Synthesis and characterization of A2 + B3-type hyperbranched aromatic polyesters with phenolic end groups

Hyperbranched (hb) aromatic polyesters with phenolic end groups were synthesized according to the A₂ + B₃ approach both, by solution polymerization and by melt (bulk) polymerization with different monomer ratios (A₂:B₃). The hb polyesters produced from solution polymerization exhibited higher yields, molar masses and glass transition temperature (T_g) compared to the products prepared in the melt. The resulting hb aromatic polyesters from the A₂ + B₃ approach were also compared with their properties to hb aromatic polymers produced from the well known AB₂ monomer 3,5-bis(trimethylsiloxy)benzoyl chloride. Both types of hb aromatic polyesters possess high T_g, high thermal stability and good solubility in common organic solvents. A typical melt viscosity behavior with shear thinning effect was also observed for both. Thus similar polymer properties compared to melt-condensed products were obtained with the A₂ + B₃ approach by solution polycondensation possessing the advantage of easy monomer availability and much milder polymerization conditions (at room temperature in solution) compared to the AB₂ approach.     

Polystyrene freeze-dried from dilute solution: Tg depression and residual solvent effects

The calorimetric glass transition temperature, T_g, was measured for both linear and cyclic polystyrenes freeze-dried from dilute solutions of 0.10, 0.05, and 0.02% of polymer by weight in benzene. Upon freeze-drying, T_g was found to be depressed by 4-15 K depending on the sample, solvent concentration, and freezing conditions. Annealing under vacuum at moderate temperatures, from 40 to 140 °C and 0.05 Torr, resulted in the shift of T_g back towards its bulk value and was accompanied by a decrease in sample weight. The data is consistent with the observed weight loss being due to residual solvent. The amount of residual solvent is a strong function of the annealing temperature and the initial freeze-drying solution concentration; exposure to vacuum at temperatures far below T_g is generally insufficient for residual solvent removal.     

Effect of phenol substitution on the network structure and properties of linear aliphatic diamine-based benzoxazines

A series of aliphatic diamine-based benzoxazines with different phenolic substitutions have been synthesized and characterized. Molecular structure of the monomers is verified by ¹H and ¹³C NMR, FTIR, and elemental analysis. The site of polymerization is regiospecified and the type of linkage in the network structure is manipulated by selectively blocking the ortho, para, and meta positions in the aromatic rings of the monomer. The controlled network structures and their properties are compared to those formed by the polymerization of aliphatic diamine-based benzoxazines prepared from unsubstituted phenol in order to provide insight into their structures. Investigation on the melting point, polymerization kinetics, gelation, and T_g and sub-T_g thermal transition is all reported on these systematically controlled structure polybenzoxazines.     

Thermal and mechanical properties of solution polymerized segmented polyurethanes with butadiene soft segments

A series of HTPBD containing polyurethanes of high molecular weight have been synthesized in solution. The value of the soft segment T_g is very close to that of the free HTPBD and independent of hard segment content indicating complete or very nearly complete phase segregation. Since the hard segments of TDI/BDO are amorphous, the driving force for phase segregation must arise from the large degree of incompatibility between the polar hard segment and nonpolar soft segment. Furthermore, in these samples there is also no opportunity for hydrogen bonding between hard and soft segments to enhance compatibility.The values of the hard segment glass transition increase with the average hard segment length following a Fox-Flory type relationship. In contrast to the segment T_g observed in bulk polymerized samples, only a single hard segment T_g occurred in the present study. This indicates that the double T_g behaviour is a result of the heterogeneous nature of the bulk polymerization.With increasing hard segment content, the properties vary from soft to rigid elastomers, and rubber roughened plastics. This variation in properties is caused by changes in the sample morphology which depends upon the relative fractions of hard and soft segments. Mechanical properties show marked improvement over the corresponding bulk polymerized samples. Unlike polyester and polyether urethanes, these materials evidence no change in the soft segment T_g following thermal treatment and no effect of thermal history on the mechanical properties.     

