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Some Observations of the lamellar Morphology in Isotactic Polypropylene Spherulites by SFM
G. Castelein, G. Coulon, M. Aboulfaraj, C. G’Sell, E. Lepleux
To cite this version:
G. Castelein, G. Coulon, M. Aboulfaraj, C. G’Sell, E. Lepleux. Some Observations of the lamellar Morphology in Isotactic Polypropylene Spherulites by SFM. Journal de Physique III, EDP Sciences, 1995, 5 (5), pp.547-555. �10.1051/jp3:1995145�. �jpa-00249330�
Classification Physics Abstracts
61.40K 68.20 81.00
Some Observations of the Lamellar
Morphology
in IsotacticPolypropylene Spherulites by
SFMG. Castelein(~), G. Coulon(~), M. Aboulfaraj(~>*), C. G'sell(2) and E. Lepleux(~)
(~) Universitd des Sciences et Techniques de Lille, Laboratoire de Structures et Propridtds de l'Etat Solide (URA CNRS 234), Bit. C6, 59655 Villeneuve d'Ascq, France
(~) Laboratoire de Mdtallurgie Physique et Science des Matdriaux (URA CNRS 155), Ecole des Mines de Nancy, Parc de Saurupt, 54042 Nancy, France
(~) Socidtd INSTRUMAT, 4 avenue des Andes, Z-A- de Courtaboeuf, 91943 Les Ulis, France (Received 4 November 1994, accepted 31 March 1995)
R4sumd. La microscopie de force atomique a dt4 utilis4e pour 4tudier la structure lamellaire d'4chantillons massifs de polypropylbne isotactique obtenus par intrusion. La microscopie de force atomique utilis4e en tapping mode se r4vble Atre un outil performant pour caract4riser la texture des sph4rolites o et fl h l'4chelle lamellaire. La structure, l'orientation et l'4paisseur des lamelles ont 4t4 d4termin4es dans les deux cas.
Abstract. The lamellar morphology of intruded bulk samples of isotactic polypropylene has been investigated by scanning force microscopy. It is shown that SFM operated in the tapping
mode is a powerful tool to characterize the texture of o and fl spherulites at the larnellar level.
Structure, orientation and thickness of the lamellae have been determined in both cases.
1. Introduction
This paper aims to report on the observation of intruded bulk samples of isotactic polypropylene (iPP) by Scanning Force Microscopy (SFM). The morphology of this polymer at the micrometer
and nanometer scales has been the object of numerous papers. Natta et al. [1], Padden and
Keith [2,3] identified first the two major crystalline forms of iPP, namely the monoclinic a form and the hexagonal fl form. Other minor forms (6,-~) have been also identified but they
appear only under specific conditions [4,5].
More recently, a quantitative structural characterization of both o and fl forms was per- formed by Norton and Keller [6] on the grounds of dramatic electron micrographs. The
spherulitic morphologies of iPP are usually classified in four distinct types [2,3]: the aI and OII types crystallize with the monoclinic structure; in addition to being formed within differ- ent temperature ranges, they also differ in their respective birefringence values: positive for
(*) Present address: Pechiney CRV, BP 27, 38340 Voreppe, France
© Les Editions de Physique 1995
548 JOURNAL DE PHYSIQUE III N°5
the former, negative for the latter. More rarely, the hexagonal fIIII and fllv types spherulites
appear to forIn within certain constraints of isothermal crystallization temperature [2,3]; they
are characterized by a strong negative birefringence. Types III and IV nucleate at a much lower rate than types I and II but they grow faster by between 20-70%. Analyzing iPP thin films by TEM, Norton and Keller [6] were able to reveal the relationship between spherulite type and the lamellar architecture. The a spherulites are constituted by a network of radial lamellae with intertwinned tangential larnellae ("cross- hatching" ), types I and II differ on the lamellar level by the relative ratio of the rudial and tangential lamellae, the amount of
tangential lamellae being higher in type I. By contrast, the fl spherulites consist only of radial lamellae either straight in the type III case or twisted in the type IV.
