DSC ofγ-irradiated ultra-high molecular weight polyethylene and high density polyethylene of normal molecular weight

The melting and the crystallization of-irradiated (doses: 0–6Mrad) ultra-high molecular weight nascent polyethylene (UHMWPE) and high density nascent polyethylene with normal molecular weight (NMWPE) were investigated by DSC. The heat of melting of the nascent UHMWPE (DSC degree of crystallinity, respectively) increases up to a dose of 3 Mrad, after which it slightly decreases. The heat of the second melting of UHMWPE and of the first and second melting of NMWPE increases slightly up to a dose of 3 Mrad, after which it does not change. The X-ray degree of crystallinity of the nascent non-irradiated and irradiated polymers was 0.62±0.02. The calorimetric crystallinity was compared to the X-ray one. The results show that radiation does not affect the polymer crystallinity, but influences the thermodynamic heat of melting. The increase ofH _m vs. dose in UHMWPE is explained in terms of processes of tie molecule scission within the amorphous regions and on the surface of the crystals, which predominate over crosslinking up to a dose of 3 Mrad. That leads to an increase in the conformational mobility of the molecules and to an increase in the enthalpy, according to Peterlin's formula. The scission of the chains at the points of entangling of the tie molecules leads to a decrease in the temperature and to an increase in the enthalpy of crystallization of UHMWPE vs. dose. In NMWPE these effects are considerably weaker.     

CPS K 843 Radiation-induced crosslinking: Effect on structure of polyethylene

Branched polyethylene irradiated (0–400 Mrad) with a Co⁶⁰ source at room temperature under vacuum was studied by density, wide- and small-angle X-ray scattering (WAXS and SAXS) measurements. The radiation effects on the structure of bulk, branched polyethylene are quite similar to those observed by others on single crystals or oriented preparations. These effects include changes in bulk density, crystallinity(w _c orv _c) and¯d ₁₀₀ and¯d ₂₀₀ spacings as a function of irradiation. A decrease in crystallinity is seen to begin at radiation dose 100 Mrad whereas lattice expansion indicating onset of an orthorhombic-hexagonal transition can begin as low as 10 Mrads. The decrease in crystallinity can be attributed to additional lattice distortions primarily introduced by the crosslinks occurring at the lateral grain boundaries, while lattice expansion can be associated with the same crosslinking mechanism which begins at the defects both within the crystals as well as those outside the crystals at the lateral grain boundaries. Strong evidence for a primary crosslinking-at-the-defects mechanism has also come from _c and _a data obtained in this study as a function of radiation dose. The same data have also led to an excellent correspondence between the measured density crystallinityv _c and the measured WAXS crystallinityw _c. Without consideration of the effects of crosslinks on _c and _a one would have obtained a divergence of the two crystallinities, especially at radiation doses greater than 100 Mrads.Dedicated Prof. Dr. R. Bonart on the occasion of his 60^th birthday     

Evolution of crystallinity in photodegraded polyethylene films studied by ft-raman spectroscopy

FT-Raman spectroscopic studies of photodegraded polyethylene films have enabled the evolution of the crystallinity process to be measured. Commercial polyethylene films of M_w=90 000 were exposed in a weathering UV-chamber under known conditions of exposure time and radiant energy. The spectral profiles were modelled using Fourier methods. The relative amounts of the orthorrombic crystalline phase, α_c, the amorphous phase, α_a and the interphase, α_b, were calculated using Raman bands at 1416 cm⁻¹ characteristic of the crystalline phase and the bands at 1080, 1305 cm⁻¹, characteristic of the amorphous phase. The interphase content can be calculated from the relationship αb= 1-(α_c+α_a). It was found that the weathering process affects only the relative intensities of the bands attributed to crystalline and amorphous fractions; the crystalline content increases at the expenses of the amorphous fraction. These results are discussed in terms of the changes in the intermolecular forces caused by radiation exposure.     

