Specification of biaxial orientation in amorphous and crystalline polymers

The fundamental concepts for specifying orientation in amorphous and crystalline polymers are reviewed. A new set of orientation factors is proposed to represent the second moments of biaxial orientation. The factors are defined both for the chain axis and for three crystallographic axes of an orthorhombic (or pseudo-orthorhombic) crystal structure. The orientation factors are defined in terms of the angles between the crystallographic axes and Cartesian coordinate reference axes defining the machine, transverse and thickness directions of films. This makes the orientation factors symmetric with respect to the machine and transverse directions unlike the Stein-Nomura-Kawai orientation factors which are defined in terms of Euler's angles. A graphical procedure for representing the state of orientation as a point inside an isoceles triangle is described. Methods of measuring the orientation factors are also reviewed. The paper concludes with examples of the application of these concepts to orientation in amorphous polystyrene films fabricated in our laboratories and to crystalline polyethylene samples discussed in the literature.     

Crystallization, spherulite growth, and structure of blends of crystalline and amorphous poly(lactide)s

The effects of incorporated amorphous poly(dl-lactide) (PDLLA) on the isothermal crystallization and spherulite growth of crystalline poly(l-lactide) (PLLA) and the structure of the PLLA/PDLLA blends were investigated in the crystallization temperature (T_c) range of 90-150 °C. The differential scanning calorimetry results indicated that PLLA and PDLLA were phase-separated during crystallization. The small-angle X-ray scattering results revealed that for T_c of 130 °C, the long period associated with the lamellae stacks and the mean lamellar thickness values of pure PLLA and PLLA/PDLLA blend films did not depend on the PDLLA content. This finding is indicative of the fact that the coexisting PDLLA should have been excluded from the PLLA lamellae and inter-lamella regions during crystallization. The decrease in the spherulite growth rate and the increase in the disorder of spherulite morphology with an increase in PDLLA content strongly suggest that the presence of a very small amount of PDLLA chains in PLLA-rich phase disturbed the diffusion of PLLA chains to the growth sites of crystallites and the lamella orientation. However, the wide-angle X-ray scattering analysis indicated that the crystalline form of PLLA remained unvaried in the presence of PDLLA.     

Melting rates of crystalline polymers under shear conditions

The melting of solids under shear conditions is an important operation in the processing of most thermoplastics. In this study, solid blocks of high density polyethylene were melted on a hot, moving surface over a range of surface temperatures and velocities. The conservation equations for mass, momentum, and energy were applied to the molten layer and then simplified by an order of magnitude analysis. A general model that included all significant terms gave rates of melting that agreed closely with the measured rates. A simpler model that was analogous to present extruder models predicted melting rates that were ten to twenty-five percent lower than the experimental values. Models for polymers with low to moderate crystallinity should be modified to account for physical removal of amorphous material by shear forces.     

Factors influencing microstructure formation in polyblends containing liquid crystalline polymers

Four isotropic polymers, poly(butylene terephthalate) (PBT), polycarbonate (PC), polyethersulfone (PES) and polysulfone (PSU), were blended by extrusion with a thermotropic liquid crystalline polymer (LCP) at different temperatures. The morphology of extrudates was observed by means of scanning electron microscopy and the intrinsic aspect ratio of LCP fibrils and particles separated from matrix resin was measured with an image analysis. Special attention was paid to the LCP fibrillation in these four matrices in a wide temperature range from 270 to 360°C and the internal relations among the effects of processing parameters, such as viscosity ratio, extrusion temperature, and LCP concentration. The results show that the viscosity ratio of the dispersed LCP phase to the continuous phase is a decisive factor determining the formation of LCP fibrils, but its effect closely relates with the LCP content. In the range of viscosity ratios investigated, 0.004 to 6.9, and lower LCP content of 10%, significant fibrillation took place only at viscosity ratios below 0.01. It is predicted that the upper limit of the viscosity ratio for LCP fibrillation will increase with increasing LCP content. A comparison of the morphologies of LCP/PES blends with different LCP concentrations reveals that the LCP phase becomes continuous at a concentration of less than 50%, and high LCP content does not always favor the formation of long and uniform LCP fibrils. The extrusion temperature has a marked effect on the size of the minor LCP domains. For fibril forming systems, the percentage of LCP fibrils with larger aspect ratios increases with increasing extrusion temperatures. All these results are explained by the combined role of deformation and coalescence of the LCP disperesed phase in the blend.     

