---
title: "Polymeric Material - MML213"
tags : "SEM4, MME"
---
# Polymeric Material - MML213
- Insulating
- Low Tensile Strength
- High Ductility
- Thermal Property higher side
- Electrical non conductive
## Ancient Polymers
- Natural Polymers
- Derived from plants and animals
- Example
- Wood
- Wool
- Leather
- Rubber
- Silk
- Cotton
- Rubber balls used by Incas
## Hydrocarbon Molecules
- Many organic materials are hydrocarbon
- Most polymers are made up of H and C
- The bonds betweem the hydrocarbon molecules are covalent
- Each Carbon atom has 4 electrons that maybe convalently bonded
### Saturated Hydrocarbons
Methane - Boiling point - -164°C
Hexane - Boiling point - 69°C
- Each carbon has a single bond to 4 other atoms
- The covalent bonds in each molecule are strong, but only weak hydrogen and van der Waals bonds exists between the molecules
- Most of these hydracarbons have relatively low melting and boiling points
- Boiling temperature ries with increasing molecular weight
### Unsaturated Hydrocarbons
- Double and Triple bonds are somewhat unstable
## Isomerism
- Two compounds with same chemical formula can have different structure (atomix arrangements)
## Synthetic Polymers
- Polymers are giant organic molecules
- Polymerization is the process by which small molecules are joined to create these giant molecules
- As the polymer molecules grow in size, the melting or softening point increases and the polymer becomes stronger and more rigid
## Bonding Mechanism
- Bonding in polymers, especially simple chains, is only by Van der Waals forces
- While there are strong covalent bonds (C-C adn C-H), this is confined to the chain
- When stresses, the simple chains tend to slide over one another and failure is by interchain separation rather than by breaking intrachain bonds
- Cross-linking covalent bonds between the chain
- Covalent bonds between the chain
- Usually developed when the unsaturated bonds (C=C to C-C) that can be used to link to other chains
- Produces high strength but limited ductility
- Side Branching
- Alters the properties of the polymer
## Molecular Structures for Polymers
The physical characteristic of a polymer depend - difference in the structure of the molecular chains (other variables are shape and weight)
- Linear Polymers
- Have repetative units joints end to end in single chain
- High crystility
- Extensive van der Waals and hydrogen bonding between the chains
- Polyethylene, PVC, nylon
- Branched Polymer
- Linear structures may sometimes have side branches this is called branched polymers
- Decreases Crystallinity
- HDPE - High Density Polyethylene - Linear polymer with minor branching
- LDPE - Low density polyethylene - contains numerous short chain branches
- Greater chain linearity and chain length tend to increase the melting point and improve the physical and mechanical properties of the polymer due to greater crystallinity
- Cross-Linked
- Adjacent linear chains are joined to one another at various positions by covalent bonding of atoms
- Rubber elastic materials
- Small molecules that form 3 or more active covalent bonds create structures cakked network polymers
- Epoxies and Polyurethanes
- Network
### Polymer Categories and Structures
Behaviour
- Thermoplastic
- Flexible Linear chains
- Addition or condensation
- Plastic, Ductile
- Formed at elevated temperature, cooled and then reheated and reformed into a different shape
- Properties and structure is unchanged during processing - Reheating and cooling
- Thermosetting
- Rigid 3D network
- Usually condensation but sometimes addition
- Generally stronger than thermoplastics
- Polymer cannot be reprocessed - part of the molecule has left the material
- Elastomers
- Linear cross-linked chains
- Usually addition produces both the chains and cross-links
- Intermediate physical behaviour
- Ability to elastically deform at large strain without permanently changing shape
## Polymerization Mechanism
- Condensation Polymerization
- Two or more different organicmolecules react to produce a chain made up of combinations of starting molecules that release a by-product, such as water
- Addition Polymerization
- One or more organic molecules containing a shared eletron bond - double bond - split by action of initiator
- Produces unpaired electrons on the starting molecule which subsequently bond with surrounding molecules of the same type by addition until a long chain is produced
### Chain Formation by the Addition Mechanism
- Functionality is the number of sites at which new molecules can be attached to the mer
- Initiator is added to first convert the reacting molecule into a mer and then allow growth
#### Growth of Addition Chain
- Growth is initially slow
- Energy is released, temperature rises, and increases growth
- When nearly complete, the remaining unattached mers must diffus a long distance begore reaching the active end of the chain
#### Termination of an Addition Chain
- Chains may terminate in two ways
- Ends of two growing chains may join to produce a single large chain - Combination
- The active end of the chain may attract an initiator group
- Active end of one chain remove a hydrogen atom from a second chain called Disproportionation
- Length of the chain is controlled by the amount of initiator
#### Degree of polymerization ($\bar X$)
Represents average chain length or Average number of monomer units in a chain
$\bar X_n$ - Number of average degree of polymerication
$\bar X_w$ - Weight average degree of Polymerization
$\bar X_n = \dfrac{\bar M_n}{\bar m}$
$\bar M_n$ - Average Molecular Weight of Polymer
$\bar m$ - Monomer Molecular Weight
$\bar X_w = \dfrac{\bar M_w}{\bar m}$
$\bar M_w$ - Weight average molecular weight of polymer
- If polymer contain same Monomer, the molecular weight of repreat unit is that of Monomer
- If polymer contain more than one type of Monomer, Molecular weight of reprat unit is the sum of the molecular weight of the monomer, less the molecular weight of by-product
12000 to 140000 g moles range
polymer is never fully crystalline it's aprox 90% crystalline
> Why the polymer is not having a fixed property
### Steps in Addition or radical Polymerization
1) Initiation
I $\rightarrow$ 2R^.^
R^.^ + m $\rightarrow$ M^.^
2) Propagation
M~1~^.^ + M $\rightarrow$ M~2~^.^
...
