Vinyl Unsaturation Research Articles

Bis(imino)-6,7-dihydro-5H-quinoline-cobalt complexes as highly active catalysts for the formation of vinyl-terminated PE waxes; steps towards inhibiting deactivation pathways through targeted ligand design.

A set of five related bis(imino)-6,7-dihydro-5H-quinoline-cobalt(ii) complexes, [2-(ArN = CPh)-8-(NAr)-C9H8N]CoCl2 (Ar = 2,6-Me2C6H3Co1, 2,6-Et2C6H3Co2, 2,6-i-Pr2C6H3Co3, 2,4,6-Me3C6H2Co4, 2,6-Et2-4-MeC6H2Co5), have been synthesized in reasonable yield by the template reaction of cobalt(ii) chloride hexahydrate, 2-benzoyl-6,7-dihydro-5H-quinolin-8-one and the corresponding aniline. The molecular structures of Co1 and Co4 highlight both the differences in the two imino-carbon environments (phenyl-capped chain vs. cyclic) and also the steric properties exerted by the bulky Nimine-aryl groups. On pre-treatment with either modified methylaluminoxane (MMAO) or methylaluminoxane (MAO), all complexes proved productive catalysts for the polymerization of ethylene. In particular, Co1/MAO was the most active reaching a very high level of 1.62 × 107 g PE per mol (Co) per h over a 30 minute run time. Owing to the presence of the imino-phenyl substituent, Co1–Co5 were able to exhibit good thermal stability by displaying appreciable catalytic activity at temperatures between 50 and 80 °C, generating polyethylenes with narrow dispersities (Mw/Mn range: 1.66–3.28). In particular, the least sterically bulky precatalysts, Co1 and Co4 formed polyethylene waxes (Mw range: 1.94–5.69 kg per mol) with high levels of vinyl unsaturation as confirmed by high temperature 1H/13C NMR spectroscopy and by IR spectroscopy.

Polysaccharide-stabilized core cross-linked polymer micelle analogues

A novel approach is presented for the synthesis of block-copolymers that resemble the architecture of a core cross-linked micelle. The polymers are synthesized from a combination of catalytic chain transfer polymerization (CCTP), thiol-Michael addition chemistry and reductive amination. A hydrophobic hyperbranched core is synthesized via CCTP of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA), which affords control over the polymer architecture and the degree of chain end-functionality. The vinyl unsaturations of the hyperbranched polymers are converted in nucleophilic pendant amines by thiol-Michael addition using cysteamine hydrochloride. A polysaccharide shell is grafted onto the hyperbranched core via reductive amination with dextran (DEX). The synthesized poly(MMA-co-EGDMA)-b-DEX polymers possess an amphiphilic character, are colloidally stable and resemble the topology of a core cross-linked micelle. The presented methodology provides a robust, modular, and tuneable approach towards the synthesis of amphiphilic core cross-linked micelle analogues.

Thermal Oxidation and Structural Changes of Degraded Polyethylene in an Oxygen Atmosphere

High-temperature bulk thermolysis of high density polyethylene was performed to obtain degraded polyethylene, which was then left in an oxygen atmosphere for oxidation reaction at 160°C at different oxidation times, yielding a series of polyethylene degradation–oxidation products. Using infrared spectroscopy, chemical titration, gel chromatography, and differential scanning calorimetry, the structural changes of the polyethylene degradation–oxidation products were investigated. Hydroxyl, carboxyl, ester, and other functional groups were introduced into the molecular chains of oxidation products, and the number of carboxyl groups sharply increased with increasing oxidation time. The breaking of molecular chains as well as combination reactions, such as esterfication, during the oxidation process lead to decrease of number-average molecular weight () and increase of both weight-average molecular weight () and molecular weight distribution, and an initial increase and leveling off of the carboxyl number per molecular chain. It is noteworthy that oxidation occurred primarily near weak bonds, mainly terminal vinyl unsaturations. Thus, compared with degraded polyethylene, the oxidation products showed little changes in the melting and crystallization behaviors; even when the oxidation time was up to 4 hr, the drop in the melting and crystallization temperature was less than 3°C. These results might lay a foundation for exploring tailored low molecular weight polymers to be used just as they are or after chemical modifications of olefinic functions.

