Controlled degradation and crosslinking of polypropylene induced by gamma radiation and acetylene

Isotactic polypropylene (iPP) undergoes crosslinking and extensive main chain scissions when submitted to irradiation. The simultaneous irradiation of PP and acetylene is able to control chain scission and produce grafting. The grafted PP further reacts with PP radicals resulting in branching and crosslinking. In this work, commercial polypropylenes (iPP) of different molecular weights were irradiated with a ⁶⁰Co source at dose of 12.5 kGy in the presence of acetylene in order to promote the crosslinking. The mechanical and rheological tests showed a significant increase in melt strength and drawability of the modified samples obtained from resins with high melt flow index. The characterization of the molecular modifications induced by gamma irradiation of isotactic polypropylenes under acetylene atmosphere proved the existence of branching, crosslinking and chain scission in a qualitative way. The G′ and G″ indicated the presence of LCB in all samples. Therefore, PP irradiation under acetylene was proved to be an effective approach to achieve high melt strength polypropylene (HMSPP).     

Thermoplastic interpenetrating polymer networks based on a poly(styrene-b-butadiene) copolymer and an ionically-terminated polybutadiene ionomer

Thermoplastic interpenetrating polymer networks (IPN) are mixtures of two physically crosslinked polymers. Thermoplastic IPNs were prepared by blending an SBS triblock elastomer with a 1,2-polybutadiene that was ionically-terminated at both ends. The morphologies of these IPNs were studied using differential scanning calorimetry and dynamic mechanical thermal analysis. It was concluded that the ionomer was incompatible with the SBS elastomer, since the T_gs of both the 1,2-polybutadiene from the ionomer and the essentially 1,4-polybutadiene from the SBS component were observable at temperatures that were close to those of the individual components. The addition of the polybutadiene material had, however, an influence on the relaxation processes of the polystyrene blocks. The polystyrene glass transition in the pure SBS copolymer is broadened by the interfacial region between polystyrene and polybutadiene. The low temperature shoulder was much more pronounced when the ion-terminated polybutadiene was present, indicating it has a preference to be located in these interfacial regions. © 1994 John Wiley & Sons, Inc.     

The morphology of irradiated isotactic polypropylene

The research in this article explores the response of semicrystalline isotactic polypropylene to gamma radiation in air, and relates the morphological changes of the polymer to corresponding changes in mechanical properties. The effect of the initial morphology of the polymer on its response to irradiation is considered using infrared spectroscopy (FTIR), small‐ and wide‐angle X‐ray scattering, dynamic mechanical thermal analysis (DMTA), and mechanical testing. The extent of chain scission and crosslinking is dependent on the dose but not the initial starting morphology. These chemical changes cause the crystallinity to increase slightly, and the glass transition temperature to rise by a few degrees in all samples, but the overall morphology is only subtly changed. In contrast, a major deterioration in mechanical properties is caused. The effects of the irradiation observed under these conditions are similar in each material and the ultimate properties determined by the properties seen in the original material. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2234–2242, 1999     

Polybutadiene modified by epoxidation: 2. The effect of epoxy groups on the crosslinking process with dicumyl peroxide

The process of crosslinking sodium and n-butyllithium polybutadienes as well as the products of their modification obtained by epoxidation has been studied. It has been found that the crosslinking efficiency of epoxy-1,2-polybutadiene is half that of the starting polybutadiene. However, the crosslinking efficiency of epoxy-1,4-polybutadiene was found to be similar to that of the starting 1,4-polybutadiene. The shift in glass transition temperature for epoxy-polybutadienes brought about by the change in the chemical composition (ΔT_g)_M was found to be 67 K for 1,4-polybutadiene, and 51 K for 1,2-polybutadiene. The effect of the epoxy groups and crosslinks on the glass transition temperatures of modified crosslinked polymers is also discussed.     

