be designed to obtain a core-shell morphology,

in which the silica shell would enclose a fluorescent RhP core). Ow and coll. [28], for

instance, recently described highly fluorescent and photo-stable core-shell nanoparticles

(size range 20-30 nm) obtained by a modified Stöber synthesis5, which are

monodisperse in solution and resulted 20 times brighter and more photo-stable than

their constituent fluorophore. They synthesized the particles for biological applications

(labeling of macromolecules for bioimaging experiments), but nothing lets imagine that

such a class of particles couldn’t be used for applications in the material field. However

it is obvious that, if we consider using photo-active core-shell silica particles (whether

the fluorophore encloses or is enclosed by silica) for Visiovis experiments, even tough

4 To be honest, during the last year of PhD research activity, we started performing some trials of cation

exchange process of Cloisite ® Na+

with a commercial low molecular weight NH2+-PDMS-NH2

+. We first

tried to reproduce the experimental protocol proposed by Schmidt [2] (melt cation exchange process) with

and without the fluorescent cationic dye but then, as we didn’t estimate this procedure sufficiently “clean”

for fluorescence applications (Schmidt assumed that the excesses of hydrochloric acid and/or surfactant

were negligible, but fluorescence doesn’t tolerate the presence of contaminants or excesses which could

affect absorption and emission), we rather started adapting the protocol set up in Chapter II to a surfactant

insoluble in water (such as the NH2+-PDMS-NH2

+ provided by Degussa). We tested the cation exchange

process protocol described in § II-2.4 with Cloisite ® Na+ and a proper amount of NH2

+-PDMS-NH2

+ in

toluene, with and without RhP: however, we won’t present any preliminary result here, since we haven’t

yet optimized the parameters of the protocol and we haven’t yet completed the required characterizations. 5 The Stöber synthesis of colloidal silica was first described in 1968 and is nowadays largely used to

obtain monodisperse nano- to micro-sized silica particles. Van Blaaderen et al. first reported the covalent

incorporation of organic fluorophores into Stöber colloidal silica and the synthesis of fluorescent silica

nanoparticles in the hundreds of nanometers size range [28].



the interactions between PDMS and filler particles will be probably enhanced, the

information retrieved would be less rich than in the case of photo-active lamellar fillers.

We amply discussed about clay morphology and the differences between distributive

and dispersive mixing during polymer-clay compounding (Chapter I): we also proposed

an experimental protocol which, performed on Visiovis, could inform about distribution

(thanks to the CCD cameras and the image processing) and probably dispersion (thanks

to spectrofluorimetry) of photo-active clays in the transparent fluid modeling a molten

polymer. As the core-shell silica particles are spherical and would likely be stabilized

by the favorable interactions established with the matrix, only distributive mixing could

be visualized.

V-2 EQUIPMENT

Configuration. As described in Chapter IV, the actual configuration of Visiovis

is the result of some reasoned changes justified by:

the complex geometry of the screw/barrel system;

the selection of the area of interest (the volume of fluid comprised between

two adjacent screw flights, the internal surface of the barrel and the screw root surface);

the set up of a planar light source for fluorescence excitation (laser sheet);

the intrinsic difficulties associated to data acquisition and processing.

All these factors have equally contributed to the modification of configuration detailed

in Chapter IV. Some of them are strictly correlated to each other and interdependent. As

several critical problems have been encountered about the compatibility of the model

fluid with the photo-active lamellar fillers (§ V-1) and would need to be promptly fixed,

we believe that Visiovis configuration shouldn’t be considered as a main concern.