Radiation-induced crosslinking: II. Effect on the crystalline and amorphous densities of polyethylene

Small-angle x-ray scattering (SAXS) was used to determine the structural changes in polyethylene induced by radiation. The changes in densities of the crystalline and amorphous phases, _c and _a , were calculated after direct determination of the mean square density fluctuation <²>. _a increases with increasing radiation dose for both linear and branched polyethylene. This accounts for the serious discrepancy between crystallinities determined from wide-angle x-ray scattering and density measurements. This study confirms our previous proposal that crosslinks occur primarily in the noncrystalline phase, most likely at the defects in the lateral grain boundary regions.Dedicated to Prof. Dr. W. Pechhold on the occasion of his 60th birthday.     

The chain length dependence of self-diffusion in melts of polyethylene and polystyrene

The self-diffusion coefficients in melts of polyethylene fractions and polystyrene standards were measured by the NMR pulsed field gradient technique and compared with those measured by other techniques. The data agree very well if one takes into account the molar mass distribution of the samples and the free volume of the matrix. For molar masses much higher than the critical molar massM _c, reptation is confirmed,D M ^–2 holds. BelowM _e=M_c/2 the self-diffusion coefficients corrected for constant free volume show approximately the dependenceD M ^–1 confirming Rouse-like diffusion. This result was also obtained by investigating the self-diffusion of the molecules with different molar masses of a polyethylene fraction with a rather broad molar mass distribution aroundM _e andM _c, i. e. diffusion in a constant matrix. In the molar mass region betweenM _c and about 3 ·M _c the observed molar mass dependence of self-diffusion can be explained by tube formation. The constraint release model of Graessley seems to slightly overestimate the self-diffusion coefficients.     

Mechanical properties and molecular mobility in oriented polyolefines

The static modulus of elasticity (E) and the correlation time of rotation ( _c ) of 2,-2,6,6-tetramethylpiperidine-1-oxyl are studied as a function of the temperature (210<T <350°K) for oriented films of isotactic polypropylene and polyethylene of high and low density.E and _c change both upon heating and polymer orientation; this result indicates that sample properties are influenced by the microstructure of the amorphous phase where probes are localized.     

Characterization of polymer networks by measurements of the freezing point depression

Based on the phenomenon of freezing point depression of a solvent byT, experimental evidence is presented to show that the distance between the junction points can be calculated fromT. Direct measurements of the temperature-time-curve of the cooling network and the Differential Scanning Calorimetry offer the determination ofT. Except the mean distances ¯d _c in dependence on cross-linking density, swelling degree, and other network parameters, the distribution of the distance between the junction pointsH(d_c) can be determined, which allows conclusions on the course of cross-linking reaction. This paper attempts to give experimental evidence of influences of the breadth ofH(d_c) on application-relevant properties.     

Dynamic shear compliance of polymer melts and networks in dependence on crosslinking density

Mechanical relaxation processes in polymer melts and networks are discussed. This is performed by decomposing master curves of the dynamic shear compliance into i) glass relaxation with its plateau complianceJ _eN ; ii) shearband process with its relaxation strengthJ _B , which is reciprocal to the total crosslink densityp _c ; and iii) flow relaxationJ _F and viscous flow (for uncrosslinked melts only). Plateau complianceJ _eN > is exponentially reduced only by effective crosslinks (p _c ^* p _c /30). This behavior is understood on the level of a meander superstructure, which includes shearbands. The observed saturation inJ _eN at higher dicumylperoxide (DCUP) crosslinking-which doesn't appear with radiation-can be explained by the lack of chemically induced effective crosslinks across the interfaces among meander cubes. This lack may be a consequence of DCUP molecules concentrating at the interfaces and thereby preventing the contact and radical recombination between chains at adjacent meander faces.Crosslink densitiesp _c (per monomer), determined from the reduction of shearband relaxation strength, vary linearly with the crosslinking agent and read: p_c2.4 · 10^–2 Dose/MGy andp _c 0.97 · 10^–2 DCUP/phr for radiation and DCUP crosslinking, respectively. This implies, e.g., that a dose of 0.4 MGy (40 Mrad) is equivalent to 1 part DCUP phr in a crosslinking polyisoprene. From activation-curve analysis it follows that3 r/d stays constant, and _s - _so (free energy of formation of a segment-dislocation) andQ _y -Q _yo (activation energy for segmental jumps) vary with the square ofP _c , as does the glass temperaturT _g -T _go from DSC measurements.     

