Entanglement Molecular Weight Research Articles

Versatile Light‐Mediated Synthesis of Degradable Bottlebrush Polymers Using α‐Lipoic Acid

AbstractBottlebrush polymers have a variety of useful properties including a high entanglement molecular weight, low Young's modulus, and rapid kinetics for self‐assembly. However, the translation of bottlebrushes to real‐world applications is limited by complex, multi‐step synthetic pathways and polymerization reactions that rely on air‐sensitive catalysts. Additionally, most bottlebrushes are non‐degradable. Herein, we report an inexpensive, versatile, and simple approach to synthesize degradable bottlebrush polymers under mild reaction conditions. Our approach relies on the “grafting‐through” polymerization of α‐lipoic acid (LA)‐functionalized macromonomers. These macromonomers can be polymerized under mild, catalyst‐free conditions, and due to reversibility of the disulfide bond in LA, the resulting bottlebrush polymers can be depolymerized by cleaving disulfide backbone bonds. Bottlebrushes with various side‐chain chemistries can be prepared through the atom transfer radical polymerization (ATRP) of LA‐functionalized macromonomers, and the backbone length is governed by the macromonomer molecular weight and solvent polarity. We also demonstrate that LA‐functionalized macromonomers can be copolymerized with acrylates to form degradable bottlebrush networks. This work demonstrates the preparation of degradable bottlebrush polymers with a variety of side‐chain chemistries and provides insight into the light‐mediated grafting‐through polymerization of dithiolane‐functionalized macromonomers.

Versatile Light-Mediated Synthesis of Degradable Bottlebrush Polymers Using α-Lipoic Acid.

Bottlebrush polymers have a variety of useful properties including a high entanglement molecular weight, low Young's modulus, and rapid kinetics for self-assembly. However, the translation of bottlebrushes to real-world applications is limited by complex, multi-step synthetic pathways and polymerization reactions that rely on air-sensitive catalysts. Additionally, most bottlebrushes are non-degradable. Herein, we report an inexpensive, versatile, and simple approach to synthesize degradable bottlebrush polymers under mild reaction conditions. Our approach relies on the "grafting-through" polymerization of α-lipoic acid (LA)-functionalized macromonomers. These macromonomers can be polymerized under mild, catalyst-free conditions, and due to reversibility of the disulfide bond in LA, the resulting bottlebrush polymers can be depolymerized by cleaving disulfide backbone bonds. Bottlebrushes with various side-chain chemistries can be prepared through the atom transfer radical polymerization (ATRP) of LA-functionalized macromonomers, and the backbone length is governed by the macromonomer molecular weight and solvent polarity. We also demonstrate that LA-functionalized macromonomers can be copolymerized with acrylates to form degradable bottlebrush networks. This work demonstrates the preparation of degradable bottlebrush polymers with a variety of side-chain chemistries and provides insight into the light-mediated grafting-through polymerization of dithiolane-functionalized macromonomers.

Raising Two More Fundamental Questions Regarding the Classical Views on the Rheology of Polymer Melts.

The current paradigm of polymer flow assumes that (i) the effect of the molecular weight of the macromolecules, M, and of the temperature, T, on the expression of the viscosity of polymer melts separate; (ii) the molecular weight for entanglement, Mc, is independent of T; and (iii) the determination of Mc by the break in the log viscosity curve against log M unequivocally differentiates un-entangled melts from entangled melts. We use reliable rheological data on monodispersed polystyrene samples from very low molecular weight (M/Mc = 0.015) to relatively high molecular weight (M/Mc = 34) to test the separation of M and T in the expression of the viscosity; we reveal that an overall illusion of the validity of the separation of T and M is mathematically comprehensible, especially at high temperature and for M > 2Mc, but that, strictly speaking, the separation of M and T is not valid, except for certain periodic values of M equal to Mc, 2Mc, 4Mc, 8Mc, 16Mc, etc. (period doubling) organized around a "pole reference" value MR = 4Mc. We also reveal, for M < Mc, the existence of a lower molecular weight limit, M'c = Mc/8 for the onset of the macromolecular behavior (macro-coil). The discrete and periodic values of M that validate the separation of the effect of M and T on the viscosity generate the fragmentation of the molecular range into three rheological ranges. Likewise, we show that the effect of temperature is also fragmented into three rheological ranges for T > Tg: Tg < T< (Tg + 23°), (Tg + 23°) < T < TLL and T > TLL' where TLL is the liquid-liquid temperature. Our conclusion is that the classical formulation of the viscosity of polymer melts is so overly simplified that it is missing important experimental facts, such as period doubling for the separation of T and M, TLL, M'c, and Mc, resulting in its inability to understand the true nature of entanglements. We present in the discussion of the paper the alternative approach to the viscoelastic behavior, "the duality and cross-duality" of the Dual-conformers, showing how this model formalism was used to test mathematically and invalidate the separation of T and M in the classical formulation of viscosity.

