Linear Chain Extension Research Articles

Material-Preserving Extrusion of Polyamide on a Twin-Screw Extruder.

In the context of plastics recycling, plastics are processed several times. With each new melting and extrusion the plastic is damaged, which can have a negative effect on product properties. To counteract material damage, special additives such as chain extenders can be used, which are intended to lead to post-polymerization during processing. A linear chain extension is important here, as branching and crosslinking can lead to uncontrolled changes in the plastic's properties. To investigate the suitability of specialized linear chain extenders for polyamides, a polyamide-6 was processed several times and the molar mass distribution was evaluated after each extrusion cycle. Three series of tests were carried out. First, the plastic was regranulated five times without additives and twice with different concentrations of chain extenders on a twin-screw extruder. The results of the study show that not only can molar mass degradation be prevented with the appropriate additive, it is even possible to achieve a material buildup during processing. In our experiments, the polydispersity of the molar mass distribution remained nearly identical despite multiple extrusions. Thus, reactive extrusion makes it possible for the corresponding plastics to be processed several times without the molar mass decreasing. If a sufficiently pure material flow can be ensured during recycling, the number of possible reprocessings of the plastic can be significantly increased without the need to add virgin material.

Tunable Photonic Materials via Monitoring Step‐Growth Polymerization Kinetics by Structural Colors

AbstractThe functional and responsive properties of elastomeric materials highly depend on crosslink density and molecular weight between crosslinks. However, tedious analytical steps are needed to obtain polymer network structure–property relationships. In this article, an in situ structure–property characterization method is reported by monitoring the structural color change in a photonic elastomeric material. The photonic materials are prepared in a two‐step polymerization process. First, linear chain extension occurs via Michael addition. Second, photopolymerization ensures crosslinking, resulting in the formation of an elastomeric photonic network. During the first step, the step‐growth polymer process can be monitored by following the photonic reflection band redshift, allowing to program the molecular weight between the crosslinks. During network formation, the crosslink density, chain length between crosslinks, and the colors are “frozen in.” These processes can be locally controlled creating both single‐layered multicolor patterned and broadband reflective coatings at room temperature. The scalability of the coating process is further demonstrated by using a gravure printing technique. Additionally, the final coatings are made responsive toward specific solvents and temperature. Here the modulus, response, and color of the coating are controlled by tuning the crosslink density and molecular weight between crosslinks of the elastomeric material.

Synthesis and characterization of chain-extended and branched polyurethane copolymers as form stable phase change materials for solar thermal conversion storage

In this work, polyethylene glycol (PEG) based polyurethane copolymer was synthesized using linear chain extension and branching methods. In linear chain extension, multiple PEG molecules were linked using urethane bonds. In branching, polyvinyl alcohol (PVOH) backbone was grafted with PEG molecules as branches and cross-linked to other PVOH molecules. 4,4′-Diphenylmethane di-isocyanate (MDI) was used as both chain extender and cross-linker. The resulting copolymers were characterized for chemical composition, microscopic morphology, crystalloid structure and thermal properties. Based on the results from characterizations, a detailed discussion on the phase change mechanism in polyurethanes is presented. 1H NMR and FT-IR results confirmed the chain extension and branching. 1H NMR spectra displayed characteristic signal of the urethane. DSC results revealed solid-solid phase change behavior of copolymers with high latent heat. Crystalline nature of spherulite lamellae structures was verified by observing birefringence using POM microscope and XRD peaks. Polyurethane copolymers had maximum latent heat absorption capacity of 144.14 kJ kg−1 at melting temperature of 51 ℃ and had maximum heat release capacity of 117.5 kJ kg−1 at freezing temperature of 33 ℃. Polyurethane copolymers were thermally stable up to 200 ℃. Polyurethane copolymers satisfied all criteria for use as form stable solid-solid phase change material (SSPCM) in solar thermal conversion storage.

Functional siloxanes with photo-activated, simultaneous chain extension and crosslinking for lithography-based 3D printing

A novel, poly(dimethyl siloxane)-based photopolymer that exhibits simultaneous linear chain extension and crosslinking was suitable for vat photopolymerization additive manufacturing. Photopolymer compositions consisted of dithiol and diacrylate functional poly(dimethyl siloxane) oligomers, where simultaneous thiol-ene coupling and free radical polymerization provided for linear chain extension and crosslinking, respectively. Compositions possessed low viscosity before printing and the modulus and tensile strain at break of a photocured, higher molecular weight precursor after printing. Photorheology and soxhlet extraction demonstrated highly efficient photocuring, revealing a calculated molecular weight between crosslinks of 12,600 g/mol and gel fractions in excess of 90% while employing significantly lower molecular weight precursors (i.e. < 5300 g/mol). These photocured objects demonstrated a 2× increase in tensile strain at break as compared to a photocured 5300 g/mol PDMS diacrylamide alone. These results are broadly applicable to the advanced manufacturing of objects requiring high elongation at break.

