Chain Entanglement Concentration Research Articles

Critical residence time in metastable region – a time scale determining the demixing mechanism of nonsolvent induced phase separation

The present work focuses on studying the relationship between the time period of a polymer solution staying in the metastable region and the resulting porous structure after phase separation of the solution. We employed the technique of FTIR microscopy to determine the composition change in the PMMA/NMP (n-methylpyrrolidone) solution after its contact with water. By plotting the composition change on the ternary phase diagram of PMMA, NMP and water, and identifying the times that the composition path intersected with the binodal and the spinodal, we then determined the residence time of the solution in the metastable region (tm). For each polymer solution, we identified a critical residence time (tmc) that played a dominant role in determining the phase separation mechanism and the resulting porous structure: with tm less than tmc, the corresponding structure was bi-continuous (typical structure resulted from spinodal decomposition); with tm greater than tmc, the corresponding structure was cellular (typical structure from the mechanism of nucleation and growth). The results show that tmc depended on the polymer molecular weight and the polymer concentration in casting solution. And the dependences can be superimposed into one curve as the polymer concentration was normalized by the polymer chain entanglement concentration. With the relationship between tmc and the normalized polymer concentration and a simple model for the effective diffusivity of water in the polymer solution, we developed a model equation to estimate the positions in PMMA membranes where structure transition occurred from bi-continuous to cellular. Good agreement was obtained between the calculated transition positions and the experimentally determined ones.

Electrospinning of poly(lactic acid): Theoretical approach for the solvent selection to produce defect‐free nanofibers

ABSTRACTIn this study an integrated methodology was proposed for the selection of solvent systems to produce electrospinnable solutions that form defect‐free poly(lactic acid) (PLA) fibers with narrow diameter distributions. The solvent systems were chosen using a thermodynamic approach, combined with electrical and rheological property criteria. More specifically, the three step methodology includes (1) initial choice of solvent by solubility evaluation to meet thermodynamic criteria, (2) electrical properties, that is, conductivity and dielectric constant adjustment by using solvent mixtures to meet electrical property criteria, and (3) critical entanglement concentration (Ce) determination by viscosity measurements, supported by elastic and plastic moduli measurements, followed by concentration adjustment to meet rheological criteria. All three criteria need to be met to ensure defect‐free nanofiber morphology. The methodology was demonstrated using PLA solutions that were characterized in terms of thermodynamic properties, conductivity, surface tension, and viscosity measurements. These data were analyzed and related to the nanofiber morphology and diameter as determined from scanning electron microscopy (SEM). Measurements of the elastic (G′) and the plastic (G″) moduli of PLA solutions showed a sharp increase of G′ at the chain entanglement concentration. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2016,54, 1483–1498

Electrospun poly lactic acid (PLA) fibres: Effect of different solvent systems on fibre morphology and diameter

The selection of an appropriate non-hazardous solvent or solvent system is essential to determine the rheological properties and electrospinnability of the solution, the productivity, and the morphology of nanofibres. In this study, poly lactic acid (PLA) solutions were prepared in various pure solvents and binary-solvent systems to investigate the effect of different solution properties on nanofibre morphology and diameter. Viscosity, conductivity and surface tension of each solution were measured. Nanofibre morphology was observed by scanning electron microscopy (SEM). Of all the solvent systems used acetone/dimethylformamide gave the highest fibre productivity and finest defect-free nanofibres. Therefore this solvent system was studied in more detail, varying the solvent ratio. Also the polymer concentration in this solvent system was varied to investigate the effect on nanofibre morphology and solution properties. Morphological investigations were done in correlation with rheological measurements: beaded nanofibrous structures were collected from solutions with concentration around the critical chain entanglement concentration (Ce), while defect-free nanofibres were produced when the concentration was increased to about twice the entanglement concentration. Further investigation of the effect of the PLA concentration on the elastic (G′) and the plastic (G″) moduli showed a sudden increase of the elastic moduli (G′) at the critical chain entanglement concentration. The results showed that the solvent properties, boiling point, viscosity, conductivity and surface tension, have a significant effect on process productivity, morphology and diameter distribution of the PLA nanofibres.