High Molar Mass Polycarbonates as Closed-Loop Recyclable Thermoplastics.

Using carbon dioxide (CO2) to make recyclable thermoplastics could reduce greenhouse gas emissions associated with polymer manufacturing. CO2/cyclic epoxide ring-opening copolymerization (ROCOP) allows for >30 wt % of the polycarbonate to derive from CO2; so far, the field has largely focused on oligocarbonates. In contrast, efficient catalysts for high molar mass polycarbonates are underinvestigated, and the resulting thermoplastic structure-property relationships, processing, and recycling need to be elucidated. This work describes a new organometallic Mg(II)Co(II) catalyst that combines high productivity, low loading tolerance, and the highest polymerization control to yield polycarbonates with number average molecular weight (Mn) values from 4 to 130 kg mol-1, with narrow, monomodal distributions. It is used in the ROCOP of CO2 with bicyclic epoxides to produce a series of samples, each with Mn > 100 kg mol-1, of poly(cyclohexene carbonate) (PCHC), poly(vinyl-cyclohexene carbonate) (PvCHC), poly(ethyl-cyclohexene carbonate) (PeCHC, by hydrogenation of PvCHC), and poly(cyclopentene carbonate) (PCPC). All these materials are amorphous thermoplastics, with high glass transition temperatures (85 < Tg < 126 °C, by differential scanning calorimetry) and high thermal stability (Td > 260 °C). The cyclic ring substituents mediate the materials' chain entanglements, viscosity, and glass transition temperatures. Specifically, PCPC was found to have 10× lower entanglement molecular weight (Me)n and 100× lower zero-shear viscosity compared to those of PCHC, showing potential as a future thermoplastic. All these high molecular weight polymers are fully recyclable, either by reprocessing or by using the Mg(II)Co(II) catalyst for highly selective depolymerizations to epoxides and CO2. PCPC shows the fastest depolymerization rates, achieving an activity of 2500 h-1 and >99% selectivity for cyclopentene oxide and CO2.

The Challenges Facing the Current Paradigm Describing Viscoelastic Interactions in Polymer Melts.

Staudinger taught us that macromolecules were made up of covalently bonded monomer repeat units chaining up as polymer chains. This paradigm is not challenged in this paper. The main question raised in polymer physics remains: how do these long chains interact and move as a group when submitted to shear deformation at high temperature when they are viscous liquids? The current consensus is that we need to distinguish two cases: the deformation of "un-entangled chains" for macromolecules with molecular weight, M, smaller than Me, "the entanglement molecular weight", and the deformation of "entangled" chains for M > Me. The current paradigm stipulates that the properties of polymers derive from the statistical characteristics of the macromolecule itself, the designated statistical system that defines the thermodynamic state of the polymer. The current paradigm claims that the viscoelasticity of un-entangled melts is well described by the Rouse model and that the entanglement issues raised when M > Me, are well understood by the reptation model introduced by de Gennes and colleagues. Both models can be classified in the category of "chain dynamics statistics". In this paper, we examine in detail the failures and the current challenges facing the current paradigm of polymer rheology: the Rouse model for un-entangled melts, the reptation model for entangled melts, the time-temperature superposition principle, the strain-induced time dependence of viscosity, shear-refinement and sustained-orientation. The basic failure of the current paradigm and its inherent inability to fully describe the experimental reality is documented in this paper. In the discussion and conclusion sections of the paper, we suggest that a different solution to explain the viscoelasticity of polymer chains and of their "entanglement" is needed. This requires a change in paradigm to describe the dynamics of the interactions within the chains and across the chains. A brief description of our currently proposed open dissipative statistical approach, "the Grain-Field Statistics", is presented.

