Polymer Science and Engineering Practice Flashcards

Flashcards covering the fundamentals of polymer science, including molecular structure, weight calculations, classification of thermoplastics and thermosets, crystallinity, mechanical behavior, and polymerization processes.

What is a polymer based on the lecture notes?

A polymer is a long chain of intertwined molecular bands built from repeated units called 'mers'.

What is the difference between the bonding from mer-to-mer and chain-to-chain?

Bonding between mer-to-mer is covalent bonding (sharing of electrons), while bonding between chain-to-chain is secondary bonding (intermolecular forces).

What is the difference between saturated and unsaturated hydrocarbon molecules?

Saturated molecules consist of singular bonds, while unsaturated molecules contain double or triple bonds ($H-C=C-H$).

How does molecular weight affect the properties of a polymer?

As the molecule size (molecular weight) increases, secondary bonding energy, boiling points, and melting points also increase.

What are the formulas for Number average ($M_n$) and Weight average ($M_w$) molecular weight?

$M_n = \sum x_i M_i$ and $M_w = \sum w_i M_i$, where $x_i$ is the fraction of total number of chains, $w_i$ is the weight fraction, and $M_i$ is the mean molecular weight.

What is the Polydispersity Index (PDI) and what does it measure?

$PDI = \frac{M_w}{M_n}$. It is a measure of the variation in molecular size within a polymer; if $PDI = 1$, the polymer is considered 'Monodisperted'.

How is the Degree of Polymerisation (DP) calculated?

$DP = \frac{M_n}{m}$, where $m$ is the repeated unit molecular weight.

What are the four main molecular structures of polymers?

1. Linear, 2. Branched, 3. Crosslinked, 4. Network.

Describe the characteristics of Linear polymers and provide examples.

Linear polymers consist of mers joined end-to-end in long flexible chains with secondary bonding between chains. Examples include PE, PVC, and Nylon.

How do Crosslinked polymers differ from Branched polymers?

Branched polymers have side branches like trees, whereas Crosslinked polymers have side chains that link to other chains via covalent bonding, often seen in Thermosetting polymers like vulcanized rubber.

What defines a Network polymer structure?

Network polymers are 3D structures where mers are tri-functional (connecting with 3 mers), creating a highly crosslinked and rigid material like Epoxies or PU.

What is the difference between Head-to-Tail and Head-to-Head molecular configurations?

In Head-to-Tail, the 'R' group connects to the head of each unit in the same direction. In Head-to-Head, 'R' groups connect directly to other 'R' groups, which is difficult to obtain due to electrostatic charge.

Define the three types of Stereoisomers based on 'tactic' arrangement.

1. Isotactic: all side groups on the same side. 2. Syndiotactic: side groups on alternating sides. 3. Atactic: side groups in random positions.

What is the difference between Cis and Trans geometric isomers?

In Cis isomers, functional groups are on the same side of the chain ($CH_3$ groups on the same side), while in Trans isomers, they are on opposite sides.

What are the primary characteristics of Thermoplastics?

Thermoplastics can be softened and reshaped when reheated because their secondary bonds are weak and break with heat. They typically have linear or branched structures (e.g., PE, PVC, PET, PS).

What are the primary characteristics of Thermosetting polymers?

Thermosetting polymers do not soften or melt upon reheating due to permanent covalent crosslinks. they have better dimensional stability and include vulcanised rubber, epoxies, and phenolics.

Name the four types of Copolymers.

1. Random, 2. Alternating ($A-B-A-B$ pattern), 3. Block (blocks of one monomer), 4. Graft (chains of different monomers attached together).

What factors influence polymer crystallinity?

1. Chain structure (linear crystallizes easier), 2. Cooling rate (slower rate allows more time for arrangement), 3. Side groups (Isotactic and smaller groups pack better), 4. Secondary bonding strength.

What are spherulites in semicrystalline polymers?

Spherulites are structures consisting of numerous chain-folded lamellae that radiate outward from a centre, separated by amorphous material.

What is the formula for percentage crystallinity of a polymer?

$\% \text{Crystallinity} = \frac{\rho_c (\rho_s - \rho_a)}{\rho_s (\rho_c - \rho_a)} \times 100$, where $\rho_c$ is crystalline density, $\rho_a$ is amorphous density, and $\rho_s$ is specimen density.

Describe the three states of viscoelastic behavior in amorphous polymers.

1. Elastic (immediate recovery, below $T_g$), 2. Viscous (liquid-like damper behavior, non-reversible), 3. Viscoelastic (both viscous and elastic, occurs above $T_g$).

How is the Relaxation Modulus ($E_r(t)$) defined?

$E_r(t) = \frac{\sigma(t)}{\epsilon_0}$, where $\sigma(t)$ is time-dependent stress and $\epsilon_0$ is the constant strain level.

What is 'Crazing' in the context of polymer fracture?

Crazing is a localized plastic deformation that occurs during tension, creating microvoids and fibrillar bridges that absorb energy before the polymer fractures.

What are the three stages of Addition Polymerisation (Chain Reaction)?

1. Initiation (formation of an active centre with a catalyst), 2. Propagation (linear growth of the chain as monomers react with the active centre), 3. Termination (two chains join or react to become stable).

How does Condensation Polymerisation (Step Reaction) differ from Addition Polymerisation?

Condensation Polymerisation involves intermolecular chemical reactions between different monomers that result in the polymer chain plus a small molecular weight byproduct like $H_2O$ or $HCl$.

What is Vulcanisation in elastomers?

Vulcanisation is a chemical process where Sulfur is added to an elastomer at elevated temperatures to create covalent crosslinks, increasing strength and stiffness.

What are common polymer additives and their functions?

1. Fillers (improve strength and stability), 2. Plasticizers (increase flexibility and lower $T_g$), 3. Stabilizers (counteract UV or oxidation), 4. Colourants (dyes/pigments), 5. Flame-Retardants.