Zusammenhänge zwischen Glasübergang,thermodynamischen und mechanischen Größen als Möglichkeit zur Voraussage über Materialeigenschaften aus der chemischen Struktur

In a recent paper the calculation of the glass transition temperature T_g of linear and crosslinked polymers from the cohesive energy E_coh normalized to the number of structural elements \documentclass{article}\pagestyle{empty}\begin{document}$\sum\limits_{\rm i} {{\rm a}_{\rm i} } $\end{document} capable of rather independent motions was described. In addition to this work, linear relations were found between T_g and the normalized molar heat C_p as well as the normalized molecular weight MW_Seg of the repeating unit. Hence linear correlations follow between C_p, E_coh, and MW_Seg without further normalization. So the cohesive energy, which is difficult to measure, may be determined from experimental values of the molar heat or even Tg. By applying the above mentioned relations a connection of thermodynamicquantities with mechanical properties was found, e. g. the ratio of specific cohesiveenergy vs. density with the torsional modulus or the yield strength of crosslinkedepoxide resins at 298 K.     

Structure–property relationships in photopolymerizable polymer networks: Effect of composition on the crosslinked structure and resulting thermomechanical properties of a (meth)acrylate‐based system

Photoinitiated polymer networks were formed by copolymerization of tert‐butyl acrylate with di(ethylene glycol) dimethacrylate (DEGDMA) or poly(ethylene glycol) dimethacrylate (PEGDMA). The degree of crosslinking was systematically varied by modifying the weight fraction and molecular weight of the dimethacrylate crosslinking agent. An increase in effective crosslink density with increasing crosslinking agent concentrations was confirmed by decreasing equilibrium swelling ratios (q) and increasing rubbery moduli (E_R). Glass transition temperatures (T_g) varied from ?22 to 124°C, increasing with increasing DEGDMA content and decreasing with increasing PEGDMA content. Tensile deformation behavior (at T_g) ranged from an elastomeric‐like large‐strain response for lightly crosslinked materials to a small‐strain brittle response for highly crosslinked networks. At low crosslinking levels, the strain‐to‐failure of the network polymers decreased quickly with increasing crosslinking agent concentration. The stress at failure demonstrated a more complex relationship with crosslinking agent concentration. The effect of composition on network structure and resulting properties (q, E_R, strain‐to‐failure) decreased as the crosslinking agent concentration increased. The results reveal trade‐offs in T_g, E_R, strain‐to‐failure, and failure stress with composition and network structure, and are discussed in light of the wide range of potential applications suggested in the literature for (meth)acrylate‐based photopolymerizable polymer networks including biomaterials and shape‐memory polymers. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis and characterization of cross‐linkable poly(phthalazinone ether ketone)s

A novel class of crosslinkable poly(phthalazinone ether ketone)s with relative high molecular‐weight and good solubility were successfully synthesized by the copolymerization of bisphthalazinone containing monomer, 3,3′‐diallyl‐4,4′‐dihydroxybiphenyl and 4,4′‐di‐ fluorobenzophenone. The synthesized polymers with inherent viscosities in the range of 0.42 to 0.75 dL/g can form flexible and transparent membranes by casting from their solution. The crosslinking reaction of these polymers can be carried out by thermally curing of the virgin polymers in or without the presence of crosslinking agent. The experimental results demonstrated that the crosslinking reaction also occurred to some extent during the polymerization. The crosslinked polymers exhibited equivalent glass transition temperature (T_g) at lower crosslinking density, and showed higher T_g than virgin polymers at higher crosslinking density. The crosslinked high‐temperature polymer can be used as the base material for high temperature adhesive, coating, enamel material, and composite matrices. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

A structure–property rationalization of some poly(carboxyl–epoxy) thermosets

The glass transition temperatures, T_g, and strength properties have been determined for an acrylic-based prepolymer containing 25 mole-% carboxyl functionality crosslinked with five different diepoxides. The T_g values vary from 100° to 200°C, and ultimate elongations of 2% to 7% at 25°C are observed with the different crosslinking agents. These variations are rationalized in terms of the structural elements present in the diepoxides. Thermosets possessing rigid structural units at the crosslink points connected by flexible segments have the best all-around combination of T_g and tensile properties. A decline in these properties was noted when the epoxide/carboxyl ratio exceeded unity owing to the formation of mixed networks and free chain ends.     

