Question 1

Cross-linked vs. linear polymers: which of the following is a classic three-dimensional, cross-linked thermoset formed from phenol and formaldehyde?

Accepted Answer

Introduction: Polymer networks created by cross-linking form rigid, insoluble thermosets. Recognizing examples of such thermosets versus linear thermoplastics is fundamental in materials selection and processing. Given Data / Assumptions: Bakelite is produced by condensation of phenol with formaldehyde. Cross-linked structure prevents melting and dissolving. Thermoplastics deform on heating; thermosets do not. Concept / Approach: Phenol–formaldehyde systems cure into a three-dimensional network. PET, HDPE, Nylon-6, and PP are linear or lightly branched thermoplastics that soften with heat and can be reprocessed under controlled conditions. Step-by-Step Solution: Identify the thermoset: phenol–formaldehyde (Bakelite) → cross-linked network.Eliminate thermoplastics: PET, HDPE, Nylon-6, PP are not cross-linked in standard grades.Select Bakelite as the correct cross-linked example. Verification / Alternative check: Handbooks classify Bakelite among oldest commercial thermosets used in electricals and molded articles. Why Other Options Are Wrong: PET, HDPE, Nylon-6, PP: predominantly linear polymers; properties derive from chain packing, not network cross-links. Common Pitfalls: Assuming crystallinity equals cross-linking; crystallinity is ordered packing of linear chains, not covalent network formation. Final Answer: Bakelite (phenol–formaldehyde)

Question 2

Feedstocks for SBR (Buna-S) manufacture: which of the following starting materials are industrial precursors for the monomers used to produce SBR?

Accepted Answer

Introduction: SBR (Buna-S) is a copolymer of styrene and butadiene. The question probes understanding of upstream petrochemical routes that supply these monomers in different regions and eras (e.g., alcohol-to-butadiene processes and ethylene-based styrene chains). Given Data / Assumptions: SBR requires styrene + butadiene. Butadiene can be produced from ethanol (historical/alternative route) or from naphtha/steam cracker C4 cuts. Styrene is typically produced from ethylbenzene (benzene + ethylene). Concept / Approach: Industrial flexibility allows multiple monomer routes. Ethyl alcohol can be transformed to butadiene via catalytic dehydrogenation/dehydration sequences (legacy processes). Ethylene is a key feed for ethylbenzene synthesis, which dehydrogenates to styrene. Both feedstocks thus link to SBR monomer supply chains. Step-by-Step Solution: Map ethanol → butadiene: catalytic pathways yield the diene monomer.Map ethylene → ethylbenzene → styrene: alkylation of benzene followed by dehydrogenation.Since SBR needs both butadiene and styrene, and (a) and (b) each provide one, choose “Both (a) and (b)”. Verification / Alternative check: Chemical process texts describe the Lebedev and related processes (ethanol to butadiene) and standard ethylbenzene–styrene routes from ethylene. Why Other Options Are Wrong: Neither (a) nor (b): incorrect; both feedstocks connect to SBR monomers. Propylene only: relates more to acrylonitrile (ABS/SAN routes), not directly to SBR's primary monomers. Common Pitfalls: Assuming only modern steam-cracking routes count; older or alternative alcohol-based routes are valid and historically significant. Final Answer: Both (a) and (b)

Question 3

Formaldehyde-based resins: which polymer family belongs to the class of formaldehyde resins used for hard, cross-linked thermosets?

