Tailoring the rigid amorphous fraction of isotactic polybutene-1 by ethylene chain defects

The effect of different amounts of ethylene co-units in the butene-1 chain, on the fold-surface structure of crystals of isotactic polybutene-1, has been probed by analysis of the rigid amorphous fraction (RAF). The exclusion of ethylene co-units from crystallization in random butene-1/ethylene copolymers and their accumulation at the crystal basal planes leads to a distinct increase of the RAF with increasing concentration of co-units. A specific RAF was determined by normalization of the RAF to the crystal fraction. While in the butene-1 homopolymer a specific RAF of 20–30% is detected, it increases to more than 100% in copolymers with 5–10 mol% of ethylene co-units, being in accordance with the previously observed increase of the free energy of the crystal fold-surface due to copolymerization. It has also been shown that the specific RAF increases with decreasing temperature of crystallization, due to formation of a fold-surface of lower perfection, containing an increased number of chain segments traversing the crystalline-amorphous interface.     

The link between rigid amorphous fraction and crystal perfection in cold-crystallized poly(ethylene terephthalate)

The rigid-amorphous fraction (RAF) in cold-crystallized and subsequently annealed poly(ethylene terephthalate) (PET) was investigated as a function of crystallinity and crystal perfection. During cold-crystallization, the amount of RAF increases non-linearly with crystallinity to a maximum of 44% at a crystallinity of 24%. Vitrification of the RAF is almost complete at the cold-crystallization temperature. Increasing the crystallinity from 24% after cold-crystallization to 44% by subsequent annealing at higher temperatures decreases the RAF. The specific RAF, i.e. the ratio of RAF to crystallinity at the glass transition temperature, T_g, of the mobile-amorphous fraction decreases from almost 2.0 after cold-crystallization to 0.75 after the subsequent annealing. The decrease in the specific RAF is attributed to crystal perfection which decreases the strain transmitted to the amorphous phase. Analysis of the reversing specific heat capacity of the annealed samples on cooling leads to the conclusion that the remaining RAF at the temperature of annealing is still glassy up to at least 490 K. An observed excess heat capacity above 490 K is due to reversible melting and is discussed within the frame of the concept of the specific reversibility of melting.     

Oxygen solubility and specific volume of rigid amorphous fraction in semicrystalline poly(ethylene terephthalate)

The existence of rigid amorphous fraction (RAF) in semicrystalline poly(ethylene terephthalate) (PET) is associated with the lamellar stack crystalline morphology of this polymer, the regions where several crystalline lamellas are separated by very thin (20-40 Å) amorphous layers. In contrast, regular or mobile amorphous fraction is associated with much thicker interstack regions. The oxygen transport properties of PET isothermally crystallized from the melt (melt-crystallization) or quenched to the glassy state and then isothermally crystallized by heating above T_g (cold-crystallization) were examined at 25 °C. Explanation of unexpectedly high solubility of crystalline PET was attributed to the formation of RAF, which in comparison with mobile amorphous phase is constrained and vitrifies at much higher than T_g temperature thus developing an additional excess-hole free volume upon cooling. Measurements of crystallinity and jump in the heat capacity at T_g were used to determine the amount of mobile and rigid amorphous fractions. Overall oxygen solubility was associated with the solubility of mobile and rigid amorphous fractions. The oxygen solubility of the RAF was determined and related to the specific volume of this fraction. The specific volume of the RAF showed a direct correlation with the crystallization temperature. It was shown that upon crystallization from either melt or glassy state, the constrained between crystalline lamellas PET chains consisting of the RAF, vitrify at the crystallization temperature and resemble the glassy behavior despite high temperature. When cooled to room temperature, the RAF preserves a memory about the melt state of polymer, which is uniquely defined by the crystallization temperature.     

Kinetics of graft polymerization of styrene on cis-1,4-polybutadiene

By measurements of polymerization rate, grafting efficiency and number-average molecular weight of free and grafted polystyrene, the α-dicumyl peroxide initiated polymerization of styrene on cis-1,4-polybutadiene at 100°C and at low extents of reaction was studied. The polymerization rate and the polystyrene molecular weights decreased with the rubber content of the solution. The grafting efficiency was found to be substantially independent of the peroxide concentration, but to increase with rubber content. A rigorous mathematical model of the reaction was developed, from which it has been possible to confirm the proposed mechanism and to establish the value of many kinetic constants.     

