Ethylene–chlorotrifluoroethylene copolymers. I. Synthesis and evaluation of compositions resistant to thermal stress cracking

Copolymerization of ethylene, chlorotrifluoroethylene, and small amounts (less than 5 mol %) of a perfluoroalkylethylene yields terpolymers in which the concentration ratio of the ethylenederived units to the units derived from the fluorine-containing monomers is unity, indicating that the perfluoroalkylethylenes are incorporated at the expense of chlorotrifluoroethylene. The resulting polymer compositions are characterized by improved resistance to thermal stress cracking.     

Structure of ethylene–chlorotrifluoroethylene copolymers

The structures of copolymers of ethylene and chlorotrifluoroethylene have been studied by infrared, nuclear magnetic resonance, and x-ray diffraction techniques. Copolymers varying in ethylene composition from 80 to 50 mole-% were prepared at a number of different temperatures with a peroxide catalyst system. Compositions of 50/50 mole ratio were found to be semicrystalline and to have melting points as high as 241°C. These materials were found to be copolymers with a high degree of one-to-one alternation. They were similar in structure to 1:1 copolymers which had been reported previously by other workers who used a triethylboron catalyst system. The x-ray evidence indicated that the copolymers prepared with the peroxide catalysts were not stereoregular. A hexagonal unit cell with a theoretical density of 1.70 g/cc was determined for the alternating one-to-one copolymer by x-ray techniques. A value of 262°C was determined for the melting point of the theoretical 100% alternating one-to-one copolymer. Values of ΔH_? = 4500 cal/mole and ΔS_? = 8.4 cal/deg-mole were also calculated for the alternating 1:1 copolymer. The preferred conformation of the material appears to be a “kinked” structure with the crystalline phase having ethylene units in one chain lining up opposite chlorotrifluoroethylene units in the adjacent chain. Polar association which can occur between fluorine and hydrogen atoms in this arrangement may account in part for the relatively high melting point of the alternating one-to-one copolymers.     

Crystallization of branched polymers. I. Ethylene–vinyl acetate and ethylene–acrylic acid copolymers

The following quantities were measured on a number of ethylene–vinyl acetate (EVA) and ethylene–acrylic acid (EAA) copolymers: (1) the small-angle x-ray scattering invariant, (2) the overall density, and (3) the crystallinity. Assuming a two-phase structure, the separate values of the densities of the crystalline and amorphous regions can be calculated from these data. Of these, the crystalline density is compared with the value obtained from the lattice constants. A systematic difference is observed which is ascribed to the presence of comonomeric side groups in the crystalline regions. For the EVA and EAA samples, their concentration is at least 0.3 and 0.5 times the overall concentration, respectively. The amorphous densities are found to be higher than the values calculated from completely amorphous copolymers by extrapolation procedures.     

Radiation-Induced Copolymerization of Chlorotrifluoroethylene with Olefins

The copolymerizations of chlorotrifluoroethylene with propylene and isobutylene were carried out at 0 ～ ?78°C by γ-ray irradiation. It was found that alternating copolymers are obtained over a wide range of monomer composition for both systems. The alternating copolymer of chlorotrifluoroethylene with isobutylene was found to be highly crystalline, but the copolymer of chlorotrifluoroethylene with propylene was found to be amorphous. The copolymerizations were considered to proceed via a radical mechanism, but in the case of the isobutylene-chlorotrifluoroethylene system a cationic polymerization also takes place simultaneously at ?78°C.     

Transport properties of poly(ethylene terephthalate) crystallized from the glassy state

Poly(ethylene terephthalate) was crystallized from the glassy state at 120 and 200°C. The structural organization of samples, after the primary and secondary crystallizations, was analyzed by density, X-rays, infrared and transport properties of dichloromethane vapor. The values of crystallinity derived with different methods do not agree, indicating that the crystallized samples cannot be considered simply biphasic. Since the fraction of the impermeable phase is much higher than the fraction of the crystalline phase, it suggested that the presence of mesomorphic form, impermeable to the vapors at low activity. With this hypothesis, the complete composition of the crystalline samples, in terms of fraction of crystalline, amorphous and mesomorphic form was derived. A value of density of the mesomorphic form of 1.39 g/cm³ was also derived.     