Effects of type of polymerization catalyst system on the degradation of polyethylenes in the melt state. Part 1: Unstabilized polyethylenes (including metallocene types)

AbstractSeveral polyethylene resins namely, high‐density polyethylene (HDPE) (Phillips metal oxide catalyst) and linear low‐density polyethylenes (LLDPE) (formed by using Ziegler‐Natta and metallocene catalyst technologies), were used in order to acquire insight into the effect of different polymerization catalyst systems on the production of degradation products during melt processing. Infrared spectroscopy, color measurement, hydroperoxide determination, and melt flow rate measurement were used to monitor the degradation as a function of the number of passes through a twin‐screw extruder. The metallocene PEs were shown to exhibit superior melt stability relative to Phillips HDPE. The latter showed high levels of hydroperoxide formation. The superior thermo‐oxidative stability of the metallocene PEs was attributed to low levels of metallic catalyst residues, together with low vinyl unsaturation content. In all of the PEs examined, the rate of crosslinking was greater than that of chain scission. IR spectroscopy indicated that crosslinking (most prevalent in the Phillips HDPE) proceeded via the addition of macroradicals to vinyl unsaturation. The Ziegler‐Natta LLDPE showed an intermediate tendency for crosslinking but notable formation of trans‐vinylidene and the most noticeable color development. J. VINYL ADDIT. TECHNOL., 2011. © 2011 Society of Plastics Engineers

Effect of natural weather on the structure and properties of polylactide/Cloisite 30B nanocomposites

The degradation of polylactide (PLA)/Cloisite 30B nanocomposites under natural weathering was investigated as a function of clay loadings (1, 3 and 5 wt.%) for up to 130 days using Fourier transform infrared (FT-IR) spectroscopy, size exclusion chromatography (SEC), nanoindentation measurements, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). For comparative purposes, the neat PLA was also considered. The FT-IR results showed that the photo-oxidation mechanism of PLA was not modified in the presence of Cloisite 30B, but only the degradation rates were accelerated. Moreover, the photo-oxidative degradation of PLA nanocomposite samples led to the formation of vinyl unsaturation, carbonyls, anhydrides and hydroperoxides groups as a result of the occurrence of several chemical mechanisms simultaneously. The decrease of the weight-average molecular weight, and the number-average molecular weight associated with an enhanced polydispersity of the nanocomposite samples indicated that chain scission was the most prominent phenomenon in natural weathering. The thermal degradation of the PLA was faster in the presence of clay. Modulus and hardness measured by nanoindentation increased slightly with exposure time for both neat PLA and PLA nanocomposite samples; the increase is also a function of the clay content. Finally, the weathering effect on the morphology of exposed samples observed by SEM revealed that the fractured surfaces exhibited many voids and cracks. These defects were much more pronounced for the PLA nanocomposites.

The effect of thiolation on the mechanical and protein adsorption properties of polyurethanes

Segmented polyurethanes are important polymers for a number of industrial and technological applications. The purpose of this work was to synthesize polybutadiene-based polyurethanes and subsequently graft carboxylate and sulfonate side chains via thiol-ene reaction. Spectroscopic investigations showed that grafting yielded good conversion for the vinyl unsaturation of the polybutadiene soft segment. DSC and tensile testing revealed that grafted polyurethanes had a better segmental compatibility and superior mechanical properties than the control polyurethane without grafting. The carboxylic side chains of the soft segment were responsible for the observed improved mechanical properties. Initial protein adsorption tests on these polymers were found to be higher than the control surface. The polyurethanes of the current study could be used for biomedical applications where protein attachment to the surface is needed for specific cell adhesion and tissue repair.

