A 13C NMR Study of End Groups and Stereoregularity Changes Induced by γ Irradiation of Molten Polypropylenes

Abstract

ABSTRACT:Samples of predominantly isotactic and syndiotactic polypropylenes have been irradiated with gamma rays to high doses in the molten state. Solution state 13 C NMR analysis has identified propyl, vinylidene, and isopropyl end groups. The yield of the former shows that the extent of chain scission in isotactic polypropylene is strongly dose dependent with an initial G value of 11.6 reducing to 5.5 at doses greater than 500 kGy. Molecular weight measurements suggest that G(crosslink) is approximately independent of dose with a values of 3.5. The triad sequence contents of the polymers after 2 MGy irradation in the molten state, show that an equilibrium triad distribution has not been achieved, even though the rates of change of triad sequences are virtually zero by this dose. Initial rate kinetics suggests that this is mainly due to the preferential formation of syndiotactic sequences from isotactic ones.KEY WORDS Irradiation / Polypropylene / End Groups / Stereoregularity Changes/ 13 C NMR / Chain Scission / Crosslinking / In recent work 1 -3 we have established that irradiation ofisotactic polypropylene, PP, with gamma or electron radiation induces relatively large changes in the stereochemistry 1 and that both the size and detailed nature of these changes are morphology 2 and dose rate 3 dependent. For example the G values for the loss of pure isotactic pentad sequences when samples of annealed PP were irradiated with gamma radiation, dose rate 4.4kGyh-1 , at ambient temperature and at 200°C (molten sample) were -94 and -283, respectively. When the same sample was irradiated in an electron beam, dose rate 720 kGy h -1 , at ambient temperature, the corresponding G value was -220. The first difference suggests that sterochemical change only occurs in the amorphous regions, as the ratio correlates well with the fraction of amorphous component in the solid state. This is reasonable when * To whom correspondence should be addressed.considering the conclusions of Keller et al. 4 which suggest that neither crosslinking nor chain scission occur within the crystalline regions of polyethylene when subjected to high energy irradiation. The large dose rate effect on the other hand, was explained by a difference in mechanism. An analysis of the full pentad sequence distributions indicated that the deposition of one quantum of energy at the high dose rate resulted in an average of five inversions on mainly alternate monomer units, whilst at low dose rate an energy quantum promoted an average of two inversions on nearby monomer units. In the latter case the distribution was more random, lying somewhere between adjacent and alternate.The detailed chemical mechanism of the inversion is unclear. The observed G values are many times greater that those observed for any other irradiation induced chemical effect in PP,

Solid-State 13C NMR Investigation of the Oxidative Degradation of Selectively Labeled Polypropylene by Thermal Aging and γ-Irradiation

“…A distribution of relatively smaller intensity was generally found between the two peaks. In previous high-temperature solution-state 13 C NMR studies of oxidatively degraded polypropylene, 11,12 a resonance at 111.5 ppm was observed and identified with the vinylidene carbon. However, the corresponding resonance of the second unsaturated carbon in the double bond, which should appear in the range 135-155 ppm, was not seen in the solutionstate NMR spectra.…”

“…Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool in the study of polymer chain structure and dynamics. , NMR has proven very useful in studying both oxidative degradation and radiation-induced changes in polyolefin thermoplastics such as polyethylene, − polypropylene, , and ethylene copolymers, , as well as other common industrial polymers. − Due to the relatively wide range of isotropic chemical shifts, 13 C NMR offers good spectroscopic resolution and effective identification of resonance peaks originating from the different oxidation products. Typically, 13 C NMR analyses of oxidation-induced functional groups in polymers have been conducted on samples in liquid solution at high temperatures (>100 °C).…”

“…Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool in the study of polymer chain structure and dynamics. 4,5 NMR has proven very useful in studying both oxidative degradation and radiation-induced changes in polyolefin thermoplastics such as polyethylene, [6][7][8][9][10] polypropylene, 11,12 and ethylene copolymers, 13,14 as well as other common industrial polymers. [15][16][17][18][19][20] Due to the relatively wide range of isotropic chemical shifts, 13 C NMR offers good spectroscopic resolution and effective identification of resonance peaks originating from the different oxidation products.…”

Identification of Oxidation Products in Selectively Labeled Polypropylene with Solid-State 13C NMR Techniques

“…Many of the products formed in polypropylene subjected to long-term oxidative degradation have been identified, 1,2 while others are in question or have yet to be realized. For example, in earlier solution-state 13 C NMR studies 3,4 of oxidatively degraded polypropylene, a 13 C resonance was observed near 112 ppm. On the basis of the isotropic chemical shift alone, this resonance could be assigned to either the vinylidene group (CH 2 dCH-), an unsaturated structure, or to dioxygenated alkyl structures (O-C-O), such as acetal and ketal groups.…”

Investigation of High-Temperature Radiation Effects on Poly(methyl methacrylate) of Specific Tacticity

“…(Similar significant changes have been obtained in the tacticity sequences for isotactic polypropylene on /-radiolysis at 200 °C. 21) The changes in the triad tacticity may be attributed to the recombination of scissioned bonds located on the quaternary carbon during /-irradiation, so that the configuration for some of the recombined units will be changed.…”