Characteristics of poly(vinyl pyrrolidone)/poly(acrylic acid) interpolymer complex prepared by template polymerization of acrylic acid: Effect of reaction solvent and molecular weight of template

Poly(vinyl pyrrolidone) (PVP)/poly(acrylic acid) (PAA) interpolymer complexes were prepared, in ethanol or dimethylformamide (DMF), by template polymerization of acrylic acid in the presence of PVP (MW: 42.5 or 1100 K) used as the template. FTIR analysis showed that the complexes were formed through hydrogen bonding between the carboxyl groups of the PAA and the carbonyl groups of the PVP. The glass‐transition temperature (T_g) of the complex, prepared in ethanol, was higher than that of the component polymers, whereas the T_g of the complex, prepared in DMF, was located between that of the component polymers. The dissolution rate of the complex was affected by the molecular weight of the PVP and the reaction solvent. The release rate of ketoprofen from the complexes showed a pH dependency, and was slower at a lower pH. The ketoprofen release rate from the complex was controlled mainly by the dissolution rate of the complex above the pK_a of PAA (4.75) and by the diffusion rate below the pK_a. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2390–2394, 2004     

Photo-DSC and dynamic mechanical studies on UV curable compositions containing diacrylate of ricinoleic acid amide derived from castor oil

A difunctional acrylate of ricinoleic acid amide (COEDA) was synthesized from castor oil, which was non-irritant, odorless and capable of fast polymerization. Cross-linked networks were prepared by photo polymerization of the COEDA monomer as well as with varying amount of reactive diluents namely tripropylene glycol diacrylate (TPGDA) and trimethylol propane triacrylate (TMPTA) with the aim of modifying the curing and viscoelastic characteristics of the networks. Conversion as well as rate of conversion of COEDA copolymers enhanced with diluents percentage. Cure kinetics indicated an auto-accelerated process in which the rate constant increased with increasing amount of diluents. Storage modulus and glass transition temperature of UV-cured films were determined by DMTA. Films containing varying amount of TPGDA showed very close T_g-values where as those with increasing amount of TMPTA exhibited higher values of T_g.     

Effect of MCM-41 nanoparticles on the kinetics of free radical and RAFT polymerization of styrene

To examine the effect of mobil composition of matter 41 (MCM-41) nanoparticles on the kinetics of free radical and 2-(dodecylthiocarbonothioylthio)-2-methylpropionic acid (DDMAT)-mediated reversible addition fragmentation chain transfer (RAFT) polymerization, the polymerization reaction using various amounts of as-synthesized MCM-41 were performed. To study the reaction kinetics, conversion, molecular weight and polydispersity index (PDI) were obtained during the polymerization. Also, differential scanning calorimetry (DSC) was used to determine the glass transition temperature (T _g) values of samples. According to the results, in free radical polymerization, conversion was increased by adding nanoparticles but the reverse trend was observed in RAFT polymerization. The same results were obtained for molecular weight values. In free radical polymerization, increasing the MCM-41 content led to higher PDI value, while in RAFT polymerization it did not appreciably affect the PDI value. In RAFT polymerization, no induction time was observed which indicates that DDMAT is an appropriate RAFT agent for styrene polymerization. Also in free radical polymerization, the addition of MCM-41 particles reduced T _g values in comparison to neat PS. On the other hand, there was an increase in T _g value up to 5 wt% of MCM-41 loading and a drastic reduction was observed in 7 wt% MCM-41 loading in the RAFT polymerization. Finally, the T _g values of nanocomposites produced by RAFT method were higher than those in the nanocomposites synthesized using the free radical method.     

Effects of ultrasonic pretreatments on thermodynamic properties,water state,color kinetics,and free amino acid composition in microwave vacuum dried lotus seeds

Abstract

Ultrasonic pretreatments were applied to lotus seeds at acoustic energy densities of 0.29, 0.40, and 0.51?W mL^?1 for 10?min. After pretreatments, lotus seeds were subjected to microwave vacuum drying (MVD). Parameters of glass transition temperature (T_g), gelatinization temperature (T_p), water state, color kinetics, and free amino acid content of microwave vacuum dried lotus seeds were determined. With increasing acoustic energy density, MVD elevated the T_g values appreciably by decreasing the content of cytoplasmic bulk water in lotus seeds tissues. The T_p had a positive relationship with the relaxation times of cytoplasmic bulk water (T₂₂), while T_g had a negative relationship with T₂₂. Color kinetics were analyzed by the divisional method during MVD due to different browning reactions, which failed to appear with ultrasonic pretreatment. Free amino acid content ranged from 517.65 to 666.13?mg/100?g dry weight at 0.51?W mL^?1.     

可用于缓控释肥聚乙烯基吡咯烷酮（K32）的合成与表征

研究了可用于缓控释肥包膜材料的聚乙烯基吡咯烷酮（PVP K32）的合成.研究了通过水溶液自由基聚合法合成PVP K32的实验技术,研究了不同引发剂、聚合工艺等对合成PVP K32的相对分子质量及残留单体含量的影响.通过优化实验获得一种分子量分布窄、残单含量低的PVP K32的聚合工艺,并对样品进行了表征及成分分析.     