Low dose γ-irradiation of some fluoropolymers: Effect of polymer chemical structure

The effect of low dose (1–20 Mrad) γ-irradiation on five fluoropolymers (PVF, PVDF, ETFE, FEP, PFA), differing in fluorine content and chain structure, was studied. The radiation effect, reflected by changes in thermal and mechanical properties, as well as NaOH etched surface morphology, was investigated. The main chain structure was found to predominate the irradiated polymers' behavior. The tensile strength of PVF and its resistance to etching increased with radiation dose, while its degree of crystallinity and melting temperature decreased. These changes were correlated with network formation by irradiation and by the incorporation of crosslinked segments into the crystalline phase during recrystallization. Solvent extraction of PVDF (no solvents were found for the other fluoropolymers) revealed increased gel formation with increasing dose. The degree of crystallinity and the tensile strength dependence on radiation dose for PVDF and ETFE reflected the competition between crosslinking and chain scission events. FEP and PFA undergo predominantly chain scission accompanied by reduction of their mechanical properties. PFA even exhibited an increase in both its melting temperature and degree of crystallinity, stemming from a massive degradation process forming shorter chains. Chemical mechanisms are discussed in an attempt to correlate the irradiation effects with the polymers' chain structure.     

Thermal crosslinking of cis-1,4-polybutadiene at ultrahigh pressures

cis-1,4-Polybutadiene (BR) samples, in the absence of crosslinking agents, were simultaneously compressed by ultrahigh pressures (>4.0 GPa) and annealed between 25 and 180°C. The BR samples were crosslinked when the annealing temperatures were above 100°C. The Vickers microhardness of the treated BR increased and the extent of swelling decreased rapidly as the annealing temperature increased, indicating that the crosslinking density increased. Solid-state carbon-13 nuclear magnetic resonance and Fourier transform infrared spectroscopy results showed that some C=C bonds in the BR were opened to form the crosslinking network structures with C—C linkages and that a partial cis–trans isomerization took place during the treatments. Differential scanning calorimetry studies revealed that the glass transition temperature increased and the crystallinity decreased as the annealing temperature increased. When the annealing temperature exceeded 140°C, the treated BR samples lost their rubbery elastic characteristics completely and became brittle materials. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:2131–2140, 1998     

The effect of photodegradation of polystyrene on its permeability characteristics

The diffusion (D) and permeability (P) coefficients for ¹⁴CO₂ in polystyrene films which had been irradiated by short-wave ultraviolet (253.7 nm) radiation under vacuum and in oxygen were determined in the temperature range of 20° to 55°C. Both D and P decreased with increasing exposure, the vacuum-irradiated sample showing significantly greater decreases, presumably on account of a larger extent of crosslinking which was possible in the absence of oxidation. A good correlation was observed between crosslinking and decreasing P and D values. The solubility coefficients for all irradiated samples increase with increasing dose. Activation energies for diffusion increase with increasing irradiation, particularly for the vacuum-irradiated samples. Reductions in P and D values are interpreted in terms of free-volume decreases due to crosslinking and other modifications, while the increasing activation energies are attributed to the increasing energy requirements for creation of free volume in modified polymers.     

Irradiation effects on the relaxation behaviour of EPDM elastomers

γ‐irradiation of ethylene–propylene diene monomer (EPDM) elastomers under oxidant atmosphere was carried out in order to change their mechanical and dielectric behaviour. Three different formulations of EPDM (70 wt% ethylene, 28 wt% propylene; diene monomer: 2 wt% norbornene) were studied: a non‐crosslinked EPDM terpolymer, a crosslinked EPDM and a crosslinked EPDM stabilized with an antioxidant. Dielectric and mechanical relaxation show a β‐sub‐glass relaxation at about ?120 °C (1 Hz) and an α‐relaxation at ?15 °C (1 Hz) associated with the glass transition but influenced by the effects of irradiation. The local mobility associated with the β‐relaxation is only weakly influenced by γ‐irradiation up to 450 kGy. The α‐process is shifted to higher temperatures as a result of crosslinking and changes in the semicrystalline structure. The amplitude of the dielectric α‐process increases as a result of the formation of oxidized species during irradiation under oxygen. In contrast, the mechanical α‐relaxation amplitude decreases as a result of physical and chemical cross‐linking. It was shown that the main factors that determine the crosslinking/chain scission balance are (1) the presence of oxygen together with the irradiation dose, (2) the dose rate and (3) the initial crosslink density of the EPDM material. As a result, the individual contribution of crosslinking and crystallization, and therefore the understanding and prediction of the properties after γ‐irradiation can only be deduced after comparison of the polymer behaviour below and above its melting temperature. Copyright © 2004 Society of Chemical Industry     

