RESEARCH INTERESTS

RESEARCH

 

My research is concerned with the development and subsequent investigation into structural properties of novel materials. Innovation of materials and processes that address the challenges of sustainability, energy supply and renewable resources play a major role in my research plan.
The major effort of my research program relate to producing materials that enable more efficient use of them in different fields, such as mechanical engineering, biotechnology, chemistry, etc. such as polymeric materials that can be modified in their structure by using different technologies of production and/or modification of their process conditions; it also includes the heat melding of microstructures. Another part is related to the development of materials used to generate low-k dielectric films for future generation of computer chips. Finally the generation of new devices that could be adapted to existing equipment in order to test new properties of materials.

 

1.-POLYMER PROCESSING

 

Development of innovative polymer microstructures
Shape and structure of materials can be manipulated by downstream mechanical processing in their molten or solid state or at the interface of these two. Downstream manipulation of normal polymer films has, in general, been extensively investigated in the past particularly in relation to the development of molecular orientation; see for example [1]. As such, a large body of literature on film extrusion, film casting and fiber spinning is of relevance to polymer materials modified by post-processing methods. Downstream drawing of polymer materials has been found to alter not only the physical shape and form of the product, but also to change the polymer’s mechanical properties. The change in elastic and other mechanical properties due to a drawing process is mainly a consequence of the molecular orientation within the polymer [1-3]. Melt drawing is another route for obtaining highly oriented products. By this method, it is possible to achieve high extension moduli and strengths, via flow induced chain extension and solid state drawing [4]. The physical and mechanical properties of solid phase polymer depend strongly on both its molecular architecture [5-8] and on the manner in which it was produced.
It has been studied by the author that by altering process conditions an isotropic/anisotropic material can be produced. From this background it has been discovered that is important to make use of the drawing properties of the materials in order to change the molecular arrangement of polymer chains. The directions in which the drawing is applied to the solid/molten polymer would modify the final molecular orientation function. Figure 1a shows an initially low level of orientation within a film and Figure 1b the development of orientation with subsequent mechanical drawing. It has been also showed that all these parameters are fundamental to obtain specific characteristics in the products. Uniaxial or biaxial mechanical drawing results in drastic modification of properties of the material "(Example: Video 1)". 
In my research project is planned to test a huge variety of polymers by using new experimental techniques and devices. By modifying the molecular structures of polymers via processing and post-processing their original molecular architecture could be possible to resemble the characteristics of expensive materials, with the advantage of managing low-costs.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Heat melding of microstructures
This research is concerned with the fabrication of new microstructures by heat
melding of plastic materials to form monoliths.
The most common processes for joining plastic components can be classified

as: mechanical joining [9], adhesive bonding [10] and welding [11] also called

melding. The melding process can be classified in terms of the heat generation

mechanism used to perform it. For example when heat is transferred to the

material either by contact, by radiation, by friction and by using an electric

current and an electromagnetic field [12].The melding, which consists of the

diffusion of polymer chains across the interface, is dependent on temperature and

can be described by the reptation theory, developed by de Gennes [13].

This theory describes the motion of individual polymer molecules

in the amorphous bulk which was derived by Edwards [14], who established

a formula for the free energy of the polymer chains and a model where the chain

is enclosed to a tube formed by surrounding chains. The theory is suitable to define

the backward or forward movements of each polymer chain which is considered to

be closely boxed in by the entanglements of contiguous chains.

 

This model is well known and can be applied to microcapillary films (MCFs) where there is one array of capillaries embedded on the film [3]. During the process it is assumed, that a perfect contact exists at the two polymer materials interfaces.
At time t = 0 all the chains have zero diffusion through the other plastic, at later time (t < tR), some of the chains have crossed the interface. When time is equal to the reptation time (t = tR), both polymer chains are totally interpenetrated creating entanglements, at this point the interface disappear becoming indistinguishable and the properties at the interface are the same as the ones at any point within the plastic [15, 16].
In the case where the parts are brought together at temperatures close to or above the melting point, known as hot plate welding [17, 18], the time to achieve maximum bond strength is approximately the same value that it takes to reach intimate contact [12].

