Grafting vinyl monomers onto silk (Bombyx mori) using different initiators: Properties of grafted silk

The graft–copolymerization of silk with methacrylamide (MAA), 2‐hydroxyethyl methacrylate (HEMA), and methyl methacrylate (MMA) was studied using three different free‐radical initiators: an inorganic peroxide [ammonium persulfate (APS)] and two azo compounds [2,2′‐azobisisobutyronitrile (AIBN) and 2,2′‐azobis(2‐methylpropionamidine) dihydrochloride (ADC)]. The rate and yield of grafting followed the order: APS > AIBN > ADC. The performance of AIBN was close to that of APS in terms of weight gain attained. The degree of yellowing of grafted silk varied as a function of the initiator–monomer system used. APS induced the highest degree of yellowing, regardless of which monomer was used, whereas silk grafted with the MAA/AIBN system displayed the lowest level of yellowing. Moisture regain of grafted silk changed as a function of the hydrophilic/hydrophobic character of the grafted polymer, regardless of the kind of initiator used. Accordingly, tensile properties showed a tendency to decrease with increasing weight gain of grafted silk, the extent of which was independent of the kind of monomer and initiator used. The different initiators did not induce any appreciable change in the fine structure of silk, as demonstrated by optical measurements. Uneven surface deposition of homopolymer was detected to variable extent with MMA and HEMA grafting, whereas the surface of MAA‐grafted silk was completely free of any foreign deposit, independently of the initiator used. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1401–1409, 2001     

Structure and properties of tussah silk fibers graft-copolymerized with methacrylamide and 2-hydroxyethyl methacrylate

Tussah silk fibers were graft-copolymerized with methacrylamide (MAA) and 2-hydroxyethyl methacrylate (HEMA) in aqueous media, using a chemical redox system as an initiator. High weight gain values were obtained with both grafting agents (up to 175%). The extent of homopolymerization was negligible for the MAA grafting system over the entire range of monomer–silk ratios examined, while polymer deposition on the fiber surface occurred when the HEMA–silk ratio exceeded 0.5% (w/w). The moisture content of poly(MAA)-grafted silk fibers was enhanced by grafting. Breaking load, elongation at break, and energy decreased at low weight gain (0–20%) and then remained rather constant. The DSC curves of poly(MAA)-grafted silk showed a new endotherm at about 280°C, due to the melting of poly(MAA) chains. The loss modulus peak of poly(HEMA)-grafted silk fibers broadened and shifted to a lower temperature, showing a tendency to split into two peaks at high weight gain. On the other hand, grafting with poly(MAA) induced a noticeable upward shift of the loss peak. The TMA curves showed that grafting with poly(MAA) resulted in a higher extent of fiber contraction from room temperature to about 250°C. Moreover, the intensity of the final contraction step at about 350°C decreased with increasing weight gain and shifted to a lower temperature. The Raman spectra of grafted fibers were characterized by overlapping of the characteristic lines of both silk fibroin and polymer, the latter showing an intensity proportional to the amount of weight gain. Among the conformationally sensitive vibrational modes of tussah silk fibroin, the amide III range was significantly modified by grafting with both poly(MAA) and poly(HEMA). © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1393–1403, 1998     

Structure and properties of bombyx mori silk fibers grafted with methacrylamide (MAA) and 2-hydroxyethyl methacrylate (HEMA)

Silk fibers were graft-copolymerized with methacrylamide (MAA) and 2-hydroxyethyl methacrylate (HEMA) in aqueous media, using a chemical redox system as an initiator. High weight gains (300%) were obtained with both monomers, the weight gain being linearly related to the amount of monomer contained in the reaction system. The reaction efficiency attained 95–100%. The extent of homopolymerization was negligible for the MAA grafting system, while large amounts of poly-HEMA covered the surface of silk fibers beyond 70% weight gain. The fiber size increased linearly with the weight gain. The moisture content of MAA-grafted silk fibers was highly enhanced by grafting. The severe grafting conditions caused a partial degradation of the tensile properties of silk fibers, as well as of the degree of fiber whiteness. Following grafting, the breaking load slightly increased, while elongation at break and energy decreased. The decomposition temperature of grafted silk fibers shifted upwards. The Raman spectra of untreated silk fibers showed strong lines at 1667 (amide I), 1451, 1227 (amide III), 1172 and 1083 cm⁻¹. Overlapping of the lines characteristic of both silk fibroin and grafted polymer was observed in the spectra of grafted silk samples. The vibrational mode of the amide III lines of silk fibroin was significantly modified by grafting. © 1996 John Wiley & Sons, Inc.     

