Molar Mass Dependence Research Articles

Selective deuteration along a polyethylene chain: Differentiating conformation segment by segment.

To improve the circularity and performance of polyolefin materials, recent innovations have enabled the synthesis of polyolefins with new structural features such as cleavable breakpoints, functional chain ends, and unique comonomers. As new polyolefin structures become synthetically accessible, fundamental understanding of the effects of structural features on polymer (re)processing and mechanical performance is increasingly important. While bulk material properties are readily measured through conventional thermal or mechanical techniques, selective measurement of local material properties near structural defects is a major characterization challenge. Here, we synthesized a series of polyethylenes with selectively deuterated segments using a polyhomologation approach and employed vibrational spectroscopy to evaluate crystallization and melting of chain segments near features of interest (e.g., end groups, chain centers, and mid-chain structural defects). Chain-end functionality and defects were observed to strongly influence crystallinity of adjacent deuterated chain segments. Additionally, chain-end crystallinity was observed to have different molar mass dependence than mid-chain crystallinity. The synthesis and spectroscopy techniques demonstrated here can be applied to range of previously inaccessible deuterated polyethylene structures to provide direct insight into local crystallization behavior.

Unraveling the Molar Mass Dependence of Shearing-Induced Aggregation Structure of a High-Mobility Polymer Semiconductor.

Aggregation-structure formation of conjugated polymers is a fundamental problem in the field of organic electronics and remains poorly understood. Herein, the molar mass dependence of the aggregation structure of a high-mobility conjugated copolymer (TDPP-Se) comprising thiophene-flanked diketopyrrolopyrrole and selenophene is thoroughly shown. Five batches of TDPP-Se are prepared with the number-average molecular weights (Mn ) varied greatly from 21 to 135kg mol-1 . Small-angle neutron scattering and transmission electron microscopy are combined to probe the solution structure of these polymers, consistently using a deuterated solvent. All the polymers adopt 1D rod-like aggregation structures and the radius of the 1D rods is not sensitive to the Mn , while the length increases monotonically with Mn . By utilizing the ordered packing of the aggregated structure in solution, a highly aligned and ordered film is prepared and, thereafter, a reliable hole mobility of 13.8 cm2 V-1 s-1 is realized in organic thin-film transistors with the moderate Mn batch via bar coating. The hole mobility is among the highest values reported for diketopyrrolopyrrole-based polymers. This work paves the way to visualize the real aggregated structure of polymer semiconductors in solution and sheds light on the microstructure control of high-performance electronic devices.

Sub-Rouse Dynamics in Poly(isobutylene) as a Function of Molar Mass

We report on the molar mass dependence of the segmental, sub-Rouse, and terminal relaxation processes of poly(isobutylene). For this purpose, seven samples were synthesized with molar masses in the range from 2.8 to 172 kg·mol–1 and investigated by temperature-modulated differential scanning calorimetry, dielectric spectroscopy, and rheology. Rheology provided access to the terminal times that were found to scale as τterm ∼ Μ3.2. On the other hand, the sub-Rouse dielectric process shows a very weak molar mass dependence (τsub – Rouse/τSM ∼ M0.1), that is, distinctly different from the Rouse and terminal dynamics. We infer that the sub-Rouse mode has a lengthscale intermediate to the statistical segment length (∼1.25 nm) and the length of an entanglement strand (∼2.3 nm).

Adsorption Kinetics of cis-1,4-Polyisoprene in Nanopores by In Situ Nanodielectric Spectroscopy

Using in situ nanodielectric spectroscopy, we studied the adsorption kinetics of cis-1,4-polyisoprene (PI) into porous alumina by following the evolution of the dielectrically active longest normal mode. We studied the influence of molar mass, nanopore diameter, and surface functionalization. Adsorption times depend strongly on the ratio 2Rg/D, where Rg is the radius is gyration and D is the pore diameter. For a given pore diameter, the characteristic adsorption times are some 8 orders of magnitude slower than the terminal relaxation times and more than 12 orders of magnitude slower than the segmental times. The extremely slow kinetics reflect the fact that exchanging chains with the pore surface have to pass through several unfavorable configurations (e.g., trains, loops). The molar mass dependence of the characteristic adsorption times (τads ∼ N2.6) is in good agreement with a scaling theory proposed by de Gennes and later refined by Semenov and Joanny. Subsequently, we investigated the imbibition of miscible PI blends by taking advantage of the difference in imbibition speeds of the respective homopolymers. We show that the shorter chains penetrate first the nanopores, whereas the longer chains enter only at the late stages of the filling process. Moreover, the long-time adsorption is dominated by an exchange mechanism involving primarily the shorter chains. The results from in situ nanodielectric spectroscopy demonstrate the capacity of the technique to provide the imbibition length, the adsorption kinetics, and, at the same time, the chain dynamics.

