Linear Macromolecules Research Articles

Termination of crystallization or ordering of flexible, linear macromolecules

This review concerns the “termination of crystallization or ordering of flexible, linear macromolecules” before the transition from the amorphous phase reaches thermodynamic equilibrium. It makes use of the precision of hindsight in interpretation of old experiments and the back-integration of more recent experiments into the knowledge gained from the well-known older experiments which had led to the paradox: Once the semi-ordered sample is produced, its disordering frequently follows a zero-entropy-production path, i.e., its latent heat is linked to the free enthalpy of the non-equilibrium phase, while on ordering, there exists a metastable temperature region of the polymer melt which cannot be broken by nuclei of the ordered phase. The classic scheme of crystallization via nucleation and growth is used to set the stage for the discussion. This scheme has been used for many years to describe the motion of single motifs to crystallize small, rigid molecules and its slow-down when approaching the glass transition. For flexible macromolecules, the ordering mechanism needs to be expanded to the description of cooperative ordering schemes of more than one motif of the molecular segments and a more complicated, multiple-step slow down when approaching the much wider glass transition region. The structural features causing the incomplete ordering of flexible macromolecules are the three-dimensional defects created at the phase boundaries between ordered and disordered phases, initially called the amorphous defects. The matter contained in these amorphous defects possesses a much broader glass transition. If this glass transition lies above the glass transition of the unrestrained, amorphous phase, the amorphous defects represent a separate nanophase, called a rigid-amorphous fraction. Modern differential scanning calorimetry (DSC), temperature-modulated DSC, and differential fast scanning calorimetry permit the study of latent heats and heat-capacity changes involved in the liquid–solid transitions of amorphous phases, crystals, and mesophases. In this more complex framework, the “termination of crystallization of flexible, linear macromolecules” is described together with the possibility of molar mass segregation by long-range and local diffusion instead of a thermodynamic mechanism.

Synthesis of tailored nanogels by means of two-stage irradiation

A new method allowing to synthesize polymeric nanogels of independently chosen weight-average molecular weight (Mw) and dimensions in an additive-free system consisting only of linear macromolecules and water is described and tested. This approach, based on generation of free radicals using ionizing radiation in oxygen-free polymer solutions, consists of two steps. In the first step, semi-concentrated polymer solution is irradiated at a moderate dose rate, so that in the stationary state the average number of radicals per chain is <1. This promotes intermolecular cross-linking with increase in Mw and coil dimensions. When the desired Mw is reached, in the second step intra-molecular recombination is induced by pulse irradiation of dilute solution (with the average number of radicals per chain >1 after each pulse), leading to a decrease in nanogel diameter while keeping Mw nearly constant. Experimental data on a model polymer – poly(N-vinylpyrrolidone) – confirm that the proposed method allows synthesizing nanogels of desired properties (independently chosen molecular weight and radius of gyration) in a pure polymer-water system, eliminating the use of monomers, cross-linking agents, or other auxiliary substances.

Controlled Release of Nanoparticles and Macromolecules from Responsive Microgel Capsules

Using a mesoscale computational model, we probe the release of nanoparticles and linear macromolecules from hollow microgel capsules that swell and deswell in response to external stimuli. Our simulations reveal that responsive microcapsules can be effectively utilized for steady and pulsatile release of encapsulated solutes. Swollen gel capsules allow steady, diffusive release of nanoparticles and polymer chains, whereas gel deswelling causes burst-like discharge of solutes driven by an outward flow of the solvent enclosed within a shrinking capsule. We demonstrate that this hydrodynamic release can be regulated by introducing rigid microscopic rods in the capsule interior. Thus, our findings disclose an efficient approach for controlled release from stimuli-responsive microcarriers that could be useful for designing advanced drug delivery systems.

