Structure as a function of relaxation in glassy polymers

Previously, amorphous glasses were simulated by carrying out energy minimization on initial conformations generated by growing polymer chains in a periodic cube. It was not known what degree of relaxation these simulated glasses possessed. The degree relaxation is determined by the thermal history of the bulk polymer and in turn determines numerous important properties. Constant stress molecular dynamics (CSMD) followed by energy minimization was used to simulate different thermal histories of an isotactic poly(propylene) glass. This simulation approach produced glasses in which the degree of energetic relaxation was a function of the thermal history. Based on the simulated cohesive energy density, the simulation using minimization alone produced a glass that was energetically annealed. However, the local geometry and x-ray structure factor indicate that it has a different structure than those obtained using CSMD followed by energy minimization and may not be structurally relaxed.     

PETN基PBX结合能和力学性能的理论研究

PETN(季戊四醇四硝酸酯)是著名的硝酸酯类猛炸药,用量子力学(QM)、分子力学(MM)和分子动力学(MD)方法,计算模拟其与高聚物组成的PBX(高聚物粘结炸药)的结合能和力学性能.以AM1-MO法和MM方法取PETN与系列高聚物的尺寸匹配原子簇模型,经几何全优化计算,发现两种方法求得的结合能彼此线性相关.对PETN超晶胞及其与系列氟聚物组成的双组分PBX,实施COMPASS力场下的分子动力学(MD)周期性模拟,求得其弹性系数、拉伸模量、体模量、剪切模量和泊松比,发现添加少量高聚物确能有效改善炸药的力学性能.     

The mechanical properties and structure of poly(m-methylene terephthalate) fibers

A study has been carried out of the differences in mechanical properties of oriented fibers of poly(ethylene terephthalate) (2GT), poly(trimethylene terephthalate) (3GT), and poly(tetramethylene terephthalate) (4GT). The properties studied include the tensile stress–strain behavior, the recovery from strain, shrinkage at 100°C and the glass-transition temperatures. The stress–strain curves of the three materials differ markedly. 2GT shows a monotonic increase in stress with increasing strain up to failure, which occurs at ～20% strain, and the oriented fibers possess a comparatively high initial modulus. 3GT shows a much lower initial modulus and there is an inflection in the stress–strain curve at about 5% strain. The stress–strain curve of 4GT shows a number of distinct features. Although the initial modulus of 4GT is similar to that of 3GT, the stress–strain curve shows a pronounced plateau in the region between 4% and 12% strain. At higher strains the stresses rise rapidly before failure. These features of the stress–strain curves in the three polymers can be related to previous studies where the x-ray diffraction spectrum and the Raman spectrum have been examined for fibers under stress. The ranking of both the recovery and shrinkage behavior of these materials is in the order 3GT > 4GT > 2GT. These results can also be understood in terms of the results of the previous structural studies, and it is concluded that the molecular conformations in both the crystalline and noncrystalline regions play a key role in determining the mechanical behavior.     

Local Transient Jamming in Stress Relaxation of Bulk Amorphous Polymers

Bulk amorphous polymers become stretched and parallel-aligned under loading stress,and their intermolecular cooperation slows down the subsequent stress relaxation process.By means of dynamic Monte Carlo simulations,we employed the linear viscoelastic Maxwell model for stress relaxation of single polymers and investigated their intermolecular cooperation in the stress relaxation process of stretched and parallel-aligned bulk amorphous polymers.We carried out thermal fluctuation analysis on the reproduced Debye relaxation and Arrhenius fluid behaviors of bulk polymers.We found a transient state with stretch-coil coexistence among polymers in the stress relaxation process.Further structure analysis revealed a scenario of local jamming at the transient state,resulting in an entropy barrier for stretch-coil transition of partial polymers.The microscopic mechanism of intermolecular cooperation appears as unique to polymer stress relaxation,which interprets the hydrodynamic interactions as one of essential factors raising a high viscosity in bulk amorphous polymers.Our simulations set up a platform of molecular modeling in the study of polymer stress relaxation,which brought new insights into polymer dynamics and the related mechanical/rheological properties.     