Molecular transitions in an alternating copolymer of ethylene and chlorotrifluoroethylene

Three transitions have been found with peaks at 130°C (α), 88°C (β), and ?65°C (γ), by mechanical relaxation techniques for a quick-quenched sample of an alternating copolymer of ethylene and chlorotrifluoroethylene. The intensity of the α transition was found to increase with an increase in crystalline content. It was observed at 150°C by infrared and x-ray diffraction techniques and by mechanical relaxation spectra on an annealed sample. X-ray diffraction and dichroic infrared measurements were conducted on oriented specimens as a function of temperature. These data showed that the β transition was accompanied by a change in lateral bonding distances in the crystalline phase and a conformational change attributed to an “unkinking” of the molecular chain. The β transition was found to be related to the amorphous phase and the onset of the β peak corresponded to the transition at 35°C found by infrared techniques and previously by a torsional modulus method. It was tentatively assigned to the glass transition. A more definitive assignment of the β transition would depend on a detailed structural analysis of the γ transition. The γ peak was not studied in detail in the present work.     

Submillimeter infrared absorption and chain conformations of poly(vinylidene fluoride) and copolymers. I. Crystalline and amorphous contributions

Far infrared spectra of poly(vinylidene fluoride) and several copolymers (with hexafluoropropylene, chlorotrifluoroethylene, and tetrafluoroethylene) have been obtained between 15 and 55 cm^?1. After correction of the interference phenomenon affecting the measurements, the absorption coefficient and the imaginary part of the dielectric constant have been calculated. We show how these parameters are affected by the chain conformation and copolymer composition. The behavior of the crystalline and amorphous phases have been studied. The effects of mechanical and electrical stresses are considered.     

How did we find the rigid amorphous phase?

The first experimental evidence of the existence of the rigid amorphous phase was reported by Menczel and Wunderlich [1]: when trying to clarify the glass transition characteristics of the first main chain liquid crystalline polymers [poly(ethylene terephthalate-co-p-oxybenzoate) with 60 and 80 mol% ethylene terephthalate units] [2], the absence of the hysteresis peak at the lower temperature glass transition became evident when the sample of this copolymer was heated much faster than it had previously been cooled. Since this glass transition involved the ethylene terephthalate-rich segments of the copolymer, we searched for the source of the absence of the hysteresis peak in PET. There, the gradual disappearance of the hysteresis peak with increasing crystallinity was confirmed [1]. At the same time it was noted that the higher crystallinity samples showed a much smaller ΔC _p than could be expected on the basis of the crystallinity calculated from the heat of fusion (provided that the crystallinity concept works). Later it was confirmed that the hysteresis peak is also missing at the glass transition of nematic glasses of polymers. When checking other semicrystalline polymers, the sum of the amorphous content calculated from the ΔC _p at the glass transition, and the crystallinity calculated from the heat of fusion was far from 100% for a number of semicrystalline polymers. For most of these polymers, the sum of the amorphous content and the crystalline fraction was 0.7, meaning that ca. 30% rigid amorphous fraction was present in these samples after a cooling at 0.5 K min⁻¹ rate. Thus, the presence of the rigid amorphous phase was confirmed in five semicrystalline polymers: PET, Nylon 6, PVF, Nylon 66 and polycaprolactone [1]. Somewhat later poly(butylene terephthalate) and bisphenol-A polycarbonate [3] were added to this list.     

Solid-state C NMR and SAXS characterization of the amorphous phase in low-molecular weight poly(ethylene oxide)s

Low-molecular weight poly(ethylene oxide)s (PEO) with extended, once or twice folded chains (as characterized by SAXS), were investigated by solid-state ¹³C NMR spectra measured under conditions to detect only the signal of the narrow line component. The direct detection and integrated intensities of the signals from hydroxy-terminated chain-end units in these spectra confirm that the narrow line component corresponds to the noncrystalline (amorphous) phase. The NMR line of PEO carbons adjacent to the hydroxy end-groups was used as an intensity standard to obtain information on the mean number of carbons per chain contributing to the amorphous phase. Assuming that amorphous phase is formed by chain ends (cilia) and folds it follows from the spectra that the length of folds is 6-7 monomer units; cilia are 2-3 monomer units long.     

