Study on biodegradable polymer materials based on poly(lactic acid). I. Chain extending of low molecular weight poly(lactic acid) with methylenediphenyl diisocyanate

Methylenediphenyl diisocyanate (MDI) was used as the chain extender for low molecular weight poly(lactic acid) (PLA) to produce high molecular weight biodegradable polymer material with a better heat resistance. PLA prepolymer with a number‐average molecular weight (M_n) of 5800 and a weight‐average molecular weight (M_w) of 9800 was produced by direct polycondensation using stannous octoate as the catalyst. After 40 min of chain extension at 175°C, the resulting polymer had a M_n of 15,000 and a M_w of 57,000. The glass transition temperature (T_g) of the low molecular weight PLA prepolymer was 48.6°C. After chain extension, the T_g of the resulting polymer was raised to 67.9°C, as determined by DSC. DMA results also indicate that the heat resistance was improved by the chain extension. The DSC spectrum and X‐ray diffraction pattern of annealed samples showed that both the crystallinity and rate of crystallization of PLA were lowered by chain‐extension reaction due to the formation of branched molecular structure. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2546–2551, 1999     

One‐pot synthesis of hyperbranched poly(aryl ether ketone)s for the modification of bismaleimide resins

Hyperbranched poly(aryl ether ketone)s with hydroxyl end groups (HBP‐OH) and high degree of branching value (83%) were synthesized via an A₂ + B₃ approach. The polymerization conditions (e.g., polymerization temperature and time, monomer concentration, stoichiometric ratio of functional groups) were explored to avoid the gelation. Allyl‐terminated hyperbranched PAEKs (HBP‐AL) with low molecular weight (M_n = 3.4 × 10³) and narrow polydispersity (PDI = 1.65) were obtained via the etherification of HBP‐OH and it has been used for the modification of bismaleimide (BMI) resins. The prepolymers showed good processibilities with a viscosity below 0.6 Pa s at 110°C, though the viscosities slightly increased as the increase of HBP‐AL contents. The cured BMI resins showed high glass transition temperatures (T_g > 320°C) and good thermal stabilities (T_d > 400°C, both in nitrogen and air). It is inspiring to note that the incorporation of HBP‐AL into BMI matrix results in a significant enhancement of toughness without any noticeable loss in modulus, processibility, and T_g. POLYM. ENG. SCI., 54:1675–1685, 2014. © 2013 Society of Plastics Engineers     

Lignin valorization by forming toughened thermally stimulated shape memory copolymeric elastomers: Evaluation of different fractionated industrial lignins

Lignin‐based thermal responsive dual shape memory copolymeric elastomers were prepared with a highly branched prepolymer (HBP, A₂B₃ type) via a simple one‐pot bulk polycondensation reaction. The effect of fractionated lignin type (with good miscibility in the HBP) on copolymer properties was investigated. The thermal and mechanical properties of the copolymers were characterized by DMA, DSC, and TGA. Tensile properties were dominated by HBP <45% lignin content while lignin dominated >45% content. The copolymers glass transition temperature (T_g) increased with lignin content and lignin type did not play a significant role. Thermally stimulated dual shape memory effects (SME) of the copolymers were quantified by cyclic thermomechanical tests. All copolymers had shape fixity rate >95% and >90% shape recovery for all compositions. The copolymer shape memory transition temperature (T_trans) increased with lignin content and T_trans was 20°C higher than T_g. Lignin, a renewable resource, can be used as a netpoint segment in polymer systems with SME behavior. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41389.     