Structural characteristics of 2-hydroxyethylmethacrylate (HEMA)/methacrylamide (MAA)-grafted silk fibers

A series of 2-hydroxyethylmethacrylate (HEMA)/methacrylamide (MAA)-grafted silk fibers obtained in various comonomer compositions was prepared and their structural characteristics were studied by X-ray diffractometry, differential scanning calorimetry, and scanning electron microscopy. HEMA/MAA-grafted silk fibers with a graft yield of about 60% obtained in a HEMA/MAA mixture system containing 20% of HEMA and 80% of MAA on a weight basis showed endothermic peaks at 280 and 420°C (shoulder form), which are attributed to the thermal decomposition of the MAA and HEMA polymers, respectively, in addition to the thermal decomposition peak of the silk fibroin fiber which appeared at 323°C. These DSC results suggest that the HEMA/MAA-grafted silk fiber showed a low compatibility in the relation between the silk fibroin molecules and HEMA and/or MAA polymers. The crystalline structure of the HEMA/MAA-grafted silk fiber remained unchanged regardless of the HEMA/MAA grafting ratio even when the graft yield value reached 120%.     

Weighting and improvement of silk properties

Silk was modified with the aim to increase the weight and improve the physical properties. A grafting chemical redox technique was adopted in this investigation by using ammonium persulfate (APS), copper sulfate as initiator and accelelerator for the reaction, glycidyl methacrylate (GM), and acrylamide (AM), or their mixture. The optimum ASP and copper sulfate concentration needed to decrease the homopolymerization reaction was noted in the case of GM and AM. The percentage graft yield was found to be highly affected by the various parameters studied, including the initiator, the monomer concentrations, the reaction time, and the temperature. The physical properties of the grafted silk fabric were done including crease recovery angles, abrasion resistance, and moisture regain. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1743–1747, 2004     

The preparation of poly[N(n-butoxymethyl) methacrylamide] grafted silk fibers by polymerization using a low pH system

The graft polymerizations of the N(n-butoxymethyl) methacrylamide (BMA) monomer onto silk fibers were effected after reducing the pH of the grafting system to 2.5 by the addition of a formic acid solution. We compared the grafting efficiencies, surface characteristics, and thermal behaviors, as well as the whiteness levels and the extent of reduction of the rate of yellowing following UV irradiation, with the equivalent features of poly(BMA)-grafted silk fibers, prepared under normal (pH 6) conditions. The grafting efficiency [as poly(BMA) weight gain] onto silk fiber that was attained was almost 100% through optimization of the pH environment in the polymerization system by the addition of formic acid. The stiffness of the silk fabrics observed, following the conventional grafting, was markedly higher than that of equivalent silks after the polymerization at pH 2.5. The rates of yellowness index increase, for these latter silk fabrics following UV irradiation were also reduced, specifically in the initial irradiation period (up to 60 h). The SEM of the grafted silk fabrics reveal the absence of granules on the surface of the grafted silk fiber, when the silk was grafted with poly(BMA), after reducing the pH of the grafting system to 2.5. These findings suggest that the BMA monomer was polymerized specifically within the silk fiber and not on the surface. It is suggested that the increase in the polymer weight gain, and the reduced adverse effect on the fabric handle, arise because of the modified polymer location. © 1993 John Wiley & Sons, Inc.     

Radiation grafting/crosslinking of silk using electron‐beam irradiation

The radiation grafting of silk with methacrylamide (MAA) was studied using an electron‐beam (EB) irradiation technique. Two irradiation processes, preirradiation and coirradiation, were compared, and some factors affecting the degree of grafting were investigated. The radiation crosslinking of silk with dimethyloldihydroxyethylene urea (DMDHEU) was preliminarily studied. The physical and mechanical properties such as whiteness, breaking strength, and resilience of the radiation‐grafted/crosslinked silk fabrics were examined. The radiation grafting of silk with MAA increases the silk weight, while the radiation crosslinking of silk with DMDHEU imparts improved crease resistance to silk. X‐ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) analysis indicate the formation of peroxy and free‐radical species on the EB‐irradiated silk. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2028–2034, 2004     

Grafting of vinyl monomers onto silk fibers: Trivalent-manganese-initiated graft copolymerization of acrylamide onto silk fibers