On the Z5.8‐dependence of viscosity of polymer rings over the range 15 &lt; Zw ≤ 300 of the entanglement number

AbstractDynamic moduli G* and zero‐shear viscosity ηo,ring data in both melt and solution states were reported for putative cyclic poly(3,6‐dioxa‐1,8‐octanedithiol) (RDODT) with molecular weights ranging from 43 to 556 kg/mol. Measurements over wide temperature ranges provide up to 18 decades of reduced frequency to characterize RDODT melts and solutions with Zw = Mw/Me,lin from 23 to 300, where Mw is the weight average molecular weight and Me,lin ≈ 1850 g/mol is the entanglement molecular weight of the linear counterparts (LDODT). The G* of all the seven RDODT melts and eight of their solutions with Zw &gt; 15 all have a rubbery plateau with the plateau moduli about same as GNo of the corresponding LDODT. The value Zw ≈ 15 marks the threshold for the change in scaling of ηo,ring from the Rouse‐like ηo,ring ∼ Zw1.0 to a much stronger dependence of ηo,ring ∼ Zw5.8 for Zw up to 300. Such a Zw5.8‐dependence has not been seen before in cyclic polymers and cannot be accounted for by current theoretical models for cyclic polymers. The Coupling Model had been used before to explain the Mw3.4 ‐dependence of linear entangled polymers and the stronger Mw ‐dependence of multi‐arm star branched polymers. It is applied to cyclic RDODT melts and solutions and the results provide an explanation of the observed Zw5.8‐dependence of ηo,ring.

Linear Viscoelastic Properties of Putative Cyclic Polymers Synthesized by Reversible Radical Recombination Polymerization (R3P)

Linear viscoelastic properties in both melt and solution states are reported for a series of poly(3,6-dioxa-1,8-octanedithiol) (polyDODT) made by reversible radical recombination polymerization (R3P) under conditions designed to produce linear (LDODT), cyclic (RDODT), and linear–cyclic mixtures (LRDODT). PolyDODT is amorphous (Tg < −50 °C) and highly flexible (entanglement molecular weight Me,lin ≈ 1850 g/mol for LDODT). PolyDODT’s low Tg and low Me,lin enable characterization over a wide dynamic range and a wide range of dimensionless weight-average molecular weight Zw = Mw/Me,lin. Measurements at temperatures from −57 to 100 °C provide up to 18 decades of reduced frequency, which is necessary to characterize RDODT melts with Zw from 23 to 300. The two highest-molecular-weight polymers in the present RDODT series have such high Mw (406k and 556k g/mol) that mass spectrometry, NMR spectroscopy, and even chemical assays for chain ends are unable to rule out up to 2 mol % of linear contaminant. By studying the samples in solution (using dilution to reduce Zw), we could compare their dynamics with those of previously established high-purity polystyrene (PS) rings (limited to Zw ≤ 13.6). RDODT solutions with Zw < 15 (concentrations <5 wt % for RDODT-406k and 556k) have dynamic moduli G* that accord with LCCC-purified PS rings in terms of the frequency dependence (including the absence of a plateau), the progression of shapes of G* as a function of Zw, and the linear scaling of their zero-shear viscosity η0 with Mw. The shape of G* as a function of Zw for solutions of RDODT-406k and -556k also accords with lower Mw RDODT melts (which have ≤1.3 mol % of linear contaminant). Thus, the measurement of the linear viscoelastic properties of appropriate concentrations of high Mw (>200k g/mol) putative cyclic polymers, in which linear chains evade spectroscopic detection, may provide an alternative means (though not fully proven) of validation of sample purity. When Zw > 15 (including all seven RDODT melts and eight of their solutions), G* has a rubbery plateau. This suggests that the onset of entanglement-like behavior in rings requires 4–5-fold greater Zw than is required for linear chains. Further, the plateau moduli of RDODT samples are indistinguishable from GNo of the corresponding LDODT (melt or matched-concentration solutions). In entangled linear polymers, the observation that GNo is independent of Zw follows from limitations on lateral fluctuations due to neighboring chains becoming independent of position along a given chain. The present results for RDODT suggest that this holds for sufficiently long endless chains, too. While the RDODTs have the same GNo as entangled LDODTs, when Zw > 60, the terminal relaxation, if reached at all, of RDODT extends to orders of magnitude lower frequency than an entangled linear polymer of the same Zw. Consequently, the viscosity of RDODT with Zw > 60 increases with Zw much more strongly than the 3.4 power observed for entangled linear polymers. Finally, these novel polymers, with a disulfide-linked backbone and broad relaxation time distribution, may prove important in relation to biodegradable elastomers and materials with exceptional low-frequency dissipation, extending at least 12 decades below the onset of the rubbery plateau.