M~x~^.^ + M $\rightarrow$ M~x+1~
3) Termination
i) Coupling or Combination
M~x~^.^+M~y~^.^ $\rightarrow$ P
ii) Disproportionation
M~x~^.^+M~y~^.^ $\rightarrow$ P~1~ + P~2~
iii) Chain transfer reaction
M~1~^.^+M $\rightarrow$ M~1~^.^ + P(Transfer to monomer reaction)
M~1~^.^+ I $\rightarrow$ R^.^ + P(Transfer to Initiator reaction)
M~1~^.^+ S $\rightarrow$ S^.^ + P(Transfer to Initiator reaction)
</b>
I - Initiator
R - Radical
M - Monomer
P - Polymer
S - Solvent
### Condensation Polymerization
- Minimum 2 different monomer are required - 2 funcational groups
- Using Catalyst
- Usually Linear Structure is formed
- Molecular weight increases slowlt at low conversion
- High extents of reaction are required to obtain high chain length
| Step-Growth polymerization| Chain-Growth Polymerization |
| -------- | -------- |
| Growth throughout matrix | Growth by addition of monomer only at one end of chain |
| Rapid loss of monomer early in the reaction | Some monomer remains even at long reaction times |
| Same mechanism throughout | Different Mechanisms operate at different stages of reaction (Initiation, propagation, termination) |
| Average Molecular weight increases slowly at low conversion and high extents of reaction are required to obtain high chain length | Molar mass of backbone chain increases rapidly at early stage and remains approximately the same through out the polymerization |\
| Ends remain active (no termination) | Chains not active terminaiotn |
| No initiator Necessary | Initiator required |
### Kinetic of Radicalic Polymerization
Addition Reaction
Initiation
I - I $\overset{k_d}{\rightarrow}$ 2I^.^ (Dissociation)
I^.^ + M $\overset{k_a}{\rightarrow}$ IM^.^ (Assosiation)
Propagation
IM^.^ + M $\overset{k_p}{\rightarrow}$ IMM^.^
...
IM~x~^.^ + M $\overset{k_d}{\rightarrow}$ IM~x~M^.^
Termination
IM~X+1~M^.^ + ^.^MM~Y+1~I $\overset{k_{td}}{\rightarrow}$ IM~X+1~ + IM~Y+1~ - Disproportionation
IM~x~M^.^ + ^.^MM~x~I $\overset{k_{tc}}{\rightarrow}$ IM~X+1~M-MM~Y+1~I - Combination
### Kinetics of Condensation Polymerization
- It occurs by consecutive reactions
- Degree of Polymerization increases as reaction proceeds - increases with time
$R_p = \dfrac{-d[A]}{dt} = \dfrac{-d[B]}{dt} = k[A][B]$
For equal feed,
$A = B = M$
$\dfrac{d[M]}{dt} = -kM^2$
$M = \dfrac{M_0}{1+M_0kt}$
In terms of fractional conversion of function group 'p'
$\dfrac{1}{1-p} = M_0kt+1$
where $\bar X_n = \dfrac{1}{1-p}$ - Degree of polymerization
$\bar X_n = M_0kt + 1$
Degree of Polymerization increases with time
## Copolymerization
- Process of polymerizing single or same monomer called homopolymerization and polymer obtained called homopolymer
- Process of polymerizing two or more monomer pairs called copolymerization
- Copolymers always show random/amorphous structure - 4 reactions
- Copolymer representation as
xA + yB $\rightarrow$ A-A-B-A-B-B-B-A-A
- Copolymerization brought by many polymerization mechanism as
- Free radical polymerization
- Polycondensation Polymerizaton
- Ionic Polymerization
- Styrene Butadiene Rubber (SBR) copolymer
- Physical Properties
- Homopolymer - Higher Density
- Ductility -
- Toughness - Homopolymer
### Types of Copolymers
- Random - A and B randomly positioned along chain
- Alternating - A and B alternate in polymer chain
- Block - Large blocks of A units alternate with large blocks of B units
- Graft - chains of B units grafted onto A backbone
Block > Alternating > Graft > Random (Physical properties)
### Free-Radical Copolymerization
As initators decomposes, the free radical formed can attack M~1~ or M~2~ as follows
R^.^ M~1~ $\rightarrow$ RM~1~^.^
R^.^ M~2~ $\rightarrow$ RM~2~^.^
Propogation reaction
### Determination of Reactivity Ratio
Monomer in the feed ($\frac{M_1}{M_2}$) = M
Monomer in the polymer ($\frac{M_1}{M_2}$) = P
- Reactivity ratios r~1~ and r~2~ computed by plotting data using
#### Mayo-Lewis Method
Copolymer equation rewritten as
$r_2 = r_1\frac {M^2}P+ \left[ \frac{M}{P} - M\right]$
Assues value of r~1~, value of r~2~ calculated for each set of values of M and P
#### Fineman-Ross Method
It utilises equation
$\qquad M-(\frac MP) = -r_2+r_1(\frac{M^2}{P})$
Plot $(M-\frac MP)$ vs $\frac{M^2}{P}$
### Ractivity ratios and copolymerization behaviour
Various Copolymerization behaviour
- r~1~ = r~2~ = 0
- r~1~ = $\frac {k_{11}}{K_{12}}$, r~2~ = $\frac {k_{22}}{K_{12}}$
- k~11~ = k~12~ = 0
- PR type 11 and 22 not possible
- M~i~ will add M~2~ and M~2~^.^ will add M~1~
- Copolymer formed-alternating type
- r~1~ = r~2~ = 1
- r~1~ = $\frac {k_{11}}{K_{12}}$, r~2~ = $\frac {k_{22}}{K_{12}}$
- k~12~ = k~11~, k~21~ = k~22~
- PR type will be 12, 21,11,22 - equally
- r~1~<1 and r~2~<1
- PR type 12 and 21 preferred over 11 and 22
- $M_1^.$ will add M~2~ and $M_2^.$ will add M~1~ simultaneous
- r~1~>1 and r~2~>1
- Does not undergo random copolymerization
- Mixture of two homopolymer
- Can be used to make block copolymers - using catalysts to promote certain reaction for a certain duration of time
## Principles of Commercial Copolymerization process
- Selection of monomer pairs with r~1~ and r~2~ values close to unity
- Maintenance of the monomer concentration by adding another Monomer
- Restriction of the copolymerization to low conversion only
## Ionic or Non-Radical Polymerization
- Unlike free radical chain Polymerization
- Ionic sites acts as active centers
- Formation of Carbonium(C^+^) or Carboanion(C^-^) ionic sites
- Catalyst used
- Environment must be pyre
- Basic Steps
- Initiation
- Propagation
- Termination
