Rejuvenation and physical ageing of a polycarbonate film subjected to finite tensile strains

Differential storage and loss tensile moduli, E′ and E″, were determined intermittently at 10 Hz on specimens of an annealed polycarbonate film during stress relaxation at static tensile strains from 1.2 to 6.25% at 50°C. It was found that E′ and 1/E″ decrease when a static strain is applied but thereafter they increase progressively with time. These changes, which increase with the applied strain until it becomes 4%, are attributed primarily to a rejuvenation of a specimen (an increase in segmental mobility) followed by physical ageing (a progressive decrease in segmental mobility). Measurements at a static strain of 3% at six temperatures from 30°C to 130°C showed, among other things, that the rates of increase of E′ and decrease of E″ are sensibly independent of temperature up to 110°C.     

Effect of hydrostatic pressure and temperature on the mechanical loss properties of polymers: 3. PET, PVAC and vinyl chloride/vinyl acetate copolymers

A torsion pendulum (1 Hz) has been used to determine the pressure dependence of the glass transition temperatures of poly(ethylene terephthalate) (PET), poly(vinyl acetate) (PVAC) and some vinyl chloride/vinyl acetate copolymers. Their glass transition temperatures are shifted upwards by amounts varying between 11 and 18°C/1000 atmospheres. In the case of PVAC it has proved possible to compare the results with theory and the observed shift is found to be much less than the predicted value. A study of poly(methyl methacrylate) (PMMA) shows the β relaxation to be displaced to a higher temperature at the rate of 4°C/1000 atm. Results are also presented for the temperature dependence of shear modulus, G′, and loss tangent, tan δ for a reinforced crosslinked polymer, Bakelite P17404, at different pressures.     

DSC analysis of ethylene/1-butene copolymers obtained with different ziegler–natta catalysts

The crystallization and melting behaviour of two sets of ethylene/1-butene copolymers have been analysed by DSC. The samples, with comonomer content in the range from 0 to 21.5 mol%, were obtained by industrial processes using both Mg/Ti-based catalyst systems. The composition dependences of melting and crystallization temperatures were found to be strictly affected by the catalyst type. Moreover, logarithmic plots of the melting and crystallization enthalpy as a function of the ethylene content (mol%) in the copolymers fitted linear relationships whose slopes have been related to the critical sequence length of crystallizable ethylene units, depending on the catalytic system. These results are compared with those reported in the literature for ethylene/1-butene copolymers synthesized by other catalysts and are accounted for by a different distribution of the comonomer units in the macromolecules of the two sets of samples.     

Thermal transitions of ethylene-vinyl chloride copolymers

Differential scanning calorimetry (d.s.c.) measurements were performed on a series of ethylene-vinyl chloride (E–V) copolymers for the purpose of studying the dependence of their thermal transitions upon their microstructure. The method of preparation, via reductive dechlorination of poly(vinyl chloride) with tributyltin hydride, resulted in a series of E–V copolymers differing only in comonomer content, sequence distribution and stereoregularity of adjacent V units. Chain length distribution and branching frequencies were identical for each member of the series.Extrapolation of glass transition temperatures, T_g, measured for our E–V copolymers to pure polyethylene (PE) predicted a T_g = ?85°C ± 10°C for amorphous PE. E–V copolymers with greater than 60 mol% E units exhibited melting endotherms characterized by melting temperatures from 20°C to 128°C and degrees of crystallinity from 12 to 63%. Observed melting temperatures were plotted against the composition of the E–V copolymers and compared to Flory's equation for melting point depression of random copolymers containing one crystallizable and one non-crystallizable monomer unit. The melting point depressions observed for our E–V copolymers were in agreement with Flory's theory, if the CH₂CH₂ moiety is considered to be the crystallizable unit and theCHmoiety is assumed to prevent the CH₂CH₂ units attached on either side from being incorporated into the crystal. This implies that among all possible comonomer triad sequences only the EEE triad may crystallize. Therefore only those E–V copolymers with average lengths of consecutive E units greater than 2 exhibit crystallinity.     