Surface-deformation characteristics of uniaxially oriented poly(ethylene terephthalate) film as evaluated from nanoscratch tests with scanning probe microscopy

The surface-deformation characteristics of uniaxially drawn poly(ethylene terephthalate) (PET) film were successfully evaluated with multiline scratch tests using scanning probe microscopy (SPM) on a nanometer scale. The PET film was prepared by compression molding from the melt, followed by quenching in ice water. The obtained amorphous film was drawn uniaxially below its glass-transition temperature, and the resultant surface roughness could be reduced to within 5 nm. A multiline scratch with the Si(3)N(4) tip of an SPM on the oriented PET surface was made parallel and perpendicular to the drawing axis under applied loads of 5-30 nN. The perpendicular scratching generated a characteristic periodic pattern on the film surface, but the parallel scratching induced a tearing of the surface. These results suggest that surface-deformation mechanisms were dominated by molecular anisotropy. The surface-deformation properties, as evaluated from scratch-angle dependences on morphological changes on a nanometer scale, were similar to the mechanical properties of the bulk.     

Tensile drawing,morphology, and mechanical properties of poly(butylene terephthalate)

The concept of the drawing of a molecular network has been employed to derive a total network draw ratio from the combination of the two deformations occurring in the production of poly(butylene terephthalate), PBT, fibers by the consecutive processes of melt spinning and cold drawing. The mechanical properties of PBT can then be more readily explained in terms of increases in this total network draw ratio. However, the preorientation and crystallization that occurs in the melt-spinning process can occur at different strain rates and temperatures, depending on the wind up speed employed, on the extensional viscosity of the polymer, and on the variation of the extensional viscosity with temperature. Therefore, for polymers such as poly(butylene terephthalate), which can exist in two crystalline forms, the morphology of the final drawn fiber might be expected to depend on the first melt-spinning stage of the process as well as on the total network draw ratio. In this work, density, birefringence, mechanical measurements, and WAXD measurements, which have been made on the melt-spun fibers and on the drawn fibers, are described. Small differences in some of the drawn yarn mechanical properties at the same overall network draw ratio are related to the crystallinity and in particular to differences in the proportion of the α and β phases present in the drawn yarn. These in turn are related to differences in the temperature and stress during melt spinning and drawing. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 2465–2481, 1997     

On the mechanical properties of poly(ethylene terephthalate). Force-field parametrization and conformational analysis for the prediction of the crystal moduli

A force-field suitable for the calculation of the mechanical properties of poly(ethylene terephthalate) and their relation with the molecular structure of the polymer has been developed. The force-field parameters were derived from quantum mechanical AM1 calculations and tested against thermodynamic and vibrational spectroscopy data available for a set of closely related small molecules. The crystal moduli of the two solid phases known for poly(ethylene terephthalate) were estimated by means of this new force-field considering both the isolated chain and the chain within the unit cell. Results were qualitatively consistent with reported x-ray data showing that the triclinic crystal form is stiffer than the mesomorphic phase provided that sample heterogeneities were taken into account. Although overestimated moduli resulted for both cases, divergences with experimental values were found to be slighter than those obtained by other theoretical methods. © 1996 John Wiley & Sons, Inc.     

Morphology of poly(butylene terephthalate)–poly(ethylene terephthalate‐co‐isophthalate‐co‐sebacate) segmented copolyesters

Segmented copolyesters, namely, poly(butylene terephthalate)–poly(ethylene terephthalate‐co‐isophthalate‐co‐sebacate) (PBT‐PETIS), were synthesized with the melting transesterification processing in vacuo condition involving bulk polyester produced on a large scale (PBT) and ternary amorphous random copolyester (PETIS). Investigations on the morphology of segmented copolyesters were undertaken. The two‐phase morphology model was confirmed by transmission electron microscopy and dynamic mechanical thermal analysis. One of the phases was composed of crystallizable PBT, and the other was a homogeneous mixture of PETIS and noncrystallizable PBT. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2257–2263, 2003     

Time-resolved SAXS studies on crystallization behavior of polyõethylene isophthalate-co-ethylene terephthalatešs