Accepted Answer

Introduction: Formaldehyde resins cover urea–formaldehyde, phenol–formaldehyde, and melamine–formaldehyde systems. They are classic thermosets used in laminates, molded parts, and surface finishes due to hardness, heat resistance, and dimensional stability. Given Data / Assumptions: We seek a resin whose chemistry explicitly reacts with formaldehyde to form a cross-linked network. Other options list common thermoplastics unrelated to formaldehyde chemistry. Concept / Approach: Melamine–formaldehyde resins result from condensation of melamine with formaldehyde and cure into hard, glossy thermosets. Teflon, PET, and polycarbonate are thermoplastics prepared by entirely different monomers and mechanisms. Step-by-Step Solution: Identify formaldehyde resin family member: melamine–formaldehyde → correct.Eliminate non-formaldehyde thermoplastics (PTFE, PET, PC).Select option (a). Verification / Alternative check: Materials references list MF resins as decorative laminates (e.g., table tops), dishes, and coatings with high hardness. Why Other Options Are Wrong: PTFE: fluoropolymer from tetrafluoroethylene; no formaldehyde. PET (Dacron): polyester from ethylene glycol + terephthalic acid. Polycarbonate: from bisphenol-A + phosgene or transesterification; not a formaldehyde resin. Common Pitfalls: Confusing “resin” as a generic term with formaldehyde-based thermoset chemistry; formaldehyde resins are a specific class. Final Answer: Melamine–formaldehyde resins

Question 4

Flooring materials: commercial “vinyl flooring” is typically laid using sheets of which polymer?

Accepted Answer

Introduction: “Vinyl flooring” is a ubiquitous term in construction and interior finishes. The question verifies the association of the name with the correct polymer used for resilient sheet and tile flooring products. Given Data / Assumptions: Vinyl flooring must be durable, water-resistant, and easily cleaned. Installation uses sheets or tiles with plasticizers and fillers. Concept / Approach: The “vinyl” in vinyl flooring refers to PVC, frequently plasticized for flexibility and processed with stabilizers, pigments, and mineral fillers. Other listed thermoplastics are not the standard base for such flooring sheets. Step-by-Step Solution: Identify the industry term: “vinyl” → PVC.Recall formulation: PVC + plasticizers + fillers → resilient sheets/tiles.Choose PVC as the correct polymer. Verification / Alternative check: Building product specifications and codes consistently denote PVC as the base for vinyl flooring products. Why Other Options Are Wrong: PP, LDPE, PVAc, PS: not the standard materials for sheet “vinyl” flooring; performance and processing do not match typical flooring requirements. Common Pitfalls: Assuming “vinyl” could mean any polymer with a vinyl group; in industry jargon it specifically denotes PVC. Final Answer: Polyvinyl chloride (PVC)

Question 5

Physical properties of polyethylene: the approximate density of high-density polyethylene (HDPE) is closest to which value (in g/cc at room temperature)?

Accepted Answer

Introduction: HDPE is a semicrystalline thermoplastic whose density exceeds that of LDPE due to higher crystallinity and linear chain structure. Recognizing the typical density range is useful in material identification and quality checks. Given Data / Assumptions: Room-temperature density values are considered. Typical HDPE density range is about 0.94–0.97 g/cc depending on grade. Concept / Approach: Higher crystallinity increases mass per unit volume. HDPE’s linear chains pack efficiently, producing densities around 0.95 g/cc, whereas LDPE is lower (~0.91–0.93 g/cc). Values near or above 1.0 g/cc are uncommon for polyolefins absent heavy fillers. Step-by-Step Solution: Recall HDPE density window: approximately 0.94–0.97 g/cc.Select the closest nominal value: 0.95 g/cc.Eliminate unrealistic options for neat HDPE (1.05, 1.18 are too high; 0.85 is too low). Verification / Alternative check: Material datasheets for general-purpose HDPE grades list densities around 0.955 g/cc (per ASTM D1505 methods), confirming the choice. Why Other Options Are Wrong: 1.18 and 1.05 g/cc: characteristic of filled polymers or other plastics (e.g., PET ~1.38), not neat HDPE. 0.99 g/cc: above typical HDPE unless heavily filled. 0.85 g/cc: too low for polyethylenes. Common Pitfalls: Confusing HDPE with PP (≈0.90–0.91 g/cc) or with filled/foamed grades that alter apparent density. Final Answer: 0.95 g/cc

Question 6

Identify Buna-S by material class: Buna-S (SBR) is best described as which type of material in engineering applications?

