Intrinsic viscosity of polymers of low molecular weight

Experimental evidence concerning the dependence of the intrinsic viscosity [η] on molecular weight M in the low molecular weight range (from oligomers to M = 5 × 10⁴) has been collected in a variety of solvents for about ten polymers, i.e., polyethylene, poly(ethylene oxide), poly(propylene oxide), polydimethylsiloxane, polyisobutylene, poly(vinylacetate), poly(methyl methacrylate), polystyrene, poly-α-methylstyrene, and some cellulose derivatives. In theta solvents, the constancy of the ratio [η]Θ/M^0.5 extends down to values of M much lower than those predicted by current hydrodynamic theories. In good solvents, and on decreasing M, the polymers examined, with the exception of polyethylene and some cellulose derivatives, show a decrease in the exponent a of the Mark-Houwink equation [η] = KM^a. This upward curvature gives rise to the existence of a more or less extended linear region where the equation [η] = K₀M^0.5 is obeyed. Below the linear range, i.e., for even shorter chains, the exponent a can increase, i.e., polydimethylsiloxane, or decrease below 0.5, i.e., poly(ethylene oxide), depending on the particular chain properties. These different dependences have been discussed in terms of: (a) variations of thermodynamic interactions with molecular weight; (b) variations of conformational characteristics (as for instance the ratio) 〈r0²/nl²〉, where 〈r0²〉 is the unperturbed mean square end-to-end distance and n is the number of bonds each of length l; (c) hydrodynamic properties of short chains.     

Synthesis and characterization of Bi-functional photorefractive polymers with high molecular weight and low glass transition temperature

Carbazole-based bi-functional photorefractive polyacrylates were prepared via free radical polymerization and post-azo-coupling reaction. The structure of polymers was characterized by Fourier transform infrared spectroscopy(FTIR) and proton nuclear magnetic resonance(1H-NMR) spectroscopy. The differential scanning calorimetry(DSC) and the thermal gravimetric analysis(TGA) were used to characterize the thermal property of polymers. The results indicate that though the glass transition temperature(Tg) of polymers increases with increasing the ratio of NLO groups, the polymers with different ratios of NLO groups still all show low glass transition temperatures around 60 °C, and good thermal stability, which are favorable to the practical application of these polymers. The gel permeation chromatographic(GPC) result indicates that these polymers all have high-molecular-weight which is favorable to the long term stability of the material. Further, these polymers have good solubility in chloroform solvent, and the solutions can easily be fabricated into optically transparent films. Gain coefficients of 75 cm-1, 185 cm-1 and 66 cm-1 can be observed at zero external electric field without any addition agent or pre-poling for polymers P-2, P-3 and P-4 respectively. The different contents of NLO groups result in the different properties of polymers P-2, P-3 and P-4.     

Flocculation by very high molecular weight polymers

Polymer-induced flocculation in which the size of polymer molecules is much greater than that of colloidal particles is investigated. A dynamic analysis is conducted which takes the transient behaviors of the adsorption of particles to a polymer molecule and the particle-particle interactions into account. We show that the number of particles adsorbed to a polymer molecule follows approximately a binomial distribution. An approximate expression for the degree of flocculation of the system under consideration is presented.     

Emulsions stabilized by stimuli-sensitive poly(N-isopropylacrylamide)-co-methacrylic acid polymers: microgels versus low molecular weight polymers

Responsive polymer microgels can be employed for the preparation of stimuli-sensitive emulsions. The microgels used in this study are based on cross-linked copolymers including N-isopropylacrylamide and methacrylic acid. We conducted the synthesis under acidic and basic conditions to investigate the effect of changes of comonomer solubility on the microgel's composition and ability to stabilize emulsions. The synthesis product was partially divided into two fractions by centrifugation. Raw product, collected supernatant, and purified microgel were characterized by means of light scattering, titration, as well as electrophoretic mobility. The ability of the three components to act as stabilizers was investigated by preparing the octanol/water emulsions and looking at their response to pH and temperature changes. The interfacial activity of the three components was characterized by means of the pendent drop technique. Furthermore, we investigated the response of the interface to dilatational stress using a pendant drop tensiometer equipped with an oscillating drop module. The results demonstrate that the pH during synthesis has a significant impact on the composition and thus the properties of the microgel and its ability to be utilized as a stimuli responsive stabilizer for emulsions. We conclude that microgels can be used as stimuli-sensitive stabilizers for emulsions, if the charges are incorporated in the microgel itself.     

Synthesis and biological activity of medium molecular weight polymers of camptothecin

A new monomer, 3,6-endo-methylene-1,2,3,6-tetrahydrophthalimidohexanoylcamptothecin (ETHCPT) was synthesized from 3,6-endo-methylene-1,2,3,6-tetrahydrophthalimidohexanoic acid. Its homopolymer and copolymer with acrylic acid (AA) were synthesized and spectroscopically characterized. The ETHCPT content in poly(ETHCPT-co-AA) obtained by elemental analysis was 37 wt.%. The number-average molecular weights of the polymers determined by gel permeation chromatography were as follows: M_n=9700 for poly(ETHCPT), for poly(ETHCPT-co-AA). The IC₅₀ value of ETHCPT and its polymers against cancer cells was much larger than that of CPT. The in vivo antitumor activity of all polymers in Balb/C mice bearing the sarcoma 180 tumor cell line was greater than that of CPT at a dose of 100 mg/kg.     

