Effect of processing on the environmental stress cracking resistance of high-impact polystyrene

Processing conditions have a strong effect on the final mechanical properties of products made of polymeric materials. Relevant phenomena most commonly include thermal stresses, physical ageing, frozen-in strains and molecular orientation. In this work, two different high-impact polystyrenes, processed by thermoforming, were considered: a “standard” one and a grade specifically resistant to Environmental Stress Cracking (ESC). The main effect induced by thermoforming was molecular orientation. The local degree of orientation was measured on a thermoformed product and its effect on the material ESC behavior in sunflower oil was studied. A Fracture Mechanics approach was applied to evaluate the fracture resistance of the two materials. Results show that a higher degree of orientation increases the fracture resistance in air but has no effect on the (expectedly lower) resistance in the active oil environment.     

Comportement des hydroperoxydes pendant la photo-oxydation d'un copolymère de styrène-butadiène

During the photo-oxidation at 254 nm of a styrene-butadiene copolymer, the quenching by hydroperoxides of singlet excited states of phenyl groups (and to a small extent of excimers) yield their photosensitized decomposition. The created radicals initiate hydroperoxide propagation reaction into the polymeric bulk. The natures of the various phases, through which this chain reaction develops, depend on the amount of available polymeric sites which can be easily oxidized (particularly allylic positions). Moreover, according to their behaviour, hydroperoxides have been placed in two classes: the “active” groups which are located at a stationary concentration in the vicinity of phenyl groups and the “inactive” groups which are outside this active sphere and remain stable after formation because they are not involved in the primary photophysical process.     

The mechanical degradation of polystyrene

The catastrophic failure of a polymeric material is preceded by a number of complex, partially understood events occurring on the molecular level. These events range from the flow of ordered regions to the cleavage of primary bonds in the chain. In recent years, stress-induced bond cleavage in polymers has received increased attention, many authors nothing the presence of free radicals and/or volatile products released upon fracture; a free-radical decomposition mechanism involving up to 10³ molecules per chain rupture also has been postulated. A special tensile stress–strain and shear apparatus was developed and located inside the ion-source housing of a time-of-flight mass spectrometer to characterize the volatile products released during mechanical degradation of polystyrene. Volatile compounds evolved during stress and fracture of polystyrene were monitored either continuously or by z-axis modulated oscilloscopic display. The polystyrene was purified by two methods: vacuum outgassing and fractional reprecipitation. Large amounts of styrene evolved from both the as-received and outgassed samples; however, essentially none was observed from the reprecipitated samples. Previous reports on monomer evolution during mechanical stress of polystyrene may be the result of residual monomer and not mechanical degradation products. The product of the surface density of primary radicals and the chain length for unzipping is less than 3 × 10¹⁰ radicals/mm² indicating a maximum radical concentration of approximately 10¹⁰ radicals/mm².     

Electronic energy transfer processes and symmetry conservation in reactions in inclusion compounds

The results are presented of a study on the electronic energy transfer (EET) processes in inclusion host-guest compounds irradiated by gamma or UV photons. The host cavity site symmetry is correlated with the guest molecular symmetry and with the stereocontrol of the reaction products formed under irradiation; a specific “heteroexcimer” or “excithrate” is suggested as an excited host-guest intermediate radical-pair; in particular, symmetry correlations are suggested in order to explain reaction pathways into cavities of inclusion compounds in the excited state.     

Vacuum and oxidative pyrolysis of poly-p-xylylene. II. Chromatographic analysis and kinetics of reaction products

Reaction products of vacuum and oxidative degradation of poly-p-xylylene have been quantitatively determined by chromatographic analysis as function of time, temperature and oxygen pressure. Respective Arrhenius parameters were also ascertained for some of the reaction products and for the sums of all products. The energies of activation for the sums agree quite satisfactorily with the energies of activation obtained previously by uninterrupted experiments in quartz-spoon reaction vessels. The results found here can be described in terms of mechanisms previously postulated on the basis of the total loss in weight (or volatile production) data. Scission of “weak” links (due to abnormal structures) takes place followed by formation of various products. The whole process is governed by the initial chain scission reaction; however, the energies of activation for each of the products do not need to be identical with that of the chain scission reaction. Each product is formed by a reaction which has its own characteristic number average kinetic chain lengths; the latter have their specific energy of activation values. Oxidative degradation produces the same organic compounds as vacuum degradation and in addition CO, CO₂, and H₂O. Oxidized intermediate compounds are apparently fairly rapidly decarboxylated and decarbonylated. Oxidative chain scission is appreciably faster than that in vacuum. Almost simultaneous “weak” link and “normal” chain scission are taking place initiating the formation of a number of products.     