Phenoxycycloalkylimine Ligated Zirconium Complexes for Ethylene Polymerization: Formation of Vinyl-Terminated Low Molecular Weight Polyethylenes with High Efficiency

Bis(phenoxyimine)Zr complexes 1−8 containing a series of cycloalkyl groups on the imine-N's were synthesized (1: cyclopropyl; 2, 3: cyclobutyl; 4: cyclopentyl; 5, 7: cyclohexyl; 6, 8: 2-methylcyclohexyl). X-ray crystallographic analyses suggested that complexes 3, 5, and 8 assume an octahedral coordination geometry with a trans-phenoxy-O, cis-imine-N, and cis-Cl disposition and that the cycloalkyl groups on the imine-N's influence steric environments around the chlorine bound sites (i.e., potential polymerization sites). Upon activation with MAO at 25 °C, these complexes produced low-to-high molecular weight polyethylenes (PEs) (Mw 1900−960000, Mw/Mn 1.6−4.9) with very high efficiency (22−290 kg of PE/(mmol of cat. h)), which is comparable to or exceeds that seen with Cp2ZrCl2/MAO (28 kg of PE/(mmol of cat. h)). The cycloalkyl group has a profound effect on both catalytic activity and product molecular weight, indicating the critical importance of the substituent on the imine-N for polymerization catalysis. The catalytic activity increased with an increase in the steric bulk of the cycloalkyl substituent, albeit too much steric bulk reduced the activity. The product molecular weight was also related to the steric bulk of the cycloalkyl group, in that increased steric bulk normally resulted in higher molecular weight PEs. The PEs produced with complexes 1−5/MAO (Al/Zr molar ratio = 1250) possess a high degree of vinyl unsaturation at one of the two polymer chain ends (Mw 2000−14000, vinyl selectivity, 90−96 mol %). Polymerizations performed at a much higher Al/Zr molar ratio of 12500 confirmed the marked preference of these complexes for β-H transfer as the chain termination mechanism (Mw 1900−14000, vinyl selectivity, 90−95 mol %). The vinyl-terminated PEs were readily transformed to the corresponding epoxy- and diol-terminated PEs, which are valuable materials for PE- and polar polymer-based block and graft copolymers. Ethylene pressure studies on complexes 1, 2, 4, and 5 revealed a first-order dependence on ethylene for both the rate of chain propagation and the rate of chain transfer. On the basis of this polymerization behavior together with X-ray analyses and DFT calculation studies, we concluded that β-H transfer to an incoming monomer is responsible for the formation of vinyl-terminated PEs. The calculations revealed that the complexes disfavor β-H transfer to the Zr metal due to the extreme instability of a metal hydride species that is produced in such a chain transfer process. Therefore, the unique polymerization catalysis and distinctive polymer formation with phenoxycycloalkylimine ligated Zr complexes were demonstrated.

Metallocene ethylene-1-octene copolymers: Influence of comonomer content on thermo-mechanical, rheological, and thermo-oxidative behaviours before and after melt processing in an internal mixer

The influence of the comonomer content in a series of metallocene-based ethylene-1-octene copolymers (m-LLDPE) on thermo-mechanical, rheological, and thermo-oxidative behaviours during melt processing were examined using a range of characterisation techniques. The amount of branching was calculated from 13C NMR and studies using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were employed to determine the effect of short chain branching (SCB, comonomer content) on thermal and mechanical characteristics of the polymer. The effect of melt processing at different temperatures on the thermo-oxidative behaviour of the polymers was investigated by examining the changes in rheological properties, using both melt flow and capillary rheometry, and the evolution of oxidation products during processing using infrared spectroscopy. The results show that the comonomer content and catalyst type greatly affect thermal, mechanical and oxidative behaviour of the polymers. For the metallocene polymer series, it was shown from both DSC and DMA that (i) crystallinity and melting temperatures decreased linearly with comonomer content, (ii) the intensity of the β-transition increased, and (iii) the position of the tan δ max peak corresponding to the α-transition shifted to lower temperatures, with higher comonomer content. In contrast, a corresponding Ziegler polymer containing the same level of SCB as in one of the m-LLDPE polymers, showed different characteristics due to its more heterogeneous nature: higher elongational viscosity, and a double melting peak with broader intensity that occurred at higher temperature (from DSC endotherm) indicating a much broader short chain branch distribution. The thermo-oxidative behaviour of the polymers after melt processing was similarly influenced by the comonomer content. Rheological characteristics and changes in concentrations of carbonyl and the different unsaturated groups, particularly vinyl, vinylidene and trans-vinylene, during processing of m-LLDPE polymers, showed that polymers with lower levels of SCB gave rise to predominantly crosslinking reactions at all processing temperatures. By contrast, chain scission reactions at higher processing temperatures became more favoured in the higher comonomer-containing polymers. Compared to its metallocene analogue, the Ziegler polymer showed a much higher degree of crosslinking at all temperatures because of the high levels of vinyl unsaturation initially present.

