Formation Of Long Chain Branches Research Articles

Suppressing the long‐chain branching in the synthesis of poly(styrene‐b‐butyl acrylate‐b‐styrene) in RAFT emulsion polymerization by tuning the interfacial properties

ABSTRACTReversible addition‐fragmentation chain transfer (RAFT) emulsion polymerization is becoming an important technique to synthesize the latex of block copolymers. A previous study showed that in the synthesis of polystyrene‐b‐poly(butyl acrylate)‐b‐polystyrene triblock copolymer via RAFT emulsion polymerization using amphiphilic oligo(acrylic acid‐styrene) macroRAFT as surfactant and mediator, the molecular weight distribution could be much broadened to PDI higher than 2. In this study, an in‐depth investigation was performed to decrease PDI. It was found that long‐chain branches could be formed in the synthesis of triblock block copolymer, leading to the appearance of a higher molecular weight shoulder in the GPC curve of the final product. The lower neutralization degree of acrylic acid (AA) units on the macroRAFT and shorter AA chains would help to suppress the formation the long‐chain branches, leading to PDI around 1.5. It is evidenced that the successful suppression is due to the promotion of radical entry as a result of decreased interfacial transport impedance. It is also evidenced that the presence of styrene during the polymerization of butyl acrylate could promote the formation of long chain branches. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1464–1473

Preparation and Characterization of Long Chain Branched Polypropylene through UV Irradiation and Coagent Use

Trimethylolpropane triacrylate as coagent was utilized along with benzophenone to modify polypropylene by generating long chain branches in the polypropylene molecular structure. The effects of trimethylolpropane triacrylate concentration, benzophenone concentration, and irradiation duration on viscoelastic properties and gel content were studied. The processing conditions that result in the greatest amount of long chain branching with minimum amount of gel content were identified. Gel permeation chromatography was used to confirm formation of branches and determine the branching content, whereas Fourier transform infrared spectroscopy confirmed the contribution of trimethylolpropane triacrylate in the formation of long chain branches.

Peroxide induced cross‐linking by reactive melt processing of two biopolyesters: Poly(3‐hydroxybutyrate) and poly(l‐lactic acid) to improve their melting processability

ABSTRACTPoly(3‐hydroxybutyrate) (PHB) and poly(l‐lactic acid) (PLLA) were individually cross‐linked with dicumyl peroxide (DCP) (0.25–1 wt %) by reactive melt processing. The cross‐linked structures of the polymer gel were investigated by nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopies. The size of the polymer crystal spherulites, glass transition temperature (Tg), melting transition temperature (Tm), and crystallinity were all decreased as a result of cross‐linking. Cross‐linking density (νe) was shown to increase with DCP concentration. Based on parallel plate rheological study (dynamic and steady shear), elastic and viscous modulus (G″ and G′), complex viscosity (η*) and steady shear viscosity (η) were all shown to increase with cross‐linking. Cross‐linked PHB and PLLA showed broader molar mass distribution and formation of long chain branching (LCB) as estimated by RheoMWD. Improvements in melt strength offer bioplastic processors improved material properties and processing options, such as foaming and thermoforming, for new applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41724.

Size exclusion chromatography of photo-oxidated LDPE by triple detection and its relation to rheological behavior

Sheets of low-density polyethylene (LDPE) were subjected to photo-oxidation in the presence of air using a xenon lamp to irradiate the samples for times between 1 day and 6 weeks. The formation of long-chain branching (LCB) up to one week of degradation and the competition between chain scission and crosslinking at longer periods of radiation were investigated by rheological characterization, Fourier transform infrared spectroscopy, and the solvent extraction method (Rolón-Garrido and Wagner. Polym Degrad Stabil 2014, 99:136, Rolón-Garrido and Wagner. J Rheol 2014, 58:199). In the present contribution the same samples are studied by size exclusion chromatographic characterization using triple detection (concentration, light scattering and viscosity). The gel content is determined by filtration followed by the analysis of the soluble polymer fraction. The influence of photo-oxidation time on the molecular weight distribution (molar masses and polydispersity), the mean square radius of gyration and the intrinsic viscosity contraction factors is discussed. The results are correlated with the model parameters (β and fmax2) of the molecular stress function (MSF) theory, used to describe quantitatively the rheological data in uniaxial elongation. It is verified that LCB occurs as an aside process, which up to one week of degradation dominates over chain scission, before gelation plays a critical role. It is confirmed that the model parameter β correlates with the gel content, which reflects the competition between chain scission and crosslinking, while fmax2 is found to correlate with the experimentally determined contraction factors. By comparing the data of this study with those obtained earlier for polystyrene comb melts with well defined structure, the influence of the branching frequency (i.e. the number of branch points per 1000 carbon atoms) on fmax2 becomes evident.

