Iodine-induced uncoiling of polyisoprene random coils in solution

Recently, we demonstrated that conjugated sequences of unsaturated double bonds can be introduced into 1,4-polyisoprene backbone through conjugation reactions induced by iodine [1]. Here, we report a structural investigation on the conjugated polyisoprene chains in solution by means of small angle X-ray scattering (SAXS). The SAXS results indicate a conformational transition from a random coil, characteristic of the pristine polyisoprene chains in solution [2], to a rod-like structure by uncoiling of the macromolecular coils due to a remarkable increase in chain rigidity associated with the formation of rigid conjugated sequences via the I₂-induced conjugation reaction.     

Branching kinetics of epoxy polymerization of 1, 4-butanediol diglycidyl ether with cis-1, 2-cyclohexanedicarboxylic anhydride

The copolymerization of an epoxy resin [1, 4-butanediol diglycidyl ether (DGEB) (Note a)] with an anhydride [cis-1, 2-cyclohexanedicarboxylic anhydride (CH)] in the presence of N, N-benzyldimethylamine (CA) as a catalyst produces a branched epoxy polymer. We show that the branching kinetics of the copolymerization reaction and the molecular weight distribution of the branched polymers can be approximated by using Smoluchowski's coagulation equation. In the simplest relevant application of this equation to our problem, the overall rate kernel w(u, v) that describes the branching probability in the equation turns out to be proportional to the sum of active sites on the two polymers with a time dependent coefficient. The molecular weight distribution (MWD) and the weight average molecular weight of the branched copolymers at different reaction stages before the gelation threshold are calculated theoretically. The calculated values are then compared with the experimental results obtained by using small angle X-ray scattering (SAXS), laser light scattering (LLS), and chemical analysis. Satisfactory agreement between experimental results and the use of the coagulation equation is attained when it is assumed that the distribution of epoxy polymer molecules is exponential in the number of branching points or, equivalently, active sites, at an early stage of the polymerization reaction.     

双烯化合物类单体合成支化聚合物的支化结构的研究

分别以二乙烯基苯(DVB)、双甲基丙烯酸二缩三乙二醇酯(tri-EGDMA)和1,6-双马来酰亚胺基正己烷(BMIH)为支化单体,采用原子转移自由基聚合合成支化聚苯乙烯;以先核后臂法合成的星状支化聚苯乙烯为参照对合成的支化聚合物的支化形态进行研究.采用气相色谱(GC)、核磁共振氢谱(1H-NMR)和三检测凝胶渗透色谱(TD-SEC)测定了苯乙烯的转化率,聚合物分子量及其分布,特性黏数和均方回转半径.实验结果表明3个支化聚合反应体系内悬垂双键是逐步消耗的,不存在明显的成核过程.反应前期,以形成带有悬垂双键的初级链和轻度支化聚合物为主,聚合物分子量随单体转化率逐步上升;反应后期,悬垂双键聚合导致的分子之间的偶合更加明显,使得聚合物分子量快速上升,合成得到的都是无规支化聚合物.     

Plasticization and cocrystallization in LLDPE/wax blends

The structure and thermal properties of linear low‐density polyethylene (LLDPE)/medium soft paraffin wax blends, prepared by melt mixing, were investigated by differential scanning calorimetry (DSC) and small‐ and wide‐angle X‐ray scattering (SAXS and WAXS). The blends form a single phase in the melt as determined by SAXS. Upon cooling from the melt, two crystalline phases develop for blends with more than 10 wt % wax characterized by widely different melting points. The wax acts as an effective plasticizer for LLDPE, decreasing both its crystallization and melting temperature. The higher melting point crystalline phase is formed by less branched LLDPE fractions. On the other hand, the lower melting point crystalline phase is a wax‐rich phase constituted by cocrystals of extended chain wax and short linear sequences of highly branched LLDPE chains. The presence of cocrystals was evidenced by standard DSC results, successive self‐nucleation and annealing (SSA) thermal fractionation and by the detection of a new SAXS signal attributed to the lamellar long period of the cocrystals. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1469–1482     