- Two types
- Cationic (Positive)
- Anionic (Negative)
Ionic Polymerization developed because
- Highest Reaction rate
- Reaction occurs in the cold
- Very good control over stereo regularization
- Block copolymer and noval polymers produced easily and economically
### Cationic Polymerization
Example - Polymerisation of Isobutylene
Mechanism
- Proton pulls the $\pi$ - electon pair towards it and positive charge of proton transfered to the farther end of monomer molecules forming carbonium ion
### Anionic Polymerisation
$\pi$-electron pair pushed to the end of the Monomer molecule forminh a carbonion ion
Example - Polymerisation of Styene Monomer
Chain propagates by insertion of additional styrene monomer between carbonion and counter ion
- Termination is not a spontaneous process
- May occur with impurities or deliberatly added ionic substances
- Called as living polymerization - Living Polymers
- Kinetics of anionic polymerization are complicated
## Kinetics of Cationic Polymerization
Not well understood but similar way to free radical chain polymerization
## Practical Polymerization Processes (Physical Aspects)
### Process Objectives
- To produce polymer to an acceptable specification at low cost
- To produce polymer of narrow molar mass range and braod distribution
- To produce polymer having consistent in character
- To produce polymer with desirable properties by controlling parameters during process
### Overall Process
- Polymerization brought adding necessary ingredients to a closed vessel or reactor
- Bringing about polymerization and product removed after reaction time
- Synthesized by batch or continous Process
1) Batch type process
Advantage
- Suitable for low product
- Low cost
Limitations
- Less suitable for mass or bulk production
- Time variation from batch to batch
- Scale up is difficult as polymerization is exothermic
- Stirring becomes difficult as size of reactor increases
2) Continuous process
Advantages
- Suitable for bulk or mass production
- No Variation in time during loading and unloading vessel
- Products are recovered continuously
Limitations
- Not suitable for low production
- High cost process
a) Continuos stirred Tank vectors
b) Horizontal Overflow Reactor
c)Vertical Mixer-Settler Reactor
d) Tower Process
Vertical Colum reactor for the continuous bulk polymerization of styrene
### 1. Bulk Polymerization
Ingredients
- Monomer
- Monomoer soluble initiator
- Perhaps a chain transfer agent
Mechanism
- Radical or condensation
- Reaction mass kept under agitation or stirring
- Temperature Control and inert (nitrogen) atmosphere
Examples - Tower Process - Styrene
Advantages
- High Yield for reactor value
- Easy polymer recovery
- Gives the highest purity Polymer
Disadvantage
- Difficult to remove the last traces of monomer
- Dissipating heat - due to polymerisation
### 2. Solution/Solvent Polymerization
- Charge Composition or Ingredient
- Monomer
- Solvent
- Initiator
- Chain transfer agent
- Netta Catalyst - Titanium trichloride with aluminium triethyl
- Monomer, initiator and resulting polymer are soluble in the solvent
- Polymerization performed in solution either batchwise or continously
- It is a homogeneous polymerization involves a pronounced increase in viscosity and evolution of heat
- Temperature - 60°C
- Pressure - 2-4 atm
- Mechaism used free radical and ionic polymerization
- Reactor design depends largely on how the heat evolved dissipated through solvent or water
- Monomer, initiator, catalyst, and resulting polymer are soluble in the solvent
- Exothermic
- The higher the conversion the higher the viscocity
Example - Propylene Monomer to Polypropylene
#### Steroisomerism or Tacticity
- Compounds have same number and kind of atom but differ in spatial relationship
Types of Tacticity
- Isotactic - R group on same side
- Syndiotactic - R grounps on alternate side
- Atactic - R groups on random position
Example
Advantages
- It reduces the heat transfer problem encounterd in bulk polymerization
- Inert solvent medium helps to control viscocity of polymer
- Relative to bulk polymerization, mixing is facilitated on the presence of the solvent reduces the rate of increase of reaction medium viscosity as the reaction progresses
Disadvantages
- Use of an inert solvent lowers the yield per reactor volume
- Reduces the reaction rate and average chain length
- Less pure than mass polymerization
- Solvent recovery, handling and separation of unreacted monomer involved additional cost
Commercial Use
- In adhesives and coating processes
- HDPE, Polybutadiene and butyl rubber
### 3. Suspension Polymerization
Charge Composition
- Water insoluble monomer
- Water as a suspension medium
- Water insoluble initiator
- Chain transfer agent
- Protective Collides (PVA)
<br>
- It is a heterogenous polymerisation where monomer is suspended in water as a droplet
- Coalscense of sticky droplets is prevented by PVA
- Temperature at 50°C
- Under nitrogen atmosphere
- Poylmerisation takes place inside the droplet and product formed being insoluble in water
- The product seprated out in the form of a spherical pearls or beads of polymer
- Hence the method is also known as Perl /Grannular/Bead Polymerization
- It is a **Hetrogenous** Polymerization
- Polymer formed in the form of spherical bead/perls is called bead polymerization
- It is isolated only by filteration and removal of protective colloids is done by water washing
- Washed and dried product can be used for moulding or as adhesive by dissolving in a suitable solvent
