CHARACTERIZATION OF γ-IRRADIATED CRYSTALLINE POLYMERS Ⅰ. CHARACTERIZATION OF γ-RADIATION INDUCED CROSSLINKED POLYAMIDE 1010 BY CRYSTALLIZATION TEMPERATURE*

The effect of ~(50)Co γ-radiation on plain polyamide 1010 (PA1010 Ⅰ) and PA1010 containing dif-ferent amount of crosslinking agent (BMI) (PA1010 Ⅱ) both in vacuum and in air at room tempera-ture was investigated with DSC. It was found that the crystallization temperature T_c of crosslinkedsample determined with DSC at constant cooling rate decreased as the radiation dose increased. Thedifference between crystallization temperatures before and after crosslinking (T_(c_o)-T_(c_R) is linearlyrelated to the radiation dose for PA1010I. Based on the Charlesby-Pinner's equation an expressionwas derived S+S~(1/2)=A+B/(T_(c_o)-T_(c_R)) where S is the sol fraction, A and B are constants. Since thereis evidence that T_c is relative to S only and independent of the way of irradiation, the equation is alsoapplicable to the enhanced γ-irradiation crosslinked PA1010 Ⅱ. Therefore, determination of T_cof crosslinked polymer by DSC offers a convenient approach to study quantitatively the random andespecially non-random crosslinking reaction of crystalline polymer.     

THERMAL BEHAVIOUR OF γ-IRRADIATED NYLON 610

The thermal behaviour of γ-irradiated nylon 610 was investigated. In DSC (differential scanning calorimetry) thermograms in addition to general features characteristic of γ-irradiatinn cross-linked crystalline polymers, a typical cold-crystallization phenomenon was observed during the sceond scan for samples that had been subjected to high radiation dose. G-values for cresslinking nylon 610 were estimated from gel fraction measurement and TMA (thermomechanical analysis), From TMA curves it was estimated that the dimensional stability of properly irradiated nylon 610 articles might be raised up to ca. 300℃.     

STRUCTURE EFFECTS ON THE SOL FRACTION-DOSE RELATIONSHIP IN RADIATION CROSSLINKING OF POLYMERS

The structure effects of polymers on the relationship between sol fraction and radiation dose arediscussed in detail. In consideration of the deviation of experimental results from Charlesby-Pin-ner's and Chen-Liu-Tang's expressions relating sol fraction to radiation dose, we introduced an ex-ponent βconcerned with the structure and property parameters of a polymer (glass transition tem-perature T_g and molecular internal rotating steric factor δ) and assumed that the fracture density isproportional to R~β(R is radiation dose). Thus, a general expression, formula (9), relating solfraction to radiation dose is derived. When β=1 andβ=0.5, this expression is reduced to the above-said two expressions. Us-ing this general expression, the experimental data of a number of polymers are treated. It is shownthat the expression is in good agreement with the experimental results, βvalue is in the range of 0. 5 to1.0.     

IN SITU OBSERVATION OF MELTING AND CRYSTALLIZATION BEHAVIORS OF POLY(ε-CAPROLACTONE) ULTRA-THIN FILMS BY AFM TECHNIQUE

The melting and crystallization behaviors of poly(ε-caprolactone) (PCL) ultra-thin films with thickness from 15 nm to 8 nm were studied by AFM technique equipped with a hot-stage in real-time. It was found that melting can erase the spherulitic structure for polymer film with high thickness. However, annealing above the melting point can not completely erase the tree-like structure for the thinner polymer film. Generally, the structure formation of thin polymer films of PCL is controlled not only by melting and crystallization but also by dewetting during thermal annealing procedures, and dewetting predominates in the structure formation of ultra-thin films. However, the presence of tree-like morphology at 75 °C may be due to the strong interaction between PCL and mica surface, which may stick the PCL chains onto the mica surface during thermal annealing process. Moreover, the growth of the dendrites was investigated and it was found that crystallization is followed from a dewetted sample, and the branches did not grow with the stems. The crystallization of polymer in the ultra-thin films is a diffusion-controlled process. Both melting and crystallization behaviors of PCL in thin films are influenced by film thickness.     

