Effects Of Confinement And Surface Roughness On Viscosity Of Polymer

The "effects of confinement and surface roughness on viscosity of polymer".

Introduction to Nanotechnology

Nanotechnology and nanoscience deal with the study and application of small (nano) materials across the field of science like chemistry, biology, physics, engineering and material science (Berne, 2005) It can make creation of new materials easy. These can be used in the medical, biomaterial, electronics and energy production field. Nanotechnology developed as an idea from Richard Feynmen, who was a physicist (Sanchez and Sobolev, 2010). He described how scientists could manipulate molecules and atoms.  Some of the fields in which nanotechnology can be practiced are highlighted below.

It encompasses medical applications of Nano electronic biosensors, nanomaterials and nanotechnology in the molecular field for imaging and therapy (Nie, 2010). Some of these include diagnosing cancer, targeting medication, improved cancer imaging, tumor homing and killing (Toy et al. 2014).

Nanofabrication involves nanoscale creation of devices. This has created an avenue for developing newer ways of capturing, storing and transferring energy. An example is a memory card and a nano-sized sim card. In the 1980s, a 2 gigabyte drive was heavy, bulky and expensive to buy and install (Brodie and Muray 2013).With the advent of nanotechnology, this has transformed to a smaller, cheaper and portable storage device (Biswas et al. 2012).

Nanomaterials have led to development of flexible electronics. One can manipulate the nanostructure the way they need to create the materials, whose properties can be made flexible (Biswas et al. 2012).

An example of its use in industries is the use in self-cleaning surface of glasses and ceramics. Therefore, they have led to an improved smoothness, heat and water resistance of household equipment (Biswas et al., 2012). Textile industries use engineered nanofibers to make clothes and foot ware. This property makes it easy to manage the textile materials since they are less frequently washed and at lower temperatures. Full surface protection from electrostatic charges for the cloth user has been made possible by integrating carbon particles membrane on the textiles. This is because of nanotechnology.

Military personnel can use nanotechnology to create sensors that can be used to detect biological agents. This happens when the particles are injected on the uniforms soldiers wear. In addition to offering protection from chemicals, high impacts and high temperatures, the material makes the soldier uniform more durable (Ramsden, 2012).

Although nanotechnology has many advantages, it needs high funding therefore long term investment. It also needs trained personnel for the production and maintenance. Nanotechnology is new in the field of technology, and is revolutionary (Pacheco-Torgal and Jalali, 2011).

Before looking at the effects of confinement and surface roughness on the viscosity of a polymer, it is important to discuss and understand the meanings of confinement, surface roughness and viscosity in relation to a polymer.

A polymer is a molecule, made from repeated subunits of other molecules, which are connected to each other by chemical bonds. Polyethene is an example. It has several properties. One of them is viscosity. Polymers depend on viscosity to exert their actions. This dependence come from transition of polymeric materials to the non-dilute form. Polymers effects are large on fluid properties like surface tension, and Non-Newtonian properties when the solvent concentration is reduced. A study was conducted to compare viscosity of two different polymers (Poly lactic acid and poly-lactidide co-glycolide). The study compared the polymer type, among other properties like concentration and atomization process on particle morphology (Xie, 2011).

Nanomedicine

 Study by Lengsfiel et al also demonstrated that gas phase nucleation caused micro particles to form. This is in contrast to nucleation when discrete liquid droplets are provided.  Perez de Diego et al also concluded that it was possible to produce polymer particles when working conditions are below the mixture critical pressure (MCP). Polymer viscosity can be calculated using manual and automatic polymer viscosity measurement systems and soft wares (Alig et al. 2012).  A ViscoSystem AVS 470 can be used to take precise and reproducible values. Others include AVS 370 system, DILUT 4 viscosity software, Winvisco software, a fully automated visometer, transparent thermostatic bath, stainless steel thermostatic bath and a manual viscometer (VISCOCLOCK).