Physicochemical properties of irradiated and loaded LDPE films with polyfunctional monomers

Polyfunctional 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     

Effect of ambient‐temperature and high‐temperature electron‐beam radiation on the structural,thermal, mechanical,and dynamic mechanical properties of injection‐molded polyamide‐6,6

Electron‐beam irradiation of injection‐molded specimens of polyiminohexamethyleneiminoadipoyl (better known as polyamide‐6,6) was carried out in air at ambient temperature (303 K) and a high temperature (393 K). Most of the irradiated specimens were tensile dumbbells, although a few were cylinders for compressive stress relaxation testing. A few representative samples were dipped in triallyl cyanurate (TAC) solution before ambient‐temperature irradiation. The gel content of the specimens increased with radiation dose and the temperature of irradiation. Moreover, the TAC‐treated specimens showed an increase in gel content over the neat specimens irradiated at the same dose levels. Wide‐angle X‐ray scattering and differential scanning calorimetry studies revealed that the crystallinity decreased with increasing radiation dose. Irradiation at the high temperature and treatment with TAC further decreased the crystallinity compared to irradiation at ambient temperature. As determined from compressive stress relaxation and mechanical and dynamic mechanical properties, the optimized radiation dose for ambient‐temperature radiation was 200 kGy. The gels had a stiffening effect, and the rate of relaxation decreased significantly. The water‐uptake characteristics of the tensile specimens were investigated; this revealed a decrease in the water absorption tendency with increasing gel content. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1633–1644, 2006     

Structure modification of isotactic polypropylene through chemical crosslinking: Toughening mechanism

Reversibly crosslinked isotactic polypropylene (iPP) was prepared in the presence of dicumyl peroxide. The effects of the peroxide oxy‐radicals in the melt were investigated in relation to the modification of the polymer. The dynamic rheology analysis of the crosslinking process was carried out by using a plastograph. The crosslinking reaction was evaluated by the Monsanto method. The resulting structure of the modified samples was studied by means of differential scanning calorimetry (DSC), wide‐angle X‐ray scattering (WAXS), microhardness, and mechanical properties. The degree of crystallinity of the modified iPP, derived from DSC and WAXS, remains almost unchanged, i.e., the crystalline structure is unaffected, though the lamellar thickness slightly decreases. The impact strength of the crosslinked iPP is greatly improved with reference to that of the unmodified material. A transition from brittle to ductile behavior appears in the modified iPP for all the crosslinking agents studied. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2968–2976, 2007     

Effect of radiation on the crosslinking and branching of polypropylene

To make crosslinked polypropylene (PP) and PP with high melting strengths, we studied the effects of radiation on the crosslinking and branching of PP in detail with the Charlesby–Pinner (C–P) equation. We studied the radiation crosslinking and branching behavior of homopolypropylene (HPP), impact copolypropylene (IPP), and random copolypropylene (RPP) and used the C–P equation for some parameter calculations. We found that (1) the C–P equation seemed applicable when PP with a multifunctional crosslinking agent was irradiated within a certain dose range in air and (2) IPP had the highest gel dose value, crosslinking G value/scission G value, and gel fraction after irradiation among HPP, RPP, and IPP; therefore, IPP is the best material for making radiation crosslinking and radiation branching PP among HPP, RPP, and IPP. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1758–1764, 2002     

Complete relaxation map of polypropylene: radiation-induced modification as dielectric probe

The molecular relaxation behaviour of isotactic polypropylene (iPP) exposed to gamma radiation in air to various absorbed doses (up to 700 kGy) has been investigated by dielectric loss (tan  d { \tan }\;\delta ) analysis; the polar (mainly carbonyl and hydroperoxide) groups that were introduced by radiation-induced oxidation were considered as tracer groups. All relaxation zones (α, β, γ and δ in the order of decreasing temperature), between 25 K and melting temperature, were studied in the frequency range from 1 kHz to 1 MHz. The changes observed in the dielectric relaxation spectra were related to the modifications in the structural and morphological parameters attributed to exposure of the iPP samples to radiation. Wide-angle X-ray diffraction was used to investigate radiation-induced changes in the crystalline structure and degree of crystallinity, since the α relaxation is connected with this phase. Infrared spectroscopy and gel measurements were used to determine the changes in the oxidative degradation and the degree of network formation, respectively. Conclusions derived using different methods were compared. This study reveals high dielectric and/or relaxation sensitivity of iPP to gamma radiation. Disappearance of the low-temperature dielectric (γ and δ) relaxations together with large changes in intensity, position and activation energy of the dielectric α relaxation are observed with gamma irradiation and are mainly connected with oxidative degradation in iPP structure.     