Transition of spherulite morphology in a crystalline/crystalline binary blend of biodegradable microbial polyesters

Crystalline/crystalline binary blend films of microbial polyesters composed of poly[(R)-3-hydroxybutyrate-co-(R)-3hydroxyhexanoate] (P(3HB-co-3HH)) and poly[(R)-3-hydroxybutyrate] (P(3HB)) that exhibit a morphological change are prepared by solvent casting. Differential scanning calorimetry measurements indicate that P(3HB-co-3HH) and P(3HB) are miscible for all blend ratios because a single glass transition temperature is observed. Polarization optical microscopy is used to investigate the transition of spherulite morphology and measure the radial growth rate of spherulites in the blend films. P(3HB-co-3HH) and P(3HB) contain positive spherulites, whereas in the binary blends, spherulite morphology changes from positive to negative. This change is related to the different growth rates of P(3HB-co-3HH) and P(3HB) lamellar crystals. Partial enzymatic degradation of the film surfaces reveals that the lamellar crystals of negative spherulites are oriented both perpendicular and parallel to the radial direction of spherulites. A new growth mechanism for spherulites in crystalline/crystalline blends is constructed from the results obtained for the blend films.     

Effect of deformation conditions of fibre-formation in spinning from heterogeneous mixtures of polymers

Conclusions Capillary diameter, shear stress, and jet stretch all affect the phenomenon of specific fibre formation in extrudates of the mixtures POM-copolyamide 548, POM-polystyrene, POM-ethylene-vinyl acetate copolymer, POM-polyvinyl alcohol, and low-pressure polyethylene-copolyamide 548.The optimum regimes found for deformation of mixture melts have been used in the development of technology for preparing ultrathin synthetic fibres and precision filters based on them.Translated from Khimicheskie Volokna, No. 3, pp. 28–31, May–June, 1983.     

Neutron scattering and amorphous polymers

During the past ten years neutron scattering has become a much more widely used technique. The use of neutron scattering to study the conformation and dynamics of polymer chains in the bulk amorphous state and in solution is reviewed here. The basic theory of neutron scattering is introduced. The types of instruments which are currently used and the factors affecting neutron scattering experiments are discussed. The following sections are each concerned with a different type of scattering experiment and the information which has been obtained. At the beginning of each of these sections the theory relating to the particular topic under discussion is introduced. The three topics covered by this review are: conformation studies of polymers; dynamics of polymer chains and studies of side group motion in polymers.     

Structure and properties of crystalline polymers

Starting with a structural study of the crystallization behaviour of poly(vinyl alcohol), the author has been analysing the crystal and molecular structures of crystalline polymers for the past 35 years. One of the characteristic points of the methods used is the co-operative use of X-ray diffraction and infrared and Raman spectroscopy (normal coordinate treatment). There are many examples of the application of this technique to a series of polyethers, polythioethers, polyesters, polymer complexes etc. Furthermore, the intra- and intermolecular energy calculations have succeeded in accelerating the structural analyses of important but complicated polymer materials and in revealing the factors governing the stable crystal structure and molecular conformation of a polymer. Poly(ethylene oxygenzoate) x form and double-stranded helices of isotactic poly(methyl methacrylate), the first double helix ever found for synthetic polymers, are used as typical for the application of this method. The structural interpretation of the mechanism of optical compensation in racemic polymers is also a good example. The energy calculations have developed to the stage where the stability of two crystal forms of polyethylene can be discussed in terms of free energy. Utilizing the structural data thus accumulated and the spectroscopically obtained interaction parameters, the structure-property relationship has been clarified quantitatively by lattice dynamical theory. The calculated crystallite moduli of polymer chains agree well with the observed ones for many polymers such as poly-p-phenylene terephthalamide, etc. The study has been advanced by a new method of calculation of the three-dimensional elastic constant tensor and its application to polyethylene, poly(vinyl alcohol), nylon 6 etc. As an extension the general method of calculating the piezoelectric constant tensor has also been derived and successfully applied to poly(vinylidene fluoride) form I, resulting in the interpretation of the origin of macroscopic piezoelectricity of this polymer.     

