Compatibilization of Polymer Blends
Polymer Morphology via Scanning Electron Microscopy
- Fig. 1: SEM-BSE images of PP-POE blend at different blend ratios without and with compatibilizer (top and bottom row, respectively). In the compatibilized cases, 10 wt% of the elastomer was replaced by compatibilizer.
- Fig. 2: SEM-BSE of large area morpology for 70-30 ratio of PP-POE blend without and with compatibilizer (top and bottom row, respectively). On the right, particle size distributions for different blends.
- Fig. 3: Phase diagram for PP-POE blend. The error bars represent the standard deviation.
Polyolefins account for more than half of total plastics consumption in the world and are potentially good for the circular economy. It is well known that control of morphology of immiscible blends is critical for tailoring of the final product properties. The effect of compatibilizer on blend morphology over a broad range of compositions was studied systematically using scanning electron microscopy. Low compatibilizer level provided significant improvement in phase dispersion.
Many plastic waste recycle streams are blends of various types of polypropylene (PP) and polyethylene (PE). When reprocessing these blends into products it is difficult to obtain good mechanical and optical properties due to the immiscibility of the components. Blend morphology is one of the leading factors for these properties. Also it is well known that morphology is governed by process conditions, material properties and interactions. Therefore control of morphology is a key challenge when turning plastic waste into valuable materials.
In this work, the effect of a novel PP-based olefin block copolymer compatibilizer on the morphology of PP – PE blends was investigated. The use of SEM analysis enabled the systematic study of the effect of polypropylene compatibilizer on phase morphology over a broad range of compositions.
Materials & Methods
Two homopolymer PPs and polyethylene-co-octene (POE) based blends were examined in this study. The homopolymer PPs from Braskem were selected to vary viscosity ratios. The polyolefin elastomers (POE) and the PP-based olefin block copolymer compatibilizer were from The Dow Chemical Company. Compression molded plaques with POE loading between 10 wt % and 40 wt % were prepared.
Blend morphology was characterized with SEM at low accelerating voltages using backscattered electron (BSE) imaging . For this method, alternative to conventional TEM, sample preparation is less demanding. Large sample cross-sections can be easily and quickly prepared.
Polymer blend samples were trimmed using cryo-microtome at –100 ºC to obtain block faces with width of 0.3 mm and length of 1 mm or approximately half the thickness of the sample plaque.
The samples were stained in RuO4 vapors overnight and then re-polished using a diamond knife at room temperature. The block faces without any further treatment were examined in FEI Nova NanoSEM 600 (Thermo Fisher Scientific) under high vacuum at 3 keV using a solid-state BSE detector in inversed contrast equivalent to TEM methodology.
SEM BSE images were aquired across the block face length allowing bulk information of phase morphology. Size distribution of elastomeric domains was obtained from image analysis. High and clean contrast allowed fully automated image processing. The gray-scale images were altered into binary images. The areas of the detected rubber particles were converted to the equivalent circular diameter. Volume-weighted mean diameter (Dv) and the rubber phase volume were calculated. Extensive statistics were obtained over large particle size population in the range of 3000 to 15000 particles per sample.
Results and Discussion
Selective staining between different polymer phases was created by absorption of RuO4 vapor. Polymer crystalline regions largely exclude the heavy metal, while lower density amorphous regions absorb strongly the metal stain. The more heavily stained phases appear brighter in the SEM-BSE images due to the higher backscattered electron yield, whereas in TEM the stained phases are darker due to electron scattering. For ease and intuitive interpretation, the results are presented in inversed BSE contrast. The elastomer phase is represented by dark and highly crystalline matrix by bright areas. The SEM-BSE images showed high contrast between these phases even at the low accelerating voltage.
The high quality of sample preparation gave little topographical contribution to the contrast formation. High resolution imaging was not a focus of this study but in general using this approach images with low nanometer detail information can be obtained as the samples do not require additional coating. The RuO4 heavy metal staining provides enough conductivity for imaging at low accelerating voltage conditions. Comprehensive morphology information can be resolved showing interfaces and differences between crystalline lamellae structure, as demonstrated in .
SEM imaging of compression-molded plaques showed that finer dispersions were obtained when adding even a small amount of compatibilizer. As an illustration, two examples of POE in PP dispersions are shown in figure 1. When the minor phase is present in only small amounts, it can be dispersed relatively easily into a droplet-in-matrix morphology by breaking up the minor phase domains. Upon increasing the amount of minor phase, coalescence of freshly formed droplets gains importance as the probability of collision increases. This results in more complex morphologies with significant co-continuity for the blend with 40 wt% minor phase.
Morphology analysis over large sample area revealed local differences in elastomeric domain size (fig. 2). This example demonstrates the need for both high number of particles to obtain good statistics and the necessity of statistically significant data containing local inhomogeneities for determination of bulk structure parameters. The histograms show particle size distribution for different blend ratios. Addition of the compatibilizer not only resulted in reduction of the droplet diameter Dv for all blend ratios, but also reduced local particle size variability. The compatibilized blends showed more uniform particle size and narrower distribution. Moreover, study of coalescence demonstrated that the presence of compatibilizer stabilized the phase dispersion and reduced coalescence.
Quantitative analysis of the SEM images allowed construction of the morphology phase diagram for the blends as presented in figure 3. The error bars represent the standard deviation and are associated with the width of the histogram.
A new PP-based olefin block copolymer compatibilizer was used in blends of PP and POE. SEM study of the blends demonstrated that addition of the compatibilizer resulted in finer blend morphologies for droplet-matrix and co-continuous systems. The compatibilizer suppressed coalescence of droplets in PP/elastomer blends and helped to stabilize fine morphology. Low compatibilizer level was sufficient for significant improvement. This is a very effective way to create valuable materials out of mixed recycle waste streams. It has been also shown that using SEM-BSE on large block face has a clear benefit for mapping representative polymer bulk morphology.
Ewa Tocha1, Sylvie Vervoort1, Krischan Jeltsch2
1Dow Benelux B.V., Terneuzen, The Netherlands
2Dow Europe GmbH, Horgen, Switzerland
 Georg Bar, Ewa Tocha, Eddy Garcia-Meitin, Clifford Todd, John Blackson: New Routes to High Resolution and Automated Polymer Morphology Microscopy via Scanning Electron Microscopy, Macromol. Symp. 282: 128–135 (2009) doi: 10.1002/masy.200950813