Helical Chain Conformation Research Articles

Multiscale Modeling of Vinyl-Addition Polynorbornenes: The Effect of Stereochemistry.

Vinyl-addition polynorbornenes are candidates for designing high-performance polymers due to unique characteristics, which include a high glass transition temperature associated with a rigid backbone. Recent studies have established that the processability and properties of these polymers can be fine-tuned by using targeted substitutions. However, synthesis with different catalysts results in materials with distinct properties, potentially due to the presence of various stereoisomers that are difficult to quantify experimentally. Herein, we develop all-atom models of polynorbornene oligomers based on classical force fields and density functional theory. To establish the relationship between chemical architecture, chain conformations, and melt structure, we perform detailed molecular dynamics simulations with the fine-tuned atomistic force field and propose simpler coarse-grained descriptions to address the high molecular weight limit. All-atom simulations of oligomers suggest high glass transition temperatures in the range of 550-600 K. In the melt state (800 K), meso chains form highly rigid extended coils (C∞≈11) with amorphous structural characteristics similar to the X-ray diffraction data observed in the literature. In contrast, simulations with racemo chains predict highly helical tubular chain conformations that could promote assembly into crystalline structures.

Comprehensive Biodegradation Analysis of Chemically Modified Poly(3-hydroxybutyrate) Materials with Different Crystal Structures.

This work presents a comprehensive analysis of the biodegradation of polyhydroxybutyrate (PHB) and chemically modified PHB with different chemical and crystal structures in a soil environment. A polymer modification reaction was performed during preparation of the chemically modified PHB films, utilizing 2,5-dimethyl-2,5-di(tert-butylperoxy)-hexane as a free-radical initiator and maleic anhydride. Films of neat PHB and chemically modified PHB were prepared by extrusion and thermocompression. The biological agent employed was natural mixed microflora in the form of garden soil. The course and extent of biodegradation of the films was investigated by applying various techniques, as follows: a respirometry test to determine the production of carbon dioxide through microbial degradation; scanning electron microscopy (SEM); optical microscopy; fluorescence microscopy; differential scanning calorimetry (DSC); and X-ray diffraction (XRD). Next-generation sequencing was carried out to study the microbial community involved in biodegradation of the films. Findings from the respirometry test indicated that biodegradation of the extruded and chemically modified PHB followed a multistage (2-3) course, which varied according to the spatial distribution of amorphous and crystalline regions and their spherulitic morphology. SEM and polarized optical microscopy (POM) confirmed that the rate of biodegradation depended on the availability of the amorphous phase in the interspherulitic region and the width of the interlamellar region in the first stage, while dependence on the size of spherulites and thickness of spherulitic lamellae was evident in the second stage. X-ray diffraction revealed that orthorhombic α-form crystals with helical chain conformation degraded concurrently with β-form crystals with planar zigzag conformation. The nucleation of PHB crystals after 90 days of biodegradation was identified by DSC and POM, a phenomenon which impeded biodegradation. Fluorescence microscopy evidenced that the crystal structure of PHB affected the physiological behavior of soil microorganisms in contact with the surfaces of the films.

Специфика статистики упругих и деформационно-прочностных свойств высокопрочных волокон полипропилена

Regularities of statistical distributions of a complex of mechanical properties, including the module of elasticity (E), strength () and strain at break (b), high-strength industrial oriented polypropylene (PP) fibers have been analyzed using the Weibull and Gauss models based on large a wide array of measurements (50 identical samples in each series). The values of the statistical Weibull modulus (m) - a parameter characterizing the scatter of the measured values of the data arrays of E,  and b – have been estimated for the PP samples of two types: single fibers (monofilaments) and multifilament fibers consisting from several hundred single fibers. For the PP multifilament fibers, a more correct description of the distributions of E,  and b has been received both in the framework of the normal distribution (Gaussian distribution) and in the framework of the Weibull distribution in comparison with the description of such distributions for the PP monofilaments. The influence of the polymer chain conformation on the regularities of the statistical distributions of E,  and b for the high-strength oriented polymeric materials with different chemical chain structures and the correctness of their descriptions in the framework of the Gauss and Weibull models have been analyzed. For this purpose, the values of m calculated in this work for PP with a helical chain conformation have been compared with the values of m determined by us earlier for ultra-high molecular weight polyethylene and polyamide-6 with the chain conformations in the form of an in-plane trans-zigzag.