Engineering plastics from lignin. XVII. Effect of molecular weight on polyurethane film properties

A series of five fractions with number average molecular weights (M?_n) between 1500 and 10,000 daltons were isolated from a Kraft hydroxypropyl lignin (HPL). From ¹H-NMR and UV analysis the chemical properties of the HPLs were found to vary slightly with molecular weight. The hydroxyl content decreased while the glass transition temperature (T_g) increased as the HPL molecular weight increased. The Fox-Flory equation adequately described the M?_n vs. T_g relationship. The HPL fractions were used as polyols for the preparation of solvent-cast polyurethane networks (PU) in film form. The T_g of the PUs increased from 40° to 120°C as the M?_n of the polyol rose from 1500 to 10,000 daltons. The molecular weight between crosslinks (M?_c) of the networks was determined by swelling. An observed decrease in M?_c with an increase in M?_n was related to the functionality of the system. The strength properties of films prepared from fractionated HPLs were superior to those prepared from nonfractionated HPLs.     

Synthesis and characterization of biodegradable lactic acid‐based polymers by chain extension

BACKGROUND: Poly(lactic acid) (PLA), coming from renewable resources, can be used to solve environmental problems. However, PLA has to have a relatively high molecular weight in order to have acceptable mechanical properties as required in many applications. Chain‐extension reaction is an effective method to raise the molecular weight of PLA. RESULTS: A high molecular weight biodegradable lactic acid polymer was successfully synthesized in two steps. First, the lactic acid monomer was oligomerized to low molecular weight hydroxyl‐terminated prepolymer; the molecular weight was then increased by chain extension using 1,6‐hexamethylene diisocyanate as the chain extender. The polymer was characterized using ¹H NMR analysis, gel permeation chromatography, differential scanning calorimetry and Fourier transform infrared spectroscopy. The results showed that the obtained polymer had a M_n of 27 500 g mol^?1 and a M_w of 116 900 g mol^?1 after 40 min of chain extension at 180 °C. The glass transition temperature (T_g) of the low molecular weight prepolymer was 47.8 °C. After chain extension, T_g increased to 53.2 °C. The mechanical and rheological properties of the obtained polymer were also investigated. CONCLUSION: The results suggest that high molecular weight PLA can be achieved by chain extension to meet conventional uses. Copyright © 2008 Society of Chemical Industry     

Effects of polycaprolactone molecular weights on thermal and mechanical properties of polybenzoxazine

Polymer blends of polybenzoxazine (PBA‐a) and polycaprolactone (PCL) of different molecular weights (M_n = 10,000, 45,000, and 80,000 Da) were prepared at various PBA‐a/PCL mass ratios and their properties were characterized. The results from dynamic mechanical analyzer (DMA) revealed two glass transition temperatures implying phase separation of the two polymers in the studied range of the PCL contents. Moreover, a synergistic behavior in glass transition temperature (T_g) was evidently observed in these blends with a maximum T_g value of 281°C compared with the T_g value of 169°C of the PBA‐a and about ?50°C of the PCL used. The blends with higher M_n of PCL tended to provide greater T_g value than those with lower M_n of PCL. The modulus and hardness values of PBA‐a were decreased while the elongation at break and area under the stress?strain curve were increased with an increase of the content and M_n of PCL, suggesting an enhancement of toughness of the PBA‐a. Scanning electron micrographs (SEM) of the sample fracture surface are also used to confirm the improvement in toughness of the blends. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41915.     

Enhanced film forming ability of benzoxazine–urethane hybrid polymer network by sequential cure method

Uniform copolymer films of benzoxazine resin (BA‐a) and urethane prepolymer (PU) were prepared at various BA‐a/PU mass ratios (100/0, 80/20, 60/40, 40/60, 20/80, and 0/100) via sequential cure method comprising of moisture cure and thermal cure steps. In the moisture cure step, Fourier Transform Infrared (FT‐IR) spectra revealed the network formation between NCO‐terminated group and moisture to firstly produce PU solid film. Then in the thermal cure step, the change of tri‐substituted benzene ring to tetra‐substituted benzene ring was observed suggesting polybenzoxazine network formation in this step. Moreover, the spectra reveal that isocyanate groups in polyurethane structure could react with phenolic hydroxyl groups of BA‐a to form biuret and allophanate groups. Dynamic mechanical analysis (DMA) confirms a synergistic behavior in glass transition temperature (T_g) of the alloys with the highest T_g value of 275°C which is uniquely observed in these alloys obtained from traditionally thermal cure method. The proposed sequential cure method above is found to be highly useful for uniform coating or film casting process which lacks in traditional, low A‐stage viscosity, benzoxazine resin. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40502.     

