Low Molecular Weight Homopolymers Research Articles

Low molecular weight poly((D,L)-lactide-co-caprolactone) liquid inks for diluent-free DLP printing of cell culture platforms.

Digital light processing (DLP) printing offers the possibility of fabricating complex objects in a fast and reproducible manner. A main requirement for DLP printing is the use of inks with low viscosities that can flow under the printing platform in a short period of time. Its exploitation in tissue engineering applications has been centered on the use of hydrogel forming materials diluted in aqueous solutions or the use of polyesters in combination with diluents and heating platforms that aid in the reduction of their viscosity. The use of diluents, however, modifies the mechanical properties and reduces the shape fidelity of the printed objects and, the use of heating platforms results in vats with heterogeneous temperatures and ink viscosities. Here, we report on the synthesis of a library of methacrylated low molecular weight (<3000 g mol-1) homopolymers ((P(D,L)LA and PCL) and copolymers (P((D,L)LA-co-CL)) of 2- and 3-arms based on (D,L)-lactide and ε-caprolactone. The resulting inks possessed low viscosity that made them printable in the absence of diluents and heating elements. DLP printing of cubical and cylindrical patterns resulted in objects with a higher shape fidelity than their counterparts fabricated using diluents and with printed features on the order of 300 μm. The printed materials were biocompatible and supported the growth of human mesenchymal stem cells (hMSCs). Moreover, the variations in the composition resulted in polymers that enabled the attachment of hMSCs to different extents, leading to the formation of well-adhered cell monolayers or loosely adhered cell aggregates.

Rapid Ordering in "Wet Brush" Block Copolymer/Homopolymer Ternary Blends.

The ubiquitous presence of thermodynamically unfavored but kinetically trapped topological defects in nanopatterns formed via self-assembly of block copolymer thin films may prevent their use for many envisioned applications. Here, we demonstrate that lamellae patterns formed by symmetric polystyrene-block-poly(methyl methacrylate) diblock copolymers self-assemble and order extremely rapidly when the diblock copolymers are blended with low molecular weight homopolymers of the constituent blocks. Being in the "wet brush" regime, the homopolymers uniformly distribute within their respective self-assembled microdomains, preventing increases in domain widths. An order-of-magnitude increase in topological grain size in blends over the neat (unblended) diblock copolymer is achieved within minutes of thermal annealing as a result of the significantly higher power law exponent for ordering kinetics in the blends. Moreover, the blends are demonstrated to be capable of rapid and robust domain alignment within micrometer-scale trenches, in contrast to the corresponding neat diblock copolymer. These results can be attributed to the lowering of energy barriers associated with domain boundaries by bringing the system closer to an order-disorder transition through low molecular weight homopolymer blending.

Polymerization Rate Considerations for High Molecular Weight Polyisoprene‐b‐Polystyrene‐b‐Poly(N,N‐dimethylacrylamide) Triblock Polymers Synthesized Via Sequential Reversible Addition‐Fragmentation Chain Transfer (RAFT) Reactions

The reversible addition‐fragmentation chain transfer (RAFT) polymerization mechanism is a powerful technique for synthesizing functional block polymers for myriad applications. Most kinetic studies regarding the RAFT mechanism have focused on low molecular weight homopolymer and block polymer syntheses using a dithiobenzoate chain transfer agent (CTA). Here, the polymerization kinetics are evaluated for a high molecular weight A‐B‐C triblock polymer system, polyisoprene‐b‐polystyrene‐b‐poly(N,N‐dimethylacrylamide) (PI‐PS‐PDMA), using a trithiocarbonate agent for application of these types of polymers. Importantly, it is demonstrated that the polymerization of polyisoprene is the step that generates the block with the largest dispersity for high molecular weight PI‐PS‐PDMA polymers. As such, the kinetics of isoprene polymerization must be altered systematically for desired nanostructures to be formed. In addition, it is established that the PS and PDMA block additions exhibit polymerization rate retardation, which is due to slow chain fragmentation of the CTA, as demonstrated by the magnitudes of the equilibrium constants for both the styrene and N,N‐dimethylacrylamide reactions, and as calculated using ab initio modeling. This elucidation of the nature of the controlled RAFT mechanism provides a critical handle for the more precise design and control of other next‐generation high molecular weight block polymer systems that are polymerized using the RAFT mechanism. image