Surface roughness of a polymer affects its properties and activities. This helps in formulating, manufacturing and applying coatings. A study was conducted to show the effect of surface roughness of a sample and its mechanical properties. This was further investigated using a commercial ultra-micro identification system (Kraft et al., 2012). The results showed that when large stresses are applied on the polymer, determining the elastic modulus of the sample could be accurately determined. The study also revealed that surface roughness is determined by looking at differences in the modulus values with penetration depth as a matter of interest. The surface roughness determined was compared and contrasted to those got by direct imaging got through tapping mode atomic force microscopy (AFM). It was also concluded that when the AFM scan sizes were close to the estimated contact surface area between the indenter and material, a reasonable relationship between surface roughness’s was identified using the microidentation method (Kalpakjian, Vijai and Schmid, 2014).

Different methods can be used to manipulate surface roughness of molecules. An example is the chemical method by grafting copolymers. This involves adding polymer chains onto the surface. In effect, the surface solution is adsorbed. In advanced methods, the polymer chain is started and integrated at the surface. Adsorption density is self-limiting because the polymerized chains used in grafting have an equilibrium dynamic volume. Grafting gives a polymer the ability to stick to the surface of a droplet or beard in the solution (Noshay and McGrath, 2013). This can be explained by the wetting of surface bead by the polymer making it become more flexible. The monomer has to be in solution and lypophilic for this property to be achieved. The polymer has to have favorable interactions with the solution, to allow it form in a linear pattern. Grafting achieves a higher density because of numerous access to chain ends.

An example of grafting method is peptide synthesis. Amino acid chains are elongated by condensation reaction series from a polymer bead surface. The peptide composition as a bonded chain confers an excellent manner in which peptide composition can be controlled because the bonded chain can be washed without adsorbing the polymer (Thakur, Thakur and Gupta, 2013). Another area of application is in polymeric coating of paints. A water-borne paint and latex particles are usually surface modified to make particle dispersion controllable, therefore giving the polymer a characteristic viscosity among other properties like environmental stability against temperature and UV radiation.

Nanofabrication

Oxidation mechanisms include flame treatment, corona treatment and plasma processing. There are oxidative chemical reactions involved. Some of them are cleaving the polymer chains and introducing carbonyl and hydroxyl functional groups. But when oxygen is incorporated into the surface, a higher surface energy for substrate coating is achieved (Celina, 2013).

In corona treatment, surface modification involves use of low temperature corona discharge to increase surface viscosity and energy of a material like natural fibers and polymers. A thin sheet of polymer is rolled in a series of high voltage electrodes. Plasma created is used to functionalize the surface (Celina, 2013).  This technique eliminates penetration depth of the polymer and preserving its viscous ability for improved adhesion. Bulk mechanical properties are also improved. In the commercial world, corona treatment has been used in dye plants to improve its adhesion before printing texts and images on plastic materials used for packaging. The process has to be well ventilated and filtration has to take place to minimize the hazardous nature of ozone remnants after corona treatment. For an efficacious flame treatment to take place, and for the final material to achieve its viscosity property, air to gas ration, surface distance, thermal output and oxidation dwell time have to be put into consideration.

Plasma processing gives larger and better injected monomer fragments than comparable surfaces. However, plasma has been shown to be thermodynamically unfavorable therefore not widely used to make surface polymers viscous. Therefore it has not been applied as oxidation in industries. The process involves ionization by depositing monomer mixture or gaseous carrier ions to produce plasma. A flamed plasma processing method was developed to overcome the slow and financial constraints of the above method. It is fast, controlled, cost-effective and is better in making surface polymer viscous.  Here, jet flames are used to ionize gaseous oxygen (Sánchez et al. 2010).

Photo grafting modifies inert polyester, polyamide and polyolefin surfaces by using vinyl monomers to increase polymer adhesion and hydrophobicity. In a large scale industry, this technique is called photo lamination. Desired surface molecules are brought together by adhesion between the two polymeric films (Liu et al, .2011).

The above described techniques need surface analytical methods. For example surface energy measurement is used as an analyzing technique in industrial corona and plasma processes. This method confirms adequate surface functionality of a surface. Infrared spectroscopy is another method used for oxidizing treatments. In this case, spectra are taken from the treated surfaces when carbonyl and hydroxyl regions are present according to a table. X-ray photoelectron (XPS) and energy dispersive X-ray spectroscopy (EDS) are used in both oxidative, corona and plasma treatment plants (Liu et al. 2011). It provides accurate surface depths and also characterize microscopic variation in surface composition. Atomic force microscopy (AFM) maps three dimensional figures and shows topical variations in atomic surfaces (Ogilvie et al. 2010). This method is favorable because lack in crystallinity leads to variations in surface topography. Grafting, corona treatment and plasma processing therefore benefit more from this method.