Lignin‐xylaric acid‐polyurethane‐based polymer network systems: Preparation and characterization

Polyurethanes (PU) were synthesized from lignin by first preparing a prepolymer from esterified sugar‐based trihydroxyl compound xylaric acid and a 20 mol % excess of methylene diphenyl diisocyanate. The prepolymer was crosslinked with 5, 10, and 15 wt % of an industrial soda lignin, and polyethylene glycol was used to bring soft segments into the material structure. The total amount of bio‐based starting materials was as high as 35%. Evidence for the reaction between the prepolymer and lignin was observed using Fourier transform infrared spectroscopic analysis and ¹³C nuclear magnetic resonance spectroscopy. The thermal properties of the materials were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The mechanical and viscoelastic properties of the materials were determined by dynamic mechanical analysis (DMA). The glass transition temperatures (T_g) obtained from DSC and DMA showed a trend of increasing T_g with the increasing amount of lignin. A similar trend was observed with TGA for the increasing thermal stability up to 550^oC with the increasing amount of lignin. The lignin‐polyurethanes obtained were stiff materials showing high Young's modulus values. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39714.     

Study of the epoxy system BADGE (n = 0)/1,2‐DCH/CaCO3 filler by DMA and DSC

The influence of an inorganic filler, CaCO₃, on the curing kinetics of an epoxy system composed of diglycidyl ether of bisphenol A [BADGE (n = 0)], 1,2‐diaminecyclohexane (1,2‐DCH), and CaCO₃ filler was studied by DMA and DSC. Different contents of filler in the range from 10 to 30%, referred to the total weight of the mixture, were tried. It was found that maximum reproducibility of the results and better performance correspond to a filler content of 20%. As usual, the T_g's obtained by DMA and DSC are different at 10–20°C. The results obtained from this study were compared with those corresponding to the system BADGE (n = 0)/1,2‐DCH without filler; the T_g for this last system is higher than that for the system with filler incorporated, whereas E′ and M_c (molecular weight between repetition units) are lower for the system without filler. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 366–370, 2002     

PMMA composites of single‐walled carbon nanotubes‐graft‐PMMA

To facilitate the dispersion of single‐walled carbon nanotubes (SWCNT) into poly(methyl methacrylate) (PMMA), SWCNT were functionalized with a RAFT chain transfer agent, and PMMA was grafted from the SWCNT by reversible addition–fragmentation transfer (RAFT) polymerization to give SWCNT‐g‐PMMA containing 6 wt % PMMA. SWCNT‐g‐PMMA in the form of small bundles was dispersed into PMMA matrices. The SWCNT‐g‐PMMA filler increased the glass transition temperature (T_g) of the composite when the matrix molecular weight M_n was less than the graft molecular weight, but not when the matrix M_n was equal to or greater than the graft M_n. The threshold of electrical conductivity of the composites as a function of weight percent SWCNT increased from 0.2% when matrix M_n was less than graft M_n to about 1% when matrix M_n was greater than graft M_n. Dynamic mechanical analyses of the composites having graft M_n less than or equal to matrix M_n showed broader rubbery plateaus with increased SWCNT content but no significant differences between samples with different grafted PMMAs. The results indicate that lower M_n matrix wets the SWCNT‐g‐PMMA whereas higher M_n matrix does not wet the SWCNT‐g‐PMMA. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39884.     