Network properties: 3. A pulsed n.m.r. study of molecular motions in some AB-crosslinked polymers

This paper presents the results of a pulsed n.m.r. study of molecular motions in poly (methyl methacrylate) (PMMA) and poly (methyl acrylate) (PMA) chains in a series of multicomponent network polymers consisting of poly (vinyl trichloroacetate) (PVTCA) crosslinked with PMMA and with PMA, with emphasis on segmental motions. Results of ancilliary broad line n.m.r. and dilatometric studies are included; the latter demonstrate that in PMA containing polymers microphase separation of the components is complete while in PMMA containing polymers a mixed microphase of PVTCA and PMMA and a pure PMMA microphase are formed. α-Methyl group rotations in PMMA chains and segmental motions in both PMMA and PMA chains are modified with respect to those in the corresponding homopolymers. Modifications to the segmental motions in the crosslinking chains are attributed to the fact that their chain ends are attached to PVTCA chains. It is considered that the comparative rigidity of PVTCA chains (T_g ～ 60°C) reduces segmental motions in at least portions of the PMA chains (T_g ～ 5°C) while the comparative mobility of PVTCA enhances segmental motions in PMMA (T_g ～ 100°C). Thus the molecular mobility of chains of one polymer is to some extent transmitted to chains of another polymer to which it is attached.     

A relationship between the glass transition temperature (Tg) and fractional conversion for thermosetting systems

An equation, based on thermodynamic considerations to relate the glass transition temperature, T_g, to compositional variation of a polymer system, is adapted in this article for modeling the T_g vs. fractional conversion (x) relationship of reactive thermosetting systems. Agreement between the adapted equation and experimental T_g vs. x data is found for several thermosetting crosslinking systems (i.e., epoxies and cyanate ester/polycyanurate) as well as for reactive thermosetting linear polymer systems (i.e., polyamic acid and esters to polyimides). The equation models the experimentally obtained T_g vs. x behavior of thermosetting systems which include competing reactions. Agreement for widely varying molecular structures demonstrates the generality of the equation. The entire T_g vs. x relationship can be predicted for a thermosetting material by using the T_g vs. x equation and the values of the initial glass transition temperature, T_g0, the fully reacted system glass transition temperature, T_g∞, and the ratio of the change in specific heat from the liquid or rubbery state to the glassy state (Δc_p) at T_g0 and T_g∞, Δc_p∞/Δc_p0. The values of T_g0, T_g∞, and Δc_p∞/Δc_p0 can be measured generally from two differential scanning calorimetric experiments. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 3–14, 1997     

Mesomorphic properties of cholesteric side‐chain liquid crystalline polysiloxanes and elastomers

A series of crosslinked liquid crystalline polymers and corresponding uncrosslinked liquid crystalline polymers were prepared by graft copolymerization. Their liquid crystalline properties were characterized by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction measurements. The results showed that the crosslinking obtained in the isotropic state and the introduction of nonmesogenic crosslinking units into a polymeric structure could cause additional reduction of the clearing point (T_i) of the crosslinked polymers, compared with the corresponding uncrosslinked polymers. The crosslinked polymers (P‐2–P‐4) with a low crosslinking density exhibited cholesteric phases as did the uncrosslinked polymers. In contrast, a high crosslinking density made the crosslinked polymer P‐5 lose its thermotropic liquid crystalline property. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 773–778, 2004     

Crosslinking of polystyrene by mono- and difunctional agents

Slightly crosslinked polystyrene networks are preferable to linear polystyrene in commercial uses where increased thermal properties are favoured. A novel method of production of macrocrosslinked networks has been developed by reaction of polystyrene with mono- and difunctional derivatives of p-xylene, durene and oligomeric chains (n<10) thereof. The reaction system consists of a common organic solvent such as acetic acid or butyl acetate and a catalyst such as H₂SO₄ or HClO₄; the reaction temperature varies between 60°C and 100°C. The degrees of crosslinking and thermal properties of the produced networks depend on the reaction system, the molar ratio of polymer to crosslinking agent and the reactivity of the functional groups; the gelation time varies between 3–12 hours for a fully crosslinked network. Promotion of the formation of regular structure networks without branches in the chains between crosslinks is achieved by the use of difunctional monomers, which favour the formation of linear polybenzylene chains between the polystyrene chains. Use of monofunctional monomers leads usually to branched and slightly crosslinked or grafted polystyrene. In both cases the regions of T_g and T_m increase up to 115°C and 350°C respectively as judged by DSC analysis. This novel crosslinking method has been also applied to crosslinking of copolymers of polystyrene and polymeric chains with aromatic structure in their backbone chain.     