Synthesis of polyrotaxanes from acetyl-β-cyclodextrin

Polyrotaxanes are intermediary products in the synthesis of topological gels. They are created by inclusion complex formation of hydrophobic linear macromolecules with cyclodextrins or their derivatives. Then, pairs of cyclodextrin molecules with covalently linkage were practically forming the nodes of the semi-flexible polymer network. Such gels are called topological gels and they can absorb huge quantities of water due to the net flexibility allowing the poly(ethylene oxide) chains to slide through the cyclodextrin cavities, without being pulled out altogether. For polyrotaxane formation poly(ethylene oxide) was used like linear macromolecules. There are hydroxyl groups at poly(ethylene oxide) chains, whereby the linking of the voluminous molecules should be made. To avoid the reaction of cyclodextrin OH groups with stoppers, they should be protected by, e.g., acetylation. In this work, the acetylation of the OH groups of β-cyclodextrin was performed by acetic acid anhydride with iodine as the catalyst. The acetylation reaction was assessed by the FTIR and HPLC method. By the HPLC analysis was found that the acetylation was completed in 20 minutes. Inserting of poly(ethylene oxide) with 4000 g/mol molecule mass into acetyl-β-cyclodextrin with 2:1 poly(ethylene oxide) monomer unit to acetyl-β-cyclodextrin ratio was also monitored by FTIR, and it was found that the process was completed in 12 h at the temperature of 10°C. If the process is performed at temperatures above 10°C, or for periods longer than 12 hours, the process of uncontrolled hydrolysis of acetate groups was initiated.

Effects of several inter-molecular interactions on the inclusion between methyl substituted β-cyclodextrin and some linear macromolecule in supercritical carbon dioxide medium

In this work, the inclusion between methyl substitute β-cyclodextrin and some linear macromolecule in supercritical carbon dioxide medium was investigated. The contributions from several factors were studied; such as the hydrogen bonding interaction between the host and guest, the hydrogen bonding interaction between the neighborhood cyclodextrin rims threaded on the polymer chain, and the matching between the host cavity and the size of the polymer axis.

Solution and melt viscoelastic properties of controlled microstructure poly(lactide)

A series of controlled microstructure poly(lactide) (PLA) samples were synthesized using a novel chiral dinuclear indium catalyst capable of living polymerization of lactide. PLAs with different ratios of L- to D- monomer ratios of 100:0, 90:10, 75:25, 50:50, and 0:100 were investigated. The relationship between intrinsic viscosity and the absolute molar mass distribution of the samples obtained by light scattering gel permeation chromatography in tetrahydrofuran gives [η] = 0.014 + 0.75 Mw, a scaling law of typical coil dimensions of linear macromolecules in good solvent. The melt rheological study includes determination of zero-shear viscosity and its relationship with the molecular weight, the relaxation spectrum, and its relation with molecular weight characteristics, as well as plateau modulus and other important rheological parameters that are helpful in predicting the linear viscoelasticity of PLA. Emphasis is placed on the uniaxial melt behavior of these polymers. At low temperatures, significant strain hardening is observed, which gradually disappears with an increase in temperature and decrease of Hencky strain rate. The K-BKZ constitutive equation is used to model the experimental data. It is concluded that in spite of their linear structure, PLAs exhibit strain hardening which is not due to strain-induced crystallization, and it is solely due to the dynamics of molecular relaxation.

Molecular Imaging and Analysis of Branching Topology in Polyacrylates by Atomic Force Microscopy

Molecular imaging by AFM provides direct and quantitative information about branching topology including length and distribution of branches, not accessible by other methods. In this paper, we report the analysis of branching in linear acrylate-based macromolecules synthesized via free radical polymerization (FRP) and atom transfer radical polymerization (ATRP). The branched structures are formed from chain transfer reactions, specifically transfer to polymer. Quantitative analysis showed that both methods produced branched species, with FRP having degree of branching of 0.035 ± 0.003% and ATRP having a degree of branching of 0.025 ± 0.002%, when measured per backbone repeat unit. The observed lower branching density in ATRP compared to FRP is consistent with a recent 13C NMR study, which also found that controlled radical methods, such as ATRP give lower degrees of branching than FRP. The absolute fraction of long chain branches measured by AFM is significantly lower than the total amount of branches evaluated by measurement of fraction of quaternary carbons using 13C NMR, indicating a predominant intramolecular chain transfer.

