Saturated Polymers Research Articles

Pore structure of affected zone around saturated and large superabsorbent polymers in cement paste

Superabsorbent polymer (SAP) has been widely used as internal curing agent for cement-based materials, duo to its high internal curing efficiency. Pore structure of affected zone around SAP is important for understanding the internal curing mechanism. This paper presents a study of the pore structure evolution of affected zone around SAP in cement paste with water/cement ratio 0.24 and 0.30 from 3 up to 28 days. Pore structure of affected zone and matrix was tested by mercury intrusion porosimetry and nitrogen adsorption and desorption. Surface fractal dimensions were determined using two different fractal models. Results indicated that SAP had no effect on the pore shape of affected zone, excluding the pore volume and pore diameter at early and late ages. Higher critical pore diameter but lower pore volume were found in affected zone at late ages. All the samples showed a multi-fractal property at micro- and macro-fractal scale, and the surface fractal dimension of affected zone was always larger than the matrix, showing a higher pore complexity. Some new findings on internal curing were achieved from an affected zone perspective. The calculated mechanical strength and permeability of affected zone from the pore structure parameters were found higher than the matrix. These new findings may give insight into the effect of internal curing with SAP from the perspective of affected zone.

The Fate of the Peroxyl Radical in Autoxidation: How Does Polymer Degradation Really Occur?

Bolland and Gee's basic autoxidation scheme (BAS) for lipids and rubbers has long been accepted as a general scheme for the autoxidation of all polymers. This scheme describes a chain process of initiation, propagation, and termination to describe the degradation of polymers in the presence of O2. Central to this scheme is the conjecture that propagation of damage to the next polymer chain occurs via hydrogen atom transfer with a peroxyl radical. However, this reaction is strongly thermodynamically disfavored for all but unsaturated polymers, where the product allylic radical is resonance-stabilized. Paradoxically, there is no denying that the autocatalytic degradation and oxidation of saturated polymers still occurs. Critical analysis of the literature, described herein, has begun to unravel this mystery. One possibility is that the BAS still holds for saturated polymers but only at unsaturated defect sites, where H transfer is thermodynamically favorable. Another is that peroxyl termination rather than H transfer is dominant. If this were the case, tertiary peroxyl radicals (formed at quaternary centers or quaternary branching defects) may terminate to form alkoxy radicals, which can much more readily undergo chain transfer. This process would lead to the creation of hydroxy groups on the degraded polymer. On the other hand, primary and secondary peroxyl radicals would terminate to form nonradical products and halt further degradation. As a result, under this scenario the degree of branching and substitution would have a major effect on polymer stability. Herein we survey studies of polymer degradation products and of the effect of polymer structure on stability and show that indeed peroxyl termination is competitive with peroxyl transfer and possibly dominant under some conditions. It is also feasible that oxygen may not be the only reactive atmospheric species involved in catalyzing polymer degradation. Herein we outline plausible mechanisms involving ozone, hydroperoxyl radical, and hydroxyl radical that have all been suggested in the literature and can account for the experimentally observed formation of hydroperoxides without invoking peroxyl transfer. We also show that oxygen itself has even been reported to slow the degradation of poly(methyl methacrylate)s, which might be expected if peroxyl radicals are unreactive toward hydrogen transfer. Discrepancies between the rate of oxidation and the rate of degradation have been observed for polyolefins and also support the counterintuitive notion that oxygen stabilizes these polymers against degradation. We show that together these studies support alternative mechanisms for polymer degradation. A thorough assessment of kinetic studies reported in the literature indicates that they are limited by their propensity to use models based on the BAS, disregarding the chemical differences intrinsic to each class of polymer. Thus, we propose that further work must be done to fully grasp the complex mechanism of polymer degradation under ambient conditions. Nonetheless, our analysis of the literature points to measures that can be used to enhance or prevent polymer degradation and indicates that we should focus beyond just the role of oxygen toward the specific chemical nature and environment of the polymer at hand.

The Effect of Polymer Structures on Complete Degradation: A First-Principles Study.