The results obtained by Norton and Keller were based on a series of delicate experiInental procedures: preparation of thin fillns by microtomy, etching by acid with the method prescribed by Olley and Bassett [7,8], replication of the samples surface for TEM observations. Despite its usefulness for the analysis of the crystalline morphology at the laInellar level, such a technique
is not appropriate to study the microstructure of bulk samples, for example to follow the evolution of lamellar morphology of a bulk sample under uniaxial tension or simple shear.
Aboulfaraj et al. [9,10] investigated the spherulitic microstructure of iPP bulk samples by
Ineans of Scanning Electron Microscopy. For as-cast intruded iPP plates, they showed that
the proportion of the a and fl phases depended on the distance from the external surfaces.
For a temperature processing of 230 °C, they found that the mid-thickness plane contained about 60% of fl spherulites, the average size of which being 120 ~m. Under tensile loading, in
sitt~ SEM investigations revealed that the a spherulites exhibit a brittle behaviour while the
fl ones deform plastically up to high deformations. In contrast, under shear loading, the a
phase cavitation disappears and both a and fl phases are capable of undergoing large strains.
However, due to the insufficient resolution of the SEM, the authors were not able to visualize
directly the larnellae (10-50 nm in thickness), consequently to detect their displacement during
the deformation tests.
In the last years, scanning force microscopy (SFM) ii1-13] has been shown to be a promising technique for the visualization of material surfaces from the microscopic level to the nanoscopic
one. SFM is a non destructive technique which is operated directly on the surface in air; it is thus easier to use than the replication technique. In the present paper, the spherulites morphology of intruded iPP plates has been investigated by SFM down to the lamellar scale
(10-50 nIn).
Up to now, only few investigations of the iPP structure by SFM have been performed.
Sch6nherr et al. [14] have studied the details of the spherulitic morphology of iPP thin films by scanning force microscopy in the contact Inode (the so-called atoInic force microscopy, AFM).
Experiments were performed using two diflerents kinds of tips. Most of their data are only
focused on the o spherulites and on the lamellar cross-hatching phenomenon they were able to detect. However, they report that, in the range between typically 10 to 200 nm, the applicability
of the AFM in the contact mode is hampered by tip effects which are related to the curvature of the apex. Recently, Crhmer et al. [15] have pointed out that, in the case of oriented isotactic
polypropylene and syntodiactic polypropylene thin films, the SFM experiments conducted in the tapping mode have a higher resolution than in the contact mode, revealing nanofibrils of 10-15 nm in width. However, their work was mainly focused on oriented films and they did not investigate in details the spherulitic structure of unstretched films.
We report here preliminary investigation by SFM in the tapping mode on the morphology of
a and fl spherulites which are present in intruded iPP plates. Furthermore, in order to check the efficiency of the tapping mode, our results have been systeInatically compared to those
obtained by Norton and Keller [6].
2. Experimental
The PP was Inanufactured by Appryl (3050 MNI). This isotactic grade has a broad Inolecular
weight distribution as assessed by gel permeation chromatography (GPC) with Mw = 75940
g/mol and Mn = 26200 g/Inol. The pellets of the material were subsequently processed by intrusion in a thick mould (300 x 200 x 15 mm~) designed for producing parallelepipedic plates.
The intrusion consists in slowly extruding the melt which is at an initial temperature of 230 °C in the mould (which is maintained at 30 °C) and continuing the mould feeding under the
extruding pressure (6 MPa) during the cooling sequence. This relatively slow cooling for about 240 s provides a quite reproducible semi crystalline structure. The samples obtained have a
negligible orientation and possess much less chain degradation than with injection moulding.
In intruded samples, the crystallization kinetics varies across the plate, the cooling rate being faster close to the external surfaces than at the core plate. Aboulfaraj et al. [9] have shown that the spherulitic structures are quite different in the two zones. In the mid-thickness plane of the plate, the fraction of fl spherulites is about 60% and their mean size is 120 ~m while
very fine spherulites of a phase are observed at the surface.
iPP samples (7 x 7 x 2 mm~) were cut out from the core of a plate. The face chosen for observations was progressively abraded by using several different emery papers and finally polished with a very fine alumina powder (0,1 ~m) until no visible scratch was detected on the surface. In order to reveal the crystalline phase of the spherulites, the samples were then immersed at room temperature in the solution recoInmanded by Olley and Bassett [7,8]
and successfully used by Norton and Keller [6] in their work on iPP: 1.3 wt% potassium permanganate KMn04, 32.9 wt% concentrated orthophosphoric acid H3P04 and 65.8 wt%
sulfuric acid H2S04. This solution etches preferentially the amorphous phase of the polyIner
in the spherulite. Subsequently, the saInples were rinsed in a dilute H2S04 solution (30%),
then in hydrogen peroxyde, in distillated water and in acetone. They were finally dried in a
vacuum oven.
