Properties of Polymers
Solubility
Water soluble polymers
interact with the water to provide sufficient energy to remove individual polymer
chains from the solid state, thus increase the viscosity of solvents at low
concentrations, swell or change shape in solution, and adsorb at surfaces. On
the other hand partially soluble polymers are used to form thin films, as film coating
materials, surgical dressings membranes for dialysis or filtration, or matrices
for enveloping drugs to control their release properties, or simply as packaging
materials. The rate of dissolution of a water-soluble polymer depends on its
molecular weight. The larger the molecule, the stronger are the forces holding
the chains together: more energy has to be expended to force the chains apart
in the solvent. The greater the degree of crystallinity of the polymer, the
lower is the rate of dissolution. The velocity of penetration, S , of a solvent
into the bulk polymer obeys the relationship: S = kM-A, where M is
the polymer molecular weight. k and A are constants. The dissolution process is
more complicated than with ordinary crystalline materials. It is frequently
observed that swollen layers and gel layers form next to the polymer. If a drug
is embedded in the polymer, the drug has to diffuse through these gel layers
and finally through the diffusion layer. It is the combination of slow solution
rate and the formation of viscous surface layers that makes hydrophilic
polymers useful in controlling the release rate of soluble drugs which are
perhaps irritant to the stomach. [1]
Crystallinity
Partial alignment of
molecular chains is associated with the
process of crystallization of polymers. Lamellae are those which are having chain fold together and form
ordered regions, which compose
spherulities. Dyeing of polymers get affected by crystallinity. Amorphous form is much more
prone to dyeing as compared to
crystalline form because the dye molecular
penetrate much easier through amorphous regions. These are being classified as-
Crystalline Polymers
Light scattering between crystalline and amorphous regions usually causes polymer
to be opaque and called as crystalline
polymers. Either for law (amorphous
polymer) or high (crystalline) degree of crystallinity the transparency is higher as because density of
such boundaries is lower. For example,
atactic polypropylene is usually
amorphous and transparent while syndiotactic polypropylene, which has
crystallinity ~50%, is opaque.
Semi crystalline Polymers
Highly ordered
molecular structures with sharp melting point
are possessed by semi crystalline materials. Semi crystalline material
rapidly change to low viscosity fluid when given quantity of heat gets absorbed and they
remain in solid form. Softens does not
vary with temperature increases. Direction flow us transverse to flow causes
less shrinking and thus material is
anisotropic in flow. Chemical resistance is excellent. Beyond their glass transition temperature. Semi
crystalline exhibit substantial improvement in HDT’s which reinforced and
retain useful levels of strength and stiffness.
Amorphous Polymers
During x-ray or
electron scattering experiments polymer
do not exhibit any crystalline structure and
those polymers were called as amorphous polymers. E.g. - using straining-induced contrast enhancement in
TEM. Formation of localised deformation
zones, such as crazes, deformation bands,
or shear bands, which are characterised by representative HVTEM micrographs, shows micromechanical
behavior of amorphous polymer.
Viscosity
The presence in
solution of large macromolecular solutes may have an appreciable
effect.Viscosity increases , the sustained drug release is prolonged.
Polymer complexes
Polymers provide ample
opportunity for the formation of complexes in solution ,e.g ,when an
aqueous solution of high molecular weight
polyacid is mixed with polyglycol. The viscosity and pH of the solution of the equimolar mixture of
polyacid and glycol remains the same with increase increase in oligomer chain
length up to a critical point. This
occurs only when the poly(ethylene glycol) moleculeas are reached a certain
size. Such macromolecular reaction are highly selective and strongly dependent
and molecular size, conformation heat etc. Biological macromolecule undergo
complex reactions, which are often vital to their activity. The studies have
estabilished a specific interacteraction between hyaluronic acid and the
proteoglycans the intracellular matrix in cartilage. Calcium (Ca2+)
is coordinated between certain uronic acid- containing polysaccharides, which
can explain the tight binding of calcium and other multivalent ions in
polysaccharide structures, and also how bivalent ions can induce gel formation
in acidic polysaccharide such as alginic acid solutions. It has been found such
interaction have dietary significance.[1]
Syneresis
The separation of
liquid from a swollen gel is known as syneresis. This is a form of instability
in aqueous and non-aqueous gels. Separation of a solvent phase is thought to
occur due to the elastic contraction of the polymeric molecules. In the swelling
process during gel formation the macromolecules involved become stretched and
the elastic forces increase as swelling proceeds. At equilibrium the restoring
force of the macromolecules is balanced by the swelling forces, determined by
the osmotic pressure.[1]
Adsorption of macromolecules
The ability of some
macromolecules to absorb at interfaces is being exploited in suspension and
emulsion stabilization. Gelatin, acacia and proteins absorb at interface.
Sometimes such adsorption is unwanted, e.g. insulin adsorption on to glass
infusion bottles. Addition of albumin to prevent adsorption is now common
practice. The albumin adsorbs at the glass or plastic surface and presents a
more polar surface to the solution, thus reducing, but not always preventing,
adsorption of the protein (e.g. insulin). The binding is considered to be a
non-specific phenomenon, which may occur on other inert materials, such as
polythene and glass. The adsorption of macromolecules at interfaces may be the
reason why molecules such as those of hyaluronic acid can act as biological
lubricants in joint fluids. [1]
Bioadhesivity of water- soluble polymers
Adhesion between a
biological surface and a biological surface and a surface of a hydrophilic
polymers or a surface to which a hydrophilic polymer has been grafted or
adsorbed, arises from interactions between the polymer chains and the
macromolecules on the mucosal surface.To achieve maximum adhesion there should
be maximum interaction between the polymer chains of the bioadhesive and the
mucus. The charge on the molecules will be important and for two anionic
polymers maximum interaction will occur when they are not charged. Penetration
and association pH must be balanced. The adhesive performance of polymers can
be excellent (e.g. carboxymethylcellulose), good (Carbopal), fair (gelatin) or
poor (povidone). Anionic poly(acrylic acid) (carbophil) derivatives and the
cationic cationic chitosans have been approved by the FDA. Polycarbophil and
carbomer (carbopol 934P) have pKa values of about 4.5 and display maximum
mucoadheivity at pH where they are undissociated. [1]
Polymer dissolution
Polymer dissolution in solvents is an important are of interest in polymer science and engineering because of its many applications in industry such as microlithography, membrane science, plastics recycling and the drug delivery. Unlike non-polymeric materials, polymers do not dissolve instantaneously, and the dissolution is controlled by either the disentanglement of the polymer chains or by the diffusion of the chains through a boundary layer adjacent to the polymer-solvent interface.[1]
References
1. N.K. Jain, Pharmaceutical product development, 1st ed: 2006;Reprint: 2008, CBS publishers & distributers, Pg .No 585 – 618.
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