For example-Polymerization of **PVC** - Poly Vinyl Chloride
**Advantages**
- Good heat transfer due to aqueous media
- Good particle size control trough agitation and dispersion agent
- Product easy to recover and transfer
- Design of reactor becomes easy through known kinetics
- Coparatively cheap - water - instead of solvent
- Purity of obtained polymer - high comparent to solution polymer
**Disadvantages**
- Yeild per reactor volume is less - Batch process
- Suspending agents containate product
- Removal of residual monomer necessary
- Polymer purity is low
- Highly agitation sensitive
- Large volume of reactor is taken up by water
### 4. Emulsion Polymerization
Ingredients
- Monomer droplet (water insoluble)
- Dispersing Medium(water)
- Initiator(water soluble, hydroperoxide)
- Emulsifying agent or surfactant(Fatty acid, sodium lauryl sulfate, soaps, etc)
- Chain transfer agent(Mercaptan)
**Emulsion**-"One liquid held in another liquid"
Monomer dispersed in water not as a discrete droplets but as a uniform emulsion
Droplet size - 0.5 to 10µm
Emulsion of monomer in water is stabalized by a emulsifier or surfactant
#### Structure in emulsion polymerization
- Surfactant
- Hydrophobic end
- Hydrophilic end
- When put in water - surfactant molecules gather together into aggregates - Miscelles - at certain concentration
- Below CMC - they do not form miscelles
- The Hydrocarbon tail
- Orients inwards - Away from water
- it's organic part
- The Hydrophilic head
- orients outwards into water
- it's inorganic part
- Miscellae - Favored as reaction site due to high monomer concentation and polymerization initiated when water soluble initaitor enters a micelle centre
Advantages
- High Molecular weight can be made at fast polymerization rates
- Finer particle - Greater surface area - greater rate of initiation
- Continous water phase
- Better heat conduction - faster polymerization
- High Yield can be obtaines
- Low tendency to agglomerate
Disadvantage
- Product obtaines is less pure
- Surfactant and other agents remain in the polymer or are difficult to remove
- Considerable technology is needed to recover the solid polymer
- Use of large quantity of water lowers the yield
### Applications
- Synthetic rubber
- Plastics
- Dispersion(i.e. polymers sold as aquesous dispersion)
# Polymer Morphology
| Semi-Crystalline | Amorphous |
| -------- | -------- |
| Molecules pack together in ordered region | Molecules are oriented randomly |
| Chains are not intertwined | Chains are intertwined |
| More Strength | Less Strength |
| More Density | Less Density |
| Less hard and brittle | More hard and brittle |
| More toughness | Less toughness |
| Opaque appearance | Transparent Appearance |
| Plyethylene, PP, etc | ABS, Polystyren, etc. |
Flexible Crystalline Exists above T~g~ so it is not important for Crystaline Polymers
## Crystallinity of Polymers
- Crystallization is easiest for simple addition polymers, such as polyethylene
- Slow cooling during solidification permits more time for chains to become aligned
- Slow deformation of the polymer between the melting and glass transition temperature may promote crystallization
- Polymers are rarely 100% crystalline
- Difficult for all regions of chains to become realigned
- Degree of crstallinity expresses as % crystallinity
- Some Physical Properties depend on % crystallinity
- Heat Treating causes crystalline region to grow and % crystallinity to increase
$\text{%Crystallinity}=\dfrac{\rho_c(\rho_s-\rho_a)}{\rho_s(\rho_c - \rho_s)}\times100$
$\rho_s$ - Density of specimen
$\rho_a$ - Density of totally amorphous polymer
$\rho_c$ - Density of perfectly crystalline polymer
Sharp Peak - Semi-Crystalline
Broad Peak - Amorphous Polymer
### Polymer Crystals
Fringed Miscelle Model
- Aligned small crystalline regions (crystallites or miscelles)
- Amorphose regions in-between platelets of crystals - 10-20mm thick
Chain-Fold Model
- Amorhphous Molecular
- Chains within platelets - back and forth
Spherulite model
- Bulk polymers solidify as small spheres(Spherulites)
- Within each such sphere, folded crystallite(lamellar) $\sim$ 10nm thick form
- Adjecent spherulites impinge on each other forming plannar bonding
- Polyethylene, Polypropylene, PVC, PTFE, Nylon
### Factors affecting Crystallization in Polymers
- Random Copolymerization
- Bulky side Groups - Decreases - disrupts the packing efficiency
- Chain Branching - Decreases - Disrupts the PF
- Tacticity(lack of Sterio regularity)
- Certain geometrical isomers (Isoprene)
- Degree of Polymerization - Longer chains are difficult to polymerize
- Cooling Rate - Faster Cooling rate decreases the crystallinity
### Properties affected by Crystallinity
- Strength
- Density
- Modulus
- Hardness
- Molecular weight
- Heat capacity
- Permeability
#### Examples of semi-crystalline polymers
| Polymers | T~g~(°C) | T~m~(°C) |
| -------- | -------- | -------- |
| Polyethylene | -20 |+120-140 |
| Polypropylene | 5 | 150-160 |
| Polyamide (nylon) | 50 | 200-215 |
|PEEK | 144 | 335 |
```PEEK- Poly Ether Ether keytone```
## Amorphous Polymer structure
- Importance of transition temperature
- Above T~g~ - Soft or rubbery phase exists
- Below T~g~ - Hard or rigid or glassy phase exists
- T~g~ - Molecular rotation in the backbone becomes possible
- Properties that change at T~g~
- Elastic Modulus
- Specific Volume
- Heat Capacity
- Thermal expansion coefficient
## Factors affecting the Mobility of chains
- Inherent stiffness of the chain
- Degree of chain entanglement