MELTING BEHAVIOR OF γ-IRRADIATED ETHYLENE-alt-TETRAFLUOROETHYLENF COPOLYMER*

The melting behavior of poly(ethylene-alt-tetrafluoroethylene) γ-irradiated in air and vacuumwas studied by DSC. From the results of DSC and density measurements the equilibrium heat offusion, △AH_(III)~o, and the density of the amorphous phase, ρ_α, of the polymer were estimated as 172J/gand 1.63g/cm~3 respectively. Thereby thc degree of crystallinity of this copolymer can be evaluatedby both density and calorimetric methods. The crystal structure was damaged gradually and eventuallycompletely destroyed with the increase in radiation dose from 0.3 to 900 Mrads. Exclusion of airwas advantageous in lessening the damage of thc crystal while increase in the degree of vacuum favoredthe formation of crosslinks. Small peaks appearing both on the DSC endotherm (600K) and onthe exotherm (540K) resulted from the presence of small amount of tetrafluoroethylene blocks alongthe main chain of poly(ethylener-alt-tetrafluoroethylene).     

HEAT SHRINKING BEHAVIOUR OF γ-RADIATION CROSSLINKED POLYETHYLENE

Heat shrinking material of γ-radiation erosslinked polyethylene is widely used for various application in industry. In this study, DSC, TMA, WAXD and density measurement techniques were used to investigate the influence of MI and thermal history of LDPE on the effectiveness of network formation. Based on the results of heat stretching and heat shrinkage tests, it is found that the formation of a network as perfect as possible is indispensable to the irradiated material if good heat shrinkage property is desired. To this end, quenching technique and polyethylene with appropriate MI must be used so that an effective radiation effect will be obtained with a minimum amount of radiation dose. In spite of that the mechanical property of the irradiated polyethylene in the rubbery state is basically in agreement with the classical expression of the theory of high elasticity, only about 90% shrinkage can be reached. Besides, the heat shrinkage temperature T_s and the % shrinkage Sare both related to the radiation dose.     

IMPACT PROPERTIES OF METALLOCENE - CATALYZED ETHYLENE-α-OLEFIN COPOLYMERS

The impact properties of two selected metallocene-catalyzed ethylene-butene copolymers and one conventionalcopolymer were evaluated using Izod impact test. It is found that the metallocene-catalyzed copolymer shows superior impactproperties. This result was explained on the basis of the more homogeneous inter-molecular composition distribution andnarrower molecular weight distribution, which leads to more homogeneous morphology with fewer defects. Stepwisecrystallization improves the impact properties, especially in the crack propagation process, to a large extent. This is due to thedecrease of entanglements by stepwise crystallization, which is advantageous for the chain slip and shear. The polymer withheterogeneous intra-molecular composition distribution exhibits a more evident improvement of impact properties understepwise crystallization.     

ROLE AND IMPORTANCE OF RADIUS OF GYRATION OF CHAINS IN THE MELT IN THE CRYSTALIZATION OF POLY(1-BUTENE)

Crystallization in polymer systems actually is a process that transfers the entangled melts into a semi-crystallinelayered structure. Whether or not a chain disentangles may result in different crystallization mechanism. When compared tothe crystal thickness (d_c), the volume occupied by the chain in the melt, i.e., the radius of gyration (R_g), plays a veryimportant role in polymer crystallization. When d_c≤R_g, crystallization does not necessitate a chain disentangling. Theentanglements are just shifted into the amorphous regions. However, as d_c>R_g, i.e., as the crystal thickness ges larger thanthe radius of gyration of the chain in the melt, it becomes necessary for a chain to disentangle. Then a change ofcrystallization mechanism occurs. Such change has been experimentally observed in the crystallization of poly(1-butene). Achange in the crystal morphologies from spherulite to quadrangle, is seen via PLM, as crystalliation temperatures increase.Even more, such a change is molecular weigh dependent and shifts to lower temperature as molecular weigh decreases.There exists a jump of crystal thickness and crystallinity associated with morphological change, as seen via SAXS. A changeof crystallization kinetics and crystallinity is further evidenced via dilatometry. The unique feature of P1b crystallization hasbeen discussed based on the radius of gyration of chain in the melt (R_g), and very good agreement is obtained.     