Synthesis of low molecular weight perfluoro oxymethylene vinyl ethers

Perfluoro oxymethylene vinyl ethers have been formed by a multi-step synthesis. The key intermediates are low molecular weight perfluoropolyether (PFPE) fluoroformates CF₃O(CF₂O)_nCOF (I) n=1-6 obtained from the photo-oxidation of perfluoro propene (HFP) in perfluorohexane. Under certain conditions the light-mediated fluorination of PFPE fluoroformates (I) gives PFPE hypofluorites CF₃O(CF₂O)_nCF₂OF (II), which can be added to sym dichlorodifluoroethene to form the dichloro adduct CF₃O(CF₂O)_nCF₂OCFClCF₂Cl (III) which, after dechlorination, gives the desired vinyl ethers CF₃O(CF₂O)_nCF₂OCFCF₂ (IV). Every reaction step has to be properly controlled as far as the reaction variables are concerned. A mechanistic scheme is presented that is consistent with the observed experimental data.     

Grafting of styrene onto low molecular weight polymers and copolymers of butadiene

The grafting of styrene onto low molecular weight polybutadienes and butadiene–styrene co-polymers was studied. A mathematical method was used for the design of experiments and for the determination of the optimum grafting conditions with respect to the conversion of styrene and the efficiency of grafting. The reaction parameters were temperature (65–105°C), time (2–10 hr), concentration of the initiator, polymer to monomer ratio (10/90–90/10) and dilution by solvent (toluene). The optimum grafting conditions were chosen under which 50–60 wt-% of styrene was grafted onto backbone polymer at a high conversion of the monomer. It was found that the reactions producing graft copolymer prevailed over the styrene homopolymerization when the temperatures employed were lower (65–85°C), and the reaction time (8–10 hr), backbone polymer/monomer ratio, and the dilution by solvent were higher. The efficiency, density, and degree of grafting were found to increase with the increase in the molecular weight of the backbone polymer. The efficiencies and densities of grafting onto low molecular weight polybutedienes were higher than those of grafting onto low molecular weight butadiene–styrene copolymers. Grafting efficiencies and grafting densities were in the ranges 37.8–61.6 wt % and 0.06–0.26, respectively, in the studied range of number-average molecular weights (M?_n = 2400–6000).     

Synthesis of ultra‐high molecular weight polymers by controlled production of initiating radicals

This study demonstrates that the gradual and slow production of initiating radicals (i.e., hydroxyl radicals here) is the key point for the synthesis of ultra‐high molecular weight (UHMW) polymers via controlled radical polymerization. Hydrogen peroxide (H₂O₂) and ferrous iron (Fe²⁺) react via Fenton redox chemistry to initiate RAFT polymerization. This work presents two enzymatic‐mediated (i.e., Bio‐Fenton‐RAFT and Semi Bio‐Fenton‐RAFT) and one syringe pump‐driven Fenton‐RAFT polymerization processes in which the initiating radicals are carefully and gradually dosed into the reaction solution. The “livingness” of the synthesized UHMW polymers is demonstrated by chain extension and aminolysis experiments. Zimm plots obtained from static light scattering (SLS) technique are used to characterize the UHMW polymers. This Fenton‐RAFT polymerization provides access to polymers of unprecedented UHMW (M_w ~ 20 × 10⁶ g mol^?1) with potential in diverse applications. The UHMW polymers made via the controlled Fenton‐RAFT polymerization by using a syringe pump shows that it is possible to produce such materials through an easy‐to‐set up and scalable process. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1922–1930     

Influence of the low molecular weight amides on the dynamic behavior of polymers

Dependence of the dynamic shear modulus G′ and mechanical damping tan δ was measured for mixtures of low molecular amides with VC/VAc copolymer and polystyrene, respectively. A new secondary relaxation was found, which in some cases depends on content of moisture. The effect of molecular structure and concentration of amide is shown.     

超声波辅助合成低分子量聚丙烯酸钠

以过硫酸铵为引发剂,在超声波辅助下引发丙烯酸单体聚合。研究了单体浓度、引发剂用量、反应温度等对单体转化率和聚丙烯酸钠分子量的影响。结果表明:在超声作用下,能够加快聚合反应速度,提高转化率,但对产物聚合度影响很小;控制单体浓度30-40%、引发剂用量1.0%、反应温度50-70℃,在超声波作用下,制得聚丙烯酸钠的相对分子量在2,000-5,000之间。     

Phase separation kinetics in mixtures of flexible polymers and low molecular weight liquid crystals