Tuning the molar mass of P3HT via direct arylation polycondensation yields optimal interaction and high efficiency in nonfullerene organic solar cells

The application and the molar mass dependence of P3HT via direct arylation polycondensation are explored in fullerene-free solar cells. The medium molar mass batch delivered a top efficiency of ∼10%.

Novel Theoretical Self‐Consistent Mean‐Field Approach to Describe the Conductivity of Carbon Fiber‐Filled Thermoplastics: Part III—Application of the Concept to Mechanical Properties of Composites and Polymer Solutions

A novel theoretical approach, which yields a nonlinear differential equation for the mechanical properties of fiber‐filled composites as a function of the volume fraction of the filler and the fiber orientation, is shown. Furthermore, the transfer to polymer solutions is shown and gives a physical explanation for various well‐known empirical relations and numerical values including the Huggins constant and the exponent of power–law molar mass dependence of the polymer melt viscosity.

Mesoscopic Simulations of Free Surfaces of Molten Polyethylene: Brownian Dynamics/Kinetic Monte Carlo Coupled with Square Gradient Theory and Compared to Atomistic Calculations and Experiment

A mesoscopic simulation approach is developed for liquid–gas interfaces of weakly and strongly entangled polymer melts and implemented for linear polyethylene at 450 K. A combined particle and field-theoretic treatment is adopted based on aggressive coarse-graining, each polymer bead representing ∼50 carbon atoms, with effective bonded interactions extracted from atomistic simulations. Nonbonded interactions in the mesoscopic model are dictated by an equation of state (here the Sanchez–Lacombe) in conjunction with a variant of gradient theory—the discrete square gradient theory. The dynamics of free films is examined in the presence and in the absence of topological constraints (modeled by slip-springs) to unveil the impact of the latter on chain self-diffusion, to assess their contribution to the interfacial free energy, and to explore how this contribution can be removed by invoking a compensating potential. The molar mass dependence of surface tension—which arises from bonded contributions among beads ...

Molar mass dependence of structure of xanthan thermally denatured and renatured in dilute solution

Double helical polysaccharide, xanthan samples with varying molar mass were thermally denatured and renatured in dilute solutions. Both the weight average molar mass, which was determined by size exclusion chromatography (SEC), and viscosity average molar mass decreased after denaturation and renaturation for samples with an initial molar mass of 106 g mol−1, but those for the samples with an initial molar mass of 105 or 107 g mol−1 decreased only slightly. Although the double helices of xanthan only partially dissociated into single chains, the molar mass distribution estimated by SEC did not broaden drastically by the denaturation and renaturation. These results can be explained by the formation of hairpin structures by the single chains, which are restricted to xanthan with a molar mass of 105 g mol−1 by the strain of the hairpin loop, and for xanthan molar masses of 107 g mol−1 by the incomplete dissociation of its long polymer chains. Double helical polysaccharide, xanthan samples with varying molar mass were thermally denatured and renatured in dilute solutions. The molar mass decreased after denaturation and renaturation for samples with an initial molar mass of 106 g mol−1, but those for the samples with an initial molar mass of 105 or 107 g mol−1 decreased only slightly. A model explaining this molar mass dependence of the denaturation and renaturation behaviors was proposed based on the experimental results.

Molar mass and temperature dependence of rheological properties of polymethylmethacrylate melt

The influence of molar mass and temperature on the rheological properties of polymethylmethacrylate (PMMA) melts was investigated by dynamical mechanical and creep-recovery experiments. The results shown that the steady state viscosities obtained from both experimental modes were in good agreement, and a well linear relationship between the activation energy and molar mass was obtained. The creep compliance of PMMA increased with the molar mass increased or temperature decreased, while the linear steady state recoverable compliance was found to be independent of the molar mass and temperature. Moreover, it was found that the master curves followed the time-temperature superposition principle well in the whole frequency range or time scale.