STUDY ON THE FORMATION MECHANISM OF MONODISPERSE PARTICLES IN THE EMULSIFIER-FREE EMULSION POLYMEAIZATION OF METHYL METHACRYLATE AND BUTYL ACRYLATE

The formation mechanism of monodisperse polymer latex particles in the emulsifier-free emulsion polymerizationof methyl methacrylate and butyl acrylate with potassium persulfate as initiator was investigated. A multi-step formationmechanism for the monodisperse polymer particles was proposed. The nucleation mechanism is considered to be thecoagulation of the precursor particles by homogeneous nucleation when the primary particles reach a critical size with highsurface charge density and sufficient stability. It had been proved by a special experiment that the early latex particles formedby the coagulation were stable. The primary particles grow by absorbing monomers and radicals in the polymerization systemand then become colloidally unstable again due to the understandable decrease of particle surface charge density, which leadsto the aggregation of the growing particles and the formation of larger latex pedicles therefrom. Aner the nucleation period,the preferential aggregation of the smaller particles in the propagation process leads to the change of the particles towards auniform size and narrower particle size distribution. The coexistence and competition of homogeneous nucleation,coagulation, propagation and aggregation result in the increase of the polydispersity index (U = D_(43)/D_(10)) in the first Stage,then its decrease in the later stage because of the competition of propagation and aggregation, and the gradual formation ofthe monodisperse particles.     

INFLUENCE OF SAMPLE THICKNESS ON ISOTHERMAL CRYSTALLIZATION KINETICS OF POLYMERS IN A CONFINED VOLUME

Isothermal crystallization process of polymers in a confined volume was simulated in the case of instantaneous nucleation by use of the Monte Carlo method. The influence of sample thickness on some kinetic parameters of crystallization was quantitatively evaluated. It was found that there was a critical thickness value. Influence of thickness on the crystallization behavior was only found for samples of thickness near and less than the critical value. For thick samples the Avrami plot showed straight lines with a turning point at the late stage of crystallization due to the secondary crystallization. When the thickness was near or less than the critical value a primary turning point appeared in the Avrami plot at the very beginning of the crystallization process. A model was proposed to explain the mechanism of this phenomenon. According to this model the critical thickness value is related to the nucleation density or the average distance between adjacent nuclei, and the primary turning point is an indication of a transformation of crystal growth geometry from a three-dimensional mode to a two-dimensional one. Analysis of experimental results of PEO isothermally crystallized at 53.5℃ was consistent with the proposed model.     

STRUCTURE-PROPERTY RELATIONSHIP OF POLYELECTROLYTES AND ITS APPLICATION IN STABILIZING DRILLING-MUD IN PRESENCE OF SALTS

A new polyelectrolyte (SPU) has been prepared. It can depress the water-loss of drilling-mud much more effective than the commonly used acrylic polyelectrolytes even in 30% NaCl solution. SPU has phenyl group in the backbone with -SO_3~- in the side chain while the acrylic polyelectrolytes have C—C and -COO~- respectively, there exists an intrinsic relationship between the structure of polymer and its tolerance to salts, it has been found: 1) The adsorption amount of polymer on clay is related closely to the flexibility of polymer chain. 2) The salt-tolerance of -SO_3~- is superior to -COO-. 3) Both SPU-mud and HPAN-mud are plastic fluids. The dependence of yield point on salts relates to the molecular weight of polymer and hydration of ionogenic group, which is quite different for SPU-mud and HPAN-mud. 4 ) The extent of raising zeta-potential of base-mud by SPU is greater than by HPAN, but the extent of dropping zeta-potential of SPU-mud by NaCI is smaller than HPAN-mud. According to these results we suppose the salt-tolerance of SPU-mud is attributed mainly to hydration of -SO_3~- and that of HPAN-mud mainly to network structure formed in the drilling-mud.     