The effect of depth and duration of UV radiation on polypropylene modification via photoinitiation

ABSTRACTLinear polypropylene (PP) was modified using UV radiation in the presence of 0.5 wt % of benzophenone photoinitiator to introduce long chain branching (LCB) to the PP backbone. Irradiation was carried out in the solid state and the temperature level was kept below 60°C. The effects of radiation duration and sample thickness on the extent of these branching modification reactions were investigated. Viscoelastic properties, molecular weight, molecular weight distribution, and gel content were determined and compared for runs having different sample thicknesses, irradiated for different times. Comparisons were also conducted with the parent PP and the PP mixed with photoinitiator. It was found that LCB decreased by increasing the thickness of the samples. Conversely, an increase in radiation duration resulted in enhanced LCB but also led to larger gel content in the samples. Based on all these measurements and observations, a mechanism was suggested to explain formation of long chain branches (LCBs) in PP in the solid state via photoinitiation. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 41021.

Efficient melt stabilization of polyethylene with quercetin, a flavonoid type natural antioxidant

The potential use of quercetin, a flavonoid type natural antioxidant, as a stabilizer in polyethylene was explored in this work. Its efficiency was compared to that of Irganox 1010, a hindered phenolic antioxidant used routinely in industrial practice, both in the presence and the absence of a phosphorous secondary stabilizer. The study was carried out with a Phillips type polyethylene and the efficiency of the additive packages was checked by various methods on samples produced by multiple extrusions. Quercetin content changed from 0 to 1000 ppm in 10 steps. The results showed that quercetin is a very efficient antioxidant. It prevents the formation of long chain branches already at a concentration as small as 50 ppm and its dosage at 250 ppm renders the polymer sufficient long term residual stability. The efficiency of quercetin is considerably better than that of Irganox 1010, the hindered phenolic antioxidant used as reference stabilizer. The difference in efficiency might be explained with the dissimilar number of active –OH groups on the two molecules, but the stabilization mechanism of quercetin may be also different from that of I1010. Quercetin interacts with the phosphonite secondary stabilizer used, which improves dispersion and increases efficiency. Besides its advantages, quercetin has also some drawbacks (very high melting temperature, poor solubility in polyethylene and strong yellow color), which must be overcome before the substance can be used in practice.

Linear and non-linear rheological characterization of photo-oxidative degraded LDPE

Abstract Sheets of low-density polyethylene (LDPE) were photo-oxidatively treated in the presence of air. Characterization methods include solvent extraction method and rheology in the melt state by shear and uniaxial extensional experiments. For photo-oxidation, a xenon lamp was used to irradiate the samples for times between 1 day and 6 weeks. Linear viscoelasticity is strongly affected by increasing photo-oxidation due to crosslinking of LDPE, as corroborated by gel fraction. For samples which were degraded for up to three days, negligible gel content was found and the non-linear parameters f max and β of the Molecular Stress Function (MSF) model indicate that the induced increase in strain hardening is due to the formation of long-chain branching, which is overcoming chain scission. Further degradation leads the parameter f max to increase to values outside the experimental window, while the parameter β was found to correlate with the gel content and with the trend followed by the strain hardening index. The competition between chain scission and crosslinking could also be followed in linear viscoelasticity by the change in activation energy and in the loss angle δ. From the constitutive analysis, it is noticed that, within the experimental window, time-deformation separability is confirmed for all samples, independent of the degree of photo-oxidation. It is demonstrated that the sensitivity of rheological characterization with respect to structural modifications, when combined with constitutive analysis, can be used to elucidate the effects of photo-degradation on LDPE.