Accepted Answer

Introduction: Buna-S is the trade name historically used for styrene–butadiene rubber (SBR), an archetypal synthetic elastomer used in tires, belts, footwear, and general-purpose molded goods. The question checks correct material classification. Given Data / Assumptions: Buna-S is a copolymer of styrene and butadiene. Products require elasticity, resilience, and the ability to be vulcanized. Concept / Approach: Elastomers display large reversible deformations and require vulcanization (cross-linking) for useful strength. Buna-S fits this description. It is not a thermoplastic, thermoset resin, reinforcement fiber, or ceramic matrix. Step-by-Step Solution: Identify composition: styrene + butadiene → elastomeric copolymer.Relate to properties: high elasticity, good abrasion resistance after compounding and vulcanization.Select “Rubbery elastomer.” Verification / Alternative check: Materials databases and standards categorize SBR under synthetic rubbers; tire treads and general rubber goods commonly use SBR blends. Why Other Options Are Wrong: Fibrous/ceramic options are entirely different material classes. Thermoplastics soften and flow on heating; elastomers like SBR require vulcanization and do not behave as typical thermoplastics. Thermoset resins are rigid, cross-linked networks (e.g., phenolics), unlike vulcanized elastomers in behavior and use. Common Pitfalls: Confusing “rubber” with “plastic”; elastomers are a separate category characterized by entropy-driven elasticity and modest modulus. Final Answer: Rubbery elastomer

Question 7

Polymer science — definition of rayon (regenerated fiber) In industrial polymer chemistry, the term “rayon” refers to synthetic fibers produced by chemically regenerating which natural precursor?

Accepted Answer

Introduction / Context: Rayon is often called a “semi-synthetic” or “regenerated” fiber because it is made by chemically processing a natural polymer into a fiber-forming solution and then regenerating it as a filament. Understanding the parent material helps learners categorize fibers and predict properties such as moisture regain and dye affinity. Given Data / Assumptions: Rayon is produced industrially via processes such as viscose, cuprammonium, and lyocell. Options include common polymer families and biopolymers. We are asked to identify the original precursor substance. Concept / Approach: Cellulose is a natural polysaccharide found in wood pulp and cotton linters. In the viscose route, cellulose is derivatized (e.g., to cellulose xanthate) and then extruded and regenerated back to cellulose filaments. Lyocell dissolves cellulose in N-methylmorpholine N-oxide and regenerates it directly. Rayon retains cellulose’s hydrophilicity and comfort attributes, distinguishing it from fully synthetic fibers like polyamides or polyesters. Step-by-Step Solution: Identify the key term “regenerated” which implies a natural polymer is chemically dissolved and reformed.Recall that all rayon variants regenerate cellulose as the final fiber.Select “cellulose” as the correct precursor. Verification / Alternative check: Standard textile references classify rayon under regenerated cellulose fibers (viscose rayon, modal, lyocell). Its chemical repeat unit in the final fiber is anhydroglucose, identical to cellulose. Why Other Options Are Wrong: Lignin: abundant in wood but not the basis of rayon processes. Polyamides: fully synthetic (e.g., nylon), not regenerated cellulose. Ethylene glycol: monomer for PET polyester, unrelated to rayon. Common Pitfalls: Confusing “synthetic” with “regenerated.” Rayon begins with a natural polymer. Assuming all man-made fibers are petrochemical-based. Final Answer: cellulose

Question 8

Elastomers — elongation and flexibility of natural rubber Among common elastomers, natural rubber shows the widest practical elongation range. Its typical ultimate elongation is of the order of what percentage range?