Specificity of statistics of elastic and strain-strength properties of high-strength polypropylene fibers

Regularities of statistical distributions of a complex of mechanical properties, including the modulus of elasticity (E), strength (sigma) and strain at break (εb), high-strength industrial oriented polypropylene (PP) fibers have been analyzed using the Weibull and Gauss models based on a large array of measurements (50 identical samples in each series). The values of the statistical Weibull modulus (m) --- a parameter characterizing the scatter of the measured values of the data arrays of E, sigma and εb have been estimated for the PP samples of two types: single fibers (monofilaments) and multifilament fibers consisting from several hundred single fibers. For the PP multifilament fibers, a more correct description of the distributions of E, sigma and εb has been received both in the framework of the normal distribution (Gaussian distribution) and in the framework of the Weibull distribution in comparison with the description of such distributions for the PP monofilaments. The influence of the polymer chain conformation on the regularities of the statistical distributions of E, sigma and εb for the high-strength oriented polymeric materials with different chemical chain structures and the correctness of their descriptions in the framework of the Gauss and Weibull models have been analyzed. For this purpose, the values of m calculated in this work for PP with a helical chain conformation have been compared with the values of m determined by us earlier for ultra-high molecular weight polyethylene and polyamide-6 with the chain conformations in the form of an in-plane trans-zigzag. Keywords: polypropylene, mechanical properties, statistical analysis, Weibull distribution, Gaussian distribution.

Ordered hexagonal columnar liquid crystalline self-assembly of mesogen-free sulfonylated side-chain chiral polyethers and their high dielectric property

To achieve large spontaneous polarization for ferroelectric liquid crystalline polymers, we have designed a series of mesogen-free isotactic polyethers with sulfonylated side chains. Different from prior studies using linear alkyl side chains, in this work a methyl branch was implemented in the alkyl side chains of the chiral polyethers. As a result of the continuous twist from the methyl branches and strong dipole-dipole interactions among neighboring sulfonyl groups, a helical chain conformation with tilted side chains was achieved. When the branched side chains were short (e.g., C5) and double chirality existed in both the main chain and side chains, a crystalline structure was obtained. When the branched side chains were long (e.g., ≥ C8) and single chirality existed in the main chain, an ordered hexagonal columnar liquid crystalline phase was induced. Their high-field dielectric property was studied by electric displacement-electric field (D-E) loop tests; however, no ferroelectric behavior was observed up to 200 MV/m. It was considered that the dipole-dipole interaction among neighboring sulfonyl groups in the helical chains was too strong to allow easy ferroelectric flipping.

Carbon molecular sieve membrane with tunable microstructure for CO2 separation: Effect of multiscale structures of polyimide precursors

Carbon molecular sieve (CMS) membranes were fabricated by designing the polyimides with different diamine moiety such as PPD, ODA, BAPP and BDAF to investigate the effects of multiscale structures of precursors on the microstructure and CO2 separation performance of the derived CMS membranes. The multiscale structures of polymeric precursor were characterized and simulated in terms of the characteristic of functional groups, the flatness of repeat units, the conformation of single chains, the packing efficiency of multi-chain network and the fractional free volume (FFV) of bulk polymers. The microstructure and CO2 separation performance of derived CMS membrane were characterized by XRD, TEM and gas permeation measurements. The CO2 permeation behavior of CMS membranes derived from different precursors was analyzed. Results indicated that the multiscale structures of polyimides significantly influenced the microstructure and CO2 separation performance of derived CMS membranes. Increased FFV (or reduced flatness of repeat units) in the precursor results in an increased disorder degree of carbon structure in the derived CMS membranes. The CMS membranes prepared by high FFV polyimide with helical chain conformation showed the looser carbon structure that are beneficial to CO2 transport compared with that prepared by polyimide with rod-like chain conformation. The larger size of the pendant functional groups (i.e. V-(CF3)2 > V-(CH3)2) in polyimide results in higher CO2 permeability of the derived CMS membranes. The BDAF-PMDA derived CMS membrane pyrolyzed at 700 °C show a high CO2 permeability (0.1 MPa of 50/50 mol.% mixed gas feed) of about 3000 Barrer and moderate CO2/CH4 selectivity, which exhibit an attractive application prospect for CO2 separation.

