Formation of spherulities in polyamides. IV. Even–odd polyamides and poly(ω-aminocarboxylic acids)

Isothermal and nonisothermal spherulitic crystallization in even–odd and poly(ω-aminocarboxylic acids) has been studied for a range of fusion conditions and solidification temperatures. The variety of spherulites so formed are classified mainly by optical microscopy. It is found that polyamides with similar types of repeat units generally exhibit similar morphological features under corresponding crystallization conditions. The basic patterns are illustrated in the text. Changes in spherulitic birefringence with temperature are also discussed. In some cases, at temperatures not far below the polymer melting point, platelet-like crystalline aggregates are formed in thin film preparations. These platelets exhibit properties characteristic of single crystals.     

Solution‐crystallization and related phenomena in 9,9‐dialkyl‐fluorene polymers. II. Influence of side‐chain structure

Solution‐crystallization is studied for two polyfluorene polymers possessing different side‐chain structures. Thermal analysis and temperature‐dependent optical spectroscopy are used to clarify the nature of the crystallization process, while X‐ray diffraction and scanning electron microscopy reveal important differences in the resulting microstructures. It is shown that the planar‐zigzag chain conformation termed the β‐phase, which is observed for certain linear‐side‐chain polyfluorenes, is necessary for the formation of so‐called polymer‐solvent compounds for these polymers. Introduction of alternating fluorene repeat units with branched side‐chains prevents formation of the β‐phase conformation and results in non‐solvated, i.e. melt‐crystallization‐type, polymer crystals. Unlike non‐solvated polymer crystals, for which the chain conformation is stabilized by its incorporation into a crystalline lattice, the β‐phase conformation is stabilized by complexation with solvent molecules and, therefore, its formation does not require specific inter‐chain interactions. The presented results clarify the fundamental differences between the β‐phase and other conformational/crystalline forms of polyfluorenes. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1492–1506     

Formation of spherulites in polyamide melts: Part III. Even-even polyamides

A preliminary study of the isothermal crystallization of “even-even” polyamides reveals striking similarities in spherulitic morphology. The observed variations in textural features of spherulites of nylon 210, nylon 66, nylon 610, and nylon 1010 show parallel changes during growth conditions and conform to a definite sequence of behavior. At least four different types of spherulites exist in each polymer. Optical and x-ray techniques were used to examine some of these spherulites. Crystalline platelets possessing single-crystal properties have been grown in thin films of these polymers near their respective melting points.     

Survey of the long spacing of polyamides crystallized from solution

A series of linear, aliphatic polyamides in which the number of carbon atoms in the repeat unit ranged from three to twenty-four was crystallized from solution. All gave lath-shaped crystallization products (usually aggregated in the form of sheaves) that were unmistakable lamellar. Sedimented mats of the crystals were examined by lowangle and wide-angle x-ray diffraction. Each polyamide had a characteristic layer thickness (fold length) which was determined by the length of the repeat unit and the number of hydrogen bonds in the lamella. The thickness was independent of other variables examined including crystallization conditions. The polyamides studied cover a wide range: they border on polypeptides at the one extreme and approach polyethylene at the other. For all these materials there emerged a unifying pattern which relates chemical structure directly to chain folding.     

Stereocontrol of diene polymers by topochemical polymerization of substituted benzyl muconates and their crystallization properties

We report the stereocontrol of diene polymers by the topochemical polymerization of alkoxy-substituted benzyl muconates in the solid state. A monomer stacking structure is controlled by the weak intermolecular interactions in the monomer crystals, depending on the structure and position of the alkoxy-substituent. The translational and alternating types of molecular stacking structures in a column provide diisotactic and disyndiotactic polymers, respectively, by the solid-state polymerization under UV and γ-ray irradiation. On the other hand, the meso and racemo structures of the resulting polymers are determined by the molecular symmetry of the used muconate monomers. The various substituted benzyl ester polymers are transformed into the same ethyl ester polymers with the four types of tacticities. The structure and crystallization behavior of the substituted benzyl ester polymers as well as the ethyl ester polymers have been revealed in detail. We clarify the effects of the tacticity on the crystallization property of the stereoregular polymuconates. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4952–4965, 2006     