Molecular dynamics simulation of cis-1,4-polybutadiene. 2. Chain motion and origin of the fast process

Molecular dynamics (MD) simulations of cis-1,4-polybutadiene in bulk amorphous state were performed to elucidate the origin of a fast relaxation process observed by quasielastic neutron scattering (QENS) measurements. The details of the torsional motion for each dihedral angle were investigated with the torsional auto- and cross-correlation functions for several temperatures in this study. Temperature dependence of the correlation between the torsional autocorrelation and cross-correlation functions is also evaluated. The origin of the fast process of cis-1,4-polybutadiene is found to be mainly the cooperative conformational transitions of two dihedral angles located at both sides of the CH₂-CH₂ bond when the bond is in the trans conformation. The cooperative conformational transitions exhibit even below the glass transition temperature of cis-1,4-polybutadiene. The cooperative motion appears at about 50 K below the glass transition temperature, corresponding to the Vogel-Fulcher temperature.     

Glass transition and free volume in the mobile (MAF) and rigid (RAF) amorphous fractions of semicrystalline PTFE: a positron lifetime and PVT study

The structure of the free volume and its temperature dependence in poly(tetrafluoroethylene) (PTFE) and of its copolymer with perfluoro(propyl vinyl ether) (PFA) was studied by pressure-volume-temperature (PVT) experiments (T=27-380 °C, P=0.1-200 MPa) and positron annihilation lifetime spectroscopy (PALS, T=−173-250 °C, P=10⁻⁵ Pa). From the analysis of these experiments we conclude on the volumetric properties of the mobile (MAF) and rigid amorphous fractions (RAF) in these semicrystalline polymers. The specific volumes of the MAF and RAF, V_MAF and V_RAF, were estimated assuming that V_MAF agrees with the specific volume of the melt extrapolated down to lower temperatures using the Simha-Somcynsky equation of state (S-S eos). V_RAF was then estimated from the specific volume of the entire amorphous phase, V_a, and the known V_MAF. The specific free volume V_f=V_a−V_occ was also estimated from V_a using the S-S eos hole fraction h, V_occ=(1−h)V_a. From the analysis of PALS data with the routine LT9.0 the mean volume, 〈v_h〉, and the width, σ_h, of the local free volume size distribution (holes of subnanometre size) were obtained. A comparison of 〈v_h〉 with V_f delivered the hole density N^′h. The volume parameters show that the RAF which is formed during crystallisation from the melt has a distinctly smaller specific free and total volume than the MAF. During cooling the contraction of the RAF slows down and finally, below room temperature, the RAF possesses a larger free volume than the MAF shows. Obviously, the restriction of the segmental mobility in the RAF by the crystals limits at high temperatures the free volume expansion and at low temperatures dense packing of the polymer chains. These conclusions from the analysis of the specific volume are confirmed by PALS experiments.     

Metal dependent control of cis-/trans-1,4 regioselectivity in 1,3-butadiene polymerization catalyzed by transition metal complexes supported by 2,6-bis[1-(iminophenyl)ethyl]pyridine

Fe(III), Cr(III), Fe(II), Co(II) and Ni(II) chloride complexes supported by 2,6-bis[1-(iminophenyl)ethyl]pyridine have been synthesized and characterized along with single crystal X-ray diffraction. These complexes, in combination with MAO, have been examined in butadiene polymerization. The catalytic activity and regioselectivity are strongly controlled by metal center and cocatalyst (MAO/Co ratio dependent in the case of Co(II) complex). The activity decreases in the order of Fe(III) > Co(II) > Cr(III) ≈ Ni (II) complexes, in consistent with the space around the metal center. Polybutadiene with different microstructure content, from high trans-1,4 units (88-95% for iron(III) and Cr(III)), medium trans-1,4 and cis-1,4 units (55% and 35%, respectively, for iron(II)) to high cis-1,4 units 79% for Co(II) and 97% for Ni(II) can be easily achieved by varying of the metal center. In addition, mechanism speculation is also presented to elucidate the dependence of catalytic behaviors on metal and cocatalyst.     

