High resolution atomic force microscopy imaging of crystalline polymers nanostructures on mica: from spherulites to single molecules

High resolution  atomic force microscopy imaging of crystalline polymers  nanostructures on mica:   from spherulites to single molecules

Prokhorov V.V.

Experimental

• AFM instruments:  Ntegra
• AFM measurements: at ambient conditions with low and moderate amplitudes of cantilever oscillations (3-30 nm) commonly in the attractive regime of probe-surface interactions.
• Samples preparation: polymers deposition on mica from semi-diluted  and  highly  diluted solutions at temperatures in a range  of 100-150OC
• The polymer grades used:  Poly(1-butene) (PB-1): M_N=85500 , M_W/M_N=2.24, isotactic polypropylene (iPP):   M_N=49000, M_W/M_N=5.3,  ultra-high molecular weight polyethylene (UHMW-PE): M_N=239000, M_W/M_N= 5.45; 

Introduction

Crystalline polymers such as widely used in industry polyolefins (polyethylene, polypropylene, poly(1-butene) are ideal objects for the demonstration of capabilities of atomic force microscopy (AFM) in a wide range of scales and physical properties. They have a rich morphology in a large scale measured by microns and tens of microns (spherulites, dendrites), in a medium scale many polymers have characteristic lamellar morphology, whereas at a small scale polymer molecules have various and generally complex  conformations with a size of submolecular details in a nm rang.  Building blocks of crystalline polymer matter formed at crystallization from both a melt and a solution are lamellae (Fig.1) in which polymer molecules fold back on themselves many times forming sheet-like lamellar crystal with a thickness of the order of 10 nm.  Despite a large progress in last decades the complex question of a polymer crystallization at a molecular level is still a subject of intensive theoretical debates  whereas the microscopic observations of isolated molecules of  crystalline  polymers such as polyolefins are very few [1]. In the present report the polymer crystallization from a solution on mica was a subject of a study in a wide range of concentrations and solution temperatures and first high-resolution AFM images of single molecules of crystalline polyolefins have been obtained.

Large scale morphology  of  bulk  specimens: spherulites  and  lamellae

Fig.2 represents the AFM height image of flat iPP spherulites with uncommon non-periodic banded structure observed in a thin polymer film grown on a glass in a quickly evaporated  concentrated  polymer  solution.   The microscopic view obtained at a micron scale  (Fig.3) demonstrates the cross-hatched structure typical for  iPP §Т-phase  whose spherulites are known  to have no any banded structure at crystallization  from  a melt. The repeated melt and re-crystallization cycles of the  specimen in Fig.   gave no banded structure as well.  The image in Fig.2  thus demonstrates that the well-known morphological similarity in solution-crystallized and melt-crystallized polymer structures observed at micro-level (lamellae) and macro-level (spherulites) is not observed in this case.  The formation of a much more complex polymer superstructure (banding) at a macroscale at a  crystallization  from  a solution is probably related with the existence of some peculiar growth regime in conditions of a diffusion-limited aggregation behavior which appears in thin films at very high crystallization rate at high solution undercooling.

 Fig. 1  A simplified model of a polymer lamella ("edge-on" ± view). In  this model two areas of  fold  consist  of close-adjacent tight folds, in reality the folds can be ordered in much lesser degree.

Fig. 2 The AFM height image of uncommon banded spherulites of isotactic polypropylene grown on glass from highly undercooled concentrated solution.

Fig. 3 The AFM image of lamellar cross-hatching in banded spherulites indicating ¦Б-phase crystallographic structure.

