Infrared: Polymer Examples

The text describes several examples, cited here briefly

• Polystyrene (atactic), used as an example of an amorphous polymer with weak intermolecular interactions (PC Fig. 6.5). van der Waals interactions are dominant.
  • Simple spectrum, considering the number of atoms in the repeat unit (and polymer)
  • Group frequency correlations reveal aliphatic and aromatic groups from the bands at 2800 to 3200 cm^-1
  • The sharper bands are dominated by conformationally insensitive local motions, such as CH stretch and breathing of the aromatic ring
  • Broader bands are more likely to be conformationally sensitive

• Poly(4-vinylphenol) (atactic), used as an example of an amorphous polymer with strong intermolecular interactions (PC Fig. 6.6), due to hydrogen bonds.
  • Bands assignable to the aromatic ring at 825, 1100, 1170, 1445, 1595, and 1600 cm^-1.
  • The region of the OH stretch, at 3100-3600^-1, is strongly temperature dependent.
  • The sharp band at 3525 cm^-1 is attributed to non-hydrogen bonded OH, and the broad band around 3360 cm^-1 is attributed to hydrogen bonded OH
  • Hydrogen bonding decreases the frequency of the OH stretch
  • Hydrogen bonds become weaker as temperature increases. Quantitative interpretation is difficult, due to the necessity to understand how the extinction coefficient depends on temperature and stength of the hydrogen bond.

• Polystyrene (isotactic), used to illustrate conformation in a semicrystalline polymer (PC Fig. 6.7).
  • Every unit in the crystalline polymer is in the same environment, in contrast to the units in the amorphous polymer
  • The isotactic chain prefers a 3₁ helix (three units per turn), which forms below its melting point (T_M about 230 deg C), if time is allowed for crystallization. Such time is not available in a quick quench, producing an amorphous glass.
  • The spectra of quick-quenched isotactic polystyrene and atactic polystyrene are similar.
  • Annealed isotactic polystyrene yields a different spectrum, approximated as the sum of "amorphous" and "crystalline"
  • A suitable difference spectrum suppresses the "amorphous" bands, and highlights the bands characteristic of the crystalline region.

• Poly(trans-1,4-isoprene), used to illustrate polymorphism (PC Fig. 6.8). The alpha and beta polymorphs have different preferred conformations, and different ir spectra. Poly(L-alanine) is another polymer with dramatic polymorphism, the two structures being known as the alpha helix and beta sheet.

• Poly[trans-1,4-(2,3-dimethylbutadiene)], used to illustrate the effect of symmetry on the intensitives of the signals (PC Fig. 6.9).
  • The vibration expected at 1660-1670 cm^-1, based on the behavior of the C=C bond in small alkenes, is absent in the infrared spectrum, but easily observed in the Raman spectrum.
  • The stretching of the C=C bond in this polymer is accompanied by very little change in dipole moment, and therefore the ir selection rule (oscillating dipole moment) is not satisfied, leading to very low intensity for the transition.
  • Raman, which uses a scattering technique, has a different selection rule, permitting observation of this band.

• Polyethylene, which is a special case due to the absence of bulky side chains, allowing close interaction of the backbones of neighboring chains (PC Fig. 6.10).
  • Stronger vibrational coupling between repeat units along the chain than in polymer with bulky side groups
  • Several of the normal vibrations are sensitive to the number of methylene units in a sequence
  • In the crystalline regions, the chain is a planar zig-zag with all trans placements at the C-C bonds
  • In the common crystal form, with an orthorhombic unit cell, there are two CH₂-CH₂ groups per unit cell.
  • Crystal field splitting, as at 733/721 and 1460/1475 cm^-1, is directly an effect of the crystal, and not just the preferred conformation of a single chain

• Nylon 11, illustrating a strongly interacting ordered polymer
  • The polymer has extensive intermolecular hydrogen bonding at ambient temperature, T_g = 45 deg C, T_M = 196 deg C.
  • Amide I region (1600-1700 cm^-1), PC Fig. 6.11
    • The Amide I vibration can be approximated as the C=O stretch
    • Highly ordered domains in the crystal permit dipole-dipole interaction of the C=O, causing the frequency of the Amide I region to be structure dependent
    • 1636-1641 cm^-1: narrow component, wich decreases in intensity and increases in frequency as tempertature increases to 190 deg C. This component disappears above T_M. Assigned to hydrogen bonded C=O groups in ordered domains.
    • 1645-1654 cm^-1: broader component, which increases in intensity and frequency as temperature increases from 40 to 220 deg C. Assigned to hydrogen bonded C=O in amorphous regions.
    • 1679-1683 cm^-1: assigned to free C=O
  • NH stretch (3300-3450 cm^-1), PC Fig. 6.12
    • Conformationally insensitive, but sensitive to hydrogen bonding
    • Free N-H near 3447 cm^-1
    • Hydrogen bonded N-H groups in a broad band at 3300-3332 cm^-1. Most of the temperature dependence comes from a reduction in the extinction coefficient as the hydrogen bonds become weaker.

• Polytetrahydrofuran, where oxidation can be monitored by following the appearance of the carbonyl band at 1737 cm^-1 (PC Fig. 6.15)

• Blend miscibility, illustrated with poly(vinyl acetate) and poly(vinyl phenol), PC Fig. 6.16
  • Monitor the relationship between the C=O groups of the poly(vinyl acetate) that are free (1739 cm^-1) and hydrogen bonded (1714 cm^-1). They can be hydrogen bonded only if very close to the poly(vinyl phenol).
  • The 1739 cm^-1 band dominates when little poly(vinyl phenol) is present.
  • The two bands are comparable in size at 80:20 poly(vinyl phenol):poly(vinyl acetate), showing intimite mixing of the two components of the blend.

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