Formation and Thermal Behavior of Polystyrene and Polystyrene/Clay Gels

DSC was used to study the thermal behavior of virgin PS as well as of PS/clay nanocomposites of exfoliated brush and of intercalated structure dissolved in carbon disulfide. Compared to virgin PS, the behavior of the brush material did not show any obvious difference, whereas the intercalated material demonstrated markedly larger heats of gelation and gel melting. Isoconversional kinetic analysis of the melting process revealed that the activation energy was independent of the conversion for the gels prepared under isothermal conditions, whereas an increase in the activation energy with the extent of conversion was observed for the gels prepared under continuous cooling conditions. The melting of the nonisothermally prepared gels apparently occurred alongside with the formation of new gel structures.

     

Structural Effect on Thermal Cure and Property of Naphthalene‐Based Epoxies: Preparation and Characterization

Summary: Three naphthalene‐based epoxy monomers containing different size of linkages between two naphthalene rings: C? C covalent bond ( 1 ), methylene ( 2 ), cycloaliphatic hydrocarbon derived from limonene ( 3 ) were prepared, and the effect of molecular structure on thermal cure behavior and property analyzed. Judged from the combination of the dynamic DSC and isothermal FTIR results, the cure reactivities were found to increase in the order of 1 > 2 > 3 when dicyandiamide was used in the curing systems, and the situation was slightly different in the monomer/4,4′‐methylenedianiline systems. The differences in reactivity as well as in thermal and mechanical properties for the epoxies can be attributed to the differences of configuration and conformational changes during the formation of crosslinked networks. These thermosets exhibited high glass transition temperature (T_g) and had excellent thermal oxidative resistance.

DSC thermograms for epoxy/dicyandiamide systems at a heating rate of 10 °C · min^?1.     

Crystallization Kinetics of PEO and PE in Different Triblock Terpolymers: Effect of Microdomain Geometry and Confinement

The isothermal crystallization kinetics of PEO and PE blocks within various triblock terpolymers were studied by DSC. The effect of the geometry of the microdomains was analyzed by studying different compositions of PE‐b‐PS‐b‐PEO triblock terpolymers. The crystallization rate decreased for decreasing block content for both crystallizable blocks. The effect of the microdomain geometry, confinement or chain tethering on the crystallization of PEO was extensively studied by comparing pairs of triblock terpolymers with differences either in the nature (crystalline, glassy, amorphous) or in the location of the other blocks in the terpolymer.

     

Differential Scanning Calorimetry (DSC) for Interdiffusion Studies in PVC/PnBMA Blend: A Quantitative Analysis

Differential scanning calorimetry (DSC) was used to study interdiffusion in an initially demixed blend, prepared as particle‐matrix system of the intrinsic miscible polymers poly(vinyl chloride) (PVC, particle, 30 wt.‐%) and poly(butyl methacrylate) (PnBMA, matrix, 70 wt.‐%) during isothermal annealing at 110 °C. A detailed interpretation of the experimental DSC curves was realised using a core‐shell model for the particle matrix system, a calculated concentration‐distance profile across the particle‐matrix interface, and a parameterised concentration dependent curve for the heat flow of mixed blend states derived from experiments. The calculated curves were fitted to the experimental data by variation of the concentration dependence of the diffusion coefficient and its concentration averaged value. Both parameters determine the concentration‐distance profile. From the evolution of the concentration‐distance profile with the annealing time the parameters of the interdiffusion kinetics were analysed. The growth of the mean width of the interfacial phase with time follows a power law with an exponent of k = 0.42. A coefficient of mutual diffusion, , increasing linearly by one order of magnitude from pure PVC to PnBMA, and its concentration averaged value of m²/s were concluded from the experiments.

Half width parameter \sigma _{{\rm av}}^* of the interphase for the initially phase‐separated blend PVC/PnBMA = 70/30 wt.‐% vs. the annealing time t_a at 110 °C.     

