Long periods in slow-cooled linear and branched polyethylene: Part II

It is shown that the long periods L in slow-cooled polyethylene materials obey the general law L = L⁰ + αr_w, where r_w is the weight average dimension of the coil before crystallization, and L⁰ is a parameter of the order of l_c, the crystalline core thickness, which increases as the cooling rate V decreases. α is a parameter independent of M and V but decreasing with the number of long-chain branches per molecule. The two terms in the above relation are, respectively, the contributions of crystalline and amorphous layers. For cooling rates from 800°C/min to 0.2°C/min, it is shown that the temperature T_c of crystallization is constant; hence the change of morphology (long period, crystalline core thickness, crystallinity) cannot be explained by supercooling. The increase in long period and crystallite thickness in slow-cooled materials with decreasing cooling rate is interpreted in terms of annealing of the crystallized materials between the crystallization temperature T_c and the secondary transition temperature T_αc. Crystallization proceeds by a two-step process of solidification and annealing. During the annealing stage, the mobility of the chains in the crystalline phase is due to defects; the kinetics of thickening is then governed by the mobility (or nucleation) of the defects appearing above T_αc. In the proposed model of crystallization, the assumption that the energy of activation is proportional to T_αc explains the observed laws L ≡ l_c ≡ log t_a, where the annealing time t_a is equal to (T_c ? T_αc)/V. The model applies also to polymers crystallized from the melt and subsequently annealed.     

Lamellar thickening in nascent poly(acrylonitrile) upon annealing

No systematic study has been reported on the lamellar thickening in atactic poly(acrylonitrile) (PAN) upon annealing because PAN, in the form of solution‐cast films or their drawn products, generally shows no small‐angle X‐ray scattering (SAXS) maximum corresponding to the lamellar thickness. In this work, PAN crystals were precipitated during the thermal polymerization of acrylonitrile in solution. The nascent PAN film, obtained by the filtration of the crystal suspension, exhibited a clear SAXS maximum revealing the lamellar structure. The lamellar thickening upon annealing of the nascent PAN films was studied in the temperature range 100–180 °C, where the degradation was minimal, as confirmed by the absence of an IR absorption band at 1605 cm⁻¹ ascribed to the cyclized nitrile groups. Above 190 °C, the degradation of the samples was significant, and the SAXS became too broad to determine the scattering maximum. The long period was significantly affected by the annealing time (t_a) and the temperature (T_a). Depending on t_a, three stages were observed for the lamellar thickening behavior. The lamellar thickness stayed constant in stage I (t_a = 0.5–3 min, depending on T_a), rapidly increased in stage II (t_a = 0.5–8 min), and stayed at a constant value characteristic for each T_a at yet longer t_a's in stage III. The lamellar thickness characteristic for T_a increased rapidly with increasing T_a at 165 °C (or higher), which was 152 °C lower than the estimated melting temperature of PAN (T_m = 317 °C). A possible mechanism for such lamellar thickening in PAN far below the T_m is discussed on the basis of the enhanced chain mobility in the crystalline phase above the crystal/crystal reversible transition at 165–170 °C detected by differential scanning calorimetry and wide‐angle X‐ray diffraction. The structural changes associated with annealing are also discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2571–2579, 2000     

Cold crystallization and thermal shrinkage of uniaxially drawn poly(ethylene 2,6-naphthalate) by solid-state coextrusion

Solid-state coextrusion has been used to prepare uniaxially drawn films from isotropic poly(ethylene 2,6-naphthalate) (PEN) of a minimum degree of crystallinity (ca. 5%) both below and above its glass transition temperature T_g. The onset of cold crystallization (T_c) of the drawn films has been studied as a function of the extrusion temperature (ET) and the draw ratio (EDR). It has been shown that T_c decreases markedly on draw, as much as 95°C, and, at constant draw ratio T_c goes through a minimum in the T_g region. For undrawn PEN, annealing below 153°C has no significant effect on T_c. To evaluate the crystallization rate constant (k) and the activation energy (E_a) of the drawn specimens, a nonisothermal DSC procedure has been used. With increasing EDR, k increases markedly and E_a goes down over threefold compared with the undrawn polymer. At high ET, strain-induced crystallization has also been shown to play an important role in lowering E_a for cold crystallization. Thermal shrinkage above T_m indicates a high elastic recovery, underlining the efficiency of deformation, ca. 93%, achieved by solid-state coextrusion.     

