The effect of short chain branching on local chain organisation in isotopically labelled blends of polyethylene

The complementary techniques of small angle neutron scattering (SANS) and infra-red spectroscopy have been used to determine features of molecular trajectory for isotopic blends incorporating linear polyethylene and copolymers containing butyl or hexyl branches. SANS data show both a more compact conformation for a copolymer guest molecule than for a linear guest and also a smaller molecular expansion with increasing molecular weight when both guest and host are copolymers. These blends also show the smallest proportion of [lcub ]110[rcub ] isolated guest stems, while wholly linear blends show the largest proportion, on the evidence of the infra-red CD₂ bending band profiles. Estimates are made of the sizes of ‘groups’ of adjacent stems, and these also show a corresponding dependence on the sample type, indicating a higher proportion of adjacent re-entry for copolymer blends than for linear blends.     

Small angle neutron scattering study of the structure of a triblock copolymer of styrene and isoprene during extension

A triblock copolymer with a styrene weight fraction of 0.41, has been examined at various extension ratios using small angle neutron scattering. The original ‘polycrystalline’ material with a face centred cubic arrangement of styrene domains acquires orientation as the extension ratio increases. Affine deformation is not obeyed at the supramolecular level and there is some evidence for non-uniform stress at this level.     

Effects of processing parameters and molecular weight distribution on the tensile properties of polypropylene fibers

The tensile properties of polypropylene fibers, produced in a short-spin line, are correlated with the parameters of the three processing stages (spinning, drawing, and annealing), and with the molecular weight distribution. In general, tensile stiffness and strength increase with increasing molecular orientation, while the elongation at break decreases. The degree of orientation is determined by the deformation ratios and temperatures of the first two stages. Tensil modulus and strength also increase with increasing annealing stage shrinkage ratio. All the tensile properties, including the elongation at break, increase with increasing average molecular weight. The mechanisms of crystallization and deformation are related to the molecular weight distribution in different ways. Hence, the tensile modulus is highest for broad distributions when the draw ratio is low, and for narrow distributions when the draw ratio is high. The tensile strength increases and the elongation at break decreases as the width of the molecular weight distribution decreases, for all combinations of processing parameters. The distribution of tensile strength, for fibers with high draw ratios, broadens as the molecular weight distribution narrows. The total draw ratio of fibers, as experienced during processing and testing, and the true stress at break, are discussed in terms of deformation rates and relaxation times. © 1994 John Wiley & Sons, Inc.     

Effect of planar extension on the structure and mechanical properties of polystyrene–poly(ethylene-co-butylene)–polystyrene triblock copolymers

Two thermoplastic poly(styrene)–poly(ethylene-co-butylene)–poly(styrene) triblock copolymers containing either spherical or cylindrical poly(styrene) microdomains were pre-oriented through extensional flow. Small angle neutron scattering (SANS) measurements revealed that the pre-oriented triblock with a spherical microstructure adopts a body-centred cubic (bcc) structure with a [111] orientation along the flow direction. For the pre-oriented triblock with a cylindrical microstructure, the cylinder axis is aligned along the extensional flow direction. Investigation of the mechanical properties showed that Young's moduli of the pre-oriented copolymers are highly anisotropic. Specimens were then subject to uniaxial deformation along the extensional flow direction and at the same time microstructural changes induced by the deformation were investigated by SANS. It was found that the deformation of the bcc lattice is affine, and that the deformation of the microstructure is reversible. For the triblock copolymer with a cylindrical microdomain structure the deformation of the lattice was found to be non-affine. In this case, SANS patterns suggest a “micronecking” and a breaking of the cylindrical domains without any change in the domain spacing.     