Graft copolymerization of acrylamide (AM) onto silk fibers, using Mn(III)–sulphate as initiator, has been investigated, in aqueous sulphuric acid in the temperature range of 30–55°C. Grafting reaction has been studied by varying the concentration of monomer, Mn(III), sulphuric acid, temperature, and also with the modified silk. The graft yield increases significantly with increase of monomer concentrations to the extent of 0.85M, after which the rate falls. With increase in Mn(III) concentration and H⁺ ion concentration the graft yield increases, but after an optimum concentration a depression in the graft yield is noticed. The rate of the reaction is temperature-dependent; with increase of temperature the graft-on increases. Among the solvent composition studied a solvent/water mixture containing 10% of the solvent seems to constitute the most favorable medium for grafting, and a further increase of solvent composition decreases the graft yield. The effect of various additives such as transition metal salts, aromatic and heterocyclic amines on grafting reaction has been studied. A suitable mechanism for grafting has been proposed. Finally physical characterization such as thermal analysis (TGA) of the grafted samples has been carried out in order to ensure grafting and to study the change in the properties of the fibers.     

Grafting vinyl monomers onto silk fibers. X. Graft copolymerization of methyl methacrylate onto silk using hydrogen peroxide–thiourea redox system

The graft copolymerization of methyl methacrylate onto silk fibers initiated by a hydrogen peroxide–thiourea redox system was investigated under various conditions. The effects of monomer, initiator, temperature, acidity of the medium, and solvent on the rate of grafting were studied. The graft yield increases with the increase of monomer and initiator concentration. The graft yield also increases with the increase of acid concentration upto 22.50 × 10^?2M and thereafter it decreases. The effect of some inorganic salts on the rate of grafting has also been investigated, and a suitable mechanism has been suggested.     

Structural analysis of methacrylamide-grafted silk fibers

The structural changes and the thermal behavior of silk fibers grafted with methacrylamide (MAA) were investigated as a function of the weight gain. The refractive index parallel to the fiber axis decreased with increasing weight gain, whereas that perpendicular remained almost unchanged. Accordingly, birefringence decreased with a steeper slope in the weight gain range 0–80%, suggesting a lower degree of average molecular orientation. Only small changes in the isotropic refractive index were detected, suggesting that the crystallinity of the fibers remained essentially unaffected by MAA grafting, as confirmed by the X-ray diffraction data. The molecular orientation in the crystalline regions remained unchanged in the weight gain range 0–60%, then sharply decreased. The strength and the initial tensile resistance of grafted silk fibers decreased both in the dry and wet states, while elongation at break increased in the dry state and remained almost constant in the wet state. The results of the thermal behavior, investigated by differential scanning calorimetry, thermomechanical and thermogravimetric analysis, and dynamic mechanical measurements, were consistent with an increased thermal stability conferred on silk fibers by MAA grafting. The cross-sectional area of MAA-grafted silk fibers increased. Moreover, ion-etched cross sections of the grafted silk fiber showed the presence of fibrils with a diameter larger than that of the untreated control. © 1993 John Wiley & Sons, Inc.     

Physical properties of 2-hydroxyethyl methacrylate-grafted silk fibers

This paper deals with the physical properties of silk fibers grafted with 2-hydroxyethyl methacrylate (HEMA). Both tensile strength and elongation measured in the dry and wet states gradually decreased with increasing weight gain. The initial modulus of the grafted silk fibers in the dry state sharply increased in the weight gain range of 0–16%, then decreased to a lower value than the reference untreated sample. The refractive indices parallel and perpendicular to the fiber axis decreased, though the former showed a steeper slope. Accordingly, birefringence and isotropic refractive index also decreased, suggesting a lower degree of crystallinity and molecular orientation of grafted silk fibers. DSC, TMA, and TGA curves of the HEMA-grafted silk fibers indicated an increased higher thermal stability of silk fibers due to the HEMA grafting. The dynamic mechanical measurements showed that the thermally induced molecular movement of both amorphous and crystalline domains of silk fibers was enhanced by HEMA grafting. X-ray diffraction curves, however, implied that the crystalline structure of the silk fibroin remained unchanged regardless of HEMA polymerization. The introduction of HEMA polymer in silk fibers was evidenced by the infrared spectra, exhibiting the absorption bands characteristic of either the grafted HEMA polymer and the fibroin molecules with ordered β structure. © 1993 John Wiley & Sons, Inc.     

