Molecular Weight Dependence Research Articles

Rheological Characterization of Sulfuric Acid Solutions of Poly(p-phenyleneterephthalamide)

Abstract Several grades of poly(p-phenyleneterephthalamide) (PPD-T) with different molecular weights were prepared. The rheological properties of sulfuric acid solutions of PPD-T were measured. In the vicinity of the critical concentration tan <5 and η'/G' exhibited a minimum value, but G'/2G" and η* showed a maximum value. High concentrations produce an anisotropic phase. Tendencies of yield behavior were observed for these solutions in the anisotropic phase. Above the saturated concentration the solution system showed gel-like properties, i. e., with increasing concentration abrupt reduction of G'/2G” and abrupt increases of η*, η'/G' and tan δ were observed. With increasing temperature the viscosity of isotropic and anisotropic phases was decreased but that of biphase was increased. The critical concentration was reduced with increasing molecular weight, but increased with increasing temperature. Use of reduced variables seemed to eliminate molecular weight dependency on the critical concentration curve. The shear viscosity appeared to be proportional to (Mw)6,7.

A β-NMR study of the depth, temperature, and molecular-weight dependence of secondary dynamics in polystyrene: Entropy-enthalpy compensation and dynamic gradients near the free surface.

We investigated the depth, temperature, and molecular-weight (MW) dependence of the γ-relaxation in polystyrene glasses using implanted 8Li+ and β-detected nuclear magnetic resonance. Measurements were performed on thin films with MW ranging from 1.1 to 641 kg/mol. The temperature dependence of the average 8Li spin-lattice relaxation time (T1 avg) was measured near the free surface and in the bulk. Spin-lattice relaxation is caused by phenyl ring flips, which involve transitions between local minima over free-energy barriers with enthalpic and entropic contributions. We used transition state theory to model the temperature dependence of the γ-relaxation, and hence T1 avg. There is no clear correlation of the average entropy of activation (Δ‡S̄) and enthalpy of activation (Δ‡H̄) with MW, but there is a clear correlation between Δ‡S̄ and Δ‡H̄, i.e., entropy-enthalpy compensation. This results in the average Gibbs energy of activation, Δ‡Ḡ, being approximately independent of MW. Measurements of the temperature dependence of T1 avg as a function of depth below the free surface indicate the inherent entropic barrier, i.e., the entropy of activation corresponding to Δ‡H̄ = 0, has an exponential dependence on the distance from the free surface before reaching the bulk value. This results in Δ‡Ḡ near the free surface being lower than the bulk. Combining these observations results in a model where the average fluctuation rate of the γ-relaxation has a "double-exponential" depth dependence. This model can explain the depth dependence of 1/T1 avg in polystyrene films. The characteristic length of enhanced dynamics is ∼6nm and approximately independent of MW near room temperature.

Comparative Investigation of Cellular Effects of Polyethylene Glycol (PEG) Derivatives.

Nowadays, polyethylene glycols referred to as PEGs are widely used in cosmetics, consumer care products, and the pharmaceutical industry. Their advantageous properties such as chemical stability, low immunogenicity, and high tolerability explain why PEGs are applied in many fields of pharmaceutical formulations including parenteral, topical, ophthalmic, oral, and rectal preparations and also in modern drug delivery systems. Given their extensive use, they are considered a well-known group of chemicals. However, the number of large-scale comparative studies involving multiple PEGs of wide molecular weight range is low, as in most cases biological effects are estimated upon molecular weight. The aim of this publication was to study the action of PEGs on Caco-2 cells and G. mellonella larvae and to calculate the correlation of these effects with molecular weight and osmolality. Eleven PEGs of different molecular weight were used in our experiments: PEG 200, PEG 300, PEG 400, PEG 600, PEG 1000, PEG 1500, PEG 4000, PEG 8000, PEG 10,000, 12,000, and PEG 20,000. The investigated cellular effects included cytotoxicity (MTT and Neutral Red assays, flow cytometry with propidium iodide and annexin V) and autophagy. The osmolality of different molecular weight PEGs with various concentrations was measured by a vapor pressure osmometer OSMOMAT 070 and G. mellonella larvae were injected with the solutions of PEGs. Sorbitol was used as controls of the same osmolality. Statistical correlation was calculated to describe the average molecular weight dependence of the different measured effects. Osmolality, the cytotoxicity assays, flow cytometry data, and larvae mortality had significant correlation with the structure of the PEGs, while autophagosome formation and the proportion of early apoptotic cells showed no statistical correlation. Overall, it must be noted that PEGs must be tested individually for biological effects as not all effects can be estimated by the average molecular weight.