Gas and vapor sorption in polymers just below Tg

Generally, sorption isotherms for gases like CO₂ in glassy polymers are concave to the pressure axis, whereas in the rubbery state these isotherms are linear for gases or sometimes convex to the pressure axis for more condensable vapors. Examples of CO₂ isotherms are reported here that show at low pressure the curvature characteristic of glasses and then become linear at higher pressures. This is observed when the glass transition temperature T_g is not much greater than the observation temperature T, and plasticization of the polymer by sorbed CO₂ causes T_g to become equal to T within the range of pressures employed in the isotherm measurement. For the sorption of vapors in glassy polymers, this can lead to sigmoidal isotherms, as discussed using an illustration from the literature.     

Atomistic molecular simulations of structure and dynamics of crosslinked epoxy resin

Many excellent thermal and mechanical performances of cured epoxy resin products can be related to their specific network structure. In this work, a typical crosslinked epoxy resin was investigated using detailed molecular dynamics (MD) simulations, in a wide temperature range from 250 K to 600 K. A general constant-NPT MD procedure widely used for linear polymers failed to identify the glass transition temperature (T_g) of this crosslinked polymer. This can be attributed to the bigger difference in the time scales and cooling rates between the experiments and simulations, and specially to the highly crosslinked infinite network feature. However, by adopting experimental densities appropriate for the corresponding temperatures, some important structural and dynamic features both below and above T_g were revealed using constant-NVT MD simulations. The polymer system exhibited more local structural features in case of below T_g than above T_g, as suggested by some typical radial distribution functions and torsion angle distributions. Non-bond energy, not any other energy components in the used COMPASS forcefield, played the most important role in glass transition. An abrupt change occurring in the vicinity of T_g was also observed in the plots of the mean squared displacements (MSDs) of the crosslinks against the temperature, indicating the great importance of crosslinks to glass transition. Rotational dynamics of some bonds in epoxy segments were also investigated, which exhibited great diversity along the chains between crosslinks. The reorientation functions of these bond vectors at higher temperatures can be well fitted by Kohlrausch-Williams-Watts (KWW) function.     

Facile tailoring of thermal transition temperatures of epoxy shape memory polymers

A critical parameter for a shape memory polymer (SMP) lies in its shape memory transition temperature. For an amorphous SMP polymer, it is highly desirable to develop methods to tailor its T_g, which corresponds to its shape memory transition temperature. Starting with an amine cured aromatic epoxy system, epoxy polymers were synthesized by either reducing the crosslink density or introducing flexible aliphatic epoxy chains. The thermal and thermomechanical properties of these epoxy polymers were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). All the crosslinked epoxy polymers with T_g's above room temperature were found to possess shape memory properties. Overall, our approach represents a facile method to precisely tune the T_g of epoxy SMP polymers ranging from room temperature to 89 °C.     

Polyurethane–polyacrylate interpenetrating polymer networks. II

Two component topologically interpenetrating polymer networks of the SIN type (simultaneous interpenetrating networks) composed of a melamine-cured polyacrylate and three different polyether-based polyurethanes were prepared. The linear polymers and prepolymers were combined in solution, together with the necessary crosslinking agents and catalysts, films were cast and subsequently chain extended and crosslinked in situ. In all cases, maxima in tensile strength significantly higher than the tensile strengths of the component networks occurred at 50% polyurethane : 50% polyacrylate. This was explained by an increase in crosslink density resulting from interpenetration. One of the interpenetrating polymer networks showed only one glass transition temperature (T_g) (measured calorimetrically) intermediate in temperature to the T_g's of the components and as sharp as the component T_g's. This is indicative of phase mixing and indicates at least partial chain entanglement (interpenetration). Some enhancement of other physical properties was also noted.     

