Radiation-induced grafting of styrene onto ultra-high molecular weight polyethylene powder and subsequent film fabrication for application as polymer electrolyte membranes: I. Influence of grafting conditions

Ultra-high molecular weight polyethylene (UHMWPE) powder was irradiated by gamma rays using a ⁶⁰Co source. Simultaneous and pre-irradiation grafting was performed in air and in inert atmosphere at room temperature. The monomer selected for grafting was styrene, since the styrene-grafted UHMWPE could be readily post-sulfonated to afford proton exchange membranes (PEMs). The effect of absorbed radiation dose and monomer concentration in methanol on the degree of grafting (DG) is discussed. It was found that the DG increases linearly with increase in the absorbed dose, grafting time and monomer concentration, reaching a maximum at a certain level. The order of rate dependence of grafting on monomer concentration was found to be 2.32. Furthermore, the apparent activation energy, calculated by plotting the Arrhenius curve, was 11.5 kJ/mole. Lower activation energy and high rate dependence on monomer concentration shows the facilitation of grafting onto powder substrate compared with film. The particle size of UHMWPE powder was measured before and after grafting and found to increase linearly with increase in level of grafting. FTIR-ATR analysis confirmed the styrene grafting. The grafted UHMWPE powder was then fabricated into film and post-sulfonated using chlorosulfonic acid for the purposes of evaluating the products as inexpensive PEM materials for fuel cells. The relationship of DG with degree of substitution (DS) of styrene per UHMWPE repeat unit and ion exchange capacity (IEC) is also presented.     

Cation exchange membranes by pre-irradiation grafting of styrene into FEP films. I. Influence of synthesis conditions

Gamma radiation-induced grafting of styrene into FEP films was investigated by the pre-irradiation method. The degree of grafting was found to be strongly dependent on the synthesis conditions, such as radiation dose, monomer concentration, crosslinker, temperature, and film thickness. The order of dependence of the rate of grafting on pre-irradiation dose and monomer concentration was found to be 0.64 and 1.90, respectively. The activation energy for the grafting in the temperature range of 50–80°C was determined to be 27.9 kJ/mol. A negative first order dependence of grafting on film thickness was observed. The results suggest that the initial grafting takes place at the film surface and proceeds to the middle by progressive diffusion of monomer through the polystyrene grafted layers. © 1994 John Wiley & Sons, Inc.     

Solvent Effects on the Rate of Radiation-Induced Grafting of Vinyl Monomers on Polymeric Films

Earlier work indicated that in the radiation-induced grafting of vinyl monomers on polymeric films, the plasticity of the film being grafted is determined by the Hildebrand solubility parameter of the grafting solution. Film plasticity affects the termination step of the grafting reaction, and thus strongly influences the overall rate of monomer grafting on the polymeric film.

In the grafting of styrene on nylon film, a sequence of irradiation runs was made at selected volume ratios of styrene/benzene/methanol, all grafting solutions having a constant solubility parameter value of 9.5 Under these conditions, a linear plot of grafting rate vs volume percent styrene in the grafting solution was obtained. A similar sequence of runs grafting pentafluorostyrene on nylon film at constant solubility parameter also produced a linear plot of grafting rate vs volume percent PFS.

Styrene was grafted on polyethylene film in a sequence of four runs using styrene dissolved in methanol, ethanol, 1-propanol, and 1-butanol, each solution having the same solubility parameter of 10.4. A straight-line plot of grafting rate vs volume percent styrene was obtained under these conditions.     

Preparation of a novel HALS polypropylene stabilizer

The radiation induced graft copolymerization of m-isopropenyl-α,α-dimethyl benzyl iso-cyanate (m-TMI) and styrene onto polypropylene was carried out. The extent of grafting increased with increasing amount of styrene and with increased radiation dose. A graft load of 180% was obtained by immersing a 50 kGy pre-irradiated film in a monomer solution containing 25 mol % m-TMI and 75 mol % styrene. The graft copolymer is suitable for covalently binding nonpolymerizable stabilizers with a suitable nucleophilic moiety. In this work the isocyanate moiety of the graft copolymer was allowed to react with 4-amino-2,2,6,6-tetramethyl piperidine, a hindered amine light stabilizer. Fourier trans-formed infrared spectroscopy confirmed the formation of an urea moiety. © 1995 John Wiley & Sons, Inc.     

