ESR study of radicals formed during tensile deformation of poly[p-(2-hydroxyethoxy) benzoic acid] fibers

Radical formation during tensile deformation of highly oriented poly[p-(2-hydroxyethoxy)benzoic acid] fibers was investigated by electron spin resonance. Stretching of fibers in vacuo and in a stream of nitrogen gas at room temperature generated a large number of radicals which increased rapidly with macroscopic strain, while stretching in air generated only a small number of radicals. The radicals formed in vacuo or in nitrogen decayed rather rapidly after introduction of air. The observed spectrum was apparently a triplet with a line separation of about 7.5 gauss and a small asymmetry. The inspection of the hyperfine structure, line separation, and total width of the spectrum and the comparison between the observed and the calculated spectrum based on a model substance proved that the observed species is a phenoxy type radical generated by rupture of main chains. A small asymmetry of observed spectrum was explained by anisotropy of the g-tensor. The alkyl end-radical generated together with one of the phenoxy type could not be detected, perhaps owing to its high reactivity.     

A molecular theory of stress–strain relationship of spherulitic materials

A primitive molecular theory for stress–strain relationship of spherulitic polymers is presented based on a consideration of changes in conformational free energy in the tie chains and floating chains located between crystalline lamellae within an ideal spherulite which is assumed to undergo an affine deformation. Numerical stress–strain curves are calculated as a function of temperature, crystallinity, and tie chain fraction.     

Polyamide 6,6 fibers: Tensile deformation behavior and chain scission

The strain dependence of the elastic, anelastic and plastic components of deformation energy was determined by means of cyclic stress-strain experiments for a set of polyamide 6,6 fibers obtained by different processing techniques. Small angle X-ray scattering experiments revealed that the deformation of the supermolecular lattices of the fibers, consisting of crystalline lamellae and amorphous regions, was identical to the macroscopic deformation of the sample.ESR experiments showed that deformation gives rise to chain rupture events obviously occurring in the amorphous regions in all fibers above a critical strain level. The strain dependence of the free radical concentration was found to agree closely with the corresponding behavior of the plastic deformation energy. This indicates that chain rupture events influence stress-strain properties, particularly at large strains. The absolute values of the experimentally determined plastic deformation energy and of the theoretical value, however, calculated from the number and energy balance of ruptured chains, disagree strongly. Possible explanations are free radical recombinations and secondary dissipative processes resulting from chain rupture.     

Structure Evolution upon Uniaxial Drawing Skin‐ and Core‐Layers of Injection‐Molded Isotactic Polypropylene by In Situ Synchrotron X‐ray Scattering

The structure evolution of the oriented layer (skin) and unoriented layer (core) from injection‐molded isotactic polypropylene samples upon uniaxial drawing is probed by in situ synchrotron X‐ray scattering. The X‐ray data analysis approach, called “halo method”, is used to semiquantitatively identify the transformation process of crystal phase upon uniaxial drawing. The results verify the validation of the stress‐induced crystal fragmentation and recrystallization process in the deformation of the injection‐molded samples under different temperatures. Furthermore, the end of strain softening region in the engineering stress‐strain curves explicitly corresponds to the transition point from the stress‐induced crystal fragmentation to recrystallization process. Basically, the skin and core layers of the injection‐molded parts share the similar deformation mechanism as aforementioned. The stretching temperature which dramatically affects the relative strength between the entanglement‐induced tie chains and the adjacent crystalline lamellae determines the crystal structural evolution upon drawing. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1618–1631     

Polymesomorphism and orientation in liquid crystalline poly(triethylene glycol p,p′-bibenzoate)

The phase transitions and the orientational behavior of liquid crystalline poly(triethylene glycol p,p′-bibenzoate) have been studied. The real-time synchrotron diffraction results indicate that, on cooling from the isotropic melt, an orthogonal SmA mesophase is formed first, and later it is transformed into a tilted SmC mesophase. However, the SmA mesophase is stable in a rather wide temperature interval, and the transformation into the SmC phase occurs at temperatures close to the glass transition, so that not very high tilting angles are attained. The uniaxial deformation of the SmC mesophase indicates that usual parallel orientation of the molecular axes in relation to the stretching direction is obtained at high strain rates, while anomalous perpendicular orientation occurs at low deformation rates, with the smectic layers aligned with the stretching direction and the molecular axes almost perpendicular. A mixture of the two types of orientation is observed at intermediate rates, with rather interesting features.     