Synthesis and characterization of high molecular weight metaloquinolate‐containing polymers

In this article, a new method to synthesize novel metaloquinolate (AlQ₃, ZnQ₂)‐containing polymers is reported. A model polymer with 8‐hydroxyquinoline ligands can be obtained by free‐radical copolymerization with methyl methacrylate (MMA), then metaloquinolate (AlQ₃, ZnQ₂)‐containing polymers are prepared by coordinating reaction with di(8‐hydroxyquinoline) aluminum (AlQ₂) chelates or mono (8‐hydroxyquinoline) zinc (ZnQ) chelates without crosslinking. The structures of products are confirmed by NMR, FTIR, ultraviolet‐visible, elementary analysis, photoluminescence spectrum, and gel permeation chromatography analysis. They are soluble in common solvents and suitable to form films. The use of AlQ₂ and ZnQ avoided the crosslinking caused by the AlQ₃, ZnQ₂ formation between different polymer chains. Different from the traditional small organic molecules in organic light‐emitting diodes (OLEDs) fabrication, the polymer can be processed by spin coating without phase separation. Compared to the PMMA or MMA‐co‐HEMA‐CH₂‐Hq, the T_g of the metaloquinolate‐containing polymers was much higher. It should be of interest for OLED applications. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1945–1952, 2006     

Catalytic activity of graphite-based nanofillers on cure reaction of epoxy resins

The strong influence of graphite oxide (GO) nanofiller on the glass transition temperature (T_g) of epoxy resins, generally attributed to restricted molecular mobility of the epoxy matrix by the nanofiller or to the crosslinking of GO layers via the epoxy chains, is investigated. The study confirms that large increases of the glass transition temperature of the nanocomposite can be observed in presence of GO. However, similar T_g increases are observed, when the filler is a high-surface-area graphite (HSAG), lacking oxidized groups. Moreover, these T_g differences tend to disappear as a consequence of aging or thermal annealing. These results suggest that the observed T_g increases are mainly due to a catalytic activity of graphitic layers on the crosslinking reaction between the epoxy resin components (epoxide oligomer and di-amine), rather than to reaction of the epoxide groups with functional groups of GO. This hypothesis is supported by investigating the catalytic activity of graphite-based materials on reactions between analogous monofunctional epoxide and amine compounds.     

Improvement of heat stability of poly(l-lactic acid) by radiation-induced crosslinking

Poly(l-lactic acid) (PLLA) has poor heat stability above its glass transition temperature (T_g∼60 °C). To improve its softing above T_g, PLLA was mixed with small amount of crosslinking agents and irradiated with various irradiation doses to introduce crosslinking between polymer chains. The most effective agent for radiation crosslinking was triallyl isocyanurate (TAIC). For melt-quenched PLLA, it was found that the most optimal conditions to introduce crosslinking were around 3% of TAIC and the irradiation dose of 50 kGy. The typically crosslinked PLLA showed very low crystallinity because of wide formation of molecular chain network that inhibited molecular motion for crystallization. Notable heat stability above T_g was given by annealing of PLLA samples. Enzymatic degradation of PLLA was retarded with introduction of crosslinks.     

Study of epoxy resin—filler interaction

A bisphenol-type epoxy resin cured with phthalic anhydride was filled with three types of SiO₂ (untreated, silanized and smaller particle size), CaCO₃, dolomite and Al₂O₃. Also, the concentration of silica was varied. Interaction of filler and resin was estimated by measuring the glass transition temperature (T_g) by different methods. Electron microscopy and x-ray diffraction were used to study the structure of the boundary layer and the bulk polymer. It was found that silica forms strong bonds with the resin and that basic fillers (CaCO₃, Al₂O₃) probably affect the crosslinking process with anhydride as hardener causing lowering of T_g.     

Bifunctional poly(ethylene glycol) based crosslinked network polymers as electrolytes for all‐solid‐state lithium ion batteries