Transformation of liquid to amorphous solid: The time to vitrify for styrene polymerization

A model is presented for the calculation of the time to vitrify vs. temperature for isothermal polymerization by the chain growth mechanism. The model is based on the glass transition temperature (T_g) rising from its initial value to the reaction temperature. The relationships between T_g and the volume fraction of polymer and monomer, the volume fraction of polymer and the extent of reaction, and the extent of reaction and time are also required. In a plot of temperature vs. time the vitrification curve is generally S-shaped; the time passes through a maximum just above the glass transition temperature. The model applies to linear polymerization in which monomer and high molecular weight polymer are the dominant species, i.e., to chain reactions. In this communication the model is applied to the bulk polymerization of styrene by the free radical mechanism.     

A study of blending and complexation of poly(acrylic acid)/poly(vinyl pyrrolidone)

Poly(acrylic acid) (PAA) and poly(vinyl pyrrolidone) (PVP) were chosen to prepare polymer complex and blends. The complex was prepared from ethanol solution and the blends were prepared from 1-methyl-2-pyrrolidone solution. DSC results show that the T_gs of the PAA/PVP blends lie between those of the two constituent polymers, whereas T_g of the PAA/PVP complex is higher than both blends and the two constituent polymers. TGA results show that degradation temperature, T_d, of PAA increases upon adding PVP in the blend, but thermal stability of the complex is higher than that of the blends as reflected by the higher T_d. Both FTIR and high-resolution solid state NMR show strong hydrogen bonding between PAA and PVP by showing significant chemical shift. The T_1ρ(H) measurement shows that the homogeneity scale for the blend is at ∼20 Å and that for the complex is ∼15 Å.     

Solvent-free copolymerization of rigid and flexible bis-1,3-benzoxazines: Facile tunability of polybenzoxazine network properties

We report the copolymerization of a flexible aliphatic-bridged bisphenol-based benzoxazine monomer comprising ten methylene units (BZ(10)BA) with two rigid benzoxazine monomers (commercially available Araldite 35600 and 35900) via a solvent-free cationic ring-opening polymerization process. The effects of monomer feed composition on polymerization behavior, thermomechanical transitions, and thermal degradation properties are reported. DSC of the ring-opening copolymerizations showed that the copolymerization behavior – in terms of polymerization onset temperature and total exothermic transition – depend greatly on the composition of the monomer feed. Samples containing larger concentrations of BZ(10)BA exhibited higher onset temperatures with lower polymerization enthalpies. The thermomechanical properties of the copolybenzoxazine networks, as evaluated by DMA, show a strong dependence on the monomer feed ratio, where higher Araldite content resulted in a higher T_g of the network. The most salient feature of benzoxazine copolymerization was revealed in the tailorability in thermomechanical properties, which were varied by 149 °C simply by changing the monomer feed ratio and the T_g was observed to be accurately predicted using the Fox equation.     

Characterization of styrene–methyl methacrylate–n-butyl acrylate terpolymers. II. Effects of sequence distribution on glass transition temperature

The effects of sequence distribution on the glass transition temperature (T_g) of the title terpolymers prepared by radical polymerization were studied. T_g was examined by thermomechanical analysis. The average diad concentrations, as estimation of sequence distributions were calculated from monomer reactivity ratios. A modified Gibbs–Dimarzio equation for binary copolymers was extended to terpolymers to explain the relation between observed T_g and average diad concentrations. The observed T_g showed good agreement with the calculated values determined by the extended equation.     

In-situ polymerization of styrene in AAO nanocavities

One of the most promising aspects of the anodic aluminium oxide (AAO) template is the ability to generate a variety of different hierarchical one-dimensional (1D) polymer morphologies with structural definition on the nanometric scale. In-situ polymerization of monomers in reduced space of porous AAO template nanocavities can give rise to the direct production of versatile polymer nanostructures. In this work, porous AAO devices of 35 nm of diameter have been obtained by a two-step electrochemical anodization process and used as a nanoreactor to study the radical polymerization kinetics of styrene (St) in confinement and the results compared to those of polymerization in bulk. SEM morphological study has been conducted to establish the final structure of obtained polymer nanostructures. Confocal Raman microscopy has been performed to study the formation of the polymer through the AAO cavities as a function of time and with this methodology it has been possible to establish the monomer conversion for styrenic polymerization in AAO devices. Polystyrene obtained in the nanoreactor was characterized by SEC, NMR, TGA and DSC and the properties compared with those of bulk polymer. It was found that both the average molecular weights and polydispersity index of nanostructured polymer are lower than those obtained for bulk polymer. NMR studies have shown that the use of a reactor with nanometric size dimensions gave the obtained polystyrene greater stereospecificity than that obtained in bulk. Thermal stability and glass transition temperature (T_g) values are higher for nanostructured than bulk polymers. Moreover, the methodology proposed in this work, using AAO nanocavities as nanoreactors for polymerization reaction, can be generalized and applied to obtain polymer nanostructures of very different chemical nature and morphology by choosing the appropriate monomer or monomer reactants and by tailoring the dimension of AAO cylindrical nanocavities, that is, diameter from 20 to 400 nm and length from a few to hundreds of microns.     