Small-angle X-ray scattering measurements using synchrotron radiation were carried out for poly(ethylene terephthalate) and poly(ethylene isophthalate-co-ethylene terephthalate)s. In addition, differential scanning calorimetric measurements were conducted. Measurements were made both on polymers undergoing isothermal crystallization and during subsequent remelting. The primary and secondary crystallization behaviors are examined. Isophthalate units were found to be excluded from the crystals into amorphous layers during crystallization. No crystal thickening was observed during isothermal crystallization, which may be due to the relatively high chain rigidity. Secondary crystallization, detected predominantly at the later stages of crystallization, causes densification and shrinkage of the amorphous layer. Considering the results, it is proposed that secondary crystallization involves the formation of short-range molecular order in the amorphous layers of a lamellar stack as well as in the amorphous regions between lamellar stacks. This short-range-ordered phase has a lower density than the lamellar crystal formed by primary crystallization.     

Effects of gamma irradiation on poly(ethylene isophthalate)

Radiation methods are largely used for polymerisation and polymer modification, since irradiation induces transformations in the structure of materials which can be exploited to improve their performance. On the other hand, combined action of ionising radiation and oxygen may lead to degradation of the polymer, with worsening of properties such as mechanical strength or electrical insulation resistance. Therefore, the change of the chemical and physical properties of polymers under irradiation is a dynamic topic of research. In this work there are discussed data on the physical features of a polyester, poly(ethylene isophthalate) (PEI), subjected to gamma irradiation up to 1 MGy. PEI is a semicrystalline polymer with a structure similar to polyethylene terephthalate. Viscosity and differential scanning calorimetry measurements were carried out which allowed the monitoring of changes in the structure in terms of variations in the molecular weight, as well as of the percentage crystallinity depending on the dose. Furthermore, positron annihilation lifetime spectroscopy supplied information on the free volume present in the amorphous phase of the irradiated polymer.     

Correlation of physico-chemical,mechanical and electrical properties of ultraviolet-degraded poly(ethylene terephthalate)

Poly(ethylene terephthalate) (PET) films have been exposed to ultraviolet radiation. Changes in physico-chemical, mechanical and electrical properties which occur in the films were examined. It was found that the specific viscosity, tensile strength, elongation at break, relative dielectric constant and dielectric strength decrease, while the dielectric loss factor, density, optical density of the ir spectrum band which refers to the trans form of the PET molecule, as well as the quantity of carboxylic end groups, increase.Least squares analysis and function straightening methods were used for the interpretation of the experimental data. Correlating relationships between measured parameters were established using the same methods.Special attention was paid to the changes in the electrical properties of poly(ethylene terephthalate), because no data are available so far. Mathematical correlations with chemical and mechanical properties were established.     

The mechanical and optical properties of oriented fibres of semicrystalline polymers

The mechanical properties of semicrystalline polymers are very much dependent on the molecular orientation. The orientation function to deal with the elastic moduli data of Hadley et al. (1969) are treated under new aspects. To determine the nature of orientation function from empirical data a relation of the form Sinθ=f (n) Sinθ′ is assumed to describe the change in the orientation angle of a unit from θ′ to θ for the draw ration. Birefringence experimental data are used to find the orientation functionf (n) which is then used to calculate elastic properties. The agreement of the experimental values ofE ₀ andE ₉₀ over the entire draw ratio range is found to be satisfactory for Nylon, low density polyethylene and polyethylene terephthalate. This furnishes strong empirical evidence for the validity of the orientational model for the above substances.     

TUNED MECHANICAL PROPERTIES ACHIEVED BY VARYING POLYMER STRUCTURE — KNOWLEDGE THAT GENERATES NEW MATERIALS FOR TISSUE ENGINEERING

By changing both the monomer composition and the polymer structure, we have varied the mechanical properties of resorbable polymers. The polymers were synthesized by ring-opening polymerization using L-lactide （LLA）, ε-caprolactone （εCL）, trimethylene carbonate （TMC） and 1,5-dioxepan-2-one （DXO） as monomers. Well-defined triblock copolymers, microblock copolymers and networks have been evaluated, and comparisons between them show that it is possible to tune the mechanical properties. Triblock copolymers with an amorphous middle block of poly（1,5-dioxepan-2- one） （PDXO） and semi-crystalline end-blocks of poly（ε-caprolactone） （PCL） were stronger and had a higher strain at break than triblock copolymers with poly（L-lactide） （PLLA） as end-blocks. Polymers with both DXO and TMC in the amorphous middle-block and PLLA as end-blocks showed a lower stress at break, but the material gained elasticity, a property which is very valuable in tissue engineering. Mechanical properties of networks, synthesized by a novel method, containing PDXO and PCL are also presented. Although it is difficult to compare them with the uncross-linked polymers, this is an additional way to modify and widen the properties.     