 

The author [19] shows the formation of a micro-capillary monolith (MCM), which is a polymer monolith of microcapillary films, containing a two-dimensional array of capillaries (Figure 2). The monolith consists of two or more layers of MCFs permanently bonded together over their full length. This multi-layer structure is created by “hot pressing” at temperatures close the melting point of the polymer. This is a delicate process because the temperature, time and load need to be precisely controlled in order to meld the interfaces and to prevent collapse of the hollow capillaries.
The process of MCM formation relies on ‘polymer welding’, in which two molten or near molten interfaces are bonded together. LLDPE MCFs were heated during the heat welding process in order to create intimate contact between the interfaces enabling polymer chain diffusion to occur across the interface allowing subsequent entanglements to achieve the bonding, see for example [20]. The welding process starts when the MCFs are at a temperature close to the melting point of the LLDPE. At this temperature a reduction in the viscosity and entropy occurs, making the mobilization of polymer chains possible, and pressure is applied to facilitate the intimate contact of the MCFs.
The degree of welding and integrity of capillaries in the MCM formation are mainly the function of three factors, the processing temperature, T, applied compression ratio, CR, and melding time, t. The MCM melding process was modeled by the author [19] using the theory of reptation developed by de Gennes [13] and followed work by Gao et al. [21].

Heat melding of microstructures is a process that can be applied a different systems [19] where a multi-layer structure is created by a ‘‘heat generation mechanism’’ at temperatures close the melting point of the polymer.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2.-ZEOLITES

 

The development of materials used to generate low-k dielectric films for future generation of computer chips


The constant development of electronic devices with continuously shrinking microprocessor feature size necessitates interconnects with metals of higher electron conductivity and insulators with lower dielectric constant (k). These improvements will reduce RC signal delay, cross talk noise and power dissipation. Many materials have been proposed in order to achieve these objectives; however none of these have been able to reduce the k value significantly to satisfy Moore’s Law predictions for the near future. Since the inception of the semiconductor industry, the material of choice for dielectric materials has been dense silica with k=4. Porous materials with different framework densities offer the opportunity to fabricate scalable dielectric constant materials. We have previously demonstrated pure silica zeolite films by spin-on can reach a k value ~2.1 [22-25]. Silica zeolite nanocrystals with monodisperse particle size distribution synthesized from a colloidal precursor solution are being used to generate hydrophobic ultrathin films on silicon wafers via spin coating.
The spin-on films from the nanoparticles suspensions are fairly smooth, and no cracks are observed. However to obtain a better quality of film it is necessary to study the rheology of this suspension which behaves as a complex fluid.
To measure the dielectric constant of the film, aluminum dots are deposited on the film using thermal evaporation deposition through a shadow mask. The film-free side of the silicon wafer was deposited with a layer of aluminum by evaporation. The dielectric constant of the film was calculated by measuring capacitance of the aforementioned metal-insulator-metal structure using an Agilent 4285A Precision LCR meter combined with a Signatone S-1160 probe station.
This measurement requires a perfectly smooth film. In order to insure a uniform film is necessary to evaluate the rheological parameters of the suspension or solutions to spun on silicon wafers.
My research consist on developing new low-k dielectric films by exploring the chemical, rheological and mechanical properties of the materials.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3.-TRIBOLOGY AND RHEOLOGY

 

Generation of new devices
In recent years the standard methods for characterizing materials have struggled to cope with the rapid development of new materials. It has therefore become apparent that new innovative devices and improved methods for measuring material properties are required. Ideally a comprehensive characterization could be carried out simultaneously with a device and method applicable to a range of different types including complex systems. One such example is the triborheometer, a device able to measure both the triborheological and rheological properties of a material. Triborheology is the study of the friction between two surfaces rubbed against each other [26, 27]. This is studied by minimizing the gap between the material surfaces, to the point that the surfaces contact. On the other hand rheology focuses on the stresses between layers of a fluid [28], and therefore there is considerable gap between the surfaces during the experiment.