X-Ray and thermal studies of nylon 5,7

The unit cell and probable space group of Nylon 5,7 has been determined. The unit cell is monoclinic with the dimensions a = 0.483 nm, b = 0.935 nm, c = 1.662 nm, and γ = 58.9°. The space group is probably P_b which is noncentrosymmetric. Rolled, annealed samples show three-dimensional orientation. The melting point peak of a rapidly cooled sample is about 213°C when it is heated at 20°C/min. Slow cooling, ≤°1°C/min generates a higher melting species, T_m = 228°C. Crystallinities are in the normal nylon range, up to 50% for a slow cooled sample.     

Investigation of rapid thermally annealed GaP(001) surfaces in vacuum

Morphology of rapid thermally annealed GaP(001) surfaces has been investigated using spectroscopic ellipsometry (SE), optical microscopy, ex situ atomic force microscopy, electron probe microanalysis (EPMA) and X‐ray photoelectron spectroscopy (XPS). The samples were annealed in vacuum for t = 2 s at temperatures T = 20–900 °C. The SE, optical microscopy and XPS spectra suggest that thermal annealing causes little influence on the GaP surface at T ≤ 600 °C; however, micro‐ and macroscopic roughening occur at T > 600 °C and T ≥ 750 °C, respectively, with a generation of Ga droplets at T ≥ 750 °C. The presence of the Ga droplets is confirmed by the EPMA measurements. The droplet density can be expressed as N_Ga ∝ exp (E_a/k_BT) with an activation energy of E_a ～ 2.3 eV. The XPS data indicate the change in the surface oxide composition from the native oxide to the Ga oxide (Ga₂O₃ and Ga₂O) after annealing at T ≥ 750 °C. Possible annealing‐induced degradation steps are proposed to provide as complete a picture as possible. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis,Spectral Characterization,and Crystal Structure of Two Polymorphs of o,o’-Dichloro Dibenzyl Disulfide

2-Chlorophenyl methanethiol undergoes air oxidation catalyzed by different selenides and yields the corresponding disulfide 1 in two polymorphic forms, 1a and 1b. In the molecular structures of the two new polymorphs of o,o′-dichloro dibenzyl disulfide, the dihedral angles between the dibenzyl groups are 82.0(1)°, (1a), and 73.7(4)°, (1b), respectively [(1a): P-1, a = 8.424(2) Å, b = 8.838(2) Å, c = 10.5823(19) Å, α = 90.122(18)°, β = 112.19(2)°, γ = 95.19(2)°, V = 725.9(3) ų; (1b): P2₁/n, a = 10.5888(10) Å, b = 9.1590(6) Å, c = 15.2489(14) Å, β = 103.072(9)°, V = 1440.6(2) ų]. MOPAC computational studies yield dihedral angles of 89.6(5)° and 71.9(9)°. Crystal packing is stabilized by weak π-ring (C?H···Cg) intermolecular interactions in both 1a and 1b and by additional weak Cg ··· Cg intermolecular interactions in 1b, which influence the bond distances, bond angles, and torsion angles of the dibenzyl groups in each polymorph. Additional characterization of each polymorph has been carried out by TEM, IR, ¹H and ¹³C NMR spectroscopy, microanalysis, and by FAB mass spectrometry. TEM studies of a sample of 1a show that it contains cigar-shaped crystallites.

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.     