Lignin valorization by forming thermally stimulated shape memory copolymeric elastomers—Partially crystalline hyperbranched polymer as crosslinks

Lignin based thermal‐responsive elastomers were produced by a melt polycondensation reaction with a long alkyl chain hyperbranched poly(ester‐amine‐amide) (B₃‐A₂‐CB₃¹). The effect of lignin content on elastomers properties was investigated. The thermal and mechanical properties of the copolymers were characterized by DMA, DSC, and TGA. The morphology of the copolymer was examined by SEM. Tensile properties were dominated by HBP <25% lignin content while lignin dominated >25% content. The copolymers glass transition temperature (T_g) increased with lignin content. The elastomer with 30% lignin content demonstrated optimal mechanical properties (tensile strength 5.3 MPa, Young's modulus 8.9 MPa, strain at break 301%, and toughness 1.03 GPa). Thermally stimulated dual shape memory effects (SME) of the copolymers were quantified by cyclic thermomechanical tests. The transition temperature (T_trans) of the polymer was able to be controlled (room to body temperature) by varying the amount of lignin added which broadens the range to medical applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41103.     

Relations of weight-average molecular weights and the thermal properties of N-cyclohexyl maleimide–methyl methacrylate

N-cyclohexyl maleimide (CHMI) was used to copolymerize with methyl methacrylate (MMA) by a suspension copolymerization method to produce heat-resistant poly(MMA) (PMMA) in this article. The copolymers were synthesized by changing the weight fractions of azobisisobutyronitrile (AIBN) and dodecanethiol (DDM), while the weight ratio of CHMI to MMA was defined. The effects of the weight fractions of AIBN and DDM on weight-average molecular weight (M_w) were studied. Meanwhile, relations between M_w and the glass transition temperature (T_g) and decomposition temperature (T_d) and M_w and the melt flow index (MFI) were described. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2001–2005, 1998     

Engineering Plastics from Lignin XIV. Characterization of Chain-Extended Hydroxypropyl Lignins

Abstract

Three series of chain-extended hydroxypropyl lignins (CEEQLs), prepared fran oqanosolv and kraft lignin, were examined regarding their chemical, molecular weight and them1 characteristics. Results showed that the molar substitution (MS) of propylene oxide, which was defined as the number of propoxy repeat units which comprise the chain attached to a single reactive site on lignin, varied and affected copolymer properties. As the MS increased from 1 to 7.2, the number average molecular weight (M_g) increased while the glass transition temperature (T_g) decreased. The actual M_g observed by GPC exceeded however that expected on the basis of mass gain by derivatization. This was attributed to changes in the apparent hydrodynamic volume in relation to MS. The change in T_g with increasing MS followed the Gordon-Taylor relationship. Differences in the chemical composition of the original lignin (organosolv or Kraft) were not obvious as the lignin content of the copolymer decreased below 50%.     

Preparation and glass transition temperature of hyperbranched poly[allyl methyl maleate‐co‐N‐propyl maleimide]

The kinetics and molecular weight averages of the hyperbranched polymers formed by the alternating copolymerization of equimolar allyl methyl maleate (AMM) and N‐n‐propyl maleimide (PMI) were investigated. The yields, molecular weight averages, and polydispersity indices as well as the branching degrees of the produced copolymers increased with increasing initiator concentrations and prolonged polymerization time. The trends of the experimental molecular weights as determined by size exclusion chromatography were in good agreement with the theoretical predictions. The molecular weight distribution indices fit the curve given by M_w/M_n = 1/(1‐x_D), and the molecular weights fit the curve given by M_w = 4076/(1‐x_D)², where x_D was the conversion of vinyl groups. DSC studies demonstrated a nonlinear relation of T_g values to the reciprocal of molecular weight (M), and T_g values decreased with the increase of molecular weight. For the T_g values of highly branched polymers in high molecular weight range, a relation of T_g = T + k/M was obtained, where T was obtained by extrapolating to infinite molecular weight and k was a constant. T was 136°C, and k = 2.9 for this work. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1941–1947, 2005     

