THE MOLECULAR WEIGHT AND ITS DISTRIBUTION OF 1, 2-POLYBUTADIENES PREPARED WITH MOLYBDENUM CATALYST SYSTEMS

The study on the molecular weight and its distribution of 1, 2-polybutadienes prepared with MoCl_4OR-i-Bu_2AlOR' catalyst systems has been carded out by viscosity and GPC methods. 1, 2-Polybutadienes with (?)=1.2-2.0can be obtained with these catalyst systems at 30—70℃. There is a linear relation between the (?) and the polymerization temperature. It was found by extrapolation that these catalyst systems may possibly initiate a living polymerization at the temperature near 18℃. The molecular weight of the polymers can be regulated either by polymerization temperature or by adding polar compounds such as allyl halides.     

COMPARISON OF GPC AND THERMAL FIELDFLOW FRACTIONATION METHODS FOR LINEAR AND STAR BRANCHED POLYSTYRENE SAMPLES

The Thermal Field-Flow Fractionation (TFFF) method was used to determine the elution volumeof a series of star branched polystyrene having different number of arms but the same arm molecularweigh and polystyrene standards with narrow distribution whose molecular weight ranged from5.0×10~4 to 8.6×10~5. Results were obtained by measuring at two temperature difference (△T=30℃and △T=50℃in THF. The same star branched samples were measured by means of GPC method.Comparison of Vr-Mrelationships obtained from TFFF and GPC showed that the displacement of V_r-M curves for star and linear polystyrene is larger than that in GPC. This difference is caused by theentirely different mechanism of separation for these two methods. As the controlling factor is hy-drodynamic volume of the polymer chain in solution for GPC, it is the diffusion coefficient of polymermolecules for TFFF. The experimental results indicate that the influence of variance of chain struc-ture on diffusion coefficient is stronger than that on the hydrodynamic volume and that TFFF tech-nique may be used as a method for characterizing branching of polymer molecules. For this pur-pose a proper theoretical model and more accurate experiments are needed.     

Effect of chemical structure of polycarbonates on entanglement spacing

The master curves of a series of aliphatic polycarbonates(APCs) with different lengths of methylene segments in the repeat unit were obtained by dynamic rheological measurements.The plateau modulus and entanglement molecular weight were determined and cross-checked by different methods.Though having distinct difference in chemical structure of repeat units,both APCs and bisphenol-A polycarbonates have the similar entanglement weight and entanglement spacing.On the other side,the plateau modulus decreases with increasing the length of the side group of aliphatic polycarbonates with different side-chain lengths in the literature.The packing length model can explain the relationship between chain structure and entanglements.     

Stability of proteins with multi-state unfolding behavior

A new model used to calculate the free energy change of protein unfolding is presented.In this model,proteins are considered to be composed of structural elements.The unfolding of a structural element obeys a two-state mechanism and the free energy change of the element can be obtained by a linear extrapolation method.If a protein consists of the same structural elements,its unfolding will displays a two-state process,and only the average structural element free energy change < △G 0 element(H2O)> can be measured.If protein consists of completely different structural elements,its unfolding will show a multi-state behavior.When a protein consists of n structural elements its unfolding will shows a(n+1)-state behavior.A least-squares fitting can be used to analyze the contribution of each structural element to the protein and the free energy change of each structural element can be obtained by using linear extrapolation to zero denaturant concentration,not to the start of each transition.The measured △G0 protein(H2 O) is the sum of the free energy change for each structural element.Using this new model,we can not only analyze the stability of various proteins with similar structure and similar molecular weight,which undergo multi-state unfolding processes,but also compare the stability of proteins with different structures and molecular weights using the average structural element free energy change < △G0 element(H2O)>.Although this method cannot completely provide the exact free energy of proteins,it is better than current methods.     

DETERMINATION OF BRANCHING DISTRIBUTION IN HIGH cis-1,4-POLYBUTADIENE BY GPC AND AUTOMATIC VISCOMETRY

Based on the methods reported by Ambler and Kraus, a method has been developed for the determination of long-chain branching distribution in polymers by the combined use of GPC and intrinsic viscosity data of polymer fractions. In this method, g_i, λ_i, G_i, m_i, the weight percentage of polymer that is branched, etc. can be used simultaneously to characterize the distribution, degree and content of branching in polymers. Some relations between molecular weight polydispersity and branching polydispersity in Nickel-based high cis-1,4-polybutadiene samples are discussed. It was found that the number of long branches λ. per unit molecular weight is a function of molecular weight and all of the samples are highly branched at a molecular weight of about 10~6.     