The combination in triborheology relies on measuring the friction coefficients measured at a range of different angular velocities and pressures. This information is essential in understanding and addressing issues of wear. There are many examples where these studies can be applied, such as knee cartilage wearing in the absence of synovial fluid [29, 30], the lubricity of asphalt [31], as well as the friction and flow of lubricating greases [32]. By studying these fundamental properties new systems could be developed using both natural and synthetic materials which may substitute or improve current systems and improve their lifetime or safety.
One of the objectives of this research is to characterize the lubricating properties of a broad spectrum of systems, both soft and hard using a single device, which could go study both the biolubrication of soft tissue and the lubrication of industrial hardware.

We are interested on an wide range of Triborheological research on the energy sector which includes:

  • Biodisel (as lubricant)

  • Wind turbines (triborheological system)

  • Development of new technologies to capture carbon ( circulant particles wear)

  • Bio-based lubricants

  • Technology to reduce vehicle friction

 

 

Bibliography
1. Ward IM: Mechanical Properties of Solid Polymers, 2nd edn. Bristol: John Wiley & Sons, Ltd; 1983.
2. Medina DI, Hallmark B, Mackley MR: Using Hollow Microcapillaries to Explore the Extrusion Rheology of Polymer Films. AIP Conference Proceedings 2008, 1027(1):54-56.
3. Medina D, Hallmark B, Lord T, Mackley M: The development of voidage and capillary size within extruded plastic films. In: Journal of Materials Science. 43(15): 5211-5221.
4. Bashir Z, Keller A: Melt Drawing as a Route to High-Performance Polyethylene. Colloid and Polymer Science 1989, 267(2):116-124.
5. Zhang XM, Elkoun S, Ajji A, Huneault MA: Oriented structure and anisotropy properties of polymer blown films: HDPE, LLDPE and LDPE. Polymer 2004, 45(1):217-229.
6.
D.I. Medina and M.R. Mackley. 2006. The evaluation of microcapillary films and microcapillary monoliths. "Structure Sensitive Mechanics of Polymer Materials: Physical and Mechanical Aspects", EUROMECH Colloquium 487, European Mechanics Society, October 10-13, 2006, Strasbourg, France. 