Light source. We mentioned that some of the factors which determined Visiovis

actual configuration are strictly correlated to each other and result interdependent: light

source is one of the constrained parameters. The first aspect limiting the choice of the

light source is, obviously, economic: powerful, monochromatic and perfectly collimated

laser sources can be quite expensive. In our case, the choice has been helped by the fact

that the cationic dye selected to perform clay photo-functionalization (Rhodamine 6G

Chapter V


Perchlorate) is one of the most common fluorophores, well known by several groups of

researchers and regularly used by biologists (see § I-4): this consideration shouldn’t be

underestimated, as it obviously implies that the laser sources adapted for RhP excitation

(as well as the required filters to separate the emission from the excitation source) are

commercially available and (only) reasonably expensive. We’ve repeatedly stressed that

RhP represents an ideal probe to study heterogeneous systems thanks to the dependence

of its absorption and fluorescence emission on the properties of the matrix: therefore,

we suggest to persevere as long as possible with such fluorophore – a choice which will

be automatic in case the photo-active clays (whose photo-functionalization protocol has

been described and optimized in Chapter II, and whose characterizations are discussed

in Chapter III) will be used as they are, but which should be rather imposed if any other

solution is found (e.g. Stöber synthesis of core-shell colloidal silica particles, § V-I).

Barrel. The transparent PMMA barrel represents, undoubtedly, the main core of

Visiovis. Its transparency symbolizes the practical interest of process visualization and,

therefore, corresponds to the aspect which should be absolutely preserved. On the other

hand, the fact that the barrel is made of plastic have represented, since the beginning,

one of the most serious limitations of Visiovis (§ IV-1.1.2). Of course, the best solution

would be to find a material (probably a special glass) totally transparent and capable of

tolerating high temperatures and high radial pressures; in the absence of such an ideal

solution, we suggest to replace the actual barrel with another PMMA barrel having the

same internal surface (cylindrical) but a parallelepipedic external surface. This wouldn’t

fix the problems due to temperature and pressure, but at least would ameliorate the

optical quality of the visualization. Some accesses for real-time sampling would also

represent a valuable amelioration and would allow a further exploitation of the actual

screw/barrel system (see § V-4).

Screw. Far from being a problem, the screw profile rather offers a big margin of

modification. In Chapter I we stressed that the industry of plastic gradually developed

its own equipments and progressively multiplied the number of applications requiring

specific processing tools. Nowadays, the trend of the market concerning the processing

tools for the industry of plastic tremendously developed the relation supply-demand, to



a point that the possible configurations of the same processing tool are as various as its

possible applications. Screw profiles are customizable – sometimes several units having

specific functions (mixing, homogenizing, etc.) can be composed to form a single screw

profile perfectly adapted to a specific application. In relation to the choice of the screw

profile, Visiovis doesn’t limit imagination – the only feature directly influenced by the

choice of the screw profile is the data acquisition and processing.

Feeding. We previously detailed the protocol for the experiments performed by

Visiovis and, in particular, we described the procedure used to inject the masterbatch

into the system: “the syringe is plunged vertically in the aperture and the injection is

rapidly achieved in the lowest accessible point” (§ IV-2). At the moment we performed

the experiments, we had no alternatives to inject the tracing masterbatch into Visiovis

screw/barrel system. Indeed, as reported by Cassagnau et al. [29]6 and confirmed by our

own experience, the feeding system may have a deep influence on the results of tracing

experiments: furthermore, the actual feeding system of Visiovis (Figure IV-F19) is far

from being practical and handy. The utilization of a syringe allowed us to perform

experiments and collect some early results – that’s surely a good point – however we

suggest, as a perspective easy to implement and probably rather inexpensive, to equip

Visiovis with a permanent, firmly positioned feeding system (eventually shaped as a

syringe) which should finally reduce any manipulation inaccuracy inevitably produced

by the operator and, thus, assure a better reproducibility of the observed phenomena.

V-3 PROCESSING OF THE ACQUIRED DATA

In Chapter IV we related Visiovis evolutions and its actual configuration, as well

as the possibilities of data acquisition and processing. We described both the categories

of experimental data provided by Visiovis (i.e. the images and the fluorescence spectra)

but we rather focused on the images, viz. we set up two methods for image processing:

6 Cassagnau et al. [29] developed a UV-fluorescence monitoring device to evaluate in situ the mixing

efficiency of an internal batch mixer (§ I-4). They observed that the fluorescence curves recorded by this

device strongly depended on the experimental conditions of injection of the tracer, thus they decided to

simply drop the tracer on the molten flow stream.

