Methylene Sequences Research Articles

Thermal and mechanical analysis of metallocene‐catalyzed ethene–α‐olefin copolymers: The influence of the length and number of the crystallizing side chains

AbstractCopolymers of ethene and 1‐octene, 1‐dodecene, 1‐octadecene, and 1‐hexacosene were carried out with [Ph2C(2,7‐di‐tertBuFlu)(Cp)]ZrCl2/methylalumoxane as a catalyst to obtain short‐chain branched polyethylenes with branch lengths of 6–26 carbon atoms. This catalyst provided high activity and a very good comonomer and hydrogen response. In this study, the influence of the length and number of the side chains on the mechanical properties of the materials was investigated. The crystalline methylene sequence lengths of the copolymers and lamellar thicknesses were calculated after the application of a differential scanning calorimetry/successive self‐annealing separation technique. By dynamic mechanical analysis, the storage modulus as an indicator of the stiffness and the loss modulus as a measure of the effect of branching on the α and β relaxations were studied. The results were related to the measurements of the polymer density and tensile strength to determine the effect of longer side chains on the material properties. The hexacosene copolymers had side chains of 24 carbons and remarkable material properties very different from those of conventional linear low‐density polyethylenes. The side chains of these copolymers crystallized with one another and not only parallel to the backbone lamellar layer, depending on the hexacosene concentration in the copolymer. The side chains crystallized even at low hexacosene concentrations in the copolymer. A transfer of these results to 16 carbons side chains in ethene–octadecene copolymers was also possible. © 2006 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1600–1612, 2006

Chain microstructure of homogeneous ethylene‐1‐alkene copolymers and characteristics of single site catalysts using a direct 13C NMR peak method I. Theory and models

AbstractTo describe the detailed microstructure of homogeneous ethylene‐1‐alkene copolymer chains and to study the characteristics of single site catalysts, Markov statistics are used to fit peak intensities of all relevant 13C NMR signals of series of copolymers. In the case of the occurrence of inverted comonomer units, a first‐order Markov terpolymer is applied, otherwise a second‐order Markov copolymer model. Chain propagation probabilities are obtained via modeling of the entire NMR spectrum. This procedure results in an accurate reproduction of the chain microstructure, including ethylene, 1‐alkene, and methylene sequence length distributions. If the experimental (co)monomer feeds are known, the reactivity ratios and the theoretical (co)monomer feeds are also found providing information about the copolymerization kinetics and the characteristics of the catalyst. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 722–737, 2006

High Speed SSA Thermal Fractionation and Limitations to the Determination of Lamellar Sizes and Their Distributions

AbstractSummary: The technique of successive self‐nucleation and annealing (SSA) has been applied using differential scanning calorimetry (DSC) to a model hydrogenated polybutadiene prepared by anionic polymerization and to a commercial Ziegler‐Natta ethylene/1‐butene copolymer. Here, it is shown that the use of high scanning rates (50 °C · min−1) in the SSA protocol can reduce the thermal fractionation time to only 78 min in length (if 6 fractions are produced with a fractionation window of 5 °C), taking into consideration the need to compensate the increment in heating rates by reducing the sample mass. This time is much shorter than those previously achieved by thermal fractionation in the literature, where fractionation times of 12 or 24 h are common. Potentially, much higher rates could be employed by further reducing the fractionation times by SSA. For the first time, a distribution of lamellar thicknesses has been obtained by transmission electron microscopy after SSA fractionation and compared to distributions calculated by the Thomson‐Gibbs equation. It is shown that the results are highly dependent on the equilibrium melting temperatures employed in the calculations, and it is recommended that the values of lamellar thicknesses obtained are checked by comparing with methylene sequence lengths derived from calibration measurements available in the literature. The Thomson‐Gibbs equation is useful for comparing lamellar thickness distributions obtained by SSA with different polymers only on a semi‐quantitative basis.Rate and mass compensation as a way to reduce fractionation time employing SSA.imageRate and mass compensation as a way to reduce fractionation time employing SSA.

