Thermal stability of poly(trimethylene terephthalate)

The first-order thermal degradation rates of poly(trimethylene terephthalate) [PTT] at 240-280 °C under non-oxidative conditions have been determined from the increase in allyl endgroups (¹H NMR) which closely match the rates determined from the decrease in molecular weight (intrinsic viscosity). Consequently, the predominant thermal degradation mechanism of PTT is consistent with concerted, electrocyclic oxo retro-ene chain cleavage under conditions pertinent to viable polymerization processes and efficient downstream extrusion and spinning into fiber. Although catalysts, additives and other reaction variables can influence the thermo-oxidative stability of polyesters including PTT, these factors have been found to have little or no effect on PTT thermal degradation rates under non-oxidative environments. The thermal stability of poly(butylene terephthalate) [PBT] has also been determined from butenyl endgroups (NMR) and molecular weight (IV). The activation energies (E_a) for both PTT and PBT thermal chain cleavage are similar to the reported E_as for poly(ethylene terephthalate) [PET] degradation, which is further supported by semi-empirical molecular orbital calculations on model compounds. However, both PTT and PBT undergo molecular weight decrease faster than PET. The apparent slower chain cleavage of PET is attributed to the contribution of productive chain propagation reactions due to unstable vinyl endgroups which alters the equilibrium stoichiometry compared to the relatively stable endgroups of PTT and PBT.     

Conformational characteristics of poly(N-vinyl pyrrolidone)

Conformational energies are calculated for poly(N-vinyl pyrrolidone) (PVP) chains as a function of stereosequence using semiempirical potential functions appropriate to peptides and n-alkanes. The planar pyrrolidone side groups are permitted to adopt both conformations which result in an eclipsed arrangement of the pyrrolidone NCH₂ or N(CO) and the C^aH^a bonds. Solvent interactions were considered in the manner used to treat other vinyl polymers bearing planar side groups. Dimensions and dipole moments were calculated using the RIS model developed for PVP from the conformational energies considering both the effects of stereosequence and temperature. Dipole moments were measured for three PVP samples with molecular weight ranging from 10 000 to 360 000. The dimensions and dipole moments calculated for atactic PVP chains agree with the dimensions reported in the literature and the dipole moments measured here.     

Conformational stability of poly(trimethylene sulphide)

CNDO/2 calculations with the spd basis set using the tight-binding approximation for polymers were performed on species of poly(trimethylene sulphide) (PTM3), assuming several typical molecular conformations. From the results of the calculations, the conformational stability of PTM3 is discussed in comparison with that of poly(trimethylene oxide) (POM3).     

聚L-丙交酯-ε-己内酯/DL-丙交酯共聚物-聚L-丙交酯热塑性弹性体的制备及表征

以辛酸亚锡为催化剂,1,4-丁二醇为引发剂,将ε-己内酯与DL-丙交酯进行开环聚合制备两端带有羟基的ε-己内酯和DL-丙交酯共聚物（PCDLA-OH预聚物）,然后以PCDLA-OH预聚物引发L-丙交酯开环聚合制备两端为聚L-丙交酯链段,中间为ε-己内酯和DL-丙交酯共聚物链段的三嵌段共聚物（PLLA-b-PCDLA-b-PLLA）,并对嵌段共聚物的结构与性能进行了测试。结果表明,PCDLA-OH预聚物中DL-丙交酯的用量越大,预聚物逐渐从部分结晶转变为无定形聚合物,玻璃化转变温度逐渐升高。当DL-丙交酯与ε-己内酯的摩尔比为3/10,PCDLA-OH预聚物与L-丙交酯的质量比为1/5时,所制备的PLLA-b-PCDLA-b-PLLA的扯断伸长率为204%,拉伸强度为4.77 MPa。     

Conformational variability of helix sense reversals in poly(methyl isocyanate)

The conformations and energies of several helix sense reversal geometries in poly(methyl isocyanate) (PMIC) have been determined using the PCFF forcefield. In an extension of previous studies, a larger conformational variability for a helix sense reversal has been investigated. In addition to the reversal geometry previously detailed by several authors that results in a relatively small angle deviation from the rod-like polyisocyanate structure, we report the discovery of reversals of similar energy with much larger angle deviations from linearity. The effect of electrostatic interactions as controlled by the value of the dielectric constant, ε, on the conformation and energy of a reversal is also shown to be important. At ε=1.0 (vacuum) the conformations of the reversals with large and small angle ‘kinks’ have similar energies. However, at ε=2.0 (non-polar organic solvent) and ε=3.5 (bulk state) the reversals corresponding to the large angle kinks have lower energies.     