Synthesis and characterizations of a vinyl-terminated benzoxazine monomer and its blending with polyhedral oligomeric silsesquioxane (POSS)

Benzoxazine was synthesized through the Mannich condensation of phenol, formaldehyde, and primary amines through ring-opening polymerization. Polybenzoxazines are phenolic-like materials that possess dimensional and thermal stability, and they release no toxic byproducts during their polymerization. To further improve the thermal stability of polybenzoxazines, a hydrosilane-functionalized polyhedral oligomeric silsesquioxane (H-POSS) was incorporated into the vinyl-terminated benzoxazine monomer (VB-a) which we then subjected to ring-opening polymerization. In addition, we also prepared hybrids from a non-reactive POSS (IB-POSS) and VB-a. The glass transition temperature (T_g) of a regular polymerized VB-a (i.e. PVB-a) is 307 °C, while the hybrid containing 5 wt% of H-POSS is 333 °C. The IB-POSS modified PVB-a hybrids, in general, results in lower T_g than the pure PVB-a due to poor missibility.     

Control of monodisperse particle size of styrenic–acrylate copolymers in dispersion copolymerization

Syntheses of monodisperse poly[(styrene)‐co‐(n‐butyl acrylate)] and poly[(styrene)‐co‐(2‐ethylhexyl acrylate)] were carried out by dispersion polymerization. The reactions were performed in the mixed solvent of ethanol–water in the presence of azo‐bisisobutyronitrile and poly(N‐vinylpyrrolidone) as the initiator and dispersant, respectively. The effects of reaction parameters, that is the type and concentration of dispersant, ratio of the mixed solvent, reaction temperature, agitation rate, monomer composition between styrene and n‐butyl acrylate or 2‐ethylhexyl acrylate, crosslinking agent and reaction time on the particle size, size distribution and average molecular weights of the resulting copolymer were thoroughly investigated. The resulting copolymer particles were smooth on their spherical surface and the sizes were in the range 0.6–1.8 µm with a narrow size distribution. In most cases, a correlation between small particle sizes with high average molecular weights was observed. The average particle size generally increased with increasing reaction temperature, time and acrylate monomer content. In contrast, the particle size decreased as the molecular weight, concentration of dispersant, polarity of the medium or agitation rate was increased. The glass transition temperature (T_g) of the copolymers can be controlled by the mole ratio of the comonomer. The T_g values decreased when the content of acrylate monomers in the copolymer increased, and T_g values of the synthesized copolymer were in the range 66–102 °C. Instead of using n‐butyl acrylate monomer in the copolymerization, 2‐ethylhexyl acrylate copolymerization with styrene resulted in insignificant changes in the particle sizes but there were significant decreases in T_g values. In this study, the monodisperse particles can be obtained by monitoring the appropriate conditions regarding PVP K‐30 (2–8 wt%), ethanol/water (90/10 wt%), the reaction temperature (70 °C) and the agitation rate (100 rpm). © 2000 Society of Chemical Industry     

Studies on high performance nonvolatile polyimides coating: Gamma ray initiated bulk copolymerization of vinyl polar monomer and maleimide-terminated polyimides with flexible backbone and the modifications

The objective of the present work was to develop a nonvolatile coating involving vinyl polar monomer and high performance maleimide-terminated polyimides (MTPs) with flexible backbone. The synthesized MTPs with different molecular weight (Mw) showed good solubility in N-vinylpyrrolidone (NVP), and the homogeneous binary systems were obtained even when the content of MTPs reached 50 wt%. NVP-MTPs copolymer resin were prepared by gamma ray initiated bulk copolymerization, followed by thermal postcuring to improve the properties. NVP-MTPs copolymer resins exhibited flexural strength higher than 76 MPa, glass transition (T_g) higher than 142 °C, thermal stability (T_5%) no less than 214 °C, but relatively poor impact strength and water resistance property. It was revealed that increasing Mw of MTPs would result in the increase of impact strength and a decrease of thermal and flexural properties of copolymer resin, accompanied with the gradual decrease of gel percentage of copolymer resin. Amino-terminated linear polyimide (ATLP) was introduced to modify NVP-MTPs copolymer resin and the C-N linkage was formed from terminal NH₂ to electron deficient double bone on maleimide ring of MTPs via Michael nucleophilic addition. Polyvinylsiloxane (PVS) were introduced to modify NVP-MTPs copolymer resin by in situ ring opening polymerization followed with copolymerization under gamma ray initiation. The introduction of both modifiers led to an appreciable increase of impact strength and a remarkable water uptake decrease, meanwhile, other properties including thermal and flexural properties of modified copolymer resins maintained and even exhibited a slight increase. Fracture surface morphology by scanning electron microscopy (SEM) technique showed that introduction of both modifiers maintained good homogeneity of copolymer resin, and led to a gradual transition from rigid deformation to plastic deformation.     