Dual ionic cross-linked double network hydrogel with self-healing, conductive, and force sensitive properties

Conventional chemically cross-linked double network hydrogels (DN hydrogels) are considered as one of the toughest soft materials. However, they lack self-healing properties due to their non-reversible covalent bonds. In this work, we propose a new design strategy to synthesize a dual ionic cross-linked polyethylene glycol (PEG)/poly(acrylamide-co-acrylic acid) (PAMAA) DN hydrogels, consisting of an ionic cross-linked linear chain extension of PEG and a PAMAA-Fe3+ as the first and second physical networks, respectively. Due to the reversible non-covalent bonds in both networks (Fe3+ coordination interactions and hydrogen bonds), the DN hydrogels can achieve high mechanical properties (σf of ∼0.36 MPa and strain of ∼1350%) and complete self-healing ability within 12 h. Moreover, due to the free ions diffusion in the water medium of the DN hydrogels, the as-prepared hydrogels also exhibited electrical conductivity and stretch sensitivity. Our results offer a new design strategy to improve the strength and self-healing ability of DN hydrogels by introducing a dual ionic cross-linking within the network, which we hope will provide an alternative focus for the design of tough and self-healing hydrogels with desirable properties.

Reactive toughening of injection-molded polylactide pieces using maleinized hemp seed oil

The present study describes the effect of maleinized hemp seed oil (MHO) on the physical performance of polylactide (PLA) pieces. To this end, PLA pieces with varying MHO contents in the 0–10 wt% range were manufactured by twin-screw extrusion (TSE) followed by injection molding. The resultant pieces were characterized in terms of their mechanical, thermal, and thermomechanical properties. The obtained properties suggested that, unlike typical plasticizers, MHO does not only induce an increment in elongation at break and impact resistance but it also enhances both elastic modulus and tensile strength. In addition, a moderate decrease in the glass transition temperature (Tg) was observed. This was ascribed to simultaneous linear chain-extension, branching, and/or cross-linking phenomena due to the reaction of the multiple maleic anhydride (MAH) groups present in MHO with the terminal hydroxyl groups of the PLA chains. Furthermore, morphology characterization revealed that, though certain phase separation occurred at its highest content, MHO was finely dispersed as submicron droplets within the PLA matrix contributing to improving toughness. The use of multi-functionalized reactive vegetable oils thus represents a highly sustainable solution to reduce the intrinsic brittleness of PLA materials without compromising their mechanical resistance and the toughened biopolymer pieces described herein can find interesting applications in, for instance, rigid packaging.

Investigations on interactions between titanium alkoxides and α,ω-dihydroxy PDMS by a combination of rheology and spectroscopy

The phenomena involved in the reactions between α-ω-dihydroxy PDMS and titanium tetraalkoxides, Ti(OR)4, were investigated by a combination of rheology and spectroscopy (NMR, IR). The increase of viscosity observed mainly originates from a linear chain extension of PDMS chains by formation of TiOSi bonds. This is mainly related to an apparent functionality of 2 of Ti(OR)4. The products formed evolve with time and are sensitive to hydrolysis due to the poor stability of TiOSi bonds. The precise understanding of the reactions involved raises interesting perspectives for the crosslinking of α-ω-dihydroxy PDMS and Si(OR)4 derivatives using titanium tetraalkoxide based catalysts.

A novel pH-responsive supramolecular polymer constructed by pillar[5]arene-based host–guest interactions

A novel pH-responsive supramolecular polymer based on the pillar[5]arene/imidazolium cation recognition motif was successfully prepared. It was demonstrated that the binding strength between the monomer molecules in solution could be reduced by adding base, thereby leading to a decrease in the polymerization degree, while a reverse switch could be achieved by the addition of acid to change the monomer back to the cationic state. These processes were confirmed by 1H NMR, DOSY experiments, viscosity measurements and theoretical calculations. Moreover, a rod-like fiber was drawn from a high concentration solution of the monomer and characterized by SEM, providing direct evidence of the formation of a supramolecular polymer with high molecular weight and high degree of the linear chain extension. In this study, a reversible transition between the high-molecular-weight supramolecular polymer and the relatively low-molecular-weight one, that is, a control of the polymerization degree, was realized by simply altering the solution pH. These results are helpful for the future fabrication of adaptive and smart supramolecular materials with stimuli-responsiveness and special functions.