Study of the thermal behaviour of alipharomatic polyesters around the glass–rubber transition region by thermomechanical analysis: the mobile and rigid amorphous fraction

The objective of this work was the study using thermomechanical analysis (TMA) of a peculiar behaviour, which was observed some years ago, around the glass–rubber transition region in some thermoplastic alipharomatic polyesters. For this purpose a series of nine alipharomatic polyesters was prepared by the two‐stage melt polycondensation method in a glass batch reactor and subjected to TMA in both penetration and expansion mode. Differential scanning calorimetry (DSC) was additionally used and the results are discussed focusing mainly on the first derivative curve of TMA thermograms in the penetration mode. From this curve, which shows two distinct peaks, the first peak could be attributed to the glass transition temperature (T_g) of the mobile amorphous fraction, since the value coincides with that obtained from DSC and is due to the abrupt shrinkage of the amorphous part of the sample. The second peak (up to 40 °C higher than T_g) is due most probably to the softening of the rigid amorphous fraction and the passage of the polymeric sample from the glass region to the cold crystallization region. When the sample is more crystalline than amorphous then the first peak is smaller or is completely absent. Copyright © 2006 Society of Chemical Industry     

Sequence distribution of cis-1,4- and trans-1,4-units in polyisoprenes

The ¹³C n.m.r. spectra of chicle polyisoprene and cis-trans isomerized 1, 4-polyisoprenes were studied. The splittings of signals were observed in the C₁, C₂, and C₄ carbon signals of the isomerized polyisoprenes. The newly appearing signals were assigned to the carbon atoms in cis-trans linkages. The fractions of the diad sequences (trans-trans, trans-cis, cis-trans, and cis-cis) were determined by using the four signals of C₁ carbon. It was found that the cis-1, 4- and trans-1, 4-units were randomly distributed in the isomerized polyisoprenes and it was confirmed that the chicle polyisoprene was a mixture of cis-1, 4- and trans-1, 4-polyisoprenes.     

Surface structure of folded-chain crystals of poly(R-3-hydroxybutyrate) of different chain length

The structure of the crystalline-amorphous interface of poly(R-3-hydroxybutyrate) (PHB) of different molar mass is evaluated by analysis of the rigid amorphous fraction and by analysis of the degree of reversible melting and crystallization. The rigid amorphous fraction of low-molar-mass PHB of 5 kDa is only 5-10%, and at best half of that of high-molar-mass PHB of almost 500 kDa, despite identical crystallinity. This result is paralleled by observation of distinctly larger degree of reversible melting and crystallization in PHB of high molar mass. The larger rigid amorphous fraction and higher degree of reversible melting and crystallization in PHB of high molar mass, consistently and independently, prove enhanced covalent coupling of crystals and amorphous structure, and/or de-coupling of segments of macromolecules which traverse between phases, respectively. The distinct isolation of crystals in PHB of low molar mass is discussed in terms of absence of wide loops/folds, long-chain cilia, and tie-molecules.     

Transition of the bounded polymer layer to a rigid amorphous phase: A computational and DSC study

The study focuses on the transition of the bounded to solid surfaces polymer layer to a rigid amorphous phase (RAP). Based on previous Monte Carlo (MC) simulation studies on bulk polyethylene (PE), we refine our numerical variable density Self Consistent Field (nSCF) method in order the calculated density in bulk to follow the predictions of the MC simulation. The proposed modification of the Sanchez–Lacombe equation of state allows us to examine thermodynamic aspects of the glass transition. By imposing a glass transition (T_g = 220 K) in the bulk we predict an earlier (during cooling), stronger transition of the bounded layer to a RAP, at ∼350 K. Also at short separation distances we record the appearance of undisturbed polymer bridges. Differential scanning calorimetry (DSC) experiments on polydimethylsiloxane (PDMS) and polyamide 6 (PA6) nanocomposites suggest that although the crystallization can be significantly suppressed by the addition of nanoparticles, the RAP layer may locally equilibrate, above T_g, for long experimental times with the melt phase. Our results support the idea that a significant free energy barrier of entropic origin appears due to the RAP formation, below the melting temperature (T_m) and above the T_g.     

