Curing of polyfurfuryl alcohol resin catalyzed by a homologous series of dicarboxylic acid catalysts. II. Swelling behavior and thermal properties

Polyfurfuryl alcohol (PFA), as a bio‐based resin made from lignocellulosic materials, was crosslinked using a homologous series of dicarboxylic acid catalysts consisting of oxalic, succinic, and adipic acids, which are different in their dissociation constants. Swelling behavior, thermal stability, and non‐oxidative char residue of the resulting networks were characterized as a function of the acids strength and their concentration. Swelling of the networks were investigated at room temperature in eight solvents differing in molecular size and solubility parameter. Using acetonitrile as a solvent, the swelling mechanism was explored by applying kinetic models to the swelling data. Dynamic swelling studies during 16–30 days supported non‐Fickian and anomalous diffusion mechanism in highly crosslinked samples supporting PFA chain rigidity and high crosslinking density of the networks. Polymer–solvent interaction parameter, molecular weight between crosslinks and crosslinking densities were also determined. According to the results, the extent of PFA crosslinking and non‐Fickian behavior of the swelling solvent diffusion through the networks are strongly dependent on the concentration and dissociation characteristics of the catalysts used. Thermal stability studies showed no significant differences between the compositions, up to 900 °C. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45770.     

Structure and swelling behaviour of hydrophilic epoxy networks investigated by SANS

Structure of hydrophilic epoxy networks swollen in deuterated water has been investigated by small-angle neutron scattering (SANS). Two series of the networks were prepared by end-linking reaction of α,ω-diamino terminated poly(oxypropylene)-block-poly(oxyethylene)-block-poly(oxypropylene) (POP-POE-POP), α,ω-diamino terminated poly(oxypropylene) (POP) and diglycidyl ether of Bisphenol A propoxylate (PDGEBA). In the first series, prepared using functionalized POP-POE-POP and PDGEBA, hydrophilicity of the networks was controlled via cross-linking density of the networks, changing the initial ratio of reactives. In the second series, POP-POE-POP was gradually substituted by POP of the same molecular weight, enabling a control of hydrophilicity of the networks at essentially constant crosslinking density in this way. Microphase separated structure of the networks swollen in deuterated water with composition variations on nanometer scale was found by SANS. The scattering data were fitted to the Percus-Yevick model, approximating the swollen network by an idealized system of spheres dispersed in the matrix. Satisfactory fits in the region of the interference scattering peak were obtained giving values 30-70 Å for the average radius of the spherical regions. Radius of the spheres increases with increasing hydrophobicity of the networks (growing POP content and/or network crosslinking density).     

Fracture and structure of highly crosslinked polymer composites

The fracture toughness of silica filled dimethacrylate resins has been found to increase with the degree of cure of the network, and was previously attributed to increased crosslinking or the concomitant reduction in unreacted monomer (sol species). Studies of the dependence of the polymerization on the photoinitiator concentration and cure time of dimethacrylate resins illustrate the interrelation of the wt % sol and the degree of cure and reveal that a considerable amount of residual monomer remains in undercured networks, as predicted by simple gelation theory. In an attempt at separating the effects of residual monomer and crosslinking on the fracture behavior, several series of filled dimethacrylate and epoxy resins were studied in which the crosslink density and sol levels were independently varied without introducing significant changes in the chemical composition of the network. Although the fracture energy and toughness were raised by increasing the crosslink density in the epoxy resins, no significant variation was found for the dimethacrylates. Saturated analogues of dimethacrylates (used to represent residual monomer) significantly impaired the fracture resistance, suggesting that the reduction in residual monomer is responsible for the improved fracture toughness observed with postcured dimethacrylate networks.     

