Nanopolymers

The word nano is derived from the Greek word for “dwarf”. It is the prefix for units of 10-9. In a nutshell, nanoscience is the study of the extremely tiny. Nanoscience is concerned with the study of the unique properties of matter at its nano level and exploits them to create novel structures, devices and systems for a variety of different uses. Particles having sizes less than 100nm are generally called nanoparticles. These have strikingly different properties due to their small size and thus are found useful in many applications. The ability to measure and manipulate matter on the nanometer level is making possible a new generation of materials with enhanced mechanical, optical, transport and magnetic properties.

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Introduction

Particles having sizes less than 100nm are generally called Nanoparticles. These have strikingly different properties due to their small sizes and thus are found useful in many applications. The ability to measure and manipulate matter on the nanometer level is making possible a new generation of materials with enhanced mechanical, optical^[1], transport and magnetic properties. They are generally crystalline in nature and thus they are called nanocrystals. They are made from microparticles by certain techniques.

Nanopolymers in simple words are nanostructured polymers. The nanostructure determines important modifications in the intrinsically properties. Multi scale nano structuring and the resulting materials properties across the hierarchy of length scales from atomic, to mesoscopic, to macroscopic is an absolute necessity. The term polymer covers a large, diverse group of molecules, including substances from proteins to high-strength kelvar fibres. A key feature that distinguishes polymers from other large molecules is the repetition of units of atoms in their chains. This occurs during polymerization, in which many monomers, polymer chains within a substance are often not of equal length. This is unlike other molecules in which every atom is accounted for, each molecule having a et of molecular mass. Differing chain lengths occur because polymer chains terminate during polymerization after random intervals of chain lengthening (propagation). The attractive forces between polymer chains play a great role in determining polymer properties. Because polymer chains are so long, these interchain forces are amplified far beyond the attractions between conventional molecules. Also, longer chains are more amorphous (randomly oriented). Polymers can be visualized as tangled spaghetti chains – pulling any one spaghetti strand out is a lot harder the more tangled the chains are. These strong forces typically result in high tensile strength and melting points.

Nanopolymers

Polymer nanocomposites (PNC) is a polymer or copolymer having dispersed in its nanoparticles. These may be of different shape (e.g., platelets, fibers, spheroids), but at least one dimension must be in the range of 1 to 50 nm. These PNC's belong to the category of multi-phase systems (MPS, viz. blends, composites, and foams) that consume nearly 95% of plastics production. These systems require controlled mixing/compounding, stabilization of the achieved dispersion, orientation of the dispersed phase, and the compounding strategies for all MPS, including PNC, are similar.

The transition from micro- to nano-particles lead to change in its physical as well as chemical properties. Two of the major factors in this are the increase in the ratio of the surface area to volume, and the size of the particle. The increase in surface are-to-volume ratio, which increases as the particles get smaller, leads to an increasing dominance of the behavior of atoms on the surface area of particle over that of those interior of the particle. This affects the properties of the particles when they are reacting with other particles. Because of the higher surface area of the nano-particles the interaction with the other particles within the mixture is more and this increases the strength, heat resistance etc and many factors do change for the mixture.

Example of nanopolymer is silicon nanospheres which show quite different characteristics like is size is 40 – 100 nm and it is much harder than silicon and the hardness of this nano-sphere lies between that of sapphire and diamond.

Properties OF Nanopolymer Composites

• The flow induced alignment of nanotubes in a polymer matrix can lead to preferential orientation of the tubes, into either ribbion fibres. Raman spectroscopy is able to determine the degree of shear orientation and the polarization direction of the nanotubes.
• Films with tunable colors can be produced depending on the nature and size of a nanorod. Metal nanorods ae espically interesting because they exhibit high electrical capacitance and the color of a colloid is affected by the effective charge of the particle.
• The nanofilms show considerable reinforcement when subjected to small deformations, where as at high elongations, the rheology approaches that of the pure nanolatex form.
• Solid state polymerization of molecularity oriented silica/monomer matrices lead to unique conductive composites. The self assembly of conductive polymer-silica hybrids as films or fibers is suitable for integration into different kinds of devices and Microsystems.
• Metal nanorods are especially interesting because they exhibit high electrical capacitance and the color of the colloid is affected by the effective charge of the particle.
• At the nanoscales, behavior of individual atoms and electrons becomes important and interesting quantum effects come into play. These fundamentally alter the optical, electrical and magnetic behavior of materials.
• Carbon nanotubes have incredible molecules have an array of fascinating electronic,magnetic and mechanical properties. They are at least 100 times stronger than steel, but only one-sixth as heavy – so nanotube fibers could strengthen just about any material.
• Also, nanotubes can conduct heat and electricity far better than copper, and are already being used in polymers to control or enhance conductivity, and in antistatic packaging.^[2]