Graft-copolymerization of methylacrylic acid onto hydroxypropyl chitosan

Summary In order to study the graft modification of chitosan derivatives, hydroxypropyl chitosan (HPCTS) was prepared and characterized by FTIR, ¹H NMR, and elemental analysis Methylacrylic acid (MAA) was grafted onto HPCTS in an aqueous solution using ammonium persuffate (APS) as an initiator. Evidence of grafting was obtained by comparison of FTIR spectra of HPCTS and the grafted copolymer as well as solubility characteristics of the products. Variations of grafting percentage and grafting efficiency with reaction time, temperature, concentraton of initiator and monomer had been investigated. Received: 25 Febuary 2002/Revised version: 7 July 2002/Accepted: 12 July 2002     

Physical properties of silk fibers grafted with vinyltrimethoxysilane

Silk fibers from Bombyx mori silkworms were grafted using a novel grafting monomer, vinyltrimethoxysilane (VTMSi), with various grafting initiators. The effects of these grafting initiators were evaluated. It was possible to successfully copolymerize VTMSi within the silk fiber matrix without disturbing the fine structure of the fiber matrix, which was shown by FTIR analysis and refractive index measurements. The physical properties of VTMSi grafted silk were analyzed and compared to fibers grafted with conventional monomers such as methyl methacrylate, methacrylamide, and 2‐hydroxyethyl methacrylate. No trend in the tensile strength and elongation at break was observed when grafting silk fibers with VTMSi. Crease recovery in the wet state improved significantly, suggesting that this new grafting technique is important for the production of washable silk fabrics. The thermal stability of VTMSi grafted silk fibers was improved as shown by the shift of the endothermic peak for the thermal decomposition toward higher temperatures. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1764–1770, 2001     

Silk grafting with methacrylamide: A near‐infrared spectroscopy study

The grafting of methacrylamide (MAA) onto silk has taken importance not only as weighting process but also as finishing operation to improve the properties of the silk fibers. It was confirmed that the MAA monomer concentration, the initiator amount, and the liquor ratio are the major parameters affecting the yield of the reaction. Varying these parameters, a large number of samples, i.e., silk yarns with different grafting yields and exhaust grafting baths with different residual MAA concentrations, were obtained. Near infrared (NIR) spectroscopy was used for the quantitative determination of MAA in silk yarns and MAA in exhaust baths after grafting. A correlation between silk yarns grafting yield and NIR spectra of MAA grafted yarns was obtained. This correlation shows a coefficient of determination (R²) of 0.96 and a standard error of calibration (SEC) of 8.37 w/w for a mean value (M) of 31.82% w/w. A second correlation between MAA in exhaust grafting baths and NIR spectra was obtained. This correlation shows a R² of 0.95 and a SEC of 2.93 g/L for a M of 17.16 g/L. Previous correlations were improved using silk samples cleaned by extraction of sizing agents and detergents which could interfere with MAA grafting and modify absorption spectra of bath solutions. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

羧甲基纤维素与甲基丙烯酸接枝共聚的研究

以硝酸铈铵－ 乙二胺四乙酸为氧化还原引发剂, 在Ｎ２ 保护下, 羧甲基纤维素钠与甲基丙烯酸进行接枝共聚反应。研究了羧甲基纤维素接枝率、甲基丙烯酸转化率与引发剂浓度、单体浓度、反应时间、反应温度之间的关系。     

Synthesis and characterization of poly(HEMA‐MAA) hydrogel carrier for oral delivery of insulin

Poly(HEMA‐MAA) hydrogel particles were synthesized by redox free‐radical polymerization using 2‐hydroxyethylmethacrylate, different concentration of methacrylic acid as monomer, ethyleneglycol dimethacrylate as crosslinking agent, and APS/TEMED as free‐radical initiator. Fourier transform infrared spectrum of poly(HEMA‐MAA) hydrogels showed intense absorption peak of carbonyl group at ～ 1700 cm^?1 due to carboxylic acid groups of MAA, peak at ～ 2960 cm^?1 due to CH stretching and vinylic peak at 1700 cm^?1 independent of MAA concentration. Highest swelling percentage 587% was observed in case of poly(HEMA‐MAA) hydrogel synthesized using 30% of MAA while lowest swelling percentage 413% was observed in hydrogel synthesized 10% of MAA at basic pH (8.0). Scanning electron micrograph of copolymeric particles showed the irregular shape of poly(HEMA‐MAA) particles with conglomeration with each due to ionization of carboxylic groups. Insulin was radiolabeled using technetium‐99m radionuclide and the radiolabeling efficiency was found to be 99%. Poly(HEMA‐MAA) hydrogel having 60% of MAA showed the highest insulin loading efficiency of 68% while lowest 37% was observed in case of 10% MAA hydrogel. Insulin release studies showed only 35–65% of insulin was released into the medium from particles at pH 2.5 in 60 min, while insulin release was significantly higher at pH 7.4. Hypoglycemic effect of the 60 and 80 I.U./kg insulin dose loaded in poly(HEMA‐MAA) copolymeric particles were carried out in fasted diabetic rats and highest decrease in blood glucose level from 506 mg/dL to 170 mg/dL was observed within first 3 h. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Structure and Thermal Analyses of MAA‐Grafted Silk Fiber Using DSC and 13C Solid‐State NMR