Thermal stability and dynamic mechanical properties of acetal copolymers

By copolymerization of trioxane with 1,3-dioxacycloheptane (DOCH) or 1,3-dioxacyclooctane (DOCO) acetal copolymers were synthesized with comonomer units which, in contrast to customary 1,3-dioxolane/trioxane copolymers, consist of sequences with four or five methylene groups, respectively. In comparison with 1,3-dioxolane/trioxane copolymers these copolymers show a significantly higher thermal stability due to the larger sequences of thermically stable C—C-bonds. Torsion pendulum measurements show that the difference of the comonomer nature influences the storage modulus, G′, which is a measure for the rigidity of the material, and the loss tangent tan δ. Obviously, G′ is dependent on the crystallinity of the sample, which again is influenced by the kind and concentration of the incorporated comonomer. Also the peak locations of the α- and γ-relaxation indicate a direct connection to the comonomer nature and content in the different copolymers. The α-maxima are shifted to lower temperatures with increasing number of methylene groups as well as with growing comonomer content. However, the γ-maxima appear at higher temperatures with increasing comonomer content, but at lower temperatures with growing length of the comonomer unit. Additionally, branched acetal copolymers were synthesized by using comonomers in which the hydrogen atom at the 4-position of 1,3-dioxolane has been substituted by alkyl groups of different length (methyl to tetradecyl). In these copolymers the nature of comonomer neither improves the thermal stability of the copolymers nor influences the α- and γ-relaxation.     

Amphiphilic copolymers of glycidol with nonpolar epoxide comonomers

To explore the industrially used copolymerization of glycidol with other epoxide species, a series of copolymers of glycidol with various comonomers (epichlorohydrin, isopropyl glycidyl ether, 1,2‐epoxybutane, propylene oxide, and glycidyl phenyl ether) has been synthesized by cationic ring‐opening polymerization in dichloromethane, using boron trifluoride diethyl etherate as the initiator. The facile synthesis proceeded at room temperature under ordinary atmosphere. The air‐ and temperature‐stable products—mostly clear, colorless, viscous liquids—are proposed to consist of a hyperbranched polyglycidol core (incorporating varying fractions of comonomer) with arms made from comonomer‐derived repeat units. These copolymers had low molecular weights and rather broad molecular weight distributions. There was wide variation in the polarity of these copolymers, depending on the comonomer used, resulting in materials that were soluble in solvents ranging from benzene to water. The glass‐transition temperatures observed for these copolymers also varied widely, ranging from ?56 to 1°C. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2290–2299, 2001     

Dielectric relaxation in vinylidene fluoride–hexafluoropropylene copolymers

The molecular mobility in copolymers of vinylidene fluoride–hexafluoropropylene VDF/HFP of 93/7 and 86/14 ratios has been investigated by means of broadband dielectric relaxation spectroscopy (10^?1–10⁷ Hz), differential scanning calorimetry DSC (?100 to 150°C), and of wide angle X‐ray diffraction WAXS. Four relaxation processes and one ferroelectric‐paraelectric phase transition have been detected. The process of the local mobility β‐ (at temperatures below glass transition point) is not affected by chemical composition of the copolymer and the formed structure. Parameters of segmental mobility in the region of glass transition (α_a‐relaxation) depend on the ratio of comonomer with lower kinetic flexibility. α_c‐relaxation is clearly observed only in VDF/HFP 93/7 copolymer, which is characterized by a higher crystallinity and a higher perfection of crystals of α‐ (α_p‐) phase. Diffuse order–disorder relaxor type ferroelectric transition connected with the destruction of the domains in low‐perfect ferroelectric phase in the amorphous regions has been detected for both copolymers. An intensive relaxation process (α‐process) was observed for both copolymers in high‐temperature region. DSC data shows that it falls on the broad temperature region of α‐phase crystals melting. It is considered to be connected with the space charge relaxation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Fluoroelastomers-dependence of relaxation phenomena on composition

The dynamic-mechanical properties of some fluoroelastomers were determined as a function of composition at low frequency (≈ 1 Hz), by means of a free oscillation torsion pendulum, between −180°C and the softening point. Vinylidenefluoride (VDF)-hexafluoropropene (HFP) copolymers of molar composition 0–39% HFP and terpolymers of VDF and HFP with up to 30mol% tetrafluoroethylene (TFE) and a constant VDF to HFP molar ratio of 3.4 were considered. Two relaxation processes typical of the amorphous phase were found. The first, located at about −87°C, is related to local motions and the transition temperature was found to be independent of composition for copolymers, while it depends on TFE molar content for terpolymers. The second is related to the glass transition and the transition temperature depends on the composition. However, for semicrystalline copolymers the double glass transition phenomenon was observed. When crystallinity goes to zero at about 20 mol% HFP, only one transition is observed. It was also found that ordered structures can take place for terpolymers when TFE molar concentration exceeds 20%. The crystal disorder transition of pure PVDF (75°C) is observed also for low HFP concentrations but the transition temperature is strongly reduced. Analogies between the VDF-HFP and E-P systems are also discussed.     