Elastic properties of poly(hydroxybutyrate) molecules

Elasticity of various poly(hydroxybutyrate) (PHB) molecules of regular and irregular conformational structure was examined by the molecular mechanics (MM) calculations. Force - distance functions and the Young's moduli E were computed by stretching of PHB molecules. Unwinding of the 2(1) helical conformation H is characterized at small deformations by the Young's modulus E = 1.8 GPa. The H form is transformed on stretching into the highly extended twisted form E, similar to the beta-structure observed earlier by X-ray fiber diffraction. The computations revealed that in contrast to paraffins, the planar all-trans structure of undeformed PHB is bent. Hence, a PHB molecule attains the maximum contour length in highly straightened, but slightly twisted conformations. A dependence of the single-chain moduli of regular and disordered conformations on the chain extension ratio x was found. The computed data were used to analyze elastic response of tie (bridging) molecules in the interlamellar (IL) region of a semi-crystalline PHB. A modification of the chain length distribution function of tie molecules tau(N) due to secondary crystallization of PHB was conjectured. The resulting narrow distribution tau(N) comprises the taut tie molecules of higher chain moduli prone to overstressing. The molecular model outlined is in line with the macroscopically observed increase in the modulus and brittleness of PHB with storage time.     

Microstructural evolution underlying the ternary stages of the elastic behaviors for poly(ether‐b‐amide) copolymer elastomers

Three stages of elastic behavior were observed during cyclic deformations for poly(ether‐b‐amide) (PEBA) segmented copolymers based on crystalline hard segments of polyamide 12 (PA12) and amorphous soft segments of poly(tetramethylene oxide) (PTMO). The underlying microstructural evolution was characterized by a combination of in situ Fourier transform infrared spectroscopy (FTIR), wide‐angle X‐ray diffraction (WAXD), and small‐angle X‐ray scattering (SAXS) technologies. The γ–α″ phase transition of crystalline PA12 occurred upon stretching, and the orientation of the α″ phase was less reversible under larger strains. PTMO chain orientation cannot be restored to the initial state, contributing to plastic deformation. Driven by the entropy effect, the strain‐induced crystallization of PTMO can fuse during sample retarding, exerting little influence on the residual strain. For PEBA with a shore D hardness of 35 D, the long period (L) can be restored to the initial L after the sample was unloaded until system fibrillation. The tie molecules between adjacent oriented lamellae can be by drawn out high stress in a PEBA material with a shore D hardness of 40 D, and the relaxation led to a second long period. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 855–864     

Evaluation of an atomic force microscopy pull-off method for measuring molecular weight and polydispersity of polymer brushes: effect of grafting density

The accuracy of the molecular weights Mn and polydispersities of polymer brushes, determined by stretching the grafted chains using atomic force microscopy (AFM) and measuring the contour length distribution, was evaluated as a function of grafting density sigma. Poly(N,N-dimethylacrylamide) brushes were prepared by surface initiated atom transfer radical polymerization on latex particles with sigma ranging between 0.17 and 0.0059 chains/nm2 and constant Mn. The polymer, which could be cleaved from the grafting surface by hydrolysis and characterized by gel permeation chromatography (GPC), had a Mn of 30,600 and polydispersity (PDI) of 1.35. The Mn determined by the AFM technique for the higher density brushes agreed quite well with the GPC results but was significantly underestimated for the lower sigma. At high grafting density in good solvent, the extended structure of the brush increases the probability of forming segment-tip contacts located at the chain end. When the distance between chains approached twice the radius of gyration of the polymer, the transition from brush to mushroom structure presumably enabled the formation of a larger number of segment-tip contacts having separations smaller than the contour length, which explains the discrepancy between the two methods at low sigma. The PDI was typically higher than that obtained by GPC, suggesting that sampling of chains with above average contour length occurs at a frequency that is greater than their spatial distribution.     