First RAFT/MADIX radical copolymerization of tert-butyl 2-trifluoromethacrylate with vinylidene fluoride controlled by xanthate

The radical copolymerization of tert-butyl 2-trifluoromethacrylate (MAF-TBE) with vinylidene fluoride (VDF), initiated by bis(4-tert-butyl cyclohexyl) peroxydicarbonate and controlled by O-ethyl-S-(1-methyloxycarbonyl) ethyl xanthate as a chain transfer agent (CTA) is presented. Molar masses ranged between 1500 and 50 000 g mol−1 while polydispersity indices (PDIs) were lower than 1.8. Various experimental conditions such as different radical initiators, solvents, initial [initiator]o/([VDF]o + [MAF-TBE]o) and [CTA]o/([VDF]o + [MAF-TBE]o) molar ratios, and their effect on the copolymerization (in terms of molar percentages of both comonomers in the copolymer, yields, molar masses, and thermal properties) were examined and discussed. These poly(VDF-co-MAF-TBE) copolymers bearing a xanthate end-group were further involved in controlled radical polymerizations of vinyl acetate (VAc) or VDF leading to poly(VDF-co-MAF-TBE)-b-poly(VAc) and poly(VDF-co-MAF-TBE)-b-poly(VDF) block copolymers, respectively. The success of these radical copolymerizations of MAF-TBE contrasts with its homopolymerization that occurred with difficulty and produced low molecular weight-homopolymers in low yields. On the other hand, the controlled character of the polymerization involving O-ethyl-S-(1-methyloxycarbonyl) ethyl xanthate was confirmed by the evolutions of log([M]o/[M]) and MAF-TBE conversion versus time and by size exclusion chromatography that were found to be linear. All the structures of the obtained intermediates and block copolymers were characterized by 1H and 19F NMR spectroscopy and size exclusion chromatography to assess the molar percentages of VDF and MAF-TBE and the molecular weights and PDIs, respectively. The thermal and surface properties of such original copolymers were determined by thermogravimetric analysis and water contact angle measurements, respectively.

Pattern Dimensions and Feature Shapes of Ternary Blends of Block Copolymer and Low Molecular Weight Homopolymers Directed To Assemble on Chemically Nanopatterned Surfaces

Ternary blends of cylinder-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) and low molecular weight PS and PMMA were directed to assemble on chemically patterned surfaces with hexagonal symmetry. The chemical patterns consisted of strongly PMMA preferential spots, patterned by electron-beam lithography, in a matrix of PS. The spot-to-spot spacing of the chemical patterns (L(s)) was varied between 0.9L(0) and 1.1L(0), where L(0) is the cylinder-to-cylinder spacing of the pure block copolymer in bulk. The homopolymer volume fraction of the blends (ϕ(H)) was varied between 0 and 0.3. In addition, chemical patterns were formed with selected spots missing from the perfect hexagonal array, such that the interpolation of domains between patterned spots could be examined on patterns where the polymer/pattern feature density ranged from 1:1 to 4:1. The assemblies were analyzed with top-down SEM, from which orientational order parameter (OP(o)) values were determined. The SEM analysis was complemented by Monte Carlo simulations, which offered insights into the shapes of the assembled cylindrical domains. It was found that, in comparison to pure block copolymer, adding homopolymer increased the range of L(s) values over which assemblies with high OP(o) values could be achieved for 1:1 assemblies. However, the corresponding simulations showed that in the 1:1 assemblies the shape of the cylinders was more uniform for pure block copolymer than for blends. In the case of the 4:1 assemblies, the range of L(s) values over which assemblies with high OP(o) values could be achieved was the same for all values of ϕ(H) tested, but the domains of the pure block copolymer had a more uniform shape. Overall, the results provided insights into the blend composition to be used to meet technological requirements for directed assembly with density multiplication.

