Molecular motions and transitions in a poly(p-hydroxystyrene) derivative

The dielectric relaxation mechanisms in a poly(p-hydroxystyrene) derivative have been studied by Thermally Stimulated Depolarization Currents (TSDC) in the temperature range between −160 and 130°C. A broad relaxation was observed at low temperatures, from −160 up to 0°C, which obeyed to the zero entropy approximation. In contrast, the glass transition relaxation showed the usual behavior, a strong departure to the zero entropy prediction. Both relaxations have been studied in detail by the technique of thermal sampling. The thermal behavior of this poly(p-hydroxystyrene) derivative was also studied by Differential Scanning Calorimetry in the temperature range between 20 and 200°C. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1921–1926, 1999     

Thermal degradation of polymers. XV. Vacuum pyrolysis studies on poly(p-methoxystyrene) and poly(p-hydroxystyrene)

The monomers p-hydroxystyrene (p-HS) and p-methoxystyrene (p-MS) have been synthesized and polymerized using azobisisobutyronitrile as free-radical initiator. The polymerization behavior of p-HS is anomalous, and a mechanism is suggested to account for this phenomenon. Poly(p-hydroxystyrene) (PHS) and poly(p-methoxystyrene) (PMS) homopolymers have been subjected to vacuum pyrolysis at temperatures in the range of 300–500°C. The products of degradation have been identified and qualitatively and quantitatively analyzed and the degradation behavior of the two systems compared with polystyrene. PHS shows anomalous behavior resulting from the high reactivity of the p-HS monomer and the facility for transfer afforded by the proton of the hydroxyl substituent. Mechanisms to account for the degradation behavior of each system are discussed.     

Radiation crosslinking of poly(vinyl chloride)

The mechanism of the electron irradiation crosslinking of poly(vinyl chloride) (PVC) with polyfunctional acrylic monomers has been investigated as part of a program to develop an insulation for distributing-frame wire. These monomers were found to undergo rapid polymerization to form a rigid, three-dimensional network onto which PVC was grafted. Polyfunctionality was necessary for crosslinking to occur, since butyl methacrylate, containing only a single unsaturated bond, did not crosslink readily. On the other hand, trimethylolpropane triacrylate and trimethylolpropane trimethacrylate, each containing three unsaturated groups, gave extremely rapid crosslinking. Trimethylolpropane triacrylate showed 40 times the radiation sensitivity of tetraethylene glycol dimethacrylate, a diunsaturated compound. The rate of disappearance of unsaturation was related inversely to the rate of gel formation. This was attributed to immobilization of unreacted pendant double bonds in the rigid crosslinked network. In the triunsaturated monomers, half the vinyl groups were left unreacted when all of the PVC was insoluble.     

Intramolecular crosslinking of poly(vinyl alcohol)

Poly(vinyl alcohol) is crosslinked in dilute solution (c=0.1 wt%) with glutaraldehyde. The reaction product is characterized by viscometry and gel permeation chromatography (g.p.c.). The intrinsic viscosity decreases with increasing degree of crosslinking and does not depend on temperature. G.p.c. reveals that the reaction product is not homogeneous, but consists of a mixture of particles with different sizes, possibly both intra- and intermolecularly crosslinked molecules. The intramolecularly crosslinked molecules are smaller in size than the initial polymer molecules and their size depends on the degree of crosslinking. They possess a narrow particle size distribution even if the initial polymer sample had a broad molecular weight distribution.     

Epoxylsilane crosslinking of rigid poly(vinyl chloride)

Crosslinking is an effective way to improve the qualities of poly(vinyl chloride). A crosslinking system consisting of R‐glycidoxypropyltrimethoxysilane (KH560) has been first used to introduce crosslinking into rigid poly(vinyl chloride). Different thermal stabilizers (organotin, Ca/Zn stearate, and Ba/Zn stearate) as well as sodium bisulfite additive were tried to promote the grafting of epoxyl group and enhance the degree of crosslinking. FTIR spectra showed that grafting and crosslinking of KH560 with poly(vinyl chloride) could take place, and a gel content of 40% could be obtained when more than 10 phr epoxylsilanes were used in the condition of 2 : 1 (parts by weight) ratio of BaSt₂/ZnSt₂ and 1 : 1M ratio of NaHSO₃/KH560, while the premature crosslinking was avoided. Thermal properties had been studied. The results showed that the Vicat softening temperature of crosslinked PVC could be improved about 10°C when 5 phr epoxylsilane was added, and thermal degradation could be delayed with increase in gel content. Therefore, epoxylsilane‐crosslinked PVC will have the potential for extensive applications © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Peroxide crosslinking of unplasticized poly(vinyl chloride)