Summary: By using DSC, ¹³C CP/MAS NMR and SEM, we studied the physical properties and chemical structure of silk fibers grafted with methacrylamide (MAA). At a given MAA concentration, the inverse of fiber weight gain linearly increased with increasing square root of the initiator concentration, and at a given initiator concentration the fiber weight gain increased with increasing MAA concentration. ¹³C CP/MAS NMR demonstrated that the primary and secondary structure remained unchanged, regardless of MAA grafting, implying the poor compatibility and the lack of new additional hydrogen bonding between the silk fiber and the MAA graft polymer. The degree of grafting in MAA‐grafted silk fiber (the accurate amount of actually loaded MAA polymer within the fiber matrix) can be evaluated from determination of the ratio of heat capacities calculated from two individual endothermic DSC peaks of silk fibroin and MAA polymer. The major endothermic peaks attributable to thermal degradation of the silk fiber and MAA graft polymer shifted to a higher temperature with increasing fiber weight gain by grafting. These findings are useful for the industrial production of grafted silk fiber with higher thermal stability.

CP/MAS spectra for poly(MAA) grafted silk and control silk fiber.     

Grafting vinyl monomers onto silk fibers,VII. Graft copolymerization of methyl methacrylate onto silk using peroxydiphosphate as initiator

Graft copolymerization of methyl methacrylate onto silk was investigated in aqueous solution using potassium peroxydiphosphate as initiator. The rate of grafting was determined by varying monomer, peroxydiphosphate ion, temperature, and solvent. The graft yield increased with increasing peroxydiphosphate ion upto 8 × 10^?3 mol/1 and with further increase of peroxydiphosphate ion the graft yield decreased. The graft yield increased with increasing monomer concentration upto 9 wt.-% and with further increase of monomer the graft yield decreased. The rate of grafting increased with the increase of temperature. The effect of acid and water soluble solvents and salts on graft yield was investigated and a suitable rate expression was derived.     

Grafting vinyl monomers onto silk fibers. XV. Graft copolymerization of methyl methacrylate onto silk using thallium (III) as initiator

The graft copolymerization of methyl methacrylate onto silk initiated by thallium (III) perchlorate was investigated in aqueous medium. The rate of grafting was evaluated varying the concentration of monomer, initiator, and acid, and the temperature. The graft yield was found to increase with increasing the monomer and initiator concentrations. The graft yield was found to decrease with increasing the acid concentration. The effect of inhibitors and various solvents on the graft yield was studied. From the Arrhenius plot the overall activation energy was found to be 4.2 kcal/mol. A suitable kinetic scheme has been proposed, and a rate equation has been derived.     

Homogeneous graft copolymerization of acrylonitrile onto high α‐cellulose in a dimethyl acetamide and lithium chloride solvent system

Homogeneous graft copolymerization of acrylonitrile (AN) monomer onto high α‐cellulose was investigated in a lithium chloride/N,N‐dimethyl acetamide (DMAc/LiCl) solvent system. Benzoyl peroxide (BPO) and azobisisobutyronitrile (AIBN) were used as radical initiators. By varying temperature, time, and monomer concentrations in grafting reactions, the optimum conditions for both initiator systems were fixed. The graft yield for the AN–BPO system was higher than that for the AN–AIBN system. The optimum conditions of reactions were at temperatures of 70 and 60°C with initiator concentrations of 0.4% (0.36 mmol) and 2% (1.24 mmol) for the AN–AIBN and AN–BPO systems, respectively, at a monomer concentration of 5% (14.1 mmol) solution. The number of grafts per cellulose chain was in the range from 2.2 to 1.1 for AN–BPO and 0.5 to 2.1 for the AN–AIBN system. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 630–637, 2003