EFFECT OF GAMMA RAYS IN THE PREPARATION OF POLYMER AND HYDROGEL FROM ACRYLAMIDE MONOMER

The formation of polymer and hydrogel from aqueous solutions having 20, 30 and 40% concentrations ofacrylamide monomer by γ-ray irradiation processing in the dose range 0.06-30 kGy using a Co-60 source and theircharacterization have been observed. Polymer conversion and gel fraction are found to depend on radiation doses. Polymerconversion increases with the increase of dose, depending on the solution concentration, where maximum conversion isachieved at 0.18, 0.16 and 0.10 kGy for 20%, 30% and 40% concentrations, respectively. On the other hand, gel fractionincreases with dose from the gel point (0.12 kGy) for all concentrations, where 100% conversion of gel occurs at doses≥5 kGy. Tensile strength, viscosity and molecular weight (M_w) of polymer samples increase with both the dose and theconcentration, showing a high value of M_w up to≈10~8. Swelling of hydrogels under water with respect to time varies due tothe variation of cross-linking density formed in the gels and the maximum swelling mainly occurs within 24 h. A remarkable change of surface morphology reveals characteristic features of monomer, polymer and hydrogel films.     

THE EFFECT OF CHAIN FLEXIBILITY OF FLUOROPOLYMERS ON THEIR RADIATION CROSSLINKING BEHAVIOR

The effect of chain flexibility of fluoropolymers on their radiation crosslinking behavior is studiedand the relationship between sol fraction and the radiation dose to the fluoropolymers given in ourprevious report is confirmed.     

EFFECT OF WATER ON THE CONDUCTIVITY OF PPQ (ClO_4~-) COMPLEX

The preparation of conducting PPQ film was first reported in the previous paper. It is very interesting that this film is highly sensitive to moisture in air. The hydration and dehydration of the film are accompanied by change not only in color but also in conductivity and UV-visible spectrum. The conducting PPQ is reduced to PPQ and loses its conductivity after being soaked in water.     

Mechanistic Insights into the Shear Effects on Isotactic Polypropylene Crystallization Containing β-Nucleating Agent

The crystalline structures and crystallization behaviors of iPP containing β nucleation agent TMB-5(iPP/TMB-5) were investigated by synchrotron radiation wide angel X-ray diffraction(SR-WAXD), differential scanning calorimeter(DSC) and polarized light microscope(PLM). It was found that α-crystallization lagged behind β-crystallization at normal temperatures, but the discrepancy reduced with increasing temperature. TMB-5 could not induce β-iPP when the nucleation agent is wrapped up with α-crystal that crystallized at high temperatures. The polymorphic composition of iPP/TMB-5 was susceptible to the introductory moment of shear. New crystallization process of β-nucleated iPP was proposed to understand the experimental phenomena which could not be explained by those reported in the literature. It was supposed that polymer crystallization initiated from mesophase, and the formations of iPP crystals involved the organization of helical conformation ordering within mesophase. It was proposed that the iPP melt contained mesophases with stereocomplex-type ordering of right-handed and left-handed helical chains which could be disturbed by shear or TMB-5, leading to different polymorphic structures.     

THE RADIATION CROSSLINKING OF EPR/SBR BLEND

The radiation crosslinking of EPR/SBR blend has been studied. A relationship between sol fraction and radiation dose for the different weight ratio polymer blends and the method to calculate βb value of EPR/SBR blend system have been established.     

Molecular dynamics study of the nucleation behavior and lamellar mergence in polyethylene globule crystallization

The site order parameter (SOP) has been adopted to analyze various orders of structure formation and distribution during the crystallization of a polyethylene globule simulated by molecular dynamics. The SOP not only identifies different orders among the structures but also different phases. The results showed that intermediate structures with medium order exist unevenly in the early stage of nucleation, which differs from traditional models. We found that the nucleation relies on crystallinity fluctuations with increasing amplitude, and a baby nucleus with different shape and increasing size suddenly appears in the fluctuations. Once its size is large enough, the baby nucleus becomes the nucleus. In the growth stage, a number of lamellar mergences were observed and their selective behaviors were related to the orientation differences between the lamellae to be merged. The SOP distribution of all atoms in the system during crystallization had two peaks: one for the amorphous phase and the other for the crystalline phase. The intermediate structures with medium orders are located between the two peaks as an order promotion pathway. The data show that the medium order structure fluctuates at the growth front and is not always available, and that the medium order structure existing at the front is not always good for development. It is possibly caused by chain entanglement. Structure fluctuation at the growth front enables the system showing thousands of the most probable configurations to approach the precursor. The growth front is thus active for a while and inactive at other times.     