Tailoring MWD of Bimodal Polyethylene in the Presence of Ziegler‐Natta Catalyst

AbstractSummary: Linear poly (ethylene‐co‐1‐butene) was produced through two‐step polymerization in one reactor using a Ziegler‐Natta catalyst, where in the first step, low molecular weight homopolymer of ethylene in the presence of hydrogen and in the next step, high molecular weight copolymer of ethylene with 1‐butene in the absence of hydrogen were produced. Molecular weight distribution of bimodal polyethylene was tailored through adjustment of polymerization time of each stage and hydrogen concentration of the first stage. Increasing hydrogen concentration shifted the molecular weight distribution curve to the lower molecular weights and broadened molecular weight distribution while interestingly increased high molecular weight incorporation of copolymer produced in the second stage due to increasing of reaction rate in the second step. To achieve bimodal molecular weight distribution, the polymerization times of the first and the second steps, which are highly dependent on the amount of hydrogen, were adjusted properly. The effects of the mentioned parameters on the processability as well as rheological properties of some samples were investigated. The rheological results showed shear thinning behavior of all specimens and confirmed the changes in molecular weight and molecular weight distribution. It was also demonstrated that the melt miscibility between low molecular weight and high molecular weight fractions improved with increasing of chains having very low molecular weight.

Homopolymer induced phase evolution in mesoporous silica from evaporation induced self-assembly process

In this paper, we report the experimental findings on the phase evolution of mesoporous silica in an evaporation induced self-assembly (EISA) approach by using low molecular weight homopolymer poly(propylene oxide) ( M n = 400, PPO400) as an additive. It is found that the addition of PPO400 can cause hydrotropic curvature-reducing effect on the mesostructures. Upon the increase of PPO400 addition amount, the structures of the obtained mesoporous silicas gradually evolve from three-dimensional (3-D) cubic Im 3 ¯ m to quasi- p6m, to hexagonal p6m, and to mixed structures consisting of lamellar and reversed cylindrical structure. The quasi- p6m mesoporous silica structure has the buckled cylindrical mesopores with intermediate curvature and acts as the metastable structure during the phase evolution from Im 3 ¯ m to p6m structure. We ascribe this curvature-reducing effect to the relatively weak hydrophobicity and good compatibility of PPO400 homopolymer. Moreover, the condition with larger homopolymer PPO4000 ( M n = 4000) as the additive has been also investigated and discussed.

Alkyne-Functional Polymers through Sonogashira Coupling to Poly(4-bromostyrene)

Alkyne-functional polymers and block copolymers were synthesized by postpolymerization modification of poly(4-bromostyrene) (PBrS) via palladium-catalyzed coupling with terminal alkynes. Several coupling methods, most notably bis(benzonitrile)palladium dichloride/tris-tert-butylphosphine catalyst systems at room temperature, were examined on well-defined polymers and copolymers of 4-bromostyrene prepared by nitroxide-mediated polymerization (NMP) with TEMPO. High conversions to alkyne-functional repeat units were obtained with low molecular weight homopolymers of PBrS (8−15 kg/mol) to afford alkyne-rich random copolymers of poly[(4-phenylethynyl)styrene]-co-poly(4-bromostyrene) and poly[(4-hexynyl)styrene]-co-poly(4-bromostyrene). Coupling reactions run on higher molecular weight polymers (70 kg/mol) or multiblock copolymers also gave high conversions to poly[(4-phenylethynyl)styrene], while coupling reactions with 1-hexyne suffered from side reactions. 1H NMR analysis of the coupled products suggests that up to 80% of terminal TEMPO groups are lost under standard coupling conditions. The polymers were purified from residual catalyst by treatment with tetrakis(hydroxymethyl)phosphonium chloride.