Thermal and wide angle X‐ray analysis of chemically and radiation‐crosslinked low and high density polyethylenes

Low and high density polyethylenes (PE) were crosslinked by two methods, namely, chemically by use of different amounts of tert‐butyl cumyl peroxide (BCUP) and by irradiation with different doses of electron beam. A comparison between the effects of these two types of crosslinking on crystalline structure, crystallinity, crystallization, and melting behaviors of PE was made by wide angle X‐ray diffraction and DSC techniques. Analysis of the DSC first heating cycle revealed that the chemically induced crosslinking, which took place at melt state, hindered the crystallization process and decreased the degree of crystallinity, as well as the size of crystals. Although the radiation‐induced crosslinking, which took place at solid state, had no significant influence on crystalline region, rather, it only increased the melting temperature to some extent. However, during DSC cooling cycle, the crystallization temperature showed a prominent decrease with increasing irradiation dose. The wide angle X‐ray scattering analysis supported these findings. The crystallinity and crystallite size of chemically crosslinked PE decreased with increasing peroxide content, whereas the irradiation‐crosslinked PE did not show any change in these parameters. As compared with HDPE, LDPE was more prone to crosslinking (more gel content) owing to the presence of tertiary carbon atoms and branching as well as owing to its being more amorphous in nature. HDPE, with its higher crystalline content, showed relatively less tendency toward crosslinking especially by way of irradiation at solid state. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3264–3271, 2006     

Sol fraction and swelling analysis of cis-1,4-polybutadiene networks produced by high-energy irradiation

Summary Soluble fraction and equilibrium swelling in toluene were investigated on cis-1,4-polybutadiene networks being crosslinked in vacuo by ⁶⁰Co-irradiation. Analysis of the sol fraction data by means of INOKUTI's equation reveals a drastic increase of the degradation-crosslink ratio for samples unpurified prior to crosslinking. Furthermore, the network crosslinking susceptibility depends on purification, although it is rather temperature insensitive for irradiation above the glass transition temperature of the rubber. The statistics of creation of crosslinks can be examined by suitable modification of INOKUTI's theory. From the present data it may be concluded that chain reaction crosslinking is negligible. The FLORY-REHNER formula for swelling equilibrium fails to reproduce the data at the cross link densities studied, but introducing a free end correction, the formula becomes applicable, with some over-correction. Hence the crosslink susceptibility could be obtained independently if the polymer-solvent heat interaction parameter were known. Additionally, the assumption about the proportionality of the crosslink density with radiation dose has been successfully tested by the swelling experiments.Financial support by DEUTSCHE FORSCHUNGSGEMEINSCHAFT and by MINISTERIUM FÜR FORSCHUNG UND TECHNOLOGIE is gratefully acknowledged. Dr. GOLDBACH from CHEMISCHE WERKE HÜLS AG we have to thank for the cis-1,4-polybutadiene.Herrn Professor Dr. K. überreiter zu seinem 70. Geburtstag gewidmet     

The effect of gamma and electron beam irradiation on the thermal and mechanical properties of injection‐moulded high crystallinity poly(propylene)

In the present study, the effect of gamma and electron beam (EB) irradiation on the thermal and mechanical properties of high crystalline polypropylene (HPP) was studied. To study the structural modifications of HPP polymer which could occur following these treatments at different doses (20, 40, 60, 80, 100, and 120 kGy) were applied to all samples. Nonirradiated HPP were used as control samples, differential scanning calorimetry analysis, thermogravimetric analysis, and Mechanical tests were carried out to evaluate the effect of both irradiation treatments (EB and gamma irradiation) on HPP samples. Irradiated samples of HPP decreases melting temperature (T_m) of matrix in EB more than in gamma rays up to 5°C. The changes of mechanical properties exhibit different radiation stability towards ⁶⁰Co‐gamma radiation and EB radiation. This difference reflects much higher penetration of the gamma radiation through the polymeric material as a function of sample thickness. The degradation on polymer properties caused by gamma irradiation was more than that caused by EB irradiation. Next, we compared the effects of gamma and EB irradiation to determine which of these two processes better maintained the integrity of the irradiated product. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers.     