EFFECTS OF COUPLING AGENTS ON THE CRYSTALLIZATION BEHAVIOR OF PP/T-ZnOw COMPOSITES

The objectives of this paper are to understand the crystallization behavior of polypropylene(PP)composites with surface modified tetra-needle-shaped zinc oxide whisker(T-ZnOw).T-ZnOw was surface modified with different coupling agents,such as silane coupling agents(KH-550,KH-560)and titanate coupling agent(NDZ-105),in order to improve the compatibility between PP and T-ZnOw.DSC and POM were used to characterize the melt and crystallization behavior and the crystalline structures of the composites,respectively.The results show that the surface modified T-ZnOw acts as a nucleating agent of PP crystallization,depending on the coupling agent used for modification.KH-550 and KH-560 have more apparent role in improving the interfacial interaction than NDZ-105 and induce PP crystallization at higher temperature and with smaller spherulites size.The results also suggest that the crystallization behavior depends on not only the content of coupling agent,but also the content of the surface modified T-ZnOw used in the composites.     

INVESTIGATION OF MICROSTRUCTURE AND CONDUCTIVE MECHANISM OF HIGH DENSITY POLYETHYLENE/CARBON BLACK PARTICLE COMPOSITE BY POSITRON ANNIHILATION LIFETIME SPECTROSCOPY

The microstructure and conductive mechanism of high density polyethylene/carbon black (HDPE/CB) compositewere investigated by positron annihilation lifetime spectroscopy (PALS). The PALS were measured in two series of samples,one with various CB contents in the composites and the other with various γ-irradiation doses in HDPE/CB compositecontaining 20 wt% CB. It was found that CB particles distribute in the amorphous regions, the CB critical content value inHDPE/CB composite is about 16.7 wtO/ and the suitable γ-irradiation dose for improving the conductive behavior ofHDPE/CB composite is about 20 Mrad. T'he result observed for the second set of samples suggests that γ-irradiation causesnot only cross-linking in amorphous regions but also destruction of the partial crystalline structure. Therefore, a suitableirradiation dose, about 20 Mrad, can induce sufficient cross-linking in the amorphous regions without enhancing thedecomposition of crystalline structure, so that the positive temperature coefficient (PTC) effect remains while the negativetemperature coefficient (NTC) effect is suppressed. A new interpretation of the conductive mechanism, which might providea more detailed explanation of the PTC effect and the NTC effect has been proposed.     

OPTIMIZATION OF THE SYNTHESIS OF ACRYLAMIDE HYDROGEL BY γ-RAY IRRADIATION

Hydrogels have been synthesized from 10%, 20%, 30%, 40%, 50% and 60% aqueous solutions of acrylamide monomer by gamma radiation employing doses in the range of 0.2-30 kGy from a Co-60 source. The effect of solution concentration, y-ray dose, pH and time was studied in order to observe the optimizing conditions in the characterization of hydrogels. Gel fraction increases with dose for all concentrations indicating hundred-percentage conversion of gel at doses ≥5 kGy for homogenous solutions in the range of 20%-50% concentration. On the other hand, 10% solution provides conversion less than 86% even at 30 kGy, whereas 60% monomer makes an inhomogeneous solution which stile gives about 100% gel fraction. Swelling of hydrogels under water with respect to time varies with both the doses and concentrations due to the change of crosslinking density in the gels. The maximum volume change of hydrogels during swelling and water desorption mainly occurs within 24 h. Swelling is also enhanced with the rise of pH due to change of ionic content of the solvent. Considering the amount of gel fraction and the properties of hydrogel, the samples prepared from 20% solution at 5 kGy show better results. Moreover, the effect of bacteria on hydrogel was found to be nil, suggesting a prohibition of growth of microorganism in it.