The widespread application of hydrocarbon polymer materials has spurred an increasing interest in the study of their dissociation mechanism, which is related to key issues such as environmental protection. In this work, the last‐step dissociation characteristics of carbon chain polymers were investigated. By using density functional theory, we considered all possible structures, including three typical normal linkage polymers and four typical abnormal linkage ones. In these structures, it can be found that the energy barrier required for the complete degradation of chain‐end saturated and unsaturated polymers are in the range of 3.42 to 4.78 eV and 0.35 to 1.31 eV, respectively. It shows that the unsaturated polymer is easier to degrade. Interestingly, as for three linkages of the polymer, the calculated results further suggest that the energy barrier of head‐to‐head, head‐to‐tail, and tail‐to‐tail linkages of the polymer dissociating to produce the monomer increase, no matter if the chain‐end is saturated or not. Therefore, we form a regular understanding of how to achieve the complete degradation of the polymer. In addition, analyses of the bond characteristics and electronic structures agree with the results of the energy barrier measurements. Meanwhile, the spin population analysis presents an obvious net spin transfer process in depolymerization reactions. We hope that the current results can provide a basic insight into polymer degradation.

Optically Active Precision Aliphatic Polyesters via Cross‐Metathesis Polymerization

AbstractPrecision synthesis of optically active poly(lactic‐alt‐caprolactic acid) is realized using cross‐metathesis polymerization (CMP) and hydrogenation. Enantiopure monomers ML and MD are generated through two consecutive esterification reactions. CMP of these two monomers in different feed ratios, followed by hydrogenation, afford saturated polymers HPL, HPD, and HPDL comprising L‐lactic and/or D‐lactic acid and ε‐caprolactone‐derived motifs with alternating sequences. CMP kinetic analysis by nuclear magnetic resonance (NMR) is performed by employing a racemic monomer Mrac. Enantiomeric polyesters HPL and HPD display opposite rotation values and mirror‐imaged cotton effects in circular dichroism (CD) spectra, whereas HPDL derived from equimolar ML and MD shows inactive optical rotation and no CD response. Stereocomplex HPD+HPL is obtained by equimolar mixing of HPL and HPD, since this specimen is the only one to exhibit a sharp melting peak on differential scanning calorimetry (DSC) curve at 45.7 °C. Hence, an amorphous, enantiomerically pure polyester becomes crystalline via stereocomplexation with its complementary enantiomer.

Stereoselective synthesis and polymerization of Exo‐5‐trimethylsilylnorbornene

ABSTRACTHerein the stereoselective two‐step synthesis of pure exo‐5‐trimethylsilylnorbornene is reported. The monomer proved to be highly reactive in both metathesis and addition polymerization. ROMP polymerization was catalyzed by the first‐generation Grubbs catalyst. High‐molecular‐weight saturated addition polymers were prepared using nickel or palladium complexes as precatalysts and Na+[B(3,5‐(CF3)2C6H3)4]− and/or MAO as cocatalysts. The obtained addition polynorbornenes are highly gas permeable and microporous materials possessing large free volume and BET surface area (up to 540 m2/g). The influence of the substituent orientation (exo‐ vs. exo‐/endo‐mixture) on polymer properties was established. The metathesis polymer based on exo‐isomer exhibits 1.5‐ to 2‐fold increase of permeability coefficients for all gases in comparison to the similar polymer based on the mixture of exo‐ and endo‐isomers. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1234–1248

Effect of chain length of polyisobutylene oligomers on the molecular motion modes of butyl rubber: Damping property

It is a promising method to obtain desirable damping materials by introducing dangling chains used as additional relaxation components. Regrettably, the strict condition of introducing dangling chains limits its application to saturated polymer or low unsaturated degree polymer, for example butyl rubber (IIR). Hence, we propose a simple yet effective method to obtain desirable IIR based damping composites by blending with miscible polyisobutylene (PIB) oligomers as additional relaxation components in this study. As expected, the damping property of IIR is easily controlled by tuning the chain length of PIB oligomers. The effects of PIB oligomers with different chain length on the molecular motions and damping mechanism of IIR were characterized by dynamical mechanical analysis and 2D correlation dynamic mechanical spectroscopy (2DC-DMS). The damping improvement of IIR was ascribed to the additional relaxation generated by introducing PIB oligomers and the movement of sub-Rouse modes and Rouse modes. It seems a universal method to enhance damping property by using miscible polymer oligomers as additional relaxation component. Furthermore, this method may give some inspirations for preparing polymer matrix specially used for damping composites.