Study on poly(ethylene terephthalate)/polypropylene microfibrillar composites. II. Solid‐state drawing behavior

A (20/80) blend of poly(ethylene terephthalate)/polypropylene (PET/PP) was solid‐state drawn to enhance the molecular orientation of the PET microfibers. Effects of drawing temperature (23–140°C) and drawing speed (max. 1000 mm/min) on the morphology and draw ratio of the blend were studied and discussed based on the drawing behaviors of the pure polymers. In cold drawing, there seemed to be a critical drawing speed below which the natural draw ratios of the polymers remained constant, but above which the draw ratios first decreased slightly because of suppression of molecular relaxation and then increased because of breakage of highly strained molecules and disintegration of lamellar crystals into finer mosaic blocks. Macroscopically, the pure PP and the PET/PP composite extrudates gave similar draw ratios at the same speeds. SEM showed that the PET microfibers suffered a smaller elongation than the PP matrix and severe voiding occurred at the PET/PP interface. Furthermore, substantial fiber breakage occurred during cold drawing at speeds above 200 mm/min. In comparison, drawing at 100°C caused no obvious interfacial voiding and fiber breakage. Furthermore, the natural draw ratio of the blend was lower than that of the pure PP extrudate, indicating that the PET microfibers had constrained the deformation of the PP matrix. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1989–2000, 2004     

Orientation and crystallization in poly(ethylene terephthalate) during drawing at high temperatures and strain rates

We review some recent research developments on structure development during drawing of poly(ethylene terephthalate) film, and we report a study of constant-load drawing of amorphous PET film at temperatures of 120°C and 132°C, including the effects of redrawing high-temperature drawn film at lower temperature. To permit constant-load drawing at high temperature without inducing crystallization in the undrawn specimen, a drawing instrument was built that permits very rapid heating of the sample, and its operation is described. The initial stage of drawing at high temperatures is characterized by polymer flow where, owing to high rates of molecular relaxation, neither molecular orientation nor crystallization occurs. Strain-rate increases sharply in the course of the deformation, reducing the time available for relaxation, and the chains start to orient at a draw ratio that depends on temperature. Orientation rapidly reaches a saturation level, which is lower at the higher draw temperature. Crystallization onset seems to lag only slightly behind orientation onset because the critical orientation for inducing crystallization is very low at these temperatures. It appears that there is time for crystallization to proceed to pseudo-equilibrium values corresponding to a particular orientation level, which differs from previous results obtained from constant-force drawing at lower temperatures, and possible reasons for this are discussed. In two-stage drawing, where film drawn at 132°C was redrawn along the same axis at 100°C, high draw ratios were obtained despite the high strain rates, and the levels of noncrystalline orientation and crystallinity were similar to the levels expected from single stage drawing at 100°C.     

Influence of molecular weight on crystallization rate of oriented,glassy nylon 6

Differential scanning calorimetry and wide angle X-ray diffractometry were used to investigate the effects of molecular weight and molecular weight distribution on the crystallization kinetics of oriented, glassy nylon 6. The samples had number average molecular weights ranging from 10,000 to 42,000 and polydispersity indices ranging from 2.0 to 3.1. Noncrystalline films were prepared by quenching molten films between plattens chilled with liquid nitrogen. These films were drawn 4X and 5-1/3X, and the resultant uniaxial orientation was observed to enhance markedly the room temperature crystallization kinetics. Although macroscopic deformation can be assumed to be affine at the molecular level, it is hypothesized that wholechain molecular relaxation occurs at rates inversely proportional to the square of molecular weight, thereby creating a distribution of extension ratios which reflect the actual molecular weight distribution. Thus, the crystallization rate and the degree of crystallinity depend on the fraction of high molecular weight molecules present in the sample. Given two samples with the same molecular weight, the one with the broader distribution crystallizes more rapidly. Similarly, samples having the larger molecular weight crystallize to a greater extent when two samples have the same molecular weight distribution.     

Ram extrusion of high-density polyethylene and polypropylene in solid state: Process conditions and properties

Highly oriented high-density polyethylene (HDPE) and polypropylene (PP) were obtained by solid state extrusion near, but below, the melting temperature. Draw ratios of 6 and 11 were obtained using properly designed dies, preformed billets and lubrication for different extrusion temperatures. Orientation induced significant changes in properties and morphology that are related to the final draw ratio. The draw ratio was observed to decrease with an increase in extrusion speed due to the molecular relaxation occurring because of local heating during deformation. An increase in the degree of crystallinity was observed for the oriented polymers as well as a large improvement in the tensile modulus. Significant molecular relaxation and elastic recoil were observed during processing. Extrusion speed strongly increases the extent of relaxation, and, consequently, reduces many of the mechanical properties.     

