Trans-vinylene Groups Research Articles

Resonance Raman Investigation of β-Cyclodextrin-Encapsulated π-Conjugated Polymers

Resonance Raman (RR) spectroscopy is used to investigate the effect of β-cyclodextrin encapsulation on the structural and photophysical properties of poly(4,4'-diphenylenevinylene). We especially focus on the thermal stability of the polymer. We find that within the range of 10-55 °C the uninsulated polymer exhibits decreased Raman intensity in all the vibrational bands with temperature, along with changes in the relative intensity of the C-C inter-ring stretch mode at 1270 cm(-1) with respect to the ring C-H in-plane symmetric bend at 1187 cm(-1), which provides evidence for conformational changes as a function of temperature. No changes are observed in the intensity of the in-phase CH out-of-plane wag of the vinylene group at 968 cm(-1). Therefore, the conformational changes involve mainly dihedral angle modification between the adjacent phenyl rings toward planarization, and little or no change in the planarity of the trans-vinylene group. The decrease of the optical absorption at 55 °C with respect to that at room temperature and the appearance of a new absorption band at lower energies explain the decrease in the RR intensities and the wavelength dependence of the relative intensities of the Raman band. We note that the conformational change into a more planar geometry, which affects a significant portion of the polymer population, is irreversible and consistent with thermally induced aggregation. Such a planarization is unexpected in view of the usually observed thermochromic behavior of conjugated polymers, which leads to an increase of the energy gap for increasing temperature, as the average dihedral angles are increased due to excitation of a larger number of vibrational modes. Interestingly, the higher threading ratio polymers are resistant to any conformational changes within this temperature range, as reflected by their unchanged RR spectra, due to the rotaxane's ability to suppress intermolecular interactions and aggregation. Interestingly, the conformation of the uninsulated polymer at room temperature appears to be the same as that in the threaded analogues, which suggests that the cyclodextrin cavity hosts the polymer without physically interacting with it.

Stereocontrolled Synthesis and Photoisomerization Behavior of All-Cis and All-Trans Poly(m-phenylenevinylene)s

All-cis and all-trans isomers of poly[(arylenevinylene)-alt-(5-octyloxy-1,3-phenylenevinylene)]s having three kinds of arylene groups (PmPVs: arylene = m-phenylene (1a), p-phenylene (1b), 4,4′-biphenylene (1c)) have been synthesized in almost perfect stereoregularity by two types of palladium-catalyzed polycondensation reactions, respectively. Suzuki−Miyaura-type polycondensation of (Z,Z)-bis(2-bromoethenyl)arenes with 5-octyloxy-1,3-benzenediboronic acid pinacolate affords all-cis 1a−c, whereas Hiyama-type polycondensation of (E,E)-bis(2-silylethenyl)arenes with 5-octyloxy-1,3-diiodobenzene forms all-trans 1a−c. The resulting polymers with relatively short effective π-conjugation lengths interrupted by m-phenylene units undergo two-way photoisomerization to give PmPVs containing cis- and trans-vinylene groups in nearly 1:1 ratios irrespective of the arylene groups of staring polymers.

Mechanistical studies on the electron-induced degradation of polymers: polyethylene, polytetrafluoroethylene, and polystyrene

Mechanisms of the electron-induced degradation of three polymers utilized in aerospace applications (polyethylene (PE), polytetrafluoroethylene (PTFE), and polystyrene (PS)) were examined over a temperature range of 10 K to 300 K at ultra high vacuum conditions (∼10(-11) Torr). These processes simulate the interaction of secondary electrons generated in the track of galactic cosmic ray particles in the near-Earth space environment with polymer material. The chemical alterations at the macromolecular level were monitored on-line and in situ by Fourier-transform infrared spectroscopy and mass spectrometry. These data yielded important information on the temperature dependent kinetics on the formation of, for instance, trans-vinylene groups (-CH=CH-) in PE, benzene (C(6)H(6)) production in PS, fluorinated trans-vinylene (-CF=CF-) and terminal vinyl (-CF=CF(2)) groups in PTFE together with molecular hydrogen release in PE and PS. Additional data on the radiation-induced development of unsaturated, conjugated bonds were collected via UV-vis spectroscopy. Temperature dependent G-values for trans-vinylene formation (G(-CH=CH-) ≈ 25-2.5 × 10(-4) units (100 eV)(-1) from 10-300 K) and molecular hydrogen evolution (G(H(2)) ≈ 8-80 × 10(-5) molecules (100 eV)(-1) from 10-300 K) for irradiated PE were calculated to quantify the degree of polymer degradation following electron irradiation. These values are typically two to three orders of magnitude less than G-values previously published for the irradiation of polymers with energetic particles of higher mass.