Characterization of hydrogen bond network of waters around polyethylene glycol by broadband dielectric spectroscopy

Polyethylene glycol (PEG) is a well-known water retention agent in biomedical products, the hydration efficiency of which is affected by its molecular weight. Using a broadband dielectric spectroscopy (100 MHz–18 THz), the hydration state of PEG aqueous solutions with various molecular weights was quantitatively evaluated. As the molecular weight increases, the restriction strength of hydration water increases in potency, while the number of hydration water molecules decreases. Owing to the opposite changes in hydration number and restriction strength, the measured collective hydrogen bond (HB) strength shows negligible molecular weight dependence. PEG with larger Mw produces a more heterogenous HB network. The internal folding and twining caused by the growth of the PEG chain obstruct the proper exposure of hydrophilic part of the monomer producing less hydration waters. The evaluation result supports an application of PEGs with low molecular weight in contact lens package solutions.

A Coarse‐Grained Model for cis‐Polyisoprene: Thermal Expansion and Glass Transition Temperature

AbstractA coarse‐grained (CG) model of cis‐1,4‐polyisoprene (cis‐PI) is developed using systematic coarse‐graining strategy. The bonded interactions of the model are captured using analytical potentials with parameters derived to match structural probability distributions of atomistic system. The nonbonded interactions are presented by Lennard‐Jones potentials with parameters specified to match the density and thermal expansion behavior obtained from experiment. The CG force field is validated through the molecular weight dependence of chain dimension and Flory−Fox scaling relationship for the glass transition temperature of the polymer. The model developed provides much faster chain dynamics and simulation speed compared to the reference atomistic one.

Chain dimension and crystallization temperature affect the II–I transition of isotactic polybutene-1

The molecular weight dependence of the II–I phase transformation shows significant differences between isothermally and non-isothermally crystallized iPB-1.

Living ring‐opening metathesis polymerization of norbornenes bay‐functionalized perylene diimides

AbstractRing‐opening metathesis polymerization (ROMP)‐derived poly(oxanorbornene imide)s bearing bay‐linked mono‐alkoxy‐M1 and 1,7‐di‐alkoxy M2 functionalized perylene diimides (PDIs) were synthesized using Grubb's third (G3) and Hoveyda‐Grubbs second generation (HG2) ruthenium‐alkylidene metathesis initiators. The mono‐alkoxy‐derived PDI‐based non‐ladderphane polymer polyM1 displayed 67% to 77% of the trans olefin content in the polymer chain depending on the initiator used for the polymerization. When using the symmetrical 1,7‐di‐alkoxy‐derived PDI‐based polymer polyM2 having the ladderphane type‐structure, this displayed a significant amount of cis and trans olefin contents in the polymer chains, irrespective of the type of initiators used for the polymerization. ROMP of both monomers M1 and M2 proceeded in a well‐controlled manner with a linear dependence of molecular weight on the monomer/initiator ratio using G3 as initiator. Optical properties of the ladderphane‐based polyM2 and non‐ladderphane‐based polyM1 were characterized in both solution and the film state. X‐ray diffraction (XRD) analysis for all the polymers showed significant π‐stacking in the thin film state with ordered molecular packing and closer values of d‐spacing for both polyM1 and polyM2. Film morphology examined by AFM elucidated homogenous smooth polymer surface for both polymers in general, but with some irregularities observed for polyM1. In addition, CV analysis revealed both polymers could be good candidates as electron‐accepting materials, with excellent film‐forming ability, and thermal stability.

Thermoplastic Elastomer Tunes Phase Structure and Promotes Stretchability of High-Efficiency Organic Solar Cells.