Insensitivity of Sterically Defined Helical Chain Conformations to Solvent Quality in Dilute Solution.

The interplay between polymer-polymer and polymer-solvent interactions as well as interactions that impose secondary structures determines the conformation of polymer chains in dilute solution. Polypeptoids-poly(N-substituted glycines) have been shown to form helical secondary structures primarily driven by steric interactions from chiral, bulky side chains, while polypeptoids with a racemic mixture of the same side chains lead to unstructured coil chains with a shorter Kuhn length. Small-angle neutron scattering (SANS) of the polypeptoids in dilute solution reveals that the helical polypeptoids are only locally stiffer than the coil chains formed from the racemic analogue, but exhibit overall flexibility. We show that chain conformations of both helical and coil polypeptoids (in terms of radius of gyration, Rg) are insensitive to solvent quality (parametrized by the second virial coefficient, A2). Potential effects from the bulky, chiral/racemic side chains dominating chain conformations are excluded by comparison with an achiral polypeptoid lacking side chain chirality. The specific interactions between polypeptoid segments are likely dominating the chain conformations in this type of polypeptoids as opposed to polymer-solvent interactions or energetic contributions from the helical secondary structure.

Grafting density-induced smectic A to hexagonal columnar transition in mesogen-free isotactic liquid crystalline polyethers with n-dodecylsulfonyl side groups

Highly dipolar mesogen-free liquid crystalline polymers are excellent candidates for achieving large spontaneous polarization under relatively low electric fields for advanced electrical and optical applications. In this report, the effect of grafting density of n-dodecylsulfonyl side chains on the mesogen-free liquid crystalline self-assembly of comb-like isotactic poly(oxypropylene) was studied. When the grafting density was 80–100%, a stable 21 helical chain conformation was induced by the strong dipole-dipole interactions among the sulfonyl groups in the side chains. Consequently, a smectic A self-assembly was induced. When the grafting density decreased to 60–80%, a poor and possibly imbalanced 21 helical chain conformation was formed due to the frequent absence of the n-dodecylsulfonyl side chains along the main chain. As a result, a hexagonal columnar self-assembly was realized. This study demonstrated that a delicate interplay between the molecular defects and dipole-dipole interactions can lead to different self-assembly structures.

Well-Defined Biomimicking Brush-Polymer Self-Assemblies Revealing Cholesterol- and Phosphorylcholine-Enriched Surface

We have newly synthesized a series of well-defined brush polyethers bearing cholesterol (Chol) and phosphorylcholine (PC) moieties in various compositions which can mimic cell membrane. They were thermally stable up to at least 230 °C and soluble in common solvents, showing good solution processability. Excitingly, they all favorably self-assembled, forming multibilayer structures with 21 chain conformation; in comparison, the brush polyether bearing only PC-bristles formed orthorhombically packed cylinder (OPC) structure with 125 helical chain conformation. Such multibilayer structure formations could be driven by a strong self-assembling ability of the Chol-bristle in extended conformation; the multibilayer structure formation was further promoted by the presence of PC-bristles. The OPC structure formation could be driven by a lateral packing ability of the brush polymer chain in the helical confirmation resulted from the minimization of repulsive interactions in the neighbored zwitterionic PC-bristles....

Characterization and bioactivities of a novel polysaccharide obtained from Gracilariopsis lemaneiformis.

Gracilariopsis lemaneiformis is a type of red alga that contains seaweed polysaccharide agar. In this study, a novel non-agar seaweed polysaccharide fraction named GCP (short of crude polysaccharide obtained from Gracilariopsis lemaneiformis) was isolated from Gracilariopsis lemaneiformis. Structural analysis showed that GCP shows triple helical chain conformation when dissolved in water and has many branches and long side chains. Also, 1→3 linkage is the major linkage and the sugar structures are galactopyranose configurations linked by β-type glycosidic linkages. Two macromolecular substance fractions (GCP-1 and GCP-2) were purified by DEAE Sepharose Fast Flow column chromatography. Moreover, a splenocyte damage assay and splenocyte proliferation assay were used to analyse the bioactivities of GCP, GCP-1 and GCP-2. It was demonstrated that polysaccharides could protect splenocyte damaged by H2O2; GCP-2 shows a greatest protection rate, that is, 92.8%, which significantly enhanced the splenocyte proliferation, and GCP showed the highest proliferation rate, 9.30%. The results suggested that this type of novel non-agar polysaccharide displayed remarkable antioxidant and immunomodulatory activities and early alkali treatment could decrease the activities. It may represent a potential material for health food and clinical medicines.