Isothermal crystallization of metallocene‐based propylene/α‐olefin copolymers

Isothermal crystallization and subsequent melting behavior of two propylene/hexene‐1 copolymers and two propylene/octene‐1 copolymers prepared with metallocene catalyst were investigated. It is found that γ‐modification is predominant in all copolymers. The Avrami exponent shows a weak dependency on comonomer content and comonomer type. At higher crystallization temperatures (T_c) the crystallization rate constant changes more rapidly with T_c and the crystallization half‐time substantially increases. Double melting peaks were also observed at high T_c, which is attributed to the inhomogeneous distribution of comonomer units along the polymer chains and the existence of crystals with different lamellar thicknesses. The equilibrium melting temperatures (T) of the copolymers were obtained by Hoffman–Weeks extrapolation. It was found that the T decreases with increasing comonomer content, but are independent of comonomer type, implying that comonomer units are excluded from the crystal lattice. Dilation of the crystal lattice was also observed, which depends on crystallization, comonomer content, and comonomer type. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 240–247, 2005     

Effects of shearing and comonomer content on the crystallization behavior of poly(butylene succinate‐co‐butylene 2‐ethyl‐2‐methyl succinate)

Poly(butylene succinate‐co‐butylene 2‐ethyl‐2‐methyl succinate) (PBSEMS) random copolymers were prepared with different comonomer compositions. The effects of shearing and comonomer content on the crystallization behavior of these copolymers were investigated at 80 °C. The thermal and morphological properties of the resulting samples were also discussed. The copolymers showed a longer induction time and a slower crystallization rate with increasing comonomer content. The promoting effect of shear on the overall crystallization behavior was more notable for those copolymers containing more 2‐ethyl‐2‐methyl succinic acid (EMSA) units. The melting temperature of ‘as‐prepared’ poly(butylene succinate) (PBS) was ca. 115 °C, while that of the copolymers varied from 112 to 102 °C. Higher comonomer contents in the copolymers gave rise to lower melting temperatures and broader melting peaks. In addition, the isothermally crystallized samples showed multiple melting endothermic behavior, the extent of which depended on the comonomer content. The copolymers showed different wide‐angle X‐ray diffraction (WAXD) patterns from that of neat PBS, depending on the comonomer content and shear applied during crystallization. With increasing comonomer content, the copolymers crystallized without shearing, showing the shifting of a diffraction peak to a higher angle, while those crystallized under shear did not show any peak shift. Copyright © 2004 Society of Chemical Industry     

Copolymers of N-alkylacrylamides and styrene as new thermosensitive materials

New thermosensitive copolymers were obtained by copolymerization of styrene with N-methylacrylamide, N-ethylacrylamide or N,N-dimethylacrylamide. The turbidity measurements showed that these polymers can undergo a thermally induced phase transition at various temperatures, depending on their chemical composition, molecular weight and concentration in water. Fluorescence measurements indicated the occurrence of aggregation through hydrophobic interactions even below the lower critical solution temperature. The enthalpy and cooperativity of phase transition determined by microcalorimetry were strongly dependent on the chemical structure of the N-alkylacrylamide comonomer.     

Influence of the polymerization conditions on the rigidity of phenylnorbornene-ethylene copolymers made using ethylene bis (indenyl) zirconium dichloride and MAO

When characterized with DMA it was found that Phenylnorbornene-ethylene copolymers with equivalent comonomer concentrations had very different storage moduli in the glassy state as well as glass transition temperatures. NMR analysis of the copolymers revealed that they had different ratios of Exo- and Endo-diastereomers even if the same comonomer composition had been used. The Exo/Endo-ratio in the polymer was surprisingly higher than in the monomer and increased with higher incorporations of the comonomer. Copolymers with higher Exo/Endo-ratios also had a tendency for lower storage moduli in the glassy state and higher tan δ peaks. These properties are valuable because more flexible and impact-resistant copolymers are obtained. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 385–393, 1998     

