Applications of the NanoRackSample Stretching Stage toa Commercial Impact Copolymer

AFMAsylum Research

Dalia G. Yablon and Andy H. Tsou, ExxonMobil Research and Engineering, Clinton, N J.

Introduction
A commercial impact copolymer (ICP), a multicomponent material typically used in automotive and appliance applications wherea balance of stiffness and toughness is needed, was studied with the NanoRack™ Sample Stretching Stage accessory on theMFP-3D™ Atomic Force Microscope to investigate material deformation and interface adhesion as a function of tensile stress.
Effects of deformation were observed within both the polypropylene and ethylene-propylene components, as well as at theinterface between the two materials. There are no other direct measurement methods available to determine interfacial adhesivestrength of polymer blends, and so AFM investigations of micro-domain deformation such as the one described here could be
used ultimately to provide a direct determination of interfacial adhesion in complex polymer containing materials such as ICP.
Studies of this kind improve our understanding of material structure-property relationships, ultimately enabling manufacture ofbetter quality products.

Application to Impact Copolymer (ICP)
The commercial impact copolymer used for this study iscomposed of a polypropylene (PP) matrix with micron-sizeddomains of ethylene-propylene (EP) rubber domains produced
in a serial polymerization reactor. Dogbone-shaped samples
were molded of the impact copolymer measuring at ~20 mm(middle straight part of dogbone) by ~4 mm in width by 0.2mm thickness. A portion of the straight part of the dogbone
was cryo-faced at -120°C with a cryomicrotome to ensure asmooth sample and to remove the thin polymer surface layerthat forms during the compression molding process (alsoreferred to as a ‘polymer skin’), leaving a small and smoothsurface area in the middle of the dogbone that was suitablefor imaging. The sample was mounted into a NanoRack
Sample Stretching Stage. The NanoRack is a high-strain,
high-travel manual stretching stage that provides two-axis
stress control of tensile loaded samples and also allows controlof the sample image region under different loads. Automatic
load cell calibration provides integrated force measurementswith MFP-3D images or other measurements and returns bothstress and strain data.
Figure 1 shows real-time stress vs. time curves of the ICP as
the sample is being pulled in the NanoRack. The baseline force
is measured at 2.55N and is a function of this particular strain
gauge. Note that the force spikes immediately upon pullingand is followed by a lengthy relaxation process that is strongly
material dependent. Even after a small initial 0.6% elongation(corresponding to pulling approximately 0.1mm on a 20mm
dogbone), this ICP sample required 2-3 hours to relax after
which AFM imaging could be conducted without significantdrift. Stiffer samples require more relaxation time while moreelastic samples may require little or no time to equilibrate aftera stretch.

1Figure 1: Stress (Newtons) vs. time (seconds) curve of ICP as it is being
stretched on the NanoRack.

As the impact copolymer stretches, the individual components(PP and EP rubber) deform accordingly in response to thetensile stress. In this note, we examine the behavior within
the EP rubber, within the PP, and at the PP/EP interface as thematerial stretches. Effects of deformation within the EP rubber
are shown in the AFM images in Figure 2 where second mode
amplitude images (from Bimodal Dual AC™ mode) are shownof the same EP rubber domain in (a) neutral position prior to
any stretching and (b) after a 3% elongation stretch wherethe stretching direction is indicated by the yellow arrow.

Applications of the NanoRackSample Stretching Stage toa Commercial Impact Copolymer
elongation and the next morning
revealed a disappearance of thestretch marks as shown in the AFM
image of Figure 3d, suggesting theyielding of the PP matrix overnight. An
overlay of amplitude mode 2 on top of
the underlying topography of Figure
3b is shown in Figure 4, showing
topographic deformation within
the EP domain but the absence of a
gap between the two components,
indicating reasonable adhesion
between the two components.
Finally, the effect of the stress within
the PP matrix at 2% elongation isshown in Figure 5. Both topography(a) and phase (b) images of a largearea (15 µm) scan size show a number of areas where cracks
have formed at the EP-PP interface and propagated into theFigure 2: Second mode amplitude images of identical EP domain within ICP material at(a) neutral position on NanoRack and (b) 3% elongation on NanoRack.

