MATS3004 · Polymer Science and Engineering 1
Spectroscopic and Structural Characterization
Week 4's spectroscopy lecture plus the four lab techniques form the identification toolkit the Group Lab Report is built on: NMR and FTIR for chemical structure, Raman as the complementary vibrational probe, XRD for morphology and DSC for thermal transitions. The examinable skill is reading a spectrum — assigning FTIR bands to functional groups — and knowing which complementary technique to reach for, because the lab requires combining all four to identify an unknown sample.
What this chapter covers
- 01FTIR: IR absorption by bond vibrations; the spectrum is a functional-group fingerprint
- 02Characteristic FTIR bands: O-H 3200-3600, N-H 3300-3500, C-H ~2850-3030, C≡N ~2250, C=O 1700-1750, aromatic C=C ~1600/1500, C-O 1000-1300, C-Cl 600-800 cm⁻¹
- 03Raman: inelastic light scattering, complementary to IR (symmetric/non-polar vibrations like C=C, C-C, S-S are strong)
- 04NMR (¹H, ¹³C): chemical shift, integration and coupling give structure, copolymer composition, tacticity and end groups
- 05XRD (Bragg's law nλ = 2d·sinθ): sharp peaks = crystalline, broad halo = amorphous; peak-area ratio → degree of crystallinity
- 06DSC: heat flow vs temperature gives Tg (step in baseline), Tm (endotherm) and crystallization (exotherm)
- 07Complementarity: FTIR/Raman → chemistry, XRD → morphology, DSC → thermal transitions
- 08Combining techniques to identify an unknown packaging polymer (the lab deliverable)
Identify two unknown polymers from their FTIR bands
- +1Sample A, 1730 cm⁻¹: a strong band in the 1700-1750 range is a carbonyl C=O stretch, and the 1240 cm⁻¹ band is a C-O stretch — together these signal an ester linkage.
- +1Sample A, 1600 and 1500 cm⁻¹: these are aromatic C=C ring stretches, so the backbone carries aromatic rings. Ester + aromatic + no N-H points to an aromatic polyester, i.e. PET.
- +1Sample B, 3300 cm⁻¹ (broad): an N-H stretch; paired with a strong band near 1640 cm⁻¹ (the amide C=O / amide I band) this is an amide linkage.
- +1Amide N-H + amide C=O identifies Sample B as a polyamide (nylon). The presence of N-H in B and its absence in A is the clean discriminator between the polyamide and the polyester.
Key terms
- FTIR spectroscopy
- Fourier-transform infrared spectroscopy measures which infrared frequencies a sample absorbs as its bonds vibrate. Each functional group absorbs at a characteristic wavenumber, so the spectrum acts as a fingerprint that identifies bonds and groups.
- Raman spectroscopy
- Measures inelastically scattered visible light to probe bond vibrations. It is complementary to IR: symmetric and non-polar vibrations (C=C, C-C, S-S) that are weak in IR are strong in Raman, so Raman is ideal for tracking C=C consumption and crosslinking/vulcanization.
- NMR spectroscopy
- Nuclear magnetic resonance (¹H, ¹³C) uses chemical shift, peak integration and coupling to reveal chemical structure, copolymer composition, tacticity and end groups; end-group integration can also give a number-average molar mass.
- X-ray diffraction (XRD)
- Probes three-dimensional order through Bragg's law nλ = 2d·sinθ. Sharp peaks indicate crystalline regions and a broad halo indicates amorphous material; the ratio of crystalline to total peak area gives the degree of crystallinity.
- Differential scanning calorimetry (DSC)
- Measures heat flow versus temperature to detect thermal transitions: the glass transition Tg (a step in the baseline / change in heat capacity), the melting temperature Tm (an endothermic peak) and crystallization (an exothermic peak).
- Complementary techniques
- The principle that no single method identifies a polymer fully: FTIR and Raman give chemistry, XRD gives morphology and DSC gives thermal behaviour, so the lab combines all four to identify an unknown sample.
Spectroscopic and Structural Characterization FAQ
How do I read an FTIR spectrum of a polymer?
Work through it by region. Bands from about 3200-3600 cm⁻¹ come from O-H and N-H stretches; the 2850-3030 region from C-H; a strong band around 1700-1750 cm⁻¹ is a carbonyl C=O; aromatic rings show C=C near 1600 and 1500 cm⁻¹; and the fingerprint region below about 1300 cm⁻¹ carries single-bond stretches like C-O and C-Cl. You assign each observed band to a bond, then combine the assignments into a functional group (for example C=O plus C-O means an ester) and finally a polymer family. Confirming a group with a second band is what makes the identification reliable.
Why use Raman as well as FTIR if they both probe vibrations?
Because they respond to different vibrations and so complement each other. IR absorption is strong for polar bonds with a changing dipole (O-H, C=O, C-O), while Raman scattering is strong for symmetric, non-polar bonds with a changing polarizability (C=C, C-C, S-S, aromatic ring breathing). A band that is weak or invisible in one is often strong in the other, so running both gives a fuller picture — Raman is especially good for following C=C consumption during polymerization and for studying crosslinking and vulcanization, which FTIR sees poorly.
What does each of the four lab techniques actually tell you?
They map onto different aspects of the sample. FTIR and Raman report the chemistry — which bonds and functional groups are present, and hence the polymer's identity. XRD reports the morphology — whether the sample is amorphous (broad halo), semi-crystalline or crystalline (sharp peaks), and gives a degree of crystallinity. DSC reports the thermal transitions — the glass transition Tg, the melting temperature Tm and any crystallization or cure. Because each answers a different question, the Group Lab Report asks you to combine all four to identify an unknown.
How is characterization examined in MATS3004?
Mostly as spectrum-reading and technique-selection: assign FTIR (or NMR) peaks to functional groups, choose the right technique for a stated question, and — in the Group Lab Report — deduce an unknown polymer's identity by combining FTIR, Raman, XRD and DSC. The lab report is worth 15% (with peer-assessment and participation components) and is submitted through Turnitin. Confirm the lab schedule and report requirements on the UNSW course outline / Moodle.
Exam move
Learn the FTIR characteristic-band table cold (O-H/N-H high, C=O ~1700, aromatic C=C ~1600/1500, C-O and C-Cl low) because band assignment is the most examinable spectroscopy skill, and practise the presence/absence discriminators — N-H separates polyamide from polyester, aromatic bands separate PET from an aliphatic polyester. Keep a one-line summary of what each technique answers (FTIR/Raman = chemistry, XRD = morphology, DSC = thermal transitions) and pair Raman with IR as complementary. For the Group Lab Report, rehearse the 'identify the unknown by combining techniques' logic on a fresh sample set, and remember Bragg's law and the DSC transition signatures (step for Tg, endotherm for Tm). Confirm the lab report structure and Turnitin requirements on the UNSW course outline / Moodle.
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