Chapter V


the first based on the standard deviation of image luminosity (§ IV-3.1.1), the second

based on the Discrete Fourier Transform of textured images (§ IV-3.1.2). We justified

our choices and we validated the selected methods for image processing (in particular

the second one) by computer simulation (§ IV-3.1.3), nonetheless a direct and precise

correlation between observed and simulated phenomena has yet to be established7, and

the data acquisition and processing methods set up during the last three years represent

a tremendously interesting starting point. Moreover, if we take into consideration the

problems encountered with the model materials (in particular, the lack of compatibility

between the model fluid and the photo-active lamellar fillers prepared from commercial

clays), it is clear that Visiovis (and the described data collection and processing) has not

yet been completely exploited – one more reason to suggest looking for a more suitable

set of model fluid and photo-active fillers before changing Visiovis configuration8 or

searching for new methods for data processing. In addition, the choice of a set of model

materials more prone to clay intercalation and or exfoliation would finally allow to take

advantage of the complementary information provided by spectrofluorimetry.

V-4 INTERPRETATION AND VALIDATION OF THE RESULTS

The interpretation and validation of the results obtained by Visiovis is certainly

priority – if compared to any possible change of configuration, or to the quest of some

new method for data processing. After having introduced the aforementioned methods

for data processing (§ IV-3.1.1 and IV-3.1.2), we provided a preliminary validation of

the results obtained by such methods, as well as of their most probable interpretation (§

IV-3.1.3). Indeed, data processing and the interpretation of the ensuing results cannot be

considered complete without an accurate and systemic experimental validation, attained

by correlating Visiovis experimental results with the evidences provided by other (more

conventional) characterization techniques such as XRD, TEM and rheology (§ I-2.2.1).

We are absolutely aware of the importance of such validation and, actually, we planned

to perform it but, as previously stressed, the choice of methyl-terminated PDMS as the

7 We’ll come back on this topic in a following paragraph (§ V-4.2).

8 About this topic, please refer to § V-2.



model fluid restricted considerably our prospects – directly (electron microscopy cannot

be performed on oils or uncured resins) or indirectly (the properties of the model fluid

aren’t specifically restraining with respect to XRD and rheology which, on the contrary,

contributed to the assessment of a visible lack of compatibility of the photo-active fillers

with the PDMS matrix). There’s no need to spell out that the best way to correlate

Visiovis results (in situ) with the information provided by traditional techniques (ex

situ) would be to perform a systemic real-time sampling of the compound evolving into

the screw/barrel system and characterize it by XRD, TEM and rheology. Manifestly,

real-time sampling represents something priority for Visiovis development, as well.

V-4.1 Real-time sampling

As explained when discussing about the calibration of Visiovis detection systems

(§ IV-1.2.4), we haven’t yet designed a practical and handy method to get some samples

of the compound evolving in the screw/barrel system – nonetheless, we acknowledge its

priority. For the moment, just to get a hint of the possible correlation between Visiovis

and the other conventional techniques for morphological characterization, we performed

some measurement tests on the masterbatches prepared for Visiovis experiments (§ IV-

2) in their initial state by XRD and rheology. We couldn’t even consider performing

electron microscopy on such masterbatches because, as previously reported, electron

microscopy imposes a few but strict conditions on the nature and physical aspect of the

samples (§ I-2.2.1). One of such conditions – probably the strictest for silicone oil – is

that the samples have to be observed in vacuum as the air molecules would significantly

scatter the electrons. Environmental electron microscopy could probably represent a

possible alternative to conventional (under high vacuum) electron microscopy (it allows

even hydrated samples to be viewed in low-pressure wet environments) but, unluckily,

we didn’t dispose of an environmental electron microscope during this PhD research

activity and, in any case, the characterization would have been surely not conventional.

Chapter V


XRD9. Schmidt et al. [16] pointed out that, because of the lack of X-ray contrast

(i.e. differences in electron density) between the silicone and the silicate (as compared

to layered silicate nanocomposites with carbon-based polymers), the XRD pathways of

PDMS-clay nanocomposites often present low intensities of the diffraction peaks and

low signal-to-noise ratios – which limits the use of XRD to materials containing at least

10% wt of clay, even if smaller amounts of clay are likely to produce better dispersion.