Top-performance organic solar cells (OSCs) consisting of conjugated polymer donors and nonfullerene small molecule acceptors (NF-SMAs) deliver rapid increases in efficiencies. Nevertheless, many of the polymer donors exhibit high stiffness and small molecule acceptors are very brittle, which limit their applications in wearable devices. Here, a simple and effective strategy is reported to improve the stretchability and reduce the stiffness of high-efficiency polymer:NF-SMA blends and simultaneously maintain the high efficiency by incorporating a low-cost commercial thermoplastic elastomer, polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS). The microstructure, mechanical properties, and photovoltaic performance of PM6:N3 with varied SEBS contents and the molecular weight dependence of SEBS on microstructure and mechanical properties are thoroughly characterized. This strategy for mechanical performance improvement exhibits excellent applicability in some other OSC blend systems, e.g., PBQx-TF:eC9-2Cl and PBDB-T:ITIC. More crucially, the elastic modulus of such complex ternary blends can be nicely predicted by a mechanical model. Therefore, incorporating thermoplastic elastomers is a widely applicable and cost-effective strategy to improve mechanical properties of nonfullerene OSCs and beyond.

Environmentally Sensitive Luminescence Reveals Spatial Confinement, Dynamics, and Their Molecular Weight Dependence in a Polymer Glass.

Polymer glasses have an irregular structure. Among the causes for such complexity are the chemically distinct chain end groups that are the most abundant irregularities in any linear polymer. In this work, we demonstrate that chain end induced defects allow polymer glasses to create confined environments capable of hosting small emissive molecules. Using environmentally sensitive luminescent complexes, we show that the size of these confinements depends on molecular weight and can dramatically affect the photoluminescence of free or covalently bound emissive complexes. We confirm the impact of chain end confinement on the bulk glass transition in poly(methyl acrylate) (pMA) and show that commonly observed Tg changes induced by the chain ends should have a structural origin. Finally, we demonstrate that the size and placement of luminescent molecular probes in pMA can dramatically affect the probe luminescence and its temperature dependence, suggesting that polymer glass is a highly irregular and complex environment, marking its difference with conventional small molecule solvents. Considering the ubiquity of luminescent glassy materials, our work lays down a blueprint for designing them with structural considerations in mind, ones where packing density and chain end size are key factors.

Estimation of linear, ring, and star polyethylene viscosity through proper orthogonal decomposition and Voronoi tessellation analysis of molecular dynamics data

AbstractWe carried out molecular dynamics (MD) simulations on polyethylene (PE) with linear, ring, and four‐arm symmetrical star structures at various molecular weights (420 − 5700 g ⋅ mol−1) and temperatures (410−450 K). We then analyzed the MD data using the technique of proper orthogonal decomposition (POD) to obtain the time correlation functions of different eigenmodes, thereby calculating the viscosity of the aforementioned macromolecules. The time correlation functions show that the ring and star PEs relax much faster than their linear counterpart. Free volume size distributions and the mean fractional free volume of the equilibrated PE melts were determined by Voronoi tessellation (VT). The mean fractional free volume and the corresponding effective pressure were then used to calculate viscosity using a polymer free volume theory recently developed in our lab. The POD and VT approaches yielded similar viscosity values. Furthermore, they both successfully predicted the crossover in the molecular weight dependence of viscosity. It is interesting to note that the difference in the free volume parameters (ϕ+ − F) of various structures always fall within the range of 0.02−0.06. Here, F and ϕ+ signify the probability of a bead having enough free volume for the activation of diffusive motion or momentum transfer and the minimum required fraction of such beads, respectively. The temperature dependence of viscosity as obtained from the POD and VT approaches gave comparable apparent activation energy () values of 5.8−6.4 and 5.4−6.4 kcal ⋅ mol−1, respectively, for the three structures.

Emulsion polymerization of vinylidene fluoride: Effects of mixing and reaction conditions on the initial rate of polymerization

AbstractIn order to contribute to a better understanding of the emulsion polymerization of vinylidene fluoride (VDF), an experimental study under conditions of temperature and pressure similar to those found in industrial processes was carried out. It is shown that the initial rate of polymerization is strongly influenced by the agitation of the reactor, with the rate of reaction increasing as the rate of agitation increases. In addition, using a more efficient impeller also increases the polymerization. It is proposed that this is due to mass transfer limitations. This idea is confirmed by a similar dependence of the average molecular weight on agitation. Experimental data also shows that particles are formed by homogeneous coagulative nucleation. It is proposed that particle nucleation occurs throughout the polymerization, and that a competition between controlled coagulation and particle generation governs the rate of reaction.

