Polymers

Thermoplastics

Melt when heated. Re-solidify when cooled. Linear or branched polymers. Ductile. Can be amorphous or semi-crystalline. Chains help with vdw or hydrogen bonds

Elastomers

Lightly cross linked. Easy to stretch. High extension ratio. Rapid recovery. Vulcanisation. Tg < room temperature

Thermosets

Network polymers. High degree of cross linking. Intractable once formed. Decompose instead of melt on heating. Hard and brittle.

Polydispersity

A measure of the distribution of the types of chains in a polymer

Number average molecular weight

The mass of the specimen divided by the number of molecules present

Configuration

An arrangement of atoms that cannot be altered except by breaking chemical bonds

Isotactic

repeating units have same stereochemical configuration

Syndiotactic

repeating units have alternating stereochemical configuration

Atactic

repeating units have no regular stereochemical configuration

Conformation

The individual, recognisable arrangement of atoms that can be altered by simple rotation around a single bond

Melting transition temperature

Occurs in crystalline polymers. Happens when the polymer chains fall out of their crystal structures and become a disordered liquid

Glass Transition temperature

Happens to amorphous polymers When the polymer is cooled below the glass transition temperature, it becomes hard and brittle, like glass

Factors controlling the Tg

Chemical composition of the polymer, chain flexibility, molar mass, branching

What affects the crystallisation

Rate of cooling. Presence of preferential orientations. Stereo-tacticity. Molar mass. Chain branches (linear polymers)

Viscoelasticity

the properties of materials that exhibit both viscous and elastic characteristics when undergoing deformation

Free radicals

species with an unpaired electron, highly reactive, and short lived

FRP: Initiation

Creation of the free-radical active centre. Homolytic. Single electron transfer

FRP: Propagation

Growth of polymer chain by addition of monomers to the active centre

FRP: Termination

Destruction of the active centre and termination of propagation. Combination. Disproportionation. Chain transfer `

Melting

Occurs in crystalline polymers. Happens when the polymer chains fall out of their crystal structures, and become a disordered liquid

Glass transition

Happens to amorphous polymers. Happens when the polymer is cooled below the glass transition temperature, it becomes hard and brittle like glass.

Heat capacity

The amount of heat required to raise the temperature of one gram of the polymer one degree Celsius

DSC

Useful for thermal characteristics. Measures the difference in heat flow rate (mW =mJ/sec) between a sample and an inert reference as a function of time and temperature.

What does DMA measure

Mechanical properties of materials as a function of time, temperature, and frequency

Spectroscopy

It is the study of the interaction between radiation and matter. The compound must interact either with the electric or magnetic component of the electromagnetic radiation to observe a spectrum.

What does Vibrational Spectroscopy: Fourier Transform Infrared Spectroscopy (FTIR) detect

Detects changes in the vibrational state of molecules. However, we are only interested in the movement of the atoms such that the molecule is distorted either by altering the bond or angles within the molecule and there is a change in the electric dipole of the molecule.

Attenuated total reflectance (ATR)

A beam of light is passed through the ATR crystal in such a way that it reflects at least once off the internal surface in contact with the sample. The penetration depth into the sample is typically between 0.5 and 2 μm. The beam is then collected by a detector as it exits the crystal.

UV-visible Spectroscopy – Electronic Spectroscopy

The absorption mechanism involves electrons transitions between partially filled orbital levels. The energies associated with these transitions falls within the ultraviolet (UV) and visible region of the electromagnetic spectrum

Resonance Spectroscopy: Nuclear Magnetic Resonance (NMR)

Determines location of atoms within a molecule from an interaction of protons and neutrons with an external magnetic field

Mass Spectroscopy 3 functions

Creates positive ions from a neutral sample

Separates the ions according to their mass/charge ratio

Measures the relative abundances of ions and their relative masses; the information being represented as a mass spectrum.

Dissolution of Polymer

Restricted to linear or branched polymers

Crosslinked polymers are not soluble

Solubility can be affected by crystallinity, hydrogen bonding,

chain branching

○ Crosslinked polymers are not soluble

○ Solubility can be affected by crystallinity, hydrogen bonding,

○ chain branching.

Two stages of dissolution

Swollen gel- Solvent molecules diffuse into polymer

Gel disintegration - Solvated polymer molecules into solution

Solubility parameters of solvent can they be equal or different

FSS < FPS < FPP or FPP < FPS < FSS

FSS ~ FPP: Mutually compatible

Free energy of mixing must be negative

Colligative properties

Depend only upon the number of solute species per unit volume and not upon the size of the species. Eg Osmotic pressure, Lowering of vapour pressure, Elevation of boiling point, Depression of freezing point

Bulk Polymerisation

adding a soluble initiator to pure monomer in liquid state

Bulk Polymerisation Advantages

Step and chain growth polymerisation, Absence of any solvent - Pure Product, High reaction rate due to high monomer concentration

Bulk Polymerisation Disadvantages

Exothermic, heat transfer - reaction difficult to control, high viscosity, low molecular weight, generation of got spots - branching/cross linking/degradation

Solution Polymerisation

Industrial polymerization. A monomer is dissolved in a non-reactive solvent that contains a catalyst.

Solution Polymerisation Advantages

Control of the chemical reaction, control of the resulting heat and viscosity, control over auto acceleration of the process

Solution Polymerisation Disadvantages

difficulty involved in the removal of excess solvent from the finished polymer, not pure because solvent

Suspension Polymerisation

used only in free radical type processes. The monomer is mechanically dispersed in a media, usually water, sometimes organic media (if polymer/monomer not soluble in it)

Suspension Polymerisation Advantages

Bulk polymerisation within a droplet , Better heat transfer compared to bulk polymerisation, Recovery of the product by mechanical separation, Water as solvent - economical

Suspension Polymerisation Disadvantages

Only for free radical process, Formation of polystyrene beads

Emulsion Polymerisation

A type of radical polymerization that usually starts with an emulsion incorporating water, monomer, and surfactant.

Emulsion Polymerisation Advantages

High polymerization rates, Good temperature control – low viscosity build, The final product can be used as it is

Emulsion Polymerisation Disadvantages

Water removal is an energy-intensive process, Significant chain transfer to polymer, Difficult to remove entrapped emulsifier

Extrusion

Moulding of a viscous thermoplastic under pressure through an open-ended die. Material propelled, melted, and formed into a continuous charge of viscous fluid.

Extrusion advantages

Long, uniform, solid or hollow, simple or complex cross-sections, wide range of dimensional tolerance, high production rate, low tool costing

Injection moulding advantages

Complex shapes of various sizes and with fine details , good dimensional accuracy, high production rate

Injection moulding disadvantages

High tool costing

Structural Foam Moulding advantages

Large parts with high stiffness to weight ratio, low production rates, less expensive tooling than in injection moulding.

Blow moulding

Heated plastic extruded into hollow tube, mould closes, compressed air blown in filling mound, trimmed and removed from mould