Evaluation of Philips and Ziegler–Natta high-density polyethylene degradation during processing in an internal mixer using the chain scission and branching distribution function analysis

The oxidative and thermo-mechanical degradation of HDPE was studied during processing in an internal mixer under two conditions: totally and partially filled chambers, which provides lower and higher concentrations of oxygen, respectively. Two types of HDPEs, Phillips and Ziegler–Natta, having different levels of terminal vinyl unsaturations were analyzed. Materials were processed at 160, 200, and 240 °C. Standard rheograms using a partially filled chamber showed that the torque is much more unstable in comparison to a totally filled chamber which provides an environment depleted of oxygen. Carbonyl and transvinylene group concentrations increased, whereas vinyl group concentration decreased with temperature and oxygen availability. Average number of chain scission and branching ( n s) was calculated from MWD curves and its plotting versus functional groups' concentration showed that chain scission or branching takes place depending upon oxygen content and vinyl groups' consumption. Chain scission and branching distribution function (CSBDF) values showed that longer chains undergo chain scission easier than shorter ones due to their higher probability of entanglements. This yields macroradicals that react with the vinyl terminal unsaturations of other chains producing chain branching. Shorter chains are more mobile, not suffering scission but instead are used for grafting the macroradicals, increasing the molecular weight. Increase in the oxygen concentration, temperature, and vinyl end groups' content facilitates the thermo-mechanical degradation reducing the amount of both, longer chains via chain scission and shorter chains via chain branching, narrowing the polydispersity. Phillips HDPE produces a higher level of chain branching than the Ziegler–Natta's type at the same processing condition.

Formation of Linear Polymers with Pendant Vinyl Groups via Inclusion Complex Mediated Polymerization of Divinyl Monomers

Ethylene glycol dimethacrylate (EGDMA) and ethylene glycol methacrylate 4-vinyl benzoate (EGMAVB) were shown to form 1:1 inclusion complexes with cyclodextrin and were characterized by instrumental techniques. Computational analysis showed that the bent conformation of the included divinyl monomer was more stable than its linear conformation. Complexation of the divinyl monomer with the first CD molecule offered substantial stabilization than with the second CD molecule. The vinyl group included in the CD cavity did not participate in polymerization. As a result, solvent soluble, linear polymers with pendant vinyl unsaturation per repeat unit were obtained. This was unequivocally established by the polymerization of a complex comprising CD and EGMAVB. The unreacted vinyl group can be polymerized in the subsequent step to yield cross-linked products.