Green chemical preparation of cellulose/high performance elastomer blend fibers by melt-spinning method

With ionic liquid namely 1-N-butyl-3-methylimidazolium chloride ([BMIM]Cl) as reaction medium in cellulose modification, maleic anhydride/glycidyl methacrylate-grafted ethylene-octene copolymer (POE-g-(MA/GMA)) was blended with cellulose by a co-rotating twin-screw extruder. Cellulose/POE-g-(MA/GMA) blend was examined by FTIR, 13C-NMR, WAXD, DSC, TGA, rheology and tensile properties. Owing to the reaction between functional groups of POE-g-(MA/GMA) and hydroxyl groups of cellulose and the high mobility of POE-g-(MA/GMA), hydrogen bond network in cellulose was destructed and cellulose-b-POE-g-(MA/GMA) copolymer was synthesized in-situ. As a result, the reduction in intermolecular force of cellulose improved the melt flowability and spinnability of cellulose blend. The formation of long-chain branches hindered the crystallization and increased the thermal stability of cellulose on one hand and obtained much more chemical bond fracture under stress action and increased the tensile strength and elongation at break of the blend fibers on the other. However, the addition of 15 wt% POE-g-(MA/GMA) led to the formation of crosslined structure, so a high degree of chain entanglement increased the dynamic viscosity and decreased the spinnability of cellulose/POE-g-(MA/GMA) blend.

Effect of solid‐state stress induced reaction on morphology and structure of metallocene polyethylene

ABSTRACTMetallocene‐catalyzed polyethylene (mPE) powder was prepared through a mechanochemical reactor in solid state at ambient temperature. The changes of morphology and structure of mPE were investigated by SEM, FTIR, DSC, viscosimetry, GPC, WAXD, dynamic rheometer, and high pressure capillary rheometer methods. FTIR measurement illustrated that the branching degree of mPE increased after mechanochemical reaction. Combined with the dynamic rheological analysis, it was proved that the rupture and combination of macromolecular chain undergo strong shear resulted in the formation of long chain branches. X‐ray diffraction analysis showed that the crystal form of mPE transferred from rhombic to monoclinic system partly, and the crystalline degree increased slightly in the initial mechanochemical reaction period. Viscometer test results exhibited that molecular weight of mPE dropped during the mechanochemical treatment. Rheology measurement indicated that the critical shear rate of the shark‐skin failure enhanced and the extruded stands exhibited smooth face for the mPE after mechanochemical treatment. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1691–1699, 2013

Revisiting the long-chain branch formation mechanism in metallocene catalyzed polyethylenes

The effect of polymerization parameters, such as pressure (ethylene concentration), temperature and comonomer type and concentration, on several long-chain branched (LCB) ethylene homopolymers and ethylene/α-olefin copolymers was investigated with respect to their degree of long-chain branching. Common long-chain branched metallocene polyethylenes (LCB-mPEs) and comb-like LCB-PEs, which have high viscosity and significantly differ from conventional LCB-mPEs, have been used to evaluate different proposed mechanisms for LCB-formation based on available characterization methods. The correlations between polymerization parameters and degree of long-chain branching were compared with the correlations predicted by different formation mechanisms (intermolecular auto-copolymerization by re-incorporation of β-hydride terminated chains, intramolecular insertion of stored vinyl terminated chains, chain walking and σ-bond metathesis reactions) to elucidate which of these mechanisms is the correct one. The intermolecular auto-copolymerization mechanism was found to be the predominant mechanism behind LCB-formation in slurry phase polymerization. However, σ-bond metathesis reactions cannot be entirely ignored.