Development of Flexible Electronics

When macromolecules are confined at a nanometer scale, the molecules exhibit dynamic properties. This has led to the use of this property in making polymers in the recent years. Investigations have been launched to confined polymers and have focused on thin films. Of particular note is the liquid glass transition temperature (Tg) with the film thickness and its dependence. It has also been shown that polymer mobility is increased when Tg is reduced upon confinement. This is because when confinement is reduced, and makes the size comparable to that of the polymer, the mobility is increased (Roth et al. 2018). However, Tg is an indirect method of measuring polymer mobility and technique dependent therefore a controversy in its use.

 A direct method of measuring this is by use of inelastic incoherent scattering (INS). This method has been explored to confirm polymer mobility. It is non-invasive method of probing local rotations of molecule side groups and proton delocalization (Chen et al, 2015). The consequences of nano polymer confinement has been elucidated using nano additives in polymer matrices and not in thin polymer films. Studies have concluded that when well dispersed nano particles are added to polymers, they closely resemble the effects of thin film confinement when measured by indirect methods like glass transition temperature (Ponting, Hiltner and Baer 2010). This can be done in a qualitative way involving reducing Tg while increasing Nano filler content or by reducing the supported thickness of the film. It has also been shown that when molecular additives are present in a polymer matrix, the effects of spatial nano confinement can be suppressed. Therefore, nanoparticle confinement is an opportunity to study polymer dynamics in a confined manner thus allowing direct exploration of dynamics of macromolecules locally.

Another study was conducted by Connie on effects of nanoscale confinement of polymer in films and nanocomposites. It involved tuning of glass transition by 100 K and the keying in diffusion coefficients by an order of magnitude. Results showed that there existed a difference between a monolayer, bilayer and trilayer polymer films. Bilayer and trilayer polymer films showed fluorescence methods sensitive and specific to glass Tg. Part of the report showed that Tg of a 14 nm thick polystyrene layers could be coupled to much of underlayer films of different polymers therefore allowing the Tg to be varied by 100 K (from 318 K to 418 K). The study further showed that an order of magnitude with confinement can influence the reduction in translational diffusion coefficient of small dye molecules within PS (Frielinghaus et al., 2013).

A journal on polymers in 2 D confinement sheds more light on the confinement of a polymer. The journal talks of a study that was conducted to observe 2 D confined polymer chains using small angle neutron scattering. The signals were recorded and interpreted after signals of unwanted background was removed. Also because of the thickness of the polymers, self-avoiding trails concept was applied. The study concluded that polymer scattering from unwanted backgrounds could be separated. Polymer conformation is usually based on random walk (Krutyeya et al, 2013). The concept is applied in Brownian motion, heat conductivity and in quantum physics. Polymers depend on viscosity to exert their actions. This dependence come from transition of polymeric materials to the non-dilute form. Polymers effects are large on fluid properties like surface tension, and Non-Newtonian properties when the solvent concentration is reduced. A study was conducted to compare viscosity of two different polymers (Poly lactic acid and poly-lactidide co-glycolide). The study compared the polymer type, among other properties like concentration and atomization process on particle morphology (Ediger and Forrest, 2013).

Industrial Application

It is evident that many studies have been done to show polymer interactions with planar interfaces. Idealized geometries of spheres and cylinders have also been applied in the studies. Group calculation and simulation studies have been conducted. The use of classical random walk model for the geometries have been applied too. The challenge comes when observing naturally occurring substances like cell membranes (Ogilivie et al, 2010). They are regular and not amenable for modelling by smooth surface models, which are conventional tools.

Theoretical and practical questions arise when talking about irregular adsorbing surfaces. Here, the surface roughness if the most important factor and therefore the questions focus on how this property can be characterized. Fractal dimension remains a model of characterizing real surface roughness using a zeroth order measure of roughness. Models of Gaussian chains, with interactions between fractal surfaces have been explored (Song et al, 2011).  A diblock copolymer is a perfect example of a block with fractal surface. An epsilon expansion together with renormalization group method forms an analytical calculation for the system.