Radiolytic Grafting of Styrene on Polymethylpentene Film

Gamma ray initiated grafting of styrene on three polymers gave rates in the order polyethylene > polypropylene > polymethylpentene, with saturation absorption of styrene in the order polymethylpentene > polypropylene > polyethylene, indicating that the plasticizing action of absorbed styrene possibly causes increased termination rate and reduced over-all grafting rate. The rate of styrene grafting on polymethylpentene was unaffected by temperature change in the range from 23 to 85°C. Above a certain critical film thickness, styrene grafting rate is proportional to film surface area rather than film weight.     

聚全氟乙丙膜电子束引发预辐射接枝AA和SSS制备阳离子交换膜的研究

利用电子束引发预辐射接枝技术,在聚全氟乙丙烯(FEP)薄膜上接枝丙烯酸(AA)和对苯乙烯磺酸钠(SSS),制备一种含羧酸基团和磺酸基团的阳离子交换膜,详细研究了反应温度、单体总浓度、pH值变化、辐照气氛及添加剂对接枝率的影响规律,明确了实验条件与接枝率的对应关系。FTIR测试证明了接枝产物是全氟乙丙烯和丙烯酸、对苯乙烯磺酸钠的接枝共聚物。     

Preparation of poly(ether ether ketone)-based polymer electrolytes for fuel cell membranes using grafting technique

Poly(ether ether ketone) (PEEK)-based polymer electrolyte membranes (PEMs) was successfully prepared by radiation grafting of a styrene monomer into PEEK films and the consequent selective sulfonation of the grafting chains in the film state. Using milder sulfonation, the sulfonation reactions proceeded at the grafted chains in preference to the phenylene rings of PEEK main chains; as a result, the grafted films could successfully transform to a PEM with conductivity of more than 0.1 S/cm. The ion exchange capacity (IEC) and conductivity of the grafted PEEK electrolyte membranes were controlled to the ranges of 1.2–2.9 mmol/g and 0.03–0.18 S/cm by changing the grafting degree. It should be noted that this is the first example of directly transforming super-engineering plastic films into a PEM using radiation grafting.     

The role of multifunctional acrylates in radiation grafting and curing reactions

Multifunctional acrylates (MFAs) are used extensively in radiation rapid cure (RRC) formulations involving u.v. and EB processing to achieve fast rates of polymerization and cross-linking if required. For certain RRC applications, the ability to achieve concurrent grafting with cure can lead to improved properties of the finished product. In the current work the effect of MFAs on concurrent grafting during the curing process is evaluated from a basic study of the role of these MFAs on a typical grafting reaction initiated by u.v. or ionizing radiation. The model used for the grafting work was the copolymerization of styren to the polyolefins and cellulose. It is found that the presence of the MFA and related polyfunctional monomers (PFMs) in additive amounts in the styrene grafting solution enhances the copolymerization yield under certain radiation conditions. Synergistic effects of the MFAs and PFMs with other additives such as mineral acids, inorganic salts (e.g. lithium perchlorate) and specific organic compounds (e.g. urea) were observed in the model grafting reactions. When additives such as silanes and fluorinated alkyl esters, which are important in RRC formulations to achieve slip and flow in the cured film, are incorporated into the model grafting monomer solution, the former additive retards copolymerization whereas the latter enhances it. The inclusion of MFAs in these monomer solutions leads to large increases in graft with both silanes and fluorinated alkyl esters. The result implies that MFAs, in addition to speeding cure and cross-linking in RRC processes, may also enhance adhesion to certain substrates because of an increase in concurent grafting during cure. A mechanism for these additive effects in concurrent grafting and curing is proposed.     