Rheology and Microscopic Heterogeneity of Poly(ethylene oxide) Solid Polymer Electrolytes

Lack of precise thermal control in poly(ethylene oxide) (PEO)‐based solid polymer electrolytes (SPEs) with molar masses well above M > 104 g mol−1 during preparatory stage often demonstrates molar mass dependence on thermal properties and ionic conductivities. In the earlier study, PEO‐based SPEs were heated under inert atmosphere above the melting temperature of PEO and then cooled down for subsequent isothermal crystallization. The system demonstrates insignificant variation with respect to molar mass of PEO at constant salt concentration for thermal properties, ionic conductivity and intermolecular interaction. Here, we report the subsequent results of rheology and microscopic heterogeneity as a function of PEO molar mass (Mɳ = 3 × 105 and 4 × 106 g mol−1) and lithium salt concentration (0–13 wt.%). Above the solubility limit of PEO, the glass transition studied using differential scanning calorimetry shows the presence of microscopic heterogeneity of the systems as indicated by the change in enthalpy of endothermic overshoot near the endset of glass transition. Rheological parameters were measured over a wide range of frequency (0.01 to 100 Hz) at different temperatures (30, 60 and 80 °C) using parallel plate geometry. Melt rheology system reveals that neat PEO at 80 °C exhibits restriction in the long‐range motion of the chains. When incorporated with salt, it exhibits further deviation from the terminal viscoelastic relaxation behaviour. This study agrees with our previous findings that influence of molar mass of PEO in this range is insignificant on the melt rheology and microscopic heterogeneity at constant salt concentration.

Influence of molar mass on the thermal properties, conductivity and intermolecular interaction of poly(ethylene oxide) solid polymer electrolytes

AbstractThe sample preparation pathway of solid polymer electrolytes (SPEs) influences their thermal properties, which in turn governs the ionic conductivity of the materials especially for systems consisting of a crystallizable constituent. Majority of poly(ethylene oxide) (PEO)‐based SPEs with molar masses of PEO well above 104 g mol−1 (where PEO is crystallizable and should reach an asymptote in thermal behaviour) display molar mass dependence of the thermal properties and ionic conductivities in non‐equilibrium conditions, as reported in the literature. In this study, PEO of different viscosity‐molar masses (Mη = 3 × 105, 6 × 105, 1 × 106, 4 × 106 g mol−1) and LiClO4 salt (0 to 16.7 wt%) were used. The SPEs were thermally treated under inert atmosphere above the melting temperature of PEO and then cooled down for subsequent isothermal crystallization for sufficient experimental time to develop morphology close to equilibrium conditions. The thermal properties (e.g. glass transition temperature, melting temperature, crystallinity) according to differential scanning calorimetry and the ionic conductivity obtained from impedance spectroscopy at room temperature (σDC ∼ 10−6 S cm−1) demonstrate insignificant variation with respect to the molar mass of PEO at constant salt concentration. These findings are in agreement with the PEO crystalline structures using X‐ray diffraction and ion − dipole interaction by Fourier transform infrared results. © 2017 Society of Chemical Industry

Physical state of cellulose in BmimCl: dependence of molar mass on viscoelasticity and sol-gel transition.

In this work, we investigated the correlation between the molar mass and the rheological properties of cellulose/1-butyl-3-methylimidazolium chloride (BmimCl) solutions, and provided the depolymerization kinetics of cellulose in BmimCl. Gel permeation chromatography was used to track the change in molar mass and kinetics as a function of the dissolution time. The molar mass of cellulose in BmimCl decreased significantly as the dissolution time increased, following a zeroth order rate law. The decrease of inter-chain friction induced by depolymerization resulted in a lower viscosity, shorter relaxation time, and lower activation energy. The activation energies for flow were distinctly different above and below the critical molar mass, which indicates that the relaxation mechanisms were not identical above and below the critical molar mass. The transition behavior of liquid crystalline phase also changed at the critical molar mass, which strongly demonstrated the effect of chain length on the formation of cholesteric phase. The exponents of Mark-Houwink-Sakurada and the radius of gyration showed that cellulose in BmimCl existed as a Gaussian chain in a theta solvent.