Structures and photoluminescence of coordination polymers assembled from bifunctional ligand containing both tetrazole and imidazole groups

Three new coordination polymers, namely, {[Cu2(IPT)(SO4)(OH)(H2O)]·H2O}n (HIPT=5-(4-(1H-imidazol-1-yl)phenyl)- 1H-tetrazolate, 1), {[Cd2(IPT)(NPA)(OH)]·H2O}n (H2NPA=5-nitroisophthalic acid, 2), and {[Zn2(IPT)(IDC)(H2O)]·3H2O}n (H3IDC=1H-imidazole-4,5-dicarboxylic acid, 3), were assembled from a bifunctional organic ligand containing both tetrazole and imidazole groups with/without the aid of carboxylate coligands. Compound 1 possesses 2D structure built by 1D [Cu2(IPT)(SO4)(OH)]n secondary building blocks and IPT− linkers. The 2D networks are linked into 3D supramolecular framework via water chains in helical conformation. Compound 2 displays 3D pillar-layer framework with 2D layers based on tetranuclear Cd(II) SBUs and NPA2− pillars. Compound 3 exhibits a 3D framework constructed from the interconnection of 1D [Zn-IDC]n chains and binuclear Zn2(IPT)2 rings. The thermal stabilities of porous compound 3 and luminescent properties of compounds 2 and 3 have also been studied in detail. They exhibit intense solid-state fluorescent emissions at room temperature.

Determination of the Triple Helical Chain Conformation of β-Glucan by Facile and Reliable Triple-Detector Size Exclusion Chromatography

Triple helical polysaccharides (t-polysaccharides) are easily gelated in water, resulting in difficult fractionation, leading to the complex and time-consuming chain conformational characterization. Moreover, the fractionation is not always successful due to the coexistence of individual chains and aggregates. In this work, we developed a facile and reliable method to rapidly and accurately characterize the chain conformation of t-polysaccharide without fractionation needed in traditional conformation characterization. A triple helical β-1,3-glucan (t-β-1,3-glucan), extracted from the fruiting bodies of Lentinus edodes, was identified to consist of a β-1,3-glucan with two β-1,6-D-glucopyranoside branchings for every five β-1,3-glucopyranoside linear linkages by one- and two-dimensional NMR and GC-MS analysis. The chain conformations of the t-β-glucan in aqueous solution and in DMSO were successfully characterized by a combination of size exclusion chromatography (SEC), multiangle static light scattering, a differential refractometer, and a capillary viscosity detector (triple-detector SEC). The results revealed that the predominate species of the t-β-glucan in a 0.15 M NaCl aqueous solution existed as a triple helical conformation with high chain stiffness, and a few aggregates (4%) coexisted here. The Mark-Houwink and ⟨S(2)⟩(1/2) versus M(w) equations of individual triple helical chains and aggregates were obtained simultaneously, and the results confirmed again the coexistence of two kinds of chain conformations. The fractal dimension indicated that the aggregate in the aqueous solution was a kind of reversible microgel with a 3D network structure. Furthermore, the chain morphology of the t-β-glucan in aqueous solution was observed directly by transmission electron microscopy and atomic force microscopy to support the worm-like chain for the individuals and 3D network for the aggregates. The triple-detector SEC technology was facile and reliable for the system with two fractions of different chain conformation, and the test time required was only 1/30 of what the traditional method needed.