Physical aging in poly(ethylene naphthalene-2,6-dicarboxylate) in relation to sorbed water,enthalpic relaxation,and mechanical properties

Amorphous poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) films (～ 220 μm thick), stored in ambient atmosphere for different periods of time and after annealing at different temperatures below T_g = 123°C, for different times, have been investigated by infrared spectroscopy (FTIR), microhardness, and differential scanning calorimetry (DSC). FTIR spectroscopy and weight measurements reveal the presence of water which is easily removed by annealing of the films. Films again recover their initial weight and absorption bands after 1-2 days storage in ambient atmosphere. Samples annealed at different temperatures T_a for different times t_a show an increasing microhardness for relatively short times of t_a. The microhardness passes through a maximum at an annealing time depending on T_a, and it decreases toward values somewhat larger than the initial ones. The changes observed in the microhardness and in the values of the excess enthalpy with storage time of the samples at room temperature depend on the physical aging as well as on the content of water of PEN films. © 1995 John Wiley & Sons, Inc.     

X‐ray diffraction study of the thermal expansion behavior of cellulose triacetate I

Highly crystalline samples of cellulose triacetate I (CTA I) were prepared from highly crystalline algal cellulose by heterogeneous acetylation. X‐ray diffraction of the prepared samples was carried out in a helium atmosphere at temperatures ranging from 20 to 250 °C. Changes in seven d‐spacings were observed with increasing temperature due to thermal expansion of the CTA I crystals. Unit cell parameters at specific temperatures were determined from these d‐spacings by the least squares method, and then thermal expansion coefficients (TECs) were calculated. The linear TECs of the a, b, and c axes were α_a = 19.3 × 10^?5 °C^?1, α_b = 0.3 × 10^?5 °C^?1 (T < 130 °C), α_b = ?2.5 × 10^?5 °C^?1 (T > 130 °C), and α_c = ?1.9 × 10^?5 °C^?1, respectively. The volume TEC was β = 15.6 × 10^?5 °C^?1, which is about 1.4 and 2.2 times greater than that of cellulose I_β and cellulose III_I, respectively. This large thermal expansion could occur because no hydrogen bonding exists in CTA I. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 517–523, 2009     

Stepwise annealing of poly(butylene terephthalate)

Annealing of poly(butylene terephthalate) (PBT) was studied by differential scanning calorimetry (DSC) and small angle X‐ray scattering (SAXS) measurement. A PBT sample was annealed at a recrystallization temperature where recrystallization occurs with a maximum rate in the heating process of the sample. In the subsequent annealing steps, the annealed sample was annealed repeatedly at the recrystallization temperatures, and the stepwise annealing sample was obtained. Peak melting temperature (T_m) and sharpness of DSC peak of the stepwise annealing sample increased with the annealing step. A high melting‐temperature sample was obtained in a short time, and T_m increased up to 238.5°C which is higher than all the T_m values that appear in the literature. The long period calculated from SAXS curves of the stepwise annealing sample increased with the annealing step. The increase of crystallite size and perfection of the crystal in the stepwise annealing process is suggested. Annealing experiment indicated that T^°_m should be higher than about 235°C. T_m increased linearly with the annealing temperature of the final step in the stepwise annealing (T_a). The equilibrium melting temperature (T^°_m) for PBT was estimated to be 247°C by the application of a Hoffman–Weeks plot to the relation between T_m vs. T_a. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2420–2429, 1999     

Structure and morphology development in syndiotactic polypropylene during isothermal crystallization and subsequent melting

Structure and morphology development during the isothermal crystallization and subsequent melting of syndiotactic polypropylene (sPP) was studied with differential scanning calorimetry (DSC), time‐resolved simultaneous small‐angle X‐ray scattering (SAXS), and wide‐angle X‐ray diffraction (WAXD) methods with synchrotron radiation. The morphology of sPP isothermally crystallized at 100 °C for 3 h was also characterized with transmission electron microscopy (TEM). Time‐ and temperature‐dependent parameters such as the long period (L), crystal lamellar thickness (l_c), amorphous layer thickness (l_a), scattering invariant (Q), crystallinity (X_c), lateral crystal sizes (L₂₀₀ and L₀₁₀), and unit cell dimensions (a and b) were extracted from the SAXS and WAXD data. Results indicate that the decreases in L and l_c with time are probably due to the formation of thinner crystal lamellae, and the decreases in a and b are due to crystal perfection. The changes in the morphological parameters (Q, X_c, L, and l_c) during subsequent melting exhibited a two‐stage process that was consistent with the multiple melting peaks observed in DSC. The two high‐temperature peaks can be attributed to the melting of primary lamellae (at lower temperatures) and recrystallized lamellae (at higher temperatures). An additional minor peak, located at the lowest temperature, was also visible and was related to the melting of thin and defective secondary lamellae. TEM results are consistent with the SAXS data, which supports the assignment of the larger value (l₁) from the correlation function analysis as l_c. WAXD showed that the thermal expansion was greater along the b axis than the a axis during melting. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2982–2995, 2001     