Engineering plastics from lignin. XV. Polyurethane films from chain-extended hydroxypropyl lignin

A series of polyurethane (PU) films was prepared from chain-extended hydroxypropyl lignins (CEHPL). In appearance, these films ranged from brittle and dark brown to rubbery and bronze. The thermal, mechanical, and network properties of these PUs were investigated by DMTA and DSC analysis. All films exhibited single T_g's which varied between ?53° and 101°C, depending on lignin content. From swelling experiments, molecular weight between crosslinks (M _c) was determined and found to vary over 2.5 orders of magnitude. The M _c's were related to the change in T_g that accompanied network formation. Stress–strain experiments showed a variation in Young's modulus between 7 and 1300 MPa. Most of the variation in material properties was related to lignin content and to a lesser extent to diisocyanate type, hexamethylene diisocyanate, or toluene diisocyanate. The source of the CEHPL had no effect on the observed properties. From these results it was concluded that the properties of PUs can be controlled and engineered for a wide variety of practical uses.     

Chain‐linked lactic acid polymers by benzene diisocyanate

Chain‐linked lactic acid polymers with high molecular weight were synthesized by two‐step polymerization method, including polycondensation and chain extending reactions. The effects of chain extender toluene diisocyanate (TDI) on the chain‐linked lactic acid polymers were studied. The polymers obtained were characterized by gel permeation chromatography, fourier transform infrared spectroscopy, ¹H NMR, and differential scanning calorimeter. Reactions between 1,4‐butanediol and lactic acid oligomers led to hydroxyl‐terminated prepolymer, which provided significant increase of molecular weight in the chain extending reaction. In addition, the glass transition temperature (T_g) and the melting temperature (T_m) were increased. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1045–1049, 2006     

Copolymers of unsaturated and saturated poly(ether ester amide)s: Synthesis,characterization, and biodegradation

A series of biodegradable random unsaturated/saturated poly(ether ester amide)s copolymers (USPEEAs) were synthesized by an active solution polycondensation of unsaturated and saturated dicarboxylic acid‐based diester monomers with diamine salts of phenylalanine and saturated oligo(ethylene glycol) (OEG). These USPEEA copolymers were obtained with fairly good yields in DMA solvent. The chemical structures of the USPEEA copolymers were confirmed by both IR and NMR spectra. The molecular weights (M_n and M_w) of USPEEAs measured by GPC ranged from 3 to 27 kg/mol with the molecular weight distribution (MWD) ranging from 1.52 to 2.13. USPEEA copolymers obtained had T_g lower than that of the pure UPEEAs but higher than that of pure saturated poly(ether ester amide)s (SPEEA). An increase in the unsaturated component in USPEEAs led to an increase in their T_g. A preliminary in vitro biodegradation property of USPEEA copolymers were investigated in both pure PBS buffer and α‐chymotrypsin solutions. The USPEEA copolymers showed a pronounced weight loss in enzyme solutions, but a smaller weight loss in a pure PBS. The biodegradation rates of USPEEA copolymers in α‐chymotrypsin solution were much slower than those of pure PEEAs. Therefore, upon adjusting monomers feed ratio, USPEEA copolymers could have controlled chemical, physical, and biodegradation properties. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Thermosetting Polyetherimides: The Influence of Reactive Endgroup Type and Oligomer Molecular Weight on Synthesis,Network Formation,Adhesion Strength and Thermal Properties