Studies on the thermal degradation of cis-1,4-polyisoprene

cis-1,4-Polyisoprene is pyrolyzed under an inert atmosphere at different temperature ranges by using differential thermal analysis and pyrolysis-gas chromatography. Fourier transform-infrared is used for investigating the pyrolysis residue. The pyrolyzed gaseous products are analyzed by gas chromatography. The effect of temperature range on the relative yields of the major decomposition products is studied. Based on these data, the thermal degradation mechanism of cis-1,4-polyisoprene is also proposed.     

Molecular dynamics simulation of cis-1,4-polybutadiene. 1. Comparison with experimental data for static and dynamic properties

Molecular dynamics (MD) calculations of cis-1,4-polybutadiene in bulk amorphous phase were performed under constant pressure and constant temperature conditions. The static and dynamic properties were evaluated from the results of MD calculations. The obtained density and coefficient of thermal expansion are in good agreement with experimental data. The feature of the calculated static structure factor is similar to the observed one. Molecular motion is examined with mean square displacements and intermediate scattering functions. An onset of a new motion, which corresponds to so-called fast process, was clearly observed in the temperature dependence of the mean square displacement above 100 K. The dynamic structure factors obtained by the Fourier transformation of the intermediate scattering functions are compared with those obtained from quasielastic neutron scattering measurements. The peaks corresponding to the elastic scattering and the low energy excitation at around 2 meV are reproduced in the dynamic structure factors. The excessive intensity observed in the dynamic structure factor, which corresponds to the fast process, is also reproduced above 140 K in our simulation.     

Transport of water in synthetic cis-1,4-polyisoprenes and natural rubber

The sorption, permeation and diffusion of water in two synthetic cis-1,4-polyisoprenes and natural rubber and of methanol in natural rubber have been measured in the range 25 to 60°C. At the higher relative pressures the diffusion coefficient in all systems decreases with concentration and activation energies for diffusion increase with concentration. The results are consistent with increased clustering of the sorbed penetrant at higher relative pressures which renders an increasing fraction of the penetrant relatively immobile. The effect of polar impurities on the sorption and diffusion process is discussed to account for differences in behaviour of the various samples at lower relative pressures.     

Effect of isothermal crystallization on the amorphous phase mobility of polycarbonate/poly(ε-caprolactone) blends

Polycarbonate/poly(ε-caprolatone), PC/PCL, amorphous miscible blends with 90/10 and 80/20 composition in weight, have been studied by X-ray diffraction and broad-band dielectric spectroscopy. The molecular dynamics for the high T_g component segmental mobility have been followed by analyzing the isothermal dielectric losses as a function of frequency with Havriliak-Negami distributions. The PCL plasticization effect is observed as a lowering of the Vogel-Tammann-Fulcher terminal temperature without any significant change in the other relaxation parameters. The isothermal crystallization process of the 90/10 PC/PCL blend has been studied in real time both by following the decrease of mobile amorphous material from the variation of the dielectric losses, and by wide angle X-ray scattering to follow the crystal growth. Quantitative comparisons show the disappearance of much higher amounts of mobile amorphous chains than those that are transferred to the crystallites. The existence of a rigid amorphous phase in the blend is established in non-negligible amounts as the isothermal crystallization of PC develops. The remaining mobile amorphous phase has unchanged dynamics.     

Constraints in semicrystalline polymers: Using quasi-isothermal analysis to investigate the mechanisms of formation and loss of the rigid amorphous fraction

The nanoscale phase behavior of a semicrystalline polymer is important for mechanical, thermal, optical and other macroscopic properties and can be analyzed well by thermal methods. Using quasi-isothermal (QI) heat capacity measurements, we investigate the formation behavior of the crystalline, mobile amorphous, and rigid amorphous fractions in poly(trimethylene terephthalate), PTT. The crystal and rigid amorphous phases comprise the total solid fraction in PTT at temperatures above T_g, the glass transition temperature of the mobile amorphous fraction. PTT was quasi-isothermally cooled step-wise from the melt which causes its crystalline fraction to be fixed below 451 K. Between the high temperature fulfillment of the T_g step and 451 K, the temperature dependent rigid amorphous fraction (RAF) is completely determined. For PTT, most of the RAF vitrifies between 451 K and T_g step by step during QI cooling after the crystals have formed. The constraints imposed by the crystal surfaces reduce the mobility of the highly entangled polymer chains. We suggest the vitrification of RAF proceeds outward away from the lamellar surfaces in a step by step manner during QI cooling. Upon reheating, devitrification of RAF occurs at a temperature above its previous vitrification temperature, due to the effects of densification brought by physical aging during the long period of quasi-isothermal treatment. Finally, we consider recent concepts related to jamming, which have been suggested to apply to filled polymer systems, and may also be applicable in describing constraints exerted by crystal lamellae upon the RAF.     