Since the etching time has a great effect on the surface roughness, samples were immersed in the acid solution during different periods of time: 0, 20, 30, 40, 60 min, 6 and 18 hours. SFM
observations of all the samples show that the lamellar structure can be revealed for etching
times greater than 20 minutes. The preliminary data we present here have been obtained on samples etched during either 20 or 40 minutes.
Systematically, we firstly observed our samples by optical microscopy. Under direct illumi- nation, a spherulites exhibit a bright contrast while fl ones show a dark aspect, due to their
respective low and high rougnesses [9]. This preliminary observation allowed us to recognize
the different types of spherulites and to choose the species deserving a detailed SFM investi-
gation, I-e- the spherulites cut in their mid-plane which provide the best characterization of lamellar Inorphology.
The first set of observations has been performed with a Nanoscope II AFM (Digital Instru-
ments). This kind of microscope operates in the contact mode: the tip located at the end of the cantilever probes the surface of the sample which is mounted on a moving piezoelectric
scanner. Square-pyramidal S13N4 nanotips, with a rudius of curvature of at least 20 nm were
used to image the surface. In the contact mode, the interaction force between the tip has two
coInponents: repulsive nor1nal and frictional lateral respectively. The measured force is the
repulsive one, typical values of the forces used in our experiments being about 10 nN. For flat surfaces, the frictional force does not affect noticeably the resolution. Unlikely, for acid-etched surfaces, the pronounced relief enhances the frictional force; the images are noisy and the res-
olution is poor. Another source of resolution loss is tip imaging: the radius of curvature of the
tip is coInparable to the laInellar thickness. As a consequence, although the contact mode is
550 JOURNAL DE PHYSIQUE III N°5
~
~,
'~W I
f~
3
Fig. 1. AFM micrograph (Nanoscope II in contact mode) of a iPP sample etched for 18 hours.
Surface area: 150 ~lm x 150 ~lm; Z range = 700 nm. Scanning frequency: 4.34 Hz. 1: fl spherulite,
2: a spherulite, 3: spherulite which has been cut out of its equatorial plane.
well adapted to the observation of the spherulites (size m 120 ~m), it is not appropriate for
observing objects whose size is comparable to the radius of curvature of the tip,
Consequently, all the observations of the crystalline lamellae presented in the following have been obtained by using the Nanoscope III scanning force microscope in the tapping mode.
That mode has been perfected by Digital Instruments [16]; in that mode, the cantilever is forced to oscillate at a frequency (331 kHz) close to its resonance frequency (343 kHz) with an
adjustable amplitude. The tip briefly contacts the surface of the sample at each oscillation, the frictional force being thus eliminated. Pure monocrystalline silicon tips model TESP,
with a radius of curvature of about 10 nm, were used to image the surfaces. Mean value of the repulsive force was about 0,1 nN. Images were obtained by using the 10 ~m x lo ~m
piezoelectric scanner and the scanning frequency was about I Hz. In the tapping mode, the microscope can be operated along two different ways: the so-called "height" and "amplitude"
modes. In the height mode, images are obtained by using the feedback loop which maintains
at a constant value the amplitude of the tips. Height rneasureInents are deduced from the
vertical displacement (z) of the piezoelectric scanner and the corresponding image reflects the
topography of the surface. In the amplitude mode, the feedback loop is not connected, the amplitude can vary and the images correspond to the variations of the amplitude. All the data we present here have been obtained in the height mode; all the images have been filtered
through the planefit procedure.
3. Results
Figure I shows the typical aspect of the spherulites in iPP as evidenced by contact mode
Nanoscope II AFM (Piezoelectric scanner: 150 ~m x150 ~m). Both a and fl spherulites can be distinguished. Numbers 1, 2 and 3 refer respectively to a fl spherulite, a a spherulite and
a spherulite cut out of its equatorial plane, showing a lesser contrast of the larnellae viewed
more or less radially.