Relationship between Morphological Change and Crystalline Phase Transitions of Polyethylene−Poly(ethylene Oxide) Diblock Copolymers, Revealed by the Temperature-dependent Synchrotron WAXD/SAXS and Infrared/Raman Spectral Measurements

The phase transition behaviors of low-molecular-weight polyethylene-poly(ethylene oxide) (PE-b-PEO) diblock copolymers with the monomeric units of PE/PEO = 17/40 and 39/86 have been successfully investigated through the temperature-dependent measurements of wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), infrared and Raman spectra, as well as thermal analysis. These diblock copolymers had been believed to show only order-to-disorder transition of lamellar morphology in a wide temperature region, but it has been found here for the first time that this copolymer clearly exhibits the three stages of transitions among lamella, gyroid, cylinder, and spherical phases in the heating and cooling processes. The WAXD and IR/Raman spectral measurements allowed us to relate these morphological changes to the microscopic changes in the aggregation states of PEO and PE segments. In the low-temperature region the PEO segments form the monoclinic crystal of (7/2) helical chain conformation and the PE segments of planar-zigzag form take the orthorhombic crystalline phase. These crystalline lamellae of PEO and PE segments are alternately stacked with the long period of 165 Angstroms. In a higher temperature region, where the PEO crystalline parts are on the way of melting but the PE parts are still in the orthorhombic phase, the gyroid morphology is detected in the SAXS data. By heating further, the gyroid morphology changes to the hexagonally packed cylindrical morphology, where the orthorhombic phase of PE segments is gradually disordered because of thermally activated molecular motion and finally transforms to the pseudohexagonal or rotator phase. Once the PE segments are perfectly melted, the higher-order structure changes from the cylinder to the spherical morphology. These morphological transitions might relate to the thermally activated motions of two short chain segments of the diblock copolymer, although the details of the transition mechanism are unclear at the present stage.

Deformation-induced highly oriented and stable mesomorphic phase in quenched isotactic polypropylene

Changes of structure and morphology of quenched isotactic polypropylene (i-PP) film during tensile deformation at room temperature have been investigated by applying in situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. SAXS patterns show that the isotropic diffuse scattering peak in the initial quenched i-PP film disappears after the film experiences necking and a strong streak, which accompanies obvious film stress-whitening, appears when strain reaches 600% and above. Explanation to the former is the dropping of density contrast between amorphous and mesomorphic phases due to deformation and that to the latter is the formation of microfibrils and/or microvoids in the film. WAXD patterns indicate that i-PP chains in the mesomorphic phase become highly oriented along the drawing direction with the strain value more than 5% and the three-fold helical chain conformation keeps in the deformed mesomorphic phase. The mesomorphic structure in the initial quenched i-PP film does not transform into crystal phase although high orientation appears during tensile deformation and the highly oriented mesomorphic phase is considered to be quite stable. Thermal behaviors of the initial quenched i-PP and deformed i-PP with the strain value of 650% are examined by modulated differential scanning calorimetry (MDSC). MDSC results show that the highly oriented i-PP chains in deformed mesomorphic phase are stable, which brings about restriction to chain mobility and prohibits from phase transformation of mesophase to crystal phase in the temperature range normally measurable for the initial quenched i-PP. A model about tensile deformation of the quenched i-PP is outlined to distinctly illuminate the changes of structure and morphology in both the amorphous and mesomorphic phases.

Carbon nanotube induced structural and physical property transitions of syndiotacticpolypropylene

In this paper we have studied the effect of increasing carbon multi-wallednanotube (CNT) concentration in composites of syndiotactic polypropylene (sPP)having the same crystalline form but different morphologies. The attention wasfocused on the form I of sPP with different degrees of perfection (in terms ofpercentages of chains in helical conformation, crystal dimensions and crystallinity)obtained using two different quenching temperatures from the melt, i.e. 25 and100 °C. We observed a decreasing effect of the crystallization temperature on increasing thenanotube content up to the samples with 10% of CNT, that show a very similar structuralorganization independent of the undercooling. Only the amorphous phase turnsout more relaxed in the samples crystallized at the highest temperature. Eitherthe thermal or the mechanical properties are improved on increasing the CNTcontent in both series of samples. The electrical conductivity increases in a similarmanner in both series of samples and between 1 and 3 wt% it shows a sizablestep of about eight orders of magnitude, a phenomenon that can be regarded asthe onset of a percolating structure for which charge transport may take place.