Preparation of Nanopolymers

Nanopolymers are prepared in many ways. The preparation of nanofibers, core shell fibers, hollow fibers, and tubes from synthetic polymers with diameters down to a few a nanometers can be done in many ways like: I. VAPOR CONDENSATION II. VACUUM EVAPORATION ON RUNNING LIQUIDS METHOD (VERL) II. ELECTRO-SPINNING

Vapor condensation process

This process is used to make the metal oxide ceramic nanopolymers. It involves evaporation of solid metal followed by rapid condensation to form nano-sized clusters. Various approaches to vaporize the metal can be used and variation of the medium in which the vapor is released affects the size of the particles. Inert gases are used to avoid oxygen while creating nanopolymers, where as the reactive oxygen is used to produce metal oxide ceramic products.

Vacuum Evaporation on Running Liquids method (VERL)

The vacuum evaporation on running liquids method (VERL) is a variation of vapor condensation method. This method is used as a thin film of a relatively viscous material, oil, or a polymer, on a rotating drum. Vacuum is maintained in the apparatus and the desired metal is evaporated or spurted in vacuum. Particles from in the suspensions in the liquid and can be grown in the process. Nano-Polymers are formed by this method.

Electro-spinning method

This method is the most useful method in the manufacture of the polymers in the nano scale compared to the above two methods that are described. Electro-Spinning is a process that utilizes electrical force to produce polymer fibers from polymer solutions or melts. The obvious advantage of the electro-spinning technique is that, it produces ultra-fine fibers, with huge surface-to –volume ratio, which have great application potentials in many fields such as protective clothing, air filtration, sensors, drug delivery system, sensors, protective textile, and as fiber templates for preparation of nanotubes.

PRINCIPLE: In the electro-spinning process, the fibres are spun under a high voltage electrical field. A polymer solution is contained in a syringe, which is equipped with a piston and a stainless steel capillary serving as an electrode. A grounded counter electrode (round metal plate at the bottom) is placed down the capillary and a high voltage is applied between the capillary and the counter electrode.

Under controlled velocity the piston on the syringe was driven down by a motor and a droplet of polymer solution is suspended by its surface tension at the tip of the capillary. If the free surface of the solution is subjected to an electric field, charge or dipolar orientation will be induced at the air-solution interface. The charge repulsion causes a force that opposes the surface tension. If the voltage surpasses a threshold value, electrostatic forces overcome the surface tension, resulting in that jets are ejected from the solution and move towards the counter electrode. During the travel to the counter electrode, the solvent in the jets evaporates and the solidified fibers are deposited on a substrate located above the counter electrode. So far, fibers with diameters ranging from as low as 5nm to several microns have been produced. It is found that the morphology and dimension of the electrospun fibers are dependent on the process parameters, including solution concentration and viscosity, electrical conductivity of the solution, surface tension of the solution, polymeric molecular weight, molecular weight distribution of polymers, vapor pressure and boiling point of the solvent, flow rate, intensity of electrical field, distance between the capillary and the substrate, temperature, humidity and atmosphere etc.

Processing parameters

In electro-spinning process, three main forces are involved:

1. Surface tension: Favors to produce as few as possible polymer jets in order to decrease surface are of the polymer droplets
2. Electrical repellent force derived from electrical charged polymer droplets: favors to form as many polymer jets as possible.
3. Visco-Elastic force coming from polymer: against the deformation of polymer droplets.