Calculation of the molecular exchanging energy of binary surfactants system on the surface monolayer of aqueous solution

By using the binary anionic/cationic surfactants system CH3(CH2)nOSO_3/CH3(CH2)nN (CH3)3 as an ex-ample, the molecular exchanging energy (ε) of adsorption on the surface monolayer of aqueous solu-tion has been studied. ε can be obtained with two methods. One is from the relationship between ε and the molecule interaction parameter (β). This relationship is founded by considering that the adsorption of mixed surfactants on the surface monolayer of solution satisfies the dimensional crystal model condition under which β can be obtained by testing the surface tension of solution. The other is directly from the molecular structure of surfactants with the Lennard-Jones formula. The results for the studied system show that these two methods coincide well.     

SEMI-QUANTITATIVE CHARACTERIZATION OF SEGMENT DISTRIBUTION IN LLDPE BASED ON THERMAL SEGREGATION

Based on successive multiple-step isothermal crystallization and self-nucleation annealing methods, a novel semi-quantitative method for the characterization of segment distribution in linear low density polyethylene (LLDPE) wasestablished by treating the thermal analysis data using the Gibbs-Thomson equation. The method was used to describe thesegment distribution of Ziegler-Natta catalyzed LLDPE (Z-N LLDPE), metallocene catalyzed LLDPE (m-LLDPE) and twoconunercial LLDPEs with wide molecular weight distribution. The differences of the results obtained from the two thermallytreated samples were compared. The results of segmeni distribution of the polymers were discussed according to theirmicrostructure data and were compared with their characteristics. It can be deduced from the results that this characterizationmethod is effective to characterize the sequence structure of the branched ethylene copolymers.     

Synthesis of heteroarm star polymers comprising poly(4-methylphenyl vinyl sulfoxide) segment by living anionic polymerization

<正>A series of 3-arm ABC and AA'B and 4-arm ABCD,AA'BC and AA′A″B heteroarm star polymers comprising one poly(4-methylphenyl vinyl sulfoxide) segment and other segments such as polystyrene,poly(α-methylstyrene), poly(4-methoxystyrene) and poly(4-trimethylsilylstyrene) were synthesized by living anionic polymerization based on diphenylethylene(DPE) chemistry.The DPE-functionalized polymers were synthesized by iterative methodology,and the objective star polymers were prepared by two distinct methodologies based on anionic polymerization using DPE-functionalized polymers.The first methodology involves an addition reaction of living anionic polymer with excess DPE-functionalized polymer and a subsequent living anionic polymerization of 4-methylphenyl vinyl sulfoxide(MePVSO) initiated from the in situ formed polymer anion with two or three polymer segments.The second methodology comprises an addition reaction of DPE-functionalized polymer with excess sec-BuLi and a following anionic polymerization of MePVSO initiated from the in situ formed polymer anion and 3-methyl-1,1-diphenylpentyl anion as well.Both approaches could afford the target heteroarm star polymers with predetermined molecular weight,narrow molecular weight distribution (M_w/M_n1.03) and desired composition,evidenced by SEC,~1H-NMR and SLS analyses.These polymers can be used as model polymers to investigate structure-property relationships in heteroarm star polymers.     

STUDIES ON STRUCTURE AND PHASE BEHAVIOR OF MULTICOMPONENT POLYMERS THROUGH RHEOLOGICAL TESTS

Rheological measurement has been a preferred approach to the characterization of the structure and phase behaviors for multi-component/multi-phase polymer systems, due to its sensitive response to the changes of structure for these heterogeneous polymers. In the present article, recent progresses in the studies on rheology for heterogeneous polymer systems including phase-separated polymeric blends and block copolymers are reviewed, mainly depending on the results by the authors＇ research group. By means of rheological measurements, not only some new fingerprints responsible for the evolution of morphology and structure concerning these polymer systems are obtained, also the corresponding results are significant for design and preparation of novel polymeric structural materials and functional materials.     