The influence of the reactive endgroup on the synthesis, cure behavior and network properties of thermosetting polyetherimides was investigated. Reactive phenylethynyl, ethynyl and maleimide terminated etherimide oligomers were prepared and characterized. Optimal reaction conditions were established to produce fully endcapped oligomers with imidized structures and controlled molecular weight. The phenylethynyl and ethynyl endcapped systems were synthesized by ester-acid methods. The maleimide endcapped system utilized an amic-acid route. Phenylethynyl endcapped oligomers had good processibility and were thermally cured at high temperatures (350–380°C). The networks exhibited good thermal and hydrolytic stability and good adhesion strength, and are candidates for “primary'' bonding adhesives. In contrast, more reactive ethynyl and maleimide endcapped systems were prepared as “secondary'' bonding materials, which could be cured at temperatures lower than that of the T _g of the primary structure. Lap shear test results obtained from NMP-cast/methanol-extracted scrim-cloth-supported precursors confirmed that good adhesion to titanium at both room temperature and at 177°C was achieved when cured at 250°C-280°C. High glass transition temperatures and good thermal stability were achieved as determined by thermal analysis (DSC, TGA and DMA). Solvent extraction measurements confirmed that very high gel fractions were obtained, which is consistent with good chemical resistance.

The influence of molecular weight between crosslinks (Mc) on thermal and mechanical behavior was also investigated for 2,3,5,7 and 10k initial M _n values. Lower molecular weight oligomers exhibited lower T _g and cure temperatures, but higher cured network crosslink densities afforded higher T _g and higher gel fractions, but with reduced toughness.     

Europium‐containing cholesteric liquid crystalline polymers in the side chain

Europium‐containing cholesteric liquid crystalline polymers were graft copolymerized using poly(methylhydrogeno)siloxane, cholesteryl 4‐(allyloxy)benzoate (M₁), cholesteryl acrylate (M₂), and a europium complexes monomer (M₃). The chemical structures of the monomers were characterized by Fourier transform infrared and ¹H‐nuclear magnetic resonance. The mesomorphic properties and phase behavior were investigated by differential scanning calorimetry, thermo gravimetric analysis, polarizing optical microscopy, and X‐ray diffraction. With an increase of europium complexes units in the polymers, the glass transition temperature (T_g) did not change significantly; the isotropic temperature (T_i) and mesophase temperature range (ΔT) decreased. All polymers showed typical cholesteric Grandjean textures, which was confirmed by X‐ray diffraction. The temperatures at which 5% weight loss occurred (T_d) were greater than 300°C for the polymers. The introduction of europium complexes units did not change the liquid crystalline state of polymer systems; on the contrary, the polymers were enabled with the significant luminescent properties. With Eu³⁺ ion contents ranging between 0 and 1.5 mol %, luminescent intensity of polymers gradually increased and luminescent lifetimes were longer than 0.45 ms for the polymers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40866.     

Ecofriendly modification of acetosolv lignin from oil palm biomass for improvement of PMMA thermo‐oxidative properties

The environmentally friendly esterification of acetosolv lignin (AL), obtained from pressed oil palm mesocarp fibers, is described, for the improvement of thermo‐oxidative properties of poly(methyl methacrylate) (PMMA) films. Acetylation of AL was performed in ecofriendly conditions using acetic anhydride in the absence of catalysts. Acetylated acetosolv lignin (AAL) was successfully obtained in only 12 min with a solvent‐free and catalyst‐free microwave‐assisted procedure. Lignins were characterized by Fourier transform infrared spectroscopy, size exclusion chromatography, thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC), confirming the efficacy of the methodology employed. AL and AAL as fillers in different concentrations (1% and 5%) were added to PMMA films. The thermal and mechanical properties of the lignin‐incorporated films were analyzed by TGA, DSC, and dynamic mechanical analysis (DMA). The films incorporated with lignin and acetylated lignin presented initial degradation temperature (T_onset) and onset oxidative temperature (OOT) values higher than pure PMMA films, contributing thus to an enhancement of thermo‐oxidative stability of PMMA. The DMA analyses showed that incorporation of AL or AAL increased the storage modulus (E′) of PMMA films, but did not affect their glass‐transition temperatures (T_g). The results indicate the potential use of oil palm mesocarp lignin to enhance the thermo‐oxidative properties of PMMA without compromising its mechanical response. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45498.