Swift heavy ion irradiation effects on carbonyl and trans-vinylene groups in high and low density polyethylene

In this work, we have studied the effects of swift heavy ion irradiation on the creation of new functional groups in high and low density polyethylene (HDPE and LDPE). Polymers were irradiated with different ions (6.77 MeV He and 47 MeV Li) and fluences. The induced changes were analyzed by Fourier transform infrared (FTIR) spectroscopy. Creation and damage cross sections for some groups were compared for two different types of PE.

Preparation and Characterization of Partially Degraded High-Density Polyethylene in an Internal Mixer

The partial degradation of high-density polyethylene was done in an internal mixer for different residence time. The oxidative thermomechanical degradation resulted in the formation of carbonyl groups in the polymer. The carbonyl index values estimated from the FTIR studies indicate the increase of carbonyl groups without any induction time. The common phenomenon during oxidative degradation of polyethylene is the increase in trans-vinylene groups and decrease in vinyl groups and these phenomena are observed in the current investigation. Owing to the chain scission occurring during the mixing at processing temperature, the change in morphology was studied by the amount of crystallinity estimated through DSC. The variation in the molar mass of the material was followed by SEC method.

Thermolysis of polyethylene hydroperoxides in the melt, Part 10: Product yields from bimolecular and pseudo-monomolecular hydroperoxide decomposition

The quantitative aspects of some specific decomposition reactions of polyethylene hydroperoxides are re-examined. New data have shown that β-scission of primary alkoxy radicals is negligible in the temperature range of the thermolysis experiments. This is important for the true bimolecular hydroperoxide decomposition for which, in a first approximation, β-scission of primary and secondary alkoxy radicals had been taken into account. The calculation shows that the yields of the main oxidation products such as secondary alcohols, ketones, trans-vinylene groups and aldehydes are not considerably affected by the change. However, the theoretical yields of some minor products such as primary alcohols and of some combination reactions are strongly affected. For the pseudo-monomolecular hydroperoxide decomposition involving a segment of the polymer, the main novelty in comparison with previous work consists in taking into account β-scission of the secondary alkoxy radicals. It allows improving the accuracy of the calculated product yields. Moreover, all the theoretical calculations are on the same level of accuracy and can be used for comparison with the experimental product yields.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 32: Formal kinetics of γ-lactone formation from secondary products. Heterogeneous kinetics

Oxidation of aldehydes and γ-hydroxy- trans-vinylene groups can yield γ-lactones. These intermediates account for γ-lactone formation in the advanced stages of polyethylene processing in air. The acyl-peroxy radical formed on free radical induced oxidation of aldehydes can abstract intramolecularly a δ-hydrogen atom to yield a peracid. Reaction of the alkyl radical formed in this reaction with the hydroperoxide group of the peracid gives a γ-lactone with simultaneous release of a hydroxyl radical. The calculated rate of γ-lactone formation according to the mechanism envisaged decreases slightly with increasing temperature (activation energy of about −5 kcal/mol). It is in agreement with the experiments that do not show significant activation energy in the high temperature range for the advanced stages of polyethylene processing. The calculated rate of γ-lactone formation is found to increase by a factor of about 2.7 if the processing experiments are performed in pure oxygen instead of in air. This is close to the experimental factor of about 2. Peroxidation of γ-hydroxy- trans-vinylene groups can also yield γ-lactones. The first possibility involves addition of a peroxy radical to the double bond followed by oxygen addition to the alkyl radical. This reaction possibly yields an α-peroxy-hydroperoxide. Intramolecular decomposition involving the two reactive groups of the α-peroxy-hydroperoxide can give an ozonide that on thermal decomposition yields among others an acid group in 4-position to the alcohol. The activation energy calculated is strongly negative so that the rate should decrease strongly with increasing temperature. Hence, the mechanism cannot contribute significantly to γ-lactone formation in the whole temperature range of the experiments. This is so in spite of the fact that the rate is estimated to increase by a factor of about 1.7 on passing from air to pure oxygen, which is close to the experimental value of approximately 2. The second possibility of transformation of γ-hydroxy- trans-vinylene groups is based on stress-induced oxygen addition to the double bond. Acid catalyzed decomposition of the allylic hydroperoxide that is formed in the reaction yields a pair of aldehydes with one of the aldehyde groups in 4-position to the alcohol group. Peroxidation of the aldehyde pair can give an acid group in 4-position to the hydroxyl group so that a γ-lactone can be formed. The activation energy calculated for the process is very small and the effect of the oxygen concentration corresponds to an increase by a factor of approximately 4.5 on passing from air to pure oxygen. It is postulated that simultaneous contribution by different mechanisms might well account for the experimental value of about 2. The heterogeneous kinetics discussed in detail allows for complementary data interpretation. It is especially suited for the understanding of the advanced stages of polyethylene processing, after some induction time.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 28: Formal kinetics of aldehyde and carboxylic acid formation in the advanced stages