GPC-VIS-MALS study of EVA copolymers: Quantification and interactions of SCB and LCB

Long-chain branching (LCB) is a structural phenomenon that affects important properties in polyethylene (PE) and some copolymers. Quantification of LCB frequency (λ) can be carried out by gel permeation chromatography dotted with detector for viscosity (GPC-VIS) or light scattering (GPC-MALS) by calculating branching indexes against a linear reference. In copolymers, interactions between LCB and SCB (short chain branching) have been described and lead to errors in quantification.In this work, ethyl vinyl acetate (EVA) copolymers of composition ranging 3–20 wt% VA have been studied. A numerical method, developed for the reduction of GPC-VIS and GPC-MALS data of PE, was used for quantifying molecular weights, intrinsic viscosities and gyration radius, as well as the confident ranges. Reliable results were obtained despite the low LCB determined values.A low density polyethylene was also included and compared. Discrepancies in the scaling laws for gyration radius and intrinsic viscosity reveal a strong effect of SCB which was confirmed by the structure factor and its dependence on molecular weight and comonomer content. However, the recently designed gpcBR index revealed to be nearly independent on the short chain branching and allowed detecting differences between apparently similar samples.     

“Super microcapsules” (SMC). I. Preparation and characterization of star polymethylene oxide (PEO)?polylactide (PLA) copolymers

A new model of molecular scale microcapsule named “super microcapsule” (SMC) obtained from a star copolymer consisting of hydrophilic and hydrophobic blocks was presented. As a first stage, 3 and 4-arm star polyethylene oxide-polylactide copolymers (s-PEO-PLA) were synthesized by the use of triethanolamine and pentaerythritol and initiating agent, respectively. The block length of PLA and PEO for the copolymers can be controlled by feed and reaction conditions. The molecular weight distributions found to be in the range of 1.3–1.7. The DTA data indicated that the phase separation behavior of s-PEO? PLA copolymers is different from that of linear PEO? PLA copolymers with comparable block length. As a evidence of SMC, the shell-core structure of s-PEO? PLA copolymers in solid state was observed by TEM. The SMC dimensions were estimated to be about 400–1000 Å     

Self-organization process of ordered structures in linear and star poly(styrene)-poly(isoprene) block copolymers: Gaussian models and mesoscopic parameters of polymeric systems

Mesoscopic simulations of linear and 3-arm star poly(styrene)-poly(isoprene) block copolymers was performed using a representation of the polymeric molecular structures by means of Gaussian models. The systems were represented by a group of spherical beads connected by harmonic springs; each bead corresponds to a segment of the block chain. The quantitative estimation for the bead-bead interaction of each system was calculated using a Flory-Huggins modified thermodynamical model. The Gaussian models together with dissipative particle dynamics (DPD) were employed to explore the self-organization process of ordered structures in these polymeric systems. These mesoscopic simulations for linear and 3-arm star block copolymers predict microphase separation, order-disorder transition, and self-assembly of the ordered structures with specific morphologies such as body-centered-cubic (BCC), hexagonal packed cylinders (HPC), hexagonal perforated layers (HPL), alternating lamellar (LAM), and ordered bicontinuous double diamond (OBDD) phases. The agreement between our simulations and experimental results validate the Gaussian chain models and mesoscopic parameters used for these polymers and allow describing complex macromolecular structures of soft condensed matter with large molecular weight at the statistical segment level.     

Branching degree and rheological response correlation in peroxide‐modified linear low‐density polyethylene

Correlations between rheological behavior and degree of long chain branching (LCB) of linear low‐density polyethylene (LLDPE) upon a peroxide (dicumyl peroxide [DCP]) modification process under various conditions are discussed in this paper. The gel content analysis revealed negligible insoluble crosslinked fraction implying that incorporation of DCP to LLDPE predominately leads to branching rather than crosslinking. The slight changes in average molecular weight and molecular weight distribution induced by peroxide modification under various conditions revealed that formation of low‐molecular‐weight fractions due to chain scission is also negligible. The changes in terminal, trans, and pendant double bonds concentration of the modified samples with different amounts of peroxide were well depicted by Fourier transform infrared spectroscopy. Considering insignificant changes in molecular weight and molecular weight distribution during peroxide modification, the deviation observed in zero‐shear‐rate viscosity (η₀) values of the modified LLDPE with that of power‐law equation related to the linear PEs could be reliably attributed to the presence of LCB in the peroxide modified samples. Increasing the DCP content at roughly constant molar mass led to increasing of η₀ values as a result of increased degree of LCB. The increase in η₀ values was ascribed to prolonged relaxation times of the polymer molecules due to the retarded reptation motion‐driven relaxation mechanism. Copyright © 2014 John Wiley & Sons, Ltd.     