Crystallization in ABC Triblock Copolymers with Two Different Crystalline End Blocks: Influence of Confinement on Self‐Nucleation Behavior

The influence of different confinements active during crystallization within polybutadiene‐block‐polyisoprene‐block‐poly(ethylene oxide) (PB‐b‐PI‐b‐PEO) and the corresponding hydrogenated polyethylene‐block‐poly(ethylene‐alt‐propylene)‐block‐poly(ethylene oxide) (PE‐b‐PEP‐b‐PEO) triblock copolymers on the self‐nucleation behavior of the crystallizable PEO and PE blocks is investigated by means of differential scanning calorimetry (DSC). In triblock copolymers with PEO contents ≤ 20 wt.‐% crystallization of PEO is confined within small isolated microdomains (spheres or cylinders), and PEO crystallization takes place exclusively at high supercoolings. Self‐nucleation experiments reveal an anomalous behavior in comparison to the classical self‐nucleation behavior found in semicrystalline homopolymers. In these systems, domain II (exclusive self‐nucleation domain) vanishes, and self‐nucleation can only take place at lower temperatures in domain III_SA, when annealing is already active. The self‐nucleation behavior of the PE blocks is significantly different compared with that of the PEO blocks. Regardless of the low PE content (10–25 wt.‐%) in the investigated PE‐b‐PEP‐b‐PEO triblock copolymers a classical self‐nucleation behavior is observed, i.e., all three self‐nucleation domains, usually present in crystallizable homopolymers, can be located. This is a direct result of the small segmental interaction parameter of the PEP and PE segments in the melt. As a consequence, crystallization of PE occurs without confinement from a homogeneous mixture of PE and PEP segments.

Self‐nucleation regimes of a block copolymer showing confined crystallization by means of DSC.     

1H and 13C NMR Study of the Intermediates Formed by (Cp‐R)2ZrCl2 Activation with MAO and AlMe3/[CPh3][B(C6F5)4]. Correlation of Spectroscopic and Ethene Polymerization Data

Using ¹H and ¹³C NMR spectroscopy, cationic intermediates formed by activation of (Cp‐R)₂ZrCl₂ (R = nBu, tBu and 1,2,3‐Me₃) with MAO in toluene were monitored at Al/Zr ratios from 50 to 1 000. The catalysts (Cp‐R)₂ZrCl₂/AlMe₃/CPh₃⁺B(C₆F₅)₄^? (nBu, tBu and 1,2,3‐Me₃) were also studied for comparison of spectroscopic and polymerization data with MAO based systems. Complexes of type (Cp‐R)₂ZrMe⁺ ← Me^?‐Al?MAO ( IV ) with different Me‐MAO^? counter anions have been identified in the (Cp‐R)₂ZrCl₂/MAO systems at low Al/Zr ratios. At Al/Zr ratios of 200–1 000, the complex [(Cp‐R)₂Zr(μ‐Me)₂AlMe₂]⁺ Me‐MAO^? ( III ) dominates in all MAO‐based reaction systems. Ethene polymerization activity strongly depends on the Al/Zr ratio (Al/Zr = 200–1 000) for the systems (Cp‐nBu)₂ZrCl₂/MAO and (Cp‐tBu)₂ZrCl₂/MAO, while it is virtually constant in the same range of Al/Zr ratios for the catalytic system (Cp‐1,2,3‐Me₃)₂ZrCl₂/MAO. The data obtained are interpreted on the assumption that complex III is the actual precursor of active centers of polymerization in MAO based systems.

Formation of cationic intermediates by activation with MAO.     

Multiple Melting Behavior of Poly(thiodiethylene terephthalate): Further Investigations by Means of X‐ray and Thermal Techniques

Summary: The complex multiple melting behavior of poly(thiodiethylene terephthalate) (PSDET) was investigated by means of differential scanning calorimetry, X‐ray diffraction and hot‐stage optical microscopy. The phenomenon was ascribed to the existence of two different crystal structures (α and β), each of them being able to melt and recrystallize under the experimental conditions adopted. Linear and nonlinear treatments were applied in order to estimate the equilibrium melting temperature for α PSDET, by using the corrected values of T_m. The nonlinear estimation led to a higher value by about 25 °C. Isothermal crystallization kinetics of each crystalline form were analyzed according to Avrami's treatment. In both cases, values of Avrami's exponent, n, close to 3 were obtained, in agreement with a crystallization process originating from predeterminated nuclei and characterized by three dimensional spherulitic growth. The rate of crystallization of each crystalline structure was found to become lower as the crystallization temperature is increased, as usual at low undercooling, where the crystallization process is controlled by nucleation.