Incorporation of salicylates into poly(l-lactide)

Summary Recent studies have indicated that complications like swelling and inflammation of the surrounding tissue may occur in the late stage of thein vivo degradation of semi-crystalline PLLA bone fixation devices. Incorporation of an anti-inflammatory drug, like a salicylate, in the poly(L-lactide) chain might be a route to prevent these complications. In this study, it has been shown that it is possible to copolymerize L-lactide with di- and trisalicylide and to use salicylic acid as an initiator for the L-lactide polymerization or the L-lactide/-caprolactone copolymerization. Furthermore, PLLA was blended with poly(salicylic acid) and Zn(salicylate)2 was synthesized and turned out to be a catalyst for the ring opening polymerization of L-lactide. The binary poly(L-lactide)/o-acetyl salicylic acid system has an eutectic composition for 52 % w/w of poly(L-lactide) in the mixture. Its eutectic melting temperature is 119 °C.     

An investigation of the synthesis and thermal stability of poly(dl-lactide)

Summary The synthesis and thermal degradability of poly (DL-lactide) were investigated. Key factors affecting the polymer molecular weight were found to be monomer recrystallization, initiator concentration and the vacuum level during drying/sealing of the polymerization reaction ampoule. It was found that poly (DL-lactide) is thermally unstable above its melting temperature. Monomer recrystallization, polymer precipitation and a low initiator content of the polymer significantly inhibited the rate and extent of thermal degradation.     

Dynamic mechanical and dielectric relaxations in poly(di-n-chloroalkylitaconates)

Dielectric and viscoelastic relaxation measurements have been carried out on poly(2-chloroethyl diitaconate) (PDCEI) and poly(3-chloropropyl diitaconate) (PDCPI) between 123 K and temperatures about 293 K above the glass transition temperatures.The two polymers exhibit three peaks, a γ-relaxation in the range from 133 to 193 K (at 1 Hz), a broad β-process, in the range from 193 to 293 K and a third peak observed in mechanical measurements at 323 K (PDCEI) and 293 K (PDCPI) probably corresponding to the α dynamic glass transition phenomena. In dielectric measurements, conductive contributions overlap the α-relaxation precluding the observation of peaks at temperatures above room temperature. The apparent activation energies for the γ-relaxation according to the mechanical and dielectric measurements are close to the values derived from the empirical force field molecular mechanics calculations. A comparison is made between the relaxational data of PDCEI and PDCPI by one hand and poly(di-n-propyl itaconate) (PDPI) and poly(di-n-butyl itaconate) (PDBI) by other. This comparison allows us to conclude the relevant role played by the chlorine atoms not only in the γ relaxations but also in the β relaxations of PDCEI and PDCPI.     

Block copolymers of poly(L-lactide) and poly(ε-caprolactone) or poly(ethylene glycol) prepared by reactive extrusion

Blends of poly(L-lactide) (PLLA) and poly(ε-caprolactone) (PCL) were prepared in a co-rotating twin screw miniextruder (40 rpm, 200°). It was attempted to prepare multiblock copolymers by allowing a controlled number of transesterification reactions. Various cat-alysts (n-Bu₃SnOMe, Sn(Oct)₂, Ti(OBu)₄, Y(Oct)₃, para-toluene sulphonic acid) were introduced to promote these transesterification reactions. However, PLLA degradation by ring-closing depolymerization was the dominant reaction in every case. Alternatively, after showing that L-lactide can be conveniently polymerized in the extruder, L-lactide and hy-droxyl functionalized prepolymers of PCL or poly(ethylene glycol) (PEG) were fed to the extruder in the presence of stannous octoate. Monomer conversions of over 90% and effective transformation of all hydroxyl end groups present were generally reached. Di-and triblock copolymers could be prepared in this way with characteristics very similar to polymers prepared in a batch-type process, but with considerably reduced reaction times in a fashion, which is, in principle, scaleable to a continuous process for the production of such block copolymers. © 1996 John Wiley & Sons, Inc.     