Accepted Answer

Introduction / Context: Elastomers are characterized by large elastic (and viscoelastic) deformations before rupture. Natural rubber (NR) is notable for its high extensibility and resilience, which is central to applications like tires, vibration isolators, and flexible mounts. The question asks for the practical order of elongation in percent. Given Data / Assumptions: Natural rubber compounds typically show ultimate elongation in the hundreds of percent. Compounding (fillers, plasticizers, crosslink density) affects the exact value. We are choosing an order-of-magnitude range rather than a single exact number. Concept / Approach: Unfilled or lightly filled natural rubber can demonstrate ultimate elongation approaching roughly 700–900% under standard tensile tests. Filled and highly crosslinked compounds reduce elongation. Therefore, a broad “up to about 1000%” range fits widely cited practice and textbook data. Step-by-Step Solution: Recall typical NR properties: tensile strength with ultimate elongation often 700–800%.Choose the option that reasonably brackets this range.Select 1–1000% as the appropriate order-of-magnitude span. Verification / Alternative check: Materials handbooks list NR ultimate elongation commonly ~500–800% depending on formulation; a top-end figure near ~1000% is often cited for soft, lightly crosslinked compositions. Why Other Options Are Wrong: 1000–1500%, 1500–2000%, 2000–2500%: exceed practical values for NR; such extensions are atypical and beyond common reported data. Common Pitfalls: Confusing elongation at break with elastic (recoverable) stretch. Ignoring the effect of fillers and crosslink density on elongation. Final Answer: 1–1000

Question 9

Polymer classification — identifying a copolymer Which of the following materials is formed from more than one monomer species and is therefore categorized as a copolymer (condensation network)?

Accepted Answer

Introduction / Context: Polymers can be homopolymers (single monomer) or copolymers (two or more different monomers). Some well-known materials are textbook examples of each category. Recognizing these helps predict properties and processing behavior. Given Data / Assumptions: PVC (polyvinyl chloride), polythene (polyethylene), and Teflon (PTFE) are common homopolymers. Bakelite is manufactured by condensation of phenol and formaldehyde. Concept / Approach: Homopolymers: PVC (from vinyl chloride), PE (from ethylene), PTFE (from tetrafluoroethylene) each derive from one monomer type. Bakelite is a thermosetting network formed by reacting two different monomers, making it a classic copolymer via condensation. Step-by-Step Solution: List the monomer(s) for each polymer.Identify whether single or multiple monomers are involved.Select Bakelite as the condensation copolymer (phenol + formaldehyde). Verification / Alternative check: Textbook classifications place Bakelite among phenol-formaldehyde resins, a two-component system forming a crosslinked network. Why Other Options Are Wrong: PVC, polythene, Teflon: homopolymers; one monomer each. Common Pitfalls: Assuming “network” implies single-monomer polymerization; here two monomers form the network. Final Answer: Bakelite

Question 10

Trade names in elastomers — identifying Buna-S Buna-S rubber is widely used in tires and footwear. By which general polymer family name is Buna-S also known?

Accepted Answer

Introduction / Context: Commercial names and abbreviations often refer to the same polymer family. Buna-S is a historical trade term, while SBR is the standard materials designation. Knowing equivalences avoids confusion in specifications and purchasing. Given Data / Assumptions: Buna-S = styrene-butadiene rubber. SBR is the accepted abbreviation in polymer engineering. Concept / Approach: SBR is a random copolymer of styrene and butadiene. It is distinct from PTFE (Teflon) and polyacrylates. The question tests recognition of synonymous naming. Step-by-Step Solution: Recall that “Buna” designations historically refer to butadiene-based rubbers.Map Buna-S → styrene-butadiene → SBR.Select SBR as the correct answer. Verification / Alternative check: Standards (ASTM D2000 family) and materials catalogs list SBR as the generic for Buna-S. Why Other Options Are Wrong: Teflon/PTFE: fluoropolymer, not an elastomer. Polyacrylates: different acrylic rubber class. Common Pitfalls: Confusing historical trade names with modern abbreviations. Final Answer: SBR

Question 11

Ion-exchange resin matrices: commercial cation/anion exchange resins are typically based on which polymer backbone?