Interaction of aluminum polyhydroxochloride sol and poly(4-vinylbenzene sulfonic acid) sodium salt

The interaction of aluminum polyhydroxochloride sol and poly(4-vinylbenzene sulfonic acid) sodium salt is studied. It is found that, in dilute aqueous solutions at pH 4.6, aluminum polyhydroxochloride sol and poly(4-vinylbenzene sulfonic acid) sodium salt form insoluble polymer-colloid complexes of steady compositions. Potentiometric titration data show that the interaction between particles of aluminum polyhy-droxochloride sol and linear macromolecules of the polyelectrolyte occurs via salt bonds formed between unlikely charged groups on the surface of particles and units of the linear polyelectrolyte. The composition of the polymer-colloid complex is determined, and the degree of conversion for the reaction of aluminum poly-hydroxochloride sol and poly(4-vinylbenzene sulfonic acid) sodium salt is estimated. The influence of various polyelectrolytes on the stability of the polymer-colloid complex is studied.

Automated Single-Molecule Imaging To Track DNA Shape

We describe a straightforward, automated line tracking method to visualize linear macromolecules as they rearrange shape by brownian diffusion and under external fields such as electrophoresis. The analysis, implemented here with 30 ms time resolution, identifies contour lines from one end of the molecule to the other without attention to structure smaller than the optical resolution. There are three sequential stages of analysis: first, "feature finding" to discriminate signal from noise; second, "line tracking" to approximate those shapes as lines; and third, "temporal consistency check" to discriminate reasonable from unreasonable fitted conformations in the time domain. Automation makes it straightforward to accumulate vast quantities of data while excluding the unreliable parts of it. We implement this analysis on fluorescence images of λ-DNA molecules in agarose gel to demonstrate its capability to produce large data sets for subsequent statistical analysis.

Complexation of Anionic Liposomes with Spherical Polycationic Brushes

Spherical polycationic brushes, consisting of polystyrene particles with linear cationic macromolecules grafted onto their surfaces, were electrostatically complexed with small unilamellar anionic liposomes. Complexation was monitored using a multimethod approach that included laser electrophoresis, dynamic light scattering, fluorescence, cryogenic transmission electron microscopy, and conductivity. Liposomes adsorb onto the outer edges of the brushes rather than penetrate into their dense polycationic layer. The integrity of the liposomes remains unaltered when the liposomes reside on the polycationic brushes. The resulting complexes (roughly 40 liposomes per brush) do not dissociate into their components upon exposure to physiological solutions. The system is potentially useful in that liposomes are gathered into well-defined clusters with a high encapsulating potential. Multicomponent constructs can be easily prepared if polycationic brushes are allowed to bind to a mixture of liposomes that encapsulate different guests. This work provides an example of "systems chemistry" whereby as many as eight components, each with its own particular location and function (i.e., polystyrene core, polycationic graft, egg lecithin, cardiolipin, two fluorescent dyes, water, and buffer), collectively self-assemble.

Kinetics of nucleation and crystallization in poly(ɛ-caprolactone) (PCL)

The recently developed differential fast scanning calorimetry (DFSC) is used for a new look at the crystal growth of poly(ɛ-caprolactone) (PCL) from 185 K, below the glass transition temperature, to 330 K, close to the equilibrium melting temperature. The DFSC allows temperature control of the sample and determination of its heat capacity using heating rates from 50 to 50,000 K/s. The crystal nucleation and crystallization halftimes were determined simultaneously. The obtained halftimes cover a range from 3 × 10 −2 s (nucleation at 215 K) to 3 × 10 9 s (crystallization at 185 K). After attempting to analyze the experiments with the classical nucleation and growth model, developed for systems consisting of small molecules, a new methodology is described which addresses the specific problems of crystallization of flexible linear macromolecules. The key problems which are attempted to be resolved concern the differences between the structures of the various entities identified and their specific role in the mechanism of growth. The structures range from configurations having practically unmeasurable latent heats of ordering (nuclei) to being clearly-recognizable, ordered species with rather sharp disordering endotherms in the temperature range from the glass transition to equilibrium melting for increasingly perfect and larger crystals. The mechanisms and kinetics of growth involve also a detailed understanding of the interaction with the surrounding rigid-amorphous fraction (RAF) in dependence of crystal size and perfection.