Molecular Dynamics of Polymyrcene: Rheology and Broadband Dielectric Spectroscopy on a Stockmayer Type A Polymer

Molecular dynamics of polymyrcene (PM) covering a wide range of molecular weights from 6.7 × 103 ≤ Mw ≤ 3.47 × 105 g/mol (weight-average molecular weight Mw) with a molar mass dispersity Đ ≤ 1.16 were investigated by oscillatory shear rheology and broadband dielectric spectroscopy. The analysis of the zero-shear viscosity (η0) versus molecular weight in a double-logarithmic plot reveals a scaling exponent of ∼1.2 below the critical molecular weight and ∼3.7 above a critical molecular weight of Mc ≅ 4.4 × 104 g/mol. However, at molecular weights greater than the reptation molecular weight Mr, a molecular weight substantially larger than the entanglement molecular weight Me, the zero shear viscosity scaling decreases. A scaling of η0 ∝ M3.0, which might be attributed to a pure tube-reptation behavior, was found. The exponent of 1.2 is attributed to a Rouse-like dynamic and is interpreted in terms of PM’s bottlebrush-like nature arising from tightly packed C6/C8 pendant groups. Dielectric spectroscopy measurements revealed two relaxation processes typical for fully amorphous glassy polymers: The first is a segmental mode process, with a dynamic glass transition process (α), due to the segmental motions and the second one is a local relaxation process in the glassy state (β). Additionally, above the glass transition temperature at low frequency, another relaxation process was observed, which appears to be the normal mode process (n) due to the fluctuation of the end-to-end vector. This polymer is assigned to be a new Stockmayer type A polymer, whose normal mode relaxation is strongly dependent on its molecular weight. These findings allow for the rare possibility to explore the global dynamics of PM by using both oscillatory shear rheology and broadband dielectric spectroscopy methods over a wide range of temperatures to investigate molecular dynamics using the time–temperature superposition principle.

Influence of Entanglements on Ultrahigh Strain Rate Deformation of Polystyrene Microprojectiles

Micron-sized polystyrene spherical projectiles made from three different molecular weight polymers spanning from below to well above the entanglement molecular weight were launched against a rigid substrate, causing ultrahigh strain rate deformation. Scanning electron microscopy, focused ion beam cross sections, and atomic force microscopy were used to elucidate the deformation mechanisms from the observed morphologies of the deformed specimens and to evaluate the important role of entanglements in the various deformation processes that dissipate the kinetic energy. Due to adiabatic heating from shock compression, the temperature at the bottom region of the polymer projectile is elevated above the glass transition temperature, enabling viscoplastic flow. The cooler portions of the unentangled sample undergo strain localization via micro-shear banding and brittle fracture, while the two entangled samples resist fracture and exhibit extensive viscoplastic flow and, in the upper, cooler regions, extensive shear banding followed by crazing. The interaction of molecular entanglements, temperature, shear rate, and total shear strain on melt viscosity influences the amount of additional plastic work and adiabatic heating occurring in the sample and hence the extent of lateral spreading of the projectile over the substrate.