Chemical kinetic mechanisms and scaling of two-dimensional polymers via irreversible solution-phase reactions.

Two-dimensional (2D) polymers are extended networks of multi-functional repeating units that are covalently linked together but confined to a single plane. The past decade has witnessed a surge in interest and effort toward producing and utilizing 2D polymers. However, facile synthesis schemes suitable for mass production are yet to be realized. In addition, unifying theories to describe the 2D polymerization process, such as those for linear polymers, have not yet been established. Herein, we perform a chemical kinetic simulation to study the recent synthesis of 2D polymers in homogeneous solution with irreversible chemistry. We show that reaction sites for polymerization in 2D always scale unfavorably compared to 3D, growing as molecular weight to the 1/2 power vs 2/3 power for 3D. However, certain mechanisms can effectively suppress out-of-plane defect formation and subsequent 3D growth. We consider two such mechanisms, which we call bond-planarity and templated autocatalysis. In the first, although single bonds can easily rotate out-of-plane to render polymerization in 3D, some double-bond linkages prefer a planar configuration. In the second mechanism, stacked 2D plates may act as van der Waals templates for each other to enhance growth, which leads to an autocatalysis. When linkage reactions possess a 1000:1 selectivity (γ) for staying in plane vs rotating, solution-synthesized 2D polymers can have comparable size and yield with those synthesized from confined polymerization on a surface. Autocatalysis could achieve similar effects when self-templating accelerates 2D growth by a factor β of 106. A combined strategy relaxes the requirement of both mechanisms by over one order of magnitude. We map the dependence of molecular weight and yield for the 2D polymer on the reaction parameters, allowing experimental results to be used to estimate β and γ. Our calculations show for the first time from theory the feasibility of producing two-dimensional polymers from irreversible polymerization in solution.

Tacky-Free Polyurethanes Pressure-Sensitive Adhesives by Molecular-Weight and HDI Trimer Design.

Polyurethane pressure-sensitive adhesives (PU-PSAs) with satisfactory tack, cohesion, and removability were newly developed through the synthetic process by reacting methylene diisocyanate, poly(ethylene glycol) (PEG), and a 1,4-butanediol chain extender based on the different HDI/HDI trimer ratios. The sticking properties of PU-PSAs depended on both the HDI/HDI trimer ratio and crosslinking-agent composition in the formulation. The molecular weight (MW) dependence of adhesion in PU-PSA was observed in the range of 1000 < Mn < 3000, suggesting that the increase in MW limits the pressure-sensitive adhesion of these samples. The differences in the crosslinking-density significantly affected the cohesion, adhesion, and tack in PU-PSA. The formulation of 50 wt.% 600PEG and 50 wt.% crosslinking-agent and an HDI/HDI trimer ratio of 1.0 led to the optimal balance between the adhesion and cohesion properties owing to the sufficient tack, high 180-peel strength, and good cohesion.

Dependence of intrinsic viscosity and molecular size on molecular weight of partially hydrolyzed polyacrylamide

AbstractAs a typical water‐soluble polymer, ultra‐high molecular weight (UHMW) partially hydrolyzed polyacrylamide (HPAM) has been widely used in various industries as thickeners or rheology modifiers. However, precise determination of its critical physical parameters such as molecular weight, radius of gyration (Rg) and hydrodynamic radius (Rh) were less documented due to their high viscosity in aqueous solution. In this work, the molecular structure of five UHMW‐HPAM samples with different MW was elucidated by 1H and 13C NMR spectroscopy, and their solution properties were characterized by both static and dynamic light scattering. It is found that all the second virial coefficient (A2) values are positive and approaching zero, indicating of a good solvent of 0.5 M NaCl for UHMW‐HPAM. The weight‐average molecular weight (Mw) dependence of molecular size and intrinsic viscosity [η] for these series of HPAM polymers with MW ranging from 4.81 to 15.4 × 106 g·mol−1 can be correlated as Rg = 3.52 × 10−2Mw0.51, Rh = 1.97 × 10−2Mw0.51, and [η] = 6.98 × 10−4 Mw0.91, respectively. These results are helpful in understanding the relationship between molecular weight and coil size of HPAM polymers in solution, and offer references for quick estimation of molecular weight and screening of commercial UHMW‐HPAM polymers for specific end‐users.

