Fabrication of high modulus films by solid state rolling

Solid state rolling of semicrystalline polymers represents a high speed process for producing oriented, high modulus films, tapes, and sheets. The important process variables include roll temperature, thickness of initial sheet, roll speed, take-up tension, roll diameter, and initial morphological state of the polymer. Roll temperature controls both the extent of maximum deformation and the rate of rolling. A minimum temperature exists for each polymer below which the orientation process is sharply limited. This condition is similar to the limitation present in the hydrostatic extrusion process, in which the alpha crystallization temperature limits the orientation process. Roll speeds as high as 20 m/min have been realized. It is apparent that film thickness and thickness reduction ratio have a strong effect on the ultimate rolling rate. The process, as currently practiced, is adiabatic, and therefore, heat transfer limited. The take-up tension influences the extant of orientation in the amorphous phse of of the polymer. This in turn affects its thermal and chemical stability. The effect of roll diameter is to limit the extent of thickness reduction by causing roll-film slippage when the roll dianmeter to thickness reduction ratio is below some as yet undetermined value. The initial morphological state of the polymer affects the amount of crystalline deformation possible, the surface texture of the rolled film, and the tear resistant of the oriented film.     

A re-examination of the elastic modulus dependence on crystallinity in semi-crystalline polymers

The elastic modulus versus crystallinity linear relationship in Polyethylene (PE) is re-examined via meticulous measurements over a wide set of PE. First, large discrepancies to linearity are observed; moreover, Raman spectroscopy revealed that the content of the so-called interphase exhibits considerable variations over the set of PE. Therefore a novel strategy based on DMA is developed for a better identification of the modulus of each phase along the temperature.On the one hand, below the α and β relaxations, the young modulus proved to be linearly dependant on the sum of the crystal and interphase content. On the other hand, between the α and β relaxations, the interphase appears surprisingly as stiff as the crystal. In addition, the quenched samples exhibit a particular behavior. A simple model has lead to the conclusion that their mechanical coupling and/or amorphous modulus are significantly different as compared with all others materials.     

Transitions in semi-crystalline polymers

Summary Transition studies in polymers by inverse gaschromatography showed a characteristic pressure-jump in the carrier with constant flow-rate, which may be related to the dilatometric behaviour of the polymer in the transition region.     

Tensile deformation of semi-crystalline polymers by molecular dynamics simulation

In this work, the microstructure evolution of semi-crystalline polymers during tensile deformation is analyzed by molecular dynamics simulation. A perfect semi-crystalline lamellar structure with crystalline/amorphous interface perpendicular to tensile direction is created with the help of coarse-grained (CG) model of poly(vinyl alcohol) (PVA). During the tensile test, two kinds of strain rates are applied to the lamellar stack to determine the stress–strain curves, yield stresses, and crystallinities. Consistent with experimental findings, two yield points were observed in the semi-crystalline sample which was corresponded to the fine and coarse crystallographic slips in the lamellar structure, where the crystal stems gradually rotated into the direction of applied stress during chain slips. After the second yielding point when the crystal stems had been rotated fully into the direction of applied stress, the lamellar structure was destroyed and it resulted in a decrease of crystallinity. In addition, the increase of the strain rate led to the acceleration of destruction of crystal structures. It is worth noting that the stress induced crystallization was observed in the interfacial region, and newly crystallized beads were belonged to the same microcrystalline domain as crystalline region due to memory effects. This work provides direct comparison of structure evolution between crystalline and amorphous region in semi-crystalline polymers during tensile deformation, and it is helpful for the design and mechanical property analysis of semi-crystalline polymers.     

Upcycling by grafting onto semi-crystalline polymers using supercritical CO2

The creation of graft copolymers by selectively grafting a second polymer to the amorphous fraction of a semi-crystalline polymer in supercritical CO₂ is demonstrated herein. The graft copolymer is synthesized by free radical polymerization of a vinyl monomer within the semi-crystalline polymer below its melt temperature. Such conditions afford selective grafting on the amorphous regions (block “B”) while leaving the crystalline domains (block “A”) unmodified. Accordingly, unique A-B, A-B-A, A-B-A-B-A, and so forth. block structures are formed. In this work, styrene is polymerized within polyamide 6, polyethylene terephthalate, and isotactic polypropylene. Purification of these material is performed to remove the un-grafted homopolymer, allowing for determination of the graft yield, the portion of polymer which covalently bonds to the semi-crystalline matrix. Grafting yields achieved in polyamide 6, polyethylene terephthalate, and isotactic polypropylene were 98%, 59%, and 15%, respectively. Property enhancements were observed upon further characterization of polystyrene-polyamide 6 copolymers, including high glass transition temperatures, the ability to be remelted, and tunable grafting molecular weight. Additionally, hydrophobicity is controlled by varying polystyrene composition. The remarkable range of accessed properties demonstrates this as a potential route to upcycling plastics.     