Intramolecular branching distributions in metallocene short‐chain branched polyethylenes examined by crystallization and annealing at high pressure

AbstractUniform branched polyethylenes produced by metallocene catalysts can be subjected to a limited degree of intramolecular fractionation by crystallization or annealing at pressures of 495 MPa. Longer stem lengths in lamellae are more achievable by this process than by treatments at atmospheric pressure. Although the intervention of the hexagonal phase allows greater lamellar stem lengths to be achieved, the small proportion of longer methylene sequence lengths in the melt imposes limitations on the degree of isothermal thickening achievable. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1986–1996, 2005

Fractionation process in TREF systems: Validation of thermodynamic model and calculation procedure by Raman LAM studies

In this paper, possible sources for the unexpected distributions of crystalline sequence lengths calculated from temperature rising elution fractionation (TREF) calibration experiments, as reported in a previous work, are investigated. With this aim, chain folding and cocrystalization phenomena were explored in the conditions of crystallization as used for TREF or crystallization analysis fractionation (CRYSTAF). Slow crystallizations were performed from xylene solutions of model low molecular weight ethylene homopolymers with narrow molecular weight distributions. The same experiments were performed with homopolymers having narrow molecular weight distributions and with blends having wide molecular weight distributions. The resulting distributions of the lengths of crystalline methylene sequences were directly studied by Raman in the so-called longitudinal acoustic mode (LAM) and by DSC. For ethylene homopolymers with molecular weights below 2000 g/mol, the results from Raman LAM indicate that slow crystallization in TREF or CRYSTAF systems occurs in the extended-chain mode. For higher molecular weights, evidence of chain folding was found. In the case of blends, independent crystallization was observed for each molecular weight when the molecular weight ranges used for the blends are relatively narrow. Cocrystallization was observed when this range was increased. Overall, these results strongly support the inverse technique calculation procedure developed by our group for the calculation of distributions of lengths of crystallizable sequences from TREF spectra. In this context, the results confirm that the unexpected crystallizable sequence lengths found in our previous work really exist and can be associated to chain folding or cocrystallization phenomena. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3083–3092, 2005

Molecular distribution analysis of melt‐crystallized ethylene copolymers

AbstractA simple technique employing differential scanning calorimetry (DSC) to investigate the molecular structure of ethylene copolymers is presented in this paper. Three commercial Ziegler–Natta catalysed linear low density polyethylenes (LLDPE) and a commercial single‐site catalysed very low density polyethylene (VLDPE) were subjected to continuous cooling at a slow rate of 0.08 °C min−1. Like other thermal fractionation techniques, the slow continuous cooling (SCC) technique segregates polymers according to their branching density, allowing the short chain branching (SCB), lamellar thickness (L) and methylene sequence length (MSL) distribution to be determined using the DSC melting curves. It was found that the single‐site catalysed VLDPE exhibits narrow SCB, L and MSL distributions, with shorter methylene sequences. In contrast, the Ziegler–Natta catalysed LLDPEs have much broader bimodal SCB, L and MSL distributions, with less SCB, and are composed of both short and long methylene sequences. LLDPEs have thicker lamellae compared with VLDPE (12.9, 12.5, 9.8 nm versus 6.7 nm) and the lamellar thickness values are consistent with the results measured by transmission electron microscopy (TEM). The slow continuous cooling and stepwise cooling techniques are complementary: the former provides a continuous distribution profile and the latter a well‐defined histogram for the lamellar thickness. The results obtained are qualitatively comparable to those gained by temperature rising elution fractionation (TREF), because the cooling rate used here is of the order of the rates used in TREF analysis. Copyright © 2004 Society of Chemical Industry

Molecular dynamics simulation of the structural and mechanical property changes in the Brill transition of nylon 10/10 crystal