Energetics of methyl branches in hydrocarbon crystals

Semi-empirical energy calculations are reported for methyl branched hydrocarbon molecules in orthorhombic and monoclinic crystalline arrays of linear molecules. The 19 molecules in the array are flexible and twenty methylene units long. Two modes of packing the defect molecule in the crystals are found and for each the mechanical relaxation criterion of there being two or more sites available to the molecule is met. However, there is no significant relaxation strength because the differences in site energies are too large. This result holds for all cases because the unfavourable orientation of the molecular stem in one of the sites makes the energy much greater than in the other for which the stem has a favourable orientation. For the case with the lowest defect energy, the unit cell dimensions are comparable to those found experimentally for the same methyl group concentration. Drawings showing the molecular distortion are presented.     

Deformation processing of PMMA into high-strength fibers

Poly(methyl methacrylate) was drawn into fibers by melt extrusion followed immediately by a transient temperature drawing process. By varying five processing variables, fibers ranging from 0.635 mm to 25 μm in diameter were produced. Heat-induced relaxation of the aligned structure was used to determine the draw ratio of the resultant fibers and therefore the degree of polymer chain alignment imposed by the deformation process. The resulting changes in length and diameter were measured and it was found that draw ratios of 5–20 had been achieved under the varying processing conditions. It was also observed that fiber diameter immediately after drawing is a good predictor of the degree of orientation present in the fiber irrespective of the processing conditions. To test the effect molecular orientation has on material properties, fibers with varying degrees of orientation were tested in tension. As expected, increasing alignment resulted in increasing tensile strength. The maximum observed true ultimate tensile strength was 225 ± 53 MPa and was seen in fibers with a draw ratio equal to 18.7 ± 4.5. Fibers with a lower degree of alignment, while not as strong in tension, exhibited significantly increased ductility. True strains of as high as 25% were observed.     

Biaxially oriented poly(ethylene terephthalate) bottles: Effects of resin molecular weight on parison stretching behavior

Uni- and biaxial stretching of poly(ethylene terephthalate) (PET) specimens of appropriate geometry at temperatures near the glass-rubber transition may lead to non-uniform deformation unless the draw ratio exceeds a critical value, the natural draw ratio, characteristic of the onset of strain hardening due to stress-induced crystallization. Experimental results obtained in the present investigation show that natural draw ratios in uni- and biaxial stretching decrease with increasing resin molecular weight and with decreasing temperature. Undesirable uneven wall thickness distribution in biaxially stretched cylindrical parisons can only be prevented if draw ratios in both orthogonal principal stretching directions exceed the corresponding natural values. The minimum thickness reduction required for uniform biaxial stretching of a cylindrical parison at 95°C may vary between 12 and 5 depending on the resin's molecular weight or viscosity and this will affect the optimum design of parison geometry. The degree of unbalanced biaxial molecular orientation in the wall of cylindrical parisons stretched up to or beyond the natural draw ratios also depends on the resin molecular weight. Unbalanced biaxial orientation has been investigated by means of wide angle X-ray diffraction and birefringence measurements as well as its effect on various properties: rigidity, yield stress, creep compliance, and dimensional stability.     