CO2 Foaming in Thin Films of Block Copolymer Containing Fluorinated Blocks

A fabricating method of nanocellular structures in single-layer polymeric thin films was developed using supercritical carbon dioxide and fluorinated copolymer templates as reported in our previous communication [Li, L.; Yokoyama, H.; Nemoto, T.; Sugiyama, K. Adv. Mater. 2004, 16, 1226−1229]. This paper presents the details of the fabrication of nanocellular thin films. The morphology in the as-cast films depends on the solvent casting process and plays an important role for the final structures after the following the scCO2 process. However, the nanocellular structures are not simply templated from the nanodomains; reconstruction of the structures during the CO2 processes is not negligible. The number density of nanocells is significantly smaller than that of PFMA nanodomains before the CO2 process. Starting from spherical domains followed by an scCO2 process at a pressure from 8 to 20 MPa, nanocells appear with an average diameter ranging from 15 to 30 nm. SEM cross-sectional views in addition to in-plane views of the nanocellular thin films prove that not open porous but closed cellular structures are formed in films irrespective of thickness. Such nanocellular formation is primarily due to swelling of PFMA domains followed by gentle removal of CO2 at reduced temperature, but expansion on depressurization still influences the size and its distribution. The size distribution of cells is effectively reduced either by lowering depressurization temperature or by adding a low molecular weight homopolymer.

Five-coordinate Schiff base complexes of gallium. Potential catalysts for the copolymerization of carbon dioxide and epoxides

We report the synthesis of the five-coordinate Schiff-base complexes, (cyc)salenGaCl, (cyc)salenGa(N 3), (et)salenGaCl, and (et)salenGa(N 3) ((cyc)salenH 2 = N,N′-bis-(3,5-di- tert-butylsalicylidene)-1,2-cyclohexyldiimine and (et)salenH 2 = N,N'-bis-(3,5-di- tert-butylsalicylidene)-1,2-ethylenediimine). The four complexes exhibit distorted square pyramidal geometry with the salen ligand forming the basal plane and the chloride or azide in the apical position. Copolymerization studies were conducted with cyclohexene oxide and carbon dioxide utilizing these derivatives as potential catalysts. In the presence of a Lewis-base co-catalyst, N-methylimidazole or phosphine, formation of polymer is not observed. In the absence of N-methylimidiazole, low molecular weight homopolymer is observed when the chloride species is the catalyst. This is in contrast to studies conducted with (cyc)salenCr(III)Cl, (et)salenCr(III)Cl, or azide derivatives thereof, which produced high-molecular-weight polycyclohexylcarbonate with CO 2 incorporation >99% in the presence of N-methylimidazole or phosphines. To cite this article: D.J. Darensbourg, D.R. Billodeaux, C. R. Chimie 7 (2004).

Tuning Substrate Surface Energies for Blends of Polystyrene and Poly(methyl methacrylate)

We compare the efficacies of three preparation methods intended to create in an easy and inexpensive way a nonpreferential surface for a two-component homopolymer blend of polystyrene and poly(methyl methacrylate). The first method is to physically absorb two different high molecular weight copolymers of styrene and methyl methacrylate onto hydroxylated silicon oxide. The second method consists of covalently bonding octyltrichlorosilane onto hydroxylated silicon oxide with gradient coverage to create a region with a neutral surface. The third method utilizes hydroxyl terminated, miscible, low molecular weight homopolymers of polystyrene and poly(methyl methacrylate) covalently attached to the substrate. We characterize the relative effectiveness of all three methods and their temporal stability using optical microscopy, atomic force microscopy, contact angle measurements, as well as near edge X-ray absorption fine structure (NEXAFS) microscopy and spectroscopy. The preparation methods explored should be extendable to a number of polymer systems.

HDPE/LLDPE reactor blends with bimodal microstructures—Part II: rheological properties

Reactor blends of polyethylene/poly(ethylene- co-1-octene) resins with bimodal molecular weight and bimodal short chain branching distributions were synthesized in a two-step polymerization process. The compositions of these blends range from low molecular weight (LMW) homopolymer to high molecular weight (HMW) copolymer and vice versa HMW homopolymer to LMW copolymer. The shear flow characteristics of these polymers in the typical processing range mostly depend on the molecular weight and MWD of the polymer and are independent of the short chain branch content. From oscillatory shear measurements, it was observed that the viscosity of HMW polymers was reduced with the addition of LMW material. For the polymers produced with this two-step polymerization process, the LMW homopolymer and HMW copolymer blends and HMW homopolymer and LMW copolymer blends were melt miscible, despite the large viscosity differences of the pure components.