Radiation-induced grafting of styrene vapor to polyethylene

The radiation-induced grafting of styrene vapor to low-density polyethylene film of 0.063 mm thickness was studied at 23°C at a dose rate of 1.98 × 10⁴ rad/hr. The concentration C of monomer in the film was measured as a function of pre-irradiation exposure time to monomer vapor. The concentration-dependent diffusion coefficient of styrene in polyethylene was calculated to be 4.9 × 10^?9 exp {2.0C/C₀} cm²/sec, where C₀ is the saturation concentration of styrene in the film, and a linear boundary diffusion coefficient for styrene vapor into polyethylene film was found to be 2.0 × 10^?7 cm/sec. The rate of grafting was determined as a function of the concentration of styrene absorbed in the film. The maximum graft yield was obtained with an initial styrene concentration in the film of 4 wt-%. Under conditions of low initial monomer concentration, the grafting rate increases with irradiation time. The results are compared with previously published data on grafting of polyethylene from methanol–styrene solutions. They are explained in terms of the viscosity of the amorphous region as a function of styrene content and the resistance to the diffusion of monomer at the film–vapor interface.     

Spectroscopic study of the penetration depth of grafted polystyrene onto poly(tetrafluoroethylene‐co‐perfluoropropylvinylether) substrates. 1. Effect of grafting conditions

This study concerns the radiation grafting of styrene onto poly(tetrafluoroethylene‐co‐perfluoropropylvinylether) (PFA) substrates and the penetration depth of the graft. Grafting was obtained by the simultaneous irradiation method, and the spectroscopic analysis was made with the micro‐Raman technique. Effects of grafting conditions such as the type of solvent, dose rate, and irradiation dose on the grafting yield were investigated. Of the different solvents used, the most efficient in terms of increasing grafting yield were dichloromethane, benzene, and methanol, respectively. A mixture of methanol and dichloromethane used as a solvent for styrene achieved a higher degree of grafting and concentration of grafted polystyrene onto the surface of PFA substrates than solutions of the monomer in the separate solvents. The degree of grafting increased with increasing radiation dose up to 500 kGy, stabilizing above this dose. However, the grafting yield decreased with an increase in the dose rate. The increase in the overall grafting yield was accompanied by a proportional increase in the penetration depth of the grafts into the substrate. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3191–3199, 2002     

Modification irréversible de la morphologie de films de poly(chlorure de vinyle) greffés d'acide acrylique sous l'action de solvants et de la température

PVC films grafted with acrylic acid were prepared with grafting ratios as high as 160% and their swelling in N/10 KOH, was measured. An earlier observation was confirmed showing that, when the films are heated in the swollen state, they undergo an irreversible transformation which leads to an increase of their swelling ratio. A study with scanning and transmission electron microscopes showed that rigid PVC as well as PVC grafted with acrylic acid up to 27% did not exhibit any specific microstructure. At high magnifications, fibrils characteristic of fragile fractures clearly appeared on the SEM pictures. For grafting ratios of 67% or more, segregated microphases of PVC and poly(acrylic acid) are clearly seen. The size of these microphases increases after the grafted films are swollen in KOH at 60°. A systematic study with PVC films grafted to 160% showed a steady evolution of the size of the microdomains during swelling in KOH. The results explain the variations of a macroscopic property of the film (its extent of swelling) by a modification of their microstructure.     

Preparation of cation-exchange membrane containing bi-functional groups by radiation induced grafting of acrylic acid and sodium styrene sulfonate onto HDPE: Influence of the synthesis conditions

Electron beam radiation induced grafting of acrylic acid (AA) and sodium styrene sulfonate (SSS) onto high-density polyethylene (HDPE) membranes was investigated by the pre-irradiation method, and a cation-exchange membrane containing bifunctional groups was synthesized. The effects of grafting conditions such as monomer concentration, radiation dose and temperature on grafting yield were studied. The dependence of grafting yield on pre-irradiation dose and monomer concentration was found to be 0.54 and 2.21, respectively. The activation energy for the grafting was calculated to be 22.2 kJ/mol. Infrared spectroscopy analysis of the grafted membrane confirmed the existence of sulfonate and carboxylic acid groups.     

