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Smart Polymers respond sharply when there are small changes in physical or chemical conditions. These small changes can induce large phase or property changes. Various stimuli can induce these dramatic behavior. These various stimuli are Temperature, Ionic Strength, Electric Field, Solvents, pH, Magnetic Field, Enzyme Substrates, Affinity Ligands, Radiation (UV, visible) and many more. These polymers have been used in the extended drug delivery systems, bio-separation, and tissue engineering, to create and maintain humid atmosphere and actuators and sensors.
Thermo- Responsive Polymeric Materials (Used in Drug Delivery)
These polymers are sensitive to changes in temperature. These polymers are most widely used and used in drug administration systems and biomaterials. There is a very sensitive balance between hydrophobic and hydrophilic groups in their structure which is liable to change with small changes of temperatures. Lower Critical Solution Temperature (LCST) is an important feature of this kind of polymers. The LCST is defined as the critical temperature at which polymer solution shows phase separation i.e it goes from one phase to another. LCST phase transition occurs at a nanometer scale, particle or aggregate dimensions change at this scale.
Example of this kind of polymers are poly(N-substituted acrylamide) polymers' family, in which most important are poly(N-isopoprylacrilamide) (PNIPAAm), poly (N,N'-diethyl acrylamide) (PDEAAm).
Fig. (a) PNIPAAm (b) PDEAAm
Other examples include poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) triple blocks of copolymers (PEO-PPO-PEO), poly (ethylene glycol)-poly (lactic acid)-poly (ethylene glycol), triple blocks of (PEG-PLA-PEG).
It is most commonly used and studied thermo sensitive polymer which can be synthesized from NIPAM. Its main feature is that it is soluble in water at room temperature. When heated above LCST (~ 32 °C), it undergoes a phase transition from a swollen hydrated state to dehydrated state (shrunken). Hydrogen bonding between polymer amide groups and water molecules gets broken above 32 °C. The LCST can increased by the incorporation of hydrophilic monomers like (acrylic acid or hydroxyethyl methacrylate) which helps in the formation of hydrogen bonds with thermosensitive monomers The co-polymers NIPAAm with hydrophilic monomers ( acrylic acid) increases LCST to around 37°C (body temperature).
Poloxamers and Derivatives
Poloxamers are nonionic triblock polymers of polyoxyethylene-polyoxypropylene polyoxyethylene (PEOn-PPOn -PEOn).These polymers are soluble in water, odorless and tasteless. These are also known as Pluronics and Tetronics. These can be used as pharmaceutical ingredients, drug carriers, food additives and tissue engineering. Pluronic (F127) and Poloxamer 407 are common examples. The aqueous solution of PF127 and P407 can turn into gels at certain temperature which can be reversibly turned into liquid at lower temperatures. This property and low toxicity makes it common polymer in drug delivery systems. Water solubility of oily substances and hydrophobics can also be increased, miscibility of two substances having different hydrophobicities can also be increased.
pH Sensitive Response Polymers
Remarkable changes in the pH can be seen in the human body, this fact can be used for directing therapeutic agents to specific tissue, body part or cell. Due to these conditions pH sensitive polymers are best suited for the delivery of therapeutic agents. The important feature of these polymers is that they can accept or donate protons when pH is changed. When the polymer is ionized in the solvent there will be a repulsive force between the charges on the polymeric chain. If in the presence of solvent polymer is not ionized then the polymeric chains will be in compact form. If these polymeric chains are hydrophobic and unionized in the presence of solvent then these chains will collapse into globules and precipitates out. Degree of ionization is controlled by pH and type of solvent used, therefore there is a change in conformation due to sudden changes of pH.
Polymers with functional Acid Groups
These polymers are polyanions which have a high number of acid groups (ionizable) like COOH or sulphonic group in the structure. Examples include polyacrylic acid (PAA) and its derivatives, polymethacrylic acid (PMAA), ploy (ethylene amine) and poly (L- lysine). These polymers have been used in drug delivery. The carboxylic groups accept protons at pH value less than pKa and donate protons at pH value greater than pKa. Therefore, polymer swells at high pH due to electrostatic repulsion of negatively charged ions.
Polymers with functional basic groups
These polymers are polycations. Examples are poly(4-vinylpyridine), poly(2-vinylpyridine) (PVP) and poly(vinylamine) (PVAm). Theses polymers accept protons at pH value greater than pKa and donate protons at pH value less than pKa.
Polymers like albumine, gelatin and chitosan are some natural polymers which are pH responsive. Chitosan is soluble in water at pH 6.2 which becomes a hydrated gel above this pH. This is used in mucosal and oral administration. It is also a good option for pharmaceutical implants because of its porosity and controlled delivery of drugs.