Polymer brushes under flow and in other out-of-equilibrium conditions

Polymer brushes are formed from flexible linear macromolecules tethered at one chain end to a solid substrate, forming a dense polymeric layer of polymer chains which are more or less stretched in the direction perpendicular to the substrate surface. These systems find interest also due to numerous applications (colloid stabilization, improvement of lubrication properties when the surfaces are exposed to shear, protection of the surface against adsorption of nanoparticles or proteins, etc.), for which often the dynamic non-equilibrium response of these brushes to external perturbation is important. The present review summarizes recent computer simulation studies pertinent to these questions. Polymer brushes exposed to shear due to flow of the solvent or a polymer melt the brush interacts with are discussed, and the friction between two brushes is discussed. Another topic that is emphasized is the interaction of polymer brushes with “nanoinclusions” (i.e., small colloidal particles or linear macromolecules down to oligomers), discussing the absorption or expulsion of these objects, and again the response to shear. The simulations are interpreted in terms of scaling concepts and other phenomenological theories wherever possible, and an outlook to related experiments is given.

Research Progress in Hemicyanine-Based Polymers

Hemicyanine is a group of acentric cation-type dyes, in which one side of the conjugated chain is composed of a quaternary nitrogen-containing heterocycle, and the other side of the conjugated chain is connected to substituted nitrogen through an aryl ring. Different chemical constitutions of hemicyanine and its various connecting modes in polymer lead to abundant topological hemicyanine-based polymers, including linear, star-shaped, brush-shaped, dendritic and hyperbranched macromolecules. These polymers have been used for applications in various roles, such as photographic sensitizers, nonlinear optical materials and biomolecular labeling probes. We will give an overview of the synthesis, structure, properties and applications of series hemicyanine-based polymers.

The Largest Synthetic Structure with Molecular Precision: Towards a Molecular Object

Pushing the limits: A 200×106 Da structurally defined, linear macromolecule (PG5) has a molar mass, cross-section dimension, and cylindrical shape that are comparable to some naturally occurring objects, such as amyloid fibrils or certain plant viruses. The macromolecule is resistant against flattening out on a surface; the picture shows PG5 embracing the tobacco mosaic virus (TMV). Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Size Separation of Macromolecules during Spreading

Spreading of homogeneous mixtures of bottle-brush and linear macromolecules of poly(n-butylacrylate) on a solid substrate has been monitored on the molecular scale by atomic force microscopy. Despite the nearly identical chemical composition and similar molecular weight, brush-like macromolecules move markedly slower than linear chains. Moreover, smaller bottle-brushes have been shown to flow faster than the larger bottle-brushes, resulting in fractionation of the macromolecules along the spreading direction. This behavior was explained by the difference in sliding friction coefficient between the bottle-brush macromolecules and linear chains with the substrate. A theoretical model of molecular size separation is in a good agreement with experimental data.

Effect of storage time and temperature on structure, mechanical and barrier properties of chitosan-based films

The mechanical (tensile strength, elongation at break, mechanical work of deformation) and barrier (water vapor permeability and water vapor uptake) properties of chitosan films produced with acetic and lactic acids have been studied as a function of storage time, molecular weight of chitosans, concentration of plasticiser and the storage temperature. It was demonstrated that mechanical properties of chitosan-based films can be improved to a great extent during storage at low temperatures in freezer and refrigerator. Transition of chitosan molecules during storage in the solid state to more extended conformations and free volume changes are considered as mechanisms for the improvement of mechanical and barrier properties of chitosan films. The best mechanical properties are achieved for chitosan films produced with acetic acid and plasticized by the addition of 20% of glycerol. Sharp decrease in water vapor permeability has been demonstrated for thinner chitosan films and related to more dense packing and orientation of linear chitosan macromolecules.

Tannin Oxidation: Intra- versus Intermolecular Reactions

Grape and apple condensed tannin fractions were autoxidized at high concentrations (5 g/L) in aqueous solutions and analyzed by thiolysis (depolymerization followed by HPLC analysis) and small angle X-ray scattering (SAXS). Structural parameters of native (unoxidized) tannin polymers were derived from SAXS according to the wormlike chain model: the length per monomer is 15 A, the length of the statistical segment 17 A, and the cross section of the macromolecule has a radius within the range 3-4.5 A. The rather short length of the statistical segment is an effect of the different location of interflavanol linkages, which cause a loss of orientational correlation between successive monomers. Oxidation created new bonds that were resistant to thiolysis, and, according to thiolysis, some of these new bonds were intramolecular. However, according to SAXS, oxidation at high tannin concentration caused the weight average degree of polymerization to increase, indicating that intermolecular reactions took place as well, creating larger macromolecules. In the case of the smaller grape seed tannins, these intermolecular reactions took place "end to end" leading to the formation of longer linear macromolecules, at least in the earlier stages of oxidation. In the case of the larger apple tannins, the SAXS patterns were characteristic of larger branched macromolecules. Accordingly, the intermolecular reactions were mainly "end to middle". This is in agreement with the higher probabilities of "end to middle" reactions arising from a higher ratio extension unit/terminal unit in the latter case.