In order to confirm the structural features in the Brill transition of aliphatic nylons, molecular dynamics calculation has been performed at the various temperatures for nylon 10/10 crystal as a model. With an increase in temperature, the torsional motions around the methylene–amide and methylene–methylene bonds were activated. These motions became more remarkable above 420 K and the interconversion between trans and gauche forms occurred frequently, resulting in the disordered conformation of the methylene sequences and the pseudo-hexagonal packing of these parts. During such a drastic structural disordering, however, the hydrogen bonds were kept alive although the bond strength became weaker. These calculations were found to be consistent with the experimental results by the temperature-dependent X-ray diffraction and infrared spectral measurements. The Young's modulus along the chain axis was also calculated as a function of temperature: it decreased remarkably from 250 GPa at 0 K to 180 GPa at 300 K due to a small contraction of the skeletal chain by only about 0.2–0.5% through the torsional motion of the skeletal chains, and the modulus decreased furthermore to 80 GPa at 550 K because of the larger conformational disordering of the skeletal chains. The Young's modulus in the direction perpendicular to the chain axis was also found to decrease remarkably in parallel to the change of the chain packing mode.

Saturation ESR spectroscopy of PA12 polyamide

This work presents the results of our EPR study of polyamide 12 (PA12) –[NH–(CH 2) 11–CO] n –NH–(CH 2) 11–CO–. In this study the spin probe method with “conventional” or “continuous saturation” at low temperatures (4–260 K) was used. The aim of our investigation was to determine if it possible to obtain the saturation effects and establish experimental conditions by using commercial ESR spectrometers. We also wanted to know what information about the relaxation processes in polymeric materials could be obtained. We have found that the information about the relaxation processes (below frequency ≈10 4 Hz) at low-temperatures in polymers could be obtained. The process, which could be associated with mobility of the spin probe is probably connected with the process called γ (in PA12 as yet unrecorded by using others methods). This process in other systems is usually connected with mobility of the methylene sequences and could be used to study the amorphous regions of PA12.

Quantitative determination of short‐chain branching content and distribution in commercial polyethylenes by thermally fractionated differential scanning calorimetry

AbstractA method for rapid quantitative analysis of the content and distribution of short chain branching (SCB) for α‐olefin/ethylene copolymers based on thermally fractionated DSC is presented. Eight commercial polyethylenes, four made with conventional Ziegler‐Natta catalysts and four made with metallocene catalysts, were analyzed by differential scanning calorimetry (DSC), after having been thermally segregated by successive nucleation annealing (SNA). The polyethylenes were also analyzed by temperature rising elution fractionation (TREF) and carbon‐13 nuclear magnetic resonance (13C‐NMR). The SNA‐DSC procedure segregates polyethylenes according to methylene sequence lengths (MSL). The relationship between DSC melting temperature and SCB content was obtained by calibration with linear hydrocarbons; TREF results were not used in the SNA‐DSC calibration. Deconvolution of the SNA‐DSC endotherms yielded estimates of the average SCB contents and SCB distributions. The SCB contents obtained from the SNA‐DSC for linear low density polyethylenes agreed very well with the SCB contents obtained by 13C‐NMR and TREF, and the SCB distributions measured by SNA‐DSC were very similar to those obtained by TREF. The SCB contents obtained by SNA‐DSC for ultra‐low density polyethylenes, made with metallocene catalysts, were about 20% lower than the values obtained by 13C‐NMR; the values obtained by TREF were even lower.