Morphological Changes of Linear,Branched Polyethylenes and their Blends during Crystallization and Subsequent Melting by Synchrotron SAXS and DSC

Summary: The crystalline structure and phase morphology of linear, branched polyethylenes and their blends during crystallization and subsequent melting were investigated, using a combination of differential scanning calorimetry (DSC), and synchrotron small angle X-ray scattering (SAXS). A linear polyethylene (PE1) with weight-average molecular weight (M_w) of 114 000 g/mol, and two branched polyethylene copolymers, containing 4.8 mol% (PE4) and 15.3 mol% (PE10) hexane, with molecular weights of 93 000 g/mol and 46 000 g/mol were used as pure samples. Two blends, PE1-4 and PE1-10, each with a weight ratio of 50/50, were prepared by solution blending. Our results indicate that in PE4 a phase separation within the branched component itself occurred, forming a broad distribution of lamellar thicknesses during the crystallization process. PE10 on the other hand did hardly crystallize because of the high degree of branching. Co-crystallization of both components took place in blend PE1-4 and liquid-liquid phase separation occurred in the melt of PE1-10. Morphological parameters were determined by using Bragg's law and the correlation function, respectively. The detected semicrystalline morphology can be well described by the lamellar insertion mode where thin lamellae develop between thicker primary lamellae. During subsequent heating, lamellae melted in the reversed sequence of their formation. The evolution of the structural parameters as a function of temperature revealed that surface melting began at first, and then the complete melting of stacks occurred until the final melting temperature was reached.     

Monodisperse macromolecules – A stepping stone to understanding industrial polymers

Polymers synthesized via anionic polymerization have proved important to our fundamental understanding of the processing, that is rheology and crystallisation, of bulk commodity polymers. The role of monodisperse hydrogenated polybutadienes as models for linear and branched polyethylene is examined. Systematic studies of the effects of long-chain branching, using well-defined “comb” materials have improved our understanding of how the number and length of branches affect the rheological properties and how these features impact on their crystallization behaviour. A combination of techniques including rheology, Small Angle X-ray Scattering (SAXS), and birefringence measurements have provided insight into role of linear long chains in the formation of oriented morphologies during the crystallization of hydrogenated polybutadiene blends of controlled polydispersity leading to the development of a quantitative model.     

Branched polymeric labels used as drag-tags in free-solution electrophoresis of ssDNA

In the framework of the classical blob theory of end-labeled free-solution electrophoresis of ssDNA, and based on recent experimental data with linear and branched polymeric labels (or drag-tags), the present study puts forward design principles for the optimal type of branching that would give, for a given total number of monomers, the highest effective frictional drag for ssDNA sequencing purposes. The hydrodynamic radii of the linear and branched labels are calculated using standard models like the freely jointed chain model and the Kratky-Porod worm-like chain model. Based on comparisons of the theory with the experimental data, we propose that the design of new branched labels should use either side chains whose length is comparable to the distance between the branching points or two long branches located near the ends of the molecule's backbone.     

A simulation model for long-chain branching in vinyl acetate polymerization: 1. Batch polymerization

A new simulation model for the kinetics of long-chain branching formed via chain transfer to polymer and terminal double-bond polymerization is proposed. This model is based on the branching density distribution of the primary polymer molecules. The theory of branching density distribution is that each primary polymer molecule experiences a different history of branching and provides information on how each primary polymer molecule is connected with other chains that are formed at different conversions, therefore making possible a detailed analysis on the kinetics of the branched structure formation. This model is solved by applying the Monte Carlo method and a computer-generated simulated algorithm is proposed. The present model is applied to a batch polymerization of vinyl acetate, and various interesting structural changes occurring during polymerization (i.e., molecular weight distribution, distribution of branch points, and branching density of the largest polymer molecule) are calculated. The present method gives a direct solution for the Bethe lattice formed under nonequilibrium conditions; therefore, it can be used to examine earlier theories of the branched structure formation. It was found that the method of moments that has been applied successfully to predict various average properties would be considered a good approximation at least for the calculation of not greater than the second-order moment in a batch polymerization. © 1994 John Wiley & Sons, Inc.     