Accepted Answer

Introduction / Context: Ion-exchange resins are widely used in water treatment, deionisation, and catalysis. The polymer backbone must be chemically robust and allow introduction of ionic functional groups. Recognising the standard matrix helps in predicting resin stability and performance. Given Data / Assumptions: Most commercial resins use a styrene–divinylbenzene (DVB) copolymer matrix. Cation exchangers are typically sulphonated; anion exchangers are amine-functionalised. Backbone must withstand aqueous environments, pH cycling, and thermal loads. Concept / Approach: Polystyrene crosslinked with DVB provides a rigid, porous network that can be functionalised. Alternative backbones exist (e.g., acrylic resins) but classical, general-purpose resins are styrenic. Bakelite (phenol-formaldehyde) is thermoset and brittle; PTFE (Teflon) is chemically inert but difficult to functionalise for ion exchange and lacks the typical porous network needed. Lucite (PMMA) is transparent acrylic, not the mainstream ion-exchange matrix. Step-by-Step Solution: Identify the standard commercial choice: polystyrene–DVB.Note functionalisation methods: sulphonation (cation), amination/quaternisation (anion).Exclude alternatives that are not typical ion-exchange matrices. Verification / Alternative check: Manufacturer datasheets and handbooks repeatedly specify styrene–DVB for strong-acid/strong-base resins. Why Other Options Are Wrong: PMMA, bakelite, PTFE: not the mainstream, general-purpose ion-exchange resin backbones. Common Pitfalls: Confusing the functional group (e.g., sulphonated) with the backbone polymer; the backbone is styrene–DVB. Final Answer: Polystyrene (crosslinked with divinylbenzene)

Question 12

Vinyl chloride–vinyl acetate copolymer: the resulting material is generally described mechanically as which type of polymer?

Accepted Answer

Introduction / Context: Copolymers of vinyl chloride with vinyl acetate modify the properties of PVC, improving flexibility and impact behaviour compared with rigid homopolymer PVC. The question asks for the general qualitative description used for such a copolymer. Given Data / Assumptions: Vinyl acetate comonomer disrupts PVC crystallinity and lowers glass transition temperature. Resulting material becomes tougher and more flexible than rigid PVC. Terminology is qualitative, reflecting feel and flexibility relative to rigid plastics and elastomers. Concept / Approach: With sufficient vinyl acetate, the PVC matrix softens and exhibits a pliable, flexible feel described in classical exam terminology as “leathery” — softer than rigid plastics but not fully rubbery/elastomeric without plasticiser levels typical of rubbers. “Fibrous” is inappropriate (that refers to materials drawn into fibres), and “hard” matches unmodified, rigid PVC rather than the copolymerised product. Step-by-Step Solution: Relate comonomer effect (VAC) to flexibility and toughness.Map qualitative categories: rigid < leathery < rubbery.Select “Leathery” to describe VC–VAC copolymer behaviour. Verification / Alternative check: Materials references describe VC–VAC copolymers and plasticised PVC products as leathery/semiflexible depending on formulation. Why Other Options Are Wrong: Fibrous: relates to drawability, not bulk mechanical feel.Rubbery: requires higher elasticity than typical VC–VAC without heavy plasticisation.Hard: closer to rigid PVC than to the copolymer’s softened state. Common Pitfalls: Assuming all VC-based materials are rigid; copolymerisation and plasticisers change mechanical character substantially. Final Answer: Leathery

Question 13

Polymethyl methacrylate (PMMA), an acrylic resin widely used as a glass substitute, is commonly called what in trade names?

Accepted Answer

Introduction / Context: Trade names often persist in engineering practice. Polymethyl methacrylate (PMMA) is a transparent thermoplastic known for its clarity and weather resistance and is frequently used as a lightweight alternative to glass. Given Data / Assumptions: PMMA is marketed under several registered names. Engineers should recognise common synonyms to avoid procurement confusion. Concept / Approach: The best-known trade names for PMMA are “Plexiglass” and “Lucite.” These denote the same base material: polymethyl methacrylate. Other names in the options refer to different polymers: Thiokol is polysulfide rubber, Dacron is polyethylene terephthalate fibre, and Teflon is polytetrafluoroethylene (PTFE). Step-by-Step Solution: Identify PMMA’s commercial aliases.Eliminate unrelated polymers by trade name.Select “Plexiglass or Lucite.” Verification / Alternative check: Materials handbooks and vendor catalogs consistently equate PMMA with Plexiglass/Lucite. Why Other Options Are Wrong: Thiokol: polysulfide; Dacron: saturated polyester (PET); Teflon: PTFE — none is PMMA. Common Pitfalls: Confusing PMMA with polycarbonate (PC); PC is tougher and more impact resistant but has different chemical resistance and trade names (e.g., Lexan). Final Answer: Plexiglass or Lucite