Energetics of low-molecular-mass products of mechanical fracture of poly(methyl methacrylate)

It is experimentally revealed that monomer and water molecules released during the fracture of PMMA have a bimodal velocity distribution. The first distribution peak for the monomer corresponds to energetic (hot) MMA molecules (0.13–0.70 eV), and the second peak is attributed to MMA molecules bearing a low energy (0.016–0.060 eV). On the basis of the results of earlier theoretical studies of the mechanically induced polymer-chain scission, it was concluded that the energetic monomer molecules are produced immediately in the event of mechanical chain rupture and are nonthermal in nature. In the bimodal velocity distribution of occluded water molecules desorbed from a subsonic crack, the first and the second peaks are attributed to “hot” and “cold” molecules with translational temperatures of 605 ± 180 K and 53 ± 5 K, respectively. A possible mechanism of the mechanically induced desorption of occluded water is discussed. The mechanodesorption of water molecules is due to the portion of vibrational energy transferred to side methyl carboxylate groups that retain water molecules by the unloading-wave front traveling along the main chain from a macromolecule scission site. Along with hot water molecules, a considerable amount of hot free hydroxyls were detected. The mechanism of formation of the latter species is associated with the double-well structure of the hydrogen bond potential and similar to the mechanism of mechanodesorption of hot molecules of water.     

“Proximity effects” in radiative transitions: a model study of the role of vibronic coupling and static crystal field interactions

We report the results of a model study of the influence of vibronic coupling involving non-totally symmetric vibrations and static crystal field interactions on the spectral properties of molecules with close-lying excited electronic states. The presented results suggests that “proximity effects” brought about by solvent perturbation arise from two sources: (i) alterations in the energy separation between vibronically coupled electronic states and (ii) crystal field mixing of the isolated molecular electronic states. It is shown that crystal field mixing leads to the breakdown of the vibronic coupling scheme for non-totally symmetric vibrations in isolated molecules. This breakdown is shown to have a very pronounced effect on the spectral properties of molecules with close-lying excited electronic states. The effect of environmental perturbations on excited state frequencies, the breakdown of symmetry and polarization selection rules, and vibrational intensity distributions is discussed.     

Knots “Choke Off” Polymers upon Stretching

Long polymer chains inevitably get tangled into knots. Like macroscopic ropes, polymer chains are substantially weakened by knots and the rupture point is always located at the “entry” or “exit” of the knot. However, these phenomena are only poorly understood at a molecular level. Here we show that when a knotted polyethylene chain is tightened, most of the stress energy is stored in torsions around the curved part of the chain. The torsions act as “work funnels” that effectively localize mechanical stress in the immediate vicinity of the knot. As a result, the knot “chokes” the chain at its entry or exit, thus leading to bond rupture at much lower forces than those needed to break a linear, unknotted chain. Our work not only explains the weakening of the polymer chain and the position of the rupture point, but more generally demonstrates that chemical bonds do not have to be extensively stretched to be broken.     

Photodissociation Dynamics of Cyclopropenylidene,c‐C3H2

In this joint experimental and theoretical study we characterize the complete dynamical “life cycle” associated with the photoexcitation of the singlet carbene cyclopropenylidene to the lowest lying optically bright excited electronic state: from the initial creation of an excited‐state wavepacket to the ultimate fragmentation of the molecule on the vibrationally hot ground electronic state. Cyclopropenylidene is prepared in this work using an improved synthetic pathway for the preparation of the precursor quadricyclane, thereby greatly simplifying the assignment of the molecular origin of the measured photofragments. The excitation process and subsequent non‐adiabatic dynamics have been previously investigated employing time‐resolved photoelectron spectroscopy and are now complemented with high‐level ab initio trajectory simulations that elucidate the specific vibronic relaxation pathways. Lastly, the fragmentation channels accessed by the molecule following internal conversion are probed using velocity map imaging (VMI) so that the identity of the fragmentation products and their corresponding energy distributions can be definitively assigned.     

Starburst Dendrimers: Molecular-Level Control of Size,Shape, Surface Chemistry,Topology, and Flexibility from Atoms to Macroscopic Matter

Starburst dendrimers are three-dimensional, highly ordered oligomeric and polymeric compounds formed by reiterative reaction sequences starting from smaller molecules—“initiator cores” such as ammonia or pentaerythritol. Protecting group strategies are crucial in these syntheses, which proceed via discrete “Aufbau” stages referred to as generations. Critical molecular design parameters (CMDPs) such as size, shape, and surface chemistry may be controlled by the reactions and synthetic building blocks used. Starburst dendrimers can mimic certain properties of micelles and liposomes and even those of biomolecules and the still more complicated, but highly organized, building blocks of biological systems. Numerous applications of these compounds are conceivable, particularly in mimicking the functions of large biomolecules as drug carriers and immunogens. This new branch of “supramolecular chemistry” should spark new developments in both organic and macromolecular chemistry.     