In situ FT‐Raman spectroscopic study of the conformational changes occurring in isotactic polypropylene during its melting and crystallization processes

AbstractThe conformational changes occurring in isotactic polypropylene during the melting and crystallization processes have been carefully investigated using FT‐Raman spectroscopy at temperatures below, at, and above the polymer melting point. Results confirmed the retention of some crystallinity up to +210 °C, which is 50 °C above the melting point. It was found that, at temperatures just above the melting point (1–10 °C), there is still some short range order of at least 12 monomer units long in certain regions of the melt. At 10 °C above the melting point, the short range order drops below 12 monomer units resulting in the disappearance of the Raman band at 841 cm–1. Vice versa, the experimental measurements show that the iPP melt system is stable when the persistence length of helical sequences is less than 12 monomer units. As soon as the helix length exceeds 12 units, the 31 helix conformation extends quickly and then crystallization occurs. These results are discussed in terms of Imai's microphase separation theory and it agreed very well with it. Also, from our observations for correlation splitting, Raman bands related to conformational states were identified. This analysis indicates the existence of three different conformational states at 808, 830, and 841 cm–1. The 808 cm–1 band was assigned to helical chains within crystals (representing crystalline phase). The 841 cm–1 band was shown to be composed of a band at 841 cm–1, assigned to shorter chains in helical conformation with isomeric defects (representing the isomeric defect phase), and a broader band at 830 cm–1 assigned to chains in nonhelical conformation (representing the melt‐like amorphous phase). This indicates the detection of a three‐phase structure in iPP, where a third phase could be due to the presence of defect regions within the crystalline region, or due to the presence of an amorphous–crystal interphase. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2173–2182, 2006

Trichroic Vibrational Analysis on the α-Form of Poly(lactic acid) Crystals Using Highly Oriented Fibers and Spherulites

Infrared and Raman spectra of the PLA α-crystal were analyzed. Detailed analysis is only possible by using highly oriented electrospun fiber mat. Infrared dichroic analysis of large two-dimensional PLA crystals was also conducted. By combining the two dichroic analyses, accurate band assignments for an α-crystal were achieved for the first time. This analysis is only consistent with a 10/3 helical chain conformation, as opposed to the more simplistic 3/1 helix generally used. Infrared and Raman activities of the PLA 10/3 single-chain helix were derived and correlated to the orthorhombic α-crystal. The orthorhombic structure, previously suggested from X-ray studies, was confirmed by vibrational analysis. Poorly understood spectral features, such as the unexpectedly large number of carbonyl stretching vibrations, are attributed to crystal field splitting. An additional Raman component has its origin due to Fermi resonance interactions.

Temperature and Orientation Induced Polymorphic Behavior of Syndiotactic Polypropylene

Syndiotactic polypropylene (sPP) was rapidly quenched from the melt to room temperature, obtaining films in the disordered form I, with the chains in helical conformation. A significant portion of chains in the trans-planar mesophase was found, as evidenced by X-ray diffraction and FTIR spectroscopy. The quenched films were drawn to a draw ratio of 6 at 25, 60, and 90 °C and analyzed before and after releasing the tension. A large shrinkage was observed unhooking the fibers, and this parameter, expressed as (lmax − lf)/lmax, almost linearly decreased on increasing the drawing temperature. The crystalline phase composition was investigated by diffraction techniques in both the fixed and relaxed fibers. The fully extended fibers drawn at all the investigated temperatures contain a significant amount of the trans-planar mesophase while the fraction of the crystalline trans-planar form III decreases on increasing the drawing temperature and it is absent in the fiber drawn at 90 °C. Upon relaxation, we found evidence that the trans-planar mesophase in the fixed fiber crystallizes into form II while form III in the fixed fibers tends to transform into the trans-planar mesophase. An infrared and dynamic-mechanical analysis on the fibers was also performed to confirm the structural results. The analysis evidenced that on increasing the drawing temperature, the mesophase in the fixed fibers more and more transforms into the helical form II. On the other hand trans-planar chains remain in the amorphous fraction of all the relaxed fibers. The experimental results allow one to better clarify the role of the trans-planar mesophase in phase transitions of sPP and, taking into account also literature data, to propose a more detailed scheme of transformations between different polymorphs of this polymer.