Online SAXS investigations of polymeric hollow fibre membranes

Polymeric membranes are used in industrial and analytical separation techniques. In this study small-angle X-ray scattering (SAXS) with synchrotron radiation has been applied for in-situ characterisation during formation of polymeric membranes. The spinning of a polyetherimide (PEI) hollow fibre membrane was chosen for investigation of dynamic aggregation processes during membrane formation, because it allows the measurement of the dynamic equilibrium at different distances from the spinning nozzle. With this system it is possible to resolve structural changes in the nm-size range which occur during membrane formation on the time-scale of milliseconds. Integral structural parameters, like radius of gyration and pair-distance distribution, were determined. Depending on the chosen spinning parameters, e.g. the flow ratio between polymer solution and coagulant water, significant changes in the scattering curves have been observed. The data are correlated with the distance from the spinning nozzle in order to get information about the kinetics of membrane formation which has fundamental influence on structure and properties of the membrane.     

大投料比下自缩合原子转移自由基共聚合中聚合物支化结构的形成与演化

研究了少量N-[4-(α-溴代异丁酰氧基)苯基]马来酰亚胺(BiBPM)与大量甲基丙烯酸-N,N-二甲氨基乙酯(DMAEMA)在CuBr/N,N,N′,N″,N″-五甲基二乙烯三胺(PMDETA)催化下的自缩合原子转移自由基共聚合(SCATRCP).分别利用气相色谱、三检测凝胶渗透色谱测定了聚合反应过程中DMAEMA的转化率、所得聚合物(PDMAEMA)的分子量与分子量分布、绝对分子量和特性黏数等随着反应时间的变化.结果表明,在以上聚合过程中,PDMAEMA的分子量随着聚合的进行而不断上升,但是支化度持续下降.由此可知,在聚合早期就形成了低分子量而高支化度的PDMAEMA,在聚合后期,主要进行DMAEMA的ATRP,导致支化度随着分子量的上升而逐渐下降.     

Size Exclusion Chromatography of Polymers With Random Tetrafunctional Branching

Abstract

The behaviour of polydisperse branched copolymers of methyl methacrylate with a small amount of randomly situated tetrafunctional ethylenedimethacrylate units was investigated by means of size exclusion chromatography (SEC). A procedure has been suggested for the conversion of apparent values of molecular parameters of real polymer branched systems (M_{n, app}; M_{w, app} obtained from SEC data by calibration of the separation system using a linear polymer) into actual values. This was made possible by off-line coupling of SEC and viscometry. The branching was characterized by the weight average number of branching sites in the macro-molecule, m_w, and the branching index, y.     

Drying Study of Siloxane-PPG Nanocomposites

This is a study of the structural transformations occurring in hybrid siloxane-polypropyleneglycol (PPG) nanocomposites, with different PPG molecular weight, along the drying process. The starting materials are wet gels obtained by the sol-gel procedure using as precursor the 3-(trietoxysilyl)propylisocyanate (IsoTrEOS) and polypropylenglycol bis(2-amino-propyl-ether) (NH₂-PPG-NH₂). The shrinkage and mass loss measurements were performed using a temperature-controlled chamber at 50°C. The nanostructural evolution of samples during drying was studied in situ by small angle x-ray scattering (SAXS). The experimental results demonstrate that the drying process is highly dependent on the molecular weight of polymer. After the initial drying stage, the progressive emptying of pores leads to the formation of a irregular drying front in gels prepared from PPG of high molecular weight, like 4000 g/mol. As a consequence, an increase of the SAXS intensity due to the increase of electronic density contrast between siloxane clusters and polymeric matrix is observed. For hybrids containing PPG of low molecular weight, the pore emptying process is fast, leading to a regular drying front, without isolated nanopockets of solvents. SAXS intensity curves exhibit a maximum, which was associated to the existence of spatial correlation of the silica clusters embedded in the polymeric matrix. The spatial correlation is preserved during drying. These results also reveal that the structural transformation during drying is governed by capillary forces and depends on the entanglement of polymer chains.     