Miscibility behavior of ternary lipidphospholipid/water systems

Differential-scanning-calorimetry was applied to study the lyotropic and thermotropic properties of the two ternary systems dimyristoylcephaline (di-(C14:0)-PE)/Palmitic acid (C₁₅COOH)/water (H₂O) and dimyristoylcephaline (di-(C14:0)-PE)/palmitic acid methyl ester (C₁₅COOMe)/water (H₂O) in dispersions with excess water (50 wt.%).The phase diagrams of both systems showed that the two systems differ in their miscibility behavior. The system di-(C14:0)-PE/C₁₅COOH/H₂O is completely miscible in its high-temperature phase. In the low-temperature phase the mixing gap was found within the concentration range of C₁₅COOH and was also indicated by a maximum value of the transition enthalpy of the pseudo-binary mixtures.In the pseudo-binary system di-(C14:0)-PC/C₁₅COOMe/H₂O, the tendency towards demixing is much more pronounced. It was observed that the incorporated C₁₅COOMe melted above its normal melting point, but below the transition temperature of di(C14:0)-PE/H₂O system; therefore, the phase transition started at lower temperature. In the low-temperature phase, both lipids are partially miscible. The demixing range of the phase diagram lies within the concentration region of C₁₅COOMe. Up to the mole fraction ofXC₁₅COOMe=0.43, C₁₅COOMe can be incorporated into theL-phase of the system di-(C14:0)-PE/H₂O.     

Spontaneous film formation from a polymer solution

An unusual continuous film formation process of lateral pentyloxy substituted poly(p-phenylene terephthalate)s (s-PPPT) and poly(carbonate) (PC) is observed. A liquid film of polymer solution creeps over the surface of water dropped into the polymer solution. By vaporization of the solvent a solid polymer film is formed on the water surface and can be removed. The driving force for the film formation mechanism is assumed by the reduction of the surface tension of water. Experiments verify this mechanism by increasing the film formation speed using a gas stream, by reducing the formation speed through lowering the surface tension by rinsing agents, and by lowering the solubility of the polymer. As expected, no effects are found by variation of the pH-value of water. Necessary conditions for the film formation process are: good solubility of the polar polymers in organic solvents having a high vapor pressure, complete phase separation, solution density higher than water density, and a surrounding gas phase unsaturated with solvent vapor.The thickness of the mechanically stable films is less than 0.5 m. The films are amorphous by microscopical, FT-IR, x-ray, and DTA investigations.     

Adhesive effect of low-density polyethylene gels on polyethylene moldings

Adhesive effect of low density polyethylene (LDPE) gels in organic solvents such as decalin, tetralin, ando-dichlorobenzene on high density polyethylene (HDPE) moldings has been investigated by shearing tests, electron microscopy, and DSC measurements. When heated at 110°C for 2 h, all of the gels showed strong adhesive strengths around 30 kg/cm², which is sufficiently strong for practical uses. It has been found that the adhesive strength increases with the heating temperature and that the temperature at which the heated gel begins to exhibit the adhesive effect depends upon solvents and is about 30° lower than that of the HDPE gels.     