Polymer segmental dynamics near the interface of silica particles in the particle/polymer composites

We demonstrate an approach to examine the local segmental dynamics of a polymer near the interface of an inorganic filler by observing the rotational dynamics of the fluorescent probe at the chain ends of polymer brushes grafted onto the surface of the filler particles. Localization of the fluorescent probe was realized by designing and synthesizing fluorophore-tethered polystyrene (PS) brushes anchored on the surface of silica particles of controlled sizes. Fluorophore-functionalized telechelic PS with an azide functionality at the other chain end was achieved via a combination of atom transfer radical polymerization and post-polymerization modification. The azide-bearing PS chains were tethered to alkyne-functionalized particles via copper-catalyzed cycloaddition reaction. The molecular weight of the grafted polymer chains was controlled to be less than the critical entanglement molecular weight, and the chain density was controlled to be low enough so that the observed dynamics was not perturbed by the polymer brush conformation and brush-matrix polymer entanglement. The polymer dynamics near the surface of the particles at low concentrations was closely examined in the bulk film geometry by employing imaging rotational fluorescence correlation microscopy (irFCM). The observed polymer dynamics near the interface were not altered in the inorganic/polymer composite geometry when the surface did not have favorable interaction with the matrix polymer. The presented rational design of the chemical route and examination of local dynamics highlight a feasible approach to construct material systems with high complexities towards a deeper understanding of composite materials.

Viscoelastic properties of comb-shaped ring polystyrenes

In this study, we examined the viscoelastic properties of a series of comb-shaped ring (RC) polystyrene samples with different branch chain lengths, i.e., the molecular weights of the ring backbone Mbb (≃ 4Me where Me is the entanglement molecular weight) and branch chains Mbr (≃ Me, 2Me, and 4Me). Even for the RC sample with the shortest branch chains, a plateau region was observed for the dynamic modulus G*(ω) in the middle angular frequency ω region, suggesting that intermolecular branch chain entanglement occurred. In the ω region between the plateau and terminal regions, G*(ω) was observed with a weaker ω dependence than the terminal relaxation. This behavior was more pronounced for RC samples with shorter branch chains and for the corresponding linear comb (LC) samples than for the RC ones. The molecular weight dependence of the zero-shear viscosity η0 and the steady-state recoverable compliance Jeo of the RC and LC samples was evaluated, and the effects of different backbone molecular structures (i.e., ring or linear) on the terminal relaxation behavior was discussed. Moreover, the G*(ω) data were analyzed with two models: the comb-Rouse model, in which the structures of the RC/LC molecules are taken into account by graph theory, and the Milner-McLeish model for entangled star-shaped polymers. The former model qualitatively described the terminal relaxation behavior of G*(ω) at low ω but failed to reproduce the plateau in the middle ω range. Conversely, the latter model described the entanglement plateau in the middle ω range, but the difference in the terminal relaxation regime for the RC/LC samples seen in the data and the comb-Rouse model disappeared.

Tuning of thermally-induced shape memory properties of low-cost biocompatible linear and chemically crosslinked isobornyl/isobutylacrylate copolymers

A series of binary low-cost biobased copolymers was elaborated from isobornylacrylate (IBOA) and isobutylacrylate (IsoBA) monomers applying UV–visible induced free radical polymerization, yielding linear and crosslinked architectures. Thermo-mechanical measurements revealed that the elaborated random copolymers exhibit single glass transition temperatures, which can be adjusted on a broad temperature range by varying the amount of IBOA- and HDDA-units. Governed by the glass transition, linear high molecular weight IBOA-IsoBA copolymers exhibit good thermally induced shape memory properties related to the occurrence of strong physical entanglements, reinforced by the presence of the voluminous isobornylene groups. Fixity and recovery ratios were found to be in the same order of magnitude when compared to crosslinked thermoplastics and thermosetting polymers. Linear IBOA-IsoBA copolymers maintain their form memory properties even after several application cycles, confirming the efficiency of the presence of physical crosslinking. These findings can be explained by the high entanglement molecular weight of 44 kg/mol providing an important elasticity as well as by an elevated glassy/rubbery moduli ratio of around 3000, facilitating deformation and fixing modes.