Thermal transitions of ethylene-vinyl chloride copolymers and their blends

Differential Scanning Calorimetry (DSC) measurements were performed on a series of ethylene-vinyl chloride copolymers (E-V) prepared via reductive dechlorination of poly(vinyl chloride) with tributyltin hydride. The copolymers were identical in chain length and branching distribution; differing only in comonomer content, sequence distribution, and stereoregularity of adjacent —V— units. Extrapolation of glass transition temperatures, T_g, measured for our E-V copolymers to pure polyethylene (PE) predicted a T_g = ?85 ± 10°C for amorphous PE. E-V copolymers with greater than 60 mol percent —E— units exhibited melting endotherms from 20 to 128°C and degrees of crystallinity from 12 to 63 percent. The melting point depressions observed for our E-V copolymers were in agreement with Flory's theory, if the ? CH₂? CH₂ moiety is considered to be the crystallizable unit and the moiety is assumed to prevent the ? CH₂? CH₂? units attached on either side from being incorporated into the crystal. In general, the E-V copolymer blends with PE were incompatible, while those with PVC were compatible only for E-V copolymers with high V contents (>80 mol percent). Blends of the amorphous E-V copolymers were found compatible if their V contents differed by less than 15 mol percent, while blends where one or both E-V copolymers are crystalline were found to be incompatible. The properties of these copolymers will be discussed in terms of their microstructure.     

Dynamic mechanical properties of polymer-fluid systems: characterization of poly(2-hydroxyethyl methacrylate) and poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) hydrogels

The complex shear moduli of crosslinked poly(2-hydroxyethyl methacrylate) and poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) hydrogels were measured as a function of water content, temperature and frequency using dynamic mechanical spectroscopy. The effect of increasing water content on both the storage and loss moduli is to lower the temperatures at which mechanical transitions occur. There is a significant decrease in the storage moduli and increase in the loss moduli dispersions with increasing water content. These data could be well correlated by a multiple-mechanism, time-temperature superposition procedure. Between 2% and 28% water content in the copolymer sample, the relaxation spectra and shift function characterizing the glassy-rubbery state transition become independent of water content when properly referenced to the observed transition temperature. At lower temperatures where the β transition occurs, the mechanical viscoelastic spectra depend much more strongly on water content although the shift factors collapse to a single curve when referenced to the observed β transition temperature.     

Some properties of copolymers of vinylidene chloride with acrylates and methacrylates,Part 1

Copolymers of vinylidene chloride (VDC) and acrylates or methacrylates were investigated by calorimetric (DSC) and infrared spectroscopic methods to determine glass transition temperature and crystallization behavior. The final crystallinity of some annealed copolymers of VDC and methyl acrylate (MA) or methyl methacrylate (MMA) was studied by means of WAXS measurements. It was found that: (i) the glass transition temperature of copolymers of VDC and acrylates goes through a maximum as a function of the composition. The maximum shifts toward higher concentrations of VDC in the copolymer with increasing length of the ester group. (ii) However, for copolymers of VDC and methacrylates one observes a monotonic rise in the glass transition temperature with increasing concentration of the methacrylate, that can be described by the equation of Gordon and Taylor. (iii) For a given comonomer, the crystallization rate increases with increasing VDC content and, moreover, for a given VDC content it is slowest with methyl acrylate and fastest with ethylhexyl acrylate. Therefore, for a given crystallization rate, the copolymers of VDC and methyl acrylate contain the highest weight fraction of VDC. However, on a molar basis the differences are rather small. Analogous behavior is observed for the copolymers of VDC and acrylates or methacrylates. (iv) The final crystallinity of the annealed copolymers decreases linearly with decreasing VDC content. From this linear decrease, the limiting VDC concentration for zero crystallinity was extrapolated to about 75 mol-% VDC. These values are slightly lower than those estimated from infrared spectroscopy. Consequently, the final crystallinity of the copolymers studied is influenced by the VDC content rather than by the comonomer type. (v) The final crystallinity of the VDC homopolymer is determined as 57 ± 2 wt.-% in agreement with literature data. The crystallinity of the copolymers of VDC with different acrylates or methacrylates can then be estimated as a function of the composition.     

Structure and properties of copolymers of vinylcyclohexane and α-methylvinylcyclohexane with acrylonitrile

A study has been made of the structure and properties of alternating equimolar and random copolymers of vinylcyclohexanne and α-methylvinylcyclohexane with acrylonitrile using infrared and ¹H-NMR spectroscopy methods, as well as x-ray diffraction and differential thermal analyses. The spectral methods allowed an estimation of the composition and distribution of comonomer units in the copolymer macrochains. The thermal stability of alternating copolymers was found to be higher than that of polyacrylonitrile. Alternating and random copolymers are amorphous products with various dimensions of crystallites. Information on the molecular motion intensity and temperatures of structural transitions in copolymer macrochains was obtained by the spin-probe technique.     