Formation of the Liquid Crystalline Glassy Phase and Cold Crystallization of a New Crystal Form from the Glassy Phase for Thermotropic Liquid Crystalline Polyether

The formation of the liquid crystalline (LC) glassy phase and phase transitions including cold crystallization associated with this phase have been investigated for main-chain thermotropic LC polyether (EDMB-10), which is composed of 3,3’-dimethyl-4,4’-biphenyl units as mesogen and 10 methylene sequences as spacer, by mainly using DSC, wide angle X-ray diffractometry, and solid–state 13C NMR spectroscopy. When the sample is quenched from the isotropic melt to ice-water, the LC glassy phase is really produced as suggested by the previous finding that the liquid crystallization temperature is almost independent of the cooling rate whereas the crystallization temperature rapidly decreases with increasing cooling rate. The liquid crystalline glass thus formed is found to undergo cold crystallization above Tg and to produce a new type of crystal form (form β) which is significantly different in structure from form α ordinarily crystallized on cooling from the isotropic melt through the nematic phase. In particular, the highest crystallinity of form β can be obtained by annealing the LC glass at 130°C for 30 min–about 2 d and then quenching to 0°C. This fact suggests that the stable nematic phase associated with the crystallization of form β may appear on rapid heating around 130°C. In addition, form α is also crystallized even on heating when the LC glass is annealed at 132–134°C and also quenched to 0°C, implying the appearance of the stable nematic phase preferentially producing form α at this temperature range. Additional phase transitions are also observed for the form β sample with the highest crystallinity and their origins are briefly discussed.

Characterization of polyethylene with partially random chlorine substitution

AbstractA series of chlorine‐containing polymers were prepared by ring‐opening metathesis polymerization (ROMP) followed by hydrogenation. This synthesis route was chosen specifically so that chain microstructures would be obtained that resembled copolymers of ethylene and vinyl chloride. The chlorine content was varied by the copolymerization of 5‐chlorocyclooctene and cyclooctene. Differential scanning calorimetry, light microscopy, tapping‐mode atomic force microscopy, wide‐angle X‐ray diffraction (WAXD), and density were employed to characterize the polymers. The copolymers had certain restrictions on the length of the methylene sequence between substituted carbons, however, ROMP copolymerization introduced enough variation in the methylene sequence length that model copolymers with the equivalent of 14 mol % vinyl chloride or less closely resembled random copolymers of ethylene and vinyl chloride. These materials organized as spherulites and exhibited the orthorhombic crystal form. Constraints on the placement of chlorine atoms strongly affected the crystallization of polymers with more than the equivalent of 14 mol % vinyl chloride. More regular chlorine substitution along the polyethylene chain translated into better ordered crystal structures with sharp melting peaks. The granular morphology of these materials at ambient temperature was interpreted as fringed micellar crystals. The WAXD patterns provided definitive evidence that chains in the fringed micelle took the hexagonal crystal form. The lower density hexagonal form facilitated the crystallization of short ethylene sequences and accommodated chlorine atoms more easily than the orthorhombic form. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2062–2070, 2003

PBT chain mobility from the ESR saturation spectra studies

Temperature and microwave power dependence of ESR spectra of the spin probes in PBT has been studied. The observed changes of ESR signal allowed to determine the long correlation times (τ 10 - 4 -10 - 5 s) as well as other parameters characterizing the saturation processes and involved relaxation processes. It was established that at low temperature range ( 120-200 K) the rotational mobility of used nitroxide radicals was related to local relaxation processes of the methylene sequences (3-4 CH 2 groups) in an amorphous region of PBT. The obtained results show the possibility of that type of investigation also for other polymers. Determination of polymer properties via analysis of parameters obtained in saturation experiments is also raised.

New interpretation of progression bands observed in infrared spectra of nylon‐m/n