Phase Equilibrium, Phase Structure, and Interdiffusion in Polystyrene–Polyphenylmethylsiloxane Polymer Mixtures

Comprehensive studies of solubility and diffusion in PS–PPMS systems are performed via optical interferometry, DSC, and SEM. The interdiffusion zones in PS–siloxane oligomer systems are studied for the first time. It is shown that the general tendency of the change in the position of the concentration profile over time is to expand without changing the nature of the dependence. A smooth change in the concentration of the siloxane oligomer in PS is observed in the limits of the solubility region. Generalized state diagrams of PS–polysiloxane systems are constructed. The effect molecular weight has on the position of the branches of the binodal curves is determined. It is shown that the upper critical solution temperature (UCST) grows along with the molecular weight of linear oligomers, while the heterogeneous region expands and the binodal dome shifts to the region of thermal destruction of the compositions. The concentration, temperature, and molecular weight dependences of the diffusion coefficients are determined for each of the studied systems. The apparent activation energies are calculated for diffusion.

In Situ AFM Observation of Folded‐Chain Crystallization of a Low‐Molecular‐Weight Isotactic Poly(methyl methacrylate) in a Langmuir Monolayer at the Molecular Level

AbstractThe formation of folded‐chain crystals (FCC) of a low‐molecular‐weight isotactic poly(methyl methacrylate) (it‐PMMA) in a Langmuir monolayer on mica is observed under high humidity by in situ atomic force microscopy at the molecular level. The size of the smallest FCC formed corresponds to the size of a crystal composed of a single chain (single‐chain crystal), and the crystals grow in a stepwise manner with steps comparable to a single chain. The crystal of it‐PMMA is composed of a double‐stranded helix, and taking into account the molecular weight dependence of the crystal growth, it is concluded that a single chain intramolecularly intertwines to form a double‐stranded helix quantitatively that further intramolecularly folds to form the single‐chain crystal, and the crystal grows with the unit of the single‐chain crystal. The crystallization process is also successfully observed in detail at the molecular level: an amorphous single chain is deposited and spread on a crystal growth plane as new stems to grow the crystal.

Effect on the processability, structure and mechanical properties of highly dispersed in situ ZnO:CS nanoparticles into PVA electrospun fibers

The main purpose of this work was to synthesize ZnO nanoparticles (NPs) in situ into a chitosan (CS) matrix to obtain highly dispersed antibacterial composites by the sol–gel technique. These composites were characterized by their thermal, structural, morphological and optical properties. NPs were subsequently used and added into polyvinyl alcohol (PVA) in order to obtain electrospun fibers. Previously, it was determined the adequate PVA/CS ratio and changes in their morphological features in dependence of molecular weight of PVA and electrospinning parameters to avoid the beads and spindle-like defects in the final fibers. Then, optimal conditions were used to fabricate PVA/in situ ZnO:CS nanofiber composites. The hydrogen bonding interaction governs the ZnO:CS system, which forms a combination between flower-type and agglomerated rod morphologies. It was found that CS promotes the crystallinity of ZnO NPs maintaining the hexagonal phase. The addition of ZnO into the CS matrix was performed via weak interactions by stabilizing the hybrid compound after the second decomposition stage when the mass loss is above 50%. Free-defects uniform fibers with an improvement in hardness as well as in the elastic modulus was obtained in the electrospun PVA/in situ ZnO:CS fibers. The presence of ZnO NPs improves the spinnability in the fiber nanocomposites with an average diameter of 223 nm. PVA/in situ ZnO:CS mats displayed an inhibition of bacterial growth of Escherichia coli and Staphylococcus aureus.