XRD patterns of PSDET samples isothermally crystallized at the indicated T_cs.     

A 13C NMR Study of the Products and Mechanism of the Thermal Oxidative Degradation of Poly(ethylene oxide)

The products and mechanism of the thermal oxidative degradation of poly(ethylene oxide) at 150 °C have been analysed using ¹³C NMR spectroscopy. The analysis was assisted by the use of distortionless enhancement by polarisation transfer spectra, longitudinal relaxation time measurements, long-range ¹³C ¹H coupling and chemical shift simulation software. The major result of degradation was chain scission resulting in two formate ester chain ends. Minor products were in-chain esters, peroxy groups, oxymethylene links and hydroxy and methoxy chain ends.

Initial steps for the mechanism of chain scission.     

Synthesis of Poly(ester amide)s Derived from Glycolic Acid and the Amino Acids: β‐Alanine or 4‐Aminobutyric Acid

The polymerization of metal salts of N‐chloroacetyl‐β‐alanine and N‐chloroacetyl‐4‐aminobutyric acid was investigated. The former gives a mixture of polymer and a seven‐membered cyclic compound constituted of glycolic and β‐alanine units, and its reaction proceeds in the solid state. However, liquefaction is observed in the second case giving rise to a polymer with a moderate molecular weight. Condensation kinetics of both sodium and silver salts of N‐chloroacetyl‐β‐alanine have been studied by differential scanning calorimetry. Copolymers of glycolic acid and β‐alanine with a molar ratio of glycolic acid/β‐alanine varying from 0.5 to 1.0 have been synthesized by thermal reaction of co‐precipitated crystals of the sodium salts of chloroacetic acid and N‐chloroacetyl‐β‐alanine. NMR spectroscopy indicates that copolymers tend to have a random distribution. The resulting new poly(ester amide)s have been characterized by spectroscopy and thermal analysis.

DSC heating runs corresponding to different mixtures of the sodium salts of chloroacetic acid and chloroacetyl‐β‐alanine.     

Thermal and Morphological Behaviour of Well‐Defined Amphiphilic Triblock Copolymers Based on Cyclohexyl and Di(ethylene glycol) Methyl Ether Methacrylates

The microphase separation and morphology of PDEG‐b‐PCH‐b‐PDEG and PCH‐b‐PDEG‐b‐PCH amphiphilic triblock copolymers have been studied by DSC, SAXS and AFM. A clear first‐order scattering peak was observed for most of the triblock copolymers, independent of the macroinitiator used. This diffraction has been ascribed to the development of a lamellar structure, which was confirmed by AFM. On the other hand, the existence of an ODT upon heating was observed for most of the triblock copolymers. The ODT location was dependent on the outer segment molecular weights, shifting at higher temperatures as the polymerisation degree increased. WAXS profiles were checked to determine the glass transition and ODT temperatures.

     

Reaction Behavior and Network Development in RAFT Radical Polymerization of Dimethacrylates

The reaction behavior and development of network microstructure in the RAFT polymerization of OEGDMA were investigated. The polymerization rate of the RAFT system was much lower than in conventional FRP, largely due to the low propagating‐radical concentration determined by the addition‐fragmentation equilibrium. A mild autoacceleration occurred as the addition reaction became diffusion‐controlled. The slow chain growth in the RAFT allowed sufficient chain relaxation and a uniform distribution of reacting species. The RAFT polymerization of OEGDMA with longer spacers yielded more homogeneous networks with a lower crosslinking density and lower glass transition temperature than the FRP.