All Polymer Diffusion Regimes Covered by Combining Field-Cycling and Field-Gradient1H NMR

Field-cycling and field-gradient 1H NMR experiments were combined to reveal the segmental mean-square displacement as a function of time for polydimethylsiloxane (PDMS) and polybutadiene (PB). Together, more than 10 decades in time are covered, and all four power-law regimes of the tube-reptation (TR) model are identified with exponents rather close to the predicted ones. Characteristic polymer properties like the tube diameter a0, the Kuhn length b, the mean-square end-to-end distance ⟨R02⟩, the segmental correlation time τs(T), the entanglement time τe(T), and the disengagement time τd(T) are estimated from the measurements and compared to results from literature. Concerning τd(T), fair agreement is found. In the case of τe, agreement with rheological data is achieved when the time constant is extracted from the minimum in the shear modulus G″(ω). Concerning the TR predictions the molar mass (M) dependence of τd is essentially reproduced. Yet, calculating τe from τd for PDMS yields agreement with experi...

Dynamics of PPI Dendrimers: A Study by Dielectric and2H NMR Spectroscopy and by Field-Cycling1H NMR Relaxometry

We investigate bulk poly(propyleneimine) dendrimers of generation (G) 2–5 by dielectric spectroscopy (DS), solid-state 2H NMR, and field-cycling 1H NMR relaxometry (FC 1H NMR) in a large temperature range (120–400 K). Three relaxation processes are identified by DS: a main (α-) relaxation (T > Tg) and two secondary processes (T < Tg). The α-process exhibits a super-Arrhenius temperature dependence typical of glass-forming liquids and changes only weakly with G, yielding Tg ∼ 200 K. The temperature dependence of the secondary relaxations is governed by an Arrhenius law. While one secondary process exhibits features characteristic for glasses, the other is atypical. Its time constant is virtually independent of G, and its spectral width does not increase with lowering temperature as is usually observed for sub-Tg relaxations. Regarding FC 1H NMR probing the dispersion of the spin–lattice rate R1 in the frequency range 200 Hz–30 MHz, transformation to the susceptibility representation, χ″(ω) ≡ ωR1(ω), and applying frequency–temperature superposition, an effective frequency range of 9 decades is covered by a master curve χ″(ωτα). In addition to the segmental time τα(T), which complements the results from DS up to high temperatures, a longer terminal relaxation τt(T) is identified. In between, an intermediate power-law regime is observed in χ″(ωτα) with an exponent of about 0.8. The broad relaxation spectrum is attributed to local dynamics, breathing modes, and overall tumbling and diffusion of the dendrimer molecule. In the low-frequency limit, R1(ω) is determined by intermolecular relaxation from which the molar mass dependence of the translational diffusion coefficient can be estimated. We find D(M) ∝ M–1.2±0.2.

Molar mass and temperature dependence of the thermodiffusion of polyethylene oxide in water/ethanol mixtures.

In this work, we study the molar mass dependence of the thermodiffusion of polyethylene oxide at different temperatures in ethanol, water/ethanol mixture (c(water) = 0.7), and water in a molar mass range up to M(w) = 180,000 g/mol. Due to the low solubility of polyethylene oxide oligomers in ethanol the measurements are limited up to M(w) = 2200 g/mol. The specific water/ethanol concentration 0.7 has been chosen, because at this weight fraction the thermal diffusion coefficient, D(T), of water/ethanol vanishes so that the system can be treated as a pseudo binary mixture. The addition of ethanol will degrade the solvent quality, so that we expect a change of the interaction energies between polymer and solvent. The analysis of the experimental data within a theoretical model shows the need of a refined model, which takes specific interactions into account.

Complementary Characterization of End Groups in Radically Polymerized Poly(methyl methacrylate) by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry and Pyrolysis-Gas Chromatography-Mass Spectrometry.

The end groups in radically polymerized poly(methyl methacrylate) samples with tert-butyl peroxy-2-ethylhexanoate as an aliphatic peroxide initiator and 1-octanethiol as a chain transfer reagent were complementarily characterized by high-resolution matrix assisted laser desorption/ionization (MALDI) spiral time-of-flight mass spectrometry (MS) and pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). The end groups comprised of three types of the initiator fragments and octylthio group originating from the chain transfer agent were confirmed by MALDI-MS measurements. In addition, their quantitative information was obtained by Py-GC-MS. Furthermore, combined with size exclusion chromatographic fractionation, the molar mass dependence of the end groups in the PMMA samples was also examined. It was suggested that the relative content of the octylthio end groups might increase with increase in the molar mass of the fractions. The observed results were interpreted in terms of the polymerization reactions of the PMMA samples.