Thermal fractionation and effect of comonomer distribution on the crystal structure of ethylene-propylene copolymers

In this paper the comonomer distributions of two series of ethylene-propylene copolymers with different propylene contents, which were prepared by a fluorinated bis(phenoxyimine) Ti catalyst (FI-EP copolymers) and a conventional Ziegler-Natta catalyst (ZN-EP copolymers), respectively, were characterized. It is found that the comonomer distribution of ethylene-propylene copolymers can still be characterized by thermal fractionation at a long scale, though the propylene units can be incorporated into the PE crystal lattice. The FI-EP copolymers exhibit a narrow and random comonomer distribution, whereas a broad comonomer distribution is observed for the ZN-EP copolymers. The crystal structures of the FI-EP and ZN-EP copolymers were studied by WAXD. The a-axis of the PE crystals of the FI-EP copolymers increases rapidly with propylene content, indicating that more propylene units are incorporated into the PE crystal lattice, whereas only a slight expansion in a-axis is observed for the ZN-EP copolymers. WAXD result also shows the presence of hexagonal phase in the FI-EP copolymers and the relative content of the hexagonal phase increases with the propylene content, while in the ZN-EP copolymers the hexagonal phase is negligible.     

Dielectric and ferroelectric properties of lead indium niobate ceramic prepared by wolframite method

The dielectric and ferroelectric properties of lead indium niobate (Pb(In_1/2Nb_1/2)O₃, PIN) ceramic prepared by an oxide-mixing method via wolframite route were investigated. The 98.5% perovskite fine-grained PIN ceramics with average grain sizes of 1–2 μm were obtained by sintering at 1050 °C for 2 h. The dielectric properties of the PIN were of relaxor ferroelectric behavior with temperature of dielectric maximum (T_m) 53 °C and dielectric constant (_r) 4300 (at 1 kHz). The P–E hysteresis loop measurements at various temperatures showed that the ferroelectric properties of the PIN ceramic changed gradually from the paraelectric behavior at temperature above T_m to slim-loop type relaxor behavior at temperature below T_m. Moreover, the P–E loop became more open at temperatures much lower than T_m. At −25 °C, the maximum polarization is found to be 8 μm/cm² at a field of 30 kV/cm, with P_r value of 2.5 μm/cm² and E_c of +7.5 kV/cm.     

Styrene copolymerization using a metallocene-MAO initiator system. Homo- and copolymerization of styrene with some cycloalkenes

The copolymerization of styrene with cyclohexene, 1-methyl-1-cyclohexene, and norbornene using ethenylbisindenylzirconium dichloride and methylaluminoxane, Et(Ind)₂ZrCl₂-MAO, initiating systems has been tested. The results obtained with each styrene-cycloalkene couple, except styrene/norbornene, indicate a less effective polymerization process compared to styrene homopolymerization, in agreement with the electronic and steric effects present in each comonomer. The electronic I+ effects of substituent groups, depending on their placement, largely improve the polymerization process, while bulky groups on or near the vinyl carbon double bond of styrene decrease its effectiveness. The present study shows that the copolymers obtained are amorphous and their composition showed a lower abundance of comonomer units with respect to the initial feed. For comparison, the results of the copolymerization of styrene/(1-octadecene) using the same initiator system and polymerization process are included, a polymerization that indicates a more reactive process, and as the proportion of octadecene in the initial feed increases, it showed a crystalline fusion temperature as well as a Tg in the styrene region which can be attributed to the formation of block styrene/octadecene copolymers.     

Synthesis, crystallization and tensile properties of poly(ethylene terephthalate-co-2,6-naphthalate)s with low naphthalate units content

A series of poly(ethylene terephthalate-co-naphthalate)s (PETN copolymers) with low naphthalate units content was synthesized. A melting point depression was observed, while the glass transition temperatures were slightly higher than that of Polyethylene terephthalate (PET). Crystallization rates of the copolymers decreased with increasing comonomer content. WAXD patterns showed that only PET crystals were formed. Co-crystallization behaviour was evaluated on the basis of the Wendling–Suter model. The tensile properties of the copolymers PETN 97/3 and PETN 94/6, Young's modulus yield stress and elongation at break was significantly improved compared to PET. WAXD showed that some crystalline precursor was generated during drawing of the specimens. DSC traces of the drawn specimens showed enhanced crystallization rates compared to that of the original amorphous specimens.