Small-angle neutron scattering from polystyrene—DVB networks containing a delta fraction of deuterated polystyrene: Evidence for aggregation during polymerization

Homopolymer polystyrene-divinylbenzene, PS-DVB, networks were made by free radical chemistry u.v. photopolymerization techniques, and characterized by small-angle neutron scattering (SANS). The networks contained a delta fraction of chains labelled with deuterium. Two different series of samples were made: Method A in which the insertion was made centrally or at the end of the polymerization; Method B in which the delta fraction of deuterated primary chains was inserted at the beginning of the polymerization. Method B samples yielded a molecular weight of about 70,000 g/mole and a radius of gyration of about 120 Å, values in the range expected for single primary chains. Method A samples, however, gave molecular weights of 1–2 million and radii of gyration ranging from 350 to 400 Å. Intrinsic viscosity and gel permeation chromatography (g.p.c.) studies on parallel fractions yielded weight-average molecular weights near 72,000 g/mole for most of the polymerization. These results strongly suggest the existence of aggregation of primary chains synthesized in the same conversion period within the network. A new method of characterizing crosslinking between network chains is proposed, since these experiments count the probability of a chain crosslinking with a chain polymerized immediately before in time.     

Control of network structure in emulsion crosslinking copolymerization

A kinetic model for network structure development during crosslinking copolymerization of vinyl and divinyl monomer is proposed. The model calculations suggest that polymer networks synthesized by free-radical copolymerization are, in general, inhomogeneous at least on a microscopic scale. By application of the same kinetic parameters as those for bulk polymerizations, it was found that the crosslinking density of the polymers formed in the earlier stages of polymerization is very high in emulsion polymerizations and polymer networks tend to be highly heterogeneous. Homogeneous networks cannot be formed even under Flory's simplifying assumptions for vinyl/divinyl copolymerization in emulsion polymerizations. The present kinetic model can be used to find semi-batch policies to control the network structure, and semi-batch policies were used to illustrate the synthesis of homogeneous polymer networks.     

Polydimethyldiphenylsiloxanes/silica interconnected networks: preparation and properties evaluation

A series of copolymers, polydimethyldiphenylsiloxane-α,ω-diols, differing by compositions and molecular masses has been prepared by equilibrium anionic ring-opening copolymerization of octamethylcyclotetrasiloxane with octaphenylcyclotetrasiloxane in presence of tetramethylammonium hydroxide as a catalyst. These copolymers were used as matrix for the in situ developing silica networks. The crosslinking of the copolymers concomitantly occurs by reaction between the ending silanol groups and tetraethoxysilane, thus the polydiorganosiloxane network interconnects with the silica one. The resulted materials processed as films (thickness 0.25–1.50 mm) were investigated by various techniques in order to evaluate morphology, optical properties, and thermal behavior. The results were compared with those obtained by model samples based on polydimethylsiloxanes/silica.     

Semi- and fully interpenetrating polymer networks based on polyurethane-polyacrylate systems. VIII. The influence of the degree of crosslinking of the second formed network on the morphology and properties of polyurethane polymethylacrylate interpenetrating polymer networks

A series of polyurethane–polymethylacrylate sequential interpenetrating polymer networks containing 40% by weight of polyurethane were prepared in which the levels of crosslinking in the second formed network—polymethylacrylate—was systematically altered over a wide range. The polymethylacrylate networks and the interpenetrating polymer networks were investigated using dynamic mechanical analysis, sonic velocity measurements, and tensile testing. In addition, the interpenetrating polymer networkds were studied using transmission electron microscopy. The interpenetrating polymer networks showed high values of the Oberst damping factor. It was concluded that tightening the second formed network does not produce the dramatic effects associated with decreasing the average molecular weight between crosslinks of the first formed network.     

Single-phase bicomponent network by random crosslinking of hydroxyl-terminated polyisobutylene/polytetrahydrofuran mixtures

Summary Random bicomponent networks were prepared from 50/50 wt % mixtures of hydroxyl-terminated polyisobutylene (HO-PIB-OH) and polytetrahydrofuran (HO-PTHF-OH) by the use of triphenylmethane triisocyanate (TTI) endlinking agent. Differential scanning calorimetry (DSC) and dynamic mechanical techniques were employed to study the networks. The glass transition temperature (T_g) of HO-PIB-OH was found to increase upon network formation. This T_g increment was used to estimate the molecular weight between crosslinking points (M_c). HO-PIB-OH/HO-PTHF-OH mixtures are immiscible and exhibit an upper critical solution temperature, however, the 50/50 network in a two-phase regime for the precursors gives a single T_g situated between the T_gs of the component networks. The single-phase network may be due to rapid nondiscriminatory endlinking and to an increase of the single-phase region of the phase diagram because of crosslinking.     