We definitely confirm the observations reported by Schmidt et al. [16]: indeed,

we performed XRD measurements on a number of masterbatches prepared with all the

available photo-active lamellar fillers, alone or with the corresponding pristine clays, by

different methods of compounding (manual stirring, mixing by a disperser TurboTest

Rayneri 33/300P, ultrasound probe) and with different clay contents (1%, 3%, 5% wt)…

whatever the clay content, whatever the nature of the filler, whatever the procedure used

for compounding, we got silent XRD pathways. Two examples of silent XRD pathways

are shown in Figure V-F2 (bold lines): they have been both obtained for a masterbatch

containing 0.25% wt of the photo-functional complex C30B 0.25MC RhP and 0.75% wt

of the pristine C30B (total amount of clay 1% wt). In particular, Figure V-F2 (a) shows

the pathways for the masterbatch obtained by manual stirring, whereas Figure V-F2 (b)

shows the pathways for the same masterbatch prepared by the disperser (20 min at 1000

rpm). Probably, we would have erroneously concluded that clay particles were perfectly

exfoliated into the polymer matrix (see considerations previously made about erroneous

interpretations of silent XRD pathways § I-2.2.1) if we didn’t dispose of the following

evidences – rather supporting the fact that clays were distributed into the PDMS, but no

dispersive mixing had occurred:

the masterbatches appeared initially (i.e. right after mixing) homogeneous, but

after some time started settling – meaning that a considerable amount of clay particles

remained agglomerated and, under the action of gravity, drifted down to the bottom of

9 The experimental protocol for XRD measurements is the same used for the photo-active lamellar fillers,

described in § II-3.1.



the pan, whereas a minor fraction of smaller clay particles10

appeared stably suspended

just because insensitive to gravity11

;

to confirm our conjecture, we centrifuged the aforementioned masterbatches

(5 min at 10000g, i.e. 15500 tr/min) in order to remove as much PDMS as possible, so

that the sediment (the sample) got enriched of silicate filler to the maximum extent. In

the case of the centrifuged samples (containing essentially the clay powder wetted by an

insignificant amount of PDMS) we got an XRD pathway showing a single low-intensity

peak roughly corresponding to the interlayer spacing of the dry filler (Figure V-F2, thin

lines). This evidence represents a further confirmation of the fact that methyl-terminated

PDMS may have some advantageous properties in terms of visualization (transparency,

inertness) but certainly isn’t a suitable fluid to model carbon-based molten polymers

compounded with clays.

Figure V-F2 XRD pathways of a masterbatch containing 0.25% wt of photo-functional

complex (C30B 0.25MC RhP) and 0.75% wt of the corresponding pristine clay (C30B),

before and after centrifugation (bold and thin lines, respectively). Two methods having

different compounding efficiencies are compared: (a) manual stirring and (b) mixing by a

disperser TurboTest Rayneri 33/300P (20 min at 1000 rpm).

10

Clay fillers can be characterized by more or less large size distributions. 11

We affirm that a minor fraction of smaller clay particles appeared stably suspended because, in spite of

decantation, the masterbatches kept their colored appearance and we never observed a clear separation of

colored (red/rose photo-active filler) sediment from a clear (transparent PDMS) supernatant.

Chapter V


Rheology further confirmed the inadequateness of methyl-terminated PDMS, as

we are going to show later on.

Rheology. The literature largely confirms that rheology is a valuable tool for the

analysis of filler dispersion in molten polymers, as reported in § I-2.2.1. Undoubtedly,

rheology can provide useful information in a variety of different measurement modes,

e.g. flow, dynamic, transient measurements. As we had already collected some negative

evidences from the XRD characterization of the masterbatches (see previous section),

we estimated unnecessarily time-consumptive to perform a complete set of rheological

characterizations just to confirm several times the same, negative result. Therefore, we

proceeded following only the most simplistic approach: we performed some dynamic

measurements in order to check whether the complex viscosity at low frequencies got

increased by the presence of a percolating filler and, additionally, whether the moduli G’