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.     

Correlation between interactions, miscibility, and spherulite growth in crystalline/crystalline blends of poly(ethylene oxide) and polyesters

Crystalline/crystalline blend systems of poly(ethylene oxide) (PEO) and a homologous series of polyesters, from poly(ethylene adipate) to poly(hexamethylene sebacate), of different CH₂/CO ratios (from 3.0 to 7.0) were examined. Correlation between interactions, miscibility, and spherulite growth rate was discussed. Owing to proximity of blend constituents' T_g's, the miscibility in the crystalline/crystalline blends was mainly justified by thermodynamic and kinetic evidence extracted from characterization of the PEO crystals grown from mixtures of PEO and polyesters at melt state. By overcoming experimental difficulty in assessing the phase behavior of two crystalline polymers with closely spaced T_g's, this work has further extended the range of polyesters that can be miscible with PEO. The interaction parameters (χ₁₂) for miscible blends of PEO with polyesters [poly(ethylene adipate), poly(propylene adipate), poly(butylene adipate), and poly(ethylene azelate) with CH₂/CO = 3.0-4.5] are all negative but the values vary with the polyester structures, with a maximum for the blend of PEO/poly(propylene adipate) (CH₂/CO = 3.5). The values of interactions are apparently dependent on the structures of the polyester constituent in the blends; interaction strength for the miscible PEO/polyester systems correlate in the same trend with the PEO crystal growth rates in the blends.     

Establishment of a solvent map for formation of crystalline and amorphous paclitaxel by solvent evaporation process

This study intended to establish a solvent map for formation of crystalline and amorphous paclitaxel by a solvent evaporation process. Crystalline paclitaxel was produced by evaporation with polar solvents (acetone, acetonitrile, ethanol, isobutyl alcohol, methanol, methyl ethyl ketone, and n-butyl alcohol) having a polarity index above 4.00. On the other hand, amorphous paclitaxel was produced by evaporation with non-polar solvents (methylene chloride, n-butyl chloride, and toluene) having a polarity index of about 4.00 or lower. The formation of paclitaxel was very closely associated with the polarity index of the organic solvent used in the solvent evaporation process. In the case of crystalline paclitaxel, the higher the polarity index and the lower the viscosity of the organic solvent (n-butyl alcohol, methyl ethyl ketone, and acetonitrile), the higher the degree of crystallinity. In the case of amorphous paclitaxel, the shape and size of particles varied according to the solvent (methylene chloride, n-butyl chloride, and toluene) used in the solvent evaporation process.     

Solvent mixtures sorption in amorphous peek

Summary The sorption kinetics and equilibrium isotherms of methylene chloride, n-heptane and of methylene chloride/n-heptane mixtures in glassy amorphous Poly(aryl-ether-ether-ketone) (PEEK) have been investigated. Ideal Fickian diffusion, anomalous non-ideal Fickian diffusion, “Case II” sorption mechanism and diffusion controlled swelling were observed depending on temperature, on solvent type and on external solvent activity. Gaschromatographic analysis performed on PEEK samples contacted with methylene chloride/n-heptane mixtures indicated that the presence of methylene chloride enhances n-heptane mobility and equilibrium sorbed amount. Dedicated to Prof. Dragutin Fleš on the occasion of his 70th birthday     

Sorption and diffusion of alcohols in amorphous polymers

The effects of polymer composition and penetrant molecular size on the solubility and diffusivity of alcohol vapors in a series of well characterized isoprene-methyl methacrylate copolymers and their corresponding homopolymers has been investigated at room temperature. The rate of sorption behavior changes progressively from Fickian to non-Fickian, to Case II to “Super Case II” transport with increasing methyl methacrylate (MMA) content in the polymers. The equilibrium solubility of the alcohols increases linearly with increasing penetrant molecular size for polymers which are above their glass transition temperature and decreases for polymers which are below their T_g. The solubility also initially increases as an approximately linear function of MMA content in the copolymers. At about 55 mole percent MMA, the sorbed concentration either levels off or passes through a maximum depending on the size of the penetrant. The apparent “diffusion coefficients” (D) decrease with increasing molecular volume of the penetrants. An exponential dependence was found between these two variables for PMMA. These “diffusion coefficients” also decrease exponentially with increasing MMA content in these polymers. However, at 55 mole percent MMA the copolymer undergoes a rubber to glass transition at the temperature of the experiments. On this basis, it is suggested that the hindered chain segmental motion contributes to the sorption process in addition to strictly thermodynamic considerations. Free volume theory can be used to explain the mechanism of diffusion through the rubbery polymers while the “hole” theory can be applied to explain the transport of the penetrants through the glassy polymers.     