A study was conducted to show the effect of surface roughness of a sample and its mechanical properties. This was further investigated using a commercial ultra-micro identification system. The results showed that when large stresses are applied on the polymer, determining the elastic modulus of the sample could be accurately determined. The study also revealed that surface roughness is determined by looking at differences in the modulus values with penetration depth as a matter of interest (Jiang et al, 2011).

It is difficult to incorporate boundary conditions into the diffusion equation. Therefore making it difficult to do a rigorous polymer treatment when it interacts with a fractal surface. A tractable model, which is simpler, has been introduced to solve the problem between polymers interacting with fractal surfaces. An Effective surface model (ESM) discusses the physics between a polymer and its interaction with fractal surface. Undertaking of the ESM needs to be done after a comparison with Monte Carlo data (Jiang et al, 2011).

Calculations have been developed to look into the problem of a surface interacting polymer. Calculations by Kosmas uses renormalization group perturbation theory, whereas those of Douglas et al involve Gaussian Chain model with renormalization group theory. A polymer is made by a continuous Gaussian chain that is specified by a position vector R(x) along the contour distance x on a chain of unit length N. Many complex calculations are involved. ES model suggests that when the fractal dimension of a surface is increased, phase transition of surface adsorption is sharpened (Wu et al, 2011).  Comparisons between the ES model and Monte Carlo data for random walks interacting with fractal surfaces led to encouraging results. The general test model for surface analogy is using block copolymer. The ES model therefore provides many quantitative predictions for polymer properties at a diffuse interface.

Other studies have been conducted on influence of surface roughness on polymer drag reduction, which is closely related to polymer viscosity. Polymer drag reduction occurs when solutions with long chain molecules are injected into turbulent boundary layers (TBL). When the polymer molecules interact with underlying turbulent flow, correlated velocity fluctuation is reduced therefore reduction in turbulent transport of momentum along the TBL. In effect, local drag reduction is achieved, up to 70%. This happens for TBL flows on smooth surfaces compared to those without polymer injection (Song et al, 2011). 

Military

Air layer reduction model has been applied to look at the effect of surface roughness on polymer viscosity. In this case, air is injected into TBL so as to create a stable gas layer with high void friction (Song et al, 2011). The liquid will be separated from solid surface, resulting into about 80% reduction in friction drag when compared with layers without air.

Drag reduction is important when applied to marine transport system. It reduces fuel costs, increasing performance, efficiency, speed and payload. A ship can counter drag from three sources. Wave, form and viscous drag. Viscous drag can be reduced by passive or active methods. Passive methods include modifying ship surface to reduce velocity gradient. However, the methods are limited since it is not practical to in real seas (Ogilivie et al, 2010).  Active methods involve injecting a substance that modifies flow conditions in near wall region from the ship. These substances include air, surfactant and polymer solutions. Active methods are able to overcome problems associated with passive methods but are limited by power needs to deliver drag reducing agent to space and flow needs of the ship. Many options are available for passive and active methods by a research group. The journal focuses on using polymer injection to reduce active drag.

Lab scale experiments have demonstrated polymer drag reactions (PDR) methods to be effective, with up to 75% reduction in skin friction. The basic principle behind it is the Newtonian flow, which argues that flow can reduce skin friction drag. Most PDR studies focused on internal flows and has produced a better understanding of factors influencing PDR. Some of them include pipe diameter, polymer molecular weight and concentration and random coil size. However, this idea has failed to sell for use in large scale since it lacks an economic solution. Studies have been conducted to look for an economic benefit for applying this scheme on large scale (Ogilivie et al, 2010). The potential between TBL and injection scheme that would make PDR economical on large scale has been probed. Many studies conducted on this subject have used PDL over smooth surfaces. The presence of roughness has been shown to increase mixing rate. Roughness has been reported to hinder polymer performance in some cases and to improve it in others. This was an exceptional study conducted. The study made a way for investigating the conditions under which surface roughness would reduce polymer breakdown.

