Polymer Defintion
Poly (MANY) + mer (REPEATED UNIT) = Many Repeated Units
Structure–Property relationships
How molecular characteristics such as chain stiffness, architecture, mass, and chemical composition are directly related to the physical (modulus, porosity (mesh size)) and chemical (hydrophilicity) properties of the macroscopic material.
Structure - Monomer Repetion
Homopolymer - same monomer repeats
Random copolymer - no discrete order of different monomers
Block copolymer - repeating sets of different monomers
Alternating copolymer - alternating pattern of different monomers
Graft copolymer - nonlinear attachment of different monomers
Structure - Polymer Architecture (how chains are arranged)
- Linear: Single molecular backbone.
- Branched: A central polymer backbone has smaller side chains extending from it.
[Branches can occur due to undesired side reactions during synthesis, or can be purposefully incorporated into the molecular structure. The type and extent of branching introduces significant changes in properties to a polymer system.]
[Branched polymers have difficulty crystallizing, since the branch sites interfere with the ability of the polymer to organize into a lattice. In cases where branch sites are purposefully incorporated into the material, they will lower the extent of crystallization] - Network: Covalently bonded linear polymer molecule to the backbone of another linear chain, many times. (a few = branched).
Techniques for verifying the chemical structure of polymers.
- Nuclear magnetic resonance (NMR) spectroscopy: an analytical technique that exploits the magnetic moments associated with isotopes that contain an odd number of protons and/or neutrons. These atoms have an intrinsic nuclear magnetic moment and angular momentum, in other words a nonzero nuclear spin, while all nuclides with even numbers of both have a total spin of zero.
- Infrared (IR) spectroscopy: a sample of interest is irradiated with IR radiation. The sample absorbs certain wavelengths, resulting in specific molecular motion or vibrations (such as C–H stretching). The IR spectrum is created by plotting absorbance versus wavelength.
Tacticity
Describes the stereochemistry of the repeat units in polymer chains.
- Tacticity can drastically affect the physical behavior of the polymer system, largely by affecting the ability of the polymer molecules to crystallize.
ie. - Atactic (methyl group is randomly located in front of and behind the polymer backbone): cannot organize themselves into a lattice due to the irregularity in their chemical structure, and are permanently amorphous.
- Isotactic (all the methyl groups are located on one side of the “stretched-out” polymer backbone).
- Syndiotactic (the methyl groups regularly alter- nate from side to side)
Molecular mass
During polymerization, polymer chains are built up from monomers to a desired molecular mass. Polymers with identical composition but different molecular mass (different chain length) may exhibit different physical properties. The number of monomer repeat units in each polymer chain is called the degree of polymerization (DP).
Distribution of molecular masses within polymer solutions
- In a traditional free radical or condensation polymerization synthesis, each polymer chain will not have the same DP (degree of polymerisation).
- Therefore most polymer systems have a distribution of molecular masses.
- As such, it is incorrect to talk about the molecular mass of a polymer system.
- Instead, polymer systems are described by average values of molecular mass. The two most commonly used averages are the number average molecular mass (Mn) and the weight average molecular weight (Mw).
Physical properties of Polymers
- The physical properties of a polymer stem
from the intermolecular interactions occurring between individual polymer molecules. - The four most fundamental molecular characteristics are:
(1) chain stiffness
(2) chain composition or polarity
(4) chain architecture or regularity
(5) molecular mass. (determine characterising temperatures Tg and Tm)
Glass transition temperature (Tg)
- Glassy –> Rubbery
- Magnitude of Tg in crystalline polymers is greatly effected by the amount of crystallinity in the system (more crystallinity –> smaller drop)
- In Crystalline polymers, after the Tg, the modulus holds steady until the melting point, illustrating that crystallinity is a way to increase the window of temperatures in which a polymer can be employed.
The Glassy State
- Hard/Stiff
- As the temperature drops, the rate of segmental motion in a polymer chain becomes slower and slower, and the chain gets stiffer and stiffer. As the system approaches the Tg the interpenetrated random coils become frozen in space. This is called the glassy state.
- Molecules in the glassy state can no longer rearrange themselves under applied stress, so deformation results in straining the secondary interactions between molecules.
The Rubbery State
- Soft, flexible, and extensible, due to the molecular motion available to the molecules.
Crystalline Melting temperature (Tm)
- Rubbery –> Liquidy
- Tm is only present when there are crystalline regions in a polymer.
- -> Melting refers to the loss of crystallinity, and since Amorphous materials are noncrystalline, they never truly melt.
Amorphous Polymers
Each random polymer coil is highly entangled with its neighbors. Polymers in the rubbery state or the glassy state have this molecular arrangement.
Crystalline Polymers
Under certain conditions, some polymers will arrange themselves into highly organized crystalline domains resulting in a semicrystalline material.
- All polymer systems form glasses at sufficiently low temperatures. However, as a melt is cooled, certain polymers have the ability to pack into a regular lattice, leading to the formation of stable crystalline domains.
Secondary Interactions between polymer chains
- van der Waals forces
- Dipole–dipole interactions
- Hydrogen bonding –> chemical cross-linking
Due to these secondary interactions, polymers are often mechanically weaker than other classes of materials; however, they can display physical behavior more similar to native tissue.
Techniques to measure the degree of Crystallinity
- X-ray diffraction (XRD): polymer sample is bombarded with X-ray radiation and the intensity of the scattered X-rays is measured as a function of scattering angle. A fully amorphous material would produce a very broad peak, covering all scattering angles. However, crystalline materials will produce sharp peaks at specific angles.
- IR spectroscopy
Interactions with Water
Biomaterials are often employed in highly hydrated environments, so their interaction with water is an important design characteristic.
- increase in polarity –> increase in hydrophobicity
- crystalline regions usually resist infiltration of water molecules.
Biodegradation
- Polymer degrades and is removed from the biological environment overtime eg. staples, pins, fracture fixation devices.
- Mainly by hydrolysis.
Biostability
- Polymer remains intact and viable with time in biological environment eg. dental implants, syringe, catheter.
- Dependent on water absorption and the susceptibility of the main chain bonds to hydrolysis.
Hydrolysis
- The reaction of the polymer backbone bonds with water, which results in the cleavage of those bonds and loss of the polymer’s mechanical properties.
- Eventually the polymer breaks up into small fragments that are metabolized and/or dissolve, and the residual molecules are eliminated from the body.
How the modulus (stiffness) of a polymer may change as the temperature applied to the polymer increases.
- Physical behaviour of polymers dependent on the morphology and type of polymer
- MORPHOLOGU:
AMORPHOUS POLYMER - the material starts with a high(er) modulus, in a glassy amorphous state
- as temperature is applied, when the polymer reaches the glass transition temperature (Tg) the modulus will decrease, and the polymer will be in a rubbery / flexible state.
- temperatures that dramatically exceed this Tg will degrade the polymer
CRYSTALLINE POLYMER - Similar to amorphous polymers, the material starts with a high(er) modulus, in a glassy crystalline state
- as temperature is applied, when the polymer reaches the glass transition temperature (Tg) the modulus will decrease, and the polymer will be in a rubbery / flexible state.
- as temperature further increases and the polymer reaches the melting temperature (Tm), the modulus further decreases as the polymer takes a liquid flow form.
- temperatures that dramatically exceed this Tm will degrade the polymer.
TYPE: - If the polymer is of thermoset type, once heated and cooled, it will become permanently rigid in an irreversible change of state (due to crosslinking).
- If the polymer is of thermoplastic type, heating and cooling is reversible for continual changes in state.