Chemical and thermal characterization of high‐ and low‐density irradiated polyethylenes

A comparative study of the dynamic mechanical relaxation spectra of high‐ and low‐density polyethylenes irradiated with γ‐radiation from a Co⁶⁰ source was performed. The irradiation doses ranged from 0 to 100 Mrad. All the samples were previously characterized by determination of the molecular weight distribution, the number of functional groups, and the crystalline fraction. All the relaxation zones between ?145°C and the melt were studied in the frequency range from 0.3 to 30 Hz. The changes observed in the mechanical relaxation spectra were related to modifications in the chemical structure and morphological parameters attributed to the exposure of the samples to the γ‐radiation. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1953–1958, 2002     

Improvement of heat stability of poly(l-lactic acid) by radiation-induced crosslinking

Poly(l-lactic acid) (PLLA) has poor heat stability above its glass transition temperature (T_g∼60 °C). To improve its softing above T_g, PLLA was mixed with small amount of crosslinking agents and irradiated with various irradiation doses to introduce crosslinking between polymer chains. The most effective agent for radiation crosslinking was triallyl isocyanurate (TAIC). For melt-quenched PLLA, it was found that the most optimal conditions to introduce crosslinking were around 3% of TAIC and the irradiation dose of 50 kGy. The typically crosslinked PLLA showed very low crystallinity because of wide formation of molecular chain network that inhibited molecular motion for crystallization. Notable heat stability above T_g was given by annealing of PLLA samples. Enzymatic degradation of PLLA was retarded with introduction of crosslinks.     

Thermal properties of proton-conducting radiation-grafted membranes

Proton-exchange membranes are required to exhibit chemical, mechanical, and thermal stability for fuel cell applications. The present investigation has been carried out to explore the thermal behavior of poly(ethylene-alt-tetrafluoroethylene) (ETFE)-based proton-conducting membranes, both uncrosslinked and crosslinked, prepared by radiation grafting and subsequent sulfonation. The influence of preparation steps (irradiation, grafting, sulfonation, crosslinking) on the thermal degradation, crystallinity, and melting behavior of membranes with varying degree of grafting was examined. ETFE base film and grafted films were studied as the reference materials. Furthermore, poly(tetrafluoroethylene-co-hexafluoropropylene)-based grafted films and membranes were investigated as well for comparison. Membrane preparation steps, degree of grafting, crosslinking, type of base polymer have considerable influence on the thermal properties of membranes. The crystallinity of the films decreases slightly by grafting, while a significant decrease was observed after sulfonation. For instance, crystallinity decreased from 37% (pristine ETFE) to 36% (uncrosslinked grafted film) and 23% (uncrosslinked ETFE-based membrane). On the other hand, the melting temperature of the base polymer was almost unaffected by irradiation and grafting. The crosslinked ETFE-based membranes exhibit a slightly higher melting temperature (262.5°C) than their corresponding grafted films (261.3°C) and the base film (260.6°C). © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Toughened polystyrene containing high cis-1,4-polybutadiene rubber

High cis -1,4 -polybutadiene has been used to prepare toughened polystyrene in an attempt to improve its low temperature impact properties. A range of physical and mechanical properties was obtained by keeping the amount of rubber and the polymerization conditions constant, and varying the rate of agitation in a purpose -built reactor system. Although a good balance of tensile and impact properties is obtained at room temperature, the rubber partially crystallizes when the polyblends are cooled to below ?40°C. This should decrease the efficiency of rubber particles to create and terminate crazes. However, it is significant that the developed crystallinity decreases with the rubber phase volume, and is suppressed almost completely at about 21% rubber phase volume (RPV). The factors influencing the RPV are discussed, and a study of the phase inversion with three different types of rubber shows that its duration depends on the viscosity of the styrene/rubber system.