Structural and Morphological Changes during UV Irradiation of the Crystalline Helical Form of Syndiotactic Polypropylene

Syndiotactic polypropylene films, in the most usual crystallographic modification (form I), were exposed to accelerated weathering in a UV device at 45 °C for increasing times. A different series of films characterized by the same structural organization were annealed at 45 °C to discern the thermal effects from those due to UV irradiation. The influence of UV irradiation and thermal treatments on the structural changes in terms of phase composition, crystallite sizes, and chain conformations was investigated. The analyzed samples were crystallized from the melt at 25 °C, obtaining the disordered crystalline form I, with chains in helical conformations, in large amounts, and a small fraction of mesophase with chains in trans-planar conformations. Diffractometric and thermal analysis have shown that UV radiation causes a strong increase in the crystallinity degree within the first 170 h of treatment. Such an increase mainly occurs by an increase in the coherent crystalline domain size in the direction perpendicular to the (200) planes. Spectroscopic data (FTIR) confirm that the increase in the crystallinity degree occurs not only by a strong reduction of the amorphous phase and a consistent increase of the conformational order of the chains in helical conformation but also by a reduction of the domains with chains in the trans-planar conformation. The trans-planar domains, very small and arranged in the amorphous matrix, are attached by UV radiation together with the amorphous phase. The observed phenomena are to be attributed to the effects of chain scission due to the photooxidation. Samples only annealed at 45 °C show only small effects of physical aging within the first 10 h of treatment.

A critical assessment of unbalanced surface stresses as the mechanical origin of twisting and scrolling of polymer crystals

Twisting of polymer lamellae manifested by e.g. appearance of concentric bands in polymer spherulites examined in a polarized optical microscope remains a topic of research and controversy. It has been interpreted variously as resulting from phenomena that take place during growth or from structural features of individual lamellae, or multilamellar aggregates. Phenomena that take place during growth are of general, or even generic character. They include non-linear diffusion processes leading to rhythmic crystallization, or self-induced compositional or mechanical fields generated near the advancing crystal front. Structural features include cumulative reorientation of lamellae at successive isochiral screw dislocations (possibly linked with surface pressure exerted by cilia) or different surface stresses on opposite fold surfaces of individual lamellae, as a result of different levels of congestion of folds.This contribution reviews evidence that has accumulated in favor of lamellar twist induced by surface stresses that result from differential congestion of fold surfaces, as suggested initially (in 1984) and advocated for many years by Keith and Padden. Such differences in fold surface structure are occasionally amenable to experimental (even if only qualitative) verification, as illustrated by polymer decoration of polyethylene single crystals. Twist is expected when a two-fold symmetry parallel to the growth direction exists in the lamellar structure (crystalline core and fold surface). This symmetry often stems from chirality: most frequently atomic (configurational) or stem (conformational) chirality but chirality (or at least asymmetry) may also be introduced by chain tilt.Possible origins of twisting in chiral polymers are also reviewed. In β sheets of fibrous proteins, the origin of twist stems from the atomic chiral centers in the crystalline core of the lamellae and its transfer to higher structural levels via the strong structural identity of the hydrogen-bonded β sheets. However, in a series of synthetic liquid-crystalline main-chain nonracemic chiral polyesters, the lamellar twist sense depends on the odd or even numbers of atoms in the aliphatic segment. For these and other more flexible chiral polymers, often with helical chain conformation, twisting appears to result from surface stresses associated with different fold structure or conformations at opposite fold surfaces, as suggested by a preliminary analysis of the Form III of isotactic poly(1-butene). Such differences in fold conformations result from, but are not directly related to, the specific helical hand of the polymer since they rest on the details of the chain conformation as it reaches the fold surface. This analysis accounts for the lack of one-to-one correspondence between configurational or conformational chirality of the polymer and lamellar twist sense (the one-to-one correspondence applies however for stereoenantiomers of a given polymer).Twist is not the only known non-planar geometry of polymer lamellae. In a few cases, the lamellae are scrolled. Scrolling of polymer lamellae is also easily accounted for by the existence of surface stresses when the two-fold symmetry parallel to the growth direction is absent. Such surface stresses are again linked to disparities in fold volume, as first suggested for poly(vinylidenefluoride) in its γ Form and later for two long paraffins substituted near their middle carbon atom and that crystallize in hairpin fashion, and for scrolled crystals of polyamide 66.The different nature and structure of polymer crystal fold surfaces, therefore, offer an unusual opportunity to decouple surface and bulk contributions and to analyze the origin of non-planar lamellar geometries at a sub-molecular level. Fold structure disparities and resulting unbalanced surface stresses provide a unified explanation for the formation of non-planar (both twisted and scrolled) lamellar crystals. They account for both the diversity of lamellar morphologies produced under the same crystallization conditions and for the similarity of lamellar morphologies produced under very different crystallization conditions.