HDPE/LLDPE reactor blends with bimodal microstructures—part I: mechanical properties

Reactor blends of polyethylene/poly(ethylene- co-1-octene) resins with bimodal molecular weight and bimodal short chain branching distributions were synthesized in a two-step polymerization process. The compositions of these blends range from low molecular weight (LMW) homopolymer to high molecular weight (HMW) copolymer and, vice versa, HMW homopolymer to LMW copolymer. The physical properties of the blends were found to be consistent with the nature of the individual components. For the tensile properties, the stiffness decreases with increasing the fraction of the copolymer, regardless of the molecular weight of the homopolymer fraction. For these blends with bimodal microstructures, it was confirmed that the degree of crystallinity governs the stiffness of the polymer. However, the energy dampening properties of the polymers benefit from the presence of the copolymer. A balance of stiffness and toughness can be obtained by altering the composition of the blends. For some blends, the presence of HMW homopolymer can dominate the tensile properties, showing little variation in the stiffness with increased addition of copolymer. It was also demonstrated that the testing conditions and thermal treatment of the polymer greatly influence the resulting elastic and energy dampening properties. Depending on the desired application, annealing these polymers (especially very low density copolymers) not only increases the crystallinity and stiffness, but also changes the frequency response of the dynamic mechanical properties.

Order−Disorder and Order−Order Transitions in Mixtures of Highly Asymmetric Triblock Copolymer and Low Molecular Weight Homopolymers

The order−disorder and order−order transitions (ODT and OOT) of a series of mixtures of a compositionally asymmetric polystyrene-block-polyisoprene-block-polystyrene copolymer (SIS) (fPS = 0.131) and low molecular weight PS homopolymers with different molecular weights have been investigated by using transmission electron microscopy (TEM), synchrotron small-angle X-ray scattering (SAXS), and rheology. ODT temperatures determined from the discontinuous change in 1/Imax and σq2 vs 1/T plots were found to be in reasonable proximity to the ones determined from the discontinuous change in log G‘ vs T plots. Various OOT's were also identified for a few of V4113/PS mixtures, as evidenced from both SAXS and rheology. This was further confirmed by TEM for the quenched samples. Spherical microdomains in liquidlike packing (LLP) observed for pure V4113 at low temperature were found not to be an equilibrium phase, which would transform completely to spheres in body-centered-cubic array (BCC) with prolonged annealing ...

Polymerization of Nitrophenyl and 3-Diethylaminophenyl Prop-2-yn-1-yl Ethers with PdCl2. Polymers and Copolymers Characterization

2-, 3- and 4-Nitrophenyl prop-2-yn-1-yl ethers were polymerized with PdCl2 in N,N-dimethylformamide (DMF) giving brown polymers soluble in DMF, dimethyl sulfoxide and N-methylpyrrolidone. Broad (mostly bimodal) molecular weight distributions were observed by size-exclusion chromatography (SEC) with maxima at molecular weight of about 4 . 103 and 1 . 105. With the same catalyst, 3-diethylaminophenyl prop-2-yn-1-yl ether gave predominantly low molecular weight homopolymer (Mn = 4 200) soluble in DMF, N-methylpyrrolidone, CHCl3 and tetrahydrofuran-soluble copolymers with 4-nitrophenyl prop-2-yn-1-yl ether (Mn about 3 000). IR, NMR and UV-VIS spectra of prepared polymers and copolymers proved the structure to consist of the polyene-type main chain with phenoxymethyl pendant groups bearing nitro or diethylamino substituents.

Modeling the Fractionation Process in TREF Systems. III. Model Validation With Low Molecular Weight Homopolymers

Low molecular weight semicrystalline homopolymers are used as a model system for temperature rising elution fractionation (TREF) analysis. An already proposed thermodynamic model for TREF analysis is used to characterize TREF fractions from low molecular weight polyethylenes Mn = 500 to 3000 and some of their mixtures. In this molecular weight range it is possible, under appropriate crystallization, conditions, to form extended-chain crystals, and therefore lamellar thicknesses become comparable to extended-chain lengths. Lamellar thicknesses calculated from TREF spectra permit calculations of the molecular weights of the fractions, up to a limit of about 142 CH2, where partially folded-chain crystallites appear under these operating conditions. Also homopolymers blends are fractionated and the TREF spectra analyzed to test model predictions. It is shown that appearance of chain folding may set a resolution limit to the analysis of commercial copolymers by TREF. © 1996 John Wiley & Sons, Inc.