Preparation of long-chain branching polypropylene and investigation on its foamability

In this work, zinc N,N-dimethyldithiocarbamate (ZDMC) was used to mediate the melt modification of polypropylene (PP) with 1,6-hexanediol diacrylate (HDDA) in an internal mixer in the presence of dicumyl peroxide (DCP). Fourier transformed infrared spectroscopy analysis revealed that HDDA was directly grafted onto PP backbone. The dependence of torque on processing time indicated that the presence of ZDMC restrained the degradation of PP, and the end-torque value increased with the addition of ZDMC. Dynamic rheological measurement indicated that the modified PP possessed higher G′ and lower tan δ at low frequency, displaying an increase in η* and disappearance of Newtonian plateau in η*–ω plot, as well as larger radius of semicircle in Cole–Cole plot. All the rheological characterizations, together with the decreased gel content with the increase of ZDMC, confirmed the formation of long-chain branching. Subsequently, the foamability of the modified samples was investigated by one-step compression–molding process. The cellular structure and morphology of the obtained foams were observed by scanning electron microscopy, and the results showed that the addition of ZDMC decreased the cell size, increased the cell density, and brought about well-defined closed cell structure. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers

Synthesis of long-chain branched isotactic-rich polystyrene via cationic polymerization

Cationic polymerization of styrene was conducted with 1-chloro-1-phenylethane (SCl)/AlCl3/phenyl methyl ether (PME) initiating system in hexane/CH2Cl2 (60/40, v/v) at −80 °C. The kinetics for cationic polymerization of styrene was investigated by in-situ ATR-FTIR spectroscopy. The isotactic-rich polystyrene (iPS) with m dyad of 81%, mm triad of 63% and mmmm pentad of 50% could be synthesized. Small amounts of crystalline regions in iPS formed after flow-induced crystallization and the crystallinity increased with increasing the molecular weight of iPS. Furthermore, the long-chain branched isotactic-rich polystyrene (biPS) with around 12 times higher molecular weight than that of corresponding iPS could be synthesized via cationic polymerization of styrene by introducing a small amount of isoprene (Ip) as a comonomer and branching sites as well. The possible mechanism for long-chain branching formation via intermolecular alkylation reaction by using Ip structural units along polymer chain as branching sites was proposed. The nucleation rate of biPS could be greatly enhanced with increasing the content of branching sites, leading to an obvious increase in crystallinity. The multi-melting temperatures from 140 °C to 237 °C were observed in DSC curves of these PS products. The tensile strength of commercial atactic polystyrene could be improved remarkably from 41.4 MPa to 55.7 MPa by adding 16.7% of biPS.

Comparison of Molecular Structure and Rheological Properties of Electron-Beam- and Gamma-Irradiated Polypropylene

Polypropylene (PP) irradiated with electrons or electromagnetic waves (gamma-rays) undergoes chain scission, and the macroradicals generated form branched molecules as shown by size-exclusion chromatography coupled with laser scattering. Information on the irradiation modification of the molecular structure of PP is still limited in the literature, especially with respect to the controlled generation of long-chain branches (LCB). This paper examines the branching structure of an electron-beam- and gamma-irradiated polypropylene. In general, appropriate irradiation conditions lead to the formation of LCB, which were analyzed in detail by rheological means. At smaller doses, the zero shear-rate viscosities η0 of the electron-beam-irradiated PP lie above the values for the unmodified PP with the same Mw but are distinctly lower than the linear reference at higher doses. This result can be interpreted by the change from a starlike to a treelike branching topography. The gamma-irradiation leads to η0 values smaller than those of the linear reference, giving rise to the assumption of the generation of treelike molecules. The measurements of the elongational viscosities support this molecular picture. An explanation of these findings is given along the line that the electron irradiation conducted at dose rates higher than the gamma-irradiation creates a larger concentration of short-living radicals, which will partially annihilate each other, but favor the growth of some long branches, which get branched themselves at higher doses. The structure of the gamma-irradiated samples is postulated to consist of a blend of linear and treelike molecules.