Polydispersity of ethylene sequence length in metallocene ethylene/α‐olefin copolymers. II. Influence on crystallization and melting behavior

In this work, crystallization and melting behavior of metallocene ethylene/α‐olefin copolymers were investigated by differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The results indicated that the crystallization and melting temperatures for all the samples were directly related to the long ethylene sequences instead of the average sequence length (ASL), whereas the crystallization enthalpy and crystallinity were directly related to ASL, that is, both parameters decreased with a decreasing ASL. Multiple melting peaks were analyzed by thermal analysis. Three phenomena contributed to the multiple melting behaviors after isothermal crystallization, that is, the melting of crystals formed during quenching, the melting‐recrystallization process, and the coexistence of different crystal morphologies. Two types of crystal morphologies could coexist in samples having a high comonomer content after isothermal crystallization. They were the chain‐folded lamellae formed by long ethylene sequences and the bundlelike crystals formed by short ethylene sequences. The coexistence phenomenon was further proved by the AFM morphological observation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 822–830, 2002     

Morphology and properties of low-density (branched) polyethylene

The different types of morphology that can be developed in a large number of low-density (branched) polyethylene whole polymers, as well as in a series of fractions, have been studied for two different extreme crystallization modes. Concomitantly, thermodynamic properties of the same samples have also been determined. After isothermal crystallization at elevated temperatures, spherulitic structures are found in all the whole polymer samples. On the other hand, after rapid crystallization a variety of different types of supermolecular structures are observed which are shown to depend systematically on the concentration of side-chain branches and the relative proportion of high molecular weight species in the sample. This temperature dependence of the morphological forms is opposite to that previously reported for linear polyethylene. The studies with the fractions show that the individual species are not the cause of this behavior; rather, the total composition is the important factor. The thermodynamic properties are also quite different from those of linear polyethylene in showing virtually no molecular weight dependence and being governed primarily by the concentration of short-chain branches. The degrees of crystallinity as determined from density and enthalpy of fusion measurements do not vary much with the two extreme crystallization conditions employed, are not sensitive to the morphology, and differ from one another, even when well-developed spherulites are formed. A major influence of the branching concentration on these properties is clearly indicated.     

Synthesis of Poly(tartar amides) as Bio‐Inspired Antifreeze Additives

Summary: Multifunctional poly(tartar amides) have been synthesized and used as bio‐inspired antifreeze additives. It is shown that these polymers strongly interfere with the crystallization process of water in comparison to commercially available commodity polymers. While the addition of the poly(tartar amides) results in minor freezing point depression, as is shown by differential scanning calorimetry, a strong change in the ice crystal morphology is evident. Wide‐angle X‐ray scattering and optical microscopy indicate that the hexagonal structure of undisturbed ice‐crystals is oriented and partly deformed.

Light microscopy image of ice crystals at 223 K after a freezing assay with poly(tartar amides) shown at a polymer concentration of 2 wt.‐%.     

A novel generation of photoactive comb‐shaped polyamides for the photoalignment of liquid crystals

A new approach to the synthesis of photoactive comb‐shaped homo‐ and copolyamides containing azobenzene, cinnamate, and coumarin side groups for photoalignment of liquid crystals was elaborated. Photooptical properties and photoorientational ability of these polymers with respect to liquid crystals were studied. It was shown that polarized UV irradiation of all spin‐coated polyamides leads to orientation of liquid crystalline molecules deposited on the polyamide thin films. The synthesized polymers containing cinnamate and coumarin side groups as well as azobenzene‐containing cyano‐ and nitro‐substituted polymers demonstrated good orientation ability in relation to liquid crystals displaying photoinduced planar orientation with high dichroism values within the range of 0.68–0.72. Contrary to the above‐mentioned polyamides, azobenzene‐containing fluorosubstituted polymers induced a homeotropic orientation of liquid crystals. It was shown that the synthesized photoactive polyamides can be considered as promising photoalignment materials for application in display technology, photonics, and other “smart” optical devices. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4031–4041     