Single  polymer lamellae of poly(1-butene)

AFM images of isolated lamellae of poly(1-butene) in Figs.4,5 demonstrate a number of interesting features. The angular distribution of lamellae long axes (not shown) has wide maxima separated by gaps of about 60 degrees indicating the epitaxial growth on mica cleaved plane which is known to have the hexagonal crystal symmetry.
In the height view (Figs.4,5) the lamellae have the characteristic “double stripe” morphology consisting of two adjacent slightly undulating stripes separated by a distance within 7-15 nm with a depression between them. The phase image (Fig.5) also reveals two characteristic areas:  sharp bright core and a featureless “negative” halo on both sides.
The interpretation of this morphology suggests that the two elevated stripes in height images correspond to two side fold areas in the lamella model in Fig.1 whereas the depression between corresponds to the crystalline core. The very low observed heights (see the section in Fig.5a) quantized with a step within 0.6-0.7 nm imply that the lamellae are formed by polymer chains folded in mono- and bi- layers on mica surface.
It is worth to notice that the appearance of “double stripe” morphology is not related with well-known artifacts of AFM imaging due to the jumps between attractive and repulsive regimes of probe-surface interactions [2]. These jumps may appear at some improper choice of operating parameters of the oscillating cantilever resulting in its bistable dynamical behavior, they may be additionally stimulated by the sample heterogeneity (the difference in visco-elasticity of crystalline and amorphous lamellae parts as well as the difference in hydrophilicity of hydrophilic mica and hydrophobic lamellae in our case). Although such artifacts due to attractive-to-repulsive transitions have also been observed at the variation of operating parameters and can even enhance the double-stripe effect it is clear from the phase section in Fig.5b that in this particular case the probe interacts with all the sample parts being in the attraction regime only (all phase values > 90O). At phase measurements the absolute phase value was tuned before phase imaging to correspond the physically relevant conditions [2]: at the resonance frequency ω0 far from the surface the phase signal was chosen to be 90O and dφ/dω >0, at such a choice phase>90O indicates the attraction, phase<90O indicates repulsion in probe-sample interactions. The appearance of a substantial phase contrast between these two lamellae parts and its interpretation in accordance with [2] indicate the difference in a degree of a local energy dissipation which is largest at the lamellae  sides and smallest for the crystalline core.
The double-striped morphology was always observed on AFM height images obtained in the attraction regime with weakest probe-surface interaction (largest set-points).  Weak non-periodic lamellae undulations seen on AFM images may be explained by invoking the idea of intralamellar chain tilt often observed in polymers (Fig.6).
AFM visualization of bulk specimens of crystalline polymers demonstrate excellent contrast, so that the crystalline parts of adjacent lamellae and amorphous gaps between them are clearly differentiated especially in phase images [4]. The images in Fig.5 show that this differentiation can be efficiently done for isolated single lamellae also and height images may have even better visibility.

 Fig. 4  The large scale AFM height image  of  isolated PB-1 lamellae deposited on mica.

Fig. 5 The comparison of AFM height (a) and phase (b) images  of  PB-1 lamellae obtained in the attraction regime. The horizontal dotted lines show the correspondence between the characteristic  lamellae  parts on height and phase  images and corresponding sections shown to the right. 

 Fig. 6 The model of lamellae  undulations  due  to  intralamellar  chain  tilt. 

Single molecules and small multimolecular nanoparticles of isotactic polypropylene (iPP) are observed in highly ordered compact crystalline conformations referred to as the nanocrystallites (Figs.6,7).  The morphological similarity between AFM images of some elongated nanoparticles such as selected by the circle and the lamellae in Fig.5 implies that polymer chains are packed in a similar way in both cases. The corresponding model of the chain packing in the simplest case of rectangular nanocrystallites is shown in Fig.6b, it implies that such nanocrystallites are in fact nanolamellae of a molecular size. The low observed AFM heights of the order of 1-3 chain diameter (see the section in Fig.7) imply that such nanolamellae are in “edge-on” orientation on mica.

It is worth to notice however that except of simple rectangular a lot of complex surface conformations are also observed (Fig. 7) indicating much more complex than lamellar chain packing in this case. The observed complexity is probably due to the absence of crystallographic matching of a polymer crystal cell and mica crystal cell what results in a complex character of chain folding at polymer molecules adsorption on mica crystal plane from a solution.