Biocatalytic Synthesis and Polymerization via ROMP of New Biobased Phenolic Monomers: A Greener Process toward Sustainable Antioxidant Polymers

Antioxidant norbornene-based monomers bearing biobased sterically hindered phenols (SHP)—NDF (norbornene dihydroferulate) and NDS (norbornene dihydrosinapate)—have been successfully prepared through biocatalysis from naturally occurring ferulic and sinapic acids, respectively, in presence of Candida antarctica Lipase B (Cal-B). The ring opening metathesis polymerization (ROMP) of these monomers was investigated according to ruthenium catalyst type (GI) vs. (HGII) and monomer to catalyst molar ratio ([M]/[C]). The co-polymerization of antioxidant functionalized monomer (NDF or NDS) and non-active norbornene (N) has also been performed in order to adjust the number of SHP groups present per weight unit and tune the antioxidant activity of the copolymers. The polydispersity of the resulting copolymers was readily improved by a simple acetone wash to provide antioxidant polymers with well-defined structures. After hydrogenation with p-toluenesulfonylhydrazine (p-TSH), the radical scavenging ability of the resulting saturated polymers was evaluated using α,α-diphenyl-β-picrylhydrazyl (DPPH) analysis. Results demonstrated that polymers bearing sinapic acid SHP exhibited higher antiradical activity than the polymer bearing ferulic acid SHP. In addition it was also shown that only a small SHP content was needed in the copolymers to exhibit a potent antioxidant activity.

Synthesis of high performance cyclic olefin polymers using highly efficient WCl6-based catalyst system

Cyclic olefin polymers (COPs) with high glass transition temperature, high transparency (higher than 80%) in the visible light range, excellent thermal stability and outstanding mechanical properties have been synthesized by effective ring opening metathesis polymerization (ROMP) of exo-1,4,4a,9,9a,10-hexahydro-9,10(1′,2′)-benzeno-l,4-methanoanthracene (HBM) and dicyclopentadiene (DCPD) or norbornene (NBE) using WCl6/i-Bu3Al/ethanol/1-hexene catalyst system, followed by hydrogenation of double bonds. 1-Hexene acted as a molecular weight controller in the polymerization reaction, tuning the number-average molecular weight (Mn) of P-HBM from 5.8 × 104 to 41.1 × 104. The monomer composition and thermal properties of the copolymers were characterized by nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The saturated polymers exhibited high decomposition temperatures (Td) around 340 °C and glass transition temperatures (Tg) in the range from 117.5 °C to 219.7 °C. What is more, tensile tests indicated that the mechanical properties of the COPs could be effectively tuned in a wide range by introducing varying amount of small cyclic olefin such as DCPD or NBE.

Effect of Branching and Molecular Weight on Heterogeneous Catalytic Deuterium Exchange in Polyolefins

Small-angle neutron scattering (SANS) is a powerful method for probing the structural properties of polymeric materials. Contrast between polymer chains can be obtained by labeling with deuterium, which provides an opportunity for analyzing individual chain behavior in bulk. A transition metal (Pt/Re)-catalyzed reaction in isooctane was used to exchange deuterium for hydrogen in various saturated hydrocarbon polymers, including a commercial polyethylene. We have investigated the role of two forms of molecular heterogeneity on the labeling reaction using narrow dispersity hydrogenated polybutadiene (hPBD) samples with controlled molecular weight and ethyl branch content (short chain branching). These materials were prepared by anionic polymerization, followed by catalytic hydrogenation. A monotonic increase of deuterium labeling from 65% to 84% was observed when molecular weight was increased from 4000 to 216 000. Increasing the molecular weight to 635 000, however, resulted in almost no exchange, which is...

Synthesis of ethylene/vinyl ester copolymers with pendent linear branches via ring-opening metathesis polymerization of fatty acid-derived cyclooctenes

Fatty acid-derived cyclooctenes, including n-hexanoic acid (M1), n-octanoic acid (M2), lauric acid (M3), and palmitic acid (M4), were prepared as monomers and polymerized by ring-opening metathesis polymerization (ROMP) using Grubbs second-generation catalyst (G2). In all the cases, the regio-irregular unsaturated polymers with pendent linear branches were obtained, which could be saturated by chemical hydrogenation with TSH/TPA in high conversion, yielding ethylene/vinyl ester copolymers with pendent linear branches on precisely every eighth backbone carbon. Both unsaturated and saturated polymers were amorphous, and their structures were characterized by FTIR, 1H and 13C NMR spectra, and elemental analysis. Differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) were used to study their thermal properties. The chain length of branches greatly affected the thermal properties of polymers. After hydrogenation, the thermal degradation stability of polymers was relatively improved. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2211–2220

Deuterium Isotope Effects in Polymerization of Benzene under Pressure.