Synthesis and micellization of a new amphiphilic star-shaped poly(D,L-lactide)/polyphosphoester block copolymer

A new amphiphilic 4-arm star-shaped poly(D,L-lactide)/poly(ethyl ethylene phosphate) (ssPLA-b-PEEP) block copolymer was synthesized by ring-opening polymerization of ethyl ethylene phosphate (EEP) with hydroxyl terminated 4-arm star-shaped poly(D,L-lactide) (ssPLA) as a macroinitiator, which was prepared by ring-opening polymerization of D,L-lactide (LA) initiated by pentaerythrite using stannous octoate as catalyst. The structures of the block copolymers were confirmed by IR, ¹H-NMR and GPC analysis. Fluorescence measurements were applied to determine the critical micelle concentration (CMC) of the copolymer micelle solutions. The diameter and the distribution of micelles were characterized by dynamic light scattering (DLS) and the shape was perceived by transmission electron microscopy (TEM). The results indicated those copolymers formed nano-micelles in aqueous solution with hydrophobic poly(D,L-lactide) core and hydrophilic poly(ethyl ethylene phosphate) shell. The CMC of the copolymer solutions increased with the increments of the proportion of PEEP segments. TEM images demonstrated that all micelles were spherical.     

The low-temperature structure of poly(di-n-hexyl germane)

The structure of poly (di-n-hexyl germane) has been investigated using X-ray and conformational energy analysis. Wide-angle X-ray diffraction analysis shows that the polymer undergoes an order/disorder transition at about 15°C. The polymer is known to exhibit thermochromism associated with this phase transition. Below the transition temperature, the backbone conformation is all-trans and the molecules are arranged in an orthorhombic unit cell. Above the transition temperature, the backbone disorders and the polymer loses long-range ordering. Fixed bonding conformational energy calculations found the lowest energy structure to be a 7/3 helix, very similar to that found for poly(di-n-hexyl silane). Most probably, the observed all-trans backbone conformation is due to intermolecular interactions which force the germanium backbone to an all-trans conformation. In order to carry out the molecular mechanics calculations, non-bonded energy parameters for germanium atoms were derived.     

Impact modification of lactic acid based poly(ester-urethanes) by blending

Branched biodegradable poly(ester-urethane)(PEU) was blended with two elastic biodegradable copolymers in proportions 5, 10, 15, and 20 wt % to investigate their effect on this hard and brittle polymer. Copolymer of L-lactide and ϵ-caprolactone, P(L-LA50/CL50), was synthesized by ring-opening polymerization and the other elastic poly(L-lactic acid-co-ϵ-caprolactone)urethane, P(LA50/CL50)U, was prepared by direct polycondensation of L-lactic acid and ϵ-caprolactone, followed with urethane bonding. In addition, four elastic biodegradable copolymers, three of them P(L-LA/CL) and one P(LA/CL)U, were blended with linear PEU to investigate their modifying effect on PEU. These compositions studied were 10, 15, and 20 wt % of P(L-LA40/CL60), P(L-LA60/CL40), P(L-LA80/CL20), and P(LA40/CL60)U in PEU. Blending was done in a batch mixer. PEU became more ductile when blended with P(L-LA/CL) and P(LA/CL)U, and its impact resistance improved markedly. In general, an addition of 15 wt % of copolymer appeared to give the most desirable mechanical properties. Moreover, the more L-lactide in the P(L-LA/CL) copolymer, the better was the miscibility of the blends, as shown by dynamic mechanical thermal analysis (DMTA) and scanning electron microscopy (SEM). One P(L-LA/CL) was also blended with poly(DL-lactide) (PDLLA) to see if the dispersion of rubbery copolymer particles was the same in PDLLA and PEU. A well-known commercial nonbiodegradable rubber [styrene/ethylene/butylene copolymer (SEBS)] was blended with linear PEU to compare its effect on impact strength. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1335–1343, 1997     

