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PS534 > Polymers, Fibres and Analysis > Flashcards

Polymers, Fibres and Analysis Flashcards

1
Q

What is a Polymer?

A

A polymer is a long chain molecule which is constructed from many like-structured molecules called monomers covalently bonded together in any conceivable pattern.

  • Linear
  • Branched
  • Cross-linked
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2
Q

What is a monosaccharide?

A
  • A monosaccharide is a simple sugar and is the basic unit of carbohydrates.
  • Building blocks of disaccharides
    • Lactose, sucrose and maltose
  • Held together through glycosidic linkages.
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3
Q

What are polysaccharides?

A
  • Polymeric chains of carbohydrate molecules, made of chains of monosaccharides.
    • e.g. cellulose, chitin and starch.
  • Held together by glycosidic linkages; a covalent bond that joins a carbohydrate molecule to another group.
    • Forms between hemiacetal carbon and hydroxyl of the adjacent molecule.
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4
Q

What are the different forms of cellulose found in the plant cell wall?

A
  • Hemicellulose
  • Paracrystalline cellulose
  • Crystalline cellulose
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5
Q

What is a semisynthetic polymer?

A

Derived from naturally occurring polymers by chemical modifications.

E.g. Rayon, Cellulose, Vulcanised Rubber and Gun cotton

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6
Q

What are Rayon and Cellophane?

A
  • Produced from cellulose by chemically treating the polysaccharide.
    • Strong base (NaOH or KOH)
    • Carbon disulfide (CS2)
    • Followed by H2SO4
  • Different processing converts to rayon or cellophane; chemically very similar.
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7
Q

What is guncotton?

A

Guncotton or nitrocellulose was originally intended as an accelerant for explosives.

Was also used for nitrate film, although was found to be too flammable and was difficult to control and handle safely.

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8
Q

What is a Synthetic Polymer?

A
  • A chain of linked monomer units.
    • Monomers may be all the same (homopolymers) or different (copolymer)
  • Have many different types of linkages.
  • May be small or complex.
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9
Q

What is the reactivity of monomers?

A

Monomers may have complimentary reactivity (react with themselves to form dimers) or react with another different polymer to perpetuate chain growth.

  • Usually, require an initiator.
  • Depending on monomer functionality, can get linear or branched structures.

Examples include PTFE and PVC.

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10
Q

What are thermal properties of polymers?

A

Thermal behaviour of polymers important for manufacturing and use, as well as forensic analysis.

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11
Q

What are thermoplastics?

A
  • Thermoplastics soften when heated.
  • No chemical changes occur during curing.
  • Are remoldable once heated.
  • Are more flexible; amorphous regions facilitate molecular motion.
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12
Q

What are thermoset plastics?

A
  • Burn when heated
  • Crosslinking occurs during curing - more than in elastomers
  • Cannot be remolded
  • Less flexible; chains linked together inhabits molecular motion.
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13
Q

What is the relationship between Tg and Tm?

A

Tg = Glass transition temperature; can be quite gradual.

Tm = Glass melting temperature; is generally quite sharp.

Generally:

  • If above Tg and Tm then viscous liquid
  • If above Tg but below Tm then rubbery solid
  • If below Tg and Tm then solid.
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14
Q

What is Lignin?

A
  • A class of cross-linked phenolic polymers
  • Very high rigidity - gives wood its structural properties.
  • Very heterogeneous - poorly defined primary structure.
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15
Q

What is cellulose?

A

Cellulose is a polysaccharide made entirely of glucose monomers.

  • Crystalline, strong, resistant to hydrolysis
  • Well defined, unbranched, long chain polymers
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16
Q

What is hemicellulose?

A
  • Polysaccharide made of many different sugar monomers.
  • Random, amorphous structure, little strength.
  • Poorly defined, branched, short chain (~500-3000 monomers)
17
Q

What is paper?

A
  • Damp cellulose fibres pressed together and dried.
    • Individual fibres c.a. 10 micrometres wide.
  • Also include fillers, additives and coatings.
    • Improve properties for specific applications
18
Q

What are the 5 steps of paper production?

A
  1. Wood is converted into woodchip.
  2. Raw pulp is created from the wood material by breaking down lignin and hemicellulose using conc. NaOH and Na2S
  3. Bleaching; remaining wood pulp is bleached to make white paper using ClO2 or H2O2
  4. Beating: Refines the wood pulp, passing it through spinning disks which allows for introduction of more water to break H bonds. Allows for introduction of fillers and additives
19
Q

How does colour and roughness of paper affect light reflectance?

A
  • White paper reflects 90% of light and absorbs 10%.
  • Black paper reflects 10% and absorbs 90% of light.
  • Glossy paper reflects light at an equal angle to the angle of incidence.
  • Matt paper reflects light in a multitude of different directions.
20
Q

How is the roughness of paper quantified?

A
  • Bendtsen roughness measures lateral air leakage; how long does it take air to escape from the gap between paper surface and a smooth metal disc?
  • Bekk smoothness; similar but air is drawn across surface under a partial vacuum.
  • Profilometry: A diamond moves across surface of paper and computer measures vertical movement of the probe caused by surface features.
21
Q

What is gloss?

A

Gloss is an optical phenomenon that is relative to shininess.

No strict definition and cannot compare absolute values reliably.

22
Q

What are the 5 different analytical techniques for fibre analysis?

A
  • Microscopy: Powerful, distinguishes fibre types, can identify wood types from paper fibres
  • AFM: Very high resolution, can analyse mechanical properties as well as imaging
  • IR spectroscopy - measures organic content of paper
  • ICP-MS: Detects metals and non-metals at trace conc.; treat samples with H2O2 and H2SO4 and digested in microwave.
    • LA-ICP-MS uses laser ablation to make sample delivery easier. Is destructive.
23
Q

What information can be obtained from synthetic polymers?

A
  • When a raw polymer is shaped, molded or drawn into fibres, property characteristics are added/altered
  • These new characteristics/properties may play an important role in forensic investigations.
  • Above and beyond any chemical analysis is due to more comparative and matchable characteristics.
24
Q

Why is GCMS a useful analytical technique?

A

Excellent at identifying a matchable trace impurity

Breakdown of polymer monomers for chromatographic analysis will be limited due to commercial abundance.

25
Q

Why is optical microscopy a useful technique in fibre analysis?

A

It is the most powerful tool in fibre analysis/comparisons.

  • Polarised Light Microscopy
  • Fluorescence Microscopy (emission analysis)
  • Hot-stage Microscopy (melting analysis).
26
Q

What are the strengths and weaknesses of the paper?

A
  • Huge variety of applications.
  • Is mass manufactured - minimal batch-2-batch variation.
  • In-depth analysis is very difficult; individualisation is difficult.

PS534 (3 decks)

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