Thermal degradation of polymers with phenylene units in the chain. I. Polyphenylenes and poly(phenylene oxides)

The thermal degradation of polyphenylenes and poly(phenylene oxides) was studied under vacuum at temperatures between 350 and 620°C. The volatile and solid degradation products were analyzed by mass spectroscopy, infrared spectroscopy, and elemental analysis. Overall mechanisms for the thermal breakdown have been proposed. Polyphenylene decomposes to form polymer carbon, while hydrogen is the major volatile product. Some ring breakdown occurs with evolution of methane. Poly(phenylene oxide) forms mainly low molecular weight chain fragments, partially with hydroxyl endgroups. Some of the ether linkages decompose with ring breakdown, yielding carbon monoxide, water, and some carbon dioxide. Pendent groups on polyphenylenes and poly(phenylene oxides) are removed at the lower temperatures. The hydroxyl group yields essentially carbon monoxide and dioxide (the carbon being supplied by the rings), the methyl group methane, and the methoxy group methane and some methanol.     

Thermal degradation and stabilization of poly(2,6-dimethyl-1,4-phenylene oxide)

Thermal degradation and stabilization of poly(2,6-dimethyl-1,4-phenylene oxide) have been examined in air in the range 100–400°. Plots of weight-average molecular weight vs time are linear, confirming random chain scission. The breakdown process has also been studied by DTA and TGA. It was concluded that thermal analysis alone was insufficient to characterize the degradation fully so the degradation products were determined qualitatively using i.r. and NMR spectroscopy. The heats of activation for the systems have been calculated and a stabilization mechanism by bis(1-phenyl-3-α-pyridyl triazeno)Cu(II) chelate has been postulated.     

Characterization of selectively degraded poly(ethylene terephthalate)

To investigate the morphology of unoriented poly(ethylene terephthalate) (PET) films and the selective character of the aminolysis of PET, 67% crystalline polymer samples were degraded with 40% aqueous methylamine at room temperature. The aminolyzed PET samples were subjected to gel permeation chromatography (GPC), viscometry, electron microscopy, and small-angle x-ray diffraction (SAXD). Weight loss and density crystallinity measurements were also made. After 24 hr of aminolysis, the amorphous regions and chain folds were completely removed. The long molecular chains in the semi-crystalline polymer were reduced to monodisperse rods having a molecular weight of 1,800. The corresponding lamellar thickness was calculated to be 101 Å, consistent with the x-ray diffraction and electron microscope (EM) measurements. The EM photographs of “stripped” crystals show the lamellar structure previously found for other selectively degraded polymeric materials. The weight of crystalline debris remaining was consistent with the initial crystallinity. After degradation the crystallinity as determined by density was 96%.     

A Mechanochemically Triggered “Click” Catalyst

“Click” chemistry represents one of the most powerful approaches for linking molecules in chemistry and materials science. Triggering this reaction by mechanical force would enable site‐ and stress‐specific “click” reactions—a hitherto unreported observation. We introduce the design and realization of a homogeneous Cu catalyst able to activate through mechanical force when attached to suitable polymer chains, acting as a lever to transmit the force to the central catalytic system. Activation of the subsequent copper‐catalyzed “click” reaction (CuAAC) is achieved either by ultrasonication or mechanical pressing of a polymeric material, using a fluorogenic dye to detect the activation of the catalyst. Based on an N‐heterocyclic copper(I) carbene with attached polymeric chains of different flexibility, the force is transmitted to the central catalyst, thereby activating a CuAAC in solution and in the solid state.     

Abiotic and biotic degradation of aliphatic polyesters from “petro” versus “green” resources

Aliphatic polyesters are degradable by abiotic and/or biotic hydrolysis. The accessibility of a polymer to degradative attack by living organisms is not dependent on its origin, but on its molecular composition and architecture. Synthetic polymers with intermittent ester linkage (e.g. polyesters, polyurethanes etc.) are accessible to biodegradative attack of esterase. On the other hand aliphatic polyesters are also quickly degraded by a pure abiotic hydrolysis. The results from abiotic and biotic hydrolyses of polycaprolactone (PCL) (from “petro” resource), poly(L-lactide) (PLLA) and polyhydroxyalkanoates (PHA) (from “green” resources) are presented and discussed with the respect to rate of degradation, molecular weight changes and degradation product pattern. For the environmental consequences, the type of formed degradation products are of importance and not the origin of the polymer.     