AbstractA series of progression bands observed in the infrared spectra of nylon‐m/n and their model compounds have been interpreted in a new manner on the basis of simply coupled oscillator models of zigzag alkyl chains. Nylon‐m/n possesses the methylene sequences of (CH2)m and (CH2)n−2, and so the effective models of m and n − 2 coupled oscillators, respectively, had previously been assumed for the methylene rocking–twisting mode, for example. However, the spectral patterns of progression bands predicted by this previously proposed model have been found to be inconsistent with those observed for many kinds of nylon samples with various m and n values. It is rather reasonable to assume that the effective numbers of oscillators should be m − 2 and n − 4 for the methylene rocking, twisting, and wagging modes of the (CH2)m and (CH2)n−2 sequences, respectively. In other words, the infrared progression bands observed for methylene local modes of nylon‐m/n may be interpreted reasonably with the data of n‐alkane molecules with the chemical formulae CH3(CH2)m−2CH3 and CH3(CH2)n−4CH3. For the CC stretching modes, the equivalent n‐alkanes are CH3(CH2)m−1CH3 and CH3(CH2)n−3CH3, respectively. In the simply coupled oscillator model, the vibrational mode of one methylene group is represented by an oscillator, for example. Our new concept is to isolate the terminal oscillator adjacent to the amide group from the other oscillators in the inner parts of the methylene zigzag sequence. This corresponds to a physical situation in which the methylene group adjacent to the amide group shows a different vibrational behavior of larger amplitude than those of the inner methylene sequence, as supported by broad‐line NMR data and molecular dynamics calculations reported in the literature. Another possibility is a difference in the electron structure of the methylene unit adjacent to the amide group from that of the inner methylene sequence, resulting in a difference in the force constant and giving a vibrational decoupling between these two types of methylene units. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1294–1307, 2003

Crystalline structure and morphology of biaxially oriented polyamide‐11 films

AbstractThe effect of the uniaxial and biaxial stretching and subsequent solution annealing of extrusion‐cast polyamide‐11 films on the crystalline structure and morphology was investigated with differential scanning calorimetry, wide‐angle X‐ray diffraction (WAXD), Fourier transform infrared spectroscopy, and small‐angle X‐ray scattering (SAXS). The extrusion‐cast polyamide‐11 films exhibited elevations in the glass‐transition and cold‐crystallization temperatures with a constant crystallinity and a constant melting point during aging under room conditions (20–26 °C and 20–31% relative humidity). WAXD and SAXS suggested that chain‐folded lamellae of coexisting α‐ and β‐crystals existed in all the stretched polyamide‐11 films. WAXD pole figures indicated that hydrogen bonds in the hydrogen‐bonded sheets of these two crystalline forms apparently formed between antiparallel chain molecules. The unit cell parameters [a = 9.52 Å, b = 5.35 Å, c = 14.90 Å (chain axis), α = 48.5°, β = 90°, and γ = 74.7° for a triclinic α form and a = 9.52 Å, b = 14.90 Å (chain axis), c = 4.00 Å, α = 90°, β = 67.5°, and γ = 90° for a monoclinic β form] for polyamide‐11 crystals were proposed according to the results of this study and the results of previous investigators. The unit cell parameters of the stretched extrusion‐cast polyamide‐11 films varied, depending on the stretching conditions (the stretch temperature and stretch ratio). As the stretch temperature and stretch ratio were increased, the crystal became more similar to the form described previously and was accompanied by an increase in the long spacing of crystalline lamellae. Annealing the stretched films in a boiling 20% formic acid solution made slightly more perfected crystals. The hydrogen‐bonding α(010) + β(002) planes, which are nearly parallel to both amide group planes and zigzag methylene sequence planes of the biaxially stretched films were found to be parallel to the film surface. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2624–2640, 2002

Partitioning of Main and Side-Chain Units between Different Phases: A Solid-State 13C NMR Inversion-Recovery Cross-Polarization Study on a Homogeneous, Metallocene-Based, Ethylene-1-Octene Copolymer

13C NMR inversion-recovery cross-polarization experiments are used to study the phase structure and partitioning of main and side-chain groups in a homogeneous, metallocene-based, ethylene-1-octene copolymer. The results provide strong evidence for a three-phase model, i.e. a rigid, (imperfect) crystalline phase, which is mainly composed of long sequences of methylene carbon atoms of the main chain, a semi-rigid, amorphous interphase (also denoted as ‘rigid amorphous’), which is enriched by chain segments bearing methylene and methine carbon atoms of the main chain, and a soft fraction of the amorphous phase (also denoted as ‘mobile amorphous’), which is largely composed of side chains and short methylene sequences of the main chain.