- Extent of cross-linking
- Degree of crystallinity
Mobility or deformation of chain in an amorphous polymer takes place by 2 processes
- Bond streching and bond angle opening
- Rotation of Segments of chain amount the chain backbone
### Factors affecting T~g~
- The Free Volume
- Higher - decreases the Deformation
- Bulky Side Group
- Rigid Structure incorporated into the back bone
- Chain length and Linearity
- Co-Polymerization
- Plasticization
- Increases - Decrease in T~g~
- Secondary Bond between the chains
- Cross-linking between the chain
- Amount of branching and side chain
- Attractive forces between the chain
- Higher - Increases the T~g~
- Internal Mobility of the Chain
- Higher Mobility - Lower T~g~
### Effect of temperatrue on thermoplstics
- Degradation Temperature
- Covalent bonds between atoms in the linear chain may be destroyed
- Amorphous Polymer (Plastic state)
- Polymer behaves in a rubbery manner, both elastic and plastic deformation can occur
- Forming temperature range
- Glassy Polymers
- Cannot be formed into useful shapes and may be too brittle for certain application
- Stronger, more rigis, dimensionally stable, and better creep resistance
### Heat Distortion Temperature, T~d~
- Associated with glass transition temperature
- Narrow range of temperature over which a stant specimen at a standard rate of heating get distorted to a fixed extent at given load
- More often used in polymer industry
### Effect of composition on T~g~
- Second component blended or added to form homogeneous mixture
- T~g~ of mixture will depend on
- T~g~ of each solution
- Amount (weight fraction) of each component
- Dependence is approximated by - Simple rule of mixture based on weight fraction
$\qquad T_g=W_1Tg_1+W_2Tg_2.....$
$\qquad\displaystyle T_g = \sum W_iT_{gi}$
T~g~ is T~g~ of blend or co polymer
W is Weight fraction of components
- T~g~ of polymer plasticized with low molecular weight additives given by Inverse Rule of Mixtures
$\dfrac{1}{T_{g1-2}} = \dfrac{W_1}{T_{g1}} + \dfrac{W_2}{T_{g2}}$ - Fox equation
- T~g~ decreases for low molecular weight polymers due to shorter chain length (more chain end)
- Less flexible chain
- Decreases specific volume
- Increases T~g~
- Increases Molecular weight
### Relation Between T~g~ and T~m~
$T_g = \dfrac 12 T_m$ - For Symmetrical polymer
$T_g = \dfrac 23 T_m$ - Unsymmetrical Polymer
### Effect of Molecular Weight on T~g~
Fox-Flory Equation
$\qquad T_g = T_g^\infty - \dfrac{K}{M_n}$
$T_g^\infty$ - Limiting Value of T~g~ at high molecular weight
K - constant for given Polymer
### Methods used to determine T~g~
1. Dialatometric
2. Differential Thermal Analysis
3. Heat Capacity
4. Modulus of Elasticity
5. Thermodynamical
#### Dialatometric Method
- Change in specif volume with temperature measured by dialatometer
#### Differential Thermal Analysis (DTA)
- $\Delta T$ - decreases at transition
- Sharp dip notices at T~m~
##### Differential Scanning Calorimetry
- When sample undergoes a physical transformation (phase transition) more or less heat will need to flow to it than the refernce
- To measure the amount of heat absorbed or released during transitions
- Measure physical changes as T~g~
- Curve rises linearly at low temperatures and steeply high temperatures
- Average Temperature where specific heat changes drastically considered as T~g~
#### Thermomechanical Method
- Amount of deformation vs Temperature
- Deformation Method
- As temperature increasesm strain increases with change demension
- Drastic change in deformation considered as T~g~
- Modulus of Elasticity method
- Modulus of elasticity vs temperature
- Modulus of elasticity depends uppon conidition or state of polymer
- An average temperature where drastic change in modulus considered as T~g~ of a polymer
#### Thermogravimetric Analysis
- Continous weighing of a sample as a fucntion of temperature
- Sample heated at a rate between 5° and 10°C/min, from room temperature to the desired temperature upton 1700°C
# Molecular Weight
- Molecular Weight - M - Mass of Mole of chain
- Sum of the atomic weights of all atoms making up a molecule
- Polymers can have various lengths depending on the number of repeat units
- During the polymerization process not all chains in a polymer grow to the same length, so there is a distribution of molecular weights due to random termination process
Molecular Weight Distribution
- Describes the relationship between the number of moles of each moles of each polymer species and the molar of hthat species
## Polymer Chain Length
- Many polymer properties are affected by the length of the polymer chains - Melting temperature increases with increasing molecular weight
- 100g/mol - Polymers - Very short chains - Liquid at RT
- 1000g/mol - Typically waxy solids amd soft resins
- Solid polymers range between 10000 and several million g/mol
- M affects the polymer's properties
- Elastic Modulus
- Strength
- The molecular weight affects the polymer's properties(examples:elastic modules & strength)
## Determination of molecular weight
- Absolute method
- Mass spectrometery
- Colligative Property
- Eng group analysis
- Light Scattering
- Ultracentrifugation
- Relative method
- solution Viscocity
- Fractionation method
- Gel Permeation Chromatography - GPC
## Average Molecular weight
- Number average Molecular Weight $(\bar M_n)$
$\qquad \bar M_n = \dfrac{\sum N_i\bar M_i}{\sum N_i}$
(colligative property and end group analysis)
- Weight average molecular Weight $(\bar M_w~)$
W~i~=N~i~M~i~---> weight of molecules