Temperature-dependence of mechanical and morphological properties of ultra-high molecular weight polyethylene cross-linked by electron beam irradiation

Cross-linking of ultra-high molecular weight polyethylene was performed with electron-beam irradiation in the range of radiation dose from 12 to 96 Mrad under nitrogen. Dry gel films and melt films were used as specimens. Two kinds of cross-linked specimens could be kept at 200°C for a prolonged time in an undeformed state and this tendency was independent of radiation dose. The elongation of the gel films hampered the heat-resistant effect and the drawn specimens were broken at temperatures lower than 175 °C. The elongation of the melt films could not be realized, because of a marked fixation of chains in the fiber network, even at a dose of 12 Mrad.     

Adhesive effect of high density polyethylene gels on polyethylene moldings

Adhesive effect of polyethylene moldings by use of high density polyethylene gels in organic solvents such as decalin, tetralin, ando-dichlorobenzene was investigated by shearing tests, electron microscope, and DSC measurements. All of the gels showed such a strong adhesive strength over 36 kg/cm² that polyethylene plates of 3 mm in thickness gave rise to necking sufficient for practical use, when heated at 120 °C for 2 h. In particular, the gel in tetralin showed a strong adhesive strength when heated at 110 °C. It was found that adhesive strength increases with the heating temperature; the temperatures at which adhesive strength begins to increase differ depending on the type of polyethylene sample and solvent. It is apparent that polyethylene gels exhibit an adhesive effect when they are heated at higher temperatures than the gel melting temperatures, and that the closer the SP values of solvents used for the gelation are to the molded polyethylene, the stronger the adhesion of the polyethylene molding.     

X-ray and NMR measurements on irradiated polytetrafluoroethylene and polychlorotrifluoroethylene

The effects of ⁶⁰Co γ-radiation on polytetrafluoroethylene (PTFE) and polychlorotri-fluoroethylene (PCTFE) have been studied for radiation doses up to 940 Mrad. The dependence of per cent crystallinity upon irradiation level has been determined from x-ray analysis. An initial increase in crystallinity in PTFE, attributable to chain scission in the amorphous phase of the material, was found, followed (above 300 Mrad) by a gradual decrease associated with a disordering of the crystallites. No initial increase was observed for annealed samples of PCTFE due to the large initial value of the per cent crystallinity. Above 200 Mrad the crystallinity was found to decrease with accumulated dose. Nuclear magnetic resonance measurements on PTFE have indicated a radiation-induced broadening of the amorphous component of the NMR line appearing to maximize above 700 Mrad. Similar measurements of PCTFE have shown a narrowing of the crystalline component of the NMR line and subsequent appearance of the amorphous component at approximately 200 Mrad. The data indicate that the radiation-induced behavior of PTFE and PCTFE is similar above 200 Mrad.     

Equilibrium and non-equilibrium melting of branched polyethylene relating to defect incorporation within crystals

New equilibrium melting point data, for polyethylene containing chain defects, are tested in the light of random copolymer predictions. A simplified expression for the melting point depression of random copolymers containing small amounts of non-crystallizable units is derived. Non-equilibrium melting data for rapidly quenched polyethylene samples are also reported. The fusion enthalpyH(X), and the surface free energy _e for crystals containing defects are evaluated using crystallinity, equilibrium meltingtemperatures and X-ray long period data. It is shown that increasing defect penetration within crystals induces a decrease ofH(X) withX in accordance with theoretical predictions. Finally _e is, similarly, shown to decrease with increasing number of chain defects attached to the crystal surface.     

Effects of uniaxial stretching and shrinkage on proton spin-lattice and spin-spin relaxation times of isotactic polypropylene

An isotactic polypropylene film was stretched at 120 °C in poly(ethylene glycol) and thermally shrunk at various temperatures. Proton spin-lattice,T ₁, and spin-spin,T ₂, relaxation times were measured using a broad line pulse spectrometer operating at 19.8 MHz in the temperature range 40 °C–100 °C. The temperature ofT ₁ minimum shifts to higher temperatures and the value ofT ₁ minimum increases in magnitude as the stretching ratio is increased. In contrast the temperature ofT ₁ minimum shifts to lower temperatures as shrinkage is increased, whereas the value ofT ₁ minimum increases in magnitude because of the increase in crystallimty during shrinkage. T_2a, the longestT ₁ associated with the mobile amorphous regions, increases during shrinkage, indicating that chain mobility in the amorphous regions increases substantially during shrinkage. It was found that an orientation function of the amorphous regions,f _a, correlates well withT _2a .Presented in part at the 52nd Annual Meeting of the Japan Chemical Society, Kyoto, April 1986.     