and G’’ crossed in correspondence of a percolation threshold – indicating a transition

from the purely viscous liquid behavior (typical of unfilled methyl-terminated PDMS)

to a solid-like behavior as the frequency decreases. The behavior of the samples (that is,

the behavior of methyl-terminated PDMS) is particularly exasperating for rheological

characterizations: the specific properties of methyl-terminated PDMS [3] make of it a

fluid which flows incredibly easy and can perfectly wet most of the surfaces12

– namely,

the metallic plate surface of a cone-plate geometry of a rheometer. These extraordinary

properties represent a serious obstacle to the formation of a regular and stable meniscus

in the gap between the cone and the plate of the rheometer – indeed, the fluid gradually

flows and the amount of measured sample diminishes, producing erroneous results. This

is the reason why the first measurement tests, performed on a stress-controlled AR1000

rheometer with the largest available cone-plate geometry (35 mm ), were unsuccessful

– in spite of the remarkable sensibility of the equipment (even to low-viscosity fluids)

and of an optimized choice of the geometry. Resolute to get at least one measurement

for confirmation, we performed few further tests on a strain-controlled ARES rheometer

with a Couette geometry (in order to prevent the sample from escaping the measurement

volume) – unsurprisingly, regardless of the reduced sensibility of the equipment with

12

Paquien [3] offered a clear and complete summary of the typical properties of polysiloxanes.



respect to the stress-controlled rheometer, these measurements were at least correct.

Nonetheless (and unsurprisingly as well), such measurements confirmed that the filler

didn’t get dispersed into the matrix (no increase of the complex viscosity has ever been

observed for any of the aforementioned masterbatches13

) and distribution was probably

inhomogeneous and certainly unstable (G’ and G

’’ never crossed). The results obtained

by dynamic rheological characterizations performed on the same masterbatches chosen

to show XRD silent pathways (Figure V-F2), as well as the homologous curves for the

neat PDMS, are shown in Figure V-F3.

Figure V-F3 Dynamic rheological behavior of the neat PDMS (a) and of the masterbatch

containing 1% wt of filler (0.25% wt C30B 0.25MC RhP and 0.75% wt C30B) prepared

by (b) manual stirring and (c) mixing by the disperser (20 min at 1000 rpm).

13

Just for rheological measurements, we even prepared a masterbatch containing 10% wt of clay!

Chapter V


V-4.2 Computer simulation

As a final point, we would like to express a few considerations about computer

simulation. In Chapter IV we affirmed having used computer simulation to justify and

partially validate the image processing applied to Visiovis experimental data. We hope

we have been sufficiently clear to convince about the pertinence of supporting the real

experiments with the results of simulations. Here we would only like to stress that, in

order for such correlation to be correct and complete, the experimental conditions and

the simulation equations and parameters must be as close as possible to each other (or,

better, as coherent as possible with each other): this is possible only if both experiments

and simulations are developed at the same time and, in particular, in the total respect of

the limitations imposed by the counterpart. The perspective of fixing Visiovis feeding

system (§ V-2), for instance, will surely help the correlation with computer simulation.

The experimental protocol could then be adjusted for the results to be as repeatable as

possible and as coherent as possible with the results of the simulation (§ IV-3.1.3). On

the other hand, computer simulation could be adapted to reproduce the flow behavior

exactly in the same geometrical plan created by Visiovis laser sheet14

, and the images

obtained by computer simulation could be rendered a little more “realistic” by applying

a standard image treatment to increase fuzziness before applying the image processing

procedures described in § IV-3.1.1 and IV-3-1-2.

A lot of work left, quite a lot of courage needed!

14

Indeed, the computer simulation performed by Yves Béreaux (§ IV-3.1.3) represents the screw channel

as it appears if observed in the direction perpendicular to the screw flight surface, whereas Visiovis laser

sheet enlightens a plan which passes by the axis of the screw/barrel system, thus is parallel to the axis and

form an angle equal to the screw helix angle of the screw (§ IV-1.2.1). So far, computer simulation and

experiments show the same phenomena – if the screw helix angle is neglected in terms of visualization.



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