$\qquad\bar M_w = \dfrac{\sum N_i\bar M_i^2}{\sum N_i M_i}$
(light scattering)
- Z average molecular weight
$\qquad \bar M_Z = \dfrac{\sum N_i M^3}{\sum N_i M_i^2}$
(ul. racentrifugation)
- General equation of average molecular weight
$\qquad \bar M = \dfrac{\sum N_i M_i^{(a+1)}}{\sum N_i M_i^a}$
$a=0 \rightarrow M_n$
$a=1 \rightarrow M_w$
## Polydispersity Index: Width of Distribution
- Polydispersity index(PI)= $\bar M_w$/ $\bar M_n m \geq 1$
- PI varies from 1.02 to 20
- When PI approaches to 1 - lower limit - 1 for special polyer with narrow $\bar M_w$ distribution
- For typical polymer - greater than 2
$PDI= \dfrac {\bar M_w} {\bar M_n}$
## Gel Permeation Chromatography(GPC)
- Fractionation Method
- Size Exclusion Chromatography (SEC)
- GPC method is modified column chromatograpghy
- Packing material - Glass or silica bead provide porus surface
- Detector - UV, IR, Light scattering
- Pumping Polymer Solution
- Fraction Collector system for elution
- By using standard (monodisperse polystyrene) can obtain $\bar M_n$ and $\bar M_w$
Sepration based on retention time
- Greater Retention - Larger Molecules(porous particals)
- Lesser Retention - Smaller Molecules(porous particals)
- Large Molecules move through quicker and Small molecules move slowly
Applications:
- For purity analysis of synthetic and biological polymers
- To determine molecular weight $\bar M_n \,\&\, \bar M_w$
# Solution Viscosity
Rheology - Study of the flow/deformation of material both in liquid and solid states
- Marterials processability measured
- Material properties under Mechanical force
- For soild - elasticity and plasticity
- For fluid viscocity
## Viscosity
- Measure of the internal friction of a fluid or it is a measure of a fluid's tendency to resist flow
- Polymer viscosity provide information about monomer molecular weight, Molecular weight distribution and other characterization Parameters.
### Newton's Law of Viscosity
Laminar viscosity profile
For laminar flow, the force is given by
$\dfrac FA = \eta \dfrac vy$
The force per unit area is proportional to the velocity decrease in the distance y
Equation rewritten
$\tau_{yx} = -\eta\dfrac{dV_x}{dy}$
Sheer stress proportional to negetive of the local velocity gradient
- Velocity Gradient measure speed at which intermediate layer move each other
- Velocity gradient can be interpreted
$\dfrac{dv}{dy} = \dfrac d{dt}(\dfrac{dx}{dy}) = \dfrac{d\gamma}{dt}$
$\dfrac{d\gamma}{dt}$ - Strain rate
$\tau = -\eta\dfrac{d\gamma}{dt}$
Shreer stress proprtional to the strain rate
- Fluid that do not obey Newton's Law of viscosity called non newtonian fluids
- For non-Newtonian fluid viscoity is dependent on the strain rate
Types of non-Newtonian fluids
| Fluid Type | Behaviour |
| -------- | -------- |
| Newtonian Fluid | Viscosity independent of strain rate |
| Dilatant Fluid | Viscosity increases with strain rate |
| Pseudoplastic Fluid | Viscosity decreases with strain rate |
| Bingham | No flow upto yeild stress |
| Thixotropy | Viscosity decreases with time under constant strain rate |
| Rheopexy | Viscosity increases with time Under constant shear rate |
## Viscometery
- A relative Method
- IUPAC
- Relative Viscosity
$\qquad \eta_{rel.} = \dfrac{\eta}{\eta_0} = \dfrac{t}{t_0}$
$\eta$ - Solution Viscosity
$\eta_0$ - Solvent Viscosity
$t$ - Flow time of solution
$t_0$ - Flow time of solvent
- Specific Viscosity
$\eta_{spe} = \dfrac{\eta - \eta_0}{\eta_0} = \dfrac{t-t_0}{t_0} = \eta_{rel}-1$
- Reduced viscocity
$\eta_{red} = \dfrac{\eta_{spe}}{c} = \dfrac{\eta_{rel} - 1}c$
c - Concentration of solution
- Intrinsic Viscosity
$\eta_{int} = \underset {c \rightarrow 0}{\text{lim}}\dfrac{n_{red}}{c}$
### Molecular Weight form intrisic viscosity Using Mark-Houwink equation
Linear relation between $'\eta'$ and 'M' as
$\qquad [\eta] = k\bar M^a$
'k'and'a' - Viscosity
- Molecular Weight Constant
- Determined from the intercept and slope of plot between viscosity vs concentration of solution
$\bar M_w > \bar M_v> \bar M_n$
- $\bar M_v$ closer to $\bar M_w$ than $\bar M_n$
- $\bar M_v$ - Viscosity average molecular weight
- $\bar M_w$ -Weight average molecular weight
- $\bar M_n$ -Number average molecular weight
# Mechanical Properties of Polymers
- Tensile Strenth
- Polymers upto 100MPa
- Metals upton 4100MPa
- Elongation
- Often elongate plastically upto 1000%
- Metals - 100%
- Modulus
- Polymer - 7MPa - $4 \times 10^3$MPa
- Metals - 48MPa - $410 \times 10^3$MPa
- Temperature Dependence
- Mechanical Properties are very temperature dependent - even close to room temperature
- Strain Rate Dependence
- Same behaviour as raising temperature
Deformation Of Polymers
- Elastic Behavior
- covalent bonds with the vhain strech or distort, allowing the chains to elongate elastically
- Time dependent changes may contribute to some elastic behavior
- Plastic Behavior
- Viscous flow or sliding of the chains past one another under load, causes permanent deformation
## Deformation of an amorphous polymer
- During tensile testing necking is observed to begin on the gauge length
- Amorphous structures are straightened out and polymer becomes more crystalline
- Polymer is locally strengthened to resist further deforemation at that location
- A sheer stress $\tau$ causes the polymer chain to slide over another by viscous flow
- $\eta=\dfrac{\tau}{dv/dz}$
- Velocity gradient $\frac{dv}{dz}$, produces a dispacement of chain that depends upon viscosity of the polymers
- Viscosity describes the case with which the chain moves and cause deformation.