Fig. 7   (a)  The  AFM height image of molecular-size  nanocrystallites of isotactic polypropylene on mica and (b) the interpretation of the submolecular structure for the nanocrystallite with a  rectangular shape.   Closely packed chain stems are separated by appr. 0.5 nm and can’t be  distinguished on AFM images however the crystalline core and  the  peripheral area of folds are clearly discriminated.  Two ‘‘simple” nanocrystallites conformations shown  by the  square and the circle have a morphology similar to that of    “double-stripe”  morphology  of PB-1  lamellae  in Figs.4,5. A lot of complex molecular conformations is also observed, the arrows show  two of  them.

The comparison of nanoparticles volumes measured by AFM with real physical volume of iPP molecules (VN=80nm3, VN=430 nm3) has been done with a  large  statistics by use specialized AFM  software [5]. The conclusion which has been done from such a comparison is that many nanocrystallites are composed of single iPP molecules.
One more unexpected finding is a fine transversally striated substructure seen for many nanocrystallites (Fig. 7) indicating the corrugation of nanocrystallites top surface. The striated morphology was often found at deposition on mica of another polymers, particularly the isolated lamellae of same iPP grade had similar striations. At the same time the lamellae of PB-1 in Figs. 4-5 have common morphology with no striations. The images in Fig. 8 also demonstrate that the resolution of a few nm can be routinely achieved at AFM measurements of nonperiodic polymer structures.

Fig. 8 The height (a) and phase (b) images of iPP nanocrystallites of same sample but taken at smaller set-point ratio. The fine transversal striations (such as shown by arrows) are clearly  resolved on both images.

Nanocrystallites of ultra-high molecular weight polyethylene

Fig. 9  The  AFM height image of UHMW-PE nanostructures on mica:  (a) single-molecule nanocrystallites, (b)  short  lamellae including  several  molecules.

In contrast to iPP the UHMW-PE nanocrystallites reveal lesser structural complexity and more perfect morphology (Figs. 9 a,b). They have predominantly simple rectangular shapes with flat tops, the peripheral fringe found for iPP wasn’t  observed here. The comparison of measured AFM volumes with real physical volumes of UHME-PE molecules (VN≈400 nm3, VW≈2180 nm3) indicates that most UHMW-PE nanoparticles even largest in size in Fig.8a are very probably composed of single polyethylene molecules. Statistics of the height distribution (Fig. 10a) reveals several strictly equidistant peaks separated by a distance corresponding to the polyethylene chain diameter (appr. 0.5 nm) and implies the chain packing in several molecular layers in accordance with the model in Fig.9b. Most probable number of layers is 3-5 in contrast to 1-3 layers  for iPP.

Fig. 10 (a) A histogram of nano-crystallites height distribution, (b)  A simplified model of a nanocrystallite structure.

The nanoparticles height quantization is accentuated by their  different colors of a chosen color palette in Fig. 9
Of two lateral crystallites dimensions one (length, L, Fig. 9a) is found to be in the range of 7-16 nm and is associated with the polymer fold length which is known to be relatively invariable. This size is not fixed however and has unexpectedly large fluctuations even within a single scan area as it may seen for example for small needle-type nanocrystallites in Fig. 9a. The reason of such large fluctuations is not clear. The second dimension (width, W) varies greatly from several nm-s to tens nm-s due to a large variation in a number of folds in accordance with the large dispersion in the molecular weight and different number of polymer molecules in multimolecular nanocrystallites which in fact turn to short lamellae in this case (Fig. 9b).

Summary

• High-resolution AFM images of single molecules of crystalline polyolefins (nanocrystallites) have been obtained for the first time.
• Not only regular geometrical shapes but also fine submolecular structure have been observed for polymer nanocrystallites demonstrating excellent AFM capability.

Opened  questions

• The reason of morphological complexity of surface conformations of molecules of crystallie polymers
• The origin of submolecular structure
• The reason of large fold length fluctuations within the statistical  ensemble of spatially close nanocrystallites.

Footnote  

The necessary prerequisite for high-resolution imaging  is a sharp cantilever. For this aim  a  severe  selection of  common commercial cantilevers was done. Alternatively cantilevers  with  super sharp diamond-like carbon tips  were  used in a  combination  with a procedure  of a “soft landing”  described  in [6].

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