Bio-hybrid polymer nanofibers

Many technical applications of biological objects like proteins, viruses or bacteria such as chromatography, optical information technology, sensorics, catalysis and drug delivery require their immobilization. Carbon nanotubes, gold particles and synthetic polymers are used for this purpose. This immobilization has been achieved predominantly by adsorption or by chemical binding and to a lesser extent by incorporating these objects as guests in host matrices. In the guest host systems, an ideal method for the immobilization of biological objects and their integration into hierarchical architectures should be structured on a nanoscale to facilitate the interactions of biological nano-objects with their environment. Due to the large number of natural or synthetic polymers available and the advanced techniques developed to process such systems to nanofibres, rods, tubes etc make polymers a good platform for the immobilization of biological objects.^[3]

Bio-hybrid nanofibres by electrospinning

Polymer fibers are, in general, produced on a technical scale by extrusion, i.e., a polymer melt or a polymer solution is pumped through cylindrical dies and spun/drawn by a take-up device. The resulting fibers have diameters typically on the 10-µm scale or above. To come down in diameter into the range of several hundreds of nanometers or even down to a few nanometers, electrospinning is today still the leading polymer processing technique available. A strong electric field of the order of 103 V/cm is applied to the polymer solution droplets emerging from a cylindrical die. The electric charges, which are accumulated on the surface of the droplet, cause droplet deformation along the field direction, even though the surface tension counteracts droplet evolution. In supercritical electric fields, the field strength overbears the surface tension and a fluid jet emanates from the droplet tip. The jet is accelerated towards the counter electrode. During this transport phase, the jet is subjected to strong electrically driven circular bending motions that cause a strong elongation and thinning of the jet, a solvent evaporation until, finally, the solid nanofibre is deposited on the counter electrode.

Bio-hybrid polymer nanotubes by wetting

Electro spinning, co-electrospinning, and the template methods based on nanofibres yield nano-objects which are, in principle, infinitively long. For a broad range of applications including catalysis, tissue engineering, and surface modification of implants this infinite length is an advantage. But in some applications like inhalation therapy or systemic drug delivery, a well-defined length is required. The template method to be described in the following has the advantage such that it allows the preparation of nanotubes and nanorods with very high precision. The method is based on the use of well defined porous templates, such as porous aluminum or silicon. The basic concept of this method is to exploit wetting processes. A polymer melt or solution is brought into contact with the pores located in materials characterized by high energy surfaces such as aluminum or silicon. Wetting sets in and covers the walls of the pores with a thin film with a thickness of the order of a few tens of nanometers. Gravity does not play a role, as it is obvious from the fact that wetting takes place independent of the orientation of the pores relative to the direction of gravity. The exact process is still not understood theoretically in detail but its known from experiments that low molar mass systems tend to fill the pores completely, whereas polymers of sufficient chain length just cover the walls. This process happens typically within a minute for temperatures about 50 K above the melting temperature or glass transition temperature, even for highly viscous polymers, such as, for instance, polytetrafluoroethylene, and this holds even for pores with an aspect ratio as large as 10,000. The complete filling, on the other hand, takes days. To obtain nanotubes, the polymer/template system is cooled down to room temperature or the solvent is evaporated, yielding pores covered with solid layers. The resulting tubes can be removed by mechanical forces for tubes up to 10 µm in length, i.e., by just drawing them out from the pores or by selectively dissolving the template. The diameter of the nanotubes, the distribution of the diameter, the homogeneity along the tubes, and the lengths can be controlled.

Applications

The nanofibres, hollow nanofibres, core-shell nanofibres, and nanorods or nanotubes produced have a great potential for a broad range of applications including homogeneous and heterogeneous catalysis, sensorics, filter applications, and optoelectronics. Here we will just consider a limited set of applications related to life science.

Tissue engineering

This is mainly concerned with the replacement of tissues which have been destroyed by sickness or accidents or other artificial means. The examples are skin, bone, cartilage, blood vessels and may be even organs. This technique involves providing a scaffold on which calls are added and the scaffold should provide favorable conditions for the growth of the same. Nanofibres have been found to provide very good conditions for the growth of such cells, one of the reasons being that fibrillar structures can be found on many tissues which allow the cells to attach strongly to the fibers and grow along them as shown.

Delivery from compartmented nanotubes

Nano tubes are also used for carrying drugs in general therapy and in tumor therapy in particular. The role of them is to protect the drugs from destruction in blood stream, to control the delivery with a well-defined release kinetics, and in ideal cases, o provide vector-targeting properties or release mechanism by external or internal stimuli.

Rod or tube-like, rather than nearly spherical, nanocarriers may offer additional advantages in terms of drug delivery systems. Such drug carrier particles possess additional choice of the axial ratio, the curvature, and the “all-sweeping” hydrodynamic-related rotation, and they can be modified chemically at the inner surface, the outer surface, and at the end planes in a very selective way. Nanotubes prepared with a responsive polymer attached to the tube opening allow the control of access to and release from the tube. Furthermore, nanotubes can also be prepared showing a gradient in its chemical composition along the length of the tube.