Rheo-optical Fourier-transform infrared spectroscopy of polymers. VII. Uniaxial deformation of heat-set poly-(ethylene terephthalate) film

Poly(ethylene terephthalate) (PET) film was uniaxially stretched to a draw ratio of 2.5. Two samples were prepared from this oriented film by heat-setting it at 493 K while free to relax and when held at constant length. The structural changes occurring during uniaxial elongation up to fracture of these two oriented crystalline samples and their stress–strain characteristics were simultaneously monitored by rapid-scanning Fourier-transform infrared (FTIR) spectroscopy. In the free-annealed sample, the orientation of the molecules in the amorphous phase shows a gradual improvement throughout the test, while chain unfolding occurs above 20% strain. This indicates that the predominant mechanism of deformation in this sample could be chain uncoiling in the amorphous phase followed by longitudinal slip processes within the sample. In the taut-annealed sample, chain unfolding occurs at low strains accompanied by slight improvements in amorphous and crystalline orientation. Thus, in this sample longitudinal slip would appear to be the main deformation mechanism. The results of the FTIR measurements are discussed with reference to the dependence of the deformation mechanisms on the initial structure of the samples.     

Application of fourier transform infrared spectroscopy to the study of semicrystalline polymers: Poly(ethylene terephthalate)

An infrared absorbance subtraction technique has been used to “isolate” bands in the composite spectrum of semicrystalline polymers according to their crystalline or amorphous character. Amorphous and crystalline spectra for annealed, melt-quenched, and solution-cast poly(ethylene terephthalate) have been separated. The spectra of the amorphous component show an increased intensity of bands associated with the trans configuration of oxygen about the C? C bond when the polymer is annealed. This increased “trans” band intensity reflects the increased proportion of trans structures as a result of annealing. The amorphous trans bands are shifted approximately 1–3 cm^?1 from their positions in the crystalline “trans” spectrum. The frequency shift of these bands can be attributed to the differences in chain interactions that exist in the amorphous phase and the crystalline lattice. We have also found that under identical anealing conditions the amorphous phase of the melt-quenched polymer contains an increased intensity of conformational trans bands compared to the sample cast from solution.     

Solid‐state NMR characterizations on phase structures and molecular dynamics of poly(ethylene‐co‐vinyl acetate)

Solid‐state nuclear magnetic resonance spectroscopy and relaxation measurements, together with DSC, have been used to elucidate the structures and molecular dynamics in poly(ethylene‐co‐vinyl acetate) (EVA). It has been found that besides immobile orthorhombic and monoclinic crystalline phases, the third mobile crystalline phase (possibly the phase) of a considerable amount (36% of total crystalline phases) appears in the EVA samples, which forms during room‐temperature aging as a result of the secondary crystallization and melts at temperature somewhat higher than room temperature. Such a mobile crystalline phase has not only the well‐defined chemical shift of its own, but also has different molecular mobility from the orthorhombic phase. The mobile crystalline phase is characterized by the rapid relaxation of the longitudinal magnetization, which is caused by conventional spin‐lattice relaxation, while the slow relaxation of the longitudinal magnetization occurring in the orthorhombic phase is originated from the chain diffusion. In addition, the amorphous phase also contains two components: an interfacial amorphous phase and a melt‐like amorphous phase. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2864–2879, 2006     

A Molecular Necklace: Threading β‐Cyclodextrins onto Polymers Derived from Bile Acids

A molecular necklace of polypseudorotaxanes was prepared by threading β‐cyclodextrins (β‐CD) onto biodegradable and thermoresponsive polyurethanes derived from bile acids. These polyurethanes were synthesized via a simple step condensation of bile acid‐based dicarbonate with poly(ethylene glycol)‐diamine. The β‐CD rings slide onto the poly(ethylene glycol) segments and selectively recognize the bile acid units of the polyurethane chains, whereas the poly(ethylene glycol) segments remain crystalline with a lower crystallinity. This bio‐compound‐derived molecular necklace can be visualized by scanning tunneling microscopy. The polypseudorotaxanes show thermosensitivity in water and the phase transition temperature may be fine‐tuned by varying the molar ratios of β‐CD to the bile acid units. Such an interesting necklace model of polypseudorotaxane constructed from natural compounds may lead to the further exploration of their applications, such as as an enzyme model, due to their biological nature.     