A 3% elongation is below the yield strain of PP. As marked byblue circles, rips within the EP rubber domain that were present
in the neutral position (a) have grown both in length and widthin (b) as a function of the stretching.
In addition, there are areas of new
growth of rips within the EP domain asshown by the region encircled in red.
The effects of tensile stress on thePP/EP interface is shown in Figure 3.
The ICP material before any pulling
occurs is shown in Figure 3a. After
1.1% elongation and 3.5 hours ofrelaxation, the image in Figure 3b
shows the “stretch marks” between
the EP rubber and PP at the interface.
The development of stretch marks may
be due to the mismatch in Poisson
ratios between EP and PP materials; EPis incompressible with a 0.5 Poisson
ratio, whereas PP has a Poisson ratio
about 0.3 - 0.35 before yielding. If theEP domain is stretched mainly along
the equatorial line as in the experimentconducted here, then stretch marks
would develop mainly at top andbottom of the EP rubber domain as
observed in the AFM image in Figures
3b,c. As shown in Figure 3, thesemarks are asymmetric about the EP
rubber domain and appear to be mostprominent at the bottom of the domain, though stretch marks
are also observed on the top portion of the domain.
These stretch marks remain throughout the pulling experimenton the first day as evidenced in the AFM image of the same
domain at 1.7% elongation and 7 hours after initial pull(Figure 3b,c). The sample was allowed to sit overnight at 1.7%

2Figure 3: Second mode amplitude images of identical EP domain within
ICP material tracked as a function of different sequential stresses exerted
by the NanoRack. Sample in neutral position shown in (a). Experimentstart time was marked at time of initial pull of 1.1% in length of total
dogbone length. Image of ICP at 1.1% elongation length shown in(b) 3.5 hours after start of experiment. ICP image at 1.7% elongation
length shown in (c) 7 hours after start of experiment. Image of ICP at
1.7% elongation length after sample in (c) equilibrated overnight for a
total of 26 hours after start of experiment shown in (d).

Applications of the NanoRackSample Stretching Stage toa Commercial Impact Copolymer
As shown in Figure 5, the larger (highlighted) cracks
propagate several microns. However, there are also
several cracks with significantly smaller dimensionsof a couple hundred nm in length and tens of nm
in width. Zooming in on the crack circled in orange(in Figure 5b) reveals tiny PP fibrils stretching acrossthe entire width of the track, as shown in thecorresponding topography (c) and phase (d) images,
at about 45 degree to the stretching direction
suggesting that the cracking is induced by shear
deformation. This particular crack is measured to be~80 nm in depth and ~600 nm in width.

Figure 4: Second mode amplitude overlaid onto 3-D topography of ICP
material rendered from Figure 3b. Interface showing stretching of the EP
domain as it adheres to the PP is circled in red.

PP matrix; some of the cracks arehighlighted in blue/orange circles.
The developments of cracks or shear
bands and micro-voids may comefrom stress amplification in the ICP
material due to the presence ofEP rubber domains. Note that thissample, whose images are in Figure
5, was pulled on a different day
than the one shown in Figure 3. In
the latter, the stress exerted on thesample did not reveal micro-voids orcracks within the PP – at least in thenarrow region that was being imaged
in Figure 3, where a single EP domain
was being followed. Maximum stressamplification by a spherical EP rubber
domain is inversely proportional
to the square root of the crack tip
radius and occurs at the poles of theEP rubber domain. All these cracks
and shear bands in Figure 3 appear
to originate at the polar locations of
the EP rubber domains, probably at
sharp corners of the rubber domain
with extremely small crack tip radii(and therefore maximum stressamplification resulting in a stresssingularity at that point). The appearance of these shear bands
and micro-voids suggests that the local stresses well exceed theyield stress despite the 2% global deformation.

Figure 5: Corresponding topography (a) and phase (b) images of15 µm x 15 µm area of PP matrix within ICP material at 2% elongation.
Regions of large crack formation are highlighted in orange and bluecircles. Zoom in on crack highlighted in orange circle in (a) is shownin corresponding topography (c) and phase (d) images. Line scan in(c) along red line measures width and depth of crack.

3

SummaryMorphology and interface adhesion of an impact copolymer(ICP) were studied using atomic force microscopy with aNanoRack tensile stage. Effects of deformation were observed
within both PP and EP components as well as at the interface
between the two materials. A continued stretching of the ICP
could lead to delamination of EP from PP matrix. The strain
required to separate the EP domains from the PP matrix could
be used as a measure of the interfacial adhesion between EP
and PP. Most importantly, the corresponding local interfacial
stretching extent or void length between EP and PP upondelamination, which can be measured directly by AFM, canbe used to calculate the interfacial strength between EPand PP. Presently, there are no direct measurement methodsavailable to determine interfacial adhesive strength of nanoand microscale domains within polymer blends, especiallyblends generated in-situ in polymerization reactors. This AFM
examination of micro-domain deformation through the usageof the NanoRack described qualitatively here could be used for
direct determination of interfacial adhesion in complex polymercontaining materials such as blends and composites.

Acknowledgements
Authors would like to thank Andy Winesett and Jean Grabowski
for sample preparation.

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