Effect of crystallization and liquid–liquid phase separation on phase‐separation kinetics in poly(ethylene‐co‐vinyl alcohol)/glycerol solution

Liquid–liquid thermally induced phase separation of the polymer‐diluent system of poly(ethylene‐co‐vinyl alcohol) (EVOH)‐glycerol was examined under light scattering. For EVOH with an ethylene content of 38 mol % (EVOH38), maxima of the scattered light intensity were observed that indicated that phase separation occurred by the spinodal decomposition (SD). The growth of the structures formed by the general liquid–liquid phase separation obeyed a power‐law scaling relationship in SD. For EVOH with an ethylene content of 32 mol % (EVOH32), the liquid–liquid phase separation resulted from the polymer crystallization. In this case, the structure growth showed the characteristic behavior in which the crystalline particles were initially formed, and then the droplets formed by the liquid–liquid phase separation induced by the crystallization grew rapidly. Furthermore, the growth of the droplet by the phase separation was followed by an optical microscope measurement at a constant cooling rate. The phase‐separated structure formed after the crystallization can grow faster than that formed by the normal liquid–liquid phase separation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 194–201, 2003     

The separation of racemic crystals into enantiomers by chiral block copolymers

A series of chiral double hydrophilic block copolymers (DHBCs) was synthesized and employed as additives in the crystallization of calcium tartrate tetrahydrate (CaT). We found that appropriate polymers can slow down the formation of the thermodynamically most stable racemic crystals as well as the formation of one of the pure enantiomeric crystals so that chiral separation by crystallization occurs even when racemic crystals can be formed. In addition, the presence of DHBCs results in major modifications of crystal morphology, creating unusual morphologies of higher complexity. Our study demonstrates the potential application of chiral DHBCs in the control of chirality throughout crystallization, in particular for racemic crystal systems, and also shows that enantiomeric excess of one enantiomer can be maximized by the kinetic control of crystallization.     

Structural evolution in microbial polyesters

The crystallization behavior of microbially synthesized poly(3-hydroxybutyrate) (PHB) and its copolymers [P(HB-co-HHx)] containing 2.5, 3.4, and 12 mol % 3-hydroxyhexanoate (HHx) comonomer and the melting of the resultant crystals were studied in detail using time-resolved small-angle X-ray scattering and differential scanning calorimetry. The polyesters were found to undergo primary crystallization as well as secondary crystallization. In the primary crystallization, the thicknesses of the lamellar crystals were sensitive to the crystallization temperature, but no thickening was observed throughout the entire crystallization at a given temperature. The thickness of the lamellar crystals in the PHB homopolymer was always larger than that of the amorphous layers. In the copolymers, by contrast, the randomly distributed HHx comonomer units were found to be excluded from the lamellar crystals into the amorphous regions during the isothermal crystallization process. This interrupted the crystallization of the copolymer chains, resulting in the formation of lamellar crystals with thicknesses smaller than those of the amorphous layers. The lamellar crystals in the copolymers had lower electron densities compared to those formed in the PHB homopolymer. On the other hand, secondary crystallization favorably occurred during the later stage of isothermal crystallization in competition with the continuous primary crystallization, forming secondary crystals in amorphous regions, in particular in the amorphous layers between the primarily formed lamellar crystal stacks. Compared to the primarily formed lamellar crystals, the secondary crystals had short-range-ordered structures of smaller size, a broader size distribution, and a lower electron density.     