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j.
o zso soo
Fig. 2. SFM micrograph (Nanoscope III in tapping mode) of iPP sample etched for 40 minutes:
cross-hatched lamellar arrangement in a tx spherulite. Surface area: 587
nm x587 nm; Z range = 80
nm. Scanning frequency: I Hz. Black and white arrows show the respective directions of some radial and tangential lamellae, "N" design a zone with a "nodular" structure.
Semi-quantitative measurements of roughnesses were mude on several spherulites of both
a and fl types observed by AFM Nanoscope II. They provide a comparative insight of relief differences between a and fl surfaces. The measurements were performed by recording the RMS roughness in a set of10 ~m x10 ~m squares among different zones covering all the surface of each spherulite studied. This procedure was performed for both a and fl spherulites.
The a spherulites exhibit a mean roughness of 257 + 36 nm and the fl ones show a roughness
value nearly twice that of o ones: 590 +131 nm. The values of the roughness are thus quite different from one type of spherulite to another. These differences in roughness induce different types of contrast in both optical and scanning electron microscopies as it has been noticed by Aboulfaraj et al. [9]. Indeed, they used these variations of contrast to distinguish the different types of spherulites.
In the following, preliminary results about both a and fl iPP structures observed by SFM
are presented. Figure 2 to Figure 5 show images which were recorded in the Tapping Mode.
a sphert~lites
Figure 2 clearly illustrates the "cross-hatched" larnellar arrangement Inentioned by Norton and Keller [6]. The sample observed here has been etched for 40 minutes. In this area, larnellae
were nearly perpendicular to the free surface. Black and white arrows show respectively the directions of radial and tangential lamellae. It has to be pointed out that the stucture appears
552 JOURNAL DE PHYSIQUE III N°5
Fig. 3. SFM 3-dimensional view of the core of a flui spherulite in iPP sample etched for 40 min.
Surface area: 10 ~lm x10 ~lm; Z range = 350 nm. Scanning frequency: 0.898 Hz.
"nodular" in some places (marked "N" in Fig. 2). Norton et al. [6] suggested that this feature represents the earliest stage of branching before the developernent of the network constituted
by both rudial and tangential larnellae. However, most of the a spherulites we have investi-
gated do not exhibit such nodular structures. More often, the texture of the o spherulites is
purely larnellar in agreement with a low fraction of tangential larnellae. Isothermal crystal- lization experiments have clearly evidenced that the ratio of tangential larnellae decreases as the crystallization temperature Tc increases [6]. Even if our data cannot be directly compared
to those obtained from isothermal crystallization experiments, the relatively small aInount of
tangential larnellae observed here in the a spherulites corresponds approximately to the Inor-
phology of iPP thin films crystallized isothermally at temperatures in the range from 135 °C to 145 °C [6,14].
Quantitative measurements have been performed on numerous a spherulites. The thickness of the lamellae is equal to 21.7 + 3,7 nm for the rudial larnellae and 19.1+ 5.6 nm for the
tangential lamellae. Since the influence of the etching time on the surface morphology has not been investigated in details to date, the thicknesses have been arbitrarily measured at the half-height of the emerging lamellae. These values agree with the larnellar thicknesses in iPP thin films crystallized isotherInally at temperatures in the range from 135 °C to 145 °C
[6,14]. However, correlating the crystallization temperature with the lamellar thickness is not
straightforward, since the crystallization kinetics in intruded iPP specimens is complex and not isothermal. Indeed, even if our measurements show that the radial lamellae are slightly
~b
~ Q
~
,.
'.
.$
.~
i
o soo
Fig. 4. SFM micrograph of iPP sample etched for 40 min: edge-on lamellae within the core of a fIIII spherulite. Surface
area = 591
nm x591 nm; Z range = 138 nm. Scanning frequency: 0.896 Hz.
The arrow indicates the radial direction of growth.
,
.,
0 a) 1 00 vm 0 b) 1 00 vm
Fig. 5. SFM micrograph of iPP sample etched for 40 minutes: flat-on lamellae in
a fIIII spherulite.
Surface area
= 1 ~lm x1 ~lm. Scanning frequency: 1 Hz a) Topography of the surface. Z range
= 350
nm b) "Error signal" image.