The enormous versatility in the properties of carbon materials depends on the content of the sp2 and sp3 covalent bonds. Under compression, if intermolecular distances cross a critical threshold, then unsaturated hydrocarbons gradually transform to saturated carbon polymers. However, the mechanism of polymerization, even for benzene, the simplest aromatic hydrocarbon, is still not understood. We used high-pressure synchrotron X-ray, neutron diffraction, and micro-Raman spectroscopy together with density functional calculations to investigate the isotope effects in benzene isotopologues C6H6 and C6D6 up to 46.0 GPa. Raman spectra of polymeric products recovered from comparable pressures show the progression of polymerization exhibiting a pronounced kinetic isotope effect. Kinetically retarded reactions in C6D6 shed light on the mechanism of polymerization of benzene. We find that C6D6-derived products recovered from P < 35 GPa actively react with moisture, forming polymers with higher sp3 hydrogen contents. Significant isotopic shift (≥7 GPa) in persistence of Bragg reflections of C6D6 is observed.

A new gas/supercritical fluid (SCF) diffusivity measurement method for CO2 saturated polymer systems using a dielectric property

A new gas/supercritical fluid (SCF) diffusivity measurement method for CO2 saturated polymer systems using a dielectric property

Ageing mechanisms of polyurethane adhesive/steel interfaces

For the achievement of a complete understanding of the ageing and corrosion of adhesively bonded steel substrates, both the ageing of a polyurethane reference adhesive and the simultaneous corrosion of a mild steel substrate were studied by combined spectroscopic, electrochemical and microscopic techniques. A corundum-blasted mild steel was used as metal substrate in combination with a cross-linked reference polyurethane adhesive free from additional additives or fillers. The ageing was performed under full immersion in a chloride-free aerated electrolyte at elevated temperature. A strong influence of the water saturated polymer phase on the change in the interfacial oxide film structure of the metal substrate was observed. The thickening and hydroxylation of the metal oxide film during the corrosive delamination of the polyurethane film indicated that the de-bonding mechanism of the polyurethane adhesive is mainly based on interfacial water enrichment and a thereby induced growth of the interfacial oxide. The measured electrochemical properties of this oxide indicate that it is highly conductive and allows for oxygen reduction as the counter reaction to metal oxidation.

Surface and Interface Modified Thermal, Structural and Charge Transport Properties in Conjugated Polymer Thin Films

Conjugated polymers are widely employed as the active layer in various electronic and optoelectronic devices. In this article, the modifications of thermal, structural and charge transport properties of conjugated polymers confined in thin film geometry are reviewed. Deviations of thermal transition temperatures of conjugated polymers are summarized and discussed through lessons drawn from conventional saturated polymer thin films. These phenomena are explained by the surface and interface effects that virtually divide a thin film into several vertically heterogeneous layers each having different physical characters. The precise determination of thermal transition temperatures as a guide for rational thermal treatments of various devices is highlighted. We then review surface and interface mediated molecular structure in conjugated polymer thin films via surface‐ and bulk‐ sensitive techniques, e.g. near‐edge X‐ray absorption fine structure and grazing‐incidence wide‐angle X‐ray scattering, and discuss how local morphology can affect charge mobility in the out‐of plane and in‐plane directions in diodes and field‐effect transistors. A thorough understanding of these phenomena and the underlying mechanism is critical for the rational design and precise control of devices for high performance.

Mechanistic insights into ozone-initiated oxidative degradation of saturated hydrocarbons and polymers.

Accurate quantum chemical calculations were employed to investigate the mechanism of ozone-initiated oxidation of C-H bonds of saturated hydrocarbons and polymers. Step wise hydrogen atom abstraction generates the first resting state the trihydroxide -COOOH, which undergoes decomposition to produce the free radical species alkoxyl -CO˙ and peroxyl ˙OOH thereby setting off a complex chain of radical processes. The H transfer from peroxyl radical to alkoxyl allows formation of inactive alcohol and the singlet excited dioxygen. Other competitive processes include the self fragmentation or β-scission of the alkoxyl -CO˙ to give rise to a carbonyl (ketone or aldehyde) and a C-centred free radical species. Tertiary C-H bonds are most susceptible to O3 oxidation followed by secondary and primary. Among the polymers studied, poly(styrene) is the least resistant to C-H bond ozonation, followed by poly(propylene), poly(methacrylate), poly(methyl methacrylate) and poly(vinyl chloride). Calculations also reveal catalytic effects of water in promoting the C-H bond oxidation process in polymer systems without competing H-bond donor groups.