Efficient chemoenzymatic oligosaccharide synthesis by reverse phosphorolysis using cellobiose phosphorylase and cellodextrin phosphorylase from Clostridium thermocellum

Inverting cellobiose phosphorylase (CtCBP) and cellodextrin phosphorylase (CtCDP) from Clostridium thermocellum ATCC27405 of glycoside hydrolase family 94 catalysed reverse phosphorolysis to produce cellobiose and cellodextrins in 57% and 48% yield from α- d-glucose 1-phosphate as donor with glucose and cellobiose as acceptor, respectively. Use of α- d-glucosyl 1-fluoride as donor increased product yields to 98% for CtCBP and 68% for CtCDP. CtCBP showed broad acceptor specificity forming β-glucosyl disaccharides with β-(1→4)- regioselectivity from five monosaccharides as well as branched β-glucosyl trisaccharides with β-(1→4)-regioselectivity from three (1→6)-linked disaccharides. CtCDP showed strict β-(1→4)-regioselectivity and catalysed linear chain extension of the three β-linked glucosyl disaccharides, cellobiose, sophorose, and laminaribiose, whereas 12 tested monosaccharides were not acceptors. Structure analysis by NMR and ESI-MS confirmed two β-glucosyl oligosaccharide product series to represent novel compounds, i.e. β- d-glucopyranosyl-[(1→4)-β- d-glucopyranosyl] n-(1→2)- d-glucopyranose, and β- d-glucopyranosyl-[(1→4)-β- d-glucopyranosyl] n-(1→3)- d-glucopyranose ( n = 1–7). Multiple sequence alignment together with a modelled CtCBP structure, obtained using the crystal structure of Cellvibrio gilvus CBP in complex with glucose as a template, indicated differences in the subsite +1 region that elicit the distinct acceptor specificities of CtCBP and CtCDP. Thus Glu636 of CtCBP recognized the C1 hydroxyl of β-glucose at subsite +1, while in CtCDP the presence of Ala800 conferred more space, which allowed accommodation of C1 substituted disaccharide acceptors at the corresponding subsites +1 and +2. Furthermore, CtCBP has a short Glu496-Thr500 loop that permitted the C6 hydroxyl of glucose at subsite +1 to be exposed to solvent, whereas the corresponding longer loop Thr637–Lys648 in CtCDP blocks binding of C6-linked disaccharides as acceptors at subsite +1. High yields in chemoenzymatic synthesis, a novel regioselectivity, and novel oligosaccharides including products of CtCDP catalysed oligosaccharide oligomerisation using α- d-glucosyl 1-fluoride, all together contribute to the formation of an excellent basis for rational engineering of CBP and CDP to produce desired oligosaccharides.

Chain Extension of Oligoamide or Polyamide in an Extruder

Polycondensates with too short chains can be chain-extended with repair molecules. In this report, the molecular weight and, thereby, the melt viscosity of polyamides is enhanced in the extruder by two chemically very different types of repair molecules. Firstly, the chains were extended linearly with a bisoxaline which is a well-known strategy. Secondly, polyamide chains were bundled by grafting onto a functionalized backbone chain, i. e., maleinated polystyrene (SMA). This less known route leads to branched rather than linear chain extension. Despite the fact that the melts of branched chains are less viscous than those of linear chains, at the same molecular weight, the viscosity of polyamides was markedly increased by grafting onto SMA. Grafting of oligoamide and polyamide chains was studied, as well as grafting of the monomer of the polyamide in combination with its polycondensation.

Study on the Shape Memory Polyamides. Synthesis and Thermomechanical Properties of Polycaprolactone-Polyamide Block Copolymer

Poly(caprolactone)(PCL)-polyamide multiblock copolymers were synthesized by the linear chain extension of PCL diol and diamine-terminated polyamide oligomer with hexamethylene diisocyanate in dimethylformamide. The thermal and mechanical properties of these block copolymers were examined and compared with the results in our previous papers about thermoplastic polyurethane elastomers (TPEs). PCL-polyamide block copolymers showed enhanced microphase separation and lower degree of fatigue by repeated deformation in shape memory behavior, and had higher rubbery plateau modulus, compared with TPEs.