## Deformation in semi-crystalline polymers
- Crystalline lamellae separated by amorphous tie chains
- Lamellae slide and tie chains exten
- Further deformation tilts the folded chains in the lamellae break into smaller blocks
- Smaller crystalline blocks become aligned
Crazing - Localized plastic deformation in a polymer
Overall Process
- Localized plastic deformation
- Chain begins to disentangle
- Voids may be created along the most deformed plane, leading to crazing
## Polymer Creep and Temperature Effect
- Continued extension or deformation - constant load
- Strain as a function of time
- Viscoelastic flow in polymer
Creep in Polymers
- Occurs at low temperatures
- Strain does not reach constant value
Creep in Metals
- Creep occurs at high temperatures
- Strain each constant value
Degree of Creep depends on several factors
- Type of plastic
- Magnitude of Load
- Temperature
- Time
- Environment
- Reinforcement and filler
- Polymer additives
- Secondary phase or copolymers
- Heat Deflection temperature under low
- Creep Modulus
## Degradation of Polymers
- Uncontrolled change in molecular weight or constitution of the polymer
- Changes in polymer properties during service or process whether for good or bad purpose
Factors affecting
- Mechanical Stress
- Temperature or heat
- Moisture
- Solar Radiation
- Oxygen
- Environments
- Chemical Environment
### Ways of degradation
#### Chain End degradation
- Starts from chain end
- Reverse of propogation (depolymerisation)
$M_n^* \rightarrow M_{n-1}^* + M$
- PMMA heated at 300°C under vacuum
#### Random degradation
- Start at any random point
- Reverse of poly condensation
- Polester under hyrolytic degradation
#### Degradation in Linear Polymers due to backbone effect
- Chain Scission
- Random rupture in main chain
- UV rays, high energy radiation
- Depolymerisation
- Reduce molecular weight
- Cross-Linking
- Occurs due to high energy radiation
- Irradiated polyethylene - increased tensile strength
- Bond Changes
- Changes backbone structure
- Side Group changes
- Properties affected - Solubility, compatibility, mechanical stregnth
### Types of Degradation
- Thermal
- Mechanical
- Degradtion by Ultrasonic waves
- Degradation by UV light
- Degradation by high energy radiation
#### Thermal Degradation
- Rupter in main chain - due to heat
- Stability depends on stability of C-C bond
- Number of substituents or side group increases, thermal stability of backbone C-C bond decreases
#### Mechanical Degradtion
- Vulnerable part ruptured due to mechanical stress
- Occurs in many processes - Mastication, Milling, grinding, agitatio, extrusion, etc
## Polymer Additives
- A substance in added small amounts (small milate jao large banate jao) to other substances to improve certain properties strength or to alter it
- Widely used for thermoplastic thermosetting and elastomers
- Additives are Available in solid ,liquid or rubbery form
### Function of additives
- Facilitate processing without degradation or decomposition
- Cater requirement - application
- Improve - mechanical and physical properties
- Increase durability of polymer
### Properties of additives
- Stable under processing condition
- Efficient
- Compatible
- Non-toxic
- Cheap
### Types of additives
- Plasticizer and softner(available in liquid form)
- Filler and reinforcing agents(solid form)
- Stabilizers()
- Flame retardants
- Blowing agents
- Cross-linking agents
- Coloring agents
#### Plasticizer and Softner
- To increase Processability
- To reduce Modulus of elasticity by lowering T~g~
- PVC became flexible otherwise brittle
Methods of Plasticization
- Eternal Plasticization
- Polermer molecules support externally
- Internal Plasticization
- Polymer molecules become part of structure
Plasticizer Efficiency
- Actual reduction in T~g~ per concentration of plasticizer mol c le
#### Filler or Reinforcements
- To inprove tensile strength
- To improve abrasion resistance
- To improve toughness
- TO reduce cost
Examples
- Carnon Black
- Silica gel
- Wood Flour
- Glass Fiber
- Limestone
- Talc
:::info
In Tyres large amounts of carbon black needs to be added in order to reduce the elasticity
:::
Fibers of different type used for application
- E-glass/S-glass Fiber -Conventional Composites
- Boron/Graphite/Kevlar- Aerospace composites
- Microfibres/Wiskers (single crystal)- Highly demanding composite application for space shuttle
Properties of fibers Used in composite applications
#### Stabilizers
- Antioxidants
- To prevent oxidation due to impact of heat and atmospheric oxygen
- Hindered Phenol
- Benzofuranone
- UV absorber or protectants
- To prevent chain scission due to impact of UV light
- Oxanildes for Polyamide
- Benzotriazoles for Polycarbonate
#### Flame Retardants
- To form the protective layer on the surface of a materials
- To inhibit or delay the spread of fuire by suppressing chemical reactions in the flame
- Substances containing chlorine, fluorine and boron
#### Blowing/Foaming Agents
- To reduce weight or density
- To save raw material
- To produce cellular structure within th epolymer
- Hydrocarbon as isobutane and isopentane
- Invert gases as CO~2~ and Nitrogen
#### Lubricant
- To allow easier processing
- To reduce friction
- Sodium Sterate
#### Colorants
- To impart specific colour
- Fyes and Pigment
# Polymer Processing
Factors affecting Selection of process
- Quantity and Production rate
- Dimensional accuracy and surface finish
- Form and details of the product