Icosahedron cage occupancy of structure-H hydrate helped by Xe -1,1-, cis -1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes

Four mixtures of 1,1-, cis-1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes (hereafter abbreviated DMCH) including H₂O and Xe have been investigated in a temperature range over 274.5 K and a pressure range up to 2.7 MPa. The 1,1-DMCH and cis-1,2-DMCH generate the structure-H hydrate in the temperature range up to 295.2 and 280.2 K, respectively. Especially, very large depression of equilibrium pressure has been observed in the structure-H 1,1-DMCH hydrate system. On the other hand, neither trans-1,2-DMCH nor cis-1,4-DMCH generates the structure-H hydrate in the present temperature range. It is an important finding that the cis-1,4-DMCH does not generate the structure-H hydrate in the presence of Xe, while the mixture of cis-1,4-DMCH and methane generates the structure-H hydrate.     

Highly efficient cis-1,4 polymerisation of isoprene using simple homoleptic amido rare earth-based catalysts

Polymerisation of isoprene was performed by means of ternary catalytic systems made of Ln[N(SiMe₃)₂]₃ (Ln = Nd (1), Y (2))/borate/Al(ⁱBu)₃ combinations (borate = [HNMe₂Ph][B(C₆F₅)₄] (HNB), [CPh₃][B(C₆F₅)₄] (TB) or B(C₆F₅)₃ (Barf)). With borate activator B(C₆F₅)₃, highly active (up to 4700 kg mol⁻¹h⁻¹) and selective catalysts are obtained with both neodymium and yttrium metals, leading up to 96% cis-1,4-polyisoprene.     

Creating aesthetics and functional values in cotton fabrics through the introduction of thermobonding amorphous polyester fibers into blends

Low‐melting thermobonding PET (copolymer) fibers were used to produce heather effects in fabrics blended with cotton, which could be classified as natural 100% cotton fabrics. PET (copolymer) fibers produced at relatively low molecular weights with intrinsic viscosities of 0.57 and 0.47 were used in this investigation. These were mostly amorphous fibers that showed a low specific gravity at 1.27 g cm^?3. The fibers were blended with cotton to produce open‐ended spun yarns without the PET component being melted. These yarns were knit into fabrics and cured. The curing was based on the thermal behavior of these fibers as observed in differential scanning calorimetry thermograms. The amount of pigmented PET copolymer needed to achieve the required heather effect was optimized by variations of the blend ratios. This approach provided another avenue for introducing additional functional properties, such as antibacterial and soil‐resistance properties, to the fabric. Both the fibers and resultant fabric properties were studied extensively, along with the finishing of these fabrics. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3747–3756, 2003     

Role of stearic acid in the strain-induced crystallization of crosslinked natural rubber and synthetic cis-1,4-polyisoprene

Strain-induced crystallization of crosslinked natural rubber (NR) and its synthetic analogue, cis-1,4-polyisoprene (IR), both mixed with various amounts of stearic acid (SA), were investigated by time-resolved X-ray diffraction using a powerful synchrotron radiation source and simultaneous mechanical (tensile) measurement. No acceleration or retardation was observed on NR in spite of the increase of SA amount. Even the SA-free IR crystallized upon stretching, and the overall crystallization behavior of IR shifted to the larger strain ratio with increasing SA content. No difference due to the SA was detected in the deformation of crystal lattice by stress for both NR and IR. These results suggested that the extended network chains are effective for the initiation of crystallization upon stretching, while the role of SA is trivial. These behaviors are much different from their crystallization at low temperature by standing, where SA acts as a nucleating agent.