Self-Exchange of Polyelectrolyte in Multilayers: Diffusion as a Function of Salt Concentration and Temperature

Polymer chain diffusion within a hydrated polyelectrolyte complex, PEC, has been measured using an ultrathin film format prepared by the layer-by-layer method. Isotopically labeled self-exchange of deuterated poly(styrene sulfonate), dPSS, with undeuterated PSS of the same narrow molecular weight distribution permitted reliable estimates of whole-molecule diffusion coefficients, D. Narrow molecular weight distribution poly(diallyldimethylammonium), PDADMA, was used as the polycation for the PEC. Extensive pretreatment of starting films was undertaken to remove residual stress, anisotropy, and layering. PSS/PDADMA “multilayers,” PEMUs, thin enough to provide substantial exchange of polyelectrolyte, even with diffusion coefficients as low as 10–16 cm2 s–1, as a function of salt concentration and temperature were measured for this PEC, which has a glass-transition temperature, Tg, close to room temperature. Two molecular weights of dPSS, about 15 and 100 kDa, presumed to be below and above the entanglement molecular weight, respectively, both diffused faster at higher temperatures with respective activation energies, Ea, of about 21 and 53 kJ mol–1, the latter about the same as Ea for the place exchange between two pairs of PSS:PDADMA. Studies of the linear viscoelastic response of macroscopic PECs showed a difference of about 8 °C in the Tg of the two lengths of PSS complexed with the same PDADMA. Increasing concentrations of NaCl influenced D of 100 kDa PSS but not 15 kDa PSS at room temperature. D was faster in the region of the film near the solution interface, again attributed to a lower Tg caused by greater water content at this interface.

A Dual-Phase Approach to Reveal the Presence and the Impact of the TLL Transition in Polymers Melts. Part I: Predicting the Existence of Boyer’s TLL Transition from the Vogel–Fulcher Equation

R.F. Boyer recognized the manifestations of a T > Tg transition-relaxation as early as 1963 and named it TLL, the liquid-liquid transition. He suggested that it was due to the melting of “local order”, a controversial issue conflicting with the dominant theories at the time, led by P. Flory, which asserted a structure-less liquid state for melts. At the same time as this controversy unrolled, de Gennes published his reptation model of polymer physics which, after some modifications and ramifications, quickly became the new paradigm to describe the dynamic properties of polymer flow. The new model of reptation has no theoretical arguments to account for a T > Tg transition occurring in the melt; hence, the current consensus about the existence of TLL is still what it was already in 1979, that it is probably an artifact only existing in the imagination of Boyer. In part I of this paper on the TLL transition we mathematically derive the existence and the characteristics of TLL from a dual-phase description of the free volume using a modification of the Vogel-Fulcher equation (VF), a well-known formulation of the temperature dependence of the viscosity of polymer melts. This new expression of the VF formula, that we call the TVF equation, permits to determine that TLL is in an iso-free volume and iso-enthalpic state when M, the molecular weight, varies. The data analyzed by the TVF equation are the dynamic rheological results for a series of monodispersed, un-entangled polystyrene samples taken from the work of Majesté. The new analysis also permits to put in evidence the existence of a new transition, which we call Mmc, approximately located at Mmc ≈ Mc/10, where Mc is the molecular weight for entanglement. A Dual-Phase interpretation of Mmc is proposed.

Hydrogenation of poly(myrcene) and poly(farnesene) using diimide reduction at ambient pressure