Quantitative bridging between full-atomistic and bead-spring models for polybutadiene and poly(butadiene-styrene) copolymers.

Despite lots of attempts on the bridging between full-atomistic and coarse-grained models for polymers, a practical methodology has not been established yet. One of the problems is computation costs for the determination of spatial and temporal conversion parameters, which are ideally obtained for the long chain limit. In this study, we propose a practical, yet quantitative, bridging method utilizing the simulation results for rather short chains. We performed full-atomistic simulations for polybutadiene and some poly(butadiene-styrene) copolymers in the melt state by varying the number of repeating units as 20, 30, and 40. We attempted to construct corresponding coarse-grained models for such systems. We employed the Kremer-Grest type bead-spring chains with bending rigidity. The stiffness parameter of coarse-grained models and the spatial conversion factor between the full-atomistic and coarse-grained models were obtained according to the conformational statistics of polymer chains. Although such a bridging strategy is similar to the earlier studies, we incorporated the molecular weight dependence of the conformational statistics for the first time. By introducing several empirical functions of the conformational statistics for the molecular weight dependence, we attained a rigorous bridging for the conformational statistics. We confirmed that the structural distribution functions of the coarse-grained systems are entirely consistent with the target full-atomistic ones. Owing to the structural conversion parameters thus obtained, we constructed the coarse-grained models that corresponded to the polymers consisting of 200 repeating units and traced the segmental diffusion. The full-atomistic simulations were also performed from the initial configurations created from the equilibrated coarse-grained models via the back-mapping scheme. From the comparison of the mean-square-displacement of the segments positioned at the middle of the chain, we obtained the temporal conversion factors.

Revisiting molecular adsorption: unconventional uptake of polymer chains from solution into sub-nanoporous media.

Adsorption of polymers from the solution phase has been extensively studied to cope with many demands not only for separation technologies, but also for the development of coatings, adhesives, and biocompatible materials. Most studies hitherto focus on adsorption on flat surfaces and mesoporous adsorbents with open frameworks, plausibly because of the preconceived notion that it is unlikely for polymers to enter a pore with a diameter that is smaller than the gyration diameter of the polymer in solution; therefore, sub-nanoporous materials are rarely considered as a polymer adsorption medium. Here we report that polyethylene glycols (PEGs) are adsorbed into sub-nanometer one-dimensional (1D) pores of metal–organic frameworks (MOFs) from various solvents. Isothermal adsorption experiments reveal a unique solvent dependence, which is explained by the balance between polymer solvation propensity for each solvent and enthalpic contributions that compensate for potential entropic losses from uncoiling upon pore admission. In addition, adsorption kinetics identify a peculiar molecular weight (MW) dependence. While short PEGs are adsorbed faster than long ones in single-component adsorption experiments, the opposite trend was observed in double-component competitive experiments. A two-step insertion process consisting of (1) an enthalpy-driven recognition step followed by (2) diffusion regulated infiltration in the restricted 1D channels explains the intriguing selectivity of polymer uptake. Furthermore, liquid chromatography using the MOFs as the stationary phase resulted in significant PEG retention that depends on the MW and temperature. This study provides further insights into the mechanism and thermodynamics behind the present polymer adsorption system, rendering it as a promising method for polymer analysis and separation.

Probing Thermoresponsive Polymerization-Induced Self-Assembly with Variable-Temperature Liquid-Cell Transmission Electron Microscopy

Summary We directly initiate and visualize the formation of polymeric nanomaterials via variable-temperature liquid-cell transmission electron microscopy (VT-LCTEM). With temperature control in the liquid cell, reversible addition-fragmentation chain transfer polymerization was thermally initiated, leading to the formation of amphiphilic block copolymers that assemble upon dispersion polymerization-induced self-assembly (PISA). In combination with traditional ex situ analyses, VT-LCTEM enabled not only the characterization of the nanoparticles in situ but also the observation of a thermal phase transition. Critically, because these assemblies form from thermoresponsive components, any temperature changes during sample preparation and analysis can alter morphologies, necessitating direct in situ characterization of reaction progression. We believe that the findings and the technique described herein will lead to unprecedented possibilities for the development and characterization of self-organizing soft matter.