The rate of acid formation at high temperature is constantly increasing but temperature independent. Two main mechanisms can account for this behavior in the advanced stages of polyethylene processing. The first mechanism is based on free radical induced oxidation of aldehyde pairs that are formed on acid-catalyzed decomposition of allylic hydroperoxides. The last will be formed essentially on mechanical stress-induced oxygen addition to trans-vinylene groups. Peroxidation of one of the aldehydes might yield an acyl-peroxy radical that is likely to abstract the labile hydrogen atom from the second aldehyde. The acyl radical formed in the reaction will abstract a hydroxyl group from the peracid formed in the same reaction. This yields an acid and an acyl-oxy radical that will give a primary alkyl radical on decarboxylation. The second mechanism involves oxidation of ketones and alcohols that accumulate in the oxidizing melt. Acid-catalyzed decomposition of the α-keto-hydroperoxides yields simultaneously an acid and an aldehyde. Formal kinetics based on each mechanism shows that they do not involve significant activation energy, as it is required by the experimental data. The dependency on the oxygen concentration deduced from the formal kinetics for the oxidation of aldehyde pairs is in agreement with the experiments.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 29: Experimental kinetics and mechanisms of γ-lactone formation

The experimental kinetics for γ-lactone formation shows more complexity than that for acids. Nonetheless, it can be concluded to the existence of a constant rate of formation from the beginning of the experiments with polyethylene melts. There is an additional term contributing to γ-lactone formation in the initial stages that is cubic in processing time. In the advanced stages of processing, in the high temperature range (170–200 °C), the concentration of γ-lactones increases linearly with the processing time. There are many mechanisms susceptible to give γ-lactones on polyethylene melt processing. Some of them are based on decomposition of intermediates formed directly on chain propagation. This is so for the α,γ-keto-hydroperoxides in 4-position to hydroxyl groups. Since decomposition of these intermediates is very fast, the reaction might account for a constant rate of γ-lactone formation from the beginning of polyethylene processing. Decomposition of the α,δ-keto-hydroperoxides formed on intramolecular reactions on chain propagation is not so fast as that of the α,γ-keto-hydroperoxides. Nonetheless, it might account for part of the delayed formation of γ-lactones. The same is valid for the mechanisms based on peroxidation of aldehydes and γ-hydroxy trans-vinylene groups that involve intermediates that are formed on polyethylene peroxidation. They might be important for explaining the cubic term as well as γ-lactone formation in the advanced stages of polyethylene processing.

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.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 23: Mechanisms and kinetics of trans-vinylene group consumption

There are many potential reactions for trans-vinylene groups in oxidizing polyethylene melts. The main possibilities are reactions with peroxy radicals, molecular oxygen, hydroperoxides and peracids. These different reactions can all contribute to the removal of trans-vinylene groups to some extent. This is especially so, for the reactions with hydroperoxides that have been found to be the dominant reactions with vinylidene and vinyl groups in the low temperature range. The reaction with peroxy radicals is thought to be as important relatively as with vinylidene groups. Therefore, the importance of the reaction is decreasing with increasing temperature. However, the most characteristic reaction for trans-vinylene groups can be detected without any doubt only in the advanced stages of processing. It is mechanical stress induced oxygen addition to the double bond. The discussion shows that the reaction should be important from the beginning of processing. The reaction cannot operate with vinyl and vinylidene groups, which are not part of the polyethylene main chain. After oxygen addition to the trans-vinylene group, the “ene” reaction yields an allylic hydroperoxide so that the double bond is not immediately removed. It is acid catalyzed hydroperoxide decomposition that leads to chain scission with aldehyde formation at the new chain ends.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 21: Improved product yields on true bimolecular hydroperoxide decomposition