Self-sustained webs of polyvinylidene fluoride electrospun nano-fibers: Effects of polymer concentration and desalination by direct contact membrane distillation

The effects of the polymer polyvinylidene fluoride (PVDF) concentration on the characteristics and direct contact membrane distillation (DCMD) desalination performance of self-sustained electrospun nano-fibrous membranes (ENMs) have been studied. Different polymer concentrations ranging from 15 to 30wt% were considered in the solvent mixture N,N-dimethyl acetamide and acetone, while all other electrospinning parameters were maintained the same. Viscosity, electrical conductivity and surface tension of the polymer solutions were measured and the effects of the PVDF concentration on fiber diameter, thickness, water contact angle, inter-fiber space, void volume fraction, liquid entry pressure, mechanical and thermal properties of the ENMs were investigated. The minimum polymer concentration, critical chain entanglement concentration, required for electrospinning beaded fibers and the concentration needed for the formation of bead-free fibers were localized. Two groups of ENMs were identified based on the surface structure of the ENMs, their void volume fraction and inter-fiber space. Bead-free ENMs, prepared with PVDF concentration higher than 22.5wt%, exhibit higher DCMD permeate flux than the beaded ENMs. Beaded ENMs can be used in desalination by DCMD. Among the prepared ENMs, the optimized membrane exhibiting the highest DCMD performance was prepared with 25wt% PVDF concentration.

Characterization and properties of electrospun thermoplastic polyurethane blend fibers: Effect of solution rheological properties on fiber formation

Abstract

Deformation of the ultra‐high molecular weight polyethylene melt in the plane‐strain compression

AbstractDeformability of molten ultra‐high molecular weight polyethylene (UHMWPE) was studied in the plane‐strain compression at the temperature from a range of 145–165°C. Two grades of UHMWPE were tested—a commercial one synthesized in a slurry process with conventional Ziegler‐Natta catalyst and the material polymerized in solution at low temperature using a homogeneous metallocene catalyst. These grades differ markedly in the initial concentration of chain entanglements in nascent powders. Specimens used for compression were produced either by compression molding or compaction of the nascent powders (sintering) at the temperature up to 130°C in order to obtain a set of samples with a range of chain entanglement concentration. Plane‐strain compression tests demonstrated a rubber‐like behavior and a good deformability of molten samples in the temperature range of 145–155°C. The ultimate true strain up to e = 3 was observed in samples prepared by compression molding, while for sintered samples of metallocene polymer it exceeded e = 4.3 (deformation ratio, λ > 74). At higher temperature of 165°C deformation became unstable. A strong correlation between topological structure of the melt and its deformation behavior was found: highly disentangled samples (from nascent powder of metallocene polymer) tend to deform to much higher strain than samples of lightly reduced concentration of chain entanglements (from nascent powder of commercial Z‐N polymer) or samples with entanglement density close to an equilibrium (prepared by compression molding), which demonstrated the lowest ultimate strain. The obtained results point out a new route of melt‐processing of UHMWPE nascent powders for fabrication of ultra‐strength fibers, alternative to the gel‐spinning technology. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Electrospinning of hyaluronic acid nanofibers from aqueous ammonium solutions.

For several reasons, the electrospinning of nanofibrous mats comprised purely of biopolymers, such as hyaluronic acid (HA) has been difficult to achieve. Most notably, due to its polyelectrolytic nature, very low polymer concentrations exhibit very high solution viscosities. Thus, it is challenging to obtain the critical chain entanglement concentration necessary for biopolymer electrospinning to ensue. While the successful electrospinning of HA fibers from a sodium hydroxide:dimethylformamide (NaOH:DMF) system has been reported, the diameter of these fibers was well above 100nm. Moreover, questions regarding the degradation of HA within the solvent system arose. These factors supported our ongoing research into determining an improved solvent system. In this study, the use of a less basic (pH 11) aqueous ammonium hydroxide (NH4OH) solvent system, NH4OH:DMF, allowed for the fabrication of HA mats having an average fiber diameter of 39±12nm. Importantly, while using this solvent system, no degradation effects were observed and the continuous electrospinning of pure HA fibers was possible.

Formation and morphological stability of polybutadiene rubber fibers prepared through combination of electrospinning and in-situ photo-crosslinking

Herein we report that fibers of polybutadiene rubber (BR) with high morphological stability were made through combination of electrospinning and in-situ photo-crosslinking. This study revealed that (1) the formation of electrospun BR fibers was correlated with the overlap/entanglement of BR macromolecules in the spinning solutions; for the formation of uniform fibers without beads and/or beaded fibers, the concentration of spinning solution had to be higher than the “critical chain entanglement concentration (Ce)”; and (2) the in-situ photo-crosslinking was an effective method to considerably improve the morphological stability of electrospun BR fibers; upon prolonged storage under ambient condition or even upon immersion in tetrahydrofuran (a good solvent for BR), the crosslinked BR fibers well retained their morphology. It is envisioned that electrospun rubber fibers could be utilized for the development of innovative composites with substantially improved toughness and/or impact strength.