The thermo-oxidative degradation of metallocene polyethylenes. Part 1: Long-term thermal oxidation in the solid state

The long-term thermo-oxidative degradation of different metallocene polyethylenes (mPEs) was investigated in the solid state (oven ageing in air at 90 °C). The mPEs differed essentially according to their initial melt index, molar mass distribution, density and ash content. Two Phillips-type HDPEs were also studied for comparison. The rate of oxidation was assessed by carbonyl index measurements. Initial vinyl unsaturation and short chain branch contents were correlated with induction times. The hydroperoxide concentrations of the aged materials and the levels of metal catalyst residues were also evaluated. Outstanding oxidative stability was exhibited for all the mPEs in the solid state relative to the Phillips HDPEs. This was believed to be due to the low concentration of both adventitious metal catalyst residues and initial vinyl unsaturation. Degree of short chain branching also appeared to influence the oxidative stability of the mPEs. Besides inherent properties, the density/crystallinity appeared to be the main factor influencing their oxidative stability. In the case of the Phillips HDPEs, the high level of vinyl unsaturation together with chromium catalyst residues may contribute to their poor oxidative stability.

The role of chain scission and chain branching in high density polyethylene during thermo-mechanical degradation

The mechanical and thermo-oxidative degradation of high density polyethylene (HDPE) was measured in a twin-screw extruder using various processing conditions. Two types of HDPE, Phillips and Ziegler-Natta, having different levels of terminal vinyl unsaturation were analysed. Mild screw profiles, having mainly conveying elements, have short mean residence times then profiles with kneading discs and left hand elements. Carbonyl and trans-vinylene group concentrations increased, whereas vinyl group concentration decreased with number of extrusions. Higher temperature profiles intensified these effects. The thermo-mechanical degradation mechanism begins with chain scission in the longer chains due to their higher probability of entanglements. These macroradicals then react with the vinyl terminal unsaturations of other chains producing chain branching. Shorter chains are more mobile, not suffering scission but instead are used for grafting the macroradicals, increasing the molecular weight. Increase in the levels of extrusion temperature, shear and vinyl end groups content facilitates the thermo-mechanical degradation reducing the amount of both, longer chains via chain scission and shorter chains via chain branching, narrowing the polydispersity. Phillips HDPE produces a higher level of chain branching than does the Ziegler-Natta type.

Radiolytic unsaturation decay in polyethylene. Part II—the effect of irradiation temperature, thermal history and orientation

The decay rate of vinyl unsaturation in high-density polyethylenes irradiated at temperatures from about 310 to 450 K, changes significantly in the melting range up to the crystalline melting point as does free radical mobility and the polymer crystallinity. However, orienting the polymer, or slow cooling or quenching from the melt, prior to irradiation, do not alter the decay process or its rate, although they do alter the rate of increase of insoluble gel and of elastic modulus in the molten state. It is suggested that, below ∼340 K, the marked deviations from a first-order decay result from the limited mobility of polymeric free radicals in the crystalline phase and from scavenging, by vinyl groups, segregated into the amorphous phase, of radiolytic hydrogen atoms (H). In the melting range, the mobility of polymeric free radicals increases as the crystallinity decreases, reducing the importance of scavenging, so vinyl decay approximates more closely to a first-order relation. In the melt, the vinyl decay relation is not changed qualitatively by H atom scavenging, but the effective vinyl concentration is lower, so the decay rate drops sharply.

Physicochemical properties of irradiated and loaded LDPE films with polyfunctional monomers

AbstractPolyfunctional monomers (PFMs), namely, trimethylol propane trimethacrylate (TMPTMA), trimethylol propane triacrylate, ethylene glycol dimethacrylate, and diethylene glycol diacrylate were blended with low‐density polyethylene (LDPE) and exposed to different doses of EB irradiation. Fourier transform infrared and ultraviolet and UV–vis spectroscopy of the unirradiated, irradiated, unloaded, and PFMs‐loaded LDPE films were studied under various irradiation doses up to 300 kGy. The degree of crosslinking and oxidative degradation, as measured by the spectroscopic parameters, were dependent on both the irradiation dose and the type of loaded PFMs. For all of the loaded monomers, the extent of crosslinking increased at different rates as a function of irradiation dose. TMPTMA monomer was the most efficient in enhancing the crosslinking of LDPE films compared to the other loaded monomers. However, the unloaded LDPE film showed the least extent of crosslinking. In addition, the EB‐radiation‐induced changes, such as trans‐vinylene formation, a decrease in vinyl and vinylidene unsaturation; and carbonyl double‐bond formation and change in crystallinity were correlated. The importance of these results on the prediction of the role of polyfunctional monomers in the production of crosslinked polymers is discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2025–2035, 2003