Size‐exclusion chromatography coupled to multiangle light scattering detection of long‐chain branching in polyethylene made with phillips catalyst

AbstractSize‐exclusion chromatography coupled to multiangle light scattering (SEC‐MALS) has been used to detect long‐chain branching (LCB) in polyethylene (PE) from Cr/silica catalysts for the first time. The observed LCB response to several catalyst and reactor variables mostly confirms earlier conclusions drawn from rheological measurements. However, SEC‐MALS has also shed additional light on a few previously unanswered questions. Above all, SEC‐MALS shows the placement of branching within the MW distribution, which was not previously known, and which may explain some of the unique molding behavior of Cr‐derived PE. This new SEC‐MALS data also provide insight into the mechanism of LCB formation, which is discussed. Like earlier studies based on rheology, this new study demonstrates that the commonly accepted view of macromer incorporation may be overly simplistic. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Enhancement of strain-hardening by thermo-oxidative degradation of low-density polyethylene

Low-density polyethylene was thermally and thermo-oxidatively degraded at 170°C and subsequently characterized by linear-viscoelastic measurements and in uniaxial extension. The elongational viscosities measured were analyzed in the framework of the Molecular Stress Function (MSF) model. For the thermally degraded samples, degradation times between 2 and 6 h were applied. Formation of long-chain branching (LCB) evidenced by enhanced strain hardening was found to occur only during the first 2 h of thermal degradation. At longer exposure times, no difference in the level of strain hardening was observed. This was quantified by use of the MSF model, which in elongation has two model parameters: $f_{\max }^2$ determining the maximum relative stretch of the chain segments, and β representing the ratio of the molar mass of the (branched) polymer chain to the molar mass of the effective backbone alone. The non-linear parameter $f_{\max }^2$ increased from $f_{\max }^2 =14$ for the non-degraded sample to $f_{\max }^2 =22$ for the samples thermally degraded for 2 up to 6 h. For the thermo-oxidatively degraded samples, i.e. those degraded in the presence of air, degradation times between 30 and 90 min were applied. Surprisingly, under these degradation conditions, the level of strain hardening increases drastically up to $f_{\max }^2 =55$ with increasing exposure times from 30 up to 75 min due to LCB formation and then decreases for an exposure time of 90 min due to chain scission dominating LCB formation. The non-linear parameter β of the MSF model was found to be β = 2 for all samples, indicating that the general type of the random branching structure remains the same under all degradation conditions. Consequently, only the parameter $f_{\max }^2$ of the MSF model and the linear-viscoelastic spectra were required to describe quantitatively the experimental observations. The strain hardening index, which is sometimes used to quantify strain hardening, was shown to follow accurately the trend of the MSF model parameter $f_{\max }^2$ .

Rheological and Thermal Properties of the PLA Modified by Electron Beam Irradiation in the Presence of Functional Monomer

Polylactic acid (PLA) has been modified by electron beam radiation in the presence of glycidyl methacrylate (GMA) to enhance the melt strength of PLA. The modified PLA was prepared by varying both the amount of GMA and the irradiation dose and was characterized by observing the thermal properties, the melt viscoelastic properties and the gel fraction. For comparison, virgin PLA was also irradiated. All irradiated virgin PLA had a lower complex viscosity and a storage modulus compared to virgin PLA due to irradiation-induced chain scission. However, these properties were remarkably improved due to formation of long chain branching and retarding chain scission if GMA was introduced in this system. The increase in melt viscoelastic property was much dependent on the irradiation dose. At optimum doses of radiation, it showed maximum complex viscosity and storage modulus. The PLA irradiated with 20 kGy in the presence of 3 phr GMA showed a complex viscosity of about 10 times higher and a storage modulus of 100 times higher than those of virgin PLA at 0.1 rad/s. Gel fraction measurement revealed that chain scission and branching was more dominant than crosslinking. The biodegradability of irradiated PLA was slightly decreased by the presence of GMA.