硅氧烷预聚体改性热塑性酚醛树脂的交联结构及其力学性能

通过不同预聚程度的环氧化硅氧烷（ES）与酚醛树脂共固化及固化物理状态的调控,构建不同拓扑结构的交联网络,探讨了不同预聚程度的硅氧烷预聚体（PES）改性热塑性酚醛树脂（NR-PES）交联网络的调控及其强韧化的构建方法。首先,合成了一种不同预聚程度的环氧化硅氧烷（PES）,通过DSC和流变分析,明确了NR-PES的固化反应和物理状态特征,在此基础上,确定了不同交联结构NR-PES的制备方法。接着,采用DMA、TGA、力学测试研究了PES的预聚程度对NR-PES交联网络和性能的影响规律。结果表明,当PES预聚程度较低时,NR-PES的交联密度较低,导致其热稳定性和弯曲强度较低;随着PES预聚程度的增加,NR-PES的交联密度不断增加,其热稳定性、弯曲强度随之增加,但K_IC不断降低。特别是,PES的预聚程度为30%时,2-NR-PES展现出优异的热稳定性、弯曲强度和断裂韧性,残炭率C_800℃为53.43%,弯曲强度为20.51 MPa,K_IC为0.389 MPa·m^1/2。此外,当PES预聚程度过高时,NR-PES的热稳定性、弯曲强度和断裂韧性显著降低。     

硅氧烷预聚体改性热塑性酚醛树脂的交联结构及其力学性能

通过不同预聚程度的环氧化硅氧烷（ES）与酚醛树脂共固化及固化物理状态的调控,构建不同拓扑结构的交联网络,探讨了不同预聚程度的硅氧烷预聚体（PES）改性热塑性酚醛树脂（NR-PES）交联网络的调控及其强韧化的构建方法。首先,合成了一种不同预聚程度的环氧化硅氧烷（PES）,通过DSC和流变分析,明确了NR-PES的固化反应和物理状态特征,在此基础上,确定了不同交联结构NR-PES的制备方法。接着,采用DMA、TGA、力学测试研究了PES的预聚程度对NR-PES交联网络和性能的影响规律。结果表明,当PES预聚程度较低时,NR-PES的交联密度较低,导致其热稳定性和弯曲强度较低;随着PES预聚程度的增加,NR-PES的交联密度不断增加,其热稳定性、弯曲强度随之增加,但K_IC不断降低。特别是,PES的预聚程度为30%时,2-NR-PES展现出优异的热稳定性、弯曲强度和断裂韧性,残炭率C_800℃为53.43%,弯曲强度为20.51 MPa,K_IC为0.389 MPa·m^1/2。此外,当PES预聚程度过高时,NR-PES的热稳定性、弯曲强度和断裂韧性显著降低。     

Transparent,methacrylate-based polymer networks with controlled crosslinker ductility

A series of transparent methacrylate-based crosslinked polymer networks are prepared in which the crosslinker length is controlled as a means to investigate the effects of network ductility on mechanical and ballistic properties. In each network the optical clarity of pure poly(methyl methacrylate) (PMMA) is retained, as well as a low value of haze. Both the glass transition temperature (T_g) and the tensile modulus of the networks are highly tunable, with network values both above and well below that of pure PMMA or the pure crosslinker network. The ballistic performance is likewise affected, with performance values of up to 400% greater than neat PMMA. We examine the effects of the crosslinker molecular weight on the impact performance, finding that, in these systems, the molecular weight between crosslinks is not a driving factor for the impact performance, and this may broadly translate to polymer networks in general. We find that improvements in ballistic performance can be realized at low molecular weight between crosslinks, provided the crosslinking agent is of sufficient ductility.     