Plastic deformation of crystalline polymers

Under uniaxial tensile load, the plastic deformation of unoriented crystalline polymers first transforms the lamellae into a fibrous structure. Usually the drawing is inhomogeneous with a neck propagating through the sample. The higher the draw ratio, the higher the axial elastic modulus as a consequence of the larger fraction of taut tie molecules in amorphous layers connecting the crystalline blocks of each microfibril. As a consequence of the almost 1/(1 ? α) times higher strain of amorphous layers under tensile load, the taut tie molecules are much more strained than the chains in crystal blocks. Hence, their contribution to elastic modulus is substantially higher than one would guess from their fraction β. This is more so in polyethylene with higher crystallinity (α = 0.8) than in nylon 6 with low crystallinity (α = 0.5). Even for the highest modulus polyethylene E = 70 GPa ～ 0.3 × E_c, one needs less than 7.5 percent of taut tie molecules. The plastic deformation of the fibrous structure markedly enhances the number of interfibrillar tie molecules in nylon 6 and to a lesser extent in polyethylene and polypropylene. Homogeneous drawing without a neck transforms the whole sample into a fibrous structure rather uniformly so that for a long while one has the lamellar and fibrillar morphology side by side. The end effect on the structure obtained does not differ appreciably from inhomogeneous drawing with neck propagation. The drawing of polymers with a liquid crystal structure yields a highly aligned fibrous structure with very few chain folds and an exceptionally high elastic modulus and strength. But the axial connection of individual highly oriented and ordered domains is affected by a relatively small fiaction of tie molecules, and this is responsible for reduction of the elastic modulus below the value of the ideal crystal lattice.     

Annealing of drawn crystalline polymers

The annealing of drawn samples mobilizes the almost fully extended amorphous tie molecules which try to assume the thermodynamically required random conformations. The sample shrinks if annealed with free ends which permits the crystal blocks on different microfibrils and connected by almost fully extended taut tie molecules to move towards the position they had before plastic deformation. Hence the annealed sample has irretrievably lost most of its high axial elastic modulus which in the sample as drawn was caused by the high fraction of taut tie molecules. With fixed ends no shrinkage is possible so that the partial relaxation of interfibrillar taut tie molecules still lets them connect far away blocks. If their fraction is large enough so that in spite of the high surface to volume ratio which drastically depresses the crystallization temperature they can crystallize they do so after cooling to room temperature. The new axial crystalline bridges restore the high elastic modulus of the material before annealing, partially stabilize the sample against shrinkage during a new annealing, but also cause the dead bend effect which is the consequence of the replacement of flexible taut tie molecules in still amorphous conformation by rigid crystalline bridges. The drawing or extrusion at high temperature produces some annealing effects comparable with those of cold drawn material annealed with fixed ends.     

Plastic deformation of crystalline polymers

Draw ratios have been measured for samples of polyethylene and trons-polyisoprene, crystallized at various temperatures and at various degrees of orientation. The values obtained range from unity, i. e., no drawing is observed, up to values of about 15X for materials crystallized in the oriented state and then drawn in a perpendicular direction. The results are in rough accord with a simple molecular network model in which network strands are incorporated into crystallites with a number of reversals of direction (folds), and the remainder of a strand between network junctions is randomly arranged. The reduction in draw ratio with increasing temperature of crystallization and with increasing orientation at the time of crystallization is then accounted for in terms of a reduction in the number of reversals (folds) per molecular strand. Differences in natural draw ratio for different polymers are attributed to variations in characteristic sequence length within a crystallite and in the number of folds per network strand.