UV- and visible light-sensitive materials (used in actuators)
These materials include hydrogels sensitive to UV or visible light. When these materials are exposed to the UV or visible light, these gels undergo reversible photomechanical change.
UV sensitive hydrogels will swell when they are exposed to UV radiation and contract when the radiation is removed. These gels are generally triphenyl-methane units or leuco derivatives. This volume transition is discontinuous in nature.
Visible light sensitive hydrogels shrinks in presence of visible light and contracts when source of light is removed. These gels can be prepared using Cu chlorophyll bound to NIPAM. These are used in photoresponsive switches, artificial muscles, and memory devices.
Cross linked PMMA and PMA dyes can also be used as photo-actuators.
Magneto responsive materials are polymer-based elastomeric materials and which undergo deformation and experience mechanical stress when magnetic field is applied. These materials are also known as magnetorheological polymers or magnetoelasts. Magnetic polymer gels are flexible cross-linked polymers which contains magnetizable particles. These materials are used in the application of diagnostic medicine, high density memory devices and spintronics.
Common materials used are poly(vinyl alcohol) (PVA) filled with magnetite particles, magnetic polystyrene, magnetic poly(dimethylsiloxane) composites (mPDMS).
Electro-active Polymers (EAP)
These polymers undergoes a change in shape or size in the presence of electric field. Electroactive polymers are used in actuators and sensors. EAP are of two types: Dielectric and Ionic.
Dielectric EAP are materials in which actuation is caused by electrostatic forces between two electrodes. Ionic EAP in which materials are actuated due to the movement of ions inside the polymer. Examples of EAP materials are ferroelectric polymers eg. Nylon 11 and Polyvinylidene fluoride, dielectric EAP, and electro- viscoelastic elastomers. Ionic EAP include conductive polymers, ionic polymers gels, carbon nanotubes and ionomeric polymer metal composites.
Polymers are made electrically active by addition of conductive fillers. Metals, carbon fibers and carbon black are the conductive fillers. Chemical modification is done to activate ionic polymeric gels. Commonly used EAPs are poly(pyridine), poly(sulfone), poly(thipene), poly(acetylene), poly(p-phenylene), Poly (methyl methacrylate),Dendritic Poly(styrene sulphonate) and poly(p-phenylene vinylene).
Methods of Preparation and Characterization
Thermo- Responsive Materials
Method of Preparation:
Thermoresponsive polymers are grafted on different substrates such as glass, silica, quartz, and polyethylene-terephthalate sheets. Grafting of these polymers on Tissue Culture grade Poly Styrene (TCPS) is common. Many types of techniques such as Plasma polymerization, Gamma radiation, Electron beam irradiation, UV irradiation and Atom transfer radical polymerization are used for grafting these materials.
Electron beam induced polymerization
In this method NIPAAm monomer is bonded on the TCPS by the irradiation of electron beam to the monomer. The nature of the bond is covalent. This method is used for thin grafting and large scale production. This method is expensive. Monomer concentration and radiation energy are used to control the thickness.
In this method Gamma irradiation is used in place of electron beam induced polymerization. This method is used for the batch processing and grafting of PNIPAAm.
In this method thermoresponsive coatings are prepared on a solid substrate. A plasma glow of NIPAAm vapor is used to deposit on to solid substrates like glass, silica or TCPS. This method is not used for the large scale production due to problems caused by continuous treatment and size.
Atom transfer radical polymerization technique (ATRP)
In this method surfaces are prepared with dense polymer brushes. Surface has immobilized ATRP initiators which facilitates this surface formation. PNIPAAm brushes are formed on poly (4 vinyl benzyl chloride) coated TCPS surface using ATRP. Thinner surfaces favor attachment/detachment process.
Solution casting method
Most of the above methods are expensive and not practiced by many researchers. Thermoresponsive polymers are coated onto TCPS surface by this method. It is a simple and cost efficient method. Thickness of the coating is in the order of few microns.
Different characterization techniques are used for evaluating physicochemical and biological properties of thermoresponsive polymers. Examples of characterization techniques are Attenuated total reflectance Fourier Transform spectroscopy (ATR-FTIR), Nuclear magnetic Resonance spectroscopy (NMR), Atomic Force Microscopy (AFM), Ellipsometry, Surface Plasmon resonance and Profilometry.
ATR-FTIR and NMR are used for the quantitative and qualitative detection whereas AFM, Ellipsometry are used for determining thickness of coatings.
PNIPAAm can be detected and quantified using ATR-FTIR on the thermoresponsive surface. The presence of NIPAAm can be detected if a peak of amide carbonyl group is present around 1650 m-1. The ratio of peak intensities (I1650/I1600) is used to determine the quantity of grafted PNIPAAm in ATR-FTIR.