Tracer diffusion of star-branched polystyrenes in poly(vinyl methyl ether) gels

The tracer diffusion of 3-, 4-, and 12-arm polystyrene (PS) stars in poly(vinyl methyl ether) (PVME) gels has been measured by dynamic light scattering (DLS). The intensity correlation functions were analyzed by two methods. One was that employed previously in a DLS study of linear PS diffusion in PVME gels [N. A. Rotstein and T. P. Lodge, Macromolecules, Vol. 24. p. 1316 (1992)], and the other was based on consideration of possible nonergodicity effects [P. N. Pusey and W. van Megen, Physica A, Vol. 157, p. 705 (1990)]. Both methods gave equivalent results, suggesting that nonergodicity plays a small role in this system. This conclusion is not unreasonable, given that the PVME gels are almost isorefractive with the solvent (toluene), and that the signal is dominated by scattering from the PS chains. The resulting star diffusivities are consistently less than or equal to those for linear probes of comparable size, with the difference increasing with molecular weight. The diffusivities are also less than or equal to those obtained for the same stars in PVME solutions. A weak dependence on the number of arms is also observed. Finally, the mobility of a given star in a gel is much more sensitive to variations in the average molecular weight between cross-links than is the mobility of a linear chain. All of these features in the data are broadly consistent with the reptation hypothesis. © 1993 John Wiley & Sons, Inc.     

Characterization of low-density polyethylene by gel permeation chromatography—III. Study on the Drott method using a whole polymer

Two features of the Drott iterative procedure for the estimation of long-chain branching have been examined, viz. the proportionality between the number of long-chain branches n per molecule and its molecular weight M, and the exponent ? relating the ratio of intrinsic viscosities of branched and linear polymers of the same weight-average molecular weight to n. The original Drott model describing the distribution of long-chain branches (LCB) has been replaced by a more general equation: n = const. M^B and an examination of the effect of the exponents B and ? on the molecular weight distribution (MWD) and LCB characteristics carried out. The analysis has revealed that, unless information on the LCB distribution is available, the method described by Drott for which B = 1 can hardly give meaningful information on MWD and LCB for commercial low-density polyethylenes.     

Effect of molecular structure on polyethylene melt rheology. I. Low-shear behavior

The melt rheological properties of both linear and branched polyethylene were investigated by use of narrow molecular weight distribution fractions and experimentally polymerized samples. Studies carried out in steady shear and in oscillatory shear yielded information concerning both the melt viscosity and the melt elasticity as a function of molecular structure, where the latter was characterized by various solution property techniques. The 3.4–3.5 power dependence of the low shear limiting viscosity on molecular weight was confirmed for linear polyethylene. The effect of long-chain branching on rheological properties was defined both at constant molecular weight and at constant molecular weight distribution and coupled with variation of molecular weight.     

Synthesis of functionalized asymmetric star polymers containing conductive polyacetylene segments by living anionic polymerization

Novel 3-arm ABC, 4-arm ABCD, and 5-arm ABCDE asymmetric star polymers comprising the conductive polyacetylene precursor, poly(4-methylphenyl vinyl sulfoxide) (PMePVSO), and other segments, such as polystyrene, poly(alpha-methylstyrene), poly(4-methoxystyrene), poly(4-trimethylsilylstyrene), and poly(4-methylstyrene), were synthesized by the methodology based on living anionic polymerization using DPE-functionalized polymers. This methodology involves the addition reaction of a DPE-functionalized polymer to a living anionic polymer followed by the living anionic polymerization of MePVSO initiated from the in situ formed polymer anion with two, three, or four polymer segments. The resultant asymmetric star polymers possessed predetermined molecular weights, narrow molecular weight distributions (Mw/Mn < 1.03), and desired compositions as confirmed by SEC, 1H NMR, SLS, and elemental analysis. After thermal treatment, the PMePVSO segment in the star polymer could be completely converted into a conductive polyacetylene segment, evident from TGA and elemental analysis. These asymmetric star polymers are expected to exhibit interesting solution properties and unique microphase-separated morphological suprastructures with potential applications in nanoscopic conductive materials. Moreover, this methodology can afford the target asymmetric star polymers with arm segments varying in a wide range and enables the synthesis of more complex macromolecular architectures.