The product yields from the reaction between two hydroperoxide groups have been re-calculated. This is a consequence of the fact that β-scission of secondary alkoxy radicals cannot be neglected in the high temperature range of the polyethylene processing experiments (170–200 °C). It must be taken into account in addition to disproportionation/combination and hydrogen abstraction by alkoxy radicals. The increased complexity caused by the additional reaction results mainly from the larger number of caged radical pairs involved in the reactions and also in the calculations. Among other products it becomes possible to calculate the yields of aldehyde and vinyl groups that would not result from hydroperoxide decomposition in the absence of β-scission. The yields of the main oxidation products such as alcohols, ketones and trans-vinylene groups are reduced to some extent in comparison with the values calculated if β-scission is neglected. The vinyl group yield corresponds to slightly more than 10% of the yield of trans-vinylene groups in the temperature range of the experiments. The aldehyde yield is significantly larger than the vinyl group yield and is important in the whole temperature range examined. Main-chain scissions are important at the temperatures of the experiments. They become more important than the sum of the different combination reactions from a temperature of 200 °C on.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 20: Additional product yields on bimolecular hydroperoxide decomposition with an alcohol group

Most products formed on polyethylene oxidation result from hydroperoxide decomposition. The product yields can be calculated for various mechanisms of hydroperoxide decomposition. This work concerns the reaction of a hydroperoxide with an alcohol group thought to be dominant in the advanced stages of polyethylene processing in the high temperature range (170–200 °C). Besides hydrogen abstraction by caged alkoxy radicals already envisaged previously, the possibility of β-scission is taken into account. This additional reaction introduces significant complexity into the reaction schemes. This is especially so because additional caged radical pairs must be included into the schemes and the calculations. It becomes possible to calculate the yields of aldehyde and vinyl groups that do not result from hydroperoxide decomposition in the absence of β-scission. The yields of the main oxidation products such as alcohols and ketones are not much affected by taking into account β-scission. The yield of aldehydes is important in the whole temperature range and increases considerably if the temperature is raised from 170 to 200 °C. It becomes more important than the ketone yield. The vinyl groups are formed in amounts corresponding roughly to 10–15% of the trans-vinylene groups in the temperature range of 170–200 °C.

Infrared spectroscopy analysis of physico-chemical modifications induced by heavy ions in ultra-high molecular weight polyethylene

Structural analysis with infrared spectroscopy was carried out on ultra-high molecular weight polyethylene (UHMWPE). The polyethylene was irradiated using Li, C, N, S and I heavy ion beams with different fluences. The analysis showed that the absorbance of the CC stretching vibration from trans-vinylene groups containing hydroperoxides presents a maximum at certain ion fluence and is correlated with the ion stopping power. The results were analyzed by a Monte Carlo simulation program that models the irradiation process from a geometrical point of view, in which the latent tracks consist in a central core zone surrounded by a larger perturbed region. This study suggests that it is possible to estimate the core dimension from absorbance measurements as a function of fluence. A possible relationship between this CC structure with the cross-link of two polymeric chains is discussed.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 18: Mechanisms and kinetics of trans-vinylene group formation

Numerous reactions can yield trans-vinylene groups on polyethylene oxidation. The first problem on data interpretation consists in the separation of the experimental data into components corresponding to well-defined mechanisms. This is achieved by fitting the experimental data into an equation comprising a linear and a parabolic term. The linear term corresponds to trans-vinylene formation at a constant rate from the beginning of the experiments. It can be attributed to trans-vinylene formation on direct decomposition of polyethylene peroxy radicals. The second term accounts for trans-vinylene group formation on cage reactions of various free radicals resulting from hydroperoxide decomposition. The first mechanism can be interpreted by formal homogeneous kinetics. Formation of trans-vinylene groups according to the second mechanism can be accounted for by the heterogeneous kinetics. It proceeds in parallel with the formation of alcohols and ketones. However, reaction of the double bonds with various reactive species in the oxidizing polymer melt does not only lead to a limiting value of the concentration in the advanced stages of polyethylene processing, but also affects the accuracy of the calculations already in the early stages.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 17. Effect of oxygen availability