Long-Chain Branches in Syndiotactic Polypropene Induced by Vinyl Chloride

Syndiotactic polypropene was synthesized using a metallocene catalyst system and traces of VC. The use of VC was found to decrease the molar mass M w of the samples distinctly and linearly, while retaining the molar mass distribution, the crystallinity, and the syndiotacticity. The VC, however, initiates the formation of long-chain branches, which were detected by GPC-MALLS and rheology. Besides their effect on the viscosity and modulus functions, a thermorheologic complexity can be observed. Also the flow activation energy E a was found to be increased. The modified samples did not obey the η 0 /M w relation for linear sPP, indicating the presence of LCBs.

Melt stabilisation of Phillips type polyethylene, Part II: Correlation between additive consumption and polymer properties

Phillips type polyethylene stabilised with combinations of 700 ppm phenolic antioxidant and different amounts of various phosphorous stabilisers (sterically hindered aryl phosphite [Hostanox PAR 24], phosphonite [Sandostab P-EPQ], and aryl–alkyl phosphine [PEPFINE]) was processed by six consecutive extrusions. The polymer was characterised by FT-IR spectroscopy, rheological (melt flow index, creep compliance), colour and oxidation induction time measurements. Films were prepared by blowing and their mechanical strength was determined by Elmendorf and dart drop tests. The consumption of the antioxidants was compared to the characteristics of the polymer and to the strength of the films. The consumption rate of both the phenolic and the phosphorous antioxidants is reduced in their combinations compared to single antioxidants. The chemical structure of the polymer is modified considerably in the first extrusion even at high antioxidant levels. The mechanism of stabilisation is determined by the type of the antioxidant(s) in further processing steps. The phenolic antioxidant does not prevent the formation of long chain branches. The phosphonite and the phosphine hinder efficiently hydrogen abstraction from the polymer chain and long chain branching. Their efficiency is similar, but the phosphonite is consumed fast, while the phosphine oxidises slowly. The investigated phosphite is less reactive; the contribution of the phenolic antioxidant to the inhibition reactions is significant in phenol/phosphite combinations, therefore long chain branching increases continuously with increasing number of processing steps.

Structure and Rheological Properties of the Products of Solid-State Graft Polymerization of Styrene in Annealed Polypropylene Reactor Granules

Solid-state grafting polymerization of styrene (St) in polypropylene (PP) reactor granules was conducted with different St load using tertiary-butyl perbenzoate as initiator. The graft copolymers showed higher weight average molecular weight than the original PP when the St load is high enough. A typical graft copolymer was fractionated into six fractions according to crystallinity, and the fractions showed similar St content. Comparing to the original PP, complex viscosity of the grafting products was significantly increased and the tan α value decreased at low frequency. Increasing the grafting degree exerts a positive effect on these changes. These phenomena prove the existence of long chain branches in the grafting products, and St played important roles in the formation of long chain branches.

Melt stabilisation of Phillips type polyethylene, Part I: The role of phenolic and phosphorous antioxidants

The role of a phenolic and three phosphorous (phosphite, phosphonite and phosphine) antioxidants in the melt stabilisation of polyethylene was studied in a Phillips type polyethylene by multiple extrusions. The polyethylene was stabilised with a single antioxidant at 700 ppm and with phenolic/phosphorous antioxidant combinations containing 700 ppm of each component. The functional groups (methyl, vinyl, vinylidene, trans-vinylene and carbonyl) of polyethylene and the residual amount of phosphorous antioxidants were analysed quantitatively by FT-IR methods developed in our laboratory. The rheological characteristics, the colour and the residual thermo-oxidative stability of the polymer were determined and compared. Blown films were prepared and their mechanical strength measured by the Elmendorf and Dart-drop tests. The comparison of the different characteristics revealed that the chemical reactions taking place during the first processing of the nascent polymer powder, as well as the chemical composition of the antioxidants determine the reactions taking place in further processing operations. The changes in the characteristics of stabilised polyethylene during processing are controlled by the phosphorous stabiliser. The effect and final result depend on the chemical structure of the given antioxidant. The phenolic antioxidant itself does not hinder the formation of long chain branches. It reduces the rate of oxidation of the various phosphorous stabilisers, but does not modify the mechanism of stabilisation of the phosphonite and the phosphine. The reactions of the phosphite are significantly modified by the presence of a phenolic antioxidant.