An analytical technique for measuring relative tie-molecule concentration in polyethylene

While consideration of the crystalline domains have long dominated research in understanding the properties of semicrystalline polymers, a satisfactory understanding of crack growth in these materials can only be realized by developing corresponding analytical tools to characterize the amorphous region. Since slow stable cracks in these materials preferentially form between crystalline lamellae, the role of tie molecules—the amorphous chains that bridge crystalline lamellae—are particularly important in this regard. Unfortunately, there is no method readily available for quantitative assessment of tie molecules. Through deformation and subsequent chlorination of polyethylene films, it is demonstrated that infrared dichroism can be used to determine relative tie-molecule concentration. Using this technique, one can a priori predict which resin in a series having comparable densities but widely varying molecular weights or comonomer distributions exhibits better crack resistance.     

Rheo-optical FT-IR Spectroscopy of LLDPE: Effect of Comonomer and Composite Materials

Summary: In order to further evaluate the potential of FT-IR spectroscopic investigations on molecular processes during tensile testing experiments, the behavior of monolayer LLDPE films, made with ethylene-butene and ethylene-octene copolymers, was studied. Additionally, multilayer LLDPE films based on the same C4 and C8 copolymers were investigated. The stress-strain data obtained from the monolayer films indicate differences in the strain hardening region. It seems that the film samples PE 469-30-2 (C4-gas phase process) and PE 469-30-5 (C8 solution process) behave similarly whereas the strain hardening for the PE 469-30-3 (C4 solution process) requires lower stress values. The orientation function changes during the stretching of the films indicating that unfolding of the polymer chains occurs at lower strain for PE 469-30-5 (C8) than in the C4 materials. In the multilayer systems the Primplast 44 material (C8) shows a lower tendency for reorientation in the strain hardening region than the Coex 82 (C4) material. In this region of the stress-strain curve the lamellar structure is already transformed into the fibrillar arrangement. Regarding the orientation behavior of the material above 200% strain, a small increase in f_b was observed, which led to a decrease of f_c. In the octene product possibly the bulky side chains influence the unfolding significantly, producing a higher resistance to unfolding and alignment along the stretching direction. In part, this is potentially caused by the more perfect lamellae in the octene copolymer, which do not include the side chains, while the butene copolymer may have weaker lamellae because they contain a fraction of the side chains which create defects. Consequently, the octene copolymer requires higher stress values to be stretched and finally results in a lower stretchability of this material, as observed on an industrial scale during pallet wrapping tests. Based on the ratio of the structural absorbance parameters of the signals at 729 and 719 cm⁻¹ changes in the crystallinity were studied. For the continuous stretching experiment, no monoclinic phase was detected even after Fourier self-deconvolution and peak fitting approaches. Literature data, however, describe that this crystalline transformation takes place as a result of mechanical deformation. Therefore, stepwise stretching experiments which allow an improvement of the spectral resolution to 1 cm⁻¹ were carried out. In the deconvoluted spectra the monoclinic, orthorhombic and amorphous LLDPE modifications could be assigned. Ultimate stretchability and stretching force of the films, both monolayer and multilayer, was well correlated to the development of crystalline orientation in the films upon stretching. Other mechanical properties like Elmendorf tear and dart impact can also be better understood with these results.     

Redrawing of oriented polyethylene film

Drawn and subsequently annealed polyethylene film was restretched along the original draw axis at various temperatures. The internal deformation was analyzed in terms of the structural parameters of a simplified model. The elementary deformations are the rotation of crystals around the b axis and shear at the crystal interface. The rigidity of the crystal plays an important role during extension; and as a result, disorientation of chains in the crystal occurs at high strain. At the same time, crystals deform in such a way that the crystalline chains tilt about the b axis along the (h00) plane. This deformation of the crystal is affected by temperature. The increase in long spacing with extension can be interpreted roughly by the changes in structural parameters. The strain in amorphous region in also discussed in relation to these parameters.     

Bond rupture in highly oriented crystalline polymers

The strength-limiting process in the fracture of semicrystalline fibers and highly oriented films is the rupture of tie molecules connecting the folded chain lamellae in the machine direction. This view is supported by the data on stress and temperature dependence of lifetime of fibers under load and on radical formation during the fracture experiment. The observed tensile strength, however, is about 10 times smaller and the number of fractured chains between 100 and 1000 times larger than expected on the basis of the known number of tie molecules in the fracture plane. This discrepancy is a consequence of the inhomogeneity of the micromorphology of fiber structure, which causes a much larger stress concentration on the most unfavorably located tie molecules than the average value one would expect in the case of perfectly uniform stress distribution on identical tie molecules. The fluctuation of amorphous layer thickness, of number and length of tie molecules, produces such a high stress concentration on some tie molecules throughout the sample that they rupture long before the average stress concentration is sufficient for chain fracture. By accumulation of damage caused by gradual chain rupture the weakening of the sample locally proceeds so far that at the maximum damage concentration, microcracks start to form, and the fiber breaks.     

Strain stiffening and negative normal stress in alginate hydrogels

Alginate hydrogels are polysaccharide biopolymer networks widely useful in biomedical and food applications. Here, we report nonlinear mechanical responses of ionically crosslinked alginate hydrogels captured using large amplitude oscillatory shear experiments. Gelation was performed in situ in a rheometer and the rheological investigations on these samples captured the strain‐stiffening behavior for these gels as a function of oscillatory strain. In addition, negative normal stress was observed, which has not been reported earlier for any polysaccharide networks. The magnitude of negative normal stress increases with the applied strain amplitude and can exceed that of the shear stress at large‐strain. Fitting a constitutive relationship to the stress‐strain curves reveals that the mode of deformation involves stretching of the alginate chains and bending of both the chains and the junction zones. The contribution of bending increases near saturation of G blocks as Ca²⁺ concentration was increased. The results presented here provide an improved understanding of the deformation behavior of alginate hydrogels and such understanding can be extended to other crosslinked polysaccharide networks. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1767–1775     

Negative thermal expansion of poly(vinylidene fluoride) and polyethylene tie molecules: A molecular dynamics study

The mechanism of thermal actuation for poly(vinylidene fluoride) (PVDF) and polyethylene (PE) tie molecules has been investigated using molecular dynamics simulations. Tie molecules are found in semicrystalline polymers and are polymer chains that link two (or more) crystalline lamellae, allowing for the transfer of force between these regions. A novel simulation technique has been developed to enable measurement of changes in the tie molecule length upon heating. We investigate the dependence of the percentage actuation observed upon heating, on the external applied force that stretches the tie molecules, the temperature range used for heating as well as the length and the number of tie molecules. Two molecular level mechanisms for actuation are identified. An entropically driven mechanism occurs at low applied forces and is applicable to all flexible polymers. A second mechanism due to conformational changes is observed for PVDF but not for PE at intermediate applied forces. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2223–2232     

Generation and decay of peroxy radicals in deformed polyethylene

The rate of peroxy radical accumulation as a function of strain at various temperatures in AC1220 high molecular weight polyethylene has been determined by EPR spectroscopy. The results of isothermal radical decay experiments are used, where appropriate, to correct the apparent accumulation rate to the actual rate. An exponential dependence of radical concentration [R], on true strain is observed at all temperatures investigated in the range from 160 to 294°K. For constant effective strain, measured from the approximate strain at which radical accumulation initiates, it is found that d[R]/de exhibits two sharp transitions as a function of temperature. One of these, at low temperature, is believed to be associated with the glass transition of the amorphous phase of the material; the other, at higher temperature, is believed to occur as a result of a change in the rate-controlling mechanism of deformation.     

Tensile deformation of high strength and high modulus polyethylene fibers

The influence of tensile deformation on gel-spun and hor-drawn ultra-high molecular weight polyethylene fibers has been investigated. In high modulus polyethylene fibers no deformation energy is used to break chemical bonds during deformation, and flow is predominantly present next to elastic behavior. Flow is reversible after tensile deformation to small strains, but becomes irreversible when yielding occurs.Stress relaxation experiments were used to determine the elastic and flow contribution to tensile deformation. A simple quantitative relation could then be derived for the stress-strain curve that directly links yield stress to modulus. Experimental stress-strain curves could be reasonably described by this relation.Flow during tensile deformation is shown to be correlated with the introduction of the hexagonal phase in crystalline domains. A mechanism of flow is proposed in which, at first, tie molecules or intercrystalline bridges are pulled out of crystalline blocks (reversible), followed by the break-up of crystalline blocks through slip of microfibrils past each other (stress-induced melting, irreversible).     

Crystal and amorphous orientation behavior of poly(chlorotrifluoroethylene) films in relation to crystalline superstructure

The relationship between the crystalline superstructure of polymer films and molecular orientation was studied in cold-drawn poly(chlorotrifluoroethylene) films by wide-angle x-ray diffraction, birefringence, and depolarized light scattering. By changing crystallization conditions, specimens with almost identical crystallinity but different crystalline superstructures were obtained; i.e., (1) a structure having a random array of crystallites, (2) a superstructure having a rod-like orientation correlation of the chains (a prespherulitic and sheaf-like superstructure), and (3) spherulitic superstructure. Upon stretching of specimens, crystallites initially randomly arranged orient with their chain axes along the stretching direction in accord with simple affine deformation. The amorphous chains also orient along the stretching direction. The orientation behavior of the specimens having the rod-like superstructure is similar to that of the specimens with a random array of crystallites, indicating that the interaction between the crystallites in the superstructure is relatively weak. The molecular orientation behavior of the spherulitic specimens, however, strongly deviates from simple affine deformation owing to strong interaction of the crystallites in the spherulites. The deviation can be interpreted in terms of spherulite deformation and of internal reorientation of chains within deformed spherulites.     

ESR-Messung von Kettenbrüchen in mechanisch beanspruchtem Polyamid

Zusammenfassung Die bei dehnungsinduzierten Kettenbrüchen in Polyamid-6-Fasern entstehenden freien Radikale wurden mit der ESR-Methode unter verschiedenen Temperaturen gemessen. Die Zahl der Kettenbrüche ist bei gleicher Dehnung um so geringer, je niedriger die beim Dehnvorgang herrschende Temperatür ist. Dieser Befund kann gedeutet werden, wenn man einen thermisch aktivierten Prozeß für den Mechanismus des Kettenbraches annimmt und auf ein einfaches Strukturmodell anwendet, nach dem feste kristalline und relativ schwache amorphe Schichten in axialer Richtung einander abwechseln. Der gemessene Verlauf der Radikalkonzentration über der Temperatur legt den Schluß nahe, daß die Verankerung der beanspruchten Kettensegmente in den kristallinen Bereichen mit zunehmender Temperatur gelockert wird, so daß bereits unterhalb der Glastemperatur dehnungsinduzierte Gefügeänderungen als Konkurrenzprozesse zu Kettenbrüchen auftreten. Der experimentelle Befund, daß zur Erzeugung meßbarer Radikalkonzentrationen mit abnehmender Temperatur höhere Probendehnung nötig ist, konnte zur Abschätzung des kinetischen Parameters verwendet werden, der den Einfluß einer mechanischen Belastung auf die Bruchzeit eines gespannten Kettenmoleküls beschreibt.

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Summary Free radicals, arising in stretched Nylon-6-fibers by scission of tie molecules, have been detected by electron paramagnetic resonance (EPR) at different temperatures. The lower the temperature during stretching of the fibers to a constant elongation, the lower is the number of strain-induced radicals. This result is explained by supposing the chain scission to be a thermally activated process occurring in the amorphous intercristalline layers. The observed dependence of radical concentration on temperature leads to the conclusion, that the junction of the stressed chain segments in the cristalline layers is detached with increasing temperature. The kinetic parameter, describing the influence of molecular stress on the lifetime of a strained tie molecule, can be derived by using the result, that the strain, at which the first chain scissions occur, is shifted to higher values with decreasing temperature.

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Wir danken Herrn Dr.J. Becht für wertvolle Anregungen zu dieser Arbeit.     

Kinetic study of in situ normal and AGET atom transfer radical copolymerization of n–butyl acrylate and styrene: Effect of nanoclay loading and catalyst concentration

The effect of clay nanolayers and catalyst concentration on the kinetics of atom transfer radical copolymerization of styrene and butyl acrylate initiated by activators generated by electron transfer (AGET initiation system) or an alkyl halide (normal initiation system) was studied. Monomer conversion was studied by attenuated total reflection–Fourier transform infrared spectroscopy, and also proton nuclear magnetic resonance (¹H NMR) spectroscopy was utilized to evaluate the heterogeneity in the composition of poly(styrene‐co‐butyl acrylate) chains. A decrease in the copolymerization rate of styrene and butyl acrylate in the presence of clay platelets was observed since clay layers confine the accessibility of monomer and growing radical chains. Considering the linear first‐order kinetics of the polymerization, successful AGET and normal atom transfer radical polymerization (ATRP) in the presence of clay nanolayers were carried out. Consequently, poly(styrene‐co‐butyl acrylate) chains with narrow molecular weight distribution and low polydispersity indices (1.13–1.15) were obtained. The linearity of ln([M]₀/[M]) versus time and molecular weight distribution against conversion plots indicates that the proportion of propagating radicals is almost constant during the polymerization, which is the result of insignificant contribution of termination and transfer reactions. Controlled synthesis of poly(styrene‐co‐butyl acrylate)/clay is implemented with the diminishing catalyst concentration of copper(I) bromide/N,N,N′,N′′,N′′‐pentamethyl diethylene triamine without affecting the copolymerization rate of normal ATRP. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 789–799, 2012     

Kinetik der thermo-mechanischen Spaltung von Kettenmolekülen in Fasern aus Polyamid 6 II. Kettenbruch unter kontinuierlicher Dehnung

Summary The kinetics of scission of stressed tie molecules, which leads to the formation of free radicals, were studied for continuously strained nylon 6 fibers by means of electron spin resonance spectroscopy. The results of experiments including different strain and temperature histories show the existence of a definite relation between temperature, strain and lifetime of stressed chain segments. The strain and temperature dependence of radical formation were interpreted on the basis of a structural two-phase-model and a well determinable length distribution of tie chain segments. The strain behaviour of the structural units connected by tie chains undergoes no observable change with temperature between –80 °C and +180°C. In the strain region between 14% and 19% slight changes in the fiber morphology seem to occur, however, which may be attributed to a stress-induced increase of the lamellar thickness. From the experimentally obtained kinetic parameters the activation energy and the activation volume for the chain scission are determined.

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Zusammenfassung Die Kinetik des mechanisch induzierten Bruchs belasteter Verknüpfungsmoleküle wurde anhand der Bildung freier Radikale mit Hilfe der ElektronenspinResonanz an kontinuierlich gedehnten Polyamid-6Fasern untersucht. Dabei wurden vergleichende Experimente mit unterschiedlicher Dehnungs- und Temperaturführung durchgeführt. Die Ergebnisse zeigen die Existenz einer eindeutigen Beziehung zwischen Versuchstemperatur, Probendehnung und der Lebensdauer belasteter Kettensegmente. Die Dehnungs- und Temperaturabhängigkeit der Radikalentstehung wird auf der Basis eines Zwei-Phasen-Strukturmodells mit einer strukturell vorgegebenen Längenverteilung von Verknüpfungssegmenten erklärt. Das Dehnungsverhalten der Struktureinheiten, welche durch die Verknüpfungsmoleküle miteinander verbunden werden, unterliegt, den Meßergebnissen zufolge, im Bereich von - 80 °C bis + 180 °C keinem nennenswerten Temperatureinfluß. Dagegen sind Anzeichen für geringfügige Änderungen der Fasermorphologie im Dehnungsbereich zwischen 14% und 19% zu erkennen, die als Folgen eines dehnungsinduzierten Kristallitdickenwachstums erklärt werden können. Aus den mit verschiedenen Methoden gemessenen kinetischen Parametern werden die Aktivierungsenergie und das Aktivierungsvolumen für den Kettenbruch bestimmt.

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Mit 11 Abbildungen und 2 Tabellen