Question 14

Perspex (used as a glass substitute) is made from which monomer? Identify the correct monomer for polymethyl methacrylate (PMMA).

Accepted Answer

Introduction / Context: Perspex is a well-known trade name for polymethyl methacrylate (PMMA), a transparent thermoplastic valued for high optical clarity, weatherability, and lower density than glass. Recognising the correct monomer for PMMA is basic polymer chemistry knowledge useful in materials selection and processing. Given Data / Assumptions: Perspex and Lucite are trade names for PMMA. We must match PMMA to its monomer. Distractor monomers correspond to other polymers (e.g., PET, PTFE, synthetic rubbers). Concept / Approach: PMMA results from addition polymerisation of methyl methacrylate (MMA). Each MMA unit adds across the carbon–carbon double bond forming the long PMMA chain. In contrast, DMT is a precursor for polyester (PET). Butadiene is a diene used for elastomers like polybutadiene or SBR. Tetrafluoroethylene is the monomer for PTFE (Teflon). Step-by-Step Solution: Map “Perspex” → PMMA.Identify PMMA monomer → methyl methacrylate (MMA).Eliminate DMT (for PET), butadiene (rubbers), tetrafluoroethylene (PTFE). Verification / Alternative check: Standard materials handbooks list PMMA as the polymer of methyl methacrylate, produced via bulk, solution, or suspension polymerisation routes. Why Other Options Are Wrong: DMT: used with ethylene glycol to make PET, not PMMA.Butadiene: elastomer precursor, not PMMA.Tetrafluoroethylene: gives PTFE, not PMMA. Common Pitfalls: Confusing trade names (Perspex/Lucite) and mixing them with polycarbonate (Lexan), which is a different polymer with different monomers and properties. Final Answer: Methyl methacrylate

Question 15

Compare low-density polyethylene (LDPE) with high-density polyethylene (HDPE): which property correctly describes LDPE relative to HDPE?

Accepted Answer

Introduction / Context: Polyethylene exists in several forms, notably LDPE and HDPE, which differ in branching, crystallinity, density, and mechanical/thermal properties. Understanding these differences guides material selection for films, containers, pipes, and packaging. Given Data / Assumptions: LDPE has more chain branching and lower crystallinity than HDPE. HDPE is denser, stiffer, and has higher softening point. Both are chemically inert polyolefins under normal conditions. Concept / Approach: Increased branching in LDPE reduces chain packing efficiency, lowering crystallinity and density. This leads to reduced stiffness and hardness but improved flexibility and clarity. HDPE, with minimal branching, packs more efficiently, producing a stiffer, harder, and higher-temperature-tolerant material. Step-by-Step Solution: Relate structure (branching) → properties (crystallinity, density).LDPE: lower density and crystallinity → more flexible.HDPE: higher density and crystallinity → harder, stiffer. Verification / Alternative check: Datasheets show LDPE tensile modulus lower than HDPE, confirming greater flexibility. Vicat softening and melting points are lower for LDPE as well. Why Other Options Are Wrong: Harder/Tougher: these describe HDPE more than LDPE for most grades.Chemically more inert: both exhibit excellent chemical resistance; not a distinguishing advantage for LDPE. Common Pitfalls: Equating “toughness” with “flexibility”; HDPE may have higher impact toughness while still being less flexible than LDPE. Final Answer: More flexible

Question 16

Which polymer type is commonly stretched into fibres for textiles and industrial yarns?