Physicochemical properties of pH-controlled polyion complex (PIC) micelles of poly(acrylic acid)-based double hydrophilic block copolymers and various polyamines

The physicochemical properties of polyion complex (PIC) micelles were investigated in order to characterize the cores constituted of electrostatic complexes of two oppositely charged polyelectrolytes. The pH-sensitive micelles were obtained with double hydrophilic block copolymers containing a poly(acrylic acid) block linked to a modified poly(ethylene oxide) block and various polyamines (polylysine, linear and branched polyethyleneimine, polyvinylpyridine, and polyallylamine). The pH range of micellization in which both components are ionized was determined for each polyamine. The resulting PIC micelles were characterized using dynamic light scattering and small-angle X-ray scattering experiments (SAXS). The PIC micelles presented a core–corona nanostructure with variable polymer density contrasts between the core and the corona, as revealed by the analysis of the SAXS curves. It was shown that PIC micelle cores constituted by polyacrylate chains and polyamines were more or less dense depending on the nature of the polyamine. It was also determined that the density of the cores of the PIC micelles depended strongly on the nature of the polyamine. These homogeneous cores were surrounded by a large hairy corona of hydrated polyethylene oxide block chains. Auramine O (AO) was successfully entrapped in the PIC micelles, and its fluorescence properties were used to get more insight on the core properties. Fluorescence data confirmed that the cores of such micelles are quite compact and that their microviscosity depended on the nature of the polyamine. The results obtained on these core–shell micelles allow contemplating a wide range of applications in which the AO probe would be replaced by various cationic drugs or other similarly charged species to form drug nanocarriers or new functional nanodevices.     

Autoxidized phospholipids in hexane: nano-self-assemblies studied by synchrotron small-angle X-ray scattering

Synchrotron small-angle X-ray scattering (SAXS) was used to analyze the structure of self-assembled autoxidized phospholipids in a very dilute solution of hexane. In addition, it was used to build a self-consistent model of the aggregates, taking into account their inner heterogeneities and polydispersity. The scattering intensity from a dilute mixture of different types of noninteracting components of the phospholipid system was represented as a linear combination of partial intensities from the components weighted by their volume fractions. Applying this approach the final model of the system was described as a mixture of polydisperse reverse micelles and aggregates with spherical and cylindrical shapes. Spherical aggregates were represented as hollow spheres with inner radius 0.7 nm (occupied by water or hexane) and outer radius 1.5 nm. Geometrical parameters of the aggregates did not change much during the oxidation process, while the ratio of reverse micelles and aggregates in solution varied. The amount of the reverse micelles increased from very low to about 80%, whereas the content of other aggregates constantly reduced. The analysis performed in this study helps one to better understand the processes of phospholipid oxidation, which may occur in biological membranes.     

Detailed structure of hairy mixed micelles formed by phosphatidylcholine and PEGylated phospholipids in aqueous media

Aqueous dispersions of mixed egg yolk phosphatidylcholine (PC) and poly(ethylene glycol) (PEG) modified distearoyl phosphatidylethanolamine (DSPE) were investigated with the purpose of determining shape, size, and conformation of the formed mixed micelles. The samples were prepared at a range of DSPEPEG to PC molar ratios ([DSPEPEG/PC] from 100:0 to 30:70) and with, respectively, DSPEPEG2000 and DSPEPEG5000, where 2000 and 5000 refer to the molar masses of the PEG chains. Particle shape and internal structure were studied using small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS). The contrast of the micelles is different for X-rays and neutrons, and by combining SANS and SAXS, complementary information about the micelle structure was obtained. The detailed structure of the micelles was determined in a self-consistent way by fitting a model for the micelles to SANS and SAXS data simultaneously. In general, a model for the micelles with a hydrophobic core, surrounded by a dense hydrophilic layer that is again surrounded by a corona of PEG chains in the form of Gaussian random coils attached to the outer surface, is in good agreement with the scattering data. At high DSPEPEG contents, nearly spherical micelles are formed. As the PC content increases the micelles elongate, and at a DSPEPEG/PC ratio of 30:70, rodlike micelles longer than 1000 angstroms are formed. We demonstrate that by mixing DSPEPEG and PC a considerable latitude in controlling the particle shape is obtained. Our results indicate that the PEG chains in the corona are in a relatively unperturbed Gaussian random coil conformation even though the chains are far above the coil-coil overlap concentration and, therefore, interpenetrating. This observation in combination with the observed growth behavior questions that the "mushroom-brush"transition is the single dominating factor for determining the particle shape as assumed in previous theoretical work (Hristova, K.; Needham, D. Macromolecules 1995, 28, 991-1002).     