Nonionic Water-Soluble Polysilynes. Synthesis and Properties of a Novel Class of Functionalized Materials

The nonionic water-soluble polysilynes poly(4,7,10-trioxaundecylsilyne) (1) and poly(4,7,-10,13-tetraoxatetradecylsilyne) (2), which are inaccessible using the conventional Wurtz-type coupling with Na in refluxing toluene, have been prepared in reasonable yields using graphite potassium C 8 K as the reducing agent in THF at 0 °C. A water-insoluble analogue of 1, viz. poly(4,7,10-trioxahexadecylsilyne) (3), is obtained in nearly quantitative yield under similar conditions. Despite the fact that 1 and 2 possess all the characteristic polysilyne-like (photo)physical properties, aqueous solutions of 1 and 2 unexpectedly exhibit thermoresponsive behavior; i.e., at 49 °C a lower critical solution temperature (LCST) is found. The presence of an LCST, which has to originate from folding/unfolding processes of the polysilyne backbone, suggests that polysilynes have a hybrid structure with a predominantly one-dimensional overall appearance consisting of linear fragments with small branches and/or incorporated (branched) cyclics, instead of the previously proposed extended sheetlike and/or hyperbranched/dendritic structures. Additional support for a hybrid structure was given by semiempirical PM3 calculations on a variety of oligomeric model compounds. The PM3 results suggest that Si-Cl moieties incorporated in oligomers will be more reactive than monomeric Si-Cl groups. The calculations further indicate that linear chain extension is preferred over branching. Cyclic voltammetry in combination with absorption/excitation spectroscopy reveals that in going from the related polysilane to the polysilyne the valence band edge shifts ca. -0.7 V, while the conduction band edge remains virtually unchanged. Furthermore, it is demonstrated that polysilynes 1 and 2 are effective photoinitiators for radical polymerizations upon excitation at λ 400 nm. This is exemplified for the conversion of methylacrylate into poly(methylacrylate).

Morphology of liquid crystalline epoxy composite matrices based on the diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene

The morphologies of various diglycidyl ethers of 4,4′-dihydroxy-α-methylstilbene-based liquid crystalline epoxy (LCE) formulations have been studied as matrices for high performance composites. Reflected light optical microscopy, selected area electron diffraction, micro-Raman spectroscopy and transmission electron microscopy of thin sections and replicas of etched surfaces were employed to probe both the micrometer-scale and nanometer-scale LCE morphology in infusion moulded, uni-weave graphite fibre composites. The results suggest that various sizes and shapes of LCE domains can be formed in the matrix resin, depending on the cure schedule and the LCE curative composition. Preferred molecular orientation along the graphite fibres can be achieved if the LCE resin is cured so as to promote extensive linear chain extension, thus giving a high concentration of mesogenic segments. Approaches for aligning mesogenic segments in the composite are discussed as well as the potential benefits of doing so.

Linear chain extension through associative termini

Poly(tetramethy1ene oxide) (PTHF) oligomers, Mn = 650-2000, have been capped with ph- thalate ester (2) and with phthalic half-ester (3). Properties of the latter are strongly influenced by hydrogen- bonding association of carboxylic acid termini. DSC revealed new, high-temperature transitions in the carboxyl- terminated oligomers (3) which decreased in temperature as PTHF molecular weight increased. Ester- terminated oligomers 2 have the rheological properties of simple liquids. Oligomers 3 exhibit melt viscosities and dynamic moduli which decrease sharply at the highest DSC transition temperature but nevertheless exceed those of 2 over the entire range studied. These carboxyl-terminated oligomers exhibit elasticity- dominated responses to small strains with a linear viscoelastic region and good recovery to large strain amplitude oscillatory shear. All observations are consistent with formation of large crystalline aggregates of carboxyl-terminated chain ends below the high-temperature DSC transition. Solid-state WNMRsuggests the presence of phthalic half-ester end groups in both crystalline and amorphous environments.

Polyamic alkyl esters: versatile polyimide precursors for improved dielectric coatings

Several approaches leading to high-solids-content polyimide precursor formulations and polyimides with multiphase morphologies have been successfully explored. These approaches were made possible by utilizing poly(amic alkyl esters), which are soluble polyimide precursors devoid of hydrolytic instability and which can be designed to imidize at significantly higher temperatures than their poly(amic acid) counterparts. The hydrolytic stability of these derivatives provides great synthetic flexibility due to solubility considerations, polymer work-up procedures, and polymer modification schemes. The synthetic flexibility is complemented by higher imidization temperatures, which provide the necessary processing window to permit the use of a condensation-chemistry-based, linear chain-extension scheme. In addition, higher imidization temperatures may further advantageously influence the development of well-defined microphase-separated morphologies in the case of block copolymers and polyimide blends. >

Influence of electron‐donor‐acceptor complex formation on the melt viscosity of some poly(dimethylsiloxane)s

AbstractA series of poly(dimethylsiloxane)s was synthesized containing either electron donor or electron acceptor groups, statistically arranged along the polymer chain or only at the chain ends (telechelic polymers). Blending of the donor and acceptor modified statistical copolymers led to a strong increase of the melt viscosity as a consequence of complex formation. In contrast to this, complex formation had no significant influence on the viscosity in blends from telechelic polymers. This different behaviour seems to be caused by the different molecular architecture favouring crosslinking by complex formation for the statistical copolymers and a linear chain extension for the telechelic polymers.