Exactly alternating silarylene–siloxane polymers. VII. Thermal stability and degradation behavior of p-silphenylene–siloxane polymers with methyl,vinyl, hydrido,and/or fluoroalkyl side groups

The thermal stability and degradation behavior of a series of nine different exactly alternating silphenylene-siloxane polymers which contained methyl, vinyl, hydrido, 3,3,3-trifluoropropyl, and tridecafluoro-1,1,2,2-tetrahydrooctyl side groups, or their combinations, were investigated by dynamic and isothermal gravimetric analyses in air and in nitrogen. Two distinctly different mechanisms were observed in these atmospheres: a complex multi-step weight loss process in air and a single-step process in nitrogen. In nitrogen all polymers produced black, insoluble, highly stable degradation residues which were characterized by high carbon content. In contrast, in air the nonfluorine containing polymers degraded to pure silica, while the fluoroalkyl substituted polymers may have formed fluorosilicates of unspecified structures. There appears to be no significant molecular weight effect on the thermal stability of these polymers, at least not above an M _w value of about 35,000. Isothermal investigations indicate that 300°C in air and 350°C in nitrogen may be possible upper use temperatures for the methylvinyl substituted, exactly alternating silphenylene–siloxane polymers for extended periods of time. A strong thermostabilizing effect by vinyl side groups on the degradation behavior of these polymers was established. The extent of stabilization depends on the content of vinyl units, but it can already be clearly seen at the 5 mol % vinyl level, and it increases exponentially with increasing vinyl concentration. In contrast to this behavior, by comparison with the parent all-methyl substituted, exactly alternating silphenylene–siloxane polymers, the hydrido and fluroalkyl side groups reduce overall polymer thermal stability in terms of the degradation onset temperature, the temperature for 50% weight loss, and the amount of degradation residue. The presence of these groups also extends the later stages of the degradation processes to higher temperatures. Based on these and previous results, an order of stability is proposed as a function of the type of the substituent side groups for the thermal degradation of these polymers.     

Extended-chain crystals. I. General crystallization conditions and review of pressure crystallization of polyethylene

This is the first paper of a series of reports concerning extended-chain crystals of flexible, linear high polymers. The general conditions for crystal growth are discussed. Polymer crystallization is described as a two-step process: nucleation of each crystallizing molecule to a folded-chain conformation, followed by an increase in fold length in a solid-state reorganization step. This reorganization step is enhanced in the case of polyethylene by crystallization at high temperature under elevated pressure. Mechanical deformation during crystallization is also able to produce extended-chain crystals. The most promising method, however, is crystallization during polymerization. Previous work on crystallization of polyethylene under elevated pressure is critically reviewed.     

Mechanism of formation of shish kebab structures

To describe the characteristic crystalline structure of polyolefins, Pennings first proposed a model consisting of a combination of an extended chain crystal (a “shish”) and folded chain crystals (“kebabs”). In Pennings' model the “shish” forms first during a crystallization process under stress and then later the “kebabs” overgrow this “shish” structure epitaxially. Because we had some doubts about such a mechanism, we undertook a series of experimental studies on linear polyethylene, particularly in regard to the crystallization process from a solution under shear. Our conclusion is that the crystals grow first by a screw dislocation mechanism, like whiskers, and then later these are deformed by the shear stress to form the shish kebab structures.     

Self‐Assembly of Aminocyclopropenium Salts: En Route to Deltic Ionic Liquid Crystals

Aminocyclopropenium ions have raised much attention as organocatalysts and redox active polymers. However, the self‐assembly of amphiphilic aminocyclopropenium ions remains challenging. The first deltic ionic liquid crystals based on aminocyclopropenium ions have been developed. Differential scanning calorimetry, polarizing optical microscopy and X‐ray diffraction provided insight into the unique self‐assembly and nanosegregation of these liquid crystals. While the combination of small headgroups with linear p‐alkoxyphenyl units led to bilayer‐type smectic mesophases, wedge‐shaped units resulted in columnar mesophases. Upon increasing the size and polyphilicity of the aminocyclopropenium headgroup, a lamellar phase was formed.     