Geminal poly(silyl ester)s: Highly labile degradable polymers

Geminal silyl ester linkages were used for the backbone construction of linear polymers, which exhibit rapid cleavage in the presence of atmospheric water. A series of poly(gem-silyl ester)s with two ester groups flanking each silicon atom were synthesized, in order to probe the effects of different silyl-substituted side-chain groups upon the physical and chemical properties. The transsilylation condensation reaction of bis(trimethylsilyl) terephthalate with dichlorodiisopropylsilane, dichlorodicyclohexylsilane, dichloromethyl-n-octadecylsilane, and dichloromethyl-4-methylphenethylsilane gave the four poly(gem-silyl ester)s with two isopropyl, two cyclohexyl, one methyl plus one octadecyl, and one methyl plus one 4-methyl-phenethyl side-chain groups per silicon, respectively. The polymers were characterized by NMR (¹H, ¹³C, and ²⁹Si), infrared spectroscopy (IR), size-exclusion chromatography (SEC), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Hydrolytic degradation studies of the polymers solvated in tetrahydrofuran and as bulk samples in the solid state were performed in the presence of atmospheric water as the nucleophilic cleavage agent, and the molecular weight loss was monitored by SEC. Poly(diisopropylsilyl terephthalate) (1a) and poly(dicyclohexylsilyl terephthalate) (1b) were found to be more stable towards nucleophilic degradation in comparison to poly(methyl-n-octadecylsilyl terephthalate) (1c) and poly(methyl-4-methylphenethylsilyl terephthalate) (1d), due to the presence of sterically bulky isopropyl or cyclohexyl groups attached to the silicon atoms. All of the polymers degraded into small molecules upon hydrolysis, with the exception that the degradation products of 1c and 1d self-condensed in the solid state to form the respective polysiloxanes. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3606–3613, 1999     

Rotational isomerism and physical properties of long-chain molecules in solid states

Rotational isomerism occurring in solid state of organic long-chain compounds, including synthetic linear polymers, have been concerned in connection with the macroscopic physical properties of bulk materials. The conformational order in the non-crystalline part of polyethylene has been investigated by Raman spectra, and related to the elastic behaviors of bulk samples. In the solid-state phase transition induced by mechanical forces of poly(butylene terephthalate) the macroscopic strain has been related directly to the conformational conversion of the molecules. Concerning the piezoelectric and pyroelectric activities of poly(vinylidene fluoride), polymorphism, phase transition, and structural change on the poling process have been investigated. A ferroelectric-paraelectric phase transition has been found for a series of copolymers of vinylidene fluoride and trifluoroethylene. On the phase transition a great change in molecular conformation is accompanied with the scrambling of the dipolar orientation. This is the characteristic of polymer ferroelectrics in which the dipolar units are linked with each other by covalent bonds in a molecular chain. Spectroscopic evidences are presented indicating that the thermodynamic stability of polymorphs of n-fatty acids is closely related to the rotational isomerism occurring in the carboxyl groups.     

Molecular dynamics simulations for pure epsilon-CL-20 and epsilon-CL-20-based PBXs