Self-Ordering of Diblock Copolymers from Solution

It is shown experimentally that the formation of two-dimensional micelles on a solid substrate is a reversible equilibrium process in very concentrated polymer solutions. The micelles form rapidly in the dipping process when the equilibrium concentration is reached. As long as the solvent is present, the polymer chains are mobile and can organize into the specific micelle structure that minimizes the total free energy. Once the solvent has completely evaporated, the polymer becomes glassy and the micelle structures are frozen. The lamellar-cylindrical-spherical transition, upon selectively swelling one of the blocks with low molecular weight homopolymer, is quantitatively similar to the results reported by Hashimoto using bulk diblock copolymers and was explained in terms of balancing the gain in conformational free energy with the cose of bending the interface. A similar mechanism seems to be the driving force in the thin films reported here, except that in this case one must also consider the absorption energies acting at the liquid and solid interfaces in order to obtain a quantitative description. 10 refs., 3 figs., 1 tab.

Endgroup analysis of polyethylene glycol polymers by matrix-assisted laser desorption/ionization Fourier-transform ion cyclotron resonance mass spectrometry.

Fourier-transform ion cyclotron resonance mass spectrometry (FTICR-MS) by external injection of matrix-assisted laser desorbed and ionized (MALDI) polymers offers good possibilities for characterization of low molecular weight homopolymers (MW range up to 10 kDa). The molecular masses of the molecular weight distribution (MWD) components of underivatized and derivatized (dimethyl, dipropyl, dibutyl and diacetyl) polyethylene glycol (PEG) 1000 and 4000 were measured by MALDI-FTICR-MS. These measurements have been performed using a commercial FTICR spectrometer with a home-built external ion source. MALDI of the samples with a 2,5-dihydroxybenzoic acid matrix in a 1000:1 matrix-to-analyte molar ratio produces sodiated molecules in a sufficient yield to trap the ions in the ICR cell. The masses of the molecular weight distribution of PEG components were measured in broad-band mode with a mass accuracy of < 5 ppm in the mass range around 1000 u and within 40 ppm accuracy around 4000 u. From these measurements, the endgroup mass of the polymer was determined by correlation of the measured component mass with the degree of polymerization. The masses of the PEG endgroups have been determined within a deviation of 3-10 millimass units for the PEG1000 derivatives and 10-100 millimass units for the PEG4000 derivatives, thus confirming the identity of the distal parts of the model compounds.

Sorption of water vapor by hydrophilic polyurethane‐ureas

AbstractSegmented polyurethane‐ureas were prepared from low‐molecular weight homopolymers or triblock (PEP) copolymers of ethylene and propylene oxides and 4,4′‐diphenylmethane diisocyanate with hydrazine as chain extender. Equilibrium sorption of water vapor at 23°C was measured at eight to nine values of relative vapor pressure, 0.08 ≤ p/p0 ≤ 0.97. The effect of the structure (composition and length) of soft segments was studied. Sorption capacities were found to be lower than the reference values computed under the assumption that the contributions to sorption by the chain constituents are additive. This finding was assigned to the shielding effect of hard segment domains on the interaction of water molecules with hydrophilic (ethoxamer) groups of soft segments. © 1993 John Wiley &amp; Sons, Inc.

Viscoelastic behavior, phase equilibria, and microdomain morphology in mixtures of a block copolymer and a homopolymer

Mixtures of a polystyrene-block-polybutadiene-block-polystyrene copolymer (Kraton 1102) and a low molecular weight homopolymer (polystyrene or poly(α-methylstyrene) with MW ranging from 1200 to 9900) were studied. Dynamic viscoelastic measurements showed that the solubility limits of pαMS in the block copolymer are greater than those of PS. Phase diagrams were constructed experimentally for Kraton 1102/PαMS, which allowed to determine the boundary between the mesophase and the homogeneous phase. Cloud point curves for liquid-liquid phase separation were determined. The results were compared with predictions made by the theory of Noolandi and coworkers