Chemical structure of poly(ethylene terephthalate)–styrene and nylon–styrene graft copolymers prepared by radiation techniques

The graft copolymerizations of styrene onto poly(ethylene terephthalate) (PET) and nylon fibers were carried out by the mutual irradiation and preirradiation methods. True graft copolymers were isolated from the products by extraction and characterized by hydrolysis and osmometry. Among the swelling agents employed, methanol was most effective for increasing the extent of grafting onto PET. In both methods of the grafting, the molecular weight of polystyrene formed in the substrate matrix was higher than one million if no chain-transfer agent was added to the monomer solution. Similar to the case of radiation grafting onto poly(vinyl alcohol) and cellulose, the isolated graft copolymer carried only one branch per copolymer molecule in both cases. Of great interest is the particularly low extent of grafting in the case of PET–styrene. This should be attributed to the low sensitivity of PET to radiation. The grafting site on the mother polymer molecule is discussed on the basis of the solution behavior of the branch polymers separated from the backbone.     

Compatibility studies on polyblends of PVC with chlororubber-20 and its graft polyblends by ultrasonics

Compatibilities of various PVC blends with chlororubber-20 and its graft polyblends have been studied on the basis of our previous findings. It has been found that chlororubber-20 makes compatible blends with PVC, while grafting of styrene, ethyl acrylate or acrylonitrile onto chlororubber-20 generates heterogeneity in their blends with PVC. The behaviour of these blends at the molecular level is reflected in the changes of ultrasonic velocity and absolute viscosity with composition.     

Radiation Graft Copolymerization of a Mixture of Styrene and n-Butyl Acrylate on Natural Rubber Latex

The radiation graft copolymerization of a mixture of styrene (St) and n-butyl acrylate (NBA) monomers on natural rubber (NR) latex has been studied. An irradiation dose of about 18 kGy was needed to attain a conversion of about 80%. The tensile strength of the grafted NR film increases with increasing irradiation dose. A film tensile strength of about 155 kg/cm² was attained by irradiation of a mixture of St, NBA, and NR latex with a dose of about 14 kGy. At low concentrations of monomer in the latex, the Mooney viscosity of the film increases with increasing irradiation dose. At higher monomer concentration, grafting and homopolymerization proceed more favorably than crosslinking, and thus the Mooney viscosity decreases with increasing dose. Thermal analysis of the film showed that the grafted NR was more heat resistant than ungrafted NR.     

Parameter Study for the Pre‐Irradiation Grafting of Styrene/Divinylbenzene onto Poly(tetrafluoroethylene‐co‐hexafluoropropylene) from Isopropanol Solution

Pre‐irradiation grafting of styrene/divinylbenzene (DVB) onto poly(tetrafluoroethylene‐co‐hexafluoropropylene) (FEP) films from isopropanol (ⁱPrOH) solution was investigated with respect to the effect of irradiation dose, film thickness, cross‐linker concentration, and reaction temperature. A mathematical model was applied to the kinetic curves to extract information on the polymerization rate, the radical‐recombination rate, and the grafting through time. It turned out that the two closely correlated reaction rates for polymerization and radical recombination can be varied over a wide range by changing the irradiation dose, the cross‐linker concentration, and the reaction temperature. On the other hand, the time until the grafting front has progressed to the center of the film is mainly affected by the film thickness and the reaction temperature. The formation of homopolymer in the grafting solution increases steeply with temperature and cross‐linker concentration.     