Molecular dynamics has been employed to simulate the well-known high energy density compound epsilon-CL-20 (hexanitrohexaazaisowurtzitane) crystal and 12 epsilon-CL-20-based PBXs (polymer bonded explosives) with four kinds of typical fluorine polymers, i.e., polyvinylidenedifluoride, polychlorotrifluoroethylene, fluorine rubber (F(2311)), and fluorine resin (F(2314)) individually. The elastic coefficients, isotropic mechanical properties (tensile moduli, bulk moduli, shear moduli, and poission's ratios), and bonding energy are first reported for epsilon-CL-20 crystal and epsilon-CL-20-based polymer bonded explosives (PBXs). The mechanical properties of epsilon-CL-20 can be effectively improved by blending with a small amount of fluorine polymers, and the whole effect of the adding fluorine polymers to improve mechanical properties of PBXs along the three crystalline surfaces of epsilon-CL-20 is found to be (100) approximately (001) > (010). The interaction between each of the crystalline surfaces and each of the fluorine polymers is different, and the ordering of binding energy for the three surfaces is (001) > (100) > (010); F(2314) always has the strongest binding ability with the three different surfaces. F(2314) can best improve the ductibility and tenacity of PBX when it is positioned on epsilon-CL-20 (001) crystal surface. The calculations on detonation performances for pure epsilon-CL-20 crystal and the four epsilon-CL-20-based PBXs show that adding a small amount of fluorine polymer into pure epsilon-CL-20 will lower detonation performance, but each detonation parameter of the obtained PBXs is still excellent.     

Towards first-principles modelling of the mechanical properties of oriented poly(ethylene terephthalate)

This paper presents a procedure for simulating the anisotropic small-strain mechanical properties of oriented amorphous poly(ethylene terephthalate) (PET) starting from an atomistic level. A technique for producing oriented amorphous simulation cells of glassy PET has been developed and closely examined against related structural and property measurement data. The simulated elastic constants of these cells, derived by energy minimisation and molecular dynamics strain fluctuation methods, show encouraging agreement with experimental data.     

Synthesis and structure of elastomeric poly[ester-block-ether]s

Different poly[ester-block-ether] (PEE) elastomers have been prepared in a two-stage process, namely transesterification followed by polycondensation in the melt. These terpolymers consisted of poly(butylene terephthalate) (PBT), poly(oxytetramethylene) (PTMO) and dimerized fatty acid (DFA). The content of PTMO and DFA blocks was changed at a constant PBT mass ratio of 50. Supermolecular structure of synthesized polymers was investigated using optical microscopy (OM) and transmission electron microscopy (TEM). Thermal and mechanical properties were examined by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMTA). An influence of dimerized fatty acid on the segment's comiscibility (phase separation) was estimated.     

Broad-line NMR and dynamic mechanical behavior of some aromatic polyesters

The dynamic mechanical properties of four aromatic polyesters were measured at temperatures in the 78–540°K region at 10³–10⁴ cps. The polymers studied were: poly(1,3 phenylene isophthalate), poly(1,4 phenylene terephthalate), poly(4,4′ diphenylene isophthalate), and poly(4,4′ diphenylene terephthalate). All four polymers had β loss peaks at about 280°K. Distinct β^* mechanical processes were found for the two terephthalate esters. Broad-line nuclear magnetic resonance measurements were carried out in the 150–440°K temperature range on the four polyesters mentioned above in addition to poly(4,4′ diphenylene 4,4′ biphenyl dicarboxylate). A change in NMR second moment takes place in the 190–330°K region, the magnitude of which is dependent on the polymer structure. The results are compared with those found for a series of aromatic polyamides and are discussed in terms of possible motional processes.     

Surface interpenetrating networks of polyacrylamide in poly(ethylene terephthalate) as a means of surface modification

Surface interpenetrating network (IPN) polymers are emerging hybrid materials in which the surface of existing polymers can be modified to preserve their chemical structure and bulk properties. A detailed structural characterization of poly(ethylene terephthalate) (PET) thin films on nanoscopically flat silicon wafers has been carried out by Scanning Probe Microscopy (SPM) and X-ray photoelectron spectroscopy (XPS). Examination of the surface of spin-coated annealed PET film by the SPM in tapping mode revealed a two-phase structure. One phase appeared as a dense crystalline fraction of the polymer while the other was identified as amorphous. These findings were supported by Differential Scanning Calorimetry (DSC), which recognized the crystallinity of annealed PET film at 30%. Modification of the PET surface with interpenetrating polyacrylamide (PAM) increased the roughness of the surface with uniform properties. The depth profiling with XPS revealed that PAM interpenetration extended down to 7.2 nm, confirming a three-dimensional character of the polymer modification, with a relative mass concentration of PAM at about 30.7% in the IPN interface.