The effects of roll drawing on the structure and properties of oriented poly(ethylene terephthalate)

The effect of processing conditions on the structure and properties of roll drawn poly(ethylene terephthalate) (PET) was examined. It was found that, when roll drawing amorphous PET at temperatures just above the glass transition, only very low draw ratios were obtained. This is probably because there were no crystallites to lock in the applied extension. Roll drawing at high temperatures, above 130°C, where there was significant thermal crystallization, produced film of high strength. At temperatures between 130°C and 190°C, the properties were almost independent of processing temperature. Mechanical tests performed on roll drawn samples, processed in this temperature range, showed that the initial modulus and the yield stress increased linearly with draw ratio. The yield strain decreased with draw ratio up to λ = 4.0, and then became almost constant. The processing temperature that produced samples with the greatest strength was 170°C. This was because the highest draw ratios were obtained at this temperature while maintaining constant width deformation. At low draw ratios, the crystallinity increased with the processing, whereas at higher draw ratios, it was independent of temperature. This constant level of crystalline fraction may have produced the constant failure strain that was observed at high draw ratios. The orientation functions were similarly unaffected by the processing temperature, although birefringence measurements did suggest that lower processing temperatures may have produced higher levels of orientation. The orientation of the trans conformers was independent of the temperature, but the overall content was increased at higher processing temperatures.     

Block copolymer aggregates with photo-responsive switches: Towards a controllable supramolecular container

Herein, we present a concept of combining host-guest chemistry with block copolymer self-assembly to fabricate an inner cross-linking block copolymer aggregate with photo-responsive switches on the basis of the reversible interaction between azobenzene and β-cyclodextrin, which can serve as a controllable supramolecular container to load and release guest molecules reversibly. The inner cross-link makes the block copolymer aggregates exhibit good stability, and the aggregates can keep their spherical morphologies during photo-irradiation treatment. When the switches are in on-state, cyclodextrins can bind with hydrophobic pyrene molecules; and when the switches are in off-state, pyrene molecules will get away from the cyclodextrins. The photo-controllable switches embedded in the aggregates endow this new supramolecular container with the capability to load and release guest molecules reversibly without structure disruption. It is anticipated that this line of work may open an avenue for fabricating new polymeric containers which can be used for controllable molecular transfer and catalysis.     

Mechanical anisotropy in oriented linear polyethylene of various crystallinities

We have studied the mechanical moduli of oriented linear polyethylene with crystallinities X varying from 0.44 to 0.63 and draw ratios λ = 1–9 by using a dynamic tensile method at 10 Hz and an ultrasonic technique at 10 MHz. Wide-angle X-ray diffraction and birefringence measurements reveal that the chains in the crystalline regions are fully aligned at λ > 4, but the degree of amorphous orientation increases steadily up to the highest draw ratio. From −180°C to the β relaxation region (near 0°C at 10 Hz) the mechanical behavior at all crystallinities is controlled by three factors: molecular orientation, weak c-shear deformation and stiffening effect of taut tie molecules. At low temperature the chain alignment in an oriented sample gives rise to an axial Young's modulus E₀ which is much larger than the transverse Young's modulus E₉₀, with the modulus for the undrawn material lying in-between. However, the results that E₄₅ < E₉₀ and C₄₄ (axial shear modulus) < C₆₆ (transverse shear modulus) imply that a weak c-shear process occurs even at low temperature. At the β relaxation where the amorphous regions are rubbery, the stiffening effect of taut tie molecules becomes prominent and leads to increases in all moduli upon drawing. For the polyethylene with the lowest cyrstallinity a strong c-shear process is activated at the α relaxation (about 50°C at 10 Hz), which gives rise to very low values of C₄₄ and E₄₅. This effect becomes weaker with increasing crystallinity and is hardly observable at X > 0.6.     

Structural evolution of uniaxial tensile‐deformed injection molded poly(ɛ‐caprolactone)/hydroxyapatite composites