Crosslinking of bimodal polyethylene

Two bimodal polyethylenes, differing only in polymerisation order, were investigated with respect to crosslinking behaviour and network properties. The crosslinked materials were examined using size exclusion chromatography (SEC), gel-content measurements, and calculations of the network density. Dynamic mechanical analyses in the melt were performed to monitor the crosslinking and to provide another measure of the network density. The experiments were performed to investigate any potential influence of the polymerisation order on the crosslinking as well as to study the network formation in the crosslinked polymers. The bimodal polyethylenes were also compared to two monomodal polyethylenes representing the short chain branched, high molecular weight fraction, and the linear, low molecular weight fraction, respectively. The SEC measurements clearly showed how the crosslinking starts with the consumption of the high molecular weight fraction. The gel-content measurement showed the importance of a high molecular weight material for the gel formation. The network density calculations demonstrated how long chains can give rise to apparent networks which are mainly due to chain entanglements. The experiments showed that the polymerisation order for the bimodal polyethylenes has no effect on the crosslinking.     

Swelling characteristics of block order networks prepared by styrene-ethylene/butylene ABA triblock copolymer and styrene homopolymer binary blends

Mixed networks based on polystyrene (PS) and styrene-ethylene/butylene triblock copolymer (SEBS) binary blends were synthesized via a postpolymerization Friedel-Crafts-type reaction employing styrene units and a bis-chloromethylated crosslinking agent. The swelling behavior of the produced gels was studied in relation to network densty, SEBS/PS mass ratio, and solubility parameter of the swelling media, in a range of aliphatic, chlorinated, and aromatic solvents. Swelling characteristics of the mixed macronets were found to differ markedly from those of pure homopolymer gels. Selective swelling was observed in three different regions in respect to the solvent solubility parameter, in contrast to pure PS networks where adsorption was favored in a single region of solvents. This phenomenon was found to depend primarily on the SEBS/PS constitution of gels, while the solvent holding capacity depended directly on the crosslinking density. © 1995 John Wiley & Sons, Inc.     

Hydrosilylation crosslinking of silicone rubber catalyzed by bis (1,5-cyclooctadiene) di-μ,μ′-chlorodirhodium

Hydrosilylation vulcanization of addition silicone rubber catalyzed by bis (1,5-cyclooctadiene) di-μ,μ′-chlorodirhodium at 150° was studied. The course of vulcanization, rubber network density, and mechanical properties of the vulcanizate are discussed with respect to the structure and molecular weight of the reactive groups of telechelic and internal vinylmethylpolysiloxanes and crosslinking methyl (hydrogen) dimethyl siloxane copolymers. Molecular weights of the polysiloxane components as well as the ration of unsaturated and Si—H groups were optimized. The results are in agreement with the theory of rubber elasticity of polymer networks and show wide application possibilities of the catalyst used.     

Interpenetrating polymer networks of poly(dimethylsiloxane): 1. Preparation and characterization

Model networks of ,ω-dihydroxy-poly(dimethylsiloxane) (PDMS) were prepared by tetrafunctional crosslinking agent tetraethyl orthosilicate (TEOS) and the catalyst stannous 2-ethylhexanoate. Hydroxylterminated chains of PDMS having molecular weights 15 × 10³ and 75 × 10³ g mol⁻¹ were used in the crosslinking reaction. Bimodal networks were obtained from a 50% (w/w) mixture of PDMS chains with M_n = 15 × 10³ and 75 × 10³ g mol⁻¹. A sequential interpenetrating network of these PDMS chains was also prepared. Physical properties of the elastomers were determined by stress-strain tests, swelling and extraction experiments, and differential scanning calorimetry measurements.     