Preparation of barium ferrite films with high Fe/Ba ratio by sol–gel method

Hexagonal BaFe₁₂O₁₉ ferrite (BaM) thin films were prepared on Si (100) substrate successfully by sol–gel technology and post annealing. The results showed the BaM phase can be formed and crystallized into c-axis textured grains even when the Fe/Ba ratio of the precursor varied from 6.5 to 9.5. However, the behavior of the saturation magnetization (M _s) and intrinsic coercivity (H _c) depended strongly on the Fe/Ba ratio and annealing temperature (T _a) varied from 700 to 900 °C. The M _s and H _c values deceased with an increase of Fe/Ba ratio, for instance, were about 290 emu/cm³ and 4,200 Oe for the Fe/Ba = 6.5 film but only 116 emu/cm³ and 2,300 Oe for the Fe/Ba = 9.5 film. The M _s and H _c values of the Fe/Ba ratio = 6.5 film increased monotonously with increasing T _a, were about 120 emu/cm³ and 2,500 Oe at T _a = 800 °C, and reached to 345 emu/cm³ and 4,600 Oe at T _a = 950 °C.     

Polymorphic structure and physical properties of the polyester/ether poly(ethylene-1,2-diphenoxyethane-p,p′ -dicarboxylate)

X-ray diffraction studies of fibers of the polyester/ether poly(ethylene-1,2-diphenoxyethane-p,p′ -dicarboxylate) (PEET) produced by high-speed melt spinning show the existence of two polymorphic forms, designated α and β, in the solid state. The α form is obtained by annealing filaments melt spun at takeup speeds below 3000 m/min and is also found in samples crystallized from the melt and from dilute solutions. The α form has a monoclinic unit cell with dimensions a = 7.83, b = 10.33, c = 18.68 Å, and β = 83.1°. The equilibrium melting temperature and heat of fusion of the α form are 288.3°C and 19.1 cal/g, respectively. The β form predominates in highly oriented filaments obtained at takeup velocities above 6000 m/min. The unit cell is orthorhombic with dimensions a = 7.28, b = 5.65, and c = 18.64 Å. The β form does not transform to the α form on annealing.     

Effects of annealing on the polarization switching of phase I poly(vinylidene fluoride)

To obtain information about microscopic processes involved in the polarization switching in uniaxially oriented poly (vinylidene fluoride) film, a least-squares estimation of nonlinear parameters was developed to yield parameters of an equation which describes the nucleation and domain growth process. Time domain measurements of polarization reversal revealed that switching times decreased as the annealing temperature T_a increased (67.0, 52.4, and 41.3 μs at –20°C under a 200 MV/m pulse field for the as-stretched samples, the samples being annealed at 120 and 160°C, respectively). The analysis showed that the value of the domain growth speed increased as T_a increased. This is consistent with x-ray diffraction data which indicated that the annealing process brought about better chain packing and increased crystallite perfection. The analysis also showed that the nucleation probability significantly increased as T_a increased. This result was interpreted in terms of a morphological transformation, which was indicated by the decrease in elastic modulus with increasing T_a with no corresponding loss of orientation. It is suggested that the annealing process brought about an increase in the number of nucleation sites as a result of a transformation from a fibrous structure to a crystal-amorphous series structure which has increased boundary zone area.     