- Nature of Materials
- Size of final products
Process consist of three phases
- Heating - To soften or melt the plastic
- Shaping/Forming - under constraint of some kind
- Cooling - retains its shape
Techniques/Processes for Thermoplastic Polymers
- Extrusion
- Blow Molding
- Injection Molding
- Thermoforming
- Calendaring
- Spinning
- Casting
Start as regular pellets or granules - suspension polymerization
Shaping is not accompanied by chemical
Techniques/Processes for Thermosetting Polymers
- Compression Molding
- Transfer Molding
- Reaction Injection Molding(RIM)
Start as liquid/syrups often called resins
Shaping is commpanied by chemical reaction
## Extrusion
- Mainly for Thermoplastic Poltmers
- Continous Process manufacturing
- Rods, sheets, pipes, film, etc
- Pelletized polymer added, compacted and melted
- Forcing soften polymer through a die with an opening
- Product formed
Advantages
- Part Cost Low
- Total Cost Low
- Production capability high
- Uniform cross section of part can be obtained
- Multiple Material are possible in th epart
Disadvantages
- Finishing or post fabricating assembly operation are often required
Application
- PVC Pipe
- Nylon 6,6 pipe used in motor vehicles
## Calendaring
- Molten plastic is poiured into sets of roll with small opening the rolls which squeeze out a thin sheet of the polymer
- Raw materialin the form of continuous strip or rod from an extruder
Advantages
- Less chance of thermal degradation
- The calender gives grater output rate
Disadvantages
- Calendars are not as versatile as extruder
- They are less suitable for short production
- They take long time to reach operating temperatures
## Spinning
- Molten thermoplastic polymer is forced through a die containing tiny holes
- The die called a spinnerate can rotate and produce yarns,filaments and fibers
## Compression Molding
- Low cost Molding as compared to injection and transfer Molding
- Molten plastic is squeezed directly into a mould cavity by the application of heat and pressure to conform to the shape of the mould
- The application of the heat and pressure increases the polymerisation process
- The application of the heat and pressure increases the polymerisation process
- Temperature of mold cavity - 130-200°C
- Hydraulic Pressuire - 7-25MPa
- Mold cavity is cooled and then opened and final product take out with the help of ejector pin
- Charge - Powde, granniles or as preformed
Factors affecting Compression Molding
1. Amount of plastic (charge)
2. Heating time and melting temperature of plastic
3. Pressure required to squeeze the material into mould cavity
4. Cooling time
Materials used
- Epoxies, urea formaldehyde phenolics, polyester, polyamide,
Before and after forming
Advantages
- Low initial setup cost and fast setup time
- Heavy Plastic parts can be molded
- Complex intricate parts can be made
- Good surface finish of the molded parts
- Molding process is cheaper
Disadvantages
- Low production rate
- Labour intensive process
- Limitation on mold depth
- Some times secondary processing (trimming, machining) required
Application
- Electronic equipments (circuit breakers, television, cabinets, radio cases, electric plugs and sockets)
- Cooking utensils, dinner plates
- Automotive parts
- Aircraft main power terminal housing pot handles
Proecss Paramters
- Heating time
- Melting temperature of the charge
- applied pressure
- cooling time
Material used
- Thermosets plastic - epoxy, polyesterm vinyl ester
Advantages
- Low Maintenance cost
- Plastic parts with metal inserts can be made design flexibility
- Large
Disadvantafes
- Wastage of material
- Production rate lower than injection molding
- Air can be trapped in the mold
Applications
- plugs
- connectors
- pins
- coils
- studs
- radio nad television cabbinets
- car and body shells
Acrylonitrile Butadiene styrene
- Structure - Amorphous
- Specific density 1.0 - 1.5 g/cm3
- Tensile Strength - 40-50Mpa
- Elongation - 27%
- Modulus - 2.4 GPa
- Water absorption - 0.05-1.8%
- Glass transition temp - 110C
- Proportions
- Acrylonitrile - 15-35%
- Butadiene - 5-30%
- Styrene - 40-50
- Number of grades of ABS
- High impact grade - Helmet
- Higher propertion of styrene
- Medium Impact Grade - Radiator Grills
- Electroplating grade - Auto-parts, TV/Radio Knobs
- High Flow grade - Plunger, hair dryer
- Extrusion Grade - Refrigerator Liners
- Heat Resistance grate - Electric cal heater
- Flame retardant grade - Parts of computer, printer and photocopier
Limitation
- Discolour - High UV
- Attacked by orgainc Solvent
- Hygroscopic - absorbs moisture
- Application
- Aircraft interior
- Comupter Housings
- Mass Transit components
- Wall coverings
- Appliances
- Automotive
Semi-Crystalline Commodity Plastics
- Low cost, stregth
Polyethylene at a glance
- Structure - Semi-Crystalline
- Chemical Structure - -[CH~2~ - CH~2~]-
- Discovered - 1900
- LDPE - Low Density Polyethylene - 1939
- HDPE - High Density Polyethylene - 1957
- LDPE - Synthesized using high pressure (1000-3000atm) and high temperature(250°C)
- HDPE - Synthesized using metal oxide catalyst at low pressure () 1000Atm and low temperature (60-100°C)
- Different types of PE - HDPE, LDPE, LLDPE, VLDPE, UHMWP
| Property | Branched Low Density | Medium Density | | High Density | Linear High Density |
| -------- | -------------------- | -------------- | --- | ------------ | ------------------- |
| Density | | | | | |
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