     

Elastomeric Poly(propylene) from “Dual‐side” Metallocenes: Reversible Chain Transfer and its Influence on Polymer Microstructure

Summary: The influence of the cocatalyst nature on the distribution of the stereoerrors along the polymer chain has been studied using either MAO or [(C₆H₅)₃C⁺] [(C₆F₅)₄B^?] to activate a C₁‐symmetric (Flu‐Ind) complex in propene polymerization experiments. The in situ activation with borate indicated the chain back‐skip as the decisive mechanism responsible for stereoerror formation. When MAO is used for activation, additionally the reversible chain transfer to aluminum occurs, which can be called into account as a second mechanism for stereoerror formation. By the combination of ¹³C NMR, DSC, WAXS and SFM, it was shown that the differences in polymerization mechanisms result in variations of stereoerror formation. Due to this, the isotactic block length n_iso as well as their distribution along the chain changes. Using MAO activation, polypropenes with crystallizable blocks consisting of 23–32 monomers in isotactic sequences were generated, which co‐crystallized in α‐ and γ‐phase lamellae. When the reversible chain transfer was occluded (in situ borate activation) the bimodal distribution of crystalline lamellae strongly referred to a homogeneous random distribution of stereoerrors. In this case, two crystalline populations were present. The prevailing one, which crystallized in the orthorhombic γ‐modification, contained 23 consecutive isotactic blocks. Additionally, small amounts of α‐phase lamellae were present consisting of longer isotactic blocks (n_iso > 35). The different crystalline modifications resulted in different polymer morphologies. These changes caused in turn variations in the mechanical properties, such as elasticity and mechanical strength. This clearly shows that, by using different cocatalysts for activating C₁‐symmetric complexes, the properties of poly(propylenes) with statistically distributed stereoerrors can be tailored.

     

Control of Molecular Weight and Tacticity in Stereospecific Living Cationic Polymerization of α‐Methylstyrene at 0 °C Using FeCl3‐Based Initiators: Effect of Tacticity on Thermal Properties

The successful stereospecific living cationic polymerization of α‐methylstyrene (α‐MeSt) using FeCl₃‐based initiator systems at 0 °C is reported. Linear first‐order ln([M]₀/[M]) vs. time and linear molecular weight vs. conversion plots suggest that the polymerization is living in nature, which is further confirmed from successful chain‐extension experiments. Poly(α‐methylstyrene)s of varying syndiotacticities (59.1% to 79.2%) and controllable molecular weights (4300–32 100 g mol^?1) with moderately narrow polydispersity indices (PDIs ≈1.3) are synthesized simply by varying the monomer‐to‐initiator ratio ([M]₀/[I]₀). A possible mechanism for this stereospecific polymerization is proposed. The glass‐transition temperature and thermal‐decomposition temperature depend on the syndiotacticity of poly(α‐methylstyrene).

     

ABA and BAB Triblock Copolymers Based on 2‐Methyl‐2‐oxazoline and 2‐n‐Propyl‐2‐oxazoline: Synthesis and Thermoresponsive Behavior in Water

Inspired by the well‐known amphiphilic block copolymer platform known as Pluronics or poloxamers, a small library of ABA and BAB triblock copolymers comprising hydrophilic 2‐methyl‐2‐oxazoline (A) and thermoresponsive 2‐n‐propyl‐2‐oxazoline (B) is synthesized. These novel copolymers exhibit temperature‐induced self‐assembly in aqueous solution. The formation and size of aggregates depend on the polymer structure, temperature, and concentration. The BAB copolymers tend to agglomerate in water, with the cloud point temperature depending on the length of poly(2‐n‐propyl‐2‐oxazoline) chain. On the other hand, ABA copolymers form smaller aggregates with hydrodynamic radius from 25 to 150 nm. The dependence of viscosity and viscoelastic properties on the temperature is also studied. While several Pluronic block copolymers are known to form thermoreversible hydrogels in the concentration range 20–30 wt%, thermogelation is not observed for any of the investigated poly(2‐oxazoline)s at the investigated temperature range from 10 to 50 °C.