7. Medina D.I, Proceedings of The Evaluation of Microcapillary Films and Microcapillary Monoliths: August 28-Sept 09, 2006 2006; Aktiengesellschaft, Ludwigshafen, Germany.
8. Medina D.I, Proceedings of The Evaluation of Microcapillary Films and Microcapillary Monoliths: 2006. CPGS (Certificate of Posgraduate Studies) thesis, University of Cambridge, UK.
9. Gao SM, Budde L: Mechanism of Mechanical Press Joining. International Journal of Machine Tools & Manufacture 1994, 34(5):641-657.
10. Morris CEM: Adhesive Bonding of Polyethylene. Journal of Applied Polymer Science 1970, 14(9):2171-&.
11. Prager S, Tirrell M: The Healing-Process at Polymer-Polymer Interfaces. Journal of Chemical Physics 1981, 75(10):5194-5198.
12. Bastien LJ, Gillespie JW: A Nonisothermal Healing Model for Strength and Toughness of Fusion Bonded Joints of Amorphous Thermoplastics. Polymer Engineering and Science 1991, 31(24):1720-1730.
13. de Gennes PG: Reptation of a Polymer Chain in Presence of Fixed Obstacles. Journal of Chemical Physics 1971, 55(2):572-&.
14. Edwards SF: The statistical mechanics of polymerized material. Proceedings of the Physical Society 1967, 92:9-16.
15. Kim YH, Wool RP: A Theory of Healing at a Polymer Polymer Interface. Macromolecules 1983, 16(7):1115-1120.
16. Wool RP, Yuan BL, McGarel OJ: Welding of Polymer Interfaces. Polymer Engineering and Science 1989, 29(19):1340-1367.
17. Jenn-Yeu Nieh LJL: Hot plate welding of polypropylene. Part I: Crystallization kinetics. Polymer Engineering & Science 1998, 38(7):1121-1132.
18. M. N. Watson MGM: Recent developments in hot plate welding of thermoplastics. Polymer Engineering and Science 1989, 29(19):1382-1386.
19. Medina DI, Chinesta F, Mackley MR: Heat melding of voided polyethylene microstructures. Polymer 2009, 50(14):3302-3310.
20. Grewell D, Benatar A: Welding of plastics: Fundamentals and new developments. International Polymer Processing 2007, 22(1):43-60.
21. Gao P, Mackley MR: The Structure and Rheology of Molten Ultra-High-Molecular-Mass Polyethylene. Polymer 1994, 35(24):5210-5216.
22. Li ZJ, Lew CM, Li SA, Medina DI, Yan YS: Pure-silica-zeolite MEL low-k films from nanoparticle suspensions. Journal of Physical Chemistry B 2005, 109(18):8652-8658.
23. Li S, Li ZJ, Medina D, Lew C, Yan YS: Organic-functionalized pure-silica-zeolite MFI low-k films. Chemistry of Materials 2005, 17(7):1851-1854.
24. Medina DI, Li ZJ, Yan YS: Nanocrystalline zeolite beta film as a low-k material. Abstracts of Papers of the American Chemical Society 2005, 229:U913-U913.
25. Lew CM, Li Z, Li S, Hwang S-J, Liu Y, Medina DI, Sun M, Wang J, Davis ME, Yan Y: Pure-Silica-Zeolite MFI and MEL Low-Dielectric-Constant Films with Fluoro-Organic Functionalization. Advanced Functional Materials 2008, 18(21):3454-3460.
26. Hutchings IM: Tribology: friction and wear of engineering materials: Edward Arnold; 1992.
27. Gellman AJ, Ko JS: The current status of tribological surface science. Tribology Letters 2001, 10(1/2):39-44.
28. Macosko CW: Rheology - Principles, Measurements and Applications. Wiley-VCH, New York, 1994
29. Bodugoz-Senturk H, Macias CE, Kung JH, Muratoglu OK: Poly(vinyl alcohol)-acrylamide hydrogels as load-bearing cartilage substitute. Biomaterials 2009, 30(4):589-596.
30. Asik M, Ciftci F, Sen C, Erdil M, Atalar A: The Microfracture Technique for the Treatment of Full-Thickness Articular Cartilage Lesions of the Knee: Midterm Results. Arthroscopy: The Journal of Arthroscopic & Related Surgery 2008, 24(11):1214-1220.
31. andrew h: asphalt lubricity test evaluation and relationship to mixture workability. 2011.
32. Heyer P, Lauger Jr: Correlation between friction and flow of lubricating greases in a new tribometer device. Lubrication Science 2009, 21(7):253-268.
33. Medina DI, Elmoumni A, Braithwaite G, McKinley G: The development of an innovative triborheometry fixture/design to study the frictional dynamics of solid-liquid systems. 83rd Annual Meeting of The Society of Rheology 2011.

 

Figure 1a shows an initially low level of orientation within a film

Figure 1b the development of orientation with subsequent mechanical drawing.

Video 1. Mechanical drawing of a plastic film in the trasversal direction.

Figure 3. Spin-on film from the HSZ BEA nanoparticles a) Top view.

a)

Figure 3. Spin-on film from the HSZ BEA nanoparticles b) Cross-sectional view.

b)

Figure 2,Polymer Monolith

Tecnológico de Monterrey 

 

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