A crosslinking system consisting of 1,1‐di‐t‐butylperoxy‐3,3,5‐trimethyl cyclohexane peroxide and trimethylolpropane trimethacrylate (TMPTMA) has been used to introduce crosslinks into unplasticized poly(vinyl chloride) (PVC). The influence of the concentration of both reagents has been investigated, and crosslinking monitored by determination of the remaining sample weight after Soxhlet extraction with tetrahydrofuran. The system used (i.e., 0.5–2.0 phr peroxide with 5 to 15 phr TMPTMA) has been shown to be effective for crosslinking PVC. Gel contents of 30–40% have been obtained, premature crosslinking during processing is largely avoided, but thermal stability still needs to be improved. Considerable improvements in elevated temperature mechanical properties can be attained using an appropriate TMPTMA/peroxide concentration. The best tensile properties were obtained with 0.5 phr peroxide and 15 phr TMPTMA. Observed increases in T_g, also achievable with only 0.5 phr peroxide, but only slightly dependent on TMPTMA concentration, represent a useful increase in service temperature for the resulting compound. Lower peroxide levels may be adequate to achieve property improvements. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2657–2666, 2000     

Peroxide crosslinking of rigid poly(vinyl chloride)

Peroxide crosslinking of unplasticized poly(vinyl chloride) with trimethylolpropane trimethacrylate was investigated. Formulations used in this work contained a nontoxic lead‐free stabilizer and showed good color and heat stability. The samples were examined by differential scanning calorimetry, and their tensile properties were measured at room temperature and at 130°C. Gel content or tetrahydrofuran‐insoluble material was measured as an indication of crosslinking. It was shown that premature crosslinking could be avoided during processing and that 190°C was the optimum processing temperature for maximum gel content. The residual unsaturation was monitored by using FTIR spectroscopy. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers     

Peroxide crosslinking of rigid poly(vinyl chloride)

The optimum conditions for crosslinking rigid poly(vinyl chloride) with trimethylolpropane trimethacrylate (TMPTMA) and peroxide have been examined. The extent of crosslinking was measured by determining gel content by Soxhlet extraction in tetrahydrofuran. Mechanical properties were measured at 130°C and dynamic viscoelastic measurements were carried out to detect changes in the glass transition temperature (T_g). It was found that 15 phr of TMPTMA and 0.3 phr of peroxide were optimum concentrations for maximizing the extent of crosslinking, tensile strength, and T_g. The lower molding temperature of 170°C was preferred to minimize thermal degradation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2904–2909, 2007     

Ultraviolet-induced crosslinking of solid poly(ethylene oxide)

Poly(ethylene oxide) (PEO) of molecular weight from 200,000 to 2,000,000 was crosslinked by exposure to ultraviolet radiation with a high-pressure 150 W mercury lamp. Photochemical crosslinking of PEO proceeds in the presence of photoinitiators such as benzophenone and acetophenone which act as a hydrogen-abstracting agent. Gel fraction yield exceeds 90%, and the number-average molecular weight between crosslinks determined by equilibrium swelling in chloroform varies from 3,000 to 100,000. The degree of crosslinking can be controlled by changing the irradiation temperature. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2299–2307, 1997     

Thermal degradation of polymers. XVI. Thermal analysis studies on poly(p-methoxystyrene) and poly(p-hydroxystyrene) in air and nitrogen

Comparative thermal degradation studies have been made in air and nitrogen on polystyrene (PS), poly(p-methoxystyrene) (PMS), poly(p-hydroxystyrene) (PHS) and p-hydroxystyrene/styrene copolymers (p-HS/S) by thermal analytical methods (TG and DSC). The degradation behavior shown by these polymers in an oxidizing or inert atmosphere is different. PMS and PS, however, degrade in a similar manner in the same test atmosphere, suggesting that similar degradation mechanisms are operative for these systems. Polymers containing p-HS behave differently to PMS and PS in both atmospheres. Their behavior in air is discussed it terms of the antioxidant effect of the phenolic hydroxyl grouping and the related crosslinking reactions. In nitrogen, the anomalous behavior is discussed in terms of the observed carbonization reaction.     

Thermal crosslinking of poly(methyl methacrylate-co-N,N-dimethylaminopropylacrylamide)

Summary Copolymers of methyl methacrylate(MMA) with N,N-dimethylaminopropylacrylamide (DMA) were heated at various temperatures, and their thermal crosslinking was investigated. A copolymer with a compsition of [MMA]/[DMA] =3 became completely insoluble in chloroform when heated at 180°C for 5 minutes or at 150°C for 20 minutes. Other copolymers containing less than 5 mol % DNA also crosslinked when heated at 170°C for 30 minutes. A mixture of polyMMA with N,N-dimethylaminopropylbenzamide also crosslinked when heated at 170°C. A copolymer of styrene with DMA also crosslinked when heated at above 140°C although the crosslinking was much slower than that of poly(MMA-co-DMA). The formation of imide linkages is proposed as the crosslinking mechanism.     