Fine structure of polyethylene prepared by γ-ray-induced polymerization in various solvents. III. Selective degradation with ozone

Ozone-degradation products of polyethylenes prepared by γ-ray-Induced polymerization in ethyl alcohol, t-butyl alcohol, and 2,2,5-trimethylhexane were analyzed by gel permeation chromatography (GPC). The ozonized polymers show two discrete peaks in the GPC traces, and the ratio of molecular weights corresponding to the peaks is close to 1:2, suggesting that these polymers are composed of folded-chain crystals similar to solution-grown single crystals. The peak profiles, however, are broad and the peaks in the chromatograms shift concertedly toward lower molecular weight in the course of degradation. These findings suggest that the fold surfaces of the polymers are coarse and that there is no unique crystalline core containing a regularly folded layer. The long period estimated from small-angle x-ray measurements increases with increasing polymerization temperature, but scarcely varies with the solvent type. This implies that when crystallization occurs during polymerization, kinetic factors may have no great effect on crystallization.     

Spherulites from polyamides with a structure characterized by three hydrogen‐bond directions

Polyamides constituted by glycine residues have a peculiar structure characterized by the establishment of intermolecular hydrogen bonds along three different directions. Spherulites of polyglycine and nylons 2/3, 2/3/3, 2/6, and 2/11 have been obtained from evaporation of concentrated solutions or from the molten state. In all cases, a negative birefringence was detected. This fact differs from the observations on spherulites of conventional nylons where intermolecular hydrogen bonds are formed along a single direction and sheet structures are postulated. Polarizing light, scanning electron, and transmission electron microscopy showed that banded spherulites are usually formed in the crystallization of nylons 2/n. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1719–1726, 2002     

Structure–dynamics relationships of the α‐relaxation in flexible copolyesters during crystallization as revealed by real‐time methods

The evolution of the α‐relaxation during an isothermal crystallization process of a series of flexible copolyesters of hydroxybutyrate (HB) and hydroxyvalerate (HV) has been followed in real‐time by wide‐angle X‐ray scattering and dielectric complex permittivity measurements. The change of the dielectric parameters with crystallization time can be phenomenologically described in terms of the Havriliak‐Negami equation. The dielectric strength follows a sigmoidal‐shaped pattern similar to that shown by the crystallinity. A reduction of the overall mobility with crystallization time of the polymeric chains in the amorphous phase has been observed. This slowing down effect depends on the HV molar content. The influence of the chain flexibility on the crystalline‐induced restriction has been discussed in the light of similar studies carried out with more rigid polymers. Dielectric experiments suggest that the progressive immobilization of polymer segments as crystallization proceeds cannot be exclusively associated with the amount of crystalline material. Differences in microstructure, depending on the HV molar content, seem to be responsible for the observed behavior. The progressive broadening and symmetrization of the α‐relaxation with increasing crystallization time has been explained as due to a restriction of the large‐scale motions of the polymeric chains, as the material is being filled in with crystals. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 37–49, 1999     

Mesogenic polymers. III. Thermal properties and synthesis of three homologous series of thermotropic liquid crystalline “backbone” polyesters

Details of the thermal behavior, including transition temperatures, transition heats and transition entropies, are reported for three series of nematic liquid-crystalline “backbone” polyesters having the general structure All polysters described above were examined by differential scanning calorimetry and were found to have reproducible thermal behavior once the polymer had been annealed by heating to the isotropic phase and then subsequently cooled. Enantiotropic nematic phases were found for all 30 polysters studied. Many polymers showed multiple endotherms on melting to the nematic phase. Plots of solid–nematic and nematic–isotropic transition temperatures versus number of carbon atoms in the diacid segment (x) for each (y) reveal an even–odd alternation reminiscent of trends in homologous series of small-molecule liquid crystals. Enthalpies for the solid → nematic and nematic → isotropic transitions do not show such a precise alternation.