Raman spectroscopic method for determining the crystallinity of polyethylene

The Raman spectrum of partially crystalline polyethylene can be described as a superposition of three components, which originate from the orthorhombic crystalline phase, a meltlike amorphous phase, and a disordered phase of anisotropic nature, where chains are stretched but have lost their lateral order. The mass fractions involved in the three phases can be derived directly from the integral intensities of characteristic bands without an additional calibration procedure. A comparison of the results obtained for a variety of samples shows agreement with the crystallinities derived from the density, and the small-angle and wide-angle x-ray diagrams. Data indicate that the disordered anisotropic phase is located at a transition zone between crystalline and amorphous layers. Application of the method in a temperature-dependence experiment permits a detailed examination of partial melting.     

Brillouin scattering studies of the effect of orientation on mechanically deformed poly(ethylene terephthalate)

Brillouin scattering is used to study the internal structure of oriented films of poly(ethylene terephthalate). Splitting in the longitudinal spectrum is observed as the film is stretched, indicating that the crystalline region is developed gradually from the amorphous region. The hypersonic velocity data obtained from these two regions are used to draw directional maps of sound velocity propagation in different directions of the film. The results are discussed and correlated with a recently proposed model. The orientational parameter in the amorphous phase is calculated from the hypersonic velocity data as a function of stretch ratio. The results are found to be in good agreement with published values obtained by a different technique.     

Conformation analysis and molecular mobility of ethylene and tetrafluoroethylene copolymer using solid-state 19F MAS and 1H --> 19F CP/MAS NMR spectroscopy

The changes in the conformation and molecular mobility accompanied by a phase transition in the crystalline domain were analyzed for ethylene (E) and tetrafluoroethylene (TFE) copolymer, ETFE, using variable-temperature (VT) solid-state 19F magic angle spinning (MAS) and 1H --> 19F cross-polarization (CP)/MAS NMR spectroscopy. The shifts of the signals for fluorines in TFE units to higher frequency and the continuing decrease and increase in the T1rho(F) values suggest that conformational exchange motions exist in the crystalline domain between 42 and 145 degrees C. Quantum chemical calculations of magnetic shielding constants showed that the high-frequency shift of TFE units should be induced by trans to gauche conformational changes at the CH2-CF2 linkage in the E-TFE unit. Although the 19F signals of the crystalline domain are substantially overlapped with those of the amorphous domain at ambient probe temperature (68 degrees C), they were successfully distinguished by using the dipolar filter and spin-lock pulse sequences at 145 degrees C. The dipolar coupling constants for the crystalline domain, which can be estimated by fitting the dipolar oscillation behaviors in the 1H --> 19F CP curve, showed a significant decrease with increasing temperature from 42 to 145 degrees C. This is due to the averaging of 1H-19F dipolar interactions originating from the molecular motion in the crystalline domain. The increase in molecular mobility in the crystalline domain was clearly shown by VT T1rho(F) and 1H --> 19F CP measurements in the phase transition temperature range.     