Thermal crosslinking of collagen immobilized on poly(acrylic acid) grafted poly(ethylene terephthalate) films

Graft polymerization of acrylic acid onto plasma‐treated poly(ethylene terephthalate) (PET) films was used to prepare surfaces suitable for collagen immobilization by dip‐coating. Such surfaces could be used as matrices for smooth muscle cell cultures in tissue engineering. Contact angle measurements showed that plasma‐treated and grafted PET films undergo considerable surface reorganization during storage under ambient conditions. However, after collagen immobilization the contact angle remained relatively stable. The amount of collagen initially attached to the film surface increased with increasing poly(acrylic acid) graft density, but subsequent washing in water led to significant collagen loss. This loss could nevertheless be substantially reduced by thermal crosslinking of the collagen in the range 110–130 °C. Atomic force microscopy (AFM) observations suggested that the washed crosslinked collagen has a very similar structure to that of the un‐crosslinked collagen. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1874–1880, 2002     

可降解聚硅基酯的合成及与苯乙烯的交联反应

采用二元羧酸与二元氯硅烷缩聚制备聚硅基酯的方法,将富马酸二叔丁基酯与1,3-二氯-四甲基二硅氧烷反应合成了黏性液态的不饱和聚(富马酸-四甲基二硅氧烷硅基酯);以偶氮二异丁腈作为引发剂,将该聚硅基酯与苯乙烯进行交联反应,黏性液态的聚硅基酯交联后转变成固态。通过傅里叶变换红外光谱、差示扫描量热法和热重法对聚硅基酯及其与苯乙烯的交联产物进行了分析,结果表明交联后的产物比未交联聚硅基酯的玻璃化转变温度明显升高,热稳定性增强。     

Characterisation and crosslinking properties of a poly(ethylene-co-divinylsiloxane)

This characterisation and crosslinking study has been carried out on a new low density polyethylene, a poly(ethylene-co-divinylsiloxane), copolymerised via a free radical mechanism in a high pressure reactor. This copolymer enabled a study of the role of vinyl groups in the peroxide crosslinking mechanism without the interference of allylic hydrogens; such interference has been present in previous studies. The copolymers contain varying amounts of siloxane vinyl groups, which depend on the concentration of divinylsiloxane in the monomer mix added in the reactor. Peroxide crosslinking experiments show that the siloxane vinyl groups are rapidly consumed, demonstrating that allylic hydrogens are not needed for the reaction of vinyl groups. The effects of a high concentration of long chain branches are also discussed.     

The heterogeneous formalization of poly(vinyl alcohol) accompanying crosslinking reaction-II

Poly(vinyl alcohol) (PVA) films were allowed to react with formaldehyde in water containing salt and an acid catalyst (the heterogeneous formalization). The content of crosslinks increased with decreasing with decreasing acid concentration under fixed formaldehyde and salt concentrations. The crosslink contents of formalized PVA were estimated in terms of the degree of swelling of formalized and acetylated PVA in chloroform. The crosslink contents were calculated by means of FLORY's method. One crosslink per 100 units of vinyl alcohol was estimated to exist for the sample of the degree of swelling about 6.     

Gelation and crosslinking characteristics of photopolymerized poly(ethylene glycol) hydrogels

The gelation and crosslinking features of poly(ethylene glycol) (PEG) hydrogels were scrutinized through the UV polymerization processes of poly(ethylene glycol) methacrylate (PEGMA) and poly(ethylene glycol) dimethacrylate (PEGDMA) mixtures. The real‐time evolutions of the elastic moduli of the prepolymerized mixtures with different crosslinking ratios of PEGMA and PEGDMA and the photoinitiator concentrations were measured during photopolymerization. The rheological properties were compared with other properties of the PEG hydrogels, including the relative changes in the C?C amounts in the mixtures before and after UV irradiation, water swelling ratio, gel fraction, mesh size, and mechanical hardness. As the portion of PEGDMA as a crosslinker increased, the final elastic modulus and gel fraction increased, whereas the swelling ratio and scratch penetration depth at the hydrogel film surface decreased because of the formation of compact networks inside the hydrogels. These results indicate that there was a good correlation between the rheological analysis for predicting the crosslinking transition during photopolymerization and the macroscopic properties of the crosslinked hydrogels. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41939.     

Mode of crosslinking of degradable poly(vinylpyridine N-oxide) gels

The degradation mechanism of gels from poly(vinylpyridine N-oxide) was investigated by differential scanning calorimetry, C, H, N analysis, infra-red spectroscopy and degradability tests. It was found that poly(2-vinyl pyridine N-oxide) gels, regardless of the persulphate initiator used, were weaker than poly(4-vinyl pyridine N-oxide) gels. C, H, N analysis and infra-red spectroscopy indicated that the initiator becomes an integral part of the gels. A mechanism for the formation of the gels is proposed.     

Reaction and characterization of crosslinking hyperbranched poly (amine-ester) with succine anhydride

Basing on hydroxyl terminated hyperbranched poly (amine-ester)s (HPAEs), the cross-linking reactions and preparation of ester-crosslinked HPAE films were investigated using succine anhydride (SA) as crosslink reagent. It was proved that the cross-linking reaction between HPAE and SA followed a two-step mechanism. This mechanism provides an efficient route to prepare HPAE/SA cross-linked films, in which, the precursor films were prepared by casting HPAE/SA solution at a lower temperature, and then curing the films at a higher temperature. By varying SA content, the solid HPAE/SA films with different cross-linking degrees were prepared successfully. The highest tensile strength of the cross-linked film could reach 59.60 MPa. With all water contact angle smaller than 74.3°, the crosslinked films demonstrated good hydrophilic properties.