Molecular orbital conformational energy calculations of the aromatic heterocyclic poly(5,5′-bibenzoxazole-2,2′ diyl-1,4-phenylene) and poly(2,5-benzoxazole)

Interest in potential high-performance polymers, leading to characterization and development of the rodlike poly(p-phenylene benzobisoxazoles) (PBO) and poly(p-phenylene benzobisthiazoles) (PBT), has recently been extended to a related group of polymers referred to as AAPBO, ABPBO, AAPBT, and ABPBT. In this study, geometry-optimized CNDO/2 molecular orbital calculations have been carried out on AAPBO and ABPBO model compounds to determine conformational energies as a function of rotation about each type of rotatable bond within the repeat units. For AAPBO, which contains two types of rotatable bonds per repeat unit, the bond between the benzoxazole group and p-phenylene group prefers the coplanar conformation with a barrier to free rotation of 2.1 kcal mol^?1, while the bond between the benzoxazole groups prefers a conformation approximately 60 degrees away from coplanarity with a barrier to coplanarity and to free rotation of 3.6 kcal mol^?1. For ABPBO, which contains only the former type of rotatable bond per repeat unit, the coplanar conformations were preferred with a barrier to free rotation of 1.6 kcal mol^?1. These results are in excellent agreement with the results of both theoretical and experimental studies on the structurally analogous PBO. They are also consistent with the liquid crystalline behavior found for ABPBO but not for AAPBO.     

Biodegradable lactone copolymers. I. Characterization and mechanical behavior of ε-caprolactone and lactide copolymers

Copolymers of ε-caprolactone and L-lactide (ε-CL/L-LA) and ε-caprolactone and DL-lactide (ε-CL/DL-LA) were synthesized with compositions 80/20, 60/40, and 40/60 (wt % in feed). The polymerization temperature was 140°C and Sn(II)octoate was used as a catalyst. The effect of the comonomer ratio on the thermal and mechanical properties of the copolymers was investigated by size-exclusion chromatography (SEC), differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR) spectrometry, and tensile testing. The copolymers differed widely in their physical characteristics, ranging from weak elastomers to tougher thermoplastics according to the ratio of ε-CL and LA in the copolymerization. Poly(L-lactide) (PLLA), poly(DL-lactide) (PDLLA), and poly(ε-caprolactone) (PCL) homopolymers were studied as references. The tensile modulus and tensile strength were much higher for PLLA, PDLLA, and PCL homopolymers than for the copolymers. The maximum strain was very low for PLLA and PDLLA, whereas the copolymers and PCL exhibited large elongation. © 1996 John Wiley & Sons, Inc.     

Macroinorganics: 8. Chelation of copper(II) ion with some new poly(amido-amines)

The effect on Cu(ll) complexing ability of the number (n) of methylenic groups between aminic nitrogens has been studied for two classes of poly (amido-amines). In order to better elucidate the mechanism of complex formation, low molecular weight models of the polymers have also been studied. The complexing abilities of polymers and models are very similar and decrease by increasing n. Indeed compounds with n = 4 fail to form complexes in aqueous solution. Some viscosimetric titrations have been performed in order to ascertain possible conformational transitions upon complexation.     