A pulsed NMR study of the effects of initial morphology on the structure of extrusion-drawn poly(ethylene terephthalate)

Proton spin–spin relaxation times and the Weibull coefficient have been measured as functions of temperature for poly(ethylene terephthalate) (PET) drawn at 50°C in both the amorphous and the semicrystalline (50%) states. Two relaxation times T_2a (long) and T_2c (short) are observed for all samples. They are ascribed, respectively, to the relaxation of the amorphous and of the crystalline components including highly strained noncrystalline segments. Effects of initial morphology are found for chain mobility in the noncrystalline regions and for the crystal perfection, evaluated from T_2a and the Weibull coefficient μ_c of the T_2c-component, respectively. For all draw ratios, T_2a for extrudates prepared from the semicrystalline polymer (C-50) is short compared to that for preparations from the amorphous (A-50) polymer. In the A-50 samples, the perfection of stress-induced crystals increase with increasing draw ratio. In the C-50 samples, the crystal orientation increases, whereas the perfection decreases with increasing draw ratio. To improve the crystal perfection, annealing at higher temperature or longer time is required for C-50 as compared with A-50. The value of μ_c correlates well with the change in crystal perfection during deformation and annealing.     

Preparation of HPEEK by Oligomer A2+B3 Approach

A series of hyperbranched poly(ether ether ketones) with different chain length between branching point (L) were prepared using a A₂+B₃ methodology, in which the A₂ is hydroxyl‐terminated PEEK oligomer. The L affects the properties of the polymers such as the inherent viscosity, the degree of crystallinity, the thermal properties of the polymers etc. The polymer with a L₂≈8 had T _g (122.4°C), T _c (200.2°C), and broad T _m (247.4°C). With the increment of L, up to the point L₂≈20 and L₂≈35, the polymers become semi‐crystalline, with a melting point of 300.9°C and 317.9°C, respectively. Their wide angle X‐ray scattering (WAXS) pattern indicated that their crystal structure is exactly the same as that of the linear homopolymer.     

High-temperature aging of Halar film. II. Analysis of melting behavior

Melting behavior of an experimental Halar film, a predominantly alternating 1:1 copolymer of ethylene (E) and chlorotrifluoroethylene (CTFE), has been studied. Differential scanning calorimetry (DSC) reveals single or double melting peaks, depending upon the thermal history. The lower-temperature melting peak T_m1 is produced only by the thermal treatment and shows a strong dependence on annealing time and temperature. On the basis of the DSC and x-ray data it can be suggested that T_m1 represents the melting of relatively small crystallites formed upon annealing. The higher-temperature melting peak T_m2 is always shown at 238°C. (Note: the specification for commercial Halar product is 240°C. The slightly lower melting temperature reported in this study is probably due to the fact that we are dealing with an experimental melt-processed material.) On the basis of the heating rate study we propose that Halar crystallizes with stable crystals (T_m2 = 238°C) regardless of the crystallization conditions, i.e., quenching, slow cooling, or even annealing. Crystals of Halar have a heat of fusion of approximately 35 cal/g or 146 kJ/kg. Detailed analysis of the melting behavior of Halar is presented.     

The temperature dependence of the viscoelastic softening and terminal dispersions of linear amorphous polymers

Thermorheological simplicity is shown to hold for poly(vinyl acetate) in the temperature range extending from T_g + 25°C to T_g + 80°C. Between T_g and T_g + 25°C the softening (glass to rubberlike) viscoelastic dispersion exhibits time-scale shift factors a_T different from those of the terminal (rubberlike to steady-state) dispersion. The a_T values calculated from zero-shear viscosities coincide with those from the terminal dispersion in the temperature range 60–154°C (T_g ? 35°C). The a_T shifts obtained from the response in the terminal dispersion can be fitted to the Williams, Landel, and Ferry equation over the entire temperature range 42–154°C. The a_T obtained from the softening dispersion is shown to exhibit a different functionality. An empirical modification of the Doolittle equation yields a very flexible relation which can be fitted to some a_Ts which cannot be represented by the usual Doolittle free-volume expression.     