Compartmented drug release systems were prepared based on nanotubes or nanofibres. Nanotubes and nanofibres, for instance, which contained fluorescent albumin with dog-fluorescein isothiocyanate were prepared as a model drug, as well as super paramagnetic nanoparticles composed of iron oxide or nickel ferrite. The presence of the magnetic nanoparticles allowed, first of all, the guiding of the nanotubes to specific locations in the body by external magnetic fields. Super paramagnetic particles are known to display strong interactions with external magnetic fields leading to large saturation magnetizations. In addition, by using periodically varying magnetic fields, the nanoparticles were heated up to provide, thus, a trigger for drug release. The presence of the model drug was established by fluorescence spectroscopy and the same holds for the analysis of the model drug released from the nanotubes.

Immobilization of Proteins

Core shell fibers of nano particles with fluid cores and solid shells can be used to entrap biological objects such as proteins, viruses or bacteria in conditions which do not affect their functions. This effect can be used among others for biosensor applications. For example Green Fluorescent Protein is immobilized in nanostructured fibres providing large surface areas and short distances for the analyte to approach the sensor protein.

With respect to using such fibers for sensor applications, the fluorescence of the core shell fibers was found to decay rapidly as the fibers were immersed into a solution containing urea: urea permeates through the wall into the core where it causes denaturation of the GFP. This simple experiment reveals that core-shell fibers are promising objects for preparing biosensors based on biological objects.

Polymer nanostructured fibers, core-shell fibers, hollow fibers, and nanorods and nanotubes provide a platform for a broad range of applications both in material science as well as in life science. Biological objects of different complexity and synthetic objects carrying specific functions can be incorporated into such nanostructured polymer systems while keeping their specific functions vital. Biosensors, tissue engineering, drug delivery, or enzymatic catalysis is just a few of the possible examples. The incorporation of viruses and bacteria all the way up to microorganism should not really pose a problem and the applications coming from such biohybrid systems should be tremendous.^[4]

Size and pressure effects on nanopolymers

The size- and pressure- dependent glass transition temperatures of free-standing films or supported films having weak interactions with substrates decreases with decreasing of pressure and size. However, the glass transition temperature of supported films having strong interaction with substrates increases of pressure and the decrease of size. Different models like two layer model, three layer model, Tg (D, 0) α 1/D and some more models relating specific heat, density and thermal expansion are used to obtain the experimental results on nanopolymers and even some observations like freezing of films due to memory effects in the visco-elastic eigenmodels of the films, and finite effects of the small molecule glass are observed. To describe Tg (D, 0) function of polymers more generally, a simple and unified model recently is provided based on the size-dependent melting temperature of crystals and lindermann criterion

Tg (D, 0)/Tg (infinite, 0) is proportional to σg2(infinite, 0)/σg2(D, 0)

where σg is root of mean-square displacement of surface and interior molecules of glasses at Tg (D, 0), α = σs2 (D, 0) / σv2 (D, 0) with subscripts s and v denoting surface and volume, respectively. For a nanoparticle, D has a usual meaning of diameter, for a nanowire, D is taken as its diameter, and for a thin film, D denotes its thickness. D0 denotes a critical diameter at which all molecules of a low dimensional glass are located on its surface.^[5]

Conclusion

The devices that utilize the properties of low dimensional objects such as nanoparticles are promising due to the possibility of tailoring a number of electrophysical, optical and magnetic properties changing the size of nanoparticles, which can be controlled during the synthesis. In the case of polymer nanocomposites we can utilize the properties of disordered systems. Here recent developments in the field of polymer nano-composites and some of their applications have been reviewed. Though there is much utilization in this field, there are many limitations also. For example in the release of drugs using nanofibres, cannot be controlled independently and a burst release is usually the case, where as a more linear release is required. Let us now consider future aspects in this field.

There is a possibility of building ordered arrays of nanoparticles in the polymer matrix. A number of possibilities also exist to manufacture the nanocomposite circuit boards. An even more attractive method exists to utilize polymer nanocomposites for neural networks applications. Another promising area of development is optoelectronics and optical computing. The single domain nature and super paramagnetic behavior of nanoparticles containing ferromagnetic metals could be possibly utilized for magneto-optical storage media manufacturing.

References

See also

• Biopolymer
• Copolymer
• Electroactive polymers
• Nanocarbon tubes