Radiation-induced graft polymerization of styrene to poly(vinyl chloride)

The radiation-induced graft polymerization of styrene to poly(vinyl chloride) (PVC) was investigated. Relations between the rate of grafting and the dose rate when the polymer is irradiated in liquid monomer or in monomer vapor, and between the rate of grafting and monomer concentration absorbed in the polymer have been investigated. The rate of grafting in monomer vapor was found to be far larger than that in liquid monomer. A high rate of grafting in monomer vapor was thought to result from a lower concentration of monomer in PVC during irradiation. An experiment carried out on PVC containing the monomer at various concentrations showed that the rate is largest at a monomer concentration of about 3.5 mole/l. and is smaller for higher and lower concentrations. On the assumption that the theory of homogeneous homopolymerization can be applied to this grafting reaction, the value of k_p²/k_t has been obtained, where k_p and k_t are propagation constant and termination constant, respectively. The value of k_t greatly increases when the monomer concentration exceeds 3.5 mole/l. This increase of k_t can be accounted for if it is assumed that the monomer absorbed in the polymer works as a plasticizer and increases the molecular motion of the polymer. A measurement of the elastic modulus of PVC containing the monomer at various concentrations showed that this is, in fact, the case.     

丙烯酸β-羟丙酯与聚氯乙烯膜的紫外光接枝改性

采用液相接枝方法 ,在紫外线辐照下 ,合成了一系列丙烯酸 β-羟丙酯 (β -HPAT) /聚氯乙烯接枝膜。讨论了引发剂浓度、单体浓度、光照时间、光照强度对接枝率的影响。结果表明 ,引发剂浓度为 6 .5 1× 10 -3 mol/L ,单体浓度 2 .0mol/L ,光照时间为 2 .0h ,且光照强度越强时 ,接枝率最大。接枝膜的结构特征通过FT -IR光谱进行了确证 ,最后 ,对接枝膜进行TGA分析 ,结果表明聚氯乙烯经过表面改性后 ,表面极性增大 ,热稳定性大大提高。     

Reaction Kinetics of the Radiation-Induced Grafting of Styrene on Polyethylene Film

Based on rate data obtained previously, an analysis has been made of the reaction kinetics of the radiation-induced grafting of styrene on polyethylene film by using various concentrations of methanol in the styrene. A new feature of the work involves the use of internal film concentrations of styrene and methanol computed from the Flory theory of polymer /solvent interaction, which possibly improves the accuracy of calculations of termination rate constant k_t at various methanol concentrations. A graphical plot of k_t dependence on internal film viscosity is computed from an equation of Smoluchowski. Techniques are defined which may lead to improved accuracy in the computation of rate constants.     

Radiation and photografting as complementary techniques for immobilizing bioactive materials

The technique of radiation grafting to immobilize a typical enzyme, trypsin, to three representative backbone polymers namely polypropylene, PVC and polystyrene, is described. The process involves the simultaneous grafting of p-nitrostyrene to the polymer, the nitro group subsequently being converted to its isothiocyanato derivative to which trypsin is attached. For this immobilization work, the utilization of additives which enhance grafting is shown to be an advantage. Typical of the additives used are mineral acids (≈0.1M) and polyfunctional monomers (≈1% v/v), the two acting in a synergistic fashion when used together. A new class of additives, namely metal salts such as LiClO₄, which also increase grafting yields has now been discovered. The reactivity of these salts in grafting enhancement is compared with that of mineral acid. A new model for the modus operandi of metal salts as well as acids in grafting enhancement is proposed whereby increased grafting yields are attributed to increased partitioning of monomer into the graft region in the presence of ionic solutes. The significance of using these additives to prepare radiation copolymers for enzyme immobilization is discussed. In addition to ionizing radiation, UV is also shown to be of value as an initiator for the preparation of graft copolymers for enzyme immobilization, however the presence of sensitizer in the copolymer may limit the photografting method. The possibility of using radiation curing processes involving electron beam (EB) and high powered (300 W/inch) UV sources for immobilization work has been explored and found to be feasible. The curing system possesses great potential for the current immobilization work since complete polymerization is achieved in a fraction of a second.