Evolutions of morphology and crystalline ordering upon annealing of quenched isotactic polypropylene

Quenched mesomorphic isotactic polypropylene precursor material has been investigated as well as the material annealed at different temperatures. At room temperature, morphology and crystalline ordering of the materials have been studied by atomic force microscopy (AFM) and wide‐angle X‐ray diffraction (WAXD). The nodular morphology of the mesomorphic precursor remains constant for annealing temperatures T_a below 120 °C. Needle‐like or leaf‐like crystals form when T_a approaches the melting temperature of the precursor. WAXD data analyzed by peak‐fitting indicate that the crystalline ordering along both the chain axis and the lateral directions quickly develops during the mesomorphic‐to‐monoclinic transition, but slowly after the transition (T_a > 140 °C). Combining the AFM and WAXD results it is proposed that two decoupled structure evolution mechanisms are occurring. During the transition, only the crystalline ordering is increasing, but not the size of the nodules. After the transition, the geometry of the nodules changes considerably, whereas there is little additional increase of crystalline ordering. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1703–1712, 2009     

Synthetic and structural studies of [6]-, [7]- and [10]metacyclophanes

A new preparative sequence from 2,3-polymethylene-2-cyclopentenone 5 to 2,6-polymethylenebromobenzenes 3 (n = 6, 7, 10) and 2,6-polymethylenephenyllithiums 6 has been found. The reaction of 6 with various electrophiles produces a number of new compounds to disclose the unique reactivity of the aryl C-Li moiety surrounded by the polymethylene chain. Photolysis of 3a and 3b provides transannular products 8, 10 and 11, all arising from the proximity between the aromatic bromine and the aliphatic hydrogen intraannularly opposed to be removed as HBr. Spectrometric study gives quantitative data of the dependence of the molecular geometry upon the chain length and the aromatic substituents. The energy barriers ΔG_c^≠ of the conformational flipping are 17·4 kcal/mol (T_c 76·5°) for [6]metacyclophane (7a), 11·5 kcal/mol (T_c ?28°) for [7]metacyclophane (7b), ·8 kcal/mol for [10]metacyclophane (7c). The lower-energy process of the aliphatic chain in [6]metacyclophane derivatives is the pseudorotation with substituent-dependent barrier ΔG_c^≠ 11·1 kcal/mol (T_c ?31·5°) for 7a, 12·4 kcal/mol (T_c ?4·5°) for 3a and 12·7 kcal/mol (T_c 1·0°) for 12a. The rather large rotational barrier is attributed to the compressed structure of each system. The benzene ring distortion of the cyclophanes is deduced from the bathochromic shift of the B-band and the diamagnetic shift of the benzene proton signals in the PMR.     

Crystal Structure and Dimorphism of Silicon Tetraisocyanate Si(NCO)4

Silicon tetraisocyanate Si(NCO)₄ was obtained by reacting SiCl₄ and AgNCO in boiling toluene. The colourless liquid was analyzed by Raman and NMR spectroscopy. Structural studies on solid Si(NCO)₄ (melting point: +26 °C) have revealed it to exist in two polymorphic modifications. According to the results of single‐crystal X‐ray diffraction, at T = –173 °C, α‐Si(NCO)₄ exhibits triclinic symmetry (P$\bar{1}$ ; a = 10.05(5), b = 10.50(2), c = 14.32(1) Å, α = 91.62(1)°, β = 92, 32(1)°, γ = 99.68(1)°; V = 1488.56(3) ų; Z = 8). Above T = –33 °C, a monoclinic phase evolves, β‐Si(NCO)₄ (P2₁/c; a = 10.78(3), b = 7.11(1), c = 10.27(5) Å, β = 99, 06(9)°; V = 777.39(1) ų; Z = 4). The charge distribution was studied for both polymorphs. In the solid state, Si(NCO)₄ is a tetrahedral molecule with the Si–N=C=O linkages bent at the nitrogen atoms.