Absorption/expulsion of oligomers and linear macromolecules in a polymer brush

The absorption of free linear chains in a polymer brush was studied with respect to chain size L and compatibility ψ with the brush by means of Monte Carlo (MC) simulations and density functional theory (DFT)/self-consistent field theory (SCFT) at both moderate, σg=0.25, and high, σg=1.00, grafting densities using a bead-spring model. Different concentrations of the free chains 0.0625≤ϕo≤0.375 are examined. Contrary to the case of ψ=0 when all species are almost completely ejected by the polymer brush irrespective of their length L, for ψ&amp;lt;0 we find that the degree of absorption (absorbed amount) Γ(L) undergoes a sharp crossover from weak to strong (≈100%) absorption, discriminating between oligomers, 1≤L≤8, and longer chains. For a moderately dense brush, σg=0.25, the longer species, L&amp;gt;8, populate predominantly the deep inner part of the brush, whereas in a dense brush σg=1.00 they penetrate into the “fluffy” tail of the dense brush only. Gyration radius Rg and end-to-end distance Re of absorbed chains thereby scale with length L as free polymers in the bulk. Using both MC and DFT/SCFT methods for brushes of different chain length 32≤N≤256, we demonstrate the existence of unique critical value of compatibility ψ=ψc&amp;lt;0. For ψc(ϕo) the energy of free chains attains the same value, irrespective of length L whereas the entropy of free chain displays a pronounced minimum. At ψc all density profiles of absorbing chains with different L intersect at the same distance from the grafting plane. The penetration/expulsion kinetics of free chains into the polymer brush after an instantaneous change in their compatibility ψ displays a rather rich behavior. We find three distinct regimes of penetration kinetics of free chains regarding the length L: I (1≤L≤8), II (8≤L≤N), and III (L&amp;gt;N), in which the time of absorption τ grows with L at a different rate. During the initial stages of penetration into the brush one observes a power-law increase of Γ∝tα with power α∝−ln ϕo, whereby penetration of the free chains into the brush gets slower as their concentration rises.

Solving the initial condition of the string relaxation equation of the string model for glass transition: part-I

The string model for the glass transition can quantitatively describe the universal α-relaxation in glassformers, including the average relaxation time, the distribution function of the relaxation time, and the relaxation strength as functions of temperature. The string relaxation equation (SRE) of the model, at high enough temperatures, simplifies to the well-known single particle mean-field Debye relaxation equation, and at low enough temperatures to the well-known Rouse–Zimm relaxation equation that describes the relaxation dynamics of linear macromolecules. However, its initial condition, necessary to the further model predictions of glassy dynamics, has not been solved. In this paper, the special initial condition (SIC) of the SRE, i.e. for straight strings and the dielectric spectrum technique that is one of the most common methods to measure the glassy dynamics, was solved exactly. It should be expected that the obtained SIC would benefit the solution of the general initial condition of the SRE of the string model, i.e. for stochastically spatially configurating strings, as will be described in separate publications.

Synthesis and Thermal Behavior of An Amorphous Solid Polymer Electrolyte

In this study the synthesis of an amorphous polymer network, poly[oxymethylene-oligo(oxyethylene)], designated as aPEO, is described. This polymer has been characterized by gel permeation chromatography, thermal analysis, conductivity measurements, evaluation of electrochemical stability and nuclear magnetic resonance spectroscopy. The synthetic procedure developed permits partial fractionation of the product of the polymerization reaction. This host appears to be a promising polymer for application in batteries and electrochromic devices since it provides access to amorphous linear macromolecules with good mechanical properties and promising electrochemical behavior.