一种硼酸酯动态交联环氧树脂的合成与性能

利用巯基与环氧的点击反应，向环氧体系中引入二醇结构，并通过硼酸酯键对其进行交联，制备了一系列不同流变性能和玻璃化转变温度的可塑环氧树脂材料。研究了不同的配方对材料性能的影响，结果表明通过改变交联剂与动态链结的比例，体系的流变性能可以被调控；另一方面，改变软硬环氧组分的比例可以使材料的玻璃化转变温度在14.6~36.5℃之间调节。制备了该可塑环氧树脂与密胺通孔泡沫的复合材料，该材料具备良好的快速自黏结性能。材料断裂后，在10 s内即可达到最大黏结强度，且经多次拆卸-黏结循环后，黏结强度可以保持在约0.06 MPa。     

Relationships in a bismaleimide resin system. Part I: Cure mechanisms

A series of bismaleimide resin systems has been examined in order to identify the molecular features responsible for the mechanical response of these materials. A range of network structures was produced both by formulation of resins with different ratios of N,N′-bismaleimido-4,4′-diphenylmethane (BMI) and methylene dianiline (MDA), and by the use of different thermal processing cycles. Spectrographic and chromatographic techniques were used to study the reactions that occurred during the cure. Two principal reactions were confirmed: a Michael addition reaction which provides linear chain growth, and an addition reaction which produces crosslinking through the double bonds of the maleimide group. In general, curing at a lower temperature or increasing the MDA content served to favor chain extension over crosslinking, which might be expected to increase molecular mobility in the resin.     

Polyaniline/polyurethane networks. II. A spectroscopic study

The interconnection of polyaniline (Pani) chains through polyurethane (PU) blocks of the same length but with different spacing has originated a series of new semi conducting networks of varying crosslinking density, with good mechanical properties. FTIR, UV-Vis-NIR, XPS and EPR were used to probe the molecular features of the conducting species, and to test the morphological model proposed in a previous publication, where a continuous Pani phase percolates a PU matrix, linked together by a mixed interphase. Blends with the same composition of the networks and a model structure of crosslinked Pani were also prepared for comparison purposes. The proposed morphological model was confirmed by the new findings obtained by the spectroscopic techniques. The differences between the blends and networks of the same composition were discussed in terms of the morphology presented by each of these systems.     

Smooth Muscle Cell Responses to Poly(ε-Caprolactone) Triacrylate Networks with Different Crosslinking Time

Poly(ε-caprolactone) triacrylate (PCLTA) is attractive in tissue engineering because of its good biocompatibility and processability. The crosslinking time strongly influences PCLTAs cellular behaviors. To investigate these influences, PCLTAs with different molecular weights were crosslinked under UV light for times ranging from 1 to 20 min. The crosslinking efficiency of PCLTA increased with decreasing the molecular weight and increasing crosslinking time which could increase the gel fraction and network stiffness and decrease the swelling ratio. Then, the PCLTA networks crosslinked for different time were used as substrates for culturing rat aortic smooth muscle cells (SMCs). SMC attachment and proliferation all increased when the PCLTA molecular weight increased from 8k to 10k and then to 20k at the same crosslinking time. For the same PCLTA, SMC attachment, proliferation, and focal adhesions increased with increasing the crosslinking time, in particular, between the substrates crosslinked for less than 3 min and longer than 5 min. This work will provide a good experimental basis for the application of PCLTA.     

Diffusion‐controlled atom transfer radical polymerization with crosslinking

The reaction behavior of atom transfer radical polymerization (ATRP) with crosslinking was studied using di(meth)acrylates as the model network‐forming system. The kinetics was followed by differential scanning calorimetry (DSC). The structural evolution of the networks was investigated by gel permeation chromatography (GPC) and extraction/swelling measurements. The linear primary chains were obtained by hydrolysis of the networks and analyzed by viscometry. It was found that the polymerization proceeded in a controlled/living manner in an early stage of the reaction with the molecular weight of primary chains increasing linearly with vinyl conversion. However, deviations from the linearity occurred at high monomer conversions due to the network structure that severely restricted mobilities of catalyst/ligand complexes in the radical activation/deactivation. The onset and degree of deviation depended on crosslinking density. The diffusion‐controlled radical deactivation caused a mild autoacceleration in the polymerization rate. Adding excess Cu(II) to the system reduced the polymerization rate but improved the linear growth of primary chains that would facilitate the synthesis of networks of controlled microstructure. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers