Crosslinking index,molecular weight distribution and rubber equilibrium shear modulus during polyfunctional crosslinking of existing polymer: Part 3, primary polymer with a cumulative flory distribution or a cumulative Schulz-Flory distribution of the molecular weights

In two previous papers theoretical and experimental results of a statistical theory were presented concerning gels formed by the polyfunctional crosslinking of an already existing polydisperse primary polymer of known, but relatively simple molecular weight distribution. The present paper deals with the calculation of network parameters during the crosslinking process of a primary polymer with a cumulative Flory or a cumulative Schulz-Flory distribution of the molecular weights. Relationships between the equilibrium shear modulus, the average crosslinking indices (in the system as a whole and the network fraction as well), the sol fraction, the fraction of ideal network, the fraction of dangling ends and the functionality of the crosslinks are given and also the molecular weights between crosslinks. With these relationships it is possible to calculate from the measured equilibrium shear modulus the network parameters, provided the number and weight average molecular weights of the polymer and the crosslink functionality are known.     

Synthetic thermally reversible gel systems. VIII. Poly(vinyl chloride)–dioxane

When interpreted by network theory, equilibrium swelling measurements on poly-(vinyl chloride) (PVC) film in dioxane and moduli measurements on the equilibrium swelled films yield values for the approximate molecular weight between thermally reversible crosslinks and for the number of these crosslinks per polymer chain. These values are in reasonable agreement with the thermodynamic analysis of PVC–dioxane gels by Takahashi, Nakamura, and Kagawa and with the premise that three-dimensional network formation in these gels occurs by crystallization of a very limited number of syndiotactic sequences per chain having a sequence length of between 8 and 10. Failure to observe fusion endotherms by DTA on PVC–dibutyl phthalate gels supports the view that PVC gels have a low crystalline crosslink density and a low heat of crosslinking. The heat of crosslinking obtained by the method of Eldridge and Ferry shows only moderate agreement with expectations based on the heat of fusion of PVC and the number of repeating units per PVC chain passing through a crystalline crosslink in a PVC–dioxane gel.     

Crosslinking index,molecular weight distribution and rubber equilibrium shear modulus during polyfunctional crosslinking of existing polymer: Part 4, recapitulation and presentation of general results of calculations for a hypothetical crosslinking process

In previous papers a statistical theory was presented concerning network formation by polyfunctional crosslinking of existing polydisperse (non-uniform) primary polymers. Relationships were derived between network parameters and the equilibrium shear modulus during crosslinking processes of polymers of various molecular weight distributions. In the present paper the various relationships obtained are compared. Moreover, results of calculations for a hypothetical crosslinking process are presented, such as the weight fractions of sol, ideal network and free or dangling ends and the molecular weights between crosslinks as functions of the equilibrium shear modulus for various molecular weight distributions. Furthermore, the results of fractionation of the primary polymer, as a consequence of the crosslinking process, are shown and also the crosslinking indexes as functions of the sol fraction.     

Microrheology of chemically crosslinked polymer gels by diffusing-wave spectroscopy

The Brownian motion of probe particles in aqueous solutions of poly(vinyl alcohol)(PVA) and in chemically crosslinked PVA gels has been studied by diffusing-wave spectroscopy (DWS). At long time scales the measurements allow us to determine the effect of the crosslinking ratio on the macroscopic viscosity of sols and the shear modulus of gels. The local shear modulus of gels as obtained from the characteristic length of the Brownian cage was found to agree with that measured by classical rheometry and dynamic light scattering (DLS). These microrheological techniques were applied to two polymer gel systems. Substrate induced gradient structure of hydrogels was studied from a microrheological point of view using DLS. It is clearly seen that hydrophobic substrate induces weakly crosslinked network formation at the interface region up to a few millimeters as expected from other experimental facts. Magnetic particle motion in gels under external magnetic field was investigated by DWS. The translational motion of the magnetic particles in gels due to the alternating magnetic force can be detected and found to be superimposed on the relaxation due to the thermal motion.     

Rheological behaviour of irradiated wound dressing poly(vinyl pyrrolidone) hydrogels

The use of hydrogels as biomaterials has increased lately. Poly(vinyl pyrrolidone) (PVP) is an example of polymer hydrogels applied for the synthesis of hydrogel to be used in different biomedical applications. This paper describes a study on rheological properties of PVP hydrogels obtained by gamma radiation techniques. PVP hydrogels were obtained by gamma radiation of PVP water solutions with different radiation doses. It was studied the influence of additives such as poly(ethylene glycol) (PEG), poly(ethylene oxide) (PEO) and glycerol on the rheological behaviour of the gel. The rheological behaviour of hydrogel samples was characterized by measuring the shear storage modulus (G′) under dynamic shear loading. Besides this, sterility and cytotoxicity tests were performed. The study on rheological behaviour of hydrogels showed that G′ of PVP gels change according to the additive used. Glycerol increases the fluidity of the gel. The influence of PEG depends on the amount and on its molecular mass. The increase on PEG amount and molecular mass cause a decrease of G′ and an increase in the crosslinking density of PVP hydrogel network. The use of high molecular weight PEO allows the increase of the elasticity of the PVP gels.     

Hyperbranched polymers with maleic functional groups as radical crosslinkers

The crosslinking performance of the unsaturated hyperbranched polyester poly(allyloxy maleic acid‐co‐maleic anhydride) (MAHP) was investigated with copolymerizations of three different monomers: styrene, vinyl acetate, and methyl methacrylate. Both styrene and vinyl acetate afforded interpenetrating‐polymer‐network copolymer gels. The gels exhibited crosslink density gradients through the polymer matrices on a macroscopic level, and density maximums were concentrated around the MAHP moieties. The heterogeneity of the gels is briefly discussed in terms of a modified two‐phase model, where one phase consists of an elastic part of low crosslinking density and the other phase consists of an inelastic dendritic part with a highly condensed bond density. Unlike the two‐phase model developed by Choquet and Rietsch, the modified two‐phase model takes into account that both phases swell in good solvents. Unlike copolymerizations employing styrene or vinyl acetate, the copolymerization of MAHP with methyl methacrylate afforded noncrosslinked starbranched copolymers that consisted of a MAHP core from which long poly(methyl methacrylate) branches were protruding. The different behaviors of the copolymerizations of the three monomers used in this study can rationally be explained by their different reactivity ratios with maleic end groups of MAHP. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 964–972, 2001     

A thermodynamic study of the crosslinking of methyl silicone rubber

The crosslinking reaction in a two component methyl silicone rubber has been studied by thermomechanical analysis (TMA) and differential scanning calorimetry (DSC). The rubber was formed from two methyl silicone prepolymers; one containing reactive hydrogens every 50 to 100 groups and the other polymer containing pendant vinyl groups at the same frequency. In the presence of a platinum catalyst above 60°C crosslinking proceeds without a loss or gain in weight. The heat of reaction, energy of activation (calculated by two methods) crosslink density and elastic modulus (Young's) were studied as a function of prepolymer concentration, dilution and swelling. A preliminary value for the heat of reaction per mole of SiH and SiCHCH₂ has been calculated. From crosslink density measurements both by hexane swelling and TMA and DSC heats of reaction a qualitative picture has been obtained of the role of entangled chains in producing effective crosslinks.     

Densely crosslinked polycarbosiloxanes. II: Thermal and mechanical properties

The thermal and mechanical properties of two densely crosslinked polycarbosiloxane systems were investigated in relation to the molecular structure. The networks were prepared from functional branched prepolymers and crosslinked via a hydrosilylation curing reaction. The prepolymers having only vinyl functionalities (poly[phenylmethylvinyl]siloxanes) were crosslinked by using crosslinking agents with reactive silicon–hydrogen groups. In prepolymers having both silicon–vinyl and silicon–hydrogen groups (poly[phenylmethylvinylhydro)]siloxanes crosslinking took place intermolecularly. The thermal and mechanical properties of the polymer networks were found to be dependent on the phenyl  Si O_3/2 (branches) content in the prepolymer, the number of elastically effective crosslinks, the elastically effective network chain density and molecular weight between crosslinks, length of the chain segments introduced by the hydrosilylation crosslinking reaction, and the number of dangling ends. As a consequence of the dense crosslinking, the mechanical properties were also strongly dependent on the glass transition temperature. A tough–brittle transition was observed around the glass transition temperature of the polymer networks. The properties of the poly(phenylmethylvinylhydro)siloxane networks were found to be superior to those of the poly(phenylmethylvinyl)siloxane networks. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1311–1331, 1997     

The modulus of randomly-crosslinked poly(dimethylsiloxane) networks

In several published studies, randomly crosslinked networks were prepared from poly(dimethylziloxane) by the selective crosslinking of vinyl side chains with a silicon-hydride crosslinking agent. Stress-strain measurements on these elastomers gave values of the elongation modulus in the limits of small and large deformations which exceeded those predicted by the Flory-Erman theory. Although these unexpectedly large values at the small-strain limit have frequently been attributed to contributions from trapped entanglements, the present analysis interprets them as simply arising from contributions from short chains inadvertently introduced from the silicon-hydride crosslinking agent. In this interpretation there is a bimodal distribution of network chain lengths and, possibly, of crosslink functionalities as well. The present analysis gives results in good agreement with experiment.     

Long branching in poly(vinyl acetate) and poly(vinyl alcohol). I. Crosslinking and crosslinking cleavage of poly(vinyl acetate)

A crosslinked network was formed by the reaction of partially saponified poly(vinyl acetate) and toluylene diisocyanate in benzene. The yield of gel was markedly dependent on the degree of saponification and the concentrations of polymer and diisocyanate. Crosslinked poly(vinyl alcohol) was obtained by treating the poly(vinyl acetate) with a catalytic amount of sodium hydroxide in methanol without any change of the urethane crosslinks. The crosslink based on the urethane linkage was quantitatively cleaved by acids, especially by hydrobromic acid, releasing polymers of the same molecular weight as the original.     

Anisotropy in Longitudinal Modulus of Polymer Gels Containing Ferrite

The effects of magnetization direction on the longitudinal modulus of magnetic gels, which consist of magnetized barium ferrite and poly(vinyl alcohol), have been investigated using 10 MHz ultrasonic waves. The modulus change due to magnetization depends on both the crosslinking density and magnetization direction. The modulus change increased and decreased when the strain direction was perpendicular and parallel to magnetization, respectively.     

聚氨酯的后交联研究进展

通过交联可以将线型聚合物连接成三维网状结构,适度交联后聚氨酯的拉伸强度、模量、耐溶剂性、耐高温性等许多性能都能得到大幅提高,而交联方法、交联的种类及交联密度的大小都是重要的影响因素,因此对交联方法、交联密度的测定、交联密度与聚氨酯性能之间的关系研究对于聚氨酯的质量控制非常重要。本文首先详细介绍了聚氨酯后交联的常用方法,然后对聚氨酯交联密度的测定方法进行了分析比较,在此基础上进一步介绍了国内外关于聚氨酯交联密度与其性能的关系方面的研究进展。     

低温水凝胶力学性能的研究

本文采用单向压缩和单向拉伸法测定了聚乙烯醇(PVA)低温水凝胶的力学性能。结果表明,其机械强度随凝胶浓度和冷冻时间的增加而增强。得到弹性模量与凝胶浓度的关系符合标度律。说明采用低温处理方法可使PVA水溶液凝胶化,制得含水量高、具有较高强度、类似于橡胶的弹性材料,认为其力学强度来自于物理交联网络的贡献。     

聚醋酸乙烯酯/聚丙烯酸甲酯互穿网络聚合物的研究——Ⅱ.强迫互容效应及形态结构

用分步法合成了交联密度不同的聚醋酸乙烯酯/聚丙烯酸甲酯互穿网络聚合物(PVAc/PMA-IPN),用DMS,TEM和SAXS等方法进行表征。发现随交联密度的增加,体系的T_g,转变由两个逐渐变为一个,清楚显示出强迫互容的贡献。模量实验值与Davies方程计算值的比较表明,所研究的IPN是一个双相连续体系,相区尺寸约为100。     

Transport of hydrogen and carbon monoxide in highly crosslinked poly(propylene glycol) networks

Previous interpretations of gas transport data in crosslinked networks have been hindered by an inability to accurately control and evaluate the network parameters. We have recently prepared a series of model networks by reacting poly(propylene glycol) with a triisocyanate crosslinking agent. The poly(propylene glycol)s had narrow molecular weight distributions and average molecular weights between 425 and 3000, so the resulting networks had uniform average molecular weights between crosslinks. Hydrogen and carbon monoxide permeabilities in membranes formed from these networks increase with decreasing crosslink density. These results indicate increased cooperative molecular motions in the networks with longer average chain lengths between crosslinks. Increasing the average molecular weight between crosslinks also reduces the discrimination between these two gases so that the separation factors decrease. For networks prepared from mixtures of poly(propylene glycol)s with different molecular weights the gas permeabilities (but not the separation factors) depend on the molecular weight distribution.     

Thermoelasticity of networks in swelling equilibrium with single component diluents: Specific polymer-diluent interactions

Using thermoelastic measurements, specific polymer-diluent interactions have been demonstrated for atactic poly(vinyl alcohol) networks swollen in water. This was done by following a thermodynamic parameter as a function of network swelling, which was controlled by varying the extent of network crosslinking. At low crosslinking (high swelling) the ratio of the energy component f_e of the force to the total force f was found to be perhaps a little less than ?0.38 for poly(vinyl alcohol) swollen in water to less than 0.36 volume fraction of polymer. The method of evaluation is new and should be applicable to other polymer single component diluent systems.     

One‐pot fabrication of robust interpenetrating hydrogels via orthogonal click reactions

Interpenetrating polymer network (IPN) hydrogels have been fabricated through a facile one‐pot approach from tetra/bifunctional telechelic macromonomers with epoxy, amine, azide, and alkyne groups by orthogonal double click reactions: epoxy‐amine reaction and copper‐catalyzed azide‐alkyne cycloaddition. Both the crosslinked networks are simultaneously constructed in water from the biocompatible poly (ethylene glycol)‐based macromonomers. The crosslinking density of each network was finely tuned by the macromonomer structure, permitting control of network molecular weights between crosslinks of the final gels. Compared to corresponding single network gels, the IPN gels containing both tightly and loosely crosslinked networks exhibited superior mechanical properties with shear moduli above 15 kPa and fracture stresses over 40 MPa. The synthetic versatility of this one‐pot approach will further establish design principles for the next generation of robust hydrogel materials. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1459–1467     

Formation of linear polymers in confined intranetwork space during synthesis of sequential semi-interpenetrating polymer networks

Sequential semi-interpenetrating polymer networks based on polyurethanes with different crosslink densities and linear polystyrene, poly(butyl methacrylate), and poly(methacrylic acid) have been studied. For the parent polyurethanes, the molecular mass of chain segments between crosslink junctions is estimated by the Flory-Rehner method. The kinetics of formation of linear polymers in semi-interpenetrating polymer networks is investigated as a function of the crosslink density of the polyurethane network. The molecular-mass distributions of linear polymers developed in semi-interpenetrating polymer networks are examined by size-exclusion chromatography. The relationship between the kinetics of formation of linear components under the conditions of confined space and their molecular mass is established.     

Structuring in poly(vinyl alcohol)-potassium iodide-iodine systems: Small-angle X-ray-scattering study

The sizes of supramolecular density fluctuations in gels and solid films obtained during formation of iodine inclusion compounds in an aqueous solution of poly(vinyl alcohol) are studied via small-angle X-ray scattering. It is shown that, in the case of iodine-containing poly(vinyl alcohol) samples prepared through drying of iodine-containing gels, several diffraction peaks that are related to the long-range order in the packing of structural fragments manifest themselves. The presence of the peaks indicates periodic density fluctuations in the packing of polymer chains with a period of 1.34 nm and a coherence length of 18.0 × 0.5 nm. The equidistance of peaks in units of scattering-vector modulus makes it possible to suggest the formation of layered systems, that is, systems with a single well-defined repeat period, during gel drying. Wet samples of the iodine-containing gel based on poly(vinyl alcohol) do not reveal any ordering. Heterogeneities in the structure of iodine-free gels may be regarded as voluminous structures with free cavities that survived after the extraction of iodine. These cavities in gels may be again filled with iodine or other complementary molecules.     

聚环氧氯丙烷聚氨酯／聚甲基丙烯酸甲酯IPN力学性能

利用改变组成比、聚氨酯ＰＵ软段的分子量、Ｒ值、异氰酸酯和两网络各自交联剂含量合成出５个系列的聚环氧丙烷聚氨酯／聚甲基丙烯酸甲酯互穿聚合物网络，利用ＩＰＮ中交联、互穿、缠结程度的不同，并结合ＤＣＳ、ＴＥＭ、动态粘弹谱讨论了ＩＰＮ力学性能。