Nuclear Magnetic Resonance Study of Water-Polymer Interactions and Self-Diffusion of Water in Polymer Films

The diffusivity and distribution of water in the butyl methacrylate and methacrylic acid copolymer films swollen in water have been investigated using the NMR relaxation and pulse field gradient (PFG) NMR techniques. The contributions of polymer matrix protons, surface water and bound water have been determined from 1H NMR spectra and relaxation functions. PFG NMR experiments showed that the echo attenuation function depends on the diffusion time indicating that water inside the swollen film is trapped in restricted confinement. The data obtained have been discussed using published physical models for diffusion of water in polymeric materials. The sizes of pores inside the film were estimated using the published model approaches giving the range of 0.8-1.0 μm. Magnetization decays as well as the spin-spin relaxation times of water saturated polymer films were also determined in this study. NMR relaxation provided additional information on the water distribution in the porous microstructure. The volume-averaged water mobility decreased with increasing hydrophobic content of the polymers.

Machine Learning for Quantum Mechanical Properties of Atoms in Molecules

We introduce machine learning models of quantum mechanical observables of atoms in molecules. Instant out-of-sample predictions for proton and carbon nuclear chemical shifts, atomic core level excitations, and forces on atoms reach accuracies on par with density functional theory reference. Locality is exploited within non-linear regression via local atom-centered coordinate systems. The approach is validated on a diverse set of 9k small organic molecules. Linear scaling of computational cost in system size is demonstrated for saturated polymers with up to sub-mesoscale lengths.

Study on Mechanism of Desorption Behavior of Saturated Superabsorbent Polymers in Concrete

The effect of several potential factors on the desorption behavior of saturated superabsorbent polymers (SAPs) in concrete was studied by simulating the internal saline concentration, alkalinity, and relative humidity. Results indicate that the desorption of SAP is mainly decided by osmotic pressure and humidity gradient. The desorption process of SAP is divided into three stages: the leading factor is the osmotic pressure during the first stage; humidity gradient during the third stage; and a relatively low-level combination of these two factors during the second stage. At the same time, the desorption is not decided by pH value, and the factor with the most effect is the osmotic pressure. The water release rate Rt of saturated SAP influenced by humidity shows the following trends: Rt indicates a clear linear trend as a function of time, and an approximate logarithm correlation exists between Rt and the relative humidity (Rt = A • ln(RH) + B).

Determination of charge-trapping sites in saturated and aromatic polymers by quantum chemical calculation

We studied the trapping of positive and negative charges in the chemical structures of polymers under a high electric field using a space charge measurement system. Positive charges accumulated in low-density polyethylene (LDPE), whereas positive and negative charges accumulated in polyimide (Kapton) and also in ethylene tetrafluoroethylene (ETFE) subjected to electron beam irradiation. To determine the charge-trapping sites in the chemical structures, a quantum chemical calculation was carried out using Density Function Theory (DFT) with Gaussian 09. The relationship between the energy band and the isosurface of orbital electrons at various energy levels was obtained. A three-dimensional (3D) electrostatic potential distribution map was obtained for positively and negatively charged polymers to determine the relationship between a trapping site and the charge accumulation center in the 3D potential distribution map. Positive and negative charges in Kapton and ETFE films are trapped in trapping sites in chemical structures and the positive charges in an LDPE film are trapped in physical defects.

Molecular modeling study of CO2 plasticization and sorption onto absorbable polyesters

Drug delivery systems are often made of porous polymer matrices. One method used to prepare a foamed polymer matrix is the controlled expansion of saturated polymers process in which amorphous polymer is exposed to CO2 at high pressure with a significant lowering of glass transition temperature. This plasticizing effect allows us to process temperature-sensitive polymers at relatively low temperatures. In the present study, computational tools were applied to estimate plasticizing effect of CO2 and calculate CO2- and H2O-loading capacities for three absorbable polyesters: polycaprolactone and two copolymers of (poly-d,l-lactid-co-glycolid)-co-polyethylenglycol. Plasticization caused by CO2 was estimated by solubility parameter and radial distribution function at several CO2 concentrations and by enlargement of free volume detected by mean square displacement of helium atoms, calculated after dynamic simulation. It was found that the maximal values of the solubility parameter and density can serve as a tool to predict saturation concentration. The loading capacities of the biopolymers that were preloaded with CO2 molecules were significantly higher than those of the nontreated polymers. Similar results were obtained for H2O molecules loading.