A dynamic Monte Carlo (MC) simulation is performed to investigate the phase behavior of mixtures of flexible polymers and low molecular weight thermotropic liquid crystals (LCs). The polymer is represented by three‐dimensional self‐avoiding lattice chains, while the LC is described by the Lebwohl‐Lasher nematogen model. The initially homogeneous rod‐coil mixture is, following a deep quench, separated into an isotropic phase rich in coils and a nematic phase rich in rods. The underlying spinodal decomposition (SD) process is then simulated and studied extensively. This is the first simulation of SD in a rod‐coil mixture where the nematic ordering is included. Concentration fluctuations with a conserved order parameter are thus coupled with orientation fluctuations with a nonconserved order parameter. It is found that the early stage SD in the rod‐coil mixture still exhibits the dominant spatial wavelength and that the scalar scattering functions in the late stage of SD obey the Furukawa scaling law. The kinetic difference between the so‐called isotropic and anisotropic SD regions is, however, much less pronounced than predicted recently by the mean‐field theory.     

Viscoelastic properties of amorphous polymers 3: low molecular weight poly(methylphenylsiloxane)

By making creep and recoverable creep measurements of a nearly monodisperse low molecular weight poly(methyl phenyl siloxane) sample, we have found on decreasing temperature towardsT _g that there is continuously a change in the viscoelastic spectrum concomitant with a decrease of the steadystate recoverable compliance. This behavior is exactly the same as previously observed in low molecular weight poly(styrene), proving that this spectacular anomaly in the viscoelasticity of low molecular weight polymers is general and deserves an explanation. Photon correlation spectroscopic measurements performed on the same sample have extended the observation of the viscoelastic response to shorter times and the result corroborates the trend of variation established by the creep data.Dedicated to Prof.Dr. E. W. Fischer on his 65th Birthday. Prof.Dr. Fischer is known for his valuable contribution to fosterine, international collaboration of research in polymer science. This work is an example of his contribution because it would not be possible without him bringing us together. One of us (KLN) would like to take this opportunity to thank Prof. Dr. Fischer for his unwaiving support of the 1st (Crete) and the 2nd (Alicante) International Discussion Meeting on Relaxations in Complex Systems     

Kinetics of spinodal decomposition in mixtures of low molecular weight liquid crystals and flexible polymers

In this paper, the time-dependent Ginzburg-Landau model for mixtures containing nematogens has been applied to mixtures of low molecular weight liquid crystals and flexible polymers. Dynamic equations for the time evolution of concentration and orientation fluctuations and the structure factors for these fluctuations are given. It is shown that the coupling between concentration and orientation fluctuations is absent in the isotropic spinodal region, thus the evolution of the structure factors for the concentration fluctuations falls into the Cahn-Hilliard classic category and it exhibits no maximum in the structure factors of orientation fluctuations. We should emphasize that, in the anisotropic spinodal region, both concentration and orientation structure factors possess a maximum but not coincide with each other and both are shifting to smaller wave numbers according to the scaling relation, q_max ˜ τ^−α, as time increases. The value of a closely correlates to the interfacial free-energy parameters.     

Precise molecular weight distributions of high polymers by semiautomatic solvent extraction

It has been found practical to fractionate relatively monodisperse polystyrenes of molecular weights up to 4 × 10⁶ by using a semiautomatic solvent extraction procedure. The fractionators consist of flow-through mixing chambers provided with settling regions for the removal of finely suspended solid material. Provision is made for accurate control of both temperature and solvent–nonsolvent ratio. Two such apparatus, operated in tandem, were used in the present work; in principle, several more stages could be employed if desired for most polymer systems. Results on polystyrenes prepared in well defined ways appear to be in good agreement with predicted distributions. Applicability to other polymer systems is discussed.     

Rapid molecular weight analysis of polymers by temperature gradient interaction chromatography

Temperature gradient interaction chromatography (TGIC) has been established as a high-resolution technique for the characterization of synthetic polymers. So far, most of the TGIC investigations focused on the high-resolution analysis and little effort has been made on the reduction of the analysis time. In this study, we examined the effect of the column heating rate, the eluent flow rate, and the column length on the TGIC analysis time. We found that the heating rate is the most important experimental parameter to control the TGIC retention time. With a C18 silica column (50 mm x 4.6mm I.D.), a set of PS standards of wide molecular weight range (5 - 648 kg/mol) could be separated within 4 min at a heating rate of 8 degrees C/min.     

High molecular weight aromatic polymers by nickel coupling of aryl polychlorides

This paper describes a totally new method for the formation of high molecular weight aromatic polymers. High molecular weight polyarylethersulfones are produced by nickel catalyzed coupling of aryl dichloride monomers. Metallic zinc is used to drive the polymerization reaction which takes place under relatively mild conditions in the presence of triphenylphosphine and a dipolar aprotic solvent. The versatility of the reaction is demonstrated, as well as the ability to provide a variety of high temperature polymers from readily available and inexpensive monomers. It is possible to use an integrated two-step process to produce aryl dichlorides from inexpensive aryl chlorides, and then to couple these directly to polymer without need for isolation and purification. This process resulted from a fundamental understanding of the chemistry and the use of statistically designed experiments to identify the important reaction parameters and to optimize the degree of polymerization.