Modeling of melt electrospinning for semi-crystalline polymers

A comprehensive model for the stable jet region in electrospinning of crystallizing polymer melts has been presented. First, the conventional flow-induced crystallization (FIC) model by Ziabicki was coupled with the non-isothermal melt electrospinning model. The modeled initial jet profiles were compared to digitized experimental images of the stable Nylon-6 melt jet near the spinneret. The final jet diameters were also compared to the average thickness of collected fibers. The results were in good agreement with the flow visualization experiments for various melt temperatures and flow rates. The modeled crystallinity predictions were also in agreement with experimental data from collected fiber mats. Then, a new FIC model that can provide microstructure information, such as crystallite number density and average size, has been proposed and validated under isothermal and non-isothermal conditions in the bulk as well as in the confined geometry of the polymer melt jet in electrospinning. Nylon-6,6 was used as the model polymer in this crystallization study, and the results are in good agreement with the widely-used Ziabicki FIC model.     

Mechanical motions in amorphous and semi-crystalline polymers

Ten areas of the field, all with some special interest to the author, were selected for predictions about the near future. In order to gain perspective, a brief review is presented of some key developments which occurred after the publication of the book ‘Anelastic, and Dielectric Effects in Polymeric Solids’ by McCrum, Read and Williams in 1967. The following predictive areas are then discussed in varying degrees of detail. (1) Apparatus: current status and the need for automation and more sophistication. (2) Extending the temperature range above T_g or T_M and the molecular weight range down to the oligomers. (3) Study of rarefied polymers (higher than normal free volume). (4) Nature of the in-chain β relaxation (T < T_g) in addition polymers. (5) Use of nitroxide probes as an auxiliary tool to study amorphous phase relaxations in highly crystalline polymers. (6) Computer simulation of molecular motion at sub-group relaxations. (7) Nature of the amorphous state and its possible effect on mechanical relaxations other than through free volume. (8) Correlation between mechanical strength of glassy polymers and secondary, glassy state relaxations. (9) The need to prepare highly crystalline polymers in the completely amorphous state. (10) The impact of new polymer types. It is concluded that mechanical spectroscopy is still a quite viable field with exciting potentialities. A synergism between automation of apparatus, new materials, new techniques (not necessarily in the field of mechanical spectroscopy) and theory is anticipated.     

The amorphous contribution to the modulus of a semi-crystalline polymer

The amorphous contribution to the Young's modulus of a semi-crystalline polymer is calculated for two morphologies: the spherulite and the stacked lamellae structure. Four types of amorphous chains are considered: bridges (or tie molecules); cilia; loops; and floating, unattached chains. The statistics of a polymer chain between two, infinite, impenetrable, parallel walls are used in the modulus calculation. It is found that for each type of amorphous chain, the Young's modulus is greater in the stacked lamellae structure than in the spherulite. The Young's modulus of a cilium, loop and floating chain all increase with increasing chain contour length while the Young's modulus of a bridge passes through a minimum value. The behavior of the Young's modulus as a function of temperature is analogous. These results are discussed in terms of the relative importance of crystalline lamellar impenetrability and the inherent elastic nature of the amorphous chains, in the Young's modulus behavior.     

Mechanisms of fatigue damage and fracture in semi-crystalline polymers

Fatigue crack profiles and fracture surfaces of poly(vinylidene fluoride) (PVDF), nylon-6,6 (N66), and poly(acetal) (PA) were studied to ascertain the mechanisms of cyclic damage and fatigue crack propagation in semicrystalline polymers. Crack tip damage is believed to begin as small trans-spherulitic and inter-spherulitic tensile crazes. However, compressive yielding within the reverse plastic zone at the crack tip crushes and elongates the spherulites in the direction of crack growth. Consequently, the microstructure of the polymer in advance of the crack front is different from the original morphology of the spherulitic bulk material as evidenced by the resulting fracture surface appearance. When the test temperature is below the glass transition temperature, however, plastic deformation is limited, and fatigue fracture occurs before significant disruption of the spherulitic structure. In this case, the fracture surface morphology reflects the original microstructure of the bulk polymer.     

Empirical determination of equation of state for zero internal pressure in rare gas solids and semi-crystalline polymers

An equation of state for zero internal pressure in rare gas solids and semi-crystalline polymers has been determined based on the empirical functions of thermal pressure coefficient γ_V with respect to volume at constant pressure. The experimental data of PVT over wide range of temperature and pressure published by Anderson and Swenson and Syassen and Holzapfel for rare gas solids and Olabisi and Simha and Zoller for semi-crystalline polymers are used to evaluate γ_V. The function of γ_V with respect to volume determined at constant pressure is given by where V₀ is the volume at 0 K, A, ? and c are constants. The function of internal pressure P_i = γ_VT − P with respect to temperature at constant pressure is determined by converting the function of γ_V(V) to a function of temperature γ_V(T). An empirical equation of state for zero internal pressure determined by pressure P^∗, volume V^∗ and temperature T^∗ at which P_i = 0 is expressed by P^∗V^∗/RT^∗=C^∗−D^∗V^∗ for rare gas and semi-crystalline polymer where C^∗ and D^∗ are constants. The practical meaning of the equation of state for P_i = 0 in the semi-crystalline polymers has been discussed.     

Shrinkage stress and thermal recovery on strain-induced semi-crystalline polymers

Summary A study of shrinkage-stress and thermal recovery on strain-induced semi-crystalline poly(ethylene-terephthalate) (PET) films has been undertaken. The residual shrinkage ratio is discussed in term of molecular disorientation in the non-crystalline region which occurs by shrinkage process, according with a composite solid model in a series coupling.     

Properties of nylon-1 polymers and copolymers in the solid state

Polymers and copolymers of alkyl isocyanates have been prepared by the method of Shashoua and their properties have been investigated. All the polymers have a significant decomposition rate at 150°C but the copolymers of butyl isocyanate with ethyl isocyanate are somewhat more stable than is the butyl homopolymer. The copolymers generally show reduced crystallinity and, with high concentrations of ethyl isocyanate, substantially amorphous polymers have been prepared. Dynamic mechanical measurements give damping peaks with a fall in modulus in the temperature range of 50°–150°C but the peaks are broader and the decline in modulus less steep than for conventional vinyl polymers. Over the same temperature range the differential scanning calorimetry (d.s.c.) curves are nearly linear and show no observable discontinuity. In mechanical tests both modulus and yield stress correlate with the NCO/hydrocarbon composition ratio. The form of the stress-strain curve in tension is similar to those of cellulose derivatives. They do not show a maximum and uniform extension occurs.     

A new strain path to inducing phase transitions in semi-crystalline polymers

A novel application of in situ neutron diffraction under applied uniaxial strain is presented; measuring the crystalline domain evolution in a semi-crystalline polymer under bulk deformation. PTFE is shown to respond to uniaxial deformation by undergoing a crystalline phase transition that is previously believed to occur only at very high hydrostatic pressure. Discovery of this phase transition under applied uniaxial strain fundamentally changes our understanding of the deformation mechanisms in semi-crystalline polymers and how they need to be modeled. Under compression parallel to the basal plane normal (i.e., parallel to the molecular axis) the modulus is ∼1000× bulk dominated by intra-polymer chain compression, providing experimental validation of theoretical predictions. Deformation parallel to the pyramidal plane normal exhibits both axial and transverse strains of the opposite sign as the applied load, suggesting that the crystalline lattice is accommodating deformation by shearing along the prismatic planes.     

Temperature dependence of some thermodynamic functions for amorphous and semi-crystalline polymers

The temperature dependence of some heat-capacity related functions is evaluated on the basis of experimental data, and a further elaboration is given for polyethylene.     

Elastic contact stress analysis of semi-crystalline polymers under normal and tangential loading

Stress distribution in a polymeric subsurface under the asperity contact is investigated assuming the well known Hertzian contact stress distribution. Elastic stress analyses for three different materials are performed using a finite element method: (1) isotropic elastic solid, (2) isotropic elastic solid with a soft layer, and (3) isotropic elastic solid with a hard layer. Highly linear polymers as high density polyethylene (HDPE), poly(tetrafluoroethylene) (PTFE), and polyoxymethylene (POM) which transfer thin wear films are modeled as the elastic solid with a soft layer. Gammaray irradiated highly linear polymers and the other ordinary semi-crystalline polymers which transfer massive lumpy wear debris are considered as the elastic solid without any heterogeneous surface layer. Helium plasma treated polymers are modeled as the elastic solid with a hard layer. Octahedral shear stress and equivalent strain contours in the subsurface are obtained for each case. The octahedral shear stress and equivalent strain distributions are examined to explain various wear behaviors of semicrystalline polymers based on the Mises yield criterion and the delamination theory of wear.     

High stiffness polymers by die-drawing

The application of die-drawing to the production of highly oriented polymers is considered, ft is shown that a variety of useful products can be obtained including not only rod and multifilament, but also tube and filled polymer. The investigations under discussion include the processing of several grades of linear polyethylene, polypropylene homopolymer and co-polymer. In favorable cases, products of very high stiffness were obtained, reaching modulus values of 50 GPa for linear polyethylene and 20 GPa for polypropylene.     

Mobile amorphous phase fragility in semi-crystalline polymers: Comparison of PET and PLLA

PET and PLLA were cold crystallised at various times and the two polymers were studied by differential scanning calorimetry (DSC), dielectric spectroscopy (DS) and thermally stimulated depolarisation currents (TSDC). The crystalline, the amorphous and the rigid amorphous fraction were quantified. The percentage of rigid amorphous fraction is very large in semi-crystalline PET and very low in semi-crystalline PLLA. From DSC, DS and TSDC data, the values of the relaxation times of four samples were obtained above and below the glass transition. The “strong-fragile” glass former liquid concept was used and the fragility of polymers was obtained. The presence of the crystalline phase and of a rigid amorphous fraction does not significantly modify PLLA fragility parameters and the polymer remains “fragile”, while for PET the semi-crystalline material goes towards a “strong character”. The coupling between phases is much weaker in PLLA than in PET.