Polymer electrolyte based lithium ion batteries represent a revolution in the battery community due to their intrinsic enhanced safety, and as a result polymer electrolytes have been proposed as a replacement for conventional liquid electrolytes. Herein, the preparation of a family of crosslinked network polymers as electrolytes via the ‘click‐chemistry’ technique involving thiol‐ene or thiol‐epoxy is reported. These network polymer electrolytes comprise bifunctional poly(ethylene glycol) as the lithium ion solvating polymer, pentaerythritol tetrakis (3‐mercaptopropionate) as the crosslinker and lithium bis(trifluoromethane)sulfonimide as the lithium salt. The crosslinked network polymer electrolytes obtained show low T_g, high ionic conductivity and a good lithium ion transference number (ca 0.56). In addition, the membrane demonstrated sterling mechanical robustness and high thermal stability. The advantages of the network polymer electrolytes in this study are their harmonious characteristics as solid electrolytes and the potential adaptability to improve performance by combining with inorganic fillers, ionic liquids or other materials. In addition, the simple formation of the network structures without high temperatures or light irradiation has enabled the practical large‐area fabrication and in situ fabrication on cathode electrodes. As a preliminary study, the prepared crosslinked network polymer materials were used as solid electrolytes in the elaboration of all‐solid‐state lithium metal battery prototypes with moderate charge–discharge profiles at different current densities leaving a good platform for further improvement. © 2018 Society of Chemical Industry     

Glass transitions of topologically interpenetrating polymer networks

Two component topologically-interpenetrating polymer networks were made of the SIN type (simultaneous interpenetrating network) composed of two polyurethanes (a polyether-based and a polyester-based) in combination with an epoxy resin, a polyacrylate and two unsaturated polyesters. The linear polymers and/or prepolymers were combined in solution and in bulk together with the necessary crosslinking agents and catalysts. Films were cast and chains extended and crosslinked in situ. All of the IPN's exhibited one glass transition (T_g) intermediate in temperature to the T_g's of the component networks, and as sharp as the T_g's of the components. This suggests that phase separation may not occur and thus some chain entanglement (interpenetration) of the two networks is involved. The observed T_g's are always several degrees lower than the arithmetic means of the component T_g's. A theory based on interpenetration is developed to account for this.     

Untersuchungen zum Einfluß der Polymermatrix auf die thermische Rückreaktion von photochromen 2,4-Dinitrobenzylpyridinen

The Influence of the Polymer Matrix on the Thermal Back Reaction of Photochromic 2,4-Dinitrobenzyl Pyridines In solid solution with polymers (polymethylmethacrylate, polyethyl acrylate and polystyrene) below T_g the thermal back reaction of the coloured forms of photochromic 2-(2′,4′-dinitrobenzyl)pyridines 1–3 can be described by second-order kinetics whereas above T_g (in polyethyl acrylate) the reaction is first order. The life time of coloured form depends on polymer structure and is longest in polar polymers of high molar mass. These results as well as the Δ^≠ and ΔH^≠ values are discussed applying the concept of reaction cavity to the solid solutions.     

Stabilized electrospinning of heat stimuli/in situ crosslinkable nanofibers and their self‐same nanocomposites

We present a strategy for stabilizing the morphological integrity of electrospun polymeric nanofibers by heat stimuli in situ crosslinking. Amorphous polymer nanofibers, such as polystyrene (PS) and its co‐polymers tend to lose their fiber morphology during processing at temperatures above their glass transition temperature (T_g) typically bound to happen in nanocomposite/structural composite applications. As an answer to this problem, incorporation of the crosslinking agents, phthalic anhydride (PA) and tributylamine (TBA), into the electrospinning polymer solution functionalized by glycidylmethacrylate (GMA) copolymerization, namely P(St‐co‐GMA), is demonstrated. Despite the presence of the crosslinker molecules, the electrospinning polymer solution is stable and its viscosity remains unaffected below 60 °C. Crosslinking reaction stands‐by and can be thermally stimulated during post‐processing of the electrospun P(St‐co‐GMA)/PA‐TBA fiber mat at intermediate temperatures (below the T_g). This strategy enables the preservation of the nanofiber morphology during subsequent high temperature processing. The crosslinking event leads to an increase in T_g of the base polymer by 30 °C depending on degree of crosslinking. Crosslinked nanofibers are able to maintain their nanofibrous morphology above the T_g and upon exposure to organic solvents. In situ crosslinking in epoxy matrix is also reported as an example of high temperature demanding application/processing. Finally, a self‐same fibrous nanocomposite is demonstrated by dual electrospinning of P(St‐co‐GMA) and stabilized P(St‐co‐GMA)/PA‐TBA, forming an intermingled nanofibrous mat, followed by a heating cycle. The product is a composite of crosslinked P(St‐co‐GMA)/PA‐TBA fibers fused by P(St‐co‐GMA) matrix. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44090.