Solid-State 13C and 15N NMR Analyses of Structure and Dynamics of Spacer Methylene Sequences and Mesogen Groups for Liquid Crystalline Polyurethanes with Different Spacer Lengths

Solid-state 13C NMR analyses of the structure and dynamics have been performed for liquid crystalline polyurethanes with different spacer CH2 lengths, which were polymerized from 3,3‘-dimethyl-4,4‘-biphenyldiyl diisocyanate, α,ω-alkanediols having various even numbers m of carbons, and 1-hexanol. Each sample was crystallized by cooling from the melt through the liquid crystalline phase at an appropriate cooling rate depending on its phase transition behavior detected by DSC. 13C spin−lattice relaxation analyses have revealed that each CH2 carbon resonance line contains three components with different T1C values which are assigned to the crystalline, medium, and noncrystalline (supercooled liquid crystalline) components. For the polyurethane samples with m = 8 and 12, it has been found that the spacer CH2 sequences for the noncrystalline component will adopt the alternate trans (t) and trans−gauche exchange (x) conformation in the central part whereas those of the crystalline and medium components may be in the all-trans conformation. Accordingly, the conformation of the spacer CH2 sequences can be generally expressed as ttx(tx)𝓁 tt (𝓁 = m/2 − 3) for 8 ≤ m ≤ 12, including the result for m = 10 reported previously. Furthermore, to examine molecular motion of the mesogen units in detail, 13C CSA spectra have been measured at room temperature for the mesogen carbons by 2D SASS 13C NMR spectroscopy and compared with those simulated by using the two-site exchange model for the flip motions of the phenylene group. As a result, it has been suggested that flip angles of fluctuations around the phenylene axis may be limited to less than about 20° without any 180° flip motion in all components, including the supercooled liquid crystalline component. Natural abundant CP/MAS 15N NMR measurements also suggest that such rigidity of the mesogen groups may be due to the formation of intermolecular hydrogen bonding between NH and CO groups, and resulting segmental assemblies will induce the characteristic spacer conformation in the supercooled liquid crystalline component.

MOLECULAR WEIGHT DISTRIBUTION OF POLYAMIDES BY GPC-VISCOMETRY. A COMPARISON BETWEEN HIGH TEMPERATURE AND LOW TEMPERATURE ELUANTS

Synthetic polyamides (PA) owe many outstanding properties to their semicristalline morphology and to the intermolecular hydrogen bonding of the amide groups. Although the strength of the Van der Waals network confers an exceptional solvent resistance to this class of polymers, it may lead to serious difficulties in their solution characterization, such as the determination of molecular weight distribution (MWD). Over the years, a large number of dissolution schemes have been developed for the GPC characterization of PA. A recent IUPAC round-robin test on the GPC of aliphatic PAs nevertheless showed large variations in the experimental data not only between laboratories employing different methods, but also between laboratories adopting similar protocols. The purpose of the present investigation is to understand the origins of these disparities by comparing GPC-Viscometry data obtained with some frequently used eluant systems: a. benzyl alcohol at high temperature; b. hexafluoro-isopropanol, pure or mixed with dichloromethane, at low temperature; c. common GPC solvents (THF, CH2Cl2,) after N-trifluoracetylation of the PA. Apart from pure HFIP, all the methods tested show consistent MW results when measured in the same laboratory. Viscometry data reveal, however, that the expected linear viscosity law is obeyed only in benzyl alcohol. With the other eluants, significant departure from the Mark-Houwink relation denotes presence of non-size exclusion effects, rendering data evaluation imprecise with the Universal Calibration. The problems are particularly acute for polyamides with short methylene sequences which can only be dissolved in mixtures containing a high concentration of HFIP. To improve accuracy, it is proposed that GPC calibration for this class of polymer be performed with absolute detection (on-line viscometer, light scattering) or with a broad MWD standard of identical chemical composition to the analyzed sample.