Synthesis of hydroxy- and dihydroxy-end-capped poly(n-butyl acrylate)s and their use as reactive stabilizers for the preparation of polyurethane latexes

Well-defined hydroxy end-functionalized poly(n-butyl acrylate)s (PBA-OH and PBA-(OH)2), were prepared by atom transfer radical polymerization (ATRP) and used as reactive stabilizers for the preparation of polyurethane in dispersed medium. PBA-OH was obtained by end-capping the growing poly(n-butyl acrylate) chains with allyl alcohol added in excess at the end of the polymerization. The two hydroxyl functions of PBA-(OH)2 were fixed at one end of the poly(n-butyl acrylate) chains either by initiation or by chain-end functionalization reactions. The latter were protected under the form of cyclic acetal and attached either to the initiator bearing a secondary bromine or to the terminating agent carrying a poorly reactive vinylic unsaturation. PBA-OH and PBA-(OH)2 have been successfully used as reactive stabilizers (surfmers) to prepare core-shell polyurethane particles in dispersed medium. The final particle size was found to be very much dependent to parameters such as the molar mass, concentration and valence of the reactive stabilizer as well as the manner of addition of the reactants during the procedure.

Coupling mechanism and fatigue behavior of vinyl triethoxysilane‐treated carbon black—styrene–butadiene rubber vulcanizate

When a furnace carbon black was treated with vinyl triethoxysilane (VTEOS) coupling agent, the carboxyl and/or lactone groups of it reacted with the ethoxy group of the coupling agent. The vinyl unsaturation thus grafted to the surface of the treated carbon black underwent crosslinking with the unsaturation present in the backbone of rubber chains during sulphur vulcanization. The vulcanizate containing silane-treated carbon black (STCB) showed a tremendous improvement in the fatigue behavior compared to the vulcanizate containing untreated furnace carbon black (UFCB). This striking difference was explained on the basis of the mechanism of attachment of carbon black to the rubber chains. The STCB anchored to the rubber chains through chemical linkages, whereas the UFCB anchored to the rubber chains by physical means. The chemical linkages of STCB through silane to the rubber chains provided a strong interface, which considerably reduced the chance of failure from the interface, which happened to be the stress concentrating zone, while the physically adhered weak interface of UFCB and rubber chains failed prematurely. The analytical tools employed were infrared spectroscopy, differential scanning calorimetry, and fatigue-to-failure testing. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2235–2241, 2000

Chemical and mechanical changes in poly(ethylene- co-1,9-decadiene) following crosslinking induced by peroxides

The response of poly(ethylene- co-1,9-decadiene) to peroxide treatment was examined and compared with a reference. The polymers were produced in a low-pressure process using a chromium-based catalyst giving a relatively high level of inherent unsaturations through thermal termination of the polymerisation. The copolymerisation with 1,9-decadiene gave an additional 63% vinyl end groups in the polymer. A great improvement in the response to dicumyl peroxide was observed for the decadiene polymers leading to a considerably smaller need for peroxide in order to obtain a certain gel content. The reason for the improved response was previously found to be the overall higher amount of vinyl groups as well as the placement of these groups along the molecular weight distribution. The remaining amount of vinyls after crosslinking was found to be exponentially dependent on the amount of peroxide added. At high peroxide levels almost all vinyl groups were consumed. The peroxide-induced consumption of vinyls was found to increase linearly with the original amount of vinyl unsaturations. At high peroxide levels, approximately the same gel content was obtained in all samples, leading to a considerably higher consumption of vinyls in the decadiene samples as compared to the reference. Mechanical measurements indicate a higher crosslinking density for the materials containing decadiene as compared to the reference. However, at lower gel contents, e.g. 40% and below, the situation is the opposite.