Solution properties of poly(diallyldimethylammonium chloride) (PDDA)

Size exclusion chromatography (SEC) with dual detection, i.e. employing a refractive index (RI), concentration sensitive, detector together with a multiangle light scattering (MALS) detector which is sensitive to molecular size, has been applied to study the solution properties of poly(diallyldimethylammonium chloride) (PDDA) in water containing different electrolytes, namely: NaCl, NaBr and LiCl, at 25 °C. The analysis of a single highly polydisperse sample is enough for obtaining calibration curves for molecular weight and radius of gyration and the scaling law coefficients. The effect of the ionic strength on the conformational properties of the polymer can also be analyzed and unperturbed dimensions can be obtained by extrapolation of the values measured in a good solvent. The values of the characteristic ratio of the unperturbed dimensions thus obtained were: 17, 11 and 17, respectively, for NaCl, NaBr and LiCl solutions. Viscosity and conductivity measurements support the results obtained by SEC. Moreover, the experimental results are in good agreement with the theoretical calculations performed by combining molecular dynamics and Monte Carlo sampling procedures.     

Blends of crystalline and amorphous poly(lactide). III. Hydrolysis of solution-cast blend films

Hydrolysis of blend films prepared from amorphous poly(DL-lactide) (a-PLA) and isotactic crystalline poly(D- or L-lactide) (c-PLA) having different c-PLA contents [X = c-PLA/(a-PLA + c-PLA)] was performed in phosphate buffered solution of pH 7.4 at 37°C. The blend films before and after hydrolysis were studied using gel permeation chromatography, tensile testing, differential scanning calorimetry (DSC), and optical rotation. The mass of the blend films remaining after hydrolysis of longer than 20 months was larger with the increasing initial X. The tensile strength of the blend films remained unchanged in the early stage of hydrolysis, followed by a rapid decrease with time, the duration of period for the tensile strength remaining unchanged was longer for the blend films of smaller X. The change in crystallinity, molecular weight, and specific optical rotation during hydrolysis of the blend films revealed that degradation took place preferentially in the amorphous region than in the crystalline region of the blend films. A double melting peak was observed in the DSC spectra of blend films with X = 0.75 and 0.5 after hydrolysis for 20 months. The time difference in the induction of reduction between the tensile strength and the mass due to hydrolysis of the blend films increased with an increase in X. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 855–863, 1997     

Conformational stability of poly(trimethylene oxide) by the CNDO/2 calculations

The CNDO/2 calculations using the tight-binding approximation for polymers were carried out on poly(trimethylene oxide), (OCH₂CH₂CH₂)_n, assuming three crystal modifications, zigzag, orthorhombic, and trigonal forms. The relative stability among the three forms in the polymer was reasonably reproduced by the present calculations. The factors governing the conformational stability were demonstrated.     

Effect of polymerization and preheating processes on poly(ethylene terephthalate) depolymerization

The depolymerization reaction of poly(ethylene terephthalate) (PET) was analyzed on the basis of experimental and numerical data obtained from molecular orbital calculations. Various types of PETs polymerized by different methods and preheated by microwave irradiation were used to investigate the depolymerization mechanism. The activation energies obtained from the optimized structures of the ground state and the transition state were compared with the experimental data. On the basis of these comparisons, it was found that not only the crystallinity but also configuration changes caused by preheating were essential for realizing a high rate of transesterification reactions, such as depolymerization. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Properties of stereo multi-block polylactides obtained by chain-extension of stereo tri-block polylactides consisting of poly(L-lactide) and poly(D-lactide)

Stereo multi-block polylactides (multi-sb-PLA) having controlled block sequences of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) were synthesized by chain-extension of stereo tri-block polylacitdes (PLLA-PDLA-PLLA) that were readily synthesized by two-step ring-opening polymerization (ROP) of D- and L-lactides. The weight-average molecular weight of the resultant multi-sb-PLAs reached about 14.3?×?10⁴ Da. These multi-sb-PLAs were found to exclusively form stereocomplex (sc) crystals without homo-chiral (hc) crystallization occurring because the quickly formed sc crystals effectively suppressed the hc crystallization as cross-linkers. Their thermal properties could be tuned by controlling the block lengths of PLLA-PDLA-PLLA in the two-step ROP. Particularly, the multi-sb-PLAs consisting of longer PLLA and PDLA blocks in 1:1 ratio exhibited higher sc crystallinity and thermo-mechanical properties after thermal annealing.