Atom transfer radical polymerization as a tool for making poly(N-acryloylglycinamide) with molar mass independent UCST-type transitions in water and electrolytes

Atom transfer radical polymerization (ATRP) of N-acryloylglycinamide has been used as a tool to make poly(NAGA) showing UCST-type phase transitions in water and electrolytes independent of molar mass and end-groups. We hypothesized that similarity in the structure of polymer chain ends with the repeat unit of poly(NAGA) could help in eliminating the effect of molar mass dependence of the cloud point especially in the low molar mass region. The monomer-like end-groups were introduced by choosing an appropriate initiator for ATRP like chloropropionamide (CPA). The catalyst system CuCl/CuCl2 and tris[2-(dimethylamino)ethyl]-amine (Me6TREN) as a ligand provided controlled polymerization of NAGA in DMSO at 45 °C with UCST-type transitions retained in water and electrolytes without being influenced by chain ends/molar mass and high concentration of salts like NaCl and Na2SO4.

Comparative thermal, biological and photodegradation kinetics of polylactide and effect on crystallization rates

A comparison is made between available literature and present results of the three major types of polylactide (PLA) degradation giving a compiled view on the details of degradation mechanisms with relation to explicit factors affecting degradation. The temporal decrease of molar mass has been analyzed under isothermal conditions at 220 °C, biological and photodegradation conditions using a polylactide (PLA) with ∼4 mol% D units. The decrease of molar mass with time during biodegradation follows a first order process (M = Moe−kt) while the molar mass of specimens tested during thermal and photodegradation follows a second order law (1/M = (1/Mo) + kt) Literature data obtained in similar degradation conditions were also adequately fitted with these equations. This allows us to conclude that the main step in the three types of degradation is a random chain excision, with some differences in the algebraic functionality. Under the degradation conditions tested, the degradation rate follows the progression thermal > photo > biological. For equivalent molar mass, the effect of degradation type on cold crystallization and melting is significant indicating that degradation cannot be explained by a solely outcome of chains breakage and molar mass reduction. This feature is especially prominent when the linear growth rates of specimens subjected to bio or photo degradation are compared. Anhydride groups that are formed during photodegradation decrease the crystallization rate compared to biodegraded specimens of equivalent molar mass. The molar mass dependence of the maximum growth rate follows a power law with exponents 1.3 for bio and 1.0 for photodegraded specimens, representative of semi-entangled systems. The temperature coefficient of the growth rate, analyzed according to secondary nucleation leads to a linear dependence for bio and photodegraded specimens, and to values of the surface free energy of crystallites that decrease from ∼85 to 55 erg/cm2 with decreasing molar mass. Combinations of molar mass characterization, FTIR, and thermal and crystallization rate analysis are proven useful strategies to assess and discriminate macroscopic changes of PLA structure induced by different types of degradation. This work also underlines the importance of analyzing the linear growth rates as a parameter that uncovers specific structural changes during degradation.

Molar Mass Dependence of Polyethylene Chain Dynamics. A Quasi-Elastic Neutron Scattering Investigation

We present a detailed analysis of the incoherent dynamic structure factor of a series of n-alkanes and high molar mass polyethylene (PE). C30H62 data indicate that two molecular processes are simul...

Molar Mass and Microstructure Analysis of PI‐b‐PMMA Copolymers by SEC‐NMR

AbstractThe online coupling of size‐exclusion chromatography and NMR is used to characterize block copolymers consisting of polyisoprene (PI) and poly(methyl methacrylate) (PMMA) regarding their distributions of molar mass (MMD) and chemical composition (CCD). Using the CCD, $\overline{M}_{\rm n}$ and $\overline{M}_{\rm w}$ are calculated on the basis of PI and PMMA homopolymer calibrations. The microstructure distribution of PMMA and the distribution of isomeric units of PI in dependence of molar mass is also demonstrated. Furthermore, a simulation analysis is presented for a bimodal eluting sample. It allows for full separation, quantification and molar mass determination of the coeluting co‐ and homopolymer fractions. The quantification of the fractions is verified by liquid chromatography at critical conditions.