Therban® – The High Performance Elastomer for the New Millennium

Hydrogenated nitrile rubber (Therban) was introduced in the early 80's by Bayer as an oil-resistant elastomer for permanent exposure to temperatures between –25°C and 150°C. By specific modification of micro- and macrostructural features the service temperature range could be extended. The introduction of bulky side groups hinders the crystallization of methylene sequences along the polymer backbone. The inability to crystallize leads to superior dynamic-mechanical properties at low service temperatures. High temperature properties are determined by the plateau of rubber elasticity and are mainly influenced by the network density and structure of the crosslinked polymer matrix as well as by the polymer-filler interactions. The temperature dependence of the latter causes a decline in the reinforcement properties of filler and polymer matrix at elevated temperatures. Above 150°C a deterioration of the polymer network is observed for standard HNBR grades. This service temperature limit can be increased by ca. 10°C being equivalent to a doubling of the service life-time by the proper use of Therban HT.

Chain Conformation of Spacer Methylene Sequences for a Liquid Crystalline Polyether As Revealed by Solid-State 13C NMR Spectroscopy

Solid-state 13C NMR analyses of the structure and chain conformation have been carried out mainly at room temperature for a main-chain thermotropic liquid crystalline polyether which was newly polymerized from 3,3‘-dimethyl-4,4‘-dihydroxybiphenyl and 1,10-dibromodecane by using a phase transfer catalyst. This sample was crystallized by cooling from the melt through the nematic liquid crystalline state. Differential scanning calorimetry measurements and polarizing optical microscopic observations have confirmed that the nematic liquid crystalline phase appears in both heating and cooling processes. 13C spin−lattice relaxation analyses have revealed that there exist three components with different T1C values, which correspond to the crystalline, medium, and noncrystalline (supercooled liquid crystalline) components. By employing the difference in T1C, the spectrum of each component is separately recorded, and the conformation of the CH2 sequence is evaluated by considering the γ-gauche effect on the 13C chemical shifts. As a result, the crystalline component is found to adopt the alternate trans (t) and trans−gauche exchange (x) conformation (txtxtxtxt), in good accord with the result separately obtained by the molecular dynamics simulation. In contrast, all the C−C bonds of the CH2 sequence for the noncrystalline component are in the rapid trans−gauche exchange conformation (xxxxxxxxx), reflecting the same conformation in the liquid crystalline phase or at the melt.

Physical characterization of poly(ω‐pentadecalactone) synthesized by lipase‐catalyzed ring‐opening polymerization

AbstractThe Candida antarctica lipase B (Novozyme‐435)‐catalyzed ring‐opening polymerization of ω‐pentadecalactone in toluene was performed. Poly(ω‐pentadecalactone) [poly(PDL)] was obtained in a 93% isolated yield in 4 h with a number‐average molecular weight of 64.5 × 103 g/mol and a polydispersity index of 2.0. The solid‐state properties of poly(PDL) were investigated by thermogravimetric analysis (TGA) coupled with mass spectrometry, differential scanning calorimetry (DSC), stress–strain measurements, wide‐angle X‐ray diffraction, and dynamic mechanical and dielectric spectroscopies. Poly(PDL) is a crystalline polymer that melts around 100 °C. The polyester shows good thermal stability, with a main TGA weight loss centered at 425 °C. Because of the high degree of poly(PDL) crystallinity, the glass transition (−27 °C) is revealed by relaxation techniques such as dynamic mechanical and dielectric spectroscopies, rather than by DSC. In addition to the glass transition, the viscoelastic spectrum of poly(PDL) also shows two low‐temperature secondary relaxations centered at −130 (γ) and −90 °C (β). They are attributed to local motions of the long methylene sequence (γ) and complex units involving water associated with the ester groups (β). The mechanical properties of poly(PDL) are typical of a hard, tough material, with an elastic modulus and yield parameters comparable to those of low‐density polyethylene. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1721–1729, 2001