Coupling mechanism and fatigue behavior of vinyl triethoxysilane-treated carbon black?styrene-butadiene rubber vulcanizate

When a furnace carbon black was treated with vinyl triethoxysilane (VTEOS) coupling agent, the carboxyl and/or lactone groups of it reacted with the ethoxy group of the coupling agent. The vinyl unsaturation thus grafted to the surface of the treated carbon black underwent crosslinking with the unsaturation present in the backbone of rubber chains during sulphur vulcanization. The vulcanizate containing silane-treated carbon black (STCB) showed a tremendous improvement in the fatigue behavior compared to the vulcanizate containing untreated furnace carbon black (UFCB). This striking difference was explained on the basis of the mechanism of attachment of carbon black to the rubber chains. The STCB anchored to the rubber chains through chemical linkages, whereas the UFCB anchored to the rubber chains by physical means. The chemical linkages of STCB through silane to the rubber chains provided a strong interface, which considerably reduced the chance of failure from the interface, which happened to be the stress concentrating zone, while the physically adhered weak interface of UFCB and rubber chains failed prematurely. The analytical tools employed were infrared spectroscopy, differential scanning calorimetry, and fatigue-to-failure testing. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2235–2241, 2000

Network structure and reaction mechanisms in high pressure and peroxide vulcanization of polybutadiene: Microstructural changes studied by13C solid-state NMR

This article is concerned with the microstructural changes during peroxide and high pressure vulcanization of polybutadiene that is unfilled and filled with carbon black. The main tool is 13C solid-state NMR; it shows that vinyl unsaturations are consumed in both filled and unfilled samples under peroxide and high pressure vulcanization. Chemical shift calculations of unvulcanized polybutadiene show good agreement with the observed peaks. Calculations of proposed structures, based on a possible reaction mechanism, suggest that a large number of peaks will appear, each at very low intensity. Nevertheless, some changes can be seen as a result of the crosslinking reaction, and the results provide support for the suggested reaction mechanism. Thus, the proposed addition crosslinking mechanism over vinyl unsaturations seems to be a reasonable explanation of the crosslink formation in high pressure vulcanization. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2799–2806, 1999

EFFECT OF THE THERMO-OXIDATION AND NATURAL WEATHER ON THE STRUCTURE, MORPHOLOGY, AND PROPERTIES OF UNSTABILIZED AND HALS-STABILIZED LDPE FILMS

ABSTRACT The aging of unstabilized low density polyethylene (LDPE) films and those stabilized with hindered amine light stabilizers (HALS) was studied as a function of the exposure time to thermo-oxidation at 90°C and to natural weathering. The HALS tested was Tinuvin 783 from Ciba-Geigy and added to the polymer in the concentration of 0.6% (w/w). Modifications in the film characteristics were investigated by using Fourier transform infrared (FTIR), scanning electron microscopy (SEM), wide angle X-ray scattering (WAXS) and mechanical testing. The FTIR spectra showed that the thermo-oxidation led to the formation of ketone groups attributed to hydroperoxide decomposition, whereas in natural weathering ketones and vinyl unsaturations are formed due to Norrish II reactions. The mechanism of stabilization involved in thermo-oxidation of the HALS-stabilized samples may be attributed to the classical radical scavenging of alkyl and peroxy radicals formed during propagation by nitroxyl radicals. The stabilizing activity of Tinuvin 783 in natural weathering may be explained by its capacity to trap the alkyl radicals resulting from the charge transfer complexes reactions between the polymer and oxygen. The effectiveness of Tinuvin 783 as heat and UV stabilizer in LDPE film was confirmed by the slow decline of the elongation at break as found in the exposed samples. The thermo-oxidation and the natural weathering caused the formation of micro-voids on the surfaces of the unstabilized films; the presence of the HALS reduced the number and the size of the micro-voids. WAXS analysis showed that the crystallinity indexes of the unstabilized samples, both thermo-oxidated and exposed to natural weather, increase significantly with the time; for the stabilized samples the increases of the crystallinity resulted less pronounced.