Accepted Answer

Introduction / Context: Many thermoplastics can be melt-spun or solution-spun into fibres. Understanding which polymer families are fibre-forming is fundamental in polymer engineering and textile technology. Given Data / Assumptions: Saturated polyesters (e.g., polyethylene terephthalate, PET) are major fibre-formers (Dacron/Terylene). Unsaturated polyesters are thermosetting resins used for composites, not typical fibres. Isoprene forms elastomers (polyisoprene) rather than structural fibres. Bakelite (phenol-formaldehyde) is a thermoset, not spinnable into fibres. Concept / Approach: Fibre-forming polymers require adequate molecular weight, chain rigidity, and ability to orient and crystallise upon drawing. PET meets these conditions, giving high-strength, high-modulus filaments. Thermosetting resins cannot be melt-spun due to irreversible crosslinking. Elastomers do not typically yield strong drawn fibres because of their low glass transition and high elasticity without crystallisable structure under draw. Step-by-Step Solution: Identify fibre-forming candidates: saturated polyester (PET).Eliminate thermosets and elastomers (unsaturated polyester, Bakelite, isoprene).Choose saturated polyester for textile fibres. Verification / Alternative check: Global fibre production statistics consistently list polyester (PET) as the dominant synthetic textile fibre. Why Other Options Are Wrong: Unsaturated polyester: crosslinking for composites; not spinnable.Isoprene: elastomeric; not a structural fibre.Bakelite: thermoset; cannot be drawn into fibres. Common Pitfalls: Confusing “polyester” as a single type; only saturated aromatic polyesters like PET are fibre-forming mainstream materials. Final Answer: Saturated polyester

Question 17

General properties of polymers: pick the correct statement about conductivity, crystallinity, vaporisation, and changes with increasing degree of polymerisation.

Accepted Answer

Introduction / Context: Polymer properties evolve with molecular weight and microstructure. Recognising qualitative trends is essential for processing and application choices in plastics, rubbers, and fibres. Given Data / Assumptions: Most common polymers are thermal and electrical insulators. Polymers may be amorphous, semicrystalline, or (rarely) highly crystalline. On extreme heating, polymers typically degrade thermally rather than vaporise cleanly. Concept / Approach: Increasing the degree of polymerisation (DP) raises molecular weight, viscosity, softening point, and mechanical strength. A typical qualitative sequence for the same chemistry is liquid → greasy/oily → waxy → rubbery/solid as chain length grows. Conductivity is generally low for organic polymers. Crystallinity depends on tacticity, branching, and chain regularity; many important polymers are only semicrystalline. Thermal decomposition precedes true vaporisation for most polymers due to covalent backbone scission. Step-by-Step Solution: Evaluate (a): False — polymers are usually poor conductors.Evaluate (b): False — many are amorphous or semicrystalline.Evaluate (c): False — decomposition occurs before clean vaporisation.Evaluate (d): True — describes the qualitative change with increasing DP. Verification / Alternative check: Polymer synthesis texts describe viscosity and softening increases with molecular weight; processing windows reflect this behaviour. Why Other Options Are Wrong: (a) and (b) contradict broad polymer behaviour; (c) ignores thermal degradation. Common Pitfalls: Confusing crystallinity with orientation; drawing may increase orientation without drastically changing crystallinity in some cases. Final Answer: A liquid polymer becomes greasy, then waxy, and finally solid as the degree of polymerisation increases.

Question 18

Viscosity trends for polymer solutions and melts: which statement is correct regarding the effects of molecular weight and temperature?

