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Some polyisoprene samples of different microstructure contents were studied by Fourier transform infrared (FTIR) and ^{1}H Nuclear magnetic resonance (^{1}H NMR). On the basis of detailed analysis of FTIR spectra of polyisoprene, the shift of absorption peaks caused by microstructure content’s variation was discussed. The contents of the polyisoprene samples’ microstructure which was determined by the ^{1}H NMR was used as the standard. Through the choice, calculation, and comparison with the corresponding absorption peaks of FTIR, a method based on the results of the analysis has been developed for the determination of the microstructure contents of
polyisoprene by FTIR.

As it is well known, polyisoprene (PIp) is one kind of important rubbers, and there are four kinds of microstructure in its molecular chain which are _{5}H_{8}, the research of synthetic polyisoprene keeps active [

Except for the synthesis of high content of

Most of the studies on microstructure of polymers are characterized by nuclear magnetic resonance (NMR) spectrophotometer and FTIR [

All the polyisoprene samples with variable microstructure content were polymerized according to [

Tensor 27 (Bruker, German) has been used in the analysis. The samples are tested by ATR-FTIR, with 4 cm^{−1} resolution, and scanned 32 times.

AV500 (Bruker, German) has been used in the analysis. ^{1}H NMR spectra of the polyisoprene in CDCl_{3} were obtained at 500.13 Hz, and chemical shifts were referred to TMS.

The ^{1}H NMR spectrum of polyisoprene which contains 1,4-, 1,2-, and 3,4-unit is shown in Figure

Microstructure contents of polyisoprene by ^{1}H NMR.

Sample | 1,4-mol% | 3,4-mol% | 1,2-mol% |
---|---|---|---|

A | 100 (cis-1,4) | 0 | 0 |

| |||

B | 99.6 (trans-1,4) | 0.2 | 0.2 |

| |||

C | 90 | 5 | 5 |

D | 53 | 24 | 23 |

E | 30 | 59 | 11 |

F | 24 | 63 | 13 |

G | 18 | 67 | 15 |

^{1}H NMR spectrum of polyisoprene.

Polyisoprene has four kinds of microstructure which are

The structural formula of polyisoprene.

Figure

FTIR spectra of polyisoprenes of variable microstructure content. (A)

The characterization and attribution peaks of FTIR spectra of polyisoprene are listed in Table

The explanation of absorption peaks of FTIR spectrum of polyisoprene.

Wavenumber/cm^{−1} |
Attribution | Wavenumber/cm^{−1} |
Attribution |
---|---|---|---|

3080 | –C–H stretching vibration of carbon-carbon double bond in 1,2-unit |
1150 | Stretching vibration of CC main chain in |

3070 | CH_{2} stretching vibration of –C=C– in 3,4-unit |
1140 | Stretching vibration of CC main chain in |

3035 | CH stretching vibration of –C=C– in 1,4- or 1,2-unit | 1044 | Stretching or wagging vibration of CH_{3}C=C in |

2727 | Sympathetic vibration | 1036 | Stretching or wagging vibration of CH_{3}C=C in |

1663 | C=C stretching vibration of 1,4-unit | 1003 | Stretching vibration of C–C in 3,4-unit |

1644 | C=C stretching vibration of 3,4- or 1,2-unit | 910 | Out-of-plane bending vibration of CH_{2} in the –CH=CH_{2} (1,2-unit) |

1413 | Bending vibration of C–H in the =CH_{2} group of 3,4- or 1,2-unit |
888 | Out-of-plane bending vibration of CH_{2} in the –C=CH_{2} (3,4-unit) |

1383 | Scissoring vibration of CH_{3} in |
843 | Out-of-plane bending vibration of C–H in the –CH=CH– group of |

1375 | Scissoring vibration of CH_{3} in |
837 | Out-of-plane bending vibration of C–H in the –CH=CH– group of |

1325 | Scissoring vibration of CH_{3} or CH in |
600 | Torsion or twisting vibration of CCC group in |

1311 | Scissoring vibration of CH_{3} or CH in |

It can be seen from Figure ^{−1} peaks can be used for quantitative calculation of microstructure content of polyisoprene. It is because that the peaks for quantitatively calculation must have moderate intensity, little interfering factors by other peaks or conditions, and so on. Except for those differences shown in Table

The change of peaks of FTIR spectra according to the microstructure contents of polyisoprene.

Explanation |
Spectrum A | Spectrum B | Spectrum C | Note |
---|---|---|---|---|

Microstructure content | ||||

3,4-unit% | 5 | 24 | 67 | 3,4-unit% increase by progressively |

1,2-unit% | 5 | 23 | 15 | 1,2-unit% increase by degrees |

1,4-unit% | 90 | 53 | 18 | 1,4-unit% decrease by degrees |

Dissymmetry stretching vibration of –CH_{3} |
2962.125 | 2964.053 | 2965.982 | Blue shift of peak (the value is 2978 cm^{−1} in |

C=C stretching vibration | 1644.982 | 1644.018 | 1643.054 | Red shift of peak |

Scissoring vibration of CH_{3} |
1376.925 | 1374.997 | 1374.033 | Red shift of peak (the value is 1383 cm^{−1} in |

Out-of-plane bending vibration of CH_{2} in the –C=CH_{2} (3,4-unit) |
889.023 | 888.059 | 887.095 | Red shift of peak |

Out-of-plane bending vibration of C–H in the –CH=CH– group of 1,4-unit | 836.955 | 840.812 | 848.849 |
blue shift of peak |

According to the Beer-Lambert law, the integrated intensity

Here,

Choosing a peak which has moderate intensity and is not affected by configuration, conformation or other structure factors as internal standard of thickness, then formula (

Here, “1” means characteristic band and “0” means internal standard band. “^{1}H NMR dates. As described above, 910, 888, and 840 cm^{−1} peaks can be used for quantitatively calculating microstructure content of polyisoprene. Table ^{−1} is used as internal standard peak. The integrated intensity

Samples of absorption values of peaks of polyisoprene.

The samples shown in Table

The absorption coefficient calculation of (a) 910 cm^{−1}, (b) 888 cm^{−1}, and (c) 840 cm^{−1} peaks of FTIR spectra of polyisoprene.

After regression of measured points of testing points based on a least-squares method, the formula of microstructure content is shown as follows:

Because of 910, 888, and 840 cm^{−1} peak overlaps partly, the errors of calculated values of integrated intensity ^{−1} peak is far stronger than 840 cm^{−1} peak when the contents of their corresponding units are the same. Therefore, the calculation of microstructure content of polyisoprene shall choose the former two peaks.

The change of microstructure content of polyisoprene can cause a lot of differences in FTIR spectra. The described method by FTIR can be used to determine the microstructure content of polyisoprene. 910 and 888 cm^{−1} peaks of 1,2- or 3,4-unit have higher absorption ability than 840 cm^{−1} peaks of 1,4-unit; therefore 910 and 888 cm^{−1} peaks shall be used first for the quantitative analysis of microstructure content of polyisoprene. The quantitative calculation formulas were also obtained.