The last issue of the “SURFACES: THE INVISIBLE UNIVERSE” focused on viewing the chemistry of surfaces through the lens of X-ray Photoelectron Spectroscopy (XPS, aka ESCA). In this issue we shift focus to the problem of surface modification and in particular polymer surface modification. This topic is highly apropos in view of the upcoming Tenth International Symposium on Polymer Surface Modification: Relevance to Adhesion, to be held at the University of Maine, Orono, June 22-24 (2015). This symposium will cover all aspects of polymer surface modification from mechanical roughening to laser modification. The meeting will be concerned with the those areas where surface modification is a key technology which allows for the processing and manufacture of products which would otherwise be unobtainable. This meeting will also delve into the realm of biopolymer materials with applications to forest products, medical implants and food processing. All readers are cordially invited to join the symposium either to present a paper on their current work in this field or to simply attend and greatly expand their awareness of current developments. Further details are available on the conference web site at: www.mstconf.com/surfmod10.htm
SURFACES: LOW VOLUME BUT HIGH IMPACT ENTITIES
An elementary calculation for just about any cube of a simple atomic or molecular material 1 centimeter on a side indicates that roughly one atom/molecule in ten million resides within ten angstroms of the outer surface. Though relatively minuscule compared to the roughly 1023 atoms present in the sample, this surface layer determines many of the most important physical and technological properties of the material. Common examples include optical properties such as reflection of light, all contact properties including friction, surface wetting and adhesion and many chemical phenomena including resistance to corrosion, biofouling and staining. As important as these properties are, however, surface related phenomena received relatively scant attention during the early decades of the 20th century mainly due to the difficulty in performing careful experiments on such small amounts of material. Well into mid century the solid state physics community was more focused on bulk properties of solids such as crystallographic structure, electronic band structure and bulk thermodynamic behavior. What made all this work possible was the relative ease in preparing well characterized samples of uniform quality. The influence of small amounts of contamination and crystallographic defects is small for bulk samples and more or less in proportion to the amount present.
In dealing with surfaces, however, we are faced with an entirely different situation. The effect of small amounts of contamination and surface defects, rather than being more or less suppressed, can actually dominate the surface properties of the material under investigation. Thus the basic problem of preparing well characterized surfaces for investigation is far more difficult than in the case of examining bulk properties. Nonetheless, even though the sensitive properties of surfaces present daunting difficulties for those who want to perform careful investigations, these same properties present opportunities for the technologist who wants to alter surface behavior in order to create useful products and devices. Nowhere is this opportunity more prevalent than for polymer materials which offer surfaces that can be modified by a number of physico-chemical methods to give specifically tailored results. What makes this technology even more attractive is the fact that surface modification leaves all the bulk properties of the material intact so that one can, so to speak, have one’s cake and eat it too. A typical example might be the case of a high modulus fiber being considered as a reinforcing agent in a particular matrix material. One often finds that the fiber and matrix material are incompatible as obtained off the shelf. However, appropriate surface treatment of the fiber can make it compatible with the matrix without sacrificing its reinforcing properties making possible the creation of a new composite material.
MICRO-ORGANISMS AND THE FIBER-MATRIX INTERFACE
One of the biologically oriented applications of surface modification technology is the removal of pathogens from food products such as berries, fruits and vegetables using atmospheric plasma technology. Work in this area is currently underway and is planned to be presented at the symposium mentioned above. However, on the flip side of this development, microorganisms, instead of being removed from a surface can also be added to improve adhesion as was discussed in a most interesting paper given at the first in the Polymer Surface Modification series.
Since ancient times mankind has taken advantage of the bio-chemical synthesis skills of micro-organisms to create a number of desirable products. Wine, beer and cheese derived from fermentation are the most recognized achievements of microbial industry. However, to the best of my knowledge the work of Pisanova and Zhandarov is the first to put the microbes to work in modifying the surface properties of fibers for use in reinforced composites. These authors worked with the following fiber materials:
- poly(caproamide) (Abbr. PCA)
- poly(p- amidobenzimideazole) (Abbr. PABI)
- poly(p-phenyleneterephthalamide) (Abbr.
- poly(m-phenyleneisophthalamide) (Abbr. PPIA)
- polyimde (Abbr. PI)
The above fiber materials were considered as reinforcements for the following matrix materials:
- polycarbonate Abbr. PC
- polysulfone Abbr. PSF
- polyethylene Abbr. PE
The fibers were exposed to the following micro-organisms:
- Bacillus vulgaris (bacterium)
- Bacillus cereus (bacterium)
- Pseudomonas (bacterium)
- Aspergillus (fungus)
The authors knew that the above listed micro-organisms could attack and degrade polymer materials. However, they reasoned that in the case of densely packed fibers the bacteria would be limited to performing their biochemical antics at the fiber surface leaving the bulk properties intact. Thus each of the fibers was immersed in a nutrient medium containing one of the micro-organisms for up to two weeks to see what changes in the surface properties could be obtained. The timing of exposure was important so as to limit the action of the organisms to just the fiber surface. The mechanical properties of the fibers were measured beforehand by standard methods and the adhesion of the fiber to the matrix material was ascertained using the fiber pullout method. The results of their experiments were quite surprising in that in many cases they saw both improved mechanical properties of the fiber and improved adhesion of the fiber to the binding matrix material. A sample of their results for the PCA/high density polyethylene (HDPE) system is given in Table I. A cursory look at the data in Table I shows that the fiber mechanical properties are moderately improved by the micro-organism action with an increase in fiber modulus by as much as 18% and the strain at break by nearly 25%. However, the bond strength of the fiber to the matrix in some cases could increase by nearly a factor of 2 as seen for the Bacillus vulgaris data.
Also notable from the data in the table is the deleterious effect of overexposure to the microbial treatment as evidenced by the drop off in properties when the exposure time for Bacillus cereus was increased a factor of 4.
|TABLE I: Effect of micro-organism treatment on fiber mechanical properties for PCA and fiber/matrix bond strength for the PCA/HDPE system
||TENSILE STRENGTH (GPa)
||ELONGATION AT BREAK %
||BOND STRENGTH PCA/HDPE COMPOSITE (MPa)
|Bacillus cereus(8 week treatment)
Though the precise details of the microbial action were not uncovered in these experiments a few things were made clear by microscopic examination. In particular the first stage of the treatment involved the filling of cavities, pores and microcracks on the fiber surface with the byproducts of the micro-organism’s metabolism which resulted in the healing of the defects and the subsequent improvement in mechanical properties.
In addition the surface chemistry of the fiber is significantly altered making it more compatible with the binding matrix and thus also improving the performance of the composite material.
As a final word it is clear that this type of work is in its very early stages and that there is tremendous scope for expansion of the types of systems which can be looked at and the extent of improvement that can be achieved. In addition, since the types of micro-organisms being used are commonly present in the environment this type of surface treatment could be considered more environmentally friendly than other methods which rely on harsh chemicals.
Dr. Robert H. Lacombe, Chairman Materials Science and Technology CONFERENCES
3 Hammer Drive, Hopewell Junction, NY 12533-6124
Tel. 845-897-1654, FAX 212-656-1016 E-mail: email@example.com
 “Modification of Polyamide Fiber Surfaces by Micro-organisms”, Elena V. Pisanova and Serge F. Zhandarov, in Polymer Surface Modification: Relevance to Adhesion, Ed. K. L. Mittal (VSP, The Netherlands, 1995) p. 417