AbstractAmbient pressure chemical hydrogenation using p‐toluene sulfonyl hydrazide (TSH) via thermal diimide formation (N2H2) permitted reduction of double bonds of poly(myrcene) (poly[Myr]) and poly(farnesene) (poly[Far]). Both pendent and backbone double bonds in poly(Myr) (Mn = 56 kg/mol) and poly(Far) (Mn = 62 kg/mol) synthesized by conventional free radical polymerization were hydrogenated to almost completion. Furthermore, TSH semi‐batch addition efficiently hydrogenated double bonds, while avoiding undesired autohydrogenation of diimides that occurred in batch mode. Thermal stability improved for hydrogenated poly(Myr) and poly(Far), where temperature at 10% weight loss (T10%) increased from 188 to 404°C for poly(Myr) and from 310 to 379°C for poly(Far). Tgs of poly(Myr) and poly(Far) also increased by about 10–25°C, indicating increased stiffness after hydrogenation. Finally, viscosities of poly(Myr) and poly(Far) were also increased after hydrogenation, and a greater increase was observed for poly(Myr) (by two orders of magnitude from 102 to 104 Pa s) due to its Mn being much higher than its entanglement molecular weight. Poly(Far) viscosity only increased by 1.5 times after hydrogenation (~104 Pa s), comparable to the poly(Myr) after hydrogenation, suggesting unsaturated poly(Far) was more entangled than unsaturated poly(Myr) because of its longer side chains.

Self-Stable Precipitation Polymerization Molecular Entanglement Effect and Molecular Weight Simulations and Experiments.

In this paper, we developed a reactive molecular dynamics (RMD) scheme to simulate the Self-Stable Precipitation (SP) polymerization of 1-pentene and cyclopentene (C5) with maleic anhydride (MAn) in an all-atom resolution. We studied the chain propagation mechanism by tracking the changes in molecular conformation and analyzing end-to-end distance and radius of gyration. The results show that the main reason of chain termination in the reaction process was due to intramolecular cyclic entanglement, which made the active center wrapped in the center of the globular chain. After conducting the experiment in the same condition with the simulation, we found that the distribution trend and peak value of the molecular-weight-distribution curve in the simulation were consistent with experimental results. The simulated number average molecular weight (Mn) and weight average molecular weight (Mw) were in good agreement with the experiment. Moreover, the simulated molecular polydispersity index (PDI) for cyclopentene reaction with maleic anhydride was accurate, differing by 0.04 from the experimental value. These show that this model is suitable for C5–maleic anhydride self-stable precipitation polymerization and is expected to be used as a molecular weight prediction tool for other maleic anhydride self-stable precipitation polymerization system.

Hydroxyethylresorcinol- and hydroxyethylhydroquinone-containing poly(ethylene terephthalate) copolymers

Melt polycondensation incorporating meta-substituted hydroxyethylresorcinol (HER) or para-substituted hydroxyethylhydroquinone (HEH) regioisomers enabled the synthesis of poly(ethylene terephthalate) (PET)-based (co)polyesters with targeted comonomer mol %. Incorporation of either HER or HEH lowered the glass transition temperature (Tg) relative to PET, but the 5% weight loss temperature (Td,5%) remained unchanged. Similarly, the semi-crystalline morphology varied as a function of comonomer selection and mol % comonomer. Dynamic mechanical analysis confirmed decreasing β-relaxation intensities upon HER and HEH incorporation, while HER impacted β-relaxation intensity more substantially at similar mol %. HER incorporation also shifted the β-relaxation temperature (Tβ) to lower temperatures considerably more than HEH. Time-temperature-superposition (TTS) and Williams-Landel-Ferry (WLF) analysis of melt rheology probed various physical polymer properties including melt flow activation energy (Ea), fractional free volume at Tg (fg), and molecular weight of entanglement (Me). HER-based (co)polyesters exhibited lower Ea compared to HEH. Likewise, HER incorporation lowered fg while HEH incorporation increased fg. HER-based (co)polyesters possessed a higher Me compared to HEH-based (co)polyesters, although both copolymers resulted in higher Me compared to PET. Tensile testing revealed HER/HEH-based (co)polyesters ultimate mechanical performance compared to amorphous PET. Both HER and HEH incorporation increased Young's moduli and yield strengths, although comonomer incorporation above 25–30 mol % yielded minimal change. Establishing structure-property relationships provided insight into the role of regioisomeric variation at various comonomer levels on thermal, rheological, and mechanical performance for PET-based copolymers.