Plastic deformation of poly(tetramethylene oxide). II. Molecular orientation as measured by x-ray diffraction and NMR measurements

The orientation of the crystalline and amorphous phases in uniaxially drawn samples of polytetramethylene oxide has been studied by x-ray and NMR methods. Pole figure measurements give the orientation of the crystalline phase. Its dependence on the draw ratio does not obey the pseudoaffine model. The crystalline fraction is derived from NMR measurements at temperatures between T_g and T_m, where the free induction decay (FID) of the unoriented sample can be analyzed in terms of a rigid (crystalline), a constrained, and an amorphous phase. Low temperature (T < T_g) measurements of the NMR second-moment anisotropy in protonated and deuterated samples give a mean orientation of the chains higher than that corresponding to the crystalline phase. The lack of anisotropy in the tail of the FID at temperatures between T_g and T_m indicates no appreciable anisotropy in the truly amorphous interlamellar phase. From this and from the ratio of the P₄/P₂ orientations, factors which obey the “most probable distribution,” it is concluded that the amorphous orientation is due to the layer of constrained chains at the surface of the lamellae.     

Effect of amorphous sequences on the longitudinal acoustic modes in partially crystalline polymers. I. Transfer matrix method

A novel theoretical scheme is developed which enables the determination of the LAM-like vibrations of polymer chains made up of crystalline and amorphous parts as they occur in partially crystalline structures. The boundary conditions effective at the junction points are formulated in terms of the compliances of the associated amorphous sequences. These compliances can be derived from their eigenfrequencies and eigenvectors in a disconnected state. The treatment uses a matrix formalism which can be extended to include bending and torsional motions in a general state of vibration of the crystalline stem. A first numerical example demonstrates that the LA mode of a crystalline stem can be strongly perturbed by the coupling to the adjacent amorphous sequences. Interpretation of frequencies and line shapes of observed LA modes should always include these coupling effects; their neglect can lead to considerable errors.     

Oxygen‐barrier properties of oriented and heat‐set poly(ethylene terephthalate)

The improvement in the oxygen‐barrier properties of poly(ethylene terephthalate) (PET) by orientation and heat setting was examined. Orientation was carried out at 65 °C by constrained uniaxial stretching to a draw ratio of about 4. Heat setting was performed at temperatures from 90 to 160 °C with the specimen taut. Orientation decreased the permeability of PET to almost one‐third that of the unoriented, amorphous polymer because of decreases in both the diffusion coefficient and the solubility coefficient. The proposed two‐phase model for oriented PET consisted of a permeable isotropic amorphous phase (density = 1.335 g/cm³) with ethylene linkages predominately in the gauche conformation and an impermeable oriented phase (density = 1.38 g/cm³) with ethylene linkages that had transformed from the gauche conformation to the trans conformation during stretching. Chain segments in the trans conformation did not possess crystalline order; instead, they were viewed as forming an ordered amorphous phase. Crystallization by heat setting above the glass‐transition temperature did not dramatically affect the permeability. However, a decrease in the diffusion coefficient, offset by an increase in the solubility coefficient, indicated that crystallization affected the barrier properties of the permeable amorphous phase. Analysis of the barrier data, assuming a two‐phase model with variable density for both the permeable and impermeable phases, revealed that the impermeable phase density increased during crystallization, approaching a value of 1.476 g/cm³. This value is consistent with previous measurements of the density of the defective crystalline phase in PET. The density of the permeable amorphous phase decreased concurrently to about 1.325 g/cm³, indicating the appearance of additional free volume. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1679–1686, 2000     

The role of crystalline, mobile amorphous and rigid amorphous fractions in the performance of recycled poly (ethylene terephthalate) (PET)

The action of thermo-mechanical degradation induced by mechanical recycling of poly(ethylene terephthalate) was simulated by successive injection moulding cycles. Degradation reactions provoked chain scissions and a reduction in molar mass mainly driven by the reduction of diethyleneglycol to ethylene glycol units in the flexible domain of the PET backbone, and the formation of -OH terminated species with shorter chain length. The consequent microstructural changes were quantified taking into account a three-fraction model involving crystalline, mobile amorphous (MAF) and rigid amorphous fractions (RAF). A remarkable increase of RAF, to a detriment of MAF was observed, while the percentage of crystalline fraction remained nearly constant. A deeper analysis of the melting behaviour, the segmental dynamics around the glass-rubber relaxation, and the macroscopic mechanical performance, showed the role of each fraction leading to a loss of thermal, viscoelastic and mechanical features, particularly remarkable after the first processing cycle.