Studies have concluded that roughness increases mixing of polymer solutions and enhanced polymer breakdown. For a very near injector, increased mixing can lead to short region of increases FDR when compared to the smooth condition. Effect of air later drag reduction over rough surface was also looked into (Song et al, 2011). It found that when enough air is injected into the TBL, the gas would condense into a monolayer under buoyancy influence. At lower speeds friction drag was reduce by 80%. At higher speeds, ALDR was possible when combined with increased gas flux.

Different studies have been conducted to show the effect of confinement on polymer viscosity. One such study uses the nano confinement during capillary rise infiltration while looking at untangled polymers. In a capillary rise infiltration (CaRI), there is thermal induction of the polymer to induce dense packing of nano particles (Pontin, Hiltner and Baer, 2010). Polymer infiltrated nano particles is formed after this, and they are usually highly concentrated. To understand this, the research team used two different polymers with different interactions with silicon oxide nano particles. Polymer infiltration process was monitored using insitu spectrometric ellipsometry. Monitoring is based on Lucas-Washburn model (Pontin, Hiltner and Baer, 2010). The results suggested that when particles are physically confined, viscosity was increased by two orders of magnitude. Confinement was also shown to increase glass transition temperatures for the polymers. Conclusion was that extreme nano confinement had more impact of surface interactions and on the viscosity than for the unconfined particles. The experiment is important in guiding making of CaRI nano composites with vast range of nano particles and polymers (Pontin, Hiltner and Baer, 2010).

A Polymer

A journal on polymers in 2 D confinement sheds more light on the confinement of a polymer. The journal talks of a study that was conducted to observe 2 D confined polymer chains using small angle neutron scattering. The signals were recorded and interpreted after signals of unwanted background was removed. Also because of the thickness of the polymers, self-avoiding trails concept was applied (Suzuki et al, 2013).  The study concluded that polymer scattering from unwanted backgrounds could be separated. Polymer conformation is usually based on random walk. The concept is applied in Brownian motion, heat conductivity and in quantum physics.

Another study looked at interferometric method to find out the dynamics of polymeric structures under strong confinement. It is a new method of studying polymer under a strong geometrical guided confinement in nano porous films. The technique involves sensing of changes of optical qualities of porous matrix as a result of polymer imbibition into pores using optical interferometry (Suzuki et al, 2013). A model is utilized in correlating time-dependent optical thickness of film while looking at the function on temperature on polymer dynamics. It was found that the confinement degree varied in approximately 2 orders of magnitude when matrices of different pore radii were used. Reduced viscosity was also observed when pore radius was reduced (Krutyeva et al 2013). The method of study enabled measuring along a wide temperature range in a single experiment. The consequences of nano polymer confinement has been elucidated using nano additives in polymer matrices and not in thin polymer films (Cuenca and Bodiguel. 2013). Studies have concluded that when well dispersed nano particles are added to polymers, they closely resemble the effects of thin film confinement when measured by indirect methods like glass transition temperature.

A different study focused on polymer glass Tg under extreme nano confinement in weakly interacting nanoparticle films. When a polymer is inside a polymer nanocomposite, their properties have been shown to deviate from their bulk properties. CaRI enables polymer infiltration into nanoparticle packing. Molecular weight ant diameter of particles influence the degree of confinement of particles (Lin et al, 2013).

Conclusion

Nanotechnology and nanoscience deal with the study and application of small (nano) materials across the field of science like chemistry, biology, physics, engineering and material science (Jancar et al, 2010).  It encompasses medical applications of Nano electronic biosensors, nanomaterials and nanotechnology in the molecular field for imaging and therapy (Bose, Khare and Moldenaers, 2010).  Nanofabrication involves nanoscale creation of devices. This has created an avenue for developing newer ways of capturing, storing and transferring energy (Luo, Nangrejo and Edirisinghe, 2010). Surface roughness of a polymer affects its properties and activities. This helps in formulating, manufacturing and applying coatings (He et al, 2012).

When macromolecules are confined at a nanometer scale, the molecules exhibit dynamic properties. This has led to the use of this property in making polymers in the recent years. It is evident that many studies have been done to show polymer interactions with planar interfaces. Idealized geometries of spheres and cylinders have also been applied in the studies. Studies have been conducted on influence of surface roughness on polymer drag reduction, which is closely related to polymer viscosity.

Surface Roughness of a Polymer

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