The acid-catalyzed polycondensation of furfuryl alcohol: Old puzzles unravelled

This paper describes mechanistic studies aimed at understanding the origin of two important side events accompanying the linear polycondensation of furfuryl alcohol in acidic media. The first process concerns the formation of conjugated sequences along the polymeric chains. The use of model monomers and compounds simulating the structure of the linear polymer provided for the first time a full understanding of the reactions leading to multiple unsaturations. The main culprits for this process are the labile hydrogen atoms on the methylene moiteties bridging the furan rings. The second anomaly in these systems concerns the formation of networks following a complex branching mechanism. Again, model structures helped to pinpoint the origin of this process and to propose plausible reactions to describe it.     

Peptide dendrimers based on polyproline helices

We present a new family of peptide dendrimers based on polyproline helices and cis-4-amino-L-proline as a branching unit. Dendrimers were synthesized by a convergent solid-phase peptide synthesis approach. The conformational transition between polyproline type I helix and polyproline type II helix was observed by circular dichroism in branched polyproline building blocks with more than 14 proline residues and in the resulting dendrimers. Both linear and dendritic polyprolines were found to be actively internalized by rat kidney cells. Preliminary results show that the antibiotic ciprofloxacin form complexes with branched polyproline chains in 99.5% propanol.     

Characterisation of structure and aggregation processes of aquatic humic substances using small-angle scattering and X-ray microscopy

Aquatic humic substances (HS), an important part of the dissolved organic carbon in freshwater systems, are polyfunctional natural compounds with polydisperse structure showing strong aggregation/coagulation behaviour at high HS concentrations and in the presence of metal ions. In this study, small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS) and X-ray microscopy (XRM) were applied to characterise the structure and aggregation processes of HS in solution. In SAXS and XRM the high brilliant synchrotron radiation was used as X-ray source. Applying small-angle scattering, information about the size distribution and shape of aquatic HS was obtained. Spherical HS units were found which were stable in a wide concentration range in a kind of "monomeric" state almost independent of pH and ionic strength. At higher concentrations they formed chain-like agglomerates or disordered HS structures. In studies on the coagulation behaviour of HS after addition of copper ions, a linear relationship between Cu(2+) concentration and the formation of large disordered HS-Cu(2+) agglomerates was obtained. By using X-ray microscopy, single "huge" particles were found in older solutions and in solutions with high HS concentrations. Over a threshold Cu(2+) concentration of approx. 300 mg/L, the formation of an extensive HS-Cu(2+) network structure was observed within a few minutes. The presented structures show the ability of the methods used to characterise processes between diluted phase and suspended matter, which play an important role particularly in the region of phase interfaces.     

Gelation of xyloglucan ID water/alcohol systems

Xyloglucan has a cellulose backbone with branched (16)--xylose or (12)--galactoxylose as a side chain. Its aqueous solution yields a gel by adding alcohol. The gel structure of xyloglucan ID various kinds of mono- or polyhydric alcohol/water systems was studied by small-angle X-ray scattering (SAXS). The gelation behavior ID strongly dependent on the type of alcohol. The SAXS from gel with monohydric alcohols indicated that the xyloglucan chains caused random aggregation, as expressed with a Debye–Bueche type scattering function. The type of alcohol added was correlated with the size of the inhomogeneity, as evaluated by SAXS results. The gelation with polyhydric alcohols resulted ID less association, which occurred as side-by-side association with a few xyloglucan chains, rather than as random aggregation.     

Segment densities in star and comb random-flight chains

Segment densities of comb and star branched random-flight chains have been computed. It is found that the commonly used gaussian differs more significantly for branched chains than for linear chains. The asymptotic results are also found to depend on the branching parameter g.