It is essential to examine the mechanisms of plastic deformation of polymer composites under external loads and large strains, especially if the material is intended to be used in a dynamic environment. This work investigated the variation of structure as well as the properties of poly(ɛ‐caprolactone) (PCL) deformed under different tensile draw ratios and strain rates. The PCL/HA composites were prepared by melt mixing the PCL with up to 10 wt% HA in a twin‐screw extruder. The deformation behavior of the PCL/HA composites revealed a strong correlation between the mechanical response and the accompanying structural transformations. It was found that the strain rate and stretching ratio played important roles in modulating the molecular orientation and crystallization of the PCL/HA composites. The increase in strain rate from 0.2 to 100 mm/min led to the variation of crystallinity from 56.81% to 67.50%. With an increase of the strain rate, the chain extension rate along the stretching direction increased faster than the chain relaxation, which improved the orientation of the polymer chains. The crystallinity and orientation of the deformed PCL/HA composites increased with an increase in draw ratio. The composites also possessed enhanced yield strength resulting from an increased strain rate. POLYM. COMPOS., 38:1771–1782, 2017. © 2015 Society of Plastics Engineers     

Diffusional and orientational dynamics of various single terylene diimide conjugates in mesoporous materials

Mesoporous silica materials are ideally suited as host–guest systems in nanoscience with applications ranging from molecular sieves, catalysts, nanosensors to drug-delivery-systems. For all these applications a thorough understanding of the interactions between the mesoporous host system and the guest molecules is vital. Here, we investigate these interactions using single molecule spectroscopy (SMS) to study the dynamics of three different terylene diimide (TDI) dyes acting as molecular probes in hexagonal and lamellar mesoporous silica films. The diffusion behaviour in the hexagonal phase is represented by the trajectories of the single molecules. These trajectories are highly structured and thus provide information about the underlying host structure, such as domain size or the presence of defects inside the host structure. The three structurally different TDI derivatives allowed studying the influence of the molecular structure of the guest on the translational diffusion behaviour in the hexagonal phase and the lamellar phase. In the lamellar phase, the differences between the three guests are quite dramatic. First, two populations of diffusing molecules – one with parallel orientation of the molecules to the lamellae and the other with perpendicular orientation – could be observed for two of the TDI derivatives. These populations differ drastically in their translational diffusion behaviour. Depending on the TDI derivative, the ratio between the two populations is different. Additionally, switching between the two populations was observed. These data provide new insights into host–guest interactions like the influence of the molecular structure of the guest molecules on their diffusional as well as on their orientational behaviour in structurally confined guest systems.     

FTi.r.-rheo-optical characterization of the molecular orientation behaviour of amine cured epoxy resins during cyclic deformation

Rheo-optical FTi.r. spectroscopy was used to study molecular orientation phenomena in highly crosslinked epoxies on the basis of the diglycidyl ether of bisphenol-A and a polyetherdiamine. In previous publications, their orientation behaviour during continuous uniaxial deformation was described. The present paper reports on the rheo-optical characterization of these resins during specific cyclic deformation experiments below the glass transition temperature (T_g). Epoxy films were subject to various successive loading—unloading cycles including elongation, recovery, annealing, and stress relaxation in order to study the reversibility of the orientation during relaxation processes. The investigations show that the orientation is only in part reversible upon unloading or stress relaxation below T_g. However, it can be annealed by heating the epoxy resin to above T_g. Furthermore, it was found that no significant fatigue due to chain scission occurs until failure of the sample. The results were discussed with respect to the mechanism of plastic deformation.     

Characterization of the adhesion of single-walled carbon nanotubes in poly(p-phenylene terephthalamide) composite fibres

Poly(p-phenylene terephthalamide)/single-walled carbon (PPTA/SWNT) composite fibres with different draw ratios have been spun using a dry-jet wet spinning process and their structure and deformation behaviour analysed using Raman spectroscopy. The dispersion of nanotube has been examined by Raman scattering intensity mapping along the fibre. The nanotubes improved the polymer orientation in composite fibre with a draw ratio of 2 but degraded the orientation at higher draw ratios. The mechanical reinforcing effect by nanotubes is related to the change of polymer orientation, suggesting a dominant role of polymer orientation in mechanical performance of the composite fibre. High efficiency of stress transfer within the strain range of 0-0.35% and breakdown of the interface at higher strains has been found in the composite fibres through an in situ Raman spectroscopic study during fibre deformation. Cyclic loading applied on the fibre has indicated reversible deformation behaviour at low strain and gradual damage of the interface at high strains.