Effects of thermal history on the rigid amorphous phase in poly(phenylene sulfide)

The rigid amorphous phase of semicrystalline poly(phenylene sulfide) (PPS) has been studied as a function of thermal history using scanning calorimetry, dielectric relaxation, density, and small-angle x-ray scattering (SAXS). Based on the new heat of fusion of perfect crystalline PPS, which is 26.7±0.8 cal/gram, the weight fraction of rigid amorphous phase is shown to be nearly twice as large as previously reported [1]. The mass fraction of the rigid amorphous phase ranges from 0.24 to 0.42 and is dependent upon thermal treatment. We have taken the approach of assuming a three-phase model for the morphology of semicrystalline PPS consisting of crystalline lamellae, mobile amorphous, and rigid amorphous components. Using this three-phase model, we determine that the average density of the rigid amorphous fraction is 1.325 g/cc, which is slightly larger than the density of the mobile amorphous phase fraction and was insensitive to thermal history. From the SAXS long period, the layer thicknesses of the mobile amorphous phase, rigid amorphous phase, and crystal lamellae were estimated. Only the lamellar thickness shows a systematic variation with thermal history, increasing with melt or cold crystallization temperature, or with decreasing cooling rate.     

Determination of degree of crystallinity of drawn polymers by means of density measurements

The well known procedure of determining the degree of crystallinity by means of measuring the density presupposes the knowledge of both the densities ρ_c and ρ_a of the crystalline and of the noncrystalline regions. By combination of small-angle and wide-angle x-ray scattering and of density measurements it can be shown that this method is not justified in the case of drawn polyethylene if the values of ρ_c and ρ_a known from isotropic material are used. Both ρ_c and ρ_a depend considerably on annealing and drawing conditions. In addition the effective density ρ_c^* of the more densely packed phase in a two-phase structure is much lower than the value ρ_c calculated from the positions of the x-ray reflections due to a large number of lattice defects. This conclusion is based on the results of three independent sets of experiments: determination of the mean-square fluctuation of density 〈η²〉 by means of x-ray small-angle scattering; x-ray wide-angle measurements of the positions of the crystal reflections and of the halo arising from the noncrystalline regions; and comparison of densities and long periods of samples treated at various annealing temperatures.     

Phase and miscibility behavior of binary lipid systems

Differential scanning calorimetry was applied to study the phase diagrams of the following binary lipid systems: myristic acid (C₁₃COOH) / pentadecanoic acid (C₁₄COOH); palmitic acid methyl ester (C₁₅COOMe) / heptadecanoic acid methyl ester (C₁₆COOMe); palmitic acid methyl ester (C₁₅COOMe) / stearic acid methyl ester (C₁₇COOMe); palmitic acid methyl ester (C₁₅COOMe) / arachidic acid methyl ester (C₁₉COOMe). A distinct succession in the phase diagram types and phase regions was observed, according to the chemical structure of the mixing components.In the systems C₁₃COOH/C₁₄COOH; C₁₅COOMe/C₁₆COOMe and C₁₆COOMe/ C₁₇COOMe, both components are completely miscible in the low- and high-temperature phase. Contrasting with these three binary lipid systems, the system C₁₅COOMe/ C₁₄COOMe shows complete miscibility only in the high-temperature phase, but almost complete demixing in the solid state. In the phase diagram an incongruent melting 11 complex is built up. This complex forms an eutectic mixture with the phase of C₁₅COOMe and a peritectic system with C₁₉COOMe.