Polyether-metal salt complexes: 1. The elevation of the glass transition temperature of polyethers by metal salts

Various metal halides have been dispersed in high and low molecular weight, amorphous poly(propylene oxide), by solution blending techniques, to give single phase polymeric complexes which remained thermoplastic even at very high salt loadings. These complexes were amorphous and showed a single well-defined glass transition temperature (T_g) by differential thermal analysis with the T_g of the complex up to 140°C greater than the T_g of the parent polymer. The T_g elevation depended upon both the amount and the type of salt added and for a given salt the elevation followed a sigmoidal curve which levelled out at high salt concentrations. The T_g data have been interpreted in terms of a chelate ring model involving the co-ordination of two adjacent ether oxygen atoms in the polymer backbone to the salt. Using this model it was possible to consider the complex as a random copolymer consisting of complexed and uncomplexed monomer units. The contribution of crosslinking by metal salts to the elevation of T_g was assessed by studying poly(tetramethylene glycol)-zinc chloride complexes in which chelate formation is entropically unfavourable. Mechanical data are reported for ZnCl₂ complexes with high molecular weight poly(propylene oxide). The results indicate that ZnCl₂ increased the rubbery modulus and this has been interpreted in terms of ZnCl₂ forming a few, weak intermolecular crosslinks.     

Photocuring and thermomechanical properties of multifunctional amide acrylate compositions derived from castor oil

The photopolymerization of multifunctional acrylate monomers synthesized from castor oil was investigated by photo-DSC. These studies revealed that the extent of photopolymerization depended on the double bond concentration and a greater degree of crosslinking occurred in monomer mixtures with higher difunctional content. The monomer mixtures displayed significantly higher maximum rate of polymerization (R_p^max) and shorter time to reach peak maximum than the pure monomers. DMTA studies of films showed good storage modulus and broad tan δ transitions indicating heterogeneity in the crosslinked networks. The films displayed sub-T_g transitions in the loss modulus curves were possibly due to the side chain motions of the monomer acrylates which increased with increasing triacrylate concentration. Glass transition temperature (T_g) of these networks depended on composition and shifted to higher values with increasing amount of triacrylate.     

Confinement, composition, and spin-coating effects on the glass transition and stress relaxation of thin films of polystyrene and styrene-containing random copolymers: Sensing by intrinsic fluorescence

The glass transition temperatures (T_gs) of polystyrene (PS) and styrene/methyl methacrylate (S/MMA) random copolymer films are characterized by intrinsic fluorescence, i.e., monomer fluorescence from an excited-state phenyl ring and excimer fluorescence from an excited-state dimer of two phenyl rings. The T_g is determined from the intersection of the rubbery- and glassy-state temperature dependences of the integrated fluorescence intensity measured upon cooling from an equilibrated state. With PS, the effects of nanoconfinement on T_g and the transition strength agree with results from studies using probe fluorescence and ellipsometry. The T_g-nanoconfinement effect is “tuned” by copolymer composition. As S-content is reduced from 100 mol% to 22 mol%, the confinement effect changes from a reduction to an enhancement of T_g relative to bulk T_g. Intrinsic fluorescence is also a powerful tool for characterizing relaxation of residual stresses. Stresses induced by spin coating affect local conformations, which in turn affect excimer and monomer fluorescence and thereby integrated intensity. The heating protocol needed to achieve apparently equilibrated local conformations is determined by equivalence in the integrated intensities obtained upon heating and subsequent cooling. While partial stress relaxation occurs upon heating in the glassy state, full relaxation of local conformations requires that a film be heated above T_g for times that are long relative to the average cooperative segmental relaxation time. For example, in thin and ultrathin films, equilibration is achieved by heating slowly (∼1 K/min) to 15-20 K above T_g. Dilute solution fluorescence of PS and S/MMA copolymers is also characterized and compared to reports in the literature.