The Mechanism of Poly styrene Degradation

Center of Applications in Polymer Science, Department of Chemistry, Central Michigan 

Atul Tiwari and Baldev Raj, Reactions and Mechanisms in Thermal Analysis of Advanced Materials, (259-268) 2015 Scrivener Pubhshing LLC 

The latter two polymers contain no head-to-head units as a consequence of the means of polymerization. The material from conventional radical polymerization contains one head-to-head unit as a consequence of polymerization termination by radical coupling [21]. The remaining polymer contains only head-to-head units. 

Evolved gas analysis for the degrading polymers was conducted by TG/MS and TG/ GC/MS. In the first case, samples were held at 280 C for 80 min with any volatile fragments formed being carried by the helium purge gas to a mass selective detector tuned to m/e 104 (styrene monomer). For the head-to-head polymer, no signal was detected nor was any measurable weight loss noted, i.e., the polymer was relatively mass stable at 280 °C over the course of the experiment (it should be noted that the time at 280 C for this experiment was much shorter than that previously utilized for TG at 280 C). In contrast, the conventional polymer lost 7.7% of its initial mass. 

While degradation of conventional poly(styrene) occurs when the polymer is subjected to 280 for even a short time and styrene monomer is evolved, the head-to-head polymer is relatively mass stable at this temperature and no styrene monomer is evolved. 

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The thermal degradation mechanisms of poly(styrene-co-methacrylonitrile) (P(S-co-MAN)) is reported in terms of the competition between the depolymerisation and backbiting reaction on the basis of the bond dissociation energies of the C-C and C-H bonds in the polymer chains [a.l88j. The activation energy of pyrolysis obtained by Ozawa s plot increased with the content of methacrylonitrile units in the copolymer chain, although the onset temperatures of loss of sample mass in the TG curves shifted to the lower temperature region. Yields of each monomer, dimer and trimer, and also those of hybrid dimers and... 

The mechanism of photo-degradation of poly(p-methylstyrene) was studied by several investigators [544-547]. A gas evolution was observed during the irradiation with ultraviolet light. This gas contains hydrogen as its major portion and methane, ethane as the minor portions. There are also traces of styrene, p-methylstyrene, and toluene [101]. The gas evolution is accompanied by cross-linking. The start of the process is pictured as follows [547] ... 

PVAc, PVA and PVB homopolymers as well as the different copolymers mentioned above all have a similar chemical motif in common. They exhibit an all carbon-carbon single bond backbone, which needs to be broken at some point in a potential biodegradation mechanism. With respect to the backbone, poly(vinyl ester)s are closely related to poly(olefin)s, poly(styrene)s and poly(acrylate)s. These three are known not to be biodegradable. Instead, they usually decompose by the impact of UV radiation, oxidation and hydrolysis reactions, which are not considered to be biological degradation. 

After the examination of the PS photooxidation mechanism, a comparison of the photochemical behavior of PS with that of some of its copolymers and blends is reported in this chapter. The copolymers studied include styrene-stat-acrylo-nitrile (SAN) and acrylonitrile-butadiene-styrene (ABS). The blends studied are AES (acrylonitrile-EPDM-styrene) (EPDM = ethylene-propylene-diene-monomer) and a blend of poly(vinyl methyl ether) (PVME) and PS (PVME-PS). The components of the copolymers are chemically bonded. In the case of the blends, PS and one or more polymers are mixed. The copolymers or the blends can be homogeneous (miscible components) or phase separated. The potential interactions occurring during the photodegradation of the various components may be different if they are chemically bonded or not, homogeneously dispersed or spatially separated. Another important aspect is the nature, the proportions and the behavior towards the photooxidation of the components added to PS. How will a component which is less or more photodegradable than PS influence the degradation of the copolymer or the blend We show in this chapter how the... 

Recently, the thermal degradation of a block copolymer of ordinary and perdeuterated styrene, poly(styrene-b-styrene-d8) (styrene/styrene-dg = -1/1) was investigated by Py-GC/MS and by pyrolysis-field ionization MS (Py-FIMS) to clarify the detailed mechanism of the dimer formation. Figure 3.19 shows the pyrograms of the poly(styrene-b-styrene-dg) at500°C. 

They found that thermal degradation of poly(4- -alkyl styrenes) followed mainly a free radical depolymerisation mechanism. The main product is a monomer similar to unsubstituted PS, i.e., 59% to 92% monomer from poly(4-n alkyl styrenes) ranging from 136,500-737,000 and = 37,000-99,000. The amoxmts of this monomer decrease with increasing length of alkyl sidechain from hexyl to decyl. This behaviour is connected with the stability of monomer under isothermal pyrolysis conditions at 600 °C. 

ABS consists of a styrene/acrylonitrile continuous phase partially grafted to a dispersed butadiene phase. Butadiene acts as an impact modifier, and imparts excellent mechanical properties to the material. Improvement of the impact-modifying properties of ABS during melt processing and product use focuses on protecting the poly butadiene phase from degradation. Polybutadiene is particularly susceptible to oxidation due to the presence of residual double bonds [37]. The properties of ABS are tabulated in Table 2.7. 

The method of differential radiation induced contrast depends on enhancement of contrast in multicomponent polymers where the components have different electron beam-polymer interactions [173]. Contrast has been observed in sections of styrene-acrylonitrile/poly(methyl methacrylate) (SAN/PMMA) polymers where the PMMA exhibits a high rate of mass loss compared to SAN, creating contrast between the phases. It is well known that electron irradiation results in chain scission and crosslinking, loss of mass and crystallinity [75]. Polystyrene, polyacrylonitrile and SAN crosslink and thus are stable in the electron beam whereas polymers exhibiting chain scission, PMMA and poly(vinyl methyl ether), degrade in the beam. It is suggested that experiments be conducted on the homopolymers to determine the expected irradiation damage mechanism in the multi-component system [173]. 

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