Specific features of the radiolysis of water and aqueous solutions of H2 and O2 by mixed n,γ-radiation with a high portion of the neutron component

A previously unknown feature of the kinetics of the radiolysis of water and hydrogen, oxygen, and hydrogen peroxide solutions has been discussed. By calculation, it has been revealed that concentration oscillations of the radiolysis products can appear during irradiation of the solution with fast neutrons or mixed n,γ-radiation with a high portion of the neutron component. The period and amplitude of the oscillations depend on the temperature, the dose rate, and the ratio of n/γ radiation components. It has been shown that oscillations cannot be excited during γ-radiolysis under any conditions. It is suggested that the mechanism of the oscillations is similar to the Belousov-Zhabotinsky reaction mechanism. A chain reaction proceeds in the irradiated system, in which the reactants H₂O₂ (“reducing agent”), “oxidizing agent” OH radicals initiating the chain, and the “catalyst” are introduced from the outside. Hydrogen molecules produced by the action of radiation play the role of the “catalyst”, and H₂O molecules formed in the secondary reactions are the “deactivated form of the catalyst”. Hydrogen atoms and hydrated electrons propagate the chain. Oxygen formed in both spurs and the secondary reactions is the “inhibitor” terminating the chain reaction.     

Dynamic mechanical thermal and infrared analyses of polyacrylonitrile “electrografted” onto a metal

Polyacrylonitrile (PAN) films have been “grafted” onto copper electrodes by cathodic polarization and analyzed by infrared spectroscopy and dynamic mechanical thermal analysis (DMTA). The dynamic mechanical response shows two or three transitions depending on the film thickness and the potential deposition. The viscoelastic properties of “ungrafted” PAN films, e.g., solvent cast films of commercially available PAN, are deeply different from those of “electrografted” films. The experimental data support that “ungrafted” chains are paracrystalline in contrast to the “grafted” ones which are essentially amorphous. Moreover, the irreversible transformation of the “grafted” PAN chains observed beyond 225°C is confirmed by Fourier transform infrared (FTIR) analysis and ascribed to an intramolecular cyclization of PAN into polyimine. This reaction occurs rapidly and at a comparatively low temperature with respect to “ungrafted” PAN, which suggests that the “grafted” chain configuration might be predominantly isotactic. The isotacticity and the amorphous structure appear to decrease as the thickness of the PAN film is increased. Literature data and the herein reported observations would suggest a dependence of the amorphous structure of PAN on the chain isotacticity. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 543–553, 1998     

Polymeric carriers of drugs for site-specific therapy

Three types of water-soluble polymeric drug carrier systems facilitating targeted drug delivery and controlled drug release were synthesized. All systems consist of an inert soluble synthetic polymer, drug and homing device (targeting moiety). In the first “classical” system, both drug and targeting moiety are bound to a nondegradable polymer by means of biodegradable oligopeptide side chains statistically distributed along the polymer chain. The second, “star-like” system contains a targeting moiety (antibody) in the centre and a hydrophilic polymer, bearing drug molecules, in the shell of the system. The third, “biodegradable” carrier system is based on block copolymers of poly(ethylene glycol) containing biodegradable oligopeptide sequences both in the main polymer chain and in the spacers between main chain and drug molecules. Strategy and details of the synthesis of all three systems are given.     

Synthesis and structural characterization of nickel(II) hexaazamacrotetracyclic complexes with phthalate ligands

Highly crosslinked polymeric networks formed by cyclodextrins (CD) have recently been shown to be highly versatile nanosponge systems, being for instance very efficient both for drug delivery and for pollutants removal. Here we report some molecular simulation results for dry and hydrated CD nanosponge models aimed to study their swelling behavior. We also report simulation results about the water mobility in these systems in terms of the calculated diffusion coefficient of “free” and of “bound” water molecules confined within the nanosponge cavities. Furthermore, we also suggest the presence of surface-constrained water molecules temporarily bound to the network surface but eventually set free in the bulk.     

An investigation of dynamic mechanical, thermal, and electrical properties of housing materials for outdoor polymeric insulators

The present paper reports the results about a study of mechanical, thermal, dynamic mechanical and electrical properties of housing (weather shed) materials for outdoor polymeric insulators. Silicone rubber, ethylene-propylene-diene monomer (EPDM) and alloys of silicon-EPDM are known polymers for use as housing in high voltage insulators. The result of dynamical mechanical measurement shows that the storage modulus of blends enhances with increase EPDM in formulation. It can be seen from the result of TGA measurement that initial thermal degradation of silicone rubber improves by the effect of EPDM in blends. The blends of silicone-EPDM show good breakdown voltage strength compared to silicone rubber. Surface and volume resistance of silicone rubber improve by EPDM content. The mechanical properties of EPDM such as strength, modulus and elongation at break improve by silicone.