Accepted Answer

Introduction / Context: Processing polymers requires understanding how viscosity responds to key variables. Melt extrusion, injection moulding, and solution spinning rely on predictable rheology as functions of temperature and molecular weight. Given Data / Assumptions: Molecular weight strongly influences zero-shear viscosity of melts and inherent viscosity of solutions. Temperature lowers viscosity for most polymer melts and solutions within non-degraded ranges. Concept / Approach: For polymer melts, zero-shear viscosity scales roughly as MW^a (a ≈ 3.4 above entanglement), so increasing molecular weight raises viscosity dramatically. Temperature provides thermal energy for chain mobility, thus decreasing viscosity (Arrhenius or WLF dependence, depending on reference temperature relative to glass transition). Step-by-Step Solution: Check MW effect: higher MW → higher viscosity (so (a) is false).Check temperature effect: higher T → lower viscosity (so (b) is true).Options (c) and (d) contradict general rheological behaviour. Verification / Alternative check: Capillary rheometry and DSC-supported processing windows consistently show decreased viscosity at elevated temperatures until degradation begins. Why Other Options Are Wrong: (a) opposes established scaling; (c) and (d) ignore temperature dependence of polymer mobility. Common Pitfalls: Overheating can induce degradation, increasing apparent viscosity irregularly or causing gelation; always stay within safe thermal windows. Final Answer: Viscosity decreases with increase in temperature.

Question 19

Identify the addition polymer among the following common engineering plastics.

Accepted Answer

Introduction / Context: Polymerisation mechanisms are broadly classified as addition (chain-growth) and condensation (step-growth). Recognising examples of each helps predict byproducts, processing routes, and typical properties. Given Data / Assumptions: Polyethylene forms via addition polymerisation of ethylene. Nylons (polyamides) form via condensation with elimination of small molecules (e.g., water). Bakelite is a condensation thermoset (phenol + formaldehyde). Concept / Approach: Addition polymerisation involves repeated addition across unsaturated bonds without byproduct formation. Polyethylene is the archetypal addition polymer formed from ethylene via radical, Ziegler–Natta, or metallocene catalysis. Condensation polymers eliminate small molecules during chain formation, as with polyamides and phenolic resins. Step-by-Step Solution: Match each polymer to mechanism: PE → addition; Nylon, Bakelite → condensation.Select polyethylene as the sole addition polymer listed. Verification / Alternative check: Introductory polymer chemistry texts classify PE, PP, PS, and PVC as addition polymers. Why Other Options Are Wrong: Nylon: condensation via diacid/diamine or lactam ring opening with water byproduct.Bakelite: condensation thermoset; crosslinked network. Common Pitfalls: Assuming ring-opening polymerisations are always addition; nylon-6 from caprolactam is often classed as step-growth-like with water participation. Final Answer: Polythene (polyethylene)

Question 20

Cold SBR (styrene–butadiene rubber) emulsion polymerisation: typical operating temperature and gage pressure are approximately what?

Accepted Answer

Introduction / Context: SBR is produced by emulsion polymerisation of styrene and butadiene. Two major variants exist: hot SBR (~50°C) and cold SBR (~5°C). Operating conditions affect molecular weight distribution, branching, and final properties like abrasion resistance and tensile strength. Given Data / Assumptions: “Cold” SBR is polymerised at low temperature near 5°C. Gage pressures are modest for emulsion reactors; 1 kgf/cm2 is close to atmospheric with small overpressure. Negative temperatures like -20°C are atypical for standard cold SBR production. Concept / Approach: Lower temperature in emulsion polymerisation reduces chain transfer and branching, giving improved mechanical properties versus hot SBR. Reactor pressure mainly maintains phase equilibrium and containment; high pressure is unnecessary in standard aqueous emulsions for SBR. Step-by-Step Solution: Identify cold SBR temperature setpoint ≈ 5°C.Confirm modest pressure near 1 kgf/cm2.Eliminate options with extreme sub-zero temperature or higher pressure not typical for cold SBR. Verification / Alternative check: Industrial references differentiate hot (≈50°C) and cold (≈5°C) SBR recipes, with cold SBR yielding better abrasion resistance for tyre treads. Why Other Options Are Wrong: -20°C: not standard; increases cost/complexity with limited benefit.0°C and 1 or 3 kgf/cm2: less characteristic than 5°C at near-atmospheric pressure. Common Pitfalls: Confusing bulk solution polymerisation conditions with emulsion conditions; SBR mainstream production is emulsion-based. Final Answer: 5°C and 1 kgf/cm2

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