The gas in contact with polyethylene has considerable impact on its oxidation. The rate of oxidation product formation is mostly larger with oxygen blanketing than in air. Similarly, the rate in air is larger than that under nitrogen blanketing. Moreover, the relative effect of the surrounding gas is depending heavily on the particular oxidation product considered. The effect on the alcohol concentration on passing from air to pure oxygen is the same as that on the hydroperoxide concentration. It is only under pure nitrogen that alcohol formation is relatively more affected than hydroperoxide formation. The overall carbonyl groups as well as the ketones show the expected ranking, i.e. faster rate in pure oxygen than in air and faster rate in air than under pure nitrogen. However, carboxylic acids are formed much faster in oxygen than in air. For the acids the results in air and under nitrogen are significantly closer in the initial stages of processing than the results obtained under pure oxygen. This is different for γ-lactones for which formation is faster in oxygen than in air where it is faster than under nitrogen. With trans-vinylene groups the situation is opposite to that observed for carboxylic acids: the rate of formation is close for the experiments performed under air and under oxygen and significantly faster than under nitrogen. The results for hydroperoxides, alcohols and ketones are easily interpreted taking into account the kinetics developed in previous work. Fitting the data to the heterogeneous kinetics shows the effect of the oxygen concentration on this kinetics. It is especially unexpected with respect to its impact on the initiation rate. It is discussed taking into account various possibilities. The only one that is compatible with all the data envisages chain initiation resulting from interaction of oxygen with strained polymer molecules.

Physico-chemical aspects of polyethylene processing in an open mixer. Part 9: complements to the experimental kinetics of oxidation product formation

Polyethylene oxidation on processing in air has been discussed several times. In spite of substantial amounts of work, there remains considerable uncertainty with respect to the mechanisms and kinetics involved. The aim of this work is presenting additional data and highlighting similarities and differences between the rates of formation of various oxidation products. The data available point to significant correlation between the rates of formation of alcohol, ketone and trans-vinylene groups. Hence, these groups should be formed in the same or, at least, in similar processes. There is no general correlation between the formation of carboxylic acids and γ-lactones. There is also no direct relation between the two last oxidation products and the other products detected in the system. The Arrhenius-plots of the experimental rates deduced from fitting the data to two elementary rate equations yield many values for the activation energy. However, usually, these values cannot be related to well-defined chemical reactions. Even if the values look reasonable, there remains considerable doubt concerning the correct attribution to some reaction. In fact, further work shows that the apparent agreement is mainly fortuitous. It seems mandatory to take a new approach at data interpretation.

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.

Very low temperature irradiation of aliphatic polymers: Role of radical migration on the creation of stable groups (O-127)

We have analysed the influence of temperature, and thus the role of radical motion on the evolution of two in-chain unsaturated bonds, transvinylene, –CH 2–CHCH–CH 2– and trans–trans diene, –CHCH–CHCH–, created in aliphatic polymers submitted to radiations. Results reported here are related to three polymers (PE, EPDM-1,4 hexadiene and EPR) irradiated with high energy ions at 8 and 298 K. Annealing samples, irradiated at 8 K, up to room temperature provides information on the radical migration effect on the creation and the stability of the studied defects. The modifications were followed on line by Fourier transform infrared spectroscopy. The trans–trans diene groups creation necessarily requires radical migration while an in-cage process is efficient for transvinylene groups creation. The radiative destruction of transvinylene groups is clearly induced by radical migration.

Thermolysis of polyethylene hydroperoxides in the melt 5. Mechanisms and formal kinetics of product formation

Many products are formed on thermolysis of polyethylene hydroperoxides. This is especially so for the carbonyl compounds involving ketones, in-the-chain ketones and metyl-ketones, carboxylic acids, aldehydes, esters and γ-lactones. Only γ-lactones can be easily measured as such by IR spectroscopy because they absorb at a wavelength different from that of the bulk of the carbonyl groups. This is also true for the secondary alcohols and the trans-vinylene groups. Formation of all the products observed can be explained by relatively simple mechanisms. Confrontation of the experimental results with formal kinetics based on the assumed mechanisms leads to improved understanding of these mechanisms. It is concluded that hydrogen abstraction by alkoxy radicals is the only significant source for alcohols on thermolysis of polyethylene hydroperoxides. The abstraction of hydroxyl radicals from hydroperoxides, very often envisaged with alkyl radicals, does not contribute to alcohol formation. Trans-vinylene groups seem to result from disproportionation of geminate alkyl radicals. Intramolecular hydrogen abstraction following intramolecular hydroperoxide decomposition might also contribute to double bond formation. Formation of γ-lactones is best explained by monomolecular decomposition of special structures formed on processing in air. The last result from reactions following intramolecular hydrogen abstraction by the alkoxy radical formed on pseudo-monomolecular hydroperoxide decomposition. The reactions proposed are in agreement with formal kinetics as well as with thermochemistry.