Ultrafine Cellulose Acetate Fibers with Nanoscale Structural Features

Nano-structural features were introduced to ultrafine cellulose acetate (CA) fibers by electrospinning of its mixtures with either poly(vinyl pyrrolidone) PVP or beta-cyclodextrin (beta-CD) in DMF, followed by dissolution of the added PVP or beta-CD. The presence of the charge-holding PVP enabled fiber formation from CA below its entanglement chain length and improved the electrospinning efficiency to produce bicomponent fibers with wide ranging diameters from 30 to 650 nm. At up to 50% contents, the PVP in the bicomponent fibers was phase-separated from CA and, upon removal, resulting in highly angulated fiber surfaces with nanometer-size spherulites and sub-micron size ridges and grooves. Adding beta-CD to CA enabled fiber formation at concentrations below the chain entanglement concentration Ce (16.5%). Hydrogen bonding between beta-CD and CA, as evident by FTIR, helped to distribute beta-CD as individual molecules in the CA matrix and producing more uniform and finer (130-150 nm in diameters) fibers, irrespective of their beta-CD contents. Removal of beta-CD from the fibers originally containing 40% beta-CD, generated nanoporous fibers with 2-nm nanopores and 70% increase in specific surface and doubled pore volume.

Electrospinning of linear homopolymers of poly(methyl methacrylate): exploring relationships between fiber formation, viscosity, molecular weight and concentration in a good solvent

A series of seven linear homopolymers of poly(methylmethacrylate) ranging from 12,470 to 365,700 g/mol M w, were utilized to further explore scaling relationships between viscosity and concentration in a good solvent at 25 °C and to investigate the impact of these relationships on fiber formation during electrospinning. For each of the polymers investigated, chain dimensions (hydrodynamic radius and radius of gyration) were measured by dynamic light scattering to determine the critical chain overlap concentration, c *. The experimentally determined c *, was found to be in good agreement with the theoretically determined value that was calculated by the criteria c *∼1/[ η], where the intrinsic viscosity was estimated from the Mark–Houwink parameters, K and a (at 25 °C in dimethyl formamide) obtained from the literature. The plot of the zero shear viscosity vs. c/ c * distinctly separated into different solution regimes, viz. dilute ( c/ c *<1), semidilute unentangled (1< c/ c *<3) and semidilute entangled ( c/ c *>3). The crossover between semidilute unentangled and semidilute entangled regimes in the present investigation occurred at c/ c *∼3, which, therefore, marked the onset of the critical chain entanglement concentration, c e, according to the procedure utilized by Colby and co-workers [Colby RH, Rubinstein M, Daoud M. J de Phys II 1994;4(8):1299–310. [52]]. Electrospinning of all solutions was carried out at identical conditions to ascertain the effects of solution concentration, molecular weight, molecular weight distribution and viscosity on fiber formation and morphological features of the electrospun material. Only polymer droplets were observed to form from electrospinning of solutions in the dilute concentration regime due to insufficient chain overlap. As the concentration was increased, droplets and beaded fibers were observed in the semidilute unentangled regime; and beaded as well as uniform fibers were observed in the semidilute entangled regime. Uniform fiber formation was observed at c/ c *∼6 for all the narrow MWD polymers ( M w of 12,470–205,800 g/mol) but for the relatively broad MWD polymers ( M w of 34,070 and 95,800 g/mol), uniform fibers were not formed until higher concentrations, c/ c *∼10, were utilized. Dependence of fiber diameter on concentration and viscosity was also determined, viz. fiber dia∼( c/ c *) 3.1 and fiber dia ∼ η 0 0.71 respectively. These scaling relationships were in general agreement with that observed by Mckee et al. [McKee MG, Wilkes GL, Colby RH, Long TE. Macromolecules 2004;37(5):1760–67. [33]].

The hydrosilylation cure of polyisobutene

A liquid polyisobutene oligomer with unsaturated chain ends undergoes hydrosilylation with HMe2SiOMe2SiOMe2SiH or Si(OMe2SiH)4 to give higher molecular weight polymers or elastomers. A major side reaction consumes SiH to give redistributed siloxane in the resulting polymers and gaseous silanes and siloxanes as by-products. A second side reaction results in loss of reactivity in the oligomer due to a shift of the terminal double bond to an internal position. If the side reactions are taken into account, it is possible to forecast quantitatively molecular weight, gel point and modulus from the conversions of >SiH, >C=CHf2 and the chain entanglement concentration reported for polyisobutene in the literature.