NMR technique gives details of structure. It also helps in understanding the polymerization mechanism better. It also provides qualitative analysis of thermoresponsive surfaces.
Differential scanning calorimetry (DSC) is used to determine the LCST. The LCST of homopolymer PNIPAAm is around 32 °C, whereas the LCST of copolymers of PNIPAAm lies above or below 32 °C depending upon the hrdrophilicity and hydrophobicity of the comonomer.
X ray Photoelectron Spectroscopy (XPS) is a qualitative technique to detect the presence of PNIPAAm on different substrates. The composition of various elements like nitrogen, oxygen and carbon on the grafted surface can be determined using this technique.
Profilometry is used to estimate surface roughness and thickness from the surface of the grafted polymer. This is also to visualize the morphology of the surface.
Ellipsometry and Surface Plasmon resonance is not used to determine the thickness because of the very close value of refractive index of PNIPAAm and polymeric substrates.
Water contact angle is measured to determine the hydrophobicity and hydrophilicity of the surface.
Method of Preparation
Preparation of magnetic polymer gels can be done similar to other filler - loaded networks. The preparation and characterization is done separately, then the polymer solution and the magnetic sol is mixed to cross link them.
Magnetite-Loaded Poly (vinyl alcohol) Gels:
First, magnetite (Fe3O4) sol (ferrofluid) is prepared by co- precipitation. The particle can be varied by using different concentrations of reactants and by varying stirring rate. X- Ray scattering is used for determining the size and size distribution of magnetite in the ferrofluid. The magnetite sol having concentration 10% is then mixed with poly(vinyl alcohol) (PVA) solution to form magnetite loaded PVA gels. Prepared samples are kept in distilled water to remove the unreacted compounds.
Small angle X-ray scattering is used as the characterization technique. This technique is used to determine the microscale and nanoscale structure of materials. This technique is accurate , only requires minimum amount of sample. It is also non- destructive.
Magnetic Polystyrene Latex (mPS)
Miniemulsion polymerization method is used to prepare mPS. The magnetite particles in a ferrofluid is put into the monomer under continuous stirring to obtain stable magnetite/styrene dispersion. Co-stabilizers are used for stabilizing the styrene/water miniemulsion. Stabilization is required to reduce coalescence and diffusional degradation. Stearyl alcohol(SA) is used as co-stabilizer. Ultrasound homogenization is used for the precipitation of miniemulsion.
Transmission electron microscopy (TEM) Analysis is also used as characterization technique in the preparation of mPS.
Electro-active Polymers (EAP)
Methods of preparation
Poly (methyl methacrylate) (PMMA)
PMMA clay nanocomposite are prepared as thermal actuators which can be used as thermal switches, in auto aeration and ventilation systems. The PMMA clay nanocomposites are prepared by melt processing on a twin roll mill, temperature is around 170 °C.
Dendritic Poly(styrene sulphonate)
This material is prepared by using ϒ radiation technique. A 60Co source is used for ϒ radiation at room temperature. This polymer is a copolymer of vinylidene fluoride and hexaflouropropylene.
Crosslinked Poly(vinylidene fluoride) (PVDF)
The monomers are vinylidene fluoride, triflouroethylene and chlorotriflouroethylene or flouroethylene. These monomers forms a terpolymer. Heating at around 160- 170°C will crosslink the polymer. Crosslinking enhances chemical resistant and thermally stable. Organic peroxides with amines is used as accelerator for crosslinking. The extent of the crosslinking is determined by differential scanning calorimetry (DSC), solubility test or thermal mechanical analysis (DMA).
The main characterization techniques are described below.
Information about the mechanical properties like brittleness, yield strength and elasticity is provides by Stress-Strain Curve. In this technique a force is applied to the polymer at a constant rate and the deformation is measured. Nature of the material like (brittle, tough) can be determined. This technique is destructive because polymer will fracture at higher stress.
Dynamic mechanical thermal analysis (DMA)
It is a nondestructive characterization technique which is used to the mechanism of deformation at molecular level. In this method a sinusoidal stress is applied to polymer which is used to determine elastic modulus and damping characteristics. Polymer is assumed as damped harmonic oscillator.
Dielectric thermal analysis (DETA)
This technique is similar to DMA but in this case alternating electric field is applied in place of alternating mechanical energy. Due to the application of the electric field the polymeric material gets polarized. If the polymeric material have permanent dipoles, dipoles will get aligned with the electric field. This can be used to measure permittivity and electric displacement field.
Differential scanning calorimetry (DSC) gives the information about the crystallinity.