Direct Observations of Segmental Dynamics at the Polymer–Substrate Interface Enabled by Localizing Fluorescent Probes with Polymer Brushes

We employed imaging rotational fluorescence correlation microscopy to directly observe the segmental dynamics of the polymer near the polymer–substrate interface using a probe-tethered polymer brush buried in bulk polymer films. The probe-tethered polymers grafted onto the polymer–substrate interface provide spatial selectivity to probe only the segmental dynamics of polymer near the polymer–substrate interface. The perturbation caused by the grafted polymer chains on the segmental dynamics of the polymer film near the interface was minimized by reducing the molecular weight of the polymer brush compared with the critical entanglement molecular weight of the polymer and grafted with a low grafting density (≈0.1 chains/nm²) using a “grafting-to” method, which allows precisely controlled and fully characterized polymer brush structures. In addition, the molecular weight of the brush polymer was further controlled to ≈10 kg/mol with the root-mean-square end-to-end distance of ≈5 nm to probe dynamics within 10 nm of the polymer–substrate interface. Polystyrene and poly(methyl methacrylate) films carrying rather unfavorable and similar surface energies, respectively, with the silane-modified silicon wafer substrates were investigated. The polymer dynamics near the interface characterized by the degrees of non-Arrhenius temperature dependence and nonexponential relaxation were not altered in both systems compared with those of the bulk, suggesting that the polymer dynamics is not sensitive to moderate differences in enthalpic interaction between the polymer and the substrate.

Exploring the entangled state and molecular weight ofUHMWPEon the microstructure and mechanical properties ofHDPE/UHMWPEblends

AbstractThree types of ultra‐high molecular weight polyethylene (UHMWPE) with different entangled state and molecular weight were blended with high‐density polyethylene (HDPE) matrix by melt blending. Rheology, 2D‐SAXS, 2D‐WAXD, DSC, and mechanical tests were used to study the evolution and difference of microstructure and mechanical properties of the blends. The addition of weakly entangled UHMWPE enhanced the chain diffusion and chain orientation ability under a specific flow field. Thus, the rheological properties and mechanical properties of the blends were improved with the mix of weakly entangled UHMWPE. The mechanical properties enhancement effect of HDPE/UHMWPE blends with weakly entangled UHMWPE was owing to the shish‐kebab structure formed in the injection molding process. The molecular chains of UHMWPE with a low degree of entanglement and high molecular weight increased the lamella size and crystallinity of the blends during processing. This leads to the formation of more oriented shish structures and more kebab lamella. Besides, the molecular chains of weakly entangled UHMWPE were better interlocked and intertwined with other polyethylene chains in the amorphous region, acting as the tie molecules, significantly improving the impact resistance.

Influence of dianhydride regiochemistry on thermomechanical and rheological properties of 3,3′- and 4,4′-polyetherimides

The design of linear high performance polyetherimides (PEIs) comprising 3,3′- and 4,4′-bisphenol-A dianhydride (bis-DA) and m-phenylene diamine (mPD) provided an opportunity to elucidate the influence of dianhydride regiochemistry on thermomechanical and rheological properties. This unique pair of regioisomers allowed the tuning of the thermal and rheological properties for high glass transition temperature polyimides for emerging engineering applications. Step-growth polycondensation enabled the production of high molecular weight PEIs for subsequent thermal and mechanical analysis. Monofunctional phthalic anhydride as an endcapper and monomeric stoichiometric imbalances enabled predictable number-average molecular weights ranging from 3,600 to 35,400 g mol−1, as confirmed with size exclusion chromatography coupled with light scattering detection for absolute molecular weight determination. The selection of the dianhydride regioisomer influenced the weight loss profile as a function of temperature, entanglement molecular weight (Me), glass transition temperature (Tg), tensile strain-at-break, zero-shear melt viscosity, average hole-size free volume (Vh), and the plateau modulus prior to viscous flow during dynamic mechanical analysis. The 3,3′- PEI composition interestingly exhibited a ~20 °C higher glass transition temperature than the corresponding 4,4′- PEI analog. Moreover, melt rheological analysis revealed a two-fold increase in Me for 3,3′- PEI, which pointed to the origin of the differences in mechanical and rheological properties as a function of PEI backbone geometry.